CA1309259C - Hydrometallurgical arsenopyrite process - Google Patents
Hydrometallurgical arsenopyrite processInfo
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- CA1309259C CA1309259C CA 538019 CA538019A CA1309259C CA 1309259 C CA1309259 C CA 1309259C CA 538019 CA538019 CA 538019 CA 538019 A CA538019 A CA 538019A CA 1309259 C CA1309259 C CA 1309259C
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- liquid
- nitrogen species
- oxidized nitrogen
- denitrating
- solid
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- Y—GENERAL TAGGING OF NEW TECHNOLOGICAL DEVELOPMENTS; GENERAL TAGGING OF CROSS-SECTIONAL TECHNOLOGIES SPANNING OVER SEVERAL SECTIONS OF THE IPC; TECHNICAL SUBJECTS COVERED BY FORMER USPC CROSS-REFERENCE ART COLLECTIONS [XRACs] AND DIGESTS
- Y02—TECHNOLOGIES OR APPLICATIONS FOR MITIGATION OR ADAPTATION AGAINST CLIMATE CHANGE
- Y02P—CLIMATE CHANGE MITIGATION TECHNOLOGIES IN THE PRODUCTION OR PROCESSING OF GOODS
- Y02P10/00—Technologies related to metal processing
- Y02P10/20—Recycling
Abstract
IMPROVED HYDROMETALLURGICAL ARSENOPYRITE PROCESS
ABSTRACT
This invention is directed to an improved process for leach treating gold and silver bearing pyritic and arsenopyritic concentrates and ores. More particularly, the improved process avoids the necessity of adding recycled neutralized solution to the leach solution, thereby alleviating difficulties in maintain-ing acid levels in the leach solution, and provides for bleeding solutions containing dissolved arsenic, iron and sulphate from the process without the loss of oxidized nitrogen species. The process for recovering valuable metals from pyritic and arsenopyritic concen-trates and ores involves decomposing the arsenopyrite or pyrite concentrates and ores in acidic solution in a common volume space which contains a gas phase and a liquid slurry (which comprises a liquid phase and a solid phase) through the action of higher valence oxidized nitrogen species in which the nitrogen has a valence of at least plus 3. The active oxidized nitro-gen species are regenerated in the same common volume space by an oxygen containing gas. The concentrate or ore is introduced into a denitrating step. The liquid and solid products of the denitrating step are subjected to a solid-liquid separation, after which the solids from the separation step are transported to a leaching vessel where the solids are treated with oxygen and nitric oxide replaced from the denitrating step.
ABSTRACT
This invention is directed to an improved process for leach treating gold and silver bearing pyritic and arsenopyritic concentrates and ores. More particularly, the improved process avoids the necessity of adding recycled neutralized solution to the leach solution, thereby alleviating difficulties in maintain-ing acid levels in the leach solution, and provides for bleeding solutions containing dissolved arsenic, iron and sulphate from the process without the loss of oxidized nitrogen species. The process for recovering valuable metals from pyritic and arsenopyritic concen-trates and ores involves decomposing the arsenopyrite or pyrite concentrates and ores in acidic solution in a common volume space which contains a gas phase and a liquid slurry (which comprises a liquid phase and a solid phase) through the action of higher valence oxidized nitrogen species in which the nitrogen has a valence of at least plus 3. The active oxidized nitro-gen species are regenerated in the same common volume space by an oxygen containing gas. The concentrate or ore is introduced into a denitrating step. The liquid and solid products of the denitrating step are subjected to a solid-liquid separation, after which the solids from the separation step are transported to a leaching vessel where the solids are treated with oxygen and nitric oxide replaced from the denitrating step.
Description
`` ~h3~ 5~3 IMPROVED HYDROMETALLURGICAL ARSENOPYRITE PROCESS
FIELD OF THE INVENTION
This invention is directed to an improved process for leach treating gold and silver bearing pyritic and arsenopyritic concentrates and oxes. More particularly, the improved process avoids the necessity of adding re-cycled neutralized solution to the leach solution, thereby alleviating difficulties in maintaining acid levels in the leach solution and provides for bleeding solutions contain-ing dissolved arsenic, iron and sulphate from the process without the loss of oxidized nitrogen species.
BACKGROUND OF THE INVENTION
In our copending application Serial No. 797,838, filed November 14, 1985, which has matured as U.S. patent No. 4,647,307, March 3, 1987, we disclose a hydro-metallurgical process for the recovery of pr~cious metalfrom a concentrate or ore containing at least some arseno-pyrite or pyrite wherein at least some of the precious metal is occluded in the arsenopyrite or pyrite. The hydrometallurgical process involves forming in a common ~olume space a gas phase and a liquid slurry which com-prises the ore or concentrate as the solid phase and acid and water as the liquid phase of the slurry. An oxidation-reduction reaction having a standard potential between about 0.90 and about 1.20 volts on the hydrogen scale is effected in the slurry between the arsenopyrite or pyrite and an oxidized nitrogen species in which the nitrogen has a valence of at least plus 3. This solubilizes in the liquid phase the arsenic, iron and sulphur in the arseno-pyrite, or the iron and sulphur in the pyrite, all as the oxidation products, and produces in the liquid phase nitric oxide in which the nitrogen has a valence of plus 2, as the , ~3~
reduction product. At least part of the nitric oxide from -the liquid phase is released into the gas phase.
The nitric oxide in the gas phase, in which a signifi-cant oxygen partial pressure is maintained by continuous addition of an oxygen containing gas, is oxidized to form an oxidized nitrogen species in which the nitrogen has a valence of at least plus 3. The total amount of oxygen that is added is at least in an amount stoichio-metrically required for solubilization in the liquid phase of the arsenic, iron and sulphur in the arseno-pyrite, or the iron and sulphur in the pyrite. The oxidized nitrogen species is absorbed into the slurry wherein the oxidized nitrogen species become available for the oxidation-reduction reaction as described. In this way, the nitrogen, in its oxide forms, functions as a catalyst for the transport of oxygen from the gas phase to the oxidation-reduction reactions in the slurry. This permits the total of the oxidized nitrogen species and nitric oxide in the system to be substan-tially less than a stoichiometric balance that would berequired for the oxidation of the arsenic, iron and sulphur. The slurry is then subjected to a solid-liquid separation to produce a solid residue and a liquid fraction. Precious metal is recovered from the solid residue.
While the process described has a number of unique features and important advantages, which are fully discussed in the co-pending application, the process has three minor shortcomings which are briefly described below:
1. A bleed solution must be utilized in order to remove impurities which build up in the solution~
Unfortunately, this bleed solution removes not only impurities but also valuable catalyst from the process.
FIELD OF THE INVENTION
This invention is directed to an improved process for leach treating gold and silver bearing pyritic and arsenopyritic concentrates and oxes. More particularly, the improved process avoids the necessity of adding re-cycled neutralized solution to the leach solution, thereby alleviating difficulties in maintaining acid levels in the leach solution and provides for bleeding solutions contain-ing dissolved arsenic, iron and sulphate from the process without the loss of oxidized nitrogen species.
BACKGROUND OF THE INVENTION
In our copending application Serial No. 797,838, filed November 14, 1985, which has matured as U.S. patent No. 4,647,307, March 3, 1987, we disclose a hydro-metallurgical process for the recovery of pr~cious metalfrom a concentrate or ore containing at least some arseno-pyrite or pyrite wherein at least some of the precious metal is occluded in the arsenopyrite or pyrite. The hydrometallurgical process involves forming in a common ~olume space a gas phase and a liquid slurry which com-prises the ore or concentrate as the solid phase and acid and water as the liquid phase of the slurry. An oxidation-reduction reaction having a standard potential between about 0.90 and about 1.20 volts on the hydrogen scale is effected in the slurry between the arsenopyrite or pyrite and an oxidized nitrogen species in which the nitrogen has a valence of at least plus 3. This solubilizes in the liquid phase the arsenic, iron and sulphur in the arseno-pyrite, or the iron and sulphur in the pyrite, all as the oxidation products, and produces in the liquid phase nitric oxide in which the nitrogen has a valence of plus 2, as the , ~3~
reduction product. At least part of the nitric oxide from -the liquid phase is released into the gas phase.
The nitric oxide in the gas phase, in which a signifi-cant oxygen partial pressure is maintained by continuous addition of an oxygen containing gas, is oxidized to form an oxidized nitrogen species in which the nitrogen has a valence of at least plus 3. The total amount of oxygen that is added is at least in an amount stoichio-metrically required for solubilization in the liquid phase of the arsenic, iron and sulphur in the arseno-pyrite, or the iron and sulphur in the pyrite. The oxidized nitrogen species is absorbed into the slurry wherein the oxidized nitrogen species become available for the oxidation-reduction reaction as described. In this way, the nitrogen, in its oxide forms, functions as a catalyst for the transport of oxygen from the gas phase to the oxidation-reduction reactions in the slurry. This permits the total of the oxidized nitrogen species and nitric oxide in the system to be substan-tially less than a stoichiometric balance that would berequired for the oxidation of the arsenic, iron and sulphur. The slurry is then subjected to a solid-liquid separation to produce a solid residue and a liquid fraction. Precious metal is recovered from the solid residue.
While the process described has a number of unique features and important advantages, which are fully discussed in the co-pending application, the process has three minor shortcomings which are briefly described below:
1. A bleed solution must be utilized in order to remove impurities which build up in the solution~
Unfortunately, this bleed solution removes not only impurities but also valuable catalyst from the process.
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2. The recycle solution in the process is satura-ted in dissolved calcium (calcium nitrate). When this solution is recycled to the leach, and leaching proceeds, the sulphate which is formed in solution tends to combine with the calcium from the recycled solution to form calcium sulphate. Such calcium sulphate in-creases the weight of solids leaving the reaction auto-clave and thereby increases the burden on the downstream cyanide circuit. The tendency to form calcium sulphate in solution causes the build-up of this material on heat exchanger surfaces, which reduces process efficiency over time.
2. The recycle solution in the process is satura-ted in dissolved calcium (calcium nitrate). When this solution is recycled to the leach, and leaching proceeds, the sulphate which is formed in solution tends to combine with the calcium from the recycled solution to form calcium sulphate. Such calcium sulphate in-creases the weight of solids leaving the reaction auto-clave and thereby increases the burden on the downstream cyanide circuit. The tendency to form calcium sulphate in solution causes the build-up of this material on heat exchanger surfaces, which reduces process efficiency over time.
3. Since the recycle solution is neutral (with regard to pH) the introduction of this recycle stream into the leach can cause difficulties in main-taining the leach solution at a sufficient acid concen-tration.
SU~ARY OF THE INVENTION
The invention is directed to an improved hydrometallurgical process for recovering valuable metals from pyritic and arsenopyritic concentrates and ores involving decomposing the arsenopyrite or pyrite concentrates and ores in acidic solution in a common ~5 volume space which contains a gas phase and a liquid slurry (which comprises a liquid phase and a solid phase) through the action of higher valence oxidized nitrogen species in which the nitrogen has a valence of at least plus 3. The active oxidized nitrogen species is regenerated in the same common volume space by an ~; oxygen containing gas. The improvement comprises intro-ducing the concentrate or ore into a denitrating step, ` along with the solution from the leach. The oxidized nitrogen species present in the leach solution react with the concentrate or ore to produce nitric oxide , i which is released into the gas phase. The liquid and solid products of the denitrating step are then sub-jected to a solid-liquid separation. The solids from the separation are transported to a leaching vessel where the solids are treated with oxygen and nitric oxide taken from the denitrating step. The liquid is a bleed to the process.
~ pecifically, the process which is intended for the recovery of precious metal from an ore or con-centrate containing at least some arsenopyrite orpyrite, wherein at least some o~ the precious metal is occluded in arsenopyrite or pyrite, comprises: (a) introducing pyrite or arsenopyrite concentrate or ore into a denltrating vessel where it is reacted with a recycled solution which is derived from the process - after gold and silver has been precipitated, (b) contin-uously removing nitric oxide gas which is generated in the gas phase; (c) continuously withdrawing the treated concentrate or ore ~rom the denitrating vessel and subjecting it to a solid-liquid separation; (d) trans-; porting the solids from the solid-liquid separation to a leaching vessel; (e~ forming in a common volume space a gas phase and a liquid slurry comprising the ore or concentrate as the solid phase, acid and water as the liquid phase oF the slurry and nitric oxide gas from step (b) above and oxygen as the gas phase; (f) ef~ect-ing in the slurry between the arsenopyrite or pyrite and an oxidized nitrogen species in which the nitrogen has a valence of at least plus 3 an oxidation-reduction reac-tion having a standard potential between about 0.90 andabout 1.20 volts on the hydrogen scale, thereby solu-bilizing in the liquid phase the arsenic, iron and sulphur in the arsenopyrite, or the iron and sulphur in the pyrite, all as the oxidation products, and producing in the liquid phase nitric oxide in which the nitrogen -~3~ 9 has a valence of plus 2, as the reduction product; (g) releasing at least part of the nitric oxide generated in the liquid phase into the gas phase; (h) oxidizing the nitric oxide in the gas phase, in which a significant oxygen partial pressure is maintained by continuous addition of an oxygen containing gas, to form an oxi-dized nitrogen species in which the nitrogen has a valence of at least plus 3, the total amount of oxygen added being at least in an amount stoichiometrically required for solubilization in the liquid phase of the arsenic, iron and sulphur in the arsenopyrite or the iron and sulphur in the pyrite; (i) absorbing the oxidized nitrogen species into the slurry wherein the oxidized nitrogen species become available for the oxidation-reduction reaction of step (f) above whereby the nitrogen, in its oxide forms, functions as a cata-lyst for the transport of oxygen from the gas phase to ; the oxidation-reduction reactions in the slurry, thereby permitting the total of the oxidized nitrogen species and nitric oxide in the system to be substantially less than a stoichiometric balance required for the oxidation of the arsenic, iron and sulphur; (j) subjecting the slurry to a solid-liquid separation to produce a solid residue and a liquid fraction; and (k) recovering precious metal from the solid residue.
~ ;
DRAWINGS
In the drawing which discloses a specific embodiment of the improved process:
Figure l illustrates a graphic flowsheet of the improved process of the invention.
i9 DETAILED DESCRIPTION OF A SPECIFIC
-EMBODIMENT OF THE INVENTION
The improved hydrometallurgical process is basically intended for the recovery of precious metal from an ore or concentrate containing arsenopyrite or pyrite wherein at least some of the precious metal is occluded in the arsenopyrite or pyrite. The improvement comprises first introducing the concentrate or ore into a denitrating step, along with the solution from the - 10 leach. The oxidized nitrogen species present in the leach solution react with the concentrate or ore to produce nitric oxide which is released into the gas phase. The liquid and solid products of the denitrating step are subjected to a liquid-solid separation. The resultant solids from the separation are introduced into the leaching vessel where the solids are treated with oxygen and nitric oxide Erom the denitrating step. -~
A gas phase and a liquid slurry are formed in the leaching vessel. The liquid is a bleed to the process. The slurry is comprised o-E -the ore or concen-trate as a solid phase and acid and water as a liquid phase. An oxidation-reduction reaction having a ~ standard potential between about 0.90 and about 1.20 ; ~ volts on the hydrogen scale is effected in the slurry between the arsenopyrite or pyrite and an oxidized nitrogen species in which the nitrogen has a valence of at least plus 3. Arsenic, iron and sulphur in the arsenopyrite, or iron and sulphur in the pyrite, are solubilized in the liquid phase as oxidation products.
Nitric oxide in which the nitrogen has a valence of plus 2 is produced as a reduction product in the liquid phase. At least part of the nitric oxide is released from the liquid phase into the gas phase. The nitric oxide in the gas phase, in which a significant oxygen partial pressure is maintained by the continuous addi-::
~ - 6 -' ' , , ~' ' ,' ' ~ ` .
- ' ' ' .. . .
`
. - ~ .,, . ., .. , ,,.
~3~59 tion of an oxygen containing gas, is oxidized to form an oxidized nitrogen species in which the nitrogen has a valence of at least plus 3. The total amount of oxygen added is at least in an amount that is stoichiometri-cally required for solubilization in the liquid phase ofthe arsenic, iron and sulphur in the arsenopyrite, or the iron and sulphur in the pyrite. The oxidized nitrogen species are absorbed into the slurry wherein the oxidized nitrogen species become available for the oxidation-reduction reaction. The nitrogen, in its oxide forms, functions as a catalyst for the transport of oxygen from the gas phase to the oxidation-reduction reactions in the slurry. This permits the total of the oxidized nitrogen species and nitric oxide in the system to be substantially less than a stoichiometric balance required for the oxidation of the arsenic, iron and sulphur. The slurry is removed from the common volume space and is subjected to a solid-liquid separation to produce a solid residue and a liquid fraction. Precious metal is recovered from the solid residue.
The arsenopyrite and pyrite are decomposed by the oxidation-reduction reaction in acid solutions in the slurry where the pH is less than about 1.0 to about 3 by the action of oxidized nitrogen species where the nitrogen has a valence of plus 3 or greater. These oxidized nitrogen species include nitrous acid and ; nitrogen dioxide. The oxidized nitrogen species are present in sufficient concentration in the liquid fraction (typically about 0.25 Molar (M~ to about 4.0 Molar (M), calculated on a nitric acid basis) to provide an adequate rate of dissolution (typically within a residence time of about 2 minutes to about 60 minutes) at the reaction temperature used (typically about 60C to about 119C for arsenopyrite concentrate and about 60C to about 180C for pyrite concentrate or '' ore). Normally, the lower oxidized nitrogen species concentrations and longer residence times are used when treating ore while the higher oxidized nitrogen species concentrations and shorter residence times are used when treating concentrates.
The main products from the oxidation-reduction reaction are soluble ferric iron species, soluble arsen-ate species, soluble sulp~ate species, minor amounts of elemental sulphur and nitric oxide.
Insoluble gangue minerals and elemental sulfur remain as solids in the slurry. The slurry is subjected to a solid-liquid separation to yield a solid residue and a liquid fraction. A major portion of the gold or other precious metal contained in the concentrate or ore remains in the solid residue. Some of the gold or other precious metal appears in the leach solution, and can be recovered with activated carbon.
Almost all of the silver present in the con-centrate will usually remain in the liquid fraction.
The silver can be recovered from the liquid fraction by using a thiocyanate compound such as sodium thiocyanate, potassium thiocyanate, or ammonium thiocyanate. Arsenic and iron can be optionally removed from the silver- Eree separated liquid fraction by elevating the temperature ~5 to precipitate ferric arsenate. In the case of pyrite, iron can be removed from the liquid fraction.
In general terms, the process of the invention can be operated at a standard potential between the arsenopyrite or pyrite and the oxidized nitrogen species of about 0.90 volts and about 1.20 volts on the hydrogen scale. At potentials below about 0.9 volts, arseno-pyrite or pyrite will not decompose efficiently. At potentials above about 1.2 volts, no significant oxida-; tion of the nitrogen species will take place because ~, 5;9 oxygen per se has a potential of about 1.23 volts on thehydrogen scale.
On the standard oxidation-reduction potential scale, the reduction of nitrous acid to nitric oxide has a standard potential of about 0.996 volts. The reduc--tion of nitrate to nitrous acid has a sta~dard potential of about 0.94 volts. Thus the former couple has a higher driving force than the latter in decomposing sulphide minerals such as arsenopyrite and pyrite.
Preferably, the process of the invention is operated at a potential greater than about 0.94 volts up to about 1.0 volts on the hydrogen scale.
The process can typically be conducted within a residence time range of about 2 minutes to about 60 minutes calculated on a plug flow basis. A process which is completed in a time less than about 2 minutes is difficult to control and basically impractical. On the other hand, a process which takes more than about 60 minutes to complete is too slow and thus uneconomical.
The process has been conducted experimentally at initial temperatures from above the freezing point of the slurry to temperatures of several hundred degrees ~; Celsius. However, temperatures falling in the range of about 60C to about 180C are preferred for economical reasons. Similarly, the process has been conducted at pH ranges of less than about 1.0 to as high as about ; 3Ø In situations where silver is not present, and the formation of basic iron sulphate or jarosite can be tolerated in the process, the process can be conducted at a pH of about 3Ø However, silver is usually present and therefore it is preferable to operate the process at lower pH ranges. Typically, a pH of about 1.0 or below is preferred because it is desirable to keep the iron and arsenic in solution. Also, the pro-:
_ g _ cess is more rapid and economical at a pH range of lessthan about 1Ø
In the process, the oxidized nitrogen species in a sense act as a transporter of oxygen. The basic process may be regarded as an oxygen leach rather than an oxidized nitrogen species or nitric acid leach. The oxidized nitrogen species serves as a carrier for the oxygen as the oxidized nitrogen species is cycled between the gas phase and the liquid phase o the slurry of the common volume space. It follows that the rate at which the reaction proceeds is proportional to the number of oxidized nitrogen species carriers that are in the process.
Sufficient oxygen must be supplied to the leaching vessel in order to completely decompose the arsenopyrite and pyrite in the slurry. Nitric oxide derived from the denitrating step is also introduced into the leach vessel. If insufficient oxygen is sup-plied, then the pressure of the nitric oxide generated increases and ultimately the reaction stops because there are no oxidized nitrogen species left in the liquid phase of the slurry.
In order to overcome the limitations of uti-lizing a bleed which removes valuable catalyst from the process, and introducing into the leach vessel a neutral recycled solution which promotes calcium sulphate forma-tion and causes difficulties in maintaining the leach solution at appropriate acid levels, while at the same time retaining and utilizing all of the unique and advantageous features of our basic arsenopyrite and pyrite treating process, we have developed a variation of the basic process which does not introduce the concentrate in the leach. The improved process is described in association with the flow sheet which is illustrated in Figure 1.
: ' ' ' ' .
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In the process as illustrated in the flowsheet o~ Figure 1, the concentrate is introduced into a pre-liminary denitrating stage along with a recycled solu-tion which is derived from the leach after gold and silver precipitation. The liquid and solids of the denitrating stage are subjected to a solid-liquid separation. The solids are transported to the leach vessel. Nitric oxide from the denitratil~g stage is also introduced into the leach vessel. The nitrate in the leach solution reacts with the arsenopyrite and pyrite concentrate according to the following general process criteria and leaching reactions:
A. Mineral Oxidations (1) Fe AsS -~ Fe+3(aq) + 1/2As2S2 + 3e ~2) FeAsS + 3H20 -~ Fe+3 + H3As03 + SO
+ 3H+ + 6e (3) FeAsS + 4H20 -~ Fe+3 + H3AsO4 -~ SO
+ 5H+ + 8e (4) FeAsS + 8H20 -~ Fe+3 + H3As04 + S04=
+ 13H~ + 14e (5) FeS2 -~ Fe+3(aq) + 2SO + 3e (6) FeS2 + 8H20 -~ Fe+3 + 2S04= + 16H+
+ 15e B. Oxidized Nitrogen Species Reduction (7) HN03 + 3H~ + 3e -~ NO(g) + 2H20 (8) HN02 ~ H+ + e -3 NO(g) + H20 :
In the oxidation of arsenopyrite, it has been found that 60-90% of the mineral's sulphur is converted to soluble sulphate species. In the oxidation of pyrite, the degree of conversion is 80-100%.
Equations A(l) and A(5) will, in principle, take place at potentials above about 0.6 volts on the hydrogen scale; however, since 1/~ (~s2S2) on arsenopyrite and 2S0 on pyrite have molar volumes larger than FeAsS and FeS2 respectively, the first submicroscopic layers of these leach products protect the mineral from further oxidation, and no substantial : reaction is observed. At potentials above about 0.94 volts on the hydrogen scale, reactions A(4) and A(6) taXe place, and the protective layers of As2S2 and S0 are eliminated by oxidation.
In the denitrating step, reaction B(7) absorbs electrons at a standard potential of 0.94 volts on the hydrogen scale, just barely adequate to remove electrons from arsenopyrite and pyrite to drive reactions A(4) and :~
A(6) at a feasible rate (as in Queneau). Reaction B(8), which takes place in the leach process, absorbs elect-: rons a~ a standard potential of 0.996 volts on the hydrogen scale, which is high enough to drive reactions A(4) and A(6) rapidly at temperatures as low as 60C.
The active nitrogen oxides are required only to act as a sink for electrons which are released by decomposition of the minerals in the concentrate or ore.
The oxidized nitrogen species should be present in sufficient concentration in the solution (typically about 0.25 M to about 3.0 M or 4.0 M) to provide an :~ adequate rate of dissolution (typically within a residence time of about 2 minutes to about 60 minutes) at the reaction temperature used (typically about 60C
to about 119C for arsenopyrite concentrate, and about 60C to about 180DC for pyrite concentrate or ore).
:
.:
:- :
: .
' Sulphuric acid may be used to form the soluble ferric iron species and under certain circumstances is produced in situ.
In the following reaction, nitrous acid is the decomposition agent for arsenopyrite with sulphuric acid ~- present.
SU~ARY OF THE INVENTION
The invention is directed to an improved hydrometallurgical process for recovering valuable metals from pyritic and arsenopyritic concentrates and ores involving decomposing the arsenopyrite or pyrite concentrates and ores in acidic solution in a common ~5 volume space which contains a gas phase and a liquid slurry (which comprises a liquid phase and a solid phase) through the action of higher valence oxidized nitrogen species in which the nitrogen has a valence of at least plus 3. The active oxidized nitrogen species is regenerated in the same common volume space by an ~; oxygen containing gas. The improvement comprises intro-ducing the concentrate or ore into a denitrating step, ` along with the solution from the leach. The oxidized nitrogen species present in the leach solution react with the concentrate or ore to produce nitric oxide , i which is released into the gas phase. The liquid and solid products of the denitrating step are then sub-jected to a solid-liquid separation. The solids from the separation are transported to a leaching vessel where the solids are treated with oxygen and nitric oxide taken from the denitrating step. The liquid is a bleed to the process.
~ pecifically, the process which is intended for the recovery of precious metal from an ore or con-centrate containing at least some arsenopyrite orpyrite, wherein at least some o~ the precious metal is occluded in arsenopyrite or pyrite, comprises: (a) introducing pyrite or arsenopyrite concentrate or ore into a denltrating vessel where it is reacted with a recycled solution which is derived from the process - after gold and silver has been precipitated, (b) contin-uously removing nitric oxide gas which is generated in the gas phase; (c) continuously withdrawing the treated concentrate or ore ~rom the denitrating vessel and subjecting it to a solid-liquid separation; (d) trans-; porting the solids from the solid-liquid separation to a leaching vessel; (e~ forming in a common volume space a gas phase and a liquid slurry comprising the ore or concentrate as the solid phase, acid and water as the liquid phase oF the slurry and nitric oxide gas from step (b) above and oxygen as the gas phase; (f) ef~ect-ing in the slurry between the arsenopyrite or pyrite and an oxidized nitrogen species in which the nitrogen has a valence of at least plus 3 an oxidation-reduction reac-tion having a standard potential between about 0.90 andabout 1.20 volts on the hydrogen scale, thereby solu-bilizing in the liquid phase the arsenic, iron and sulphur in the arsenopyrite, or the iron and sulphur in the pyrite, all as the oxidation products, and producing in the liquid phase nitric oxide in which the nitrogen -~3~ 9 has a valence of plus 2, as the reduction product; (g) releasing at least part of the nitric oxide generated in the liquid phase into the gas phase; (h) oxidizing the nitric oxide in the gas phase, in which a significant oxygen partial pressure is maintained by continuous addition of an oxygen containing gas, to form an oxi-dized nitrogen species in which the nitrogen has a valence of at least plus 3, the total amount of oxygen added being at least in an amount stoichiometrically required for solubilization in the liquid phase of the arsenic, iron and sulphur in the arsenopyrite or the iron and sulphur in the pyrite; (i) absorbing the oxidized nitrogen species into the slurry wherein the oxidized nitrogen species become available for the oxidation-reduction reaction of step (f) above whereby the nitrogen, in its oxide forms, functions as a cata-lyst for the transport of oxygen from the gas phase to ; the oxidation-reduction reactions in the slurry, thereby permitting the total of the oxidized nitrogen species and nitric oxide in the system to be substantially less than a stoichiometric balance required for the oxidation of the arsenic, iron and sulphur; (j) subjecting the slurry to a solid-liquid separation to produce a solid residue and a liquid fraction; and (k) recovering precious metal from the solid residue.
~ ;
DRAWINGS
In the drawing which discloses a specific embodiment of the improved process:
Figure l illustrates a graphic flowsheet of the improved process of the invention.
i9 DETAILED DESCRIPTION OF A SPECIFIC
-EMBODIMENT OF THE INVENTION
The improved hydrometallurgical process is basically intended for the recovery of precious metal from an ore or concentrate containing arsenopyrite or pyrite wherein at least some of the precious metal is occluded in the arsenopyrite or pyrite. The improvement comprises first introducing the concentrate or ore into a denitrating step, along with the solution from the - 10 leach. The oxidized nitrogen species present in the leach solution react with the concentrate or ore to produce nitric oxide which is released into the gas phase. The liquid and solid products of the denitrating step are subjected to a liquid-solid separation. The resultant solids from the separation are introduced into the leaching vessel where the solids are treated with oxygen and nitric oxide Erom the denitrating step. -~
A gas phase and a liquid slurry are formed in the leaching vessel. The liquid is a bleed to the process. The slurry is comprised o-E -the ore or concen-trate as a solid phase and acid and water as a liquid phase. An oxidation-reduction reaction having a ~ standard potential between about 0.90 and about 1.20 ; ~ volts on the hydrogen scale is effected in the slurry between the arsenopyrite or pyrite and an oxidized nitrogen species in which the nitrogen has a valence of at least plus 3. Arsenic, iron and sulphur in the arsenopyrite, or iron and sulphur in the pyrite, are solubilized in the liquid phase as oxidation products.
Nitric oxide in which the nitrogen has a valence of plus 2 is produced as a reduction product in the liquid phase. At least part of the nitric oxide is released from the liquid phase into the gas phase. The nitric oxide in the gas phase, in which a significant oxygen partial pressure is maintained by the continuous addi-::
~ - 6 -' ' , , ~' ' ,' ' ~ ` .
- ' ' ' .. . .
`
. - ~ .,, . ., .. , ,,.
~3~59 tion of an oxygen containing gas, is oxidized to form an oxidized nitrogen species in which the nitrogen has a valence of at least plus 3. The total amount of oxygen added is at least in an amount that is stoichiometri-cally required for solubilization in the liquid phase ofthe arsenic, iron and sulphur in the arsenopyrite, or the iron and sulphur in the pyrite. The oxidized nitrogen species are absorbed into the slurry wherein the oxidized nitrogen species become available for the oxidation-reduction reaction. The nitrogen, in its oxide forms, functions as a catalyst for the transport of oxygen from the gas phase to the oxidation-reduction reactions in the slurry. This permits the total of the oxidized nitrogen species and nitric oxide in the system to be substantially less than a stoichiometric balance required for the oxidation of the arsenic, iron and sulphur. The slurry is removed from the common volume space and is subjected to a solid-liquid separation to produce a solid residue and a liquid fraction. Precious metal is recovered from the solid residue.
The arsenopyrite and pyrite are decomposed by the oxidation-reduction reaction in acid solutions in the slurry where the pH is less than about 1.0 to about 3 by the action of oxidized nitrogen species where the nitrogen has a valence of plus 3 or greater. These oxidized nitrogen species include nitrous acid and ; nitrogen dioxide. The oxidized nitrogen species are present in sufficient concentration in the liquid fraction (typically about 0.25 Molar (M~ to about 4.0 Molar (M), calculated on a nitric acid basis) to provide an adequate rate of dissolution (typically within a residence time of about 2 minutes to about 60 minutes) at the reaction temperature used (typically about 60C to about 119C for arsenopyrite concentrate and about 60C to about 180C for pyrite concentrate or '' ore). Normally, the lower oxidized nitrogen species concentrations and longer residence times are used when treating ore while the higher oxidized nitrogen species concentrations and shorter residence times are used when treating concentrates.
The main products from the oxidation-reduction reaction are soluble ferric iron species, soluble arsen-ate species, soluble sulp~ate species, minor amounts of elemental sulphur and nitric oxide.
Insoluble gangue minerals and elemental sulfur remain as solids in the slurry. The slurry is subjected to a solid-liquid separation to yield a solid residue and a liquid fraction. A major portion of the gold or other precious metal contained in the concentrate or ore remains in the solid residue. Some of the gold or other precious metal appears in the leach solution, and can be recovered with activated carbon.
Almost all of the silver present in the con-centrate will usually remain in the liquid fraction.
The silver can be recovered from the liquid fraction by using a thiocyanate compound such as sodium thiocyanate, potassium thiocyanate, or ammonium thiocyanate. Arsenic and iron can be optionally removed from the silver- Eree separated liquid fraction by elevating the temperature ~5 to precipitate ferric arsenate. In the case of pyrite, iron can be removed from the liquid fraction.
In general terms, the process of the invention can be operated at a standard potential between the arsenopyrite or pyrite and the oxidized nitrogen species of about 0.90 volts and about 1.20 volts on the hydrogen scale. At potentials below about 0.9 volts, arseno-pyrite or pyrite will not decompose efficiently. At potentials above about 1.2 volts, no significant oxida-; tion of the nitrogen species will take place because ~, 5;9 oxygen per se has a potential of about 1.23 volts on thehydrogen scale.
On the standard oxidation-reduction potential scale, the reduction of nitrous acid to nitric oxide has a standard potential of about 0.996 volts. The reduc--tion of nitrate to nitrous acid has a sta~dard potential of about 0.94 volts. Thus the former couple has a higher driving force than the latter in decomposing sulphide minerals such as arsenopyrite and pyrite.
Preferably, the process of the invention is operated at a potential greater than about 0.94 volts up to about 1.0 volts on the hydrogen scale.
The process can typically be conducted within a residence time range of about 2 minutes to about 60 minutes calculated on a plug flow basis. A process which is completed in a time less than about 2 minutes is difficult to control and basically impractical. On the other hand, a process which takes more than about 60 minutes to complete is too slow and thus uneconomical.
The process has been conducted experimentally at initial temperatures from above the freezing point of the slurry to temperatures of several hundred degrees ~; Celsius. However, temperatures falling in the range of about 60C to about 180C are preferred for economical reasons. Similarly, the process has been conducted at pH ranges of less than about 1.0 to as high as about ; 3Ø In situations where silver is not present, and the formation of basic iron sulphate or jarosite can be tolerated in the process, the process can be conducted at a pH of about 3Ø However, silver is usually present and therefore it is preferable to operate the process at lower pH ranges. Typically, a pH of about 1.0 or below is preferred because it is desirable to keep the iron and arsenic in solution. Also, the pro-:
_ g _ cess is more rapid and economical at a pH range of lessthan about 1Ø
In the process, the oxidized nitrogen species in a sense act as a transporter of oxygen. The basic process may be regarded as an oxygen leach rather than an oxidized nitrogen species or nitric acid leach. The oxidized nitrogen species serves as a carrier for the oxygen as the oxidized nitrogen species is cycled between the gas phase and the liquid phase o the slurry of the common volume space. It follows that the rate at which the reaction proceeds is proportional to the number of oxidized nitrogen species carriers that are in the process.
Sufficient oxygen must be supplied to the leaching vessel in order to completely decompose the arsenopyrite and pyrite in the slurry. Nitric oxide derived from the denitrating step is also introduced into the leach vessel. If insufficient oxygen is sup-plied, then the pressure of the nitric oxide generated increases and ultimately the reaction stops because there are no oxidized nitrogen species left in the liquid phase of the slurry.
In order to overcome the limitations of uti-lizing a bleed which removes valuable catalyst from the process, and introducing into the leach vessel a neutral recycled solution which promotes calcium sulphate forma-tion and causes difficulties in maintaining the leach solution at appropriate acid levels, while at the same time retaining and utilizing all of the unique and advantageous features of our basic arsenopyrite and pyrite treating process, we have developed a variation of the basic process which does not introduce the concentrate in the leach. The improved process is described in association with the flow sheet which is illustrated in Figure 1.
: ' ' ' ' .
q~
In the process as illustrated in the flowsheet o~ Figure 1, the concentrate is introduced into a pre-liminary denitrating stage along with a recycled solu-tion which is derived from the leach after gold and silver precipitation. The liquid and solids of the denitrating stage are subjected to a solid-liquid separation. The solids are transported to the leach vessel. Nitric oxide from the denitratil~g stage is also introduced into the leach vessel. The nitrate in the leach solution reacts with the arsenopyrite and pyrite concentrate according to the following general process criteria and leaching reactions:
A. Mineral Oxidations (1) Fe AsS -~ Fe+3(aq) + 1/2As2S2 + 3e ~2) FeAsS + 3H20 -~ Fe+3 + H3As03 + SO
+ 3H+ + 6e (3) FeAsS + 4H20 -~ Fe+3 + H3AsO4 -~ SO
+ 5H+ + 8e (4) FeAsS + 8H20 -~ Fe+3 + H3As04 + S04=
+ 13H~ + 14e (5) FeS2 -~ Fe+3(aq) + 2SO + 3e (6) FeS2 + 8H20 -~ Fe+3 + 2S04= + 16H+
+ 15e B. Oxidized Nitrogen Species Reduction (7) HN03 + 3H~ + 3e -~ NO(g) + 2H20 (8) HN02 ~ H+ + e -3 NO(g) + H20 :
In the oxidation of arsenopyrite, it has been found that 60-90% of the mineral's sulphur is converted to soluble sulphate species. In the oxidation of pyrite, the degree of conversion is 80-100%.
Equations A(l) and A(5) will, in principle, take place at potentials above about 0.6 volts on the hydrogen scale; however, since 1/~ (~s2S2) on arsenopyrite and 2S0 on pyrite have molar volumes larger than FeAsS and FeS2 respectively, the first submicroscopic layers of these leach products protect the mineral from further oxidation, and no substantial : reaction is observed. At potentials above about 0.94 volts on the hydrogen scale, reactions A(4) and A(6) taXe place, and the protective layers of As2S2 and S0 are eliminated by oxidation.
In the denitrating step, reaction B(7) absorbs electrons at a standard potential of 0.94 volts on the hydrogen scale, just barely adequate to remove electrons from arsenopyrite and pyrite to drive reactions A(4) and :~
A(6) at a feasible rate (as in Queneau). Reaction B(8), which takes place in the leach process, absorbs elect-: rons a~ a standard potential of 0.996 volts on the hydrogen scale, which is high enough to drive reactions A(4) and A(6) rapidly at temperatures as low as 60C.
The active nitrogen oxides are required only to act as a sink for electrons which are released by decomposition of the minerals in the concentrate or ore.
The oxidized nitrogen species should be present in sufficient concentration in the solution (typically about 0.25 M to about 3.0 M or 4.0 M) to provide an :~ adequate rate of dissolution (typically within a residence time of about 2 minutes to about 60 minutes) at the reaction temperature used (typically about 60C
to about 119C for arsenopyrite concentrate, and about 60C to about 180DC for pyrite concentrate or ore).
:
.:
:- :
: .
' Sulphuric acid may be used to form the soluble ferric iron species and under certain circumstances is produced in situ.
In the following reaction, nitrous acid is the decomposition agent for arsenopyrite with sulphuric acid ~- present.
(9) FeAsS + 1/2H2S04 -t 14HN02 -~
1/2Fe2(S04)3 + H3As04 + 14NOtg) + 6H2 Sufficient sulphuric acid was supplied with arsenopyrite and was consumed to form soluble ferric iron species. Without such acid, compounds will precipitate from solution.
In the reaction detailed below, the sulphuric acid is generated from the decomposition of pyrite.
1/2Fe2(S04)3 + H3As04 + 14NOtg) + 6H2 Sufficient sulphuric acid was supplied with arsenopyrite and was consumed to form soluble ferric iron species. Without such acid, compounds will precipitate from solution.
In the reaction detailed below, the sulphuric acid is generated from the decomposition of pyrite.
(10) FeS2 + 15HN02 -~ 1/2Fe2(S04)3 +
1/2H2S04 ~ 15NO(g) + 7H20 In the preceding reactions, the active nitro-gen oxides are reduced to nitric oxide which may then be regenerated by an oxidant. ~ useful oxidant is oxygen which reacts with nitric oxide in the presence of water to form nitrogen dioxide, nitrous acid and nitric acid as shown in the reactions set forth below.
1/2H2S04 ~ 15NO(g) + 7H20 In the preceding reactions, the active nitro-gen oxides are reduced to nitric oxide which may then be regenerated by an oxidant. ~ useful oxidant is oxygen which reacts with nitric oxide in the presence of water to form nitrogen dioxide, nitrous acid and nitric acid as shown in the reactions set forth below.
(11) ~0 + 1/2 2 ~- N2 (12~ NO + N02 ~ H20 ~- 2HN2 (13) 3HN02 ~ HN03 + H20 -~ 2NO
While the leach process is in continuous operation, the generation of nitric acid (reaction (13~) :
~:~
is not desirable and is to be avoided. This is accomp-lished by conducting reactions A(4) and A(6), B(8) and reactions (11) and (12) in a common volume space where the nitrous acid can be readily consumed by reactions (9) and (10) so as not to form nitric acid according to reaction (13). The regeneration of nitric oxide to the higher valence states is done concurrently with the decomposition of pyrite in the common volume space.
It is clear from equations (11) to (13) that HNO2 is the principal dissolved oxidized nitrogen species arising from the gas phase oxidation of NO and dissolution of the resulting NO2. Reaction (13) is rather slow, and HNO2 is therefore the principal dissolved oxidized nitrogen species that is able to react with the oxidizable minerals (reactions (9) and (10)). Oxygen is used for nitrogen oxide regeneration.
The rate of regeneration varies directly with oxygen partial pressure. Any oxygen partial pressure above ambient is adequate, but oxygen partial pressures of about 50 psig to about 100 psig are preferred. The regeneration step is carried out with an oxygen containing gas concurrently with the decomposition reaction(s) (reactions A(4) and A(6)). The overall stoichiometry of arsenopyrite reacting with sulphuric acid and oxygen utilizing the oxidized nitrogen species as a catalyst (transporter) is illustrated by the reaction below.
(14) FeAsS + 1/2 H2SO4 ~ 1/2 Fe2(SO4)3 +
+ 7/2 2 + H2O H3AsO~ + 14 HNO2 + 14 HNO2 Since the active oxidized nitrogen species are regenerated during the decomposition step in the common volume space, the quantity of these species present at any time may be quite small.
It is apparent from equation calculations that the H~03 concentrations initially added are far too low to completely decompose so much arsenopyrite. If the initially present HN03 were the only oxidant, and remained the only oxidant, stoichiometric calculations would show that a minimum of 5 moles of HN03 would have been required to completely decompose the 1 mole of arsenopyrite. This is evidenced by the following equation:
(15) FeAsS ~ 5H~03 ~' 1/3FE2(S04)3 ~ 1/3FE(N03)2 + H3As04 + H20 + 5NO(g) Yet, the mineral has been found to be completely decom-posed by as little as 0.5 M HN03, or 1/10 of the stoichiometric requirement, for example, oxidized nitro-gen species cycled ten times. This illustrates the highly catalytic property of the oxidized nitrogen specles.
At oxidized nitrogen species concentrations of 0.25 M or less, the decomposition rate is too slow to be a practical consideration. At oxidized nitrogen species concentrations of about 3.0 M, the reaction rate is ver~
; rapid and hence sufficient for most purposes. Greater concentrations than about 3.0 M do not provide greatly improved reaction rates.
The mineral decomposition and oxidized nitro-gen species regeneration steps are both exothermic.Thus, in conducting the reactions, the slurry in the ~ common volume space and the denitrating step must be ; cooled in order to maintain a constant operating temper-ature.
.
' ~ :
, The decomposition leach can be carried out over a wide range of solid-liquid ratios. The solid-liquid ratio in the denitrating step is dictated by the mass balance of the overall process.
In the denitrating step, the objective is to treat the liquid recycle from the silver precipitation with feed concentrate or ore, in the absence of oxygen, in order to produce a bleed solution free of oxidized nitrogen species. This is accomplished by some of the leaching reactions which have been discussed above.
Temperatures and pressures are generally similar except that the pressure in the denitrating step should be slightly greater than in the leaching step, eg. 20 to 50 psig greater, in order to drive the generated nitric oxide over to the leaching process in the common volume space. The time duration of the denitrating step is similar to the leaching step.
Following the denitrating step, a solid-liquid separation is effected. The partially leached solids from this separation step are transported to the lsach vessel in order to be reacted according to the reactions which are fundamental to our basic leach process as discussed above. A part of the nitrate-free solution forms the partial recycle of the bleed solution which is added to the concentrate or ore to for~ a slurry which is pumped into the denitrating step. The remainder of the nitrate-free solution forms the bleed solution of the process which can be discarded or treated. The nitrate-free solution is neutralized with limestone and/or lime in order to reject the waste materials according to the normal procedures of our basic process.
Our improved process, among other things, has the advantage that any desired amount of solution can form a bleed to the process. The quantity of solution ::~
which is treated in the denitrating step relative to the amount of solution which is recycled directly to the leach as the optional recycle will depend on factors such as the iron and arsenic content of the concentrate.
Another advantage of the preferred process, ie. no precipitation of iron, arsenic or sulphate from the liquid recycle, is that no neutralized solution is introduced into the leach.
The choice of oxidized nitrogen species concentration, decomposition temperature and time for leaching is governed by the nature of the material to be leached. Convenient initial sources of the oxidized nitrogen species are nitric oxide gas or nitric acid.
The solids are decomposed in a single pass and no recycle of solids is required. When the decomposition reactions are complete, a solid-liquid separation is carried out to produce a solid residue containing the majority of the gold and a clarified liquid fr~ction which may contain some of the gold and silver.
Our process offers the option of producing high purity arsenic trioxide. The conditions of the leach can be varied to maximize the presence of the extracted arsenic as soluble arsenite. Arsenic trioxide can then be precipitated by cooling the filtered decom-position solution. By using a low decomposition temper-ature (70C) and a low concentration of oxidized nitro-gen species (0.5 M HN03 for 1.25 M FeAsS) and then cooling the filtered decomposition solution to 10C, it wa5 found that 35 percent of the extracted arsenic was recovered as As203. ~ormally, however, when arsenic trioxide production is not required, process conditions are chosen so as to maximize to oxidiation of arsenic to the arsenate state.
The separation of silver from the acidic liquid fraction which contains iron, arsenic, sulphate `"` ~3~ i9 and oxidized nitrogen species represents another inven-tive aspect of the process.
A portion of the silver present in the concen-trate or ore reports to the liquid fraction. The silver may be recovered as a thiocyanate compound with the addition of one mole of thiocyanate per mole of silver.
The reaction time involved is very short, typically about one minute. Thiocyanate compounds which can be used are sodium -thiocyanate, potassium thiocyanate or ammonium thiocyanate~
At high solution temperatures, thiocyanate is oxidized by the oxidized nitrogen species present in the solution. In a solution which is three molar in nitrate ions, oxidation of the thiocyanate occurs at an in-creased rate at temperatures in excess of about 80C.Therefore, if the leach is conducted at a temperature of lOO~C, for example, the liquid fraction should be cooled to about 80C or lower, eg., down to 60nC, in order to avoid decomposing the thiocyanate. An important and unique feature of the silver removal process is that the thiocyanate added in excess of that required for silver removal reacts with the ferric iron present to form soluble ferric thiocyanate complexes which have an intense red colour. The presence of this red colour acts as an indicator to show that sufficient thiocyanate has been added. A solid and liquid separation is car-ried out to recover the silver thiocyanate precipitate.
The silver can be recovered from the precipitate by smelting or by conventional hydrometallurgical treat-ment. The liquid separated is suitable for recycle tothe denitrating step. Although the inclusion of the denitrating step eliminates the need to remove dissolved arsenic, iron and suplhate, removing these dissolved substances enables the process to be operated with less ~ J~ ~
make-up water being added. This could be an important feature in water conservation areas.
Dissolved arsenic can be removed from solution with dissolved iron in the form of ferric arsenate. The following reaction shows the formation of ferric arsenate from ferric nitrate and arsenic acid (arsenate)0 tl3) Fe(N03)3 + H3As04~` 3 HNO3 + FeAsO4 2H20 ~ 2 H20 Ferric arsenate is produced, virtually quanti-tatively from an equimolar solution of ferric nitrate and arsenate at all temperatures above ambient. How-ever, the rate can be controlled by temperature regula-tion and by the addition of nucleating a~ents. With an -~ unnucleated solution at room temperature, complete precipitation (> 95 percent removal of iron and arse-nate) requires several months, at 100C, precipitation requires several hours; and at 200C, precipitation occurs in less than one hour. When nucleated by fine ferric arsenate, the rates become more rapid.
The ferric arsenate produced is a crystalline solid which shows the X-ray diffraction pattern of FeAsO4 2H20. The solubility of this material, when mixed with water, is very low (less than 1 ppm arsenic). The crystalline ferric arsenate is unique in that it precipitates from a strong nitric acid solùtion.
For example, a ferric arsenate precipitate has been produced in 5 M HNO3.
The crystalline ferric arsenate obtained from this process is distinctly different from the ferric arsenate that is produced from the neutralization of acidic ferric nitrate and arsenate solutions. The latter material is colloidal and shows no X-ray diffrac-, `
, , :
- ,.....
~3~
tion pattern. ~en mixed with water, the solubility of the amorphous ferric arsenate is in excess of 20 ppm arsenic. The amorphous ferric arsenate is difficult to filter and can contain ferric hydroxide which also tends to be colloidal and hence difficult to filter.
It has been discovered that the presence of sulphate in solution hampers the formation of crystal-line ferric arsenate. Sulphate must be removed from solution prior to ferric arsenate precipitation. A
solution which is 1 M in ferric nitrate and arsenate is stable at 100C in the presence of 0.8 M sulphate as H2S04 .
A calcium-bearing substance such as calcium oxide, calcium hydroxide or calcium carbonate, or a barium-bearing substance such as barium carbonate, can be used to remove sulphate in order to facilitate crystalline ferric arsenate precipitation. The calcium and barium bearing substances also partially neutralize the solutions, however amorphous ferric arsenate is not produced if the rate of addition of the neutralizing agent is slow. The mixture of crystalline ferric arsenate and calcium or barium sulphate filters very well.
Because of the inhibiting effect of sulphate on the formation of ferric arsenate, the rate of ferric arsenate precipitation is dependent on the rate of calcium sulphate precipitation. At high temperature, e.g. over 150C, 95 percent of the iron and arsenic is removed in less than one hour. At 100C, while some sulphate is present, in the absence of a nucleation agent, the rate of iron and arsenic removal is slower, i.e. 95 percent removal requires in excess of 12 hours.
At 100C, when sulphate removal is complete, and nucle-ation is provided by recycling previously formed ferric arsenate, 95 percent removal can be achieved in one hour. Arsenic removal proceeds at a satisfactory rate at temperatures below 100C when sulphate removal i5 complete and a nucleation agent is provided.
When treating pyritic concentrates, ferric iron can be removed from solution by the formation of insoluble iron compounds e.g. ferric hydroxide or basic iron sulfate through the neutralization of the solution.
The tendency for silver to be bound up with jarosite results in silver losses if jarosite precipi-tates are formed during the decomposition step of theprocess. E~owever, jarosites do not form promptly from supersaturated solutions since they are a crystalline, filterable solid that nucleates very slowly. A high acid level suppresses the formation of jarosite. The applicants have found that it is possible with the process to conduct the decomposition step in such a way that all the iron, and arsenic, and most of the sulphur, are dissolved long before the precipitation of jarosite becomes rapid. It is also possible to complete the decomposition step, separate the gold bearing solid residues, precipitate any dissolved silver, and then reheat the liquid fraction (without necessarily addi-tional neutralization) to precipitate jarosite free of ~; precious metals.
Various trace elements such as copper, mag-nesium, zinc, bismuth or tellurium may be present in the concentrate being treated. While some of these trace elements will report to the solid residue or waste precipitation residues, some may build up in the liquid phase or the liquid fraction and have to be bled-off.
-~ When trace elements are present in sufficient concen-tration, their recovery may be economically justified.
The process is effective in treating arseno-pyritic and pyritic ores which contain carbonaceous material. Some of this carbonaceous material may be ~;
- , , , ' -active and thus interfere with precious metal recovery.
The process renders such carbonaceous material inactive so that the material does not interfere with subsequent gold recovery.
The practice of the invention is not limited to the treatment of gold and silver bearing pyritic and arsenopyritic concentrates and ores. Kunda ~U.S. Patent ~o. 4,331,469) discloses a process for the recovery of silver values from silver - containing material which also contains iron and arsenic. Kunda's process uti-lizes a nitric acid solution to leach the concentrate.
His process suffers from the fact that the solutions can not be recycled and thus represents a loss of valuable reagent. The liquor leaving the process has a high content of ammonium nitrate and ammonium sulphate and therefore requires the plant to be associated with a fertilizer plant.
Utilizing our process, the nitric acid will react with the concentrate to Eorm nitric oxide. In the oxidation leach this nitric oxide will be regenerated with oxygen to provide for complete oxidation of the sulphide and arsenide minerals. In the denitrating step the nitrates are recycled to the oxidation leach. A
nitrate-free solution is produced for the recovery of dissolved metals and neutralization Example 1 A batch test was done to demonstrate the use of the denitrating step for the removal of oxidized nitrogen species from the leach solution by contact with the feed concentrate.
A leach discharge solution was prepared by reacting a pyritic concentrate with a nitric acid solution in an autoclave. Oxygen was introduced into the autoclave as the leach proceeded in order to oxidize the nitric oxide so that the oxidized nitrogen species would be regenerated. After completion of the leaching the solution contained 12,000 mg per litre oxidized nitrogen species.
500 ml of this leach solution was placed in an autoclave with 100 g of the pyritic concentrate. The autoclave was sealed and then heated to 100C. The denitrating leach was allowed to proceed until the pressure in the autoclave was at 170 psig due to nitric oxide production. At this point, the nitric oxide gas was vented from the autoclave to maintain an operating pressure of 160 psig. The test was continued for 30 minutes during which time the pressure was decreased to 120 psig as the nitric oxide gas was vented.
Following this denitrating leach, the solution was analyzed and found to contain 94.6 mg per litre oxidized nitrogen species. This demonstrates 99.2%
removal of nitrogen species from the leach solution.
This solution was further sparged with air to remove dissolved nitric oxide. After air sparging the solution contained 30 mg per litre oxidized nitrogen species indicating an overall removal of 99.7%.
Example 2 A batch test was conducted to demonstrate the various steps in the process. The concentrate used for this sequence contained pyrite and arsenopyrite and had the following composition:
Fe = 40.8%, As = 11.1%, S = 40.6%
3,300 g of this concentrate was leached in an autoclave according to the normal practice of our invention with 22 litres of solution containing 42,000 mg per litre of oxidized nitrogen species as nitric acid and sufficient oxygen to satisfy the requirements of the sulphide `: , : :
'L~ 9 oxidation reactions. The solution from this leach had the following composition:
56.5 g/l Fe, 15.5 g/l As, 160 g/l SO4 6,900 g of this leach solution together with 1,050 g of water were com~ined with 1,050 g of fresh concentrate.
A denitrating leach was done for 30 minutes at lOO~C
with venting of the nitric oxide gas to maintain an operating pressure of 150 psig.
The discharge slurry from the denitrating leach was filtered to produce a partially leached residue and a solution. The solution had the following composition:
60.6 g/l Fe, 18.2 g/l As, 163.5 g/l So4, 480 mg/l nitrates Following purging with nitrogen for 5 minutes the nitrate content of the solution was reduced to 180 mg/l. This demonstrates 99.5% removal of oxidized ~ nitrogen species in the denitrating leach.
; The residue from this denitrating leach had the following composition:
Weight = 823 g F3 = 38.3%, As = 8.6%, S - 43.9%
300 g of this residue was leached with 1,700 g of solution containing 3 m/l oxidized nitrogen species as nitric acid. Leaching was done in an autoclave under normal conditions for the process with oxygen being supplied to satisfy the oxidation requirements. The ~ residue from this leach was analyzed as follows:
; Weight = 35.3 g Fe = 6.6%, As = 9.8%
The overall iron extraction from the concen-trate through the denitrating step plus oxidizing leach step was 98.5%. The arsenic extraction was 91.9%.
The above results demonstrate the sequence of steps in the process flowsheet. While the example .
:
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~ 3~
utilized a nitric acid solution to leach the residue from the denitrating step, it has previously been demonstrated that under continuous or batch conditions, such leaching can be done utilizing the nitric oxide gas from the denitrating step together with oxygen.
As will be apparent to those skilled in the art in the light of the foregoing disclosure, many alterations and modifications are possible in the practice of this invention without departing from the spirit or scope thereof. Accordingly, the scope of the invention is to be construed in accordance with the substance defined by the following claims.
,,. . ,;, , .
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' '
While the leach process is in continuous operation, the generation of nitric acid (reaction (13~) :
~:~
is not desirable and is to be avoided. This is accomp-lished by conducting reactions A(4) and A(6), B(8) and reactions (11) and (12) in a common volume space where the nitrous acid can be readily consumed by reactions (9) and (10) so as not to form nitric acid according to reaction (13). The regeneration of nitric oxide to the higher valence states is done concurrently with the decomposition of pyrite in the common volume space.
It is clear from equations (11) to (13) that HNO2 is the principal dissolved oxidized nitrogen species arising from the gas phase oxidation of NO and dissolution of the resulting NO2. Reaction (13) is rather slow, and HNO2 is therefore the principal dissolved oxidized nitrogen species that is able to react with the oxidizable minerals (reactions (9) and (10)). Oxygen is used for nitrogen oxide regeneration.
The rate of regeneration varies directly with oxygen partial pressure. Any oxygen partial pressure above ambient is adequate, but oxygen partial pressures of about 50 psig to about 100 psig are preferred. The regeneration step is carried out with an oxygen containing gas concurrently with the decomposition reaction(s) (reactions A(4) and A(6)). The overall stoichiometry of arsenopyrite reacting with sulphuric acid and oxygen utilizing the oxidized nitrogen species as a catalyst (transporter) is illustrated by the reaction below.
(14) FeAsS + 1/2 H2SO4 ~ 1/2 Fe2(SO4)3 +
+ 7/2 2 + H2O H3AsO~ + 14 HNO2 + 14 HNO2 Since the active oxidized nitrogen species are regenerated during the decomposition step in the common volume space, the quantity of these species present at any time may be quite small.
It is apparent from equation calculations that the H~03 concentrations initially added are far too low to completely decompose so much arsenopyrite. If the initially present HN03 were the only oxidant, and remained the only oxidant, stoichiometric calculations would show that a minimum of 5 moles of HN03 would have been required to completely decompose the 1 mole of arsenopyrite. This is evidenced by the following equation:
(15) FeAsS ~ 5H~03 ~' 1/3FE2(S04)3 ~ 1/3FE(N03)2 + H3As04 + H20 + 5NO(g) Yet, the mineral has been found to be completely decom-posed by as little as 0.5 M HN03, or 1/10 of the stoichiometric requirement, for example, oxidized nitro-gen species cycled ten times. This illustrates the highly catalytic property of the oxidized nitrogen specles.
At oxidized nitrogen species concentrations of 0.25 M or less, the decomposition rate is too slow to be a practical consideration. At oxidized nitrogen species concentrations of about 3.0 M, the reaction rate is ver~
; rapid and hence sufficient for most purposes. Greater concentrations than about 3.0 M do not provide greatly improved reaction rates.
The mineral decomposition and oxidized nitro-gen species regeneration steps are both exothermic.Thus, in conducting the reactions, the slurry in the ~ common volume space and the denitrating step must be ; cooled in order to maintain a constant operating temper-ature.
.
' ~ :
, The decomposition leach can be carried out over a wide range of solid-liquid ratios. The solid-liquid ratio in the denitrating step is dictated by the mass balance of the overall process.
In the denitrating step, the objective is to treat the liquid recycle from the silver precipitation with feed concentrate or ore, in the absence of oxygen, in order to produce a bleed solution free of oxidized nitrogen species. This is accomplished by some of the leaching reactions which have been discussed above.
Temperatures and pressures are generally similar except that the pressure in the denitrating step should be slightly greater than in the leaching step, eg. 20 to 50 psig greater, in order to drive the generated nitric oxide over to the leaching process in the common volume space. The time duration of the denitrating step is similar to the leaching step.
Following the denitrating step, a solid-liquid separation is effected. The partially leached solids from this separation step are transported to the lsach vessel in order to be reacted according to the reactions which are fundamental to our basic leach process as discussed above. A part of the nitrate-free solution forms the partial recycle of the bleed solution which is added to the concentrate or ore to for~ a slurry which is pumped into the denitrating step. The remainder of the nitrate-free solution forms the bleed solution of the process which can be discarded or treated. The nitrate-free solution is neutralized with limestone and/or lime in order to reject the waste materials according to the normal procedures of our basic process.
Our improved process, among other things, has the advantage that any desired amount of solution can form a bleed to the process. The quantity of solution ::~
which is treated in the denitrating step relative to the amount of solution which is recycled directly to the leach as the optional recycle will depend on factors such as the iron and arsenic content of the concentrate.
Another advantage of the preferred process, ie. no precipitation of iron, arsenic or sulphate from the liquid recycle, is that no neutralized solution is introduced into the leach.
The choice of oxidized nitrogen species concentration, decomposition temperature and time for leaching is governed by the nature of the material to be leached. Convenient initial sources of the oxidized nitrogen species are nitric oxide gas or nitric acid.
The solids are decomposed in a single pass and no recycle of solids is required. When the decomposition reactions are complete, a solid-liquid separation is carried out to produce a solid residue containing the majority of the gold and a clarified liquid fr~ction which may contain some of the gold and silver.
Our process offers the option of producing high purity arsenic trioxide. The conditions of the leach can be varied to maximize the presence of the extracted arsenic as soluble arsenite. Arsenic trioxide can then be precipitated by cooling the filtered decom-position solution. By using a low decomposition temper-ature (70C) and a low concentration of oxidized nitro-gen species (0.5 M HN03 for 1.25 M FeAsS) and then cooling the filtered decomposition solution to 10C, it wa5 found that 35 percent of the extracted arsenic was recovered as As203. ~ormally, however, when arsenic trioxide production is not required, process conditions are chosen so as to maximize to oxidiation of arsenic to the arsenate state.
The separation of silver from the acidic liquid fraction which contains iron, arsenic, sulphate `"` ~3~ i9 and oxidized nitrogen species represents another inven-tive aspect of the process.
A portion of the silver present in the concen-trate or ore reports to the liquid fraction. The silver may be recovered as a thiocyanate compound with the addition of one mole of thiocyanate per mole of silver.
The reaction time involved is very short, typically about one minute. Thiocyanate compounds which can be used are sodium -thiocyanate, potassium thiocyanate or ammonium thiocyanate~
At high solution temperatures, thiocyanate is oxidized by the oxidized nitrogen species present in the solution. In a solution which is three molar in nitrate ions, oxidation of the thiocyanate occurs at an in-creased rate at temperatures in excess of about 80C.Therefore, if the leach is conducted at a temperature of lOO~C, for example, the liquid fraction should be cooled to about 80C or lower, eg., down to 60nC, in order to avoid decomposing the thiocyanate. An important and unique feature of the silver removal process is that the thiocyanate added in excess of that required for silver removal reacts with the ferric iron present to form soluble ferric thiocyanate complexes which have an intense red colour. The presence of this red colour acts as an indicator to show that sufficient thiocyanate has been added. A solid and liquid separation is car-ried out to recover the silver thiocyanate precipitate.
The silver can be recovered from the precipitate by smelting or by conventional hydrometallurgical treat-ment. The liquid separated is suitable for recycle tothe denitrating step. Although the inclusion of the denitrating step eliminates the need to remove dissolved arsenic, iron and suplhate, removing these dissolved substances enables the process to be operated with less ~ J~ ~
make-up water being added. This could be an important feature in water conservation areas.
Dissolved arsenic can be removed from solution with dissolved iron in the form of ferric arsenate. The following reaction shows the formation of ferric arsenate from ferric nitrate and arsenic acid (arsenate)0 tl3) Fe(N03)3 + H3As04~` 3 HNO3 + FeAsO4 2H20 ~ 2 H20 Ferric arsenate is produced, virtually quanti-tatively from an equimolar solution of ferric nitrate and arsenate at all temperatures above ambient. How-ever, the rate can be controlled by temperature regula-tion and by the addition of nucleating a~ents. With an -~ unnucleated solution at room temperature, complete precipitation (> 95 percent removal of iron and arse-nate) requires several months, at 100C, precipitation requires several hours; and at 200C, precipitation occurs in less than one hour. When nucleated by fine ferric arsenate, the rates become more rapid.
The ferric arsenate produced is a crystalline solid which shows the X-ray diffraction pattern of FeAsO4 2H20. The solubility of this material, when mixed with water, is very low (less than 1 ppm arsenic). The crystalline ferric arsenate is unique in that it precipitates from a strong nitric acid solùtion.
For example, a ferric arsenate precipitate has been produced in 5 M HNO3.
The crystalline ferric arsenate obtained from this process is distinctly different from the ferric arsenate that is produced from the neutralization of acidic ferric nitrate and arsenate solutions. The latter material is colloidal and shows no X-ray diffrac-, `
, , :
- ,.....
~3~
tion pattern. ~en mixed with water, the solubility of the amorphous ferric arsenate is in excess of 20 ppm arsenic. The amorphous ferric arsenate is difficult to filter and can contain ferric hydroxide which also tends to be colloidal and hence difficult to filter.
It has been discovered that the presence of sulphate in solution hampers the formation of crystal-line ferric arsenate. Sulphate must be removed from solution prior to ferric arsenate precipitation. A
solution which is 1 M in ferric nitrate and arsenate is stable at 100C in the presence of 0.8 M sulphate as H2S04 .
A calcium-bearing substance such as calcium oxide, calcium hydroxide or calcium carbonate, or a barium-bearing substance such as barium carbonate, can be used to remove sulphate in order to facilitate crystalline ferric arsenate precipitation. The calcium and barium bearing substances also partially neutralize the solutions, however amorphous ferric arsenate is not produced if the rate of addition of the neutralizing agent is slow. The mixture of crystalline ferric arsenate and calcium or barium sulphate filters very well.
Because of the inhibiting effect of sulphate on the formation of ferric arsenate, the rate of ferric arsenate precipitation is dependent on the rate of calcium sulphate precipitation. At high temperature, e.g. over 150C, 95 percent of the iron and arsenic is removed in less than one hour. At 100C, while some sulphate is present, in the absence of a nucleation agent, the rate of iron and arsenic removal is slower, i.e. 95 percent removal requires in excess of 12 hours.
At 100C, when sulphate removal is complete, and nucle-ation is provided by recycling previously formed ferric arsenate, 95 percent removal can be achieved in one hour. Arsenic removal proceeds at a satisfactory rate at temperatures below 100C when sulphate removal i5 complete and a nucleation agent is provided.
When treating pyritic concentrates, ferric iron can be removed from solution by the formation of insoluble iron compounds e.g. ferric hydroxide or basic iron sulfate through the neutralization of the solution.
The tendency for silver to be bound up with jarosite results in silver losses if jarosite precipi-tates are formed during the decomposition step of theprocess. E~owever, jarosites do not form promptly from supersaturated solutions since they are a crystalline, filterable solid that nucleates very slowly. A high acid level suppresses the formation of jarosite. The applicants have found that it is possible with the process to conduct the decomposition step in such a way that all the iron, and arsenic, and most of the sulphur, are dissolved long before the precipitation of jarosite becomes rapid. It is also possible to complete the decomposition step, separate the gold bearing solid residues, precipitate any dissolved silver, and then reheat the liquid fraction (without necessarily addi-tional neutralization) to precipitate jarosite free of ~; precious metals.
Various trace elements such as copper, mag-nesium, zinc, bismuth or tellurium may be present in the concentrate being treated. While some of these trace elements will report to the solid residue or waste precipitation residues, some may build up in the liquid phase or the liquid fraction and have to be bled-off.
-~ When trace elements are present in sufficient concen-tration, their recovery may be economically justified.
The process is effective in treating arseno-pyritic and pyritic ores which contain carbonaceous material. Some of this carbonaceous material may be ~;
- , , , ' -active and thus interfere with precious metal recovery.
The process renders such carbonaceous material inactive so that the material does not interfere with subsequent gold recovery.
The practice of the invention is not limited to the treatment of gold and silver bearing pyritic and arsenopyritic concentrates and ores. Kunda ~U.S. Patent ~o. 4,331,469) discloses a process for the recovery of silver values from silver - containing material which also contains iron and arsenic. Kunda's process uti-lizes a nitric acid solution to leach the concentrate.
His process suffers from the fact that the solutions can not be recycled and thus represents a loss of valuable reagent. The liquor leaving the process has a high content of ammonium nitrate and ammonium sulphate and therefore requires the plant to be associated with a fertilizer plant.
Utilizing our process, the nitric acid will react with the concentrate to Eorm nitric oxide. In the oxidation leach this nitric oxide will be regenerated with oxygen to provide for complete oxidation of the sulphide and arsenide minerals. In the denitrating step the nitrates are recycled to the oxidation leach. A
nitrate-free solution is produced for the recovery of dissolved metals and neutralization Example 1 A batch test was done to demonstrate the use of the denitrating step for the removal of oxidized nitrogen species from the leach solution by contact with the feed concentrate.
A leach discharge solution was prepared by reacting a pyritic concentrate with a nitric acid solution in an autoclave. Oxygen was introduced into the autoclave as the leach proceeded in order to oxidize the nitric oxide so that the oxidized nitrogen species would be regenerated. After completion of the leaching the solution contained 12,000 mg per litre oxidized nitrogen species.
500 ml of this leach solution was placed in an autoclave with 100 g of the pyritic concentrate. The autoclave was sealed and then heated to 100C. The denitrating leach was allowed to proceed until the pressure in the autoclave was at 170 psig due to nitric oxide production. At this point, the nitric oxide gas was vented from the autoclave to maintain an operating pressure of 160 psig. The test was continued for 30 minutes during which time the pressure was decreased to 120 psig as the nitric oxide gas was vented.
Following this denitrating leach, the solution was analyzed and found to contain 94.6 mg per litre oxidized nitrogen species. This demonstrates 99.2%
removal of nitrogen species from the leach solution.
This solution was further sparged with air to remove dissolved nitric oxide. After air sparging the solution contained 30 mg per litre oxidized nitrogen species indicating an overall removal of 99.7%.
Example 2 A batch test was conducted to demonstrate the various steps in the process. The concentrate used for this sequence contained pyrite and arsenopyrite and had the following composition:
Fe = 40.8%, As = 11.1%, S = 40.6%
3,300 g of this concentrate was leached in an autoclave according to the normal practice of our invention with 22 litres of solution containing 42,000 mg per litre of oxidized nitrogen species as nitric acid and sufficient oxygen to satisfy the requirements of the sulphide `: , : :
'L~ 9 oxidation reactions. The solution from this leach had the following composition:
56.5 g/l Fe, 15.5 g/l As, 160 g/l SO4 6,900 g of this leach solution together with 1,050 g of water were com~ined with 1,050 g of fresh concentrate.
A denitrating leach was done for 30 minutes at lOO~C
with venting of the nitric oxide gas to maintain an operating pressure of 150 psig.
The discharge slurry from the denitrating leach was filtered to produce a partially leached residue and a solution. The solution had the following composition:
60.6 g/l Fe, 18.2 g/l As, 163.5 g/l So4, 480 mg/l nitrates Following purging with nitrogen for 5 minutes the nitrate content of the solution was reduced to 180 mg/l. This demonstrates 99.5% removal of oxidized ~ nitrogen species in the denitrating leach.
; The residue from this denitrating leach had the following composition:
Weight = 823 g F3 = 38.3%, As = 8.6%, S - 43.9%
300 g of this residue was leached with 1,700 g of solution containing 3 m/l oxidized nitrogen species as nitric acid. Leaching was done in an autoclave under normal conditions for the process with oxygen being supplied to satisfy the oxidation requirements. The ~ residue from this leach was analyzed as follows:
; Weight = 35.3 g Fe = 6.6%, As = 9.8%
The overall iron extraction from the concen-trate through the denitrating step plus oxidizing leach step was 98.5%. The arsenic extraction was 91.9%.
The above results demonstrate the sequence of steps in the process flowsheet. While the example .
:
.
~ 3~
utilized a nitric acid solution to leach the residue from the denitrating step, it has previously been demonstrated that under continuous or batch conditions, such leaching can be done utilizing the nitric oxide gas from the denitrating step together with oxygen.
As will be apparent to those skilled in the art in the light of the foregoing disclosure, many alterations and modifications are possible in the practice of this invention without departing from the spirit or scope thereof. Accordingly, the scope of the invention is to be construed in accordance with the substance defined by the following claims.
,,. . ,;, , .
.
' '
Claims (17)
1. In a process for recovering precious metals from pyritic and arsenopyritic concentrates and ores involving decomposing the arsenopyrite or pyrite con-centrates and ores in acidic leach solution in a common volume space which contains a gas phase and a liquid slurry, which comprises a liquid phase and a solid phase, through the action of higher valence oxidized nitrogen species in which the nitrogen has a valence of at least plus 3, the active oxidized nitrogen species being regenerated in the same common volume space by an oxygen containing gas, the improvement which comprises introducing the concentrate or ore into a denitrating step along with solution from the leach solution, react-ing essentially all the oxidized nitrogen species in the leach solution with concentrate or ore to produce in the denitrating step nitric oxide, which is released into a gas phase and is transported to the gas phase of the common volume space, subjecting the liquid and solid products of the denitrating step to a solid-liquid separation, transporting the solids from the separation to the solid phase of the common volume space where the solids are treated with oxygen and nitric oxide, and bleeding the remaining liquid free of oxidized nitrogen species from the process.
2. A hydrometallurgical process for the recovery of precious metal from an ore or concentrate containing arsenopyrite or pyrite wherein at least some of the precious metal is occluded in arsenopyrite or pyrite, which process comprises:
(a) introducing pyrite or arsenopyrite concen-trate or ore together with a liquid fraction which contains oxidized nitrogen species from step (m) below into a denitrating vessel where the concentrate or ore is subjected to an oxidation-reduction reaction to produce a solution which is essentially free of oxidized nitrogen species, a partially oxidized concentrate or ore, and a nitric oxide gas in a gas phase;
(b) continuously removing the nitric oxide gas from the denitrating vessel;
(c) continuously withdrawing the partially oxidized concentrate or ore and the solution which is essentially free of oxidized nitrogen species from the denitrating vessel and subjecting it to a solid-liquid separation;
(d) recycling part of the oxidized nitrogen species free solution from the solid-liquid separation to the pyrite or arsenopyrite concentrate or ore of step (a) above to prevent the pyrite or arsenopyrite concen-trate or ore from oxidizing until it is introduced into the denitrating vessel, and discarding from the process a remaining bleed solution which is essentially free of oxidized nitrogen species;
(e) transporting the solids from the solid-liquid separation to a common volume space;
(f) forming in the common volume space a gas phase and a liquid slurry comprising the solids from step (d) above as the solid phase, acid and water as the liquid phase of the slurry, and nitric oxide gas from step (b) above and oxygen as the gas phase;
(g) effecting in the liquid slurry between the arsenopyrite or pyrite and an oxidized nitrogen species in which the nitrogen has a valence of at least plus 3 an oxidation-reduction reaction having a standard potential between about 0.90 and about 1.20 volts on the hydrogen scale, thereby solubilizing in the liquid phase the arsenic, iron and sulphur in the arsenopyrite, or the iron and sulphur in the pyrite, all as the oxidation products, and producing in the liquid phase nitric oxide in which the nitrogen has a valence of plus 2, as the reduction product;
(h) releasing at least part of the nitric oxide generated in the liquid phase into the gas phase;
(i) oxidizing the nitric oxide in the gas phase, in which an oxygen partial pressure is maintained by continuous addition of an oxygen containing gas, to form an oxidized nitrogen species in which the nitrogen has a valence of at least plus 3, the total amount of oxygen added being at least in an amount stoichio-metrically required for solubilization in the liquid phase of the arsenic, iron and sulphur in the arseno-pyrite or the iron and sulphur in the pyrite;
(j) absorbing the oxidized nitrogen species into the liquid slurry wherein the oxidized nitrogen species become available for the oxidation-reduction reaction of step (g) above whereby the nitrogen, in its oxide forms, functions as a catalyst for the transport of oxygen from the gas phase to the oxidation-reduction reactions in the liquid slurry, thereby permitting the total of the oxidized nitrogen species and nitric oxide in the system to be less than a stoichiometric balance required for the oxidation of the arsenic, iron and sulphur;
(k) subjecting the liquid slurry to a solid-liquid separation to produce a solid residue which contains precious metal and a liquid fraction which contains oxidized nitrogen species;
(l) recovering precious metal from the solid residue; and (m) recycling the liquid fraction to step (a) above of the process.
(a) introducing pyrite or arsenopyrite concen-trate or ore together with a liquid fraction which contains oxidized nitrogen species from step (m) below into a denitrating vessel where the concentrate or ore is subjected to an oxidation-reduction reaction to produce a solution which is essentially free of oxidized nitrogen species, a partially oxidized concentrate or ore, and a nitric oxide gas in a gas phase;
(b) continuously removing the nitric oxide gas from the denitrating vessel;
(c) continuously withdrawing the partially oxidized concentrate or ore and the solution which is essentially free of oxidized nitrogen species from the denitrating vessel and subjecting it to a solid-liquid separation;
(d) recycling part of the oxidized nitrogen species free solution from the solid-liquid separation to the pyrite or arsenopyrite concentrate or ore of step (a) above to prevent the pyrite or arsenopyrite concen-trate or ore from oxidizing until it is introduced into the denitrating vessel, and discarding from the process a remaining bleed solution which is essentially free of oxidized nitrogen species;
(e) transporting the solids from the solid-liquid separation to a common volume space;
(f) forming in the common volume space a gas phase and a liquid slurry comprising the solids from step (d) above as the solid phase, acid and water as the liquid phase of the slurry, and nitric oxide gas from step (b) above and oxygen as the gas phase;
(g) effecting in the liquid slurry between the arsenopyrite or pyrite and an oxidized nitrogen species in which the nitrogen has a valence of at least plus 3 an oxidation-reduction reaction having a standard potential between about 0.90 and about 1.20 volts on the hydrogen scale, thereby solubilizing in the liquid phase the arsenic, iron and sulphur in the arsenopyrite, or the iron and sulphur in the pyrite, all as the oxidation products, and producing in the liquid phase nitric oxide in which the nitrogen has a valence of plus 2, as the reduction product;
(h) releasing at least part of the nitric oxide generated in the liquid phase into the gas phase;
(i) oxidizing the nitric oxide in the gas phase, in which an oxygen partial pressure is maintained by continuous addition of an oxygen containing gas, to form an oxidized nitrogen species in which the nitrogen has a valence of at least plus 3, the total amount of oxygen added being at least in an amount stoichio-metrically required for solubilization in the liquid phase of the arsenic, iron and sulphur in the arseno-pyrite or the iron and sulphur in the pyrite;
(j) absorbing the oxidized nitrogen species into the liquid slurry wherein the oxidized nitrogen species become available for the oxidation-reduction reaction of step (g) above whereby the nitrogen, in its oxide forms, functions as a catalyst for the transport of oxygen from the gas phase to the oxidation-reduction reactions in the liquid slurry, thereby permitting the total of the oxidized nitrogen species and nitric oxide in the system to be less than a stoichiometric balance required for the oxidation of the arsenic, iron and sulphur;
(k) subjecting the liquid slurry to a solid-liquid separation to produce a solid residue which contains precious metal and a liquid fraction which contains oxidized nitrogen species;
(l) recovering precious metal from the solid residue; and (m) recycling the liquid fraction to step (a) above of the process.
3. A process as defined in claim 2 wherein the oxidation-reduction reaction has a standard potential of at least 0.94 and less than about 1.0 volts on the hydrogen scale.
4. A process as defined in claim 3 wherein the nitrogen in the oxidized nitrogen species has a valence of +3 or +4.
5. A process as defined in claim 4 wherein the liquid fraction from the solid-liquid separation is treated for gold and silver recovery before the liquid fraction is recycled to step (a) of the process.
6. A process as defined in claim 4 wherein the solubilized iron, arsenic and sulphur are precipitated from the liquid fraction of step (k) and the precipi-tated iron, arsenic and sulphur are removed from the process before the liquid fraction is recycled to step (a) of the process.
7. A process as defined in claim 4 wherein the oxidation-reduction reaction in the common volume space and the denitrating vessel is conducted at a temperature of about 60°C to about 180°C.
8. A process as defined in claim 4 wherein the oxidation-reduction reaction in the common volume space and the denitrating vessel is conducted at a pH of less than about 3.
9. A process as defined in claim 4 wherein the oxidation-reduction reaction in the common volume space and the denitrating vessel is conducted at a pH less than or equal to about 1.
10. A process as defined in claim 4 wherein the oxidized nitrogen species concentration in the common volume space is between about 0.25 M and about 4.0 M.
11. A process as defined in claim 4 wherein the oxidized nitrogen species concentration in the common volume space is between about 0.5 M and about 3.0 M.
12. A process as defined in claim 4 wherein the oxygen partial pressure in the common volume space is at least about 50 psig.
13. A process as defined in claim 4 wherein the partial pressure of the nitric oxide in the denitrating vessel is greater than the pressure in the common volume space.
14. A process as defined in claim 2 wherein solubilized iron, arsenic or sulfur withdrawn in the liquid from solid-liquid separation step (b) is precipi-tated as jarosite and ferric arsenate from the liquid fraction by raising the temperature of the liquid fraction to a temperature of about 100°C and removing precipitated solids from the liquid fraction.
15. A process as defined in claim 2 wherein solubilized iron, arsenic or sulfur withdrawn in the liquid from solid-liquid separation step (b) is precipi-tated as jarosite, ferric arsenate, and anhydrite from the liquid fraction by neutralizing surplus acid gen-erated during sulphide leaching, and removing precipi-tated solids from the liquid fraction.
16. A process as defined in claim 15 wherein a calcium bearing substance is used to remove solubilized sulphur from the liquid fraction, ferric arsenate is added as a nucleating agent, and the liquid fraction is heated to precipitate ferric arsenate.
17. A process as defined in claim 1 wherein part of the remaining liquid free of oxidized nitrogen species is recycled to the concentrate or ore to prevent the pyritic or arsenopyritic concentrate or ore from oxidizing until the concentrate or ore is introduced into the denitrating step.
Applications Claiming Priority (2)
| Application Number | Priority Date | Filing Date | Title |
|---|---|---|---|
| US86802986A | 1986-05-29 | 1986-05-29 | |
| US868,029 | 1986-05-29 |
Publications (1)
| Publication Number | Publication Date |
|---|---|
| CA1309259C true CA1309259C (en) | 1992-10-27 |
Family
ID=25350953
Family Applications (1)
| Application Number | Title | Priority Date | Filing Date |
|---|---|---|---|
| CA 538019 Expired - Fee Related CA1309259C (en) | 1986-05-29 | 1987-05-26 | Hydrometallurgical arsenopyrite process |
Country Status (2)
| Country | Link |
|---|---|
| AU (1) | AU582961B2 (en) |
| CA (1) | CA1309259C (en) |
Family Cites Families (1)
| Publication number | Priority date | Publication date | Assignee | Title |
|---|---|---|---|---|
| CA1219132A (en) * | 1983-01-18 | 1987-03-17 | Morris J.V. Beattie | Hydrometallurgical arsenopyrite process |
-
1987
- 1987-04-30 AU AU72250/87A patent/AU582961B2/en not_active Ceased
- 1987-05-26 CA CA 538019 patent/CA1309259C/en not_active Expired - Fee Related
Also Published As
| Publication number | Publication date |
|---|---|
| AU582961B2 (en) | 1989-04-13 |
| AU7225087A (en) | 1987-12-03 |
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