EP3555327A1

EP3555327A1 - Processing of industrial metal-containing waste materials

Info

Publication number: EP3555327A1
Application number: EP17880716.0A
Authority: EP
Inventors: Pertti Koukkari; Jussi Rastas; Pekka Saikkonen; Väinö HINTIKKA
Original assignee: Valtion Teknillinen Tutkimuskeskus
Current assignee: Valtion Teknillinen Tutkimuskeskus
Priority date: 2016-12-15
Filing date: 2017-12-15
Publication date: 2019-10-23
Also published as: EP3555327A4; FI128281B; FI20165972L; WO2018109283A1

Abstract

The present invention relates to a process for separating metals using a series of precipitations from waste materials of the zinc or steel industry, or both. The method is characterized in carrying out the precipitations using hydroxides or sulfur-containing chemicals selected from sulfates, sulfides, sulfur oxides and sulfites, or both, in order to obtain metal precipitates as well as an aqueous sulfur-containing solution, the latter optionally being recycled in the process, where after a final precipitate is carried to a thermal step, for forming

Description

PROCESSING OF INDUSTRIAL METAL-CONTAINING WASTE MATERIALS

Background of the Invention Field of the Invention

[0001] The present invention concerns the hydro metallurgical processing of industrial waste materials in order to separate fractions containing valuable metals therefrom.

[0002] Particularly, the materials to be processed are obtained from the zinc or steel industries, or both. Suitable waste materials that can be processed according to the invention, either separately or combined, are jarosite and goethite rejects of the zinc industry, as well as the zinc-containing dusts (such as electric arc furnace dusts, i.e. EAF dusts) of the steel industry.

Description of Related Art

[0003] Presently, a main part (85 %) of the world's zinc production takes place by an electrolytic process. After the year 1970, several zinc factories started to operate using so-called jarosite and goethite methods, whereby the recovered yield of zinc in these processes increased from less than 90% to 97-98%. Although the obtained yield of zinc increased, a negative result of these new processes was the resulting large amount of waste, in the form of jarosite and goethite residues.

[0004] Jarosite is a basic hydrous sulfate mineral of iron (A[Fe3S0₄)2(OH)₆], A =

H₃0, Na, K, NH₄), which is formed in ore deposits by the oxidation of iron sulfides, and is, as mentioned, produced as a byproduct during the purification and refining of zinc. [0005] Goethite is a hydroxide mineral of iron (FeOOH), which is found in soil and other low-temperature environments. It exists as an iron ore, which is commonly found in waste materials of the steel industry, but is, as mentioned above, also formed during zinc production. [0006] For these residues, the zinc processes require sufficiently large waste areas close to the factories, and these waste areas need to be fitted with impermeable

foundations. Also other environmental requirements need to be considered. In many cases also valuable elements contained by the ore end up in these waste residues.

[0007] Constantly increasing amounts of waste have become a serious issue for zinc producers, and an environmental problem. On the other hand, the valuable elements contained in the waste also constitute a considerable potential value. [0008] In the electrolytic zinc process, there is thus a considerable need for a procedure for removing or at least considerably reducing the continuously growing waste problem in an economical manner, and simultaneously recovering valuable substances from the waste. [0009] In US3871859A there is described a process for utilizing jarosite waste by acidifying and crystallizing, but this process fails to separate the different metals contained in the jarosite from each other. Instead, the product is used as a slightly purified combination of components for fertilization purposes. [0010] About half of the currently produced zinc is used in prevention of corrosion, whereby a major portion will be returned with steel waste into the electromelting-type processes of the steel industry. During this processing, the zinc becomes evaporated and oxidized, and is carried away from the process with the formed dusts. [0011] The zinc content of these dusts of the steel factories varies between 30 and 40%. The dusts are commonly processed using the Walz process (see e.g. EP0709472B1), which is not a simple process, and which includes the problem that a portion of the halogens added to the process remains in the product, i.e. in the Walz oxide. The halogens are usually removed from the Walz oxide using a two- or three-fold wash with a Na₂CC"3 solution, before the thus treated oxides can be fed to the electrolytic zinc process. It would pose a considerable advantage if the dusts of the steel industry could be processed in a manner facilitating early removal of halogens, taking place in a close vicinity to the zinc manufacturer.

[0012] The research on jarosite waste has to the present date been focusing on the utilization of metallic sulphates, among others, in the construction material industry, while the possibility of using these in combination with enrichment sands in landfills has been determined (see e.g. Moors & Dijkema 2006, Rathore & al. 2014). These utilizations involve a problem, relating to the toxicity of the soluble heavy metals contained in these materials, as well as the sulfur and arsenic.

[0013] However, storing the waste at permanent industrial waste disposal sites requires extraordinary measures for eliminating possible leakage and runoff, as well as other detrimental environmental effects. Thus, this storage alternative would cause high costs and would be difficult to implement.

[0014] While the demand for raw materials increases, an increasing amount of attention is still focused on the recovery of critical metals contained in jarosite waste. The various possibilities linked with the recovery of metallic value from jarosite waste have been emphasized, among others, in the report from the year 2013 by the UN

Environmental Program (UNEP), concerning the recycling of metals. For example in Korea, it has been attempted to develop a high-temperature pyrometallurgical process based on the so-called Ausmelt technique (see the UNEP report), where the roasting of zinc waste, or early reductive melting, is combined with the recovery of metals. [0015] Applying a high-temperature melting technique to the treatment of jarosite waste is, however, technically challenging and, due to its high demand of energy, uneconomical. Some possibilities relating to early thermal treatments and subsequent hydrometallurgical processing have also been presented in the research, but the suggested techniques have not resulted in commercial solutions.

[0016] In the late 1970's in Finland, particularly Outokumpu Oy began to pay attention to the processing of jarosite (both the jarosite obtained from the process and the jarosite stored in the waste area. However, the used procedure focused mainly on a sulfidization and flotation in order to separate lead, silver and gold from the material, while other components remained as a waste (US 4,385,038).

Summary of the Invention

[0017] It is an object of the present invention to solve at least some of the problems related to the prior art. [0018] Thus, according to a first aspect of the present invention, there is provided a multistep process for separating metals from a waste material of the zinc or steel industry, or combined waste materials.

[0019] According to a second aspect of the present invention, there is provided a use of said process in separating valuable metals, such as zinc, lead, iron, silver and gold, from waste materials of the zinc or steel industry, or combined waste materials. [0020] The suggested hydrometallurgical processing of the invention makes it possible to eliminate the halogen problem formed in connection with using the Walz process, while in a unique manner providing a procedure for the zinc industry for utilizing the jarosite precipitate, which has up to the present date been stored as hazardous waste, in connection with zinc production. Further, a concentrate containing significant amounts of lead, silver and gold is also obtained in the present process.

[0021] In the present process, several types of industrial waste, including waste dust of the steel mills and jarosite or goethite waste of the zinc mills, can be processed by hydrometallurgy, either separately or combined. The products include the valuable metal fractions that these wastes contain, recovered as utilizable concentrates.

[0022] The invention provides an advantageous and environmentally friendly solution for recycling the zinc-containing waste dust of the steel mills in connection with the recovery of metals from the jarosite precipitate formed as a waste in the zinc mills.

[0023] Using the present invention, it is possible to utilize, in an advantageous and cost-efficient manner, not only the main components of jarosite (i.e. zinc, iron and lead), but also the critical metals it contains in smaller concentrations (such as silver, gold, indium and gallium), and similar metals of other waste materials.

Brief Description of the Drawings

[0024] FIGURE 1 is a block diagram illustrating the processing steps in accordance with at least some embodiments of the present invention. [0025] FIGURE 2 is an alternative block diagram illustrating the process steps in accordance with at least one embodiment of the present invention. Embodiments of the Invention

[0026] Definitions

The "hydrometallurgical processing" of the invention is intended to cover a multistep procedure for separating at least the valuable components from the starting material, i.e. the waste, the procedure including steps of acidifying, precipitating, concentrating and heat treating, as well as one or more steps of metals recovery.

The term "waste" is intended to cover all by-products of metal production industries, particularly the metal-containing by-products of the zinc and steel industries.

In said context, the term "metal" is intended to encompass the elements of the periodic table of elements that belong to the transition metals, post-transition metals and metalloids, the groups of transition and post-transition metals having the highest significance.

At least one precipitation step of said process is carried out using a "sulfur- containing chemical", which is intended to cover sulfates, sulfides, sulfur oxides and sulfites, which generate chemicals that easily can be reacted into a suitable form to be recycled, and optionally used in a sulfur dioxide treatment. [0027] Thus, the present invention relates to a process for separating metals from waste materials of the zinc or steel industry, or both. The material to be processed can be a metal-containing waste material or a combination of two or more such waste materials.

[0028] The process includes a series of precipitations using e.g. sulfur-containing chemicals selected from sulfates, sulfides, sulfur oxides and sulfites, and hydroxides, in order to obtain an aqueous sulfur-containing solution, which optionally is recycled in the process, whereafter a final precipitate is carried to a thermal step, for forming and separating solid oxides from the sulfates remaining in the solution phase. [0029] Figure 1 illustrates a process scheme in accordance with an embodiment of the invention.

[0030] According to this embodiment, a sulfuric acid treatment using hot concentrated sulfuric acid is first carried out on an industrial zinc-containing dust, such as an electric arc furnace dust (an EAF dust).

[0031] Preferably, the acid is heated to a temperature of > 100°C, particularly to about 200°C, and is mixed with the preheated (e.g. 100-150°C) dust. The temperature of the formed mixture then rises, typically to more than 250°C. As a result, the oxides in the dust are sulfatized to form a sulfate phase, while the halogenides also contained therein are decomposed and sulfatized, generally at least to a degree of 70%. The water and the halogen hydrides of the formed mixture are transferred to the gas phase, from where they can be removed, e.g. by compressing, preferably using water washing. [0032] The reactions taking place in this process step include one or more, preferably all, of the following listing:

[0033] By continuing the treatment of the sulfatized dust with a heat treatment at a temperature of 400-600°C, the halogens can be removed from the solid fraction. When using temperatures at the higher end of this range, the halogen removal is almost complete (at 600 °C, 98% of the chlorides are removed and 95% of the fluorides). Such

dehalogenated sulfatized dusts can be carried as such to be used in zinc processes.

[0034] However, when treated further as herein described, such an almost complete halogen removal is not required.

[0035] The obtained solid sulfatized dust, optionally mixed with further metal- containing waste materials, such as jarosite and/or goethite waste, are fed to a S0₂ dissolution step. [0036] The temperature during said dissolution step is preferably >50°C and

<100°C, most suitably about 90°C, whereby one or more of the reactions of the following listing take place, typically all of the reactions, as long as the relevant metals are present in the treated waste material.

[0037] During the dissolution, a vast amount of the metal components of the raw material(s) is/are dissolved, thus ending up in the formed S0₂ solution phase, and the iron (Fe³⁺) is reduced to its Fe²⁺ form, which has higher potential in the subsequent reactions. Said reduction also produces sulfuric acid (see e.g. reaction (17)), which causes further dissolution of components of the raw material(s), which require harsh dissolution conditions (e.g. ferrite).

[0038] After the S0₂ dissolution step, the formed phases are separated, the obtained solid residue is washed and the washing solution is added to the S0₂ solution phase.

[0039] This S0₂ solution phase is, according to the embodiment described in Fig. 1, processed further in later described steps, while the solid residue is carried to a

sulfidization and flotation step for concentrating and recovering a metals fraction.

[0040] In this sulfidization and flotation, the dissolution residue is first suspended into water to form a slurry. Subsequently, sodium sulfide, or another similar sulfide reagent, is added to the sludge (see reactions (30) and (31)) in an amount equivalent to the lead and silver present in the residue, and the mixture is floated to give a first fraction of metal sulfides and a first S0₄ solution.

[0041] In this step, the following reactions take place:

(30) PbS04 (s) + Na₂S (aq) => PbS (s) + Na₂S04 (aq)

(31) Ag2S04 (s) + Na₂S (aq) => Ag₂S (s) + Na₂S04 (aq)

[0042] Typical products of this step are concentrates containing lead, silver and gold. The remaining waste materials are preferably discarded as a sulfide waste residue, while the S0₄ solution can be recycled or combined with the previously obtained S0₂ solution.

[0043] In the flotation, it is assumed that the yield of lead in comparison to the dissolution residue is 97%, and the yield of silver and gold is 95%>. No significant amounts of minor components of the dissolution residue, such as gypsum and Si0₂, are included in the sulfide residue, although trace amounts are inevitably carried there.

[0044] In the following step shown in Fig. 1 , the indium (In) and gallium (Ga), and possibly germanium (Ge) are separated from the S0₂ solution (optionally combined with the first S0₄ solution) by adjusting the pH to a level of 3.5-4, preferably using a solution containing magnesium hydroxide (Mg(OH)₂) as the pH adjustment agent (causing precipitation). The temperature of the solution is between 80 and 90°C. Other possible pH adjustment agents are zinc oxide (ZnO), Walz-oxide (or the ZnO therein), calcium oxide (CaO), calcium hydroxide (Ca(OH)₂) and calcium carbonate (CaC0₃).

[0045] Due to the possibility to use a hydroxide as the pH adjustment agent, this step can be called a hydroxide precipitation step. The fractions obtained in this step are thus a second S0₄ solution and a solid phase containing a first residue of metal hydroxides.

[0046] The solubility product values of the obtained hydroxides vary to some extent, depending on their source. If the solubility product values for the indium and gallium hydroxides are equal to or lower than 10(exp(-36)), and if the corresponding value for aluminium hydroxide is 10(exp(-31)), it is possible to obtain a sharp distinction. If the pH adjustment range is 3.5-4, the precipitate will, however, contain also aluminium hydroxide.

[0047] When assuming that indium, gallium and aluminium hydroxides are precipitated in a pure form, and that germanium is precipitated in the form of its hydroxide, the following reactions take place:

[0048] Preferably, the precipitated hydroxides are separated from the second S0₄ solution, and are washed, whereby the washing solution can be added to the original second S0₄ solution. The thus recovered precipitate will contain In, Ga, Ge and Al hydroxides. [0049] According to another option, the Indium, Gallium and Germanium can be separated using a liquid-liquid extraction.

[0050] The following step according to Fig. 1 is a sulfide precipitation, which is carried out by adding hydrogen sulfide (H₂S) to the second S0₄ solution obtained in the previous step, while adjusting the pH of the solution, for example using Mg(OH)₂, so that no significant amounts of iron (Fe²⁺) is precipitated. The reactions taking place during this step of the process preferably include the following:

[0051] Thus, a third S0₄ solution is obtained, which can then be carried to the following step shown in Fig. 1, while also a sulfide precipitate is obtained, which contains the sulfides shown in the above reactions.

[0052] This precipitate can then be treated further with a polysulfide solution, preferably an ammonium polysulfide solution, whereby the sulfides of the precipitate can be separated into a solid phase, containing a third fraction of metal sulfides, and a solution phase. Thus, AS2S3, Sb₂S3 and SnS dissolve as in the following reactions, whereas CuS, CdS and ZnS remain in solid form.

[0053] The obtained solid and solution phases are then separated, whereby the solid phase is washed using a solution based on ammonium polysulfide, and the washing solution is combined with the solution phase. [0054] The solution phase and the dissolved metals therein are can then be treated by an addition of sulfuric acid, whereby the sulfides of arsenic, antimony and tin are precipitated (reactions (46) - (48)).

[0055] After these separations, a sulfide solution remains, which can be recycled and reused.

[0056] The above mentioned third SO₄ solution is, according to the procedure shown in Fig. 1, then concentrated by a multiphase evaporation crystallization, where the formed steam phase can optionally be cooled, compressed and returned to the S0₂ dissolution step for reuse.

[0057] The salt phase remaining after the evaporation, after removal of the steam phase, is, according to Fig. 1, carried to a thermal step, where the following reactions preferably take place:

[0058] Thus, a remaining portion of the metals form a mixture of solubilized sulfates and non-soluble oxides. In typical conditions, the iron, manganese, nickel, cobalt and aluminum of said salt phase are turned into their oxide forms during said thermal step, whereas magnesium remains as a sulfate. [0059] Due to their S0₂ content, the gaseous products also formed are preferably returned to the above described S0₂ dissolution step.

[0060] The solid residue obtained in the thermal step is preferably carried to a water washing step, where the soluble components are transferred to the solubilized sulfate phase, and the non- soluble oxide phase (mostly containing Fe₂0₃) forms an iron concentrate.

[0061] Based on the above, according to a preferred embodiment of the invention, the overall process includes the following steps:

- a sulphuric acid treatment using hot concentrated sulphuric acid, preferably carried out on a waste dust obtained from the steel industry,

- optionally mixing one or more further waste materials with the acid-treated

material, preferably including jarosite or goethite, or both, particularly being jarosite,

- a sulphur dioxide (S0₂) dissolution step, where a solid residue and a S0₂ solution phase are formed,

- sulfidization and flotation of the solid dissolution residue, to obtain a first fraction of metal sulfides and a first S0₄ solution, which metal sulfides can be recovered

- hydroxide addition and subsequent precipitation of metal hydroxides from the combined solution phases of the dissolution step (the S0₂ solution) and of the sulfidization and flotation step (the first S0₄ solution), whereby a solid phase and a second S0₄ solution phase are formed, the solid phase containing a first fraction of metal hydroxides, which can be recovered,

- sulfide addition and subsequent precipitation of a precipitate from the hydroxide solution phase, the step also yielding a third S0₄ solution,

- a polysulfide treatment of the sulfide precipitate obtained from the polysulfide treatment, in order to provide dissolved sulfides and a second fraction of metal sulfides, which latter can be recovered,

- a sulfuric acid treatment of the dissolved sulfides obtained from the polysulfide treatment, whereby also the sulfides therein are precipitated, and can be recovered as a third fraction of metal sulfides, - a step of concentrating the thus obtained solution phase in order to form a salt phase, followed by carrying out a thermal step on the formed salt phase, where a remaining portion of the metals form a mixture of metal sulfates and metal oxides, and

- finally a washing step carried out on the mixture of sulfates and oxides, whereby a non- soluble metal oxide phase and a solubilized sulfate phase are obtained, and can be recovered.

[0062] Figure 2 illustrates an alternative process scheme in accordance with an embodiment of the present invention. The process scheme of this Figure includes a step of roasting the solid phase obtained from the dissolution step, before sulfidization and flotation.

[0063] Said roasting step is intended to oxidize any elemental sulphur present in the solid phase obtained from the dissolution step, according to the following reaction (54):

(54) S(s) + 0₂(g) => S0₂(g)

[0064] This step can be essential in certain cases, since many zinc processes recently developed result in jarosite fractions that are rich in elemental sulphur, while sulphur in its elemental form would have a negative effect on the subsequent sulfidization and flotation step.

[0065] It is to be understood that the embodiments of the invention disclosed are not limited to the particular structures, process steps, or materials disclosed herein, but are extended to equivalents thereof as would be recognized by those ordinarily skilled in the relevant arts. It should also be understood that terminology employed herein is used for the purpose of describing particular embodiments only and is not intended to be limiting. [0066] Reference throughout this specification to one embodiment or an

embodiment means that a particular feature, structure, or characteristic described in connection with the embodiment is included in at least one embodiment of the present invention. Thus, appearances of the phrases "in one embodiment" or "in an embodiment" in various places throughout this specification are not necessarily all referring to the same embodiment. Where reference is made to a numerical value using a term such as, for example, about or substantially, the exact numerical value is also disclosed.

[0067] As used herein, a plurality of items, structural elements, compositional elements, and/or materials may be presented in a common list for convenience. However, these lists should be construed as though each member of the list is individually identified as a separate and unique member. Thus, no individual member of such list should be construed as a de facto equivalent of any other member of the same list solely based on their presentation in a common group without indications to the contrary. In addition, various embodiments and examples of the present invention may be referred to herein along with alternatives for the various components thereof. It is understood that such embodiments, examples, and alternatives are not to be construed as de facto equivalents of one another, but are to be considered as separate and autonomous representations of the present invention.

[0068] Furthermore, the described features, structures, or characteristics may be combined in any suitable manner in one or more embodiments. In this description, numerous specific details are provided, such as examples of lengths, widths, shapes, etc., to provide a thorough understanding of embodiments of the invention. One skilled in the relevant art will recognize, however, that the invention can be practiced without one or more of the specific details, or with other methods, components, materials, etc.

[0069] While the forgoing examples are illustrative of the principles of the present invention in one or more particular applications, it will be apparent to those of ordinary skill in the art that numerous modifications in form, usage and details of implementation can be made without the exercise of inventive faculty, and without departing from the principles and concepts of the invention. Accordingly, it is not intended that the invention be limited, except as by the claims set forth below. [0070] The following non-limiting examples are intended merely to illustrate the advantages obtained with the embodiments of the present invention. EXAMPLES

[0071] In these examples, each step of the overall process is described with quantitative details of the composition, starting with the content of the raw materials, shown in Tables 1 and 2.

[0072] The annual feed amounts of jarosite and dust have in the below calculations been selected to be 400,000 t/a and 20,000 t/a, respectively, and the processing time 8000 h/a.

Example 1 - H₂S0₄ treatment

[0073] A sulphuric acid treatment is carried out on the dust. The sulphuric acid is heated to a temperature of 200°C and mixed with the preheated (100-150°C) dust. The temperature of the formed mixture thus rises to more than 250°C. As a result, the oxides in the dust are sulphatized, the halogenides are decomposed and sulphatized at least to a degree of 70%. The water and the halogen hydrides are transferred to the gas phase, from where they are compressed using water washing. The reactions of this sulphuric acid treatment are further described below, in Tables 3-5. Table 3. Reactions with sulfuric acid:

[0074] In addition to the sulphates, also NaCl, KF and CaF₂ are considered to belong to the sulphate phase.

Table 5. Halogenides

Example 2 - S0₂ dissolution

[0075] Jarosite and the sulphatized dust are fed to an S0₂ dissolution step. The temperature in said dissolution is about 90°C, whereby the reactions of the following listing take place, affecting the components of the jarosite and the components of the sulphatized dust. In this reaction listing, it has been assumed that all reactions (16) - (29) have a reaction degree between 0.95 and 1.00.

Example 3 - Sulphidization of the dissolution waste and flotation of the sulphide phase

[0077] In the sulphidization and flotation step, the following reactions (30) and (31) take place:

(30) PbS04 (s) + Na₂S (aq) => PbS (s) + Na₂S04 (aq)

(31) Ag2S04 (s) + Na₂S (aq) => Ag₂S (s) + Na₂S04 (aq) [0078] The solid dissolution waste is first suspended into water to form a slurry.

Subsequently, sodium sulfide is added to the sludge (see reactions (30) and (31)) in an amount equivalent to the lead and silver, and the sludge is floated. The sulfide phase and the gold are floated. [0079] In the flotation, it is assumed that the yield of lead in comparison to the sulfide phase is 97%, and the yield of silver and gold is 95%>. No significant amounts of minor components of the dissolution waste, such as gypsum and Si0₂, are included in the sulfide waste, although trace amounts are inevitably carried to the sulfide phase. [0080] Solution 1 (dissolution solution) and 2 (sulfidization and flotation solution) are combined to form solution 3 (see the following Table 8).

Example 4 - The separation of indium, gallium ja germanium

[0081] The indium and gallium are separated from Solution 3 by a hydroxide precipitation, by adjusting the pH to a level of 3.5-4, using Mg(OH)₂ as the pH adjustment agent. The temperature of the solution is between 80 and 90°C. Other possible pH adjustment agents are ZnO, Walz-oxide (or the ZnO therein), CaO, Ca(OH)₂ or CaC0₃.

[0082] The hydroxides In(OH)₃ and Ga(OH)₃ are less soluble compared to Al(OH)₃, which is also one of the least soluble hydroxides of the solution phase (when no ferric ions are present).

[0083] The values of the solubility products of the hydroxides varies to some extent, depending on their source. If the solubility product values for the indium and gallium hydroxides are equal to or lower than 10(exp(-36)), and if the corresponding value for aluminium hydroxide is 10(exp(-31)), it is possible to obtain a sharp distinction. If the pH adjustment range is 3.5-4, the precipitate will contain also aluminium hydroxide.

[0084] Germanium (and gallium) can be precipitated completely from the solution in the form of a tannine. When assuming that indium, gallium and aluminium hydroxides are precipitated in a pure form, and germanium is precipitated in the form of its hydroxide, the following reactions take place:

(32) I (S04)3 (aq) + 3 Mg(OH)₂ (s) => 2 In(OH)₃ (s) + 3 MgS04 (aq)

(33) Ga2(S0₄)3 (aq) + 3 Mg(OH)₂ (s) => 2 Ga(OH)₃ (s) + 3 MgS0₄ (aq)

(34) GeS04 (aq) + Mg(OH)₂ (s) => Ge(OH)₂ (s) + MgS04 (aq)

(34') Al₂(S0₄)₃(aq) + 3Mg(OH)₂(s) => 2Al(OH)₃(s) + 3MgS0₄(aq)

(35) H₂S04 (aq) + Mg(OH)₂ (s) => MgS04 (aq) + 2 H₂0

[0085] The precipitated hydroxides are separated from the solution, and are washed. The washing solution is added to the solution phase. Thus, a precipitate containing In, Ga and Ge hydroxides is obtained, and a Solution 4.

Example 5 - Sulfide precipitation

[0086] The sulfide precipitation is carried out by adjusting the pH of the solution, for example using Mg(OH)₂, so that no significant amounts of iron (Fe²⁺) is precipitated. The 5 reactions taking place during the precipitation include the following:

(36) 2H₃As03 (aq) + 3H₂S (aq) => AS2S3 (s) + 6 H2O

(37) 2HSb02 (aq) + 3H₂S (aq) => Sb₂S₃ (s) + 4 H2O

(38) SnS0₄ (aq) + H2S (aq) => SnS (s) + H₂S04 (aq)

(39) CuS04 (aq) + H2S (aq) => CuS (s) + H2SO4 (aq)

(40) CdS0₄ (aq) + H2S (aq) => CdS (s) + H₂S04 (aq)

(41) ZnS04 (aq) + H2S (aq) => ZnS (s) + H2SO4 (aq)

(42) H2SO4 (aq) + Mg(OH)₂ (s) => MgS04 (aq) + 2 H2O

[0087] The obtained sulfide precipitate 1 is treated with an ammonium polysulfide solution, whereby AS2S3, Sb₂S3 and SnS dissolve, whereas CuS, CdS and ZnS remain in solid form.

(43) AS2S3 (s) + 3 (NH4 S (aq) + 2 S => 2 (NH₄)₃AsS4 (aq)

(44) Sb₂S3 (s) + 3 (NH4 S (aq) + 2 S => 2 (NH₄)₃SbS4 (aq)

(45) SnS (s) + (NH4 S (aq) + S => (NH₄)₂SnS3 (aq)

(Ammonium polysulfide: (NH4)2Sn (n = 2 ... 5))

[0088] The solid and solution phases are then separated, whereby the solid phase is washed using a solution based on ammonium polysulfide, and the washing solution is combined with the solution phase, i.e. with Solution 5. [0089] Sulphuric acid is added to the thus obtained solution phase, whereby the sulfides of arsenic, antimony and tin are precipitated from the solution (reactions (46) - (48)). After these steps, sulfide precipitates 2 and 3 are obtained, as well as a sulfide solution, which can be recycled and reused.

(46) 2 (NH4 AsS4 (aq) + 3 H2SO4 (aq) => AS2S5 (s) + 3 (NH4 S04 (aq) + 3 H2S (aq)

(47) 2 (NH₄)₃SbS4 (aq) + 3 H2SO4 (aq) => Sb₂S₅ (s) + 3 (NH₄)₂S04 (aq) + 3 H2S (aq)

(48) (NH₄)₂SnS3 (aq) + H₂S04 (aq) => SnS2 (s) + (NH₄)₂S04 (aq) + H2S (aq)

Example 6 - Evaporation crystallization

[0090] Solution 5 is concentrated by a multiphase evaporation crystallization. The steam phase is cooled, compressed and returned to the

S0₂ dissolution.

Table 13

Example 7 - Thermal treatment

[0091] The salt phase is carried to a thermal phase, where the following reactions take place:

(49) (NH₄)₂S04 (s) + O2 (g) => N2 (g) + SCh (g) + 4 H2O (g)

(50) 2 FeS04-H₂0 (s) => Fe₂03 (s) + 2 SO2 (g) + 1/2 O2 (g) + 2 H2O (g)

(51) MgS04-H₂0 (s) => MgS04 (s) + H2O (g)

(52) MeS04-H₂0(s) => MeO (s) + SCh (g) + 1/2 02 (g) + H2O (g)

(Me = Mn, Ni, Co)

(53) Ak(S04)3 -6 H2O (s) => AI2O3 (s) + 3 SO2 (g) + 3/2 O2 (g) + 6 H2O (g)

[0092] In the calculations it has been assumed that the iron, manganese, nickel, cobalt and aluminum are completely turned into their oxide forms, whereas magnesium remains as a sulfate.

Table 14

[0093] The reaction product obtained after the thermal step is carried to a water washing step, whereby the soluble components are transferred to the solution phase, and the non- soluble oxide phase (mostly containing Fe₂C"3) forms an acceptable iron concentrate. Table 15

Table 16. Water fed to the washing step Example 8 - Recirculation

[0094] Certain components can be recirculated in the process, particularly in case all of the above mentioned process steps are carried out in series as described. These components include sulfur dioxide (S0₂), sulfuric acid (H₂SO₄) and the sulfide solution containing ammonium sulfate ((NH₄)₂S0₄) and hydrogen sulfide (H₂S).

Table 17. Recirculation

Example 9 - Product yields

[0095] In case all of the above mentioned process steps are carried out in series as described, the following products can be obtained, for example in the following yields, as obtained in the experiments carried out by the inventors.

Table 18. Products

[0096] Certain separated fractions can be utilized after some further treatments

(separations or purifications). These include the following.

Table 19. Fractions that rovide useful roducts after further se arations

[0097] Silver and gold can, in turn be separated from the final waste, by treating it further using conventional techniques.

Example 10 - Total procedure

[0098] Using the complete processing scenario as described in Fig. 2, the complete process was then performed in lab scale. .

[0099] The raw materials are shown in the following Table 20.

[00100] The Jarosite material used was selected from those containing large amounts of elemental sulfur, and obtained from the process applications, which utilize direct dissolving procedures.

[00101] The process steps in the following are thus based on the flowsheet as described in Fig. 2.

Table 21. H₂SO₄ treatment

Table 22. S0₂ dissolution

[00102] After the dissolving with S0₂, the liquid and solid phases are separated and the wash waters from washing the solids are returned to the liquid phase.

[00103] The solid fraction is treated with thermal, calcination, treatment at about 500 °C to remove the elemental sulfur (Reaction (30)), and ZnS reacts according to Reaction (31).

(30) S (s) + 02 (g) = => S0₂ (g)

(31) ZnS (s) + 3/2 02 => ZnO (s) + S02 (g)

[00104] After those reactions there are two phases; a solid, called calcine and gas phase with its S0₂-content. able 24. Solid calcine phase

[00105] The following steps are the sulfidization of the dissolution waste and flotation of the sulfide phase (reactions (32) and (33)). (32) PbS04 (s) + Na2S (aq) => PbS (s) + Na2S04 (aq)

(33) Ag2S04 (s) + Na2S (aq) => Ag2S (s) + Na2S04 (aq)

[00106] The treated dissolution waste is dissolved with water. To the amount of the contained lead and silver, the equivalent amount of Na₂S is added (ref. Reactions (32) and

[00107] The solid phase is flotated, with the sulfide phase and gold collected froth.

[00108] Solutions 1 and 2 are combined to get Solution 3.

Table 25. Solution3

[00109] In the following hydroxide precipitation, magnesium hydroxide is used as a neutralization agent at the temperature of 90 °C with very accurate pH-control.

(34) In₂(S0₄)₃ (aq) + 3 Mg(OH)₂ (s) => 2 In(OH)₃ (s) + 3 MgS0₄ (aq)

(35) Ga₂(S0₄)₃ (aq) + 3 Mg(OH)₂ (s) => 2 Ga(OH)₃ (s) + 3 MgS0₄ (aq)

(36) GeS0₄ (aq) + Mg(OH)₂ (s) => Ge(OH)₂ (s) + MgS0₄ (aq)

(37) H₂S0₄ (aq) + Mg(OH)₂ (s) => MgS0₄ (aq) + 2 H₂0 (XI) A1₂(S0₄)₃ (aq) + 3 Mg(OH)₂ (s) => 2 Al(OH)₃ (s) + 3MgS0₄ (aq) (X2) 2 FeS0₄ + H₂0₂ (aq) + H₂S0₄ (aq) => Fe₂(S0₄) (aq) + 2 H₂0

(X3) Fe₂(S0₄)₃ (aq) + 3 Mg(OH)₂ (s) => 2 Fe(OH)₃ (s) + 3 MgS0₄ (aq)

[00110] The precipitated hydroxides are separated and washed. The washing water added to the solution phase, resulting in a hydroxide precipitate and a Solution 4.

Table 26. Hydroxide precipitate and Solution 4.

[00111] The sulfide precipitation is performed by using pH-control with aid of Mg(OH)₂, in such amounts that the ferrous iron (Fe²⁺) will not substantially precipitate. (38) 2 H₃As0₃ (aq) + 3 H₂S (aq) => As₂S₃ (s) + 6 H₂0

(39) 2HSb0₂ (aq) + 3 H₂S (aq) => Sb₂S₃ (s) + 4 H₂0

(40) SnS0₄ (aq) + H₂S (aq) => SnS (s) + H₂S0₄ (aq)

(41) CuS0₄ (aq) + H₂S (aq) => CuS (s) + H₂S0₄ (aq)

(42) CdS0₄ (aq) + H₂S (aq) => CdS (s) + H₂S0₄ (aq)

(43) ZnS0₄ (aq) + H₂S (aq) => ZnS (s) + H₂S0₄ (aq)

(44) H₂S0₄ (aq) + Mg(OH)₂ (s) = => MgS0₄ (aq) + 2 H₂0

Table 27. Sulfide precipitation

Reagents (NH₄)₂S H₂SO,

g/mol 68,136 32,064 98,078

3,562 1,150 5,127

mmol 52,280 35,854 52,280

[00112] The sulfidic precipitate is treated with ammonium polysulfide solution, whereby AS2S3, Sb₂S3 and SnS dissolve, but CuS, CdS and ZnS remain undissolved.

(45) As₂S₃(s) + 3(NH₄)S(aq) + 2S => 2 (NH₄)₃AsS₄ (aq)

(46) Sb₂S₃(s) + 3(NH₄)₂S(aq) + 2S => 2 (NH₄)₃SbS₄ (aq)

(47) SnS(s) + (NH₄)₂S(aq) + S => (NH₄)₂SnS₃ (aq)

(Ammoniumpolysulfide: (NH₄)₂S_n (n = 2...5))

Table 28. Sulfide reci itate treatment

[00113] The solid and liquid phases are separated. The solid phase is washed with liquid containing ammoniumpolysulfide. The washing solution is connected with the main liquid phase, which is treated with sulfuric acid. In this stage, the sulfides of Arsenic, Antimony and Tin are precipitated (Reactions (46) . .. (48)) into Sulfide precip. 2 and 3, leaving a Sulfidic solution to be treated further. (48) 2 (NH₄)₃AsS₄ (aq) + 3 H₂S0₄ (aq) => As₂S₅ (s) + 3 (NH₄)₂S0₄ (aq) + 3 H₂S (aq)

(49) 2 (NH₄)₃SbS₄ (aq) + 3 H₂S0₄ (aq) => Sb₂S₅ (s) + 3 (NH₄)₂S0₄ (aq) + 3 H₂S (aq)

(50) (NH₄)₂SnS₃ (aq) + H₂S0₄ (aq) => SnS₂ (s) + (NH₄)₂S0₄ (aq) + H₂S (aq)

Table 29. Sulfidic solution 5

[00114] Solution 5 is then concentrated by evaporation. The vaporized phase is cooled down and returned back to the S02-solution.

Table 30. Concentration

[00115] The Salt Phase is then treated thermally according the following reactions:

10

(51) (NH₄)₂S0₄ (s) + 0₂ (g) => N₂ (g) + S0₂ (g) + 4 H₂0 (g) (52) 2 FeS0₄ H₂0 (s) => Fe₂0₃ (s) + 2 S0₂ (g) + 1/2 0₂ (g) + 2 H₂0 (g)

(53) MgS0₄ H₂0 (s) => MgS0₄ (s) + H₂0 (g)

(54) MeS0₄ H₂0(s) => MeO (s) + S0₂ (g) + 1/2 0₂ (g) + H₂0 (g) (Me = Mn, Ni,

(55) Al₂(S0₄)₃ -6 H₂0 (s) => A1₂0₃ (s) + 3 S0₂ (g) + 3/2 0₂ (g) + 6 H₂0 (g)

Table 31. Thermal reaction

[00116] After the thermal treatment, the solid phase is washed with water. At this stage all the soluble components are washed out and the insoluble residue, which contains mainly Fe₂0₃, can be collected as it is suitable for use as Iron making concentrate.

Table 32. Further products

The contents of the water fed to the washing step is shown in the following

Table 33. Washing water

Industrial Applicability

[00118] The present invention provides, among others, an environmentally friendly solution for recycling the zinc-containing waste dust of the steel mills in connection with the recovery of metals from the jarosite precipitate formed as a waste in the zinc mills.

[00119] With the present invention, it is possible to utilize, not only the main components of jarosite (i.e. zinc, iron and lead), but also the critical metals it contains in smaller concentrations (such as silver, gold, indium and gallium) Citation List

Patent Literature

EP0709472

US3871859

US4385038

Non-Patent Literature

United Nations Environment Programme, (2013), Metals Recycling Full Report Moors & Dijkema, Technological Forecasting & Social Change 73 (2006) 250-265 Rathore & al, Int. J. Civil Engineering & Technology, 5 (2014), Issue 11, pp. 192- 200

Claims

1. A process for separating metals from waste materials of the zinc or steel industry, or both, characterized by

- carrying the optionally pre-treated waste material(s) through a series of 2 - 5

precipitations, using hydroxides and/or sulfur-containing chemicals selected from sulfates, sulfides, sulfur oxides and sulfites, in order to obtain

o metal precipitates, which can be separated from the other obtained fractions, as well as

o one or more, separate or combined, aqueous sulfur-containing solution(s), which solution(s) optionally are recycled in the process,

- whereafter a final precipitate is carried to a thermal step, for forming and separating solid oxides from sulfates in the solution phase, and

- when using a waste dust as a raw material, by carrying out a sulfuric acid treatment on said dust using hot concentrated sulfuric acid.

2. The process according to claim 1, characterized by the following steps being carried out on the waste materials, including at least a waste dust obtained from the steel industry

- a sulfuric acid treatment using hot concentrated sulfuric acid, carried out on said waste dust, to obtain an acid-treated material,

- optionally mixing one or more further waste materials with the acid-treated

material, preferably including jarosite or goethite, or both,

- a sulphur dioxide (S0₂) dissolution step of the acid-treated material, or of the

mixture containing the acid-treated material and jarosite or goethite, or both, whereby a solid sulfate residue and a sulfate solution are formed, and

- carrying out one or more further precipitations on either one of the obtained phases, to obtain different metal precipitates, whereafter a final precipitate is carried to a thermal step, for forming and separating solid oxides from dissolved sulfates.

3. The process according to claim 1 or 2, characterized by the following steps being carried out on one or more of the waste materials

- a sulphuric acid treatment using hot concentrated sulphuric acid, preferably carried out on a waste dust obtained from the steel industry, - optionally mixing one or more further waste materials with the acid-treated material, preferably including jarosite or goethite, or both, particularly being jarosite,

- a sulfuric acid treatment of the dissolved sulfides obtained from the polysulfide treatment, whereby also the sulfides therein are precipitated, and can be recovered as a third fraction of metal sulfides,

- a step of concentrating the thus obtained solution phase in order to form a salt phase, followed by carrying out a thermal step on the formed salt phase, where a remaining portion of the metals form a mixture of metal sulfates and metal oxides, and

4. The process according to any preceding claim, characterized by carrying out the sulfuric acid treatment using sulfuric acid that has been heated, preferably to a temperature of about 200°C, and mixing this heated acid with a preheated raw material, preferably heated to a temperature of 100-150°C.

5. The process according to any preceding claim, characterized by carrying out a dissolution step using S0₂, and roasting the obtained solid phase, to obtain an undissolved residue containing metal(s).

6. The process according to claim 5, characterized by carrying out a sulfidization and flotation step on the undissolved residue from the dissolution step by adding any sulfidizising agent, such as sodium sulfide, and subsequently floating the mixture.

7. The process according to any preceding claim, characterized by carrying out a hydroxide precipitation on a metal-containing solution, preferably using magnesium hydroxide (Mg(OH)₂), in order to separate indium and gallium, and possibly germanium from the solution.

8. The process according to any preceding claim, characterized by carrying out a sulfide precipitation step using H₂S , while adjusting the pH of the thus formed mixture, preferably using magnesium hydroxide (Mg(OH)₂).

9. The process according to any preceding claim, characterized by carrying out an ammonium polysulfide treatment of a metal-containing precipitate, in order to separate the arsenic sulphide (AS₂S₃), antimony sulfide (Sb₂S3) and tin sulfide (SnS), which are dissolved in the NH₄ solution, from the copper sulfide (CuS), cadmium sulfide (CdS) and zinc sulfide (ZnS), which remain as a precipitate.

10. The process according to claim 9, characterized by carrying out a sulfuric acid treatment on a NH₄ solution, whereby the sulfides of arsenic, antimony and tin are precipitated.

11. The process according to any preceding claim, characterized by concentrating a dilute solution phase by evaporation, from which a major portion of the metals have already been crystallized, followed by a thermal step, where the iron, manganese, nickel, cobalt and aluminum of the concentrate are turned into their oxide forms, whereas magnesium remains as a sulfate.

12. The process according to claim 11, characterized by using a temperature of >300°C, particularly 500-1200°C, in the thermal phase, sufficient to oxidize at least a major portion, e.g. >50% by weight, of the metals in the material to be treated.

13. The process according to claim 11 or 12, characterized by washing the thermally treated remaining residue, whereby a non- soluble oxide phase (mostly containing Fe₂C"3) obtained, and a solution phase, which can be discarded or recycled, or treated further to separate further components.

14. The process according to claim 1, characterized by the following steps being carried out on a jarosite or goethite residue of the zinc industry

- feeding the residue directly to a sulphur dioxide (S0₂) dissolution step, where a solid phase and a S0₂ solution phase are formed,

- sulfidization and flotation of the solid dissolution residue, to obtain a first fraction of metal sulphides and a first S0₄ solution, which metal sulfides can be recovered

15. The process according to any preceding claim, characterized by carrying out a sulfuric acid treatment, on a waste dust obtained from the zinc industry, combining this treated dust with a jarosite or goethite waste material, and precipitating metal fractions from this combined material.

16. Use of the process according to any preceding claim in separating valuable metals from waste materials of the zinc or steel industry, these valuable metals including gold, silver, zinc and iron.

17. The use of claim 16 in separating further metals from waste materials of the zinc or steel industry, these metals including at least a major portion of the following: lead, gallium, germanium, indium, copper, cadmium, zinc, arsenic, antimony, tin and iron.