GB1573685A - Recovery of metal values - Google Patents

Abstract

For recovering metals adsorbed as ionic alkaline earth metal complexes on a carrier, in particular gold, silver, nickel or copper, an alkali metal cyanide solution, an alkali metal hydroxide solution or a mixture thereof is used for pretreating the carrier. The said solution preferably contains alkali metal cyanide in a concentration of 1 to 10% by weight and alkali metal hydroxide in a concentration of 1 to 20% by weight, the concentration of cyanide ions in general exceeding the concentration of hydroxyl ions. The process leads to high yields with respect to the recovery of especially rare metals.

Description

(54) RECOVERY OF METAL VALUES (71) We, ANGLO AMERICAN CORPORATION OF SOUTH AFRICA LIMITED, a company registered according to the laws of the Republic of South Africa, of 44 Main Street, Johannesburg, Transvaal, Republic of South Africa, do hereby declare the invention for which we pray that a patent may be granted to us, and the method by which it is to be performed, to be particularly described in and by the following statement: This invention relates to metal recovery, in particular to the recovery of gold, silver, nickel or copper metal values.

A method of recovering the above metals from supports of the charcoal type on which they are adsorbed as complexes is covered by U.K. Patent No. 1450486. According to this method, the metal values are desorbed by contacting the support with water of low metal cation concentration. The water preferably has a low-multi-charged cation (e.g. alkaline earth metal cation) concentration. Concentrations less than 100 ppm, preferably 50 ppm, are preferred. Suitable waters are for example distilled, deionised and softened waters.

The above method has application to metal values in the form of ionic complexes, the metal value forming the anionic portion of the complex.

It is also disclosed that when the cation of the complex is an alkaline earth metal, the complex is adsorbed on to the support very much more strongly than when the cation is an alkali metal, particularly when the complex is an aurocyanide one. Accordingly, if the cation of the complex is an alkaline earth metal, particularly calcium, the support is preferably pre-treated with an alkali metal salt solution prior to water desorption.

This pretreatment results in an exchange reaction between the alkali metal and the alkaline earth metal. The exchange reaction is enhanced by providing a salt the anion of which forms an insoluble or substantially insoluble salt with the alkaline earth metal so that the latter is effectively removed from the system as exchange takes place. Suitable salts for this purpose are carbonates, oxalates, sulphites, and fluorides, the alkali metal preferably being sodium, potassium, or lithium.

A preferred pretreatment solution according to the above patent is a potassium carbonate/potassium hydroxide solution having a basic pH.

We have now discovered that improved results are obtained with other pretreatment solutions.

According to the invention there is provided a method of recovering gold, silver, copper and or nickel metal values from a support of the charcoal type having one or more of these metal values adsorbed thereon in the form of an alkaline earth metal ionic complex, the metal value forming part of the anionic portion thereof including the steps of contacting the support with a solution which is an alkali metal cyanide solution, an alkali metal hydroxide solution or a mixture thereof, and then followed by desorbing the metal values from the support with water having a low concentration of metal cations.

The solution may be one containing alkali metal cyanide of concentration in the range 1 to 10 percent by weight and alkali metal hydroxide of concentration in the range 1 to 20 percent by weight. It is preferred that the concentraton of cyanide ions exceeds the concentration of the hydroxyl ions in solution. The preferred mixture is sodium cyanide and sodium hydroxide.

One particularly suitable pretreatment solution contains an alkali metal cyanide concentration of about 10 percent by weight and an alkali metal hydroxide concentration of about 1 percent by weight.

Another suitable pretreatment solution is a sodium hydroxide solution. It has been found that concentrations not exceeding 10 percent by weight provide satisfactory results.

As disclosed in the above mentioned patent, the preferred concentration of metal cations in the water is less than 100 ppm. Initially the metal values are adsorbed on to the support in the form of an alkaline earth metal ionic complex. The preferred support is activated carbon. The desorbing, after treatment of the support with the selected solution, is carried out under the conditions and using waters as described above and in the above mentioned patent. It is important to note that the process of desorbing involves only physical adsorption/desorption action and does not involve ion exchange.

The alkaline earth metal will generally be calcium and the complex will generally be a cyanide ionic complex. The invention has particular application to the recovery of gold and other metal values from solutions containing these values obtained in the cyanidation of gold bearing ores with, for example, a sodium cyanide/calcium hydroxide leach solution as described in the above mentioned patent.

The invention is illustrated in the following Examples: Example I: Type G210 granular activated carbon (Le Carbone (Pty) Ltd) was laden using gold plant cyanidation effluent. The laden charcoal analysed as follows: Gold 2,9 kg/t Silver 64 g/t Nickel 2,2 kg/t Copper 90 g/t These metals were present on the charcoal in the form of calcium cyanide ionic complexes, e.g. Ca(Au(CN)2)2.

The original capacity constant of the virgin charcoal was 22 mg gold per gram of charcoal.

Various runs were conducted using different pretreatment solutions and operating conditions. Except where otherwise indicated, in each case 16 g of charcoal were laden using a vibrator into a small glass elution column, to provide a charcoal bed of 1 cm internal diameter and 25 cm in length.

The bed was pretreated by pumping a half-bedvolume of pretreatment solution at a rate of approximately 1 bedvolume per hour (apparent flow velocity 9, 2x10-3 cm/sec) through the bed, after first draining the column of any surplus water.

The pretreated bed was then eluted using deionised water at 90"C, at elution rates in the range 0,5 to 2,0 bedvolumes per hour.

10 bedvolumes were then collected and analysed. Finally the charcoal was removed from the column, ovendried, and analysed and activity tested.

In all the experiments conducted, the pretreatment solution comprised a mixture of sodium cyanide and sodium hydroxide.

(a) Runs 1 to 11 In a first series of experiments, the effect of variations in the relative proportions of these two components were studied. The results of these experiments are set out in Tables I and II hereunder.

TABLE I Effect of change in sodium hydroxide concenhation used as a pretrearnient reagent Residual Elution peak charcoal concentration Charcoal residues activity No. Pretreatment reagent/ (g/t) (glut) (mg Au/g conditions Au Ag Ni Au Ag Cu Ni charcoal) 1 10%,N,aCN at 90"C 940 40 1600 16 < 2 4 64 13 2 " / 1% NaOH '' 862 37 1930 15 < 2 7 100 12 3 " / 5%NaOH +825 25 1430 23 3 8 57 12 4 " /14% NaOH ' 820 25 1350 18 5 1 70 13 5 " /20G/aNaOH " 770 23 1260 19 11 - 40 12 TABLE II Effect of change in sodium cyanide concentration used as a pretreatment reagent Residual Elution peak charcoal concentration Charcoal residues activity No. Pretreatment reagent/ (g/t) Ni (g/t) (mg Au/g conditions Au Ag Ni Au Ag Cu Ni charcoal) 6 14% NaOH at 90"C 550 11 - 42 22 17 300 13 7 " / 1% NaCN: 630 21 1250 17 5 -- 90 13 8 " / 3%NaCN " 705 28 1560 26 33 18 88 13,5 9 " / 5% NaCN " 670 25 1360 18 11 14 64 13 10 " / 8%NaCN " 740 24 1560 18 9 6 30 13 11 " /10% NaCN " 820 25 1350 18 5 1 70 13 It is seen that as the sodium hydroxide concentration was decreased, and the sodium cyanide concentration increased, the recovery of metal values improved, the best results being obtained using a pretreatment solution comprising 10 percent by weight sodium cyanide and 1 percent by weight sodium hydroxide.

(b) Runs 12 to 14 In a second series of experiments (Runs 12 to 14) the efficiency of the process with regard to high gold loadings was studied. The results are set out in Table III.

G 215 activated charcoal was laden to 4-6 percent gold using clarified gold plant pregnant solutions. The metal values in the solutions were in the form of cyanide ionic complexes.

The laden charcoal was treated with various sodium cyanide/sodium hydroxide reagents, and eluted at 90"C with deionised water.

In Run 12 the analysis of the laden charcoal was: Gold 4 percent Silver 600 g/t Nickel 2 600 g/t The pretreatment solution comprised 10 percent sodium cyanide/14 percent sodium hydroxide Recoveries of 98,6% gold, 83% silver, and over 99% nickel were obtained.

After 6 bedvolumes of deionised water (12 hours) the gold eluate contained only 6 g/t of gold.

In Run 13 the analysis of the laden charcoal was: Gold 6 percent Silver 2 000 g/t Nickel 6 000 g/t The pretreatment solution comprised 12 percent sodium cyanide/1 percent sodium hydroxide.

Recoveries of 99,9 percent gold, 97,1 percent silver, and 99,9 percent nickel were obtained. The lower hydroxide concentration is seen to provide better results.

Similar results were obtained in Run 14, wherein the charcoal analysis was 4 percent gold, 1 000 g/t silver, and 1 700 g/t nickel.

TABLE III Elution data at higher gold loadings Residual Original charcoal Charcoal No. Pretreatment conditions loading Elution peak residue activity (%) (g/t) (g/t) % Recovery (mg Au/g Au Ag Ni Au Ag Ni Au Ag Ni Au Ag Ni charcoal) 12 Bedvalume 4,0 0,06 0,26 8500 140 970 550 125 9 98,8 83 99,6 21 14% NaOH/10% NaCN at Bedvolume/h elution Temperature 90 13 Bedvolume 6 0,2 0,6 14000 1000 3500 77 58 8 99,9 97,1 99,9 16 12% NaCN/1% NaOH at Bedvolume/h elution Temperature 90 C 14 As above 4 0,1 0,17 8800 470 720 78 93 12 99,8 91,0 99,3 17 (C) Runs 15 to 17 In a third series of experiments the process of the invention was compared with two other processes, namely potassium carbonate pretreatment followed by elution with deionised water at 90 C (Run 15), and elution at 65 C with sodium sulphide sodium hydroxide solution (Run 16).

The pretreatment reagents, conditions and results for Runs 15, 16, and 17 are set out in Table IV hereunder and graphically illustrated in Figures 1,2 and 3 which respectively show plots of the number of bedvolumes collected from the elution columns against the gold, silver and nickel concentrations present in the bedvolumes collected.

TABLE IV Comparison of different methods used for elution of gold from loaded charcoal Residual Elution peak charcoal No. Pretreat elution reagent conditions concentration Charcoal residue activity (g/t) (g/t) (mg Au/g Au Ag Ni Au Ag Cu Ni charcoal) 15 1 Bedvolume 10% K2CO3/5% KOH at Bed- 478 12,6 - 27 7 10 340 13,5 volume/h followed by deionised water at 90 C 16 10 Bedvolumes of 35 Na2S/3% NaOH/% Na2SO3 1050 0,2 300 81 40 50 85 10 elution at 65 C at 2 Bedvolumes/h followed by 10 Bedvolumes H2O Followed by 4 Bedvolumes of 5% HNO3 Followed by 10 Bedvolumes of H2O 17 Bedvolume 10% NaCN/1% NaOH 862 37 1930 15 < 2 7 100 12 Followed by deionised water at Bedvolume/h at 90 C h = hour The process employing the sodium cyanide sodium hydroxide pretreatment (Run 17) is seen to be superior to both the other processes.

In the case of gold recovery, the Na2S/NaOH/Na2SO3 provided marginally better recovery than NaCN/NaOH, but the unpleasantness of handling NaS/NaOH solutions and the costs thereof make it unacceptable as a pretreatment reagent.

There is no precipitation of by-products on the charcoal and close to 100 percent recoveries are possible. Costs of reagents are relatively low since the regenerated sodium cyanide and sodium hydroxide solution may be reused in other parts of the gold circuit; elution is flexible in that it may be effected at a rate between one-half and one bedvolume per hour in 5 to 7 bedvolumes of eluate at 90 C, or 4 to 5 bedvolumes at 125 C (3 hours elution time); and the higher the gold loading on the charcoal, the more efficient becomes the elution.

Example II In this example the effect of using sodium hydroxide pretreatment solutions was investigated. The procedures and materials used were similar to those used in Example 1.

The metal rich solution, as in Example I, was a cyanidation effluent from a gold recovery process.

Various sodium hydroxide solutions were investigated and the results of these investigations can be found in Table V. From these results it can be seen that there is no appreciable advantage to be gained by using sodium hydroxide solutions of concentration greater than 10 percent by weight. It should also be noted that in all of runs 18 to 21, the loaded charcoal was acid washed with hydrochloric acid to remove calcium carbonate from the carbon prior to treatment with sodium hydroxide.

TABLE V Elution peak Carbon concentration Residue Recovery No. Pretreatment conditions (g/t) (g/t) (%).

Au Ag Ni Cu Au Ag Ni Cu Ca Au (Au) Ag Ni Cu Ca 18 Bedvolume 10% NaOH. 247 9 140 146 68 42 240 488 462 96,4 (89,9) 58 91,3 49,3 97,8 Eluted for 12 hours 19 Bedvolume 20% NaOH. 301 10 17 192 86 18 238 448 490 95, (91,3) 82 91,2 @3,4 97,7 Eluted for 12 hours 20 Bedvolume 30% NaOH. 327 5 24 197 90 36 316 468 530 95,2 (91,5) 64 88,6 51,2 97,6 Eluted for 12 hours 21 Bedvolume 10% NaOH. 310 5 69 210 132 50 232 430 442 93 (93) 50 91,6 55,2 98,0 In contact for 16 h.

Eluted for 8 hours () * Calculated Au recovery after 8 bedvolumes

Claims (11)

WHAT WE CLAIM IS:

1. A method of recovering gold, silver, copper and for nickel metal values from a support of the charcoal type having one or more of these values adsorbed thereon in the form of an alkaline earth metal ionic complex, the metal value forming part of the anionic portion thereof, including the steps of contacting the support with a solution which is an alkali metal cyanide solution, an alkali metal hydroxide solution or a mixture thereof, and then followed by desorbing the metal values from the support with water having a low concentration of metal cations.

2. A method as claimed in claim 1, wherein the solution contains alkali metal cyanide of concentration in the range 1 to 10 percent by weight and alkali metal hydroxide of concentration in the range 1 to 20 percent by weight.

3. A method as claimed in claim 2, wherein the concentration of cyanide ions exceeds the concentration of hydroxyl ions.

4. A method as claimed in claim 2 or claim 3, wherein the solution contains sodium cyanide and sodium hydroxide.

5. A method as claimed in claim 1, wherein the solution is a sodium hydroxide solution of concentration not exceeding 10 percent by weight.

6. A method as claimed in any one of the preceding claims, wherein the water has a metal cation concentration of less than 100 ppm.

7. A method as claimed in any one of the preceding claims, wherein the water is deionised water or softened water.

8. A method as claimed in any one of the preceding claims, wherein the support is activated carbon.

9. A method as claimed in any one of the preceding claims, wheren the complex is a cyanide ionic complex.

10. A method as claimed in claim 1, substantially as herein described with reference to Runs 1 to 14 and 17 of Example I, and to Example II.

11. Metal values recovered by a method as claimed in any one of the preceding claims.