Method of preparing a gold-containing solution and process arrangement for recovering gold and silver

Abstract

The invention relates to a method for preparing a gold-containing solution by chloride leaching from gold-bearing raw materials. A further object of the invention is to provide a method for recovering gold and optionally silver from the prepared gold-containing solution. The invention relates also to a process arrangement for recovering gold and optionally silver.

Claims

1. A method of preparing a gold-containing solution from gold-bearing raw materials, wherein the method comprises (a) a gold leaching step, wherein the gold-bearing raw material is subjected to oxidative chloride leaching in an aqueous leaching liquor in the following conditions: (i) the aqueous leaching liquor contains dissolved cupric ions (Cu.sup.2+), chloride (Cl.sup.) and bromide (Br.sup.), (ii) pH is within a range wherein the cupric ion does not precipitate, and (iii) the oxidation potential is at least 450 mV Pt vs. Ag/AgCl, to provide a gold-containing solution optionally further containing silver, (b) a liquid/solid separation step, wherein the gold-containing solution is separated from undissolved solid material; (c) a gold recovery step, wherein gold is recovered from the gold-containing solution from step (b) to obtain a gold-bearing organic solution and a gold-depleted leach solution containing Cu.sup.2+, Cl.sup. and Br.sup.; and (c) an evaporation step, wherein gold-depleted leach solution from step (c) containing Cu.sup.2+, Cl.sup. and Br.sup. is evaporated for removing excess water; and (d) a circulation step, wherein the gold-depleted leach solution obtained from the evaporation step containing Cu.sup.2+, Cl.sup. and Br.sup. is circulated to the leaching step (a).

2. The method as claimed in claim 1, wherein the gold-bearing raw material is selected from ores, concentrates, scraps, recycled materials, leach residues, tailings, flotation tailings, and refractory gold materials, which refractory gold materials have been pretreated by pressure oxidation, roasting and/or bacterial leaching.

3. The method as claimed in claim 1, wherein free-milling gold ores/concentrates and pretreated refractory gold concentrates are used as the raw material and they have been selected from sulfidic and oxidic materials.

4. The method of claim 1, wherein the gold-bearing raw material is selected from refractory gold concentrates and the pretreatment of the refractory gold concentrates used as the raw material has been performed by a method selected from pressure oxidation, bioleaching and roasting.

5. The method of claim 4, wherein the pretreatment of the refractory gold concentrates used as the raw material has been performed by pressure oxidation.

6. The method of claim 1, wherein the gold-bearing raw material is selected from refractory gold concentrates which have been pretreated by pressure oxidation.

7. The method of claim 1, wherein the oxidative chloride leaching is performed at atmospheric pressure.

8. The method of claim 1, wherein the oxidative chloride leaching is performed at the temperature of at least 50 C.

9. The method of claim 1, wherein the pH value wherein the cupric ion does not precipitate is less than 2.6.

10. The method of claim 1, wherein the oxidative chloride leaching is performed by providing an oxidizing source.

11. The method as claimed in claim 10, wherein the oxidizing source is an oxygen feed, an oxygen-enriched air feed or an air feed to the solution.

12. The method of claim 1, wherein the oxidation potential is 450-800 mV Pt vs. Ag/AgCl.

13. The method of claim 12, wherein the oxidation potential is 450-700 mV Pt vs. Ag/AgCl.

14. The method of claim 1, wherein the concentrations of Cu.sup.2+, Cl.sup. and Br.sup. in the leaching liquor are 10 110 g/l Cu.sup.2+, 50-300 g/l Cl.sup. and 1-100 g/l Br.sup. expressed as NaBr.

15. The method of claim 1, wherein the acid concentration of the leaching liquor is 5-20 g/l HCl.

16. The method of claim 1, wherein the gold recovery step (c) is performed by liquid-liquid extraction.

17. The method according to claim 16 wherein the extractant contains a diester of 2,2,4-trialkyl-1,3-pentanediol or 2-ethylhexanol or both.

18. The method of claim 1, wherein the method further comprises a gold stripping step, wherein gold is stripped from the gold-bearing organic solution obtained in step (c) to obtain an aqueous gold-bearing solution.

19. The method of claim 1, wherein the method comprises a further gold recovery step, wherein gold is recovered from the gold-bearing organic solution or from an aqueous gold-bearing solution.

20. The method as claimed in claim 19, wherein the said further gold recovery step is performed by a method selected from reduction with oxalic acid, ascorbic acid or glucose, adsorption on activated carbon, cementation and direct burning of the gold-bearing organic reagent.

21. The method of claim 1, wherein a part of the leaching solution is fed to the evaporation step or wherein all of the leaching solution is fed to the evaporation step.

22. The method as claimed in claim 21, wherein 5-99 wt-% of mass of the leaching solution is fed to the evaporation step after gold recovery step.

23. The method of claim 1, wherein the method further comprises a silver recovery step, where silver is recovered from at least part of the gold-depleted leach solution obtained from the gold recovery step (c).

24. The method as claimed in claim 23, wherein the silver recovery step comprises a method selected from liquid-liquid extraction and precipitation.

25. The method as claimed in claim 24, wherein the silver recovery step comprises liquid-liquid extraction with triisobutylphosphine sulphide, stripping with an aqueous thiosulfate salt solution and cementation of silver from the thiosulfate solution with metal powder.

26. The method as claimed in claim 24, wherein the silver recovery step comprises sulfide precipitation.

27. The method as claimed in 23, wherein the gold-depleted leach solution from step (c) is returned to the leaching step (a) after the silver recovery step.

Description

BRIEF DESCRIPTION OF THE SEVERAL VIEWS OF THE DRAWINGS

(1) In the following the invention will be described in greater detail by means of preferred embodiments with reference to the attached drawings, in which

(2) FIG. 1 is a block diagram of an exemplary embodiment of the invention, showing also silver recovery;

(3) FIG. 2 depicts the effect of the copper concentration on the gold concentration in the each solution of test 3 of Example 1;

(4) FIG. 3 depicts the effect of the bromide concentration on the gold concentration in the leach solution of test 4 of Example 1;

(5) FIG. 4 depicts the effect of the leaching time on the gold concentration in the leach solution of test 1 of Example 2;

(6) FIG. 5 depicts the effect of the copper concentration on the gold concentration in the leach solution of test 3 of Example 2;

(7) FIG. 6 depicts the effect of the bromide concentration on the gold concentration in the leach solution of test 4 of Example 2;

(8) FIG. 7 depicts the effect of the chloride concentration on the gold concentration in the leach solution of test 5 of Example 2;

(9) FIG. 8 depicts the gold concentration in the leach solution vs. leaching time in the comparative cyanide test of Example 3;

(10) FIG. 9 depicts the effect of the leaching time on the gold concentration in the each solution of tests 1 and 2 of Example 3;

(11) FIG. 10 is an example embodiment of the process arrangement.

DETAILED DESCRIPTION OF THE INVENTION

(12) The invention relates to a method of preparing a gold-containing solution from gold-bearing raw materials, wherein the method comprises (a) a gold leaching step, wherein the gold-bearing raw material is subjected to oxidative chloride leaching in an aqueous leaching liquor in the following conditions: (i) the aqueous leaching liquor contains dissolved cupric ions (Cu.sup.2+), chloride (Cl.sup.) and bromide (Br.sup.), (ii) pH is within a range wherein the cupric ion does not precipitate, (iii) the oxidation potential is at least 450 mV Pt vs. Ag/AgCl, to provide a gold-containing solution optionally further containing silver, (b) a liquid/solid separation step, wherein the gold-containing solution is separated from the undissolved solid material; (c) a gold recovery step, wherein gold is recovered from the gold-containing solution from step (b) to obtain a gold-bearing organic solution and a gold-depleted leach solution containing Cu.sup.2+, Cl.sup. and Br.sup.; and (c) an evaporation step, wherein gold-depleted leach solution from step (c) containing Cu.sup.2+, Cl.sup. and Br.sup. is evaporated for removing excess water; and (d) a circulation step, wherein the gold-depleted leach solution obtained from the evaporation step containing Cu.sup.2+, Cl.sup. and Br.sup. is circulated to the leaching step (a).
Starting Materials

(13) The gold-bearing raw materials for the method of the invention may be selected from ores, concentrates, scraps, recycled materials, leach residues, tailings, such as flotation tailings, and refractory gold materials, which refractory gold materials have been pretreated by pressure oxidation, roasting and/or bacterial leaching. The gold-bearing raw materials used as the starting material in the present invention may also contain silver.

(14) Typically, the starting materials for the method of the invention may be selected from free-milling gold ores/concentrates and pretreated refractory gold concentrates.

(15) Free-milling gold ores/concentrates refer to materials, from which gold can be recovered using cyanide leaching or physical separation methods. Free-milling materials can be sulfidic or oxidic ores. The main minerals of sulfidic ores are pyrite and pyrrhotite. The oxidic ores mainly contain silicates and ferric/ferrous oxides.

(16) Refractory gold concentrates refer to materials, wherein the gold recovery in conventional cyanide leaching process is less than 80%. In most refractory gold concentrate materials, gold is associated with sulphide minerals as very fine inclusions or as invisible gold in the mineral lattice. Decomposition of gold-bearing minerals is required for gold liberation. Examples of refractory gold concentrates are ores based on arsenopyrite, pyrite and/or silicate.

(17) The pretreatment of the refractory gold concentrates used as the starting material may be performed by a method selected from pressure oxidation, bioleaching and roasting, preferably pressure oxidation.

(18) In one embodiment of the invention, the gold-bearing raw materials used as the starting material are selected from refractory gold concentrates which have been pretreated by pressure oxidation.

(19) Method

(20) In the gold leaching step (a), the gold-bearing raw material is subjected to oxidative chloride leaching in an aqueous leaching liquor in the following conditions: (i) the aqueous leaching liquor contains dissolved cupric ions (Cu.sup.2+), chloride (Cl.sup.) and bromide (Br.sup.). (ii) pH is within a range wherein the cupric ion does not precipitate, (iii) the oxidation potential is at least 450 mV Pt vs. Ag/AgCl, to provide a gold-containing solution.

(21) In the leaching also silver may be leached and therefore the gold-containing solution may optionally further contain silver.

(22) The method further comprises (b) a liquid/solid separation step, wherein the gold-containing solution is separated from the undissolved solid material; (c) a gold recovery step, wherein gold is recovered from the gold-containing solution from step (b) to obtain a gold-bearing organic solution and a gold-depleted leach solution containing Cu.sup.2+, Cl.sup. and Br.sup.; and (c) an evaporation step, wherein gold-depleted leach solution from step (c) containing Cu.sup.2+, Cl.sup. and Br.sup. is evaporated for removing excess water; and (d) a circulation step, wherein the gold-depleted leach solution obtained from the evaporation step containing Cu.sup.2+, Cl.sup. and Br.sup. is circulated to the leaching step (a).

(23) The oxidative chloride leaching is typically performed at atmospheric pressure. Further, the oxidative chloride leaching is typically performed at the temperature of at least 50 C., more typically at the temperature of 88-100 C.

(24) In the oxidative chloride leaching the pH value, i.e. the range wherein the cupric ion does not precipitate, is less than 2.6, typically less than 2.2.

(25) The oxidative chloride leaching is typically performed by providing an oxidizing source into the chloride leaching liquor. Typically, the oxidizing source is an oxygen feed, an oxygen-enriched air feed or an air feed to the solution. The purpose of the oxidizing source is to allow the copper in the solution to maintain in a 2+ ion form.

(26) The oxidation potential is typically 450-800 mV Pt vs. Ag/AgCl, typically 450-700 mV Pt vs. Ag/AgCl.

(27) In the leaching step, the gold ore/concentrate is introduced into the leaching liquor, which contains Cu.sup.2+, Cl.sup. and Br.sup., typically in the following concentrations: 10-110 g/l Cu.sup.2+, 50-300 g/l Cl.sup. and 1-100 g/l Br (expressed as NaBr). Typically the leaching liquor is aqueous. Furthermore, the leaching liquor as a rule contains acid, in a typical concentration of 5-20 g/l HCl. Typically, oxygen is fed into the leaching liquor. Copper (Cu.sup.2+) is required to provide high oxidation power in order that gold can be oxidized together with the oxygen feed. Chloride and bromide (Cl.sup. and Br.sup.) form aqua complexes with gold so that gold is dissolved in the leaching solution. The presence of Br.sup. in the leaching liquor enables to maintain gold in a stabile form in the liquor, in other words the presence of Br.sup. enables the formation of a more stable complex of gold compared to if only Cl.sup. was present. pH of the leaching liquor is kept at a level wherein the cupric ion does not precipitate. Typically the pH of the leaching liquor is less than 2.6, more typically less than 2.2. In one embodiment of the invention, the pH is less than 2.2 but higher than 1.5. It was found that only about 0.5 hours may be sufficient for gold dissolution for example from oxide-based free-milling materials and from refractory concentrates pretreated by pressure oxidation.

(28) In a typical embodiment of the invention, the leaching liquor containing Cu.sup.2+, Cl.sup. and Br.sup. is circulated gold-depleted and optionally silver-depleted leach solution obtained from subsequent steps of the method after the recovery of gold and after optional recovery of silver. Before introducing the circulated leaching solution into the leaching step, the solution is as a rule subjected to evaporation to remove excess water. It is also possible to feed only a part of the solution to the evaporation step. Another option is to feed all of the solution to the evaporation step. The portion of the solution to be evaporated can vary in a wide range, depending on the raw material type, moisture and environmental regulations. In a typical embodiment of the invention 5-99%, preferably 20-70%, more preferably 40-60% of the leaching solution is fed to the evaporation step after gold recovery step. Heat to the evaporation is typically provided by steam, live steam condensate typically returning at 90-100 C. The amount of evaporation depends on the chemical and mineralogical composition of the raw material. In one application (calculated by HSC Sim 7.1) 200 t/h evaporation was needed to balance 300 t/h free-milling ore feed into the process. The evaporated leaching solution may be introduced to the leaching as such or, if necessary, after optional regeneration. In the regeneration, the circulated leaching solution is supplemented with fresh leaching chemicals (make-up chemicals) containing Cu.sup.2+, Cl.sup. and Br.sup.. The addition of the make-up chemicals is suitably performed in connection with the leaching.

(29) The present method and process arrangement are suitable for a wide variety of starting materials. By performing an evaporation step (c), wherein part or all of the gold-depleted leach solution from step (c) containing C.sup.2+, Cl.sup. and Br.sup. is evaporated for removing excess water; and a circulation step (d), wherein the gold-depleted leach solution obtained from the evaporation step containing Cu.sup.2+, Cl.sup. and Br.sup. is circulated to the leaching step (a); there is no need to add additional copper ions to the process. This further enables the use of a wide variety of starting materials in a very sustainable way because no new copper is added (or needed to be present in the starting material) and therefore no copper-containing bleed to the environment is formed. Furthermore the chemicals circulation in the process is maximized resulting in savings in costs as well as small chemicals consumption. For being able to recover and recycle the chloride/bromide, water needs to be removed in order to maintain the water balance. High chloride concentrations cannot be sustainably discharged to the environment and even if this would be possible the cost of make-up chloride addition would leave the process uneconomical.

(30) The leaching provides a gold-containing solution with a gold yield of more than 70%, preferably more than 85%, still more preferably more than 95% and even more than 98% and up to 99.7% based on solid analysis. In addition to gold, the leach solution as a rule contains silver and other components, such as e.g. arsenic, aluminium, iron, cobalt, nickel, and magnesium.

(31) In the liquid/solid separation step (b), the gold-containing solution is separated from the solid material. The solid/liquid separation may be performed by filtering, solid/liquid settling, evaporation or centrifugation, for example.

(32) The gold-containing solution is introduced into the gold recovery step (c), after optional washing.

(33) The gold recovery step (c) may be performed by liquid-liquid extraction (also known as solvent extraction). The liquid-liquid extraction is performed by using an extraction agent, which is selective for gold. Typically the extractant contains a diester of 2,2,4-trialkyl-1,3-pentanediol or 2-ethylhexanol or both. In one embodiment of the invention, the liquid-liquid extraction is performed by using a diester of 2,2,4-trialkyl-1,3-pentanediol as the extraction agent. In another embodiment of the invention the liquid-liquid extraction was performed by using a diester of 2,2,4-trialkyl-1,3-pentanediol blended with 2-ethylhexanol as the extraction agent. The extraction may be performed using the method disclosed in WO 2011/154603 A1. It was found that even 99.9% gold recovery was achieved.

(34) Gold can be stripped from the gold-bearing organic solution obtained in step (c) to obtain an aqueous gold-bearing solution. Water or an aqueous salt solution may be used as the stripping agent.

(35) Furthermore, the method may comprise a further gold recovery step, wherein gold is recovered from the aqueous gold-bearing solution obtained above or even directly from the organic gold-bearing solution obtained in step (c). The further gold recovery step may be performed by a method selected from reduction with oxalic acid, ascorbic acid or glucose, adsorption on activated carbon, cementation and direct burning of gold bearing organic reagent. In one embodiment of the invention, the reduction of gold is performed with oxalic acid.

(36) In the circulation step (d), the gold-depleted leach solution from step (c) containing Cu.sup.2+, Cl.sup. and Br.sup. is circulated to the leaching step (a) after optional regeneration and/or after evaporation for removing excess water. Due to the efficient circulation and optional regeneration of the gold-depleted leach solution, accumulation of impurities disturbing gold dissolution is avoided. Typically solution is recycled in its entirety.

(37) In one embodiment of the invention, the method further comprises a silver recovery step, where silver is recovered from at least part of the gold-depleted leach solution obtained from the gold recovery step (c). Naturally the silver recovery step is not necessary if silver is not present in the starting material. The silver recovery may be performed by a method selected from liquid-liquid extraction and precipitation. In one embodiment of the invention, the silver recovery comprises liquid-liquid extraction with Cyanex 471X (a commercial liquid-liquid extraction reagent where the active component is triisobutylphosphine sulphide), stripping with an aqueous thiosulfate salt solution, such as sodium thiosulfate solution, and cementation with metal powder, such as Cu powder. For the liquid-liquid extraction. Cyanex 471X is typically used as a 0.1-0.7 mol/l solution in a hydrocarbon solvent. The solution may also contain a modifier. The silver recovery may also be performed by sulphide precipitation. Silver is typically recovered with a yield of more than 70%.

(38) In one embodiment of the invention, the gold-depleted leach solution from step (c) is circulated to the leaching step (a) after the silver recovery step.

(39) In the following, the invention is illustrated by referring to FIG. 1, which is an exemplary embodiment of the invention where gold leaching is combined with gold recovery and further with silver recovery. The example embodiment is not meant to restrict the invention in any way. Naturally, if silver is not present in the starting material, the silver recovery step is not necessary. It should also be noted that only a part of the solution may be subjected to evaporation. A gold concentrate (Au concentrate 5) is introduced into leaching 10, together with oxygen feed 7 and a leaching solution containing Cu (Cu.sup.2+), Cl.sup. and Br.sup.. In the leaching 10, gold is solubilized forming a gold-containing solution. Leaching is followed by solid/liquid separation (S/L separation 20), where the gold-containing solution is separated from the undissolved solid material (leach residue 105) The gold-containing solution obtained from the leaching is introduced into mid extraction (Au extraction 30), which produces a gold-bearing organic solution and a gold-depleted leach solution containing Cu.sup.2+, Cl.sup. and Br.sup.. The gold-bearing organic solution is further introduced into gold stripping (Au stripping 40), where gold is stripped with water from the gold-bearing organic solution to obtain an aqueous gold-bearing solution. Gold is finally recovered as elemental gold from the aqueous gold-bearing solution by reduction with sodium oxalate (Na oxalate 45) by introducing the aqueous gold-bearing solution to reduction (Au reduction 50). The reduction product is subjected to solid/liquid separation (S/L separation 60), from which gold is recovered as a solid product (Au product 65). The liquid product from the solid/liquid separation contains some impurities, such as iron and copper. The impurities are removed from the solution (effluent treatment 70) by hydroxide precipitation. The slurry from the effluent treatment is introduced into solid/liquid separation (S/L separation 80), from which a hydroxide precipitate 85 and a solution bleed 90 are obtained.

(40) The gold-depleted solution containing Cu.sup.2+, Cl.sup. and Br.sup. from Au extraction 30 is introduced into silver recovery, comprising silver extraction (Ag extraction 100), silver stripping (Ag stripping 110), silver cementation with Cu powder 125 (Ag cementation 120) and solid/liquid separation (S/L separation 130), from which a solid silver product (Ag product 135) is obtained. The liquid product from the solid/liquid separation 130 is introduced into effluent treatment 70 (together with the liquid product from gold recovery).

(41) Part of the leaching solution containing Cu.sup.2+, Cl.sup. and Br.sup. from the silver extraction 100 is introduced into evaporation 160 to remove excess water as steam 165. After evaporation to a suitable consistency, the remaining solution 170 containing Cu.sup.2+, Cl.sup. and Br.sup. is circulated to leaching 10.

(42) A small part of the solution from the silver extraction is introduced into effluent treatment 140, where some impurity metals such as zinc and lead are removed by hydroxide precipitation. The effluent treatment is followed by solid/liquid separation (S/L separation 150). The liquid product from the solid/liquid separation is introduced into evaporation 160. The solid product from the solid/liquid separation is recovered as a CuZn precipitate 155.

(43) The invention relates also to a process arrangement for recovering gold and optionally silver from gold-bearing raw material, wherein the arrangement comprises (a) a gold leaching unit adapted for subjecting the gold-bearing raw material to oxidative chloride leaching in a leaching liquor containing cupric ions (Cu.sup.2+), chloride (Cl.sup.) and bromide (Br.sup.), for dissolving gold and optionally silver, (b) a liquid/solid separation unit, adapted for separating the gold- and optionally silver-containing solution from the undissolved solid material; (c1) a gold recovery unit, adapted for recovering the gold from the gold- and optionally silver-containing solution from unit (b) to obtain a gold-bearing organic solution and a gold-depleted leach solution; (c2) optionally a silver extraction unit adapted for recovering the silver from the gold-depleted leach solution; (c3) an evaporation unit adapted for evaporating excess water and optionally an effluent treatment unit for treating liquid obtained from silver extraction unit; and (d) a circulation unit (d), adapted for circulating the gold-depleted leach solution containing Cu.sup.2+, Cl.sup. and Br.sup. to the leaching unit (a).

(44) It is possible to subject only a part of the solution or all of the solution to the evaporation unit.

(45) The liquid-solid separation unit may be selected from filter, solid/liquid settler, evaporator or centrifuge, for example.

EXAMPLES

(46) The following examples illustrate the invention without limiting the invention in any way.

(47) In the following examples 1-3, leaching tests with different gold concentrates were carried out in order to see whether it is possible to obtain as high extraction of gold with chloride leaching as with cyanide leaching.

Example 1

(48) Recovery of Gold from a Sulfidic Gold Concentrate

(49) The sulfidic gold concentrate used as the starting material was a free-milling concentrate. In the concentrate, most of the gold exists as pure metal and it can be dissolved directly with cyanide. The amount of liberated gold in the material is 80.85%. The material contains also locked gold.

(50) Chemical composition of the sulfidic concentrate is presented in Table 1.

(51) TABLE-US-00001 TABLE 1 Chemical composition of sulfidic concentrate Sulfidic concentrate Ag, ppm 35.90 Au, ppm 114.60 Cu, % 0.94 Fe, % 41.70 S, % 46.60 SiO.sub.2, % 7.84

(52) Chloride-bromide leaching test conditions are presented in Table 2. The redox potential varied between 550-676 mV Pt vs. Ag/AgCl.

(53) TABLE-US-00002 TABLE 2 Conditions of chloride-bromide leaching tests Solid conc., T, [Cl.sup.].sub.aq, [NaBr].sub.aq, [Cu.sup.2+].sub.aq, pH, Acid g/l C. g/l g/l g/l conc, g/l Test 1 400 98 225 100 100 1.7 Test 2 200 98 225 100 100 10 Test 3 200 98 225 100 50-100 10 Test 4 200 98 225 10-100 100 10

(54) The results are presented in Table 3.

(55) TABLE-US-00003 TABLE 3 Extractions of gold and silver Au, % Ag, % Test 1 76.4 87.1 Test 2 83.9 97.4 Test 3 78.4 96.5 Test 4 74.4 94.0

(56) Furthermore, the effect of copper concentration on gold dissolution was studied in test 3. During the test, the copper concentration was increased step by step. Used copper concentrations were 50 g/l, 67 g/l, 83 g/l, and 100 g/l. Addition of copper was made every two hours.

(57) The copper concentration affected gold dissolution. 50 g/l copper dissolved part of gold in half hour (FIG. 2). Higher copper concentrations increased gold dissolution.

(58) Additionally, the effect of bromide concentration on gold dissolution was studied in test 4. The sodium bromide concentration was changed every second hours. The sodium bromide concentration effected dissolution of gold (FIG. 3). Kinetics of gold dissolution was faster with higher bromide concentrations.

(59) Relatively good extractions of gold (83.9%) were obtained with chloride-bromide leaching. Kinetics was fast.

Example 2

(60) Recovery of Gold from a Silicate Based Gold Ore

(61) The silicate based gold ore used as the starting material was also a free-milling ore.

(62) The chemical analyses of the ore is presented in Table 4.

(63) TABLE-US-00004 TABLE 4 Chemical analysis of the used gold ore Silicate based ore Al, % 5.61 Au, ppm 5.054 Cu, % 0.025 Fe, % 0.851 SiO.sub.2, % 51.3

(64) Chloride-bromide leaching test conditions are presented in Table 5. The redox potential varied between 450-682 mV Pt vs. Ag/AgCl.

(65) TABLE-US-00005 TABLE 5 Conditions of chloride-bromide leaching tests Solid Acid conc., T, [Cl.sup.].sub.aq, [NaBr].sub.aq, [Cu.sup.2+].sub.aq, pH, conc, g/l C. g/l g/l g/l g/l Test 1 200 98 225 100 100 1.7 Test 2 400 98 225 100 100 1.7 Test 3 400 98 133-197 100 0-75 1.7 Test 4 400 98 225 1-60 100 1.7 Test 5 400 93 26-125 15 50 1.7 Test 6 400 90 150 8 15 10

(66) A cyanide leaching test for comparison purposes was carried out in the following conditions: solid concentration 33 w-%, pH 11, air feed 500 ml/mm. NaCN concentration 3 g/l, and leaching time 24 h.

(67) The results from all tests are presented in Table 6.

(68) TABLE-US-00006 TABLE 6 Gold extractions in chloride-bromide leaching tests and cyanide leaching test. Au, % Test 1 98.6 Test 2 98.4 Test 3 91.0 Test 4 97.0 Test 5 100.0 Test 6 95.1 Cyanide 98.0

(69) Furthermore, the gold concentration of the solution vs. leaching time is presented in FIG. 4.

(70) In test 3, the effect of copper concentration was also studied. During the leaching, copper concentration had to be so high that redox potential is over 450 mV Pt vs. Ag/AgCl. Copper concentration was increased step by step from 0 g/l to 75 g/l during the test. Used concentrations were 0, 1, 10, 30, 60, and 75 g/l. It seems that 10 copper was a sufficiently high concentration for gold dissolution (FIG. 5). Required copper concentration depends on the mineralogy of feed material.

(71) Additionally, the effect of sodium bromide concentration on gold dissolution was studied in test 4 (FIG. 6). Used sodium bromide concentrations were 0, 1, 5, 10, 40, and 60 g/l.

(72) Furthermore, the effect of chloride concentration on gold dissolution was studied (FIG. 7). Chloride concentration 100 g/l increased dissolution of gold. 25 g/l chloride dissolved some of the gold, but most of gold remained in solid. Leaching of gold needed quite high chloride concentrations. According to the test 5, a suitable chloride concentration would be at least 125 g/l for this material. Extraction of gold was 99.98%.

(73) Results of the cyanide leaching test are presented in FIG. 8. Extraction of gold was 98.0% for the used silicate based gold ore.

(74) During the leaching tests it was found that the mineral composition of the material as well as copper, bromide, chloride and acid concentrations affect the gold leaching. Relatively good extractions of gold (over 95%, Table 6) were obtained with chloride-bromide leaching. Kinetics was fast in chloride-bromide leaching. In cyanide leaching, the retention time was 24 hours. After 25 hours, the extraction of gold was 98%.

(75) The chloride leaching was possible with low copper and sodium bromide concentrations. The temperature needs to be at least 90 C.

Example 3

(76) Recovery of Gold from Pretreated Refractory Gold Concentrate

(77) In the used concentrate, most of the gold is inside sulfide minerals.

(78) The chemical composition of the used refractory gold concentrate before pretreatment is presented in Table 7. The gold is mostly locked in pyrite.

(79) TABLE-US-00007 TABLE 7 Chemical composition of the used refractory concentrate Test material Ag, ppm 19.6 Au, ppm 29.4 As, % 6.9 Cu, % 0.062 Fe, % 31.3 S, % 32.7 SiO.sub.2, % 12.7

(80) Chloride-bromide leaching test conditions are presented in Table 8. The redox potential varied between 568-681 mV Pt vs. Ag/AgCl. In tests 1-2, the material was pretreated with POX (pressure oxidation). POX treatments were made in the following conditions: solid concentration was 150-300 g/l, temperature was 210 C., and the oxygen overpressure was 6 bar (total pressure was about 26 bar). After POX treatment, hot cure treatment was done. The temperature was decreased to 90 C. and the overpressure was released (to atmospheric pressure).

(81) TABLE-US-00008 TABLE 8 Conditions of chloride-bromide leaching tests Solid Acid conc., T, [Cl.sup.].sub.aq, [NaBr].sub.aq, [Cu.sup.2+].sub.aq, pH, conc, g/l C. g/l g/l g/l g/l Test 1 50 98 225 100 100 1.7 Test 2 315 98 150 8-100 20 10

(82) The chloride-bromide leaching results are presented in Table 9.

(83) TABLE-US-00009 TABLE 10 Extractions of gold and silver in chloride-bromide leaching Au(s), % Ag(s), % Test 1 99.5 Test 2 99.7 97.3

(84) Relatively good extractions of gold (over 99%) and silver (about 97%) were obtained with chloride-bromide leaching for POX treated material. In test 2 (FIG. 9), the effect of sodium bromide concentration was also studied. Sodium bromide concentration did not affect gold extraction in the sodium bromide concentration range used in this case. In FIG. 9, the gold concentration of the solution varied widely depending on solid concentration and the amount of solid.

Example 4

(85) Gold leaching was modeled using HSC SIM 7.1 process simulation program for free-milling ore with 300 t/h ore feed into the chloride-bromide leaching. Process parameters similar to the patent examples were used.

(86) A feature of a chloride based gold processes is that chloride/bromide concentration is kept high. This is needed to keep gold stable in the solution.

(87) Hence to recover and recycle the chloride/bromide, water need to be removed in order to maintain the water balance. High chloride concentrations often cannot be sustainably discharged to the environment but even if it could, the cost of make-up chloride addition would leave the process uneconomic.

(88) For that reason the evaporation and water balance is an essential innovative feature of the process which makes the process environmentally and economically sustainable.

(89) It was observed that with 300 t/h ore feed, 203 t/h of water accumulated into the process e.g. without evaporation stage >200 m.sup.3/h of solution with high chloride and bromide concentrations should be bled out of the process. This would be not environmentally sustainable. Evaporation stage (evaporation 203 t/h water) allows the chemicals such as Cl, Br and also Cu to be recycled and reused in the process. Then only pure water (vapor) is released and can be used in the process if needed.

(90) It will be obvious to a person skilled in the art that, as the technology advances, the inventive concept can be implemented in various ways. The invention and its embodiments are not limited to the examples described above but may vary within the scope of the claims.