Chloride process for the leaching of gold

Abstract

A process for the extraction of gold from a gold-bearing ore or concentrate, comprising the steps of leaching the gold-bearing ore or concentrate with a lixiviant of hydrochloric acid and magnesium chloride at atmospheric pressure at a temperature of at least 90° C. and an Eh of at least 900 mV. After a liquid/solids separation step, the solution obtained is subjected to an organic solvent extraction step using an oxime to obtain a solution of organic solvent containing gold, which is stripped with sodium thiosulphate to recover gold. The extraction may be operated to extract gold with or without iron. Materials used in the process may be recycled. The process avoids environmental and other hazards associated with the use of cyanide to extract gold.

Claims

1. A process for the extraction of gold from a gold-bearing ore or concentrate, consisting the steps of: a) leaching the gold-bearing ore or concentrate with a lixiviant of hydrochloric acid and magnesium chloride at atmospheric pressure at a temperature of at least 90° C. and an Eh of at least 900 mV; b) subjecting the solution obtained in step a) to a liquid/solids separation step; c) subjecting the liquid obtained in step b) to an organic solvent extraction step to selectively extract gold from the liquid and obtain a pregnant gold-bearing organic solvent extractant solution; d) stripping gold from the pregnant organic solvent extractant solution obtained in step c) with sodium thiosulphate; and e) recovering gold; in which the organic solvent extraction of step c) is carried out using an oxime.

2. The process of claim 1 in which the solvent extraction of step c) is a selective extraction step to obtain gold.

3. The process of claim 1 in which the solvent extraction of step c) is a co-extraction step to obtain gold and another metal.

4. The process of claim 1 in which, in step d), gold is recovered by precipitation.

5. The process of claim 1 in which the oxime is 5-nonylsalicylaldoxime.

6. The process of claim 1 in which, subsequent to the stripping of step d), gold is precipitated from solution by addition of oxalic acid.

7. The process of claim 6 in which, subsequent to the precipitation of gold, the solution obtained is subjected to a liquid/solids separation step and the liquid obtained is recycled to step a).

8. The process of claim 1 in which the raffinate from step c) is subjected to steps to remove iron, and magnesium chloride solution so obtained is recycled to step a).

9. The process of claim 1 wherein the gold-bearing ore or concentrate is in the form of tailings from another process or a roasted or calcined ore or concentrate.

Description

DETAILED DESCRIPTION OF THE INVENTION

(1) The present invention relates to a process for leaching of gold from a gold-bearing ore or concentrate. The gold-bearing ore or concentrate is leached with a lixiviant of hydrochloric acid and magnesium chloride under conditions such that the gold is leached into solution. After a solids/liquids separation step, the gold may be extracted from the liquid in a solvent extraction step and recovered. In particular, the gold may be recovered by stripping an organic solution of gold with sodium thiosulphate. The hydrochloric acid and magnesium chloride of the lixiviant may be recovered and recycled to the leaching step.

(2) The present invention particularly relates to a process for leaching of gold-bearing ores or concentrates. Such ores may contain iron. The process is operated at atmospheric pressure. While the concentration of hydrochloric acid may be varied, the preferred concentration of hydrochloric acid is not more than about 20% (mass ratio). Such a concentration of acid may be obtained by azeotropic distillation of hydrochloric acid solution, for example in recycle of hydrochloric acid solution in the process e.g. using pyrohydrolysis. As discussed herein, such a low concentration of hydrochloric acid provides advantages to the process, including in recycle steps in the process and in requirements for disposal of effluent.

(3) The gold-bearing ore material may be ore per se, but is preferably a concentrate thereof. Techniques for treating gold-bearing ore to form a concentrate or for beneficiation of the ore are known. The ore may also be in the form of tailings from another process or a roasted or calcined ore or concentrate. Flotation processes may be used to obtain concentrates for the process of the present invention. It is understood that the expression “ore” also includes any other form of the ore, and that mixtures of the various forms of the ore may be used. In particular embodiments of the invention, the process is operated on tailings, waste or other material obtained from another process. The process may also be operated on refined or other products that are relatively high in gold content. In addition, the process may be operated on gold-bearing materials that also contain carbonaceous material. Other sources will be apparent to persons skilled in the art. The material fed to the process should be comminuted or ground to improve the contact between lixiviant and gold to be leached. All of these forms are regarded as ore or concentrate to be fed to the process. It is understood that some sources may not be appropriate for the acid conditions of the leach e.g. sources containing cyanide from a cyanidation leach, unless steps have been taken to reduce hazards to an acceptable level.

(4) The process may be operated with or without roasting or reduction of the ore. However, in embodiments, the ore or concentrate may have been subjected to roasting or calcination.

(5) In the method of the present invention, ore in a form as discussed above is fed to a leaching step in which the ore is contacted and leached with a lixiviant comprising magnesium chloride and hydrochloric acid, optionally also containing an oxidant. Regeneration and recycling of ingredients used in the leach and in subsequent extraction and recovery steps is an important aspect of the process, and use of other chlorides or mixtures of chlorides e.g. alkali metal chlorides, may adversely affect such recovery and recycling. Thus, the lixiviant is restricted to hydrochloric acid with magnesium chloride being the only added metal chloride, with oxidant to obtain the Eh (electric potential versus SHE (standard hydrogen electrode) levels discussed herein. Such oxidant is selected from the group consisting of alkali metal peroxide, alkali metal perchlorate, ammonium perchlorate, magnesium perchlorate, alkali metal chlorate, magnesium chlorate, alkali metal hypochlorite, chlorine, hydrogen peroxide and other non-sulphur containing oxidants, and mixtures thereof. Examples of alkali metal peroxide are sodium peroxide and potassium peroxide. Examples of alkali metal perchlorates are sodium perchlorate and potassium perchlorate. Ammonium perchlorate, magnesium perchlorate and magnesium chlorate may also be used. Examples of alkali metal chlorates are sodium chlorate and potassium chlorate. An example of an alkali metal hypochlorite is sodium hypochlorite. Other oxidants are non sulphur containing oxidants; the presence of sulphur in oxidants is to be avoided. The preferred oxidants are chlorine and sodium chlorate.

(6) The gold-bearing ore is subjected to a leach with a lixiviant of hydrochloric acid and magnesium chloride. The leaching may be conducted as a co-current step, a countercurrent step or in another manner, and is most conveniently carried out at atmospheric (ambient) pressure. It is not necessary to conduct the leaching step under pressure. According to the invention, the leach is carried out at a temperature in the range of from 90° C. up to the boiling point of the leaching solution, and especially at a temperature in the range of 90-100° C. In addition, the oxidation potential (Eh) should be at least 900 mV and especially in the range of 1050-1150 mV. The leaching step is preferably carried out with a magnesium chloride concentration of at least 150 g/L, for instance 150-400 mg/L and especially in the range of 200-250 g/L. The hydrochloric acid used in the lixiviant has a maximum concentration of 18% (mass ratio). In embodiments, the concentration of hydrochloric acid is at least 12% and especially in the range of 14-18% (mass ratio); for instance the hydrochloric acid may be 3.5-4.0N. The chloride concentration in the lixiviant is most preferably in the range of 100-400 g/L, and especially 200-300 g/L. The Mg/HCl (magnesium to hydrochloric acid) ratio expressed in terms of mass percentage (m/m) in the leach is preferably adjusted to optimize the leach, based on for example the particular ore or concentrate being leached and temperature. The Mg/HCl ratio in the lixiviant is generally in the range of 0.1-2.0, especially 0.4-1.0. The leach may be a single stage leach or a multi-stage leach. In addition, the leaching steps may be conducted co-current or countercurrent, in whole or in part.

(7) Subsequent to the leach, the leaching solution is subjected to a liquid/solids separation step. The liquid is then subjected to steps to extract the gold from the leaching solution, especially using solvent extraction, as discussed below. The solids may be discarded or may be subjected to further steps to extract gold or other value metals that may be in the solids.

(8) As noted above, the leaching solution from the gold leach is subjected to a liquid/solids separation step with solids being recycled for further leaching or discarded. The pregnant liquid from the gold leach is subjected to steps to recover gold. In particular, the liquid is treated to a solvent extraction step using an organic solvent to extract the gold into the extractant. In particular, the extractant of the solvent extraction step may be an oxime. An example of an oxime is LIX 622N i.e. 5-nonylsalicylaldoxime. Other extraction agents may be used, for example a crown ether, phosphinic acid, ester or oxide, or tertiary or quaternary ammonium salt. An example is Alamine™ 336 alkyl amine. The extractant is preferably mixed with a diluent and a modifier. Examples of the diluent are organic phase diluents, for instance kerosenes e.g. CF-231 kerosene. Examples of the modifier are alcohol surfactants e.g. EXXAL™ 13 tridecylalcohol.

(9) The liquid subjected to solvent extraction may contain value metals other than gold, especially iron. In the leaching step with hydrochloric acid and magnesium chloride, iron will be leached into solution. The solvent extraction may be carried out such that relatively high or relatively low amounts iron are extracted into the organic solution of the solvent extraction. Thus, the pregnant gold-bearing organic solvent extractant solution may have a high iron content or a low iron content. The latter is preferred, as it facilitates the recovery of gold of higher purity. For instance, if the extractant is LIX 622N and the solvent extraction is carried out with LIX 622N, EXXAL™ 13 tridecyl alcohol and CF-231 kerosene in a ratio of Jan. 20, 1979 by volume, then gold is extracted in preference to iron. Alamine™ 336 alkyl amine gives co-extraction of iron and gold. Selective extraction of gold but not iron is preferred as it simplifies subsequent recovery of gold and allows for the use of smaller quantities of an extractant that selectively extracts gold than of an extractant needed for co-extraction of both gold and iron.

(10) The liquid from the solvent extraction is separated into a loaded organic phase and an aqueous phase. The loaded organic phase is sent to a stripping step; the phase may be subjected to a scrubbing step prior to stripping, using selective or co-extraction techniques. The loaded organic phase may be stripped, in particular, with an aqueous solution of sodium thiosulphate, water or sulphuric acid. Sodium thiosulphate is preferred. The resultant aqueous phase, which contains gold, is subjected to a gold-precipitation step. Oxalic acid may be used to effect precipitation of gold. The gold-bearing solids are separated in a liquid/solids separation step, with the solids being subjected to calcination to recover gold and the liquid recycled to the gold stripping step.

(11) The raffinate from the gold extraction step may be treated for recovery of iron. For instance, the raffinate may be subjected to solvent extraction with a dialkyl ketone. In particular, the alkyl groups of the dialkyl ketone may be C.sub.1-C.sub.6 alkyl groups, especially C.sub.1-C.sub.4 alkyl groups, examples of which are methyl, ethyl, propyl and butyl including isopropyl and isobutyl. Preferred ketones are methylisobutylketone (MIBK) and diisobutylketone (DIBK). The dialkyl ketone is preferably mixed with a diluent and a modifier. Examples of the diluent are organic phase diluents, for instance kerosenes e.g. CF-231 kerosene. Examples of the modifier are alcohol surfactants e.g. EXXAL™ 13 tridecylalcohol. In an embodiment, methylisobutyl ketone is used in a ratio of May 10, 1985 of MIBK/EXXAL™ 13 tridecyl alcohol/CF-231 kerosene. Iron is stripped from the loaded organic solution obtained e.g. by addition of water, and may be recovered by pyrohydrolysis to give Fe.sub.2O.sub.3 which may be offered for sale. The raffinate from iron pyrohydrolysis contains hydrochloric acid which may be recycled to the leaching step. The raffinate from the gold extraction step contains hydrochloric acid and magnesium chloride which, subsequent to iron extraction, may be recycled to the leaching step. Methods for purification and recycling of such liquids are known.

(12) In an embodiment of the invention, the gold ore or concentrate contains other value metals. It may be advantageous to subject the gold ore or concentrate to a first extraction step using a lixiviant of hydrochloric acid and magnesium chloride at a lower temperature e.g. a temperature in the range of 65-80° C., especially 70-75° C., followed by a liquid/solids separation step. The liquid may be subjected to steps to recover value metals therein. The solids may be subjected to the steps described herein for the recovery of gold.

(13) A particular advantage of the process of the present invention, and its embodiments, is that high rates of extraction of gold are obtained in a leaching step that operates at atmospheric pressure. The use of sodium thiosulphate in the stripping of gold allows for recovery of gold from the organic solution in a simple and effective manner. The use of atmospheric pressure results in substantial economic advantages, especially in capital costs. The use of chloride chemistry offers advantages in operating and capital costs of the process. Leaching agent may be regenerated and recycled, especially using a pyrohydrolysis step with additional hydrochloric acid being formed from chlorine if required. Magnesium chloride may be recycled to the leaching step.

(14) The present invention is illustrated by the following Examples.

Example I

(15) A concentrate of a gold-bearing ore was subjected to a leach using a lixiviant of hydrochloric acid, magnesium chloride and an oxidant. The concentrate contained 5.34 mg/L of Au and 6.7 g/L of Fe. The lixiviant contained 32 g/L of magnesium chloride and the hydrochloric acid was 1N. The resulting leach solution was subjected to a liquid/solids separation step, and the liquid was subjected to extraction with LIX 622N, which is 5-nonylsalicylaldoxime, in EXXAL™ 13 tridecyl alcohol and CF-231 kerosene with the oxime being 56.5±2.3% (w/w). The ratio of organic:aqueous in the extraction solution was 1:1, the temperature was room temperature and the time of contact between the extractant and liquid from the leach was 3 minutes.

(16) The results were as follows:

(17) TABLE-US-00001 Metal Feed (mg/L) Raffinate (mg/L) Extraction (%) Au 5.34 2.27 57.5 Fe 6700 6700 0

(18) The results show very effective separation of gold from iron in the leach solution, in a one-step extraction. Repetition of the extraction step lowered the amount of gold in the raffinate.

Example II

(19) Solution obtained from Example I was subjected to stripping with 0.1M sodium thiosulphate (Na.sub.2S.sub.2O.sub.3) solution. The results obtained were as follows:

(20) TABLE-US-00002 Organic Phase Stripping Solution Stripping Efficiency Au (mg/L) Au (mg/L) (%) 3.07 2.65 86.3

(21) The results show good stripping of gold from solution obtained in the extraction of gold from the leach solution.

Example III

(22) Samples of a gold-bearing ore, in the form of a concentrate, were subjected to a two-stage leach at atmospheric pressure with a lixiviant of hydrochloric acid (4N) containing magnesium chloride at a concentration of 250 g/L. The Eh was 1150 mV. In the first stage of the leach, the temperature was 70° C. and in the second stage the temperature was 90° C. In each of the first and second stages of the leach, the leaching time was 2 hours.

(23) Two separate runs of the leach were made, each using 50 g of a gold-bearing ore that assayed 3.48 ppm of gold. In each of the two runs, the solids obtained after a liquid/solids separation step weighed 17 g; the assay of gold in the solids was 0.20 and 0.19 ppm, respectively. Thus the extraction of gold from the gold-bearing ore in the two runs was 98.12% and 98.05%, respectively.

Example IV

(24) A concentrate of a roaster calcine-magnetic concentrate obtained from a gold-bearing ore was subjected to a leach using a lixiviant of hydrochloric acid and magnesium chloride followed by a liquid/solids separation step. The liquid obtained contained 45.7 mg/L of gold and 22300 mg/L of iron. The liquid was subjected to extraction with LIX 622N, which is 5-nonylsalicylaldoxime, in EXXAL™ 13 tridecyl alcohol and CF-231 kerosene, with the ratio being Jan. 20, 1979. The ratio of organic:aqueous in the extraction solution was 1:1, the temperature was room temperature and the time of contact between the extractant and liquid from the leach was 3 minutes. The feed and raffinate solutions were analyzed for gold and iron. The results obtained are shown in Table 1:

(25) TABLE-US-00003 TABLE 1 Metal Feed (mg/L) Raffinate (mg/L) Extraction (%) Au 45.7 19.2 58.2 Fe 22300 22300 0

(26) The pregnant solvent was then subjected to stripping with 0.1M sodium thiosulphate using a ratio of organic:aqueous of 1:1. The results obtained were as follows:

(27) TABLE-US-00004 TABLE 2 Organic phase (Au mg/L) Stripping Soln (Au mg/L) Stripping Efficiency (%) 26.6 22.5 84.6

(28) The results show that sodium thiosulphate is effective in stripping gold from the pregnant solvent solution.

Example V

(29) Samples of a gold-bearing concentrate were subjected to a leach using a lixiviant of hydrochloric acid (4N) and magnesium chloride (250 g/L). The Eh of the lixiviant was adjusted to 1150 mV. The leach was conducted in two stages, firstly at a temperature of 70-75° C. and then at 90-95° C., with a liquid/solids step between stages. The time of leaching in each stage was 2 hours. After a liquid/solids separation step, gold was recovered from the liquid using the method of Example IV. The first sample subjected to the leach was a calcined sample of a gold ore and the second sample was a concentrate of a gold ore.

(30) Analysis subsequent to the second leach showed that 99% of the gold was extracted from the calcined sample and 98% of the gold was extracted from the concentrate sample.

Example VI

(31) To illustrate an alternate extraction step, liquid obtained from a liquid/solids separation step as described in Example IV was subjected to extraction with Alamine™ 336 alkyl amine in EXXAL™-13 tridecyl alcohol and CF-231 kerosene, with the ratio being 35/20/45. The ratio of organic:aqueous in the extraction solution was 1:1, the temperature was room temperature and the time of contact between the extractant and liquid from the leach was 10 minutes. The feed and raffinate solutions were analyzed for gold and iron. The results obtained are shown in Table 3.

(32) TABLE-US-00005 TABLE 3 Metal Feed (mg/L) Raffinate (mg/L) Extraction (%) Au 99.9 0.08 99.9 Fe 14800 1.35 99.9

Example VII

(33) A first sample of the loaded organic solution obtained in Example VI was subjected to selective stripping, in two stages, using 0.05N HCl at an organic:aqueous ration of 1:1 at room temperature for 10 minutes in each stage. A second sample was stripped with 2% thiourea solution at an organic:aqueous ratio of 0.33 at room temperature for 5 minutes.

(34) The results obtained were as follows:

(35) TABLE-US-00006 Organic Phase Stripping (%) Stripping Method Au (mg/L) Fe (mg/L) Au (mg/L) Fe (mg/L) HCl 99.82 14800 0.0 87.8 Thiourea 99.82 1819 95.27 3.5

(36) The results showed that hydrochloric acid stripped iron from the loaded organic solution whereas thiourea stripped gold from solution, thereby illustrating preferential stripping of iron or gold from the loaded organic solution.