Process of leaching gold

Abstract

Disclosed is a process of leaching gold that can be inexpensively and efficiently carried out from a mixture containing sulfur and gold, typically from the material containing sulfur and gold that is an intermediate product recovered by the flotation method in the hydrometallurgical method. Specifically, disclosed is a process of leaching gold from the mixture containing elemental sulfur and gold, the process involving combining the mixture containing elemental sulfur and gold and an aqueous solution of hydroxide of one or more of metals of alkali metals and alkaline earth metals, the hydroxides reacting with elemental sulfur to form thiosulfate of alkali and/or alkaline earth metal, and the gold thereby being leached by reaction with the thiosulfate.

Claims

1. A process of leaching gold from floats containing a mixture of elemental sulfur and gold, said floats recovered by a flotation method from a post-leaching residue obtained after a gold-containing copper concentrate is leached, the process comprising: contacting said mixture with an aqueous solution of one or more metal hydroxide, wherein said one or more metal is one or more metal selected from the group consisting of alkali metal and alkaline earth metal, reacting said one or more metal hydroxide and elemental sulfur to produce one or more metal thiosulfate, and leaching gold by the produced one or more metal thiosulfate.

Description

BRIEF DESCRIPTION OF DRAWINGS

(1) FIG. 1 is a figure showing an example of a flow of process of leaching gold according to the present invention.

(2) FIG. 2 is a figure showing the effects of temperature and time on the leaching of gold from the floats mainly constituted of sulfur recovered by a flotation in Example.

MODE FOR CARRYING OUT THE INVENTION

(3) The object substance of the present invention is not particularly restricted, as long as it is a mixture containing elemental sulfur and gold. However, typically, it is the floats recovered by a flotation from the post-leaching residue obtained after hydrometallurgically leaching objective metal from ore or concentrate containing gold and sulfide of objective metal. While the objective metals are not particularly restricted, for example, copper and zinc are mentioned. In the typical embodiment of the present invention, the object substance is the floats recovered by a floatation from the post-leaching residue obtained by hydrometallurgy of copper.

(4) According to the present invention, said mixture is firstly contacted with an aqueous solution of hydroxide of one or more metals selected from the group consisting of alkali metals and alkaline earth metals, and thus, said hydroxide is reacted with sulfur so that thiosulfate of the corresponding metal is produced. By said reaction, an effect that elemental sulfur covering the surface of gold is decreased, and thus, the leach rate of gold will increase is obtained. In addition, since the produced thiosulfate has an action of leaching gold, the leaching of gold can be carried out without adding any other agent for leaching gold.

(5) As for hydroxides of alkali metals, there is not any particular restriction, and therefore, any of caustic soda (sodium hydroxide), potassium hydroxide, rubidium hydroxide, cesium hydroxide, and francium hydroxide can be used. However, among them, caustic soda is preferable on the cost front.

(6) As for hydroxides of alkaline earth metals, there is also no restriction, and therefore, calcium hydroxide, strontium hydroxide, barium hydroxide and radium hydroxide can be used. However, hydrated lime (calcium hydroxide) is preferable because it is easily available. Hydroxides of beryllium and magnesium which are elements belonging to Group II elements like alkaline earth metals have a very poor water solubility, and therefore, they are not adequate.

(7) On the basis of the leach rate of gold, total amount of hydroxides of alkali metal(s) and alkaline earth metal(s) is preferably 0.5 equivalent or more, more preferably 1.0 equivalent or more, and most preferably 2.0 equivalent or more. However, on the basis of saturation of the leach rate and cost, no excessive amount is required to be added, and thus, practically, the amount is preferably 5.0 equivalent or less, more preferably 3.0 equivalent or less, and yet more preferably 2.5 equivalent or less.

(8) As for the method for contact, there is no particular restriction, and thus, for example, a method wherein the mixture is soaked in said aqueous solution, a method wherein said aqueous solution is sprayed to the mixture, a method wherein said aqueous solution is poured onto the mixture, and the like can be mentioned. On the basis of the efficiency of leaching, the method wherein the mixture is soaked in said aqueous solution is preferable. In order to increase the efficiency of the reaction, the mixture is preferably provided in powdered or granular form. Stirring the mixture is also preferable. Further, it is also preferable to heat the mixture or to blow in an oxygen-containing gas into the mixture.

(9) When the mixture is heated, the temperature of the mixture is preferably 40 C. or more, more preferably 60 C. or more, and yet more preferably 70 C. or more. The temperature is preferably 75-85 C. (e.g., about 80 C.) because too much heating increases energy cost and the amount of evaporation of water is increased.

(10) As for the oxygen-containing gas, pure oxygen, industrial oxygen purified by pressure swing adsorption and the like, air and the like can be mentioned. Air is preferable because of cost/effect tradeoffs.

(11) One embodiment of the process according to the present invention is shown in FIG. 1. Along with the flow sheet, the process is explained below. The process according to the present invention can be preferably applied to the post-leaching residue obtained by the hydrometallurgy of copper as indicated, but is not limited to it.

(12) The post-leaching residue of the gold-containing copper concentrate is separated into the floats and tailings by a floatation. Gold is recovered in the floats together with sulfur. The grade of the floats as the raw material is, for example, 30-55 g of gold/t, 0.3-1.0 mass % of copper, 9.2-14 mass % of iron, and 50-70 mass % of sulfur.

(13) When the floats recovered are soaked in an aqueous solution of caustic soda and stirred, sodium thiosulfate is produced. This reaction is represented by the following formula.
6NaOH+6S.fwdarw.2Na.sub.2S.sub.2+Na.sub.2S.sub.2O.sub.3+3H.sub.2O
Na.sub.2S.sub.2.fwdarw.Na.sub.2S+S

(14) The amount of caustic soda added in this process is typically about 1-1.2 times of stoichiometric amount. In addition, it is considered that blowing oxygen into said reaction will cause oxidation of Na.sub.2S as shown in the following formula, and therefore, sodium thiosulfate will be produced, and thus, the leach rate will further increase, and therefore the efficiency of the reaction will be improved.
2Na.sub.2S+2O.sub.2+H.sub.2O.fwdarw.Na.sub.2S.sub.2O.sub.3+2NaOH

(15) When further stirring is continued, the reaction of leaching of gold by sodium thiosulfate produced will progress. This reaction is represented by the following formula.
Au+O.sub.2+4S.sub.2O.sub.3.sup.2+H.sub.2O.fwdarw.2Au(S.sub.2O.sub.3).sub.2.sup.3+2OH.sup.

(16) Further, it is also considered that since said reaction also requires oxygen, the leach rate will increase and the efficiency of the reaction will be thus increased by blowing in oxygen.

(17) A dissolved solution of gold obtained by solid-liquid separation after the reaction of leaching gold is then sent to the process for recovering gold, and it is purified by known means such as activated carbon adsorption, solvent extraction, reduction, cementation, electrolytic refining and the like, and thus, gold product can be obtained. For example, while S value exists in the post-leaching solution in the forms of thiosulfate, sulfide and elemental sulfur and the like, these can be separated from gold by solvent extraction.

EXAMPLE

(18) While the present invention will be further explained below in detail, the present invention is not limited by these examples at all.

(19) The method for analyzing metals used in examples was carried out by ICP Emission Spectroscopy. In the analysis of gold, gold was separated out by cupellation and then quantitative analysis was performed by ICP Emission Spectroscopy.

Example 1

Effects of Temperature and Time on the Rate of Leaching Gold

(20) The post-leaching residue obtained after copper was leached with sulfuric acid from copper concentrate was separated, by a floatation, into the floats and tailings. The grade of the floats thus obtained was 45.1 g of gold/t, 0.6 mass % of copper, 14 mass % of iron, and 62 mass % of sulfur.

(21) 75 g of said floats were projected into an aqueous solution of caustic soda of 2.8 mol/L so that the pulp concentration would be 70 g/L, and thus, Au was leached by the produced sodium thiosulfate in each condition described in Table 1. In this case, the amount of caustic soda added is 2 equivalent.

(22) As the result, while the leach rate of gold for 100 hours was 6% at 25 C., the reaction being promoted by increasing the temperature. At 40 C., the leach rate was 28% for 1 hour, and was 49% for 10 hours. At 60 C., the leach rate was 46% for 1 hour and was 67% for 10 hours. Further, when the temperature was 80 C., the leach rate of gold reached 89% for 10 hours. These results are shown in Table 1 and FIG. 2.

(23) The leach rate of gold was determined by calculating the amount distributed into the aqueous solution based on the weight and grade of the floats and post-leaching residue, provided that the weight of gold in the float is 100%.

(24) At 25 C., the leach rate of sulfur for 100 hours was 98%. The reaction was promoted by increasing temperature, and thus, at 60 C., the leach rate was 98% for 1 hour and was 99% for 10 hours. At 80 C., the leach rate was 88% for 10 hours. The reason why the leach rate of sulfur at 80 C. is lower than that at 60 C. is assumed that elemental sulfur was produced by the degradation of thiosulfate.

Example 2

Effects of Blowing in Air

(25) 75 g of the same floats as used in Example 1 was projected into 2.8 mol/L caustic soda solution so that the pulp concentration would be 70 g/L, and then Au was leached by the produced sodium thiosulfate produced at a solution temperature of 60 C., with air being blown into the solution at the rate of 1.0 L/min for 1 hour. The result is shown in Table 1. In this case, the leach rate was 56%, which was improved by 10% or more compared to the case where air was not blown.

Comparative Example 1

Effect of Removing Sulfur on Leaching Gold

(26) The leach rate of gold was examined by performing the leaching of gold with sodium thiosulfate on the raw material floats in a similar manner.

(27) The grade of the raw material floats was 45.1 g of gold/t, 0.6 mass % of copper, 14 mass % of iron, and 62 mass % of sulfur.

(28) 75 g of said floats was projected into 0.5 mol/L of an aqueous solution of sodium thiosulfate so that the pulp concentration would be 70 g/L, and thus, Au was leached.

(29) The condition for leaching was the same condition as in Example 1 which achieved the maximum leach rate, namely, the temperature of the solution being 80 C. and the reaction time being 10 hours. Using sodium carbonate, pH was adjusted to 10. During the test, pH was constant. In this case, the leach rate was 10% for 1 hour, and was 14% for 10 hours. These are remarkably lower values compared to Example 1.

(30) It is considered this is because removal of sulfur was performed at the same time as gold was leached in Example 1, and therefore, covering of gold by sulfur and disturbance against the leaching by hydrophobicity was avoided.

(31) Accordingly, it is clear that the process according to the present invention for leaching gold from the material containing sulfur and gold which is recovered by the floatation from the post-leaching residue obtained through the hydrometallurgical process, is preferable as a process for leaching without using cyan.

(32) TABLE-US-00001 TABLE 1 Temperature of the solution 25 C. 40 C. 60 C. 80 C. Leaching Leach Leaching Blowing of air solution rate time no no no yes no Remarks Aqueous Leach 1 hour 28% 46% 56% Examples solution rate of 5 hours 48% (1-2) of Au 10 hours 49% 67% 89% caustic 100 hours 6% soda Leach 1 hour 85% 98% rate of 5 hours 99% sulfur 10 hours 98% 99% 88% 100 hours 98% Aqueous Leach 1 hour 10% Comp. sol. of rate of 10 hours 14% Example sodium Au (1) thiosulfate

Example 3

Effect of the Amount of Caustic Soda on the Leach Rate of Gold

(33) 150 g of the same floats as used in Example 1 was projected into the caustic soda solution such that the concentration of pulp would be 70 g/L. The concentration of caustic soda was adjusted to have the value of equivalent against sulfur as described in Table 2. Then, under the conditions described in Table 2, without blowing in air, Au was leached with sodium thiosulfate produced, leaching time being 10 hours. The results are shown in Table 2. It can be seen that the leach rate of Au increases according to the increase in the equivalent of caustic soda against sulfur.

(34) TABLE-US-00002 TABLE 2 Leaching Temp. of Equivalent of NaOH (to sulfur) solution Leach rate sol. ( C.) 0.5 1 2 Aqueous Leach rate 60 55% 58% 67% (Example 1) sol. of of Au 80 50% 70% 89% (Example 1) caustic Leach rate 60 98% 99% 99% (Example 1) soda of sulfur 80 98% 99% 88% (Example 1)