Alkaline and acid pressure oxidation of precious metal-containing materials

1. A method, comprising:

receiving an aqueous leach solution comprising a leaching agent;

passing the aqueous leach solution through a membrane filter to form an aqueous retentate comprising at least most of the leaching agent and an aqueous permeate comprising some of the leaching agent;

recycling the aqueous retentate to a leaching step;

destroying the at least most of the leaching agent in the aqueous permeate to form an aqueous recycle stream substantially free of leaching agent; and

recycling the aqueous recycle stream to a unit operation upstream of the leaching step.

2. The method of claim 1 , wherein the aqueous leach solution comprises a precious metal, wherein the leaching agent is at least one of cyanide, thiosulfate, and thiourea, and wherein the membrane filter is at least one of a semi-permeable reverse osmosis filter, a nanofilter, an ultrafilter, and a microfilter.

3. The method of claim 2 , wherein the aqueous leach solution is passed through only a single stage of membrane filter.

4. The method of claim 2 , wherein the destroying step includes an oxidation process to destroy the leaching agent, and further comprising before recycling of the aqueous recycle stream: contacting the aqueous recycle stream with a reductant to react with at least most of any excess of oxidant of the oxidation process.

5. The method of claim 1 , wherein the leaching step comprises the substeps:

pressure oxidizing, at a pH of less than pH 6.5, an acid generating feed material comprising a valuable metal and a sulfide sulfur to form an acid discharge slurry comprising a sulfate sulfur and having a first pH of less than pH 6.5;

pressure oxidizing, at a pH of at least pH 6.5, an acid consuming feed material comprising the valuable metal and the sulfide sulfur to form an alkaline discharge slurry comprising the sulfate sulfur and having a second pH of at least pH 6.5; and

combining the acid and alkaline discharge slurries to provide a combined discharge slurry having a third pH greater than the first pH and less than the second pH.

6. The method of claim 5 , wherein the valuable metal is a precious metal, wherein the acid generating feed material comprises more total moles of acid generators than acid consumers, and wherein the acid consuming feed material comprises more total moles of acid consumers than acid generators.

7. The method of claim 6 , wherein a first equivalent carbonate level in the acid generating feed material ranges from about 0.1 to about 1% by weight and wherein a second equivalent carbonate level in the acid consuming feed material is at least about 0.7% by weight.

8. The method of claim 5 , wherein the acid discharge slurry comprises at least about 2 g/L free sulfuric acid while the alkaline discharge slurry comprises no more than about 1 g/L free sulfuric acid and acid equivalents.

9. The method of claim 5 , wherein at least most of the sulfide sulfur in the acid generating and acid consuming feed materials is oxidized in the leaching step and wherein the sulfate sulfur comprises at least most of the sulfide sulfur oxidized in the acid and alkaline discharge slurries.

10. The method of claim 5 , wherein the acid discharge slurry comprises from about 2 to about 150 g/L free sulfuric acid and acid equivalents, wherein the alkaline discharge slurry comprises no more than about 1 g/L free sulfuric acid and acid equivalents, wherein the combined discharge slurry has an acidic pH and contains less free sulfuric acid and acid equivalents than the acid discharge slurry and more free sulfuric acid and acid equivalents than the alkaline discharge slurry, and wherein the valuable metal is gold and further comprising: after the combining step, adjusting the combined discharge slurry to have a fourth pH having a fourth pH value equal to or higher than the third pH; and recovering gold from a solid phase of the combined feed slurry to form a gold product and the aqueous leach solution.

11. The method of claim 5 , wherein the acid discharge slurry comprises from about 2 to about 150 g/L free sulfuric acid and acid equivalents, wherein the alkaline discharge slurry comprises no more than about 1 g/L free sulfuric acid and acid equivalents, wherein the combined discharge slurry has an alkaline pH and contains less acid consumers than the alkaline discharge slurry and more acid consumers than the acid discharge slurry, and wherein the valuable metal is gold and further comprising: after the combining step, adjusting the combined discharge slurry to have a fourth pH having a fourth pH value higher than the first, second, and third pH's; and recovering gold from a solid phase of the combined feed slurry to form a gold product and the aqueous leach solution.

12. The method of claim 1 , wherein the leaching step comprises the substeps:

pressure oxidizing, at a pH of less than pH 6.5 and in an acid autoclave, an acidic precious metal-containing feed material comprising an acid generator and acid consumer to form an acid discharge slurry comprising a generated acid and having a first pH of less than pH 6.5;

pressure oxidizing, at a pH of at least pH 6.5 and in an alkaline autoclave, an alkaline precious metal-containing feed material comprising the acid generator and acid consumer to form an alkaline discharge slurry comprising a sulfate sulfur and having a second pH of at least pH 6.5; combining the acid and alkaline discharge slurries to provide a combined discharge slurry having a third pH greater than the first pH and less than the second pH; and

thereafter recovering the precious metal from a solid phase of the combined discharge slurry to form a gold product and the aqueous leach solution.

13. The method of claim 12 , wherein the acid generating feed material comprises more total moles of acid generators than acid consumers and wherein the acid consuming feed material comprises more total moles of acid consumers than acid generators.

14. The method of claim 13 , wherein a first equivalent carbonate level in the acid generating feed material ranges from about 0.1 to about 1% by weight and wherein a second equivalent carbonate level in the acid consuming feed material is at least about 0.7% by weight.

15. The method of claim 12 , wherein the acid generator is primarily a sulfide sulfur and wherein the acid discharge slurry comprises at least about 2 g/L but less than about 25 g/L free sulfuric acid and acid equivalents while the alkaline discharge slurry comprises no more than about 1 g/L free sulfuric acid and acid equivalents.

16. The method of claim 15 , wherein at least most of the sulfide sulfur in the acid and acid consuming feed materials is oxidized in the leaching step and wherein the sulfate sulfur comprises at least most of the sulfide sulfur in the acid and alkaline discharge slurries.

17. The method of claim 12 , wherein the acid discharge slurry comprises from about 2 to about 150 g/L free sulfuric acid and acid equivalents, wherein the alkaline discharge slurry comprises no more than about 1 g/L free sulfuric acid and acid equivalents, wherein the combined discharge slurry has an acidic pH and contains less free sulfuric acid and acid equivalents than the acid discharge slurry and more free sulfuric acid and acid equivalents than the alkaline discharge slurry, and wherein the valuable metal is gold and further comprising: after the combining step and before the thereafter recovering step, adjusting the combined discharge slurry to have a fourth pH, the fourth pH having a fourth pH value equal to or higher than the third pH.

18. The method of claim 12 , wherein the acid discharge slurry comprises from about 2 to about 150 g/L free sulfuric acid and acid equivalents, wherein the alkaline discharge slurry comprises no more than about 1 g/L free sulfuric acid and acid equivalents, wherein the combined discharge slurry has an alkaline pH and contains less acid consumers than the alkaline discharge slurry and more acid consumers than the acid discharge slurry, and wherein the valuable metal is gold and further comprising: after the combining step and before the thereafter recovering step, adjusting the combined discharge slurry to have a fourth pH, the fourth pH having a fourth pH value being higher than the first, second, and third pH's.