Hydrometallurgical method for silver recovery

Abstract

A process for recovering silver from silver-bearing gold concentrate or other silver-bearing material which may comprise adding oxygen, water, and/or acid to an acidulated concentrate slurry of an input silver bearing material and reacting them together in an autoclave at an elevated pressure and temperature in a pressure oxidation step; processing the oxidized concentrate slurry in a post pressure oxidation conditioning step; applying a first solid/liquid separation and wash step and a filter and wash step to form a first washed slurry/solid and first acid-containing solutions; reacting the first washed slurry/solid with sulfur dioxide in a reductive leach step; applying a second solid/liquid separation and wash step to form a second washed slurry/solid and second acid-containing solutions; and applying an optional surface cleaning step, to produce a free-milling silver-bearing material, which is amenable to conventional cyanidation to recover the silver therefrom.

1 . A process for recovering silver from a silver-bearing material, comprising:
 subjecting an aqueous slurry of the silver-bearing material to a pressure oxidation step, wherein the aqueous slurry of the silver-bearing material is reacted with oxygen and an acid in an autoclave at an oxidizing pressure and at an oxidizing temperature, to form an oxidized concentrate slurry;   subjecting the oxidized concentrate slurry to a post pressure oxidation conditioning step, wherein the oxidized concentrate slurry is discharged from the autoclave and maintained for several hours at a temperature in a range of from about 50° C. to about 100° C., to form a conditioned slurry;   subjecting the conditioned slurry to a first solid/liquid separation and wash step, wherein the first solid/liquid separation and wash step comprises at least one conventional technique for facilitating the separation of a slurry into solids and a solution, and washing the resultant solids, to form a first washed slurry/solid and at least one first acid-containing solution;   subjecting the first washed slurry/solid to a reductive leach step, wherein the first washed slurry/solid is reacted with sulfur dioxide to form a leached concentrate slurry;   subjecting the leached concentrate slurry to a second solid/liquid separation and wash step, wherein the second solid/liquid separation and wash step is one of a number of techniques for facilitating the separation of a slurry into solids and a solution, and washing the resultant solids, to form a second washed slurry/solid and at least one second acid-containing solution; wherein the second washed slurry/solid is a free-milling silver-bearing material that is amenable to leach treatment to extract the silver.   
     
     
         2 . The process of  claim 1 , wherein prior to the pressure oxidation step, the silver-bearing material is subjected to an acidulation step, wherein an acid is added to the silver-bearing material, to form an acidulated concentrate slurry; 
     
     
         3 . The process of  claim 2 , wherein in the acidulation step, the acid is concentrated sulfuric acid or an aqueous solution of sulfuric acid. 
     
     
         4 . The process of  claim 1 , wherein in the pressure oxidation step, the aqueous slurry of the silver-bearing material is reacted with the oxygen and acid for from about 30 minutes to about 120 minutes in the autoclave, and wherein the oxidizing pressure is in a range of from about 200 psig to about 600 psig total pressure, the partial pressure of oxygen is in the range of from about 15 psi to about 250 psi, and the oxidizing temperature is in the range of from about 190° C. to about 240° C. 
     
     
         5 . The process of  claim 4 , wherein in the pressure oxidation step, the aqueous slurry of the silver-bearing material is reacted with the oxygen and acid for from about 60 minutes to about 90 minutes in the autoclave, and wherein the oxidizing pressure is in a range of from about 430 psig to about 530 psig total pressure, the partial pressure of oxygen is in the range of from about 25psi to about 100 psi, and the oxidizing temperature is in the range of from about 220° C. to about 230° C. 
     
     
         6 . The process of  claim 1 , wherein the first solid/liquid separation and wash step, comprises one or more of: applying a thickening step, to form an underflow stream of thickened oxidized concentrate slurry and an overflow stream of a first acid-containing solution; applying a countercurrent decantation wash step, to form an underflow stream of washed oxidized concentrate slurry and an overflow stream of a first acid-containing solution; and applying a filter and wash step, to form the first washed slurry/solid and a filtrate stream of a first acid-containing solution. 
     
     
         7 . The process of  claim 1 , wherein the first solid/liquid separation and wash step comprises: applying a thickening step; applying a countercurrent decantation wash step; and applying a filter and wash step. 
     
     
         8 . The process of  claim 1 , wherein at least one of the acid-containing solutions from the first solid/liquid separation and wash step is recycled to the acidulation step and/or to the pressure oxidation step. 
     
     
         9 . The process of  claim 1 , wherein in the reductive leach step, the first washed slurry/solid is reacted with sulfur dioxide and at least one of sulfuric acid and water. 
     
     
         10 . The process of  claim 1 , wherein in the reductive leach step, the first washed slurry/solid and sulfur dioxide are reacted together for from about 1 hour to about 10 hours, at a temperature of from about 50° C. to about 150° C., at a total pressure of from about 1 psig to about 150 psig, and at a partial pressure of sulfur dioxide from about 1 psi to about 50 psi. 
     
     
         11 . The process of  claim 10 , wherein in the reductive leach step, the first washed slurry/solid and sulfur dioxide are reacted together for from about 3 hours to about 6 hours, at a temperature of from about 70° C. to about 100° C., at a total pressure of from about 1 psig to about 30 psig, and a partial pressure of sulfur dioxide of from about 1 psi to about 30 psi. 
     
     
         12 . The process of  claim 10 , wherein in the reductive leach step, a catalyst of copper sulfate is added to enhance the reaction rate. 
     
     
         13 . The process of  claim 1 , wherein the second solid/liquid separation and wash step, comprises one or more of: a thickening step, to form an underflow stream of thickened leached concentrate slurry and an overflow stream of a second acid-containing solution; a countercurrent decantation wash step, to form an underflow stream of washed thickened slurry/solid and an overflow stream of a second acid-containing solution; and a filter and wash step, to form the second washed slurry/solid and an filtrate stream of a second acid-containing solution. 
     
     
         14 . The process of  claim 1 , wherein the second solid/liquid separation and wash step comprises: a thickening step; a countercurrent decantation wash step; and a filter and wash step. 
     
     
         15 . The process of  claim 1 , wherein the at least one second acid-containing solutions from the second solid/liquid separation and wash step is recycled to the acidulation step and/or to the pressure oxidation step. 
     
     
         16 . The process of  claim 1 , wherein before the pressure oxidation step, the silver-bearing material undergoes mechanical regrinding in a regrinding step. 
     
     
         17 . The process of  claim 1 , wherein following the second solid/liquid separation and wash step, the second washed slurry/solid is subjected to a surface cleaning step to form the free-milling silver-bearing material, wherein the surface cleaning step is selected from treatment with an oxidizing agent, treatment by extended aeration, and treatment by regrinding. 
     
     
         18 . The process of  claim 17 , wherein the oxidizing agent is selected from a group consisting of hydrogen peroxide, ozone, pure oxygen, and hypochlorite. 
     
     
         19 . The process of  claim 1 , further comprising a silver extraction step, wherein the free-milling silver-bearing material is leach treated with a lixiviant to extract the silver. 
     
     
         20 . The process of  claim 17 , wherein the lixiviant is selected from a group consisting of sodium cyanide, calcium thiosulfate, ammonium thiosulfate, thiourea, and glycine.