Recovery of gold and/or silver from scrap

Abstract

Gold and silver are recovered selectively such that gold and silver are separated from non-silver and non-gold material within the scrap. Gold and silver are recovered from scrap material using mixtures of acids, in some instances. The mixture comprises nitric acid and at least one supplemental acid, such as sulfuric acid or phosphoric acid. The amount of nitric acid within the mixture are relatively small compared to the amount of sulfuric acid or phosphoric acid within the mixture. The recovery of gold and silver using the acid mixtures are enhanced by transporting an electric current between an electrode and the gold and silver of the scrap material. Acid mixtures are used to recover silver from particular types of scrap materials, such as scrap material comprising silver metal and cadmium oxide and scrap material comprising silver metal and tungsten metal.

What is claimed is: 
     
         1 . A method of recovering gold and/or silver from a scrap material, comprising:
 exposing the scrap material comprising the gold and/or silver and at least one base metal to a mixture comprising nitric acid and at least one supplemental acid; and   recovering at least a portion of the gold and/or silver from the scrap material,   wherein the amount of nitric acid within the mixture is less than or equal to about 10 wt %.   
     
     
         2 . The method of  claim 1 , wherein the ratio of the weight of the at least one supplemental acid to the weight of the nitric acid is at least about 3:1. 
     
     
         3 . The method of any one of  claims 1 - 2 , wherein the supplemental acid comprises phosphoric acid and/or sulfuric acid. 
     
     
         4 . The method of any one of  claims 1 - 3 , comprising transporting an electric current between an electrode and the gold and/or silver of the scrap material such that at least a portion of the gold and/or silver is removed from the scrap material. 
     
     
         5 . The method of  claim 4 , wherein the mixture comprises nitric acid and sulfuric acid, and the weight ratio of sulfuric acid to nitric acid within the mixture is at least about 12:1. 
     
     
         6 . The method of  claim 4 , wherein the mixture comprises nitric acid and phosphoric acid, and the weight ratio of phosphoric acid to nitric acid within the mixture is at least about 11:1. 
     
     
         7 . A method of recovering gold and/or silver from a scrap material, comprising:
 exposing the scrap material comprising the gold and/or silver and at least one base metal to a fluid comprising an oxidant having the ability to dissolve gold and/or silver; and   recovering at least a portion of the gold and/or silver from the scrap material;   wherein the amount of the oxidant within the fluid is less than or equal to about 10 wt %.   
     
     
         8 . The method of  claim 7 , wherein the fluid comprises at least one supplemental acid. 
     
     
         9 . The method of  claim 8 , wherein the supplemental acid comprises phosphoric acid and/or sulfuric acid. 
     
     
         10 . The method of any one of  claims 7 - 9 , wherein the oxidant comprises an oxide of manganese, nickel, lead, and/or chromium. 
     
     
         11 . The method of  claim 10 , wherein the oxidant comprises manganese dioxide (MnO 2 ). 
     
     
         12 . The method of any one of  claims 1 - 11 , wherein the amount of water in the mixture is less than or equal to about 17 wt %. 
     
     
         13 . The method of  claim 12 , wherein the mixture comprises sulfuric acid. 
     
     
         14 . The method of  claim 13 , wherein the amount of water within the mixture is less than about 8 wt %. 
     
     
         15 . The method of any one of  claims 12 - 14 , wherein the mixture comprises phosphoric acid. 
     
     
         16 . A method of recovering gold and/or silver from a scrap material, comprising:
 exposing the scrap material comprising the gold and/or silver and at least one base metal to a mixture comprising sulfuric acid and nitric acid and/or to a mixture comprising phosphoric acid and nitric acid; and   transporting an electric current between an electrode and the gold and/or silver of the scrap material such that at least a portion of the gold and/or silver is removed from the scrap material.   
     
     
         17 . A method of recovering gold, comprising combining water and a gold-containing solution comprising dissolved gold, nitric acid and sulfuric acid, and/or nitric acid and phosphoric acid to form a mixture, such that solid gold is dissolved within the mixture. 
     
     
         18 . A method of recovering silver, comprising exposing a scrap material comprising the silver and cadmium oxide to a mixture of sulfuric acid and nitric acid and/or to a mixture of phosphoric acid and nitric acid such that the silver is dissolved by the mixture. 
     
     
         19 . A method of recovering silver, comprising exposing a scrap material comprising the silver and tungsten to a mixture of sulfuric acid and nitric acid and/or to a mixture of phosphoric acid and nitric acid such that the silver is dissolved by the mixture. 
     
     
         20 . The method of any one of the preceding claims, wherein the scrap material comprises a coating comprising silver and/or gold over a substrate material comprising at least one base metal. 
     
     
         21 . The method of  claim 20 , wherein the mixture comprising sulfuric acid and nitric acid or phosphoric acid and nitric acid dissolves the gold and/or silver such that the ratio of the mass of the coating that is dissolved to the mass of the substrate material that is dissolved is at least about 5:1, at least about 10:1, at least about 25:1, at least about 50:1, or at least about 100:1. 
     
     
         22 . The method of  claim 20 , wherein the mixture comprising sulfuric acid and nitric acid or phosphoric acid and nitric acid dissolves the gold and/or silver such that the ratio of the mass of the coating that is dissolved to the mass of the substrate material that is dissolved is up to about 10,000:1. 
     
     
         23 . The method of any one of  claims 1 - 16  and  20 , wherein gold and/or silver is dissolved from the scrap material. 
     
     
         24 . The method of any one of  claims 17 - 19  and  21 - 23 , comprising forming a silver-containing solid and/or a gold-containing solid from the dissolved gold and/or silver. 
     
     
         25 . The method of  claim 24 , wherein the silver-containing solid comprises silver metal and/or a silver salt. 
     
     
         26 . The method of  claim 24 , wherein the gold-containing solid comprises gold metal. 
     
     
         27 . The method of any one of  claims 24 - 26 , wherein the silver-containing solid and/or the gold-containing solid is at least partially separated from the mixture. 
     
     
         28 . The method of any one of  claims 1 - 27 , wherein the mixture contains supplemental acid in an amount of at least about 50 wt %. 
     
     
         29 . The method of any one of  claims 1 - 28 , wherein the mixture contains sulfuric acid in an amount of at least about 50 wt %. 
     
     
         30 . The method of any one of  claims 1 - 29 , wherein the base metal comprises iron, nickel, lead, zinc, copper, manganese, tin, antimony, and/or aluminum. 
     
     
         31 . The method of  claim 30 , wherein the base metal comprises copper, iron, nickel, lead, and/or zinc. 
     
     
         32 . The method of  claim 31 , wherein the base metal comprises copper. 
     
     
         33 . A system for the recovery of gold and/or silver from scrap material, comprising:
 a rotatable container positioned within a vessel configured to contain a liquid having a pH of less than about 2; and   an electrically conductive pathway configured such that, when the scrap material is contained within the rotatable container, the electrically conductive pathway remains in electrical communication with the scrap material when the container is rotated.   
     
     
         34 . The system of  claim 33 , wherein the liquid having the pH of less than about 2 comprises nitric acid and at least one supplemental acid. 
     
     
         35 . The system of  claim 34 , wherein the liquid having the pH of less than about 2 comprises nitric acid and sulfuric acid. 
     
     
         36 . The system of any one of  claims 34 - 35 , wherein the amount of nitric acid within the liquid is less than or equal to about 10 wt %. 
     
     
         37 . The system of any one of  claims 35 - 36 , wherein the weight ratio of sulfuric acid to nitric acid within the liquid is at least about 12:1. 
     
     
         38 . The system of any one of  claims 33 - 37 , wherein the amount of water in the liquid is less than or equal to about 17 wt %.