Precious metals bearing waste materials are generated in a great variety by many metal working industries. A substantial amount of these materials are defined as hazardous and are strictly regulated under state and federal laws. Over 300 Million tons of hazardous wastes are generated each year in the States. Although any movement of this material for disposal needs shipping manifest, its ultimate disposition is difficult to trace. Most materials receive final treatment off site, and typically find their way into hazardous waste dumps. Valuable resources in the form of gold, silver and PGM contained in materials sent to waste dumps are irretrievable lost, while the cradle to grave responsibility of the waste generator continues.
Electronic scrap in the form of circuit boards, both with and without devices, IC’s, both plastic and ceramic encased, is often shipped abroad for recovery of devices and metallic content. Unlike the above types of waste material, electronic scrap is not required to be shipped out under manifest, thus making it difficult to quantify electronic scrap production.
Recycling of precious metals from hazardous waste using both hydrometallurgy and pyrometallurgy is economically possible when done in conjunction with electronic scrap. However, it’s imperative that an incoming fee from $150 to $900 per ton be imposed on hazardous waste. These fees are competitive with landfills. The generator is absolved from ongoing liability because waste has been recycled.
Incoming hazardous wastes are classified in five groups. These wasted plus circuit board scrap, IC’s (plastic and ceramic) and other plated scrap such as connectors, become the feed stock for any recycling process. Waste types listed in next table may contain different proportions of one or several of the following metals or metal salts as solids or in solution; copper, chromium, nickel, tin, lead, cadmium, zinc, cobalt, vanadium, neodymium, rhenium. Additionally some salts of calcium and sodium as chlorides, sulphates, nitrates, phosphates and cyanides which are routinely found in hazardous wastes.
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Type |
Description |
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1 |
Alkaline solutions typically produced from boiler blow downs. These contain very small amounts of metals and some sodium salts. pH > 7 |
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2 |
Cyanide solutions generated by metal finishing industries. These contain both sodium hydroxide and sodium cyanide plus cyanide salts of various metals. May contain precious metals. pH > 8 |
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3 |
Acid solutions, typically produced by the metals industry, these contain metallic salts as well as free acids from such common industrial processes as picking, plating, galvanizing. pH < 7 |
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4 |
Chromium solutions and solids, generally produced by the plating industry. Chromium solids are by products of alloys manufacture such as bag-house dust and grindings |
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5 |
Metal hydroxide cake produced from alkaline precipitation of wastewater from plating or other operations. May contain precious metals. |