Recovery of Trace Precious Metals in Strongly Acidic Wastewater

The strongly acidic wastewaters generated in the process of refining and recycling of precious metals are generally treated by hydrometallurgical routes. For example, wastewaters containing precious metals such as gold, platinum, palladium and silver at concentrations of around 10 mg/L, and other metals such as copper and iron at concentrations ranging from several mg/L to more than 10,000 mgg/L, are usually treated by neutralization. However, wastewaters containing a large amount of ammonium ion (NH(4)(+)) cannot be treated by general methods of neutralization due to the formation of metal-ammine complexes with increasing pH. The objective of this work is to recover trace precious metals in strongly acidic wastewater containing a large amount of ammonium ions. When wastewater was neutralized at pH 6, the majority of metals such as copper, lead and iron were precipitated, while precious metals were not precipitated. After neutralization, precious metals remaining in the filtrate were precipitated by NaBH(4) reduction at pH 7.5. In the case of platinum in particular, the reduction was improved by deammoniation with sodium hydroxide. Precious metals were concentrated in copper-metal by combining neutralization, deammoniation, reduction and fusion. Recoveries of gold, silver, and palladium were more than 91%, whereas that of platinum was about 71%.