Recovery of Gold from Incinerated Sewage Sludge Ash by Chlorination

In a series of studies into the development of an efficient and selective recovery process for gold that eliminates the need for water, the release behavior of gold from an incinerated sewage sludge ash heated to between 100 and 1000 degrees C under a chlorine gas stream was investigated. A release of gold was observed above 600 degrees C, with all gold being transferred to the gas phase at 1000 degrees C. The addition of solid carbon to the incinerated ash reduced this initial release temperature to 400 degrees C, with all gold being released into both the gas phase and solid carbon at 700 degrees C. Subsequent optimization of the reaction conditions to maximize the amount of gold reporting to the solid carbon determined that the mixture of incinerated ash and carbon should first be heated to a terminal temperature of 800 degrees C under a nitrogen gas stream. Upon reaching this temperature, the gas stream should be switched from chlorine to nitrogen, and the mixture held at 800 degrees C for 1 h. Scanning electron microscopy and X-ray diffraction measurements confirm that this produces a distribution of fine metallic gold over the carbon surface.