Precious metals in urban waste

Samples of incinerator ash from sewage operations in the UK have been analysed and all contain concentrations of over 1 ppm gold (Au), ranging up to 7 ppm, showing a remarkable consistency of enrichment for all the 9 incinerators sampled. Samples analysed from 2005 to 2007 and in 2014 all have elevated values showing a consistency of high Au grades over time as well as between incinerators. One dried sewage sample also has elevated Au values averaging 721 ppb. Rounded grains of Au, 2-3 microns in diameter, have been located in the incinerator ash. Our results indicate that this ash produced from incinerated sewage contains Au, with associated platinum-group elements (PGE) and silver (Ag). This forms a polymetallic resource which may be economic to recover and recycle. Tests to extract these precious metals from the ash are ongoing.