Exploitation of gold in a historic sewage sludge stockpile, Werribee, Australia: resource evaluation, chemical extraction and subsequent utilisation sludge - Reply

Sewage sludges are potential targets for economic extraction of Au because of the documented Au content of sewage sludges worldwide, which are of the order of some ore deposits currently mined for Au. The sewage of Melbourne, Australia, was stockpiled in large, closed, lagoonal tanks from 1898 until 1980. Reeves, Plimer and Foster, 1999, have conducted, and published, an e xtensive and exhaustive study of the Werribee sewage reserves utilising RNA A, INAA, GFAAS, ICPMS, and FLAAS to determine 31 elements, including Au, Ag , Sb, As, Cd, tig, Zn, Cu, and Pb. The study was initiated to determine Au, Ag and other metal variations in both space and time and to investigate th e economics of chemical extraction of the precious metals. A total of 149 s amples from over 50 hand-auger drillholes to a depth up to 4m were analysed from the stockpiles, with Au assays yielding remarkably consistent results . Average grades of 0.77 g/t Au and 18.8 g/t Ag have been documented for a measured resource of 770,000 m(3). Laboratory-based extractive metallurgy o f the Werribee sludges has demonstrated that Au, Ag, and Zn can be removed with relative ease by heap-leaching using modified conventional technology, albeit with prohibitive reagent consumption. The extraction of the preciou s metals also results in the variable removal of contaminant metals such as Cd, As, Sb, Hg and Cr which may render the sludges fit for sale as agricul tural fertiliser, provided organic pollutants and pathogenic organisms are below governmental environmental protection limits. (C) 2001 Elsevier Scien ce B.V. All rights reserved.