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

Sewage sludges are dewatered end products of human sewage waste and are rec ognised repositories of organic pollutants and heavy metals. They may be co nsidered targets for economic extraction of Au because of the documented Au content of sewage sludges worldwide which are of the order of some ore dep osits currently mined for Au. They are also highly nutrient enriched (nitro gen and phosphorus) and therefore amenable to use as agricultural fertilise r or as covers for mine wastes. The sewage of Melbourne, Australia, a city with a current population of 3.3 million, was stockpiled in large, closed, lagoonal tanks from 1898 until 1980. In 1995 Echidna Mining, an Australian gold exploration company, acquired the exploration rights to the ground sur rounding the historic sludge reserves and commenced a program of resource e valuation, utilising RNAA, INAA, GFAAS, ICP-MS and FLAAS to determine 31 el ements, including Au, Ag, Sb, As, Cd, Hg, Zn, Cu, and Pb. The study was ini tiated to determine Au, Ag and other metal variations in both space and tim e and to investigate the economics of chemical extraction of the precious m etals. A total of 149 samples from over 50 hand-auger drillholes to a depth up to 4 m have been analysed from the stockpiles, with Au assays yielding remarkably consistent results. Average grades of 0.77 g/t Au and 18.8 g/t A g have been documented for a measured resource of 770,000 m(3) (of an estim ated 1.6-2.5 million m(3) contained) at st density of 1.0 g/cm(3) and an av erage moisture content of around 40%. Laboratory-based extractive metallurg y of the Werribee sludges has demonstrated that Au, Ag and Zn can be remove d with relative ease by heap-leaching using modified conventional technolog y, albeit with prohibitive reagent consumption. The extraction of the preci ous 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 agric ultural fertiliser, provided organic pollutants and pathogenic organisms ar e below governmental environmental protection limits, an area beyond the sc ope of this paper. Another potential avenue of the exploitation of sewage s ludges is discussed: that of the utilisation of sludges to extract contamin ant metals from waste water and contaminated mine waters, which we demonstr ate on pure aqueous synthetic samples. This paper presents a study of the e xploitation of an historic sludge resource for its contained Au and residue post-metal extraction. (C) 1999 Elsevier Science B.V. All rights reserved.