6/9/2004

Gold leach plant tailings is the last place where we would normally look at installing a gravity plant, for very obvious reasons. At first glance, the only justification for such an installation is that it is extremely easy to substantiate any benefit - if anything can be had from the tailings, it is an unarguable addition to recovery.

The not-so-obvious justification to a tailings gravity installation is that of economy of scale. A single Knelson Concentrator will apply in most cases, with unit capacity of up to 400 tph solids. (Although the Knelson KC-XD70 concentrator is nameplate rated at up to 650 tph with a maximum theoretical capacity of 1000 tph, this rating applies in a closed milling circuit, and there is a need to be conservative in a single-pass application such as leach tailings.)

In most tailing installations the Knelson can be gravity fed from the leach plant, and tails can gravity flow to existing pumps, thus avoiding major pumping costs. Effectively therefore such an installation comprises only the gravity concentrator, fluidising water supply and a relatively small concentrate pumping system. The concentrate of course also must be treated, optionally by return to the primary grind circuit, regrind and leach, dedicated intense leach, etc.

Fortunately all of the process stages from gravity recovery via semi-continuous or continuous Knelson to concentrate treatment method can be easily tested at bench scale, and thereafter at pilot scale. Justification becomes a very simple issue.

Peacocke & Simpson is carrying out a major investigation on potential tailings installations, and has to date completed primary gravity testwork on tailings from several major South African gold mines, as well as a few mines in Zimbabwe and West Africa. Early indications are that in many cases gravity scavenging of tails will prove viable despite very low gold content in these streams - as low as 0.2 g/tAu in some cases.

Concentrate grades of up to 20 g/t have been achieved, and although these are generally in mass yields of less than 0.2%, they represent recoveries of up to 20%, or 0.04-0.05 g/tAu. Whilst these are very low numbers, returns become appreciable when coupled with high tonnage and low capital cost.

Financial modeling of a hypothetical case at 300 tph throughout and only 0.04g/t recovery indicates installation payback of about 8 months, with a project IRR of 90%.