A Standardized Test to Determine Gravity Recoverable Gold
A.R. Laplante; McGill University, Montreal
April 18th, 2000
Using gravity to supplement either flotation or cyanidation is a well established practice in the gold industry. Gravity differs from other recovery methods in that most of the gold recovered by gravity would be recovered by the circuit downstream, be it flotation or cyanidation, should gravity be by-passed. The economic justification of gravity is therefore based on small margins (for example, a net smelter return of gravity gold of 99%, as opposed to 94% for flotation). It was easily demonstrated when either flotation or cyanidation were relatively inefficient processes, and labour costs low (as gravity can be labour intensive).
However, over the past thirty years the introduction of better flotation machines (flash, column, high capacity), more effective collectors, and better control systems has increased flotation's metallurgical performance, thereby decreasing the incentive for gravity recovery. Cyanidation technology has undergone similar changes, with the advent of activated carbon, oxygen and lead nitrate addition, and improved impeller design.
Today, gravity can remain an attractive option only inasmuch as it can be implemented with very low capital and operating costs. This has resulted in a relative shift away from gold gravity recovery (except for alluvial deposits), in the seventies and eighties. For example, as of the early nineties, gold gravity recovery has disappeared from the typical South African flow sheet. The advent of the Knelson Concentrator, at the beginning of the eighties, foreshadowed a resurgence of gravity recovery, as gold's grinding and classification behaviour (Banisi, Laplante and Maro is, 1991) makes it possible to achieve adequate gold recoveries with very simple, Knelson based, gravity circuits (Laplante et al., 1994).
Consider for example the Hemlo recovery circuit (Honan, 1996; Laplante, Vincent and Luinstra, 1996), consisting of a single 61 cm x 122 cm (2' x 4') screen, an automated 76 cm (30") Knelson feeding a Gemeni table, to produce a 70- 80% Au concentrate which accounts for approximately one fourth of the gold production (about 3 million grams per year). The low capital and operating costs make it possible to justify the gravity circuit on the basis of savings in other operating costs (e.g. fewer carbon stripping cycles). Any improvements in recovery, which are at Hemlo difficult to measure, would be additional benefits. Such a circuit is rapidly becoming a standard in Canadian and Australian operations.
The justification for installing a gravity circuit must be first based on an estimate of how much gold it will recover (irrespective of what the potential benefits are). This is a function of the nature of the gravity circuit to be installed; however, Knelson circuits, by their very simplicity and efficiency, limit the options. This leaves the amenability of the ore to gravity recovery as the single most important factor in predicting gravity recovery. Various approaches can be used to characterize this response (Woodcock, 1994). The present work sought a route which would fulfil the following criteria....
Download the whole report:
