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Lakefield, Canada

Fleming C.A.,SGS Minerals
Minerals and Metallurgical Processing | Year: 2010

Refractory gold concentrates often contain submicroscopic gold that is encapsulated within the crystal matrix of iron sulfide minerals, such as pyrite, pyrrhotite and arsenopyrite. To recover the gold, the host mineral must generally be broken down chemically by oxidative processes, such as roasting, pressure oxidation or bacterial leaching, which expose the gold for subsequent recovery by leaching in cyanide solution. The focus of attention in these pretreatment processes is usually the oxidation of the sulfides to elemental sulfur, sulfur dioxide gas or sulfate ions. Less attention is paid to the deportment of iron and the changes in its oxidation state, although this can have a profound effect on gold and silver liberation, as well as downstream operating costs. Iron sulfide minerals break down completely during pressure oxidation, and dissolve in the sulfuric acid solution that is generated from oxidation of the sulfides. This dissolution liberates the tiny gold particles that were originally trapped in the sulfide crystals, and gold recovery during subsequent cyanidation is usually very high (>95%). Iron goes into solution in the oxidation process, initially as ferrous sulfate, but this compound is rapidly oxidized to ferric sulfate, which then hydrolyzes and reprecipitates. The form of the precipitate varies depending on the operating conditions in the autoclave and the presence of certain metal cations. When the acidity in the autoclave is quite low (<20 g/L H2S04) and the temperature is high (>200° C), the formation of hematite is favored. When the acidity is high (>20 g/L H2S04) and the temperature is relatively low (160 to 200° C), the form ation of basic iron sulfate is favored. If the ore or the leach solution contains significant levels of certain cations (such as Na+, K+, NH4+, Ag+ or Pb2+) and the acidity is high (>20 g/L H2S04), jarosite compounds are favored. Hematite is the desired iron product in the autoclave discharge, for both metallurgical and environmental reasons, but it is difficult to operate an autoclave under the conditions required for effective liberation of gold without converting some of the iron to basic iron sulfate and/or jarosite. These compounds fall into a category of iron compounds known generically as iron hydroxy sulfates, all of which can cause significant processing and environmental problenis in the downstream gold process. This paper deals specifically with basic iron sulfate: the conditions under which it is formed in an autoclave, the problems that are caused by its presence in the feed to a cyanidation plant and possible remedial strategies that can be adopted, both in the autoclave and downstream. Copyright 2010, Society for Mining, Metallurgy, and Exploration, Inc.

Fleming C.A.,SGS Minerals | Mezei A.,SGS Minerals | Bourricaudy E.,SGS Minerals | Canizares M.,SGS Minerals | Ashbury M.,SGS Minerals
Minerals Engineering | Year: 2011

The carbon in pulp (CIP) and carbon in leach (CIL) processes became firmly established in the gold mining industry in the 1980s, initially in South Africa and Australia, from where they spread rapidly to all the gold producing regions of the world. The percentage of annual global gold production by activated carbon-based processes grew from zero in the 1970s to almost 70% by the turn of the century, which represented a phenomenal rate of acceptance of a new technology by a traditionally conservative industry. The main reason for this rapid acceptance of the new technology was the fact that the first few large industrial plants in South Africa convincingly demonstrated better gold recoveries than the traditional filtration/Merrill Crowe process, with lower capital and operating costs. And as the plants developed an operating track record over their first few years of life, they proved to be remarkably robust mechanically, and highly tolerant of plant upsets, changes in feed composition and solution phase contaminants that had caused great problems in Merrill Crowe plants. These stellar attributes of the carbon-based gold plants have led to complacency and laziness in the industry, both at the new plant design stage, and with on-going optimization of existing plants. In many cases, basic "rules of thumb" that were developed as design criteria for the early CIP plants are still used today, with no appreciation of the factors that may cause one plant to perform quite differently from another. There seems to be little incentive to improve performance when it is well known that most CIP and CIL plants operate quite well with minimal optimization and, in many cases, minimal understanding of the factors that influence performance. Consequently, almost all CIP and CIL plants are overdesigned at the construction stage and are then operated sub-optimally. This can lead to higher gold losses and/or higher capital and operating costs than necessary. This paper examines the factors that influence CIP and CIL plant design and performance, and demonstrates a very simple methodology that can be used to arrive at something close to an optimum plant design. It can also be used as an on-going tool by plant metallurgists to transform a fairly well run plant into an exceptionally well run plant. © 2010 Elsevier Ltd. All rights reserved.

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