Factors influencing the rate of gold cyanide leaching and adsorption on activated carbon, and their impact on the design of CIL and CIP circuits

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.