Gold recovery from cyanide leach solutions using anion exchange resins has been commonly practiced in the former Soviet Union nations for 30 years1, and Western gold mining companies are now beginning to realize the economic benefits of ion exchange technology.

Resins are more versatile than activated carbon because they can be customized to improve features such as selectivity, loading capacity and resistance to poisoning by organics. They can also be eluted at much lower temperatures, and unlike activated carbon do not require thermal regeneration. The main disadvantage of gold-selective resins is that they are more expensive than activated carbon. However, despite their higher unit costs, the enhanced metallurgical performance can result in significant overall plant capital and operating cost savings.

MINIX ion-exchange resin was developed by Mintek as an alternative to granular active carbon for gold recovery. The resin, brand-named DOWEX XZ 91419, is manufactured and supplied under license by the Dow Chemical Co. It is a strong-base resin with excellent metallurgical and physical characteristics, and is suitable for both resin-in-pulp and resin-in-solution applications. MINIX resin found its first niche application in recovering gold from carbonaceous preg-robbing ores. It is used in this role at Avocet Mining’s Penjom mine in Malaysia, and formerly at the Barbrook gold plant near Barberton in South Africa (both resin-in-pulp operations).

More recently, Anglo Asian Mining plc selected MINIX ion-exchange resin for gold recovery at its Gedabek gold-copper project in Azerbaijan, which is situated approximately 300 km northwest of the country’s capital of Baku. Gedabek, which is the first operating gold mine in Azerbaijan, was officially opened and produced its first gold in May 2009. The operation consists of an open-pit mine and heap-leach facility, with gold recovery by resin-in-solution and electrowinning. The mine is expected to produce more than 300,000 oz of gold during its initial six-year mine life.

At Gedabek, ore is crushed, screened and agglomerated before stacking in conventional heaps. The pregnant leach solution (PLS) is fed to four adsorption columns in parallel containing 6.5 m3 per column of MINIX ion-exchange resin. Due to the fast kinetics of loading, the adsorption residence time is only 10 minutes. The gold content of the barren solution from each column is monitored to determine the onset of gold breakthrough, at which point the column is taken off line and the resin is transferred hydraulically to the elution circuit.

The elution circuit is as simple as practically possible. The eluant, consisting of 0.2M sulphuric acid and 1M thiourea at 50°C, is pumped down-flow through the column, exiting to the electrowinning cell and pumped back into the eluant tank. Gold is recovered by removing the cathodes from the electrowinning cell and washing with water hoses in a small tank.

The gold and copper content of the PLS is typically 1-1.5 g/t and 1,100 g/t respectively, giving a copper-to-gold ratio of approximately 1,000:1. Despite the very high copper content, the typical loadings of gold and copper on the MINIX ion-exchange resin are 2,000 g/m3 and 2,600 g/m3 of dry resin, respectively. These figures illustrate the high selectivity of the resin for gold over copper.

The Gedabek plant is the first application of MINIX resin in a leach solution containing such high copper concentrations. Several important classes of gold deposit contain appreciable quantities of cyanide-soluble copper minerals that can build up in the metallurgical process solutions, resulting in gold lock-up in the circuit and reduced recoveries. MINIX makes low-cost heap leaching viable where, due to technological or economic considerations, it is not practicable to remove the copper by flotation or acid leaching prior to cyanidation.

References
1 Kotze, M, B. Green, M. Mackenzie, and M. Virnig, “Resin-in-Pulp & Resin-in-Solution” in Advances In Gold Ore Processing 15, edited by M.D. Adams, Elsevier, ISBN-13: 978-0-444-51730-2.

This information was provided by Dr. Roger Paul ([email protected]), general manager–business development at Mintek, Randburg, South Africa. Mintek  acknowledges the support and cooperation of Farhang Hedjazi, vice president–technical, and Mehrdad Etemad, senior vice president, of Azerbaijan International Mining Co.

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