The economic benefits of minimized mill gold lock-up at AngloGold Ashanti Kopanang gold plant

• Published in: Journal of the South African Institute of Mining and Metallurgy 2005-05, Vol.105 (5), p.301-309
• Main Author: Van der Merwe, M.F., Viljoen, J. & Van Drunick, W.I.
• Format: Article
• Language: English
• Subjects: Anglogold Ashanti; gold lock-up; gold lock-up elimination; gold lock-up minimisation; grid liner system; Kopanang gold plant; mill gravity concentrates; ore milling; wave liners
• ISSN: 0038-223X

Anglogold Ashanti�s Kopanang Gold Plant (previously No. 9 Gold Plant) treats ore from Kopanang and Tau Lekoa shafts. The plant is twin streamed and has dedicated treatment circuits, from milling to final product, for each shaft. Waste rock material is used to compensate for the mill gap on both streams. Each stream consists of three 4.85 m x 9.15 m ROM mills in closed circuit with a single stage hydrocyclone, milling an average of 7 000 t per day. The liner configuration in each mill consists of six rows of 30 grid liners along with rectangular lifter bars. The topic of gold lock-up is not new and has been investigated and reported on many times, both within Anglogold Ashanti and the broader mining industry. Although the lock-up mechanism is understood, its measurement is neither practical nor precise. The initial lock-up in a new liner and the release as the liner wears down cannot be accurately quantified. Previous test work carried out on Kopanang Gold Plant (with a �gold scanner�) measured the natural gamma radiation emitted by uranium. This radiation is in proportion to the mass of concentrate locked up in the grid liner pockets, as well as between the cast-on backing plate and the shell. The ratio of concentrate lock-up was estimated as 35% behind the liners and 65% in the pockets. Mill gravity concentrates are recovered during routine relining operations from behind the grid liners at Kopanang Gold Plant. Prior to relining, the liners tend to release the concentrate from the pockets as the liners wear. Historical Kopanang data from actual �mill gold� smelted indicates that an average of 29 kg gold is recovered from behind each row. This indicates a total lock-up of 250 kg gold per mill. This locked-up gold should preferably be minimized or eliminated for the following reasons: * The cyclic build-up and release of gold in mills complicates accounting * Gold concentrate arising from liners is a security risk and creates a major mechanism for gold theft * Loss of interest on the value of 250 kg fine gold is significant and was a major driver for the project. These factors led to the need to develop techniques to eliminate or at least minimize the gold lock-up in the mills. Wave liners, with rubber inserts, have been successful in eliminating lock-up, and were tested at Kopanang Gold Plant in November 2000. However, due to increased load-slip and high wear rates associated with these liners, this option proved less economical in terms of liner