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MEI Online: Plant Operation News: Africa: October 23rd 2002


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:: Ashanti Goldfields Benefits from Early Adoption of ILR Technology

Operations within the Ashanti Goldfields group are adopting the InLine Leach Reactor (ILR) technology of Australia ’s Gekko Systems and ILRs are already working successfully at Bibiani, Obuasi and Geita. Apart from improving gold recoveries, the technology lowers the security risks traditionally associated with gravity circuits.

George Potter, Ashanti ’s Managing Director - Metallurgy and Suppliers, was one of the first key individuals within the mining industry to recognise the cost and recovery advantages of installing intensive cyanidation to treat gravity gold concentrates.For a number of years industry operators, particularly in Africa, had identified three key problems with gravity circuits in gold operations.These were poor unit recovery, the volume of concentrate produced and the security issues associated with the handling of concentrates.

The early InLine Leach Reactor units for Ashanti were installed at Bibiani in Ghana, which pioneered the technology within the group, Obuasi, also in Ghana, and Geita, the Ashanti/AngloGold joint venture in Tanzania.Other early installations of this technology in Africa include Gold Fields ’Damang mine in Ghana and AngloGold ’s Morila operation in Mali, where the circuit was designed by South Africa ’s MDM.More recently, Ashanti has invested in two further units for Freda Rebecca (Zimbabwe)and Iduapriem (Ghana).

Across the board, the ILR provides benefits in lowering operating costs, increasing recovery in gravity circuits and improving security.These benefits are achieved by replacing the tabling upgrade step in treating gravity concentrates. The ILR is a simple rolling drum that provides highly efficient, oxygenated leach environment.

Utilising high cyanide and oxygen levels, extremely high recoveries on coarse mineral are achieved, says Gekko.The pregnant liquor is either pumped to electrowinning, carbon columns or resin.Spent eluate is recycled to the feed of the ILR thus providing an environmentally improved leaching process. Recoveries by the unit on free gold concentrates are traditionally of the order of 98 %plus.Excellent recoveries are also achieved on many “difficult to leach ”sulphide ores.

The ILR can treat concentrate volumes ranging from below 100 kg/h (typical of batch centrifugal concentrator installations)up to 20 t/h (utilising continuous gravity devices such as jigs and spirals)and concentrate grades from 50 g/t up to 30 000 ppm. Particle sizes of gold as large as a thumbnail are successfully leached using the technology. The ILR1000 Auto Continuous Unit was installed at the 240 000-ounce-a-year Bibiani mine in mid- 2000.The unit was justified for a number of reasons but the key factors included de-bottlenecking of the gravity circuit and eliminating recycling of gold losses to the mill circuit from the existing gravity upgrade circuit.As a result of the easier concentrate handling, excellent recoveries and higher through- put, the primary gravity device, batch centrifugal concentrator, can be more frequently discharged. Commenting on the performance of the ILR at Bibiani, the mine ’s Process Manager Quintin McLennan points out that the shaking tables used prior to retrofitting the ILR to the gravity circuit gave a very variable recovery ranging from 60 %to 90 %.”With the ILR, the recovery is about 98 %- we’re feeding about 10 000 g/t to the ILR, which is pushing out about 50 to 200 g/t, ”he says.“In our view, this is the first commercially viable gravity concentration system using cyanide."

McLennan is undertaking an on-site research programme to determine if the use of continuous gravity devices such as the InLine Pressure Jig in conjunction with the InLine Leach Reactor can provide incremental recoveries on “difficult to leach ”sulphide ore components.
Obuasi and Geita have installed n ILR2000 and ILR1000 respectively. Both units are continuous models each treating around 100 –200 kg of concentrates per hour.At Geita, Tanzania ’s biggest gold mine, the ILR was included in the circuit at design stage and achieves recoveries of the order of 96 –98 %.At Obuasi, which hosts the world ’s largest BIOX sulphide ore treatment plant, the ILR is incorporated in the Sulphide Treatment Plant (STP).The ILR at Obuasi achieves the highest recovery on site, says Gekko. Comments Obuasi ’s Sam Oti-Atakorah:“We’re very happy with the installation and we’re currently looking at an additional unit to be installed in the BIOX circuit, which will enable us to recover fine gold which is currently lost from the table operations and then to preg robbers.The installation of the ILR in this circuit should pay dividends."

At Freda Rebecca in Zimbabwe Ashanti is currently installing a batch model ILR100 to replace an alternative intensive leach technology that was used by the mine.The previous unit had struggled to provide recovery and reliability.

Advantages of the ILR system include low operating costs and the fact that it offers fully integrated electronic control of all critical leach parameters and is fully automated.The complete system, which is modular and can be located adjacent to the plant, can be retrofitted to any grinding circuit.There are no high speed/high wear components in the system and maintenance costs are therefore low.

Commenting on the success of Gekko ’s technology, the company ’s Managing Director, Elizabeth Lewis-Gray, says:“We are excited by the potential for continuous gravity devices such as the IPJ (InLine Pressure Jig)and intensive cyanidation to positively impact on the costs, recovery and financial returns of gold circuits.This particularly relates to complex ore bodies where recoveries can be significantly improved and in greenfield sites where there is potential to eliminate the CIP circuit in its entirety and dramatically minimise capital investment.".




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