The development of a process flowsheet for the new Anglo

129 Introduction The Platreef is a geological region of the Bushveld Complex—the largest source of primary platinum group metals, PGMs, in the world. PGM mineralization in the area was first reported by Dr Hans Merensky in 1924. The operation is part of the Anglo Platinum operations Potgietersrust Platinums Limited, ‘PPL’, being a wholly owned subsidiary. The operations are situated on the Northern Limb of the Bushveld complex some 270 kilometres north of Johannesburg. The expansion project will lead to a new mining operation and new concentrator plant. The Anglo Platinum mineral leases extend for a strike of over 15 kilometres on the Tweefontein and Tweefontein North properties—not currently mined; Sandsloot and Zwartfontein South properties—currently mined and the contiguous extension to the North—Zwartfontein North and Overysel properties, which will be mined in the new project. The Platreef is a thick package of partly ‘footwall’contaminated pyroxenite rocks, which are currently defined as a separate intrusion of the Bushveld Complex. The orebody is the only Bushveld ore deposit that is mined using large-scale open-pit mining techniques. Limited, open-pit mining of sub outcrops of the UG2 and Merensky reefs is practised in the industry. The Sandsloot operations commenced production in 1993 at a design throughput of 200 ktpm. This has subsequently been expanded to a current rated capacity of 385 ktpm. In the financial year, 2005, the PPL operation milled 4.5 million tons, producing 201 000 oz troy platinum and 214 000 oz troy palladium with byproduct base metals of 4 600 tons nickel and 2 700 tons copper. The PPRust North project will increase the annual milling rate to 11.8 million tons adding a further 230–240 000 oz troy of platinum and 260–270 000 oz troy palladium to current production levels. The 1993 Sandsloot flow sheet was defined after comprehensive laboratory, pilot plant testing and batch processing trials through a nearby plant. This included pilot scale milling on selected bulk samples through an AG Ball mill circuit, utilizing a 1.75 m inside liners AG mill. On start-up it was soon evident that the chosen AG ball milling circuit was not ideal to cater for the variable ore rock characteristics and mine fragmentation. Periods of low throughput and reduced metals recovery resulted (the primary mills achieving only 90 tph versus design 145 tph). Ore characteristics varied significantly in some areas of the orebody. A period of optimization and modification of the circuits began immediately. However, because the project was commercially very successful the opportunity arose to rectify the circuit deficiencies. The circuit upgrades and expansions were sequentially: