Modern Mining August 2018

CHROME

chromite ore, is mined by ACF in an opera- tion which is possibly unique on a large scale. What is unusual about the resource is that it is alluvial in nature – the result of weathering over long periods of time of the chrome seams of the Great Dyke, a process which has resulted in chrome fines being deposited in valleys on the flank of the dyke. The material of interest to ACF occurs at depths ranging from a few mil- limetres up to about 3 m from surface. Says Leon Richardson, ACF’s COO: “In essence, ACF runs a huge strip-mining opera- tion. The mining – which currently takes place at seven sites – is simplicity itself. There is no drilling or blasting required – we simply scoop up the material using excavators. Given the very shallow depth of our excavations, rehabili- tation – which is carried out concurrently with mining – is straightforward and easily accom- plished. The grade of the alluvial deposits is very low compared to most ‘in-situ’ chrome deposits but this is compensated for by the low-cost nature of the mining and processing operation.” Richardson, a metallurgist by training who was previously CEO of Tharisa Minerals in South Africa before joining ACF in May last year, adds that in certain areas where the washable component of the resource is very low, ACF carries out pre-screening using Pilot Crushtec mobile plants to avoid trucking exces- sive amounts of material for processing. The mining is carried out in-house by ACF which uses a mining fleet which totals over 50 machines (excluding LDVs). It includes 15 excavators, 38 rigid tipper trucks (of Chinese manufacture), six Bell B40 articulated dump trucks, as well as ancillary equipment such as dozers, water bowsers and graders. Because ACF’s tenements extend for a dis- tance of around 30 km along the Great Dyke,

a South African company specialising in pyrometallurgical engineering, providing some specialist assistance. It was brought on line just over 10 months from ACF receiv- ing approval for the project from Zimbabwe’s Environmental Agency. According to Drummond, the specific ‘rec- ipe’ and know-how of the aluminothermic plant is proprietary to ACF. In principle, at least, the process is simple and involves high-grade chro- mite, aluminium powder and accelerants being blended and then ignited in a crucible inside a reaction chamber. The ‘burn’ takes just two or three minutes – with the temperature reaching around 2 000 deg C – and results in two prod- ucts being produced – the Ultra Low Carbon High Grade Ferrochrome, which is the primary product, and a high-grade alumina slag which can be used as a refractory material. The dust from the process is collected in a baghouse and also has value as it can be used as a cement additive in refractory applications. The main feedstock for the process, the

Leon Richardson, COO of African Chrome Fields, explains the new plant to President Emmerson Mnangagwa. In the background (from left) are Moti Group executives Zunaid Moti (CEO), Nadia Mahne and Ashruf Kaka while Minister Winston Chitando (wearing a hard hat) can be seen on the left.

A night view of Plant 1. Each plant is made up of 45 t/h spiral plant modules (photo: ACF).

with the mining sites being scattered over a large area, the company has elected not to have a central plant for processing the ore. “A single processing facility would not be economically feasi- ble, given the distances involved,” explains Richardson. “So we’ve chosen to rather place wash plants at each mining site. We’ve gone for a modular, semi-mobile design as generally our sites only have a life of three to five years before we move on to fresh mining areas. “The plants – which typically treat ores mined within a 5,5 km radius – provide for scrubbing,

28  MODERN MINING  August 2018