ACF — the great fields of chrome

By | 2018-07-04T16:43:15+00:00 July 4th, 2018|Mine Excursion|

The Moti Group has established a unique chrome operation in the Zimbabwean Midlands, writes Leon Louw.

Four years ago, few international investors would have braved the politically loaded landscape of Zimbabwe. Back then, Robert Mugabe was still firmly entrenched, and the plot that would eventually lead to his demise was probably just an unspoken thought in the minds of one or two progressive politicians.

At the time, foreign investment had become an intermittent trickle, but despite the clear political and economic risks, the Johannesburg-based Moti Group navigated the political minefield and established a chrome mine in the Midlands Province of Zimbabwe in less than a year. The company spent more than USD250-million to develop a mine that would produce an expected 750 000 tonnes (t) of chrome ore per annum.

President Emmerson Mnangagwa, who hails from the Midlands, has been a frequent visitor to the alluvial deposit in the KweKwe region, and is clearly an ardent supporter of the operation.

“President Mnangagwa backed this project from the start. But remember, we entered Zimbabwe when Mugabe was still in full control,” says Ashruf Kaka, CEO of the Moti Group and national project liaison director of African Chrome Fields (ACF). “The positive political change, however, is clear to see just looking at our production charts, which have been moving north since Mnangagwa’s inauguration,” adds Kaka.

The rise of ACF

The Moti Group constructed subsidiary ACF’s first semi-mobile processing plant in 2014. The chrome deposit, located in the platinum-rich Great Dyke of Zimbabwe, occurs on surface and therefore, strip mining is the most appropriate method to use. The area has been subject to extensive exploration work in the past, and Anglo American developed a pilot plant in the 1990s, which is still in operation today. Since commissioning its first plant (Plant 1) in 2014, ACF has built five more, and Plant 7 is in the pipeline. Plant 6 was commissioned towards the end of April 2018. The chrome deposits are found in several valleys spread over a large area, which resulted in management’s decision not to use a central plant. Instead, each of the processing plants is the central unit around which mining takes place in a radius of three to five kilometres. This also allows for easier relocation to new mining centroids, as each plant is dismantled after three to five years and moved to another chromite-rich deposit identified during the exploration programme.

According to Leon Richardson, chief operation officer at ACF, some of the chromite will be processed into low and ultra-low carbon ferrochrome in the aluminothermic facility, currently under construction, to be used in the alloy and stainless-steel industries. The company’s main chrome processing facility is located at a site near Chinyika in the Lalapanzi Black Rhino Conservation area, located north of the Sebakwe Dam, south of the Munyati River.

In a country where the unemployment levels are estimated to be between 90% and 95% (depending on the source), ACF’s project has been a saviour for many families in the region. Kaka says the mine employs more than 1 100 local Zimbabweans and about 36 expatriates, and with skills transfer programmes in place, Zimbabweans are set to reap more benefits in the future. “Our recruitment programme specifically targets local employees from the Midlands area and only specialised skills are sought outside the Midlands Province,” says Kaka.

Stripping the chrome

The big advantage of strip mining is of course that there is no drilling and blasting involved, so the impact on the environment is minimal. Moreover, it reduces the operating cost significantly. The mine is really a large earthmoving project and as a result, it requires substantial infrastructure development and maintenance. ACF has constructed about 87km of gravel roads in the past four years to access the mining units. Negotiating harsh road conditions impacts heavily on equipment and haul trucks and requires a stringent maintenance regime, but that cost is offset by the straightforward mining and processing methods and the fact that the strip ratio is zero, although the grade is not the best in the world.

ACF’s chrome is so close to surface that it can be detected without digging. “It is an alluvial deposit, and we do strip mining in the enriched valleys based on extensive geological prospecting. We define the resource in terms of a split — that means in two components, namely a ‘washable’ portion and a ‘non-washable’ portion. The ‘non-washable’ portion does not have economic value,” says Richardson.

The vegetation in the area is normally a good indicator of the quality of the specific deposit. According to Barry Jones, group geologist, tall grasses are a sign of better splits: “Big trees indicate a very poor split; in other words, most of the material in that area is bigger than 1mm. In most cases though, the deposit is heterogenous, where the split can be anything from 20% ‘washable’ and 80% ‘non-washable’, to 85% ‘washable’ and 15% ‘non-washable’.” The chrome content is defined at a minimum of 7% chrome in some places, and more than 20% in others.

Basic mining methods

According to Richardson, the mining cycle is reasonably straightforward: “It starts with the definition of the resource, a description of the vegetation type, and then we do a complete evaluation. After mine planning, we start with bush clearing, primary extraction, pre-screening, transportation to the washing plant, return of the discard to the mining site, and then the rehabilitation,” says Richardson. Haul trucks load the stripped material, it is processed in the washing plants, and the slurry is then stored in a dam and left to dry, after which it is returned to the mining site, where it is used again to fill the mined-out area. This entire process takes about 18 months. “However,” says Richardson, “the long time-lag is one of the reasons why we introduced new thickeners. We aim to improve the process with each new plant we build, and through good engineering and innovation, Plant 6 is kitted out with the most advanced technology, including the Magra thickeners. The thickeners will accelerate the process tremendously,” says Richardson. (Ed’s note: Plant 6 was started-up when Mining Mirror visited the mine.)

At the moment, the tailings in ACF’s slurry dams contain about 17% solids, while the thickeners will increase that figure to approximately more than 65%. This will reduce the drying time of the slurry dramatically, and as a result the material will be ready for rehabilitation sooner.

All mining functions at ACF are performed in-house, though a number of suppliers and original equipment manufacturers (OEMs) are represented on site. These include Multotec, who supplies the spirals, trommels, and screens; Caterpillar and Bell trucks, dozers, and excavators, which have a small presence; while all the rigid dump trucks (RDTs), most excavators, and some other equipment are Chinese manufactured. According to Schalk Engelbrecht, general project manager at ACF, the Chinese RDTs have really impressed with their reliability, robustness, and performance. PLC refurbishes old equipment while the mine has five Pilot Crushtec mobile units with which pre-screening is done on site where the splits are poor. Battlemax and Multotec provide the majority of the slurry pumps.

Mining in the valleys

At each mining site, exploration is done in grid spacings of either 300m × 300m, 100m × 100m, or 50m × 50m, with infill pitting of =25m × 25m. This data indicates the grade, split, and depth, which determines whether it will be profitable to mine that specific area. “It has to be noted that different areas have different grades, which means we often have to blend material from different plants,” continues Richardson. “Therefore, the information needs to be accurate. After identifying an area and agreeing that this is where we are going to mine, we clear the site for the surveyor to determine the topography and demarcate the mining area,” he adds.

Each site has a ‘checker’ who records the cycle time of the equipment, which includes the time that trucks wait before loading at the mining site; how long it takes to load the truck, dump it on the stockpiles at the washing plant, and then return the slurry material back to site for rehabilitation, and the payload on each truck. The average distance from the mining site to the plant is normally between two and three kilometres. If the distance is 2 600m to the plant, for example, the trucks will take about 26 minutes from loading to dumping the rehabilitation material. “On average, we take nine to 10 scoops and four minutes with the excavator to load the dump truck, and that gives us 19–21 tonnes in the bin,” explains Richardson.

The same cycle takes place at all seven sites and the team allocates four trucks for one excavator. However, this ratio can change, depending on the hauling distance. If the hauling distance is less than one kilometre, only two trucks will be sufficient. On the other hand, if the distance increases to more than three kilometres, five or six trucks are deployed. The trucks consume between seven and 10 litres of fuel per hour, which is a lot more economical than the OEM recommendation of 10–12 litres per hour.

Two different sizes of Chinese-manufactured tipper trucks are used. The smaller MT50 has a payload of about 21 tonnes, while the bigger MT86 loads, on average, have close to 35 tonnes into the bin. According to Engelbrecht, the bigger trucks have performed exceptionally well. Equipment used on the mine include four dozers to assist in clearing and rehabilitation; 15 prime-movers (excavators); 38 RDTs, all of them Chinese manufactured (RTT); and six ADTs (Bell). In addition, there are numerous support vehicles, such as fuel bowsers, water carts, graders, and LDVs.

Screening out the splits

Where the split is poor, the mine uses scalping screens. However, where a large percentage of the material is ‘washable’, the mined material is sent to the plant directly. The processing plants consist of a configuration of scrubbers, screens, spirals, and magnetic separators to liberate and extract the chromite from the -1mm fraction.

According to Marike Barnard, process manager at ACF, the new thickeners are being installed to improve water recovery and efficiency. “With the new thickeners, we are recovering about 80% of our water, which reduces the environmental impact, and we don’t source as much water from the boreholes as we used to. The slurry that is pumped out now dries out faster and we are able to return it to the rehabilitation areas a lot quicker. Not only does it reduce our environmental footprint, but it also ensures better water quality and better production and efficiencies in the plant, and eventually, a better-quality product,” says Barnard. ACF has purchased thickeners for all the plants, but they are still to be installed at all the plants. A second thickener will be commissioned at Plant 6 soon, and after that Plant 5, Plant 1, and Plant 4 will be boosted by new thickeners. All of these thickeners are manufactured and installed by South Africa-based Magra.

According to Kaka, the clarified process water recovery from the six plants amounts to 1 774m³/h, resulting in massive savings and a recovery of 82% of the process water directly from the thickener. “Processing of thickened tailings will amount to an additional 112m³/h of process water recovered from the dewatering screens, with the benefit of having a conveyable virtual dry tailings product at 20% moisture content. This product can be loaded and transported together with the discard rock to the mining area for rehabilitation and full utilisation of what was practically an empty truck,” says Kaka. The total estimated water recovery is about 87%, including water recovery from the screens. Only a fractional 13%, amounting to 284m³/h of borehole make-up water, is required for all six plants. Module water specifics vary from plant to plant and depends significantly on the split.

ACF operates a unique project in a unique country. Zimbabwe is on the recovery path after Mugabe’s devastating rule. Foreign investment is on the rise, and many new projects are in the pipeline. However, the reward for companies that entered Zimbabwe when others fled will be so much better. ACF is now established in the country and according to Kaka, the mine has acquired enough claims to keep it going for at least another 12 years. They rode out the tough times and is now on a growth path next to none. They have a good relationship with the new government and plans are afoot for several new projects in Zimbabwe, including in the gold mining as well as platinum and lithium space.