chrome
The chrome that ACF mines is amenable to strip mining using excavators

Privately-owned mining company African Chrome Fields is proving its commitment to Zimbabwe by expanding its already impressive alluvial chrome mining operations along the Great Dyke of the Southern African country with the aim of establishing a long and prosperous future.

Zimbabwe has garnered a lot of attention recently, this time for the right reasons, as former president and long-time ruler Robert Mugabe was ousted and replaced by his deputy, Emmerson Mnangagwa.

AUTHOR: Mining Review Africa's senior deputy editor, Chantelle Kotze.

As a result of decades of economic degradation Zimbabwe has been left with a skeleton of an economy and a cash crisis that impedes any major company from doing business in the country.

This article first appeared in Mining Review Africa Issue 6 2018

The new dispensation however, bodes well for investors and project developers looking to tap into the country’s vast mineral resources, as Mnangagwa has adopted an “open for business” approach to foreign companies as he tries to bring about an economic revival within the country.

Johannesburg-based Moti Group, who owns and is the holding company of ACF, has taken a decision to strike while the iron is hot and double its investment in the country before the next election, which may cause asset prices to rise, as the country stabilises.

“Moti Group plans to spend US$250 million over the next four years across several of the group’s different business ventures, including ACF’s chrome mining operation, having already invested about $250 million to date, mainly in mining,” says Moti Group CEO Ashruf Kaka.

The slow but sure move by Mnangagwa to open Zimbabwe up for business bodes well for ACF as it begins implementing its expansion plans at its chrome operation.

Tapping into Zimbabwe’s chrome potential

Zimbabwe, while not only home to a significant 19% of global chrome ore reserves, the country’s chrome ore resources are also higher grade than in other parts of the world, with an average chrome to iron ratio of 2:1, explains ACF COO and deputy CEO Leon Richardson.

ACF has a very unique resource, in that it is one of very few alluvial chromite deposits.

Formed as a result of erosion and weathering, the company’s chromite deposits are located in the valleys along the flanks of the centre part of the Great Dyke, at surface no more than 3 m deep and dispersed no more than 500 m from the foot of the Great Dyke formation.

Not only has African Chrome Fields already invested in building a presence in Zimbabwe, but is continuing to do so through an expansion of its current operations, which will see further investment flowing into the country.

Kaka notes that there are multiple seams within the Great Dyke, ranging in thickness from stringers to approximately 300 mm, occurring in the virgin unweathered rock formations.

The Great Dyke’s alluvial chrome deposits averages approximately 800 mm in thickness and is the result of weathering. This weathered product has been deposited in localised chrome enriched valleys.

The weathering has caused the chrome crystals to be liberated from the mafic host rock, resulting in high grade (+50%) chrome with low silica (<2%), the final product of which attracts a premium price in the market.

While the ACF deposits primarily host alluvial chrome, potential for lumpy chrome exists further below surface but which has not been explored by the company to date.

“The benefit of this type of chrome deposit is that the chrome is easily accessible without the need for drilling and blasting as it lies at surface within valleys.

This means that our operation has less of a negative impact on the environment enabling ACF to backfill and rehabilitate our mined out sites as we mine,” says Richardson.

The geology also lends itself to easy, low cost mining and processing methods. The chrome is amenable to strip mining using excavators.

Once pre-screened on site at the deposit to remove over-sized material, the chrome-bearing ore is hauled to the plant where it scrubbed and screened to remove the washable component.

The chromite is then extracted in a five-stage spiralling circuit, followed by magnetic separation, to produce a final product in the -1 mm size fraction.

ACF, which currently produces about 30 000 tpm of chromite ore from seven chrome-rich deposits across its 11 000 ha land holding, first began operations in the country in 2014 and was awarded National Project Status by the Zimbabwean Government in 2015 – often awarded to projects that have a large capital cost and significant development potential.

ACF has six chrome processing plants as well as one pilot plant.

Dubbed the ‘old lady’ or Plant 0, the pilot plant was the first plant on site and was subsequently taken over by ACF and refurbished.

This was followed by the construction of the remainder of the processing plants – Plant 1 through to Plant 6.

The plants are modular and designed to be semi-mobile because each of the six deposits currently being mined only has a life of mine of between three to five years based on each plant’s current design capacity.

Most recently, the company has established a new mining area for Plant 6 having already pre-mined 48 000 t prior to the commissioning of the plant.

Based on the deposits that ACF currently owns and is in the process of acquiring, the company has an estimated life of mine of 12 years.

The mining fleet consists of four dozers to assist in clearing and rehabilitation, 15 excavators and 38 rigid dump trucks, as well as six articulated dump trucks which are mainly used for dam cleaning operations and rehabilitations as it is more suited for the wet conditions.

The primary mining fleet is supplemented by several support vehicles, including fuel bowsers, water carts, graders and LDVs.

Expansion projects to bring long term benefit

ACF commissioned its newest wash plant in April 2018 and is in the process of a stage implementation of tailings thickeners at each of the processing plants.

Moreover, the company is also underway with construction of an exothermic ultra-low carbon ferrochrome operation which will become operational in mid-2018.

Latest wash plant makes its debut

Construction of Plant 6, the seventh wash plant to on site, began in August 2017 and was commissioned in April.

The four-module processing plant, which is double the size of AFC’s standard two-module plant, is the largest to be established to date and can process about 400 tpd.

“Not only will the new plant increase production to 40 000 tpm from the current 30 000 tpm, the plant features the best available technology and is a significant improvement on the existing plants on site, designed to be robust and improve efficiencies and production,” says Kaka.

Staged tailing thickener installations

All of the processing plants are being retrofitted in a staged manner with high rate tailings thickeners to improve the recovery of process water and accelerate the rehabilitation process by reducing the drying time of the discard washable portion significantly  through the significant reduction in moisture content of the tailings.

The first plant to be fitted with the tailings thickener is Plant 2.

Currently being commissioned, the tailings thickener recovers approximately 80% of water from the tailings stream for reuse in the processing plant.

The future goal for the recovered washable tailings will be to dewater it further using ultra-fine screening technology to produce an approximate 20% moisture tails, which will be suitable for direct transportation to the rehabilitation site.

The installation of the tailings thickeners help to minimise ACF’s dependence on borehole water thus preserving ground water from boreholes while also reducing the footprint of the tailings dams.

Once installed on all the plants, ACF expects the water recovery process to result in a significant saving across the board, which ultimately means that only a fraction of borehole make-up water will be required for the plants.

“All the high rate thickeners for the respective wash plants are already on site and civil work is well advanced, the thickener at Plant 6 is already installed and will be commissioned soon, following the installation of HDPE pipe work,” Richardson notes.

Aluminothermic beneficiation plant under development

ACF is currently under way with construction of a US$15 million proprietary aluminothermic processing plant to process chrome fines by mid-2018.

“The exothermic process uses high quality chromite blended with aluminium and an accelerator. Once ignited, the exothermic process results in the exothermic reduction of the chromite and as no traditional reductants or fluxes are being uses, very few trace elements such as phosphorus or sulphur reports to the metal, making this a niche product for specialised steels.

"Upon cooling, a perfect metal slag separation takes place, producing ultra-low carbon ferrochrome alloy necessary for the production of some high quality steel grades specially the corrosion-resistant and high-temperature oxidation-resistant steels,” explains Richardson.

The plant will have the capacity to process 1 344 tpm of chromite, producing 600 tpm of LCFeCr and will operate at 70% at inception and reach full capacity within three months.

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