A large percentage of the world’s economically mineable chrome ore reserves are located in the Bushveld Igneous Complex (BIC), a saucer-shaped deposit in the northern and north-eastern provinces of South Africa
. Chrome ore is mined along the eastern and western rims of the complex.
Eastern Chrome Mines
The Samancor Chrome mines in the eastern belt include, but are not restricted to, Doornbosch, Lannex, Tweefontein and Steelpoort (Winterveld) and stretch over a distance of 100 kilometres in the Steelpoort region (some 200 kilometers from Lydenburg in Mpumalanga). Total capacity at Eastern Chrome Mines is over 2.0, million tons of run of mine ore per annum. Actual production varies according to market demands.
Western Chrome Mines
In the Western belt, Samancor Chrome’s mines include Millsell and Mooinooi (and we are in the process of establishing a third underground mine), stretching over a distance of 50 kilometers between Rustenburg and Brits in the North West Province
Western Chrome Mines has a capacity of 2.0 million tons of run of mine ore. The main products of the mines are metallurgical and chemical concentrates, with some foundry grades.
Two underground mining methods are employed at the Samancor Chrome mines, namely conventional scraper mining and trackless mechanised mining.
The former method involves the ore being drilled and blasted, then scraped mechanically to loading points from where it is transported to shaft tips by locomotives and hoppers. From here it is hoisted to surface by conveyor belts installed in the inclined shafts.
In trackless mechanised mining the seam, which is contaminated by an internal waste parting, is drilled and blasted by conventional means. The blasted rock consisting of some 70 percent chrome ore and 30 percent waste, is then loaded by load-haul-dump units and transported to conveyor loading points from where it is then hoisted from the mine workings. At these sections, about 19 percent of the chrome ore is left intact as pillars to stabilize the excavation and act as regional support.
Samancor Chrome has three fully-integrated and self-reliant alloy plants. The alloys produced at these plants are transported by rail to the ports of Durban
and Richards Bay
for export to producers of stainless steel and speciality steels worldwide.
Ferrometals, the largest single ferrochrome producer in the western world, is situated just outside of eMalahleni (formerly known as Witbank) in Mpumalanga
. The intermediate carbon ferrochrome (IC3) plant was constructed in 1986 and relies on the furnaces for a supply of charge chrome. The IC3 plant produces intermediate carbon ferrochrome for the foundary and special steel-producing markets. FMT also includes Ferroveld, a joint venture with Elkem.
Tubatse is located about 300 kilometres north-east of Johannesburg
and has the economic advantage of being close to extensive chrome ore deposits.
Middelburg Ferrochrome comprises a chrome direct reduction plant that ensures that Samancor Chrome remains at the technological forefront in the chrome alloy industry and continues to be a cost competitive alloy producer.
Run of mine ore is crushed to size when it reaches the surface. A certain percentage may be milled further. The ore is then sorted to different size fractions by means of a dense media separation process. Silica is removed from the finer fractions by means of gravity and elutriation separation techniques. Further screening may be needed for specialised grades. Final products are stored on concrete floors prior to shipment. The majority of the final product is in the form of lump and metallurgical concentrate used in the metallurgical industry for the production of ferrochrome. This is largely used by the Samancor Chrome alloy works, and is ideal for the production of charge chrome.
Samancor Chrome produces a significant tonnage of chemical grade product for use by the chrome chemical industry, where it is the feedstock for the production of sodium dichromate. This grade has a low silica content and has been well-accepted by the industry.
Foundary Grade products are used as moulding sand in the casting industry. Chrome is useful here as it has a low coefficient of thermal expansion, and has good heat transfer properties. These grades have a low silica content and are screened to give a specific size distribution. Acid demand values as well as pH (potential of hydrogen value indicating acidity or alkalinity) of these products are also important to the user.
A range of refractory grades are produced, for use in the production of magnesia-chrome bricks and other refractory products. Silica content is low, and a variety of specially-sized products are available.
Ferrochrome is an essential ingredient for the production of stainless steel, and is an alloy of iron and chromium with a chromium content of between 50 and 55 percent. Stainless steel depends on chrome for its appearance and its resistance to corrosion.
Samancor Chrome produces three grades of ferrochrome: charge chrome, intermediate carbon ferrochrome and low carbon ferrochrome. Ferrochrome production is essentially a carbothermic reduction operation taking place at high temperatures. The ore – an oxide of chromium and iron – is reduced by coal and coke to form an iron-chromium alloy called ferrochrome. The heat for this reaction comes from the electric arc formed between the tips of the electrodes in the bottom of the furnace and the furnace hearth. This arc creates temperatures of about
2 800°C. In the process of smelting, over 3 500 kilowatt-hours of electricity are consumed for each ton of ferrochrome produced.
Tapping takes place intermittently. When enough smelted ferrochrome has accumulated in the hearth of the furnace, the tap hole is drilled open and a stream of molten metal and slag rushes down a trough into a chill or ladle. The ferrochrome solidifies in large castings, while the slag is separated and stockpiled for further processing.
The product is crushed and screened to exact customer specifications. Alternatively, the ferrochrome is granulated into a flowing stream of water.
With a total capacity of approximately one million tons per year, Samancor Chrome ranks as one of the largest producers of ferrochrome in the world. In addition, it is also a substantial producer of metallurgical ferrosilicon, a silicon-rich alloy used in steel production.
Two decades ago South African chromite was not regarded as suitable for charge chrome production. Since then pioneering achievements have been made in the areas of raw material preparation, furnace design and operation, downstream beneficiation and optimisation. By means of upgrading and redesign, Samancor has grown to be one of the world’s largest producers of charge chrome.
Charge chrome is used in the stainless steel industry and as an additive in the steel industry. Charge chrome is used for the manufacture of more than 170 different types of stainless steels whose main properties include resistance to most types of wet and dry corrosion. For the manufacture of special steels, chromium imparts special properties to the steel, including an improvement in the tempering quality of steel, particular hardness and resistance to wear, and heat resistance.
To improve total ferrochrome recovery while maintaining a high-quality product, alloy recovery plants are utilised to recover the ferrochrome from the slag produced during the charge chrome process. The slag, which has a metallic content of approximately 4 percent, is processed through a series of crushers and broken down to minus 15mm material. It then moves through a jigging plant where the chrome and slag are separated by means of gravity. The slag chips are sold for road building and concrete work.
Intermediate Carbon Ferrochrome
The intermediate carbon ferrochrome plant (IC3) erected at Ferrometals was the first of its kind in the world. With a capacity of 70 000 tons a year, IC3 produces a product range of ferrochrome with a 1,5 percent to 6 percent carbon content, supplementing the 6 percent to 8 percent carbon ferrochrome produced. The IC3 plant uses the liquid charge chrome from the East Plant at Ferrometals as its basic raw material. The process takes place in a pear-shaped convertor where the liquid charge chrome is bottom blown by oxygen and steam through tuyeres to a specific carbon content. The final product, an intermediate carbon ferrochrome, is produced in granulated form.
The tailor-made alloy is used specifically in the foundary industry; as a trimming addition to certain stainless steels – specifically some ferritic grades; in tool steels; and in alloy steels such as beating, spring, high speed and valve steels.
Low Carbon Ferrochrome
Low carbon ferrochromium is produced at Middelburg Ferrochrome by means of a three-stage “dry” process whereby molten and solid ferrosilicon-chromium are added to the lime / chrome ore melt in a two-phase reaction.
The typical products produced have chrome contents of between 59 percent and 61 percent respectively, with carbon content ranging from 0,02 percent to 0,1 percent. The alloy is used for trimming additions in stainless steel production as well as in other specialised applications.