A large artificial freshwater pond is created in the dunes, on which floats the dredger and concentrator plant. While the dredge removes the material from the front end of the pond, the tailings generated by the separation process is stacked at the back; as a result the pond continuously moves in a forward direction.
Burrowing into the mining face of the dune, the dredger advances at a rate of two to three metres per day, depending on the height of the dune. As the sand face is undermined it collapses into the pond forming slurry, which is sucked up and pumped to a floating concentrator. At this point, the heavy minerals are separated from the sand by exploiting differences in mineral density via a multi-stage circuit of sluices.
A portion of the magnetite and the chromium-containing minerals are removed magnetically, and the resulting heavy mineral concentrate (HMC) is stockpiled for transportation by road to the mineral separation plant.
Upon arrival at the mineral separation plant, located at the smelter site, the heavy mineral concentrate is re-slurried and pumped into the feed preparation circuit. Here the slurry is passed over successive stages of low- and high-intensity magnets to remove the ilmenite that is set aside as feedstock for the smelter.
The non-magnetic materials, including zircon and rutile, are concentrated for further processing in the dry mill. These two minerals are separated and upgraded in a series of circuits comprising a number of stages of high-tension electrostatic separation, magnetic separation, gravity separation, and screening.
Essentially, rutile and zircon are separated by their difference in conductivity while residual gangue is removed by magnetic and gravity separation circuits.
At this point, the zircon and rutile can be dispatched and sold in their raw form as mineral sands. Some zircon is upgraded to produce a higher-grade product by removing various impurities.
Ilmenite, as mined, has a high Cr2O3 content which makes is unsuitable for direct smelting to titania slag.
Some of this Cr2O3 is removed at the mine when the ilmenite is passed through a magnetic separation step in which the highly susceptible Cr2O3-rich fraction of the ilmenite is removed. The remaining minerals containing Cr2O3 are not readily separable from the ilmenite by magnetic means as their magnetic susceptibility is almost identical to that of ilmenite. The separation is therefore affected by subjecting the ilmenite to an oxidising roast that alters the magnetic susceptibility of the ilmenite while leaving the Cr2O3-containing minerals unchanged.
The roasting process is carried out in two three-stage fluidised bed roasters operated in the temperature range of 730°C to 800°C.
After being roasted and cooled to ambient temperature, the roaster product is passed over low-intensity drum magnets to separate out the now, more magnetic low-chromium fraction of ilmenite, yielding a feed material suitable for the smelter.
Anthracite is dried on two Peabody grate-type units to produce a reductant for the furnaces. A portion of the reductant is screened out for use as a re-carburising agent for the iron.
The smelting technology used at RBM was originally developed and proven at Quebec Iron and Titanium (QIT Fer et Titane) in Sorel, Canada, where coarse ilmenite is smelted to produce a high-TiO2 slag and pig iron in similar furnaces. This technology was adapted for RBM to process the fine ilmenite concentrate mined on the north coast of KwaZulu-Natal.
This is due to the grade of ilmenite produced at RBM being of too low a grade to be used directly in the production of pigment or synthetic rutile. The TiO2 content is increased by smelting the ilmenite with anthracite to produce a slag containing approximately 85 per cent titanium dioxide and a high-purity, low-manganese pig iron as a co-product.
The process generates very little in the way of waste products. The ilmenite (FeTiO3) is partially reduced with char to yield a low-manganese iron, a slag containing 85 per cent TiO2 (which is the primary product) and a gas containing roughly 85 per cent CO and 12 per cent H2 according to the reaction: FeTiO3 + C = TiO2 (l)+ Fe (l)+ CO (g).
The gas is cooled, scrubbed, pressurised, and used around the site as a fuel for heating and drying. Any excess smelter gas is burnt in a flare stack. The small amount of dust that is scrubbed from the furnace off-gas is the only discard produced.
No fluxes are added to modify the slag properties such as density, fluidity, melting point, or electrical conductivity, because this would dilute the titania in the slag and more reductant would be required to provide the required degree of reduction to yield the 85 per cent titania tapped slag.
The smelter consists of four of the world’s largest six-in-line electric arc furnaces. The furnaces are rectangular in shape, being 18 metres long and eight metres wide, and have six electrodes in line. The process is highly energy intensive, with each pair of electrodes being supplied by one 35 MVA transformer, giving a total of 105 MVA per furnace.
The furnace power is controlled by the electrode regulator, which moves the electrodes to achieve the target power level. This is affected by controlling the arc lengths under each electrode to maintain balanced electrode voltage and power.
The slag produced is highly aggressive towards the furnace refractories. For this reason, control of the thermal balance is essential, with the furnace being operated to form a protective frozen layer of material along the side and end walls of the furnace.
The furnaces are operated continuously with a relatively constant inventory of slag and iron being maintained, with tapping of the slag and iron being done intermittently.
The slag and iron tap holes are located along the same (tapping) side of the furnace with each furnace having two slag tap holes and four iron tap holes. Molten slag or iron is removed from only one of these holes at a time. The slag leaves the furnace at a temperature of approximately 1700°C and is tapped into four 20 ton moulds mounted on a specially designed mould car.
Shortly after the slag tap-hole has been plugged, the mould car is pulled by track mobile to a weighbridge and then to a holding area to allow it to cool further. The iron is tapped into 60-ton preheated refractory-lined ladles mounted on specifically designed ladle cars. As with the slag, the ladle car is pulled away shortly after the tap hole is plugged.
These furnace products are further upgraded in subsequent processes. The titanium dioxide slag is crushed and classified according to particle size and sold largely to pigment manufacturers.
Slag and iron processing
Upon receipt of the iron ladle at the iron processing plant, the ladle is weighed and the iron temperature taken with a dip thermocouple. The ladle is placed on a ladle tilter and an injection hose connected to an angled tuyére in the ladle hood.
Nitrogen is fed through the tuyère and the ladle tilted until the tuyère is suitably submerged. Injection reagents are then fed sequentially into the nitrogen stream until processing is complete. As a general rule, the more stringent quality iron grades are produced from the larger taps of hot iron.
Fine char is added to increase the carbon content, and calcium carbide is added to reduce sulphur. Small quantities of ferrosilicon are added for de-oxidation and improved physical quality, while larger quantities are added if high-silicon iron is required.
On completion of the injection process, the iron is cast into pigs on a twin strand pig-casting machine. Several grades of iron are produced, and individual heats are either stockpiled on site or loaded onto rail cars for transport to Richards Bay Harbour or to customers in South Africa.
The cooled slag is crushed and then ground and dried in an Aerofall mill. The mill product is classified to produce the size fractions required by the chloride and sulphate slag markets. The slag is then stored in silos ready for dispatch by rail to the harbour or to the local customer.