The smelter retrofitting will result in the following main changes to the smelting process:
- Phasing out the old primary smelting technology namely, Reverberatory furnace and replacing it with a Double-Sided Blown (DSB) electric furnace in order to improve process efficiencies and gas capture to comply with the Environmental Emissions legislations.
- Converting the sulphuric Acid Plant from single contact and single absorption into double contact and double absorption in order to increase SO2 conversion.
- An oxygen plant with a capacity of 8066Nm3/hr oxygen to supply the DSB furnace with oxygen.
- Converter slag cooling plant for handling converter slag to prepare it for recycling.
Primary smelting furnace
The new primary smelting technology employs a double sided blown (DSB) electric furnace fitted with 3 electrodes. Refractory lined furnace walls are cooled by copper water jackets. The DSB feed material comprises of sulphide copper concentrate, coal and fine quartzite. The raw materials are blended according to the required proportions through Distributed Control System. The blend is continuously fed into the furnace. Oxygen enriched air is blown through an array of ten tuyères on either longitudinal sides of the furnace. The tuyères are completely submerged in the molten slag phase. Smelting energy is generated by the exothermic reaction, which occurs because of the interaction of sulphides and coal with oxygen. The resulting melt separates into two layers in the hearth section of the furnace. The denser molten matte layer contains copper-iron sulphides below the unwanted upper slag layer which contains waste oxides.
Matte with 55% copper content is periodically tapped through an 80mm taphole which is at an angle with the horizontal. The weight of the melt exerts pressure to the base of the furnace hearth and thus pushing matte upwards through the matte channel. Matte flows via a launder into a ladle and transferred by a matte winch system and overhead crane to one of three tilting Pierce-Smith converting furnaces. The slag is continuously skimmed through the skimbay and gravitates along an inclined launder where it is intersected by a jet stream of pressurised water thus granulating, cooling and transporting the slag into a pond. Cold granulated slag is loaded into locomotive wagons by a grab crane and transported to the slag dump where it is discarded.
Pierce-Smith Converters are operated batch-wise, by blowing natural air at a rate of 650Nm3/min through a set of 48 tuyères submerged into molten matte to oxidise the sulphur and the iron, and then adding silica flux to form an iron-rich fayalite slag. Once all of the iron has been removed in the form of slag, further air blowing without flux oxidises the rest of the sulphur, and ultimately produces blister copper that is 98% copper. The converting process is exothermic and generates energy which is used to melt internal recycle materials, spent anodes and other copper scrap from the refinery. The blister copper is transferred to the Anode making section for further processing. Converter slag is skimmed into big ladles which are carried by a slag haulers to transport from the converter aisle to the slag cooling plant. The slag is cooled down using water which is poured into the ladle full of slag. After 48 hours of cooling, the slag is dislodged from the ladle, crushed, screened and recycled through milling and flotation circuit at the Concentrator section to make concentrate.
Fire Refining and Anode Casting
Blister copper is transferred to one of two anode casting furnaces or holding furnace, where the last traces of sulphur are removed by blowing air through the molten blister copper. During blowing of air into the liquid blister copper, some of the oxygen dissolves in the copper. This is followed by an injection of hydrocarbon fuel to reduce oxygen concentration to very low levels. 99.5% copper is cast into copper anodes by means of a single rotating anode casting wheel. Water sprays on the casting wheel are used to cool the anodes. The anodes are further cooled in water quench tanks. They are loaded into racks for sorting. Final quality control on the anodes is conducted once the anodes have been loaded into the locomotive trucks. They are then transferred by rail to the refinery. The 'holding' anode furnace is also sometimes used to supplement production by melting anode scrap that has returned from the refining process.
Metallurgical Sulphuric Acid Plant
Off-gases from the DSB furnace passes through a waste heat boiler to extract the heat and a balloon flue before it reaches further cleaning in the electrostatic dust precipitation. The extracted heat is used for electricity generation at the Power Plant. Higher strength off-gas from each of the converters is passed through separate electrostatic precipitators for gas cleaning. The clean off-gases from the DSB furnace and Pierce-Smith converters are combined and treated in a double contact and double absorption Sulphuric acid plant to produce 98% sulphuric acid for sale to the domestic market and for internal use at the Refinery Tankhouse.
Tankhouse electrorefining process produce 99.99% copper cathode and byproduct anode slime from 99.5%copper anode. Smelter supplies the Tankhouse with two types of copper anodes that are stripper and commercial, with the stripper anodes used to make starter sheet and commercial anodes are used for cathode plating or deposition. The copper anodes and starter sheets are loaded into commercial sections, each consisting of 40 electrolytic cells. Voltage is applied across the anode-cathode pair to stimulate copper ions to get electrons and deposited on the cathode surface. Copper-loaded cathodes are harvested periodically with desired pull weight and current efficiency. The precious metal from copper anode reports to anode slime settling down to the cell bottom, which is processed through settling tanks, filter press and dried in the oven. As the electrorefining process going, electrolyte requires regular purification and control by adding reagents and bleeding off some of the electrolyte to remove accumulated impurities, which can negatively influence the cathode quality. The Tankhouse is equipped with the following sub-systems including anode preparing units, automatic crane, starter sheet stripping & assembling, anode scrap pulling and packing unit, short-circuit monitoring system.
The Liberators in the copper Tankhouse are used as impurity management system to provide a bleed stream supplying the Nickel Plant with its feed solution. Solution is pre-treated in the acid retardation unit and copper ion exchanger to remove excess acid and residual copper that may interfere with nickel recovery. The nickel recovery make use of a continuous carousel type ion exchange system, consisting of uniform bead size form of the Dow XUS resin to concentrate nickel. Iron guards are employed to purify nickel sulphate eluate by removing traces of iron, which is then crystallized and dried to produce dry powder of nickel sulphate hexahydrate crystals. The nickel sulphate hexahydrate final product is packaged in 1 ton lots consisting of forty 25kg bags packed on a pallet to various customers such as steel-making industry.
Rod Casting Plant
Rod casting plant receives refined copper cathode from the Tankhouse with 99.99% copper purity that is melted in the shaft furnace by combusting LPG under strict process monitoring. During melting, the oxygen content in molten copper is controlled by adjusting LPG and air ratio. The molten copper at 11000C is then transferred through launders, holding furnace and tundish to the casting system, where copper is cast into an endless bar with temperature range of 890-9100C. The still glowing bar enters the rolling mills, which is made up of 12 roll stands powered by electric motors via gearboxes. Through the mills, rod of 7.9mm diameter is attained by constantly reducing the cross-section of rod from 23.5mm while lubricating rolls surface with soluble oil. The 400-5000C rod is then surface-treated and cooled by isopropyl alcohol at constant speed. The temperature is subsequently reduced to 400C. After it is dried, it is treated with protective wax coating. The rod will go through a coiler then incline conveyor for stacking and packaging.