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 feeds material comprising of sulphide copper concentrate, coal and fine quartzite. The raw materials are blended according to the required proportions by Distributed Control System (DCS). 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. Slag is continuously skimmed through the skimbay and gravitates along an inclined launder where it is intersected by a stream of pressurised water granulating, cooling and transporting the slag into a pond. The granulated slag is further cooled in a pond full of water. Cool slag is loaded into a locomotive wagons by a grab crane and transported to the slag dump where it is discarded.


Matte Smelting

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. Converter slag is skimmed into big ladles which are carried by 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 in the concentrator to make concentrate.


Fire Refining and Anode Casting

Blister copper is then 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 cool 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 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 and a balloon flue before it reaches further cleaning in the electrostatic dust precipitation. The flue dust is packaged in one tonne bags. 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.

Power Plant

During primary smelting in the DSB furnace, hot flue gasses are produced at the temperature of about 12500C. The waste heat boiler is used for harnessing waste heat and converted into electrical energy. The DSB furnace flue gas enters waste heat boiler from the radiation zone to convection zone. The water inside boiler tubes gets heated up to produce steam. The steam is superheated and then used for driving a turbine to produce electricity of about 4 MVA and the cooled gases at about 3500C are channelled to the Acid plant for sulphuric acid production after dust has been removed.