The copper drying, converting and cleaning processAs a first step, dried copper concentrate is blended with fine quartz flux and process materials, which are recycled from upstream. It is then discharged into a continuous, coal-fired reverberatory furnace, operating at up to 1 400°C. The resulting melt separates into two layers in the hearth section of the furnace. The denser molten matte layer captures the copper-iron sulphides below the unwanted upper slag layer, which is skimmed off periodically via skim bays and launders into large pots destined for rail transportation to the slag stockpile.
Matte with 42% copper content is tapped off periodically via open launders into ladles, which are transferred by rail and overhead crane to one of three tilting converter furnaces. These are operated batch-wise, by blowing hot air through the matte to oxidise the sulphur and the iron, and then adding silica flux to form an iron-rich slag before returning it to the reverberatory furnace. Once all of the iron has been removed, further air blowing without flux oxidises the rest of the sulphur, and ultimately produces blister copper that is 98% pure. The converting process is exothermic and the excess heat is used to melt internal recycle materials and copper scrap from the refinery.
Blister copper is then transferred to one of three anode furnaces, where the last traces of sulphur are removed by blowing air through the molten metal. This is followed by an injection of hydrocarbon fuel to reduce oxygen to very low levels. 99.5% copper is cast into copper anodes by means of a single rotating anode casting wheel. Anodes are hoisted into water quench tanks and racks for sorting and cooling; and then transferred by rail to the copper refinery. A 'melting' or 'holding' furnace is used to supplement production by melting anode scrap that has returned from the refining process.
Off-gas from the reverberatory furnace passes through two waste heat boilers and a balloon flue before it reaches a final process of cleaning by electrostatic dust precipitation. The flue dust is returned to the furnace and the low concentration off-gas can be discharged directly into the atmosphere via a 152 meter high concrete stack. It can also be treated in the wet gas scrubbing plant, if the precipitator is off-line. In this instance, once scrubbed, the off-gas is released to the atmosphere via a 70 meter high clean gas stack. Steam generated by the waste heat boilers is used to pre-heat primary combustion air for the coal pulverisers and to direct secondary air to the furnace. Excess steam is used to drive a 9.6 MW turbo-generator. Higher strength off-gas from each of the converters is passed through separate electrostatic precipitators for gas cleaning. The gas is then treated in a single contact sulphuric acid plant to produce 98% sulphuric acid for sale to the domestic market.