Smelting and Converting
Pyrometallurgical processing of the concentrate consists of smelting, converting, and fire refining. Smelting consists of melting the sulfide concentrate in an oxidizing atmosphere, which produces a copper rich (35-70% Cu) molten sulfide phase called matte, a low copper silicate slag, and offgas with SO2. The capture of SO2 is environmentally important and economically significant due to the production of H2SO4. Smelting is carried out either in a reverberatory or flash furnace. Flash furnaces are replacing older reverberatories and account for approximately 75% of the world's current smelting capacity .
Converting is a two step process in which matte is made into "blister" copper. The first stage of converting is the removal of iron in a slag and the generation of an SO2 bearing offgas. The second stage involves the further oxidation of the remaining copper sulfide to liquid or "blister" copper. Converting has traditionally been performed batch style in a Peirce-Smith converter. Recent developments have led to continuous converting, but these technologies are not widely used.
The final pyrometallurgical step is fire refining. Fire refining consists of an oxidation step followed by reduction. The "blister" copper is oxidized to lower the sulfur content of the copper to approximately 0.001%. Following oxidation, oxygen is removed by the introduction of a reducing agent such as natural gas or ammonia. A traditional reducing agent was a tree trunk or pine pole giving this stage the common name of "poling". The final oxygen content is typically between 1500 and 3500 ppm.
The removal of sulfur and oxygen is imperative to ensure a flat, thin casting needed for the last process in the production of pure copper, electrorefining. Most industrial casting involves the use of an anode casting wheel. The molten copper from fire refining is poured into a tiltable tundish where the amount of copper is weighed to ensure proper anode weights. Upon achieving the desired weight, the copper is poured into an anode shaped mold on the casting wheel. There are twenty to thirty such molds on the wheel. The wheel is then rotated and copper is poured into the next mold. As the process continues, the copper anode is cooled within the mold due to water cooling of the wheel and water spray on top. After about a one-half rotation, the anodes are removed from the mold. This is achieved by an automatic raising pin and a lifting machine. The lifting machine places the anodes in a quench tank which lowers their temperature so they can be transported to the electrorefinery. The empty molds are sprayed with a barite-water wash to prevent sticking of the next anode.
A few smelters (four of the fifty-three in Schleon and Davenport's survey) use a Hazelett continuous caster instead of a casting wheel. The continuous caster uses two water cooled steel belts (one on top, the other on the bottom) and stationary edge dams to contain the molten copper. As the belts rotate, the copper is moved through the caster and cooling occurs. When the copper leaves the caster, it is a solid continuous strip with the correct anode thickness. Anodes are made from the strip by shearing. If the reader desires more information on the pyrometallurgical processing of copper, Biswas and Davenport provide an excellent overview of the topic.
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