Smelting

    Base metal concentrates from the flotation or other physical beneficiation processes are shipped to a plant which is known as a smelter for the actual recovery of the contained metals. The term smelting is referred to pyrometallurgical processes – the ones which use heat to reach the desired separation.

    Maybe the least complicated non-ferrous smelter is one that treats copper sulphide ores for the production of blister copper, which will very probably be further refined electrolytically before entering manufacture.

    When following a typical copper refining and smelting process, commencing with the arrival of the copper concentrates. We will assume the grade of the concentrate is about 26% copper, which are linked with iron and sulphur in the form of the mineral chalcopyrite.

    The first step in the refining and smelting process is to roast the concentrate, which, in the following example, is 26% copper and still is linked with iron and sulphur in the form of the mineral chalcopyrite.

     Roasting involves heating the concentrate to a very high temperature while in contact with air or air that is enriched with oxygen. This burns off part of the sulphur; more importantly, it changes the copper-iron-sulphur complexes to chemical forms which are more amenable to the smelting or reduction process. Some of the sulphur actually acts as a fuel in roasting and, in some roasters, little or no additional fuel need be added once the system has been brought to incandescence (when the heat makes it glow). The product of the roaster is known as a calcine, and in our example its grade of copper would be increased to an amount of about 31%.

    The calcine is mixed with a variety of reagents known as fluxes. Different fluxes are used for ores which have different mineral compositions. For example, silica is used for an ore high in lime.

    The flux-ore mixture is fed into a reverberatory furnace — a long, flat chamber, in which a flame is shot from one end of the chamber to the other, where a flue system removes the hot gases. the fluxes react with the gangue minerals to form a variety of low-melting silicate minerals known here as slags in the furnace.

    The copper, and a lot of the iron and sulphur that remain, form a matte. The matte also picks up and dissolves any precious metals that may be found at the time. In the furnace, which operates at temperatures above 2,000°F (1,100°C), the slag floats on top of the heavier matte and is tapped off at times to be sent to the dump.

    The matte which is molten, which is now grading about 46% copper, is also tapped off and poured into a converter, together with more fluxes and reducing agents, where it is blown with air. In this final furnace treatment, the iron becomes completely oxidized and unites with the fluxes to form a slag. The copper becomes reduced to its elemental state, and the last of the sulphur is driven off in the form of sulphur dioxide. The slag which is rich in iron floats on top of the molten mass and is at times sent back to the reverberatory furnace until it no longer contains any copper.

    The converter produces blister copper, which is an average of about 99% pure.