Smelting and refining

MINING EXPLAINED

— The following is an excerpt from Mining Explained, published by The Northern Miner.

Base metal concentrates from flotation or other physical beneficiation processes are shipped to a plant known as a smelter for the actual recovery of the contained metals. The term smelting refers to pyrometallurgical processes — those which use heat to achieve the desired separation.

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

Let’s follow a typical copper smelting and refining process, beginning with the arrival of the copper concentrates. We’ll assume the grade of the concentrate is about 26% copper, linked with iron and sulphur in the form of the mineral chalcopyrite. The first step in the smelting and refining process is to roast the concentrate.

Roasting involves heating the concentrate to a very high temperature while in contact with air, or oxygen-enriched air. 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. In roasting, some of the sulphur actually acts as a fuel and, in some roasters, little or no additional fuel need be added once the system has been brought to incandescence (when the heat causes it to glow). The product of the roaster is known as a calcine, and in our example its copper grade would be increased to about 31%.

The calcine is mixed with various reagents known as fluxes. Different fluxes are used for ores of different mineral composition. For instance, 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 to the other, where a flue system removes the hot gases. In the furnace, the fluxes react with the gangue minerals to form various low-melting silicate minerals known here as slags.

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

The molten matte, 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 fully 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 iron-rich slag floats on top of the molten mass and is periodically sent back to the reverberatory furnace until it no longer contains any copper.

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

Refining — After a further, slight fire-refining, the copper is cast into shapes known as anodes. These are shipped to an electrolytic refinery for the purification of the copper to commercial specifications, and for the recovery of the precious metals the copper has gathered to itself in the converter.

In the electrolytic plant, the copper anodes are placed in tanks containing copper sulphate. Thin sheets of pure copper are also placed in the tanks to act as cathodes. Electric current is passed through the system of anode-electrolyte-cathode, and the copper is carried from the anodes to build up on the cathodes in a highly purified form.

Eventually, the cathodes are removed and melted down for casting into various commercial shapes. Any precious metals in the copper anodes fall to the bottom of the tank with the last of the impurities and become contained in a muddy deposit, whence they are recovered and refined in a separate process.

The smelting of complex, nickel-copper sulphide ores and the refining of their contained metals involves a much more complicated suite of processes. Nevertheless, the same, general ideas outlined above for the smelting of copper are to be followed.

In the case of lead derived from galena (lead sulphide) ores, a lead concentrate is roasted in a sintering machine, then fed to a blast furnace along with coke, which acts as a fuel and as a reducing agent, and various fluxes. The actual reduction to metallic lead is carried out entirely in this furnace, which produces an impure lead bullion. The bullion is refined electrolytically and the precious metals contained are recovered in much the same way as in refining copper.

The treatment of zinc concentrates follows the pattern of roasting to a form in which all of the sulphur has been driven off, and the resulting calcine consists of zinc oxides and sulphates, iron oxides and sulphates and various gangue minerals.

The calcine is leached with sulphuric acid to dissolve the zinc as a sulphate. The leach solution is purified chemically so that a pure zinc sulphate solution may be fed to an electrolytic tank for its final reduction to the pure metal.