Pyrometallurgical Copper Production

- Smelting of concentrate to matte

Matte smelting entails melting concentrate in a large furnace at about 1,200°C in an oxidising atmosphere. The objective of the smelting is to oxidise some of the sulphur and iron in the concentrate and to produce a molten sulphide phase (mat­te) rich in copper and to separate the rest as slag.

The products of the smelting are molten sulphide matte, 35 to 70 % Cu, con­taining most of the copper in the concentrate feed and a molten oxide phase (slag) as free of copper as possible. The molten matte is subsequently converted in a con­verting furnace to form impure copper metal. The slag is discarded, usually after a copper recovery step.

Sulphur dioxide bearing gas is also generated. SO₂ is harmful to the environ­ment and must be removed before the off gas is released. This is almost always done by recovering the SO₂ as sulphuric acid or liquid SO₂.

An important objective of matte smelting is to produce a slag which contains as little copper as possible. This is done by keeping the slag near silica saturation and by keeping the furnace sufficiently hot that the slag is molten and fluid.

- Flash furnaces

Smelting is most often performed in flash furnaces. It used to be carried out to a considerable extent in reverbatory furnaces, but this older technology is be­ing replaced.

The oxidation reactions provide much or all of the energy for heating and melt­ing of the products. Fossil fuel burners are often employed to control furnace tem tem­peratures, but mostly the process is autogeneous. The oxygen enrichment degree is used for the temperature control. The molten matte and slag are tapped sepa­rately from the furnace as they accumulate.

There are three types of industrial flash furnaces in use: the Outokumpu type, which uses oxygen enriched air as the oxidant, the Inco type, which uses technical oxygen as the oxidant; and the Contop application, which uses cyclone burners to introduce oxygen enriched air and concentrate into the smelting fur­nace. This application will be closed in Spring 1999.

Flash furnaces accounted for about 47 % of the world’s copper concentrate smelting in 1995.

- Reverbatory Furnace Smelting

Reverbatory furnace smelting accounted for most of the world’s copper smelting as late as 1975. However, the last reverbatory smelter was built in 1976 and the existing furnaces are gradually being replaced by flash and other smelting furnaces.

The reverbatory furnace is a hearth furnace in which a solid charge of con­centrates and flux is melted at about 1,200°C by hot combustion gases sweeping over the hearth. The combustion gases are provided by burning fossil fuel in end- wall or roof burners. The products of the process are molten matte and molten slag.

Reverbatory furnace smelting is primarily a melting process in contrast to flash smelting which is an oxidation/melting process. Reverbatory smelting makes lit­tle use of the energy from sulphur and iron oxidation for heating and melting. It also produces large quantities of fossil fuel combustion gas containing about 1 % SO₂. It is difficult to remove SO₂ at this low concentration which means that the off-gases are an air pollution problem.

- Other smelting processes

In recent decades many new processes have been developed. These processes, like flash smelting, use the oxidation of iron and sulphur in the concentrate feed to provide energy for heating and melting.

The Isasmelt, Noranda, Teniente, Mitsubishi, Vanyukov and Baiyin process­es use bath smelting. Oxygen enriched air is blown to the furnace with vertical lances or through submerged tuyeres. All these processes are discussed later in Chapter 4.

The Noranda process was commissioned at the company’s Horne smelter in 1997. The process can produce blister copper when using concentrates with low impurity levels. The normal operating practice is to produce copper-rich matte to undergo further conversion.

The Teniente process is similar to the Noranda process. The main differences are the addition of “seed” matte, which is periodically added to the furnace to maintain the heat balance; the injection of dry concentrate through some of the tuyeres and the charging of moist concentrate above the bath; and the fact that no coal is included in the furnace charge. Six smelters use Teniente furnaces in con­junction with reverbatory furnaces.

The Isasmelt process uses moist feed that is fed to the furnace continu­ously with fluxes. Fuel oil or coal and oxygen are injected into the furnace through a steel lance, which is submerged in the molten bath. The lance stirs the bath and produces a slag along with matte.

Converting

Copper converting consists of air oxidation of the molten matte from smelting. Converting removes the iron and sulphur from the matte to produce crude mol­ten metallic blister copper (99 % Cu) which is sent forward to pyro- and electrore­fining.

Liquid matte is transferred from the smelting furnace in large ladles and poured into the converter through a large central mouth. The oxidising blast is then turned on and the converter is rotated forcing the blast into the matte through a line of tuyeres along the length of the vessel. The heat generated in the converter by iron and sulphur oxidation is sufficient to make the process autogeneous.

Converting is done in two stages. The first is the slag forming stage, in which iron and sulphur are oxidised. The second stage is the copper-making stage, in which the sulphur in the CuS, is oxidised to SO₂.

Peirce-Smith converters account for 90 % of copper matte smelting. This is due to their simplicity and high chemical efficiency. There is, however, significant leak­age of SO₂ into the atmosphere during charging and pouring. Several alternatives have therefore been developed. The Hoboken converter has an improved gas col­lection system. In the flash converter solidified and ground matte is smelted with oxygen enriched air in a closed flash smelting furnace.

The copper smelting furnace slag can be cleaned either in an electric furnace or in a slag concentration plant.

Electric furnace

Copper smelting furnace slag is tapped through slag tap holes along launders into the electric furnace. Coke and reverts are added through the furnace roof. Slag from the electric furnace is tapped into granulation or cast in blocks and sold or trans­ported to the slag storage area. Matte from the electric furnace is periodically tapped through tapping holes and launders into ladles and transferred by cranes to converters for further treatment.

There are three or six electrodes in the electric furnace to supply energy into the furnace. Electric power is needed for the reduction reactions, heat losses and smelting of reverts. The reductive agent is coke. Electric furnace off-gases are first burned in an incinerator, then cooled and taken through a bag filter or a scrubber to the stack.

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