Copper Production Processes

Copper can be produced either pyrometallurgically or hydrometallurgically. The hydrometallurgical route is used only for a very limited amount of the world’s copper production and normally only considered in connection to in-situ leaching of copper ores; from an environmental point of view this is a questionable production route.

Several different processes can be used for copper production. The old traditional process is based on roasting, smelting in reverbatory furnaces or electric furnaces for more complex ores, producing matte and converting for production of blister copper which is further refined to cathode copper.

This route for production of cathode copper requires large amounts of energy per ton of copper (30-40 million Btu per ton cathode copper). It also produces furnace gases with low sulfur dioxide concentrations from which the production of sulfuric acid or other products is less efficient. The sulfur dioxide concentration in the exhaust gas from a reverbatory furnace is about 0.5-1.5% and from an electric furnace is about 2-4%.

So called flash smelting techniques have therefore been developed which utilize the energy released during oxidation of the sulfur in the ore. The flash techniques reduce the energy demand to about 20 million Btu/ton of produced cathode copper. The sulfur dioxide concentration in the off gases from flash furnaces is also higher, over 30%, and less expensive to convert to sulfuric acid (note: the INCO process results in 80% sulfur dioxide in the off gas). Flash processes have been in use since the 1950s.

In addition to the above processes there are a number of newer processes such as the Noranda, Mitsubishi, and Contop which replaces or was intended to replace roasting, smelting and converting, or processes such as ISA-SMELT and KIVCET which replaces roasting and smelting. For converting, the Pierce-Smith and Hoboken converters are the most common processes.

The matte (copper-iron sulfide) from the furnace is charged to converters where the molten material is oxidized in the presence of air to remove the iron and sulfur impurities (as converter slag) and to form blister copper.

Blister copper is further refined as either fire-refined copper or anode copper (99.5% pure copper), which is used in subsequent electrolytic refining. In fire-refining, molten blister copper is placed in a fire-refining furnace, a flux may be added and air is blown through the molten mixture to remove residual sulfur.

Air blowing results in residual oxygen which is removed by the addition of natural gas, propane, ammonia or wood. The fire-refined copper is cast into anodes for further refining by electrolytic processes or is cast into shapes for sale. [1]

In the most common hydrometallurgical process the ore is leached with ammonia or sulfuric acid to extract the copper. These processes can operate at atmospheric pressure or as pressure leach circuits. Copper is recovered from solution by electro winning – a process similar to electrolytic refining. The process is most commonly used for leaching low grade deposits in situ or as heaps.

The raw materials for copper winning contain, besides copper, numerous other elements like nickel, lead, tin, zinc and iron. During the refining procedure of the copper these elements are removed by using different techniques like selective vaporization and oxidation as well as refining electrolysis.

Nearly all copper which is ‘won’ using the pyrometallurgical method (about 85 %) passes through the copper refining electrolysis. In the electrolysis the impure copper is anodically dissolved and crystallized at the cathode without impurities. The space time yield (currently about 0.03 t/m³) and the specific energy consumption (about 0.4 kWh/kg Cu) represent the main key figures of the process.

In order to guarantee an economical process operation it is therefore necessary to optimize those two operating figures to the highest possible extent. By increasing the current density we face the problem of anode passivation, so that the electrochemical dissolution nearly stops.

The consequences are a lower electricity yield as well as higher potential drops which in turn result in an increased specific energy consumption. Due to the necessity of remelting the remaining anodes, a large amount of copper has to be fed again to the anode furnace. The passivation behavior of the anodes is strongly dependent on their chemical composition, and in this context the contents of accompanying elements like As, Bi, Sb, Pb, O and Ni are of great importance.

In many companies, but especially in recycling plants, the removal of those elements is very difficult since the raw material and accompanying elements are more or less given. To economically process scrap it is often even necessary to feed low grade material.

Considering these aspects, there is an absolute necessity to realize further optimizations in the field of pyrometallurgical refining in the anode furnace. In this context the behavior and reactions between metal and slag as well as the conditions for a volatilization are of great importance. This is because those conditions directly influence the composition of the refined copper and the anodes respectively and as a consequence also the composition of the anode slimes.

During the converting period of black copper and during the following pyrometallurgical refining of all base metals as well as a part of the copper are oxidized, so that slags with high contents in different metal oxides are generated.

The latter together with the oxygen potential strongly influence the liquidus area of the slag. At present these slags are recycled to the shaft furnace where the accompanying elements either accumulate in the flue dust or are transferred into the black copper. In order to break up this closed loop of several elements and to discharge them from the process it is necessary to try and reduce the metal oxides contained in the anode furnace- and converter slag.

This reduction step is becoming increasingly important since the quality of the scrap is continuously decreasing and therefore makes, with respect to the necessity of unloading the refining electrolysis, a further optimization of the pyrometallurgical refining process absolutely necessary. Additionally it is of special interest that the slags are very homogenous and have a low viscosity, so that a high mass transfer, ensuring high reaction rates, can be guaranteed. [2]

Figure 1: Furnace Copper tapping [3]