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   WB00941_3.GIF (1211 bytes)  TECHNICAL NOTES  WB00941_3.GIF (1211 bytes)


Pyrometallurgical processes
Electrolytic processes



Apart from gold and silver, lead bullion contains many other metallic impurities including antimony, arsenic, copper, tin and zinc.

Copper is the first of the impurities to be removed and a liquation process is used. The lead bullion is melted and held just above its melting point when solid copper rises to the surface and is skimmed off. Sulphur stirred into the melt facilitates the operation by producing a dry powdery dross which is more readily removed.

Once copper has been removed, there are a number of processes available for the extraction of the other impurities from the bullion. These include pyrometallurgical techniques where elements are removed one or more at a time in several stages and electrolytic processes that deal with most of the impurities in one operation. Although electrolytic methods are used in large scale production, they only account for the smaller part of the world refined lead production, the pyrometallurgical techniques being more widely employed.


Pyrometallurgical processes

Removal of antimony, arsenic and tin

After the removal of copper, as described above, the next step is to take out antimony, arsenic and tin. There are two methods available - the softening process (so called since these elements are standard hardeners for lead) or the Harris process.

The softening process

This is carried out in a reverberatory furnace in which the lead is melted and agitated with an air blast. This causes preferential oxidation of the impurities which are then skimmed off as a molten slag.

Harris process

This employs a molten flux of sodium hydroxide and sodium nitrate or some other suitable oxidising agent. Using a rotary stirrer the molten bullion and the flux are churned into a whirlpool which thoroughly mixes them at the vortex. Sometimes, instead of the stirring method, lead is pumped out from the bottom of the pot and sprayed on to the top of the flux cover through which it passes owing to its greater density. After some hours the impurities have left the lead and are suspended in the alkali flux in the form of sodium antimonate, arsenate and stannate and any zinc has been removed in the form of zinc oxide. This flux is then separated from the softened lead and the impurities are extracted in a separate process.

Removal of gold and silver

After the removal of antimony, arsenic and tin, the softened lead may still contain silver and gold and perhaps bismuth. The removal of the precious metals by the Parkes process depends on them both being more soluble in zinc than in lead.

Parkes process

The lead is melted and mixed with zinc at 480oC. At this temperature there is little interaction between lead and zinc but the precious metals are much more soluble in zinc than in lead. The temperature of the melt is gradually lowered to below 419.5oC at which point the zinc (now containing nearly all the silver and gold) begins to solidify as a crust on the surface of the lead and can be skimmed off.

Port Pirie process

An alternative process, used at the Port Pirie refinery in Australia, is based on similar metallurgical principles. The molten lead is introduced into the top of a vertical cylinder, flows down it and returns up a concentric inner pipe. The top of the outer cylinder is maintained at 650oC and zinc is introduced into the hot incoming stream of lead with which it quickly mixes. The bottom of the cylinder is kept at a temperature of 318.20C (the freezing point of the lead/zinc eutectic) and as the stream of metal flows down the outer tube it cools so that the zinc separates and floats upwards taking the silver and gold with it. As more lead and zinc are added, so the hot zone becomes saturated with zinc. As a result, molten zinc containing silver and gold floats on top of the liquid lead - because of the difference in their densities and can be removed. The lead flowing up the inner tube is thus free of the precious metals.

Removal of zinc

The removal of the precious metals results in the contamination of the lead with zinc, which must then be removed. This can be done either by oxidation with gaseous chlorine or by vacuum distillation. This latter process involves melting the lead in a large kettle covered with a water cooled lid under vacuum. The zinc distills from the lead under the combined influence of temperature and reduced pressure and condenses on the underside of the cold lid.

Removal of bismuth

The only likely remaining impurity is bismuth, though it is not always present in lead ore. It is easily removed by electrolysis and this accounts for the favouring of electrolytic methods in Canada, where it is a frequent impurity. Where pyrometallurgical methods of refining are used, bismuth is dealt with by adding a calcium-magnesium alloy to the molten lead, causing a quaternary alloy of lead calcium-magnesium-bismuth to rise to the top of the melt where it can be skimmed off.


Electrolytic processes

In the Betts process, massive cast anodes of bullion are used in a cell containing an electrolyte of acid lead fluosilicate and thin cathode "starter sheets" of high purity lead. The lead deposited on the cathodes will still contain tin and perhaps a small amount of antimony and these must be removed by melting and selective oxidation. For many years the Betts process was the only one that removed bismuth efficiently.

A more recently developed electrolytic process, first used in the 1950s in Italy, employs a sulphamate electrolyte. This is claimed to be an equally efficient refining method with the advantage that the electrolyte is easier to prepare.

By combining a number of the processes described above to build up a complete refining scheme, it is possible to produce lead of a very high degree of purity. Most major refiners will supply bulk quantities of lead of 99.99% purity and for very special purposes it is possible to reach 99.9999% purity by additional processing.

WB00860_.GIF (262 bytes) Technical notes


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