Electrolytic Refining of Minerals

Using the process of electrolysis, when a metal (mainly copper) is refined, the whole process is termed Electrolytic Refining. Talking about the mechanism of the process, during electrolysis, a huge slab of impure metal is used as the anode, and a thin strip of pure metal is used at the cathode. 

In metallurgy, the purifying of an impure metal constitutes the refining. We can distinguish it from other processes like smelting and calcining. In these two, there is an involvement of chemical change to the raw material, while in refining, the original and final material is chemically identical. Pyro-metallurgical and hydro-metallurgical techniques are used with many other types of processes. 

Electrorefining of Lead

Many processes are involved, such as Cupellation, Pattinson process and Parkes process. 

Cupellation 

Cupellation is one ancient process for extracting silver from lead. Air is blown across the surface of a bone ashed ‘test’ or ‘cupel’ where lead is melted. The silver and gold remain unoxidised, whereas the lead is oxidised to litharge and also the other base metals get oxidised. 

Using a reverberatory furnace, the process was carried on in the 18th century. Still, it differs from the usual type where the air was blown over the surface of the molten lead from bellows or blowing cylinders. 

Pattinson process

In 1833, Hugh Lee Pattinson invented this process and described it as “An improved method to separate silver from lead.” The first thing is to solidify out of the melt, which is lead, and in molten lead containing some traces of silver (Ag), the remaining liquid left is rich in silver contents. 

A row of up to 13 iron pots heated from below constitutes Pattinson’s equipment. Some lead containing a small amount of silver gets loaded into the central pot and then melted. Then this is allowed to cool. The lead is solidified, and large perforated iron ladles are used to remove it and move it to the next pot in another way. The remaining metal is now rich in silver, which is then transferred to the next pot in the opposite direction. This process is then repeated from one pot to the next. The metal-enriched in silver accumulates in one end of the pot, and the lead is collected at the other end of the pot. 

Typically, the process gets stopped around 600-700 ounces/ton, and the level of enrichment possible is limited by lead-silver eutectic, so cupellation is done for further separation. This process was economical for lead, containing at least 250 grams of silver per ton. 

Parkes process

In 1850, Parkes’ process was patented, and it uses molten zinc. The zinc is immiscible with lead when the two molten metals are mixed, the zinc gets separated, and it floats at the top carrying only 2% lead. Zinc floating to the top carrier a significant proportion of the silver as silver preferentially dissolves in zinc. The melt is coupled until the zinc solidifies and the zinc crust is skimmed off. By volatilising the zinc, the silver is then recovered. Pattinson process was replaced by Parkes process except where insufficient silver is present in the lead. In this case, Pattinson’s approach provides a method to enrich the silver to about 40-60 ounces per ton, and then at this concentration, it could be treated using the Parkes process. 

Electrorefining of Copper

Electrolytic refining is used to obtain the purest form of copper, which is done using the anode, a slab of impure copper (Cu), and the cathode, which is a thin sheet of pure copper (Cu). Acidified copper sulphate (CuSO4 + H2SO4) is used as an electrolyte. The copper(Cu) gets deposited on the cathode, and it gets dissolved from the anode by the passage of Electricity through the cell. Impurities remain in solution or collect as an insoluble sludge. When the dynamo was invented, this process became possible, and it was first used in South Wales in 1869. 

Electrorefining of Iron

Wrought Iron

Pig iron is the product of a blast furnace, and it contains 4-5% carbon and usually some silicon. A further process was needed to produce a forgeable product, and it is generally described as fining rather than refining. Puddling (in a puddling furnace) replaced this at the end of the 18th century. By the production of mild steel by the Bessemer process, puddling was superseded. 

Refined Iron

Using the term refining is a narrower context. Henry Cort’s original puddling process worked when the raw material was white cast iron. The general raw material for finery forges is grey pig iron. To remove silicon and use the grey pig iron, there was a requirement for a preliminary process for refining. A running out furnace is used to melt the pig iron, and then it runs out into a trough. Using this process, the silicon is oxidised to form a slag, which floats on the iron and then a dam at the end of the trough is lowered to remove the floating iron. The white metal is the product of this process, known as finer metal or refined iron.

Conclusion 

Minerals contain many impurities; therefore, to purify them, the preferred method used is Electrolytic Refining. In this method, the concept of electrolysis is involved. That is, there are two electrodes, and an electrolyte is present. An anode is a slab made up of impure metal, and a cathode is a thin sheet of pure metal. The electrolyte is the soluble salt solution of the metal. 

Many minerals are purified using this technique, such as Copper, Iron, Silver, Gold etc.