Smelting or converting operations produces metals with some degree of impurities. Broadly speaking, the term Refining means removals of impurity elements and undesirable compounds to the extent possible in commercial production. For specific applications, each metal or its alloy, after processing into the desired form, has to meet certain specified ranges of properties, apart from the composition.
These properties include mechanical and tensile properties, electrical conductivity, resistance against corrosion under load and at different temperatures etc. Such properties are functions of not only composition but also of the concentration of impurities and how they are distributed in the metal.
Since every individual metal in common use has its own typical extraction route, the concentration and the characteristics of impurities vary widely. In addition, the requirements of properties of engineering metals from the application point of view are getting more stringent. This, in turn, demands stricter control of the extent of refining that can be achieved on a commercial scale.
Objectives of Refining an Impure Metal
The purpose of refining is
- To minimize the concentration of non-metallic impurities
- To modify the shape and size, or ‘morphology’ of impurities in the refined metal to make them less harmful.
- To reduce the concentration of dissolved gases in the melt impairs mechanical properties of the metal or its alloy.
General Methods of Refining
Refining is the process of elimination of one or more solute elements from the principal metal by creating suitable thermodynamic and kinetic conditions. Such conditions of temperature, pressure and or addition of specific agents are created to ensure that the solutes form a more stable phase, with lowering of free energy, after removal from the parent solvent phase.
Refining, therefore, requires that suitable thermodynamic and kinetic conditions should be created for the transfer of specific unwanted solutes from the metal to another phase in which the solute is more stable. Often these solutes are removed from the metallic phase as stable compounds. At present, due to increasing demand for metals with higher purity and more strict specification of the upper limit of certain solutes, refining step in extractive metallurgy have acquired great significance.
Refining steps establish very clearly the importance of the theoretical aspects of thermodynamics and kinetics when applied on an industrial scale. Refining step can involve any one or more of the following methods:
(A) Physical Methods
- Selective distillation – depends on the difference in boiling point;
- Liquation – depends on the difference in melting point and density
- Removal of dissolved gases from a melt by reducing partial pressure – by inert gas
purging or by vacuum treatment.
(B) Chemical Methods
These methods, used more frequently, maybe grouped as follows:
- Selective oxidation or chlorination of impurities and removal of the reaction products in slag phase: Fire refining of copper, removal of zinc from lead by chlorination
- De-oxidation of metallic bath: Dissolved oxygen gives rise to insoluble oxides in the metal after solidification which is harmful with respect to mechanical properties. Suitable agents such as Ferro-silicon and Aluminium having a strong affinity to oxygen are added in the steel bath to tie up with oxygen to form oxides. These oxides can float to the top to be removed. Oxide removal is faster and better when an inert gas is blown (purged) through the melt from the bottom. In secondary steelmaking, argon purging has become a common step to clean up the melt, to remove suspended impurities and also they remove dissolved gas.
- Addition of suitable solid agents to combine with specific solutes: An affinity for the solute of the agent must have higher than the affinity for the solute of the solvent metal. In other words, the free energy of formation of the compound formed by the interaction of the solute and the added reagent should have a high negative value. Example – secondary refining of steel from primary steelmaking units such as oxygen converter or electric arc furnace, sulphur removal from steel, de-oxidation of steel, removal of gold and silver form impure lead (lead bullion) in Parke’s process.
- Selective volatilization as an intermediate chemical compound and their subsequent decomposition – such as forming nickel carbonyl vapour in Mond process and getting nickel by decomposition of the carbonyl vapour.
There are numerous examples of purification by chemical methods. In the following sections, applications of the above methods with selected examples shall be discussed.
(C) Electrolysis Method – ‘Electrolytic refining’ (Electro-refining)
The electrolytic method is based on the transfer of metal ions from an impure anode, through a suitable electrolyte so that these metal irons with positive charge migrates to the cathode where they get deposited. Faraday’s laws are applicable here, with certain additional factors affecting electrolysis. As a result, the amount of metal deposited does not exactly obey Faraday’s laws. This method produces extremely pure cathode deposits. For example, refining of impure copper, nickel, silver, tin are carried out by electro-refining. As the anode, made of impure metal, gradually dissolves, more active solutes tend to dissolve in the electrolyte and more noble constituents as residual ‘anode slime’ underneath the electrode.
D) Zone Refining
For a binary system of the solvent metal and the impurity as a solute, solidification always produces preferential segregation of the solute in the solid or the liquid phase, according to solidus and liquidus lines. Repeated melting and solidification thus would yield a purified solute because the solute would be transferred to the liquid phase.
The conditions for the removal of sulphur from steel: As observed from the above reaction equilibrium expressions, are as follows:
- The high basicity of the slag gives the high activity of CaO.
- The high temperature of the metal, since the process is endothermic.
- Reducing conditions, which means low FeO content in the slag
Apart from the above three thermodynamic factors, other factors such as good fluidity of the slag and good mixing of the reagent with steel also should be favourable. For industrial applications, fluorspar is added to improve fluidity and fine-sized solid desulphurises are injected by a carrier gas for thorough mixing.
Special Electric Remelting Techniques
Vacuum Arc Remelting(VAR): For refining, reactive metals and metals with high melting points, such as titanium, zirconium this method is applied. Like ESR, the impure metal as an electrode kept in a vacuum chamber. As the electrode melts, impurities with low boiling points are removed from the droplets. The refined metal solidifies as the purified electrode. Dissolved gases are also removed.
FAQs about Refining
Q.1 What are the functions of refining slag in ESR?
Answer: The functions of refining slag in ESR are- i) to remove sulphur by reaction with CaO, forming CaS. ii) Act to trap the inclusions in the molten droplets.
Q.2: Name three metals that can be purified by the liquation method?
Answer: By the liquation method lead, tin and bismuth can be purified.
Q.3: Why Fluorspar is added in case of the removal of sulphur from steel?
Answer: Fluorspar improves the fluidity of the molten metal.