Metallurgy (Refining)

Refining Of Metals

Refining is the process of purifying a crude metal obtained after the extraction process to obtain a metal of high purity. Different refining methods are used depending on the metal and the nature of impurities.

1. By Heating in Absence of Air (Liquation):

This method is used for metals that have a lower melting point than their impurities. The crude metal is heated on a sloping surface. The metal melts and flows down, leaving the impurities behind.

Example: Refining of Tin (Sn) and Lead (Pb).

2. By Displacement with Other Metals:

This method is used when the impure metal is less reactive than the metal used for displacement. For example, impure copper can be purified using a more reactive metal like Zinc, or more commonly, by electrolytic refining.

Example: Refining of Silver (Ag) from lead by Parkes process or electrolytic refining.

3. By Heating in Presence of Air (Roasting):

This method is used for metals whose oxides are more stable than their sulphides. The impure metal is heated in the presence of air. Impurities are oxidized and removed.

Example: Refining of Copper. Impurities like Zn, Fe, Ni, Ag, Au are present. The crude copper is melted and heated in the presence of air. The more reactive metals form oxides which float on the surface as scum and are removed. The copper oxide formed is then reduced by heating in the absence of air.

4. By Electrolysis:

This is a highly effective method for purifying metals, especially those that are difficult to refine by other means. The impure metal is made the anode, a thin strip of pure metal is made the cathode, and a solution of the metal's salt is used as the electrolyte.

Example: Electrolytic refining of Copper.
Anode: Impure Copper
Cathode: Pure Copper strip
Electrolyte: Acidified $CuSO_4$ solution.
At Cathode: $Cu^{2+} + 2e^- \rightarrow Cu$ (pure copper gets deposited)
At Anode: $Cu \rightarrow Cu^{2+} + 2e^-$ (impure copper dissolves)
Impurities like iron and zinc dissolve as ions in the electrolyte. More noble metals like gold, silver, platinum settle down as anode mud.

Refining

Refining is a critical step in metallurgy to obtain pure metals from the extracted crude metal. Several advanced methods are employed for this purpose:

Distillation:

Principle: This method is used for refining metals with low boiling points. It is based on the difference in the boiling points of the metal and the impurities.

Process: The impure metal is heated in a distillation flask. The metal vaporizes, and the vapours are condensed in a separate receiver, leaving behind the non-volatile impurities.

Used for: Metals like Zinc (bp 1180°C), Mercury (bp 357°C), and Cadmium (bp 767°C).

Example: Refining of Zinc. Crude zinc is heated; it vaporizes and is condensed to obtain pure zinc, while impurities like lead and iron, which have higher boiling points, remain behind.

Liquation:

Principle: This method is based on the difference in melting points between the metal and its impurities. It is applicable to metals that have low melting points and form alloys with impurities having higher melting points.

Process: The crude metal is melted on a sloping hearth. The molten metal flows down, leaving behind the solid, high-melting-point impurities.

Used for: Refining of Tin (Sn), Lead (Pb), Bismuth (Bi).

Example: Crude tin containing copper (higher melting point impurity) is heated on a sloping hearth. Tin melts and flows down, while copper remains solid.

Electrolytic Refining:

Principle: Based on electrolysis. Impure metal is used as the anode, a thin strip of pure metal as the cathode, and a solution containing ions of the metal as the electrolyte.

Process:

At the anode, the impure metal dissolves.
At the cathode, pure metal ions from the electrolyte deposit as pure metal.
Less reactive impurities settle down as anode mud. More reactive impurities remain in the electrolyte.

Used for: Copper, Zinc, Nickel, Gold, Silver.

Example: Refining of Copper. Impure copper anode dissolves, pure copper deposits on the cathode. Silver, Gold, Platinum settle as anode mud, while Zinc and Iron remain in the electrolyte.

Zone Refining:

Principle: This method is based on the principle that impurities are generally more soluble in the molten state of the metal than in the solid state.

Process: A molten zone is created by heating a rod of impure metal with a heating coil. The coil is slowly moved along the length of the rod. As the zone moves, impurities concentrate in the molten zone, and pure metal crystallizes out behind it. Repeating this process results in higher purity.

Used for: Obtaining ultra-pure metals required for semiconductors and electronic industries.

Used for: Germanium (Ge), Silicon (Si), Boron (B), Gallium (Ga).

Vapour Phase Refining:

Principle: This method involves converting the impure metal into a volatile compound (like oxide, carbonyl, or halide) which is then decomposed at a higher temperature to obtain the pure metal. The compound must be stable at low temperature and decompose at high temperature.

Conditions:

The metal should form a suitable volatile compound.
The volatile compound should decompose easily at a higher temperature or by some other method.

Examples:

Mond's Process for Nickel (Ni): Impure Nickel is heated with carbon monoxide (CO) at about 330-350 K to form volatile Nickel carbonyl ($Ni(CO)_4$). This is then decomposed at about 450-500 K to deposit pure Nickel.
- $Ni(s) + 4CO(g) \xrightarrow{330-350 K} Ni(CO)_4(g)$
- $Ni(CO)_4(g) \xrightarrow{450-500 K} Ni(s) + 4CO(g)$
Decomposition of Van Arkel Method for Zirconium (Zr) and Titanium (Ti): The impure metal is heated with iodine to form volatile metal iodide. The pure metal is then obtained by decomposing the iodide on a hot filament.
- $Zr(s) + 2I_2(s) \rightarrow ZrI_4(g)$
- $ZrI_4(g) \xrightarrow{1800 K} Zr(s) + 2I_2(g)$

Chromatographic Methods:

Principle: This method is based on the difference in adsorption of different components of a mixture onto an adsorbent. Various chromatographic techniques like column chromatography can be used for metal refining.

Process: A solution containing the metal ions is passed through a column packed with an adsorbent material. Different metal ions are adsorbed to different extents, allowing for their separation.

Used for: Separation and purification of metals that are difficult to separate by other methods, especially in the context of rare earth metals.

Example: Column chromatography is used for separating and purifying alkali metals and alkaline earth metals from their mixtures.