Refining of metals: poling, electro-refinement

Purification of the metal is the last step in metallurgy.

Refining is based on the difference between the properties of metals and their impurities.

Liquation

This method is used for refining metals having a low melting point.

e.g., tin, lead, bismuth

Liquation

Process

• The metal to be refined is placed over the sloping hearth of a furnace

• The temperature of the furnace is maintained slightly above the melting point of the metal

• Pure metal melt and flow down

• Impurities, having higher melting point, are left behind

Distillation

This method is used for volatile metals having boiling points lower than their impurities. e.g., zinc, mercury.

Process

• The metal to be refined is heated above its boiling point

• Impurities do not vapourise

• Pure metal vapourises and is condensed

• Impurities are left behind

Electrolysis

This method is used for refining copper, silver, tin, nickel.

Refining of the metal

Process

• A block of impure metal is made the anode of an electrolytic cell containing an aqueous solution of the metal salt

• A thin sheet of pure metal is made the cathode of the electrolytic cell

• When electric current of a suitable voltage is passed, metal ions from the electrolyte get deposited on the cathode as pure metal M⁺ⁿ + ne^- g M

• Metal ions from the anode enter the electrolyte M g M⁺ⁿ + ne^-

• Impurities present in the anode settle down as anode mud under the anode

• Anode finally disintegrates while the cathode gains in weight due to the collection of pure metal
• The metals so extracted are further refined by suitable methods. The electrolytic method is the most suitable method for refining the non-ferrous metals. The metals produced by any method given above are not very pure. These need further refining.

  This can be done by any of the following methods:

  Liquation method

  This method is used when the melting temperature of the metal is lower than that of the impurity and the impurities are not miscible with the metal. Metals like Bi, Sn, Pb, Hg etc., are purified by this method. The sloping floor of a reverberatory furnace is used to melt crude metal, when the pure metal flows down, the impurities are left behind.

  

  Fig: 10.7 - The Liquation method

  Distillation

  When metals have low boiling points e.g., mercury, zinc, cadmium etc., most of the impurities can be separated by distillation. Their vapours, on condensation in a suitable condenser give the pure metal.

  Poling

  Oxide impurity of impure metals can be purified by this method. Poles of green wood are used to stir the molten crude metal sample. The hydrocarbon organic matter in the poles acts as a reducing agent and reduces the oxide impurities. This method is used for refining copper.

  Oxidation or cupellation

  Impurities that can be oxidised to volatile products can be removed by oxidation. For example, the impurity of lead from silver is removed when lead is oxidized and led away by a current of air. Shining silver is left behind.

  Molten pig iron is also purified by this technique in a Bessemer converter. The impurities in the form of volatile oxides, escape out.

  Electrolytic method

  A large number of metals e.g., copper, silver, gold, nickel etc. are refined by this method. In an electrolytic cell the impure metal is made anode and a thin strip of pure metal is made cathode. A solution of a suitable salt of the concerned metal is used to fill the electrolytic cell. On passing electricity, the anode undergoes dissolution while the pure metal gets deposited at cathode. Insoluble impurities fall below the anode in the form of anode mud.

  

  Fig: 10.8 - Electrolytic purification of a metal

  Zone refining method

  When metals are required in a ultra pure state, the zone refining method is used. The principle-employed states that the impurities, which lower the melting point of a metal remain preferentially dissolved in the liquid phase and purer metal will emerge in the solid phase.

  In actual practice, a cylinder of impure metal is kept in a tubular furnace and the heater is made to move in one direction at a very slow speed. The solid material crystallizes as the heater moves along the tube, and the advancing zone contains liquid with higher impurity content. The process is repeated a number of times till the desired level of purity is obtained.

  

  Fig: 10.9 - The Zone refining method

  Van Arkel method

  In the Van Arkel method impure metal is first converted into one of its unstable compounds. Heating the unstable compound of the metal at higher temperatures, decomposes it to give pure metal. For example, pure titanium can be obtained from impure titanium by converting it into tetraiodide, and then decomposing this tetraiodide to pure Ti.

  

  8. How do you purify ores, which have easily oxidizable impurities?

  Solution

  The method used to purify impurities that are easily oxidizable is cupellation. Silver is refined by this method. The impure metal is fused in small boat shaped dishes made of bone ash called as cupels. The cupels are heated in a suitable furnace by a blast of air blown over them. The impurity like lead is easily oxidized to lead monoxide and is carried away by the blast, while pure silver is left behind.

  9. How is blistered copper purified?

  Solution

  Removing the reducible cuprous oxides of copper by the method of poling purifies blistered copper. The metal ore is initially melted in a shallow reverberatory furnace and the molten copper is stirred with logs of green wood. The blistered copper with cuprous oxide gets reduced to its metallic state by the gases given off by green wood; the impurities get removed either in the form of gases such as SO₂, As₂O₃ etc., or they form scum over the molten metal, which is then skimmed off, leaving behind the pure metal.