VAN ARKEL Method

Ultra pure metals are being prepared by the Van Arkel Method. Crude metal is heated with a suitable substance so that the pure metal present in it may be converted into stable volatile compound leaving behind impurities. The compound so formed is then decomposed by heating to get the pure metal. Van Arkels method is used to purify crude titanium metal. It is heated with iodine to about 500K to form volatile compound. Til₄ leaving behind the impurities .Til₄ is further heated to 1700K when it decomposes to give pure titanium.

Electro Refining

This method is based upon the phenomenon of electrolysis. The crude metal is made anode whereas the thin sheet of a pure metal is made cathode. The electrolyte is the solution of soluble salt of the same metal, preferably a double salt on passing electricity the metal from anode goes into solutions as ions due to oxidation while the pure metal gets deposited at the cathode due to reduction of metal ions the impurities settle down below the anode as Anode Mud.

Zone-Refining

This method is particularly used when metals are required in high degree of purity. It is based on the principle that when an impure metal in a molten state is allowed to cool, only the metal crystallizes while the impurities remain present in the molten state (mass) or melt.

                                    The impure metal converted into a rod which is heated at one end with a circular heater. A narrow zone of metal is melted. The heater is slowly moved along the rod. The pure metal recrystallizes out of melt while the impurities remain in the melt which moves along with the melted zone of the rod with the movement of heater. The process is repeated several times. The end of rod where impurities have collected is cut off. This method is employed for the purification of germanium, silicon, gallium etc, which are used in semi – conductors.

Oxidation

Oxidation method is applied for those metals in which the impurities can be easily oxidised metals like Cu, Ag etc. can be purified by oxidation method. Oxidation can be done in following way;

            (a).       Cupellation:  This method is used in the refining of a crude metal which contains the impurities of another metal capable of getting oxidised to a volatile oxide. This method is used in refining of silver containing lead as impurities. A cupel is boat – shaped dish made up of bone ash or cement. The crude metal is heated in a cupel placed in reverberate furnace and a blast of air is blown over the molten surface. The impurities are converted into volatile oxides which are swept away by the air current. In case of silver, lead is converted into litharge (PbO) which being volatile leaves behind pure silver.

            (b).      Bessemerisation:  In this method the impure or crude metal is taken in a specially designed furnace called Bessemer Converter into which hot blasts of air and sand is blown impurities are removed as volatile oxides which escapes leaves the metals in the pure state. Impurities of As, Mn, P etc present in pig iron can be removed by this method.

Poling

This method is applied to those metals which contain impurities as their own oxides. The crude metal is melted in a big container and is then stirred with green poles of wood. Gaseous hydrocarbons (CH₄, C₂H₆, C₂H₄, etc) are released from green poles which reduces the oxides of metals to the pure state by taking up oxygen. Copper is purified by Poling Process.

Distillation

This process is applied to the metals having low boiling points such as Bi, Hg and Cd. They can be recovered from crude metals by distillation. They readily change into vapours leaving behind the impurities. The vapours are collected and then cooled to get pure metals.

Liquation

This process is applied to the metals having low melting points such as Pb, Sn, Bi, Hg etc. The impure metal is placed on the sloping hearth maintained at the temperature above the melting point of the metals. The metal melts and flows down the slopping hearth into a receiver leaving behind the solid impurities.

The various methods for Purification or Refining used are as follows:

1.                      Liquation

2.                      Distillation

3.                      Poling

4.                      Oxidation

a.       Cupellation

b.      Bessemerisation

5.                      Zone – Refining

6.                      Electro Refining

7.                      Van – Arkel Method

Purification or Refining

The metal extracted by any of the above method is not completely pure and is known as Crude Metal. The metal extracted still contains some impurities are:–

·                     Other metals originally present in ore.

·                     Unreduced oxides or sulphides of metal.

·                     Residual slag or flux

·                     Non-metals like carbon, silicon, phosphorous etc.                 

                     The process of removal of impurities from the crude metal is known as Refining. The method used for refining of the metal depend upon the nature of impurities.

Hydrometallurgy

Some metals like gold and silver are extracted from their concentrated ores by leaching. They are dissolved in suitable reagents like acids or bases leaving behind insoluble impurities. The metal is recovered from the solution by crystallization. For Example, Silver ore is leached with dilute solution of sodium cyanide. Silver dissolves forming a complex, Sodium Argento Cyanide. The solution is further treated with scrap zinc to get the precipitate of silver.

Similarly, native gold is leached with potassium cyanide solution and it is recovered from the solution by addition of scrap zinc.

Leaching or Chemical Separation

It is a chemical method for the concentration of the ore. In this process the powdered ore is dissolved selectively in certain acids, bases or other suitable reagents. The impurities remain undissolved as Sludge. The solution of ore is filtered and ore is recovered by precipitation or crystallization.

Magnetic Separation

This method is used in those cases where either ore or the impurities are of magnetic in nature. In this method, the powdered impure ore in the form of thin layer is allowed to fall on a rubber belt which moves horizontally over two rollers, one of which has electromagnetic attached it. As the ore particles roll over the belt, the magnetic component in the ore gets attracted towards the magnet. It gets collected in a heap while the non – magnetic component forms a separate heap.

Chromite Fe(CrO₂)₂ an ore of chromium is magnetic in nature and contains non – magnetic impurities casseterite (SnO₂) is non – magnetic in nature while the impurities of iron and chromium are magnetic in nature. These can be separated by magnetic separation.

Froth Flotation Process

The method is extensively employed for the concentration of Sulphide ores. The process is based on the principle of preferential wetting of ore particles by the oil and gangue by water.

The impure crushed ore is taken in the big tank provided the resulting mixture is agitated vigorously by passing compressed air through a perforated pipe. The ore particles are preferentially wetted by oil which rises up to the surface with froth or foam formed by the oil and air. The gangue impurities being wetted by water settle to the bottom of the tank. The foam gets transferred to the other side of the tank where it slowly subsides. It is transferred to another tank and is repeatedly washed with water to recover are particles free from impurities. Sometimes other substance known as Collectors are also added. These substance increases the binning properly of ore particles and foam. The commonly used collectors are Ethyl Xanthate and Potassium Ethyl Xanthate.

Gravity Separation or Lavigation

This method of concentration of ore is based on the difference in densities of the metallic ore and gangue particles powdered ore is treated with a stream of running water when the lighter gangue particles are washed away and heavier ore particles are left behind.

Two common methods for Gravity separation are as follows:

(i).        Wilfley’s Table Method:  A Wilfley Table used for the purpose is a slopping table with top corrugated or riffled in the direction of the slope. The crushed ore is fed on the top of table, which is kept vibrating all the time. A stream of water is made to flow across the table at right angles to the slope. At each jerk the lighter gangue particles are thrown upward and are carried across the table by the stream of water. At the same time the heavier particles (ore) move towards the bottom of table. A good deal of separation takes place by the time the ore particles reach the bottom. This method is particularly suitable for dense eres like cassiterite (SnO₂) and hematite (Fe₂O₃).

(ii).       HydrulicClassifier Method:  Hydrulic Classifier is a conical reservoir fitted with a hopper at the top and a pipe at bottom, Powered ore is dropped through the hopper and the powerful stream of water is forced in through the pipe below. Lighter gangue particles are carried force in, through the pipe below. Lighter gangue particles are carried away by the current of water and the heavier ore particles collect in the apex of conc. the conical shape helps in reducing the velocity of water and thus ore particles are prevented from being carried away by water.

Hand Picking

This method can be employed where the ore particles are distinct from gangue and are visible to naked eyes. The ore particles are selectively picked up. This method is used in the concentrate of gold.

Various methods of concentration of ore

1. Hand Picking
2. Gravity Separation Or Lavigation
1. Wilfley’s Table Method
2. Hydrulic Classifier Method
3. Froth – Floatation
4. Magnetic Separation
5. Leaching Or Chemical Separation