How to extract sodium from its ore?
Extraction of Sodium
The two methods used for extracting sodium metals are,
* Castner's process: By the electrolysis of fused caustic soda.
* Down's process: By the electrolysis of fused sodium chloride.
Castner's process
Molten sodium hydroxide (caustic soda, NaOH) is electrolyzed using an iron cathode and nickel anode. A nickel wire gauze cylinder prevents the electrodes from touching each other. On passing an electric current through the melt, sodium is liberated at the cathode and oxygen at the anode. The liberated sodium metal floats over the surface (lighter than fused caustic soda) and collects inside the receiving vessel where hydrogen prevents sodium from oxidation. Excess of the gas escapes from the outlet. Sodium is removed from time to time with the help of perforated spoons and kept under kerosene oil. The various reactions taking place during electrolysis are:
NaOHmelting−−−−→Na++OH− (dissociation)
At cathode:
Na++e−→Na
At anode:
OH−→12H2O+14O2+2e−
Most of the water formed in the reaction gets evaporated, and the rest is electrolyzed into hydrogen and oxygen. Therefore, metallic sodium and hydrogen are liberated at the cathode and oxygen at the anode.
Down's process
In this process, molten sodium chloride (common salt) is electrolyzed using a graphite anode and a ring-shaped iron cathode. The two electrodes are separated by a wire gauge partition to avoid the mixing of sodium and chlorine so formed. As Sodium chloride melts at a very high temperature of 1085 K, a mixture containing sodium chloride, potassium chloride and potassium fluoride (NaCl + KCl + KF) is employed. This mixture melts at about 850-875 K.
The reasons for lowering the temperature are:
* It is difficult to maintain a high temperature of 1085K.
* Sodium is volatile at this temperature and so a part of the metal formed may vaporize.
* At this high-temperature chlorine produced as a by-product corrodes the vessel.
* Metal at this temperature will be in a colloidal state and its separation will be difficult.
On passing electric current, chlorine is liberated at the carbon anode and escapes through the dome-shaped steel hood outlet. Sodium rises from the cathode and remains in the wire gauze shell. The sodium produced is in a molten state. Being lighter than the electrolyte it rises to the surface. As the level of molten sodium arises, it is forced into the receiver. The process is continuous and fresh salt is introduced to maintain the level of molten electrolyte high enough to allow sodium to rise into the iron pipe. Chemical reactions involved in this process are as follows:
2NaClmelting−−−−→2Na++2Cl−
At cathode:
2Na++2e−→2Na
At anode:
2Cl−→cl2+2e−
Advantages of Down's process
* Sodium metal obtained has a high degree of purity (99.5%).
* The starting material, sodium chloride is very cheap.
* Chlorine is obtained as a useful by-product.
Extraction of Sodium
The two methods used for extracting sodium metals are,
* Castner's process: By the electrolysis of fused caustic soda.
* Down's process: By the electrolysis of fused sodium chloride.
Castner's process
Molten sodium hydroxide (caustic soda, NaOH) is electrolyzed using an iron cathode and nickel anode. A nickel wire gauze cylinder prevents the electrodes from touching each other. On passing an electric current through the melt, sodium is liberated at the cathode and oxygen at the anode. The liberated sodium metal floats over the surface (lighter than fused caustic soda) and collects inside the receiving vessel where hydrogen prevents sodium from oxidation. Excess of the gas escapes from the outlet. Sodium is removed from time to time with the help of perforated spoons and kept under kerosene oil. The various reactions taking place during electrolysis are:
NaOHmelting−−−−→Na++OH− (dissociation)
At cathode:
Na++e−→Na
At anode:
OH−→12H2O+14O2+2e−
Most of the water formed in the reaction gets evaporated, and the rest is electrolyzed into hydrogen and oxygen. Therefore, metallic sodium and hydrogen are liberated at the cathode and oxygen at the anode.
Down's process
In this process, molten sodium chloride (common salt) is electrolyzed using a graphite anode and a ring-shaped iron cathode. The two electrodes are separated by a wire gauge partition to avoid the mixing of sodium and chlorine so formed. As Sodium chloride melts at a very high temperature of 1085 K, a mixture containing sodium chloride, potassium chloride and potassium fluoride (NaCl + KCl + KF) is employed. This mixture melts at about 850-875 K.
The reasons for lowering the temperature are:
* It is difficult to maintain a high temperature of 1085K.
* Sodium is volatile at this temperature and so a part of the metal formed may vaporize.
* At this high-temperature chlorine produced as a by-product corrodes the vessel.
* Metal at this temperature will be in a colloidal state and its separation will be difficult.
On passing electric current, chlorine is liberated at the carbon anode and escapes through the dome-shaped steel hood outlet. Sodium rises from the cathode and remains in the wire gauze shell. The sodium produced is in a molten state. Being lighter than the electrolyte it rises to the surface. As the level of molten sodium arises, it is forced into the receiver. The process is continuous and fresh salt is introduced to maintain the level of molten electrolyte high enough to allow sodium to rise into the iron pipe. Chemical reactions involved in this process are as follows:
2NaClmelting−−−−→2Na++2Cl−
At cathode:
2Na++2e−→2Na
At anode:
2Cl−→cl2+2e−
Advantages of Down's process
* Sodium metal obtained has a high degree of purity (99.5%).
* The starting material, sodium chloride is very cheap.
* Chlorine is obtained as a useful by-product.
