Sodium Hydroxide Manufacture
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Sodium Hydroxide manufacture is world-wide, with Europe, North America and Asia manufacturing over fifty per cent of the total world production. In America there are several main producers including the Dow Chemical Company, Oxychem, PPG, Olin and Formosa.
As a method of Sodium Hydroxide manufacture, the Chloralkali Process is a major production method which uses an aqueous solution of Sodium Chloride. This is then electrolysed to produce Sodium Hydroxide with additional products of Chlorine and Hydrogen. Reaction at the cathode is :-
2NaCl + 2H2O + 2e = H2 + 2Cl + 2NaOH
During this process it is important that the Sodium Hydroxide and Chlorine produced do not react with each other. Three other methods of Sodium Hydroxide manufacture are employed to prevent this.
The Castner-Kellner or Mercury Cell Process
A Mercury cathode is used to form an amalgam with Sodium metal which has beeen produced from the reduction of Sodium ions. The second stage to produce Sodium Hydroxide is to react Sodium with water. Mercury may be released to the environment through this process.
Diaphragm Cell Process
In this process a permeable diaphragm separates the anode and a steel cathode. Brine enters the process via the anode compartment and continues through the diaphragm to the cathode. Following reaction, the product leaves as a diluted Caustic Brine solution. Salt is removed from the product which can be used to saturate dilute Brine.
A large evaporation process is employed to concentrate the dilute Sodium Hydroxide solution, which removes approximately three times the amount of steam per unit of product. Asbestos fibres are commonly used to construct the diaphragm.
Membrane Cell Process
This process is the most economic in terms of energy consumption and produces a better quality of Sodium Hydroxide. It also involves the separation of anode and cathode as with the Diaphragm process but uses e a Nafion membrane. This type of material has benefits from health, safety, environmental and financial perspectives. Nafion can withstand the harsh corrosive environment, high temperatures and electrical currents of the process.
Sodium ions transfer from one cell to the other through the Nafion with minimal chemical resistance. Additional membranes are used as a reinforcement to prevent gas product mixing and reduce the liklihood of Chlorine and Hydroxide ions transferring back.
Nafion was discovered in the late 1960's by Walter Grot and is a sulphonated tetrafluoroethylene.
Sodium Hydroxide solutions are generally evaporated to a concentration of approximately 50% before sale.
Nelsons Process
This is similar to the Diaphragm Cell process but involves a U-shaped steel tube lined with asbestos suspended in a rectangular tank. This acts as the cathode, while a carbon rod is used as the anode. Brine (saturated Sodium Chloride in water) is placed in the Cell.
As Sodium Hydroxide forms, it collects at the bottom of the tank while Chlorine escapes via an outlet in the top. The product is evaporated to form a concentrate or solidified as pellets or flake.
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