Electrolysis of brines with chlorine production at the anode and hydrogen together with sodium or potassium hydroxide at the cathode.
Two types of cells: diaphragm
The diaphragm cells contain a porous asbestos diaphragm to separate the anode from the cathode. This allows ions to pass through by electrical migration but reduces diffusion of products.
Anodes are graphite or Titanium (DSA).
Cathodes are made from steel wire.
NaCl + H2O – NaOH + 1/2 H2 + 1/2 Cl2
If OH* reach the anode hypochlorites are formed with subsequent loss of chlorine.
The following occurs:
2OH* – H2O + 1/2 O2 + 2E
The oxygen will attack the graphite and chlorinated HC are entrained in chlorine gas.
In a mercury cell, brine is partly decomposed in the electrolyzer between a graphite anode and a moving mercury cathode forming chlorine gas at the anode and sodium amalgam (NaHg) at the cathode.
The amalgam flows to the decomposer box where it becomes the anode to a short circuited iron or graphite cathode in an electrolyte of sodium hydroxide solution.
Purified water is fed to the decomposer; hydrogen is formed and NaOH concentration increases. Hg is formed and recycled to the electrolyzer.
electrolyzer 2Na Cl + Hg – Cl2 + 2Na(Hg)
decomposer 2Na(Hg) + 2H2O – 2NaOH + H2 + Hg
Hydrogen is usually compressed for miscellaneous usage.
NaOH is entrained from cell.
If mercury cell Hg sprays and vapour are present in the gas stream.
Typical load NaOH : Maxi: 2,000 mg/Nm³
Hg : up to: 200 mg/Nm³
Filters are installed after cell to collect NaOH and Hg mists which:
* impact H2 purity
* corrode compressor
* decrease life of active charcoal system
– to remove mercury vapour (mercury cell).
Candle Filters Type HT1 or HT2 or HT3 with 316L Stainless Steel structure and PP13.5 Polypropylene fibre.
Option of acceleration plate for increasing velocity of high viscous mercury.