Electrolysis of brines with chlorine production at the anode and hydrogen together with sodium or potassium hydroxide at the cathode.
Types of cells:
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
Chlorine leaving the cell is cooled down by water, dried with sulfuric acid, compressed and liquefied.
NaCl is entrained from cell; chlorinated HC (if graphite anode) are also entrained. Water (when direct cooling) saturates the gas stream.
Typical load: 800 – 3,500 mg/Nm³
Filters are installed before the drying tower to collect:
Candle Filters Type HT1 or HT2 or HT3 with GRP / FRP or Titanium structure and TGW15 glass or PT12 Polyester / Terylene fibre.
Lowest pressure drop is recommended (Air/Cl2 explosive mixture)