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Caustic sodaand chlorine

Industrial uses

Caustic Soda• in the production of many useful organic chemicals• inorganic chemicals like paints, glass and

ceramics, fuel cell production and cosmetics• paper, pulp and cellulose industries• food industry• water treatment (for the flocculation of heavy metals and

acidity control)• soaps and detergents sectors• textile sector (as a bleaching agent)• mineral oils (preparation of greases and fuel additives)

and the synthesis of the synthetic fibre rayon• in the process of refining aluminium from its ore bauxite• synthesis of pharmaceutical compounds• rubber recycling

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Industrial uses

Chlorine• used for producing safe drinking water• as a disinfectant in the form of the liquid hypo• in the syntheses of synthetic rubber and PVC

(polyvinyl chloride)• in agrochemicals and pharmaceuticals• in bullet-proof vests, ultra-pure silicon chips

for solar panels and computer chips• in solvents like chloroform and carbon

tetrachloride• in dyestuffs, petroleum products, medicines,

antiseptics, insecticides, foodstuffs and paints• in paper and pulp, explosives and pesticides

Production

• Sodium hydroxide (NaOH), lye or caustic soda is a strong metallic base available in pellets, flakes, granules, and as 50% saturated solution.

• NaOH and Cl2 are produced as co-products by the electrolysis of brine.

• This yields sodium NaOH solution, Cl2, and H2 in the mass ratio 1: 0.88: 0.025 in accordance with the following overall equation:2 NaCl + 2H2O→2NaOH + Cl2 + H2

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Production

Various commercial cells have been developed in order to keep the anode and cathode products separate from one another.

– Diaphragm cell (Griesheim, Dow, Glanor, Hooker, HU Monopolar, OxyTech)

– Mercury cathode cell (Castner – Kellner, Uhde, De Nora cell, Olin – Mathieson, Solvay, Krebs Paris)

– Membrane cell (Asahi Kasei, Chlorine Engineers, Krupp Uhde, EL-Tech, Ineos Chlor)

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Diaphragm cell process

• anode reaction2Cl− → Cl2 + 2e−

• cathode reaction is2H2O + 2e −→ H2 + 2OH−

Na+ + OH− NaOH

Mercury cell process

• anode reaction2Cl− → Cl2 + 2e−

Cl2 + H2O H++ Cl- + HOCl3Cl2 + 6OH− ClO3

−+ 5 Cl−+3 H2O• cathode reaction is2H2O + 2e −→ H2 + 2OH−

Na+ + Hgx + e- NaHg x• decomposers2 NaHg x + 2H2O 2NaOH + 2Hg x + H2

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Membrane cell process

• anode reaction2Cl− → Cl2 + 2e−

Cl2 + H2O H++ Cl- + HOCl3Cl2 + 6OH− ClO3

−+ 5 Cl−+3 H2O

• cathode reaction is2H2O + 2e −→ H2 + 2OH−

Na+ + OH− NaOH

styrene–divinylbenzene K/Na

Membrane characteristics

• High selectivity for the transport of sodium or potassium ions

• Negligible transport of chloride, hypochlorite, and chlorate ions

• Zero back-migration of hydroxide ions• Low electrical resistance• Good mechanical strength and properties

with long term stability

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Production• The first step in all three processes is to purify the feed salt

brine. • Brines contain many contaminants such as calcium,

magnesium, barium, and sulfate ions which are detrimental to the electrolytic process.

• Removal of brine contaminants accounts for a significant portion of overall chlor–alkali production cost, especially for the membrane process since it requires a higher degree of brine purity.

• Brines are treated with sodium carbonate to precipitate calcium carbonate, followed by treatment with sodium hydroxide to precipitate magnesium hydroxide.

Production• Most trace metal impurities are also precipitated during the

process. • The precipitates are allowed to settle in a clarifier where most of

the solids are removed as a mud. • The brine is then filtered by sand filters and precoat polishing

filters (rotary vacuum filter). • At this point the brine contains less that 4 ppm calcium and 0.5

ppm magnesium ions, which is satisfactory for the mercury and diaphragm processes.

• The membrane process however requires additional ion exchange to reach hardness levels below 20 ppb.

• The treated brine feed is also usually acidified with hydrochloric acid to reduce oxygen and chlorate formation in the anolyte.

• HCl neutralizes residual hydroxide and carbonate in the brine and prevents their reaction with chlorine formed at the anodes

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Production• After treatment, the brine is sent to the electrochemical

cell. • This liquor is concentrated to 50% NaOH in a series of

evaporative crystallizers. • The crystallized NaCl is recovered and recycled.• Sodium hydroxide remains in the liquid phase.• Membrane cells produce 30–35% NaOH solutions which

are evaporated to 50% in a single evaporation step.• Seventy three percent caustic containing very little salt is

made in the mercury cell decomposer. • The product is filtered to remove entrained mercury and

further processing to meet final product specification on concentration are done as needed.

Production• The trend for new installations is to use membrane

cells since they give good performance with low energy requirements.

• Environmental, health and safety issues have led to a long term move away from mercury cells.

• The chlorine coproduct from electrolysis is dried in a packed tower using concentrated sulfuric acid (96–98 wt%) to absorb water vapor.

• After drying, the chlorine is compressed. • The hydrogen coproduct from electrolysis is relatively

pure and can be used for a wide variety of chemical uses with only minimal additional processing.

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Diaphragm cell

• Advantage– low purity rock salt, saving the need for purification

before electrolysis

• Disadvantage– less efficient in terms of energy– chlorine contains oxygen and must often be purified

by liquefaction and evaporation– safety and disposal of asbestos diaphragms reach the

end of their life. The cell can, however,.

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Mercury cell

• Advantage– NaOH obtained is highly pure. – The process is very efficient. – Possible reaction between NaOH and Cl2 is avoided as

NaOH is obtained in a separated chamber.

• Disadvantages – High electricity consumption– Environmental pollution due to escape of Hg vapours

Process flow diagram

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Production

• Owing to imbalances in the chlorine and caustic markets, commercial interest in production of sodium hydroxide by the lime soda process:

Na2CO3 + CaO + H2O → 2NaOH + CaCO3