• Document: Soda Ash ( Sodium carbonate) Manufacture
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Soda Ash ( Sodium carbonate) Manufacture Pertinent properties Mol. Wt. 106 M.P. 851deg.C. B.P. Decomposes Soluble in water 8.9 gm/100gm at 20 deg.cel. Grade s: 99% sodium carbonate washing soda ( Na2CO3 .10 H2 O) Methods of production: (i) Solvay or ammonia soda process (ii) dual process (iii) natural soda ash deposits (i) Solvay or ammonia soda process Reactions: 2 NaCl + CaCO3 → Na2CO3 + CaCl2 Raw material : They are salt, coal and limestone Quantitative requirements : Basis : 1 ton of Na2CO3 salt 1.55 tons Limestone 1.2 tons Coke 0.09tons High pressure steam 1.35 tons Low pressure steam 1.60 tons Ammonia losses: 1.5 kg Cooling water 40-60 tons Electric power 210 KWH Plant Capacity: 200,-2000 tons/day Process Description: In 1869 Solvay discovered that NH3 dissolved in an NaCl solution and then reacted with CO2 . A precipitate of NaHCO3 was obtained which could be calcined to produce high purity Na2CO3. Saturated salt brine is purified in series of wash towers with NH3 and then with CO2 to remove Ca, Mg and sludge. The purified brine is pumped to the ammonia absorber tower where it dissolves NH3 with liberation of heat . Some CO2 dissolves in this tower. The ammoniated partially carbonated brine is cooled to 30 deg. cel. and pumped to the carbonating tower that is on cleaning duty. Weak CO2 gas is admitted below the tower. This gas serves to further carbonate the liquor to just below precipitation point. The carbonating towers are about 22-25 m high and 1.8 – 2.5 m in diameter and constructed of cast iron. During precipitation cycle the tower temperature gradients is maintained as 20-25 deg. C. at both ends and 45-55 deg. C. in middle. The remainder process concerns ammonia recovery and recycle . Chemistry of Solvay process The Solvay process results in soda ash (predominantly sodium carbonate (Na2CO3)) from brine (as a source of sodium chloride (NaCl)) and from limestone (as a source of calcium carbonate (CaCO3)). The overall process is: 2 NaCl + CaCO3 → Na2CO3 + CaCl2 The actual implementation of this overall reaction is done is four steps. In the first step in the process, carbon dioxide (CO2) passes through a concentrated aqueous solution of sodium chloride (NaCl) and ammonia (NH3). NaCl + CO2 + NH3 + H2O → NaHCO3 + NH4Cl (I) Chemistry of Solvay process The carbon dioxide required for reaction (I) is produced by heating ("calcination") of the limestone at 950 - 1100 °C. The calcium carbonate (CaCO3) in the limestone is partially converted to quicklime (calcium oxide (CaO)) and carbon dioxide: CaCO3 → CO2 + CaO (II) Chemistry of Solvay process The sodium bicarbonate (NaHCO3) that precipitates out in reaction (I) is filtered out from the hot ammonium chloride (NH4Cl) solution, and the solution is then reacted with the quicklime (calcium oxide (CaO)) left over from heating the limestone in step (II). 2 NH4Cl + CaO → 2 NH3 + CaCl2 + H2O (III) CaO makes a strong basic solution. The ammonia from reaction (III) is recycled back to the initial brine solution of reaction (I). Chemistry of Solvay process The sodium bicarbonate (NaHCO3) precipitate from reaction (I) is then converted to the final product, sodium carbonate (Na2CO3), by calcination (160 - 230 C), producing water and carbon dioxide as byproducts: 2 NaHCO3 → Na2CO3 + H2O + CO2 (IV) Chemistry and process step The carbon dioxide from step (IV) is recovered for re-use in step (I). When properly designed and operated, a Solvay plant can reclaim almost all its ammonia, and consumes only small amounts of additional ammonia to make up for losses. The only major inputs to the Solvay process are salt, limestone and thermal energy, and its only major byproduct is calcium chloride, which is sold as road salt. Major engineering problems : (a) Development of suitable calcining equipment: Moist NaHCO3 will cake on sides of klin, preventing effective heat transfer through the shell. The klin must be equipped with heavy scraper chain inside and wet filter cake must be mixed with dry product to avoid caking. These problems can be avoided by using fluidized bed calciners in newer installations. Major engineering problems contd : (b) Economic balance on tower design: the tower height, pressure and temperature are optimized, giving approximately 75% yield of NaHCO3 from NaCl. (c) Ammonia recovery: NH3 inventory costs 4-5 times that of Na2 CO3 inventory so losses must be kept low. (d) plant modernization (e) waste disposal. USES OF SODA ASH Soda ash is used in many industrial processes, and its production is sometimes used as an indicator of economic health. The principal current uses include: Glass making: More than half the worldwide production of soda ash is used to make glass. Bottle and window glass (Soda-lime glass) is made by melting a mixture of sodium carbonate, calcium carbonate and silica sand (silicon dioxide (SiO2)). USES OF SODA ASH

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