CHLOR-ALKALI INDUSTRY
The chlor-alkali industry represents of three major industrial chemicals: Soda ash (sodium carbonate-na 2 CO 3 ) Caustic soda (sodium hydroxide-naoh) Chlorine (Cl 2 ) These chemicals have as widespread distribution in industry as does sulfuric acid.
The electrolysis of NaCl brine furnishes over 90% of the chlorine produced plus its co-product sodium hydroxide. This alkali is competitive in many areas with soda ash also produced from brine by a non-electrolytic process. The use of a common raw material, NaCl, together with similarities in alkalinity and or co-production of chlorine, is the reason why the above-outlined chemicals form the basis for the chlor-alkali industry.
This industry is worthy of study because of these production achievements. Quarries and transports vast quantities of limestone (for soda ash production) Extracts NaCl mineral from open-water brine or deep wells Processes products to exceptional purity Operates 100% indigenously no imports required Walks an economic tightrope shipping products such as lowpriced soda ash
Process description The principal discovery of Solvay in 1869 was that NH 3 dissolved in an NaCl solution and then reacted with CO 2. A precipitate of NaHCO 3 was obtained which could be calcined to produce high purity Na 2 CO 3. Saturated salt brine is purified in a series of wash towers with NH 3 and then with CO 2 to remove Ca, Mg and Fe as a sludge.
The purified brine is pumped to the ammonia absorber tower where it dissolves NH 3 with liberation of heat [reaction (e)]. Some CO 2 also dissolves in this tower. The ammoniated partially carbonated brine is cooled to 30 C and pumped to the carbonating tower that is on cleaning duty. In order to hasten the cleaning process, weak CO 2 gas is admitted at the bottom of the tower. This gas serves to further carbonate the liquor to just below the precipitation point.
The carbonating towers are about 22-25 m high, 1.8-2.5 m in diameter and constructed of cast iron. During the precipitation cycle, the temperature gradient is maintained as 20-25 C at both ends and 45-55 C in the middle. The tower gradually becomes fouled as bicarbonate cakes on the cooling surfaces. Cleaning is done to remove fouling. The liquor from a cleaning tower is passed to a series of four to five remaining towers in a production line.
A tower is generally on the make part of a cycle for three days and cleaning portion for twelve hours. In the making portion of a tower run, lean lime kiln gases are injected near the middle of the tower and rich CO 2 gas from the bicarbonate calciner is recompressed and pumped to the bottom of the tower. In the make towers, reactions (f), (g) and (h) take place. The towers are constructed with a series of cooling boxes and sloped baffles so that the NaHCO 3 precipitate settles to the bottom and is then pumped as a magma or slurry to a rotary filter. The solids from the rotary filter are calcined at about 200 C in a calciner which may be gas-fired or be a steam-heated unit.
The average daily production of a modern carbonating tower is about 100 tons as finished soda ash. The remainder of the process concerns ammonia recovery and recycle. Gases from the calciner are cooled and returned to the carbonating tower. The filtrate liquor from the pressure type rotary filter is sent to a pair of ammonia stills. In the first still, free ammonia in solution is driven off by distillation using a steamheated reboiler. The bottoms, containing combined ammonia, is fed to the lime still where reaction (j) releases NH 3 gas and the liquor effluent contains largely CaCl 2 which must be disposed off. The product from the calciner is light soda ash. To produce the dense grade required by the glass industry, sufficient water is milled in to form a monohydrate and the mixture is recalcined.
Kinetics Reactions : Reactions (e)-(h) are useful in explaining the kinetics of the precipitation reaction.
The rate controlling steps are (f) and (g), the recovery of CO 2. Furthermore, the key to the process, as discovered by Solvay, was that the reactions must occur in the order shown. If NH 4 HCO 3 is prepared and brine added, no precipitation of sodium bicarbonate occurs. In other words, ammonia must be absorbed in brine first, then carbon dioxide added.
Major engineering problems (a) Development of suitable calcining equipment Moist NaHCO 3 will cake on sides of the kiln, preventing effective heat transfer through the shell. (b) The kiln must be equipped with heavy scraper chain inside and wet filter cake must be mixed with dry product to avoid caking. (c) These problems can be avoided by using fluidized bed calciners in newer installations.
(b) Economic balance on tower design The tower height, pressure and temperature are optimized, giving approximately 75% yield of NaHCO 3 from NaCI. (c) Ammonia recovery NH 3 inventory costs 4-5 times that of Na 2 CO 3 inventory so losses must be kept low. By proper choice of equipment design and maintenance, losses are less than 0.2% of recycle load (0.5 kg/kg product) or about 1 kg/ton of Na 2 CO 3.
(d) Plant modernization Three Solvay plants were built prior to 1947 in India. These had to be modernized: Substituting better materials of construction in replacement maintenance Use of automatic control (e) Waste disposal Find uses for large quantities of CaCl 2 -NaCI liquor or dispose as waste.
Process description The principal modification of the dual process is the recovery of NH 4 Cl as a co-product rather than liberation of the contained ammonia for recycle as in the Solvay process. The liquor from the bicarbonate filters is mixed with washed salt feed to aid in precipitation of the NH 4 Cl which is crystallized in a refrigerated tank-continuous unit at 0 C. The slurry is centrifuged and the NH 4 CI crystals are dried in a rotary drum hot air dryer, then packaged in bags for shipping.
Major engineering problems In addition to calcining and economic balance on tower design, (a) Salt purification Solid salt, used to obtain better crystallization yields of NH 4 Cl, cannot be purified as with brine feeds in Solvay. Only purification is mechanical washing and dewatering. (b) Corrosion NH 4 Cl solution is quite corrosive in equipment involved in crystallization and solids recovery. Durmet 20 or rubber-lined units specified. (c) Refrigeration Economic balance on yield of NH 4 Cl versus refrigeration costs with temperature of crystallization as a variable. Note that at economic temperature of around 0 C the electric requirements are still twice that of the Solvay operation.
CHLORINE-CAUSTIC SODA
These two chemicals are being discussed in combination as they are produced as coproducts in the electrolysis of brine. This process accounts for 80% of caustic soda and >95% of chlorine production in India. 1. Pertinent Properties of Chlorine (Cl 2 ) Mol. wt. 70.9 M.P. 101.6 C Critical temp. 146 C B.P. 34.6 C Critical pressure 93.5 atms Liquefaction point 5.7 atms and 15 C Toxic gas 0.35-2.0 ppm is max. conc Grades Technical (99.0%) 2. Pertinent Properties of Sodium Hydroxide (NaOH) Mol. wt. 40.00 B.P. 1,390 C M.P. 318 C Very soluble in water with high exothermic heat of solution Grades: Available in solid form of flakes, granules, sticks, lumps, pellets and aqueous solutions (50 and 73% Na0H). Purity of solid forms ranges from 60% Na 2 0 (77.4% NaOH) to 76% Na 2 0 (98% Na0H).
Methods of Production For many years since its discovery in 1853, the "lime causticization" method of manufacturing caustic soda was used which involves reaction of slaked lime and soda ash. Na CO + Ca(OH) 2 NaOH + CaCO 2 3 2 3 In 1892, the electrolysis of brine was discovered as a method for making both sodium hydroxide and chlorine. This rapidly grew in importance and since the 1960's it has been the only method of manufacture. Among electrolytic industries it is the second largest consumer of electricity, aluminum manufacturing being the largest.
Classification of Processes Electrolytic process producing chlorine, sodium hydroxide, and hydrogen as co-products; accounts for 80% of production. (a) Diaphragm electrolytic cell uses saturated NaCI solution and produces 10-12% caustic which must be concentrated. Being replaced by membrane cells. (b) Mercury electrolytic cell uses saturated NaCl solution with solid salt make-up, gives 70% caustic solutions directly. Chlorine processes without co-products (a) HCI- air oxidation with Fe 2 O 3 catalyst (b) HCI- air-cl 2 oxychlorination processes, e.g., production of ethylene dichloride from ethylene (c) HNO 3 - NaCl-air process NaOH process with no Cl 2 co-product Na 2 CO 3 - Ca(OH) 2 ; no further investments allocated as process not competitive.
Process description A combination of the diaphragm and mercury cell processes will be described. Brine solution flows through pipelines to a storage reservoir and then through a brine treatment system. Caustic soda, soda ash, and/or barium carbonate removes calcium, magnesium and iron salts which would clog up diaphragms. This purified, saturated brine (25-28 % NaCl) is heated and electrolyzed in a diaphragm cell. The cell, operating at 45-55% decomposition efficiency, discharges a 10-12% solution of caustic soda with about an equal concentration of NaCl.
Multiple effect evaporation concentrates the cell liquor to 50% NaOH solution. The precipitated salt is separated, centrifuged, washed, and then slurried with treated brine. Salt separator overflow is 50% caustic soda product containing 2% NaCl and 0.1-0.5 % NaClO on a dry basis. This commercial caustic grade can be evaporated to produce saturated 73% NaOH liquor or fused to flake, granular or stick caustic. Purified grade can be produced by a combination treatment of CaCO 3 to remove colloidal Fe and liquid NH 3 counter-current extraction to take out chloride and chlorate impurities.
Mercury Process: Advantages: Pure 50% sodium hydroxide solution (without evaporation) Pure chlorine gas Disadvantages: Higher voltage than with the diaphragm process and hence 10 to 15% higher electrical energy consumption More stringent brine purification requirements Stringent mercury contamination avoidance measures required
Diaphragm process: Advantages: Utilization of less pure brine Lower voltage than in the mercury process Disadvantages: Sodium hydroxide produced is both dilute and chloridecontaminated, evaporation required Chlorine gas contains oxygen Rigorous measures required to avoid asbestos emission
Membrane process: Advantages: Pure sodium hydroxide Electrical energy consumption only about 77% of that of the mercury process No mercury or asbestos used Disadvantages: Sodium hydroxide content only ca. 33% by weight Chlorine gas contains oxygen Very high purity brine required Present high cost and short lifetime of the membranes