Chemical Recovery of Caustic Soda-Indian Scenario
CAUSTIC SODA Caustic soda (sodium hydroxide) is a general utility chemical and constitutes one of the important segments of the chemical process industry in India. The caustic soda industry, with a Rs 36-bn turnover, accounts for about 5% of the output of the chemical process industry. Caustic soda and chlorine are co-products, which are generated in the ratio of 1:0.88, the combination known as an electrochemical unit (ECU). The caustic soda industry has made rapid strides in the last 30 years, despite the country's shortage of power, which is the most important input in the manufacture manufacture of caustic soda. Industry Structure The domestic industry has around 40 players, most wi th merchant plants, while a few are integrated manufacturers manufacturers that sell only surplus production, production, after captive c aptive consumption. The top eight players constitute about 45% of the installed capacity. The average plant size is in the range of 50,000 tons per annum (tpa), as against the global average size of 0.2-0.25 mn tpa. The industry industry leaders are Gujarat Alkalis and Chemicals, Grasim Industries, Standard Industries, Mardia Chemicals and Punjab Alkalis and Chemicals. Most of these companies are from the west zone, except for Punjab Alkalis and Chemicals. Applications Caustic soda is used in diverse sectors such as paper, water treatment chemicals, chemicals, soaps and detergents, aluminum and man-made and c otton fibres.
Caustic soda: User Profile
Manufacturing Processes Caustic soda is derived by electrolysis of natural salt and water, p ower and salt forming the key inputs. It is derived in the liquid form, known as caustic soda lye, which is then dried to give solid caustic soda flakes. The principle method for its manufacture is electrolytic dissociation of sodium chloride; chorine gas is a coproduct. Caustic Soda is produced mainly by one of these three processes: Diaphragm, Membrane or Mercury Cell. Diaphragm - Named for its use of a Diaphragm to separate the Caustic Soda from the Chlorine. It is the preferred grade in pulp and paper, alumina and refining. Mercury Cell (Rayon) - This grade was originally created for the rayon fiber industry. It is of higher purity than Diaphragm due to the lower levels of Sodium Chloride and Iron. The negatives of this process include high electrical consumption and the possibility of traces of mercury in the product. Membrane - Is the newest process and its level of purity is between Diaphragm and Mercury Cell. It is a higher grade material than Diaphragm but due to the levels of Sodium Chloride and Iron, is not as pure as Mercury cell. Membrane Grade is used primarily in applications such as food, photography and ion-exchange regeneration. The membrane cell technology has become the most widely accepted global p roduction process. This technology is the most energy efficient (power is the largest component of cost of production) and least polluting. However, the capital costs for membrane technology based plants are high, Rs. 800-1000 mn for a 33,000 tpa unit. 10% of the Chlor-Caustic Plants use Membrane Cell Technology, which will find higher usage, as no new capacities are allowed for the mercury cell process S.No. 1. 2. 3.
Process Diaphragm Process Mercury Cell Membrane Cell
Power Consumption (kwh/mt) 3500 3300 2900
http://www.myiris.com/shares/sectors/sectorReport.php?secode=CHEMICAL&peercode =&fname=./data/chemical/chemical.htm Since 1986, the government has made it mandatory that new caustic soda units be set up using membrane cell technology only. Indian manufacturers are slowly shifting from mercury to membrane cell process, and, at present, about 66% of the domestic capacity is based on membrane cell technology, a level next only to Japan.
Membrane process: In the process of manufacturing Caustic Soda with membrane Process, the main raw material required is salt and other important input is power. Lean brine (salt solution) returning from the process plant is saturated in saturators by adding salt. This saturated brine is t reated to remove impurities in primary and secondary brine is treatment sections. The desired ultra purity of the brine is achieved in secondary brine treatment by means of ion exchange columns. Ultra pure brine is fed to anode compartment of the membrane cell. The cell consists o f two compartment separated by membrane. One compartment is called cathode and other compartment is called as anode. Dilute Caustic is fed to cathode compartment. Concentrated Caustic is collected from the cell as the main product. The lean brine from the cell is sent b ack to saturators after dechlorination. Caustic Soda, Chlorine and Hydrogen is produced in Membrane Cell by ion exchange Membrane process using selective ion exchange properties of the Membrane, which is the main component of the Membrane Cell. A Membrane Cell consists of cell elements. Each cell element is made up of anode and cathode assembly separated by means of Membrane Cell. Pure brine is fed to anode compartment through inlet nozzles under electrolytic condition salt (NaC1) gets splitted into Na+ and CL ions. The Na+ ions travel through Membrane element into cathode compartment. CL ions evolve as CL 2 gas from anode compartment. Dilute Caustic is fed to the cathode compartment through inlet nozzles. Here, H2O gets splitted into H+ and OH ions. The OH ions combines with Na+ ions (from anode compartment) thereby produces Caustic H+ ions evolves as H2 gas cathode compartment. Hydrogen from cell is cooled and sent to H2 has holder. Chlorine from cells is cooled and dried in chlorine treatment sections. The dry chlorine is then compressed and sent to the liquefier where it is liquefied. This liquid chlorine is taken to liquid chlorine storage tank by gravity and packed in one tonne capacity container for sale. Further the excess chlorine gas which is not liquefied is sent to HCL section where CL 2 gas with Hydrogen and it is mixed with water to make liquid Hydrochloric Acid.
Chemical Recovery Process Most of the agro residue based paper industries in India do not have a soda recovery plant, which one of the prime factors for crippling the economy of the paper industry in India. It is estimated that the small-scale paper industry of about 50 T PD discharges waste soda liquor equivalent to Rs.30,0 millions and Rs.3.3 million as thermal energy annually, at the same time releasing pollution load equivalent to a 100 tpd mills equipped with a recovery unit. The energy loss also accounts for 12000 MW per annum at the national level, assuming the energy consumption as 2000 kWh per tonne of caustic production as envisaged by any caustic plant. The presence of silica in black liquor is a problem in the development of suitable chemical recovery process for agro-based paper industries. The objective of the chemical recovery system is to recover the organic pulping chemical. The main process units in the chemical recovery process are the evaporator, recovery boiler, smelt dissolver and causticizing plant. The concentration of dry solid in waste black liquor, obtained from the pulping process is about 8%. This concentration is increased to upto 60-62% in the evaporator. The caustic rich liquor can be used again, thereby saving on the caustic cost which is very high. Condensate from black liquor evaporators will vary in their degree of contamination according to origin. They are treated in a stripper column that is normally integrated with black liquor evaporation by using steam. Stripping condensate makes it possible to reuse the condensate in washing unbleached pulp and in the causticizing plant. This concentrated black liquor then undergoes combustion in the recovery boiler, so as to obtain sodium and sulphur content in a chemical form suitable for regeneration. The smelt obtained from the boiler is dissolved in water in the smelt dissolver to produce ‘green liquor’. Green liquor consisting of sodium sulphide and sodium carbonate is sent to the causticizing plant where it is clarified and causticized with lime. Thus sodium carbonate is converted to sodium hydroxide for pulping.
The process flow diagram is as follows:
Weak Black Liquor from pulping process
Evaporator
Concentrated Black Liquor Recovery Boiler
Smelt (Na2CO3 + Na2S) Smelt Dissolver
Green Liquor Lime Mud
Causticizing Plant
White Liquor (NaOH +Na 2S)
