Caustic Soda Industry

Apart from the above three types there are custom built rubber products such as expansion joints, flexible cell covers and large size rubber foils for the caustic soda industry, and many inflatables, fabric reinforced products and thick moulded sheets for specialty applications in certain process plants. These are all hand formed in aluminium or cast iron moulds or forms by laying up process and then cured in autoclave. Here the flow of the un-vulcanized rubber during cure is not very important as the shape is already formed rather the green strength and the stiffness of rubber stock with a low scorch time are the important requisites. A rubber expansion joint made by a hand layup method and cured in autoclave is shown in the following figure 14.1. 

In order to understand how anti-corrosive rubber linings are used in the caustic soda industry it is useful to have a broad understanding of the design, construction and operation of the process, mainly about the cell house where corrosion is severe. A brief description of design, construction and operation of mercury cells in the caustic soda industry is given next [11]. 

This brief description of the mercury cells in the caustic soda industry reveals how rubber can play a vital role as an anti-corrosive protective material in all the critical equipment and connected piping systems handling acidic and alkaline solutions gases and fumes. 

Flexible Cell Covers in the Caustic Soda Industry... 

When anticipated, flexing is of constant amplitude, extension modulus should be kept as low as possible in products like flexible cell covers and hoses in caustic soda industries. Another important factor is the degree of cure. Prolonged vulcanisation cycles will degrade thermally susceptible rubbers. 

Many moulded components are affixed on the rubber-lined surface such as anode sleeves in mercury cells used in caustic soda industry. These moulded components are either made from natural soft or ebonite rubbers or Neoprene rubber compounds. While moulding, the flow characteristics of the rubber compound and shrinkage need to be taken into consideration. Some aspects of mould designs are described next. 

Vertical-type natural circulation evaporator. In this type of evaporator, vertical rather than horizontal tubes are used, and the liquid is inside the tubes and the steam condenses outside the tubes. Because of boiling and decreases in density, the liquid rises in the tubes by natural circulation as shown in Fig. 8.2-lb and flows downward through a large central open space or downcomer. This natural circulation increases the heat-transfer coefficient. It is not used with viscous liquids. This type is often called the short-tube evaporator. A variation of this is the basket type, where vertical tubes are used, but the heating element is held suspended in the body so there is an annular open space as the downcomer. The basket type differs from the vertical natural circulation evaporator, which has a central instead of annular open space as the downcomer. This type is widely used in the sugar, salt, and caustic soda industries. 

United States Caustic Soda Production. In 1987 U.S. production of caustic soda increased to 10.4 million tons (fig. 1), more than 10% over that of the previous year, furthermore, 1988 production was up another 6.7% to 11.1 million tons. The demand for caustic soda has been very strong in recent years as evidenced by both increased U.S. consumption and a strong export demand. In 1987 the United States exported 1.5 million tons, 14.5% of the total caustic soda production (6), representing a 25.5% increase over exports in 1986. Then, in 1988, caustic soda exports grew by another 4.1%. A weak doUar helped boost the 1987 exports. Growth slowed in 1988, however, as a result of an industry (and world) wide caustic soda shortage, which was caused by lower U.S. chlorine consumption and forced allocations. Because industries switched from caustic to soda ash where possible, the lower 1988 export growth was not indicative of caustic soda s export potential. 

Caustic Soda Markets. Forecasted changes in caustic soda consumption (Table 24) are addressed below in descending order of the importance of the major industries, based on 1987 demand. Consumption patterns for many derivatives cannot be accurately quantitied and must be estimated. 

Petroleum and Natural Gas. Over 90% of the 428,000 t of caustic soda used in the petroleum and natural gas industry is used to process oil and gas into marketable products, especially by removing acidic contaminants. The remainder is used primarily to decrease corrosion of drilling equipment and to increase the solubiUty of drilling mud components by maintaining an alkaline pH (6). 

Other Organic Processes. Solvent extraction has found appHcation in the coal-tar industry for many years, as for example in the recovery of phenols from coal-tar distillates by washing with caustic soda solution. Solvent extraction of fatty and resimic acid from tall oil has been reported (250). Dissociation extraction is used to separate y -cresol fromT -cresol (251) and 2,4-x5lenol from 2,5-x5lenol (252). Solvent extraction can play a role in the direct manufacture of chemicals from coal (253) (see Eeedstocks, coal chemicals). 

Mona.Zlte, The commercial digestion process for m on a site uses caustic soda. The phosphate content of the ore is recovered as marketable trisodium phosphate and the rare earths as RE hydroxide (10). The usual industrial practice is to attack finely ground m on a site using a 50% sodium hydroxide solution at 150°C or a 70% sodium hydroxide solution at 180°C. The resultant mixed rare-earth and thorium hydroxide cake is dissolved in hydrochloric or nitric acid, then processed to remove thorium and other nonrare-earth elements, and processed to recover the individual rare earths (see... 

Alkalies. In the 1960s, 3.2-34 x 10 t /yr of lime was captively produced by the U.S. alkaH industry for manufacturing soda ash and sodium bicarbonate via the Solvay process. Electrolytic process caustic soda and natural soda ash (trona) from Wyoming have largely replaced the Solvay process. Three of the trona producers in Wyoming now purchase quicklime for producing caustic soda. 

Alumina. A pure although not necessarily a refractory grade of alumina is obtained from bauxite by the Bayer process. In this process, the gibbsite from the bauxite is dissolved in a caustic soda solution and thus separated from the impurities. Alumina, calcined, sintered, or fused, is a stable and extremely versatile material used for a variety of heavy industrial, electronic, and technical appHcations. 

Inorga.nicNIa.teria.ls. These include acids (sulfuric, nitric, hydrochloric, and phosphoric), bases (caustic soda, caustic potash, soda ash, sodium carbonate, ammonia, and lime), salts (sodium chloride, sodium nitrite, and sodium sulfide) and other substances such as chlorine, bromine, phosphoms chlorides, and sulfur chlorides. The important point is that there is a significant usage of at least one inorganic material in all processes, and the overall toimage used by, and therefore the cost to, the dye industry is high. 

The electrolysis of NaCl brine for the production of chlorine and caustic soda is one of the oldest and certainly one of the most important industrial electrochemical processes (22—26). The overall reaction is... 

Mercury is widespread in the environment, originating both from natural (natural gassing of the Eailh s cmst, leaching from rocks) and industrial sources (e.g. production of caustic soda and chlorine, electrical industry, biocides, etc.). 

Electrochemical corrosion protection of the internal surfaces of reaction vessels, tanks, pipes and conveyor equipment in the chemical, power and petroleum industries is usually carried out in the presence of strongly corrosive media. The range stretches from drinking water through more or less contaminated river, brackish and seawater frequently used for cooling, to reactive solutions such as caustic soda, acids and salt solutions. 

Six caustic soda evaporators were anodically protected against stress corrosion in the aluminum industry in Germany in 1965 [27]. Each evaporator had an internal surface area of 2400 m. The transformer-rectifier had a capacity of 300 AJ 5 V and was operated intermittently for many years. Automatic switching on of the protection current only took place in case of need when the drop in potential reached... 

Evaporators have performed successfully in a number of industrial applications. Typical materials that are processed in evaporators include Caustic Soda, Caustic Potash, Sodium Carbonate, Sodium Dichromate, Sodium Nitrate, Ammonium Nitrate, Phosphoric Acid Superacid, Potash, Urea, Glue, Glycerine,... 

The paper and pulp industries consume taif/e quantities of Ca(OH)2 and precipitated (as distinct from naturally occurring) CaCOs. The largest application of lime in pulp manufacture is as a causlicizing agent in sulfate (kraft) plants (p. 89). Here the waste NajC solution is reacted with lime to regenerate the caustic soda used in the process ... 

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