Soda ash, Na2C03, is available in two forms. Light soda ash has a bulk density range of 35 to 50 lb/cu ft and a working density of 41 lb/cu ft. Dense soda ash has a density range of 60 to 76 lb/cu ft and a working density of 63 lb/cu ft. The pH of a 1 percent solution of soda ash as 11.2. It is used for pH control and in lime treatment. The molecular weight of soda ash is 106. Commercial purity ranges from 98 percent to greater than 99 percent Na2C03. Soda ash by itself is not particularly corrosive, but in the presence of lime and water caustic soda is formed which is quite corrosive.
Dense soda ash is generally used in municipal applications because of superior handling characteristics. It has little dust, good flow characteristics, and will not arch in the bin or flood and feeder. It is relatively hard to dissolve and ample dissolver capacity must be provided. Normal practice calls for 0.5 lb of dense soda ash per gallon of water or a 6 percent solution retained for 20 minutes in the dissolver.
The dust and solution are irritating to the eyes, nose, lungs, and skin and therefore general precautions should be observed and the affected areas should be washed promptly with water.
Soda ash is usually stored in steel bins and where pneumatic-filling equipment is used, bins should be provided with dust collectors. Bulk and bagged soda ash tend to absorb atmospheric C02 and water, forming the less active sodium bicarbonate (NaHC03). Material recommended for unloading facilities is steel. Feed equipment as described for dry alum is suitable for soda ash. Dissolving of soda ash may be hastened by the use of warm dissolving water. Mechanical or hydraulic jet mixing should be provided in the dissolver. Materials of construction for piping and accessories should be iron, steel, rubber, and plastics.
Anhydrous caustic soda (NaOH) is available but its use is generally not considered practical in water and wastewater treatment applications. Consequently, only liquid caustic soda is discussed here. Liquid caustic soda is generally shipped at two concentrations, 50 percent and 73 percent NaOH. The densities of the solutions as shipped are 12.76 lb/gal for the 50 percent solution and 14.18 lb/gal for the 73 percent solution. These solutions contain 6.38 lb/gal NaOH and 10.34 lb/gal NaOH, respectively. The crystallization temperature is 53° F for the 50 percent solution and 165°F for the 73 percent solution. The molecular weight of NaOH is 40. The pH of a 1 percent solution of caustic soda is 12.9. Truckload lots of 1,000 to 4,000 gallons are available in the 50 percent concentration only. Both shipping concentrations can be obtained in 8,000, 10,000 and 16,000 gal carload lots. Tank cars can be unloaded through the dome eduction pipe using air pressure or through the bottom valve by gravity or by using air pressure or a pump. Trucks are usually unloaded by gravity or with air pressure or
Liquid caustic soda is received in bulk shipments, transferred to storage, and diluted as necessary for feeding to the points of application. Caustic soda is poisonous and is dangerous to handle. U.S. Department of Transportation Regulations for "White Label" materials must be observed. However, if handled properly caustic soda poses no particular industrial hazard. To avoid accidental spills, all pumps, valves, and lines should be checked regularly for leaks. Workers should be thoroughly instructed in the precautions related to the handling of caustic soda. The eyes should be protected by goggles at all times when exposure to mist or splashing is possible. Other parts of the body should be protected as necessary to prevent alkali bums. Areas exposed to caustic soda should be washed with copious amounts of water for 15 minutes to 2 hours. A physician should be called when exposure is severe. Caustic soda taken internally should be diluted with water or milk and then neutralized with dilute vinegar or fruit juice. Vomiting may occur spontaneously but should not be induced except on the advice of a physician. Liquid caustic soda may be stored at the 50 percent concentration. However, at this solution strength, it crystallizes at 53° F. Therefore, storage tanks must be located indoors or provided with heating and suitable insulation if outdoors. Because of its relatively high crystallization temperature, liquid caustic soda is often diluted to a concentration of about 20 percent NaOH for storage. A 20 percent solution of NaOH has a crystallization temperature of about - 20° F. Recommendations for dilution of both 73 percent and 50 percent solutions should be obtained from the manufacturer because special considerations are necessary. Storage tanks for liquid caustic soda should be provided with an air vent for gravity flow. The storage capacity should be equal to 1.5 times the largest expected delivery, with an allowance for dilution water, if used, or two-weeks supply at the anticipated feed rate, whichever is greater. Tanks for storing 50 percent solution at a temperature between 75°F and 140°F may be constructed of mild steel. Storage temperatures above 140°F require more elaborate systems.
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