Ash, soda

Dense soda ash is generally used in munieipal applications because of superior handling charaeteristies. It has little dust, good flow eharaeteristics, and will not areh in the bin or flood and feeder. It is relatively hard to dissolve and ample dissolver capacity must be provided. Normal practiee 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 preeautions should be observed and the affeeted areas should be washed promptly with water. 

Soda ash is usually stored in steel bins and where pneumatie-filling equipment is used, bins should be provided with dust collectors. Bulk and bagged soda ash tend to absorb aonospheric COj and water, forming the less aetive sodium bicarbonate (NaHCOj). Material reeommended 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. Mechanieal or hydraulie jet mixing should be provided in the dissolver. Materials of construetion for piping and accessories should be iron, steel, rubber, and plastics. 

Formerly, most sodium carbonate was made by the famous Solvay process, which has been used since 1869 but is no longer competitive with trona. Nevertheless, the Solvay process merits study as a classic example of chemical engineering practice. The net Solvay reaction 

Reaction 11.6 is exothermic as well, so cooling to 20 °C is required. The precipitated NaHC03 is filtered. The reaction is stopped at about 75% completion, otherwise NH4HCO3 is also precipitated. 

The Solvay process consumes only brine (aqueous NaCl) and limestone, which are inexpensive, and energy. The only waste product is calcium chloride, which can be used for deicing roads. Inevitably, there are complications in particular, the plant design must provide for the elimination of solid contaminants such as clay minerals (from the limestone) and CaS04 (from the sulfate ion usually present in brines). 

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Sillimanite, see Aluminum silicon oxide (1/1) Smithsonite, see Zinc carbonate Soda ash, see Sodium carbonate Spelter, see Zinc metal Sphalerite, see Zinc sulflde Spherocobaltite, see Cobalt(II) carbonate Spinel, see Magnesium aluminate(2—)... 

The %w/w Na2C03 in soda ash can be determined by an acid-base titration. The results obtained by two analysts are shown here. Determine whether the difference in their mean values is significant at a = 0.05. 

The material should be stored in corrosion-resistant containers, away from alkaline or strong oxidizing materials. In the event of a spill or leak, nonsparking equipment should be used, and dusty conditions should be avoided. Spills should be covered with soda ash, then flushed to drain with large amounts of water (5). 

Since the 1960s the commercial development of continuous countercurrent processes has been almost entirely accompHshed by using a flow scheme that simulates the continuous countercurrent flow of adsorbent and process Hquid without the actual movement of the adsorbent. The idea of a simulated moving bed (SMB) can be traced back to the Shanks system for leaching soda ash (58). 

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. 

Chlorine Caustic Soda Chlorine Caustic Potash ) Chlorine Sodium Soda Ash ) Chlorine Magnesium Chlorine, Caustic Soda Caustic Potash... 

The only caustic soda production process besides electrolysis is the soda—lime process involving the reaction of lime with soda ash ... 

Historically, soda ash was produced by extracting the ashes of certain plants, such as Spanish barilla, and evaporating the resultant Hquor. The first large scale, commercial synthetic plant employed the LeBlanc (Nicolas LeBlanc (1742—1806)) process (5). In this process, salt (NaCl) reacts with sulfuric acid to produce sodium sulfate and hydrochloric acid. The sodium sulfate is then roasted with limestone and coal and the resulting sodium carbonate—calcium sulfide mixture (black ash) is leached with water to extract the sodium carbonate. The LeBlanc process was last used in 1916—1917 it was expensive and caused significant pollution. 

These disadvantages prompted Ernest Solvay (1838—1922) to develop and commercialize a procedure using ammonia to produce soda ash from salt and limestone. The first plant using the Solvay process was built in 1863 this process or variations are in use in much of the world in the 1990s. 

The basic Solvay process remains the dominant production route for soda ash. Its continued success is based on the raw matedals, salt and limestone, being more readily available than natural alkaU. AH soda ash processes are based on the manipulation of saline phase chemistry (6,7) an understanding of which is important both to improving current processes and to the economic development of new alkaU resources. 

Brine Preparation. Sodium chloride solutions are occasionally available naturally but they are more often obtained by solution mining of salt deposits. Raw, near-saturated brines containing low concentrations of impurities such as magnesium and calcium salts, are purified to prevent scaling of processing equipment and contamination of the product. Some brines also contain significant amounts of sulfates (see Chemicals FROMBRINe). Brine is usually purified by a lime—soda treatment where the magnesium is precipitated with milk of lime (Ca(OH)2) and the calcium precipitated with soda ash. After separation from the precipitated impurities, the brine is sent to the ammonia absorbers. 

Hbls Process. Chemische Werke Huls AG has developed a process to produce soda ash and hydrochloric acid from salt via an amine—solvent system (12). A potential advantage of the Huls process is that, under some market conditions, hydrochloric acid may be more easily sold than either ammonium or calcium chloride. 

Akzo Process. Akzo Zout Chemie has developed a route to vinyl chloride and soda ash from salt usiag an amine—solvent system catalyzed by a copper—iodide mixture (13). This procedure theoretically requires half the energy of the conventional Solvay processes. 

Another mining process involves the recovery of sodium carbonate decahydrate from alkaline ponds. EMC mines this material from its solar evaporation pond using a bucket wheel dredge. The decahydrate slurry is dewatered, melted, and processed to soda ash. 

Trona Purification Processes. Two processes, named the monohydrate and sesquicarbonate according to the crystalline intermediates, are used to produce refined soda ash from trona. Both involve the same unit operations only in different sequences. Most ash is made using the monohydrate process. Eigure 2 shows simplified flow diagrams for each. 

Eig. 2. Simplified flow diagrams for soda ash from trona. 

Sodium carbonate monohydrate crystals from the crystallizers are concentrated in hydroclones and dewatered on centrifuges to between 2 and 6% free moisture. This centrifuge cake is sent to dryers where the product is calcined 150°C to anhydrous soda ash, screened, and readied for shipment. Soda ash from this process typically has a bulk density between 0.99—1.04 g/mL with an average particle size of about 250 p.m. 

Lake Texcoco. Lake Texcoco, a few miles northeast of Mexico City, is in the lowest part of the Valley of Mexico. The lake is mostly dry and alkaH is recovered from brine weUs that have been drilled into the underlying stmcture. The brine is concentrated first in a spiral flow solar evaporation pond and further in conventional evaporators. This strong brine is carbonated and then cooled to crystallize sodium bicarbonate which is subsequently filtered and calcined to soda ash. Purity of this product is similar to Magadi material (9,29). 

Sua Pan, Botswana. A soda ash plant is under construction at Sua Pan ia Botswana (32). The plant will recover ash from an alkaU brine via a process similar to that at Sead.es Lake (29). 

Natural and synthetic soda ash capacity is shown ia Table 4. As iadicated ia Table 5, eight companies represent about 75% of the Western wodd s soda ash capacity. 

Capital and operating costs for soda ash production are extremely site specific (29,10). Key factors iaclude iafrastmcture development, freight to consumers, local energy and labor costs, and by-product saleabiUty. 1990 Hst price of bulk natural soda ash was 108/t, F.O.B. Wyoming. 

Sodium Bicarbonate. Many soda ash plants convert a portion of their production to sodium bicarbonate [144-55-8], NaHCO. Soda ash is typically dissolved, carbonated, and cooled to crystallize sodium bicarbonate. The mother Hquor is heated and recycled. The soHd bicarbonate is dried in flash or tray driers, screened, and separated into various particle size ranges. Bicarbonate markets include food, pharmaceuticals, catde feed, and fire extinguishers. U.S. demand was approximately 320,000 t in 1989 world demand was estimated at one million metric tons. 

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