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Table 6.2 Average Operating Conditions for Cells Cell Type Brine Concen- tration (g//l) Limits on Impurities Temperature oF pH CaO MgO SO2 Diaphragm 315-330 5 ppm 0.8 ppm 0-0.3 g/l 194-221 10.5 - 11 Mercury 300-320 < 5 ppm < 3 ppm < 2g/l 165 -185 3-5 Membrane ~ 445 nila nila nil 165 - 195 7 a less than 20 parts per billion of calcium and magnesium combined. Sources: IND CHEM 1990, Ayres 1997. 6.1.3 Manufacture of Soda Ash Trona Ore Is the Feedstock for U.S. Soda Ash Production In 1864, a Belgium chemist named Ernest Solvay invented the ammonia-soda or Solvay process for manufacture of sodium carbonate (soda ash). While this process is still popular world-wide, in the United States all soda ash is produced from natural trona ore. In the 1940s, large deposits of this ore were found in Wyoming, which ultimately led to the closing of the last Solvay process plant in 1986. Conversion from the synthetic Solvay process to natural soda ash has been touted as one of the most successful transformations of the chemical industry during the 1970s and 1980s (Chenier 1992). Trona ore, also called sesquicarbonate, is a complex of sodium, carbon, hydrogen, and oxygen (2NA2CO3•NAHCO3•2H20). Upon heating it produces soda ash and carbon dioxide (see Figure 6-8). Approximately 1.8 tons of trona ore are required to produce one ton of soda ash. There are four methods currently used to mine Wyoming trona ore: 1) solution mining, 2) room- and-pillar, 3) longwall, and 4) shortwall. In solution mining, dilute sodium hydroxide is injected into the trona ore to dissolve it. The solution is then treated with carbon dioxide to precipitate out sodium carbonate. The crystals are then dissolved again in water, precipitated with carbon dioxide, and filtered. Calcining is then employed to produce a dense soda ash. All other methods of trona ore mining require blasting. A combination of prilled ammonium nitrate and fuel oil is the most commonly used blasting agent. In this process the trona ore is crushed, dissolved in water, filtered to remove insoluble impurities, subjected to vacuum crystallization to precipitate sesquicarbonate, then calcined to produce sodium carbonate. Alternatively, a monohydrate process is employed. In this process crushed trona ore is calcined in a rotary kiln, yielding dense soda ash and carbon dioxide as byproducts. The calcined material is combined withwater to settle out impurities, and is then concentrated using triple effect evaporators or mechanical vapor recompression. For every pound of soda ash created in this process, 0.42 pounds of carbon dioxide are also produced and vented to the atmosphere (Brown 1996, EPA 1997g, EIA 1998). 189PDF Image | The Chlor-Alkali Industry
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