Synthetic soda ash

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. 

The chemical category of inorganic salts encompasses many substances that dissociate completely in water, but only one salt, sodium chloride, is referred to by the common name, salt. Sodium chloride is ubiquitous in both its occurrence and its many uses. To date, there are over 14,000 uses for salt.1 Salt is used as a feedstock for many chemicals including chlorine, caustic soda (sodium hydroxide), synthetic soda ash (sodium carbonate), sodium chlorate, sodium sulfate, and metallic sodium. By indirect methods, sodium chloride is also used to produce hydrochloric acid and many other sodium salts. In its natural mineral form, salt may take on some color from some of the trace elements and other salts present, however, pure sodium chloride is a white to colorless crystalline substance, fairly soluble in water.2 Also known as halite, the substance... 

In countries with high demand and insufficient reserves of the natural product, the demand for soda ash must be met either with imports or by producing the so-called synthetic product. Deposits of salt and limestone, the primary raw materials needed to produce the synthetic product, are readily available in many parts of the world. In 2003, China became a major world producer of synthetic soda ash, surpassing the United States, the world s leading producer for over 100 years, by producing 11.1 million tons.7 Data on soda ash production and reserves are given in Table 26.3. 

Calcium chloride occurs in nature as tach-hydrite and in some other minerals. It is mainly produced by refining of natural brines (mixtures of salts of Ca, Mg and Na) and from the synthetic soda ash production due to the following simplified scheme 2NaCl + CaCOj + NH3 -> Na2C03 + CaCl2. 

A chemical reaction of an aromatic amine with a chloronitro compound went out of control due to ferric chloride catalysed side reactions when the reaction mass became acidic. Natural soda ash used as an acid acceptor in the non-aqueous system was ineffective. The exothermic side reactions developed pressures above those normally encountered in the process. Synthetic soda ash had been used for all previous batches over a 20-year period. The difference in crystallinity made the natural soda ash less effective than the synthetic type normally used and it acted like an undercharge of soda ash. This permitted an acid build-up which formed ferric chloride as the reactor was made of stainless steel. 

The use of sodium carbonate is considered to be environmentally safe. There are concerns, however, regarding synthetic soda ash production, which can result in calcium chloride as well as ammonium and heavy metal compounds in the wastewater. The high alkalinity of sodium carbonate is also a consumer safety concern [16]. 

Soda ash can be produced synthetically by one of several processes that use commonly available raw materials such as limestone, salt, and coke. Some of the these processes include the Leblanc process, Solvay process, electrolytic process, caustic carbonation process, Caprolactum pyrolysis, ammonium chloride process, new Asahi (NA) process, Akzo process, Ormiston Mining process, and Huls process. The Solvay process is the most commonly used method for the manufacture of synthetic soda ash. This method is discussed in the following text. For details of all the processes see Ref 17. 

In Europe and in many other parts of the world, the anunonia-soda process (Solvay process see Chapter 21) is used to produce synthetic soda ash according to Equation 22.2 ... 

The production cycle starts with the extraction of sodium chloride. About 20% of the world s salt consumption goes into soda ash production [24]. The next step after rock salt mining is the production and purification of brine yielding a concentrated aqueous sodium chloride solution [8,25-27]. A parallel step is the production of carbon dioxide gas by calcination of limestone. The brine is treated with ammonia and carbon dioxide under precipitation of the less-soluble sodium hydrogen-carbonate. Ammonia is recovered by mixing the mother liquor with calcium hydroxide and stripping off the ammonia with steam. Thermal decomposition of sodium hydrogencarbonate yields synthetic soda ash [8,20,22,23,28-38]. The output of soda ash produced by the ammonia-soda process amounts to about two-thirds of the world production [22,23]. 

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