Let’s discuss the various consumption levels and costs related to ammonia-alkali and double-alkali processes

Sodium hydride: 345 units of ammonia consumed; 80 for washing. I forgot the values for the other items

Steam consumption: 1.66; Salt consumption: 1.20; Maintenance costs: 1.85; Alkali-soda plant

This post was last edited by Qingtan Xiaolong on 2010-9-26 19:07. Introduction to the ammonia-alkali process and the double-alkali process? Anhydrous sodium carbonate, commonly known as soda ash. It has a wide range of applications in the chemical industry, serving as an important raw material for industries such as glass manufacturing, paper production, soap and detergent production, textiles, and leather processing. It is a key product in the basic chemical industry, and its industrial production methods mainly include the ammonia-soda process and the combined soda process. I. The ammonia-alkali process, also known as the Solvay process, is a method for producing soda ash invented in 1892 by the Belgian engineer Solvay (1838–1922). Soda ash is produced using table salt (sodium chloride), limestone (which, when calcined, produces quicklime and carbon dioxide), and ammonia as raw materials. First, ammonia is introduced into saturated salt water to form an ammonium salt solution; then carbon dioxide is added, resulting in the formation of sodium bicarbonate precipitate, which has a lower solubility, and an ammonium chloride solution. ??NaCl + NH3 + H2O + CO2 = NaHCO3↓ + NH4Cl?? The tiny NaHCO3 crystals obtained after filtration and washing are then heated and calcined to produce soda ash. ??2NaHCO3 → Na2CO3 + H2O + CO2↑?? The carbon dioxide gas produced can be recovered and reused. The filtrate containing ammonium chloride is mixed with lime milk [Ca(OH)2] and heated; the ammonia gas released can be recovered and reused. ??CaO + H2O = Ca(OH)2 (slaked lime)??2NH4Cl + Ca(OH)2 → CaCl2 + 2NH3↑ + 2H2O??The simple process for its industrial production is shown in Figure 1. The advantage of the ammonia-alkali process is that the raw materials (table salt and limestone) are inexpensive ； The purity of the soda ash product is high ； Both by-products, ammonia and carbon dioxide, can be recovered and reused ； The manufacturing process is simple, making it suitable for large-scale production. However, the ammonia-alkali process also has many disadvantages: firstly, only half of the components in the two raw materials are utilized – the sodium ions (Na+) in table salt and the carbonate ions (CO32–) in limestone combine to form sodium carbonate. Yet, the chloride ions (Cl–) in table salt and the calcium ions (Ca2+) in limestone combine to form calcium chloride (CaCl2), which has little practical use. As a result, finding a way to deal with calcium chloride becomes a significant problem. The biggest drawback of the ammonia-alkali process is that the utilization rate of the raw material, table salt, is only 72% to 74%; the remaining table salt is discarded as waste liquid along with the calcium chloride solution, which represents a significant loss. ?? Figure 1? Process flow for ammonia-alkali method production?? II. Combined alkali production method, also known as the Hou’s alkali production method?? It was developed in 1943 by Chinese chemical engineering expert Hou Debang (1890–1974). It is a method that combines the ammonia-alkali process and the synthetic ammonia process to produce both soda ash and ammonium chloride. The raw materials are table salt, ammonia, and carbon dioxide – the waste gases produced when hydrogen is generated using water gas in ammonia synthesis plants: ???C + H2O (gas) → CO + H2; CO + H2O (gas) → CO2 + H2. The combined alkali production process involves two steps: the first step is identical to that in the ammonia-alkali process, where ammonia is introduced into saturated saltwater to form an ammonia-salt solution, after which carbon dioxide is added to produce sodium bicarbonate precipitate. Through filtration and washing, fine crystals of NaHCO3 are obtained, which are then calcined to yield soda ash. The filtrate consists of a solution containing ammonium chloride and sodium chloride. The second process is the crystallization of ammonium chloride from the filtrate containing ammonium chloride and sodium chloride. Since ammonium chloride has a higher solubility at room temperature than sodium chloride, its solubility is lower at low temperatures. Moreover, the solubility of ammonium chloride in concentrated solutions of sodium chloride is much lower than its solubility in water. Therefore, under low-temperature conditions, by adding fine powdered sodium chloride to the filtrate and introducing ammonia gas, ammonium chloride can be precipitated as a separate crystal; after filtration, washing, and drying, the ammonium chloride product is obtained. At this point, the filtrate obtained after filtering out the ammonium chloride precipitate is essentially saturated with sodium chloride and can be recovered for reuse. ??The simple process of its industrial production is shown in the figure. ?Process flow for production by the combined soda-making method?? Compared with the ammonia-soda method, the greatest advantage of the combined soda-making method is that it allows the utilization rate of table salt to exceed 96%, enabling the production of more soda ash using the same amount of table salt. Furthermore, it makes use of carbon dioxide from ammonia plants and chloride ions from alkali plants, simultaneously producing two valuable products—soda ash and ammonium chloride. The carbon dioxide from the ammonia plant’s waste gases is converted into a key raw material for soda ash factories, thereby saving the large number of lime kilns used in those factories to produce carbon dioxide ； The useless component in alkali plants, chloride ions (Cl–), is used to replace the more expensive sulfuric acid in ammonia plants to fix ammonia and produce the nitrogen fertilizer ammonium chloride. As a result, calcium chloride, which is of little use and difficult to handle, is no longer produced; this reduces environmental pollution. Moreover, it **lowers the costs of soda ash and nitrogen fertilizers, fully demonstrating the advantages of large-scale integrated production.