Key technologies for the high-value utilization of by-product ammonium chloride in the soda ash industry

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The production process of soda ash generates large amounts of ammonium chloride as a by-product; it is currently mainly used as an agricultural fertilizer, and its potential for higher-value utilization is worth exploring. This paper proposes a new process for producing chloromethane by reacting ammonium chloride with methanol and achieving the recycling of ammonia. Catalysts were screened and modified through experiments, and process conditions such as reaction temperature, space velocity, and raw material ratios were investigated. 1. Introduction: During the production of caustic soda, a large amount of by-product ammonium chloride is generated. Currently, ammonium chloride is primarily used as a fertilizer in agriculture, and the surplus supply has kept its price at low levels. The development of downstream products based on it has therefore been a focus of attention. The chlorine content in ammonium chloride molecules is 66%. Theoretically, each ton of ammonium chloride produces 0.68 tons of hydrogen chloride or 0.66 tons of chlorine gas. By reacting ammonium chloride with organic compounds to introduce chlorine from its molecules into those of the organic compounds, the decomposition process allows only ammonia to be recovered, which simplifies the processes of purifying, compressing, and recovering hydrogen chloride or chlorine. It also helps to reduce corrosion of the equipment. Methyl chloride is mainly used as a raw material for the production of organosilicon polymers and for further chlorination to other halogenated hydrocarbons. In recent years, the rapid expansion of silicone production capacity has posed new demands on chloromethane production. If it becomes possible to produce chloromethane and ammonia through the reaction of ammonium chloride, a by-product of the ammonia-soda process, with methanol, this will not only promote the healthy development of China’s soda ash industry; it will also change the current situation where silicone manufacturers are obliged to build dedicated chloromethane production facilities. In the early 1950s, Olin Mathieson Chemical Company invented a method for producing chloromethane and ammonia by reacting ammonium chloride with methanol, proposing two types of reaction processes. In the 1980s, Mitsubishi Gas Chemical Company of Japan made improvements to the reaction process for producing chloromethane and ammonia through the reaction of ammonium chloride and methanol in its published patents; it also conducted further research on catalyst selectivity and methods for product separation. In the aforementioned published patents, fixed-bed or fluidized-bed reactors are used, and ammonium chloride is first decomposed before being fed into the reactor. Although the yield of chloromethane in some embodiments can reach a relatively high level, since the decomposition of ammonium chloride and the synthesis of chloromethane take place in two separate reaction stages, hydrogen chloride generated by the decomposition of ammonium chloride may, if it does not react quickly with methanol to be consumed, recombine with ammonia during transport due to temperature fluctuations to form ammonium chloride again, thereby blocking the pipes. Additionally, the energy consumption throughout the process is relatively high, and the corrosion of equipment caused by hydrogen chloride is also quite severe. In light of the current research status, this paper proposes a process for producing chloromethane through a one-step reaction using methanol and ammonium chloride as raw materials, under the action of a catalyst, in order to achieve high-value utilization of ammonium chloride and the recycling of ammonia. II. Experimental Principles and Procedures When ammonium chloride reacts with methanol, the main reaction that occurs is as follows: When both processes are carried out in a single reactor, the hydrogen chloride generated from the decomposition of ammonium chloride readily reacts with methanol. This prevents issues such as equipment corrosion and recombination of gases after decomposition, which would otherwise occur if ammonium chloride were decomposed separately. The experiment was conducted in a small fluidized bed reactor. After the reaction mixture was dried, the content of each component was analyzed by gas chromatography. III. Results and Discussion 3.1 Catalyst Screening Figure 1 shows the changes in the yield of chloromethane obtained using different catalysts as the temperature varies. The results show that the supported activated carbon catalyst F exhibits good activity, and the optimal reaction temperature is 370°C. Figure 1 shows the yield of chloromethane for different catalysts. Figure 3 shows the yield of chloromethane at various raw material ratios. Figure 2 illustrates the trend in chloromethane yield as the ratio of ammonium chloride to methanol changes; generally, the yield of chloromethane increases as this ratio rises, but the change is relatively gradual. Taking into account factors such as reaction pressure, overall raw material flow rate, and cost, a suitable raw material ratio is 1.1–1.5 of ammonium chloride to methanol. 3.3 Chloromethane yield at different space velocities Figure 3 shows the trend in chloromethane yield as the space velocity changes. It can be seen from the figure that when the space velocity exceeds 0.7 s-1, the yield of chloromethane decreases significantly. Since this reaction is a rapid reaction at high temperatures, the reaction time has little impact on the yield of the target product; therefore, the appropriate space velocity range should be 0.4–0.7 s-1. 3.4 Modification of the catalyst Based on the experiments above, the activated carbon catalyst was modified. The results show that when using an activated carbon catalyst loaded with 4% silica gel and 2% metal nitrate, both the yield and selectivity of chloromethane are relatively high, reaching 71.59% and 78.34%, respectively. The above studies show that it is technically feasible to produce chloromethane using ammonium chloride, a by-product of the soda ash industry, as raw material. Once the engineering challenges are addressed, this process can not only supply chloromethane as a raw material for the silicone industry, thereby increasing the value of its products, but it can also allow ammonia to be recovered and reused in the soda ash production process. This approach could serve as a model for the development of a circular economy in China’s soda ash production sector.