Abstract: Aluminum chloride anhydrous is an important industrial raw material, and its preparation method has received widespread attention. The preparation methods of anhydrous aluminum trichloride at home and abroad were reviewed, the characteristics of the preparation methods such as aluminum ingot method, alumina method, aluminum-containing resource chlorination method, crystalline aluminum chloride hexahydrate high temperature atmosphere protective dehydration method and crystalline aluminum chloride hexahydrate organic salt dehydration method were analyzed, and the preparation progress of anhydrous aluminum trichloride was elaborated. At present, the preparation of anhydrous aluminum chloride from fly ash is a hot spot in research, and the development of simple, economical and reasonable processes and supporting industrial production equipment will be the difficulties and priorities of future research.
0 Introduction
Anhydrous aluminum trichloride is an important inorganic chemical raw material, mainly used in the manufacture of detergent alkylators, synthetic drugs, synthetic dyes, synthetic rubber, detergents, plastics, fragrances and so on. At the same time, anhydrous aluminum trichloride is also a very important catalyst, especially as a catalyst for the Fred-Kleinford reaction. In addition, anhydrous aluminum trichloride is expected to be used in the production of metallic aluminum, and aluminum electrolysis using anhydrous aluminum trichloride as a raw material will have great potential in energy conservation and emission reduction and greenhouse gas emission reduction [1]. Due to the wide application field and potential market prospects of anhydrous aluminum trichloride, researchers at home and abroad have carried out a lot of research work around the preparation of anhydrous aluminum trichloride and developed many kinds of production processes, which will be introduced in this paper. At the same time, due to the emergence of new resources, scientific researchers have proposed some new methods and ideas for the preparation of anhydrous aluminum chloride.
1 Preparation process of anhydrous aluminum trichloride
1.1 Aluminum ingot method
The aluminum ingot method is also known as the metal aluminum method, that is, the chlorine gas is directly through the molten metal aluminum, and the two are in direct contact to form anhydrous aluminum trichloride, and the reaction equation is:
The reaction temperature is generally controlled at about 800 °C, and the anhydrous aluminum trichloride product enters the product trap at about 400 °C, and the anhydrous aluminum trichloride product is obtained by natural condensation crystallization. The exhaust gas is washed and absorbed by dilute alkali or lime emulsion and then emptied.
The aluminum ingot method for producing anhydrous aluminum chloride has a simple process, less equipment, and small investment per product, so its fixed cost is low. However, the method aluminum source is metallic aluminum, resulting in relatively high production costs. In order to reduce the cost of raw materials, some processes use miscellaneous aluminum or part of the miscellaneous aluminum as raw materials. After the use of miscellaneous aluminum, due to the complex composition of impurities in the raw material, more impurities will be caused in the product [2].
1.2 Alumina method
The alumina method is to produce anhydrous aluminum chloride by reacting alumina, chlorine and carbon as raw materials. Foreign researchers have studied the reaction mechanism of alumina method anhydrous aluminum chloride in the late 1960s, and they believe that the reaction formula of alumina method of anhydrous aluminum trichloride is:
m+2n=3 in equation (2), when m=0, n=3/2, equation (2) becomes:
When n=0 in equation (2), m=3, equation (2) becomes:
Equations (3) and (4) are both exothermic reactions, while formula (4) consumes twice as much carbon as equation (3). It can be seen that if the reaction can be carried out according to formula (3), the best state of the reaction is at this time; If the reaction is carried out according to equation (4), the worst state of the reaction is at this time; Reactions can also be performed between the best and the worst. Therefore, the design of a suitable reactor to carry out the reaction vector (3) has become the key to the development of alumina method of alumina [2].
The reaction equipment used is as follows: (1) fixed bed, the use of this equipment to prepare anhydrous aluminum chloride with alumina powder as the main raw material, coal as a reducing agent, pulp waste liquid as a binder, chlorine as an oxidant, reaction at 850 ~ 950 °C. The furnace gas with dust is cooled to obtain the coarse product AlCl3, and the coarse AlCl3 is refined to obtain a qualified purity product. The exhaust gas contains a small amount of chlorine, which is treated with a lye or sodium sulfite solution and emptied. Relatively speaking, the process of preparing anhydrous aluminum trichloride using this equipment is long, and ingredients and balls are required before chlorination. (2) Fluidized bed, the average particle size of 0.079mm alumina powder and 0.14mm petcoke into the roaster according to a certain proportion, into the chlorine and oxygen, temperature 950 °C, alumina powder in the presence of reducing agent carbon, reacts with chlorine, the gas phase product generated by the pre-cooling separator and pre-cooling tower, the entrained ash and chloride impurities in the gas are removed. The purified gas is cooled to about 250 °C and passed into four series of jacketed water-cooled traps, and then cooled and crystallized to obtain a powdered anhydrous aluminum trichloride, and discharged from the trap outlet. The non-condensing gas is washed in three tandem exhaust gas treatment towers and entered the turbulent tower sprayed with an aqueous solution of sodium sulfite to remove the unreacted chlorine gas in the tail gas and the hydrogen chloride gas generated by the hydrolysis of anhydrous aluminum trichloride. Compared with the fixed bed reactor, the fluidized bed reactor has the advantages of short process flow, high utilization rate of chlorine, low cost, low labor intensity, and small amount of three wastes[2]. (3) Salt bath, foreign countries after the 1970s attach importance to energy saving, so there are more studies on the preparation of anhydrous aluminum chloride by alumina method, of which the representative salt bath method is the salt bath method. The method uses alumina powder as the source of aluminum, substances containing carbon and chlorine as reducing agents and chlorides, and commonly used reducing agents and chlorides as coke and chlorine. Three kinds of raw materials are reacted in a salt bath to obtain anhydrous aluminum trichloride, and the resulting anhydrous aluminum trichloride is volatilized in a gaseous form. The main components of the salt bath used are AlCl3 and NaCl. In order to increase the chlorination rate, some metals or compounds thereof can be added to the salt bath system, including iron, chromium, copper, europium and cerium. The production of aluminum chloride by salt baths has the characteristics of simple process, less equipment and low investment. However, because the reaction is carried out in a salt bath system, the corrosion resistance of the reaction equipment is required to be high [3-4].
Zhou Xiaosong et al. [5] proposed a method for producing anhydrous aluminum chloride and applied for a patent. The method is: (1) the raw material containing more than 30% aluminum is leached with hydrochloric acid, the aluminum trichloride solution is obtained, and the concentrated crystallization is obtained to obtain aluminum chloride crystals hexahydrate. (2) Calcination of aluminum chloride hexahydrate above 300 °C to obtain primary alumina. (3) Primary alumina and chlorine gas react in the chlorination furnace when the reducing agent carbon is present, and the chlorination temperature is 800~1 200 °C. (4) The chlorination products were separated by three-stage cooling, the first two steps were cooled to remove ferric chloride, and the anhydrous aluminum trichloride was collected during the third step of cooling, and the three-stage cooling temperatures were: (280±10) °C, (260±5) °C and (160±2) °C. (5) The exhaust gas discharged after cooling is absorbed and treated.
1.3 Chlorination of aluminum-containing resources
The principle of the chlorination process of aluminum-containing resources is the same as that of the alumina powder chlorination method, due to the large number of impurities in the ore and the complex composition, it is necessary to pretreat the ore or carry out a complex purification treatment of the crude aluminum trichloride generated by chlorination. In the chlorination process for aluminium-containing minerals, many studies revolve around ore pretreatment and purification of crude aluminum trichloride. Holliday et al. prepared anhydrous aluminum trichloride from aluminum-containing minerals after two-step treatment. The treatment steps of bauxite are: (1) at 400~750 °C, iron is removed in the form of iron sulfide in CO and SO2 atmospheres. (2) At 430~750 °C, the remaining iron in bauxite is volatilized and removed in the form of gaseous ferric chloride. The loss of aluminum in the pretreatment process is less than 3%, and the iron content of anhydrous aluminum trichloride prepared with treated bauxite can be as low as 0.05% after general purification[6].
Due to the small difference between the boiling point of anhydrous ferric chloride and anhydrous aluminum trichloride, ferric chloride is the focus of removal in the purification process of crude aluminum trichloride. The main separation methods are as follows: (1) the chlorinated gas is condensed to obtain solid ferric chloride and aluminum trichloride, and then the required purity of the aluminum chloride product is obtained by fractionation with a scraped condenser. The scraped condenser equipment is huge, and the investment cost and production cost are high. (2) First remove other impurities, to obtain aluminum trichloride containing ferric chloride, and then use aluminum powder to reduce ferric chloride to ferrous chloride or even elemental iron, aluminum chloride and ferrous chloride (or iron) boiling point difference is large, can be fractionated to obtain high-purity aluminum trichloride. The process is easy to control, but it consumes a considerable amount of aluminum, and the energy consumption in the process is large. (3) First remove other impurities with large boiling point differences, obtain aluminum trichloride containing ferric chloride, according to the difference in solubility of the two in titanium tetrachloride (aluminum trichloride soluble, ferric chloride insoluble) ferric chloride is removed, and then aluminum trichloride is separated from titanium tetrachloride. The method is also easy to control, but the process requires a large amount of titanium tetrachloride as a solvent. (4) The steam after chlorination is initially cooled, and the aluminum trichloride and ferric chloride in the steam are harvested with a suitable solvent, and other components can be separated and removed without effect with the solvent used. The collectors are available in KCl-NaCl-AlCl3, NaAlCl4, NaCl-AlCl3, KClAlCl3, MoCl6-FeCl3, SbCl3-AlCl3, AlBr3 and SbCl3. Collectors containing aluminium trichloride are separated by distillation and can be recycled after treatment [7]. The production of aluminum trichloride by aluminum-containing mineral chlorination requires huge equipment, which can use minerals that are not suitable for the preparation of alumina as raw materials, which is conducive to the comprehensive utilization of resources [8].
Fly ash is a solid waste after coal combustion, and its main chemical components are Al2O3, SiO2, Fe2O3, TiO2, CaO, MgO and so on. Among them, the alumina content in high-alumina fly ash is as high as more than 40%, which is comparable to the alumina content in low- and medium-grade bauxite in mainland China. At present, the mainland emits about 100 million tons of high alumina fly ash per year, and high alumina fly ash has become a potential resource for the mainland alumina industry to replace bauxite. In recent years, the preparation of anhydrous aluminum trichloride from high alumina fly ash has become a research hotspot in China, and many new processes have been proposed. Referring to the preparation process of titanium tetrachloride, Ma Jiayu et al. proposed the process of fly ash drying, magnetic separation of iron, charcoal boiling and chlorination, and cooling and separation. The chlorination conditions of the main components in fly ash are thermodynamically calculated, and the conditions can be controlled according to the order of chlorination of different substances, and it is possible to achieve selective chlorination of alumina, and the recommended chlorination temperature is 800 °C. The refining of anhydrous aluminum chloride recommends a two-stage cooling combined with distillation and distillation, and the process conditions and equipment at each stage need to be further determined [9]. Microwave heating has the advantages of fast heating speed, high thermal efficiency and small equipment footprint, while microwave heating is selective, which is conducive to the dissociation of minerals and plays a role in strengthening the reaction process. Lu Guozhi et al. introduced microwave heating into the chlorination process [10].
Aluminium chloride
1.4 Crystalline aluminum chloride hexahydrate high temperature atmosphere protection dehydration method
Anhydrous aluminum trichloride is more difficult to prepare, while aqueous aluminum chloride is easier to prepare, so it can be prepared from aluminum-containing raw materials to obtain aqueous aluminum chloride, and then dehydrated to obtain anhydrous aluminum chloride. Sinha and Hilld invented a method for preparing anhydrous aluminum chloride from aluminum hexahydrate, first heating aluminum chloride hexahydrate at 200 to 450 °C until the hexahydrate compound was basically dehydrated, and the dehydration product was reacted with a chlorine containing 40% to 50% (volume fraction), 30% to 50% carbon monoxide, 5% to 15% carbon dioxide and 5% to 15% hydrogen gas mixture to produce anhydrous aluminum chloride gas [1]. This method can also prepare crystalline aluminum chloride from mineral raw materials, making full use of the advantages of resources, and the disadvantage is that the production equipment is large.
1.5 Crystalline aluminum chloride hexahydrate organic salt dehydration method
On the basis of drawing on the dehydration process of magnesium chloride alcohol hexahydrate and the dehydration process of the compound salt method, Ma Jiayu proposed the dehydration process of aluminum chloride hexahydrate and the dehydration process of aluminum chloride alcohol hexahydrate. The dehydration process steps of aluminum chloride hexahydrate compound salt method are: (1) aluminum chloride hexahydrate fluidization drying to remove most of the crystalline water. (2) The dried aluminum chloride product is dissolved in the alcohol to prepare the alcohol solution of aluminum chloride, and a certain amount of ammonium chloride is added at the same time, and then the water in the aluminium chloride alcohol solution is removed by vacuum distillation method. (3) The dehydrated aluminium chloride alcohol solution reacts with ammonia gas countercurrent to form AlCl3· NH3 crystal. (4) The products after ammonization are washed, filtered and dried, and the dried crystals are heated and decomposed to obtain a mixture of ammonia and anhydrous aluminum trichloride, and after separation, a solid anhydrous aluminum trichloride is obtained, and ammonia is recycled. The characteristics of this process are that the drying and dehydration process of aluminum chloride hexahydrate is easy to operate and the process conditions are wide range; Anhydrous organic media has a small amount of circulation, high yield and low energy consumption. The dehydration process of aluminium chloride ammonia method mainly includes: (1) the synthesis of organic hydrochloride, amine substances, imidazole or pyridine react with hydrochloric acid to obtain crude organic hydrochloride. Organic hydrochloride is obtained after the crude product is dried. (2) Synthesis of complex salts, dried organic hydrochloride and aluminum chloride hexahydrate and water, heating and stirring reaction, and then cooling precipitate crystals, hydrated amine aluminum chloride compound salt, imidazole hydrate aluminum complex salt or hydrated pyridine aluminum chloride compound salt crystal obtained by filtration. (3) Compound salt decomposition, compound salt dehydration, to obtain non-crystalline water-free amine aluminum chloride compound salt, imidazole chloride aluminum compound salt or pyridinium chloride compound salt. The reaction temperature of the complex salt of the crystalline water was controlled to remove the aluminum trichloride, and the sublimated aluminum trichloride gas was captured to obtain anhydrous aluminum trichloride, and the organic hydrochloride was recycled[1]. Ma Jiayu et al. proposed the dehydration process and related conditions of aluminum chloride hexahydrate compound salting method as follows: (1) Aluminum chloride hexahydrate 110~170 °C fluidized drying dehydration to crystalline water content less than 21.2%; (2) The dried aluminum chloride product is dissolved in the alcohol with an alcohol solution of aluminum chloride, and a certain amount of ammonium chloride is added at the same time, and then the water in the aluminium chloride alcohol solution is removed by vacuum distillation method of less than 1.5%. The alcohol used may be methanol, ethanol, n-propanol, n-butanol, ethylene glycol and glycerol one or more, ammonium chloride added amount of aluminum chloride content of 10% to 15%. Distillation temperature 100 ~ 150 °C, vacuum degree 500 ~ 700mmHg. (3) The dehydrated aluminium chloride alcohol solution reacts with ammonia in reverse contact with the fluidized bed to generate AlCl3· NH3 crystal. The crystallization temperature is 20~50°C, and the residence time is 30~150min. (4) The products after ammonization are washed, filtered and dried, and the dried crystals are heated and decomposed at 300~400 °C to obtain a mixture of ammonia and anhydrous aluminum trichloride, and after separation, solid anhydrous aluminum trichloride is obtained, and ammonia is recycled. The characteristics of this process are: the drying and dehydration process of aluminum chloride hexahydrate is easy to operate, the process conditions are widely controlled, the circulation of anhydrous organic media is small, the yield is high, and the energy consumption is low [11]. At the same time, Ma Jiayu also proposed the relevant process conditions of the aluminium chloride ammonia method hexahydrate and applied for a patent [12]. Liu Xiquan proposed a preparation method of anhydrous aluminum trichloride, which takes aluminum chloride hexahydrate crystal and thionyl chloride as raw materials, both dehydrated at below 76 °C, under atmospheric pressure or micro negative pressure, in the process, crystalline water reacts with thionyl chloride to generate sulfur dioxide and hydrogen chloride, and aluminum chloride hexahydrate is converted into anhydrous aluminum trichloride. The slurry formed during the dehydration process is filtered to obtain pure anhydrous aluminum trichloride, and the filtered slurry continues to be used. Sulfur dioxide and hydrogen chloride gases produced during dehydration are treated with water absorption. The process is simple and easy to industrialize [13]. Crystalline aluminum chloride hexahydrate organic salt dehydration method can recycle organic raw materials in the whole process, reducing production costs and reducing pollutant emissions.
2 Conclusion
Anhydrous aluminum chloride is an important chemical raw material with a wide range of applications and a huge potential market. High alumina fly ash is a potential resource for the mainland alumina industry to replace bauxite, in recent years, the preparation of anhydrous aluminum chloride from high alumina fly ash has become a domestic research hotspot, and many new processes have been proposed. Researchers have carried out detailed theoretical research and laboratory practice on various methods, and many processes are feasible, but there is still a long way to go before large-scale industrial production. Therefore, the development and design of production equipment suitable for the corresponding process is the focus of future research.