Aluminum Chloride Production Process

Aluminum chloride, an inorganic compound with chemical formula ALCL 3, is a compound of chlorine and aluminum. Aluminum chloride has a very low melting and boiling point, and it will sublimate and become a covalent compound. Molten aluminum chloride does not conduct electricity easily, unlike most salts containing halogen ions, such as sodium chloride.

AlCl3 is a "YCl3" structure with Al3+ cubic dense accumulation layer structure, while Al3+ occupies the adjacent tetrahedral gap of Br− dense accumulation frame in AlBr3. On melting, AlCl3 forms a volatile dimer, Al2Cl6, containing two three-center four-electron chlorine Bridges. At higher temperatures, Al2Cl6 dimer dissociates to form planar triangular AlCl3, similar in structure to boron trifluoride (BF3).

Aluminum chloride is a white crystalline powder. The vapor of aluminum chloride exists as a covalent dimeric molecule (Al2Cl6) when dissolved in nonpolar solvents or in molten state. Soluble in water and many organic solvents. An aqueous solution is acidic. In the presence of aromatic hydrocarbons, aluminum chloride mixed with aluminum can be used to synthesize di (aromatic) metal complexes. For example, chromium diphenyl is prepared by Fischer-Hafner synthesis of specific metal halides.

Aluminum chloride is an important inorganic chemical raw material, mainly used in the manufacture of detergent alkylating agent, synthetic drugs, synthetic dyes, synthetic rubber, detergent, plastics, spices and so on. At the same time, anhydrous aluminum chloride is also a very important catalyst, especially as the catalyst of the Freider-Crawford reaction has been widely used. In addition, aluminum chloride is expected to be used in the production of metal aluminum. 

If anhydrous aluminum chloride is used as raw material for aluminum electrolysis, it will have great potential in energy saving, emission reduction and greenhouse gas emission reduction. Due to the wide application field and potential market prospect of anhydrous aluminum trichloride, researchers at home and abroad have carried out a lot of research work on the preparation of anhydrous aluminum TRICHloride, and developed many kinds of production processes, this paper will introduce these processes. At the same time, due to the emergence of new resources, researchers put forward some new methods and ideas to prepare anhydrous aluminum trichloride.

1. The method of aluminum ingots

Aluminum ingot method is also known as aluminum metal method, that is, chlorine gas directly through molten aluminum metal, the two direct contact reaction to produce anhydrous aluminum chloride, reaction equation is:

2Al+3Cl2→2AlCl3                           (1)

The reaction temperature is generally controlled at about 800℃, and the gas phase product anhydrous aluminum chloride enters the product trap at about 400℃. After natural condensation and crystallization, anhydrous aluminum chloride is obtained. The tail gas is washed and absorbed by dilute alkali or lime milk water before being emptied.

The production of anhydrous aluminum trichloride by aluminum ingot method has the advantages of simple process, less equipment and less investment per unit product, so its fixed cost is low. However, the aluminum source of this method is metal aluminum, which leads to relatively high production cost. In order to reduce the cost of raw materials, some processes use mixed aluminum or part of mixed aluminum as raw materials. After the use of hybrid aluminum, due to the complex composition of impurities in the raw material, will lead to more impurities in the product.

2. Alumina

Alumina method is based on the reaction of alumina, chlorine and carbon to produce anhydrous aluminum chloride. In the late 1960s, foreign researchers have studied the reaction mechanism of anhydrous aluminum trichloride made by alumina. They believe that the reaction formula of anhydrous aluminum trichloride made by alumina is as follows:

Al2O3+(m+n)C+3Cl2=2AlCl3+mCO+nCO2          (2)

In Equation (2), m+2n=3. When m=0, n=3/2. In this case, Equation (2) becomes:

Al2O3+1.5C+3Cl2=2AlCl3+1.5CO2                  (3)

When n=0 in Equation (2), m=3, Equation (2) becomes:

Al2O3+3C+3Cl2=2AlCl3+3CO                    (4)    

Equations (3) and (4) are exothermic reactions, and the carbon consumption of Equation (4) is twice that of Equation (3). It can be seen that if the reaction can be carried out according to Equation (3), it is the best state of the reaction. If the reaction proceeds according to Equation (4), it is the worst state of the reaction. The reaction can also be between the best and the worst. Therefore, the key to the development of aluminum trichloride by alumina method is to design an appropriate reactor to make the reaction proceed to Equation (3).

3. Chlorination of aluminum-containing resources

The principle of chlorination process of aluminum-containing resources is the same as that of aluminum oxide powder chlorination process. Due to the large amount of impurities and complex composition in the ore, it is necessary to pretreatment the ore or purify the crude aluminum trichloride generated by chlorination. In the chlorination process of aluminum-containing minerals, many researches focus on ore pretreatment and crude aluminum trichloride purification. Holliday et al. prepared anhydrous aluminum trichloride from aluminum-containing minerals after two steps of treatment. 

The treatment steps of bauxite are as follows: (1) at 400~750℃, iron is removed in the form of iron sulfide in the atmosphere of CO and SO2; (2) At 430~750℃, 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 from the treated bauxite can be as low as 0.05% after general purification.

Because the boiling point difference between anhydrous ferric chloride and anhydrous aluminum chloride is small, ferric chloride is the key object to be removed in the purification process of crude aluminum chloride. The main separation methods are as follows: (1) condense the chlorinated gas to obtain solid ferric trichloride and aluminum trichloride, and then fractionation with scraper condenser to obtain the desired purity of aluminum trichloride products. Scraper condenser equipment is large, the investment cost and production cost are high. (2) first remove other impurities, get aluminum chloride 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 chloride. 

The process is easy to control, but a considerable amount of aluminum is needed, and the energy consumption is large in the process. (3) First remove other impurities with large boiling point difference, get aluminium trichloride containing ferric chloride, according to the two in titanium tetrachloride solubility difference (aluminium trichloride soluble, ferric chloride insoluble) to remove ferric chloride, and then aluminium trichloride 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 cooled preliminarily, and aluminum trichloride and ferric trichloride are captured in the steam with appropriate solvent. 

Other components can be separated and removed without the effect of solvent. The collector can choose KCl-NaCl-AlCl3, NaAlCl4, NaCl-AlCl3, KClAlCl3, MoCl6-FeCl3, SBCL3-ALCL3, AlBr3 and SbCl3. The collector containing aluminium trichloride was separated by distillation, and the collector could be recycled after treatment. The production of aluminium trichloride by the method of chlorination containing aluminium minerals requires a large amount of equipment. In this method, minerals that are not suitable for alumina production in industry can be used as raw materials, which is conducive to the comprehensive utilization of resources.

Fly ash is the solid waste after coal combustion, and its main chemical components include Al2O3, SiO2, Fe2O3, TiO2, CaO, MgO, etc. The content of alumina in high alumina ash is more than 40%, which is comparable to that in low grade bauxite in China. At present, China emits about 100 million tons of high alumina powder coal ash annually. High aluminum powder coal ash has become a potential resource to replace bauxite in alumina industry. In recent years, preparation of anhydrous aluminum trichloride from high-alumina coal 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 flow of fly ash drying, iron removal by magnetic separation, boiling chlorination by adding carbon and cooling separation. The thermodynamic calculation of the chlorination conditions of the main components in fly ash is carried out. According to the chlorination sequence of different substances, the conditions can be controlled, and it is possible to realize the selective chlorination of alumina. The recommended chlorination temperature is 800℃. It is suggested that the method of two-stage cooling combined with distillation and rectification should be adopted for refining anhydrous aluminum trichloride. 

The process conditions and equipment of each stage need to be further determined. Microwave heating has the advantages of fast heating speed, high thermal efficiency and small equipment footprint. At the same time, microwave heating is selective, which is conducive to the dissociation of minerals and plays a role in strengthening the reaction process. Lv Guozhi et al. introduced microwave heating into chlorination process.

4. Crystal aluminum chloride hexahydrate high temperature atmosphere protection dehydration method

The preparation of anhydrous aluminum chloride is difficult, while the preparation of aqueous aluminum chloride is easier. Therefore, aqueous aluminum chloride can be prepared from aluminum containing raw materials first, and then dehydration to obtain anhydrous aluminum chloride. Sinha and Hilld invented a method to prepare anhydrous aluminum trichloride from aluminum chloride hexahydrate. Firstly, aluminum chloride hexahydrate was heated at 200~450℃ until the hexahydrate was basically dehydrated. The dehydrated products were produced at 350~600℃. Anhydrous aluminum trichloride gas is produced by reaction with a gas mixture containing 40%~50% (volume fraction) chlorine, 30%~50% carbon monoxide, 5%~15% CARBON dioxide and 5%~15% hydrogen. This method can also be used to produce crystalline aluminum chloride from mineral raw materials, making full use of the advantages of resources, but the disadvantage lies in the large production equipment.

5. Crystal hexahydrate aluminum chloride organic salt dehydration method

On the basis of referring to the dehydration process of magnesium chloride alcohol hexahydrate ammonia method and double salt method, Ma Jiayu proposed the dehydration process of aluminum chloride hexahydrate double salt method and aluminum chloride hexahydrate ammonia method. The dehydration process of aluminum chloride hexahydrate double salt method is as follows:

(1) Aluminum chloride hexahydrate fluidized drying to remove most of the crystal water.

 

(2) The dried aluminum chloride product was dissolved in alcohol to prepare the alcohol solution of aluminum chloride, and a certain amount of ammonium chloride was added at the same time, and then vacuum distillation method was used to remove the water in the aluminum chloride alcohol solution. 

(3) AlCl3·NH3 crystal was formed by countercurrent contact reaction between the dehydrated aluminum chloride alcohol solution and ammonia gas. 

(4) The products after ammoniation are washed, filtered and dried, and the dried crystals are heated and decomposed to obtain ammonia and anhydrous aluminum trichloride mixed gas. After separation, solid anhydrous aluminum trichloride is obtained, and ammonia is recycled. 

The characteristics of the process are that the drying and dehydration process of aluminum chloride hexahydrate is easy to operate and the range of process conditions is wide. Anhydrous organic medium has less circulation, high yield and low energy consumption. The dehydration process of aluminum chloride alcohol ammonia hexahydrate mainly includes: 

(1) synthesis of organic hydrochloride, amine substances, imidazole or pyridine reacts with hydrochloric acid to obtain crude organic hydrochloride. Organic hydrochloride was obtained after the crude product was dried. 

(2) Synthesis of double salt, dried organic carbonate, aluminum chloride hexahydrate and water, heated stirring reaction, after cooling precipitation crystal, obtained by filtration of hydrated aluminum amines chloride double salt, hydrated aluminum imidazole chloride double salt or hydrated aluminum pyridine chloride double salt crystal. 

(3) the decomposition of double salt, double salt dehydration, to obtain chlorinated amine aluminum double salt, imidazole chloride aluminum double salt or pyridine chloride aluminum double salt without crystal water. 

The reaction temperature was controlled by removing the double salt of crystal water to remove aluminum trichloride from the double salt, and the sublimated aluminum trichloride gas was captured to obtain anhydrous aluminum trichloride, and the separated organic carbonate salts were recycled. Ma Jiayu et al. put forward the dehydration process and related conditions of aluminum chloride hexahydrate double salt process as follows: 

(1) Aluminum chloride hexahydrate fluidized drying dehydration at 110~170℃ until the crystal water content is less than 21.2%; 

(2) The dried aluminum chloride product is dissolved in the alcohol solution of aluminum chloride, and a certain amount of ammonium chloride is added at the same time, and then the vacuum distillation method is used to remove the water content of aluminum chloride alcohol solution is less than 1.5%. The alcohol used can be methanol, ethanol, n-propanol, n-butanol, ethylene glycol and glycerol one or more, ammonium chloride added 10%~15% of the content of aluminum chloride. Distillation temperature 100~150℃, vacuum 500~700mmHg. 

(3) AlCl3·NH3 crystal was formed by countercurrent contact reaction between the dehydrated ALCL 3 alcohol solution and ammonia gas in a fluidized bed. The crystallization temperature was 20-50 ℃, and the residence time was 30-150 min. (4) The product after ammoniation is washed, filtered and dried, and the dried crystal is decomposed by heating at 300~400℃ to obtain ammonia and anhydrous aluminum chloride mixture gas. After separation, solid anhydrous aluminum chloride is obtained, and ammonia is recycled. 

The characteristics of the process are as follows: the drying and dehydration process of aluminum chloride hexahydrate is easy to operate, the range of process conditions is wide controlled, the amount of anhydrous organic medium circulation is small, the yield is high, and the energy consumption is low. At the same time Ma Jiayu also put forward the related process conditions of aluminum chloride alcohol ammonia hexahydrate and applied for patent. Liu Xiquan proposed a preparation method of anhydrous aluminum trichloride. The method uses aluminum chloride hexahydrate crystal and thionyl chloride as raw materials, which are dehydrated under normal pressure or slightly negative pressure under 76℃. 

In this process, the crystal water reacts with thionyl chloride to produce sulfur dioxide and hydrogen chloride, and aluminum chloride hexahydrate is transformed into anhydrous aluminum trichloride. The slurry formed in the process of dehydration is filtered to obtain pure anhydrous aluminum trichloride, and the filtered slurry continues to be used. The sulfur dioxide and hydrogen chloride gas produced in the process of dehydration are treated by water absorption. The process is simple and easy to industrialize. Organic raw materials can be recycled in the whole process of crystalline aluminum chloride organic salt dehydration, which reduces the production cost and the emission of pollutants.

Anhydrous aluminum trichloride is an important chemical raw material, which has a wide range of applications and a huge potential market. High-alumina coal ash is a potential resource to replace bauxite in alumina industry in China. In recent years, the preparation of anhydrous aluminum chloride from high-alumina coal ash has become a research hotspot in China, and many new processes have been proposed. Researchers have carried out detailed theoretical research and laboratory practice on various methods. Many of the techniques are feasible, but there is still some distance from large-scale industrial production. Therefore, the development and design of production equipment suitable for the corresponding process is the focus of future research.