Anhydrous aluminum trichloride is an important inorganic chemical raw material. It is also expected to produce aluminum metal by direct electrolysis, which has great potential in energy saving and emission reduction and greenhouse gas emission reduction.
Fly ash is a solid waste after coal combustion. Its main chemical components are Al203, SiO2, Fe203, Tio2, Ca0, Mg0 and many other trace elements. Among them, Al203+SiO2+Ca0+Fe203 generally accounts for 90% (mass fraction, the same below), Al203 can reach 15%～40%, especially high alumina fly ash with Al203 greater than 30% is a good resource for preparing alumina. . At present, the use of high alumina fly ash to prepare alumina technology is still in the laboratory research and industrialization test stage. The main processes are divided into acid method, alkali method and acid-base combination method, but various methods have limitations. For example, the acid process has equipment corrosion, low dissolution rate, and the produced alumina quality index does not meet the requirements of electrolytic aluminum; the alkali process has complex process, high energy consumption, large residue, difficult comprehensive utilization of residues, poor comprehensive benefits, etc. .
In view of the above situation, we have proposed a new research idea: using anhydrous fly ash to obtain anhydrous aluminum trichloride, and then directly electrolyzing aluminum aluminum with anhydrous aluminum trichloride. The complete process is divided into three parts: fly ash is leached by hydrochloric acid, decontaminated, concentrated and crystallized to obtain aluminum chloride hexahydrate (ie, crystalline aluminum chloride), and dehydrated aluminum chloride hexahydrate to prepare anhydrous aluminum trichloride and Electrolytic aluminum trichloride is electrolyzed to produce metal aluminum. Among them, fly ash wet preparation of hexahydrate aluminum chloride has been researched and industrialized application demonstration report; anhydrous aluminum trichloride electrolysis method to produce metal aluminum as early as the 1970s, Alcoa has done a lot of research, and The production line of 15,000 t/a anhydrous aluminum trichloride was put into operation and it was actually operated for 4 years. The key to the success of this process is the dehydration of hexahydrate aluminum chloride to prepare anhydrous aluminum trichloride technology. At present, there are few reports on the preparation of anhydrous aluminum trichloride by dehydration of hexahydrate aluminum chloride.
The direct heating dehydration reaction of aluminum chloride hexahydrate (AlCl3.6H2O) is: AlCl3·6H2O=AlCl3+6H20(1) At the same time, AlCl3.6H2O will undergo hydrolysis reaction under high temperature conditions: 2(AlCl3.6H20)=Al203+ 6HCl + 9H20 (2) thermodynamic calculation of the reactions (1) and (2), the results are shown in Table 1. From the Gibbs free energy of Table 1, the reaction (2) can occur at 300 ° C, and the reaction (1) can occur at 600 ° C. That is, during the heating process, the hydrolysis reaction first occurs, and the higher the temperature, the more likely the hydrolysis reaction occurs. Therefore, direct pyrolysis of AlCl3.6H20 can only obtain Al203, and anhydrous aluminum trichloride is not obtained.
In order to inhibit the occurrence of the hydrolysis reaction, it has been proposed in the literature that AlCl3·6H20 is dehydrated under a hydrogen chloride atmosphere to prepare anhydrous aluminum trichloride. It can be seen from the data in Table 1 that when the temperature is 600 ° C, the dehydration and hydrolysis reactions proceed simultaneously, and the hydrolysis reaction tends to be much larger than the dehydration reaction. In a hydrogen chloride atmosphere, it is equivalent to increasing the partial pressure of hydrogen chloride in the reaction gas phase, thereby suppressing the hydrolysis reaction. The nature of anhydrous aluminum trichloride was investigated, and its melting point was 190 ° C (at 0.25 MPa), and sublimation occurred at 178 ° C. Under the high temperature condition of 600 °C, the anhydrous aluminum trichloride gas and water vapor generated by dehydration are sublimated at the same time. In the process of condensing gas, since anhydrous aluminum trichloride has strong water absorption, it will occur with water vapor. The reaction again produces AlCl3.6H2O, and anhydrous aluminum trichloride is not obtained. In addition, it has been reported in the literature that the dehydration of AlCl3.6H20 under a hydrogen chloride atmosphere does not result in anhydrous aluminum trichloride.
In addition, the literature reports on the dehydration of aluminum chloride hexahydrate in a protective atmosphere such as chlorine. The main method is as follows: heating the AICl3.6H20 basic dehydration at 200-450 °C, and then adding 40% to 50% chlorine gas, 30% to 50% CO, 5% to 15% CO2 and 5% to 15% at 350-500 °C. The hydrogen mixture is reacted to produce anhydrous aluminum trichloride gas. The essence of the method is to first obtain a mixed raw material containing anhydrous aluminum trichloride and aluminum oxide, and then chlorinate the alumina in the mixed raw material under the action of chlorine gas or the like to finally obtain an anhydrous product. However, the method uses toxic gas chlorine gas, requires high temperature and corrosion resistant production equipment and strict production control conditions, and the environmental pollution is serious. Therefore, the law is currently unable to achieve industrial applications.
The method uses aluminum chloride hexahydrate as a raw material, firstly formulating aluminum chloride hexahydrate into an aqueous solution, and then adding an alcohol to the solution. When the crystals in the alcohol solution are separated and solid-liquid separated, the crystal is anhydrous aluminum trichloride. Such as Zhang Haolong’s two patents is this idea, its patent 1 program is: AlCl3.6H2O is formulated into an aqueous solution, adding ethanol, propanol, etc., after the crystal is charged and analyzed, the solid-liquid separation is carried out in a centrifugal separator. The white crystal is obtained as anhydrous aluminum trichloride. The scheme of the patent 2 is: filling the ion exchange column with aluminum chloride hexahydrate, introducing ethanol into the liquid inlet of the exchange column, detecting the water content of the liquid at the outlet of the exchange column, and the water content of the liquid at the outlet of the exchange column When the water content is the same, it indicates that the dehydration is complete, and the alcohol is stopped. After the outlet of the exchange column is no longer dripping the liquid, the solid in the exchange column is taken out, and anhydrous aluminum trichloride is obtained.
Both of the above schemes directly remove the crystal water on the aluminum chloride hexahydrate through the alcohol solution. However, since aluminum chloride has a strong adsorption property to water, the added alcohol itself also contains a certain amount of water. Therefore, the characteristic that the water content does not change before and after washing only with the alcohol solution does not indicate that the crystal water is washed off (for example, the crystal water is not removed). Also, the water is not adsorbed, and the water content in the alcohol is also found to be constant). In addition, the separation of AlCl3.6H20 from crystal water is an intermolecular force that needs to overcome adsorption, requiring a large amount of energy, and the control condition for providing such energy is usually heating. Furthermore, the AlCl3 aqueous solution is mixed with the alcohol solution without significant heat release or exothermic heat to overcome the intermolecular force of AlCl3.6H20. Obviously, the above two inventions do not provide any external energy, and the purpose of dehydration cannot be achieved by merely adsorbing the water of AlCl3·6H20 by alcohol to overcome the interaction between molecules. Therefore, the above two process schemes are not feasible.
The dehydration of AlCl3.6H20 is still in the exploratory stage. There are few literatures. The dehydration method reported in the literature is not feasible at present. It is necessary to redesign the dehydration process of aluminum chloride hexahydrate. During the literature research, it was found that the properties of AlCl3.6H20 and MgCl2.6H20 have some similarities. The dehydration research of MgCl2.6H20 has done a lot of work, has a sound theoretical foundation, and has been successfully applied in industrial practice. In particular, the dehydration of ammonium chloride hexahydrate and the double salt method are environmentally friendly and have low energy consumption, which has become a hot topic in current research. In 2004, East China University of Science and Technology and Qinghai Salt Lake Group jointly built the first 1500t/a anhydrous magnesium chloride alcohol ammonia dehydration industrial demonstration unit and successfully produced 98.5% anhydrous magnesium chloride. On the basis of the dehydration process of ammonium chloride hexahydrate and double salt method, we briefly designed the dehydration process of hexahydrate aluminum chloride.
The method comprises two steps. The first step is to complex the aluminum chloride hexahydrate with an alcohol to cause the alcohol to replace most of the water of crystallization in the hydrate. However, due to the dehydration of pure alcohol, the incomplete dehydration of aluminum chloride and the difficulty in solid-liquid separation, the method adds a second step after the alcohol complexation, that is, the alcohol-containing aluminum chloride mixture and some The substance is chelated, and the added substance further attenuates the bonding force between the water and the aluminum chloride molecule in the aluminum chloride hydrate. The key to this method is to find alcohols that can complex with aluminum chloride, as well as substances that can chelate with aluminum chloride. After preliminary experiment and exploration, and considering the price and boiling point of the alcohol, the initially selected alcohol is ethylene glycol, and the substance chelated with aluminum chloride is ammonia gas, thereby designing the hexahydrate chlorination shown in Fig. 1. Alcohol ammonia dehydration process.
1) The aluminum chloride hexahydrate is removed by fluidized drying technology to remove most of the crystal water, and it is not necessary to prevent hydrolysis in the process.
2) After drying, the aluminum chloride is dissolved in an alcohol and an alcohol solution of aluminum chloride is added, and a certain amount of ammonium chloride is added to eliminate the influence of the hydrolysis by-product, and then the aluminum chloride is removed by vacuum distillation. Moisture in the solution.
3) The dehydrated aluminum chloride solution is operated countercurrently with ammonia gas to form AlCl3 in a fluidized bed reaction crystallizer. NH3 crystal.
4) Washing, filtering and drying to obtain AlCl3. The NH3 crystal is heated and decomposed by the dried crystal, and a mixed gas of ammonia and anhydrous aluminum trichloride is collected, and the mixed gas is cooled to room temperature to obtain solid anhydrous aluminum trichloride, and the separated ammonia is recycled.
1) The drying and dehydration process of aluminum chloride hexahydrate does not need to prevent hydrolysis, and it is easy to operate and has a wide range of process conditions.
2) Since the aluminum chloride hexahydrate has first removed most of the crystal water by drying, the amount of the organic solvent (alcohol) required for preparing the unit weight of anhydrous aluminum trichloride can be reduced, and the organic product of the unit product can be reduced. The amount of medium circulation increases the yield of aluminum chloride per unit of organic solvent and reduces energy consumption.
3) Reaction crystallization to prepare AlCl3. Before NH3, using dry and vacuum distillation method, almost completely remove water, eliminating moisture on AlCl3. The effect of NH3 crystal quality.
4) AlCl3. The NH3 reaction crystallization process uses a fluidized bed reaction crystallizer to enhance the mass transfer and heat transfer of the process and improve the uniformity of crystal particle size distribution. Of course, the process conditions of the various processes and the realization of the entire process still require further in-depth research and experimental verification.
This method is similar to the method in which dehydration of magnesium chloride hexahydrate by synthesis of carnallite or aniline hydrochloride (C6H5NH2.HCl). The key to this method is to find a compound that can form a double salt with hydrated aluminum chloride. The purpose of forming a double salt is to weaken the bonding force between the water molecule and the aluminum chloride molecule. Based on the above ideas, the dehydration process of the hexahydrate aluminum chloride double salt method shown in Fig. 2 was designed.
The law mainly includes:
1) Synthesis of an organic hydrochloride: an amine substance, imidazole or pyridine is added to a reaction crystallization tank, and then hydrochloric acid is added to carry out a reaction, followed by filtration to obtain a crude organic hydrochloride. The crude product was further dried to give the organic hydrochloride.
2) Synthesis of double salt: The dried organic hydrochloride is added to the reaction crystallization kettle, then adding aluminum chloride hexahydrate and water, and the contents of the reaction vessel are dissolved into a homogeneous phase under stirring and heating, and then cooled to The crystal is precipitated at room temperature, and the crystal obtained by filtration separation is dried to obtain a hydrated ammonium chloride-based aluminum double salt, a hydrated imidazolium chloride double salt or a hydrated pyridine aluminum complex double salt.
3) Double salt decomposition: The obtained double salt is added to a fluidized bed reactor for dehydration to obtain an amine chloride-based aluminum double salt containing no crystal water, an imidazolium chloride double salt or a pyridinium chloride double salt. Then, the double salt for removing the crystal water is added to another fluidized bed reactor, the temperature of the reactor is controlled to remove the aluminum trichloride from the double salt, and the sublimed aluminum trichloride gas is trapped to obtain anhydrous trichlorination. Aluminum, the separated organic hydrochloride is used for recycling.
The characteristics of this process are:
1) The double salt can be thermally decomposed at a lower temperature (200 to 240 ° C), which greatly reduces the cost of preparing anhydrous aluminum trichloride and saves energy.
2) The double salt does not produce corrosive gases such as HC1 and NH3 during the decomposition process, and is environmentally friendly.
3) The organic hydrochloride obtained after the decomposition of the double salt can be recycled, which greatly reduces the cost of producing anhydrous aluminum trichloride. Of course, the process conditions of each process of the new process require further exploration of the test.
1) At present, the dehydration method of hexahydrate aluminum chloride is mainly reported in the dehydration and alcohol dehydration under protective atmosphere, but it is not feasible under the current conditions.
2) The properties of aluminum chloride hexahydrate and magnesium chloride hexahydrate have some similarities. The dehydration process of magnesium chloride hexahydrate can provide reference for the dehydration of hexahydrate aluminum chloride.
3) It is recommended to use the alcohol ammonia method and the double salt method to dehydrate the aluminum chloride hexahydrate, but it needs to be thoroughly studied and tested.
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