The main preparation methods of silane can be divided into silicon-magnesium alloy method, sodium aluminium hydride method, trichlorosilane disproportionation method and so on.


The main industrial processes for large-scale application include the trichlorosilane disproportionation method invented by the United Carbide Corporation of the United States (UCC) and the sodium aluminium hydride method developed by the Ethyl Company of the United States (now MEMC Company of the United States). The annual output is more than 1000 tons of silane. Up to now, the annual output has exceeded 30000 tons.


In the 1960s, the research on silane method was also carried out in the polycrystalline silicon battle of Beijing Chemical Plant II. The main route used was the silicon-magnesium alloy method. In the 1970s, a small-scale silane preparation device was developed by Zhejiang University. A high purity detection-grade polycrystalline silicon was prepared by the adsorption method of 4A and 5A molecular sieves developed by Zhejiang University. However, due to the high cost of silane production and the lack of large-scale industrialization, the domestic silane industry has not been able to develop, and the domestic demand for silane has always been dependent on foreign imports.


At present, there are only a few silane preparation technologies that have been used in mass production.
L Magnesium silicide process
L Sodium Aluminum Hydride Method
Disproportionation of L trichlorosilane


Introduction of Magnesium Silicide Process


Magnesium silicide method uses magnesium silicide powder and ammonium chloride to produce silane in liquid ammonia environment. Magnesium silicide process, also known as Komatsu process, is the earliest industrialized silane preparation technology in the world and one of the most mature preparation technologies in China. Because of the high cost, this process has not developed a production scale of 100 tons. The chemical reaction equation is as follows:


Mg2Si+NH4CL--MgCL2.6NH3+SiH4


The process includes the synthesis of magnesium silicide, silane synthesis, molecular sieve adsorption, ammonia tail gas absorption, etc. The main raw materials are silicon powder, magnesium powder, ammonium chloride, ammonia water, etc. These raw materials are well supplied in the market and relatively cheap. At the same time, the process is simple to operate, temperature (-20 C) and pressure (0.2MPA) are easy to reach.


But the shortcomings of this technology are also obvious.
First, if the drying of ammonium chloride is not complete, the yield of silane will be greatly affected. At the same time, because magnesium powder is easy to react with ammonia gas and other chemical reactions, the drying of magnesium powder and ammonium chloride is extremely strict to the environment, which virtually increases the cost.


Secondly, it is not easy to recover and utilize a large number of by-products, magnesium hexaammonia chloride.


Thirdly, because the reaction is carried out in the environment of liquid ammonia, the consumption of liquid ammonia is large, all of which increase the production cost and require high treatment of three wastes. Moreover, the single set of magnesium silicide process is not easy to be large-scale and suitable for small-scale production.


Introduction of Sodium Aluminum Hydride Process


The typical representative of metal hydride process is American MEMC Company. Silane gas was synthesized by reaction of sodium aluminium hydride with silicon tetrafluoride gas. The main reactions are:


The crude silane gas produced by the reaction is purified and refined by adsorption tower, deadweight tower and deadweight tower. The purity of crude silane gas is raised to more than 6N of high purity electronic grade silane gas. The liquid silane produced by low temperature liquefaction treatment is stored in the product silane storage tank. The liquid silane gas is converted into normal temperature silane gas by evaporation for use in the polycrystalline silane reduction section.


Characteristic:
In the manufacture of silicon tetrafluoride, sodium fluosilicate, a by-product of fertilizer enterprises, can be used as raw material, even if the raw material is made by fluorite sulfuric acid method, it is easy to obtain cheap raw material.


Sodium aluminium fluoride, a by-product, can be used as raw material of electrolytic aluminium (instead of cryolite), and the sulfate industry can be sold.


In liquid phase reaction, HF, the pyrolysis product of fluosilicic acid, is not discharged systematically, but returned to the first reactor as a fluorine source.


MEMC Company of the United States has been used for more than 1000 tons of scale 20 years ago, and its technology is mature.


Questions:
It consumes a large amount of high-price raw materials, sodium and aluminium powder, and the cost is relatively high.

Ultra-high purity semiconductors are difficult to use.

Introduction of Disproportionation Process of Silicon trichloride

The chlorosilane disproportionation method was developed by United Carbide Company (UCC Company) of the United States.

The main preparation method of silane is hydrogenation of silicon tetrachloride to produce trichlorosilane, and then the trichlorosilane is disproportionated to dichlorodihydrosilane, which is then disproportionated to silane again. The reaction equation is as follows:

The main characteristics of the process are as follows:


1. Disproportionation reaction is carried out in a tower reactor, which is very suitable for large-scale production with high production efficiency, low power consumption and low cost.
2. Closed circuit circulation of the whole system, with little discharge, is conducive to environmental protection and high utilization rate of materials;


3. Reaction products can be easily separated from silane, and high purity silane (9-11N) can be obtained.


4. All kinds of silicon hydrochloride remaining from silane distillation can be reacted in disproportionation column.


5. Reaction conditions are mild, energy consumption is low and easy to operate and control.


6. Silane, silicon tetrachloride, silicon dichloride and other silicon source materials can be produced, and the proportion of products can be matched at will.


This method has the highest purity of silane. Polycrystalline silicon with purity as high as 9-11N can be obtained by pyrolysis. This method has achieved the production level of 1000 tons scale. REC Corporation (formerly Asimi), the largest silane manufacturer in the world, uses this method to prepare silane gas.


Comparison of the advantages and disadvantages of three silane production processes


No matter what method is used to prepare silane gas, there are three main industrial processes for silane production in the world, namely Komatsu method, MEMC method and UCC method.


The first two processes are dangerous, by-products are environmentally friendly, pressure is high, and production costs are relatively high. Domestic enterprises have introduced their processes, but the current operation is not ideal.


Hydrodechlorination of trichlorosilicon disproportionation process is in line with large-scale chemical production, low cost, less environmental pollution, suitable for the expansion of polycrystalline silicon industry chain and recycling of by-products, as well as the expansion of product chain of trichlorosilicon plant. It is the best choice for Chinese polycrystalline silicon and trichlorosilicon enterprises in the future.