PROCESS FOR THE PRODUCTION OF SILICON TETRACHLORIDE

Abstract: A silica-containing material such as silica sand, a carbonized biomass or biomass ash is converted into carbon tetrachloride by reaction with chlorine gas at an elevated temperature in the presence of a carbonaceous material such as coke, and sulfur or a sulfur compound. The reaction may be suitably performed in the further presence of a potassium compound.

Description: This invention relates to a process for the production of silicon tetrachloride by chlorination of a silica-containing material. Silicon tetrachloride which is an important raw material for the production of organosilicon compounds, silicone resins, fine particulate silica, high purity artificial quartz, silicon nitride, silicon carbide, etc is generally prepared by the following three methods: (1) a method in which elemental silicon powder is chlorinated by reaction with HCl to produce trichlorosilane as a main product and silicon tetrachloride as a by-product; (2) a method in which ferrosilicon or silicon carbide is reacted with chlorine; and (3) a method in which a mixture of a silica-containing material such as silica stone and carbon is reacted with chlorine (see US-A-3197283). These conventional methods involve the following problems. Method (1) in which silicon tetrachloride is produced as a by-product has a problem because the economics thereof depends on the demand for trichlorosilane produced as the major product. Further, elemental silicon is not available at low cost since its production, generally performed by reduction of silica at a temperature of 2000°C or more in an electric furnace, consumes much electrical energy. Method (2) is also disadvantageous from the standpoint of economy since the raw material, i.e. ferrosilicon or silicon carbide is expensive because the production thereof requires much electrical energy. Method (3) has a problem because the reactivity of a silica-carbon mixture with chlorine is low and requires a high reaction temperature, though the raw materials are easily available at a low price. EP-A-167156 teaches a method of making silicon tetrachloride wherein a mixture of silica and carbon and a catalyst such as phosphoroxychloride is reacted with chlorine. Furthermore, US-A-4082833 discloses the use of sulfur and various sulfur compounds, including carbon disulfide, to promote carbochlorination reactions, such as the carbochlorination of calcined clay according to the following equation: Al₂O₃.2SiO₂ + 7C + 7Cl₂ → 2AlCl₃ + 2SiCl₄ + 7CO The present invention seeks to provide an improved method which can produce silicon tetrachloride with a high yield using a silica-containing material such as silica stone as a raw material. In accordance with the present invention, there is provided a process for the production of silicon tetrachloride, comprising reacting a silica-containing material with chlorine in the presence of (a) a carbonaceous material, (b) sulfur or carbon disulfide and (c) a potassium compound selected from potassium carbonate, potassium chloride, potassium hydrogensulfate, potassium hydroxide, potassium nitrate or potassium sulfate. The silica-containing material to be used as a raw material in the process of the present invention may be, for example, silica sand, silica stone or combustion residues (ash) or carbonized materials of a biomass of silica-containing plants, for example leaves, stalks, straw and chaff of various silica-containing plants. Illustrative of suitable biomass are wheat chaff, rice hull, straw of wheat and rice plant, and leaves and stalks of corn, sugar cane, bamboo and rush. The use of ashes of biomasses as starting material for the production of silicon tetrachloride is known from JP-62/17012. The biomass combustion residues may be obtained by combusting the biomass at a temperature of 500-1100°C, preferably 600-900°C. Since the silica in the ash obtained from a high temperature combustion tends to form crystals and since the reactivity of silica is lowered when it forms crystals, the combustion is desirably performed at a temperature below 900°C. The carbonized biomass which can be used as the silica-containing material in the process of the present invention may be obtained by carbonizing the biomass at a temperature of 300-1200°C, preferably 600-1000°C in an inert atmosphere such as nitrogen, argon or helium. The silica contained in the silica-containing material is preferably amorphous though crystalline silica such as cristobalite, tridymite or quartz may be used. In the present invention, the chlorination of the silica-containing material is performed in the presence of a carbonaceous material, sulfur or carbon disulfide and one of a number of potassium compounds. The carbonaceous material to be used in the present invention may be, for example, pitch, coke, carbon black, activated carbon or a carbonized biomass and may be in the form of a liquid or solid. When a silica-containing, carbonized biomass is used as the silica-containing material, the use of a separate carbonaceous material may be omitted. The carbonaceous material is preferably used in such an amount as to provide a weight ratio of SiO₂ to carbon of about 10:1 to 1:1, more preferably 2.5:1 to 1.5:1. Although sulfur itself can be used we prefer to employ carbon disulfide which, like sulfur itself, may be in the form of a gas, liquid or solid. Since, carbon disulfide can provide carbon necessary in the chlorination of silica, a separate carbonaceous material may be omitted or the amount thereof may be decreased when the chlorination of the silica-containing material is performed in the presence of carbon disulfide. The amount of sulfur or carbon disulfide is preferably 1:1000 to 1:1, more preferably 1:1000 to 1:2 in terms of the weight ratio thereof to the chlorine fed for the chlorination to the reaction zone. It is an advantageous feature of the present invention that the chlorination is performed in the presence of a potassium compound which improves the yield of the silicon tetrachloride. The potassium compounds used herein are potassium carbonate, potassium chloride, potassium hydrogensulfate, potassium hydroxide, potassium nitrate and potassium sulfate. The potassium compound may be in the form of a solid or liquid, and is preferably used in an amount of 0.05-50 weight %, more preferably 0.1-10 weight % based on the weight of silica in the silica-containing material. The chlorination of the silica-containing material is preferably performed at a temperature of 400-1100°C, more preferably 600-1000°C and may be carried out in various ways. For example, the carbonaceous material, the sulfur or carbon disulfide and the potassium compound may be mixed with the silica-containing material prior to the commencement of the chlorination. It is preferred that the mixture be well commingled by means of, for example, a ball mill prior to the chlorination for reasons of improved conversion of the silica-containing material. In a preferred embodiment according to the present invention, the above mixture is pulverized so that the particle sizes of the silica-containing material and the carbonaceous material are, for example, about 10 µm or less and about 50 µm or less, respectively, more preferably about 4 µm or less and 15 µm or less, respectively. The mixture which may be in the form of powder or may be shaped into pellets is supported in the form of a bed in a reaction zone to which a chlorine-containing gas which may be a pure chlorine gas or a chlorine gas diluted with nitrogen or other inert gas is fed for contact with the bed. The bed may be a fixed bed or a fluidized bed. This method may be adopted even when one or more of the carbonaceous material, the sulfur or carbon disulfide, and the potassium compound are liquid. Chlorine is generally used at least in a stoichiometric amount. When the sulfur or carbon disulfide is in a gaseous form, it may be supplied to the reaction zone simultaneously with the chlorine-containing gas for contacting with the bed of a mixture of the silica-containing material, carbonaceous material and potassium compound. The sulfur or carbon disulfide even when it is in the form of a liquid or a solid may also be fed to the reaction zone simultaneously with the chloride-containing gas using a liquid pump or a powder feeder. The gas discharged from the reaction zone and containing unreacted chlorine and silicon tetrachloride is fed to a separator where the silicon tetrachloride is separated, for example, by condensation. The gas from which silicon tetrachloride is separated may be recycled to the reaction zone, if desired. The following examples will further illustrate the present invention. In the examples, combustion residues of rice hull, a carbonized product from rice hull and natural silica sand were used as silica-containing raw materials. The combustion residues of rice hull were those obtained by combusting rice hull in a furnace at a temperature in the range of 800-1000 °C at an excess air ratio with a rice hull feed rate of 450 kg/hr and with a residence time of 4 hours. The rice hull residues (ash) were found to contain 96.5 % by weight of SiO₂ and 2.1 % by weight of residual carbon. The carbonized rice hull was that obtained by heating rice hull (20 g), packed in a quartz glass tube with an inside diameter of 55 mm, at 700 °C for 1 hour under a nitrogen stream supplied at a rate of 2 liter/minute. The carbonized product was found to contain 47.4 % by wight of SiO₂ and the balance of essentially carbon. The natural silica sand contained 97.7 % by weight of SiO₂. The SiO₂ content herein was analyzed after treating the sample with nitric acid and perchloric acid. The carbon content was determined by ignition loss by thorough combustion in air. Oil coke (petroleum oil coke) having the following analytical values or the above-mentioned carbonized rice hull was used as carbonaceous material. Proximate analysis (equilibrated sample) Elementary analysis (water- and ash-free sample) The silica-containing materials and the carbonaceous materials were each ground for 5 minutes with a planetary ball mill having an inside volume of 12 ml. Comparative Examples 1-12 A quantity of the silica-containing material as shown In Table 1 was mixed with a quantity of the carbonaceous material as shown in Table 1 to obtain a mixture having a weight ratio of SiO₂ to C of 2:1. The mixing was performed with a planetary ball mill for 1 minute (Comparative Examples 1-3 and 6) or 10 minutes (Comparative Examples 4, 5 and 7-12). Each mixture (or carbonized rice hull in Comparative Examples 7 and 8) was then subjected to chlorination, in the presence of sulfur in Comparative Example 5 and in the presence of CS₂ in Comparative Examples 1-4, 7, 9 and 11. Thus, the above mixture (1 g) was put in a boat made of alumina and this boat was set in the center of a quartz glass reaction tube with an inside diameter of 30 mm. The quartz glass tube supporting the mixture-containing boat therein was heated to 900°C at a heating rate of 26°C/minute in an argon gas stream flowing at a rate of 100 ml/minute through the reaction tube. After maintaining the reaction tube at 900°C for 30 minutes, the feed of argon gas was stopped and chlorine gas was fed at a rate of 100 ml/minute for 1 hour while maintaining the reaction tube at 900°C. In Comparative Examples 1-4, 7, 9 and 11, CS₂ was also introduced into the reaction tube, simultaneously with the feed of the chlorine gas, by means of a liquid pump at a feed rate as shown in Table 1, in which the amount of the CS₂ is given as percentage based on the weight of the chlorine feed and calculated on elemental sulfur basis. In Comparative Example 5, the reaction tube was set in a vertical position and elemental sulfur was supplied by means of a powder feeder. The downstream of the reaction tube was connected to a condenser (methanol-dry ice trap) so that the gas discharged from the reaction tube was cooled for the recovery of silicon tetrachloride. After the termination of the chlorination, the reaction mixture remaining in the alumina boat was allowed to be cooled to room temperature while feeding argon and then analyzed for determining the conversion of SiO₂ into SiCl₄. The conversion was determined from the contents of SiO₂ before and after the chlorination. The results are summarized in Table 1. Examples 1-6 Comparative Example 4 was repeated in the same manner as described except that each of the potassium compounds shown in Table 2 was further incorporated into the mixture of the rice hull ash and the coke in an amount of 10 % based on the weight of the SiO₂ contained in the rice hull ash. The results were as shown in Table 2 together with the result of Comparative Example 4. Example 7 Comparative Example 7 was repeated in the same manner as described except that potassium hydrogensulfate was mixed with the carbonized rice hull in an amount of 10 % based on the weight of the SiO₂ contained in the carbonized rice hull. The result is shown in Table 2 together with that of Comparative Example 7. Example 8 Comparative Example 9 was repeated in the same manner as described except that potassium hydrogensulfate was further incorporated into the mixture of the natural silica sand and the oil coke in an amount of 10 % based on the weight of the SiO₂ contained in the silica sand. The results were as shown in Table 2 together with the result of Comparative Example 9. From the results shown in Tables 1 and 2, it will be apparent that while the addition of sulfur or CS₂ improves the conversion, the addition of a potassium compound improves the conversion still further. Examples 9-13 Example 1 was repeated in the same manner as described except that the amount of the coke in the mixture was varied to provide a SiO₂/C weight ratio as shown in Table 3 and that the mixing time was changed from 10 minutes to 1 minute. In Example 13, no coke was used. The results are shown in Table 3. Examples 14 and 15 Examples 14 and 15 Example 13 was repeated in the same manner as described except that the feed rate of the CS₂ was increased as shown in Table 3. The results are shown in Table 3. Comparative Example 13 Example 13 was repeated in the same manner as described except that the CS₂ was not supplied. The result was as shown in Table 3. From the results shown above, it will be seen that the conversion becomes higher as the amount of the coke is increased. Even if no coke is used, the conversion can be improved by increasing the amount of CS₂.

Claims: 1. A process for the production of silicon tetrachloride, comprising reacting a silica-containing material with chlorine in the presence of (a) a carbonaceous material, (b) sulfur or carbon disulfide and (c) a potassium compound selected from potassium carbonate, potassium chloride, potassium hydrogensulfate, potassium hydroxide, potassium nitrate or potassium sulfate. 2. A process as claimed in Claim 1, wherein said potassium compound is used in an amount of 0.1-50 % by weight based on the weight of the silica contained in said silica-containing material. 3. A process as claimed in Claim 1 or Claim 2, wherein said reaction is performed at a temperature of 400-1100°C. 4. A process as claimed in any preceding claim, wherein said carbonaceous material is used in an amount providing a SiO₂/C weight ratio of 10:1 to 1:1. 5. A process as claimed in any preceding claim, wherein the reaction is performed with a weight ratio of the sulfur or carbon disulfide to the chlorine of 1:1000 to 1:1. 6. A process as claimed in any preceding claim, wherein said chlorine is used at least in a stoichiometric amount. 7. A process as claimed in any preceding claim, wherein said carbonaceous material is pitch, coke, carbon or a carbonized biomass. 8. A process as claimed in any preceding claim, wherein said silica-containing material is silica sand, silica stone, a carbonized biomass or combustion residues of a biomass. 9. A process as claimed in any preceding claim, wherein the silica of said silica-containing material is in the form of amorphous silica, cristobalite, tridymite or quartz. 10. A process for the production of silicon tetrachloride, comprising reacting a silica-containing material with chlorine in the presence of carbon disulfide and a potassium compound selected from potassium carbonate, potassium chloride, potassium hydrogensulfate, potassium hydroxide, potassium nitrate or potassium sulfate.

Inventors: NAKATA, YOSHINORI, OKUTANI, TAKESHI 2-17, TSUKISAMU HIGASHI, SUZUKI, MASAAKI 2-6, TSUKISAMU HIGASHI
Applicants: DIRECTOR-GENERAL OF THE AGENCY OF INDUSTRIAL SCIENCE AND TECHNOLOGY
Application year: 1988
Main classification: C01B33
IPC codes: C01B33