FR2544719A1 - Process for the preparation of monohydrotrihalosilane from residual silicon - Google Patents

Abstract

Process which increases the degree of use of silicon which takes part in the synthesis of organohalosilanes by the direct process. It consists in bringing the residual silicon into contact simultaneously with gaseous hydrogen halide and gaseous alkyl halide to form a contact mass of residual silicon and in heating the contact mass of residual silicon at a temperature below the temperature at which significant alkylation of the contact mass of residual silicon takes place, by virtue of which the residual silicon reacts at this temperature to provide reaction products containing monohydrotrihalosilane. Application to the synthesis of monomethyltrichlorosilane.

Description

 The present invention relates to a process for treating residual silicon powder, and more particularly to a process for the production of halosilanes from residual silicon powder.

The current commercial method of making organohalosilanes is well known and is described in the US Pat.
US Pat. No. 2,380,995. This patent describes the direct reaction of an organohalogenide, such as methyl chloride, with silicon particles to produce organo-chlorosilanes. Copper particles are mixed with these silicon particles, thus forming a reactive mass or a reactive contact mass. In commercial practice, this reaction is generally carried out in one of three types of equipment: the reactor type stirred bed as described in U.S. Patent No. 2,449,821 the fluidized bed reactor as described in US Pat.
United States of America No. 2,389,931; or the oven turning.

Organotrichlorosilanes and diorganodichlorosilanes are the two basic products of the direct process reaction described above. These compounds are used in the production of organopolysiloxane resins as described in US Pat.
United States of America Nos. 2,258,218 to 2,258,222. Other products include organopolysiloxane fluids as described in US Pat. Nos. 2,469,888 and 2,469,890, as well as elastomers. organopolysiloxane described in US Patent 2,448,756.

Currently, it is recommended to produce diorganodichlorosilanes commercially because they are generally used to produce linear polysiloxane fluids and polymers used in the production of thermally vulcanized rubber elastomers and room temperature vulcanizable silicone rubber compositions of various types.

 At a certain stage in the production of organochlorosilanes by the direct process, the reactive mass or reactive contact mass containing silicon particles becomes less reactive, and it is desirable to replace the reactive contact mass. Thus, used or less reactive silicon particles are removed from the reactor, and new silicon particles are added after which the reaction is restarted. When the reactive contact mass becomes less reactive or is worn, and is replaced by a new charge of silicon, this contact mass used or less reactive by residual silicon, residual silicon powder, contact mass containing residual silicon or residual contact mass, and these terms are used interchangeably.A known solution to this problem was to reject the residual contact mass, however, there is a lot of reactive silicon remaining in the residual contact mass, and It is desirable for reasons of economy and to avoid waste disposal problems, to use at least a portion of the silicon remaining in the residual contact mass.

Much research has been done to find a method to further utilize the residual silicon particles in the reactor used to effect direct organochlorosilane synthesis, such that the weight ratio of organotrichlorosilanes (known as
T) to diorganodichlorosilanes (designated D) can be maintained at a desired level for a longer period of time, thus resulting in maximum utilization of the silicon particles to produce diorganodichlorosilanes. In the production of the organochlorosilanes by the direct process, the The weight ratio of triorgancchlorosilanes to diorganochlorosilanes (T / D) is desirably about 0.1 during organochlorosilane production and preferably does not exceed about 0.35. However, it has been found that in most commercial manufacturing operations the ratio will be about 0.15 when the reaction is started with new materials, but after a reaction period it will rise to more than 0.2. One of the breakthroughs in this field is the method disclosed by US Patent No. 3,133,109. This patent discloses that silicon particles can be more fully utilized and can be maximized. the amount of diorganodichlorosilanes by passing spent particles from a fluidized bed reactor into an external fluid energy mixer. As a variant of the external fluid energy mixer, this patent describes the passage of the spent silicon particles which are recycled from the reactor into a set of sonic jets located at the base of the reactor to create a reduction of the particles or the fragmentation of the silicon particles by following the collision of the particles against each other or on the walls of the reactor.

 It was found that by using the process of this patent, a larger amount of diorganodichlorosilanes could be obtained from the same amount of silicon so that the ratio could be maintained near the desired level of 0.15 and would remain below 0. , 35 for a longer period. However, it has also been found that this process causes the underutilization of about 12 to 15% of the silicon that has been introduced into the reactor and that must be removed from the process as silicon waste. It was also considered that this silicon was worn or exhausted, and therefore was no longer usable.

 It is desirable to use this rejected silicon to prevent problems normally encountered in waste disposal and for reasons of economy. In addition, it is still desirable to find novel processes which can utilize substantially all of the valuable silicon remaining in the residual silicon-containing contact masses after the T / D ratio has risen to an undesirable level.

 U.S. Patent No. 4,481,149 discloses a method for treating silicon particles in such a silicon reactor system and thereby improving the utility of the metal silicon particles. The method comprises treating silicon particles generally having an average diameter of less than forty micrometers, after which these particles are abraded to remove the surface coating and these abraded particles can be sent to the reactor for further use.

 U.S. Patent 4,307,242 discloses another method for recovering and recycling silicon particles in an organochlorosilane reactor system.

The described method consists in classifying the contact mass of the direct process by particle size after which the more strongly poisoned or impure (spent) silicon particles are separated from the relatively non-poisoned silicon particles, and only these non-poisoned particles are recycled, thus improving the use of silicon. Thus, instead of rejecting the total mass of spent silicon particles from the direct process, only a small fraction of the spent silicon particles must be discarded at any given time. In U.S. Patent No. 4,307,242, the effluent powder (residual contact mass or residual silicon) is sent to one or more mechanical cyclones for recovery. This effluent powder is generally the spent reaction mass from the reactor that produces the organotrichlorosilanes and diorganodichlorosilanes. The crude products T and D are recovered at the top of the cyclones and these products may contain small amounts of very fine entrained particles. The remainder of the reaction mass is pneumatically treated in Hechanical cyclones and is sent to a receiving hopper for rejection. It is desirable to obtain useful products at any described step and, in particular, the relatively non-poisonous fraction of the reactions. fine particles of the secondary cyclone resulting from the method of classification of this US Pat. No. 4,307,242. It is also desirable to obtain useful products from the residual silicon contact masses obtained from any other source.

Such a useful product that can be obtained from various reactions with silicon is trichlorosilane also referred to herein as monohydrotrichlorosilane (HSiCl 3). A study of the effect of HC1 on the synthesis of methylchlorosilanes is shown on page 138 in voorhoeve Organohalosilanes,
Precursors to Silicones, (organohalosilanes, precursors of silicones) published by Elsevier in 1967. Voorhoeve mentions reactions made with metallurgical grade silicon in the presence of a copper catalyst with different molar ratios of methyl chloride: hydrogen chloride ranging from 6: 1 to 1: 6. However, all of these reactions were performed at 300 C, and there is no indication of any temperature selectivity made by Voorhoeve.

 U.S. Patent No. 2,488,487 disclosed increased yields of the most valuable "monoalkyl silicon halides" by the simultaneous introduction of a hydrogen halide, for example HCl, together with a halide. when it comes into contact with silicon or an alloy or mixture of silicon with metal at an elevated temperature, Although this patent indicates a temperature range of 2000C to 5500C, it indicated that the mixture of hydrogen chloride with starting methyl chloride resulted in increased yields of demethyl silicon chloride, and it did not indicate improvements or high yields of monohydrotrihalosilane or any effect of temperature selectivity.

 In a method of treating spent metal reaction masses from the process for the direct production of organohalosilanes, US Pat. No. 2,803,521 mentions that the reaction of methyl chloride and silicon at 2000-5000C , usually in the presence of copper or copper chloride, and often with HCl as the added reagent, is the best known and most widely used application in the direct process trade.According to this patent, this particular reaction produces various methylchlorosilanes, such as CH3SiCl3, (CH3) 2SiCl2, (CH3) 3SiCl and CH3HSiCl2. However, this patent treats reaction masses containing spent (residual) silicon by dispersing the reaction mass in water or dilute hydrochloric acid and contacting the dispersed mass with a source of chloride at a temperature of 20 ° C. at 1000C. The silicon particles settle and are separated from the supernatant liquor, and the metal salts in the supernatant solution are precipitated, collected and reused as a fresh catalyst. This patent does not suggest the preparation of monohydrotrichlorosilane from the metallic reaction mass. used, nor does it mention any effect of selectivity of the temperature.

 Among the well-known uses of trichlorosilane are the hydrosililation reaction of trichlorosilane (monohydrotrichlorosilane) to make organofunctional silanes, and the use of trichlorosilane as a feedstock in the manufacture of hyper-pure silicon as described in US Pat. GB Patent Application 2,028,289, issued March 5, 1980. Recent developments in the semiconductor industry have created a growing demand for hyper-pure silicon for electronic devices, photovoltaic solar cells, etc. GB patent application 2,028,289 further discloses that trichlorosilane and silicon tetrachloride are made by the reaction of silicon and hydrogen chloride. In addition, United States Patent No. 2 380,995 mentions the reaction of hydrogen chloride with silicon as described by Combes in Compt. makes. 122, 531 (1896) where a mixture of about 80% trichlorosilane and 20% silicon tetrachloride was obtained by passing hydrogen chloride through an iron tube filled with silicon heated to 3000C-4400C. Thus, it is desirable to provide additional sources of trichlorosilane, to provide economical processes for the manufacture of trichlorosilane and to provide a synthesis of trichlorosilane production in which the yield of trichlorosilane is improved. It is also desirable to provide improved methods of management of the contact mass containing residual silicon obtained during the preparation of organohalosilanes by the direct process.

Accordingly, the present invention aims to
to provide a method of using the residual silicon obtained during the preparation of the organochlorosilanes
to provide an improved process for obtaining trichlorosilane from silicon,
providing a process for preparing trichlorosilane from residual silicon contact masses;
to provide a process for improving the amount of monohydrotrihalosilane (trihalosilane) obtained by reacting silicon with an alkyl halide in the direct metal-catalyzed process
to provide a process for improving the yield of monohydrotrichlorosilane (trichlorosilane) obtained from residual silicon-containing contact masses from the organochlorosilane preparation by direct metal catalyzed processes.

 According to the present invention, monohydrotrihalosilanes are prepared from the residual silicon from the preparation of the organohalosilanes by the direct metal-catalyzed process, which comprises contacting the residual silicon simultaneously with the gaseous hydrogen halide and with the gaseous alkyl halide to form a residual silicon contact mass and to heat the residual silicon contact mass to a temperature below the temperature at which a significant alkylation of the residual silicon contact mass occurs, whereby the residual silicon reacts at this temperature to produce reaction products containing monohydrotrihalosilanes. According to the present invention, the choice of the temperature at which the residual silicon contact mass is heated is critical, and must be a temperature at which the alkylation of the silicon contact mass is inhibited or delayed and at which the halogenation of the residual silicon contact mass is favored. In general, the temperature at which the hydrohalogenation of silicon in the residual silicon contact mass takes place is between about 2000C and about 350 C. The selectivity of the temperature may be otherwise expressed as the temperature below the temperature at which takes place a significant alkylation of the residual silicon contact mass.

 According to certain objects of the present invention, there is also described a process for improving the amount of monohydrotrihalosilane obtained by reaction of silicon with alkyl halide in the direct metal-catalyzed process, which consists in contacting the silicon simultaneously with gaseous alkyl halide and with a gaseous hydrogen halide to form a silicon contact mass, select a temperature between about 200BC and about 3500C at which the alkylation of the silicon contact mass is inhibited or delayed and at which the hydrohalogenation of the silicon contact mass takes place or is favored; and heating the silicon contact mass to that selected temperature, whereby the silicon reacts with the gaseous alkyl halide and the gaseous hydrogen halide at the selected temperature to provide improved yields of monohydrotrihalosilane.

 By simultaneously introducing gaseous hydrogen chloride and gaseous methyl chloride to a residual silicon contact mass, it has been demonstrated by the process of the present invention that the alkylation reaction between methyl chloride and silicon is suppressed when this reaction is carried out at the selected reduced temperatures. Although there is some sacrifice in the use of silicon using the method of the present invention compared to the use of silicon of a reaction between metallurgical grade silicon and gaseous hydrogen chloride at room temperature. the absence of methyl chloride, the process of the present invention remains desirable in cases where sources of methyl chloride are readily available.However, it has been demonstrated that there is increased use of silicon by the process of this invention. The invention relates to the use of silicon when using methyl chloride in the absence of hydrogen chloride.

It has also been found that silicon tetrachloride as well as monohydrotrichlorosilane is produced together with various methyltrichlorosilanes. However, by choosing a reduced reaction temperature, suppression of methylchlorosilane production was achieved by suppressing the alkylation while promoting hydrochlorination to produce monohydrotrichlorosilane.

 The present invention relates to a process for improving the yield of monohydrotrihalosilanes (HSiX3) where X is a halogen, by reacting silicon and in particular residual silicon obtained in the preparation of organohalosilanes by the direct metal-catalyzed process, with gaseous alkyl halide in the presence of gaseous hydrogen halide. Until now, the reaction was a residual alkylation reaction resulting in the production of alkyl silicon halides and only minor amounts of hydrohalogenation.

According to some of the objects of the present invention, and by controlling the temperature, ie, by carrying out the reaction at the lower selective temperatures, the yield of monohydrotrihalosilanes is improved, and in certain embodiments the product is predominantly monohydrotrihalosilane.

To obtain improved yields of monohydrotrihalosilanes, the temperature selectivity is critical, and the silicon or the residual silicon contact mass is heated to a temperature below the temperature at which the high alkylation of silicon or residual silicon contact mass.

 The predominant silicon alkyl halides resulting from the alkylation of silicon are clearly indicated in US Pat. Nos. 2,488,487 and 2,803,521. The results of alkylation and hydrohalogenation by contact with metallurgical grade silicon with methyl chloride and hydrogen chloride in various ratios at a constant temperature are also illustrated in the previously mentioned British patent application.

It can be seen from this US patent application and from examples of US Pat. No. 2,488,487 that without the use of temperature selectivity, silicon alkylation is the predominant reaction when Methyl and hydrogen chloride come into contact with silicon in the presence of a metal catalyst at elevated temperatures. The patent of the States
United States of America No. 2,803,521 discloses the various methylchlorosilanes obtained by reacting methyl chloride and silicon at 200 ° C.-500 ° C. in the presence of copper and hydrogen chloride as added reagents. In the process of the present invention, the alkylation reaction is suppressed, inhibited or retarded while promoting the hydrohalogenation reaction by carefully selecting and carefully adjusting the reaction temperature. The British patent also shows that this phenomenon can take place in altering the molar ratio of methyl chloride to hydrogen chloride while maintaining a constant temperature at 3000C. Thus, by increasing the amount of hydrogen chloride, the British patent shows that the amount of trichlorosilane in the product increases, while the amount of various methylchlorosilanes decreases.

 When methyl chloride is used as the gaseous alkyl halogenide and hydrogen chloride as gaseous hydrogen halide, and the reaction is carried out using the direct process using a suitable catalyst, such as that, copper, the product is generally composed of silicon tetrachloride (SiC14), monohydrotrichlorosilane, otherwise designated herein by trichlorosilane (HSiCl3), methylhydrodichlorosilane (CH3HSiCl2), methyltrichlorosilane (CH3SiCl3), dimethyldichlorosilane ((CH3) 2SiCl2) , trimethylchlorosilane ((CH3) 3SiCl) and a residue generally consisting of several components including hexachlorodisilane (C13Si-SiC13) and various siloxanes.The residue is generally characterized as the high boiling fraction and comprises all component having a boiling point greater than 70bC. As indicated above, according to the process of the present invention, and according to a preferred embodiment in which the gaseous alkyl halide and methyl chloride is the gaseous hydrogen halide and the hydrogen chloride, the expected product is monohydrotrichlorosilane.

 In the preferred embodiments of the process of the present invention, the silicon is a residual silicon, such as a contact mass containing residual silicon obtained from the synthesis by the direct process or the direct process preparation of organochlorosilanes which uses a metal catalyst in the reaction. In the process of the present invention, the residual silicon is a silicon that contains a metal catalyst because it is the spent contact mass of the direct process that uses a metal catalyst, or a metal catalyst can be added to the mass of the catalyst. silicon contact if there is no metal catalyst, or if it is desirable to supplement insufficient amounts of metal catalyst in the residual contact mass.In general, according to the method of the present invention, the mass of Residual silicon containing contact may contain other adjuvants and additives which have been added during the reaction in the direct process, or additives and adjuvants which do not interfere with the process of the present invention but which are added to promote the hydrohalogenation reaction or for any other desirable purpose.

 The synthesis or the direct process preparation of the organosilanes is described in US Pat. No. 2,380,995. The present invention can be carried out without further treatment of the remaining residual silicon-containing contact mass, for example , after.

However, in the recommended embodiments, the contact mass containing residual silicon is further treated to remove any part of the residual contact mass which poisons or interferes with the hydrohalogenation reaction, for example, by reduction. separation, treatment with agents to dissolve or extract non-reactive substances, etc.

In a preferred embodiment, the contact mass containing residual silicon obtained by the direct process synthesis can be subjected to the process described in US Pat. No. 4,307,242. According to the process of this patent, sends the silicon-containing contact mass referred to as "effluent contact mass powder", or also "fine silicon particles", to an aerodynamic centrifugal classifier. Such a classifier is an apparatus capable of classifying and separating fine particles of the cyclone into distinct fractions according to the particle size. In the most general case, the relatively coarse particles are recycled to the organochlorosilane reactor, and the thinnest fraction is discarded or otherwise used. However, according to US Pat. No. 4,307,242, the thinnest contains the highest percentages of impurities other than silicon, and a determination must be made to establish what size fraction will be rejected and what fraction will be recycled. Thus, in a process for purifying the silicon contact mass from a reaction system for organohalosilanes by the direct method according to the US Pat.
United States of America No. 4,307,242, the particle size of a portion of the contact mass of the reactor is analyzed; the mass of contact analyzed is classified into a relatively pure first fraction and a second relatively impure fraction; and the first and second fractions are separated.

The silicon particles or powder of the effluent contact mass can be collected in a secondary cyclone and sent into a receiving hopper and then into a transfer hopper from which these silicon particles pass into a mechanical classifier which preferably is an aerodynamic or centrifugal classifier.

These particles are defined here as silicon particles collected from a secondary cyclone used to separate the spent silicon particles obtained during the production by the direct organochlorosilane process.

In the preferred embodiments of the present invention, the relatively pure second cyclone classified particles are contacted simultaneously with hydrogen halide gas, such as hydrogen chloride, and with halide. gaseous alkyl, such as methyl chloride, to form a residual silicon contact mass; at a temperature between about 2000C and about 3500C at which the alkylation of the residual silicon-containing contact mass is inhibited or delayed and wherein the halogenation of the residual silicon-containing contact mass is promoted; and heating the residual silicon-containing contact mass to the selected temperature, whereby the silicon reacts with the gaseous alkyl halide and the gaseous hydrogen halide at the selected temperature to produce improved yields of monohydrotrihalosilane.

In another embodiment, the residual silicon source is the reduced silicon particles from the process and apparatus described in the US Pat.
No. 3,133,109. In this patent, the silicon-containing spent particles of a fluidized bed reactor are passed through an external fluid energy mill or passed through a set of sonic jets. located at the base of the reactor to cause a reduction or fragmentation of the silicon particles as a result of the collision of the particles with each other or on the walls of the reactor. Thus, any contact mass containing residual silicon which has been ground, pulverized or disintegrated by particle shock of the silicon-containing mixture by compression, percussion, grinding, attrition, tc., may be used in the process of the present invention.

 In some cases, it is possible to use a virgin silicon powder that has never been used in a process such as those described above as a source of silicon. Thus, any finely divided silicon, preferably free of any impurities which may cause contamination of the final product, may be used in the process of the present invention together with a suitable catalyst such as the well known catalyst of the present invention. prior art.

 The size of any residual silicon or silicon particles that can be used in the process of the present invention is not critical, and the residual silicon or silicon may be in any finely divided form, and for optimum results, the silicon in the reactor generally has an average particle diameter of between about 20 microns and about 200 microns. Preferably, at least 25% by weight of the silicon particles have actual diameters between about 20 microns and about 200 microns. As indicated above, it is possible to use in the process of the present invention a wide variety of silicon alloys, residual silicon alloys, or mixtures of silicon or residual silicon and metals.Among silicon alloys or mixtures that can be used include calcium-silicon, calcium-manganese-silicon, manganese-zirconium-silicon, iron-silicon, titanium-silicon, zirconium-silicon, copper-silicon, intimate mixtures of silicon with copper, nickel, tin or silver, etc.

 Although there is no limitation for the apparatus or device in which the method of the present invention can be implemented, this apparatus or apparatus must be an apparatus or device in which the residual silicon or silicon may be suitably be contacted with the alkyl halide and the hydrogen halide and maintained or adjusted at the temperature (s) selected for the hydrohalogenation reaction. The reactor or reaction chamber may be suitably sized and shaped and preferably made of a material which is not corroded or which does not react with the reagents and products and / or by-products. The apparatus or device must be capable of heating the silicon or the silicon-containing contact mass to the critical temperature or within the critical temperature range as previously discussed. The process of the present invention can be carried out continuously, intermittently or discontinuously. Any suitable technique of the prior art can be used to carry out the method.

 One method for carrying out the process of the present invention is to bring the mixed alkyl halide with hydrogen halide above and through a fixed bed of residual silicon maintained at the selective temperature at which the The alkylation is delayed or inhibited and the hydrohalogenation favored. In another embodiment, the process can be carried out by simultaneously passing alkyl halide and hydrogen halide vapors through an externally heated rotating barrel containing the residual silicon while keeping the barrel heated to selective temperatures.

Yet another mode of carrying out the process of the present invention is to preheat the vapor mixture to the selective temperature at which hydrohalogenation is favored and the alkylation is inhibited prior to contacting with residual silicon.

 The introduction of gases and / or vapors into the reaction chamber is not critical and the hydrogen halide gas and the alkyl halide gas or vapor can be mixed before their introduction into the reaction chamber or tank or they can be introduced into the reaction chamber or tank separately. Any conventional metering or measuring means may be used to provide a suitable ratio of gases in the reaction chamber or vessel. It is within the scope of the present invention to provide multiple gas feed streams to the reactor. In a preferred embodiment, the process is carried out by forming a fluidized bed of residual silicon or silicon, and the residual silicon or silicon is brought into contact with the appropriate gases or vapors while the residual silicon or silicon is in the Fluidised bed mode. Various embodiments can be used to maintain a fluidized bed of residual silicon such as the use of a gas or vapor of sufficient velocity to maintain the residual silicon in a fluidized state in the reactor. This can be done by injecting the alkyl halide and / or the hydrogen halide into the particulate residual silicon at a rate suitable for fluidizing the particulate product. Optimum gas and / or vapor velocity can be determined by the skilled artisan and depends on various operating conditions. When using a stirred bed reactor, or when using a rotary kiln, the velocity of the gas or The flow rate may be less than the minimum rate necessary for the fluidization of silicon in the reactor, and may more accurately reflect the stoichiometric amount of hydrogen halide and alkyl halide required for the reaction with silicon. Of course, other conventional fluidization means can be used to maintain the residual silicon bed in the fluidized state while it is in contact with the alkyl halide and the hydrogen halide, and it enters also in the context of the present invention to use a combination of means for maintaining the residual silicon bed in the fluidized state. Although this is not generally desired because requiring significant ventilation means, it is possible to use an inert gas, for example, nitrogen, for fluidizing or completing other fluidization means, the contact mass containing residual silicon. U.S. Patent 2,389,931 discloses a conventional fluidized bed reactor. Other means for contacting residual silicon with gaseous materials include a stirred bed reactor as described in the US Pat. No. 2,449,821 or using a conventional rotary kiln. It is also possible, by any conventional means, to provide means for heating the residual silicon bed in the reactor and to maintain this residual silicon bed at the selective temperature at which the hydrohalogenation reaction is favored and the alkylation reaction is inhibited. and both internal and external sources of heating, and / or cooling including heating of gases and / or vapors can be used in the process of the present invention. The documents cited above provide examples of conventional ways of heating beds.

 In the preferred embodiment of the present invention, the alkyl halide is methyl chloride (CH 3 Cl). Generally, the alkyl halides that can be used in the process of the present invention include those having the formula RnH2n + 1X where X is halogen, and n is an integer of 1 to 4. The alkyl group having 1 at 4 carbon atoms can be linear or branched.

The halogen can be chlorine, bromine, iodine or fluorine.

In the process of the present invention, a single alkyl halide in the reaction or a mixture of alkyl halides can be used.

 The recommended hydrogen halide that can be used in the process of the present invention is hydrogen chloride. However, in various embodiments, hydrogen bromide, hydrogen iodide or hydrogen fluoride can be used. It is also possible to use mixtures of hydrogen halides in the process of the present invention.

 The amount or proportions of hydrogen halide and alkyl halide used in the process of the present invention are not critical. Usually from 0.5 to 20 parts by volume of hydrogen halide per part of vaporized alkyl halide is used, but the hydrogen halide may be used in larger or smaller proportions as desired. In some embodiments, the molar ratio of alkyl halide to hydrogen halide is from about 6: 1 to about 1: 6. However, in some preferred embodiments, the molar ratio of alkyl halide to hydrogen halide is from about 3: 1 to about 1: 3.

In a preferred embodiment of the present invention, when the alkyl halide is methyl chloride, and when the hydrogen halide is hydrogen chloride, the molar ratio of methyl chloride to hydrogen chloride is about 3: 1. As mentioned above, it has been stated in the British patent cited that as the amount of hydrogen chloride increases, the amount of trichlorosilane increases during the reaction of hydrogen chloride and methyl chloride with silicon at a temperature of 3000C in the presence of a copper catalyst.

 The flow of gas or vapor in the process of the present invention is not critical, however, in preferred embodiments, it is adjusted to cause the consumption of about 0.5 or more of the alkyl halide. in a single pass through the residual silicon bed.

Since it is sometimes difficult to achieve a high conversion in a single pass, in which case a considerable portion, for example, from about 0.5 to about 0.8, vapors passing through the bed can be recycled through the bed so to cause additional consumption of the alkyl halide. Another advantage of this vapor recycle is that the temperature in the reaction zone is more uniform, and tends to require the reaction to take place over a larger portion of the residual silicon-containing bed rather than in localized portions of the bed. . When operating in this mode, some of the vapors passing through the residual silicon bed, leaving the reactor, can be removed from the reaction system and cooled to condense the contained products. The operation can be continued until the silicon in the residual silicon bed is consumed to the desired point. As explained above, batchwise modes are also possible, and in some cases the reactor may be initially charged with the alkyl halide and the gaseous hydrogen halide and maintained at the desired selective temperature until reaction is completed.

Completion of the reaction or substantially completion of the reaction can be readily determined by the analysis of the products shown in more detail below.

 As indicated above, the reaction temperature of the alkyl halide and the hydrogen halide with the residual silicon is critical and must be made selective to achieve the desired hydrohalogenation of the silicon. The contact mass of the residual silicon, that is, the residual silicon in the presence of hydrogen halide and alkyl halide, is selectively heated to a temperature between about 2000C and about 3500C depending on the variables of the process. Thus, to obtain the improved yields of monohydrotrihalosilane according to the process of the present invention, the residual silicon contact mass is heated in the presence of the gaseous hydrogen halide and the gaseous alkyl halide at a temperature below the temperature at which the high alkylation of silicon takes place or is favored.In general, during the preparation of monohydrotrichlorosilane, the residual silicon is heated in the presence of methyl chloride and hydrogen chloride at a temperature between about 2000C and about 250 ° C, the temperature being below the temperature at which substantial alkylation of the residual silicon occurs or at a temperature at which significant hydrohalogenation of the silicon takes place. Generally, it is considered that significant hydrohalogenation has been achieved when about 15% by weight, as determined by gas chromatography, of the product is monohydrotrihalosilane. In general, it is considered that the alkylation is important when the distribution of organohalosilanes in the product totals about 70% by weight or more as determined by gas chromatography.Thus, by improving the amount of monohydrotrihalosilane in the reaction product between the silicon of a residual silicon contact mass and the alkyl halide and the hydrogen halide, a temperature between about 2000C and about 3500C must be selected at which the alkylation of the silicon contact mass is inhibited and at which hydrohalogenation takes place this silicon contact mass, and the silicon contact mass is heated to this selected temperature whereby the silicon reacts with the gaseous alkyl halide and the halogenide of hydrogen gas at the selected temperature to produce the improved yields of monohydrotrihalosilane.

 The selectivity of the temperature can easily be determined by the product analysis by controlling either the product stream, the gas recirculation stream, the gas in the reaction chamber, etc., by gas chromatography or by any other means. equivalent means of measuring the composition of the gases. This control of the appropriate current or the chamber can be intermittent or continuous, and it can be carried out using any well known conventional, disposative and technical equipment. When the alkylation reaction takes place, and the organohalosilanes appear in undesirable amounts, that is to say in significant amounts in the stream of the controlled chamber, the temperature is lowered to suppress alkylation and promote hydrohalogenation silicon in the residual silicon mass. In addition, in some cases it may be necessary to adjust the temperature in a range that suppresses alkylation and promotes hydrohalogenation. For example, when the reaction becomes exothermic, after being initiated, it may be desirable to control the temperature in a range that suppresses silicon alkylation and promotes the hydrohalogenation of silicon. This can also be determined by controlling the gases in the streams and / or chambers as described above, and appropriate steps can be taken to stop the heating and / or to provide conventional cooling to maintain the selective temperature.From the foregoing, it is obvious that the selective temperature can also encompass a temperature range in which the alkylation reaction is suppressed and the hydrohalogenation reaction is favored.

 Certain variables that will affect the reaction between the silicon in the residual silicon contact mass with the alkyl halide and the hydrogen halide include the particular alkyl halide and hydrogen halide used as the and the conditions in which the reaction is carried out, such as the contact time of the reagents, the quality and grain size of the silicon, etc. In addition, the rate constant, K, differs for each reaction and under different conditions, and this has an effect on temperature selectivity, although from a practical point of view the temperature selectivity is more readily determined. for any set of reaction conditions, reagents, reactor equipment, etc., by regulating the product gases as described above.

 The reaction is usually carried out at atmospheric pressure, however, lower or much higher pressures, e.g., as much as 14 kg / cm 2 can be used.

If desired, it is also possible to add diluents, such as carbon dioxide, carbon monoxide, silicon tetrachloride, nitrogen, and various other inert gases.

 The products formed by the process of the present invention can be collected and separated by conventional methods and devices. Thus, the reaction product of the present invention can be readily recovered and the monohydrotrichlorosilane separated therefrom by any suitable technique.

A mode of recovery is to condense in a conventional condenser cooled to about -200C with a suitable refrigerant, for example, a methanol-based refrigerant.

This causes condensation of the monohydrotrichlorosilane, silicon tetrahalide, organohalosilanes and other by-products and the residue. They are collected appropriately, and can then be separated by any suitable separation technique, including fractional distillation.

 Generally, it is desirable to carry out the process of the present invention under anhydrous conditions to prevent the formation of hydrolysis byproducts and other undesired byproducts which decrease the yield of monohydrotrihalosilane. Therefore, in the recommended embodiments, arrangements will be made to exclude moisture from the system and reagents.

EXAMPLES
A 2.54 cm electrically heated stirred bed reactor made of a 2.54 cm diameter and 45.7 cm long stainless steel tube with special end plugs was used as the reactor. and a spiral stirrer driven discontinuously by an electric motor for agitating the bed. The section of the tube was divided into an upper reaction zone and a lower reaction zone, and each reaction zone was fitted into an independent heating element to maintain the zones at the desired temperature setting. The cylindrical tube or reactor was sufficiently insulated to prevent heat loss and thermal variations. An inlet at the bottom of the tube was provided for the source of gas and / or vapors, i.e., for the feed. alkyl halide and hydrogen halide. An outlet at the top of the tube was provided and equipped with a suitable condenser to ensure the collection of liquid trichlorosilane and any by-products and a suitable vent for the evolution of gas. Glass and plastic fittings and tubes suitable for reducing the corrosive effect of the reaction gases have been used. For these examples the reactions were carried out discontinuously.

 The residual silicon-containing contact mass used in the following examples consisted of secondary cyclone fine particles described in the method of U.S. Patent No. 4,307,242.

The fine residual silicon particles of the secondary cyclone were analyzed and contained about 2.1% by weight of carbon, 3.8% by weight of copper, 2.7% by weight of iron, 1.7% by weight of aluminum, 1.0% by weight of zinc, 69.0% by weight of silicon, the balance being amounts of chlorine, hydrogen and traces of other elements. The designated amount of fine particles of the secondary cyclone was placed in the stirred bed reactor system described above. The temperature of the reaction zones was maintained at the selected temperature for the duration of the experiment. Methyl chloride and / or anhydrous hydrogen chloride were metered into the reaction zone at the rates indicated for each example using linear mass flow meters.

The reactions were carried out at three temperatures, namely 3000C, 2500C and 2000C. Before the study of the reaction, the internal temperature profile of the reactor under nitrogen flow was established using a thermocouple probe. In this way, the settings of the temperature controller were determined. Under the reaction conditions, the external reactor temperatures (for both reaction zones) were continuously monitored.

 The flow rate of methyl chloride was standardized using a rotameter in series. The hydrogen choride co-feeds were performed using a linear mass flow meter with an appropriate correction factor.

 Reaction rates were determined by recovering the crude product per unit time. Crude fractions of methylchlorosilane were corrected, i.e., normalized, for unreacted methyl chloride based on gas chromatographic analysis. The use of silicon was calculated by subtracting the silicon consumed to form the product from the initial silicon level as determined by the alkaline melting method of the sample of the contact mass.

Any suitable silicon analysis can be used for this purpose.

 The crude product distribution was determined by gas chromatography. The residue or content of high boiling fractions was also determined by gas chromatography. In the case of comparative overall yields with feeds of methyl chloride, hydrogen chloride or hydrogen chloride / methyl chloride mixed at various temperatures, the contact masses were reacted until exhausted, ie that is, until there is no more formation of additional raw products.

Example 1
A sample of described secondary cyclone fine particles weighing 50.0 grams was placed in the reactor described above, and anhydrous hydrogen chloride was charged to the reactor at standard 100 cc. The reactor was kept at ambient pressure and heated to 3000C. The product was analyzed as described above, and about 76% of the secondary cyclone fines were converted into products of which about 94.4% were trichlorosilane and silicon tetrachloride.

About 4.6% by weight of the effluent was considered a high boiling residue, that is, it had a boiling point above 700C, about half of which was hexachlorodisilane. The results are shown for comparison purposes in Tables 1 and 2 below.

Example 2
All reaction conditions and amounts were the same as in Example 1 except that the stirred bed of the reactor was maintained at 2500C at ambient pressure.

The analytical data for this example are shown in Tables 1 and 2 below.

Example 3
The reaction conditions and amounts were the same as in Example 1 except that the stirred bed reactor was maintained at 2000C at ambient pressure. The data for this example are shown in Tables 1 and 2 below.

Example 4
In the same stirred bed reactor system as described for Examples 1 and 3 above, secondary cyclone residual silicon fine particles taken in the same batch as in the previous examples were reacted using a chlorine feed. methyl gas at 3000C and at ambient pressure as in Example 1 above.

Anhydrous hydrogen chloride was not used in this example. About 36% silicon utilization was achieved with about 92% of the effluent consisting of methyltrichlorosilane and dimethyldichlorosilane; 2.6% by weight of the product was a high boiling residue of which about 85% by weight was 1,1,2-trichloro-trimethyldisilane and 1,1,2,2-dimethyldisilane tetrachloro. The data in this example are shown in Tables 1 and 2 below.

Example 5
Another identical sample of secondary cyclone fines was processed according to the method of Example 4 except that the reaction was carried out at 2500C at ambient pressure. The data for this example are shown in Tables 1 and 2 below.

Example 6
Secondary cyclone residual silicon fine particles weighing 50.0 grams taken in the same batch as for the previous examples were reacted in the same reactor system at 3000C and at ambient pressure using both gaseous methyl chloride. and gaseous anhydrous hydrogen chloride so that the total molar flow rate of gas per unit of time is the same as that of the previous examples. The gases were assayed to obtain a 3: 1 molar ratio of methyl chloride to hydrogen chloride. Overall, about 64% silicon utilization was achieved.

About 22% by weight of the effluent was silicon tetrachloride and combined trichlorosilane. About 59% by weight of the effluent was methyltrichlorosilane and dimethyltrichlorosilane combined, and the remainder consisted predominantly of about 12% by weight of methylmonohydrodichlorosilane, about 2.5% by weight of trimethylchlorosilane, and about 3.1% by weight of residue and dimethyl monohydrochlorosilane. The data for this example are shown in Tables 1 and 2 below.

Example 7
The reaction conditions and amounts were the same as for Example 6 except that the reaction temperature was 2500C at ambient pressure.

The overall silicon utilization was 50 t. The data for this example are shown in Tables 1 and 2 below, and demonstrate that the use of silicon in desirable products, trichlorosilane and silicon tetrachloride, was approximately double by reducing the reaction temperature, ie to say, 13.2% at 3000C and 25.5% at 2500C.

Example 8
The reaction conditions and amounts were the same as in Examples 6 and 7 above except that the reaction temperature used was 2000C. The overall silicon utilization was 31%. The data for this example are shown in Tables 1 and 2 below. TABLE 1
RECAPITULATION OF PRODUCTS AT VARIOUS TEMPERATURES

Temperature <SEP> Distribution <SEP> of <SEP> products
<tb> Example <SEP> Gas
<tb> of <SEP> reaction
<tb> N <SEP> Power Supply <SEP> C <SEP> 2A <SEP> 3B <SEP> 4C <SEP> 5D <SEP> 6E <SEP> 7F
<tb> 1 <SEP> HCl <SEP> 300 <SEP> 68.08 <SEP> 26.38 <SEP> 0.10 <SEP> trace <SEP> 0.58 <SEP> 0.11
<tb> 2 <SEP> HCl <SEP> 250 <SEP> 69.82 <SEP> 25.63 <SEP> 0.04 <SEP> trace <SEP> 0.28 <SEP> 0.10
<tb> 3 <SEP> HCl <SEP> 200 <SEP> 74.20 <SEP> 16.39 <SEP> 0.01 <SEP> no <SEP> 0.06 <SEP> 0.09
<tb> determined
<tb> 4 <SEP> 1MeCl <SEP> 300 <SEP> trace <SEP> trace <SEP> 1.07 <SEP> 3.73 <SEP> 19.66 <SE> 71.88
<tb> 5 <SEP> 1MeCl <SEP> 250 <SEP> trace <SEP> trace <SEP> 0.15 <SEP> 3.49 <SEP> 10.83 <SEP> 80.14
<tb> (3: 1)
<tb> 6 <SEP> MeCl: HCl <SEP> 300 <SEP> 16.27 <SEP> 5.39 <SEP> 12.39 <SEP> 2.48 <SEP> 36.37 <SEP> 22.85
<tb> (3: 1)
<tb> 7 <SEP> MeCl: HCl <SEP> 250 <SEP> 38.13 <SEP> 10.89 <SEP> 8.32 <SEP> 0.67 <SEP> 20.57 <SEP> 14.33
<tb> (3: 1)
<tb> 8 <SEP> MeCl: HCl <SEP> 200 <SEP> 64.17 <SEP> 13.97 <SEP> 2.81 <SEP> trace <SEP> 9.21 <SEP> 1.36
<tb> 1 - MeCl represents methyl chloride 2 - A is trichlorosilane (HSiCl3) 3 - B is silicon tetrachloride (SiCl4) 4 - C is methyldichlorosilane 5 - D is trimethylchlorosilane 6 - E is methyltrichlorosilane 7 - F is dimethyldichlorosilane TABLE 2
SUMMARY OF REACTION CONDITIONS AND PRODUCT RATES

Temper- <SEP>% Si
<tb> Gas <SEP> ture <SEP> of <SEP>% If <SEP> used <SEP>% Si
<tb> Example <SEP> of food <SEP> reaction <SEP> use <SEP> for <SEP><SEP> pro <SEP> used
<tb> N <SEP> tation <SEP> C <SEP> total <SEP> dump <SEP> A + B2 <SEP> for <SEP> MCS * <SEP> 2A / B <SEP> 2E / F <SEP> % R **
<tb> 1 <SEP> HCl <SEP> 300 <SEP> 76.5 <SEP> 72.3 <SEP> 0.60 <SEP> 0.39 <SEP> --- <SEP> 4.60
<tb> 2 <SEP> HCl <SEP> 250 <SEP> 77.7 <SEP> 74.2 <SEP> 0.33 <SEP> 0.37 <SEP> --- <SEP> 4.05
<tb> 3 <SEP> HCl <SEP> 200 <SEP> 64.2 <SEP> 58.0 <SEP> 0.10 <SEP> 0.22 <SEP> --- <SEP> 9.16
<tb> 4 <SEP> 1MeCl <SEP> 300 <SEP> 38.2 <SEP> --- <SEP> 36.8 <SEP> --- <SEP> 0.27 <SEP> 2.94
<tb> 5 <SEP> 1MeCl <SEP> 250 <SEP> 38.4 <SEP> --- <SEP> 36.3 <SEP> --- <SEP> 0.14 <SEP> 5.39
<tb> (3: 1)
<tb> 6 <SEP> MeCl: HCl <SEP> 300 <SEP> 78.1 <SEP> 16.9 <SEP> 57.8 <SEP> 0.33 <SEP> 1.59 <SEP> 3.08
<tb> (3: 1)
<tb> 7 <SEP> MeCl: HCl <SEP> 250 <SEP> 58.6 <SEP> 28.7 <SEP> 25.7 <SEP> 0.29 <SEP> 1.44 <SEP> 6.45
<tb> (3: 1)
<tb> 8 <SEP> MeCl: HCl <SEP> 200 <SEP> 33.6 <SEP> 26.3 <SEP> 4.5 <SEP> 0.22 <SEP> 6.77 <SEP> 8, 06
<tb> * MCS represents methylchlorosilane ** R represents the residue (fraction boiling above 70 ° C) 1 - MeCl represents methyl chloride 2 - A, B, E and F are defined in Table 1.

From the above data, it can be seen that the secondary cyclone residual silicon fine particles and the hydrogen chloride readily react at 30 ° C., 25 ° C. and 200 ° C. to predominantly produce trichlorosilane and silicon tetrachloride. At comparable temperatures, the reaction with hydrogen chloride results in a much higher use of silicon compared to the reaction with methyl chloride (even at 200 C). Although the decrease in temperature from 3000C to 250C does not affect the total use of silicon in terms of reaction with methyl chloride or hydrogen chloride alone, there are marked differences in selectivity in the lower temperature reaction of methyl chloride, namely, a significantly lower overall E / F batch, reduced formation of methyl monohydrodichlorosilane and increased residue formation. A co-feed of reagent as exemplified by molar ratio 3: 1 methyl chloride: hydrogen chloride shifted the results as follows
a) at 25O0C, although the overall use of silicon
was lower than the 100% chlo mode
hydrogen, it was higher than that of the
100% methyl chloride mode.

(b) at 3000C, the overall use of silicon was
comparable to the reaction with a diet to
100% hydrogen chloride.

(c) at 3000C, a mixed feed of chlorine
methyl: hydrogen chloride was translated into
the higher use of silicon to form
methylchlorosilanes than a 100% reaction of
methyl chloride at the same temperature.

d) the reaction at 200 C clearly suppressed the reaction
alkylation, that is to say,
methyl and silicon, or resulted in a
sacrifice in the global use of silicon.

(e) the inclusion of hydrogen chloride results in
marked increase in E / F of methylchlorosilane product.

f) Residue formation increases when the
reaction temperature is decreased.

 From the foregoing, it is clear that hydrogen chloride and methyl chloride react with silicon-copper mixtures to yield trichlorosilane and silicon tetrachloride at selected low temperatures.

 In accordance with at least one of the subject-matter of the present invention, a process for the preparation of trichlorosilane in large quantities has been developed by means of a hydrochlorination reaction carried out under selected temperature conditions which suppress the alkylation reaction between methyl chloride and silicon. By changing the reaction temperature and carefully adjusting this temperature, improved yields of trichlorosilane and silicon tetrachloride are obtained. Moreover, the reaction in the mode of coalification, that is to say, feeding both with hydrogen chloride and methyl chloride in the presence of silicon at low temperatures between about 200 ° C. and about 3000 ° C., has shown an ability to selectively suppress the reaction of obtaining methylchlorosilane, that is, that is to say, an alkylation reaction, while continuing the hydrochlorination of silicon.

 Although the process has not been carried out using fresh or virgin silicon, the process of the present invention can be generalized using such species, and fresh or virgin silicon can be reacted with gaseous halide. at a temperature chosen between about 2000C and about 250 ° C at which the alkylation of silicon is inhibited and at which the hydrohalogenation of silicon takes place, whereby the silicon reacts with the alkyl halide gas and the halogenide of hydrogen gas at the temperature chosen to give improved yields of monohydrotrihalosilane

Claims (23)

 1. Process for the preparation of monohydrotrihalosilane from residual silicon obtained by the preparation of organohalosilanes by a direct metal-catalyzed process, characterized in that it consists in bringing the residual silicon into contact simultaneously with the halogenide. hydrogen gas and gaseous alkyl halide to form a residual silicon contact mass and to heat the residual silicon contact mass to a temperature below the temperature at which significant alkylation of the contact mass of residual silicon, whereby the residual silicon reacts at this temperature to provide reaction products containing monohydrotrihalosilane.  2. Method according to claim 1, characterized in that the gaseous hydrogen halide and the gaseous alkyl halide are brought into contact with a fluidized bed of reductive silicon.  3. Process according to claim 1, characterized in that the gaseous alkyl halide is chosen from the group comprising chloride, bromide, iodide or fluoride of an alkyl group having from 1 to 4 carbon atoms.  4. Process according to claim 1, characterized in that the gaseous hydrogen halide is chosen from the group comprising hydrogen chloride, hydrogen bromide, hydrogen iodide and hydrogen chloride.  5. Method according to claim 1 or 2, characterized in that the gaseous alkyl hydrogen halide is hydrogen chloride, the gaseous alkyl halide is methyl chloride, and the monohydrotrihalosilane is monohydrotrue ichlorosilane.  The process according to claim 1, characterized in that the molar ratio of the alkyl halide to the hydrogen halide is from about 6: 1 to about 1: 6.  7. Process according to claim 1 or 2, characterized in that the residual silicon contact mass is heated selectively at a temperature of between about 200 ° C. and about 300 ° C.  8. The method of claim 1 characterized in that it further comprises the collection of the reaction products and the preparation of monohydrotrihalosilane.  9. Process for the preparation of monohydrotrihalosilane from residual silicon obtained during the preparation of organochlorosilanes by the direct process, characterized in that it consists in supplying gaseous methyl chloride and gaseous hydrogen chloride to a fluidized bed. of residual silicon in a molar ratio of methyl chloride to hydrogen chloride of between about 6: 1 and about 1: 6 and heating the fluidized bed of residual silicon to a temperature between 200 C and 3000C, this temperature being lower at the temperature at which substantial alkylation of the residual silicon occurs to produce a product containing significant amounts of monohydrotrihalosilane and silicon tetrachloride.  10. The method of claim 9, characterized in that the molar ratio of methyl chloride to hydrogen chloride is between 3: 1 and 1: 3 approximately.  11. The method of claim 9, characterized in that the residual silicon is composed of fine particles of silicon collected from a secondary cyclone used to separate fine particles of spent silicon obtained during the process for the direct preparation of organochlorosilanes.  12. The method of claim 9, characterized in that it further comprises the collection of the product and the separation of the monohydrotrichlorosilane.  13. The method of claim 9, characterized in that the molar ratio of methyl chloride to hydrogen chloride is about 3: 1; the temperature is from about 200 ° C to about 250 ° C; and the predominant product is monohydrotrichlorosilane.  14. Process for improving the amount of monohydrotrihalosilane obtained by reaction of silicon with an alkyl halide in the direct metal-catalyzed process, characterized in that it consists in contacting the silicon simultaneously with the alkyl halide gaseous and with the gaseous hydrogen halide to form a silicon contact mass; selecting a temperature between about 2000C and about 3500C at which the alkylation of the silicon contact mass is inhibited and at which the hydrohalogenation of the silicon contact mass takes place; and heating the silicon contact mass to the selected temperature, at which point the silicon reacts with the gaseous alkyl halide and the gaseous hydrogen halide at the selected temperature to give improved yields of monohydrotrihalosilane.  15. Process for improving the amount of monohydrotrihalosilane obtained by reaction of residual silicon obtained as a result of the preparation of organohalosilanes by a direct metal-catalyzed process, characterized in that it consists in bringing the residual silicon into contact simultaneously with the gaseous alkyl halide and the gaseous hydrogen halide to form a residual silicon contact mass; selecting a temperature between about 2000C and about 3500C at which the alkylation of the residual silicon contact mass is inhibited and at which the hydrohalogenation of the residual silicon contact mass takes place; and heating the residual silicon contact mass to the selected temperature, whereby the silicon in the residual silicon contact mass reacts with the gaseous alkyl halide and the gaseous hydrogen halide at the selected temperature to give improved yields of monohydrotrihal osilane.  16. The method of claim 14 or 15, characterized in that the gaseous alkyl halide and the hydrogen halide gas are contacted with a fluidized bed of silicon or residual silicon.  17. The method of claim 14 or 15, characterized in that the gaseous alkyl halide is selected from the group consisting of chloride, bromide, iodide or fluoride of an alkyl group having 1 to 4 carbon atoms.  18. The method of claim 14 or 15, characterized in that the gaseous hydrogen halide is selected from the group consisting of hydrogen chloride, hydrogen bromide, hydrogen iodide and fluoride. hydrogen.  19. The method of claim 14 or 15, characterized in that the hydrogen halide gas is hydrogen chloride; the gaseous alkyl halide is methyl chloride; and the monohydrotrihalosilane is monohydrotrichlorosilane.  20. The method of claim 14 or 15, characterized in that the molar ratio of the alkyl halide to the hydrogen halide is between about 6: 1 and about 1: 6.  21. A process according to claim 14 or 15, characterized in that the molar ratio of alkyl halide to hydrogen halide is from about 3: 1 to about 1: 3.  22. Process according to claim 14 or 15, characterized in that the hydrogen halide gas is hydrogen chloride; The alkyl halide is methyl chloride; the molar ratio of alkyl halide to hydrogen halide is from 3: 1 to 1: 3; and the temperature selected is from about 200 to about 2500 C, whereby the silicon reacts with methyl chloride and hydrogen chloride to provide a product consisting essentially of monohydrotrichlorosilane.  23. The method of claim 14 or 15 further comprising collecting a reaction product and separating the monohydrotrihalosilane therefrom.