GB2304104A

GB2304104A - Process for producing boron trichloride

Info

Publication number: GB2304104A
Application number: GB9516557A
Authority: GB
Inventors: Anthony Copeland Jones; Graham Williams; Albert Barry Leese
Original assignee: Epichem Ltd
Current assignee: Epichem Ltd
Priority date: 1995-08-12
Filing date: 1995-08-12
Publication date: 1997-03-12
Also published as: GB9516557D0

Abstract

A process for producing boron trichloride comprises reacting a carbide of boron eg B 4 C or B 6 C with chlorine gas at an elevated temperature in a reactor (10) tube containing the carbide of boron (16) in finely divided form and having an inlet (12) for chlorine gas, an outlet (14) for vaporised product and means (18) eg a susceptor for heating the carbide of boron to a desired reaction temperature.

Description

TITLE: Process for producing boron trichloride DESCRIPTION This invention concerns a process for producing boron trichloride.

Boron trichloride is a useful starting material for various purposes. Boron trichloride has been used as a starting material in the production of, for example, boron nitride, lanthanum boride and catalysts for cationic polymerisation. Various processes have been described for preparing boron trichloride. In U.S.

Patent No. 2097482 boron trichloride was prepared by passing chlorine gas through a mixture of an oxygencontaining boron compound and free carbon heated to a temperature within the range of about 400 to about 7000C, the proportion by weight of oxygen-containing boron compound to free carbon being within the range of about 0.6 to about 1.5.

According to U.S. Patent No. 2369212 boron trichloride was prepared by passing chlorine through a mixture of boron oxide and carbon and heated to elevated temperatures above about 12000C to eliminate therefrom all moisture and absorbed atmospheric gases contained therein and to effect the consolidation of the mixture into a porous sintered product consisting largely of boron sub-oxides before chloridising the mixture.

In U.S. Patent No. 3019089 boron trichloride was prepared by reacting an oxygen-containing boron compound selected from boric oxides, boric acids and sodium borates with chlorine and carbon at an elevated temperature to produce a gaseous mixture comprising boron trichloride which upon cooling to a temperature below about 2500C deposited solids consisting primarily of a boron trichioride-boric oxide complex and spraying the gaseous mixture at a temperature above about 2500C with a liquid comprising essentially boron trichloride, whereby boric oxide solids-forming constituents of the gaseous mixture were removed therefrom.

In U.S. Patent No. 3152869 a process for making boron trichloride is described wherein a mixture of a boron-oxide containing material and a solid carbonaceous reducing agent consisting essentially of carbon is devolatilized and dehydrated and the mixture so formed is treated in a vertical moving bed reaction zone with chlorine at a temperature of between about 500 and 14000C, wherein the level of solid carbonaceous reducing agent relative to boron-oxide containing materials in the initial mixture is adjusted to at least about 1.75 times the stiochiometric requirements to produce entirely CO from the carbonaceous reducing agent and the boron oxide containing material; a coke-forming binder is added to the mixture at the time of formation thereof, the mixture is formed into agglomerates having a dimensional range between about 1/16 of an inch and about 4 inches and dehydrating and devolatizing by heating the mixture at a temperature of between about 100 and 7000C for a time sufficient to remove substantially all water and other volatile material and to produce a coking bond therein, said forming of agglomerates and heating preceding contacting said mixture with chlorine, the agglomerate so formed being charged into the top of a vertical reaction zone and passing the agglomerates downwardly while passing chlorine into the bottom of the reaction zone and allowing the chlorine to pass upwardly therethrough and boron trichloride is recovered from the top of the reaction zone.

More recently it has been proposed in U.S. Patent No. 4125590 to produce boron trichloride by introducing boron containing ore consisting essentially of sodium tetraborate into a reaction zone; introducing carbon into the reaction zone; heating the reaction zone to an elevated temperature to produce a molten reaction mixture consisting essentially of the boron-containing ore and carbon; introducing chlorine gas into the molten reaction mixture to form molten by-products and gaseous products comprising crude boron trichloride within the reaction zone; maintaining the reaction zone at an elevated temperature to effect a separation of the molten by-products from the molten reaction mixture to form a molten by-product-rich upper phase and a molten reactant-rich lower phase in the reaction zone; withdrawing at least a portion of the molten by-product rich upper phase; and recovering at least a portion of the gaseous products formed within the reaction zone.

The recovery of at least a portion of the gaseous products preferably further includes the steps of partially condensing the gaseous products to produce at least a liquid phase comprising crude boron trichloride; separating the liquid phase from gaseous products; subjecting the liquid phase to a two stage distillation to produce at least a light phase comprising purified boron trichloride; and recovering the purified boron trichloride.

Finally, from U.S. Patent No. 4327062 the production of boron trichloride may be achieved by heating activated carbon particles having a supportive aqueous solution of a boron compound thereon at a temperature of from 300 to 10000C and then reacting the resultant activated carbon particles with chlorine.

Typically boron oxide and sodium tetraborate are used as the boron compound.

A problem with all of the above processes is that they utilise an oxygen containing boron compound, such as boron oxide, boric acid or sodium tetraborate, which can lead to the incorporation of impurities such as carbonoyl chloride, carbon monoxide and carbon dioxide in the resultant boron trichloride. These impurities are difficult to remove and can be detrimental to the performance of the boron trichloride in its further applications.

An object of the invention is to provide an improved process for producing boron trichloride.

According to a first aspect of the invention there is provided a process for producing boron trichloride, which comprises reacting a carbide of boron with chlorine gas at an elevated temperature.

According to another aspect of this invention there is provided apparatus for producing boron trichloride from chlorine gas and a carbide of boron comprising a reactor tube containing the carbide of boron in finely divided form, an inlet for chlorine gas, an outlet for vaporized product and means for heating the carbide of boron to a desired reaction temperature.

Preferably B4C will be the carbide of boron used but it may be possible to use B6C as another source of boron.

The carbide of boron is preferably heated to a temperature of at least 8000C. Heating of the carbide of boron is preferably achieved using a susceptor, i.e.

a material that couples with radio frequency (RF) to achieve the desired temperature. A useful susceptor is made of silicon carbide.

The process according to the invention results in an exothermic reaction which produces vaporized boron trichloride, which is of high purity, and leaves carbon as a residue. The boron trichloride is preferably collected as a vapour, liquified and may be purified using conventional techniques. Typically purification will be carried out by fractional distillation.

The reaction is preferably carried out in a reactor tube, especially of quartz and the reactor tube is preferably arranged vertically, so that chlorine gas can be passed through it in a downward direction. The carbide of boron is preferably packed into the reactor tube and the heating source, typically a silicon carbide susceptor, is placed at the top of the reactor in contact with the boron carbide.

The reactor is preferably gas tight and air or moisture is preferably substantially removed prior to reaction. Typically the reactor will be subjected to reduced pressure to draw off moisture and will preferably be purged with a non-oxygen containing gas, such as helium.

The rate of flow of chlorine through the reactor is preferably controlled to match the rate of reaction.

Typically the rate of flow of chlorine gas through a reactor will be at least 3.0 Kg/hour and preferably between 3.0 and 8.0 Kg/hour.

The boron carbide used preferably has a mesh size of between 2 and 5.

This invention will now be further described, by way of example only, with reference to the accompanying drawing which shows apparatus for producing boron trichloride from a carbide of boron and chlorine gas.

Referring to the accompanying drawing, apparatus for producing born trichloride from a carbide of boron and chlorine gas comprises a quartz reactor tube 10 of 10cm internal diameter and 1 metre in length. The reactor tube 10 has in inlet 12 for chlorine gas and an outlet 14 for vaporized reaction product consisting primarily of boron trichloride. The reactor is held vertical and gas flow therethrough is downwards. The reactor tube is sealed to be gas tight to avoid entry of oxygen or oxygen containing gases. The reactor is filled with a carbide of boron 16, usually B4C, of mesh size 2 to 5cm and a 5cm square silicon carbide susceptor 18 is placed in the top of the reactor in contact with the boron carbide.The susceptor is connected to a radio frequency source which causes the susceptor to become heated until it glows red, typically at about 8000C. The susceptor promotes the reaction of chlorine with the boron carbide to produce boron trichloride in accordance with the following equation: B4C + 6C12

4BC13 + C The carbon remains in the reactor and the boron trichloride exits the reactor as a vapour for collection and purification, usually by distillation.

This invention will now be further described by means of the following example.

EXAMPLE This Example used the apparatus described with reference to the accompanying drawing. The quartz reactor tube of 10 cm internal diameter and 1 metre in length was filled with 6 kg. boron carbide (B4C) having a mesh size of between 2 and 5. A silicon carbide susceptor, approximately 5 cm square, was placed in intimate contact with the boron carbide at the top of the reactor. The reactor was vacuumed and purged at least four times with high purity helium gas to remove residual traces of air and moisture.

The silicon carbide susceptor was heated using a coupled radio frequency source to red heat (at least 8000C) and chlorine gas was then passed through the reactor from the top and through the boron carbide at a rate of at least 3.0 kg/hour.

Resultant vaporize boron trichloride was liquified and purified by traditional distillation techniques. Analysis showed that the reaction took place at an efficiency of greater than 80%. The resultant boron trichloride was substantially free of oxygen containing impurities.

Claims

1. A process for producing boron trichloride, which comprises reacting a carbide of boron with chlorine gas at an elevated temperature.

2. A process as claimed in claim 1, wherein B4C is the carbide of boron used.

3. A process as claimed in claim 1, wherein B6C is used as the source of boron.

4. A process as claimed in claim 1,2 or 3, wherein the carbide of boron is heated to a temperature of at least 8000C.

5. A process as claimed in claim 4, wherein heating of the carbide of boron is achieved using a susceptor, i.e. a material that couples with radio frequency (RF) to achieve a desired temperature.

6. A process as claimed in claim 5, wherein the susceptor is made of silicon carbide.

7. A process as claimed in any one of claims 1 to 6, wherein boron trichloride is collected as a vapour.

8. A process as claimed in claim 7 further comprising the steps of liquefying the collected boron trichloride and purifying same by distillation.

9. A process as claimed in any one of claims 1 to 8, wherein the reaction is carried out in a reactor tube.

10. A process as claimed in claim 9, wherein the reactor tube is of quartz.

11. A process as claimed in claim 9 or 10, wherein the reactor tube is arranged vertically, so that chlorine gas can be passed through it in a downward direction.

12. A process as claimed in claim 9, 10 or 11, wherein the carbide of boron is packed into the reactor tube and the heating source is placed at the top of the reactor in contact with the boron carbide.

13. A process as claimed in any one of claims 9 to 12, wherein the reactor is gas tight and air or moisture is substantially removed prior to reaction.

14. A process as claimed in claim 13, wherein the reactor is subjected to reduced pressure to draw off moisture and is purged with a non-oxygen containing gas.

15. A process as claimed in claim 14, wherein the purging gas is helium.

16. A process as claimed in any one of claims 9 to 15, wherein the rate of flow of chlorine through the reactor is controlled to match the rate of reaction.

17. A process as claimed in claim 16, wherein the rate of flow of chlorine gas through the reactor is at least 3.0 Kg/hour.

18. A process as claimed in claim 17, wherein the rate of flow of chlorine gas through the reactor is between 3.0 and 8.0 Kg/hour

19. A process as claimed in any one of claims 1 to 18, wherein the boron carbide used has a mesh size of between 2 and 5.

20. A process as claimed in claim 1 and substantially as hereinbefore described with reference to the foregoing Example.

21. Apparatus for producing boron trichloride from chlorine and a carbide of boron comprising a reactor tube containing the carbide of boron in finely divided form, an inlet for chlorine gas, an outlet for vaporised product and means for heating the carbide of boron to a desired reaction temperature.

22. Apparatus as claimed in claim 21, wherein the reactor is a quartz tube.

23. Apparatus as claimed in claim 21 or 22, wherein the means for heating the carbide of boron is a susceptor coupled to radio frequency.

24. Apparatus as claimed in claim 23, wherein the susceptor is of silicon carbide.

25. Apparatus for producing boron trichloride substantially as hereinbefore described with reference to and as illustrated in the accompanying drawing.