Patent GB-919018-A (GB)
Title: Production of silicon dioxide
Inventor: EVANS ARTHUR WALLACE
IPC: C01B33/18
Filed: 1958-03-04
Abstract: <PICT:0919018/III/1> Silicon dioxide is produced by reacting silicon tetrachloride vapour with oxygen in the presence of a fluidized bed of inert solid materials, the silicon dioxide being produced within the bed and being discharged in entrainment in the outgoing gases. The reactants are separately introduced through inlet ducts in a perforated base of the fluidized bed, the perforations being provided with inserts forming constrictions such that the pressure drop thereacross is at least one half of the pressure drop from the bottom to top of the fluidized bed.  The bed is of sufficient size to act as a heat reservoir to avoid the necessity of supplying extra heat.  It is desirable however to preheat the oxygen to avoid condensation of the silicon tetrachloride vapour. The velocity of the fluidizing gas is preferably between 2 and 50 times the minimum required for fluidization and the pressure drop across the constrictions will generally be more than 2 p.s.i. Excess reaction heat may be withdrawn by conventional coolers or by the employment of gaseous coolants such as chlorine, nitrogen, carbon dioxide, or cooled recycled tail gases or by liquid chlorine injected into or sprayed upon the bed; additional cooling may be effected by progressively introducing relatively cool sand or other inert material to the bed whilst discharging hot material thereby maintaining the temperature from 500 DEG -1300 DEG C. and more particularly at about 1000 DEG C.  The silica reaction product, separated from the gases by means of cyclones may be freed from adsorbed or combined chlorine by heating to 300 DEG -600 DEG C. by passing air countercurrent to the silica reaction product in a rotary tube or by fluidizing the hot reaction product with cold gas.  If oxygen is employed for this purpose it may then be fed as a pre-heated reactant to the main oxidation process. The after-treatment gas may also contain ammonia or water vapour to assist in the removal or neutralization of the adsorbed chlorine in the silicon dioxide product.  The silicon chloride and oxygen reactants are generally employed in stoichiometric amounts but the oxygen tetrachloride ratio may vary from 1:1 to 5:1 on a molar basis. Moisture is preferably, but not necessarily, avoided. The apparatus employed may be that described and claimed in Specification 919,020. Thus, as shown in Fig. 1, a heat insulated reaction chamber is provided with a perforated base having inlets 112 connected to a manifold 26 for the supply of silicon tetrachloride and inlets 212 and 312 for oxygen connected to manifold 25.  Each inlet is provided with a constriction plate 43 which produces the requisite pressure drop.  Reaction products are withdrawn through line 126 and cool sand is fed from a fluidized bed 73 through pipe 70 to the main reaction bed whilst excess sand is withdrawn through a line 77 to a bed fluidized with air fed through line 85 and leaving through a pipe 84. The sand is finally withdrawn through line 83.  An alternative base plate construction is shown in Fig. 5, where separate wind boxes 404 and 405 are fed with oxygen and the tetrachloride respectively through lines 406 and 407.  The fluidized bed is supported on a perforated refractory base plate 11 having ceramic inserts 400 provided with downwardly directed outlets 410.  The lower end of the ceramic inserts are provided with screw plugs 402 having pressure reducing orifices 403.  Silica reaction product entrained in the gases leaving through line 126 (Fig. 1) are first separated in a cyclone separator from the chlorine-containing reacted gases, and then treated to remove undesired acidity for example by feeding to a fluidized bed where they are treated with air or oxygen, containing ammonia with or without water vapour, whereby the pH of the product is above 3,5.  According to a typical example, silica sand of average diameter 250 microns constituted the main reaction bed to which oxygen was fed together with silicon tetrachloride through a steam jacketed vaporizing tube.  The tetrachloride oxygen molar ratio was 1-1,5.  The reaction temperature was maintained at 1000 DEG -1050 DEG C. by continuous slow replacement of the silica which was heated to 400 DEG C. in the fluidized bed 71 before passage to the main reactor.  The silica product had an average particle size of 0,004 microns and a bulk density of 4 1/2 pounds per cubic foot.  The surface area was 260 square metres per gram and its pH after heat treatment was 4,1.  Specification 796,735 also is referred to.