GB903992A - Treatment of gases containing titanium dioxide in suspension - Google Patents

Abstract

Gases containing suspended titanium dioxide particles, and particularly those resulting from the vapour phase oxidation of titanium tetrahalides, are cooled by passing the gaseous suspension through a bed of solid particles to form a fluidized bed and simultaneously cooling the fluidized bed so formed with heat transfer surfaces. The bed is preferably composed of titanium dioxide particles at least 20 times, and preferably at least 100 times, as large as the suspended particles. The gas velocity of the suspension to be cooled is preferably within the range of 1 1/2 to 3 times the critical fludizing velocity. Heat transfer may be effected by cooled inner walls of the vessel containing the fluidized bed or cooled tubes may be inserted in the bed. Stagewise cooling through a plurality of fluidized beds may be employed, the average particle size, or preferably the horizontal cross-sectional area of the beds, decreasing in sequential beds in the direction of flow of the cooled suspension. The invention is particularly applicable to the cooling of gas resulting from the oxidation of titanium tetrahalides (other than the tetrafluoride) at a temperature of at least 800 DEG C. in the presence of a fluidized bed of titanium dioxide particles. Such particles are preferably also employed in the subsequent fluidized beds which serve to cool the suspension from the reactor, and it is then desirable that the critical fluidizing velocity of the particles in the cooling bed be substantially equal to the critical fluidizing velocity of those in the reaction bed and the sieve analysis of the particles of both beds is preferably substantially the same. To this end the particle size of the reaction bed is preferably controlled as in Specification 866,363. According to an example, the titanium dioxide was produced by oxidation of titanium tetrachloride in a fluidized bed of titanium dioxide particle of 14 to 36 mesh B.S.S. size. The reaction was effected at 1,040 DEG C. in the presence of carbon monoxide which was burnt to counteract heat losses. 60% of the titanium dioxide produced was retained on the bed as a coating on the particles and the remainder in suspension had a mean particle size of 0,86 microns. The effluent suspension from the reaction bed was passed into a water-jacketed aluminium tube containing a bed of titanium dioxide particles. The temperature of the effluent gas from this cooling bed was 226 DEG C.