Patent GB-868711-A (GB)
Title: Improvements in or relating to the manufacture of phosphors and fluorescent lamps
Inventor: MCKEAG ALFRED HAMILTON
IPC: C09K11/08
Filed: 1956-05-14
Abstract: 868.711. Pneumatic separating. GENERAL ELECTRIC CO. Ltd. July 22, 1957 [May 14, 19561. No. 14964/56. Class 117 [Also in Group XL(a)] Crystalline inorganic phosphor in the form of a finely divided powder in which substantially all the particles are of size less than about 25 microns are manufactured by a method in which the component materials are fired to form a sintered mass which is subjected to light crushing to produce an intermediate powder containing appreciable proportions of particles of sizes both less and greater than 25 microns, and the intermediate powder, or a powder derived therefrom by sieving, is subjected to an elutriation treatment which separates out mainly particles of less than 25 microns. In the elutriation treatment the powder is entrained in a stream of gas or liquid whose conditions of flow are such that particles of size appreciably greater than 25 microns fall downwards whilst particles of size appreciably less than 25 microns are carried by the stream and deposited on a collector, possibly assisted by electro-static means. The intermediate powder can be sieved through a 150 mesh silk sieve to remove particles of size greater than about 60 microns and yield a powder in which about 10% by number of the particles are of size within the range of 25-60 microns with the mode of the particle size distribution at about 7 microns, and the sieved powder is subjected to the elutriation treatment so that not more than about 4 per cent by number of the particles in the resultant powder are of size greater than 25 microns. The air separator shown in the Figure includes a vertical cylindrical column 1 into which an upwardly directed stream of air is directed through a tube 2 having a disperser outlet 3 which may be fitted with vanes to impart a spiralling motion to the air stream. The tube 2 communicates with a funnel 4 branched from a part 5 of the tube 2 formed as a Venturi tube. The phosphor particles to be separated are fed through the funnel 4, and carried into the column 1 by the air stream produced by the impeller 6. The velocity of the air stream is so adjusted and the apparatus so dimensioned and arranged that in the column 1 particles of size appreciably greater than 25 microns fall to the bottom whilst particles of size appreciably less than 25 microns are carried up by the air stream to be deposited, possibly assisted by electrostatic precipitation means, in a collection chamber 7. Particles of size appreciably less than 3 microns are not readily deposited and are carried away. The particles falling to the bottom of the column 1 include some of the finer particles and are subjected to a further separation treatment by passing down the tube 11 to the Venturi tube 12 and being carried by the air stream produced by the impeller 6 into the column 8 through the tube 9 and dispenser 10. In the column 8 most of the particles of size less than 25 microns are carried upwards into the tube 2 and column 1, and the relatively large particles fall to the bottom of the column 8 and, are collected in the chamber 13 and can be crushed and added to a further batch of phosphor component materials prior to firing. Dimensions of the apparatus are stated. The rate of air flow can be 30 cubic feet per minute. Specification 578,192 is referred to.