GB453646A - Improvements in the treatment of raw phosphates - Google Patents

Abstract

<PICT:0453646/III/1> <PICT:0453646/III/2> <PICT:0453646/III/3> In decomposing raw phosphates by heat treatment at above 1000 DEG C. in the presence of silica, but in the absence of alkalis, whilst scouring with gases containing steam, the fluorine containing-waste gases are carried off from the main reaction zone, and further contact with the material to be or being decomposed is avoided. The production of an undesirable easily-fusible enamel-like compound from the silicon fluoride of the gases and the lime of the phosphate is thereby avoided. In Fig. 1, the phosphate is fed through orifice e through an inclined rotary retort m, and is heated at one end by a powdered coal, oil or gas flame k. Fluorine-containing gases are drawn off through a tube c secured on struts d. The phosphate is preheated by this tube. The phosphate may be fed to the retort through tube c, the gases being withdrawn on the outside of this tube. Fans may be disposed at the end a of the retort. The decomposed material is discharged at orifice f. In another retort, a cooled suction pipe enters in the same direction as, and immediately below, the flame. In Fig. 3, powdered coal or gas n and air o produce the flame k. The lower part q contacting with the material is sucked back by fans through outlet r, carrying with it the fluorine escaping from the decomposing material. The decomposed material is discharged at orifice s. In a modification, the firing of the retort may be so controlled and the main reaction zone may be moved so far in the direction of the gas outlet that the temperature is higher than that at which fluorine is reabsorbed by the material. The hot gases, after removal of fluorine by treatment with basic substances, may be used for preheating the charge. In Fig. 4, the material is preheated in a retort m<1>, care being taken that no sintering occurs. Fluorine is removed to some extent in this retort. The material passes through a short channel 8<1> to a second retort m<11>, wherein the temperature is maintained above that at which fluorine is absorbed. The material may be preheated by means of a travelling grate similar to that used in the Dwight-Lloyd method. The reaction mixture should be finely ground, for instance, to pass through a 2500 mesh sieve, or even so as to leave a 10 per cent residue on a 10,000 mesh sieve. Grain sizes of 0,3 mm. or over are also referred to. The total amount of silica present should not be less than 15 per cent and may amount to one third of the phosphate. Up to 15 per cent of sulphuric, nitric, hydrochloric or phosphoric acid or an acid calcium phosphate may be added. When a product having a composition 5CaO.P2O5.SiO2 is desired, lime may be added. Heating is preferably effected in two operations, first at 1000--1200 DEG C., then at 1300--1450 DEG C. The product is soluble in citric acid and ammonium citrate. The Specification as open to inspection under Sect. 91 comprises also melting of the furnace charge at 1600--1650 DEG C. The melt is cooled by allowing it to fall in drops into a sheet metal chamber. The mixed charge may be wet ground. Precipitated silicic acid, clay, kaolin, blast furnace slag, sand or quartz are specified as sources of silica. This may, however, be replaced by compounds of aluminium, iron, titanium, boron, molybdenum, chromium, vanadium, nickel, &c., producing volatile fluorides. Aluminium fluoride subliming, for example, may be used in the ceramic industry, or for producing artificial cryolite. An oxyhydrogen or acetylene flame may be employed. The reaction mixture may be admixed with powdered coal and blown into the flame. If the fluorine-containing waste gases are allowed to contact with the material to be decomposed, the fluorine concentration is reduced by dilution with inert gases, or by removal with basic absorbents. Basic substances specified for extraction of fluorine from the gases are slaked lime, sodium hydroxide and alumina. This subject-matter does not appear in the Specification as accepted.