Method of Obtaining Granulated and Defluorated Phosphogypsum

This method is related to the obtaining or granulated and defluorated phosphogypsum.

There is a well-known method, related to the defluoration of gypsum alone, according to which, first of all a full separation of the free water is implemented (drying) and a partial or full separation of the crystalline water during the heating process, then, after the addition of sulphuric acid and at a novel heating at 200.degree. to 500.degree. C. the defluoration itself occurs. The main drawback of the method lies in the necessity of a full drying of the phosphogypsum and the separation or a part or of the whole quantity of the water, which is chemically bound with gypsum, what in turn is connected with elevated power-expenditures. Besides that, the method features a major drawback in that polydispersed & defluorated phosphogypsum is obtained, which is merely unsuitable for thermal treatment in general and in fluid bed specifically.

The scope of the invention is to elaborate a method of obtainment of granulated & defluorated phosphogypsum, suitable for thermal treatment/or processing, as well as of gas, free of fluoric compounds.

This goal is reached by the following way:

The humid phosphogypsum is submitted to a preliminary drying at 100.degree. to 150.degree. C., unless a residual moisture-content of 0.5 to 10 percent is reached. Then, the same phosphogypsum is granulated at simultaneous introduction of sulphuric acid and binding additives. The concentration of the sulphuric acid is in the range of 55 to 98% and in such a form it is added, i.e. 0.5 to 15 weight parts to a batch of 100 weight parts of dried phosphogypsum. As a binding additive one can use carbamide, ammonium nitrate, tylose, calcium nitrate, polyethylene oxide and distillating fluid from the soda-production which are introduced in quantities, ranging from 0.01 to 6 percent. With the addition of sulphuric acid and binding additives, some water, necessary for the granulation of the phosphogypsum is also added. The humid granules are submitted to a thermal processing at 170.degree. to 700.degree. C. for 15 to 120 minutes, wherein they are dried, mechanically-strengthened and finally defluorated. The main fluorine-content of the output phosphogypsum is separated during the process of granulation and drying of the humid granules.

The so obtained defluorated and granulated phosphogypsum meets all the requirements for a further thermal processing in a fluid bed, wherein a gas, non-containing fluoric compounds is obtained, and out of which the corresponding sulphuric acid can be easily produced.

The method, according to the invention, can be better illustrated by the following example: A certain amount of phosphogypsum, containing 3.70% of P.sub.2 O.sub.5, 0.44% of F and 42.3% of moisture is dried at 110.degree. C. unless a free residual moisture of 3.5% is reached. In the volume of a drum-type granulator, containing 100 weight parts of dried phosphogypsum are inserted 13 weight parts of a 90% sulphuric acid-solution and 3 weight parts of water-diluted carbamide in order to obtain an overall humidity of the mass, equal to 21%. The granulation process takes 4 min. at a rotation speed of the drum equal to 40 r.p.m. The so obtained granules are dried at 170.degree. C. during 60 min, resulting in a fluor-content of 0.07%. At an additional thermal processing of these granules at 450.degree. C. for 25 minutes, the fluor-content falls to 0.02%.

The method proposed herewith, features a great deal of advantages over the known methods of phosphogypsum-defluoration. It permits a high rate of the defluoration to be achieved, partically up to a full separation of fluor, with a possibility of controlling the desired rate of defluoration. The process features simplicity and easiness of implementing of a large number of operations regarding the phosphogypsum process, makes possible a better blending of the phosphogypsum with the sulphuric acid-additive, combined with a simultaneous granulation of the mixture, hence--to a better defluoration.

The binding substances and the sulphuric acid used here are cheap and attainable, i.e. almost constantly available, and the simultaneous mixing and granulation allows their insertion in the form of aqueous solutions together with the necesaary for the granulation process water-amount, this making opportune the additional metering and mixing of said additives with phosphogypsum.

At the preliminary-drying stage of phosphogypsum, according to the present method, a full separation of free moisture is not foreseen (this also regards the part of chemically-bound water), but only a part of said free-moisture is separated in order to secure the proper granulation of the material.

In accordance with all yet known methods of phosphogypsum-defluoration, polydispersion is obtained-a mostly powderlike material, utterly unfit for any further processing in fluid-bed apparatus. When reprocessed in other equipments, the latter causes a large dusting of the gas and a considerable dust-ablation. The method proposed, combines the defluoration with granulation and besides that ensures the obtaining of granulated and defluorated phosphogypsum, which is very suitable for a further reprocessing, particularly in the highly intensive, and more and more used fluid-bed apparatus. The defluorated and granulated phosphogypsum obtained by this method is particularly suitable for a thermal reprocessing into lime and gas in fluid-bed furnaces. The gas,--containing sulphur dioxide, is wherefrom sulphuric acid can be easily obtained.