Method of purifying phosphoric acid of heavy metals

Details Method of purifying phosphoric acid of heavy metals Method of purifying phosphoric acid of heavy metals

1981-01-10

1983-03-29

Vertiz, O. R.

Chemistry of inorganic compounds

Phosphorus or compound thereof

Oxygen containing

423 34, 423 87, 423 92, 423101, 423140, C01B 2516

Patent

active

043783406

DESCRIPTION:

BRIEF SUMMARY
Phosphate raw materials contain varying quantities of heavy metals which, when digesting the phosphate raw material in acid, such as sulphuric acid, are dissolved and remain in the resultant phosphoric acid solution. When using the phosphoric acid in, for example, the production of fertilizers, animal foods and other more qualified end products, the heavy metals will be present in the end product. Studies made in the agriculture industry on the problem of heavy metals over recent years have shown that more attention should be paid to the cadmium situation. The bonds by which cadmium ions are bound to the ground are relatively weak and consequently cadmium can be readily taken up by plant growing in the ground and, in this way, passed to the nutrient chains via the ground, the animal food and the vegetables intended for human consumption. The element cadmium is considered particularly dangerous, due to its long biological half life. It has been found that the long term exposure of cadmium to animals has, for example, resulted in proteinuri and kidney damage. Another known sickness promoted by cadmium is the so-called Itai-Itai-sickness in Japan, caused by the release of industrial cadmium and resulting in kidney damage and serious changes in bone structure. Thus, there are strong reasons to prevent, as far as possible, the entrance of cadmium into the various steps of the nutrient chain.
As before mentioned, cadmium and other heavy metals are present in the majority of phosphate raw materials and the heavy metal composition of some of such raw materials is listed below in table 1.


TABLE 1 ______________________________________ Khouribga Florida Florida Kola phosphate phosphate phosphate Element apatite BPL* 70/72 Taiba 72% BPL* 75% BPL* ______________________________________ P.sub.2 O.sub.5, % 39.1 31.6 36.9 33.3 34.5 As, .mu.g/g 0.7 12.5 2.2 3.6 5.3 Cu, .mu.g/g 29 39 51 7.0 7.0 Cd, .mu.g/g <0.2 14 66 8.0 9.0 Pb, .mu.g/g 2.0 3.0 4.0 15 15 Hg, .mu.g/g <0.01 0.04 0.5 0.12 0.10 ______________________________________ *BPL = Bone Phosphate Lime (% Ca.sub.3 (PO.sub.4).sub.2) remain, to a large extent, in the phosphoric acid prepared from the phosphate raw materials. Table 2 below illustrates a heavy-metal content of phosphoric acid, which content is depending upon the raw material.


TABLE 2 ______________________________________ Khouribga Khouribga phosphoric acid Kola phosphoric acid Element (1) phosphoric acid (2) ______________________________________ P.sub.2 O.sub.5, % 53.0 53.5 31.4 As, .mu.g/g 14 1.2 9.2 Cu, .mu.g/g 45 14 18 Cd, .mu.g/g 14 0.5 9.2 Pb, .mu.g/g 0.4 2.2 0.2 Hg, .mu.g/g 0.002 0.005 0.001 ______________________________________ N.b. (1) Evaporated phosphoric acid, hemihydrate process (2) Nonevaporated phosphoric acid, dihydrate process.
Various methods of purifying phosphoric acid with respect to its heavy metal content are known to the art. Normally the various heavy metals, with the exception of cadmium, can be readily precipitated out and removed from the phosphoric acid, by, for example, adding hydrogen sulphide, sodium sulphide solution, potassium sulphide solution or an ammonium sulphide solution, or by adding calcium and barium sulphide. Cadmium is much more difficult to precipitate out than the other heavy metals mentioned.
In the known technique there are only three methods for removing cadmium from phosphoric acid which, according to results shown, provide acceptable residual contents of cadmium in the purified phosphoric acid. One method (disclosed in Japanese patent specification No. 7575-115) is based on the concept of adding a large quantity of sodium sulphide to the phosphoric acid, cadmium sulphide being precipitated out in an autoclave at overpressure. The overpressure is probably necessary in order to reduce the solubility of the resultant cadmium sulphides in the phosphoric acid. The method requires a large surplus of sulphide in

REFERENCES:
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patent: 3205589 (1965-09-01), Fies et al.
patent: 3685964 (1972-08-01), Treitler
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