Chemistry of inorganic compounds – Phosphorus or compound thereof – Oxygen containing
Patent
1992-01-06
1994-01-18
Langel, Wayne
Chemistry of inorganic compounds
Phosphorus or compound thereof
Oxygen containing
423306, C01B 2516
Patent
active
052798062
DESCRIPTION:
BRIEF SUMMARY
The present invention relates to a process for the treatment of phosphoric acid. More particularly, it concerns a process for treating concentrated phosphoric acid to eliminate certain heavy metals and also fine sludges which accumulate therein.
Concentrated phosphoric acid is to be understood as an acid whose concentration, expressed as P.sub.2 O.sub.5 (by weight) according to the usage in the phosphate industry, is between about 50 and about 60% (unless otherwise stated, the noughts are not considered to be significant figures). These values are approximate and depend especially on the process used for the conversion of phosphates into phosphoric acid. They also depend on the impurities which, like magnesium, can affect the viscosity.
The above contents are rounded up and relate, in particular, to acids which may be about 48%.
These acids, after having been formed by reaction, have been subjected to an evaporation in order to reach the above concentrations. In general, the P.sub.2 O.sub.5 concentration is between 50 (two significant figures) and 57%. This phosphoric acid is generally low in dissolved calcium, but it can contain relatively large quantities of sulfuric acid. In general, these acids clarified before the concentration-evaporation stage are not treated immediately at the outlet from the evaporation and are in a state of disequilibrium of suspended and dissolved salts, which is sufficient for considerable quantities of calcium sulfate and other compounds to settle out in any storage locations, necessitating both major and costly treatment.
Among the large number of impurities present in phosphate minerals and especially in apatites, uranium and the rare earths, and also iron, aluminum and titanium may be mentioned. Numerous processes for the recovery of uranium and, in some cases, the rare earths have been proposed in the period when uranium prices were particularly high.
Among the numerous other elements present in the phosphate mineral and hence in the phosphoric acid, divalent metals (Sr, Ba) and more particularly cadmium may be mentioned.
While certain phosphate minerals are very low in cadmium (less than a few ppm), other minerals are fairly rich in this element and can sometimes reach values much higher than 100 ppm.
For environmental reasons, it is becoming less and less acceptable to use phosphate fertilizers which contain an element reputed as toxic, namely cadmium.
Numerous countries have issued, or are about to issue, regulations which limit the cadmium content of fertilizers.
This is the reason why it is important and even essential for the mining of deposits with cadmium-containing zones to be able to effect a removal of cadmium from their minerals or from the phosphoric acid obtained therefrom.
Numerous cadmium removal processes have been proposed such as, for example, solvent extraction, sulfide precipitations, removal by ion exchange, and electrolytic techniques. Nevertheless, the volumes of phosphoric acids to be treated are so great relative to the small cadmium content that most of the proposed processes, even though they are chemically efficient, represent a heavy economic burden.
More recently, a process proposed in European Patent Application No. 0,253,454 suggests precipitating the anhydrite and accomplishing by means of this anhydrite precipitation an entrainment of cadmium, thus allowing purification of the phosphoric acid.
Nevertheless, the cadmium contents of the anhydrite in this process are too small to allow a further treatment and lead to depositions of considerable quantities of anhydrite in controlled landfills, with a risk of pollution due to the cadmium content.
Moreover, the process as proposed does not appear to be applicable to highly concentrated phosphoric acids. In fact, this process, as described in general terms in the description and more precisely in the examples, aims essentially to precipitate anhydrite from phosphoric acid of medium concentration of, for example, 44% of P.sub.2 O.sub.5 by adding an excess of sulfuric acid which causes the calcium
REFERENCES:
patent: 4457848 (1984-07-01), Shimmel et al.
patent: 4822582 (1989-04-01), Weterings et al.
patent: 4986970 (1991-01-01), Haraldsen
Langel Wayne
Office Togolais des Phosphates
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