The present invention relates to a method for preparing phosphoric acid by a wet method, so called extraction phosphoric acid, from calcium phosphate ores, e.g. phosphorites and apatites and sulphuric acid.
In processes for production of extraction phosphoric acid mainly ground mineral phosphorites or phosphorites enhanced by concentration processes are used. These contain generally fluoroapatite, frequently supplemented with small quantities of mixed apatites and, depending on the origin, various mineral contaminants such as quartz, calcite, dolomite, loamy matter and variously mineralized organic compounds and/or organic compounds introduced in the course of concentration.
From the chemical point of view natural calcium fluorophosphates are always accompanied by great number of various salts of magnesium, aluminium, iron and other cations including rare earth and radioactive elements. Organic contaminants of phosphorites comprise mainly humus and bituminous compounds with a small concentrations of sulphur (II), phosphorus (III) and nitrogen compounds.
In the process for production of extraction phosphoric acid the basic reactions of decomposition of apatite with sulphuric acid are accompanied by a number of various simultaneous side reactions of said contaminants. These reactions influence greatly the quality of phosphoric acid and the course and yield of the process for production of the same.
Organic contaminants of phosphate ores exert a particularly adverse effect on the process for production of phosphoric acid. Already at the extraction stage they are adsorbed on the surfaces of gypsum crystallisation, block active sites of crystals and limit capabilities of their growth. In consequence of this suspensions hard to filter appear having sticky structure of a filtered precipitate and high content of residual phosphoric acid hard to wash out.
The substantial part of organic contaminants is dissolved in resulting phosphoric acid giving it a typical dark-brown colour and enlarging its viscosity and surface tension. Gaseous reaction products, generally carbon dioxide, under this conditions bring about an abundant and stable foam which lowers the rate of extraction and mass yield of the reactor.
Some known and most frequently used mechanical and chemical methods for quenching foam are not always useful and frequently ameliorate the arduousness of the occurrence to a small degree only.
At high temperatures of reaction with phosphoric acid and sulphuric acid the organic compounds contained in some phosphorites liberate to the flue gases vapours of both saturated and unsaturated fatty acids, various organic amines, sulphides and phosphines distinguishing with their strong, repulsive odour. Having been vented with flue gases to the atmosphere, the compounds can be detected even in trace amounts in the distance of tens of kilometers from the phosphoric acid production facility. Attempts to deodorise the vented gases by known methods, e.g. thermal and catalytic oxidation, adsorption on surface active materials, or destruction in bio-reactors failed to achieve success, though the methods are successfully utilised in another branches of industry, e.g. heavy chemical synthesis, tobacco industry, food industry etc.
According to the current state of knowledge the only known efficient method of making phosphorites with high content of organic contaminants suitable for the process for production of extraction phosphoric acid is to precalcine phosphorite in oxidizing conditions at temperatures of alkaline earth carbonates decomposition. However, the method is energy-consuming and applied in special cases, e.g. to produce special extraction phosphoric acid completely devoid of organic contaminants for further purification by solvent extraction. Most frequently such phosphorites are not processed, though they are cheap and readily available, and raw materials more expensive, but less troublesome in processing are used.
Therefore, the object of the present invention is to make independent the process for preparing phosphoric acid by a xe2x80x9cwetxe2x80x9d method independence of the type and kind of calcium phosphate ore, and particularly of the type and amount of organic contaminants contained therein, at eliminating the emission of offensive odours, at the distinct reduction of reaction pulp foaming and at improving the filtrating properties of resulting phosphogypsum.
This object is achieved through a method for preparing phosphoric acid from calcium phosphate ores, especially the ores highly contaminated with organic compounds, by decomposing the ore with sulphuric acid, wherein according to the invention the process of decomposition of the calcium phosphate ores with sulphuric acid and circulating phosphoric acid is carried out in oxidizing conditions by a constant supply of ozone to the reaction slurry, preferably to the zone of commencing decomposition of the phosphate ores.