Chemistry of inorganic compounds – Treating mixture to obtain metal containing compound – Group vib metal
Patent
1992-10-22
1994-12-20
Langel, Wayne
Chemistry of inorganic compounds
Treating mixture to obtain metal containing compound
Group vib metal
423596, C01G 3714
Patent
active
053744080
DESCRIPTION:
BRIEF SUMMARY
The present invention concerns the oxidative disaggregation of chromium minerals, that is to say the preparation of hexavalent chromium compounds by means of the oxidation of trivalent chromium compounds and, in particular, of minerals containing trivalent chromium.
The extraction of trivalent chromium from the natural minerals which contain it (minerals which will henceforth be referred to with the generic term "chromite") involves oxidizing trivalent chromium present in the minerals to hexavalent chromium and then extracting it, by means of subsequent leaching, in the form of hexavalent chromium soluble salts.
Conventional methods for the oxidative disaggregation of chromite involve finely grinding the mineral, which is then oxidized in the presence of sodium carbonates and/or sodium hydroxide or of other alkaline metals, at a temperature of between 600.degree. and 1500.degree. C.
In addition, thinning materials and sometimes oxidants are added to the mixture. Generally ferrous oxides and leaching residues are used as diluents.
The high temperatures necessary for the oxidation reaction are obtained by heating the mixture directly, that is to say by putting the mass to be disaggregated in direct contact with the flame and with the combustion products of the burner usually diluted in the atmospheric air necessary for oxidation.
Various working methods or compositions of the mix have been proposed with the aim of improving process yields.
For example, German patents Nos. DE-25 57 403 and DE-26 07 131 concern the disaggregation of chromite minerals lean in chromium but particularly rich in silica; German patent No. DE-25 42 054 proposes a multi-phase process which, in its preferential form, foresees three hot oxidation cycles (roasting), with the aim of raising the yields of the transformation of the CR.sub.2 O.sub.3 of the chromite into hydrosoluble chromate to values to between 70 and 85%.
European patent No. EP-A-027 868 describes feeding furnaces with a mixture of minerals pelletized in an aqueous liquid, obtained in practice by the use of wet leaching residues.
The main disadvantage with all these known processes described above is the difficulty of obtaining a concentration of O.sub.2 in the oxidizing gases which is sufficient for the complete oxidation of the trivalent chromium present in the mineral, since the oxygen present in the combustion fumes enriched with air is usually no more than about 8-10% of the total.
In addition, the fact that both the combustion and dilution gases are brought into direct contact with the mixtures leads to the entrainment of the particles from the furnace. These particles must be separated, with consequent and not indifferent ecological damages, since they contain hexavalent chromium also.
In order to improve the characteristics of the atmosphere inside the furnace, attempts have been made to increase the oxygen content of the oxidizing gases. For example, Japanese patent No. 75905 (Nippon K. K.), which describes the admission of an oxygen-rich gas under the flame of a directly heated rotating tubular furnace. Such a solution leads however to an increase in the entrained particles and in the formation of rings of fouling without any valuable yields improvements.
In an alternative process, known since the beginning of the century, (we cite for example, German patents No. DE-163814 and No. DE-171089), the oxidation reaction of chromium minerals is carried out at a relatively low temperature (400.degree.-700.degree. C.) by using low flux mixtures, generally obtained thanks to the presence of high quantities of alkaline hydroxides in the mixture. Oxidation is usually assisted either by injecting oxygen-containing gases into the melting bath or by the addition of oxygen donors (such as nitrogen acid alcohol salts, manganese oxides, lead oxides and the like) to the mixture.
A further problem arising from the known technique is due to the fact that the monochromate which forms during the reaction melts at the roasting temperature of the mineral.
It has been found that the mel
Bruzzone Giuseppe
Parodi Alfredo
Perrone Diego
Langel Wayne
Luigi Stoppani S.p.A.
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