| 摘要 |
Substantially iron free aluminium nitrate solutions are prepared from contaminated solutions, which contain free Al2O3 in a ratio of free Al2O3 to Al(NO3)3 (expressed as Al2O3) of at least 1 : 100 and at least 0.10 part by weight of iron (expressed as Fe2O3) per part by weight of aluminium (expressed as Al2O3) present as free Al2O3 and Al(NO3)3, by heating to above 140 DEG C. and separating the iron containing precipitate, the heating being carried out for such a time that it produces after separation of the precipitate an Al(NO3)3 solution having a ratio by weight of aluminium (expressed as Al2O3) to iron (expressed as Fe2O3) of at least 2000 : 1. The contaminated Al(NO3)3 solution preferably contains at least 6% by weight of aluminium (expressed as Al2O3) and preferably 0.2 to 0.5 part by weight of iron (expressed as Fe2O3) per part by weight of aluminium (expressed as Al2O3). The heating may be carried out under autogenous pressure (5.6 to 10.5 kg/cm.2) and preferably at 140-220 DEG C. The contaminated Al(NO3)3 solution may be prepared by the digesting of an aluminous ore (which may be finely divided) with aqueous HNO3 of 30% by weight concentration or more, preferably at 90-120 DEG C., the HNO3 being used in an amount such that not more than 98% (preferably 50-90% of the alumina in the ore is converted to Al(NO3)3 and for such a time as to leave no free acid in solution. If the contaminated nitrate solution contains less than the requisite amount of iron, sufficient acid soluble iron containing material may be added to the ore before digestion or the Al(NO3)3 solution after digestion to provide the desired ratio of iron to aluminium, and if necessary alumina may be added to the Al(NO3)3 solution to ensure that the ratio of free Al2O3 to Al(NO3)3 remains above the desired level after exhaustion of the HNO3. The digestion of the ore and the heating of the solution may be combined into a single step by heating with aqueous HNO3 the aluminous ore which contains at least 6% by weight, based on the weight of ore, of Al2O3 and at least 0.1 part by weight of iron (expressed as Fe2O3) per part by weight of Al2O3. If the ore contains less than the requisite amount of iron sufficient acid soluble iron containing material may be added to the ore to provide at least 0.1 part by weight of iron (expressed as Fe2O3) per part by weight of Al2O3. The ore stage process is preferred when the ore is low in silica i.e. less than 3% for example a bauxite and the two stage extraction preferred when the ore is low in iron but particularly when it is high in silica i.e. more than 30% for example a clay. The ore may alternatively be leached with ferric nitrate solution all or part of the HNO3 being formed by thermal decomposition of the ferric nitrate. The acid soluble iron containing material which may be added to ores or to the Fe(NO3)3 solutions which are deficient in iron may be Fe2O3 obtained as the residue from the leaching of ore having a high iron content or from the thermal decomposition of Fe(NO3)3 or may be an aluminous ore high in iron. The substantially iron free Al(NO3)3 solution which preferably contains less than 0.02% by weight of iron (expressed as Fe2O3) based on the weight of aluminium (expressed as Al2O3) is suitable for the production of cell grade alumina, which is produced by crystallizing Al(NO3)3 . 9H2O from the solution, heating it (preferably at 300-500 DEG C. to convert it to partially hydrated alumina and calcining (preferably at 1000-1250 DEG C.) to produce cell grade alumina. A continuous process for the production of cell grade alumina from bauxite is described. |
