The Bureau of Mines investigated the recovery of aluminum by nitric acid leaching of calcined kaolinitic clay. Most iron impurities were leached from the calcined kaolin during the extraction of aluminum and must be removed before or during the crystallization of aluminum nitrate nonahydrate in order that the subsequent thermal decomposition step will yield an alumina product of acceptable iron analysis. Solvent extraction miniplant studies using di(2- ethylhexyl) phosphoric acid in kerosine diluent have demonstrated that iron content in aluminum nitrate pregnant liquors can be decreased to levels of less than 0.0007 pct (7 ppm), expressed as fe2o3. A number of the miniplant tests were interrupted or terminated because of an increase in the viscosity of the solvent extraction circuit flowstreams. Under these conditions circuit control was difficult, high alumina loss occurred, and iron impurities increased in the recycled solvent. An addition of less than 0.5 Mol of nitric acid per liter increased the free acid content of the pregnant liquor and eliminated the viscosity problem in solvent extraction. Bench-scale research was not confirmed because most miniplant testing was not at best operating parameters.