Thermal Decomposition of Magnesium Chromate

A wet process for the preparation of MgCrO4⋅7H2O, which is applicable to a binder of basic refractories as an excellent substitute for MgCl2⋅ or MgSO4, was investigated.The thermal decomposition of magnesium chromate hydrates and anhydrate was studied by using DTA, TGA and X-ray diffraction.The following results are obtained(1) MgCrO4⋅7H2O is prepared by crystallization from a solution of MgO, Mg(OH)2 or MgCO3 in chromic acid, which is thermally decomposed to give MgCrO4.This method is preferable to a dry process which needs oxygen pressure of 12 atm. and heating temperature of 420°C.(2) In dehydration of the heptahydrate, the four separate reactions distinguished in this work have been attributed to the following:MgCrO4⋅7H2O=MgCrO4⋅5H2O+2H2O (DTA peak temp. 33° & 58°)MgCrO4⋅5H2O=MgCrO4⋅1.5H2O+3.5H2O (DTA peak temp. 127° & 150°)MgCrO4⋅1.5H2O=MgCrO4⋅H2O+0.5H2O(DTA peak temp. 196° & 215°)MgCrO4⋅H2O=MgCrO4+H2O(DTA peak temp. 335°)(3) The decomposition of MgCrO4 anhydrate begins at about 600°C and obeys a first-order law. The value of activation energy determined by isothermal decomposition is about 74kcal/mol.