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.