Properties and acid dissolution of metal-substituted hematites

The dissolution in 1 M HCl of Al- Mn-, and Ni-substituted hematites and the influence of metal substitution on dissolution rate and kinetics of dissol ution were investigated. The inhomogeneous dissolution of most of the hemat ites investigated was well described by the Avrami-Erofe'ev rate equation, kt = root[-ln(1 - alpha)], where k is the dissolution rate in time, t, and alpha is the Fe dissolved. Dissolution of Al-substituted hematite occurred mostly by edge attack and hole formation normal to (001), with the rate of dissolution, k, directly related to surface area (SA). Dissolution of rhomb ohedral Mn- and Ni-bearing hematites occurred at domain boundaries, crystal edges, and corners with k unrelated to SA. The morphology of Mn- and Ni-su bstituted hematites changed during dissolution with clover-leaf-like forms developing as dissolution proceeded, whereas the original plate-like morpho logy of Al-bearing hematite was generally retained. Acid attack of platy an d rhomboidal hematite is influenced by the direct (e.g., metal-oxygen bond energy, hematite crystallinity) and indirect (e.g., crystal size and shape) affects associated with incorporation of foreign ions within hematite.