Stability of iron phases presently used for disposal from metallurgical systems-a review

The current phases used for the disposal of iron and arsenical wastes, jarosite and scorodite, are examined from a thermodynamic viewpoint using critically assessed data. For scorodite, it is demonstrated that the differences in the solubility reported in the literature are due to starting phases of differing crystallinity. Contour plots of solubility vs pH and activity are presented for both scorodite and jarosite and show that the minimum solubility is achieved for pH 4-7. However, neither phase is stable towards transformation to goethite with scorodite only stable below pH 4 and jarosite below pH 7. The practicalities and implications of the disposal of these phases is discussed and recommendations for future disposal options are made.