Application of a selective dissolution protocol to quantify the terminal dissolution extents of pyrrhotite and pentlandite from pyrrhotite tailings

- The developed protocol selectively dissolves the pyrrhotite fraction from an upgraded pyrrhotite tailings sample
- Selective dissolution is achieved at an optimum acid strength of 15 wt% HCl, and at 80 °C
- The dissolution of Fe was correlated with the dissolution of Ni from pyrrhotite as functions of acidity and temperature
- Pyrrhotite and pentlandite dissolve at essentially similar rates during oxic ferric leach

The recovery of valuable base metals from mining rejects presents an economical alternative to conventional hydrometallurgical processes. The reject of interest investigated in the present work is a nickeliferous, upgraded pyrrhotite tailings produced by Vale Base Metals in the Sudbury Basin of Ontario. A QEMSCAN™ analysis of the tailings showed a total Ni content of 1 wt%, with 59% of the total Ni deported to pyrrhotite and 40% associated with pentlandite. The initial part of this study involved the development of a selective dissolution protocol to quantify the differential dissolution extents of the two Ni-bearing minerals. Application of this protocol to the pyrrhotite tailings sample showed that an acidity of 15 wt% HCl, and a temperature of 80 °C was adequate to selectively dissolve 96% of pyrrhotite from the tailings. Subsequently, an anoxic acidic leach of the tailings at pH 1.5, and at 5% (w/v) solids loading showed minimal dissolution of Fe and Ni from pentlandite in tests with and without pH control. The extent of Ni dissolution was next evaluated as a function of pH during an oxic acid leach. The testing protocol involved leaching the tailings under two pH regimes: fixed and uncontrolled pH. The results showed that operation at a fixed pH increases the extent of Ni dissolution due to the higher concentration of the oxidant Fe(III) in solution. Finally, an oxic ferric leach showed that the terminal dissolution extents of pyrrhotite and pentlandite were comparable at 46% total Ni extraction.