The effect of copper on the precipitation of scorodite (FeAsO4·2H2O) under hydrothermal conditions: evidence for a hydrated copper containing ferric arsenate sulfate-short lived intermediate.

The effect of copper sulfate on scorodite precipitation and its mechanism of formation at 150 °C was investigated. Scorodite was determined to be the dominant phase formed under all conditions explored (0.61 < Fe(III)/As(V) < 1.87, 0.27-0.30 M Fe(SO(4))(1.5), 0-0.3 M CuSO(4), 0-0.3 M MgSO(4), at 2.5 h and 150 °C). The produced scorodite was found to incorporate up to 5% SO(4) and ≤1% Cu or Mg in its structure. The precipitation of scorodite was stoichiometric, i.e. the Fe/As molar ratio in the solids was equal to one independent of the starting Fe/As ratio in the solution. The presence of excess ferric sulfate in the initial solution (Fe/As>1) was found to slow down the ordering of the H-bond structure in scorodite. Precipitation under equimolar concentrations (As = Fe = Cu = 0.3 M), short times and lower temperatures (30-70 min and 90-130 °C) revealed the formation of a Cu-Fe-AsO(4)-SO(4)-H(2)O short lived gelatinous intermediate that closely resembled the basic ferric arsenate sulfate (BFAS) type of phase, before ultimately converting fully to the most stable scorodite phase (96 min and 138 °C). This phase transition has been traced throughout the reaction via elemental (ICP-AES, XPS), structural (PXRD, TEM) and molecular (ATR-IR, Raman) analysis. ATR-IR investigation of an arsenic containing industrial residue produced during pressure leaching of a copper concentrate (1 h and 150 °C) found evidence of the formation of an arsenate mineral form resembling the intermediate basic ferric arsenate sulfate phase.