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

Abstract

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, 0.27–0.30M Fe(SO4)1.5, 0–0.3M CuSO4, 0–0.3M MgSO4, at 2.5h and 150°C). The produced scorodite was found to incorporate up to 5% SO4 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=0.3M), short times and lower temperatures (30–70min and 90–130°C) revealed the formation of a Cu–Fe–AsO4–SO4–H2O 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 (96min 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 (1h and 150°C) found evidence of the formation of an arsenate mineral form resembling the intermediate basic ferric arsenate sulfate phase.