Defining the Paragoethite process for iron removal in zinc hydrometallurgy

The previously ambiguous Paragoethite process and residue are defined, and the importance of a fundamental understanding of iron-phase precipitation in hydrometallurgical processing is demonstrated. A review of iron removal in zinc hydrometallurgy and in the Paragoethite process extends previous Paragoethite process studies and provides a brief overview of the iron-phase ferrihydrite, including recent experimental evidence. It is demonstrated that commonly referred to ‘amorphous iron phases’ are likely to be the nanoscale minerals ferrihydrite and/or schwertmannite. In the case of the Paragoethite process, 6-line ferrihydrite constitutes around 40–50% of the residue. Through further characterization and continuous crystallization studies the remaining precipitated components of Paragoethite process residues were found to be solid-solution jarosite phases (containing Pb), silica (containing iron), and at lower pH, poorly crystalline goethite. By using continuous crystallization, the effect of pH on phase formation and on the properties of the residue is documented; emphasizing filterability constraints and the role and reaction of residual calcine. Such studies have permitted a more fundamental understanding of the simultaneous precipitation of ferrihydrite, goethite, and jarosite from acidic hydrometallurgical liquors, including the interplay in this kinetically controlled system, where ferrihydrite is favoured. Reasons for poor filterability are suggested, indicating that the rate of crystallization, and hence, supersaturation, governs phase formation and residue properties. The aggregation-dominated and kinetically favoured nanoscale ferrihydrite particles dictate the physical properties of the residue.