Resource-recycling and energy-saving innovation for iron removal in hydrometallurgy: Crystal transformation of ferric hydroxide precipitates by hydrothermal treatment

In hydrometallurgy industry, the accumulation of iron removal residues containing heavy metal elements and toxic elements poses great threats to ecological systems. We propose a novel method to prevent the production of hazardous iron removal residues: firstly, neutralization precipitation is used to purify iron ions in solution; after sedimentation of the obtained suspension, only dense underflow is subjected to hydrothermal reaction, in which ferric hydroxide transforms into hematite crystal. Results showed that ferric hydroxide precipitated into a thin sedimentation layer at temperature greater than 60 degrees C. For hydrothermal treatment of the sedimentation layer, a high hydrothermal reaction temperature was conducive to complete transformation of ferric hydroxide into hematite. The precipitated ferric hydroxide firstly changed from the crystallite of goethite or lepidocrocite to amorphous particles, and then gradually formed spherical alpha-Fe2O3 monocrystalline with diameter of around 50 nm, as indicated by TEM and XRD results. At 200 degrees C, hematite precipitates with iron content of about 65% can be obtained. For iron-containing zinc/nickel/cobalt sulfate solution, controlling hydrothermal reaction temperature and acidity of the underflow solution can effectively avoid the generation of zinc/nickel/cobalt hydroxides or subsulfates in the hematite precipitates, thereby significantly reducing the loss of those valuable metals.

Automated summary

In the hydrometallurgy industry, the accumulation of iron removal residues containing heavy metal and toxic elements poses a threat to ecological systems. A novel method was proposed to prevent hazardous iron removal residues by purifying iron ions through neutralization precipitation and converting ferric hydroxide into hematite through hydrothermal reaction. Results showed that controlling hydrothermal reaction temperature and acidity can effectively reduce the loss of valuable metals in the precipitates.