Ferric Hydroxide Recovery from Iron-Rich Acid Mine Water with Calcium Carbonate and a Gypsum Scale Inhibitor

Abstract

The focus of this study was to improve the Reverse Osmosis Cooling (ROC) process by using CaCO3 for neutralization and selective recovery of Fe(OH)3 at pH 3.5. By using a specific inhibitor, ferric hydroxide was recovered separately from gypsum and other metals present in mine water. Ferric hydroxide was processed to pigment, a product that is imported and used as colorant in paints and tiles. In addition to pigment recovery, aluminum hydroxide and calcium carbonate can also be recovered from mine water. The following conclusions were made: (i) the rate of gypsum crystallization, in the absence of Fe3+, is influenced by the over saturation concentration in solution, the seed crystal concentration and temperature; (ii) gypsum crystallization from an over-saturated solution, in the presence of Fe(OH)3 sludge, required an inhibitor dosage of 100 mg/L to keep gypsum in solution for a period of 30 min; (iii) gypsum crystallization from an over-saturated solution, in the presence of both Fe(OH)3 sludge and CaCO3 reactant, required a higher inhibitor dosage than 100 mg/L to keep gypsum in solution for a period of 30 min. A dosage of 200 mg/L kept gypsum in solution for the total reaction period; (iv) when only Fe(OH)3 is present in the slurry, gypsum inhibition is more effective when Fe(OH)3 sludge is allowed to settle after the initial mixing; (v) when both Fe(OH)3 and CaCO3 are present in the slurry, gypsum inhibition is more effective when the inhibitor is added over a period of time (10 min) rather than applying the total dosage at time zero; (vi) Fe(OH)3 can be changed to yellow pigment (Goethite) by heating to 150 °C and to red pigment (Hematite) by heating to 800 °C. Pigment of nano particle size was produced; (vii) in the case of Na2CO3, the TDS increased from 12,660 mg/L in the feed to 13,684 mg/L due to the replacement of metal ions (Fe3+, Al3+, Fe2+, Mn2+ and Ca2+) with Na+ in solution. In the case where CaCO3 was used for the removal of Fe3+ and Al3+, Ca(OH)2 for the removal of Fe2+, Mn2+, and Na2CO3 for the removal of Ca2+, the TDS dropped from 12,661 mg/L to 2288 mg/L, due to gypsum precipitation. The alkali cost in the case of calcium alkalis amounted to ZAR29.43/m3 versus ZAR48.46/m3 in the case of Na2CO3.