Dehydroxylation - Crystallization (Aging) and Oxidation Rate in the One-Step Ambient Temperature Ferrite Process: Removal of Non-Ferrous Metals from Acid Mine Drainage
Report No. 1454/1/05
August 2005
Executive Summary

Dehydroxylation – crystallization (aging) and oxidation rate in the one-step ambient temperature ferrite process for the removal of metals from acid mine drainage

A novel one-step ambient temperature ferrite process for the removal of metals from acid mine drainage was previously reported (WRC Report No: 1244/1/03, ISBN: 1-86845-825-5). The present report describes further research undertaken to (1) better characterize and quantify aspects of the process and (2) test the capacity of the process for non-ferrous metals removal from acid mine drainage (AMD).

In the one-step ambient temperature ferrite process (ATFP) lime is used to raise the pH to 10.5 so that Fe2+ present in AMD precipitates as ‘ferrous intermediate’ (FI). FI is then oxidized to magnetite (Fe3O4). For this reaction to occur, magnetite seed and a threshold FI concentration are required. Magnetite differs from other iron oxides in that Fe is present in both divalent and trivalent forms. Magnetite formation therefore requires that the oxidation rate of ferrous to ferric does not exceed the rate at which ferrous iron is incorporated into the crystal structure (dehydroxylation-crystallization). If the oxidation rate is too high, ferric-only oxides are formed, an effect exacerbated by calcium (Ca : Fe = 1 : 1 from use of lime).

It was found that magnetite formation in the presence of calcium requires a high magnetite seed concentration (~20 g/L), a threshold FI concentration ≥ ~1700 mg/L, an oxidation rate ≤ 8.5 mg/L/min and an aging period ≥ 3 hours. High FI concentrations are easily maintained by a contact stabilization reactor - settler sequence whose further advantage is the early separation of the metals-containing AMD stream from the bulk AMD volume, resulting in significantly smaller infrastructure needs.

Results for heavy metals removal in the presence of calcium for the test-cases of (1) cobalt (Co : Fe = 1: 10), (2) Co, Ni, Zn (Co : Ni : Zn : Fe = 2.5 : 2.5 :2.5 :100) and (3) Co, Ni, Zn, Mn (Co : Ni : Zn : Mn : Fe = 2.5 : 10 : 2.5 : 2.5 : 100) are also presented.

The 1-step ATFP here described can be used, not only for the removal of metals from AMD, but is in fact applicable to any industrial waste-stream containing dissolved metals (even if iron is absent). The process therefore has great potential to contribute to the improvement of water quality in South Africa and abroad.