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.