International Slag Valorisation SymposiumLeuven6-7/4/2009 - Third ...

Characterisation of leaching processes
The pH-dependent leaching characteristics of both the fresh and carbonated steel slag
samples were determined in a pH-stat system. Eight suspensions of a slag sample and
Nanopure-demineralised water, at an initial liquid to solid (L/S) ratio of 10 L/kg, were
stirred for 48 h in closed Teflon reaction vessels at room temperature. For seven vessels,
the pH was controlled automatically ± 0.2 pH units around a pre-set pH-value (pH = 2-
12.5) by the addition of 1 or 5 M HNO3 and 1 M NaOH. For one vessel, the pH was not
adjusted and leaching was performed at the native pH of the sample. After 48 h, the pH
and redox potential of the suspensions were determined and the suspensions were
filtered through 0.2 µm filters.
Column leaching (percolation) tests were performed according to CEN/TS 14405. The
steel slag was added to a borosilicate glass column (inner diameter 5 cm) in layers of a
few cm and packed by shaking and pushing gently with a rod to a filling height of ± 20
cm. Nanopure demineralised water was used as the leachant. The packed columns were
water-saturated and pre-equilibrated for 72 hours, as prescribed by CEN/TS 14405, after
which the influent was pumped in up-flow direction. Computer-controlled flow
controllers assured a constant flow velocity during the experiments. Fractions were
collected automatically at cumulative L/S values of 0.1, 0.2, 0.5, 1, 2, 5 and 10 (l/kg).
Effluent fractions were collected in acid-cleaned PE bottles. Shortly after collection of
each effluent fraction, pH, redox potential (Eh) and conductivity were determined, and
sub-samples for chemical analysis were taken and filtered through 0.45 µm membrane
filters.
The filtered leachates from the pH-stat and column experiments were analysd for a large
number of major and trace elements. 1,3)
Geochemical Modelling
Geochemical modelling was performed on pH-stat leachates and process water samples
from the carbonation experiments to identify the leaching processes. Details of the
model set up in the geochemical modelling framework ORCHESTRA, considering
aqueous speciation, mineral dissolution/precipitation and sorption processes, are
provided by Huijgen & Comans. 3) Selective chemical extractions were performed on the
fresh and carbonated pH-stat samples to obtain model input parameters for the amounts
of reactive amorphous and crystalline Al-, Fe- and Mn-(hydr)oxide minerals in the steel
slag matrix, which were considered to potentially control sorption processes. 3) The
element concentrations in the leachates were modelled over the entire pH range. For all
sorbates except Mo, the measured concentration at pH = 2 was taken as an estimate of
their availability (i.e., the maximum fraction of an element that can be leached), since it
was assumed that the available element fraction is completely leached at this pH. For
(anionic) Mo, the highest concentration was measured at alkaline pH-range and taken as
1 st International Slag Valorisation Symposium│Leuven│6-7/4/2009
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