The leaching curves in Fig. 4 clearly show that Ca and S
may be depleted from the solid residues at L/S ratios (200–
900 l/kg) lower than most other elements. Conversely,
these L/S ratios correspond to rather high pH-levels: about
pH 10–11 for the non-carbonated residues and pH 8–10 for
the carbonated residues. This suggests that discussing Ca
and S leaching at low pH-values, for example pH 4–6,
may not be relevant for actual leaching scenarios as both
Ca and S will most likely be depleted before the pH reaches
that low. Similarly, it may be realized that heavy metals
like Cd, Ni and Zn may also be depleted before the pH
reaches the acidic range. In Fig. 5, Cd, Ni and Zn appear
to be depleted around L/S 4000–6000 l/kg. From Fig. 3,
it can be seen that L/S 5000 l/kg corresponds to a pH-value
of about 6. Thus, these heavy metals may be removed by
the leachate before reaching very low pH-values, such as
pH 4. Therefore, the high leachability of cationic heavy
metals at very low pH levels may not pose a hazard.
This illustrates that it is very important to discuss leaching
test results in the context of actual leaching scenarios
before applying conclusions regarding leachate concentrations
and leaching control at pH-values in the acidic range
to full-scale leaching.
3.8. Implications