Use of MgSO4 during washing
It is also possible to diminish the release of heavy
metals by decreasing the pH, which is controlled by
Ca-salt solubility. Most of the heavy metals show the
minimum solubility of their (hydroxides within the
pH range of 9–11. In this case, the pH may be corrected
by the addition of additives during the washing process.
The use of diluted mineral acids (nitric acid or chloride
acid) increases the metal extraction during the washing
steps, leading to highly contaminant wastewater. Moreover,
the reaction of the mineral acid with APC ash is
always very exothermic, adding high anionic concentration
to wastewater and APC residue. Nevertheless, the
addition of a weak acid substance will enhance the reaction
with Ca(OH)2 and decrease the pH to within the
optimum range where the metal solubility is minimum.
In this way, the heavy metals will remain stabilized into
the final residue.