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 (hydr)oxides 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.
Use of MgSO4 during washingIt is also possible to diminish the release of heavymetals by decreasing the pH, which is controlled byCa-salt solubility. Most of the heavy metals show theminimum solubility of their (hydr)oxides within thepH range of 9–11. In this case, the pH may be correctedby the addition of additives during the washing process.The use of diluted mineral acids (nitric acid or chlorideacid) increases the metal extraction during the washingsteps, leading to highly contaminant wastewater. Moreover,the reaction of the mineral acid with APC ash isalways very exothermic, adding high anionic concentrationto wastewater and APC residue. Nevertheless, theaddition of a weak acid substance will enhance the reactionwith Ca(OH)2 and decrease the pH to within theoptimum range where the metal solubility is minimum.In this way, the heavy metals will remain stabilized intothe final residue.
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