Method for treating solid residues containing heavy metals from combustion plants

A method of treating solid residues containing heavy metals from combustion plants, said method comprising the steps of:

• washing the residues with water in a washing stage to dissolve water-soluble compounds including heavy metals therein into solution;
• thereafter separating insoluble solid residues from the solution of heavy metals in a dewatering stage;
• removing the separated insoluble solid residues from the dewatering stage to a storage station;
• correcting the pH of the solution containing heavy metals from the dewatering stage by adding at least one alkaline solution in a neutralizing stage to adapt the pH to the specific metal impurities;
• precipitating hydroxides of the heavy metals from said pH-adapted solution in a precipitation stage;
• thereafter adding a precipitating agent to the pH-adapted solution to form a mixture containing insoluble heavy metals;
• separating a sludge containing heavy metal precipitate from the mixture by passing the mixture through a cross-current diaphragm filter in a filter stage to separate purified liquid from the mixture;
• concentrating the dewatered mixture from the filter by a sedimentation stage while removing the purified liquid from the filter;
• and
• re-cycling the sludge containing heavy metals from the sedimentation stage to the dewatering stage to separate the insoluble heavy metals therein for delivery to the storage station.