The incineration of municipal solid waste generates mineral residues which contain soluble chlorides and heavy metal pollutants. The mineral residue is currently landfilled after a cement solidification. An alternative treatment, the new patented by Solvay process uses phosphoric acid to stabilise mineral residue by the formation of stable minerals such as calcium phosphates which are insoluble in natural environments and can incorporate heavy metals inside their crystalline structure. Two different water-washed municipal solid waste incinerator mineral residues are investigated during the phosphate treatment. Phosphoric acid reacts exothermically with mineral residues with rapid kinetics of the dissolution-precipitation type. Second order rate is observed with respect to the phosphate concentration and activation energies for the phosphate reaction are found to be small (near 20 kJ/mol). Mineral residues react with phosphoric acid mainly as calcium carbonate does. Precipitated amorphous calcium phosphates coat the more inert particles (silicoaluminates, silicates, and calcium sulphates…). The two mineral residues with different compositions show similar behaviour. This indicates the phosphate stabilisation procedure can be widely applied. This chemical treatment can be considered as a first essential step in the effective insolubilisation of extractable trace heavy metals in a mineral material that could find in the future an beneficial reuse after further processing as secondary raw material.