Removal of toxic and alkali/alkaline earth metals during co-thermal treatment of two types of MSWI fly ashes in China

•Most alkali/alkaline earth metals could be removed.•Cd and Pb can be nearly completely removed under all conditions.•Alkali/alkaline earth metals can promote removal of Cu by formation of CuCl2.•The removal of Zn and Cr was low due to formation of stable compounds.•Removal of Zn was accelerated and Cu was hindered in N2.