Abstract: Method and device for separating harder from less harder substances of a compact mass block (1) consisting of hard and less hard substances, such as magnesium or aluminum from slag and refuse containing magnesium or aluminum, thereby directing at least one fluid beam having at least 400 bar pressure against the mass block (1) for separating the harder from the less harder substances by breaking out pieces from block, whereby the nozzle or nozzles (11) and the mass block (1) are moved in relation to each other and the substances are cooled with water to reduce metal loss by oxidation.