The present invention provides a method of simultaneously recycling plastics and detoxifying chromite ore processing residue with residual heat from a steel slag. By heating and gasifying plastics with steel slag, followed by catalytically split-decomposing the plastics with catalysts such as chromite ore processing residue, the plastics are thoroughly converted into a energy gas under water vapor gasification. The surface coking of Chromite Ore Processing Residue is avoided. Meanwhile, the energy gas reduces Cr.sup.6+ in Chromite Ore Processing Residue into Cr.sup.3+, and the energy gas is cooled, and CO.sub.2 and Cl in the energy gas are adsorbed by alkaline substances in Chromite Ore Processing Residue. With this method, chromite ore processing residue is detoxified, and steel slag is cooled, furthermore, energy is saved and a energy gas is obtained.

The liquid steel slag (2) is poured in at least four successive layers (6-8), comprising a lowermost layer (6), an uppermost layer (7) and at least two intermediate layers (8), in a reservoir (3) and the layers of liquid steel slag are allowed to solidify. After having applied the uppermost layer (7), the solidified steel slag is cooled down more quickly by means of water. The average temperature of each of the intermediate layers (8) is kept at least until the start of the water cooling step, and this for at least one hour, above a minimum temperature which is equal to or higher than the temperature at which -dicalcium silicate is formed. In this way, more crystalline phases different from dicalcium silicates are formed so that the formation of fines by the transition of into -dicalcium silicates can be considerably reduced. A glassy material and/or a phosphorus containing compound is preferably added to the liquid slag to further reduce the formation of fines.

A method for producing hydrogen and/or other gases from steel plant wastes and waste heat is disclosed. The method comprises the steps of providing molten waste from steel plant like molten slag in a reactor. The molten slag is contacted with water and/or steam in the presence of a reducing agent to form a stream of hydrogen and/or other gases. The hydrogen and/or other gases can then be extracted from the stream of gases from the reactor.