Process for preparing a catalyst composition for ethylene polymerization or copolymerization, with the steps of: (a) treating at least one magnesium alkoxide compound with the effluent waste from a process for production of a polypropylene catalyst of the Ziegler-Natta type, the effluent waste being mainly composed of about 10 to about 20 wt.-% transition metal based compounds, about 20 to about 50 wt.-% solvents, about 0 to about 2 wt.-% electron donors and impurities; (b) washing the product of step (a) with an inert hydrocarbon solvent; and (c) drying the material for use as the catalyst composition.