process for preparing a catalyst composition for ethylene polymerization or copolymerization , with the steps of : 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 ; washing the product of step with an inert hydrocarbon solvent ; and drying the material for use as the catalyst composition .