A slurry polymerization process for the preparation of polyethylene in a reactor cascade of two or more polymerization reactors comprising the steps of: a) feeding to a polymerization reactor amounts of ethylene, of a Ziegler catalyst, of fresh aluminum alkyl and of a diluent and optionally amounts of hydrogen and optionally amounts of one or more C 3 to C 10 alpha-olefins; b) contacting in the polymerization reactor at a reactor temperature from 60°C to 95°C and a reactor pressure from 0.15 MPa to 3 MPa, the amounts of the ethylene, the Ziegler catalyst, the fresh aluminum alkyl, the diluent and optionally the hydrogen and the C 3 to C 10 alpha-olefins, thereby forming a slurry product comprising particulate polyethylene and a suspension medium; c) withdrawing the slurry product from the polymerization reactor; d) feeding the slurry product to a second polymerization reactor of the reactor cascade, e) feeding to the second polymerization reactor additional amounts of ethylene, of diluent and optionally additional amounts of hydrogen and of C 3 to C 10 alpha-olefins; f) contacting in the second polymerization reactor at a reactor temperature from 60°C to 95°C and a reactor pressure from 0.15 MPa to 3 MPa, the slurry product fed to the second polymerization reactor and the additional amounts of ethylene, diluent, and optionally hydrogen and C 3 to C 10 alpha-olefins, thereby forming an additional amount of polyethylene in the slurry product; g) and withdrawing the slurry product from the second polymerization reactor, wherein the ethylene is first passed through an ethylene purification unit, which reduces at least the concentration of carbon monoxide, carbon dioxide, oxygen, acetylene and water contained in the ethylene, before it is fed to the two or more polymerization reactors of the reactor cascade. a) feeding to a polymerization reactor amounts of ethylene, of a Ziegler catalyst, of fresh aluminum alkyl and of a diluent and optionally amounts of hydrogen and optionally amounts of one or more C 3 to C 10 alpha-olefins; b) contacting in the polymerization reactor at a reactor temperature from 60°C to 95°C and a reactor pressure from 0.15 MPa to 3 MPa, the amounts of the ethylene, the Ziegler catalyst, the fresh aluminum alkyl, the diluent and optionally the hydrogen and the C 3 to C 10 alpha-olefins, thereby forming a slurry product comprising particulate polyethylene and a suspension medium; c) withdrawing the slurry product from the polymerization reactor; d) feeding the slurry product to a second polymerization reactor of the reactor cascade, e) feeding to the second polymerization reactor additional amounts of ethylene, of diluent and optionally additional amounts of hydrogen and of C 3 to C 10 alpha-olefins; f) contacting in the second polymerization reactor at a reactor temperature from 60°C to 95°C and a reactor pressure from 0.15 MPa to 3 MPa, the slurry product fed to the second polymerization reactor and the additional amounts of ethylene, diluent, and optionally hydrogen and C 3 to C 10 alpha-olefins, thereby forming an additional amount of polyethylene in the slurry product; g) and withdrawing the slurry product from the second polymerization reactor,