Olefin polymerization process

A process for the preparation of an ethylene polymer having a density, determined according to DIN EN ISO 1183-1:2004, Method A at 23° C. of from 0.918 g/cm3 to 0.970 g/cm3 and a melt flow rate MFR21 determined according to DIN EN ISO 1133:2005 at a temperature of 190° C. under a load of 21.6 kg from 0.1 g/10 min to 100 g/10 min, comprising polymerizing ethylene or copolymerizing ethylene and one or more other olefins in the presence of a Phillips-type chromium catalyst and an antistatic agent in a gas-phase polymerization reactor containing a reactor bed of particulate polymer, which gas-phase polymerization reactor is equipped with a cycle gas line for withdrawing reactor gas from the reactor, leading the reactor gas through a heat-exchanger for cooling and feeding the reactor gas back to the reactor, wherein the reactor gas comprises one or more C4-C6 alkanes in a content from 1 vol. % to 10 vol. % and the reactor gas returned to the polymerization reactor through the cycle gas line has been partly condensed and the amount of liquid in the reactor gas returned to the polymerization reactor is from 0.5 wt. % to 10 wt. %, the polymerization is carried out at a temperature from 108° C. to 125° C., and an aluminum alkyl of formula AlR3 or of formula AlRnR′m, in which R is a C4-C12-alkyl, R′ is a C4-C24-alkanediyl group which is bridging two aluminum atoms, and n+m=3, is fed into the polymerization reactor in an amount in the range of from 0.0025 mole to 0.1 mole per ton of dosed ethylene.