The disclosure concerns both a process and an apparatus for producing wood pulp. Wood to be pulped is fed into a housing where the wood is ground. The housing is pressurized above atmospheric pressure. Water at a temperature over 100 DEG C. is fed to the pressurized housing in the grinding zone. The wood pulp produced by grinding is collected and is conveyed through a worm conveyor which causes the water in the pulp to be removed therefrom, while the pulp and the water removed therefrom are maintained at elevated pressure. The water under pressure is returned to the grinding zone in the housing. The wood pulp is thereafter returned to atmospheric pressure. Various pressure lock devices for transmitting the wood pulp from being under pressure to atmospheric pressure are disclosed including a nozzle, a pressure lock device, a hydrocyclone and/or a disc refiner.
