a method of producing chemical cellulose pulp from sawdust utilizing a static down - flow retention vessel comprising the steps of continuously pressurizing a flow of sawdust using a progressive cavity pump to produce a pressurized sawdust feed for a treatment vessel , passing the pressurized sawdust feed at super - atmospheric pressure downwardly in the treatment vessel , adding treatment white black liquor to the treatment vessel , and treating the pressurized sawdust feed in the treatment vessel with treatment white black liquor to form a treated sawdust , and discharging treated sawdust from the treatment vessel .