Medium consistency (MC) unit operations are becoming increasingly widespread in pulping and bleaching processes. Many MC devices, notably MC pumps, mixers andscreens,relyon creation of a fluid-like state in the pulp suspension. This fluidization requires the dissipation of considerable energy and exposes the pulp fibres to mechanical treatment. The treatment alters pulp physical properties.
The changes in properties of a never-dried semi-bleached softwood kraft pulp with mechanical treatment were determined in a batch operated concentric-cylinder device. A fluid-like state was created in medium and low consistency pulp suspensions with treatment energy varying up to 10 MJ/kg pulp. The treatment energies encompassed the range used in mill and batch laboratory operations and the extensive exposure given during batch MC fluidization studies. MC treatment was found to beat, curl and microcompress the pulp fibres. The implications of MC treatment in mill devices are discussed.