Determination of contaminant particles in an aqueous pulp

A method for determining contaminant particles in an aqueous paper-making pulp comprising

• a) providing a screen assembly comprising a screen supported in a housing, said housing having first and second zones separated by said screen,
• a first port in said housing communicating with said first zone, and a second port in said housing communicating with said second zone,
• b) feeding an aqueous paper-making pulp containing contaminant particles tangentially into said first zone through said first port, thereby producing a separation of heavy contaminants from lightweight contaminants in said first zone, feeding pulp fibres and lightweight contaminants along a first flow path adjacent said screen in said first zone, and feeding the pulp fibres through said screen to deliver a flow of a screened aqueous suspension of pulp fibres from said first zone to said second zone while retaining the heavy and lightweight contaminants present in said aqueous pulp, in said first zone,
• c) withdrawing the screened suspension through said second port from said second zone,
• d) discontinuing steps (b) and (c), feeding a rinse fluid into said first zone to entrain residual pulp fibres in said first zone, and flush them through said screen as a flushed fibre suspension and withdrawing the flushed fibre suspension through said second port from said second zone, while retaining said heavy and lightweight contaminants accumulated in said first zone,
• e) discontinuing step (d), feeding a flush fluid into said first zone to entrain contaminant particles accumulated in said first zone, and flowing said entrained particles out of said first zone through said first port, and
• f) recovering the contaminant particles from said flush fluid.