Paper Mill Prethickening Options

December 12, 2000

In some pulp & paper screw press applications there is no need to prethicken the flow to the press. This is the case both with rejects from pressure screens and knots from shaker tables. Similarly, most clarifier underflow has 2% to 3% solids consistency, which, especially in virgin fiber mills, is adequate for dewatering in a screw press.

Even in these easy applications it is recommended that a simple sidehill screen be installed over the inlet to the screw press. This assures proper press operation even under upset or shut-down conditions where the flow can become very dilute. (In dilute flow conditions the high flow of water through the press screen washes the solids through the screen so that the capture rate becomes unacceptable.)

Where a paper mill wastewater flow is being coagulated and/or flocculated, there is an aversion to using a sidehill screen. This is because the surface of the screen will become coated with the polymerized sludge, blinding the screen.

In these installations there are a number of prethickening machines that can be used. The simplest is a rotary drum screen. The use of continuous sprays on the outside of the drum keep the screening surface clear. FKC is a competitor who makes frequent use of these devices. Where the mill specifies a rotary drum screen ahead of the press, Vincent has included equipment designed and manufactured by IPEC Industries in British Columbia.

Many times the mill will specify that a gravity table (or table thickener) be supplied with the press. These are a specialty of Andritz, a very strong competitor that offers the Dupps screw press. A gravity table is in principle the upper deck of a belt press.

Probably the most effective prethickening machine is a belt press. Cake solids from a belt press will range from a low of 20% where there is a high content of secondary sludge, to a high of 35% or 40% solids where there is no biological content. For the last five years Vincent presses have been offered in conjunction with Phoenix belt presses.

In one mill Vincent screw presses were supplied along with gravity tables whose cake was fed to belt presses before being fed to the Vincent presses. The final cake is burned.

Issue 113