Pressure filtration is used to improve white liquor and weak wash quality in the recausticizing process. There are a few common problems that affect the performance of these filters. McFarlen Engineering has helped many mills “tune” there systems for best results. For additional information and tips to improve pressure filter operation please call our Vancouver offices or visit www.recaust.com.
Common Causes of High Pressure:
High pressure occurrences can be the result of several different conditions and operational choices. Here are three of the common causes;
1. Over-Liming – pressure rises slowly over a long time-period – occurs when more than the stoichiometric quantity of lime (CaO) is added to green liquor (specifically Na2CO3, the controlling reagent) at the Slaker overloading the Causticizer train with slaked lime, often referred to as free lime. This Calcium hydroxide is a smaller, flatter, particle as compared to that of the larger, more spherical calcium carbonate. These excess Ca(OH)2 particles settle poorly and easily plug filter media in the pressure filters unlike CaCO3 which provides excellent liquid-solid separation. Recovery from over-liming can occur quickly if the proper equipment and controls are in place and crucial actions are taken to ease or eliminate the effects of an excursion. Over-liming may be prevented by understanding and controlling critical process elements around the plant.
2. High Mud Level – pressure rises quickly over a few filtration cycles – a result of a more closed positon on the underflow valve than required. Better control or observation of body mud level and recovery protocols should be in place for rapid return to normal operation. Underflow valve position and sizing are crucial to prevent frequent valve replacement and line repair.
3. Dregs Carry Over – GORE® filter socks are much more forgiving for dregs excursions because they utilize “surface filtration” via a patented PTFE membrane that is bonded to a polypropylene felt substrate. Non-membrane and “look-a-like” socks which provide “depth loading filtration” are much less effect because they allow dregs to penetrate the sock felt easily blinding the media. If a dregs excursion is extensive a sock change will be necessary.
Dregs impact the recausticizing system in many ways. Our next blog will address some of these issues.
Timing, backwash volume and valve speed all contribute to good Pressure Filter operation. They should be “tuned” before placing them in operation. This promotes smooth and easy operation, extends sock life and periods between acid washes.