Dregs Carry Over Operational Trouble Shooting
Dregs contamination can cause major operational impacts including; poor liquor quality, reduced lime availability, decreased causticizing efficiency, decreased filter-ability in pressure filters and drum filters, increased soda carry-over, increased corrosion to and deterioration of refractory brick lining in the lime kiln and increased total system dead load. In systems which rely on white liquor clarifiers dregs can make a significant contribution to dirt in the finished product i.e. pulp/paper. Dregs particles are formed in the recovery boiler and largely consist of unburned carbon, iron, silica, alumina, magnesium and metal sulfides. Green Liquor and Dregs have similar densities making it challenging to settle Dregs and easy to re-suspend them. For additional information and tips to reduce Dregs carry over please call our Vancouver offices or visit www.recaust.com .
Common Causes of Dregs Carryover:
Common causes of Dregs carry over that can go overlooked on a day to day basis.
1. Increased Production Demand – Increases rise rate and reduces the slip velocity between the up flowing green liquor and down flowing Dregs particles. Reduced slip velocity translates into increased Dregs carryover. Old style feed-well and bustle pipe designs also impact Dregs settling as they do not remove the angular momentum effectively from the incoming feed. New designs like McFarlen Engineering’s ClearFlow™ feed-well and bustle pipe system are designed using 3D fluid computational methods that eliminate turbulence as well as angular momentum. The ClearFlow™ bustle pipe system reduces localized clarifier velocity gradients that can reduces clarifier efficiency.
2. Air or Vapor Entrainment – Will cause Dregs particles to be floated, suspended or agitated. It is important to clean and monitor the feed-well vent line to remove excess vapor. If the vent line is plugged vapor and air can build up inside the feed well reducing efficiency and causing the feed well to “burp”.
3. Clarifier Rake Speed – Due to the similar densities between Dregs and Green Liquor, rake tip speed can stir-up settled Dregs particles causing carry-over. Rake speeds will vary depending on the diameter of the clarifier. To insure the rake speed is within range it’s important to calculate the rake tip speed. It’s typically half that of a white liquor clarifier.
4. Flush Water – Leaving underflow flush water on to thin the sludge stream causes water to move counter current to the out-coming slurry. Due to its lighter density hot water can flow back through the sludge well and carry Dregs particles upwards toward the bustle pipe.