Mike Ryan, a chemical engineer and consultant with extensive international expertise in rotary kilns, pulp mill lime kilns, and recausticizing processes, shares his insights on the critical role of green liquor dregs in bleach plant performance.
Drawing on decades of experience in system optimization and industrial process improvement, he demonstrates how tighter green liquor clarification can reduce chemical demand, prevent scaling, and improve operational efficiency—showing that effective mill housekeeping translates directly into cost savings and greater reliability.
Case Study: Facing cost pressure at a 1,500 odtpd bleached pulp mill, we targeted the bleach plant, hypothesizing that excessive green liquor dregs containing C, Ca, Mg, Mn, Fe, Ba, and Al were a primary culprit of high bleaching chemical demand.
When green liquor clarification is poor, these non-process elements slip into the white liquor, reaching the fiber line with severe consequences: catalytic metals (Mn, Fe) consume ClO₂ and H₂O₂, forcing chemical overdoses to maintain brightness. Calcium and barium form tenacious oxalate scale, clogging equipment and increasing downtime. Unstable chemistry also drives brightness reversion, prompting over-bleaching.
To quantify this, we executed a 14-day paired trial: 7-day baseline at 225 ppm followed by 7 days of optimized dregs control at 50 ppm via polymer overdosing. We tracked metals in white liquor and pulp, alongside key operational metrics like ClO₂ demand and peroxide half-life, all normalized to odt.
The analytical work was designed to trace the fate of the contaminants and their specific impacts:
Inductively Coupled Plasma Optical Emission Spectrometry (ICP-OES) was used to quantify metals (Mn, Fe, Ca, Ba, Al) in liquors and in pulp via DTPA extraction.
Ion Chromatography (IC) measured anions like chloride and sulfate.
Standard titrations confirmed white liquor quality (alkalinity, sulfidity).
Performance tests included turbidity, TSS, COD, and, crucially, bench-scale chlorine dioxide demand and peroxide half-life measurements on pulp filtrate to directly assess the catalytic effect of metals.
Statistical analysis using t-tests and CUSUM plots was employed to identify significant shifts in chemical demand between the trial periods. Daily KPIs were tracked and visualized to provide the operating team with actionable data in near real-time.
The results were definitive. Enhanced dregs capture drastically reduced transition metal loads. This directly correlated with a drop in oxidant consumption from 13.8 to 8.5 kg CLO2/odt. The trial projected annual chemical savings in the high six figures, with added gains from reduced scaling, improved uptime, and superior product stability. This confirms that tight green liquor clarification is essential housekeeping, offering a fast ROI by relieving chronic pressure on the bleach plant.
Dregs and non process elements increase the energy demand at the lime kiln. Poorly removed green liquor dregs carry calcium, magnesium, and other solids into black liquor, where they form scale and deposits in evaporators, reducing heat-transfer efficiency and increasing maintenance and steam costs. This also exacerbates non-process element buildup downstream, driving higher chemical demand in the bleach plant.
Some of your mill’s best investments are in green liquor clarification.
About Mike Ryan
Mike Ryan is recognized for enhancing operational reliability and efficiency, combining deep technical expertise with a strong focus on training and process improvement across a wide range of industrial settings.
He can be reached 24/7 by mobile at +1 (740) 649-2272 or by email at
