Concurrent neutralization and flocculation of acidic pulp bleaching effluent using phosphorylated cellulose dissolved in sodium hydroxide

Pulp mills are significant water consumers, and the main culprit in a modern mill is commonly the acidic stage of pulp bleaching at 5–10 m 3 per air-dry ton of pulp. A major limiting factor in the water reuse from this bleaching stage are calcium and oxalate ions, which begin to precipitate if they accumulate in the process water causing operational and quality problems. When the acidic bleaching effluent is neutralized with NaOH, a portion of calcium oxalate crystals remain suspended in the liquid and the effluent turbidity is high. We found that phosphorylated microcrystalline cellulose (PMCC) can be dissolved in NaOH solution, and the solution can be used to neutralize the effluent to reduce the turbidity caused by suspended calcium oxalate. When bleaching effluents were neutralized to pH 7–8 with PMCC/NaOH solution >90% reduction in turbidity was achieved. The source of turbidity was analyzed with SEM and XRF. The turbidity after neutralization was predominantly caused by suspended calcium oxalate crystals that were captured on the surface and inside the regenerating PMCC particles during mixing and settling. This finding suggests that pulp mills could reduce the amount of calcium oxalate in bleaching effluents using a cellulose-based solution manufactured with safe and affordable chemicals to avoid scaling problems related to reuse of acidic process waters and to reduce the overall water consumption of the mill.