Acidic Pretreatment
High-temperature dilute acid treatment applied to unbleached chemical pulp hydrolyzes hexenuronic acid groups from the xylan backbone prior to modern bleaching stages. Performing hot acid hydrolysis lowers the consumption of chemical oxidants in elemental chlorine free bleaching plants. The process operates within a temperature window of 85 to 110 degrees Celsius at pH levels maintained between 3.0 and 3.5 using sulfuric acid.
Pulp slurries pass through specialized pressurized retention vessels for 60 to 180 minutes. The treatment selectively cleaves acid-labile carbon-oxygen bonds without excessively degrading resistant crystalline cellulose polymers. Hydrolyzed fragments and solubilized transition metals are washed out before subsequent bleaching stages.
Chemical Mechanism
Hexenuronic acid groups formed during high-temperature alkaline kraft cooking contribute significantly to pulp kappa numbers and consume bleach chemicals uselessly. Integrating hot acid hydrolysis converts these unsaturated uronic acid units into furan derivatives such as 2-furoic acid and 5-formyl-2-furoic acid. The selective removal of these structures reduces the overall unbleached kappa number by two to five units without expending chlorine dioxide or ozone.
Pulp brightness stability improves because unsaturated hexenuronic structures, which are notorious for post-bleaching optical reversion and yellowing, are systematically removed. Careful acid dosing and temperature control prevent unwanted acid-catalyzed cellulose depolymerization and fiber strength loss.
Pulp Quality
Paperboard grades specified for archival print media, premium packaging and photographic backings benefit from raw pulps subjected to hot acid extraction. Pulp treated via hot acid hydrolysis exhibits higher ultimate brightness ceiling potential and reduced brightness reversion during thermal boxboard drying processes. Papermakers achieve required tensile, burst and tear specifications using lower total bleaching chemical charges per air-dry tonne of finished pulp.
Controlling scale precipitation, specifically calcium oxalate deposition on downstream washer surfaces, represents the principal operational constraint in continuous commercial operations. Acid hydrolysis remains a cornerstone pre-bleach technology in modern hardwood kraft pulp mills.
