Hydrogen peroxide and caustic soda: Dancing with a dragon while bleaching

When hydrogen peroxide is mixed with caustic soda, an auto-accelerating reaction can lead to gen­ eration of significant amounts of heat and oxygen. On the basis of experiments using typical pulp mill process con­ centration and temperatures, a relatively simple kinetic model has been developed. Evaluation of these model results reveals that hydrogen peroxide-caustic soda systems are extremely sensitive to hydrogen peroxide:caustic soda ratio, transition metal contamination, and temperature. Small changes in initial conditions can result in a close system becoming explosive. Analysis of model results was used to develop guidelines for safer application of hydro­ gen peroxide in a mill setting. Application: Because peroxide at 10% cannot generate an explosive mixture with sodium hydroxide, mills that either install new or retrofit bleaching systems should consider installing a dilution system that will take the peroxide from the storage concentration of 50% to 10% before introduction into the mixer. ydrogen peroxide is an environmentally compatible ready source of oxygen; thus, typical extinguishing technolHbleaching chemical that has been employed in kraft ogy that uses exclusion of oxygen as a mode of action (e.g., mills for many years. As part of a bleaching sequence, foams or dry powders) is much less effective. hydrogen peroxide brightens and provides brightness Runaway decomposition of hydrogen peroxide has resultstabilization, if used as the last stage in a multistage bleach ed in catastrophic events in several mills over the last 15 years. plant [1-4]. Because hydrogen peroxide bleaching does Two bleach plants experienced pump explosions when hy­ not delignify fibers, it does not produce as many soluble drogen peroxide came into contact with caustic soda (NaOH) by-products as other popular bleaching chemicals; thus, it in the absence of pulp and resulted in serious injuries [12,13]. can reduce organic load to wastewater treatment plants. Both mills had been using 50%-concentration hydrogen perHydrogen peroxide is delivered to pulp mills via tank truck or railcar. Truck deliveries are near 50% conOther incidents, one in Europe and one in North centration [5]. Railcar deliveries may be either at America, resulted from rapid decomposition with­ 50% or 70% concentration. If a mill receives hy­ in a storage tank when an organic chelant was drogen peroxide deliveries at 70% concentra­ mistakenly mixed with hydrogen peroxide tion, it is routinely diluted to 50% concentration [8,14]. In those cases, the explosions were likeas it is unloaded. Thus, industrial-strength hy­ ly caused by a combination of alkaline-catalyzed drogen peroxide employed in kraft pulp bleachdecomposition and rapid oxidation of an organic ing is generally near 50% concentration. chemical. Alkali-catalyzed decomposition of hydrogen per­ oxide follows second-order reaction kinetics, as shown in Eq. (1) [15]. The rate of decomposition depends on the concentration of both hydrogen ide decomposes at a rate of about 1% per year [6]. Traces of transition metals (e.g., iron, copper, and manganese) increase hydrogen peroxide decompo­ sition [7], and suppliers add stabilizers to sequester these metals. Fifty-percent-concentration hydrogen peroxide solutions repre­ tential for rapid pressurization within tanks, pipes, pumps, and/ or storage vessels; potential for fire due to formation of oxygen during decomposition; hydrogen peroxide-organic vapor phase explosions; and runaway decomposition [8-11]. During startup conditions, fires resulting from the reaction of hydrogen peroxide with partially dried pulp on standpipe walls have been reported. Industrial-strength hydrogen per­ oxide fires are extremely dangerous because they provide a oxide without incident for several years.