Wet strength paper repulping

Recovered paper is the most important raw material in the world for the production of paper and paperboard (Hoke, 2009). The availability of recycled fibre is coming under pressure from competition for biomass for different uses, such as construction materials, biorefinery applications and the production of energy (Hoke, 2009). As the demand for secondary fibre increases, paper made with wet strength resins are becoming a more important source of fibre. Currently, wet strength paper entering traditional recycled paper mills ends up in the waste stream (Hoke, 2009). So not only is this a lost opportunity, but also a significant processing cost for its removal and disposal. Therefore it will become increasingly necessary to ensure that wet strength products are firstly removed from comingled recycling streams, and then secondly utilised in an economical manner. This investigation used a wet strength product, Coated Carrier Board (CCB) from Amcor Cartonboard Petrie Mill to investigate variables including chemical type, chemical concentration, temperature and pulping time with the aim of studying the process economics using conventional equipment and chemicals. Pulp tests include Canadian Standard Freeness, visual assessment and rejects %wt. Experimentation was completed in three stages, Scoping Laboratory Trials to investigate chemical types, Laboratory Trials to investigate the effects of variables and finally Industrial Pilot Scale Trials to observe real world effects. This study has presented an experimental methodology, investigated the effect of variables, and then considered their economic outcome. Finally, and most importantly it has been demonstrated that there is more than one way to achieve a desired rejects level. By observing the line of fit for the lowest cost for reject level, these optimum combinations can be selected on their merit on the basis of other operational aspects. Anecdotal evidence of the usable nature of the pulp was gained in industrial pilot scale trials. Use of chemical aids was found to be necessary for repulping CCB to achieve satisfactory rejects levels. Hydrogen peroxide was ineffective at repulping, expensive and hazardous. Hypo produced successful repulping results in tap water, however did not in mill white water, these were therefore eliminated as candidates. Potassium Persulphate used in conjunction with Sodium Hydroxide (Caustic Soda) exhibited a small but positive effect in laboratory trials, however failed to provide a positive effect in industrial pilot scale trials, therefore suggesting it is not an effective additive for CCB repulping. Of the chemical combinations trialled, it was therefore concluded that Caustic Soda was most effective and economical. The experimentation undertaken successfully observed the effects of the selected variables on rejects level. It was observed that time had the greatest and most consistent effect on reducing reject content. In Laboratory trials from 10mins to 20mins pulping caused an approximate 75 percent reduction in rejects and this was repeated in Industrial Pilot Scale Trials with thirty to ninety minute trials exhibiting approximately the same reduction. The repulping process was determined to follow first order kinetics with respect to rejects %wt. It was also established that even though the addition of chemicals and increased temperature contributes to increased cost per Accepts Tonne, when aiming for low rejects level these costs are necessary. Increasing pulping time, therefore energy, was found to have the greatest effect and paid for itself up to approximately 550kWh/Tonne. The variable of second greatest effect was temperature, where an approximately 50 percent reduction of rejects was observed from 45°C to 75°C. In the social, economic and environmental climate requiring increasing use of recycled feedstock, it is an important result that repulping wet strength paper is feasible with existing paper mill recycling equipment and commercially available chemicals, and can likely be done below the cost of traditional wastepaper.