Environmental and strength properties evaluation of bleached recycled pulp with calcium hydroxide

The aim of the current study was to investigate the effect of using different charges of calcium hydroxide (as an alternative alkali source for sodium hydroxide) in hydrogen peroxide bleaching system of recycled pulp on paper strength properties and chemical oxygen demand (COD) of wastewater. The pulp mixture including recycled 70% newspaper and 30% magazine was deinked using a conventional (chemical) system and then bleached using a conventional hydrogen peroxide system under specified process conditions. The results indicated that the use of calcium hydroxide was relatively effective in enhancing the paper strength properties so that at the usage range of 2-4% calcium hydroxide, no significant reduction was observed in strength properties. On the other hand, with increasing the charge of calcium hydroxide to more than 4%, the strength properties of the paper increased relatively compared to the control sample (conventional peroxide bleaching system including sodium hydroxide alkali). Maximum values of tensile (35.64 Nm/g) and burst (2.98 kPa.m2g-1) indices were determined in experimental run C5 (10% calcium hydroxide) and maximum tear index (6.88 mN. m2g-1) was observed in experimental run C1 (2% calcium hydroxide). The amount of wastewater COD was much less than the control sample when sodium hydroxide was removed completely and replaced by calcium hydroxide in hydrogen peroxide bleaching, so that with increasing calcium hydroxide usage from 2% to 10%, the wastewater COD amount was still less than the control sample. Also, the results of the evaluation of the scale amount in the experimental run included 2% calcium hydroxide in comparison with the control sample indicated that the amount of scale in the bleaching wastewater was 145.24 mg/ml with 2% calcium hydroxide and in the control sample was obtained 32.87 mg/ml.