SUMMARY:The potential of using acid hydrogen peroxide for lowering the electrical energy consumption during production of Black spruce (Picea mariana) thermomechanical pulp (TMP) was investigated. The chemical system, which consisted of ferrous sulphate, hydrogen peroxide and optionally an enhancer [ethylenediaminetetraacetic acid (sodium salt), 3,4-dimethoxybenzyl (veratryl) alcohol or oxalic acid/sodium oxalate], was evaluated as an inter-stage treatment where the primary refiner was used as a mixer. The approach has the advantage of minimising the capital investment needed for implementation, thus being directly applicable in a thermomechanical pulping process consisting of two or more refiners in series.The results obtained in a pilot plant trial revealed that is was possible to significantly reduce the specific energy consumption by approximately 20 and 35% to a freeness value of 100 ml CSF by using 1 and 2% hydrogen peroxide, respectively. The energy reduction could be obtained without any substantial change in fibre length, fractional composition of the pulp or tensile strength of the paper. The tear strength was slightly reduced however, as was the pulp yield. The major drawback with the acid hydrogen peroxide system was a reduction in brightness by at least 6 brightness units. The addition level of ferrous sulphate was too high and the possibility to reducing the discoloration should be considerable when the chemical system is optimized.

We investigated the bleaching efficiency of the final chlorine dioxide (D2) stage in the D0(EP)D1D2 bleaching sequence, focusing on the effect of pH in relation to the bleaching history of pulp samples. The samples used were unbleached kraft Eucalyptus grandis pulps with kappa no. 14.8 and the same pulp oxygen-delignified to kappa nos. 12 and 9.8. The samples were bleached according to the D0(EP)D1 sequence to a brightness of about 86% ISO and then submitted to the final D2 stage under identical conditions (e.g., chlorine dioxide charge, time, temperature, and final pH). The target final brightness was 90.5% ISO. Changes in the kappa number, brightness, viscosity, and contents of hexenuronic acid, 4-O-methylglucuronic acid, and the total amount of carboxylic acid (COOH) groups in pulps were monitored during the bleaching sequence. The final brightness of eucalyptus kraft pulps increased when the terminal pH of the D2 stage was raised from 3.0 to 6.5. The 90.5% ISO target brightness in the D2 stage was achieved for all pulps within a pH range of 4.5–6.5, but this required adjusting final pH for individual samples. The optimal pH value with respect to pulp viscosity was between 3 and 5. Despite dissimilar conditions applied in previous bleaching stages, the samples after the D1 stage revealed similar residual lignin contents as shown by kappa number analysis. The content of hexenuronic acid in the samples, however, varied broadly, from 2 to 26 mmol/kg. Conductometric titration showed different amounts of carboxylic acid groups in pulps after the D1 stage, of which hexenuronic acid accounted for only a minor part. The variations in the fiber charge resulted from the different bleaching conditions applied before the D2 stage. The fiber charge affected the alkali demand in the final D2 stage, whereas variations in the alkali demand affected the initial pH and associated process kinetics. Lower total fiber charge was found to be beneficial for improved final brightening and viscosity when bleached at higher final pH.

SUMMARY:The acid hydrogen peroxide system has the potential to significantly reduce the specific energy consumption in the production of softwood thermomechanical pulps (TMPs). A drawback of the chemical system is discoloration of the pulp during refining. The work presented in this study evaluates the possibility to regain the lost brightness by washing, chelating and sodium dithionite or hydrogen peroxide bleaching of the treated pulps.A washing or chelating procedure can reduce the metal ion content of the chemically treated TMPs considerably, though brightness can be increased by a maximum of two ISO units. The amount of iron can be further reduced to a level similar to that of untreated pulps by performing a reducing agent-assisted chelating stage (Q Y ) with dithionite. The discoloration cannot, however, be completely eliminated. The brightness decrease of the treated pulps is thus not only caused by higher iron content in the pulp, but is also dependent on the type of iron compound and/or other coloured compounds connected with the acid hydrogen peroxide treatment.Oxidative bleaching with hydrogen peroxide (P) is more effective than reductive bleaching with sodium dithionite in regaining the brightness lost during the energy reductive treatment. By using a Q Y P sequence, a hydrogen peroxide charge of 3.8% was needed to reach an ISO brightness of 75% for the chemically treated pulps. The corresponding hydrogen peroxide charge for the untreated TMP reference was 2.5%.

This paper presents a life cycle assessment (LCA) of bleached eucalyptus kraft pulp production in Brazil. The entire production system was investigated, starting with forestry and ending with bleached pulp at the gate of the pulp mill. Alternative bleaching sequences were compared for three different scenarios using somewhat different elemental chlorine-free (ECF) sequences: Dhot(EPO)DD, Dhot(EPO)DP, and aZeDP. The main difference between the scenarios investigated was the magnitude of the carbon footprint contribution from bleaching. For the base case and chemical island scenarios (both reflecting Brazilian conditions), the contribution was 15%-18% of the total carbon footprint. For the ecoinvent scenario, the corresponding share was 34%-41%. The ecoinvent scenario represents generic LCA data for bleaching chemicals. Ecoinvent is a public database commonly included in commercial LCA software. For each scenario, the alternative bleaching sequences studied resulted in similar carbon footprints of the bleached pulp. A comparison of the data from the different scenarios showed a large range of carbon footprints for the chemicals used for pulp bleaching. It is crucial to select data sets that are relevant in terms of geography and technology. The most dominant contributors to the carbon footprint of the unbleached pulp were forestry and pulp production. Although the focus has been on carbon footprints, the contributions to other environmental effects commonly included in LCAs were also assessed and only minor differences between the alternative bleaching sequences were found.

Ten industrial fully bleached hardwood pulps representing very different brightness reversion profiles were evaluated. The pulps were characterized before and after brightness reversion. The aging method (dry, wet, high/low RH, high/low temperature, short/long time) had, in most cases, no effect on overall brightness reversion trends, but affected the absolute reversion values significantly. Relative humidity in the range of 50 to 90% influenced reversion much more than temperatures in the range from 30 to 45°C. Dry and wet heat-induced brightness reversion correlated positively and significantly with hexenuronic acid (HexA) content in the pulp. The brightness reversion causes a significant decrease in pulp viscosity values, and HexA thus causing decrease of kappa number. Modern pulps produced by TCF and light ECF bleaching technologies were more prone to brightness reversion than regular ECF pulps, partly due to the higher content of HexA. At a pH level above 8, the brightness reversion process was strongly retarded for pulps with a very high HexA content.