Thermal yellowing of hornbeam chemi-mechanical pulps bleached with hydrogen peroxide and sodium dithionite

The thermal yellowing of hornbeam chemi-mechanical pulps (CMP) after bleaching with ‎hydrogen peroxide and sodium dithionite was investigated. The hornbeam chips were randomly chosen ‎from Mazandaran wood and paper industries. The CMP pulps prepared with 85% yield were separately ‎bleached with diethylenetriamine pentaacetic acid (DTPA), without DTPA, and hydrogen ‎peroxide. Some pulps were bleached with sodium ‎dithionite. The optical ‎properties of prepared hand-sheets of 60 g/m2 after spraying with 0.5% DTPA were measured using TAPPI standard methods. All prepared papers were thermally aged ‎separately in an oven at 105 °C for 0, 10, 20, 30, and 40 h. The optical properties, such as brightness, ‎yellowness, coefficient of absorption, k/s ratio, post ‎color (PC) number, and a* factor, before and after ‎thermal aging were measured. The results showed that from 0 to 40 h, the optical properties ‎of paper increased except brightness and ‎greenness. ‎This increase was more extensive up to 15 h. Additionally, among the various treatments, DTPA treatment in long-term thermal aging and the use of ‎sodium ‎dithionite and hydrogen peroxide in the short-term aging had noticeable effects on brightness ‎‎durability and decrease in the color reversion. Thus, there was an increase in the durability of the paper ‎against thermal deterioration.

Energy efficiency in mechanical pulping – definitions and considerations

Production of mechanical pulps requires high specific electrical energy compared to many other attrition processes. In Scandinavia, the lowest specific refining energy for production of thermomechanical pulp is around 1800 kWh/t for newsprint quality, which is roughly 60 times higher than for crushing of stone to a similar size distribution. The high specific energy demand for refining has naturally motivated large efforts in the search for improved efficiency. It is always practical to be able to quantify improvements in efficiency for comparison of process designs and of different machine types. However, there is no commonly accepted definition of efficiency for mechanical pulping processes. In published work within mechanical pulping, energy efficiency has been presented in different ways. In this paper, we discuss definitions of energy efficiency and aspects that ought to be considered when energy efficiency is presented. Although focus of this work is on energy efficiency for refiner processes, the principles can be applied to other types of mechanical pulping processes such as stone groundwood.

Theoretical analysis of LC-refining – pressure screening systems in TMP

LC refining of mechanical pulps has proven to save energy in the production of TMP pulps. However, the specific role of LC refining as part of a TMP system has not been thoroughly studied since it is difficult to conceive any particular system at industrial-scales and impractical at pilot-scales. In this study, pressure screening and LC refining models that describe fibre length distributions, together with correlations to predict refining power were used to model three basic refining systems. From the simulation results, the impact of important variables such as reject ratio, refiner gap and refining net-power was studied. Performance curves of length-weighed average fibre length were generated from simulation results and were used to assess each system behaviour and also to make comparisons between systems. Data from an industrial scale TMP mill sub-system was gathered and compared to simulation results showing relative errors between 0–18 % on the predicted variables.

Power–gap relationships in low consistency refining

Distance between stationary and rotating refining plates, gap, has a direct and significant impact on refining power. Gap is almost universally used to control power in low consistency refining operations. The relationship between power and gap are affected by refiner size, pulp type, plate pattern and refining conditions. In this study, a correlation was developed to describe the power–gap relationships at a wide range of refining conditions and furnish. The correlation was developed using pilot-scale refining data of mechanical pulps. Results showed that a properly defined dimensionless power number is suitable to describe refining power as well as to compare different refiners under the same grounds. The developed correlation was also used to predict mill-scale refining data showing good agreement with between predicted and measured values. Finally, experimental data from force sensor measurements supports the correlation’s theoretical assumptions.

Average fibre length as a measure of the amount of long fibres in mechanical pulps – ranking of pulps may shift

The purpose of this study was to investigate the applicability of different ways of calculating the average fibre length based on length measurements of individual particles of mechanical pulps. We have found that the commonly used average length-weighted fibre length, which is based on the assumption that coarseness is constant for all particles, as well as the arithmetic average, may lead to erroneous conclusions in real life as well as in simulations when used as a measure of the amount of long fibres. The average length-length-weighted fibre length or a weighting close to that, which to a larger extent suppresses the influence of shorter particles, is a relevant parameter of the “length” factor, i. e. amount of long fibres. Our findings are based on three studies: refining of different assortments of wood raw material in a mill; data from LC refining in mill of TMP, including Bauer McNett fractionation; mixing of pulps with different fibre length distributions. If the acceptable average fibre length for different products can be lowered, the possibility of reducing the specific energy input in refining will increase. Therefore, we need a reliable and appropriate way to assess the “length” factor.

Formation of oxalic acid in alkaline peroxide treatment of different wood components

Oxalic acid (OA), formed as an oxidation product in alkaline peroxide bleaching (P) of mechanical pulps, can form a sparingly soluble salt with calcium and cause severe scale deposit problems. The focus of this work was the question how much OA is formed from the different components of spruce (Picea abies (L.) Karst.), i.e., from cellulose, lignin, hemicelluloses, extractives, and bark, during the P-stage of bleaching. Isolated wood components and monosaccharides typical for spruce hemicelluloses and pectins were treated with chemicals of P-stage for 90 min at 73°C, and the released OA was analyzed by ion chromatography. Most OA was formed from galacturonic and glucuronic acids, xylans, and lignin. Taking into account the content of these substances in spruce wood, lignin can be regarded as the major source of oxalate. Untreated spruce bark was found to contain substantial amounts of oxalate, and an additional amount of oxalate was formed in the P-stage. It is evident that effective debarking is essential, not only for pulp brightness, but also for the control of calcium oxalate formation.