Mid Sweden University

Thin sections of wood, i.e., shavings, were used in spectroscopic measurements studying the formation and elimination of chromophores under mechanical pulping conditions. Light absorption coefficients, light scattering coefficients, and brightness were monitored.The method was applied to study how temperature in the absence of mechanical energy affects wood brightness. An increase in absorption coefficient at 420 nm indicates formation of ortho-quinone structures. By applying mechanical pulp processing conditions, the light absorption coefficient increased in the 400-500-nm wavelength region, probably due to the formation of chromophores in the lignin. Comparison of heat treated wood shavings with mechanical pulps indicated that additional chromophores were created during the mechanical pulping processes.The method was also used in studying how high-temperature hydrogen peroxide bleaching of the wood shavings affects the light absorption coefficients. The light absorption spectra reveal information about chromophore elimination in wood chemical components during hydrogen peroxide bleaching at slightly pressurized conditions. This is relevant to processes, such as the alkaline peroxide mechanical pulp process, in which the wood chips are impregnated with bleaching chemicals before refining. The easy penetration of chemicals into the wood shavings ensures rapid and even distribution of the treatment chemicals. A change in light absorption coefficient at shorter wavelengths (400-500 nm) was observed.

Hydrogen peroxide is a widely used bleaching chemical for mechanical pulps and is particularly useful when high brightness levels are required. The objective of this work was to study fines as a limiting factor for reaching higher brightness levels in high consistency hydrogen peroxide bleaching of Norway spruce (Picea abies) thermomechanical (TMP) and stone groundwood (SGW) pulps. The hydrogen peroxide bleaching efficiency (i.e. light absorption coefficient reduction) was studied as a function of temperature, alkali charge and fines content using an experimental design based on MODDE software (Umetrics AB).It is known that all types of fines contain more lignin, hemicelluloses, pectins, metals and less cellulose than long fibres. The light absorption coefficient was greater for unbleached TMP than for unbleached SGW pulp and an increased quantity of fines increased the light absorption coefficient for both pulp types. The increase was, however, most pronounced for the TMP. Furthermore, the data suggests that both the fibre fraction and the fines fraction are more coloured in the thermomechanical pulp. In most cases, increased amounts of fines in the pulp had a negative impact on the bleached pulp brightness in spite of the improved light scattering ability accompanying the addition of fines. A level of fines exceeding 50% was especially detrimental resulting in lower pulp brightness due to a higher light absorption coefficient.Increasing the bleaching temperature did not improve the bleaching efficiency. The optimal bleaching temperature at a hydrogen peroxide charge of 4.5% was found to be 70^°C within the tested interval of 70-110^°C. For a fines content of 25%, the stone groundwood pulp displayed a brightness of 83% whereas the thermomechanical pulp achieved a brightness of 79%.

The influence of Mg2+, Ca2+, Fe3+ ions and glucuronic acid on the dry and humid heat-induced (120180C) discoloration of cotton linter sheets was investigated. The quantity of the added metal ions was similar to that of metal ions found in wood and pulp. Heat-induced ageing (120150C, pH 5) under dry conditions caused only slight discoloration of the reference sheets and of the sheets impregnated with the metal ions. However, during ageing at 180C, the presence of Mg2+ ions accelerated the discoloration. An initial pH in the range of 38 had no great influence on the discoloration of the reference sheets, but a low pH in combination with Mg2+ ions increased the discoloration drastically. Under humid ageing conditions at 180C (<30 min), the sheets discolored approximately twice as much as under dry conditions; the difference in discoloration caused by the metal ions was less than in dry ageing. Adding glucuronic acid to the sheets caused extensive discoloration under dry ageing conditions (120180°C), while Ca2+ and Mg2+ ions decreased the glucuronic acid induced discoloration. Under humid conditions (180°C), however, Mg2+ and Ca2+ ions did not prevent the discoloration caused by glucuronic acid. An increase in the absorption coefficient was observed at 285 nm, most likely due to the formation of 5-hydroxy-2-methyl-furaldehyde (HMF) and 2-furaldehyde (furfural). GC-MS analysis reveals increased formation of HMF in the sheets impregnated with Mg2+ ions. Degradation of the cellulose in which the alkaline earth metals act as Lewis acids is proposed.

We investigated the influence of Mg2+, Ca2+, and Fe3+ ions and glucuronic acid on the viscosity, after dry or humid ageing, of two different cellulosic materials, cotton linters and dissolving pulp. The quantity of added metal ions was similar to that found in pulp. In the cotton linters sheets, especially in those containing Mg2+, increased 5-hydroxy-2-methyl furfural (HMF) and 2-furaldehyde (furfural) concentrations were accompanied by decreased viscosity under dry conditions at 180C (pH 5). This indicates that Mg2+ ions can promote the Lewis-acidcatalyzed degradation of cellulose. For cotton linters and dissolving pulp sheets, adding Ca2+ and Mg2+ ions is detrimental when ageing at 180C under dry conditions (pH 5). Adding glucuronic acid increases the viscosity of cotton linters sheets, especially when Ca2+ and Mg2+ ions are present. Due to formation of complexes between the Ca2+ and Mg2+ ions and glucuronic acid, these ions reduce pulp viscosity to a smaller degree.

High-temperature (HT)-peroxide bleaching of a thermomechanical pulp made of Norwegian spruce was investigated; to that end, one-stage reference bleaching at 70°C and one- and two-stage bleaching at 105°C at different total alkali charges (TAs) using a wing defibrator as a mixer were performed. Two-stage HT peroxide bleaching produces a brighter pulp than one-stage HT peroxide bleaching does. The TA should be kept low (10�15 kg/t) and the total alkali/peroxide ratio should be low in the first stage to minimize the COD load and obtain higher residual peroxide. Increased TA does not improve brightness. The bleaching time in HT bleaching at 105°C can be short, since the maximum brightness is reached after only 2.5 min. Preimpregnation of the pulp with hydrogen peroxide before a two-stage alkali addition can produce a pulp with a brightness and residual peroxide comparable to those obtained in conventional one-stage peroxide bleaching at the same hydrogen peroxide charge and at a low TA. The COD load was, however, higher than in conventional bleaching.

A laboratory method was developed, which facilitates to investigate how the temperature profile during a TMP-process including a bleaching stage influences pulp brightness. Thin shavings are prepared by slicing a fresh and frozen block of wood with a microtome. The shavings are rapidly dried in an air-stream at room temperature and the optical properties are measured in an Elrepho instrument. After that the shavings are placed in a stainless steel container that can be pressurized with steam, where the temperature profile in a TMP-process or a bleach plant can be simulated. Addition of bleaching chemicals can be done during the simulation. The optical properties are measured and can be compared to those of the same shaving before treatments. The temperature profile in a TMP-process has a large impact on the optical properties of the pulp. Many studies have been done to show how brightness is affected, when temperature/pressure in TMP processes are varied. However, pilot-scale or full scale-trials can often be performed only under certain limitations. With this new method it is easy to simulate different processing conditions and measure the kinetics of the brightness change on a shaving sample before and after treatments. To get as high final brightness as possible it is necessary to limit the brightness reduction as much as possible in manufacturing of the unbleached pulp.

Cellulose sheets impregnated with metal chlorides were treated at high temperatures in either a dry or a humid environment. The influence of Mg2+, Ca2+ and Fe3+ on the heat-induced discolouration and on the viscosity of cellulose was investigated. Addition of metal ions to cellulose sheets did not significantly influence the colour of the un-aged sheets. At 120 and 150C there are only small increases in the discolouration during heat-ageing. The samples with iron are somewhat more discoloured than the others. After 3,5 hours at 150C Mg2+ also showed an increased discolouration. When the sheets are aged at 180C for short time no large difference between the cellulose reference and the metal impregnated cellulose can be observed. After longer time there is a significant discolouration caused by Ca2+ and Mg2+. Samples with Ca2+ and Mg2+ are 4 and 10 times, respectively, more discoloured than cellulose reference and cellulose impregnated with Fe3+. Under humid conditions at 180C and after 30 minutes Ca2+ and Mg2+ impregnated cellulose are more discoloured than the cellulose reference and the Fe3+ impregnated cellulose. Moisture more than doubles the effect of temperature. pH has no large influence on the discolouration of the cellulose but acid in combination with Mg2+ increases the discolouration 20 times compared to the un-aged reference. The k-values are related to the amount of chromophore formed during ageing. The largest increase in k-values was observed at 285 nm, which was due to the formation of 5-hydroxymethyl-2-furaldehyde (HMF) from wood hexoses formed during hydrolysis of cellulose. Mg2+ and Ca2+ caused a large decrease in DP and increased formation of HMF, compared to the cellulose reference with subsequent formation of chromophores upon heating.

High-temperature (HT)-peroxide bleaching of a thermomechanical pulp made of Norwegian spruce was investigated; to that end, one-stage reference bleaching at 70°C and one- and two-stage bleaching at 105°C at different total alkali charges (TAs) using a wing defibrator as a mixer were performed. Two-stage HT peroxide bleaching produces a brighter pulp than one-stage HT peroxide bleaching does. The TA should be kept low (10�15 kg/t) and the total alkali/peroxide ratio should be low in the first stage to minimize the COD load and obtain higher residual peroxide. Increased TA does not improve brightness. The bleaching time in HT bleaching at 105°C can be short, since the maximum brightness is reached after only 2.5 min.Preimpregnation of the pulp with hydrogen peroxide before a two-stage alkali addition can produce a pulp with a brightness and residual peroxide comparable to those obtained in conventional one-stage peroxide bleaching at the same hydrogen peroxide charge and at a low TA. The COD load was, however, higher than in conventional bleaching