Development of Wood Grinding 1. Grinding Model
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CELLULOSE CHEMISTRY AND TECHNOLOGY DEVELOPMENT OF WOOD GRINDING 1. GRINDING MODEL BRUNO LÖNNBERG Pulping Technology, Åbo Akademi University, Åbo, Finland ✉Corresponding author: Professor Emeritus B. Lönnberg, [email protected] Dedicated to Prof. Dr. Cr. I. Simionescu, on his 100 th birth anniversary The aim of this study was to develop and test a model expected to be useful in interpretation of stone grinding of wood. Since stone grinding has been in use for about two centuries, it is surprising that some grinding mechanisms still remain undiscovered. The application of an energy balance set-up for the wood and grindstone surfaces, valid for wood grinding under conditions presenting continuity, resulted in a useful model. The theoretical model developed suggests that the ratio of the compression and tension powers, called the power ratio, depends linearly on the specific production. The experimental grinding data tested in this context follow the theoretical model. Hence, it would be a valuable tool in further evaluation of grinding and groundwood data. Keywords : balance, energy, grinding, mechanism, modelling, power, wood INTRODUCTION Mechanical fiberising of wood has been heated wood and stone surfaces. The friction developed for years in the direction of higher between the stone and wood surfaces produces demand of fiberising energy and higher pulp heat, which modifies the visco-elastic properties strength, but unfortunately lower pulp yield. of the wood surface and its fibres. The total However, increasing energy prices will favour mechanical energy and heat introduced per unit of classical stone grinding, which has been in use for time equal the heat leaving the balance area. 1,2 about two centuries. Stone grinding converts the Mechanical compression and simultaneous wood almost totally into paper-making fibres and tension treat the wood surface in the balance area. fines, and accordingly, it appears as a potential Hence, the corresponding powers are also mechanical fiberising method in the future, if introduced into the balance area, along with the further improved. The improvement would be specific heat of the moist wood, and finally, the probable if the mechanism of grinding is specific energy of the heated fibres and the understood properly. moisture leave the balance area: In fact, the development of the stone grinding F v + F v + Ġ c T + Ġ c T = process requires knowledge of not only the c c t t w w w m m m Ġ Ġ fiberising mechanism, but also the grindstone = w cw Tp + m cm Tp + Q (1) surface and the wood raw material properties, as In Equation (1), the symbols represent: F – well as of their role in the grinding mechanism. force, v – speed, Ġ – mass flow, c – specific heat, Specific information has been collected about T – temperature and Q – heat losses, while the wood grinding on a mill scale and in laboratories, subscripts have the following meaning: c – but proper understanding of the process seems to compression, t – tension, w – wood (fibres), m – be still lacking. This research paper is intended to moisture in wood and p – pulp of fibres. As the start scientific evaluation of the stone grinding of energy balance is valid for continuity, the effects wood for technical improvements. of shower water quantity and temperature may be neglected for the energy balance suggested. DEVELOPMENT OF A MODEL If Q ≈ 0 as in continuity of a permanent process, and if the temperatures of the entering A theoretical energy balance that is descriptive wood and its moisture are assumed equal, as well for stone grinding of wood should include the as the temperatures of the wood fibres and their Cellulose Chem. Technol., 54 (9-10), 939-941(2020) BRUNO LÖNNBERG moisture leaving the balance area, the model Examples clarify the relationship between the changes as follows: power ratio and the specific production of wood F v + F v + ( Ġ c + Ġ c ) T ≈ grinding data, provided that they were produced c c t t w w m m w by a technically advanced experimental grinder, Ġ Ġ ≈ ( w cw + m cm ) Tp (2) by selecting the wood samples carefully, by When Fc vc is replaced by Pc, the compression application of a well-conditioned grindstone and power, and Ft vt by P t, the tension power, and by following standardised preparation and testing when ( Tp – Tw) is replaced by ∆Tp-w, the model methods. The wood grinding apparatus was can be simplified as follows: successfully planned and constructed, and it was P + P ≈ ( Ġ c + Ġ c ) ∆T (3) presented together with the first grinding results c t w w m m p-w in a Master’s thesis. 3 Forest spruce ( Picea abies If all the terms are divided by Pt and the L.) was used in the grinding studies. Its moisture Ġ Ġ moisture-to-dry wood ratio ( m/ w) is replaced content and basic density were typically around by X, the model changes further: 55% and 350 kg/m 3, respectively. In this (Pc / Pt) + 1 ≈ ( cw + X c m) ∆Tp-w ( Ġw / Pt) (4) particular case, the grindstone was a Norton Ġ A601-N7VG, suitable for grinding of newspaper where ( w/Pt) represents the specific production pulp, sharpened with a spiral roll no. 10/28º to a in units of (kg/h)/kW or kg/kWh, which, in fact, is depth of 0.6 mm and opened with a pitch roll no. the inverse of the specific energy, kWh/kg. 24, and finally conditioned with SiC powder in two stages by 120 and 180 mesh powder, MODEL TESTING respectively. Unpressurised grinding was The obtained model would be useful in the performed under the following conditions: shower interpretation of wood grinding, provided that the water – 80 °C, stone speed – 10, 20 and 30 m/s, basic experiments have been performed under and the wood feeding rate was varied between 0.5 controlled conditions. The model is indicating that Ġ and 1.0 mm/s. (Pc /Pt) as a function of ( w/Pt) would represent a In accordance with the model, Figure 1 linear function y = a x + b, where the gradient a = ∆ suggests that there exists a linear relationship (cw + X c m) Tp-w. The power ratio expresses between the power ratio and the specific simultaneously slow and rapid movements, production in stone grinding at 30, 20 and 10 m/s. produced by the stone surface during grinding. The gradient a = 0.41, the intercept b = – 0.17 and Theoretically, mainly slow and perpendicular the coefficient of determination R 2 = 0.80. compression by the grindstone is produced in the Unexpectedly, however, there were no clear wood surface, which probably generates differences between the various stone speeds used viscoelastic movements in the fibres. in this investigation, but further studies, also Simultaneously, mainly rapid and tangential including pulp and paper properties, may provide tension is causing breakage in the outmost fibres new insights. and distortion in the inner fibres of the fibre layer Another example of grinding is based on the heated. Accordingly, the power ratio might be data collected in a Master’s thesis 4 that focused descriptive also for basic groundwood pulp and on pressure grinding of spruce wood at moisture paper properties, such as fibre length distribution contents of 65, 58, 44, 28 and 15%, respectively. and “fines” particles of the pulp, as well as for the Figure 2 indicates that fresh wood represented, in tensile strength and light scattering of the paper this context, by 65 and 58% moisture content sheet. (MC) forms, surprisingly, together with 15% MC, If the experimental grinding data obtained for Ġ a specific linear function for grinding with a = wood, plotted in a (Pc/Pt) vs ( w/Pt) diagram, 0.41, R 2 = 0.93 and b = – 0.017. The slightly drier form a linear relationship, it indicates that the wood samples represented by 44 and 28% MC model developed is probably descriptive for stone again have a = 0.29, b = 0.026 and R 2 = 0.91. The grinding of wood. If so, the gradient of the linear clearly higher gradient of the fresh wood and one ∆ function is represented by ( cw + X c m) Tp-w. Since dry wood sample indicates that grinding now is the specific heats of wood and water are nearly more sensitive to changes in specific production, constant, only X, the moisture-to-wood ratio and ∆ than in the case of wood of relatively low Tp-w, the temperature increase of the fibre pulp in moisture content, the limits of which, however, grinding, are potentially applicable quantities. are still unknown. 940 Grinding Figure 1: Grinding of spruce wood (stone speed 30, Figure 2: Pressure grinding of spruce wood 20 or 10 m/s) grinding process can be developed further to meet CONCLUSION the demands of the future, where wood and This study indicates that an energy balance energy will apparently become more expensive. may be useful for the development of a model for Also, to be successful in the future, stone grinding evaluating stone grinding data. As suggested by of wood requires deeper knowledge about the the developed model, grinding data applied for effects of wood quality, stone material and its calculation of the specific production ( Ġw/Pt) = x pattern, as well as grinding conditions. and the power ratio ( Pc/Pt) = y probably form linear functions, y = a x + b . The initial tests REFERENCES applying the developed energy balance appeared 1 B. Lönnberg, in Procs . 9th Fundamental successful for the grinding data used in this Mechanical Pulp Research Seminar , Trondheim, context. Norway, May 19-20, 2015, Extended abstract 2 B. Lönnberg, O. Tuovinen and P. Fardim, in Procs . It seems that grinding with widely varied stone th speeds (30, 20 and 10 m/s) did not affect the 10 Fundamental Mechanical Pulp Research Seminar , 2 Jyväskylä, Finland, June 13-14, 2017, Extended linear function significantly, since R was as high abstract, https://afrc.fi/fmprs2017/ as 0.80.