Refining Hemp Fibers for Papermaking

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Refining Hemp Fibers for Papermaking

Olev Trass* and Constantin Delibas†

Hemp, an annual plant that grows well in the temperate climate is an alternative fibre source.

While there are data on its disk refining there is little available about its behavior in other pulping

equipment. This paper focuses on a series of tests conducted with the Szego MillTM SM 440

equipped with four plastic rollers to determine its suitability in refining hemp fibers. The

resulting pulp was mixed with Kraft pulp and then the strength and optical properties were

measured. A comparison in the performance of the mill in refining wood chips and hemp is

presented with respect to fiber properties and paper quality.

Keywords: non-wood fibers, wood fibers, mechanical pulping, paper properties

* Olev Trass Professor, B.S.E., Sc.D. (MIT), F.C.I.C., P.Eng. Department of Chemical Engineering & Applied Chemistry University of Toronto 200 College Street Toronto ON Canada M5S 3E5 Phone: 416-978-6901; e-mail: trass@chem-eng.utoronto.ca † Constantin Delibas Ph.D., eng. Department of Chemical Engineering & Applied Chemistry University of Toronto 200 College Street Toronto ON Canada M5S 3E5 Phone: 416-536-0680; e-mail: cndelib@chem-eng.utoronto.ca

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INTRODUCTION

The modeling of the process of chemimechanical refining of wood chips with the Szego

Mill (Delibas, 2003) has established the operating characteristics and mill design that give the

best product. However, that study did not consider the effect of the type of material on mill

performance. In addition, it showed that the energy consumption is about three times lower in a

Szego Mill than that in a disc refiner for the same amount of mechanical damage produced, but

the fibers are shorter in the Szego Mill. The mill has also been used successfully in grinding of

cellullosic materials such as newsprint (Molder and Trass, 1996) and wood wastes (Trass and

Gravelsins, 1987).

This preliminary study tried to establish if a material with longer fibers than those in black

spruce chips would result in longer fibers and consequently a stronger paper.

DESCRIPTION OF SZEGO MILLTM‡

The planetary ring-roller mill known as the Szego Mill was designed by General

Communition Inc. and researchers in the Department of Chemical Engineering at the University

of Toronto (Austin and Trass, 1997). As Figure 1 shows, it consists of a number of rollers with

helical grooves and ridges, rolling inside a vertical cylinder called a stator. The number, diameter,

and length of the rollers depend on the internal diameter of the stator (in mm) that is incorporated

into the name of the mill (for example, the Szego Mill 440 has four 170 mm diameter rollers with

a height of 380 mm in a 440 mm stator).

‡ Szego MillTM is a trade mark of General Comminution Inc. for its planetary ring-roller mills

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The material to be ground is fed by gravity, or pumped into a top feed cylinder, and is

discharged continuously at the bottom of the mill. It is subjected to crushing and shearing

between the rollers and the stator. The grooves aid the transport of material through the mill.

The force can be controlled by the speed of rotation around the central shaft and the mass of the

rollers.

Figure 1 Diagram of the Szego Mill

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The quality of the product after one pass through a Szego Mill with a given diameter is

determined by the number, length and diameter of the rollers, the feed rate, the angular velocity,

the material of the grinding surfaces, and the consistency of the material. Typically, the angular

velocity is between 250 and 1200 rpm, depending on the equipment size, which translates into a

roller peripheral velocity of 5 to 10 m/s.

The mill is used for dry or wet grinding of materials of average hardness such as coal and

cereals; processing of cellulosic materials (chemimechanical pulping of wood chips, grinding of

hemp, grinding of wood waste); and mixing of polymers with cellulosic fillers.

EQUIPMENT, MATERIALS, AND PROCEDURE

All grinding tests were carried out in the SM - 440 Szego Mill equipped with four plastic

rollers. Each roller weighs 10 kg, and has equal ridges and grooves of 8 mm in length. The mill is

driven by a 40 HP motor.

The material used in these tests was hemp straws that were previously broken so that the

remaining skin joined chips that did not exceed 5 cm in length. This material was treated with

saturated steam at 1 bar for one hour before the actual grinding.

To prevent bridging and reduce feed fluctuations, the chips were continuously mixed in the

hopper. During the experiment water was added to help the transport of the material to and into

the mill.

However, because of the long skin joining the chips, the material tended to build on the top of

the mill. This is why more water was added than previously intended. The bridge of material had

to be hit with a long pole from time to time to solve the problem, but did not give a constant flow

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rate of material through the mill. Thus, the specific energy, flow rate, and consistency were not

measured accurately in these experiments.

The CPPA Standard Test methods were used in measuring the following pulp and handsheet

properties: pulp freeness (CSF), density, tear index, opacity, and light scattering coefficient

(LSC) of the paper. The fiber length of the samples was measured using a Kajaani FS-200,

performed on the accepts from a 0.006" screen plate at the Alberta Research Council Inc.,

Edmonton.

The fiber and pulp properties were measured only for the hemp slurry whereas the

strength and optical properties of the paper were measured for paper made of the mixture hemp

pulp and Kraft pulp. Kraft pulp was added to make possible the formation of the paper.

The effect of the amount of mechanical treatment (as described by CSF) and Kraft

content, in percentage, (K) on the selected paper indices were quantified using regression

analysis. The qualitative trends are illustrated using two- dimensional plots.

RESULTS AND DISCUSSION

The comparison between mill performance in grinding hemp and wood chips is made for

the same level of freeness. Based on the data in Tables 1 and 2 one can infer that for the same

level of freeness the fibers are longer in the case of hemp refining than in the case of wood chips

from chemimechanical pulping.

Paper Quality

The data in Tables 3 and 4 show that the paper made of hemp and Kraft pulp is lighter but

with a higher tear index than that made of black spruce pulp. The optical properties of this paper

are superior to those of the black spruce paper.

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Figure 2 shows that paper density, tear and opacity are not significantly affected by the

amount of mechanical treatment, whereas the scattering coefficient increases with this variable.

Figure 3 shows that paper density increases with the Kraft content, the tear index decreases with

an increase in Kraft content, and the scattering coefficient and opacity are not significantly

affected by this variable.

The graphical analysis of the data allows one to draw only qualitative conclusions

regarding the effect of one independent variable on the dependent variable investigated at a time.

However, both independent variables were simultaneously varied in this study. To obtain the

effect of each independent variable, as well as that of the possible interaction between them or

that of a quadratic term, a statistical analysis was performed.

In the following section the regression models relating the handsheet properties to the

amount of mechanical treatment and Kraft content are discussed with a view to determining the

significant variables for paper quality from a set of variables that includes both first and second

order terms, even though the number of data points is limited.

Paper Density

The regression model in Table 5 explains 94 % of the variability in the data (R2 = 0.942).

The first column shows the independent variables in the model, the second shows the estimates of

their regression coefficients, and the absolute magnitude of the t statistics in the third column

shows the relative importance of the variables in the model. The variables in all models have

been centered to eliminate the collinearity between the predictor variables.

Using the estimates of the regression coefficients, paper density is expressed as:

Density = 271.6 - 0.031⋅(K - 48.75)⋅(CSF - 432.4) + 0.198⋅(K - 51.25)2 or

Density = 271.6 - 0.031⋅k⋅csf + 0.198⋅k2

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where K is the content of Kraft pulp and CSF is the Canadian Standard Freeness. A lower case

letter denotes that that variable has been centered (the mean value of all observations for that

variable is subtracted from each individual observation). Thus, the value of the intercept, row

one, is the predicted value of the paper density when all independent variables take their mean

value for all observations. The point that has as coordinates the mean values of the independent

variables is called the design center.

The model shows that paper density increases with a decrease in pulp freeness as

expected. As fibers become finer, freeness decreases and paper density increases. Figure 2 shows

that the pure Kraft pulp is quite influential; the highest density value is that of Kraft pulp.

Based on the correlation, for Kmin = 15.025 + 0.078⋅CSF the paper reaches the minimum density.

For example, at CSF = 125ml, Kmin = 24.77%. The implication is that paper density will decrease

with the Kraft content up to these values and then it will increase with an increase in the Kraft

content as observed in Figure 3.

Tear index

The model in Table 6 shows that an increase in mechanical treatment (described by lower

values of pulp freeness) leads to an increase in tear index. The Kraft content that maximizes the

tear index is determined by:

Kmax = 13.12 + 0.0824⋅CSF.

Up to these values, the tear index will increase with the addition of Kraft pulp and then it will

decrease.

Opacity

The optical indices of the pure Kraft paper were not measured. Figure 2 shows that paper opacity

decreases sligthly as pulp freeness increases. This conclusion is confirmed also by the model in

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Table 7. Kraft content does not affect significantlly the opacity of the paper when it is varied

from 20 % to 50% and this is why it does not enter the regression model.

LSC

Using the estimates of the regression coefficients in Table 8, LSC is expressed as:

LSC = 41.222 - 0.0042⋅(K - 41.43)⋅(CSF - 414.43) + 0.00331⋅(K - 41.43)2

The model shows that an increase in the amount of mechanical treatment increases the

light scattering coefficient of paper as expected. Based on the above correlation the paper has the

minimum light scattering coefficient for:

Kmin = 15.16 + 0.0634⋅CSF.

Therefore, the light scattering coefficient of paper will decrease with the Kraft content up to these

values and then it will increase as observed in Figure 3.

CONCLUSIONS

The comparison between mill performance in refining hemp straws and chemically pretreated

black spruce wood chips shows that hemp gives a pulp with longer fibers. This in turn leads to a

better tear index of the paper. The statistical analysis of the data shows that there is interaction

between the mechanical treatment of the fibers and the content of Kraft pulp in the mixture.

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REFERENCES

Austin L. G., and Trass O. (1997). Size Reduction of Solids Crushing and Grinding Equipment

in “Handbook of Powder Science and Technology” 2-nd edition, editors Fayed E. M. and Otten

L., Chapman & Hall, New York, 586-634.

Delibas C., (2003). Modelling of the Dynamics and Performance of the Szego Mill, Ph.D. Thesis,

University of Toronto, Toronto, 2003.

Molder, T., and Trass O. (1996). Grinding of Waste Paper and Rice Hulls with the Szego Mill for

Use as Plastic Fillers, International Journal of Mineral Processing, 44-45, 583 – 595.

Trass, O., and Gravelsins, R. (1987). Fine Grinding of Wood Chips and Wood Wastes with the

Szego Mill, Proc. Sixth Canadian Bioenergy R&D Seminar, Vancouver, B.C., 198 – 204.

APPENDIX

TABLE 1 PROPERTIES OF THE HEMP PULP

Mass of a roller, kg 10 10 10 10 10 10

Freeness, mL 125 216 285 335 620 638

L (lwt’d.), mm 0.85 0.87 0.99 1.19 1.34 1.28

Rotational velocity, rpm 640 640 640 640 500 350

TABLE 2 PROPERTIES OF THE BLACK SPRUCE PULP

Mass of a roller, kg 32 32 18 18 18 18 10 10

Freeness, mL 130 180 347 395 414 481 541 619

L (lwt’d.), mm 0.67 0.72 0.89 0.95 0.94 1.06 1.15 1.16

Rotational velocity, rpm 550 550 550 550 550 550 600 600

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TABLE 3 PROPERTIES OF THE PAPER MADE FROM HEMP AND KRAFT PULP

Agrifiber/Kraft 50/50 80/20 80/20 50/50 50/50 50/50 50/50 0/100

Freeness, mL 125 216 285 335 620 638 688 558

Density, kg/m3 348 251.9 296.6 261.8 247 257 233.1 591.7

Tear index, 5.93 7.23 6.35 8.23 6.42 6.90 6.56 3.21

LSC, m²/kg 57.6 37.92 44.90 40.99 39.16 30.44 38.07

Opacity, iso 97.9 97.6 96.20 95.34 93.28 88.32 93.39

TABLE 4 PROPERTIES OF THE PAPER MADE FROM BLACK SPRUCE PULP

Freeness, mL 130 180 347 395 414 481 541 619

Density, kg/m3 575 526 501 515 454 450 375 386

Tear index, mN⋅m²/g 3.93 4.27 5.02 5.24 5.42 5.92 6.1 6.55

LSC, m²/kg 30.5 30.9 32.3 28.5 31.7 30.9 32.3 32.4

Opacity, iso 93.4 92.8 96 92.9 95.5 95.2 94.2 95.1

TABLE 5 REGRESSION OF PAPER DENSITY ON KRAFT CONTENT AND FREENES

Predictor coefficient t ratio R2 Intercept 271.596 17.83 0.942

k⋅csf -0.031 -3.817

k2 0.198 7.574

TABLE 6 REGRESSION OF TEAR INDEX ON KRAFT CONTENT AND FREENESS

Predictor coefficient t ratio R2 Intercept 6.783 19.298 0.795

k⋅csf 3.774E-4 -2.0357

k2 -2.290E-3 -3.8045

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TABLE 7 REGRESSION OF OPACITY ON KRAFT CONTENT AND FREENESS

Predictor coefficient t ratio R2 Intercept 99.6174 55.2736 0.712

CSF -0.0122 -3.1433

TABLE 8 REGRESSION OF LSC ON KRAFT CONTENT AND FREENESS

Predictor Coefficient t ratio R2 Intercept 41.222 15.2991 0.772

k⋅csf -0.0042 -3.6846

k2 0.0331 2.3893

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Figure 2 Effect of mechanical treatment on handsheet properties

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Figure 3 Effect of Kraft content on handsheet properties