The invention relates to a method for reducing pitch trouble in mechanical pulp and/or papermaking pulp containing mechanical pulp by a cellulase/hemi¬ cellulase treatment.

Mechanical pulps include groundwood pulp, refiner mechanical pulp, pressure groundwood, thermo- mechanical pulp and chemi-thermomechanical pulp (CTMP). In the production of mechanical pulp, fibres are detached from the wood mainly mechanically by utilizing heat. The fibre is subjected to stress so that the lignin binding fibres together is softened, and the fibres are detached from each other when the elasticity of lignin fails.

Wood contains about 1 to 10% of pitch and extractants soluble in organic solvents in addition to its main components (cellulose, hemicellulose and lignin). Pitch contains fatty acids, resin acids, glycerides, etc. It is well-known that the pitch content in softwood, which is the primary raw material of mechanical pulp, is high as compared with hardwood, for instance. At the pulp production stage, pitch components are separated from the pulp into a free space, e.g., in the white water. Pitch suspended in the white water is in the form of particles 0.2 to 2 μm in diameter, also known as colloidal pitch.

In papermaking, pitch may deposit on pipes, containers, wires or presses, causing such pitch troubles as inferior paper quality (e.g. holes and spots) and paper breakages. It may also block felts and wires, hampering the removal of water from a paper web. Pitch troubles may also lead to long production stoppages. They occur frequently

especially when stock containing plenty of mechanical pulp is used as raw material.

Traditionally, wood has been stored outdoors for long periods of time (3 to 6 months or more) in an attempt to avoid pitch trouble. In this way, pitch components are degraded mainly by the oxidizing effect of air. Such inorganic substances as talc and anionic surfactants that disperse pitch particles have also been used in the prevention of pitch trouble.

FI Patent Application 870072 discloses a pitch prevention method which utilizes certain water- soluble polyquaternary amines. These compounds are added to the pulp or paper making system to avoid pitch trouble.

FI Patent Application 900679 discloses a method for reducing the pitch content of wood by means of fungi degrading the pitch and resin components present in wood. FI Patent Application 895901 discloses a method for avoiding pitch trouble associated with mechanical pulp by adding acylglycerol lipase enzyme to the stock or white water. This enzyme degrades tri- glycerides contained in pitch. The use of enzymes, including cellulase and hemicellulase, for improving the properties of pulp is known per se. For example, FR Patent Specification 2557894 discloses a method for treating cellulose pulp by xylanase in order to shorten the beating time. CA Patent Specification 758488 concerns a method for improving the beatability of pulp by a cellulase/pectinase/lipase treatment. FR Patent Spe¬ cification 2571738 concerns a method in which pulp is provided with special pulp properties by cellulase treatment.

FI Patent Application 874113 (corresponds to FR Patent Application 8613208) relates to a method for improving the properties of recycled pulp, for instance, (in this particular case a pulp containing plenty of chemical pulp) by cellulase/hemicellulase treatment. FI Published Specification 81394, in turn, aims at improving the drainability of mechanical pulp by hemicellulase treatment (no cellulase).

In addition, FI Patent Application 890214 describes treatment of the white water of a paper¬ making system by enzymes for degrading components dissolved or dispersed from the pulp, such as hemicellulose.

The object of the invention is to develop a method for reducing pitch trouble associated with the production of mechanical pulp, especially papermaking pulp containing mechanical pulp. Methods known from the prior art have been used with varying success, and problems have not been completely avoided. It has now been unexpectedly discovered that the pitch trouble associated with the production of mechanical pulp can be reduced substantially by a cellulase/hemicellulase treatment. In the pulp filtrate water, there occurred an abrupt decrease in the extractant concentrations indicating the pitch content, especially in the fatty acid, resin acid and sterol concentrations, and the turbidity of the filtrate water was reduced.

In accordance with the invention, the pitch trouble associated with the production of mechanical pulp and/or papermaking pulp containing mechanical pulp is solved by treating the mechanical pulp, papermaking pulp containing mechanical pulp and/or white water by an enzyme preparation containing cel- lulase/hemicellulase enzyme activity.

It was fully unexpected that the cellulase and hemicellulase enzymes, which, as is well-known, de¬ grade cellulose and hemicellulose, also affect the main components of pitch, that is, fatty acid, resin acid and sterol type substances.

In practice, the reduction in the pitch trouble become apparent e.g. in that the runability of the paper machine was improved, the wires and felts re¬ mained clean, and the number of holes and spots in paper was decreased.

The enzyme treatment had also other advantageous effects. For example, the drainability of pulp was improved. The enzyme treatment did not either deteriorate the optical or printing properties of the pulp but its brightness, light scattering co¬ efficient, compressibility and smoothness were improved.

The enzyme treatment according to the invention can be performed at any pulp production stage after the mechanical detachment of fibres. The enzyme can be added e.g. to a pulp storing container, storage tower or metering chest. The enzyme treatment can be performed before the bleaching of pulp, in connection with a pulp bleaching process or after the bleaching. The enzyme can also be added to the white water.

The cellulase/hemicellulase enzymes for the use in accordance with the invention can be produced in a known manner by means of actinomycetes, bacteria and fungi. It is also possible to use commercial cellulase/hemicellulase preparations, such as Liftase A40 (manufacturer Genencor International Europe Ltd.), produced by the fungus Triciiodenπa longi- brachiatum and having a CMCase activity (carboxy- methyl cellulase activity) of 2,500 U/ml, a filter

paper activity (FPU activity) of 110 U/ml and a xylanase activity of 500 U/ml. The carboxymethyl cel¬ lulase activity and the filter paper activity de¬ scribe the cellulolytic activity, and the xylanase activity describes the hemicellulolytic activity.

The determination of the filter paper activity is described in Ghose, T.K., Patnak, A.N., Bisaria, V.S., Symposium of Enzymatic Hydrolysis of Cellulose, Bailey, M., Enari, T.M., Linko, M. , Eds. (SITRA, Aulanko, Finland, 1975), 111 - 136; the determination of the CMCase activity is described in Mandels, M. , Weber, J. , Adv. Chem. Ser. 95 (1969) 391-413; and the determination of the xylanase activity is described in Khan, A.W., Tremblay, D., LeDuy, A., Enzyme Microb. Technol., 8 (1986) 373 - 377.

Other cellulase/hemicellulase preparations of the same manufacturer (Genencor International Europe Ltd), such as Multifect L250 and Cytolase 123, and cellulase/hemicellulase preparations from other manufacturers can also be used.

Suitable enzyme dosages given as enzyme activities per kg of pulp dry solids are within the following limits (U - activity unit):

Cellulases:

Filter paper activity 1 - 20,000 U/kg pulp CMCase activity 10 - 500,000 U/kg pulp Hemicellulases: Xylanase 0 - 2,000,000 U/kg pulp

Preferred enzyme dosages are:

Filter paper activity about 20 - 600 U/kg pulp

CMCase activity about 500 - 10,000 U/kg pulp

Xylanase activity about 500 - 100,000 U/kg pulp. The enzyme treatment is usually carried out

within the pH range from about 2 to 10, preferably within the range from about 4 to 8. The temperature of the enzyme treatment may range from about 10 to 90°C, preferably from about 25 to 70°C. In the following the invention will be described more closely by means of working examples based on laboratory experiments and mill test runs. The examples are merely illustrative and they are not intended to restrict the invention. The measurements were performed in compliance with the SCAN standards, if not stated otherwise. Example 1

Thermomechanical pulp (TMP) produced from spruce (Picea abies) and having a consistency of 4%, pH 4.9, a freeness value of 69 ml CSF and ISO bright¬ ness of 66% was taken from a mill process and treated with an enzyme preparation called Liftase A40. Liftase A40 (manufacturer Genencor International Europe Ltd. ) is produced by means of the Triciioderπza long±brachiatυm microorganism and the principal activities contained in it are as follows: CMCase activity 2,500 U/ml

FPU activity 110 U/ml

Xylanase activity 500 U/ml Liftase A40 enzyme was added under careful mixing to the pulp at 55°C in an amount corresponding to 2.5 1/ton of pulp dry solids.

The dosages of the added enzyme preparation given as cellulase/hemicellulase activities per kg pulp dry solids were as follows:

CMCase activity 6,250 U/kg

FPU activity 275 U/kg

Xylanase activity 1,250 U/kg

The enzyme was allowed to react with the pulp at 55°C and pH 4.9 while mixing the pulp at 150 rpm for one hour altogether. Samples were taken from the pulp at uniform intervals, and the samples were determined for turbidity and the final sample also for extractant concentrations. In the liquid fraction of TMP, turbidity is caused by extractants (pitch), suspended carbohydrates and other small components detached from the pulp. Accordingly, the turbidity describes approximately changes associated with all the above-mentioned components. Separate analysis of the extractants gives a more accurate result spe¬ cifically as far as pitch is concerned. For turbidity measurement, 250 g of the pulp suspension was centri- fuged (1,800 rpm, 20 min). The supernatant (liquid fraction) was recovered and measured immediately for the turbidity with a Novasine Analite NTM-150 turbidity meter. Extractants present in the super¬ natant were measured by gas chromatography. A control sample was treated in a similar way as the enzyme-treated sample but without enzyme. The measuring results are shown in Table 1.

Table 1 Effect of enzyme treatment on the turbidity and extractant concentrations of the liquid fraction of TMP

CONTROL ENZYME TREATED

TURBIDITY, NTU

- 0 min 320 320

- 10 min 300 190

- 30 min 310 100

- 60 min 330 25

EXTRACTANTS, mg/1 60 min

- fatty acids 65 6.2

- resin acids 21 2.5 - sterols 13 1.2

The results show that a significant reduction in the turbidity of the liquid fraction of the pulp occurs as soon as after a reaction time of 10 minutes. After a reaction time of 60 min, the liquid fraction is almost clear, that is, has a turbidity of 25 NTU, and the concentration of extractants in the liquid fraction has decreased to 1/10 of the original value. This indicates that the extractants are deposited on or adhere to the surface of the fibre due to the enzyme treatment. Example 2. The practical significance of the reduction in the pitch content of the filtrate water of mechanical pulp was to be determined, that is, a mill test run was carried out. The test run was carried out on an LWC machine applying on-machine coating.

The mechanical pulp used in the machine was peroxide bleached TMP, and as the experiments in Example 1 were performed on unbleached TMP, a laboratory check experiment series was performed before the test run. Peroxide bleached TMP was derived directly from a process with a consistency of 3.2%. The pH of the pulp was 5.5 and temperature 45°C. Under these conditions, the pulp was subjected to an enzyme treatment by using the Liftase A40 enzyme in an amount of 2 1/t. The pitch content was estimated by a turbidity measurement. For measure- ment, the pulp was filtered through a Macherey-Nagel

MN 640 w filter paper, and turbidity was measured with the meter mentioned in Example 1. The results with different reaction times are shown in the table below. Reaction time Turbidity

(min) (NTU)

0 323

40 110

120 28 As is to be seen, the enzyme was extremely effective, and the mill test run could be started.

The duration of the test run was 6 days and the enzyme was Liftase A40 with a dosage of 2 1/t TMP. The enzyme was dosed to the suction side of a pump pumping peroxide bleached TMP from a storage tower into a so-called refiner mechanical pulp chest. The enzyme treatment conditions in the chest were as follows:

- pH 5.5 - temperature, ^β C 45

- consistency, % 3.5

- reaction time, min 42

After the refiner mechanical pulp chest the peroxide bleached TMP was mixed with chemical pulp and reject in a mixing chest. Then the pulp mixture was pumped into a machine chest and further after wire water dilution into the head box of a paper machine. During the test run the pitch content was estimated by measuring the turbidity of the filtrate water of TMP, in addition to which the freeness of TMP was measured. The results are shown in the table below.

CONTROL ENZYME

Filtrate turbidity, NTU

- before refiner pulp chest 370 370 - after refiner pulp chest 420 250

Freeness, ml

- before refiner pulp chest 62 62

- after refiner pulp chest 59 65

It appears from the results that the enzyme is effective also in mill conditions and the obtained reduction in turbidity is 170 units, which indicates a marked decrease in the pitch content in the fil- trate, i.e. that the pitch has adhered to or deposited on the fibres. It is also to be seen that the freeness of TMP has increased by 6 units, which means that the removal of water is easier.

During the test run the following observations suggesting that the pitch trouble was eliminated/ reduced were made on the machine:

The runability of the machine was exceptionally good throughout the enzyme period.

- The wires and felts and the entire wire section remained very clean.

- The number of holes and spots in base paper decreased clearly as compared with a normal run.

The results of the test run were verified by a repeat test run having a duration of 10 days. As to the pitch and runability, the results complied with those of the first test run. In addition, it was observed that the enzyme treatment has a positive effect on the function of the conventional retention agent, as the target level of retention was achieved with a dosage of retention agent which was 20 to 30%

smaller than normally. Example 3

The increase in the freeness of mechanical pulp observed in the mill test run raised a question about its effect on the optical and printing properties of the pulp primarily in view of the use of the enzyme- treated pulp in uncoated paper qualities, such as SC paper and newsprint. To study this, a sample was obtained from a typical mechanical pulp i.e. groun - wood for SC paper. The pulp was subjected to an enzyme treatment under conditions given in Table 2 with different dosages of Liftase A40. After the enzyme treatment, laboratory sheets were prepared from the pulp. Properties measured from the sheets both immediately and after calendering are also shown in Table 2. As can be seen from the results, the enzyme treatment did not deteriorate the optical or printing properties of the pulp even though the in¬ crease in the freeness value was as high as 25 units; in fact, the effect was the opposite as

- the light absorption coefficient decreased, which indicates improved brightness,

- the light scattering coefficient improved,

- the compressibility increased (density after calendering), and the smoothness improved (roughness decreased).

Accordingly, enzyme-treated pulp can well be used in uncoated paper qualities. The enzyme treat- ment not only reduces the pitch trouble but it can also be expected to improve the quality of the paper either directly or at least in that the fineness of the mechanical pulp can be increased and inferior water removal can be compensated for by enzyme treat- ment.

Example 4.

To verity the obtained results, tests were carried out both on unbleached TMP and dithionite bleached TMP, and not only the effect on the pitch content (turbidity) but also the effects on the water removal (freeness), water retention ability (WRV) and sheet properties were measured. This time the sheets were prepared by using a so-called white water sheet mould. The enzyme treatment conditions and the obtained results appear from Table 3. As is to be seen, the effect of the enzyme treatment on pitch content (turbidity) also became apparent with these pulps. In addition, the following positive effects were observed: - water removal properties are improved

(freeness increases and WRV decreases); optical properties are improved (light scattering coefficient and brightness increase),

- printing properties are improved (roughness decreases and compressibility increases, that is, density after calendering increases). Example 5.

As it was observed that the enzyme treatment has a positive effect on the brightness of mechanical pulp, it was decided to study the effect of enzyme treatment carried out before bleaching. For this pur¬ pose, the pulps described in Example 1 (control and enzyme-treated) were peroxide bleached with different amounts of peroxide under normal peroxide bleaching conditions (consistency 16%, temperature 60°C, react¬ ion time 90 min, DTPA 0.2%, MgS0 -7H ₂ 0 0.5% and Na- silicate 5% on pulp). After bleaching the pulps were measured for brightness immediately and after ageing. The results are shown in Figure 1. It can be seen that the brightness has improved and the improvement

is to be seen also after ageing. As a consequence, it can be stated that the enzyme treatment has a positive effect also on the bleachability of mechanical pulp. Example 6.

The positive effect of the enzyme on the function of the conventional retention agent observed in the mill test run was verified by a laboratory experiment. Unbleached TMP from an intermediate con- tainer of mechanical pulp was treated with Liftase A40 under the following conditions:

- pH 5.0

- temperature 50°C

- reaction time 1 hour - pulp consistency 3.7%

- enzyme dosage 0, 1, 2 and 4 1/t TMP.

Corresponding enzyme dosages given as cel¬ lulase/hemicellulase activities per kg pulp dry solids were as follows:

After the enzyme treatment, TMP was mixed with chemical pulp and kaolin under the following con¬ ditions: - TMP 42%

- chemical pulp 24%

- kaolin 33%.

The pulp mixture was diluted to a concentration of about 10 g/1. The dry solids content and fine solids content of the mixture were determined as well

as its total, fine solids and ash retention with a Dynamic Drainage Jar device in accordance with the TAPPI T261 pm-80 method. The retention measurement was carried out on the mixture as such and after an addition of the retention agent.

The obtained results appear from Table 4. It is seen that the enzyme treatment does improve the retention, that is, in practice, the function of the conventional retention agent is made more effective, and when aiming at constant retention, the amount of the conventional retention agent can be decreased.

Table 2.

Pulp: 100% SC groundwood, 52 CSF

Enzyme treatment conditions:

Temperature: 40°C pH 5.0

Reaction time: 1 h

Consistency: 2%

Liftase A40, 1/t

Freeness after enzyme treatment Laboratory sheets/before calend gram age, g/m density, kg/nr* tear index, mNm ^z /g tensile index, Nm/g elongation, % burst index, kPam ² /g

G-H porosity, S/100 ml light scattering coefficient, light absorption coefficient,

Laboratory sheets/after calendering (2 x 200 kN/m) grammage, g density, kg roughness,

Bendtsen ai

Cobb-Unger

Table 3,

Enzyme treatment of unbleached and dithionite bleached TMP

Treatment conditions: pH 4.9 (unbleached TMP); and 5.0 (dithionite bleached TMP), temperature 55°C, reaction time 1 h, consistency 3%.

Unbleached TMP Dithionite bleached TMP

Enzyme dosage, 1/t

Turbidity, NTU

CSF, ml

WRV, %

Grammage, g/m ³

Density, kg/m ³

Tear index, mN /σ

Burst index, kPam ³ /g

Tensile index, Nm/g

Elongation, %

Porosity, Gurley-Hill, s/100 ml

Brightness, % ISO

Light scattering coefficient, πv /kg

Light absorption coefficient, m ² /kg

After calendering (2 x 200 kN/m):

Unbleached TMP Dithionite bleached TMP

Grammage, g/m; Density, kg/m ² Roughness, PPS^ _Q , m

- top

- wire

Air permeability, Bendsten. ml/min Oil absorption, Unger, g/m ²

Table 4.

Effect of enzyme treatment on retention.

Enzyme treatment conditions: pH 5.0, temperature 50°C, reaction time 1 h, consistency 3.7%, enzyme dosage 0-4 1/t TMP.

Liftase A40 Retention agent* Retention % 1/t TMP

1 1

2 2

4 4

* Fennopol K 211 155 g/t dry solids