New Ways to Manage Drainability, Fiber Properties and Shive Content of Pulp

NEW WAYS TO MANAGE DRAINABILITY, FIBER PROPERTIES AND SHIVE CONTENT OF PULP

Authors Pasi Keinänen, Esa Piirainen, Janne Määttä, and Emerson Armani New Ways to Manage Drainability, Fiber Properties and Shive Content of Pulp

Metso Automation Oy, Pasi Keinänen, Esa Piirainen and Janne Määttä Kajaani, Finland

Metso Automation do Brasil Ltda. Emerson Armani, Sorocaba, Brazil

1. ABSTRACT

Pulp and paper producers are increasingly developing their processes and pulp qualities in order to achieve certain specific, optimum paper properties. This trend, together with the need for more stable final product quality and decreased production costs, has increased the demands for high capacity, accurate, reliable pulp quality measurements.

Traditional laboratory measurements cannot determine pulp properties frequently enough for accurate control, and the use of older quality analyzers in process management has been limited by unreliability problems. Recent advances in computer and camera technology have enabled the development of a new revolutionary method to measure fiber properties and shive content from pulp. These new measurements, together with Canadian Standard Freeness (CSF) measurement, are now included in the kajaaniMAP on-line analyzer. The analyzer is suited for all pulp and paper making processes and has been designed to exceed the present quality, capacity and functional expectations of pulp and paper producers.

This paper presents experiences from several pulp and paper mills where the new on-line pulp quality analyzer was utilized in process management.

Keywords: Freeness, Schopper Riegler, Fiber, Shive, on-line analyzer, Refining, Screening, Dewatering, Drain- age.

1. Resumo

Os produtores de papel e celulose estão desenvolvendo sempre mais seus processos e a qualidade em celulose, a fim de conseguirem algumas ótimas propriedades específicas de papel. Essa tendência, juntamente com a necessidade de uma qualidade final de produto mais estável e custos de produção reduzidos, aumentou as demandas para medições confiáveis, precisas e de alta capacidade da qualidade da celulose.

As medições tradicionais de laboratório não podem determinar as propriedades da celulose freqüentemente suficientes para um controle preciso, e o uso de analisadores de qualidade antigos na gestão do processo foi limitado por problemas de falta de confiabilidade. Avanços recentes em tecnologia da computação e câmera possibilitaram o desenvolvimento de um novo método revolucionário para medir as propriedades da fibra e fragmentar o conteúdo da celulose. Essas novas medições, juntamente com a medição pelo método Canadian Standard Freeness (CSF), estão agora incluídas no analisador online kajaaniMAP. O analisador é adaptado para todos os processos de fabricação de papel e celulose e foi projetado para superar a qualidade atual, a capacidade e as expectativas funcionais dos produtores de papel e celulose.

Este documento apresenta experiências realizadas em fábricas de papel e celulose, onde o novo analisador online da qualidade de celulose foi utilizado na gestão do processo.

Palavras-chave: Coeficiente de refino; Método de teste Schopper Riegler; Analisador online; Refinação; Filtragem; Esgotamento, Drenagem.

2. INTRODUCTION

Globalization is one of the driving forces behind the recent changes in forest industry: the real prices of certain paper grades have gone down while at the same time competition has increased. Success in the international competition requires strict control of production costs in order to ensure high productivity. Many companies have decided to increase their competitiveness by specializing in certain products to achieve optimum paper properties. As productivity and quality targets are getting more crucial, the need for fast, accurate and reliable on-line pulp quality analyzers is increasing.

Paper quality control often relies on measurements from the dry end of the machine: basis weight, moisture, ash content, thickness, etc. In many cases the level of instrumentation in the wet end is far less sophisticated, even though paper quality is determined primarily in the stock preparation and wet end processes [1, Leiviskä]. Freeness is one of the most frequently measured pulp quality variables in mill laboratories. Freeness gives an estimate of what kind of fibers the pulp contains and what kind of paper can be made from it [2, Riippa]. It affects paper machine runnability and several paper properties, for instance tensile, burst and tear strength, bulk, stiffness, gloss and formation. Freeness control can also be used to fix certain quality defects in the finished product.

However, identical drainability values do not always translate into identical paper properties, especially when comparing different pulp grades. Other measurements of fiber properties are therefore needed. Fiber measurements, especially fiber length, are often used e.g. for monitoring raw material variations and for refining and blending optimization. Shive content, especially with mechanical pulps, has also been a very important measurement when evaluating pulp quality. Discontinuities like shives, holes, or dirt specks affect paper machine runnability and local strength of the wet web and can cause web breaks and production losses. [1, Leiviskä]. Shive measurement is therefore utilized in the quality control of refining and screening and when determining finished pulp quality.

Manual freeness, fiber and shive measurements in a laboratory are often unable to meet the requirements of modern pulp production, and substitutive measurements must be found. Traditional measurements are also plagued by accuracy problems when measuring the very low shive levels that we have today. In the past few years, fiber measurements have evolved toward more accurate determination of fiber size and shape [3, Tiikkaja]. New computer and camera technology gives an opportunity to develop multifunctional, fast measurements of fibers and shives. These modern fiber and shive measurements are now combined with a reliable standard freeness measurement in one analyzer, which improves the control possibilities in different pulping processes and opens a new window into paper machine operation.

3. ONLINE QUALITY ANALYZER kajaaniMAP (Modular Analyzer Platform), the analyzer discussed in this paper, was first introduced in 2005. The analyzer is able to measure crucial pulp quality parameters from mechanical pulping, chemical pulping, recycled pulping processes and from paper machine stock preparation. The analyzer is specially designed for on-line control purposes, and a lot of attention has been paid to ensure reliable, accurate and fast operation.

The analyzer consists of separate Freeness, Fiber and Shive content measurement modules that can be freely selected according to each mill's needs (Fig.1). The expandable construction allows new measurement modules to be easily added later on.

The analyzer provides shive content measurements that are 15 times faster and fiber measurements that are up to 500 times faster than previous-generation devices. The total measurement time is 4–6 minutes per sample, and about 300–350 freeness, fiber and shive analyses are performed daily.

3.1 Process sampling and automated analysis

In many cases the most error-prone stage of measurement is sampling from pulp slurry, and therefore representative, reliable sampling is an essential part of a reliable analysis [4, Ämmälä]. It also requires deep knowledge of the materials and techniques used to transport and condition the sample. The

2 analyzer's measurements are available from 100 meters' distance, from up to ten sampling points and up to 15% consistency.

The sample is first delivered to the sample handling unit, where sample consistency is adjusted to 0.3% as in Tappi standard Freeness measurement. Consistency-controlled samples are then fed to the modules for freeness, fiber and shive analyses. If required, a 0.3% consistency sample from the same batch is also available for laboratory validation. The analyzer utilizes the 30-year experience ac- cumulated in Metso Automation’s Kajaani product line in taking and processing samples from various locations in the pulp and paper process. Examples of sampling points in mechanical pulping and stock preparation processes are shown in Figures 2 and 3.

Figure 1. The analyzer consists of sample handling unit (left), freeness module (middle) and fiber/shive module.

kajaaniMAP

Figure 2.Typical sampling points of the analyzer in TMP process.

3 W G P M T T AF KR KE O BR

Figure 3. Typical sampling points of the analyzer in stock preparation.

3.2 Standard Freeness measurement kajaaniKSF was the first on-line analyzer to measure freeness according to Canadian Standard method, and nowadays nearly 100 units are in operation [2, Riippa]. The kajaaniMAP freeness measurement is an improved version of the kajaaniKSF, and it uses the consistency, chamber dimensions, screen plate and measurement sequence of the Tappi T227 standard.

After consistency calibration the analyzer performs freeness analysis in standard 0.3% consistency, and thus the freeness measurement itself requires no time-consuming calibration. During freeness measurement the volume and temperature of the pulp suspension are recorded and freeness or Schopper-Riegler value is calculated. Freeness measurement accuracy is according to Tappi standard. Repeatability is excellent over a wide freeness range and even at very low freeness levels (~20 ml) due to the completely automatic system that prevents any systematic or human errors typical of manual sampling and freeness measurement. Figure 4 shows an example of freeness measurement during a follow-up period with SW kraft pulp.

600 240 Pine+ 220 550 200

180 500 160 CSF [ml] 140 SEC [kWh/t] 450 120

400 100 20:00 4:13Time 10:18 16:04

kajaaniMAP freeness [ml] Lab freeness [ml] Specific energy consumption

Fig. 4. Freeness (CSF) follow up period with softwood kraft pulp at different SEC levels and during a grade change from pine to pine+ (mixture of sawmill pine and spruce).

4 3.3 Fiber and Shive module

Fiber and shive measurement is based on modern image analysis technology, which allows the same device to simultaneously and accurately analyze and classify both very large and very small objects – such as shives and fines. The module measures the amount of light received by the camera's imaging cell by applying revolutionary subpixel calculation which provides more accurate particle size determination than traditional methods (Fig. 5). In addition to better accuracy, this new technique also provides considerably faster measurement and better repeatability. The same measurement method is also used in the new kajaaniPulpExpert analyzer [5, Kauppinen].

The module takes some of the consistency-adjusted sample from the sample handling unit and further dilutes it to a very low consistency suitable for fiber and shive measurement. The sample then flows through the measurement cell, where a camera takes up to 50 photographs per second using light scattering optics.

Fiber measurements include several important fiber size and shape measurements: - Arithmetic, length weighted and weight weighted average fiber length (mm) - ISO fiber length, range 0.2–7.0 mm, without short fibers - Fiber length distribution - Arithmetic, length weighted and weight weighted average fiber width (µm) - Fiber width distribution - Fines content (%) - Fiber curl (%) - Softwood-hardwood ratio (%) - Fiber Kink index - Vessel cells content (n/m, n/1000) - Fiber coarseness (mg/m) - Measuring range, width 2–500 µm, length 0.01–7.6 mm

The fiber module is able to measure about 8000 fibers per second, the total number of fibers in one analysis being from two hundred thousand up to several million. Such high capacity ensures that the repeatability for average fiber length is better than 1.5% of the length level as shown in tables 1 and 2. Typically a representative fiber result is obtained in only 30–40 seconds. Figure 6 shows an example of the fiber length measurement during a follow-up period with SW kraft pulp.

Figure. 5. Subpixel calculation improves considerably the accuracy of particle size determination.

5 Table 1. Example data, repeatability test with TMP pulp. Sample Lc(l) Lc(l) ISO Width Curl fines A Kink kink angle Coarseness no. Mm Mm Μm % % 1/m Deg mg/m 1 0.74 1.04 32.2 4.45 23.9 418 39.3 0.41 2 0.74 1.04 32.2 4.51 23.1 422 39.3 0.41 3 0.73 1.03 32 4.49 24.0 407 39.0 0.41 4 0.74 1.04 32.3 4.40 23.5 409 39.3 0.41 5 0.74 1.04 32.2 4.44 24.9 410 39.5 0.41 6 0.74 1.04 32.3 4.42 22.7 416 39.7 0.41 7 0.74 1.05 32.2 4.40 23.5 408 39.6 0.41 8 0.73 1.03 32.1 4.45 24.1 417 39.2 0.41 9 0.73 1.04 32 4.31 24.6 403 39.2 0.4 10 0.74 1.04 32.1 4.42 23.2 412 39.6 0.41 Average 0.74 1.04 32.2 4.43 23.7 412 39.4 0.41 St. dev. 0.00 0.01 0.11 0.06 0.7 6 0.2 0.00 CV [%] 0.66 0.55 0.33 1.24 2.89 1.44 0.55 0.77

Table 2. Example data, repeatability test with chemical pulp. Sample Lc(l) Lc(l) ISO Width Curl fines A Kink kink angle Coarseness no. mm Mm Μm % % 1/m Deg mg/m 1 1.89 2.13 34.8 14.0 7.2 1374 43.3 0.41 2 1.89 2.13 34.8 14.0 7.1 1356 43.3 0.41 3 1.9 2.14 34.9 13.8 7.6 1349 42.9 0.41 4 1.91 2.14 34.9 13.7 7.3 1329 42.9 0.42 5 1.89 2.13 34.9 13.8 7.6 1368 43.1 0.42 6 1.9 2.14 34.9 13.9 7.6 1355 42.9 0.42 7 1.9 2.14 34.9 13.4 7.5 1342 42.6 0.41 8 1.9 2.13 34.8 13.7 7.4 1350 42.9 0.41 9 1.88 2.13 34.8 13.7 7.7 1342 42.8 0.41 10 1.9 2.14 34.8 13.9 7.6 1335 42.7 0.41 Average 1.90 2.14 34.9 13.8 7.5 1350 42.9 0.41 St. dev. 0.01 0.01 0.1 0.18 0.2 13.9 0.2 0.00 CV [%] 0.44 0.25 0.2 1.34 2.6 1.0 0.5 1.17

2.4 240 Pine+ 220

200 2.3 180

160

2.2 SEC [kWh/t]

length [mm] 140

length weighted fiber 120

2.1 100 20:00 4:13 10:18 16:04 Time fiber length kajaaniMAP [mm] fiber length Fiberlab [mm] Specific energy consumption [kWh/t]

Fig. 6. Fiber length follow-up period with softwood kraft pulp at different SEC levels and during a grade change from pine to pine+ (a mixture of sawmill pine and spruce).

Shive content is presented as a weight percentage and as shives per gram, in many different formats and separately for each freely adjustable shive class: - Shive content (%, n/g, n, g) - shive classes (mini, wide, long and coarse shives)

6 - Shive size matrix 4 * 4 and colored shive size distribution map - Measuring range, width 75–2,000 µm, length 0.3–20 mm

The shive module is able to analyze about 1 g of dry pulp per minute, and depending on the shive content typically 3–6 g of pulp is analyzed during one analysis. The large sample volume ensures that at shive content levels of over 400 #/g the measurement repeatability is less than 5% of the shive content level. Repeatability remains good even at very low shive content levels, as shown in table 3. The shive module classifies the shives according to their dimensions, and the results correlate well with Sommerville and Pulmac shive analyzers based on mechanical screening. When both fiber properties and shive content are measured, one analysis takes less than 6 minutes

Table 3. Example data of repeatability tests with different shive content levels of chemical and mechanical pulps. Sample type Average shive content Stdev. CV n/g n/g % A 26 1.7 6.5 B 396 16 4.1 C 661 24 3.6 D 1101 26 2.3 E 1754 33 1.9 G 6880 124 1.8

3.4 Cleanliness is essential

One of the main problems with traditional on-line pulp quality analyzers has been their high manual cleaning and maintenance requirement, which limits their availability and prevents their active use in process management. The new pulp quality analyzer has been designed especially to provide reliable analysis results with minimum maintenance. Essential features of the analyzer are advanced self- diagnostics and efficient automatic cleaning with several different methods after each measurement. Experience has shown that the manual maintenance or cleaning interval has varied from one month to several months depending on the application, and availability of over 99% is achievable.

To ensure good measurement performance, each of the analyzer's units is automatically washed with water after every measurement. Water washing can be enhanced with air mixing and with two different, automatically dosed cleaning chemicals, and the wire of the freeness module is ultrasound- cleaned between every sample. In addition, the cleanliness of the freeness measurement chamber is checked after each analysis by measuring the freeness of pure water [2, Riippa]. A carefully thought- out fiber and shive module structure prevents measurement cell blockages, and advanced calculation provides accurate results even when impurities or air are present in the cell.

3.5 Easy calibration

The analyzer’s consistency measurement is sweep-calibrated to ensure quick and reliable calibration for different pulp grades. The Freeness module measures according to the T227 standard and thus requires no time consuming calibration.

The fiber and shive measurement method can be calibrated without calibration fibers or reference pulps that are usually needed to provide the image analysis parameters. The new method is easy and quick to calibrate using just two calibration tools: one to adjust image sharpness, the other to determine the physical dimensions of the image (= pixel height and width). This calibration method ensures excellent reproducibility between different fiber and shive modules [5, Kauppinen].

3.6 Result reporting

Results from freeness, fiber and shive modules are presented in a user-friendly operating interface that can be used either at the analyzer, in the control room, or over a remote connection. Measure- ments are displayed graphically and numerically, showing averages, distributions and matrix presen- tation. The analyzer also provides versatile communication possibilities to the mill network.

7 4. STOCK PREPARATION

For an existing mill, the key factors for success include optimization of the product range and quality, high utilization rate of design capacity, high productivity and operating efficiency. Manufacturing costs are reduced through working capital optimization, by avoiding overhiring in the mill, and by constantly focusing on energy and furnish costs [6, Paulapuro]

Uncontrolled variations in the flow of any material component (furnish components, fine and filler, colloidal material, etc.) can cause variations in the dewatering mechanism on the paper machine, which in turn may cause runnability problems. Variations in raw materials add to the complexity: for example the fiber distribution and the amount of dissolved and colloidal materials may change as a function of wood quality and freshness. A well-designed process and an efficient mill is better equipped with on- line instrumentation to reduce the possible fluctuations caused by variations in raw material characteristics. [6, Paulapuro]

Paper quality results from the control of such variables as refining degree, pulp blending, and handling of additives [1, Leiviskä]. A pulp quality analyzer suitable for freeness and fiber measurement is bene- ficial when the goal is to produce stable quality paper with the minimum use of expensive pulps.

Drainage rate is one of the key measurements affecting the stability of stock preparation and wet end. With reliable and accurate freeness measurement it is possible to close the refining and proportioning control loops and to achieve both optimal web formation and maximum efficiency. Freeness control makes it possible to maintain optimum drainage of pulp. It facilitates the control of wet end and stabi- lizes in the long term many quality variables: opacity, different strength properties, bulk, etc. [7, Ojala]

When a fiber length measurement is available, changes in wood quality can be detected. Shive content is another variable that has an effect on paper machine runnability. Discontinuities like shives, holes, or dirt specks can affect the local strength of the wet web and cause web breaks and production losses.

4.1. Blending

Paper machine control is disturbed by two sources of variability: the raw materials entering the process, and the process itself. The first of these is caused by actual variations in the raw material and poor mixing. Different species have totally different effects on paper properties, and thus the exact recipe used in blending together different pulps, recycled fibers, and other raw materials is extremely important. Blending controls require accurate measurements of pulp flow rate and consistency. In order to control the blending and to meet specific fiber requirements, drainage and fiber length measurements from all fiber flows are also needed [1, Leiviskä]. Accurate blending control makes it possible to achieve savings in raw material costs while at the same time maintaining the desired furnish quality.

The analyzer is useful when the target of the mill is to reduce deviations in paper properties (e.g. formation, air permeability, tensile and tear strength) and the consumption of higher-price pulps. The measurement of fiber properties, particularly fiber length, can be utilized to detect changes in wood quality and to optimize the percentages of kraft pulp and mechanical pulp in the furnish. Studies have shown that even small changes in fiber length and width can be detected, such as the difference between softwood species (e.g. Northern pine and spruce) or between the same species grown in different areas. For instance the differences in the fiber properties (length, fines, vessel cells content, curl, coarseness, kink etc.) of different eucalyptus pulps can be detected very clearly, as illustrated by figure 7.

How microscopic fiber features translate into macroscopic properties of sheet – e.g. strength, bulk or softness – is relatively well understood. Fiber length has a strong effect on paper formation, and the fiber length of a furnish can be controlled either by controlling the fiber length of the raw material pulps, or by changing the mixing ratio of short-fiber and long-fiber materials. Fiber measurement can indicate certain changes in pulp better than traditional drainage measurement. Figures 4 and 6 illus- trate that fiber measurement has detected the change in pulp quality whereas freeness has remained stable during a grade change from pine to pine+, a mixture of long fiber sawmill pine and spruce pulps. Figure 8 shows another example of a grade change situation in stock preparation. In this case the furnish properties after machine chest mainly followed the properties of CTMP pulp, the main component of the furnish.

8 0.93 pulp A pulp B pulp C 0.92 0.91 0.9 0.89 [mm] 0.88 0.87 0.86 length weighted fiber length length fiber weighted length 0.85 0 5 10 15 20 25 30 35 hours

Figure 7. Fiber length measurements of three different eucalyptus pulps from Europe and South America.

2.4 Grade 1Grade 2 Grade 1 2.2

1.8

1.6

1.4 fiber length [mm] 1.2

0.8 012Days SW Fibel length L[l] CTMP Fibel length L[l] Machine Chest Fiber length

Fig. 8. Fiber length measurement of CTMP, SW Kraft and machine chest (mixture of CTMP 65%, SW kraft 25% and broke).

4.2 Refining

Refining is the key process where fibers are treated to meet the target properties of drainability, bonding ability, etc. Without reliable drainage control, fiber properties may be deteriorated or modified in such a way that stable quality cannot be reached anymore.

Stock Freeness/SR (Schopper Riegler) Control reacts to variations in the refining process and in stock beatability. Variability can be caused e.g. by changes in the pulping process or raw material. If left uncontrolled, these factors have been observed to make the drainage drift off target over a few hours. Laboratory drainage analysis is not frequent enough for feedback control. On-line drainability measurement with a 4-minute measurement interval gives a good idea of the drainage trend, even when one analyzer handles several sampling lines.

The analyzer’s freeness measurement has been successfully used in stock preparation for pulp refining control, to stabilize the effect of refining on fibers and product quality. The reliable measurement makes it possible to apply a simple control loop. Pulp freeness or SR value is measured directly after a refiner, and the analyzer's drainage value is compared to the drainage target given by operator. The specific energy setpoint of the refiner is then corrected to eliminate deviations between target and measurement. Stock Freeness/SR Control can be used to: - keep drainage at the desired range, - reduce drainage variation, - minimize the effect of variations in fiber properties and beatability, - shorten paper grade changes or start-up times

Figure 9 shows an example of drainage control with pine kraft pulp. During the control period, SR variation was 0.51 ml –considerably less (>55%) than with traditional specific energy control which cannot ensure stable drainability when changes occur in the raw material.

30 oSR Control OFF oSR Control ON 28

o 26 SR Setpoint SR

o 24

22 oSR Pine Kraft 20 0 5 10 15 20 25 30 35 Days Figure 9. Refined pine pulp drainage before and after automatic control

The same control method is applicable for low consistency refiner processes of different pulp types. For instance with hardwood kraft pulp, the variation of Schopper-Riegler number during a control period was less than 0.4 oSR.

Refining is normally used to modify those fiber properties that contribute to the desired drainage, fiber flexibility, and surface and optical properties of paper. Standard drainage measurements (CSF, SR) are traditionally used to predict drainage of pulp, strength properties, stiffness etc. On the other hand, in the refining environment it is possible to obtain good drainage without really knowing how much fiber length has changed during the process, and in some cases fiber cutting may actually be the primary target. Freeness versus fiber length information is a good tool for an operator to understand the fiber properties in more detail. The goal is to stay in the target area where both the drainage value and fiber length meet their target values.

5. MECHANICAL PULPING

Because pulp and paper products are mostly made of fibers, fiber morphology of the original wood raw material can be expected to provide the basis for final sheet properties and energy consumption in mechanical defibering. Measuring the important wood and fiber properties is often time-consuming and thus traditionally seldom practiced in mill conditions. The first real pulp measurement method was the Schopper-Riegler dewatering test that was later modified to create the Canadian Standard Free- ness test. Drainage properties of pulp suspensions change when the fiber length distribution is changed, and also when the properties of fractions are changed. The drainage ability often correlates inversely with the pulp's bonding ability and its capability to form smooth, dense sheets. This is why freeness measurement has become so widely used and why it still remains the most important test for mechanical pulps [6, Paulapuro].

The main requirements for newsprint quality mechanical pulp are sufficient strength properties and sufficiently low shive content, to ensure good runnability in the paper machine and in the printing press. Knowing the shive content of mechanical pulp is essential in order to avoid serious problems on the paper machine or at the printing house. Increased shive content causes linting problems in offset printing and significantly reduces the visual quality of paper. Fiber and shive measurements are vital in the optimization of refining and screening processes and in monitoring of raw material variations [5, Kauppinen].

Some quality analyzers have been available for mechanical pulping processes, but only a few automatic pulp quality controls are in active use, mainly due to the unreliability of older analyzers. The tar-

10 get is to keep the process at the desired operating point. If this works in the long term, optimization of the operating point becomes possible. Another problem is that only freeness has been controlled: if the disturbances are caused for instance by chip moisture variations and freeness variation is compensated by controlling the plate gap, fiber length variation can in fact increase. [8, Sundholm]

The new pulp freeness, fiber and shive analyzer has been designed to exceed the current requirements for measurement reliability, performance and capacity, and it combines the most crucial measurements of mechanical pulps.

5.1 Screening

Typical sampling points in mechanical pulping include the inlet, accept and reject lines of different screening stages. The task of screening and cleaning is to remove impurities (sand, metal, etc.) and to separate fiber bundles (shives) and undeveloped coarse fibers from the prime quality pulp.

Pressure screens are the most common type of screening equipment in mechanical pulp processing. The accept and reject quality is mainly controlled by the flow conditions inside the pressure screen basket. Stock consistency, rotor speed, pulp throughput and reject ratio are the most important operating parameters. Pulp characteristics, such as freeness, fiber length distribution, ash content, con- taminant content, and the size distribution of contaminants, also influence the screening result [6, Paulapuro].

Nowadays good performance of the screening room is essential when manufacturing top quality paper, since one single harmful shive in the accept pulp can cause runnability or quality problems at the paper machine. The quality of refiner pulps is described using three factors: freeness, fiber length distribution (in practice: long fiber content), and shive content. The first two factors can be controlled by changing refiner operation, while shives control usually means changes in the pulp flow to the reject refiner. [1, Leiviskä]

Figure 10 shows an example of the correlation between analyzer’s freeness and shive measurements from the reject screening inlet and accept lines. Good repeatability of freeness, fiber and shives makes it possible to optimize the screening stages and to improve final pulp quality.

Line 2 and 4 CSF and Shive content

140 2.0 Freeness reject screening feed 1.8 120 1.6 100 1.4 Freeness reject screening accept 80 1.2 Shive% reject screening feed 1.0 60

CSF [ml] 0.8 0.6 40 Shive content [%] 0.4 20 Shive% reject screening accept 0.2 0 0.0 29.11 30.11 1.12 3.12 4.12

Fig. 10. Freeness and shive content in inlet and accept of reject screening.

Figures 11 & 12 show analyzer's freeness, fiber length and fiber width measurements from the reject screening inlet and accept lines of a TMP plant. Most of the time the freeness signal follows fiber properties, but there is a period where the trends differ, probably due to changes in raw material.

11 Line 2 CSF, Fiber length

120 1.74 115 Fiber length 110 1.72 105 1.7 100 1.68 95

CSF [ml] CSF 90 1.66 85 Freeness 1.64 80 1.62 75 length weighted fiber length [mm] length fiber weighted length 70 1.6 29.11 30.11 1.12 3.12 4.12

Fig. 11. Freeness versus fiber length in the reject screening accept of a TMP plant.

Line 4 CSF, Fiber width

135 0.037

130 Freeness

125

120 0.036 115 CSF [ml] CSF 110 Fiber width 105 length weighted fiber width [µm] width fiber weighted length 100 0.035 29.11 30.11 1.12 3.12 4.12

Fig. 12. Freeness versus fiber width in the reject screening inlet of a TMP plant.

5.2 Mechanical pulp refining

The analyzer's measurements are also essential for refining process control. Mills often want to minimize freeness variations between grade changes. The new pulp quality analyzer makes it possible to monitor freeness, fiber properties and shive content in several process points continuously, and allows the operators to react to changes much faster than with traditional laboratory freeness measurement.

The control of thermomechanical pulp (TMP) plants is affected by two different disturbances: the slowly occurring wear of refiner plates, and the more rapid variations in raw material quality. Raw material, chip moisture and chip size variations cause changes in consistency and in the mass flow rate into the refiner. High quality TMP pulp production requires that these variations are compensated, and for this we need to measure the variations and their influence on quality variables. [1, Leiviskä].

Refiner control requires accurate flow and consistency measurements, otherwise the measurement and control of specific energy will not be possible. Refining control based on a TMP consistency analyzer (TQA) can be used together with a pulp quality analyzer to optimize TMP plant operation. [9, Jo- ensuu]

The basic idea of TMP control is to first stabilize refining consistency and motor load in both stages. The pulp quality analyzer is then used to maintain freeness and fiber length at the desired level by adjusting motor loads and primary stage consistency. Fiber length can be kept above the correct level by adjusting the freeness target. TMP control has been able to stabilize TMP process variables considerably, for example freeness stability has improved by up to 50%. [9, Joensuu]

12 6. Summary

Tight competition in the forest industry has increased the mills' need to achieve better competitiveness and lower unit costs. Nowadays companies must simultaneously ensure high product quality and improved productivity by better organizing work, pay levels and working hours. In consequence, new on- line pulp quality analyzers are gaining more attention, as they improve the possibilities to control pulping processes and open new windows into paper machine operation. Paper machine drainage speed is one of the deciding factors for final paper properties. Measurements of fiber properties and pulp shive content are essential when the target is to optimize production costs, process runnability and end product quality. kajaaniMAP is a new automatic pulp quality analyzer that is able to measure freeness, fiber properties and shive content from up to 10 sampling points. The analyzer is specially designed for control purposes, to supply reliable, accurate and fast measurement data with minimum service need. It can be used with all paper making pulps, and its typical applications include the control and optimization of refining, screening and blending. The analyzer makes it possible to keep drainage rate at the target level and enables mixture optimization to ensure the desired end product quality. It also improves productivity by decreasing raw material costs and reducing the need for manual work.

The main benefits of kajaaniMAP compared to older fiber, shive and freeness measurements are its high availability and measurement capacity, low maintenance need, better measurement performance and ability to take samples from more locations. Due to its modular design the analyzer can be equipped according to needs, and new modules can be easily added also later on.

7. BIBLIOGRAPHY

1. Gullichsen J & Paulapuro H (Series editors); Leiviskä K (Book editor), Papermaking Science and Technology, Book 14. Process Control. Jyväskylä 1999, Fapet Oy. 361 p.

2. Riippa T, Piirainen E, kajaaniKSF – Automated Standard Freeness Analyzer. APPITA 2003, Mel- bourne, Australia.

3. Tiikkaja, E, It's all about fibers. Metso Automation, Automation Magazine 1/2006, pp.25-28

4. Ämmälä A, Hietanen T, Niinimäki J & Ylinen R (1999c) Sampling of pulp slurry. Proc. TAPPI Pulping Conference, Orlando, USA, 1199-1209.

5. Kauppinen M, Lehtikoski P, Riippa T, and Salopuro A, kajaaniPulpExpert, on-line automatic pulp laboratory with revolutionary new fiber and shive measurement. APPITA 2004, Melbourne, Aus- tralia.

6. Paulapuro, H.; Papermaking Science and Technology Book 8: Papermaking Part 1, Stock Prepa- ration and Wet End; Finnish Paper Engineers' Association and Tappi Press: 87-122 (2000).

7. Ojala, T., Zynger, L., Barbosa, F., Armani, E., Furley, M., Roccha, C.Ripasa: modern online wet end controls for fine paper machine – achieved benefits for runnability, speed and paper quality. EXPOCORMA (EXPOCELPA), 13th International Fair Forestry Cellulose and Paper

8. Gullichsen J & Paulapuro H (Series editors); Sundholm J (Book editor), Papermaking Science and Technology, Book 5. Mechanical Pulping. Jyväskylä 1999, Fapet Oy. 427 p.

9. Joensuu, I, Karlsson, L., Myllyneva, J., Fuzzy logic quality control in hallsta paper mill’s TMP plant. International Mechanical Pulping Conference 2001 June 4-8.6.2001 Helsinki, Finland