On the Development of the Refiner Mechanical Pulping Process

Nordic Pulp & Paper Research Journal 2020; 35(1): 1–17

Review paper

Christer Sandberg*, Jan Hill and Michael Jackson On the development of the refner mechanical pulping process – a review https://doi.org/10.1515/npprj-2019-0083 When reviewing all the process combinations and re- Received September 24, 2019; accepted December 31, 2019; previ- fner types that have been utilized, several questions arise: ously published online February 11, 2020 What has been the driving force behind all these process Abstract: This paper is a review of the development of the combinations? Is there a structured development, or have mechanical pulping process with focus on refner-based the processes arisen merely due to trial and error? Can all processes. The intention is to provide an overview of the these system solutions be justifed by product specifca- trends and the major advances in the development of the tions, raw material quality or energy efciency? Is it the mechanical pulping (MP) process. The focus is on the de- mills or the suppliers that have directed the development? velopment of the entire MP process, rather than the re- We may be unable to answer all questions objectively, fner as such. However, when discussing the MP process but we will try to shed some light on the relatively short development, it is inevitable to consider the development history of refner-based mechanical pulping. of the refner unit operation briefy. Processes for printing We will discuss mainly the processes for printing pa- papers based on softwood is mainly discussed, but board pers based on softwood, but processes for board pulp man- processes are discussed briefy as well. ufacture will be discussed briefy as well. Keywords: mechanical pulp; process development; refn- There are of course local constraints that afect the ing; RMP; TMP. process design. The available wood raw material has a large impact on both product quality and process design. Generally, the spruce species are easier to handle and re- Introduction quire less energy to attain a given set of strength properties compared to pine and fr species (Härkönen et al. 1989, This paper is a review of the development of the me- Varhimo and Tuovinen 1999). Furthermore, for the various chanical pulping (MP) process with focus on refner-based sub species of pine, the handling of extractives must be processes. The intention is to provide an overview of considered more extensively. the trends and the major advances in the process development. Wood handling, chipping, water circuits, and bleaching concepts will not be discussed. Concerning the The refner stone groundwood (SGW) process, only the rejects system will be considered since it has afected the refner-based The frst refner-like equipment (i. e. machines with rotat- process design. ing grinding-discs) that was used in wood based pulp- The focus is on the development of the whole mechanical pulping process, rather than the refner as such. How- ing processes, was the machine developed by Voith to ever, when discussing the MP process development, it is make pulp out of SGW rejects in 1859 (Carpenter 1989). inevitable to consider the development of the refning unit It was called rafneur and it was initially equipped with operation briefy. A more comprehensive description of the sandstone discs. The frst metal discs were used for refn- th refner development and principles can be found in e. g. ing of brown groundwood rejects in the end of the 19 th Carpenter (1989) and Sundholm (1999). century (Carpenter 1989). During the frst half of the 20 century, refner-based processes operating at high tem- *Corresponding author: Christer Sandberg, Holmen Paper, perature (170–190 °C) were developed for the production Norrköping, Sweden; and Mid Sweden University, SE-851 70 of hard board from wood chips (Asplund 1934). During Sundsvall, Sweden, e-mail: [email protected] the 1950’s the frst attempts were made to utilize refners Jan Hill, QualTech, Tyringe, Sweden, e-mail: [email protected] Michael Jackson, Michael Jackson Consulting Inc., Vancouver, for production of newsprint (Eberhardt 1955). Reading the Canada, e-mail: [email protected] paper presented by Asplund (1953), it is quite clear that

Open Access. © 2020 Sandberg et al., published by De Gruyter. This work is licensed under the Creative Commons Attribution 4.0 Public License. Brought to you by | Mittuniversitetet / Mid Sweden University Authenticated Download Date | 3/17/20 8:18 AM 2 | C. Sandberg et al.: On the development of the refner MP process he experimented early with lower refning temperatures that, at least for spruce, it was possible to raise the refn- adapted for printing paper pulp production. ing pressure (temperature) if the preheating temperature At an early stage, two major types of refners were was reduced (Nunn and Thornton 1978, Huusari and Syr- developed: double disc (DD) refners equipped with two jänen 1981, Sundholm and Mannström 1982, Jackson and counter rotating discs by Bauer, later also by SCA/Sunds Åkerlund 1983). The increased refning pressure made it Defbrator, and single disc (SD) refners by Defbrator, Jyl- possible to recover steam from the refners for paper dry- hävaara, Sprout Waldron and Hymac (Carpenter 1989). ing without steam compressors. The Kaipola mill was frst Two design concepts were introduced to increase the ca- to install a reboiler without steam compressors in 1980 pacity of a single refner. The frst was the Twin refner, es- for the production of clean steam (Huusari and Syrjänen sentially two SD refners in one, developed by Sprout Wal- 1981). Steam recovery made the production cost for refner ὔὔ dron. The frst mill installation involved a 45 refner in the pulp competitive with SGW (Ulander 1985). The frst two- Grand´Mère Mill, Canada, (Jones 1968). The second con- stage pressurised TMP line was developed by Jylhävaara cept was the conical disc (CD) refner developed by Def- and the frst mill installation was made at the Kaipola Mill, brator. The frst mill installation in the Hallsta mill, Swe- Finland, in 1976 (Huusari and Syrjänen 1977). ὔὔ den, involved a 70 CD refner (Tistad and Görfeldt 1981). All MP lines for printing paper started up after 1990 in During this early period of refner mechanical pulp- Scandinavia (Saugbrugs, Kvarnsveden, Hallsta, Braviken ing, the process was often referred to as refner ground- and Ortviken) have atmospheric preheating. Some pro- wood or super groundwood. Later, the process and the cesses employ only a short preheating time (5–10 s) at pulp produced were referred to as refner mechanical pulp high pressure (5–6 bar) (Sabourin et al. 1997, Nelsson et al. (RMP). The frst commercial RMP process for printing pa- 2017). Processes with atmospheric preheating should actu- per was started-up in 1956 at the Lyons Falls Mill (hard- ally be referred to as PRMP (Pressurized Refner Mechan- wood) and Diamond Match Mill (Softwood), USA (Evans ical Pulp), but the acronym TMP is normally used for all 1956). In the beginning, refners for paper production were pressurized refner processes today. The frst mill instal- non-pressurized (open discharge) and operated at rela- lation of a single stage PRMP refner was started up in tively low consistency (below 15 %), but it was soon re- the Braviken mill, Sweden in 1982 (Jackson and Åkerlund alized that higher refning consistency was benefcial for 1983). pulp quality development (Holzer et al. 1962). The TMP expansion has been recorded by Leask and The breakthrough for refner-based processes oc- later Barnet in a yearly report published in Pulp & Paper curred when pressurized chip refners were introduced Magazine Canada. At the end of the 1970’s, printing pa- (Asplund and Bystedt 1973, Charters and Ward 1973). per applications accounted for 82 % of the installed TMP Strength properties of the produced pulp were much refners, with the remainder almost equally split between higher, and the shives content was 70–90 % lower com- paperboard and market pulp (Data from Leask reviews). pared with RMP. The largest step in the refner devel- During the 1980’s, the major refner development was opment during the 1970’s was the introduction of pres- machine size and thereby capacity increased from approx. surized preheating prior to pressurized primary refning. 5 adt/h to 18 adt/h for two stage processes (Leask 1989). This process was called thermomechanical pulping (TMP) Control systems for refners also improved considerably (Asplund and Bystedt 1973). In the beginning, open dis- during this decade (Dahlqvist and Ferrari 1981, Oksum charge machines were utilized in the subsequent refner 1983). stages. The frst TMP refner for market pulp production In the 1990’s the manufacturers of high consistency was started in the Rockhammar Mill in Sweden 1968 (Ahrel (HC) refners had been consolidated to Valmet and An- and Bäck 1970). The frst large TMP-lines for newsprint dritz. During this decade, Valmet developed the large con- were started-up simultaneously in 1974 in the Hallsta Mill, ical disc refner, (CD82) and the large double disc refner Sweden (Arnesjö and Dillén 1975) and in the Newberg Mill, (RGP68DD), whereas Andritz developed the Twin refner USA (Strom 1975). For a period of time, some mills operated up to Twin-66. Andritz presented the RTS process in 1997 a process with pressurized preheating followed by open based on optimized Retention time, refning Temperature discharge refning, sometimes referred to as TMR (Wood and rotational Speed (Sabourin et al. 1997) and the frst and Karnis 1975, Peterson et al. 1975, Oksum 1983). mill installation was started-up in the Perlen Mill, Switzer- It was considered that the optimal TMP refning tem- land, in 1996 (Aregger 1997). perature was between 120 and 130 °C (Atack 1972, Asplund Since the mid 1990’s, not much has happened with and Bystedt 1973, Higgins et al. 1978). Later, it was realized the HC refner design, except that Andritz introduced the

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Figure 1: Symbols used in block diagrams.

Figure 2: Overview of a typical TMP process. Latency chests are denoted “Lat.” Red lines are steam and blue are water.

largest HC Twin refner model TX68. However, process de- Chip pre-conditioning sign, segment design and refner control have improved refner performance in terms of loadability, operational sta- The wood chips must be hot and moist to attain proper bility, production capacity and pulp quality (e. g. Vikman fbre separation in the refning process. Therefore, wood et al. 2003, Karlström et al. 2018). chips are presteamed and then immediately submerged in the chip washer to remove heavy debris such as sand, grit, gravel, etc. After the chip wash, chips can be preheated one more time before refning. The presteaming and chip Mechanical pulping systems washing operations have not changed much over the years and will not be discussed further. Symbols used in block diagrams are explained in Figure 1. As an integral part of chip conditioning, water impreg- Almost all refner-based MP lines consists of four nation of chips using a heavy duty plug screw feeder or an major process sections shown in Figure 2: chip pre- Impressafner feeding into an impregnating column has conditioning, main line refning, fractionation/cleaning and reject treatment. An important additional unit pro- been used to advantage in various parts of the world. It cess is the latency treatment after all HC refners. The pro- is one way to deal with fuctuations in the moisture con- cess sections will be discussed in detail below. Franzén tent of the incoming chips and ensure that chips with a (1986) described the status of the mechanical pulping pro- more or less constant moisture content are fed to the refn- cess at the time, covering the mentioned process sections ers (Hartler 1977, Fournier et al. 1991). This kind of heavy as well as other aspects, such as raw material selection and compression pretreatment of chips before refning can also bleaching. reduce specifc refning energy by approximately 10 % and In the block diagrams below, the presteaming, chip removes 30–40 % of the wood extractives as well as some washer, plug screw feeders, chests, and steam separators brightness lowering substances (Robinson 1964, Thornton have been omitted to make the fgures easier to read. Re- and Nunn 1978, Costantino and Fisher 1983, Tanase et al. jects dewatering is symbolized by a screw press only. 2010, Nelsson et al. 2012). A comprehensive review of me-

Brought to you by | Mittuniversitetet / Mid Sweden University Authenticated Download Date | 3/17/20 8:18 AM 4 | C. Sandberg et al.: On the development of the refner MP process chanical chip pretreatment has been made by Gorski et al. (2010). Chemical treatment in conjunction with refner pulping has been used since the Asplund process was introduced (Asplund 1953). Chemicals have been added in trials and in mill operations in almost all imaginable pro- Figure 3: Early RMP process with second stage MC/LC refning and cess positions. However, it is beyond the scope of this screen rejects fed back to LC refning; Redrawn from Eberhardt review to provide a complete picture of all work in this (1955). area. The intention with chemical additions has mainly been to improve pulp quality and/or reduce specifc energy. In addition, adding alkaline sulphite to the refner re- Main line refning duced segment corrosion, which was a problem with the alloys used in the early days of RMP production (Beath In the early RMP lines, main line refning was performed et al. 1973, Rankin 1977). By far the most utilized chem- in two stages after chip pretreatment in a Pressafner, Fig- icals in mechanical pulp production are bisulphite or ure 3. The primary refning was done at higher consis- sodium sulphite. Impregnation of chips with charge lev- tency (frst at 15 %, later above 20 %) and the secondary refning was performed at low consistency (LC, 4–6 %) or els of 10–30 kg/bdt (as Na2SO3) before pressurized preheating (120–135 °C) and refning is usually referred to medium consistency (MC, 7–15 %) (Eberhardt 1955). Dur- as chemithermomechanical pulping (CTMP), which is not ing the 1960’s it became more common to install processes discussed in detail in this review. During the 1980’s the with two stages of HC refning often followed by a third low CTMP production capacity started to expand rapidly, espe- consistency refning stage, but without a Pressafner. cially in Canada (Sharman 1989). In the early days of RMP The change to two stage HC refning was driven by the and TMP some mills added sulphite in the process (e. g. fact that it was difcult, at least with the early DD refn- Cochrane and Crotogino 1965, Beath et al. 1973). ers, to apply the load needed to achieve reasonable pulp Chemicals have been added to the process in diferent quality in a single non-pressurized HC stage. To be able to positions, e. g. in an impregnator or to the refner dilution maintain high refning consistency, Atack and Wood (1973) water (Dorland et al. 1962, Costantino and Fisher 1983). Ad- pointed out that it was necessary to perform the main line dition of sulphite to the refner dilution water was later re- refning (non-pressurized) in two or three stages. On the ferred to as dilution water sulphonation (DWS) (Richard- other hand, Mills and Beath (1975) showed that it was pos- son et al. 1990). Chemical treatments in conjunction with sible to perform the main line refning in one stage at high MP have been more comprehensively summarized by e. g. consistency. Lindholm (1979) and Mackie and Taylor (1988). When sul- It was realized early in the RMP history that the pro- phite is added before the primary refner, the light scat- cess conditions during the initial fbre separation have a tering of the produced pulp is reduced (Atack et al. 1977, large impact on the attainable pulp properties. Therefore, Nelsson et al. 2017). This efect is lower when sulphite is the design of the main line refning is crucial (Atack 1972, added after the primary refner stage, referred to as Inter- Peterson and Dahlqvist 1973, Falk et al. 1987, Heikkurinen stage sulphonation (Heitner et al. 1982), or the OPCO pro- et al. 1993, Stationwala et al. 1993). An expression some- cess (Barnet et al. 1980). The lowest reduction of light scat- times used is that the primary refner sets the “fnger- tering is attained if the chemical treatment is performed print” of the produced pulp. The idea of separating def- on a long fbre fraction (e. g. screen rejects) prior to refn- bration and fbrillation into diferent refning stages was ing, referred to as the Monopulp process by Gavelin (1980) suggested early (Neill and Beath 1963). However, at the and as long fbre sulphonation by Gummerus (1983) and time, it was not known which refning conditions were Franzen (1983). The loss of light scattering from sulphite optimal for the two processes. Research during the 80’s chip pretreatment can also, to some extent, be reduced by showed that it was benefcial to do initial fbre separa- an increased refning intensity (Sandberg et al. 2018b). tion at relatively low temperature and fbrillation at higher Chemical treatments were not common in MP mills for temperature (Salmén and Fellers 1982). Based on this con- printing papers from the late 1980’s and forward. However cept, Valmet developed the Thermopulp process (Höglund more recent work has shown that it can be benefcial for et al. 1997) and later Andritz developed the Advanced TMP energy efciency and pulp properties to include a chemi- (ATMP) process, Figure 4, in cooperation with Norske Skog cal addition in the process (Johansson et al. 2011, Nelsson (Sabourin et al. 2003, Hill et al. 2009). An ATMP process 2016). was started in Steyrermühl, Austria in 2011. In contrast to

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refning, whereas the installations outside North America are more varied consisting of: – 36 % single stage operations dominated by double disc refners Figure 4: Example of ATMP process, Redrawn from Hill et al. (2009). – 45 % two stage operations, Figure 5a – 4 % two primary refners feeding to one secondary refner, Figure 5b this, it has been shown that high quality pulp can be pro- – 14 % more complex confgurations – usually four pri- duced with low specifc energy in a single stage DD68 re- mary refners feeding three secondary refners, Fig- fner operated at high temperature (Muhic et al. 2010). ure 5c Wood can be characterized as a viscoelastic material, and therefore the strain rate that the wood and fbre mate- In North America, where the power grid frequency is rial is subjected to will afect property development and 60 Hz, single disc refners rotate at 1800 rpm while large energy efciency (Becker et al. 1977, Salmén and Fellers double disc refners rotate at 2 × 1200 rpm, which gives a 1982). Diferent concepts for improved efciency, based on Δrpm = 600 (DD-SD). In Europe and many other parts of this fact, have been presented over the years. Miles et al. the world, where the power grid frequency is 50 Hz, sin- (1991) suggested that it was benefcial to apply a relatively gle disc refners rotate at 1500 rpm, unless they have been low amount of energy in the primary refning stage at high equipped with a gear box to increase the rotational speed, intensity and the major part of the energy at low intensity while double disc refners rotate at 2 × 1500 rpm, which in the second refning stage. On the other hand, Sabourin gives a Δrpm = 1500 (DD-SD). These diferences between et al. (2003) suggested that initial defbration should be North American and European operation are sufcient to made at low intensity and in the second stage, the ma- account for some of the variation in observations regard- jor part of the energy should be applied at high intensity. ing pulp quality and specifc energy between North Amer- Intensity is a concept that is not easy to defne exactly ican and European refning installations (Bergström et al. but it can be described as the harshness or severity of re- 1970, Sundholm et al. 1988, Cort et al. 1991). fning and can to some extent be connected to the strain rate (Sandberg et al. 2018b). It was frst introduced for low consistency refning (Lewis and Danforth 1962) and later Single stage main line refning adapted for HC refning (Miles and May 1989). Main line refners have been combined in several The frst RMP lines were usually operated with single stage ways, Figure 5. Since the chip refners usually could be HC refning followed by a second LC refning stage. Later loaded more than the second stage refners of the same most of them were converted to multi-stage HC refning size, one concept was to supply one second stage refner since the refners usually were more stable and had longer with pulp from two primary refners, Figure 5B or three sec- segment life when operated at higher production rate ondary refners fed by four primary refners, Figure 5C. The in multi-stage confgurations (Holzer et al. 1962, Gavelin main purpose of these confgurations was to be able to use 1966). one motor size and one refner size and load all stages as At the time, it was difcult to load the DD refners to the much as possible. level required to reach the desired pulp freeness in a sin- Considering the main line design, there are rather dis- gle stage, especially for the larger DD refners (Atack and tinct diferences between North American practices and Wood 1973). Mannström modifed the feeding to the Bauer the rest of the refner world. Today, almost all processes DD refners, which made it easier to run pressurised sin- in North America have two (or more) stages of main line gle stage DD refners to low freeness values (below 100 ml

Figure 5: Three examples of main line refner confgurations.

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Figure 7: An example of a modern CTMP process design with LC refning as second stage in the main line and for reject refning; Redrawn from Peng et al. (2018). Figure 6: Example of process with single stage main line refning, normally there are several parallel chip refners.

(Levlin 1980, Robinson et al. 1985), however, Kurdin (1974) CSF) (Mannström 1980). The frst pressurized single stage pointed it out earlier. TMP line was started in the Summa mill, Finland, in 1977 During the mid-1990’s the number of LC refners em- (Skinnar 1979). ployed in TMP lines increased, mainly installed for third It has been pointed out that single stage processes are stage main line refning (Musselman et al. 1997). At the easier to control and maintain (Mills and Beath 1975). With time, the incentives for such installations were a cost ef- DD refners, single stage operations also have higher en- cient 10–15 % increase of production capacity and reduced ergy efciency (Peterson and Dahlqvist 1973). On the other specifc energy to a given freeness. LC refning of mechan- hand, Wild and Steeves (1972) and Mihelich et al. (1972) re- ical pulp has been thoroughly described e. g. by Welch ported that there was no signifcant diference in energy (1999), Luukkonen 2007, Andersson (2011). For LC refn- efciency and pulp quality for single versus two stage re- ing, the energy consumption for a given tensile index in- fning with the same refner type. crease or freeness reduction is approximately half com- Today, single stage main line refning, for printing pa- pared to HC refning. However, LC refning develops fbres per, is almost exclusively utilized in processes with high diferently compared with HC refning. Fibres are straight- intensity chip refning such as DD or RTS. A single stage ened in LC refning and fbre walls are not peeled and ex- design with a modern slotted screening system is shown ternally fbrillated to the same extent as in HC refners, re- in Figure 6. Examples of such systems are Perlen (RTS) sulting in pulps with lower light scattering compared to HC (Aregger 2001) and Kvarnsveden (DD) (Ferritsius 2019). For refning (Ferritsius et al. 2016, Sandberg et al. 2017a). It is printing paper, only a few single stage SD refners have preferable to combine LC refning with high intensity chip been in operation. Some examples are: Kenogami (Mills refning to attain light scattering level equivalent to that and Beath 1975), Donnacona (Wood and Karnis 1975), Hall- achieved by SD HC refning (Andersson et al. 2012, Sand- sta CD70 and RLP58 (Tistad and Görfeldt 1981), Kvarnsve- den CD82 (Ferritsius et al. 2016), Braviken RLP58 (Sand- berg et al. 2017a). berg et al. 2017a). The process in Braviken is in production. Modern CTMP lines, mainly based on hardwood, utilize LC refning in the main line and for the treatment of rejects, Figure 7 (Guangdong et al. 2011, Peng et al. 2018). Main line low consistency refning

LC refning was used at an early stage for reject refning Latency treatment and post refning in SGW lines (Klemm 1957, Richardson 1969). It was however believed to be impossible to suf- After HC refning, fbres are twisted and curled as well as ciently develop fbre properties in LC refning (Kurdin 1974) entangled in focs. When the pulp leaves the high temper- and the incentive was mainly for shives reduction and for ature conditions, the deformations are frozen into the fbre a fnal freeness adjustment. structure and the phenomenon is referred to as “latency” In the early days of RMP, LC refning was used for (Beath et al. 1966). Usually, latency is removed in agitated second or third stage refning in the main line, often in chests at approximately 3 % consistency, 70–90 °C for 20– combination with screen rejects refning, Figure 3, (Eber- 60 minutes. Recent research has shown that free fbres are hardt 1955) but it was also used for separate reject refn- straightened in seconds in hot water and that the defoccu- ing (Gavelin 1966). During the 1980’s it was widely ac- lation is the rate controlling mechanism in latency removal cepted that mechanical pulps required lower intensity dur- (Gao 2014). The defocculation rate depends strongly on ing LC refning to avoid extensive fbre length reduction the applied energy intensity. Thus, latency can be removed

Brought to you by | Mittuniversitetet / Mid Sweden University Authenticated Download Date | 3/17/20 8:18 AM C. Sandberg et al.: On the development of the refner MP process | 7 more efciently in a smaller volume with lower energy consumption by intense mechanical treatment at high temperature under a short time. This was also proposed earlier by Karnis (1979), but the mechanisms were not established at the time. Beath et al. (1966) and Welch (1999) showed that conventional latency treatment can be replaced with an LC refning stage at high enough temperature. Figure 8: Rejects system at Lufkin Mill, USA; Redrawn from Witherell (1968). Fractionation

Mechanical pulp has almost always been further treated after the main line process. The pulp leaving the main line refners or grinders usually contains material, such as unseparated fbres (shives, slivers), poorly treated f- bres, sand, bark, etc. that must be upgraded or discarded from the process. Depending on the quality demands on Figure 9: Examples of groundwood fne screening and rejects re- the produced pulp, more or less advanced equipment is fning systems A: One of the most common early confgurations. B: needed for separation of the unwanted material (Martin System with a separate rejects screen; Redrawn from Beath (1971). 1982, Hautala et al. 1999). The separation of pulp into two streams, or more, with diferent properties is referred to as fractionation. Fractionation can be made according to any 2003). Undoubtedly, the screen room is the process section pulp property, such as fbre length, shives content, fbre that have had the largest number of confgurations. wall thickness, etc. Usually a specifc fraction of the pulp Since similar system designs have been utilized for is removed for upgrading prior to recombination with the SGW, RMP and TMP, we will discuss them together. main fow or for utilization in another product. A special For a long time, groundwood pulps were only fraction- case of fractionation is cleaning, in which a small fraction ated with non-pressurized screens (Centrifugal screens) of debris, which cannot be upgraded to usable material most often ftted with hole screen plates. Pressure screens (e. g. bark, sand, etc.), is removed from the process. Be- were introduced in the SGW processes in the late 1950’s low, we will use the term fractionation for processes where (e. g. McLenaghen et al. 1959). SGW processes always have a larger part of the pulp is separated for rejects refning and two screening systems; In the frst, larger wood residues cleaning for process where the reject is sewered. We use the (slivers) are removed with coarse screens (bull screens) for term screen room for the whole fractionation and cleaning further treatment. After coarse screening, the pulp is frac- systems. tionated in a fne screen system. One example of a com- Over the years, two major equipment categories, plete groundwood screen room is shown in Figure 8. screens and hydrocyclones, have been utilized for frac- For the fne screening, numerous combinations have tionation. Hydrocyclones are often referred to as clean- been utilized (Steenberg 1953), however, the most com- ers, which is misleading, since hydrocyclones can also be mon fne screening confguration until the 1990’s was two- used for fbre fractionation. Thus, both screens and hydro- stage screening, Figure 9A, with the second stage accepts cyclones can be used for cleaning and for fractionation. fed back to the primary screen feed or to the bull screen However, in MP processes, screens are seldom used for feed. In processes producing pulp for rotogravure, dou- cleaning, as they are in recycled fbre processes. Gener- ble screening (two screens in series) was often utilised in ally, screens separate particles according to size, e. g. fbre the fne screening system (Chapman and Allan 1970, Dillén length, and hydrocyclones according to specifc surface et al. 1973). and density (Wood and Karnis 1979, Karnis 1982, Sandberg In processes producing pulp for rotogravure, double et al. 1997, Ouellet et al. 2003). Two major types of aper- screening (two screens in series) was often utilised in the tures have been utilized in screens; holes and slots. Slot- fne screening system (Chapman and Allan 1970, Dillén ted screen baskets have higher shives removal efciency et al. 1973). (Hooper 1987). Both screens and hydrocyclones are nor- In SGW processes, refned rejects were often directed mally arranged in two or more stages to improve the se- to the second stage screen feed. However, Beath (1971) lectivity of the separation (Steenberg 1953, Friesen et al. showed that it was more efcient (debris reduction vs. re-

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Figure 12: Main line hydrocyclone system utilised for both fraction- Figure 10: Groundwood fractionation and cleaning system with no ation and cleaning, TMP line at Hallsta Mill, Sweden; Redrawn from screening, just cleaning of the refned reject; Redrawn from Richard- Arnesjö and Dillén (1975). son (1969).

Figure 13: Early Bauer RMP process at the Kenogami Mill, Canada; Figure 11: Groundwood screen room with hydrocyclones only on Redrawn from Neill and Beath (1963). second stage screen accepts, Laurentide Mill, Canada; Redrawn from Racine (1968).

fning energy) to have a dedicated rejects screen as shown in Figure 9B. When rejects refning was improved during the late 1960’s, it was realised that reject screen accepts could be fed forward, Figure 8 (Witherell 1968). Figure 14: A Sprout Waldron RMP process at the Kawerau Mill, New During the 1950’s hydrocyclones were introduced in Zealand; Redrawn from Rankin (1977). the SGW screen rooms, which improved the removal of specks, chops (small cubical shives) and sand (Klemm 1957). The hydrocyclone systems were either utilised only for cleaning, Figure 8, or for fractionation and cleaning, Gavelin 1966). In the early processes, rejects were usually Figure 10. After HC rejects refning had been introduced in fed backwards to the main line after primary refning, Fig- the late 1960’s, Richardson (1969) recommended omitting ure 13. Later, it became more common to install dedicated screening and only have cleaning of the refned rejects to rejects refners, Figure 14. minimize fbre length reduction, Figure 10. Since the non-pressurized refners produced a rela- Usually hydrocyclones were arranged with the accepts tively large amount of chops, and the removal efciency of fed backwards in cascade to the previous stage, as in Fig- those was low for screen baskets equipped with holes, hy- ures 8 and 10. In some installations, at least with RMP pro- drocyclones were usually needed to achieve a sufciently cesses, accepts were fed forward as shown in Figure 14. In clean pulp. some mills (both SGW and RMP) hydrocyclones were only When TMP was introduced, shives content was re- utilised in the reject system, Figure 11. duced to such an extent that it was discussed whether The hydrocyclone systems sometimes have a broken screening was necessary at all (Kurdin 1979). A few mills cascade in which the second or third stage accepts are were actually operating without screens in the pulp mill, combined with screen rejects before rejects refning, Fig- at least for a period of time (Butcher 1975, Skinnar 1979). ure 12. It was also suggested to replace fractionation and clean- When refners were introduced for production of me- ing with LC post refning (Gustafsson and Vihmari 1977). chanical pulp, fractionation systems similar to those That concept was however not pursued further until Sand- used for SGW were adopted (Klemm 1957, Rydholm 1965, berg et al. (2017b) showed that newsprint for modern high-

Brought to you by | Mittuniversitetet / Mid Sweden University Authenticated Download Date | 3/17/20 8:18 AM C. Sandberg et al.: On the development of the refner MP process | 9 speed ofset printing can be produced without any fractionation or cleaning in the pulp mill. It has also been suggested that more energy should be applied in the main line and thereby the screens could be omitted and only hydrocyclones used for fractionation (Ahrel 1973, Hofmann 1975). Even earlier, Holzer et al. (1962) showed that RMP for newsprint could be produced Figure 15: Inter stage screening (ISS) process. Fibres and fnes are with a three stage HC-HC-LC process (DD HC refners) with- separated in the frst screening system and shives/coarse fbres out screens and using only hydrocyclones. However, such are removed in the second screening stage; Redrawn schematically a concept will require larger disc flter capacity. from Bousquet et al. (2016). A large improvement in the development of the screening process occurred when wedge wire slotted screen baskets were introduced during the 1990’s. The These processes are more complicated and thus have large improvement in shives removal efciency ren- higher capital and maintenance costs, but might be benef- dered the hydrocyclone systems superfuous, at least for cial for pulp quality and energy efciency. There are some newsprint, and they were removed in many mills (Cannell results showing an energy reduction potential of the order 1999). Some mills removed the hydrocyclones earlier, e. g. of 10–20 % (Sandberg et al. 2001, Nurminen and Liukko- the Hylte Mill, Sweden, (Ulander 1985), the Ortviken Mill, nen 2001, Bousquet et al. 2016). Sweden, (Engstrand et al. 1987) and the Summa Mill, Fin- land, (Nopanen 1989). With the introduction of modern slotted screens, the system design of the screening pro- Rejects refning cess also changed from feed backward to feed forward of second stage accept which led to the most common screen Refning of rejects for recombination with screen accepts confguration of today, Figure 6. was not common practice in groundwood mills until the 1930’s. Before this time, the screen rejects were sewered, at least in North America (Edwards 1964). The early re- Advanced fractionation systems fners were gravity fed open discharge units, usually operating in the consistency range 4–15 % (de Montmorency A few examples of processes incorporating more com- and Koller 1957, Klemm 1957). On the other hand, Gardiner plicated fractionation systems are worthy of mention. and Staford (1955) stated that it was better for refner sta- The Ortviken mill, Sweden, has a complex multi-stage bility and pulp quality to perform the rejects refning at screening system using a combination of hole and slotted very low consistency, approx. 1 %. However, most studies screens, which according to Asplund et al. (2009) is ad- showed that it was preferable to perform the rejects refn- vantageous for the removal of coarse fbres. The Braviken ing at high consistency. It should be noted that before the Mill, Sweden (Sandberg et al. 1997) and the Saugbrugs 1960’s, 15 % was considered as high consistency in rejects Mill, Norway, (Kure 2018) use a combination of screens refning, whereas today 7–15 % is considered medium con- and fractionating hydrocyclones which is benefcial for the sistency and > 30 % is considered high consistency. removal of thick-walled latewood fbres (Sandberg et al. In the early RMP processes, rejects refning was per- 1997, Kure et al. 1999). Another process based on extreme formed at low to medium consistency (4–15 %), Figure 3. fractionation is the interstage screening process (ISS), in From the late 1960’s, rejects refning was usually per- which fbres and fnes are separated already after primary formed at high consistency for the most efcient pulp qual- refning and the fbre fraction is subsequently refned in a ity development, (e. g. Wild 1971). As TMP was introduced, second HC refning stage after dewatering, Figure 15 (Fer- it was found that a large proportion of the fbres contained luc et al. 2010, Bousquet et al. 2016). After the fbre frac- in the long fbre fractions of main line TMP were poorly de- tion refning, the pulp is processed in a conventional re- veloped and that these fbres needed to be removed from jects system, or the rejects (shives/coarse fbre) from bar- the main stream and reprocessed separately in a dedicated rier screening are refned in the fbre fraction refner stage. refner in order to improve their bonding potential. (e. g. In this connection, it is very important to understand at Jackson and Williams 1979, Mohlin 1989, Corson and Lee what freeness level the primary refning should be termi- 1990). nated and this will, of course, depend on the wood species The concept with combined rejects and second stage involved and fnal pulp quality requirement. main line refning that was performed with LC refners

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even better in printing than the normal newsprint produced with a conventional SD TMP process.

Figure 16: TMP system from 1980’s with screen rejects fed back to Post refning second stage main line HC refning; Redrawn from Ulander (1985). Post refning of groundwood pulp with pump through refners was introduced during the 1960’s, mainly as a f- nal adjustment of paper quality. An applied specifc en- in the early Bauer RMP installations, was also applied in ergy of 40 kWh/t gave a reduction in shives content of 50 % HC refner lines supplied by Sprout Waldron (Jones 1968, and increased burst and surface smoothness by 10–20 % Ulander 1985, Fjerdingen et al. 1997). The main line con- (Richardson 1969, Kurdin 1974). Post refning was also a sisted of two or three refners in series and the screen low-cost alternative to increase groundwood production rejects were introduced before the second refning stage, (Hoholik 1958). It was also used to upgrade a newsprint Figure 16. This process concept however is no longer in furnish to higher quality grades (Brandal and Hauan 1970). widespread use. Kilpper and Baumgartner (1977) showed in pilot scale that For printing paper, the most common rejects treat- it was better to only post refne the longest fbres. ment today is single stage HC refning. Other approaches In some mills, news grade pulp is upgraded to light- include two stage HC refning (Viljakainen et al. 1997, weight coated (LWC) and super calandered (SC) quality Amiri et al. 2016, Sandberg et al. 2017a) and a combination by HC post-refning and peroxide bleaching (e. g. Asplund of HC and LC refning. The combination of HC and LC refn- et al. 2009). ing of rejects has been shown to improve pulp properties and efciency (tensile index – specifc energy) (Lansford 1979, Sandberg et al. 2017a). Viljakainen et al. (1997) evaluated LC-HC refning on rejects, but found no positive ef- Process design approaches fects of the combined LC and HC refning. However, the ap- Some process concepts can be distinguished from the most plied specifc energy in that LC refner was low compared common process design shown in Figure 2. We have tried to the specifc energy reported by Sandberg et al. (2017a). to group these designs according to, in our judgement, HC-LC refning of rejects was also applied in some ground- an intention or approach in the choice of process confg- wood mills (e. g. Witherell 1968, Ferritsius and Ferritsius uration. This is our subjective classifcation, and another 1997). grouping might of course be possible. As mentioned above, LC refning of rejects was common in the early RMP lines. Nowadays, this is rare and for printing paper, the TMP mill in Skogn, Norway, is the only Main line approach mill in operation, to our knowledge, that utilises LC refners for rejects refning (Imppola et al. 2013). However, in One idea is that the most economical process with good the Skogn mill, LC refning is carried out on mixed main fbre development is attained by doing as much of the f- line pulp and rejects. In Braviken, the same concept has bre development as possible in the main line refning and been introduced recently, but the screen accepts are fur- only having a low reject rate with LC refning on the re- ther fractionated in hydrocyclones with the rejects being ject. Such a process, with three CD82 HC refners in series, refned in HC refners. was in operation for several years in the Port Hawkesbury In contrast to the established idea of mechanical pulp- Mill, Canada, Figure 17, (Mokvist et al. 2005). Nowadays, ing processes containing rejects separation and refning, the mill has a more advanced two stage HC reject refning Ferritsius et al. (2014) showed, based on thorough pulp quality comparisons of a large number of TMP lines, that the same pulp quality can be reached after single stage high intensity main line refning as fnal pulp from a TMP line with two stage SD refning, screening and rejects refning. Sandberg et al. (2017b) produced paper on a high- speed paper machine with a high efciency TMP process Figure 17: Example of process with rejects LC refning – Port Hawkes- without any fractionation. The produced paper performed bury, Canada; Redrawn from Mokvist et al. (2005).

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Figure 18: TMP process in Skogn, Norway; Redrawn from Imppola et al. (2013).

Figure 20: Noss Advanced Mechanical Pulping Process; Redrawn from Shagaev and Bergström (2005).

Figure 19: Fractionation and refning of coarse groundwood pulp at the Manistrique Mill, USA; Redrawn from Hoholik (1958). achieve good fractionation and thereby much larger disc flter capacity is required. (Karnis 1982, Mohlin 1989, Sand- berg et al. 1997, Kure et al. 1999, Wakelin et al. 1999, Sand- system for the production of SCA++ on the same refning berg et al. 2001, Ouellet et al. 2003). line (Amiri et al. 2016). The Skogn Mill, Norway, has a sim- One extreme example of this approach is the Noss Ad- ilar process, but with two stage HC refning (CD+SD) and vanced Mechanical Pulping Process (NAMP), Figure 20, LC refning on combined main line pulp and rejects, Fig- which was developed to maximize fractionation efciency ure 18, (Imppola et al. 2013). and optimize the refning conditions for diferent fbre fractions (Shagaev and Bergström 2005). The concept utilizes Fractionation approach LC refning on the intermediate hydrocyclone fraction (en- riched in transition wood fbres) which can withstand a Much research has been made on concepts where the re- high load in LC refning, whereas late wood fbres (third fning is separated into a primary refning, after which the stage hydrocyclone rejects) and screen rejects are refned pulp is fractionated into a coarser fraction sent to further at HC. The process has been evaluated in mill scale, (Sand- refning and a fner fraction that is fed directly to dewa- berg and Shagaev 2011) but is not in continuous opera- tering. Such a process concept was frst suggested for re- tion. fner processes by Luhde (1966). However, it was used even earlier in groundwood production where a coarse groundwood pulp was produced, coarse-screened and fraction- Defbration/fbrillation approach ated using a vibrating screen, Figure 19. The screen rejects (roughly 50 % of the bull screen accepts) were refned in Another approach is the idea of performing defbration three DD refners (Hoholik 1958). The refned rejects were and fbrillation in separate machines using diferent con- combined with the vibrating screen accepts and fed to the ditions, as mentioned in the “Main line refning” section fne screens. The fne screen rejects were fed back to the above. This concept was proposed by Neill and Beath vibrating screen feed. (1963). Based on similar ideas, Valmet developed the Ther- The refning process is strongly heterogenic (Reyier mopulp process (Höglund et al. 1997) and later, Andritz de- Österling 2015), and the thought has been that it ought veloped the ATMP process, Figure 4, in cooperation with to be more energy efcient to do limited initial refning Norske Skog (Sabourin et al. 2003, Hill et al. 2009). The and separate the poorly developed fbres for additional re- Thermopulp concept was difcult to run due to low disc fning (Gavelin 1966, St. Laurent et al. 1993, Corson et al. gap in the second stage refner operating at 7–8 bar pres- 1995). One major problem with this idea is how to achieve sure (authors experience from the Braviken mill) and there the fbre fractionation with high efciency. Screens have are unfortunately no reports, so far, from the only mill in- a low ability to separate fbres with regard to fbrillation stallation of a full ATMP process. However, the concept of and bonding ability but can to some extent separate fbres separating the defbration and fbre development seems according to coarseness (Ämmälä 2001). Hydrocyclones to be a good way to further reduce the specifc refning have higher separation efciency, but they sufer from the energy. More mill experiences are however needed in this large disadvantage that very low consistency is needed to area.

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is being reduced and the demand for stable product quality is increasing. Systems such as those in Figures 5c and 21 where pulps from diferent refners are mixed and where the production from individual refners could not be moni- Figure 21: CTMP process;Redrawn from Jussila et al. (1999). tored on-line are not ideal for automatic control. Processes with a small standpipe or pulper after each refner and a minimal number of large chests, illustrated in Figure 22, are ideal for process control (Blanchard and Fontebasso 1993). TMP processes normally contain large volumes of pulp in diferent chests, which makes the system very slow.

Figure 22: Simplifed process concept. Of course, at least one large chest is needed to minimize the efect of shorter stops in parts of the process. In regions where the electricity has a large variation in price depending on the time of the day, larger pulp storage volumes can System simplifcation approach be economically motivated. Reducing process complexity has, in some cases, repre- Another way to classify the processes could be ac- sented a driving force for system development, motivated cording to fnal product quality, e. g. newsprint (Coldset), by reduced capital cost and easier process control (e. g. SC/LWC (Rotogravure), etc. We have not chosen to do this Ulander 1985). Some TMP and CTMP lines were built with classifcation since there are several diferent design con- the process confguration outlined in Figure 21. (Jussila cepts that have been utilized for high quality grades, e. g. et al. 1999). It is a simplifed process with low capital cost extended main line refning (three stages), more advanced since there is no separate rejects screening or rejects la- fractionation or extensive post refning. tency chest. Even though this process is compact, it is not One thing that is striking when earlier work is exam- easy to have an on-line pulp quality control of the two HC ined is the creativity of engineers in designing the fraction- refners. ation systems. Were all these confgurations motivated by The TMP process utilized in Skogn, Figure 18, is also thorough development work? Probably not. Since screens an example of a design that simplifes the process. Sand- and hydrocyclones separate particles according to difer- berg et al. (2018a) have shown that it is preferable to utilize ent properties, it is not a good idea to combine hydrocy- high intensity refning, such as DD or RTS, for this process clones and screens in an arbitrary way. It is, however, be- concept. A process incorporating single stage HC refning yond the scope of this work to elaborate on what the opti- and LC refning of primary stage pulp and screen rejects, mal design is. Of course, the fnal product and raw material Figure 22, reduces system complexity and refning energy can infuence the fractionation system design. as well as improves process control. Such a process was Proximity to an equipment supplier can afect the evaluated by Strand et al. (1993) and by Sandberg et al. choice of process equipment. A mill might prioritize good (2018a), the latter involved single stage HC refning with service from a local supplier before lowest price or best RGP68DD refners. A similar process has also been in oper- performance. ation at the Boyer Mill, Tasmania, with smaller DD refners The future remains uncertain and complex. The (Bonham et al. 1983). shrinking market for printing papers will continue but it will be ofset to some degree by an increased production of board and packaging grade pulps, partly due to an in- Discussion creasing demand for this type of pulp but amplifed due to pressure from both the public and private sectors to dras- As has been presented above, there are many system de- tically reduce the use of non-biodegradable plastics. signs for MP production. What have been the driving forces In the early days, considerable efort was made by for the process development? There have clearly been the pulp and paper producers to develop the refner me- many diferent views on what constitutes the best solution chanical pulping process, sometimes in cooperation with for refning efciency and achieving good pulp quality. machine suppliers, see e. g. Jones (1968), Mihelich et al. One approach that, in most cases, has not been uti- (1972), Peterson and Dahlqvist (1973), Pearson et al. (1977) lized is controllability. Automatic control of refner sys- and Huusari and Syrjänen (1977). In addition, all major tems is becoming more and more important since manning R&D Institutes directed considerable eforts towards re-

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fner mechanical pulping. Nowadays there are few indus- Aregger, H.J. (1997) The TMP system at Perlen using the RTS try initiatives and rather limited research activities. concept. In: Int. Mech. Pulping Conf. Stockholm, Sweden. Usually, investments in new production capacity of- pp. 251–255. Aregger, H.J. (2001) Operation experience with single stage main fers an opportunity to integrate process and machine de- line refning in RTS operation. In: Int. Mech. Pulping Conf. velopment. The equipment suppliers have a vital role in Helsinki, Finland. pp. 229–237. contributing to the progress of new and improved technol- Arnesjö, B., Dillén, S. (1975) Thermomechanical pulping at Hallsta ogy. In recent years the investment in refner mechanical papermill. In: Int. Mech. Pulping Conf., San Francisco, USA, pulping lines has declined drastically. In summary, it is Vol. 1. pp. 97–103. hard to deny that this will risk limiting the rate of progress Asplund, A. (1934) Method of manufacture of pulp, Patent 2,008,892, Sep 19, 1934. of mechanical pulping despite its renewable, high wood Asplund, A. (1953) The origin and development of the Defbrator yield product characteristics. It will be interesting to see process. Sven. Papp.tidn. 56(14):550–558. if the pulp and paper industry will manage to reinvest in Asplund, A., Bystedt, I. (1973) Development of the both the human capital and the industrial capital neces- thermo-mechanical pulping method. In: Int. Mech. Pulping sary to build a fourishing business based on renewable Conf. Stockholm, Sweden, Pres. 15. Asplund, P., Bäck, R., Åhlund, A., Gannå, A., Jonsson, S. (2009) mechanical pulp. Screening of TMP is an essential key process in Ortviken paper mill. In: Int. Mech. Pulping Conf. Sundsvall, Sweden. Acknowledgments: Jan-Erik Berg at Mid Sweden Uni- pp. 147–151. versity is acknowledged for valuable discussions of the Atack, D. (1972) On the characterization of pressurized refner manuscript and Meaghan Miller at the University of British mechanical pulps. Sven. Papp.tidn. 75(3):89–94. Columbia is thanked for assistance in the search for early Atack, D., Heitner, C., Stationwala, M.I. (1977) Ultra high yield references. pulping of eastern black spruce. In: Int. Mech. Pulping Conf. Helsinki, Finland, Pres. 7. Atack, D., Wood, P.N. (1973) On the high consistency operation of Funding: This work was supported by Holmen Paper AB large double rotating disc refners. In: Int. Mech. Pulping Conf. and the Knowledge Foundation, grant No. 20170305. Stockholm, Sweden, Pres. 12. Barnet, A.J., Leask, R., Shaw, A.C. (1980) The OPCO process – Part I Confict of interest: The authors declare no conficts of in- and II. Pulp Pap. Can. 81(10):T255–T260. terest. Beath, L.R. (1971) Groundwood rejects refning: Shive Reduction – specifc energy relationships. In: CPPA Technical Section Proceedings, D92–D95. 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