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 proper- ties 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 devel- opment. 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 mechan- ical 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.