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OULU 2012 C 425 ACTA Mika Körkkö UNIVERSITATIS OULUENSIS C TECHNICA ON THE ANALYSIS OF INK CONTENT IN RECYCLED PULPS UNIVERSITY OF OULU GRADUATE SCHOOL; UNIVERSITY OF OULU, FACULTY OF TECHNOLOGY, DEPARTMENT OF PROCESS AND ENVIRONMENTAL ENGINEERING ACTA UNIVERSITATIS OULUENSIS C Technica 425 MIKA KÖRKKÖ ON THE ANALYSIS OF INK CONTENT IN RECYCLED PULPS Academic dissertation to be presented with the assent of the Doctoral Training Committee of Technology and Natural Sciences of the University of Oulu for public defence in Arina-sali (Auditorium TA105), Linnanmaa, on 27 July 2012, at 12 noon UNIVERSITY OF OULU, OULU 2012 Copyright 2012 Acta Univ. Oul. C 425, 2012 Supervised by Professor Jouko Niinimäki Doctor Ari Ämmälä Reviewed by Doctor Mahendra Doshi Professor Harald Grossmann ISBN (Paperback) ISBN (PDF) ISSN (Printed) ISSN (Online) Cover Design Raimo Ahonen JUVENES PRINT TAMPERE 2012 Körkkö, Mika, On the analysis of ink content in recycled pulps. University of Oulu Graduate School; University of Oulu, Faculty of Technology, Department of Process and Environmental Engineering, P.O. Box 4300, FI University of Oulu, Finland Acta Univ. Oul. C 425, 2012 Oulu, Finland Abstract The amount of printing ink in a pulp suspension produced from recovered paper and its impact on overall brightness is commonly estimated from the reflectance-based ink content measured at a wavelength of 700 nm or 950 nm. The method uses a light scattering coefficient that can be measured from a slightly translucent test medium, i.e. of an opacity less than 97%. This is the case with machine-made papers in most instances. Alternatively, suitable opacity can be achieved by preparing a standard low-grammage sheet on a wire screen, but this results in poor retention of fibre fines, mineral fillers and printing inks, which is especially detrimental to ink measurement when the pulp suspension contains substantial amounts of printing inks. Hence opaque pads are often prepared on filter paper to achieve high retention. Unfortunately their high opacity prevents measurement of the light scattering coefficient, and thus a constant coefficient must be used for the determination of ink content. The aim of this thesis was to clarify the effects of retention and fine material changes on the light scattering coefficient in ink content measurement. The results showed that the light scattering properties of pulp in the wavelength region used for ink content analysis do not remain constant when the fine material content varies. The grade of the recovered paper, hyperwashing and flotation alter the fine material content and thus affect the light scattering. Printing ink also affects light scattering, but its practical impact is smaller than that of fibre fines and mineral fillers. The light scattering coefficient used for each ink content measurement needs to be representative, otherwise a systematic bias in ink content measurements may result from changes in the nature of the fine material and in its content. It is recommended that the light scattering coefficient should be measured in order to avoid this. The measurement should preferably be performed from a low-grammage sheet prepared on filter paper, as this ensures high retention and a measured value that represents better the initial state of the pulp suspension. Keywords: deinking, effective residual ink concentration, ERIC, fibre fines, fine material, light scattering, mineral filler, recovered paper, recycling, residual ink Körkkö, Mika, Jäännösmusteen mittaus kierrätysmassoista. Oulun yliopiston tutkijakoulu; Oulun yliopisto, Teknillinen tiedekunta, Prosessi- ja ympäristötekniikan osasto, PL 4300, Oulun yliopisto Acta Univ. Oul. C 425, 2012 Oulu Tiivistelmä Mustepitoisuusmittaus perustuu hajaheijastukseen joko 700 nm tai 950 nm aallonpituudella ja sitä käytetään usein arvioitaessa keräyspaperista valmistetun massasuspension soveltuvuutta painopaperien valmistukseen ja painomusteen vaikutusta massan vaaleuteen. Mustepitoisuuden mittauksessa käytetään valonsirontakerrointa, joka voidaan mitata hieman läpikuultavasta näytteestä eli näytteen opasiteetin on oltava pienempi kuin 97 %. Tämä opasiteettiraja toteutuu useimmiten paperikoneella tehdyille painotuotteille. Riittävän alhainen opasiteetti saavutetaan myös valmistamalla standardin mukainen matalaneliömassainen arkki viiralle, mutta tämä johtaa kuitumaisten hienoaineiden, mineraalisten täyteaineiden ja painomusteiden alhaiseen retentioon. Matala retentio on erityisen haitallinen piirre mustemittauksen kannalta massoilla, jotka sisältävät huomattavia määriä painomusteita. Siten usein valmistetaan läpikuultamattomia arkkeja suodatinpaperin päälle, joiden retentio on korkea. Korkeasta opasiteetista johtuen näistä arkeista ei voida määrittää valonsirontakerrointa, jolloin mustepitoisuuden määritys perustuu vakiokertoimeen. Tämän väitöskirjan tavoitteena oli selvittää retention ja hienoaineiden muutoksien vaikutuksia valonsirontakertoimeen ja mustemittaustulokseen. Tutkimuksen tulokset osoittivat että valonsirontakerroin, joka mitataan mustepitoisuuden yhteydessä, ei pysy vakiona hienoainepitoisuuden muuttuessa. Hienoainepitoisuuteen ja siten valonsirontakertoimeen vaikuttavat keräyspaperin laji, hyperpesu ja vaahdotus. Myös painomuste vaikuttaa valonsirontakertoimeen, mutta käytännössä vaikutuksen suurusluokka on pienempi kuin hieno- ja täyteaineilla. Mustepitoisuuden määrityksessä käytetyn valonsirontakertoimen on oltava edustava, muutoin arvot voivat olla systemaattisesti virheellisiä hienoainemäärän tai laadun muuttuessa. Virheen välttämiseksi olisi suositeltavaa määrittää valonsirontakerroin mustepitoisuusanalyysin yhteydessä. Tämä olisi mahdollista tehdä suodatinpaperin päälle valmistetusta matalaneliömassaisesta arkista, jolloin saavutetaan korkea retentio ja näin mitattu arvo edustaa paremmin massasuspension alkuperäistä tilaa. Asiasanat: ERIC, hienoaine, keräyspaperi, kierrätys, kuitumainen hienoaine, siistaus, täyteaine, valonsironta Acknowledgements The research work reported in this thesis was carried out at the University of Oulu in the Fibre and Particle engineering laboratory during the years of The work was conducted in co-operation with the Finnish Graduate School in Environmental Science and Technology (EnSTe). Funding from Finnish Society of Automation, Tauno Tönning Research Foundation, Niemi Foundation, Alfred Kordelin Foundation, Walter Ahlström Foundation, the Association of the European Fibre and Paper Research Organisation, European Cooperation in Science and Technology, Kemira Oyj and Metso Corporation is greatly appreciated. My sincere thanks to Professor Jouko Niinimäki for an opportunity to work on such an interesting field of deinking and for supervising the thesis. In addition, thanks belong also to Dr. Ari Ämmälä for his efforts. I highly appreciate the reviewing work done by Dr. Mahendra Doshi, the executive editor of Progress in Paper Recycling, and Professor Harald Grossmann from the Dresden University of Technology. I would like to express my gratitude also to Malcolm Hicks for revising the English language of this thesis. I would like to thank my colleagues and friends; Henrikki Liimatainen, Antti Haapala, Ossi Laitinen, Liisa Mäkinen and Sari Vahlroos-Pirneskoski: the discussions during the writing of articles were very valuable. Also, thanks to Kalle Kemppainen, our guru in statistical significance; laboratory assistants Jani Österlund and Jarno Karvonen, your help was irreplaceable; and many students, especially for Pirita Huotari, for participating in the experiments. Finally, I would like to thank my girlfriend Saana for understanding and encouragement. My warm thanks to my parents, sisters and brothers: Annikki and Eero, Marja and Pertti, Tapani and Susanne, and Tuomo and Anja, it has always been enjoyable to discuss with you. Also the parents and sister of Saana: Hilkka and Seppo, and Anna and Timo, thank you for a number of occasions in spending evening. The literature-based experimental-approach on the mystery of Scotch has been fascinating. And very finally, I wish to thank all my friends, especially Petri Reponen, Miia and Janne Asikkala, and dancing teachers Sari and Jari Aaltonen, it has been very nice to spend time and dance in so many places in Finland and as far away as Canary Islands. Oulu, June 2012 Mika Körkkö 7 8 Abbreviations APPITA CIE CIE LAB CPPA CTP EN GCC HW HWK INGEDE ISO L&W LWC od ONP OMG PAPTAC PCC PTS SC SFS SWK TAPPI TMP Australasian Pulp and Paper Industry Technical Association International Commission on Illumination (Commission Internationale de l Eclairage) CIE 1976 colour space system: L*, a* and b* coordinates Canadian Pulp & Paper Association, nowadays known as PAPTAC Centre Technique du Papier European Norm Ground calcium carbonate Hyperwashed Hardwood kraft pulp International Association of the Deinking Industry (Internationale Forschungsgemeinschaft Deinking-Technik) International Organization for Standardization Lorentzen & Wettre, a spectrophotometer supplier Lightweight coated paper Oven dry Old newsprint Old magazines Pulp and Paper Technical Association of Canada Precipitated calcium carbonate Paper Technology Specialists Supercalendered paper Finnish Standards Association Softwood kraft pulp Technical Association of the Pulp and Paper Industry Thermomechanical pulp cs Consistency [%] DEM f Deinkability factor based on brightness or luminosity [%] DEM LAB Deinkability factor based on CIELAB colour system [%] dmc Dry matter content [%] E r Removal efficiency [%] ERIC Effective residual ink concentration, ink content [ppm] ERIC 700 Effective residual ink concentration at 700 nm [ppm] ERIC 950 Effective residual ink concentration at 950 nm [ppm] 9 IE Ink elimination [%] k Light absorption coefficient [m 2 /kg] ṁ Mass flow rate [kg/min] p Student s t-test distribution Q K Karnis s selectivity index Q N Nelson s selectivity index Q V Volumetric flow rate [L/min] r TAPPI repeatability R 0 Single-sheet reflectance factor R Intrinsic reflectance factor, reflectivity RI Residual ink, measurement of ink content [ppm] RI L&W Residual ink, a special procedure issued by Lorentzen & Wettre [ppm] RR m Mass reject ratio [%] s Light scattering coefficient [m 2 /kg] std Standard deviation w Grammage of the paper sheet [g/m 2 ] x Content of component [%] ẋ Mass flow rate of component [kg/min] X Tristimulus factor red Y Tristimulus factor green, intrinsic luminance factor, luminosity Z Tristimulus factor blue %r TAPPI repeatability ratio [%] 10 List of original publications This thesis is based on the following publications, which are referred throughout the text by their Roman numerals: I Körkkö M, Laitinen O, Vahlroos S, Ämmälä A & Niinimäki J (2008) Components removal in flotation deinking. Prog Pap Recycl 17(4): II Körkkö M, Laitinen O, Haapala A, Ämmälä A & Niinimäki J (2011) Scattering properties of recycled pulp at the near infrared region and its effect on the determination of residual ink. TAPPI J 10(6): III Körkkö M, Haapala A, Liimatainen H, Ämmälä A & Niinimäki J (2011) Challenges in residual ink measurement Effect of fibre fines and fillers. Appita 64(1): IV Körkkö M, Haapala A, Mäkinen L, Ämmälä A & Niinimäki J (2011) Comparison of test medium preparation methods for residual ink analysis. TAPPI J 10(10): All the listed publications were written by the author of this thesis, whose main responsibilities were the experimental design, data analysis and reporting of the results. 11 12 Contents Abstract Tiivistelmä Acknowledgements 7 Abbreviations 9 List of original publications 11 Contents 13 1 Introduction Background The research problem Aim and hypotheses Outline of the thesis Present understanding of optical properties in relation to ink content measurements History of light and colour measurement The Kubelka-Munk theory Measurement of ink content Machine-made paper Low-grammage sheets formed on a wire screen Opaque pads formed on filter paper Low-grammage sheets formed with a closed water loop Summary of ink content measurement Materials and Methods Sample preparation Pulping Flotation Hyperwashing and washing Addition of fresh fillers Preparation of test media for optical analysis Opaque pads on filter paper Low-grammage sheets on a wire screen Low-grammage sheets on filter paper Analyses Ash content and measurement of the added filler Mass fractions Fine material content 3.3.4 Reflectivity, light scattering coefficient and ink content Calculations Free ink content Mass reject ratio, removal efficiency and selectivity Mass balance calculation of ink content TAPPI repeatability t-test statistical analysis Results and Discussion Removal of fine materials Effect of fine materials on light scattering at 700 nm Fresh mineral fillers Fresh fibre fines Fine materials from unprinted and printed papers Correlation between light scattering coefficients Effect of fine materials on measured ink content Ink content with constant light scattering Ink content with measured light scattering Calculation of free ink content and removal efficiencies Influence of the test medium preparation method Fine materials Light scattering coefficient at 700 nm Reflectivity at 700 nm Ink content at 700 nm Repeatability Conclusions 63 References 65 Original publications 71 14 1 Introduction 1.1 Background Paper is one of the everyday commodities that it would be very difficult imagine the world without. In the past, graphic paper manufacturing was based on virgin fibres and minerals, but the importance of recovered papers has been constantly increasing during the last four decades. In fact, the volume of recovered paper surpassed the use of virgin raw materials a few years ago (Ervasti 2010). In the production of graphic papers from recovered paper, the impurities that detract from the quality of the product need to be removed in order to achieve a clean pulp with sufficiently high optical properties (Schabel 2010). The recovered paper stream contains all the materials that became attached to the paper during its production and printing, e.g. fibres, fibre fines, mineral fillers, printing ink and adhesives. In addition, a certain amount of extraneous matter, e.g. sand, glass, metal and plastic originating from recovered paper collection, handling and storing are also present in the recovered paper stream. These impurities are removed in the deinking process, in which the first unit operation is pulping. This causes the fibres to disintegrate, detaches impurities from the recovered paper and removes large-sized foreign bodies. Smaller-sized impurities remaining in the pulp suspension are removed in the subsequent screening, cleaning, flotation and washing operations. (Schabel 2010). Flotation causes the highest material losses in the deinking line producing printing and writing papers, while in the case of tissue production the highest losses are the result of pulp washing (Hamm 2010). Printing ink has the highest impact on the optical properties of pulp produced from recovered papers (Carré et al. 2010, McKinney 1998). This is removed during flotation (and washing), but only free ink particles can be removed without sacrificing the yield. The optical properties of pulp can also be improved by bleaching the fibre material a yellowish colour and this is normally done in the production of high grades. The fact that the bleaching response is higher for deinked than non-deinked pulps (Rangamannar 2003) highlights the importance of printing ink removal. Determination of the ink content of a pulp suspension is based on reflectance measurements in a wavelength region higher than 650 nm, where the yellowish colour of the fibre material has a negligible impact on light absorption (Jordan & Popson 1994). By this means the reflectance-based ink content is unaffected by 15 the changes in the colour of the fibre material induced by alkali darkening or bleaching. The ISO and TAPPI T 567 standards specify the use of a wavelength of 950 nm, yielding a result that is referred as the Effective Residual Ink Concentration, ERIC 950 or simply ERIC. Since an extended-range spectrophotometer is needed for measurements in this near-infrared region (Villforth et al. 2010), a modified version of ERIC which can be measured using spectrophotometers of a kind generally to be found in paper industry laboratories has become widely accepted. This modified version uses reflectance measurements at 700 nm and is known as ERIC 700, Residual Ink and RI. Ackermann & Göttsching (2002a) have shown that the light absorption coefficient measured at 700 nm responds linearly to black printing ink equally as well as the coefficient measured at 950 nm. The reading can be different, however, depending on the wavelength chosen, because cyan-coloured printing inks are also accounted for when reflectivity is measured at 700 nm (Ackermann & Göttsching 2002b, Jordan & O Neill 1994). But even though there is a difference between the wavelengths, both methods are applicable for the analysis of ink content in practical instances. Reflectance-based ink content can be determined by means of the Kubelka- Munk theory, by a method similar to the optical measurements applied in paper production that use blue or luminous light reflectance. The reflectivity measured indicates the sum effect of light scattering and absorption, whereas the light absorption coefficient indicates the magnitude of light absorption alone. In blue and luminous light reflectance the fine materials are known to alter the light scattering properties of the pulp, and therefore the absorption properties of the pulp can be estimated more accurately using the light absorption coefficient rather than reflectivity. (Pauler 2008, Vaarasalo 1999, Walmsley & Silveri 1999). The ISO and TAPPI T 567 standards lay down that the ERIC has to be determined in accordance with Kubelka-Munk theory, i.e. the light scattering coefficient has to be measured during the determination of ink content. For this purpose, the single-sheet opacity must remain below 97% (ISO and TAPPI T 567). Determination of the ink content of machine-made papers requires no test medium preparation. The light scattering coefficient can be measured in most instances, as the opacity of machine-made papers remains below 97%. Test medium preparation is needed when the ink content of the pulp suspension is to be determined. The sheets are prepared on a wire screen (nominal aperture µm) in accordance with ISO and TAPPI T 567, even though this 16 results in washing out the printing ink and other fine materials such as fibre fines and mineral fillers (Dorris 1999, Haynes 2000, Lévesque et al. 1995, Lévesque et al. 1998, McKinney 1988, Scott 1993). The loss of printing ink during sheet preparation is considered acceptable where deinked pulps are concerned, because then the ink content can be determined by means of the measured light scattering coefficient (ISO and TAPPI T 567). The loss of printing ink remains only slight, because the deinked pulps contain low amounts of printing ink. The evaluation of ink content in deinking is also widely used for non-deinked and partially deinked pulps. In these instances the pulp suspensions contain substantial amounts of printing ink, which would be largely rinsed out during sheet preparation on a wire screen. For this reason, INGEDE Method 2 recommends the preparation of opaque pads on filter paper for improving the retention of printing ink (pad preparation is similar to that in ISO 3688 and TAPPI T 218). The preparing of opaque pads resolves the retention issue, but measurement of the light scattering coefficient is prevented due to the high opacity of the pads and the determination of ink content has to be based on a constant value for the light scattering coefficient. T
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