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PROYECTO FIN DE CARRERA INGENIERÍA QUÍMICA ESTUDIO DE LA FLOTACIÓN PARA EL DESTINTADO DE LA FRACCIÓN DE FIBRA CORTA EN PULPA PROCEDENTE DE PAPEL RECUPERADO Autor: Gabriel Cisneros Barriga Director: Ari Ämmälä, Universidad de Oulu Ponente: Alberto Gonzalo Callejo, Universidad de Zaragoza Fibre and Particle Engineering Laboratory, Department of Process and Environmental Engineering, University of Oulu Zaragoza, Febrero de 2012 2 AGRADECIMIENTOS El presente proyecto se llevó a cabo en el Laboratorio de Ingeniería de Fibra y Partículas, Universidad de Oulu. Me gustaría agradecer a toda la gente del departamento del Laboratorio de Ingeniería de Fibra y Partículas de la Universidad de Oulu por su amabilidad y por hacer el invierno más frío de mi vida un poquito más cálido. Especial agradecimiento para mis supervisores Liisa Mäkinen y Mika Körkkö por su actitud calmada y positiva, y por su incalculable ayuda y disponibilidad durante todo el proyecto. Sincero agradecimiento para mi director Ari Ämmälä por su acogida en el departamento y por sus consejos durante todo el trabajo. Agradecer también a Alberto Gonzalo su amabilidad, disponibilidad y predisposición desde el primer minuto en que le propuse ser mi ponente. Finalmente, me gustaría agradecer a mi familia, especialmente mis padres, por su paciencia y por darme la oportunidad de estudiar en el extranjero, y a mi hermano, por ser mi apoyo y fuerza durante los buenos y los malos momentos. También me gustaría agradecer a todos los amigos que he hecho durante mi Erasmus en Oulu, un año repleto de recuerdos inolvidables. Especial agradecimiento para mis amigos del colegio, de la universidad y de mi pueblo Malanquilla por su cariño, amistad y por todas las risas compartidas. 3 TABLA DE CONTENIDOS AGRADECIMIENTOS .................................................................................................... 2 TABLA DE CONTENIDOS ............................................................................................ 3 1 INTRODUCCIÓN Y OBJETIVOS .......................................................................... 5 2 ANTECEDENTES .................................................................................................... 7 2.1 LÍNEA DE DESTINTADO CONVENCIONAL .............................................. 7 2.1.1 Pulpeado ..................................................................................................... 7 2.1.2 Depuración por tamizado............................................................................ 8 2.1.3 Limpieza por centrifugado.......................................................................... 8 2.1.4 Flotación ..................................................................................................... 9 2.1.5 Dispersión ................................................................................................... 9 2.1.6 Blanqueado ................................................................................................. 9 2.1.7 Eliminación de agua ................................................................................. 10 2.2 LÍNEA DE DESTINTADO CON FRACCIONAMIENTO ............................ 10 2.2.1 Diferencias entre fraccionamiento y depuración por tamizado ................ 11 2.2.2 Propiedades de las fracciones ................................................................... 12 2.3 FUNDAMENTOS DE LA FLOTACIÓN ....................................................... 12 2.3.1 Fases de la flotación ................................................................................. 13 3 MATERIALES Y MÉTODOS ............................................................................... 15 3.1 Materiales ......................................................................................................... 15 3.1.1 Materia prima ........................................................................................... 15 3.1.2 Reactivos químicos ................................................................................... 15 3.2 Métodos ........................................................................................................... 16 3.2.1 Descripción del proceso............................................................................ 16 3.2.2 Análisis ..................................................................................................... 18 3.3 Desarrollo de la parte experimental ................................................................. 19 3.3.1 Experimentos de referencia ...................................................................... 19 3.3.2 Estudio del nivel de flotación y flujo de aire durante la flotación ............ 21 3.3.3 Estudio del acondicionamiento previo a la flotación................................ 22 3.3.4 Estudio del efecto del pH en la flotación .................................................. 23 3.3.5 Estudio de la adición de diferentes reactivos químicos de flotación ........ 24 4 4 RESULTADOS Y DISCUSIÓN ............................................................................. 26 4.1 Experimentos de referencia ............................................................................. 26 4.2 Nivel de flotación y flujo de aire durante la flotación ..................................... 28 4.3 Acondicionamiento previo a la flotación ......................................................... 29 4.4 Efecto del pH en la flotación ........................................................................... 31 4.5 Adición de diferentes reactivos químicos de flotación .................................... 35 5 CONLCUSIONES .................................................................................................. 38 6 BIBLIOGRAFÍA ..................................................................................................... 39 ANEXOS ANEXO 1. Cisneros Barriga, Gabriel, 2011, Study of DIP fines fraction flotation, University of Oulu. 5 1 INTRODUCCIÓN Y OBJETIVOS El reciclado de papel es un proceso cada vez más importante dentro de la industria papelera, debido tanto a razones económicas como medioambientales. De hecho en los últimos 15-20 años, ha aumentado el uso de papel reciclado para la fabricación de papel comercial: como el usado en pañuelos, periódicos, etc. Este papel reciclado requiere unas condiciones mínimas de algunas de sus propiedades para su uso en papeles de alta calidad, como por ejemplo brillo, contenido de cenizas o tinta residual. Actualmente, alrededor de la mitad del papel y el cartón producido mundialmente se obtiene con papel recuperado. El principal componente del papel recuperado que ha de retirarse es la tinta, debido a que produce una reducción en el brillo del papel y su blancura. Por ello, los procesos de destintado de papel han sufrido progresivamente mejoras, y las líneas de reciclado cada vez son más complejas. Como parte de dichas mejoras, diversas alternativas de destintado están siendo consideradas con el objetivo de mejorar el proceso. En este proyecto, se realizó un estudio en detalle de la flotación para la eliminación de la tinta en la fracción de fibra corta de pasta procedente de papel recuperado, dentro de una nueva propuesta de proceso de destintado. Esta propuesta se muestra esquemáticamente en la Figura 1. Básicamente, el papel recuperado (mezcla entre periódicos y revistas) es pulpeado durante un tiempo muy corto y posteriormente separado en fracción de fibra corta (o fracción de finos) y fracción de fibra larga. Con este corto pulpeado se evita que la tinta sufra una excesiva fragmentación y vuelva a depositarse en las fibras. Tras el fraccionamiento, la fracción de fibra larga pasa por un segundo pulpeado, y la fracción de fibra corta se limpia mediante un proceso de flotación. Esta fracción de fibra corta ya flotada se puede combinar con la fracción de fibra larga y ser realimentada a una segunda parte del proceso de producción de papel, para mejorar la limpieza y el brillo de la pasta de papel. (Körkkö y cols., 2010). 6 Pulpeado Fraccionamiento Fracción Fibra corta FlotaciónPulpeado Fracción Fibra larga A 2º lazo de destintado Figura 1. Diagrama del destintado alternativo estudiado (Körkkö y cols., 2010). El objetivo de este proyecto fue obtener más información sobre la flotación de la fracción de fibra corta dentro de la propuesta de destintado de la Figura 1. Para ello, se realizaron diferentes experimentos de flotación variando sus propiedades en búsqueda de los valores óptimos, con los que la muestra consiguiera un mayor brillo con la menor pérdida de fibras posible. Todo el trabajo se llevó a cabo en el Departamento de Ingeniería de Procesos y Medio Ambiente de la Universidad de Oulu (Finlandia), dentro del Laboratorio de Ingeniería de Fibra y Partículas, y su memoria original se puede consultar en el Anexo 1 de este proyecto fin de carrera. 7 2 ANTECEDENTES 2.1 LÍNEA DE DESTINTADO CONVENCIONAL El propósito del destintado es eliminar la tinta y otras substancias contaminantes que pueden afectar el proceso de fabricación de papel o sus propiedades finales. Por este motivo, los procesos en los que se emplea papel recuperado son más complejos que los que usan fibras vírgenes, ya que el papel recuperado es una mezcla de varios tipos de papel con cierta cantidad de contaminantes. Las propiedades finales de la suspensión de pasta de papel dependen directamente de la eliminación de contaminantes, de modo que una baja separación de contaminantes lleva a valores bajos de brillo y calidad del producto final (Lassus 2000 pp. 241, Holik 2000 pp.91). En la página 9 del Anexo 1 se presenta un diagrama con las etapas del proceso de fabricación de papel de periódico. Las diferentes etapas de una línea convencional de destintado son las siguientes: pulpeado, depuración por tamizado, limpieza por centrifugado, flotación, dispersión, blanqueado y eliminación de agua. Durante el pulpeado se aplican fuerzas mecánicas, térmicas y químicas para despegar las impurezas de las fibras, como la tinta. Otras etapas como la depuración por tamizado, la limpieza por centrifugado y la flotación son etapas de separación en las que se elimina la tinta libre de la suspensión (Eul y cols. 1990, Doshi 1997 pp.3). A continuación se explican más detalladamente estas etapas. 2.1.1 Pulpeado El objetivo del pulpeado es desintegrar el papel en fibras individuales para obtener una suspensión que pueda ser bombeada. Las fuerzas de desintegración aplicadas durante el pulpeado deben de ser mayores que la resistencia del papel o las fuerzas de unión de los contaminantes a las fibras. Pero estas fuerzas de desintegración no deben de ser excesivas para evitar que los contaminantes (como la tinta) se rompan en partículas demasiado pequeñas, lo cual dificulta su posterior eliminación. (Holik 2000 pp. 95-97) 8 Se necesita que durante el pulpeado se consiga una buena separación entre las partículas contaminantes y las fibras para que posteriormente la suspensión de pasta de papel sea tratada eficientemente. Las fuerzas mecánicas separan la tinta mediante fricción entre fibras y los reactivos químicos consiguen que algunas partículas se vuelvan más hidrófobas. El objetivo es liberar la tinta de las fibras evitando que se vuelvan a adherir a ellas o que se puedan fragmentar. Cuanto más baja es esta fragmentación de la tinta, mayor brillo tiene la suspensión de papel tras la flotación (McKinney 1999 pp. 107, Beneventi y cols. 2005, Holik 2000 pp. 151-153). En las páginas 10 y 11 del Anexo 1, se amplía la información sobre el proceso de pulpeado. 2.1.2 Depuración por tamizado El objetivo de la depuración por tamizado es eliminar los contaminantes sólidos de la pasta de papel recuperado. Se emplean diferentes pantallas o tamices según el tamaño de partícula y la forma de los contaminantes. Habitualmente se tamiza la misma suspensión varias veces, para evitar en la medida de lo posible la pérdida de fibras. En las páginas 11 y 12 del Anexo 1 se amplía la información de esta fase. 2.1.3 Limpieza por centrifugado La limpieza por centrifugado es otro proceso de separación que complementa a la depuración por tamizado, y su objetivo es eliminar partículas de la suspensión que puedan afectar la calidad del papel. Como partículas contaminantes nos referimos a arena, piezas de metal, grapas o materiales plásticos. Para una eliminación eficiente, es necesario que los contaminantes tengan una densidad mayor a la del agua. Gracias a la limpieza por centrifugado se pueden eliminar partículas más pequeñas que con la depuración por tamizado. (Holik 2000 pp. 134-135) 9 2.1.4 Flotación La flotación es el proceso más común para realizar la separación de la tinta de las fibras sobre las que se encuentra depositada. Es un proceso de separación que se basa en la probabilidad de que burbujas de aire se adhieran a las partículas de tinta y las lleven a la superficie de la suspensión formando una capa de espuma. Este proceso es posible gracias al comportamiento hidrófobo de las partículas permitiendo que puedan ser eliminadas, mientras que las fibras son retenidas en la suspensión. (Holik 2000 pp. 151- 152, 241) Una vez que se ha conseguido una buena separación de la tinta y las fibras en el pulpeado, se introduce un flujo de aire a la suspensión y las partículas repelentes al agua se adhieren a estas burbujas de aire formando una capa de espuma en la superficie de la suspensión, partículas contaminantes como tinta, cargas, pigmentos, etc. La espuma formada se retira de la suspensión o mecánicamente, o por rebose o por extracción al vacío (Holik 2000 pp. 151-153, 244). La flotación se presenta más a fondo tanto en el Apartado 2.3 de este proyecto, como en las páginas 13 y 14 del Anexo 1. 2.1.5 Dispersión El objetivo de la dispersión es romper los contaminantes a un tamaño en el que no interfieran en el proceso y evitar que se vean a simple vista. La dispersión hace que ciertos contaminantes se hagan más pequeños y se favorezca su eliminación por flotación, ya que mezcla reactivos blanqueantes y despega la tinta todavía adherida a las fibras. (Holik 2000 pp. 185-186) 2.1.6 Blanqueado La etapa de blanqueado mejora las propiedades ópticas de la pasta de papel destintada, como por ejemplo el brillo y la luminosidad. Los reactivos más utilizados son peróxido de hidrógeno (H2O2), hidrosulfito de sodio (Na2S2O4), cloro (Cl2), dióxido de cloro 16 3.2 Métodos 3.2.1 Descripción del proceso Producción de la pasta de papel (Pulping) La pasta fue producida empleando un desintegrador (pulper) Hobart (H600) con un mezclador helicoidal (Figura 4). En cada preparación de pasta se emplearon 2.500 g de papel recuperado, el cual se rasgó en tiras de unos dos centímetros de ancho y se mezcló con agua caliente (45 ºC) para obtener un contenido en sólidos (consistencia) del 15%. El tiempo de pulpeado total fue de 14 minutos. Figura 4. Desintegrador Hobart (H600). 17 Fraccionamiento El fraccionamiento se llevo a cabo empleando el dispositivo piloto de fraccionamiento PULA del laboratorio, consistente en un tanque de 300 litros, una bomba y una cesta con ranuras de 0,15 mm de anchura. La pasta se diluyó en el tanque de alimentación con agua caliente del grifo (45 ºC) hasta obtener una consistencia aproximada de 0,8%. La pasta diluida se separó empleando una frecuencia de 50 hercios en el rotor de la cesta en dos fracciones: fracción de fibra corta (aceptado) y fracción de fibra larga (rechazo). Flotación La flotación se llevó a cabo empleando un equipo de flotación Voith Delta 25 equipado con caja de nivel para mantener constante el volumen de líquido mediante el aporte de agua al dispositivo durante la flotación. Tanto el aire como el agua se introducen al equipo de manera continua, mientras que la suspensión de pasta de papel se alimenta de manera discontinua. Las fracciones de fibra corta (o finos) de todos los experimentos de fraccionamiento se emplearon como alimentación para la célula de flotación. Estas muestras de fibra corta se mantuvieron a 45 ºC empleando un calentador eléctrico hasta su introducción en dicho equipo de flotación. Figura 5. Equipo de flotación Voith Delta 25. 18 3.2.2 Análisis Consistencia y cenizas La consistencia de las muestras de pasta de papel se realizó según la normativa SFS EN ISO 4119. El contenido en cenizas se determinó mediante ignición de las muestras a 525 ºC según la normativa ISO 1762. Estos datos se emplearon para calcular otras propiedades como la ratio de rechazo de masa (RRm) y la eliminación de cenizas, según las ecuaciones de la página 34 del Anexo 1. Hojas de mano Éstas son muestras de papel, producidas a partir de la pasta obtenida en cada experimento y realizadas en el laboratorio. A lo largo de este proyecto se ha trabajado con dos clases diferentes de formetas u hojas de mano para la realización de ensayos. Las hojas de mayor tamaño (165 mm x 165 mm) y bajo gramaje (30 g/m2) se prepararon en un molde depositando las fibras de papel sobre un papel de filtro de alta retención (Körkkö y cols., 2010); por otra parte, para las hojas de mano de menor tamaño (ø 39 mm) y mayor gramaje (225 g/m2) se emplearon unos papeles de filtro de membrana de nitratos de celulosa, que evitaban la pérdida de tinta (INGEDE Method 11p, 2009). Una vez preparadas, todas las hojas de mano se aclimataron en el laboratorio antes de las mediciones ópticas, siguiendo la normativa SFS EN 20187. Mediciones ópticas Se midieron propiedades ópticas como el brillo o la tinta residual de las hojas de mano empleando un espectrofotómetro L&W Elrepho. Los datos se calcularon con la media entre dos mediciones de cada hoja. - Brillo: Definida como la reflectancia de la luz azul. El brillo ISO es un factor intrínseco de reflectancia determinado con un medidor cuya sensibilidad a la luz concuerda con la normativa ISO estándar 2470. - Tinta residual: La presencia de tinta varía el brillo y el color del papel reciclado. La concentración de tinta fue determinada mediante el método ERIC (concentración efectiva de tinta residual) según la normativa ISO 22754. Para calcular ERIC se emplean las fórmulas de las páginas 35 y 36 del Anexo 1. 19 Hiperlavado (hyper washing, HW) El hiperlavado se llevó a cabo lavando la pasta de papel con un caudal de agua de 8 L/min durante 20 minutos mediante un equipo de depuración por tamizado de tipo Sommerville con un tamiz de 0.15 mm de malla, según la normativa Tappi T 275. Variables como la retención en el hiperlavado y la eliminación de finos se calcularon empleando las ecuaciones de la página 36 del Anexo 1. 3.3 Desarrollo de la parte experimental La parte experimental del proyecto se dividió en cinco diferentes grupos de experimentos de flotación. El primer grupo fueron los experimentos de referencia, tanto para la flotación como para el fraccionamiento. Con el segundo grupo de experimentos se determinaron los valores óptimos de nivel de agua y flujo de aire para la flotación de la fracción corta de papel. En los otros tres grupos de experimentos se utilizaron diferentes reactivos químicos durante la flotación de la fracción corta de papel. 3.3.1 Experimentos de referencia Este bloque de experimentos se dividió en dos partes: flotaciones y fraccionamiento. De esta manera se obtuvieron muestras de referencia para poder comparar con los resultados obtenidos por muestras de otras flotaciones o fraccionamientos realizados en diferentes condiciones. Flotaciones de referencia Pulpeado Flotación Rechazo Aceptado Pasta de papel Figura 6. Diagrama de las flotaciones de referencia. 20 Se realizaron 5 flotaciones empleando muestras de pasta de papel sin fraccionar (se denominan en el proyecto como “flotaciones de referencia”, pese a que los valores del ensayo nº 41 son los que se emplean como referencia en los resultados). Después del pulpeado, la pasta sin fraccionar se diluyó con agua caliente del grifo (45 ºC) hasta una consistencia de 1,2% y se añadieron 6 mg de jabón por cada gramo de pasta seca en la célula de flotación. En estos 5 experimentos, el nivel de flotación fue de 19 litros y el flujo de aire de 7,4 L/min. Se estudió la influencia del tiempo en la flotación, por lo que se obtuvieron progresivamente una mayor cantidad de rechazo en cada flotación hasta que no se obtuvo más cantidad de rechazo. Las variables de estos experimentos están presentadas en la Tabla 1 y el diagrama del proceso en la Figura 6. Tabla 1. Variables de las flotaciones de referencia. Nº de test Volumen [L] Flujo de aire [L/min] Jabón [mg/g] Rechazo [g] 37 19 7,4 6 545 38 19 7,4 6 1022 39 19 7,4 6 1533 40 19 7,4 6 2024 41 19 7,4 6 2508 Fraccionamiento Después del pulpeado, la pasta sin fraccionar fue diluida con agua caliente del grifo (45 ºC) hasta una consistencia de 1,2%. Para realizar las posteriores flotaciones utilizando únicamente la fracción de fibra corta y empleando reactivos químicos (5.3.3, 5.3.4 y 5.3.5), se llevaron a cabo tres experimentos de fraccionamiento como el presentado en la Figura 7. Se estudió uno de estos fraccionamientos en el cual no se emplearon reactivos durante el pulpeado y se analizaron muestras tanto de la alimentación (feed, F), fibra corta (short fibre, SF) y fibra larga (long fibre, LF). Las muestras de alimentación y fibra corta también fueron hiperlavadas para comparar resultados. Figura 7. Diagrama del fraccionamiento. 21 3.3.2 Estudio del nivel de flotación y flujo de aire durante la flotación Pulpeado Fraccionamiento SF Flotación Rechazo Aceptado LF Figura 8. Diagrama del proceso para el etudio del nivel de flotación y flujo de aire. Después del pulpeado y del fraccionamiento, se llevaron a cabo 12 experimentos de flotación separados en 4 grupos de 3 experimentos cada uno. En cada uno de los grupos se modificó el nivel de flotación y el flujo de aire, buscando los valores óptimos de ambas variables (se emplea “nivel de flotación” en el proyecto pese a que se refiere al volumen de carga dentro de la célula de flotación). Una vez fijados tanto el nivel de flotación como el flujo de aire para cada grupo, se realizaron las tres flotaciones. En ellas se estudió la influencia del tiempo en la flotación, por lo que se obtuvo progresivamente una mayor cantidad de rechazo hasta que no se obtuvo más cantidad de rechazo. No se emplearon reactivos químicos en estas flotaciones. Los valores de estas variables se muestran en la Tabla 2 y el diagrama del proceso en la Figura 8. Tabla 2. Valores de nivel de flotación y flujo de aire para las diferentes flotaciones. Nº de test Volumen [L] Flujo de aire [L/min] Rechazo [g] 1 19 7,4 610 2 19 7,4 1174 3 19 7,4 1705 4 20 7,4 671 5 20 7,4 1150 6 20 7,4 1720 7 19 13 554 8 19 13 1008 9 19 13 1512 10 20 13 668 11 20 13 1180 12 20 13 1737 22 3.3.3 Estudio del acondicionamiento previo a la flotación Pulpeado Fraccionamiento LF SF Flotación Rechazo AceptadoTanque Reactivos Figura 9. Diagrama de las flotaciones con acondicionamiento previo. A continuación se realizó un estudio de la influencia del acondicionamiento previo a la flotación de la pasta de papel. Como se puede observar en el diagrama de la Figura 9, después del pulpeado y del fraccionamiento se añadió como reactivo una dosis 9,5 mg de jabón por gramo de pasta seca a la fracción de fibra corta. Cinco flotaciones se llevaron a cabo a diferentes tiempos de contacto entre el reactivo y la fracción de fibra corta. El nivel de flotación y el flujo de aire se fijaron en 19 litros y 7,4 L/min (ver apartado 4.2). Los parámetros de estas flotaciones se muestran en la Tabla 3. En cada flotación se recogieron muestras a diferentes tiempos de flotación, obteniendo diferentes cantidades de rechazo que se fueron acumulando hasta que no se obtuvo más cantidad de rechazo. El test 13 se empleó como referencia para los restantes experimentos. El clorato cálcico se añadió en todos los experimentos en la célula de flotación menos para el experimento 18 en el que el CaCl2 y el jabón se añadieron a la vez. Tabla 3. Parámetros para las flotaciones con acondicionamiento previo. Nº de test Reactivo Tiempo [min] Rechazo [g] 13 - - 771, 1328 14 Jabón 0 230, 474, 491 15 Jabón 20 202, 438, 553 16 Jabón 40 234, 461, 673 17 Jabón 60 243, 484, 794 18 Jabón + CaCl2 60 714, 1208, 1655 23 3.3.4 Estudio del efecto del pH en la flotación Cuatro experimentos de flotación diferentes se llevaron a cabo controlando el pH: en las dos primeras flotaciones se empleó pasta de papel sin fraccionar, y en las dos últimas se empleó la fracción de fibra corta. Durante el pulpeado se añadió como reactivo una dosis de 6 mg de jabón por gramo de pasta seca, como se presenta en la Figura 10. En cada uno de los 2 primeros experimentos, 1.520 g de pasta de papel se diluyeron en la célula de flotación hasta un nivel de flotación de 19 litros (consistencia del 15%). En la primera flotación se midió el pH de la alimentación (en torno a 7,5), mientras que el pH de la segunda flotación se ajustó a 8,5 en la celda de flotación empleando una solución 0,2M de NaOH. Pulpeado Flotación Rechazo Aceptado Reactivos Pasta de papel Figura 10. Diagrama de las flotaciones de pasta sin fraccionar con control de pH. Las otras dos flotaciones se llevaron a cabo con la fracción de fibra corta (SF) obtenida tras el fraccionamiento, como se detalla en la Figura 11. En la primera flotación se midió el pH de la alimentación (en torno al 7,9), mientras que el pH de la segunda flotación se ajusto a 8,5 empleando una solución 0,2M de NaOH. Pulpeado Fraccionamiento LF SF Flotación Rechazo Aceptado Reactivos Figura 11. Diagrama de las flotaciones de fibra corta con control de pH. 24 Las variables de todas estas flotaciones se muestran en la Tabla 4. En cada flotación se recogieron muestras a tres diferentes tiempos de flotación, consiguiendo diferentes cantidades de rechazo que se fueron acumulando hasta que no se obtuvo más cantidad de rechazo. Tabla 4. Variables para las flotaciones con control de pH. Nº de test Voumen [L] Flujo de aire [L/min] Nota Rechazos [g] 19 19 7.4 Sin fraccionar 506, 1091, 2651 20 19 7.4 Sin fraccionar – pH 8.5 566, 1141, 2746 21 19 7.4 Fibra corta 613, 1257, 1756 22 19 7.4 Fibra corta – pH 8.5 573, 1219, 1621 3.3.5 Estudio de la adición de diferentes reactivos químicos de flotación Pulpeado Fraccionamiento LF SF Flotación Rechazo Aceptado Reactivos Figura 12. Diagrama de las flotaciones con reactivos químicos. Como se puede ver en la Figura 12, después del pulpeado y el fraccionamiento las flotaciones se llevaron a cabo con la fracción de fibra corta (SF). Se emplearon tres reactivos químicos diferentes en seis experimentos de flotación: jabón, surfactante #1 y surfactante #2. Todos los reactivos químicos se añadieron en la célula de flotación antes de comenzar la misma. Para el jabón, se añadieron dosis de 6 y 9 mg de jabón por gramo de pasta seca respectivamente. Para los surfactantes, las dosis añadidas fueron de 0,6 y 0,9 gramos por gramo de pasta seca. En cada flotación se recogieron muestras tanto de aceptado y como de rechazo a diferentes tiempos de flotación, con diferentes cantidades de rechazo para cada toma que se fueron acumulando hasta que no se obtuvo más cantidad de rechazo. Los parámetros de estas flotaciones se muestran en la Tabla 5. 25 Tabla 5. Parámetros para las flotaciones químicas. Nº de test Reactivo químico Dosis [mg/g] Rechazo [g] 23 Jabón 6 238, 382 24 Jabón 9 148, 265 25 Surfactante #1 0,6 293, 485, 858 26 Surfactante #1 0,9 337, 591, 832 27 Surfactante #2 0,6 346, 590, 1020 28 Surfactante #2 0,9 388, 616, 1151 32 Figura 19. Brillo ISO para las flotaciones de pasta sin fraccionar con control de pH. Figura 20. Contenido en cenizas de las flotaciones de pasta sin fraccionar con control de pH. Figura 21. Tinta residual para las flotaciones de pasta sin fraccionar con control de pH. A41 44 45 46 47 48 49 50 51 52 53 0 10 20 30 40 ISO Brightness [%] RRm [%] Reference flotations Entire pulp Entire pulp, pH 8.5 A41 0 5 10 15 20 25 30 35 0 10 20 30 40 Ash 525ºC [%] RRm [%] Reference flotations Entire pulp Entire pulp, pH 8.5 A41 400 600 800 1000 1200 1400 1600 1800 0 10 20 30 40 Residual ink [ppm] RRm [%] Referenc flotations Enitre pulp Enitre pulp, pH 8.5 33 Figura 22. Eliminación de cenizas para las flotaciones de pulpa sin fraccionar con control de pH. En la Figuras 19, 20, 21 y 22 se observa una pequeña diferencia entre las muestras de la flotación con control de pH respecto a las no controladas. Los valores finales de brillo, contenido en cenizas y en tinta residual fueron similares excepto para el test 20 (Entire pulp, pH 8,5) que obtuvo un valor de RRm máximo inferior. Comparando las flotaciones con control de pH con las flotaciones de referencia (Test 41), se observa que estas últimas obtuvieron valores de brillo levemente mayor (un 1% mayor), menor contenido en cenizas y similar contenido en tinta residual pero con menor ratio de masa de rechazo. Sin embargo, al añadir los reactivos químicos a la desintegradora se consiguió aumentar la flotabilidad de las partículas de tinta. Como se puede ver en la Figura 21, los valores de tinta residual tuvieron un drástico descenso desde 1630 hasta 625 en la muestra de la flotación con el pH controlado. Flotaciones de la fracción de fibra corta Se realizaron dos flotaciones de fracción de fibra corta, ajustando el pH de una ellas a 8,5. Los valores de brillo obtenidos gracias a mediciones ópticas de las hojas de mano pequeñas (filtros de membrana) de cada flotación (muestras de alimentación y fracción de fibras corta post-flotada), se representaron frente a RRm en la Figura 23. Otras variables como el contenido en cenizas, la eliminación de cenizas y la eliminación de finos también fueron estudiadas, pero se obtuvieron unas diferencias mínimas entre ambas flotaciones. Las gráficas de estas variables se pueden ver en las páginas 49 y 50 del Anexo 1. R41 0 10 20 30 40 50 60 70 80 0 10 20 30 40 Ash removal [%] RRm [%] Reference flotations Entire pulp Entire pulp, pH 8.5 34 Figura 23. Valores de brillo para las flotaciones de fracción de fibra corta con control de pH. Se observa que ajustar el pH no tuvo casi efecto en las flotaciones de la fibra corta. El RRm máximo de ambas flotaciones fue aproximadamente el mismo (32%), y la única diferencia fue en el valor de brillo de la muestra sin el pH controlado que resultó un 1% mayor que la muestra controlada, como se puede ver en las Figura 23. Comparando el valor máximo de brillo de estas flotaciones con los resultados obtenidos por las flotaciones de nivel de flotación y flujo de aire, se observa un aumento de 2% unidades de brillo ISO. Por lo tanto, al añadir los reactivos en la desintegradora se consigue una mejor separación entre la tinta y las fibras de papel. Sorprendentemente, ajustar el pH a 8,5 no tuvo ningún efecto sobre las propiedades estudiadas. Según el método INGEDE 11, al final del pulpeado el valor de pH debe de ser de 9,5 y se ha de emplear sosa para alcanzarlo (si fuera necesario), por lo que la bibliografía se sugiere que cuanto mayor es el pH se obtienen mejores resultados. Sin embargo, queda demostrado que las flotaciones se pueden llevar a cabo con condiciones neutras de pH. 42 43 44 45 46 47 48 49 50 51 0 10 20 30 40 ISO Brightness [%] RRm [%] Fines fraction Fines fraction, pH 8.5 35 4.5 Adición de diferentes reactivos químicos de flotación El efecto de los reactivos químicos (jabón, surfactante #1 y #2) durante la flotación fueron estudiados conjuntamente para comparar los resultados en variables como el brillo y el contenido en cenizas. Estas variables se obtuvieron gracias a mediciones ópticas de hojas de mano pequeñas (filtros de membrana) obtenidas a partir de la pasta de cada flotación (muestras de alimentación y fracción de fibras corta post-flotada), y están representados frente a RRm en las Figuras 24 y 25. Figura 24. Valores de brillo para las flotaciones con diferentes reactivos químicos. Figura 25. Contenido en cenizas para las flotaciones con diferentes reactivos químicos. En la Figura 24 se observa que las muestras del test 23 (6 mg/g) y de las flotaciones con surfactante #2 obtuvieron los valores más altos de brillo. En la Figura 25 las flotaciones 42 43 44 45 46 47 48 49 50 0 5 10 15 20 25 ISO Brightness [%] RRm [%] Soap, 6 kg/t Soap, 9 kg/t Surfactant #1, 0.6 kg/t Surfactant #1, 0.9 kg/t Surfactant #2, 0.6 kg/t Surfactant #2, 0.9 kg/t 15 20 25 30 35 40 45 50 55 0 5 10 15 20 25 Ash 525ºC [%] RRm [%] Soap, 6 kg/t Soap, 9 kg/t Surfactant #1, 0.6 kg/t Surfactant #1, 0.9 kg/t Surfactant #2, 0.6 kg/t Surfactant #2, 0.9 kg/t 36 con surfactante #2 obtuvieron los valores más bajos de contenido en cenizas. El primer dato de contenido en cenizas de la flotación con jabón (6 mg/g) es erróneo, debido a que la materia prima no puede variar tanto entre las distintas flotaciones. Según Dorris y cols. (2011), los jabones de ácidos grasos se emplean como colectores de flotación para la eliminación de tinta para mezclas de ONP/OMG. En nuestras flotaciones, el valor más alto de brillo ISO se obtuvo en la flotación con la dosis de 6 mg/g de jabón. Una dosis de jabón baja consiguió una mejor aglomeración entre la tinta y el jabón creando agregados más hidrófobos. No hubo una gran diferencia entre las flotaciones con el surfactante #1 o el #2. Según Dorris y cols. (2011), la principal ventaja de usar surfactantes no iónicos es que se necesitan pequeñas cantidades para mezclas de ONP/OMG, por lo que se reduce la dosis. La velocidad de flotación fue mayor cuando se emplearon los surfactantes, debido a que la espuma se formó rápidamente en la superficie de la suspensión. Basándonos en la cantidad de rechazo obtenido para cada flotación, se calcularon variables como la eliminación tanto de cenizas como de finos (fibra corta) y se representaron frente a RRm en las Figuras 26 y 27. En ambas figuras se observa que los valores más altos de eliminación de cenizas y de eliminación de finos fueron obtenidos por las flotaciones con surfactante #2. Figura 26. Eliminación de cenizas para las flotaciones con diferentes reactivos químicos. 0 5 10 15 20 25 30 35 40 45 0 5 10 15 20 25 Ash removal [%] RRm [%] Soap, 6 kg/t Soap, 9 kg/t Surfactant #1, 0.6 kg/t Surfactant #1, 0.9 kg/t Surfactant #2, 0.6 kg/t Surfactant #2, 0.9 kg/t 37 Figura 27. Eliminación de finos para las flotaciones con diferentes reactivos químicos. Comparando todas las flotaciones químicas, el experimento con una dosis de jabón de 6 mg/g consiguió los mejores resultados según la Figura 24 (mismos valores de brillo pero con menor RRm). No hubo gran diferencia al usar una dosis mayor o menor de surfactante, por lo que el uso de una dosis menor (0,6 mg/g) consigue ahorrar gastos. 0 5 10 15 20 25 30 35 40 0 5 10 15 20 25 Fines removal [%] RRm [%] Soap, 6 kg/t Soap, 9 kg/t Surfactant #1, 0.6 kg/t Surfactant #1, 0.9 kg/t Surfactant #2, 0.6 kg/t Surfactant #2, 0.9 kg/t 38 5 CONLCUSIONES Antes de comenzar las conclusiones, es necesario recordar que el uso de materia prima antigua (ONP/OMG) empeora el destintado de la pasta de papel y por consiguiente, los valores de propiedades como el brillo son peores que los obtenidos con materia prima fresca. Sin embargo, en nuestros experimentos, a través de la flotación de pasta de papel sin fraccionar (Entire pulp), el valor de brillo fue aumentando moderadamente cuanto mayor cantidad de rechazo se eliminaba de la suspensión. Por lo tanto, los experimentos de flotación funcionaron bien pese al uso de materia prima antigua. Las conclusiones tras la realización de este proyecto son las siguientes: El fraccionamiento empleado en este proyecto resultó ser adecuado para los posteriores experimentos. Los valores de brillo de la fracción de fibra larga fueron levemente inferiores debido a la presencia de tinta libre en la suspensión. La mayor presencia de cenizas en la fracción de fibra corta confirma que el fraccionamiento fue correcto. Los resultados de las flotaciones de fibra corta muestran que cambiar parámetros como el flujo de aire o el nivel de flotación en el equipo no tuvo mucho efecto en las propiedades de la suspensión tras la flotación. Por otro lado, incrementar el tiempo de acción de los reactivos en la suspensión antes de la flotación sí que tuvo efecto en las propiedades de la fracción de fibra corta. A mayor tiempo, mayor valor tanto de brillo como de ratio de masa de rechazo, con la consiguiente pérdida de fibras. Las propiedades de la pasta de papel no fueron modificadas por el uso de reactivos químicos para controlar el pH, por lo que aparentemente, estas flotaciones se pueden llevar a cabo en unas condiciones de pH neutras. Basándose en estos resultados, se recomienda el uso de una dosis de jabón de 6 mg/g en el equipo de flotación. Gracias a esta dosis se consiguió una mejor aglomeración entre la tinta y el jabón creando aglomerados más hidrófobos. Se obtuvieron los datos más altos de brillo con menor ratio de masa de rechazo, en comparación con otros reactivos colectores como los surfactantes. 39 6 BIBLIOGRAFÍA Ackermann C (2000) Bleaching of deinked pulp. In: Göttsching L & Pakarinen H (eds.) Papermaking Science and Technology, Book 7, Recycled Fibre and Deinking. Helsinki. 1st edition, Finland, Fapet Oy, pp. 306-356. ISBN 952-5216-07-1. Beneventi D, Carré B & Gandini A (2005) Physico-chemical aspects of deinking. 7th CTP/PTS Advanced Training Course on Deinking. Grenoble, France, May 31st/June 1- 2nd. Carré B & Galland G (2007) Overview of deinking technology. 8th CTP/PTS Deinking Training Course. Grenoble, France, May 29-30-31st. Chabot B, Daneault C, Sain MM & Dorris GM (1997) The adverse role of fibres during the flotation of flexographic inks. Pulp & Paper Canada, 98, (12), pp. T451-T456, ISSN: 0316-4004. Dorris G, Ben Y & Ricard M (2011) Overview of Flotation Deinking. Progress in Paper Recycling, 20, (1), 41. ISSN: 1061-1452. Dorris GM & Nguyen N (1995) Flotation of Model Inks. Part II: Flexo Ink Dispersions Without Fibres. Journal of Pulp and Paper Science, 21, (2), pp. J55-J62. ISSN: 0826- 6220. Doshi MR (1997) Overview – Deinking & Bleaching. IN: Doshi MR & Dyer JM (eds.) Paper recycling challenge. Volume II. Deinking & bleaching. Appleton, WI, Doshi and Associates Inc, pp. 3-5. ISBN: 0-9657447-1-X. Eul W, Meier J, Arnold G, Berger M & Suess HU (1990) Fractionation prior to flotation – A new approach for deinking technology. Proceeding of the TAPPI Pulping conference. Atlanta, GA, TAPPI Press, pp. 757-765. ISBN: 0-89852-742-2. 40 Eul W, Süss HU & Helmling O (1989) Fibre fractionation and post-treatment of deinked pulp. Pulp & Paper Canada, 90, (10), pp. T391-397. ISSN: 0316-4004. Holik H (2000) Unit operations and equipment in recycled fiber prosessing. In: Göttsching L & Pakarinen H (eds.) Papermaking Science and Technology, Book 7, Recycled Fiber and Deinking. 1st edition, Helsinki, Finland, Fapet Oy, pp. 90-209. ISBN: 952-5216-07-1. INGEDE Method 11 (2009) International Association of the Deinking Industry. Method 11p: Assessment of Print Product Recyclability – Deinkability Test. [pdf-file]. 12/2009. Available at: http://www.ingede.de/ingindxe/methods/ingede-method-11p-2009.pdf Julien Saint Amand F (1999) Hydrodynamics of Deinking Flotation. International Journal of Mineral Processing, 56, (1-4), pp. 277-316. ISSN: 0301-7516. Julien Saint Amand F (2005) Ink Removal by Flotation and Washing: Hydrodynamic and Technological Aspects. Part 1: Flotation. 7th CTP/PTS Advanced Training Course on Deinking. Grenoble, France, May 31st/June 1-2nd. Kraschowetz H, Hertl E and Aregger HJ (2008) Fractionation in DIP-Lines – A further step in waste paper recycling. Proceedings form 62nd APPITA Annual Conference, Carlton, Australia, Appita, pp. 177-183. ISBN: 978-0-95757469-4-4. Körkkö M, Bussini D, Laitinen O, Elegir G & Niinimäki J (2010) True-neutral fractional deinking for flexographic and offset newsprints. Proceedings from the 65th Appita Annual Conference, Appita, Carlton: pp. 23-30. ISBN: 978-0-9757469-9-5. Körkkö M, Haapala A, Mäkinen L, Ämmälä A, & Niinimäki J (2010) A Novel Sheet Preparation Method for Ink Content Measurement in Paper Recycling. Proceedings of the TAPPI PEERS conference and 9th Research Forum on Recycling, Atlanta, GA, TAPPI Press, Conf. CD. ISBN: 978-161782196-7. 41 Körkkö M, Laitinen O, Vahlroos S, Ämmälä A & Niinimäki J (2008) Components Removal in Flotation Deinking. Progress in Paper Recycling, 17, (4), pp. 15-22. ISSN: 1061-1452. Lapierre L, Pitre D & Bouchar J (2003) Fines from deinked pulp: Effect of contaminants on their bleachability and on the pulp final brightness. Pulp and paper Canada, 104, (8), pp. T208-T211. ISSN: 0316-4004. Lassus A (2000) Deinking chemistry. In: Göttsching L & Pakarinen H (eds.) Papermaking Science and Technology, Book 7, Recycled Fiber and Deinking. 1st edition, Helsinki, Finland, Fapet Oy, pp. 240-265. ISBN: 952-5216-07-1. Mäkinen L, Ämmäla A, Vahlroos-Pirneskoski S, Körkkö M, Sarja T and Niinimäki J (2010) Fractionation prior to deinking – a study of optical properties of fractions. Ipw no. 4-5, pp. 14-20. ISSN: 1615-1720. Matzke WH, Selder HH (1996) New Development in Deinking and Bleaching of Secondary Fibers. Journal of Korea Tappi, 28, (1), pp. 73-79. ISSN: 0253-3200. McKinney R (1999) Flotation Deinking Overview. In: Doshi MR & Dyer JM (eds.) Paper Recycling Challenge. Volume III. Process Technology. Appleton, WI, Doshi and Associates Inc, pp. 99-114. ISBN: 0-9657447-3-6. Meltzer FP (1999) Fractionation. In: Doshi MR & Dyer JM (eds.) Paper Recycling Challenge. Volume IV. Process Control & Mensuration. Appleton, WI, Doshi and Associates Inc, pp. 153-162. ISBN: 0-9657447-4-4. Vahlroos-Pirneskoski S, Körkkö M, Rosencrance S and Niinimäki J (2008) Postbleaching Response of Pulps Deinked using an Alkaline Fatty Acid Soap or Reduce Alkaline Surfactant Blend Chemistry. Progress in Paper Recycling, 17, (3). ISSN: 1061- 1452. Anexo 1 7 1 INTRODUCTION Recycling is an old tradition of the paper industry. Recycling process was developed at the very beginning of the 20th century and in 1955 first recycling line was started (Kemper et al. 1999). In the early days, the processed waste paper was only used for low paper grades. Thanks to the consistent development of the machines and systems (such as screening, dispersing, and particularly flotation), the use of the processed waste paper was extended to papers with the highest quality requirements. Nowadays, more or less half of the paper and board produced worldwide are produced with recovered papers. Since 15-20 years there has been a drastic increase in the use of recovered papers to produce, through deinking, white grades such as newsprint, tissue, market pulp and also more recently magazine papers (SC & LWC). By using the deinking technology, white grade papers can be produced from post-consumer, or post-industrial recovered papers. This means that the components which cause a reduction of brightness – the inks – must be removed, as well as all the contaminants of the pulp such as additives used during the paper converting and packaging: staples, plastic films, etc. (Carré and Galland 2007) Due to the exacting demands on the final stock, the complexity of the machines increased constantly. Nowadays process lines are equipped with several screening and cleaning stages, pre- and postflotation, and up to two dispersing systems with oxidative and reductive bleaching stages to achieve the required final stock properties. (Kraschowetz et al. 2008) In order to improve the deinking process, other deinking lines are being considered. In this Master’s Thesis, DIP fines fraction flotation is studied in detail, as part of a fractional deinking concept that is considered as an alternative deinking process. Fractional deinking concept in relation to fines flotation is shown schematically in Figure 1. Basically, recovered paper (a mixture between old newsprints and magazines) is pulped and then separated into short fibre fraction (also called fines fraction) and long fibre fraction in order to avoid excessive fragmentation and redeposition of ink. Pulping time used is very short and after fractionation long fibre fraction has a secondary Anexo 1 8 pulping. Flotation of the fines fraction is suggested to improve the yield for further combination of pulp streams that can be fed forward to the paper mill. (Körkkö et al. 2010) Figure 1. Block diagram of the studied fractional deinking (Körkkö et al. 2010). The objective of this work was to obtain more information about the less studied fines fraction flotation and to optimize its properties. Theoretical part of this thesis is divided in three big chapters: in the first one, all the steps of a conventional deinking line are presented; in the second one, fractional deinking line concept is introduced; and finally fundamentals of flotation are viewed in detail. Anexo 1 9 2 CONVENTIONAL DEINKING LINE The purpose of deinking is to remove printing ink and other substances that might affect the papermaking process or final properties of paper. For that reason, recycled fibres used in a deinking process require more complex process systems than virgin fibres because the recovered paper usually is a mixture of various paper grades and usually, it also contains a certain proportion of contaminants. The final stock properties depend on the removal of contaminants from the pulp. Unsatisfactory separation of contaminants results a low brightness and quality for the end product. (Lassus 2000 pp. 241, Holik 2000 pp. 91) Some of the ink, like newsprint ink, is relatively weakly bonded to the fibres, and can be detached rather easily. On the other hand, toner or UV-cured ink may be strongly bonded to the fibres and cannot be detached easily in a pulper. Mechanical dispersion units or kneaders are needed to handle these inks. The production of newsprint and improved paper grades are nowadays usually carried out by using the process concept presented in Figure 2. Figure 2. Concept for newsprint and improved paper grades (Schwarz 2000 pp. 213). The unit operations of a deinking line are the following ones: pulping, screening, cleaning, flotation, dispersion, bleaching and dewatering. In pulping, mechanical, chemical and thermal forces are used to detach impurities from the fibres, such as ink. Anexo 1 10 This is usually carried out in pulpers where strong agitating provides shear force. Steam or hot water and deinking chemicals are added to help dislodge ink from the fibres. Screening, cleaning and flotation are separation stages. The detached ink is removed from the stock by screening, cleaning, flotation and washing stages. (Eul et al. 1990, Doshi 1997 pp. 3) 2.1 Pulping The purpose of pulping is to slush the raw material into individual fibres as much as possible to form a suspension that can at least be pumped. During pulping, large, solid contaminants such as strings, plastic, stickies and printing ink, are removed from the pulp suspension. The disintegrating forces applied in the pulper must be greater than the strength of the raw material or the adhesion force of contaminants to the fibres. But pulping forces should not be excessive or contaminants will break excessively which leads to a less efficient removal. For that reason, it is desirable to remove solid contaminants at an early stage. To save energy, the first step is often limited to coarse slushing and heavy particle separation. (Holik 2000 pp. 95-97) Chemicals are most often used in the pulping stage in order to improve ink release from the fibres even if there is a recent trend to move to neutral pulping conditions. For instance, NaOH swells fibres and increase slushing rate during pulping. Bleaching chemicals, such as hydrogen peroxide are also often used in this stage. The most important and oldest peroxide application in context of deinking is at the pulping stage. In industrial deinking operations, this peroxide application invariably occurs in an alkaline environment. To prevent yellowing of the mechanical fibres of the recycled pulp, peroxide has to be used in alkaline conditions. (Carré & Galland 2007, Ackermann 2000 pp. 322) During pulping, a good detachment of the particles from the fibres is needed for further efficient treatment of the pulp suspension. Mechanical forces remove the ink through friction between fibres, and chemicals make some particles hydrophobic. Pulping variables such as pulper type or pulper chemistry influence the ink detachment most, because ink is mainly separated from the fibres during pulping. The aim is to free the Anexo 1 11 ink from the fibre and to prevent reattachment and fragmentation of the ink particles. The lower the ink fragmentation during pulping is, the higher will be the brightness after flotation. Chemicals during pulping and pulp pre-treatment using kneaders or dispersers improve ink detachment. (McKinney 1999 pp. 107, Beneventi et al. 2005, Holik 2000 pp. 151-153) The presence of metal ions catalyses the undesired peroxide decomposition. The consequence is an increased addition of stabilizing chemicals such as water glass and possibly chelating agents. For an efficient use of hydrogen peroxide in pulper, the standard conditions are 15% to 18% consistency, 40 °C and 20 minutes pulping time. (Ackermann 2000 pp. 322, Eul et al. 1990) Pulpers operate at high consistencies HC (up to 19%), at medium consistencies MC (up to 12%), and at low consistencies LC (up to 6%) depending on raw material. LC pulpers and HC drum pulpers usually operate continuously. HC pulpers (Helico) operate in batch mode. (Holik 2000 pp. 99) 2.2 Screening The purpose of screening is to remove debris and solid contaminants from the recycled fibre pulp. Screening uses appropriate screen configurations according to particle size, shape, and deformability of the contaminants. Nondeformable contaminants whose dimensions exceed the screen opening size are retained while fibres go through the openings (slots or holes) of the screen. At the beginning, the screening devices were open vibrating screens, today the screens are closed and pressurised to increase capacity (Holik 2000 pp. 109-111, Carré & Galland 2007). The principle of screening is shown in Figure 3. Anexo 1 12 Figure 3. Principle of screening (Carré & Galland 2007 pp. 3). Complete screening of recycled fibre pulp in a single stage is normally not possible. Avoiding fibre loss during screening is impossible as well. For these reasons, rescreening the first stage screen rejects in a second, third, or even fourth stage can reduce such losses significantly and improve the screening of recycle fibre pulp. The last stage (tailing screen) determines the overall fibre losses of the screening system. (Holik 2000 pp. 109) Different types of screening machines have use at different points in the system depending on raw material, debris particles, flake content, and stock consistency. Coarse screening is used for pulp slurries with a high trash and flake content. Fine screening occurs in the LC range and partially in the MC range following coarse screening. For instance, large particles are removed by coarse screening while small particles (such as stickies) are removed by fine screening. Screen surfaces can be flat (disk screens) or cylindrical (cylindrical screens). Hole perforations are about 0.8 - 1.5 mm diameter for cylindrical screens and 2.0 - 3.0 mm diameter for flat screens. Slotted screens are generally in the cylindrical form with slot widths from 0.1 - 0.4 mm. (Holik 2000 pp. 119-120, 124) 2.3 Cleaning Centrifugal cleaning is a separation process that complements other separation methods such as screening, and its purpose is to remove particles from the suspension that affect Anexo 1 13 paper quality or cause excessive wear in subsequent processing machinery. These contaminants may include heavy weight particles such as sand, metal pieces, and shives or light weight particles including plastic foam or other plastic material. (Holik 2000 pp. 134-135) For an efficient removal of contaminants, their density must differ from water. Centrifugal cleaning can remove smaller particles than screening. The centrifugal field generated in these machines forces the heavy particles outward while the light ones move to the centre. The flow streams containing primarily heavy or light contaminants are separated from the main flow. (Holik 2000 pp. 134-135) 2.4 Flotation Flotation is the most common method used for separating ink and fibres from each other. It is a separation process for suspension cleaning based on probability using air bubbles that attach themselves to the particles and transport them to the surface of the suspension. Deinking flotation has been used in recycled pulp processing for removing contaminants from the suspension, thanks to the different surface hydrophobicity of the particles to be removed and the fibres to be retained (Holik 2000 pp. 151-152, 241). The principle of flotation is shown in Figure 4. Figure 4. Principle of flotation (Carre & Galland 2007 pp. 4). Anexo 1 14 Once a good detachment of the particles from the fibres is achieved through pulping, air is introduced into the suspension and the water-repelling particles attach to the air bubbles and rise to the surface forming a foam layer at the top of the suspension, while the hydrophilic fibres remain in the water phase. These hydrophobic particles include printing ink, stickies, fillers, coating, pigments and binders. The foam can be removed mechanically, by overflow or by a vacuum extraction. (Holik 2000 pp. 151-153, 244) In flotation systems air is introduced into a diluted fibre suspension of 0.8 - 1.5% consistency. The relative air load is mostly about 300% or more, expressed as total air volume flow to total suspension volume flow. pH should be about 7 - 9 when using fatty acid surfactants and temperatures of 40 – 70 ºC. The main application of selective flotation in the paper industry is deinking. This improves pulp brightness by removing ink particles and enhances cleanliness by removing dirt specks (Holik 2000 pp. 151, 157, 244). Flotation is discussed more deeply in Chapter 4. 2.5 Dispersion The purpose of dispersion is to break down contaminants to a size where they usually no longer interfere or detach them from the fibres if necessary to remove them in a subsequent process. Dispersion makes dirt specks and stickies smaller and floatable, mixes in bleaching agents and detaches ink still adhering to the fibres. Two kinds of dispergers are used in the dispersion stage: disk and kneading dispergers. (Holik 2000 pp. 185-186) Recycled pulp is dispersed at consistency of 25 - 30% and temperatures of 60 - 95 °C. These process conditions and the high shear environment are ideal for admixing of peroxide. Dispersion can be done also without NaOH and hydrogen peroxide. (Ackerman 2000 pp. 232) 2.6 Bleaching Bleaching contributes decisively to the optical characteristics of DIP such as brightness and luminance. The bleached deinked pulp is usually used in the production of printing Anexo 1 15 papers such as super calendared (SC) and office papers. Especially with hygienic papers the microbial activity can be stopped with hydrogen peroxide. Bleaching chemicals are divided into two groups: lignin preserving such as hydrogen peroxide (H2O2), sodium dithionite (Na2S2O4) and formamidine sulphinic acid (FAS), and lignin degrading such as chlorine (Cl2), chlorine dioxide (ClO2), sodium hypochlorite (NaOCl), ozone (O3) and oxygen (O2). Lignin preserving chemicals are normally used for wood containing papers, and lignin degrading chemical (if used) for wood free papers. (Ackermann 2000 pp. 307) The choice of bleaching chemicals and processes depends on the demanding quality of the final product. Besides fibre composition, this also includes different types and proportions of contaminants and detrimental substances still present in the deinked stock. (Ackermann 2000 pp. 307) The bleaching of deinked pulp at the end of stock processing is post-bleaching. The deinked pulp is thickened and heated with steam to a temperature of approximately 60 °C. After adding bleaching chemicals, the pulp is stored in a bleaching tower at a consistency of about 15% for 1 - 3 hours. (Ackermann 2000 pp. 324) 2.7 Dewatering Dewatering is a filtration process with mechanical pressing and its purpose is to remove process water from the pulp slurry and increase the stock consistency simultaneously. Dewatering makes subsequent processes at the production level more cost-effective and saves space in further storage. HC dispersion is an example of these subsequent processes that can be improved thanks to dewatering. Stock dewatering characteristics are normally expressed in the paper industry as Schopper-Riegles value (SR) or Canadian Standard Freeness (CSF). Dewatering without further mechanical pressing is often called thickening. The most common dewatering machines used in recovered paper processing are belt filters, drum filters, disk filters and screw presses. During filtration, a filter cake or filter mat is accumulating on the filter base such as a wire or screen. (Holik 2000 pp. 168, 169, 171, 172) Anexo 1 16 3 FRACTIONAL DEINKING LINE Fibre fractionation and separate bleaching of the fractions (long and short fibre) could be an efficient way to upgrade deinked pulp and its properties. In fact, not only cleanliness of the stock with regard to stickies and cubic impurities is important, but optical properties such as brightness, dirt specks and bleachability of the stock are a challenge for deinking process to reach the exacting demand of the final stock. (Eul et al. 1989) Fractional deinking lines are a development of conventional deinking lines. In fractional deinking lines, deinked pulp is separated into a long fibre fraction and a short fibre fraction (or fines fraction) through fractionation. Fractionation is a special screening application that splits a given fibre quality into two fractions, so these fractions can be treated separately and used for different paper grades. Fractionation can also be done by washing. The principle of fractionation is shown in Figure 5. Accept = Short fibre fraction Reject = Long fibre fraction LC or MC fractionation Feed = Waste Paper Furnish Figure 5. Principle of fractionation (Kraschowetz et al. 2008). The fractionation concept has been introduced successfully in deinking lines. The main reason of this development and adjustment of fractionation into the deinking process is a worldwide increasing use of recovered paper to produce paper and board for environmental reasons. Increased collection of recycled material has resulted in increased quantities of non-fibrous materials or contaminants. Therefore, fractionation creates a superior long fibre fraction and minimizes the cleaning and screening effort by concentrating contaminants in one stream. (Kraschowetz et al. 2008, Meltzer 1999 pp. 153) Anexo 1 23 iv) Solid drainage. Water drainage can convey down to the pulp suspension solid particles of the froth. Figure 10. The four main solid transfer mechanisms used to describe the deinking flotation process. i) True flotation of hydrophobic particles, ii) mechanical entrainment of suspended solids, iii) water drainage, iv) drainage of solids dispersed in the froth (Beneventi et al. 2005). 4.1 Flotation phases Many of the steps needed for a successful flotation are beyond the control of flotation deinking cell manufacturers. These steps are affected by several factors such as consistency, pH, temperature, size of air bubbles, air ratio, etc. A successful flotation requires: Anexo 1 24 - Ink detachment from fibre (shown in Chapter 2.1), - a collision between an air bubble and a free ink particle, - the formation of an air bubble-ink particle complex, - migration of the complex to the surface, - froth removal of air bubble-ink particle complex from the flotation cell. 4.1.1 Collision between an air bubble and a free ink particle Air bubble and ink particle has to be close enough to collide and form an air bubble/ink particle complex. A particle must be within the radius of the streaming tube, which is defined by the diameter of the air bubble and fluid streamlines. Particle and bubble size are factors that affect the probability of collision. The higher the particle size is and the smaller the bubble size is, the higher the probability of collision is. The effect of bubble size on flotation efficiency is shown in Figure 11. Flotation cell design influence the air bubble size and the degree of mixing and turbulence. (McKinney 1999 pp. 107-108, Julien Saint Amand et al. 2005) Figure 11. Effect of bubble size and particle size on flotation efficiency (Julien Saint Amand et al. 1999). Anexo 1 25 4.1.2 Formation of an air bubble-ink particle complex The most important factor that influences the formation of an air bubble/ink particle complex is the surface chemistry of the ink particle and the air bubble. Other factors are ink particle size and shape. Chemicals added are generally referred to as ink collectors and are surface active agents. It is important that small ink particles are agglomerated before flotation to improve the flotation efficiency. Traditional chemicals used are non saturated fatty acid soap, usually a blend of soaps with variations in chain length. This soap reacts with the ink particles and calcium ions (determined by water hardness) to form a strongly hydrophobic complex that attached to the air bubble. Other surfactants are also used as ink collectors, with lower addition rates and less dependence of calcium hardness. The effect of the hydrophobic chain length of the soap in the pulp brightness is shown in Figure 12. Conditions for flotation should be alkaline with fatty acid soaps, typically in the pH range 8 - 10. (McKinney 1999 pp. 108-109, Beneventi et al. 2005) Figure 12. Effect of the hydrophobic chain length of the soap on the flotation efficiency (Beneventi et al. 2005). Ink particle size is affected by the type of ink, age of ink and the treatment prior to flotation. Increasing ink particles size decreased the ink removal efficiency. For an efficient flotation of ink particles, their size range must be 10 – 250 mm. Regarding to Anexo 1 26 ink particle shape and according to Schmidt and Berg (1996), flat shaped particles did not float as well as spherical particle because of the low attachment probability of the disc shaped particles after collision between particle and bubble. (Holik 2000 pp. 153, McKinney 1999 pp. 108, Julien Saint Amand et al. 2005) 4.1.3 Migration of the complex to the surface Even if ink particle/air bubbles complexes are successfully formed, there is no guarantee that the ink will be removed from the flotation cell. It depends on factors such as the stability of the complex, consistency, turbulence, position relative to the surface, bubble complex rise rate, etc. Adhesion probability and migration to the surface is higher with smaller bubble rising velocity. Also small bubbles shown to be best, due to the higher amount of bubbles and surface area. (McKinney 1999 pp. 109, Beneventi et al. 2005) 4.1.4 Froth removal The air bubble/ink particle complex has to be stable enough to resist the forces applied by the froth removal system. Froth removal is accomplished by the overflow, sometimes assisted by a rotating mechanical scraper. The probability of aggregate stability depends on several parameters: gravity and centripetal acceleration which tend to separate heavyweight particles from the bubble, shear forces and attachment forces due to capillary force on air bubble/ink particle complex. For instance, large particles detach from the air bubbles especially at high turbulence. The presence of fibres in the pulp suspension is another factor that influences the probability of aggregate stability. (McKinney 1999 pp. 109, Beneventi et al. 2005) 4.2 Flotation yield During flotation, valuable materials such as ink, ash, filler and fibres are also removed with the froth. Below is presented the variables affecting fibres and fines losses and an introduction of fines fraction flotation. Anexo 1 27 4.2.1 Fibres and fines losses Fibres and fines losses are influenced by several factors such as chemicals and air bubble size. Chemicals improve the selectivity of the flotation process making particles more hydrophobic attracting air bubbles. Without this selectivity, the flotation process would remove particulate matter in a non-selective manner (as in DAF) leading to the loss of valuable fines and fillers by entrainment rather than true flotation. This entrainment phenomenon in flotation deinking is particularly harmful. (Dorris et al. 2011) A high calcium concentration has a negative influence on fibre losses. At high Ca2+ concentration, calcium soap flakes can deposit on ink particles, fillers, fibres and fines, increasing their tendency to attach to air bubbles. Nevertheless, sodium silicate has a positive influence on fibre losses. Fibre losses decreased if sodium silicate is used during flotation, due to its dispersing effects based on electrostatic and steric repulsion. (Beneventi et al. 2005) Air bubble size is a major factor affecting on pulp losses. If 0.01 mm to 0.1 mm sized air bubbles were created in flotation deinking cells, all the solids, including fibre, would float, and there would be no separation of ink from fibre. In froth flotation, air bubble sizes range from about 0.1 mm up to 5.0 mm, and differences in properties between the suspended solids (such as surface chemistry) assist the separation of solids (McKinney 1999 pp. 100). According to Körkkö (2008), ash selectivity in removal was practically unaffected by bubble size and fibre fines loss was higher with larger bubbles. The effect of bubble size on pulp loss and fines and fillers loss are represented in Figures 13 and 14. According to this figures, the amount of flotation rejects increases as bubbles size is decreased. Anexo 1 28 Figure 13. Pulp losses vs. air ratio for different bubble sizes (Julien Saint Amand et al. 1999). Figure 14. Fillers and fines losses vs. air ratio for different bubble sizes (Julien Saint Amand et al. 1999). The mechanisms involving losses of fibre, filler, fines and inorganic components during flotation are represented in Figure 15. Anexo 1 29 Figure 15. Mechanisms of fibre and filler losses during flotation (Beneventi et al. 2005). 4.2.2 Fines fraction flotation The study of flotation of deinked pulp samples without fibres could help for further developments in deinking line processes but nowadays there are not a lot of articles and information about this short fibre or fines fraction flotation. According to Dorris (1995), flexographic inks, that are prone to float in normal deinking flotation conditions, were found to be floatable under various conditions when there was no fibre in the system, conditions such as low pH or high concentration of calcium ions. The use of conventional calcium soaps of fatty acids was effective as a flotation collector for oil-based inks and also for flexographic ink. According to Chabot (1997), the efficiency of flotation of flexo-printed paper decreased as fibre concentration increased. Fibres have an adverse role during the flotation of flexographic inks, affecting the formation and growth of flexographic ink-calcium soap aggregates. Anexo 1 30 Matzke (1996) proposed a new development in deinking and bleaching through flotation of washing filtrate. It was shown that flexographic inks can be floated from a wash filtrate because of its low viscosity and the absence of fibres. By this mean the yield could be increased while retaining a high brightness level. Anexo 1 31 5 EXPERIMENTAL The experimental part of this work was divided in five different scenarios of flotation experiments. The first scenario was the reference flotation experiments with the whole pulp sample. The second scenario was performed to determinate the optimum variables for air flow and flotation level at the fines flotation. And the other three different cases were performed using several chemicals during the fines fraction flotation. Before a deeply description of the flotation experiments, an introduction about the materials and methods used is needed. 5.1 Materials 5.1.1 Raw material The recovered paper used in the experiments was a mixture of old Finnish newsprint (ONP) and old magazines (OMG). The feeding ratio for every pulping was 50% ONP and 50% OMG. The OMG feed consisted half of super calendared paper (SC) and half of light weight coated paper (LWC). The recovered paper used is considered as a very old raw material because it is older than three months. All papers used were sorted by hand in order to get batches of uniform quality for pulping. 5.1.2 Chemicals Three different commercial chemicals were used during the process: surfactant #1, surfactant #2 and soap. Surfactant #1 was a non-ionic surfactant, surfactant #2 was a slightly different non-ionic surfactant and the soap was a mixture of sodium soaps of fatty acids (i.e. anionic surfactant). Every experiment in which soap has been used, 7.2 g of CaCl2 was also added to adjust the hardness. The dosing place of these chemicals and its dosage were different in several flotation experiments. Anexo 1 32 5.2 Methods 5.2.1 Process description Pulping The pulp was produced using a Hobart (H600) pulper with a hook impeller as presented in Figure 16. Batch size in Hobart pulping was 2500 g. All the newsprints and magazines were torn into about two centimetre wide strips and mixed with warm tap water (45 °C) before pulping to adjust the consistency to 15%. Pulping time was 14 minutes while in every pulping experiment the first two minutes were performed at speed #1, the next two minutes at speed #2 and the last ten minutes at speed #3. Figure 16. Hobart (H600) pulper. Anexo 1 39 Table 3. Parameters for delay chemical flotation. Test Run Chemical Delay [min] 13 - - 14 Soap 0 15 Soap 20 16 Soap 40 17 Soap 60 18 Soap + CaCl2 60 5.3.5 Effect of pH in flotation Four different flotation experiments were carried out controlling the pH. During pulping, a chemical dosage of 6 kg/t of commercial soap were added into the pulper. In each of the first two flotation experiments, 1520 g of entire pulp (15% consistency) were diluted in the flotation device until the flotation level was 19 litres. The pH of the second flotation experiment was adjusted to 8.5, using 0.2M NaOH to reach it. The other two flotation experiments were done with the short fibre fractionation obtained after fractionation. The pH of the second flotation experiment was set at 8.5 using 0.2M NaOH. In every flotation experiment, three different samples of accept and reject were taken and 7.2 g of CaCl2 were added into the flotation device at the beginning of every flotation experiment. The variables of pH flotation experiments are presented in Table 4. Table 4. Variables of pH flotation experiments. Test Run Flotation level (L) Air level (L/min) Note 19 19 7.4 Entire pulp 20 19 7.4 Entire pulp – pH 8.5 21 19 7.4 Fines fraction 22 19 7.4 Fines fraction – pH 8.5 Anexo 1 40 5.3.6 Comparison of flotation chemicals Three different chemicals were used in six flotation experiments. Commercial soap, surfactant #1 and surfactant #2 were used in three different groups of two flotation experiments each, in which the dosage was changed. After pulping and fractionation, the short fibre fraction was used for the flotation experiments. All the chemicals were added in the flotation cell before starting flotation experiments. For the commercial soap, dosages of 6 and 9 kg/t were used (adding 7.2g of CaCl2 every time soap was used). For the surfactants, dosages of 0.6 and 0.9 kg/t were added respectively. The parameters for the chemicals flotation experiments are shown in Table 5. Table 5. Parameters for chemicals flotation experiments. Test run Chemical Dosage [kg/t] 23 Commercial soap 6 24 Commercial soap 9 25 Surfactant #1 0.6 26 Surfactant #1 0.9 27 Surfactant #2 0.6 28 Surfactant #2 0.9 Anexo 1 41 6 RESULTS The results are presented in the same five different scenarios of flotation experiments explained in Chapter 5.3. Several pulp properties were studied while brightness and ash content were the variables studied in every flotation experiment. 6.1 Reference flotation Brightness, ash content and residual ink values were obtained by optical measurement of sheets from every sample, and the results are presented versus RRm in the following figures. Figure 18. ISO brightness for entire pulp feed and accepts. A = accept. FEED A37 A38 A39 A40 A41 44 45 46 47 48 49 50 51 52 53 0 5 10 15 20 25 30 ISO Brightness [%] RRm [%] Anexo 1 42 Figure 19. Ash content for entire pulp feed and accepts. A = accept. Figure 20. Residual ink content for entire pulp feed and accepts. A = accept. Figure 21. Ash removal for entire pulp feed and accepts. R = reject. FEED A37 A38 A39 A40 A41 0 5 10 15 20 25 30 0 5 10 15 20 25 30 Ash 525 [%] RRm [%] FEED A37 A38 A39 A40 A41 400 600 800 1000 1200 1400 1600 0 5 10 15 20 25 30 Residual ink [ppm] RRm [%] FEED R37 R38 R39 R40 R41 0 10 20 30 40 50 60 70 80 0 5 10 15 20 25 30 Ash removal [%] RRm [%] Anexo 1 43 According to Figures 19 and 20, accept samples with a higher mass reject are the ones with less content in residual ink and ash. The higher the mass reject is the higher the brightness, despite losing some fibre because of higher mass reject rates. 6.2 Fractionation The values from membrane pads of feed, accept (short fibre) and reject (long fibre) are presented in Table 6. After fractionation, brightness values were similar but the short fibre fraction had the highest ash content value. Feed and long fibre fraction samples had similar retention values. ISO brightness of hyperwashed pulps were 44.0% and 47.0% for feed and long fibre fraction, respectively. Table 6. Brightness, ash content and retention of feed (entire pulp), accept (short fibre fraction) and reject (long fibre fraction) from fractionation. Feed Short fibre Long fibre ISO Brightness [%] 44.3 44.0 42.7 Ash 525 [%] 24.0 32.9 21.9 Retention [%] 61.0 38.6 59.1 HW Brightness [%] 44.0 NA 47.0 6.3 Flotation level and air flow during flotation Four groups of three flotation experiments in each were carried out in order to obtain the optimum values for flotation level and air flow during flotation. Optical properties were measured from pads of every flotation feed, accept and reject sample. Brightness and ash content values are presented in Figures 22 and 23. Anexo 1 44 Figure 22. Brightness values for flotation level and air flow flotation experiments. Figure 23. Ash content values for flotation level and air flow flotation experiments. According to Figure 22, only 1.0% unit difference in ISO brightness was achieved with the studied range of air flow and flotation level. In Figure 23, there is a small difference in ash content between the experiments. 6.4 Conditioning prior to flotation Three different amounts of reject sample were taken in every flotation experiment in order to observe the effect of chemical delay on brightness and ash content in feed and accept samples. In Figures 24 and 25 these variables are represented vs. RRm. 41 42 43 44 45 46 47 48 49 0 5 10 15 20 25 30 ISO Brightness [%] RRm [%] Level 19, Air 7.4 Level 20, Air 7.4 Level 19, Air 13 Level 20, Air 13 24 26 28 30 32 34 36 0 5 10 15 20 25 30 Ash 525ºC [%] RRm [%] Level 19, Air 7.4 Level 20, Air 7.4 Level 19, Air 13 Level 20, Air 13 Anexo 1 45 Figure 24. Brightness values for delay chemical flotation experiments. Figure 25. Ash content for delay chemical flotation experiments. Highest values of brightness are obtained by the samples with higher chemical delay but also with the higher reject mass rate which leads to a loss of fibre on the yield. In Figure 25, lowest ash content was achieved by test 15 (Soap, Delay 20 mins), with lower fibre loss on the yield compared to samples with higher delay. Values from test 21, in which soap was added in the pulper and there was no delay, were added to these figures to compare results. Based on the amount of reject obtained in every flotation experiment and further calculations, ash and fines removal values of all delay chemical flotation experiments were calculated. Ash removal and fines removal are shown in Figures 26 and 27. 40 42 44 46 48 50 52 0 10 20 30 40 ISO Brighness [%] RRm [%] No chemicals Soap, Delay 0 mins Soap, Delay 20 mins Soap, Delay 40 mins Soap, Delay 60 mins Soap + CaCl2, Delay 60 mins Soap (Pulper) 20 24 28 32 36 40 0 10 20 30 40 Ash 525ºC [%] RRm [%] No chemicals Soap, Delay 0 mins Soap, Delay 20 mins Soap, Delay 40 mins Soap, Delay 60 mins Soap + CaCl2, Delay 60 mins Soap (Pulper) Anexo 1 46 Figure 26. Ash removal for delay chemical flotation experiments. Figure 27. Fines removal for delay chemical flotation experiments. In Figure 26, a similar value of ash removal was obtained by the last four flotation experiments, with differences in mass reject rate. In Figure 27, flotation experiments with higher chemical delay were the ones with higher yield loss of fines. 6.5 Effect of pH in flotation Comparison between entire pulp pH flotation and reference flotation experiments Brightness, ash content, residual ink and ash removal values were obtained by optical measurement from sheets of every flotation experiment (feed and accepts samples), and represented in Figures 28, 29, 30 and 31. 0 10 20 30 40 50 60 0 10 20 30 40 Ash removal [%] RRm [%] No chemicals Soap, Delay 0 mins Soap, Delay 20 mins Soap, Delay 40 mins Soap, Delay 60 mins Soap + CaCl2, Delay 60 mins Soap (Pulper) 0 10 20 30 40 50 0 5 10 15 20 25 30 35 40 Fines removal [%] RRm [%] No chemicals Soap, Delay 0 mins Soap, Delay 20 mins Soap, Delay 40 mins Soap, Delay 60 mins Soap + CaCl2, Delay 60 mins Soap (Pulper) Anexo 1 47 Figure 28. Brightness values for entire pulp pH flotation experiments. A = accept. Figure 29. Ash content for entire pulp pH flotation experiments. A = accept. Figure 30. Residual ink values of entire pulp pH flotation experiments. A = accept. A41 44 45 46 47 48 49 50 51 52 53 0 10 20 30 40 ISO Brightness [%] RRm [%] Reference flotations Entire pulp Entire pulp, pH 8.5 A41 0 5 10 15 20 25 30 35 0 10 20 30 40 Ash 525ºC [%] RRm [%] Reference flotations Entire pulp Entire pulp, pH 8.5 A41 400 600 800 1000 1200 1400 1600 1800 0 10 20 30 40 Residual ink [ppm] RRm [%] Referenc flotations Enitre pulp Enitre pulp, pH 8.5 Anexo 1 48 Figure 31. Ash removal for entire pulp pH flotation experiments. R = reject. In Figures 28, 29, 30 and 31 there was a small difference between the pH controlled sample and the non-controlled one. Final values for brightness, ash content and residual ink content were similar but flotation test 20 (Entire pulp, pH 8.5) had a lower mass reject rate. Comparing pH flotation experiments with reference flotations, higher brightness, lower ash content and similar residual ink values were obtained by the reference flotation experiments but with lower mass reject rate. Fines fraction flotation experiments Two fines fraction flotation experiments were carried out, one of them adjusting the pH of the flotation feed sample to 8.5. Brightness and ash content values were obtained by optical measurement from pads of every flotation experiment (feed and accepts samples), and were represented versus RRm in Figures 32 and 33. Figure 32. Brightness values for fines fraction pH flotation experiments. R41 0 10 20 30 40 50 60 70 80 0 10 20 30 40 Ash removal [%] RRm [%] Reference flotations Entire pulp Entire pulp, pH 8.5 42 43 44 45 46 47 48 49 50 51 0 10 20 30 40 ISO Brightness [%] RRm [%] Fines fraction Anexo 1 55 Adding CaCl2 at the beginning of the delay time increased the mass reject rate by making fillers, ash and fines more hydrophobic as shown in Figures 24, 25, 26 and 27. Comparing these values with the ones obtained by test 21, in which the soap dosage was added into the pulper, there was no effect on where the chemical was added. Adding CaCl2 at the beginning of the delay had a negative effect on flotation, because the yield loss was higher. 7.5 Effect of pH in flotation Entire pulp flotation experiments According to Figure 28, ISO brightness values around 51% were obtained by pH flotation experiments, fairly similar values compared to reference flotations (only 1% unit difference). Adding chemicals in the pulper made ink particles more floatable. Residual ink values had a drastic decrease from 1630 to 625 ppm in the pH controlled flotation experiment, according to Figure 30. Fines fraction flotation experiments Adjust of the pH to 8.5 did not have so much effect in fines fraction flotation. Ash content, ash removal and fines removal of both experiments had very similar values as shown in Figures 33, 34 and 35. According to Figure 32 the highest value of brightness is obtained by flotation test 21, 1% unit higher than the pH controlled test 22. When adding chemicals to pulping, a better separation between ink and fibres was achieved: an increase of 2% units in ISO brightness can be seen compared to flotation level and air flow flotation experiments carried out without chemicals. Surprisingly, adjusting the pH to 8.5 did not have an effect in the properties measured. There is a widespread belief that the higher the pH is, the better results are achieved. According to INGEDE method 11, at the end of pulping pH value has to be 9.5 and adjust using sodium hydroxide has to be done if the pH after pulping is too low. However, this flotation experiments can be carried out in neutral pH conditions. Anexo 1 56 7.6 Comparison of flotation chemicals Commercial soaps of fatty acids are used as flotation collectors for ink removal in ONP/OMG blend for newsprint production (Dorris et al. 2011). The highest value of brightness was obtained with lower soap dosage (6 kg/t). Lower dosage of commercial soap led to a better agglomeration of ink and soap and made the ink-soap aggregates more hydrophobic. There was not a big difference between flotation experiments using either surfactant #1 or surfactant #2. The main advantage of using non-ionic surfactants in ONP/OMG blends is that small quantities are needed, reducing the chemical dosage (Dorris et al. 2011). The speed of flotation was higher when surfactants were used because the froth was quickly formed and removed from the top of the sample. Comparing all the chemicals flotation experiments, highest brightness value was obtained by the lower soap dosage (6 kg/t) flotation test according to Figure 36 (same brightness with lower RRm). There were no big differences between higher or lower dosage of surfactants, so the best dosage was 0.6 kg/t which lead to cost savings. According to Dorris (2011), if too much surfactant is added, the adhesion of ink particle to air bubble will be too weak to permit flotation. It was supposed that the surfactants would have worked better than soap during flotation, however soap worked very well. For that reason lower soap dosage is recommended because higher value of ISO brightness was obtained compared to surfactants flotation tests. After comparing all the different fines fraction flotation experiments carried out, the use of lower soap dosage (6 kg/t) in the flotation cell is recommended to improve pulp properties. Comparing this flotation with flotation level and air flow flotation (Figure 22) and with fines fraction pH flotation (Figure 32), a ISO brightness value of 48.9% is obtained with the lowest mass reject rate of all, losing less amount of fibres and fillers with the reject, and with no need of adjusting the pH. Anexo 1 57 8 CONCLUSIONS AND RECOMMENDATIONS The use of old raw materials worsens the deinkability of the pulp and therefore pulp properties such as brightness values are lower than values obtained using fresh raw material. Through entire pulp flotation, however, the brightness value increased moderately while the more amount of reject was removed from the sample. Thus, flotation experiments worked very well despite of the old raw material. Fractionation performed here was found to be adequate for these experiments. Brightness value of long fibre fraction was a little low because of the presence of free ink in the sample. However, short fibre fraction had the highest content of ash after fractionation, which indicates that the pulp was fractionated. Fines fraction flotation results suggest that changing flotation cell level and air flow do not have so much effect on pulp properties after flotation. On the other hand, increasing the delay of chemicals prior to flotation has an effect on fines fraction properties. Longer delay increases brightness but also mass reject rate which lowers the yield. The use of chemicals to adjust pH prior to flotation do not have an effect in pulp properties, so apparently this flotation experiments can be carried out in neutral pH conditions. Based on these results, the use of a soap dosage of 6 kg/t in the flotation cell is recommended. Lower dosage of soap leads to a better agglomeration of ink and soap and made the ink-soap aggregates more hydrophobic. Highest value of brightness is reached with lower mass reject rate, compared to other flotation collectors such as surfactants. Anexo 1 58 9 SUMMARY The target of this work was to obtain more information about the less studied fines fraction flotation and to optimize its variables. The work was divided in five different flotation scenarios in which a mixture of recovered paper was used as feed. The feeding ratio was 50% ONP and 50% OMG (OMG consisted in half SC and half LWC). 9.1 Reference flotation Five entire pulp flotation experiments were carried out in the flotation device. The consistency of the feed was 1.2%, after diluting the pulp slurry from pulping with warm tap water. The flotation level and the air flow were set to 19 litres and 7.4 litres per minute, respectively. Residual ink and ash content decreased through flotation while ISO brightness increased from 45.1% to 51.8%. 9.2 Fractionation Fractionation was performed with pressure screening in order to obtain 35% of long fibres and 65% of short fibres. Because high accept consistency was wanted due to the short fibre flotation right after fractionation, the feed consistency of the fractionation was around 0.8%. Three samples from a fractionation were taken: feed, short fibre fraction and long fibre fraction. Through fractionation, high free ink content was achieved in the long fibre fraction and high ash content in the short fibre fraction. 9.3 Flotation level and air flow during flotation Flotation level and air flow values were changed in different fines fraction flotation experiments to obtain the optimum values. These changes did not have so much effect on pulp properties and therefore flotation level and air flow were set to 19 litres and 7.4 litres per minute for the rest of flotation experiments. Results showed that ISO brightness of the fines fraction increased to values around 48% through flotation. Anexo 1 59 9.4 Conditioning prior to flotation Soap was added in this fines fraction flotation experiments to determine the effect of different delay time of chemicals in the pulp properties. The use of chemicals improved the brightness values: delay chemical flotation results were better than results of flotation level and air flow flotation experiments. Longer delay increases brightness, but also RRm which lowers the yield. 9.5 Effect of pH in flotation pH of different flotation experiments were adjust to 8.5 to compare the difference between the controlled and the non controlled pH test. Controlling the pH was not an important factor for pulp properties. 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Anexo 1 64 Block diagram of the flotation experiments APPENDIX 1 (1/3) Reference flotation - Flotation tests 37-41 Pulping Flotation Reject Accept Entire pulp - Fractionation samples Pulping Fractionation LF SF F Flotation level and air flow during flotation - Flotation tests 1-12 Pulping Fractionation LF SFF Flotation Reject Accept