Classifications

• • D—TEXTILES; PAPER
  • D21—PAPER-MAKING; PRODUCTION OF CELLULOSE
  • D21C—PRODUCTION OF CELLULOSE BY REMOVING NON-CELLULOSE SUBSTANCES FROM CELLULOSE-CONTAINING MATERIALS; REGENERATION OF PULPING LIQUORS; APPARATUS THEREFOR
  • D21C9/00—After-treatment of cellulose pulp, e.g. of wood pulp, or cotton linters ; Treatment of dilute or dewatered pulp or process improvement taking place after obtaining the raw cellulosic material and not provided for elsewhere
  • D21C9/10—Bleaching ; Apparatus therefor
• • D—TEXTILES; PAPER
  • D21—PAPER-MAKING; PRODUCTION OF CELLULOSE
  • D21H—PULP COMPOSITIONS; PREPARATION THEREOF NOT COVERED BY SUBCLASSES D21C OR D21D; IMPREGNATING OR COATING OF PAPER; TREATMENT OF FINISHED PAPER NOT COVERED BY CLASS B31 OR SUBCLASS D21G; PAPER NOT OTHERWISE PROVIDED FOR
  • D21H15/00—Pulp or paper, comprising fibres or web-forming material characterised by features other than their chemical constitution
  • D21H15/02—Pulp or paper, comprising fibres or web-forming material characterised by features other than their chemical constitution characterised by configuration
• • D—TEXTILES; PAPER
  • D21—PAPER-MAKING; PRODUCTION OF CELLULOSE
  • D21H—PULP COMPOSITIONS; PREPARATION THEREOF NOT COVERED BY SUBCLASSES D21C OR D21D; IMPREGNATING OR COATING OF PAPER; TREATMENT OF FINISHED PAPER NOT COVERED BY CLASS B31 OR SUBCLASS D21G; PAPER NOT OTHERWISE PROVIDED FOR
  • D21H17/00—Non-fibrous material added to the pulp, characterised by its constitution; Paper-impregnating material characterised by its constitution
  • D21H17/63—Inorganic compounds
  • D21H17/70—Inorganic compounds forming new compounds in situ, e.g. within the pulp or paper, by chemical reaction with other substances added separately
• • D—TEXTILES; PAPER
  • D21—PAPER-MAKING; PRODUCTION OF CELLULOSE
  • D21H—PULP COMPOSITIONS; PREPARATION THEREOF NOT COVERED BY SUBCLASSES D21C OR D21D; IMPREGNATING OR COATING OF PAPER; TREATMENT OF FINISHED PAPER NOT COVERED BY CLASS B31 OR SUBCLASS D21G; PAPER NOT OTHERWISE PROVIDED FOR
  • D21H17/00—Non-fibrous material added to the pulp, characterised by its constitution; Paper-impregnating material characterised by its constitution
  • D21H17/63—Inorganic compounds
  • D21H17/67—Water-insoluble compounds, e.g. fillers, pigments
  • D21H17/675—Oxides, hydroxides or carbonates
• • D—TEXTILES; PAPER
  • D21—PAPER-MAKING; PRODUCTION OF CELLULOSE
  • D21H—PULP COMPOSITIONS; PREPARATION THEREOF NOT COVERED BY SUBCLASSES D21C OR D21D; IMPREGNATING OR COATING OF PAPER; TREATMENT OF FINISHED PAPER NOT COVERED BY CLASS B31 OR SUBCLASS D21G; PAPER NOT OTHERWISE PROVIDED FOR
  • D21H25/00—After-treatment of paper not provided for in groups D21H17/00 - D21H23/00
  • D21H25/08—Rearranging applied substances, e.g. metering, smoothing; Removing excess material
  • D21H25/12—Rearranging applied substances, e.g. metering, smoothing; Removing excess material with an essentially cylindrical body, e.g. roll or rod
  • D21H25/14—Rearranging applied substances, e.g. metering, smoothing; Removing excess material with an essentially cylindrical body, e.g. roll or rod the body being a casting drum, a heated roll or a calender

Definitions

• the present invention relates to methods for treating pulp for use in the production of paper and paper products, in particular super calendered (SC) paper.
• SC super calendered
• the present invention also relates to products obtained by said methods.
• the term "paper” should be understood to mean all forms of paper, including board, card, paperboard, and the like.
• any improvements with respect to the bleaching process should preferably not result in a significant deterioration in the performance of the paper. Balancing these aspects has hitherto proven difficult.
• One of the other environmental issues relating to the production of paper concerns the aqueous waste streams produced by paper mills, which generally comprise suspensions of fine organic microfibres, for example cellulosic fibres which typically have a length of no greater than about 75 ⁇ m, and other organic materials usually in association with inorganic particulate materials.
• the solids in such streams have, in some cases, proven difficult to dewater; in addition the waste streams are environmentally and economically undesirable to discharge.
• Aqueous wastes from paper mills generally comprise solids which are so-called fines which may be organic and/or inorganic in nature and which are defined in TAPPI Standard No. T261 cm-90 "Fines fraction of paper stock by wet screening” (1990).
• This document describes a method for measuring the fines of paper making stock and specifies that fines are those particles which will pass through a round hole of diameter 76 ⁇ m.
• particles includes minute materials of all types including inorganic particles, organic microfibres and particles and fine minerals.
• the present invention is based on the finding that more economical and environmentally friendly methods for bleaching pulp can be obtained when the pulp to be treated in the bleaching process is divided into fractions determined primarily by the size of the fibres or particles in the pulp.
• the pulp is divided into a coarse and a fine fraction followed by separate treatment of each of the fractions.
• the amount of bleaching material required is decreased.
• This improvement in known bleaching techniques is achieved without a significant compromise in the qualities of the resulting paper products and in some respects, the qualities of the paper products are improved.
• the present invention is particularly designed to operate in a continuous fashion at an integrated paper mill. In such an operation, delays between the various steps can be kept to a minimum and the potential so-called alkali darkening of mechanical pulp can be avoided or at least lessened.
• a fibre pulp mixture comprising:
• the coarse fraction and/or the fine fraction are dewatered prior to (b) and (c) respectively.
• Separating the fibre pulp mixture into a coarse and fine fraction separates the fibres of the wood pulp in the fibre pulp mixture by any one of a number of parameters related to the size of the fibres and may be carried, out using known and standard screening techniques. Separation may, for example, be according to the length of the fibre, so that fractions comprising mainly long and short fibres respectively are obtained in each fraction.
• the fractionation may be achieved by separating the fractions into fibres and particles having an average size above and below a certain value, for example, about 50 ⁇ m according to the TAPPI Standard No. T261 cm-90 mentioned previously.
• the fine fraction may comprise about 15-50 % by weight, preferably 20- 30% by weight of the fibres or particles making up the fibre pulp mixture prior to fractionation and will have an average particle size of less than about 50 ⁇ m.
• the fine fraction will consist predominantly of organic fines which will pass through a substantially round hole of diameter 50 ⁇ m.
• the precipitated alkaline earth metal carbonate will usually form an aggregate with the fibres and/or particles of the fine fraction.
• the aggregated material resulting from step (c) may be supplied for use as a filler or pigment in a filler or pigment containing composition.
• the wet precipitate of alkaline earth metal carbonate and entrained fine material is added to a paper making composition to provide filler particles for the paper making fibres.
• the wet material may be dewatered by any conventional method, for example by pressure filtration or in a centrifuge.
• some or all of the mixed aggregated material resulting from step (c) may be combined with the bleached coarse fraction resulting from step (b) according to the first aspect of the invention, in order to make paper.
• This aspect of the invention advantageously results in minimising the various waste streams and helps to maintain the balance of the plant in which the paper making process is being carried out. There are numerous other advantages associated with the present invention.
• the bleaching equipment may be smaller in size compared to conventional bleaching units, due to the smaller mass to be bleached.
• the devices for carrying out the dewatering may also be reduced in size.
• the bleaching chemical consumption for a given weight of material is proportional to the surface area of the material.
• the removed fine fraction will in general have a higher specific surface area than the coarse fraction and consequently the bleaching chemical consumption for the coarse fraction is lower for a unit of weight than that for the original pulp; it is estimated by about 10 - 15%.
• the investment costs for a bleaching plant according to the process of the present invention may be significantly less than the costs for the current type of plant; typically up to about 30% less. It is estimated that the running costs, mainly due to the decreased use of resources, for example, chemicals and electricity would be about 15% lower than for a conventional plant.
• the present invention is concerned with treating mechanical pulp. It is well known to persons of ordinary skill in the art of making pulp and paper how to make a mechanical pulp.
• the first step in the mechanical pulping process is the grinding or refining of wood.
• the Stone GroundWood (SGW) process involves making pulp by pressing logs and chips against an abrasive rotating surface. Originally, the grinding surface used was an actual stone. In current practice, specifically designed "artificial" pulp stones are available for the grinding.
• a Pressurized Ground Wood (PGW) process is where the grinding operation is completely pressurized.
• RMP Refiner Mechanical Pulp
• TMP Thermo Mechanical Pulping
• TRMP Thermo Refiner Mechanical Pulping
• DIP De-inked pulp
• the paper is disintegrated into fibres in a mechanically operating pulping drum, followed by screening, flotation, washing and bleaching.
• the mineral content of the fines fraction in DIPs can be as high as about 50wt%.
• the pulp is divided or screened into coarse and fine fractions; this may involve using any of the known fractionating or screening methods used in pulp processing, for example a wire screen possessing a particular, or range, of hole size or a number of wire screens possessing a range of suitable hole sizes.
• a wire screen possessing a particular, or range, of hole size or a number of wire screens possessing a range of suitable hole sizes Numerous methods are known and are suitable for separating fibres of varying size.
• a method, apparatus and a screen for screening mechanical fibre pulp is described which are suitable for use in the present invention. Further examples include Dorr Oliver's DSM curved screen and Krofta's spray screen.
• a mechanically manufactured fibre pulp mixture containing fibres of varying lengths is screened into at least two fractions containing fibres of mainly varying lengths.
• the invention described in US 2004/001 1483 allows for the accurate separation of a fibre fraction that is shorter than a fraction representing a particular length from the fibre pulp mixture.
• the apparatus described in US 2004/0011483 comprises a gap screen to separate short fibres comprising a convergent gap and at least one wire restricting the convergent gap.
• the fibre pulp mixture to be screened is fed into the convergent gap so that it flows through the gap screen, in the same direction as the wire, towards the convergent end of the gap so that the shorter fibres and some water exit through the openings in the wire and the remaining part of the fibre pulp mixture exits from the gap screen through the output port at the convergent end of the gap.
• Other methods and apparatus suitable for separating the pulp into fractions of differing size are well known to those skilled in the art and are suitable for use in the present invention. There are numerous ways in which particles, and fibres may be measured. According to the present invention the screening of the fibre pulp mixture will, for example, result in the coarse fraction being 20-30% by (dry) weight less when compared to the original fibre pulp mixture.
• the screening method will be such that preferably the fine fraction will consist predominantly of fibres and/or particles that are about 50 ⁇ m or less; i.e. most of the 20-30wt% loss will be substantially in the form of fines according to TAPPI Standard No. T261 cm-90. Bleaching the coarse fraction
• the coarse fraction is bleached in a standard manner.
• the coarse fraction Prior to bleaching, the coarse fraction may be dewatered and optionally washed using any of the known standard procedures, for example, by use of a belt press. Dewatering of the coarse fraction may advantageously give rise to better economy and efficiency of the overall process. Depending, for example, on the type of solids present, dewatering of the coarse fraction typically results in the solids content in the coarse fraction increasing from a range of about 0.01 to 0.1wt% to a range of about 1.5 to 6.5wt%. Due to the reduced mass of the original pulp mixture, the equipment needed for the bleaching process may advantageously be smaller than standard bleaching equipment.
• Bleaching processes are well known to those skilled in the art and those processes suitable for use in the present invention will be readily evident.
• a reducing bleaching agent such as a dithionite salt, for example sodium or zinc dithionite.
• reducing bleaching agent include zinc dust, thiourea dioxide (i.e. formamidine sulphinic acid) and sulphur dioxide.
• Oxidising bleaching agents may also be used and hydrogen peroxide is a particularly preferred bleaching agent.
• Other suitable oxidising bleaching agents include peracetic acid and ozone.
• the amount of the reducing bleaching agent used is preferably in the range of 1.5g to 7.5g of the reducing bleaching agent per kilogram of dry fractionated pulp material and in the case of oxidising bleaching agents the corresponding amount is preferably in the range of 1.0g to 4.0g of the oxidising agent.
• the fine fraction may be in the form of a dilute suspension. It is generally unacceptable, for environmental and economic reasons, to allow such a suspension of fine particles to be discharged to rivers or lakes, and as a result such unwanted suspensions of very fine particles are ultimately often retained in lagoons, thus occupying large areas of land which could more profitably be used for other purposes.
• the fine fraction may be thickened to a suitable level, for example by a known dewatering method, for example by using Dissolved Air Flotation (DAF) Unit(s) and a mineral filler is co-precipitated on its surface using techniques known to those skilled in the art.
• DAF Dissolved Air Flotation
• the applicant's rECCIaimTM method is a known method and precipitates an alkaline earth metal carbonate (preferably calcium carbonate) on the surfaces of paper making fines in controlled conditions and is suitable for use in the present invention.
• alkaline earth metal carbonate preferably calcium carbonate
• Anions other than carbonate may also be used in the present invention.
• An example of such an anion is silicate. Dewatering of the fine fraction may advantageously result in a better quality carbonation product and provides cleaned water for recycling in the process of the present invention.
• the solids content of the fine fraction is generally increased from about 0.03wt% to 6.5wt%, more particularly to a range of about 0.2wt% to about 4wt%, for example to about 1.5wt%; in particular this increase may be achieved with the use of DAF units.
• the use of a combination of dewatering units, such as DAF units, may advantageously lead to the use of units of reduced size and/or the amount of waste solids in the overall process being reduced.
• the so-called fines material is generally considered to become entrained in the alkaline earth metal carbonate to form a mixed aggregated material.
• the alkaline earth metal carbonate precipitate may be formed by introducing into the suspension of the fine fraction a source of alkaline earth metal ions and a source of carbonate ions. This will form the desired precipitate of alkaline earth metal carbonate in situ which will entrain the particulate or fibrous material.
• the first reagent which is added should preferably be uniformly distributed throughout the aqueous based fine fraction to avoid local concentration gradients. When the first reagent is sparingly soluble, as is the case with calcium hydroxide, thorough mixing is desirable.
• the fine fraction which may be in the form of a suspension, should be agitated while the second reagent is added in order to ensure an even distribution of the precipitate. It is preferred to add the source of alkaline earth metal ions and the source of carbonate ions sequentially. The source of alkaline earth metal ions may be added first followed by the source of carbonate ions or the source of carbonate ions may be added first followed by the source of alkaline earth metal ions.
• US 5665205 includes a description of a process wherein carbon dioxide is brought into contact with a fibre slurry prior to the introduction of a calcium hydroxide slurry.
• the source of alkaline earth metal ions is conveniently the alkaline earth metal hydroxide (known as milk of lime, when the alkaline earth metal is calcium), but it may alternatively be a water-soluble alkaline earth metal salt, for example the chloride or nitrate.
• the alkaline earth metal hydroxide may be added to the aqueous suspension already prepared, or alternatively may be prepared in situ, for example by slaking an alkaline earth metal oxide (e.g. quicklime when an aqueous suspension of calcium hydroxide is desired) in the suspension.
• the source of carbonate ions is conveniently carbon dioxide gas, which is introduced into the fine fraction and, depending on the order of addition, the source of the alkaline earth metal ions.
• the carbon dioxide gas may be substantially pure as supplied in gas cylinders or may be present in a mixture of gases such as flue gases.
• the source of carbonate ions may be an alkali metal or ammonium carbonate. Sodium carbonate is particularly preferred. It is particularly preferred that the source of alkaline earth metal ions is introduced into the fine fraction either by slaking an alkaline earth metal oxide, for example calcium oxide or quicklime, in the fine fraction or by adding to the fine fraction a separately prepared suspension of an alkaline earth metal hydroxide.
• the mixed aggregated material from step (c) is combined with the bleached coarse fraction resulting from step (b) according to the first aspect of the invention, in order to make paper.
• the combined product when used in a conventional paper-making process will be mixed with chemical pulp in a paper machine mixing chest in order to make paper stock. From there, the paper stock is pumped to a machine chest, via cleaning and de-airing systems and is then pumped to a headbox. Mineral fillers and paper-making chemicals may be added to the paper stock at appropriate points during this process.
• the mixed aggregated material from step (c) and the coarse fraction from step (b) may be used to make paper in separate paper making processes.
• paper suitable for books, magazines, newspapers and the like may be calendered or super calendered as appropriate; for example super calendered magazine paper for rotagravure and offset printing may be made according to the present methods.
• Paper suitable for light weight coating (LWC), medium weight coating (MWC) or machine finished pigmentisation (MFP) may also be made according to the present methods.
• LWC light weight coating
• MWC medium weight coating
• MFP machine finished pigmentisation
• typical types of paper in addition to those mentioned above, include so-called woodfree paper.
• the aggregated material may be blended in various proportions with conventional filler materials, e.g.
• the ingredients and composition being selected according to the quality of the paper required to be produced. In general these materials are likely to be in a slurry form when they are mixed.
• the paper maker will normally select the concentration of the aggregate material in aqueous suspension and the delivery rate of the suspension at the point of addition to the paper making composition.
• the paper making composition may typically comprise, in aqueous suspension and in addition to the optional aggregated material referred to in the methods of the present invention, cellulosic fibres and other conventional additives known in the art.
• a typical paper making composition would contain up to about 35% by weight of filler material of the total dry contents and may also contain a cationic or an anionic retention aid in an amount in the range from 0.1 to 2% by weight, based on the dry weight of the filler material. It may also contain a sizing agent which may be, for example, a long chain alkylketene dimer, a wax emulsion or a succinic acid derivative. The composition may also contain dye and/or an optical brightening agent.
• the paper products obtained according to the present invention may be coated.
• a paper coating composition will include, in aqueous or non-aqueous suspension, an adhesive and, optionally, other filler materials.
• the formula of the paper coating composition will typically depend upon the purpose for which the coated paper is to be used. Calendering is a well known process in which paper smoothness and gloss is improved and bulk is reduced by passing a coated paper sheet between calender nips or rollers one or more times.
• the methods according to the present invention are particularly suited for the production of super calendered paper. Methods of coating paper and other sheet materials, and apparatus for performing the methods, are widely published and well known. Such known methods and apparatus may conveniently be used for preparing coated paper. For example, there is a review of such methods published in Pulp and Paper International, May 1994, page 18 et seq.
• the coating is usually added by a coating head at a coating station.
• paper grades are uncoated, single coated, double coated and even triple coated.
• the initial coat may have a cheaper formulation and optionally less pigment in the coating composition.
• a coater that is applying a double coating i.e. a coating on each side of the paper, will have two or four coating heads, depending on the number of sides coated by each head. Most coating heads coat only one side at a time, but some roll coaters (e.g., film press, gate roll, size press) coat both sides in one pass.
• Figure 1 is a flow diagram of a conventional method for bleaching a mechanical pulp.
• Figure 2 is a flow diagram showing an embodiment of the process according to the present invention.
• Figure 1 which illustrates a conventional method for bleaching a mechanical pulp
• the mechanical pulp typically TMP is led through pipeline (50) as an aqueous solid suspension at about 0.6wt% solid content to a dewatering device (1 ) such as a disk filter.
• the thickened pulp is led through pipeline (51) to an unbleached pulp tank (2) at about 8wt% solid content in an aqueous suspension.
• the water from the dewatering device (1 ) is led via pipelines (52) and (53) to clear (3) and cloudy (4) water tanks respectively.
• Cloudy water is further led through pipeline (54) via a feed tank (5) to a micro flotation system (6) where the suspended solids are collected and dumped via pipeline (55).
• the cleared water is fed back from the microflotation tank (6) to the mechanical pulp water chest (7) via pipeline (56) for re-use. Any extra water required for the system is taken from paper machine press section (6a) to the feed tank (5) through pipeline (56b) and then to microflotation tank (6). This operation also serves to dump the solids in the press section water. Extra water may also be taken from a paper machine clear filtrate chest (6b) to the clear water tank (3) via pipeline 56a.
• the thickened pulp from the unbleached pulp tank (2) is fed through pipeline (57) to a thickener (8), for example a belt press, to be thickened, i.e.
• Thickened, washed and bleached pulp is diluted to, for example, 8 wt % solids after passing through pipeline (72) in to mixer (11 ) using process water via pipeline (73). The diluted pulp is then led via pipeline (74) to bleached pulp storage chest (12) for further use.
• Water from thickener (10) is fed through pipeline (71 ) to water tank (3b).
• the water from thickener (8) is fed to the water tank (3a) via pipeline (70).
• Water from the water tank (3a) is fed via pipeline (70a) to cloudy water tank (4).
• Water chest (7) supplies water to the clear water tank (3) and to the pulping process via pipelines (74) and (75) respectively.
• the mechanical pulp is fed to a screening/separating device (15), for example an OptiThick TM washer (commercially available from Metso Corporation and as described in US 2004/0022483) via pipeline (50) and fractionated into coarse and fine fractions.
• the coarse fraction is fed from the screening device (15) to a bleaching process (9), via (optionally) a thickener (8) at, for example 8% by weight solids, through pipelines (51 ), (57) and (65). Bleaching is performed in the conventional manner.
• the fine fraction leaves the screening device (15) as a dilute suspension at, for example 0.2% by weight solids via pipeline (80).
• the separation to a cloudy leg and clear leg is not necessary.
• the fine fraction is optionally thickened, for example, in a dissolved air flotation (DAF) unit (16).
• DAF dissolved air flotation
• the solids from the DAF unit (16) are fed via a pipeline (81) to a unit (17) that will co-precipitate the alkaline earth metal carbonate with the fine fraction.
• the co-precipitate is formed, for example, by adding milk of lime to agitated fines slurry, and bubbling carbon dioxide containing gas through the mix until slightly acid conditions are reached.
• Pipeline (16b) feeds process water to bleaching plant thickeners.
• the water from the air flotation unit (16) has a very low suspended solids content, typically below 50 ppm, and is particularly useful process water and is led to the mechanical pulp water chest (7) or pipeline 16b via pipeline (16a).
• Part of the water from the air flotation unit (16) is stored in pulp plant water chest (7) and supplies water for the screening/separating device (15) via pipeline (74), and clear water to the pulping process via pipeline (75).
• the other feeds to the water chest (7) come from the microflotation tank (6) and from the paper machine clear filtrate chest (6b) via pipelines (56) and (56a) respectively.
• the carbonator feed solids are typically about 4wt% resulting in a fines/calcium carbonate co-precipitate of about 6wt% solids.
• Pipeline (85) takes the co-precipitate to a storage tank (18).
• the co- precipitate may be combined with the bleached coarse fraction from storage tank (12) in a paper making process.
• the combined product resulting from storage tank (18) and storage tank (12) when used in a conventional paper-making process will, for example, be mixed with chemical pulp in a paper machine mixing chest in order to make paper stock. From there, the paper stock is pumped to a machine chest, via cleaning and de-airing systems and is then pumped to a headbox. Mineral fillers and paper-making chemicals may be added to the paper stock at appropriate points during this process. Examples
• the ISO brightness of coated paper was measured by means of an Elrepho Datacolour 2000TM brightness meter fitted with a No. 8 filter (457nm wavelength), according to ISO 2470: 1999 E.
• Brightness cakes or pads were made according to standard methods. More specifically, the pads are formed by taking about 5g by dry weight of sample and diluting to about 1%. The sample is dewatered on a 15cm diameter Buchner funnel to give pads of approximately 280g basis weight. The pads are then pressed on a handsheet press for about 2 minutes, using three standard blotters per pad. The sequence on the press is: 3 blotters; filter paper; sample; metal plate. After pressing, the samples are dried between two fresh blotters on a drying drum with a surface temperature of 80°C. The filter paper is removed from the dry pad and the brightness is measured from each side wherein the top is the side away from the filter paper and filter is the side against the filter paper. Reflectance measurements were carried out on the brightness pads.
• the light scattering coefficient represents the proportion of light that is scattered from a surface. Light scattering coefficients were measured according to ISO 9416: 1998 E.
• the light absorption coefficient is a measure of the portion of incoming light that is absorbed by a surface. It is inversely related to the surface brightness and directly to its opaqueness. Light absorption was measured according to ISO 9416: 1998 E.
• the yellowness was measured according to the procedure described above for the brightness measurements. The yellowness is reported as the value obtained when the reflectance at 457nm is subtracted from the reflectance at 571 nm.
• Paper strength was assessed using standard tensile and tear tests. Surface strength measurements and tensile stiffness measurements were also carried out. Tensile index measurements and tensile stiffness index measurements were carried out according to ISO 5270: 1924-2 on an L&W Alwetron instrument. The tear index was measured according to ISO 5270: 1998 E on an L&W Elmendorf instrument. Freeness Tests
• the freeness test which measures drainability, is a traditional method for characterising pulp.
• Common test methods for characterising pulp are the Canadian Standard Freeness (CSF) test method and the Schopper-Riegler method.
• the Bauer-McNett classification is a well-known method for characterising both chemical and mechanical pulps, although it is generally considered more suitable for the latter.
• the Bauer-McNett apparatus has been shown to classify pulp primarily according to fibre length. The results are represented as the portion of fibres having different freeness values and were obtained by making measurements according to SCAN-M 6:69. For example, in Table 1 below, with regard to the feed SGW, the percentage of fibres by weight having a freeness value of 30 or greater is 8.4% whilst the percentage of fibres by weight having a freeness value ranging from 50 to 30 is 13.9%.
• the values 30, 50 and 200 are the size of the meshes on the screening device.
• B-McN +30 (%) in Table 1 indicates the fraction (% by weight) that remains on a 30 mesh wire and B-McN +50 (%) indicates the fraction that remains on a 50 mesh wire but passes through the 30 mesh wire.
• the size of the holes in the mesh wires are as follows:
• Pulp splitting was carried out using the OptiThick TM device which is commercially available from Metso in Inkeroinen, Finland, under the following conditions:
• SGW Stone GroundWood
• DPTA diethylenetriaminepentaacetate
• 25wt% solids 25wt% solids
• a series of chemical doses of hydrogen peroxide was used to bleach 30g samples in plastic bags. Bleaching was carried out at 70°C (maintained in a water bath), using 25 wt% solids and a reaction time of 90 minutes. Following this, the samples were acidified to pH 5.5 with H 2 SO 4 . Brightness cakes of each of the samples obtained were made and the results according to Table 2 were obtained which compare the original feed with the coarse fraction.
• DPTA diethylenetriaminepentaacetate
• a mobile carbonation unit was used to study the effect of carbonation on the optical properties of de-inking plant micro flotate.
• the unit consisted of: two feed stock tanks each of 750 litres capacity; a slaked lime preparation module which comprised a 100 litre stirred vessel, a centrifugal slaked lime slurry transfer pump, a 300 mesh screen for the lime slurry and a 100 litre stirred lime storage tank; a carbonation unit which comprised feed pumps for micro flotate feed and milk of lime, a transfer pump for the precipitated product, carbon dioxide vaporizer and flow control equipment all operating with respect to a 10 litre turbine mixed carbonator vessel, that could be heated via an inside hot water coil; and a product storage tank.
• the unit possessed all of the controls necessary for carrying out the required operations and a small laboratory for quality control work.
• the micro flotate samples were taken two and a half hours after coarse and fine screen rejects had been stopped from entering the micro flotate.
• Typical batch carbonation operating parameters were as follows:
• Feed pulp volume (litres) 8.5 Mass of feed solids (kg) 0.2
• Feed solids (adjusted in feed stock tank) (wt%) 2.35
• Table 5 shows the feed samples bulked to one daily sample for three different daily runs.
• the number in parentheses after the run number indicates the total number of runs making up the bulk number.
• Example D Paper production Samples of super calendered magazine quality SGW pulp as referred to above were acquired from the Kajaani paper mill in Finland. The sample was split into a coarse and a fine fraction using an OptiThick TM device in the manner described in Example A. The fine fraction had an ISO brightness of 62.4 and a yellowness of 17.6. Samples of the original SGW pulp and the coarse fraction were peroxide bleached in the manner described in Example B. The fine fraction was co-precipitated according to standard procedures producing a 50/50 product (50wt% original feed plus 50wt% of newly grown carbonate) with an ISO brightness of 67.5 and a yellowness of 17.6.
• Hand sheets (330mm by 330mm) were made using an automatic sheet moulder with re-circulation following standard procedures. The target grammage was 52g/m 2 .
• a second set of sheets referred to as the trial sheets, were made combining the bleached coarse fraction and the co-precipitated fine fraction so that the fibre composition matched that of the original Stone GroundWood pulp.
• the hand sheets were super calendered with a Gradek 1-nip laboratory calender (hard and soft paper cylinder) under standard calendaring conditions:

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