This invention relates to improvements in processes for coating paper and board, and in particular concerns the use of equipment for controlling the size and quantity of large particles in the coating colour during the process of making paper. In typical embodiments of the invention, a decanter centrifuge is employed in the coating colour loop of the paper-making machine in order to control the build-up of large particles in the recycle coating colour. Benefits of the invention include the reduction of wear of components of the paper coating machine and reduced damage to the surface of the paper. This allows a longer running time of the paper coating machine between stoppages to replace elements of the coating applicator and other associated wear parts, such as backing roll, holder for the applicator, and bearings for the applicator.

Background of the Invention

In the well-known industrial process for the manufacture of paper, a paper coating machine is employed to apply a coating on to the surface of a continuous web of paper which is moving through the machine. Typically, a coating of a fluid paper coating composition is applied to the surface of the paper web, dried, and then optionally subjected to further processing steps in order to provide the final finished surface. Application of the paper coating composition to the paper surface is accomplished by way of a coating applicator which introduces the coating composition onto the paper surface at a location upstream of a metering element of the paper coating machine, such as a lateral blade or rotatable rod. In use, the coating applicator can be (i) a stationary element (e.g. a blade) which defines a narrow slit opening with respect to the moving surface of the paper web through which the coating composition is carried and applied as a smooth coating over the surface of the web; (ii) a metered size press (MSP), sometimes referred to as a metering size press or film press coater. In the MSP process, a coating is first applied ("metered") by a metering rod onto an applicator roll. The applicator roll is then pressed against a paper in the nip of a size press roll. This transfers the coating to the paper. Unlike blade coating, MSP coating has no stationery elements (e.g. a blade) that are in contact with the paper. Thus, the stress on paper is minimal as compared to blade coating. Excess coating composition from the coating applicator is collected and reused, typically being mixed with fresh coating composition and subsequently being recycled via a closed loop in the paper-coating machine.

Whilst the pigments used in paper coating compositions are typically supplied to the paper maker in a form with a negligible amount of oversize particles which could damage the paper coating machine, it has been found that a build up of oversize particles in the paper coating composition occurs in the paper coating machine leading to increased wear in components of the paper coating machine (such as the aforementioned stationary coating element of the coating machine and/or the backing roller which supports the paper web on the opposite side of the paper web from the coating element). In addition, the surface of the coated paper web may be damaged. It is currently thought that these deleterious particles include contaminants derived from the shipping of the pigment, a negligible amount from the pigment itself, contaminants from other raw materials used for making the coating colour, as well as particles which are stripped or otherwise removed from the paper web during the paper coating process. Such particles may, for example, include particles which are derived from the media/systems used to grind the fibres employed in the paper making process. A particular complication is that, if unchecked, the presence of oversize particles in the paper coating composition can build up extremely rapidly (e.g. > 500 fold in 24 hours), and even over a relatively short period of operation cause significant problems in the running of the paper coating machine, leading to downtime in the paper mill, which is extremely undesirable from an economic point of view.

Hitherto, screening systems have been used to screen the excess coating composition collected from the coating applicator. Examples of prior art publications disclosing such screening systems are US-A-4102299, US-A- 5401899, US-A-6074700 and EP-A-0733734, the contents of which are hereby incorporated by reference in their entirety for their teaching of paper coating systems utilizing circulation systems for the coating. The present invention provides a solution to the aforementioned problems by treating at least a portion of the excess coating composition collected at the coating applicator using a centrifuge in order to remove particles which are of such a size that their passage through the gap between paper surface and coating applicator is difficult. Typically, such particles will be those which are larger than 5 μm. The use of screening devices is commercially impractical at such particle sizes. Summary of the Invention

In accordance with a first aspect of the present invention, there is provided a process for coating a continuous web of paper in which a paper coating composition is applied to the surface of the web of paper to form a coating thereon and excess coating composition collected for re-use, wherein at least a portion of the collected, excess paper coating composition is subjected to centrifugation to produce a product stream having a reduced concentration of particulate matter, prior to re-use in the process.

In accordance with a second aspect of the present invention, there is provided a method for reducing paper damage and wear at the coating applicator of a paper coating machine of the type in which a paper coating composition is continuously applied to the surface of a web of paper to form a coating thereon and excess coating composition collected for re-use, wherein at least a portion of the collected, excess paper coating composition is subjected to centrifugation to produce a product stream having a reduced concentration of particulate matter, prior to re-use.

Centrifugation may be accomplished in one or more centrifuge devices.

In accordance with a third aspect of the present invention, there is provided a process for coating a continuous web of paper with a paper coating composition, said process comprising: providing a circulation system for circulating paper coating composition to a paper coating applicator and recycling excess paper coating composition; applying paper coating composition from the coating applicator to a surface of the web of paper which is moving relative to the coating applicator; collecting excess coating composition not applied to the web of paper; mixing the collected excess coating composition with fresh coating composition to replace the coating composition applied to the web of paper to prepare a mixed paper coating composition which comprises fresh paper coating composition and the excess coating composition; and recycling the mixed paper coating composition in the circulation system; wherein a portion of the excess coating composition is separated from the circulation system, subjected to centrifugation in a centrifuge to produce a product stream having a reduced concentration of particulate matter, and the product stream returned to the circulation system. In accordance with a fourth aspect of the present invention, there is provided a paper coating machine which comprises:

(a) a coating applicator for applying a paper coating composition to a surface of a web of paper;

(b) a collector situated to collect excess paper coating composition from the coating applicator for recycling to the coating applicator;

(c) a recycle loop having an upstream end in fluid communication with said collector and a downstream end in fluid communication with the coating applicator for delivering recycled coating composition to the coating applicator; (d) one or more centrifugation devices situated in the recycle loop or a branch thereof, for treating at least a portion of the recycled coating composition to remove particles greater than 20 μm therefrom and returning the treated coating composition from which the said particles have been removed to the recycle loop for delivery to the coating applicator; and

(e) a supply line for supplying a source of fresh coating composition for mixing with the treated recycle coating composition prior to delivery to the coating applicator.

Brief Description of the Drawings In the drawings, which are provided by way of illustration only, Figure 1 is a schematic diagram of a paper coating apparatus according to the present invention.

Detailed Description of the Invention

In a typical paper coating operation, a coating composition is supplied in excess to a coating applicator where it is applied, in a manner known per se, to the surface of a continuous web of paper (this term also including "board"). The excess coating composition not applied to the paper surface is recycled to the coating applicator via a circulation system for re-use, usually in combination with fresh coating composition. However, it has been found that this can lead to the build up of oversize particles in the paper coating composition which is supplied to the coating applicator for coating onto the paper, leading to wear in the applicator and possible damage to the paper surface. By subjecting a portion of the excess coating composition to centrifugation in accordance with the present invention, a proportion of larger particles is removed and an improvement in runnability of the paper coating operation is achieved, both in terms of reduced wear in components of the coating applicator and also in the quality of the coated paper obtained.

The amount of the excess coating composition which needs to be treated in the loop and the degree of centrifugation required in order to achieve the improvements referred to will vary between machines. However, in general, the portion of the excess coating composition to be treated and the centrifugation conditions applied should be such that the amount of +20 μm particles in the coating composition recycled from the coating applicator before a portion is fed to the one or more centrifuges is maintained at a stable level, not exceeding 1200ppm, or not exceeding l,000ppm or not exceeding 800ppm or not exceeding 600ppm.

In the present invention, the skilled person will understand that the centrifugation step removes particles across the whole range of particle sizes in the paper coating composition, including particles which are smaller than 20 μm. Indeed, the inevitable removal of particles smaller than 20 μm is important in obtaining good runnability of a paper coating machine (reduced damage to the coating applicator and the coated paper), since the particles smaller than 20 μm (e.g. in the range 5 to 20 μm) are damaging in the paper coating process. The 20 μm dimension by which the process of the present invention is defined above represents a convenient dimension which is measurable accurately across a wide range of paper coating compositions using methods well known in the art. Further, removal of substantial amounts of the particles larger than 20 μm from the recirculating paper coating composition correlates well with reduced damage to the coating applicator and the coated paper.

It is to be further understood that removal of all of the potentially damaging particles from the recirculating coating composition is not essential to the successful operation of the present invention. A finding underlying the present invention is that, in a given paper coating operation, the damaging consequences on the paper coating machine and the coated paper arise from the rapid build-up of larger particles in the recirculating paper coating composition above a certain threshold. Typically, this may be evidenced by the build up of particles larger than 20 μm. In particular, the inventors found that by using a centrifuge to continuously remove a proportion of the particles larger than 20 μm from the recycled coating composition, the absolute level of particles larger than 20 μm delivered to the coating applicator (including the fresh make-up coating composition) could be maintained below the empirically determined threshold which correlated with an improved runnability. This threshold will be dependent on the particular paper coating machine (specifically the gap in the coating applicator) and the paper coating composition in use. Thus, the absolute amount of particles larger than 20 μm which is effective to give good runnability in one machine (for example 1,000 ppm by weight) may not be the same as another machine, or where a different paper coating composition is used on the same machine. In practicing the present invention using a particular paper coating machine and paper coating composition, therefore, the skilled person will need to determine, by routine experimentation, the precise centrifugation conditions to be used as well as the proportion of the excess coating composition collected from the coating applicator which is to be treated in the centrifugation step, which avoid a rapid build up of large particles and provide reduced damage to the coating applicator and the coated paper. By way of guidance, however, a beneficial reduction in blade wear is found when the flow rate to the centrifuge matches the flow rate onto the web of paper and the reduction in the amount (by weight) of +20 μm particles in the feed to the centrifuge is 90% or more. A flow rate to the centrifuge which is higher than the flow rate onto the paper web will require a lower efficiency of separation for the same effect and a flow rate to the centrifuge which is lower than the flow rate onto the paper web will require a higher efficiency of separation for the same effect.

This may be generally expressed using the following equations in which the rate of flow (F) of excess coating composition to the centrifuge is determined in accordance with the following relationship, where C is the rate at which coating composition is applied to the web of paper and R is the removal efficiency (expressed as a percentage) of particles larger than 20μm:

F = (C x 90)/R

If the rate of flow of excess coating composition from the coating applicator is E, the portion (P) of the excess coating composition to be treated in the centrifuge is F/E, and hence

P = (C x 90)/(R x E).

As discussed below, the excess coating composition to be treated in the centrifuge may be diluted to facilitate the centrifugation. The afore-mentioned calculations should be carried out on an "undiluted" basis.

In general it is desirable to operate the centrifuge at the highest possible efficiency as this requires a smaller proportion of the excess coating composition to be treated in the centrifuge, which leads to economic advantages in terms of the size of centrifuge required. Thus, for example, the centrifuge may be operated such that at least 95 wt% of the +20 μm particles removed, or such that at least 98 wt% of the +20 μm particles are removed, or such that at least 99 wt% of the +20 μm particles are removed. In these circumstances, the amount of the excess coating composition fed to the centrifuge or centrifuges may be correspondingly reduced. There may be situations where the centrifuge can not be operated effectively to remove 90% or more of the +20 μm particles. In this case, the centrifuge may be operated such that at least 70 of the +20 μm particles removed, or such that at least 75 wt% of the +20 μm particles are removed, or such that at least 80 wt% of the +20 μm particles are removed, or such that at least 85% of the +20 μm particles are removed. In these circumstances, the amount of the excess coating composition fed to the centrifuge or centrifuges may be correspondingly increased.

In embodiments of the invention, the excess coating composition is not subjected to a screening operation, that is to say an operation where the excess coating composition, or a portion of it, is passed through a screen or mesh in order to screen particles above a certain dimension.

As stated above, it is not essential, in the operation of the process of the present invention, for the entire volume of excess coating composition from the coating applicator to be treated, although there may be embodiments in which the entire volume is so-treated. Instead, it has been found that a beneficial outcome is obtained when only a portion of the excess coating composition is treated. Furthermore, regarding the degree of separation of oversize particles, as represented by particles larger than 20 μm, in the centrifugation step, this is controlled, in conjunction with the amount of the recycle coating composition to be treated, in order to give the desired outcome in terms of the runnability of the paper coating machine. Thus, a balance will exist between the proportion of the excess coating composition to be treated, and the degree of separation of oversize particles carried out, which balance can be determined by monitoring the build up of oversize particles in the coating loop. In the event that oversize particles continue to build up over time, the removal of oversize particles is too low, and either the amount of the recycle coating composition to be treated should be increased, or the degree of separation of the oversize particles should be, or a combination of both. On the other hand, if the amount of oversize particles in the coating composition reaches a steady state which is acceptable from a runnability point of view, then a suitable balance has been obtained. By way of example, the portion of the excess coating composition treated may vary between 1 to 20% by volume (based on the proportion of the stream being treated), for example, 2 to 10% by volume, or for example from 3 to 8% by volume. In embodiments of the invention, the paper coating composition is treated using one or more centrifuge either in series or parallel, which may be situated in the recycle loop of the paper coating machine, or a branch thereof. The treated paper coating composition may be mixed with a portion of the excess coating composition which has not been treated and/or with fresh coating composition, before being re-introduced to the coating head.

The centrifugation treatment step of the methods of the present invention may be carried out using one or more decanter centrifuge devices, also known as a continuously discharging decanter centrifuge. The decanter centrifuge is a well known device which is used on an industrial scale to effect a separation on a suspension of a solid in a liquid, which in the present invention is the excess paper coating composition from the paper coating head. Typically, the bowl of the centrifuge is spun at high speed, so that solids in the suspension accumulate at the walls of the bowl from where they are removed as a "residue". The liquid phase from which solids have been separated is, in the present invention, the treated paper coating composition having a reduced amount of oversize particles, and is destined to be recycled to the paper coating head for re-use.

The precise dimensions of the one or more centrifuge used in the methods of the present invention may be determined by the skilled person depending on the volume of material which is to be treated, as will the precise operating conditions of the centrifuges to achieve the desired degree of separation. Typically, the centrifuge used will perform at between 1,000 x g and 4,000 x g.

In the centrifugation treatment step to reduce the amount of particles in the recycle coating composition, the viscosity of the coating composition may be adjusted to facilitate the separation operation. Normally, because the coating composition is highly viscous as a consequence of the presence of the coating pigment and binder (typically a latex), this adjustment in viscosity is a reduction in viscosity, which may be achieved by a dilution of the coating composition with an aqueous medium, typically water. Of course, the minimum amount of dilution necessary to achieve the desired viscosity reduction is preferred, since after treatment the viscosity will need to be re-adjusted (as discussed below) before reuse. After treatment, the excess coating composition is recycled back to the coating head. Whilst it may be possible to store the treated coating composition and re-use it in batches, normally there will be a continuous process in which excess coating composition is treated and then re -used in a closed coating colour loop. Before re-use, the treated excess coating composition is mixed with fresh coating composition and also with untreated excess coating composition, in those embodiments where only a portion of the excess coating composition has been treated. The formulation of the fresh coating composition may be selected so that it has a viscosity (solids content) higher than the desired viscosity at the coating head so that the viscosity of the mixed composition obtained by mixing with the treated recycled composition (and untreated composition) is the desired viscosity and solids content for operation of the coating head.

The nature of the coating composition used in the present invention is not critical. That is to say, the present invention may be used in respect of diverse paper coating operation using a variety of different types of paper coating compositions. However, typical coating compositions comprise an aqueous suspension of the coating pigment and a binder together with other typical components such as rheology modifiers, thickeners, optical brighteners, cross- linkers, dyes, biocides, insolubilizers, or pH control agents. The coating pigment may be any pigment known in the art of paper coating, including, for example, a kaolin clay or a calcium carbonate pigment, or titanium dioxide, or gypsum or calcined clay, or a mixture thereof. Typically, calcium carbonate and kaolin clay may be used in amounts up to 100 wt%, whilst calcined clay, gypsum and titanium dioxide are used in amounts of up to 20wt% in blends with another pigment. The solids content of the coating composition is typically 45-75wt%.

The amount of binder used is usually at least 4pph. Typically the binder is a latex or starch or PVOH.

With regard to the paper coating machine used in the present invention, this is not critical, and the present invention can be used in all manner of machines in which there is an excess of paper coating composition applied at the coating applicator, and a recycle loop included for re-use of the excess. For example, the paper coating applicator may either be of the stationary element or metered size press (MSP) as mentioned in the background section herein. The slit width is not critical. However, the problems of wear and paper damage are more noticeable where the slit width is narrow, for example less than 10 μm, although this may vary depending on the coat weight used and the amount of solids. Measurement of the amount of particles larger than 20 μm may be determined using a sieving method employing a sieve having apertures of nominally 20 μm (635 mesh British Standard Certified). A suitable method is set forth in Appendix A.

A schematic diagram of a paper coating apparatus is shown in Figure 1. The apparatus comprises two paper coating stations 1,2 each of which applies a paper coating composition to a moving web of paper (not shown). Each station 1,2 is supplied with paper coating composition from a recycle tank 3a, 3b which is situated in a loop with the paper coating station. A branch 4a, 4b in each loop, downstream of the paper coating station, leads to a single feed tank 5 which feeds a decanter centrifuge 6. The decanter centrifuge 6 is operated to remove a portion of the oversize particles as described in more detail herein. The stream from which oversize particles has been separated is returned to the recycle tanks 3a, 3b. Additional fresh coating composition is also fed to the recycle tanks 3a, 3b from make down tanks 8a, 8b and feed tanks 7a, 7b located outside the main coating colour loops.

In an example of a paper coating operation using the apparatus of Figure 1, each coating station may be supplied with coating colour at a rate of 15 1/s from the recycle tank. An amount of coating colour of about 1 1/s may be applied to the web of paper at each station, and thus 14 1/s is returned in the loop in the direction of the recycle tank. A side stream of 1 1/s is taken from the main coating colour loop and is diluted with water (not shown) and fed to the decanter centrifuge, where it is treated to remove oversize particles. The coating colour is returned to the recycle tank and made up with 1 1/s of fresh coating colour from the make down tank. Coarse residue particles from the centrifuge are collected as a waste stream.

In a typical operation of the centrifuge, the feed from the recirculation line from the coating station may contain about 58% solids. The side stream may be diluted to a solids content of about 44% and then treated in the centrifuge to remove 90% of the particles larger than 20 μm. A residue to waste having a solids content of about 68% may be obtained together with a product stream having a solids content of about 42%, which is returned to the holding tank. In order to adjust the solids content of the coating composition back to the desired value of 58%, the solids content of the make up coating colour may be adjusted as necessary.

A paper coating operation was carried out using a paper coating machine of the type generally shown in Figure 1, under the operating conditions stated above for this machine, with the exception that, on Day 13, the centrifuge was not operational or was operating intermittently. The fresh coating composition used was one comprising:

Samples were taken for analysis over a time span of a month and a half, and analysed for the amount of coarse material in the -45 +20 μm range and in the +45 μm range. The results obtained are set forth in Figures 2 and 3 respectively.

The +45μm fractions was found to contain mainly fibre, the remainder being 'colour mix' and quartz/minerals along with traces of magnetics. The -45+20μm fractions contained mainly colour mix with the remainder consisting of quartz and traces of fibre and magnetics. Although all samples contained magnetics there appeared to be less in the samples collected in later batches.

From Figures 2 and 3 it can be seen that initial values are low in the first 24 hours with all values of -45+20 μm less than lOOppm (average = 62). A slight upward trend follows for the remainder of the testing period with an average of -45+20μm = 268ppm. During this time the blade wear was reported to be acceptable and manageable. On Day 13, the centrifuge was not operational or was operating intermittently. On this day, the amount of residue in the coating colour at Station 1 peaked dramatically. The amount of residue at Station 2 was found to be low, which is the expected result since Station 2 applies paper coating composition to an already coated sheet of paper, and thus the base paper is no longer a potential source of coarse, deleterious particles.

A one-off long term check sample for both coating stations at Day 49 showed levels below the average for the whole at 222ppm.

Overall, the use of the centrifuge has contributed significantly in keeping the residue at consistent and therefore manageable levels.

Selected coating colours having predetermined amounts of +20 μm residue were studied in a laboratory-scale wear study using a laboratory paper coating machine and a standard reel of base paper. Blade wear was measured accurately and the results obtained are set out in Figure 4. These results indicate that levels of '-45+20um residue' above 500ppm will lead to a significant increase in wear of contact parts.

Appendix A

Method for the determination of the concentration of particles larger than 20 micron in the coating composition

The following method is a method for determining the concentration of particles larger than 20 micron in the coating composition by measuring the +20 micron residue which is retained on a sieve with apertures of nominally 20 microns (635 mesh British Standard certified) when a dispersed slurry is passed through a sieve.

Sample Size

The sample size should ideally be within the range of 100 g to 1000 g.

Apparatus Required

British Standard sieve; aperture nominally 20 microns BS 410-1 (ISO 3310-1), available from Endecotts Limited, London, England. Disperser; ultrasonic bath or ultrasonicator (bath or probe), e.g. Vibra Cell, Sonic

Materials Inc., Kenosia Avenue, Danbury, Connecticut, USA. Dispersion vessels.

Balance; capable of weighing 1 kg ± 0.1 g. Balance; capable of weighing 150 g ± 0.001 g.

Hotplate or oven; capable of maintaining a temperature of 8O ⁰ C ± 5 ⁰ C. Measuring cylinders; 100 ml, 1 litre and 2 litre. Beakers; various sizes. Stirring rod. Small brush.

Reagents Required

A source of clean water.

Proprietary wetting agent (e.g. Surfynol TG).

Proprietary defoamer (e.g. Foamaster NDW). Method

1. Mix the slurry to homogeneity with either the bench stirrer or rotating rollers, or by shaking the container and contents vigorously.

2. Measure the solids content ¹ . Calculate the volume required to give the required sample weight.

3. Select a test portion and measure its volume. If the slurry is a product of a known solids specification, assume it to have a solids concentration equating to the mid point of the specification range.

4. Add this volume of slurry to a beaker.

5. Record the weight of solid material contained in the test slurry to within 0.1 g.

6. Carefully pour the slurry from the beaker onto the sieve a little at a time and wash the slurry through, taking care to avoid flooding it.

7. Rinse the beaker to ensure complete transfer of the test material.

8. Continue washing until the washings are visibly clear.

9. Transfer the oversize fraction to a 150 ml beaker, using a wash bottle to wash the sieve, ensuring complete transfer. Dilute the contents of the beaker to approximate 75-100 ml and disperse using one of the following methods: a. Ultrasonic probe 40 % amplitude (pulsed) setting for 3 minutes ± 10 seconds. (Processor model: Sonics & Materials VCX500, Probe model: CV33). b. Ultrasonic bath - sonicate for 4 minutes ± 10 seconds at the maximum setting.

If an ultrasonic bath is being used, the sample should be contained in a glass beaker.

10. Carefully pour the dispersed sample from the beaker onto the test sieve and re- wash it through the sieve.

11. Observe the amount of foam that is apparent on the sample residue and if approximately 25% or less remains from that of the original, wash the

¹ The solids content is defined as the percentage mass of material remaining after the sample has been dried to zero moisture under conditions at which the pigment is stable. residue from the sieve into an evaporating dish, ensuring minimal but sufficient rinsing to ensure complete transfer.

NOTE: As the amount of foam generated can vary from sample to sample, if significant levels still remain, it is likely to include some hydrophobic material, therefore add sufficient drops of wetting agent (Surfynol TG) and Defoamer (Foamaster NDW), stirring adequately to ensure complete mixing and continue from 6, repeating the process as required, until approximately 25% or less of the total foam originally identified in 11, remains in the test sample.

12. Dry the dish and its contents on a hot plate or in an oven. NOTE: If using a hotplate, place the dish on a ceramic support. Do not place it directly on the hotplate as 'spitting' may occur.

13. When dry, remove the dish and its contents and allow to cool.

14. Brush the residue from the dish onto a tared watch glass.

15. Weigh and record the weight of the residue to within 0.001 g.

Expression of Results

The percentage mass of material larger than 20 micron M is calculated as follows:

M = W2 x 100 Wl where Wl is the dry weight equivalent of sample taken recorded in 5. W2 is the weight of residue recorded in 15.

Precision

For a test material with a +20 micron residue content of 0.0025 % (25 ppm) the standard deviation is 0.0005 % (5 ppm).

Equipment Check and Calibrations

Equipment should be controlled to be ISO9001 compliant.