WO2008154961A1

WO2008154961A1 - A method and a mill system in connection with coating and filling a web

Info

Publication number: WO2008154961A1
Application number: PCT/EP2007/056212
Authority: WO
Inventors: Riku Pihko; Petri Pihko; Teemu Turunen
Original assignee: Metso Paper Oy
Current assignee: Valmet Technologies Oy
Priority date: 2007-06-21
Filing date: 2007-06-21
Publication date: 2008-12-24
Anticipated expiration: 2009-12-21
Also published as: DE112007003516T5

Abstract

The present patent application relates to a web coating process, a coated web obtainable by the web coating process, a web coating device as well as a web mill system comprising said web coating device, wherein precipitated calcium carbonate is manufactured during the web coating process or in the web coating device.

Description

DESCRIPTION

A METHOD AND A MILL SYSTEM IN CONNECTION WITH COATING AND

FILLING A WEB

Conventional web mill systems such as paper mills for coated or uncoated printing and writing paper have to deal with producing good quality web products. In the recent years, the brightness, opacity and printability of coated or uncoated paper have been obtained by replacing the traditional kaolin clay, titanium dioxide and calcium clay with precipitated calcium carbonated, thus significantly improving the paper or web quality.

Precipitated calcium carbonate (PCC) is a widely used mineral pigment for paper production. It is valued for its high brightness and light-scattering characteristics and paper filling and coating applications. Generally, PCC is produced through a reaction process that utilizes quicklime (CaO), water and carbon dioxide. The reaction produces very pure carbonate crystals and water. The crystals can produce a variety of shapes and sizes, depending on the specific reaction process that is used.

Usually, PCC is manufactured in an offsite plant and is, after dewatering by the merchant plant, shipped to the paper mill. The combined costs of dewatering and transporting are high. Thus, satellite PCC plants on sites of the web mills using local quicklime and carbon dioxide have been suggested in recent years. Examples of such satellite PCC processes are described in DE 10 2005 015 490 Al and WO 2005/005 726 Al.

Furthermore, usual sources of CO2 used for PCC production according to the prior art are stack gas of a power plant, recovery boiler or lime kiln. However, the power plant flue gas contains many components which are harmful for the PCC production. For example, the power plant flue gas comprises sulfur dioxide (SO2) , nitrogen oxides (NO_x) , carbon monoxide (CO), hydrocarbons (C_xH_y) and particulates. Recovery boiler flue gas contains many components which are harmful for PCC production such as sodium sulfate and sodium carbonate with small amounts of chlorides and potassium. Lime kiln flue gas contains many components which are harmful for PCC production such as sulfur dioxide, TRS, dust, and in many countries today nitrogen oxides. TRS emission consists primarily of hydrogen sulfide .

Impingement or air flotation dryers using natural gas combustion in web mills such as paper mills produce carbon dioxide which is generally blown into the atmosphere. In recent years, however, it has been noticed that carbon dioxide is causing a greenhouse effect and, thus, should significantly be reduced. For the time being, the carbon dioxide containing exhaust gas of such dryers in web mill systems are not utilized economically profitable in web making processes such as paper, board, and tissue-making processes .

A further disadvantage of the conventional web mill systems is that the heat exchangers have a quite low efficiency and most of the thermal energy remains unutilized, even though the carbon dioxide containing exhaust gases are let to the outside through heat exchangers.

In the light of the above described drawbacks of the conventional processes in web mill systems, the inventors of the present patent application searched for possibilities to utilize the carbon dioxide contained in the exhaust gases of impingement and air flotation dryers economically and to find environmentally friendly processes. In addition, the inventors sought for web coating processes for producing coated webs having an improved quality.

According to a first aspect of the present invention, the above-mentioned problems are solved by a web coating process comprising a step of manufacturing precipitated calcium carbonate.

Preferably, the web coating process according to the first aspect comprises the steps of: a) coating a web with calcium hydroxide, b) blowing carbon dioxide containing gas onto the web surface, thus precipitating calcium carbonate onto the surface of the web, and c) optionally drying the obtained precipitated calcium carbonate on the web with heat.

The web coating process according to the first aspect of the invention may preferably be a paper or board coating process .

The calcium hydroxide coating on a web may preferably be in the form of a solution or paste, optionally in admixture with carbon dioxide. Binders and/or surface size and/or coating additives such as coating chemicals or particles optionally may be co-precipitated in the coating step a) .

Preferably, the carbon dioxide containing gas used in the web coating process according to the first aspect of the present invention may be an exhaust gas from gas burning dryers of a paper or board mill, and more preferably from impingement dryers or air flotation dryers.

The web coating process of the present invention may additionally comprise a recycling step for the carbon dioxide containing gas.

The second aspect of the present invention relates to a coated web obtainable by the web coating process of the first aspect.

The coated web according to the second aspect of the present invention may be alternatively characterized in that acidic residues of the web are substantially neutralized by the reaction with calcium hydroxide or carbon dioxide at least in the vicinity of the web surface during the steps a) and/or b) .

The third aspect of the present invention relates to a web coating device comprising at least means for manufacturing precipitated calcium carbonate.

In a preferred embodiment thereof, the web coating device comprises a means for coating a web with calcium hydroxide, a means for blowing carbon dioxide containing gas onto the web surface, and optionally a means for drying the obtained coated web. The web coating device according to the third aspect of the invention may further comprise a means for guiding exhaust gas from impingement dryers or air flotation dryers to the means for blowing carbon dioxide containing gas .

In a preferred embodiment of the third aspect of the invention, the means for manufacturing precipitated calcium carbonate comprises a closed web run and/or a web transfer portion confined from the outer atmosphere. Preferably, this web coating device comprises an impingement hood containing said means for blowing carbon dioxide containing gas and intake nozzles for recycling residual carbon dioxide .

The web coating device according to the third aspect of the invention may further comprise a spray coater, blade coater, roll coater, or size press as means for coating the web with calcium hydroxide.

When PCC is directly made on the web, it is possible to use both supported and unsupported web runs :

A supported web run is used until the end of the drying section (the wire/press/dryer sections) , preferably the last section thereof, in combination with the impingement dryers. The supported web run is preferably used near by or more preferably after a coating step.

An unsupported web run is already used in coating/size press solutions of the prior art. This is an important solution for making PCC on the web. According to the present invention, before coating, the web is dried to minimize the moisture variations and make it more resistant to open draws of unsupported web runs. With unsupported web runs, the use of air flotation dryers for simultaneous drying of both sides of the web is preferred. The above unsupported arrangement is possible with paper and/or coating machines in which the PCC is generated online, and with paper and/or coating machines machines in which the PCC is generated off-line and then coated on the web.

In the preferred embodiment of the coating device according to the third aspect of the present invention using a supported web run, the web may be supported by a metal belt or polymer band which is provided at least in the coating zone for the calcium hydroxide and/or the carbon dioxide as a back-side supporting member for the web.

The fourth aspect of the present invention relates to a web mill system comprising a unit for manufacturing precipitated calcium carbonate. Preferably, the web mills system comprises a reactor unit for manufacturing a precipitated calcium carbonate and a means for guiding exhaust gas from impingement dryers or air flotation dryers to said reactor unit. The web mill system according to the fourth aspect of the present invention may comprise the web coating device according to the third aspect of the present invention as that unit for manufacturing precipitated calcium carbonate.

In an alternative embodiment of the fourth aspect of the present invention, PCC prepared from CO2 generated as exhaust gas from impingement dryers or air flotation dryers of the mill can be added as a filler of the paper web or as a component of a coating paste in a conventional coating device. Then, the advantage of this web mill system is that the site of the production of the PCC is near by or on the site of the web mill (for example within a paper and/or coating machine) . Only short pipes are needed from the CO2 source (generally from impingement dryers or air flotation dryers of the mill) to the PCC generating site and to the web mill system using the PCC. Thus, this alternative embodiment leads to reduced costs, a lower environmental pollution and a better product as described above.

In the above mentioned embodiment, the PCC used in the form of a filler mill solution is preferably fed to the paper machine raw material before the head box and the coating PCC is preferably fed to the coating paste before the coating device.

Furthermore, when the PCC is directly produced from exhaust gas from impingement dryers of air flotation dryers of a paper mill, the PCC production amount is in relation to the paper production amount because the more paper is produced and dried with gas burning dryers the more PCC is produced due to a higher amount of available CO2. Thus, according to the mill system of the present invention the amount of PCC needed and generated can easily be adjusted dependent on the amount of paper to be produced.

Further preferred embodiments, combinations of features, modifications and related advantageous effects are described in the following.

The web coating process according to the present invention comprises a first step of coating a web with calcium hydroxide, optionally binders, surface size and coating additives such as usual coating chemicals or particles by means of a common coating method. For example, the calcium hydroxide can be in a paste or solution and can be coated on the web by means of a spray coater, blade coater, roll coater, or size press. The term solution means a real solution or a slurry of the respective compounds in a suitable solvent usually used in web coating processes. A preferred example of such a suitable solvent is water. The calcium hydroxide may be prepared from CaO received from a pulp mill or bought directly before the coating step. The reaction of the preparation of calcium hydroxide can be expressed by the following equation:

CaO + H₂O = Ca(OH)₂

The slurry dryness of the calcium hydroxide solution or paste, when applied on the web, may be adjusted within the range of 10 to 75%, preferably 40 to 70%. The calcium hydroxide may optionally be mixed with carbon dioxide, binders, dispersants or conventional additives etc.

The solution or paste can preferably contain latex as a binder in the range of 5 to 20% of the weight of the coating pigment.

During the coating of the calcium hydroxide on the web, the temperature of the calcium hydroxide solution or paste may preferably be between 20 to 100⁰C, more preferably about 35 to 55°C.

In a further preferred embodiment of the present invention, carbon dioxide added for the precipitation of calcium carbonate can be mixed with the calcium hydroxide already before putting the mixture on the web. In this case, the temperature of the carbon dioxide should not increase the temperature of the coating materials. If latex is used as the binder, the temperature range should not be above 35 to 55°C because latex works in this temperature range well in the coating materials circulation process. However, this is no restriction in the case of the preparation of PCC on the surface of the web. The pressure conditions during the application of the calcium hydroxide solution or paste on the web (step a) ) is the highest when the coating is put on the web in the coating nip formed by rolls or blades used in the respective coating method. The pressure is near the blade tip preferably adjusted to be about 120 kPa (dwell time of 0.01 ms) . The pressure in the roll nip of a size press is preferably adjusted to be about 400 kPa (dwell time 1 to 4 ms) .

In the step of blowing carbon dioxide gas onto the web surface, the temperature of the carbon dioxide can preferably be increased in order to have a fast PCC reaction. According to a preferred embodiment of the invention, the carbon dioxide produced in gas dryers of a web mill system is already at a higher temperature and thus, a fast PCC reaction can be carried out without an additional heating step of the carbon dioxide. Thereby, water from the web may simultaneously be evaporated during the precipitation reaction due to the heat energy of the heated carbon dioxide gas.

In addition, the PCC reaction as shown by the following equation produces heat energy which can suitably be used for increasing the reaction rate of the PCC reaction:

Ca ( OH ) ₂ + CO₂ = CaCO₃ + H₂O + 1 12 , 6 kJ/mo l

Thus, the water evaporation from the web during this step is further intensified by the PCC reaction. Thereby, the heat energy contained in the carbon dioxide containing gas and produced during the PCC manufacturing reaction can efficiently be utilized for increasing the drying of the coated web. Therefore, instead of wasting most of the heat energy of the carbon dioxide exhaust gases of a web mill, the process of the present invention allows the use of this energy in the web coating process. Thus, an additional drying step of the precipitated calcium carbonate on the web with heat is only optional, if the dryness of the coated web is not sufficient.

If necessary, an additional drying step of the coated web with heat may be applied in the process of the invention.

Hence, according to the invention the harmful carbon dioxide produced in gas dryers of a web mill is economically transformed into PCC directly on the web surface. There is no need for a separate PCC plant such as a satellite plant on the web mill location.

In addition, the exhaust gas from gas dryers of a web mill is clean compared to the exhaust gas of power plants. For example, usual sources of carbon dioxide used for PCC production according to the prior art are stack gas of a power plant, recovery kiln or lime kiln. These gases contain many components which are harmful for the PCC production such as sulfur dioxide (SO2) , sodium sulfate (Na2SO₄) , sodium carbonate (Na2CC>3) , nitrogen oxides (NO_x) , carbon monoxide (CO) , hydrocarbons (C_xH_y) , chlorides, and particles. In contrast thereto, the exhaust gases produced in the web mill system such as in impingement and air flotation dryers of a web mill do not contain these harmful components or contain them only in low or trace amounts. Due to the low sulfur and smoke content of the high quality carbon dioxide available from natural gas burners, the PCC obtained from these gases will be whiter, cleaner and more homogenous .

Generally, gas refiners are used for cleaning the gases before the use in the PCC manufacturing. According to the present invention, cheaper gas refiners or no refiners are necessary. Simultaneously, cleaner PCC can be obtained at lower costs compared to the conventional techniques.

The combustion process in the gas burner is homogenous and therefore the composition of flue gas is homogenous, which is good for the PCC production directly on the web surface, because homogenous flue gas produces homogenous and good quality PCC.

Therefore, it is possible to rapidly adjust grade changes of the PCC, because the PCC manufacturing process according to the present invention can be tuned very fast to be suitable for different grades.

The direct PCC production reaction on the web surface is expected to be very rapid as a result of a large surface area. The reaction stoichiometry can be controlled by adjusting the pressure, velocity and the contact area of the carbon dioxide feed. Furthermore, the drying of the web coating is very fast due to the increased temperature of the coating on the web surface by the reaction heat of the PCC reaction.

In a particular preferred embodiment of the process of the invention, the temperature conditions during the step (b) are adjusted to be as high as 400⁰C. The velocity of the carbon dioxide gas when blown on the web is up to 100 m/sec. The pressure during the blowing of the carbon dioxide is adjusted within the range of 0 to 5 kPa. It is also possible to arrange an extended nip to the coating process or to apply a spray coating.

If necessary, the drying of the obtained precipitated calcium carbonate on the web is usually done with heat after the step of PCC precipitation. In a further preferred embodiment of the present invention, the web is supported by a metal belt or a polymer band which is provided at least in the coating zone for the calcium hydroxide and/or the carbon dioxide as a back-side supporting member for the web. Optionally, this metal belt or polymer band can be heated in order to speed up the drying of the precipitated calcium carbonate on the web from the back-side of the supporting member. The supported web run is especially suitable in case the web is still weak or too weak for the use of an unsupported web run.

In a further preferred embodiment, the coating of the web with calcium hydroxide and carbon dioxide is carried out in a closed web run and/or web transfer portion confined from the outer atmosphere in a suitable web coating device. Thus, it is easier to recycle non-reacted carbon dioxide gas from the confined inner atmosphere of the web coating device. This recycling of residual carbon dioxide makes the process more economic and environmentally preferred.

The pressure difference between the inner atmosphere and the outer atmosphere can, for example, be adjusted such that a leakage of the carbon dioxide from the inner atmosphere is minimized without any special sealing means. Similar means as in impingement drying devices can be used.

The inventors have found that the use of precipitated calcium carbonate produced with the novel coating method directly on the web surface improves the web brightness, gloss, opacity, smoothness, strength and decreases production costs and blistering tendency. This appears to be due to the fact that the PCC forms controlled a smooth, white layer with an appropriate porosity on the web. PCC manufactured at a web mill from the CO2 produced at another site of the mill, wherein the CO2 is a greenhouse gas and generally produces emission trade costs to the mill, is a much cheaper raw material (e.g. filler material) compared to the commonly used PCC and granulated calcium carbonate .

In the web coating process of the present invention, calcium hydroxide penetrates at least to the web surface and neutralizes acid components of the web and produces a web, which has good aging properties, a good strength and color over many decades. Thus, an ideal coated web for archiving purposes, libraries etc. can be produced according to the process of the present invention.

Furthermore, it is expected that the coated webs obtainable by the process according to the present invention need less consumption of PCC for the same properties or better/more even properties with the same amount of PCC. Thus, in order to produce standard products, raw materials can be saved. Thus, this new process yields to standard products, whereby even 5 to 10% less use of costly bulk filler/coating raw material is necessary. This significantly reduces the production costs, wherein the same properties ("standard products") are achieved with the novel product.

With regard to the coated web obtained by the process according to the present invention, it is now possible to direct CO2 flow through and on a calcium hydroxide layer by applying a higher flow velocity, temperature, and pressure to create an appropriate PCC particle structure suitable for the respective web grade. Because the PCC is generated on the web surface (during the reaction the PCC structure is strongly attached to the other web components and other PCC particles) , the new PCC structure can improve strength properties of the web. Moreover, the precipitated calcium carbonate particles/crystals will fill the voids in the web in a way that a normal PCC application process cannot do.

Thus, the end product, the coated web (such as a coated paper or card board, etc.) has better physical properties than a web according to the prior art. Especially, by introducing the PCC directly by a controlled precipitation reaction on the web, the precipitation process on the web can be controlled and adjusted much more rapid than by switching over two different PCC products.

In the light of the above, it can be concluded that the new coating process allows at least the following benefits:

Due to the higher strength properties of the web, it is possible to reduce the amount of expensive strength increasing fibers (soft wood craft) . Thus, raw materials can bee saved. Therefore, the production costs can be lowered. Alternatively, it is also possible to leave the surface sizing process away, thus reducing the investment and production costs. In addition, it is possible to create a web surface with less web surface adjusting, thus improving the web quality.

Due to the PCC concentration on the surface of the web during the coating process, the PCC is at the location where it is supposed to be, i.e. on the surface of the web. Therefore, it is possible to decrease the amount of PCC or other filler particles put to the web through the head box. The improved web surface properties by the use of less filler particles also lower the raw material production costs . Additionally, due to the low sulfur and smoke content of the high quality carbon dioxide used for the precipitation of calcium carbonate which is available from natural gas burner or impingement or air flotation dryers, the PCC will be whiter, cleaner and more homogenous compared to currently most easily used PCC grades produced by the CO2 from power plants, recovery boiler or lime kiln.

Hence, the coated web according to the present invention has indeed better characteristics than the coated webs produced from PCC added as external filler during the coating process.

The web coating device according to the present invention comprises at least a means for manufacturing precipitated calcium carbonate directly on the web surface. In a preferred embodiment, the web coating device comprises a means for coating a web with calcium hydroxide, a means for blowing carbon dioxide containing gas onto the web surface and optionally a means for drying the obtained coated web. Preferably, the carbon dioxide containing gas is produced in impingement dryers or air flotation dryers and guided by a means for guiding exhaust gas from impingement dryers or air flotation dryers to the means for blowing carbon dioxide containing gas.

In a particularly preferred embodiment of the present invention, the means for manufacturing precipitated calcium carbonate comprises a closed web run and/or a web transfer portion confined from the outer atmosphere. Confined in the meaning of the present invention means that the web run and/or the transfer portion part in which the coating of calcium hydroxide or the blowing of carbon dioxide is carried out under an atmosphere which is confined from the outer atmosphere. This can, for example, be achieved by providing an impingement hood where there is a means for blowing carbon dioxide on the web and intake nozzles for recycling non-reacted carbon dioxide and removal of other exhaust gases such as evaporated water. By a pressure difference between the hood and the air of the machine hall (outer atmosphere) , the leakage of the carbon dioxide gas can be minimized without any special sealing means.

In a further preferred embodiment of the web coating device according to the present invention, a metal belt or polymer band is provided on the back-side of the web during the direct PCC coating process as a back-side supporting member for the web. The use of such a metal belt as a back-side supporting member for the web during the coating process is advantageous, because the product obtained has a hard, smooth, even surface. Thus, even coating layers having good web cross-directional and machine-directional profiles can be produced by the use of such a back-side supporting member .

Furthermore, if a heatable back-side supporting member is provided, a very high drying rate after the coating can be obtained. Thus, a short, profitable drying unit can be used in order to decrease the time required for drying and the costs for an additional drying means.

Additionally, when a good adhesion of the metal belt to the web is possible, no or less web stealing problems, no or less web breaks, a good web quality, the possibility to produce low weight and low strength webs can be mentioned. Thus, the production costs are lowered by the use of such a back-side supporting member.

Furthermore, many web making process steps on one metal belt can be carried out or combined such as pressing, drying, sizing, pre-calendaring, coating, after drying, calendaring.

In a preferred embodiment of the device of the invention, the coating of the carbon dioxide is carried out by means of an extended coating nip. After the coating of the calcium hydroxide layer on the web, a subsequent carbon dioxide containing impingement process can be carried out or both coatings can be carried out at the same time. Immediate CO2 containing impingement drying after spreading calcium hydroxide on the web is preferred due to an overall easier construction of the web coating device.

The web coating device preferably uses a spray coating technique for the coating of the calcium hydroxide and/or carbon dioxide, and optionally binders and other components on the web.

In the web mill system according to the present invention, the system comprises a unit for manufacturing precipitated calcium carbonate. Preferably, a reactor unit for manufacturing precipitated calcium carbonate and a means for guiding exhaust gas from impingement dryers or air flotation dryers to said reactor unit are provided, wherein the reactor unit is the web coating device according to the present invention or, alternatively, a separate reactor for generating PCC on site. Thereby, it is possible to use the carbon dioxide containing gas from the impingement or air flotation dryers as the carbon dioxide source of the PCC reaction process on the web or in the production of PCC as, e.g., filler material or coating solution. Generally, the drying section is located after the PCC reaction site on the web solution, preferably adjacent to it. By the above-described embodiments of the present invention, the above-mentioned object is properly solved and the present invention provides an economically and environmentally friendly process for web coating as well as a coating device for such a process.

Claims

1. Web coating process comprising a step of manufacturing precipitated calcium carbonate.

2. Web coating process according to claim 1 comprising the steps of: a) coating a web with calcium hydroxide, b) blowing carbon dioxide containing gas onto the web surface, thus precipitating calcium carbonate onto the surface of the web, and c) optionally drying the obtained precipitated calcium carbonate on the web with heat.

3. Web coating process according to claim 1 or 2, wherein the process is a paper or board coating process.

4. Web coating process according to any of the preceding claims, wherein the calcium hydroxide is in the form of a solution or paste, optionally in admixture with carbon dioxide .

5. Web coating process according to any of the preceding claims, wherein in the coating step a) binders and/or surface size and/or coating additives are co-precipitated.

6. Web coating process according to claim 5, wherein the coating additives are coating chemicals or particles.

7. Web coating process according to any of the preceding claims, wherein the carbon dioxide containing gas is an exhaust gas from gas dryers of a paper board mill.

8. Web coating process according to claim 7, wherein the carbon dioxide containing gas is an exhaust gas from impingement dryers or air flotation dryers.

9. Web coating process according to any of the preceding claims, wherein the process comprises a recycling step for the carbon dioxide containing gas.

10. Coated web obtainable by any one of the web coating processes of claims 1 to 9.

11. Coated web according to claim 10, characterized in that acidic residues of the web are substantially neutralized by the reaction with calcium hydroxide or carbon dioxide at least in the vicinity of the web surface during the steps a) and/or b) .

12. Web coating device comprising at least means for manufacturing precipitated calcium carbonate.

13. Web coating device according to claim 12, comprising a means for coating a web with calcium hydroxide, a means for blowing carbon dioxide containing gas onto the web surface, and optionally a means for drying the obtained coated web.

14. Web coating device according to claim 12 or 13, further comprising a means for guiding exhaust gas from impingement dryers or air flotation dryers to the means for blowing carbon dioxide containing gas.

15. Web coating device according to any of the claims 12 to 14, wherein the means for manufacturing precipitated calcium carbonate comprises a closed web run and/or web transfer portion confined from the outer atmosphere.

16. Web coating device according to claim 15 comprising an impingement board containing said means for blowing carbon dioxide containing gas and intake nozzles for recycling residual carbon dioxide.

17. Web coating device according to any of claims 13 to 16, wherein the means for coating the web with calcium hydroxide is a spray coater, blade coater, roll coater, or size press.

18. Web coating device according to any of the claims 12 to 17, wherein the web is supported on a metal belt or plastic band which is provided at least in the coating zone for the calcium hydroxide and/or the carbon dioxide as a back-side supporting member for the web.

19. Web mill system comprising a unit for manufacturing precipitated calcium carbonate.

20. Web mill system according to claim 19, comprising a reactor unit for manufacturing precipitated calcium carbonate and a means for guiding exhaust gas from impingement dryers or air flotation dryers to said reactor unit .

21. Web mill system according to claims 19 or 20, wherein the unit for manufacturing precipitated calcium carbonate comprises a web coating device according to any of claims 12 to 18.