Field of the invention

The invention relates to a method for preventing the soiling of the surface of a roll, and in particular the surface of a centre roll, in a paper or board machine, and for keeping the same clean.

Background art

A very common problem with paper and board machine rolls and in particular with centre rolls is that the roll is quickly soiled irrespective of what surface material has been used in the roll. The soiling of the centre roll becomes evident in the paper or board machine in the deterioration of the operation of the roll, in the impairment of separation of the web from the centre roll and in the increase of web breaks. The soiling of the centre roll occurs in the course of time and it increases in amount once the first dirt layer has formed on the surface of the centre roll. The centre roll is in contact with a wet paper web, the process water removed from the web, the jet waters cleaning the roll, and with the doctors that clean the roll mechanically.

The components soiling the centre roll come to the process waters of the paper and board machine from several different sources, for example, with mechanical pulp, recycled fibres, and coated broke. The components soiling the centre, roll are, regardless of their origin, anionic, colloidal or neutral hydrophobic components by nature.

In addition to cellulose and hemicelluloses, mechanical pulp also contains plenty of lignin and extractives, which tend to dissolve from fibres in colloidal form or as dispersed in small droplets from the surface of fibre into the aqueous phase during the manufacturing process.

Chemi-mechanical pulps contain colloidal material that causes dirt because chemi-mechanical pulp preparation converts lignin and extractives into a more soluble form. Hemicelluloses are water soluble and may act like anionic disturbing substances, lignin derived substances are separated and dissolve in a manner similar to that of carbohydrates, and with increasing pH the water solubility of the lignin derived substances increases.

The extractives in wood-containing pulps comprise triglycerides, stearyl esters, diterpenes, and resin acids. Resin and fatty acids are compounds which are anionic and stable in water, and which are neutralized with increasing pH and converted into soluble alkali salts when they are saponified with sodium. The thus produced soaps are poorly soluble and may dissolve neutral pitch as small droplets in water.

Recycled fibres bring with them components of the tacky type, and they cannot be completely removed from the pulp during the deinking process. The tacky components can be synthetic in origin, such as latexes, sticker adhesives, hot melts or natural substances, such as extractives and pitch. Deinked pulp may also bring with it inorganic materials, such as pigments, dissolved salts and readily soluble organic components, of which may be mentioned lignin-based substances, hemicelluloses and starch, and some other soluble components, such as dispersing agents, anti-foaming agents and cleaning agents. When coated broke is used in papermaking as raw material, process waters will receive coatings that contain binding agents, such as anionically stabilized latexes and starch as well as different pigments, which are often suspended by using anionic dispersing agents.

Attempts have been made to reduce and prevent the sticking of dirt by means of several prior art arrangements. Centre rolls are cleaned continuously using doctors, ultrasonic washers, carbon dioxide snow jets or oscillating cleaning heads, or the cleaning operation is performed in connection with a shutdown. The purpose of these methods is to remove mechanically the dirt layer already formed on the roll surface but the formation of a new dirt layer is hardly affected at all by these methods. Based on the foregoing it can be noticed that there exists an obvious need for a method that prevents dirt and disturbing substances from sticking and staying attached to the surface of a roll, such as a centre roll, and keeps the surface of the roll free of dirt and disturbing substances during the process.

Object of the invention

The object of the invention is a method for preventing the soiling of a roll, in particular a centre roll, in a paper or board machine and for keeping the surface of the roll clean during the process.

The characteristic features of the method according to the invention for preventing the soiling of a roll and for keeping the surface of the roll clean, in particular the surface of a centre roll, are set forth in the claims.

In the method according to the invention, a charge state repelling dirt particles is produced at the surface of the roll. The charge state of the roll surface is made dirt-repellent by manufacturing the surface of the roll out of a material having a dirt-repellent charge state, and/or the surface of the roll is regenerated such that a dirt-repellent charge state is continuously returned to the surface of the roll, and/or the surface of the roll is provided with a dirt-repellent charge state from outside.

Description of the invention

The above-mentioned objects can be attained and the problems of the arrangements known from prior art can be avoided or at least substantially reduced by means of the method according to the invention. It has been found that the charge state of the roll and the charge state of disturbing substances mainly determine the sticking of dirt and disturbing substances and staying attached to the surface of the roll. Thus, the problem with the soiling of the roll can be prevented when the chemical and physical properties of the surface of the roll are adjusted to be repellent of components causing soiling, i.e. the charge state of the surface of the roll is adjusted.

When the soiling components are hydrophobic and neutral, soiling can be minimized by means of a hydrophilic roll coating. In an aqueous environment, a hydrate layer is formed on the hydrophilic surface, and the hydrophobic component is then not able to spread or attach itself to the surface.

When the soiling components are anionically charged, the roll coating shall also have a negative charge, whereby an electrostatic repulsion is created between the roll and the dirt components, and no soiling occurs because the dirt particles are not able to attach themselves to the surface of the roll.

In accordance with the invention, a soil-repellent roll coating is manufactured that is both hydrophilic and has a charge of the same sign as dirt. The majority of dirt particles are anionic by nature, such as, for example, pitch, deinking residues, several fibres, and fillers. Li addition to the properties of the surface itself, the charging behaviour of the surface is affected by the environment where the surface is. Thus, the surface charge of a roll, in particular a centre roll, in a paper and board machine is also dependent on the prevailing process conditions, such as the pH and electrical conductivity of process waters.

An almost ideal roll surface material is natural granite, which is strongly anionic, but for practical reasons the use of granite rolls is rather limited and attempts to make a coating exactly like granite have so far failed. Ceramic surfaces are often relatively hydrophilic, so when they are in an aqueous solution, a hydrate layer is formed on their surface. The more hydrophilic the ceramic surface, the thicker the hydrate layer formed on the surface. The hydrate layer prevents hydrophobic dirt from adhering to the ceramic surface of the roll itself. Since the charge state of the roll surface is dependent on pH, the surface should be negative at least in the pH range of the process.

When a solid substance having a surface charge is in an electrolyte solution, an electrical double layer is formed on it. A stationary layer is formed on the surface of the solid substance out of ions having a charge of an opposite sign to that of the surface. On the surface of the stationary surface, a second layer is formed which contains ions charged with both an opposite sign and an equal sign such that the concentration of the ions is reduced the longer the distance from the surface, and they can move in the direction of the plane of the surface. This layer is called a diffusion layer. The boundary surface between the stationary layer and the diffusion layer is called a slipping plane. It is not possible to measure the value of the absolute surface charge of the solid substance, but, instead, the surface charge of the slipping plane can be measured and this value is called a zeta potential.

The isoelectric point (IEP) is a pH value at which the zeta potential of the surface is zero. At pH values higher than the isoelectric point, the zeta potential is negative and, correspondingly, at pH values lower than the isoelectric point, the zeta potential is positive. Thus, the isoelectric point of a roll having a negative charge shall be below 3, in which case the surface can be considered to be suitable for a roll and in particular for a centre roll from the viewpoint of the prevention of contamination. In addition to the isoelectric point, the value of the zeta potential must also be adjusted to a correct level. In that connection, the roll shall have a negative surface charge and the values of the zeta potential of the roll shall be of the same order of magnitude as that of the zeta potential of disturbing substances in order to generate electrostatic repulsion. In accordance with the invention, the charge state of the roll surface can be adjusted to be as desired by means of the following methods or combinations of them:

1) The surface of the roll is originally made as desired 2) The surface is regenerated as desired 3) The surface is provided with a desired charge state from outside.

The charge state of the roll surface can be made as desired preferably using the following methods. According to the charge state, a desired roll coating is manufactured out of ceramic, plastic or a combination of them preferably on a cast-iron body. The roll coating can also be polymer-sealed ceramic, a polymer reinforced with ceramic particles, a mixture of ceramic and metal with added polymer, or a diamond-like coating. In this procedure, the surface material is selected to be in accordance with the desired charge state.

As a second alternative, a porous ceramic material and a suitable sealing agent are selected for the surface such that a desired total charge state is imparted to the surface. The porosity of the ceramic surface can also be affected by selecting suitable ceramic spraying parameters.

As a third alternative, the charge state of the adhesion layer of the ceramic surface is selected such that the charge state of the ceramic surface can be made as desired, the charge state of the adhesion layer affecting the charge state of the roll surface through the porous ceramic.

The roll surface can be regenerated to have a desired surface charge preferably using the following methods. New material is supplied to the surface of the roll by means of doctor material, whereby the charge state of the surface can be changed selecting the doctor material appropriately with respect to the surface material of the roll.

The charge state of the roll surface is changed continuously by spraying a chemical to the surface of the roll. The chemical to be sprayed is selected such that it is a compound or a mixture of compounds that adheres poorly to the surface of the roll but strongly to dirt. When the dirt contains a sufficient amount of the chemical, the aggregates forming the dirt will fall off the roll.

The surface of the roll can be provided with a desired charge state from outside by means of an external voltage source. Voltage is brought, for instance, between the centre roll and a counter roll, such as a grooved roll, or an adjacent paper guide roll or a spray, which may optionally also contain the above-mentioned chemical, electrolytes, in that case, however, bearing housings and attachments must be insulated carefully.

Combinations of the above-mentioned methods can also be used for producing a dirt-repellent charge state at the surface of the roll.

Advantages of the invention

One of the advantages of the method according to the invention, in which the charge state of the surface of a roll and preferably that of a centre roll is adjusted to be ideal from the viewpoint of the prevention of soiling of the roll, is that in the method dirt is removed even before large amounts of it are produced on the roll surface. Dirt is removed continuously in small amounts, and the soiled roll does not cause a need for a shutdown.