A method and an apparatus for treating paper pulp

Abstract

 A method and an apparatus for increasing form factor and tensile index in a dry paper pulp for sale which per se is produced in a paper pulp mill and comprises the steps of: raw materials intake, possible debarking and chipping, cooking, as well as bleaching of the paper pulp obtained therefrom to brightnesses of between 70 and 95 per cent ISO, whereafter the product is dried. According to the invention, the paper pulp is mechanically processed in the wet state after the bleaching and before the drying in a fibre shearing apparatus (1-6 weight per cent concentration) at a moderate pressure of from 100 to 750 kPa.

Description

[0001] The present invention relates to a method and an apparatus for straightening chemically extracted pulp fibres in pulp for sale and thereby increasing the form factor of the fibres. This also results in an increase in the tensile index of such dried paper pulp.

[0002] Pulp for sale normally has a total solids of between 80 and 100 per cent. Preferably of the order of magnitude of 90 per cent. The form factor is a dimension of the straightness of a wood fibre, a completely straight fibre possesses a form factor of 100 per cent while a double folded fibre has a form factor of 50 per cent. Tensile index is a measurement of the load-absorbing capacity of a paper pulp per unit of weight and depends to some degree on the form factor of the fibres (increased form factor gives increased tensile index) and is expressed in kNm/kg.

[0003] Chemical paper pulp production encompasses, in daily parlance, most generally the following process steps: wood intake, debarking, chipping, cooking, bleaching and drying. In practice, it is often the case that the dried paper pulp (so-called pulp for sale) is produced at one site by a pulp producer, whereafter this pulp product is refined at another site by a paper mill. In the pulp production process, a gradual deterioration and reduction of form factor and tensile index, respectively, take place. Generally, the deterioration is considered as a consequence of the chemical and mechanical effects exercised on the fibre during the pulp producing process. Tensile index is an important paper pulp property in final use. In order to increase tensile index, the pulp is beaten in connection with the paper making. The fibres in the pulp are then straightened out to some degree, whereby the tensile index is increased. In connection with this beating, a considerable quantity of energy is consumed. The beating capacity for the paper maker may be a tight sector which could result in reduced production or poorer quality if the paper pulp in question requires a higher level of beating energy than capacity allows. It is therefore one object of the present invention to realise a method and an apparatus which give a straightening of the fibres. Hereby, the energy requirements are optimised for the entire process from raw materials to finished paper, and the intermediate product, so-called pulp for sale, will be obtained which is considerably more attractive for a paper maker. For the paper maker, the tensile index level at a specific beating energy input is an essential parameter.

[0004] According to the present invention, the paper pulp is processed in the fibre line with mechanical processing in the wet state prior to drying and after the bleaching step of the process. As a result, the risk is reduced that the fibres in the paper pulp are subjected to negative effects after the mechanical processing.

[0005] In another further developed embodiment of the method according to the present invention, it is expressly specified that the concentration of the pulp in the processing may be from 1 to 6 weight per cent and that the mechanical processing should be put into effect in a paper pulp processing plant which includes a stator provided with bars and one or more rotors which are mutually adjusted so that a gap is formed between them of between 0.05 and 1 mm, preferably of the order of magnitude of about 0.2 mm. As a result, the processing will preferably take place at a low to moderate pressure of between 100 and 750 kPa. The bars may consist of steel, but other materials such as, for example, ceramics of different compositions may function just as well. With a thus defined processing plant, straightened fibres will be obtained from the process.

[0006] The present invention will now be described in greater detail hereinbelow, with reference to embodiments which are shown in the accompanying Drawings. In the accompanying Drawings:

Fig. 1 shows a simple flow diagram of the process for the production of paper pulp for sale;

Fig. 2 shows a flow diagram corresponding to that of Fig. 1 which, however, according to the present invention encompasses an additional step with mechanical processing;

Fig. 3 shows the relationship between obtained form factor as a function of the energy supplied in the mechanical processing;

Fig. 4 shows the tensile index as a function of the form factor; and

Fig. 5 shows the relationship between tensile index of a reference pulp and of a pulp mechanically processed in accordance with the present invention.

[0007] A raw materials intake (see Fig. 1)handles log wood (or alternatively chips) and stores it. During a debarking step, the bark is removed from the log wood, whereafter the wood is conveyed through a chipper in order to be converted into wood chips. The wood chips are cooked with the intention of removing lignin from the fibres, whereafter the fibres are exposed and pulp is formed. Thereafter, the pulp is bleached in order to increase the pulp brightness and, last but not least, the pulp is dried whereafter it is packed in the form of pulp bales which are then sent to paper makers.

[0008] The present invention entails that a novel process step is introduced between cooking and drying, in which the fibres are mechanically processed in the wet state. Fig. 2 illustrates the process schematically in the form of an expanded flow diagram.

[0009] As a result of the processing, the fibres are straightened and so that the form factor of the pulp increases and so in accordance with the foregoing definition, increased tensile index is obtained in the pulp. In the novel process step, use is made of a mill or similar fibre straightening equipment which processes the fibres in the gap between a rotor and a stator. The rotor and stator, respectively, display steel bars which between them create shearing forces which act on the fibres at the same time as the fibres are kneaded against one another. The shearing and kneading reduce the curving in the fibres and thus impart to the fibres the increased form factor, as is apparent from Fig. 3.

[0010] The form factor of the fibre has a major effect on the tensile strength in the paper. A less curved fibre is capable of as good as taking up a load immediately, while a more curved fibre must first be straightened out by the tensile loading in order to take up a corresponding load. This implies that a less curved fibre gives a higher strength initially compared with a more curved fibre and which also entails that a higher strength will be attained for a straight fibre compared with a curved, as is apparent from Fig. 4.

[0011] Mechanically processed pulp before drying according to the present invention attains a higher tensile index than conventional pulp for sale. Compared with the conventional pulp for sale, the tensile index in both the unbeaten and the beaten state is higher. Consequently, the customer is given an opportunity either to achieve higher tensile strength in the pulp after the addition of a certain beating energy, or be able to reduce the beating energy in order to attain a given, predetermined tensile strength, as is apparent from Fig. 5.

Claims

1. A method of increasing form factor and tensile index in a pulp for sale which is produced in a paper pulp mill, comprising the steps of: raw material intake, cooking, bleaching of the paper pulp obtained therefrom to brightnesses between 70 and 95 per cent ISO and thereafter drying, characterised in that the paper pulp is mechanically processed in the wet state in a mill or the like between the bleaching and drying steps after dilution to a low concentration amounting to between 1 and 6 weight per cent, at a moderate pressure amounting to between 100 and 750 kPa.

2. The method as claimed in Claim 1, characterised in that said low concentration processing takes place at a pressure which amounts to between 100 and 300 kPa.

3. The method as claimed in Claim 1 or 2, characterised in that the wet paper pulp is processed mechanically at a concentration of between 2.5 and 4.5 weight per cent.

4. The method as claimed in any of the preceding Claims, characterised in that the processing is carried out in a mill comprising a stator provided with bars and one or more rotors which are adjusted so that a gap of 0.05 to 1 mm is formed between them.

5. An apparatus for increasing form factor and tensile index in a pulp for sale which is produced at a paper pulp mill where the pulp mill includes the process steps: raw materials intake, cooking, bleaching step and drying, characterised in that the paper pulp mill includes, in addition to the above mentioned parts, a mechanical processing step which is carried out in a fibre shearing apparatus, such as a mill or the like, for straightening the fibre in the wet state after bleaching and before drying, whereby the fibres present in the paper pulp raw material obtain increased form factor and increased tensile index.

6. The apparatus as claimed in Claim 5, characterised in that the fibre shearing apparatus constitutes a process step wherein the paper pulp fibres are sheared and kneaded at a concentration of between 1 and 6 weight per cent in a pressure of between 100 and 750 kPa in order to achieve increased form factor and an increased tensile index in the pulp.

7. The apparatus as claimed in Claim 6, characterised in that the fibre shearing apparatus comprises a stator provided with bars, and also one or more rotors provided with such bars, they being adjusted so that a gap of between 0.05 and 1 mm is formed between them.

Drawing