GB1591777A - High temperature low consistency refining of thermo-mechanical pulp - Google Patents

Description

(54) HIGH TEMPERATURE, LOW CONSISTENCY REFINING OF THERMO-MECHANICAL PULP (71) We, BELOIT CORPORATION, a corporation organised and existing under the laws of the State of Wisconsin, United States of America, of Beloit, Wisconsin 53511, United States of America, do hereby declare the invention for which we pray that a patent may be granted to us, and the method by which it is to be performed, to be particularly described in and by the following statement:- This invention relates to the refining of a thermo-mechanical pulp immediately prior to passing the same to the head box of a paper making machine, and involves refining the pulp at a high temperature and low intensity to reduce the bulk of the fibres and to reduce the amount of breakage of the fibres in the refiner.

The thermo-mechanical pulping process has become widely accepted in the paper industry for producing mechanical pulp from chips. To a large extent, it has replaced the purely mechanical process in which the chips were ground by means of stone grinding wheels and chemical processes involving the use of sulphates or sulphites.

Pulps produced by the thermo-mechanical pulping process exhibit substantially higher strength than pulps produced by the stone groundwood process. While the chemical pulping methods produce a higher quality pulp, they require extensive capital investment for high pressure treating vessels and the like.

One of the difficulties with thermomechanical pulps, however, is the higher bulk (lower density) in papers produced from such pulps. This high bulk causes problems in printing and converting operations and may itself be a problem if finished sheet thickness, for a given basis weight, is critical. Additionally, stronger sheets can usually be obtained from a given pulp if the bulk is reduced, for example, by higher pressing pressures.

The processing of mechanical pulp suspensions by conventional "post refining methods" in a refiner at a low consistency of about 3 to 50/, has not proven completely satisfactory because of the tendency to cut the fibres during. refining. Although some reduction in bulk can be achieved in this way, the reduced fibre length caused by the broken fibres results in excessive freeness drop, and possible loss in sheet strength properties.

According to the invention there is provided a method of refining a thermomechanical pulp fibre suspension in which the fibres are characterised by the presence of lignin and being substantially free from chemical residues, which method comprises refining such a suspension having a consistency of 3 to 5 /a at a temperature of 200 to 3000F in a refiner operating at an intensity less than 40x 10-4 horsepowerminutes/inch contact.

The following is a more detailed description of one embodiment of the invention, reference being made to the accompanying drawings in which: Figure 1 is a partial block diagram illustrating some of the preceding steps and the post-refining operation with which the present invention is concerned, Figure 2 is a fragmentary view with portions broken away to illustrate the interior of a refiner which can be used for the purposes of the invention, and Figure 3 is a greatly enlarged fragmentary cross-sectional view of the confronting rotor and stator plates which form the preferred embodiment of the invention.

Figure 1 of the drawings is a flow diagram of a typical sequence which includes the post-refining step of the present invention.

Specifically, the stock may be treated in one or more refiners 10 and then passed to one or more mixing chests 11 where suitable additives such as clay and the like may be added through a line 12. The thus treated stock is then passed to one or more machine chests 13 and ultimately to a post refiner 14, the details of which will be subsequently described. The stock entering the post refiner 14 is a suspension having a consistency of 3 to 50/,. The thermomechanical pulp is characterised by the presence of lignin and by substantial freedom from chemical residues. In the post refiner stage 14, the stock is treated at a temperature of 200 to 3000F, and the refiner operates at an intensity of less than 40x 10-4 horsepower-minutes/inch contact.

Preferably, the intensity is less that 15xl0-4 horsepower-minutes/inch contact.

Conventional refiners all act on the fibres by means of attrition. There are various types of refiner configurations, but most of them make use of metal attrition members in the form of bars. The intensity of refining is defined as the net refining power divided by the length of bar crossings per unit time.

Since the refining action of the fibre takes place at the leading edge of the filling element, the number of inch contacts/minute can be determined from the following: Inch contacts/minute= Total length of bars in rotor x total length of bars in stator x refiner speed in rpm.

A typical refiner which can be used for the purposes of the present invention is illustrated in Figure 2 of the drawings. It includes a base 15 on which there is mounted a shaft 16 supported for rotation in a bearing 17. The shaft is driven from a coupling 18 connected to a suitable motor (not shown). The stock is introduced into an inlet 19 and then flows into a refining cavity 20 whereupon the stock flow is split up by passing through a pair of perforated rotor elements 21 and 22 secured to a rotating head 23. Details of such rotor elements will be found described in U.S. Patent Specification No. 3,438,586.

The rotor elements 21 and 22 are in confronting relation with stator plates 24 and 25 on opposed sides thereof. The spacings between the rotor plates and the stator plates is adjusted by means of a worm gear 26 which operates in conjunction with an adjusting screw 27 which in turn is connected to a gear motor 28.

A more detailed view of the refiner plates is shown in Figure 3 of the drawings. As illustrated in that figure, the rotor element 22 is provided with a series of alternating bars 29 and grooves 30, the widths of the bars and grooves not exceeding 1/8 inch.

Similarly, the stator 25 is provided with ribs 31 and grooves 32 in which the widths of the bars and the grooves do not exceed 1/8 inch.

In the preferred form of the present invention, the rotor element 22 is made of stainless steel and the stator plate 25 is made of a synthetic resin such as a polysulphone resin.

Fibre length reduction which is achieved in normal low consistency "post refining" of mechanical pulps can be attributed to the relatively high stiffness of the fibre. At normal refining temperatures of 110 to 140"F, the brittle fibres are fractured when impacted by the impinging edges of the refiner. In accordance with the present invention, the refiner is operated at an increased temperature so that the lignin softening temperature is approached or exceeded. At these higher temperatures, the fibres are much more flexible and have a tendency to be cut in the refiner.

To reduce fibre damage further, the high temperature processing is carried out under conditions of minimum practical intensity in the refiner. To minimise the intensity, the refiner is operated at relatively high peripheral speeds, in excesss of about 6,000 feet/minute. This high speed, -coupled with the very fine pattern of bars in the stator and rotor, results in maximum practical levels of inch-contacts/minute. Consequently, when relatively low power is applied to the refiner, the intensity will be at the lowest practical level.

Since the refiner operation is at relatively high speed, a considerable circulating load due to hydraulic losses will exist. This load is reduced substantially by reducing the draft of the rotor plates, and this reduced draft becomes practical because of the lower wear rate of the steel rotor when operated against a synthetic resin stator. This reduction in load, therefore, contributes to power savings.

As an illustration of the differences involved between the operation of the refiner described above with reference to the drawings and conventional practice, it might be noted that the normal operation of a 42 inch diameter refiner makes use of bars and grooves which are 3/16 inch in width, a rotational speed of about 514 rpm, and a net applied horsepower of 600. In the refining operation of the refiner described with reference to the drawings, the bar and groove width is reduced to 1/8 inch or less, a rotational speed of at least 600 rpm is used, and the net applied horsepower is 300 or less.

WHAT WE CLAIM IS: 1. A method of refining a thermomechanical pulp fibre suspension in which the fibres are characterised by the presence of lignin and being substantially free from chemical residues, which method comprises refining such a suspension having a consistency of 3 to 5 /n at a temperature of 200 to 3000F in a refiner operating at an intensity less than 40x 10-4 horsepowerminutes/inch contact.

2. A method according to Claim 1, wherein said refiner is a disc refiner.

**WARNING** end of DESC field may overlap start of CLMS **.

Claims (7)

**WARNING** start of CLMS field may overlap end of DESC **. presence of lignin and by substantial freedom from chemical residues. In the post refiner stage 14, the stock is treated at a temperature of 200 to 3000F, and the refiner operates at an intensity of less than 40x 10-4 horsepower-minutes/inch contact. Preferably, the intensity is less that 15xl0-4 horsepower-minutes/inch contact. Conventional refiners all act on the fibres by means of attrition. There are various types of refiner configurations, but most of them make use of metal attrition members in the form of bars. The intensity of refining is defined as the net refining power divided by the length of bar crossings per unit time. Since the refining action of the fibre takes place at the leading edge of the filling element, the number of inch contacts/minute can be determined from the following: Inch contacts/minute= Total length of bars in rotor x total length of bars in stator x refiner speed in rpm. A typical refiner which can be used for the purposes of the present invention is illustrated in Figure 2 of the drawings. It includes a base 15 on which there is mounted a shaft 16 supported for rotation in a bearing 17. The shaft is driven from a coupling 18 connected to a suitable motor (not shown). The stock is introduced into an inlet 19 and then flows into a refining cavity 20 whereupon the stock flow is split up by passing through a pair of perforated rotor elements 21 and 22 secured to a rotating head 23. Details of such rotor elements will be found described in U.S. Patent Specification No. 3,438,586. The rotor elements 21 and 22 are in confronting relation with stator plates 24 and 25 on opposed sides thereof. The spacings between the rotor plates and the stator plates is adjusted by means of a worm gear 26 which operates in conjunction with an adjusting screw 27 which in turn is connected to a gear motor 28. A more detailed view of the refiner plates is shown in Figure 3 of the drawings. As illustrated in that figure, the rotor element 22 is provided with a series of alternating bars 29 and grooves 30, the widths of the bars and grooves not exceeding 1/8 inch. Similarly, the stator 25 is provided with ribs 31 and grooves 32 in which the widths of the bars and the grooves do not exceed 1/8 inch. In the preferred form of the present invention, the rotor element 22 is made of stainless steel and the stator plate 25 is made of a synthetic resin such as a polysulphone resin. Fibre length reduction which is achieved in normal low consistency "post refining" of mechanical pulps can be attributed to the relatively high stiffness of the fibre. At normal refining temperatures of 110 to 140"F, the brittle fibres are fractured when impacted by the impinging edges of the refiner. In accordance with the present invention, the refiner is operated at an increased temperature so that the lignin softening temperature is approached or exceeded. At these higher temperatures, the fibres are much more flexible and have a tendency to be cut in the refiner. To reduce fibre damage further, the high temperature processing is carried out under conditions of minimum practical intensity in the refiner. To minimise the intensity, the refiner is operated at relatively high peripheral speeds, in excesss of about 6,000 feet/minute. This high speed, -coupled with the very fine pattern of bars in the stator and rotor, results in maximum practical levels of inch-contacts/minute. Consequently, when relatively low power is applied to the refiner, the intensity will be at the lowest practical level. Since the refiner operation is at relatively high speed, a considerable circulating load due to hydraulic losses will exist. This load is reduced substantially by reducing the draft of the rotor plates, and this reduced draft becomes practical because of the lower wear rate of the steel rotor when operated against a synthetic resin stator. This reduction in load, therefore, contributes to power savings. As an illustration of the differences involved between the operation of the refiner described above with reference to the drawings and conventional practice, it might be noted that the normal operation of a 42 inch diameter refiner makes use of bars and grooves which are 3/16 inch in width, a rotational speed of about 514 rpm, and a net applied horsepower of 600. In the refining operation of the refiner described with reference to the drawings, the bar and groove width is reduced to 1/8 inch or less, a rotational speed of at least 600 rpm is used, and the net applied horsepower is 300 or less. WHAT WE CLAIM IS:

1. A method of refining a thermomechanical pulp fibre suspension in which the fibres are characterised by the presence of lignin and being substantially free from chemical residues, which method comprises refining such a suspension having a consistency of 3 to 5 /n at a temperature of 200 to 3000F in a refiner operating at an intensity less than 40x 10-4 horsepowerminutes/inch contact.

2. A method according to Claim 1, wherein said refiner is a disc refiner.

3. A method according to Claim 1 or

Claim 2, wherein said refiner operates at an intensity of less than 15x10-4 horsepowerminutes/inch contact.

4. A method according to any Claims 1 to 3, wherein said refiner includes confronting rotor and stator surfaces each of which has a series of alternating bars and grooves, the widths of said bars and grooves in each instance not exceeding 1/8 inch.

5. A method according to Claim 4, wherein said rotor surfaces are made of steel and said stator surfaces are made of synthetic resin.

6. A method according to any of Claims 1 to 5, wherein said refiner operates at a peripheral speed in excess of 6,000 feet/minute.

7. A method of refining a thermomechanical pulp fibre suspension substantially as hereinbefore described with reference to the accompanying drawings.