Especially it relates to the treatment of a pulp suspension intended for production of an end product with high porosity, i. e. with low Gurley number, preferably for production of craft sack paper. The invention is also applicable to pulp suspensions for other products. The invention also relates to a paper or paper board product produced according to the method of the invention.

PRIOR ART In the production of paper or paper board, bulk is a very important property for a lot of products. A high bulk, i. e. a low density is often aimed at, e. g., for porous sheets.

Another important parameter for such products, especially craft sack paper, is porosity.

High porosity is aimed at because it implies high air permeability (for example measured in Gurley number), a low Gurley number corresponding to a high air permeability. The reason for aiming at low Gurley numbers for especially craft sack paper is that when the sacks, often industrial sacks, are filled a certain amount of air comes along with the material into the sack being filled. Especially at industrial filling of sacks its important that these amount of air can find its way out very fast.

A problem in connection with wet pressing of a paper or paper board web is that conventional roll pressing results in both increased density and reduced porosity as the dry solids content is increased. If the web is heated in the press section, e. g. steam boxes, during very short periods of time-typically less than 50 ms-the dry solids content is certainly increased but the bulk deteriorates, i. e. the density increases.

However, when shoe press technology is compared to conventional roll press technology, the following can be said. The density increase and porosity reduction that comes together with the increased dry solids content of the web is slower when shoe press technology is used.

However, there are still great benefits to get and great advantages to obtain, even if shoe press technology results in an improvement, by further reduced density and increased porosity at given dry solids contents of the web. Also for other paper and paper board

products there are great benefits to get and advantages to obtain with increased bulk at a given dry solids content.

DISCLOSURE OF THE INVENTION One object of the invention, is in the production of a paper or paper board web from a chemically produced pulp suspension, to achieve reduced density and/or increased porosity, by comparison with a corresponding paper or paper board web produced from a reference pulp, i. e. a corresponding pulp suspension not treated according to the invention.

According to the invention, this is achieved by heating a beated pulp suspension in a heat treating step to a treating temperature of at least 60° C yielding a reduced density and/or increased porosity in a paper or paper board product produced from the treated pulp suspension.

It has surprisingly been found that a heat treatment of the stock/pulp suspension, after beating of it at least at 60° C, preferably at least at 70° C and more preferably at least at 80° C, but preferably under 100° C, results in reduced density, i. e. increased bulk and increased porosity in the finished, dried web, compared to reference. It may appear especially surprisingly because the experiments mentioned above using steam heating of the produced web, with steam chests in the press section, on the contrary have revaled that the density follows the increase of the dry solids content.

According to the invention, it is sufficient that the heating continues just until the intended treating temperature has been attained after which the pulp suspension is lead further in the line. However, the suitable treating time at said treating temperature is at least 30 seconds, preferably at least 1 minute and more preferably 1-60 minutes and most of all 1-30 minutes.

The surprising result, according to the invention, can be used for increased quality when the product is craft sack paper. For the product paper board the grammage can be reduced thanks to the reduced density. Alternatively, at maintained level of density/bulk, at least some percent (e. g. about 1 %) higher dry solids content can be obtained before the end drying, which may be said to correspond to some percent (e. g. about 4 %) increase of the production and corresponding lower specific steam consumption.

From US 4,431,479 it is indeed known to heat treat a pulp suspension after mechanic treatment at high consistency. However, the process described therein differ considerably from the invention concerning both the type of product/pulp, the object of the treatment and the way the treatment is carried out. Accordingly, in US 4,431,479 the treatment of high yield pulp or mechanical wood pulp intended for newsprint paper is described, the object of the treatment being to improve the runability of the paper making machine and the efficiency in the printing office. Further, it is mentioned that the object of the heat treatment is to"fix"the fibre curling produced at mechanical treatment at high consistency. According to US 4,431,479 the heat treatment is carried out at temperatures above 100° C, typically 100-170° C.

According to one aspect of the invention, the heat treatment, which preferably is carried out at atmospheric pressure by heating with steam, can be carried out both at low consistency and high consistency. Accordingly, in a first embodiment of the invention the pulp suspension is heat treated at a consistency of 1-12 %, preferably 2-8 %.

According a second embodiment of the invention the pulp suspension is instead heat treated at a consistency of 10-50 %, preferably 10-40 % and more preferred 13-40 %.

The latter case may have economical advantages: the less water the pulp suspension contains the less steam is consumed to heat the pulp to desired temperature. Moreover, a high consistency in the treatment has the advantage that the treatment, if desired, may take place directly at the consistency of the pulp after the mechanical treatment of the fibres at high consistency, so called HC-beating, i. e. at least 20 %, preferably 25-40 % and more preferably 25-35 %. Alternatively, less removal of water is needed after HC- beating before heat treatment at high consistency. Also, in the case when the heat treatment, according to the invention, is carried out at low consistency, it is preferred that the beating is carried out as HC-beating, at least when the product is craft sack paper. The beating is carried out to a beating degree of 15-25° Shopper-Riegler, preferably under 20° Shopper-Riegler.

According another aspect of the invention the pulp suspension is produced according to the sulphate method (craft) or modifications of it. Preferably the pulp suspension is intended to be used in the end product craft sack paper showing high porosity, i. e., a low Gurley number, preferably a Gurley number under 20 s, more preferably a Gurley number under 10 s and most preferably a Gurley number under 5 s. The grammage of the product is typically 60-120 g/m2.

Further, for the product craft sack paper it is required that the pulp suspension constitutes of pulp for craft sack paper, or more specifically of long-fibred chemical pulp of soft wood. As the craft sack paper is unbleached, the pulp suspension shows a relatively high kappa number during the heat treatment.

The heating according to the invention is carried out preferably at atmospheric pressure, the steam having a temperature of 100° C. Also a pressurised method may be possible, the pressure of the treatment being above 1 bar (abs) and the temperature being above 100° C. Preferably, the pulp suspension is stirred mechanically during the heating.

After the heat treatment the pulp suspension is diluted with white water as required, suitably to a consistency of under 2 %, preferably under 1 %, suitably some tenth %, and is led to a headbox for formation of a web on a wire. During the dilution/supplying to the headbox, the temperature of the pulp suspension can be maintained at the treating temperature or reduced by at most 20° C compared to a temperature during the treatments according to the invention, preferably with at most 5-15° C. The web is pressed and dried in a conventional way after formation of the wire, as well modern as older press and drying methods being possible to use.

BREIF DESCRIPTION OF THE DRAWINGS The following detailed description will describe the invention by reference to the drawings, wherein: Fig. 1 shows schematically a flow diagram for a possible accommodation plan of the apparatus for realization of the invention, Fig. 2 shows a diagram over the density as a function of dry solids content for a test series according prior art, Fig. 3 shows a diagram over density as a function of dry solids content for a first test series according the present invention, Fig. 4 shows a diagram over density as a function of dry solids content for a second test series according to the present invention, Fig. 5 shows a diagram over porosity as a function of dry solids content for said second test series according to the present invention.

DETAILED DESCRIPTION OF THE DRAWINGS Fig. 1 shows a possible accommodation plan of the apparatus for a embodiment of the invention according to production of craft sack paper. Part number 1 in Fig. 1 shows an

atmospheric compensation tank to which a pulp suspension 2 is led after beating 12. To the compensation tank 1 in which the consistency of the pulp is about 5 %, is also added steam 3 having a temperature of 100° C for heating the pulp to about 90° C under mechanic stirring. The dwell time in the compensation tank 1 is about 15 minutes. After the compensation tank 1, the pulp suspension is diluted with process liquid 4 having a temperature that is 0-20° C below the temperature in the compensation tank, the liquid consisting preferably of so called white water. The temperature of the pulp suspension decreased by as most 20° C when the consistency is regulated to about 3-4 %. After this the pulp suspension is pumped 5 to a so called machine chest 6 acting as a buffer and to which possible chemicals are added. After the machine chest 6, the pulp suspension is pumped 7 further to be subsequently additionally diluted 8 with white water from a white water tank 9 provided with white water 13 from the wire tank 14. The consistency is regulated at the dilution 8 to about 0,2 % and also the flow of pulp is regulated after which the pulp suspension is pumped by a mixing pump 10 to further be led to a headbox 11. In a conventional way the pulp suspension is formed from the headbox 11 on the wire (not shown) in order to the be led further through wet press section and drying.

EXAMPLE 1 (prior art) In a test series according prior art, a paper web was heat treated with low pressure steam by means of two steam boxes, between first and second press, and before couch roll.

Line 1 in Fig. 2 constitutes the reference showing how the density of the end product is dependent of dry solids content at the pressing of the web without treatment with headboxes. In principle the density increases proportionally to the dry solids content.

The line 2 shows how the density in the end product is dependent of the dry solids content at the pressing of the web including the treatment by means of steam boxes. For all test points the white water temperature was the same. In principle, as can be seen, line 2 constitutes a continuation of line 1, i. e. the density continues to increase with increasing dry solids contents which is achieved by means of treatment with headboxes.

Accordingly, higher dry solids contents in the press section is achieved at the price of higher density in the end product.

EXAMPLE 2 In a first test serie according to the invention, a pulp suspension of craft sack paper pulp, produced by means of the sulphate method, was treated according to the invention. Line 1 in Fig. 3 shows how the density depends on the dry solids content of a reference sample in which the pulp had a conventional temperature of about 40° C after the

beating. The pulp was diluted with white water having a temperature of 44° C, formed on the wire by means of the headbox, pressed and dried. After beating, in the serie according to line 2, the pulp suspension has been treated with steam to a temperature of about 60° C after which the pulp was diluted with white water having a temperature of about 50° C, formated on a wire, pressed and dried. Apparently, a lower density was achieved in the end product at a given dry solid content in the press section compared to the reference line 1. In the serie according to line 3, after beating, the pulp suspension has been treated with steam to a temperature of about 90° C. The after-treatment where carried out in the same way as for the serie according to line 2. Apparently, the further increased temperature gives a much lower density value (line 3) for the product compare to line 1 and 2.

EXAMPLE 3 For the purpose of making sure that it is the heat treatment giving the desired effect and not a higher temperature out from headbox, a second test serie was carried out. In the test according to example 2, the increased white water temperature has given an increased temperature of the pulp suspension leaving the headbox for the tests according to line 2 and 3, compared to tests according to line 1.

Consequently the object of the second test serie was to investigate: A. The influence of increased white water temperature (80° C) at increased pulp temperature (80° C), i. e. the heat treatment according to the invention, and an increased white water temperature.

B. The influence of merely increased pulp temperature (80° C) with constant white water temperature (40° C), i. e. heat treatment according to the invention, but dilution with white water with conventional mainly constant temperature.

The problem to study this is that the white water temperature can't be held constant because it increases with the pulp temperature during the run. It was possible to achieve, for case B above, a white water temperature of 48-63'C (intended=40* C) and for case A above, the white water temperature of 66-70° C (intended= 80° C). Reference series (case C) with a pulp temperature of 40° C and a white water temperature of 40° C (conventional) was also carried out.

The results, in the form of how the density is dependent on dry solids content, is showed as line A, B and C in Fig. 4. Apparently, the lowest density was achieved at given dry solids content when the temperature of the high consistency pulp was 80° C, line A and

B. Consequently a temperature of 80° C of the high consistency pulp gave a better result than the reference, a temperature of 40° C of the high consistency pulp, line C. Further, from line A it can be established that increased temperature out from a headbox, owing to increased white water temperature, does not give a better density (or bulk) than a lower temperature, line B.

In Fig. 5 porosity values as a function of dry solids content is shown for the test A, B and C. Also here the best results have been achieved when the white water temperature was 48-63° C (line B), which shows that it is the heat treatment giving the desired effect. However, it is noted that also at an increased white water temperature (66-70° C) and with that increased temperature out from the headbox (line A), the effect of the treatment, according to the invention, is better than the results of the reference sample (line C).

The invention is not limited to the above described embodiments but can be varied within the scope of the following claims. Consequently, a man skilled in the art realizes for example that the heat treatment may be carried out in many other ways than with direct steam, for example by use of indirect steam or in other ways. The process is not limited to be atmospheric. Further, the pulp suspension may also be sulphite pulp, at least if the product is not craft sack paper.