Paper making and fiber liberation – Processes of chemical liberation – recovery or purification... – With squeezing – compression – rubbing – kneading
Reexamination Certificate
2002-06-07
2004-11-16
Fortuna, José A. (Department: 1731)
Paper making and fiber liberation
Processes of chemical liberation, recovery or purification...
With squeezing, compression, rubbing, kneading
C162S055000, C162S020000, C241S021000, C241S024100, C241S029000, C241S030000
Reexamination Certificate
active
06818099
ABSTRACT:
The present invention relates to stock, a method for preparing it, the use of the stock as a raw material for producing printing paper, especially newsprint, and a printing paper. The stock produced in accordance with the method of the present invention can be used as a raw material for producing different papers, such as SC paper (supercalendered) comprising both offset and gravure grades, coated paper having a low grammage or LWC paper (light weight coated) comprising both offset and gravure grades, and newsprint or corresponding printing papers. Newsprint also comprises other grades of paper than those used in newspapers, e.g. catalogue papers and gravure papers.
A known method for producing mechanical pulp is presented in patent publication U.S. Pat. No. 5,145,010, corresponding to international application WO 8906717 and Swedish patent publication SE 459924. The method comprises the following phases:
impregnating softwood chips with water and chemicals
primary refining of the treated chips
fractionating the refined softwood pulp into accept and reject stock portions, whereby the reject portion comprises 15-35% of the refined stock
refining of the reject stock portion in two steps, whereby the stock consistency in the first step is approximately 20-35% and in the third step approximately 5%, and
the above-mentioned stock is fractionated to form an accept stock portion and a reject stock portion. The fractionation is carried out with a screen.
A known method for producing mechanical pulp is presented in patent publication U.S. Pat. No. 4,938,843. The process involves the production of chemi-thermomechanical stock. The chips impregnated with chemicals and treated with heat are refined to a freeness value of 100-700 ml CSF, usually in a two-phase refining process and screened to form a first accept stock portion and a first reject stock portion, so that at least 30% of the stock goes into the reject stock portion. The consistency of the stock during the screening is approximately 2%. The first accept stock portion is screened for a second time, whereby a second accept stock portion and a second reject stock portion are formed. The first and the second reject stock portions are combined, creating a long-fibre fraction with a freeness value of 200-750 ml CSF, which can be used separately to produce coarse-fibred products, for example cardboard, or it can be further refined and returned to the first screening.
A known method is the method for producing stock described in the introductory part of patent claim 1 of the present application, in which method the process is begun with two-phase refining. The chips are fed into the first refiner, from which they are fed into the second refiner after the first refining is complete. After the second refiner, the freeness value of the stock is about 120 ml CSF. The consistency is typically 50% at the first refiner and 45% at the second refiner. After the first refiner, the measured average fibre length, when using spruce as the raw material, is approximately 1.7 mm, and after the second refiner the average fibre length when using the same raw material, is approximately 1.5 mm. After the second refiner there is a latency chest, in which the fibres are straightened by diluting the consistency to 1-2%. The fibres are treated in the latency chest for one hour. The fibres are conveyed to the first screen, which fractionates the stock into an accept portion and a reject portion. The freeness value of the accept stock portion is about 20 ml CSF. Water is removed from the reject stock portion to obtain a consistency of 45%. The reject stock portion, which constitutes 40-50% of the total stock, is conveyed to the third refiner, from which the reject stock diluted to a consistency of 1%, is transported on to a second screen. Again the stock is fractionated into an accept stock portion and a reject stock portion. The reject stock portion is conveyed, after the removal of water, at a consistency of 45%, to a fourth refiner and after being diluted to a consistency of 1%, on to a third screen. The reject stock portion from this screen is fed again to the fourth refiner. The stock obtained from the process has a freeness value of 30-70 ml CSF. advantageously about 50 ml CSF. The pressure used in the refiners is 350-400 kPa. The process consumes about 3.3 MWh/t of energy (using spruce as the raw material), 0.3 MWh/t of which is used for regulating the consistency so as to be suitable for every stage of the process.
In the process mentioned above, regulating the consistency to a suitable level consumes 9-10% of the total energy used in the process. In the present application, consistency refers to the amount of stock as a percentage by weight in the mixture of pulp and water. The water can be either in vapour or in liquid form.
After the refining, a latency chest, in which the fibres remain, is needed to straighten the fibres. Regulating stock consistency requires the use of suitable equipment, for example presses to press water out and pumps to pump water into the process. This means that the process is prolonged and the equipment for the process is complex. In addition, the problems of the known processes include high energy consumption, a relatively short average fibre length of the obtained stock, and mainly due to this, deficiencies in the tensile strength and tear resistance of the printing paper produced from the stock.
The above-mentioned problems can be reduced by the method of the present invention for producing stock, the stock itself, the use of the stock in producing printing paper and the printing paper itself. The method of producing stock in accordance with the present invention is characterised in that the stock is screened at a consistency of no less than 10%. The stock produced in accordance with the present invention is characterised in that at least 40% by weight of the fibres do not pass through a Bauer-McNett screen with a mesh size of 28. The printing paper produced in accordance with the present invention is characterised in that it has been made of stock that has been produced by the method in accordance with the present invention, or stock that has the same fibre distribution as the stock produced by the method of the present invention.
In the method of producing stock in accordance with the present invention, the stock is screened at a high consistency, whereby it is not necessary to change the consistency to suit each refining step between the refiner and the screen, but the refining and screening can be done at essentially the same consistency. The amount of energy that is consumed for pumping water into the process and pressing it out of the stock can thus be saved. When using the new screening method, there is no need for pumps to pump water, presses to remove water or a latency chest between the refining and screening steps of the process, whereby the process becomes simpler.
When using the new type of screen that screens stock at a high consistency, the quality of the stock improves because the screen efficiently separates the coarser fibres that need further refining into a reject fraction, and flexible, long fibres into an accept portion. In this way the printing paper produced from the long-fibre stock has good formation. The resin remains in the fines because, due to the high consistency, it cannot spread onto the surface of the fibres.
The screen is simple and does not contain moving parts and therefore its manufacture and maintenance costs are low. The size of the screen is small because the screening process is carried out at a high speed. Due to its small size, the manufacturing costs of the screen are low. The screen can utilise the steam produced in the refiner as the screening force, as a result of which no separate sources of power are necessarily required.
In addition, thanks to the new refining process, the energy consumption is lower than in the known methods which aim at the same freeness value. In this patent application, freeness refers to Canadian Standard Freeness, the unit of which is ml CSF. Freenes
Gummerus Markku
Tienvieri Taisto
Fitch Even Tabin & Flannery
Fortuna Jos'e A.
UPM-Kymmene Corporation
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