FI74051B - FOERFARANDE FOER BEHANDLING AV MEKANISK MASS FOER ATT FOERBAETTRA DESS DRAENERINGSFOERMAOGA OCH VAOTTOEJNING. - Google Patents

Description

1 74051

Method for treating mechanical pulp to improve its dewatering and wet stretching properties

The invention relates to a method for treating a mechanical wood pulp which has acquired its fibrous shape by mechanical action and which is suitable for forming a sheet of paper, with the intention of improving its dewatering and wet stretching properties.

Usually, newsprint is made from a paper stock containing about 3 parts wood pulp and 1 part cellulose. Wood chips, made by pressing logs against a rotating grindstone, are obtained in almost 15 100% additional yields and are the cheapest ingredient in the paper raw material. In addition to economy, wood chips provide several inexpensive properties to a sheet of paper. In particular, the high light binding coefficient of wood chips causes the opacity of the paper and allows the use of thinner sheets. The strong oil absorption capacity of wood chips improves the ink reception capacity during printing. The pollution problems associated with the production of wood chips are minor.

The cellulosic ingredient is usually prepared either ·. : by the sulphate or sulphite process with yields ·. *. 25 45 to 65% by weight.

. Cellulose is expensive, places high demands on the pulp mill's wood stocks and causes considerable pollution problems. Newsprint is therefore currently being used to reduce the use of cellulose.

30 Cellulose is used because it gives the paper raw material properties that improve its processability. Workability refers to a combination of properties that allow the wet weight to be transferred at high speed through the forming, pressing, and drying section of the paper machine and allow the dry paper to be wound on a roll and printed with only an acceptable number of breaks. Workability is a crucial factor in paper machine and weight efficiency, which in turn determines productivity. Despite the above-mentioned disadvantages of cellulose, it remains essentially an indispensable ingredient in newsprint raw material for processability reasons.

Surprisingly, the mechanism by which cellulose pa-5 enhances processability is not well understood. Evidence is constantly emerging that the critical properties for efficient use of a high speed paper machine are high wet web elongation and water removal rate. The ability to stretch, both wet and dry, prevents the formation of breaks, preventing the accumulation of stresses around the failure points of the paper from which the breaks usually originate. The high water removal rate reduces the water content, resulting in a less brittle web. These phenomena have recently been addressed in articles by D. Y. Harvey, A. J. Barnet, and L. M. Lyne (Pulp and Paper Canada (7) T 154-6, July 1977); and D. F. Rutland, A. Y. Jones, P. M. Shallhorn, J. Tichy, and A. Karnis (Pulp and Paper Canada 78 (4) T 99-105, April 1977).

20 In conventional wood chips, wet stretching is achieved at the expense of water removal capacity, i. the wet elongation of well-purified groundwood is high but the dewatering capacity is low, while in poorly cleaned groundwood the thing is: vice versa. Wood chips can also be produced in refining • • 25 rolling mills in both open discharge types and printing. i in nested units, at higher temperatures. Received; * ’And chips called refined wood chips (RMP) and hot-ground wood chips (TMP) are of better quality than ordinary wood chips and are obtained in yields of more than 30% with less than 95% yields with low pollution problems. However, they are not a substitute for cellulose and are generally not accepted as the sole raw material for newsprint machines.

Efforts have been made to further improve the quality of wood chips by chemical pre-treatment of wood chips prior to processing. Numerous such methods are currently being developed and are referred to as chemical-mechanical (CMP) and thermochemical (TMCP) methods. These treatments generally increase production. Although useful properties have been developed at a certain cost, no substantial improvements have generally been observed in properties that are of paramount importance for handling, namely, dewatering and stretching when wet.

It has now surprisingly been found that the dewatering and wet stretching properties of wood pulp pulps can be improved by controlled chemical treatment of these pulps after their formation.

The process according to the invention for treating a certain type of mechanical wood pulp is characterized in that the pulp is boiled at a consistency of 5 to 50% with an aqueous solution of sodium sulphite in an amount of about 1 to 25% by weight of the mechanical pulp at an elevated temperature sufficient to provide reaction with the pulp and improve the dewatering and wet stretching properties of the pulp, but not enough time to reduce the pulp yield to less than about 90% by weight, wherein the aqueous sodium sulfite solution contains enough alkali to maintain the pH above 3 during cooking.

. ···. The improvements in the dewatering and wet stretchability of the mechanical wood pulp achieved by the method according to the invention make it possible to treat the abrasives thus treated; . the use of pulps in newsprint to provide good usability without or with a reduced proportion of cellulose.

The process according to the invention can be carried out on all the above-mentioned types of wood chips, namely ordinary wood chips, RMP, TMP, CMP and TCMP sand chips, and the improvement in dewatering ability and wet elongation properties is achieved with all these types of wood chips.

*. ·. As used herein, "wood chips" means wood that has been comminuted into a fibrous form by mechanical action and that is suitable for forming into a sheet of paper.

... "35 Wood chips are often prepared by a two-step refining process. In the production of a hot-ground ground sand, for example, a printed refining step is usually followed by refining in air. The chemical treatment of this invention can be applied to wood chips between refining steps.

The method according to the invention can also be used for separate parts separated from 5 wood chips. For example, wood chips are generally separated into "passed" and "rejected" fractions by means of screens or hydrocyclones before use. The process according to the invention can be applied separately to one or both of the nobles obtained in these separations.

In particular, the process of the invention comprises treating wood chips with sodium sulphite in combination with or without sodium bisulphite or additional alkali such as sodium hydroxide or sodium carbonate at an elevated temperature for a time sufficient to react with the wood chips but not so long as to cause substantial lignin leaching.

It has been found that as the reaction proceeds, the pH of the medium probably decreases as a result of the hydrolysis of the acetyl groups of the wood chips. If the pH falls below 3, there is a risk of damage to the fibers due to the hydrolytic effect, resulting in a decrease in strength. Wood fibers exposed to mechanical action,; weft refined fibers are particularly susceptible to acid degradation. Thus, it is essential to the present invention that sufficient alkali be present; in the chemical charge to prevent the pH from dropping below 3 during treatment. The exact amount of alkali required depends on the magnitude of the acetyl content of the wood raw material and cannot be precisely determined, but can be easily estimated experimentally.

The alkali requirement can be met by sodium sulfite alone. However, since only half of the sodium in sodium sulfite is available for neutralization, it is usually more economical to form part of the required alkali - *** by adding sodium hydroxide or sodium carbonate.

; : However, the pH of the mixture is preferably kept lower than 12, since the hemicelluloses are soluble in the fibers at pH values of 5: 74051, which correspondingly causes a loss of yield.

The amount of sodium sulphite used in the preferred embodiment of the process according to the invention is in the range from 4 to 5% by weight of the wood chips, although lower concentrations up to about 1% by weight can be used when the advantageous effect then decreases. If the sodium sulfite content is less than 1% by weight, the improvements are too small compared to the treatment cost. Similar improvements are observed with up to 25% of the weight of the groundwood, but the additional costs are unreasonable compared to the additional improvements achieved. Thus, about 1 to 25% by weight, preferably about 4 to 10% by weight, of the chemical charge, based on the wood chips, is generally used.

The chemical charge preferably has a pH between about 9-12 and contains sodium sulfite and sufficient alkali to maintain the pH greater than 3 during the reaction.

It is preferred to carry out the reaction at degrees of consistency between 5 and 50%, preferably about 25%. Lower degrees of consistency increase equipment size and energy costs and reduce the reaction rate due to dilution of the reactants, necessitating longer reaction times. and. Degrees of consistency greater than 50% cannot be easily handled; mechanically compressed.

The time and temperature required for the reaction may vary; within wide limits. The essential requirements are that the reaction must be allowed to affect the rate of dewatering and wet elongation, and must not go too far to achieve substantial lignin dissolution, as this would reduce the yield of groundwood and cause contamination problems. In practice, the application of the process according to the invention is limited to treatments in which yields of more than 90% by weight of the starting wood pulp are obtained.

*. According to the invention, the minimum reaction time to obtain improvements in wet strength exceeding 10% corresponds to about 30 minutes at 100 ° C, with shorter times being used at higher temperatures. The effects of time and temperature are combined with a descendant called the "H-factor" presented by K. E. Vroom in Pulp and Paper Mag. Can. 58 (3) (1957), p. 228. H-factor can be used to change time and temperature simultaneously to achieve the desired degree of cooking. For the purposes of the invention, the wood chips are thus subjected to a minimum degree of cooking corresponding to 30 minutes at 100 ° C and a maximum degree of cooking corresponding to a wood chips yield of 90% by weight.

The invention is illustrated by the following examples.

Example 1 This example illustrates the application of the invention to various wood chips.

Wood chips were treated with sodium sulfite solutions at pH 9 and pressed to a consistency of 25%. The amount of chemical remaining in the wood chips was 4% by weight based on the dry fiber content. The wood chips were then heated at 140 ° C for 30 minutes in an autoclave. Test sheets were made by hand and paper properties were measured.

The results obtained are shown in Table I below.

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From the results in Table I, it can be seen that the chemical treatment of each wood chippings improved dewatering and elongation in each case without adversely affecting other properties.

Example 2 This example shows the effect of reaction temperature on the properties of wood chips.

Wood chips impregnated with 4% by weight of sodium sulfite at pH 9 were heated for 30 minutes 10 at different temperatures. The properties of the obtained grinds are shown in the following Table II.

Table II

15 Λ. TMP TCMP

Feature --------

__before 100 ° C 140 ° before 100 ° C 140 ° C

Dewatering (s) 1.32 1.19 0.95 0.71 0.89 0.63 Wet elongation (%) 4.7 5.0 7.9 3.5 3.8 6.0

Tearing 22.0 22.4 18.0 16.3 17.1 15.8 20 Tearing length 3600 3740 3050 3150 3100 3000

Tearing 105 103 108 107 108 99

Dry elongation (%) 1.94 2.24 2.50 1.62 1.98 2.00 pH after reaction_ - 6.02 4.19 - 6.77 6.10 25

The results in Table II show improved ve-! dewatering and wet stretching properties with respect to TMP at 140 ° C compared to a reaction temperature of 100 ° C.

Example 3 30 This example shows the effect of the amount of chemical on the properties of wood chips.

The TMP powder impregnated with 4% by weight and 10% by weight of sodium sulfite at pH 9 was heated for 30 minutes at 140 ° C; 35 test sheets were prepared by hand and the properties were measured.

"The results are shown in Table III below.

i.

9 74051

Table III

Property Before 4% 10% 5 Dewatering (s) 1.32 0.95 1.16 Wet elongation (%) 4.7 7.9 7.9

Tearing 22 18 23.7

Tear length 3600 3050 3820

Tear 105 108 96 10 Dry elongation (%) 1.94 2.50 2.92 pH after reaction - 4.19 6.83

The values in Table III show a beneficial effect at both chemical levels. The main effect of the higher chemical level is to maintain a higher pH during the reaction, which leads to an improvement in tear and tear length.

Example 4 This example shows the effect of pH during the reaction on the properties of the groundwood.

The wood chips were heated with 4% by weight of sodium sulfite at different pH values.

The results are shown in Table IV

25 Table IV

V TMP__TCMP_

Property__before_Before

Initial pH 7.0 79 30 Reaction temperature ° C 140 140 140

Dewatering (s) 0.81 0.64 0.71 0.61 0.63 Wet elongation (%) 4.4 10.6 3.5 7.6 6.0

Tearing 17.2 8.7 16.3 9.9 15.8. Tear length 2970 1810 3150 2020 3000 35 Tear 117 106 107 112 108

Dry elongation 1.61 2.05 1.62 1.89 2.00 pH after reaction - 2.51 - 2.72 6.10 10 74051

From the results in Table IV, it is apparent that when the pH drops below 3, heating is associated with a severe deterioration in rupture and tear length. It was also found that this deterioration in properties was greater at higher temperatures and longer reaction times.

The present invention thus provides a method of treating wood chips which improves both its dewatering and wet stretching properties. Modifications 10 are possible within the scope of the invention.

Claims (4)

11 74051 A process for treating a mechanical wood pulp which has taken its fibrous shape by mechanical action and which is suitable for forming a sheet of paper to improve its dewatering wet stretching properties, characterized in that the pulp is boiled at a consistency of about 5 to 50% with aqueous sodium sulfite solution. in an amount of about 10 to 25% by weight of the mechanical pulp, at elevated temperature, a time sufficient to react with the pulp and improve the dewatering and wet stretching properties of the pulp, but not sufficient to reduce the pulp yield by about 90% by weight. below, where the aqueous sodium sulfite solution contains enough alkali to keep the pH higher than 3 during cooking. Process according to Claim 1, characterized in that the sodium sulphite solution has an initial pH of about 9 to 12 and an alkali content of 20 is obtained in part by means of sodium hydroxide and / or sodium carbonate. Process according to Claim 1 or 2, characterized in that the amount of sodium sulphite is from about 4 to 10% by weight of the mechanical pulp. - 25 Process according to Claim 1, 2 or 3, characterized in that the minimum cooking time corresponds to a cooking time of 30 minutes at about 100 ° C.