NO137650B - PROCEDURE FOR PREPARING A PREPARED MASS WITH GOOD TENSION STRENGTH WITHOUT REDUCED STRENGTH, FROM A SIMILAR CELLULOSE-VEGETABLE MATERIAL - Google Patents

Description

Vegetable materials, e.g. wood, straw, bamboo or sugar cane, which are used or can be used for the production of fiber materials, consist of several main components. Generally, vegetable fiber materials consist of approx. 15 to 30% lignin and extractives, e.g. resins, and the rest is approx. 70 to 80% carbohydrates. The carbohydrate part of the vegetable fiber material is approx. 10 to 30% hemicellulose and the rest is cellulose, and of the cellulose part of the carbohydrates, alpha cellulose accounts for approx. 45 more

55% and other celluloses approx. 5%. All percentages are given on a wood basis. As is known, one of the first steps for converting vegetable fiber material into fibers for use in the manufacture of paper or paper-like materials is a pulping process. The primary objective of the method is to remove most of the lignin from the vegetable fiber material and separate the remaining carbohydrate fibers into individual fibers. In known pulp production processes, e.g. kraft and sulphite, where an attempt is made to remove practically all lignin from the vegetable mass., ^most of the hemicellulose is lost, and the remaining cellulose' or hemicellulose fibers are chemically and/or mechanically damaged. This results in a significant yield loss and a large reduction in the strength of the paper or paper-like products due to fiber damage. With the power method, the normal yield is e.g. about. 45 percent by weight. But if only lignin and extractives were removed from the vegetable fiber material, the yield obtained would be 70-80%.

The present invention relates to a method by which a pulp with good tensile strength is produced from a lignin-containing cellulose vegetable material, without a reduction in tear strength.

The method according to the invention mainly removes only lignin and extractives from vegetable fiber materials and leaves the cellulose and hemicellulose in the material in an essentially undamaged state, and this results in cellulose and paper products that have new and unknown properties and exceptionally high strength figures. since the method according to the invention is selective and mainly only removes lignin and extractives,

yields are exceptionally high and in the range from 55 to 85%. The method according to the invention comprises a basic series consisting of chlorine dioxide treatment, alkali treatment and a chlorine dioxide treatment, and this basic series is carried out after a chemical and a mechanical pre-treatment of the vegetable fiber chips. Each of the steps in the basic series is followed by a wash with water, and this can be carried out with liquid that comes from other places in the process, e.g. by countercurrent washing which also has preservative benefits.

The present invention therefore relates to a method for producing a pulp from a lignin-containing cellulose vegetable material, with good tensile strength without a reduction in tear strength, and the method is characterized by a combination of the following features: (A) eh mild pre-treatment which includes (1) an alkaline boiling followed by a (2) mechanical treatment, the boiling (1) being carried out with neutral sulphite, according to the sulphate method or with sodium hydroxide, and where the mild pre-treatment ends when the yield of pre-treated material on a dry weight basis is 64-95% by weight , based on the inserted material, and where the mechanical treatment (2) is a paint with eh power application of 0.5-5 horse power days per 907.2 kg of dry vegetable material, and (B) treatment of the intermediate material from (A) for lignin oxidation and removal with chlorine dioxide in a series of

two to three chlorine dioxide oxidation steps where the total chlorine dioxide used is from 4 to 13%, calculated on the dry weight of the dry vegetable material, and where the chlorine dioxide may contain a small amount of chlorine, on an equivalent oxidizing agent basis that is generally formed when chlorine dioxide is produced, and where the amount of chlorine dioxide is smaller in each subsequent oxidation step, as each chlorine dioxide oxidation is followed by a water wash, and a caustic ex-

traction at 50 to 75°C, followed by a water wash.

When carrying out the method according to the invention, the water washing after the last chlorine dioxide treatment can be used as washing water for the previous alkali treatment, etc. countercurrent to the flow of fiber materials through the process to the first washing water resulting from the first, chlorine dioxide treatment; from this point the wash water can be sent to the sewer or treated for the recovery of chemicals.

When carrying out the method according to the invention, the chemical pretreatment is preferably carried out with neutral sulphite at a given chemical concentration and cooking cycle.

Pulp produced by the method according to the present invention has a higher polymerization degree, higher hemicellulose content, higher carboxyl content and lower carbonyl content than conventionally bleached kraft pulp from the same wood mixture. In addition, less energy is needed to refine than with a conventionally bleached kraft pulp from the same wood mixture. Laboratory sheets made from the pulp have superior wear resistance and tensile strength when compared to sheets from conventionally bleached kraft pulp from the same wood mixture.

The pulp properties mentioned above relate directly to paper produced from the pulp. such paper has higher tensile strength, wear resistance, burst strength, number of folds, tear strength and burst factor.

Figures 1 to 3 are flowcharts, and figures 4 to 13 are curves. Figure 1 indicates the basic series of the method according to the present invention. Figure 2 shows a particularly preferred embodiment of the method according to the present invention, and Figure 3 shows a particularly preferred chemical pretreatment step in the method according to the present invention. Figure 4 shows the yield of pretreated material with spent alkali in each step. Figure 5 compares G.E. brightness with alkali consumed in each step. Figure 6 compares percent reject with consumed alkali. Figure 7 shows total yield compared to pre-treatment yield. Figure 8 compares the consumption of chlorine dioxide with the pretreatment yield. Figure 9 shows a comparison between grinding degree and grinding time. Figure 10 compares the Schopper fold number with the effect of the cation treatment. Figure 11 shows a comparison of MIT false numbers with Schopper false numbers. Figure 12 indicates the Schopper fold number in relation to the number of days of ageing, and Figure 13 indicates the Schopper fold number in relation to the number of years of ageing.

Reference is now made to Figure 1, which fiber chips of which

any vegetable fiber material is first taken to a pre-treatment step 10. After the pre-treatment, the pre-treated chip is taken to a first chlorine dioxide treatment 11 where it is brought into contact with chlorine dioxide, either in aqueous solution or as a gas. The chlorine dioxide-treated material is then washed with water 12 to bring the mixture back to substantially neutral pH; after washing with water, the washed, chlorine dioxide-treated material is subjected to an alkali treatment 13 for 1 1/2 to 2 hours. The treated material is washed again with water to bring the material to a substantially neutral pH, which is denoted by 14, and to remove

ne water-soluble materials that have arisen in the alkali treatment step. Then the treated fiber material, which has been washed a second time, is subjected to another chlorine dioxide treatment 15, either aqueous or gaseous, and the material which has been treated with chlorine dioxide a second time is washed with water 16 to produce a pulp in high yield and with good brightness . Yields from this basic series vary from 55 to 85% at a G.E. lightness (TAPPI Standard T 217 m-48) of approximately 80 to 90.

Reference is now made to Figure 2, which relates to a particularly preferred embodiment of the present invention. Vegetable fiber chip is taken to a pre-treatment step 20 where it is subjected to a combined chemical/mechanical pre-treatment for easier removal of lignin and extractives. After the pretreatment step, the pretreated material is subjected to a chlorine dioxide treatment 21, a washing with water 22, alkali treatment 23, washing with water 24, chlorine dioxide treatment 25, washing with water 26, alkali treatment 27, washing with water 28, chlorine dioxide treatment 29 and washing with water 30.

In a particularly preferred embodiment, the water is only added to the last water wash 30 through line 40 and circulates countercurrently to the flow of materials as indicated by line 41 to the penultimate washing stage 28, and from there through

ning 42 to the preceding washing stage 26, then through line 43 to washing stage 24 and from this washing stage to the first washing stage 22 through line 44, and then to sewage or chemical recovery through line 45. It is of course clear that fresh water or make-up water can be added to which any of the washing stages through the lines 46,47,48,49 and that the water can be led to sewage or chemical recycling from any or all of the washing stages through the lines 50,51,52 and 53.

Chlorine dioxide, either in aqueous solution or in gaseous form, can be added to each of the chlorine dioxide stages through lines 60, 61 and 62, and an aqueous solution of alkali can be added to each of the alkali treatment stages through lines 63 and 64. The pulp produced by the method according to the invention, is extracted at 70 in a high yield of approx. 5.5 to 85% and with a G.E„ brightness of

about. 80 to 90.

A particularly preferred embodiment according to the present invention is a method in five steps which successively consists of chlorine dioxide treatment, alkali treatment, chlorine dioxide treatment, alkali treatment and chlorine dioxide treatment, and with washing with water between each step, and that the first chlorine dioxide treatment takes place after a chemical/mechanical pretreatment.

Figure 3 shows a flow chart of a preferred chemical/mechanical pretreatment. The vegetable fiber chips are added to a chemical treatment step 70 where the pre-treatment takes place. This pre-treatment can have the character of a weak mass formation up to a high yield using e.g. a neutral sulfite method. During the pre-treatment, the fiber chip is pre-treated to a yield of at least 64 percent by weight based on the dry weight of the chip of the vegetable material. After the chemical treatment, the pretreated material is refined in step 71, washed in step 72 and dewatered as indicated at 73 to provide a combined chemical/mechanical pretreated material ready for treatment in the first chlorine dioxide treatment step as indicated at 21 in Figure 2.

The method according to the present invention is intended for the production of a new cellulose and paper product from any vegetable fiber material containing lignin.

As is necessary in all pulp production methods, foreign materials should be removed from the vegetable material before the present method is carried out. When it e.g. applies to wood, it must be debarked before use. In the following description, the vegetable fiber material will be wood; however, it will be understood that the method according to the invention is applicable to all vegetable fiber materials.

The bark of wood, whether hardwood or conifer, can be transferred to chips with a "carthage multiknife chipper" or equivalent equipment. The tiles should have a length of approx. 15 to 75 mm, a width of 10 to 40 mm and a thickness of 0.5 to 20 mm. During the chemical-mechanical pretreatment, it is preferable that the tiles have a length and width that roughly corresponds to what is stated above and that the thickness is from 2 to 5 mm. After the chipping, the manufactured tiles are pre-treated in a pre-treatment step.

The pre-treatment step is, as mentioned, a combination of chemical and mechanical treatment. During the mechanical pre-treatment, the vegetable fiber chip is subjected to a known unraveling effect, refining or splitting effect with a "Pallman knife ring flake", which, when cut, reduces chips of conventional size to thin flakes while maintaining the chip length and thickness, or in a standard disc refiner or equivalent . It is also known that chips can be treated with water or steam before the splitting or refining, either under vacuum or under pressure, and after the splitting or refining the resulting fibers or fiber bundles should be as small as possible without much damage to the fibers. The optimal size depends on the equipment used for the splitting and refining.

The refining step can be carried out with standard disc refiners or other equivalent equipment and is carried out so that a minimum particle size is obtained without significant fiber damage. Although it is desirable to refine the chemically pre-treated chips as much as possible, so that the subsequent washing can be carried out more efficiently and that the reaction with chlorine dioxide can proceed smoothly and result in a uniform mass that is free of chips, however, it is clear that too much refining at this stage will not only break up the wood structure into fibers as desired, but it will also destroy and break up the individual fibers. This will reduce the quality of the final bleached pulp and this is undesirable. The primary aim is therefore to break up the wood structure, but not the fiber structure, so that the particle size after refining is as fine as possible, in accordance with the least possible fiber destruction. In a particularly preferred embodiment of the method according to the present invention, this is achieved by refining at an elevated temperature, preferably in one pressure refiner. Temperatures of up to 100°C can be achieved in a conventional disk raf f iner. Higher preferred temperatures necessitate a pressure purifier. A temperature of from 66°C to approx. 200°C with a to-2 2 corresponding pressure of from 1.0 kg per cm to 17.2 kg per cm is suitable; a temperature of from 100°C to approx. 143°C at an equivalent pressure of from approx. 1.0 kg per cm to approx. 4.3 kg per cm is preferred. In general, it is preferred to carry out the mechanical refining at a pressure above atmospheric pressure and at a temperature of from 100°C to 170°C.

Temperatures above the preferred in a pressure refiner tend to reduce pulp quality. A power consumption of from 12.1 to approx. 121 horsepower hours (0.5-5 HP days) per 907.2 kg of dry pre-treated wood chips are used. There is a clear saving in power consumption with pressure refining compared to conventional refining, in addition to the improvement in pulp quality. Generally, it is not necessary to use higher power consumption on the pressure refiner, since the resulting refined material is finely divided and well suited for washing and for rapid and efficient conversion in the chlorine dioxide bleaching process according to the present invention.

It has been shown that if one uses a chemical pre-treatment rather than just a mechanical pre-treatment, the amount of fines during the refining is reduced, the optimal diameter of the fiber bundles is reduced, the energy consumption during the refining is reduced, the amount of chlorine dioxide necessary for the pulp production to a desired lightness is reduced, the quality of the resulting pulp

is improved, and the yield of the pulp is increased.

After the pretreatment step, the pretreated refined material goes to the first chlorine dioxide treatment step. In this step, the unraveled pulp of fiber bundles from the pretreatment refining has a concentration of from 5 to 50 percent by weight based on the total weight of unraveled pulp and water. If chlorine dioxide is used as an aqueous solution, it can be added as approx. 1 percent by weight in aqueous solution, and, depending on "the desired chlorine dioxide concentration, which is indicated below, additional water may be added to produce a mixture of the desired concentration. If gaseous chlorine dioxide is used, an inert diluent may be required, eg air, to avoid the risk of explosion.

Any conventional treatment tower can be used for the chlorine dioxide treatment step, and heat can be added if necessary. Additional heat can also be applied to reduce the time that the unraveled pulp is in contact with chlorine dioxide,

and the time is from approx. 10 minutes to 2 hours, depending on the concentration, temperature and product yield resulting from the pretreatment step. Usually, the unraveled mass of fibers is allowed to be in contact with chlorine dioxide until the added chlorine dioxide has essentially been used up. The pH of this system can initially vary from 4.0 to 8.0, and when chlorine dioxide is consumed, the pH value of the treated solution will be 0.5 to 3.0. After the chlorine dioxide treatment, the resulting mass is washed with water on a conventional vacuum cylinder washer or equivalent.

After the first washing with water, and when the material has a practically neutral pH value, the washed material is subjected to a first alkali treatment in a conventional treatment tower that is commonly used in the area. In the alkali treatment, any water-soluble alkaline material can be used, e.g. sodium hydroxide, ammonium hydroxide, sodium carbonate, ammonia gas or mixtures of these - however, an aqueous solution of sodium hydroxide is preferred. During the treatment, the alkali addition should be approx. 4% based on oven-dry weight of fibrous material, and sufficient water may be removed or added to produce an aqueous mass having a concentration of from .5% to about 50 percent by weight based on the total weight of unraveled pulp and water. The alkali treatment should continue for at least approx. 1/2 hour at a temperature of from 50°C

to approx. 75°C, and a temperature of 65°C is preferred. ■ After the alkali treatment, the alkali-treated material is subjected to another washing with water under substantially the same conditions as the first washing with water, to remove extracted material and remaining chemicals.

The second chlorine dioxide treatment can be carried out in a conventional treatment tower as described for the first chlorine dioxide treatment, and the desired concentration of material in the tower is essentially the same for the second chlorine dioxide treatment as for the first. Either gaseous chlorine dioxide or an aqueous one, approx. 1% by weight solution can be added to the second treatment step. In this step, the pH is initially from 4.0 to 8.0 and at the end approx. 2.0; the chlorine dioxide treatment continues until substantially all of the chlorine dioxide added to the treatment step is used up. The temperatures for the second chlorine dioxide treatment are adjusted to 40°C to 60°C to keep contact times to a minimum of from 30 minutes to approx. 4 hours to consume added chlorine dioxide. After the second chlorine dioxide treatment, the treated material is subjected to a third washing with water under essentially the same conditions as the first and second washing with water. After the third washing with water, another alkali extraction is carried out, followed by a washing with water under substantially the same conditions as in the first alkali treatment and washing. The washed material in this step of the process can be screened, if desired, to remove chips from fibrous materials that may remain, and these chips are scrapped or returned to the first chlorine dioxide treatment step.

The treated material, which may or may not be silt, is subjected to a third chlorine dioxide treatment under the same conditions of concentration and chlorine dioxide concentration as the first and second chlorine dioxide treatment steps for approx. 2 to 6 hours, depending on the desired lightness of the manufactured product. The temperature in this third chlorine dioxide treatment step is from 40°C to 80°C, and after the third chlorine dioxide treatment, the treated material is subjected to a fifth and final washing with water under the same conditions as the previous washings with water.

The total concentration of chlorine dioxide used in the multi-stage process, whether it is two or three chlorine dioxide stages, depends on the yield of the product obtained from the pre-treatment stage and the desired brightness of the resulting product from the last treatment stage. Generally, the total chlorine dioxide consumption at multiple stages, regardless of the number of stages used, is from 1.0 to 15.0 weight percent based on the total dry weight of fibrous material added to the pretreatment stage. It has been found that the total concentration of chlorine dioxide used must be from 4 to 13 percent by weight based on the total weight of dry fiber material added to the pretreatment step.

The amount of chlorine dioxide that is added to each chlorine dioxide stage depends on the number of chlorine dioxide stages used and on the yield during the pre-treatment. For a given total amount of chlorine dioxide used, it has been shown that approx. twice the amount used in the last step should be added to the chlorine dioxide step that precedes the last and twice the amount used in the preceding step should be added to the next preceding one, etc. E.g. in a process with three chlorine dioxide stages, this means that approx. 4/7 of the total chlorine dioxide will be added to it. first stage, approx..2/7 of the total chlorine dioxide will be added to the second stage and approx. 1/7 to the third step.

As previously indicated, the preferred chemical pretreatment in the method according to the present invention is carried out with neutral sulphite. Among available neutral sulphite pre-treatments, pre-treatment with neutral sulphite with sodium as a base is preferred. As is known, a standard neutral sulphite treatment comprises boiling fibrous, vegetable material for 10 to 15 minutes, -at 1-77°C at a concentration of approx. 10% sodium sulphite and approx. 3% sodium carbonate, the addition being based on calf weight. Although this standard neutral sulphite treatment has advantages, in the method according to the present invention it is preferred to use a special neutral sulphite treatment. concentration of from 5

to 30% sodium sulphite and from 3 to 25% sodium carbonate so that the ratio between sodium sulphite and sodium carbonate is approx. 1.2 or more. Particularly preferred concentrations are from 7 to 20% sodium sulfite and from 5 to about 18% sodium carbonate, all percentages being based on the dry weight of the vegetable material. A special one

preferred ratio of sodium sulfite to sodium carbonate is from 1.2 to 1.5. The specified relationship between time and temperature takes

aim to provide a suitable impregnation of the liquid into the tile before reaching a temperature of approx. 1"+9°C. This ratio depends on the type of wood and the type of chip and also the previous treatment of the chip. When a chemical pre-treatment is carried out. in accordance with the

as stated above, higher final yields and better quality products are obtained compared to other mechanical and chemical pretreatments.

In the following, all tests on the paper and paper products are carried out at a gram weight of 60 g/m 2 if nothing else is stated.

The pulp produced by the invention is chemically unparalleled in that it has a higher degree of polymerization, a higher hemicellulose content, a higher carboxyl content and a lower carbonyl content than conventional pulp produced from the same mixture. Due to the high overall yield compared to conventional bleached kraft pulp, it contains more hemicellulose. At the same time, however, the average viscosity polymerization degree of the pulp is higher than with conventional bleached kraft pulp. The conclusion is that the method according to the present invention breaks down wood cellulose less than conventional methods when going from wood to cleaned pulp. The carboxyl content of the pulp produced according to the present invention is at least twice as high as the carboxyl content of conventionally bleached pulps and has a carboxyl number (TAPPI Standard T 237su-63) that is greater than 6, preferably greater than approx. 9, and especially higher than approx. 12, and as high as e.g. 20 and even higher. At the same time, the carbonyl content is only half to a third of the carbonyl content of conventionally bleached pulp from the same batch.

It is known that the brightness stability of a mass is in a

certain relation to the carbonyl content. in the mass. The higher the carbonyl content, the higher the loss of brightness during ageing. As the product according to the present invention has a very low carbonyl content, it is quite stable and loses little brightness with ageing. By

by comparison, pulps with a high carbonyl content which are not produced according to the present invention result in rather poor lightness stability.

The mechanical properties of the pulp produced by the invention due to its unique chemical properties are ease of refining, fiber wear resistance and the ability to form fiber-fiber bonds in dry sheets. The pulp according to the present invention consumes only 1/3 to 1/4 of the energy consumption necessary to grind a conventionally bleached kraft pulp from the same wood mixture to the same level of freeness. The grinding speed of the pulp according to the present invention is 4.2 times greater than the speed of the corresponding bleached kraft pulp when it is produced from hardwood mixtures from the northern states and 3 times greater when using softwood mixtures from the southern states. Since the required grinding time to grind a mass is directly proportional to the energy consumption to grind the mass, the mass according to the present invention provides significant savings in refining to reach a given level of freeness. The speed of mechanical refining (ml. Canadian St per minute when grinding according to TAPPI Standard T 200ts-66) can be greater than 15, preferably greater than 20, and particularly preferably greater than 25, and can go up to e.g. 50 and even higher. When paper is produced from the pulp according to the present invention on a paper machine, faster dewatering, increased ability to retain fibres, increased wet strength and increased drying speed are obtained in comparison with conventional pulps produced from the same wood.

Sheet strips of pulp according to the present invention have superior wear and tear strength compared to conventional bleached kraft pulp from the same wood mixture. This is surprising since pulp with higher wear resistance usually has lower tear strength. That the mass has both a superior wear resistance and a superior tear strength shows another unique physical property of the mass according to the present invention; it also has superior individual fiber abrasion resistance.

The properties of these pulps directly affect the papers produced from them. Machine-made paper from pulp according to the present invention provides high wear resistance, tear strength, burst strength, number of folds, tear-out strength and splitting strength. The fat density (TAPPI Standard T 454ts-66) of the paper according to the present invention can be higher than approx. 500 seconds, preferably higher than approx. 1000 seconds, and can go up to e.g. 1800 seconds and even higher; the wear resistance (TAPPi Standard T 404ts-66) for paper from hardwood pulp can be greater than approx. 80%, preferably greater than approx. 100%, and preferably greater than approx. 120% and can go up to e.g. 200% and higher, and for paper from softwood pulp it can be greater than approx. 120%, preferably greater than approx. 140%, and especially greater than approx. 160% and can go up to e.g. 250% and even higher; burst strength (TAPPI Standard T 403 ts-63) for paper from hardwood pulp can be greater than approx. 140%, preferably above 190%, and up to 250% or higher. For paper from softwood pulp, the burst strength can be greater than 160%, preferably above 230%, and up to 300% or higher. Tear strength (TAPPI Standard T 414ts-65) for paper from hardwood pulp can be greater than 160%, preferably above 300%, and up to 400% or more, and for paper from softwood pulp it can be greater than 320%, preferably above ziono/ 4.-1 JCrø fold count, TAPPI Standard T 423 su-68), 420%, and up to 600% or more. Fold number/y:or paper from hardwood pulp can be greater than 500, preferably above 1500, and up to 3000 or higher, and for paper from softwood the fold number can be greater than 1000, preferably greater than 4000 and up to 6000 or higher.

The following examples illustrate the invention.

Example 1

To demonstrate the advantages of using several chlorine dioxide-alkali treatment steps in the delignification compared to a single chlorine dioxide step, experiments were carried out with finely ground conifer flour from the southern states, which was then reacted with aqueous chlorine dioxide solutions at an initial pH of 4 and at 70°C with a reaction time of 1 hour, and this led to complete consumption of chlorine dioxide.

After the reaction-extraction series, the total yield and the lignin content (Klason) were determined for each experiment. The carbohydrate content was calculated by difference. Two factors were determined:

Optimum conditions are achieved with a high value of E, keeping the value of R as close to 1 as possible. By comparison, a bleached softwood kraft pulp would have an R-value of approx. 0.55.

The table below shows the results of these tests for a single chlorine dioxide step, a single step followed by an alkali treatment and a three step series. It will be seen that by using an alkali treatment step (case 2) the effect is doubled without reducing the retention or the selectivity factor to any particular extent. Since the products of the alkali treatment step are very dark, a chlorine dioxide step is required at the end to give a bleached product. The high E and relatively high R are maintained through the second chlorine dioxide stage, and this shows the superiority of the multi-stage experiment.

With the chlorine dioxide-alkali treatment-chlorine dioxide series as shown above, the resulting product does not have as high a brightness as it is possible to achieve. When a fibrous material obtained from mechanical or chemical-mechanical pretreatment of wood (as opposed to the fine sawdust as indicated above) is processed in a three-step series, the resulting mass contains some chips and fiber bundles. By using more chlorine dioxide in each of the two steps and also by using somewhat stronger alkali treatment, the results are improved. However, this leads to an increase in chlorine dioxide consumption and a decrease in R. The preferred method of producing a bleached pulp with a negligible chip content is therefore carried out by extending the series to five stages of alternating chlorine dioxide-alkali treatment (with intermediate washing). . Multiple steps give the alkali additional opportunity to soften and disperse fiber bundles, and also to remove additional alkali-soluble lignin materials, thereby reducing total chlorine dioxide consumption.

Example II

To show the preferred distribution of chlorine dioxide between the steps, hardwood chips were chemically pretreated with neutral sulfite with sodium as a base to approx. 85% yield, and followed by mechanical refining in an 8" laboratory disc refiner and finally well washed.

Representative examples of the resulting pretreated material were subjected to various treatments with chlorine dioxide and alkali. The following conditions were kept constant in these experiments: First chlorine dioxide stage: 10% concentration; the reaction got

run all the way out;

First alkali treatment step: 4% sodium hydroxide based on

the mass; 12% concentration; 65 C treatment for 1 hour;

Second chlorine dioxide stage: 10% consistency; 65°C turnover to

chlorine dioxide was used up;

Second alkali stage: Same conditions as in the first

alkali stage;

Third chlorine dioxide stage: Same conditions as in the second chlorine dioxide stage.

It was washed with water in all stages. First, the distribution between the first two chlorine dioxide stages was investigated with five combinations of chlorine dioxide in these stages by keeping the total amount constant at 5%, based on pre-treated material, eliminating the last (third) chlorine dioxide stage and determining yield and lignin.

The results are shown in the following:

It appears from the table that maximum delignification is achieved with at least half of the added chlorine dioxide in the first stage. A maximum yield is assumed to be approx. twice as much chlorine dioxide in the first stage as in the second.

The experiments were repeated using a five-stage series (chlorine dioxide-alkali treatment-chlorine dioxide-alkali treatment-chlorine dioxide), using a fixed addition of 4% chlorine dioxide in the first stage and 5% distributed between the second and third chlorine dioxide stages as shown below. In all experiments, added chlorine dioxide is based on pre-treated starting material.

The lignin figures in the table again show that optimal treatment requires more than half of the 5% chlorine dioxide to be supplied in the second chlorine dioxide step; there is no significant change in lignin-free yield between experiments 4 and 5.

From these two comparisons, it appears that if you want to make the consumption of chlorine dioxide as low as possible and the yield of lignin-free (bleached) as high as possible, the addition of total chlorine should be distributed between the stages by approx. 4/7 in step 1, 2/7 in step 2 and 1/7 in step 3.

Example III

To demonstrate the advantages of using sodium hydroxide instead of sodium carbonate or ammonium hydroxide in the alkali treatment steps, the following comparisons were made and these show that sodium hydroxide is more effective in extracting lignin than sodium carbonate or ammonium hydroxide. At the same time, sodium hydroxide is at least as effective as the other two when it comes to retaining the carbohydrate content of the pulp.

The first two experiments below show a comparison between sodium hydroxide and sodium carbonate. Mixed hardwood chips from the southern states are chemically pretreated with a neutral sulphite treatment and mechanically treated in a laboratory disc refiner to a resulting pretreatment yield of 84%. After washing with water, the pre-treated material was treated with 5.5% chlorine dioxide (wood base), the alkali treatment was carried out at 10% concentration and 65°C, and the effect of the different extraction conditions was determined by measuring the yield and lignin content of the washed pulp . The alkali treatment conditions and the experimental results were as follows:

That shows that the same alkali treatment time results in much lower extraction of lignin with carbonate than with hydroxide. The hydroxide does not remove more carbohydrate than the carbonate does. An increase in the treatment time with carbonate increases lignin extraction, but does not give equivalent results as with hydroxide. Longer carbonate treatment results in increased extraction of carbohydrate and this is undesirable.

Similar experiments were carried out using ammonium hydroxide and sodium hydroxide. The starting material was a 73% neutral sulphite hardwood compound which had been pre-treated in a disc refiner and with a preceding 4% chlorine dioxide treatment step. The treatment conditions and results are shown below.

The results show that the ammonium hydroxide treatment, even at a higher chemical level, is less effective in removing lignin. Ammonium hydroxide also offers no benefits when it comes to retaining the carbohydrates.

Example IV

To demonstrate the preferred conditions of temperature and added alkali to be used in the alkali treatment steps of the chlorine dioxide-alkali treatment series, a pretreated wood as prepared in Example X was delignified using the following four-stage series:

It was washed with water after each of the steps. The resulting masses were analyzed for yield, G.E. brightness (TAPPI Standard T 217 m-48), and sieve rejects (amount returned on a 0.006" vibrating laboratory plan sieve). The results of two extraction temperatures (20 and 65°C) and an added amount of alkali from 1 to 6%

(mass basis) is shown in figure 4 (yield), figure 5 (lightness) and figure 6 (rejects).

Figures 4 and 5 show that yield losses and lightness are relatively little affected by temperature. However, while the loss in yield is approximately proportional to added alkali, there is little benefit for brightness and reject amounts to using more than 4% sodium hydroxide at 12% concentration. In order to therefore keep the yield and achieve maximum lightness and minimal reject amounts, approx. 4% added alkali and 12% concentration are used. Figure 6 shows that a higher temperature (65°C) is advantageous for reducing the reject quantities. Since this has no adverse effect on the yield, preferred temperature conditions for the glkali treatment are above room temperature in the range of 65 C.

Example V

In order to show the preferred yield range of pretreatment with neutral sulfite with sodium as a base, a series of pretreatments with neutral sulfite with sodium as a base was carried out. In all trials, a sufficient (and constant between trials) liquid impregnation of hardwood chips from the southern states was allowed before the pretreatment temperature was raised to the maximum temperature of 168°C. After chemical pretreatment, the hardwood material was refined in an 8" laboratory disc refiner and then thoroughly washed. Pulp was then prepared which was bleached to 80 G.E. brightness (TAPPI Standard T 217 m-48) by the preferred five-step chlorine dioxide-alkali treatment series, using total amounts of chlorine dioxide corresponding to the level of the pretreatment yield shown in Example IX The pretreatment yield ranges from 61% to 86%.

Yield figures, strength figures of standard laboratory sheets and some figures for the degree of polymerization (D.P.) of the resulting bleached pulp are given in the following table.

The table shows that there is a tendency towards increasing (all) strength properties when the pretreatment yield is increased. It is also clear that this increase is usually more prominent in the yield range from 61 to 70%, and then an almost constant high level of strength is maintained as the yield is further increased.

This tendency can be deduced from the figures for the degree of polymerization (DP). When bleached yield increases (under otherwise constant conditions), the pulp contains less low-DP hemicellulose and (relatively) more high-DP cellulose. If no unwanted chemical degradation therefore took place, one would expect a constant increase in DP as the pretreatment yield increases. In reality, the opposite occurs, and shows that the chemical pretreatment (especially with pretreatment yields in the area below 70%) has a chemically degrading effect on retained carbohydrates.

From both the experimental results on measurements of mass DP and molecular lengths, it is clear that maximum mass strengths are favored at higher chemical pretreatment yields. This effect is also more pronounced at the low end of the pretreatment yield scale, and suggests that preferred conditions will include pretreatment yields above approx. 64%.

An upper limit has been determined for pretreatment yield by looking at the relationship pretreatment yield-final bleached yield-chlorine dioxide consumption. Figure 8 shows the relationship between pre-treated yield and chlorine dioxide consumption, as 100% pre-treatment yield corresponds to no chemical treatment. In reality, the neutral sulfite line is followed until a pretreatment yield of 95% is achieved; at this point consumption increases rapidly from this line and passes through the point of no chemical pretreatment. With pretreatment yields of over 95%, a disproportionately large amount of chlorine dioxide is thus consumed. This is of course very undesirable.

Figure 7 shows a parallel behavior for bleached yields. In the case of pretreatment yields above approx. 9 5%, actually decreases the bleached yield to the point equivalent to mechanical pretreatment only. Since maximum yield is desired, such a reduction is undesirable.

A further reason for keeping the pretreatment yield below 95% is seen in example IX, where it is shown that 100% pretreatment yield gives poor paper strength compared to paper in areas below 95% pretreatment yield.

Example VI

In order to show preferred conditions for neutral sulphite treatment with sodium as a base, it is shown in the following that suitable impregnation of the tiles with pre-treatment liquid before heating is necessary to give maximum strength of the laboratory sheets and reduce the content of fiber bundles and tiles. It has also been shown that suitable ratios of pretreatment chemicals are required to provide maximum strength and light pulping and bleaching in the subsequent chlorine dioxide-alkali treatment series.

Experiments were carried out using two extremes of liquid impregnation before the chemical pre-treatment, and with different ratios of sodium sulphite to sodium carbonate in the pre-treatment liquid. In all the experiments, sufficient pre-treatment chemicals were added so that the final liquid pH was above 7. If you let it fall below this value, you get a weak mass. Raw-

the material was a hardwood chip mix from the southern states. After chemical pretreatment, the produced materials were refined under con-

constant conditions in an 8" laboratory disk refiner, and then washed well with water, and then treated in a five-stage chlorine dioxide-alkali treatment series for pulping and bleached to 80 G.E. brightness (TAPPI Standard T 217 m-48). The total chlorine dioxide to obtain this lightness was determined and analyzes were carried out on laboratory sheets of bleached pulp.

The critical experimental conditions and the indicated results are shown in the following. Experiments 1 and 2, which were carried out at the same pretreatment yield, show the effects of variations in added chemicals. While the consumption of chemicals (chlorine dioxide) is constant, a significant increase in the strength of the laboratory sheets is obtained when the sulphite/carbonate ratio is reduced from approx. 5 to something over 1.

Experiments 3 and 4 show the effect of a further reduction in this base ratio from 1.2 to 1.0. You get a significant reduction in the strength figures of the laboratory sheets together with a large increase (approx. 25% based on the normal chemical consumption) in the consumption of chlorine dioxide. While there are thus optimal conditions at a sulphite/carbonate ratio of something above 1, it is critical that this ratio is not allowed to go below approx. 1,2, due to the adverse effects with regard to both physical properties and chemical consumption.

Tests 1, 3 and 5 show the importance of suitable liquid impregnation when pre-treating the tile. Since the chemical ratio in experiment 5 lies between those in experiments 1 and 3, the observed, marked reduction in physical properties of the laboratory sheets is due to insufficient chip impregnation. Optimum impregnation conditions depend on the type (wood species) and chip dimensions of the raw material. Sufficient pretreatment chemical must be added to maintain the pH at 7 or above during pretreatment.

The ratio between added sodium sulphite and sodium carbonate should be kept close to approx. 1.5 to get the highest mass strength. Higher values result in reduced strength figures; values below approx. 1,2 gives weaker mass and increased chemical consumption (chlorine dioxide).

Adequate penetration of the pretreatment liquid into the raw material must be achieved, otherwise the strength properties will suffer and the chip content / will increase. Optimal conditions depend on the raw material structure and particle dimensions.

Example VII

In order to show the extent to which chlorine can replace chlorine dioxide in the present mass production bleaching series, and because of the potential economic benefits, it was investigated how desirable it might be to replace other selective delignifiers instead of at least part of this chlorine dioxide.

Usually, with an equivalent addition of bleaching chemicals, a higher bleached brightness is obtained by using mixtures than by using either chlorine or chlorine dioxide alone when bleaching kraft pulp. Since, however, the starting material with a bleachable kraft pulp is very different from neutral sulphite pretreated materials in this case, and since a desired effect of chlorine or chlorine dioxide additions to conventional kraft pulp is mostly to bleach (the lignin content of the pulp at the start maybe 2%), while chlorine dioxide in the present case is used to remove a large amount of lignin which is chemically different from the aforementioned (lignin content in pre-treated mass approx. 15%) but also to bleach, previously known results are not directly transferable to the present invention.

Since some systems for the production of chlorine dioxide result in the simultaneous production of chlorine, and since the efficient utilization of this chlorine is essential for good economy in the production of chlorine dioxide, attempts were made to investigate the possibility of replacing chlorine with chlorine dioxide at certain points in the the mass production process. Hardwood chips from the southern states which had been pre-treated with sodium neutral sulphite as a base were mechanically refined in a laboratory disc refiner and washed well with water before the mass production bleaching series. In another experiment, all chlorine was replaced with chlorine dioxide. Chemical addition and the results in terms of pentosan content in the pulp, lightness, sieve rejects (amount returned on

a vibrating laboratory plan sieve with o.2 mm slits) and brightness of the laboratory sheets can be seen from the following:

It is clear from the very low lightness, low pentosan content, low DP and high sieve rejects that chlorine cannot completely replace chlorine dioxide.

A series was then carried out where all or half of the normally added chlorine dioxide in the first step (equivalent oxidation base) was replaced with chlorine. This time the raw material was also hardwood chips from the southern states which had been pre-treated with neutral sulphite, mechanically refined in an 8" laboratory disc refiner; the pre-treatment yield was 85%. The test conditions and the lightness and strength figures of the manufactured laboratory sheets from the resulting pulp are shown in the following:

From the results it was concluded that there was no synergistic effect of chlorine-chlorine dioxide mixtures, as has been indicated for such mixtures when bleaching kraft pulps; the brightness is somewhat lowered by 50% chlorine substitution (oxidizing equivalent basis), and is significantly influenced in a negative direction by using only chlorine in the first stage of the pulp production; if only chlorine is used in the first step, this results in non-bleachable, non-defibrable chips or fiber bundles; and the strength properties are not affected to any particular extent by substituting only half, but are reduced very significantly when only chlorine is used in the first stage.

While using chlorine in any ratio according to the present invention offers no advantages in relation to pulp quality, certain amounts of chlorine can be used in the first chlorine dioxide stage without harmful effects. An upper limit for the use of chlorine is approx. 25 to 30% of the total required chlorine dioxide on an equivalent oxidation basis; this maximum corresponds to a weight ratio of approx. 50% chlorine and 50% chlorine dioxide.

Example VIII

In order to show the advantages of using neutral sulphite pretreatment with sodium as a base (sodium sulphite and sodium carbonate) rather than neutral sulphite with either nitric acid or ammonia as a base, these three combinations were investigated.

The three chemical-mechanical pretreatments were achieved by

to use the conditions shown below and by using hardwood chips from the southern states as raw material. In all the experiments, the pretreatment was followed by refining in an 8" laboratory disc refiner, well washed, and using the series: chlorine dioxide-alkali treatment-chlorine dioxide-alkali treatment-chlorine dioxide treatment. The experimental conditions and results appear from the following:

It appears that the pre-treatment with nitric acid results in both a lower bleached yield and a lower strength of the laboratory sheet, and this indicates too great a breakdown of the carbohydrates during this pre-treatment. Sodium sulphite with sodium as a base gives a stronger mass than pretreatment with ammonia as a base. With pre-treatment with ammonia as a base, it is also more difficult to delignify and bleach, and the pulp contains somewhat more chips and fiber bundles.

It appears from this comparison that none of the neutral sulphite bases provide a better pre-treatment than nitric acid. Sodium base is also superior to neutral sulfite with ammonium as base, when other factors are not taken into account. The ammonium base gives a very acceptable bleached pulp with a high yield, and where such factors as contaminants are of great importance, the ammonium base is very attractive.

Example IX

In order to demonstrate the advantages of using neutral-sulfite pretreatment over a high-yield power pretreatment or a mechanical pretreatment, the following experiments were conducted.

The raw materials were a mixture of hardwood chips from the southern states (approx. 1/3 oak, 1/3 yellow poplar and 1/3 rubber trees). During the chemical pre-treatments (kraft and neutral sulphite) sufficient time was allowed for liquid impregnation before heating to maximum temperature. The chemically pre-treated material (and the leaf tea chip after evaporation in the case of mechanical pre-treatment only) was then refined in an 8" laboratory disc refiner to provide a feedstock for the subsequent treatment. The pre-treatment was analyzed as follows:

The selectivity of various chemical x-treatment yields was determined as follows:

The table above shows that in a high-yield pulp production method, the neutral sulphite pretreatment gives the greatest selectivity and therefore the greatest potential opportunity in the final bleached or semi-bleached pulp yield. As the pretreatment yield approaches that normally achieved in the pulping part of a pulp bleaching series, the selectivities approach each other.

When pulping and bleaching this pretreated material to 80 G.E. brightness (TAPPI Standard T217 m-48) by using a chlorine dioxide-alkali treatment-chlorine dioxide-alkali treatment-chlorine dioxide treatment series, one obtains the selectivities shown in the following:

It turns out again that at the higher bleaching yields (over 55%), a pre-treatment with neutral sulphite is more selective than with kraft.

Since the chlorine dioxide series itself is quite selective, the use of only a mechanical pretreatment followed by a chlorine dioxide series can be expected to give a very high selectivity value. However, the figures show that a high-yield neutral sulfite pretreatment followed by the chlorine dioxide series is more selective than the case without pretreatment.

These results are shown graphically in Figure 7. For any given pretreatment yield, the neutral sulfite treatment gives a significantly higher bleached final yield, indicating its increased selectivity. The case of mechanical pretreatment (shown as 100% pretreatment yield) is far below the extrapolated chemical treatment cases, and this indicates the relatively low degree of selectivity that has been achieved.

The relatively low selectivity of the kraft pretreatment suggests that more lignin is left at a given pretreatment yield and therefore needs more pulp treatment chemicals (chlorine dioxide).

The table below shows that this is the case, the neutral sulphite pre-treatment thus gives a higher bleached yield and lower chlorine dioxide consumption than the kraft pre-treatment gives; chlorine dioxide savings of up to 24% are possible with higher pretreatment yields.

These results and also the relatively high consumption of chlorine dioxide during mechanical pretreatment are shown graphically in Figure 8.

From these comparisons, it is clear that neutral sulphite treatment with sodium as a base gives a higher bleached yield and lower chemical costs (chlorine dioxide) than power. Pure mechanical pretreatment also results in higher chlorine dioxide consumption and lower yield than extrapolated neutral sulphite figures. Seen against the background of yield and chemical economy, neutral sulphite is preferred over these two alternatives.

The table below shows the standard strength characteristics of neutral-sulphite, power and mechanically pretreated pulp, each treatment using the five-step chlorine dioxide-alkali treatment series:

It turns out that the power pretreatment results in lower wear and tear than the neutral sulphite pretreatment, and that mechanical pretreatment is also inferior in strength.

Overall, it can be concluded that neutral sulfite pretreatment with sodium as a base is preferred over high-yield power or mechanical pretreatment, since it results in higher bleached final yield, lower chlorine dioxide consumption and higher strength of the pulp sheets.

Example X

To demonstrate the uniqueness of the sodium neutral sulfite pretreatment - chlorine dioxide pulping series compared to conventional bleaching of neutral sulfite pulps, the strength properties of paper made from Southern hardwood chips by the process of the present invention were compared to the properties of a bleached pulp made by a conventional neutral sulfite pulping step followed by a conventional bleaching series consisting of chlorine treatment - alkali treatment - hypochlorite bleaching, and the last case is typical for current bleaching of neutral sulphite pulp. The differences that were achieved appear in the following table:

From the table it appears that the present invention produces a pulp with a very high yield, and at the same time gives a paper of very high physical strength. The difference in the number of folds is particularly noteworthy, since the number of folds is the most important physical paper property for paper for books and printed matter.

Example XI

To demonstrate the superiority of the sodium neutral sulphite - chlorine dioxide treatment over conventionally bleached kraft pulps,

the experiments below were carried out. Previous tests have shown the superiority of the process product from the sodium neutral sulphite-chlorine dioxide process in relation to conventionally bleached neutral sulphite and also in relation to pulps subjected to the series of pre-treatment-chlorine dioxide where the pre-treatment is one of the following: mechanical, neutral sulphite with ammonium as base, high yield power and nitric acid.

By using a constant raw material consisting of mixed hardwood chips from the Southern States, pulp samples were produced using two methods: the series of sodium neutral sulphite pretreatment - chlorine dioxide according to the present invention and a conventionally produced and bleached kraft pulp. These pulps were refined to 300° Canadian standard freeness (TAPPI Standard T227 m-58) and paper was produced on a small 30 cm paper machine so that the results could be more comparable to the properties of machine-made paper. The physical properties and surface smoothness of these two papers were compared in the following table:

It thus appears that paper produced according to the neutral sulphite-chlorine dioxide process is very superior in physical properties such as bursting, tearing, wear, stretching and M.I.T. fold number. The "wax pick" test showed that the new pulp was very superior in internal binding strength, a property of great importance for papers to be printed. The smoothness measurements showed that the new pulp had a higher smoothness, and this is beneficial for printing papers and for a number of other applications.

The superiority of the new pulp both in wear resistance and tear strength of the laboratory sheet shows that the new method produces a pulp of very superior fiber strength; if the bonds alone were involved, a high wear resistance would result in a low tear strength and vice versa. To demonstrate the superiority in fiber strength, zero-span abrasion tests were performed on laboratory sheets made from two pulps. The results are shown below:

Since this analysis, which is carried out on standard sheets of constant basis weight, provides a direct means of comparing fiber strengths, it can be concluded that the method of the present invention has produced a very unique fiber having a strength per unit fiber mass that is over 60% greater than with a conventionally produced fiber.

Example XII

To show that the pulp, in addition to the superior properties of the pulp according to the present invention when producing laboratory sheets or paper on a paper machine, shows certain unique properties that are related to the treatment before or after the production of the paper sheet, e.g. light refining, increased dewatering during sheet formation, ability to retain filler very effectively, superior wet strength on the paper machine and that the paper dries more easily, the following comparison was made.

How easily the pulp can be refined can be seen from the figures i

the table below and also of figure 9. A comparison of the masses was carried out when refining after a standard TAPPI grinding trial. Neutral sulphite-chlorine dioxide pulps are compared with conventionally bleached kraft, in one case with pulp produced from hardwood chips from the Southern States and in the other case by using hardwood chips from the Northern States. The slope of the curves in Figure 9 shows that the relative grinding speed, new mass versus force, is 3.0 for pulp from wood from the Southern States and 4.2 for pulp from wood from the Northern States. Since the time required to grind a pulp under these standard conditions is directly proportional to the energy consumption for refining to a given degree, it turns out that the power consumption required for refining for the new pulp is only 1/3 or 1/4 of what is required for power mass.

When paper was produced on the small paper machine,

it was observed that at a constant freeness of 300° Canadian standard,

only half as much vacuum was required for the dewatering area of the paper machine wire to dewater free water from the neutral-sulphite-chlorine dioxide mass than was required with the

correspondingly bleached kraft pulp (when in both cases the dewatering was forced to take place over a constant stretch of the paper machine wire). This illustrates the possibilities for increasing machine speeds with the new mass.

In the same paper machine tests, filler was added to the pulp and the retention of this filler was measured during sheet forming. These results, which are shown in the following,

illustrates the very marked superior retention that the new pulp has compared to conventionally bleached kraft. In reality, this effect is even more dramatic than the present retention percentages seem to indicate, since 50% more filler was .il-

put in the experiment with the new mass. It is known that % retention decreases when the added amount of filler increases under otherwise constant conditions.

When the wet sheet is lifted from the paper machine wire and goes to the first press, there is a danger that the sheet may burst when it is in a wet and weak state. This tendency to fracture is dependent on the wet strength of the sheet. In the tests on the small paper machine and when the machine was run under identical conditions, it was observed that the kraft sheet tended to break frequently, while the new pulp sheet was produced without difficulty. While these results are not quantitative, they demonstrate the superior wet strength of the new compound.

In the same experiments on the paper machine and when the machine ran at a constant speed, it was observed that only half as large a temperature difference was needed in the drying section to dry the new pulp sheet in relation to the kraft pulp. In other words, the new pulp sheet had twice the drying speed of conventional power.

Light refining, combined with faster dewatering, higher wet strength and that the pulp is easy to dry, indicate the ability of the new pulp to increase paper production compared to conventional pulp. The significantly increased filler retention also leads to significantly improved economy, and particularly when it comes to sheets that have a high content of filler.

Example XIII

In order to show the chemical characteristics of the pulp, comparisons were made of the degree of polymerization and the hemicellulose content of different pulps from the same wood mixture as indicated in the following table:

The carboxyl content of pulp treated with the neutral sulphite chlorine dioxide series is at least twice as high as the carboxyl content of conventionally bleached pulps. At the same time, the carbonyl content is only half to a third as great as the carbonyl content of conventionally bleached pulp from the same batch. The pretreatment part with neutral sulphite in the treatment series is necessary to keep the carbonyl content low, as can be seen when comparing the figures for pulps with and without neutral sulphite treatment.

Example XIV

To show the effect of the pulp's chemical properties on the lightness stability and ion exchange capacity, two pulps from the chlorine dioxide series were compared with conventionally bleached power as shown in the table below. In the experiments, the raw materials were a mixture of Sydstatsløvved. It turned out that the pulps can be listed in the following order: neutral sulphite > mechanical pretreatment > chlorine dioxide > conventional kraft according to lightness stability, and kraft is the worst even if it has an average carbonyl content (high percent decline or high post color number indicates low lightness stability).

The carbonyl content of the chlorine dioxide mass is so high that it can act as an ion exchange material. If this mass is treated with a salt solution, the resulting mass will absorb metal atoms. This results in a higher ash content and many distinctive chemical properties. Lightness and opacity and also lightness stability are strongly influenced by the nature of the cations bound to the pulp. The table shows the effect of trace amounts of zinc, sodium, aluminum and potassium ions on the optical properties of the mass. It turns out that significant changes occur that do not take place when conventional kraft mass is treated in a similar way.

Example XV

To show the durability of masses treated with neutral sulphite chlorine dioxide, fold number measurements were carried out on four cation-treated neutral sulphite chlorine dioxide masses after zero, one, two, three and seven days of aging at 105°C. The results were compared with typical paper for books and with a special alkali-treated paper made from softwood.

It was found that laboratory sheets made from neutral sulphite chlorine dioxide pulp which had been treated with either sodium, lead or no cations were very durable. Laboratory sheets made from pulp according to the present invention and treated with alum were less durable, although they were more durable than typical book papers treated with resin and alum.

The pulp used was made from southern hardwoods. The wood mixture was given a neutral sulphite boil and refined in a high pressure refiner to 85.5% yield. The pulp was then washed and bleached in a five-step chlorine dioxide extraction treatment series. All chlorine dioxide steps were carried out with gaseous chlorine dioxide. The mass was tumbled in a 12 liter polyethylene reactor during the reaction period. A total of 8.1% chlorine dioxide (based on wood) was consumed. All chlorine dioxide steps were carried out at room temperature without the addition of buffer.

The treated pulp was ground for 11 minutes to 300 ml of Canadian standard freeness (TAPPI Standard T227 m-58) in a valley Dutcher. Three cation-treated series of lab sheets were prepared and one series was prepared with distilled water only. They . treated laboratory sheets were prepared by adding 1.2 g of the salt or base in question to a slurry containing 12 g of pulp (oven dry base). Sufficient slurry was then added to the lab sheet former (using distilled water) to produce a standard 1.2 g lab sheet. The cations chosen were sodium in the form of sodium hydroxide, lead in the form of lead acetate, aluminum in the form of aluminum sulphate and the control was carried out with distilled water.

Two by two of the laboratory sheets from each of the four groups were aged at 10 5°C for zero days, one day, two days, three days and seven days. Each laboratory sheet was then cut into four strips and analyzed for breakage in a Schopper fold count meter. All the strips were of course conditioned in the test laboratory before the analysis. The figures obtained are shown in the table below and in Figure 10.

Figure 11 shows the relationship between M.I.T. and Schopper fold numbers based on the numbers of mass according to the present invention

which has been analyzed according to both methods.

Figure 12 shows that typical paper for books that has been treated with alum is destroyed very quickly. Neutral sulphite chlorine dioxide masses which have been replaced with aluminum ions from alum are destroyed less quickly. Neutral sulphite chlorine dioxide mass which has been replaced with sodium is very resistant to fold number loss. The last curve shows the behavior of a special long-grain softwood pulp which has been treated with alkali to develop a very high degree of stability. The neutral sulphite chlorine dioxide pulp with sodium treatment (or no treatment) is almost as durable as the special pulp. Even with alum treatment, the neutral sulphite chlorine dioxide pulp is much more durable than modern paper for books.

In Figure 13, the same masses are compared with regard to their fold number durability on the basis of years under standard conditions. The half-life (the time in years to reduce the fold number to half of its original value) of neutral sulphite chlorine dioxide pulps is very high compared to conventional book paper. The half-life of neutral sulphite chlorine dioxide mass which has been treated with alum is approx. 18 years. The typical modern paper for books has a half-life of approx. 6 years.

From this, one can draw the conclusion that neutral sulphite chlorine dioxide mass according to the present invention is very durable without special treatment or with lead or sodium treatment. Aluminized neutral sulfite-chlorine dioxide pulps are less durable than lead, sodium, or untreated pulps, but are more durable than typical alum-treated book papers.

Example XVI

A neutral sulfite pretreatment with sodium as base was carried out as indicated below. Hardwood chips from the southern states (approx. 1/3 oak, 1/3 yellow poplar and 1/3 rubber trees) at approx. 50% moisture was heated at a liquid to dry wood ratio of 3:1 in a solution containing 10% sodium carbonate and 12% sodium sulfite, both based on the dry weight of treated wood. The following time-temperature cycle was applied using a laboratory reboiler with forced liquid circulation and indirect heating:

30 minutes from ambient temperature to 134°C

60 minutes at 134°C,

30 minutes from 134°C to 168°C,

55 minutes at 16 8°C.

The yield of pre-treated wood chips was 80.5% based on dry starting wood. The softened chip, still saturated with pretreatment liquor at a pH of 9.5, was then refined in various ways using a small laboratory refiner (Example 1), pressure refiner (Examples 2-5) and a conventional disc refiner (Examples 6-7). The refining conditions that were used are indicated in the following table:

Summary of refining conditions

The pulps refined in two industrial-sized refiners in Examples 2 to 7 were then characterized by a standard classification (TAPPI Method T233 su-64). These figures are shown in the following table: Classification of refined pre-treated hardwood fractions) than the pressure refiner at approx. 1/3 of the power consumption (examples 2 and 3). 2. When the power consumption in conventional refiners is increased to the usual level used in the industrial refining of pulps that have been treated with neutral sulphite with sodium as a base (example 7), the highest fines production is obtained. 3. Increased temperature in the pressure refiner (compare experiments 4 and 5 with experiments 2 and 3) gives less coarse and less fine material. These refined pretreated hardwood samples were treated with a chlorine dioxide-alkali treatment series. Constant conditions were kept with regard to chemical addition, concentration, reaction temperatures and reaction times. The bleached masses produced had a lightness of approx. 85 G.E. (TAPPI Method T217 m-48), and a bleached total yield of 62%. The bleached pulps were refined in a laboratory Valley Dutcher (TAPPI Method T200 ts-66), and standard laboratory sheets were prepared from these pulps and analyzed. The strength values are compared at two freeness levels (freeness according to TAPPI Method T227 m-58) in table III. No values are shown for 450 in freeness in example 7, as the unrefined, bleached pulp had a freeness of 345 ml.

Comparison of the strength properties of bleached pulp

If one compares the pulps refined in a conventional, commercial disc refiner (Examples 6 and 7) and in a laboratory refiner (Example 1), it appears by comparing wear and burst strengths between Examples 1 and 6 that the latter pulp does not undergo any significant fiber destruction . As the power consumption is increased to a typical industrial level (Example 7), the low starting freeness and very low wear resistance and burst strength show that a very significant fiber destruction has been caused by over-refining. Heavy refining produces a lot of fines, and this results in a drop in freeness without a simultaneous increase in strength. The use of conventional refiners at conventional levels of power consumption should therefore be avoided.

A comparison of four pressure-refined pulps (Examples 2-5) shows that milder conditions (Examples 2 and 3) are preferred to a higher temperature (Examples 4 and 5). Weak refining (Examples 2 and 3) produces a pulp superior to weak refining in a conventional disc refiner (Examples 1 and 6).

Claims (2)

1. Method for the production of a pulp from a lignin-containing cellulose vegetable material with good tensile strength without reduction in tear strength, characterized by a combination of the following features: (A) a mild pre-treatment which includes (1) an alkaline boiling followed by a (2) mechanical treatment, the boiling (1) being carried out with neutral sulphite, by the sulphate method or with sodium hydroxide, and wherein the mild pretreatment ends when the yield of pretreated material on a dry weight basis is 64-95% by weight, based on the material inserted, and where the mechanical treatment is a paint with a power application of 0.5-5 horsepower days per 907.2 kg of dry vegetable matter, and (£>) treating the intermediate material from (A) for lignin oxidation and removal with chlorine dioxide in a series of two to three chlorine dioxide oxidation steps where the total chlorine dioxide used is from 4 to 13%, calculated on the dry weight of the dry vegetable material, and where the chlorine dioxide may contain a small amount of chlorine, on an equivalent oxidizing agent basis which is generally formed when chlorine dioxide is produced, and where the amount of chlorine dioxide is smaller in each subsequent oxidation step, each chlorine dioxide oxidation being followed by a water wash, and a caustic extraction at 50 to 75°C, followed by a water wash. 2. Method as stated in claim 1, characterized in that the mechanical treatment in step (A) (2) is carried out at a pressure above atmospheric pressure and at a temperature of from 100°C to 170°C.