US2061205A - Pulping process - Google Patents

Description

Patented Nov. 17, 1936 PATENT OFFICE rmrmc raooass Fredrich Olsen, Alton, 11L, assignor, by mesne assignments, to The Cellulose Research Gorporation, a. corporation of Delaware No Drawing. Application October 31, 1931,

. Serial N0. 572,443

8 Claims.

This invention relates to the pulping of cellulose material and particularly to cellulose in fibrous form mixed with noncellulosic material, such as lignins and the like.

The object of the invention is to free the cellulose fibers by softening or dispersing the noncellulosic material and in most cases removing it in desired degree from the cellulose, so that the fibers of the latter are formed into a pulp while at the same time avoiding any degradation of the cellulose and maintaining the fibers in substantially natural condition, particularly as to their cellulose content. I

A further object of the invention is to cause ing medium for the noncellulosic material to be uniform and homogeneous throughout the mass being treated and throughout each individual unit thereof.

Another object of the invention is to restrict the softening and dispersing action of the treating medium to the noncellulosic material and maintain the cellulose content of the wood or other treated substance substantially unchanged controlled to obtain the desired characteristics of the final cellulose pulp. Y

The pulps to be produced under this invention comprise a range in which substantially all of the cellulosic material and a very large percentage of the noncellulosic material are retained so that the fiberable pulps to the extent of ninety percent or more of the dry weight of the original wood can be recovered. At the other end of this range high grade pulps, such as substantially pure chemical cellulose or pulps of very high alpha cellulose content, e. g. 95% or better, are produced. Intermediate pulps will consist of the types commonly employed in wrapping, book papers, writing papers, etc. The pulps produced by this process within these ranges will in many cases be new in the proportions of their various ingredients. Due to the accurate controls obtainable and the precise regulation of the action 45 on the cellulose and on the noncellulosic ingredients, substantially any desired composition may be obtained.

As an example of the application of the process to wood material in the form of flakes, chips 50 or blocks, these are. first thoroughly combined with a chemical agent preferably carried in solution in water. This combination of the. material with the treating medium. is obtained by a soaking action in which the water or other fluid medium carries the active agent into the innerthe softening or dispersing action of the treator changed only in predetermined manner and mostinterstices of the wood particles, the action being so arranged and controlled as to subject every particle of each unit of the wood to very nearly the same action. Assuming that the chemical used in treating is chosen to have the '5 selective or preferential action upon the noncellulosic content of the wood, then the first combining step of this process will associate this chemical with this noncellulosic material in such away as to place it in position to exert this selec- 10 tive action upon a change, for instance, in the temperature conditions.

With the proper composition of soaking liquor and with a time temperature schedule adequate togive conditions of equilibrium of impregnation 15 within .the wood and with the correct amount of chemical to react with the cementing material of the wood, the cooking of the wood with the proper amolmt of reagent and in the absence of excess liquor insuresthe minimum solubility 20 of cellulose, and hence maximum yields. There are also provided the necessary conditions for extremely rapid reaction since the combination of the chemical, for instance, caustic soda, with the hydroxyl radical of the complex organic sub- 5 stances takes place most favorably when only very limited quantities of water are present.

If, for instance, in the treatment of the wood with a live percent solution of caustic soda, the wood is left in the solution sufliciently long and 30 under proper conditions as to temperature and pressure and with or without agitation and with or without prior conditioning of the wood, the fibers of the wood will tend to adsorb the caustic at a rate which tends to decrease as an apparent 35 point of saturation is approached. This point can also be described as a point of equilibrium between concentration of the liquor within the wood and the concentration of the reagent in the liquor in which the wood is being soaked. The 40 condition of equilibrium thus set up does not involve equality of concentration of the reagent in the liquid within and without the chip, but will usually involve an appreciably higher concentration of the reagent within the liquor in the wood.

Without going too deeply into theories as to the cause of this withdrawal of the active chemical from the solution by the wood, it may be considered as due in part to the adsorption of the reagent in solution on the interior surface of the porous wood and possibly to a physical mixing of the ingredients of the wood with the solution in highly variable manner. For instance, the nonccllulosic material within the wood probably being a highly complicated intermixture of a number of ingredients would tend to mix as to some one or more of these ingredientswith the treating solution so that there is an actual per meation of the solution, and this treating chemical into the noncellulosic material along intricate paths following or crossing the wood fibers. Whatever the explanation, the observed result is a very intimate combination of the reagent-in the wood material and in quantity which I have found may be controlled to give a very thorough reaction with the noncellulosic material under proper subsequent conditions of treatment. This combination of the wood with the treating chemical produces at least a physical change in the wood, and probably particularly in the noncellulosic content thereof in that athin piece of wood is distinctly flexible in comparison to its natural condition. It is also resilient and not easily permanently distorted, and unless sub-' jected to considerable pressures, it tends to resume its former shape after bending. In this condition the wood tends to remain without substantial change for long periods of time so that it-may be kept in stock for use in subsequent op erations as desired.

The treating liquor may be removed by being drained away, and the woody material in combination with the retained solution will now constitute a mass which by reason of the now favorable association of these substances will be in condition to react when, for instance, the temperature is raised to the digestion point, such as the point of delignification. During this reaction, the retained solution shows a pronounced preference for the noncellulosic material, tending to react with this and reduce it to other forms generally soluble so as to cause a disintegration of the woody structure which releases the cementing bonds between the cellulose fibers and so pe'rmits the wood to pulp. In such digestion or delignification of this previously prepared material, I have found that the action on the cellulose content of .the fibers may be very accurately restricted and controlled. Under certain conditions the cellulose fibers may be preserved in substantially their natural condition and with full proportions of their alpha, beta and gamma cellulose. With some treating liquors and under predetermined conditions of temperature and the i like, there are reactions altering the cellulose content of the fibers, but only in desired and predetermined manner to produce the characteristics of the celulose suitable for the final product in which the pulp is to be used.

I have determined that the amount of chemical, for instance, alkali, which is taken up by the wood will depend upon (a) the concentration of the liquor in which the wood is soaked, (b) the temperature at which the soaking takes place and (c) the duration of the soaking. For exampleflif poplar chips of the form commonly employed are soaked in 11.5% caustic soda solution for five days at room temperature of about 25 C. then a product results containing 36.4 grams of caustic soda per grams of dry wood. The ratio of the absorbed liquid to the wood is two to one afterthe surrounding liquid has been removed by draining. The concentration of the caustic in the liquor within the chip is approximately eighteen percent anq upon heating the resultant combined wood and caustic for twenty minutes while continuously raising the temperature from 25 C. to about 170 C., the digestion or delignification results in a pulp with a yield of 545.2% of screened pulp, whose lignin contentis 2.2% and mately seven percent concentration in caustic and the chips are soaked in this weaker liquor for about five days, the result is a combination of about '18 grams of the caustic per 100 grams of the dry wood, and a concentration of the liquor within the wood. of about seventeen percent when the'chips are centrifuged to remove or wring out their contained solution to the point of retaining substantially equal portions of solution and wood. In this. form the physical appearance of the chips is that of merely dampened wood, there being no visibly free liquor present with the wood. After the digestion period involving twenty minutes treatment in which the temperature is raised from about 25 C. to 155 C. followed by sixty minutes cooking at 155 C. a yield of screened pulp of fifty two percent containing 3.3% lignin is obtained. The consumption of caustic soda in this case amounts to 13.8 grams of caustic soda for each 100 grams of dry wood treated. In this case the Cross and Bevan cellulose content of the pulp is about 96.5%.

Similarly, spruce chips soaked seven days at 20 C. in 7% sodium sulflte solution can be completely defibered by cooking for one hour at C., addingSOz during the cooking either as a gas rapidly.

or in aqueous solution and holding the ratio of liquor to wood within 10:1. Assuming a composition of the cooking .liquor 0.6% combined SO: and 3.2% free on the basis of the amount of chemicals per 100 grams dry wood, these values for the composition of the solution being 5% combined and 27.4% free S02, 8. high yield of approximately 55% may be obtained with resultant pulp of good appearance. Thus in the case of sodium sulflte either alone or mixed with sodium bisulfite or the combination of either of these with sodium bicarbonate, similar delignification actions will be produced if the reagent is uniformly disseminated throughout the mass prior to temperature conditions being raised to the point at which the reaction with the lignin can proceed In these typical instances as in the process in general, the cellulose fibers and the noncellulosic material bound together are first reduced to the desired form of flakes, chips, blocks or the like; and then subjected to a soaking in the treating liquor for sumcient time under the surrounding conditions to give a complete permeation of the material by all constituents of the liquor. In this soaking action the interior portions of the wood apparently have a preferential action on the caustic in the solution tending to withdraw the caustic and so to lower the relative concentration of the liquor outside of the wood. Within the wood the relatively concentrated liquor is to be very evenly distributed so that there will be no substantial variation in impregnation between the inner portions and the outer portions. With this even distribution of a reacting agent having a preferential action on thenoncellulosic content of the material, these noncelluloses become softened and changed probably acting by swelling in colloidal manner and rendering the wood relatively resilient, tending from the start to weaken the bonds between the fibers.

An important further advantage is secured in the case of cooking with bisulfite solutions conor sodium base, e. g., calcium bisulfite or sodium bisulfite with every particle of lignin. When, therefore, sulfonation is promoted through the agency of sulfurous acid, the conditions encountered in the conventional sulfite cooking are absent, namely, that in the conventional process free $02 penetrates the wood more rapidly than the calcium or sodium bisulfite and can produce conditions of excessive local acidity especially in that range of temperatures of about 11o-125 C; during which not only sulfonation proceeds most actively butthe formation of organic acids apparently takes place most readily.

It is apparent from the examples given that with the treating liquor so absorbed and adsorbed, there is as a result during the later step of cocking or deligniflcation an automatic reaction between the ingredients of the treated material whereby the noncellulosic material is changed and dispersed to release the cellulose fibers for pulping. Where the association of the active constituent of the treating liquor is sufiiciently intimate and homogeneous throughout the cellulose structure, the subsequent reaction with the noncellulose material takes place of itself as the temperature is raised and without any supply of outside liquid or reagent. I have found not only that the cellulosic material will draw to itself and distribute within itself automatically the proper amount of, the reacting agent and hold this in intimate unionwithin the woody structure, so as to provide conditions for the subsequentautomatic deligniflcation entirely within the materials themselves, but that such action takes place in an extremely preferential manner in that the reactive agent eventually combines almost exclusively with the noncellulosic material, leaving the cellulose fibers undegraded and substantially unchanged.

Whatever may be the true explanation, the action proceeds as though the interior surfaces of the woody material are more largely formed by noncellulosic material surrounding and protecting the cellulose fibers. Then when the treating liquor penetrates into contact with these surfaces, there is adsorption and also perhaps'a mixing of the liquor and the noncellulosic materials tending to draw in more and more of the more active agent from the outside solution and distributing it progressively and gradually and finally unir formly throughout the mass of the wood.

Upon raising the temperature the reaction is confined practically exclusively to the noncellulosic material, no doubt because of the close prior association of this material with the absorbed solution and also because of a natural tendency of the selected reagent to react with these non-celluloses. The result is that throughout the entire structure of each unit of the woody material, the

noncellulosic material takes-up the reagent and in efiect protects the cellulose fibers from its action. Since the prior treatment has associated with this noncellulosic material the desired predetermined amount of reagent, there is no substantial excess of the reagent'sufiicient to cause" any substantial action on the cellulose, and at the same time, the original impregnating liquor having been removed there is no further source of reagent in contact with or .available for the cellulose fibers, so that they are loosened and freed in substantially their natural condition. Since the amount of chemical adsorbed and held within the wood depends upon the concentration of the surrounding liquor, the quantity available for delignification may be controlled by the amount of thereagent. The impregnation particularly at "lower temperatures will usually require relatively long periods of time, seven or nine days or even longer.

Itmay also be advantageous to first soak the wood in fairly strong solutions of caustic soda until a condition of equilibrium has been reached as far as the concentration of alkali within the chips and the concentration of the alkali in the soaking liquor is concerned, but in which the alkali content of the chips will be perhaps considerably in excess of that required for proper delignification.

The chips are then soaked in a weaker solution which resultsin the displacement of the concentrated liquor throughout the chips with a weaker liquor, the process being so designed that after equilibrium has been again established, the correct amount of alkali is now within the chips.

The principle which is probably involved in this practice is that stronger caustic liquors apparently have greater penetration ability than weaker liquors. This may be due to the fact that alkaline liquors eat their way through the masses of cementing material which is designated broadly as lignin, preferentially dissolving or associating with only the more soluble or reactive portions of the lignin. The 'use of stronger soaking liquors permits more intimate penetration of the cementing matter by the alkali presumably because a greater number of the constituents of the cementing material are capable of associating either chemically or through solution or perhaps by adsorption with the stronger alkaline solution. Once the cementing material has been thoroughly permeated by the alkaline solutions, consideration is given the cementing mass as having been thoroughly channeled by these liquors which are then capable of being replaced by weaker alkaline liquors which would not of themselves have been able to so thoroughly penetrate the mass. Consequently' by a second soaking process, the chip now contains the appropriate quantity of reactant in a degree of uniformity which would not have been obtainable had the stronger liquor not first been used.

I claim:

1. .The process of treating ,wood material composed of cellulose fibers and'noncellulosic mate rial bound together comprising providing said material in desired subdivision, submerging said material in soaking liquor including a chemical adapted to be adsorbed by the interior capillary surfaces of the wood at temperatures below the reaction point, continuing said treatment for at least three hours at said temperatures and until the concentration of the liquor chemical within the wood exceeds the concentration of said chemical in said treating liquor, and then subjecting said material to a second impregnation with a diiferent liquor to control the chemical content 'of said material within predetermined limits.

the wood exceeds the concentration of said chem ical in said treating liquor, and then subjecting said material to a second impregnation with a different liquor to reduce the chemicalcontent of said material within predetermined limits.

3. The process of treating wood material composed of cellulose fibers and noncellulosic material bound together comprising providing said material in desired subdivision, submerging said material in soaking liquor including a chemical adapted to be adsorbed by the interior capillary surfaces of the wood at temperatures below the reaction point, continuing said treatment for a time sufilcient to deposit said chemical onsaid interior capillary surfaces in desired amount, and then subjecting saidmaterial to a second impregnation with a different liquor to control the chemical character of the deposits within said wood material and prepare said material for digesting action when the temperature is raised to the reaction point. p 4. The process of treating wood material composed of cellulose fibers and noncellulosic material bound together comprising providing said material in desired subdivisions, submerging said material in soaking liquor including the chemical adapted to be adsorbed by the interior capillary surfaces of the wood at a temperature below the reaction point, continuing said treatment fora time sufllcient to deposit said chemical on said interior capillary surfaces in desired amount, arid then subjecting said material to a second impregnation with a different liquor to control the chemical character of the deposit ing said treatment for a time suflicient to deposit said chemical on said interior capillary surfaces in desired amount, and then subjecting said material to a second impregnation with a different liquor to control the chemical character of the deposit within said wood material and prepare said material for digesting action, said action being capable of completion within the time re-' quired to raise the temperature to 150 to 170 C.

6. The process of pulping wood chips, flakes or blocks composed of cellulose fibers and noncellulosic material comprising treating said material in a relatively strong chemical solution-at pressures not exceeding atmospheric for a sufficient time to evenly distribute through the material the chemicals in said solution, then subjecting said material to a relatively weaker solution for a suflicient time to lower the concentration of chemicals within said material, and subsequently changing the conditions of said material with said chemical therein to' carry on a delignifying'action to desired degree. Y

7. The process of digesting wood material to produce pulp comprising providing said material in desired subdivision, subjecting said subdivisions. to soaking for five or more days in a digesting liq uor and producing a concentration of the liquor within the subdivisions'substantially exceeding the concentration 1 of the soaking Iiquor, subsequently raising the temperature of said material to not over 170 C. and effecting digestion of said material to easily defiberable condition by maintaining said temperature one hour or less.

8. The process of digesting wood material to produce pulp comprising providing said materal in desired subdivision, subjecting said subdivisions to soaking for five or more days in an alkaline digesting liquor and producing a concentration of the liquor within the subdivisions exceeding the concentration of the soaking liquor by or more, subsequently raising the.temperature of said material to not over C.

and effecting digestion of said material to easily -defiberable condition by maintaining said temperature one hour or less.

FREDRICH OLSEN.