US2956918A - Chemically assisted mechanical wood pulp - Google Patents

Description

mmMrcALLY ASSISTFgi MncnANroAr. woon P P Francis H. Snyder, Newtown, Conn., assignor to Francis H. Snyder and Associates, incorporated, New Milford, Conn., a corporation of Connecticut No Drawing. Filed July 9, 1957, Ser. No. 670,656

19 Claims. (Cl. 162-90) Many kinds of alkaline treatment of wood for the formation of pulp are already known. This invention relates to an alkaline treatment departing from the known methods in certain significant respects, characterized by substantial economies.

Briefly, my process employs cheap alkalies at atmospheric temperature and pressure, certain other conditions being appropriately controlled, followed by a mechanical fiber separation. Very substantial savings in chemical and power costs are achieved yet the product obtained is actually superior for many purposes.

For convenience of description, my process may best be described in terms of the Cold Soda process developed by the U.S. Forest Products Laboratory at Madison, Wisconsin. The two major stages in the cold soda process are 1) soaking the chips in an approximately solution of the swelling agent, sodium hydroxide, at atmospheric conditions or under moderate pressure and (2) fiberizing the softened chips in an attrition mill. According to one modification of the cold soda process, the chips have been soaked in the caustic soda solution for approximately 2 hours before milling; more recently, it has been proposed to subject the chips to a mechanical kneading operation in the presence of the caustic soda with only a brief holding period before milling. The power consumption of this process is materially less than that utilized in the familiar groundwood process where large quantities of Water are also required in order to carry away the heat generated in the milling process.

I have found that the treatment of wood chips with ordinary slaked lime to which has been added a fractional amount of either soda ash or caustic soda will, first of all, achieve significant economies in chemical cost. The chemicals are preferably employed in a sulficiently thick slurry so that the requisite amountof re-, agent deposits on the chips without intentional drainage. Ordinarily, the amount of time needed for contact with the wood of the chips is not greater than the normal hold-up of the materials in process. If it is desired to employ lime alone, without soda, a storage period before milling is desirable. A further economy is efiected during the grinding operation because chips treated with lime are significantly more friable than those treated with soda alone. The ground pulp also responds to the action of bleaching agents more readily than that from the comparative process.

My process is especially suited to the treatment of hard woods. It is becoming increasingly difficult to locate large stands of the soft woods formerly considered essential to the production of mechanical pulps. It is relatively easier to buy mixed stands of hard wood of inferior lumbering quality including such species red oak, white oak, maple, beech, birch, aspen, cottonwood and willow. Furthermore, since my process is well suited to operation upon chipped or bogged woods, a

Patented Oct. 18, 1960 mill utilizing my process may advantageously be located where there'is available lumbering and saw mill waste.

The wood is chipped or hogged and screened so that the wood particles are retained on a 20-60 mesh screen and passed through a round-cut screen having V4 to V2 inch holes or a square mesh of about /2 to inch. Chips of a slivery rather than blocky character are preferred. The operation should be controlled to yield chips having a maximum single dimension of /2 inch and combined radial and tangential dimensions of about /1 inch.

The chips may be immersed in or sprayed with a slurry containing calcium hydroxide and sodium carbonate in the CaO/Na O ratio of from 10/1 to 20/1. Since much of the commercial lime is of the dolomitic type, it is to be understood that mixtures of calcium and magnesium compounds are to be considered as if they were the corresponding calcium compound with no other allowance than is required for the matter of combining weights. Also, if desired, potash may be substituted for soda in the appropriate combining weight.

For convenience, the concentrations are adjusted so that the wood receives a dosage of Ca(OH) which is calculated for any given wood species or mixtures of species by the formula:

percent acetyl (CH CO-) X40 =lb. Co(OH) per ton O.D. wood This relationship has been established empirically, providing an excess of lime over acetyl and constitutes my preferred practice. The acetyl value of Wood is a well known relation-ship, more or less related to the particular species of wood. The method of Phillips found in Industrial and Engineering Chemistry, Analytical Edition, volume 6, page 321, is generally accepted.

For example, in the case of southern red oak, the acetyl value is about 3.5%. The quantity of Ca(OH) required is, therefore, 3.5 X40: lb./ton O.D. wood. With this is employed Na CO or NaOH in an amount equivalent to a ratio of CaO/Na O between 10/1 and 20/1. In the case of red oak and certain other species, there are acid substances present in addition to acetyl such as tannins, etc. For such woods, ratios of Na O in the higher range are to be preferred such as 12.0 CaO/ 1.0 Na O or about 17 lb. Na CO per 140 lb. Ca(OH) A slurry may be made up as follows:

Water 400-600 (3:1 011) 140 N32C03 The quantity of water used will vary with the moisture content and porosity of the wood being treated. The soda ash is first dissolved and the lime added slowly with good agitation. One ton of DWS in the green condition (35004200 lb. moist wood) will take up this quantity of slurry without leaving significant excess of liquid.

The slurry may be put on the chips in such conventional apparatus as a tumbling drum or a ribbon mixer. The coated chips are then delivered to a storage bin or chip silo. Although they may be milled immediately or retained for periods as great as 48 hours, a mod erately short retention time is conveniently consistent with ordinary mill practice.

It should not be overlooked that the reaction:

does not go to completion at room temperature.

In the system and with the proportions of reactant uronic acids.

stated above, the relation of pH to temperature is given in the following table:

When wood chips are brought into the presence of this system, NaOH will be consumed by reaction with wood constituents forming sodium salts of acetic, tannic and The result will be to a shift of the equilibrium to the right.

What has not been realized up until the present, is that these organic sodium compounds are re-causticized by lime within the temperature range normally employed in grinding: 30-60 C. As long as there is lime present in substantial excess, the pH of the system will remain fairly constant between 11.5 and 11.8. In this manner, a steady, very low level of causticity is maintained during the course of the process. This pH is in the vicinity of that reached by lime alone. It is especially advantageous for the pulping operation since it brings about conversion of the lamellar substance of the wood to a friable condition Without material removal of the pentosans from the wood.

In endeavoring to distinguish my process from those which rely upon treatment of the wood with NaOH, I

v have come to a realization that certain events take place in such wood treatment, that have not previously been recognized, so far as I know.

I believe that it is impossible to treat wood uniformly with dilute aqueous NaOH. This reagent attacks acetyl, polyuronides, pentosans, hexosans and lignin, the latter, slowly (at room temperature). If the total quantity of NaOH present is insufficient to react with all of these components, the result is that, for the most part, the surface layers are attacked and the interior mass hardly at all. When steeping is carried out in a 5% solution of NaOH, in excess of that required to react with all of the reactable constituents, the outer layers of the wood become impregnated with organic sodium compounds that are colloidal, viscous substances rather than true solutions, and these impede or prevent complete penetration of the chips. Chips treated commercially with 15 to 25% of their own weight of NaOH as a 3 to 5% solution are made up of a mixture of under-reacted wood, over-reacted wood, fibers in various degrees of reaction and substantial quantities of solubilized substances, the latter being lost to the paper making process.

The exterior layers of wood treated with NaOH solu- 'tion become extremely soft and are easily defibered. De- I pending upon the size of chip, and to some extent upon the time of treatment, the interior portions become rubbery and elastic, resisting defibering to a greater extent than does untreated wood. As a laboratory project, it may be possible, by repeated cycles of treatment with NaOH, followed by extraction with hot water, to obtain removal of pentosans and hemicelluloses, thus to produce a uniformly treated product but this would have no commercial value. There would be insuperable difiiculties in recovery of unused caustic from dilute solutions, consumption of energy in handling, the use of much hot water as well as the need for much added equipment.

The high causticity involved in the use of NaOH, leads to the formation of sodium addition compounds of lignin which, in turn, are susceptible to oxidation by atmospheric exposure. These oxidation compounds are dark in color and thereby lead to excessive requirements of bleaching agents and, even so, are not whollybleachable to the original wood color.

At the causticity and pH levels necessary for reasonable completion of treatment of wood with NaOH, from 15 to 22% of the wood substance is lost in commercial practice. Particularly, I believe that the pentosans are consumed in this manner.

It is reported that a swelling action on the Wood by caustic soda is essential to the cold soda process. Contrary to this experience, I endeavor to avoid swelling in my process and find that excellent results are obtained thereby.

To demonstrate the relative loss in weight between the use of caustic soda and my process, the following comparison was made:

Two samples of red oak each weighing 1000 grams were taken. The first of these were steeped in NaOH solution containing 30 grams per liter. The other sample was treated with the 12:1 mixture of limezsoda ash. The treated samples were in each case extracted with dilute acetic acid and with hot water until neutral and practically ash free. The wood treated with the NaOH had lost 19.6% in weight while that treated with limezsoda ash lost only 5.6% by weight. This value is almost exactly that predictable on the assumption that acetyl, tannin and CO from the polyuronides, would be removed.

Although, from the standpoint of economy, balanced among chemicals, energy and equipment, lime and soda ash provided the best balance, there are several variations available in the broadest aspect of my process. As stated, potash is the equivalent of soda ash in performance but its attractiveness is limited by the relatively greater cost usually involved. Whether caustic or soda ash is em ployed initially with the lime, the effective ingredient is NaOH, supplied by the recaustization of the organic sodium compounds formed. When soda ash is used, the control of the process under more extreme conditions, is more readily maintained.

From the foregoing explanation, it will be obvious that the chips may be first treated with a slurry of lime and thereafter, the alkali in solution may be added to the lime coated chips, care being exercised to avoid washing the lime from the chips. As much as 2%, based upon the weight of the dry wood, of alkali may be used, although ordinarily lesser amounts are necessary. Provided the lime is applied first, the concentration of alkali in water may reach 10% but lower concentrations between 2 and 5% are preferred. Obviously, the reverse procedure of first applying the alkali, followed by coating with lime is not satisfactory, since the lime, when applied in this manner, does not counteract the swelling previously caused by the alkali.

Instead of carbonates and hydroxides of sodium and potassium, other compounds of these metals, falling between the carbonates and hydroxides in alkalinity, may be used but I have not succeeded in using compounds of these metals having less alkalinity than soda ash.

Under suitable conditions, the use of lime alone is effective to facilitate grinding of the wood chips. However, lime is soluble in water to the extent of only 0.08% so that its action upon the lamellar constituents of the wood is quite slow. Probably, as the lime reacts, particularly with the acetyl constituents of the wood, the rate of reaction is somewhat accelerated but I have found that from one to two weeks moist storage of lime coated chips is necessary to attain satisfactory grinding. While the product of lime treatment alone may be distinguished from that produced by cold soda treatment, the storage requirement is not ordinarily economical.

In order to make a comparison of the new process with the prior art, mixed eastern hardwoods were chipped and screened to obtain chips passing through /2 inch holes of a round-cut screen and retained on a 20 mesh screen. A limezsoda ash slurry of 12:1 ratio was applied to one batch of chips and the chips retained in storage for four hours. A second batch of chips was steeped in a caustic soda solution having an initial concentration of 50 grams 7 per liter, also for four hours.

The following is a table of comparative results:

New Gold Process Soda HP days per on .002 inch thick Settin 18 32 Mullen 0. 48 0. 32 Screenings Loss, per n 2. 2 17 Freeness 475 450 Refining of Screenings HP days/ton 24. 44. 0 Bleach (N aOCl) Demand to 70% GE Reflectance,

percent 2. 3 10. 0 Maximum Reflectance Na Ol Bleach (3%) 87 7 The pulp after grinding, was screened to remove coarse fiber bundles, using a slot width of .012 inch. The screened stock, at about 0.8% consistency, was passed through a series of dirt traps and to a thickener. The thickened stock at about 10% consistency was treated with unbufiered NaOCl yielding 2.2 to 2.6% available chlorine. At a temperature of 35-45 C., the bulk of the bleach had been consumed in about 10 to minutes. Although, in the example herein given the stock was self buffering by the sequesterment of acidic groups normally present in the hemicelluloses of wood, it is contemplated that an optimum pH between 9.5 and 10.0 may be maintained by the addition of small amounts of alkali or buffer salts, preferably sodium metasilicate.

The quantity of bleach employed is sutficiently small so that no further washing following bleaching is necessary. For fine printing papers, a second refining step may be included and a final screening, just ahead of the paper machine is standard practice. For commoner paper grades such as newsprint, coated book paper, tissues and the like, a single stage of refining on the original chips followed by the usual light refining ahead of the paper machine is all that is necessary. Papers of high tensile and bursting (Mullen) strength can be made by a secondary refining stage ahead of the paper machine. This is best accomplished by adding the bleach liquor to the unbleached, screened stock and doing the refining in the presence of the bleach.

As will be seen from the table appended to the foregoing example, the power consumption utilizing my process amounts to less than 40% of that used in the prior art in the fiberizing operation. The low loss on screening is even more dramatic. Further savings are yielded in the refining of the screenings to accumulative energy saving.

The low bleach demand that characterizes the product of this invention, distinguishes it from all other mechanical wood products whether straight mechanical or chemically assisted.

The novel character of the wood fibers produced by the present process is further demonstrated by their response to bleaching with sodium peroxide. It is well known that mechanical pulps are brightened when treated with Na O together with such substances as MgSO and Na SiO which serve as activators and/or stablizers.

It is possible to bleach cold-soda pulps with Na O to reflectance levels in the range from 74-77% GE using about 3% Na O on the dry weight of the fiber followed by treatment with about 1% sodium or zine hydrosulphite, actually the compounds used are the hyposulfites, salts of hyposulphurous acid (H S O The fibers produced by the practice of the present process, treated with 3% Na O followed by treatment with 1% sodium hydrosulphite produced a reflectance of 87% GE. At this level the color is pure white and entirely free of yellow. This result is known to be unprecedented and of an entirely new order.

Paper made from pulp based upon the foregoing process is characterized by substantially greater opacity than paper derived from the cold soda process. When used with bleached sulphite pulp in the manufacture of book papers, the higher strength, and brightness of the new pulp, added to its opacity, makes it possible to use up to, 50% more of this pulp compared to cold soda pulp. Thus, although substantial savings are achieved through reduced chemical and power costs, additional savings are attained through higher yield per ton of wood and through lowered demand of more expensive pulp in the furnish to the machine. Although comparisons have been made between the present process and the cold soda process, even more advantageous comparisons might be made with other chemically assisted mechanical pulping processes having even less efiiciency.

From the substantial reduction in the amount of energy necessary to separate the wood fibers, coupled with the relatively small loss of original wood substance, I believe that a significantly different pulp product is obtained by the practice of my invention.

It is well known that strong caustic dissolves the pentosan constituents of the hemicellulose. The degradation products of such reaction are easily-recovered from the spent extracting liquid. Very low concentrations of caustic, of the order of those obtainable by the use of the reagents disclosed by me and low concentrations of alkaline substances having a lower pH than sodium hydroxide, do not have this action.

The low causticity and controlled, alkalinity merely account for the increase in yield of pulp; as such, they do not account for the sharp reduction in energy required to separate the fibers. This energy saving is apparent, even to the uncritical human observation. A wood chip, shortly after being treated in accordance with the invention, will be felt, between the fingers, to fall apart easily. It is my belief that, when the alkaline liquid is suitably restrained, as disclosed herein, from vigorously attacking the wood to swell it, the lamellar structure of the wood is broken down through the combination of the alkali with the acetyl groups of the wood so that the fibers are easily released to form pulp. Thus it may be said that my process is more selective than those heretofore known in retaining more of the paper making value of the wood at minimum cost of destruction of the fraction of the wood acting as a cementing medium for the cellulose fibers.

Because of the selective operation of my process the fiber or pulp product derived from the process, diifers intrinsically from the previous pulp products. With due regard for variations in strength of reagents and the amount of time intervening between the application of the reagent and the separation of the wood fibers, on a commercial scale, from 65 to 75%, or even more of the tannins and acetyl groups are removed from the wood and, in addition, a substantial portion of the polyuronide structure is broken down. Again, allowing for occasional hot spots the pentosans are scarcely attacked at all. In no instance, have I observed as much as /3 loss of these valuable constituents. The result, as stated, is an entirely new distribution of wood constituents in a paper making pulp to produce a material of superior economic value in the industry.

I claim:

1. The process of obtaining a chemically assisted mechanical wood pulp comprising reducing the wood to chips, applying to the wood chips a slurry of lime in sufficient water to distribute the bulk of the lime over the chips with only minor drainage, storing the chips until the acetyl value has been largely consumed and subjecting the chips to a mechanical fiber separation operation.

2. The process of claim 1 wherein the chips have a maximum dimension of /2 inch and combined maximum radial and tangential dimensions of inch.

3. The process of claim 1 wherein the amount of lime used is from 10 to in excess of that required to satisfy the acetyl value of the wood.

4. The process of claim 1, wherein there is used with the lime, an alkali selected from the class consisting of sodium and potassium compounds at least as alkaline as Na CO in proportions of a CaO:Na O ratio between :1and :1.--

5. The process of claim 4 wherein the alkali is Na CO 6. The process of claim 4 where the storage period is in the range of 4 to 24 hours.

7. The process of claim 4 carried out between 20 and 80 C.

8. The process of claim 7 wherein the grinding operation is carried out at from 30 to 60 C.

9. The process of claim 4 carried out on hardwoods.

10. The process of claim 9 when the wood is selected from the class consisting of red oak, white oak, maple, beech, birch, aspen, cottonwood, willow and mixtures thereof.

11. The process of claim 4 wherein the ground stock 'is diluted, screened to remove coarse fiber bundles and dirt and then bleached.

12. The process of claim 11 wherein the bleaching is carried out with simultaneous refining.

13. The process of claim 1 wherein the stored chips 'are rinsed before fiber separation and the rinse liquid is ficient water to distribute the bulk of the lime over the chips with only minor drainage, storing the chips for at least a week and subjecting the chips to a mechanical fiber separation operation.

16. The process for obtaining a chemically assisted, mechanical Wood pulp, comprising reducing thewood to chips, applying to the wood chips a slurry of lime and an alkali selected from the class of sodium and potassium compounds at least as alkaline as Na CO in proportions of CaO:Na O between 10:1 and 20:1, in suflicient water to distribute the bulk of the slurry over the chips with insignificant drainage and subjecting the chips to a mechanical fiber separation operation.

17. The process of claim 16 wherein the alkali is Na CO 18. A chemically assisted, mechanical Wood pulp, derived from hardwood, characterized by its attainment of a GE reflectance of with the aid of not more than 2.6% Cl as NaOCl.

19. A wood pulp, produced by mechanical action with chemical assistance, characterized by its attainment of a GE reflectance of at least with the aid of not more than 3% sodium peroxide (or equivalent) and 1% sodium or zinc hydrosulfite.

References Cited in the file of this patent UNITED STATES PATENTS 223,670 Farrell Jan. 20, 1880 329,370 Cushman Oct. 27, 1885 2,061,205 Olsen Nov. 17, 1936 2,169,473 Olsen Aug. 15, 1939

Claims (1)

1. THE PROCESS OF OBTAINING A CHEMICALLY ASSISTED MECHANICAL WOOD PULP COMPRISING REDUCING THE WOOD TO CHIPS, APPLYING TO THE WOOD CHIPS A SLURRY OF LIME IN SUFFICIENT WATER TO DISTRIBUTE THE BULK OF THE LIME OVER THE CHIPS WITH ONLY MINOR DRAINAGE, STORING THE SHIPS UNTIL THE ACETYL VALUE HAS BEEN LARGELY CONSUMED AND SUBJECTING THE CHIPS TO A MECHANICAL FIBER SEPARATION OPERATION.