TECHNICAL FIELD This invention relates to a novel method and system which may selectively produce a substantially fiberized, substantially delignified, high yield cellu- losic pulp.

BACKGROUND ART In general, pulping to produce cellulose fib¬ ers is accomplished by defibering and delignifying lignocellulose by various well-known pulping techniques . One object in pulping is to defiber the lignocellulose, i.e., liberate the cellulosic fibers from the lignocel- lulose. In chemical pulping the lignocellulose is de- fibered with a chemical pulping liquor. Another object is delignification of the lignocellulose, i.e., removal of substantially all of the lignin which surrounds the individual cellulosic fibers, to produce a substan- tially lignin-free cellulosic fiber without substantial degradation of the cellulose (polysaccharide) structure. ^• Substantial degradation of the cellulose during pulping reduces the strength of the pulped cellulose fibers and lowers the pulp yield. The prior art describes various pulping pro¬ cesses employing acid, neutral, and alkaline pulping agents, respectively. Acid pulping tends to reduce the strength properties of the cellulose pulp more than alkaline pulping, causing the disadvantages previously described. Therefore, alkaline pulping, which is represented for the most part by the kraft (sulfate) pulping process, is extensively employed and produces pulps having yields of about 46-48% by weight at a lignin content equivalent to a kappa number of about 30. The kappa number is determined by TAPPI T-236.

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In an effort to conserve lignocellulose due to its accelerating price, alkaline pulping processes which produce cellulosic pulp at higher yields than by the kraft process have become quite attractive. One of these higher yield pulping processes employs ammoni¬ um sulfide as the pulping agent. The use of ammonium sulfide in pulping was originally described in U. S. Patent 1,891,337 to Seaman, and U. S. 1,817,525 to Richter. The use of ammonium sulfide as a pulping chemical was more recently studied at the Pulp and

Paper Research Institute of Canada. This work is de¬ scribed in two articles in the Pulp and Paper Magazine of Canada. The first article, which was authored by J. E. Stone and L. F. Nickerson, appeared in the Sep- tember 1961 issue, beginning at page T-4-29. A second article was written by J. E. Stone, A. M. ^' Scallan and H. H. Atilla, and appeared in Volume 74, No. 6, in the June 1973 Edition, beginning at page 75. The first article concludes that ammonium sulfide is an effect- ive pulping agent and that the composition of the pulps and the physical properties of the handsheets prepared therefrom compared favorably with both kraft and neutral sulfite pulps. The second article con¬ cludes that the optimum conditions for ammonium sulfide pulping of spruce wood are an ammonium sulfide concen¬ tration of 0.5 M at 170°C. The pulp yields, at a given lignin content, were much higher than for the same pulp prepared by the kraft process. However, this method produces pulp of dark color, with high residual lignin levels, particularly in the case of pulps made from softwoods.

Another method proposed for delignifying ligno¬ cellulose is described in U. S. 1,856,567 and U. S. 3,585,104, both to Kleinert. The Kleinert processes describe the use of a mixture of water and a water-

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miscible, volatile organic solvent, preferably a lower aliphatic alcohol or ketone, or mixture thereof, as a pulping agent. Screened pulp yields of 50-57%, using ethanol as the pulping agent, were reported by Kleinert in the August 1974 Edition of TAPPI, Volume 57, No. 8, beginning at page 99. In U. S. 4,100,016 to Diebold et al., which purports to be an improvement of the Kleinert processes, in column 1, beginning at line 7, alcohol pulping, as provided by the above Kleinert pat- ents, is described. It is stated in the Diebold patent that processes such as Kleinert ". . . have shown seri¬ ous limitations with respect to lignin removal, quality and ease of bleachability of the crude pulp . . ." .

DISCLOSURE OF INVENTION In contradistinction to the previously de¬ scribed prior art systems, and methods for pulping lig¬ nocellulose, the selected method and system of this invention may produce a substantially fiberized, sub¬ stantially delignified cellulosic pulp which has a substantially higher pulp yield at a given lignin con¬ tent than pulps produced by, for example, kraft or soda-anthraquinone pulping methods, or pulps made accord¬ ing to the teachings of J. E. Stone et al. (ammonium sulfide per se) and Kleinert (alcohol per se) , respect- ively. As for the two latter references, the subject method and system may also exhibit substantially higher physical properties as well.

The subject method contemplates the formation of an aqueous alkaline pulping system comprising ligno- cellulose, and a pulping liquor including water and a pulping agent. The pulping agent comprises a water- miscible organic reagent, and a sulfide or bisulfide compound selected from the group consisting of alkali metal sulfides and bisulfides, and ammonium sulfide and ammonium bisulfide, and mixtures thereof. The initial

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pH of the alkaline pulping system is preferably greater than a pH of about 7.0, up to a pH of about 12.5. Pulping occurs when the lignocellulose and pulping liquor are heated to an elevated temperature for a period of time sufficient to selectively produce the substantially fiberized, substantially delignified cellulosic pulp.

In the preferred case, the water-miscible organic material is selected from the group consisting of aliphatic alcohols, aliphatic ketones, and aliphatic glycols, and mixtures thereof. Of particular prefer¬ ence are aliphatic alcohols, aliphatic ketones, and aliphatic glycols having from 1 to 6 carbon atoms, the aliphatic alcohols being the more preferred of this group. However, the most preferred compounds for use as water-miscible organic solvents are methanol and/or ethanol.

Of the sulfide or bisulfide compounds describ¬ ed above, sodium sulfide and/or ammonium sulfide are the most preferred.

MODES FOR CARRYING OUT THE INVENTION Almost any lignocellulosic material can pro¬ vide the source of cellulose for the method and system of this invention. More particularly, the lignocellu- lose starting material can comprise the usual species of coniferous pulpwood (softwood) , including spruce, hemlock, fir, pine, and the like, as well as deciduous pulpwood (hardwood) such as oak, poplar, birch, cotton- wood, alder, etc., generally in the form of wood chips, as well as other lignocellulosic materials including cotton linters, bagasse, cornstalks, esparto, flax, jute, kenaf, and the like. It should be noted with respect to the above pulpwoods that the deciduous vari¬ eties are easier to pulp since they contain less lig- nin, and the lignin, itself, is more responsive to

pulping in the case of hardwoods than in their soft¬ wood counterparts. Thus, although certain processes, such as the previously described ammonium sulfide process of Stone et al., have exhibited some degree of effectiveness with respect to hardwoods, theix effectiveness with respect to pulping softwoods is limited.

The pulping process of the present invention is performed in an aqueous environment. The relation- ship of water to the lignocellulose and to the pulping agent will hereinafter be described.

The pulping liquor of this invention com¬ prises two components. One of the components is a water-miscible organic reagent, or a mixture of such reagents. Typical materials for use as the water- miscible organic reagent are selected from the group consisting of aliphatic alcohols, aliphatic ketones, aliphatic glycols, and mixtures thereof. Preferably, the above aliphatic reagents are those compounds having organic moieties comprising 1 to 6 carbon atoms, and more preferably aliphatic alcohols having from 1 to 6 carbon atoms. Of the preferred aliphatic alcohols, methanol and/or ethanol are the most preferred.

Generally, for ease of operation, a solution of water containing the water-miscible organic reagent is first formulated. Although the amount of water- miscible organic reagent can be quite high, the practi¬ cal aspects suggest limiting its use. Preferably, the ratio of water to water-miscible organic reagent, on a volume percent basis, is 30:70, up to about 90:10, and more preferably from about 50:50, up to about 80:20.

The second component in the system which forms the pulping agent is a sulfide or bisulfide com¬ pound selected from the group consisting of alkali

metal sulfides and bisulfides , and ammonium sulfide and ammonium bisulfide, and mixtures thereof. Of the above described sulfide and bisulfide compounds , how¬ ever, ammonium sulfide and/or sodium sulfide are the most preferred. Preferably, the amount of sulfide compound in the aqueous pulping system, which is con- troled by the cost of the pulping agent, is from about 0.10 M to about 2.0 M, and more preferably from about 0.15 M to about 1.5 M, and most preferably from about ^■ 0.20 M to about 1.0 M.

The water and pulping agent together form the "pulping liquor" for delignifying and fiberizing the lignocellulosic starting material. The ratio of pulp¬ ing liquor to lignocellulose is maintained at a level sufficient for effective delignification and fiberiza- tion without substantially degrading the cellulose, and without extending the pulping processes beyond a reas¬ onable time for commercial purposes. Preferably, the pulping liquor-to-lignocellulose ratio is from 3.5:1, up to 15:1, and more preferably from about 4:1, up to about 10:1.

The initial pH of the aqueous pulping system is preferably σontroled from greater than about 7.0, up to a pH of about 12.5. This minimizes the degrada- tion of the polysaccharide components of the cellulose fiber structure. More preferably, the pH of the aque¬ ous pulping system is from a pH of from about 8.0, up to a pH of about 12.0, and a pH of from about 8.5, up to about a pH of 11.0 being the most preferred. The lignocellulose is added to the pulping liquor described above and is pulped at an elevated temperature for a period of time sufficient to select¬ ively produce said substantially fiberized, substan¬ tially delignified cellulosic pulp. Selectivity herein is defined as the ability to substantially defiber the

lignocellulose, and to substantially delignify same, without substantial degradation of the polysaccharide structure of the cellulosic fiber. Preferably, the temperature for pulping the lignocellulose is from about 150°C, up to about 190°C, and more preferably from about 155°C, up to about 180°C, and most prefer¬ ably from about 160°C, up to about 170°C.

The subject time period to complete the above described pulping method is variable, depending upon the temperature, the aqueous pulping system employed, and the desired lignin content of the cellulosic pulp product. From a practical standpoint, a period of time in excess of 8 hours would probably be limiting from a commercial standpoint. The further preferred limits for pulping time are from about 0.5 hour, up to about 6 hours, and most preferably from about 1 hour, up to about 5 hours .

EXAMPLE 1 To illustrate the unexpected increase in total yield achieved by the method and system of the present invention, a hardwood (cottonwood) pulp was produced by similar methods, including the use of ammonium sulfide as a component of the pulping liquor, except that the aqueous pulping system in a first experiment included 50% by volume of a water-miscible organic reagent in place of that portion of water, i.e., ethanol, and in the second experiment, water only was employed. In fact, at least twice the molar concentration of ammonium sulfide was employed in Experiment 2 than was employed in Experiment 1.

In Experiment 1, an autoclave supported in an insulated, rocking heating block was charged with 5 grams of oven-dried cottonwood wafers and 50 milliliters of a pulping liquor comprising water and ethanol in a 50%:50% ratio by volume and an amount of ammonium sul-

fide sufficient to produce a pulping liquor having a 0.5 molar concentration. The autoclave was sealed and directly heated to a temperature of 170°C. After a three-hour reaction period at the 170°C temperature, the autoclave was cooled, opened, and the contents added to about 150 milliliters of ethanol and water. The above mixture was then disintegrated in a Waring Blendor, filtered, washed once with ethanol and water, and then washed with water only, and air-dried. The kappa number of the pulp produced according to the above method was 19.5, and the total pulp yield was 66.5%.

The above experiment was repeated in Experi¬ ment 2, except that no ethanol was included in the pulp- ing liquor. After collecting the pulp and air-drying same, as previously described, it was determined that the total yield at 30 kappa number of this latter pulp was about 60%*. .

Therefore, a substantial, unexpected differ- ence in the total yield, i.e., 6%, resulted when ethanol was added as a pulping agent to the aqueous ammonium sulfide-containing pulping liquor, at a substantially lower kappa number, i.e., 19.5 versus 30.

The above procedure was again repeated in two additional experiments at a temperature of 160°C and a pulping time period of 6 hours. In the first experi¬ ment, 0.25 M ammonium sulfide in 50:50 ethanol-water was employed, while in the second experiment, a four¬ fold greater amount of ammonium sulfide solution (1.0 M) in water only was provided. In spite of the above dis¬ parity in sulfide compound usage, the ammonium sulfide- ethanol-water pulping liquor produced a substantially fiberized, substantially delignified pulp having a high yield (66.5% versus 61%), at a lower kappa number (27.5 versus 36.5).

EXAMPLE 2

Cottonwood wafers were pulped, according to the procedure outlined in Example 1, Experiment 1, except that the ammonium sulfide was not present in the pulping liquor. A series of three pulping runs was conducted at a temperature of 180°C for a period of time of 0.75 hour (Run No. 1) , 1.0 hour (Run No. 2) , and 1.5 hours (Run No. 3) , respectively. The kappa number and total yield for these reactions are shown below in Table 1.

TABLE 1 Run No. Kappa No. Total Yield

1 69 56.8 ^'

2 37 52.0 3 37 49.7

It should be noted that total yield for cottonwood wafers employing the conventional kraft pulp¬ ing process at a kappa number of 37 is about 55%. There¬ fore, it is clear that the total yield employing a 50% volume ratio of ethanol and water per se as the pulping agent, without ammonium sulfide, produces a total yield not only substantially lower than provided by the method and system of the present invention, but even lower than that which is produced by the kraft process. EXAMPLE 3

In order to examine the system and method of the present invention with respect to softwood, which, as previously described, is far more difficult to pulp than hardwoods such as cottonwood, and the like, Douglas fir softwood chips were pulped according to the follow¬ ing experimental procedure:

Using a 12-liter circulating liquor pulping digester, 1 kilogram of air-dried Douglas fir chips was added to 8 kilograms of 0.51 M ammonium sulfide (Experi- ment 1) . Pulping was conducted at _a temperature of

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180°C for 5 hours. The liquor-containing pulp was then blown down into a second digester to which about 8 kilograms of water at 250°C had previously been added and the mixture circulated for about 2 minutes. This procedure was repeated and the washed pulp re¬ covered. The pulp was then broken up by mild agita¬ tion (about 1,750 rpm) with a 10-centimeter diameter plate stirrer.

The above experiment was repeated (Experi¬ ment 2) , except that, instead of the ammonium sulfide solution containing only water, a 50:50 volume percent water-ethanol solution was employed. The results of these two experiments are summarized in Table 2 below:

TABLE 2

Experiment 1

Total yield 67.4%

Screenings 18.8%

Kappa No. • 141.7

Canadian Standard Freeness 600 500 400

Brightness, Elrephc > 6.1 6.5 6.5

Burst Factor, m 44.5 51.6 55.5

Tear Factor, dm 2

110.9 105.7 99.2

Fold, MIT 342 496 570 -

Breaking Length, meters 8,548 9,167 9,447

Stretch, % 2.76 3.07 3.17

TEA, Kg met/met 7.21 8.74 9.98

TABLE 2 (continued) Experiment 2

Total yield 62.3.

Screenings 3.0%

Kappa No. ^' 52.7

Canadian Standard Freeness 600 500 400

Brightness, Elrepho 7.7 7.6 7.4

Burst Factor, m 65.3 69.1 72.9 Tear Factor, d -m2

108.0 99.8 91.6

Fold, MIT 461 521 581

Breaking Length, meters 9,588 9,954 10,320

Stretch, % 3.48 3.44 3.40

TEA, Kg met/met 12.47 12.45 12.43

It can be concluded from the above results that pulp produced by the method of the present inven¬ tion (Experiment 2) effectively delignified and fiber- ized the Douglas fir chips. This is clear from the fact that less than 5% screenings, which is the normal meas¬ ure of acceptable defiberizing, and, in fact, only 3% screening was produced employing the method of the pre¬ sent invention. This is contrasted to 18.8% screening present in Experiment 1 in which ammonium sulfide and water only were employed as the pulping liquor. Fur- ^' thermore, the kappa number of 141.7 in Experiment 1 clearly indicates that substantially no delignification occurred with the ammonium sulfide-water liquor system, as compared to the ammonium sulfide-ethanol-water pulp¬ ing system in which a kappa number of 52.7 was achieved. Finally, the physical properties of the screened pulp produced in Experiment 2 were far superior to those of the ammonium sulfide-water system, further verifying the nature of the results which -were obtained herein.

Of most significance are the average increases in burst (37%), stretch (15%), and TEA C44%) .

EXAMPLE 4 The procedure of Example 3 was repeated, using 50% by volume methanol instead of ethanol with softwoods. The reaction was conducted at a tempera¬ ture of 170°C. The total pulp produced was refined in a PFI mill and the results of experiments using Douglas fir (Experiment 1) and hemlock (Experiment 2) are sum¬ marized below and compared to typical, unbleached Douglas fir pulps prepared by the conventional kraft pro¬ cess.

TABLE ^" 3 Experiment 1

Species Douglas fir

Yield 68%

Kappa No. 77

Canadian Standard Freeness 600 500 400

2

Burst Factor, m 57 60 61

Tear Factor, dm 91 82 75

Breaking Length, meters 8,500 9,000 9,300

Stretch, % 3.1 3.3 3.4

Experiment 2

Species Hemlock

Yield 70%

Kappa No. 85

Canadian Standard 600 500 400 Freeness

2

Burst Factor, 71 73 73

Tear Factor, d -m2 78 70 66

Breaking Length, meters 10,680 10,880 10,733

Stretch, % 3.1 3.2 3.2

- ϋ-

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TABLE ^' 3 Ccontinued)

^• ' ^• Kraft Process

Species Douglas fir

Yield 55%

Kappa No . 85

Canadian Standard Freeness 550-600

2

Burst Factor, m 50

2 Tear Factor, dm 190

Breaking Length, meters 6,500

Stretch, % 2.5

* Typical physical properties for unbleached kraft handsheets.

It is clear from examining the results above that pulp produced by the subject process compared favorably to kraft pulping in most of the physical pro¬ perties listed above and had a 13% or better yield improvement over kraft pulps.

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