US3368935A - Sulfite pulping process with urea - Google Patents

Description

United States Patent 3,368,935 SULFITE PULPING PROCES WITH UREA Alfred M. Heald and Julius D. Robertson, Hartsville, S.C., assignors to Sonoco Products Company, Hartsville, S.C., a corporation of South Carolina No Drawing. Filed Oct. 12, 1964, Ser. No. 403,384 7 Claims. (Cl. 162-72) This invention relates to a pulping process and more particularly to a process for digesting cellulosic raw material such as wood chips by the sulfite process.

The pulping of cellulosic raw materials such as wood chips by conventional sulfite pulping processes has always been characterized by certain drawbacks which continuous efforts have failed to overcome. While many solutions have been proposed for the elimination of these drawbacks, efforts to date have culminated in procedures which, rather than eliminating these drawbacks, serve merely to compensate in part for such drawbacks and, at the same time, are expensive and time consuming. For instance, in a conventional sulfite digestion of wood chips using an acid sulfite or bisulfite cooking liquor, the initial pH of the liquor is generally within the range of approximately 1.5 to 4.5. Utilizing such a low pH during the initial stages of digestion has the advantage of rapid sulfonation of the lignin but, at the same time, it promotes undesirable acid hydrolysis of the carbohydrates in the chips. Furthermore, in such a pulping process, the release of wood acids dur-. ing digestion further lowers the pH of the cooking liquor, increasing its acidity and promoting this acid hydrolysis of carbohydrates. This acid hydrolysis has been partially compensated for in such an acid sulfite or bisulfite pulping process by maintaining the digestion temperature to a level of below approximately 145 to 150 C. While such a low digestion temperature suppresses acid hydrolysis to an extent, it, at the same time, retards the rate of lignin sulfonation and greatly increases the time of digestion required to obtain the desired degree of delignification and pulping. In addition, in such an acid sulfite pulping process, there is a release of substantial amounts of gaseous sulfur dioxide during digestion which gaseous sulfur dioxide must be removed and recovered for reuse by a complicated and expensive process in the cooking liquor manufacturing plant.

In conventional neutral or alkaline sulfite pulping processes, the common practice is to utilize a cooking liquor having an initial pH within the range of approximately 9.0 to 10.0 or even higher to provide for neutralization of the wood acids formed during the digestion and to obtain a digestion pH and/ or terminal pH of 7.0 or higher. The use of such a high initial pH for the cooking liquor has many disadvantages in that it produces alkaline degradation of certain short chain carbohydrates (hemi-celluloses) in the wood chips and retards lignin sulfonation increasing the digestion time and/ or requiring a higher cooking temperature. Furthermore, the use of cooking liquors containing magnesium, calcium and other similar insoluble bases for such neutral or alkaline sulfite pulping processes is restricted even to the extent of eliminating for all practical purposes the use of at least one of such bases (calcium) because of the tendency of these bases to form insoluble monosulfites which is generally referred to as liming out under alkaline or near alkaline conditions. For instance, using present sulfite pulping practices, precipitation of insoluble monosulfites when using these bases can be avoided only when the ambient liquor pH during the initial digestion period does not exceed a maximum of approximately 6.0 for magnesium base liquors and even lower (1.5 to 2.0) for calcium base liquors. Present-day practices used to obtain neutral or alkaline sulfite pulping with liquor containing these insoluble bases include either the injection of a suitable alkali dur- "ice ing digestion or the draining of the acid liquor of the initial digestion stage and subsequent introduction of an alkaline liquor for a subsequent digestion stage. Both of these practices have the serious disadvantage that the added alkali must migrate by the slow process of diffusion and liquid exchange into the wood chips which are saturated within an acid liquor permitting acid hydrolysis of the carbohydrates in the wood chips to occur while this slow migration is taking place. The practice of utilizing an alkaline liquor for a second digestion stage has the additional disadvantage that acid hydrolysis of carbohy drates will have occurred before the introduction of the alkaline liquor.

Accordingly, a primary object of this invention is to provide a new and novel sulfite pulping process which permits the liquor pH during pulping to be closely controlled whereby optimum pulping conditions may be obtained during the pulping of a wide variety of cellulosic materials using a wide variety of liquor bases.

Another object of this invention is to provide a new and novel sulfite pulping process which eliminates virtually all of the undesirable effects of wood acids bydrolytically released from the pulped material during the process.

Still another object of this invention is to provide a new and novel sulfite pulping process which permits pulping at either a substantially neutral or even slightly alkaline terminal pH to be obtained without alkaline degradation of certain short chain carbohydrates at the start of the digestion, and at an initial pH sufiiciently low to permit a rapid rate of lignin sulfonation.

A further object of this invention is to provide a new and novel sulfite pulping process for pulping a wide variety of cellulosic raw materials utilizing a wide variety of cooking liquors without risk of precipitation of insoluble monosulfites from insoluble base liquors containing calcium, magnesium, and the like.

Still another object of this invention is to provide a new and novel sulfite pulping process that permits the maintaining of an essentially neutral liquor pH throughout the entire digestion and thereby eliminates the necessity for the high initial pH with attendant alkaline degradation of carbohydrates that is characteristic of conventional neutral sulfite digestions.

A further object of this invention is to provide a new and novel sulfite pulping process which permits the use of a low initial liquor pH for an acid sulfite or bisulfite process as Well as neutral or alkaline sulfite process with substantial elimination of acid hydrolysis of carbohydrates in the pulped material and which permits the conversion of sulfur dioxide liberated during an acid sulfite or bisulfite pulping process into non-gaseous sulfur compounds.

A still further object of this invention is to provide a new and novel sulfite pulping process in which the initial pH of the cooking liquor can be varied throughout a wide range from a low pH to substantially neutral pH and the digestion and/or terminal pH varied to any value within a range between the initial pH and substantially neutral pH or even higher with an accelerated digestion time, with virtually no acid hydrolysis of carbohydrates and without the time consuming and expensive steps of injecting additional alkali during digestion or the use of multiplicity of digestion stages.

Still another object of this invention is to provide a new and novel sulfite pulping process which permits the use of sulfurous acid as a cooking liquor to obtain rapid lignin sulfonation and a terminal pH at or near neutrality without the customary lignin resinification, chip burning and acid degradation of carbohydrates. I

Other objects and advantages of the invention will become apparent from the following description.

The objects stated above and other related objects in this invention are accomplished by reacting a selected cellulosic raw material such as Wood chips with a sulfite cooking liquor having an initial pH selected in accordance with the particular type of sulfite pulping process employed. The cooking liquor may have any suitable base such as magnesium, calcium, sodium and even ammonia. An alkali liberating compound such as urea is added to the cooking liquor in an amount which is calculated to produce a predetermined terminal pH in the pulped mixture at the completion of the digestion. Chips are digested at a selected temperature which is no lower than the temperature at which ammonia is liberated from the urea whereby the wood acids hydrolytically released during the digestion are neutralized by the ammonia to maintain a selected digestion pH and/or to produce a desired terminal pH in the pulped mixture at the completion of the digestion.

The novel features which are believed to be characteristic of the invention are set forth with particularity in the appended claims. The invention itself, however, both as to its organization and method of operation may be best understood by reference to the following description.

As specifically illustrative of the invention, the alkali liberating constituent of the sulfite cooking liquor is urea and urea only will be referred to in the examples to follow. It should be understood, however, that while the outstanding results of the invention are obtained with the use of urea, certain other alkali liberating compounds may be employed such as biuret.

In the chemical reaction which occurs in the practice of the invention, the urea constituent of the cooking liquor is converted to ammonia and carbon dioxide at temperatures within the range of approximately 110 C. to 170 C. in accordance with the following reactions:

During sulfite pulping process, wood acids such as formic and acetic acids are hydrolytically released from the cellulosic raw material such as wood chips which are neutralized by the ammonia formed in reactions Nos. 1, 2 by the following reaction:

(3) RCOOH+NH RCOONH Taking reactions 1-3 together, the neutralization of wood acids by urea can be summarized by the following reaction:

As previously referred to, sulfur dioxide is generated during an acid sulfite or bisulfite pulping process and the neutralization of sulfur dioxide by urea to form ammonium sulfite can be summarized as follows:

In general, the amount of urea combined with the cooking liquor in the sulfite pulping process of the invention is selected in accordance with the amount of wood acids which are to be neutralized in order to arrive at a desired terminal pH in the pulped mixture. More specifically, the amount of cellulosic raw material to be pulped, the constituents and amount of cooking liquor employed as well as the initial, digestion and terminal pH of the liquor are all factors which must be taken into consideration when determining the amount of urea to be added. Furthermore, when using a cooking liquor having as an active ingredient an alkali monosulfite, it is well known that one half of this alkali monosulfite or combined alkali is available for the neutralization of the wood acids released during the digestion. The reaction by which the alkali monosulfite is made available for wood acid neutralization is best illustrated as follows:

Thus, in determining the total amount of alkali needed to obtain a desired terminal pH in the pulped mixture, only that amount of urea need be added to the cooking liquor which in combination with one half of the alkali combined as monosulfite, if such is present as a constituent, will produce the desired neutralization of the woods acids produced during pulping to the desired degree.

In order to clearly illustrate the outstanding results of the invention, various examples were conducted from which handsheets were prepared utilizing both the prior art practices as well as the process of the invention. In all of these examples with the exception of one wherein pine chips were employed, wood chips comprising a mixture of black gum, sweet gum, tupelo and red maple were digested in a digester using a sulfite cooking liquor having the indicated constituents.

In carrying out the various digestion processes, the chips, which were carefully measured to a weight of 7,000 grams based on bone dry wood substance, were placed in the digester and pre-steamed. Then sulfite cooking liquor was pumped into the digester and continuously circulated through the chips. The liquor was brought to the temperature indicated in each of the examples within a period of 120 minutes and this temperature, with constant circulation of the liquor through the chips, was maintained for the digestion time indicated in each example. At the end of the digestion, the liquor was removed, the cooked chips weighed, and their bone-dry fiber content was determined to establish yield. The chips were then given a preliminary refining in a laboratory disc mill and a sufficient quantity of the resulting prerefined chips was introduced into a standard Valley Beater to constitute a bone dry fiber charge of 454 grams. The total charge of the beater was then adjusted by the addition of water to 22,700 grams or a slurry of 2% consistency. The initial Schopper-Riegler freeness was determined and 5.5 kilograms of weight were placed on the bedplate arm. Beating of the charge was then carried out and pulp samples of standard Schopper- Riegler freeness of 750 cc., 500 cc., 250 cc., and 150 cc. were caught and the time to obtain each of these freenesses was measured. Handsheets having a basic weight of pounds per 3,000 square feet were prepared from the pulps at these standard freenesses. After a conditioning period of 24 hours at 50% relative humidity and 73 F., the various handsheets were subjected to a GE. Brightness, 6" Ring Crush (Riehle), Tear, Mullen and Zeta Tensile Tests.

The following two examples were conducted to illustrate present day sodium base sulfite pulping processes:

Example I Schopper-Riegler Freencss, cc .i 750 500 250 150 G.E. Brightness, percent 42 0 Minutes Beating i. 14 39 51 Density, Grams/cc 0. 532 0. 647 0. 728 0. 775 6 Ring Crush, Pounds i 68 93 107 112 Tear, Grams 9t) 95 91 76 Mullen, Pounds 54 88 111 114 Zeta Tensile, Pounds 37 115 188 H...

Example 2 As an example of a conventional high yield sodium base bisulfite digestion process, the wood chips were Schopper-Riegler Freeness, cc 750 500 250 150 G.E. Brightness, percent 380 Minutes Beating 8 42 63 Density, Grams/cc O 490 625 0.720 0.808 6 Ring Crush, Pounds 60 88 104 117 Tear, Grams 94 101 107 98 Mullen, Pounds 50 74 75 97 Zeta Tensile, Pounds 43 92 125 The tollowing three examples were conducted to illustrate the novel process of this invention as applied to sodium base sulfite pulping:

Example 3 This example is an illustration of the sulfiite pulping process of the invention utilizing a sodium base bisulfite liquor. The active ingredients of the cooking liquor were 7 16 grams of sodium bisulfite and 294 grams of urea at an initial pH of 3.9. The digestion was maintained at 170 C. for 150 minutes. The terminal pH was 7.0, and the yield, based on the original bone dry wood, was 74.5%. The handsheet properties were as tfollows:

Schopper-Riegler Freeness, cc 750 500 250 150 G.E. Brightness, percent. 37 Minutes Beating 11 24 4 55 Density, Grams/cc 0.525 0 620 0.732 0.776 6 Ring Crush, Pounds 65 77 86 92 Tear, Grams 118 144 128 118 Mullen, Pounds 62 87 98 104 Zeta Tensile, Pounds 56 97 154 Example 4 Schopper-Riegler Freeness, cc 750 500 250 150 G.E. Brightness, percent 41 Minutes Beating 17 44 61 Density, Grams/cc.-." 0.610 0. 705 0.730 0.795 6 Ring Crush, Pounds 73 92 111 100 Tear, Grams 114 112 103 90 Mullen, Pounds 77 98 114 118 Zeta Tensile, Pounds 90 153 181 Example 5 This example is similar to Example 4 but with reduced amounts of pulping chemicals. The active ingredients of the cooking liquor were 378 grams of sodium sulfite, 308 grams of sodium bisulfite, and 150 grams of urea, with an initial pH of 7.5. The digestion was maintained at 170 C. for 100 minutes. The terminal pH was 7.4, and the yield, based on the orginal bone dry wood, was 76.6%. The handsheet properties were as follows:

Schopper-Riegler Freeness, cc 750 500 250 150 G.E. Brightness, percent 26 Minutes Beating 14 28 50 74 Density, Grams/em 0.575 0.675 0.732 0. 775 6" Ring Crush, Pounds. 71 96 96 100 Tear, Grams 91 118 115 92 Mullen, Pounds 61 85 108 113 Zeta Tensile, Pounds 76 128 161 The tfiollowing example was conducted to illustrate a present day magnesium base bisultfite pulping process:

Example 6 This example demonstrates the results obtained with a conventional high yield magnesium base bisulfite digestion of hardwood. The active ingredient of the cooking liquor was 620 grams of magnesium bisulfite, and the initial pH was 4.9. The digestion was maintained at 150 C. for 180 minutes. The terminal pH was 3.9, and the yield, based on the original bone dry wood, was 6 8.7%. The handsheet properties were as follows:

Schopper-Riegler Freeness, cc 750 500 250 150 G.E. Brightness, pereent 37 Minutes Beating 15 39 Density, Grams/ce 0.625 0.740 0.811 0.870 6' Ring Crush, Pounds 81 94 113 Tear, Grams 122 113 114 106 Mullen, P0unds 76 87 102 107 Zeta Tensile, Pounds 88 147 200 The following three examples were conducted to illustrate the novel process of this invention as applied to magnesium base bisulfite pulping? Example 7 This example is an illustration of the sulfite pulping process of the invention utilizing a magnesium base bisulfite liquor which is terminated at a relatively high pH. The active ingredients of the cooking liquor were 620 grams of magnesium bisulfite and 290 grams of urea, with an initial pH of 4.7. The digestion was maintained at 170 C. for 105 minutes. The terminal pH was 6.1, and the yield, based on the original bone dry wood, was 71.5%. At one time during the digestion a pH of 7.1 was observed with no evidence of liming out. The handsheet properties were as follows:

Schopper-Riegler Freeness, cc 750 500 250 150 G.E. Brightness, Percent 83 Minutes Beating 9 33 58 85 Density, Grams/cc 0. 695 0. 745 0.820 0.862 6 Ring Crush, Pounds. 77 92 92 101 Tear, Grams" 126 123 100 Mullen, Pounds 95 109 128 111 Zeta Tensile, Poun 85 134 200 Example 8 erties were as follows:

Schopper-Riegler Freeness, cc 750 500 250 G.E. Brightness, Percent 21 Minutes Beating 5 22 40 68 Density, Grams/cc. 0.560 0.660 0.763 0.830 6" Ring Crush, Pou 50 68 87 113 Tear, Grams... 87 120 97 95 Mullen, Pounds 84 92 104 110 Zeta Tensile, Pound 55 131 171 Example 9 This example is an illustration of a magnesium base bisulifite pulping process in accordance with the invention on which the pulped cellul-osic material is southern pine chips. The active ingredients of the cooking liquor were 930 grams of magnesium bisulfite and-360 grams of urea, with an initial pH of 4.3. The digestion was maintained at C. for 210 minutes. No evidence of liming out was observed, although the terminal pH was 7.3. The yield, based on the original bone dry wood, was 84.3%. Little or no evidence of objectionable pitch or tacky sub- 67.3%. There was no evidence of burning or lignin resinification. Handsheet properties were as follows:

stances was observed in the pulp. Handsheet properties Schopper-Riegler 13110110557 66 750 509 250 150 were as follows: 5

G.E.Brightn ess, Percent 32 .i Schopper-RieglerFreenescc 750 500 250 159 1B r?$ ;j;jj 0. 544 0.715 0.155 0.770 0" Ring Crush, Pound 56 fig fit F Percent 8 5 g l i ifiiien i t i u i'ias 5 J3 120 mu e" ea 111g...

. 0.610 (L640 0750 0770 Zeta Tens1le,Poun 51 140 14a 52 74 69 69 32 ,33 32 8% In order to provide a clear comparison of the results Zeta Tensile. Pound 77 90 125 obtained in the various examples previously referred to and thereby contrast the novel results obtained with the The following example was conducted to illustrate the process of the invention with the prior art, a tabulation novel pulping process of this invention as applied to 6211- of the results of Examples 1-11 have been complled 1n inm b a id lfite pulpin the table below ident1fied as Table I. Only the test re- Exam 10 sults obtained at Schopper Riegler Freenesses of 750 cc.

p and 500 cc. are included in this table, as the general com- The active ingredients of the cooking liquor were 202 mercial use of these Schopper Riegler Freenesses is congrams of calcium bisulfiate, 242 grams of free S0 and sidered to provide sufiicient comparison of the test results.

TABLE I Example No 1 2 3 4 5 6 7 8 9 10 11 Type of Wood Pulped Pine ase (I) Mg Mg Mg Mg Ca None Sodium Sulfite, gms 882 0 0 590 378 0 0 0 U 0 0 Bisulfite,gms 0 693 716 198 308 620 620 465 930 202 0 Free SOe,gms 0 O 0 0 0 0 0 0 0 242 427 Bufler Used 0 Urea Urea Urea 0 Urea Urea Urea Urea Urea Amount of Bnner, gms. 159 0 294 143 150 0 290 270 360 470 550 InitielDigestionpH- 10.6 5.3 3.9 7.0 7.5 4.9 4.7 4.6 4.3 1.9 1.8 TerminalDigestion pH. 7.4 4.7 7.0 7.5 7.4 3.9 6.1 6.2 7.3 7.2 7.15 Cooking Temp, 0.. 170 150 170 170 170 150 170 170 170 170 170 Cooking Time, Mins" 180 250 150 150 100 180 105 90 210 60 Yield, Percent 75.0 72.3 74.5 72.2 76.6 68.7 71.5 74.9 84.3 75.8 72.2 G.E. Brightness, Percent. 42.0 38.0 37.0 41.0 26.0 37.0 33.0 21.0 32.0 29.5 32.0 750 sRDens'y 0.532 0.490 0.525 0.610 0.575 0.625 0.695 0.560 0.610 0.534 0.544 6" Ring Crush. 68 73 71 81 77 60 52 53 56 Tear, gms 90 94 118 114 91 122 126 87 183 106 93 Mullen, lbs 54 56 62 77 61 76 95 84 83 57 52 Zeta Tensile, lbs 37 43 56 90 76 88 85 55 77 58 51 500 SR-Density. 0.647 0.625 0.620 0.705 0.675 0.740 0.745 0.660 0.640 0.589 6.715 6 Ring Crush. 93 88 77 92 96 94 92 68 74 90 'leur,gms 95 101 144 112 118 113 135 120 170 128 113 Mullen, lbs... 88 74 87 98 87 109 92 98 73 03 Zeta Tensile, lbs 115 92 97 153 128 147 134 131 75 145 1 Hardwoods. 2 Sodium. NBzCO 470 grams of urea. The initial pH was 1.9. The time con- As has been referred to above, Examples 1, 2 are consumed in heating the charge to 170 C. was extended to ventional neutral sulfite and sodium bisulfite pulping proc 330 minutes to avoid possible resinification of the lignm esses using a sodium base with Examples No. 3, 4, and 5 by the highly acid liquor. The digestion was maintained carried out in accordance with the novel process of the at 170 C. for 60 minutes. The terminal pH was 7.2, and 50 invention utilizing a sodium base. Example No. 6 is a conat no time during the digestion was there visible evidence ventional pulping process using a magnesium base and of liming out. The yield, based on the original bone dry Examples N0. 7, 8 are magnesium base sulfite pulping wood, was 75.8%, with no evidence of burning. Handprocesses showing the novel results of the process of the sheet properties were as follows: invention as applied to a magnesium base process. Ex-

55 ample N0. 9 illustrates the results of magnesium base Schopper-RieglerFreeness,cc 756 500 250 150 pulping utilizing pine as the pulped material. Example No. 10 is a calcium base sulfite pulping process and Ex- (1.12. Brightness, Percent 29.5 ample No. 11 is a further embodiment of the invention Minutes Beating 16 39 68 100 f so th K Density, Grams/cc 0.534 0.589 0.668 0.877 1 g Tee 2 as 6 8 i 6" Ring Crush, Pounds 53 80 85 8 0 One of the outstandlng features of the invention is the Tear, Grams 106 128 128 121 b r Mullen, pounds 57 73 109 Sll stantlal red ction in cooking time over prior art proc- Zeta Tensile. Pounds 58 75 135 esses using the process of the invention, Thus c n id bl cost savings are effected by reduced production time while The following example was conducted to illustrate the simultaneously improving the quality of the product. In novel pulping process of this invention as applied to sul- 65 the sodium base pulping processes of the invention, cookfurous acid pulping. mg times of 150 minutes for Examples 3, 4, and minutes for Example No. 5 compare quite favorably with Example 11 I 180 mmutes and 250 minutes for prior art Examples The active ingredients of the cookmg liquor were 427 No. 1,2. The magnesium base sulfite pulping processes grams of free S0 and 550 grams of urea. The 1n1t1al pH 70 0f the 1nvent1on show cooking times of and 90 minwas 1.8. The time required to bring the change to 170 C. was extended to minutes to avoid possible burning by the highly acid liquor. The change was maintained at C. for 45 minutes. The terminal pH was 7.15 and the yield, based on the original bone dry wood, was

utes for Examples 7, 8 respectively as compared with the conventional magnesium pulping process in Example No. 6 of minutes.

At the Schopper Riegler Freeness of 750 cc., the sodium base Examples No. 3, 4, and 5 show substantial increases in 6" Ring Crush, Tear, Mullen and Zeta Tensile over the prior art sodium base process of Examples No. 1, 2. At Schopper Riegler Freeness of 500 cc., substantially the same improved results were obtained with unusually high Tear in Example No. 3 and Zeta Tensile in Example No. 4.

In the magnesium base sulfite pulping Examples No. 7, 8 and at 750 cc. Schopper-Riegler freeness, the 6" Ring Crush, Tear and Zeta Tensile compare favorably with the prior art Example No. 6 and with a substantial improvement in Mullen being obtained. At Schopper-Riegler freeness of 500 cc., essentially the same results in 6" Ring Crush, Zeta Tensile and Tear as the prior art were obtained but with a considerable improvement in Mullen. A significantly high Tear was obtained in Example No. 7 as compared with prior art Example No. 6 at 500 cc. Schopper. What is particularly unusual regarding the test results of Examples Nos. 7, 8 is that although a terminal pH approaching neutral was involved including an even higher pH at one time during digestion, there is no evidence of liming out or precipitation of monosulfites such as is common in the present day magnesium base sulfite pulping processes.

Example No. 9 was included to show the advantageous use of the process of the invention in the magnesium base sulfite pulping of pine. Test results shown for Example No. 9 compare quite favorably with the test results of the conventional magnesium base pulping of hardwood in Example No. 6 with an unusually high Tear and Mullen being obtained. As in Examples Nos. 7, 8, there was no evidence of liming out in Example No. 9 and the process of the invention apparently prevented the forming of objectionable pitch or other tacky substances in the pulp.

Example No. 10 shows that the process of the invention may be used to accomplish what is virtually impossible with prior art practices in that with an initial pH of 1.9 in a calcium base sulfite pulping of hardwoods, a terminal pH of 7.2 was reached in a cooking time of only 60 minutes without any evidence of liming out. The test results show the producton of a pulp of more than adequate strength at 750 cc. and 500 cc. Schopper-Riegler freeness.

Example No. 11 shows another unusual result of the process of the invention wherein a highly acid free S was used as a cooking liquor buttered with urea in accordance with the invention and at an initial pH of 1.8 and a terminal pH of 7.15. The common drawbacks of S0 pulping such as ligin resinification, chip burning, and acid degradation of carbohydrates did not appear and a pulp was obtained with unusually good test results.

In view of the foregoing examples in which the effectiveness of a wide variety of sulfite bases or even sulfurous acid as pulping agents has been demonstrated, it logically follows that ammonia or less conventional bases may be employed without departing from the spirit of this invention.

The results obtained with the novel process of the invention in which the cooking liquor is buffered with an alkali liberating compound show that a pulp may be produced utilizing any of a wide selection of bases including sodium, magnesium, and calcium so as to produce a pulp at least equal to and in many cases far superior to sulfite pulps produced utilizing prior art processes. The well known tests of 6" Ring Crush, Tear, Mullen and Zeta Tensile which were performed on handsheets produced in accordance with the invention are considered to clearly reveal all of the desired strength factors of paper produced from pulp. In addition to the production of superior pulp, the cooking time in each example in the pulping of hardwoods was reduced considerably over that of the prior art. What is of particular significance is that the process of the invention permits the pH of the cooking liquor to be controlled to a very close degree throughout the entire pulping process and eliminates the severe drawbacks of alkaline degradation and acid hydrolysis heretofore encountered with the necessary use of cooking liquors at a pH Within the alkaline range of 10 and above or in the acid range for prolonged periods. Of additional significance is the elimination of the precipitation of monosulfites when magnesium or calcium bases are utilized as Well as the conversion of sulfur dioxide to ammonium sulfite during acid sulfite pulping using the process of the invention which eliminates an additional problem of the prior art processes where gaseous S0 was formed requiring its disposal.

While there has been described what at present is considered to be the preferred embodiment of the invention, it will be understood by those skilled in the art that various changes and modifications may be made therein without departing from the invention and, therefore, it is the aim of the appended claims to cover all such changes and modifications as fall within the true spirit and scope of the invention.

Having thus described the invention, what is claimed is:

1. A sulfite pulping process comprising .the steps of mixing cellulosic raw material with a sulfite cookingliquor having urea as a constituent in an amount sufficient to produce a predetermined terminal pH in the mixture, reacting said liquor and said cellulosic raw material at a temperature no lower than the temperature at which ammonia is formed from said urea for neutralizing the wood acids hydrolytically released during said reaction and continuing said reaction to completion at said predetermined terminal pH to form for processing into fibrous sheet material.

2. A sulfite pulping process comprising the steps of, mixing cellulosic raw material with sulfite cooking liquor having urea as a constituent in an amount sufficient to produce a substantially neutral terminal pH in the mixture, digesting said cellulosic raw material by reacting said liquor and said cellulosic raw material at a temperature no lower than the temperature at which ammonia is formed from said urea for neutralizing the wood acids hydrolytically released during the said digestion, controlling the neutralization of said wood acids by said ammonia to maintain a substantially neutral digestion pH, and continuing said digestion to completion at said substantially neutral terminal pH to form pulp for processing into fibrous sheet material.

3. A sulfite pulping process comprising the steps of, mixing cellulosic raw material with a sodium base sulfite cooking liquor having urea as a constituent in an amount sufficient to produce a predetermined terminal pH in the mixture, digesting said cellulosic raw material by reacting said liquor and said cellulosic raw material at a temperature no lower than the temperature at which ammonia is formed from said urea for neutralizing the Wood acids hydrolytically released during said digestion, controlling the neutralization of said wood acids by said ammonia to maintain a selected digestion pH and continuing said digestion to completion at said predetermined terminal pH to form pulp for processing int-o fibrous sheet material.

4. A sulfite pulping process comprising the steps of, mixing cellulosic raw material with a magnesium base sulfite cooking liquor having urea as a constituent in an amount sufficient to produce a predetermined terminal pH in the mixture, digesting said cellulosic raw material by reacting said liquor and said cellulosic raw material at a temperature no lower than the temperature at which ammonia is formed from said urea for neutralizing the Wood acids hydrolytically released during said digestion, controlling the neutralization of said Wood acids by said ammonia to maintain a selected digestion pH and continuing said digestionto completion at said predetermined terminal pH to form pulp for processing into fibrous sheet material.

5. A sulfite pulping process comprising the steps of, mixing cellulosic raw material with a calcium base sulfite cooking liquor having urea as a constituent in an amount sufficient to produce a predetermined terminal pH in the mixture, digesting said cellulosic raw material by reacting said liquor and said cellulosic raw material at a temperature no lower than the temperature at which ammonia is formed from said urea for neutralizing the wood acids hydrolytically released during said digestion, controlling the neutralization of said wood acids by said ammonia to maintain a selected digestion pH and continuing said digestion to completion at said predetermined terminal pH to form pulp for processing into fibrous sheet material.

6. A sulfite pulping process comprising the steps of, mixing cellulosic raw material with a sulfite cooking liquor having a base selected from the group consisting of sodium, magnesium and calcium and having urea as a constituent in an amount sufiicient to produce a predetermined terminal pH in the mixture, digesting said cellulosic raw material by reacting said liquor and said cellulosic raw material at a temperature no lower than the temperature at which ammonia is formed from said urea for neutralizing the Wood acids hydrolytically released during said digestion, controlling the neutralization of said wood acids by said ammonia to maintain a selected digestion pH and continuing said digestion to completion at said predetermined terminal pH to form pulp for processing into fibrous sheet material.

7. A sulfite pulping process comprising the steps of, mixing cellulosic raw material with a solution of free sulfur dioxide as a cooking liquor having urea as a constituent in an amount sufiicient to produce a predetermined terminal pH in the mixture, reacting said sulfur dioxide and said cellulosic raw material at a temperature no lower than the temperature at which ammonia is formed from said urea for neutralizing the wood acids hydrolytically released during said reaction, and continuing said reaction to completion at said predetermined terminal pH to form pulp for processing into fibrous sheet material.

References Cited UNITED STATES PATENTS 1,880,046 9/1932 Richter 16283 2,071,304 2/1937 Hirschkind 162-72 X 2,361,639 10/1944 Loughborough 162-72 X 3,161,562 12/1964 Gillaspie 162--72 DONALL H. SYLVESTER, Primary Examiner.

H. CAINE, Assistant Examiner.

Claims (1)

1. A SULFITE PULPING PROCESS COMPRISING THE STEPS OF MIXING CLEEULOSIC RAW MATERIAL WITH A SULFITE COOKING LIQUOR HAVING UREA AS A CONSTITUENT IN AN AMOUNT SUFFIECIENT TO PRODUCE A PREDETERMINED TERMINAL PH IN THE MIXTURE, REACTING SAID LIQUOR AND SAID CELLULOSIC RAW MATERIAL AT A TEMPERATURE NO LOWER THAN THE TEMPERATURE AT WHICH AMMONIA IS FORMED FROM SAID UREA FOR NEUTRALIZING THE WOOD ACIDS HYDROLYTICALLY RELEASED DURING SAID REACTION AND CONTINUING SAID REACTION TO COMPLETION AT SAID PREDETERMINED TERMINAL PH TO FORM FOR PROCESSING INTO FIBROUS SHEET MATERIAL.