Chapter 2

Because of the similarity of hemp hurds to other materials which have been tested by the Office of Paper-Plant Investigations, semicommercial tests were conducted in cooperation with a paper manufacturer without preliminary laboratory tests. Laboratory pulp and paper tests are regarded only as a preliminary to semicommercial tests and therefore are not employed unless the material in question presents new features which should receive investigation before larger sized tests are undertaken.

The advantages of cooperative mill tests are many, among which may be mentioned the counsel and advice of the mill management and employees, the services of specialized and skilled labor, facilities for comparing the processes and the results of tests with commercial processes and results, and the use of commercial or semicommercial types and sizes of machinery. Tests conducted in this manner and on this scale are of a different quality than is possible in those conducted in a laboratory, and the results are susceptible of commercial interpretation with a fair degree of reliability. It is found, in general, that the cost of securing such equipment and service for a complete and comprehensive test does not exceed $500, while the installation of an equally satisfactory equipment alone would cost at least $50,000 and in many cases very much more. Tests conducted in this manner constitute a direct demonstration to the manufacturer, and the results obtained are found to carry more weight when presented to other manufacturers for consideration.

It is well known that the method of conducting tests necessarily varies with the size of the test. In the matter of yield determination, for example, laboratory tests may be on such a small scale that the weighing and sampling of the resulting cellulose fibers may be conducted by means of chemical laboratory apparatus and analytical balance, while in tests involving a matter of 5 to 10 pounds of material larger and different types of equipment are necessary. When the tests are so increased in size as to employ 300 or 400 pounds, still other types of equipment are necessary for the treatment of the material and for a determination of the yield of fiber. In tests involving tons of material the equipment involves the use of machines. Accuracy in degree of control and in results will vary materially with the size of the test. As the size of the test increases, certain factors will vary in a beneficial manner, while others will vary in a detrimental manner, so it is a question for each investigator to decide, after taking all factors into consideration, as to the size of test which will give the most satisfactory results. In work of this nature it is found, on the whole, that better results are obtained in large tests, although the control of the factors and the determination of the yield of fiber are more difficult than in smaller tests.

In the tests described in this bulletin, the Department of Agriculture employed a rotary digester of its own design,[2] comprising a shell 5 feet 5 inches in length by 4 feet in diameter, capable of holding about 300 pounds of air-dry hurds. It is believed that a test of this size is large enough to give satisfactory results and that the results are susceptible of commercial interpretation, while at the same time they are sufficiently small for complete control and to afford fiber-yield figures which are both accurate and reliable. Two such rotary charges gave enough fiber for one complete paper-making test.

[Footnote 2: For a description of this rotary digester, see Brand, C. J., and Merrill, J. L., Zacaton as a paper-making material, U. S. Dept. Agr. Bul. 309, p. 28, 1915.]

=OPERATIONS INVOLVED IN A TEST.=

A complete test on hurds comprises seven distinct operations, and the method will be described, operation by operation, in the order in which they were conducted.

_Sieving._--The hurds for the first test were not sieved to remove sand and dirt, but the resulting paper was so dirty that sieving was practiced in all subsequent tests. The hurds were raked along a horizontal galvanized-iron screen, 15 feet long and 3 feet wide, with 11-1/2 meshes per linear inch, the screen being agitated by hand from below. Various amounts of dirt and chaff could be removed, depending on the degree of action, but it was found that if much more than 3 per cent of the material was removed it consisted chiefly of fine pieces of wood with practically no additional sand or dirt; in most of the tests, therefore, the material was screened so as to remove approximately 3 per cent. It became apparent that a finer screen would probably serve as well and effect a saving of small but good hurds.

_Cooking._--Cooking is the technical term for the operation by which fibrous raw materials are reduced to a residue of cellulose pulp by means of chemical treatment. In these tests about 300 pounds of hurds were charged into the rotary with the addition of a caustic-soda solution, such as is regularly employed in pulp mills and which tested an average of 109.5 grams of caustic soda per liter, or 0.916 pound per gallon, and averaged 85 per cent causticity. Sufficient caustic solution was added to furnish 25 or 30 per cent of actual caustic soda, calculated on the bone-dry weight of hurds in the charge. After closing the rotary head, it was started rotating at the rate of one-half revolution per minute, and in about five minutes steam at 120 pounds per square inch was admitted at such a rate that the charge was heated in one hour to 170° C., which is the theoretical equivalent of 100 pounds of steam pressure per square inch. It was found, however, that when the temperature reached 170° C. the pressure was usually 115 or 120 pounds instead of 100 pounds, due to air and gases inclosed in the rotary. At this point the rotary was stopped and steam and air relieved until the pressure dropped to 100 pounds, or a solid steam pressure. The temperature was maintained at this point for the number of hours required to reduce the hurds, which was found to be about five, after which the rotary was stopped and steam relieved until the pressure was reduced to zero, when the head was removed and the stock was emptied into a tank underneath, measuring 5-1/2 by 6 by 2 feet deep, where it was drained and washed. Samples of waste soda solution or "black liquor," which were taken from some of the "cooks" for analysis, were drawn while the stock was being thus emptied into the drainer.

_Determination of yield._--For determining the yield of cellulose fiber the stock in the drain tank was washed with water until free from waste soda solution, when, by means of a vacuum pump communicating with the space between the bottom and the false perforated bottom, the water was sucked from the stock, leaving the fiber with a very uniform moisture content throughout its entire mass and in a condition suitable for removing, sampling, and weighing for a yield determination. Tests have shown that it is possible to sample and calculate the yield of bone-dry fiber within 0.05 per cent of the actual amount.

It has been found that stocks from different materials vary greatly in their ability to mat in the drain tank, thereby enabling a good vacuum to be obtained, some stocks permitting a 25-inch vacuum to be obtained, while others will not permit more than 5 inches. For this reason the moisture content of the stock will vary from 65 to 85 per cent.

_Washing and bleaching._--Washing and bleaching were performed for the purpose of bleaching the brown-colored cooked stock to a white product, since it was regarded as highly probable that the fiber would be suitable for book-paper manufacture. The colored stock was charged into a 400-pound beating and washing engine of regular construction and washed about one hour, the cylinder washer being covered with 60-mesh wire cloth in order to remove fine loose dirt and chemical residues. The washer was then raised, the stock heated by steam to about 40° C., and a solution of commercial bleaching powder was added in the quantity judged to be necessary, after which the stock was pumped to a large wooden tank, to remain and bleach over night. If the stock was bleached sufficiently white it was drained and washed from bleach residues, and if not more bleach was added until a good color was obtained. The bleaching powder used was estimated to contain 35 per cent of available chlorin, as this is the commercial practice, and the amount required was calculated to the bone-dry weight of the unbleached stock. More bleach is required for undercooked stock than for stock which is properly cooked or overcooked; therefore, the percentage of bleach required is an indication of the quality of the cooked stock. Since bleaching is usually more expensive than cooking, it is desirable to cook to such a degree that the consumption of bleach will be held within certain limits, depending on the raw materials used and the quality of paper to be produced. In these tests it was desirable so to cook the hurds that the consumption of bleach would not be over about 10 per cent of the fiber.

_Furnishing._--Furnishing is the operation of charging the beating engine with the desired kind or kinds of fiber in the proper proportion and amount and the adding of such loading and sizing agents as may be necessary. As shown in the record of results, the furnish in these tests consisted of hurd stock alone and of various proportions of hurds, sulphite fiber, and soda fiber. The percentages to be given in the record of the furnishes refer to the percentage of the total fiber furnish, and this likewise applies to the loading and sizing agents. In case sulphite or soda fiber was used, the commercial product in the dry state was charged into the beating engine and disintegrated, after which the hurd stock was added in the wet condition.

_Beating._--Beating is that operation concerning which the paper makers often say "there is where the paper is really made," and although the statement may not be literally true it contains a great deal of truth. It is the operation whereby the fibers are separated from each other, reduced to the proper lengths, and put in such a physical or chemical condition that they felt properly and form into a satisfactory sheet. It is probable that the quality of the sheet depends more upon the proper beater action than upon any other single operation. The action consists in drawing a water suspension of the fiber between two sets of rather blunt knives, one set being located in the bottom of a circulating trough and the other set on the periphery of a roll revolving just above the former set of knives. It is during this operation that the loading and sizing agents are incorporated and the whole furnish is tinted either to produce a satisfactory white or the desired color.

The term "paper making," as used in this publication, means the operation of forming the finished sheet of paper from stock which has been furnished and prepared in the beater. In these tests a 30-inch Fourdrinier machine of regular construction was used, a machine which often is used for the production of paper for filling regular commercial orders. The machine is designed to cause the water suspension of fibers to flow on to a traveling wire cloth, whereby the water drains away. More water is removed by passing the wet sheet through a series of press rolls, after which the sheet is dried on steam-heated drums and passed through polished iron rolls, which impart a finish to the sheet. A Jordan refining machine was employed in conjunction with the machine to improve further the quality of the fiber, and a pulp screen was used in order to remove coarse and extraneous materials from the fiber.

=DESCRIPTION OF TESTS.=

The nature of each complete paper test and the dependence of each operation on the others were such that it does not seem advisable to submit the results of the seven tests in tabular form. The numerous cooks, however, which furnished the pulp for the paper tests are presented in Table I in all essential detail.

TABLE I.--Data on cooking hemp hurds.

+-----------+------------+----------+--------------------+----------- | | | | | | | | | Cooking | Yield of | Caustic |Strength of | | | bone-dry | soda used |caustic soda|Causticity+--------+-----------+ fiber Cook |(percentage| (grams per | of soda | | |(percentage No. |of bone-dry| liter). | solution.| Time |Temperature|of bone-dry | hurds). | | |(hours).| (°C.) | unsieved | | | | | | hurds). ------+-----------+------------+----------+--------+-----------+----------- 293 | 20.6| 100 | 75.3| 3| 166| 294 | 21 | 100 | 75.3| 3| 166| 295 | 21.6| 100 | 75.3| 3| 166| 296 | 20.3| 100 | 75.3| 3| 166| 301 | 21.9| 100 | 82.5| 4| 166| [3] 302 | 24.4| 100 | 82.5| 4| 166| 303 | 24.2| 100 | 84.3| 4| 166| 44.1 304 | 25 | 100 | 84.3| 4| 170| 39.5 305 | 25 | 100 | 84.3| 5| 170| 39.4 306 | 27.8| 107.5| 84.3| 4| 166| 36.5 307 | 26.7| 107 | 84.4| 5| 170| 38.1 308 | 26 | 107 | 84.4| 5| 170|\_ 37.3 309 | 27.3| 107 | 84.4| 5| 170|/ 310 | 27.1| 107 | 84.4| 6| 170| 37.0 311 | 27.2| 107 | 84.4| 6| 170| 36.8 312 | 28.3| 116.5| 85.5| 5| 170| 35.9 313 | 29.1| 113.1| 84.9| 5| 170|\_ 35.2 314 | 29.1| 109 | 83.9| 5| 170|/ 315 | 29.4| 109 | 83.9| 5| 170| 34.9 316 | 30 | 109.5| 84.9| 5| 170| 37.2 317 | 29.6| 109.5| 84.9| 5| 170| 37.0 318 | 29.6| 107 | 84.8| 5| 170| 37.7 319 | 29.4| 107.5| 84.2| 5| 170|\_ 35.4 320 | 29.3| 107.5| 84.2| 5| 170|/ ------+-----------+------------+----------+--------+-----------+-----------

[Footnote 3: Stock not used; dirty.]

Discussion of the various cooks will be given in connection with the descriptions of those paper tests in which the stocks from the cooks were used, since a stock and its cooking condition can be judged adequately only after it has been put through the various processes and into the finished sheet of paper.

The first test consisted in making four separate cooks, Nos. 293, 294, 295, and 296, of approximately 300 pounds each, dividing the total stock into two parts and making two separate paper tests. The first test was made primarily in order to learn some of the qualities and characteristics of the stock and to get the machinery equipment adjusted properly. The yield of fiber was not determined in this preliminary test, since the knowledge of it was not essential at this stage of the work. The cooked stock which was emptied into the drainer to be washed free from black liquor was composed largely of whole pieces of hurds, but only slight pressure between the fingers was required to crush the pieces. In the case of wood, this condition ordinarily would indicate undercooking, but might not in the case of hurds. Further observation on the action of the cooked stock during subsequent processes was necessary in order to judge of its quality or the suitability of the cooking conditions. The total cooked stock, about 500 pounds, was divided into two portions of 200 and 300 pounds, respectively, and work was continued on them separately. The 200-pound test, designated as run No. 135, was put into a 350-pound washing engine, washed one hour, and given a total light brush of 2-1/4 hours. The washing removed a great amount of dirt, but the engine did not reduce the hurd stock as much as was desired. After heating the stock in the beater to 40° C., it was bleached with bleaching-powder solution, 94 gallons at 0.418 pound bleach per gallon, equivalent to 19.7 per cent of the fiber. This percentage of bleach is regarded as too high for stock intended for book-paper manufacture, and subsequent cooks therefore were given harder treatment in order to reduce this figure. After draining and washing free from bleach residues, the stock was furnished in the beater with 13 per cent of clay, 1 per cent of resin size, and 2.5 per cent of alum, was tinted blue, given one hour's light brush, and pumped to the stock chest. When running it on the paper machine, the Jordan refiner seemed to have little effect in reducing shives of undertreated wood, which indicated further the necessity of harder cooking. The furnish acted well on the paper machine at 70 feet per minute, but appeared somewhat too "free" on the wire. The paper produced from this test is of very low quality, due to the improper preparation of the stock, lack of sufficient bleach, the use of too small an amount of blue tinting, and the presence of an excessive amount of dirt, sand, and shives. The excessive amount of dirt and sand suggested the sieving of the hurds before cooking, and this was performed in all subsequent cooks.

The finish of the sheet is very poor, due to the fact that the calender stack was composed of very light rolls which did not have a satisfactory surface, yet the stack is known to be able to produce better finishes if the proper stock is employed.

Run No. 136 was made on the 300-pound portion of stock from cooks Nos. 293, 294, 295, and 296, and in essentially the same manner as run No. 135. The stock was washed one hour, but given a brush of three hours, and this brush was harder than in run No. 135. Bleach to the extent of 19.8 per cent of the fiber was used, assisted by 1 pint of oil of vitriol, and the resulting color was an improvement over that of run No. 135. After adding 13.5 per cent of clay and sizing with 1.1 per cent of resin size, the furnish was given one-half hour's light brush, tinted, and run on the machine, which was set at 70 feet per minute. This stock acted better on the wire and gave no trouble on the machine, but it still seemed to be impossible to reduce the wood shives by manipulation of the Jordan refiner. The resulting sheet is an improvement over that produced by No. 135, but is far from satisfactory.

Run No. 138 was made from hurds which, as in all subsequent tests, were sieved on a 11-1/2-mesh wire screen until practically all the loose dirt and sand was removed, which operation caused a loss averaging 3 per cent of the hurds. Stock from cooks Nos. 302 and 303 was used for this run and the increased amount of caustic soda and the increase in the time of cooking gave a stock of better appearance than those of preceding tests.

The stock, amounting to 231 pounds dry weight, was washed and at the same time given a light brush for one hour only, after which it was bleached with 17 per cent of bleach without the addition of acid. Since the preceding paper appeared somewhat weak and had a low tearing quality, it was decided to use a furnish of 15.7 per cent bleached sulphite and 84.3 per cent bleached hemp-hurd stock. After loading with 13.1 per cent of clay and sizing with 1.1 per cent of resin size, the furnish was given a medium brush for one hour, tinted, and run on to the machine at 70 feet per minute. The stock gave no trouble on the machine, but it was impossible to judge the effect of the Jordan refiner, because through an oversight the machine chest had not been cleaned since previous use on an unbleached yucca material. It is believed, however, that sheet No. 138 shows improvement in the preparation of the hurd pulp.

Run No. 139 was made from stock of cooks Nos. 304 and 305, in which still more caustic soda was employed and the time and temperature of cooking were increased, giving a yield of total fiber of 40.7 per cent of the sieved or 39.4 per cent of the unsieved hurds. The cooked stock still seemed to be undertreated, but it must be remembered that in working with any new raw material it is impossible to know in advance how the properly treated material should appear. A washing of one hour was given while the roll was lowered from a light to a medium brush, after which the stock was bleached with 17.1 per cent of bleach without the aid of acid. Since sulphite stock improved the previous paper, this bleached stock was used in a furnish of 16.6 per cent sulphite and 83.4 per cent hurds, loaded with 16.7 per cent clay, sized with 1.4 per cent resin size, given a medium brush of two hours, tinted, and run on to the machine at 70 feet per minute. The Jordan refiner seemed to have little effect in reducing shives and was therefore left "just off." No trouble was experienced with the stock on the machine, and the sheet is an improvement over previous samples.

Run No. 140 was made from cooks Nos. 306 and 307, in which more caustic soda was employed than in any previous cooks and at a higher concentration, the fiber yields of which averaged 37.3 per cent of the unsieved hurds. Not much improvement was apparent in the cooked stock, in spite of the increased severity of cooking. The stock was washed and given a medium brush for one hour, bleached with 11.9 per cent of bleach, assisted with one-half pint of oil of vitriol, and made into a furnish of 14.9 per cent sulphite and 85.1 per cent of the hurd stock. After loading with 14.7 per cent of clay and sizing with 1.28 per cent of resin size, the furnish was given two hours' medium brush, tinted, and run on to the paper machine at 70 feet per minute. Again the Jordan refiner did not seem to reduce the wood shives sufficiently, and it was left "just off." No trouble which could be attributed to the stock was experienced on the paper machine. The color of the resulting paper is due to the use of too little blue in tinting and probably in some measure to the use of too low a percentage of bleach.

Run No. 141 was made from the stock of cooks Nos. 308 and 309 in practically the same manner as run No. 140. The stock was washed and brushed one hour, bleached (the record of the amount of bleach was lost), made into a furnish of 14.7 per cent of sulphite and 85.3 per cent of hurd stock, loaded with 14.9 per cent of clay, sized with 1.26 per cent of resin size, given one hour at a medium brush, tinted, and run on to the machine. The Jordan refiner was able to reduce the wood shives to a somewhat greater degree than in previous runs and was held at a medium brush. The stock acted well on the machine and produced a sheet of better quality than any preceding, with the exception of the color, which was due to using too small a quantity of blue.