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Improved pulp cooking methods


THE principles followed in the cooking of wood have remained the same since the inception of the chemical pulp industry. The sulphite and sulphate processes are the standard methods followed. Other methods are of minor importance from the point of view of the quantity of pulp produced.

However, many improvements in technique have been developed during recent years. These have resulted in improved quality and yield, together with reduction in time, labor, chemicals, and maintenance cost. There is also a steady trend towards the pulping of more species of wood and of other cellulosic materials. The work of the United States Forest Products Laboratory in this field is particularly noteworthy.

In general, for the pulping of hardwoods and little-used softwoods, no radical departure from the standard sulphite and sulphate processes are necessary, although certain variations are desirable. It is generally recommended that widely different woods be pulped separately and that other species be pulped in mixtures of a constant ratio of composition.

Investigators continue to probe into the chemistry of the sulphite and sulphate processes. Complete understanding of the mechanism of these processes, however, awaits the solution of the lignin problem. Much progress is being made in research on these problems by Hägglund and Erdtman in Sweden, by Brauns, Harris, McCarthy, and others in the United States, and by other lignin chemists throughout the world.

Interest in investigations of other means of pulping cellulosic materials continues. The chlorine and the nitric acid processes have become established in some places, the former particularly for the pulping of straw and plants other than wood. At present, the most active new development outside the standard pulping processes is in the field of semichemical pulping. Interest in semichemical pulping has grown considerably during the past few years and already an appreciable volume of semichemical pulp is being produced.

The sulphite process

In the sulphite process the most notable trend in cooking practice has been the use of higher acid concentrations. A total sulphur dioxide content of 7-8 percent is not uncommon; in fact, the normal trend seems to be to cook with liquors containing approximately 7 percent free SO2 and 1.2 percent combined SO2. An extreme of 10 percent free SO2 is used at one mill. Some mills use a higher combined and a lower free percentage but maintain the total SO2 at around 7 percent. In one example the acid composition has been changed from 3.3 percent of free and 2.1 percent of combined SO2 in 1939 to 4.5 and 2.62.7, respectively, in 1945-46.

The maximum free SO2 content depends upon the allowable digester pressure and the acid system employed. Stronger acid permits faster cooking and a lower maximum cooking temperature. which result in a higher yield of pulp. To use acids of high SO, content requires digesters with strong shells to keep the additional sulphur dioxide in solution. For a given pressure there is a maximum efficient SO2 content. For example, at 95 lb. per sq. in. (6.3 kg. per cm2) maximum pressure, the efficient free SO2 is 6.5 percent, according to Holzer. Normal cooking pressures have been raised to 80-85 lb. per sq. in. (5.6 to 6 kg. per cm2) gauge pressure.

Investigations have shown that an increase of combined SO2 up to an amount corresponding to 5 percent of the weight of the wood slows the; rate of cooking, which decreases the attack on the cellulose and thereby increases the yield of pulp. The increased concentration of the bisulphite ion reduces the partial pressure of the sulphur dioxide. Higher combined SO2 contents than 4.5-5.5 percent of the weight of the wood are needless.

The use of stronger cooking acid, lower maximum temperatures, and higher pressures has brought about improved pulp yields and quality and has reduced the time of the pulping operation. These changes in cooking have been accompanied by other improvements in technique including (1) the use of hot acid, (2) chip packing, (3) digester circulation with or without outside heating, and (4) better digester control by instrumentation.

Most of these techniques are interdependent. For example, high acid concentration requires a hot acid system. Chip packing and forced circulation go hand in hand. External heating permits better instrumentation. In fact, best results are obtained when all of these techniques are used and properly co-ordinated.

Hot acid systems have become common and permit the use of higher acid concentrations. The acid from the Jenssen system is passed through a pressure tower in which it is brought into contact with cooled burner gas at pressures up to 10 lb. per sq. in. (0.7 kg. per cm2) This acid is further fortified by the chemi-pulp system, using digester relief gases under pressure. In Scandinavia and on the North American continent a system for the recovery of accumulator over-gas has come into use. The overgas from the low pressure accumulator goes to two absorption towers in series where it is absorbed by raw acid fed from the top. These absorption towers relieve the acid plant of fluctuations and make possible a high acid concentration as well as a saving of sulphur. The increased acid temperature, which is possible by the use of these recovery systems, results in a substantial saving of steam. The modern acid plant and digester relief system are practically automatic by instrument control.

The use of hot acid (usually 75°-90° C.) permits more rapid penetration of the chips. Precirculation with the hot acid before steaming is practiced by some mills and is claimed to secure nearly complete penetration with the strong acid by overcoming the initial dilution caused by the moisture in the wood. Precirculation also removes air from the wood. Other advantages claimed from precirculation of hot acids are increased uniformity of pulp and reduced screenings, decreased cooking temperature and time cycle and increased yield.

Chip packing is widely used in Scandinavia and is coming into greater use in North America. Proper chip distribution or packing increases the amount of wood per digester charge and hence increases the daily pulp output. One mill reports that the pulp produced per cooking increased from 12.88 tons to 14.76 tons by the installation of mechanical chip distributors. Improved steam and sulphur consumption also resulted.

Forced circulation permits increased chip packing. In fact, chip packing is desirable where circulation is used, since the even distribution and density of the chips prevents channelling and, vice versa, forced circulation is desirable when a digester is heavily charged by the use of chip packers.

The use of indirect heating, together with forced circulation; is common in Scandinavia and is becoming more widespread in North America. External heating permits closer digester control by instrumentation. The advantages of hot acid systems, chip packing, forced circulation, and outside heating can all be combined, resulting in a substantial saving of steam and sulphur, greater yield and output, and improved pulp quality. The steam saving is around 35 percent, the yield increase about 5-6 percent, and the daily increase of output as much as 35 percent.

There is a tendency towards full automatic control in sulphite cooking. With uniform wood automatically weighed and of known moisture content, with uniformly controlled acid, and with outside heating and circulation, automatic digester control is possible. Where wood of diverse species and varying moisture content is used, the control is partly automatic and partly manual.

Stainless steel valves and fittings have been in use for some time. A recent development, however, is the building of a stainless steel digester for commercial operation.

The most notable innovation in sulphite cooking procedure has been the use of ammonia, sodium, and magnesium bases in place of the usual calcium base. The ammonia base process was developed by Cross and Engelstad and put into operation in Norway about 12 years ago at a mill producing ammonia from atmospheric nitrogen. An acid of about 8 percent total and 1 percent combined SO2 is used. There are two mills in the United States which have the ammonia process, both installations being recent. The advantage of the ammonia process is that it permits easy disposal of the waste liquor by evaporation and by burning the dissolved organic matter as a fuel.

One mill in Sweden uses a sodium base sulphite liquor. Chemical and heat recovery are carried out in conjunction with the sulphate mill's soda plant. A new mill in Canada has proposed the use of a similar system.

The magnesium base system is to be put in operation at a plant in the United States. Hatch, in a series of articles, points out that the magnesium base sulphite process has the advantage of the relatively simple recovery of heat and the original chemicals without the troublesome scale problems encountered in evaporating waste calcium base liquors. The organic matter can be converted to process steam and power, and 90 percent of the magnesium and of the sulphur used can be recovered.

The soluble bases are said to give pulps with fewer screenings and a higher unbleached brightness. The time for penetration and diffusion is also-claimed to be less important, so that the cooking time can be appreciably shortened. Chemical recovery and the elimination of the waste liquor problem, however, are the two most important advantages of the soluble bases.

During the war the sulphite cooking technique was modified in Scandinavia to produce fodder cellulose and to give waste liquors with a high sugar content for alcohol production. These modifications were only of wartime duration.

Continuous sulphite pulping has been investigated. The theoretical aspects of the chemistry involved indicate that such a process can be developed, although the engineering difficulties are great. 4

The production of sulphite semichemical pulp is being studied by a number of mills. The principle involved is to prepare a raw or partly cooked stock, which is then de-fibered by a refiner. High yields can be obtained in this way, giving a pulp suitable for certain types of papers.

The sulphate process

In the sulphate process the cooking treatment given the wood depends upon whether a bleaching or an unbleached grade of pulp is desired. For producing bleaching grades it has been the general practice to cook comparatively soft stocks (TAPPI [Technical Association of the Pulp and Paper Industry] Nos. ranging from 12 to 18). There is some indication of an extension in cooking time to level the nonuniformity in cooking. Most mills making bleached pulp are designed for a cooking time of about four hours. In Scandinavia, uniformity for both bleaching and unbleached grades is achieved by the use of forced circulation and indirect heating, and there is a trend in this direction in the United States of America. For bleaching grades there is a tendency to increase the chemical ratio and some mills are using a larger liquor/wood ratio. including mills cooking for stronger grades of pulp. Where indirect heating and circulation are employed, it has been shown that lower sulphidity ratios (15-20 percent) are adequate.

For bleaching grades some mills are no longer cooking to such soft stocks as formerly. There is also experimentation on the cooking of very hard stocks, almost chippy in nature, followed by refining and the use of exceedingly large amounts of chlorine, up to as much as 30 percent, in the bleaching operation. The high chemical cost is believed to be offset by the increased yield.

For both bleaching and unbleached grades, there is now an extensive use of hardwoods in the kraft process, particularly in the southern United States. For the finer grades of paper, mills usually cook the hard woods separately, but for tonnage products such as liner, board, etc., the species are cooked together.

For semibleached and unbleached grades of pulp, there is a trend toward higher yield pulps, especially for kraft liner stock. This is brought about by short cooking schedules. Cooking cycles of only one and one-half to two hours are used by some mills. This trend toward higher yields is carrying over into the semichemical range, where the incompletely cooked brown stock is further refined mechanically by disc refiners.

Another trend is towards larger digesters equipped with forced circulation and indirect heating. Chip packing is also employed more widely. Recording and controlling instruments are now being widely used. These improvements have helped to increase quality and uniformity and to aid in steam economy. There is also an increased use of alloy steels, and alloy steel digesters have recently been made.

Nearly all new installations have multiple-stage vacuum washing, except where raw semichemical pulps are made. The latter handle better in diffusers. Commonly, three to four stages are used in vacuum washing, but recently a five-stage system has been installed. The chemical loss has been cut to 40-50 lb. (18-23 kg.) of sodium sulphate per short ton of pulp by vacuum washing. Some mills with few stages are increasing the number of stages. The use of blow tanks is standard practice in vacuum washing installations. This facilitates the recovery of the blow steam and provides an ample supply of hot water for the washing stage.

The increasing use of Cottrell precipitators in the recovery operation is, together with multiple-stage washing, an important factor in the reduction of chemical losses. Lime recovery has also improved during the past few years. The fresh lime addition is usually below 50 lb. (23 kg.) per short ton of pulp and some mills claim it to be as low as 10-15 lb. (4.5 to 6.8 kg.) per short ton of pulp.

Continuous kraft pulping is still being investigated, but operations remain on a small scale, essentially the pilot plant type, both in the U. S. A. and in Sweden.

Prehydrolysis by dilute sulphuric acid was developed in Germany during the early years of the war for beech and other woods. The operation is claimed to give a pulp of high alpha content especially suitable for staple fiber and rayon. Less alkali is required in the bleaching operation and the yield losses on bleaching are low. The acid hydrolyzate contains sugars claimed suitable for torula yeast production or for alcohol. One Scandinavian mill using pine has installed a prehydrolysis stage.

Semichemical process

Semichemical pulping is generally defined as a two-stage pulping process, involving chemical treatment to remove part of the lignin or fiber-bonding substance and mechanical refining to complete the separation of the fibers. Under this definition a wide variety of pulping methods could be included, such as, for example, the steaming of wood chips under pressure followed by mechanical refining in interplane refiners. Strictly speaking, this method, which is used to produce wallboard stock, is a semichemical process, since water at elevated temperatures and pressures has a chemical and plasticizing effect on wood. Generally, however, the semichemical process is considered to include pulping by a chemical pulping agent such as sulphite, soda, sulphate, neutral sulphite, sodium carbonate, and various mild alkaline reagents. So-called high-yield sulphite and sulphate pulping operations, giving 55-60 percent yields of pulp, are not usually considered as producers of semichemical pulp, even though the brown stock requires mechanical defibering.

The neutral sulphite semichemical process, developed at the United States Forest Products Laboratory at Madison, Wisconsin, and reported in 1926, is the most widely used semichemical process today. This process is particularly suited for hardwoods, since an especially strong sheet is produced from these short-fibered woods. Furthermore, recent developments have shown that it is possible to produce a bleachable pulp and, by conventional chlorine-hypochlorite bleaching, produce a fully bleached hardwood pulp of exceptional strength and high yield. The bleached pulps are stronger than the unbleached and are similar in many respects to softwood sulphite pulps. Bleached pulps in yields of 58 percent from aspen have been reported by Simmonds and Kingsbury and 60 percent from birch by Phelps. One mill is expected shortly to produce bleached semichemical aspen pulps for book paper.

The semichemical process is undergoing most rapid development in the southern United States, with hardwoods being mainly used. A variety of techniques have been developed, but. neutral sulphite and modified sulphate liquors are used for the most part. Interplane grinders of various manufacture are used for the second stage of pulping. From the southern United States oak is being pulped by the hard kraft semichemical process for corrugating board.

Work on a new type of semichemical pulp has been reported by Libby at the New York State College of Forestry. Pulpwood logs are first chemically treated under pressure and then ground by conventional stone grinders. Pulps of very high strength values and high yields from hardwoods are reported.

Recent investigations on the recovery of chemicals from semichemical spent liquors may lead to the industrial development of chemical recovery processes. Such a development should reduce costs and result in increasing the expansion of semichemical pulping.

Other pulping processes

Research interest has been maintained in the possibilities of pulping wood and other cellulosic materials by entirely new techniques. The use of the Pomilio chlorine process has expanded, especially in Italy, the Union of South Africa, the Philippine Republic, and Argentina, where it is mainly used to pulp cellulose materials other than wood. A plant using a modified chlorine process is now planned for Mexico.

Research on the nitric acid process has been greatest in Germany (for beech), Japan (for straws and wood) and in the U. S. A., where Aronovsky and co-workers have carried out extensive investigations on the nitric acid pulping of agricultural residues.

In Germany a sodium chlorite pulping process was developed by Jayme and others. In the U. S. A. at the Institute of Paper Chemistry, Wise has been studying the step-by-step removal of lignin by sodium chlorite.

The work of Ritter and his associates at the United States Forest Products Laboratory on the holocellulose process has been of considerable significance. Indus trial adaptations, especially in bleaching, are being developed.

Considerable research on the removal of lignin by organic reagents has been reported in the literature. The processes studied, however, appear to be too expensive for practical application.


The principal new developments and trends in the cooking of wood may be summarized as follows:

1. Increased yield and increased digester output in the sulphite and sulphate processes by chip packing and shorter cooking cycles.

2. Improved quality of sulphite and sulphate pulps by use of liquor circulation systems, outside heating, and instrument control.

3. Use in the sulphite process of high acid concentrations, together with the hot acid system, improved acid-making methods, and automatic control of acid making and sulphur dioxide recovery.

4. Use of metal bases other than calcium in the sulphite process.

5. Improved chemical recovery in the sulphate process by better pulp washing, the use of Cottrell precipitators, and improved soda house techniques and controls.

6. Adaptation of the sulphite and sulphate processes to a wide variety of species, especially hardwoods and new coniferous woods, Douglas fir, Pseudotsuga taxifolia, and western red cedar, Thuja plicata.

7. Development of a wide variety of techniques and the use of various chemicals in the semichemical process, and the development of semichemical bleached pulps.

(See Recent References on Improved Pulp Cooking Methods.)

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