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The Soda Process

( Originally Published 1920 )



THE first chemical process for reducing wood to pulp was the soda process, so-named because it uses caustic soda as the cooking agent. This process was developed in 1851 by Hugh Burgess and Charles Watt in England, who secured an American patent in 1854. The first mill to use this process was built on the Schuylkill River near Philadelphia, and began operations in 1855 under the direction of Burgess, who served as manager of the mill for nearly forty years.

By 1883 it was reported that the Penobscot Chemical Fiber Co., at Great Works, Maine, was completing one of the largest pulp mills in the country. It was to have five digesters with a total capacity of 20 tons of pulp per day, and was said to cost about $320,000. The entire output of this mill was, therefore, distinctly less than that of a single digester in a modern plant.

The success of the soda process depends on the solubility of certain constituents of the wood in the alkaline cooking solution, and the decomposition of other constituents causing the formation of acid products, which are at once brought into solution as sodium salts. Both of these actions neutralize the alkali and make it useless for further work until it is regenerated. The degradation or destruction of that portion of the wood which is dissolved is fairly complete, and it has not proved possible to prepare from it any useful by-products, though a considerable amount of sodium acetate, and a small amount of sodium formate are known to be present in solution.

The action of caustic soda on wood is very appreciable at ordinary temperatures, and is much more rapid and complete as the temperature is raised. It is, therefore, customary to cook the wood in the form of chips in closed digesters at high temperature and pressure. The digesters in which the cooking is accomplished have undergone considerable evolution. At first they were small globular or cylindrical rotary vessels into which the chips and cooking liquor were charged through a manhole, while the steam for cooking entered through the trunnion as the vessel was turning. Such digesters were of relatively small capacity and much time was lost in filling, and in blowing down pressure so the manhole lid could be removed, and in emptying the charge by rotating the digester. However, they did have the advantage of very complete mixing, or circulation, during the cook so that all parts of the charge were equally treated.

When these rotary digesters had reached a size beyond which their construction was impractical, vertical, stationary digesters came into use. These were at first of not much greater capacity than the rotary type, but have gradually increased in size until a digester holding 15 cords of wood is not at all unusual. Such a digester generally consists of a tall cylindrical section with a cone-shaped bottom and a dish-shaped top, as shown on page 58. In this the steam enters through numerous openings in a pipe encircling the digester, and the liquor for circulation is taken from under a false bottom; (the relief valve for the escape of gases is not shown) . A 15-cord digester of this type would have a capacity of about 3300 cubic feet, and might be about 11 feet 10 inches in diameter and 27 feet high in the cylindrical part with an additional inverted cone 7 feet high at the bottom. The older types of digesters were built of riveted plates and it was found difficult to keep them from leaking; modern ones are electrically welded with much more satisfactory results. Since the cooking liquor has almost no action on iron and steel it is not necessary to give the digesters any protective lining, but because there is no inner lining an insulating outer covering of about 3 inches is required, both for steam economy and operating comfort.

Vertical digesters are filled with chips through a top opening which can be closed with a heavy lid, held down by swing bolts. The chips are fed in from hoppers overhead or are brought to the digesters directly on belt conveyors. The cooking liquor, which is essentially a solution of caustic soda, is often run in at the same time as the chips. The lid is then fastened down and cooking is started by blowing live steam into the bottom of the digester. This is continued until the desired temperature and pressure are reached, after which these conditions are maintained as long as is considered necessary to complete the cooking of the chips. During this period, and especially at first, a valve is opened at the top of the digester to allow the escape of air which was present in the chips and in the digester. This "relief," as it is called, aids the circulation of liquor in the digester and gives more uniform cooking conditions throughout the charge. Uniformity is sometimes insured by pumping the liquor from the bottom of the digester and discharging it on top of the chips. A variation of this procedure is to pump the cooking liquor through an exchange heater, instead of blowing steam directly into the charge; this prevents dilution of the liquor by condensed steam and permits a stronger waste liquor to go to the evaporators in the soda recovery plant, but the time for heating is longer.

The cooking conditions vary to some extent in different mills, but the important factors are the same in all. The percentage of caustic soda on the dry weight of the wood has the greatest influence on the quality of the fiber. Below a certain amount, the cooked chips are raw and dark colored, and the fiber is full of "shives," which are small bundles of partially cooked fibers which will not break apart readily. Such fiber bleaches only with great difficulty. If too large an amount of caustic is used the wood is overcooked, the fiber likely to be tender, and the yield is somewhat reduced. The proper amount of soda to use depends somewhat on the kind of wood used, and in general it is greater for coniferous than for deciduous woods. For the latter the amount of caustic soda used will range from about 18 to 22 per cent of the dry wood, and for coniferous wood it may be as much as 26 per cent. Not all of this is used up in the cooking process, as the liquor remaining at the end of a cook may contain as much as 10—15 per cent of the caustic soda originally present. This is not a complete waste, as it appears to perform the necessary function of preventing the dissolved organic matter from separating out and again contaminating the fibers.

The other important factors in cooking are the temperature, which, of course, depends on the steam pressure used, and the time during which it is maintained. These are to some extent inter-changeable, a longer cook at lower temperature giving about the same results as a shorter one at higher temperature. Cooking can be carried out successfully at as low a steam pressure as 70 pounds per square inch if the caustic is increased above the normal, but under average mill conditions a steam pressure of 110 pounds per square inch (corresponding to a temperature of 335°F.) is common, and some mills even go as high as 130 pounds per square inch (354°F.). The time during which this temperature is maintained varies considerably in different mills, and is often more than is actually needed to pulp the wood completely. It is difficult to set an average figure, but with 22 per cent caustic and a steam pressure of 110 pounds per square inch, four hours at full pressure should be ample to produce well cooked fiber from nearly any of the deciduous woods.

When a digester charge is considered to be sufficiently cooked it is discharged through a valve in the bottom, and is blown by the pressure in the digester through a pipe into a separator which collects the fiber while allowing the steam to pass off. The escaping steam usually goes to some form of device which permits its utilization in heating water to wash the pulp. Unfortunately there is no rapid way to determine when the chips in a digester are cooked, so the time at which a cook should be blown has to be established by previous experience. Occasionally a charge will not blow out clean, and this can only be discovered by removing the lid and inspecting the inside of the digester. If much of the charge remains it is sometimes necessary to replace the lid, again steam to pressure—possibly after adding water or black liquor—and re-blow. The cause of such poor blows is not always easy to discover but they can be caused by local poor circulation or by the use of too little caustic soda to give complete cooking of the chips.

For a digester holding about 15 cords of poplar wood the data for a cook would be about as follows:

Weight of wood charged, dry basis 34,500 lbs.
Caustic soda used 7,600 lbs.
Volume of cooking liquor 11,200 gals.
Time for filling with chips 20 min.
Steaming to pressure 120 min.
Time at full pressure 180 min.
Time for discharging 20 min.

The fibers which are collected by the separator, or blow tank, are thoroughly saturated with the liquor in which over half the weight of the original wood has been dissolved. The liquor is called "black liquor" because of its color and its removal from the fiber is essential because the caustic soda has to be recovered for reasons of economy, and because even small amounts of it remaining in the pulp make its bleaching difficult or impossible. Washing the fiber is accomplished in several ways, the oldest of which is by placing it in open tanks with perforated bottoms, through which the black liquor drains. The stock in the tank is first flooded with weak black liquor, which forces the strong black liquor ahead of it by downward displacement. When the liquor draining away reaches a certain strength—generally determined by specific gravity—water is substituted for the weak black liquor and the washing continued. This wash is collected to be used again as weak black liquor on the next tank of stock. When this reaches a low strength, beyond which it does not pay to evaporate it for soda recovery, it is allowed to go to the sewer, and the washing is continued until the color of the water flowing away indicates that the fiber is sufficiently clean for bleaching.

Another, and more modern method of washing is to dilute the stock from the blow pit with strong black liquor and pump it to continuous rotary filters. The first of these removes as much black liquor as possible; then the stock discharged from it is again diluted either with weak black liquor or water, and goes to a second filter similar to the first. Washing in this way saves much floor space and time and is said to send a stronger black liquor to the recovery plant while using less wash water, and washing the fiber more completely.

After completing the washing, the pulp goes to a chest where soaking in water permits a little more soda to be removed by diffusion from the inside of the fibers. It then passes through "knotters," which are usually centrifugal screens with openings from % to % inch in diameter, and then through other screens with finer openings. These final screens may be either centrifugal or flat types, and the size of the openings must be selected according to the type of fiber being made, longer fibers requiring larger openings than short ones. After the final screening the fiber is ready for the bleaching operation, if white pulp is being made, or for making into paper if the product is unbleached or brown in color.

The recovery of the soda used by this process is a very vital factor in the cost of the fiber—without good recovery the process could not be operated at a profit. Recovery involves (1) the evaporation of the black liquor to such a concentration that the organic matter present will burn, (2) the dissolving of the resulting soda after burning, and (3) the conversion of the soda to caustic soda by treatment with lime.

Evaporation is universally carried out in multiple "effect" evaporators. These consist of a series of large vessels fitted with tubes through which the liquor passes, and a steam jacket surrounding the tubes. Evaporators operate on the principle that the boiling point of the liquor is lowered by reducing the pressure under which it boils. By this means the only heat required from outside sources is the steam entering the jacket of the first vessel, or "effect." After this the steam formed by the evaporation of the liquor in that "effect" boils the liquor in the next, and so on through the series, which may include five or more "effects." Operating on black liquor from the soda process such an evaporator will raise the solid content of the liquor from about 16 per cent on entering to about 60 per cent in the discharge. This is not quite concentrated enough to support its own combustion, so some further evaporation is necessary.

The older method of recovery involved the burning of the black liquor in rotary furnaces, known as black ash furnaces, the general design of which is shown on page 67. In this the strong black liquor from the evaporators enters at the back of the brick-lined furnace and gradually works forward to the discharge end as the furnace revolves. Some heat has to be supplied by burning wood or coal in the traverse furnace, which is a fire box on wheels so that it can be drawn back to enable repairs to be made to the furnace proper. In passing through the furnace the black liquor becomes more concentrated and finally burns, dropping into the ash car in a red-hot, or sometimes flaming, condition. This "black ash" then has to be leached with water to dissolve the soda, the solution of which is then separated from the residue of carbon, known as black ash waste. The soda ash solution goes to the causticizing plant for con-version to caustic soda, but the black ash waste finds no practical use and so has to be disposed of as a waste. It is chiefly carbon with small amounts of soda and mineral impurities.

Modern recovery methods utilize tall, rectangular furnaces very similar to present-day steam boilers which use oil or powdered coal as fuel. The walls are of water-tubes, lightly coated with a plastic refractory for protection of the metal. Once such a furnace is brought to the right temperature by burning oil, or other fuel, black liquor may be sprayed in and will burn with no further fuel consumption provided it has been evaporated to a sufficiently high solids content. Since this concentration is not usually reached in the ordinary multiple-effect evaporators the final concentration to 70-80 per cent solids is carried out in a single-effect, forced-circulation evaporator, using high pressure steam, or in a disc evaporator, in which revolving discs carry the liquor up into the hot gases passing from the furnace. In furnaces of this type practically all of the organic material in the black liquor is burned out and the soda flows from the bottom of the furnace in a red-hot, molten stream, through a water-cooled spout into a tank of water, in which it is dissolved. This type of recovery furnace utilizes the heating value of the black liquor much more fully than the black ash furnaces, even though the latter have boilers attached to absorb the heat, and large amounts of steam at high pressure are produced. The much higher temperature in the newer type furnaces causes a considerable loss of soda by volatilization and unless some form of precipitator or scrubber is provided to take this soda out of the flue gases very appreciable financial loss is suffered, and a neighborhood nuisance is created.

In order to complete the cycle of operations the soda ash recovered from the burning of the black liquor has to be converted to caustic soda. This is accomplished by treating the soda ash solution with slaked lime which causes the precipitation of calcium carbonate and the formation of caustic soda in solution. Since this reaction is never complete, there is always more or less soda ash in the cooking liquor. This is not desired, but cannot be avoided when making caustic soda in this way. It does no particular harm in the cooking process but is just so much excess material which has to be carried all through the system, and which is subject to the losses which take place all along the line. The causticizing of the soda ash is carried out either by a batch operation or in continuous systems, the latter being the more modern and preferred. The caustic soda is separated from the calcium carbonate, or lime sludge, as it is called, either by settling or by continuous filters, and the sludge is washed with water to remove as much as possible of the caustic soda. In some plants the lime sludge is reburned in rotary kilns to recover the lime for reuse.

During all these operations there is a loss of soda, and this must be kept at a minimum in order to hold the cost of fiber at a reason-able figure. Such losses are caused by incomplete washing of the fiber, by the soda content of the final wash water which it does not pay to evaporate, by the soda in the lime sludge, by volatilization from the recovery furnaces, and by any leaks or spillages all along the line. In the very best of mills the percentage of soda which is recovered for reuse is said to be 95, but this is seldom reached and 84 per cent would be much nearer the actual figure, especially for the older mills.

The use to which soda fiber is put depends on the kind of wood from which it is prepared. If it is made from coniferous woods the fibers are long and can be used in papers requiring good strength. Most soda fiber is made from hardwoods and is classed as short, hence gives paper of low strength, especially tearing strength, if used alone. When used in mixture with long fibers, soda fiber aids in producing a smooth surface and giving a well formed, uniform appearance on looking through the sheet.

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