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Bleaching

( Originally Published 1920 )



NONE of the methods used for producing papermaking fibers yields a perfectly pure cellulose or one which is sufficiently lacking in color to make paper of a high degree of whiteness. In many products, such as wrapping paper, wall-board, box-boards, etc., a bright color is not essential and no bleaching is attempted; in newsprint a reasonably good color is desired, but this is generally attained by the use of light-colored woods for grinding, and by adding coloring matters to give the paper a more pleasing hue, though not making it any whiter in the real sense of the word. For the great majority of writing and printing papers whiteness is of great importance up to a certain point, since it aids in giving a good contrast with the printing ink, and hence improves the appearance of the finished prints, especially of halftones. Unfortunately the demand for extreme whiteness often exceeds this really desirable point, and is caused by the papermaker himself, rather than by the customer. Just a little edge over a competitor in the whiteness of a paper gives the salesman an obvious talking point and aids his sales. The competitor then comes back with a little whiter paper, and thus the spiral of increasing whiteness and brilliance goes on far beyond the point where it is otherwise of any real value. As a matter of fact, probably not one reader in a thousand ever notices the color or brightness of the paper or book he is reading, or even knows whether it is really white or just seems to be so because it has been made bluer by addition of dyes.

Before discussing the bleaching of fibers the term "brightness," as applied to both pulp and paper, should be explained. It is deter-mined by an instrument which compares the pulp or paper with a block of magnesium carbonate coated with magnesium oxide as a standard. This must be done under very carefully controlled conditions, with the sample illuminated by a blue light. Such a block is considered as having a brightness of 100, and the brightness of the sample being compared with it is expressed as a percentage of this value.

The relation of brightness values of commercial pulps to the trade names commonly applied to them is about as follows:

Trade Name Fiber Brightness
Full bleached Sulfite 80-85
Full bleached Sulfate 75-85
Full bleached Soda 77-82
Semi-bleached Sulfite 72-78
Semi-bleached Sulfate 65-70
Slightly bleached Sulfate 40-50
Unbleached Sulfite 61-68

Like all other technical phases of papermaking, bleaching has undergone many changes, practically all within the last half century. Before that time the only practical bleaching agent was calcium hypochlorite, which was almost always obtained as bleaching powder. This was converted to a useful form by mixing with water and settling out the insoluble lime, which had to be present in excess in order to make the powder reasonably stable. Modern methods by-pass the powder stage and obtain the same bleaching compound by passing chlorine into milk of lime under carefully controlled conditions of temperature and ratio of chlorine to lime. Solutions so prepared also contain an excess of lime and must be settled or filtered to obtain a clear solution to use.

In the older methods the bleach solution was applied to the fibers and the action allowed to continue until the desired degree of whiteness was reached. If this was not readily attained more bleach was added, and the action sometimes hastened by adding a little alum or sulfuric acid. This method of bleaching had its limitations, for each fiber seemed to have a certain degree of whiteness, beyond which it could not be forced without seriously impairing its strength and papermaking value. This not only pre-vented the use of certain kinds of fiber for white papers—strong sulfate fibers, for example—but held down the degree of obtain-able whiteness to a relatively low figure, according to current standards. Modern methods have overcome these handicaps so that strong kraft pulps, bleached to a beautifully white color with-out much loss of strength, now form one of the chief ingredients of many white papers.

During the period when only rags were used in making paper the whiteness of cloth from which they were derived was an important matter, and this was often attained by "grass-bleaching," which was essentially spreading the cloth on the grass and allowing the sun, dew and air to bring it to a sufficiently white condition. Colored rags were from cloth dyed with natural colors and these were relatively easy to bleach, with the possible exception of indigo blue, which was much more difficult to eliminate. The general procedure was to cook with lime or other alkali, and follow by a washing and bleaching process. This took care of indigo and most of the early coal tar colors, but eventually some dyes were developed which were practically impossible to bleach in this way, and rags colored with such dyes can generally be used only for colored papers. For rags with dyes of these types there are sometimes special methods for the removal of color, but these are complicated and are not generally used unless the quantity and value of available rags of one type of color makes it desirable.

The cooking of rags and their washing in breaking engines has already been described. The bleaching begins where the washing stops, the bleach generally being added in the breaking engine after the completion of the washing. When the bleaching action has continued for a sufficient time the stock, still containing some bleach, is dumped into chests or drainers which have false bottoms. It remains there for a considerable time to continue the bleaching action; then the bottom valve of the drainer is opened, and the exhausted, or nearly exhausted bleach is drained off, and the stock is washed by flooding with water, which is allowed to pass downward through the fiber. After several such floodings to complete the removal of the bleach and the soluble impurities, the stock is drained as dry as possible and is then ready to be dug out by manual labor and conveyed to the beaters.

The bleaching of chemically cooked wood fibers followed very similar lines in the early days of its use, but as paper production increased and larger and larger quantities were used such methods of handling became too slow and costly. They were therefore superseded by some form of continuous operation in which the fiber and bleach passed successively through a series of tanks. A continuous bleaching system of four tanks arranged in series was used at Cumberland Mills, Maine, for soda poplar fiber somewhat before 1894, but in this case the bleaching was not entirely finished in these tanks and the fiber was run into deep, brick drainers in which it stood for some time for the bleach to be exhausted. Washing was accomplished by flooding with water and draining, and the fiber was sluiced from the drainers by a high-pressure stream of water. This system required considerable time, but the drainers served also as storage tanks so that a good supply of fiber was always on hand. It also allowed the fiber to season and mellow, which at that time was thought to be important.

About 1904, at least one bleaching plant for sulfite fiber was in operation as a continuous process. This was also at Cumberland Mills, Maine, and consisted of eleven brick tanks about 10 feet in diameter and 27 1/2 feet deep, each containing an agitator. These were so arranged that the stock flowed through them in series by gravity, except for the first and fourth tanks, into which it was pumped. The fiber, which was received in the dry condition, was reduced to slush form by working up in water in breaking engines, where about a third of the necessary hypochlorite bleach was added, after which it was then pumped to the first tank. By the time it had passed through three tanks, the bleach was used up and the fiber went to a washer which removed the spent bleach and impurities. The fiber was then returned to the fourth tank, where the remainder of the bleach was added, and then was passed along through the other eight tanks. At the end of the series it was again passed over a washer and was then ready to deliver to the beaters. The intermediate washing saved about 10 per cent of the bleach which would normally be required by the fiber. It was found necessary to have a small amount of bleach left in the fiber when it went to the final washer. If this was not done the color of the fiber deteriorated because some of the yellowish coloring matter in the surrounding water was again absorbed. High temperature hastened the bleaching action, but increased the danger of going back in color because the last portions of the bleach were more likely to be fully used up.

This bleaching plant had many of the essential features of modern systems, but when using hypochlorite only, such a plant, even though it had several stages of bleaching with intermediate washing, could not overcome the difficulties of bleaching to highwhiteness without too much loss of strength. When chlorine became available at reasonable cost, and chemical engineering progress made it possible to handle it safely and conveniently, the story became quite different and far superior results are now obtained.

Present systems usually involve chlorination by passing chlorine gas into the mixture of fiber and water, then an alkaline extraction to remove the chlorinated impurities, and finally a bleaching with hypochlorite. Each of these steps is followed by a washing to take out the impurities which are made soluble by the reactions involved. This is about the simplest of the modem systems and serves where a moderate degree of whiteness and strength retention is sufficient. If very strong pulp of the highest whiteness is desired the procedure should be expanded to include at least six stages: (1) chlorination, (2) caustic extraction, (3) soak in water, (4) hypochlorite bleach (partial), (5) hypochlorite bleach (final), (6) adjustment of pH to 6.0—6.5 and soak. Each of these stages is concluded by a washing. The final adjustment of pH before the final soaking may be made with iron-free hydrochloric or sulfuric acid, or better yet, with sulfur dioxide. This latter removes the last traces of bleach and improves the final color of the fiber. The soaking periods are necessary because impurities are contained within the fibers and have to diffuse into the surrounding liquid before they can be washed out. Variations of this series of bleaching steps are, of course, in operation; one breaks the chlorination into two stages, each followed by a caustic extraction; another includes a bleaching with chlorite or with chlorine dioxide.

The quantities of chemicals used in multi-stage bleaching will depend on the type and quality of the fiber being treated. The caustic soda for extraction of chlorinated impurities need not exceed 2 per cent on the weight of the fiber, though some plants use as much as 4 per cent. The amount of chlorine, whether in the elemental form or combined as hypochlorite, will vary greatly with the degree of cooking the fiber has undergone. In expressing the amount of bleach used it is customary to use the term "avail-able" or "active" chlorine, regardless of whether chlorine or hypo-chlorite is used. Since standard bleaching powder contains 35 per cent of available chlorine this offers a ready means of converting modern figures for active chlorine into the older conception of bleaching powder. On this basis a very easy bleaching sulfite would require no more than 2.5 per cent of available chlorine, while a sulfate fiber cooked for high strength might require 15 per cent or perhaps even more.

Obviously much chemical engineering is involved in constructing and operating a bleach plant of such a complicated nature. The tanks, piping and washers for the chlorinated fiber must be of acid-proof and chlorine-proof materials, such as tile, rubber, or chrome-nickel steel. The size of the tanks has to be sufficient to retain the stock long enough for the desired action to be completed. According to the amount of fiber to be treated per day and its concentration, this may mean tanks 30 or more feet deep and 10 to 20 feet in diameter. At present, chlorination is carried out at about 3 to 4 per cent fiber concentration, and caustic extraction and hypochlorite bleaching at 12 per cent or higher.

The reaction involved in bleaching by hypochlorite is chiefly one of oxidation, though under certain conditions some chlorination may take place. The treatment with chlorine is for the purpose of chlorinating the lignin remaining in the fiber, so that it can be removed by the action of the caustic. Neither chlorine nor caustic is a whitening agent, but their removal of some of the impurities enables the final oxidation with hypochlorite to be much more effective than it could possibly be if used alone. Oxidizing agents other than hypochlorites can be used for the final whitening. Permanganates give excellent results, but are more costly; chlorites and chlorine dioxide are being developed and may soon become important bleaching agents. They have the advantage of producing a high white color with relatively little loss in fiber strength; apparently they attack cellulose itself less than does hypochlorite.

The chief factors affecting bleaching by oxidizing agents are the temperature and concentration of the stock and its pH value (acidity or alkalinity). High temperatures and stock concentrations hasten the bleaching action, but in the case of temperature, at least, there is danger of weakening the fiber if the action is hastened too much. When bleaching at a fiber concentration (stock density) of 12 per cent or higher the temperature should not exceed 85°F. if the strength of the fiber is to be maintained. At a concentration of 3 to 4 per cent of fiber considerably higher temperatures are safe, but even then it is not wise to exceed about 125°F., and care must be taken that the bleach does not become completely exhausted, lest the fiber go back in color.

The length of the bleaching period is greatly influenced by the concentration of the stock and its temperature. When bleaching with hypochlorite alone at 104—122°F., the time required may be as much as 5—6 hours when the stock consistency is 3—4 per cent, but if the consistency is raised to 12—16 per cent the required time is cut to 2 4 hours, even at a temperature as low as 75—95°F. The reaction with chlorine is much more rapid than that with hypo-chlorite, and the chlorination period need not exceed an hour, even at a temperature as low as 45—50°F.

Equally important in its effect on the strength of the fiber is the pH value during bleaching. Neutrality is indicated by a pH of 7, and safe bleaching, so far as strength is concerned, is carried out well on the alkaline side of this, possibly at a pH of 9.0 to 11.0 at the start. The bleaching action tends to make the conditions more acid, and if the pH is allowed to go below 7 during any consider-able part of the bleaching time both color and strength of the fiber will be likely to suffer. Strange to say, bleaching at a pH of 7.0 exerts a more harmful influence on the strength of the fiber than at either a more acid or alkaline condition. Maintaining a high pH therefore improves the color and strength of the fiber, but may increase its time of bleaching very materially. The size of the available equipment and the quantity of fiber it is necessary to bleach sometimes make it impossible to carry the alkalinity as high as is desirable, and under such conditions the fiber suffers a considerable loss in strength.

In any multistage bleaching, the apportionment of chemicals among the various steps is a matter of some importance, so far as the efficiency of their action is concerned. With two stages of bleaching, both using hypochlorite, the greatest saving of bleach is effected if 40 to 60 per cent of the bleach is in the first stage. For soda pulp, using both chlorine and hypochlorite, 30 to 50 per cent of the total active chlorine may be added in the chlorination stage. In bleaching sulfate pulps the proportioning of the chlorine and hypochlorite is even more important than with soda or sulfite fibers. As the portion used in chlorination increases, the portion required as hypochlorite decreases, and the strength of the fiber increases up to a maximum. The ratio of the two depends on the color desired in the pulp and the arrangement of the bleaching system. In a 4-stage system, bleaching to a moderate brightness, 60 per cent of the total chlorine may be used in the chlorination stage; in an 8-stage system, bleaching to the same brightness, as much as 82 per cent may be used in chlorinating.

In all bleaching operations loss in weight of fiber occurs from chemical action and during the mechanical processes of washing, thickening, etc. In single stage bleaching of sulfite fiber the purely chemical losses may range from 1.5 to 6.0 per cent of the original weight of the fiber. This loss depends on the degree of cooking that the fiber received and on the color to which it is bleached; harder cooking and lower final color decrease the loss in the bleaching operation. Chemical losses in multistage bleaching are doubtless higher than this, but figures are not available, probably because of the difficulty of carrying out such operations on a scale small enough to permit accurate determinations to be made. Mechanical losses would, of course, depend so much on the conditions of the washing and concentrating equipment that no figures can be quoted. Doubtless such losses are considerably greater than those due to purely chemical action.

The bleaching of groundwood has always been considered desirable, but until recently it was thought to be impossible because of the idea that all the impurities present would have to be oxidized and removed. It is true that the first attempts to improve its color were with reducing agents rather than by oxidation. These early attempts were generally made by moistening the wet ground-wood as it was made into laps, or bundles, with a solution of acid sulfite. On standing, the sulfurous acid caused a distinct improvement in color, but unfortunately this was only temporary, and when the bleached fiber was made into paper it turned yellow fully as rapidly as that made from unbleached groundwood.

More recent work indicates that bleaching of groundwood can be accomplished by either peroxides or hypochlorites, both of which are oxidizing agents, but that the choice of the reagent must be governed to some extent by the kind of groundwood to be treated. It appears that successful bleaching of groundwood with 10 per cent of available chlorine in the form of calcium or sodium hypochlorite can be accomplished if the reaction can be slowed down sufficiently at the start. Normally the reaction is extremely rapid, but by operating at low fiber concentration and temperature, and high alkalinity, good results can be obtained. Of the three factors mentioned alkalinity is the most critical; at the start it should be high enough to give a pH value of 11 to 12, and at the end of the treatment it should not be below 8.0. The addition of sulfur dioxide at the end of the reaction period generally improves the brightness of the fiber, but not its stability to light. This is what would be expected from the early bleaching work which was done with bisulfites.

Under the described conditions of bleaching, groundwood produced from hardwoods is raised on an average about 17 per cent on the brightness scale. Not all hardwoods respond equally well, and the gains vary from 8 to 23 per cent. Softwoods do not respond as well, and the gains in brightness average only about 3 per cent, with the maximum for the five woods which were tested being about 6.5 per cent.

The earliest reference to peroxide bleaching was a German patent of 1905, but there were many early references to the use of sodium peroxide, especially in the textile industry where peroxides have found considerable use for many years. Its use for bleaching groundwood has been a very recent growth, as the first extensive mill trial was made in 1941 as a low consistency, batch operation. Even today the action of peroxides on wood is not fully understood, but the study which has been given to the process has permitted a practical and very useful development of the method.

Three basic operations are involved: (1) rapid and complete mixing of the peroxide solution with the pulp, (2) retention in the reaction vessel long enough to complete the bleaching, and (3) reduction and neutralization to the desired pH. Either sodium peroxide or hydrogen peroxide can be used equally well and the choice depends on their relative availability and cost. If sodium peroxide is used the solution may be prepared by dissolving in water a small amount of some agent to inhibit the catalytic effect of traces of metals. Sodium silicate is then added to act as a deter-gent and buffer, and prevent corrosion of metal equipment. The sodium peroxide is next dissolved, and finally sulfuric acid is added to bring the whole to the desired pH value. This stock solution is added to the groundwood in sufficient amount to give 1 to 2 per cent of sodium peroxide on the dry weight of the pulp. The best pH at the start is 10 to 10.5, and the total alkalinity should equal 1.2 to 1.9 per cent expressed as caustic soda.

The time required for bleaching increases as the temperature drops—5 to 6 hours would be required at 90°F., but only about 3 hours at 115°F. Stock concentration during bleaching also has an influence; the time required drops and the brightness of the fiber increases as the fiber concentration rises. The final reduction of any remaining peroxide, and the adjustment of pH to about 6 to 7 is done with sodium bisulfite, or with sulfite cooking liquor; this reaction is practically instantaneous.

Under the best conditions a gain of 12 to 13 points in brightness may be expected. This, of course, will vary somewhat with the kind and condition of the wood, and with its brightness before bleaching is started, as well as on the amount of peroxide used and the other bleaching conditions.

The only real reason for bleaching groundwood is the improvement in color, but this is sufficient to enable it to be used in grades of paper in which unbleached groundwood would not normally be used. No other change in the properties of the fiber is enough to justify the bleaching operation. Such bleaching does not improve the stability of color to sunlight or ultraviolet light over that of the unbleached pulp, but it does make it more stable during its passage over the driers of the paper machine. Neither does it change the properties imparted by the wood, or by the grinding process. The strength of the bleached pulp is practically the same as that of the unbleached and only a little loss in weight is caused by the peroxide; and while the freeness test discloses no difference in the rate at which water drains from bleached or unbleached groundwood, the bleached material is found to drain better on the paper machine wire. Papers in which bleached groundwood is used to any extent have the improved printability imparted by all groundwoods, but to not much greater extent than with ordinary unbleached groundwood.

The materials most suitable for equipment which comes in contact with peroxide are stainless steel, rubber lined steel, or concrete. Lead and copper should be avoided as they act as catalysts and cause loss of the peroxide.

During the early days of bleaching, especially when treating rags in breaking engines, it was occasionally desired to destroy an excess of bleach remaining in the fiber after the desired color was reached. This was done by adding an "anti-chlor" which reacted with the bleach to prevent any further oxidizing action. Materials used were sodium thiosulfate, sodium sulfite, sulfite cooking liquor, or even hydrogen peroxide. These, with the exception of the peroxide, tended to set up acid conditions which were harmful to the paper machines, and they had to be used with much care. Modern methods of bleaching make the use of antichlors practically obsolete.

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