US 2968589 A
Description (OCR text may contain errors)
2,968,589 CONTINUOUS PULP BLEACHING METHOD Filed Mar. 17, 1958, Ser. No. 721,967 2 Claims. (Cl. 162-19) The present invention is concerned With an improvement in the manufacture of cellulosic products and more particularly with a continuous method and suitable apparatus for the bleaching of cellulosic pulp.
Hypochlorite has been a standard bleachin the Wood pulp processing industry for many years. The hypochlorite bleaching of Wood pu'lp was first developed as a batch process in which an aqueous Suspension of Wood pulp fibers or, "stock as it is usually called, having a consistency of approximately 3 percent Was bleached by the introductlon of hypochlorite into the stock contained in a large vessel. As the hypochlorite reacts With the cellulose pulp, acids are liberated and it is desirabfle that these acids be neutralized in order to avoid degradation of the pulp and a loss of efliciency of the bleaching agent because of side reactions. Reactions Were therefore carried out at low consistencies of approximately 3 percent and also at fairly low temperatures of the order of 30-40 C. The reaction rate of the hypochlorite with the cellulose pulp at this temperature is slow but this slow reaction rate permitted adequate control of the neutralization of the acids formed, which is accomplished by the addition of alkaline materials such as calcium hydroXde or sodium hydroxide. Because of the slowness of the reaction long periods of time were required for completion of a bleaching operation and very large tank capacity was required for holding the pulp the necessary time.
Attempts were made to improve the low consistency bleaching process by employing continuous methods. However, as pointed out in The Bleaching of Pulp, TAPPI Monograph No. 10, page 79, low consistency bleaching whether batch or continuous Was a slow process requiring long periods of time for completion and very large tank capacity for holding the pulp the necessary time.
Atternpts were then made to improve hypochlorite bleaching by using bleaching processes and equipment with which much higher pulp consstencies were employed, of the order of 15-25 percent. These continuous processes, however, in general, required large vessels and much increased horsepowerV for agitation of the high consistency stock and transfer of the stock from one vessel to another. These high consistency processes have therefore been more applicable to multi-stage bleaching methods employing several different chlornaton agents and designed to produce large changes in the brightness value of the pulp than to single stage hypochlorite bleaching Operations.
It is an object of the present invention to provide an tates Patent tiu improved continuous method of bleaching cellulosic pulp contained in a low consistency Suspension by a hypochlorite bleaching agent.
It is an additional object of the present invention to provide an improved apparatus capable of automatic operation, for the continuous hypochlorite bleaching of a cellulosic pulp.
It has been found that very rapid bleaching of cellulosc pulp contained in a low consistency stock can be obtained by a continuous method in accordance With the process of the present invention. The process comprises injectiug a hypochlorite bleaching agent into a stream of pulp stock having a consistency of about 2 to 8 percent and a temperature of between about 45 and C. and preferably about 50-60 C., and injecting an alkali buffer agent into the stream in a quantity sufiicient to neutralize a portion of the acids released by the hypochlorite bleaching reaction. The stream of pulp stock is then flowed through a plurality of reaction sections each comprising a successive injection zone, an agltation zone and a calrning zone. In each section one or more chemical agents are injected into the Suspension, the Suspension is then agitated and finally calmed for a predetermined retention time. In all except the final reaction section sufiiicient alkali buffer is injected into the suspenslon in the injection zone to neutralze the acids released in the succeeding agitation zone and calming zone so as to maintain the hydrogen ion concentration of the Suspension at a pH of 9.5-l0.5 throughout that section. In the final reaction section, an antichlor and an acidic agent are injected into the Suspension in the injection zone in quantities suficient to destroy any remaining hypochlorite and to acidify the Suspension to a pH of 4-7. Automatic regulation is applcable to the present processes as Will be pointed out, thus permitting adequate control of the bleaching operation at the high temperatures employed. The use of a high reaction temperature in turn permits very rapid bleaching and together With the use of a continuous fiow process avoids the necessity of the use of the large veseels generally employed in prior art methods.
The single stage continuous hypochlorite bleaching process of the present invention is particularly applicable to the bleaching of mechanically disintegrated Wood pulp such as groundwork pulp or high yield .Semi-chemical Wood pulp. Groundwood pulp is a pulp obtained by grinding Wood to reduce the Wood to Wood fibers. Semichemical Wood pulp is a pulp obtained by first treating the Wood With a brief chemical treatment and then reducing the resultant product to Wood pulp by an attrition operation. Both pulps are relatively h'gh in lignin content. While the present process is particularly app'licable to the bleaching of such pulps it may, however, be applied to any Wood pulp or other cellulosic pulp which is conventionally treated by a single hypochlorite 'bleaching operation. The present process may also be applfed as a hypochlorite bleaching stage in a multi-stage bleaching operation employing similar or other types of bleaching for the additional stages. For example, the present' bleaching process might be used as either or both hypochlorite bleaching stages of conventional three or five stage bleaching systems.
The pulp treated by the present method is a cellulosc pulp suspended in an aqueous Suspension or pulp stock having a consistency of between about 2 to 8 percent and preferably about 2.5 to 6 percent and a temperature of about 45-70 C. and preferably about 50-60 C. The pu'lp stock is adjusted to this consistency and temperature preferably by automatic control methods whcreby steam or hot and cold water are mixed with the stock. The order of the temperature and consistency regulation is not important but there is preferably compensation between the two controls. v
Following the temperature and consistency regulation the stream of pulp stock is fiowed through a number of reaction sections each of which is comprised of a chemical injection zone, an agitation Zone and a calming zone. One or more chemicals are injected into the pulp stock in the injection zone, the stock is then agitated in the agitation zone and then calmed in the calming zone. A positive pressure is maintained on the pulp stock stream so that an even, continuous flow of the stock is maintained through all of the reaction Sections. The retention period of the stock in each reaction section is thus determined by the length and volume of the section.
The bleaching agent employed in the present process is a hypochlorite bleaching agent. Suitable hypochlorite bleaching agents include calcium hypochlorite and sodium hypochlorite although other hypochlorite agents may be used. Sufficient hypochlorite is employed to achieve the desired brightness increase in the pulp. Where automatic controls are employed with the process the hypochlorite addition to -the pulp may be made to be responsive to the brightnes of the pulp in the final stage of the process. The requisite amount of hypochlorite is injected into the pulp stream in the first injection zone immediately adjacent to and preceding the first agitation zone. A quantity of an alkali buffer is also injected into the stream in the first injection zone. The alkali buffer may be calcium hydroxide, sodium hydroxide or other suitable alkaline neutra'lizing agents. The pulp stock is then forced through an agitation zone where the injected chemicals are thoroughly agitated with the pulp stock and the pulp stock then flows through a calming zone before entering the succeeding chemical injection zone. The alkali buffer which is injected in the first injection zone is sufficient to neutralize the acids released by the reaction of the pulp with the hypochlorite throughout the first reaction section so that the pH of the pulp stock as the pu-lp leaves the first calming zone is between about 9.5-10.5. The normal method of operation is to inject sufficient alkali bufer into the stock to raise its pH to approximately 10.5. The retention period of the reaction section is then so fixed that the pH of the stock has fallen to about 9.5 as the stock leaves the section and enters the succeeding section, where the process is repeated. The quantity of hypochlorite injected into the pulp stock stream in the first injection zone may be automatically regulated by a pH control device such as an automatic pH meter in the first calming zone. Alkali bufer is injected into the stock in the injection zone of each succeeding reaction section except the final reaction section in quantity sufiicient to maintain the hydrogen ion concentration of the pulp stock throughout that reaction section in the desired pH range. These alkali additions can be regulated by automatic control means in the calming zone. No additional hypochlorite, however, s added after the initial hypochlorite injection. As many of these reaction sections as are required to obtain the desired brightness are employed. The final step of the present method is the destruction of the excess hypochlorite and acidification of the Suspension to a terminal pH of about 4 to 7. This final step destroys any hypochlorite which may remain with the pulp stock and tends to prevent reversion of the bleached stock. An acidiomaterial such as gaseous S02 or aqueous sulfurous acid is preferably employed since this acts both as an antichlor and an acidifying agent. Separate mineral acids such as sulfuric acid may however be employed for pH control. The acidic material is agitated in the final agitation zone and the stream then fiowed through a final calming zone. The quantity of acidic material added to the stream may be automatically controlled by a pH control means in the final calming zone.
The present process can be operated as an automatic continuous process. The regulation of the temperature and consistency of the initial pulp stock stream can be made automatic. The fiow of the pulp stock can be regulated by a pulp stock fiow control which will automatically maintain a given flow. The injection of the hypochlorite can be. automatically controlled by the pulp stock fiow control and can also be responsive to a brightness meter monitoring the brightness of the bleached pulp. The injection of the alkali bufier in each reaction zone may be made responsive to the pH of the stock vin the calming zone of that reaction zone and the addition of acdic material in the final reaction zone may be made responsive to an automatic pH control means in the fina'l calming zone. Thus all Operations of the bleaching system can be automatically controlled.
The process permits the efiicient bleaching of low consistency pulp stocks in periods of time comparable to continuous high consistency bleaching methods without the disadvantages inherent in the transportation and agitation of high consistency stocks. The times required for bleaching in the present process are much shorter than those required with conventional low consistency bleaching methods because of the high temperatures employed and the complete control maintained over each increment of stock flowing through the reaction Sections. The high temperature is possible because of the precise control of the addition of hypochlorite and of alkali buffer afr'orded by the continuous fiow of stock and the pH monitoring of each reaction section. The use of comparatively small volumes of the stock stream flowing through alternating agitation and calming zones permits essentially homogeneous conditions to be maintained throughout the procms. The present process also results in a much more efiicient use of chemicals since the incremental addition of alkaline neutralizing buifer permits the effective neutralization o-f the acids formed throughout the reaction while tending to limit the darkening effect of excess alkali and the decomposition of bleach solution which takes place at low pH. In addition, the process is capable of automatic control.
Now that the process has been generally described it may be further illustrated by the following example.
Example 1 A wood pulp obtained from Southern oak by the cold caustic Semi-chemical method of pulping and having a yield of percent, a freeness of 200 Canadian Standard and a GE brightness of 43.4 was diluted to a consistency of 3 percent by weight of the oven dried pulp and the temperature of the stock adjusted to 50 C.
The pulp stock was then passed through 3 bleaching zones and a final zone of hypochlorite reduction and acidification. A calcium hypochlorite bleach having 40 grams of available chlorine per liter of bleach and prepared in accordance with United States Patent 2,587,845, Heller et al., issued March 4, 1952, was added to the pulp stock prior to the first agitation zone in an amount of 8 percent available chlorine based upon the oven dry weight of the pulp. A calcium hydroxide bulfer was added to the pulp in 3 separate increments of 0.67 percent calcium hydroxide, based upon the oven dry weight of the pulp. The first increment was added to the pulp stock with the calcium hypochlorite. The pulp containing the hypochlorite and calcium hydroxide was agitated and then calmed for a total period of 21/2 minutes during which time the hydrogen ion concentration of 'the pulp fell to a pH of 9.5. The second increment of calcium hydroxide was added prior to the second agitation. The second agitation and calming period was 6.5 minaeeaese utes and the hydrogen ion concentration during this period fell from 10.4 to 9.6 pH. The third agitaton was preceded by the addition of the third increment of caleium hydroxide which raised the pH of the stock to 10.6. A 6 minutes' agitation and calming period followed during which the pH fell to 9.5. Two percent of sulfur dioxide based ou the oven dry Weight of the pulp was added preceding the final stage in which the pulp was agitated and then calmed for a total of 3 minutes. The final GE brightness of the pulp was 60.9.
The apparatus employed in the practice of the present nvention may be varied in form and arrangement. The main components of the system are low density mixers and retention pipes. The pipes are conventional units and may be straight or looped and of varying diameters and lengths to suit the particular retention times desired. The mixers may also be conventional commercial units. In addition, one or more pumps are required to circulate the bleach stock through the various units. The bleaching system is flexible because pipes, mixers control, etc., may be conventional commercial units instead of the large specially designed vessels which have heretofore been used. The process is adaptable to various bleaching conditions since the flow of pulp stock and chemical additions are subject to automatic control. In addition, the retention times in the various bleaching units may be changed by changing the lengths of pipe used. The bleaching apparatus of the present invention occupies a much smaller space than conventional bleaching apparatus because of the use of pipes instead of large vessels as retention units. In addition, the capital investment required to construct and to house the bleaching apparatus is much reduced.
The preferred form of the apparatus is illustrated in the accompanying drawing in which the figure is a diagrammatic view illustrating in schematic form the bleaching apparatus.
The unbleached pulp stock is introduced to the apparatus through the unbleached pulp stock pipe or line 10. The unbleached pulp stock may come from the grinders in the case of groundwood, or from the screens, from a repulper, or a refined pulp chest, or an earlier stage of a similar or a different bleaching process. 'Ihe pulp stock normally enters the present system at a temperature and a pulp consistency in the range in which it is desired to operate the bleaching apparatus. However, additional means are provided for adjusting the temperature of the pulp stock to the desired temperature. The means may comprise a conventional temperature controller 12 automatically responsive to a temperature sensing element 14 which is also located in the unbleached stock line 10. The temperature controller 12 controls a flow of steam or hot or cold water through steam line 16 into the unbleached stock line 10. The temperature controller may, for example, be one of the self-operated type such as is described on page 269 of Instruments. for Measurement and Control, Holzbock, Reinhold, 1955. The consistency of the stock is controlled by a stock consistency control means 18 which maintains control of a flow of water from water line 20 into the unbleached stock line and so maintains the consistency of the stock at the desired co-nsistency level. While the consistency control 18 is shown as an in-line flow control such as the Consistorator, manufactured by Fisher & Porter, other type of flow controllers such as those in which a portion of the unbleached stock is circulated to a separate feed back circuit may also be employed.
The unbleached stock having the desired tempera*ure and consistency is then introduced into a pump 22. While a centrifugal type pump is shown in the illustratipon other types of pumps such as rotary or reciprocating types may be used. In the system shown a single pump is used to force the unbleached pulp stock throughout the entire bleaching system. While multiple pumps can be used throughout the system in place of the single pump, the single pump is preferred since a pump in this location of the system operates on a substantially neutral liquid so that the corrosion on the pump is little problem, whereas pumps inserted in the system after the injection of chemicals into the stock operate in either an alkaline or an acid environment and so are much more liable to corrosion. Because the pump is not required to perform any agitation service and because of the relatively small volume of liquid moving through the system, relatively small pumps may be used. The pump maintains a positive pressure on the stock throughout the system at all times.
The unbleached stock is forced by the pump into a stock mixing pipe 24. This stock mixing pipe 24- contains a stock flow control 26. The stock flow control, controls the flow of stock into the bleaching section and may be operated manually or may be made responsive to the demand for bleached stock. The stock fi'ow control may, for example, be comprised of a Ratosleeve flow meter and Ratogate stock control valve, as described in Fischer & Porter catalogues 10-A-43 and 60-l2. The stock mixing pipe 24 terminates in a chemical injection pipe 28a which is connected to a mixer 30a. A hypochlorite line 32 enters the chemical injection pipe 28a adjacent to the juncture of chemical injection ppe 28a and the low density mixer 30a. The hypochlorite line 3 2 contains a hypochlorite flow control means 34. This hypochlorite flow control means 34 is responsive to the stock control means 26 so that a change in the stock flow will automatically cause a corresponding change in the hypochlorite flow. An alkali supply line 36a terminating in the chemical injection pipe 28a adjacent to the juncture of the chemical injection pipe 28a and the low density mixer 30a is also provided. This flow of alkali buffer through the alkali supply line 36a is controlled by an alkali control 33a. The alkali is supplied to the alkali supply line 36a by' a main alkali supply line 40 which contains a specific gravity control 42. Fresh water is supplied to the specific gravity control 42 by a water supply pipe 44.
The function of the mixer 30a is to thoroughly mix the injected chemicals With the stock. In this first unit hypochlorite bleach and alkali buffer are the chemicals employed. This mixer 30a may be any conventional low density mixer. A suitable mixer for a ton o f pulp per day bleaching plant has been found to be one having 6 inch I.D. inlet and Outlet pipes, and 15 inch I.D. mixing Chamber containing 2 impellers with 4 vanes each, operated by a 40 horsepower, 1800 r.p.m. motor. The mixer is connected with a retention pipe 46a. This retention pipe 46a provides a calming zone for the agitated pulp stock. The retention pipe may be of any length or diameter and may be straight or looped in any desired configuration in order to meet the desired pulp retention time requirements and the existing space limitations. The retention pipe 46a is essentially the same as retention pipes 46h, 46a and 46d, although the length or the volume of pipes may be different to provide difierent pulp retention times in the various units. Because the mixer and the retention pipe normally contain either an alkaline or an acid solution these components are usually of corrosion resistant materials such as a nckel alloy or stainless steel. In a conventional 150 ton per day bleachery, the retention pipe may be an 18 inch LD. stanless steel pipe and the first retention pipe may provide for a retention time of the stock in the bleaching unit of 11/2 minutes. The retention time is the time required for an increment of stock to pass through a bleaching unit comprising a chemical injection pipe, mixer, and retention pipe. The hydrogen ion control means 48a continuously samples the stock in the retention pipe 46a through meter connecting lines 50. This hydrogen ion control means may be a control system including a conventional pH meter, a redox meter or other` similarl meter, an electrical to pneumatic transducer and a pneumatically controlled valve. This hydrogen ion control means 48a controls the flow of alkali through the alkali control meter 38a and alkali line 36a to the chemical injection pipe 28a. The purpose of this hydrogen ion control meter is to control the flow of alkali so that the pH of the stock is maintained at about 9.5-10.5 in the bleaching unit.
' The chemical injection pipe 28a, mixer 30a, a retention pipe 46a and hydrogen ion control means 48a form a bleaching unit. There are a plurality of these bleaching units. In each of these bleaching units, one or more chemical agents are injected into the pulp stock in the chemical injection pipe, the pulp stock and injected chemicals are agitated in the mixer, the agitated stock then flows through the retention pipe for a period determined by the volume of the retention pipe, and the hydrogen ion concentration of the stock in the retention pipe is maintained by a hydrogen ion control means which in turn controls the amount of chemical agent primarily affecting the pH, injected into the chemical injection pipe of that bleaching unit. The components in these bleaching units are similar or identical although the functions may differ somewhat as will be shown.
The intermediate bleaching units are identical with the first bleaching unit in that they are comprised of a chemical injection pipe 2811, 28c immediately preceding a low density mixer 30h, 300 and having a retention pipe 46b, 46c connected to the low density mixer, the retention pipe having a hydrogen ion control means 48b, 48a` capable of controlling the flow of alkali bulfer from an alkali supply line 361), 36a containing an alkali control 38h, 38c into the bleaching unit in response to the hydrogen ion concentration of the stock in the retention pipe. Tne volume and configuration of the retention pipes may vary With the section which they are used; for example in a 150 ton per day bleachery the retention pipe in the second section may provide a retention time of 21/2 minutes for that bleaching unit and the retention pipe in the third unit may provide a retention time of 2 minutes. Two or more of these intermediate bleaching units may be provided, the number depending upon the design of the particular bleachery.
The final bleaching unit is essentially identical in configuration and components with the preceding bleaching units having a chemical injection pipe 28d, a mixer 30d, a retention pipe 46d and a hydrogen ion control means 48z, but is operated in a different manner. The bleaching of the pulp is completed by the time the pulp stock reaches the final bleaching section so that the final section is used to destroy any residual hypochlorite and to acidify the bleached stock so that reversion will be prevented. This may be done by injecting an antichlor agent and an acid. Separate agents may be used, but it is preferable to employ sulfurous acid or a sulfurous acid forming substance such as gaseous S02 since sulfurous acid acts as both an antichlor and an acidifying agent. The sulfurous acid is injected into the final chemical injection pipe 28d through the sulfurous acid supply line 52. The flow of sulfurous acid is controlled by the sulfurous acid controller 54 which is responsive to the hydrogen ion control means 48z in the final bleaching section. The final retention pipe Ll-d terminates in a bleached stock pipe 56 which conveys the bleached stock to a storage chest (not shown) for subsequent Operations. A brightness meter control means 58 such as the Allegany Instrument Co. Automatic Bleach Control Model 12 is provided for continuously sampling the bleached stock in the bleached stock line 56. This brightness meter controls the flow of hypochlorite into the system through the hypochlorite flow control 34 in response to any variations in the brightness of the bleached stock. The hypochlorite flow control, since it is responsive to the signal from the stock flow control 26 and the brightness meter 58, may be of .the multiplying relay controller type. The instrument control lines shown inthe figure are unnumbered.
' Conventional, commercially available, automatic devices, for the control of the flow of fluids in response to temperature, consistency, flow rate, liquid level, specific gravity, hydrogen ion concentration, and brightness of pulp may be used throughout the present. apparatus. These automatic control devices which usually consist of a measuring means, a controllerand a final control element or valve, may be operated pneumatically, hydraulically, magnetically, electricallyV or electronicallyl Suitable automatic control devices are described' in Instruments for Measurement and Control, Werner G. Holzlock, Reinhold, 1955, and Instrumentation Systems for the Pulp and Paper Industry, Bul. -130-10, Fischer 8: Porter Co., 1956.
Now, while the completely automatic version of the apparatus has been described it is of course to be apprecia'ted that manual control may be substituted for the automatic control in individual instances. It is apparent, however, that the apparatus is capable of providing a hypochlorite bleaching system which can be completely automated, in that the consistency and temperature of the stock can be controlled to satisfactory limits, the pH can be controlled throughout the entire system to the Optimum level and the entire system can be made to operate automatically in response to the brightness of the pulp produced. Furthermore, since a relatively small amount of stock is in the system at any one time the system has greatly increased flexibility over conventional systems in that the bleaching conditions can be changed in response to changes in the' stock with a minimum of delay and this can be accomplished automatically. As previously pointed out the capital investment for this system is very low since the retention of the bleaching stock is in conventional pipes rather than in massive vessels. The use of pipes as retention devices also permits the most efficient use of space since the pipes may be twisted or folded in any desired configuration without affecting the efliciency of the system. Because of the low volume of stock in the system at any one time and the use of a plurality of agitation zones, essentially homogeneous conditions of mixing of the stock and chemicals can be maintained at all stages of the system. This combination of the maintenance of homogeneous mixtures and the automatic regulation of pH throughout the system results in a most eflicient system which provides the maximum points of brightness per pound of chlorine used.
Now, that the process and apparatus have been Vdescribed it is apparent that numerous modifications can be made within the spirit of the invention whichis to be limited only by the appended claims. What is claimed is:
1. A continuous pulp bleachingl method comprising the Steps of adjusting the consistency of a stream of an aqueous Suspension of cellulose fibers to between about 2 to 8 percent, adjusting the temperature of the Suspension to about 45 to 70 C., injecting hypochlorite into the Suspension, alternately agitatng and calming the suspension while fiowing the Suspension through a plurality of bleaching zones in sequence, said bleaching zones comprising in sequence an agitation zone and a calming zone, injecting sufficient alkali into the Suspension prior' to its entry into each bleaching zone to maintain the pI-I of the Suspension between about 9.5-10.5 through Veach agitation zone and succeeding calming zone, injecting antichlor and acid into the Suspension prior to the suspensions entry into a final agitation zone in quantity sufficient Vto destroy the excess hypochlorite and acidify the Suspension to a terminal pH of about 4-7, and agitating and calming the resultant Suspension. g p
2. A method for the continuous bleaching of pulp comprising the steps of continuously flowing a pulp` stream 'consisting of an aqueous Suspension of'cellulose fibers through a conduit, continuously maintaining the consistency of said stream in said conduit at about 2.5 to 6%,
-trwa mantaning the temperature of said stream within the conduit at about 50 C. to 60 C., injecting a controlled amount of hypochlon'te into said heated stream, directing the hypochlorite-containng stream of pulp in sequence first through a zone where there is injected a Controlled amount of alkali then through an agtatng zone and then through a calming zone, sequentially repeating the steps of alkali injection, agtation and calmng, then injecting acid into said stream to destroy the unused hypochlorte and lower the pH of said stream to about 4 to 7, and then causing the bleached pulp stream to flow through a final agitating zone and then through a final calming zone.
References Cited n the file of this patent UNITED STATES PATENTS Richter Dec. 29, 1931 Richter Sept. 27, 1932 Martin et al Sept. 27, 1949 Lirnerck Apr. 8, 1952 Jones et al May 12, 1953 Evans Nov. 27, 1956 Wayman et al Apr. 21, 1959 FOREIGN PATENTS Canada June 16, 1953