MXPA00001134A - Method and apparatus for pulp yield enhancement - Google Patents

Abstract

The process of the present invention purposefully precipitates a portion of the dissolved lignin onto pulp fibers to improve pulp yield of unbleached pulp. The resulting retention of lignin on the pulp creates an increase in pulp yield. Washing the pulp in a series of washer stages sequentially removes entrained lignin. Between each of the washer stages, adding dilution water repulps a pulp mat that exits from a prior washer stage and creates a pulp stream for a next washer stage. After at least one of the washer stages, adding an acidifying agent to the pulp stream forms a pulp product by precipitating the entrained lignin onto cellulosic fibers contained in the pulp stream. Finally, the process removes the pulp product from the series of washer stages with the pulp product having at least about a 1 unit increase in Kappa number.

Description

METHOD AND APPARATUS FOR IMPROVING PULP PERFORMANCE Field of the Invention • This invention relates to the reduction of the cost of pulp and to the improvement of pulp yield by the precipitation of lignin on cellulose fibers during the production of unbleached paper products. BACKGROUND OF THE INVENTION The Kraft firing process is a chemical method for • 10 prepare pulp for wood and non-wood sources to produce cellulose fibers. Essentially, the Kraft process involves cutting untreated wood logs and cooking them in a digester with sodium hydroxide and sodium sulfide (collectively known as white liquor) at a specific temperature and pressure.

The resulting reaction product is separated into cellulosic fibers (usually called pulp) and cooking chemicals are used, along with most of the lignin, the organic material that binds • fibers. During the cooking reaction, lignin dissolves and becomes part of the liquor, along with the use of cooking chemicals.

The cooking chemicals used and the dissolved lignin are collectively known as mother liquor Kraft cooking can generally be separated into two categories: the cooking of bleached products and the cooking for unbleached products. The difference in the two categories is the The amount of cooking chemicals (mother liquor) used, the temperature at which the cooking is carried out, and the time the chips are exposed to the cooking liquor. Depending on the desired degree of pulp that will be produced, the cooking process is operated to achieve • pulp of a degree of delignification, normally measured as a 5 Kappa number. The Kappa number test was used to determine the amount of remaining lignin on the pulp after cooking. The Kappa number is defined as the number of millimeters of 0.1N potassium permanganate solution consumed by a degree of pulp and recovered for the 50% consumption of the potassium permanganate initially added (TAPPI Test Method T236 cm-85, Normal CPPA G.18). The following Table 1 gives the values of the normal Kappa number, the% of lignin and the yield of the pulps produced for various paper products. 15 labia 1 Pulp produced Paper without wood without bleaching whitened bleaching • Kappa number 20-35 35-120 40-120% Lignin in 2.9-5.1 5.1-18 6-18 Pulp Total yield 44-46% 46-50% 50-58% Yield 41-44% 45-56% 48-56% sieving The degree of cooking is also indicative of the amount of lignin that dissolves in the cooking liquor. This can be measured by taking the cooking liquor from a given Kappa cooking, • acidifying at a low pH (< 3) and recovering and measuring the weight of the resulting precipitate. Kraft's cooking process recycles used cooking chemicals through a process known as the recovery cycle. The cooking chemicals used and the dissolved lignin were removed from the pulp product via washing with water • 10 counter current. The washed pulp was recovered as solids and diluted, the cooking chemicals used and the dissolved lignin recovered as a liquid known as the weak black liquor. The weak black liquor was evaporated to a high concentration of suspended solids and incinerated in a recovery heater in where some of the heat of burned lignin was recovered as energy and current and the used cooking chemicals recovered as a melt. The cooking chemicals used afterwards were processed • to convert Na2CO3 to NaOH together with a small amount of Na2S, collectively known as white liquor. 20 Raw materials represent a substantial cost of any pulp Improvements in pulp yield can dramatically affect the economics of processes Therefore, even the smallest improvements in pulp yield can translate into substantial economic benefits and increase production Higher yields can be achieved by various methods to prepare pulp, one of which pulp is prepared mechanically so that the work simply grinds the raw material into pulp. The Kraft process, however, has a relatively low yield but pulp procedures having high strength. Yield is defined as the amount of pulp, by weight, that is produced from a given amount of raw material, expressed as the percentage of the given amount of raw material. For example, a yield of 70% means that 70g of pulp is produced from 100g of raw material. One reason for the high strength of the Kraft pulp is that the cellulose fibers are not damaged relatively by the cooking process, in opposite manner to be developed in small pieces as was done in the mechanical preparation of pulp. On the other hand, the low yield of the Kraft process results from the lignin that was extracted from the wood, effectively reducing the yield between 41% and 44%. The pulps produced for unbleached products are generally performance pulps superior to bleached pulps due to the loss of lignin which was dissolved in the cooking liquor and washed in the subsequent chemical recovery step. The difference between the Total Performance and the Screening Performance is the wood under cooking removed in the sieving (an operation carried out to remove the bundles of fiber under cooking from the pulp stream) Increasing the severity of cooking increases the yield of Screening in the Total Performance expense. There are many methods to improve the performance of Kraft pulp. Generally, performance improvements are achieved by 5 one or more of three methods: process modifications, pulp-forming additives, and method changes. (a) A method for improving pulp performance involves the addition of additives to the cooking liquor in the digester in an attempt to protect the pulp fibers of the pulp. • 10 degradation. Said additives include anthraquinone (AQ) and polysulfide. The improved performance results because the additives protect the cellulosic fibers from degradation. (b) Slight changes in the process can also improve performance. The most common process modification, called "superior Kappa pulp formation", was developed from the environmental requirements and the proliferation of oxygen delignification. The modification involved in the • cooking conditions, as measured by factor H, so that the lignin content in the final pulp product is higher than normal. The factor H is determined by indicating the relative reaction rate against the reaction time in hours and measuring the area under the curve Parsad demonstrates the formation of superior Kappa pulp by modifying the H factor of several Kraft cooks, these results show that by increasing the Kappa number, also increases performance (Ver- Parsad, Bpjender, and others, "High asr £ * i * * .X 5% Kappa Pulping and Extended Oxygen Delignification Decreases Recovery Cycle Load. "Tappi Journal, Vol. 77, No. 1 1 (November 1994).) This improved performance method occurs in the digester area Furthermore, lignin is not precipitated in the fibers as is done by the invention described below, instead of lignin breaking under and dissolving in the cooking liquor for washing removal. It is intended to be used with the delignification of oxygen, which subsequently removes lignin in a final process step by the oxidation and dissolution of lignin.This method has the additional disadvantage in producing less Total Yield.If the cooking is not carried out at a sufficient degree, all the cuts can not be broken under the individual fibers, allowing some fibers tied together, known as fragments. The fragments can adversely affect the appearance of the final product and physical properties due to relatively poor fiber-to-fiber bonds. The fragments are removed and recycled in the digester in a cleaning step known as sieving, effectively reducing the capacity of the digester (c) Another method to improve the performance of Kraft cooking is known as "sorption cooking" and has researched by N ils Hartler of the Swedish Forest Products Research Laboratory (See Hartler, "Sorption Cooking Yield I ncrease for Unbleached Alkal Pulps Through Sorption of Organic Substance from the Black Liquor." Svensk Papperstidn (October 1978); the Patent of E.U.A. No. 3,937,647. This method involves a decrease in the pH of the black liquor and the end of the cooking process to precipitate the lignin in the fibers. An acid, preferably CO2, is used as the lower part of the pH of the liquor at 8.0 as the result of the yield is improved by 1% to 2%. Hartler uses an acid, preferably H2SO4, at the bottom of the pH below 11.0 and much more than 5.6. This improved performance method is similar to the invention which will be described below only in that it involves the precipitation of lignin with an acid. The acid is used to reduce the pH of the cooking liquor at the end of a Kraft cooking where the lignin concentrations are higher, while the process of the present invention uses a lower acid than the pH pulp of a diluted lignin which contains the current during washing. (d) Although it is not a method designed to increase performance, in the U.S. Patent. 5,429,717, Bokstrom addresses the problem of washing efficiency increased by the use of CO2 to lower the pH of the wash water to increase the efficiency of chemical recovery and to maintain the lignin solution. In the Bokstrom process, the pH of the pulp is lower than between 6.8 and 9.4 during the washing step, resulting in the desorption of the sodium binding and a decrease in the dissolved lignin and the used cooking chemicals carried out on the bleaching plant.

Bokstrom refers to the problems that result when the pH of the pulp is the bottom too fast, but fails to observe the important benefits that can be gained through its activities. In fact, Bokstrom helps certain pH conditions due to undesirable reactions with residual lignin (col.2, line 14). Bokstrom balances the desorption of sodium with the removal of lignin to wash the pulp with more efficient uses of chemicals. In a paper using the discussion white of the Bokstrom technique, Blanco notes that the addition of CO2 occurs at the end of the washing line to aid lignin precipitation (p.54).

(See: White, "Carbon Dioxide on Pulp During Washing in the Minimum Impact Mili. "Pulp Washing '96, Tappi (October 1993) In the prior art described above, improved performance solutions require significant changes for existing equipment, eg, the use of additives to protect cellulose, pulping to retain the lignin instead of being precipitated and, in the sorption cooking, decreasing the pH of the black liquor at the end of the cooking to precipitate the lignin.Therefore, it is an object of the invention to improve the yield of the lignin. unbleached pulp emerging from the Kraft baking process It is a further object of the invention to improve the economic means to increase the pulp yield in unbleached pulp mills, without requiring substantial modifications in the mill equipment. Invention • The process of the present invention determines the precipitates 5 of a portion of the dissolved lignin in the pulp fibers to improve the pulp yield of the unbleached pulp. The resulting retention of lignin on the pulp creates an increase in pulp yield. The washing of the pulp in a series of washing steps sequentially removes the entrained lignin.

Between each of the washing stages. Adding the repulps of • dilution of water in a pulp mat coming out of a previous washing step and creating a pulp stream for a next washing step. The pulp stream contains the introduced lignin. After at least one of the washing steps, The addition of an acidifying agent in the pulp stream forms a pulp product by precipitating the introduced lignin in the cellulose fibers contained in the pulp stream. Finally, the process removes the pulp product from the series of washing steps with the pulp product that has at least approximately 1 unit increased in the Kappa number. This increases a high Kappa number of the precipitation induced by the introduced lignin acid. Brief Description of the Drawings Figure 1 is a schematic diagram of the washing system of pulp for carrying out the invention.

Figure 2 illustrates the effect of the increased Kappa number on the brightness of the resulting product, for several starting Kappa numbers of the original pulp. Figure 3 is a graph of the Kappa number versus yield for pulp samples having original Kappa numbers of 60, 80 and 100, respectively. Figure 4 illustrates the changes in the Kappa number, which result against changes in the pH of the product, when the acidifying agent is added in the pulp stream according to the invention. Detailed Description of the Preferred Modes In the case of pulp production for bleached products, lignin is undesirable due to its darkening characteristics and is intentionally removed from the process before bleaching. This invention offers a sample, low cost and controllable way to increase pulp yield without bleaching in a Kraft mill. Referring to Figure 1, a pulp washing system 10 incorporating the method of the invention is shown. The pulp washing system 10 includes three washers 12, 14 and 16. Each washer comprises a sieved circular drum (e.g., 18) onto which a slurry of pulp is placed. A vacuum is applied inside each drum, causing fluids in the slurry of the pulp to be under sieving and feeding via a filtering line (v. Gr, 26) into a respective sealing tank (vgr. 28).

For example, the washing step 12 includes a screening drum 18 onto which a pulp flow is placed from the inlet 20. The pulp flow 20 comprises a mixture of solids of • 2-4% pulp / water exhibiting a highly basic pH of about 12. A plurality of shower heads 22 feed a sealing tank sprinkler stream with an immediate result (e.g., 24) on the mat of pulp containing the sieving drum 18. The water from the shower that comes out from the heads of the shower 22 causes the entrance of both lignin and • 10 of the sodium compounds that can be washed out of the pulp mat and can be fed via a filtration line 26 into the sealing tank 28. A recirculation pump 30 removes the black liquor from the sealing tank 28 and feeds a portion of it, via the pipe 32 in an evaporator (not shown), where the sodium chemicals and the energy of lignin combustion are recovered. A portion of the black liquor is fed to the rear in a mixing region 34 to mix with the incoming pulp flow 20. The washing steps sequentially remove the introduced lignin from the cellulosic fibers. When the pulp mat is first introduced onto the screen drum 18, it exhibits a solids content of 2-4%. However, after the pulp mat arrives at a reamer 36, it exhibits a composition of 20% solids / 80% liquids. The pulp mat is reamed towards • yfir * - out in and out of a repulper 38 where dilution of water is received via line 40 from seal tank 24. Within repulper 38, the pulp mat again liquefies in solids of 2. -4%, pulp / water mixture and then fed into a feed column 42. The lignin content of the flow in the feed column 42 is advantageously in the range of 0.2 to 5 grams of lignin per liter of the liquid. The remainder of the pulp flow lignin 20 is now contained in a sealing tank 28. The process described above was repeated in the washing machines 14 and 16 with the flow of the repulper from the repulper 44 and 46 exhibiting lignin concentrations of about 0.2. and around 5 grams per liter. This concentration of lignin facilitates the efficient use of acid to achieve effective lignin precipitation. More advantageously, lignin concentrations vary from about 0.5 to about 2 grams per liter. For example, the addition of carbon dioxide gas to wash water containing 1 to 1.7 grams per liter provides particularly effective lignin precipitation with acid added by means of carbon dioxide gas. In a similar way, the lignin content in the sealing tank 24 is considerably less than that found in the sealing tank 28. Similarly, the lignin content in the sealing tank 48 is considerably less than that found in the tank sealed 24 | ggjgygg »S For multi-stage systems having at least four washing steps, it is advantageous to precipitate the introduced lignin in at least two washing machines. In addition, it is more It is advantageous to precipitate enough lignin in each of these washers to increase the Kappa number by at least about 1 unit. The use of multiple lignin precipitation provides an effective increase in pulp yield without a major drop in pulp properties. Due to the very high alkalinity and the mass of the mat pulp, the pH of the filtrate washes are fed into the tank • sealed on the scale of approximately 10.5 to 12, regardless of the levels of acid added thereto, during the practice of the method of the invention. It is observed that the ratio of the dilution water in the shower water is approximately 90/10, indicating that the main amount of recirculating water is used in the repulping process, while only a minor portion is used in the shower process. • A slight lignin precipitation in the pulp flow in the flow region where the lignin concentration is low, causes the precipitated lignin to adhere to the cellulosic fibers and results in an increase in the output weights in the resulting pulp feed. Said precipitation is achieved by adding sufficient acidification material to a repulping mixture to cause a minor lignin precipitation. From importantly, the location of the acid addition is limited in sr a point in the washing stages where there is a relatively low concentration of lignin. We have found that the addition of sufficient acidification chemicals in the flow of repulping • between the last stages of washing makes possible a 5 increasing decrease in the pH of the pulp mat by approximately 0.5 to about 2.0 pH points, and the results in a 2-5 increase in the output pulp weight. This is accomplished without incurring the deleterious effects on washing or the subsequent pulp processing steps that may result from the precipitation of excess lignin. • A preferred method for the addition of the acidifying chemicals is via the application of a carbon dioxide flow at the outlet of the sealing tank 48, which outlet is used as a water feed dilution for the repulper 44. As shown in FIG. indicated above, the concentration of lignin in repulper 44 is from about 0.2 to about 5 grams per liter. This concentration of lignin facilitates efficient precipitation in cellulose fibers. Similarly, the partial acidification of the slurry precipitates of a modest amount of lignin in the cellulose fibers. For example, an increasing reduction in pH from about 0.5 to about 2.0 can provide the effective precipitation of lignin. Then, when the pulp flow is fed into the final washer 16, the amount of lignin that is washed out of the pulp mat is consequently reduced (due to the binding of lignin / cellulose fibers).

It should be noted that the addition of acidifying chemicals can occur at a point in the washing process where the concentration of lignin is relatively low, since, somehow, acidification results in excessive precipitation of lignin. This could be avoided. In addition, the acidification amount of the pulp flow again is maintained within a light scale to avoid excessive lignin precipitation. Although CO2 is the preferred additive to achieve acidification of the pulp flow, other acids, e.g., H2SO4, can be employed. In order to measure the amounts of the bound lignin in the flow of the pulp washing system 10, the Kappa numbers of the washed pulp output were measured in the laboratory tests. The tests were carried out at the University of Vicosa, Brazil, where the pulp samples were prepared at different Kappa numbers, that is, 60, 80 and 95, which are normal for different grades of the unbleached pulp. The pulp samples were acidified with carbon dioxide at different pH levels in the presence of the diluted black liquor and the resulting Kappa number was measured. In each case, it is possible to increase the Kappa number of the treated sample to cause an increase in the effective yield of 2-5%. Then, several physical properties were measured and compared. Figure 2 illustrates the effect of the increased Kappa numbers on the brightness of the resulting product, for several starting Kappa numbers of the original pulp. Figure 3 is a graph of Kappa numbers versus yield for pulp samples having the original Kappa numbers of 60, 80 and 100, respectively. Figure 4 illustrates the changes in the Kappa number that results against changes in the pH of the product, when the acidification is added to the pulp stream according to the invention. Comparing the pulps of the last equivalent Kappa numbers, it is observed that the pulps produced with the method for improving the pulp yield (MRP) of the invention generally improve the physical properties (see Table 2) over those produced by the traditional method. Table 2 index index of Resistance Index Absorption Voltage in Modules of Stress Burst Energy Rupture Limits of Stress Elasticity Property N.m / g kPa.m2 / g mN.m2 / g% J / m2 MPa MN.m / kg Kappa 80 73 6.4 13.7 3.2 99 18.2 6.2 MRP 77.1 6.6 13 3.1 103.6 20.1 6.8 Kappa 80 Kappa 95 75 6 4 12.3 3.4 108 18 6.1 MRP 72.5 6.5 12.5 2.9 88 18.1 6.4 Kappa 105 Kappa 62.6 5.6 10.6 2.7 71.4 16.2 5.8 120 MRP 73.9 6 5 11 4 2 9 88.3 18 6 5 Kappa 120 Table 2. Equivalent Kappa Pulps Showing Improved Physical Properties for Producing Pulps according to the invention.

• Observing the prepared pulps at the same initial Kappa number and comparing them with the pulps of improved yield, equivalent physical properties are observed (see Table 3). Table 3 Index Index of Resistance Index Absorption Voltage in Voltage Modules Burst Energy Rupture limits of Tension Elasticity Property • N.m / g kPa.m / g mN.m2 / g% J / m2 MPa MN.m / kg Kappa 60 75.7 6.7 13.5 3.3 108 19 6.4 MRP 77.1 6.6 13 3.1 103-6 20.1 6.8 Kappa 80 Kappa 80 73 6.4 13.7 3.2 99 18.2 6.2 MRP 72.5 6.5 12.5 2.9 88 18.2 6.4 Kappa 105 Kappa 95 75 6.4 12.3 3.4 108 18 6.1 MRP 73.9 6.5 11.4 2.9 88.3 18 6.5 Kappa 120 Table 3. Pulps Produced at Various Kappa Numbers and 10 Corresponding Improved Performance Pulps Showing Equivalent Physical Properties Based on common industry knowledge, we have expected that lignin precipitation in the pulp could result in less physical properties. desirable This is based in part on the theory that cellulose pulp fibers bind electrochemically with one another, resulting in a • strong union. In contrast, the lignin / cellulose pulp junction is made through the mechanical bond, it is not similar to a wood / glue bond. The physical strength properties of the resulting improved yield pulp gave an unexpected result. The measurements show the improved performance pulp of lignin so that it has strength properties • 10 effectively equivalent in the control pulp, to which no lignin is added (some initial Kappa number). Pulps produced at different Kappa numbers (ie the lower Kappa pulp increased their Kappa number by lignin precipitation) showed improved physical properties for improved performance pulp. In short, an acid such as CO2, SO2 or sulfuric acid, is injected into a stream of dilute lignin, such as the dilution of • water, during the washing stage under the mother pulp in the composition of fresh water in the washing system. The pH of the The diluted lignin stream is reduced to a level sufficient to cause the precipitation of lignin in the pulp fiber and to increase the Kappa number to at least about 1 point. For the purposes of this specification, the increase in the Kappa number is measured compared to the test pulp taken from an untreated pulp stream with acid in the same place of washing. A sufficient lignin flush to increase the Kappa number by point 1 provides a commercially important improvement in pulp production.

Advantageously, the precipitation of lignin increases the number 5 Kappa by about 2.5 to about 50 points and more advantageously by about 5 to about 30 points providing a dramatic increase in pulp yield. The additional acid can be added to the stream of the fresh water composition in order to cause the precipitation of lignin • 10 enough. Initial grinding tests indicate that an addition of 10 to 20 kilograms of carbon dioxide per tonne of pulp dried by air achieves an increase of 1.5% to 3% yield. By adding sufficient acid, the required amount of lignin is removed from the solution and precipitated into the pulp to improve the pulp yield in the grind tests by between 3 to 4%, but not so much to cause the formation of cakes or obstructions in the pipe or in the washing machine. • Although vacuum drum washers are preferred, the process of the present invention is carried out in other types or washers, including, but not limited to, diffusion washers, pressure washers, presses, and band washers. In fact, the process of the present invention can be used in washing lines using any combination of washing equipment, for example, a diffusion washer followed by a washing machine. single stage vacuum drum The process of the present invention is available in single or multi-stage coffee raw material pavers. The process of the present invention is available for all wood species, including, but not limited to, hardwoods, soft woods and eucalyptus. Although wood is the preferred raw material, any raw material that can form pulp by the Kraft process can be used. Examples of non-wood materials that can benefit from the present invention are bagasse and sugarcane. It should be understood that the foregoing description is only illustrative of the invention. Various alternatives and modifications can be made by those skilled in the art without departing from the invention. Accordingly, the present invention is intended to encompass all alternatives, modifications and variations that fall within the scope of the appended claims.

Claims (10)

1. A method to process alkaline cellulose pulp to cause the precipitation of lignin in the pulp fibers, which • comprises the steps of: 5 (a) washing the pulp in a series of washing steps to sequentially remove the lignin introduced therein; (b) between each washing step, add the dilution of water to repulp a pulp mat that exists from the previous washing step and to create the pulp stream for the next 10 washing stage, the pulp stream containing the lignin • entrained, (c) after at least one of the washing steps, add an acidifying agent in the pulp stream to form a pulp product by precipitating the introduced lignin 15 in the cellulose fibers contained in the pulp stream, and (d) removing the pulp product from the washing step sequence with the pulp product having at least one • increase of 1 unit in Kappa number, the increase in Kappa number rising from the precipitation of lignin

Introduced 2 The method according to claim 1, wherein the acidifying agent is added to the pulp stream having a concentration of the lignin introduced on a scale from about 02 to about 5 grams per liter.

3. The method of acBfe with claim 1, wherein a sufficient quantity of the introduced lignin is precipitated in the pulp stream to increase the Kappa number of the pulp product from about 2.5 to about 5 units.

4. The method according to claim 1, wherein the acidifying agent is carbon dioxide. The method according to claim 1, wherein adding the acidification agent precipitates the introduced lignin after at least two washing steps and increases the Kappa number by at least about one after at least every two stages of washing. 6. The apparatus for processing the alkaline cellulose pulp to cause the precipitation of lignin in the pulp fibers, including a series of washing steps to wash the pulp in a series of washing steps to sequentially remove the introduced lignin therefrom, the apparatus comprising: first means placed between each washing step to add the dilution water in order to repulp a pulp mat coming out of a previous washing step and to create a pulp stream for a next washing step, the pulp stream containing the introduced lignin; and second media placed after at least one of the washing steps by adding an acidifying agent in the pulp stream to form a pulp product by precipitation of the lignin introduced into the cellulosic fibers in the pulp stream, and leaving The pulp product is removed from the series of washing stages containing at least approximately 1 unit increase in the Kappa number, the • Increase in the Kappa number rises from the precipitation of the introduced lignin. The apparatus according to claim 6, wherein the acidifying agent is carbon dioxide. The apparatus according to claim 7, wherein the second means precipitate a sufficient amount of the lignin 10 introduced into the pulp stream to increase the number • Kappa pulp product from approximately 5 to around 30 units. The apparatus according to claim 7, wherein the second means add acidifying agent to the stream 15 of pulp having a concentration of lignin introduced on a scale of about 0.5 to about 2 grams per liter. 10. The apparatus according to claim 6, wherein • the second media add the acidification agent for the precipitation of the introduced lignin after at least two 20 washing steps and e increase the Kappa number by at least about 1 after at least every two washing steps. % ^ 2, * * \