MXPA96002927A - Procedure for whitening pulp paste kr - Google Patents

Abstract

A method for bleaching pulp containing lignocellulose that has been chemically formed in a pulp is described, the method comprising the steps of sequentially treating the pulp with a reducing people, a chelating agent and an oxidizing agent.

Description

PROCEDURE FOR WHITENING KRAFT PULP PASTA FIELD OF THE INVENTION The invention relates to a process in which a Kraft pulp is bleached first with a chemical bleaching agent, reducing agent such as sodium hydrosulfite, and is treated with a chemical oxidizing bleaching agent such as CO 2. hydrogen peroxide. The combination of a reducing treatment step followed by an oxidative treatment step imparts special properties to the drug that are not available with each treatment alone or with treatments in the reverse order.

BACKGROUND OF THE INVENTION The paper and pulp industry faces many challenges in the modern world. One of these is the development of new strategies to whiten pulp that are truly benign. The industry has made significant gains in this area especially in the use of clerks and chlorine-containing compounds. It is generally accepted that molecular chlorine is replaced, for example, by roro nitrogen. However, chlorine dioxide also has its known risks and therefore much attention has been paid to alternative chemicals that do not contain chlorine. The Examples of these include hydrogen peroxide, oxygen, o.io and percents. Most of these are either in current use or have been demonstrated at the level of a pilot plant or higher. All these chemical agents act as oxidizing agents. There is another class of chemical agents, the reducing agents, which have received very little attention compared with oxidants. An example of a reducing bleaching agent is sodium hydrosulfite. Comparatively little work has been done to investigate its use in pulps for bleaching with chemical agents. The use of sodium hydrosulfite to whiten mechanical pulps is widely known. Hyryrosulfite in various forms has been used in this application, since the 40s. Sodium hydrosulphite combinations with other chemical bleaching agents are also known. See, for example, U.S. Patent 4, ß04,440; Furopeo case EP 0 1 &7 k77 Ai; and PCT case WO 90/11403. In an article by Barbe, et al., 1989 TflPPI PULPING CONFERENCE PRQCEEDINGS. 627-640, the use of hydrocosulfite and peroxide in various combinations for bleaching mechanical puffs and qui io gases is discussed. The use of a reducing agent is described before a multi-step peroxide treatment. Liebergott, and others, TflPPI JournaL. &7 (&): 76- &Q1964) and 1983 TflPPI PULPING CONFERENCE PROCEEDINGS. 323-332, describes the use of sodium hydrosulphite as a final bleaching step after the use of oxidants to brighten Kraft puffs. In WO 94/12721 and WO 94/12722 PCT a process is described for delignifying the pulp containing lignocejuosa, with a peracid followed by a bleaching step in which the bleaching agent without chlorine, such as dithionite Sodium can be used. The conditions for s ».j use are not defined. In the case WO 92/07994 PCT a procedure is described in which the wood becomes pupa and is bleached e »? the same reactor displacing the various reactive solutions. This is a variation of U.S. Patent 4,576,149 which discusses oxidative methods. The hydrosulfite is included in the discussion. However, no details are given as to the particular conditions necessary to use it or other reducing agents. In Canadian Patent 1,154,205 a method is described for the treatment of Kraft pulp together with a first stage of ozone bleaching. The European case EP 0 514 901 A2 (also US Pat. No. 5,211,609 and other references listed therein) describes a process for washing the strand of secondary fibers using oxygen in combination with peroxide, ozone and / or hydrosulfite. A final stage of hidrssuJ f ito. The European case EP 0 606 667 A1 describes a process for increasing the whiteness of bleached cellulose without clack, with an aqueous solution of a reducing agent in the presence of chelating agents. An article by Ducharme, and others, 1993 TflPPI PULPING CQNF? ENGE PRQC ?? PI ßS, 777-796 describe the optimal conditions for the use of hydrosulphite co or the final bleaching stage for Kraft puPs. WO 92/07139 PCT describes a three-stage pro-bed for use in Kraft bags, in which a step of reducing whiteness is used between two oxidizing stages, the first being acid and the second alkaline. Due in part to the extreme variability found in blanquea bleach applications, the need still exists for alternative bleaching pro ducts, which do not involve chemical agents that have chlorine. Thus, it is an object of this invention to provide an efficient method for using a reducing agent to whiten the pulp. It is another object of this invention to provide a method that whiten the pupa with improved bleaching results. It is yet another object of this invention to define the particular conditions of temperature, retention time and pH of the pulp under which the polishing efficiency of reducing whitening chemical agents such as sodium hydrosulfite is increased. Fstos and Other objects of the invention will be apparent from the wish of the invention.

BRIEF DESCRIPTION OF THE INVENTION This invention provides a method for bleaching pills containing lignocejujuose with a three-step sequence reducing / chelating / oxidant. L? Reductive stage of the sequence is carried out before the oxidant stage. The oxidizing step can be done in one stage or in multiple stages. The selected temperature and pH conditions as well as the doses of bleach, bleach additives and chelating agents are also used. The effect of this combination of the reductive and oxidant stages is a pulp containing lignacellula a with improved properties that are not available with any single or ca. the combination d »to stages in an alternative order.

DETAILED DESCRIPTION OF THE INVENTION For the purposes of this invention, the pulps containing 1 ignocelulosa (hereinafter also referred to CO or "pulp") should refer to any fibrous material formed from the chemical digestion of wood or any other plant material.

The ignorant material for which this invention is applicable is derived preferably from wood of various species including varieties of hardwood and coniferous wood. The wood must have been chemically pulped by one of many procedures, including but not limited to sulphate, suffix, caustic soda, or organic solvent processes. Wood processed using the sulphate process (also called Kraft) is preferred. The wood treated with the Kraft and the variety of coniferous wood is especially preferred. The raft can also be processed with a stage that uses oxygen gas. The purpose of this stage is to reduce the Kappa number of the bid. The number Kappa is a qualitative measure of the lignin component of the Kraft pulp. It was measured by a method such as that given in the FrueHba TAPPI Method 236 cm-65, incorporated herein by reference in its entirety. The K.appa number of the pupa used in that procedure should be in the range of 5 to 100. Preferably, the pulp used in the process of this invention should be treated once with an oxygen delignification step. such that there is a Kappa number in the range of 5 to 20. The Kraft pulps treated in this oxygen stage are usually also treated ad icionally with chemical agents such as chlorine, chlorine dioxide, peroxide »> of hydrogen and sodium hydroxide in one or more stages Subsequent to whiten the pulp. The bleaching process has the objective of eliminating all or most of the residual lignin and eliminating the color of the puja. In the process of this invention, the K.raft pulp is first treated with a reducing agent. The reducing agent is selected from the group consisting of salts of the hydrosulfite ion (including the salts of s-hatred or zinc), sodium borohydride, sodium bisulfite formaldehyde, glyoxal sodium bisulfite, amidinsulfinic acid, dioxide Sulfur and sodium bisulfite. Preferably sodium hydrosulphite is used. The bid adjusts to a consistency of approximately 1 to approximately 20 percent. The pulp consistency is defined as the amount (dry weight in the oven) of solid material, mainly pulp, in a given mixture of solids and water. Although the puja consistency may be in the range of i to 20 percent e-n based on the solid material, the preferred range is from 3 to 15 percent and the most preferred range is from 3 to 5 percent. If it is not at the preferred temperature, the mixture of the pulp is heated to a temperature in the range of approximately 40 ° C to 100 °, but preferably in the range of 65 ° C to 90 ° C. The heating is critical to the procedure. The pulp should be at the desired temperature before the network agent »Jctor is added. In a continuous pulp production process, the steam can be added to the pulp stream between the steps to bring the pulp to the pulp. temperature. Secondarily, the pH of the pulp is adjusted before the addition of the reducing agent. The pH adjustment of the pulp before the addition of the reducing agent is critical and can be in the range of 4.5 to 10.0 with a preferred range that is 6.0 to 7. 5. Then the reducing agent is added; and the mixture is kept at the temperature for a period of time. The dose of the added reducing agent, for example, sodium hydrasulfite, is based on the weight of the solid material present in the solid / water mixture. Therefore, a dose quoted as 1 percent is 1 gram of solid sodium hydrosulphite per 100 grams of pulp &each in the oven. The reducing agent, for example sodium hydrosulfite, may be added in an amount of about 0.1 to 3 percent based on the dry weight in the pulp furnace, preferably in an amount of 0.5 to 2.0 percent, and greater preference in an amount between 1 and 2 percent. As will be appreciated by those skilled in the art, the addition of a reducing agent such as sodium hydrosulfite, preferably must be done in an atmosphere that is substantially free of oxygen. A suitable point of addition of the reducing agent, for example, is the suction side of a pump that carries the pump through the process. The right types of sodium hydrosulfite that can be used include »! sodium hydrosulphite mixtures sold commercially under the trademarks V- BRITER and VIRWITER from Hoechst Celapese Corporation, Samerville, New Jersey. The retention time for the treatment with the reducing agent is a little dependent on the chosen temperature; In this way, the slight decrease in operation at lower temperatures can S G formed retain the Kraft pulp for longer periods of time. The retention time can vary between 10 and 360 minutes. With a temperature at the lower end of the preferred range, such as 65 ° C, the preferred retention time range is J20-160 minutes. At 90 ° C the time is shortened to 60-120 minutes. Next, the pulp is washed. It is important that the pulp be thoroughly washed between the chemical treatment stages. The purpose of the washing step is to remove undesirable chemical components such as dissolved lignin or other argonic material. Such polluting material can consume and / or interfere with the chemical agents added in subsequent treatment stages or it can be redeposited on the fibers of the pulp and become difficult to eliminate in subsequent stages of the treatment. Washing refers to a process where the liquid phase of the pulp / water suspension is displaced by means of suction or pressing. The washing liquid can be tap water or water provided by some other stage in the pulp treatment process while not Contains materials that may be harmful to the SE-T stage of subsequent treatment. Usually the procedure proceeds by adding water to the pulp / water suspension with good mixing followed by a redrix of the amount of water, now in the pulp / water suspension by filtration or pressing through a sieve device. The water eliminated orn this form is either sent to the waste or is reused in a suitable location in the treatment process of 1 to pulp. The thickened pulp is sent to a treatment stage s »jhsig» jie e. After the step of the reducing treatment and after the washing step, the pulp is subjected to a chelating treatment to eliminate the undesirable metal ions which will interfere with the acceptable functioning of any chemical oxidizing bleaching agent. The treatment should be designed specifically to remove undesirable metal ions such as manganese but from intact desirable metal ions such as magnesium and calcium. For this purpose, a chelating agent is used which is selected from the group consisting of nitrogen-containing polycarboxylic acids, nitrogen-containing polyphosphonic acids or polya-nitrogenous alcohols. Examples of nitrogen-containing polycarboxylic acids include diethylenetriaminepentacetic acids (DTPA), ethylenediaminetetraacetic acid (EDTA) or nitric acid lat. (NTA). Preferably, DTPA or EDT are used. Examples of nitrogenous polyphosphonic acids include dietary acid. riamino- pentamet ilphosphonic (DTPMA). Other composite laughs, such as sa < These are known to those skilled in the art, can be used without significantly detracting from the spirit and scope of the invention. The conditions under which the chelator treatment is carried out greatly affect the efficiency of the metal ion removal and therefore the efficiency of the subsequent oxidation treatment steps. After the washing step, the pH of the pulp is adjusted to be in the range of 2.5 to 11.0, but preferably in the range of 4.5 to 9.0 as noted in the following. The amount of added chemical chelator should be in the range of 0.01 to 1.0 percent based on the dry weight in the pulp furnace with a preferred range that is 0.1 to 0.5 percent. The temperature during the treatment may be in the range of 20 ° C to 100 ° C, but is preferably selected to be between 55 ° C and 90 ° C. Although the pH of the pulp may be in the range of 2.5 to 11.0, the pH in the range of dc-be made to measure to realize the type and level of elimination of the desired metal ion. For example, to remove most manganese, but leave most of the magnesium and calcium, the preferred range is 4.5 to 9.0. A suitable retention time for this stage is 15 to 60 minutes. After the chelation stage, there must be an efficient washing step that will remove the metal ions attached in this way, so that they will not interfere with Jas Subsequent bleaching stages which will be oxidants by nature. This can be done by techniques known to those skilled in the art and which have been previously described. The third stage of the chemical treatment uses a chemical agent that reacts by means of an oxidizing mechanism. Oxidizing chemical agents useful in the practice of this invention include those selected from the group consisting of (a) inorganic peroxides such as hydrogen peroxide and sodium peroxide; (b) organic peracids and their salts, selected from the group consisting of aliphatic peracids of C ^ -Ca such as peracetic and perforic acid and aromatic peracids of CA-C; 3; (c) inorganic peracids and their salts, selected from the group consisting of peroxycarbonate, perborata and persulfates such peroxymonosulfuric acid bed (Caro's acid); and (d) compounds containing clear such chlorine bed, chlorine dioxide, sodium hypochlorite and hypochlorous acid. For salts, the cations are preferably sodium and potassium, especially sodium. Preferred peracids are peracetic acid and p roxomonosulfuric acid or their salts. Preferably, the peroxide will be hydrogen peroxide. Combinations of the above chemical agents can also be used such as oxygen with hydrogen peroxide. For certain of the oxidizing agents described above, the pH of the pulp must be adjusted as it will be.appreciated by those with skill in the art. For example, if hydrogen peroxide is selected as the oxidizing agent, then the following conditions should be used. The pH of the reaction should be in the range of 7-13 and, more preferably, be between 9.5 and 11.5. The temperature of the reaction should be between 40 ° C and 100 ° C, preferably between 75 ° C and 90 ° C. The pulp should be held at that temperature for 20 to 360 minutes, but preferably 90 to 120 minutes. . The consistency of the reaction mixture should be between 1 and 50 percent based on the dry weight in the pulp furnace in the pulp / water suspension, but preferably between 10 and 20 percent. The amount of hydrogen peroxide present should be between 0.1 and 5 percent based on the dry weight in the pulp furnace, but preferably between 1 and 3 percent. An additional amount of the chelate material, such as those described in the preceding paragraphs, may also be added at the same time as the hydrogen peroxide. The amount of this addition can be between 0.01 and 1 percent e »? base to the sack weight in the pulp oven, but preferably it is selected to be between 0.1 and 0.5 percent. An amount of a protective metal cation can also be added, especially if the preceding stage of cheating was not effective. Examples of protective cationic metals include calcium, but preferably magnesium. The amount of the magnesium ion to be added is preferably in the range of 0.01 and 0.1 per hundred. A common source of magnesium is the common Epsom salts or magnesium sulfate. For peroxides (such as hydrogen peroxide) to become chemically active, an alkali source must also be added. The alkali may be chosen from a group consisting of the alkali metal hydroxides and carbonates, for example, sodium hydroxide or potassium hydroxide. A mixture of one or more of the hydroxides and / or carbonates may also be used. The amount of the alkali metal hydroxide to be added to the bleaching step with hydrogen peroxide will, to a large extent, depend on the amount of hydrogen peroxide used, thus bed also the pH of the pulp, which is the same. »It enters the stage and therefore must be carefully optimized for each situation. For example, the alkali can be used in an amount between 0.05 and 4 percent based on the dry weight in the pulp furnace, but is preferably selected to be between 0.5 and 3.5 percent (if the amounts are used of peroxide described herein). The final treatment step should be a neutralization step, in which the remaining chemical agent or unreacted oxidation chemicals are reacted to remove it from the final pulp product. For hydrogen peroxide, this chemical neutralization agent can be chosen from the group consisting of sulfur dioxide, sodium sulfite, sodium bisulfite or sodium hydrosulfite or the alternative salts of any of these chemical agents. The Specific conditions and techniques for this stage are not critical and are generally known to those skilled in the art. The criteria for success at this stage will be the elimination of the majority of the chemical agent or residual oxidation agents, as determined by appropriate chemical testing, for example, titration of iodine iodine standard. . After the end of the treatment of the pulp with the process of the invention, the resulting pupa may be treated as an ap- stages blanqt-iead > to additional, directly used for the production of paper products, or thickened in such a way that it is cheaper to ship it to a location where the bid can be used in the production of paper products. The invention and these advantages over conventional treatment strategies, will be illustrated in more detail by the examples given in the following. In these examples, the brightness of the pulp are those measured on the sample sheets formed in accordance with Standard Method T 205 or -β of TAPPI which is incorporated in its entirety for reference in the present. Brilliance is measured according to Standard Method T 525 om-92 of the TAPPI, which is incorporated in its entirety for reference herein. The machine used to measure the diffuse brilliance was manufactured by Tech idyne Corporation and was a Micro TB-1C Technibrite model.

EXAMPLES The following examples are illustrative of the invention, but should not be considered as limitations of the invention. Note that the commonly recognized symbols will be used to represent the different stages of bleaching. These san: And for a sodium hydrosulfite stage, Q for the quilatation and P for the hydrogen peroxide stage. For example, in Example 1 the sequences YQP and QPY will be compared.

EXAMPLE 1 QPY Sequences The purpose of Examples 1 and ÍA is to show The advantage in using a sodium hydrosulfite treatment as the first stage of a three-stage bleaching sequence, in which the other two stages are a quilatation and a bleaching with hydrogenated peroxide. A kraft pulp of caniferous wood from a malignant in the North American West that has been ignifiable every ton oxygen to a Kappa number of 6.6 was used. The initial brightness of the pulp was 36.6 percent JSO.

Stage Q For the phase of quilatation, a portion of the pulp (approximately 60 grams (dry weight in the oven)) was taken and it was adjusted to 10 percent consistency with deionized water. The pulp is placed in a plastic beaker and heated in a microwave oven at 90 ° C. The pulp is then transferred rapidly to a Habart mixer and while mixing at low speed, the pH of the heated pulp is adjusted to 6.0 with dilute sulfuric acid. Once a pH of 6.0 is obtained, an amount of diethylene-penta-acetic acid (DTPA) equal to 0.5 percent is added based on the dry weight in the oven of the pulp used. DTPA was a commercially available product called Versenex ™ 60 produced by the Dow Chemical Company. After mixing the dietary chelate in the pulp for 2 minutes, the pulp was quickly transferred to a polyethylene plastic bag, sealed and placed in a hot water bath set at 90 ° C. The complete procedure of adjusting the pH and adding the chelating agent took no more than 5 minutes and the temperature of the pulp did not decrease significantly. The pulp is left to rest in the hot water bath for 60 minutes. After 60 minutes of retention, the pulp is removed from the plastic bag, placed in a beaker of 4 liter plastic precipitate and diluted to 2.5 percent consistency with deionized water. This is a washing stage. The diluted pulp is mixed with a propellant type mixer for 2 minutes then filtered on a 170 mesh screen with vacuum. The filtered pulp is removed from the calocad sieve again on the plastic precipitate of 4 liters, is rediluted to 2.5 liters. 100% consistency, mixed again for 2 minutes and finally filtered on a 170 mesh sieve. The final consistency of the filtered pulp was in the range of 20 to 25 percent. A portion of this pulp is taken for the measurement of brightness.

Stage P Next p < Jlpa is bleached with hydrogen. The pulp at a consistency of 20 to 25 percent is adjusted to a consistency of 12.5 percent by adding deionized water. This pulp is placed in a plastic bag, then placed in a second plastic bag and placed in a hot water bath for 60 minutes. After 60 minutes of the retention time, the temperature of the pulp was 90 ° C. The pulp is quickly transferred to a Hobart mixer and the juice starts at low speed. A solution of sodium hydroxide, magnesium sulphate and hydrogen peroxide is made in such a way that once the volume of this solution adds pulp consistency to the pulp, the consistency of the pulp is increased. it would be .10 percent. The amount of each added chemical agent was: 1.5 percent sodium hydroxide, 0.1 percent magnesium sulfate and 2.5 percent hydrogen peroxide, where all percentages are based on the dry weight in the pulp furnace. Hydrogen peroxide, sodium hydroxide and magnesium sulfate are reactive hardener chemicals, such as are commonly available from local supply of chemical products. Hydrogen peroxide was obtained as 50 percent by weight by weight of the solution. The solution of the chemical bleaching agents are added to the pulp as required »! mixed and the mixing is continued for 2 minutes. Then the pulp is quickly transferred to a plastic bag and play to a second plastic bag which is sealed. Then the pulp is placed in a hot water bath set at 90 ° C. The pulp was retained in the water bath for 120 minutes. Pe riodically, during retention, the pulp is removed and stirred or mixed for a short time. At the end of the 20 minutes, the pulp is removed from the plastic bags and washed as described in the previous paragraph. At the end of the second l, the pu pa was tested for the residual hydrogen peroxide by testing the ag > .? filtered with peroxide sensitive indicator strips (3.1 Baker Inc., catalog number 4416-01). No residual peroxide with a limit of detection of 1 ppm was found. A sample of this washed pulp is taken to measure the brilliance.

Stage Y The pulp is bleached with sadia hydrosulfite (V-BRITER, 6000, a mixture containing sodium hydrosulfite, available from Hoechst Celanese Corporation). The pulp is taken from the peroxide washing stage and diluted in polycarbonate plastic bottles to a consistency of 3.5 percent. The deionized water is used for the digestion. L bottles they are covered and placed in a hot water bath set at 90 ° C for 60 minutes. The bottles are placed in the water bath in staggered times to facilitate equal exposure to the hot water bath. After 60 minutes, a bottle is removed from the water bath and alternately the bottle and its contents are placed in vacuum and nitrogen gas, the air is purged from the pulp. After this, the bottle is placed on an apparatus which permits a propellant of the mixing blade type and a pH probe is placed inside the bottle. The bottle is capped and a stream of nitrogen is introduced. Mixing is started and after a short period of time dilute sulfuric acid or sodium hydroxide is added to adjust the pH of the tablet to 6.5. Immediately after this step, 2 percent sodium hydrosulphite is added based on the seo weight, in the pulp oven. The sodium hydrosulfite was in the form of a 2 percent by weight solution in deionized water. Then, add the sodium hydrosulfite, the pupa is mixed for an additional minute, then removed from the mixing apparatus and sealed. The covered bottle is placed in the hot water bath at 90 ° C for 120 minutes. After 120 minutes of retention time, the pulp is removed from the bottle and the bed is washed as previously described. A sample of this pulp E-e takes for the measurement of brightness. Using the techniques described above, the bleached pulp in the QPY sequence is made in sheets by hand TAPPI standard and brightness is measured with 62.6 percent ISO.

EXAMPLE ÍA YQP Sequence It is repeated from the process described in Example 1, except that the order of the steps was changed to YQP; that is, the treatment can be carried out first instead of last. In this way, a sample of Kraft coniferous wood pulp (approximately 60 grams (dry weight in the furnace)) from a North American West mill, which had si or delignified with oxygen to a Kappa number of 6.6, is diluted to a 3.5 percent consistency. The procedure described in Example 1 under Step Y is repeated. Next, a portion of the pracac-tiie pulp from stage Y is taken and adjusted to a 10 percent consistency with deionized water. The procedure described in Example i under Step Q is repeated. Next, the pulp is adjusted to a 12.5 percent consistency by adding deionized water and the procedure described in Example 1 is repeated under the Stage P- The resulting pulp gives this sequence had a brightness of 67.3 percent ISO, which is an improvement of 4.5 points when the treatment with sodium hydrosulfite was the initial treatment.

EXAMPLE 2 Sequence YP The need for the stage of elasticity was shown in this axis where the stage of quilatation was omitted. Here, the sequence YP (sodium hydrosulfite is followed by hydrogen peroxide) was carried out by repeating the procedure of Example IA, but omitting Stage Q. For the YP sa sequence it obtained a pulp with brightness of only 56.9 by hundred. This is 6.4 points less than the corresponding YQP sequence and even smaller than the reverse sequence QPY. In this way, the qualatation step before the oxidation blanching step is necessary.

EXAMPLE 3 Sequence QPY For this example, he used a kraft pulp of coniferous wood from a North American mill. This pulp was also delignified with oxygen and had an initial Kappa number of 14.6 and a brightness of 31.2 percent ISO. The pulp was treated with the QPY sequence using the method as described above in Example 1. The brilliance of the handmade leaf is measured using the TAPPI method described in the above. The brightness was 61.0 percent ISO.

EXAMPLE 3A The procedure described for Example J.A was repeated except that the pulp used was of the type described in. Example 3. The brightness was 67.0 percent ISO, 6 points higher than the pulp described in Example 3.

EXAMPLES 4 AND 4A Sequence QPY and Sequence YQP A sample of hardwood Kraft pulp was used from an eastern mill that had not been delignified with oxygen. This pulp has an initial ^ ppa number of 9.4 and an initial brightness of 37.1 percent TSO. The procedures described in Examples 1 and IA were repeated and the handmade sheets are made using the methods described in the above. The brightness is measured using the eti-las described in the above. BriJlantez values were 62.6 percent ISO for the sequence QPY and 70.5 percent ISO for the preferred sequence YQP - a difference of 7. 7 points.

EXAMPLES 5 AND SA The pulp of Examples 4 and 4A is bleached again but with different conditions for stages Q and P in an attempt to improve the response obtained from these steps.

EXAMPLE 5 The etada of Example 1 was followed by its animeme, in In the same way, except that the pulp was diluted to 2% 'of consistency, the pH was adjusted to 5.0, the temperature of the reaction was 50 degrees C and the chelating agent or used was the ethereal inert acid (EDTA). at a dose of 0.6-% 'based on the dry weight in the pulp furnace. The interetapa washing stage was the same. Stage P is changed in such a way that 0.05 & of magnesium sulfate and 0.2% DTPA based on the dry weight in a pellet furnace added to the hydrogen peroxide bleach solution. The temperature of stage P remains at 90 degrees C pe or the retention time is extended to 240 minutes. Using these changed conditions and following a treatment of stage Y as described in Example 1, the brightness of the final pulp obtained was 77.2 ISO. This brightness is 14.4 points higher than the pulp made in Example 4, indicating that the changes in the procedure resulted in a brighter pulp.

EXAMPLE 5A The method of Example IA was followed with the changes described in the above for Example 5. The final brightness obtained using this sequence of the bleaching steps was 79.6% ISO, which is 2.4 points higher than Jos; results achieved in Example 5. Thus, even when the process conditions of step Q and P are improved, there is still an advantage in using the YQP sequence when compared to the QPY sequence of the bleaching stages. The following Table I summarizes the data in the previous examples. All brightness units are in percent of ISO.

TABLE 1

Claims (3)

NOVELTY OF THE INVENTION CLAIMS

1. A method for bleachpulp containlignocellulose, the pulp which has been chemically pulped, in which the method is characterized because it comprises the steps of sequentially treatthe pulp with a reducagent, a chelatagent and an agent oxidant

2. The method accordto claim 1, characterized in that it comprises: < a) adjust the consistency of the pulp to be 1 - 20 percent; (b) heat the pulp to be at a temperature in the range of? +0 degrees C -100 degrees C; (c) adjust the pH of the pulp so that it is in the range of h .5 - 10.0; (d) .Add a reducagent in an amount of approximately 0.1-3 percent, based on the pulp kiln dry weight for a retention time of 10. 360 minutes; (e) washthe pulp; (f) adjust the pH of the pulp to be in the range of 2.5 - 11; (g) treatthe pulp with a chemical chelatagent in an amount of 0.01 - 1.0 percent by weight based on the dry weight in the pulp furnace, while keepthe pulp at a temperature of 20 degrees - 100 degrees C; (h) washthe pulp; < i > treat the pulp with a chemical oxidizagent; (j) neutralize the pulp to remove excess chemical oxidizagent.

3. The method accordto claim 1 or 2, characterized in that the reducagent is selected from the group consistof sodium hydrosulfite, zinc hydrosulfite, sodium borohydride, sodium bisulfite fmaldehyde, sodium bisulfite gliaxa.l, formamidine sulfinic acid, sulfur dioxide and bisulfite sodium. M-. The method accordto claim 1 or 2, characterized in that the reducagent is sodium hydrosulfite. 5. The method accordto the rei indication 1 or 2, characterized in that the pH of the pulp for the treatment with the chemical chelatagent, is adjusted to be in the range of .5 to 9.0. 6. The method accordto claim 5, characterized in that the chemical chelatagent is selected from the group consistof nitrogenated polycarboxylic acids, nitrogenated polyphosphonic acids, and trogen-free polyols. 7. The method accordto claim 6, characterized in that the chemical chelatagent is selected from the group consistof ethylenediaminetetraacetic acid, acetic acid diethylenetriabine acid, and diethylene glycine-pen ta et i 1 phosphonic acid. & . The method accordto claim 2, characterized in that in step (i) the chemical oxidizagent is selected from the group consistof: (a) inorganic peroxides; (b) organic peracids and their salts, selected from the group consistof aliphatic peracids of CA-C3 and aromatic processes of Ca.-C3; < c > inorganic peracids and their salts, selected from the group consistof peraxycarbonates, perborates and persulfates, in which the salts are formed with a cation selected from the group consistof sodium and potassium; and (d) chlorine-containcompounds selected from the group consistof clear, chlorine dioxide, sodium hypochlorite and hypochlorous acid. 9. The method accordto claim 5, characterized in that the inorganic peroxide is selected from the group consistof hydrogen peroxide and sodium peroxide. 10. The method accordto claim ß, characterized in that the organic acid is selected from the group consistof peracetic and perforic acids. 11. The method accordto claim ß, characterized park the inorganic peracid is acid perox onosul fúri o. 12. The method accordto claim 9, further characterized in that it comprises the step of addin the oxidation stage, 0.05 ~ 4 percent based on the dry weight in the pulp furnace, of an alkali selected from the group that It comprises sodium hydroxide, potassium thioxide, sodium carbonate and potassium carbonate. 13. The method of compliance with the rei indication 12, to use hydrogen peroxide co or the oxidant stage, characterized in that: (a) the consistency of the pulp in step (i) is from 10 to 20; (b) the temperature of the pulp in step (i) is 75 to 90 degrees C; (c) the oxidizing agent is hydrogen peroxide in an amount of 0.1 to 5.0 percent, based on the dry weight in the pulp furnace and includes the addition of sodium hydroxide in an amount of 0.5 to 3.5 percent by weight in it bases to the dry weight in furnace of the pulp, to produce a pulp with pH in the range of 9.5 to 11.5; and < d > The magnesium ion is added at a dose of 0.01 to 1 percent by weight based on the dry weight in the pulp oven during a retention period of 90-120 minutes.