Process for bleaching of lignocellulose-containing material

Abstract: The present invention relates to a process for delignification and bleaching of chemically digested lignocellulose-containing pulp, where the pulp is treated with a complexing agent at a pH between 3.1 and 9.0, whereupon the pulp is bleached with ozone. The initial treatment with a complexing agent removes the ions of certain metals detrimental to the subsequent ozone bleaching, while retaining in the pulp the desirable ions, primarily of alkaline earth metals. Thereby, the selectivity in the delignification is increased and the strength of the pulp maintained. The pulp can be bleached with peroxide before the ozone step and/or after the treatment according to the invention, to obtain the desired final brightness and completely avoid formation and discharge of chlorinated organic compounds.

Description: The present invention relates to a process for delignification and bleaching of chemically digested lignocellulose-containing pulp, where the pulp is treated with a complexing agent at a pH between 3.1 and 9.0, whereupon the pulp is bleached with ozone. The initial treatment with a complexing agent removes the ions of certain metals detrimental to the subsequent ozone bleaching while retaining in the pulp the desirable ions, primarily of alkaline earth metals. Thereby, the selectivity in the delignification is increased. The pulp is bleached with peroxide before and after the ozone step, to obtain the desired final brightness and completely avoid formation and discharge of chlorinated organic compounds. Background In the production of chemical pulp of high brightness, wood chips are first cooked to separate the cellulose fibres. Part of the lignin holding the fibres together is thus degraded and modified, such that it can be removed by subsequent washing. However, in order to obtain sufficient brightness, more lignin has to be removed, together with brightness-impairing (chromophoric) groups. This is frequently effected by delignification with oxygen, followed by bleaching in several stages. For environmental reasons, it has become increasingly common to treat chemical pulp with chlorine-free bleaching agents as early as in the first bleaching steps, thereby drastically reducing the discharges of chlorinated organic compounds detrimental to the environment. Ozone is a very suitable bleaching agent from an environmental point of view. Furthermore, ozone is very effective when attacking the lignin but also when attacking the cellulose chains in the pulp. Thus, the pulp obtained has an extremely high brightness with but a small charge of ozone, but the inadequate selectivity in the delignification brings about a pulp of insufficient strength. It is known to use chlorine-free bleaching agents, such as hydrogen peroxide and ozone, as early as in the prebleaching. However, the delignification and the consumption of the bleaching agent become less effective than with chlorine-containing bleaching agents, unless the pulp is pretreated. Thus, an ozone treatment is disturbed by the presence in the pulp of ions of certain metals, such as Mn, Cu and Fe. These metal ions cause disintegration of the ozone and/or degradation products, which tend to considerably reduce the strength properties of the pulp, such as the viscosity. This can be counteracted by pretreating the pulp at a low pH by means of a so-called acid wash, e.g. according to Germgård et al, Svensk Papperstidning, 88(15), R127-132 (1985). The pulp may also be treated at a low pH directly in the first step of the bleaching sequence, by bleaching with chlorine-containing chemicals, such as chlorine dioxide, e.g. according to US-A-4,959,124. Such treatment reduces the concentrations of all types of metal ions. The European patent publication EP-A-0 402 335, which is our own application, relates to a two-step process wherein the pulp is treated with a complexing agent and subsequently bleached with a peroxide-containing substance. The only bleaching agents suggested in addition to peroxide are chlorine and chlorine dioxide. EP-A-0 402 335 does not contain any statements concerning ozone bleaching and thus, there is no indication in said published publication that ozone would be a suitable bleaching agent in a bleaching stage immediately following the initial peroxide stage. The US-A-4 372 812 relates to the general problem of formation and discharge of chlorinated organic compounds when bleaching lignocellulosic pulps. Said US patent relates especially to the use of exclusively chlorine-free bleaching agents for bleaching pulps to brightness above 80% G.E.. From table II of said publication it can be seen that with use of the bleaching steps illustrated in the said table, the viscosity of the pulp is considerably reduced as compared to the starting Kraft Pulp of Northern hardwood. By means of the process according to the present invention the viscosity values of the bleached pulps are considerably less reduced while there is a large increase in the last step of the brightness values. The invention The invention provides a process in which lignocellulose-containing pulp is treated under the conditions disclosed in the claims, whereby the initial treatment with a complexing agent effectively removes the metal ions detrimental to the subsequent ozone bleaching, while retaining in the pulp the desirable metal ions. By this process, the lignin in the pulp is attacked more selectively in the subsequent ozone bleaching. The invention concerns a process for delignification and bleaching of chemically digested lignocellulose-containing pulp, wherein the trace-metal profile of the pulp is altered by treatment with a complexing agent at a pH in the range from 3.1 up to 9.0, whereupon the pulp is washed to remove non-desirable complexed metal ions, then is subjected to a bleaching step with a peroxide-containing compound at a pH in the range from about 8 up to about 12, characterised in that after the peroxide bleaching step the pulp is bleached with ozone and finally is bleached with a peroxide-containing compound in alkaline solution. The main difference between prior-art ozone techniques and the invention is that the present process makes possible a maintained pulp strength, e.g. measured as viscosity, while a high degree of brightness is obtained. It has thus been found that the treatment with a complexing agent at an almost neutral pH instead of a highly acidic treatment with acid wash or chlorine-containing bleaching steps, entails that certain desirable ions in the pulp are maintained, both as to concentration and position. These ions, primarily of alkaline earth metals, such as magnesium and calcium, slow down the attack by ozone and its degradation products on the cellulose chains in particular. Thus, the selectivity in the delignification is increased and the shortening of the cellulose chains is counteracted, the latter giving a strong pulp. Despite the slowing-down of the ozone attack, use of the present process means a rapid bleaching course, since the ozone in itself is one of the most energetic bleaching chemicals hitherto known. In the process according to the invention, the treatment with a complexing agent is carried out at a pH of from 3.1 up to 9.0, suitably from 4 up to 8, preferably from 5 up to 7. The ozone bleaching is carried out at a pH in the range from about 1 up to about 8, suitably in the range from 1 up to 4. In a process according to the invention where the initial bleaching with ozone is followed by a bleaching step with a peroxide-containing compound, the pH suitably is kept in the range from 5 up to 7 in the treatment with a complexing agent. In the bleaching with ozone the pH is suitably retained within the range from 5 up to 7, to maintain the optimal trace-metal profile for the subsequent treatment with a peroxide-containing compound. The treatment with a peroxide-containing compound takes place within the range from 8 up to 12. The treatment according to the invention is carried out with a bleaching step with a peroxide-containing compound before the ozone bleaching of the pulp. It has been found that the detrimental effect of the ozone on the viscosity of the pulp, is considerably reduced if the ozone step is preceded by a peroxide step. In addition, the brightness of the pulp is further improved. Peroxide-containing compounds relate to inorganic peroxide compounds, such as hydrogen peroxide and sodium peroxide, and organic peroxide compounds, such as peracetic acid, separately or in optional mixtures. The effect of the peroxide-containing compound may also be reinforced by the presence of oxygen. Preferably, use is made of hydrogen peroxide or a mixture of hydrogen peroxide and oxygen. If the peroxide-containing compound is hydrogen peroxide, the pulp is suitably bleached at a pH of from about 8 up to about 12, preferably at a pH of from 10 up to 12. Treatment with the other peroxide-containing compounds mentioned above, is carried out within the normal pH range for each bleaching agent, which are well-known to the person skilled in the art. The complexing agent used is primarily chosen from nitrogenous polycarboxylic acids, suitably diethylenetriaminepentaacetic acid (DTPA), ethylenediaminetetraacetic acid (EDTA) or nitrilotriacetic acid (NTA), preferably DTPA or EDTA, polycarboxylic acids, suitably oxalic acid, citric acid or tartaric acid, or phosphonic acids, suitably diethylenetriaminepentaphosphonic acid. The treatment according to the invention is carried out with a washing step after the treatment with a complexing agent, such that the non-desirable complexed ions of certain metals are removed as completely as possible from the pulp suspension prior to the treatment with ozone or, optionally, a peroxide-containing compound. After the treatment according to the invention, the pulp can be dewatered and the spent liquor be recycled in order to lower the pulp concentration before the ozone step. Also, the pulp can be washed with water after the ozone step, and this washing water can also be recycled to a position before the ozone step. The treatment with a complexing agent, a peroxide and ozone can be carried out either immediately after digestion of the pulp or after an oxygen step. The process according to the invention is preferably applied to pulp that has been delignified in an oxygen step prior to the treatment. Lignocellulose-containing pulps relate to chemical pulps of softwood and/or hardwood digested according to the sulphite, sulphate, soda or organosolv process, or modifications and/or combinations thereof. Use is suitably made of softwood and/or hardwood digested according to the sulphate process, preferably sulphate pulp of hardwood. The treatment according to the invention can be applied to lignocellulose-containing pulps having an initial kappa number within the range from about 5 up to about 40, suitably from 7 up to 32, preferably from 10 up to 20. Here, the kappa number is determined according to the standard method SCAN-C 1:77. The amount of complexing agent (100% product) charged, lies in the range from about 0.1 up to about 10 kg/ton of dry pulp, suitably in the range from 0.5 up to 5 kg/ton of dry pulp and preferably in the range from 1 up to 2.5 kg/ton of dry pulp. The amount of ozone charged, lies in the range from about 0.1 up to about 20 kg/ton of dry pulp, suitably in the range from 0.5 up to 10 kg/ton of dry pulp and preferably in the range from 2 up to 10 kg/ton of dry pulp. In preferred embodiments employing hydrogen peroxide as the peroxide-containing compound before the ozone step, the amount of hydrogen peroxide, lies in the range from about 0.5 up to about 50 kg/ton of dry pulp, calculated as 100% hydrogen peroxide. The upper limit is not critical, but has been set for reasons of economy. The amount of hydrogen peroxide suitably lies in the range from about 2 up to about 50 kg/ton of dry pulp and preferably from 3 up to 35 kg/ton of dry pulp, calculated as 100% hydrogen peroxide. It is especially preferred with an amount of hydrogen peroxide within the range from 4 up to 25 kg/ton of dry pulp, calculated as 100% hydrogen peroxide. Hydrogen peroxide is also used after the ozone step, at which the charges of hydrogen peroxide in both steps are adapted to give the desired brightness of the pulp. In the process according to the invention, the treatment with a complexing agent is carried out at a temperature of from about 10 up to about 100°C, suitably from 26 up to 95°C, preferably from 40 up to 90°C, and for a period of time of from about 1 up to about 360 min, preferably from 5 up to 60 min. The ozone bleaching is carried out at a temperature of from about 10 up to about 100°C, preferably from 25 up to 90°C, and for a total residence time of from about 1 up to about 120 min, preferably from 10 up to 60 min. The time of contact between ozone and the pulp influences the brightness as well as viscosity and is among other things dependent on the pulp concentration. Thus, a contact time of from about 1 up to about 2 min is suitable at a pulp concentration of about 35 percent by weight. The contact time is suitably from about 10 up to about 30 seconds at a pulp concentration of about 10 percent by weight. In the treatment with a complexing agent and in the ozone bleaching, the pulp concentration may be from about 1 up to about 40% by weight, suitably from 3 up to 35% by weight, preferably from 5 up to 15% by weight. In the treatment with a complexing agent and the ozone bleaching, the pH value can be adjusted by means of sulphuric acid or residual acid from a chlorine dioxide reactor. In the ozone bleaching, the pH value can also be adjusted by recycling acidic spent bleach liquor from the ozone step. In the peroxide treatment in alkaline environment, the pH is suitably adjusted by adding, to the pulp, an alkali or an alkali-containing liquid, e.g. sodium carbonate, sodium hydrogen carbonate, sodium hydroxide, oxidized white liquor or magnesium hydroxide slurry. Suitably, the magnesium hydroxide slurry is taken from the chemical handling system in the production of sulphite pulp with magnesium as base, so-called magnefite pulp. In bleaching with hydrogen peroxide as the peroxide-containing compound before and after the ozone step, the pulp is bleached at a temperature of from about 30 up to about 100°C, preferably from 60 up to 90°C, and for a period of time of from about 30 up to about 300 min, suitably from 60 up to 240 min. The pulp concentration may be from about 3 up to about 35% by weight, preferably from 10 up to 25% by weight. Treatment with the other peroxide-containing compounds mentioned above, is carried out within the normal ranges of temperature, time and pulp concentration for each bleaching agent, which are well-known to the person skilled in the art. The pulp is finally bleached to the desired higher brightness, by treatment in one or more steps with chlorine-free bleaching and extraction agents. In this way, the formation and discharge of chlorinated organic compounds are completely eliminated. The final bleaching is carried out with a peroxide-containing compound in alkaline solution in one or more steps, optionally reinforced with oxygen. By the treatment according to the invention, the lignin content has been reduced to a sufficiently low level before any chlorine-containing bleaching agents are used. Therefore, chlorine dioxide and/or hypochlorite may well be used in one or more final bleaching steps without causing formation of large amounts of chlorinated organic compounds. In a bleaching process for chemical pulps, the aim is a high brightness, a low kappa number and a sufficiently high viscosity, the latter meaning that the strength of the pulp will correspond to the demands of the market. When use is made of the process according to the invention, the strength of the pulp, measured as viscosity, is higher than in prior-art techniques for ozone bleaching. This means that the pulp contains such long cellulose chains, that a sufficiently strong product can be obtained. Furthermore, the resulting pulp has a higher brightness and a lower kappa number, than pulp from processes in which the trace-metal profile has not been adjusted before the ozone treatment or at a pH outside the range of the present process. Thus, as is apparent from Example 1, use of the present process in the sequence Step1 - P₁ - Z - P₂ makes it possible to obtain a sulphate pulp of hardwood with a final brightness of more than 89% ISO at a viscosity exceeding 800 dm³/kg. Here, only chlorine-free bleaching agents has been used in each bleaching step, which means that this process is superior to prior-art bleaching techniques from an environmental point of view. The invention and its advantages are illustrated in more detail by the Examples below which, however, are only intended to illustrate the invention and are not intended to limit the same. The kappa number, viscosity and brightness of the pulp were determined according to SCAN standard methods. The percentages and parts stated in the description, claims and examples, refer to percent by weight and parts by weight, respectively, unless anything else is stated. Example 1 Oxygen-delignified sulphate pulp of birchwood having a kappa number of 13.0, a brightness of 47.1% ISO and a viscosity of 1120 dm³/kg was treated according to the invention in the sequence Step1 - P₁ - Z - P₂, in which P₂ represents final bleaching with hydrogen peroxide. The conditions in Step1 and Z, and P₁, in which Step1 represents a complexing agent, P₁ represents hydrogen peroxide, and Z represents ozone (O₃), were the following: In the treatment with a complexing agent, 2 kg EDTA/ton of dry pulp was charged, the temperature was 70°C, the pH was 6, the residence time was 60 min, and the pulp concentration was 10% by weight. The pulp was washed after Step1, whereupon it was bleached in step P₁ with varying amounts of between 15 and 30 kg hydrogen peroxide/ton of dry pulp at a pH of 11.0, a temperature of 90°C and for 240 min, the pulp concentration being 10% by weight. After washing, the pulp was treated with 5 kg ozone/ton of dry pulp, at a pH of 2.0, a temperature of 25°C, the pulp concentration being 30% by weight. The total residence time, including subsequent washing, was 30 min in the treatment with ozone. The contact time between ozone and the pulp was about 1 min. The pulp was finally bleached (P₂) with 5 kg hydrogen peroxide/ton of dry pulp at a pH of 10.8, a temperature of 60°C and for 75 min, the pulp concentration being 10% by weight. The results after each step appear from the Table below. Amount of H₂O₂ charged in P₁ (kg/ton of pulp) Step Kappa No. Viscosity (dm³/kg) Brightness (% ISO) 15 P₁ 8.5 1010 76.0 Z 3.3 885 82.3 P₂ 1.4 900 86.0 20 P₁ 7.9 995 79.4 Z 3.1 870 84.6 P₂ 1.3 890 87.6 30 P₁ 7.7 955 82.2 Z 2.8 850 86.0 P₂ 1.1 850 89.1 As is apparent from the Table, a sulphate pulp of birchwood can be finally bleached to full brightness by a charge of hydrogen peroxide of 30 kg/ton of dry pulp in the P₁ step. This is possible, while at the same time diminishing the viscosity reduction normally obtained in bleaching steps involving ozone and eliminating the viscosity reduction normally obtained in final bleaching steps involving hydrogen peroxide. Example 2 Oxygen-delignified sulphate pulp of softwood having a kappa number of 14.0, a brightness of 37% ISO and a viscosity of 1040 dm³/kg was treated in the sequences Step1 - Z - P₁, Step1 - P₁ - Z and, according to the invention, Step1 - P₁ - Z - P₂. In the treatment with a complexing agent (Step1), 2 kg EDTA/ton of dry pulp was charged, at a temperature of 60°C, a pH of about 6, a pulp concentration of 10% by weight and a residence time of 30 min. In the ozone step (Z) 3.5 kg of ozone/ton of dry pulp was charged, at a temperature of 25°C, a pH of about 2, a pulp concentration of about 35% by weight and a contact time between ozone and the pulp of about 1 min. In the first step with hydrogen peroxide (P₁), 20 kg of hydrogen peroxide/ton of dry pulp was charged at a temperature of 90°C, a pH of 11, a pulp concentration of 10% by weight and a residence time of 240 min. In the second step with hydrogen peroxide (P₂), 5 kg of hydrogen peroxide/ton of dry pulp was charged at a temperature of 70°C, a pH of 11, a pulp concentration of 10% by weight and a residence time of 60 min. For comparative purposes, the same pulp was also treated in the sequence Step1 - Z - P₁ without using EDTA in Step1, to show the influence of a complexing agent before the ozone step. The results after each sequence appear from the Table below. Sequence pH in Step1 Kappa No. Viscosity (dm³/kg) Brightness (% ISO) Step1-P 6.2 7.4 932 71.6 Step1-Z-P 2.0 5.8 811 60.5 Step1-Z-P 4.8 4.8 814 73.3 Step1-P-Z 6.2 3.9 801 77.3 Step1-P-Z-P 6.2 1.7 828 82.1 Step1-Z-P 5.7 6.9 743 53.1 As is apparent from the Table, a treatment of the pulp with a complexing agent and ozone according to the present process in sequences where hydrogen peroxide is used before and after the ozone step, gives very good results as regards kappa number, viscosity and brightness.

Claims: A process for delignification and bleaching of chemically digested lignocellulose-containing pulp, wherein the trace-metal profile of the pulp is altered by treatment with a complexing agent at a pH in the range from 3.1 up to 9.0, whereupon the pulp is washed to remove non-desirable complexed metal ions, then is subjected to a bleaching step with a peroxide-containing compound at a pH in the range from about 8 up to about 12, characterised in that after the peroxide bleaching step the pulp is bleached with ozone and finally is bleached with a peroxide-containing compound in alkaline solution. A process according to claim 1, characterised in that the treatment with a complexing agent is carried out at a pH in the range from 4 up to 8. A process according to claim 2, characterised in that the treatment with a complexing agent is carried out at a pH in the range from 5 up to 7. A process according to claim 1, characterised in that the complexing agent consists of nitrogenous polycarboxylic acids. A process according to claim 1, characterised in that the peroxide-containing compound consists of hydrogen peroxide or a mixture of hydrogen peroxide and oxygen. A process according to claim 5, characterised in that the bleaching with a peroxide-containing compound is carried out at a pH in the range from 10 up to 12. A process according to claim 1, characterised in that the pulp is a sulphate pulp, preferably of hardwood. A process according to claim 1, characterised in that the amount of ozone added in the ozone bleaching, lies in the range from about 0.1 up to about 20 kg/ton, preferably 0.5 to 10 kg/ton of dry pulp. A process according to claim 1, characterised in that the ozone bleaching is carried out at a pH in the range from about 1 up to about 8, preferably 1 up to 4.

Inventors: ANDERSSON LENNART, BASTA JIRI, HOEOEK JAN, HOLTINGER LILLEMOR, ANDERSSON, LENNART, BASTA, JIRI, HOEOEK, JAN, HOLTINGER, LILLEMOR, Höök, Jan
Applicants: EKA NOBEL AB
Application year: 1992
Main classification: D21C9
IPC codes: D21C9