TREATMENT OF PULP WITH CHELATING AGENT IN AT LEAST TWO MIXING STEPS WITHOUT INTERMEDIATE WASHING

Abstract: A method of chlorine-free bleaching of pulp where metals are removed by means of a chelating agent. The chelating agent is admixed to the pulp in at least two subsequent mixing steps without intermediate washing. The stay-time between the mixing steps shall be less than 15 minutes, and the pulp is washed after the last mixing step.

Description: TREATMENT OF PULP WITH CHELATING AGENT IN AT LEAST TWO MIXING STEPS WITHOUT INTERMEDIATE WASHING This invention relates to the bleaching of chemical pulp with chlor¬ ine-free bleaching chemicals. The invention, more precisely, refers to the treatment of pulp by chelating agents in connection with the bleaching. Totally chlorine-free bleaching (TCF) of chemical papermaking pulp has become a very great success, and the " making of chlorine-free pulp in the world will soon have reached five millions tons per year, which is a highly exceptional development considering that this bleaching method was introduced less than five years ago. Bleaching is carried out in most cases by first treating the paper- making pulp to be bleached with a chelating agent of the type EDTA or DTPA. The chelating agent has the object to remove from the pulp certain metal ions, particularly manganese, copper, iron a.o. which would affect the bleaching result negatively if they remain in the pulp. The chelating agent step ususlly is designated by the letter Q. After the Q-step the pulp is washed to eliminate the exposed metal ions from the pulp as completely as possible. Thereafter follows the bleaching, which normally is carried out with hydrogen peroxide, at times in combination with other bleaching steps and intermediate washing steps where ozone or peracids can be used. Hydrogen peroxide steps usually are designated by the letter P. According to a thumb rule, the more harmful metal ions are eliminated from the pulp in the Q-step, the more effective the subsequent bleaching will be. A method, at which the pulp is treated in a Q-step and thereafter in a P-step,, usually is called the "Lignox"-method, as disclosed, for example, in EP-A-402 335. The original pulp normally is oxygen delignified to a relatively low kappa number prior to the QP-bleaching. A low kappa number implies that the obtainable maximum brightness increases and the bleaching chemical demand decreases. The QP-bleaching is carried out in mills most often in such a way, that the pulp first is passed through a pump, which pumps the pulp suspension through the treatment steps. Thereafter the chelating agent is admixed homogenously to the pulp in a mixer. The next step is a bleaching tower where the pulp without additional activation slowly passes through and where all remaining reaction takes place. The pulp finally is directed to a washing step where the exposed and complexed metal ions are washed out. The exposed liquid enriched on metal ions most often is discharged. The conditions normally applied in a Q-step are a temperature of 70-90 C, a stay-time of 1-2 hours, a pulp concentration of 10-14%, a pH of 5-7 and a charge of chelating agent of 1-3 kg/ton pulp. The Q-step, from a strict bleaching point of view, actually is an unnec- cessary treatment step, because no increase in brightness takes place here. The Q-treatment, however, is necessary for a successful chlor¬ ine-free bleaching, as appears from above. If the Q-step could be simplified without thereby reducing the efficiency of metal eliminat¬ ion, great savings could be made, because a complete bleaching step with bleaching tower is expensive. The present invention is directed to a solution of the aforesaid problems. By the invention, thus, the Q-step is made more efficient by eliminating the metals effectively at the same time as the Q-step is simplified, resulting in reduced investment costs in relation to a conventional Q-step. The invention is based on the surprising discovery that the efficiency of the Q-step is affected considerably less by increased reaction temperature than by increased retention time. This indicates, accord¬ ing to classic chemical kinetics, that the Q-step is controlled by diffusion and not by reaction. The Q-step, consequently, should be carried out so that diffusion is facilitated. If this is done effect¬ ively, the total retention time can thereby be reduced considerably. The 1 characterizing features of the invention are apparent from the attached claims. The best way of reducing the diffusion resistance is by decreasing the diffusion distance for active reactants and released reaction products. This is achieved on technical scale by effective mixing, preferably in a mixer. The retention time in the Q-step, thus, can be reduced by repeated effective mixing. The number of mixing steps should be at least two, preferably at least three. For economic reasons, however, the number of mixing steps, and thereby the number of mixers, must be held on a reasonable level. A suitable number can be 2-4 steps. A cert¬ ain retention time between the mixing steps is of good use, but this time need not be long, because the diffusion rate without mixing still is much slower than what is obtainable by mixing, for example in a mixer. The stay-time between the mixing steps, thus, shall be short, less than 15 minutes, suitably less than 10 minutes, and preferably less than 5 minutes. The mixing steps should be carried out one after the other, without intermediate washing. The washing required for re¬ moving the metals from the pulp suspension shall be carried out after the last mixing step. The chelating agent preferably is of the type EDTA (ethylene diamine tetraacetic acid) or DTPA (diethylene triamine pentaacetic acid) , but other types of chelating agent can be used, for example NTA (nitrilo- triacetic acid) or DTPM, J (diethylene triamine pentamethylene phosphonic acid). The total amount of added chelating agent should be 0,5-5 kg/ t pulp (counted as pure chelating agent), preferably 1-3 kg/ t pulp. The entire charge of chelating agent can be added in the first mixing step, whereby subsequent mixing steps bring about only repeated mix¬ ing. Alternatively, the chelating agent can be added in several of the mixing steps. The charging then should be made substantially in the first step, and in subsequent steps the added amount should be less than 2 kg/ t pulp per step. The temperature in the mixing steps should be above 70 C and suitably above 80°C. A preferable temperature interval is 80-120°C. The pressure in the entire Q-step preferably should be 1-10 bar. An especially effective admixture can be achieved by uisng highly in¬ tensive mixers, which break-up the fiber network in the pulp and bring about close contact between fibers and chelating agent. The invention, of course, is not restricted to the embodiments described above, but can be varied within the scope of the invention idea.

Claims: Claims 1. A method at chlorine-free bleaching of pulp where metals are re¬ moved by means of a chelating agent, c h a r a c t e r i z e d i n that the chelating agent is admixed to the pulp without use of a bleaching tower in at least two subsequent mixing steps without washing, that the stay-time between the mixing steps is less than 15 minutes, and that the pulp is washed after the last mixing step. 2. A method as defined in claim 1, c h a r a c t e r i z e d i n that the entire charge of chelating agent is made in the first mix¬ ing step, whereby subsequent mixing steps bring about only repeated mixing. 3. A method as defined in claim 1, c h a r a c t e r i z e d i n that the chelating agent is added in several mixing steps. 4. A method as defined in any one of the preceding claims, c h a r a c t e r i z e d i n that the entire charge of chelating agent amounts to 0,5-5 g/ton pulp, preferably 1-3 kg/ton pulp. 5. A method as defined in any one of the preceding claims, c h a r a c t e r i z e d i n that the temperature in the mixing steps is above 70 C. 6. A method as defined in any one of the preceding claims, c h a r a c t e r i z e d i n that the chelating agent is of the type EDTA or DTPA. 7. A method as defined in any one of the claims 1 - 5, c h a r a c t e r i z e d i n that the chelating agent is of the type NTA or DTPMP. 8. A method as defined in any one of the preceding claims, c h a r a c t e r i z e d i n that the admixing of the chelating agent is made by highly intensive mixers, which break up the fiber network in the pulp and bring about close contact between fibers and chelating agent. 9. A method as defined in any one of the preceding claims, c h a r a c t e r i z e d i n that the mixing steps are carried out at a pressure of 1 - 10 bar.

Inventors: GERMGAARD ULF, LINDQUIST JONAS, GERMGAARD, ULF, LINDQUIST, JONAS
Applicants: GERMGAARD ULF, LINDQUIST JONAS, SUNDS DEFIBRATOR IND AB, GERMGAARD, ULF, LINDQUIST, JONAS, SUNDS DEFIBRATOR INDUSTRIES AB
Application year: 1996
Main classification: D21C9
IPC codes: D21C9