This invention relates to the bleaching of chemical pulp with chlorine-free chemicals. The invention, more precisely, refers to a method of pretreating the pulp with chelating agents in combination with the bleaching.
Totally chlorine-free bleaching (TCP) of chemical papermaking pulp has become a success, and the world production of chlorine-free pulp will soon have got up to five million year-tons, which is an exceptional development considering that the bleaching method was introduced less than six years ago. There exist, besides, variants of this theme, where the prebleaching is chlorine-free and the final bleaching is either of ECF (elementary chlorine-free) or TCF (totally chlorine-free) type.
Bleaching is carried out in most cases by treating the papermaking pulp to be bleached with chlorine-free chemicals first 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 affect the bleaching result negatively, if they remain in the pulp. The chelating agent usually is designated by the letter Q.
The pulp is washed after the Q-step, so that the exposed metal ions are eliminated from the pulp as completely as possible. Thereafter the bleaching takes place, which normally is carried out with hydrogen peroxide, at times in combination with oxygen. Hydrogen peroxide steps are designated with the letter P, and P-steps reinforced with oxygen are designated with OP or PO where the dominating chemical is mentioned first. The thumb rule is that the greater the number of harmful metal ions eliminated from the pulp is, the more effective the subsequent bleaching will be.
A method, at which the pulp is treated in a Q-stage and thereafter bleached in a P-stage, usually is called the "Lignox"-method, which is 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 before the bleaching implies that the obtainable maximum bleaching result increases in the subsequent step, and that the bleaching chemical demand is reduced.
In mills, the QP-bleaching most often is carried out in such a way, that the pulp first is passed through a pump, which pumps the pulp suspension through the treatment steps. Thereafter follows a mixer where the chelating agent is added to the pulp. The next step is a bleaching tower, where the pulp without additional agitation slowly passes through and where all remaining reaction takes place. The pulp finally passes to a washing step where the exposed and complexed metal ions are washed out. The freed 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 retention time of 1-2 hours, a pulp consistency of 10-14%, a pH of 5-7 and a charge of chelating agent of 1-3 kg per ton pulp. By these conditions a very good metal separation can be achieved, which is necessary if subsequent bleaching is carried out on pulp with low lignin content (the kappa number is below about 5) and if a large amount of hydrogen peroxide is used in the stage (more than 15 kg/t). If, however, the pulp still contains a considerable amount of lignin (kappa number above about 5), and the hydrogen peroxide charge in the subsequent step is limited to about 5-10 kg/t, the demand for a complete metal elimination is less critical. Another case when it is sufficient that less metal is eliminated in the stage, i.e. less than 60% reduction, is for example the case when the metal content before the Q-step is so low in relation to the metal content required after the Q-step. In such positions a somewhat simplified Q-process should, therefore, be possible.
From a strict bleaching point of view, the Q-step per se is an unnecessary treatment step, because no proper bleaching takes place there. The Q-treatment, however, is necessary for a following successful P- or PO-bleaching, as appears from the aforesaid. If the Q-step could be simplified without reducing the effectiveness in the elimination of metals, great savings could be made, because a complete bleaching stage is expensive.
The present invention is directed to a simplification of the Q-step, especially for pulps with kappa number above about 5, and for chelating agent treatments which require less than 60% metal reduction. The simplification implies, that the investment cost decreases in relation to a conventional Q-step.
The invention is based on the surprising observation, that the effectiveness of the Q-step is considerably less affected by increased reaction temperature than by increased retention time. This indicates, according to classical chemical cinetics, that the Q-step is diffusion controlled and not reaction controlled. This means, that the Q-step should be carried out so that the diffusion is facilitated, and if this is made effectively, the total retention time can thereby be reduced substantially.
The characterizing features of the invention are apparent from the attached claims.
The best method to reduce the diffusion resistance is to decrease the diffusion distance for active reactants and released reaction products. On a technical scale this is made by effective mixing, for example in a mixer with long retention time. Thus, by effective mixing, the retention time in the Q-step can be shortened. The number of mixing steps is not critical, but the total mixing time must be above a certain critical level, alternatively at least two mixing apparatuses shall be used. For economic reasons, however, the number of mixing steps must be held on a reasonable level. For a pulp with kappa number above about 5, which shall be bleached further with at maximum about 5-10 kg hydrogen peroxide exclusive possible oxygen, a suitable number of mixing steps is 1-2. The same applies, if the metal reduction to be achieved in the chelating agent step is limited and need not be above 60%.
When two mixing steps are used, a certain retention time between the mixing steps is favourable, but this time need not be long, because the diffusion rate without mixing yet is much slower than what can be achieved by mixing, for example in a mixer. The stay-time between the mixing steps, thus, shall be short, less than 10 minutes and preferably less than 5 minutes. The mixing steps then shall be carried out one after the other without intermediate washing. The washing required for removing the metals from the pulp suspension shall be carried out less than 15 minutes after the last mixing step.
When more than one mixing step is used, the entire charge of chelating agent shall be added in the first mixing step, and subsequent mixing steps bring about only a renewed mixing. To divide the charge in such a way that at maximum one third of the chelating agent charge is added in the last step, however, is not detrimental, if the subsequent mixing is sufficiently effective.
The chelating agent preferably is of the type EDTA (ethylene diamine tetraacetic acid) or DTPA (diethylene triamine pentaacetic acid), but also other types of chelating agent can be used, for example NTA (nitrilotriacetic acid) or DTPMP (diethylene triamine pentamythylene phosphoric acid). The total amount of added chelating agent should be 0.5-3 kg per ton pulp (calculated as pure chelating agent), preferably 1-2 kg per ton pulp.
The temperature in the Q-step should be above 75°C, suitably over 80°C and more preferably over 85°C. A preferred temperature interval is 85-110°C. The desired pH interval is pH 5 to pH 11. The pulp consistency should be between 2 and 16%.
An especially effective mixing can be achieved by using high-intensity mixers or with another type of mixer, where the mixing is effective and the retention time is sufficient, so that a homogenous mixture of fibres and chelating agent is obtained.
In the Table below some results from a one-stage Q-treatment are shown, which was carried out with a high-intensity mixer for 3 seconds and with a subsequent retention time without mixing for 40 seconds before the pulp washing, and where the treatment was carried out at 95°C. For comparison, data are also shown from a conventional Q-treatment with conventional lab mixing of chelating agent with the pulp and a subsequent retention time of 60 minutes, where the step was carried out at 90°C.
The starting pulp was oxygen bleached and of coniferous sulphate type with kappa number 10.1, viscosity 975 dm3/t and brightness 43.0% ISO. The bleaching was carried out according to Q(OP) with 10 kg H2O2/t.
The invention, of course, is not restricted to the embodiments described above, but can be varied within the scope of the patent claims.