SE526241C2 - Procedure for acidifying lignocellulosic pulp before ozone bleaching - Google Patents

Abstract

A method for ozone bleaching pulp at high pulp concentration and a temperature in the interval 20-60°C in an ozone stage (9), which is preceded by an acidification reactor (4), in which the pulp is acidified at a pulp concentration of 15-25%, and in the later part of the same reactor is cooled via dilution liquid added in a dilution zone (5), so that the temperature of the pulp out from the reactor is lower than the temperature during the acidification and adapted to the temperature desired in the ozone stage. The acidification is carried out at a temperature of above 70°C, but at the highest 95°C at an acidification time of 30 minutes and at the highest 80°C at an acidification time of 120 minutes, where the highest temperature at staying-times between 30 and 120 minutes is obtained by linear interpolation.

Description

15 20 25 5 ïso Üíïss. . ø. 1. 526 241 2 ozone consumption for a given bleaching work, and partly undesirable side reactions, e.g. cellulose degradation, occur to an increasing extent. In prior art ozone bleaching processes, the bleaching is performed at a temperature of 20-70 ° C.

It has been found that the efficiency of the ozone step can be significantly improved if the acidification is intensified, ie higher temperature and longer residence time. But then the difficulty arises in achieving a sufficiently low temperature in the ozone step, which is advantageous in order to minimize side reactions that lead to reduced ozone utilization.

SE518808 discloses a bleaching process in which the pulp is treated at a high temperature and then bleached in at least one bleaching step. Although it is stated that the hot A-step (acid step) can be followed by a Z-step (ozone step), there is no indication of how to solve the temperature problem that arises in this case.

Thus, several difficulties arise when in an integrated ozone bleaching process at high mass concentration one tries to combine a more effective acidification step which requires a higher temperature and longer residence time than the usual acidification step with a subsequent ozone step.

The relatively high temperature of the acidification stage does not correspond to the requirements of the low temperature ozone stage. The low temperature in the ozone step is necessary so that the pulp is not exposed to unnecessarily large decomposition with reduced viscosity and lower pulp strength as a result. In addition, the low temperature is necessary to get as good ozone utilization as possible.

The reason is, as previously mentioned, that ozone tends to decompose to oxygen to an increased extent at higher temperatures.

A new method that meets these seemingly conflicting requirements is to perform the acidification step in a manner that intensifies the acidification without severe degradation of the pulp by delivering a pulp with a sufficiently low temperature to the ozone step. This can be achieved by dividing the acidification into two phases. The 10 15 20 25 ïfïzaø. 235 n. «« U. 526 241 in the first 3 phases is carried out at a significantly higher mass concentration than is usual. In the second phase, the pulp is diluted and cooled.

Because the first phase is carried out at a high pulp concentration, the heat content in the pulp is relatively low, which practically makes it possible to cool the pulp to a suitable temperature with diluent before it is dewatered and transferred to the ozone step. The pulp thus does not need to be cooled to a temperature suitable for the ozone step in a separate step, but this can be done by dilution to a later part of the acidification step. If the pulp had been acidified at a lower concentration, the heat content of the pulp would have been so great that it would have been practically impossible to lower the temperature sufficiently by dilution directly to the reactor.

The temperature in the acidification step is adjusted to intensify the acidification treatment without deteriorating the pulp properties. You do not want too high a temperature partly because this can mean increased decomposition of the cellulose and partly because an excessively high temperature would make it impossible to lower the temperature sufficiently in a simple way to meet the temperature requirements in the ozone reactor. By raising the temperature during the acidification treatment compared to the standard procedure where the temperature is 20 - 70 ° C but still not so high that it becomes practically impossible to lower the pulp temperature before the subsequent ozone bleaching step is achieved an improved integrated ozone bleaching process.

The invention thus solves the problems of effectively combining an intensified acidification step which requires a higher temperature with a subsequent ozone step without the need for any more expensive intermediate steps in order to give the pulp the temperature required for the ozone step. The acidification time can be 30-120 minutes. By having a temperature in the acidification step which is above 70 ° C but not more than 95 ° C at an acidification time of 30 minutes and not more than 80 ° C at a residence time of 120 minutes and where the maximum temperature at residence times in between is obtained by linear interpolation, an intensified phase is achieved. 10 15 20 25 ». Acidification step which is adapted for the subsequent ozone bleaching step both in terms of temperature and cellulose degradation.

The procedure is described in detail below. Pulp is washed and dewatered to a pulp concentration of about 30% - 35%). which is mixed with the pulp so that the pulp concentration is lowered (25) Then a tempered liquid, containing acid, is added to about 20% (15-25%), a temperature of at least 70 ° C and a pH of 2 - 3.5 is obtained. large decomposition of the mass, the temperature should not exceed 95 C at 30 minutes of acidification time and 80 ° C at 120 minutes of acidification time.

Maximum temperature in the acidification step at times between 30 and 120 minutes is obtained by linear interpolation.

After acidification, the pulp is diluted with temperature-controlled washing liquid in the reactor dilution zone to a pulp concentration suitable for transport to subsequent steps (3 - 12%).

The temperature after the dilution zone, Tmv must be lower than the temperature in the reactor, TR, and adapted to the temperature requirement in the ozone reactor, 20 - 60 ° C. The mass is dewatered to> 35% in a press and bleached with ozone in a known manner. After reaction with ozone, the pulp is washed for the next treatment step. The pulp may be alkalized before washing.

The described ozone bleaching process may be preceded and / or followed by one or more delignification or bleaching steps.

The presses used for dewatering and washing can e.g. be of different types, constriction washing press, drainage press, double wire press or screw press.

This process achieves a much more efficient use of ozone than in existing processes.

Example 526 241 ~ .u ~ ua u -. v now Table l shows results from ozone bleaching where the intensity of the acidification step has varied.

Table 1. Ozone bleaching of hardwood sulphate pulp; kappatal 11.5, viscosity 983 dm ° / kg, brightness 53.4% ISO Pretreatment Standard HC Improved bleaching process Mass concentration% 10 20 20 20 20 Retention time min 20 80 50 120 120 Temperature ° C 20 85 90 80 80 HZSO. kg / od! 9 9 9 12 9 End QH 2.8 2.8 2.8 2.4 2.7 Ozone step MâSSaKšlnCullllnuull% 42 42 42 42 42 Residence time sec 95 94 94 93 92 Temperature ° c 40 40 40 40 40 O; kg / odt 4.98 4.99 4.98 4.98 4.99 O, rest kg / odt 0.08 0.07 0.08 0.17 0.17 O; consumption kg weights 4.90 4.92 4.90 4.81 4.82 Alkalization Mass concentration% 12 12 12 12 12 uphills fi d min 10 10 10 10 10 Temperature ° C ss ss ss ss ss NaOH kg / odt 12 12 12 12 12 § | utpH 10.6 11.4 11.2 11.5 11.6 Analyzes Brightness% ISO 67.6 69.5 68.7 71.8 71.4 kappa number 3.7 2.6 3.0 2.0 2.1 Vískosiket mllg 857 775 798 740 742 Delta kappa (kappa number reduction) 7.8 8.9 8.5 9.5 9.4 Delta kappa / O, consumption kappa / kg O, ptp 1.59 1.81 1.73 1.98 1.95 Results in Figure 4 where ozone consumption per reduced kappa number unit has been plotted against temperature in the ozone step. The figure shows that ozone utilization deteriorates with increasing temperature in the ozone stage and that the intensified acidification leads to improved ozone utilization.

In the reference Figure made at is performed as follows, the invention will be described in more detail with reference to Figures 1-4. 1 shows a commercial process for ozone bleaching high pulp concentration (Standard HC). The acidification is an integral part of the ozone bleaching process at a pulp concentration of about 10% for a time of less than about 20 minutes. At the same time, the temperature in the acidic sub-stage -n ~ .on is regulated. 1 - a nu-1 ~; c. ~ N 1 coon.- 15 20 25. . -. .. 526 241 6. . . . _. c. so that the desired temperature in the ozone reactor is reached. With reference to the numerals of the figure, the acidification is performed in (13). to a concentration of about 10 6 with filtrate returned from a reactor The mass is diluted before the treatment in the reactor a press (15) located after the acidification reactor (13). The filtrate (14) (12) is mixed into the mass. After acidification, the pulp is dewatered in a (15) (16). then before any extraction step (17) filtrate whose temperature is adjusted via a temperature control (18). (19). temperature is regulated and acid is added before it is pressed and then taken to the ozone step. The mass can be diluted with The mass is dewatered and then washed in a press step (acid step with short residence time) was performed at 5% pulp concentration, pH = 2.5 and time 20 minutes. The ozone addition was 5.7kg O3 / odt and the time about 5 min. After the ozone bleaching, an e-step (alkaline extraction step) was performed at 12% pulp concentration, time 10 minutes and pH = 11.5. It can be seen from the figure that a lower temperature during ozone bleaching results in a lower loss of viscosity.

Figure 3 schematically shows the improved ozone bleaching process according to the invention. The mass is washed and dewatered in a press (1) to a pulp concentration of 25-35%. Then liquid and acid are added so that the pulp concentration is lowered to 15-25% and a temperature of over 70 ° C and a pH of 2-3.5 is obtained. The addition can be made by a tempered liquid containing acid or by separate additions of diluent (2), acid and steam (3).

The pulp is acidified in a reactor (4). The highest temperature during the acidification depends on the acidification time. At an acidification time of 30 minutes the temperature should not exceed 95 ° C and at an acidification time of 120 minutes the temperature should not exceed 80 ° C. After the acidification phase, diluent is added to a dilution zone (5) in the latter part of the reactor. The diluent must have such a temperature that the temperature of the pulp out of the reactor is lower than during the acidification and adapted to the temperature requirement in the ozone reactor. The temperature of the diluent is regulated via a temperature control (6). The pulp concentration from the reactor should be 3-12%, preferably 4-10%. The pulp is then dewatered in a press (7) so that a pulp concentration of 35-50% is achieved.

The pulp is then bleached with ozone at a temperature of 20-60 ° C in one (8) - (9) before the next treatment step. Optionally, the pulp can be alkalized (10) reactor. After the reaction, the pulp is washed one step at a time before being washed. The mass can then be diluted with (9) (ll).

Figure 4 shows partly how ozone utilization deteriorates with increased filtrate from the press, which can be temperature-regulated temperature in the ozone step, and partly how the intensified acidification provides more efficient ozone utilization. The dots connected by a dashed line show experiments where the acidification took place at a pulp concentration of 5%, pH 2.5, temperature 20 ° C and time 20 minutes (ie the standard procedure).

The circled dots above the line represent dots with intensified acidification according to Table 1.

The invention is of course not limited to the embodiment shown but can be varied within the scope of the claims.

Claims (5)

10 15 20 25 30 35 526 241 8 Patent claims Process for ozone bleaching of lignocellulosic pulp where a) the pulp is dewatered to a pulp concentration of 25-35% in a press (1) after which liquid and acid are mixed with the pulp b) the pulp is acidified in a reactor (4) where the pH is in the range 2 3.5 c) the pulp is dewatered in a press (7) to a pulp concentration of 35-50% and then d) bleached with ozone in a step (8) where the temperature is in the range 20 - 60 ° C, characterized in that the acidification is carried out at a pulp concentration of 15-25% and at a temperature above 70 ° C but not more than 95 ° C at an acidification time of 30 minutes and not more than 80 ° C at an acidification time of 120 minutes, where the highest temperature at residence times between 30 and 120 minutes are obtained by linear interpolation and that diluent is supplied to a dilution zone (5) in the later part of the acidification reactor (4) so that the outgoing mass has a concentration of 3-12% and a temperature lower than the temperature during the acidification and adapted to the desired the temperature in ozone the bleaching step (8). A method according to claim 1, that the pulp after (9). k o n n e t e c k n a t a v ozone bleaching step is washed in a press A method according to claim 2, characterized in that the pulp is alkalized in one step before it is washed in the press (9). 4. A method according to claim 1, characterized in that the ozone bleaching process can be preceded by one or more delignification or bleaching steps. Process according to Claim 1 or 4, characterized in that the ozone bleaching process is followed by one or more bleaching steps.