FI90256B - Method for washing alkaline mass - Google Patents

Description

1 90256

Method for washing alkaline mass

The present invention relates to a process for washing an alkaline cellulose pulp, the washing being carried out in one or more successive steps.

Many processes in the pulp industry, e.g. in the production of sulphate pulp and chemico-mechanical pulp, use enormous amounts of relatively expensive chemicals. For these processes to be profitable, it is essential that these treatment chemicals be recycled and regenerated as much as possible. Another, and now increasingly significant, reason for the widespread recycling of processing chemicals is the strict regulations issued by central and local government authorities on low emissions of chemicals into the environment.

In order to achieve washing results that meet the requirements of these authorities, the cost of capital invested has had to be a consideration for operating costs.

As long as the energy price was low, it was possible to increase the amount of wash water as the requirements for wash wastewater losses became stricter. It was then possible to evaporate the large amounts of water thus obtained. Now that the price of energy is many times higher, a concerted effort must be made to provide more efficient washing equipment and more efficient washing methods that release the pulp to a considerable extent from its treatment chemicals.

It is an object of the present invention to reduce washing losses in the washing of alkaline pulp.

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Another object of the present invention is to release a basic cellulose mass of sodium ions.

A third object of the present invention is to improve the separation of substances from the basic cellulose pulp which contribute to the oxygen demand (COD) of the artificial alloy.

These objects are achieved according to the present invention in the washing of alkaline cellulosic pulp by lowering the pH during the washing step in one or more steps, or in at least one of the washing steps, wherein the most preferred reagent to be added to the washing step or steps is carbon dioxide.

According to one embodiment of the present invention, the pH is lowered to approximately 9.

According to another embodiment of the present invention, the pH is lowered with carbon dioxide in one or more washing steps, resulting in improved separation from the pulp of substances that contribute to chemical oxygen demand (COD).

The most preferred reagent for carrying out the improved washing of the pulp is carbon dioxide, which is added to the washing water used for washing the pulp and / or the pulp suspension before the washing steps. Carbon dioxide may also be added to the washing step itself.

Carbon dioxide is a particularly preferred reagent for improved washing results. Carbon dioxide does not contain any substances that are harmful to the environment, such as chlorine or sulfur.

However, to reduce the pH, other acids can be added, e.g. sulfuric acid, which of course contains sulfur, since the pulp to be washed according to the present invention has been treated with a sulphate-containing treatment solution before washing. If used, sulfuric acid is preferably added together with carbon dioxide to achieve a clearer decrease in pH. It should be mentioned here that the pH must not be lowered too much, as this may lead to undesired reactions with residual lignin.

Above all, the pulp that can be washed using the addition of carbon dioxide or another pH-lowering reagent is the softwood and hardwood sulphate pulp and also the chemical-mechanical pulp, CTMP and CMP pulp.

Since the washing operations are normally performed in several steps, the pH-lowering reagent is added to one or more steps, at least in which case the reagent is not added to the first washing step.

According to a particularly preferred embodiment of the present invention, the pH is lowered to approximately 5, for example, in the last step of the three or four washing steps. In this case, in addition to carbon dioxide, mineral acid, in particular sulfuric acid, can also be used.

The nature of the present invention and its aspects will be more readily understood from the following brief description of the accompanying drawings and the related presentation.

In the accompanying drawings: Figure 1 is a schematic representation of a laboratory facility for practicing the present invention; Fig. 2 is a schematic representation of a laboratory plant for countercurrent washing of pulp; and Figure 3 is a schematic representation of a slightly modified plant for washing alkaline pulp in four steps.

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Referring to Figures, reference numeral 51 in Figure 1 indicates a source of carbon dioxide connected via line 57 to rotameter 52, manometer 53, and vessel 54. The bottom 58 of vessel 54 is a sintered body that finely distributes gas from source 51. During the addition of carbon dioxide, the contents of the vessel 54 are agitated by a motor mixer 55 driven by a motor 56.

To 25 g of dry mass was added 200 ml of black liquor. The mixture was allowed to stand for 48 hours.

The mixture was then diluted with deionized water to one liter, te. mass concentration 2.5%. The suspension was stirred for one hour before the filtrate was removed. The pulp was then subjected to a four-stage wash. In each step, deionized water was added to the pulp to a pulp concentration of 2.5%. Each washing step lasted one hour, during which it was stirred. All filtrates were stored for analysis. The amount of sodium in the wash water was determined using atomic absorption and COD according to the method of Dr. Lange.

In the series of experiments described, carbon dioxide was added to the second wash step over 5, 10, 20 and 40 minutes, respectively.

The carbon dioxide flow was constant and steady in all experiments. Due to the poor efficiency of carbon dioxide dissolution, the amount of CO2 has not been calculated in these experiments. In industrial and other installations, equipment familiar to those skilled in the art can of course dissolve carbon dioxide in the wash water and the pulp suspension.

Table 1 shows that the total amount of sodium separated is higher when CO 2 is added to the second scrubbing step. The effect becomes clear already at the stage where the + addition of carbon dioxide took place, te. in the second stage. Na differs more efficiently as a result of the decrease in pH resulting from the addition of CO2.

5 90256 The COD separation event is more complex. The total amount of COD separated is higher when CO2 is added in the second wash step. However, the effect is delayed and the improved wash-ho does not appear until the third wash step.

It is not clear why COD separation is improved in this way. One possible explanation is based on the nature of surface chemistry. In this case the following applies: (RCOO> 2 Ca -> RCOONa (RCOOH) resin and fatty acids co2 + h2o -> h2co3 2+ 2-

Ca + CO 3 -) CaCO 3

CaCOg is less soluble than <RCOO) 2Ca 6 90256

Table 1 Addition of CC> 2 in the second wash step (CO2 * flow in constant addition) CO2 CO2 CO2 CO2 0 Sample 5 min 10 min 20 min 40 min

Na <tot)

Na (1) 1.14 1.26 1.23 1.18 3.28

Na <1 + 2)

Na (1) 1.10 1.21 1.19 1.16 1.22

Na <1 + 2 + 3)

Na (1) 1.13 1.25 1.21 1.17 1.24 pH <2) 10.8 7.5 7.4 7.2 5.7 pH (4) 9.4 7.6 7, 8 7.6 7.1 COD (tot) COD <1) 1.20 1.29 1.26 1.32 1.32 COD <1 + 2) COD (1) 1.11 1.13 1.13 1 , 12 1.07 COD (1 + 2 + 3) COD (1) 1.17 1.21 1.19 1.23 1.25

Na (tot) shows the total amount of Na separated and the quotient of the Na separated in the first Na (1) washing step.

Correspondingly, the example below shows the quotient of the total COD (1) in the first and second stages of COD (1+ 2) and the quotient of COD separated in stage 1.

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In step 1, no carbon dioxide was added. pH <2) refers to the pH after the second washing step.

Calcium carbonate is temporarily precipitated when calcium is “inactivated.” Calcium soaps are insoluble in nature and do not form lamellar states to the same extent as sodium beans. The surface activity in the system increases, which improves the removal of organic matter.

An experiment has been performed to wash birch kraft masse upstream on a laboratory scale. The purpose of this experiment is to get closer to full-scale conditions using the countercurrent method and to obtain a higher eluate content for this washing of birch kraft pulp than with pine kraft pulp.

The washing steps are shown schematically in Figure 2. In the continuity state, a batch of pulp suspension is fed to the first wash step 61 and washed using the filtrate from the second step 62. The washed pulp in the first step is then fed to the second step 62 and washed with the third step 63 filtrate. The pulp washed in the second step is then fed to the third step, where the pulp is washed with the filtrate from the fourth step 64. In the fourth step, the pulp is washed with the filtrate from the fifth step 65. In the fifth or final step, the mass 65 is washed with deionized water.

A total of 700 ml of deionized water was used to wash each batch as a washing liquid. The washing of each step was performed in a measuring cylinder and the pulp suspension was stirred for several minutes. The pulp slurry was then filtered using a Buechner funnel and 700 mL of filtrate was separated.

For each 25 g batch of dry birch kraft pulp suspension, 100 ml of black liquor was mixed. The suspension was allowed to stand for at least 2 days before washing was started at step 61. During this time, steady state was reached or almost reached.

Two sets of experiments were performed, one with carbon dioxide and the other without carbon dioxide. In the first series, no carbon dioxide was used.

To form a continuous wash system, 16 samples were washed in each series. Batch No. 1 was first washed in step 61 using 700 ml of deionized water. The resulting filtrate (700 ml) was discarded. Batch # 1 was then transferred to step 62 and washed with 700 mL of deionized water. The resulting 700 ml filtrate was stored and used as batch No. 2 wash in step 61. In batch No. 1, third wash step 63, the pulp was washed again with 700 ml of deionized water, the resulting filtrate 700 ml was stored and used as batch No. 2 in step 62. in step 62, batch No. 3 was then used as the washing liquid in step 61. The filtrate of 700 ml from step 1 was discarded. Batch No. 1 was then washed in step 64,700 ml with deionized water and finally in step 65,700 ml with deionized water. The filtrates, 700 ml from batch no. 1 in step 65, were then used as washing liquid for the next batch. Thereafter, deionized water was added only to step 65 and the filtrate from this step was used to wash the next batch in step 64, etc. The filtrate from step 61 of the first twelve batches was discarded. The last four filtrates from step 61 were then stored, pooled and analyzed.

In the second set of experiments, carbon dioxide was added to step 65. Carbon dioxide was added during the wash, which lasted a few minutes and occurred while stirring the straw suspension in deionized water. Continuity conditions were established in the same manner as in the comparison series of experiments without carbon dioxide. The last four filtrates from the sixteen filtrates of step 61 obtained were combined and analyzed.

9 90256 In addition, the pH of the filtrates of all steps was measured when the state of continuity was reached. In the following, filtrate 61 describes the filtrate from step 61, filtrate 62 describes the filtrate from step 62, and so on.

Table 2 shows the filtration pH values after the state of continuity has been reached.

Table 2

Filtrate pH without CC> 2 with pH CC> 2 65 10.3 6.8 64 10.6 9.4 63 11.0 10.9 62 11.6 11.8 61 12.1 12.6

Washed mass, you. mass from batches 12-16, were combined into a single sample and filtrate 61 was further analyzed. Filtrate 61 describes the last four filtrates taken from step 61 and pooled as an average sample in each series, which contained 16 batches. The results of the filtrate analysis are shown in Table 3.

Table 3 without CC ^ with CC ^

Na Cg / 1) 3.59 3.81 2+

Ca (g / l) 0.033 0.033 COD (g / l> 17.1 17.9 COD = chemical oxygen demand 10 90256

The results of the mass analysis are shown in Table 4.

Table 4 without CO2 with CC 2

Na (g / kg> 2.00 1.34 2 +

Ca (g / kg) 1.10 1.17 COD (g / kg) 10.19 9.37 DCM extract (%) 0.10 0.08 DCM = dichloromethane The addition of CO2 to the final washing step has resulted in: more sodium ions and COD have come to be washed from the pulp. The calcium ion concentration has not changed significantly. The extract content (dichloromethane) in the pulp is lower after washing with the addition of carbon dioxide.

Figure 3 shows a modified plant for washing pulp in sulphate cooking.

According to Figure 3, the pulp comes from a cooking pot (not shown) along a pipe 1 to a pulp pit 3. The pipe 2 leads to a blowing condenser. The pulp in the pulp well 3 is diluted with a weak (thin) lye along a pipe 4 from the low lye tank 19. The mixer in the pulp well 3 is indicated by the reference number 5.

The pulp from the pulp pit 3 goes along the pipe 29 to the branch screen 6. The pulp then passes to the first wash filter 15 and then to the next three wash filters 16, 17 and 18. The filtrate from the first filter 15 is collected in the low liquor tank 19 and the filtrate from the other three filters in the wash liquor tanks 20, 21 and 22, respectively. The washed mass starts from the fourth filter 18 at reference number 13. As the mass passes from one filter to the next, the filter cake is ground to a dry decomposer 11la 8. The filtrate from one filter is used as the liquid for washing the previous filter 90256 and the diluent in the same filter. Normally, clean water is added to the last filter 1Θ as a washing solution via a pipe 12.

According to the invention, carbon dioxide is introduced into pipes 31 and / or 32 via pipes 23 and 24, respectively. The carbon dioxide immediately dissolves in the washing solution and is passed to the filters 17 and 18 in a similar manner through pipes 31 and 32 and through pipes 10 and 11. The filtrate from the filter 16 passes along the pipe 26 to the tank 20. The pH sensor device is placed either in the tank 20 or in the pipe 26, whereby the sensor monitors the supply of carbon dioxide to the pipe 31 via the monitoring and control device. the sensor is connected to a control device that maintains the pH at a predetermined level by setting the CCl2 increase through the tube 24.

Water is added to the last wash filter 18, whereby carbon dioxide and / or sulfur dioxide are mixed with this water.

A pH sensor device can also be placed in the tube 28.

It has been found that it is possible to reduce washing losses, calculated as Na 2 SO 4 by approximately 1 kg per kg of carbon dioxide charged. A suitable amount of carbon dioxide is approximately 6 kg per tonne of pulp, thus making it possible to reduce the washing loss by 6 kg per tonne of pulp.

By further lowering the pH by adding mineral acid, preferably sulfuric acid and thus lowering the pH to 6 or less, even lower washing losses are achieved.

In the manufacture of CTMP and CMP pulp, the cellulosic material is pretreated with an alkaline treatment grade and ground in one or more mills, usually plate mills. The pulp suspension then passes into the screen space. The resulting pulp is then transferred to a washing plant unit, e.g. of the type described in Figure 3.

Using the method of the present invention, it is possible to improve the washing results of all alkaline cellulosic pulps, regardless of whether the pulp is hardwood / softwood pulp or any other type of pulp, e.g. made from sugar cane waste.

The present invention is not to be construed as limited to that described above and shown in the drawings, many modifications being possible without departing from the spirit and scope of the appended claims.

Claims (4)

1. A composition comprising an alkaline, cellulosic pulp, colored by means of a solid or carbonaceous mixture, which can be used to determine the pH of the mixture or a mixture of solids. 25 A method for washing an alkaline cellulosic pulp, the washing being carried out in one or more successive steps, characterized in that the pH is lowered in the washing step or in at least one washing step with carbon dioxide. 2. A method according to claim 1, comprising the following figures: The composition of the colloidal oxide is based on the composition and / or mass suspension of the composition. 30 3. In the case of patent claim 2, the color of the sample is determined, the reference to which is provided in the case of a core account. Process according to Claim 1, characterized in that the carbon dioxide is added to the washing water and / or the pulp suspension immediately before the washing step. 10 A method according to claim 2, wherein the washing is carried out in at least two steps, characterized in that carbon dioxide is added in all steps except the first step. 15 Process according to Claims 1 to 3, characterized in that the pulp is a sulphate pulp and / or a chemical-mechanical (CTMP, CMP) pulp. 20 Patentkrav 4. A preferred composition according to claims 1-3, which comprises 35 pulps of sulphate pulp and / or chemical chemistry (CTMP, CMP) pulp.