JP3425280B2 - Pretreatment of oxygen bleached pulp - Google Patents

Description

Detailed Description of the Invention

[0001]

TECHNICAL FIELD The present invention relates to a pretreatment method for oxygen bleached pulp in a kraft pulp manufacturing process.

[0002]

2. Description of the Related Art Oxygen bleaching in a conventional kraft pulp manufacturing process will be described with reference to FIG. 2. The unbleached pulp sheet that has been washed by the unbleached washing filter 1 in the preceding step of the oxygen bleaching tower 6 is crushed by a repulsor 14. Meanwhile, white liquor, oxidized white liquor, or caustic soda is added as an alkali to form a pulp slurry, which is sent to the steam mixer 2. Steam is blown in the steam mixer 2 and the temperature is raised to about 98 ° C.
Then, it is sent to the oxygen mixer 5 via the suction tank 3 and the high-concentration pump 4.

In the oxygen mixer 5, steam is blown again into the unbleached pulp slurry and the temperature is raised to about 103 ° C. At the same time, high-purity oxygen gas having an oxygen concentration of 93% or more is press-added and sent to the oxygen bleaching tower 6. In the oxygen bleaching tower 6, delignification proceeds in the unbleached pulp under a predetermined residence time, and oxygen bleaching is completed. The pulp that has been subjected to oxygen bleaching is sent to the degassing tower 7.

The oxygen bleached pulp sent to the degassing tower 7 is degassed in the degassing tower 7, diluted with the filtrate in the oxygen tower washing filtrate tank 11 and washed with the oxygen tower washing filter 8, and then the high-concentration chest. It is stored in 9 and sent to the next bleaching process. Exhaust gas 12 discharged from the degassing tower 7 is sent to a deodorizing device 16 by an exhaust fan 13 to be processed and released to the atmosphere, or burned in a combustion facility.

As a problem of the oxygen bleaching process, since the exhaust gas 12 of the degassing tower 7 is a malodorous gas, it is released to the atmosphere after deodorization or sent to a combustion facility and burned, but the exhaust gas 12 was analyzed. As a result, it was found that a large amount of nitrogen content was included, and this nitrogen content is because the ambient air is entrained when the unbleached pulp is crushed by the repulsor 14 because the unbleached cleaning filter 1 is an open type. It was This is because an open type washing machine is generally used in the case of a washing filter for washing unbleached pulp.

A high-purity oxygen gas having an oxygen concentration of 93% or more is press-fitted and added to the unbleached pulp in the oxygen mixer 5, but the high-purity oxygen gas is press-fitted and added in the oxygen mixer 5 by the entrainment of ambient air having a low oxygen concentration in the repulsor 14. There is a problem that the high-purity oxygen gas is diluted to lower the oxygen concentration. Also,
As a result, there is a problem that delignification in the oxygen bleaching tower 6 does not proceed.

Generally, in the medium concentration oxygen bleaching delignification method, the unbleached pulp slurry supplied to the oxygen bleaching tower 6 has a pulp concentration of 8 to 15% and a slurry temperature of 70 to 140.
C, the top pressure of the oxygen bleaching tower is atmospheric pressure to 7 Kg / cm ² ,
The residence time of the pulp in the tower is 10 to 90 minutes, the amount of oxygen added is 0.7 to 1.5 Kg / Kappa number reduction amount 1 / absolute dry pulp t, and the amount of NaOH added is 1.0 to 1.7 Kg / kappa. The value reduction amount is 1 / absolute dry pulp t.

[0008]

SUMMARY OF THE INVENTION The present invention solves these problems and provides a method for increasing the oxygen concentration after the oxygen mixer 5 to a higher level than before without changing the amount of oxygen gas in the oxygen mixer 5. It is intended.

[0009]

SUMMARY OF THE INVENTION The present invention is an oxygen bleaching tower 6
In the previous step, the upper part of the unbleached cleaning filter 1 is covered with a hood to make it a hermetic type , and the oxygen composition is higher than the oxygen concentration in the air so that the pressure in the hood is atmospheric pressure or higher. Is a pretreatment method for oxygen bleached pulp, which is characterized by supplying a gas having

As a gas having an oxygen concentration higher than the oxygen concentration in the air, the exhaust gas 12 (from the deaeration tower 7 after oxygen bleaching, which is conventionally deodorized and released to the atmosphere or sent to a combustion facility for combustion, An oxygen concentration of about 70-80%) is preferred,
As other used, vaporized gas of liquefied oxygen, oxygen gas produced by oxygen PSA, or oxygen-enriched air obtained by mixing these oxygen gases with air can be used,
It is also possible to mix it with the exhaust gas 12.

[0011]

According to the present invention, the upper part of the unbleached cleaning filter 1 in the previous step of the oxygen bleaching tower 6 is covered with a hood to form a closed type, and the pressure in the hood becomes equal to or higher than the atmospheric pressure. The exhaust gas 12 of the deaeration tower 7 after oxygen bleaching is supplied. Therefore, when the unbleached pulp sheet is crushed by the re-pulper 14 into a pulp slurry, a gas higher than the oxygen concentration in the air is supplied to the pulp slurry, so the amount of oxygen gas added by the oxygen mixer 5 is not changed. In addition, unlike the conventional open type cleaning filter, the surrounding air is not trapped, and the present invention can increase the oxygen concentration in the pulp slurry after the oxygen mixer 5.

As a result, the oxygen concentration in the oxygen bleaching tower 6 becomes higher than before, the reaction in the oxygen bleaching tower 6 progresses, delignification progresses, and the bleaching chemical unit in the bleaching step following oxygen bleaching Improvement and reduction of wastewater pollutants discharged from the bleaching process and reduction of wastewater treatment cost.

[0013]

BEST MODE FOR CARRYING OUT THE INVENTION The present invention will be described in more detail with reference to the following examples.

(Experimental example) When the unbleached pulp is crushed by the repulsor 14 of the conventional open type unbleached cleaning filter 1, the nitrogen content in the exhaust gas 12 of the degassing tower is calculated in order to calculate the amount of air involved. Was measured. Exhaust gas amount of degassing tower 7 is 11
The nitrogen content in the exhaust gas was 12.24% at 1 m ³ N / Hr. Calculation based on this revealed that about 17 m ³ N / Hr of air was entrained. It should be noted that the high-purity oxygen gas of 93% or higher added by the oxygen mixer 5 contains 0.9% of the nitrogen content, but since it is a very small amount, it was ignored.

(Embodiments 1 and 2) Embodiment 1 of the present invention is shown in FIG.
The unbleached cleaning filter 1 in the previous step of the oxygen bleaching tower 6 is a closed type whose upper part is covered with a hood. The unbleached cleaning filter 1 is a suction type, and it is necessary to increase the cleaning efficiency by keeping the pressure inside the sealed hood above the atmospheric pressure and not reducing the pressure. Therefore, in the present embodiment, the degassing tower 7 after oxygen bleaching is performed in the closed hood as a gas having an oxygen composition higher than the oxygen concentration in the air.
The exhaust gas 12 having an oxygen concentration of 70% (Example 1) and 78.6% (Example 2) of 17 m ³ N /
Hr was supplied. The pulp sheet washed with the unbleached washing filter 1 and peeled from the cylinder is crushed by the re-pulper 14 while 16 kg of oxidized white liquor (NaOH
(Converted) / absolute dry pulp t was supplied.

The crushed pulp slurry having a concentration of 12% is sent to the steam mixer 2, where steam is supplied and the pulp slurry is heated to 98 ° C. and sent to the suction tank 3. Then, it is sent to the oxygen mixer 5 by the high-concentration pump 4.

In the oxygen mixer 5, steam is again supplied to the pulp slurry to raise the temperature to about 103 ° C. and at the same time 93
% High purity oxygen gas of 200 m ³ N / Hr was added under pressure, then sent to the oxygen bleaching tower 6 and retained for 60 minutes for delignification.

The oxygen bleached pulp which has been subjected to the oxygen bleaching is sent to the deaeration tower 7 where it is deaerated. As described above, the exhaust gas from the degassing tower 7 is supplied into the hood of the unbleached cleaning filter 1 by the exhaust fan 13.

The oxygen bleached pulp is degassed in the degassing tower 7, diluted with the filtrate of the oxygen tower washing filtrate tank 11, washed with the oxygen tower washing filter 8, and then stored in the high-concentration chest 9 for the next bleaching step. Sent to. Table 1 shows the processing conditions and the measurement results.

Comparative Example 1 Using the apparatus shown in FIG. 2, the unbleached cleaning filter 1 is an open type, and the exhaust gas from the degassing tower 7 is not exposed.
Example 1 except for deodorizing and releasing to the atmosphere without supplying to
I went under the same conditions. The results are shown in Table 1.

[0021]

[Table 1]

As is clear from the results shown in Table 1, Comparative Example 1
Then, since 17 m ³ N / Hr of air is entrained when the pulp sheet is crushed by the repulsor 14, 93% or more of high-purity oxygen gas supplied by the oxygen mixer 5 is contained in the pulp slurry after the oxygen mixer 5. Oxygen concentration is 87.4
%, But in the embodiment, the exhaust gas 12 of the degassing tower 7 after oxygen bleaching is stored in the hood of the unbleached cleaning filter 1 as a gas having an oxygen composition higher than the oxygen concentration in the air. Since the exhaust gas having an oxygen concentration of 70% is supplied in Example 1 and the exhaust gas 12 having an oxygen concentration of 78.6% is supplied in Example 2, the oxygen concentration in the pulp slurry after the oxygen mixer 5 is The concentration was 91.2%, and in Example 2, the oxygen concentration was 91.9%, which was higher than in the conventional case, so that delignification proceeded and the Kappa number could be reduced.

[0023]

EFFECTS OF THE INVENTION According to the present invention, the oxygen concentration in the pulp slurry after the oxygen mixer 5 can be made higher than before without changing the amount of oxygen gas charged in the oxygen mixer 5, so that in the oxygen bleaching tower 6, The delignification progressed and the Kappa number could be reduced, and the bleaching chemical unit consumption in the bleaching step after the oxygen bleaching step was improved, and the wastewater pollutants discharged from the bleaching step were reduced to reduce the wastewater treatment cost. Degassing tower 7
In addition, deodorizing equipment and combustion equipment are not required when exhaust gas is released to the atmosphere, which contributes to cost reduction.

[Brief description of drawings]

FIG. 1 is a schematic flow sheet diagram of oxygen bleaching according to the present invention.

FIG. 2 is a schematic flow sheet diagram of conventional oxygen bleaching.

[Explanation of symbols]

1 Unbleached Cleaning Filter 2 Steam Mixer 3 Suction Tank 4 High Concentration Pump 5 Oxygen Mixer 6 Oxygen Bleaching Tower 7 Degassing Tower 8 Oxygen Tower Cleaning Filter 9 High Concentration Chest 10 Cleaning Filtration Tank 11 Oxygen Tower Cleaning Filtration Tank 12 Exhaust Gas 13 Exhaust Fan 14 Repulsor 15 Oxygen gas (vaporized gas of liquefied oxygen or PSA oxygen gas) 16 Deodorizer

─────────────────────────────────────────────────── ─── Continuation of the front page (72) Nobuhiko Takahashi Nobuhiko Takahashi 4-320 Tsukagoshi, Sachi-ku, Kawasaki City, Kanagawa Nihon Oxygen Co., Ltd. (56) References JP-A-51-109307 (JP, A) (58) Field (Int.Cl. ⁷ , DB name) D21C 9/147 D21C 9/06

Claims (2)

(57) [Claims] 1. An oxygen concentration in air so that the upper portion of the unbleached washing filter in the previous step of the oxygen bleaching tower is covered with a hood to make it a closed type , and the pressure in the hood is equal to or higher than atmospheric pressure. A pretreatment method for oxygen bleached pulp, which comprises supplying a gas having a higher oxygen composition. 2. The pretreatment method for oxygen bleached pulp according to claim 1, wherein the gas having an oxygen composition higher than the oxygen concentration in the air is exhaust gas from the degassing tower.