JPH10292278A - Production of polysulfide cooking liquor - Google Patents

Abstract

PROBLEM TO BE SOLVED: To obtain a polysulfide cooking liquor excellent in storage stability and useful for production, etc., of pulp without requiring facility for separating lime mud by oxidizing a sodium sulfide-containing chemical with an oxygen- containing gas under specific conditions. SOLUTION: A sodium sulfide containing chemical having 5-20 g/l (as S) sodium sulfide concentration is oxidized with an oxygen-containing gas fed in 2-3 G/L ratio expressed in terms of pure oxygen in the presence of a calcium compound (generally called as lime mud) produced in a cooking liquor-recovering step and a transition metal compound (a ratio added to sodium sulfide-containing chemical is preferably 0.001-1 g/100 cc), e.g. an oxide or sulfide of manganese to provide the objective polysulfide cooking liquor.

Description

DETAILED DESCRIPTION OF THE INVENTION

[0001]

BACKGROUND OF THE INVENTION 1. Field of the Invention The present invention relates to a method for producing a cooking liquor for producing pulp by digesting a lignocellulosic material with a cooking liquor containing sodium sulfide and / or polysulfide.

[0002]

2. Description of the Related Art Kraft (hereinafter abbreviated as KP) digestion is the world's main method of producing chemical pulp by digesting lignocellulosic substances mainly composed of wood with a mixture of caustic soda and sodium sulfide. It is used in most chemical pulp mills. Polysulfide (Na- (S-) x-Na: x = 2 ~) instead of sodium sulfide (Na-S-Na)
The polysulfide (hereinafter abbreviated as PS) digestion method using 5) has a higher pulp yield than the KP method at the same degree of digestion, and reduces the environmental burden of pulp bleaching wastewater.
It has attracted attention as a method for reducing residual lignin in unbleached pulp. The PS method has been evaluated for its ability to reduce residual lignin without lowering the pulp yield as compared with the current KP method, and has been put to practical use in several plants in Japan.

[0003] As a method for producing a cooking liquor of the PS method, (1)
Method for producing PS by adding elemental sulfur to sodium sulfide
(Hagglundt, Svensk Papperstidn. 49 (9): 191 (1946)
), (2) Method of mixing KP white liquor with black liquor and oxidizing with air (Landmark et al., Norsk Skogind. 14 (5): 221 (1961)
), (3) Oxidation of sodium sulfide with a manganese oxide catalyst (Barker, Tappi 53 (6): 1087 (1970)), (4) Air oxidation with a water-repellent activated carbon catalyst (Smith et al.,
US Patent 4,024,229 (1977)), (5) A method of air oxidation with a granular activated carbon catalyst having a limited pore size and pore volume ratio of activated carbon, an average particle size, etc. (Suzuki et al., JP-A-61-25
9754), (6) A method of air-oxidizing green liquor or white liquor with lime mud (Dorris, US Patent 5,082,526 (1992)) and the like have been proposed.

In the method (1) for producing PS, since the chemical balance of sodium / sulfur after sulfur addition is greatly changed, there is a need for a method for removing sulfur in a separate step, and there is a problem in operability such as corrosion. It has not been put into practical use due to problems such as a complicated apparatus, a high degree of sulfuration and a strong odor.

In the method (2), the amount of air for oxidation and the concentration of black liquor are not described, but the black liquor is converted into white liquor by 20 to 7 times.
By adding 0%, a PS concentration of 1.5 to 2.2 g / liter as S was obtained. According to the report by the inventors of (5) (Paper Pulp Technology Times 32 (8): 1 (1989)), the improvement effect of pulp yield by PS digestion was 0.8% as per wood.
The pulp yield improvement clearly exceeds 1% at the PS addition rate of S, confirming the effect of PS. If the active alkali concentration of the white liquor is a general concentration of 100 g / liter as Na ₂ O, the active alkali addition rate is 20%, and the addition amount of the PS solution is the same as the white liquor, the PS concentration is calculated to be 4.0 g / Liter as S or more is required. At a PS concentration of 1.5 to 2.2 g / l as S, the PS concentration is about half that of 4.0 g / l as S, which improves pulp yield, and is low in concentration. Since it is a digested waste liquid and contains a large amount of foaming components, the method of oxidizing by blowing with air is extremely intense foaming and has a problem in operability, and has not been put to practical use.

In the method (3), a white liquor is supplied to a reaction tower and oxidized with an oxygen-containing gas while circulating a catalyst slurry such as manganese dioxide, or the white liquor and an oxygen-containing gas are supplied to a catalyst packed tower. Although a method is presented, according to the examples, it takes 3 hours and a long time to generate PS, so that a large reactor is required, and a step of separating the catalyst and white liquor is required. It has problems and has not been put to practical use.

The method (4) is a method practically used in Japan as the Moxy method, but it is said that there are problems with the life and filtration performance of the activated carbon catalyst used. Although some improvements have been made over the method of 4), it has been put into practical use in Japan, but the equipment costs of both methods (4) and (5) are high,
There is a problem that the activated carbon catalyst needs to be periodically washed with an acid to re-activate the catalyst.

[0008] The prior arts (1) to (5) can be summarized as follows: the chemical balance of sodium / sulfur is broken, the equipment is complicated, the reaction takes a long time, and large equipment is required. And the PS concentration is not sufficient.

The inventors presume that the method (6) generates PS by some catalytic action of lime mud. However, air oxidation using lime mud alone does not provide sufficient selectivity of the oxidizing action of the catalyst. Rather than the inventor's report in (6) (Pulp
According to Paper Can., 95 (10): T394 (1994), a method of adding MnO ₂ to lime mud to increase the selectivity of PS generation is proposed.

When lime mud is added to green liquor or white liquor and air-oxidized, a step of separating the lime mud from the PS-containing chemical solution is required. In general, a white liquor clarifier is used as a separation device, but it takes 12 to 36 hours depending on the scale of the factory until the liquor is collected there and used for digesting chips. It is known that PS is extremely unstable at high temperatures.
It is a problem whether the PS concentration that can exhibit the pulp yield improving effect of the cooking can be maintained. The inventor of (6) pointed out this point and made it a subject for further study.

[0011]

DISCLOSURE OF THE INVENTION The present inventors have established a PS digestion method which can be put to practical use with simple additional equipment changes without major changes in the current chemical recovery equipment. . When lime mud was added to green liquor or white liquor and air-oxidized, sufficient PS could not always be obtained to improve the pulp yield depending on the experimental conditions.
This is considered to be due to the influence of fluctuations in the particle size, shape, specific surface area, impurities and the like of the lime mud. Further, in this method, by using MnO ₂ as a catalyst, PS
Although the production efficiency is improved and a sufficient PS concentration can be ensured immediately after the oxidation, 90 ° C. set as the retention condition until use is set.
The PS concentration after storage for 24 hours was insufficient for practical use. Therefore, in order to put the lime mud method to practical use, the storage stability of the chemical solution containing oxidized PS was dramatically improved,
P sufficient for manifestation of the effect of PS digestion when served for cooking
A technique for ensuring an S concentration of 4.0 g / liter as S or more is required, and the present invention is to solve these problems.

[0012]

In order to solve the above problems, the present invention employs the following constitution. (1) Oxidizing a chemical solution containing sodium sulfide with an oxygen-containing gas in the presence of a calcium compound (collectively called lime mud) and a transition metal compound generated in a cooking chemical recovery step,
A method for producing a polysulfide cooking liquor, wherein the oxygen-containing gas is supplied and oxidized in a G / L ratio of 2 to 3 in terms of pure oxygen in a method for producing a polysulfide cooking liquor. (2) The sodium sulfide concentration of the chemical solution containing sodium sulfide is 5 to 20.
g / liter as S, and the transition metal compound is an oxide or sulfide of manganese, and the addition rate to the sodium sulfide-containing chemical solution is 0.001 to 1 g / 100 cc ( The method for producing a polysulfide cooking chemical according to 1).

[0013]

DETAILED DESCRIPTION OF THE PREFERRED EMBODIMENTS The chemical solution containing sodium sulfide used in the present invention is mainly a green liquor or a white liquor produced in a KP chemical recovery process.

The temperature of green liquor and white liquor handled in the chemical recovery process of KP is generally 70 to 110 ° C.
No heating or cooling is required during the oxidation reaction. The reaction time in the present invention is sodium sulfide concentration of the sodium sulfide-containing chemical solution, lime mud, transition metal oxide, the amount of quinone compound added,
Although it depends on the processing conditions such as the amount of air supplied, it is 30 to 240 minutes, and the residence time of the green liquor in the causticizing tank is generally 60 to 240 minutes.
Since this is 180 minutes, it is suitable to apply this residence time to the reaction time. When the existing white liquor clarifier is used in the step of separating lime mud and the generated PS-containing chemical solution, the residence time there is 12 to 36 hours,
The temperature there is 70-100 ° C. In the storage stability test in the present invention, the conditions were set at 90 ° C. for 24 hours.

As lime mud, there is a calcium compound generated in the cooking chemical recovery step. The amount of lime mud added is in the range of 50 to 500 g / liter, and the lime mud concentration in the causticizing tank is generally high. Is 100-125 g / l
Therefore, the operating conditions in the normal KP cooking chemical recovery step are sufficient. The lime mud having a particle size of 1 to 15 μm used in a usual KP cooking chemical recovery step is sufficient, and a specific surface area of 1 to 10 m ² / g is sufficient. However, the smaller the particle size and the larger the specific surface area, the greater the catalytic action of the lime mud and the reaction is accelerated, but the excessive oxidation reaction is easily caused, and the sedimentability of the lime mud deteriorates There is a problem.

Considering the simplification of the device as described later,
It is considered more reasonable to target green liquor.In this case, it is appropriate to use a lime mud mainly composed of calcium oxide calcined at 800 ° C. or higher. In the case of white liquor, it is appropriate to use an unfired lime mud after causticization. The concentration of sodium sulfide in the chemical solution containing sodium sulfide is 1 to 40 g / liter as S, which is 5 to 20 g / liter as the concentration of the cooking chemical used in ordinary KP cooking.
Since it is S, there is no need to specially concentrate and dilute it. However, when the sodium sulfide concentration is low, it is necessary to relax other reaction conditions, and when the concentration is high, it is necessary to strengthen the treatment conditions.

Among the oxidation catalysts used in the present invention, manganese oxides or sulfides have excellent oxidation catalytic activity. However, the storage stability of the obtained PS-containing chemical solution is poor,
The amount of PS after storage is a small value for the effect of PS digestion to clearly appear. These addition rates are 0.001 per chemical
~ 1 g / 100cc, more preferably 0.001 ~ 0.1 g / 100c
However, if it is too small, the oxidation rate is low and P
The S selectivity decreases. If the amount is too large, the oxidation reaction proceeds excessively, and the generated PS is oxidized to lower the PS selectivity. 0.
Around 05 g / 100cc is appropriate. Manganese oxides include MnO, MnO ₂ , Mn ₂ O ₃ , Mn ₃ O ₄ , Mn ₂ O
_7, but MnO ₂ is particularly preferred. Further, a sulfide generated by adding a water-soluble salt of manganese to a chemical solution containing sodium sulfide can also be used. Further, for the purpose of improving the PS selectivity, another transition metal compound such as nickel can be mixed and used with the manganese compound.

An oxygen-containing gas is used to oxidize the chemical solution containing sodium sulfide used in the present invention. As the oxygen-containing gas, air is sufficient, but pure oxygen can be used, and PSA oxygen having an oxygen concentration lower than that of pure oxygen can also be used. Using high concentration oxygen, such as pure oxygen or PSA oxygen, can reduce the amount of use compared to using low concentration oxygen, such as air. By using a higher concentration of oxygen, the oxygen partial pressure in the drug solution increases, and the solubility of oxygen in the drug solution increases,
Since the oxidation efficiency increases, the PS concentration increases. However, it is necessary to select conditions under which excessive oxidation reaction is prevented and PS is not further oxidized. However, since the use of high-concentration oxygen requires a small volume of gas to be supplied, it is possible to oxidize with pressurized pure oxygen or pressurized PSA oxygen in a pipeline reactor. In order to control an excessive oxidation reaction, it is also effective to use a low-concentration oxygen-containing gas having a lower oxygen concentration than air, for example, air diluted with PSA oxygen purge gas.

The supply amount of the oxygen-containing gas varies depending on the sodium sulfide concentration of the sodium sulfide-containing chemical solution, but from the viewpoint of the storage stability of the PS described later, the G / L ratio is 2 to 2 in terms of pure oxygen.
3 is required. Here, the G / L ratio is a standard state equivalent volume (liter) of gas with respect to the volume (liter) of the sodium sulfide-containing chemical solution.

The oxidation reaction in which PS is generated by oxidizing a chemical solution containing sodium sulfide with an oxygen-containing gas is represented by the following reaction formula (a).
However, excessive oxidation should be prevented because excessive oxidization produces sodium thiosulfate that is ineffective for digestion due to the reaction formulas (b) and (c). is important. 4Na ₂ S + O ₂ + 2H ₂ O = 2Na ₂ S ₂ + 4NaOH (a) 2Na ₂ S + 2O ₂ + H ₂ O = Na ₂ S ₂ O ₃ + 2NaOH (b) 2Na ₂ S ₂ + 3O ₂ = 2Na ₂ S ₂ O ₃ (c) In the above equation (a), PS is Na to simplify the reaction equation.
2S2, but actually, as described above, Na ₂ Sx (x
= 2 to 5). The reaction formulas (b) and (c) show that if the oxidation reaction goes too far, sodium sulfide and PS in the reaction solution are oxidized to sodium thiosulfate.

Since sodium thiosulfate is a substance that does not exhibit a cooking effect, it is necessary to minimize the production of PS cooking liquor. In other words, in the PS generation reaction using the oxygen-containing gas, it is necessary to generate sodium sulfide at the highest possible concentration of PS and sodium thiosulfate at the lowest possible concentration. As an indicator of the PS generation reaction, the oxidation rate, which is the ratio of oxidized sodium sulfide after the reaction to the sodium sulfide before the reaction, the PS generation rate, which is the ratio of the amount of PS generated after the reaction to the sodium sulfide before the reaction, The PS selectivity, which is the ratio of the post-PS production to the post-reaction oxidized sodium sulfide, is used. The larger the PS generation rate and PS selectivity, the better,
The PS generation rate is desirably 30% or more, and the PS selectivity is desirably 40% or more. The oxidation rate is given by the following equation 1, the generation rate is given by the following equation 2,
The PS selectivity is shown by the following Equation 3, respectively.

[0022]

(Equation 1)

[0023]

(Equation 2)

[0024]

(Equation 3)

PS is a very unstable substance under high alkali and high temperature conditions, and is said to be decomposed by the disproportionation reaction as shown in the formula (d) without oxygen. In order to simplify the reaction formula, x is shown as x = 3. Since _{2Ns 2 S 3 + 2NaOH + H} 2 O = 4NaSH + Na 2 S 2 O 3 (d) storage stability of the PS is the better the higher the sodium sulfide concentration, excessive from the viewpoint of storage stability Oxidation should be prevented. In order to secure a sufficient amount of PS immediately after the oxidation, it was important to increase the oxidation rate while maintaining the PS selectivity. However, considering the storage stability of PS, it is disadvantageous to increase the oxidation rate. The feature of the present invention is that the oxygen-containing gas is converted into pure oxygen equivalent G
By supplying the / L ratio in the range of 2-3, the oxidation rate is reduced, and the sodium sulfide concentration immediately after oxidation is maintained at a high level. When the G / L ratio in terms of pure oxygen is less than 2, storage stability is excellent, but the PS concentration immediately after oxidation is low, and even after storage, the concentration is insufficient for the PS digestion effect to be exhibited. If it exceeds 3, the PS concentration immediately after oxidation is high, but the storage stability decreases.

Although the present invention has been described above, the present invention will be described below in more detail with reference to examples, but these examples do not limit the present invention in any way. In this example, the following evaluation items were used for evaluation. <Evaluation items> PS concentration: PS concentration (g / liter as S) with respect to the amount of the chemical solution containing sodium sulfide Oxidation rate: described in the above formula 1 PS selectivity: described in the above formula 3

<Example 1> Green liquor (Na ₂ CO ₃ 105 g / L as Na ₂ O, Na ₂ S 25 g / L) heated to 90 ° C. in a stainless steel container which can be heated and maintained by a rubber heater a
s Na ₂ O) Add 2.4 liters, add the following A, and raise the temperature
Maintain at 95 ° C for 1 hour with a 4 cm diameter 4 stirrer
The mixture was stirred at 0 RPM to carry out a causticization reaction. Then, air was supplied from the bottom of the vessel at a flow rate of 1.0 liter / min (converted to standard conditions), and a 4-blade 4-cm diameter stirrer was stirred at 500 RPM to obtain an O ₂ -converted G / L ratio of 2.3. Oxidation reaction was carried out until reaching. A: MnO ₂ 1.2 was added to 259 g of lime mud that had been dried in advance (corresponding to 108 g / liter per chemical solution containing sodium sulfide).
g (equivalent to 0.05 g / 100 cc per sodium sulfide-containing chemical) and baked at 1200 ° C for 2 hours. Immediately after the completion of the oxidation reaction and after storage at 90 ° C for 24 hours, the PS concentration of the chemical, etc. The evaluation is shown in Table 1.

<Example 2> and <Comparative Example 1> to <Comparative Example 2> A PS was manufactured in the same manner as in Example 1 except that the conditions shown in Table 1 were changed.

[0029]

[Table 1]

[0030]

The advantages of the present invention as compared with the above-mentioned known prior art are as follows. (1) Lime mud and a single transition metal oxide and / or transition metal sulfide are allowed to coexist in a sodium sulfide-containing chemical solution,
In a method for producing a polysulfide digester by oxidizing with an oxygen-containing gas, the oxygen-containing gas is converted into pure oxygen equivalent G
By supplying the / L ratio in the range of 2 to 3, a PS-containing chemical solution having excellent storage stability can be prepared. A conventional white liquor clarifier may be used in the step of separating the lime mud and the PS-containing chemical solution, and no new lime mud separation equipment is required. (2) Manganese and nickel used here are not expensive noble metal oxidation catalysts. In addition, their addition rates are as small as 0.001 to 0.1 g / 100cc per sodium sulfide-containing chemical solution, and they are reused by circulating the cooking chemical recovery process together with lime mud. Since it is a catalyst in principle and is not consumed in the reaction, it is economical because only a small amount of loss needs to be replenished.

Claims (2)

[Claims] 1. A calcium compound (collectively referred to as lime mud) produced from a chemical solution containing sodium sulfide in a cooking chemical recovery step.
And oxidizing with an oxygen-containing gas in the presence of a transition metal compound to produce a polysulfide digestion liquid, wherein the oxygen-containing gas has a G / L ratio of 2 to 3 in terms of pure oxygen.
A method for producing a polysulfide digester, characterized in that the solution is supplied and oxidized in the range described above. 2. The sodium sulfide-containing chemical solution having a sodium sulfide concentration of 5%.
~ 20 g / liter as S, and as the transition metal compound,
The method for producing a polysulfide cooking chemical according to claim 1, wherein the addition rate to the chemical liquid containing manganese oxide or sulfide and containing sodium sulfide is 0.001 to 1 g / 100 cc.