JPH0532099B2 - - Google Patents

Description

[Detailed description of the invention]

[Industrial Application Field] The present invention relates to an oxidation treatment method for converting a sodium sulfide-containing aqueous solution into a polysulfide-containing aqueous solution and a catalyst thereof. The present invention can be particularly applied to the oxidation reaction treatment of a sodium sulfide-containing aqueous solution in the Kraft method recovery process of pulp manufacturing, and provides excellent effects. It can also be used as a method for producing sodium hydroxide substitutes containing sodium thiosulfate by completely oxidizing white liquor, green liquor, and weak liquor from the Kraft method recovery process. Furthermore, various alkaline cleaning solutions containing sodium sulfide that generate foul-smelling gas (H ₂ S) are completely oxidized,
It can be made odorless and can also be used for waste liquid treatment to prevent pollution. [Prior art] The kraft method as a pulp manufacturing method has the following advantages: it has a complete energy and chemical recovery method, it can be applied to a wide range of tree species such as softwood, hardwood, and southern wood, and it can produce high-quality pulp. It is known to be an excellent method suitable for mass production because it takes a short time. However, this method has the disadvantage that the pulp yield is lower than that of the sulfite method. For this reason, various methods for improving the pulp yield with respect to the kraft process have been investigated. As one of them, Hagglund E [Svensk]
It has been widely known since the research of Papperstidn 49(9)191 (1946) that cooking with sodium polysulfide improves the pulp yield from wood. This method oxidizes and stabilizes the aldehyde groups at the ends of hemicellulose in wood, thereby suppressing the elution of hemicellulose by alkali, improving the cooking yield, and increasing the yield. Taking advantage of this advantage, polysulfide cooking methods were developed.
However, this method was not widely adopted because it was difficult to create fibers with a balance of sodium and sulfur, the amount of sulfur added was large, and it was difficult to solve the problem of bad odors caused by sulfides. I didn't need it. Therefore, various methods of producing sodium polysulfide without changing the balance of sodium and sulfur using the Kraft method were subsequently investigated (for example, USP Nos. 3216887, 3470061, 3653824).
No., Special Publication No. 53-25043, Japanese Patent Publication No. 56-149304
Publications, etc.). These all relate to oxidizing agents that oxidize sodium sulfide. Furthermore, methods other than those described above are also known in which sodium sulfide is oxidized using activated carbon as a catalyst. For example, Yoshida (Netsusokutei 8(1)1981,
2-5) reported that the surface functional groups of carbon black were involved in promoting the sodium sulfide oxidation reaction as a result of dispersing carbon black in an aqueous sodium sulfide solution and causing a reaction. However, when considering use on an industrial scale, the method of adding powdered activated carbon to a sodium sulfide-containing aqueous solution and blowing air or oxygen into it while stirring to cause an oxidation reaction is the most effective method. It is very difficult to separate, recover, and reuse it, and it cannot be called an industrial method. On the other hand, a method has also been proposed that aims to eliminate the above-mentioned drawbacks when powdered activated carbon is used industrially and to efficiently oxidize sodium sulfide. That is, in Catalyst Vol. 23, No. 4, P293-295 (published on 4th, 1981), activated carbon is hydrophobically modified with a water-insoluble substance such as polytetrafluoroethylene or polyethylene, and the hydrophobized activated carbon is used to produce activated carbon. It is described that the separation and recovery of the compound becomes easier, and at the same time, the catalytic activity of the compound is improved. Furthermore, according to Japanese Patent Publication No. 50-40395, carbon or activated carbon can be used as polytetrafluoroethylene,
A method has been proposed for producing sodium polysulfide and sodium hydroxide from sodium sulfide and sodium hydrosulfide using a catalyst that has been partially hydrophobized with a hydrophobic substance such as polyethylene, polystyrene, or fluorocarbon resin. [Problems to be Solved by the Invention] In general, activated carbon has a high surface area, excellent corrosion resistance, and electrical conductivity. It is used as. Here, when considering the preferable conditions required for a catalyst for oxidizing an aqueous sodium sulfide solution, the following can be considered. That is, (1) the oxidation reaction activity of sodium sulfide is high; (2) Ensuring sufficient supply of oxygen to the catalyst surface. (3) The catalyst has electrical conductivity because the oxidation reaction of sodium sulfide is a redox reaction that involves the transfer of electrons. (4) When processing pulp cooking white liquor, it must be alkaline resistant as it is strongly alkaline. (5) When considering industrial use, special separation, recovery, and regeneration operations of the catalyst are not required. (6) The catalyst is easily economically available. Among these conditions, conditions 3, 4, and 6 are fully satisfied by conventional activated carbon, and condition (5) is almost solved by the hydrophobically treated activated carbon. However, when hydrophobically treated activated carbon is used as an oxidation catalyst, supply of oxygen to the active sites on the catalyst surface becomes a problem. In order to ensure smooth oxygen supply, the hydrophobic treatment applied to activated carbon must be only partial. On the other hand, the partial hydrophobization treatment is special, and it is not clear whether the partial hydrophobization effect can be uniformly exhibited. Furthermore, in the oxidation reaction of sodium sulfide, under conditions that allow a sufficient supply of oxygen to the reaction surface on the catalyst, hydrophobic treatment is undesirable because it may crush the active sites on the activated carbon catalyst surface with hydrophobic substances. , regarding point (1) above, it would actually be a hindrance. Furthermore, when hydrophobization is applied partially, depending on the amount of hydrophobic substances, even the outer surface of the catalyst may be covered, making it impossible to effectively utilize the inner surface of the pores of the activated carbon. A problem occurs with the oxygen supply shown in . [Configuration of the Invention] According to the present invention, there is provided a catalyst for oxidizing a sodium sulfide-containing aqueous solution for converting sodium sulfide in an aqueous solution made of activated carbon fibers into sodium polysulfide. Further, according to the present invention, the sodium sulfide-containing aqueous solution is brought into contact with air or oxygen in the presence of activated carbon fibers to convert the sodium sulfide in the aqueous solution into sodium polysulfide. A method for oxidizing an aqueous solution is provided. In the present invention, the activated carbon fiber used as a catalyst for the oxidation reaction treatment of an aqueous solution containing sodium sulfide is thermosetting rayon-based, PAN (polyacrylonitrile)-based, phenol-based, thermoplastic PVA (polyvinyl alcohol)-based, or pitch-based. It is possible to use products manufactured from various raw materials such as The non-surface area of activated carbon fibers produced from these raw materials varies depending on the type of raw material and the degree of activation, but from the standpoint of the number of reactive active sites, basically the larger the better. However, activated carbon fibers that have been activated in order to increase their specific surface area have the disadvantage of weak strength, and the specific surface area of the activated carbon fibers used in the present invention is in the range of 300 to 2500 m ² /g. Preferably. The specific surface area of the activated carbon fiber was measured using an automatic gas adsorption device manufactured by Carlo Erba, Italy.
The nitrogen isothermal adsorption curve was measured using Sorptomatic 1800 and calculated by the Cranston-Inkly method. Since the activated carbon fiber used in the present invention has not been subjected to hydrophobic treatment, its outer surface is covered with a film of an aqueous solution such as sodium sulfide in a reaction state.
Therefore, oxygen is supplied to the catalyst active sites through the coating covering the outer surface. Therefore, the larger the outer surface area of the activated carbon fiber, the better. Here, the outer surface area is the surface area on the outer shape of the catalyst, excluding the inner surface area of the catalyst pores, and is calculated from the average thickness of the fibers with a true specific gravity of 1.8 g/cc. but,
Increasing the outer surface area results in activated carbon fibers becoming thinner and less strong, which poses a problem in industrial use. Therefore, the outer surface area of the activated carbon fiber used in the present invention is preferably in the range of about 0.1 to 5 m ² /g. The form of the activated carbon fiber having these properties is appropriately selected depending on the use, such as chop, yarn, felt, sheet, cloth, mat, etc. Furthermore, when filling a reactor and using it, it can be used as is, but it can be further processed into a honeycomb shape, felt wrapped into a cylindrical shape, etc. It can be used in any suitable shape. The sodium sulfide-containing aqueous solution used as the raw material to be treated in the present invention can be any of various aqueous solutions containing sodium sulfide, and may also contain other substances such as sodium disulfide, sodium hydrosulfide, and carbonates. I can do it. Examples of such aqueous solutions include white liquor, green liquor, and weak liquor from the Kraft method recovery process. Next, the oxidation reaction of a sodium sulfide-containing aqueous solution, which is the object of the present invention, will be explained. in this case,
The main reactions consist of main reaction 1, which is an oxidation-reduction reaction that produces polysulfides, and side reactions 2 and 3, which produce sodium thiosulfate. 4Na ₂ S+O ₂ +2H ₂ O=2Na ₂ S ₂ +4NaOH (1) 2Na ₂ S+2O ₂ +H ₂ O=Na ₂ S ₂ O ₃ +2NaOH (2) 2Na ₂ S ₂ +3O ₂ =2Na ₂ S ₂ O ₃ (3) In addition, in the above formula, sodium polysulfide is shown as sodium disulfide (Na ₂ S ₂ ), but in general, it is represented by the formula Na ₂ Sx, and compounds in which X = 2 to 5 exist. It is believed that. As can be seen from the above equation, when a sodium sulfide-containing aqueous solution is subjected to an oxidation reaction, the reaction of equation (1) in which sodium polysulfide is produced and the reaction of equation (2) in which sodium thiosulfate is produced occur simultaneously. Also,
Also important is the reaction of formula (3) in which the produced sodium polysulfide further produces oxidized sodium thiosulfate as a by-product. In other words, in order to obtain sodium polysulfide by oxidizing sodium sulfide with air or oxygen, proceed with the main reaction of equation (1) among the three reactions above, and then proceed with the main reaction of equation (1) and
It is necessary to suppress side reactions of the formula. As is clear from the above reaction equation, the ratio of oxygen to sodium sulfide is large in the reactions of equations (2) and (3), so by appropriately selecting the reaction conditions, the selectivity of sodium polysulfide can be increased to some extent. can be done. In other words, the main reaction of formula (1) must proceed efficiently in the presence of a relatively small amount of oxygen, and the region where sodium polysulfide is produced must be free of oxygen. When using a flow-through reaction tube,
This allows for efficient gas-liquid contact, as well as
This is made possible by the use of highly active catalysts. From the above, in the present invention, the contact method when bringing an aqueous solution containing sodium sulfide into contact with air or oxygen is to fill a fixed bed reaction vessel with an activated carbon fiber catalyst and supply gas/liquid from the top of the reactor. , it is preferable to use a cocurrent trickle flow of contact. This is important in order to form a thin liquid film on the activated carbon fiber catalyst having a large external surface area and to ensure smooth supply of oxygen overall. vessel·
In a contact method in which the liquid is supplied from the bottom of the reactor and air is bubbled through the liquid, it is difficult to carry out the reaction efficiently even if activated carbon fibers with a large external surface area are used. In addition, as shown in Side Reaction 3 above, polysulfides easily react with oxygen to produce sodium thiosulfate, so co-current contact, in which the oxygen partial pressure is lower in the later stages of the reaction, is more effective than counter-current contact. preferable. Next, the preferred redox reaction conditions are described below when a sodium sulfide-containing aqueous solution is fed cocurrently with air or oxygen from above into a fixed bed reactor filled with an activated carbon fiber catalyst. . The reaction is carried out at a temperature range of about 50-150°C.
The temperature of the white liquor and green liquor in the Kraft recovery process generally ranges from 70 to 100°C. Therefore, if the reaction is carried out at the same temperature, there is no need for special heating or cooling for the reaction, which is economical. Moreover, the reaction pressure condition is preferably in the range of 0 to 10 kg/cm ² ·G. In particular, in the present invention, since oxygen is supplied through the liquid film formed on the outer surface of the fibers of the activated carbon fiber catalyst, the reaction pressure is preferably higher. However, as shown above, it is not preferable to unnecessarily increase the reaction pressure due to the production of sodium thiosulfate in side reaction 2 or the disappearance reaction of sodium polysulfide in side reaction 3. Therefore, the range of reaction pressure conditions described above is preferred. The air or oxygen/liquid ratio is preferably in the range of about 5 to 500 Nl/l based on the fixed bed reactor inlet. The air or oxygen/liquid ratio determines the air (oxygen) diffusion resistance of the liquid film formed on the outer surface of the activated carbon catalyst, and the larger this value, the more oxygen can be supplied. However, unnecessarily increasing the size promotes the progress of side reactions, which is not preferable. The liquid hourly space velocity on a weight basis can be varied in the range of 5 to 500 Hr ^-1 . Advantageous conditions are selected depending on the activity of the catalyst, the above-mentioned reaction conditions, and the desired product and its amount. [Effects of the Invention] The activated carbon fiber catalyst of the present invention for oxidizing a sodium sulfide-containing aqueous solution with air or oxygen and its method of use are particularly superior to the previously known methods described above as follows. It is. (1) By using activated carbon fibers as a catalyst, the external surface area of the catalyst was increased, making it easier to supply oxygen to the surface of the catalyst pores. (2) In addition, since the fiber diameter of activated carbon fibers is small, there is no need for oxygen to diffuse through secondary pores on the order of 100 to 1000 Å and tertiary pores on the order of several μm that exist in general activated carbon. This allows direct and easy access to pores on the order of 10 Å, and as a result, the reaction proceeds smoothly.
Moreover, the reaction products can now be easily separated. (3) By not applying hydrophobic treatment, the inner surface of the pores of the activated carbon fiber can be completely utilized for the reaction.
Furthermore, pretreatment such as hydrophobic treatment is no longer necessary. (4) For these reasons (1) to (3), the activity per unit weight of the catalyst could be increased by several times or more than that of the conventional catalyst (see Table 3 below). (5) Since the activated carbon fiber itself does not change due to the reaction, there is no need for regeneration treatment for activation as in the case of conventional quinone-stabilized redox resins or manganese dioxide. (6) Since the activated carbon fiber catalyst is used in a fixed bed type reaction vessel, there is no need for separation operations from reactants as in the case of adding powdered activated carbon or quinones. (7) Activated carbon fibers have good moldability and can be easily processed into various shapes, so they can be used in shapes that match the purpose of use. (8) Since the activated carbon fiber catalyst is highly active, the packing density in the reaction vessel can be reduced. Therefore, the number of voids in the reactor increases, the flow resistance of gas and liquid is small, and the power energy cost is reduced. (9) In addition, it is extremely resistant to clogging in the reactor due to suspended solids (hereinafter referred to as SS) contained in the white liquor and green liquor during the Kraft method recovery process. (10) Since the outer surface area of the fibers is large, it is resistant to a decrease in catalytic activity due to the precipitation of calcium carbonate and sodium carbonate contained in white liquor or green liquor, and is also easily regenerated by acid. (11) Since activated carbon fibers are made of carbon, they are chemically resistant to corrosion and have electrical conductivity necessary for redox reactions, making them excellent for use in the present invention. (12) Since activated carbon fiber has a fiber shape, there is less risk of generating dust like crushed charcoal. [Example] The present invention has been described above, and examples for explaining the present invention in further detail will be described below. Example 1 Reaction tests were carried out using various activated carbon fiber catalysts in a small flow reactor having a glass fixed bed reactor with a diameter of 26 mmφ and a height of 50 cm. The prepared white liquor composition and reaction test conditions used in the reaction test are shown in Table-1 and Table-2, respectively, and the catalyst activity test results are shown in Table-2.
Shown in 3. The activity of the catalyst was shown by calculating the rate constant using the weight-based liquid space velocity, with the rate of decrease in sodium sulfide as a secondary reaction. Further, the properties of the tested activated carbon fibers, such as specific surface area and outer surface area, are as shown in Table 3.

【table】

[Table] Comparative Example 1 A reaction test was conducted on PAN-based carbon fiber (not activated) in the same manner as in Example 1, and the results are shown in Table-3. Comparative Example 2 500 g of an emulsion solution containing 60 g of polytetrafluoroethylene was mixed and impregnated with 1 kg of activated carbon particles having an average particle diameter of 2.9 mmφ manufactured by Nippon Carbon Co., Ltd. while stirring the granular activated carbon. After that, it was placed in a hot air dryer and the moisture was dried at a drying temperature of 190°C. This dried product was used as a hydrophobically treated granular activated carbon catalyst, and its performance was tested under the same conditions as in Example 1. The test results are also shown in Table 3.

[Table] Example 1 and Comparative Examples 1 and 2 shown in Table 3 above
The results show that the activated carbon fibers of the present invention have significantly superior catalytic activity compared to conventionally known hydrophobically treated granular activated carbon catalysts and carbon fibers having a similar shape, and can also produce polysulfides, which is the object of the present invention. It was found that the product was produced efficiently, and it can be said to be an excellent catalyst for the sodium sulfide oxidation reaction. Example 2 Using the activated carbon fiber catalyst shown in Experiment No. 4 of Example 1, the activity was tested under various reaction conditions. The reaction test was carried out using a small flow reactor with a stainless steel fixed bed reaction vessel with a diameter of 1.25 inches and a length of 50 cm, in which the factory white liquid shown in Table 4 was brought into contact with air in parallel flow as a trickle flow. Catalytic activity was tested under various reaction conditions. The test results are summarized in Table-5. The amount of activated carbon fiber catalyst charged into the reaction vessel was 4.7 g. Table 4 (Composition of industrial white liquor) Na ₂ S (g/, Na ₂ O equivalent): 31.4 NaOH (g/, Na ₂ O equivalent): 74.8 Na ₂ CO ₃ (g/, Na ₂ O equivalent): 17.6 Na ₂ S ₂ O ₃ (g/, S conversion): 3.0 Na ₂ S ₂ O ₃ (g/, S conversion): 0.8 SS (wt-ppm): 140

[Table] Example 3 Industrial white liquor with the composition shown in Table 4 was filtered and SS
was set at 10 wt-ppm, the weight-based liquid hourly space velocity was set at about 50 under the reaction conditions shown in Table 2, and a 1000-hour active life test was conducted using the same reaction apparatus as in Example 2. FIG. 1 shows the relationship between the amount of sulfide produced and the processing time, and FIG. 2 shows the relationship between the catalyst activity and the treatment time as graphs. From the results shown in FIGS. 1 and 2, it was found that the activated carbon fiber catalyst can maintain high activity over a long period of time and can also stably generate polysulfides. Furthermore, no clogging of the catalyst packed bed was observed despite the presence of SS content. From these facts,
It can be seen that the activated carbon fiber catalyst is an excellent catalyst for the oxidation reaction of sodium sulfide and is extremely effective as an industrial catalyst.

[Brief explanation of the drawing]

FIG. 1 is a graph showing the relationship between polysulfide production amount and treatment time, and FIG. 2 is a graph showing the relationship between catalyst activity and treatment time.

Claims (1)

[Claims] 1. A catalyst for oxidizing a sodium sulfide-containing aqueous solution for converting sodium sulfide in an aqueous solution made of activated carbon fibers into sodium polysulfide. 2 Activated carbon fiber has a specific surface area of 300 to 2500 m ² /g
The catalyst according to claim 1, which is large in size and has an external surface area of 0.1 to 5 m ² /g. 3. A method for oxidizing a sodium sulfide-containing aqueous solution, which comprises contacting the sodium sulfide-containing aqueous solution with air or oxygen in the presence of activated carbon fibers to convert the sodium sulfide in the aqueous solution into sodium polysulfide. 4 Activated carbon fiber has a specific surface area of 300 to 2500 m ² /g
The method according to claim 3, wherein the method has an outer surface area of 0.1 to 5 m ² /g. 5. Claims 3 or 4 supplying the sodium sulfide-containing aqueous solution and air or oxygen in parallel flow to the fixed bed reactor filled with the activated carbon fibers.
The method described in section.