JPS6339716B2 - - Google Patents

Description

Claim 1. A kraft process comprising converting kraft black liquor to green liquor and then oxidizing the green liquor in the presence of an oxygen-containing gas and an effective amount of a spent neutral sulfite semichemical black liquor additive. How to dispose of used black liquor from the law. 2. The method of claim 1, wherein the additive is 1-15% by weight of NSSC spent liquor. 3 The oxidation is carried out at atmospheric pressure in the presence of air at 50 to 150
2. A method according to claim 1, which is carried out in the temperature range of .degree. 4. Recovering spent black liquor from the Kraft process, converting the Kraft black liquor to green liquor, and converting the green liquor containing sodium sulfide into an oxygen-containing gas and an effective amount of spent neutral sulfite semi-chemical (NSSC). By oxidizing the black liquor in the presence of additives, the sodium sulfide of the green liquor is converted to sodium thiosulfate and the thus oxidized green liquor is added to the cooking liquor in the neutral sulphite semichemical (NSSC) pulping process. Craft methods including using it as part of a method for reusing used black liquor. 5. The method of claim 4, wherein the additive is 1-15% by weight NSSC. 6 The oxidation is carried out at atmospheric pressure and in the presence of air.
5. The method according to claim 4, which is carried out in the temperature range of .degree. 7 The process of pulping lignocellulosic material by both the Kraft method and the NSSC method, the step of converting the black liquor from this Kraft method into green liquor containing sodium sulfide, and the process of pulping the lignocellulosic material by both the Kraft method and the NSSC method, and the step of converting the black liquor from the Kraft method into green liquor containing sodium sulfide, and A process of oxidizing the green liquor with gas-containing oxygen using sulfite semi-chemical black liquor as a catalyst to convert the sodium sulfide of the green liquor into sodium thiosulfate, and pulping this oxidized green liquor in the NSSC process. A method for pulping a lignocellulose material, which comprises a step of adding it to a liquid. DESCRIPTION OF THE INVENTION This invention relates to the recovery and use of chemicals used in the production of paper pulp by the Kraft process and the neutral sulfite semichemical (NSSC) pulping process. Kraft processes for producing paper pulp from wood and other lignocellulosic materials require cooking these materials with aqueous solutions of sodium hydroxide and sodium sulfide. This includes a chemical recovery step in which spent liquor ("black liquor") containing dissolved organic matter from the lignocellulose as well as inorganic pulping chemicals is concentrated and burned in a furnace. By this method, the heat equivalent of organic material, which corresponds to about half the weight of the lignocellulosic feedstock, is recovered in the form of water vapor, and the inorganic chemicals are recovered as a melt. This melt consists essentially of a mixture of sodium carbonate and sodium sulfide,
When dissolved in water it forms "green liquor" and then when treated with slaked lime it again forms the pulping chemicals, namely sodium hydroxide and sodium sulfide (white liquor). This sodium sulfide can thus theoretically be recycled in a closed system. In reality, there is always a loss from such a system, so
These must be replenished by the addition of sodium and sulfur containing chemicals, the most common such chemical being sodium sulfate or "mirabilite". Pulping equipment for producing pulp for the production of container materials, e.g. corrugated boxes, in addition to kraft mills produces high yields of pulp suitable for producing corrugated substrates and possibly for inclusion as a component of liners. including a neutral sulfite semi-chemical (NSSC) mill that produces at a high rate. The methods available to recover pulping chemicals from the NSSC process are complex and expensive, and therefore the spent liquor obtained from the NSSC process cannot be used as a "replenishment" of the kraft mill to compensate for losses in the kraft mill. Usually done. This implementation is satisfactory if the kraft mill and the NSSC mill are "matched" so that the amount of chemical loss of the former is similar to the spent chemical content obtained from the latter. This may be the case if the pulp output from the kraft mill is significantly greater than that from the NSSC mill (eg, about 3 times) or if the losses from the relatively smaller kraft mill are unusually high. However, it is common for an imbalance to exist between the two mills, especially as craft mills have been affected due to pollution control regulations that have become more stringent in recent years, and the sodium output from such mills has been affected. and sulfur losses were reduced. It is usually found that the content of inorganic chemicals in NSSC spent liquor is significantly higher than that required as replenishment in kraft mills. Under such conditions, the total stock of sodium and sulfur in the kraft system will increase to the point that the chemicals must be extracted from the system and sold or disposed of. Disposal of such chemicals is difficult due to their adverse environmental impact. Selling is also difficult at times because there is not much demand for the type of chemicals that can be produced by this method. In Australian Patent No. 473185 it was proposed to use Kraft black liquor as a catalyst to oxidize Kraft white liquor and then use the Kraft white liquor for gas refining, oxygen pulp bleaching, ion exchanger regeneration and other applications. . Another method considered is to recirculate the kraft mill chemicals, at least in part, to the NSSC mill;
Thus, the overall amount of new chemicals entering the Kraft-NSSC system is reduced. For example, this can be accomplished by withdrawing the green liquor from the kraft recovery circuit prior to causticization and using this green liquor as the chemical in the NSSC mill. Green liquor is an aqueous solution of sodium carbonate and sodium sulfide, generally in a ratio of about 4:1, which in theory can replace some and all of the sodium carbonate used in neutral sulfite pulping. can. In the production of boiling chemicals for the latter, sodium sulfite is formed by treating a sodium carbonate solution with sulfur dioxide ("sulfitization"), a process that converts approximately 75-95% of the carbonate into sulfite. It is usually stopped after it has been converted.
The remaining 5-25% of the carbonate acts as a so-called "buffer" to neutralize the wood-derived acids during the early stages of pulping. Green liquor can act as a "buffer" by mixing it with sodium sulfite produced by complete sulfitization of a sodium carbonate solution, or because it replaces the sodium carbonate solution before sulfitization, i.e. NSSC Can be used to provide total sodium for the mill. Whichever method green liquor is used, it creates significant environmental problems. If “buffer”
If used as a fuel, it generates hydrogen sulfide, which is toxic and malodorous at the near-neutral PH normally encountered in NSSC pulping. When used in sulfite formation, hydrogen sulfide is expelled by the action of sulfur dioxide. In either case, the results are environmentally unsuitable. These problems can be overcome by converting sodium sulfide to sodium thiosulfate (usually the main product), sodium sulfite and sodium sulfate by oxidizing the green liquor, any of which can be used for NSSC pulping. or does not generate hydrogen sulfide under the conditions normally used for sulfite formation. Oxidation of green liquor can be carried out using oxygen gas or air. When using air, oxidation is somewhat slow and it is difficult to achieve complete destruction of the sulfides in a reasonable amount of time without the use of high temperatures that require a pressure vessel. Thus, in order to efficiently use the spent chemicals of the Kraft process, it is necessary to use them or to convert them into a form that does not generate H ₂ S and is carried out economically. We need to create a way to do it. The present invention provides such a method, which recovers spent black liquor from a kraft process, converts the kraft black liquor into green liquor, and converts the green liquor into an oxygen-containing gas and an effective amount of spent NSSC. oxidizing the black liquor in the presence of additives and using the thus oxidized green liquor in a neutral sulphite semichemical (NSSC) pulping process. The oxidation of green liquor according to the present invention is significantly accelerated compared to air oxidation alone. The oxidation-containing gas is preferably air and the reaction is carried out at a preferred temperature range of 50-150°C. With this invention, sulfites can be completely destroyed in a relatively short time at moderate temperatures at atmospheric pressure and without the need for a pressure vessel. The preferred spent delignification liquor as catalytic additive to green liquor in the present invention is 1 to 15% by weight, preferably 3 to 10% by weight of spent NSSC black liquor.
To illustrate the advantages of the present invention, air oxidation of green liquor was performed using the NSSC spent liquor (spent water) of the present invention.
A comparison was made between the use of NSSC black liquor as a catalytic additive and the use of kraft black liquor, spent soda liquor from the soda process, and quinone compounds as known additives. Table 1 and Figure 1 show the absence of additive (line A)
Compared to giving. These results show that the use of NSSC spent liquor has complete sulfide removal in about half the time required with kraft black liquor, but all additives (lines B to E) This shows a significant improvement in reaction time compared to air oxidation alone. All tests were carried out in a reactor consisting of a glass tube 1 m long and 80 mm internal diameter containing four 10 mm stainless steel buckles. The air was placed directly below the center of the stainless steel turbine type agitator.
into the bottom of the reactor through a glass tube. The entire reactor was placed in a water bath maintained at 80°C. Charge green liquor (1.5) to the reactor and increase the air flow rate to 1/min.
The stirring speed was 2100 rpm. The sodium sulfite content in grams per liter was determined at various stages of the oxidation.

【table】

Table 1 and Figure 1 show that in the absence of additives, oxidation is extremely slow under the very moderate conditions used, ie at about 80° C. and atmospheric pressure. This is the case even when using high air flow rates. In the absence of NSSC spent liquid, complete oxidation would likely require many days of reaction time. Green liquor cake had only little effect on promoting the oxidation rate. The effect of different concentrations of NSSC liquid on achieving complete removal of sulfides is shown in Figure 2, where the vertical axis represents the time to complete oxidation of Na ₂ S in minutes, and the horizontal axis represents the time to complete oxidation of Na 2 S. The axis represents the percentage concentration of NSSC black liquor. For example, a 4% concentration of NSSC spent fluid is equally effective as a 9% concentration of Kraft fluid. Although the reason for the promoting effect of the NSSC spent solution is not known, it is thought that this may be due in part to the surfactant properties of the solution containing lignosulfonates, as it does. however,
Commercially available anionic surfactants such as "Comprox" or "Teepol" are ineffective in promoting oxidation, and other properties of the spent liquor other than surface activity are likely responsible. It is known that lignin is partially demethoxylated during NSSC pulping, resulting in the formation of atechol-type groups. It is hypothesized that these groups are active in promoting or catalyzing sulfide oxidation, possibly via peroxide-type intermediates. The effect of using oxidized Kraft green liquor as a buffer or as a total chemical solution in NSSC pulping is shown in Table 2. The NSSC pulping liquor equivalent is formed when the sulfitized and oxidized green liquor is sulfitized.

【table】

[Table] The results given in Table 2 clearly show that oxidized green liquor can be successfully used as a complete replacement for buffering agents in NSSC pulping without significantly affecting the pulp quality (Examples Example 2 compared to Example 1 and Example 5 compared to Example 4). In the case of pulping with mixed hardwood species, increasing the buffer content from 15 to 30% had no significant effect on pulp color or strength other than slightly improved tear length (Example 4). and Example 6 compared to 5). Furthermore, in the case of mixed hardwood species, the "sulfited" and oxidized green liquor can be used as a complete chemical replacement without any adverse effect on pulp strength or color (Example 7 compared to Examples 4 and 5). . From the above, the present invention effectively uses the spent liquor from the delignification process to avoid waste of the used liquor, and
It is found that recovery and recycling of excess kraft green liquor can be provided for oxidation and use in NSSC pulping. Regeneration of kraft black liquor by this method is simple and convenient and provides a product that can be used in associated pulping processes. Compared to the process of Australian Patent No. 473185, the present invention is more convenient as it eliminates the need to convert green liquor to white liquor before oxidation, and furthermore the oxidation product of the present invention is more demanding than oxidized white liquor. It is a product with a lot of
Can be used as NSSC pulping liquid. Another advantage provided by this invention is that the used NSSC black liquor
It is more active than conventional Kraft fluids or quinone-based compounds in oxidizing Na ₂ S-containing solutions.