JPS6219557B2 - - Google Patents

Description

[Detailed description of the invention]

The present invention relates to a method for producing an alkaline pulp cooking liquor that makes it possible to significantly reduce energy consumption in the entire pulping system. This invention relates to a method for producing an alkaline pulp cooking liquor using a sodium salt of an anion that is more acidic than carbonate ions, dissolving smelt in dilute black liquor before concentration, and eliminating the need for a causticizing step. Due to the recent sharp increase in energy costs, so-called energy saving technology that aims to reduce energy consumption has become an important issue in all industries. In the pulp industry, chemical recovery and energy saving measures in the cooking liquor manufacturing process are of utmost importance in chemical pulping, especially in alkaline pulping methods such as the soda method and the kraft method. In other words, when producing alkaline pulp, approximately half of the organic matter in the raw wood components is eluted into the black liquor, so most of the energy required for the pulping system is consumed by the amount of heat generated by the combustion of organic matter in the recovery boiler as a combustion process. are covered. However, in the process of producing cooking liquor, the smelt produced by condensing and burning black liquor is dissolved, and the sodium carbonate in the liquor is causticized using quicklime and converted into sodium hydroxide, a cooking chemical. After cleaning the calcium carbonate, it must be calcined in a rotary kiln or calciner to return it to quicklime, so it is necessary to supply the fuel for calcining from outside the system. Therefore, as an energy saving measure, it is important to take measures that can save fuel or omit the causticizing process. Against this background, the following various proposals have been made as techniques that can omit the causticizing step. All of these methods are methods for creating a state in which sodium carbonate does not exist in the smelt, and this proposal, called the autocausticizing method [E.Kiiskila, Paperi ja
Puu, Vol.60NO.3 P.129 (1978)] can be broadly divided into two methods. The first method is to add iron oxide or titanium dioxide to the smelt; in either case, 1000
It reacts with sodium carbonate in smelt at a high temperature close to ℃ as follows to release carbon dioxide. Na ₂ CO ₃ +Fe ₂ O ₃ →2NaFeO ₂ +CO ₂ Na ₂ CO ₃ +TiO ₂ →Na ₂ TiO ₃ +CO _2Next , when the smelt is dissolved in water, sodium hydroxide is produced by the following reaction. 2NaFeO ₂ +H ₂ O → 2NaOH + Fa ₂ O ₃ Na ₂ TiO ₃ +H ₂ O → 2NaOH + TiO ₂ Therefore, if Fe ₂ O ₃ or TiO ₂ is separated, this solution can immediately be used as an alkaline pulping cooking liquor, The separated Fe ₂ O ₃ or TiO ₂ can be recycled and reused. However, although this technique is theoretically interesting, it has practical problems. First, if the method of adding iron oxide is applied to a kraft pulping method, iron sulfide is produced by the reaction between sodium sulfide in the smelt and iron, which is not preferable. Furthermore, although the method of adding titanium dioxide does not produce sulfides when applied to the kraft pulping method, it has the disadvantage that titanium dioxide produced by dissolving smelt cannot be easily separated. That is, titanium dioxide particles are very fine and difficult to recover. This is well known because it is difficult to separate and remove titanium dioxide that has flowed out into papermaking wastewater. The second method is an alkaline pulping method that uses sodium salts such as boric acid, phosphoric acid, silicic acid, and aluminic acid as cooking chemicals. Taking the example as follows. 2Na ₂ CO ₃ +B ₂ O ₃ →Na ₄ B ₂ O ₅ +2CO ₂ Na ₂ CO ₃ +Al ₂ O ₃ →2NaAlO ₂ +CO ₂ Na ₄ B ₂ O ₅ , NaAlO ₂ , etc. produced in the atmosphere are water-soluble; Since it becomes alkaline upon hydrolysis, it is theoretically possible that this solution can be subjected to an alkaline pulping cooking liquor. However, in reality, compounds other than the sodium salt of boric acid are unstable compounds, and many problems still remain to be solved before their aqueous solutions can be used as alkaline pulping cooking liquors. It is therefore preferred to use the sodium salt of boric acid as the cooking chemical, and this process will be described in more detail. Although the above chemical formula shows the reaction between sodium carbonate and boric acid oxide for generalization, the reaction that actually occurs in the recovery boiler can be understood as follows. First, sodium carbonate and sodium metaborate are produced, and the two react as follows. Na ₂ CO ₃ +2NaBO ₃ →Na ₄ B ₂ O ₅ +CO _2When this smelt is dissolved in water, it becomes Na ₄ B ₂ O ₅ +H ₂ O→2Na ₂ HBO ₃ , and further hydrolyzed to Na ₂ HBO ₃ +H ₂ O →NaOH+NaH ₂ BO ₃ . That is, 1 mol of disodium monohydrogen borate (Na ₂ HBO ₃ ) acts as an alkali equivalent to 1 mol of sodium hydroxide. The above reaction occurs regardless of the presence of sodium sulfide in the smelt. Therefore, one part of the sodium hydroxide in the Kraft cooking liquor was replaced with one part of boric acid.
It is also possible to substitute with disodium hydrogen.
In industrial implementation, the smelt is dissolved in water and the undissolved components (so-called dregs) in the smelt are separated and removed and used as a cooking liquor, and the cooking liquor, or black liquor, is used in the conventional alkaline pulping method. It can be concentrated and burned in a recovery boiler in the same way as in the previous case. To replenish chemicals, borax (Na ₂ B ₄ O ₇ ) and sodium hydroxide corresponding to the insufficient amount of chemicals may be added to the black liquor. In this way, in the alkaline pulping method, that is, the automatic causticizing method, which uses sodium salt of boric acid as the cooking chemical, it is possible to completely omit the causticizing step of the conventional alkaline pulping method, and the causticizing step in the conventional alkaline pulping method can be completely omitted. This is an excellent method that eliminates the need for heat energy. However, this method is still insufficient as an energy saving measure for the following reasons. In other words, although the inorganic content in the black liquor is higher than in the conventional alkaline pulping method by the amount equivalent to NaH ₂ BO ₃ , the relationship between black liquor concentration and viscosity is the same as in the conventional method. , it can only be concentrated to the same solids concentration. Therefore, the calorific value of the black liquor is lower than in the conventional method due to the lower organic content. Therefore, combustion in the recovery boiler requires auxiliary combustion with heavy oil, and even if the thermal energy is saved in the causticizing process, a portion of it will eventually be offset by the use of fuel. Next, we will consider energy saving measures during the concentration process in an alkaline pulping system. In this process, dilute black liquor with a solid content concentration of 10 to 20% is mixed with 50 to 70%
It requires a huge amount of thermal energy to concentrate. In order to save this energy, it is necessary to increase the solid content concentration of the black liquor before concentration by some means or to reduce the amount of black liquor subjected to the concentration process. For this purpose, we have previously applied for a method in which the dilute black liquor generated in the alkaline pulping system is used as water for the causticizing process, but this method significantly reduces the amount of dilute black liquor that goes to the concentration process. At the same time, it has the advantage that the concentration of organic matter gradually increases due to the circulation of black liquor, and the concentration of black liquor that finally goes to the concentration process increases, and the energy saving effect is extremely large, but in actual implementation, It may be necessary to make some changes to the current causticizing process or subsequent cleaning process equipment. In other words, since the organic matter in black liquor generally acts as a dispersant, the separation and removal of calcium carbonate (lime mattow) produced by the causticizing reaction is insufficient with the current sedimentation separation method, and a different type of equipment has been introduced. may also be necessary. Taking these points into consideration, the present inventors came up with a combination with the above-mentioned automatic causticizing alkaline pulping method, and as a result of combining this with the use of diluted black liquor to dissolve the smelt, a This project succeeded in establishing an innovative energy-saving alkaline pulping system. That is, while each of these methods still has insufficient energy saving and some problems, the present invention, which combines these methods, solves these problems all at once. In order to demonstrate the effects of the present invention, the energy saving effect in the dilute black liquor concentration process will be explained by examining the water balance in an alkaline pulping system. FIGS. 1 and 2 show the flow and water balance in the conventional alkaline pulping method and the present invention. In the flow of the conventional method (Figure 1), chips are cooked in a digester, the cooked pulp is separated from the pulp in a washing process, the diluted black liquor is concentrated and burned in a recovery boiler, and the resulting smelt is dissolved and causticized. In the process, sodium carbonate is converted to sodium hydroxide to produce white liquor, which is used for cooking. On the other hand, quicklime produced by washing and firing the calcium carbonate produced in the causticizing process is used to causticize sodium carbonate in the causticizing process. The fresh water used for washing the calcium carbonate is used as water for the above-mentioned smelt dissolution process. On the other hand, in the flow of the present invention (Fig. 2), the steps from cooking to smelt dissolution are the same, but if the diluted black liquor obtained in the washing step is used as water for dissolving smelt and the smelt is dissolved, automatic causticization is performed. It can be used as a white liquor and the causticizing step can be omitted. Regarding the water balance, in the conventional method (Figure 1), water accompanying the chips (2.4 m ³ /t. pulp), fresh water for washing the pulp (9.2 m ³ /t. pulp), and fresh water for washing the calcium carbonate. The water that enters the system as (3.8 m ³ /t. pulp) is the water that comes out with the pulp (6.0 m 3 /t. pulp).
m ³ /t.Pulp), the black liquor concentration process (8.2m ³ /t.
pulp), a recovery boiler (0.9 m ³ /t. pulp), and the calcium carbonate calcination process (0.3 m ³ /t. pulp).
In Figure), the incoming water is chip entrained water (2.4m ³ /
t.pulp) and pulp washing water ( ^9.2m3 /t.pulp)
On the other hand, the water that comes out is pulp entrained water (9.2m ³ /t. pulp), and the water that comes out is the black liquor concentration process (4.7m ³ /t.pulp).
t. pulp) and the evaporated water in the recovery boiler (0.9 m ³ /t. pulp). Comparing the amount of evaporated water in both methods, the conventional method yields 8.2m ³ /t.pulp + 0.9m ³ /t.pulp + 0.3m ³ /t.
Pulp = 9.4m ³ /t.pulp, whereas in the method of the present invention, 4.7m ³ /t.pulp + 0.9m ³ /t.pulp = 5.6
m ³ /t. pulp, a reduction of 40%, and the savings in thermal energy are enormous. That is, in the present invention, which uses sodium salt of boric acid as the cooking liquor and diluted black liquor as water for dissolving smelt, the loss due to the increase in auxiliary fuel oil in the recovery boiler is reduced by increasing the concentration of black liquor through circulating use. This is more than compensated for by the savings in thermal energy in the black liquor concentration process, and the problem of separating and removing lime mud that may occur when adding dilute black liquor to the conventional cooking liquor manufacturing process is automatically eliminated. In the causticized alkaline pulping method, the causticizing step itself is advantageously omitted, so there is no need to consider it at all. In other words, by dissolving smelt with black liquor to produce cooking liquor in the automatic causticizing alkaline pulping method, it is possible to realize the advantages of both technologies. As mentioned above, in practical implementation, the smelt is dissolved in black liquor, the Dregs is separated and removed, and then used as cooking liquor, and a clarifier is generally used as a device for this purpose. . However, when dissolving smelt with black liquor, the organic matter in the black liquor acts as a dispersant, so the clarifier may not be able to separate the dredges sufficiently. In this case, a filtration method or a centrifugal separation method may be used to separate the Dregs. As mentioned above, in an automatic causticizing alkaline pulping system that uses sodium boric acid as the cooking chemical, revolutionary energy savings can be achieved by dissolving smelt with black liquor to produce cooking liquor.
It is clear from the pulping examples shown below that the obtained pulp is comparable to conventional kraft pulp. Example of pulping: Comparison between the method of the present invention and the Kraft method using the cooking liquor (sulfidity 25%) shown in the following table: Packing the equivalent of 700 g of bone-dry chips prepared from domestically produced mixed hardwood into a stationary autoclave. Pulping was carried out. In both pulping methods, the liquid ratio is 4, and the cooking temperature is
The cooking time was 90 minutes at 165°C, and the properties of the unbleached pulp and bleached pulp were as shown in the table below. In addition, the bleaching property and
There was no difference in the beatability between the two pulps using the PFI mill.

【table】

【table】 [Brief explanation of the drawing]

FIG. 1 is a diagram showing the flow and water balance of the conventional method in an alkaline pulping system, and FIG. 2 is a diagram showing the flow and water balance of the method of the present invention. In the figure, solid line: flow containing water, dotted line: flow containing only other substances.

Claims (1)

[Scope of Claims] 1. In an alkaline pulping system consisting of a cooking process, an unbleached pulp washing process, a black liquor concentration process, a black liquor combustion process, and a cooking chemical manufacturing process from smelt, the cooking chemical is alkaline and carbonated. A method for producing alkaline pulping cooking liquor characterized by using a sodium salt of an anion that is more acidic than ions and dissolving smelt in dilute black liquor before concentration. 2. The method for producing an alkaline pulp cooking liquor according to claim 1, in which a sodium salt of boric acid is used as the sodium salt of an anion that is more acidic than carbonate ions.