Method for recovering white liquor and agent for promoting white liquor separation

Abstract

The present invention provides a method for recovering white liquor and a white liquor separation accelerator that makes it possible to reduce the amount of white liquor contained in the lime mud after recovering the white liquor from the emulsion, thereby enabling the separation and recovery of more white liquor from the lime mud. [Solution] The method for recovering white liquor includes a white liquor treatment step of recovering white liquor by separating the lime sludge from the emulsion, which is a mixture of white liquor and lime sludge generated during the manufacture of kraft pulp, by filtration, and further includes a separation enhancement step of adding a white liquor separation enhancer, which contains at least a nonionic surfactant and promotes the separation of lime sludge and white liquor in the emulsion, to the emulsion before or during the white liquor treatment step.

Description

Technical Field

[0001] The present invention relates to a method for recovering white liquor and a white liquor separation accelerator, and particularly to a method for recovering white liquor in which lime mud is separated from an emulsion, which is a mixed liquid of white liquor and lime mud, generated during the production of kraft pulp by filtration to recover white liquor, and a white liquor separation accelerator for separating lime mud from the emulsion by filtration.

Background Art

[0002] As methods for separating lime mud from an emulsion generated during the production of kraft pulp to recover white liquor, sedimentation separation and filtration are generally used. Here, a clarifier is used as sedimentation separation type white liquor treatment equipment, and an atmospheric pressure filtration type clarifying equipment, a pressure filtration type clarifying equipment, or a pressure disk type clarifying equipment is used as filtration type white liquor treatment equipment.

[0003] Among these, the clarifier has the advantage that its structure is simple and maintenance is easy, but in recent years, the pressure disk type clarifying equipment, which is a filtration type white liquor treatment equipment, has been increasing because its separation efficiency is lower than that of the filtration type.

[0004] Among filtration types, as a method using a pressurized filtration type white liquor treatment facility, for example, Patent Document 1 describes a pulverization step in which finely crushed lignocellulose material is pulverized with a pulverizing chemical called white liquor to separate the cellulose component; a green liquor clarification step in which the smelt obtained by diluting and purifying the black liquor produced by dissolving the lignin component in the white liquor is concentrated and calcined to obtain green liquor; a smoking and causticization step in which the green liquor and quicklime react to obtain an emulsion, which is a mixture of white liquor and lime sludge; and a white liquor clarification step in which the lime sludge is separated from the emulsion by sedimentation to separate the white liquor and lime sludge. A method for recovering chemicals in a kraft pulp manufacturing plant is described, characterized in that the green liquor is added in two or more stages during the smothering and causticization stage to increase the particle size of the lime sludge. This method increases the particle size of the lime sludge, which consists of calcium carbonate and the like, by adding the green liquor in two or more stages during the smothering and causticization stage. This method promotes the sedimentation and separation of lime sludge particles in a white liquor clarification device, thereby improving the efficiency of the white liquor clarification stage and the subsequent dewatering stage. [Prior art documents] [Patent Documents]

[0005] [Patent Document 1] Japanese Patent Publication No. 2005-179785 [Overview of the project] [Problems that the invention aims to solve]

[0006] The method described in Patent Document 1 can increase the settling rate for sedimentation-type white liquor treatment equipment and improve the filtration resistance for filtration-type white liquor treatment equipment by improving the caustication reaction. However, there was room for further improvement in order to further enhance these effects and to improve the separation efficiency of white liquor and lime sludge from the emulsion in recent operations where it is necessary to pulverize a larger amount of white liquor using difficult-to-pulverize chips. More specifically, there was room for improvement in reducing the amount of white liquor contained in the lime sludge after separation, thereby enabling the separation and recovery of a larger amount of white liquor from the lime sludge.

[0007] In particular, separating and recovering a larger amount of white liquor from the lime sludge is useful in several ways: it increases the separation capacity of the white liquor processing equipment, thereby improving the productivity of kraft pulp; it reduces the amount of pulping chemicals added to the white liquor that could not be recovered and reused, thereby lowering operating costs; and it helps to suppress problems such as damming in the rotary kiln, deterioration of the lime calcination rate, and increased wastewater load caused by white liquor carried into the later stages of the manufacturing process.

[0008] This invention has been made in view of the above circumstances, and aims to provide a method for recovering white liquor and a white liquor separation accelerator that makes it possible to reduce the amount of white liquor contained in the lime mud after recovering the white liquor from the emulsion, thereby making it possible to separate and recover more white liquor from the lime mud. [Means for solving the problem]

[0009] The inventors diligently conducted research to solve the above-mentioned problems. As a result, they discovered that by adding a white liquor separation accelerator containing at least a nonionic surfactant to the emulsion before or during the white liquor treatment process, which involves recovering the white liquor from the emulsion (a mixture of white liquor and lime mud), the surface of the calcium carbonate particles contained in the lime mud in the emulsion is modified, improving the drainage of interparticle water and promoting the separation of lime mud and white liquor from the emulsion. This reduces the amount of white liquor contained in the lime mud after the white liquor has been recovered, thus completing the present invention. In other words, the gist of the present invention is as follows:

[0010] (1) A method for recovering white liquor, comprising a white liquor treatment step of recovering the white liquor by separating the lime mud from an emulsion, which is a mixture of white liquor and lime mud generated during the manufacture of kraft pulp, wherein the method further comprises a separation enhancement step of adding a white liquor separation accelerator, which contains at least a nonionic surfactant and promotes the separation of the lime mud and the white liquor in the emulsion, to the emulsion before or during the white liquor treatment step.

[0011] (2) The method for recovering the white liquor as described in (1), wherein the white liquor separation accelerator is added to the water sprayed onto the emulsion in the filtration type white liquor treatment facility that performs the white liquor treatment step.

[0012] (3) The method for recovering the white liquor, further comprising a smothering and causticization step of generating the emulsion by adding quicklime to the green liquor generated during the production of the kraft pulp, thereby causing a smothering reaction and a causticization reaction, wherein the white liquor separation accelerator is added to the emulsion before the white liquor treatment step and during or after the smothering and causticization step.

[0013] (4) A method for recovering the white liquor according to any one of (1) to (3) above, wherein one or more nonionic surfactants selected from the group consisting of polyoxyethylene alkyl ether, polyoxyethylene polyoxypropylene alkyl ether, polyoxyethylene alkylphenyl ether, polyoxyethylene polyoxypropylene glycol, and polyoxyethylene fatty acid ester are added to the emulsion.

[0014] (5) A method for recovering the white liquor as described in (4), wherein at least one of the polyoxyethylene alkyl ether, polyoxyethylene polyoxypropylene alkyl ether, polyoxyethylene alkylphenyl ether, and polyoxyethylene fatty acid ester is a random polymer or a block polymer having a linear, branched, or cyclic alkyl group having 8 to 18 carbon atoms, and the number of moles of ethylene oxide added is in the range of 2 to 20, and the number of moles of propylene oxide added is 35 or less.

[0015] (6) The method for recovering the white liquor according to (4) or (5) above, wherein the polyoxyethylene polyoxypropylene glycol is an EO / PO / EO type or PO / EO / PO type block polymer in which the number of moles of ethylene oxide added is in the range of 5 to 20 and the number of moles of propylene oxide added is in the range of 24 to 35.

[0016] (7) The method for recovering white liquor according to any one of (1) to (6) above, wherein the amount of the white liquor separation accelerator added to the emulsion is such that the amount of the nonionic surfactant added to the emulsion is in the range of 10 mg / L to 1000 mg / L.

[0017] (8) A white liquor separation accelerator used when separating the lime sludge from an emulsion, which is a mixture of white liquor and lime sludge generated during the manufacture of kraft pulp, by filtration and recovering the white liquor, and containing at least a nonionic surfactant, which promotes the separation of the lime sludge and the white liquor in the emulsion.

[0018] (9) The white liquor separation accelerator described in (8) above, which is added to the spray water sprayed in a filtration-type white liquor treatment facility that separates and recovers the white liquor from the emulsion, and is used for addition to the spray water to the emulsion in the filtration-type white liquor treatment facility that performs the white liquor treatment step.

[0019] (10) The white liquor separation accelerator as described in (8) or (9) above, which is added to the emulsion after causing a slaking reaction and a causticization reaction by adding quicklime to the green liquor generated during the production of the kraft pulp.

[0020] (11) The nonionic surfactant is one or more selected from the group consisting of polyoxyethylene alkyl ether, polyoxyethylene polyoxypropylene alkyl ether, polyoxyethylene alkyl phenyl ether, polyoxyethylene polyoxypropylene glycol, and polyoxyethylene fatty acid ester. The white liquor separation accelerator according to any one of (8) to (10) above.

[0021] (12) At least any one of the polyoxyethylene alkyl ether, the polyoxyethylene polyoxypropylene alkyl ether, the polyoxyethylene alkyl phenyl ether, and the polyoxyethylene fatty acid ester has a linear, branched, or cyclic alkyl group in the range of 8 to 18 carbon atoms, and the added molar number of ethylene oxide is in the range of 2 to 20, and the added molar number of propylene oxide is 35 or less. The white liquor separation accelerator according to (11) above, which is a polymer.

[0022] (13) The polyoxyethylene polyoxypropylene glycol is a block polymer of the EO / PO / EO type or the PO / EO / PO type, in which the added molar number of ethylene oxide is in the range of 5 to 20, and the added molar number of propylene oxide is in the range of 24 to 35. The white liquor separation accelerator according to (11) or (12) above.

Advantages of the Invention

[0023] According to the present invention, it is possible to reduce the amount of white liquor contained in the lime mud after recovering white liquor from the emulsion, and thereby it is possible to separate and recover more white liquor from the lime mud. A method for recovering white liquor and a white liquor separation accelerator can be provided.

Brief Description of the Drawings

[0024] [Figure 1] FIG. 1 is a schematic diagram showing an example of the apparatus configuration from the process of forming green liquor and emulsion from black liquor and recovering white liquor until the lime mud after recovering the white liquor is reused in the kraft pulp manufacturing process. [Figure 2] FIG. 2 is a graph showing the water content of the dewatered cake when the addition amount of the nonionic surfactant to the emulsion is changed within the range of 0 mg / L to 2000 mg / L. [Figure 3] FIG. 3 is a graph showing the water content of the dewatered cake when an emulsion containing lime mud with different particle sizes (median diameter) in the range of 22.4 μm to 26.2 μm is used.

Mode for Carrying Out the Invention

[0025] Hereinafter, specific embodiments of the present invention will be described in detail. However, the present invention is not limited to the following embodiments, and can be implemented with appropriate modifications within the scope of the object of the present invention.

[0026] <Method for Recovering White Liquor> The method for recovering white liquor according to the present invention is a method including a white liquor treatment step of separating lime mud by filtration from an emulsion, which is a mixed liquid of white liquor and lime mud generated during the production of kraft pulp, and recovering the white liquor, and further includes a separation promotion step of adding a white liquor separation promoter containing at least a nonionic surfactant to promote the separation of lime mud and white liquor in the emulsion before or during the white liquor treatment step.

[0027] In the white liquor recovery method of the present invention, by adding a white liquor separation accelerator containing a nonionic surfactant to the emulsion before or during the white liquor treatment process, the surface of the hydrophilic calcium carbonate particles constituting the lime mud in the emulsion is modified to be hydrophobic. As a result, when filtering the emulsion to which the white liquor separation accelerator has been added, water can more easily pass through the gaps between the lime mud particles. Consequently, after separating the lime mud from the white liquor by filtration using a filtration-type white liquor treatment facility and recovering the white liquor, the desorption of adsorbed water from the resulting dehydrated cake containing calcium carbonate is promoted, improving dewatering efficiency. As a result, more white liquor can be separated from the lime mud and recovered.

[0028] The present invention provides a method for recovering white liquor, comprising at least a white liquor treatment step of recovering white liquor by separating the lime sludge from the emulsion, which is a mixture of white liquor and lime sludge generated during the production of kraft pulp, by filtration, and a separation enhancement step of adding a white liquor separation enhancer to the emulsion before or during the white liquor treatment step.

[0029] The emulsion used in the white liquor treatment process is generated during the production of kraft pulp and is a mixed aqueous solution containing white liquor, which mainly consists of sodium hydroxide and sodium sulfide, and calcium carbonate. Here, kraft pulp has a pulping process in which wood chips and the like are pulped with white liquor to extract the cellulose component. After the cellulose component has been extracted, the black liquor, which contains dissolved lignin, is reduced in a black liquor recovery furnace 11 to recover smelt in a black liquor reduction process, for example in the apparatus configuration shown in Figure 1. Next, the obtained smelt is mixed with weak liquor in a dissolving tank 12 to produce crude green liquor in a green liquor production process. After that, a green liquor clarification process is performed in which dregs are separated from the crude green liquor in a green liquor clarifier 13 to purify the green liquor L1, and the obtained green liquor L1 is stored in a green liquor tank 14. Quicklime is added to the green liquor L1 in a slaker 15, causing a slaking reaction and a caustic reaction in the slaker 15 and the subsequent caustication tank 16, thereby performing a slaking and caustication process to produce an emulsion. The emulsion L2 obtained in the slaking and caustication process is stored in a caustication tank 17 as needed, and then a white liquor treatment process is performed in a filtration-type white liquor treatment facility 18, where lime sludge is separated from the emulsion L2 by filtration and the white liquor L4 is recovered. In the white liquor treatment process, the white liquor L4, which is the dispersion medium, is recovered by filtration, and lime sludge S1, which is the dispersed phase, is obtained.

[0030] Here, the separation acceleration step can be performed before or during the white liquor treatment step, and more specifically, it can be performed by adding a white liquor separation accelerator to the emulsion between the caustication tank 16 and the filtration-type white liquor treatment equipment 18. In this case, the white liquor separation accelerator may be added to the emulsion in the caustication tank 16 during or after the suffocation and caustication step. On the other hand, from the viewpoint of stabilizing the reaction conditions in the separation acceleration step by adding the white liquor separation accelerator to the emulsion L2 after the suffocation and caustication reactions in the suffocation and caustication step have been completed, it is preferable to add the white liquor separation accelerator to the emulsion located downstream of the caustication tank 16. More specifically, it is preferable to add a white liquor separation accelerator to at least one of the following: the emulsion stored in the caustication tank 17, the emulsion stored in the filtration-type white liquor treatment equipment 18, the emulsion flowing through the piping between the caustication tank 16 and the caustication tank 17, and the emulsion flowing through the piping between the caustication tank 17 and the filtration-type white liquor treatment equipment 18.

[0031] Among these, it is preferable to add the white liquor separation accelerator to the water sprayed onto the emulsion (shower water) in the filtration-type white liquor treatment equipment 18 that performs a white liquor treatment process to separate and recover the white liquor from the emulsion. The filtration-type white liquor treatment equipment 18 may have a mechanism to spray water to reduce clogging by lime sludge S1 when filtering the emulsion L2 and to wash the lime sludge S1 after filtration. In particular, in such cases, by adding the white liquor separation accelerator to the water sprayed onto the emulsion, the water containing the white liquor separation accelerator is sprayed onto the emulsion, which has a reduced amount of white liquor. This makes it easier for the white liquor separation accelerator to reach the lime sludge S1, and thus the amount of white liquor contained in the lime sludge after the white liquor has been recovered from the emulsion can be further reduced.

[0032] On the other hand, in configurations that perform a smoking and causticization process, it is also preferable to add the white liquor separation accelerator to the emulsion during or after the smoking and causticization process, either inside the causticization tank 16, inside the causticization tank 17, or in the piping between the causticization tank 16 and the causticization tank 17. By adding the white liquor separation accelerator to the emulsion inside the causticization tank 16, inside the causticization tank 17, or in the piping between the causticization tank 16 and the causticization tank 17, the contact time between the lime sludge S1 contained in the emulsion and the white liquor separation accelerator is increased, allowing the white liquor separation accelerator to mix more uniformly in the liquid, thus enabling more efficient filtration of the emulsion L2 in the filtration-type white liquor treatment equipment 18.

[0033] Furthermore, the "emulsion" to which the spray water is applied within the filtration-type white liquor treatment equipment 18 is not limited to the emulsification supplied to the filtration-type white liquor treatment equipment 18 in the separation acceleration process, but also includes emulsification in which the components of the lime sludge S1 have been concentrated by filtration.

[0034] The white liquor separation accelerator added to the emulsion contains at least a nonionic surfactant. These nonionic surfactants have a hydrophilic portion and a hydrophobic portion, and the hydrophilic portion of the nonionic surfactant can be adsorbed onto the surface of the hydrophilic calcium carbonate particles constituting the lime mud in the emulsion, thereby modifying the surface of the calcium carbonate particles to be hydrophobic. In particular, from the viewpoint of making it easier to modify the surface of the calcium carbonate particles to be hydrophobic, it is preferable that the nonionic surfactant is a compound having both a hydrophilic portion and a hydrophobic portion within its molecule. Here, as the hydrophilic portion, for example, there can be a portion to which multiple ethylene oxides are attached, and among these, it is preferable that there are 2 to 20 moles of ethylene oxides attached, more preferably 2 to 12 moles of ethylene oxides attached, and even more preferably 3 to 8 moles of ethylene oxides attached. Thus, by limiting the number of moles of ethylene oxide added to the hydrophilic portion to 20 or less, the hydrophilicity of the nonionic surfactant does not increase unnecessarily, and the amount of white liquor contained in the lime mud after recovering the white liquor from the emulsion can be further reduced. Furthermore, as for the hydrophobic portion, for example, there can be a portion to which propylene oxide is added and a portion having at least one of linear, branched, or cyclic alkyl groups in the range of 8 to 18 carbon atoms. Among these, it is preferable to have one or both of the portion to which propylene oxide is added in a number of moles of 35 or less and a portion having a linear or branched alkyl group in the range of 8 to 18 carbon atoms. It is more preferable to have one or both of the portion to which propylene oxide is added in a number of moles of 21 or less and a portion having a linear or branched alkyl group in the range of 8 to 18 carbon atoms. It is even more preferable to have one or both of the portion to which propylene oxide is added in a number of moles of 7 or less and a portion having a linear or branched alkyl group in the range of 10 to 13 carbon atoms. These hydrophilic and hydrophobic portions may be connected by either or both ether bonds and ester bonds.When the hydrophilic and hydrophobic portions are bonded by an ester bond, the hydrophobic portion may be an ester of a fatty acid in which a carboxyl group is bonded to at least one of linear, branched, or cyclic alkyl groups having 8 to 18 carbon atoms. Furthermore, one or both of the hydrophilic and hydrophobic portions may be composed of copolymers of alkylene oxides, i.e., at least one of block copolymers and random copolymers, for example, a copolymer of ethylene oxide and propylene oxide. In addition, the hydrophobic portion of a nonionic surfactant constituting a white liquor separation accelerator may be an alkyl group bonded to a benzene ring. Examples of nonionic surfactants in which the hydrophobic portion is an alkyl group bonded to a benzene ring include polyoxyethylene alkylphenyl ethers such as polyoxyethylene nonylphenyl ether.

[0035] Here, as a nonionic surfactant, from the viewpoint of further reducing the amount of white liquor contained in the lime mud after recovering the white liquor from the emulsion, it is preferable to add one or more selected from the group consisting of polyoxyethylene alkyl ether, polyoxyethylene polyoxypropylene alkyl ether, polyoxyethylene alkylphenyl ether, polyoxyethylene polyoxypropylene glycol, and polyoxyethylene fatty acid ester to the emulsion.

[0036] Of these, at least one of polyoxyethylene alkyl ethers, polyoxyethylene polyoxypropylene alkyl ethers, polyoxyethylene alkylphenyl ethers, and polyoxyethylene fatty acid esters is preferably a polymer having a linear, branched, or cyclic alkyl group with 8 to 18 carbon atoms, with an addition mole of ethylene oxide in the range of 2 to 20 and an addition mole of propylene oxide in the range of 35 or less, from the viewpoint of further reducing the amount of white liquor contained in the lime mud after recovering the white liquor from the emulsion. In particular, the alkyl group of this polymer is preferably a linear, branched, or cyclic alkyl group with 8 to 18 carbon atoms, more preferably a linear or branched alkyl group with 8 to 18 carbon atoms, and even more preferably a linear or branched alkyl group with 10 to 13 carbon atoms. Furthermore, the addition mole of ethylene oxide in this polymer is preferably in the range of 2 to 20, more preferably in the range of 2 to 12, and even more preferably in the range of 3 to 8. Furthermore, the number of moles of propylene oxide added to this polymer is preferably 35 or less, more preferably 21 or less, and even more preferably 7 or less. Here, when both ethylene oxide and propylene oxide are added, the polymer may be either a random polymer or a block polymer.

[0037] On the other hand, from the viewpoint of further reducing the amount of white liquor contained in the lime mud after recovering the white liquor from the emulsion, it is preferable to use an EO / PO / EO type or PO / EO / PO type block polymer of polyoxyethylene polyoxypropylene glycol in which the number of moles of ethylene oxide added is in the range of 5 to 20 and the number of moles of propylene oxide added is in the range of 24 to 35. In particular, the number of moles of ethylene oxide added in this block polymer is preferably in the range of 5 to 20, more preferably in the range of 5 to 14, and even more preferably in the range of 5 to 10. Furthermore, the number of moles of propylene oxide added in this block polymer is preferably in the range of 24 to 35, and more preferably in the range of 30 to 35.

[0038] The white liquor separation accelerator may contain components other than nonionic surfactants. Examples of such components include one or more of acrylic acid polymers, maleic acid polymers, and higher alcohol-based emulsion defoamers.

[0039] When adding a white liquor separation accelerator to an emulsion, it is preferable to add an amount of nonionic surfactant to the emulsion such that the amount added is in the range of 10 mg / L to 1000 mg / L. In particular, by adding 10 mg / L or more of nonionic surfactant to the emulsion, when filtering the emulsion to which the white liquor separation accelerator has been added, water can pass through the gaps between the lime sludge particles more easily. For this reason, the amount of nonionic surfactant added to the emulsion is preferably 10 mg / L or more, more preferably 30 mg / L or more, even more preferably 50 mg / L or more, and even more preferably 100 mg / L or more. On the other hand, if the amount of nonionic surfactant added to the emulsion is greater than 1000 mg / L, it becomes difficult to obtain further effects. In this case, by keeping the amount of nonionic surfactant added to the emulsion at 1000 mg / L or less, it is possible to suppress the increase in material costs due to the excessive use of nonionic surfactant.

[0040] Furthermore, when adding the white liquor separation accelerator to the emulsion, the temperature of the emulsion is preferably in the range of 70°C to 90°C, from the viewpoint of further promoting the separation of lime sludge and white liquor by the white liquor separation accelerator and further improving the filtration efficiency when filtering the emulsion.

[0041] The white liquor recovered in the white liquor treatment process is reused in the pulping process described above (pulping process 19 in Figure 1). Furthermore, the lime mud S1 recovered in the white liquor treatment process can be reused for addition to the green liquor L1 in the slaker 15 by removing impurities other than calcium carbonate using the lime mud washer 21 and lime mud filter 22 in the apparatus configuration shown in Figure 1, and then calcining it in the kiln 23. This process converts the lime mud S1 into quicklime S2 and also thermally decomposes the white liquor separation accelerator.

[0042] <White liquor separation accelerator> The white liquor separation accelerator according to the present invention is used to separate the lime sludge from the emulsion, which is a mixture of white liquor and lime sludge generated during the production of kraft pulp, by filtration and recover the white liquor. It contains at least a nonionic surfactant and promotes the separation of lime sludge and white liquor in the emulsion. The composition of the white liquor separation accelerator is the same as that of the white liquor separation accelerator described above.

[0043] By using such a white liquor separation accelerator, the surface of the hydrophilic calcium carbonate particles that make up the lime sludge in the emulsion is modified to be hydrophobic. This makes it possible to reduce the amount of white liquor contained in the lime sludge after the white liquor has been recovered from the emulsion, thereby allowing more white liquor to be separated from and recovered from the lime sludge. [Examples]

[0044] The present invention will be specifically described below with reference to examples, but the present invention is not limited to these examples.

[0045] [Examples 1-68 of the present invention] An emulsion (emulsion A) was collected during the typical scavenging process in the manufacture of kraft pulp. This emulsion A consists of calcium carbonate sludge dispersed in a dispersion medium composed mainly of an aqueous solution containing sodium hydroxide and sodium sulfide. The particle size (median diameter) of the calcium carbonate sludge dispersed in emulsion A was measured using a particle size distribution analyzer (Horiba, Ltd., model number: LA-300), and the particle size (median diameter) of the calcium carbonate sludge was measured to be 22.4 μm.

[0046] 50 ml of emulsion A was placed in a poly bottle and heated in a 80°C hot water bath for more than one hour. Then, in a procedure simulating the separation acceleration process, the white liquor separation accelerator consisting of nonionic surfactants as described in Tables 1 and 2 was added to the heated emulsion in the amounts described in Tables 1 and 2. The poly bottle was then held by hand and shaken vigorously for 10 seconds.

[0047] Next, Kiriyama funnel filter paper (NO.5A, manufactured by Kiriyama Seisakusho Co., Ltd., 50 mm in diameter) was placed in a Kiriyama funnel (model: SB-40, manufactured by Kiriyama Seisakusho Co., Ltd.), which simulates a filtration-type white liquor treatment facility. The entire volume of emulsion A was then poured onto this filter paper, and the white liquor treatment process was simulated by suction filtration of emulsion A, to which a white liquor separation accelerator had been added, for 60 seconds using a suction bell (model: VKB-200, manufactured by Kiriyama Seisakusho Co., Ltd.) and a pump-type aspirator (model: A-1000S, manufactured by Tokyo Rikakikai Co., Ltd.), while maintaining a temperature of 80°C at a suction pressure of -0.089 MPa. The mass of the dewatered cake (wet mass a), which corresponds to lime sludge and remains on the filter paper after suction filtration, was measured. Subsequently, the obtained dewatered cake was dried in a dryer at 105°C for more than 12 hours, and the mass of the dewatered cake (dry mass b) was measured again. From these wet masses a and dry masses b, the moisture content of the dewatered cake (remaining on the filter paper) before drying was calculated using the following formula (I). The results are shown in Tables 1 and 2. Moisture content c = (wet mass a - dry mass b) ÷ wet mass a × 100...Formula (I)

[0048] [Examples 69 and 70 of the present invention] Emulsion B was collected during the general scavenging process in the manufacture of kraft pulp, and was produced under different conditions than emulsion A described above. Like emulsion A, emulsion B also consists of lime sludge made of calcium carbonate dispersed in a dispersion medium consisting of an aqueous solution mainly containing sodium hydroxide and sodium sulfide. Furthermore, the particle size (median diameter) of the lime sludge dispersed in emulsion B was measured using the same apparatus as in Examples 1 to 68 of the present invention, and the particle size (median diameter) of the lime sludge was measured to be 24.7 μm.

[0049] 50 ml of emulsion B was placed in a poly bottle and heated in a 80°C hot water bath for more than one hour. Then, to simulate the separation acceleration process, the white liquor separation accelerator consisting of a nonionic surfactant as described in Table 2 was added to the heated emulsion in the amount described in Table 2. The poly bottle was then held by hand and shaken vigorously for 10 seconds.

[0050] Next, in the same manner as in Examples 1 to 68 of the present invention described above, emulsion B to which the white liquor separation accelerator was added was subjected to suction filtration. The wet mass a and dry mass b of the dewatered cake, which corresponds to lime sludge and remains on the filter paper after suction filtration, were measured, and the moisture content of the dewatered cake before drying (remaining on the filter paper) was calculated from these wet mass a and dry mass b. The results are shown in Table 2.

[0051] [Examples 71 and 72 of the present invention] Emulsion C was collected during the general scavenging process in the manufacture of kraft pulp, and was produced under conditions different from those of emulsions A and B described above. Like emulsion A, emulsion C also consists of calcium carbonate sludge dispersed in a dispersion medium consisting of an aqueous solution mainly composed of sodium hydroxide and sodium sulfide. Furthermore, the particle size (median diameter) of the calcium carbonate sludge dispersed in emulsion C was measured using the same apparatus as in Examples 1 to 68 of the present invention, and the particle size (median diameter) of the calcium carbonate sludge was measured to be 26.2 μm.

[0052] 50 ml of emulsion C was placed in a poly bottle and heated in a 80°C hot water bath for more than one hour. Then, to simulate the separation acceleration process, the white liquor separation accelerator consisting of a nonionic surfactant as described in Table 2 was added to the heated emulsion in the amount described in Table 2. The poly bottle was then held by hand and shaken vigorously for 10 seconds.

[0053] Next, in the same manner as in Examples 1 to 68 of the present invention described above, emulsion C to which the white liquor separation accelerator was added was subjected to suction filtration. The wet mass a and dry mass b of the dewatered cake, which corresponds to lime sludge and remains on the filter paper after suction filtration, were measured, and the moisture content of the dewatered cake before drying (remaining on the filter paper) was calculated from these wet mass a and dry mass b. The results are shown in Table 2.

[0054] [Comparative Example 1] 50 ml of the aforementioned emulsion A was placed in a poly bottle and heated in an 80°C hot water bath for more than one hour. Then, without adding the white liquor separation accelerator that simulates the separation acceleration process, the amount listed in Table 2 was added, and the mixture was shaken vigorously by hand for 10 seconds.

[0055] Next, in the same manner as in Examples 1 to 68 of the present invention described above, emulsion A to which the white liquor separation accelerator was added was subjected to suction filtration. The wet mass a and dry mass b of the dewatered cake, which corresponds to lime sludge and remains on the filter paper after suction filtration, were measured, and the moisture content of the dewatered cake before drying (remaining on the filter paper) was calculated from these wet mass a and dry mass b. The results are shown in Table 2.

[0056] [Comparative Examples 2-6] 50 ml of the above-mentioned emulsion A was placed in a poly bottle and heated in an 80°C hot water bath for more than one hour. Then, instead of the white liquor separation accelerator consisting of a nonionic surfactant, an anionic surfactant or cationic surfactant listed in Table 2 was added to the heated emulsion in the amount listed in Table 2. The poly bottle was then held by hand and shaken vigorously for 10 seconds.

[0057] Next, in the same manner as in Examples 1 to 68 of the present invention described above, emulsion A to which the white liquor separation accelerator was added was subjected to suction filtration. The wet mass a and dry mass b of the dewatered cake, which corresponds to lime sludge and remains on the filter paper after suction filtration, were measured, and the moisture content of the dewatered cake before drying (remaining on the filter paper) was calculated from these wet mass a and dry mass b. The results are shown in Table 2.

[0058] [Table 1] [Table 2]

[0059] As a result, it can be seen that in the white liquor recovery methods of Examples 1 to 72 of the present invention, the water content of the dewatered cake made of lime mud after suction filtration of the emulsion was 26.1% by mass or less, and because the amount of white liquor contained in the lime mud was small, it was possible to separate and recover a large amount of white liquor from the lime mud. On the other hand, in the white liquor recovery methods of Comparative Examples 1 and 2, the water content of the dewatered cake made of lime mud after suction filtration of the emulsion did not reach an acceptable level, and it was difficult to separate and recover a large amount of white liquor from the lime mud.

[0060] From the results of Examples 1-8 and Comparative Example 1 of the present invention, the relationship between the amount of nonionic surfactant added to the emulsion and the water content of the dehydrated cake was investigated when the amount of nonionic surfactant added to the emulsion was varied within the range of 0 mg / L to 1000 mg / L. As a result, as shown in Figure 2, in the white liquor recovery methods of Examples 1-8 of the present invention, in which an amount of nonionic surfactant of 10 mg / L or more was added to the emulsion, the water content of the dehydrated cake was lower compared to Comparative Example 1, in which no nonionic surfactant was added, making it possible to separate and recover more white liquor from the lime sludge. Furthermore, in the white liquor recovery method of Example 8 of the present invention, in which an amount of nonionic surfactant of 2000 mg / L or more was added to the emulsion, the water content of the dehydrated cake was slightly higher compared to Example 7 of the present invention, in which an amount of nonionic surfactant of 1000 mg / L was added to the emulsion, so no further effect was obtained compared to Example 7 of the present invention.

[0061] Furthermore, based on the results from Examples 4, 69, and 71 of the present invention, we investigated the relationship between the particle size (median diameter) of the lime mud and the water content of the dewatered cake when using emulsions containing lime mud with different particle sizes (median diameters) in the range of 22.4 μm to 26.2 μm. As a result, as shown in Figure 3, in the white liquor recovery methods of Examples 69 and 71 of the present invention, where the lime mud particle size (median diameter) was 24.7 μm or larger, the water content of the dewatered cake was lower compared to Example 4 of the present invention, where the lime mud particle size (median diameter) was 22.4 μm, making it possible to separate and recover more white liquor from the lime mud. Furthermore, in the white liquor recovery method of Example 71 of the present invention, where the particle size (median diameter) of the lime mud is 26.2 μm, it can be seen that, compared to Example 4 of the present invention, where the particle size (median diameter) of the lime mud was 24.7 μm, the water content of the lime mud is lower, making it possible to separate and recover a larger amount of white liquor from the lime mud. [Explanation of Symbols]

[0062] 11 Black Liquor Recovery Furnace 12 Dissolving Tanks 13 Green Liquid Clarifier 14 Green liquid tank 15 Slaker 16 Causticization tank 17 Caustic Tank 18. Filtration-type white liquor treatment equipment 19 Cooking process 21 Lime Mud Washer 22 Lime Mud Filter 23 Kilns L1 Green liquid L2 emulsion Water sprayed onto emulsion in L3 filtration-type white liquor treatment facility L4 white liquor S1 lime mud S2 quicklime

Claims

1. A method for recovering white liquor, which includes a white liquor treatment step of recovering the white liquor by separating the lime mud from the emulsion, a mixture of white liquor and lime mud generated during the production of kraft pulp, by filtration, A method for recovering white liquor, further comprising a separation enhancement step of adding a white liquor separation enhancer, which contains at least a nonionic surfactant and promotes the separation of the lime mud and the white liquor in the emulsion, to the emulsion before or during the white liquor treatment step.

2. The method for recovering white liquor according to claim 1, wherein the white liquor separation accelerator is added to the water sprayed onto the emulsion in a filtration-type white liquor treatment facility that performs the white liquor treatment step.

3. The method for recovering the white liquor further includes a smothering and causticization step in which quicklime is added to the green liquor generated during the production of the kraft pulp to induce a smothering reaction and a causticization reaction, thereby producing the emulsion. The method for recovering white liquor according to claim 1, wherein the white liquor separation accelerator is added to the emulsion before the white liquor treatment step and during or after the scavenging and causticization step.

4. The method for recovering a white liquor according to claim 1, wherein one or more nonionic surfactants selected from the group consisting of polyoxyethylene alkyl ether, polyoxyethylene polyoxypropylene alkyl ether, polyoxyethylene alkylphenyl ether, polyoxyethylene polyoxypropylene glycol, and polyoxyethylene fatty acid ester are added to the emulsion.

5. The method for recovering a white liquor according to claim 4, wherein at least one of the polyoxyethylene alkyl ether, the polyoxyethylene polyoxypropylene alkyl ether, the polyoxyethylene alkylphenyl ether, and the polyoxyethylene fatty acid ester is a polymer having a linear, branched, or cyclic alkyl group having 8 to 18 carbon atoms, and the number of moles of ethylene oxide added is in the range of 2 to 20, and the number of moles of propylene oxide added is 35 or less.

6. The method for recovering a white liquor according to claim 4, wherein the polyoxyethylene polyoxypropylene glycol is an EO / PO / EO type or PO / EO / PO type block polymer in which the number of moles of ethylene oxide added is in the range of 5 to 20 and the number of moles of propylene oxide added is in the range of 24 to 35.

7. The method for recovering white liquor according to claim 1, wherein the white liquor separation accelerator is added to the emulsion in an amount such that the amount of the nonionic surfactant added to the emulsion is in the range of 10 mg / L to 1000 mg / L.

8. A white liquor separation accelerator used when separating the lime sludge from an emulsion, which is a mixture of white liquor and lime sludge generated during the production of kraft pulp, by filtration and recovering the white liquor, and containing at least a nonionic surfactant to promote the separation of the lime sludge and the white liquor in the emulsion.

9. The white liquor separation accelerator according to claim 8, which is added to spray water sprayed in a filtration-type white liquor treatment facility for separating and recovering the white liquor from the emulsion.

10. The white liquor separation accelerator according to claim 8, which is added to the emulsion after a saturation reaction and a caustic reaction have been caused by adding quicklime to the green liquor generated during the production of the kraft pulp.

11. The white liquor separation accelerator according to claim 8, wherein the nonionic surfactant is one or more selected from the group consisting of polyoxyethylene alkyl ether, polyoxyethylene polyoxypropylene alkyl ether, polyoxyethylene alkylphenyl ether, polyoxyethylene polyoxypropylene glycol, and polyoxyethylene fatty acid ester.

12. The white liquor separation accelerator according to claim 11, wherein at least one of the polyoxyethylene alkyl ether, the polyoxyethylene polyoxypropylene alkyl ether, the polyoxyethylene alkylphenyl ether, and the polyoxyethylene fatty acid ester is a random polymer or block polymer having a linear, branched, or cyclic alkyl group having 8 to 18 carbon atoms, and having a number of moles of ethylene oxide added in the range of 2 to 20 and a number of moles of propylene oxide added of 35 or less.

13. The white liquor separation accelerator according to claim 11, wherein the polyoxyethylene polyoxypropylene glycol is an EO / PO / EO type or PO / EO / PO type block polymer having an addition mole number of ethylene oxides in the range of 5 to 20 and an addition mole number of propylene oxides in the range of 24 to 35.

Images