Method for recovering white liquor and white liquor separation accelerating agent

Abstract

To provide a method for recovering white liquor and a white liquor separation accelerating agent that make it possible to reduce an amount of white liquor contained in lime mud after the white liquor is recovered from milky liquid, thereby allowing more white liquor to be separated from the lime mud and recovered. A method for recovering white liquor includes treating white liquor by separating lime mud by filtration from milky liquid generated during production of kraft pulp to recover the white liquor, the milky liquid being a mixed liquid of the white liquor and the lime mud; and further includes accelerating separation by adding a white liquor separation accelerating agent that includes at least a nonionic surfactant and that accelerates separation of the lime mud and the white liquor in the milky liquid to the milky liquid before or during the treating of the white liquor.

Description

METHOD FOR RECOVERING WHITE LIQUOR AND WHITE LIQUOR SEPARATION ACCELERATING AGENT

[0001] The present invention relates to a method for recovering white liquor and a white liquor separation accelerating agent and, in particular, to a method for recovering white liquor by separating lime mud by filtration from milky liquid generated during production of kraft pulp, the milky liquid being a mixed liquid of the white liquor and the lime mud, and to a white liquor separation accelerating agent that separates lime mud from milky liquid by filtration.

[0002] Sedimentation separation and filtration have been commonly used to separate lime mud from milky liquid generated during production of kraft pulp to recover white liquor. Here, a clarifier has been used as a sedimentation separation type white liquor treatment facility, and an atmospheric pressure filtration type clarification facility, a pressure filtration type clarification facility, or a pressurized disk type clarification facility has been used as a filtration type white liquor treatment facility.

[0003] Among these, the clarifier has the advantage of simple structure and easy maintenance, but in recent years, the pressurized disk type clarification facility, which is a filtration type white liquor treatment facility, has been on the rise since the clarifier has a separation efficiency lower than that of the filtration type clarification facility.

[0004] For example, Patent Document 1 describes as a method using a pressure filtration type white liquor treatment facility which is of the filtration type, a method for recovering a chemical in a kraft pulp manufacturing plant characterized in that, in a kraft pulp manufacturing plant for implementing a process including a cooking step in which a finely crushed lignocellulosic material is cooked with a cooking chemical solution called white liquor to separate a cellulose component; a green liquor clarification step in which smelt obtained by concentrating and calcining black liquor produced by dissolving a lignin component in the white liquor is diluted and purified to obtain green liquor; a slaking and causticization step in which the green liquor and quick lime undergo slaking and causticization reactions to obtain milky liquid which is a mixed liquid of white liquor and lime mud; a white liquor clarification step in which lime mud is separated from the milky liquid by sedimentation to separate the white liquor and the lime mud; a weak liquor clarification step in which the thus-separated lime mud is washed with a washing liquor and the thus-washed lime mud is separated from a mixed liquid of the lime mud and the washing liquor by sedimentation to thereby separate the lime mud and weak liquor which is the washing liquor after washing the lime mud; a lime mud dehydration step in which the thus-washed lime mud is dehydrated; and a lime calcination step in which the thus-dehydrated lime mud is calcined to obtain quick lime, a particle diameter of the lime mud is increased by adding the green liquid in two or more stages in the slaking and causticization step. Patent Document 1 describes that this method increases efficiency of the white liquor clarification step and the subsequent dehydration step by adding the green liquor in two or more stages in the slaking and causticization step to increase a particle diameter of the lime mud formed of, for example, calcium carbonate, thereby accelerating sedimentation separation of particles of the lime mud in a white liquor clarification device, etc.

[0005] [PTL 1] Japanese Unexamined Patent Application, Publication No. 2005-179785Problems to be Solved by the Invention

[0006] The method described in Patent Document 1 can increase a sedimentation rate for a sedimentation separation type white liquor treatment facility and improve filtration resistance for a filtration type white liquor treatment facility by improving a causticization reaction. However, there is room for further improvement in terms of enhancing these effects and also in terms of increasing efficiency of separation of white liquor and lime mud from milky liquid even in recent operations that require cooking with more white liquor using less-cookable chips, and more specifically in terms of reducing an amount of white liquor contained in lime mud after separation, thereby separating and recovering more white liquor from the lime mud.

[0007] In particular, separation and recovery of more white liquor from lime mud is also useful in terms of increasing a separation capacity of a white liquor treatment facility to improve kraft pulp productivity, reducing an operating cost by reducing an amount of a cooking chemical to be added for compensating for a portion not recovered or reused as white liquor, and preventing damming in a rotary kiln, deterioration of a lime calcination rate, or an increase of effluent load caused by white liquor carried over to a subsequent stage of a production process.

[0008] The present invention was made in view of such circumstances, and an object thereof is to provide a method for recovering white liquor and a white liquor separation accelerating agent that make it possible to reduce an amount of white liquor contained in lime mud after the white liquor is recovered from milky liquid, thereby separating and recovering more white liquor from the lime mud.Means for Solving the Problems

[0009] The present inventors conducted extensive studies to solve the above problem. As a result, the present invention has been completed based on findings that addition of a white liquor separation accelerating agent containing at least a nonionic surfactant to milky liquid before or during a white liquor treatment step in which white liquor is recovered from the milky liquid which is a mixed liquid of the white liquor and lime mud modifies surfaces of calcium carbonate particles included in the lime mud in the milky liquid to improve drainage of gap water between the particles and promotes separation of the lime mud and the white liquor from the milky liquid, thereby reducing an amount of the white liquor contained in the lime mud after the white liquor is recovered. That is, a summary of the present invention is as follows.

[0010] A first aspect of the present invention relates to a method for recovering white liquor, the method including: treating white liquor by separating lime mud by filtration from milky liquid generated during production of kraft pulp to recover the white liquor, the milky liquid being a mixed liquid of the white liquor and the lime mud,; and accelerating separation by adding a white liquor separation accelerating agent that includes at least a nonionic surfactant and that accelerates separation of the lime mud and the white liquor in the milky liquid to the milky liquid before or during the treating of the white liquor.

[0011] A second aspect of the present invention relates to the method for recovering white liquor according to the first aspect, wherein the white liquor separation accelerating agent is added to water to be sprayed on the milky liquid in a filtration type white liquor treatment facility in which the white liquor is treated.

[0012] A third aspect of the present invention relates to the method for recovering white liquor according to first or second aspect, further including slaking and causticizing by adding quick lime to green liquor generated during the production of kraft pulp to cause slaking and causticization reactions, thereby producing the milky liquid, and in which the white liquor separation accelerating agent is added to the milky liquid before the treating of the white liquor and during or after the slaking and the causticizing.

[0013] A fourth aspect of the present invention relates to the method for recovering white liquor according to any one of the first to third aspects, in which 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 are added to the milky liquid as the nonionic surfactant.

[0014] A fifth aspect of the present invention relates to the method for recovering white liquor according to the fourth aspect, in which at least one of the polyoxyethylene alkyl ether, the polyoxyethylene polyoxypropylene alkyl ether, the polyoxyethylene alkyl phenyl ether, or the polyoxyethylene fatty acid ester is a random polymer or a block polymer having a linear, branched, or cyclic alkyl group with 8 or more and 18 or less carbon atoms and having a number of moles of ethylene oxide added in a range of 2 or more and 20 or less and a number of moles of propylene oxide added of 35 or less.

[0015] A sixth aspect of the present invention relates to the method for recovering white liquor according to the fourth or fifth aspect, in which the polyoxyethylene polyoxypropylene glycol is an EO / PO / EO- or PO / EO / PO-type block polymer having a number of moles of ethylene oxide added in a range of 5 or more and 20 or less and a number of moles of propylene oxide added in a range of 24 or more and 35 or less.

[0016] A seventh aspect of the present invention relates to the method for recovering white liquor according to any one of the first to sixth aspects, in which the white liquor separation accelerating agent is added to the milky liquid in an amount so that the nonionic surfactant is added to the milky liquid to a concentration in a range of 10 mg / L or more and 1000 mg / L or less.

[0017] An eighth aspect of the present invention relates to a white liquor separation accelerating agent including at least a nonionic surfactant, the white liquor separation accelerating agent being used to accelerate separation of lime mud and white liquor in milky liquid generated during production of kraft pulp, when the lime mud is separated from the milky liquid by filtration to recover the white liquor, the milky liquid being a mixed liquid of the white liquor and the lime mud.

[0018] A ninth aspect of the present invention relates to the white liquor separation accelerating agent according to the eighth aspect, in which the white liquor separation accelerating agent is added to water to be sprayed in a filtration type white liquor treatment facility in which the white liquor is separated from the milky liquid and recovered, and is used for addition of water to be sprayed on the milky liquid in the filtration type white liquor treatment facility in which the white liquor is treated.

[0019] A tenth aspect of the present invention relates to the white liquor separation accelerating agent according to the eighth or ninth aspect, in which the white liquor separation accelerating agent is added to the milky liquid after slaking and causticizing reactions caused by adding quick lime to green liquor generated during the production of kraft pulp.

[0020] An eleventh aspect of the present invention relates to the white liquor separation accelerating agent according to any one of the eighth to tenth aspects, in which 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.

[0021] A twelvth aspect of the present invention relates to the white liquor separation accelerating agent according to the eleventh aspect, in which at least one of the polyoxyethylene alkyl ether, the polyoxyethylene polyoxypropylene alkyl ether, the polyoxyethylene alkyl phenyl ether, or the polyoxyethylene fatty acid ester is a polymer having a linear, branched, or cyclic alkyl group with 8 or more and 18 or less carbon atoms and having a number of moles of ethylene oxide added in a range of 2 or more and 20 or less and a number of moles of propylene oxide added of 35 or less.

[0022] A thirteenth aspect of the present invention relates to the white liquor separation accelerating agent according to the eleventh or twelvth aspect, in which the polyoxyethylene polyoxypropylene glycol is an EO / PO / EO- or PO / EO / PO-type block polymer having a number of moles of ethylene oxide added in a range of 5 or more and 20 or less and a number of moles of propylene oxide added in a range of 24 or more and 35 or less.Effects of the Invention

[0023] The present invention can provide a method for recovering white liquor and a white liquor separation accelerating agent that make it possible to reduce an amount of white liquor contained in lime mud after the white liquor is recovered from milky liquid, thereby enabling more white liquor to be separated and recovered from the lime mud.

[0024] Fig. 1 is a schematic diagram showing an exemplary equipment configuration for formation of green liquor and milky liquid from black liquor, recovery of white liquor, and recycling of lime mud after the white liquor is recovered in a kraft pulp production process;Fig. 2 is a graph showing a water content of a dehydrated cake when an amount of a nonionic surfactant added to milky liquid is varied in a range of 0 mg / L to 2000 mg / L; andFig. 3 is a graph showing a water content of a dehydrated cake when a milky liquid containing lime mud with a particle diameter (median diameter) varying in a range of 22.4 μm to 26.2 μm is used.PREFERRED MODE FOR CARRYING OUT THE INVENTION

[0025] Although specific embodiments of the present invention will be described hereafter in detail, the present invention is not limited to the embodiments below in any way and can be implemented with modifications as appropriate within the scope of the object of the present invention.

[0026] <Method for recovering white liquor> 　A method for recovering white liquor according to the present invention includes a white liquor treatment step in which lime mud is separated by filtration from milky liquid generated during production of kraft pulp to recover the white liquor, the milky liquid being a mixed liquid of the white liquor and the lime mud; and further includes a separation acceleration step in which a white liquor separation accelerating agent that includes at least a nonionic surfactant and that accelerates separation of the lime mud and the white liquor in the milky liquid is added to the milky liquid before or during the white liquor treatment step.

[0027] In the method for recovering white liquor of the present invention, addition of a white liquor separation accelerating agent containing a nonionic surfactant to milky liquid before or during the white liquor treatment step causes surfaces of hydrophilic calcium carbonate particles that constitute lime mud in the milky liquid to be modified to be hydrophobic, which makes it easier for water to pass through gaps between the particles of the lime mud when the milky liquid to which the white liquor separation accelerating agent has been added is filtered. As a result, after the white liquor is recovered by separating the lime mud from the white liquor by filtration using a filtration type white liquor treatment facility, desorption of adsorbed water from the resulting dehydrated cake containing calcium carbonate is accelerated to enhance drainability. Thus, more white liquor can be separated from the lime mud and recovered.

[0028] A method for recovering white liquor of the present invention at least includes a white liquor treatment step in which lime mud is separated by filtration from milky liquid generated during production of kraft pulp to recover the white liquor, the milky liquid being a mixed liquid of the white liquor and the lime mud; and a separation acceleration step in which a white liquor separation accelerating agent is added to the milky liquid before or during the white liquor treatment step.

[0029] The milky liquid to be used in the white liquor treatment step is generated during production of kraft pulp, and is a mixed aqueous solution that includes calcium carbonate and white liquor including sodium hydroxide and sodium sulfide as main components. Here, production of kraft pulp includes a cooking step in which chips such as wood chips are cooked with white liquor to extract a cellulose component. For example, in the equipment configuration shown in Fig. 1, black liquor in which a lignin component after the cellulose component is extracted is dissolved is subjected to a black liquor reduction step in which the black liquor is reduced in a black liquor recovery furnace 11 to recover a smelt. Thereafter, a green liquor production step in which the resulting smelt is mixed with weak liquid in a dissolving tank 12 to produce crude green liquid, and then a green liquid clarification step in which the crude green liquid is purified to green liquor L1 by separating dregs from the crude green liquid in a green liquid clarifier 13 are performed. The resulting green liquid L1 is stored in a green liquid tank 14. Then, a slaking and causticization step in which quick lime is added to the green liquor L1 in a slaker 15 to cause a slaking reaction and a causticization reaction in the slaker 15 and a subsequent causticization bath 16, thereby producing a milky liquid is performed. Milky liquid L2 obtained by the slaking and causticization step is stored in a causticization tank 17 as necessary and then undergoes a white liquor treatment step in which lime mud is separated by filtration from the milky liquid L2 in a filtration type white liquor treatment facility 18 to recover white liquor L4. In the white liquor treatment step, white liquor L4 serving as a dispersion medium is recovered by filtration to obtain lime mud S1 serving as a dispersoid.

[0030] The separation acceleration step can be performed before or during the white liquor treatment step. More specifically, the separation acceleration step can be performed by adding a white liquor separation accelerating agent to milky liquid between the causticization bath 16 and the filtration type white liquor treatment facility 18. The white liquor separation accelerating agent may be added to the milky liquid in the causticization bath 16 during or after the slaking and causticization step. On the other hand, from the viewpoint of stabilizing reaction conditions in the separation acceleration step by adding a white liquor separation accelerating agent to the milky liquid L2 after the slaking reaction and the causticization reaction in the slaking and causticization step are completed, it is preferred to add the white liquor separation accelerating agent to the milky liquid at a position downstream from the causticization bath 16. More specifically, a white liquor separation accelerating agent is preferably added to at least one of milky liquid stored in the causticization tank 17, milky liquid stored in the filtration type white liquor treatment facility 18, milky liquid flowing through a pipe between the causticization bath 16 and the causticization tank 17, or milky liquid circulating in piping between the causticization tank 17 and the filtration type white liquor treatment facility 18.

[0031] Among these, the white liquor separation accelerating agent is preferably added to water to be sprayed on the milky liquid (shower water), which is sprayed in the filtration type white liquor treatment facility 18 in which the white liquor treatment step is performed, that is, the white liquor is separated and recovered from the milky liquid. The filtration type white liquor treatment facility 18 may have a mechanism for spraying water to reduce clogging caused by the lime mud S1 during filtration of the milky liquid L2 and to wash lime mud S1 after filtration. Especially in such a case, by adding the white liquor separation accelerating agent to water to be sprayed on the milky liquid, water containing the white liquor separation accelerating agent is sprayed on the milky liquid containing a decreased amount of white liquor, which allows the white liquor separation accelerating agent to easily reach the lime mud S1. As a result, an amount of the white liquor included in the lime mud after the white liquor is recovered from the milky liquid can be further decreased.

[0032] On the other hand, especially in a configuration in which a slaking and causticization step is performed, the white liquor separation accelerating agent is also preferably added to the milky liquid in the causticization bath 16 during or after the slaking and causticization step, in the causticization tank 17, or in a pipe between the causticization bath 16 and the causticization tank 17. Thus, addition of the white liquor separation accelerating agent to the milky liquid in the causticization bath 16, in the causticization tank 17, or in the pipe between the causticization bath 16 and the causticization tank 17 brings the lime mud S1 included in the milky liquid into contact with the white liquor separation accelerating agent for a longer time to mix more evenly the white liquor separation accelerating agent in the milky liquid, which enables more efficient filtration of the milky liquid L2 in the filtration type white liquor treatment facility 18.

[0033] Note that, the "milky liquid" to be sprayed with water in the filtration type white liquor treatment facility 18 is not limited to milky liquid to be supplied to the filtration type white liquor treatment facility 18 in the separation acceleration step, and also includes milky liquid in which components of the lime mud S1 are concentrated by filtration.

[0034] The white liquor separation accelerating agent to be added to the milky liquid contains at least a nonionic surfactant. The nonionic surfactant has a hydrophilic portion and a hydrophobic portion, and the hydrophilic portion of the nonionic surfactant can be adsorbed onto surfaces of hydrophilic calcium carbonate particles that constitute lime mud in the milky liquid, thereby modifying the surfaces of the calcium carbonate particles to be hydrophobic. In particular, from the viewpoint of facilitating hydrophobic modification of the surfaces of the calcium carbonate particles, the nonionic surfactant is preferably a compound with both hydrophilic and hydrophobic portions in its molecule. Here, the hydrophilic portion may be, for example, a portion to which a plurality of ethylene oxides is added. Among these, the hydrophilic portion is preferably a portion to which ethylene oxides in a number of moles of 2 or more and 20 or less are added, more preferably a portion to which ethylene oxides in a number of moles of 2 or more and 12 or less are added, and further preferably a portion to which ethylene oxides in a number of moles of 3 or more and 8 or less are added. Such a hydrophilic portion having a number of moles of ethylene oxides added of 20 or less does not increase hydrophilicity of the nonionic surfactant beyond necessity. As a result, an amount of white liquor included in lime mud after the white liquor is recovered from the milky liquid can be further decreased. The hydrophobic portion may be, for example, a portion to which propylene oxide is added and a portion having at least any of linear, branched, and cyclic alkyl groups each having 8 or more and 18 or less carbon atoms. Among these, the hydrophobic portion preferably has one or both of a portion to which propylene oxide in a number of moles of 35 or less is added and a portion having a linear or branched alkyl group having 8 or more and 18 or less carbon atoms, more preferably has one or both of a portion to which propylene oxide in a number of moles of 21 or less is added and a portion having a linear or branched alkyl group having 8 or more and 18 or less carbon atoms, and further preferably has one or both of a portion to which propylene oxide in a number of moles of 7 or less is added and a portion having a linear or branched alkyl group having 10 or more and 13 or less carbon atoms. The hydrophilic portion and the hydrophobic portion may be bonded by one or both of an ether bond and an ester bond. When the hydrophilic portion and the hydrophobic portion are bonded by the ester bond, the portion having an alkyl group in the hydrophobic portion may be an ester of a fatty acid in which a carboxy group is bonded to at least any of linear, branched, and cyclic alkyl groups having 8 or more and 18 or less carbon atoms. One or both of the hydrophilic portion and the hydrophobic portion may be composed of a copolymer of an alkylene oxide, i.e., at least one of a block copolymer or a random copolymer, for example, a copolymer of ethylene oxide and propylene oxide. Furthermore, the hydrophobic portion of the nonionic surfactant constituting the white liquor separation accelerating agent may be an alkyl group bonded to a benzene ring. A nonionic surfactant of which hydrophobic portion is an alkyl group bonded to a benzene ring may be a polyoxyethylene alkyl phenyl ether such as polyoxyethylene nonylphenyl ether.

[0035] 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 are preferably added to milky liquid as the nonionic surfactant from the viewpoint of further decreasing an amount of white liquor included in lime mud after the white liquor is recovered from milky liquid.

[0036] Among these, at least one of the polyoxyethylene alkyl ether, the polyoxyethylene polyoxypropylene alkyl ether, the polyoxyethylene alkyl phenyl ether, or the polyoxyethylene fatty acid ester is preferably a polymer having a linear, branched, or cyclic alkyl group with 8 or more and 18 or less carbon atoms and having a number of moles of ethylene oxide added in a range of 2 or more and 20 or less and a number of moles of propylene oxide added of 35 or less from the viewpoint of further decreasing an amount of white liquor included in lime mud after the white liquor is recovered from the milky liquid. In particular, the alkyl group that the polymer has is preferably a linear, branched, or cyclic alkyl group having 8 or more and 18 or less carbon atoms, more preferably a linear or branched alkyl group having 8 or more and 18 or less carbon atoms, and further preferably a linear or branched alkyl group having 10 or more and 13 or less carbon atoms. A number of moles of ethylene oxides added in the polymer is preferably in a range of 2 or more and 20 or less, more preferably in a range of 2 or more and 12 or less, and further preferably 3 or more and 8 or less. A number of moles of propylene oxides added in the polymer is preferably 35 or less, more preferably 21 or less, and further 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, the polyoxyethylene polyoxypropylene glycol is preferably an EO / PO / EO- or PO / EO / PO-type block polymer having a number of moles of ethylene oxide added in a range of 5 or more and 20 or less and a number of moles of propylene oxide added in a range of 24 or more and 35 or less from the viewpoint of further decreasing an amount of white liquor included in lime mud after the white liquor is recovered from milky liquid. In particular, a number of moles of ethylene oxides added in the block polymer is preferably in a range of 5 or more and 20 or less, more preferably in a range of 5 or more and 14 or less, and further preferably 5 or more and 10 or less. A number of moles of propylene oxides added in the polymer is preferably in a range of 24 or more and 35 or less and more preferably in a range of 30 or more and 35 or less.

[0038] The white liquor separation accelerating agent may include a component other than a nonionic surfactant. Examples of such a component include, for example, one or more of an acrylic acid polymer, a maleic acid polymer, and a higher alcohol emulsion defoaming agent.

[0039] The white liquor separation accelerating agent is preferably added to the milky liquid in an amount so that the nonionic surfactant is added to the milky liquid to a concentration in a range of 10 mg / L or more and 1000 mg / L or less. In particular, addition of the nonionic surfactant to the milky liquid to a concentration of 10 mg / L or more makes it easier for water to pass through gaps between particles of lime mud when the milky liquid to which the white liquor separation accelerating agent has been added is filtered. Therefore, the nonionic surfactant is preferably added to the milky liquid to a concentration of 10 mg / L or more, more preferably 30 mg / L or more, further preferably 50 mg / L or more, and still preferably 100 mg / L or more. On the other hand, addition of the nonionic surfactant to the milky liquid to a concentration of more than 1000 mg / L makes it difficult to obtain a further effect. An increase in material cost due to excessive use of the nonionic surfactant can be suppressed by keeping the amount of the nonionic surfactant added to the milky liquid equal to or lower than 1000 mg / L.

[0040] When the white liquor separation accelerating agent is added to the milky liquid, the milky liquid preferably has a temperature of 70°C or more and 90°C or less from the viewpoint of further acceleration of separation of lime mud and white liquor by the action of the white liquor separation accelerating agent and thus further improvement of a filtration efficiency when the milky liquid is filtered.

[0041] The white liquor recovered in the white liquor treatment step is reused in the above-described cooking step (cooking step 19 in Fig. 1). The lime mud S1 recovered in the white liquor treatment step is converted to quick lime S2 and, simultaneously, the white liquor separation accelerating agent is pyrolyzed by removing an impurity other than calcium carbonate using a lime mud washer 21 and a lime mud filter 22 and then calcining the resultant in a kiln 23, for example, in an equipment configuration shown in Fig. 1. Thus, the lime mud S1 can be reused for addition to green liquid L1 in a slaker 15.

[0042] <White liquor separation accelerating agent> 　A white liquor separation accelerating agent according to the present invention contains at least a nonionic surfactant, and is used to accelerate separation of lime mud and white liquor in milky liquid generated during production of kraft pulp, when the lime mud is separated by filtration from the milky liquid to recover the white liquor, the milky liquid being a mixed liquid of the white liquor and the lime mud. A composition of the white liquor separation accelerating agent is the same as that of the above-described white liquor separation accelerating agent.

[0043] Such a white liquor separation accelerating agent modifies surfaces of hydrophilic calcium carbonate particles that constitute lime mud in milky liquid to be hydrophobic, which makes it possible to reduce an amount of white liquor contained in lime mud after the white liquor is recovered from milky liquid, thereby allowing more white liquor to be separated and recovered from the lime mud.Examples

[0044] Hereinafter, the present invention will be specifically described with reference to Examples, but the present invention is not limited thereto.

[0045] <Examples 1 to 68> 　Milky liquid (milky liquid A) produced during a common slaking and causticization step in production of kraft pulp was taken. This milky liquid A is one in which lime mud composed of calcium carbonate is dispersed in a dispersion medium composed of an aqueous solution containing sodium hydroxide and sodium sulfide as major components. A particle diameter (median diameter) of the lime mud dispersed in the milky liquid A was determined to be 22.4 μm as measured by a particle diameter distribution analyzer (model number: LA-300, manufactured by HORIBA, Ltd.).

[0046] After heating 50 mL of the milky liquid A contained in a polyethylene bottle in a warm water bath at 80°C for 1 hour or more, a white liquor separation accelerating agent composed of a nonionic surfactant as described in Tables 1 to 4 was added to the thus-heated milky liquid in amounts as described in Tables 1 to 4 to simulate the separation acceleration step. The bottle was held by hand and shaken vigorously for 10 seconds.

[0047] Next, a KIRIYAMA ROHTO (model number: SB-40, manufactured by Kiriyama Glass Works Co.) simulating a filtration type white liquor treatment facility was lined with filter paper for KIRIYAMA ROHTO (No. 5A, manufactured by Kiriyama Glass Works Co., 50 mm in diameter), all the milky liquid A was placed on this filter paper, and the milky liquid A to which the white liquor separation accelerating agent had been added was subjected to suction filtration at a suction pressure of -0.089 Mpa for 60 seconds while maintaining a temperature of 80°C using a Bell Jar (model No.: VKB-200, manufactured by Kiriyama Glass Works Co.) and a pump-type aspirator (model number: A-1000S, manufactured by TOKYO RIKAKIKAI CO., LTD.) to simulate a white liquor treatment step. A mass (wet mass a) of a dehydrated cake, which corresponded to lime mud, remaining on the filter paper after suction filtration was measured. The resulting dehydrated cake was then dried in a dryer at 105°C for 12 hours or more, and a mass (dry mass b) of the dehydrated cake was measured again. A water content of the dehydrated cake before drying (remaining on the filter paper) was calculated from the wet mass a and the dry mass b according to the following Equation (I). The results are shown in Tables 1 to 4. Water content c = (Wet mass a - Dry mass b) / Wet mass a x 100 ... Equation (I)

[0048] <Examples 69 and 70> 　Milky liquid (milky liquid B) produced during a common slaking and causticization step in production of kraft pulp under conditions different from those of the milky liquid A described above was taken. Like the milky liquid A, this milky liquid B is also one in which lime mud composed of calcium carbonate is dispersed in a dispersion medium composed of an aqueous solution containing sodium hydroxide and sodium sulfide as major components. A particle diameter (median diameter) of the lime mud dispersed in the milky liquid B was determined to be 24.7 μm as measured by the same analyzer as in Examples 1 to 68.

[0049] After heating 50 mL of the milky liquid B contained in a polyethylene bottle in a warm water bath at 80°C for 1 hour or more, a white liquor separation accelerating agent composed of a nonionic surfactant as described in Table 4 was added to the thus-heated milky liquid in amounts as described in Table 4 to simulate the separation acceleration step. The bottle was held by hand and shaken vigorously for 10 seconds.

[0050] Next, suction filtration was performed on the milky liquid B to which the white liquor separation accelerating agent had been added in the same manner as in Examples 1 to 68. A wet mass a and a dry mass b of a dehydrated cake, which corresponded to lime mud, remaining on the filter paper after the suction filtration were measured. A water content of the dehydrated cake before drying (remaining on the filter paper) was calculated from the wet mass a and the dry mass b. The results are shown in Table 4.

[0051] <Examples 71 and 72> 　Milky liquid (milky liquid C) produced during a common slaking and causticization step in production of kraft pulp under conditions different from those of the milky liquid A or the milky liquid B described above was taken. Like the milky liquid A, this milky liquid C is also one in which lime mud composed of calcium carbonate is dispersed in a dispersion medium composed of an aqueous solution containing sodium hydroxide and sodium sulfide as major components. A particle diameter (median diameter) of the lime mud dispersed in the milky liquid C was determined to be 26.2 μm as measured by the same analyzer as in Examples 1 to 68.

[0052] After heating 50 mL of the milky liquid C contained in a polyethylene bottle in a warm water bath at 80°C for 1 hour or more, a white liquor separation accelerating agent composed of the nonionic surfactants as described in Table 4 was added to the thus-heated milky liquid in amounts as described in Table 4 to simulate the separation acceleration step. The bottle was held by hand and shaken vigorously for 10 seconds.

[0053] Next, suction filtration was performed on the milky liquid C to which the white liquor separation accelerating agent had been added in the same manner as in Examples 1 to 68. A wet mass a and a dry mass b of a dehydrated cake, which corresponded to lime mud, remaining on the filter paper after the suction filtration were measured. A water content of the dehydrated cake before drying (remaining on the filter paper) was calculated from the wet mass a and the dry mass b. The results are shown in Table 4.

[0054] <Comparative Example 1> 　After heating 50 mL of the above-described milky liquid A contained in a polyethylene bottle in a warm water bath at 80°C for 1 hour or more, the bottle was shaken vigorously by hand for 10 seconds without adding a white liquor separation accelerating agent for simulating the separation acceleration step.

[0055] Next, suction filtration was performed on the milky liquid A in the same manner as in Examples 1 to 68. A wet mass a and a dry mass b of a dehydrated cake, which corresponded to lime mud, remaining on the filter paper after the suction filtration were measured. A water content of the dehydrated cake before drying (remaining on the filter paper) was calculated from the wet mass a and the dry mass b. The results are shown in Table 4.

[0056] <Comparative Examples 2 to 6> 　After heating 50 mL of the milky liquid A contained in a polyethylene bottle in a warm water bath at 80°C for 1 hour or more, an anionic surfactant or a cationic surfactant as described in Table 4 was added to the thus-heated milky liquid in amounts as described in Table 4 instead of a white liquor separation accelerating agent composed of a nonionic surfactant. The bottle was held by hand and shaken vigorously for 10 seconds.

[0057] Next, suction filtration was performed on the milky liquid A to which the anionic surfactant or the cationic surfactant had been added in the same manner as in Examples 1 to 68. A wet mass a and a dry mass b of a dehydrated cake, which corresponded to lime mud, remaining on the filter paper after the suction filtration were measured. A water content of the dehydrated cake before drying (remaining on the filter paper) was calculated from the wet mass a and the dry mass b. The results are shown in Table 4.

[0058]

[0059]

[0060]

[0061]

[0062] The results show that the water content of the dehydrated cake composed of lime mud after suction filtration of the milky liquid was 26.1% by mass or less in the methods for recovering white liquor of Examples 1 to 72, indicating that the lime mud contained a smaller amount of white liquor, in other words, a larger amount of the white liquor could be separated and recovered from the lime mud. On the other hand, in the methods for recovering white liquor of Comparative Examples 1 and 2, the water content of the dehydrated cake composed of lime mud after suction filtration of the milky liquid did not reach an acceptable level, indicating that it was difficult to separate and recover a large amount of white liquor from the lime mud.

[0063] Among these, a relationship between an amount of a nonionic surfactant added to milky liquid and a water content of a dehydrated cake was investigated when the nonionic surfactant was added to the milky liquid to a concentration varying from 0 mg / L to 1000 mg / L, based on the results of Examples 1 to 8 and Comparative Example 1. As a result, as shown in Fig. 2, the water content of the dehydrated cake in the methods for recovering white liquor of Examples 1 to 8 in which the nonionic surfactant was added to the milky liquid to a concentration of 10 mg / L or more was lower than that in Comparative Example 1 in which no nonionic surfactant was added, indicating that more white liquor could be separated and recovered from lime mud. Meanwhile, the water content of the dehydrated cake in the method for recovering white liquor of Example 8 in which the nonionic surfactant was added to the milky liquid to a concentration of 2000 mg / L or more was slightly higher than that in Example 7 in which the nonionic surfactant was added to the milky liquid to a concentration of 1000 mg / L. Thus, there was no additional effect in Example 8 compared to Example 7

[0064] A relationship between a particle diameter (median diameter) of lime mud and a water content of a dehydrated cake was investigated when milky liquid contains lime mud with particle diameters (median diameters) varying from 22.4 μm to 26.2 μm, based on the results of Examples 4, 69, and 71. As a result, as shown in Fig. 3, the water content of the dehydrated cake in the methods for recovering white liquor of Examples 69 and 71 in which the particle diameter (median diameter) of the lime mud was 24.7 μm or more was lower than that in Example 4 in which the particle diameter(median diameter) of the lime mud was 22.4 μm, indicating that more white liquor could be separated and recovered from lime mud. Furthermore, the water content of the lime mud in the method for recovering white liquor of Example 71 in which the particle diameter (median diameter) of the lime mud was 26.2 μm was lower than that in Example 4 in which the particle diameter (median diameter) of the lime mud was 24.7 μm, indicating that more white liquor could be separated and recovered from lime mud.EXPLANATION OF REFERENCE NUMERALS

[0065] 11 Black liquor recovery furnace 12 Dissolving tank 13 Green liquid clarifier 14 Green liquid tank 15 Slaker 16 Causticization bath 17 Causticization tank 18 Filtration type white liquor treatment facility 19 Cooking step 21 Lime mud washer 22 Lime mud filter 23 Kiln L1 Green liquid L2 Milky liquid L3 Water to be sprayed on milky liquid in filtration type white liquor treatment facility L4 White liquor S1 Lime mud S2 Quick lime

Claims

1. A method for recovering white liquor, the method comprising: 　treating white liquor by separating lime mud by filtration from milky liquid generated during production of kraft pulp to recover the white liquor, the milky liquid being a mixed liquid of the white liquor and the lime mud; and 　accelerating separation by adding a white liquor separation accelerating agent that comprises at least a nonionic surfactant and that accelerates separation of the lime mud and the white liquor in the milky liquid to the milky liquid before or during the treating of the white liquor.

2. The method for recovering white liquor according to claim 1, wherein the white liquor separation accelerating agent is added to water to be sprayed on the milky liquid in a filtration type white liquor treatment facility in which the white liquor is treated.

3. The method for recovering white liquor according to claim 1, further comprising slaking and causticizing by adding quick lime to green liquor generated during the production of kraft pulp to cause slaking and causticization reactions, thereby producing the milky liquid, 　wherein the white liquor separation accelerating agent is added to the milky liquid before the treating of the white liquor and during or after the slaking and the causticizing.

4. The method for recovering white liquor according to claim 1, wherein 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 are added to the milky liquid as the nonionic surfactant.

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

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

7. The method for recovering white liquor according to claim 1, wherein the white liquor separation accelerating agent is added to the milky liquid in an amount so that the nonionic surfactant is added to the milky liquid to a concentration in a range of 10 mg / L or more and 1000 mg / L or less.

8. A white liquor separation accelerating agent comprising 　at least a nonionic surfactant, 　the white liquor separation accelerating agent being used to accelerate separation of lime mud and white liquor in milky liquid generated during production of kraft pulp, when the lime mud is separated from the milky liquid by filtration to recover the white liquor, the milky liquid being a mixed liquid of the white liquor and the lime mud.

9. The white liquor separation accelerating agent according to claim 8, wherein the white liquor separation accelerating agent is added to water to be sprayed in a filtration type white liquor treatment facility in which the white liquor is separated from the milky liquid and recovered.

10. The white liquor separation accelerating agent according to claim 8, wherein the white liquor separation accelerating agent is added to the milky liquid after slaking and causticizing reactions caused by adding quick lime to green liquor generated during the production of kraft pulp.

11. The white liquor separation accelerating agent 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 alkyl phenyl ether, polyoxyethylene polyoxypropylene glycol, and polyoxyethylene fatty acid ester.

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

13. The white liquor separation accelerating agent according to claim 11, wherein the polyoxyethylene polyoxypropylene glycol is an EO / PO / EO- or PO / EO / PO-type block polymer having a number of moles of ethylene oxide added in a range of 5 or more and 20 or less and a number of moles of propylene oxide added in a range of 24 or more and 35 or less.

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