JP2016040419A - Method for producing carbon fiber - Google Patents

Abstract

PROBLEM TO BE SOLVED: To provide such a technique that when the surface of a carbon fiber is treated, a tar-like substance or the like in an electrolytic solution is removed efficiently so that the electrolytic solution is reused while suppressing an increase of a production cost thereof.SOLUTION: A method for producing the carbon fiber satisfies the following steps (1)-(4). At the step (1), a polyacrylonitrile-based carbon fiber precursor fiber bundle is introduced into a flameproofing furnace and flameproofing treatment is performed on the introduced fiber bundle at the temperature range of 200-300°C to obtain a flameproofing fiber bundle. At the step (2), the obtained flameproofing fiber bundle is introduced into a carbonization furnace and carbonization treatment is performed on the introduced flameproofing fiber bundle at the temperature range of 300-2,500°C to obtain a carbon fiber bundle. At the step (3), the obtained carbon fiber bundle is immersed in the electrolytic solution in an electrolytic oxidation treatment tank to perform electrolytic oxidation treatment thereon. At the step (4), a polyacrylonitrile-based fiber is immersed in the electrolytic solution by a ratio of 10 g/L or higher so that absorbance of the electrolytic solution is made equal to or lower than 0.4 and the resulting electrolytic solution is circulated through the electrolytic oxidation treatment tank and then reused.SELECTED DRAWING: None

Description

  The present invention relates to a method for producing a polyacrylonitrile-based carbon fiber.

  Carbon fiber is widely used in aircraft parts, railway vehicle parts, ship parts, sports equipment, etc. due to its mechanical and chemical properties and light weight, and in recent years, it is also used in general industrial fields such as automobiles. It has been expanded. For this reason, the level of mechanical properties desired for carbon fibers is increasing, and there is a strong demand for a reduction in price by improving productivity.

  However, carbon fibers often have insufficient wettability, affinity, and adhesiveness with the matrix resin, and it has been difficult to obtain satisfactory characteristics when used as a composite material. For this reason, the carbon fiber after firing is usually subjected to a surface treatment and further subjected to a sizing treatment to improve the wettability, affinity, and adhesion of the matrix resin to the carbon fiber.

  As a method for surface treatment of carbon fiber, liquid phase oxidation treatment such as electrolytic oxidation treatment and chemical solution oxidation treatment, and gas phase oxidation treatment are known. By subjecting the carbon fiber surface to an oxidation treatment, oxygen-containing functional groups are introduced to the fiber surface and the surface area of the fiber is increased, so that the wettability, affinity, and adhesion of the matrix resin to the carbon fiber are improved. In particular, electrolytic oxidation treatment is easier than chemical oxidation treatment or gas phase oxidation treatment from the viewpoint of ease of treatment, ease of treatment condition control, ease of introduction of oxygen-containing functional groups on the carbon fiber surface, and the like. Has been widely used industrially as a practical and effective surface treatment method.

  On the surface of the carbon fiber, a carbon fiber precursor fiber or a pyrolysis product derived from an oil agent adheres to the fiber bundle, a tar-like deposit, a deposit made of a low crystalline carbonized product, or thermal damage to the fiber bundle. Alternatively, there is a highly fragile heterogeneous structure (hereinafter abbreviated as “tar-like substance”) generated by mechanical damage. When such a carbon fiber is immersed in an electrolytic bath for electrolytic oxidation treatment, tar-like substances and the like are detached from the surface of the carbon fiber and accumulated in the electrolytic bath or deposited in the electrolytic solution. As a result, the production yield of the carbon fiber is lowered and the quality of the carbon fiber product is lowered.

  In order to solve the above problems, for example, Patent Document 1 (Japanese Patent Laid-Open No. Sho 63-315668) discloses a technique of circulating and reusing an aqueous electrolyte solution containing an alkaline electrolyte while filtering it with a filter. Yes. Patent Document 2 (Japanese Patent Application Laid-Open No. 2-200867) discloses a technique for circulating and reusing the electrolyte while adsorbing and removing the tar-like substance in the alkaline electrolyte with an anion exchange substance. Has been.

Japanese Unexamined Patent Publication No. Sho 63-315668 JP-A-2-200787

  However, in the method described in Patent Document 1, although the solid tar-like substance detached from the carbon fiber can be removed with a filter, the tar-like substance dissolved in the aqueous electrolyte solution cannot be removed. The aqueous electrolyte solution could not be washed sufficiently.

  Further, in the method described in Patent Document 2, tar-like substances and the like in the alkaline electrolyte can be removed with an anion exchange substance, but a large amount of anion exchange substance is required for the carbon fiber, and the electrolyte washing There was a problem that capital investment costs increased.

  From the above, when selecting a method for cleaning and reusing the electrolyte for surface treatment, the effects of raw material costs, capital investment costs, utility costs, etc. on the production cost of carbon fiber for various surface treatment methods and cleaning methods. Therefore, it is necessary to select an optimal cleaning method.

  An object of the present invention is to provide a technique for efficiently removing tar-like substances and the like detached from the surface of a carbon fiber into the electrolytic solution and reusing the electrolytic solution while suppressing an increase in manufacturing cost. There is.

The present invention relates to a method for producing a carbon fiber bundle that satisfies the following (1) to (4).
(1) A polyacrylonitrile-based carbon fiber precursor fiber bundle is introduced into a flameproofing furnace and subjected to a flameproofing treatment in any temperature range of 200 ° C to 300 ° C to obtain a flameproofing fiber bundle.
(2) The flame-resistant fiber bundle is introduced into a carbonization furnace and carbonized in any temperature range of 300 ° C to 2500 ° C to obtain a carbon fiber bundle.
(3) The carbon fiber bundle is immersed in an electrolytic solution in an electrolytic oxidation treatment tank and subjected to electrolytic oxidation treatment.
(4) The electrolytic solution is circulated and used while being washed with polyacrylonitrile fiber having a single fiber fineness of 1.0 dtex or more and a total fineness of 30000 dtex or more.

  The polyacrylonitrile fiber is preferably immersed in the electrolytic solution in the electrolytic oxidation treatment tank at a rate of 10 g / L or more.

  Further, the polyacrylonitrile fiber is immersed in the electrolytic solution extracted from the electrolytic oxidation treatment tank, and the absorbance at 230 nm measured by the following absorbance measurement of the electrolytic solution is set to 0.4 or less, and then the electrolytic oxidation treatment. It is preferable to send the solution to a tank.

  According to the present invention, in the step of surface treatment (electrolytic oxidation treatment) of carbon fibers, tar-like substances and the like in the electrolytic solution can be efficiently removed by an inexpensive method. As a result, the electrolytic solution can be reused while being circulated, and a reduction in carbon fiber production yield and a reduction in carbon fiber quality can be improved.

  The following is a detailed description of the present invention.

(Polyacrylic precursor fiber)
The PAN-based polymer used for the spinning solution of the polyacrylonitrile-based precursor fiber for carbon fiber (hereinafter, “polyacrylonitrile” is abbreviated as “PAN”) of the present invention includes a copolymer component when converted into carbon fiber. In order to reduce defects caused by the above and improve the quality and performance of the carbon fiber, it is preferable to polymerize acrylonitrile at 90% by weight or more, preferably 96% by weight or more.

  The PAN-based polymer used in the present invention has no restrictions on the copolymer component, molecular weight distribution, stereoregularity, etc., and promotes flame resistance as a copolymer component in order to promote flame resistance treatment for carbon fiber. It is preferable to copolymerize 0.1 to 5 mol% of the monomer having As the flame resistance promoting component, those having at least one carboxyl group or amide group are preferably used. Further, as the flame resistance reaction becomes higher, the flame resistance treatment can be performed in a shorter time, and the productivity can be increased. Therefore, it is desirable to increase the copolymerization amount of the flame resistance promoting component. However, on the other hand, as the amount of copolymerization increases, the exothermic rate increases and the risk of runaway reaction may occur. Therefore, the range is preferably not more than 5 mol%, more preferably 0.5 to 3 mol%. Preferably, it is more preferable to set it as 1-3 mol%.

  As specific examples of the monomer having a flame resistance promoting action, acrylic acid, methacrylic acid, itaconic acid, crotonic acid, citraconic acid, ethacrylic acid, maleic acid, mesaconic acid, acrylamide, methacrylamide and the like are preferably used. Acrylamide and methacrylamide are more preferably used from the viewpoint of promoting flame resistance in the firing step and improving solubility in solvents.

  In order to produce the PAN-based polymer used in the present invention, conventionally known solution polymerization, suspension polymerization, emulsion polymerization and the like can be applied. Examples of the solvent used for preparing the acrylic polymer solution include dimethyl sulfoxide, dimethylacetamide, dimethylformamide, an aqueous zinc chloride solution, and nitric acid.

(Spinning of PAN-based precursor fiber)
In the method of the present invention, the above-described spinning solution is spun from a die by a wet spinning method or a dry-wet spinning method and introduced into a coagulation bath to coagulate the fibers. From an industrial viewpoint, a wet spinning method excellent in productivity is preferable.

  In the present invention, the coagulation bath is preferably an aqueous solution containing materials used for the spinning dope, and is set so as to reduce the porosity of the coagulated yarn by adjusting the concentration of the contained solvent. Generally, depending on the solvent used, for example, when DMAc is used, the concentration of DMAc is 50 to 80% by weight, preferably 60 to 75% by weight. The temperature of the coagulation bath is preferably low, and is usually 50 ° C. or lower, more preferably 40 ° C. or lower. If the temperature of the coagulation bath is lowered, a denser yarn can be obtained. However, if the temperature is lowered too much, the take-up speed of the coagulated yarn is lowered and the productivity is lowered.

  In the method of the present invention, the swollen yarn obtained above is washed and drawn in the washing and drawing steps. In addition, about the order of washing | cleaning and extending | stretching, you may perform washing | cleaning first and may carry out simultaneously. Although there is no restriction | limiting in particular as a washing | cleaning method, The method of immersing in the water generally used, especially warm water is good.

  The stretching method may be a method of stretching while being immersed in water or warm water, a dry heat stretching method in the air using a hot plate or a roller, or stretching in a box furnace where hot air is circulated. However, it is not limited to these. From an economical viewpoint, it is preferable to carry out in warm water. The draw ratio is preferably 1 to 8 times. However, when performing secondary stretching later, it is preferable to set in consideration of the stretching ratio.

  In the method of the present invention, the fiber bundle after washing and stretching obtained above in the oil agent application step is guided to an oil bath containing a silicone oil agent, and the silicone oil agent is applied to the fiber bundle. As the oil agent, a silicone-based oil agent containing a silicone compound is used. As the silicone oil, dimethyl silicone oil or organically modified silicone oil is preferably used, and amino-modified silicone oil having high heat resistance is more preferable. Usually, a silicone compound and a nonionic emulsifier are mixed and emulsified. In some cases, an antioxidant, various additives, and an organic substance not containing a silicone atom can be mixed.

  In the method of the present invention, the fiber bundle provided with the silicone-based oil obtained above in dry densification is dry densified. As a method for drying and densifying, it is generally performed by contacting with a hot plate or a heating roller, and drying with a heating roller is preferably used. The higher the drying temperature, the more the crosslinking reaction of the silicone oil agent is promoted, and it is also preferable from the viewpoint of productivity. Therefore, it can be set high within a range in which fusion between single fibers does not occur. Specifically, 150 ° C. or higher is preferable, and 180 ° C. or higher is more preferable. Further, it is preferable that the drying time is a time for sufficiently drying the fiber bundle.

  In the method of the present invention, the fiber bundle after drying and densification obtained above can be secondarily stretched as necessary. Examples of the secondary stretching method include dry heat stretching and steam stretching.

  In the present invention, the single fiber fineness of the PAN precursor fiber for carbon fiber to be obtained is preferably 1.0 to 3.0 dtex, and more preferably 1.0 to 2.5 dtex. The number of filaments of the PAN-based precursor fiber is preferably 30,000 to 300,000, more preferably 30,000 to 200,000, and even more preferably 30,000 to 100,000. If the single fiber fineness is too small, the processability of the yarn-making process and the firing process may decrease due to a decrease in spinnability and the occurrence of yarn breakage due to contact between rollers and guides. In addition, if the single fiber fineness is too large, the difference between the inner and outer structures of each single fiber after flame resistance becomes large, and the process passability in the subsequent carbonization process, the tensile strength of the obtained carbon fiber, and the tensile elastic modulus may decrease. is there.

(Flame resistance treatment)
The PAN-based precursor fiber is made flame resistant fiber by subjecting it to flame resistance treatment by the method described below. Flame resistance is an atmospheric heating method, and flame resistance treatment is performed in any temperature range of 200 ° C to 300 ° C. As the apparatus, a hot-air circulating furnace that circulates a heated oxidizing gas can be suitably employed. Usually, in a hot-air circulating furnace, a fiber bundle that has entered the furnace is once taken out of the furnace, and then folded back by a folding roll disposed outside the furnace and repeatedly passed through the furnace. As the atmosphere, a known oxidizing atmosphere such as air, oxygen, and nitrogen dioxide can be adopted, but air is preferable from the viewpoint of economy.

(Pre-carbonization treatment and carbonization treatment)
The flame-resistant fiber is made into carbon fiber by pre-carbonization treatment and carbonization treatment at any temperature range of 300 ° C. to 2500 ° C. by the method described below. First, the flame resistant fiber is put into a first carbonization furnace and pre-carbonized. An inert atmosphere having a temperature of 300 to 800 ° C. circulates in the first carbonization furnace, and the flame resistant fiber is pre-carbonized while traveling in the inert atmosphere. In addition, the flow of the inert atmosphere which circulates in the 1st carbonization furnace may be a parallel direction with respect to the to-be-processed fiber, or a perpendicular direction, and is not specifically limited. As the inert atmosphere, a known inert atmosphere such as nitrogen, argon, or helium can be adopted, but nitrogen is desirable from the viewpoint of economy.

  Next, the pre-carbonized fiber is put into a second carbonization furnace and carbonized. An inert atmosphere having a maximum temperature of 1000 to 2500 ° C. circulates in the second carbonization furnace, and the pre-carbonized fiber is carbonized while traveling in the inert atmosphere. The In addition, the flow of the inert atmosphere which circulates in the 2nd carbonization furnace may be a parallel direction or a perpendicular direction with respect to the to-be-processed fiber to drive, and is not specifically limited. The inert atmosphere can be selected from the known inert atmospheres exemplified above, but nitrogen is desirable from the viewpoint of economy.

(Carbon fiber surface treatment)
The manufacturing method of the carbon fiber of this invention includes surface-treating the carbon fiber obtained by the method mentioned above using the electrolytic oxidation treatment method. The electrolytes used in the electrolytic solution for electrolytic oxidation include (1) inorganic acids such as sulfuric acid, nitric acid, phosphoric acid, boric acid and carbonic acid, (2) organic acids such as acetic acid butyric acid, oxalic acid and maleic acid, ( 3) Alkali metal salts and ammonium salts of these inorganic or organic acids can be mentioned. Said (1)-(3) may be used individually or may be used as a 2 or more types of mixture.

(Cleaning of electrolyte)
On the surface of the carbon fiber, a carbon fiber precursor fiber or a pyrolysis product derived from an oil agent adheres to the fiber bundle, a tar-like deposit, a deposit made of a low crystalline carbonized product, or thermal damage to the fiber bundle. Alternatively, there is a highly fragile heterogeneous structure (such as a tar-like substance) caused by mechanical damage. When such a carbon fiber is immersed in an electrolytic bath for electrolytic oxidation treatment, tar-like substances and the like are detached from the surface of the carbon fiber and accumulated in the electrolytic bath, or deposited in the electrolytic solution, This causes a decrease in the production yield of carbon fibers and a decrease in the quality of carbon fiber products.

  The present inventor has studied a method for adsorbing and removing tar-like substances and the like in an electrolytic solution. It has been found that tar-like substances and the like in the electrolyte can be efficiently adsorbed and removed by soaking. As such a fibrous substance, a PAN-based fiber bundle that has high adsorptivity to tar-like substances and the like and is easily available to those who manufacture carbon fibers is optimal.

  In the present invention, the PAN-based precursor fiber described above can be used for the PAN-based fiber bundle used for cleaning the electrolytic solution. The fineness of the single fiber constituting the PAN-based fiber bundle is preferably 1.0 to 3.0 dtex, and more preferably 1.0 to 2.5 dtex. The number of filaments in the PAN-based fiber bundle is preferably 30,000 to 300,000, more preferably 30,000 to 200,000, and even more preferably 30,000 to 100,000. If the single fiber fineness and the number of filaments of the PAN-based fiber bundle are too small, the surface area of the fibrous material is small and the tar-like material cannot be sufficiently removed by adsorption.

  In the present invention, it is preferable that the PAN fiber used for washing the electrolytic solution is not provided with an oil agent. When the oil agent adheres to the PAN-based fiber, the oil agent itself causes the electrolytic solution to become dirty, and the tar-like substance cannot be sufficiently adsorbed and removed.

  In the present invention, the amount of the PAN-based fiber used for washing the electrolytic solution is preferably 10 g / L or more, more preferably 15 g / L or more with respect to 1 L of the electrolytic solution. When the amount of PAN fiber used is small relative to the electrolyte, the tar-like substance in the electrolyte cannot be sufficiently adsorbed and removed. This causes a decrease in production yield and a decrease in the quality of the carbon fiber. There is no particular upper limit on the amount of PAN fiber used, but 40 g / L is sufficient.

  As a specific aspect of washing of the electrolytic solution, the PAN-based fiber is immersed in the electrolytic solution extracted from the electrolytic oxidation treatment tank so that the absorbance (described later) of the electrolytic solution is 0.4 or less, and then the electrolytic oxidation treatment tank And a method of circulating and feeding the solution.

(Absorbance)
In order to sufficiently wash the electrolytic solution, the absorbance of the electrolytic solution measured by the following method is preferably set to a certain value or less. If the absorbance is 0.4 or less, it can be considered that tar-like substances and the like in the electrolytic solution have been removed to the extent that they do not affect the carbon fiber production yield and the quality of the carbon fibers.
Absorbance measurement
The electrolytic solution Xg is taken out, and diluted in a beaker containing distilled water Yg for spectroscopic analysis using the electrolytic solution as a cleaning solution. (However, it is assumed that “X: Y = 1: 9”) The cleaning liquid after the treatment was subjected to a spectrophotometer (manufactured by HITACHI, U-3300) under the conditions of a scanning speed of 300 nm / min, a sampling interval of 1 nm, and a slit of 2 nm. Absorbance measurement is performed using a quartz cell having a cell length of 10 mm. First, a reference measurement using distilled water for spectroscopic analysis is performed, and the resulting transmittance at a wavelength of 230 nm is T ₀ . Subsequently, the absorbance of the cleaning liquid is measured in the same manner, and the resulting transmittance at a wavelength of 230 nm is T. Absorbance A = −log ₁₀ (T / T ₀ ) calculated from the transmittance is defined as the amount of foreign matter present.

  Hereinafter, the present invention will be described more specifically with reference to examples. In this example, various characteristics were measured as follows.

<Total fineness of PAN fiber>
The total fineness of the PAN-based fiber used for washing the electrolytic solution was measured according to JIS R 7605.

<Absorbance>
The electrolytic solution Xg is taken out, and diluted in a beaker containing distilled water Yg for spectroscopic analysis using the electrolytic solution as a cleaning solution. (However, it is assumed that “X: Y = 1: 9”) The cleaning liquid after the treatment was subjected to a spectrophotometer (manufactured by HITACHI, U-3300) under the conditions of a scanning speed of 300 nm / min, a sampling interval of 1 nm, and a slit of 2 nm. Absorbance measurement is performed using a quartz cell having a cell length of 10 mm. First, a reference measurement using distilled water for spectroscopic analysis is performed, and the resulting transmittance at a wavelength of 230 nm is T ₀ . Subsequently, the absorbance of the cleaning liquid is measured in the same manner, and the resulting transmittance at a wavelength of 230 nm is T. Absorbance A = −log ₁₀ (T / T ₀ ) calculated from the transmittance is defined as the amount of foreign matter present.

[Example 1]
An acrylonitrile-based polymer composed of 96% acrylonitrile units, 3% acrylamide units and 1% methacrylic acid units (the amount of carboxylic acid groups is 7.0 × 10 ⁻⁵ equivalent, the intrinsic viscosity [η] is 1.7), The acrylonitrile polymer was dissolved in DMAc so that the total solid content concentration was 21.2% by weight to obtain a spinning dope for acrylic precursor fibers for carbon fibers. Using a spinneret having a hole diameter of 0.006 mm and a number of holes of 30000, a DMAc aqueous solution (coagulation bath) having a temperature of 38 ° C. and a concentration of 68% was discharged using a wet spinning method to obtain a coagulated yarn. Next, the coagulated yarn was stretched 7 times while removing the solvent in warm water of 60 ° C to 98 ° C. After the drawn yarn was immersed in a 1% aqueous solution of an aminosilicon-based oil, it was dried and densified with a heating roller at 180 ° C. to obtain a carbon fiber precursor fiber having a single yarn fineness of 1.0 dtex, a filament number of 60000, and a total fineness of 60000 dtex. .

The resulting acrylic precursor fiber bundle was heated in air at 230 to 250 ° C. under tension for 70 minutes to obtain a flameproof fiber bundle having a density of 1.37 g / cm ³ .

  This flame-resistant fiber bundle was heated under tension at a maximum temperature of 700 ° C. in a nitrogen atmosphere for 1 minute to obtain a pre-carbonized fiber bundle. The temperature rising rate at 400 to 500 ° C. in this carbonization treatment was 200 ° C./min. The obtained pre-carbonized fiber bundle was heated for 1 minute under tension at a maximum temperature of 1350 ° C. in a nitrogen atmosphere to obtain a carbonized fiber bundle. The rate of temperature increase at 1000 to 1200 ° C. in this carbonization treatment was 400 ° C./min.

  The obtained carbonized fiber bundle was subjected to an electrolytic oxidation treatment for 30 seconds using an aqueous solution of ammonium bicarbonate having a concentration of 0.1 N as an electrolytic solution and carbon fiber as an anode so that the amount of electricity per 1 g of carbon fiber was 30 coulombs. went.

  The electrolytic solution was impregnated with 10 g / L of polyacrylonitrile fiber having a single fiber fineness of 1.0 dtex and a total fineness of 30000 dtex to adsorb tar-like substances and the like in the electrolytic solution. Immediately after the start of electrolytic oxidation at this time and after 1 hour, 2 hours, 3 hours, 24 hours, and 48 hours, the absorbance of the electrolyte was measured. Further, the degree of contamination of the electrolytic solution and the degree of aggregation were visually observed to examine the time for the formation of aggregates. 50 ml of the electrolytic solution thus obtained after electrolytic oxidation was sampled and used as an evaluation sample. The absorbance at 230 nm of the fiber bundle was 0.38.

[Examples 2-3, Comparative Examples 1-4]
The electrolytic solution was washed under the conditions shown in Table 1. Table 1 shows the absorbance of the electrolyte solution after the washing treatment.

  When Examples 1 and 3 are compared with Comparative Examples 1 and 2, the amount of PAN-based fibers immersed in the electrolyte is 10 g / L or more, so that tar-like substances in the electrolyte are adsorbed and removed. A sufficient cleaning effect can be obtained.

Moreover, when Example 1 and Comparative Examples 3 and 4 are compared, if the single fiber fineness of the PAN-based fiber is small or the total fineness is small, the tar-like substance and the like in the electrolytic solution are not sufficiently adsorbed and removed. The cleaning effect cannot be obtained.

Claims (3)

The manufacturing method of the carbon fiber bundle which satisfies the following (1)-(4).
(1) A polyacrylonitrile-based carbon fiber precursor fiber bundle is introduced into a flameproofing furnace and subjected to a flameproofing treatment in any temperature range of 200 ° C to 300 ° C to obtain a flameproofing fiber bundle.
(2) The flame-resistant fiber bundle is introduced into a carbonization furnace and carbonized in any temperature range of 300 ° C to 2500 ° C to obtain a carbon fiber bundle.
(3) The carbon fiber bundle is immersed in an electrolytic solution in an electrolytic oxidation treatment tank and subjected to electrolytic oxidation treatment.
(4) The electrolytic solution is circulated and used while being washed with polyacrylonitrile fiber having a single fiber fineness of 1.0 dtex or more and a total fineness of 30000 dtex or more.   The method for producing carbon fiber according to claim 1, wherein the polyacrylonitrile fiber is immersed at a rate of 10 g / L or more with respect to the electrolytic solution in the electrolytic oxidation treatment tank. The polyacrylonitrile fiber is immersed in the electrolytic solution extracted from the electrolytic oxidation treatment tank, and the absorbance at 230 nm measured by the following absorbance measurement of the electrolytic solution is set to 0.4 or less, and then the electrolytic oxidation treatment tank The method for producing a carbon fiber according to claim 2, wherein the solution is fed to the liquid.