Method for washing chlorine bypass dust with water

Abstract

The present invention provides a method for washing chlorine bypass dust with water, which can reduce the selenium concentration in the wastewater discharged when chlorine bypass dust is washed with water, while keeping costs for equipment and chemicals down. [Solution] A method for washing chlorine bypass dust, comprising a slurrying step of adding washing water to chlorine bypass dust to form a slurry, and a solid-liquid separation step of separating the liquid phase from the slurry, wherein the slurry further contains municipal solid waste incineration fly ash in addition to the chlorine bypass dust, and the municipal solid waste incineration fly ash accounts for 15% by mass or more and 80% by mass or less of the total amount of the chlorine bypass dust and the municipal solid waste incineration fly ash.

Description

Technical Field

[0001] The present invention relates to a method for washing chlorine bypass dust with water.

Background Art

[0002] Components such as chlorine, alkali, and sulfur contained in cement raw materials volatilize during the process of firing the cement raw materials in a cement kiln, circulate, and accumulate in the system. In particular, chlorine causes problems such as clogging of the preheater, so chlorine is selectively discharged outside the system. Conventionally, as a method for discharging this chlorine outside the system, a chlorine bypass system has been adopted in which a part of the combustion gas is extracted from the kiln exhaust gas flow path from the kiln bottom of the cement kiln to the lowest cyclone, cooled, condensed, and solidified to recover chlorine. It has been disclosed that the chlorine bypass dust recovered by the chlorine bypass system is washed with water, and the residue after the water washing treatment can be reused as a cement raw material (Patent Document 1).

[0003] Such chlorine bypass dust contains selenium, which is a harmful component, and selenium elutes from the chlorine bypass dust into the wastewater discharged during the water washing treatment. Therefore, it is necessary to remove selenium from the wastewater. As a method for removing selenium from wastewater, a coagulation precipitation method, an ion exchange method, etc. are known. As an example of the coagulation precipitation method, for example, Patent Document 2 discloses a method in which selenium-containing wastewater is evaporated and concentrated, and the concentrated water is treated at 60°C or higher by contact with metallic iron or addition of an iron (II) salt. Further, for example, Patent Document 3 discloses a method in which sulfuric acid is added so that the sulfuric acid concentration of a selenium-containing solution becomes 3.5 to 6N, a reducing agent composed of metal powder or metal ions is added, and the treatment is carried out at a temperature of 80°C or higher to precipitate selenium as a selenide of the reducing agent metal. As an example of the ion exchange method, for example, Patent Document 4 discloses a method for removing selenate ions contained in wastewater, in which the wastewater is irradiated with light in the presence of a semiconductor photocatalyst and an organic reducing agent, and the selenate ions are reduced to solid selenium and / or gaseous hydrogen selenide.

[0004] However, the above-mentioned methods for removing selenium have the problem of being economically burdensome due to the large expenditures required for wastewater treatment facilities and treatment work. Therefore, in the water washing treatment of chlorine bypass dust, attempts have been made to reduce the elution rate of selenium from chlorine bypass dust and lower the selenium concentration in the wastewater. As a method for reducing the elution rate of selenium and lowering the selenium concentration in the wastewater, for example, Patent Document 5 discloses a method for water washing cement kiln extraction dust in which only chlorine bypass dust with a free lime content of less than 30% by mass is water washed, and for example, Patent Document 6 discloses a method for water washing cement kiln extraction dust using washing water to which acid has been added in advance. [Prior art documents] [Patent Documents]

[0005] [Patent Document 1] Japanese Patent Application Publication No. 11-100243 [Patent Document 2] Japanese Patent Publication No. 2000-167571 [Patent Document 3] Japanese Patent Application Publication No. 10-218611 [Patent Document 4] Japanese Patent Application Publication No. 09-239377 [Patent Document 5] Japanese Patent Publication No. 2020-147469 [Patent Document 6] Japanese Patent Publication No. 2021-59472 [Overview of the project] [Problems that the invention aims to solve]

[0006] However, the method described in Patent Document 5 involves sorting the chlorine bypass dust to be washed based on the amount of free lime, which requires a measurement procedure and measuring device for the amount of free lime. Furthermore, depending on the specifications and / or operating conditions of the chlorine bypass equipment, storage equipment for chlorine bypass dust according to the amount of free lime and / or mixing equipment to adjust the amount of free lime may be required. In addition, the method described in Patent Document 6 involves adding acid (chemical) to the washing water, which incurs equipment and chemical costs.

[0007] Therefore, the object of the present invention is to provide a method for washing chlorine bypass dust with water that can reduce the selenium concentration of wastewater discharged when chlorine bypass dust is washed with water, while keeping costs for equipment and chemicals down. [Means for solving the problem]

[0008] To achieve the above objective, the present invention provides a method for washing chlorine bypass dust with water, comprising a slurrying step of adding washing water to chlorine bypass dust to form a slurry, and a solid-liquid separation step of separating the liquid phase from the slurry, wherein the slurry further contains municipal solid waste incineration fly ash in addition to the chlorine bypass dust, and the municipal solid waste incineration fly ash accounts for 15% by mass or more and 80% by mass or less of the total amount of the chlorine bypass dust and the municipal solid waste incineration fly ash.

[0009] According to the above-described method for washing chlorine bypass dust with water, by adding municipal solid waste incineration fly ash to the chlorine bypass dust and washing it with water, the amount of selenium leached into the liquid phase after solid-liquid separation can be kept low. This reduces the selenium concentration in the wastewater discharged when chlorine bypass dust is washed with water, and consequently reduces the costs required for equipment and chemicals used in selenium removal.

[0010] In the above-described method for washing chlorine bypass dust with water, it is preferable to include a selenium concentration measurement step in which the selenium concentration in the liquid phase separated in the solid-liquid separation step is measured, and to adjust the content of the municipal solid waste incineration fly ash subjected to the slurrying step according to the measurement results obtained in the selenium concentration measurement step.

[0011] The above configuration allows for a more efficient reduction of selenium concentration in wastewater.

[0012] In the above-described method for washing chlorine bypass dust with water, it is preferable to recover the liquid phase separated in the solid-liquid separation step and use it as the washing water used in the slurrying step.

[0013] According to the above configuration, the amount of washing water used can be reduced, and consequently, the amount of wastewater discharged from the washing process can be reduced. [Brief explanation of the drawing]

[0014] [Figure 1] This is a flowchart illustrating one embodiment of the water washing method for chlorine bypass dust provided by the present invention. [Figure 2] This is a flowchart illustrating another embodiment of the chlorine bypass dust washing method provided by the present invention. [Figure 3] This is a flowchart illustrating yet another embodiment of the chlorine bypass dust washing method provided by the present invention. [Figure 4] The first example shows a graph illustrating the effect of the composition of municipal solid waste incineration fly ash on the amount of selenium leached from chlorine bypass dust, and the second example shows the results of measuring the selenium concentration in the filtrate. [Figure 5] This figure shows the results of an investigation into the effect of the composition of municipal solid waste incineration fly ash on the amount of selenium leached from chlorine bypass dust in Test Example 1, and also shows the results of calculating the selenium leaching ratio based on Comparative Example 1. [Modes for carrying out the invention]

[0015] In this specification, "chlorine bypass dust" is usually synonymous with the meaning understood by those skilled in the art. That is, for the purpose of reducing chlorine circulating in the cement kiln, a part of the exhaust gas generated in the cement kiln is extracted, and it refers to the dust (powder) generated when the extracted exhaust gas is cooled. Generally, chlorine bypass dust contains chlorine at a concentration of about 10% to 40% by mass. In addition to chlorine, heavy metals such as selenium and lead are also contained in a gaseous state.

[0016] In this specification, "municipal solid waste incineration fly ash" is usually synonymous with the meaning understood by those skilled in the art. That is, it is the dust generated when waste (typically household waste) is incinerated, and usually contains dust containing heavy metals such as lead volatilized in the incinerator, strongly acidic gases such as hydrogen chloride and sulfur oxides, and moisture.

[0017] In the method for washing chlorine bypass dust provided by the present invention, by adding municipal solid waste incineration fly ash to the chlorine bypass dust and performing a washing treatment, the elution amount of selenium eluted in the liquid phase after solid-liquid separation can be kept low. Thereby, the selenium concentration of the drainage discharged when the chlorine bypass dust is washed can be reduced, and ultimately the costs required for facilities, chemicals, etc. for the selenium removal treatment can be suppressed.

[0018] Here, the selenium concentration of the drainage can be measured by a well-known method. For example, the 3,3'-diaminobenzidine spectrophotometric method in accordance with JIS K 0102 "Test Methods for Industrial Wastewater" is preferably exemplified. Alternatively, it can also be measured by a hydride generation atomic absorption method, a hydride generation ICP emission spectroscopic analysis method, an ICP mass analysis method, etc.

[0019] Hereinafter, the present invention will be described more specifically with reference to the drawings.

[0020] Figure 1 shows one embodiment of the water washing method for chlorine bypass dust provided by the present invention. As shown in Figure 1, the water washing method for chlorine bypass dust according to the present invention comprises a slurrying step of adding washing water to chlorine bypass dust to form a slurry, and a solid-liquid separation step of separating the liquid phase from the slurry. Furthermore, the slurry contains not only chlorine bypass dust but also municipal solid waste incineration fly ash.

[0021] The means of forming the slurry described above are not limited to, but can be used, for example, by using a powder dissolution tank equipped with a container that can contain chlorine bypass dust, municipal solid waste incineration fly ash, and washing water together, and at least a stirring means for mixing them to form a slurry.

[0022] During the slurrying process described above, chlorine bypass dust is mixed with municipal solid waste incineration fly ash in a predetermined ratio. While not limited to this, for example, by equipping the container holding the chlorine bypass dust and municipal solid waste incineration fly ash with a weighing device such as a hopper scale, the chlorine bypass dust and municipal solid waste incineration fly ash can be supplied to the powder dissolution tank in a predetermined ratio.

[0023] There are no particular restrictions on the timing of mixing the chlorine bypass dust and the municipal solid waste incineration fly ash. The predetermined amounts of chlorine bypass dust and municipal solid waste incineration fly ash may be mixed in a mixer before adding the washing water. Alternatively, the predetermined amounts of chlorine bypass dust, municipal solid waste incineration fly ash, and washing water may be added simultaneously and mixed by stirring. Alternatively, the washing water may be added to the predetermined amounts of chlorine bypass dust and municipal solid waste incineration fly ash separately before mixing.

[0024] In the present invention, the slurry contains 15% to 80% by mass of the total amount of chlorine bypass dust and municipal solid waste incineration fly ash, with the municipal solid waste incineration fly ash content being preferably 60% to 80% by mass. If the municipal solid waste incineration fly ash content is less than 15% by mass, the effect of reducing the selenium elution rate cannot be sufficiently obtained, and if the municipal solid waste incineration fly ash content exceeds 80% by mass, a large amount of municipal solid waste incineration fly ash is required for the water washing treatment of the chlorine bypass dust, which actually makes it less efficient. If the municipal solid waste incineration fly ash content is 60% to 80% by mass, the selenium elution rate can be further reduced.

[0025] The amount of washing water added for slurry formation should be sufficient to allow chlorine to dissolve when added to the chlorine bypass dust. For example, the amount of washing water may be 1 to 10 parts by mass, 1 to 8 parts by mass, or 1 to 4 parts by mass per 1 part by mass of the total of chlorine bypass dust and municipal solid waste incineration fly ash.

[0026] In the slurry formation process, chlorine is eluted by allowing the slurry to stand or be stirred for a predetermined time. The required time is preferably 30 minutes or more, and more preferably 45 minutes or more, in order to thoroughly treat the chlorine bypass dust with washing water. The temperature can be 10°C to 60°C, 20°C to 50°C, or even room temperature.

[0027] In any non-limiting embodiment of the present invention, a CO2-containing gas may be introduced in the slurrying process described above. That is, by introducing a CO2-containing gas and adjusting the pH to the slightly alkaline to acidic side, chlorine can be eluted more efficiently. There are no particular limitations on the method of introducing the CO2-containing gas; for example, combustion exhaust gas from a rotary kiln of a cement manufacturing facility or combustion exhaust gas from a municipal solid waste incineration facility can be blown into the slurry. The CO2-containing gas only needs to contain carbon dioxide, but the carbon dioxide concentration is preferably 10% by volume or more, and more preferably 20% by volume or more.

[0028] The means for the above solid-liquid separation process can be any means that separates the liquid phase from which chlorine and selenium have leached from the slurry, thereby obtaining a washed product from which they have been removed. There are no particular restrictions, but it can be carried out using general solid-liquid separation equipment such as a pressurized dewatering machine, a vacuum dewatering machine, or a centrifugal dewatering machine. A pressurized dewatering machine or a vacuum dewatering machine is preferred from the viewpoint of dewatering ability.

[0029] In the solid-liquid separation apparatus described above, the resulting washed material usually contains water, and chlorine dissolved in the liquid phase of the slurry may remain with the liquid phase. To reduce the chlorine remaining with the liquid phase, if necessary, a cake washing process may be performed by adding water to the cake-like washed material after solid-liquid separation. This will almost completely replace the liquid phase of the slurry with water, thereby further reducing the chlorine remaining in the resulting washed material. The amount of water used for cake washing may be 1 to 8 parts by mass, 2 to 7 parts by mass, or 3 to 6 parts by mass per 1 part by mass of the cake-like washed material from which the liquid phase has been removed in the solid-liquid separation process.

[0030] As shown in Figure 1, in this embodiment, the liquid phase separated in the solid-liquid separation step may be subjected to wastewater treatment to remove selenium and other heavy metals in the liquid phase, if necessary. Wastewater treatment is not limited to but can be carried out by general heavy metal treatment methods such as coagulation-sedimentation or ion exchange.

[0031] As shown in Figure 1, in this embodiment, the washed material separated in the solid-liquid separation process may be recycled as a cement raw material if necessary. This allows for the effective utilization of both chlorine bypass dust and municipal solid waste incineration fly ash without disposal.

[0032] In the chlorine bypass dust washing method provided by the present invention, the movement of slurry and liquid can be carried out using fluid supply devices, supply tanks, fluid supply pipes, predetermined storage tanks, etc., which are well known to those skilled in the art.

[0033] Figure 2 shows another embodiment of the water washing method for chlorine bypass dust provided by the present invention. In this embodiment, a selenium concentration measurement step is provided to measure the selenium concentration in the liquid phase separated in the solid-liquid separation step, and the amount of municipal solid waste incineration fly ash treated in the slurrying step is adjusted according to the measurement results obtained in the selenium concentration measurement step. According to this embodiment, the selenium concentration of wastewater can be reduced more efficiently.

[0034] The above selenium concentration can be measured, but is not limited to, methods such as 3,3'-diaminobenzidine spectrophotometric analysis, hydrogen compound generation atomic absorption spectrometry, hydrogen compound generation ICP emission spectrometry, and ICP mass spectrometry, which are based on JIS K 0102 "Test Methods for Industrial Wastewater".

[0035] When adjusting the selenium content of municipal solid waste incineration fly ash, if the selenium concentration obtained in the selenium concentration measurement process is high, the selenium content can be increased to bring the selenium concentration within the desired range. On the other hand, if the selenium concentration obtained in the selenium concentration measurement process is low, the selenium concentration can be maintained or decreased to bring the selenium concentration within the desired range. The target selenium concentration is preferably 0.25 mg / L or less, more preferably 0.15 mg / L or less, even more preferably 0.1 mg / L or less, and particularly preferably 0.05 mg / L or less. The permissible limit for selenium in general wastewater standards is 0.1 mg / L, and if the selenium concentration is 0.1 mg / L or less, selenium removal treatment becomes unnecessary, thus reducing the equipment and treatment costs.

[0036] Figure 3 shows yet another embodiment of the chlorine bypass dust washing method provided by the present invention. In this embodiment, the liquid phase separated in the solid-liquid separation step is recovered and used as washing water supplied to the slurrying step. That is, in the example shown in Figure 3, the liquid phase separated in the solid-liquid separation step is used to prepare a slurry containing chlorine bypass dust and municipal solid waste incineration fly ash. On the other hand, the liquid phase separated in the solid-liquid separation step may be divided into either being reused as washing water or proceeding to wastewater treatment, as needed. The choice of whether to reuse as washing water or proceed to wastewater treatment is not particularly limited and may be decided arbitrarily, but may be determined, for example, by the concentration of chlorine, selenium, etc. in the liquid phase, or by the amount of wastewater discharged in the chlorine bypass dust washing treatment. In particular, it is preferable to use it as washing water when the selenium concentration of the liquid phase is 0.1 mg / L or higher, which is the permissible limit of general wastewater standards. According to this embodiment, the amount of wastewater discharged in the chlorine bypass dust washing treatment can be reduced.

[0037] Furthermore, if a method is employed in which cake washing is performed as a solid-liquid separation method and the cake washing liquid is discharged, the cake washing liquid may be collected together and used as the washing liquid supplied to the slurry process described above. In addition to the liquid phase separated in the solid-liquid separation process, the washing wastewater used for cleaning the equipment may also be reused as washing water in the slurrying process. In this manner, the amount of wastewater discharged from equipment and facilities that utilize the water washing treatment of chlorine bypass dust can be further reduced.

[0038] The present invention is not limited to the embodiments described above, and various combinations and modifications are possible within the scope of disclosure herein, and such embodiments are also included in the technical scope of the present invention. [Examples]

[0039] The present invention will be described in more detail below with reference to test examples. However, the scope of the present invention is not limited by these test examples.

[0040] <Test Example 1> Chlorine bypass dust generated at a cement plant (chlorine concentration: 11.2% as determined by the fundamental parameters of X-ray fluorescence analysis) and municipal solid waste incineration fly ash generated at a waste incineration plant (with the chemical composition shown in Table 1) were mixed in various proportions as shown in Table 2 below to obtain powder samples.

[0041] [Table 1]

[0042] 100 g of the powder sample and 400 g of water were placed in a beaker and stirred with a stirrer for 30 minutes to form a slurry. This slurry was separated into solid and liquid using quantitative filter paper No. 5C to obtain the residue and filtrate.

[0043] The selenium concentration of the filtrate obtained for each powder sample was measured in accordance with JIS K 0102 "Test Methods for Industrial Wastewater". Furthermore, using Comparative Example 1, in which chlorine bypass dust was 100% by mass, as a reference, the selenium elution ratio of chlorine bypass dust, taking into account dilution by municipal solid waste incineration fly ash, was calculated from the selenium concentration of the filtrate obtained for each powder sample using the following formula.

[0044] Selenium elution ratio = For each powder sample, the selenium concentration of the filtrate obtained (mg / L) ÷ (selenium concentration of Comparative Example 1 (mg / L) × percentage of chlorine bypass dust in each powder sample (mass%) ÷ 100) The selenium elution ratio in Comparative Example 1 was 1.0, and a smaller selenium elution ratio indicates a lower selenium elution rate from chlorine bypass dust.

[0045] The selenium concentration and selenium elution ratio of the filtrate obtained for each powder sample are shown in Table 2 and Figures 4 and 5.

[0046] [Table 2]

[0047] (evaluation) As a result, the following became clear:

[0048] (1) As seen in the results of Examples 1 to 3, compared to Comparative Example 1, in which only chlorine bypass dust was slurryed and then subjected to solid-liquid separation, when municipal solid waste incineration fly ash was added to the chlorine bypass dust so that its content in the powder sample was between 15% by mass and 80% by mass, the selenium elution ratio into the filtrate decreased when solid-liquid separation was performed after slurrying.

[0049] (2) As seen in the results of Examples 2 and 3, when the content of municipal solid waste incineration fly ash in the powder sample was 60% by mass or more, the selenium concentration in the filtrate after solid-liquid separation following slurry formation was less than 0.1 mg / L, which is the permissible limit for general wastewater standards. Since the selenium concentration is below the general wastewater standard value, selenium removal treatment of the wastewater becomes unnecessary, and it is thought that the equipment and treatment costs for this purpose can be reduced.

[0050] From the above, it was found that the selenium concentration in wastewater can be reduced by adding municipal solid waste incineration fly ash to chlorine bypass dust and washing it with water so that the fly ash content is between 15% and 80% by mass.

Claims

1. A method for washing chlorine bypass dust with water, comprising a slurrying step of adding washing water to chlorine bypass dust to form a slurry, and a solid-liquid separation step of separating the liquid phase from the slurry, A method for washing chlorine bypass dust, wherein the slurry further contains municipal solid waste incineration fly ash in addition to the chlorine bypass dust, and the municipal solid waste incineration fly ash accounts for 15% by mass or more and 80% by mass or less of the total amount of the chlorine bypass dust and the municipal solid waste incineration fly ash.

2. The process includes a selenium concentration measurement step for measuring the selenium concentration in the liquid phase separated in the solid-liquid separation step, A method for washing chlorine bypass dust according to claim 1, wherein the content of the municipal solid waste incineration fly ash subjected to the slurrying step is adjusted according to the measurement results obtained in the selenium concentration measurement step.

3. A method for washing chlorine bypass dust with water according to claim 1 or 2, wherein the liquid phase separated in the solid-liquid separation step is recovered and used as washing water for the slurrying step.

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