Recovery system of calcium carbonate and recovery method of calcium carbonate

Abstract

To provide a recovery system of calcium carbonate and a recovery method of calcium carbonate which are capable of recovering calcium carbonate from an incineration ash easily and efficiently.SOLUTION: A recovery system of calcium carbonate 10 includes a main ash washing part 13 in which a main ash is washed and main ash washing water 31 is recovered, a fly ash washing part 11 in which a fly ash 15 is washed and fly ash washing water 29 is recovered, and a calcium carbonate recovery part 33 in which the main ash washing water 31 is blended with the fly ash washing water 29 and a gas containing carbon dioxide is supplied to precipitate calcium carbonate 41 to recover the calcium carbonate 41.SELECTED DRAWING: Figure 1

Description

Technical Field

[0001] The present invention relates to a system for recovering calcium carbonate from combustion ash and a method for recovering calcium.

Background Art

[0002] In a landfill for disposing of incineration ash and the like, in order to stabilize the incineration ash (so that the elution of heavy metals, alkali components, etc. from the incineration ash is below the allowable limit), the incineration ash is sprinkled with water for washing. However, some heavy metals such as lead and zinc are difficult to efficiently wash out and continue to elute over a long period of time, delaying stabilization. At the same time, the calcium that is eluted simultaneously forms scale in the washing water after washing, hindering the safe operation of the washing water treatment device.

[0003] As a method for solving this problem, by spraying fine bubble water containing carbon dioxide, calcium and lead are insolubilized as carbonates (CaCO 3 and PbCO 3 ) and a technique for actively immobilizing them in the waste has been proposed (for example, Patent Document 1). In addition, a technique for performing such carbonation treatment before landfill to form a low-solubility carbonate has also been proposed. For example, there is the FAST-BOX system (registered trademark) of Fujita Corporation (URL: https: / / www.fujita.co.jp / solution-technology / 3130 / ).

Prior Art Documents

Patent Documents

[0004]

Patent Document 1

Summary of the Invention

Problems to be Solved by the Invention

[0005] However, these existing technologies have two problems. The first problem is that it is difficult to reliably and stably insolubilize both calcium and heavy metals in incineration ash and prevent them from eluting. First, calcium carbonate generated to fix calcium in incineration ash has low solubility if the pH is sufficiently high, but it tends to dissolve when the pH becomes low. For this reason, when water containing carbon dioxide is continuously sprinkled and the elution of alkaline components in the incineration ash progresses, the pH decreases, and the calcium carbonate fixed in the incineration ash dissolves. For example, if it is necessary to continue sprinkling because other water quality items are above the allowable limit even though the pH is within the allowable limit, there is a risk that the pH will drop below 7 and the calcium carbonate will dissolve.

[0006] On the other hand, when water containing carbon dioxide is sprinkled on incineration ash, calcium is insolubilized as calcium carbonate, and at the same time, heavy metals such as lead and zinc are insolubilized and fixed as carbonates or hydroxides.

[0007] Since lead and zinc are amphoteric metals, they can be immobilized with a decreasing solubility when the pH decreases in the strong alkaline region of pH 11 - 13, but they have the property of dissolving when the pH increases. Therefore, the desired pH is contradictory for the generated calcium carbonate and the hydroxides of lead and zinc to remain insolubilized, and it is difficult to achieve a balance. Also, when there is a large amount of incineration ash, it is difficult to uniformly control the overall pH, and there is a risk that the state of immobilization in the locally pH - fluctuating parts is different from that of the surrounding area. Furthermore, even if elution can be allowed to be below the allowable limit and stabilization can be completed once, if the pH of the incineration ash fluctuates due to some factor later, re - elution is a concern and the risk is high.

[0008] The second problem is that calcium carbonate generated in incineration ash cannot be recovered. Calcium carbonate is in great demand as a raw material for low - carbon concrete such as CO2 - SUICOM (registered trademark). Therefore, it is desirable to separate and recover calcium in incineration ash as calcium carbonate and recycle it as a resource.

[0009] However, it is difficult to separate and recover calcium carbonate from incineration ash after carbonation treatment of the incineration ash.

[0010] Also, even if washing water containing a high concentration of dissolved calcium is obtained by washing incineration ash, it is difficult to efficiently carbonate calcium. The reason for this is that hydroxide ions tend to be insufficient in the treated water during the carbonation treatment. Specifically, in order to react calcium ions in the treated water with carbon dioxide to obtain calcium carbonate, the reaction shown in the following (Equation 1) must proceed. Ca 2+ +CO 2 +2ОH - →CaCO 3 +H 2 О (Equation 1) As shown in (Equation 1), twice as much hydroxide ion is required as calcium ion. For example, in 1 L of treated water, in order to react approximately 0.5 g (12.5 mmol) of calcium ions and obtain approximately 1.25 g (12.5 mmol) of calcium carbonate, it is essential that the concentration of hydroxide ions is 25 mmol / L or more, and the pH of the treated water before carbonation treatment must be 12.4 or more. Additionally, in the above example, after carbonation treatment, hydroxide ions are consumed and the pH of the treated water decreases. As described above, calcium carbonate dissolves more easily in water as the pH decreases, so it cannot be recovered as a precipitate. Therefore, in order to increase the pH of the treated water after carbonation treatment, an even larger amount of hydroxide ions is required.

[0011] In this regard, it is possible to supplement hydroxide ions by adding general alkaline chemicals (such as potassium hydroxide and sodium hydroxide) to the treated water, but the cost of the chemicals is incurred. Furthermore, a large amount of cations (such as potassium ions and sodium ions) of the added chemicals remains in the treated water after calcium carbonate recovery, so new equipment for appropriately treating these cations must be additionally introduced, which is not desirable. Therefore, a method for efficiently carbonating and recovering calcium in incineration ash without such extra chemical addition and equipment is desired.

[0012] The present invention has been made in view of the above-described problems, and an object thereof is to provide a calcium carbonate recovery system and a calcium carbonate recovery method capable of easily and efficiently recovering calcium carbonate from incineration ash.

Means for Solving the Problems

[0013] In order to achieve the above-described object, a first invention is a calcium carbonate recovery system from combustion ash, comprising: a main ash washing unit that washes main ash and recovers main ash washing water; a fly ash washing unit that washes fly ash and recovers fly ash washing water; and a calcium carbonate recovery unit that mixes the main ash washing water with the fly ash washing water, supplies a gas containing carbon dioxide, precipitates calcium carbonate, and recovers calcium carbonate.

[0014] According to the first invention, since the fly ash washing unit washes only fly ash containing heavy metals, calcium, and alkaline substances at a high concentration, concentrated and efficient washing can be performed. Further, the fly ash washing water obtained at that time has a high pH, which is convenient for the carbonation reaction. In addition, lead and zinc, which are particularly emphasized as washing targets, are amphoteric metals and are easily dissolved when the pH is high, so a particularly high washing effect is exhibited on them as compared with the prior art. Furthermore, since the main ash washing unit washes only the main ash that hardly contains contaminants, alkaline main ash washing water having a low concentration of heavy metals and the like can be obtained. By mixing this with the fly ash washing water to supply hydroxide ions, the recovery amount of calcium carbonate can be easily increased without increasing the cost and the number of steps.

[0015] It has an impurity removal unit that supplies a gas containing carbon dioxide to the fly ash washing water to precipitate and recover carbonates or hydroxides. It is desirable that the calcium carbonate recovery unit can mix the main ash washing water with the fly ash washing water from which carbonates or hydroxides have been removed by the impurity removal unit, supply a gas containing carbon dioxide to precipitate calcium carbonate, and recover calcium carbonate. Since the impurity removal unit removes impurities, a large amount of calcium carbonate can be recovered in a highly pure state.

[0016] In the calcium carbonate recovery unit, it is desirable that it is possible to add seed crystals and precipitate calcium carbonate by the pellet method. Since seed crystals are added and calcium carbonate is precipitated by the pellet method, calcium carbonate easily grows into granular bodies with the seed crystals as nuclei, and calcium carbonate can be recovered without problems.

[0017] A second invention is a method for recovering calcium carbonate from combustion ash, comprising: a step a of washing main ash and recovering main ash washing water; a step b of washing fly ash and recovering fly ash washing water; and a step c of mixing the main ash washing water with the fly ash washing water, supplying a gas containing carbon dioxide to precipitate calcium carbonate, and recovering calcium carbonate.

[0018] According to the second invention, first, only the fly ash containing heavy metals, calcium, and alkaline substances at high concentrations is washed, so that concentrated and efficient washing can be performed. In addition, the fly ash washing water obtained at that time has a high pH and is convenient for the carbonation reaction. In addition, lead and zinc, which are particularly emphasized as the objects to be washed, are amphoteric metals and are easily dissolved when the pH is high, so a particularly high washing effect is exerted on them compared with the conventional method. Furthermore, since only the main ash containing almost no pollutants is washed separately from the fly ash, an alkaline main ash washing water with a low concentration of heavy metals and the like can be obtained. By mixing this with the fly ash washing water to supply hydroxide ions, the recovery amount of calcium carbonate can be easily increased without increasing the cost or the number of steps.

[0019] It is desirable that the amount of washing water per unit main ash amount in the step a is less than the amount of washing water per unit fly ash amount in the step b. Since the amount of washing water per unit main ash amount is less than the amount of washing water per unit fly ash amount, the main ash containing only a small amount of heavy metals and calcium is not washed with an unnecessarily large amount of water, and the pH of the obtained main ash washing water can be increased, and when mixing with the fly ash washing water, hydroxide ions can be efficiently supplied.

[0020] Before the step c, there is a step d of supplying a gas containing carbon dioxide to the fly ash washing water to precipitate and recover carbonates or hydroxides. In the step c, it is desirable to mix the main ash washing water with the fly ash washing water from which carbonates or hydroxides have been removed in the step d, and supply a gas containing carbon dioxide to precipitate calcium carbonate and recover calcium carbonate. Since there is a step d of recovering impurities before the step c of recovering calcium carbonate, a large amount of calcium carbonate can be recovered in a highly pure state.

[0021] The step d is terminated under the conditions that the pH drops by 0.1 or more from the start of the step d and the pH does not become 7 or less. By mixing the main ash washing water in the step c, the pH is raised once, and then the step c is terminated under the conditions that the pH drops by 0.1 or more from the start of the step c and the pH does not become 7 or less. Since the step d for recovering impurities is terminated at the stage where the pH drops by a predetermined amount, the impurities can be preferably removed in the pH range with a high removal effect, and the decrease in the amount of calcium in the fly ash washing water can also be minimized. Also, in the step c for recovering calcium carbonate, the pH is raised once, and the blowing of the gas containing carbon dioxide is terminated in the range where the pH does not become 7 or less. Thus, calcium carbonate with high purity can be efficiently recovered in the pH range where the generated calcium carbonate is hardly soluble. Note that the standard of the pH when terminating the step d is determined according to the required quality of calcium carbonate. For example, if the combustion ash contains a large amount of heavy metals and high-purity calcium carbonate is required, it is necessary to lower the pH to 11 or less to firmly remove the heavy metals. Conversely, if there is no problem even if the calcium carbonate contains a small amount of heavy metals, the step d may be terminated at the stage where the pH becomes 12.2 or less. Also, as the step c, for example, it may be terminated at a stage where the pH exceeds 7 and is lower than the pH at the end of the step d.

[0022] In the step c, it is desirable to add seed crystals and precipitate calcium carbonate by the pellet method. Since seed crystals are added and calcium carbonate is precipitated by the pellet method, calcium carbonate easily grows into granular form with the seed crystals as nuclei, and calcium carbonate can be recovered without problems.

[0023] It is desirable that the gas containing carbon dioxide supplied in the step c uses the gas released from the semi-aerobic landfill structure. Since the gas containing carbon dioxide uses the gas released from the semi-aerobic landfill structure, a gas containing a high concentration of carbon dioxide can be easily used, which is also preferable from the viewpoint of environmental protection.

[0024] The above-mentioned step a and step b are performed before the landfill of the combustion ash, and the washed combustion ash discharged in step a and step b may be landfilled. Since the main ash and fly ash are washed before the landfill of the combustion ash, and the washed main ash and fly ash are landfilled, there are fewer restrictions on the location and equipment, and the degree of freedom is increased.

[0025] The combustion ash is landfilled by being divided into fly ash and main ash, and watering and washing are performed on each division. In step a and step b, the washing water discharged from each division may be recovered. By landfilling after dividing into fly ash and main ash, watering and washing each division, and recovering the washing water discharged from each division as fly ash washing water and main ash washing water, it can also be used in a conventional landfill site where the ground has been excavated.

Advantages of the Invention

[0026] According to the present invention, it is possible to provide a calcium carbonate recovery system and a calcium carbonate recovery method capable of easily and efficiently recovering calcium carbonate from incineration ash.

Brief Description of the Drawings

[0027]

Figure 1

Figure 2

Figure 3

Figure 4

Figure 5

Figure 6

Figure 7

Figure 8

Figure 9

Figure 10

Figure 11

Figure 12

Embodiments for Carrying Out the Invention

[0028] Hereinafter, embodiments of the present invention will be described in detail with reference to the drawings.

[0029] (First Embodiment) Hereinafter, a first embodiment of the present invention will be described. FIG. 1 is a diagram showing the steps of a calcium carbonate recovery system 10 according to the first embodiment. The calcium carbonate recovery system 10 includes a main ash washing unit 13, a fly ash washing unit 11, and a calcium carbonate recovery unit 33. The main ash washing unit 13 washes the main ash and recovers the main ash washing water 31. The fly ash washing unit 11 washes the fly ash 15 and recovers the fly ash washing water 29. The calcium carbonate recovery unit 33 mixes the main ash washing water 31 with the fly ash washing water 29, supplies a gas containing carbon dioxide, precipitates calcium carbonate 41, and recovers the calcium carbonate 41.

[0030] Here, first, the main ash and the fly ash will be described. The incineration ash discharged from the waste incineration facility is roughly classified into two types: main ash and fly ash. The main ash is the ash that comes out from the bottom of the boiler after the waste and the like have burned out. On the other hand, fly ash is fine ash that has risen during incineration, accumulates in a dust collector, a filter, etc., and is collected and discharged.

[0031] Although the amount of fly ash generated is small compared to the main ash, it contains a large amount of heavy metals such as lead and zinc that are easily evaporated at the operating temperature of the incinerator (about 750 to 950 ° C) as fine aggregates. Furthermore, the fly ash contains a large amount of slaked lime or the like that has been blown in for the purpose of removing chlorine gas or the like during waste incineration in a state where it has chemically reacted at high temperature or in an unreacted state, contains a large amount of calcium, and the pH of the water it touches becomes extremely high.

[0032] Table 1 shows the generation amounts of fly ash and main ash, the pH of the washing water, and the elution amounts of each ion into the washing water when the same amount of ash is washed with the same amount of water for fly ash and main ash respectively.

[0033]

Table 1

[0034] The generation amount of the main ash is about three times that of the fly ash. Also, when the main ash or fly ash is washed, the pH of the washing water is about 12.0 for the main ash washing water and about 12.4 for the fly ash washing water when washed with water having a weight 20 times that of the ash weight. When the weight ratio of ash to water is the same, the pH of the fly ash washing water is higher. The elution amount of each ion into the washing water is 50 to 1000 times that of the main ash washing water compared to the fly ash washing water.

[0035] When fly ash and main ash having such properties are washed with water in a mixed state without distinction, the pH of the washing water becomes lower compared to the washing water when only fly ash is washed. Also, the main ash containing only very small amounts of calcium, lead, and zinc compared to fly ash accounts for 75% of the mixed ash at three times the amount of fly ash, making efficient washing impossible. In contrast, in the calcium carbonate recovery method of the calcium carbonate recovery system 10 of the first embodiment, the fly ash and the main ash are washed in separate steps.

[0036] Next, the calcium carbonate recovery method of the first embodiment will be described. The calcium carbonate recovery method of the first embodiment includes a step of washing the main ash and recovering the main ash washing water 31, a step of washing the fly ash 15 and recovering the fly ash washing water 29, and a step of mixing the main ash washing water 31 into the fly ash washing water 29, supplying a carbon dioxide-containing gas, and precipitating calcium carbonate 41 to recover the calcium carbonate 41.

[0037] First, the step of washing the fly ash 15 and recovering the fly ash washing water 29 will be described. That is, the fly ash washing unit 11 washes the fly ash 15 (S101), separates the fly ash 15 and the fly ash washing water 29, and recovers the fly ash washing water 29 (S103).

[0038] Figure 2 is a diagram showing an example of each step in the fly ash washing section 11. Figure 2(a) is a diagram showing an example of step S101 of washing fly ash 15. As shown in the left diagram of Figure 2(a), a predetermined amount of fly ash 15 and water 17 are placed in a container 19, and a mixed liquid 21 is obtained as shown in the right diagram. At this time, the mixed liquid 21 may be stirred or shaken as necessary. Figure 2(a) is only an example, and any means may be used as long as the fly ash 15 can be washed by bringing it into contact with water 17.

[0039] Figure 2(b) is a diagram showing an example of step S103 of separating fly ash 15 and fly ash washing water 29 and recovering the fly ash washing water 29. As shown in the left diagram of Figure 2(b), a filtration filter 23 is arranged above a washing water container 25, and the mixed liquid 21 is transferred from the container 19 into it. Then, as shown in the right diagram, the filtration filter 23 is sealed so that the mixed liquid 21 does not leak out, and it is squeezed with a squeezing machine 27. Then, the fly ash washing water 29 accumulated in the washing water container 25 is recovered. The washed fly ash 15 remains in the filtration filter 23 after squeezing. Figure 2(b) is only an example, and any means may be used as long as the mixed liquid 21 can be separated into fly ash 15 and fly ash washing water 29.

[0040] Also, it is desirable to repeat steps S101 and S103 (Figure 2(a) and Figure 2(b)) a plurality of times using the washed fly ash 15 remaining in the filtration filter 23 after squeezing in S103 (Figure 2(b)). That is, the washed fly ash 15 may be put into the container 19 again together with a predetermined amount of water for a second washing. At this time, if the pH of the fly ash washing water 29 obtained in the first and second S103 is measured respectively, it is possible to confirm how far the washing of the fly ash has progressed. Instead of pH, the concentration of calcium or heavy metals may be measured. When steps S101 and S103 are repeated a plurality of times and the pH of the recovered fly ash washing water 29 falls within a predetermined range, it may be determined that the washing is sufficient, and the repetition of S101 and S103 may be terminated. At that time, the washed fly ash 15 is landfilled at a disposal site or the like (S105 in Figure 1).

[0041] In this way, only the fly ash 15 is washed in the fly ash washing section 11. As described above, since the fly ash 15 contains particularly large amounts of calcium and heavy metals such as lead and zinc, concentrated and efficient washing can be performed. In addition, since the fly ash 15 contains a large amount of alkaline substances derived from slaked lime used during waste incineration, the pH of the fly ash washing water 29 becomes 12.4 to 12.8, which sufficiently contains hydroxide ions and is advantageous for the carbonation reaction. In addition to this point, lead and zinc, which are particularly emphasized as the objects to be washed, are amphoteric metals, and the higher the pH, the greater the solubility. Therefore, when the pH of the fly ash washing water 29 is high in this way, lead and zinc are less likely to be insolubilized (precipitated) as hydroxides and dissolve into the fly ash washing water 29. Thereby, a particularly high washing effect is exhibited with respect to lead and zinc.

[0042] Next, a step of washing the main ash and recovering the main ash washing water 31 will be described. That is, the main ash washing section 13 washes the main ash (S107) and separates the main ash and the main ash washing water 31 to recover the main ash washing water 31 (S109). Each step S107 and S109 of the main ash washing section 13 can be performed in the same manner as S101 and S103 of the fly ash washing section 11.

[0043] At this time, in the step S107 of washing the main ash, it is desirable that the amount of water used to wash a predetermined amount of the main ash is less than the amount of water 17 used to wash the same amount of fly ash 15 in the step S101 of washing the fly ash 15. That is, it is preferable that the amount of washing water per unit main ash amount in the step of washing the main ash and recovering the main ash washing water 31 is less than the amount of washing water per unit fly ash amount in the step of washing the fly ash and recovering the fly ash washing water. Further, when S101 and S103 (and S107 and S109) are repeated a plurality of times, it is sufficient that the total amount of washing water per unit main ash amount is less than the total amount of washing water per unit fly ash amount.

[0044] In this regard, compared with the fly ash 15, the main ash has a very small content of calcium and heavy metals, and can be washed with a smaller amount of water than the fly ash 15. Also, although the main ash does not contain alkaline substances as highly concentrated as the fly ash 15, it contains alkaline substances. If the main ash is washed with an excessive amount of water that is not necessary, the pH of the obtained main ash washing water 31 will decrease, but the pH can be adjusted to 12.0 - 12.5 by reducing the amount of water. In addition, as described above, the main ash washing water 31 has a low content of heavy metals, and there is little excess generation of cations (such as potassium ions and sodium ions) that require subsequent treatment, as is the case when adding general alkaline chemicals (such as potassium hydroxide and sodium hydroxide).

[0045] In this way, the main ash washing water 31 is recovered in S109, and the washed main ash is landfilled (S111).

[0046] As described above, in this embodiment, the step of washing the main ash and recovering the main ash washing water 31, and the step of washing the fly ash 15 and recovering the fly ash washing water 29 are performed before landfilling the combustion ash. It is advisable to landfill the washed combustion ash discharged in the step of washing the main ash and recovering the main ash washing water 31, and the step of washing the fly ash 15 and recovering the fly ash washing water 29. By doing so, the restrictions on location and equipment are less, and the degree of freedom can be increased.

[0047] Next, the process of mixing the main ash washing water 31 with the fly ash washing water 29, supplying a carbon dioxide-containing gas to precipitate calcium carbonate 41, and recovering the calcium carbonate 41 will be described. That is, the calcium carbonate recovery unit 33 precipitates the calcium carbonate 41 (S113), separates the calcium carbonate 41 from the mixed washing water 37 (S115), and recovers the calcium carbonate 41 (S117).

[0048] Figure 3 is a detailed flowchart of the process S113 for precipitating the calcium carbonate 41. In S113, first, the fly ash washing water 29 and the main ash washing water 31 are placed in a container 35 (S201), and then a carbon dioxide-containing gas is blown in (S202).

[0049] FIG. 4 is a diagram showing an example of each step in the calcium carbonate recovery section 33. FIG. 4(a) is a diagram showing an example of step S201 of putting the fly ash washing water 29 and the main ash washing water 31 into the container 35. As shown in FIG. 4(a), the fly ash washing water 29 and the main ash washing water 31 recovered in S103 and S109 are put into the container 35. The fly ash washing water 29 and the main ash washing water 31 may be put into the container 35 simultaneously, or either one of them may be put in first. Also, the fly ash washing water 29 and the main ash washing water 31 may be put into the container 35 alternately in a plurality of times in predetermined amounts. Also, the container 35 may be any thing as long as the subsequent steps can be performed. For example, the washing water container 25 (see FIG. 2(b)) containing the fly ash washing water 29 after S103 may be used as the container 35 as it is, and the main ash washing water 31 may be put in it.

[0050] FIG. 4(b) is a diagram showing the mixed washing water 37 of the container 35 after S201. As shown in FIG. 4(b), the fly ash washing water 29 and the main ash washing water 31 are mixed in the container 35 to become the mixed washing water 37.

[0051] By mixing the fly ash washing water 29 and the main ash washing water 31 in this way, a state can be achieved in which sufficient hydroxide ions are present in the mixed washing water 37. As described above, in the reaction for generating calcium carbonate, a large amount of hydroxide ions are required, and the pH must increase after the reaction in order not to dissolve the generated calcium carbonate. Therefore, it is desirable to replenish hydroxide ions as much as possible. Although the pH of the fly ash washing water 29 is as high as 12.4 to 12.8 as described above, it is not sufficient when a large amount of calcium ions in the fly ash washing water 29 are to be carbonated and the recovery amount is to be increased. Therefore, in step S107 of washing the main ash, the amount of washing water per unit main ash amount is suppressed to obtain the main ash washing water 31 having a pH of 12.0 to 12.5 and mixing it to efficiently replenish hydroxide ions to the fly ash washing water 29.

[0052] In addition, when the pH of the main ash washing water 31 is higher than the pH of the fly ash washing water 29, the pH of the mixed washing water 37 will be higher than the pH of the fly ash washing water 29 before mixing, and a clear effect can be obtained. On the other hand, when the pH of the main ash washing water 31 is lower than the pH of the fly ash washing water 29, the pH of the mixed washing water 37 will be lower than the pH of the fly ash washing water 29 before mixing. However, by keeping the pH of the main ash washing water 31 as high as possible, the decrease in pH can be minimized, and hydroxide ions can be supplied to the fly ash washing water 29.

[0053] For example, when 1 L of fly ash washing water 29 with a pH of 12.4 (containing about 25 mmol of hydroxide ions) is mixed with 1 L of main ash washing water 31 with a pH of 12.0 (containing 10 mmol of hydroxide ions), the 2 L of mixed washing water 37 contains about 35 mmol of hydroxide ions, and its pH (calculated from the amount of hydroxide ions in 1 L) is about 12.25, and the pH becomes lower. However, in order to cause a carbonation reaction of calcium ions to precipitate calcium carbonate, the total amount of hydroxide ions in the 2 L of mixed washing water 37 is important. Since this value has increased from about 25 mmol to about 35 mmol, hydroxide ions can be supplied, and more calcium carbonate can be recovered.

[0054] Next, Fig. 4(c) is a diagram showing an example of the step S202 of blowing a carbon dioxide-containing gas into the mixed washing water 37. As shown in Fig. 4(c), a carbon dioxide-containing gas is blown in using a carbon dioxide-containing gas blowing pipe 39. It is most desirable to use a high-purity gas for the carbon dioxide-containing gas for high efficiency, but considering the cost, other gases may be substituted as long as they can supply carbon dioxide to the mixed washing water 37. Air can be used as an inexpensive and convenient example. In this regard, in the step of supplying a carbon dioxide-containing gas to precipitate calcium carbonate and recovering calcium carbonate, it is preferable to use the gas released from the semi-aerobic landfill structure as the supplied carbon dioxide-containing gas. The gas released from the semi-aerobic landfill structure contains a high concentration of carbon dioxide and is relatively stably available, which is convenient.

[0055] FIG. 4(d) is a diagram showing the mixed washing water 37 and calcium carbonate 41 after the start of the step S202 of blowing a carbon dioxide-containing gas into the mixed washing water 37. After the start of the blowing of the carbon dioxide-containing gas, calcium ions in the mixed washing water 37 undergo a carbonation reaction, and calcium carbonate 41 is formed in the mixed washing water 37. FIG. 4(d) is a diagram showing an example when a precipitate of calcium carbonate 41 is formed. In the illustrated example, calcium carbonate 41 has precipitated, but calcium carbonate 41 may float in the mixed washing water 37 without sinking while the carbon dioxide-containing gas is being blown in, etc.

[0056] In this regard, before FIG. 4(c), that is, before the step S202 of blowing a carbon dioxide-containing gas into the mixed washing water 37, it is preferable to add seed crystals to the mixed washing water 37 so as to precipitate calcium carbonate by the pellet method. The pellet method is a technique generally used also when precipitating the hardness components of water as calcium carbonate in water supply facilities and the like. When seed crystals are not added, the generated calcium carbonate 41 has a small particle size and is difficult to precipitate. Moreover, once it sinks to the bottom of the container 35, it may stick as it is and become difficult to handle. On the other hand, with seed crystals, the generated calcium carbonate 41 adheres around the seed crystals and easily grows into relatively large granular bodies. At this time, it is preferable that the seed crystals are made of calcium carbonate so that the seed crystals do not become impurities.

[0057] Note that FIGS. 4(a), 4(b), and 4(c) show an example in which the main ash washing water 31 is mixed with the fly ash washing water 29 (FIG. 4(a)), the mixed washing water 37 is obtained (FIG. 4(b)), and then a carbon dioxide-containing gas is blown into the mixed washing water 37 (FIG. 4(c)). However, the method of mixing the main ash washing water 31 is not limited to this. For example, the main ash washing water 31 may be mixed with the fly ash washing water 29 during the blowing of the carbon dioxide-containing gas. Also, the main ash washing water 31 may be mixed in predetermined amounts at any plurality of timings from FIGS. 4(a) to 4(c). Further, the carbon dioxide-containing gas may be blown intermittently, and the main ash washing water 31 may be mixed during the pause in the blowing. That is, before the end of the blowing of the carbon dioxide-containing gas, the main ash washing water 31 may be appropriately mixed with the fly ash washing water 29.

[0058] Next, proceed to S203 in FIG. 3. S203 measures the pH of the mixed cleaning water 37 at a predetermined timing, and checks whether the pH has decreased by at least 0.1 or more compared to the start of S202 and whether the pH is within a predetermined pH (for example, 9.0) arbitrarily set in the range above 7. At this time, if the pH is not the predetermined pH (9.0), it is determined as No, and the process returns to S202 to continue blowing the carbon dioxide-containing gas. If the pH is the predetermined pH (9.0), it is determined as Yes, the blowing of the carbon dioxide-containing gas is terminated, and the process proceeds to S115 described later.

[0059] In this regard, as described above, calcium carbonate 41 is more likely to dissolve as the pH decreases. Therefore, when the carbon dioxide-containing gas is continuously blown into the mixed cleaning water 37 and the pH decreases, even if the blowing of the carbon dioxide-containing gas is continued, it becomes difficult for new calcium carbonate 41 to be generated in the mixed cleaning water 37. That is, even if new calcium carbonate 41 is generated, since the pH is low, it will almost immediately dissolve, and the recovery amount of calcium carbonate 41 cannot be increased much with respect to the blowing amount of the carbon dioxide-containing gas. This tendency generally becomes prominent when the pH is 7 or less. Therefore, when the pH of the mixed cleaning water 37 reaches the predetermined pH set in the range above 7, it is advisable to terminate the blowing of the carbon dioxide-containing gas. If the decrease in pH is less than 0.1, almost no reaction has occurred, so sufficient recovery cannot be achieved.

[0060] Next, proceed to S115 in FIG. 1. S115 is a step of separating calcium carbonate 41 and the mixed cleaning water 37 as described above. For separation, for example, the same method as in FIG. 2(b) may be used. The separated calcium carbonate 41 is recovered (S117), and the mixed cleaning water 37 is treated at a water treatment facility or the like (S119).

[0061] According to the calcium carbonate recovery method in the calcium carbonate recovery system 10 of the first embodiment described above, first, the fly ash washing unit 11 only washes the fly ash 15 that contains heavy metals, calcium, and alkaline substances at a high concentration, so that concentrated and efficient washing can be achieved. In addition, the obtained fly ash washing water 29 has a high pH and contains a large amount of hydroxide ions necessary for the reaction to produce calcium carbonate, which is convenient for the carbonation reaction. In addition, it exhibits a particularly high washing effect on lead and zinc, which are amphoteric metals and are easily dissolved when the pH is high.

[0062] Furthermore, since the main ash washing unit 13 only washes the main ash that contains almost no contaminants compared to the fly ash 15, alkaline main ash washing water 31 with a low concentration of heavy metals and the like is obtained. By mixing this with the fly ash washing water 29 to replenish hydroxide ions, the recovery amount of calcium carbonate can be easily increased without increasing the cost and processes.

[0063] Also, if the amount of washing water per unit main ash amount in the process S107 of washing the main ash is made less than the amount of washing water per unit fly ash amount in the process S101 of washing the fly ash, the main ash containing only a very small amount of the object to be washed is not washed with an unnecessary large amount of water, and the pH of the obtained main ash washing water 31 can be increased to as high as 12.0 - 12.5. Therefore, when mixing with the fly ash washing water 29, hydroxide ions can be efficiently replenished.

[0064] Also, in the process S113 of precipitating calcium carbonate, seed crystals are added and calcium carbonate is precipitated by the pellet method, so that calcium carbonate 41 can be recovered without problems.

[0065] In addition, if the gas released from the semi-aerobic landfill structure is used as the carbon dioxide-containing gas, it is convenient because it contains a high concentration of carbon dioxide and is also preferable from the viewpoint of environmental protection.

[0066] Also, before landfilling the combustion ash, a step of washing the main ash and fly ash 15 and recovering the main ash washing water 31 and the fly ash washing water 29 is performed, and the washed main ash and fly ash 15 are landfilled, so there are few restrictions on the location and equipment and the degree of freedom is high.

[0067] (Second Embodiment) Hereinafter, another example of the present invention will be described as a second embodiment. The differences between the second embodiment and the first embodiment will be described, and the description of the same configurations will be omitted by attaching the same reference numerals in the drawings and the like.

[0068] FIG. 5 is a diagram showing the steps of the calcium carbonate recovery system 10a of the second embodiment. The calcium carbonate recovery system 10a has, in addition to the fly ash washing section 11, the main ash washing section 13, and the calcium carbonate recovery section 33 similar to those of the first embodiment, an impurity removal section 45. The impurity removal section 45 supplies a gas containing carbon dioxide to the fly ash washing water 29 to precipitate and recover carbonates or hydroxides. Further, the calcium carbonate recovery section 33 of the second embodiment mixes the main ash washing water 31 with the fly ash washing water 29 from which carbonates or hydroxides have been removed by the impurity removal section 45, supplies a gas containing carbon dioxide, and precipitates calcium carbonate 41 to recover calcium carbonate 41.

[0069] Next, the calcium carbonate recovery method of the calcium carbonate recovery system 10a of the second embodiment will be described in detail. The calcium carbonate recovery method of the second embodiment has the same steps as those of the first embodiment. Further, before the step of supplying a gas containing carbon dioxide to precipitate calcium carbonate 41 and recovering calcium carbonate 41, a step of supplying a gas containing carbon dioxide to the fly ash washing water 29 to precipitate and recover carbonates or hydroxides is included. In the second embodiment, in the step of recovering calcium carbonate 41, after the carbonates or hydroxides are removed in the step of supplying a gas containing carbon dioxide to the fly ash washing water 29 to precipitate and recover carbonates or hydroxides, the main ash washing water 31 is mixed with the fly ash washing water 29, a gas containing carbon dioxide is supplied, and calcium carbonate 41 is precipitated to recover calcium carbonate 41.

[0070] First, a process of supplying a gas containing carbon dioxide to the fly ash washing water 29 to precipitate and recover a carbonate or a hydroxide will be described. That is, the impurity removal unit 45 precipitates the impurities 51 in the fly ash washing water 29 recovered in S103 (S301), and separates the impurities 51 from the fly ash washing water 29 (S303). S301 and S303 are performed after the process S103 in which the fly ash washing unit 11 separates the fly ash 15 from the fly ash washing water 29 and recovers the fly ash washing water 29, and before the process S113 in which the calcium carbonate recovery unit 33 precipitates the calcium carbonate 41.

[0071] FIG. 6 is a detailed flowchart of the process S301 of precipitating the impurities 51 in the fly ash washing water 29 recovered in S103. In S301, first, the fly ash washing water 29 is put into a container 47 (S401), and then, a gas containing carbon dioxide is blown into the fly ash washing water 29 (S402).

[0072] Here, FIG. 7 is a diagram showing an example of each process in the impurity removal unit 45. FIG. 7(a) is a diagram showing an example of the process S401 of putting the fly ash washing water 29 into the container 47. As shown in FIG. 7(a), the fly ash washing water 29 recovered in S103 is put into the container 47. The container 47 may be any material as long as the subsequent processes can be performed. For example, the washing water container 25 (see FIG. 2(b)) containing the fly ash washing water 29 may be used as the container 47 as it is.

[0073] FIG. 7(b) is a diagram showing an example of the process S402 of blowing a gas containing carbon dioxide into the fly ash washing water 29. As shown in FIG. 7(b), a gas containing carbon dioxide is blown using a gas blowing pipe 49 containing carbon dioxide. The gas containing carbon dioxide can be the same as that used in the process S113 of precipitating the calcium carbonate 41 in the first embodiment. In particular, it is preferable to use the gas released from the semi-aerobic landfill structure.

[0074] FIG. 7(c) is a diagram showing the fly ash washing water 29 and the impurities 51 after the start of the step S402 of blowing the carbon dioxide-containing gas into the fly ash washing water 29. After the start of the blowing of the carbon dioxide-containing gas, lead ions and zinc ions, which are contained in a large amount as impurities in the fly ash washing water 29, undergo a carbonation reaction to form lead carbonate and zinc carbonate. Separately from this, as the pH decreases due to the blowing of carbon dioxide, the solubility decreases and lead hydroxide and zinc hydroxide are formed. In addition, a small amount of calcium ions also undergoes a carbonation reaction to form calcium carbonate. FIG. 7(c) is a diagram showing an example when a precipitate of the impurity 51 is formed. Note that the impurity 51 may float in the fly ash washing water 29 without sinking while the carbon dioxide-containing gas is being blown in.

[0075] Note that before FIG. 7(b), that is, before the step S402 of blowing the carbon dioxide-containing gas into the fly ash washing water 29, seed crystals may be added to the fly ash washing water 29 so that the generated impurities 51 grow with the seed crystals as nuclei. Since the seed crystals at this time are later processed together with the impurities 51, they may be other than calcium carbonate and may be made of any material.

[0076] Next, proceed to S403 in FIG. 6. S403 is a step of measuring the pH of the fly ash washing water 29 at a predetermined timing and checking whether the pH is 0.1 or more lower than that at the start of S402 and within a predetermined pH range (for example, 12.0) exceeding pH 7. At this time, if the pH is not the predetermined pH (12.0), it is determined as No, and the process returns to S402 to continue blowing the carbon dioxide-containing gas. If the pH is the predetermined pH (12.0), it is determined as Yes, the blowing of the carbon dioxide-containing gas is terminated, and the process proceeds to S303 described later.

[0077] Here, the reason for doing this will be explained. First, the fly ash washing water 29 after S103 in FIG. 5 contains not only calcium but also a large amount of heavy metal ions such as lead and zinc. If carbon dioxide-containing gas is blown in as it is, together with calcium carbonate, lead and zinc will also form carbonates and hydroxides and precipitate, resulting in a decrease in the purity of the obtained calcium carbonate. Therefore, it is desirable that the calcium carbonate recovery unit 33 removes lead ions and zinc ions in the fly ash washing water 29 as much as possible before the step S113 of precipitating calcium carbonate 41.

[0078] First, the characteristics of lead carbonate and zinc carbonate will be described. Lead carbonate and zinc carbonate are characterized by having a significantly smaller solubility product compared to calcium carbonate. Therefore, when both lead ions (or zinc ions) and calcium ions are present in the fly ash washing water 29, when carbon dioxide is blown into it and an environment for carbonation reaction can occur, lead carbonate (zinc carbonate) is much more likely to be generated than calcium carbonate and selectively precipitates preferentially. At that time, although a small amount of calcium ions that would not originally be desired to be lost also forms calcium carbonate and precipitates and is lost, the removal effect of lead ions and zinc ions using the carbonation reaction is very large. Moreover, the generated lead carbonate and lead carbonate are not easily redissolved under the influence of pH and can be stably removed.

[0079] Furthermore, the characteristics of lead hydroxide and zinc hydroxide will be described. As described above, when carbon dioxide-containing gas is blown into the fly ash washing water 29, the pH decreases. As a result, ions of amphoteric metals such as lead and zinc have a reduced solubility and form insoluble hydroxides. These hydroxides have a solubility that changes with pH and the amount that precipitates changes.

[0080] In this regard, FIG. 8(a) is a graph showing the logarithm of the saturation molar concentration C with respect to pH. This graph shows that the larger the value on the vertical axis (higher in the graph), the easier it is to dissolve, and the smaller the value (lower in the graph), the easier it is to precipitate as a hydroxide.

[0081] The data of calcium, lead, and zinc are represented by solid line, dashed line, and dotted line respectively. Hydroxides of lead and zinc are more likely to dissolve at high pH values, which is disadvantageous for recovering precipitates to remove lead and zinc ions. On the other hand, they are more likely to precipitate at low pH values, and are most likely to precipitate around pH 10. In this regard, as shown by the white arrow, in the range where pH is greater than 12, the slope is particularly large. By simply lowering the pH slightly, the precipitation amount can be significantly increased, and the effect of precipitating and removing lead and zinc as hydroxides is particularly high.

[0082] It should be noted that although it is also possible to continuously blow in carbon dioxide-containing gas to lower the pH to the pH (approximately 10) at which the hydroxide is most likely to precipitate, in the range where pH is 12 or less, the slope becomes gentler as it approaches 10. Therefore, even if the pH is continuously lowered, a large removal effect cannot be obtained. In addition, as the carbon dioxide-containing gas is blown in, calcium ions in the fly ash washing water 29 precipitate as calcium carbonate and are lost. Therefore, it is desirable to set the pH to 10 - 12. Note that if the decrease in pH is less than 0.1, almost no reaction proceeds, and sufficient removal of impurities cannot be achieved.

[0083] Figure 8(b) is a graph showing the measurement results of the concentrations of various ions in the fly ash washing water 29 when a carbon dioxide-containing gas is blown into the fly ash washing water 29 to lower the pH. The fly ash washing water 29 had a pH of 12.44 before the carbon dioxide-containing gas was blown in, and the carbon dioxide-containing gas was blown in to lower the pH to 12.00. The results of calcium, lead, and zinc are shown by the solid line of white circle plots, the dashed line of black circle plots, and the dotted line of black square plots respectively. By lowering the pH from 12.44 to 12.00, approximately 87% of lead ions and approximately 86% of zinc ions were removed. (The lead ion concentration decreased from 6.06 to 0.81 (mg / L), and the zinc ion concentration decreased from 0.25 to 0.04 (mg / L)). On the other hand, the decrease in calcium ion amount remained at approximately 15%. (The calcium ion concentration only decreased from 2466 to 2093 (mg / L).)

[0084] Thus, in S403 of FIG. 6, when the predetermined pH set in a range lower than the original pH is reached, the blowing of the carbon dioxide-containing gas is terminated, thereby minimizing the reduction amount of calcium ions in the fly ash washing water 29 while effectively removing a large amount of lead ions and zinc ions as carbonates or hydroxides.

[0085] Next, proceed to S303 in FIG. 5. As described above, S303 is a step of separating the impurity 51 and the fly ash washing water 29. For separation, for example, the same method as in FIG. 2(b) may be used. The separated impurity 51 is processed (S305).

[0086] Next, proceed to S113. As described above, S113 is a step in which the calcium carbonate recovery unit 33 precipitates calcium carbonate 41 using the fly ash washing water 29 separated in S303.

[0087] Note that since the carbon dioxide-containing gas was blown into the fly ash washing water 29 separated in S303 in S301 (S402 in FIG. 6), a part of the hydroxide ions was consumed and the pH decreased. However, as described above, in S113, the main ash washing water 31 is added as shown in FIG. 4(a) to increase the pH (hydroxide ions are replenished as described above), so there is no shortage of hydroxide ions. After that, calcium carbonate 41 is recovered in S117 in the same manner as in the first embodiment. In the step S113 of precipitating calcium carbonate 41 in the second embodiment, since the carbonation reaction is carried out with the fly ash washing water 29 from which lead and zinc have been removed, high-purity calcium carbonate 41 can be recovered.

[0088] That is, in the impurity removal section 45, since S403 of S301 in FIG. 5 (see FIG. 6) ends under the condition that the pH is 12 or less, a large amount of lead ions and zinc ions can be effectively removed while minimizing the reduction amount of calcium ions in the fly ash washing water 29. In addition, in the calcium carbonate recovery section 33, in S113 of FIG. 5, by mixing the main ash washing water 31 as in S201 of FIG. 3 (FIG. 4(a)), the pH is increased once, and S203 of FIG. 3 ends under the condition that the pH drops by 0.1 or more from the start of S202 and is in the range exceeding pH 7. Therefore, the recovery amount of highly pure calcium carbonate 41 can be greatly increased. In consideration of the impurity removal efficiency and the calcium carbonate recovery efficiency, it is desirable that the pH at the end of the calcium carbonate precipitation step in S203 thereafter is lower than the pH at the end of the impurity removal step in S403.

[0089] As described above, according to the calcium carbonate recovery method in the calcium carbonate recovery system 10a of the second embodiment, in the step S301 where the impurity removal section 45 precipitates the impurities 51 in the fly ash washing water 29 recovered in S103, a carbon dioxide-containing gas is blown into the fly ash washing water 29 to remove impurities such as lead and zinc as carbonates or hydroxides. At this time, the pH of the fly ash washing water 29 decreases, but in the step S113 of precipitating the calcium carbonate 41, the main ash washing water 31 is mixed and the pH increases again, so that there is no shortage of hydroxide ions, and a large amount of calcium carbonate can be recovered in a highly pure state.

[0090] In addition, since the step of supplying a gas containing carbon dioxide to the fly ash washing water 29 to precipitate and recover a carbonate or a hydroxide is terminated under the condition that the pH has decreased by a predetermined amount from the initial pH, lead and zinc can be suitably removed while minimizing the decrease in the amount of calcium ions in the fly ash washing water 29. In addition, in the step of mixing the main ash washing water 31 with the fly ash washing water 29, supplying a gas containing carbon dioxide to precipitate calcium carbonate 41 and recovering the calcium carbonate 41, the pH is increased once by mixing the main ash washing water 31, and further terminated under the condition that the pH is in the range exceeding 7, so that the recovery amount of highly pure calcium carbonate 41 can be greatly increased.

Example

[0091] First, the effect of separately washing the main ash and the fly ash was confirmed.

[0092] FIG. 9 is a diagram showing the measurement results of each washing water at each elution repetition number when elution is repeated 10 times for only fly ash, undifferentiated mixed ash obtained by mixing the main ash and the fly ash, and only the main ash. FIG. 9(a) is a graph showing the measurement results of pH.

[0093] First, E (solid line of black circle plot) in FIG. 9(a) is the measurement result of the pH of the washing water of 400 g of only fly ash. After mixing the fly ash with 2 L of water and shaking for 6 hours, solid-liquid separation was performed by centrifugation, and the obtained water was used as the washing water for the first elution repetition number. The separated ash was again subjected to the same procedure with 2 L of water to obtain the washing water for the second elution repetition number. The above was repeated 10 times, and the pH of the fly ash washing water at each elution repetition number was plotted. In the case of only fly ash of E, the pH remained as high as 12.4 to 12.8 even after repeated elution. Therefore, it was found that the fly ash contains a large amount of alkaline substances and needs to be washed intensively.

[0094] F (the dashed line of the white triangular plot) represents the measurement results obtained in the same manner as E using 400 g of undifferentiated mixed ash, which is a mixture of 300 g of main ash and 100 g of fly ash. In the case of the mixed ash of F, the pH decreases starting from the 3rd elution repetition, and becomes about 12.0 - 12.2 after the 4th time. Compared with the fly ash of E, the pH decreases when the elution is repeated.

[0095] G (the solid line of the white circle plot) represents the measurement results obtained in the same manner as E using only 400 g of main ash. In the case of only the main ash of G, the pH rapidly decreases from the 1st to the 3rd elution repetition, and remains low at 11.6 - 11.8 after the 3rd elution repetition. Therefore, in the case of only the main ash of G, it was found that the washing was almost completed by the 3rd elution repetition, and the amount of water required for washing the main ash could be less than that in the cases of the fly ash of E and the mixed ash of F.

[0096] Figure 9(b) is a graph showing the measurement results of the elution concentration of lead, and Figure 9(c) is a graph showing the measurement results of the elution concentration of zinc. Almost no lead was eluted from the washing water of only the main ash of G, and only a very small amount of zinc was eluted. On the other hand, in the washing water of only the fly ash of E, when the plot values for each time were summed up and the total amount eluted per 1 L of washing water was determined for 10 elution repetitions, about 42.7 mg of lead and 5.6 mg of zinc were eluted, indicating a large amount of elution.

[0097] Regarding this point, when focusing on the mixed ash of F, the values are 3.3 mg of lead and 0.9 mg of zinc. Simply considering, since the condition of only the fly ash of E contains 4 times as much fly ash as the mixed ash of F, the elution amounts of lead and zinc in E should remain at about 4 times that of F. However, in the experimental results, it is significantly larger than 4 times as described above. Therefore, it was found that by washing the fly ash and the main ash separately, lead and zinc can be washed more efficiently than in the state of mixed ash.

[0098] The reason for this is considered to be that, as in the case of E in FIG. 9(a), even if the elution of the fly ash washing water is repeated, the pH remains high. On the other hand, when the mixed ash of F is washed, the pH immediately decreases. Since lead and zinc are amphoteric metals, they are less likely to precipitate as lead hydroxide or zinc hydroxide and are more likely to dissolve when the pH is high. Therefore, if only the fly ash is washed as in E, lead and zinc can be effectively dissolved in the high-pH fly ash washing water, and a high washing effect can be obtained.

[0099] Next, as in the step (S113 in FIG. 1) where the calcium carbonate recovery unit 33 of the first embodiment of the present invention precipitates calcium carbonate 41, the main ash washing water 31 was mixed with the fly ash washing water 29 to confirm the effect of increasing the recovery amount of calcium carbonate.

[0100] FIG. 10 is a graph showing the ratio of the total amount of dissolved calcium in Example 1 and the comparative example when the total amount of dissolved calcium contained in a predetermined amount of fly ash washing water 29 after S103 in FIG. 1 is set to 100%.

[0101] In FIG. 10, the result indicated as after S103 in FIG. 1 is obtained by measuring the total amount of dissolved calcium contained in 1 L of a sample (hereinafter referred to as fly ash leachate) simulating the fly ash washing water 29 after S103 in FIG. 1 and setting it to 100%. The fly ash leachate was obtained by shaking and washing 400 g of fly ash with 2 L of water and recovering the washing water by filtration. The pH of the fly ash leachate was 12.4.

[0102] Next, the result of Example 1 in FIG. 10 is shown as a ratio obtained by creating a sample simulating the mixed washing water 37 after S115 in FIG. 1 and measuring the total amount of dissolved calcium therein.

[0103] Here, the preparation method of Example 1 will be described. First, as a sample simulating the main ash washing water 31 after S109 in FIG. 1 (hereinafter referred to as the main ash leachate), 1200 g of main ash was shaken and washed with 2 L of water, and the washing water was collected. The pH of the main ash leachate was 12.5. Then, 1 L of the main ash leachate was mixed with 1 L of the fly ash leachate to raise the pH, and then a carbon dioxide-containing gas was blown in. The blowing was terminated when the pH reached 9.0. After that, about 2 L of the mixed leachate after filtering the precipitate was taken as Example 1.

[0104] A in FIG. 10 is the amount that could be recovered as calcium carbonate by reacting the hydroxide ions contained in 1 L of the fly ash leachate and 1 L of the main ash leachate. As in A, in Example 1, by mixing the main ash leachate with the fly ash leachate, about 65% of the calcium contained in the original fly ash leachate could be recovered.

[0105] On the other hand, the result of the comparative example in FIG. 10 is the result when the main ash leachate was not mixed. That is, in the comparative example, a carbon dioxide-containing gas was blown into 1 L of the fly ash leachate, the blowing was terminated when the pH reached 9.0, and it is approximately 1 L of the fly ash leachate after filtering the precipitate.

[0106] B in FIG. 10 is the amount that could be recovered as calcium carbonate by reacting the hydroxide ions contained in 1 L of the fly ash leachate. As in B, in the comparative example, since the main ash leachate was not mixed, only about 35% of the calcium contained in the original fly ash leachate could be recovered.

[0107] Next, the effect of increasing the purity of the recovered calcium carbonate by the step (S301 in FIG. 5) in which the impurity removal unit 45 of the second embodiment of the present invention precipitates the impurities in the fly ash washing water 29 recovered in S103 was confirmed.

[0108] FIG. 11 is a graph showing the ratio of the total amount of dissolved calcium in Example 2-1, Example 2-2, and the comparative example when the total amount of dissolved calcium contained in a predetermined amount of the fly ash washing water 29 after S103 in FIG. 5 is set to 100%.

[0109] In FIG. 11, the result marked as "after S103 in FIG. 5" is the total amount of dissolved calcium contained in 1 L of a sample (fly ash leachate) that mimics the fly ash washing water 29 after S103 in FIG. 5, with this amount being set as 100%.

[0110] Next, the result of Example 2-1 in FIG. 11 is shown as a ratio obtained by creating a sample that mimics the fly ash washing water 29 after S303 in FIG. 5 and measuring the total amount of dissolved calcium in it. Example 2-1 is approximately 1 L of fly ash leachate after blowing carbon dioxide-containing gas into 1 L of fly ash leachate until the pH reaches 12.0, terminating the blowing, filtering the precipitate, and then having approximately 1 L of fly ash leachate.

[0111] C in FIG. 11 is the amount by which some calcium ions have precipitated as calcium carbonate together with impurities due to the carbon dioxide-containing gas blown in to remove lead and zinc. Like C, approximately 11% of the calcium contained in the original fly ash leachate precipitates as calcium carbonate and is lost together with the impurities.

[0112] Next, the result of Example 2-2 in FIG. 11 is shown as a ratio obtained by creating a sample that mimics the mixed washing water 37 after S115 in FIG. 5 and measuring the total amount of dissolved calcium in it. Example 2-2 is approximately 2 L of mixed leachate after mixing 1 L of main ash leachate with approximately 1 L of filtered fly ash leachate from Example 2-1 to raise the pH, then blowing carbon dioxide-containing gas until the pH reaches 9.0, terminating the blowing, and filtering the precipitate.

[0113] D in FIG. 11 is the amount that can be recovered as high-purity calcium carbonate by mixing 1 L of main ash leachate with approximately 1 L of filtered fly ash leachate from Example 2-1. D is approximately 54% of the calcium contained in the original fly ash leachate, which is more than the approximately 11% lost as C. In this regard, D is less than approximately 65% of A, but it is desirable in that impurities such as lead and zinc have been removed and high-purity calcium carbonate can be recovered.

[0114] B in Fig. 11 is the same as B in Fig. 10. In the comparative example, neither impurity removal nor mixing of the main ash leachate is performed. Therefore, only about 35% of the calcium contained in the original fly ash leachate can be recovered as in B, and moreover, the recovered calcium carbonate contains impurities. The calcium carbonate recovered in B of Fig. 11 (B of Fig. 10) is inferior in terms of recovery amount and purity compared to the calcium carbonate recovered in A of Example 1 in Fig. 10 and D of Example 2-2 in Fig. 11.

[0115] Here, Table 2 shows the weight percentages of calcium carbonate, lead, and zinc in the calcium carbonate obtained in A of Example 1 in Fig. 10 (without an impurity removal step) and D of Example 2-2 in Fig. 11 (with an impurity removal step).

[0116]

Table 2

[0117] Compared with A of Example 1 (without an impurity removal step), the calcium carbonate obtained in D of Example 2-2 (with an impurity removal step) had a higher composition ratio of calcium carbonate and lower levels for lead and zinc, indicating an increase in the purity of the calcium carbonate.

[0118] As described above, the preferred embodiments of the present invention have been described with reference to the accompanying drawings, but the present invention is not limited to such examples. It is obvious that those skilled in the art can conceive of various modification examples or correction examples within the scope of the technical idea disclosed in this application, and it is naturally understood that those also belong to the technical scope of the present invention.

[0119] For example, the fly ash washing unit 11 and the main ash washing unit 13 washed the fly ash 15 and the main ash before landfilling and landfilled the washed fly ash 15 and main ash, but they may be washed after landfilling.

[0120] FIG. 12 is a diagram showing another example of the present invention. As shown in FIG. 12, in a landfill site 73 where the ground 71 is excavated, fly ash 15 and main ash are first landfilled in a fly ash landfill area 77 and a main ash landfill area 79 divided by a boundary 75, respectively. Then, watering units 83a and 83b connected to a watering facility 81 water the fly ash landfill area 77 and the main ash landfill area 79, respectively. After that, the washing water discharged from each area is collected by a fly ash washing water recovery unit 85 and a main ash washing water recovery unit 87 and used as fly ash washing water 29 and main ash washing water 31.

[0121] In this way, the steps of landfilling the combustion ash by dividing it into fly ash and main ash, watering and washing each division, washing the main ash, recovering the main ash washing water, and washing the fly ash and recovering the fly ash washing water may collect the washing water discharged from each division. It can also be used in a landfill site as in the prior art.

[0122] Further, the incineration ash used in the present invention is not limited to that discharged from a general waste incineration plant, and the same effect can be obtained with incineration ash from a thermal power plant or a biomass power plant. In the present invention, an example in which only the main ash washing water 31 is used as a supply source of hydroxide ions is shown. However, when the main ash washing water 31 alone is insufficient in hydroxide ions to generate a desired amount of calcium carbonate 41 from the fly ash washing water 29, it is also possible to additionally add a general alkaline chemical separately. Even in this case, since the amount of the alkaline chemical used can be reduced as compared with the case where the main ash washing water 31 is not mixed, less cost and water treatment equipment are required.

[0123] Further, when using incineration ash of waste that has not been separated for aluminum, the main ash washing water 31 contains aluminum ions. In this case, it is also possible to blow a carbon dioxide-containing gas only into the main ash washing water 31 without mixing the main ash washing water 31 with the fly ash washing water 29 to precipitate aluminum hydroxide and recover aluminum.

Explanation of symbols

[0124] 10, 10a......... Calcium carbonate recovery system 11………Fly ash washing section 13………Main ash washing section 15………Fly ash 17………Water 19………Container 21………Mixture liquid 23………Filter 25………Washing water container 27………Press 29………Fly ash washing water 31………Main ash washing water 33………Calcium carbonate recovery section 35………Container 37………Mixed washing water 39, 49………Carbon dioxide-containing gas injection pipe 41………Calcium carbonate 45………Impurity removal section 47………Container 51………Impurities 71………Ground 73………Landfill site 75………Boundary section 77………Fly ash landfill area 79………Main ash landfill area 81………Sprinkler equipment 83a, 83b………Sprinkler section 85………Fly ash washing water recovery section 87………Main ash washing water recovery section

Claims

1. A calcium carbonate recovery system from combustion ash, comprising: a main ash washing section for washing main ash and recovering main ash washing water; a fly ash washing section for washing fly ash and recovering fly ash washing water; a calcium carbonate recovery section for mixing the main ash washing water with the fly ash washing water, supplying a gas containing carbon dioxide to precipitate calcium carbonate, and recovering calcium carbonate; A calcium carbonate recovery system, characterized by comprising the above.

2. An impurity removal section for supplying a gas containing carbon dioxide to the fly ash washing water to precipitate and recover carbonate or hydroxide is provided. The calcium carbonate recovery section can mix the main ash washing water with the fly ash washing water from which carbonate or hydroxide has been removed in the impurity removal section, supply a gas containing carbon dioxide to precipitate calcium carbonate, and recover calcium carbonate. The calcium carbonate recovery system according to claim 1, characterized in that it is as described above.

3. The calcium carbonate recovery system according to claim 1, characterized in that in the calcium carbonate recovery section, it is possible to add seed crystals and precipitate calcium carbonate by the pellet method.

4. A method for recovering calcium carbonate from combustion ash, comprising: step a of washing main ash and recovering main ash washing water; step b of washing fly ash and recovering fly ash washing water; step c of mixing the main ash washing water with the fly ash washing water, supplying a gas containing carbon dioxide to precipitate calcium carbonate, and recovering calcium carbonate; A method for recovering calcium carbonate, characterized by comprising the above.

5. The method for recovering calcium carbonate according to claim 4, characterized in that the amount of washing water per unit amount of main ash in step a is less than the amount of washing water per unit amount of fly ash in step b.

6. Before step c, there is step d of supplying a gas containing carbon dioxide to the fly ash washing water to precipitate and recover carbonate or hydroxide. Step c is to mix the main ash washing water with the fly ash washing water from which carbonate or hydroxide has been removed in step d, supply a gas containing carbon dioxide to precipitate calcium carbonate, and recover calcium carbonate. The method for recovering calcium carbonate according to claim 4, characterized in that it is as described above.

7. The step d is terminated under the condition that the pH drops by 0.1 or more from the start of the step d and the pH does not become 7 or less, and the pH is increased once by mixing the main ash washing water in the step c, and further the step c is terminated under the condition that the pH drops by 0.1 or more from the start of the step c and the pH does not become 7 or less. The method for recovering calcium carbonate according to claim 6, characterized in that.

8. The method for recovering calcium carbonate according to claim 4, characterized in that in the step c, seed crystals are added and calcium carbonate is precipitated by the pellet method.

9. The method for recovering calcium carbonate according to claim 4, characterized in that the gas containing carbon dioxide supplied in the step c uses the gas released from the semi-aerobic landfill structure.

10. The method for recovering calcium carbonate according to claim 4, characterized in that the step a and the step b are performed before landfilling the combustion ash, and the washed combustion ash discharged in the step a and the step b is landfilled.

11. The method for recovering calcium carbonate according to claim 4, characterized in that the combustion ash is separated into fly ash and main ash for landfilling, each section is sprinkled and washed with water, and the step a and the step b recover the washing water discharged from each section.

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