JP3981761B2 - Method for removing heavy metals from incineration ash - Google Patents

Description

[0001]
BACKGROUND OF THE INVENTION
The present invention relates to a method for effectively removing heavy metals such as Pb, Zn, and Cd in incineration ash discharged from a municipal waste incinerator or the like.
[0002]
[Prior art]
Approximately 10% of the treated waste is discharged as ash from municipal waste incinerators. There are two types of ash: incineration main ash which is incineration residue of the incinerator and incineration fly ash which is a mixture of dust entrained in the exhaust gas during incineration and dust condensed with gasified metal salt. These ashes all contain harmful heavy metals and dioxins. Currently, research on dioxins treatment has progressed, and for the removal of dioxins, for example, a technique called a heat dechlorination method has been put into practical use. However, there are few practical methods for removing heavy metals in ash, and most of them rely on suppression of elution.
[0003]
One method that has been proposed as a method for removing heavy metals in ash is a method called a chlorination volatilization method. This method is intended to actively utilize the following reactions that are partly occurring in waste incinerators.
[0004]
PbO + 2HCl (g) = PbCl ₂ (g) + H ₂ O (g) (1)
ZnO + 2HCl (g) = ZnCl ₂ (g) + H ₂ O (g) (2)
CdO + 2HCl (g) = CdCl ₂ (g) + H ₂ O (g) (3)
[0005]
The concentration of heavy metal chloride in the gas in these reactions is related to the concentration of HCl and H ₂ O coexisting in the gas phase. For example, the concentration of HCl in the waste incinerator exhaust gas is 1000 (ppm), the concentration of H ₂ O Is 20 (%), at a temperature of 200 to 300 or higher, any chloride may be present in the gas phase as 1 (ppm) or more as a gas. Thus, the solid vapor pressure of these materials from the next reaction will determine the concentration in the gas.
[0006]
PbCl ₂ = PbCl ₂ (g) (4)
ZnCl ₂ = ZnCl ₂ (g) (5)
CdCl ₂ = CdCl ₂ (g) (6)
[0007]
By these reactions, the temperature at which the equilibrium concentration in the gas exceeds 1000 (ppm) is roughly as follows.
[0008]
PbCl ₂ (g): 530 ° C
ZnCl ₂ (g): 415 ° C
CdCl ₂ (g): 530 ° C
[0009]
[Problems to be solved by the invention]
An object of the present invention is to provide a technique for gasifying and separating heavy metal from ash at a lower temperature than the chlorination volatilization method.
[0010]
[Means for Solving the Problems]
FIGS. 1 to 3 compare the equilibrium concentrations of heavy metal chlorides in formula (4), (5), and (6) with the iodide and bromide equilibrium concentrations. As is clear from these figures, the temperature at which all heavy metals exhibit the same concentration in the gas is lower in iodide and bromide than in chloride.
[0011]
The present invention intends to remove heavy metals from the ash by vaporizing heavy metals in the ash discharged from the waste incinerator using the properties of these substances.
[0012]
In the method for removing heavy metals in incineration ash according to the present invention, when removing heavy metals in incineration ash discharged from a municipal waste incinerator or the like, an alkali metal or alkaline earth metal iodide or bromide is reacted with the ash to be treated. Thus, heavy metals are gasified as iodides or bromides, which are recovered by wet or dry dust collection or wet smoke cleaning.
[0014]
The following forms are also preferable.
[0015]
i) A solid or aqueous solution of an alkali metal or alkaline earth metal iodide or bromide is mixed with the ash to be treated, and then the mixture is heated to gasify the heavy metal as an iodide gas or bromide gas.
[0016]
ii) Mixing a solid or aqueous solution of an alkali metal or alkaline earth metal iodide or bromide with an ash to be treated and a solid or sol containing an organosilicon compound or silicon oxide, and then heating the mixture to form a heavy metal Is gasified as iodide gas or bromide gas.
[0017]
iii) Mixing a solid or aqueous solution of an alkali metal or alkaline earth metal iodide or bromide with an aluminum oxide solid or sol to the ash to be treated, and then heating the mixture to convert the heavy metal to an iodide gas or Gasifies as bromide gas.
[0018]
DETAILED DESCRIPTION OF THE INVENTION
The present invention will be specifically described with reference to examples.
[0019]
Reference example 1
Heavy metals in ash discharged from waste incinerators are mainly composed of chlorides and oxides. Therefore, when these ashes are brought into contact with gaseous iodide and bromide in a heated state, heavy metal iodide and bromide gases are generated and separated from the ash into the gas by the following reaction.
[0020]
(Reaction with chloride)
PbCl ₂ + 2HI (g) = PbI ₂ (g) + 2HCl (g) (7)
PbCl ₂ + 2HBr (g) = PbBr ₂ (g) + 2HCl (g) (8)
ZnCl ₂ + 2HI (g) = ZnI ₂ (g) + 2HCl (g) (9)
ZnCl ₂ + 2HBr (g) = ZnBr ₂ (g) + 2HCl (g) (10)
CdCl ₂ + 2HI (g) = CdI ₂ (g) + 2HCl (g) (11)
CdCl ₂ + 2HBr (g) = CdBr ₂ (g) + 2HCl (g) (12)
[0021]
The concentrations of iodide gas and bromide gas of heavy metals by the reactions of formulas (7) to (12) when the concentrations of HI, HBr, and HCl in the gas phase are 1000 (ppm) are shown in FIGS. 6 and the temperature showing the same concentration is lower than that in the case where all of chlorides exist alone.
[0022]
(Reaction with oxide)
PbO + 2HI (g) = PbI 2 (g) + H 2 O (g) (13)
PbO + 2HBr (g) = PbBr 2 (g) + H 2 O (g) (14)
ZnO + 2HI (g) = ZnI ₂ (g) + H ₂ O (g) (15)
ZnO + 2HBr (g) = ZnBr ₂ (g) + H ₂ O (g) (16)
CdO + 2HI (g) = CdI 2 (g) + H 2 O (g) (17)
CdO + 2HBr (g) = CdBr 2 (g) + H 2 O (g) (18)
[0023]
These reactions tend to proceed from normal temperature to the right, as in the case of formulas (1) to (3), and the reaction from right to left, that is, hydrolysis of chloride occurs at high temperatures. However, when both are compared at the same temperature, the concentration that can exist as iodide and bromide is higher than the concentration that can exist as chloride. Therefore, it can be said that iodide and bromide are more easily gasified.
[0024]
As described above, when the heavy metal form of the ash discharged from the incinerator is chloride or oxide according to the description of this reference example, when removing heavy metal from these ash as 1000 (ppm) gas,
1) When volatilizing with chloride
PbCl2 (g): 530 ° C
ZnCl2 (g): 415 ° C
CdCl2 (g): 530 ° C
Temperature is required,
2) When heated while passing 1000ppm of HI gas,
PbI 2 (g): 220 ° C
ZnI2 (g): 230 ° C
CdI2 (g): 150 ° C
3) When heated while passing 1000ppm of HBr gas,
PbBr2 (g): 340 ° C
ZnBr2 (g): 260 ° C
CdBr2 (g): 310 ° C
Then, the heavy metal can be separated from the ash as a gas at a lower temperature.
[0025]
Example 1
The method of Reference Example 1 is a method in which gaseous iodide and bromide are reacted directly with ash to gasify heavy metals to separate them from ash, but the method of Example 1 is an alkaline earth metal iodide. , A bromide solid or a solution in which the bromide is dissolved is reacted with the ash to be treated, and then heated to separate the resulting heavy metal iodide and bromide from the ash. That is, the reaction is represented by the formulas (19) to (24). After the reaction, the reaction mixture is heated to separate heavy metals from the ash as iodide and bromide.
[0026]
(Reaction with chloride)
_{PbCl 2 + CaI 2 (g)} = PbI 2 (g) + CaCl 2 (19)
_{PbCl 2 + CaBr 2 (g)} = PbBr 2 (g) + CaCl 2 (20)
ZnCl ₂ + CaI ₂ = ZnI ₂ (g) + CaCl ₂ (21)
ZnCl ₂ + CaBr ₂ (g) = ZnBr ₂ (g) + CaCl ₂ (22)
CdCl ₂ + CaI ₂ = CdI ₂ (g) + CaCl ₂ (23)
CdCl ₂ + CaBr ₂ = CdBr ₂ (g) + CaCl ₂ (24)
[0027]
Figures 7 to 9 show the concentrations of heavy metal iodide and bromide generated by these reactions in the gas. In any case, it is understood that when heavy metal having the same concentration is obtained in the gas, conversion to iodide or bromide can be obtained at a lower temperature than chloride.
[0028]
(Reaction with oxide)
Considering the case where the heavy metal form in the ash is an oxide, it is better to consider the following reaction in which SiO ₂ coexists. Of course, SiO ₂ is contained in the ash, but if a more reliable reaction is desired, heavy metal oxides can be converted to alkaline earth metal iodides, bromides, diatomides, silica-based sols, or siloxanes. A better result is obtained when the reaction is carried out in a state in which such organic silicon is mixed.
[0029]
2PbO + 2CaI ₂ + SiO ₂ = 2PbI ₂ (g) + 2CaO * SiO ₂ (25)
_{2PbO + 2CaBr 2 + SiO 2 =} 2PbBr 2 (g) + 2CaO * SiO 2 (26)
2ZnO + 2CaI ₂ + SiO ₂ = 2ZnI ₂ (g) + 2CaO * SiO ₂ (27)
2ZnO + 2CaBr ₂ + SiO ₂ = 2ZnBr ₂ (g) + 2CaO * SiO ₂ (28)
2CdO + 2CaI ₂ + SiO ₂ = 2CdI ₂ (g) + 2CaO * SiO ₂ (29)
2CdO + 2CaBr ₂ + SiO ₂ = 2CdBr ₂ (g) + 2CaO * SiO ₂ (30)
The vapor pressures of heavy metal iodides and bromides generated by these reactions are from room temperature to over 1 atm. The case of ZnI ₂ (g) and ZnBr ₂ (g) generated by the reaction with ZnO according to the equations (27) and (28) having the lowest vapor pressure is shown in FIG.
[0030]
In the reaction with ZnO, which has the lowest vapor pressure, ZnCl ₂ alone has a concentration of 1000 ppm (ppm) in the gas as described above, while the temperature in the gas is 415 ° C, while the reaction of formulas (27) and (28) If used, the temperature will be lowered to 170 ° C and 260 ° C, respectively.
[0031]
Example 2
In Example 1 , a method was described in which heavy metal chloride was reacted with Ca iodide and bromide to convert to heavy metal iodide gas and bromide gas, and this was separated from ash. Similar reactions occur not only with Ca but also with alkali metals and Mg compounds. As an example, the concentrations of iodide and bromide in the gas by the following reaction are shown in FIGS.
[0032]
PbCl ₂ + 2KI = PbI ₂ (g) + 2KCl (31)
PbCl ₂ + 2NaI = PbI ₂ (g) + 2NaCl (32)
_{PbCl 2 + CaI 2 = PbI 2} (g) + CaCl 2 (19)
_{PbCl 2 + MgI 2 = PbI 2} (g) + MgCl 2 (33)
PbCl ₂ + 2KBr = PbBr ₂ (g) + 2KCl (34)
PbCl ₂ + 2NaBr = PbBr ₂ (g) + 2NaCl (35)
_{PbCl 2 + CaBr 2 = PbBr 2} (g) + CaCl 2 (20)
PbCl ₂ + MgBr ₂ = PbI ₂ (g) + MgCl ₂ (36)
[0033]
As is clear from these results, conversion of heavy metal chlorides to iodides and bromides is easier than gasification and gasification at a lower temperature and separation from ash.
[0034]
Example 3
In Example 1 , the reaction in which Ca iodide, bromide and SiO 2 are reacted with heavy metal oxide is described. However, the same effect can be obtained even if aluminum oxide (alumina) powder or alumina sol is used in place of SiO 2. It is done. As an example, the heavy metal concentration in the gas by the following reaction is shown in FIG.
[0035]
PbO + CaI ₂ + Al ₂ O ₃ = PbI ₂ (g) + CaO * Al ₂ O ₃ (37)
As is clear from this example, the iodide can be formed at a low temperature in the presence of alumina rather than gasifying PbCl ₂ alone.
[0036]
【The invention's effect】
As described in the embodiments, the effects of the present invention are as follows.
[0037]
(1) There is a method called the chlorination volatilization method that volatilizes and removes heavy metals from incinerated ash, but the heavy metals already present as chlorides in the ash as in the present invention are alkali metals and alkaline earths. By adding a metal iodide or bromide, it can be volatilized at a lower temperature by converting it to a heavy metal iodide or bromide form.
[0038]
Also, heavy metals present in the form of oxides in the ash can be volatilized at a lower temperature by reacting iodides and bromides of alkaline earth metals.
[0039]
In this case, when a powder or sol of a silicon oxide compound or an aluminum oxide compound coexists, it is volatilized at a lower temperature.
[0040]
(2) In this way, heavy metal chlorides and oxides can be converted to heavy metal iodides and bromides by adding alkaline earth metal iodides and bromides, and volatilized and removed from ash at lower temperatures. It is the gist of the invention. It has been confirmed that almost the same effect can be obtained even when alkali metal iodide or bromide is used. However, when the form of heavy metal in the waste incinerator ash is mostly oxide, the effect of lowering the volatilization temperature of heavy metals by alkali metal iodide and bromide may be small.
[Brief description of the drawings]
FIG. 1 is a graph showing the equilibrium concentration in gas when lead chloride, iodide and bromide are present alone.
FIG. 2 is a graph showing the equilibrium concentration in gas when zinc chloride, iodide and bromide are present alone.
FIG. 3 is a graph showing the equilibrium concentration in gas when cadmium chloride, iodide and bromide are present alone.
FIG. 4 is a graph showing the equilibrium concentration of lead iodide and bromide in the gas according to reaction formulas (7) to (8).
FIG. 5 is a graph showing the equilibrium concentrations of zinc iodide and bromide in the gases according to reaction formulas (9) to (10).
FIG. 6 is a graph showing the equilibrium concentrations in the gas of cadmium iodide and bromide according to reaction formulas (11) to (12).
FIG. 7 is a graph showing the concentrations of iodide and bromide in gas when calcium chloride and bromide are reacted with lead chloride.
FIG. 8 is a graph showing the concentrations of iodide and bromide in a gas when zinc iodide and bromide are reacted with zinc chloride.
FIG. 9 is a graph showing the concentrations of iodide and bromide in a gas when calcium iodide and bromide are reacted with cadmium chloride.
FIG. 10 is a graph showing the concentrations of ZnI ₂ and ZnBr _{2 in} a gas when zinc iodide and calcium bromide are reacted with zinc oxide in the presence of silicon oxide.
FIG. 11 is a graph showing the concentration of lead iodide gas in the gas when alkali metal iodide or alkaline earth metal iodide is reacted with lead chloride.
FIG. 12 is a graph showing the concentration of lead bromide gas in the gas when alkali metal bromide and alkaline earth metal bromide are reacted with lead chloride.
FIG. 13 is a graph showing the concentration in a gas of lead iodide gas generated when calcium iodide is reacted with lead oxide in the presence of alumina.

Claims (4)

When removing heavy metals in incineration ash discharged from municipal waste incinerators, etc., heavy metals are gasified as iodides or bromides by reacting ash to be treated with iodides or bromides of alkali metals or alkaline earth metals, A method for removing heavy metals from incinerated ash, which is recovered by wet or dry dust collection or wet smoke cleaning.   A solid or aqueous solution of an alkali metal or alkaline earth metal iodide or bromide is mixed with the ash to be treated, and then the mixture is heated to gasify the heavy metal as an iodide gas or bromide gas. The method of claim 1. When removing heavy metals in incineration ash discharged from municipal incinerators, etc., solid or aqueous solution of alkali metal or alkaline earth metal iodide or bromide and solid containing organic silicon compound or silicon oxide in the ash to be treated Or incinerated ash, characterized in that heavy metal is gasified as an iodide gas or bromide gas by mixing the sol and then heating the mixture, and recovering it by wet or dry dust collection or wet smoke cleaning. Method for removing heavy metals inside. When removing heavy metals in incineration ash discharged from municipal incinerators, etc., the solid or aqueous solution of alkali metal or alkaline earth metal iodide or bromide and aluminum oxide solid or sol are mixed with the ash to be treated. And then gasifying the heavy metal as an iodide gas or bromide gas by heating the mixture and recovering the heavy metal by wet or dry dust collection or wet smoke cleaning method. .

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