CH616601A5 - Process for removing heavy or toxic metals from exhaust gases - Google Patents

Description

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PATENT CLAIM Process for removing heavy or toxic metals from exhaust gases flowing from an incinerator, characterized by the following process steps: washing the exhaust gas by means of a washing liquid which contains at least water, contacting at least a part of the waste water which results from the washing process with additional waste gas which has not yet been subjected to washing in order to enrich the waste water mentioned, adding ferro-ions and alkali to the enriched waste water and bubbling an oxidizing gas into the enriched waste water, so that either ferrite fails alone or ferrite and FeO (OH) , or magnetite and ferrite of the formula MxFe3_x04, where O <xg 1 and the precipitate has heavy or toxic metal (M) to be removed.

The invention relates to a method for removing heavy or toxic metals from exhaust gases flowing from an incinerator.

Exhaust gases from the municipal waste incinerator or from another incinerator contain a large number of toxic substances. Therefore, these toxic substances should be extracted from the exhaust gas before it is released into the air to prevent air pollution. An electric precipitator has been widely used for this purpose. In addition to or in place of this electrical precipitator, a scrubber can be used in which exhaust gases are washed and cleaned by a washing liquid such as. B. water or alkaline solutions. Although such a washer does a lot to clean exhaust gases, removing the waste water from the washer poses other serious problems. Where exhaust gases contain heavy or toxic metals, these metals are absorbed by the wash solution and the discharge of the waste water containing these metals can cause secondary contamination, e.g. B. the rivers, the lakes or the sea. However, it is very difficult to effectively remove heavy or toxic metals from the wastewater.

A method is thus to be created for removing heavy or toxic metals from exhaust gases, according to which exhaust gases which flow from urban waste incineration plants or other incinerators can be cleaned, while avoiding the possibility of secondary contamination.

The process according to the invention is characterized by the following process steps: washing the waste gas by means of a washing liquid which contains at least water, bringing at least a portion of the waste water which is produced by the washing process into contact with further waste gas which has not yet been subjected to the washing to enrich the said waste water, adding ferro ions and alkali to the enriched waste water and bubbling an oxidizing gas into the enriched waste water, so that either ferrite alone precipitates or ferrite and FeO (OH), or magnetite and ferrite of the formula MxFe3_x04, where O <x 1 and the precipitate has heavy or toxic metal (M) to be removed.

The invention is described in more detail with reference to the accompanying drawing, which represents a schematic block diagram.

An escaping gas 11 at high temperature, which contains at least one heavy or toxic metal such. B. Cr, Fe, Co, Ni, Cu, Zn, As, Ag, Ba, Cd, Sn, Hg, Pb, Ti, Mn or Bi, is from a combustion furnace 1 directly or via a heat recovery boiler, an electrical precipitator or other known devices (not shown) passed to the heat exchanger 2, which will be described later, and then passed through a pipe 12 into a washer 3. The washer 3 used in this invention can be an absorption tower with trays in it or a packed tower, a falling film distillation tower or a venturi washer. In particular, such a device is preferred, which has a high effectiveness in gas-liquid contact, z. B. a BEFLEX washer (trade name, manufactured by Seitetsu Kagaku Co., Ltd., Osaka / Japan), which is provided with sound elements. Wash water is passed via pipe 21 to washer 3, before or during the washing process. In many cases, alkali is constantly added to the scrubber via pipe 21 to remove hydrochloric acid and sulfurous acidity in the exhaust gas 12. Wash water which has absorbed impurities such as heavy or toxic metals in the exhaust gas 12 through the gas-liquid contact reaction in the scrubber 3 is discharged from the scrubber 3 as waste water 22. Part of the washing water is preferably discharged as waste water 22 and the remaining water is used repeatedly in the washer 3. Cleaned and cooled gas 13 is discharged from the scrubber 3 and, if necessary, passed through a smoke screen (not shown), through a chimney 4, from where the gas is released into the air.

Waste water 22 from the washer 3 is passed to the heat exchanger 2 and brought into direct or indirect contact with the high-temperature exhaust gas 11, being enriched and heated to a temperature of 20 to 100 ° C, preferably 50 to 100 ° C, which is necessary is for the reaction in the reactor 6, which will be described later. The exhaust gas 12 is cooled to a temperature at which the washing can be carried out with high efficiency. As a heat exchanger 2, any known device such. B. an immersion tube arrangement can be used. The heat exchanger 2 can be installed separately from other devices. Instead of an independent installation of the heat exchanger, a waste heat boiler, a cooling tower or a quenching device for the exhaust gas and / or a washer 3 can be used as the heat exchanger 2.

Where the heat exchanger 2 is combined with the washer 3, the function of the exchanger 2 can be realized by collecting waste water 22 from the washer tower 3 in the lower part of the tower, namely by introducing the exhaust gas 11 directly into this water tank. In any event, the wastewater 22 is preferably so enriched that the concentrations of sulfate, carbonate, chloride and other salts present in the wastewater 22 are at or near the points of crystal separation from such salts, the wastewater remaining liquid . The enriched wastewater 23, which contains various salts and heavy and toxic metals, is derived from the heat exchanger 2 and led to the reactor 6. In this case, if possible, it is preferable that part of the treated wastewater 23 is discharged and the remaining one is circulated in the heat exchanger to obtain a sufficiently concentrated wastewater 23. The concentrated wastewater 23 can be returned to the scrubber 3 via the line 23 'and can be returned to the exchanger 2 via the line 22. Thus, the waste water 22 of the washer 3 can be circulated twice or more times through the devices 2 and 3 and then discharged from the exchanger 2 via the line 23 or from the washer 3 via the line 23 ". Since the temperature of the exhaust gas 12, which in the scrubber is introduced is usually less than 100 ° C and since the gas 11 which is introduced into the exchanger 2 is more than 100 ° C, preferably more than 300 ° C

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in many cases, it is advantageous to increase the concentration of the waste water by repeatedly circulating the waste water through the heat exchanger 2.

The concentrated waste water 23, which contains various salts and heavy and toxic metals, has a suitable temperature for the reaction in the reactor 6 and is introduced into the reactor 6 in a liquid state. For enriched waste water 23 on the way to the reactor 6 or in the reactor 6 ferro-ions 31, such as. B. Ferro salt, e.g. B. ferrosulfate, ferrochloride and ferronitrate or an aqueous solution thereof, and alkali 32, such as. B. hydroxides or carbonates of alkali-containing metals and alkali-containing earth metals, alkali-containing substances containing nitrogen, such as. B. NH4OH or an aqueous solution thereof added. The ferro-ion addition amount should be at least twice the total molar amount of heavy or toxic metal or metals contained in the enriched wastewater. Examples of such heavy or toxic metals treated in this invention are Cr, Fe, Co, Ni, Cu, Zn, As, Ag, Cd, Sn, Hg, Pb, Ti, V, Mn, Bi and Ba. The amount of alkali should be in the range of 0.9 to 1.2 equivalents and more, preferably 0.98 to 1.05 equivalents, of the acid radicals which are present in the enriched waste water after the addition of the ferro ions.

As a result of the addition of ferro-ions and alkali to the waste water, a suspension of ferrous hydroxides and in many cases hydroxides of other heavy and toxic metals that are present in the waste water is formed. The suspension is then oxidized at a temperature between 20 and 100 ° C while stirring the hydroxides. For the oxidation, an oxidation gas 33, such as. B. air or oxygen, bubbled into the reactor 6 until the hydroxides disappear. In this process, the ferro-ions which have been dissolved in the waste water in equilibrium with the hydroxides are largely oxidized, and crystal particles which contain ferro-ions are precipitated out of the water. The hydroxides continuously dissolve in the wastewater to supplement the ferro ions, with the oxidation progressing. The hydroxides gradually dissolve in the waste water and may glaze. Thus, only the crystal particles containing ferro ions are obtained as a precipitate. The chemical composition, the crystal structure and the particle size of the precipitate, which contains ferro ions, are dependent on various conditions, such as e.g. B. the ratio of the amount of alkali to the acid radical, the temperature during the oxidation and the duration of the oxidation. If alkali of one equivalent to the ferrous salt is added, a suspension of 9 to 10 pH is obtained. From this suspension magnetite Fe304 and ferrites with the composition (M "0) x-Fe3_x04_x, where the factor M" is a heavy metal or metals other than iron, and 0 <xS 1, at a temperature of 60 ° C or precipitated more, preferably at 60 to 90 ° C. A mixture of ferrous Fe304 and FeO (OH) is precipitated at a temperature of less than 60 ° C. Some heavy metals, such as Mn, Cd and Zn become slightly magnetic to form ferrites in an amount of one mole or less (x = 1) with respect to the ferro ions of two moles. Some other heavy metals, such as B. Pb, Cr, Hg and Ni are combined in a small amount in the magnetite. These metals do not form a perfect solid solution or ferrite. Other heavy metals are combined in the magnetite in an intermediate amount between the above amounts. Some heavy metals can be found in a very small amount in FeO (OH) ferric oxyhydrate. The remaining amounts of heavy metal are adsorbed by magnetite, ferrite or FeO (OH). Since they cannot be removed from magnetite, ferrite or FeO (OH), not even by ultrasonic rinsing in pure water, it is assumed that they are not connected to magnetite, ferrite or FeO (OH) by simple adhesion, but by another relatively strong bond.

The precipitated ferrites, magnetite or FeO (OH) have a large particle size. So z. B. Ferrites and magnetites of 0.05 micron obtained by oxidation at 50 ° C for one hour and 0.1 to 0.2 microns at 70 ° C for three hours. As for FeO (OH), a particle size of 0.3 micron is obtainable by oxidation at 15 ° C for one hour. If the time and the temperature of the oxidation are longer or higher, the deposited crystals have a larger particle size.

Since the excretions have a large particle size, they can easily be separated from the water by means of ordinary sedimentation and filter techniques. Ferrites and magnetite can be removed using a magnet. It has been found that a significant amount of FeO (OH) is attracted to the magnet together with the ferrites or the magnetite.

A large number of heavy or toxic metals contained in the waste water 23 are strongly adsorbed by the ferritic oxides (ferrites or magnetite) and in smaller cases by the iron hydroxide FeO (OH) to give insoluble, stable precipitates. Even if various salts are contained in the waste water 23 and also if sea water is used for the washing solution 21, the oxidation reaction in the reactor 6 is not influenced.

The concentrated waste water 23 from the heat exchanger 2 can be subjected to the sedimentation in advance, with a resultant excess being introduced into the reactor 6.

The reacted wastewater 24 is discharged from the reactor 6, and ferrite sludge 41, which in some cases contains FeO (OH), is separated from the water in a separator 7 by one or a combination of filtering, sedimentation and magnetic attraction. Salts continue to be separated from the wastewater, which is now free of heavy or toxic metals, and the purified water 25 can then be discharged into the rivers or into the sea or can serve as a source for domestic or industrial water supply. The ferrite slurry 41 can also be used as a material for preventing interference with radio waves, for forming magnets or as a magnetic fluid.

According to the present invention, heavy or toxic metals contained in the exhaust gas from an incinerator can be effectively removed without the possibility of secondary contamination. The size of the devices, in particular the reactor 6, can be reduced because the wastewater to be treated is sufficiently concentrated. In addition, the uneconomical heating of the waste water can be avoided by using the waste heat of the exhaust gas 11.

example 1

The exhaust gas was treated in accordance with the present invention from an urban waste incinerator 1. The furnace 1 with the combustion capacity of 180 tons / day emits 42,000 normal cubic meters per hour (amount of dry gas) exhaust gas 11, which has a temperature of 800 to 1000 ° C. The exhaust gas 11 passes through a waste heat boiler (not shown) and its temperature drops to approximately 300 ° C. The exhaust gas of approximately 300 ° C passes through an electrical precipitator (not shown) and is passed into a cooling tower 2 where its temperature rises about 80 ° C is lowered. The cooled gas 12 is in a washer

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3 washed (in the BEFLEX washer already mentioned) in which a washing solution 21 of 36,000 kg per hour circulates, the pH of which is maintained at about 8, by adding 25% NaOH solution. This washing reduces the hydrogen chloride contained in the gas from 1300 to 10 ppm. The cleaned gas 13, which is still at a temperature of approximately 80 ° C., is removed via a chimney 4. Part of the washing solution 22 circulates through the cooling tower 2 and the washer 3 via the lines 22 and 23 '. In the cooling tower 2, the washing solution is in contact with the exhaust gas. The wastewater 23 of 470 kg per hour with a temperature of about 80 ° C and such a concentration that the

Salinity is 30% by weight, is deducted from the washer 3. Ferrosulfate 31 of 47 mol / hour is added to this wastewater 23 and 25% NaOH solution 32 of approximately 1 equivalent. The resulting wastewater is introduced into a reactor 6 in which air 33 is bubbled in to effect the oxidation reaction for 30 to 90 minutes. Under these conditions, ferrite crystals containing heavy metals are precipitated. These ferrite deposits 41 are filtered out and the io contents of the heavy metals in the supernatant are measured. The results are shown in Table 1 together with the heavy metal contents of the concentrated waste water 23.

Table 1

Metals in the concentrated in the converted wastewater

Wastewater (after reaction)

CD

8 ppm

Ed

5 ppm

Total Cr

85 ppm

Cr6 +

1 ppm

Pb

93 ppm

As

2 ppm soluble Mn

35 ppm

Zn

12 ppm soluble Fe

5 ppm

Cu

15 ppm less than 0.01 ppm less than 0.02 ppm less than 2 ppm less than 0.05 ppm less than 0.1 ppm less than 0.05 ppm less than 10 ppm less than 5 ppm less than 10 ppm less than 3 ppm

Example 2

Phosphate ore has been found to contain Cd, Zn, Mg and other heavy metals. To remove these heavy metals, phosphate ore was crushed and calcined in a combustion furnace 1 at a temperature of 1100 ° C by burning heavy oil. The exhaust gas 11 from the furnace 1 is passed through a cyclone, a waste heat boiler and an electrical precipitator and its temperature is reduced to 300 ° C. This gas is further cooled by means of a cooling tower 2 to 70 ° C., washed in a washing tower 3 and released via a chimney 4 (chimney). Sea water 2, which contains salts of approximately 3% by weight, is introduced into the washing tower 3 and serves as a washing solution. The washing solution 22 circulates from the washing tower 3 to

Cooling tower two or more times, with the scrubbing solution in contact with the exhaust gas in the cooling tower, and the concentrated wastewater 23 of approximately 70 ° C and having a Cd content of 200 ppm, Zn, Mg and other heavy metals, is removed from the cooling tower 2 withdrawn and passed to the reactor 6. Then a FeS04 solution 31 of 0.15 mol / liter and an NaOH solution 32 of 0.33 mol / liter are fed to the reactor 6 and air is bubbled into the reactor 6 for 40 minutes for 60 minutes. Under these conditions, ferrites containing heavy metals are eliminated. After the separated ferrites 41 have been separated, the purified water contains 25 Cd of only 0.01 ppm and other heavy metals in small amounts.

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1 sheet of drawings