Method of removing mercury in exhaust gases from a waster incinerator

A method of removing mercury from the exhaust gases of a waste incinerator. The method is applied to a gas-liquid contact-type smoke cleaner so that the exhaust gases emitted from the waste incinerator are washed with a washing liquid to which an oxidizing agent is added so as to attain an oxidation-reduction potential or COD value such that mercury existing in the form of mercuric chloride and mercury forming complex ions with chlorine in the washing liquid are not reduced by reducing substances present in the washing liquid.

BACKGROUND OF THE INVENTION 
1. Field of the Invention 
The present invention relates to a method of removing mercury from waste 
incinerator exhaust gases and more particularly to a mercury removing 
method in which exhaust gases containing as constituents mercury and 
hydrochloric acid are washed with a washing liquid and the mercury is 
dissolved in the form of mercuric chloride and mercury forming complex 
ions with chlorine into the washing liquid thereby removing the mercury 
from the exhaust gases with high efficiency. 
2. Description of the Prior Art 
The harmful substances contained in the exhaust gases from a waste 
incinerator have heretofore been governed by the regulations from the 
standpoint of environmental pollution prevention. The harmful substances 
have included hydrogen chloride (HCl) and sulfur dioxide (SO.sub.2) and 
thus these harmful substances have been removed by harmful gas cleaning 
apparatus employing the dry process, the semi-dry process and the wet 
process. 
Then, it has recently been found that the exhaust gases from waste 
incinerators have contained mercury in an amount exceeding the 
environmental guideline of 0.015 mg/m.sup.3 provided by the World Health 
Organization (WHO) and this has become an object of public concern. 
In fact, however, the existing waste incinerator harmful gas cleaning 
apparatus of the dry, semi-dry and wet processes have all been designed to 
mainly remove hydrogen chloride (HCl) and sulfur dioxide (SO.sub.2) thus 
making the removal of mercury difficult and only the apparatus of the wet 
process have been capable of removing a trace amount of mercury. 
On the other hand, the known means of removing the mercury from gases 
include the techniques of removing mercury from the hydrogen gas produced 
during the production of caustic soda by the mercury method. However, 
these techniques have been designed to remove the mercury entering as 
mercury vapor into the hydrogen gas from the electrolytic cell or the 
mercury accompanying the hydrogen gas produced. In other words, these 
techniques have been designed to remove the mercury present in the form of 
metal mercury in the hydrogen gas. On the contrary, the waste incinerator 
involves various chlorine generating factors, such as, chlorine gas and 
hydrogen chloride gas and these gases react with mercury. Thus, the 
mercury is mostly contained in the form of mercuric chloride in the 
exhaust gases. As a result, the known techniques of removing mercury from 
hydrogen gas, which have been used with the caustic soda production 
processes by the mercury method, cannot be applied as such to the removal 
of mercury, particularly mercuric chloride contained in the exhaust gas 
from a waste incinerator. 
SUMMARY OF THE INVENTION 
The present invention has been made as a result of various studies and 
research works made with a view to overcoming the foregoing deficiencies 
in the prior art, and it is an object of the invention to provide an 
improved method of removing mercury from the exhaust gases of a waste 
incinerator which is operated so that the exhaust gases containing mercury 
and hydrochloric acid as constituents are washed with a washing liquid and 
an oxidizing agent is added in such a manner that reducing substances 
dissolved into the washing liquid from the exhaust gases are prevented 
from reducing the mercury present as mercuric chloride and mercury forming 
complex ions with chlorine in the washing liquid while preventing any 
excessive addition of the oxidizing agent.

DESCRIPTION OF THE PREFERRED EMBODIMENTS 
The present invention is readily applicable to not only the existing waste 
incineration processes but also to any waste incineration process designs 
to be newly built and the invention is intended to remove mercury from the 
exhaust gases in the proper range from the operation and maintenance point 
of view of the actual apparatus. 
Since the waste materials involve various chlorine generating factors such 
as chlorine gas and hydrogen chloride gas, the mercury in the exhaust 
gases emitted from a waste incinerator reacts with these chlorine gas and 
hydrogen chloride gas and almost all the mercury is converted to mercuric 
chloride. Then, the solubility of mercuric chloride in water is 3.6 g/100 
ml at 0.degree. C. and 61.3 g/100 ml at 100.degree. C. and there is known 
a method of washing the exhaust gases with a large quantity of water to 
dissolve the mercuric chloride in the water and thereby removing the 
mercury in the exhaust gases. However, since the exhaust gases from the 
incinerator contain hydrochloric acid and sulfur dioxide, the washing of 
the exhaust gases with an alkaline solution is necessary and moreover 
washing the exhaust gases with a large quantity of water for removing the 
mercuric chloride is practically difficult since this requires the use of 
means for the disposal of a large quantity of diluted alkaline solution. 
Also, a method is known in which a large quantity of powerful oxidizing 
agent is used to decompose and convert mercuric chloride to ionic mercury 
and the ionic mercury is washed by utilizing the polarity of water. 
However, the addition of such powerful oxidizing agent in any excessive 
amount cause corrosion of the apparatus and the means involving such 
corrosion of the apparatus is not desirable from the operation and 
maintenance standpoint of the apparatus. 
Thus, in the case of the existing waste incineration processes, the smoke 
cleaner utilizes a method in which the exhaust gases are brought into 
contact with a suitable amount of washing liquid or alkaline solution in a 
washing tower such as a spray tower or tray tower so that the harmful 
substances in the exhaust gases, e.g., hydrochloric acid and sulfur 
dioxide are reacted with the alkali and the resulting salts are extracted 
from the bottom of the tower. It has been considered that in such a case, 
the mercuric chloride is dissolved completely into the washing liquid 
owing to its extremely high solubility and that, while the mercuric 
chloride in the form of simple salt is unstable, the mercuric chloride 
forms and stabilizes as complex salts in the presence of soluble chlorides 
such as ammonium chloride (NH.sub.4 Cl) and sodium chloride (NaCl) and it 
exists in the form of stable mercuric chloride in the washing liquid. 
Moreover, it has been confirmed that 10 to 15% of various salts such as 
sodium chloride (NaCl) and sodium sulfate (Na.sub.2 SO.sub.4) coexist in 
most alkaline solution washing liquids subjected to the washing of such 
waste incinerator exhaust gases and also there is the coexistence of salts 
of soluble chlorides which form complex salts with mercuric chloride, and 
it has been considered that the mercuric chloride forms complex salts and 
dissolves in a stable form in the washing liquid. 
However, various studies made by the inventors has shown that the mercuric 
chloride in the exhaust gases dissolves in the form of mercuric chloride 
and mercury forming complex ions with chlorine in the washing liquid and 
that the mercury chloride is easily reduced by various reducing substances 
such as sulfite existing in the washing liquid to become metal mercury 
having practically no solubility in water and the metal mercury is 
volatilized and scattered into the atmosphere from the washing liquid. 
Thus, in accordance with the invention the reducing substances in the 
washing liquid are oxidized so as to cause no corrosion of the apparatus 
and the reduction of mercury in the form of mercuric chloride and mercury 
forming complex ions with chlorine is prevented thereby exhausting the 
mercury in its stable dissolved form in the washing liquid. 
FIG. 1 is a block diagram of a waste incineration process incorporating a 
smoke cleaner to which the present invention is applied. Waste materials 
are charged into an incinerator 1 so that the waste materials are burnt at 
900.degree. C. to 1200.degree. C .and the resulting exhaust gases are 
cooled to about 300.degree. C. in an exhaust gas cooler 2. Then, after the 
dust has been removed by an electric dust collector 3, the exhaust gases 
are introduced into a harmful gas cleaner or smoke cleaner 4 through its 
lower part so that in the smoke cleaner 4 comprising a counter 
current-type or parallel current-type spray tower, tray tower, packed 
tower or the like, the exhaust gases are contacted with an alkaline 
solution such as caustic soda and the harmful substances such as 
hydrochloric acid and sulfur dioxide are reacted with the alkaline thereby 
removing them in the form of salts such as sodium chloride (NaCl) and 
sodium sulfate (Na.sub.2 SO.sub.4). On the other hand, the mercury emitted 
from the incinerator 1 is almost entirely converted to mercuric chloride 
(HgCl.sub.2) due to the coexistence of chlorine and hydrochloric acid 
inherent to the incinerator exhaust gases and the mercuric chloride is 
brought into contact with the washing liquid or the alkaline solution in 
the smoke cleaner 4. Regarding the alkaline solution, a 20 % caustic soda 
solution is for example added and the weak alkaline solution is used. 
The alkaline solution washed and exhausted from smoke cleaner 4 is not 
discharged to prevent environmental pollution. Therefore, the alkaline 
solution is cleaned first by means of washing liquid treatment unit 6 and 
is then discarded. On the other hand, the incineration exhaust gases are 
cleaned by smoke cleaner 4 first, and are then discharged from chimney 5 
into the atmosphere. 
In accordance with the invention, under these conditions of the washing 
tower in the existing smoke cleaner 4, the reducing substances tending to 
reduce the mercuric chloride and mercury forming complex ions with 
chlorine in the washing liquid are oxidized. The reducing substances are 
oxidized in such a manner that only the reducing substances present in the 
washing liquid are oxidized and the smoke cleaner is not corroded by the 
addition of an oxidizing agent in an excessive amount. For this purpose, 
the amount of the reducing substances in the washing liquid is measured 
and a controlled amount of the oxidizing agent corresponding to the amount 
of the reducing substances is added to the washing liquid. While it is 
conceivable, as the method for this purpose, to directly measure the 
amount of such reducing substance as sulfite serving as a reducing agent 
in the washing liquid, the direct measurement of sulfite tends to be 
affected by the coexisting gas components and also it is difficult to 
determine the concentrations of the exhaust gas components for each of 
different incinerators thus making this attempt inpractical. It is also 
apparent that there exist other reducing substances than sulfur oxides, 
such as, sulfite and thus the measurement of sulfite cannot be used as a 
guide required directly for adding an oxidizing agent in an amount 
required to prevent the reduction of mercuric chloride. 
On the other hand, the studies and research made by the inventors of this 
invention have shown that there is a correlation between the amount of 
reducing substances tending to reduce mercuric chloride and mercury 
forming complex ions with chlorine in a washing liquid and variations in 
the oxidation-reduction potential of the washing liquid. Thus, in 
accordance with the invention, noting the fact that the reducing 
substances also enter the washing liquid from the exhaust gases and the 
amount varies depending on the waste composition and burning conditions of 
each waste incinerator, after preliminarily measuring the 
oxidation-reduction potential of the washing liquid, adding an oxidizing 
agent to the washing liquid until the mercuric chloride and the mercury 
forming complex ions with chlorine are no longer reduced and measuring the 
corresponding variations in the oxidation-reduction potential and then 
determining that value of the oxidation-reduction potential which prevents 
these mercury from being reduced to form metal mercury, the oxidizing 
agent is added to the washing liquid so as to maintain this value of the 
oxidation-reduction potential. Thus, it is apparent that not only the 
mercury is allowed to dissolve as mercuric chloride and mercury forming 
complex ions with chlorine but also the entry of an oxidizing agent in any 
excessive amount is prevented thus preventing the occurrence of any 
corrosion of the smoke cleaner, while preventing the mercury from being 
volatilized and scattered due to the addition of the oxidizing agent in 
any excessively small amount. 
It is to be noted that the oxidation-reduction potential can be measured by 
using an electrode i.e., a sensing electrode composed of a combination of 
different metal electrodes, made by a combination of silver chloride and 
platinum (Pt), an electrode made of silver chloride and gold (Au) or the 
like. 
From the results of the measurement by a composite electrode of silver 
chloride and platinum of the oxidation-reduction potential of a washing 
liquid in a washing tower of a certain incineration process it has been 
found that the oxidizing agent should be added at a potential of about 300 
mV or higher. A proper composite electrode for the washing liquid is used 
by suitably selecting it in dependence on the substances which enter and 
coexist in the washing liquid and a composite electrode of silver chloride 
and platinum may be used with the ordinary washing liquid. 
As regards the COD value of the washing liquid, due to the fact that not 
only the amount of reducing substances such as sulfite but also the amount 
of reducing substances such as organic matter are measured, theoretically 
it has not been made clear how the COD value is related with the 
oxidation-reduction potential of the washing liquid. However, the studies 
and research made by the inventors of this invention, etc., have shown 
that there is an approximate correlation between the COD value and the 
oxidation-reduction potential of the washing liquid as shown in FIG. 2 and 
that the COD value can be utilized for the purpose of controlling the 
amount of addition of the oxidizing agent in the like manner as the 
oxidation-reduction potential. 
Thus, by measuring the COD value of the washing liquid and then adding the 
oxidizing agent to the washing liquid and measuring the COD value of the 
washing liquid at which mercuric chloride is no longer reduced and 
volatilely scattered as metal mercury, it is possible to control the 
amount of addition of the oxidizing agent to the washing liquid in 
accordance with the COD values in this range. It has been found that with 
the washing liquid of a washing tower in a certain waste incineration 
process, by adding an oxidizing agent so as to limit the value of COD to 
100 mg/l or less, it is possible to prevent the reduction of mercuric 
chloride and mercury forming complex ions with chlorine. 
Also, the inventors of this invention have investigated into the 
concentration of chlorine in the exhaust gases after the washing with a 
washing liquid containing hypochlorite as an oxidizing agent and it has 
been found that the quantitative relation between the concentration of 
chlorine in the exhaust gases and the concentrations of reducing 
substances, oxidizing agent and chlorine in the washing liquid varies 
while maintaining a definite relation as shown in FIG. 3. 
In the Figure, symbols A and B indicate the same washing liquid and C the 
exhaust gases There is a proportional relation between the reducing 
substance concentration and COD value of the washing liquid A and the 
amount of the reducing substances in the washing liquid B and the 
concentrations of the oxidizing agent and chlorine in the washing liquid B 
vary while maintaining a proportional relation. 
It has been found that the chlorine concentrations of the exhaust gases 
washed with the washing liquids A and B and the amounts of the reducing 
substances in the washing liquids A and B vary with quantitative 
proportional relations. While the waste materials of the waste 
incinerators differ from one incinerator to another with the resulting 
variations in the concentrations of the exhaust gas components, the 
results of the long-term analysis of the exhaust gas compositions of the 
waste incinerators in various regions have shown that the variations in 
exhaust gas composition among the waste incinerators of any given region 
are not so large. 
Thus, where the exhaust gases are washed with a washing liquid containing 
hypochlorite as an oxidizing agent, it is also possible to preliminarily 
measure the variations in the amounts of reducing and oxidizing agents and 
chlorine in a washing liquid and the chlorine concentration of the exhaust 
gases after washing and present the definite relations as data as shown in 
FIG. 3, so that thereafter the chlorine concentration of the washing 
liquid or the exhaust gases after washing is measured continuously and the 
amount of addition of the oxidizing agent is controlled in accordance with 
the chlorine concentration of the washing liquid or the chlorine 
concentration of the exhaust gases after washing at which the mercuric 
chloride and mercury forming complex ions with chlorine in the washing 
liquid are no longer reduced. 
The oxidizing agent used with the invention may comprise an alkaline 
solution, acid solution or the like which discharges oxygen in the washing 
liquid. It is to be noted that the oxidizing agent is decomposed to form 
oxygen and given ions in the washing liquid with the resulting giving and 
receiving of electrons and thus a consideration must be given to their 
reactions with the salts and ions coexisting in the washing liquid. From 
this point of view, the use of hypochlorite or hydrogen peroxide is 
preferred. While the hypochlorite is a salt of hypochlorous acid (HClO) 
and a variety of salts may be used, sodium hypochlorite is well suited 
from the handling point of view. 
In particular, the hydrogen peroxide produces no corrosive gas upon 
decomposition and therefore its use is preferred in the case of a washing 
tower made of materials having no corrosion resistance. 
It is to be noted that the production of oxygen takes place as shown by the 
following formulas. In the case of hypochlorite which is alkaline 
EQU 2ClO.sup.- .fwdarw.2Cl.sup.- +O.sub.2 (1) 
In the case of hydrogen peroxide 
EQU H.sub.2 O.sub.2 .fwdarw.1/2 O.sub.2 +H.sub.2 O (2) 
In this way, the reducing substances are oxidized and the reduction of 
mercuric chloride and mercury forming complex ions with chlorine is 
prevented. 
While the hypochlorite and hydrogen peroxide may each be added in the form 
of a single substance, the addition in the form of an aqueous solution is 
preferred from the handling point of view. 
While the smoke cleaner to which the invention is applied may be of either 
the batch type or the continuous type, the actual apparatus should 
preferably be of the continuous type from the standpoint of ease of 
operation. While the continuous-type smoke cleaner may be any apparatus 
provided that it provides a gas-liquid contact continuously, particularly 
the use of a spray tower or tray tower is preferred. 
The oxidizing agent may be added into the washing liquid staying in the 
lower part of the tower or any other suitable place within the tower or 
alternatively the oxidizing agent may be added to the washing liquid which 
has been removed from the tower and stored in a separate tank. Also, while 
the washing liquid in the form of an alkaline solution can wash 
hydrochloric acid, etc., in the exhaust gases and thus is suitable for the 
purposes, the washing liquid may be neutral or acidic. 
FIG. 4 is a schematic diagram of an apparatus used in making experiments on 
the invention. In the Figure, numeral 7 designates an exhaust gas outlet 
of an electric dust collector, and 8 a pipe connected to the outlet 7 for 
the purpose of extracting experimental exhaust gases. Numeral 9 designates 
a sampling point for analyzing the exhaust gases before their entry into a 
washing apparatus. Numeral 10 designates a glass container having an 
effective volume of 1 l and forming the experimental washing apparatus, 
and 11 the washing liquid extracted from the washing liquid used in the 
washing tower of the smoke cleaner in the actual waste incineration 
process and stored in the container 10. Numeral 12 designates a valve 
fitted to the container 10 for sampling purposes to measure the COD value 
of the washing liquid as occasion demands. Numeral 13 designates a 
temperature controller for maintaining the temperature of the washing 
liquid at the same temperature as in the actual apparatus. 
Numeral 14 designates an air-diffusing bowl connected to the pipe 8 to blow 
diffusely the exhaust gases into the washing liquid. Numeral 15 designates 
an electrode of an oxidation-reduction potential measurer 16 and it 
comprises a composite electrode of silver chloride (AgCl) and platinum 
(Pt) in this embodiment. Numeral 17 designates a container containing an 
aqueous solution of sodium hypochlorite (or any other oxidizing agent) and 
the aqueous solution of sodium hypochlorite is added to the washing liquid 
by a pump 18. Numeral 19 designates a pipe for discharging the exhaust 
gases washed with the washing liquid 11 and the gases are discharged after 
circulating within a container 20. Mounted at the end of the pipe 19 is a 
sampler 21 for analyzing the washed exhaust gases and also suspended in 
the container 20 are test pieces of steel for structure and stainless 
steel materials, respectively, for the purpose of performing a corrosion 
test on the materials of the apparatus. 
A washing liquid comprising an aqueous solution containing salts (NaCl, 
Na.sub.2 SO.sub.4, etc.) and having a concentration of about 10% was 
supplied into the container 10 of the experimental apparatus constructed 
as mentioned above. Then, the exhaust gases were blown at the rate of 
about 1 l/min into the washing liquid and were washed while maintaining 
the pH at 8. Sodium hypochlorite or hydrogen peroxide was continuously 
added to the washing liquid under manual control while continuously 
measuring the oxidation-reduction potential of the washing liquid and also 
extracting the washing liquid at intervals of 30 minutes and measuring the 
COD value by the method of JIS K 0102. Each continuous running time for 
experiment was 6 hours and the test pieces in the container 20 were judged 
by visual observation with the exposure time of 30 hours. 
The percentage removal of mercury from the exhaust gases was determined 
from the ratio between the mercury concentrations of the exhaust gases 
measured before and after the washing with the washing liquid 11. Also, 
the chlorine was measured by measuring outlet exhaust gases 21 by the 
method of JIS K 0106. 
The results of the above experiments were as shown in the following tables. 
TABLE 1 
__________________________________________________________________________ 
Washing Liquid Exhaust gases 
Degree of 
Oxidation- Percentage 
Outlet corrosion of 
reduction 
Oxidizing 
removal of 
concentration 
materials by 
Test No. 
potential 
p.sup.H 
agent mercury 
of chlorine 
exhaust gases 
__________________________________________________________________________ 
1 530 mV 
8 Sodium 80% Undetected 
Slight 
hypochlorite 
2 105 mV 
8 None 12% " Slight 
3 820 mV 
8 Sodium 81% Detected 
Scattered 
hypochlorite corrosion 
__________________________________________________________________________ 
TABLE 2 
__________________________________________________________________________ 
Washing Liquid Exhaust gases 
Degree of 
Percentage 
Outlet corrosion of 
removal or 
concentration 
materials by 
Test No. 
COD .sup.p H 
Oxidizing agent 
mercury 
of chlorine 
exhaust gages 
__________________________________________________________________________ 
4 11 mg/l 
8 Sodium 88% -- Scattered 
hypochlorite corrosion 
5 30 mg/l 
8 Sodium 74% -- Slight 
hypochlorite 
6 105 mg/l 
8 Sodium 22% -- Slight 
hypochlorite 
7 150 mg/l 
8 Sodium 10% -- Slight 
hypochlorite 
8 20 mg/l 
8 Hydrogen peroxide 
85% -- Slight 
__________________________________________________________________________ 
With the tests in the above Tables 1 and 2, the tests 1 to 3 controlled the 
amount of addition of the oxidizing agent in accordance with the 
oxidation-reduction potential and the tests 4 to 8 controlled the amount 
of addition of the oxidizing agent in accordance with the COD value. Also, 
with the washing liquid in the tests 1 to 3, the oxidation-reduction 
potential was 710 mV in a condition where the mercuric chloride was not 
reduced, whereas with the washing liquid in the tests 4 to 8, the COD 
value was 15 mg/l in a condition involving no reduction of the mercuric 
chloride. 
From the results of the tests it will be seen that if the exhaust gases are 
washed by adding an oxidizing agent to the washing liquid in such a manner 
that the oxidation-reduction potential or the COD value of the washing 
liquid approximates a value at which the mercuric chloride and mercury 
forming complex ions with the chlorine are no longer reduced, the mercury 
in the exhaust gases is removed with a high yield and the corrosion of the 
apparatus is reduced. 
From the foregoing description it will be seen that in accordance with the 
invention, by virtue of the fact that the amount of addition of an 
oxidizing agent is controlled in accordance with the oxidation-reduction 
potential or the COD value of the washing liquid, the invention is 
applicable to the washing device of the harmful gas cleaner in newly-built 
waste incineration processes as well as the existing ones. Also, the fact 
that the mercuric chloride is dissolved into the washing liquid in the 
washing device has the effect of simplifying the washing liquid treating 
step and preventing the mercury from being volatilized and scattered into 
the atmosphere thereby contributing toward the prevention of environmental 
pollution.