CA1294759C - Method of removing mercury in exhaust gases from a waste incinerator - Google Patents

Abstract

ABSTRACT
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.

Description

INCINERATOR

BACKGROUND OF THE INVENTION
Field of the Invention The present invention relates to a method of removing mercury from waste inc;nerator exhaust gases and more particularly to a mercury removing method in wh;ch 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.

Descript;on 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 env'ironmental pollution 20 prevention. The harmful substances have included hydrogen chloride (HCl) and sulfurdioxide (S02) and thus these harmful substances have been removed by harmful gas cleaning apparatus employing the dry p-rocess, 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 m9/m3 provided by the World Health Organ;za,tion (WHO~ and this has become ; an object of public concern.

~e ~29~7~9 In fact, however, the ex;sting waste incinerator harmful gas cleaning apparatus of the dry, semi-dry and wet processes have all been des;gned to mainly remove hydrogen chloride (HCl~ and sulfur dioxide (S02) thus making the removal of mercury diff;cult 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 electro lytic 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 ;n 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 conta;ned in the form of mercuric chloride in the exhaust gases. As a result~ the known techniques of remov;ng 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.

' ~L2~475g The present invention has been made as a result of various studies and research works made with a view to overcoming the foregoing deficiencies ;n 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.

BRIEF DESCRIPTION OF THE DRAWINGS
Fig. 1 is a block diagram of a waste incineration process incorporating a smoke cleaner to w'hich the present invention is applied.
~ig. 2 is a graph showing the relation between the COD
value (the chemical oxygen démand) and the oxidation-reduct;on potential of the washing liquid in the smoke cleaner~ , ' Fig. 3 is a graph showing the relation between the reducing substances in the washing liquid of the smoke cleaner and the COD value, the relation between the concentrations of the oxidizing agent and the reducing agents in the washing liquid in a case where the exhaust ~Z947~9 1 gases are washed with the washing liqu;d conta;ning the ox;d;z;ng agent and the relat;on between the concentrat;on of the chlorine ;n the exhaust gases or the washing l;quid and the concentrat;on of the reduc;ng agents in the wash;ng liquid in the same case.
Fig. 4 ;s a schematic'diagram of an exper;ment apparatus for carrying out the invention~

DESCRIPTION OF THE PREFERRED EM~ODIMENTS
The present invention ;s 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 ac'tual apparatus.
S;nce the wasle materials involve various chlorine generating factors such as chlorine gas and hydrogen chloride gas, the mercury in the exhaust gases emitted from a waste ;nc;nerator reacts w;th these chlorine gas and hydrogen chloride gas and almost all the mercury is converted to mercur;c chloride. Then, the solubility of mercuric chloride in water is 3.6 g/100 ml at 0C and 61.3 g/100 ml at 100C and there is known a method of washing the exhaust gases with a large quantity of water to dissolve the mercuric chlor;de ;n the water and thereby removing the mercury in the exhaust gases. However, since the exhaust gases from the ;ncinerator contain hydrochloric acid and sulfur diox;de, the washing of the exhaust gases with an alkaline solution is necessary and moreover washing the ~L29~759 1 exhaust gases with a large quantity of water for removing the mercur;c 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 ox;dizing agent ;s used to decompose and convert mercuric chloride ~o 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 arnount cause corrosion of the apparatus and the means involving such corrosion of the apparatus is not desirable from the opera-tion and maintenance standpoint of the apparatus, Thus, in the case of the existing waste ;ncineration processes, the smoke cleaner utilizes a method in wh;ch the exhaust gases are brought into contact with a suitable amount of washing liqu;d 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 ;nto the washing liquid owing to its extremely h;gh 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 (NH4Cl) and sodium chloride (NaCl) and ;t exists in the form oF stable mercuric chlor;de in the washing liquid.

lZ99~759 1 Moreover~ it has been conf;rmed that 10 to 15 % of various salts such as sod;um chloride tNaCl) and sodium sulfate (Na2S04) 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 chlor;des which form complex salts with mercur;c chloride and it has beeri considered that the mercuric chloride forms complex salts and dissolves in a stable form ;n the wash;ng 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 subw stances 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 pre-sent invention is applied. Waste materials are charged into an incinerator 1 so that the waste materials are burnt at 31 2~7~

1 900C to 1200C and the resuLting exhaust gases are cooled to about 300C 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 hydrochlor;c acid and sulfur dioxide are reacted with the alkaline thereby removing them in the form of salts such as sodium chloride tNaCl) and sodium sulfate (Na2S04). On the other hand, the mercury em;tted from the incinerator 1 is almost entirely converted to mercuric chloride (HgCl2) due to the coexistence of chlorine and hydrochloric ac;d inherent to the inci71erator 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.
In accordance w;th 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 form;ng complex ions with chlorine in the washing l;quid 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 1 in an excessive amount. For this purpose, the amount of the reducing substances ;n the wash;ng liquid is measured and a controlled amount of the oxidizing agent corresponding to the amount of the reducing substances is added to the ; 5 washing liquid. Wh;le it is conceivable, as the method for th;s purpose, to d;rectly measure the amount of such reduc-ing substance as sulfite serving as a reduc;ng agent in the washing liquid, the d;rect mea;urement of sulf;te tends to be affected by the coex;sting gas components and also it 1û ;s difficult to determ;ne the concentrations of the exhaust gas components for each of different ;nc;nerators thus mak;ng this attempt ;npract;cal. It ;s also apparent that there ex;st other reduc;ng substances than sulfur oxides, such as, sulfite and thus the measurement of sulf;te cannot be used as a guide required directly for add;ng an oxid;zing agent in an amount required to prevent the reduction of mercuric chloride.
On the other hand, the stud;es and researches made by the inventors of this invention have shown that there is a correlation between the amount of reduc;ng substances tending to reduce mercuric chloride and mercury form;ng complex ions with chlorine in a washing liquid and variat;ons in the oxidation-reduction potential of the wash;ng liquid.
Thus, ;n accordance with the invention, not;ng the fact that the reducing substances also enter the wash;ng liqu;d from the exhaust gases and the amount varies depend;ng on the waste composition and burning conditions of each waste incinerator, after preliminarily measuring the oxidation reduction potential of the washing liquid, adding an ~Z9~'75~

1 oxid;zing agent to the washing liquid until the mercuric chloride and the mercury forming complex ions w;th chlorlne are no Longer reduced and.measuring the correspond-;ng variations in the oxidat;on-reduct;on potential and then determining that~value of the oxidat;on-reduction potential which prevents these mercury from be;ng reduced to form metal mercury, the oxidizing agent is added to the washing liquid so as to mainta;n th;s value of the oxidation-reduction potent;al. Thus, ;t ;s apparent that not only the mercury is allowed to dissolve as mercur;c 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 a composlte electrode made by a combination of silver chloride and platinum (Pt), a ` composite electrode made of silver chloride and gold (Ag) or the like.
From the results of the measurement by a compos;te electrode of s;lver chlor;de and platinum of the oxidat';on-reduct;on potent;al of a washing liqu;d ;n a wash;ng towerof a certain incinerat;on process ;t has been found that the oxidizing agent should be added at a potent;al of about 300 mV or h;gher~ A proper compos;te electrode for the washing liquid is used by suitably select;ng ;t ;n 1 dependence on the substances which enter and coexist in the wash;ng liqu;d and a composite electrode of silver chlor;de and platinum may be used with the ordinary washing liquid.
As regards the COD value of the washing liqu;d, due to the fact that not only the amount of reduc;ng substances such as sulfite but also the amount of reduc;ng substances such as organic matter are measured~ theoret;cally ;t has not been made-clear how the COD value ;s related w;th the oxidation-reduction potential of the washing liquid. How-ever~ the studies and researches made by the inventors of this invention~ etc~, have shown that there is an approx;mate 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 oxidat;on-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 liqu1d 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 ~ . .

~294759 1 of mercuric chloride and mercury forming complex ;ons w;th chlorine.
Also~ the inventors of this invention have ;nvestigated into the concentration of chlorine in the exhaust gases after the washing with a washing liquid containing hypochlorite as an oxidizing agent and ;t has been found that the quant;tat;ve relation between the concentration ot chlor;ne ;n the exhaust gases and the concentrations of reducing substances, oxid;ng agent and chlor;ne ;n the wash;ng l;qu;d var;es wh;le mainta;ning a definite relation as shown ;n Fig. 3.
In the Figure, symbols A and B indicate the same washing liquid and C the exhaust gases. There is a propor-tional relation between the reducing substance concentration and COD value of the washing liqu;d A and the amount of the reducing substances in the washing liquid B and the concentrations of the ox;dizing agent and chlorine in the washing liquid B vary while maintaining a proportional relation.

It has been found that the chlor;ne 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 inc;nerators differ from one incinerator to another w;th the resulting variations in the concentrations of the exhaust gas components, the results of the long-term analys;s of the exhaust gas compos;t;ons of the waste ;ncinerators in various regions have shown that the variat;ons ;n 4~

~ 12 1 exhaust gas composition among the waste ;ncinerators of any g;ven region are not so large.
Thus, where the exhaust gases are washed w;th a wash;ng liquid containing hypochlorite as an ox;dizing agent, ;t ;s also possible to preliminarily measure the variations in the amounts of reducing and oxidizing agents and chlor;ne ;n a washing liquid and the chlorine concentration of the exhaust'gases after washing and present the defin;te 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 ;s controlled in accordance w;th the chlorine concentration of the washing liquid or the chlor;ne concentration of the exhaust gases after wash;ng at wh;ch the mercuric chloride and mercury forming complex ;ons with chlor;ne ;n'the washing liquid are no longer reduced.
The oxidizing agent used with the invent;on may comprise an alkaline solution, acid solution or the like wh;ch discharges oxygen in the washing liquid. It is to be noted that the ox;dizing agent is decomposed to form oxygen and given ions in the wa'sh;ng liquid with the result;ng giving and rece;ving of electrons and thus a cons;derat;on must be given to their reactions w;th the salts and ;ons coexisting in the washing liquid. From this po;nt of view, the use of hypochlorite or hydrogen peroxi'de ;s 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 handl;ng point of v;ew.

~25~7~9 1 In particular~ the hydrogen peroxide produces no corrosive gas upon decomposition and therefore its use ;s preferred in the case of a wash;ng 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 2ClO --~ 2Cl + 2 ~1) In the case of hydrogen peroxide H22 ~ ~ 1/2 2 ~ H20 ..... ~2) In this way, the reducing substances are ox;d;zed and the reduction of mercur;c chlor;de and mercury forming complex ions with chlorine is prevented.
While the hypochlorlte and hydrogen peroxide may each be added in the form of a s;ngle substance, the add;t;on ;n 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 ty~e 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 i~ 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 1 l;qu;d stay;ng in the lower part of the tower or any other su;table place within the tower or alternat;vely the ox;d;z;ng agent may be added to the washing liquld which has been removed from the tower and stored ;n a separate tank. Also, wh;le the wash;ng liquid in the form of an alkal;ne solution can wash hydrochloric ac;d, etc., ;n the exhaust gases and thus 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 invent;on. In the F;gure, 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 extract;ng exper;mental exhaust gases.
Numeral 9 designates a sampling po;nt for analy~ing the exhaust gases before their entry into a washing apparatusO
Numeral 10 designates a glass conta;ner having an effective volume of 1 l and forming the experimental washing apparatus, and 11 the washing liquid éxtracted from the washing liqu;d used ;n the wash;ng tower OT the smoke cleaner ;n the actual waste incineration process and stored ;n the conta;ner 10.
Numeral 12 designates a valve fitted to the conta;ner 10 for sampl;ng purposes to measure the COD value of the wash;ng l;qu;d as occasion demands. Numeral 13 des;gnates a temperature controller for ma;nta;n;ng the temperature of the washing l;quid at the same temperature as ;n the actual apparatus.
Numeral 14 des;gnates an a;r-d;ffusing bowl connected to the p;pe 8 to blow d;ffusely the exhaust gases ;nto the wash;ng liquid. Numeral 15 designates an electrode of an lZ94759 1 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 contain;ng 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 w;th the washing liquid 11 and the gases are dis.charged 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 SS and SUS 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, Na2S04~ etc.~ and having a concentra-tion 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 ra-te 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 cont.inuously 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 contlnuous running time for exper;ment was 6 hours and the test pieces in the container 20 were judged - l~g~759 1 by visual observation w;th the exposure time of 30 hours.
The percentage remova.l 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 follow;ng tables.

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~29 3L7~9 1 W;th the tests ;n the above Tables 1 and 2, the tests 1 to 3 controlled the amount of add;t;on of the ox;dizing agent in accordance w;th the ox;dation-reduction potent;al and the tests 4 to 8 controlled the a~ount of addition of the oxid;z;ng agent ;n accordance w;th the COD value. Also, w;th the wash;ng l;quid in the tests 1 to 3, the oxidation-reduct;on potential was 710 mV in a condition where the mercuric chloride was not reduced, whereas with ~he washing liquid in the tests 4 to 8, the COD value was 15 mg/l in a cond;t;on involv;ng no reduction of the mercur;c 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 l;quid ;n such a manner that th.e ox.;dat;on-reduct;on potent;al of the COD value of the wash;ng liquid approximates a value at which the mercuric chloride and mercury forming complex ;ons with the chlorine are no longer reduced, the mercury in the exhaust gases is removed with a high yield and the corros;on of the apparatus is reduced.
From the foregoing description it will be seen that ;n accordance with the ;nvent;on, by virtue of the fact that the amount of addition of an oxid;z;ng agent is controlled in accordance with the oxidation-reduct;on potent;al or the COD value of the wash;ng l;qu;d, the invention is applicable to the wash;ng dev;ce of the harmful gas cleaner in newly-built waste ;nc;nerat;on processes as well as the existingones. Also, the fact that the mercuric chloride is dissolved into the wash;ng liquid in the washing device has the effect of simplify;ng the wash;ng l;quid treat;ng step and preventing the mercury from be;ng volatilized and scattered ~g47S9 into the atmosphere thereby contributing toward the preven-tion of environmental pollution.

Claims (3)

1. A method of removing mercury contained in exhaust gases from a waste incinerator comprising the steps of:
(a) providing smoke cleaning means for washing the exhaust gases emitted from said waste incinerator by contacting the same with a washing liquid;

(b) adding an oxidizing agent to said washing liquid to such extent that mercuric chloride and mercury forming complex ions with chlorine dissolved into said washing liquid by the washing of said exhaust gases are no longer reduced by reducing substances contained in said washing liquid;

(c) measuring at least one of an oxidation-reduction potential, COD value, chlorine concentration and oxidiz-ing agent concentration of said washing liquid, these objects to be measured being varied with the addition of said oxidizing agent, for measuring a concentration of chlorine in said exhaust gases after said washing, said concentration of chlorine being varied with the addition of said oxidizing agent;

(d) obtaining at least one of an oxidation-reduction potential, COD value, chlorine concentration and oxidizing agent concentration of said washing liquid in which said mercuric chloride and mercury forming complex ions with chlorine are not reduced, or obtaining a chlorine concentration of said exhaust gases after said washing; and (e) washing said exhaust gases with said washing liquid in which the amount of said oxidizing agent is controlled in accordance with the value of said at least one of an oxidation-reduction potential, COD value, chlorine concentra-tion and oxidizing agent concentration of said washing agent or the chlorine concentration of said exhaust gases after washing in such a manner that the value of said at least one parameter or the chlorine concentration of said exhaust gases after washing is in a range between one attained without addition of said oxidizing agent and another attained when said oxidizing agent is added so that the mercuric chloride and mercury forming complex ions with chlorine in said washing liquid are no longer reduced.

2. A method according to claim 1, wherein said oxidation-reduction potential is measured by measuring means comprising a composite electrode made of silver chloride and platinum or silver chloride and gold.

3. A method according to claim 1, wherein said oxidizing agent comprises a substance selected from the group consisting of hypochlorite and hydrogen peroxide.