CA2150529A1

CA2150529A1 - Method and apparatus for continuously separating mercury from flowing gases

Info

Publication number: CA2150529A1
Application number: CA 2150529
Authority: CA
Inventors: Volker Scherer; Monika Greczmiel
Original assignee: Patent Treuhand Gesellschaft fuer Elektrische Gluehlampen mbH
Current assignee: Osram GmbH
Priority date: 1994-06-28
Filing date: 1995-05-30
Publication date: 1995-12-29
Also published as: DE4422661A1

Abstract

A method and apparatus for the continuous separation of mercury from flowing gases, in particular from exhaust gases from thermal processes, e.g. melting of mercury containing glas shards from fluorescent lamps. At least one halogen, in particular iodine and/or bromine, is added to the gases when they flow into a reaction vessel. The halogen reacts with the mercury to form mercury halide, which is separated in the solid state from the gases prior to their outflow from the reaction vessel, for example by deposition on a cooled metal surface or by filtration.
Excess halogen is separated by means of adsorption. A regulating device is optionally provided which readjusts the feed rate of the halogen in such a way that a predetermined small mercury concentration is set in the outflowing gases, e.g. less than 0.2 mg per m3 of gas.

Description

IN THE UNITED STATES PATENT AND TRADEMARK OFFICE
"METHOD AND APPARATUS FOR CONTINUOUSLY
SEPARATING MERCURY FROM FLOWING GASES"
Reference to related patent and appllcation, the dlsclosures of whlch are hereby lncorporated by reference, U.S. 4,729,882, Ide et al.;
Internatlonal Appllcatlon PCT/EP 92/01557, publlshed WO
93/02773, U.S. designated, Kurzlnger et al.

Reference to related patent 8:
European 0 289 810 Al, Vogg;
German 41 40 969 Cl, Korner et al.
* * * * * * *
FIELD OF THE INVENTION.
The present lnventlon relates to an apparatus and method for contlnuously separatlng mercury from flowlng gases, ln partlcular from exhaust gases of thermal processes. Such gases arlse durlng operatlon of a glass meltlng lnstallatlon for fragmented mercury-contalnlng llghtbulbs, e.g. fluorescent lamps or metal hallde lamps.

BACKGROUND.
Conventlonally, ~ome mercury separatlon methods employ actlvated charcoal for blndlng the mercury. For example, the actlvated charcoal may be ln the form of fllters or screens, and 21~0S29 .
possibly used for pre-cleaning (for example Wo 93/02773, Kurzinger et al.). Since activated charcoal is highly flammable, its employment is limited to cleaning gases at an appropriately low temperature.
The following methods are suitable for higher gas temperatures. Lime, to which green coke has been added, is used as the reaction agent for binding mercury, see EP 0 289 810. The mercury-green coke-adsorbate being generated is separated in a precipitator and stored. A large amount of special waste is generated, which is disadvantageous for this type of method.
Another method involves bringing the mercury- containing gases into contact with precious metal halides. In such a method, the precious metals are reduced and the corresponding -mercury halide is separated. However, high costs are incurred because of the use of precious metals.
Some amalgam-forming methods are known. In the simplest case, a sheet metal plate forming an amalgam is disposed in the gas flow.~ The relatively small surface and the resultant short service times are disadvantageous. To avoid this problem, it is proposed in the German Patent Disclosure Document DE-OS 41 40 969 to apply amalgam-forming metals, for example gold, silver, copper, tin, zinc, palladium, iridium, or rhodium, on a substrate with a large inner specific surface (up to approximately 90 m2/g). However, the cost of these metallized substrates is high.
A method for cleaning mercury-containing gases from municipal waste incineration plants is disclosed in US Patent No.
4,729,882, Ide et al. In a first method step the mercury is converted to water-soluble mercury chloride by the addition of chlorine gas. The mercury is washed out of the gas in a subsequent wet-chemical method step by means of a washing solution. However, the high chemical aggressiveness of chlorine is disadvantageous. Furthermore, wet washing causes an additional pressure loss in the gas flow which is equivalent to an increased energy outlay.

-THE INVENTION.
It is an object of the invention to provide an efficient method and apparatus for separating mercury (Hg) from gases on a large industrial scale and so that the threshold values for mercury concentrations in exhaust air fall below those prescribed in the relevant regulations, while overcoming the previously mentioned disadvantages.
Briefly, at least one halogen X, in particular iodine and/or bromine, is admixed directly and continuously at a controllable feed rate to mercury-containing gas in order to thereby trigger a quantitative reaction of Hg + X2 - > HgX2. By means of this reaction the concentration of elementary mercury in the gas flow is directly reduced. The mercury halide(s) HgX2 generated in the course of the reaction is or are subsequently separated in the solid state from the gas flow, possibly together with further mercury compounds present in the gas flow.
Iodine is a preferred reactant, because it reacts well with mercury to form mercury iodide. It is furthermore advantageous in that as compared to chlorine and fluorine, iodine is clearly less chemically aggressive. In addition, mercury iodide has a lesser water solubility (approximately 0.006%) than mercury bromide (approximately 0.62%), mercury chloride (approximately 6.9%) and mercury fluoride (dissociates), as well as a relatively high evaporation temperature (approximately 354C). Because of this, the humidity and temperature of mercury-containing exhaust gases have a lesser effect on the separation of the mercury iodide than with the other mercury halides mentioned. In spite of this, however, it is possible in principle to employ halogens other than iodine as reactants, provided a reduced efficiency of the separation is acceptable.
It is also possible to use two or more different types of halogens simultaneously as reactants.
In a first embodiment of the invention, the halogen is -.

continuously supplied in manually preset amounts. Optionally, the feed rate of the halogen(s) is increased to such a degree that the gas flow shows an excess of halogen. In this manner, the elemental mercury can be completely converted to the greatest extent into mercury halide(s) and separated from the gas flow.
The excess halogen contained in the gas flow after the reaction is separated in a further method step.
In a second embodiment of the method, the mercury concentration is selectively and continuously measured following the reaction. The feed rate of the halogen(s) is readjusted as a function of the measured value in such a way that a preselectable threshold value of the mercury concentration occurs, particularly less than 0.2 mg mercury per m3 of exhaust gas. In a preferred embodiment of the method, the mercury concentration is adjusted to a value which, although lying below a prescribed threshold value, lies above zero. Overdosing with halogen is prevented in this manner, even with fluctuating mercury concentrations in the inflowing~gas. Accordingly, in this embodiment it is possible to omit the additional method step for separating excess halogen, which is generally required with the uncontrolled method. A
further advantage is the flexible adaptation of the supplied amount of halogen to fluctuating mercury concentrations. Halogen use is reduced to a minimum as well. This embodiment is particularly efficient.
A basic apparatus which carries out the method in accordance with the invention has a reaction vessel open on both ends, wherein the mercury-containing gases flow in through its first open end and flow out through its second open end. The flow can be created by thermal convection or by means of a blower. The reaction vessel is preferably pipe-shaped and the cross section can be arbitrary, for example circular or polygonal, e.g.
rectangular. The reaction vessel has a supply or feed device for supplying at least one halogen or for placing the halogen into the reaction vessel.
In addition, the apparatus provides a separating device, in which the mercury halide(s) can be separated from the gas flow in the solid state, possibly together with further mercury compounds present in the gas flow. In the process, the mercury halide either (a) cools to a temperature below the evaporation temperature in the separating device or (b) flows at an already appropriate temperature into the separating device. When employing two or more halogens, cooling is performed to below the lowest evaporation temperature of the mercury halides used. For example, for mercury iodide and for mercury bromide the evaporation temperatures are approximately 354C and 319C, respectively. Thus, if both halogens are used, the temperature must be lower than 319C.
In a preferred form of the first embodiment, a heat exchanger is used as the separating device. The advantage here is that by this step the energy balance is improved at the same time and in this way the efficiency of the entire system increased. In conventional installations the heat exchanger is primarily used in a manner known per se for the partial return of the process heat contained in the exhaust gas. For this purpose the heat exchanger has a large effective surface cooled by a fluid. The mercury halide(s) is (are) deposited thereon.
Because of this, the effective flow cross section of the heat exchanger is increasingly reduced and as a consequence the flow resistance is increased. If it exceeds a threshold value, the heat exchanger must be cleaned. The time when a change is due is determined by means of two pressure probes, one in the inflow and the other in the outflow of the heat exchanger. With the flow speed being constant, the pressure difference between the two probes, i.e., the pressure loss caused by the heat exchanger, is a direct measurement of the flow resistance.
In the second embodiment, a filter is used as the separating -unit, for example an electrostatic filter (E-filter) or a mechanical-one. The advantage of the E-filter is a reduced pressure loss and conse~uently a reduced additional energy outlay. However, a mechanical filter, for example a conventional fibrous or textile filter, is cheaper.
To assure intimate mixing and reaction of mercury and halogens, a minimum distance must be maintained between the region where the halogen(s) enter(s) and where the corresponding mercury halides are separated from the gas flow. In individual cases this depends on the respective flow conditions and the type of admixture of the halogen(s). In a conventional exhaust gas installation the minimum distance is six times the hydraulic diameter of the exhaust gas conduit. It may be advantageous to assist admixing by suitable elements for the generation of flow eddies disposed in the interior of the reaction vessel.
The apparatus may have a second separating device. By means of this second separating device, halogen, which may be in excess after the reaction, is separated from the gas chamber, for example by means of an adsorption agent.
Preliminary tests with iodine have shown that aluminum chips are well suited as an adsorption medium. With dry gases it was possible to achieve an iodine absorption of 60 percent by weight, and with moist gases, such as is the general case with fossil-fired thermal processes, an iodine absorption of 40 percent by weight, at least, related to the adsorption medium. In comparison therewith, a clearly lesser iodine absorption was measured in the case of zinc chips (with dry gases, only 15 percent by weight). Aluminum chips, such as occur as waste products in machining operations, for example, are suitable.
However, prior to their use as an adsorption agent, it is necessary to remove adhering lubricants which are often present.

In contrast, aluminum needles have not proven themselves useful because of the high apparent weight. Also, the air resistance of a corresponding filter is too great and the needles tend to adhere together, particularly with moist gases.
An apparatus for realizing the method in accordance with the invention with controlled halogen metering additionally has an Hg-measuring device for measuring the concentration of elemental mercury, as well as a control and regulating unit connected with the Hg-measuring device and acting on the feed device for the halogen. The measuring sensor of the Hg-measuring device is disposed at a distance downstream from the place where the halogen is added; the distance corresponds to at least six times the hydraulic diameter of the reaction vessel. This assures that at the place of measuring, the previously metered and supplied halogen has reacted essentially completely with the mercury and that therefore the actual remaining concentration of elemental mercury is measured.
The second separating device for separating excess halogen in the controlled apparatus may be omitted. The control unit preferably is adjusted in such a way that although the remaining concentration of mercury in the exhaust gas lies below a prescribed threshold value, it still lies above zero. In this way overdosing with halogen is impossible, i.e., the outflowing gas is sufficiently free of halogen even without an appropriate separating device.
DRAWINGS:
The invention will be explained below by means of some exemplary embodiments.
Fig. 1 is a schematic flow diagram of the method for the continuous separation of mercury from flowing gases in accordance with the invention. The additional method steps required for a controlled operation (second embodiment) are those framed by dashed lines.
Fig. 2 is schematic representation of an apparatus for 21~052~

executlng the directed embodlment of the method of Flg. 1.
Flg. 3 ls a schematlc representatlon of an apparatus for executlng the controlled embodlment of the method of Flg.l.
Flg. 4 ls a schematlc fragmentary view illustratlng the separatlng devlce in form of a heat exchanger.

DETAILED DESCRIPTION
Flg. 2 shows an apparatus 8 for performing the method of the flrst embodlment of the invention. The apparatus uses iodine as the reactant for the mercury and comprises the following components a tube-shaped reaction vessel 2 that ls open at both ends and has a circular cross sectlon, and a feed devlce 3-4-5 for lntroduclng the iodine lnto the reaction vessel. The feed device lncludes at least one reservoir 3, which contalns at least one halogen such as solld lodlne and whlch ls connected to a halogen holder 4, e.g. an evaporatlon trough, for charglng the holder 4 with the halogen. The holder is displaceable (e.g., in a dlrectlon transverse to that of the gas flow) vla a dlsplacement unlt 5 drlven by a motor M. The apparatus also lncludes wlthin the reaction vessel 2 a separation device 6, such as a flbrous or textlle fllter or an electrostatlc fllter 6a (Flg. 3), and an adsorptlon agent 7.
The feed device lntroduces at least one halogen lnto the reactlon vessel 2, so that a predetermlned amount of the approprlate halogen vapour ls admlxed wlth the gas flow. The evaporatlon holder 4 ls introduced into vessel 2 by drlvlng - 21S052~
the dlsplacement unlt 5, e.g. a supply belt, wlth the motor M.
The reservolr 3 ls external to the reactlon vessel 2; the temperature of the halogen wlthln the reservolr 3 may be approxlmately 30C whlch ls less than the temperature of the gas flow wlthln the reactlon vessel 2.
The reactlon vessel 2 ls-placed lnto an exhaust alr condult of a glass meltlng lnstallatlon (not shown) for mercury-contalnlng fragments of llght bulbs. Mercury-contalnlng gases flow through lt ln the dlrectlon of the arrow, whereln the gases flow ln through the flrst open end of the reactlon vessel 2 and, after havlng passed through the ad~oining reaction area 8, flow out through the second open end of the reactlon vessel, essentlally free of mercury.
The evaporatlon holder 4 of the feed devlce ls located ln the area of the first end of the reactlon vessel 2 and ls charged wlth solld iodlne from the reservolr 3. The feeding rate of gaseous lodlne to supply the gas flow corresponds to the evaporatlon rate of the lodlne and can therefore be affected by the temperature of the evaporation holder 4 and by the lodlne surface exposed lnslde the reactlon vessel, i.e., by the manually preselectable length of the partial area of the evaporatlon holder 4 lntroduced lnto the reactlon vessel 2. A heat exchanger 25 can be located upstream of the feed device lf the temperature of the Hg-laden gases entering vessel 2 ls excesslvely hlgh.
Because of the gas flow, the lndlvldual gas components are mlxed wlth each other relatlvely qulckly ln the ad~olnlng reactlon reglon 8 and the lodlne comblnes wlth the g 2 1 ~

mercury to form mercury lodlde.
Wlthln the reactlon area 8, whose length L
corresponds to approxlmately slx tlmes the dlameter D of the reactlon vessel 2, the reactlon vessel 2 may have a slmple and therefore cost-effectlve separatlon devlce 6 ln the form of a flbrous or textlle fllter, whlch fllters out the mercury lodlde. The lnstallatlon posltlon of the apparatus 1 ln relatlon to an exhaust gas condult (not shown) and the connectlon posltlon of the upstream heat exchanger 25, brlng the mercury lodlde flowlng through the vessel 2 and then through the flbrous or textlle fllter 6 to a temperature below the evaporatlon temperature of the lodlde (approxlmately 354C). The downstream-connected adsorptlon agent 7 ls made of alumlnum chlps at whose surfaces posslble excess lodlne ls adsorbed.

- 9a -The particular advantage of this apparatus lies in its simple and cost-efficient design. However, the evaporation rate and therefore also the metering of the iodine is possibly affected by the temperature of the gas flowing around the evaporation trough 4. In that case it is not possible to use very small metered amounts and it may be necessary to accept overdosing.
In a particularly simply designed and therefore cost-effective embodiment, the supply or feed device consists of a wide-meshed metal net. It extends preferably over the entire cross section of the reaction vessel and is partially covered with solid iodine. The gases flow around the iodine through the uncovered mesh of the metal net, heat it and generate the desired vapor pressure. This embodiment is suitable for uses which do not require the exact control of the feed rate of halogen.
Fig. 3 schematically illustrates an apparatus 9 for carrying out the method in accordance with the second embodiment of the invention. The basic design is similar to that in Fig. 2.
The flow direction has again been marked by arrows. The difference with respect to the embodiment of Fig. 2 lies in a changed feed device for the iodine, an additional regulating device and the omission of an adsorption agent.
The feed device uses a thermostat- controlled reservoir 11 of solid iodine 12, a suction line 13 provided with a blowback safety valve 14 and drying cartridge 15. The suction line 13 terminates in the reservoir 11 via a controllable pressure pump 16 and a continuous pressure line 17. A supply pipe 18 connects the reservoir 11 with the interior of the reaction vessel 10.
The pressure pump 16 sucks ambient air through the drying cartridge 15 and the suction line 13 and introduces it via the pressure line 17 into the reservoir 11. The reservoir 11 is located inside a thermostat-controlled housing 19 which keeps the iodine 12 at a temperature of approximately 30C. A suitably 2150~29 high vapor pressure of the iodine in the reservoir 11 is thereby created. The compressed air flows over the surface of the iodine 12 and is enriched with iodine vapor. The iodine-air mixture subsequently reaches the reaction vessel 10 through the supply pipe 18. The feed rate of the mixture can be controlled by means of controlling the flowing volume of the compressed air as well as the temperature and therefore the vapor pressure of the iodine 12.
The regulating device essentially includes an Hg-measuring device 20 for the selective measurement of the mercury concentration and a regulating device 21 connected therewith and controlling the controllable pressure pump 16. A measuring probe 22 of the Hg-measuring device 20 is disposed inside the reaction vessel 10 downstream of the supply pipe 18 at a distance A which approximately corresponds to six times the diameter D of the reaction vessel 10. A separation device 6 such as a fibrous or textile filter 6 (Fig. 2) or an electrostatic filter 6a (Fig. 3) is placed~downstream of the measuring probe 22.
Preliminary tests have shown that this embodiment can react rapidly to fluctuations in the mercury content of the gas flow, so that with a suitable setting of the control 20, 21, no iodine overdosing occurs. For this reason an adsorption agent for iodine was omitted here, in contrast to Fig. 2.
Fig. 3 also shows, highly schematically, the separating device in form of an electrostatic precipitator 6a, connected to a voltage source 6b, the other terminal of which is connected to the vessel 10.
Fig. 4 shows, highly schematically, the separating device in form of a heat exchanger 26, located in the stream of gas flow, for use if the temperature of the gas and/or of the mercury halide goes above the evaporation temperature of the halogen being used. Heat recovered can be re-used. Cooling fluid is connected to couplings 27, 28.

Varlous changes and modlflcatlons may be made, and any features descrlbed hereln ln connectlon wlth any one of the embodlments may be used wlth any of the others, wlthln the scope of the lnventlve concept.

Example of an oPeratlve embodlment:
Inslde dlameter D of the reactlon vessel 2, 10:
1.10 m temperature of ga~es at lnlet end of reactlon vessel 2, lO: 350C gas flow rate through the reactlon vessel: about 4.2 m3/sec temperature of gases ln reglon of lntroductlon of halogen (Flg. 2: 4I Flg. 3: 18): 150C rate of supply of halogen vapour from condult 18 (Fig. 3) ls such that the remalnlng concentratlon of mercury wlll be about 10 ~g/m3 temperature of gase~ wlth mercury halldes at separator 6, 6a, 26: 140C.

Claims

1. A method for continuously separating mercury from a gas flow, comprising the steps of:
adding a reactant to the gas flow to form a reaction product with the mercury and thereby separate the mercury from the gas flow, wherein, in accordance with the invention, the reactant is at least one halogen, reacting with the mercury in the gas to form a reaction product, the reaction product including at least one mercury halides converting the mercury halide into solid state; and separating the mercury halide from the gas flow.

2. The method of claim 1, wherein the step of converting includes bringing the at least one mercury halide into contact with a surface having a temperature below the evaporation temperature of the at least one mercury halide so that the at least one mercury halide deposits on this surface.

3. The method of claim 1, wherein the reaction product includes a plurality of mercury halides; and the step of converting includes cooling the mercury halides to a temperature below a temperature which is lower than the lowest one of the evaporation temperatures of the mercury halides.

4. The method of claim 1, wherein the separation of the mercury halide from the gas flow comprises electrostatic filtering.

5. The method of claim 1, wherein the separation of the mercury halide from the gas flow comprises mechanical filtering.

- 13a -

6. The method of claim 1, further comprising the steps of selectively measuring the mercury concentration in the gas, and readjusting the feed rate of the halogen as a function of the measured mercury concentration in such a way that a preselectable threshold value of the mercury concentration is set; and wherein the steps of selectively measuring and readjusting are carried out after the reaction product is formed.

7. The method of claim 6, wherein the preselectable threshold value is less than 0.2 mg of mercury per m3 of the gas.

8. The method of claim 1, further comprising the step of separating excess halogen from the gas flow.

9. The method of claim 8, wherein the step of separating excess halogen comprises an adsorption step.

10. The method of claim 1, wherein the at least one halogen includes an element selected from the group consisting of iodine and bromine.

11. An apparatus for continuously separating mercury from a gas flow, carrying out the method of claim 1, comprising a reaction vessel (2, 10) having a first open end through which the gas flows in and a second open end through which the gas flows out, said reaction vessel (2, 10) having a feed device (3-4, 11-17) in the vicinity of said first open end, for supplying and placing at least one halogen into the reaction vessel, and said reaction vessel further having a separating device (6, 6a, 26) in a vicinity of said second open end for separating mercury halide formed in the reaction vessel (2, 10) from the gas flow.

12. The apparatus of claim 11, wherein the separating device includes a heat exchanger (26).

13. The apparatus of claim 11, wherein the separating device includes an electrostatic filter (6a).

14. The apparatus of claim 11, wherein the separating device includes a mechanical filter (6), optionally a fibrous or textile filter.

15. The apparatus of claim 11, wherein the reaction vessel (2, 10) is substantially tubular in shape and the separating device and the feed device are disposed at a distance (A) from each other which corresponds to at least six times the hydraulic diameter (D) of the reaction vessel (2, 10).

16. The apparatus of claim 11, wherein said reaction vessel (2, 10) has an additional separating device (7) located in the vicinity of said second end, said additional separating device containing an adsorption agent for separating excess halogen from the gas flow.

17. The apparatus of claim 16, wherein the adsorption agent consists essentially of aluminum chips (7) when the halogen comprises essentially iodine.

18. The apparatus of claim 11, wherein the feed device includes at least one halogen reservoir (3) containing the halogen, and means (4, 5) for receiving the halogen from said reservoir (3) and for controllable feeding said at least one halogen into the reaction vessel (2), so that a predetermined amount of the halogen vapor emitted from said at least one halogen is admixed with the gas flow, for reaction with the mercury within said gas flow.

19. The apparatus of claim 11, wherein the feed device has at least one reservoir (11) containing the at least one halogen (12);
means (13-17) for introducing pressurized gas into the reservoir (11); and a halogen supply conduit (18) extending from the reservoir (11) into the reaction vessel (10) to provide a predetermined amount of the respective at least one halogen for admixture with the gas flow.

20. The apparatus of claim 19, wherein the pressurized gas introduction means includes a controlled element (16);
a halogen measuring device (20, 22) measuring the concentration of elemental mercury within the gas flow;
and a control circuit (21), responsive to the measuring device (20, 22) as a function of the concentration of elemental mercury, and controlling said controlled element (16) to thereby control the amount of halogen admixed to the gas flow.

21. The apparatus of claim 20, wherein the mercury measuring device (20, 22) has a measuring probe located in the the gas flow downstream of the supply conduit (18) where the at least one halogen is admixed into the gas, said probe being spaced from the conduit (18) by a distance (A) of at least six times the hydraulic diameter (D) of the reaction vessel (10).