WO2010009803A1

WO2010009803A1 - Method for precipitating mercury or compounds thereof from exhaust gases

Info

Publication number: WO2010009803A1
Application number: PCT/EP2009/004831
Authority: WO
Inventors: Rico Kanefke; Uwe Listner; Volker Damrath
Original assignee: Currenta Gmbh & Co. Ohg; Bayer Materialscience Ag
Priority date: 2008-07-19
Filing date: 2009-07-04
Publication date: 2010-01-28

Abstract

The present invention relates to a method for precipitating elemental mercury and mercury compounds from different process or room exhaust gases, particularly from chlorine-alkali electrolysis, comprising the following steps: a) discharging an exhaust gas flow comprising mercury into a reaction chamber, b) oxidizing the elemental mercury species in the discharged exhaust gas flow using at least one gaseous halogen to form mercury-(II)-halides, c) separating the formed mercury-(II)-halides from the oxidized exhaust gas flow formed in step b) in a washer unit, while at the same time removing the residues of the gaseous halogen used for the oxidation and in the presence of a reduction agent such as sodium thiosulfate, and d) discharging the purified air flow and the waste water containing mercury.

Description

Process for the separation of mercury or its compounds from venting

The present invention relates to a method for the separation of elemental mercury and mercury compounds from various process or Rauma, especially from the chlorine-alkali electrolysis.

Exhaling processes or rooms or halls may contain mercury and / or its compounds through direct or indirect handling in the production process. This is the case, for example, in the chlor-alkali electrolysis according to the amalgam method or in dental laboratories. Depending on the underlying process, these flash offs can occur as point sources or as diffuse sources and also contain amounts of other constituents.

The mercury pollution in the air results in an increased mercury pollution for persons staying in it. The mercury levels are usually not so high that acute poisoning occurs. However, mercury also has a chronic toxic effect. Once absorbed into the body, it remains there for a long time, i. it is accumulated. In order to reduce health risks, occupational exposure limits are prescribed in many countries, and due to the accumulation of even the smallest quantities in the body, a minimization requirement should be better applied - similar to that for carcinogenic substances.

To lower the mercury concentration in the room air, the air must either be directly sucked off and cleaned at a point source, or a sufficiently large air change must be realized in the entire room.

The extracted mercury-containing air must be cleaned before discharge into the environment, since for example in the TA Luft in Germany a minimization requirement according to the prior art is given. Currently, this cleaning is usually done by Aktivkohlefϊlter. In particular, in large exhaust air streams and in the case of elemental mercury large apparatuses are required for this because of poor adsorbability, which lead to high operating costs. In addition, so-called "hot spots" can form in the activated carbon filter, which can lead to a (smoldering) fire.Therefore, a very high expenditure with regard to the fire protection must be operated and, if necessary, a CO monitoring for the detection of fire or embers must be installed Another disadvantage of common processes is the large amount of garbage, and good activated carbons can produce up to 4% by mass Adsorb mercury before they are saturated with mercury. This results in a waste mass that is at least 24 times higher than the actual mercury mass. Taking into account the respective densities (activated carbon: 600-800 kg / m ³ , Hg = 13.55 kg / 1), the waste volume is even 400 times the actual mercury volume. These large quantities of loaded refuse charcoal are usually spent in underground storage.

DE 44 22 661 Al describes a process for the continuous separation of mercury from flowing gases, wherein the elemental mercury is oxidized by means of a halogen, in particular bromine or iodine, continuously to mercury halides. These mercury halides are separated from the gas stream in a further step of the process. The control of the oxidation step is carried out by measuring the mercury concentration following the reaction and readjustment of the feed rate of the halogen (s). The mercury halides formed are separated from the gas stream by physical or chemical means. In the first case, the gas stream is cooled to a temperature below the vaporization temperature of the mercury halides; the used separator is a heat exchanger or a filter. Alternatively, a wet chemical scrubber may be used as a separator. However, the corresponding washing process is not described in detail. Preferably iodine or bromine are used as halogens because of their low water solubility and relatively high evaporation temperature.

H. Braun et al. (Chem.-Ing.-Techn. 60 (1988) No. 2, p. 135) report that, despite oxidation and precipitation of the resulting HgCl ₂ in a wet scrubber, there are still high levels of elemental (metallic) mercury in the clean gas. In the wet scrubber, however, the absorbed mercury is present as slightly soluble HgCl ₂ . H. Braun et al. Therefore, these results lead to dissolved sulfur dioxide in the wash water, which leads to the formation of Hg (I) and subsequently disproportionates to Hg (U) and Hg (0). H. Braun et al. propose an improvement in the separation of mercury from flue gases of waste incineration by means of a venturi scrubber. In addition, the addition of oxidizing substances in the flue gas scrubbing water should cause a significant reduction of the metallic discharge.

H. Braun et al. also teach "Other measures to optimize the operation of the scrubber, such as changes in differential pressure (1), Flue gas / Vapor ratio (2), pH (3), chloride content (4) and temperature reduction (5) do not lead to even better mercury reduction ".

Thus, there is still a need for an effective and at the same time cost-effective method for removing mercury from exhaust air.

The present invention is therefore based on the object to provide a method for reliable removal of mercury compounds available, which ensures the lowest or lowest operating costs and significantly reduced amounts of waste the same or better deposition of mercury than the prior art.

Surprisingly, it has now been found that this problem is solved by a combined oxidation-AVEscherverfahren which can be controlled automatically and / or continuously based on various process parameters.

The present invention therefore provides a process for the separation of elemental mercury [Hg (el) or Hg (O)] and mercury compounds from exhaust air, comprising the following steps:

a) discharge of a mercury-containing exhaust air stream into a reaction chamber,

b) oxidation of the elemental mercury species in the derived exhaust air stream with at least one gaseous halogen to form mercury (II) halides,

c) separation of the mercury (II) halides formed from the oxidized waste air stream formed in step b) in a scrubber unit with simultaneous removal of residues of the gaseous halogen used for the oxidation and

d) Derivation of the purified air stream and the mercury-containing wastewater.

The extraction of Hg (el) -containing exhaust air can take place directly at the emission source or from the room containing this emission source.

In one embodiment of the invention, which is illustrated in FIG. 1, the Hg (el) -containing exhaust air (All) is first sucked off, for example, by means of a suction draft (3) and fed to a reaction chamber (1). There the mercury gets through by little Concentrations of gaseous halogen, preferably bromine or chlorine or a mixture thereof, more preferably chlorine at temperatures of less than 500 ⁰ C, preferably at temperatures of 20 ⁰ C to 100 ⁰ C, in water-soluble mercury (II) halides, in particular mercury (II) chloride converted according to equation [I].

[l] Hg ^el + Cl ₂ «-» HgCl ₂

Preferably, the addition of the gaseous halide is adjusted to the Hg (el) concentration and controlled automatically and / or continuously.

Preferably, to control the addition of the gaseous halogen, the Hg (el) concentration is already in the Hg (el) -containing exhaust air stream (All), i. before addition of halogen in the reaction chamber (1), measured by means of a measuring device. Especially preferred are continuous measuring devices, such as e.g. the Hg-Monitor 3000 from Seefelder Messtechnik GmbH or Mercury Vapor Monitor VM 3000 from Mercury Instruments GmbH.

On the other hand, continuous measurements of mercury by means of cold vapor atomic absorption spectroscopy in a halogen-containing (eg chlorine-containing) exhaust gas stream lead to incorrect measurements, which makes targeted control of the oxidation impossible.

In order to achieve a complete, preferably complete oxidation of the elemental mercury by the addition of the halogen, a mass ratio of halogen to mercury of 0.35 to 10,000, preferably 20 to 500, particularly preferably 20 to 100 is usually used. If the halogen is added in large excess, it does not adversely affect the oxidation, but this should be avoided for cost reasons.

By means of the aforementioned continuous mercury measurement, the extracted amount of exhaust air can also be varied in order to achieve the desired exhaust air concentration in the space to be extracted.

In a particular embodiment of the method according to the invention, therefore, the suctioned volume flow is adjusted on the basis of predetermined upper and lower limit values in the space to be suctioned off. If the value in the room to be extracted exceeds the upper limit value, the exhaust air volume flow is increased to comply with the specifications. In return, falls below the lower limit, the Suction - also for reasons of energy saving - throttled. The lower and upper limits can also be identical. This ensures a constant Hg concentration and may even turn off the ventilation of the room to be extracted if the Hg concentration in the exhaust air is less than the limit.

As a further variant, it is also possible to work with a fixed number of air changes in the space to be sucked off, whereby the Hg concentration in the exhaust air flow then varies, but for this the exhaust air volume flow remains constant.

Also possible is a combination of the two variants described above, i. The space to be extracted is normally vented with a continuous exhaust air flow (constant air exchange rate), which is temporarily increased with increased Hg concentration in the room to be extracted.

The reaction chamber (1) leaves an exhaust air stream containing oxidized mercury (for example HgCl ₂ ) (A12). This exhaust air stream A12 also still contains residues of the added halogen (for example chlorine), since this is preferably added in excess.

In the reaction chamber (1) downstream scrubber unit (2) of the exhaust air stream A12 is simultaneously freed of excess halogen and mercury (II) halide.

As a scrubber unit or wet scrubber, in particular a packed column scrubber is advantageous because of its low operating costs and good separation efficiency.

The wash solution present in the scrubber unit contains one or more halides, in particular chlorides and / or bromides, and at least one reducing agent.

Targeted control of the wash solution composition is advantageous. Divalent mercury halides are physically dissolved in water and have a vapor pressure over the solution. This vapor pressure limits the minimum achievable Hg concentration in the withdrawing clean gas flow (A13). This vapor pressure is lowered significantly when complexing occurs in the wash solution of mercury takes place. The complexation is carried out, for example, and preferably by halides (preferably bromides or chlorides). Suitable sources of these halides are all customary salts, in particular sodium chloride and sodium bromide

The temperature of the washing solution is usually in the range from 4 to 90 ^° C., preferably from 10 to 30 ^° C., more preferably at room temperature or in open-air conditions at ambient temperature (but frost-free).

For the achievable minimum clean gas concentration of oxidized mercury, the following empirical relationship [2] was found:

[2]

B

'Hgox gas = A - (I - S / 213,15) - e (5 + 273,15). C c

Hgox _ liquid _ total (10 * - + D)

wherein c'Η _gox ^ _as the mimmal achievable clean gas concentration in g / m ³ and cΗ _g oχ_ _fl lockingly i _n $ total in the scrubber, the total dissolved mercury (π) concentration m mg / is 1 and AD smd different adaptation factors in Depending on the mercury Halogemd system must be determined experimentally. For some systems, the experimentally determined values are given in Tables 1 and 2 below. Temperature & stands for the temperature of the wash solution at ⁰ ° C.

Table 1: Experimentally determined adaptation coefficients A and B for the equation (3-10) (independent of the additional ligand present for complexation)

Table 2: Experimentally determined adaptation coefficients C and D for the equation (3-10) (independent of the mercury (H) compound to be complexed)

By way of example the resulting Hg are indicated concentrations of the pure gas in Figure 3 for the system HgCl ₂ / Cl- / H ₂ O and 30 ⁰ C scrubber temperature.

In the case where chloride is used as the halide in the washing solution, the chloride concentration is usually more than 35 g / L, preferably more than 50 g / L.

The higher the chloride concentration in the wash solution, the lower the

Pure gas concentrations of mercury. The chloride concentration should therefore preferably be controlled to a constant high value as a function of the specifications.

For this purpose, the chloride concentration can be determined either via a chloride-selective electrode or via a combined conductivity and pH measurement in the solution, cf. Measurement QIA in Figure 1 and Figure 4. The values determined here are the

Basis of continuous control.

For automatic control of the chloride concentration in the washing solution, preferably a fresh water addition (P3) and wastewater discharge (P6), as well as an addition of possibly missing chloride in the form of a NaCl solution addition (P5) are used.

Simultaneously with the deposition of the mercury chloride, the excess of halogen, in particular chlorine, introduced in the reaction chamber is deposited. This is done by reducing the halogen with at least one reducing agent present in the washing solution. In principle, all known soluble reducing agents are suitable. Particularly preferred is sodium thiosulfate.

In the case of using chlorine as the halogen and sodium thiosulphate as the reducing agent, a reaction according to equation [3] is carried out, hydrogen chloride and sodium hydrogen sulfate being formed as further reaction products: [3] Na ₂ S ₂ O ₃ + 4Cl ₂ + 5H ₂ O → 2 NaHSO ₄ + 8HCl

The reduction of the excess halogen results in a basic concentration of halide, in particular chloride.

Preferably, the sodium thiosulfate addition (P3) is also controlled automatically

By a preferably carried out pH-controlled addition of base, in particular sodium hydroxide solution (P4) is formed by their reaction with the further reaction products of hydrogen chloride and sodium hydrogen sulfate sodium chloride and sodium sulfate. The pH is usually maintained in the range of 5-8.

The mercury-containing wastewater (P6) can be introduced into a downstream wash water treatment or treatment plant, where it is freed, for example by precipitation of mercury. The mercury filter cake is usually deposited underground, with the volume and mass of the waste only 2-5% of the amount of waste, which is obtained in an activated carbon filter is.

The discharged into the environment exhaust air (withdrawing clean gas) (A13) is preferably monitored by a continuous mercury meter and a continuous chlorine meter, which - integrated for Störgrößenkompensation in the scrubber control - in case of breakdowns of mercury or chlorine due to e.g. sudden changes in concentration adjust the scrubber parameters.

The invention will be explained in more detail with reference to diagrams and figures.

Figure 1 shows the exemplary structure of the deposition process according to the invention in the form of an R + I scheme. From the continuous measuring systems, control lines run to the corresponding actuators (control valves for addition and wastewater discharge and the room air blower). Reference numerals:

I Hg ^el- containing exhaust air

π Purified exhaust air

IE discharge

Pl: Chlorine gas / halogen gas addition

P2: adding fresh water

P3: Sodium thiosulfate addition

P4: Sodium hydroxide addition

P5: Sodium chloride addition

P6: wastewater discharge

All: exhaust air flow (Hg (el) -containing)

A12: exhaust air stream (Hg (ox) -containing, in particular HgCl ₂ -containing)

A13: withdrawing clean gas flow

(1) reaction chamber for the reaction of Hg ^el with Cl ₂

(2) scrubber unit

(3) Blower for removing the contaminated air

(4) scrubber circulation pump

QlAl mercury measurement

QIA2 Measurement of chloride or halide concentration in the wash solution by conductivity (Lf) and / or pH.

Q1A3 mercury measurement

Q1A4 halogen measurement

M control element (fan motor) Figure 2 shows the function for complete oxidation of elemental mercury by chlorine (Cl ₂ ). In order to achieve a complete oxidation of elemental mercury, depending on the Hg (el) concentration, various excesses of chlorine are necessary. The control function is indicated.

Figure 3 shows a state diagram for the clean gas concentration (calculated according to the empirically determined equation 2) for the scrubber system HgC12 / chloride / H2O is applied at a scrubber temperature of 30 ⁰ C. The Hg (II) concentration in the exhaust air flow (A13) ( Ordinate) as a function of the chloride concentration in the scrubber (abscissa). With increasing chloride concentration in the scrubber, the clean gas concentration in the exhaust air stream A13 decreases. The clean gas concentration is also dependent on the concentration of dissolved mercury (Hg (π) aq) contained in the scrubber cycle. This is regulated by the fresh water addition (P2) and the wastewater discharge (P6). The higher the dissolved mercury concentration, the higher the concentration in the evacuating clean gas of the exhaust air stream A13.

FIG. 4 shows the conductivity of a solution as a function of the chloride content in mol / l. It is irrelevant whether the chloride concentration results from sodium chloride, calcium chloride or from an aqueous hydrochloric acid solution. One mole of chloride in one liter of water has a conductivity of 76.374 mS / cm. With commercially available conductivity probes, however, only the conductivity of the entirety of the ions can be measured. The other ions (eg Na +, Ca ²⁺ , H ₃ O ⁺ , OH) must be calculated out beforehand so that the correct chloride concentration in the scrubber can be determined and regulated. For example, the pH value measurement is used to determine the concentration of hydronium or hydroxide ions in the scrubber water and their conductivity (eg hydronium ions: y = 13.603x3 - 63.001x2 - 46.58 lx + 349.8 with y = conductivity in mS / cm and x = molar concentration) subtracted from the total conductivity.

Example:

From a room X (0, I m ³ - 0.2 m height / 0.5 m width / I m length), a Hg ^el - containing exhaust air stream with a concentration of elemental mercury of 5Oμg / m3 with a volume flow of ImVh was continuous derived. The

Concentration of the Hg ^el was by means of a device of the type "HM 1400 wet" the company

DURAG / Verewa determined and forwarded to control the addition of chlorine in the reaction chamber to a mass flow controller. At an excess of 167 μg C12 / μgHg, complete oxidation of Hg in the reaction chamber was achieved. Complete chlorination was also achieved for all excesses> 167 μg C12 / μgHg.

The mercury (H) halide-containing exhaust air stream was introduced into a scrubber unit downstream of the reaction chamber into a halide-containing scrubbing solution (consisting of NaCl, sodium thiosulfate 5 g / l, water at room temperature). The pH was maintained at 7-8 with dilute sodium hydroxide solution (5%). The chloride concentration of the washing solution was determined by means of a chloride-selective measuring probe from IRAS, Hermsdorf and adjusted to 50 g / l by automatically controlled addition of a 3 molar NaCl solution into the washing solution. The optimal complexation of Hg (H) was achieved from a concentration of 50 g / l chloride.

Under these conditions, a concentration of Hg (II) in the clean gas of a maximum of 3 μg / m ^{3 was} achieved.

The concentrations of Hg ^el , required chlorine excess and chlorine concentrations in the washing solution were varied. The results of these experiments are shown by way of example in FIGS. 2 and 3.

Claims

claims:

1. A method for the separation of elemental mercury and mercury compounds from exhaust air comprising the following steps:

a. Derivation of a mercury-containing exhaust air stream in a reaction chamber,

b. Oxidation of the elemental mercury species in the discharged exhaust air stream in the reaction chamber with at least one gaseous halogen to form mercury (II) halides,

c. Separating the mercury (II) halides formed from the oxidized waste air stream formed in step b) in a scrubber unit with a halide-containing scrubbing solution with simultaneous removal of residues of the gaseous halogen used for the oxidation and

d. Derivation of the purified air stream and the mercury-containing wastewater.

2. A process according to claim 1, wherein the mass ratio of halogen to mercury in the reaction chamber is from 0.35 to 10,000.

3. The method according to claim 1 or 2, wherein the temperature in the reaction chamber is less than 500 ⁰ C, preferably from 20 ⁰ C to 100 ⁰ C.

4. λ'erfahren according to any one of the preceding claims, wherein the addition of the gaseous halogen is adapted to the mercury concentration of the exhaust air and this concentration is determined before addition of halogen in the reaction chamber by means of a measuring device.

A process according to any one of the preceding claims, wherein the halogen is chlorine.

6. The method according to any one of claims 1 to 5, wherein as the halide in the halide-containing washing solution, chloride is used and the chloride concentration is more than 35 g / l.

7. The method according to claim 6, wherein in the halide-containing washing solution is additionally contained a reducing agent.

8. The method of claim 7, wherein the reducing agent Natriumtm ^'is osulfat.

9. The method according to any one of claims 1 to 6, wherein the temperature of the halide-containing washing solution in the range of 4 to 90 ⁰ C, preferably 10 to 30 ⁰ C.

A process according to claim 9 wherein the pH of the halide-containing wash solution is maintained in the range of 5-8.