DE19520127A1 - Quantitative and selective mercury@ sepn. from waste air flows e.g. from incineration plants - Google Patents

Abstract

Hg is selectively separated from gases by passing the gases over a carrier coated with metal salts.

Description

The invention relates to a method for selective Separation of mercury from gases, especially from Exhaust air flows.

Exhaust air streams from waste and special waste incineration, but also from numerous other industrial plants, can contain a variety of impurities in gas and dust-bound form, which must be removed for environmental reasons. The most common toxic pollutants are acidic gases such as SO₂, HCl, NO _x , heavy metals such as mercury, thallium, cadmium or lead, as well as organic compounds such as chlorinated dibenzofurans, dioxins, polycyclic aromatic hydrocarbons and chlorinated biphenyls. Because of its high volatility and toxicity, mercury occupies a special position among heavy metals because, unlike lead or cadmium, for example, it is converted almost quantitatively into the gas phase and thus into the exhaust air stream during thermal processes. Mercury is therefore an exhaust gas problem.

It is known to use different methods of mercury from the exhaust air from incinerators or other gases to remove. The known methods for removing the Mercury can be classified according to the different Sorption mechanisms or according to the physico-chemical Structure the properties of the materials used:

• - Adsorption on various types of carbon filters
  [J. Bewerunge, G. Ritter: Lecture on the occasion of the GVC conference, December 4-6, 1989 in Baden-Baden;
  B. Kassebohm, P. Asmuth, G. Wolfering: VGB Kraftwerkstechnik 69 (1989) 88-94]
• - Adsorption on impregnated activated carbon
  [Y. Matsumura: Atmospheric Environments. 8 (1974) 1321-1327;
  H. Krill, H. Wirth, G. Rittinger, V. Hohmann:
  German Offenlegungsschrift 26 03 807;
  R. Jockers, H. Ruppert, K. Keldenich:
  Exhaust air purification by mercury separation on sulfur-impregnated activated carbons, report by Bergbau Forschung GmbH, 1987;
  K. Bergk, F. Wolf, S. Eckert: Z. Chem. 17 (1977) 85-89]
• - dry washing process
  [H. Mosch: Energie Spektrum 12 (1987) 20-25;
  R. Wilder: Chem.-Techn. 13 (1984) 22-26]
• - wet washing
  [F. Siefert: Reports from the sewage technology. Association 37 (1987), wastewater engineering. St. Augustin Association, Stuttgart, ATV Annual General Meeting 1986;
  DO Reimann: Special substance problems in waste management, collection of papers in 1984, municipal waste disposal association Saar]
• - Adsorption on impregnated synthetic materials
  [Removal of mercury from stack gas, JP 56 026 533, March 14, 1981 Showa;
  DM Sheth: Indian J. Environ. Prot. 1 (1981) 26-30]
• - Selenium filter
  [B. Lindquist: Chemistry Environment Technology (1990) 93-97]
• - Deposition through amalgam formation
  [K. Fichtel: VDI Reports No. 429 (1982) 307;
  A. Keller: Sorption of gaseous mercury on metal-coated porous sorbents, series 15:
  Environmental protection technology No. 116, VDI Verlag, Düsseldorf 1993;
  W. Weisweiler: Offenlegungsschrift DE 41 40 969 A1 dated 12.12.1991, process for the separation and extraction of mercury and mercury compounds from gases].

The known methods have the following disadvantages:
Mercury can only be removed to a certain extent from the exhaust air by the known exhaust gas purification processes. However, the separation is not selective on the one hand, so that mercury accumulates elsewhere in the cleaning process, for example in the wet washing in the washing water and in the dry washing process in the solid filter residues. On the other hand, depending on the process used, up to 70% of the mercury passes through the cleaning stage and thus gets into the environment. There is generally no provision for the recovery of mercury.

A way to selectively separate mercury and to recover through thermal desorption Amalgam formation with metals. For this purpose Metal salts applied to porous supports and then reduced to metals in the hydrogen stream. With these supports coated with metal in this way can be coated with copper, Silver or gold achieved very good separation performance will. The reduction step is disadvantageous Hydrogen an additional preparation step, the should be left out.  

The invention relates to a method for selective Separation of mercury from gases, in particular Exhaust air flows, which is characterized in that one the gases over carriers coated with metal salts are leads.

Al₂O₃ / SiO₂ ceramics can be used as carriers. The carriers can have a porous structure.

Al₂O₃, in particular γ-Al₂O₃; TiO₂; SiO₂; Fe₂O₃; ZrO₂ and their compounds, in particular Silicates and aluminosilicates; Zeolites; Glasses as well as porous Glasses; also Ca and Mg compounds, especially oxides and carbonates.

Salts of the following cations can be used as metal salts are used: alkali, alkaline earth metals, Fe, Co, Ni, Cu, Zn, Ru, Rh, Pd, Ag, Ir, Pt, Au, Al, Ga as well Metal complexes of the metals Ag, Pd, Pt.

As metal salts can be salts with the following anions are used: halides (F⁻, Cl⁻, Br⁻, I⁻), sulfides, Selenides, sulfates, sulfites, phosphates, nitrates, nitrites, Carbonates, hydrogen carbonates.

Mixtures of at least two of the abovementioned can also be used Metal salts are used.

In particular, salts of the metals can be used as metal salts Copper, aluminum, zinc and / or calcium can be used. In particular, the nitrates, Chlorides and / or sulfates are used.

The coating of the support with metal salt can be preferred by impregnation with metal salt solutions. In a Another embodiment of the invention can be the coating also by physical vapor deposition like Vacuum evaporation or cathode sputtering (sputtering) or chemical vapor deposition take place.  

The amount of metal salt on the support can be 0.5 to 25 Mass% metal salt on the carrier.

The method according to the invention can advantageously be used for exhaust gases of any composition which contain mercury in an amount of 50 to 5,000 µg / Nm³. The method according to the invention can, for example, form the third cleaning stage after the acidic and basic washing stage. The temperature of the exhaust gas can be 20 to 250 ° C depending on the sorbent. The flow conditions in the exhaust gas RG = 1,000-50,000 h ^-1 .

The method according to the invention has the advantage that Mercury by means of the metal salt carrier can be removed quantitatively from the gases. The Reduction of the metal salts to the elementary metal by means of Hydrogen need not be carried out.

Examples

The apparatus used to carry out the method according to the invention ( FIG. 1) is designed so that the carrier is exposed to a gas stream which contains a constant mercury concentration in the range from 200 to 500 μg / m 3. Due to the high vapor pressure of the mercury, such concentrations can already be achieved by passing an air stream over a mercury bath at room temperature. The concentration of the mercury is measured quasi-continuously before and after the reactor (cein and caus) using an atomic absorption spectrometer [cold vapor technique (AAS)].

For the sorptive binding of mercury to metal salts basically three options conceivable. First of all, it can Mercury to be removed from the exhaust air is purely physisorptive to the carrier surface impregnated with metal salts  be bound. Second, there is a possibility that forms a kind of amalgam, for example

2 Hg + AgNO₃ → HgAg + Hg (NO₃) ₂ (1)

and third is a chemical reaction of the mercury from the gas phase with the metal salt on the Carrier surface conceivable, such as

CuCl₂ + Hg → Cu + HgCl₂ (2)

Zn (NO₃) ₂ + Hg → Zn + Hg (NO₃) ₂ (3)

If one calculates the standard reaction enthalpies ΔH _r from the standard formation enthalpies of the salts ΔH _f (Table 1), it is found that under normal conditions such reactions turn out to be strongly endothermic and therefore will not occur. However, if it is assumed that the salts have largely lost their water of crystallization at a temperature of 150 ° C., chemisorption on the support surface is conceivable, especially for the reaction of copper chloride with mercury (Table 2).

Table 1

Standard reaction enthalpies for the reaction of some salts with mercury under normal conditions

Table 2

Standard reaction enthalpies for the reaction of some dewatered salts with mercury under standard conditions

If the experimentally determined sorption curves of the various metal salts ( FIGS. 2 and 3) are compared with the calculated reaction enthalpies, the sorption performances falling from chloride to nitrate to sulfate verify the presumed chemisorption. If, on the other hand, the cations of the corresponding salts are varied, the discrepancies between sorption curves and reaction enthalpies do not allow one to speak of pure chemisorption; for example, aluminum salts sorb mercury better than copper and zinc salts, although according to the enthalpies they should show the worst deposition rates. The affinity of the mercury to the cations (amalgam effect) and transport phenomena in the pores of the carrier presumably play a not insignificant role here and thus contribute to the overall observed sorption performance of a metal salt.

To investigate the influence of the pore structure on the Adsorption performance four selected Al₂O₃ / SiO₂- Ceramics of the designation No. 1 to No. 4 examined.

With the help of mercury porosimetry, the proportion of the specific surface is measured in the respective pore size interval ( FIG. 4). The mercury porosimetry used here only detects pores with a smallest pore radius of 3.7 nm; the values obtained have a measurement error of approx. 10%. A comparison with BET measurements shows that there are smaller pores, which experience has shown to have a not insignificant influence on the adsorption performance; therefore, the results can only be interpreted with reservations. Comparing the sorption rate of the various Al₂O₃ / SiO₂ carriers ( Fig. 5), characterized by their specific surface area and broken down by pore radius distributions ( Fig. 4), there is a clear relationship between the proportion of pores in the range from 3.7 to 5 , 0 nm and the sorption rate. This means that the mercury sorption takes place mainly in this pore radius range. The narrower this pore radius range can be set, the more effective the mercury sorption will be.

The carriers No. 3 and No. 4 with a larger specific Show surface in the pore radius range 3.7 to 5.0 nm significantly higher mercury sorption rates than the samples No. 1 and No. 2. Only between carrier no. 1 and No. 2 occurs a slight discrepancy in the specific Surface that is most likely due to measurement inaccuracies can be returned; on the other hand is the one mentioned Range of the specific surface between 3.7 and 50 nm Pore size is not necessarily the sole determining factor in the sorption behavior towards mercury.

The dependence of the sorption performance on the temperature is determined, for example, on the Al₂O₃ / SiO₂ ceramic No. 4 impregnated with copper chloride, copper nitrate and copper sulfate, starting at room temperature up to a temperature of 600 ° C. ( FIG. 6).

The experimentally determined result that the Sorption curve of copper chloride at room temperature,  that of copper nitrate only at approx. 270 ° C and that of Copper sulfate is only sufficiently high at over 300 ° C Sorption rate shows again points to the dominant one Influence of chemisorption on mercury sorption Metal salts.

If the temperature of the reactor is increased above the maximum sorption capacity, desorption gradually begins. Sorption and desorption are in equilibrium and are dependent on the partial pressure of the mercury in the gas phase. With increasing temperature, the equilibrium shifts towards desorption, the sorption rate drops sharply ( Fig. 6).

If mercury is desorbed in a hot stream of nitrogen, a course as shown in FIG. 7 is found.

While coated with copper nitrate and copper sulfate Desorb atomic mercury from carriers copper chloride-containing surface of mercury oxide desorbed. The latter is expressed in one experiment yellow / red precipitate at the cold end of the reactor. After The three investigated sorbents desorbed Mercury again with almost the same sorption rates as on the first try.

Over a period of 7 days, the CuCl₂-containing carrier sorbed almost completely mercury from the <5 gas stream. If one assumes a mercury concentration of approx. 400 µm / m³ in the sample gas and a space velocity of 5000 h ^-1 corresponding to a volume flow of 55 l / h, the result is a low reaction rate of 10% between mercury and copper chloride (at a Carrier occupancy of 5.70 mmol CuCl₂ this corresponds to a possible mercury sorption of 0.57 mmol) a hypothetical service life of the carrier of approx. 5600 operating hours ( Fig. 8):

The sorption experiments show that mercury can be Coating the Al₂O₃ / SiO₂ carrier used certain metal salts almost completely from exhaust air can be sorbed. While the chlorides are mercury sorb very well even at room temperature Nitrates from 250 ° C and the sulfates only from 330 ° C good Sorption performance. An explanation for this deliver different reactivities of the salts largely the same graded standard Reaction enthalpies of an assumed chemisorption. Discrepancies between determined types of separation and calculated enthalpy values can be explained that there is no pure chemisorption, but also that Affinity of mercury to amalgam-forming metals or metal ions influences the sorption. The strong one The influence of chemisorption confirms in particular that different reactivities of chlorides, nitrates and Sulphates when choosing the sorption temperature. All in all very good sorption performances show AgNO₃, CuCl₂ and AlCl₃; as well are those of Cu (NO₃) ₂, Al (NO₃) ₃ and CaCl₂ to classify.

A further improvement in mercury sorption can can be achieved through a suitable choice of carrier materials. The larger the specific surface in the pore radius area fails from 3.7 to 5.0 nm, the higher the deposition rate can be achieved with the coated ceramics.

If the temperature of the adsorbent is increased, it starts at 470 ° C the desorption of mercury. For example with Copper nitrate and copper sulfate coated carriers increased then the mercury content in the sample gas becomes strong. The metal can be recovered by condensing. Porters who are coated with copper chloride, do not desorb  Mercury, but mercury oxide. That I Mercury oxide precipitates at the cold end of the reactor this with much less technical effort be recovered.

The results of the tests are in the figures represented graphically. Show it

Fig. 1 shows the structure of the measuring apparatus;

. Figure 2 shows the sorption of various copper salts of mercury, gas temperature 150 ° C, gas flow rate / bulk volume of the sorbent [1 / h] = space velocity RG = 4.500 h ^-1;

FIG. 3 shows the sorption behavior of different metal nitrates for mercury, the gas temperature is 150 ° C, RG = 4.500 h ^-1;

Figure 4 shows the pore structure of Al₂O₃ / SiO₂ ceramics No. 1 to 4 after impregnation with aluminum nitrate.

Figure 5 shows the sorption rates of the aluminum nitrate coated Al₂O₃ / SiO₂ ceramics No. 1 to No. 4, gas temperature 150 ° C, RG = 4,500 h ^-1 ;

Fig. 6 shows the sorption behavior of the impregnated with copper salts Al₂O₃ / SiO₂ carrier No. 4 with variation of the adsorption temperature, RG = 4,500 h ^-1 , measuring time at the respective temperature setting 20 min;

FIG. 7 shows the desorption of mercury from the coated copper nitrate Carrier No. 4, gas temperature after approximately 15 min to 600 ° C, RG = 4.500 h ^-1.

Figure 8 shows the hypothetical service life of the support containing copper chloride No. 4 at a reaction rate Hg / CuCl₂ of 10%. c _Hg : 370 µg / m³, gas temperature 150 ° C, RG 5,000 h ^-1 .

Claims (1)

Process for the selective separation of mercury from gases, in particular exhaust air streams, characterized in that the gases containing mercury are passed over supports which are coated with metal salt.