DE3816599C1 - Process for eliminating arsenic compounds from exhaust gases - Google Patents

Abstract

As a sorbent for eliminating arsenic compounds from exhaust gases, activated charcoal is known which can only be used up to exhaust gas temperatures of 200@C. The novel process is to be applied over a broad temperature range, in particular above 200@C. The arsenic elimination from exhaust gases is carried out with a sorbent based on high-surface-area titanium dioxide, that is even at exhaust gas temperatures of above 200@C.

Description

The invention relates to a method for the removal of arsenic compounds from exhaust gases.

Emissions of arsenic compounds occur in the smelting of many ores and in the flue gas of coal power plants. It is known that a Removal of arsenic oxide from an ore resulting from ore smelting Exhaust gas by adsorption on different materials, particularly advantageous on activated carbon, is possible. The adsorption typically takes place at 200 ° C (Environmental Science & Technology, Vol. 13, No. 1, Jan. 1987, p. 93-97). Only for activated carbon as a sorbent is also the regeneration by heating in nitrogen flow at 400 ° C described.

However, this method of arsenic removal is only in one narrow temperature range, since between 200 and 400 ° C the Desorption begins to predominate. This means activated carbon as Sorbent arsenic removal from hot exhaust gases, such. B. from hot Flue gases from coal power plants, can not be used.

Furthermore, it is still in the use of activated carbon as a sorbent necessary for the thermal regeneration of the same due to their easy flammability to use expensive nitrogen as purge gas. In addition, the high energy costs have to heat up for the regeneration as well as the long regeneration time - are typical 24 hours - and you are forced to do that To deposit arsenic oxide from the purge gas.

The invention is therefore based on the object, a method for removal to develop from arsenic compounds from exhaust gases that over a wide temperature range, in particular over 200 ° C, apply is.  

It has now surprisingly been found that arsenic oxide on a Sorbent is sorbed on the basis of high-surface titanium dioxide, and even at exhaust gas temperatures of over 200 ° C.

The subject of the present invention is therefore a method for removal of arsenic compounds from exhaust gases, characterized is that the arsenic-containing exhaust gas via a sorbent based on surface-rich Titanium dioxide is passed, in the case of non-oxidic Arsenic compounds preceded by the Sorbent Abgasnachverbrennung becomes.

The inventive method is equally suitable for the already known application area with exhaust gas temperatures of less than 200 ° C as Also for use with hotter exhaust gases with temperatures of over 200 ° C, d. H. in the latter case, the exhaust gas is at a temperature passed over the sorbent at over 200 ° C. For example, the amount Temperatures of exhaust gases from power plants in front of the electrostatic precipitator approx. 300 up to 450 ° C. For this range of higher exhaust gas temperatures of over 200 ° C has not existed any technically useful process.

The method according to the invention is preferably suitable for removal Of arsenic oxides from exhaust gases, but also generally for the removal of non-oxidic Arsenic compounds, it is applicable by adding the sorbent an exhaust gas afterburning upstream, in which the arsenic compounds be converted into oxides.

Suitable sorbents are, for example, those surface-rich Titanium dioxide types, as they are made in known manner from Ortho- or metatitanic acid by drying or by drying with the following Calcining produces. Also pyrogenic titanium dioxide is suitable. Such surface rich titanium dioxide types exist for most, preferably practically complete, of Anatas. Preferably one starts from a metatitanic acid suspension, as in the Sulfate process of titanium dioxide production is obtained as an intermediate.  

This starting material still contains 6 to 10 wt .-% sulfuric acid, based on titanium dioxide, either remaining in the material or by basic laundry partially or largely removed or by Addition of a barium salt in the sparingly soluble BaSO₄ is transferred.

The metatitanic acid is dried, if appropriate after filtration, preferably in the temperature range of 100 to 200 ° C; the drying can without Limitation to a particular process, such as hordes, in the paddle dryer, in the rotary tube or by means of a spray dryer. Subsequent calcination is at 300 to 800 ° C, preferably at 400 to 600 ° C, for example on trays, in a fluidized bed or in Rotary tube performed. The duration of calcination is typically between 0.5 and 12 hours, in the preferred range of temperature between 2 and 8 hours. Drying and calcination can also be done in be summarized in one operation.

Furthermore, the sorbent, for example, additionally Zirkenoxid and / or clay and / or silica and / or aluminosilicates and / or Containing alumina. Alumina as an additive is preferably used in Absence strongly acidic gas components used.

Such sorbents can be obtained by co-precipitation of suitable precursors, z. As sulfates, oxysulfates, chlorides, alkoxides or gels, with subsequent thermal treatment can be made, as for example DE-OS 34 38 367 describes.

Before or after calcining the raw material for the sorbent can an addition of or treatment with an acid, e.g. Saltpeter, Sulfuric, acetic or oxalic acid, carried to hardness and cohesion to improve the material. In general, it is necessary that calcine calcined material before further processing.

The sorbent can be used in powder form. Preferably it will to tablets, extrudates, moldings, such as rings or saddles, molded, or it is extruded into honeycomb bodies. For this purpose, it is necessary to use processing aids or plasticizing auxiliaries, such as water, higher alcohols, methylcellulose,  Polyalkylene oxides, oils and the like a. m. add. Shares of heat resistant Fibers of z. As glass, ceramic, asbestos, aramid and / or additives of clay, silica sol and / or metatitanic acid improve the mechanical Strength of the final product.

The molded bodies are in turn at room temperature to about Dried 200 ° C and calcined at temperatures of 300 to 650 ° C. Alternatively, the sorbent can also be on supports such as metal or ceramic balls or other shaped bodies, ceramic honeycomb or metal plates applied become.

The sorbent may additionally contain transition metal oxides, e.g. B. such as those in catalysts for nitrogen oxide reduction after the SCR (selective catalytic reduction) methods are present.

By way of example, the oxides are here as additional transition metal oxides of vanadium and / or molybdenum and / or tungsten.

The above manufacturing characteristics for the inventively used Sorbents based on surface-rich titanium dioxide are in themselves of Process for the preparation of catalysts known forth, for. B. by the DE-PS 24 58 888, DE-PS 25 54 198, DE-PS 26 58 569 or DE-PS 29 27 246.

When arsenic removal from dusty exhaust gases is the use of the sorbent in the form of coated metal plates or nets or coated expanded metal or in the form of extruded honeycomb bodies preferred, since all these forms pass the dust largely unhindered to let. In addition, they only cause a small pressure drop.

By a suitable choice of the manufacturing conditions, the sorbent can be customize special purpose. So lead higher calcination temperatures and times, acid treatment or those mentioned above Strength improvers for good mechanical strength regarding Pressure and abrasion resistance of the sorbent. If you renounce the straight On the other hand, a softer, d. H. fewer mechanically loadable, but more surface-rich sorbent with larger Arsenic absorption capacity.  

The inventively used sorbent based on surface-rich Titanium dioxide has a BET surface area of 40 to 300 m³ / g. typical Powder values range from 150 to 300 m² / g, uncalcined Powder pressed and calcined at low to medium temperature Moldings at 100 to 200 m² / g and for medium to high temperature calcined plate and honeycomb forms, preferably made from precalcined powder, at 40 to 150 m² / g, preferably at 60 to 120 m² / g.

The inventive method can be equally in the temperature range but also above 200 ° C, but has special Advantages in the range above 200 ° C; where the sorbents according to the state of Technology can no longer be used. The upper temperature limit becomes due to the tendency of titanium dioxide, under strong thermal Stress on BET surface to lose and finally the surface poor Modification rutile form. This inclination also depends on the exposure time of the temperature and of stabilizing foreign components, such as As sulfates or destabilizing, such as. B. Vanadium oxides and rutile traces from. Pure or stabilized titanium dioxide tolerates temperatures up to 650 ° C, for a few hours to 800 ° C. Typical operating temperatures are 200 to 500 ° C.

Significant desorption of arsenic oxides even reaches temperatures not exceeding 800 ° C and more.

With the help of the inventively used sorbent, it is the first time possible, arsenic compounds from hot exhaust gases, such as. B. flue gases of power plants, remove, so that the activity of catalysts to remove nitrogen oxides from exhaust gases no longer by arsenic poisoning is impaired.

Another, particular advantage of the sorbent used in the invention based on high-surface titanium dioxide is that it in the presence of ammonia as a reducing agent contained in the exhaust gas Nitrogen oxides reduced to nitrogen and water. The sorbent catalyzes so in the presence of ammonia at the same time the reductive distance of nitrogen oxides from the exhaust gas.  

This also does not interfere with sulfur oxides, z. B. in most exhaust gases of combustion plants occur, provided that one to avoid of ammonium bisulfate deposits an operating temperature of about 300 ° C is not below for a long time. This ability For the catalytic reduction of nitrogen oxides not only have the Sorbents which additionally transition metal oxides, such as. B. oxides of Vanadiums and / or molybdenum and / or tungsten, but also surface-rich titanium dioxide alone.

The catalytic property of the sorbent used according to the invention Although decreases by the recordings of arsenic oxides, but shuttles to a certain level. A residual activity of 30 to 40% of the Initial value remains with complete loading of the sorbent Arsenic oxides still obtained.

The method according to the invention will now be described with reference to the following example explained in more detail:

example

Metatitanic acid from the sulphate process is filtered.

To produce the raw material of type A, one calcines a part of the filter cake directly at temperatures up to 500 ° C.

The type B raw material is made as follows:
The other part of the filter cake is mixed to immobilize about 40% of its sulfate content with barium carbonate, then transferred by the addition of concentrated nitric acid in the sol state, then spray-dried and calcined at temperatures up to 600 ° C.

After milling, 74.2% by weight of the type A raw material, 17.4% by weight of the type B raw material, 2.7% by weight plastic clay and 5.7 wt.% E glass fibers of 3 mm length mixed together and after addition of polyethylene oxide as plasticizer and water as well as neutralization with concentrated ammonia water to a plastic Mass kneaded. The resulting plastic mass deformed to cylindrical extrudates of 3 mm diameter and 5 mm Length, dried for 20 hours at 100 ° C and then calcined for 5 hours at 550 ° C. The BET surface area was after calcining 70 M² / g.

A tubular reactor was filled with 400 cc of extrudates and at a temperature of 400 ° C of 0.5 Nm³ / h test gas of the following composition flows through:

5 vol.% O₂ 1000 ppm by volume SO₂ 1000 ppm by volume NO _x (<95% NO) 1000 ppm by volume NH₃ 81 mg As / Nm³ (feed as As₄O₆).

The space velocity was 1250 h ^-1 .

After 7 hours of operation, the sorbent was removed, pulverized and analyzed. The arsenic content was 0.14 wt .-%, calculated as As, which is an average arsenic removal from the test gas of 99.8% corresponds.

Claims (9)

1. A process for the removal of arsenic compounds from exhaust gases, characterized in that the arsenic-containing exhaust gas is passed through a sorbent based on high-surface titanium oxide, wherein in the case of non-oxidic arsenic compounds, the exhaust gas after-burning is preceded. 2. The method according to claim 1, characterized, the arsenic compounds are arsenic oxides. 3. Process according to claims 1 and 2, characterized, that the sorbent additionally contains transition metal oxides. 4. The method according to claim 3, characterized, that the sorbent as additional transition metal oxides oxides of Vanadium and / or molybdenum and / or tungsten contains. 5. Process according to claims 1 and 4, characterized, that the sorbent additionally zirconium oxide and / or clay and / or silica and / or aluminosilicates and / or alumina. 6. Process according to claims 1 to 5, characterized, that the sorbent additionally contains heat-resistant fibers. 7. Process according to claims 1 to 6, characterized, that the sorbent based on surface-rich titanium oxide a BET surface area of 40 to 300 m² / g.   8. Process according to claims 1 to 7, characterized, that the exhaust gas at a temperature of about 200 ° C over the sorbent is directed. 9. Process according to claims 1 to 8, characterized, that the sorbent in the presence of ammonia at the same time the reductive Catalytic removal of nitrogen oxides from the exhaust gas.