Abstract:
A sorption agent comprising activated carbon and an impregnation of the activated carbon for the sorption of air pollutants, wherein the impregnation contains a zinc compound and a molybdenum compound, as well as a gas-filtering element, such as a respirator canister, containing said sorption agent.

Description:
CROSS REFERENCE TO RELATED APPLICATIONS 
     This application claims the benefit of priority under 35 U.S.C. §119 of German Patent Application DE 10 2009 019 737.0 filed May 2, 2009, the entire contents of which are incorporated herein by reference. 
     FIELD OF THE INVENTION 
     The present invention pertains to a sorption agent comprising an impregnated activated carbon for the sorption of air pollutants, wherein the impregnation has a zinc compound and a molybdenum compound, and to a gas-filtering element, e.g., a respirator canister, containing the above-mentioned sorption agent. 
     BACKGROUND OF THE INVENTION 
     Broad band filters in respirator canisters or in gas-filtering elements offer the user comprehensive protection against a great variety of harmful gases. Such a comprehensive protective action of a gas filter imposes high requirements on the chemical performance of the activated carbons used. To enable such activated carbons to remove the greatest possible variety of chemically different reactive harmful gases, these activated carbons must be chemically impregnated. 
     A large number of inorganic and organic compounds, for example, basic copper and zinc carbonates, copper and zinc sulfates, sodium dichromate, sodium molybdate, sodium vanadate, ammonium chloride, ammonium carbonate, silver nitrate as well as TEDA (triethylenediamine) or tartaric acid and salts thereof are known for impregnating activated carbons for use in gas filters. Chromate, which is often classified as carcinogenic by experts, has been regularly eliminated in impregnations of activated carbons for use in gas canisters. The necessary chemical action is ensured, instead, among other things, by molybdates and vanadates. 
     Copper-containing activated carbons are known as well. An essential drawback of the copper-containing activated carbons is the possibility of formation of cyanogen upon admission of HCN after storage of the filter. Cyanogen possesses toxic properties comparable to those of HCN, but it cannot be perceived by humans. Consequently, the user could not recognize the breakthrough of his or her breathing filter in an environment contaminated with HCN and could even run the risk of becoming poisoned with cyanogen without noticing it. To ascertain that the user of a breathing filter can perceive the breakthrough of his or her filter, which may possibly have already been stored for a rather long time, in an HCN-contaminated environment, the activated carbon contained in the filter must not form cyanogen at any time. 
     Cyanogen is formed from HCN in the presence of copper compounds in a basic environment, more precisely by Cu(II) salts, which react while undergoing reduction to Cu(I). The formation of cyanogen by Cu 2+  is as follows:
 
Cu 2+ +2 HCN−&gt;Cu II (CN) 2 +2H + 
 
Cu II (CN) 2 −&gt;Cu I (CN) 2 +½ (CN) 2 .
 
     The presence of a sufficient quantity of suitable oxidants, for example, chromates, molybdates or vanadates, on the copper-containing activated carbon can prevent the release of cyanogen. 
     Should the ratio of copper to the oxidant decrease, e.g., due to aging effects, the risk of release of cyanogen will increase (Table 1). It has now been found that the formation of cyanogen can be avoided by eliminating the use of Cu(II) compounds. 
     Besides the prior-art activated carbons, which are free from chromates but do, however, contain molybdate, there also are impregnated activated carbons in the area of broad band filters, which activated carbons do entirely without oxo anions of groups V and VI. Impregnations that contain neither oxo anions of groups V and VI nor copper salts are known as well. The activated carbons are impregnated with small quantities of very expensive silver salts in both cases and sometimes also contain larger quantities of zinc salts. However, the silver salts used for the impregnation predominantly react on the activated carbon by forming elemental silver, which has formally no oxidizing action. Since neither copper salts nor oxidants are present on such an activated carbon, such activated carbons have a markedly poorer performance with respect to SO 2  and H 2 S. Even though this property can be partially compensated by increasing the percentage of water on the activated carbon, the separation capacity for organic non-polar harmful gases is lowered by this measure. Furthermore, the storage of filters with activated carbon with high water content should be considered with markedly more criticism. Drying out of the activated carbon leads to major loss of performance, e.g., in case of the adsorption of SO 2  or NH 3 . 
     SUMMARY OF THE INVENTION 
     The object of the present invention was consequently to make available a sorption agent for gas filtration, which contains no chromium and/or copper, minimizes or eliminates the risk of release of cyanogen and is nevertheless as versatile as possible in terms of the sorption of various harmful gases. In addition, correspondingly equipped gas filters shall still be able to be used reliably even after longer storage. 
     The object is surprisingly accomplished according to the invention by a sorption agent comprising activated carbon and an impregnation of the activated carbon for protection against air pollutants or gaseous harmful substances, the impregnation containing: 
     0.5 wt. % to 15 wt. % of at least one zinc compound and 
     0.5 wt. % to 15 wt. % of at least one molybdenum compound, wherein the molybdenum compound is an oxo anion, dioxo anion or polyoxo anion compound, each wt. % being relative to the dry weight of the impregnated activated carbon, and 
     wherein the sorption agent contains: 
     no chromium and no chromium compound, and 
     no copper and no copper compound. 
     According to a further aspect of the invention a gas-filtering element is provided containing a sorption agent as noted. Further a respirator is provided comprising a respirator structure and a gas-filtering element containing a sorption agent as described. 
     The various features of novelty which characterize the invention are pointed out with particularity in the claims annexed to and forming a part of this disclosure. For a better understanding of the invention, its operating advantages and specific objects attained by its uses, reference is made to the accompanying drawings and descriptive matter in which preferred embodiments of the invention are illustrated. 
    
    
     
       BRIEF DESCRIPTION OF THE DRAWINGS 
       In the drawings: 
         FIG. 1  is a schematic view of a respirator with a gas-filtering element containing a sorption agent; 
         FIG. 2  is a diagram showing preparation steps and validation for the sorption agent; and 
         FIG. 3  is a diagram showing an example of a possible design for testing the filters with respect to HCN. 
     
    
    
     DESCRIPTION OF THE PREFERRED EMBODIMENTS 
     Referring to the drawings in particular,  FIG. 1  presents a schematic showing of a simple respirator generally designated  10 . The respirator  10  includes a respirator body or respiration structure  12  as well as a gas filtering element support  14  with a filter  7  containing a sorption agent according to the invention. 
     Zinc compounds are capable of binding HCN in a basic environment permanently and without the formation of the toxic cyanogen. They do not require oxo anions to be present. The following shows the irreversible binding of HCN to Zn 2+  in a basic environment
 
Zn 2+ +2 HCN −&gt;Zn II (CN) 2 +2H + .
 
     Copper-free, zinc-containing impregnations are therefore still able to offer a sufficiently good protection against HCN even after longer storage (artificial aging at elevated temperatures). 
     
       
         
               
               
             
               
               
               
             
           
               
                   
                 TABLE 1 
               
             
             
               
                   
                   
               
               
                   
                 Retention time for HCN 
               
               
                   
                 (desired value according to the EN 
               
               
                   
                 14387 standard) 25 minutes 
               
             
          
           
               
                 Impregnation 
                 not aged 
                 after artificial aging 
               
               
                   
               
               
                 Cu/Zn/Cr* 
                 36 minutes 100% HCN 
                 19 minutes 100% (CN) 2   
               
               
                 approx. 7 wt. % 
               
               
                 Cu/Zn/Mo* 
                 36 minutes 100% HCN 
                 28 minutes 100% (CN) 2   
               
               
                 approx. 7 wt. % 
               
               
                 Zn/Ag* approx. 
                 45 minutes 100% HCN 
                 28 minutes 100% HCN 
               
               
                 12 wt. % 
               
               
                 Zn/Mo approx. 
                 45 minutes 100% HCN 
                 35 minutes 100% HCN 
               
               
                 7 w. % 
               
               
                   
               
               
                 *Not according to the present invention 
               
             
          
         
       
     
     To ensure that the copper-free, zinc-containing impregnations can offer sufficiently good protection against SO 2  and H 2 S in the less wet state as well, impregnation with oxo anions, such as molybdate, is markedly more effective on activated carbon than silver-containing impregnations. 
     
       
         
               
               
             
               
               
               
             
           
               
                   
                 TABLE 2 
               
             
             
               
                   
                   
               
               
                   
                 Retention time according 
               
               
                   
                 to the EN 14387 standard 
               
             
          
           
               
                 Impregnation and percentage of water 
                 20 minutes SO 2   
                 40 minutes H 2 S 
               
               
                   
               
               
                 Cu/Zn/Cr H 2 O* approx. 7 wt. % 
                 26 minutes 
                 147 minutes 
               
               
                 Cu/Zn/Mo H 2 * approx. 7 wt. % 
                 25 minutes 
                 127 minutes 
               
               
                 Zn/Ag H 2 O* approx. 12 wt. % 
                 21 minutes 
                  43 minutes 
               
               
                 Zn/Mo H 2 O approx. 7 wt. % 
                 25 minutes 
                  70 minutes 
               
               
                   
               
               
                 *Not according to the present invention 
               
             
          
         
       
     
     A formula according to the present invention for preparing a zinc- and molybdate-containing impregnation as well the chemical performance of an activated carbon thus impregnated with respect to harmful gases will be presented below as an example. 
     EXAMPLE 
     740 g of zinc carbonate, 320 g of ammonium chloride, 290 g of ammonium carbonate and 250 g of sodium molybdate are dissolved. Then, 4.5 kg of activated carbon are impregnated with this impregnating solution and the pourable impregnated activated carbon thus obtained is carefully heated and dried in a drier (for preparation, see  FIG. 2 ). 
       FIG. 2  shows the steps for the preparation and validation of the impregnated activated carbon. The steps include preparation of the impregnating solution  21 , weighing of activated carbon  22 , impregnation of the activated carbon  23  and the drying of activated carbon  24 . The steps continue with the preparation of filter  25  and the testing of filter  26 . 
       FIG. 3  shows an example of a possible design for testing the filters with respect to HCN and SO 2 . Test air is sent in a controlled manner into a mixing vessel  5  via a test air supply  1  through a metering unit  4 ; HCN can be admitted into said mixing vessel  5  by an in-situ reaction of a sulfuric acid  3  and a KCN solution  2 , likewise via a respective metering unit  4 . Filter  7  is accommodated in a test container  6 . The retention capacity of the filter  7  can be recorded by a measuring device  8 . 
     The exemplary retention time of a Zn/Mo-impregnated activated carbon according to the present invention with approx. 7 wt. % of H 2 O against 5,000 ppm harmful gas in a class 2 filter according to EN 14387 is shown in Table 3. 
     
       
         
               
               
               
               
             
           
               
                   
                 TABLE 3 
               
               
                   
                   
               
               
                   
                 5,000 ppm 
                   
                 Retention time according to 
               
               
                   
                 harmful gas 
                 Retention time 
                 the EN 14387 standard 
               
               
                   
                   
               
             
             
               
                   
                 C 6 H 12   
                 47 minutes 
                 35 minutes 
               
               
                   
                 HCN 
                 45 minutes 
                 25 minutes 
               
               
                   
                 Cl 2   
                 30 minutes 
                 20 minutes 
               
               
                   
                 SO 2   
                 25 minutes 
                 20 minutes 
               
               
                   
                 H 2 S 
                 70 minutes 
                 40 minutes 
               
               
                   
                 NH 3   
                 43 minutes 
                 40 minutes 
               
               
                   
                   
               
             
          
         
       
     
     The present invention offers a technically meaningful solution to the known problem of cyanogen formation by chromate-free and copper-containing activated carbons, according to which Cu(II) compounds can be eliminated in the impregnation for the activated carbon. The storage stability of the impregnated activated carbon is markedly improved by the elimination of copper compounds. Sufficiently good protection against SO 2  and H 2 S is offered by the use of oxo anions of molybdenum on the activated carbon even at a lower water content in the sorption agent. Furthermore, the zinc compound are preferably in the form of an oxide, carbonate, halide, hydroxide, sulfate and/or oxo or polyoxo anions of molybdate. 
     While specific embodiments of the invention have been described in detail to illustrate the application of the principles of the invention, it will be understood that the invention may be embodied otherwise without departing from such principles.