Chromium-Free Impregnated Activated Carbon for Adsorption of Toxic Gases and/or Vapors

The present invention relates to impregnated activated carbon adsorbents for removal of toxic materials in respiration and collective filter applications and, in particular, to a chromium free impregnated carbon adsorbent for the removal of toxic gases and/or vapors.

Activated carbon has long been used to remove toxic gases and vapors from a stream of gas. These activated carbons are useful in respirators, collective filters and other applications, and often involved the use of special impregnants to remove gases that would not otherwise be removed through the use of unimpregnated activated carbons. These impregnated activated carbon adsorbents for removal of toxic gases and/or vapors have been known and used for many years. The prior art formulations contain copper, chromium and silver impregnated on an activated carbon. These adsorbents are effective in removing a large number of toxic materials.

Generally, it is known that removal of cyanogen chloride, hydrogen cyanide and cyanogen require the presence of chromium and copper on the activated carbon for effective removal. Copper and silver have been shown to be effective in the removal of arsine and phosphine. Chlorine, hydrogen chloride, hydrogen fluoride and hydrogen sulfide are also removed by the presence of copper impregnates on activated carbon.

In addition to a number of other inorganic materials which have been impregnated on activated carbon, various organic impregnants have been found useful in military applications for the removal of cyanogen chloride. Examples of these include triethylenediamine (TEDA) and pyridine-4-carboxylic acid. Tartaric acid has also been found to be used with other materials in various applications.

A search of the prior art has uncovered patents which disclose a variety of agents impregnated in activated carbon to remove toxic vapors from a gas stream.

British patent No. 2,187,725 discloses activated charcoal impregnated with transition metal salts preferably consisting of cobalt, nickel or zinc acetate.

U.S. Pat. No. 3,355,317 to Keith et al. discloses the use of the oxides of cobalt, copper, zinc, iron and molybdenum on activated carbon to remove hydrogen cyanide from tobacco smoke.

U.S. Pat. Nos. 2,920,050 and 2,920,051 both to Blacet et al. describe the preparation of whetlerite type filters which include copper, chromium, silver and molybdenum impregnants.

U.S. Pat. No. 4,801,311 to Tolles discloses a whetlerite type activated carbon filter material impregnated with copper, silver, and molybdenum and/or vanadium salts together with triethylenediamine.

U.S. Pat. No. 4,802,898 to Tolles discloses a whetlerite type activated carbon filter material impregnated with triethylenediamine, and optionally with copper and silver.

U.S. Pat. No. 4,531,953 to Groose discloses an activated carbon impregnated with copper, silver and/or chromium mixtures and triethylenediamine.

In addition to the foregoing prior art patents, other references disclose that activated carbon impregnated with triethylenediamine (TEDA) has been found to be particularly effective for gas masks for removing cyanogen chloride, without the need for chromium. References also disclose that activated carbon may be impregnated with copper and/or silver for removal of additional toxic gases or vapors often required for military protective mask applications.

Other references, including the 1946 NDRC report by Grabenstetter et al. describe the use of copper, silver, chromium and molybdenum or vanadium impregnants on activated carbon to remove hydrogen cyanide and cyanogen chloride. Other organic base impregnations of charcoal are disclosed, including the use of amines such as diethylene triamine and others are also found in prior art.

The use of impregnated activated carbon adsorbents in respirators and collective filters, either for military or industrial applications, require special considerations regarding the toxicity and carcinogenicity of the impregnants to the user. These considerations eliminate a large number of prior art potential impregnants from use in respirator and collective filter applications. This is especially the case with hexavalent chromium, which involves potential health hazards.

Accordingly, it is an object of the present invention to provide an activated carbon that has the advantages of a chromium-free adsorbent while remaining effective in removing toxic gases and/or vapors through the impregnation of the activated carbon adsorbent with copper and zinc. It is a further object of the invention to provide an activated carbon absorbent for use in both military and industrial respirator and collective filter applications.

Generally, the present invention comprises an activated carbon adsorbent impregnated to contain, depending on the particular characteristics desired, a final composition by weight containing a mixture 6-16% total copper and zinc, within the range of 0% to 14% copper and 2% to 16% zinc. Silver, triethylenediamine, and/or tartaric acid may or may not be required, depending on the desired application.

Specific embodiments of the present invention comprise an activated carbon adsorbent impregnated so as to result in a composition containing from about 6% to 8% copper, 6% to 8% zinc, and optionally 0.03% to 0.05% silver and/or 2% to 4% triethylenediamine. Zero to 10% tartaric acid may also be added during preparation of this composition. The adsorbent of this embodiment of the present invention has been found to be effective for removing gases and/or vapors including chlorine, hydrogen chloride, hydrogen cyanide, cyanogen, cyanogen chloride, sulfur dioxide, hydrogen sulfide, formaldehyde and others, which are hereinafter referred to as "toxic gases and/or vapors".

The present invention is useful in respirator and collective filter applications, and meets both military and industrial toxic gas and/or vapor performance criteria. The formulation of the impregnants can be varied to adjust the performance characteristics to achieve the desired enhanced removal of specific toxic gases and/or vapors and extended shelf life. Other advantages of the present invention will become apparent from the following detailed description of the presently preferred embodiments of impregnant compositions.

Samples were impregnated by the "addition-titration" method where solution is slowly added nearly to the point that the activated carbon appears wet. In this method, the pore volume is almost filled with solution. Other methods of impregnation can also be utilized. One or two impregnation "cycles" can be used; sample metal loadings are calculated based on preparation conditions, not actual analysis. Samples are then checked by nitric acid extraction and atomic absorption analysis. The calculated loadings are typically accurate to about +/-0.5% weight. Samples tested herein have been dried in a forced air draft convection oven on shallow trays. Other methods of drying the activated carbon (including at elevated temperatures in N2) can also be employed, again according to the desired characteristics of the activated carbon. The drying temperatures used vary, and include the methods as provided in the specific data below.

The samples were prepared in batches of approximately 1 Kg and dried in a laboratory oven. The composition of a typical impregnation solution used is shown in Table I. As used herein, ASZ refers to activated carbon impregnated with copper, zinc and optionally silver. ASZ-TEDA refers to the same formulation additionally impregnated with TEDA as set forth in Table I.

Two impregnation cycles were generally used. The first impregnation solution was added to dry coal-based activated carbon in approximately 25 ml aliquots at the ratio of 70 ml per 100 g activated carbon. The activated carbon was shaken during addition to avoid pooling of the solution. The partially impregnated activated carbon was then dried in thin beds in a forced air convection oven for 30 min. at 100.degree. C., and 30 min. at 130.degree. C. After cooling, the activated carbon was reimpregnated with approximately 55 ml solution per 100 g activated carbon in a similar manner. The drying profile used for the twice impregnated product was: 100.degree. C., 30 min; 130.degree. C., 30 min; 160.degree. C., 45 min; 180.degree. C., 45 min. The cooled activated carbon was impregnated with TEDA according to the method of U.S. Pat. No. 4,531,953. Addition of tartaric acid to the impregnation solution gives additional improvement in the HCN performance of the activated carbon.

The results of hydrogen cyanide, cyanogen chloride and aging tests of a test sample of ASZ-TEDA and a production sample of ASC (prior art chromium impregnated activated carbon) are shown in Table II.

The hydrogen cyanide and cyanogen test results in Table II indicate that performance is relatively insensitive to humidity. The test lives are also comparable to the ASC sample used as a reference. The cyanogen chloride performance of ASZ-TEDA is quite dependent on humidity. Test life is relatively low under dry conditions, and increases as moisture is added to the system during preconditioning. This behavior is different than that of ASC. ASC has better dry performance, reaches peak lifetime at 50/50 conditions, and decreases as more water is added. At 80/80 conditions, cyanogen chloride life of ASZ-TEDA is only slightly lower than ASC.

The most striking improvement in performance between ASZ-TEDA over ASC is that cyanogen chloride performance of ASZ-TEDA does not deteriorate with aging at 45.degree. C. and 85% relative humidity. The ASC reference lost 33% of its effectiveness under these same conditions.

The results of physical property tests and chemical analysis of the product are shown in Table III.

As shown in Table IV, incorporation of up to 10% tartaric acid in the solution used to impregnate the product resulted in noticeable improvement in the hydrogen cyanide and cyanogen life of the adsorbent of the present invention.

The improvements that result from the addition of tartaric acid are most noticeable at higher metal loadings for adsorbents impregnated with 8% Cu and 6% Zn. For these adsorbents, hydrogen cyanide, cyanogen life increased from 21-23 minutes to 28-32 minutes. (Table IV). The improvement in performance for adsorbents containing 6% Cu and 6% Zn was not as marked; in this case, performance improved from 24-26 minutes to 28 minutes. At both the 6% and 8% Cu levels, little effect on cyanogen chloride life was found. Performance improvements with less than 10% tartaric acid were less dramatic.

Adsorbents of the present invention were prepared using other organic acids to improve performance. However, as shown in Table V, only maleic acid indicated any benefit for hydrogen cyanide, cyanogen performance.

With respect to maleic acid, the degree of improvement was similar to that found for tartaric acid. Cyanogen chloride performance of the adsorbent prepared with maleic acid was lower than for adsorbent containing tartaric acid.

To understand the response of hydrogen cyanide, cyanogen and cyanogen chloride to variations in the compositional impregnant regions of 0-12% An, and 0-6% TEDA, tests were performed in these ranges. The results of these tests are set forth in Table VI.

It has been found that the major contributor to cyanogen chloride life is TEDA in conjunction with copper and zinc. Interactions between copper and TEDA and zinc and TEDA are important, and contribute significantly in improving performance at higher metal levels.

Hydrogen cyanide/cyanogen protection is provided by copper and zinc, with zinc being more effective at lower metal levels.

While presently preferred embodiments of the invention have been described in particularity, the invention may be otherwise embodied within the scope of the appended claims.

As used herein, AZ refers to copper and zinc impregnated activated carbon formulations prepared in the same manner as the ASZ-TEDA (Cu-Zn-Ag-TEDA) composition described in Table I, but without the addition of Ag or TEDA. AZ samples were also prepared in the same manner as the aforementioned ASZ-TEDA (Cu-En-Ag-TEDA) composition as described in Table I, but again without the addition of Ag or TEDA, which are often not required for non-military toxic gas and/or vapor applications.

The AZ embodiments of the present invention are comprised of an activated carbon adsorbent impregnated with (depending on the particular characteristics desired) compositions resulting in a metals content of 0% to 14% copper and 2% to 16% zinc in the impregnated activated carbon. The formulations of the impregnants are varied, and demonstrate the that the performance characteristics of the chromium-free compositions in the present invention can be tailored to achieve enhanced removal of specific toxic gases.

Table VII sets forth the results of sample gas life challenges of Chlorine, Hydrogen Chlorine, Sulfur Dioxide, Formaldehyde and Hydrogen Sulfide for a typical AZ formulation.

Table VIII sets forth the results of tests on various AZ activated carbon formulations challenged with sulfur dioxide.

Laboratory studies show that well-balanced performance in all critical areas is attained with the present invention composition containing from 6-8% Cu, 6-8% Zn, 0.03-0.05% Ag and 2-4% TEDA. When this composition is used, the impregnated activated carbon meets all current military specification tests for ASC Whetlerite. In addition, hydrogen cyanide, cyanogen and cyanogen chloride performance did not deteriorate under tropical aging conditions.

Laboratory studies also show that good performance is attained in all critical areas with the present invention in specialized compositions containing a mixture of 0% to 14% copper and 2% to 16% zinc. Silver and/or triethylenediamine may also be added, depending on desired performance characteristics. Tartaric, maleic and/or other organic acids may also be incorporated into the preparation to improve performance. These compositions can be tailored within the described boundaries to meet specific desired performance characteristics for any number of industrial, military and other needs.