Effect of ageing and moisture on the retention of hydrogen cyanide by impregnated activated charcoals

Abstract

The effect of ageing and humidity on the uptake of hydrogen cyanide (HCN) and cyanogen ((CN) 2) by activated charcoal granules (ACG) and cloth (ACC) impregnated with copper and chromate was studied. Accelerated ageing was achieved by means of storage at 80% RH and 60°C for several days. The presence of sodium hydroxide, (above 2.8% w/w) as a third impregnant, improved the primary performance of impregnated activated charcoal granules, but had little effect on ageing. Granules age with respect to HCN and (CN) 2 breakthrough times, while ACC ages mainly with respect to the (CN) 2 breakthrough time. Thus, ageing appears to manifest itself with respect to pore size distribution. Chromium (VI) which can be adsorbed in all pores is gradually reduced to chromium (III) ions with time. Copper (II) impregnant only ages in ACG that has large enough pores (macro- and mesopores) to take it up. The effect of moisture (80% RH) and its absence (<1% RH) in air streams containing HCN or (CN) 2 challenge gas on the uptake of these gases by activated charcoal cloth (ACC) impregnated with copper and chromium of various valencies and sodium hydroxide was investigated. It was found that moisture was generally detrimental to HCN uptake. Only in the presence of Cu (II), capable of catalysing a chain mechanism removing HCN, was the presence of moisture in the gas stream advantageous and then only if Cr (VI) was available to remove the cyanogen (CN) 2 that had been formed in the process. ACC impregnated with Cu (II) was more effective than Cu (I) or Cu (0) in removing HCN, while chromate, moisture and NaOH catalysed the formation of HCN from (CN) 2. The results can be explained in terms of a circuit of interlinked reaction mechanisms. An empirical formula is proposed grading the impregnated ACC filter material: [HCN held − (CN) 2formed] × t shortest breakthrough time.