The thermal decomposition of hydrated calcium hypochlorite (UN 2880)

Although this material has been generally regarded as prone to exothermic s elf-decomposition with very rapid release of heat, oxygen and gaseous chlor ine derivatives it has not been the subject ol an orthodox thermal ignition study such as that performed on anhydrous calcium hypochlorite(UN 1748), s tudied in the standard manner in 1978. This paper describes such rests on s amples varying in mass from 5 g up to 200 kg in cylindrical containers of v arying aspect ratio and various materials including stainless steel gauze, high-density polyethylene and coated fibre. The heat transfer coefficients of all the containers were measured accurately by both steady-state and coo ling curve procedures and the thermal conductivity of the hypochlorite was measured independently. These measurements revealed that the Blot numbers o f the samples tested were: in a range where the critical ambient temperatur es were sensitive to the values and thus sensitive to the convective air fl ow around the test bodies. With these factors taken into account the usual Frank-Kamenetskii (F-K) plot gave a good straight line in the range 115 deg rees C upwards. However, below this temperature, i.e. in the range 40-115 d egrees C very strong deviations occur. Nevertheless, the F-K theory still h olds as the low-temperature points also fall on a good straight line with a ctivation energy 48.5 kJ/mol which compares with the high-temperature activ ation energy much greater than this. These results indicate a sharp change in the rate determining step for heat generation at temperatures around 100 -120 degrees C. In temperature ranges both above and below this area the te mperature-lime traces for the samples behave classically as would be expect ed from the F-E; theory but inside this range the traces show much more com plex behaviour with some oscillatory characteristics, also some evidence of endothermic behaviour. The consequences of this unexpected behaviour for s afety (particularly in the bulk; marine shipping context) are serious. The deviations predicted from simple extrapolation of the high-temperature resu lts indicate much lower critical ambient temperatures for large quantities of this material than previously thought. Our low-temperature results obtai ned by orthodox ignition methods are entirely consistent with independent d irect isothermal calorimetric measurements already present in the literatur e but not used quantitatively in this context. (C) 2000 Elsevier Science Lt d. All rights reserved.