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Carriage of Calcium Hypochlorite.

SSM Roundel

Steamship Mutual

Published: January 01, 2009


The very first issue of the old style Sea Venture, published in February 1978 included an article on calcium hypochlorite. In a six year period prior to 1978 there were at least twelve serious incidents related to the carriage by sea of calcium hypochlorite. These resulted in fires or explosions during loading, whilst sailing or discharging, sadly with loss of life in some instances. 

       The major producers at that time were the U.S. and Japan and the majority of the problems related to high strength chemicals produced by Japanese manufacturers. The losses generated large claims against underwriters and P&I Clubs which prompted the Clubs in London to set up a sub-committee to study the phenomenon .  Minton, Treharne & Davies Ltd was represented on this committee. Following extensive travel to the U.S. and Japan, the committee gathered so much information on the potential problems with the product that they found it necessary to arrange a detailed research programme at the Royal Armament Research and Development Establishment at Woolwich. 

      Calcium Hypochlorite is used extensively world wide for purifying water supplies and to a great extent in swimming pools. It is relatively inexpensive and is required in bulk for disaster relief in areas where the ambient temperature is generally high. Transport costs can represent a significant proportion of the final invoice value.  

     The chemical readily decomposes to release chlorine and oxygen, the components which make it useful as a disinfectant product. A side effect of this process is the reaction with combustible materials leading to spontaneous ignition and severe fires, promoted by the oxygen released from the chemical which renders ineffective the gas smothering systems typically used for fire fighting in ships holds. This characteristic as an oxidising agent had led to calcium hypochlorite being classified within Class 5.1 of the International Maritime Dangerous Goods (IMDG) Code. The studies undertaken at Woolwich, some of which were repeated in Japan, also confirmed that some drums of calcium hypochlorite had a propensity to undergo a self-accelerating decomposition reaction that concluded with an explosion releasing large clouds of thick white “smoke”. This occurs when the heat produced by the decomposition reaction is unable to escape from inside the chemical package (because it is big, well insulated or in a hot environment). The temperature at which decomposition was initiated was found to be about 70oC for drums of good quality material. Manufacturers experimented with the addition of water (as chemical hydrates) to the product in an attempt to reduce the reactivity with combustible materials, although the Woolwich tests indicated that an increase in the water content of the chemical could decrease the temperature at which the self accelerating decomposition could start, as could the presence of several different types of contaminants.  

Calcium hypochlorite was subsequently classified under 3 headings, UN1748, the high strength, dry material (with more than 39% available chlorine); UN2880, hydrated material with between 5.5% and 10% moisture, and UN2208, low strength material (typically known as bleaching powder) with between 10% and 39% free chlorine. Material with less than 10% available chlorine was not classified within the IMDG Code. UN1748 material was only to be carried on deck and “away from” sources of heat. UN2880 and UN2208 material could be carried under deck, UN2880 material also being stowed “away from” sources of heat “where temperatures in excess of 55oC for a period of 24 hours or more will be encountered”.  

During the 1990s the proportion of calcium hypochlorite carried in containers increased and the manufacturers increased the drum size of “bulk” packages. This resulted in large quantities of calcium hypochlorite being “bulk” carried in unventilated containers through areas of the world where the ambient temperature was very high.  

Between 1997 and 1999 there were 6 very large (multi-million pound loss) incidents on container ships involving calcium hypochlorite in various forms, including the “Contship France”, “Maersk Mombasa”, “Sea Express”, “DG Harmony”, “Aconcagua” and the “CMA Djakarta”. At about the same time there were fires and explosions in several warehouses in which large quantities of swimming pool chemicals were stored. Most of these cases were examined in detail either at arbitration or in court. As a part of the investigations, detailed theoretical and practical studies were undertaken of the temperatures at which the thermal decomposition of large volumes of calcium hypochlorite could start to self accelerate. One such study was performed for the International Group of P&I Clubs and was submitted to the sub-committee on Dangerous Goods, Solid Cargoes and Containers as an argument for stricter measures on the carriage of the chemical. The study found that self-accelerating decomposition in a 40ft container could start at temperatures in the order of 37oC, a temperature well within the range of temperatures that could be expected during ocean voyages. The International Group made several recommendations, including a limitation on package size and type and the possibility of reclassification to include calcium hypochlorite in Class 4.1 self-reacting materials, to warn of the self-heating characteristics. Other suggestions included forced ventilation or carriage in temperature controlled containers. 

During the investigations, many shipping companies restricted the carriage of calcium hypochlorite and as questions arose concerning the safety and classification of the chemical as a cargo, the variety of names under which the cargo was declared increased. These included: chloride of lime, lime chloride and hy-chlor, none of which appeared by name in the index to the IMDG Code. 

Following these studies and discussions in the Dangerous Goods sub-committee, the IMO recommended that all forms of calcium hypochlorite should be carried “On Deck Only”. Furthermore this deck cargo should be shaded from direct sunlight, stowed away from any heat sources and should have an adequate air circulation throughout the stow. The IMDG Code currently has 3 entries for calcium hypochlorite: 

  • UN 1748  Calcium Hypochlorite, Dry or Calcium Hypochlorite Mixture, Dry with more than 39% available chlorine (8.8% available oxygen) 
  • UN 2208  Calcium Hypochlorite Mixture, Dry with more than 10% but not more than 39% available chlorine
  • UN 2880 Calcium Hypochlorite Hydrated or Calcium Hypochlorite Hydrated Mixture with not less than 5.5% but not more than 16% water 

The Code notes that these materials are liable to exothermic decomposition at elevated temperatures and shall be shaded from direct sunlight and all sources of heat and placed in adequately ventilated areas. It also warns that decomposition can be initiated by impurities. The changes came into effect in January 2001. 

Many shipping companies have considered whether they should carry calcium hypochlorite (as well as several other chemicals with similar self-heating characteristics). This is particularly the case when the manufacturers are based in China and transport to Europe involves passage through the high ambient temperatures experienced in the Tropics and the Red Sea. It is difficult to assess what temperature the contents of a container will experience and what impurities may be present in the chemicals (from manufacture) to depress the “safe temperature” above which self-heating is likely. Although not required by the Code, temperature controlled carriage (i.e. refrigerated containers) have been seen to provide the safest option. 

There has been a major reduction in the number of fire incidents involving calcium hypochlorite reported in recent years. This may be the result of the revised methods of carriage recommended by the IMO, the clear warnings given regarding the risk of self heating or possibly because many ship owning companies have excluded the cargo in their charter parties. Despite all of these precautions, however, there has still been the occasional fire attributed to calcium hypochlorite, whether declared or not. The carriage of containerised cargo relies on the shippers’ full and accurate description of the unit’s contents. Only with this information can proper carriage conditions and safe transport, be provided. 


With thanks to Dr Geoffrey Bound of Minton, Treharne and Davies Ltd for preparing this article.

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