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Dealing with Bleaching Powder
S K Khurana, Chlor - Alkali & Business, Development Consultant. Stable Bleaching Powder (SBP) is widely used in Disinfections, Sanitation, Bleaching Oxidation, etc. It is manufactured by Chlorination of high-grade Hydrated Lime (Slaked Lime) using liquid chlorine. Handling and storage of the product become a Herculean task due to two imperative factors; i.e. to ensure effective stability and preventing fire and explosion hazards. The article highlights factors affecting the stability of the product and explains an effective way to store SPB in godowns.

Among the various uses of chlorine, an important one is for the purpose of sanitation. The quantity of chlorine required for sanitation is small and the place of application keeps on changing. The persons employed for this purpose are mostly unskilled. Therefore, we need a chemical which can be handled in small quantity, should be safe for persons who are handling it and can be taken easily at places to be sanitised. Stable Bleaching Powder meets all these requirements as discussed above and has been found to be the best option. In addition to small sanitation jobs in the municipalities, it plays a very major role in the control of epidemics when there are natural calamities i.e. earthquake and floods etc.

Stable Bleaching Powder is manufactured by the chlorination of hydrated lime.

2Ca (OH)2 + 2Cl2 = Ca (OCL)2 + CaCL2 + 2H2O.

This is an exothermic reaction. The absorption of 1 Kg of chlorine evolves 264 Kcal of heat. This heat is partly removed by cooling water, partly by evaporation of water formed in the reaction and partly by the heat of evaporation of liquid chlorine (the use of chlorine gas has been stopped since last many decades)

The chlorination reaction is carried out in a jacketed cylindrical drum mounted horizontally and fitted with moving scrapers. The drum can be cooled (by circulating cooling water) or heated (by steam) as per process requirement. It is operated under vacuum. Chlorination of hydrated lime is a batch process. In this process only about 60 per cent of Ca (OH)2 present in the hydrated lime reacts with chlorine that is why the finished product contains approximately 35.0 per cent Available chlorine. It is packed in laminated HDPE bags with a LDPE liner (25 Kg capacity) or 50 Kgs M.S./G.I. Drums with liner.

A good quality bleaching powder should show good stability; i.e. low loss of available chlorine during storage over long duration in properly packed conditions and under changing climatic conditions.

A quick test for determining the stability of bleaching powder is by heating the sample to 100 +2 OC for two hours. The loss of available chlorine should not exceed 1/15 (2.0 gpl maximum). However, it is desire able to carry through a storage test over an extended period of time in order to establish the proper correlation between the two tests. The long time storage test is the most dependable method of establishing the stability of the bleaching powder since this test is carried out under conditions those of actual storage.

Factors Effecting Stability
The following factors affect the stability of bleaching powder.
a) High moisture content in the product: It should be 0.50 per cent maximum (lower the better). However this limit can be higher for countries with cold climate. Low moisture content of the final product is essential to prevent splitting off of chlorine.

Ca (OCL)2 + Ca CL2 + 2H2O = Ca (OH)2 + 2Cl2.

This equation represents the reversed formation reaction of the bleaching powder.

b) Presence of heavy metals in hydrated lime: The heavy metals like Nickel, Cobalt, Manganese, copper and Iron accelerate the decomposition of bleaching powder. Their activity decreases in the same order. The source for these contaminants is the raw material calcium hydroxide. Therefore extreme care should be taken in the selection of the calcium hydrate.

c) Exposure to air: The product is packed in airtight containers to avoid exposure to CO2 present in air.

d) Storage temperature: The storage temperature of packed bleaching powder plays a very important role in the stability besides avoiding fire hazards.

Fire and Explosion Hazards
Fire in a bleaching powder storage godown can be due to one or more of the following causes.
a) Impurities in the hydrated lime (raw material)
b) Storage of the material for a long time
c) Poor air circulation in the storage godown
d) Contact with incompatible materials

Bleaching powder is liable to exothermic decomposition at elevated temperatures. Decomposition can be initiated by heat or impurities in lime.(iron, manganese, cobalt, magnesium and their compounds.) This is to be controlled at manufacturerĘs end.

When involved in a fire, bleaching powder decomposes without burning to release oxygen, which will intensify the fire. Also if it is mixed with organic materials such as sawdust or oil, it can result in fire without the need of an external ignition source.

One other important characteristic is that it is unstable and self-reactive. At normal encountered temperatures calcium hypochlorite decomposes only very slowly and releases heat. However at higher temperatures the rate of decomposition increases and if the heat is not able to escape from within the material then its temperature increases and so on. As such the reaction can runaway and result in a violent decomposition of calcium hypochlorite.

Incompatibles: Calcium hypochlorite is a strong oxidiser. Reacts with water and acids giving off chlorine gas. Forms explosive compounds with organic materials, nitrogen compounds and combustibles materials.

Conditions to Avoid: Heat, flame, moisture, dusting, sources of ignition, shock and incompatibles.

Fire Hazards: Not Combustibles, but the substance is a strong oxidiser and its heat of reaction with reducing agents or combustibles may cause ignition. Thermally unstable at higher temperatures, may undergo accelerated decomposition with release of heat and oxygen.

Explosion Hazards: Sealed containers may rupture when heated. An explosion can occur if a carbon tetrachloride or a dry ammonium compound fire extinguisher is used to extinguish a fire involving calcium hypochlorite. Sensitive to mechanical impact.

Fire Extinguishing Media: Use flooding quantities of water as fog or spray. Use water spray to keep fire exposed containers cool. Avoid direct contact with water, reacts with water releasing chlorine gas. Fight fire from maximum possible distance wearing full protective clothing and self contained breathing apparatus with full-face piece.

Storage of Bleaching Powder
A) The flooring of the godown should be impervious with proper drainage.
B) The bags can be stacked about 10 high on the floor.
C) The material should be stored in 200 - 250 bags (25 kgs weight) lot in line with the batch size.
D) There should be a gap of 15 inches or so between two lots to facilitate movement of personnel.
E) Bags must be stored 8 - 10 inches away from the walls to facilitate air circulation.
F) A good number of exhaust fans should be installed in the godown for better ventilation. Only minimum number of them should be run during winter.
G) Humidity in air during monsoon does not affect packed bleaching powder due to double packing; i.e. HDPE laminated bag with liner.
H) During periods of lean sales it is desire able to curtail production rather than over loading the godown.
I) Material should be dispatched from the godown in the order it is received; i.e. material entering first should go out first so that duration of its stay in the godown is optimum.
J) The godown should have two additionaldoors (facing each other), which should have frames made of M.S. flats / small diametre pipes & M.S, angels to facilitate cross ventilation.
K) Temperature of the material in the bags should be measured with a temperature gun or temperature gauge daily during summer and 2-3 times a week in winter. Any abnormal rise in temperature in a bag indicates an abnormality. It should be removed from the godown.
L) Consumers of bleaching powder can limit the godown capacity to one truckload plus 25- 50 per cent extra or three months requirement which ever is more. It is a good practice to keep the stocks low during summer. They should Insist on the manufacturer to give the batch No and date of manufacture of the material supplied.
M) Chlorine will be released if bleaching powder comes in contact with water.