This column is based on “Calcium Hypochlorite Production via Sodium Method – Cost Analysis,” a report published by Intratec. It can be found at: www.intratec.us/analysis/calcium-hypochlorite-production-cost.
Calcium hypochlorite, Ca(OCl)2, is among the few metal hypochlorites that is stable enough to be produced as a solid salt. Because of this fact, it is the main bleaching agent produced commercially as a solid (sometimes referred to as bleach powder).
The following paragraphs describe the production of calcium hypochlorite (70 wt.%) from slaked lime, sodium hydroxide and chlorine, a typical, so-called “sodium process.” Figure 1 presents a simplified flow diagram of the process.
Reaction. Initially, caustic soda solution is mixed with recycled bleach mother liquor — composed of calcium hypochlorite and sodium chloride dissolved in water — in the caustic mixing vessel. The resulting solution is sent to the lime-mixing vessel, where slaked lime is fed to form a milk-of-lime slurry. This slurry is forwarded to the salt nucleator vessel, where it is mixed with chlorinator liquor recycled material and recycled bleach mother liquor. The resulting solution is supersatured with sodium chloride and calcium hypochlorite, which initiates the precipitation of the crystals. The mixture from the salt nucleator vessel is fed to the chlorinator along with gaseous chlorine, which reacts with calcium hydroxide and sodium hydroxide to form calcium hypochlorite and sodium chloride. In this way, further crystals of calcium hypochlorite and sodium chloride are formed and growth of pre-existing crystals occurs.
Separation. The chlorinator outlet slurry is fed into the a two-stage, upward-flow cylindrical elutriator-classifier at its midpoint. Bleach mother liquor from the calcium hypochlorite centrifuge is fed into the bottom portion of the lower stage of the elutriator as the elutriating fluid. From the upper portion of the elutriator, an aqueous slurry containing most of the calcium hypochlorite is withdrawn and sent to calcium hypochlorite centrifuge. An aqueous slurry containing denser sodium chloride crystals are removed. The bottom product of the elutriator, an aqueous slurry containing the denser sodium chloride crystals is sent to the salt centrifuge.
The sodium chloride slurry from the bottom of the elutriator is centrifuged, and the sodium chloride wet cake is then conveyed to an intermediate tank. The salt-centrifuge mother liquor is sent to the Ca(OCl)2 centrifuge.
Finishing. The wet cake from the calcium hypochlorite centrifuge is dried in a turbo dryer, and gases from the dryer are scrubbed and released. The dry calcium hypochlorite is squeezed, prilled and then classified. The oversized and undersized solids are returned for further processing, while the calcium hypochlorite product is directed to packing.
Ca(OCl)2 production pathways
Calcium hypochlorite was initially produced by passing chlorine gas over slightly moist calcium hydroxide. Currently, calcium hypochlorite is mainly produced from slaked lime and sodium hydroxide. Figure 2 presents different pathways for calcium hypochlorite production.
The total operating cost (raw materials, utilities, fixed costs and depreciation costs) estimated to produce calcium hypochlorite was about $1,120 per ton of calcium hypochlorite in the fourth quarter of 2013. The analysis was based on a plant with capacity to produce 25,000 metric ton per year of calcium hypochlorite.
Edited by Scott Jenkins
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