National Iodine Deficiency Disorders Control Program (NIDDCP)

 
PROCESS OF IODISATION OF SALT
Iodisation of salt could be accomplished by one or more of the following processes :-
 
bullet Spray mixing process  
 
bullet Drip feed process
 
bullet Dry mixing

Spray Mixing Process :

In the Spray Mixing Process, salt in crystalline or in crushed form, is dumped into a feed hopper. A  stainless steel wire-mesh (3 x 36) fitted to the hopper screens off lumps of salt and prevents gunny bags/baskets being draw-in. From the feed hopper the salt is carried by an inclined rubber belt conveyor moving at a speed  of 100 ft. per minute. The Salt is discharged from the belt conveyor into a mixing chamber at the rate of 5 tonnes per hour.
            A 3 to 4% aqueous solution of KIO3 is sprayed through special type of stainless nozzles designed to deliver a flattened spray that spreads over the entire width of the salt stream falling from the belt. The iodate solution is held under pressure in a stainless steel drum of 80 litres capacity. The pressure in the storage drum is maintained at 20 p.s.i with the help of an air compressor equipped with a regulator.
          The salt crystals unevenly wetted with KIO3 solution are mixed thoroughly by a stainless steel screw (9 dia.) conveyor which pushes out the iodised salt through twin outlets for bagging.
         Nowadays mobile salt iodisation units are used widely for spray process which can be taken from salt works to salt works and within salt works in salt storage area.  Crushers can also be fitted below the feed hopper for producing crushed and powdered variety of iodised salt. 

Drip-Feed Process :

Common Salt can also be iodised by Drip-Feed Process. In this process Kurkutch variety of common salt is fed into the grinder, where 3-4 percent solution of potassium iodate is fed through hollow needle at the inlet of the crushing zone. Mixing of the solution with salt crystals and grinding of salt crystals take place simultaneously. The ground salt is fed to screw conveyor underneath the roller grinder for effecting homogeneous mixing of potassium iodate.  Here the flow of potassium iodate solution is being monitored and controlled through appropriate regulating device.
            The salt manufactured by the above method does not remain free flow and cakes during the long storage.

Dry Mixing Process:

Nowadays this process is not adopted.  However when the iodisation program started this method was adopted by Sambhar Salts Ltd. In this Dry Mixing Process, a stock mixture of Potassium Iodate and anti-caking chemicals like Tri-calcium phosphate or Calcium Carbonate is prepared in proportion 1:10. The stock mixture is again mixed with 10 parts of free flowing Sodium Chloride, the entire pre-mix passing through 180 micron 15 sieve. The salt to be iodised is fed into a hopper of bulk controller and passes into an enclosed worm-screw mixer. At a point near the base of the bulk-controller a mixture of Potassium Iodate and anti-caking agent is fed in to a worm-screw mixer conveyor through a process feeder so as to give the desired quality of Iodised Salt.

Refining Process

Raw Salt which has many impurities, like soluble and insolubles in water, which adhere to it at the time of harvesting, has to be removed to obtain better quality edible salt. The process which is adopted to remove both soluble and insolubles is a counter current washery with liquid-solid separation etc.
            Raw Salt is first fed through a volumetric belt Feeder for size reduction to less than 2 mm in presence of brine. This slurry by gravity is fed to a series of tanks with a jet of brine in the upward direction. This effects in the removal of solubles and insolubles by Flotation. Fresh water is fed at various points of the washery to keep the solubles at working limits. This slurry is then fed to the certifuge, the filtrate by gravity goes to the filtrate tank. This filtrate is then pumped back into the washery to keep the slurry concentration at 60%. The Bleed brine is sent to the tank for the removal of insolubles by giving sufficient setting time. The pure brine is again pumped back into the system to maintain the slurry concentration as well as maintain and remove solubles and insolubles so that they are in working limits. The cake which is continuously discharged from the centrifuge is taken on a belt conveyer. Potassium Iodate solution of 5% and Potassium Ferrocynide is sprayed on to the salt which is fed in to the fluidized bed dryer. The moisture maintained at the outlet of the dryer is below 0.2%. The dried product is sieved into different particles sizes. The oversize is size reduced and recycled for size separation. The acceptable particle size is stored in the product silo.  The iodised alt is mixed with an anticaking aged  and packed by Form Fill and seal machines or manually in retail pouches.

Iodised Vacuum Salt Process

Vacuum Salt:
By far the largest proportion of evaporated salt is produced by boiling the salt brines in an evaporator. There may be only one evaporator body (single effect) or up to four evaporators can be connected in series so that the steam produced in the preceding evaporator body is used as the heating steam in the following evaporator (multiple effect evaporation).  The solar evaporated brine of 24 degree baume is fed to sophisticated multiple effect evaporators.  The evaporation is effected by indirect heating with 50 psig steam in the evaporators working under vacuum.  With the increase in the concentration due to evaporation, the precipitation of salt takes place in the evaporators and Magma is drawn out and centrifuged to get vacuum salt.  The condensate recovered can be used as feed water for other purposes.  The centrifuged wet salt is dried in fluidized Bed Dryers using hot air and dired vacuum salt thus produced is then packed.

Iodised Vaccum Salt:

Iodised salt is produced by injection of potassium iodate solution in a controlled manner into the wet salt from the centrifuge which is then dried in fluidized bed dryers and packed.

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