How is salt iodized?
Iodine is added as potassium iodate to salt after refining and drying and before packing. Iodization can often be linked with existing production and/or refining lines. This can be done by adding a solution of potassium iodate to the salt or by adding dry potassium iodate powder. Potassium iodide is only appropriate in certain areas with high quality packaging.
In the wet method, potassium iodate is first dissolved in water to make a concentrated solution. This solution can be either dripped or sprayed on the salt at a uniform rate.
In the dry method, the potassium iodate is first mixed with a filler such as calcium carbonate and/or dry salt and the powder is then sprinkled over the dry salt. In both methods, thorough mixing of the salt after the addition of the potassium iodate is necessary to ensure even penetration of the potassium iodate. If the mixing is insufficient, some batches of salt will contain too much iodine and others too little.
U.S. salt producers use potassium iodide to iodize salt because of high-quality packaging. Everywhere else, iodine is added to salt as the more stable potassium iodate after refining and drying and before packing. Iodization can often be linked with existing production and/or refining lines. This can be done by adding a solution of potassium iodate to the salt (wet method) or by adding dry potassium iodate powder (dry method). In the wet method, potassium iodate is first dissolved in water to make a concentrated solution. This solution can be either dripped or sprayed on the salt at a uniform rate. In the dry method, the potassium iodate is first mixed with a filler such as calcium carbonate and/or dry salt and the powder is then sprinkled over the dry salt. In both methods, thorough mixing of the salt after the addition of the potassium iodate is necessary to ensure even penetration of the potassium iodate. If the mixing is insufficient, some batches of salt will contain too much iodine and others too little.
The potassium iodate is mixed with an anti-caking agent like calcium carbonate, tricalcium phosphate, or magnesium carbonate in a ratio of 1:9. One part of this stock mixture is then mixed with 10 parts of salt to form the "premix" which is introduced onto a screw conveyor at a constant rate ((Fig 8.1). Salt is also introduced onto the conveyor and mixing takes place as the material moves through. This process is suitable for fine and even-grained salt with a grain size of less than 2 mm. It has been adopted in several countries of South and Central America including Argentina, Bolivia, Guatemala and Peru.
In China, a unique and compact type of dry mixing machine has been developed. Essentially, it consists of an inclined screw conveyor with a feed hopper at its lower end through which salt is fed. A slide within the feed hopper controls the rate of salt addition. A premix of KIO3 with salt at an approximate ratio of 1:2000 is prepared separately and fed onto the bulk salt at a controlled rate by a rotating arm within a conical feeder, or by a screw within the screw feeder located close to the salt feed hopper above the conveyor. The salt and premix are mixed as they move up the screw conveyor and the desired content of iodine in the salt is achieved. The mixture is then homogenized by passing through a roll crusher or pin mill that grinds it to a uniform size of 1-3 mm. After crushing, the iodized salt is passed through a second inclined screw for further mixing before packing.
Drip Feed Addition
This process is commonly used for iodization of salt crystals. The crystals are manually fed into a hopper that discharges at a uniform rate onto a belt conveyor, about 35-40 cm wide and 5.5 m long inclined at a slope of about 20 degrees. The conveyor is equipped with a tensioning device. The feed hopper has a capacity of about 300 kg and the rate of salt flow onto the conveyor is controlled by a slide valve. Flexible rubber curtains on three sides shape the salt into a narrow band 10-12 cm wide and 2 cm deep on the conveyor belt and prevent it from spilling over the edge. The KIO3 solution is stored in two 200 litre polyethylene stock tanks with discharge valves at the bottom to permit the filling of two 25 litre feed bottles, mounted to ensure a continuous circulation of solution from the main tank to the feed bottles. Thus the solution continuously drips at the desired rate onto the salt crystals. The iodized salt falls into a discharge hopper for collection in bags. For continuous operation the hopper should have a twin spout with a diversion valve. Experience has shown that a capacity of 5 tons per hour is ideal for a drip feed system, which requires only a low pressure head to maintain the required flow rate. This method is used in some Asian countries, for example, Indonesia. The drip feed system is simple and cheap and is often used for iodizing moist crude salt crystals and even refined powder salt.
In a simplified system used in India, the drip system is introduced into the feed point of a salt grinder. The drip feed system followed by grinding often yields consistent iodate dispersion.
Often, iodization is to be integrated with existing salt production and refining systems. Typically salt slurry from a thickener is dewatered in a centrifuge and then dried in a rotary or fluid bed drier. Into this system a sensor installed on the thickener can send a signal to the solution dosing pump that sprays iodate solution at a rate proportional to the flow rate of solids to the centrifuge.
In more conventional operations where refining equipment is not available, salt iodization plants will need to be established. Salt in crystal form is crushed to a coarse powder in a roller mill and manually fed into a feed hopper fitted with a wire mesh screen or grating at the top to prevent large lumps of salt from falling into it. A second shaft with four plates is fitted in the outlet of the hopper and regulates the flow onto an inclined conveyor belt. Both these shafts are driven by a variable speed drive system and the rate of rotation is adjusted to give the required throughput.
The sheet of salt discharging from the belt into the spray chamber receives a fine atomized spray of potassium iodate solution from two nozzles, at a pressure of 1.4 kg/cm2. The spray nozzles are designed to deliver a flattened spray that spreads over the entire width of the salt stream. The concentration of solution and the spray rates are adjusted to yield the required dosage of iodate in the salt. The iodate solution is kept under pressure in two stainless steel drums, each of about 80 liters capacity. The pressure in the drums is maintained constant by an air compressor with a regulator. The salt with added potassium iodate falls into a screw conveyor 20-25 cm wide and 2.5-3.0 m long. Travel through the screw provides uniformity of mixing. The screw conveyor discharges into twin outlets where bags are kept ready for filling. The spray mixing plant can be powered by electricity or diesel engines. All the parts coming into direct contact with salt are made of stainless steel, to minimize corrosion. The plant can also be made mobile for operational convenience. A spray mixing type of plant built to UNICEF specifications operates at 6 tons/hour or about 12,000 tons per year. This method is being increasingly preferred in Asia, South America, and Africa.
The standard spray-mixing plant configuration described above has been simplified with the elimination of the belt conveyor and making the screw conveyor inclined.
A batch-type version has been developed in India for small manufacturers who cannot afford or do not need continuous spray mixing plants. It consists of a ribbon blender fitted with an overhead drip or spray arrangement. A pre-weighed quantity of salt is fed into the blender. The blender is operated and a prefixed quantity of iodate is sprayed through overhead nozzles using a hand pump or compressor as mixing proceeds. After iodization, the batch is discharged and packed.
The blenders are powered by suitable motors or diesel engines. The rotation is reduced through a suitable gear box to give a speed of 20-30 rpm. The speeds and power requirements of blenders of different capacities are given below:
|125 kg ||2.2 KW ||3 HP
|250 kg ||3.7 KW ||5 HP
|500 kg ||3.7+1.5* KW ||5+2* HP|
*For vertilift conveyor to feed salt to blender. In smaller blenders, salt is fed manually
The quantity of salt that can be produced by a batch blender is determined by the cycle time and batch capacity. Assuming:
- Loading: 3 min
- Mixing: 10 min
- Unloading: 2 min
- Slack: 5 min
Total time for a single batch would be 20 minutes. There will be 3 cycles per hour, yielding a daily 8-hour iodization capacity reflected in the following table:
|125 Kg ||0.375 ton ||3 ton ||750 ton
|250 Kg ||0.750 ton ||6 ton ||1500 ton
|500 Kg ||1.500 ton ||12 ton ||3000 ton|
The capacity will obviously increase if the blenders run for longer hours or if more batches are produced hourly.
This method is simple to operate in the capacity range 0.5-3 tons/hour. It is already being used in India, Peru, Vietnam, and may also be applicable in several other countries needing small iodization plants close to salt production sites or at strategic points in the distribution network.
The spray system atomizes the iodate solution and disperses it uniformly on the salt crystals, thus ensuring much more uniform mixing when compared to the drip feed system for all kinds of salt. The equipment requirements for the spray system and their maintenance are a little more elaborate.
This table shows the amount of KIO3 solution in L/hr for 5 ton/hr spray mixing plant:
|20 ppm ||100 ||10.0 ||5.0 ||3.3
|30 ppm ||150 ||15.0 ||7.5 ||5.0
|40 ppm ||200 ||20.0 ||10.0 ||6.7
|50 ppm ||250 ||25.0 ||12.5 ||8.3
|60 ppm ||300 ||30.0 ||15.0 ||10.0
|70 ppm ||350 ||35.0 ||17.5 ||11.7
|80 ppm ||400 ||40.0 ||20 ||13.3
|90 ppm ||450 ||45.0 ||22.5 ||15.0
|100 ppm ||500 ||50.0 ||25.0 ||16.7|
This is how much of a KIO3 solution is required (ml) to introduce 50 ppm in a batch of salt:
|125 Kg ||625 ||313 ||208
|250 Kg ||1250 ||625 ||416
|500 Kg ||2500 ||1250 ||833|
Comparison of methods
The table below compares the different salt iodization methods, showing the relative advantages and restrictionsof each. Dry mixing of salt with KIO3 is possible only if the salt is dry and finely ground. Otherwise the KIO3, having a finer particle size and being heavier than salt, will settle at the bottom of the container. This method is therefore not recommended for the unrefined coarse salt that is commonly used in developing countries.
The drip-feed system is the simplest and cheapest. It is suitable for coarse salt with uniform particles of a size up to 1 cm and a moisture content of up to 5%. However, when the particle size of the salt is very fine (less than 2 mm), the drip feed system is not suitable because it does not disperse the iodate solution with sufficient uniformity. In such cases, the spray-mix method is better because it atomizes the iodate solution and sprays it as a mist, thus mixing it uniformly with the salt. The spray-mix method is also preferable to the drip feed system when the salt varies in particle size and moisture content, as frequently occurs when the iodizing plant receives salt from a number of different sources.
|Salt Type: Refined powder, dry ||++ ||+++ ||+++
|Salt Type: Unrefined powder, dry ||++ ||+++ ||+++
|Salt Type: Unrefined powder, moist ||++ ||++ ||++
|Salt Type: Unrefined crystals, dry ||++ ||++ ||+
|Salt Type: Unrefined crystals, moist ||+ ||++ ||+
|Cost: Capital cost ||Medium ||Med ||High
|Cost: Operating cost ||Medium ||Medium ||High
|Cost: Cost to consumer ||Medium ||Medium ||High|
The choice of salt iodization method depends upon the conditions prevailing in a particular location. The less developed countries frequently are hampered by salt of uneven purity, humidity, and unreliable packing material, and for them the preferred method will usually be crushing the salt and iodizing it by spray mixing with KIO3. Small producers should either set up individual small batch iodization plants or form co-operatives for centralized iodization and packing. Iodization units portable from one field to another should also be considered.
Simple Methods for Salt Iodization at the Village Level
Salt iodization, like most industrial procedures, is most efficient when operated on a large scale, as described in the previous methods. A large producing unit can use sophisticated equipment, optimize employee activity, consolidate laboratory and other quality control measures, and facilitate packaging and transport. Each of these steps promotes greater efficiency, reliability, and economy. Since salt production is usually concentrated at a limited number of sites, most iodization interventions will be located near these sites.
Despite these advantages of the type of salt plants described in the preceding paragraphs, simple salt iodization at the village level is occasionally valuable, and will be considered briefly here.
Perhaps the simplest method is the dry mixing of sodium chloride with KIO3, or with KI. For an iodization level of 50 ppm (1:20,000), one needs to add 84 mg of KIO3 for each kg of sodium chloride. This is best added slowly while the dry sodium chloride is being mixed in a bowl. The amount of KIO3 being added is minuscule compared to the volume of the salt, so that even with sustained manual mixing and pouring back and forth, the distribution of the KIO3 within the sodium chloride will always be somewhat uneven. The salt and KIO3 can be mixed simply with a large wooden spoon, and including pouring back and forth between two vessels if feasible. This process is both inefficient and unreliable, and we would not recommend it except as a temporary measure when no other means of achieving iodization is available. Since the amount of KIO3 is so small, it will have to be supplied in pre-weighed packets because accurate balances will virtually never be available in villages choosing this approach.
Other methods use manual spraying of a KIO3 solution on to salt as it is being stirred by hand. The spray bottle approach permits greater dispersion of the iodate than is achieved with manual dry mixing. In a system developed by Professor Romsai Suwanik in northern Thailand, 24 grams of KIO3 (containing 14.2 grams iodine) are dissolved in 725 ml of distilled water, to give a concentration of 2 mg iodine/ml. For spraying, 60 ml of this concentrated KIO3 solution are diluted to 240 ml in a spraying bottle and spray mixed by hand with 24 kg of salt in a plastic basin. The uniformity of distribution of iodine throughout the salt in this procedure depends on the evenness of the spraying and the vigor of the mixing.
A larger scale manual procedure has also been developed and applied in the north of Thailand by Professor Romsai. This uses a spray mixing table which houses two large salt-containing basins that can be emptied from the bottoms. Spray bottles are hand operated at the rate of 50 kg in ten minutes or 300 kg/hour. By this technique, about 18 tons of salt can be iodized in 20 working days. The equipment expense is minimal, consisting only of the table, costing about US $50. As in the procedure described above, the success of this procedure will depend on the evenness of spraying and vigor of mixing of the salt.
These village methods cost virtually nothing in equipment and put the village in direct control of its own salt iodization. These advantages must be balanced against the difficulty of achieving even distribution of iodate in the salt, the general labor inefficiency of these small operations, and the dependence on constant reliable operation. These programs are probably best viewed as an inexpensive way to initiate an iodization program and obtain community interest, in preparation for a more conventional and sustainable salt iodization program over a larger geographical area.
Iodization equipment can often be housed in an existing salt storage warehouse which can be done by reallocating the space available or by adding a separate room. If, however, a new building is to be added, it should be done on a very functional basis. For a 5 tons/hr plant working 8 hours a day, the daily requirement of salt is 40 tons. Assuming a raw salt stock of 15 days and a finished product stock of 15 days, the total storage capacity should be 1200 tons. On this basis, the building area is estimated in this table which estimates the required building area for 5 ton/hr, single shift plant
|Salt storage, 1200 tons (4 ton/m²) ||300
|Alleyways and passages (add 30% of storage) ||90
|Iodization plant and processing room ||100
|Stores for spares, etc. ||16
|Switch room ||6
|Bay for loading and unloading ||150
|Total Area Required ||687 m²|
Operations Control - Spray Mixing Process (batch and continuous)
The success of the spray-mixing process depends on ensuring a steady and uninterrupted flow of salt from the conveyor belt. The spray should cover the entire width of the sheet of salt falling into the chamber. The movement of the belt must be even and the speed uniform. The belt tension should be checked to ensure that the throughput rate does not fluctuate owing to belt slippage. Care should be taken to see that the air pressure is maintained at the desired level (25 psi).
The KIO3 solution should be prepared by dissolving the pre-weighed quantity of iodate powder in water(preferably distilled) and filtering it through a fine cloth to avoid clogging the nozzles with any undissolved crystals and extraneous matter, and then analyzed to check the concentration. For a quick check, the concentration can also be measured with a precision hydrometer. An iodate solution concentration of 25-30 gms per litre is recommended. Evaporation of the iodate solution and crusting in the nozzle present the danger of clogging the nozzles. It is therefore advisable to check the nozzles every day and to clean them by immersion in boiling distilled water for 30 minutes whenever necessary, or at least once a week. The nozzles may have to be replaced once a year.
The chemist should collect samples of iodized salt at regular intervals as it flows out of the chutes. He should analyze them immediately for iodine content, and advises the plant operator to take corrective measures, as needed, by adjusting the flow of salt/spray. This analysis must be very prompt to permit effective control of levels.
Iodized salt should be collected into bags directly as it flows out of the chutes instead of allowing it to fall onto the ground, because any moist crystals may pick up dust and dirt.
Maintenance of Iodization Equipment
All parts of the plant that are not stainless steel should be regularly cleaned with rags to brush away salt particles and given a coat of anti-corrosive paint periodically as outlined below.
Maintenance painting is necessary to protect equipment of mild steel (carbon steel) from saline corrosion. This involves surface preparation and coating. Surface preparation and application are as important to the successful life of a coating as the coating itself. Without proper surface preparation, the most resistant coating will fail in service. After hand cleaning the steel of scale, dirt and grease, the surface has to be sand blasted to near white metal (NACE (National Association of Corrosion Engineers) No. 2 finish). Where the surface is small, it has to be thoroughly scraped with a steel wire brush.
For coating, a paint based on epoxies, neoprene chloroprene, chlorinated rubber, polyvinylidene chloride, or polyvinyl chloracetates is recommended. The proper coating procedure begins with two primer coats, including a first coat of 2-component cold-curing epoxy resin-based primer with zinc chromate, color grey and a second coat of the same primer, but of grey colour with bluish tinge, to give a total thickness of two coats not less than 80 microns, followed by two final coats of two- component cold-curing epoxy-based paint.
All the bearings should be well greased so that the operational efficiency of the plant is maintained. Jammed rollers and slow-moving belt conveyors will give uneven iodization.
All electrical connections and control points should be periodically checked to prevent short-circuiting from corrosion.