Commercial questions about salt  

 

1. What are the best mixing/blending techniques for salt additives? 
2. What are the best manufacturing practices for preventing salt dust when loading and unloading or transferring loose salt outside?  Inside?
3. How much and what form of iodine (as iodate, or iodide) is permitted to be formulated in food grade salt in a various country?
4. What are the best ways to improve the purity of solar salt? 
5. How does one keep stockpiled salt from forming clumps or from setting up as hard as rock?
6. How can salt be kept from drawing moisture in packaging?
7. What are storage recommendations or guidelines for packaged products? 
8. What is total moisture and how does that differ from surface moisture?  How is this value measured?
9. How does kosher salt differ chemically and physically from other salts and how do these characteristics affect its functionality in different applications?
10. What are the food additives that are permitted in the U.S. to be added to salt and what are their Federal Regulation citations? 
11. How can you avoid material segregation in bulk salt handling? 
12. Why is the direct method of sodium and chloride analysis so prone to error and which alternative methods show the most promise? 

References/ Footnotes:

 

 

 

 

What are the best mixing/blending techniques for salt additives? 

First and foremost, the type of salt to which additives are being introduced needs to be determined.  It is also important to determine the type of process as batch or continuous.  Once the process type has been established, then you need to examine the ingredients and salt type for particle size homogeneity.  Proper mixing procedures depend upon the above-mentioned factors and the best blended mixtures will take these factors into consideration.

 Evaporated salt

For dry, evaporated salt where the additives have similar particle size as the salt, the additive (or additive mixture) can be added to a screw conveyor or auger if a continuous process is in use. As the mixture of additives and salt travel along the length of the screw conveyor or auger, the motion of the screw mixes and blends the ingredients.  The longer the screw conveyor is, the better the blending process, because the retention time inside the screw conveyor enhances the blending process.

For batch mixing, a commercial stirring device or even a clean, dedicated cement mixer can be employed.  Contact time is important.  Initially, one should test the timing of the blending process by taking several samples during the blending to determine if the additives are being well mixed, and what the optimum timing is.  Once the optimum timing is determined, procedures should state specifically the proper timing for optimum mixing and periodic tests should be performed to assure the quality and homogeneity of the mixture.

 Dry, homogeneous particles can also be mixed together by blending at a point where the mixture will be traveling and mixing, such as moving across several transfer points.  

 Sea salt (ungraded) and Rock salt

Other salt types, such as sea salt and rock salt are not necessarily homogeneous in particle size distribution and therefore tend to segregate by particle size.  If salt is transferred during processing, such as falling into a bin from a conveyor, falling into a truck or even transferring from conveyor into a pile, segregation takes place.  This is particularly important when salt additives need to be evenly dispersed but are smaller in particle size than the salt to which it is being added.  When this is the situation, it is often necessary to consider spraying on the additives as a liquid or water solution, using as little water as possible unless the salt is already quite moist.  Here again, it is important to use adequate blending techniques as mentioned above. Liquids can be added to screw conveyors and augers as well as dry ingredients.  If the salt is to be dried, such as kiln drying, the moisture will evaporate leaving the additives adhering to the larger salt crystals.  Be sure to investigate the impact of applying heat to the additives.  For example, are you adding ingredients that are heat sensitive, such as some iodine-containing compounds?  In such instances, it would be a better plan to add the ingredients to dry, warm or cool salt, rather than risking the loss of iodine in the heating process.  Another alternative would be to select another type of more heat-stable, iodine-containing additive, as long as it is approved safe for food use.

For batch processes, the ingredients are measured by weight or volume and added to a mixing or blending container.  Even a dedicated cement mixer may be used.  The time needed to obtain good mixing can be determined by taking periodic samples and assaying for the additive.  If the mix initially meets the set limits, then take earlier samples. 

 

What are the best manufacturing practices for preventing salt dust when loading and unloading or transferring loose salt outside?  Inside?

Transferring salt inside or outside, for whatever reason, can be a very dusty operation if measures are not taken to contain fine particles.  Even when there is no wind, fine particles of salt dust will still escape into the atmosphere, causing a potential contamination incident.  Many communities have regulations on airborne particles (nuisance dust) that can contribute to anything from smog and haze to film on metal vehicles and possibly causing corrosion to vehicles and metal equipment, and possibly dusting neighboring crop fields. Inside a plant, salt dust can corrode machinery, cause a nuisance and might even lead to employee complaints.  There are ways to contain or control dust, keeping it to a minimum. 

Outside

Filling a vehicle from a bin.  If the transferring is accomplished by gravity feed, the chute should have a flexible, telescoping snout extending well into the vehicle cargo area that will contain the salt.  This will keep the salt flowing, but minimize the wind currents from carrying the smaller particles and dust off into the wind.  As the vehicle cargo area is filling, the telescope mechanism can be lifted or shortened to keep the chute from being buried in the salt as it fills the container.

Building a stockpile.  The act of transferring salt (or even other bulk materials) from a conveyor belt while building a salt pile is another place that salt dust can become airborne.  Here again, it is important to cover the conveyor belt, especially at the transfer point.  If covering is not possible, then the dust can be controlled by minimizing the height or distance that the salt is allowed to fall.  This means that it is important to keep the distance from the ground to the end of the conveyor as short as possible.  As the pile builds, the conveyor can be lifted to build a higher pile, but here again, the space between the top of the pile and the end of the conveyor should be kept to a minimum.  If at all possible, building a pile on a windy day should be avoided.

 Inside

Be sure all transfer points are enclosed.  Enclosing all conveyors and transfer points will minimize the escape of dust.  Often plants build and utilize containment covers to protect the process and keep it sanitary/safe or to contain dust, but often, employees will need to remove covers to either repair or to inspect/sample from within the equipment.  It is at these times that one is exposing the atmosphere to salt dust.  Often workers/employees will neglect putting back covers into their proper places, especially if they do not understand the consequences of such negligence.  Each work site should have written procedures for all employees to follow and understand that states that these covers, doors, openings, etc., must be returned to their closed state and any spillages must be contained and cleaned up as quickly as possible.

3. How much and what form of iodine (as iodate, or iodide) is permitted to be formulated in food grade salt in various countries?

The U.S. Food and Drug Administration (FDA) approved potassium iodide (KI) and cuprous iodide (CuI₂) as the only sources of supplemented iodine for human consumption in food. Currently, cuprous iodide is not used for supplementation around the globe, most likely due to the price differential between cuprous iodide and potassium iodide, which is much cheaper and more readily available.  The amount of potassium iodide is listed in the Current Federal Register under 21CFR100.155 and is approved as a nutrient supplement in salt.  The maximum limit is 0.01 % as potassium iodide.

 

The most frequently used source of iodine supplementation globally is potassium iodate, due to stability issues that are adversely impacted by humidity, heat and cold, and long periods of storage.  In the U.S., potassium iodate is approved for animal consumption as a source of dietary iodine, but is not approved for human consumption.

 

Most companies that produce iodized salt set a target amount of KI as between 0.006 and 0.008 %.  This range allows for ample dosing and prevents exceeding the allowable limit due to improper mixing and blending.  The potassium iodide should be buffered to prevent the breakdown of KI to free iodine which will sublime (convert from a solid chemical to a gas) and leave the salt matrix.  Some buffering agents currently in use are dextrose, sodium thiosulfate, sodium carbonate, and similar food grade additives. 

4. What are the best ways to improve the purity of solar salt? 
5.  

There are two distinct types of impurities in solar salt:  1) soluble impurities and 2) insoluble impurities.  Each type takes know-how and skill and we will discuss some ways in which your facility can improve its salt purity.    

Soluble impurities

In more primitive times, the production of solar salt made a very impure salt product.  The reason for this was the entrapped brine batch was reduced nearly to dryness.  Thus, it contained all the salts making up sea water.  The majority of these salts were roughly sodium chloride and magnesium salts, now known in the salt industry as bittern salts, primarily for their bitter taste.¹  In order to remove the bitter salts from the solar salt, a process known as fractional crystallization is used. 

The concentration of dissolved salts in sea water does vary by depth of the water and location, but on average it is about 3.5 wt % (3.6^o Bé).²  The makeup of the total dissolved solids in seawater runs around 77% sodium chloride.  Calcium carbonate (CaCO₃) is less soluble in concentrated brine and when the brine reaches that point, calcium carbonate precipitates, leaving other solutes dissolved in the brine.  Usually, by 13^o Bé, soluble iron, calcium and magnesium carbonates are crystallized.  Next, the calcium sulfate is crystallized between 13^o 25.4^o Bé.  Crystallization of sodium chloride (NaCl) begins once the concentrated seawater reaches 25.4^o Bé.  Ideally, sodium chloride precipitation should occur between 26^o Bé and 29^o Bé. 

Since sodium chloride is less soluble in concentrated sea water than the magnesium salts, the optimum process is to allow only the sodium chloride to crystallize, and then to drain off the bitterns containing mainly soluble magnesium salts, such as chloride and sulfate, plus other soluble components such as potassium chloride, leaving the sodium chloride to crystallize in the crystallizing ponds.  If one waits until above 30^o Bé has been reached, the high levels of magnesium in solution reduce the evaporation rate to unacceptable levels.

Once the bitterns have been removed, other concentrated salt brine is added to the crystallizer pond and the crystallization process is allowed to continue.  Proper brine control during concentration and crystallization results in salt of purity >99.7 % NaCl on a dry basis.³  

Insoluble impurities

Several techniques contribute to impurities in solar salt or sea salt.  Some of the impurities exist in the intake seas and should be kept from entering the condensers by using grates or filters.  These are sizable impurities and would include sea creatures, such as fish, eels, and other aquatic life. 

Other types of impurities could come from the base (floor) materials used to build the condensers and crystallizers.  The floor materials should be non-porous (to prevent leaking or influx of more dilute seawater.  Often, calcium sulfate and clay are materials used to build a non-porous floor layer.  On top of this “primer floor,” several inches of sodium chloride crystals are allowed to grow.  These form a seed bed and barrier between the calcium sulfate and the new growth that will be harvested.  This layer of salt is left as a base and is not harvested from year to year.

Another way that impurities enter into the salt is from airborne contamination.  Wind can carry dust and contaminants into the condensers and crystallizers, and depending upon the stage of salt crystallization, the particles can either become occluded into the crystals or can be deposited on the crystal surfaces. 

Vehicle traffic can also stir up dust on roadways connecting the ponds, so efforts should be made to control dust and prevent it from become a contamination problem.  Salt washing enhances the purity of solar salt because it reduces the level of surface insolubles adhering to the crystals. 

After the crystallizers are drained of bitterns, the salt is harvested and sent to a salt washing location.  There the salt is mixed with clean, nearly saturated brine to remove particles and also to replace magnesium brine clinging to the salt crystals.  To make the wash brine, fine salt which is free of magnesium and sulfate is dissolved to make a brine.  This brine is recycled by adding seawater or other weak brine that dissolves the fine salt collected during the salt washing process, however production losses are increased by this method, although purity is improved.  Also limited rainfall will improve salt quality once the salt is stockpiled.  The salt will drain of most moisture with time in stockpile until it reaches a moisture level of ~3.5 %.  Kiln drying will improve the level of dryness, especially if the product is to be packaged.

Some solar salt plants perform further processing on the salt, even re-dissolving it and using the brine to feed evaporators, thus producing a table salt similar other brands made from underground sources of sodium chloride. 

5. How does one keep stockpiled salt from forming clumps or from setting up as hard as rock?

Cover the pile

Inside storage is the best way to keep clumping to a minimum.  This keeps precipitation off the pile.  If this is not a viable solution, then an outside pile can be covered with a water-shedding tarp.

Additive addition

Another method is to add anti-caking or free-flowing agents.  Common types of such additives are:  YPS, Prussian Blue, calcium and magnesium chlorides.  The first two are crystal modifiers while the second two work best as antifreeze for the moisture adhering to the surface of the crystals.  

Use the proper product for the task

Buy product that meets ASTM specifications for Ice control salt.  The current specification is D 632-01. 

The specifications for ice control salt is up for review.  To be certain the new specification does not adversely impact your production due to sieve size changes or additive changes /moisture levels, take an active approach to the ASTM committee that sets this policy.  This specification also impacts AASHTO which is the other specification that departments of transportation use to assay received salt products for deicing. 

YPS Addition

The additive is available in dry form as sodium ferrocyanide decahydrate.  It can be applied as a solid or as a solution to dry salt.  Amounts added can vary greatly from community to community or from state to state, even though guidelines are provided by the regulating agencies, ASTM and AASHTO.  Canadian agencies also have varying required levels and maximums.  It is important to keep levels high enough to perform their function of keeping the salt from setting up, but at the same time, low enough to prevent any perceived or real negative environmental effects.  The topic has been debated as to the environmental safety of both the additive and the salt.  Currently, both are established as being safe for the environment.

6. How can salt be kept from drawing moisture in packaging?
7.  

The humidity in the air will impact salt even in some porous packaging, especially as it cycles above and below relative humidity of 75%.  That is the critical relative humidity for salt.

 

The best way to keep packaged salt from drawing moisture is to prevent the cycling of relative humidity. 

 

For Salt Institute members other helpful methods are explored. Avoid porous fiber containers, like burlap.

Paper packaging

When using paper bags, using several plies with a thin layer of polyethylene between the plies will help keep the humidity away from the salt product.

Consider plastic packaging if it suits your type of product.

Store all packaged material on pallets off the ground or floor, where they can draw moisture.  This allows air to circulate below the pallets as well.

Store packaged product in humidity-controlled areas.  Storing salt packages in areas of high humidity is not as serious as storing them in areas where the humidity will rise and fall above and below a relative humidity of 75%.  As the salt is exposed to high humidity, the salt adsorbs surface moisture and sometimes so much moisture is present, it dissolves the salt to form brine on the surface of the crystals.  When the humidity falls below 75%, the moisture moves away from the surface of the salt and the brine recrystallizes, sometimes forming “bridges” between crystals that can be seen as clumping or caking.

Use of an anti-caking or free-flowing agent or a combination of these can eliminate most caking and clumping.  The anti-caking agent works by coating the salt crystals with an absorbent and allows the salt crystals to slide against one another without sticking or forming bonds.  The free-flowing agents are usually crystal modifiers that change the crystal structure so that the bond formation is much weaker, thus minimizing clumping and caking.

7. What are storage recommendations or guidelines for packaged products? 

Whatever method you eventually use, your business should practice FIFO (first in, first out) production and storage.  This means that you should ship or use the oldest product first and proceed to the next oldest date and so on.  In order to practice FIFO, planned storage must be maintained. 

The U.S. and Canadian salt plants produce palletized packages that are held in place with plastic stretchwrap, whether it is case-packed material or 50 lb. bags, or 1000 lb flexible sacks with liners.  The stretch wrap should include a portion of the pallet to keep the load stable and from experiencing shifting in transit.  Palletized bags should not be stacked higher than 6 rows of bags or for a total weight of no more than one ton.  When storing the pallets in a warehouse, the pallets can be stacked on top of one another if they are fairly level.  Often wooden spacers are used between stacked pallets to protect the bags from tearing and to assist in keeping stacks level.  The height pallets can be stored is subjective.  Safety must be foremost.  The danger of pallet stacks falling is a serious one and death can result from being crushed.   Pallets should never be stored against a wall.  It is recommended that an 18” to 2-foot space be left clear between the wall and the pallets to allow for inspection and for pest control. 

Rodents and other vermin, if present, will keep to the perimeter of the building for movement and this is an excellent area to set up a pest control system for containment and abolishment.  It is also recommended that wooden pallets be inspected for insect infestation, and that pallets can be reused, however, food grade shipments should be limited to either new or food-dedicated pallets.  Cross contamination can occur if pallets shipped to chemical plants are mixed in with food grade pallets.

8. What is total moisture and how does that differ from surface moisture?  How is this value measured?

Total moisture is a combination of surface moisture or the moisture held on the surface of the crystals, and interstitial moisture, the moisture inside the crystals of sodium chloride.  These two types of moisture are released under different physical conditions.  To measure these two types of moisture the following procedures are used:

Surface moisture is held with only moderately weak bonds and can be driven off by heating a salt sample in an oven to a temperature above the boiling point of water to drive off surface moisture.  If the sample is weighed prior to heating, in a previously tared vessel, and then after 2 hours at 110^o C, cooled in a desiccator, and then reweighed, the loss of moisture can be calculated as a percent of the total weight of the sample. 

Interstitial moisture is then determined on the previously dried sample at a much higher temperature that would release water held in molecular interstices.  The difference in sample weight before and after each heating method determines the amount and type of moisture present.

Total Moisture

Weigh approximately 100 g of salt sample (or sufficient quantity to fill about half of the weighing vessel) into the weighing dish. 

Use a nickel or porcelain crucible as the weighing vessel if both moisture determinations are needed.  Results for surface and interstitial moisture will be derived from the same initial salt sample.  Record all weights to the nearest tenth of a milligram.  Do not handle cooled (or hot) vessels by hand.  Use tongs or wear white cotton gloves to avoid moisture from the skin being transferred to the weighing vessel.

Surface Moisture

Weigh an empty dried aluminum dish (nickel or porcelain if both moisture analyses are needed) with lid.  Heat at 104^o C for 2 hours with the lid off center.  Cool to ambient temperature in desiccator (30 minutes or longer) and reweigh.

Interstitial Moisture

Follow entire surface moisture procedure using a nickel or porcelain weighing vessel.  After surface moisture is determined, place in 625^o C muffle furnace for 2 hours with the lid firmly in place.  Cool to ambient temperature in a desiccator (1 hour or longer ) and reweigh.

Note:  Solar salts contain considerable interstitial moisture.  Crystals burst upon heating at 625o C.  Therefore, it is recommended that the lids be weighed down with a metal weight or several lids stacked on top of each dish.

Calculations: 

% Surface Moisture = ((A + B) – C)  ÷  B  x 100

% Interstitial Moisture =  (C – D) ÷  B  x 100

 Where     A = weight of dish and lid

                        B = initial salt weight

C = weight of dish, lid, and salt after heating at 104^o C

D = weight of dish, lid, and salt after heating at 625^o

Ref. “Official Methods of Analysis”, Association of Official Analytical Chemists (AOAC), Arlington, Virginia, XX edition, (1984).

 

How does kosher salt differ chemically and physically from other salts and how do these characteristics affect its functionality in different applications?

Kosher Salt is a type of salt sold in supermarkets that goes by the special name, Kosher Salt, and this product should not be confused with any other food grade salt product that has been “kosher certified” and bears the emblem of the rabbinical certifying body that certified it.  Kosher Salt is also “kosher certified”, but this is not the only distinguishing feature of this type of product.  Mainly it is pure salt and contains no additives. 

Kosher Salt not only has no food additives, it also has a coarser crystal structure than ordinary food grade salt products.  Not all types of kosher salt look similar due to the manufacturer’s process that is used to produce the Kosher Salt crystal.  Some crystals are the coarsest product of the Alberger process of salt recrystallization which involves an open-pan method of formation, while other companies use a compaction technique to produce the coarser crystal, combined with grinding that forms a resulting coarser evaporated salt format.  Kosher Salt can also be produced from solar salt.  Other than the characteristic larger grain format, the product, Kosher Salt, contains no additives.

10.  
11. What are the food additives that are permitted in the U.S. to be added to salt and what are their Federal Regulation citations?

All of these additives are approved for food addition in their purified forms. The allowable amount for sodium thiosulfate is <0.01 %, and it is considered GRAS at this level (21CFR 184.1807).  YPS is defined for addition to table salt as GRAS at <13 ppm calculated as the anhydride.  It is allowable as an anticaking agent in salt (21CFR 172.490).  Dextrose is considered GRAS (21CFR 84.1857).  Polysobate 80:  listed as <10 ppm in finished sodium chloride; used as a processing aid (21CFR).  Sodium aluminosilicate: GRAS < 2% (21CFR 182.2727).  Tricalcium phosphate: GRAS < 2%.  Tricalcium silicate: GRAS < 2% (21CFR 182.2906).

11. How can you avoid material segregation in bulk salt handling?

Salt that is varied in particle size distribution is prone to segregation upon handling due to the physical properties of the material.  Bulk salt is no exception.  The more it is handled, the more one will experience segregation, meaning that the particle sizes will end up with the large particles in one place and the finer particles in another.  To provide a similar example, any dry packaged cereal or similar product will have the coarser particles on the top with the finer particles settled to the bottom.  This is the perfect example of one form of segregation.  The best way to avoid material segregation is to continuously mixing the salt while handling it, or to avoid rough handling.  If you drop salt from a conveyor belt and from a height to the ground or into a truck, for example, a conical pile will begin to form.  If you sample this growing pile, you will discover that the fine particles are located in the middle of the cone and the coarser particles are located at the edges of the cone as they will roll farther down the slope of the forming cone.

The degree of segregation can be reduced significantly by avoiding these long distance drops of salt to build a pile.  When beginning the pile, set the stacker conveyor so that it is closer to the ground, adjusting it upwards as the pile builds.  This will not completely eliminate the formation, but it will reduce it.  One reason why you want to avoid segregation in salt piles is that segregated salt will clump more readily than will heterogeneous salt.  The fine grades of salt pick up more moisture and then may freeze when the temperature drops or if the moisture dries out, then bridging will occur.

Use a front end loader to mix segregated piles by taking one scoop from the outside of the pile and the next scoop from an area where the center of the pile has been exposed.  Continue alternating the loader buckets and this will help to provide a mixed outgoing load.  Many times, a customer will complain that he/she has received a too coarse or too fine load and that the shipment does not meet the product specifications.  Proper blending and mixing of segregated piles will avoid such complaints.

12. Why is the direct method of sodium and chloride analysis so prone to error and which alternative methods show the most promise? 

The analysis of sodium and chloride in salt is not a simple and straight forward assay because of several issues:

-       Dilution error

-       Ion Interferences

Due to factors such as dilution error and ion interferences, it is best to have analyses provided with laboratories experienced in working with concentrated brine solutions.  If you have an onsite lab, the technique can be learned.  For example, the most common method for determining sodium chloride in low concentrations in a food or in water is to use the Mohr method⁴ (colorimetric) or the direct potentiometric method of silver nitrate titration.  In order to perform either of these methods on a brine sample, one would have to dilute a representative dry or brine sample down so significantly that it would introduce a large error of magnification.  Once diluted, then it would be exceedingly difficult to determine what the impurities present in the salt were.  Ion interferences occur with sodium masking the assays of other constituents such as potassium, aluminum, titanium, vanadium and several transition metals, especially when performing analysis by atomic absorption spectrophotometry, graphite furnace and ion specific electrodes.

The most accurate method for determining sodium chloride concentrations of a salt or brine solution is to use the indirect method:  determine the nature and amount of the trace elements and measure them after determining the total moisture and water soluble levels and subtracting them from the total⁵.  By using the indirect method of trace element assay, one can arrive at a relatively accurate level (with good precision) for the sodium chloride purity. 

References/ Footnotes:

Kaufmann, Dale W. ed., Sodium Chloride: The Production and Properties of Salt and Brine, American Chemical Society Monograph Series No.145, Hafner Publishing Company, New York, 1968, p. 96.

Feldman, S. R., “Sodium Chloride, Halite”, Kirk-Othmer’s Encyclopedia of Chemical Technology, John Wiley & Sons, New York, 2004.

 “Annual Book of ASTM Standards,” American Society for Testing and Material, Philadelphia, PA (2005).

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