SOLAR SALT ENGINEERING
A SERIES OF NOTESPrepared over the years to familiarize people who are not too familiar with solar salt production as done in the modern salt plants to produce high quality salt at high production rates efficiently.
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Basic necessities for a properly functioning wash plant and refinery (if the salt is made and harvested properly) are discussed below
The first section is titled "SOLAR SALT" and is a discussion of how a plant must be operated to achieve high purity or to control the purity at the desired level.
The second section is concerned with the different types of "Harvesting" and harvest transport systems to harvest the salt and get it to a wash plant without contamination with insolubles (mud).
Salt is a general term used for Sodium Chloride (NaCl). Its most common form is table salt used in food preparation, canning, preservation and adjusting taste of foods. However that is only a small fraction of the many uses of Salt such as Chemical Manufacture, highway deicing, water softening, hide tanning. There are thousands of end use products from the chemical industry use of salt such as plastics, glass, paints and on and on.
A major portion of the salt used today is Solar Salt. All the mined salt in the world is from ancient sea beds where salt was deposited and later covered with deposits of clay , sand or rock. Since these deposits were formed naturally, they all contain various levels of impurities. Mined salt, normally called rock salt requires extensive washing processes to refine to a level suitable for human use. In most cases the salt is dissolved in place and recrystallized in vacuum evaporating plants for human use. Solar Salt produced from sea water can be controlled to produce salt that is virtually as pure and clean as vacuum evaporated salt. Vacuum salt is about 99.9 % pure NaCl, Solar Salt is routinely produced by the worlds major salt plants at purity of 99.7 % pure NaCl. The operating procedures to achieve that level of purity will be explained later in this discussion.
Salt has been recovered from the sea since the beginning of time, first in small natural pools along the sea shore where the highest tides of the year filled the pools and then would dry up before the next high tides would refill the pool and dissolve the salt. Salt would be crystallized during that period and could be recovered by simply picking it up. Small hand built levees were added and logs laid over the openings to the sea to trap additional sea water and gradually the ponds were made larger with ditches, gates and pumps of different types were used. The early pond systems consisted of very small ponds, sometimes lined with clay tiles or wood, the salt was raked up after deposition, taken out and the ponds refilled. Primitive windmills were and are still used in some of these systems in the islands in the Atlantic Ocean.
The modern solar salt plant usually consists of a large series of ponds, reducing in size progressively from the sea water intake point to the crystallizing ponds. In plants where the rainfall is seasonal but not too severe the partially concentrated sea water brines are simply left in the "concentrating" ponds over the rainy season. The "crystallizing" ponds are drained, the salt remaining after the harvest dissolved by the rain and the ponds, dried, graded and compacted for the next years crop. In climates where there is minimal rain in the rainy season the plants operate year round only interrupting harvesting salt for a few days when it rains. In climates that are very hot with heavy evaporation and fairly significant rain but not all concentrated in a short period they also operate year round, just avoiding harvesting during the heaviest rainy periods. In climates where there is heavy evaporation and very heavy rains during the rainy season, the entire pond system is drained and restarted each year.
Solar Salt Production Process
The oceans of the world contain many chemicals, some in very minute quantities. The predominate chemicals are Sodium, Chloride, Magnesium and Sulphur. These combine during crystallization to form various compounds as water evaporates in a situation where the compounds will crystallize out of solution.
Fortunately these compounds form in what the chemists call fractional crystallization, that is they come out of solution sequentially. The various compounds crystallize at different times as the solution increases in density although they all overlap to some extent.
The various density measurements are shown on the bottom of the graph, we will use Degrees Baume--deg BE.
The curves on the "A" graph are labeled CaS04,(Gypsum) NaCl,(Salt) MgCl,( Magnesium Chloride) MgS04 (Magnesium Sulphate) and KCL.(Potassium Chloride).
The Gypsum crystallizes first and it can be seen that a very large part of the Gypsum has crystallized before Salt starts to crystallize at 25.9 deg Be. This feature is used in the production of high quality Solar Salt by not allowing brine of less than 25.9 deg Be to be fed to the crystallizing ponds.
On the graph the vertical lines show a crystallizing section B and C. The section labeled B ends at 29.5 deg Be and is the point that the strong brines must be drained from the crystallizers to produce the 99.7% pure NaCl required by the Chemical Industries. The C section of the crystallizing area on the graph ends at 32 deg Be and produces salt suitable for human use but contains more of the Magnesium chemicals and gives the salt a slight sting to the taste. The gypsum is crystallized with the salt on the front end of the crystallizing period in many primitive solar salt plants in the world (by feeding under saturated brine to the crystallizing ponds. To people receiving high purity salt for the first time it doesn't taste as salty because it lacks the sting they are used to. The Salt still in solution at 29.5 deg Be can be recovered by taking the "Bittern" to a separate pond, crystallizing the salt on to 32 deg Be, then dissolving it with sea water and feeding that "made" brine back into the concentrating system--it will make good salt.
Normally the concentrating ponds are set up in a 10 pond series of progressively smaller sizes to prevent back mixing of brines and to prevent time delay in concentrating a large body of water.
The "D" graph shows the salt content of the brine in the crystallizers and the decline in evaporation rates as the brine is concentrated to higher strength in the bittern range. Less salt is produced the higher the bittern discharge salinity.
The common practice of retaining bittern ( salt brines in the crystallizers above 32 deg baume) must be changed and not carried over from crop to crop. Salt produced from this type operation is not as firm as normal salt and will not support harvesting or transport equipment. Additionally this practice actually reduces that amount of salt that would be produced by draining away the bittern and refilling with new brine.
As can be seen from the ¡° D¡± graph:
1. The amount of salt in the brine decreases significantly in the high salinity ranges as shown on the upper graph.
At saturated conditions -- 25.9 deg be there is 2.177 lbs/gal of NaCl in the brine, at 32 deg be there is only 0.865 lbs /gal.
2. The evaporation rate deterioriates rapidly in the high be ranges as shown on the lower graph where the evaporation of an open body of fresh water is compared to various salinity brines in open ponds.
This is a major double negative effect, greatly reduced evaporation trying to make salt from brine that has very little salt anyway. The salt that is visible as crystallizing from high strength brine is probably mostly magnesium salt and not sodium chloride. This visible effect is the major reason most operating personnel are reluctant to drain bittern. The visible jump in measured salinity when adding bittern back into a virgin brine is a false impression, the resultant brine will not make salt at the normal 25.6 deg baume but will have to be higher before it starts making salt.
One other effect not so obvious is the fact that with the greatly reduced evaporation there is little demand for fresh brine ( containing salt) to be added to the crystallizing area.
Measured evaporation and rainfall values at the site are used to determine the amount of land required for concentrating and crystallizing ponds.
All the machinery required for the concentrating pond part of the system is adequately sized propellor pumps, similar to those used for irrigation. Ditches and gate structures between ponds and pipes under roads etc are also used. The same is true for the crystallizing ponds as well until the salt is made.
Harvesting and Washing of Solar Salt
Salt made in accordance with the previous procedures will be over 99% pure NaCl right in the Crystallizing ponds and only a simple washing process using brines available in the pond system is required to wash the salt¨Cs small amount of fresh, brackish or sea water is used as a final spray to flush away some residual brine as the salt leaves the washer.
With the salt washer located adjacent to the crystallizing ponds, all the soluble impurities are recovered back to the pond system¨Ca small settling pond is used to deposit the insoluble impurities removed by the washer.
The Harvest methods employed depend on many factors, many of them controlled by the climate. If the rain is small and the evaporation high enough, a layer of salt is laid down as a floor and each years crop is made and harvested from the top of that floor which is never removed. This is the most desirable of all the methods used in harvesting because the entire operation is so much simpler, reliable, low cost and produces clean salt. Transporting salt from any harvesting device to the pond shore and roadway places severe pressures on the pond floor below the salt. In order to cope with these problems in climates where mud floors are necessary, small portable railroads, conveyors, small rubber tired trains and wheel barrows are used in many plants around the world
Depending on the effectiveness of the harvest method and its effect on the amount of insolubles in the salt, a finished product of 99.7 pure NaCl can be achieved if all the procedures listed are followed.
Solar Salt Refining
The Salt produced in this manner requires very little refining, just drying crushing , screening and packaging with appropriate additive mixing systems.
Guy Wilkins, Professional Engineer