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).
SOLAR SALT
Introduction
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.

Graph ¡°A¡±
demonstrates which compound crystallizes over what range of density (click
for larger view).
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.

Click graph for larger view
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