Sodium chloride or common salt is thechemical compound NaCl. It
occursnaturally in many parts of the world as themineral halite and as
mixed evaporites in saltlakes. Seawater contains an average of2.6% (by
weight) NaCl, or 26 million metrictons cubic kilometer (120 million short
tonsper cubic mile, an inexhaustible supply.
Underground salt deposits are found in both sedimentary and domal
Properties of Pure Sodium Chloride
Molecular weight - NaCl58.4428
Atomic weight - Na22.989768 (39.337%)
Atomic weight - Cl35.4527 (60.663%)
Eutectic composition23.31% NaCl
Freezing point of eutectic mixture-21.12° C (-6.016°F)
Crystal formisometric, cubic
Colorclear to white
Index of refraction1.5442
Density or specific gravity2.165 (135 lb/ft3)
Bulk density, approximate (dry, ASTM D 632gradation)1.154 (72 lb/ft3)
Angle of repose (dry, ASTM D 632 gradation)32°
Melting point800.8° C (1,473.4° F)
Boiling point1,413°C (2,575° F)
Hardness (Moh's Scale)2.5
Critical humidity at 20 °C, (68° F)75.3%
pH of aqueous solution neutral
Sodium chloride is sold in several different particle sizes (gradation)
and forms, depending onthe intended end use. Fine granulesare typical of
table salt and even finer popcorn salt. Kosher salt, pickling salt and ice
cream saltare slightly coarser. Small compressed pellets are used in water
softeners and large salt blocksare used as salt licks for livestock.
When viewed under strong magnification, all sodium chlorideis crystalline.
Very large cubic crystals, of two, three or more inches in size, can be
seen insome salt mines. They are transparent and cleave into perfect cubes
when struck with a hard object.
atomic structure of sodium atom.
It becomes an positive ion when it loses an electron
formation of sodium chloride (salt)
+positive charge on Na+(sodium ion) attracted to -negative charge on
of rock salt produced varies depending on the type of salt(evaporated,
rock, solar) and on the source. Rock salt typically ranges between 95% and
99%NaCl, and mechanically evaporated salt and solar salt normally exceed
99% NaCl. Evaporatedsalt made with purified brine has the highest purity,
in some cases 99.99% NaCl. Voluntarystandards, such as those developed by
the American Society for Testing and Materials(ASTM), the American Water
Works Association (AWWA) assure appropriate quality for theintended use.
Mandatory specifications for food grade, drug/medical and analytical use
includeFood Chemicals Codex, U.S. Pharmacopoeia, and Reagent Grade
Common salt or sodium chloride is considered assafe for its intended use
as a food additive. TheMerck Index refers to sodium chloride as "(n)ot
generally considered poisonous." Manysubstances in everyday use can be
toxic in high concentrations, even water. reports
Toxic levelsof sodium chloride are reported as:
Human; TDLo: 12,357 mg/kg/23 D-C
Mouse; LD50: 4,000 mg/kg
Rat; LD50: 3,000 mg/kg
Rabbit; LDLo: 8,000 mg/kg
Acute aquatic toxicity
Rana Breviceps (frog); No observed effect concentration (NOEC): 400
Daphnia pulex 48-hour LC50 or EC50: 1,470 mg/L
Daphnia magna (water flea); 48 hour EC50: 3,310 mg/L
Myriophyllum spicatum (water milfoil); Phytotoxicity (EC50 for growth):
Pimephales promealas (fathead minnow); 69-hour LC50: 7,650 mg/L
Lepomis macrochirus (Bluegill) LC50 or EC50: 7,846 mg/L
Anguilla rostrata (American eel) 48-hour LC50 or EC 50: 13,085 mg/L
The chlorides of calcium, magnesium and potassium are generally more toxic
tofresh water species than sodium chloride.
Sodium and chloride occur naturally in soils and waters, and are added by
residential,commercial and industrial activity. Aquatic organisms and
vegetation, including crops androadside grasses, shrubs and trees,
tolerate various concentrations of sodium and chloride.
Thefollowing classification is used by the U.S. Department of Agriculture
to indicate the degree ofhazard of saline soils to food crops. It is based
on conductivity and salinity hazard.(Conductivity can be converted to
approximate mg/L dissolved solids).
USDA Salinity hazard ratings:
Low: 70 - 175 mg/l
Medium: 176 - 525 mg/l
High: 526 - 1,575 mg/l
Very high: more than 1,575 mg/l
Factors that affect the degree of salinity hazard are: soil texture, soil
permeability,drainage, quantity of water applied and the salt tolerance of
Uses of Salt
Every day, each of the earth's 5.5 billion inhabitants uses salt. Annual
saltproduction has increased over the past century from 10 million tons to
about 190million tons today. Nearly 100 nations have salt producing
facilities ranging fromprimitive solar evaporation to advanced,
multi-stage evaporation in salt refineries.
Humans need salt to live. Prehistoric man obtained salt from the meat of
huntedanimals. When man developed agriculture, salt was added to
supplement thevegetable and cereal diet and the quest for salt became a
primary motivation in history. In the mid-1800s, salt's value as an
important raw material for thechemical industry was established when the
Solvay process in Belgium convertedsalt to synthetic soda ash. Salt is,
today, the largest mineral feedstock consumedby the world chemical
Solar Salt Production
Solar salt is produced by the action of sun and wind on seawater or
natural brine in large ponds.
The water evaporates in successive ponds until the brine is fully
concentrated and saltcrystallizes on the floor of the crystallizing ponds.
Solar salt plants must be located in areas oflow rainfall and high
evaporation rates, and where suitable low-cost is available.
Seawater contains about 3.5% (by weight) dissolved minerals. Sodium
chloride is 77%
Solution Mining for Salt
Solution mining of salt or halite deposits is just like it sounds. Fresh
and recycled water isinjected through a well (or wells) drilled into an
underground salt bed, usually between 150 and1,500 meters (500 to 5000
feet) deep. Dissolution of the salt forms a void or cavern in the
saltdeposit. Salt brine is withdrawn from the cavern and transported by
pipeline to an onsiteevaporating plant to make dry salt, or to a chemical
processing plant for chlor-alkali or otherchemical production. Solution
mines located at the site of chemical plants are called captivebrine
Some salt solution mines consist of a single well with concentric casings
extending into the saltcavern. Others consist of several adjacent wells
extending into a single large cavern.
Brine iswithdrawn either through the outer concentric casing in a single
well cavern, or through aseparate casing in a multiple well salt cavern.
The size and shape of solution mined caverns canbe measured and controlled
with well logging devices and operating techniques, thus minimizingthe
potential for surface subsidence. After the end of use for salt or
chemical production,solution-mined salt caverns are often used to store
petroleum or other products.
Vacuum Pan Salt Refining
Table salt is typical of the fine, granulated-evaporated salt produced in
vacuum pane vaporators.. Prior to mechanical evaporation, the brine may be
treated to remove minerals that can cause scaling in the evaporators and
adversely affect salt purity.
Chemical treatment of the brine, followed by settling, reduces levels of
dissolved calcium,magnesium and sulfate. Sulfuric acid treatment or
chlorination may be used to remove hydrogen sulfide, and hydrochloric acid
will neutralize brine used in diaphragm cell production of chlorineand
Brine purification has become increasingly important to produce high
puritysalt for use in chlor-alkali production, particularly in Europe
where dry salt is used extensively for this purpose.
Water is evaporated from purified brine using multiple-effect or vapor
Multiple-effect systems typically contain three or four forcedcirculation
evaporating vessels connected together inseries. Steam from boilers
supplies the heat forevaporators and is fed from one evaporator to thenext
to increase energy efficiency in the multiple effect system.
Vapor recompressionforced-circulation evaporators (pictured below)consist
of a crystalliser, compressor and vapor scrubber. Feed brine enters the
crystalliser vessel where salt is precipitated. Vapor is withdrawn,
scrubbed and compressed for reuse in the heater.
Recompression evaporators are more energy efficient than multiple effect
evaporators,but require higher cost electrical power for energy input. The
development of single stage compressors has significantly reduced costs.
Ultimately, weak brine from either process isrecycled to the solution
mined cavern.Crystallized salt is produced as slurry which is dewatered
first by centrifuging or vacuum drying andthen in kiln or fluidised-bed
dryers where moisture content of the final product is reduced to 0.05%
orless. During this century, salt producers have made significant advances
in lowering energy consumption and in reducing salting and scaling in
Rock Salt Mining
Salt occurs naturally in underground deposits, and occasionally in surface
deposits in arid areas,as the mineral halite. Ancient salt deposits are
widespread. There are ten major salt basins in the western hemisphere. A
number of these salt deposits are mined for halite, commonly known asrock
Salt deposits formed as horizontal salt beds in ancient oceans and were
later buried deeply beneath sediments as mountains eroded. Later, some of
these buried salt deposits were geologically deformed by tectonic forces
within the earth. Salt domes (diapirs) are a major typeof salt structure
resulting from tectonic deformation. Both bedded salt deposits and salt
domes or diapirs are mined by drilling and blasting.
Bedded salt deposits are nearly horizontal, although some contain fault
zones and other anomalies. Salt beds range in thickness from a few tens of
feet to several thousands of feet. Saltdomes or diapirs formed as salt
flowed plastically (because of pressure and heat) upward through overlying
sediments. The result is a vertically elongated salt deposit of a mile or
more in diameter and perhaps 15,000 to 20,000 feet in vertical length. The
tops of some U.S. gulf coast salt domes are very near the surface.
Mining of both types of salt deposit is similar. The method of mining is
called "room and pillar"because the salt is excavated by blasting and
loading out a series of rectangular entries and cross cuts. Rectangular
pillars in a checkerboard-like pattern, typically 35% to 50% of
theoriginal salt, remain to support the mine roof. Rooms in bedded salt
mines are 10 ft to 45 fthigh, while rooms in domal mines can exceed 100 ft
When a new mine is constructed, two shafts are excavated down to the salt
deposit, usually between 500 feet to more than 2,000 feet deep. Shafts are
about 20 ft in diameter and lined with concrete. After the shaft is sunk
to the salt, rooms are mined in a planned pattern byundercutting, drilling
and blasting. An undercutter cuts a horizontal slot or kerf along the
floor ofthe advancing room to provide a second free face for blasting. A
drilling rig drills a series ofholes into the face, and an ammonium
nitrate-fuel oil mixture (ANFO) is pneumatically placed into the holes.
The room is then blasted, creating a "muck" pile of salt ready to be
transported to an underground crushing and screen station. After loose
pieces of salt are removed from the roof and pillars for safety reasons,
the blasted salt is loaded into large trucks by front end loaders, or
loaded by load-haul-dump units (LHDs) of lesser capacity that pick up the
salt and haul it to the crusher.
After crushing, the salt is transported by conveyor belts to salt
"pockets" at the shaft bottom for loading and hoisting to the surface. Two
counter balanced salt skips of up to twenty short tons each are loaded
automatically for the 1,000 ft/min trip. At some mines, the skip
loading,hoisting, and surface dumping operations are managed completely by
On the surface the salt is rescreened to remove fines, bagged,
palletised and prepared for shipment to the salt customer. Most rock salt
from underground mines in North America is transported in bulk by ship,
barge, truck, or rail. Some is packaged in ten to 80 lb bags or compressed
into salt blocks for animal nutrition and water softening.
Rock salt mines often have rich histories.
Wieliczka -site of the oldest working salt mine in Europe
In Cracow's proximate vicinity, 14 km south-east from the city center near
Route 4 (E22), liesa well known country wide and abroad, city of Wieliczka
with population of 20 thousand. The city is famous for having the oldest
operational salt mine in Europe which has been working for over 700 years.
This mine, a centuries old tourist attraction, cannot be compared to
anything else in the world.
The mine has been visited from its beginning by the famous and the mighty
of this world.Kopernik, Goethe, Humboldt, Paderewski, Mendelejew, Baden
Powell, Karol Wojtyla - who later became Pope John Paul II, ordinary
people and crowned heads of state, all have visitedthis remarkable site.
Since 1978, the mine is on UNESCO's list of World Class Landmarks of
Cultural and NaturalHeritage - where it was listed among the top twelve
attractions in the world.
The tourist route open to sightseers is only a small fraction of the
entire mine. The mine includes7.5 million square meters of post-excavation
space on nine levels, each between 64 and 327meters.
The tourist route extends to level three only - to the depth of 135
During the two and half hour tour, visitors travel underground
approximately 3.5 km., along passages totalling more then 320 km. During
this time they will visit 30 of the more than 2148 chambers.
The most popular attractions are the enormous excavation chambers, the
tallest being 36 meters high, and two others reaching 30 meters each. The
tour includes a visit to three chapels, theoldest more than 300 years old
and youngest a mere 100 years old. They are richly decoratedwith salt and
wooden sculptures. The oldest salt sculptures date from the end of the
The largest of the chapels - Blessed Kinga - is located 101 meters below
the surface, it is over50 meters long, 15 meters wide, 12 meters high, and
has the volume of 10,000 square meters.It is entirely made out of salt,
richly furnished with sculptures, bas-reliefs, and large chandeliers made
from salt crystals.
The three underground salt lakes are a real attraction, the deepest
reaching seven meters. Signs of ancient mine works, wooden structures,
machines and equipment hundreds of years old, may all be noticed
throughout the tour. All this is located within the underground scenery,
remarkably rich in geological specimens which illustrate the complicated
structure of this deposit. In many places, this beauty is most tangible.
An hour and a half into the tour, it is time to take a break in the
"Warszawa" chamber which often serves as a sport, ball, theatre and even
an opera hall!
The second part of the tour covers 10 chambers representing an underground
exhibition of Cracow's Salt Mine Museum .
The collection includes antique tools, machinery and equipment,lighting
equipment, as well as an exposition of surface archaeological excavations
and interesting geological specimens.
The exhibition also includes painting portraying the work of miners in the
old days, old models of buildings and mining structures, and a miniature
model of Wieliczka 350 years ago as well as the cross section of the
old mine. No description, no matter how detailed, will convey the
charm of this mine and its underground attractions. An additional
advantage of the mine is its underground microclimate which is
especially beneficial for asthma sufferers. Sanatorium stays have
even been organized here periodically.