“Modeling and Simulation of Solar Evaporative Crystallization of Lake Brine” (The Case of Lake Abijata/Salla)
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Date
2006-07
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Addis Ababa University
Abstract
Abijata Lake water constitutes the raw material for the production of salt in solar salt
works. The salts are:
Industrial salts------Sodium carbonate (Na2CO3)
Edible salt-----------Sodium chloride (NaCl)
The concentration of the Lake water through solar evaporation results in the successive
crystallization of less soluble salts (Na2CO3, NaHCO3) first, followed by NaCl. Salt work
uses the empirical Baume (OBe) Scale, to measure the concentration of the brines.
According to that scale the Lake water concentration is 4.22 OBe (specific gravity of
1.03).The crystallization of Trona (Na2CO3.NaHCO3.2H2O) begins at 26.13 OBe (specific
gravity of 1.22) and sodium chloride (NaCl) crystallize at 39.16 OBe (specific gravity of
1.37)
Solar crystallization Processes
Producing salt from the lake water involves the selective recovery of pure Trona
(Na2CO3.NaHCO3.2H2O) and sodium chloride (NaCl), free of other soluble or non-soluble
salts and other substances. To this end, an appropriate quantity of Lake Water is
concentrated through natural evaporation, which leads to the fractional crystallization of all
salts contained: a process based on their varying solubility. The basic step in the processes
of salt recovery from the Lake water was made with the division of the evaporation basins
in to two:
The first basin, usually called nurse pond (pre-concentration pond), was used for the
production of saturated brine, which was feed in to second basin, usually called crystallizer.
What basically elevates Salt works to ecosystem is the fact for Lake Water to be
concentrated up to the point salt crystallization. 90% of its salt content has to be evaporate,
thus requires a vast surface. For this reason ponds (pre-concentration ponds) take up 90%
of the salt work area. Crystallizers take up approximately 10% of the salt work area.
Crystallizes take up the remaining 10% of the area. These basins are especially designed
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and have their bottom leveled and concentrated, aiming to facilitate and optimize the
collection of salt with machinery.
The first pond of the salt work is feed with Lake Water (raw material) usually via pumping.
As the lake water flows along the pond, its concentration raises continuously through
natural evaporation. The evaporation (concentration) of brine is achieved by exposure to
solar radiation and with the help of the prevailing microclimate in the area, especially the
winds, rainfall, air temperature and humidity and duration of sunshine. So an increasing
salinity (concentration) gradient is created throughout the ponds of the salt works with a
simultaneous and continuous reduction of the volume of Lake Water, which initially
entered the system of ponds. This is the physicochemical process of salt production.
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Mechanical Engineering