A State of the Art Paper on Improving Salt Extraction
from Lake Katwe Raw Materials In Uganda
Kasedde H.1,2, Kirabira J.B.2, Bäbler M.U.3, Tilliander A.1, Jonsson S.1
1. Department of Materials Science and Engineering, Royal Institute of Technology
KTH, Brinellvägen 23, SE-100 44 Stockholm, Sweden.
2. Department of Mechanical Engineering, College of Engineering, Design, Art and
Technology, Makerere University, P.O Box 7062, Kampala, Uganda.
3. Department of Chemical Engineering and Technology, Royal Institute of
Technology KTH, Teknikringen 42, SE-100 44 Stockholm, Sweden.
Abstract
The characteristics of Katwe salt lake are briefly discussed. The lake is the largest of the eight
saline lakes in the Katwe-Kikorongo volcanic field and is a major source of salt production in
Uganda. Today, salt production at the lake is carried out using traditional and artisanal
mining methods. Attempts to mechanize the production of domestic and commercial grade
salt at the lake were unsuccessful due to the use of a wrong technology. In this paper, the
most common available technologies for salt extraction from brine are described. These are
divided into four broad categories, namely thermal, membrane, chemical and hybrid
processes. A review of the state of the art, previous research and developments in these
technologies is presented. A detailed analysis of the processes used was done based on
studies reported in the literature. From the analysis, it was observed that thermal salt
production processes, especially distillation and solar evaporation have the highest share in
installed capacities worldwide. Membrane technologies such as Electro-dialysis, Reverse
Osmosis and chemical technologies have not found wide application in the commercial salt
industry. Electro-dialysis and Reverse Osmosis have been used mainly as pre-concentration
processes for subsequent thermal processes. Prospects for application of hybrid systems for
salt production through integration of thermal desalting processes should be investigated for
better performance efficiencies and recoveries at the salt lake.
Keywords:
1.
Lake Katwe, salt, salt recovery and purification, separation processes.
Introduction
Salt, also known as sodium chloride, the most common evaporate salt is an ionic chemical
compound which has a chemical formula NaCl. It is an inexpensive bulk mineral also known
as halite which can be found in concave rocks of coastal areas or in lagoons where sea water
gets trapped and deposits salt as it evaporates in the sun. Originally, salt was produced for
human consumption purposes and later on its significant applications were discovered. This
has made salt one of the most important commodities for centuries, comparable to the
importance of oil in the present times (Korovessis and Lekkas, 2009). The use of salt as a
food preservative together with its economic significance began to decline after the industrial
revolution hence finding its extensive use in the chemical industry and other applications.
There are more than 14,000 reported usages of halite, and it, along with other salts, has
played a very important role in human affairs (Kilic and Kilic, 2005).
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Today, the annual world production of salt has increased to an estimated 270 million tonnes
(US Geological Survey, 2011). Statistics also indicate that the chemical industry accounts for
about 60 percent of the total production followed by 30 per cent for human consumption and
lastly 10 per cent for other applications such as road de-icing, water treatment, production of
cooling brines, and in agriculture. The chemical industry uses the salt as a raw material for
production of chlorine, caustic soda and soda ash for petroleum refining, petro-chemistry,
organic synthesis and glass production (Sedivy, 2009).
Almost two thirds of the world countries have salt producing facilities, ranging from
traditional solar evaporation in salt pans to advanced, multi-stage evaporation in salt
refineries (Abu-Khader, 2006). Over 100 countries produce a significant amount of salt with
many others on a small scale. The Chinese salt industry is the largest world producer of salt
(US Geological Survey, 2011). In the year 2011, its production accounted for about 22.2
percent of the world’s total production followed by the United States at 16.7 percent. Other
major producers are India, Australia, Mexico, Germany and Canada.
There are three methods used to produce dry salt based on the method of recovery (AbuKhader, 2006).
(a) Underground mining: Also known as rock salt mining, this process involves
conventional mining of the underground deposits through drilling and blasting
whereby solid rock salt is removed. Mining is carried out at depths between 100 m to
more than 1500 m below the surface.
(b) Solar evaporation method: This method involves extraction of salt from oceans and
saline water bodies by evaporation of water in solar ponds leaving salt crystals which
are then harvested using mechanical means. Solar energy of the wind and sun is used
in the evaporation process. The method is used in regions where the evaporation rate
exceeds the precipitation rate. Over a third of the total global annual salt production is
produced using this method.
(c) Solution mining: Evaporated or refined salt is produced through solution mining of
the underground halite deposit and removing the water from the saline brine which is
pumped to the surface. The water is evaporated from the brine using mechanical
means such as steam-powered multiple effect or electric powered vapour compression
evaporators. In the process, thick slurry of brine and salt crystals is formed.
Among the above methods, solar evaporation is the most widely used salt production
technique at Lake Katwe in Uganda. The process is carried out following ancient traditional
and artisanal methods leading to low qualities and limited production. An attempt to
mechanize the production of domestic and commercial grade salt from the lake was
conceived in 1975 with a chemical plant being installed to enable purification of the brines.
The plant was commissioned, worked for a few years and later collapsed mainly due to a
poor design that subsequently led to corrosion of the poorly designed heat exchanger tubes
(Mathers, 1994). Several attempts to revamp salt production through re-establishment of the
plant or a process that would recover the salt together with its purification failed and thus the
plant has remained inactive implying that the country’s salt resources have not been fully
exploited to effectively contribute towards the country’s economic growth.
2
The objective of this study is to gain an understanding of the available salt mining and
extraction technologies, determine the previous and current state of research in the salt
recovery techniques, highlighting benefits and limitations of the processes with a view to
devise suitable practical and sustainable methodologies that could be implemented in order to
realize the commercial potential of Lake Katwe salt raw materials.
2.
Lake Katwe general characteristics
2.1
Location and Access
Lake Katwe is the largest of the eight saline crater lakes within the Katwe-Kikorongo
volcanic field in Western Uganda and situated on the floor of the western rift valley and
south-east of the Rwenzori mountains (Figure 1). Preliminary investigative fieldwork studies
indicate that the lake contains the best salt reserves evident in its brines and evaporate
deposits. There are 22.5 million tonnes of crystalline salts in Lake Katwe which can sustain a
plant for over 30 years at 40,000 tonnes/a NaCl production (UDC, 1997).
Figure 1: Location of Katwe salt deposit and surrounding areas (Hinton, 2011)
Apart from Lake Katwe, it is reported that limited quantities of salt for human and animal
consumption have been extracted from the hot spring waters at Lake Kibiro located in the
Albertine region and at Lake Kasenyi on the shores of Lake George. At Kibiro, salt is
produced from the waters of the spring and the saline muds around the lake. Reserves for
industrial salts such as Trona also occur at Katwe, Kasenyi and Kibiro respectively. They
however exist on a larger scale at Lake Katwe and Kasenyi (Mathers, 1994).
2.2
Geological setting
The salt lake lies on the floor of an explosion crater formed in tuffs with about 230 meters of
rock separating Lake Edward from the crater at the closest point. The explosion crater ejected
pyroclastics, tuffs with abundant granite and gneissic rocks from the basement which
dominate the area (Figure 2). The volcanic rocks are mainly composed of pyroclastics and
3
ultramafic xenoliths which are deposited on the extensive Pleistocene lacustrine and fluvial
Kaiso beds and in some places directly on Precambrian rocks. The salinity of Lake Katwe
and the other closed saline lakes is derived mainly by evaporative concentration of mineral
spring waters (Arad and Morton, 1969). The saline spring waters filled the crater during
Pleistocene vulcanism. Suggestions have indicated that salt has also been leached out of the
surrounding tuffs by water percolating from Lake Edward into Katwe crater which lies about
30 m below the fresh lake level (Barnes, 1961).
Figure 2: The geology of the Katwe-Kikorongo volcanic field and surroundings
(Bahati and Natukunda, 2008)
2.3
Climate
Although there is not much detailed meteorological data available, the region experiences
two dry seasons between January to March and July to September every year. It is within
these periods that salt is traditionally mined and extracted from the lake.
2.4
Chemistry of the deposit
A number of studies have been done on the Lake Katwe salt deposit mainly in the chemistry
of the mineral springs, feasibility of commercial production through estimation of the salt
reserves and chemical characterization of the salt resources.
Arad and Morton, 1969 investigated the mineral springs and saline lakes of the Western rift
valley. Analysis of the brine samples from various depths in the evaporite deposit at Lake
Katwe reported various concentrations in the solutes. Interstitial brines from over 6 meters
deep were poorer in K+, Cl- and Br+ than those near the surface at about 2 meters from the top
of the deposit. There was also a high proportion of sodium carbonate and bicarbonate in the
evaporate deposit. In addition, the mineral waters within the region possess relatively high
4
sulphate contents as compared to those of the Eastern Rift. They also possess lower fluoride
contents hence suitable for usage in production of food grade salt.
In another investigation, Morton, 1969 carried out a sample chemical analysis of salt grades
produced at the lake. It was observed that Lake Katwe salt varies in composition due to
presence of a number of impurities. It was observed from the analyses that the chief minerals
present in the Lake Katwe salts were Halite (NaCl), Hanksite (9Na2SO4·2Na2CO3·KCl),
Burkeite (Na2CO3·2Na2SO4) and Trona (Na2CO3·NaHCO3·2H2O). Indeed, composition
varies considerably even within the same grades of the salts produced. The best quality
(No.1) salt grade contains over 90 % NaCl but the remainder contains as much as 50 % of
Na2CO3, Na2SO4, KCl and mud. No purification was carried out as the complex chemistry of
the deposit prevents this from being done by simple methods (Dixon and Morton, 1970).
Morton, 1973 reported systematic sampling of the Lake Katwe brine done in 1967 at different
depths of the lake both in the dry and wet season respectively. It was noted that, the salt lake
brine had its ion composition mainly dominated by Na+ and Cl- with lesser amounts of SO42-,
K+, and CO32- ions. Furthermore, it was noted that the compositional extremes resulted from
the effects of dilution during the wet and evaporation during the dry season. There was no
determination of the mean ion concentration of the salt lake brine in a yearly cycle and thus
no detailed study was done to determine the mineralogical and chemical composition of the
raw material.
Nielsen and Dahi, 1995 carried out a study on the fluoride contamination and mineralogical
composition of a rock salt sample from Lake Katwe. Through chemical and mineralogical
analysis, it was concluded that the salt lake contains no fluoride holding minerals. This was
attributed to a very low fluoride concentration of about 0.02 mg/g. In addition, the salt lake
raw material also consists of Trona mixed with Burkeite and Halite. The evaporite from Lake
Katwe contains much more halite than that from other saline lakes in East Africa such as
Lake Natron and Lake Magadi. This is attributed to the high chloride concentration of Lake
Katwe brine which have an equivalent Cl/TAL ratio of 2-3 as compared to 0.23-1.0 for Lake
Magadi and Lake Natron brines.
3.
Salt extraction technologies
To date, several technologies can be used to extract valuable domestic and commercial salts
and minerals from saline water. Based on the type of separation process, the technologies are
classified into four categories:
(a) Thermal processes: These are phase change processes involving use of thermal
energy to evaporate feed water to generate steam which is then condensed, a process
known as distillation. Alternatively, saline water is frozen followed by the separation
of pure ice and salts. Distillation can use any heating source such as fossil fuels,
nuclear energy with solar energy being a low-tech option. Application of this
technique includes solar distillation (SD), multi-effect distillation (MED) or multiple
effect evaporation (MEE), multi-stage flash distillation (MSF), mechanical vapour
compression (MVC), eutectic freezing crystallization (EFC) and open pan boilers.
5
(b)Membrane processes: They involve separation of dissolved salts from the feed water
by mechanical or chemical/electrical means using a selective membrane barrier
between the feed water and product. This principle is applied in Reverse Osmosis
(RO) and Electro-dialysis/Electro-dialysis Reversal (ED/EDR). These processes do
not lead to a phase change.
(c) Chemical change processes: These involve application of chemical techniques to
extract salts from feed water. The principle is applied in the ion exchange, reactive
precipitation and calcination processes.
(d)Hybrid processes: They involve combination of the above processes in a single unit
or in sequential unit or in sequential steps. Examples include: membrane distillation
(MD), membrane crystallization (MCr), RO with MSF or MED (Gude et al., 2010).
In the salt crystallization plants, saturated brine or rock salt and solar salt can be used as a
raw material for the process. A summary of the possible processes for the production of
crystallized salt based on rock salt deposits is shown in Figure 3. Processes that are used in
the production of vacuum salt from sea water or lake brines as a raw material are also shown
in Figure 4. The present technologies together with emerging technologies for salt production
are discussed next.
Figure 3: Processes for production of crystallized salt based on rock salt deposits
(Westphal et al., 2010)
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Figure 4: Processes for salt production from brine (Westphal et al., 2010)
4.
Thermal processes
4.1
Distillation
Several forms of distillation are used for salt recovery and are classified into conventional
and commercial salt production techniques. Processing methods other than solar evaporation
are used commercially in the production of salts, usually in the making of refined (table salt)
and purification of crude salt. These involve MSF, MEE, MVC and open pan boilers. Such
techniques are used to make refined salt in many countries where the climate is temperate or
cold (Aral et al., 2004). The operating principle behind these processes is that the vapour
pressure of the feed water within the unit is lowered for boiling to occur at lower
temperatures without further heat addition.
Conventional distillation
Solar distillation of saline water is a conventional salt production process practised widely all
over the world. It is a simple operation carried out in specially designed shallow salt pans or
ponds. It is within the shallow ponds that the sun evaporates most of the water. Within the
ponds, brine from the sea or lake evaporates due to the effects of the sun and wind followed
by the crystallization process. Here, the salt crystals begin to grow to a point when the salt
layer is thick enough for harvesting. After harvesting, the salts are washed to meet the
required standards and stockpiled to drain and dewater. Notable areas where the method is
predominantly used for salt production on a large scale include Great Salt Lake in the United
States, Dead Sea in Jordan, and Salar de Atacama in Chile (Aral et al., 2004).
Open pan evaporation is another conventional salt production technique involving addition of
heat in open pans. The large and shallow open pans or boilers are made in dimensions of 6 m
x 12 m x 1 m. Evaporated salt forms on the brine surface supported by surface tension. This
7
is an outdated technology that was replaced by wood burning furnaces which have also been
observed to be unsustainable in terms of the energy use (Bauschlicher, 1983).
Commercial distillation
The Multi-Stage Flash process is based on the principle where saline feed water is heated and
evaporated by pressure reduction as opposed to temperature increment. It involves
regenerative heating where saline water flashing in each flash stage gives up some of its heat
to the saline water going through the flashing process. Heated water from an initial stage
passes to another stage at a lower pressure hence forming vapour which is led off and
condensed to pure water using cold brine which feeds the first heating stage. Concentrated
brine is then passed to a second chamber at a lower pressure hence more water evaporates
with vapour being condensed. The principle of operation of the process is shown in Figure 5.
The process is repeated through other stages up to atmospheric pressure. It can contain from 4
to about 40 stages. The process is considered to be the most reliable and most widely used.
Recrystallization is a similar technique as MSF involving saturation of under saturated recirculating brine with solid solar or rock salt at about 100oC and fed downstream to several
flash crystallizers (Westphal et al., 2010).
Figure 5: Schematic diagram of a basic multi-stage flash (MSF) system
(Evans and Miller, 2002)
Multiple Effect Evaporation is similar to Multi Stage Flash in principle except that energy
from an external source is used to heat up the saline water in only the initial stage. The
resulting vapour or steam is used in the subsequent stages to evaporate the water and the
brine is used to cool and condense the vapour in each successive stages hence having a
temperature fall across each stage (Figure 6).
8
Figure 6: Schematic diagram of multiple-effect evaporation technique
(Hartzikioseyian et al., 2011)
In the Mechanical Vapour Compression process, the heat for evaporation of the saline water
comes from the evaporator. The process operates on the principle of compressing vapour to
increase its temperature and pressure. The saline water feed is heated by steam, and part of it
is vapourised. A mechanical compressor and a steam jet are used to condense water vapour
which produces sufficient heat to evaporate the incoming saline feed water (Figure 7).
Figure 7: Schematic diagram of a mechanical vapour-compression system
(Evans and Miller, 2002)
Multiple distillation methods have been used to extract salts from a salt lake as reported by
Kilic and Kilic, 2005. In their study, sodium chloride (NaCl) was produced from the lake
9
brine through evaporation up to a specific gravity of 1.240. This was followed by cooling to 10oC thus forming Thenardite and Bloedite. Further evaporation produced Carnalite, Halite,
Langbeinite and Sylivite. At the last stage, Bischofite and Kieserite were obtained. The same
technique demonstrated by Nie et al., 2009 recovered salts from a salt lake using multiple
isothermal evaporation technique at 25oC. In another similar laboratory investigation, Wang
et al., 2011 performed multiple-evaporation after freezing of salt lake brine to recover salts.
In their study, brine was first frozen at 263.15±0.2 K for a period of two days to a specific
gravity of 1.1115 g/cm3 where Natron (NaCO3·10H20) and Hydrohalite (NaCl·2H2O)
precipitated. Isothermal evaporation at 288.15±0.2 K was then performed on the mother
liquor to recover more salts with increasing specific gravities.
In previous studies, salts have been recovered from desalination plants effluent using this
technique. Mohammadesmaeili et al., 2010, recovered mixed salts through isothermal
evaporation of RO concentrate. Specifically, sodium sulphate, sodium chloride and potassium
salts were produced through continuous evaporation of the desalination concentrate. The
feedstock concentrate had been previously treated with a lime-soda process before
evaporation. In addition, isothermal and isobaric evaporation of reject brine of a desalination
plant has also been carried out to recover various salts for industry and agriculture. Various
salts such as NaCl, KCl, CaSO4, MgSO4·7H2O were recovered from the process (Hajbi, et al.
2009, 2011).
Further research studies have also demonstrated the feasibility of extracting commercial salts
from saline effluent using the SAL-PROC technology. SAL-PROC is an integrated process
for the selective recovery of dissolved elements from saline waters in the form of valuable
salt minerals, slurries and liquid compounds. It involves controlled chemical reactions in
combination with evaporation and/or cooling stages, thus causing precipitation and
crystallization followed by conventional mineral and chemical processing. The process is
particularly suitable for brine with high levels of dissolved sulphate, potassium and
magnesium salts and can be operated at a commercial scale (Ahmed et al. 2001, 2003).
Ahmed et al., 2003 did not implement the feasibility study experimentally.
Sedivy 2000, describes the THERMOSAL salt processing technology developed at Sales
Monzon in Spain as an option for the enhancement of natural evaporation of brine in open
ponds through the use of residual waste heat. When combined with SALEX salt purification
process, the process is the economic alternative to vacuum crystallization for production of
top quality industrial and food grade salt. The process is applicable whenever the raw salt is
available as brine.
Domestic and commercial salts have been produced from geothermal brines in Iceland using
the vacuum evaporation technique with the geothermal wells being the source of salt-brine
and process steam. Specifically, multiple effect evaporation is used to concentrate the brine
before crystallization (Kristjánsson, 1992).
A refrigerator heat pump desalination scheme has been used for recovery of fresh water and
salt from sea water as proposed by Reali, 1984. In the investigation, sea water was
continuously refilled in a salt-water chamber via atmospheric pressure. Evaporation of the sea
water into a vacuum chamber was done hence allowing the water vapour to condense on top
of the fresh water chamber while the salt precipitated at the bottom of the evaporation
chamber.
10
The evaporation and cooling technique has also been used as an initial or final stage in the
recovery of salts from lake brine and sea waters and for disposal of the effluent from
desalination plants (Vergara-Edwards and Parada-Frederick, 1983, Al-Harahsheh and AlItawi, 2005, Ravizky and Nadav, 2007). An advanced solar dryer has been demonstrated to
optimize the recovery of salts from brine effluent of a desalination plant (Pereira et al. 2007).
A number of processes for salt recovery based on the evaporation and cooling technique have
been developed. Winkler, 1985 developed a process for the treatment of brine elutriate drawn
off after evaporation of the brine at an elevated temperature in order to separate some of the
sodium chloride. In the investigation, crude brine containing sodium sulphate and potassium
chloride is cooled in a chamber whereby NaCl, KCl and Na2SO4 salt mixture is separated off
by crystallization. The remaining parent solution can be returned for renewed evaporation to
recover NaCl without disturbing the equalized balance of the secondary salts in the brine to
be processed. The NaCl present in the mixture is recovered by deposition with refrigerated
water and separation of Na2SO4.
Ninawe and Breton, 1989 invented a process for the production of salt in which sodium
chloride is crystallized in form of spheres by evaporation of sodium chloride brine and the
spheres obtained are then broken up. The salt produced by the process is particularly suitable
for the food industry. Extension of the research by Ninawe, et al., 2004 developed a process
for the production of sodium chloride crystals from sodium chloride brine contaminated by
potassium chloride and sulphate ions. In their research, a calcium compound is added to brine
to crystallize Glauberite (Na2CaSO4)2, which is isolated. The resulting solution is then
subjected to evaporation in order to crystallize sodium chloride which is then collected, and
the mother liquor from the crystallization is subjected to cooling hence crystallization of
Glaserite (K3Na(SO4)2).
Vohra, et al., 2004 developed a process for recovery of common salt, potassium chloride,
concentrated magnesium chloride with enriched bromide, and high purity magnesia from
brine in an integrated manner. Chirico, 1979 also presented a continuous process for the
recovery of chemicals in saline water which involves converting the sulphates in the saline
water feed to sodium sulphate, separating and recovering in the oxide forms, essentially all of
the magnesium and calcium from the saline water feed, preparation of sodium chloride
fortified solution by mixing the feed with recycled sodium chloride, crystallizing and recrystallizing and then separating sodium chloride crystals in two evaporative crystallization
processes.
4.2
Eutectic Freezing Crystallization
The eutectic freezing crystallization separates aqueous solutions of inorganic solutions into
pure water and pure salt (Van der Ham et al., 1999). The process is characterized by
operation near the eutectic point of the solution where both ice and salt crystallize
simultaneously (Vaessen et al., 2003). Figure 8 shows a typical EFC process scheme.
11
Figure 8: Schematic representation of EFC for production of pure water and salt from
waste or process streams (Van der Ham., 1998)
Compared with the conventional evaporation and cooling techniques, this process is capable
of reducing energy costs by up to 70%. However, the high investment costs together with the
scale limitations make the process unfavourable but can be overcome with time (Van der
Ham et al., 1998). Experimental investigations of the eutectic temperatures of various
proportions of ions commonly found in natural waters show that the process operates well at
temperatures not lower than -25oC (Bardurn and Manudhane, 1979).
The process has been used to demonstrate separation of ice and KCl crystals from brine based
on the direct cooling technique (Stepakoff et al., 1974). In another investigation, Van der
Ham et al., 2004 demonstrated separation of ice from CuSO4·5H20 crystals in a cooled disk
column crystallizer based on indirect cooling. The process has also been applied at an
industrial scale to study the recovery of purified MgSO4·H2O from a magnesium sulphate
industrial stream emitted from a flue gas desulphurization process (Himawan et al., 2006).
Experimental techniques have also been used to validate theoretical models developed for
predicting alternative pathways for seawater freezing and crystallization (Marion et al.,1999).
However, according to Westphal et al., 2010, the freezing process for the production of salt
from saline water has not got practical importance.
5.
Membrane processes
These processes use a selective synthetic polymeric membrane or resin to recover dissolved
salts when subjected to a pressure gradient or an electrical potential across the membrane
surface. This technique is currently used in RO and ED/EDR. Other pressure driven
membrane processes similar to the RO principle are Ultrafiltration (UF) and Nanofiltration
(NF). ED/EDR is an electro-membrane process.
5.1
Pressure driven membrane processes
Osmosis is the transfer of water through a semi-permeable membrane from a lowly
concentrated to a highly concentrated solution until an equilibrium osmotic pressure is
achieved. In the RO process, pressure greater than the osmotic pressure is applied to the
12
highly concentrated side of the membrane hence leading to the diffusion of water through the
membrane to the fresh water side leaving dissolved salts behind with an increase in salt
concentration. Higher salt concentration in the feed water would require higher pressure. An
illustration of both processes is shown in Figure 9.
Figure 9: Reverse osmosis vs. osmosis (Krukowski 2001 cited in Younos and Tolou,
2005)
NF and UF, in concept and operation are similar to RO. NF is used to partially soften water
and is successful at removing low total dissolved solids as well as dissolved organic carbon.
It uses a coarse form of RO membranes hence has feed pressures lower than those used in RO
systems. UF is a technique for separation of dissolved molecules on the basis of size implying
that larger molecules than the membrane pore size are retained at the surface of the
membrane. The only difference between UF, NF and RO is the size of the molecules that
each process retains. Pre-treatment is very important for RO processes since the membrane is
prone to fouling due to dissolved solutes and impurities. UF is used for the pre-treatment of
Seawater Reverse Osmosis (SWRO) plants (Van Hoof, et al. 1999). Commercial RO
membranes are manufactured from modern plastic materials in the form of sheets or hollow
fibres.
Pressure driven membrane processes are widely being used for separation and recovery of
salts from water. However, such processes do not recover salts from water to the low levels
achieved in distillation processes (Clayton, 2006). An assessment of the capacities of NF and
RO high-pressure membrane systems to separate a high-concentration water solution was
done by Karelin et al., 1996. In their study, RO membranes were capable of retaining NaCl at
a very high concentration in an aqueous solution even when the concentration is near the
saturation level. In addition, NF membranes were capable of separating multi-component
solutions into binary ones, from which salts can be recovered by vaporization. In other
previous works, the membrane separation technique has been used for salt recovery from
waste brine at a sugar decolourisation plant on a pilot-scale (Wadley, et al. 1995), dye-salt
separation using weak acid polyelectrolyte membranes (Xu and Spencer, 1997), brackish
water desalting (Almulla, et al. 2002, Lee, et al. 2004), separation of salts from concentrated
organic/inorganic salt mixtures (Freger, et al. 2000), salt recovery from industrial effluents
resulting directly from a dyeing industry (Allègre, et al. 2004).
13
5.2
Electro-chemical separation processes
ED/EDR is an electro-membrane process which consists of a stack of ion-exchange
membranes (Figure 10) which are arranged between an anode and a cathode. These
membranes are selective to positive and negative ions. Under the influence of an applied
electric potential difference, the ions are forced to migrate towards the electrodes. The cations
pass through a cation membrane while the anions pass through the anion membrane thus
converting the feed saline water into two streams of concentrated brine and fresh water.
Fouling of the ion exchange membranes by dissolved impurities is overcome by EDR which
involves reversing the direction of the electric current. The process is applicable for preconcentration of brine in arid regions unsuitable for solar ponds and having no rock salt
deposits (Bauschlicher et al, 1983).
Figure 10: An Electrodialysis stack (Brunner, 1990 cited in Younos and Tolou, 2005)
The production of table salt from sea water by use of ED to concentrate NaCl up to 200 g/l
prior to evaporation is a technique developed and used exclusively in Japan. More than
350,000 tons of table salt are produced annually using this technique thus requiring more than
500,000 m2 of installed ion-exchange membranes (Noble and Stern, 1995). The process has
also been used for salt production in the Middle East (Kobuchi et al., 1983). ED-MSFcrystallization was also demonstrated for a dual purpose desalination-salt production system
(Turek, 2002). ED-EDR double step electrodialytic pre-treatment and pre-concentration of
coal-mine brine was examined for purposes of salt production (Turek et al, 2005).
Furthermore, the technique was also demonstrated for table salt production using waste brine
accompanied with a mining process (Kawahara, 1994), brine discharged from a RO seawater
desalination plant (Tanaka et al., 2003).
ED has been observed to be more economically advantageous compared to the evaporation
process according to Tanaka, 1999. In this analysis, it was observed that to produce a ton of
NaCl, ED would consume 151.3 kWh as compared to 1000 kWh for evaporation. However,
Turek and Dydo, 2003 noted that electricity as an energy carrier is about 7-10 times more
expensive than in the form of steam.
14
6.
Chemical processes
6.1.
Ion exchange
This process involves a chemical reaction involving exchange of ions between a liquid phase
and a solid phase. The solid phase, also known as an organic resin is contained in a column
where the liquid phase is passed hence leading to the exchange of ions. For salt recovery
purposes, the feed saline water is passed over the organic resin beds where dissolved Na+ , Cland other ions are exchanged for other ions of similar charge. The beds are normally arranged
in series as shown in Figure 11. Ion exchange materials are classified as either cationic or
anionic, implying that the cation resins interchange H+ ions for positively charged ions while
the anion resins exchange OH- ions for negatively charged Cl- ions hence producing water
and salt. It should be noted that the number of charges on the ions removed from solution
must be equivalent to number of charges on the ions exchanged from the material.
Figure 11: Schematic diagram of the ion exchange process (Al-Kharabsheh, 2003)
This technique is environmentally friendly, has cheap maintenance and long life of the resins.
None the less, it has limitations in that the volume of desalted water produced in the process
is inversely proportional to the ionic concentration in the water. In addition, the higher
salinity of the water requires larger ion exchange equipment which at times is not economical
due to the high cost of the organic resins. This therefore implies that the process is suitable
for low salinity water (Guyer, 2010). The process is also prone to fouling of the resin due to
organic matter or Fe3+ ions. The technique also has environmental implications in that the
waste water for disposal after regeneration contains all the minerals removed from the water
and the salt. Previous studies, investigations and assessments have demonstrated the
feasibility of this process for recovery of salt from saline water (Ginde and Chu, 1972,
Kobuchi et al., 1983, Tanaka, 2003, Ghaly and Verma, 2008).
6.2.
Chemical extraction techniques
Saline water is an inexhaustible source of common salt together with other valuable salts such
as sodium sulphate, magnesium chloride, magnesium sulphate, calcium sulphate etc.
Chemical process techniques have been developed and applied to purify crude salts from
15
saline waters and their preparation as feedstock for production of value added products from
the salts (Aral et al., 2004). Attractive chemical processes involve calcination which is a
thermal decomposition process and reactive precipitation. Chemical processes are
conventionally applied in extraction of salts having similar chemical properties which would
make it difficult for high purity extraction (Lee and Bauman, 1980). The technique is not
applicable in extraction of sodium, potassium and magnesium chlorides since they somewhat
possess similar chemical properties. Sodium and potassium chlorides would be extracted out
of solution by the evaporation and cooling technique (Kilic and Kilic, 2005).
Recovery of magnesium hydroxide, Mg(OH)2 from samples of waste brine or bitterns at a
salt processing plant in Ghana by precipitation has been demonstrated by Lartey, 1997. In this
study, the waste brine samples were precipitated with NaOH followed by washing to yield
products which contained some NaCl and Potassium impurities. Calcination of Mg(OH)2
yielded MgO which also contained some impurities. Rigorous washing of Mg(OH)2 was
suggested for quality improvement. Reactive precipitation has also been applied to recover
CaCO3 from nanofilitration retentate by adding NaHCO3/Na2CO3 aqueous solutions to the
retentate. In this investigation, the sodium (bi) carbonate solutions had been produced by
reactive absorption of CO2 into sodium hydroxide solutions (Drioli et al., 2004). The same
technique has also been used in the precipitation of magnesium sulphate from lake brine
(Estefan and Nassif, 1976), precipitation of basic magnesium carbonate from high sulphate
content brines followed by calcinations at 1100oC to produce high quality magnesia (Estefan
and Nassif, 1980).
The recovery of potassium chloride from Dead Sea brines by precipitation and solvent
extraction has been demonstrated by Epstein et al., 1975. In their study, Potassium was
recovered from brines by precipitation as potassium perchlorate, followed by conversion to
potassium chloride by liquid anion exchange with a tertiary amine in the form of its
hydrochloride. The Dow chemical process has also been observed as a classical economic
process of MgCl2 extraction from desalination brines. It is accomplished by addition of lime
to sea water bitterns to precipitate Mg(OH)2 followed by addition of HCl to precipitate MgCl2
(Al Mutaz and Wagialia, 1990). Chemical precipitation techniques have been coupled with
evaporation-crystallization processes for more efficiency and purity of recovered salts (Atashi
et al., 2010).
7.
Hybrid technologies
Conventional salt recovery technologies are both energy and cost intensive if they were to
operate stand-alone (Gude, 2010). In this regard, several conventional techniques have been
combined in order to hybridize the salt recovery process to increase system performance
efficiency and reduction of final disposable brine hence maintaining zero-discharge.
Examples would involve integration of UF-NF-MSF-crystallization or UF-NF-RO-MSFcrystallization systems (Turek, 2002), MCr-NF-RO (Drioli et al., 2006). Such processes have
been applied to desalt solutions of extremely high concentration which other conventional
techniques such as RO are incapable of handling.
MD and crystallization have been used to obtain pure crystalline products and pure water
from solutions of NaCl and MgSO4 of concentrations near to saturation (Mariah, 2006).
Drioli et al., 2004, demonstrated an integrated system for recovery of CaCO3, NaCl and
16
MgSO4·7H2O as solid products from a NF retentate stream. Specifically, chemical
precipitation techniques were used to precipitate Ca2+ ions followed by a final membrane
crystallization unit to recover both NaCl and Epsom salts. Pre-filtration, neutralization, NF
and RO have been used for recovery and re-use of salts present in dye house effluent
(Allègre, 2004). Innovative techniques have developed to desulfate brine according to Bader,
2007. The techniques are an alternative to NF as a stand-alone process for desulfating brine
due to scale and hydraulic boundaries at the membrane surface. The proposed techniques
integrate NF with liquid-phase precipitation (LPP) and MD with LPP.
Hybrid techniques involving RO, chemical separation and crystallization have been applied
for salt production from coal mine brines in Poland (Ericsson and Hallmans, 1996). Further
research investigations by (Turek et al., 2005, 2008) have demonstrated that ED/EDRevaporation-crystallization and NF-evaporation-crystallization as opposed to a stand-alone
evaporation-crystallization method can be applied to the recovery of salt from coal mine
brines. In both investigations, it was observed that there were considerable improvements in
the plant outcome. There was a reduction in the unit energy consumption and reduced amount
of salt in post-crystallization lye.
8.
Discussion and synthesis of the findings
This section provides a discussion and synthesis of the literature review findings of this
paper. The discussion includes a comparison of the different salt extraction processes and
technologies. The discussion primarily addresses the following fundamental questions:
(a) How do the processes and technologies compare with each other in terms of their
performance characteristics?
(b) What are the potential challenges and benefits of the technologies?
8.1
Comparison of the processes and techniques
The review elucidated the different available salt production technologies. In the evaluation
of the different processes, it is important to select a technique that is suitable for a particular
operation. The important factors that should be considered for such a selection in particular
are the following (Wade, 1993, Gude et al., 2010, Westphal et al., 2010):
(a)
(b)
(c)
(d)
(e)
(f)
(g)
Availability of raw materials (brine, raw salt),
Impurities of the raw material,
Availability, reliability and energy costs (power, steam, primary energy),
Water situation (quantity and quality),
Commercial readiness,
Environment (waste products disposal, use of chemicals),
Economic feasibility (financing, capital costs, operating costs).
Based on the above criteria, a comparative summary of the relative pros and cons of the
identified technologies together with their characteristics as applied to salt production from
brine is provided in Table 1. A few notable salt production plants based on the extraction
technique are highlighted in Table 2.
17
Table 1: Comparison of the salt production processes (Bauschlicher, 1983, Ericsson and Hallmans, 1996, Kilic and Kilic, 2005, Turek
and Dydo, 2003, 2008, Westphal et al., 2010, Blumenthal and Losordo, 2011)
Brine initial Energy
Process
Size of
Output
Main key features
application concentration consumption quality,
(Benefits and limitations)
(ppm)
(kWh/t of
%
salt)
(Purity
NaCl)
MSF
Medium10,000-50,000 45
99.97
• High initial capital investments
large
(Output 20
• Requires highly qualified personnel and
tons /hour)
intense process control.
• Undersaturated brine causes serious
problems
• Prone to corrosion and erosion
MEE
Small
- 10,000-50,000 25
99.97
• Has moderate to good efficiency
medium
(Output 150
• Easy to operate since it’s a once through.
tons /hour)
• Preferred for simple process operation and
dual purposes
Thermal
• Expensive to start
Processes
• Prone to corrosion and erosion
MVC
Small
10,000-50,000 155
99.97
• Has high energy efficiency
(Output 30
• Requires electricity, high maintenance and
tons /hour)
pressure for operation
• Great potential for corrosion and erosion.
Solar
Small30,000
155
98.54
• Uses free energy, requires no maintenance
Evaporation
medium
(Output 36
(un
and is environmentally friendly. Simple and
2
kg/m /year)
washed)
cheap technique.
99.75
• Can achieve up to 72% salt retrieval
(washed) • Difficult to get high quality salt
• Has significant waste of salt
• Requires a lot of space and time.
18
Membrane
Processes
Chemical
Processes
Hybrid
Processes
Open pan
evaporation
Small to 30,000
medium
EFC
-
-
2,708
99.98
(Output of 5
tons/hour)
-
RO
Small
1,000-10,000
-
-
ED/EDR
Smalllarge
1,000-5,000
151.3
97.47
Ion exchange
Small
1,000-5,000
-
97.47
Precipitation
-
-
-
-
(1)PretreatmentRO-SeparationCrystallization
(2)NFEvaporation etc
Small
large
- 10,000-30,000
318
>99.6
(Output 12.5
tons/hour)
450
19
• Process is energy inefficient, costly
outdated and unsustainable.
• Cannot be operated in high humidity region.
• Process capable of reducing energy costs by
up to 70% compared to distillation.
• High investment costs and scale limitations.
• Process has not got commercial application.
• Used as a pre-concentration stage for
evaporation crystallization. Mainly for
desalination
• More pre-treatment of brine needed than
thermal process
• Uneconomical for salt production
• Concentrates brine from 3.5 to 15-20%
NaCl before thermal evaporation.
• Energy use proportional to salt recovered.
• Long membrane lifetime and high
efficiency
• High capital and operational costs
• Environmental limitations with disposal of
chemical resins
• Impractical for high saline water hence
uneconomical for salt production
• No commercial importance in table salt
manufacturing industry.
• Low energy consumption and high
performance efficiencies.
• Able to overcome problems of scale
formation caused by CaSO4, CaCO3 and
Mg(OH)2
Table 2:
Some notable commercial salt production plants
Plant Location
Total
capacity
tonnes/year
Process and remarks
Reference
Poland
(Debiensko)
109,500
Hybrid: First stage-Pretreatment and RO
Second stage: Post- treatment of permeate
Third stage: Evaporative crystallization
Ericsson and Hallmans, 1996
Japan, Korea, Taiwan
1,722,990
ED with exchange membranes and MEE
Dead Sea, Jordan
25,000
Solar evaporative crystallization
Mexico
(Guerrero Negro)
Spain
1. Sals Monzon
2. Torrevieja
17.5 x 106
Solar evaporative crystallization
Kobuchi et al., 1983, Kawate et al.,
1983, Noble and Stern, 1995
Tanaka et al., 2003
Abu-Khader, 2006, Bashitialshaaer
et al., 2011
Baja Quest, 2011, Westphal et al.,
2010
120,000
700,000
1.2 x 106
Brine purification-crystallization ponds-SALEX. Crystallization
based on THERMOSOL technique.
Evaporation due to solar radiation and wind
MEE
550,000
820,000
Solar evaporative crystallization
Brine pre-treatment-Crystallization driven by MVR
600,000
400,000
Hybrid: MVC-MSF
Solar evaporation
Westphal et al., 2010,
260,000
Recrystallization also similar to MSF
Westphal et al., 2010
Netherlands
Italy
(Magherita Di Sovoia)
United States
(Louisiana)
France
1. (Varangeville)
2. Aigues-Mortes
Algeria, Bangladesh,
Germany, Greece,
Iran, and Turkey
20
Sedivy, 2000
Nolan, 2011
Westphal et al., 2010
Zeno, 2009
Westphal et al., 2010, Shintech,
Inc, 2011
Considering Tables 1 and 2 above, it can be observed that thermal processes are in a mature
phase of technical development and have gained commercial readiness in the salt industry.
However, there is scope for further improvement in the materials selection in order to address
issues regarding to corrosion and erosion which would lead to break downs and huge
maintenance costs. MED has the least energy consumption among the thermal processes. The
technology is less complex than MSF and MVC in terms of auxiliary equipment and pumps
hence well suited for remote applications. Thermal methods are more effective than
membrane methods in terms of production of highly concentrated brines (Turek and Dydo,
2003).
The procedures of ED and RO have not found commercial application in the salt industry
though. They remain restricted to special cases, since they involve high energy costs most
especially in driving the process pumps. In places where they have been applied for salt
production purposes, they have only been used for production of concentrated brine
(Westphal et al., 2010). This brine is fed to crystallization plants to avoid expensive
evaporation of water thus maintaining high energy efficiencies. According to Wade, 1993,
these processes have considerable scope for future technology in regard to the membrane
technology, pre-treatment and detail engineering. Membrane technologies are not tolerant of
the initial feed water concentrations than distillation technologies. In this regard, membrane
performance and life can be adversely affected by contamination of the feed water thus
having a big effect on reliability and availability. More research efforts are being done in
integrating the different salt production technologies (Ericsson and Hallmans, 1996, Ahmed
et al., 2001, Turek and Dydo, 2003, Turek et al., 2008).
9.
Conclusions and outlook
Various salt production technologies from brine have been reviewed and analysed in
literature with respect to energy consumption, raw material treatment needs, cost, availability,
reliability and commercial readiness. It was observed from the literature that distillation
involving evaporation either by solar or thermal processes, and the cooling techniques are the
most widely used technologies for both traditional and commercial salt production worldwide
today. Membrane, chemical and hybrid salt production technologies are also emerging.
Several research efforts in these areas have been conducted most especially at demonstration
scale thus their full scale commercialisation for salt production has not been fully realized.
The use of solar evaporation technologies and distillation techniques mainly multiple-effect
evaporation would be appropriate in terms of sustainability. Thermal processes have the
largest share in the installed capacity of commercial salt production implying that these
technologies have gained technical maturity over the years. With increasing demand for
usage of renewable energy technologies for sustainability, there is need to investigate the
feasibility of integrating solar thermal technologies with proven distillation processes such as
MEE for commercial salt production, most especially in areas with favourable climatic
conditions such as Lake Katwe. This would ensure better recovery and process efficiencies,
low costs and simple brine pre-treatment procedures, hence sustainable salt production.
21
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