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SOLAR SALTWORKS PRODUCTION PROCESS
EVOLUTION - WETLAND FUNCTION
Chemical Engineer M.Sc. Technical Director, HELLENIC SALTWORKS S.A. 1, Asklipiou str, 10679 Athens, Greece
ABSTRACTS
Solar saltworks are very well known plants, mainly because of their product. Salt is one of the world's best-known minerals and the chemical substance most related with the history of human civilization. Its significance for the creation of life itself on the planet and its importance as a commodity are paramount. Nevertheless, the development of a unique saline ecosystem in parallel with the salt production process has not always been understood. The biological process that develops along with the increasing salinity gradient in the evaporating ponds and crystallisers of saltworks, produces excellent food for many kinds of birds, which for this reason rest, feed and breed in saltworks. The basic steps in the evolution of solar salt production process are identified, where the final one corresponds to modern saltworks operation. It is shown that especially modern saltworks are not just salt production plants but they also function as integrated saline wetlands. Their ecological importance consists in the fact that they comprise the characteristics of both regular and hypersaline wetlands. Modern saltworks are also compared with natural saline ecosystems, taking as an example the case of Kalloni Saltworks in Lesvos island and ìAlikiî lake, located in the nearby island of Lemnos.
INTRODUCTION
Salt, the common name for the compound of sodium (Na +) and chloride (Cl-), is the first substance after water to have attracted humans' attention in their
by Nicholas A. Korovessis
Themistokles D. Lekkas Rector of the University of the Aegean 30, Voulgaroktonou str., 114 72 Athens, Greece
evolution from wilderness to civilisation. Both its significance in the creation of life itself on the planet and its importance as a commodity are paramount. It is common knowledge that life began in the oceans, where the first monocel- lular organisms were created. Although some creatures left their marine envi- ronment after a long evolutionary process, they continued being dependent on salt (Young 1977). Nowadays, we know that sodium chloride is the basic extra- cellular electrolyte of the human body and that the salinity levels of the envi- ronment where human foetuses develop are similar to those of the sea! Therefore, salt has remained a necessary element for the survival and prolifer- ation of not only herbivorous animals, which take the necessary quantity of salt by licking the salty soil, but also for carnivorous ones, which ensure the neces- sary intake of salt from the blood of their prey. The time when humans began engaging in farming activities and became settlers coincides with their search for salt, which is provided by nature in abundance. Salt along with water, cereals (bread) and the meat of domestic animals constituted the staple basis of human society in its infancy. Humans must have found salt where it can still be found, that is, in concave rocks of coastal areas or in lagoons where seawater gets trapped and deposits salt as it evaporates in the sun. At a cer- tain point in their history, humans must have copied nature and produced salt on their own by evaporating seawater via either solar energy or ebullition. Originally, salt was used to cater for the needs of human diet but later, it was dis- covered that it had significant food preserving properties. This particular prop- erty made salt one of the most important commodities for centuries. It is under- stood that in pre-classical Greece not knowing the use of salt was considered a
Salt stockpile at Kitros Saltworks
in the chemical industry have increased dramatically salt consumption world- wide, with annual figures reaching 200 million tonnes nowadays. One third of this is produced in solar saltworks. About 20% of the international salt produc- tion is destined for human consumption, whereas 55% is used in the chemical industry and 15% is spread on roads to thaw ice or snow in winter.
SEAWATER COMPOSITION
Seawater constitutes the raw material for the production of salt in solar salt- works. It is well known that this raw material is inexhaustible, amounting to approximately 5x10^16 tonnes. It is worth noting that, the relative ratio of the various ions contained in seawa- ter is almost independent from its overall salinity, it is practically the same on every coast of all open seas. However the overall salinity of seawater changes, as a result of the different evaporation rates for each sea or ocean. The concentration of seawater through solar evaporation results in the succes- sive crystallisation of the less soluble salts (CaCO 3 , CaSO 4 ) first, followed by NaCl and finally Magnesium salts. Saltworkers use the empirical Baume (∞Be) scale, to measure the concentration of brines. According to that scale the sea- water concentration is 3.5∞Be. The crystallisation of CaCO 3 begins at 4.6∞Be and that of CaSO 4 at 13.2∞Be. NaCl crystallises at 25.7∞Be, followed by the more soluble Mg salts at 30∞Be.
J. Usiglio, published the first scientific paper studying the fractional crystallisa- tion of all salts contained in seawater through its gradual evaporation under controlled conditions, in 1849. It is a classic study, which inspired Vant Hoff in his phase rule studies (Baas-Backing 1931). Subsequent studies offered only a fragmentary approach to the issue. It was only in 1974 that G. Baseggio pre- sented a comprehensive study of the composition of seawater and its concen- tration, in the 5th International Conference on Salt. The present paper has used data from the aforementioned study.
SOLAR SALT PRODUCTION PROCESS
Producing salt from seawater involves the selective recovery of pure NaCl, free of other soluble or non-soluble salts and other substances. To this end, an appropriate quantity of seawater is concentrated through natural evaporation, which leads to the fractional crystallisation of all salts contained; a process based on their varying solubility.
As already mentioned, originally, humans found salt in coastal concavities or in lagoons where seawater was trapped, evaporated in the sun and deposited its salt content. It can be deduced that, after a long period of observation and knowledge-building, humans eventually copied nature and began producing salt in quantities meeting their personal and social needs, thus moving away from nature's production rates. This is therefore, the initial stage and constitutes the first form of the solar sea salt production process hereby described.
This method has certain disadvantages since the salt produced contains all the ingredients of seawater and it is very difficult to produce relatively pure NaCl (in fact, it requires great experience). Moreover, this method of salt production is a batch process, with limited production rates.
The second step in the process of salt recovery from seawater was made with the division of the evaporation basin into two (figure 1). The first basin, usually called nurse pond, was used for the production of saturated (in NaCl) brine, which was fed into the second basin, usually called crystalliser.
Thus, it was made possible to: ï achieve continuous salt production (crystallisation) and to unbound the salt production rate, ï eliminate those seawater salts, with less solubility than NaCl (i.e. CaCO 3 and CaSO 4 ), since these crystallise in the first basin and remain there.
The third and most decisive step concerned the division of the nurse pond into several interconnected basins. With this design, seawater enters the first basin and, as it flows through the next ponds and evaporates in the sun, its concen- tration increases. Thus, by the time it reaches the last basin, which has now
Figure 1. Basic stages in solar salt production process evolution
Evaporating pan dikes ó Kalloni Saltworks
Flock of terns on a saltworks islet (Kalloni)
in our times: they combine their production process with the conservation of the environment. This is so because such process is not only environment-friendly, but also saltworks themselves constitute integrated ecosystems. They consist of a system of shallow ponds (15-60 cm deep), connected mainly in series, and their natural bottom has the appropriate clay composition to ensure very low water permeability. Their operation principle is basically no different from the one described in the third stage of the previous chapter. The only dif- ferences that have occurred since the method was first applied, concern its opti- misation as well as the means by which brine is transferred and salt is collected, resulting from subsequent technological progress. According to this method, the ponds are divided into two basic groups. The first group, usually called evaporating ponds or ponds, is where seawater is concen- trated up to saturation point in terms of NaCl (25.7∞Be). The second group, called crystallisers, consists of the basins where salt is crystallised and produced via further evaporation of the brine up to 28-29∞Be. What basically elevates saltworks to ecosystems is the fact that for seawater to be con- centrated up to the point of salt crystallisation, 90% of its water content has to evaporate, thus requiring a vast surface. For this reason, ponds take up approximately 90% of the saltworks area. Their bottom is totally natural without any inter- vention and the concentration of contained brine covers the whole range from 3.8∞Be (almost sea- water) to 25.7∞Be, corresponding to the last pond which feeds the crystallisers continuously with
Washing and stockpiling of salt - Kalloni Saltworks
lated with the mathematical model (Pancharatnam 1972) given below:
with boundary values:
where: hG,L: heat transfer coefficients Òw: brine density kG: mass transfer coefficient Tw: brine temperature h: brine depth Ta: air temperature RN: direct and indirect solar radiation Tsg: ground temperature ·: refraction index pa: partial pressure of water in air Cp: specific heat capacity p*: saturation vapor pressure of brine Kg: ground thermal conductivity Ï: thermal diffusivity Î: latent heat of vaporization t: time
However, apart from the physicochemical process described above, a biological process develops in the evaporating and crystallising ponds, which is equally important to the production of salt. Surprisingly enough, despite rising salinity, life in the basins of the saltworks does not stop. Seawater organisms gradually disappear as they move from the initial pan to the hostile environment of the others. However, other organisms develop in their place and, as there is no competition, they proliferate. Such large populations are able to survive in areas with different concentration levels (that is, in different pans) because of their varying sensitivity to the ion composition of the medium they inhabit. Thus, in parallel with the physicochemical process, a chain of microorganisms is devel- oped in the evaporating ponds system, constituting the biological process of the salt production process. Such a chain is similar to those of naturally saline or hypersaline coastal ecosystems.
Figure 2. Pond model.
w pw w RN ( T^ w^460 )^4 hG ( Tw T ) kG ( p * p ) hL ( Tw T sg^ ) dt
dT h ρ C =α −εσ + − − α −λ − α − −
2
2
x
T
t
T (^) g g
g ∂
μ
x
T
x
x
T
x h T T Q K
g
x
g L w sg^ α sg g g
The biological process of saltworks is a sensitive process and depends on: ï the prevailing conditions in the basins of the saltworks (temperature, depth and turbidity of brine), ï the rational control of the natural (physicochemical) process during salt production and, ï the overall design of the saltworks.
As can be seen on the very indicative diagram below (figure 3, J. Davis), the small crustacean Artemia Salina, also called brine shrimp, is the key organism in this biological chain.
It constitutes the link between the organisms living in low concentration pans and those of high concentration pans. Organisms developing in saltworks that operate efficiently constitute a biological system or ecosystem, which interacts with the physicochemical process and is vital to the production of salt. The first attempt to interrelate in detail the physicochemical and the biological process developed in solar salt production, is presented in figure 4 (J. Davis).
The biological system is in admirable harmony with the production process of the saltworks, in three ways: ï it produces the appropriate quantity of organic matter, which is a source of energy for the various organisms, and reduces the permeability of the bottom of the ponds, thus minimising brine losses, particularly at low concentrations, ï it colours red the brines in the crystallisers, thus maximising the evapora- tion rate, by maximising the rate of solar energy absorption and eliminat- ing solar radiation reflection from the white saltbed. The red colour of the brines in the crystallisers is due to Halobacterium and to the monocellular seaweed Dunaliella salina and,
Figure 3. Biomass of main organisms in saltworks ponds and crystallisers.
Coloured brine in crystalliser
ï finally it creates and maintains the appropriate conditions in the evapora- tion ponds and the crystallisers, for the continuous and maximal produc- tion of high quality salt, which is characterised by clear, compact and main- ly thick granules, low in Ca2+^ (0.03-0.06%), Mg 2+^ (0.003-0.05%), SO 4 2- (0.10-1.2%) and admixtures of soil (0.01-0.02%).
When the biological system of saltworks is upset - due to either negligent oper- ation and generally deficient design, or to the pollutants carried in the seawa- ter, which is fed into the saltworks - an excessive quantity of organic matter is produced. Thus, the biological chain is altered and the saltworks become down- graded with the reduction of the surface of the ponds and increased viscosity of the brine resulting in the production of bad and sometimes potentially not mar- ketable quality salt. Therefore, it is clear why the optimal operation of modern saltworks is impossible without maintaining, at the same time, a healthy and stable ecosystem. This was very difficult to achieve in traditional saltworks, the operation of which was fragmentary and the control of the brine flow negligent. We finally end up with the following surprising, for a production process, para- dox, that modern saltworks are better and more stable ecosystems than the traditional ones.
Artemia adults and cysts in a Kalloni evaporating pond
However, the ecological importance of the saltworks is mainly connected to its ornithological interest. Basic organisms of the biological system described abo- ve constitute excellent food for a large number of birds living in the saltworks for this matter. Certain species of birds, especially the Avocet, the Black-necked Grebe, the Kentish Plover etc., depend directly on the productivity of the salt- works, since their diet is exclusively based on Artemia salina. Artemia is also part of the diet of the beautiful flamingos and it is the main reason for the orange colour of their feathers. On average, more than 100 species of birds have been observed in each of the Company's saltworks (188 in the saltworks Kitros in 1990), many of which have been identified as endangered species, or are protected by Greek, European Union or international conventions. It is worth noting that saltworks are totally free of pesticides or other chemical compounds used in farming.
Considering the case of Kalloni saltworks, located in the north Aegean Sea Island of Lesvos, which was recently redesigned and modernised, we can make the following remarks: ï there was a remarkable increase in bird species and population, ï Ornithologists reported a movement of many flamingos from lake ìAlikiî of Lemnos island to Kalloni saltworks (Lemnos is another Aegean sea island located north of Lesvos), ï Flamingos built nests in Kalloni saltworks for the first time in Greece, ï Ecotourism is developed in the area, especially in April and May.
Aerial view of Kalloni Saltworks during its reconstruction
Pelicans on a Kalloni Saltworks dike
Phoenicopterus ruber (flamingoes) - Kalloni Saltworks
that, in the first case the salinity gradient develops with respect to time, where- as in saltworks with respect to area. This means that what takes place through- out the year in the lake of Lemnos in terms of the physicochemical and biolog- ical processes, in the case of saltworks it occurs at any moment without a drying up period. Obviously this difference is in favour of the saltworks, which consti- tute a stable ecosystem throughout the whole year.
Furthermore saltworks are areas free of chemical contaminants, fertilisers etc., used by agriculture, whereas natural wetlands are not. This is true because, in the case of saltworks, all the effluents of the surrounding area go through their surrounding protective channel directly to the sea. Another difference, which is derived from what has already been explained, is that saltworks consist from more than one, interconnected ponds (lakes). This inter- vention results in two more advantages (assuming properly designed saltworks): ï birds can use the constructed dikes for nesting ï small birds find more shallow waters, comparing with the case of one big lake, where they can feed.
Nests and chicks ó Kalloni Saltworks
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People use to bathe in concentreted brine to snoothe skin and bone diseases.
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