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International Journal of Environmental Studies

ISSN: 0020-7233 (Print) 1029-0400 (Online) Journal homepage: http://www.tandfonline.com/loi/genv20

Critical density index for the solar production of bittern from seawater G. M. Ayoub , M. El‐Fadel , A. Acra & R. Abdallah To cite this article: G. M. Ayoub , M. El‐Fadel , A. Acra & R. Abdallah (2000) Critical density index for the solar production of bittern from seawater, International Journal of Environmental Studies, 58:1, 85-97, DOI: 10.1080/00207230008711318 To link to this article: http://dx.doi.org/10.1080/00207230008711318

Published online: 24 Feb 2007.

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Date: 28 January 2016, At: 04:34

© 2000 OPA (Overseas Publishers Association) N.V. Published by license under the Gordon and Breach Science Publishers imprint. Printed in Malaysia.

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Intern. J. Environ. Studies, 2000, Vol. 58, pp. 8 5 - 9 7 Reprints available directly from the publisher Photocopying permitted by license only

CRITICAL DENSITY INDEX FOR THE SOLAR PRODUCTION OF BITTERN FROM SEAWATER G. M. AYOUB*, M. EL-FADEL, A. ACRA and R. ABDALLAH Department of Civil and Environmental Engineering, American University of Beirut, Lebanon (Received in final form 16 February 2000) Solar extraction of common salt from seawater has been practiced for centuries in a rather crude manner because the process was not performed in stages to obtain a pure salt. The common practice in developing countries is to carry through with the evaporation process to the stage of dryness to facilitate harvesting the salt mix. If salt is harvested prior to the deposition of impurities during the evaporation process then the resulting bittern would provide chemicals which could have many commercial applications particularly in treatment industries. This paper describes an effective procedure for the separation of common salt and impurities. Sufficiently sensitive indicators that would signal the start and deposition of impurities in the course of the solar evaporation process were established. These indicators are particularly useful in regions where advanced desalting technology is not readily available. Keywords: Bittern; common salt; solar production; seawater

INTRODUCTION Seawater generally contains about 3.7% w/w (3.6% w/v) of dissolved solids, and about 2.8% w/w (2.6% w/v) of common salt. When evaporated close to dryness, the product would be a mass with bitter taste, caused by the high content of bittern salts, and contains not more than 65% sodium chloride (NaCl), about 28% of hydrated *Address for correspondence: American University of Beirut, Faculty of Engineering and Architecture, 850 Third Avenue, New York, NY 10022, USA. 85

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magnesium salts in the form of magnesium sulfate (MgSC>4), magnesium chloride (MgC^) and magnesium bromide (MgBr2), 4% gypsum (CaSO4), and smaller amounts of other compounds such as, potassium chloride (KC1) and sodium bromide (NaBr) [1]. The first deposits to appear during the evaporation process are compounds other than common salt which need to be separated and discarded. Therefore, evaporation should be performed in stages each to be effected in a different evaporation container. Considering the purity of the solar salt produced, Lozano [2] reported that the salt harvested at densities of 1.250 to 1.285 g/mL consists of 98.5% and 96.4% NaCl, respectively, whereas salt harvested at a density of 1.310 g/mL has a NaCl content of 88.4%. The rate of water evaporation from the ponds decreases as the concentration of the brine increases, evidently due to the increase in salinity and to the decrease in the vapor pressure of the solution [1]. In this study, an experimental investigation is performed with the objective of evaluating the phases of salt deposition and the establishment of practical and sensitive indicators that would lead to the harvesting of different salts and particularly bittern from the solar evaporation of seawater. Lack of appropriate indicators or indices that would denote the beginning and end of crystallization of each salt present in seawater led to investigating the possibility of establishing such indices which could also be used to demarcate the limits of the various phases pertaining to the seawater evaporation process.

EXPERIMENTAL SET-UP Three separate metallic evaporation pans ( l m x lmx0.15m), with internal surfaces painted black to enhance evaporation, were constructed and installed in the open. The pans were filled with 125L of seawater at the beginning of the evaporation process (Fig. 1). Average constituent concentrations from chemical analysis of seawater samples are summarized in Table I. During the evaporation process, complete deposition of the calcium salts (CaCC>3 and CaSO4) occurred at a density of 1.225 g/mL in about a week from the start of the process. The supernatant liquid in each pan was then drained through a connection pipe into smaller plastic

87

SEA WATER BITTERN 125 L CaSO 4

12 L NaCL

12 L NaCL

125 L CaSO 4

Seawater

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=>

125 L CaSO 4

=>

12 L NaCL

Large Pans

=>

Small Pans

=>

Flask

FIGURE 1 Flow diagram showing the seawater evaporation process.

TABLE I Average concentrations of major seawater constituents Constituent

C\~

Na+ SOJ2 + Mg Ca 2

+

Concentration mg/L 20,900 11,780 2,926 1,400

534

pans where the evaporation process continued. NaCl crystallization and precipitation occurred at a density of 1.297 g/mL. The corresponding supernatant liquid was then transferred into a 4-L flask. This constituted the bittern which is highly concentrated with Mg +2 salts (70,000 mg/L as Mg+ 2) and is relatively free of residual sodium (Na + ) or potassium ( K + ) salts. During the evaporation process, a sample of the progressively concentrated solution was collected daily from each of the large metallic pans and the smaller plastic pans, thus yielding six samples per day. The samples were used for the daily triplicate determination of the physico-chemical characteristics. The assay also covered triplicate measurements of the same parameters on the bittern contained in the small pan. In addition to temperature and pH measurements, the samples were analyzed for total dissolved solids (TDS), conductivity, salinity, Ca2 + , Mg 2 *, Na + , chloride (Cl~), and sulfate (SO2;"). All analysis were conducted in accordance with Standard Methods [3], using

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methods 2510 B, 2510 B, 2520 B, 2340 C, 2340 C, 3500 Na + , 4500 Cl ~ , 4500-SO^ E, and 2550 B. Density was measured by adjusting the temperature of a specific volume of the samples to 20°C.

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RESULTS AND DISCUSSION A set of conditions noted during the evaporation process was the diurnal/nocturnal fluctuation of the seawater temperature from high levels at daytime to low levels at night-time. This is apt to govern the factors involved in the evaporation process such as the evaporation/ concentration rates, and the temperature/solubility determinants, both of which play a significant role especially as the evaporation process progresses. The solubility of salts in seawater undergoing solar evaporation is largely dependent on temperature. With increasing temperature, the solubility of MgCl2 increases appreciably (four times that of NaCl), while that of NaCl increases relatively little [4]. Other salts with similar solubility properties include: MgSO4, KI, and KC1 [5]. The opposite condition known as inverted solubilty is exemplified by calcium sulfate, the solubility of which decreases slightly with temperature above 38°C. In addition, the presence of other salts, as in the case of saline water, often exerts a strong influence on the solubility of a specific salt. For example, calcium sulfate and calcium carbonate are more soluble in saline water than in fresh water, whereas magnesium chloride is relatively less soluble [4]. As the pure water continues to evaporate, the concentration of salts increases gradually, and eventually reaches the saturation level with respect to one or more of the constituent salts which would consequently precipitate. In addition, the heating of water saturated with a salt characterized by an inverted solubility pattern such as calcium sulfate, coupled with the fact that the solubility limit (saturation point) would eventually be exceeded, results in a supersaturated calcium sulfate solution because of the decreased solubility at high temperatures. Since supersaturated solutions are unstable, immediate crystallization and precipitation may occur, particularly if brought into contact with some crystals of the same salt [4]. In the light of these explanatory considerations, it is reasonable to assume that the

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SEAWATER BITTERN

89

daytime solar irradiation and heating of the seawater is conducive to the formation of saturated or supersaturated solutions. In effect then, it is expected that crystal formation and deposition at daytime would exceed that at night time. But such a phenomenon is of no practical consequence in this particular case. At the initial stage of the solar evaporation process, the progressive evaporation and concentration is the dominant factor. As the evaporation increases during daytime, salts that attain, or exceed, their respective saturation limit would obviously crystallize and settle out of solution. The persistent salts with higher saturation limits would tend to form crystals at a later stage as evaporation continues and the concentration increases steadily to reach specific saturation limits. Daily sampling was conducted between 7:30 and 8:00 am. to overcome the possible impacts of these factors on the analytical results, and no samples were collected at night for analysis and comparative purposes. Identification of Evaporation Phases

Experimental results were reduced in terms of the mean values of the triplicate measurements recorded for the daily samples. Moving averages for the recorded data for each parameter were then computed to smooth out random variations [6]. The moving averages of each parameter are depicted in Figure 2 as a function of sample density of the seawater concentrate. This approach was conceived on the ground that, by plotting the concentration values of each parameter (dependent variables) versus the corresponding density value (independent variables), it would allow: • Tracing and comparing the trends in each case; • Pinpointing the density at which each of the major salts in seawater would crystallize out of the solution and settle; • Establishing practical indices for application in the commercial production of common salt and Mg-rich liquid bittern from seawater. Differentiation of the phases relating to the progressive concentration of seawater by solar evaporation is of practical value. The criteria for this purpose are based on the delineation of the limits indicative of the

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| 16

A, I1

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- • — TDS (X104)

|

12

I

8

-•-Ca -A-Na

^

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1.07

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Density (g/mL)

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20

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ft 17

200 -•-a

150 |

/

a. 100

i

I

50

1\ V

6 1.02

1.07

1.12

1.17 1.22 Density (g/mL)

1.27

1.32

FIGURE 2 Variation of seawater parameters with density.

starting and ending of crystallization of the various salts contained in the seawater. Accordingly, this process was divided into four distinctive phases as described below.

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Phase I: In this initial phase no precipitation of any salt occurs because the saturation level has not been reached. The density of this phase ranges from 1.027 to 1.106 g/ml. Phase II: This is the phase when calcium salts (CaSO4 and CaCO3) precipitate out of solution. Precipitation of these salts was observed at densities ranging from 1.106g/mL to 1.222 g/mL. Phase III: In this phase, 94.1%w/v of the NaCl initially contained in seawater crystallized and settled out of solution within the density levels of 1.222 and 1.269 g/mL. The Na + ion concentration starts to decrease after a density of 1.222 g/ mL (Fig. 2), then it increases again to a maximum level at a density of 1.243 g/mL. Thereafter, it decreases sharply until the concentrate reaches a density of 1.269 g/mL, which marks the end of this phase. The chloride ions in seawater undergoing solar evaporation behave in a similar manner. Phase IV: In this phase, the liquid bittern containing highly concentrated magnesium salts was produced and retained in liquid form, or in the form of the dried bittern containing the residual salts of magnesium obtained by drying the liquid bittern. Phase Delineation The range of densities corresponding to the deposition of the various salts present in the concentrates of seawater was found to be a fairly sound basis for the differentiation of the evaporation phases. However, this concept falls short of being precise because it is impractical to observe the beginning and ending of the deposition in each case. Parametric ratios as a function of density represent a better criterion for this purpose. Ratios pertaining to Phases I to III indicate that the values tend to peak and then decline to a level that is indicative of the end of the specific phase, and the start of the next one. On the basis of this concept, it is realistic to consider the densities marking the start and end of each evaporation phase as the critical diagnostic indicator denoted by the term Critical Density Index (CDI). For instance, the CDI values for Phase II are 1.069 and 1.220 g/mL, which implies that deposition of certain salts (i.e., CaCC>3 and CaSO4) occurs as the density of the concentrate increases from 1.069 to

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1.220 g/mL during the evaporation process. This concept is illustrated in Figure 3 from which the following inferences of practical importance can be deduced:

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Phase

Stage

CDI

Description

I

Evaporation and concentration

1.027-1.106

II

Deposition of calcium salts

1.106-1.220

HI

Deposition of common salt

1.220-1.269

IV

Generation of liquid bittern

1.269-1.297

Evaporation and concentration of seawater progresses concurrently without deposition of any salt. This occurs entirely in this phase as it is evident from the declining curve depicting the Ca2+/SC$ ratio as a function of density. Complete deposition of slightly soluble CaCO3 and CaSO4. The small portion of NaCl remaining in solution form, part of the constituents of the liquid bittern. The coneshaped curves characterizes this phase. This highly concentrated Ilquor starts to form just after the deposition of the calcium and sodium salts in the preceding phases.

Validation of the CDI Concept The validity of the CDI indicative concept was conducted in two approaches. The first approach focussed on the density levels at which the salt crystals in the concentrates are generated and precipitated out of solution. For this purpose a comparison with densities indicating the start and end of salt precipitation as reported in similar studies (Tab. II) was the initial step. The data indicate that the density values, reported by Rothbaum [7] and Kaufmann [1], are consistent with the corresponding densities found in this study. The second approach scrutinized the trends depicted in Figure 3 which reflect fairly accurately the densities at which the generation and deposition of the salt crystals occur in sequence. The coneshaped sections that fall within the CDI limits of Phase III (1.220 and

SEAWATER BITTERN

93

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1.269 g/mL) are indicative of the crystallization and deposition of the NaCl. A corollary of this phenomenon involves the inclination of each curve to attain a peak level, followed by a declining slope as depicted in Figure 2. These features may be interpreted in terms of

-0.1 1.02

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15 CI/TDS

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1.02

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FIGURE 3 Parametric ratios as a function of density with CDI demarcation limits.

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G. M. AYOUB et al.

18 16 14 12 •210

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38 6 4 2 0

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