Available on line at
Association of the Chemical Engineers of Serbia AChE
www.ache.org.rs/CICEQ
Chemical Industry & Chemical Engineering Quarterly 18 (1) 8388 (2012)
RADOUANNE FEZEI
HALIM HAMMI
ADEL M’NIF
Technological pole of Borj Cedria,
National Center of Research in
Materials Sciences, Valorization
Laboratory of Useful Materials,
Soliman, Tunisia
SCIENTIFIC PAPER
UDC 66.06:543.3(611)
DOI 10.2298/CICEQ110815049F
CI&CEQ
EXTRACTIVE PROCESS FOR PREPARING
HIGH PURITY MAGNESIUM CHLORIDE
HEXAHYDRATE
This paper reports a method for the preparation of magnesium chloride hexahydrate (bischofite) from Sebkha el Melah of Zarzis Tunisian natural brine. It is
a five-stage process based on crystallization by isothermal evaporation and
chemical precipitation. The first two steps were dedicated to the crystallization
of sodium chloride and potassium-magnesium double salts, respectively. Then,
the resulting liquor was desulfated using calcium chloride solution. After that,
another isothermal evaporation stage was implemented in order to eliminate
potassium ions in the form of carnallite, KCl·MgCl2·6H2O. At the end of this
step, the recovered solution, primarily composed of magnesium and chloride
ions, was treated by dioxan in order to precipitate magnesium chloride as
MgCl2·6H2O·C4H8O2. This compound dried at constant temperature of 100 C
gave good quality magnesium chloride hexahydrate. Besides this salt, the various by-products obtained from the different treatment stages are also useful.
Keywords: process; extraction; magnesium chloride; natural brine.
Brines are seawater-type solutions often assimilated to the quinary system Na+, K+, Mg2+/Cl-, SO42–
//H2O [1,2]. These solutions, which represent a nearly
endless reserve of dissolved salts, are expected to
play an important role in the economic sector of some
countries and especially those in developing such as
Tunisia. Thus, several works were developed on Tunisian natural brines. The first ones studied geological
aspects as well as solubility diagrams related to the
previously mentioned system [1,3-6]. Then, investigations were extended to the modeling of phase diagrams [7-14]. Recent studies are rather directed towards the extraction of interesting salts [6,15-18].
Among these salts, magnesium chloride is of great
utility in industry as well as agriculture.
The present work is a contribution to the vaporization studies of Tunisian brines. In particular, it deals
with the preparation of magnesium chloride hexahydrate salt from the naturally occurring brine of Sebkha
el Melah of Zarzis.
Correspondening author: R. Fezei, Technological pole of Borj
Cedria, National Center of Research in Materials Sciences, Valorization Laboratory of Useful Materials, Tourist road of Soliman B.P. 73 - Soliman 8027, Tunisia.
E-mail: [email protected]
Paper received: 15 August, 2011
Paper revised: 10 October, 2011
Paper accepted: 11 October, 2011
Process description
Choice of the different treatment stages adopted
in the conceived process was directed by some previous work [2, 19-22]. The corresponding flowchart is
given by Figure 1. It is a five stage process based
mainly on isothermal evaporation and salts crystallization as well as chemical precipitation.
The first step consists of evaporating the considered brine at 35 C in order to crystallize sodium
chloride (halite). During the second treatment, the resulting solution is further concentrated by isothermal
evaporation at the same temperature. The precipitated salts are expected to be sodium chloride and
substantial amounts of kainite (KCl·MgSO4·3H2O).
The third step of the process is a chemical precipitation reaction of sulfate ions using calcium chloride
solution. After removing calcium sulphate precipitate,
the mother liquor is subjected to another 35 C isothermal evaporation in order to eliminate partially potassium ions as potassium-magnesium double salt,
carnallite (KCl·MgCl2·6H2O). The resulting brine, composed essentially of magnesium and chloride ions, is
treated by 1,4-dioxan (C4H8O2), which is expected to
precipitate the magnesium chloride salt as
MgCl2·6H2O·C4H8O2 [22-25]. This compound was dried
at constant temperature (100 °C) to eliminate dioxan
83
R. FEZEI, H. HAMMI, A. M’NIF: EXTRACTIVE PROCESS FOR PREPARING HIGH PURITY…
CI&CEQ 18 (1) 8388 (2012)
Figure 1. Flowchart of the process conceived for magnesium chloride hexahydrate extraction from
Sebkha el Melah natural brine (the given compounds are expected theoretically).
used: calcium chloride dehydrate (99%) and 1,4-dioxan (99%) supplied by Fluka.
molecules and thus obtain magnesium chloride hexahydrate, MgCl2·6H2O.
Apparatus and analysis methods
EXPERIMENTAL
All treatments were performed at constant temperature (35 °C) in a thermostatic bath, except the
third and fifth ones. These two treatment stages were
carried out in a jacketed glass reactor where temperatures (35 and 25 °C, respectively) were controlled by
water circulation. In both steps agitation was assured
by a mechanic stirrer.
Chemicals
The principal raw material considered in this
study is natural brine sampled from Sebkha El Melah
of Zarzis. Its chemical composition in major ions is
given in Table 1. Besides this highly concentrated
aqueous solution, two commercial reagents were also
Table 1. Ionic composition of brines (g L-1)
Brine
Na
+
K
+
Mg
2+
Ca
2+
-
2-
Cl
SO4
S0 (initial)
41.32
7.36
53.95
0.40
205.14
30.50
S1
21.62
10.95
77.76
0.00
234.57
43.81
S2
13.14
12.77
90.52
0.00
265.97
51.89
S3
6.64
7.55
70.35
0.00
224.73
6.09
S4
4.02
5.88
100.89
0.00
299.35
8.25
84
R. FEZEI, H. HAMMI, A. M’NIF: EXTRACTIVE PROCESS FOR PREPARING HIGH PURITY…
CI&CEQ 18 (1) 8388 (2012)
of the 35 °C isotherm of the quinary system Na+, K+,
Mg2+/Cl-, SO42-//H2O usually used to describe brines
equilibrium states. In this diagram, the coordinates of
a considered solution are expressed as follows:
Brines and solid phases collected from the different treatment stages of the process were chemically
analyzed. The chemical analysis methods applied for
sulphate, chloride, magnesium, and alkaline ions
were respectively gravimetry, potentiometry (Titrino
DMS 716 of mark Ω Metrohm), EDTA complexometry
and flame atomic absorption spectroscopy (SAAF).
Furthermore, X-ray diffraction analysis using a
Philips instrument (PW 3040 generator, 3050/60 θ/2θ)
and CoK radiation ( = 1.789 Å) were performed.
%K 2 
100nK 2
,
D
100nMg
,
%Mg 
D
100n SO4
,
%SO4 
D
D  nK 2  nMg  nSO4 ,
RESULTS AND DISCUSSION
where ni is the mole number of the i entity.
Geometrical positions of the original brine (S0)
and the mother liquors resulting from the four considered treatment stages (S1, S2, S3 and S4) in the
equilibrium diagram show the brine’s evolution towards the crystallization field of magnesium chloride
hexahydrate (MgCl2.6H2O).
Regarding the recovered solid phases (Sdi),
their ionic compositions are given in Table 2. Besides
chemical analysis, they were also characterized by X-ray diffraction. Results of this characterization are
represented by Figures 3 and 4.
Ionic composition of the original brine (S0) and
circulating solutions (Si, i refers to the corresponding
treatment) are summarized in Table 1.
As expected, the first four treatment stages gave
a magnesium chloride solution (S4) containing smaller
amounts of sodium chloride, potassium chloride and
magnesium sulphate. Thus, such solution may be suitable for magnesium chloride salt precipitation by dioxan as MgCl2·6H2O·C4H8O2.
Chemical composition evolution of the treated
brine is well seen in Figure 2. It represents a section
Figure 2. 35 °C Isotherm of the quinary system Na+, K+, Mg2+/Cl-, SO42-//H2O (NaCl saturated).
Table 2. Ionic compositions of solid phases (mass%)
Brine
Na
+
K
+
Mg
2+
Ca
2+
-
2-
Cl
SO4
2.55
Sd1
35.90
0.26
1.02
0.00
56.25
Sd2
35.10
0.37
1.27
0.00
55.29
2.42
Sd3
1.92
2.32
1.53
21.06
2.15
50.46
Sd4
0.62
14.02
8.13
0.00
38.32
0.89
Sd5
0.06
0.05
7.94
0.00
23.10
0.00
Sd6
0.11
0.09
11.61
0.00
34.05
0.00
85
R. FEZEI, H. HAMMI, A. M’NIF: EXTRACTIVE PROCESS FOR PREPARING HIGH PURITY…
CI&CEQ 18 (1) 83-88 (2012)
Figure 3. Diffractograms of the solid phases Sd1 (a), Sd2 (b), Sd3 (c) and Sd4 (d).
86
R. FEZEI, H. HAMMI, A. M’NIF: EXTRACTIVE PROCESS FOR PREPARING HIGH PURITY…
CI&CEQ 18 (1) 8388 (2012)
Figure 4. XRD Patterns of the solid phase precipitated by dioxane a) before drying and b) after drying.
Solids resulting from the first two treatment
stages (Sd1, Sd2) are sodium chloride salts containing
less than 5% impurities (mainly ions from coating solution). The chemically precipitated compound, Sd3, is
almost pure gypsum. Salt Sd4 is basically carnallite
(KCl·MgCl2·6H2O) containing little quantities of halite
(NaCl). Solid Sd5 precipitated by dioxan corresponds
to the compound MgCl2·6H2O·C4H8O2 including substantial amounts of sodium, potassium and sulphate
ions. Finally, the end-product obtained by drying Sd5
solid phase is an extremely pure magnesium chloride
hexahydrate salt. Its purity is more than 99.5%. When
treating one liter of brine, 155 grams of this salt were
recovered, i.e., an overall process yield of 40%.
Considering the information provided by the experimental results regarding the solid phases reco-
vered, only one difference was noted in comparison
with our predictions. It is the absence of double salt
KCl·MgSO4·3H2O (Kaïnite) in Sd2 solid phase. Lack of
this salt could be explained by its low crystallization
rate. In fact, magnesium ions exist in aqueous solutions predominantly as [Mg(H2O)6]2+ which promote the
formation of salts having six water molecules in their
structures.
To show the importance of prepared salt, we
studied its industrial application in Sorel cement manufacture. The compressive strength and setting time
of the obtained cement paste were determined and
were 86 MPa and 62 min, respectively. These values
are comparable to those found when using commercial magnesium chloride hexahydrate.
87
R. FEZEI, H. HAMMI, A. M’NIF: EXTRACTIVE PROCESS FOR PREPARING HIGH PURITY…
CI&CEQ 18 (1) 8388 (2012)
CONCLUSIONS
[9]
L. Zayani, R. Rokbani, M. Trabelsi-Ayadi, J. Therm. Anal.
Calorim. 57 (1999) 575–585
This work is a contribution to the vaporization
studies of Tunisian natural resources dealing with the
extraction of magnesium chloride hexahydrate salt
from Sebkha el melah natural brine. To perform this
task, a five stage process based on isothermal evaporation and chemical precipitation was performed.
The conceived process gave good quality magnesium
chloride hexahydrate (purity higher than 99.5%) which
was successfully checked in Sorel cement manufacture. Besides this chloride, all the collected by products are also useful.
[10]
H. Hammi, A. M’nif, R. Rokbani, J. de physique IV 11
(2001) 157-163
[11]
H. Hammi, J. Musso, A. M’nif, R. Rokbani, J. Calphad 27
(2003) 71-77
[12]
H. Hammi, J. Musso, A. M’nif, R. Rokbani, Desalination
158 (2003) 215-220
[13]
H. Hammi, J. Musso, A. M’nif, R. Rokbani, J. de physique
IV 113 (2004) 119-123
[14]
H. Hammi, J. Musso, A. M’nif, R. Rokbani, Desalination
166 (2004) 205-208
[15]
C. Djebali, L. Zayani, A. M’nif, R. Rokbani, J. Société Chimique Tunisie 4 (1998) 233–244.
[16]
L. Zayani, R. Rokbani, J. Therm. Anal. Calorim. 59 (2000)
885–892
REFERENCES
[1]
A. M’nif, Contribution to the study of equilibrium diagram
for the valorization of natural brines, Ph.D. Thesis, Faculty of Sciences of Tunis, Tunisia, 1984
[17]
A. M’nif, R. Rokbani, Asian J. Chem. 12 (2000) 58–68
[18]
R. Fezei, H. Hammi, A. M’nif, Lat. Am. Appl. Res. 39
(2009) 375-380
[2]
R. Cohen-Adad, Chr. Balarew, S. Tepavitcharova, D. Rabadjieva, Pure Appl. Chem. 74 (2002) 1811-1821
[19]
Berthon, R., Représentation des équilibres de solubilité et
utilisation des diagrammes, Gauthier-Villars, Paris, 1962
[3]
A. Chaabouni, Extraction of KCl from Chott el Jerid, DEA,
Faculty of Sciences of Tunis, Tunisia, 1980
[20]
[4]
N. K. Guerfal, System: K , Na / Cl , SO4 / H2O at 10 °C,
Ph.D. Thesis, Faculty of Sciences of Tunis, Tunisia, 1981
H. Hammi, Contribution to the study of salts crystallization from Sebkha El Melah natural brine and their
transformation, Ph.D. Thesis, Faculty of Sciences of Sfax,
Tunisia, 2004
[5]
N. Kalai, Contribution to the study of the Tunisian south
chotts brines, Ph.D. Thesis, Faculty of Sciences of Tunis,
Tunisia, 1986
+
+
–
2–
[21]
A. M’nif, R. Rokbani, Cryst. Res. Technol. 39 (2004) 40-49
[22]
R. Fezei, H. Hammi, A. M’nif, Chem. Eng. Res. Des. 89
(2011) 367-372
[6]
A. M’nif, R. Rokbani, J. Société Chimique Tunisie 4
(1998) 221-231
[23]
J.C. Barnes, T.J.R. Weakley, J. Chem. Soc. Dalton
Trans. (1976) 1786–1790
[7]
L. Zayani, Study of a Tunisian south brine, crystallization
sequence and modelling, Ph.D. Thesis, Faculty of Sciences of Tunis, Tunisia, 1999
[24]
M. Bester Rogac, D. Dolar, Acta Chim. Slov. 46 (1999)
463–470
[25]
[8]
D.B.H. Chehimi, Contribution to the study and the modelling of salts systems, Ph.D. Thesis, Faculty of Sciences of
Tunis, Tunisia, 1997
J.A. Weicksel, C.C. Lynch, J. Am. Chem. Soc. 72 (1950)
2632–2636.
RADOUANNE FEZEI
HALIM HAMMI
ADEL M’NIF
Technological pole of Borj Cedria,
National Center of Research in
Materials Sciences, Valorization
Laboratory of Useful Materials, Soliman,
Tunisia
NAUČNI RAD
DOBIJANJE MAGNEZIJUM HLORID
HEKSAHIDRATA VISOKE ČISTOĆE POMOĆU
PROCESA EKSTRAKCIJE
U ovom radu je predstavljena metoda za pripremu magnezijum-hlorid heksahidrata (bischofite) iz prirodne slane vode Sebkha el Melah u Zarzisu, Tunis. Proces se sastoji iz
pet faza, koji je u suštini zasnovan na kristalizaciji pomoću izotermnog isparavanja i hemijske precipitacije. Prve dve faze su kristalizacije natrijum hlorida i dvogube soli kalijuma i magnezijum. Zatim je dobijena smeša tretirana rastvorom kalcijum-hlorida radi
desulfatizacije. Nakon toga, izvršena je još jedna faza izotermnog isparavanja u cilju eliminisanja jona kalijuma u obliku karnalita, KClMgCl26H2O. Na kraju ove faze, dobijen
rastvor, primarno sastavljen od jona magnezijuma i hlorida, tretiran je dioksanom radi
precipitacije magnezijum-hlorida u obliku MgCl26H2OC4H8O2. Dobijeno jedinjenje je sušeno na konstantnoj temperaturi od 100 C, pri čemu je dobijen je kvalitetan magnezijum
-hlorid heksahidrat. Pored ove soli, iz različitih faza procesa dobijeni su sporedni proizvodi, koji su, takođe, korisni.
Ključne reči: proces; ekstrakcija, magnezijum hlorid, prirodna slana voda.
88