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Chemical Industry & Chemical Engineering Quarterly 18 (1) 8388 (2012) CI&CEQ

83

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

EXTRACTIVE PROCESS FOR PREPARING HIGH PURITY MAGNESIUM CHLORIDE HEXAHYDRATE

This paper reports a method for the preparation of magnesium chloride hexa-hydrate (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 va-rious by-products obtained from the different treatment stages are also useful.

Keywords: process; extraction; magnesium chloride; natural brine.

Brines are seawater-type solutions often assi-milated to the quinary system Na+, K+, Mg2+/Cl-, SO4

2–

//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 Tu-nisian natural brines. The first ones studied geological aspects as well as solubility diagrams related to the previously mentioned system [1,3-6]. Then, investiga-tions were extended to the modeling of phase dia-grams [7-14]. Recent studies are rather directed to-wards 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 vapo-rization studies of Tunisian brines. In particular, it deals with the preparation of magnesium chloride hexahyd-rate 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, Va-lorization Laboratory of Useful Materials, Tourist road of Soli-man B.P. 73 - Soliman 8027, Tunisia. E-mail: radouanne.fezei@yahoo.fr 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 pre-vious 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 crystalli-zation as well as chemical precipitation.

The first step consists of evaporating the con-sidered brine at 35 C in order to crystallize sodium chloride (halite). During the second treatment, the re-sulting solution is further concentrated by isothermal evaporation at the same temperature. The precipi-tated salts are expected to be sodium chloride and substantial amounts of kainite (KCl·MgSO4·3H2O). The third step of the process is a chemical precipi-tation reaction of sulfate ions using calcium chloride solution. After removing calcium sulphate precipitate, the mother liquor is subjected to another 35 C iso-thermal evaporation in order to eliminate partially po-tassium ions as potassium-magnesium double salt, carnallite (KCl·MgCl2·6H2O). The resulting brine, com-posed 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

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molecules and thus obtain magnesium chloride hexa-hydrate, MgCl2·6H2O.

EXPERIMENTAL

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

used: calcium chloride dehydrate (99%) and 1,4-di-oxan (99%) supplied by Fluka.

Apparatus and analysis methods

All treatments were performed at constant tem-perature (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 tempera-tures (35 and 25 °C, respectively) were controlled by water circulation. In both steps agitation was assured by a mechanic stirrer.

Figure 1. Flowchart of the process conceived for magnesium chloride hexahydrate extraction from Sebkha el Melah natural brine (the given compounds are expected theoretically).

Table 1. Ionic composition of brines (g L-1)

Brine Na+ K+ Mg2+ Ca2+ Cl- SO42-

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

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Brines and solid phases collected from the diffe-rent 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.

RESULTS AND DISCUSSION

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 suit-able for magnesium chloride salt precipitation by di-oxan as MgCl2·6H2O·C4H8O2.

Chemical composition evolution of the treated brine is well seen in Figure 2. It represents a section

of the 35 °C isotherm of the quinary system Na+, K+, Mg2+/Cl-, SO4

2-//H2O usually used to describe brines equilibrium states. In this diagram, the coordinates of a considered solution are expressed as follows:

2K2

100%K

n

D ,

Mg100%Mg

n

D ,

4SO4

100%SO

n

D ,

2 4K Mg SOD n n n ,

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 consi-dered treatment stages (S1, S2, S3 and S4) in the equilibrium diagram show the brine’s evolution to-wards 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.

Table 2. Ionic compositions of solid phases (mass%)

Brine Na+ K+ Mg2+ Ca2+ Cl- SO42-

Sd1 35.90 0.26 1.02 0.00 56.25 2.55

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

Figure 2. 35 °C Isotherm of the quinary system Na+, K+, Mg2+/Cl-, SO42-//H2O (NaCl saturated).

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Figure 3. Diffractograms of the solid phases Sd1 (a), Sd2 (b), Sd3 (c) and Sd4 (d).

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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 so-lution). 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 sub-stantial 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 ex-perimental 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 solu-tions 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 ma-nufacture. 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 com-mercial magnesium chloride hexahydrate.

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CONCLUSIONS

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 eva-poration 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 manufac-ture. Besides this chloride, all the collected by pro-ducts are also useful.

REFERENCES

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Ph.D. Thesis, Faculty of Sciences of Tunis, Tunisia, 1981

[5] N. Kalai, Contribution to the study of the Tunisian south chotts brines, Ph.D. Thesis, Faculty of Sciences of Tunis, Tunisia, 1986

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RADOUANNE FEZEI

HALIM HAMMI

ADEL M’NIF

Technological pole of Borj Cedria,National Center of Research in

Materials Sciences, ValorizationLaboratory 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 (bi-

schofite) 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 he-

mijske precipitacije. Prve dve faze su kristalizacije natrijum hlorida i dvogube soli kali-

juma i magnezijum. Zatim je dobijena smeša tretirana rastvorom kalcijum-hlorida radi

desulfatizacije. Nakon toga, izvršena je još jedna faza izotermnog isparavanja u cilju eli-

minisanja jona kalijuma u obliku karnalita, KClMgCl26H2O. Na kraju ove faze, dobijen

rastvor, primarno sastavljen od jona magnezijuma i hlorida, tretiran je dioksanom radi

precipitacije magnezijum-hlorida u obliku MgCl26H2OC4H8O2. Dobijeno jedinjenje je su-

šeno 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 proiz-

vodi, koji su, takođe, korisni.

Ključne reči: proces; ekstrakcija, magnezijum hlorid, prirodna slana voda.