Process technology for recovery of magnesia from brines

Powder Technology, @ Elsevier Sequoia

27 (1980)

Sk,

Process Technology S. F. ESTEFAN, National

Research

(Received

233 - 240

Lausanne -Printed

for Recovery

F. T. AWADALLA Centre,

May 14,198O;

Cairo

233

in the Netherlands

of Magnesia

from Brines

and A. A. YOUSEF

(Egypt)

in revised form July 29, 1980)

SUMMARY

Laboratory tests for the utilization of natural trona mineral for the recouery of magnesium from high sulphate brines, in the form of basic magnesium carbonate, were conducted at a fixed temperature of 60 “C. The role of the precipitation pH on the chemicalconstitution of theprecipftated magnesium was investigated_ The prepared precipitates were examined by X-ray, DTA, chemical analysis, loss on ignition at 1OOO”Cand spectrographic analysis of the minor impurities which may influence the refractory properties of the magnesia produced. When the basic magnesium carbonate, obtained under optimum conditionsofpH= 1 Oand ata temperature of 60 “C, was calcined at 1100 “C pure magnesium oxide assaying 99.4% MgO was obtained_

INTRODUCTION

Magnesium comprises the sixth most abundant element in the earth’s crust and the third most abundant eIement in sea water. Being one of the most reactive metals, magnesium is never found in nature in the elemental state. As part of the rock-forming minerals like magnesite, doiomite, brucite, olivine and serpentine, it is combined in virtually insoluble forms. As part of soluble minerals like leonite, kainite, shoenite, langbeinite and camallite, it is found in evaporite deposits, generally of marine origin, deep within the earth’s C~LI&.Finally, as solute in marine-like brines, magnesium is common in surface and subterranez+n waters throughout the world. The oceans, which contain 0.13% Mg, ilhxtrate the inexhaustible natural source of the reserve.

Recovery

of

magnesium

from

the

rock-

forming minerals, usually by thermal decomposition, and from sea water by precipitation with lime, is a frequent industrial practice. Other methods of recovering magnesium from sea water have been reported, but none seem as attractive as precipitation in the form of basic carbonate [l - 43 by proper adjustment of pH where carbonate is present. Recovery of magnesium in the form of magnesium chloride from either evaporites or naturally concentrated brines has been of little industrial importance for technical and economic reasons. However, recovery of magnesium chloride from the waste brine effluent of potash operation is due to receive more attention in the near future in view of current demand for that raw material of cell feed for electrolytic production of magnesium metal. While the United States has relied Freponderantly on the sea-water magnesia process as a source of magnesium chloride for cell feed, some of the European practice is oriented more towards use of the by-product magnesium chloride, obtained mostly from the potash industry. Many magnesium products are made from the hydroxide or carbonate_ Probably the largest in tonnage is the oxide, which is obtained by dead-burning [1, 2, 5, 61 for the manufacture of refractories. Also commercialized is the production of a variety of magnesium salts [7 - lo] and magnesium metal by electrolysis [ll - 133 of the fused anhydrous magnesium chIoride. The aim of the present work is to investigate the optimum conditions favouring the utilization of the Egyptian trona deposits in Wadi El-Natzun for precipitation of basic magnesium carbonate from high sulphate content brines, which on calcination at 1100 "C produceshighqualitymagnesia.

234 EXPERIMENTAL

A volume of 20 litres of Qarun Lake brine havinga concentrationof3.8" Beand a pH = 8.2 was cleaned from suspended matter by filtration and then adjusted at pH = 6 by using sulphuric acid for decarbonation. The brine was evaporated at 70 “C up to a concentration of 23” Be_ An amount of 10 p_p.m. of soluble starch was added as an aqueous solution to behave as a nucleating agent for calcium sulphate and evaporation continued until 26” Be was reached. Tne brine was boiled for 10 minutes and filtered quickly while hot. The filtrate was checked for calcium ions qualitatively and there was no evidence for the existence of any detectable calcium. The filtrate was made up to 2 litres and analysed for its magnesium content. Aliquots of 100 ml of the calcium-free brine were used for the recovery of magnesium at a fixed temperature of 60 “C and at different pH vaIues when both trona and caustic soda solutions were used as precipitating agents_ A sampIe of trona presented from Wadi El-Natrun, Egypt, was dried at 120 “C for two hours and analysed and then calcined at 500 “C for two hours whereby the bicarbonate fraction was decomposed to carbonate and the complex coloured organic matter was carbonized. The calcined trona, dirty black in colour, when dissolved in water gave a solution with the carbonaceous matter in suspenr%n. The solution when filtered gave a clear filtrate free from any colloidal organic matter, and when checked for calcium, it was realized that it is calcium-free. This solution was used together with caustic soda for the precipitation of magnesium from brine. The role of the sequence of addition of trona and caustic soda, and the effect of the final precipitation pH on the chemical constitution of the magnesium precipitate was studied. The magnesium preci%tates were left overnight to settle down, then filtered, thoroughly washed with distihed water and dried for 24 hours at 60 “C and were then examin ed by X-ray using copper target element, by DTA using AlzOs as a reference material, by loss on ignition at 1000 “C and by chemical analysis_ When the products were calcined at 1100 “C, pure magnesia was obtained. The results were compared with that obtained from the precipitation of magnesium

using chemical grade sodium carbonate and caustic soda at a pH = 10. The mother liquor left after the recovery of magnesium was used for the recovery of technical-grade sodium sulphate and edible sodium chloride.

RESULTS

AND

DISCUSSION

Lake Qarun at Faiyum, Egypt. which contains an estimated 7 million tor_nes of magnesium sulphate and which is annually fed with incremental amounts of salts carried in by the agricultural wastewater, is one of the principal resources for planned expanded domestic refractory magnesia and marine chemicals production. For several years, Estefan [l, 14 - 201 has continued investigations of the comparative merits of alternative methods for processing the lake brine to recover diversified mineral salts and by-product chemicals [14,16 J _ Chemical analysis of the major constituents of the lake brine, presented in Table 1, indicates its very high sulphate content relative to sea water. Spectrochemical analysis of the evaporite residue of the lake brine and its phase chemistry behaviour along the evaporation route have been presented elsewhere [ 14, 151. Although the total salinity of Lake Qarun is susceptibIe to smaU variations throughout the different seasons of the year as a result of the feed of agricultural wastewater, the relative concentrations of the individual salts are fairly constant. Exploitation of the mineral wealth of a lake brine with as high a sulphate content as Lake Qarun reflects technical and economical problems. However, there are several schemes involving step evaporation and phase rule manipulation through which such products as gypsum [lSJ , sodium sulphate [19], sodium chloride [16], potassium chloride and magnesium chloride can be fractionally crystailized. Recent studies in our Institute have been focused on evaluating alternative methods adopted for recovery of high-quality magnesia for pharmaceutical, refractory and metallurgical purposes. Application of the conventional technique of precipitating seawater magnesium in the hydroxide form by using slaked lime [Zl] to the virgin brine of Lake Qarun may be inadequate in regard to

236 TABLE

2

Chemical analysis of the trona ore Component

A by weight

cos2HC03clSO,_ ;z$

12.5 2.8 5.1 25.4 0.03 nil

Component

A1203

B203 SiO2 Water insolubles % loss at 120 “C Q loss at 800 “C

56 by weight 0.019 0.03 0.064 0.10 1.24 2.18

(2) The brine, after treatment with trona, has become highly enriched in sodium sulphate as a result of two individual factors: (a) the additive fraction of sodium sulphate already contaminating the trona; (b) the displacement of the magnesium snlphate component in the brine by sodium sulphate through the pmcipitation of basic magnesium carbonate. Therefore, recovery of sodium sulphate from the lake brine could be effectively practised on an economic basis [ 191. Basic magnesium carbonate was obtained when precipitation of magnesium present in the lake brine was carried out by using both trona and caustic soda solutions at a thermostatically controlled temperature of 60 “C and a controlled pH of lo_ Figure 1 represents

Fig. 1. DTA

records of basic magnesium

carbonate.

the DTA records of the recovered basic magnesium carbonate. Both DTA records for hydromagnesite Mg5(C03)4.(0H)2.4H,0 and synthetic basic magnesium carbonate Mg,(C03)3.(0H)23H20 are quite similar [24, 25) . The diverging endothermic peak detected at 310 “C is characteristic for loss of water moiecuies of crystahization, the endothermic doublet at about 440 “C which merges with the subsequent effect for loss of hydroxyl water, and the endothermic doublet at 540 “C for loss of carbon dioxide. Curve (1) (Fig. 1) represents the DTA record for the precipitate obtained when cakined trona was firstly added to pH 9.5 followed by caustic soda solution to a final precipitation pH of 10. The lowering in the temperature of the endothermic peak associated with the loss of carbon dioxide to a value of 500 “C may be attributed to contamination with trace amounts of sodium chloride. It has been observed [26] that addition of about 1% sodium chloride lowers the magnesite peak temperature by about 50 “C, a finding confirmed also by Webb [27] _ Chemical analysis of the sample dried at 60 “C indicated that it contains 95.07% basic magnesium carbonate. The loss on ignition at 1000 “C was found to be 56.28%, which is in good confirmity with the calculated loss on ignition of 56.04% for

237

pure basic magnesium carbonate Mg,(CO&.(OH)2.3Hz0. Curve (2) in Fig. 1 represents the DTA record for the precipitate obtained when caustic soda solution was firstly added to pH 9.5 followed by uncalcined trona solution to a final precipitation pH of 10. The anomalous exothermic peak detected at 500 “C, just before the loss of carbon dioxide, is frequently observed with hydromagnesite samples [ZS] , preceding the peak associated with the loss of carbon dioxide, and was also observed with magnesite samples [27,29 - 34). De Vlesschauwer [35] suggested that this exothermic peak may be due to shrinkage of the sample from the walls of the sample well after dissociation, whereas Wilbum et al. [36] have indicated that this effect is attributed to thermal diffusivity of the constructional material and to the design of the specimenholder. The first interpretation [35] seems to be more logical since all the present samples were examined in one and the same specimenholder, while this anomalous exotbermic peak was only observed with the sample represented in curve (2). Webb 1271 observed that this effect is not reversible and is not due to any detectable phase changes. Base-line displacement after the main decomposition peak is markedly distinct for this sample. Chemical analysis of a sample dried at 60 “C indicated

Fig.

2. X-ray

patterns

of magnesium

precipitates.

thatit assays 94.95% basic magnesium carbonate. Curve (3) in Fig. 1 represents the DTA record for the precipitate obtained when caustic soda solution was firstly added to pH 9.5 followed by chemical grade sodium carbonate to a final precipitation pH of 10. This curve is presented here for the sake of comparison with the results obtained with natural trona ore. This curve has the familiar shape of the DTA record for hydromagnesite [ZS] _The loss on ignition at 1000 “C for this sample was found to be 56.18%. However, the results presented above indicate that the sequence of addition of trona and caustic soda solutions has a minor effect on the physical nature of the precipitates formed as long as the final precipitation pH does not exceed 10. The yield of basic magnesium carbonate was in the range 4.5 - 5.0 g/L of the lake brine. Examination of the filter cake product, dried at 60 “C, by X-ray diffraction (Fig. 2, pattern 3) indicated conclusively that it is basic magnesium carbonate hydrate having the crystalline form Mg,(C0s)3.(OH)2.3H.r0. When basic magnesium carbonate was calcined at 1100 “C in a rotary kiln, pure magnesia assaying 99.4% MgO was obtained. On the other hand, when precipitation of magnesium by trona and caustic soda was

238

conducted at high pH values of 11 - 12, magnesium hydroxide and not basic magnesium carbonate was obtained. Figure 3 presents the DTA records of the magnesium hydroxide precipitates obtained at the pH values 11 and 12. The endothermic peak diverging at 130 “C corresponds to loss of hygroscopic water. Zeolitic water is lost at 270” - 300 “C. The deep endothermic peak detected at temperatures in the range 405” - 425 “C 12 characteristic for the loss of constitutional hydroxyl water. These records are in good conformity with the results reported by Bran&i and his coworkers [37] _ They have observed that at about 500 “C 91.5% of the initial water is eliminated and a metastable form of magnesia is formed. At 780 “C this metastable form is converted to periclase c~~~stals.Webb [27] has reported that the DTA record for magnesium hydroxide shows an endothermic peak at 440 “C. However, Berak et al. [38] have found that the decompos;tion temperature of magnesium hydroxide decreases slightly in the presence of carbonate impurities. The occurrence of periclase MgO and brucite Mg(OH)P in nature indicates that magnesium oxide and magnesium hydroxide do not readily carbonate, but on the basis of extensive studies Murray and Fischer 1253 have concluded that despiw the inconclusive evidence, there is some probability that carbonation of magnesium hydroxide can occur. Braniski and his

co-workers [37] have observed two distinct decarbonations in the DTA record for magnesium hydroxide precipitated from sea water with calcined dolomite. Webb 1271 has presented evidence for the very limited carbonation of magnesium hydroxide in a carbon dioxide atmosphere, but he could not confirm whether the reaction product was basic magnesium carbonate or magnesite. Curve (1) (Fig. 3) represents the DTA record for the precipitate obtained when uncalcined trona solution was firstly added to pH 9.5 followed by caustic soda solution to a final precipitation pH of 11. In this record, the endothermic peak for the loss of zeolitic water is not clearly identified but it merges with the preceding effect, to loss of hygroscopic water_ Chemical analysis of a sample dried at 60 “C indicated that it con’bins 64.78% MgO. The loss on ignition at 1000 “C was found to be 42.42%. The calculated loss on ignition for chemically pure Mg(OH)* is 30.88%. However, the high percentage loss on ignition for the prepared sample may be attributed to loss of some adsorbed water molecules as indicated by the endothermic peaks scattered on the DTA record at temperatures of 100° - 200 “C. It should be noticed that the prepared samples were dried only at 60 “C for 24 hours. Curve (2) in Fig. 3 represents the DTA record for the precipitate obtained when caustic soda solution was firstly added to pH

240

and Dr. Sabah N. Boulos, National Research Centre, for their sincere cooperation and help.

20 21 22 23

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14 15 16 17 18 19

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