The Preparation of High Purity Magnesium Oxide Part 1

NO 786/C

REPORT No 786/С

THE PREPARATION OF HIGH PURITY MAGNESIUM OXIDE PART 1. THE SEPARATION OF MICROQUANTITIES OF CALCIUM FROM MACROQUANTITIES OF MAGNESIUM

A. DĘBSKA - HORECKA . T. PUKA5 . I. STROŃSKI

KRAKOW report baa been reproduced directly from the best available copy

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Available from: HUCLEAR ENERGY INFORMATION CENTER of the Polish Government Commissioner for Use of Nuclear Energy Palace of Culture and Science Warsaw, POŁAM)

Drukuje i rozprowadza: OŚRODEK IflSOMfcACJl 0 EflSRGII JĄDROWEJ Pełnomocnika Rządu d/s Wykorzystania Energii Jądrowej Warszawa, Pałac Kultury i

Wydaje Instytut Fizyki Jądrowej

520 1 7 ^*i •«*•:•••<**»*£ ark. wyd. 1-8 f Ark. druk. - , Dat! ** "Maszynopisu przez autora 10.1,1972 f Oddano do druku ,Druk ukończono X 71 ,SP-09/250/66, Zam. nr 12/72 THE PREPARATION OP HIGH PURITY MAGNESIUM OXIDE i. THE SEPARATION OF MICROQUANTITIES OP CALCIUM PROM MACROQUANTITIES 0,? MAGNESIUM

PREPARATYKA TLENKU MAGNEZU O WYSOKIM STOPNIU CZYSTOŚCI i. ROZDZIELANIE MIKROILOŚCI WAPNIA CD MAIfflOILOŚCI MAGNEZU

ПОЛУЧЕНИЕ ВЫСОКОЧИСТОГО ОКСВДА МАГНИЯ

I. ОТДЕЛЕНИЕ МИКРОКОЛИЧЕСТВ КАЛЬЦИЯ ОТ МАКЮКОЛЯЧВСТВ МАГНИЯ

Antonina DEBSKA-HORECKA, Tadeusz PUKAS Institute of Inorganic Chemistry and Technology, Technical University, Gliwice

Ignacy STROŃSKI Laboratory of Chemistry and Radiochemistry, Institute of Nuclear Physics, Krakdw-Bronowic/e

CRACOW, December, 1971 II

The results of an investigation of processes of co-precipitation, crystallization, and liquid-liquid extraction of ions of calcium for the preparation of high purity salts of magnesium, by means of the ra- dioactive Isotope of Ca, are presented. fjj\ Ij])

Stosując promieniotwórczy izotop Ca badano proceey współstrącania, krystalizacji i ekstrakcji jonów wapnia w celu otrzymania soli magnezu o wysokim stopniu czystości.

.Используя радиоактивный изотоп *^Са были исследованы методы соосаждении, кристаллизация и экстрак- ции ионов кальция с целю получения высокой чистоты солей магния. The main impurity of magnesium preparations, analytically pure (A.P.) J..s calcium (iO~2%). The fundamental problem in investigations concerning the preparation of high-purity magnesium oxide is, then, the removal of microquantities of calcium from the niacr©quantities of magnesium. The removal of trace anounts of calcium from magnesium compounds is connected with enormous difficulties, due to the fact that the magnesium and calcium belong to the same group of the periodic system, and because of the isomorphic structure of the correspondent compounds, eg. CaO and MgO both crystallize according to the regular pattern, and final- ly because of the occurrence of calcium in nature in large quantities, so that a secondary contamination of the magnesium preparations is always possible. From literature we know methods of separating calcium from magnesium, though in the majority of cases this concerns compounds in which the concentration of 1/ 2/ these tv?o elements is nearly equal ' * ' . The aim of this research was to investigate the effect of various methods of purifying magnesium pre- - 2 -

parations from calcium, viz. a. by precipitating the macro-component in the form of a poorly soluble sediment in the presence of an agent complexing the raicro- oomponeut, b, by co-precipitating the micro-component together with the partial precipitation of the macro- component used as medium, с by crystallization, and d. by extracting the micro-component.

Experimental

The effect of purification was tasted radiometri- cally, applying the isotope Ca fa solution ol tho chloride Ca from the Radiochamical Centre at Amersham, with an activity of 1.9 ia'Ji/ml, Tt/2 = i36 days). For all the experiments an initial solution had been prepared (further on it will be called sample '7;, do that 10 ml 1 M A.P. magnesium nitrate was sampled and 0.05 ml Ca solution dropped in. The racUo- metric measurements were taken by теапз of an elec- tronic computer of the type LL-i and a G-M BAT-2 meter selecting for those measurements three 0.1 ml samples of the dried-out solution. The Eiean value of these reckonings made it possible to calculate the total activity. The sediments were solved in 10 ml - 3 -

2 М НС1, and the filtrates and organic phases were vaporized until a volume of 10 ml was reached» The magnesium was determined by complexometric 3/ tltration with sodium verse.nate '»

a/ Precipitation of magnesium in the form of a poorly soluble sediment in the presence of a calcium completing agent. In order to separate the calcium ions from the magnesium ions the fundamental component was precipitated from the solution of 1 M magnesium nitrate in the form of hydroxide, carbonate, and oxalate. It was found that mopt of the calcium contained in the solution was co-precipitated together with the sediments (Table i) To prevent the co-precipitation of traces of calcium together with the sediment of magnesium hydroxide, various ' have suggested removing the magnesium hydroxide by means of basic sodium in the presence of sodium verenate so that the whole amount of calcium will be left in the solution* The suggested methods have been checked, with the modification that ammonium salt of ethylenediaminetetraacetio acid (EDTA) was used in order to eliminate the possibility of introducing additional impurities in the form of Na+ ions. Also ТаЪ1е_1

Co-proelpitatlon of trace amounts of ealolum nlta poorly soluble magnesium compounds

1 Radioactivity Degree of oounts/min ml precipita- of calcium tion of ma- No Vest co-precipi- gnesium in precipi- filtrate tated to- the sedi- tate gether with ment % the sediment i 2 Ъ 4 5 6 1 Precipitation of magnesium 6270±200 260^40 hxdroxide_with 2 If NaOH 96 39 la Precipitation of magnesium hydroxide with 2 U NaOH in 50-10 5280±120 1 99 the presence of EDTA 1b Precipitation of magnesium hydroxide with 2 M NaOH 60-18 4900^200 1 99 in the presence of EGTA 2 Precipitation of magnesium hydroxide by means of 4O±10 49ОО±16О 1 97 ammonia Table 1

1 2 3 4 5 6

3 Precipitation of magnesium 4840-128 carbonate 1100*96 81 98 I За Precipitation of magnesium СП carbonate in the presence 70*14 5400*172 1 90 I of EDTA ЗЪ Precipitation of magnesium carbonate in the presence 50*10 4500*160 1 98 of EGTA

4 Precipitation of magnesium 950*87 82 86 oxalate 4440-105 4а Precipitation of magnesium oxalate in the presence 20-10 4850*165 1 84 of EDTA 4b Precipitation of magnesium oxalate in the presence 10*10 4570*96 1 86 of EGTA Notes In test 1 - pH = 12.3, test 2 pH в 9.6, test 3 and 4 - pH = 9.3 - 6 - investigated «as the adaptability of БОТА as a calcium masking agent while the magnesium carbonate and magnesium oxalate are being precipitated. These compounds are more' advantageous in preparing MgO than magnesium hydroxide, due to the possibility of precipitating the easily washed out precipitates, Some authors 7~" *' have suggested using ethylene- glycolbisamino-ethyl-ether-tetra-acetic add (EGTA) as a reagent for the determination of calcium as compared with magnesium. Unlike magnesium in the range 7.5 - 8.5 pH calcium forms with EGTA a stable, negatively charged chelate; therefore an attempt was made to precipitate magnesium hydroxide, magnesium carbonate, and magnesium oxalate in the presence of EGTA. Attempts were also made to Isolate magnesium hydroxide by means of ammonia. The following tests were carried out:

Precipitation of %(0H)2 by 2 M NsiOH To the sample W some drops of indigo carmine were added ; the whole was stirred while it was heated up to the boiling point} 2 U NaOH was gradually dropped In until the oolour changed from blue to yellow. The precipitate was left to settle, after which it was filtered

and rinsed with hot H20. In the same way tests were carried out for samples la and lb (of. Table!), except - 7 - that 5 ml 0,15 M EDTA and 5 ml 0.15 M EGTA respectively were here added to the sample.

Precipitation of Mg(0H)2 with ammonia 500 ml of A*P. 25% ammonia was placed in a desicc- ator 250 mm in diameter. Above the surface of the solution was placed a shallow polyethylene vessel 80 mm in diameter, containing the sample W. After 48 hours a sediment was obtained in the form of flakes. The sediment was rinsed with hot water. Precipitation of magnesium carbonate The sample W was brought to about 3.5 pH by 0,1 M ammonia; the whole was heated and 10 ml of 3 M ammonium carbonate were added, as well as 2 ml of concentrated ammonia solution and 2 ml of ethyl alcohol. This was put aside until its temperature reached room temperature, and it was then filtered and rinsed with hot water. In the same way tests were carried out for samples 3a and 3b (Table 1), except that in this case to the samples being provided 5 ml 0,15 M EDTA and 5 ml 0.15 U EGTA respectively were added. Precipitation of magnesium oxalate The tests were carried out similarly as in the case of precipitating magnesium carbonate, applying 30 ml - 8 - of 0.5 U A* P. ammonium oxalate. Also the tests for samples 4a and 4b were carried out In this nay (Table 1), but to the sample W б ml of 0.15 M EDTA and 5 ml of 0.15 M SGTA were added respectively. The obtained results are assembled in Table 1. Investigations «ere aloo carried out concerning the co-preolpltation of calcium with magnesium hydroxide, aooordlng to the pH (Table 2). b. Co-precipitation of calcium ions Tilth the partial presipitatlon of magnesium as a collector Angelov et al. ' suggested removing the calcium from a magnesium chloride solution by co-precipitating It together with magnesium carbonate. The precipitation of magnesium by means of ammonium carbonate nas repeated four times. Each time 10% of the magnesium «as removed in the form of carbonate. The remaining magnesium was removed by means of a stoichiometrie amount of ammonium carbonate* The magnesium carbonate thu3 obtained was calcimated, as a result of which a spectral- ly pure magnesium oxide «as obtained* In the paper mentioned above the oo-preoipitation of calcium with magnesium oxalate was also investigated. In order to participate the magnesium, both ammonium oxalate and oxalic were used. The tests did not reveal a sufficient - 9 -

fable a

Co-precipitation «ИГ tree* emouote ot oftleltm t*g*4%«r with magnesium hy

% of calolnm % Of ДО"» oo-precipitated oipitated Test pH with tit» preci- magneeiiiB pitate of шасвв* sina npurosifB ^ Precipitation of magnesium hydroxide by means of 9.6 1 ammonia

Precipitation of magnesium hydro- 9,3 «Л xide with а Ы NaQH 11 23 12*3 96 9S - 10 - degree of purfication froni calcium. , On the basis of data taken from? literature arid on preliminary tests, Investigations were carried .out concerning the co-precipitation of calcium ions, where part of the magnesium (the carrier) was precipitated by means of ammonium carbonate, salicylhydroxamic a©id and aranonium carbonate, picrelonic acid, and 8-hydroxyquinoline. Poddar et al, ' state that salicylhydroxamic acid precipitates the calcium quantitatively in the range

3.5 - 10.5 pHt while the magnesium remains in the solution* Picrolonic acid forms with the calcium and magnesium ions stable complexes of the constitution 1:2, appearing in the form of a yellow deposit ~ 14/C This acid is

13 applied in the determination of calcium /,15/,i6/e The optimum pH for calcium precipitation in the form of pi- krolonate amounts to 3 - 717% 8-hydroxyquinoline forms, together with the calcium and magnesium ions, poorly soluble sediments in an alkaline aiedium; henoe it is used for the determination of

18 i9 both magnesium and calcium /i /# The expremlnets were carried out in such a way that the sample W was brought by means of 0.1 M Щ.0Н to the adequate pH. The solution was stirred and gently heated, - 11 - after which the precipitating reagent

o* Crystallization of magnesium salts The crystallization of A.P. magnesium chloride, nas investigated earlier ' and it «as found that after a two-fold crystallization the oalclua concentration decreases 60 times, the degree of crystallization amounting to 50$. In order to supplement these investigations, attempts «ere made to crystallize A,P. magnesium sulphate and magnesium nitrate. The effectiveness of purification is generally knosra to decrease due to the isomorphism of the micro- and the macro-component* The formation of mixed crystals depends on smcfe coefficients as the radius and the charges of the ions, the symmetry, the parameters of the crystal lattice, end the analogy of chemical formulae (Table 4). Data taken from the literature (Table 5) indicate the possibility of magnesium purification on account of the differente solubilities of the corresponding oaloiam and magnesium salts either in the case of the crystallis- ation of magnesium nitrate from methyl alcohol or la that of the crystallization of magnesium sulphate front Tablę_3 Removal of trace amounts of oalolum from 1 M magneeium nitrate by means of oo-precipltation

Radioactivity % of calcium Degree of- counte/min ml co-precipi- the precipi- Precipitating tated with tation of Test pH preci- filtra- the sediment magnesium reagent pitate te from the solution % 1 0.5 ml 3 M ammonium oarbonate + 5 ml H«0 8.6 940-56 4210-84 13 6 i re-distilled water 2 5 ml 0.08 M salicyl- hydroxamio acid 8.5 3150±150 36 6 + 0.5 ml 3 U ammonium 1810-56 carbonate 3 5 ml 0.05 M aqueous alcoholic solution of 6 650-100 4800*246 12 4 picrolonio acid 4 5 ml 0.2 M 8-hydroxyquinoline in ethyl 410-39 4540*165 6 8 alcohol + 5 ml ammo- 9 nia 1 i 1 Note: In test 3 the solution was not heated; in the case of test 4 it was found additionally that from 8-hydroxyquinoIate such impurities as Mn, Pb, Cu, N1, Ag are co-precipitated; this will be the subject of further investigations. Table 4 20/ 21/ The structure of calcium and magnesium salt crystals *

Crystallo- Size of the elementary cell Salt graphic structure /system/ a, ь. c, Angle + В I Mgso4 . 7 H20 orthorhombic 11.91 "12.02 6.87 u CaS04 . 2 H20 monoclinio 10.47 15.15 6.61 151°33»

Ug(NO3) 2 . 6 H20 monoclinic 5.19 10.00 9.69 92°56» Ca(NO ) monoclinic 6.54 10.00 8.97 9I°10f 3 2 • * H20

MgCl2 . 6 H20 nionoclinic 9.90 7.15 6.10 94°00•

CaCl2 . 6 н2о trigonal 7.86 .... 3.90 Tafcie «5

Solubility of oalolum and magnesium salts In water and methanol in g/100 g solution

Water Methanol Salt 16° 20° 100° 20° 60°

Magnesium nitrate 40.4 71.7 14.5 25.9 Calcium nitrate 54.9 78.4 57.2 61.2 * Magnesium sulphate 25.2 40.6 Calcium sulphate 0.21 0.16

Magnesium chloride 35.3 42.2 - Calciuii; i oride 42.7 61.4 wateri in $H$ Latter case, together with the crystal- 11 zed magnesium sulphate, the lees soluble calcium sulphate should appear. In this case the tests were carried out in such a way that 0.1 male of A,P. magnesium nitrate or magnesium sulphate was solved' in a suitable amount of solvent, 0,05 ml Ca was added and the whole heated, after which it was filtrated whan it cooled down again to room temperature, fhe results are shown, in Table 6. d. Extraction of calcium ions from a magnesium nitrate solution

The literature does not give many selective reagents for the extraction of calcium. On the whole 23/ 24/ extraction occurs in alkaline media * , which finds no application if there are large amounts of magnesium, because then magnesium hydroxide is precipitated. Zolotov et al.55^ state that 0.G2 M pierolonie acid in either cyclohexanone or benayl alcohol extracts 90 to iOO;» calcium at a pH of 4 to 7. A reagent which extracts 100$ calcium at а рЯ of 6.5 to 8.5 is thenoyltrifluoroacetone {ПА). Quinoy developed a method of obtaining a high purity. Tąblę_6

Practionation of calcium during the crystallization of magnesium nitrate and magnesium sulphate

Radioaotivity % Ca in % Mg oounts/min ml the obtai- in the ned crys- obtained No of the of the crystals Test obtained post- tal crystals er ye ta- ll i za- tion. lye 1 Crystallization of magnesium 1320*54 2990*85 30 : 63 en nitrate from 0.7 ml water i 2 Crystallization of magnesium nitrate from concentrated HN0„ 260*24 4250*53 5 60 «i 3 ..Crystallization of magnesium ni tratę from t mi methyl aloohol ' 1720*90 2620*75 39 62

4 Crystallization of magnesium 3990*60 9. sulphate from 2 ml water 400*11 20

Notei Sample 2/ was solved in 0*3 ml of water, and heated, after which 2.5 ml of oonoentrated BN0„ was added| then the whole was rinsed with 0.5 ml oaf concentrated HNO - 17 -

oxide, based on purification of the magnesium nitrate solution by raeans of extraction with 0.5 51 TTA solution in methyl i 30butylke tone 'MI3JCJ at a pH of 5.5. ОЯ / Akaza " reports that the efficiency of extraction by meaii3 of TTA in UIBK decreases in the series Mg, Ca, v"'r, Da, their distribution coefficients being too similar to one another to warrant an effective distribution. In our experiments attempts were carried out to oxtract calcium from a solution of magnesium nitrate with pikrolonic acid in cyoiohexanone; it was also checked whether at the pH given by the authors" ^ crlcium ions raay bo extracted by means of 0.5 M TTA solution in UIBK.

Extraction with 0.02 II pikrolonic acid in cyclohexanone.

The sample W was brought to рП = 6 and extracted twice, 5 ml each time, with 0.02 Ы pikrolonic acid in oyclohexanone, and then rinsed twice :7ith 5 iiil oyclohexanone .

extraction with 0.05 M ТТЛ in ВДВК at a pH of 5.5 and 8.5,

The sample XI was brought in both cases to the respective pll, and then 10 ml were extracted with 0.5 II TTA in MIBK. The whole was rinsed twice with 5 ml UIBK- - 18 -

The obtained results are assembled in Table 7

The best separation of magnesium from calcium was achieved by magnesium precipitation in poorly soluble compounds in the presence of an agent masking the calcium, ;cf. Table i; . It was found that in the presence of EDTA and EGTA practically the whole amount of 45Ca remains in the solution and does not co-precipitate with the sediments of magnesium hydroxide 'precipitated with a solution of NaOHy, magnesium carbonate, and magnesium oxalate. The applied reagents favour the selective separation of magnesium from calcium because of the considerable difference in the stability of the respective complexes. The calcium complexes are much more stable than the magnesium ones (cf. Table 3j• Calcium also does not co-precipitate with the sediment (even if no complexons are used], when the magnesium hydroxide is formed by using ammonia. The accomplished tests (Table Z) showed that the co-precipitation of calcium with magnesium hydroxide increases with the increase in pH. In the test with pH a 12.3 the co-precipitation of calcium with magneeium Table 7

Extraction of trace amounts of calcium from a magnesium nitrate solution

Radioactivity % of magne- counts/min ml % of calcium ex- sium in the of the traction aqueous No Test PH of the phase after aqueous organic extraction phase extract

1 Extraction with CD 0.02 M picrolonic — I acid in cyclo- 6 4320±77 0 100. hexanone

2 Extraction with 0.5 U ТТЛ in 5.5 4360±95 120 2 100 MIBK

3 Extraction with 0.5 M TTA in 8.5 358O±1OO 20±10 1 100 MIBK Tatole_8

Logaritms of the stability constants of oaloium and magnesium complexes7 '/

Coraplexing x K *« к - ig ^ reagent « Ca Са

EDTA . 10.7 8.7 2 I» о

EGTA 10.9 5.3 5.6 - 21 -

hydroxide is in fact complete. In the case of precipitating magnesium hydroxide with ammonia at a pH of 9.6 the concentration of the hydroxide ions ia too low for the calcium hydroxide tc oo-precipitate with the

Mg(pHJ2, tout it is high enough to allow a practically complete precipitation of magnesium. The solubility products of magnesium hydroxide and caloium hydroxide

a 6 12 L e 3 10 5 amount to L^ (0H)2 • *°~ > Ca (OH) 2 ' ' ' The removal 01' magnesium hydroxide with ammonia is more to be recommended than with sodium hydroxide, as it does not lead to any additional contamination with sodium ions. In the case of precipitating magnesium carbonate in the presence of EDTA or EGTA and when, mag* nesium hydroxide is precipitated with ammonia, separation from calcium together with precipitation of about 90u of magnesium is obtained. Thus the methods described above will be applied In the further preparation of high purity magnesium oxides. Together with magnesium carbonate, magnesium pioro- lonate, and 8-hydroxyquinolinate, only slight traces of calcium are co-precipitated. (Table 3). In order to achieve a sufficient purity the co-precipitation should be repeated several times, which is connected with considerable losses of magnesium. Only the addition of - 22 - salicylhydroxamic acid, a compound that forms stable oomMnations only with calcium, increases the amount of the co-precipitated calcium from 18% (when only cunmoniua carbonate is used] to 36$. The crystallization of magnesium nitrate and magnesium sulphate makes io possible to remove the calcium at least to some exent (Table 6J . The best result •were obtained when magnesium nitrate was salted out by means of concentrated nitric acid, because then only 5fS of calcium was left in the obtained crystals. An efficiency of crystallization amounting to 60;&, however, renders this method less advantageous than the precipitation of magnesium hydroxide with ammonia or magnesium carbonate in the presence of EDTA. The investigated methods of extraction do not allow the separation of calcium from magnesium. It was found [Table t) that if the mole ratio to Ca: Mg = l : 5000, the calcium ions are not extracted either by means of 0.02 M picrolonic acid at a pH = 6, or 0.50 M TTA in MIBK with a pH * 5.5 and 8.5.

The authors wish to thank Prof. A.Z. Hrynkiewicz, Director of the Institute of Nuclear Physics in Kraków for his interest and encouragement, and to Mr. J. Kwaś- nik for his technical assistance. - 23 -

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