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Structural evidence on the beryl-sodium fluorosilicate reaction
J. Inorg. Nucl. Chem., 1961 Vol. 19, pp. 237 to 244. Pergamon Press Ltd. Printed in Northern Ireland
S T R U C T U R A L E V I D E N C E O N THE B E R Y L - S O D I U M FLUOROSILICATE REACTION K. R. HYDE,P. L. ROBINSON,M. J. WATERMAN and J. M. WATERS Atomic Energy Research Establishment, Harwell, Didcot, Berks. (Received 25 November 1960)
Abstract--An X-ray examination has been made of the products of the reaction of beryl with sodium fluoride, silicon tetrafluoride and sodium fluorosilicate. The evidence, in conjunction with that from chemical analysis, has enabled certain stages in the reaction of beryl with sodium fluorosilicate to be recognized. The first of these leads to a soluble fluoroberyllate, cryolite and cristobalite, and the second to the felspar albite. The third stage, occurring when all the beryl has been destroyed, produces albite, sodium fluoride and silicon tetrafluoride by a reaction between cryolite and cristobalite. The albite incorporates beryllium ions in its lattice and these cease to be extractable by water. SINCE it was advocated in 1919, the Copaux process, ~x) sometimes in a modified form, has been used for the extraction of the beryllium from beryl; a recent instance is by Murex Ltd. ~ We have described an investigation of the beryl-fluorosilicate reaction on which the Copaux extraction is based, directed to establishing conditions furnishing a product which gave the maximum proportion of beryllium extractable by aqueous leaching, c3~ Here we deduce something of the mechanism of this reaction, largely from an X-ray identification of certain intermediate and final products. The beryl was ground material from the U.K.A.E.A. Milford Haven factory, almost entirely free from other minerals and, judged by the proportion of beryllium (BeO = 11.9 per cent), of high quality. Thus the reaction can be considered as occurring between two well-characterised chemical compounds, an aluminosilicate, empirically 3BeO'AlzO3"6SiO2, and sodium fluorosilicate, Because more or less pure beryl has generally been used and because the reaction is remarkably specific in that water extracts from the reaction products almost exclusively the elements sodium, beryllium and fluorine and leaves silicon and aluminium in the leached residue, rather precise courses for the reaction have been advanced, such as: 3BeO'A12Oa'6SiO 2 q- 3Na2SiF 6 --~ 3BeF~ q- 2A1Fa ÷ 9SiO 2 ÷ 6NaF or or
3BeO-A1203'6SiO 2 ÷ 6Na2SiF ~ ~ 3Na2BeF4 q- 2NaaA1F s -~ 9SiO 2 ÷ 3SiF 4 2(3BeO'AlzOa'6SiO2) + 6NazSiF 6 --~ 6NazBeF 4 -q- 2AlzO3 q- 15SiO2 + 3SiF4
There does not appear to have been much stoicheiometric justification for any of these equations, or even identification of the compounds. The mechanism is certainly complex and recognition of individual products not always possible, but a combination of chemical and X-ray observations has given us some insight into the course of the reaction. First a non-crystalline sodium fluoroberyllate glass is formed along with tl~ H. COPAUX, C.R. Acad. Sci., Paris 168, 610 (1919). ~2~ p. S. BRYANT, Beryllium production at Milford Haven. Extraction and refining o f the rarer metals (London: Inst. Min. Metall., 1957) 310. ta~ K. R. HYDE, P. L. ROBINSON, M. J. WATERMAN and J. M. WAXERS, Trans. Inst. Min. Metall., 70, 397, (1960-61). 237
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K . R . HYDE, P. L. ROBINSON, M. J. WATERMANand J. M. WATERS
insoluble, crystalline cryolite, NasA1Ft, and ~-cristobalite, SiOz. This is followed by a re-arrangement of the ions A13+, Si4+ and 02- and their combination with Na + to produce crystalline albite, NaA1SiaOs. Finally there is a reaction between ~-cristobarite and cryolite to form more albite. Some of the beryllium is incorporated in the albite lattice and ceases to be soluble. There is reason to suppose the ~-cristobalite is derived from the silicon of the fluorosilicate.
The structure of beryl and its behaviour when heated Beryl has a structure (4) consisting of sheets of hexagonal rings of six silicon-oxygen tetrahedra bound to one another in the individual sheets and to rings in contiguous sheets by bridging aluminium and beryllium ions, the former being hexa-coordinated and the latter tetra-co-ordinated to the oxygen ions. Although the arrangement appears layered in a diagram, the bonding is so uniform that in place of a marked basal cleavage there is a poor and irregular subconchoidal fracture. The mineral has a hexagonal unit cell with fifty eight atoms (% = 9.215/~, e0 ---- 9"192/~) and a sharp X-ray diffraction pattern; the latter is useful both for detecting structural differences and for recognising its presence, even in low proportions, in mixtures. The structure remains unchanged after the crystal has been heated to 800° and rapidly cooled. Extra, unascribed lines appear, however, when the temperature has been to 1100°, and beryllia is detected when it has reached 1200°. The amount of beryllia is larger at 1400°, and the beryl has lost much of its crystallinity when it has been heated to 1600°. Within the temperature range employed for the fluorosilicate reaction, the expansion of beryl is slight up to about 300° and somewhat greater in both a and c directions between that point and 800°. The change is completely reversible and there is no evidence of structural alteration that might facilitate chemical attack.
Sodium fluorosilieate The effect of heat on sodium and other fluorosilieates has been examined by CAILLAT(5) who gives dissociation pressures for the sodium salt. Although some of Caillat's results have been questioned/e) we have our own evidence that liberation of silicon tetrafluoride occurs at 500°, a temperature at which a reaction with beryl is barely detectable after 24 hr, and that it becomes very considerable at 600°, a temperature at which most of the mineral is consumed in 4 hr. Thus the question arises of the respective parts played by sodium fluoride, silicon tetrafluoride and undissociated sodium fluorosilieate in the attack on beryl. When heated in an open system, the salt loses silicon tetrafluoride without melting and leaves mixtures in which the proportion of sodium fluoride increases with the time of heating. The reaction is reversed in an atmosphere of silicon tetrafluoride. Thus the reactant always contains a certain proportion of sodium fluoride, a proportion which depends not only on time, but also upon the atmosphere over the solid. (4) W. L. BRAGGand J. WEST,Proc. Roy. Soc. A l l l , 691 (1926); N. V. BELOVand R. G. MATVEENA,Dokl. Akad. Nauk SSSR 73, 299 (1950). (5) R. CAILLAT,Ann. Chim. 20, 368, (1945). Atomic Energy Commission. (8) I. G. RYss, The Chemistry of Fluorine and its Inorganic Compounds Moscow (1956). Translated from a publication of the State Publishing House for Scientific, Technical and Chemical Literature, Moscow, (1956). AEC-tr-3927 (Pt. 1) and (Pt. 2). United States Atomic Energy Commission, Technical Information Service (1960).
Structural evidence on the beryl-sodium fluorosilicatereaction
239
Sodium .fluoride and beryl Sodium fluoride melts about 990 ° at which temperature the vapour-pressure is low. With beryl:fluoride ratios less than 1:4, the mixture melts below 700 ° and leaves, on cooling, a hard slag, At 600 ° all mixtures form a frit; with a 1 : 3 ratio about 90 per cent of the beryl is consumed in 4 hr, but the proportion of beryllium rendered soluble in water is low. The crystalline products which have been identified show that at 600 ° the reaction proceeds mainly with the formation of albite, very little ct-cristobalite is produced, and cryolite is not detected. Clearly there is a fluorine attack on some of the silicon atoms co-ordinated with oxygen which leads to a release of the four ions Be2+, A13+, Si~- and 03- and allows the last three to rearrange with Na + as albite, NaA1SizO 8. The plagioclase felspar lattice can accommodate beryllium ions and the low extraction of beryllium from the roast compacts by water is, in part at least, due to this cause. But neither sodium fluoride nor fluoride ion appear to convert the aluminium to fluoride and there may be a similar difficulty with beryllium. The failure to produce aluminium fluoride accounts for the absence of cryolite. The absence of ~t-cristobalite is significant and suggests that this compound, when formed, is derived from the silicon of the fluorosilicate; for this we have other evidence.
Silicon tetrafluoride and beryl Silicon tetrafluoride boils at --65 ° and the gas does not react with beryl below 800 °. At 900 °, however, interaction produces ~-cristobalite in appreciable quantity, and also, though less evidently, aluminium fluoride. The appearance of ~-cristobalite suggests a fluorine oxygen interchange: SiF 4 ÷ 202- (beryl) --* SiO 2 Jr 4F-, followed by the formation of aluminium fluoride; 3F- + A1a+ (beryl) --~ AIF a. It is pertinent that reactions similar to these, at least as regards their products, take place at a temperature 200 ° lower when sodium fluorosilicate or sodium fluoride and silicon tetrafluoride are present.
Silicon tetrafluoride and beryl mixed with sodium fluoride Sodium fluoride and silicon tetrafluoride react to give mixtures of the former with sodium fluorosilicate. Accordingly the behaviour of sodium fluoride and silicon tetrafluoride when together with beryl might be expected to resemble that of sodium fluorosilicate itself rather than of either of the reactants separately. This proves to be true. In 4 hr at 700 ° all the beryl is consumed, much 0~-cristobalite is produced, cryolite is formed but, significantly, only a trace of albite is found. With lower proportions of sodium fluoride, the aluminium appears in the alternative fluoroaluminate chiolite, NasAlaF14, rather than in cryolite. In effect, these conditions are equivalent to maintaining a higher concentration of Na~SiF6 during the whole period of heating, with a consequentially greater replacement of oxygen by fluorine.
Sodium fluorosilicate and beryl Reaction in this mixture between 500 and 600 ° produces, according to the conditions, albite, cryolite, 0~-cristobalite and an unidentified, soluble fluorine compound of
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K.R. HYDE,P. L. ROBINSON,M. J. WATERMANand J. M. WATERS
beryllium. Albite is formed at 600 ° in mixtures with beryl:fluorosilicate ratios up to 1 : 3. But at the higher ratio of 1 : 4, albite is absent because more time is required for an equivalent dissociation of the fluorosilicate and thus the proportion of sodium fluoride is lower in the earlier stages. However, at 800 °, where a more rapid dissociation of the fluorosilicate increases the concentration of sodium fluoride, albite persists in mixtures with ratios up to 1:4.5. Always, longer heating periods encourage the formation of albite. Cryolite is the first crystalline phase, other than residual beryl, to be detected at 500 °. Although at 500 and 600 ° the cryolite formed does not increase with the proportion of sodium fluorosilicate, at 800 ° it does; probably because there is a partial conversion of cryolite to chiolite at the lower temperature where the concentration of fluorosilicate is higher. Silica, an early product is found at all temperatures. It appears in X-ray photographs as ~-cristobalite--the term used for it throughout this paper--but is probably produced in the fl form and remains as such down to about 300 °. We believe the silicon for it comes from the fluorosilicate. In a separate study of the reaction of beryl with sodium fluoroferrate, NaaFeFt, in a 1 : 3 molecular ratio at 700 °, the first products were sodium fluoroberyllate, 0~-ferricoxide and cryolite which were followed, later, by albite. It is notable that 0c-cristobalite, or other form of crystalline silica, is absent and reasonable to assume that its place has been taken by 0c-ferric oxide also derived from the anion of the sodium salt, this time FeF6 z-. The results of operating with a beryl: sodium fluorosilicate ratio of 1 : 3 at temperatures above 600 ° are, briefly, that all the beryl is consumed in 2 hr at 700 °, in 1 rh at 750 °, and in 15 min at 800 °. Cryolite and ~t-cristobalite are invariably produced. Under suitable conditions, namely a 1:3 ratio of reactants, a temperature of 700°C and a heating time of 4 hr, 96 per cent of the beryllium in beryl is rendered soluble. With shorter periods, the release of beryllium is incomplete and, with even an hour longer, it has fallen to 94 per cent. After 19 hr heating there is a further fall to ca. 80per cent. This fall coincides with the appearance of more albite and, as some natural plagioclase felspars are known to carry beryllium, the incorporation of Be2+ ions in the albite lattice is understandable. Of the nature of the soluble beryllium compound only indirect evidence has been obtained. Both NaBeF s and v-Na~BeF4 are known. These compounds and also NaBegF5 and BeF2 have been prepared and X-ray diffraction photographs have been taken; but none of the lines characteristic of any of these substances has been observed in our reaction products. Furthermore, the diffraction pattern of the residue after leaching is identical with that of the virgin material which suggests that the compound in question occurs as a glass. As, however, only a trace of sodium fluoride, which is readily identified, is found in the roast material before leaching, and, as sodium, beryllium and fluorine in leach liquors, giving maximum extraction of beryllium, are present in a ratio corresponding to NaBeF 8, it is possible that this fluoroberyllate is formed, tS) but glassy mixtures of Na~BeF4 and BeF 2 are not excluded. DISCUSSION OF THE BERYL-FLUOROSILICATE REACTION AT 750 ° The presence of undissociated sodium fluorosilicate appears to be necessary in an attack on beryl which is to release soluble beryllium, although it most probably operates through the agency of the sodium fluoride produced in its thermal
Structural evidence on the beryl-sodium fluorosilicatereaction
241
decomposition. Silicon tetrafluoride, so long as it remains within the system, serves the reaction by maintaining the concentration of fluorosilicate; otherwise it seems to take no part. It is possible to give a tentative picture of the main course of this complex reaction as it occurs in compacts heated to about 750 ° under conditions not especially designed to conserve the silicon tetrafluoride liberated. Three consecutive, somewhat overlapping, stages may be recognized. In the first, during which little silicon tetrafluoride has been lost, the proportion of sodium fluoride to fluorosilicate in the reacting mass remains low. The product species first to be formed are sodium fluoroberyllate glass, cryolite, N%A1Fe, and 0c-cristobalite, SiO3. A remarkable feature is that more than half the beryllium is rendered soluble in as short a period as 15 min. During the second stage the proportion of sodium fluoride to fluorosilicate is higher and, as would be expected from our examination of the reaction of fluoride with beryl, the change is accompanied by the appearance of albite, NaA1SisOs. It is possible that some albite is formed earlier, but, if so, then in quantities insufficient for recognition. It seems reasonable to suppose that most, if not all, the albite formed at this stage is derived from fluoride and beryl because, so long as the later remains available, there is no evidence of a reduction in the amounts of cryolite and ~-cristobalite. The third stage, however, is marked by a decrease in the amounts of cryolite and ~-cristobalite and an increase in the amount of albite. Notably, there is a progressive fall in the proportion of beryllium which can be extracted by water, although none of it has been lost from the roast. We have shown that cryolite and silica, when heated together at the process temperature, give albite and sodium fluoride and, furthermore, that when this reaction occurs in the presence of beryllium ions some of these become incorporated in the albite crystals. Thus the third stage is entirely disadvantageous. Indeed, it is probable that albite formed at any time during the complex reactions of beryl with sodium fluorosilicate includes beryllium in its lattice and thereby hinders the aqueous extraction of the element. These three stages, which must all include other less important reactions and which undoubtedly overlap one another to a certain extent, are shown schematically. First stage (with low sodium fluoride) Na + -k Be3+ + 3F- ~ NaBeF 3 (glass) 3Na + + A13+ -k 6F- --~ N%A1F 6 (crystalline) Si4+ + 203- ~ SiO3 (crystalline) Second stage (with higher sodium fluoride) Na + + A13+ -~- 3Si4+ + 803- --~ NaA1Si30 s (crystalline) Third stage (in the absence of beryl) Na3A1F6 ÷ 4SIO3 --* NaA1Si308 (crystalline) + 2NaF (crystalline) + SiF4 Although the chemical changes involved are more numerous and mechanistically of greater complexity than has been indicated, it is not unreasonable, on the evidence, ito speculate about the steps in the first stage. The ready fritting of sodium fluoride
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K . R . HYDE,P. L. ROBINSON,M. J. WATERMANand J. M. WATERS
and beryl, at temperatures as low as 500 °, suggests the early formation of a molten or semi-molten film of fluoride-beryl flux round the mineral particles. Through this film Na + and F- ions pass inwards and 02- ions outwards. The first two take part in the formation of NaBeF3 and N%A1F6 and the 02- ion reacts with the SiFt9- ion of the fluorosilicate to produce SiO2 and F-. This implies that the ~-cristobalite is derived from silicon in the fluorosilicate rather than from that in the beryl. The suggestion is consonant with the implications of the behaviour of sodium fluoroferrate with beryl where we have shown 0c-cristobalite is not formed, its place being taken by ferric oxide which must originate in the FeF6a- ion. Further attack on the mineral by fluorosilicate may be assumed to proceed by the interfacial film moving inwards on the surface of a diminishing core of residual beryl. The bearing of these findings on the commercial extraction of beryllium from beryl by the Copaux process have been discussed elsewhere. (a) EXPERIMENTAL
Materials. The beryl was crushed, good quality material of sieve-size --200 mesh, from the U.K.A.E.A. agency factory at Milford Haven. Microscopic and X-ray inspection did not disclose any other mineral present. It was dried at 110°C for 24 hr, but no attempt was made to remove combined water and the reactions described must be assumed to have occurred in the presence of some moisture. The sodium fluorosilicate, from two commercial sources, was dried at ll0°C for 24 hr. The sodium fluoride was AR salt. The silicon tetrafluoride was made by heating dried sodium fluorosilicate. Chemical operations. The components were weighed-out into a screw-capped glass bottle and mixed by rolling. About 2'5 g of the mixture was compressed in a Millen press at 3000 lb/inL to give a ½in. × ½in. cylindrical compact. This was weighed in a silica boat and introduced into a silica furnace-tube which was long enough to allow the boat to be drawn into a cold zone for cooling. The tube was protected from atmospheric moisture and could be provided with the required atmosphere, either streaming or stagnant. It was heated by a Variac-controlled electric furnace. Heating times were defined as the period in the hot zone, the temperature of which was measured by a thermocouple. After cooling and reweighing, the sample was transferred to a glovebox, crushed and ground to --200 mesh. Some of the powder was mounted for X-ray examination and a 1 g portion was subjected to the standard extraction. This consisted of stirring with 10 ml cold water in a centrifuge tube for 15 rain, filtering the extract through a weighed, fritted-glass crucible, and washing and drying the residue at 110°. The residue was weighed; the beryllium in it was determined by chemical analysis and sometimes an X-ray photograph was taken. The filtrate was analysed for sodium, beryllium and fluorine. The chemical work has been already described in detailJ s~ X-ray examination. For the X-ray photographs of beryl at high temperatures a Unicam S-150 camera and CuK~ radiation were used. The capillaries holding the specimens were coated with metallic platinum, the thermal expansion of which allowed the temperature attained by the specimen to be calculated. The diffraction patterns for beryl had sharp but weak lines where sin 0 was large. The X-ray photographs of the powdered sinters and the extracted residues, used for the identification of crystalline phases produced in the various reactions, were taken with monochromatised CuK~ radiation on a Guinier focusing camera. The specimens were prepared by sprinkling a very shallow layer of powder on cellotape supported by means of a small brass disc with a hole in the centre. To avoid health hazard through the powder becoming detached from the cellotape, it was covered with a thin film of a non-scattering adhesive, Bostikote, before removal from the glove-box. Most of the photographs given by these solids had a great number of lines varying widely in sharpness and reflecting a great range of crystallinity among the product compounds. Despite the presence of glassy material among them, the background scatter was, in general, not high. The complexity of the patterns encountered is illustrated by the typical one given in Table I ; this shows the observed and calculated values of sins 0 for severa! phases. Action of heat on beryl. The lattice constants of beryl were derived by the least-squares refinement of sin2 0, with the assumption that the systematic errors would be expressed by the Nelson-Riley
Structural evidence on the beryl-sodium fluorosilicate reaction
243
function, from diffraction lines of general indices hkl. A plot of the cell constants against temperature showed that the thermal expansion in the direction of both (c) and (a)-axes occurred in two stages; it is very slight up to about 250 ° and greater from that temperature up to 800 °. A least squares treatment of the data gave a = ao(1 + xt) where x is the average linear expansion coefficient between the temperatures; (i) 20-250°: a = 9.181(1 + 1.7 × 10-6 t); c = 9.208(1 4 1 . 4 × 10-6 t); (ii) 250-800°: a = 9.175(1 + 4"7 × 10-6 t); c = 9.193(1 + 6.5 × 10-e t). Of these (i) is derived from results at three temperatures and (ii) from those at six, the mean standard deviations being 0-001 for a and 0.002 for c. Values for a and e at 20 ° derived for the higher temperature measurements are 9.176 A. and 9-194 A, which are close to those found at 20 ° for a specimen of beryl which had been previously heated to TABLE 1.--TYPICAL
DIFFRACTION PATTERN OF A REACTION PRODUCT
(Compact beryl-sodium fluorosilicate in molecular ratio 1 : 3, heated at 600 ° for 24 hr) Sin s 0 (observed) 0092 0140 0279 0290 0303 0362 0374 0396 0429 0459 0496 0563 0584 0596 0656 0725 0768 0795 0818 0936 0964 1011 1071 1098 1113 1166 1217
W-W W W S W W W W W m
w+ w w+ rH-W
w+ W W--
w+ W W W
w+ W W
1293
W
1315 1368 1405 1451 1495 1588 1641 1851 1971
W
* w =
W W-14'-W-n'l-W-W-W
weak,
m =
Compounds present
Sin ~ 0 (calculated)
beryl albite beryl cryolite cryolite 0~-cristobalite, albite beryl cryolite, albite albite albite cryolite beryl albite 0~-cristobalite beryl beryl, ~t-cristobalite, albite cryolite, albite cryolite cryolite beryl ct-cristobalite, cryolite cryolite cryolite cryolite beryl cryolite beryl beryl c~-cristobalite, albite cryolite, albite beryl ct-cristobalite beryl cryolite, 0~-cristobalite cryolite beryl beryl
0093 0145 0280 0290 0302 0364, 0365 0372 0393, 0399 0437 0442 0490 0559 0581 0606 0653 0723, 0731, 0723 0763, 0763 0786 0810 0933 0965, 0964 1001 1064 1089 1117 1155 1213 1286 1332, 1314 1360, 1376 1399 1454 1494 1574, 1592 1646 1846 1961
medium,
s =
strong.
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K . R . HYDE, P. L. ROBINSON,M. J. WATERMANand J. M. WATERS
1500°, namely 9.175 A and 9.194 A. Probably the alkali metals known to be present in this beryl expand the lattice and give a false coefficient at lower temperatures. The alkalis are removed by heating and normal values are then observed. Of the extrapolated ao and co values, namely 9.183 and 9.205 A, that for ao is shorter than the reported values 9.200 and 9.215 A, but that for co agrees with FRANK-KAr~r,mTSKUand SOSEDKO'StT~9"209 A for beryl which contained alkali metal. A c k n o w l e d g e m e n t s - - W e are indebted to Dr. R. W. M. D'EYE for X-ray facilities and other help, to
Dr. E. WAIT for preparing the Mercury Computer Programme for ceil-constant calculations and useful discussion, and to Mr. R. S. STREETwho took the high-temperature X-ray photographs. c~ V. A. FRANK-KAMENETSKIIand T. A. SOSEDKO,DokL Akad. Nauk S.S.S.R. 118, (4), 815 (1958).
S T R U C T U R A L E V I D E N C E O N THE B E R Y L - S O D I U M FLUOROSILICATE REACTION K. R. HYDE,P. L. ROBINSON,M. J. WATERMAN and J. M. WATERS Atomic Energy Research Establishment, Harwell, Didcot, Berks. (Received 25 November 1960)
Abstract--An X-ray examination has been made of the products of the reaction of beryl with sodium fluoride, silicon tetrafluoride and sodium fluorosilicate. The evidence, in conjunction with that from chemical analysis, has enabled certain stages in the reaction of beryl with sodium fluorosilicate to be recognized. The first of these leads to a soluble fluoroberyllate, cryolite and cristobalite, and the second to the felspar albite. The third stage, occurring when all the beryl has been destroyed, produces albite, sodium fluoride and silicon tetrafluoride by a reaction between cryolite and cristobalite. The albite incorporates beryllium ions in its lattice and these cease to be extractable by water. SINCE it was advocated in 1919, the Copaux process, ~x) sometimes in a modified form, has been used for the extraction of the beryllium from beryl; a recent instance is by Murex Ltd. ~ We have described an investigation of the beryl-fluorosilicate reaction on which the Copaux extraction is based, directed to establishing conditions furnishing a product which gave the maximum proportion of beryllium extractable by aqueous leaching, c3~ Here we deduce something of the mechanism of this reaction, largely from an X-ray identification of certain intermediate and final products. The beryl was ground material from the U.K.A.E.A. Milford Haven factory, almost entirely free from other minerals and, judged by the proportion of beryllium (BeO = 11.9 per cent), of high quality. Thus the reaction can be considered as occurring between two well-characterised chemical compounds, an aluminosilicate, empirically 3BeO'AlzO3"6SiO2, and sodium fluorosilicate, Because more or less pure beryl has generally been used and because the reaction is remarkably specific in that water extracts from the reaction products almost exclusively the elements sodium, beryllium and fluorine and leaves silicon and aluminium in the leached residue, rather precise courses for the reaction have been advanced, such as: 3BeO'A12Oa'6SiO 2 q- 3Na2SiF 6 --~ 3BeF~ q- 2A1Fa ÷ 9SiO 2 ÷ 6NaF or or
3BeO-A1203'6SiO 2 ÷ 6Na2SiF ~ ~ 3Na2BeF4 q- 2NaaA1F s -~ 9SiO 2 ÷ 3SiF 4 2(3BeO'AlzOa'6SiO2) + 6NazSiF 6 --~ 6NazBeF 4 -q- 2AlzO3 q- 15SiO2 + 3SiF4
There does not appear to have been much stoicheiometric justification for any of these equations, or even identification of the compounds. The mechanism is certainly complex and recognition of individual products not always possible, but a combination of chemical and X-ray observations has given us some insight into the course of the reaction. First a non-crystalline sodium fluoroberyllate glass is formed along with tl~ H. COPAUX, C.R. Acad. Sci., Paris 168, 610 (1919). ~2~ p. S. BRYANT, Beryllium production at Milford Haven. Extraction and refining o f the rarer metals (London: Inst. Min. Metall., 1957) 310. ta~ K. R. HYDE, P. L. ROBINSON, M. J. WATERMAN and J. M. WAXERS, Trans. Inst. Min. Metall., 70, 397, (1960-61). 237
238
K . R . HYDE, P. L. ROBINSON, M. J. WATERMANand J. M. WATERS
insoluble, crystalline cryolite, NasA1Ft, and ~-cristobalite, SiOz. This is followed by a re-arrangement of the ions A13+, Si4+ and 02- and their combination with Na + to produce crystalline albite, NaA1SiaOs. Finally there is a reaction between ~-cristobarite and cryolite to form more albite. Some of the beryllium is incorporated in the albite lattice and ceases to be soluble. There is reason to suppose the ~-cristobalite is derived from the silicon of the fluorosilicate.
The structure of beryl and its behaviour when heated Beryl has a structure (4) consisting of sheets of hexagonal rings of six silicon-oxygen tetrahedra bound to one another in the individual sheets and to rings in contiguous sheets by bridging aluminium and beryllium ions, the former being hexa-coordinated and the latter tetra-co-ordinated to the oxygen ions. Although the arrangement appears layered in a diagram, the bonding is so uniform that in place of a marked basal cleavage there is a poor and irregular subconchoidal fracture. The mineral has a hexagonal unit cell with fifty eight atoms (% = 9.215/~, e0 ---- 9"192/~) and a sharp X-ray diffraction pattern; the latter is useful both for detecting structural differences and for recognising its presence, even in low proportions, in mixtures. The structure remains unchanged after the crystal has been heated to 800° and rapidly cooled. Extra, unascribed lines appear, however, when the temperature has been to 1100°, and beryllia is detected when it has reached 1200°. The amount of beryllia is larger at 1400°, and the beryl has lost much of its crystallinity when it has been heated to 1600°. Within the temperature range employed for the fluorosilicate reaction, the expansion of beryl is slight up to about 300° and somewhat greater in both a and c directions between that point and 800°. The change is completely reversible and there is no evidence of structural alteration that might facilitate chemical attack.
Sodium fluorosilieate The effect of heat on sodium and other fluorosilieates has been examined by CAILLAT(5) who gives dissociation pressures for the sodium salt. Although some of Caillat's results have been questioned/e) we have our own evidence that liberation of silicon tetrafluoride occurs at 500°, a temperature at which a reaction with beryl is barely detectable after 24 hr, and that it becomes very considerable at 600°, a temperature at which most of the mineral is consumed in 4 hr. Thus the question arises of the respective parts played by sodium fluoride, silicon tetrafluoride and undissociated sodium fluorosilieate in the attack on beryl. When heated in an open system, the salt loses silicon tetrafluoride without melting and leaves mixtures in which the proportion of sodium fluoride increases with the time of heating. The reaction is reversed in an atmosphere of silicon tetrafluoride. Thus the reactant always contains a certain proportion of sodium fluoride, a proportion which depends not only on time, but also upon the atmosphere over the solid. (4) W. L. BRAGGand J. WEST,Proc. Roy. Soc. A l l l , 691 (1926); N. V. BELOVand R. G. MATVEENA,Dokl. Akad. Nauk SSSR 73, 299 (1950). (5) R. CAILLAT,Ann. Chim. 20, 368, (1945). Atomic Energy Commission. (8) I. G. RYss, The Chemistry of Fluorine and its Inorganic Compounds Moscow (1956). Translated from a publication of the State Publishing House for Scientific, Technical and Chemical Literature, Moscow, (1956). AEC-tr-3927 (Pt. 1) and (Pt. 2). United States Atomic Energy Commission, Technical Information Service (1960).
Structural evidence on the beryl-sodium fluorosilicatereaction
239
Sodium .fluoride and beryl Sodium fluoride melts about 990 ° at which temperature the vapour-pressure is low. With beryl:fluoride ratios less than 1:4, the mixture melts below 700 ° and leaves, on cooling, a hard slag, At 600 ° all mixtures form a frit; with a 1 : 3 ratio about 90 per cent of the beryl is consumed in 4 hr, but the proportion of beryllium rendered soluble in water is low. The crystalline products which have been identified show that at 600 ° the reaction proceeds mainly with the formation of albite, very little ct-cristobalite is produced, and cryolite is not detected. Clearly there is a fluorine attack on some of the silicon atoms co-ordinated with oxygen which leads to a release of the four ions Be2+, A13+, Si~- and 03- and allows the last three to rearrange with Na + as albite, NaA1SizO 8. The plagioclase felspar lattice can accommodate beryllium ions and the low extraction of beryllium from the roast compacts by water is, in part at least, due to this cause. But neither sodium fluoride nor fluoride ion appear to convert the aluminium to fluoride and there may be a similar difficulty with beryllium. The failure to produce aluminium fluoride accounts for the absence of cryolite. The absence of ~t-cristobalite is significant and suggests that this compound, when formed, is derived from the silicon of the fluorosilicate; for this we have other evidence.
Silicon tetrafluoride and beryl Silicon tetrafluoride boils at --65 ° and the gas does not react with beryl below 800 °. At 900 °, however, interaction produces ~-cristobalite in appreciable quantity, and also, though less evidently, aluminium fluoride. The appearance of ~-cristobalite suggests a fluorine oxygen interchange: SiF 4 ÷ 202- (beryl) --* SiO 2 Jr 4F-, followed by the formation of aluminium fluoride; 3F- + A1a+ (beryl) --~ AIF a. It is pertinent that reactions similar to these, at least as regards their products, take place at a temperature 200 ° lower when sodium fluorosilicate or sodium fluoride and silicon tetrafluoride are present.
Silicon tetrafluoride and beryl mixed with sodium fluoride Sodium fluoride and silicon tetrafluoride react to give mixtures of the former with sodium fluorosilicate. Accordingly the behaviour of sodium fluoride and silicon tetrafluoride when together with beryl might be expected to resemble that of sodium fluorosilicate itself rather than of either of the reactants separately. This proves to be true. In 4 hr at 700 ° all the beryl is consumed, much 0~-cristobalite is produced, cryolite is formed but, significantly, only a trace of albite is found. With lower proportions of sodium fluoride, the aluminium appears in the alternative fluoroaluminate chiolite, NasAlaF14, rather than in cryolite. In effect, these conditions are equivalent to maintaining a higher concentration of Na~SiF6 during the whole period of heating, with a consequentially greater replacement of oxygen by fluorine.
Sodium fluorosilicate and beryl Reaction in this mixture between 500 and 600 ° produces, according to the conditions, albite, cryolite, 0~-cristobalite and an unidentified, soluble fluorine compound of
240
K.R. HYDE,P. L. ROBINSON,M. J. WATERMANand J. M. WATERS
beryllium. Albite is formed at 600 ° in mixtures with beryl:fluorosilicate ratios up to 1 : 3. But at the higher ratio of 1 : 4, albite is absent because more time is required for an equivalent dissociation of the fluorosilicate and thus the proportion of sodium fluoride is lower in the earlier stages. However, at 800 °, where a more rapid dissociation of the fluorosilicate increases the concentration of sodium fluoride, albite persists in mixtures with ratios up to 1:4.5. Always, longer heating periods encourage the formation of albite. Cryolite is the first crystalline phase, other than residual beryl, to be detected at 500 °. Although at 500 and 600 ° the cryolite formed does not increase with the proportion of sodium fluorosilicate, at 800 ° it does; probably because there is a partial conversion of cryolite to chiolite at the lower temperature where the concentration of fluorosilicate is higher. Silica, an early product is found at all temperatures. It appears in X-ray photographs as ~-cristobalite--the term used for it throughout this paper--but is probably produced in the fl form and remains as such down to about 300 °. We believe the silicon for it comes from the fluorosilicate. In a separate study of the reaction of beryl with sodium fluoroferrate, NaaFeFt, in a 1 : 3 molecular ratio at 700 °, the first products were sodium fluoroberyllate, 0~-ferricoxide and cryolite which were followed, later, by albite. It is notable that 0c-cristobalite, or other form of crystalline silica, is absent and reasonable to assume that its place has been taken by 0c-ferric oxide also derived from the anion of the sodium salt, this time FeF6 z-. The results of operating with a beryl: sodium fluorosilicate ratio of 1 : 3 at temperatures above 600 ° are, briefly, that all the beryl is consumed in 2 hr at 700 °, in 1 rh at 750 °, and in 15 min at 800 °. Cryolite and ~t-cristobalite are invariably produced. Under suitable conditions, namely a 1:3 ratio of reactants, a temperature of 700°C and a heating time of 4 hr, 96 per cent of the beryllium in beryl is rendered soluble. With shorter periods, the release of beryllium is incomplete and, with even an hour longer, it has fallen to 94 per cent. After 19 hr heating there is a further fall to ca. 80per cent. This fall coincides with the appearance of more albite and, as some natural plagioclase felspars are known to carry beryllium, the incorporation of Be2+ ions in the albite lattice is understandable. Of the nature of the soluble beryllium compound only indirect evidence has been obtained. Both NaBeF s and v-Na~BeF4 are known. These compounds and also NaBegF5 and BeF2 have been prepared and X-ray diffraction photographs have been taken; but none of the lines characteristic of any of these substances has been observed in our reaction products. Furthermore, the diffraction pattern of the residue after leaching is identical with that of the virgin material which suggests that the compound in question occurs as a glass. As, however, only a trace of sodium fluoride, which is readily identified, is found in the roast material before leaching, and, as sodium, beryllium and fluorine in leach liquors, giving maximum extraction of beryllium, are present in a ratio corresponding to NaBeF 8, it is possible that this fluoroberyllate is formed, tS) but glassy mixtures of Na~BeF4 and BeF 2 are not excluded. DISCUSSION OF THE BERYL-FLUOROSILICATE REACTION AT 750 ° The presence of undissociated sodium fluorosilicate appears to be necessary in an attack on beryl which is to release soluble beryllium, although it most probably operates through the agency of the sodium fluoride produced in its thermal
Structural evidence on the beryl-sodium fluorosilicatereaction
241
decomposition. Silicon tetrafluoride, so long as it remains within the system, serves the reaction by maintaining the concentration of fluorosilicate; otherwise it seems to take no part. It is possible to give a tentative picture of the main course of this complex reaction as it occurs in compacts heated to about 750 ° under conditions not especially designed to conserve the silicon tetrafluoride liberated. Three consecutive, somewhat overlapping, stages may be recognized. In the first, during which little silicon tetrafluoride has been lost, the proportion of sodium fluoride to fluorosilicate in the reacting mass remains low. The product species first to be formed are sodium fluoroberyllate glass, cryolite, N%A1Fe, and 0c-cristobalite, SiO3. A remarkable feature is that more than half the beryllium is rendered soluble in as short a period as 15 min. During the second stage the proportion of sodium fluoride to fluorosilicate is higher and, as would be expected from our examination of the reaction of fluoride with beryl, the change is accompanied by the appearance of albite, NaA1SisOs. It is possible that some albite is formed earlier, but, if so, then in quantities insufficient for recognition. It seems reasonable to suppose that most, if not all, the albite formed at this stage is derived from fluoride and beryl because, so long as the later remains available, there is no evidence of a reduction in the amounts of cryolite and ~-cristobalite. The third stage, however, is marked by a decrease in the amounts of cryolite and ~-cristobalite and an increase in the amount of albite. Notably, there is a progressive fall in the proportion of beryllium which can be extracted by water, although none of it has been lost from the roast. We have shown that cryolite and silica, when heated together at the process temperature, give albite and sodium fluoride and, furthermore, that when this reaction occurs in the presence of beryllium ions some of these become incorporated in the albite crystals. Thus the third stage is entirely disadvantageous. Indeed, it is probable that albite formed at any time during the complex reactions of beryl with sodium fluorosilicate includes beryllium in its lattice and thereby hinders the aqueous extraction of the element. These three stages, which must all include other less important reactions and which undoubtedly overlap one another to a certain extent, are shown schematically. First stage (with low sodium fluoride) Na + -k Be3+ + 3F- ~ NaBeF 3 (glass) 3Na + + A13+ -k 6F- --~ N%A1F 6 (crystalline) Si4+ + 203- ~ SiO3 (crystalline) Second stage (with higher sodium fluoride) Na + + A13+ -~- 3Si4+ + 803- --~ NaA1Si30 s (crystalline) Third stage (in the absence of beryl) Na3A1F6 ÷ 4SIO3 --* NaA1Si308 (crystalline) + 2NaF (crystalline) + SiF4 Although the chemical changes involved are more numerous and mechanistically of greater complexity than has been indicated, it is not unreasonable, on the evidence, ito speculate about the steps in the first stage. The ready fritting of sodium fluoride
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K . R . HYDE,P. L. ROBINSON,M. J. WATERMANand J. M. WATERS
and beryl, at temperatures as low as 500 °, suggests the early formation of a molten or semi-molten film of fluoride-beryl flux round the mineral particles. Through this film Na + and F- ions pass inwards and 02- ions outwards. The first two take part in the formation of NaBeF3 and N%A1F6 and the 02- ion reacts with the SiFt9- ion of the fluorosilicate to produce SiO2 and F-. This implies that the ~-cristobalite is derived from silicon in the fluorosilicate rather than from that in the beryl. The suggestion is consonant with the implications of the behaviour of sodium fluoroferrate with beryl where we have shown 0c-cristobalite is not formed, its place being taken by ferric oxide which must originate in the FeF6a- ion. Further attack on the mineral by fluorosilicate may be assumed to proceed by the interfacial film moving inwards on the surface of a diminishing core of residual beryl. The bearing of these findings on the commercial extraction of beryllium from beryl by the Copaux process have been discussed elsewhere. (a) EXPERIMENTAL
Materials. The beryl was crushed, good quality material of sieve-size --200 mesh, from the U.K.A.E.A. agency factory at Milford Haven. Microscopic and X-ray inspection did not disclose any other mineral present. It was dried at 110°C for 24 hr, but no attempt was made to remove combined water and the reactions described must be assumed to have occurred in the presence of some moisture. The sodium fluorosilicate, from two commercial sources, was dried at ll0°C for 24 hr. The sodium fluoride was AR salt. The silicon tetrafluoride was made by heating dried sodium fluorosilicate. Chemical operations. The components were weighed-out into a screw-capped glass bottle and mixed by rolling. About 2'5 g of the mixture was compressed in a Millen press at 3000 lb/inL to give a ½in. × ½in. cylindrical compact. This was weighed in a silica boat and introduced into a silica furnace-tube which was long enough to allow the boat to be drawn into a cold zone for cooling. The tube was protected from atmospheric moisture and could be provided with the required atmosphere, either streaming or stagnant. It was heated by a Variac-controlled electric furnace. Heating times were defined as the period in the hot zone, the temperature of which was measured by a thermocouple. After cooling and reweighing, the sample was transferred to a glovebox, crushed and ground to --200 mesh. Some of the powder was mounted for X-ray examination and a 1 g portion was subjected to the standard extraction. This consisted of stirring with 10 ml cold water in a centrifuge tube for 15 rain, filtering the extract through a weighed, fritted-glass crucible, and washing and drying the residue at 110°. The residue was weighed; the beryllium in it was determined by chemical analysis and sometimes an X-ray photograph was taken. The filtrate was analysed for sodium, beryllium and fluorine. The chemical work has been already described in detailJ s~ X-ray examination. For the X-ray photographs of beryl at high temperatures a Unicam S-150 camera and CuK~ radiation were used. The capillaries holding the specimens were coated with metallic platinum, the thermal expansion of which allowed the temperature attained by the specimen to be calculated. The diffraction patterns for beryl had sharp but weak lines where sin 0 was large. The X-ray photographs of the powdered sinters and the extracted residues, used for the identification of crystalline phases produced in the various reactions, were taken with monochromatised CuK~ radiation on a Guinier focusing camera. The specimens were prepared by sprinkling a very shallow layer of powder on cellotape supported by means of a small brass disc with a hole in the centre. To avoid health hazard through the powder becoming detached from the cellotape, it was covered with a thin film of a non-scattering adhesive, Bostikote, before removal from the glove-box. Most of the photographs given by these solids had a great number of lines varying widely in sharpness and reflecting a great range of crystallinity among the product compounds. Despite the presence of glassy material among them, the background scatter was, in general, not high. The complexity of the patterns encountered is illustrated by the typical one given in Table I ; this shows the observed and calculated values of sins 0 for severa! phases. Action of heat on beryl. The lattice constants of beryl were derived by the least-squares refinement of sin2 0, with the assumption that the systematic errors would be expressed by the Nelson-Riley
Structural evidence on the beryl-sodium fluorosilicate reaction
243
function, from diffraction lines of general indices hkl. A plot of the cell constants against temperature showed that the thermal expansion in the direction of both (c) and (a)-axes occurred in two stages; it is very slight up to about 250 ° and greater from that temperature up to 800 °. A least squares treatment of the data gave a = ao(1 + xt) where x is the average linear expansion coefficient between the temperatures; (i) 20-250°: a = 9.181(1 + 1.7 × 10-6 t); c = 9.208(1 4 1 . 4 × 10-6 t); (ii) 250-800°: a = 9.175(1 + 4"7 × 10-6 t); c = 9.193(1 + 6.5 × 10-e t). Of these (i) is derived from results at three temperatures and (ii) from those at six, the mean standard deviations being 0-001 for a and 0.002 for c. Values for a and e at 20 ° derived for the higher temperature measurements are 9.176 A. and 9-194 A, which are close to those found at 20 ° for a specimen of beryl which had been previously heated to TABLE 1.--TYPICAL
DIFFRACTION PATTERN OF A REACTION PRODUCT
(Compact beryl-sodium fluorosilicate in molecular ratio 1 : 3, heated at 600 ° for 24 hr) Sin s 0 (observed) 0092 0140 0279 0290 0303 0362 0374 0396 0429 0459 0496 0563 0584 0596 0656 0725 0768 0795 0818 0936 0964 1011 1071 1098 1113 1166 1217
W-W W W S W W W W W m
w+ w w+ rH-W
w+ W W--
w+ W W W
w+ W W
1293
W
1315 1368 1405 1451 1495 1588 1641 1851 1971
W
* w =
W W-14'-W-n'l-W-W-W
weak,
m =
Compounds present
Sin ~ 0 (calculated)
beryl albite beryl cryolite cryolite 0~-cristobalite, albite beryl cryolite, albite albite albite cryolite beryl albite 0~-cristobalite beryl beryl, ~t-cristobalite, albite cryolite, albite cryolite cryolite beryl ct-cristobalite, cryolite cryolite cryolite cryolite beryl cryolite beryl beryl c~-cristobalite, albite cryolite, albite beryl ct-cristobalite beryl cryolite, 0~-cristobalite cryolite beryl beryl
0093 0145 0280 0290 0302 0364, 0365 0372 0393, 0399 0437 0442 0490 0559 0581 0606 0653 0723, 0731, 0723 0763, 0763 0786 0810 0933 0965, 0964 1001 1064 1089 1117 1155 1213 1286 1332, 1314 1360, 1376 1399 1454 1494 1574, 1592 1646 1846 1961
medium,
s =
strong.
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K . R . HYDE, P. L. ROBINSON,M. J. WATERMANand J. M. WATERS
1500°, namely 9.175 A and 9.194 A. Probably the alkali metals known to be present in this beryl expand the lattice and give a false coefficient at lower temperatures. The alkalis are removed by heating and normal values are then observed. Of the extrapolated ao and co values, namely 9.183 and 9.205 A, that for ao is shorter than the reported values 9.200 and 9.215 A, but that for co agrees with FRANK-KAr~r,mTSKUand SOSEDKO'StT~9"209 A for beryl which contained alkali metal. A c k n o w l e d g e m e n t s - - W e are indebted to Dr. R. W. M. D'EYE for X-ray facilities and other help, to
Dr. E. WAIT for preparing the Mercury Computer Programme for ceil-constant calculations and useful discussion, and to Mr. R. S. STREETwho took the high-temperature X-ray photographs. c~ V. A. FRANK-KAMENETSKIIand T. A. SOSEDKO,DokL Akad. Nauk S.S.S.R. 118, (4), 815 (1958).