T H E BINARY SYSTEM SODIUM METASILICATE-SILICA BY G . W. MOREY AND N. L. BOWEN
This investigation of the melting point relations in the binary system sodium metasilicate-silica was undertaken primarily because of its importance as a n end-member of more complicated systems of direct petrologic bearing; in particular, those formed by the addition pf alumina and water, which are a t present being studied by us. Fublication of this material by itself was, however, deemed advisable becauee of the considerable technologic interest in this system, not only by reason of its bearing on the problem of the iniportant soda-lime glasses, but also as constitut,ing in itself a major chemical industry. The manufacture of silicate of soda has grown to be one of the largest chemical industries, and one whose ramifications rival those of sulfuric acid. 7 he study of the ternary system formed by adding water to the sodium silicate-silica system is well under way, and a separate study of the mixtures richer in alkali than the metasilicate, both of sodium and of potassium, is planned. Altho this system is of considerable technical importance, little has been published in regard to it. l k e two compounds which appear, sodium metasilicate, NazO.SiOz and sodium disilicate, NazO.zSiOz, were prepared by Moreyl, and described by him. Niggliz, in studying the reaction between Na2C03and SiOz, also prepared sodium metasilicate, and includes some crystallographic and optical data by Fenner. Niggli also states that, “When sodium carbonate was heated with more than one equivalent of quartz, the product was, in my experiments, always sodium metasilicate mixed with quartz,” bot he also points out that his experiments give “no evidence against the formation under other conditions of an anhydrous sodium silicate containing more than one equivalent of silica,’’ and mentions the preparation of sodium disilicate under hydrothermal conditions by Morey. Kultascheff 3 determined the melting point of sodium metasilicate and gives a value of 1007’. He did not analyze his material, tho he cites experiments which show that a melt of this composition at about its melting point loses 4.8 per cent NaeO in an hour. His low result is accordingly to be ascribed to the presence of a considerable excess of SOz. Van Klooster4 determined the melting point of sodium metasilicate, using a material prepared by ignition of Kahlbaum’s crystalline hydrated sodium metasilicate. He also did not analyze his preparation, and his value, 1056”,is doubtless low. Several lots of Kahlbaum’s crystalline sodium metasilicate analyzed by us showed the J. Am. Chem. Soc., 36, 215-30(1914). P. b iggli: J. Am. Chem. Soc., 35, 1693 (1913). 3 X. V. Kultascheff: Z. anorg. Chcm., 35, 186-93(rgq). H.S. van Klooster: Z. a.norg. Chem., 69, 135-57 (1910). 1
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G. W. MOREY AND N. I,. nOWEN
presence of an excess of sodium oxide, and it is probable that his preparation was more soda-rich than the metasilicate. Wallace' studied this binary system, but did not succeed in determining any of the melting point curve, nor did he prepare the disilicate. He gives 1018' as the melting point of sodium metasilicate, but fails to give analyses of his material, and probably was not dealing with a mixture of exactly the zomposition of the metasilicate. In working with the alkali oxides a t high temperatures, account must be taken of the volatilization of alkali, especially before decomposition of the carbonate or hydroxide is complete. Wallace also concluded that Na zSi03 takes SiOz into solid solution, but his evidence is little more than conjectural. No evidence of solid solution was obtained in this study, either between sodium metasilicate and sodium disilicate, or between sodium disilicate and quartz, altho the method of study was particularly favorable to the detection of any such solid solution formation. Jaeger? made a determination of the melting point of sodium metasilicate on material carefully prepared and analyzed, and his value, 1088"~ has been confirmed by us.
Preparation and Analysis of Materials The raw materials used throughout were quartz and sodium carbonate. The former was a well washed sample, I O g. of which showed a residue of 0.0077 g. after evaporation with HF and H2S04and subsequent heating with (NH4)&03. The sodium carbonate contained but 0.9 mg. FezO3+Al2O3in I O grams. The materials were mixed in the desired proportions, and melted in platinum, either over a Meker burner or in a gas or electric furnace. The charges were usually calculated to give I O or 2 5 grams of glass. After the glass appeared clear and free from bubbles it was quickly cooled, either by placing the crucible on an iron plate or by holding it in water, then the glass was broken out of the platinum crucible and pulverized. It was tested for homogeneity by examination with the petrographic microscope, using an immersion liquid matching it to within 0.005 in refractive index, and if not homogeneous again heated and powdered. The onIy glasses that required more than two heatings to become homogeneous were those containing go and 95 per cent Si02, which required three. Glasses of exactly the composition of the disilicate were prepared by weighing the carefully analyzed quartz into a weighed platinum crucible, adding a slight excess of sodium carbonate, and melting to a clear glass, taking care to avoid loss from spattering. The cooled melt was then weighed, broken out of the crucible and pulverized. The powder, of known composition, was again weighed into a weighed crucible, a slight excess over the calculated amount of sodium carbonate necessary to bring the mass to the desired composition added, and the charge again melted to a clear glass. Heating was then continued until the charge had the. desired composition, as determined by the weight after cooling, after which it was again broken 1Z. 2
anorg. Chem., 63,1-48(1909).
J. Wash. Acad. Rci., 1, 49-53 (1911).
SODIUM METASILI CATE-SILI C.4.
I 169
up and tested for homogeneity. If not homogeneous, the same procedure was followed until a homogeneous melt of the desired composition was obtained. The composition was always checked by analysis. The resulting glasses differed widely in their tendency to crystallize. Sodium metasilicate and mixtures near it in composition crystallize readily. While no difficulty was experienced in obtaining glasses with the small charges used, it is doubtful if large meltings could be obtained. A glass of the composition of the metasilicate-disilicate eutectic is easily obtained; on the other hand, a few hours’ heating at the appropriate temperature suffices to crystallize it almost completely. The compound sodium disilicate, which has not been prepared, hitherto, in the dry way, is easy to crystallize, a few hours’ heating being sufficient to obtain a homogeneous melt to that composition in the cryetalline condition. Mixtures slightly richer in silica than the disilicate-quartz eutectic are the most difficult to crystallize, but even they may be completely devitrified by heating below the eutectic temperature for a day or two. For example, a mixture containing 32 per cent NazSiOa, 68 per cent Si02 was heated for 3 days a t 750’, and the product was entirely crystallized as Naz0.2SiOzand high-quartz. In spite of the high viscosity of the melt a t this temperature, the quartz was in the form of well-terminated dihexahedrons characteristic of high-temperature quartz. This is the first\ time that quartz has been obtained in the dry way without the aid of fluxes. Some of these crystals were isolated by first digesting with water, then removing the coating of gelatinous silica by rapid treatment with dilute HF. It is noteworthy that prolonged digestion of this material on the steam bath with water alone or with sodium carbonate resulted in complete solution of the quartz. Methods for the separation of amorphous silica from quartz based on a difference in the rate of solution are evidently to be used with caution.’ These mixtures were also crystallized by heating in bombs with , water, as described in a previous publication2. Approximately 5 grams were placed in a gold crucible and heated with 5 cc. of water in a volume of about 80 cc. a t 500’ over night; the product was entirely crystalline. Mixtures in which tridymite and cristobalite are solid phases proved easier to crystallize, probably because of the lower viscosity a t the higher temperature. Mixtures very rich in silica, such as that containing 95 per cent SiOz, were difficult to obtain in the form of glasses, cristobalite separating out even when a I O g. melt was plunged as quickly as possible into water. Glass of this composition was, however, obtained by quenching a few milligrams in mercury from a temperature above its liquidus. Incidentally, it should be noted that in one case a charge of this composition, which was removed from the furnace above the liquidus temperature and quenched by means of a blast of air, developed crystals of quartz. I Cf. Hillehrand: “The Analysis of Silicate and Carbonate Rocks.” U. S. Geol. Survey Bull. 700, pp. 244-5. * Morey: J. Am. Chem. Sor., 36, 215-30 (1914).
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The analytical methods used were simple. It was shown by Niggli that even a t 956' all the COZis expelled from Na2CO3by heating with one equivalent of SiOz. With the higher temperatures used in preparing our melts, and th.eir greater Si02 content, no difficulty was found in obtaining COz-free mixtures. As the ingredients were of high purity, and all the meltings were made in platinum, which does not contaminate the melt in mixtures containing so large a proportion of Si02, the glasses were truly binary mixtures, and hence determination of one constituent should suffice to fix the composition. Nevertheless, many glasses were analyzed by first digesting with water, to decompose them as completely as possible, then dehydrating the silica by two evaporations with strong HC1, with intervening filtration, as described by Hillebrand'. The precipitate was always checked by treatment with HF and H2804, and it was found that the amount of residue could be diminished by digesting the first silica with water, the filtrate being added to the first filtrate. The filtrate and washings from the second evaporation were collected in a I O O cc. gold crucible, in which the NaC1 was weighed after evaporation and careful ignition. I n the other glasses NazO was determined by weighing as NazSiFe, as described in connection with the analysis of potassium silicates in a previous paper2. With sodium silicates it is advisable to decompose the more siliceous glasses by heating with water, before adding HF.
Experimental Methods The melting point curve of the system NazSi03-Si02 was determined by the quenching method used so extensively in this LaboratoryS Small charges, a few milligrams, are wrapped in platinum foil, and held a t a constant known temperature until equilibrium is reached. The charge is then quenched by dropping into mercury, usually by means of an electrical device or, in the case of glasses difficult to crystallize, by merely removing from the furnace and cooling in air. It was found that quenching was made more certain with substances which crystallize with extreme facility by fastening the charge to a small platinum weight, heavy enough to completely submerge the charge in the mercury. With some of the mixtures 15 minutes heating at constant temperature was sufficient to secure equilibrium, and identical results were obtained whether the mixture consisted initially of crystals or glass. With other mixtures, however, several hours were necessary, and in one case initial quartz persisted for several hours, zoo above the liquidus for that composition. I n mixtures such as these the furnace regulator devised by Roberts4 was a n invaluable aid. With it temperatures could be maintained constant within ha.lf a degree for long periods. Mixtures close to the sodium disilicate-quartz eutectic were usually heated over night for the final run.s. With one mixture (99.23 per cent NazSi03, 0.77 per cent SiOz) a comparison was made between Op. cit., p. 99. Morey and Fenner: J. Am. Chem. Soc., 39, 1173-1229 (1917). Shepherd and Rrtnkin: Am. J. Sei., 28, 293 1909). H. S. Roberts: J. Wash. Acad. Sci., 11, 401-409(1921).
SODIUM METASILI CATE-SILICA
1171
the melting point determined by quenching and determined by the usual time-temperature curve method, and the greater ease and certainty of interpretation with the quenching method’ confirmed. The thermoelement used was frequently calibrated, using as fixed points the melting points of NaCl and NazS04, determined by the heating curve method2, and the melting points of LizSi03and anorthite3, determined by the quenching method. Quenching of the LizSiOawas greatly facilitated by the use of the platinum weight previously mentioned. The melting point of the 95 per cent Si02 mixture was determined by J. W. Greig, with a thermoelement calibrated by comparison with an element previously calibrated at the anorthit,e point and at the melting point of platinum.
TABLEI Designation
Analysia
NazO
50.40 45.88 44 * 92 39.55 37.83 37.59 35.90 34.04 33.99 33.26 32.83 29.20 27.32 25.78 24.81 19.54 11.67 4.07
Pi02
Mol% Ka8i03
Mol 70 Fi02
Melting point
99.23 82.32 79.27 63.42 60.85 58 48 54.29 50.03 49.91 48.44 47.32 39.97 36.44 33.27 31.99 23.55
0.77 17.68 20.73 36.58 39.15 41 * 52 45.71 49.97 50.09 51.69 52.68 60.03 63.56 66.73 68.01 76.45 87.49 94.88
1086.5 1031 . o
-
12.51
5.12
1001.
863. 847, 859. 871. 873.5 873 872.5 868. 831. . Q
802.
830. 841. 1145. 1457. 1596.
Solid phase
Na2SiOa Na2Si03 NazSi03 NazSi03 NazSiz05 NazSi2O5 Na2SizO5
NazSiz05 NazSizOr, ?SazSizOr, NazSi2 0 5 NazSizOb NazSizOj High quartz High quartz Tridymite Tridymite Cristobalite
The results of this investigation are presented in Tables I and 11, and in Fig. I . Table I gives for each mixture the reference number, the weight per cent NazO or of both NazO and SiOz, as determined, the calculated weight percentage of NazSiO, and SiOz, the composition of the primary solid phase, and the liquidus temperature. I n Table I1 are given for each mixture the essential experimental data upon which the thermal dat’a of Table I are based, including for each run the temperature, the durat,ion of the heating a t that Morey: J. Wash. Acad. Sei., 13, 326-9 (1923). Roberts: Phys. Rev., 23, 386-95 (1924). 3 DRYand Sosman : “High-t,emperature gas thermometry.” Carnegie Inst. Washing t.on, Publ. 157.
1172
C,.
W. MOREY AND N. L. BOWEN
TABLEI1 Glass
2154-4
2330A
Temperature
Time
1085.6 1086.4 1086.5 1086.7 1086.5
15 min
1025.7 1029.I
T S min
ii
11 (1
iI (1
1032.6 1031. 2142A
998* 5
20
min 16
1000.2
i
