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MacKenzie: Effect of High-Temperature D.C. Electrolysis on Hydrous Ahminosilicates

EFFECT OF HIGH-TEMPERATURE DIRECT-CURRENT ELECTROLYSIS ON PROPERTIES OF HYDROUS ALUMINOSILICATES By K. J. D. MacKENZlE

The application of a d.c. potential to kaolinite and halloysite pellets during firing under reduced pressure has been found to produce significant improvements in the hardness, tensile strength, and density of the product. Studies of these properties as a function of voltage, electrolysis time and temperature, and fuing atmosphere are reported. A'-Ray and i.r. studies of mullite formation at the electrode faces indicate enhanced crystallinity at the negative electrode. The electrolysis mechanism is thought to involve the removal of residual protons normally present even at high temperatures.

Introduction In the past, little attention has been paid to the effects of d.c. electric fields on the solid-state reactions of silicates and particularly hydrous aluminosilicates. In the field of mineral synthesis, Hawkins & Roy' have shown that well crystallised kaolinite and montmorillonite can be prepared hydrothermally from andesite and dasite glass at 185-360" and 15,OOO lb/in2 by electrolysis at 30V d.c. Unelectrolysed samples gave analcite rather than kaolinite. The anodic deposition of kaolinite was thought to involve the formation of (HSi03)- and (Al(OH),)- or (Al(OH)6)3- followed by the expulsion of water and protons from these species. Electrolysis apparently facilitates this step without the formation of polysilicate anions. The effect of d.c. electrolysis on the tridymite-cristobalite conversion has been reported.2 Synthetic tridymite was electrolysed at 900-1300" and 250V. The electrode products were studied by X-ray microscopy and chemical analysis. Above 1050" the anode material was cristobalite and the cathode material tridymite. Below 1050", quartz was formed at the anode. Depletion of impurity ions occurred at the anode, and the results were explained in terms of impurity stabilisation of the tridymite structure. After experiments by Garino-Canina & Priqueler3 in which vitreous silica was electrolysed at 1050", a number of workers similarly studied other types of vitrous silica. The results are reviewed by Hetherington et aL4 who themselves electrolysed a number of silicas of varying hydroxyl and metal ion content, and Observed 3 depletion of protons at the anode and the enhanced formation of cristobalite and quartz a( the cathode. This was attributed to the anodic replacement of alkali ions by protons, which migrated via the hydroxyls to the cathode. Here, the formation of water vapour enhanced the devitrification process. In other silica experiments5 electrode metals such as gold, silver, palladium and copper (but not platinum) were found to replace the anodic alkali cations more readily than did protons. The present study of the electrolysis of kaolinite minerals was initiated by the observation that kaolinite itself generates appreciable electric potentials when heated between two similar platinum electrodes.6 In several preliminary electrolysis experiments on kaolinite, significant modification of the physical and mineralogical properties occurred, and the following systematic study of the effect of d.c. fields on bulk density, hardness, tensile strength, degree of mullite formation

and crystallinity was undertaken. Information about possible electrolysis mechanisms in hydrous oxide and silicate systems was also sought. Experimental Materials The kaolinite used in this work was supplied by English China Clays Ltd., Cornwall. The analyses for the principal constituents of Cornish kaolinite are: Si02 48.2, A 2 0 3 37-0,Fe,03 0.49, Ti02, 0-04, MgO 0.3, CaO 0.2, K 2 0 1.5 and N a 2 0 0.08%. The weight loss at 950" was 12.18 % and the calculated quartz and mica contents were 0.7% and 9-8%, respectively. X-Ray and d.t.a. investigations revealed no abnormalities in the sample, which has the characteristics of a reputable china clay. Small cylindrical sample pellets 7-5 mm dia. by 4 mm thick were formed in a brass mould from a stiff kaolinite paste prepared with distilled water. After drying at 80" the flat surfaces were trued up with a scalpel in preparation for electrolysis. High-temperature electrolysis Most of the electrolyses were carried out in the simple apparatus shown schematically in Fig. 1. The alumina work-tube was sufficiently long to enable rubber bungs to be used in the ends without charring. The sample was clamped between two spring-loaded alumina rods carrying platinum foil electrodes. The platinum lead-out wires were brought out through the rubber bungs and re-sealed with black wax at the start of each experiment. The work-tube could be either flushed with a firkg gas or evacuated to about 1 mm mercury pressure by continuous pumping with a rotary backing pump. mm Hg) was Electrolysis at lower pressures (10-4-10-6 carried out in a modified silica conductance cell, shown schematically in Fig. 2.

Fig. 1. Schematic diagram of medium-pressure electrolysis cell A. Pellet sample; B. Platinum electrodes on alumina bearers; C. Platinum lead-out wires; D. Alumina work tube; E. Phosphorbronze springs; F. Black-wax seals; G. Rubber bungs; H. Copper cooling coils J. appl. a m . , 1W0, Vol. 20, March

MacKenzie: Eflect of High-Temperature

D. C. Electrolysis on Hydrous Ahminosilicates

voc

Fig. 2. Schematic diagram of low-pressure electrolysis cell A. Sample pellet; B.Platinum electrodes on silica bearers; C . Platinum lead-out wires; D. Tungsten-Pyrex vacuum seals; E. Phosphor-bronze spring; F. Fused silica support tube; G.Silica thermocouple sheath with Pt-Rh thermocouple; H. Silica Pyrex graded seals; I. Body tube (fused silica) The d.c. supply was obtained from a Labgear type D4019/8 stabilised E.H.T. unit delivering up to 2 kV. The sample was raised to working temperature at 20 deg c/min and held to k 5" by a proportional controller. Five sets of experiments were carried out as follows: at 1 m m pressure, and isothermal firing voltages of M S O V , time of 2 h at lOSO"-higher voltages were not used because of arcing caused by the breakdown of the surrounding dielectric medium; at times of 15 min-8 h, 450V, 1050" and N 1 mm pressure; at temperatures of 650-1150", 450V, 1 mm pressure for 2 h; at 450V, 1 mm pressure for 2 h at a soaking temperature of 1050O-the electrolysis voltage was applied at 100, 650, 980 and 1050", these temperatures representing stages at which the various reactions in the sample begin; and under atmospheres of nitrogen, oxygen, carbon dioxide, argon, (10% Hz +90% N2),compressed air, still air, reduced pressure (- 1 mm), hard vacuum 10-4mm) and water vapour, at 450V, 1050" for 2h-the water vapour was introduced in a stream of nitrogen carrier gas saturated at 80". After electrolysis, the samples were air-quenched and submitted to the following tests:

-

N

-

Measurement of bulk density. The mercury method of Clark & White' was used, giving results reliable to within 0.02 g/ml

81

pellet were X-rayed in a Philips goniometer and the major diffraction peaks of all the expected phases scanned. The relative crystallinity of the mullite in each face was estimated by cutting out the peaks from the chart and weighing. No quantitative measurements could be made in this way, but the ratio of the peak areas of the two faces ( M - / M + ) provided an index of the variation in crystallinity and orientation of the mullite at each end of a particular sample. Infra-red determination of percentage mullite formation. Small scrapings from each electrode face were analysed for mullite by an infra-red frequency shift method.*O The results were independent of the crystallinity of the mullite and depended only on the extent of development of the mullite crystal structure. The method gives an accuracy of about k 3%. X-Ray determination of bulk mullite content. The pellets were ground and analysed by an X-ray powder method" using CaF, as an internal standard.'2 The accuracy of the method is f 3-5%.

Results and discussion Effect of voltage The voltage dependence of bulk density, tensile strength, bulk mullite percentage and hardness is shown in Fig. 3. All these parameters show a similar drop at very low voltages, but a marked improvement with increasing voltage. The low voltage effect may arise because the applied field is sufficient to interfere with normal ionic progress, but not strong enough to boost ions out of their P.E. wells and impart a drift direction. A gradiation in hardness from one electrode face to another is most marked at higher voltages (Fig. 4), tKe hardness of the negative face being extremely voltage dependent. At higher voltages the X-ray crystallinity of mullite in the negative face also becomes much greater than in the positive face, as shown by the steep rise in the X-ray ratio M-/Mi.

after temperature corrections were applied. These were not measurements of true mineralogical density, but gave an indication of the degree of compaction and coalescence of the particles. Vickers microhardness determinations. The microhardness of the positive and negative electrode faces was determined with a Leitz miniload tester.8 The average of a number of tests on each face is quoted (H+and H-), but the accuracy was not better than 15% in most cases because of the variation from one surface crystallite to the next, and because the unpolished surface had to be viewed under oblique lighting with a low-powered objective in order to see the indentation. The ratio of hardness (H-/H+) was also evaluated. Measurement of tensile strength. A compressional force was applied across the pellet diameter in an Instron model TT universal tester at a crosshead speed of 0.005 cmlmin. The output from the load cell is recorded directly and the breaking load ohtained from the stress-strain curve. Following the stress analysis of such a system given by Frocht? the tensile strength is given by T = 2 P / ~ d...................... t , (1)

.......

where P is the breaking load, d the diameter and t the pellet thickness. X-Ray examination of the electrodefaces. The ha% of each

J. appl. Chem., 1970, Vol. 20, March

L

0

I 100

I 200

I 300

ELECTRlLYSIS VOLTAGE, V

Fig. 3. Variation of bulk density, tensile strength, X-ray mullite percentage and cathode hardness with electrolysis voltage 2.0 h at 1050°c, 1 mm

-

82

MacKenzie: Effect of High-Temperature D.C. Electrolysis on Hydrous Aluminosilicates

I I _

0

ELECTROLYSIS TIME, h

I 2m ELECTROLYSIS WLTAGE ,V

-7.k-

Fig. 5 . Variation of bulk density, tensile strength, X-ray mrrllite percentage and cathode hardness with electrolysis time 450V, 1O5O0c, 1 mm N

Fig. 4. Variation of hardness, differential X-ray mullire crystallinity and i.r. mullite content with electrolysis voltage 2.0 h at 1050"c, 1 mm

-

+

z i\

Also plotted on Fig. 4 are the absolute percentages of mullite at each face, determined by the i.r. method, and therefore independent of crystallinity. These values show a real increase with increasing voltage, but the constancy of the i.r. mullite ratio for the two faces of each pellet in contrast to the rise in the X-ray ratio indicates that the X-ray difference between the two faces is due to crystallinity and orientation effects alone, and not to differences in the absolute amount of mullite formed. The factors affecting X-ray crystallinity are discussed in a later section.

Effect of electrolysis time Fig. 5 shows the physical parameters of kaolinite as a function of the electrolysis time. Again, these curves are of the same general shape, suggesting that the maximum at t = 2 h is real. This may be due to an unusual cation configuration initially produced by the electrolysis which is normalised or annealed by prolonged firing. A similar but smaller peak is seen in the hardness of the positive face, and reflected in the ratio H - / H + (Fig. 6). The i.r. mullite analysis also shows this trend but the i.r. ratio M - / M + indicates no preferential cathodic mullite development at any stage of the firing. Effect of electrolysis temperature An improvement would be expected in the physical properties with increasing temperature even in the absence of an electric field, but the steepness of the curves at higher temperatures indicates increased efficiency of the electrolysis process with increasing ionic mobility (Fig. 7). At higher temperatures, the disparity between H- and H+ increases (Fig. 8) but this trend is not followed by the i.r. mullite content. The drop in M - / M + (X-ray) at higher temperatures is caused by the appearance at 850-950" of a strong peak at d = 3.46 A at the negative face. The ratio is decreased by mullite growth at the positive face at higher temperatures.

I

0

I

L ELECTROLYSIS TIME,h

I

8

Fig. 6 . Variation of hardness, differential X-ray mullite crystallinity and i.r. mullite content with electrolysis time 450V, 1050", 1 mm

-

Effect of temperature of voltage application This variable would be expected to influence the efficiency of electrolysis, since the various thermal reactions begin at well-defined temperatures. Electrolysis is most beneficial when begun at high temperatures, i.e. when the ions are mobile (Fig. 9). If electrolysis is started during any one of the reactions preceding mullite formation, the preceding phase is apparently stabilised, to the detriment of further reaction. This is particularly noticeable at 98W, the formation temperature of a defect spinel13 susceptible to cation (or proton) stabilisation (Fig. 10). J. appl. Chem., 1970, Vol. 20, March

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MacKenzie: Efect of High-Temperature D.C. Electrolysis on Hydrous Ahminosilicates

650

I I 850 1050 ELECTROLYSIS TEMPERATUREpc

I

Fig. I. Variation of bulk density, tensile strength, X-ray mullite percentage and cathode hardness with electrolysis temperature 2.0h, 450V, 1 mm N

ELECTROLYSIS

TEMPERATURE:^

Fig. 8. Variationof hardness, diferential X-ray mullite crystallinity and i.r. mullite content with electrolysis temperature 2.0 h, 450V, 1 mm

-

J. appl. chela, 1970, Vol. 20, March

I 150

I 550

I

950

TEMPERATURE OF VOLTAGE AFPLICATION~C

Fig. 9. Variation of bulk density, tensile strength, X-ray mullite percentage and cathode harhess with electrolysis starting temperature 2.0 h, 1050"~ soaking temperature, 450V, 1 mm

-

I

I I I 1% 550 950 TEMPERATURE OF VOLTAGE APPLICATION.'C

Fig. 10. Variationof hardness, differential X-ray mullite crystallinity and i.r. mullite content with electrolysis starting temperature 2.0 h, 1050"~soaking temperature, 450V, 1 mm N

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MacKenzie: Efect of High-Temperature D.C. Electrolysis on Hydrous Aluminosikates

Effect of firing atmosphere The results, shown graphically in Figs 11 and 12, reveal no simultaneous trend in all the measured properties, but rather suggest the operation of several competing factors. A previous study of the effects of firing atmosphere on the formation of mullite from une1ectrolys:d kaolinitet4 has established that the benefiial effect of various atmospheres falls off in the order: HzO > vacuum > Hz > N2 > air > O2 > COz. Mullite formation and hardness follows a broadly similar sequence under the present electrolysis conditions, but the order of the other physical properties varies considerably. The greatest enhancement of all properties is obtained under reduced pressure, although it is apparently unnecessary to employ high vacua. If, as previously suggested,14 reduced pressure serves to remove gaseous species as they occur, thus assisting the formation of the hightemperature phases, the electrolysis process apparently attenuates the initial elimination of such spe5es from the lattice by a mechanism discussed below. The presence of water vapour would be expected to compete with this elimination mechanism, and so the beneficial effect of water on mullite formation and hardness must be due to another phenomenon, namely enhanced nucleation by proton attack on the Si-0 bond.14 The other atmospheres investigated apparently also suppress the gas elimination reaction, without having any compensating nucleation effect, X-Ray intensity measurements of the pellet faces

Comparison of the i.r. mullite ratios of the two electrode faces of each pellet with the X-ray intensity ratios (Figs 4, 6 and 8) shows that whereas the absolute mullite content as measured by i.r. does not differ significantly, between the faces of each pellet, the X-ray parameter shows marked I

A ‘P

High

vacuum

I

I

I

I

Low SliN H21N2 C02 N2 v.xuum o1r REPCTION AlTHOSPHERE

Ar

variations. Since X-ray measurements are dependent on crystallite size and crystallinity as well as percentage mullite development, the discrepancy between the two techniques can be accounted for on the basis of differences in the crystalline properties between the two faces of each pellet. Detailed examination of the X-ray traces of each pellet face reveals in the cathode faces of samples fired at reduced pressure and 450V a very strong peak at d = 3.46 A, slightly below the 120 mullite peak at d = 3.42 A. The 3.46 A peak occurs weakly during the first stages of mullite growth under normal conditions, but under reduced pressure and electrolysis it occurs at temperatures as low as 65W,and grows anomalously, apparently at the expense of the true 120 reflection, since the 120 peak never develops fully in samples containing the 3.46A peak (Fig. 13). The 3.46A peak occurs weakly in samples electrolysed in still air, and may be present in samples electrolysed in carbon dioxide. Examination of the complete X-ray traces of all samples containing the 3.46 A peak reveals no extra peaks other than those of mullite, quartz and cristabolite. Therefore the 3.468, peak must be associated with a rnullite-like phase, and indeed, its growth behaviour with time and temperature (Fig. 14) is similar to that of normal mulli te. It seems reasonable to suppose that the 3.46 A peak is the precursor of the mullite 120 peak and has developed preferentially as a result of the electrolysisprocess. Reference to the mullite structure of B ~ r n h a m ‘shows ~ that the 120 plane contains normally co-ordinated tetrahedral Al atoms, tetrahedral Si atoms and octahedral A1 atoms, together with certain shared oxygens. The ‘normal’ 120 plane does not, however, contain ‘removable’ oxygens or ‘removable’ tetrahedral Si or A1 atoms, or even those atoms whose co-ordination has been made abnormal by the removal of random oxygen atoms. If, however, the electrolysis process had the effect of replacing the tetrahedral vacancies with other tetrahedral cations, while the vacuum conditions had the effect of removiug extra oxygen, then a situation may arise in

Campmsod air

Fig. 1 I . Variation of bulk &nsity, tensile strength, X-ray mullite percentage and cathode harhess with $ring atmosphere 2.0 h, 1050”C, 450V

REACTW ATMOSPHERE

Fig. 12. Variation of hardness, differentialX-ray mullite crystallinity and i.r. mullite content with firing atmosphere 2.0 h, 1050”~, 450V

J. appl. Chem., 1970, Vol. 20, March

MacKenzie: Efect of High-Temperature D.C. Electrolysis on Hydrous Ahminosilicates Ibl

lo1

LIB

'28 26.5

85

255

ELECTROLYSIS TIME, h

255

265

I

26.5

I

255 ELECTROLYSIS TEMPEPAlURE,%

Fig. 14. Growth of the 3.46 d; X-ray peak (a) as a function of electrolysis time, at 1050"c, 450V, 1 m m (b) as a function of electrolysis temperature, at 450V, 2 h, 1 mm

kl

N

N

Ji 5

2!

Fig. 13. a-f Stages in the evolution of a normal mullite X-ray peak, unelectrolysed, in air a. 980"c; b. llOOoc, 1 h; c. 11OO"c, 2 h; d. 11OO"c,4 h; e. llOOoc, 16 h; f. 1100"~.36 h g-1 Stages in the evolution of an abnormal mullite peak at 1 mm, 450V, for 2.0 h (cathodeface) g. 650%; h. 750%; i. 8 5 0 " ~ ;j. 950"~;k. 1050"~;1. 1150"~

which a pseudo-120 plane of strong reflection containing an abnormal number of tetrahedral cations may occur, but at a slightly distorted d value caused by the presence of the extra cations and the loss of extra oxygen. The 210 plane would not be expected to be similarly affected, being further away from the 'centres of ionic unsaturation'. The phenomenon would be expected at the cathode only, since it is in this region that surplus cations would be found.

Mechanism of the electrolysis process If one accepts that metakaolinite and the subsequent phases are anhydrous, the only possible cationic migrating species are Si and Al (and the various impurity cations present). Of the structural cations, Si may be the most mobile, particularly under vacuum conditions, where the formation of SiO is likely. Again, a separation of silica from the reacting system involves the movement of Si, although it is not clear whether this is in the form Si4+,Si, SiO or SiO,. There is, however, an increasing weight of evidence that protons persist in the system up to temperatures of mullite formation. Weight-loss measurements on kaolinite show a

J. appl. em., 1970, Vol. 20, March

small but steady decrease in weight over the range 700-950", attributed to the loss of residual water.I6 1.r. work by Pampuch" has confirmed the presence of hydroxyls on metakaolinite up to 600°, and there is evidence of proton retention at higher temperatures. Water vapour, hydrogen and oxygen have been detected by the present author by mass spectrometry in the reaction atmosphere of kaolinite at 1100°.18 If, then, protons are retained at these temperatures,they would be expected to preferentially migrate in an electric field gradient, by a hopping mechanism such as proposed by Hetherington et aL4 for the electrolysis of vitreous hydroxyl-containing silica. Once the protons have reached the cathode, a recombination of water may occur in the early stages of the reaction, and it has been suggested by Hetherington et al. that the presence of water vapour at the cathode promotes enhanced nucleation of silica (and aluminosilicate) phases at that electrode. As the reaction proceeds at low pressures, however, molecular hydrogen is evolved in preference to water. This is probably due to the depletion of 'removable' oxygen at the cathode, leaving the protons with no alternative but to recombine. The electrons involved in this process would be supplied by the variable valence impurities such as iron and titanium, present in good quantity in the natural minerals. At the same time, a small amount of atomic oxygen is evolved, either by an electron-transfer process at the anode, or from the disproportionation of SiO. The gasphase recombination of the hydrogen and oxygen explains the sudden dramatic rise in water vapour at 1100°.18 The enhancement of mullite formation and the associated physical properties is therefore seen to depend on the removal by electrolysis and pumping of certain gaseous species, and the subsequent rapid formation of a suitably defect mullite structure, which may, however, revert to a more normal cationic structure under the influence of prolonged thermal treatment. At higher temperatures the vaporisation of SiO probably becomes increasingly important; SiO and Si have been detected by mass spectrometry.1S

MacKenzie: Effect of High-Temperature D.C. Electrolysis on Hydrous Aluminosilicates

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Hetherington, G., Jack, K. H., & Ramsay, M. W., Physics

Acknowledgments Part of this work was carried out under a New Zealand University Grants Committee Postdoctoral Fellowship and part under a Science Research Council Assistantship. The author is indebted to Prof. J. F. Duncan and Dr. B. C. H. Steele for helpful discussion. Dept. of Ceramics with Refractories Technology, University of Sheffield Received 22 August, 1969

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I 1 l2

Chem. Glasses, 1965, 6, 6 Dunn, T., Hetherington, G., & Jack, K. H., Physics Chem. GIusses, 1965,6, 16 MacKenzie, K. J. D., Nature, Lond., 1969,222,469 Clark, P. W., &White, J., lkzns. Br. Ceram. SOC.,1950,49, 305 Fleurence, A., Bull. SOC.fr. Ckram., 1966, No. 70, 51 Frocht, M. M., ‘Photoelasticity’, 1948, Vol. 2, p. 121 (New York: Wiley) MacKenzie, K. J. D., J. appl. Chem., Lond., 1969,19,65 Sane, S. C., & Cook, R. L., J. Am. Ceram. SOC.,1951,34, 145 Duncan,J. F., MacKenzie, K. J. D., & Foster, P. K., J. Am. Ceram. SOC.,1969,52,74 Brindley, G. W., & Nakahira, M., J. Am. Ceram. SOC.,1959, 42, 311

References Hawkins, D. B., & Roy, R., J. Am. Ceram. SOC.,1962, 45, 507 Floerke, 0. E., Naturwissenschaffen,1956, 43, 419 Garino-Canina, V., & Priqueler, M., Physics Chem. Glasses, 1962,3,43

l4 l6 I7

MacKenzie, K. J. D., Trans. Br. Ceram. SOC.,1969.68, 103 Burnham, C. W., Yb. Carnegie Instn Wash., 1964, 63,223 Rieke, R., & Mauve, L., Ber. dt. keram. Ges., 1942,23, 119 Pampuch, R., Pr. Mineralog., Polska Akad. Nauk, Krakow, 1965, No. 6, 53

MacKenzie, K. J. D., in the press

J. appl. Chem., 1970, VoL 20, March