THE SOLUBILITY OF CORUNDUM IN BASIC HYDROTHERMAL SOLVENTS

By R. L. BARNS, R. A. LAUDISE AND R. M. SHIELDS

Bell Telephone Laboratories, Incorporated, Murray Hill, New J ersey

Received September 24-, 1962

The solubility of corundum has been measured in sodium hydroxide and in sodium carbonate solutions from about 400 to 600° at pressures up to about 2800 bars. In sodium hydroxide, the solubility was independent of pressure but dependent almost linearly on base concentration up to 10 m N aOR. In sodium carbonate, the solu­bility depended almost linearly on pressure up to about 1370 bars beyond which it was almost independent of pres­sure. The solubility in Na2COa depended almost linearly on base concentration up to 4.3 m beyond which it was independent of base. The temperature coefficient of solubility was large in Na2C03 and nearly zero in NaOR. The solubility in LiOR, KOR, CsOR, K 2COa, Ce2coa, and several other mineralizers is reported. The solubility data suggest that in (OR )- the predominant species is (Al02)- or (Al03)- while several species are present in (CO)-2.

Introduction Alpha-AbOa has been grown on a seed crystal from

dilute aOH and Na2C03 solutions at temperatures above 400 0 and pressures from about 500 bars upward. The method of preparation has been reported by Laudise and Ballman l and more recently by Bashuk, et aU It is of interest to determine the solubility of corundum in the hydrothermal solvents from which it has been or might be grown. Such solubility data will aid in elucidating the mechanisms of hydrothermal crystal growth in general and hopefully will suggest conditions for the more rapid growth of corundum.

The hydrothermal solubility of refractory oxides is of general interest and a considerable body of theoretical work has been done on the thermodynamics of hydro­thermal systems. 3-5 However, the only experimental solubility data under hydrothermal conditions in existence was for a-quartz.5- 7 In general, theoretical treatments of solubility have restricted themselves to systems containing only water and solute. For the growth of crystals under hydrothermal conditions, the

(1) R. A. Laudise and A. A. Ballman, J. Am. Chern. Soc., 80, 2655 (1958). (2) R . P . Basbuk, V. P. Basnev, R . B. Tsadkina, and S. A. Faivusovich,

KriBtallografiya , II, 666 (1960). (Eng. Translation-Soviet Phy";c. CrYBtal­lography. lI. 638 (1961).

(3) E . U. Franck, Z. phll,ik. Chem. (Frankfurt), 6, 345 (1956). (4) G. J. Wasserburg, J. Geol., 66. (5 ),559 (1958). (5) R. A. Laudise Ilnd A. A. BaUman. J . PhYB. Chem., 611,1396 (1961). (6) G. W. Morey and J. M. Hes.elges.er, Am. J . Sci .. Bowen Volume, 343

(1952) . (7) A more complete compilation of references of theoretical and experi­

mental publioations on quartz solubility is given in "Hydrothermal Syn­thesis of Single Crystals." R. A. Laudise in "Progre.s in Inorganic Chem­i8try" Vol. III. edited by F . A. Cotton, Interscience, New York, N. Y., 1962, p. Iff.

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solubility usually must be increased by the addition of a mineralizer. Previous work in the system SiOr NaOH-H20 has shown that the function of the (OH)­mineralizer is to promote the formation of new silica containing species other than those found in pure water.6•7 It would be of interest to test hydrothermal solubility theories and postulates concerning the role of a mineralizer in a system containing some other refractory oxide besides Si02• Consequently, it was decided to study the hydrothermal solubility of AbOa in the presence of mineralizers. Further work has ex­tended these solubility studies to ZnO.8

Experimental Hydrothermal equipment. has been discussed elsewhere,·

so only a brief description will be given here. Previous hydro­thermal solubilities have been determined by (1) sampling the fluid phase at the chosen p-t conditions or (2) determining the loss in weight of the solid phase following rapid quenching to room temperature. Method (2) is satisfactory provided the solid phase does not recrystallize on the solid piece during the quench and provided but one fluid phase co-exists in equilibrium with the crystalline phase. Method (2) was used in our previous determinations of solubility in the SiOr Na20-R20 system.5

The hydrothermal vessel had a volume of about 100 cm.! and was arranged in a rocking furnace to accelerate equilibrium. Recently we have found that with a small sacrifice in precision and much less effort. it is possible to carry out solubility determi­nations. at least where the solubility is above about one per cent. in welded platinum capsules whose volume is one cm.' or less.

The procedure used was as follows . One end of a thin walled

(8) R. A. Laudise and E. D . Kolb. Am. M ineralogia!. in pre.s. (9) R. A. Laudise and J . W. Nielsen. "Solid State Physics," Vol. 12, ed.

by F. Seitz and D . Turnbull, Academic Press, New York, N. Y., 1961, p. 149ff·

35

30

>-' !:: 25 ...J

III :::> ...J o III 20 fo-Z W u ffi 15 Il.

f0-r <.!I w 10 ~

5

o o

I

V I

I v /

/ /

I 2 4 6 8 10 12 16 18

MOLALITY OF Na OH.

Fig. I.-Solubility of corundum as a function of NaOH concen­tration at 430° and 1450 bars.

~ fo-::::; 6 iii :3 o III 4 fo-Z W U a: 2 w Il.

j: 0 <.!I 0 iii ~

.J --L

I~ ./' r X~

r/ V ·

500 1000 1500 2000 PRESSURE I N BARS .

9-fW .-

2500 3000

Fig. 2.-S01ubility of corundum as a function of pressure in 3.4 m Na,C03 at 430°.

C.p. plat,inum tube (typically 0.19 in. o.d. X 1.5 in. long X 0.007 in. wall ) was crimped and welded with a d.c. arc welder. The tube was then weighed. A length of single crystal corundum (Linde Co. white sapphire rod ) was weighed. 'I he rod and the desi red solvent were loaded into tbe tube and the open end was crimped and welded . The whole capsule was then weighed. The capsule was tested for leaks by placing it in an oven at 225° for at least 10 min., after which it was reweighed . The capsule was placed in a 0.25 in. i.d. Tuttle cold seal test~tube type auto­clave. tO The autoclave was attached to an air driven intensifier (which pumped wat.er at any desired pressure up to 4,000 bars) and a pressure gage (calibrated to ±10 bars with a dead weight tester).

Water was pumped into tbe autoclave up to the desired pres­sure (controlled within ±30 bars) and a furnace (controlled to ±3°) by a conventional contactor controller with a chromel­alumel thermocouple was placed around the autoclave. Another thermocouple was inserted in a cavity in the autoclave wall and its output was recorded.

At the end of the run, the autoclave was quenched with cold water and the capsule was reweighed to determine if any leaks had occurred. The capsule was then opened and the contents carefully examined for evidence of foreign phases. The sapphire rod was then rinsed in 6 N HCI, carefully dried and weighed .

In all cases where a solubility is reported , the rod surface was smooth with no evidence of redeposition. In those cases where corundum was found not to be the stable phase, the rod usually had It rough, chalky surface.

Reagent purity chemicals were used in all experiments and solutions were standardized against potassium acid phthalate. All mineralizer concentrations are e:-''jJressed in terms of molality, m.

(10) The autoclaves and associated pressure and furnace system were pu rchased from Tempres., Inc., State College, Pa.

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Using five replicate determinations of solubility in 3.4 m Na,C03 at 430° and 1,450 bars, the standard deviation of a single observation was found to be 0.19% at an average value of 4.94 wt. % solubility . The bars on the graph indicate the ±3 u limits for either single or multiple determinations. For mUltiple determinations the bar height shows the ±3 u limits for the mean of the several determinationR.

Results The apparent solubility of corundum was deter­

mined as a function of time under representative condi­t ions from 5 to 100 hours and it was found that equilib­rium was achieved in five hours. All experiments were for at least 18 hr. Phase equilibrium experiments were performed by using as starting material "Ab03, Al­(OH)a· nli20, or both. Solubilities are reported only for those conditions where the stable solid phase was ob­served to be a-Ab03. Per cent solubility was calcu­lated as wt . of corundum lost X 100/wt . of water + wt. of mineralizer. Solid phases were always identified by X-ray powder patterns and verified in some cases by petrographic microscopy.

Solubility in N aOH.-Certain pertinent solubility data are shown ill Table I. As can be seen, and as

TABLE I

SOLUBILITY OF CORUNDUM IN NaOH-WATER SOLUTIONS No. of

Base deter-conen., Temp., Pressure, Solubility, mina.-

m °C. bars wt. % tions

0.5 400 310---2,680 2.0 5 lndep. of p

2.0 430---600 1 ,450 7.5 at 4300 10 Slightpos. 8.0 at 6000 slope

2.0 430 1 ,380---2 ,760 7 .5 7 Indep. of p

2.0 600 1 ,380---2,760 8.0 2 Ind.p. of p

10 .0 431HlOO 1 ,450 30.5 at 4300 12 Slight po • . 31. 5 at 600 0 slope

10.0 430 140---2,760 30.5 9 Ind.p. of p

other data not shown here substantiate, the solubility from 0.5-10.0 m NaOH in the temperature range from 400- 600° is apparently independent of pressure for pressures from 140 to 2,760 bars. The solubility has a slight positive temperature coefficient and other data not shown here indicate that the Van't Hoff equation is obeyed in 10 m NaOH with a pressure independent tlH of about 0.2 kcal./ mole between 430 and 600°. The slope in 2.0 m NaOH is probably similar, but the data scatter rather badly.

Figure 1 shows the dependence of solubility on base concentration to be nearly linear up to 10 m NaOH.

Solubility in Na2C03.-The dependence of solubility on pressure in 3.4 m Na2C03 at 4300 is shown in Fig. 2, while Fig. 3 shows the dependence on temperature at 1450 bars. Figure 4 shows the solubil ity dependence on Na2C03 concentration at 1450 bars and 430° . As can be seen, the solubility depends almost linearly on pressure up to 1380 bars above which the pressure co­efficient is either zero or slightly negative. The tem­perature coefficient of solubility is large, while the Van't Hoff equation is only imperfectly obeyed with tlH 574 0 ~

1 kcal. / mole while tlli450 0 ~ 5 kcal./ mole . The solu­bility increases with increasing base concentration up to about 3.4 m beyond which it is independent of base concentration until sodium aluminate becomes the stable phase.

Solubility in Other Mineralizers.- A number of other mineralizers for Al20 3 were investigated, and the results are summarized in Table II.