Hydrothermal dissolution reactions of magnesium-aluminium-spinel in alkaline solutions

Journal

of Crystal Growth 7 (1970) 97-101 62 North-Holland

HYDROTHERMAL

DISSOLUTION

Publishing

REACTIONS

IN ALKALINE

W. FRANKE Institut

Received

ftir

Minernhgie

8 December

1969; revised

OF MAGNESIUM-ALUMINIUM-SPINEL

SOLUTIONS

and

der Freien

Co.

R. HEIMANN

Unit’ersifiif

manuscript

Berlin,

received

Berlin,

Germany

16 February

1970

Single crystal spheres of spine1 of 4 mm diameter having a molar ratio of MgO/Al,03 1 : 5, were dissolved in 0.1 M solutions of K,CO,, NaF, KF, Na3P0, and K,P04 under a pressure of 2 kbar and at temperatures between 390 “C and 630 “C. The final shape of the dissolved spheres are cubes. The reaction layers formed during dissolution varied in porosity and consisted of

Mg(OH), in alkali carbonate solutions, MgFZ in alkali fluoride and Mg,(PO& in alkali phosphate solutions. The porous Mg(OH)* layer hardly affected the material transport, so that the relationship between log u, and l/ris a linear function, which was not found for the less porous layers of MgFz and Mg,(PO,),. In the case of alkali fluoride the rate of reaction is limited by the diffusion of fluoride ions.

1. Introduction

sphere was 25 “C, determined without pressurizing the vessel using two thermocouples. The pressure was kept constant during the heating up of the vessel by manual control of the overflow valve. The durations of the experiments were between 15 and 30 hr, at a constant pressure of 2 kbar with the temperatures varying between 390 “C and 630 “C.

of SiO- in melts and aqueous The dissolution has been recently published’**). alkaline solutions The kinetics and morphology of the hydrothermal dissolution of Mg-Al-spine1 under supercritical conditions will be reported here. The diffusion of the corrosive component through the insoluble reaction product which is formed at the phase boundary, will be discussed. 2. Materials and experimental

3. The chemical analysis of the investigated

spine1

It is known3-‘) that the composition of Mg-AIspinels synthesized by the Verneuil process, deviates greatly from the stoichiometric molar ratio of the natural MgO-Al,O, (1: 1) spinels. Beyond a critical limit of MgO-Al,O, (I :2.5), an interphase very rich in alumina is deposited, monoclinic in structure6-‘). The spinels used in the present investigation showed segregations. From X-ray diffractions we could identify the strongest lines of the monoclinic interphase. The lattice spacing constant of the spine1 was calculated to be 7.955 A. A chemical analysis using an X-ray fluorescence spectrophotometer gave a value of 83.7 mole”/, for Al,O,. The spine1 has a composition of MgO-AI,O, in the ratio of 1:5, in agreement with previous results6’r0*’ I). The density, calculated from the chemical composition and the lattice constant, gave a value of 3.629 g/ cm3 which agrees very well with the pyknometric determination of 3.626 g/cm3.

procedure

The 0.1 M solutions of K,CO,, NaF, KF, Na,PO, and K,PO, were prepared using A.R. grade chemicals manufactured by Merck in Darmstadt, Germany. Ground polished single crystal spheres of synthetic spine1 (supplied by Litzenberger, Idar-Oberstein, Germany), 4 mm in diameter have been prepared and located on a stainless steel spiral inside a pressure vessel of 15 mm diameter and an internal volume of 27 cm3. The vessels were delivered by Tern-Pres Research, State College, Penn., USA and consisted of Rent steel. Runs were made without using noble metal liners, Variation in temperature was never larger than 4 “C. The temperature difference between the inner and the outer wall of the pressure vessel at the level of the 97

98

W.

FRANKE

AND

4. Kinetics of the dissolution

excess of potassium carbonate. brucite layer was 0.75 g/cm3.

4.1. THE REACTION WITH 0.1 M K,CO,

The

density

of the

SOLUTION

The rate of dissolution given in mg/mm2 * hr was obtained from the amount of dissolved spinel, the time of the experiment and the change in surface area. The temperature dependence of the rate of dissolution is shown in fig. 1 for 0.1 M K,CO, solution. The sample retained its spherical shape during the experiment, the diameter changing only a very small amount. This sphere consisted of an outer white flaky reaction layer composed of Mg(OH), type brucite12); in the inner part a cubic shaped crystal of unaltered spine1 remained. We can assume the following dissolution reaction: MgO .5Al,O, + Mg(OH),

R. HEIMANN

+ 5K,CO, + H,O + + (10KA102), + 5C0,.

The X-ray spectrum of a sample obtained by evaporating the aqueous solution and melting the residue at 1100 “C showed potassium aluminate as well as an

4.2. THEREACTION WITH 0.1M NaF AND KF SOLUTIONS A strong adhering white reaction layer of MgF, was formed on the remaining cubic shaped crystal of spine1 preserving pseudomorphically the form and the size of the original sphere. A small amount of 2NaAl0, . 3H,O was detected when the attacking agent was NaF. Other expected reaction products like aluminium fluoride, alkali aluminium fluoride or aluminium hydroxide could not be detected by X-ray diffraction in the evaporated residue, despite the colloidal turbidity of the solution suggesting the presence of such products. The X-ray diagram of the 1100 “C calcined reaction layer showed traces of x-AI,O,, from which it can be concluded that aluminium hydroxide is primarily formed. By plotting of log u, against the reciprocal of temperature for a constant pressure, no linear dependence was shown because of the varying layer

1

T 2

10-l

z 3 s ._ 5

10-z

P VI G 6 al 3

10-a

7 P E

10-b

100

110

1.20

1.30

1.40

150

1 7 .103CoK-')

Fig. 1. Log U, plotted

against

the reciprocal

of Tat

a pressure

of 2 kbar

for 0.1 M KzCOJ

solution.

160

HYDROTHERMAL

10-E

.

DISSOLUTION

REACTIONS

99

OF MAGNESIUM-ALUMINIUM-SPINEL

1

8

0

NaF

0

KF

10-g

100

110

1 20

1 30

1.40

150

$ 103L0K-'I Fig. 2. Log D plotted against the reciprocal of T at a pressure of2 kbar for 0.1 M NaF and KF solutions. For further details see text.

thicknesses and the differing times of runs. From this it is concluded that the material transfer is hindered by the reaction layer. By solving

Fick’s 2nd law we obtain

D = S2/2t, i.e.

Z-

Jt,

where D, fi2 and t are the diffusion coefficient, the mean displacement square and the time respectively, for c(x,O)=f(x) c(0, t) = c,

for for

O 0,

where c is the concentration and c, the saturation concentration. The boundary conditions refer to a constant diffusion coefficient, a constant concentration of the solution and a diffusion system which is realized only in a semispace with an impermeable boundary kept at a constant concentration’3). In the present case for dilute solutions this can be assumed to be true. The diffusion coefficient, calculated from the thickness of the reaction layer and the time of the run, is plotted against the reciprocal of T (fig. 2). The points for NaF and KF can be approximated roughly to a straight line. This means that the rate of reaction is mainly determined by the diffusion of the fluoride ion.

4.3.

THE REACTION WITH

0.1

M Na,PO,

AND

K,PO,

SOLUTIONS

By the reaction of tertiary alkali phosphates, a voluminous layer of Mg3(P0,)2 was formed, which was partly chipped off from the final solution form. Most of the outside surface of the crystals after dissolution were blue, due probably to the presence of traces of vivianite formed by the reaction of the phosphates with the pressure vessel. The plottings of log v, or log D against the reciprocal of the temperature gave no smooth curve (fig. 3). It is supposed that the change in kinetics is due to the reaction layer being chipped during the reaction. 5. Morphology of the final solution form Fig. 4 shows the general view of the final solution form after 21 hr at 500 “C and 2 kbar in 0.1 M K,CO, solution taken using a Stereo-Scan electromicroscope. Fig. 5 shows a greatly enlarged edge between two “planes” of the cube. From the photograph it can be seen that the edge in the direction [OlO] is relatively sharp, but curved. The (lOO)-planes have, on the other hand, a rough and coarse structure. From the experimental conditions it cannot be concluded definitely whether this is due to a selective dissolution of the

100

W.

FRANKE

AND

R.

HEIMANN

1o-6

1o-7 7

A

” m

-; v 0

A A

lo-@

A

A A

A

Na3P0,

A

K,PO,

1o-g

1 00

110

130

120

140

150

;lO+K-‘1

Fig. 3. Log

D plotted

against

the reciprocal

of T at a pressure

of 2 kbar

for 0.1

M Na3P0,

Fig. 5. View of the edge between Fig. 4. Final solution form obtained by the dissolution of a single crystal sphere of spine1 at a pressure of 2 kbar in 0.1 M K2C03 solution ( x 30). Stereo-Scan (Cambridge).

Al,O, component or to a local hindering of the reaction by the topochemically implanted Mg(OH), layer. Small octahedra1 surfaces are formed composed of etch pits approximately triangular in shape. These

ness of the

faces

(X 150).

and

K,PO,

solutions.

two cube faces. Note the roughStereo-Scan

(Cambridge).

(11 I)-faces occurred in all experiments at the beginning of the dissolution process. In the course of dissolution the (Ill)-planes were suppressed by formation of the planes of the cube. When dissolving a convex body, the faces formed

HYDROTHERMAL

DISSOLUTION

REACTIONS

are those that have the highest relative rate of dissolution in the particular direction14.’ ‘). Therefore it can be concluded that ~(100, ’ V(lll). The fact that the [lOOI-direction is the direction of the highest rate of dissolution is explained by considering the strength of the 0-Mg-0 binding forces in the spine1 lattice. Magnesium is much more electropositive than aluminium and therefore the Mg-0 bond is the strongest one in the lattice. The breaking of these bonds is the limiting step for dissolving the spine]. The direction of the Mg-0 bond is similar to the direction of the C-C bond in a diamond lattice. The O-atoms form a tetrahedron around the central Mgatom. When the interaction is considered only between the nearest neighbours, then using Hartman-Perdok’s (ref. 16) or Honigmann’s”) notations respectively in a diamond-type lattice, (Ill) is an F- or AZ-plane, (110) is an S- or At-plane and (100) is an K- or A,plane. The (I 11) is the only equilibrium form plane, (I 10) and (100) are not equilibrium form planes. A lattice element can be removed out of a (lOO)-plane at any place by breaking two bonds. This binding energy corresponds to the “Halbkristallage”L8~‘9). An element in a (1 IO)-plane can only be removed at the very end of a chain by cracking two bonds, indicating the [I IO]-direction. Therefore the removal of atoms takes place row after row and should go slower than the removal of atoms in a (lOO)-plane. In the case of a (11 I)-plane the elements can be only disconnected from a corner and then one row after another can be removed, so that the following relationship should be true : “(100) ’

qllo)

’

V(lll).

OF

MAGNESIUM-ALUMINIUM-SPINEL

101

Acknowledgements The authors are indebted to Dr. R. Lacmann, FritzHaber-lnstitut fur physikalische Chemie (Max-PlanckGesellschaft), Berlin-Dahlem for the helpful theoretical discussions. We wish to thank Dr. G. Schneider for the X-ray fluorescence investigations. The Deutsche Forschungsgemeinschaft has supported this work with equipment and materials. Thanks are due to Miss Susan Oxford for translation.

References R. Heimann, W. Franke and A. Willgallis, Neues Jahrb. Mineral. Monatsh. (1969) 413. W. Franke and A. Willgallis, Neues Jahrb. 2) R. Heimann, Mineral. Mona&h. (1970) 74 3) G. A. Rankin and H. E. Met-win, Z. Anorg. Allgem. Chem. 96 (1916) 291. 4) F. Rinne, Neues Jahrb. Mineral. A Beil.-Bd. 58 (1928) 43. Z. Physik. Chem. B 29 (1935) 5) G. Hagg and G. Siiderholm, 88. and H. Jagodzinski, Z. Krist. 109 (1957) 87. 6) H. Saalfeld Z. Krist. 109 (1957) 388. 7) H. Jagodszinski, 8) H. Arnold, Z. Krist. 114 (1960) 23. K. M. Shmukler, B. Ya. Sukharevskii and 9) A. S. Frenkel, N. V. Gulko, Dokl. Akad. Nauk SSSR 130 (1960) 1095. Am. J. Sci. 251 10) D. M. Roy, R. Roy and E. F. Osborn, (1953) 337. 11) L. Navias, J. Am. Ceram. Sot. 44 ( 1961) 434. 12) D. M. Roy and R. Roy, Am. J. Sci. 255 (1957) 579. in und an festen Stoffen, 2. Auflage 13) K. Hauffe, Reaktionen (Springer, Berlin, 1966) P. 390. 14) W. Kleber, Neues Jahrb. Mineral. A Beil.-Bd. 65 (1932) 447. 62 (1958) 587. 1% W. Kleber, Z. Elektrochem. 16) P. Hartman and W. G. Perdok, Acta Cryst. 8 (1955) 49, 521, 525. Gleichgewichtsund Wachstumsformen con 17) B. Honigmann, Kristallen (Steinkopff, Darmstadt, 1958). IS) W. Kossel, Nachr. Ges. Wiss. Gottingen, Math.-Naturw. KI. (1927) 135. 19) I. N. Stranski, Z. Physik. Chem. 136 (1928) 259.