Clay Minerals (1982) 17, 271-283
REACTIONS
KAOLINITE
G. B A Y E R ,
OF A M M O N I U M
SULPHATES
AND OTHER SILICATE AND
MINERALS
G. K A H R *
WITH
OXIDE
AND M. M U E ( L L E R - V O N M O O S
*
Institute of Crystallography and Petrography and *Institute of Foundation Engineering and Soil Mechanics,
ETH-Zentrum, CH-8092 Ziirich, Switzerland
(Received I October 1981; revised 23 February 1982)
Interactions of ammonium sulphates and various silicate and oxide minerals
have been studied by X-ray heating methods and simultaneous TG-DTA. Kaolinite and other
clay minerals were found to be very susceptibleto these sulphating treatments, (NH4)3Al(SO4)3,
(NH4)AI(SO4)2 and A12(SO4)3 being formed successivelywith increasing temperature from 350
to ~550~ The same reaction products were obtained on heating mixtures of (NHn)zSO 4 and
y-AI203,AI(OH)3and bauxite. Fe-, Mg- and Ti-silicates and oxides gave NH4-doublesulphates
below 400~ and the correspondingmetal sulphates above this temperature.
A BSTR A CT :
INTRODUCTION
There is considerable interest in bauxite substitutes as sources of alumina for the
production of metallic aluminium (Ziegenbalg, 1979). The most promising alternatives so
far are clays, anorthosite and alunite. A technological comparison of six processes for the
production of reduction-grade alumina from such non-bauxitic raw materials has recently
been published by Bengtson (1979). In particular, the extraction of alumina from kaolinitic
clay by acid treatment (HC1, H2SO4,H2SO 3 or HNO3) appears to be technically feasible
and attractive, although it is not economical at present. The important point here is that the
rate of solution of clay in such acids is greatly increased by prior calcination. This
'activation' treatment is usually carried out at 650-800~ The abundance of clay, its high
Al-content, and the possibility of rejecting the unwanted silica and most of the impurities
without reagent consumption therefore make kaolinitic clay a preferred alternative raw
material for the acid extraction of alumina.
Apart from such direct leaching treatments, other processes have been devised where
kaolinite is mixed with alkali salts or lime, heated to the reaction temperature and then
leached. Heller-Kallai (1978) found that heating kaolinite and alkali salts between
460-600~ produced modified metakaolin phases incorporating alkali ions. At higher
temperatures, however, these Li, Na- or K-salts react further with kaolinite to give stable,
insoluble alkali-alumino-silicates such as eucryptite, carnegieite or kaliophilite, respectively.
The technically promising processes involve heating mixtures of clays and ammonium
salts, preferably ammonium sulphates (Grim, 1962; Ullmann, 1954), because water-soluble
aluminium salts are formed as the reaction products. These processes involve the following
steps: (i) sulphating the alumina of the clay by heating with (NH2)2SO 4 at ~400~ or with
(NH4)HSO 4 solution in an autoclave at 200~ (ii) extracting the Al-sulphates by leaching
with water and converting them to aluminium hydroxide by precipitation with ammonia,
9 1982The Mineralogical Society
272
G. Bayer et al.
(iii) calcining the aluminium hydroxide to alumina. In spite of the simplicity of this process,
there are certain problems related to the removal of impurities such as Fe 3+, Na + and K +,
and the filtration characteristics of the AI(OH) 3. Furthermore, the composition and stability
of the M-sulphate-containing reaction products depends strongly on the type of clay
(kaolinite or illite) and on the heating procedure. This may also be the reason for the
discrepancies in the literature with respect to the intermediate NH4-Al-sulphate compounds
formed. In a recent investigation by Badyoczek (1978) on the system kaolinite-ammonium
sulphate, only the formation of A12(SO4)3 was reported, whereas for the reaction between
iron oxides and ammonium sult~hate, intermediate double salts, e.g. (NH4)3Fe(SO4) 3 and
(NH4)Fe(SO4) 2, were found.
The object of the present paper is to clarify certain aspects of the reaction between
kaolinite and ammonium sulphate using thermoanalytical and X-ray methods. In
particular, the proposal made by Badyoczek (1978) that ammonium sulphate decomposes
at 300-400~ to molten (NH4)HSO4, which is the reactive component in the formation of
A1- and Fe-sulphates, is critically assessed. Details of the reactions of (NH4)zSO 4 with
other clay minerals and A1- or Fe-containing minerals are also presented.
EXPERIMENTAL
Materials
The kaolinite used was 'Supreme China Clay' supplied by English Clays Lovering
Pochin & Co. Ltd. The other materials were standard mineral samples from the collection
of the Institute for Crystallography and Petrography, ETH-Zfirich; most of these were
chemically analysed and checked for purity by X ray powder photography. The
ammonium sulphates (NH4)zSO 4 and (NH4)HSO 4 were of analytical grade. All materials
were used in finely powdered form (<20 gin). Mixtures were prepared by grinding under
acetone and evaporating to dryness.
Methods
Thermoanalytical investigations (TG and DTA) were carried out with a Mettler
Thermoanalyzer TA 1 in an oxidizing atmosphere. The heating rate was usually 1 ~
X-ray photographs of the starting materials and of the reaction products were taken with a
Guinier powder camera using Cu-Kccradiation. Reactions of the mixtures on heating were
studied with an Enraf Nonius Guinier-Lenn6 camera. The heating rate was 1 ~
Most
of the samples were leached with H20 after heating to various temperatures for further
investigations of the residue and of the evaporated extract.
RESULTS
Of primary interest in these investigations was the leachability of aluminium from
Al-containing silicates and oxides after heating these minerals with (NH4)2SO 4 at the lowest
temperature possible. The solid sulphate reaction products were separated from the
SiO2-rich residue by leaching with water. The alumina was recovered from these solutions
either by evaporation or by precipitation with ammonia, followed by heating to about
IO00~
Reactions of NH4-sulphates with silicates
273
FIG. 1. X-ray heating photograph of NH4HSO4 and (NH4)2SO4.
Experiments on the decomposition of (NH4)zSO 4 and its reaction with various silicates
and oxides were carried out in air--in open crucibles for D T A - T G (solid-state synthesis)
and in a Pt-wire mesh for heating X-ray studies. Some discrepancies in the data on melting
and decomposition of the various ammonium sulphates obtained from these two techniques
are due to differing experimental conditions. For example, the heating X-ray photographs
(Fig. 1) prove that (NH4)zSO 4 is transformed first to (NH4)3H(SO4) 2 above 170~ and
this, in turn, melts at ~240~ The heating X-ray pattern only shows the high-temperature
modification of (NH4)3H(SO4) 2. NH4HSO4, on the other hand, melts without decomposition at ~150~ Both sulphates decompose on further heating above 350~ (Fig. 2) to
N H 3, H20, SO z and probably 02 (N 2 according to Halstead (1970) and Nakamura
(1980)). The endothermie melting and decomposition peaks for (NH4)3H(SO4) 2 were at
235 and >330~ respectively, and at 150 and 340~ for NH4HSO 4.
The compound (NH4)3H(SO4) 2 was prepared by heating an equimolar mixture of
(NH4)2SO 4 and NH4HSO 4 at 180~ for 20 h. It was less hygroscopic than NH4HSO 4, but
still not as stable as (NH4)2SO 4. The phase transformation of (NH4)3H(SO4) z at 130~
far below the melting point, can be seen in the heating photograph (Fig. 3). Confusion with
respect to the decomposition steps of (NH4)2SO 4 (Suzuki, 1978) is due to this
polymorphism of (NH4)3H(SO4) 2 and to the similarity between the X-ray patterns of
high-(NH4)3H(SO4) 2 and (NH4)2S~07. In principle all three sulphates, i.e. (NH4)2SO 4,
NH4HSO 4 and (NHn)3H(SO4)2, could be used for sulphating reactions of AI-, Fe-, Mg-, Tiand Zr-containing minerals, because of their similar decomposition behaviour. The low
melting point of NH4HSO 4 would lower to some extent the starting temperature of the
reactions.
Reaction of kaolinite with ammonium sulphates
The thermal decomposition behaviour of kaolinite and other clay minerals is well known
and has been the subject of many investigations (Grim, 1963: Smykatz-Kloss, 1974).
Using DTA-TG, it can be shown that dehydroxylation of kaolinite occurs between
274
G. Bayer et
al.
~E
.2
o
(
i
i ~ --~--
\~ - - ~
e4
Reactions of NH4-sulphates with silicates
275
FIG. 3. X-ray heating photograph of (NH4)3H(SOa) 2.
500-700~
The resulting amorphous, finely dispersed mixture of A1203.2SIO 2
(metakaolin) is highly reactive and suitable for leaching the aluminium with acids. The
metakaolin phase also reacts easily with other materials, e.g. oxides or carbonates, leading
to the formation of aluminosilicates at higher temperatures. If alkali sulphates are used,
intermediate alkali-aluminium double sulphates are formed which may be leached out. Of
special interest here are the ammonium sulphates since not only the NH4-Al double
sulphates, but also their decomposition product, A12(SO4)3, are water-soluble.
The reaction between kaolinite and (NH4)2SO 4 may be characterized by the following
steps (Table 1), corresponding to formation of the compounds (NH4)3AI(SO4)a-,
(NH4)AI(SO4)EOA1E(SO4)s--,A12Ov The heating X-ray photograph (Fig. 4) proves that the
reaction starts immediately after melting of (NHa)3H(SO4) 2 at ~230~
with partial
formation of (NH4)3AI(SO4) 3. This compound has a very limited stability and decomposes
above 250~ to (NH4)AI(SO4) 2. This double sulphate changes gradually to A12(SO4)3,
which remains stable up to ~650~ when it decomposes to A120 s. Fig. 5 shows this
reaction sequence as recorded by simultaneous DTA-TG, the curves also recording the
melting (~230~
and decomposition (330-370~
of (NH4)3H(SO4) 2. The extended
stability range of (NH4)AI(SO4) 2 is of advantage for the synthesis of this phase by
isothermal heat treatment, Mixtures of 1 part kaolinite to 4 parts ( N H 4 ) 2 S O 4 heated to
340-380~ for 20 h show only the X-ray pattern of pure (NH4)AI(SO4) 2 and can be
leached with water. The residue is colloidal silica; the solution can be evaporated and
crystallizes as ammonia alum (Fig. 6). Heating kaolinite-(NH4)2SO 4 mixtures isothermally
between 420--600~ leads directly to the formation of A12(SO4)a. Fig. 7 shows more
clearly the thermal decomposition of synthetic NH4-Al-sulphates to A12(SO4)a. This X-ray
photograph was recorded at a slower heating rate of 0.5 ~
Heating a mixture of 1 part kaolinite to 6 parts N H 4 H S O 4 gave slightly different results.
(NH4)3H(SO4) 2 did not appear as an intermediate product. The reaction between kaolinite
and molten NH4HSO 4 starts just above 150~ and leads directly to NH4AI(SO4) 2 without
TABLE 1. Reaction of kaolinite with (NH4)2SO 4.
AI203. 2SiO 2. 2H20 § 6(NH4)2SO 4 = 2(NH4)3Al(SO4) 3 + 2SiO 2 + 6NH 3 + 5H20
kaolinite
2(NH4)3Al(SO4) a = 2(NH4)AI(SO4) 2 + 4NH 3 + 2H20 + 2SO 2 + 02
2(NH4)AI(SO4) 2 = A12(SO4)3 + 2NH 3 + H20 + SO 2 + 89
A12(SO4) 3 = A1203 + 3SO z + 1 . 5 0 2
276
G. Bayer et al.
FIG. 4. X-ray heating photograph of a mixtureof 1 part kaolinite + 6 parts (NH4)2SO 4.
formation of the 3:1 double sulphate. The decomposition of the NH4-Al-sulphate to
A12(SO4) 3 also occurs here at ~430~
Therefore, from practical considerations it would
be more economical to use ratios of 1 part kaolinite to 4 parts (NH4)2SO 4 or 4 p a r t s
NH4HSO 4 if NH4AI(SO4) 2 was the desired leachable phase, or a ratio 1 : 3 if A12(504) 3 was
required. The reaction temperatures for the isothermal heat treatment would be ~380~
and ~500~ respectively.
Reaction of various sheet silicates and aluminosilieates with ammonium sulphates
Since kaolinite showed pronounced reactivity with NH4-sulphates, other sheet silicates
were also investigated with respect to their reaction behaviour. Chrysotile was mixed with
(NH4)2SO 4 (molar ratio I : 12) and heated to 800~ Both D T A - T G curves and the X-ray
heating photograph (Fig. 8) proved that in this case also at least two reaction steps may be
distinguished. After decomposition of (NH4)2SO 4 to (NH4)3H(SO4)2, the reaction starts
during melting of the latter at ~230 ~ C, leading to the formation of (NH4)2Mg2(SO4) 3. This
double sulphate remains stable on further heating to ~340~
Above this temperature it
decomposes to MgSO4 which is stable to at least 600~ Almost identical behaviour was
shown by mixtures of sepiolite and ammonium sulphate. The only differences were that the
formation of (NH4)2Mg2(SO4) 3 had already started at ~210~ and that the products
Reactions of NH4-sulphates with silicates
277
1 KAOLIN. 'L!NN,~ so,
100 mg
800~
t
600
500
DTA
t
lOmg
t
TG
DIG
FIG. 5. DTA-TG curves of a mixture of 1 part kaolinite + 4 parts (NH4)2SO 4.
FIG. 6. X-ray powder patterns of NH4AI(SO4)2 a n d of its hydration and decomposition
products.
FIG. 7. X-ray heating photograph of NH4-Al-sulphates.
278
G. Bayer et al.
Fx6. 8. X-rayheating photographof a mixtureof 1 part chrysotile+ 12 parts (NH4)2SO 4.
formed had a somewhat poorer crystallinity, as deduced from their slightly broadened
X-ray reflections.
Talc showed practically no reaction with (NH4)2SO 4 during heating. This is probably
due to the high thermal stability of this mineral, since it does not decompose below 800~
The isostructural Al-mineral, pyrophyllite, which is intermediate in thermal stability
between kaolinite and talc, only reacted to a small extent with (NH4)2SO 4 during heating.
Only traces of the NHa-Al-sulphate and of A12(SO4)3 could be detected in the X-ray
heating pattern.
Reactions between illite and ammonium sulphates on heating were similar to those of
pure kaolinite, except that the rather unstable phase (NH4)3AI(SO4) 3 was not observed for
illite. Montmorillonite showed a much weaker reaction with ammonium sulphates, amounts
of (NH4)AI(SO4) 2 and of A12(SO4)3 formed during heating being much smaller. This may
be due to the more complex chemical composition of montmorillonite, especially the
presence of Na + ions which may affect the decomposition temperature of the
NH4-sulphates. It should be emphasized that the SiO2 formed by decomposition of these
sheet silicates and also of other silicates during the sulphating reactions remains
amorphous at least up to the highest temperatures used in these experiments (900~
Hectorite showed the expected reaction to (NH4)zMg2(SO4) 3 and then MgSO 4 when
heated with (NH4)2SO 4. Vermiculite, on the other hand, showed the formation of both
NHa-A1 and NHn-Mg double sulphates with corresponding decomposition. This was also
Reactions of NH4-sulphates with silicates
279
observed on heating chlorite (penninite) with ammonium sulphate, except that the
Mg-sulphate phase was present in much smaller amounts compared to the Al-sulphate
phase. Micas, e.g. muscovite, biotite and phlogopite, did not show any pronounced
reactivity with (NH4)2SO 4. Sulphating reactions did occur, however, to some extent after
prolonged isothermal heating between 300--400 ~C.
Aluminosilicates, especially feldspars, did not react readily on heating with ammonium
sulphate. Of the three polymorphs of AI2SiOs, only sillimanite reacted with (NH4)2SO 4,
for instance after isothermal treatment for 50 h at 400~ (when NH4AI(SO4) 2 was formed)
and at 500~ (when A12(SO4)3 was the reaction product). Mg-silicates, e.g. forsterite and
enstatite, also reacted with ammonium sulphates. The SiO 2 released during these various
sulphating reactions, which was separated by leaching out the sulphates, was again very
fine grained and amorphous. It could be recrystallized to cristobalite on heating above
1150~
Reaction of Al- and Fe-containing oxide minerals with ammonium sulphates
This investigation and results reported in the literature (e.g. Semin et al., 1966; Badyoczek,
1978; Nakamura et al., 1980) proved that A1- and Fe-containing minerals, especially sheet
silicates, react strongly with (NH4)-sulphates when heated between 250-600~
It was
considered of interest, therefore, to study also the reaction behaviour of non-silicates, e.g.
A1- and Fe-containing oxide and hydroxide minerals. As expected, a~A1203 did not react
with (NH4)2SO4, whereas ~A1203 reacted easily, forming the same sequence of sulphate
phases as kaolinite. However, (NH4)3AI(SO4) 3 showed a much increased stability region
(220-280~
changing to (NH4)AI(SO4) 2 between 260-420~
This decomposed to
A12(SO4)3 (stable from 420-640~
which, in turn, decomposed to A1203 above 650~
The corresponding reaction steps were found also for a~Fe203 (hematite) but these
occurred at lower temperatures, e.g. (NH4)3Fe(SO4) 3 (210-280~
(NH4)Fe(SO4) 2
(240-370~
Fe2(SO4) 3 (340-550~ and a-Fe20 a (>550~
A comparison of the reaction behaviour of y-A1203 and of ff-Fe203 with (NH4)ESO 4 is
shown in Fig. 9. Both thermoanalytical and X-ray data were obtained on 1:4 molar
mixtures of the oxide with (NHa)ESO 4. Ilmenite showed the same reaction steps as the iron
oxide, these occurring at slightly lower temperatures (Fig. 10). The TiOE-Component of this
mineral crystallized to anatase on heating to 800~ Pure TiO 2 (anatase), on the other
hand, reacted completely with (NH4)2SO 4 on heating to 900~ The final product was a
mixture of anatase and rutile formed by decomposition of the intermediate NH aTi-sulphates. This reaction requires further investigation, since at least two new Ti-sulphate
phases were found.
Aluminium hydroxide (gibbsite) reacted similarly with (NH4)2SO 4 (molar ratio 1:3) as
~-AI203. However, the amount and the stability region of (NHa)aAI(SO4) 3 was very small,
whereas the primary decomposition product of AI(OHa), namely boehmite (?-A1OOH),
was stable over the range from about 200-500~
There was some overlapping with
(NH4)AI(SO4) 2 (240-420~ and with A12(SO4)3 (400-620~
For comparison, one part
of bauxite (from Gove, Australia) was mixed with 2 parts of (NH4)2SO 4 and heated. The
intermediate formation of y-A1OOH was not observed in this case, but a very similar
reaction behaviour to ?-A1203 was observed, i.e. (NH4)3AI(SO4)3~(NHa)AI(SO4)2 ~
A12(SO4)3--,AI203. Finally, sulphating reactions were investigated for alunite, the natural
hydroxide sulphate, which has been considered as a possible substitute for bauxite. Again
280
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<
o"
s.
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E
[-
Reactions o f NH4-sulphates with silicates
281
FIG. 10. X-ray heating photograph of a mixtureof 1 part ilmenite + 4 parts (NH4)2SO4.
the reaction with (NH4)2SO 4 led to the formation of the NH4-Al-sulphates and to A12(SO4) 3
at higher temperature. In addition to (NH4)AI(SO4) 2, KAI(SO4) z was also formed and this
latter phase remained stable up to ~640~ at the expense of A12(SO4) 3 which was present
in small amounts. KAI(SO4) 2 could he leached out with water in a similar way to
NH4AI(SO4) 2 and formed the corresponding alum by crystallization of the aqueous
solution.
DISCUSSION
The melting and decomposition behaviour of (NH4)zSO 4 and of NH4HSO 4 were
investigated since conflicting data are given in the literature. According to the most recent
studies by Nakamura et al. (1980), the heating procedure and atmosphere determine the
course of the decomposition. These authors found that heating (NH4)2SO 4 at a rate of
10~
leads to the formation of (NH4)3H(SO4)2, NHaHSO 4 and (NH4)2S207 as
intermediate products, and to the gaseous compounds NH3, SO 2 and H20 above 400~ If
(NH4)2SO 4 is mixed with kaolin, A1203 or Fe203 (Grim, 1962; Semin et al., 1966;
Badyoczek, 1978; Nakamura et al., 1980) sulphating reactions take place at 350-550~
which transform the A1203 or Fe203 to water-soluble sulphates. In such reactions it is
therefore important to know the intermediate reaction steps which are governed by the
282
G. B a y e r et al.
ratio (NH4)2SO 4 to silicate or oxide mineral, by the heating rate and time, and by the
process technology (closed or open reaction vessel, fluidized bed etc.). All our experiments
proved however that (NH4)2SO4, either pure or in mixtures with various silicates, does not
decompose to (NH4)HSO 4 but rather to (NH4)aH(SO4)2 . Ammonium hydrogen sulphate
itself melts at 147~
In his studies of the reaction between kaolinite and (NH4)2SO 4, Badyoczek (1978) was
mainly concerned with A12(SO4) a formation and its effect on mullitization and there is no
reference to any of the intermediate NHa-Al-sulphates reported by other authors, e.g.
Semin et al. (1966); their stability regions have been described in the present investigations.
On the other hand, Badyoczek (1978) did find such NH4-double sulphates during the
reaction of Fe20 3 with (NH4)2SO 4. A detailed investigation of this reaction has also been
carried out recently by Nakamura et al. (1980).
CONCLUSIONS
Kaolinite and other clay minerals can be transformed either to NHa-Al-sulphates or to
A12(SO4) 3 by heating with ammonium sulphates in the temperature range 300-500~
Thermoanalytical and X-ray investigations on the thermal stability of (NH4)2SO 4 and of
NH4HSO 4 proved that either (NH4)3H(SO4) 2 or NH4HSO 4 melts are involved in these
reactions. These melts decompose gradually to N H 3, SO 2, H20 and 0 2 at temperatures
above 300~
The SiO 2 liberated in the sulphating reactions of various silicates, which
can be separated after leaching out the sulphates, is very fine grained and amorphous. It
recrystallizes to cristobalite only after heating above 1150~
Feldspars and micas are
resistant to such treatment; only siltimanite reacted with (NH4)2SO 4. Many Fe-, Mg- and
Ti-containing silicate and oxide minerals react with ammonium sulphates.
Some main features of these combined chemical-thermal treatments are the following:
1. The reaction mixtures can be prepared in the dry state.
2. No previous thermal activation treatment is necessary for clay minerals and other
sheet silicates.
3. Sulphating by decomposition of ammonium sulphates leads to solid reaction products
which can be leached with water.
4. The kind of sulphate reaction product depends on the ammonium sulphate addition
and on the temperature. Either NH4-double sulphates of A1, Fe, Mg and Ti are formed
below 400 ~ C or the corresponding metal sulphates above this temperature.
5. After leaching, the metal hydroxides can be precipitated from solution with ammonia
and the metal oxides recovered by calcination.
REFERENCES
BADYOCZBKH. (1978) Phasenumwandlungen von Kaolinmineralien unter dem Einfluss einer W~irmebehandlung mit Ammoniumsulfat.~Sprechsaa1565-566.
BENGTSONK.B. (1979) A technological comparison of six processes for the production of reduction-grade
alumina from non-bauxiticraw materials.Light Metals 1,217-282.
GRIMR.E. (1962) Applied Clay Mineralogy, pp. 335-345. McGraw-Hill,New York.
HALSTEADW.D. (1970) Thermal decompositionof ammonium sulphate,J. appl. Chem. 129-132.
HELLER-KALLAIL. (1978) Reactions of salts with kaolinite at elevatedtemperatures I. Clay Miner. 221-235.
NAKAMURAH., HARA Y. & OSADAH. (1980) The thermal decomposition of ammonium sulfate and its
reaction with iron(IlI)-oxide,Nippon Kagaku Kaishi 706-710.
Reactions o f NH4-sulphates with silicates
283
SEMn~ E.G., DrMITPaEVI.A., ZA1NULrNY.G. & SrAR1KOVV.M. (1966) Reaction of aluminium and iron oxides
with ammonium sulfate, Tr. Ural Politekh. Inst. 148, 11-19 (Chem. Abstr. 68 (1966), 108292 g)
SMYKATZ-KLOSSW. (1974) Differential Thermal Analysis, pp. 64-87. Springer-Verlag, Berlin, Heidelberg &
New York.
SuzuKi S. & MAgrrA Y. (1978) The crystal structure of triammonium hydrogen disulphate, (NH4)3H(SO4)2,
Acta Cryst. B34, 732-735.
ULLMA~ B. (1954) Encyklopiidie der teehnischen Chemie Bd. 3, 401-420. 3. Auttage, Urban &
Schwarzenberg, Miinchen & Berlin.
ZIt~G~NBALGS. (1979) Tonerdegewinnung aus nichtbauxitischen Rohstoffen. Neue Hiitte 44-50.
R E S U M E : On a &udi6 l'interaction de sulfate d'ammonium et de divers mintraux, silicates,
oxydes, lors de chauffage simultantment par diffraction des RX, par TG et ATD. La kaolinite et
les autres mintraux argileux se rtvtlent tr~s susceptibles / t c e s traitements sulfates:
(NH4)3AI(SO4)3, (NH4)AI(SO4) 2 et Ale(SO,) 3 se forment successivement lorsque la temptrature crok de 350 ~ ~ 550~ On obtient les m~mes produits de rtaction par chauffage de
mtlanges de (NH4)2SO 4 et 7-A1203, AI(OH) 3 et bauxite. Les oxydes et silicates de fer, Fe, Mg,
Ti, donnent des sulfates doubles (NH4) en dessous de 400~ et les sulfates m&alliques
correspondants au-dessus de cette temptrature.
K U R Z R E F E R A T : Durch Rtntgenbeugungsuntersuchungen nach Erhitzung sowie simultane TGA-DTA wurde nachgewiesen, dal3 Kaolinit und andere Tonminerale sehr empfindlich
auf die Behandlung mit Ammoniumsulfat reagieren. Dabei bildeten sich bei steigender
Temperatur im Bereich zwischen 350 and 550~ nacheinander (NH4)3Al(SO4)3, NH4AI(SO4)2
und A12(SO4)3. Die gleichen Reaktionsprodukte entstanden bei der Erhitzung von Gemischen
aus (NH4)2SO4 und ?-A12Oa, AI(OH) 3 sowie Bauxit. Fe-, Mg- and Ti-silicate ergaben unterhalb
yon 400~ Doppelsulfate und oberhalb dieser Temperatur die entsprechenden einfachen Sulfate.
R E S U M E N : Se han estudiado las interacciones del sulfato am6nico con varios silicatos y
6xidos mediante las ttcnicas de difracci6n de rayos X, termogravimetria y ATD. La caolinita y
otros minerales de la arcilla son muy sensibles a estos tratamientos, form/mdose sucesivamente
(SO4)3Al(NH4)3, (SO4)2AI(NH4) y (SO4)3A12 a media que la temperatura aumenta de 350 fi
~550~ Los mismos productos de reacci6n se obtienen calentando mezclas de SO4(NH4) 2 y
~,-A1203, AI(OH)3 y bauxita. Los silicatos de Fe, Mg y Ti asi como sus 6xidos dan lugar a
sulfatos dobles de amonio por debajo de 400 oC y a los correspondientes sulfatos de los metales a
temperaturas superiores.