Technology
STUDYING PROCESSES OF
HIGH-TEMPERATURE DIGESTION OF
SUBR AND TIMAN BAUXITES
High-temperature digestion (HTD) of monohydrate bauxites is
currently becoming the prevailing tendency in Russia in the
reconstruction of existing and creation of new alumina refineries
T.S. LYAPTSEVA based on the Bayer technology. The advantages of the high-
/ Engineer, temperature digestion process (HTD process) are obvious. They
Uralaluminium include: high speed of the process and increased extraction of
OJSC, Kamensk- bauxite alumina, as well as significant reduction of the impact of
Uralsky recycled liquor concentration if the temperature is increased.
The main type of alumina-containing raw material at
Bogoslovsk and Urals aluminium smelters are bauxites from
V.S. ANASHKIN SUBR (North Ural Bauxite Mine) and Timan (Bezhayu-
/ Ph.D. in Technical Borykvinskoye Bauxite Mine) deposits. With the lapse of time,
Science, Senior the quality characteristics of the used bauxites change. Thus, for
Research Officer, proper application of the HTD process at Urals plants, it was
Uralaluminium necessary to specify the process parameters for bauxites in
OJSC, Kamensk- current shipments and forecast bauxites.
Uralsky We have conducted laboratory studies of the HTD process for
bauxites from the SUBR and Timan deposits. During these, we
determined the major process parameters of this process,
E.S. FOMIN demonstrated the performance of bauxite impurity components at
/ Managing high (260-265°C) digestion temperatures. We made a special
Director, focus on the issue of digesting Timan bauxites with a high
Uralaluminium content of chamoisite, which degrades the performance indicators
OJSC, Kamensk- of alumina refining from this type of raw material.
Uralsky For studying these bauxites, the temperature of 260°C was
selected. Further increase of temperature is not practicable due to
the fact that higher supersaturation of liquor occurs at higher
temperatures, which leads to increased alumina loss in subsequent
I.A. KUZNETSOV desilication and thickening within the flow diagrams existing at
/ Junior Research the plants.
Officer, The chemical properties, structure and mineralogy of bauxite
Uralaluminium have a considerable impact on bauxite extraction (process
OJSC, Kamensk- efficiency) and alkali loss with waste red mud.
Uralsky SUBR bauxite has the following mineralogy characteristics:
• main alumina-containing mineral: diaspore
characterised by the lowest solubility (leachability);
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………………………………………...…. • silica-containing minerals: kaolin and chamoisite with
prevailing content of the first component;
• significant content of impurity components: carbonates
(calcite and siderite) and sulphur sulfide (pyrite).
Middle-Timan Bauxite Mine (STBR) bauxite has the following
mineralogical characteristics:
• main alumina-containing mineral: boehmite;
• silica-containing minerals: chamoisite and kaolin with
prevailing content of the first component;
• insignificant content of carbonates (calcite) with almost
zero sulphur content.
It was determined that during processing of STBR bauxites in the
HTD process, the optimal fineness is -0.315 mm – 100%, whereas
for SUBR bauxites, finer grinding is required (-0.1 mm – 100%).
This is related to the differences in the bauxite structure, which is
……………………………………………
mainly oolitic coarse powder for SUBR diaspore and polymorphic
boehmite for STBR bauxite.
We determined equilibrium caustic modules of aluminate liquor
…………………………………………… in high-temperaure digestion (260°C) of SUBR and STBR bauxites
with recycled liquor, whose caustic alkali concentration is C = 200-
220 g/dm3. For STBR bauxites, the caustic module of liquors
obtained (αk) was 1.3-1.32. If the amount of ferrous iron was
increased, the equilibrium module of liquor increased up to 1.33. In
HTD of SUBR bauxite, the values of the equilibrium caustic module
(αkequil) reached 1.37. The determination of the maximum alumina
extraction allows us make the conclusion that conducting high-
temperature autoclave digestion allows almost all alumina contained
in SUBR bauxite to be extracted.
Studying high-temperature digestion of SUBR bauxite
demonstrated (see Table 1) that for efficient extraction of alumina
from it, it requires quite concentrated recycled liquor (CNazOk ~ 220
g/l – caustic alkali concentration). This is related to the fact that
increasing CNazOk in the digestion area increases the intensity and
…………………………………………… depth of the calcite decomposition process CaCO3 Æ CaOactive,
which requires input of a less activating calcium-containing additive
from the outside. The lime dosing should be at least 1% to the
bauxite weight, since if there is not additional input of calcium
……………………………………………
oxide, alumina extraction decreases rapidly by ~5%.
It was noted that the HTD process for SUBR bauxite, compared
to the similar processing of Timan bauxites ensures low content
(loss) of alkali in waste red mud: The ratio of Na2O:SiO2 is 0.24
units with CaO dosing = 1.0% by weight and 0.18 units with CaO
dosing = 2.0%, respectively, while in the HTD process of Timan
bauxite, the value of Na2O:SiO2 is twice higher and averages 0.45-
0.50 units.
Using thermogravimetric and X-ray-structural analysis
techniques, we studied the performance of silica minerals
(chamoisite and kaolin) in the temperature range of 100-280°C and
determined the degree of their decomposition, i.e. the content of
reactive silica in SUBR and STBR bauxites, and also studied the
performance of titanium and temperature intervals of perofskite
…………………………………………… generation for SUBR and STBR bauxites.
It is interesting that aluminate-alkali liquor after the HTD process
for SUBR bauxite has a lot higher silica module (μSi) than the same
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liquor after the HTD process for STRB bauxite. μSi values are 280-
300 and 150-180, respectively. This is related, firstly, to a high
content
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Table 1
Impact of process parameters on alumina extraction and sodium oxide loss with red mud
during high-temperature digestion of SUBR bauxite
Parameter Other
3
measured process Analysis of liquid phase, g/dm Analysis of solid phase, mass % Extraction of Al2O3 Ratios in red mud
parameters
Na2Ok Al2O3 Αk SiO2 ΜSi Al2O3 Fe2O3 SiO2 CaO Na2O η1 chemical η2 (η1 / η2 )- CaO Al2O3 Na2O
theoretical 100 SiO2 SiO2 SiO2
Post- t = 260°C,
desilication τ = 1 hour,
slurry dosing CNa2Ok = 200
3
module g/dm ,
CaOactive =
1.40 2.0 wt % 187.5 219.1 1.41 0.59 369.8 10.5 46.3 6.47 14.7 1.12 90.72 93.92 96.59 2.27 1.62 0.173
1.35 184.2 214.2 1.41 0.68 315.0 15.6 43.9 6.25 14.0 1.17 85.46 93.92 91.0 2.24 2.50 0.19
1.30 184.5 211.7 1.43 0.60 306.8 20.5 40.3 5.69 14.2 1.08 79.19 93.92 84.20 2.50 3.60 0.19
Lime dosing, t = 260°C,
% τ = 1 hour,
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CNa2Ok = 190
3
g/dm ,
0.0 αk – 1.40 175.1 187.0 1.54 0.81 216.2 16.45 42.75 6.50 9.7 1.95 84.50 93.92 0.900 1.49 2.53 0.30
1.0 174.2 194.9 1.47 0.70 278.4 14.61 43.27 6.53 12.4 1.56 86.43 93.92 0.920 1.90 2.24 0.24
2.0 173.7 198.4 1.44 0.62 320.0 12.69 45.06 6.71 14.6 1.20 88.70 93.92 0.944 2.18 1.89 0.18
Alkali t = 260°C,
concentration τ = 1 hour,
dos
in recycled αk – 1.40,
liquor, g/l CaOactive –
180.0 1.0 wt % 164.8 172.7 1.57 0.60 288.0 18.7 41.8 6.15 11.4 1.67 82.04 93.92 0.874 1.85 3.04 0.272
202.0 184.2 211.9 1.43 0.68 315.0 13.4 46.4 6.63 12.8 1.72 88.84 93.92 0.946 1.93 2.02 0.260
220.0 198.1 238.8 1.40 0.84 284.3 10.0 48.5 6.79 13.1 1.69 91.70 93.92 0.976 1.93 1.473 0.249
Duration of t = 260°C,
digestion, min. CNa2Ok = 220
3
g/dm ,
CaOactive –
30 1.0 wt %, 198.6 230.1 1.42 1.02 225.5 11.0 46.9 6.56 12.9 1.61 90.60 93.92 0.965 1.97 1.682 0.245
dos
45 αk – 1.40 198.0 231.0 1.41 0.93 248.4 10.6 47.9 6.64 13.1 1.64 90.94 93.92 0.968 1.97 1.600 0.247
60 198.1 238.8 1.40 0.84 284.3 10.0 48.5 6.79 13.1 1.69 91.70 93.92 0.976 1.93 1.473 0.249
transfer
sulphur.
Thus,
of
1.40;
aluminate-alkali
• bauxite
g/dm3);
weight;
aluminium hydrate.
Technology
at least 70%).
sufficient and necessary:
sulphur
higher liquor desilication.
process: 1 hour;
• post-desilication
• temperature: 260°C;
Due to the fact that at present,
The following HTD process
the HTD of SUBR bauxite,
CaOactive: 1.0% by
conditions for SUBR bauxite are
high-temperature
(degree of sulphur transfer to
content of 0.1 mm
even more advantageous for
liquor is about 10%) will
fineness:
digestion area ~ 200
• concentration of Na2-
It was determined that during
to sulphur impurity present in
of kaolin in SUBR bauxite,
slurry dosing module:
and low alkali loss with waste
digestion of SUBR bauxite
promote the reduced transfer of
red mud (Na2O:SiO2 ~ 0.25), to
chemical alumina extraction ≥
allows, with high degree of
SUBR bauxite, which leads to
(60-90 μm category –
91% (>97% of theoretical value)
which easily transforms into
50-55% during digestion. It was
significantly increases – up to
into
• duration of digestion
desilication products; secondly,
• additional dosing of
of raw slurry, notably, 4 hours
found that prolonged pre-soaking
~ 220 g/dm3 (in the
Ok in digestion liquor
obtaining coarse-grained (sandy)
obtain aluminate liquor with
liquor
grain content – 100%
caustic module 1.40, which is
Technology
37
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Table 2
………………………………….. Studying process parameters during digestion of STBR
bauxite with a varying content of chamoisite
The chemical properties,
structure and mineralogy of Bauxite Variable αk of Alumina Alkali loss
sample No. parameters aluminium extraction with red
bauxite have a significant silicate (to mud,
influence on alumina extraction liquor theoretical) Na2O
,% SiO2
and alkali loss with waste red Digestion STBR (9% of 250 1.36 97.0 0.48
temperature,°C chamoisite)
mud. 260 1.35 97.6 0.47
STBR (13% of 250 1.42 95.8 0.47
chamoisite) 260 1.37 96.3 0.48
………………………………….. Dosing of STBR (9% of 0 1.38 96.5 0.48
CaOactive, % to chamoisite) 1 1.35 97.6 0.47
bauxite weight 2 1.35 97.1 0.46
STBR (13% of 3 1.36 96.2 0.41
chamoisite) 0 1.39 95.6 0.51
1 1.37 96.3 0.48
2 1.35 96.4 0.46
3 1.38 95.0 0.46
Concentration STBR (9% of 165 1.38 97.1 0.47
of CNa2Ok in chamoisite)
recycled liquor, 180 1.35 97.3 0.47
g/dm3 200 1.34 97.6 0.46
STBR (13% of 165 1.45 92.3 0.51
chamoisite)
180 1.38 96.1 0.50
200 1.34 97.0 0.51
Duration of STBR (9% of 30 1.37 97.0 0.45
digestion, min. chamoisite)
45 1.34 97.5 0.47
60 1.35 97.6 0.47
STBR (13% of 30 1.42 92.7 0.46
chamoisite)
45 1.39 94.7 0.44
60 1.37 96.3 0.48
the silica module in STBR bauxite is gradually alumina loss. Therefore, it is necessary to
decreasing from μSi = ~ 9.0 to μSi = ~ 7.5 due to increased perform further research into the issue of
content of the main silica-containing mineral chamoisite, chamoisite decomposition in production.
the process parameters of the HTD process for this Laboratory research of high-temperature
bauxite were specified depending on the content of digestion of SUBR and Timan bauxite was
chamoisite therein. performed. Process parameters were
Table 2 presents the data on the impact of variable determined for the optimal processing of this
parameters on alumina extraction and alkali loss with raw material. Positive and negative sides of
cryolite mud during hydro-chemical processing of STBR the digestion process at 260°C were singled
bauxite with a varying chamoisite content. Changing one out. These studies can be used for
of the process parameters, the other parameters were modernisation and construction of new
adopted as follows: alumina refining capacities of UC RUSAL.
• duration of process: 60 min; Photo by: I.A. Kuznetsov
• synthetic recycled liquor with CNa2Ok ~180 ……………………………………………….
g/dm3;
• caustic module of recycled liquor αkr.l. = 3.15;
• dosing caustic module αkdos. = 1.35;
• 1% addition of CaOactive+ to the bauxite
weight;
• process temperature: 260°C.
As one can see from Table 1, a high content of
chamoisite requires more rigid digestion conditions. At
the same time, it was determined that the content of
Al2O3 in chamoisite was ~15%, and in sodium
hydroaluminium silicate formed during chamoisite
decomposition, its content is ~34%. Thus, it is obvious ……………………………………………….
that full decomposition of chamoisite will lead to high
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Technology
RUSAL Technical & Economic Bulletin No. 18
