Note: Descriptions are shown in the official language in which they were submitted.
2002172 7875/0~100
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METHOD FOR BAUXITE TREATMENT
TECHNICAL FI~LD
This invention relates to a method for reducing the
amount of organic carbon substances present in Bauxite prior to
dissolving the bauxite in a Bayer process caustic aluminate
liquor, while also reducing the conversion to sodium oxalate of
the remaining organic contaminants. The method also converts
any gibbsite present in the bauxite to a more soluble form.
B~CKGROUND
In the Bayer process for the production of alumina, a
1~ bauxite ore is contacted with recycled causSic aluminate liquor
at elevated temperatures and pressures to extract the alumina
content. An undissolved "red mudn residue, consisting
primarily of iron oxides, such as goethite ~alpha-FeOOH) and
hematite (alpha-Fe203)~, are first separated, such as by
filtration, with aluminum hydroxide precipitated by cooling the
remaining caustic aluminate liquor. A part of the aluminum
hydroxide precipitate is recycled to act as seed in subsequent
precipitation steps, with the remainder recovered as product.
The spent caustic aluminate liquor is recycled in the process
for further alumina recovery from fresh bauxite.
Approx. 85% o~ the western world's bauxite supplies are
located in tropical regions (IBA Review - Sept.-Dec., 1987).
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Z002172
~lumina occurs in tropical bauxites m~inly in the forms
gibbsite (alpha-aluminum trih~droxide) and boehmite (alpha-
aluminum monohydrox~de). Both forms di~fer considerably in
their solubility behavior in hot caustic aluminate liquor. The
predominant alumina form is gibbsite, which is usually
dissolved at temperatures of about 140-150-C and pressures of
about 6-7 bar. Boehmite, on the other hand, is much less
soluble in caustic aluminate liquors than gibbsite, requiring
higher extraction temperatures (240-280-C) and pressures (35-50
bar).
If a bauxite ore contains greater than 1.5-2% by weight
boehmite, it is generally considered economically desirable to
warrant the extra cost of recovering the boehmite. In other
words, the cost for extracting boehmite at the higher tempera-
ture is justified by the additional alumina recovery.
The technical and economic problems of the Bayerprocess however go beyond the initial choice as to the most
appropriate temperature/pressure conditions for bauxite
extraction. A major problem of the Bayer process is the
contamination of the caustic aluminate liquor that occurs from
the dissolution and accumulation of organic carbon substances
derived from the starting bauxite.
Bauxites recovered from tropical regions contain
organic carbon substances generally within the range 0.1-0.6
by weight. These organics usually dissolve during the
extraction step of the process, and accumulate, leading to
steady-state concentrations of about 3-30 g/l in the caustic
aluminate liquor. The disadvantages of dissolved organic
contaminants on the operation of the Bayer process are well-
known. These include a decreased settling rate of the 'redmud', foaming of the liquor, and organic carbonation which
leads to a loss of caustic due to formation of sodium car-
bonate.
Under the influence of the high caustic concentration
3S and elevated temperatures encountered during bauxite extrac-
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200217Z
tion, the ~issolved orgAnic carbon substances degrade to lower
molecular weight compounds. Thus, the organic carbon compounds
in the caustic aluminate liquor vary from high molecular weight
humic-type organics (e.g., humus, soil, plant elements, etc.)
to the ultimate degradation products of such organics, for
example, sodium oxalate and sodium carbonate; see, K. Yamada,
T. Harato and }~. Kato, nOxidation of Organic Substances in the
Bayer Process". Liqht Metals Conf. Proc., February, 1981.
Sodium oxalate presents a special problem. Approx-
imately 3-20% of the organic carbon in the starting bauxite is
converted to sodium oxalate during bauxite extraction, with
sodium oxalate being the only degradation product which
accumulates to a concentration exceeding its solubility in
solution. Difficulties arise during product precipitation, as
the dissolved sodium oxalate crystallizes at the temperatures
and caustic concentrations which occur in the product aluminum
trihydroxide precipitation circuit. The crystalline sodium
oxalate may interfere with the agglomeration mechanism for
particle size enlargement of the product Al(OH)3, stimulating
instead the formation of fine new crystals of Al(OH)3. The
presence of crystallized sodium oxalate therefore has a
deleterious effect on the particle size of the product Al(OH)3.
It has long been recognized that an effective way of
solving the organics problem in general would be to preheat the
bauxite to a temperature high enough to thermally eliminate the
organic carbon substances before bauxite extraction, that is,
destroy the organics before they enter the process. Elimina-
tion of the organics in tropical bauxite, however, requires
heating to temperatures of at least 500-C; see, for example,
30 T.G. Pearson, "The Chemical Backaround of the Aluminum In-
dustrY", The Royal Institute of Chemistry, London, 1955.
Unfortunately, such thermal conditions convert gibbsite
to boehmite and other alumina species, such as gamma-alumina,
which are less soluble and dissolve more slowly in caustic
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2002~7~
aluminate liquor than gibbsite; see, for example, ~ussell,
~wards and Taylor, J. of M~tals, pp. 1123-1128, October 1955.
Consequently, thermal pretreatment as a means of
eliminating organic substances in bauxite has generally been
c:onsidered unattractive on technical and economic grounds.
Modern practice of the Bayer process recognizes,
however, that it is neither a practical proposition nor a
clesirable technical objective to aim for a complete elimination
of organic carbon substances from caustic aluminate liquors.
The presence of relatively small concentrations of organics in
p~c ~ the baux-i*~ (3-6 g~l, for example) can
~ ~t stabilize caustic aluminate liquors against premature
z~ 8 precipitation of the Al(OH)3 between the extraction and
precipitation steps of the process; and
stabilize the particle size distribution of product
Al~OH)3 against excessive formation of new crystals via
the secondary nucleation mechanism. See, for example,
N. Brown, ~Kinetics and Mechanism of Secondary
Nucleation", Liaht Metàls Conf. Proc., Feb. 1977.
Thus, what is needed in the art is a method for
lowering the amount of organic carbon substances in bauxite,
obtaining a disproportionately large reduction i~ the sodium
oxalate generating ability of the caustic aluminate liquors,
without converting gibbsite to less soluble forms of alumina.
S~MMARY OF THE INVENTION
It is an object of the present invention to provide a
method for reducing the amount of organic carbon substances in
bauxite.
It is another object of the present invention to reduce
the amount of organic substances in bauxite without converting
gibbsite to less soluble forms of alumina.
It is another object of the present invention to reduce
the amount of organic substances in the caustic aluminate
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200217Z
liquor o th~ ~ayer proc~ss, and to do so in an economic
fashion.
Another object of the invention is to convert any
gibbsite present to a more soluble form.
These and other objects of the present invention are
achieved by utilizing a process comprising:
heating the bauxite to a predetermined temperature and
for a predetermined time to destroy a portion of the
organic carbon, without converting any gibbsite present
to a form that îs less soluble in a caustic aluminate
liquor.
The present invention provides an improved method for
the thermal treatment of bauxite which achieves substantially
the aforementioned objectives.
DETAILED DESCRIPTION OF THE INVENTION
In accordance with the present invention, when tropical
bauxites are thermally pretreated, i.e., heated prior to
introduction into the Bayer process, at relatively low tempera-
tures, preferably within the range of about 300-400-C, optimal-
ly about 360'C, for a relatively short period of time of about
10-120 minutes, pxeferably about 20-30 minutes,
the amount of organic carbon substances in bauxite can
be decreased by up to approximately 70%;
the sodium oxalate generating ability of the bauxite is
decreased by up to a factor of ten;
the gibbsite content of the bauxite converts to an
alumina species which is at least 25% more soluble in
caustic liquor than gibbsite.
Referring to Table 1, the chemical and mineralogical
compositions of four tropical bauxites are shown, on which the
work of the present invention is based. The data in brackets
a~e the actual analyses obtained after thermal treatment of the
bauxites at 360-C for 30 min.
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2002172
Th~ chemical analyses presented in Table 1 show that
or~anic carbon in bauxite i5 reduced to within the range 0.10-
0.1~% (~ased on the ori~inal bauxite wei~ht) by heating to
360 C for 30 min. The table also shows that the greater the
percent organic carbon in the bauxite, the greater is the
percent reduction obtained as a result of thermal treatment,
with the organic carbon content of the various bauxites reduced
within the range of from 41.2% to 70.8~.
The mineralogical analyses indicate, ~urprisingly, that
only in the case of Bo~e bauxite is there a significant amount
of new boehmite formed on thermal treatment. Moreover, the
gibbsite in the starting bauxites is converted (except for Boke
bauxite) to a more soluble X-ray amorphous phase with trace
conversion to chi-alumina. In addition, goethite (alpha-FeOOI~)
is found to convert, advantageously, to hematite (alpha-Fe2O3),
the better settling iron mineral phase.
Where boehmite is formed (in Bok~ bauxite), it is
already present at 300'C and is stable up to temperatures of
approximately 400-C, beyond which it begins to convert to
gamma-alumina.
The temperature-time combination can be varied within
the range 300-400'C to obtain the same reduction in organic
carbon as at 360-C-30 mins. For example, a higher temperature-
shorter time equivalent would be 380 C-20 min, whereas a lower
2S temperature-longer time equivalent would be 330-C-120 min.
Under the latter conditions, however, the goethite to hematite
transformation would not be obtained. Temperatures less than
340-C, while usable, are actually of little interest because
the rate of destruction is much too slow and the goethite to
hematite transformation does not occur to any significant
degree.
Table 2 lists the temperature/time relationship for a
typical bauxite pretreatment:
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Table 2
TemPeratureTime (min.
330 C 120
340-C 60
360-C 30
380-C 20
400-C 10
The temperature-time combinations were determined
experimentally at 330-C, 360-C and 380-C for each bauxite by
lo measuring organ;c carbon (%) in bauxite as a function of time
(min) at each temperature. Other temperature-time combinations
may be extrapolated from the experimental data using a plot of
temperature vs.time.
An advantage of the temperature-time combinations in
15 ~able 2 is that rotary kilns may be used to achieve the
required degree of heat treatment. Other variations on the
basic method w~uld include heating at higher temperatures i.e.
greater than 400-C, but for appropriately reduced retention
- times. Generally, a rotary kiln may be used for times greater
20 than 10 minutes; a fluid bed for times 1-10 minutes; and a
~lash calcinator for times less than one minute. For example,
at very high temperatures, e.g., lOOO-C, a flash calcinator
would be used where the bauxite falls through a hot zone in a
vertical tube reactor.
The same four types of bauxites were tested to compare
the extraction and conversion of organic substances to sodium
oxalate between treated and untreated bauxite. Samples of each
bauxite were extracted at 250-C at 40 bar in 50 ml of a 5 N 2~8
NaOH solution ~5~-m~ in a stainless steel autoclave, before
30 and after thermal treatment at 360-C for 30 min, to determine
the amount of organic substances which dissolved and the
percentage of organic carbon which transformed to sodium
oxalate. The extraction was carried out at a temperature of
250C for 15 min, with a charging ratio of Na20:A1203 = 1.~
35 and assuming 97~ extraction efficiency. zz.~ 8
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The data obtained are ~iven in Table 3, with the data
in brackets representin~ the changed behavior due to thermal
~retreatment of each bauxite at 360 C for 30 min. All data are
calculated in terms of the organic carbon substances present in
the starting bauxites.
TABLE 3
Source Organic Organic CarbonOrganic Carbon
of CarbonExtractedConverted to Sodium Oxalate
Bauxite ~%1 _ (%) (~)
Australia 0.34 56 12
- Gove ~0.14)*(34)* (1.1)*
Australia 0.24 80 18
- Weipa(0.11) (35) (4.3)
Africa 0.17 58 9.4
- Bok~ (0.10) (37) (1.3)
Jamaica0.48 85 20
(0.14) (40) (3.9)
* Data in brackets obtained after thermal treatment, calcu-
lated on an original bauxite basis.
As shown in Table 3, the percent organic carbon
extracted from thermally treated bauxites was broadly in line
with the reduction in orgànic carbon following thermal treat-
ment. For example, there was a 59% reduction in organic carbon
in the Gove bauxite through thermal treatment. In other words,
the behavior of the organic carbon substances in terms of their
percent extraction was not significantly affected by thermal
treatment. This similarity in behavior does not, surprisingly,
extend to the percent organic carbon which converts to sodium
oxalate. One would expect the conversion of the remaining
organic carbon to be roughly the same percentage. Yet the
conversion to sodium oxalate is substantially reduced, for Gove
bauxite from 12 to 1.1%, a 90% reduction. The bauxite thermal
treatment therefore substantially removes that part of the
organic carbon content responsible for the formation of sodium
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oxalate, with consequent benefits in improved product recovery
from the Bayer process.
The present invention is further described below in
specific examples which are intended to illustrate the inven-
t;ion without limiting its scope.
xam~le 1 :'~
Organic substances were extracted from each of fourtypes of bauxite both before and after a thermal treatment,
with an analyses of the "1umic Extracts' and the organic
carbon to sodium oxalate conversion performed. The thermal
treatment was performed at 360-C for 30 minutes. The initial
bauxite charge was 2000 g/l, with the initial thermally-
treated bauxite charge being 1500 g/l. The extraction was
performed at 85-C for 30 min in 5 N NaOH under a nitrogen atmo-
sphere (1 atmosphere). Solids remaining were separated fromthe extract by centrifugation. Each extract was then subjected
to a hydrothermal treatment, heating to 250-C and holding at
that temperature for 15 min. The results are shown in Table 4 ~
below. '
Table 4
Untreated Bauxite Thermally-Treated Bauxite
organic Sodium Oxalate Organic Sodium Oxalate
Carbon in Liquor (g/l) Carbon in Liquor (g/l)
in Liquor in Liquor
(g/l) 85'C 250'C (g/l)85~C 250'C
Australia -
- Gove 1.25 1.1 1.2 1.1< 0.020.05-0.10
Australia
- Weipa 0.94 0.80 1.0 1.1< 0.02 0.10
Africa
- Bok~ 1.09 0.30 0.40 1.1< 0.020.05-0.10
Jamaica 1.30 1.1 1.3~ 1.4< 0.02 0.15
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The data in Table 4 shows the extent ~f organic carbon
conversion to sodium oxalate at both 85-C and 250 C, il-
lustrating that the sodium oxalate concentrations, present in
the liquors of thermally-treated bauxites from all sources, was
S reduced to less than 0.02 g/l at 85-C, a greater than 90%
reduction in all cases. A similar reduction was realized at
250-C.
Analyses by high pressure liquid chromatography (~PLC)
indicated that the organic carbon extracts could be classified
as 'humic' extracts. In other words, there was little or no
degradation of the organic carbon substances other than to
sodium oxalate. Note in particular that approx. 75-90% of the
sodium oxalate ultimately formed in the untreated bauxite (i.e.
at 250-C) was already present at 85-C.
On the other hand, the thermally-treated bauxites
surprisingly produced no measurable sodium oxalate in caustic
liquor at 85-C. Thermal treatment of bauxite clearly destroys
the organic carbon responsible for the rapid conversion to
sodium oxalate at 85'C, while the sodium oxalate generating
ability of the organics remaining in bauxite after thermal
treatment is relatively low.
E~amDle 2
Weipa bauxite was used as an example to demonstrate
further advantageous aspects of the present invention. More
particularly, further tests relating to the temperature-time
conditions for bauxite thermal treatment and the subsequent
dissolution behavior of the bauxites in caustic aluminate
liquor of the Bayer process were carried out.
Samples of Weipa bauxite were ground to < 63 um and
thermally treated to determine the effects on organic carbon
content and dissolution behavior in caustic aluminate liquor,
with the starting liquor composition including Na2Ofree - 144
g/l; Na2Ocarb - 19.6 g/l; A12O3 ~ 61.0 g/l.
This bauxite was treated in accordance with Example 1.
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The results are presented in Table S be~ow.
Table 5
Temp. Time Organic Dissolution in Caustic Aluminate
( C) (min) Car~on Liquor (g/l) at 85 c *
(%) 30 60 120 240 (min)
380 200.16 124.2 134.9140.0 147.8
400 300.12 97.0 108.1120.5 134.6
400 600.05 90.5 96.3106.6 115.0
500 30< 0.0~ 84.6 89.2 92.7 99.6
600 30~ 0.0Z 83.3 88.0 92.0 96.8
Bauxite - 0.25 120.2 --- --- 120.5
* Data relative to Na20free - 138 g/l.
The results show that the thermal treatment of Weipa
bauxite at temperatures < 400-C converted gibbsite to a more
soluble form of alumina (147.8 vs. 120.5). The solubility was
also higher after thermal treatment at 400-C for 30 minutes
although the dissolution rate is significantly slower. Longer
heating periods at 400-C and higher temperatures lead to
reduced solubility and reduced dissolution rate. -
As shown in the data presented in Tables 1 to 5 above,
thermal treatment of tropical bauxites at relatively low
temperatures ~i.e., 300-C - 400DC) not only lowered the organic
carbon levels, producing a disproportionately large decrease in
sodium oxalate generating ability, but also converted the
gibbsite content of the bauxites substantially to an alumina
species which was at least 25% more soluble then gibbsite in
caustic aluminate liquor.
By the method of the present invention, tropical
bauxites containing a relatively wide range of organic carbon
contents, i.e., 0.17 - 0.48%, have these organic carbon levels
lowered to within the narrow range 0.10 - 0.14% (original
bauxite basis). Moreover, a disproportionately large decrease
in sodium oxalate generating ability is achieved to within the
range of 1.1 - 4.3%. The importance of these narrow ranges is
that bauxites from a variety of sources with differing organic
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200~
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carbon contents and behavior with respect to sodium oxalate are
reduced by thermal treatment to a condition whereby proces-
sability and plant operation are relatively immune to bauxite
source and organic carbon content,'with the additional benefit
of improved dissolution with the conversion of gibbsite to a
more soluble form.
Thus, the method of the present invention has large
scale industrial benefits.
The invention has been described above by reference to
preferred embodiments. It is understood, however, that many
additions and modifications will be apparent to one of ordinary
skill in the art in the light of the present descriptions
without departing from the scope of the invention.
