2002172 7875/0~100 ~ .
METHOD FOR BAUXITE TREATMENT
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.
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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~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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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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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 , . ~
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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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.
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 ~B
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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.
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 ..
20C~Z1~2 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 z~
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112OZ~72 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 .
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.