IE913108A1 - Improvements to alumina calcination - Google Patents

Abstract

28426-1596/18190 The conversion of Al(OH)3 particles to alumina is accelerated by utilizing particulate Al(OH)3 having inclusions comprising .alpha.-A1203 in amounts sufficient to enhance the rate of conversion of Al(OH)3 to .alpha.-A1203, preferably in amounts of from 1% to 20% by weight of the particles.

Description

The present invention relates to improvements in alumina calcination. More particularly, the invention relates to a method for accelerating the conversion of aluminum trihydroxide Al(OH)3 to α-alumina, a-Al2O3.

BACKGROUND OF THE INVENTION The final step in the production of alumina from 10 bauxite by the Bayer process is the calcination of A1(OH)3 to alumina. The characteristics of the calcined alumina produced are controlled mainly by the time and temperature of calcination. When A1(OH)3 is heated in air at atmospheric pressure, it undergoes a series of compositional and structural changes before being finally converted to aalumina. See, for example, K. Wefers and C. Misra, Oxides -1IE 913108 28426-1596/18190 and Hydroxides of Aluminum, Alcoa Technical Paper No. 19, Revised, 1987.

In modern practice, 15-20% of smelter grade alumina is required to be in the alpha form. To achieve such a product, the calcination is carried out at temperatures in the range of about 1050’C to 1200°C depending on the equipment used. In this respect, rotary kilns are gradually being replaced by stationary calciners which have a higher thermal efficiency and hence can operate at lower temperatures and reduced retention times.

In industrial calciners, the particles undergoing calcination are constantly in motion. In rotary kilns, for example, moist A1(OH)3 enters the upper (cold) end and slowly tumbles toward the lower end travelling against a hot stream of gas formed by combustion of oil or natural gas at the discharge end. In stationary calciners, on the other hand, the A1(OH)3 particles are suspended and transported in burning gas or oil.

Energy is one of the largest cost elements in alumina production. Depending on the equipment used, the energy cost associated with calcination varies between one quarter and one third the total energy cost of the alumina production process. See, for example, G. Lang, K. Solymar and J. Steiner, Prospects of Bayer Plant Energy Conservation, Light Metals, pp. 201-214 (1981).

In the aluminum industry, a continuing source of effort is directed at ways and means of reducing the specific energy costs of calcination.

The present inventors have unexpectedly discovered a means of achieving the aforementioned technical objective -2IE 913108 28426-1596/18190 whereby the temperature and/or time of calcination can be lowered. It has been found that by incorporating previously calcined alumina within the particle structure of A1(OH)3 agglomerates via precipitation from sodium aluminate liquor (as, for example, during the precipitation step of the Bayer process), the subsequent conversion of the precipitated A1(OH)3 to a-Al2O3 is significantly faster.

SUMMARY OF THE INVENTION The present invention relates to a method for accelerating the conversion of A1(OH)3 particles to alumina by utilizing particulate A1(OH)3 having inclusions compris15 ing q-A12O3 in amounts sufficient to enhance the rate of conversion of A1(OH)3 to a-Al2O3, preferably in amounts of from 1% to 20% by weight of the particles. Preferred inclusions are of 0.1 to 25 micrometers in size.

In one embodiment of the invention, relatively finely divided Q-Al203-cont'aining particles are coated with Al(OH)3 by precipitation in, for example, the sodium aluminate liquor of the Bayer process. The coated a-Al2O3containing seed particles may be agglomerated to the relatively coarse particle sizes.preferred for the calcina25 tion step associated with the Bayer process. The precipitation conditions employed promote crystal growth and optionally, agglomeration to the exclusion of secondary nucleation. Thus, in this embodiment, the method of the present invention comprises the following steps: (a) providing o-Al203-containing seed of appropriate particle size; -3IE 913108 28426-1596/18190 (b) precipitating A1(OH)3 on the seed particles in sodium aluminate liquor under conditions favorable for coating and agglomerating the seed; i.e., relatively low seed charge, high precipitation temperature and, optionally, the presence of an added calcium-containing compound as agglomeration aid; (c) filtering the product and washing with deionized water followed by drying at 110°C; and (d) calcination of the a-Al2O3-containing A1(OH)3 particles at an elevated temperature, i.e. > 900?C.

BRIEF DESCRIPTION OF THE FIGURES Fig. 1 shows graphically the relationship between the amount of α-alumina in the product and calcination time; and Fig. 2 shows graphically the relationship between the amount of α-alumina in the calcination product and in the seed crystal inclusions.

DETAILED DESCRIPTION OF THE INVENTION The basis of the present invention is the observation that the presence of previously calcined alumina in intimate contact with Al(OH)3 particles accelerates the transformation of the A1(OH)3 to a-Al2O3 during subsequent high temperature calcination.

The effect was first observed in static bed laboratory calcination experiments in an electric furnace. The presence of α-alumina or α-alumina-containing particles in a shallow bed of A1(OH)3 particles during calcination -4IE 913108 28426-1596/18190 slightly but unmistakenly promotes the conversion of the Al(OH)3 to a-alumina.

Such an effect is more or less excluded in industrial calcination equipment because the particles are in constant motion. Thus, to exploit the basic invention in industrial practice the α-alumina-containing seed particles had to be built into the structure of the A1(OH)3 particles as inclusions. In other words, intimate contact is maintained despite the motion of the calcining particles. In practice, there are two ways of achieving this objective.

In the first instance, the most desirable effect is that A1(OH)3 precipitates directly on the a-alumina-containing seed particles. In this way, each of the A1(OH)3 particles would be seeded from within by an inclusion comprising a-Al2O3. The second effect, which is most desirably coupled with the first, is that the seeded A1(OH)3 particles then come together and build relatively massive agglomerates.

This would enable the use of relatively fine a-aluminacontaining seed particles as inclusions, which would be evenly dispersed throughout the A1(OH)3 agglomerates, and yet achieve sufficiently large particles for use in Bayer process calcination.

Advantageously, it has been found that both effects occur when sodium aluminate liquor is seeded with α-alumina or α-alumina-containing particles. In addition the process is aided by the addition of a small amount of CaCO3 which not only prevents the α-alumina seed particles nucleating fine new Al(OH)3 crystals but aids agglomeration of the α-alumina-containing A1(OH)3. -5IE 913108 28426-1596/18190 The development of the present invention is illustrated by the following series of examples, which, however, are not to be interpreted as limiting the invention.

Example 1 Four separate powders were prepared for calcination experiments at elevated temperatures in a laboratory electric furnace (Netzsch Geratebau GmbH, Selb, Federal Republic of Germany) fitted with a Novocontrol system.

Powder #1 Coarse A1(OH)3 of average particle size 92 pm was prepared in the laboratory by seeding Bayer process sodium aluminate liquor (Na20F - 126.6 g/1; Na20c - 16.3 g/1; Al2O3 - 151.4 g/1) with Al(OH)3 seed crystals of average particle size 82 pm and carrying out A1(OH)3 precipitation over the temperature-time conditions 76*C 52eC. The yield of precipitated A1(OH)3 was 117 g/1.

Particle size analyses were carried out using a Malvern Master Sizer.

Powder /2 Powder /2 was prepared by mixing Powder fl with ground calcined alumina of average particle size 4.6 pm in the weight ratio 4:1 in a powder mixing apparatus (W.A. Bachofen Maschinenfabrik, Basle, Switzerland) for 10 minutes.

The ground calcined alumina had an a-Al2O3 content of 98% as determined by X-ray diffraction using a Phillips PW 1820 Vertical Diffractometer equipped with a PW 1825 Generator and APD software. -6IE 913108 28426-1596/18190 Powder #3 Powder /3 was prepared by carrying out a precipitation experiment using 20 g/1 of the aforementioned ground calcined alumina (98% a-Al2O3) as seed charge. Bayer process sodium aluminate liquor (Na2OF - 130 g/1; Na2Oc 18.0 g/1; A12O3 - 155 g/1) was used under precipitation . . 25h temperature-time conditions of 76’C -► 68°C. The yield of A1(OH)3 was 76.5 g/1, so that the end product contained .0% of a-Al2O3. The average particle size was 72 μπι indicating that under the present experimental conditions even q-A12O3 can function as an acceptable seed for A1(OH)3 precipitation.

Powder /4 This was prepared in the same manner as Powder /3 except for carrying out precipitation twice, the second precipitation being carried out on a portion of the product from the first. The a-Al2O3 content of the end-product was lower at 11.1%. The average particle size of the end20 product was 79 μη.

The four powders were then calcined at temperatures in the range 1050-1200’C for periods of between 0.5 and 4.0 hours. After calcination, the powders were analyzed for % a-Al2O3 content by X-ray diffraction.

The results, given Table 1, show that the presence of q-A12O3 simply mixed amongst A1(OH)3 particles, i.e., Powder /2, in a static bed slightly accelerates the transformation of the A1(OH)3 to a-Al2O3. However, when the q-A12O3 is used as seed in A1(OH)3 precipitation, then the same amount of q-A12O3, i.e., 20%, has a very significant -7IE 913108 28426-1596/18190 accelerating effect on the transformation of precipitated Al(OH)3 to a-Al2O3 during subsequent calcination.

Thus, A1(OH)3 by itself in laboratory static bed calcination requires ca. 1.0 hour at 1200eC to reach 17% a-Al2O3, whereas the seeded system (Powder /3) achieves the same degree of calcination at a temperature of about 1075°C with the same residence time. The greatest effect/benefit of seeding with q-A12O3 clearly is obtained at the low levels of a-Al2O3 present in normal Bayer calcined alumina. -8IE 913108 28426-1596/18190 Λ -P n — * -H O ?, 'T a (n « H p „ ->« o ω x α o a o 2 ο «η O CM < H Al Al σι Al <Λ n in a* in CALCINATION OF Al(OH)3 USING GROUND ALUMINA [98% q-A12O3] AS SEED E CM .4 < w 4J *n «Η O Φ £ CM r™4 E O 1 X a σ ό o «0 Λ» ·— »—I o < Al Λ * P n (N •H o 'tfc 5 CM rH X n < u 1 Q X a £ o H Ο '—Λ’ ΛΗΟ 2 < W υ « M O. r·I ΊΜ m X W O Q X «Η o < σι Al r* r> in o vo cm VO Al Al Ar4 VO O M· CO n CPi in CM o O O O O O O o 2 u in O in O o O O o U o o rK H Al Al Al Al Al «Η cH H H rH rH i-l «Η ω O O o in o O O o •-4 rd r4 o H Al Minus the α-ΑΙ,Ο·, seed -9IE 913108 28426-1596/18190 Example 2 In Bayer plant practice, the alumina used as seed would be recycled product material of reduced particle size. The a-Al2O3 content would be of the order of 20%, with the remainder consisting of transition aluminas which would be expected to form, at least partially, further a-Al2O3 on further calcination, thereby enhancing the seeding effect of the recycled alumina.

To check the seeding effectiveness of Bayer plant 10 alumina, three further powders were prepared in a manner similar to that described in Example 1.

Powder #5 The alumina seed was separated from normal Bayer plant alumina (% a-Al2O3 - 20; average particle size - 90 pm) by a wet sieving technique. The average particle size i.._ of the alumina thus separated was 21 pm.

A precipitation experiment was then carried out under the same conditions qsed in the preparation ofPowder /3. The A1(OH)3 yield was 83.9 g/1 using a 20 g/1 seed charge of the aforementioned alumina. Thus, the end-product contained 3.9% a-Al2O3. The average particle size was 70 pm.

Powder #6 Powder /6 was prepared in the identical manner to Powder /5 except that 60 mg/1 CaO (added as CaCO3 equivalent) was added to the alumina seed particles at the start of precipitation. The yield of A1(OH)3 was 76.3 g/1 giving an end product of a-Al2O3 content of 4.2%. The average particle size of the agglomerated product was however coarser at 98 pm. -10IE 913108 28426-1596/18190 Powder #7 For this powder, the alumina seed was prepared by milling a portion of Bayer plant alumina in a ball-mill down to an average particle size of 10 μτα.

A precipitation experiment was then carried out under the same conditions used in the preparation of Powder /6. The A1(OH)3 yield was 70.2 g/1 using a 20 g/1 seed charge (+ 60 mg/1 CaO) of the aforementioned alumina. Thus, the end-product contained 4.4% a-Al2O3. The average particle size was 88 μτα, indicating that milled alumina functions as an effective seed for A1(OH)3 preparation.

Powder #8 The preparation of Powder /8 was carried out in the same manner as described for Powder /7 except that no CaCO3 was added with the milled alumina seed particles. In < the absence of added calcium, the yield of A1(OH)3 increased to 102.3 g/1. Thus, the end product contained 3.3% a-Al2O3. The average particle size was finer at 75 μτα.

The three powders of Example 2 were then calcined at temperatures in the range 1100-1200eC for periods of up to 4.0 hours. After calcination, the powders were analyzed for % a-Al2O3 content by .X-ray diffraction.

The results, given in Table 2, indicate that fine milled alumina (10 gm average particle size) is more effective at seeding the A1(OH)3 —» a-Al2O3 transformation than is the unmilled alumina (21 μτα average particle size) separated from Bayer plant product alumina by classification. There is no evidence that the use of added calcium, « which promotes the agglomeration of the a-alumina-containing seed particles has any significant effect on the A1(eOH)3 —► -11IE 913108 28426-1596/18190 a-Al2O3 transformation, at least under the precipitation conditions used.

The data in Table 2 is corrected for the changing -a-Al2O3 content of the seed within the A1(OH)3 particles as calcination proceeds. For example, after 1.0 hour at 1200°C the a-Al2O3 content of the starting seed increased to 55% a-Al2O3 (starting powder basis). In other words, for the three powders /4, /5 and /6, at 1.0 hour calcination time and 1200eC, the % α-content of the seed was 11.6%, 12.1% and 9.1% respectively.

Thus, the data in Table 2 at 1.0 hour and 1200°C can be compared with the corresponding data in Table 1, Powder #4, which contained 10% a-Al2O3 stable during calcination. There is no appreciable difference in the magnitude of the results obtained.

I -12IE 913108 28426-1596/18190 ci M Q — * —I t-ι i! 3 £ . O e mJ β (0 Lti 33 o e O' SO -- a. — 2 r-M O σι o «r Cl M· in in Cl in CALCINATION OF A1(OH)3 USING BAYER PLANT ALUMINA [20% a-Al2O3] AS SEED 4 2 H x3 < 1 4-f 55 *7 I * •H H *5 2 *£> 5 £ « Μ» 5 g o Ej X mj-l ο O » <0 J W x 20 Q o e ? X z*\ — a.i + _i r\ io H CM ►4 w u « M CM * in *> Ci W Q X O CM < +J 2 Η H > S ~ m nJ®8 — < e S ™ 3.S + *C CM O' M O o CM I m X o co CM CM T Γ T CM in co m *e* u> in CM o O O O o O O £ o o in O o o O o u o r-l rH CN ΓΜ CM CM CM r-l rH r-l r-I r-l r-l r-l w X W Λ o o in o o O o r-( r-l o r-l CM r> Minus the a-Al2O3 content of the seed at each temp-time combination. -13IE 913108 28426-1596/18190 Example.-2 A source of very fine partially calcined alumina in a Bayer alumina plant is Electrostatic Precipitator (ESP) dust. ESP dust has a higher o-Al203 content than the calcined alumina product, particularly where the dust is recycled through the calciner. Because of its behavior in the calciner, however, the dust generally contains undercalcined as well as over-calcined material. Consequently, small amounts of gibbsite and boehmite may be present in addition to the transition aluminas. ESP dust calcined at 1200’C for 1.0 hour developed an a-Al2O3 content of 59%j calculated on the starting powder basis.

Three powders were prepared in which ESP dust was used as the seed material to promote the A1(OH)3 —► a-Al2O3 transformation in precipitated A1(OH)3. The ESP dust had an (. . average particle size of 3.4 pm and an a-Al2O3 content of 29%.

Powder /8 Ar Ζ2·2.Ί° fat, To prepare Powder /8, 20 g/1 of ESP. dust was added to Bayer process sodium aluminate liquor (Na2OF - 135 g/1 Na2Oc - 23.0 g/1; A12O3 - 159 g/1) along with 60 mg/1 of CaO (added as CaO equivalent) to aid in the agglomeration to coarse particle size dimensions. Precipitation conditions were 76’C 56eC, giving a yield of Al(OH)3 of 68.4 g/1.

Thus, the a-Al2O3 content of the end-product was 6.6%. The average particle size was 71 pm.

Powder /9 To prepare Powder /9, a portion of Powder /8 was 30 initially mixed with-a portion of the seed crystals used in the preparation of Powder /1 in the ratio 2:5. Precipitar -14IE 913108 28426-1596/18190 tion was then carried out in Bayer process sodium aluminate liquor [Na2OF - 127 g/1; Na2Oc - 14.4 g/1; A12O3 - 154 g/1] with a seed charge of 140 g/1 under temperature-time 50h conditions of 76°C -► 52°C to give Powder /9, heterogeneous in that not all of the product particles contained ESP dust as seed within. The yield of A1(OH)3 was 119 g/1 giving an a-Al2O3 content of 1.0%. The average particle size of Powder /9 was 104 pm.

Thus, Powder /9 could be considered to represent an industrial process where the ESP dust is agglomerated initially in a separate step and then fed to the main precipitation step of the Bayer process for strengthening and consolidation of the agglomerates.

Powders /8 and /9 were then calcined at temperatures in the range 1050°-C to l200eC for periods of up to 4.0 hours. After calcination, the powders were analyzed for % a-Al2O3 by X-ray diffraction.

The results, given in Table 3, indicate that a-Al203-containing ESP dust seeds the A1(OH)3 —* a-Al2O3 transformation in precipitated A1(OH)3 as effectively as does the a-Al2o3-containing Powders /3-#7 of Examples 1 and 2.

The results obtained for Powder /9 are surpris25 ingly good considering the very low amount of a-Al2o3 present, i.e., - 1% (rising to - 2% at 1200eC/lh), and the heterogeneity of the sample. -15IE 913108 28426-1596/18190 * H E * _ W -o % 4J Q -Η ε a r>° W o Q n CALCINATION OF Al(OH)3 OSING FINE ALUMINA DUST [29% Q-Al2O3] AS SEED < X H Ο X *4 0, O I o rH £ X VO *4 • M3 * ® c.

X> Ή X o, ►4 *4 X w u oi w CU o co r» vo <«· tO *0 O 4» Ω E r,° o ΓΊ C *—% X < O I a rH *, x w o Q X O < P< vo o co «-< m n ci in c o H X> <0 G •H XI β o υ v β •Η X> I D. β X> >r i ο o r·' rH ft ft m P< - o o O O O O X o in o in O O O W o o «-Ι t-4 Cl Cl Cl rH •H c-t rH rH «Η Μ 1 S -d o o O to o o H -C • • • • « • C*«H rH o Cl (0 X» (0 Ό V V) •p X» G V x> c o υ n O CM ft *4 I a X» Ul G •H -16IE 913108 28426-1596/18190 Thus, the present invention provides a method for accelerating the conversion of precipitated A1(OH)3 to a-Al2O3 with its advantageous implications in terms of reducing the energy costs of the alumina production process.

The effectiveness of the method of the present invention derives from the seeding technique employed whereby fine α-alumina-containing seed particles are incorporated within the structure of relatively coarse A1(OH)3 agglomerates as can be prepared, for example, via the precipitation step of the Bayer process. Although the formation of coarse agglomerates facilitates the handling of the Al(OH)3 during calcination, the decisive step is in fact the crystal growth of A1(OH)3 directly on the surfaces of the e-Al2O3-containing seed particles. This intimate contact between a-Al2O3 and precipitated A1(OH)3 initiates the subsequent transformation of A1(OH)3 —► a-Al2O3.

Advantageously, the effect of seeding is greatest at low temperature-time combinations. See, for example, Figure 1, which shows the development of q-A12O3 as a function of calcination time at 1200°C for Powders #1, #3, /4 and /9. Unseeded Al(0H)3 requires about 3.0 hours to reach 40% a-Al2O3 whereas the Powder /3 (containing initially 20% a-Al2O3) achieves the same degree of calcination in about 10 minutes.

Also, the finer the a-Al203-containing seed, the more effective is the subsequent transformation of A1(OH)3 to a-Al2O3, the extent of the transformation increasing with the amount of seed used (see Figure 2).

This invention has been described by reference to preferred embodiments. It is understood, however, that many -17IE 913108 28426-1596/18190 additions, deletions and modifications will be apparent to one of ordinary skill in the art in the light of the present description without parting from the scope of the invention, as claimed below.

Claims (16)

1. WE CLAIM: 1 1. A method of preparing a-Al 2 O 3 comprising the

2. Step of calcining particulate A1(OH) 3 , wherein the

3. Particulate A1(OH) 3 has inclusions comprising a-Al 2 O 3 in

4. Amounts sufficient to enhance the rate of conversion of

5. A1(OH) 3 to a-Al 2 O 3 . 1 2. A method according to claim 1, wherein the a2-· A1 2 O 3 in the inclusions constitutes from 1% to 20% by weight 3 of the particulate A1(OH) 3 . 1 3. A method according to claim 1, wherein the 2 particulate Al(OH) 3 is formed by precipitating Al(OH) 3 onto 3 the surface of a-Al 2 0 3 -containing seed crystals to form 4 Al(OH) 3 -coated a-Al 2 0 3 -inclusion containing particles. 1 4. A method according to claim 3, wherein the 2 precipitation of A1(OH) 3 is performed by adding as seed 3 crystals an a-Al 2 0 3 -containing solid powder to 4 supersaturated sodium aluminate liquor. 1 5. A method according to claim 4, wherein the a2 Al 2 0 3 -containing solid is selected from the group consisting 3 of a-Al 2 O 3 , milled o-A1 2 O 3 and partially calcined aluminas 4 containing a-Al 2 O 3 in combination with other alumina phases. 1

6. A method according to claim 4, wherein the 2 precipitation is performed at a temperature of greater than 3 60°C. -19IE 913108 28426-1596/18190 1

7. A method according to claim 4, wherein the a2 AljOj in the inclusions constitutes from 1% to 20% by weight 3 of the particulate Al(OH) 3 . 1

8. A method according to claim 1, wherein the 2 inclusions have an average size of from 0.1 to 25 3 micrometers. 1

9. A method according to claim 3, wherein the 2 seed crystals have an average size of from 0.1 to 25, 3 micrometers. 1

10. A method according to claim 4, further 2 comprising the step of adding an agglomeration aid during 3 the precipitation step. 1

11. A method according to claim 10, wherein the 2 agglomeration aid is CaO or CaC0 3 . 1 12. A method according to claim 3, wherein the 2 Al(OH) 3 -coated a-A

l 2 O 3 -containing particles are washed with 3 deionized water. 1

13. A method according to claim 4, wherein the 2 calcination is carried out at a temperature in excess of 3 900°C. -20IE 913108

14. A method of preparing alpha-alumina, substantially as described herein by way of Example.

15. A method of preparing alpha-alumina, substantially as 5 described herein by way of Example and with reference to the accompanying drawings.

16. Alpha-alumina prepared by the method of any of Claims 1 to 15.