CA1325708C - Alumina products and their production process by removing water from aluminum hydroxide - Google Patents

Abstract

ABSTRACT OF THE DISCLOSURE
Disclosed is a composition of matter comprising particles, each particle containing boehmite and gamma and/or X-ray indifferent alumina pseudomorphous with aluminum hydroxide, an absence of alpha-alumina, and having a reduced amount of parallel fissuring as compared to alumina from atmospheric pressure calcination of aluminum hydroxide, each particle containing a plurality of crystals, the LOI (300-1,200°C) of the particles being from 1 to 12%, their surface area being 10-100 m2/g. Also disclosed is a process for producing the composition by removing water from Al(OH)3 in a decomposer at a pressure higher than one atmospheric gage pressure. The product is stronger than alumina produced by calcining at atmospheric pressure.

Description

~ 3 2 ~ 7 ~ ~ 60828-1199D

This is a divisional application of Serial No.
456,444 filed June 13, 1984 and entitled "Steam Producing Process and Products".
An object of the invention of the parent applica-tion is to provide a method of producing steam for steam-consuming processes.
According to the invention of the parent application, steam is produced by heating a water-containing, solid substance for generating steam.
According to the invention of the parent application, there is provided in a process utilizing steam, the method of producing at least a portion of the steam, comprising ~a) heating a solid substance at a temperature greater than 250C
in a decomposer at a pressure greater than one atmosphere ~age pressurel i. for decomposing the substance to drive off chemically combined water, and ii. for yielding the water in the form of steam at a pressure greater than one atmosphere gage pressure, ~b) removing the steam from the decomposer, and (c) capturing the steam for the process, the process being external to said decomposer.
The invention of the parent application also provides a method comprising piling particles of a sub~tance on top of one another, whereby the particles form a bed, and removing water from the particles, with the water being evolved in the gaseous state and self-fluidizing the bed.
One embodiment of the invention of this divisional application provides a composition of matter comprising particles, each particle containing hoehmite and gamma and/or X-ray indifferent alumina pseudomorphous with aluminum hydroxide, an absence of alpha-alumina, and having a reduced amount of parallel fissuring as compared to alumina from atmospheric pressure calcination of aluminum hydroxide, each ~ 3 ~ ~ 7 0~ 60828-1199D

particle containing a plurality of crystals, the LOI (300-1,200C) of the particles being from 1 to 12%, their surface area being 10-100 m2/g. Preferably, paral.lel fissuring is absent.
Another embodiment of the invention of this divisional application provides alumina having a surface area in the range 10-100 m2/g, an attrition index in the range 1-20, a reduced amount of parallel fissuring as compared to alumina from atmospheric pressure calcination of aluminum hydroxide and an.absence of alpha-alumina. Preferably, the surface area is in the range of 10-70 m2/g. Preferably, the attrition index is not more than 2. Preferably, LOI (300-1,200C) is less than 1%.
A still another embodiment of the invention of this divisional application provides a process for producing the alumina product, which comprises: heating aluminum hydroxide which may be moistened at a temperature greater than 250C in a decomposer at a pressure higher than one atmosphere gage pressure, i. for decomposing the aluminum hydroxide to drive off at least a part of the chemically combined water, and ii.
yielding water in the form of steam at a pressure greater than one atmosphere gage pressure, and removing the steam from the decomposer.
In the following description, the expression "the invention" or "the present invention" is employed both for that of the parent application and that ~f this divisional application.
The invention has particularly advantageous applica-tion to con~unction with water removal from aluminum hydroxide to prepare alumina particularly as feed for the Hall-Heroult electrolytic process for producing aluminum metal. The ~32~708 60828-1199D

aluminum hydroxide, in a moist or dry condition, is heated in a decomposer under self-fluidizing conditions, or fluidi~ed by steam, and without con~act by exhaust gases from fuel combustion, so that pure steam is obtainedO Pressures are typically in the 20 to 500 psia range, in order that the steam will be at pressure suitable for use. This steam is available for use as process steam in a Bayer refining plant and also for power generation in usual steam engines or turbines. New alumina products are obtained. An important advantage achieved is that there is less breakage of particles during water removal than is the case during the flash or kiln calcination, prior art techniques of water removal. An additional advantage is that an alumina product can be obtained which is characterized by a reduced amount of the parallel fissuring typifying alumina from atmospheric pressure, flash and kiln calcination of aluminum hydroxide. The alumina product of this invention is stronger than alumina produced in atmospheric pressure calcination processes, as indicated by lower attrition indices. As an added benefit, differential calorimetry tests show the process using about 10~ less energy as compared to flash or kiln calciners, this saving being in addition to the energy savings achieved by utilizing the steam coming from the aluminum hydroxide.

The production of essentially pure steam ~rom aluminum hydroxide (alumina trihydrate, or "hydrate" for short) can be carried out using a variety of energy sources. Both direct and indirect heating methods are possible. The heating method most applicable for retrofit to an existing calciner is the use of hot combustion ~asses from the furnace section of the calciner to indirectly heat the hydrate. Other heating methods for indirect heat transer include electrical resistance heating, hot oil or salt baths, heating by inductance, lasers, plasmas, 10 combustion of coal, microwave radiation, nuclear and chemical reactors. The heat source may originate from another process, such as the use of hot gasses from a coal gasification unit to decompose hydrate, or the hydrate may be indirectly decomposed while acting ~s a eoolant to control a highly exo~hermic reactor.
Direct heat transfer methods for drying and decomposition of hydrate to form an alumina product and pure steam include the use of in-bed, electrical resistance heat~ng, microwave generators, lasers and~or superheated steam.
2~ Figures lA to lC compose a process flow schematic of an embodiment of the invention.
Figure 2 is a schematic of an alterna~ive apparstus for use in the process of Figure 1.
Figures 3 and 4 are scanning electron micrographs, at 12,000 times magnification; Figure 3 being of a product of the invention and Figure 4 being o~ a product of the prior art.
A. Continuous Process The invention find~ ~ preferred setting in conjunc~ion with the Bayer process for producing alumina from bauxite. The 3~ Bayer process ~tilizes steam, for instance, in i~s digester, where bauxite ore previously crushed in a grinder is treated ~ith sodium hydroxide solution to dissolve aluminum values.
_ 4 _ ~ .

~ ~ 2 5 7 0 8 Steam provides the heat needed to maintain the temperature and pressure conditions of the slurry in the digester. These are typically in the 100 to 300C and 100 to 500 pounds per square inch absolute pressure (psia) ranges.
Steam is also u~ed, e.g., in the evaporator of a Bayer plant.
Taking the dlgester as an example, the steam may heat the slurry directly by being lnjected righ~ into the slurry - for instance, through a pipe opening below the slurry 10 surface. Alte~natively, the steam may heat ~he slurry indirectly by supplying heat to a heat exchanger in contact with the slurry.
In general, it is of advantage that ~he s~eam be completely H2O, thus containing no diluting gases such as air.
For instance, the steam should be at least 50 volumP-% water, preferably at least 75 volume-% water, and more preferably at least 94, or even 100, volume-% water. One particular advantage of steam which is pure H2O is its characteristic of condensing a~ a constant temperature, such constant temperature being a 20 function of pressure. This is of use for temperature control in the process consuming the steam. For instance, 110 psia pure-H2O steam condenses at about 175C and can be used to maintain a digester temperature of about 150~C, the 2SC
difference being provided ~or heat transfer driving force.
If the steam is diluted with other gases, such as air, which is uncondensable at temperatures of interest for steam-consuming processes, the H2O in the steam condenses over a range of temperatures (as will be evident from the known variation of dew point with water vapor partial pressure).
3~ Additionally, the unconden~ahle gases represent a dilution of the ~reat heat release obtained as H2O condenses, the heat 13 2 5 7 0 ~ 60828-1199D
released by the uncondensable gases as they drop in temperature being relatively insignificant.
A diluting gas such as C02, for instance resulting from combustion of a fuel containing carbon, is extremely disadvantageous in the case of direct steam heating in a Bayer process digester, because the C02 reacts with the NaOH needed to dissolve the aluminum values in the bauxite. The NaOH loss occurs by reaction of the type 2NaOH + C02 Na2C03 + H20 l~ In the present invention it is proposed to carry ou~
at least the initial part of the calcination of aluminum hydroxide, "hydrate", by indirectly or directly heating the hydrate in a suitable, pressurized container vessel. This enables collection of steam released under pressure by the evaporation of the free moisture and removal of the chemically bound water. The separated steam can then be used in other areas of the alumina refining plant, resulting in considerable energy savings. The partly calrined alumina product obtained from the pressure decomposition vessel may then be calcined in ~ conventional calcination equipment such as a rotary kiln or a stationary calciner.
Thus, according to one embodiment of the present invention, s~eam for the Bayer process i5 obtained by heating aluminum hydroxide, Al(OH)3, to at least partially calcine it, with the evolved steam being captured for transmission, e.g., to the digester. This is unli~e previous methods oE calcining A](01~)3j where, rather than being steam, the gases coming from the calcining are fuel exhaust con~aining water from the burning of the fuel and from the Al(OH)3, an example of an analysis 3~ being, in volume-%, 8% C02, 55.3% N2, 2.5% 0~, and 34.2% H20, as taken from the table on page 34 of Sch11hmann's Metallurgical En~ineerin~, Volume l, En~ineerin~ Principles, Addislon Wesley ~ 3 ~ 5 7 o ~ 60828-llggD

Press Inc., Cambridge, Massachusetts tl952). In such processes, the burning, or burned, fuel-air mixture direc~ly contacts the Al(O~l)3 as it is being dehydrated.
A particular advantage of Al(OH)3 as a solid used as a source of steam is that the steam is essentially 100%, i.e.
pure, H2O. This s ~o be con~ras~ed with, e.g., saw dust, where the steam might be contaminated with organic compounds also volatilizing during the liberation of ~he steam.
It is estimated that about one-third of the steam lO requirement for a Bayer process can be supplied from the water in the Al(OH)3 product.
A principal equipment of the process is a decomposer vessel. Though the decomposition process may be carried out by any combination of a pressure vessel and indirect heatin~, an efficient method for carrying out this process is by the use of a fluidized bed. Our investigations have shown that a bed of the hydrate exhibits self-fluidizlng behavior on heating due to steam release on decomposition. High rates of hea~ transfer to the decomposing hydrate can thus be attained by utilizing this 2~ self-fluidizing characteristic.
With reference now to Figure 1, it illustrates an embodiment where the process of the invention is used in conjunction with a Bayer process, The segments of Figure 1 show ~s follows:
Figure lA - A Bayer process utilizing steam from the process of the invention;
Figure lB - Generating steam according to the invention;
Figure lC - A flash calciner for bringin~
3~ partially calcined alumina from Figure lB to a final, desired water-content suitable for use as metallurgical alumina.

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~ ~32~7~

First with respect to Figure lA, ~auxi~e frolll bauxite stockpile 10 is ground in grinder 12 and then sent to digester 14, where it contributes to the solids portion of the slurry in the digester. The liquid portion of the slurry is a suitably concentrated, for example by evaporator 16, NaOH-containing, aqueous solution.
Followlng digestion, the slurr~ i6 fed to residue separation equipment 18, which removes solids remalning from the digestion. Then, solution, free of solids, is fed to 1~ precipitator 20, where aluminum hydroxide, Al(011)3, crystalline form glbbsite, is precipi~ated. The resul~ing slurry is run through filter ~2, to yield a solids portion through line 24 and a liquids portion through line 26 going to evaporator for concentration for recycle back to digester 14. The Al~OH33 -filter cake solids in line 24 will have both free moisture, to the extent of 8-16 weight-%, and chem~cally bound water at 34.5 weight-~ on dry Al(OH~3.
Other s~eps may, o course, be in the Bayer process, for instance a causticization using lime. Thus, Figure lA is 20 for the purpose of illustrating how the steam generating process of the invention may be combined with a Bayer process in general, as an example of a steam consuming process, rather than to go into the many fine points of a Bayer process.
According to the invention, steam is generated for supply to the digester and evaporator portions of the Bayer process through lines 28 and 30 by hea~ing Al(OH)3 in decomposer 3~ (Figure lB) to drive off free moisture and chemically combined water ~lom ~he Al(0~1)3. The equilibrium sta~e of the removed water is the gaseous state. The decomposer is operated 3~ under pressure of sui~able magnitude above one atmosphere (14.7 psi3 gage pressure to give the temperature desired during condensation of the steam in the Bayer process. The pressure _ 8 _ ' 132~708 may, for instance, be in ~he range 20 to 25n psig (n~un~.~ per square inch gage pressure).
Feeder 34 is provided to transfer the Al(OH)3 from essentially atmospheric pressure in line 24 to the elevated, e.g. 20 to 250 psig, pressure of the decomposer 32. Feeding equipment of the type described in pages 99 to 120 of the publication "Workshop on Critical Coal Conversion Equipment" for continuous feeding of coal rom a low pres~ure state to a high pressure state may be used for feeder 34 For example, a 10 suitable feeder would be the Lockheed Kinetic Extruder shown in Slide 6.22 on page 113 of that publication. Alternatively, the feeder could be a pair of lock hoppers, with one feeding under elevated pressure while the other is filling under atmospheric pressure, and vice versa, as illustrated by the Ducon Coal Feed ~ystem in Slide 6.26 on page 115. Further identifying information on this publication is as follows: Huntington, W.V. - October 1-3, 1980, Harry W. Parker, Editor, The ~ -Engineering Societies Commission on Energy, Inc., 444 North Capitol Street, N.W., Suite 405, Washington, D.C. 20001, Date 20 Published - January, 1981, Prepared for ~he United States Department of Energy, Under Contract No. EF-77-C-01-2468;
FE-2468-88, Dist. Category LIC-9OD.
These feeders permit decomposer 32 to be a continuously, or semicontinuously in the case of e.g. lock hopper-type systems, operating pressure ~essel, where Al(OU)3 entering chamber 36 falls into a number of tubes 38, of which only three have been shown for schematlc purposes. Tubes 38 are at a high temperature, for instance 250 to 650~, so tha~ free moisture and chemica]ly combined water are driven off as steam.
3~ This eYolution of water in the gaseous state fluidizes the ~l~OH)3 partic~es in ~ubes 38. Residence ~ime in the tubes is _.g. 10 to 120 minutes.

';' ~32~g The high temperature in the tubes is achieved by hot gases entering chamber 39 through line 40 and leaving through line 42. Chamber 39 is enclosed by side walls 39a, ~op wall 39b ar~d bottom ~alL 39c.
As the particles of Al(OH~3 fall into tubes 38, ~hey pile on top of one another ~o form a bed. The water evolved from them in the gaseous state flows upwardly toward the vent ~o line 44 and is sufficient to fluidize the particle beds in the tubes. This is termed self-fluidizing, in that the fluidizing 1~ gas comes from the particles themselves. During start-up, steam from an auxiliary steam source may be injected into chamber 52 through line 53 to fluidize particles in the tubes 38, until self fluidization is achieved, after which line S3 may be closed. To the extent that the evolved water may not be enough to achieve desired fluidization, such may be supplemented by s~eam injection through line 53 during water removal from ~he Al(OH~3. Steam feedback from line 44 to line 53 is another possibility. Feedback can be used, or example, to maintain fluidization, or prevent bed particles from interbonding, when 2~ the process might be placed in idle, as when the need for steam in the Bayer plant is temporarily diminished.
The steam product of the invention moves through line 44 to a solids separator such as cyclone 46, wi~h solids being returned to the decomposer through line 48 and steam going to the Bayer process through line 50 to lines 28 and 30. It is estimated that steam from the decomposer will make up one-third of the total steam needed by a Bayer pr~cess. The remaining two-thirds will enter line 50 through line 49 and will come from one or ~ore water boilers (not shown). In general, the steam 3~ coming from the decomposer will be in a superheated condition, and this may be cared for by heat loss in the lines or by a he~t recovery desuperheater ~not shown~ to bring the steam to its 2~7 0~

saturation temperature at its location of use. For example, condensate returning to water boilers feeding line 49 may be preheated by running through such a desuperheater. Such condensate may also be used to get more heat from the gases in line 42.
The aluminum-bearing product resulting from removal of water flom the Al(OH)3 in ~ubes 38 collects in the chamber 52.
It now comprises par~icles of boehmite and/or gan~na alumina and X-ray indifferent ~no well-defined di~fraction pattern) alumina 10 pseudomorphous with aluminum hydroxide. Gam~a alumina is somewhat on the borderline of being X-ray indifferent in that its diffraction pattern is not very well-defined. The product loss on ignition (LOI) (300 to 1200C) is in the range from 1 to 12%, its surface area in the range 10 to 100 m2/g.
Interestingly, in the pressurized decomposer, there i8 less breakage of particles during water removal than is the case during the flash or kiln calciner, prior art techniques of water removal. In laboratory scale tests, there was no particle breakage as dPtermined by "before" and "after" weights of the 20 -325 mesh particle size fraction; in similar laboratory scale tests for atmospheric pressure, flash or kiln calciner tests, it is typical to find that the -325 mesh size fraction increases in weight by 2 to 5% due to particle breakage.
The aluminous product in chamber 52 is next conducted through pressure-reducing feeder 54 (of the same construction as feeder 34) and line 56 to the flash calcining operation of Figure lC for ~urther water removal to produce an alumina suitable for electInlysis in the Hall-Heroult process for t producing aluminum meta~.
3~ Alternatively, in the case where ~he higher temperature portion of the 250 to 650~C range in the decomposer has been used, the product in line 56 may be collected as feed _ 11_ ~ ~325~g f~r a Hall-Heroult electrolysis. Thus, Example I below shows that sufficiently low LOI values can be achieved ~or this purpose, The flash calclner may be built on the principles described in `'Alumina Ca]cination in the Fluid-Flash Calciner'`
by William ~. Fish, I.ight Metals 1974, Volum 3, The Metallurgical Society of the American Insti~ute of Mining, Metallurgical and Petroleum Engineers1 Inc., New York, New York, pages 673-682, and in "Experience with Operation of the Alcoa 10 Fluid Flash Calciner" by Edward W. Lussky, Li~ht Metals 1~80, The Metallurgical Society of AIME, Warrendale, PA, pages 69-79.
Alternative equipment are ~1) the F. L. Smidth calciner as described in a paper presented at the AIME annual meeting in Dallas in February, 1482, by B. E. Raahauge et al and (2) the Lurgi/VAW calciner as described in the Proeeedings of the Second International Symposium of ICSOBA, Volume 3, pages 201-214 (1971).
The flash calciner schematically il~ustrated ln Figure lC receives, e.g, boehmite particles through line 56 and brings 20 them to temperatures in the range 950 to 1220C by the action of the fuel through line 58 and the ho~ gas (line 60) which supports the combu~tion of the fuel. Alumina product is collected through line 62. Hot exhaus~ gases leave through line 64 and are cleaned of entrained alumina in separa~or 66, wlth solids being directed through line 68 to line 62 and hot exhaust traveling through line 40 to heat the decomposer.
Batch Process The proce~ of Fig~res lA to lC operates continuously.
The portlon in Figure lB may be operated in a batch mode as 3~ illustrated in Figure 2. Figure 1 numerals have been retained where the equipment is the same. This modification is achieved by substitution of a ba~ch decomposer 70 and suitable storage :

~ l32~7as60~28~ 99D

hoppers 72 and 74 to accommodate the still continuous feeds through line 24 from the Bayer process and through line 5~ to the flash calciner.
Batch decomposer 70 includes a lid 76, which may be moved between a pressure-sealing position (shown) and an open position (not shown) by releasing cla~p 78 and rotating counterclockwise, say 95, about hinge 80. On the lower end of the decomposer is a 100r 82. Lid 76 and floor 82 form, together with side walls 84, a chamber 86. With floor 82 in the 10 closed position, as shown, pressure-sealing of chamber 86 is :~
complete. Floor 82 can be rotated counterclockwise about hinge 85, upon release of clamp 86, to open chamber 86 for emptying a batch of product.
Chamber 86 is heated by heat exchanger 88, which is located within chamber 86 and provided with a flow of hot gas through lines bO and 42. Electric heating is another option.
Back pressure regulator or valve 90, firstly, acts to prevent steam in line 50 from flowing back into decomposer 70 when the pressure in the decomposer is still too low and, 20 secondly, opens line 50 to steam only after the desired pressure, e.g. llO psia, has been achieved in the decomposer.
In operation of the batch process, lid 76 is opened, and AltOH)3 from hopper 72 is loaded into chamber 86 through line 25. The resulting bed self-fluidizes from the action of heat exchanger 88 in driving free moisture and chemically combined water from the Al(0~)3 particles.
As ~ure and more water is driven off as steam, the pressu~e wit~i.rl chamber 86 r~ses. Steam moves through line 44 to ~yclone 46 as explained above for continuous operation. When 3~ the pressure on valve 90 builds up to a sufficient value, valve 90 opens and steam i8 supplied through line 50 to the Bayer process.

1 3 2 5 7 0 ~ 6 0 8 2 8--1 1 9 9D

When sufficient water has been removed, floor 82 is opened and e.g. boehmite product released from chamber 86 in direction 73 into hopper 7ll.
Before valve 90 opens, the Bayer process can be supplied from any auxili~ry steam source supplying steam of desired press~re. The auxiliary source can e.g. include a second decomposer 70 run in alternation with that illustrated in Figure 2, C. Products The steam produeing process of the inventlon as applied to Al(OH)3 gives new products.
Operating conditions of the pressure decomposer are: temperature: 250-650C; pressure (steam): 20-250 psig;
and residence time: 10-120 minutes. The hydrate decomposes under these conditions releasing water as steam under pressure and producing a partially calcined alumina. Our inves~igations - have shown that the dehydration of alumina hydrate under steam pressure progresses by initial conversion of the trihydrate (gibbsite) to the oxide-hydroxide boehmite. The boehmite formed 20 then decomposes to gamma and/or X-ray indifferent, or amorphous, alumina partially or fully depending on the temperature and residence time of the material in the decomposer.
Thus, the material coming from the decomposer ls boehmite and gamma and/or X-ray indifferent alumina pseudomorphous with aluminum hydroxide. The boehmite conten~ is usually rela~ively high, in the 10 to 50~ range. Each particle is composed of a plurality of crrs~als. Loss on ignition (LOI) (300-1200C) is in ~he range 1 ~o 12Z, ~he lower water content ma~erial being obtained using the high portion of the e.g. 250 3~ ~o 650C ~emperature range in the decomposer. Surface area will be in the r~n~e 10-100 m2/g (square meters per gram). Surface area data herein is BET, N2 adsorption.

60828~ 39D
~32s7as In the case where a high temperature, atmospheric pressure, i.e. at most 10 psig (resulting from blowers to move the gases), calcina~ion follows the decomposer, the resulting alumina attains an LOI (300-1200C) of less than 1%, a surface area in the range of 10-100 m2/g, and a modiied Forsythe-Hertwig attrition index of 1 to 20.
Attri~ion index data herein is determined according to the method of Forsythe and Hertwig, "Attrition Characteristics of Fluid Cracking Catalysts", Ind. and Engr. Chem. 41, ~
10 1200-1206, modified in tha~ samples are attrited for only 15 -minutes, this time having been found to be better for showing di~ferences in aluminas.
Attrition Index I - 100(X - Y)/X where X = percent plus 325 mesh before attrition, Y = percent plus 325 mesh after attrition.
The lower ~he attrition index, the higher is the resistance to attrition.
In general, the alumina product of the invention i5 much s~ronger, e.g. has a lower attrition index, than alumina 20 made by calcining Al(OH33 a~ atmospheric pressure.
There are beneflts in combining an initial pressure decomposi~ion with a terminal a~mospheric-pressure calcination.
For insta~ce, the terminal atmospheric-pressure calcination can be run at 850C, or less, to bring LOI (300-1200C) to lZ or below. This is to be compared to ~he over 950C required previously.
An addi~ional benefit is the ability to arrive at an alumina product of 1% LOI, or less, and of surface area in the range 10-lO0 m2fg, withou~ p~o~ucing any alpha-alumina, a ~rystalline form o~ poor solubility in the molten salt bath of a llall-Heroult cell. In general, the atmospheric-pressure calcination converts the boehmite of the intermediate product to 5 7 0 ~

ga~Tuna alumi~a. lilus ~ so/o boehmit~ content in the product of the pressure decomposer will mean about 50% gamma alumina content in the product obtained by submitting the pruduct of the pressure decomposer to an atmospheric-pressure calcination.
In both cases, deco~poser only or decomposer plus calciner, the product is characteri~ed by at least a reduced amount, and even an absence, of the parallel fissures so characteristic of the alumina that results from aluminum hydroxide calcined at the zero, to perhaps at most the 7, or 1~ 10, psig pressures previously used in dewatering the feed for Hall-Heroult cells.
Figure 3, compared with Figure 4, illustrates this important characteristic of products accordlng to the invention, namely the reduced amount, or even absence, parallel fissuring.
The prior art product (Reference No. AP T182B~ of Figure 4 was prepared by calcining Al(OH)3 in a flash calciner as described in the above-cited articles of Fish and Lussky. It measures l to 5~ boehmite, rest amorphous. As is evident from Figure 4, ~he prior art product is characterized by parallel 20 fissuring. This prior art alumina product had an LOI
(300-1200C) of .94%, a surface area of 84 m /g, and an attrition index of 15.
Thé product (Reference No. AP 4052-H2) of Figure 3, which is a product according ~o the invention, was prepared by heating Al(OH)3 at 500C, 120 psig pressure, for one hour under self-fluidizing conditions and then a~ 850C, atmospheric pressure ~0 psig), for one hour (the 850C treatment corresponding to a flash calcining operation). The product res~lLing from this treatment was about 50% gamma alumina, rest 3~ amorphous, and had an LOI of 0.5%, a surface area of 51 m2/g and an attri~ion index of 6. Clear from a comparison of Figure 3 with Figure 4 is, in this example, a complete absence of parallel fissuring in the product of the invention.

~ ~2~7~

It is thought that a beneficial result of the lack of parallel fissuring i5 an ability of the product of the invention to be handled and transported with a lesser production of fines as compared with material having the parallel fissurlng. In any event, the product of the invention is stronger, as evidenced by characteris~ically lower values of the modified Forsythe-Hertwig attrition index~ Thus, or a given Al(OH)3 feed, it is possible to achieve consistently lower attrition indices for the alumina feed for Hall-Heroult cells~ For example, for a given Al(OH)3 lO feed, the attrition index will be at least 2 units low~r for the product of the present invention than it would be for a product of the same LOI achieved in an atmospheric pressure calcination.
Furthermore, attrition indlces lower than those of previous commercial alumina products are obtainable in the presen~
invention. Thus, it is known that different Al(O~l)3 feeds give different attrition indices in atmospheric pressure calcination.
Lower attrition indices are said to result from well crystallized Al(OH)3 while higher indices come rom weakly crystallized Al(OH)3. Using well crystallized Al~OH)3 in t 20 atmospheric pressure calciners, the lowest attrition indices pre~iously obtained were 4 or ~. In the present inven~lon, well crystallized Al(OH)3 yields attrition indices of 2 or les5.
Further illustrative of the invention are the following examples:
Exa~ple I
A product (Reference ~o. AP 4203-6)`was decomposed according to the present invention by transferring heat ~hrough the walls of a container and into a bed of Al(OH)3 in the contalner. The ~emperature in the bed was 6009C. Hea~ing was 3~ for a perlod of one hour. Pressure was 120 psig. The bed self-fluidized under the action of the gaseous water, steam, coming off the particles. There was no particle breakage during ~ ~325~
this process, 3 .e~ ~n incre~n~ h~ wei~ht of the -325 mesh size fraction. The product obtained by this procedure had an LOI (300-1200C) of 1.5% 7 a surface area of 94 m2/g and an attrltion index of 4. X-ray diffraction analysis gives the following results: about 2% boehmite (X-ray diffraction pattern matching those of cards 5-01~0 ~nd 21-1307 of the Joint Committee on Powder Diffraction Standards, Swarthmore, PA3, rest gamma and amorphous.
Example II
A product (Reference No. AP 4064) was prepared according to the present invention by transferring heat through container walls and into a bed of Al(OH)3 in the decomposer con-tainer. Red conditions were 500C, 120 psig pressure, one hour residence time. The material in the bed self-fluidized under the action of the gaseous water being evolved during decomposition. A product containing 28% boehmite resulted.
Subsequently, the product of this high pressure treatment was heated at 850C, atmospheric pressure (Q psig), for one hour (this 850C treatment corresponding to a flash calcining 20 operation). The final product had an LOI (300-1200C) of 0.5%, a surface area of 60 m2~g and an attrition index of 2. X-ray diffraction analysis gave the following results: 0% boehmite, rest gamma and amorphous.
Fxample III
Products of the invention were prepared as in Example II, except for the following di~ferences. Decomposer bed temperature was 400~C. Analysis of the product from the decompose~: ~urEace area 40 m2lg, ~OI ~3.4ZJ ~t~rition index 5, %-boehmite 447. Analysis o~ th~ pro~uct from the 850C, 3~ atm~spheric-p~essure calcining: LOI 0.5%, %-boehmlte 0%, surface area 41 m2/g.

~32~70 r. amp e T~
Products of the invention were prepared as ln Exan~yle II, except for the following differences. Decomposer bed temperature was 400C, pressure 200 psig, time at pressure 3/4 hour. Analysis o the product from the decomposer: surface area 19 m /g LOI 16~, at~rition index 9, ~-boehmite 44Z.
Analysis of the product from the 850C, atmospheric-pressure calcining: LOI 0.4%, %-boehmite 0%, surface area 30 m2~g, attri~ion index 9.
xample V
Products of the invention were prepared as in Example II, except for the following differences. Decomposer bed temperature was 400C, pressure 60 psig, time at pressure 2 hours. Analysis of the product from the decomposer: surface area 63 m /g, LOI 11.3Z, ~-boehmite 30~. The atmospheric-pressure cal~ining was at 750C, again for 1 hour. Analysis of the product from this calcining was: LOI 0.9%, surface area 65 m /g and attrition index 6.
Various modiflcations may be made in the invention without departing from the spirit thereof, or the scope of the claims, and, therefore, the exact form shown is to be taken as illustrative only and not in a limiting sense, and i~ is desired that only such limitations shall be placed ~hereon as arP
imposed by the prior art, or are specifically set forth in the appended claims .

- 19 _

Claims

THE EMBODIMENTS OF THE INVENTION IN WHICH AN EXCLUSIVE
PROPERTY OR PRIVILEGE IS CLAIMED ARE DEFINED AS FOLLOWS:

1. A composition of matter comprising particles, each particle containing boehmite and gamma and/or X-ray indifferent alumina pseudomorphous with aluminum hydroxide, an absence of alpha-alumina, and having a reduced amount of parallel fissuring as compared to alumina from atmospheric pressure calcination of aluminum hydroxide, each particle containing a plurality of crystals, the LOI (300-1,200°C) of the particles being from 1 to 12%, their surface area being 10-100 m2/g.

2. A composition of matter as claimed in claim 1, wherein there is an absence of parallel fissuring.

3. Alumina having a surface area in the range 10-100 m2/g, an attrition index in the range 1-20, a reduced amount of parallel fissuring as compared to alumina from atmospheric pressure calcination of aluminum hydroxide and an absence of alpha-alumina.

4. Alumina as claimed in claim 3, wherein surface area is in the range 10-70 m2/g.

5. Alumina as claimed in claim 3, wherein there is an absence of parallel fissuring.

6. Alumina as claimed in claim 3, wherein the attrition index is less than or equal to 2.

7. Alumina as claimed in claim 3, wherein LOI
(300-1,200°C) is less than 1%.

8. A process for producing the composition as defined in claim 1, which comprises:

heating aluminum hydroxide which may be moistened at a temperature greater than 250°C in a decomposer at a pressure higher than one atmosphere gage pressure, i. for decomposing said aluminum hydroxide to drive off at least a part of the chemically combined water, and ii. yielding water in the form of steam at a pressure greater than one atmosphere gage pressure, and removing said steam from the decomposer.

9. A process for producing alumina having a surface area in the range 10-100 m2/g, an attrition index in the range 1-20, a reduced amount of parallel fissuring as compared to alumina from atmospheric pressure calcination of aluminum hydroxide and an absence of alpha-alumina, which process comprises the process of claim 8 under such conditions that substantially all aluminum hydroxide is converted to alumina or flash calcining of the composition produced by the process of claim 8.