GB2184715A - The production of alumina - Google Patents

Abstract

High purity alumina may be produced by heat-decomposing an aluminium halohydrate for example aluminium chlorohydrate.The decomposition may be achieved by distributing the chlorohydrate in finely divided form, whether as a liquid, slurry, or solid into a hot chamber, e.g. the chamber of a spray drier having an inlet temperature of, for example, 830 DEG C or 1000 DEG C and a residence time of, for example, 8 seconds followed by calcination. The calcined product may have purity in excess of 99.95%wt purity, HCl<150 ppm, Na<150 ppm, total organic carbon <200 ppm, surface area of 1-30m<2>/g, an elementary particle size of 0.05-0.1 mu and an agglomerate particle size of <10 mu (100%). The alumina is suitable for use as a high grade polishing alumina or as an ingredient in ceramic manufacture.

Description

SPECIFICATION The production of alumina This invention relates to the production of alumina.

Alumina may be produced by heat-decomposing aluminium chloride solutions. One example of such a process is the so-called "fast-heat" process according to which highly pure aluminium is dissolved in hydrochloricacidtoform a solution of aluminium chloride with the evolution of hydrogen,the aluminium chloride solution is directly contacted with hot reaction gases, to evaporate water and hydrochloric acid and to concentrate the solution, and the concentrated solution is pressurised and sprayed into a directly fired furnace capable of giving temperatures in the range 750 Cto 900 C so asto decomposethe aluminium chloridetoproduceacrudealumina which typically may still contain up to about 1000 ppm of chloride.

The aluminium chloride concentration stage ofthe above process involves a high energy requirement, and at concentrations above about 10%, expressed on a weight basis as Al2O3, a tendency to precipitation ofthehexahydrate makes itdifficultto spraythe concentrated aluminium chloride solution efficiently.

The present invention relates to a relatively low energy process for the production of alumina and/orto a process for the production of a very pure grade of alumina for lens polishing applications where a purity of at least 99.5% by weight is generally requi- red or as an ingredient in ceramicsforwhich a purity of at least 99.95% and often at least 99.99% by weight may be required.

The present invention provides a processforthe production of alumina by the heat-decomposition of an aluminium halohydrate. The term "aluminium halohydrate" is used herein to denote compounds having a general formula corresponding to aluminium halide in which a deficiency of halogen atoms is counter-balanced, at least partly by a content of hydroxyl ions. The general formula may therefore be expressed as Al(OH)rnX3rn.yH2O or a multiple thereof wheres is a halogen atom, mis a number less than 3 andyis a number art least equal to O. Such compounds may be characterised by their hydroxyl content which is expressed as a % of the theoretical content of the corresponding aluminium hydroxide, referred to in the art and hereafter as "% basicity".It is preferred to utilise aluminium chlorohydrate according to the invention.

The present invention may be put into effect by heat-decomposing the aluminium halohydrate in the form of solid particles or in the form of a solution or in the form of a solution containing a particulate solids phase.

Aluminium halohydrate in the form of solid part icles n may be heat-decomposed according to the in- vention bythe use of a gas conveyance heater, the particles of alumina so produced being collected by, for example, cyclone means. A scurry comprising a solution of aluminium halohydrate containing a substantial proportion of a particulate solids phase, for example more than 2% by weight ofthe slurry, may also be heat-decomposed by means of a gas conveyance heater. Such heaters are in common use in in dustry and the selection of a suitable model and its operation is within normal skiil in the art.

Preferably, the aluminium halohydrate heatdecomposed according to this invention is in the form of a concentrated solution or a dilute slurry con tainingasmall proportion,forexample2%byweight or less, and preferably no more than 1%, by weight of a particulate solids phase. Such a solids phase may comprise undissolved aluminium halohydrate and/ orotherinsoluble aluminium compounds such as hydrous alumina. Such high concentration solutions or dilute slurries may be sprayed, without undue difficulty, into the free space of a suitable heating chamber.

Stable aluminium halohydrate solutions of high concentration may readily be produced and this fact gives rise to substantial energy advantages ofthe presentprocess.Aluminium halohydratecon- centrations in the range ofgreaterthan 10% preferably greaterthan 12% by weight to about 26% by weight of dissolved halohydrate may be achieved although at concentrations above about 24% by weightthere may be a tendency to gel which may give rise to some problems of handling.All such concentrated solutions may be utilised according to the present invention although a particularly preferred concentration range is 15% by weight to 24% by weight. The above stated concentrations of halohydrate are on a calculated Al203 basis.

The processing of such concentrated solutions can give rise to a great energy saving in comparison with the normal maximum of about 10% by weight concentration calculated as Al2O3 at which aluminium chloride solutions are available.

Preferably the aluminium halohydrate utilised in the practice of the present invention has a basicity of at least 30% and preferably at least 50% for example from 65% to 85%. Such compounds are highly soluble in water and provide,for higher basicities, a greater concentration of aluminium. A higher basi city halohydrate gives a smaller evolution of halide on decomposition and therefore a smaller recovery facility is required and a lower energy requirement is involved.

Aluminium chlorohydrate or other aluminium halohydrate in solid form for use according to this invention may be produced by any suitable means.

One preferred means of producing it is bycontacting an aqueous liquor, containing aluminium halide, in atomised form, with a hot gas having a temperature immediately before contact with the liquor of above 1 900C, preferably of at least 500 C, while controlling the contact parameters to avoid complete loss of halide from the product. Preferably the temperature is less than about 1200 C. In order to achieve a satisfactory degree of control over the heating process to achieve the required basicity it is highly desirable to use a reactor which allows the establishment of a short and measurable residence time. Particularly preferred for this purpose is a spray drier or a pneumatic conveyor drier designed for contacttimes of less than 60 seconds and capable of achieving contact times of below 15 seconds for examplefrom 4 seconds to below 15 seconds, since such contact times are generally, although not necessarily invariably, required.

The inlet temperature, ofthe hot gas is preferably at least 500"C particularly preferably at least 700"C and is, further, preferably less than about 120000, particularly preferably less than 1050"C.

The outlettemperature of the hot gas, now mixed with water vapour and hydrochloric acid vapour, is preferably above 1 50"C. It will generallybefoundde- sirable forthe outlettemperature to exceed this and to be at least 1 750C.

It is preferred to perform one or more "sighting" runs in the reactor followed by the analysis oftie production its content of halogen and aluminium and the adjustment of the parameters of operation of the reactor, for example its inlettemperature, or the rate of gas flowthrough the reactor, to identifythe appropriate parametersforthe production of a pro duct having a given basicity. The heating para meters, e.g. the inlet temperature and/or the resid ence time may be controlled in response to the halogen and aluminium contents of the productto achieve a desired basicity.

The metal halide aqueous liquor used as a raw material forthe production of the aluminium halo hydrate is preferably relatively concentrated to re ducethe heat load on the process and enable a suit ably short residence time to be achieved. Preferably such solutions contain metal halide in at least 1%for example up to about 12% particularly suitably in from about 5% to about 11% by weight. Aqueous metal halide liquors may contain a proportion of the corresponding acid. Aluminium chloride solutions as described above may typically contain from 1% to 3% of hydrochloric acid. This is not detrimental to the use of the liquor as a ravv material.

The metal halohydrate product is produced in solid finely divided form and can have an average particle size less than 10 microns which is ideally suited for the production of alumina for ceramics app lications, which productmay be separated from the gases leaving the reactor by means of a cyclone. Que to the cooling effect cf vaporisation the temperatu re of such solids and of the gases in which they are, or have been, in contactwill generally be within the range suitable for processing by available high tem perature cyclone equipment having, for example, stainless steel dust collectors.

The product is a dry free-flowing finely divided easily water-soluble metal halohydrate particularly suitable as a raw material e.g. in the form of a solution having the required concentration in, for ex ampleåemineralisectwaterforthe production of highly pure alumina. This mode of carrying outthe present invention is relatively effective to reduce the quantity or organic impurities which may be present leading to the possibility of using, as a source ofthe aluminium chloride, industrial liquors which may, as a result of recovery from organic synthetic processes using aluminium containing Freidel Crafts catalysts, contain a high organic carbon content.

Aluminium chiorohydratefor use according to this invention may alternatively be produced in the form of a concentrated aqueous solution prepared by the dissolution of aluminium metal in aqueous hydro chloric acid. This process requires little heat input and may be operated at relatively low temperatures, for exam pie from 80"C to the refl ux temperature of the aqueous dissolution medium, which generally would be in the region of 105into 1 100C.While aluminium metal of 95% byweight, or greater, purity may be utilised it is preferred that at least 99.9% and particularly preferred that at least 99.99% by weight pure aluminium metal be used to avoid undue contamination ofthe alumina eventually to be produced therefrom. The concentration of the hydrochloric acid in the reaction medium is preferably in the range of about 5% to about 25% by weight, con centrations being in the range of about 10% to 20% by weight particularly suitable. The quantity ofthe hydrochloric acid may suitably be substantially stoichiometricfortheformation of aluminium chlor ohydrate having a desired basicity, and is preferably within 10% and particularly preferably within 5% of the stoichiometric amount.Aluminium chlorohyd rate solutions having concentrations of up to 26% by weight, expressed asAI203, may be achieved by the above process. The formation of aluminium chloro hydrate by the acidic dissolution of aluminium metal may be accelerated if desired by the addition of small quantities of suitable additives which may be suit able organic or inorganic compounds. Examples of suitable organic additives are aldoses or ketoses or their corresponding carboxylic acid or hydroxyl derivatives for example D-glucose, D-fructose, lactone, maltose,, sucrose, galactose, L-ascorbic acid or starch.Additives may suitably be used, if desired, in from 0.001% to 1.0% although, to reduce'organic contamination ofthe chlorohydrate, organic addit ives are preferably used in no more than 0.5%. The use of additives to achieve a reduction in dissolution time,from about 10 hoursto no morethan 6hours and even down to less than 5 hours, for example, is more fully disclosed in UK patent specification No.

2113666.

The utilisation of aluminium chlorohydrate produced bytheacidicdissolution of aluminium metal in the production of alumina according to this invention represents a particularly low cost process by virtue of the low energy requirements ofthefirst stage and the high concentration of aluminium in the chlorohydrate which is heat decomposed.

The heat-decomposition ofthe aluminium halohydrate according to the present invention is preferably accomplished at a temperature, before contact, of at least 600"C, particularly prefera biy at least 750"C and up to even 120000 or more, using suitable contact parameters to achieve removal of the majority of the halide and of the water, which may be present as a separate phase or bound in the halohydrate molecule as water of crystallisation, in the form of the corresponding acid. Since the quantity of water can vary greatly, according to the method of production,from the presence of a substantial aqueous phase where a solution of halohydrate is being processed to as iittle as less than 0.5 molecules of waterfor each molecule of halohydrate where the halohydrate is being processed in solid form, the required contact parameters may vary greatly. Suitably the contact para meters, notably inlettemperature and outlet temperature or residence time are determined by reference to the residual halide in the alumina product, and in the gaseous process effluent, as determined in trials. The residence time is preferably at least 5 seconds particularly preferably 5 to 60 seconds and/ ortheoutlettemperature at least 300"C.

The particular apparatus preferably used to achieve heat-decomposition ofthe aluminium halohydrate, preferably chlorohydrate according to this invention may be determined by the physical form of the halohydrate. Where aluminium chlorohydrate or other halohydrate in the form of a solution is used it is preferably sprayed, or otherwise projected in the form of fine droplets into a heating chamberwhich may be directly fired by combustion gases. One form of such an apparatus is a spray drier marketed bythe firm Drytec Limited. Where the chlorohydrate or other halohydrate is being processed in solid form a rotary calciner may be used. Suitable types of furnace may be determined by those skilled in the art from their background knowledge. Suitably the outlet ofthe heater is connected to a suitable cyclone operable at the elevated temperatures involved.

Typically the residual halide content, expressed as the corresponding acid, ofthe alumina productdirectly afterthe heat decomposition step, which may be conducted in one or more passes, may be from about 0.05% to about 10% on a weight basis depending on the temperature used, contents of halide in the lowered ofthis range being achievable if decomposition temperatures in the range of about950 C to 1 2000C be used.

It is a further and preferred feature of this invention thatthe content of halide may be reduced to below 150 ppm, for example to as little as about 100 ppm or even down to 10 ppm or below, by a final calcination step conducted at a temperature and for a duration effective to produce a sufficiently low impuritycon- tent Such calcination is preferably conducted at a temperature of from 600"C to 1 3000C particularly pre ferablyfrom 700"Cto 1200 C for at least 15 minutes, for example for 30 minutes to 90 minutes but may be varied up to several,forexample 5, hours oreven up to 1 or2 days if required to achieve a particularly low halide content.

The product of such a heat decomposition as above described, irrespective of whether a solid, solution orslurry input of halohydrate is used is finely divided free flowing, high purity alumina (e.g. upto over 99.95% or even over 99.99% by weight purity) which will usually also contain less than 200 parts per million oftotal organic carbon even where an organic accelerator is used to prepare the chlorohydrate starting material, or even down to 10 ppm or below oftotal organic carbon, although it may be preferred to restrict the quantity of accelerator in this case, for example to no more than, or below, 0.1% by weight of the reaction mixture. It is noted that such accelerators can be effective in quantities down to below0.005% byweightofthe reaction mixture.The alumina product of this invention may also contain below 100 ppm of soda, calculated as Na.

High purity alumina for lens polishing applications is generally required to have a surface area of about 10-20m2/g and for ceramics application of about 1 to 10 m2/g and to comprise, mainly, alpha-alumina or corundum. It is a feature ofthe present invention that the surface area and the type of alumina produced may be varied by control ofthe temperature and residence time used to achieve decomposition ofthe halohydrate. The alumina so produced may have a surface area in the range of about 1 to 30 m2/g but may readily be controlled at or near a given value within that range by sighting trials two achieveforexample the surface areas required for either of the two applications mentioned above.At surface areas in the range of 1 to 20 m2/g the alumina mainly comprises alpha alumina or corundum although above that surface area the content of gamma alumina increases. The product may have a density in the range of about 0.1 to 0.75 glce depending on particle size and comprise elementary particles in the 0.5 to 0.1 micron size range and agglomerates of which, depending on deagglomeration treatment, 100% may be below 10 microns or even below 4 microns, and at least 85% below 1 micron in diameter. The uniformly fine particle size of the alumina product of this invention makes it suitable, possibly after deagglomera- tion and sieving, for magnetic tape use.Deagglomeration, for example by grinding, is preferably performed in alumina-lined vessels to avoid contamination by undesired metals.

The invention will now be illustrated by means of the following Examples. Examples 1 and 2 show the preparation of a suitable aluminium chlorohydrate starting material for use according to the invention.

Examples 3 and 4 are examples ofthe invention.

Example 1 The production of a concentrated aluminium chlorohydrate solution bythe dissolution of aluminium metal.

438 g of 36 wt % hydrochloric acid is diluted with 445g demineralisedwaterand 117 g ofturningsof aluminium of purity in excessof99.9%wtareintro- duced. The reaction medium is heated to 950C and maintained at that temperature. Agitation of the reaction mixture by means of a nitrogen sparge is commenced as soon as any ofthe aluminium metal shows a tendency to float indicating hydrogenevol- ution.

When all ofthe aluminium has substantiallydissolved, judged visually, the reaction mixtureisfilte- red and the content of aluminium chlorohydrate in the filtrate and its basicity determined. The filtrate is a solution of aluminium chlorohydrate having a concentration of 20.2% by weight expressed as A1203 and a basicity of 64%. This corresponds to a 92% utilisation of aluminium.

Example2 The production of solid aluminium chlorohydrate by heating an aqueous liquor of aluminium chloride.

An aqueous aluminium chloride liquor having a concentration equivalent to 10.76% by weight expressed as Awl203, an acidity of 1.69% by weight expressed as HCl, and a total organic carbon content of 2035 parts per million is sprayed into a direct fired spray drier having an inlettemperature of 7750C and arranged to give a residence time of 8 seconds and an outlettemperature of 194"C. The solid product is collected and on analysis gives a composition correspondingtotheformula AI(OH)2,o7CI0.93.0 4H20 and a total organic carbon content calculated as C of only 177 parts per million. This product is a good quality aluminium chlorohydrate powder, having a particle size of less than 10 microns. which could dissolve readily in water.

Example3 The production of high purity alumina from solid aluminium chlorohydrate.

The dry powder product of Example 2 is fed into the same spray drier apparatus (Drytec Limited) using a powder feeder. The inlettemperature of the spray drier is 830"C, the residence time 8 seconds and the outlettemperature 310 C. The productana lyses 89.1% aluminium expressed as A1203.This product is then calcined in a rotary calcinerfor 1 hour at 750 C to give a highly pure ( > 99.5%v'eightAl2O3) finely divided free-flowing alumina having an agglomerate size of below 10 microns with an elementary particle size below 0.1 microns and containing only 109 parts per million of chloride, expressed as HCI, and little or no organic carbon the content ofwhich is further reduced from the input value of 177 parts per million, a soda content below 150 ppm calculated as Na and a surface area in the range 1 to 20 microns.

This product is suitable after suitable sizing and sieving operations for magnetictape use.

Example4 The production of high purity alumina from aluminium chlorohydrate liquor.

Aluminium chlorohydrate liquor is produced at a concentration of 15% by weight calculated as A1203 by dissolving aluminium metal having a purity of 99.98% by weight in Analar (Trade Mark) grade hydrochloric acid. The aluminium chlorohydrate has a basicity of 70%. The aluminium chlorohydrate liquor so produced is sprayed into the free space of a furnace at an inlettemperature of 1000 C for a resid- encetime of 8 seconds and the resulting entrained particles of alumina are withdrawn from the furnace at an outlet temperature of490'C. The alumina dir- ectly so produced has, before any further calcination, an alumina content of 99.9% by weight and a chloride content below 0.1% byweight calculated as Cl, a surface area in the range 5 microns to 20 microns, a bulk density afterdeagglomeration of 0.2 g/cc and essentially consists of alpha-alumina.

Claims (13)

1. A process forthe production of alumina by the heat decomposition of an aluminium compound the process being characterised in thatthe aluminium compound is an aluminium halohydrate.

2. A process as claimed in claim 1 wherein the aluminium halohydrate is aluminium chlorohydrate.

3. A process as claimed in claim 1 or2wherein the basicity ofthe aluminium halohydrate is from 50% to 85%.

4. A process as claimed in any preceding claim wherein aluminium halohydrate is heat decomposed by projecting it in the form of droplets of a solution or of particles of a solid into the free space of a heating chamber, controlling the residence time ofthe droplets or particles within the said space to achieve decomposition of the halohydratetherein and removing the resulting particles of alumina from the chamber.

5. A process as claimed in claim 4 wherein the halohydrate is in the form of a solution which contains from 0% to 2%, by weight of the solution, of solid particles.

6. A process as claimed in claim 5 wherein the solution of aluminium chlorohydrate has a concentration of above 12% by weight calculated asAI203.

7. A process as claimed in any one of claims 4 to 6 wherein the residence time of the droplets or part icles within the said space is controlled in the range5 seconds to 60 seconds.

8. A process as claimed in any preceding claim wherein the heat decomposition of the aluminium halohydrate is conducted at atemperature of from 600 C to 1200 C.

9. A process as claimed in any preceding claim wherein the heat decomposition of the aluminium halohydrate is controlled to achieve a residual chloride content in the alumina product directly produced thereby of below 10% by weight calculated as HCI.

10. A process as claimed in any preceding claim wherein the alumina product directly produced thereby is calcined at a temperature of from 600 C to 1300 Cforat least 15 minutes and so asto producea high purity alumina having a chloride contentcalculated as HCl of below 150 ppm, a total organic carbon content of below 200 ppm, an alumina content of at least 99.5% wt, a surface area of 1 to 30 m2/g, and a particle size substantially 100% below 10 microns.

11. A process as claimed in claim 1 and substantially as described herein.

12. A process as claimed in claim 1 and substantially as described herein with reference to Example 3 or4 herein.

13. Alumina having a chloride content calculated as HCI below 150 ppm, a soda content calculated as Na below 150 ppm, a total organic carbon content below 200 ppm, an alumina content of at least 99.5% and a surface area offrom 1 to 30 m2/g characterised by an elementary particle size of from 0.05 to 0.1 microns and a particle size of 100% below 10 microns.