WO2002079090A1

WO2002079090A1 - Process for producing synthetic spinel

Info

Publication number: WO2002079090A1
Application number: PCT/AU2002/000409
Authority: WO
Inventors: Halil Aral; Robert Charles Flann
Original assignee: Commonwealth Scientific And Industrial Research Organisation
Priority date: 2001-03-29
Filing date: 2002-03-28
Publication date: 2002-10-10
Also published as: AUPR409701A0

Abstract

A process for producing synthetic spinel includes the steps of forming a mixture (for instance as a slurry) of magnesium chloride with an aluminous material, and roasting the mixture to temperature and to a time sufficient for the magnesium chloride and the aluminous material to react to form a spinel. The magnesium chloride source is advantageously bittern, containing 20 to 40% MgCl2. A relatively impure waste material of the alumina industry such as ESP (Electro Static Precipitate) can be used as the aluminous material. This waste material has a superfine particle size resulting in the production of a very fine particle size spinel product (less than 30 νm) which can be used without further grinding in most commercial applications.

Description

PROCESS FOR PRODUCING SYNTHETIC SPINEL

The present invention relates to a process for producing synthetic spinel. The spinel group includes mixed metal oxides with cubic crystal structures which have the generalised formula AB₂0_4/ where A and B represent a wide range of divalent (Fe²⁺, Zn²⁺, Co²⁺, g²⁺, etc) and trivalent (Al³⁺, Cr³⁺, Fe³⁺) elements. The natural minerals of the spinel group are rarely seen in pure form and minor impurities exist in most cases. The common spinel is magnesium, aluminium spinel (MgAl₂0) or simply spinel as referred here in general.

Recently, high temperature furnaces have come to be operated under increasingly severe conditions and often furnace refractories became damaged as a result of erosion caused by FeO, MnO or CaO contained in the molten metal or slag or as a result of structural spalling induced by the penetration of slag. Spinel refractories perform well in a high-temperature and corrosive environment, and can be substituted for other refractories such as chrome- containing refractories whose disposal presents an enviroπmenta1 hazard.

Application of spinel containing refractories in the steel industry started in the late 1980 's in Japan, driven by the development of secondary steelmaking, which imposed very aggressive conditions on zircon castables used in ladle linings. Traditional zircon castables were found to be unsatisfactory under these conditions. Low cement high alumina spinel castables with superior performance were developed in late 1980's and replaced zircon castables in steel plants in Japan. However, the expansion of the spinel market did not grow in other parts of the world as fast as in Japan. The wider use of spinel is hampered by its relatively high cost which is determined by expensive raw material and high manufacturing and processing costs. Many attempts to produce low cost magnesium aluminium spinels of high quality have not resulted in commercial success .

Alumina-spinel monolithic refractories are high performance refractory material, resistant to acidic and basic attack and thermal shock. They are used as refractories for various applications, notably for cement kilns, middle checker section of glass furnaces, castables for lining the inner walls of ladles in steel plants, vacuum degassing vessels, hot-metal mixers, blast-furnace troughs and their covers, hot-metal mixer cars, tundishes and the like.

Natural and synthetic spinels were used as catalysts for catalytically cracking petroleum for more than 50 years. They were employed because they have catalytically active acid sites distributed over their extensive pore surfaces. Synthetic spinels, aside from their petroleum cracking capabilities, are also used as catalysts in reducing levels of noxious sulfur oxides such as S0₂ and S0₃, which are emitted in the course of burning fossil fuels such as petroleum. In this application magnesium-rich spinels are used in preference to those rich in alumina.

Natural and synthetic spinels are also used as a colour pigment in plastics, paint and ceramics industries. The coloured synthetic spinels are usually prepared by heating magnesia, alumina and the colouring agent at elevated temperatures in the presence of mineralisers such as alkali chlorides and fluorides.

Numerous patents have been published disclosing different techniques for preparing spinels. Solid state and hydrothermal processes are the two major routes to synthesise spinels. Most procedures employ metal oxides or oxidisable compounds, both of which are converted to a spinel by firing or fusion with or without pressure. In solid state spinel making processes, a magnesium compound and an aluminium compound are mixed to give the requisite molecular constitution, ground wet, and fired at temperatures up to about 1660°C as in U.S. Patent 2,618,566, or shaped before firing into pellets as in U.S. Patent 2,805,167. Others use pure magnesia and alumina mixtures which are then fired at 2150° C and cooled slowly overnight (e.g. U.S. Patent 3,516,839). U.S. Patent 3,950,504 employed ESP (electrostatic precipitator) dust, as in our invention, as a source of aluminous material, but differed significantly in other aspects. Their process comprised grinding ESP with finely divided magnesium hydroxide at a magnesium:aluminium weight ratio of about 0.4 to 0.8 and heating this mixture to a temperature of from about 1600 to 2100°C for a period of time to produce a spinel having high density and low sodium oxide content.

Romanian Patent 87,564 claimed a solid state process of making spinel suitable for fabrication of lining for metallurgical furnaces. The steps of this process were: (1) milling a mixture of 20-90% calcined alumina or corundum and 10-80% magnesite, Mg(0H)₂, sintered magnesia, or MgO to a particle size <0.04 mm as a MgO/Al₂0₃ ratio of (0.1-4.0) :1; adding 0.1-5.09& HCl, MgCl₂, HF, and/or MgS0₄ as a temporary binder, mineralisation agent, and crystallisation modifier; drying at 110-180°C, calcining at 1700-1900°C (preferably at 1820°C) to obtain a product having a density of 3.50-3.56 g/cm³, apparent density of 3.1-3.2 g/cm³, and apparent porosity of 0.5-10%. This invention differs from the current invention by employing magnesium chloride in smaller quantities as a mineralising agent, a temporary binder and crystallisation modifier. It is quite common in the art to use small quantities of mineralisers to improve the reaction rate. For example, Bakker and Lindsay (1967) report that a high density spinel body can be made from Mg(OH)₂ and Al(OH)₃ if 1.5% AlF₃ is added as a mineraliser.

Several hydrothermal processes are disclosed as an alternative to solid state synthesis of spinels. Researchers investigated the nature of metal double hydroxides formed by copreσipitation, some of which are shown to convert to a spinel upon calcination.

Bratton (1969a and 1969b) reported the coprecipitation of numerous magnesium and aluminium chloride solutions and oxalates which on heating, drying, calcining or firing, exhibited a spinel x-ray diffraction crystallographic pattern. Bratton in U.S. Patent 3,567,472 also disclosed coprecipitation of a magnesium and aluminium chloride from a solution having a pH from 9.5 to 10, drying or firing to obtain a light-transmitting spinel by adding CaO.

Dragunova and co-workers (1991) investigated the phases formed at a temperature range of 400-1100°C during heat treatment of precipitates of aqueous aluminium- magnesium solutions . The aluminium-magnesium spinel was prepared by hydrothermal processing by the addition of 2.5M solution of MgCl₂ or MgO in a solution of basic aluminium chloride Al₂ (OH)₄.₈Clι.₂.

In U.S. Patent 4,9120,78, spinels, Mg0.nAl₂0₃ (n=l and 2), are synthesised hydrothermally using commercially available hydroxide reactants. The synthesis is carried out in an aqueous suspension within an autoclave under a pressure of 4 MPa and at a corresponding saturated steam temperature of 270° C. The product characterisation has shown that the particle size ranges from 2 to 10 um. Sintering of preformed green bodies comprising compressed powder, without the use of additives, is then carried out at 1620°C at one atmosphere. The resulting product was 100% spinel, MgAl₂0₄, with a density greater than 94% of theoretical for spinel. The examination of the early work also show that spinels could be made richer in one of its components, namely magnesium or aluminium. The "magnesium-rich" spinels are those where the magnesium to aluminium ratio is greater than about 0.5. Conversely, if their ratio of magnesium to aluminium were less than about 0.5, the resulting spinels could be characterised as "aluminium rich" spinels . U.S. Patent 4,471,070, teaches methods of making "magnesium rich" spinels wherein the atomic ratio of magnesium to aluminium is purposely held >0.5 in order to enhance the spinel's SO_x catalytic activity. The process of making "magnesium rich" spinel in this patent involves precipitation of magnesium hydroxide and aluminium hydroxide from the mixture of magnesium nitrate and sodium aluminate at a pH of 9.5. The reaction is induced by the addition of sodium hydroxide to the Mg(N0₃)₂ - NaA10₂ mixture.

In the invention described in U.S. Patent 4,400,431, spinel was prepared by coprecipitating metal compounds, that is the metal halides, sulfates, formates, hydrogen phosphate, hydroxides, acetate, nitrate, carbonate, bicarbonate and the like, or mixtures thereof including hydroxycarbonate, chlorohydroxide, the halogenated carboxylates at pH in the range of from about 9 and 9.5. The product slurry may be treated with an alkaline solution before being filtered and washed. This alkaline wash may be used to increase the Mg/Al ratio of the coprecipitate by the selective dissolution of Al from the coprecipitate. The coprecipitate is then dried and calcined at a temperature of from 400° C to 1400° C. thereby forming the crystal lattice of the spinel structure with little or no segregated phases of either metal. The so-formed spinel can be sintered at a higher temperature.

This review of the literature shows that solid state heating of oxides or hydroxides of magnesium and aluminium to synthesise spinel requires high temperatures, often above 1600°C. This is not unexpected, as thermodynamic predictions (eg. Hallstedt, 1992) show reaction between MgO and Al₂0₃, to make synthetic spinel, requires significantly high temperatures. Hydrothermal synthesis of spinels is a complicated, lengthy process and requires costly reagents and mineralisers . Hydrothermal processes often require pH adjustments and addition of sodium hydroxide to provide precipitation of spinels. Most of the processes proposed in the art are laboratory scale demonstrations and far from being realistic for any commercial application. Therefore, there is a need for a simpler, straightforward and cost-effective process that can produce spinel in commercial scale operations.

Accordingly, the present invention provides a process for producing a synthetic spinel, the process including the steps of forming a mixture of magnesium chloride and an aluminous material and roasting the mixture at a temperature and for a time sufficient for the magnesium chloride and the aluminous material to react to form a spinel.

Preferably, the aluminous material is of a fine particle size, normally less than 53 μ and preferably less than 30 μm. The reason for the selection of the small particle size is to increase the surface area of the reagent and to obtain a small grain sized product. In the most common applications for the spinel product, a very fine (<20 μm) particle size is desired. However, in situations where a coarser grained product is desired, a coarser aluminous material could be used. A coarser aluminous material could also be used to produce a coarse product which could then be crushed to produce fine spinel. The mixture may be prepared as a slurry, in which case, the slurry is preferably dried before the resultant mixture is roasted. Drying and dehydration is appropriately conducted at a temperature of less than 500°C, more preferably less than 250°C. 500°C is the minimum temperature for conducting the roasting step. Whilst the slurry could be dried as a part of the roasting step by raising the temperature to greater than 500°C, this is not preferred since the rapid water evaporation leads to frothing. Consequently, gentle heating to remove water (and hydrogen chloride) is preferred. The process of the invention may also include one or more of the following steps:

(i) collecting evaporated water for use in washing impurities out of the aluminous material;

(ii) heating at a higher temperature to expel remaining water and chloride as hydrogen chloride; and (iii) scrubbing hydrogen chloride gas by condensation for use in manufacture and recycling of magnesium chloride.

Magnesium chloride forms hydrates with 2, 4, 6, 8 and 12 molecules of water. The 6-hydrate and 12-hydrate forms are of primary commercial importance, therefore they can also be used in the process of this invention. Aqueous magnesium chloride , another major source of magnesium for the production of spinel may be obtained from natural brines (such as sea water) or can be prepared synthetically by reacting magnesium oxide, magnesium hydroxide or magnesium carbonate with hydrochloric acid, pure, or recycled by-product of this invention. Bittern is a natural liquor rich mainly in magnesium chloride. It is a by-product of salt (NaCl) production from saline waters. The higher the magnesium chloride content of the bittern the better. Bitterns that contain 20 to 40% MgCl₂ are found suitable for this invention. The use of natural bittern as a cheap source of magnesium is one of the most preferred options of this invention.

Aluminous material used in this invention is generally a solid, comprising either an aluminium hydroxide or an aluminium oxide or a mixture of both in varying proportions. Aluminium hydroxide includes crystalline minerals such as gibbsite, bayerite, nordstrandite and boehmite, and amorphous varieties. Aluminium oxides include calcined aluminium hydroxides. The aluminium oxides could have degrees of crystallisation that vary according to the temperatureand the duration of the calcination.

Aluminous material may be a technical (semi-pure) or a reagent grade (highly pure) gibbsite bayerite, nordstrandite, boehmite and amorphous varieties, or relatively impure waste material of the alumina industry such as ESP (Electro Static Precipitate) dust. ESP is a superfine particle size (<20 μm) aluminous material collected in the dust extraction systems of alumina plants during calcination of alumina. It is generally a mixture of gibbsite and alumina (corundum) and contains small amounts of impurities (mainly hydroxides, sulphates and carbonates of magnesium, sodium and calcium) . ESP is often discarded and stored in waste dumps of enormous size around the world. It is an expense to the alumina producers to maintain the dump-sites as dry ESP becomes airborne easily causing dust problems. Therefore, the use of ESP is one of the most preferred options of this invention. Besides its attractiveness as a low cost material, its superfine particle size, high surface area and thermal reactivity are other physical properties that make ESP a valuable raw material for preparation of high density spinel products.

The utilisation of by-product and waste materials is increasingly imperative in today's environmentally and economically aware society, and complete utilisation of resources is a target to which all mining industries aspire.

The invention will now be explained with reference to the accompanying figure. Figure 1 represents a flow diagram of a preferred embodiment of the invention. As shown in the drawing, magnesium chloride and aluminous material, generally in stoichiometric amounts, are charged to the pre-heater 1, in which the moisture is expelled at a temperature less than 500°C and a hard, dry product is obtained. Colour giving agents may be incorporated into the feed material. The product is transferred to the heater 2, in which the remaining water and chlorides are expelled by heating above 500°C. The final product is a free flowing material with a particle size of less than 30 μ .

The water vapour generated during pre-heating may be collected by condensation and used for washing the aluminous material. Similarly, the hydrogen chloride gas generated in heater 2 may be condensed and used later to dissolve magnesia, magnesium hydroxide or magnesium carbonate to make more magnesium chloride. The final product of this invention shown in the flow diagram is a spinel, MgAl₂θ₄, or alternatively, a spinel containing an excess of either magnesium or aluminium oxide, or a colour stain depending on the required final product.

The process may comprise one, or several, or all of the above steps. Some of the steps may be combined into one. Heating at the lower and higher temperatures could be done separately in two different furnaces or in one furnace with adjustable temperature setting. Magnesium chloride solution of any concentration, natural or synthetically prepared, may be sprayed onto hot aluminous material in a suitable manner so that part or all of the heating steps may not be required. Condensation of water is preferred but found not essential in cases where water is easily accessible. Spinel produced by heating magnesium chloride and aluminous material according to this invention may be used to prepare sintered or fused spinels of marketable quality. Also, the spinel produced according to this invention may be heated at a temperature of up to 1400°C in the presence of excess aluminous or magnesium material to make spinel that contains excess alumina or magnesia.

Synthetic spinel produced from magnesium chloride and fine particle size aluminous material in the presence of colour-giving agents may be used as colour stains. For the preparation of stoichiometric spinel, one mole of Al₂0₃ is reacted with one mole of MgO. There may often be some variation in the ratio of Al₂0₃ to MgO in the reagent mixture, but generally this should be kept to less than ± 25%, or more preferably less than ± 10%, from the molar equivalent amounts.

The magnesium chloride and fine particle size aluminous material may alternatively be mixed in non- stoichiometric proportions and heated as prescribed in this specification to enhance the catalytic and S0_X scrubbing properties of the final product or to make refractory materials that contain spinel only as a partial component of the final product. Stoichiometric spinel or spinel containing higher amounts of magnesia or alumina may be prepared for this purpose by heating the appropriate amounts of magnesium chloride and aluminous material.

Heating at the lower temperature may be conducted below 500°C. The heating at the higher temperature may be conducted at a temperature above 500°C and preferably between 700°C to 1400°C. In general, 2 to 24 hours heating time is sufficient to react aluminous material and aqueous magnesium chloride completely. Longer heating time is required if the heating temperature is chosen below 1000°C. Gradually lesser heating time is needed at temperatures above 1000°C. Two hours heating time was found sufficient to react aluminous material and aqueous magnesium chloride completely at 1400°C. Generally, the temperatures and heating time should be selected to ensure a complete or partly complete reaction of the magnesium chloride and aluminous material. The temperature and the heating time should be such that the spinel made according to this invention should contain only acceptable amounts of moisture, chlorides, sulphates and other volatile impurities for the specific end use.

The heating process may be conducted in any suitable conventional manner, including utilising batch or continuous processing in a furnace. The initial (low temperature) heating (moisture evaporation) stage may be carried out in part or all together in solar ponds. The heating steps of the process may be conducted in a furnace of any size, shape and type including classic and circulating type fluidised beds, and static, rotating or vibrating type furnaces. These furnaces may be heated by employing electric resistance or gas (propane and similar) or any other suitable heat source. Alternatively, the slurry of magnesium chloride solution and aluminous solid material may be sprayed into a chamber that is heated to suitable temperature (s) . The heating and cooling of the heated material may be carried out in a way that it provides maximum crystallisation of the end product, spinel. The heating and cooling regimes during the heating process may be such that the spinel made according to this invention contains minimum amount of volatile substances and other impurities . Spinel produced according to this process is a fine powder, white or coloured. Those white or cream- coloured spinels, made according to this invention, can be used for manufacturing sintered and fused spinels, spinel- containing refractories and for other suitable applications. Coloured spinels are suitable for being used as a pigment in plastic, paint and ceramics industries. In the following description the invention is more fully described with reference to the accompanying examples. It should be understood, however, that the following description is illustrative only and should not be taken in any way as a restriction on the generality of the invention described above.

EXAMPLE 1 This example demonstrates that calcined alumina

(corundum) does not react to any large extent with caustic calcined magnesia at temperatures employed in this invention.

A 5.1g of BDH's finely ground (<53 μm) calcined reagent grade Al₂0₃ (purchased from BDH) was mixed with 4.8g reagent grade (UniLab) MgO (<70 μm) by mortar and pastle grinding to form a uniform mixture. The mixture was then heated at 1000°C for 16 hours and quenched in air to cool the product. A white fine powder was obtained. The XRD pattern indicated no significant reaction between the two. The mineral phases identified in this Example are shown in Table 1 to allow comparison with other examples. EXAMPLE 2

This example demonstrates the formation of spinel when natural bittern was used as a source of magnesium instead of magnesia.

7.5g natural bittern containing about 35 % MgCl₂ and 6.0g of BDH's finely ground (<53 μ ) calcined A1₂0₃ was mixed to form a uniform slurry and heated at 110°C in an oven to dryness to obtain a hard, partially dehydrated mixture of the two. This product, without any further treatment, was heated in the same vessel at 1000°C for 16 hours and quenched in air. A white, soft powder was obtained. The XRD work identified the crystallisation of mainly corundum and spinel, and to a minor extent periclase. The relative peak intensities of the identified mineral phases are shown in Table 1 to allow comparison with other examples.

EXAMPLE 3 This example demonstrates further improvement in the extent of spinel formation (compared to Example 2) when the dry mixture was heated for 4 hours.

Calcined alumina (corundum) was prepared by calcining 150. Og of fine powder (<7 μm) gibbsite at 1350°C for 1 hour. 40.9g of this freshly calcined alumina was mixed with 167g natural bittern containing about 35% MgCl₂ to form a uniform slurry and heated at 110°C in an oven to dryness to obtain a hard, partially dehydrated mixture of the two. This product, without any further treatment, was heated in the same vessel at 1000°C for 4 hours and cooled in the switched-off furnace. A fine cream-white coloured powder was obtained. The XRD work identified spinel and the presence of periclase and corundum. The relative peak intensities of the identified mineral phases are shown in Table 1 to allow comparison with other examples. EXAMPLE 4

This example demonstrates complete conversion to spinel when appropriate amounts of gibbsite and natural bittern were used.

7.41g natural bittern containing about 35 % MgCl₂ and 5.6 g of fine (<7 μm) powder of Al(OH)₃ were mixed to form a uniform slurry and heated at 110°C in an oven to dryness to obtain a hard, partially dehydrated mixture of the two. This product, without any further treatment, heated in the same vessel at 1000°C for 16 hours and cooled in the switched-off furnace. In both cases fine cream-white powder of spinel of different crystallinity were obtained. The sharper XRD pattern peaks for 1000°C heating indicated improvement in crystallinity compared with heating at a higher temperature. The relative peak intensities of the identified mineral phases are shown in Table 1 to allow comparison with other examples.

EXAMPLE 5

This example demonstrates partial formation of spinel when ESP dust and natural bittern were heated.

7.1g natural bittern containing about 35 % MgCl₂ and 5.6 g of fine (<20 μm) powder of ESP were mixed to form a slurry and heated at 110°C in an oven to dryness to obtain a hard, partially dehydrated mixture of the two.

This product, without any further treatment, was heated in the same vessel at 1000°C for 16 hours and quenched in air.

A white, soft powder was obtained. The XRD work identified the crystallisation of spinel and corundum. The relative peak intensities of the identified mineral phases are shown in Table 1 to allow comparison with other examples.

EXAMPLE 6 This example demonstrates making of an "alumina- rich" spinel when less bittern (compared to Example 4) is used. 3.65g natural bittern containing about 35% MgCl₂ and 5.64g of fine (<7 μm) powder of Al(OH)₃ were mixed to form a uniform slurry and heated at 110°C in an oven to dryness to obtain a hard, partially dehydrated mixture of the two. This product, without any further treatment, was heated in the same vessel at 1000°C for 16 hours and quenched in air. A white, soft powder was obtained. The XRD pattern showed the formation of spinel and corundum. The relative peak intensities of the identified mineral phases are shown in Table 1 to allow comparison with other examples .

Table 1. Relative XRD peak intensities representing major mineral phases in Examples 1-6.

* 2θ value is given as d-spacing value in angstroms.

EXAMPLE 7

This example demonstrates the increasing crystallinity of spinel with progressive heating.

6.6g of reagent grade anhydrous magnesium chloride (BHD) and 11.lg of finely divided gibbsite were mixed with a spatula and heated progressively from 1000°C up to 1350°C in a muffle furnace. Samples were collected at 1000°, 1200° and 1350°C for x-ray diffraction analysis. The XRD peak intensities of mineral phases at select peak positions are shown in Table 2 for each sample. Table 2. The effect of temperature on the crystallinity of spinel .

* 2θ value is given as d-spacing value in angstroms.

The data in Table 2 show the improvement in the crystallisation of spinel with increasing heating temperature.

EXAMPLE 8

This example demonstrates that the amount of chloride (in the form of hydrogen chloride gas) released is temperature dependent, and shows that a greater amount of chloride is released at higher temperatures .

A bulk sample of MgCl₂ solution used in the above tests and fine particle size gibbsite were evaporated to complete dryness by heating in an oven at 180°C overnight. Sub-samples of this material were taken to determine if the hydrogen chloride given off when the solid is heated at 700 and 1000°C (according to Equation 1) could be determined analytically.

MgCl₂ + 2Al(0H) ₃ === MgAl₂0₄ + 2HC1 Equation 1

The calcination tests were done such that the HC1 gas is scrubbed with water. The scrubbing solution was titrated against a standard base using bromocresol green as an indicator to determine how much HC1 gas was given off. In this particular case, the titration data on a 1000°C heated sample showed 70% of the chloride was recovered as HCl. The XRF analyses of the starting material and the reaction product from each reaction are given in Table 3.

Table 3. The effect of temperature on the composition of spinel.

EXAMPLE 9

This example demonstrates production of a blue pigment by adding cobaltous chloride to Example . 7.41g natural bittern containing about 35 % MgCl₂ and 5.6 g of fine (<7 μm) powder of Al(OH)₃ were mixed to form a uniform slurry. This slurry is mixed with l.Og of reagent grade (AnalR) crystalline cobaltous chloride (CoCl₂.6H₂0) and heated on a hot plate while stirred with a spatula to dryness to obtain a hard, partially dehydrated, blue coloured product. This product, without any further treatment, heated in the same vessel to 1200°C for 1 hour and naturally quenched in air. The cooled product was a vivid blue coloured fine powder suitable for being used as a pigment without a further grinding.

EXAMPLE 10

A synthetic spinal was produced using the procedure outlined in Example 5, with the exception that the roast was conducted at 1050°C for 20 hours. The product was slightly non-stoichiometric; the excess Al₂0₃ was α-alumina. The product had a particle size of 20 μ and the following composition:

A1₂0₃(%) 76.5

MgO(%) 22.0

Cl(%) 0.56

Na₂0(%) 0.59

CaO(%) 0.51

K₂0(%) 0.06

Fe₂0₃ (%) 0.02

Si, Cr, Mn, Zr levels all <0.005%.

Claims

THE CLAIMS DEFINING THE INVENTION ARE AS FOLLOWS:

1. A process for producing a synthetic spinel, the process including the steps of forming a mixture of magnesium chloride and an aluminous material and roasting the mixture at a temperature and for a time sufficient for the magnesium chloride and the aluminous material to react to form a spinel.

2. The process as claimed in claim 1, wherein the step of forming a mixture of magnesium chloride and the aluminous material involves forming a slurry of magnesium chloride and the aluminous material .

3. The process as claimed in claim 2, wherein the slurry is dried before the resultant mixture is roasted.

4. The process as claimed in claim 3, wherein the slurry is dried at a temperature of less than 500°C.

5. The process as claimed in claim 3 or claim 4, wherein water evaporated in the drying step is collected and used for washing impurities out of the aluminous material prior to formation of the slurry.

6. The process as claimed in any one of claims 3 to 5, wherein, following drying, the mixture is heated at a temperature greater than the drying temperature to expel remaining water and chloride as hydrogen chloride.

7. The process as claimed in claim 6, wherein hydrogen chloride gas expelled is scrubbed by condensation for use in manufacture and recycling of magnesium chloride.

8. The process as claimed in any one of the preceding claims, wherein the roasting step is conducted at a temperature above 500°C.

9. The process as claimed in claim 8, wherein the roasting step is conducted at a temperature between 700°C to 1400°C.

10. The process as claimed in any one of the preceding claims, wherein the roasting time is between 2 and 24 hours.

11. The process as claimed in any one of the preceding claims, wherein the magnesium chloride used to form the mixture is in the 6-hydrate and/or 12-hydrate form.

12. The process as claimed in any one of the preceding claims, wherein the magnesium chloride source is bittern.

13. The process as claimed in claim 12, wherein the bittern is obtained from natural brine.

14. The process as claimed in claim 12, wherein the bittern is prepared by reacting magnesium oxide, magnesium hydroxide or magnesium carbonate with hydrochloric acid.

15. The process as claimed in claim 14, wherein the hydrochloric acid is recycled from the process for producing synthetic spinel.

16. The process as claimed in any one of the preceding claims, wherein the magnesium chloride is provided as an aqueous solution containing 20 to 40% MgCl₂.

17. The process as claimed in any one of the preceding claims, wherein the aluminous material is an aluminium hydroxide or an aluminium oxide or a mixture of both.

18. The process as claimed in claim 17, wherein the source of the aluminium hydroxide is selected from the group consisting of gibbsite, bayerite, nordstrandite and boehmite, each of which may be crystalline or amorphous.

19. The process as claimed in claim 17, wherein the aluminium oxide is derived from calcined aluminium hydroxide .

20. The process as claimed in any one of the preceding claims, wherein the average particle size of the aluminous material is less than or equal to 60 μm.

21. The process as claimed in any one of the preceding claims, wherein the average particle size of the aluminous material is less than or equal to 30 μm.

22. The process as claimed in any one of the preceding claims, wherein the aluminous material contains or comprises a waste material of the alumina industry.

23. The process as claimed in any one of the preceding claims, wherein the aluminous material is ESP (Electro Static Precipitate) dust.

24. The process as claimed in claim 23, wherein the ESP has a particle size of less than or equal to 20 μm.

25. The process as claimed in any one of the preceding claims, wherein the product immediately following the roasting stage has an average particle size of less than or equal to 30 μm.

26. The process as claimed in any one of the preceding claims, wherein a colouring agent is incorporated into the feed material.

27. The process as claimed in any one of the preceding claims, wherein the spinel produced is MgAl₂0₄, or a spinel containing an excess of either magnesium or aluminium oxide, or a colour stained spinel.

28. Use of bittern in the production of synthetic spinel .

29. Use of Electro Static Precipitate dust in the production of synthetic spinel.

Dated this 28th day of March 2002.

COMMONWEALTH SCIENTIFIC AND INDUSTRIAL RESEARCH

ORGANISATION

By their Patent Attorneys

GRIFFITH HACK Fellows Institute of Patent and

Trade Mark Attorneys of Australia