GB1569033A - Production of metallurgically pure alumina - Google Patents

Description

(54) PRODUCTION OF METALLURGICALLY PURE ALUMINA (71) We, ALUMINIUM PECHINEY, a French body corporate, of 28, rue de Bonnel, 69433 Lyon Cedex 3, France, do hereby declare the invention for which we pray that a patent may be granted to us and the method by which it is to be performed to be particularly described in and by the following statement: The present invention is concerned with the production of metallurgically pure alumina, i.e. alumina containing less than 0.03 % of Fe203, less than 0.5% of Na2O or K2O and less than 0.003% of Ti, by sulphuric acid attack upon a silico-aluminous substance containing titanium, iron and less than 0.1% of K2O.

Sulphuric acid attack upon an aluminous substance to extract the alumina from it was first proposed many years ago. Mention might be made, for example, of French Patent No.

574,983, which describes the attack of sulphuric acid on an ore, treatment of the resulting solution with hydrochloric acid and decomposition by heat of the hexahydrated alumium chloride thus formed. However, this document gave no details regarding the means to be used for eliminating the impurities which accompany the aluminium in the ore and which differ very greatly from ore to ore.

In later documents, for example in French Patents No.s 1,558,347 and 71 42 250, we proposed cyclic methods, each of which uses means for eliminating certain impurities: iron and potassium are largely eliminated in the form of a ferric potassium double sulphate Fe2(SO4)3. K2SO4; the greater part of the titanium remains insoluble and is contained in the insoluble residue resulting from the attack by sulphuric acid.

The liquor resulting from the attack contains only a small portion of the initial titanium and of the initial ferric iron; it also contains ferrous iron if this was present in the ore when the latter was subjected to attack. The methods described do not enable titanium to be readily separated when the ore contains little potassium and if it contains a relatively high proportion of titanium as compared with its iron content.

The method in accordance with the present invention is well suited to the treatment of natural or residual silico-aluminous substances containing iron and titanium and practically no potassium; such substances include, for example, certain siliceous bauxites and kaolin clays, the latter being characterized by the presence of kaolinite as the main constituent, by their very low contents of the alkali metals sodium and potassium, and by the presence of considerable quantities of titanium and of variable quantities of iron compounds. These substances may be in the crude wet state when treated in accordance with the present invention, thus saving energy used in drying and calination.

Such ores do not contain enough potassium to ensure elimination of iron by the method described in the above-mentioned French Patents No.s 1,558,347 and 71 42 250; the introduction of potassium sulphate, which would permit of such elimination of iron, would not permit of easy elimination of the titanium compounds solubilized during attack.

The present invention is based on a number of discoveries. It was first unexpectedly found that, contrary to what had previously been believed, sulphuric acid attack on raw clays and shales, e.g. the above-mentioned initial crude substances, can, in certain circumstances, lead to concentrated solutions containing dissolved aluminium sulphate. The concentration of such solutions may be as high as about 150 g/l of Al203, say 10 to 12% by weight of Al203, when they are obtained at atmospheric pressure and temperatures up to 140"C.

It was also found that the conditions necessary for obtaining these high concentrations include the use of a high ratio of weight of ore to volume of sulphuric solution used, and a relatively low content of free sulphuric acid in the solution resulting from attack; these two conditions have to exist simultaneously.

It was also found that ammonium sulphate is an agent which, when added to the sulphuric acid solutions used for the attack under certain conditions, permits of precipitation of iron and titanium and their subsequent recovery; moreover, the sulphate and ammonium ions can be readily recycled. The quantity of ammonium sulphate to be introduced into the sulphuric acid attacking solution must be related to the quantities of iron and titanium that are solubilized when the ore is attacked; as regards iron, account will be taken not only of the solubilized iron in the ferric state, but also the ferrous compounds, a proportion of which is eliminated directly and the remainer of which is converted into very soluble ferric compounds before separation of the aluminium sulphate.

In order to achieve adequate elimination of the iron and titanium, it is necessary for the attack solution to contain an excess of ammonium sulphate over the stoichiometric quantities corresponding to the formation of a double sulphate of the type Fe2(SO4)3.(NH4)2SO4 and of a titanium compound containing one mole of (NH4)2SO4)2SO4 for each mole of TiO2, and this is achieved by the proportions used in accordance with the present invention.

The main features of the method in accordance with the invention include introducing the silico-aluminous substance containing iron and titanium and less than 0.1% by weight of potassium, calculated as K20, into an aqueous solution containing 49 to 59% by weight of free sulphuric acid (i.e. that which is available to link with metal ions) and 6 to 8 % by weight of ammonium sulphate, together with residual quantities of aluminium sulphate and sulphates of the various metals constituting impurities in the material being treated, thus forming a suspension of fine particles of this material in the solution, the quantity of solids per unit volume of the attacking solution increasing with the aluminium sulphate content required in the liquor resulting from the attack and being, for example, about 370 kg of crude ore having a moisture content of 20% per m3 of attack solution for obtaining a solution containing 6.8% of dissolved aluminium sulphate, calculated as A1203; heating the resulting suspension at a temperature in the range 125 to 135"C for from 1 to 5 hours; separating the siliceous residue resulting from the sulphuric-acid attack, which residue is impregnated with mother liquors, from the rest of the liquid; treating this residue to remove the mother liquors with which it is impregnated by means of a limited quantity of a recycled sulphuric acid solution containing only a small quantity of alumina, and recombining the resultant liquid with the liquor from the acid attack washing the said siliceous residue with water and discarding it, recombining the remainder of the mother liquor from the aluminium acid sulphate with the washings from the said siliceous residue, concentrating the resulting mixed liquor and holding it at 80"C for from 2 to 3 hours in order to precipitate a complex sulphate of iron, titanium and ammonium with other materials, decomposing the complex sulphate to form a solid residue containing principally iron and titanium compounds and a gaseous mixture including ammonia and oxides of sulphur, recycling the gaseous mixture to form ammonium sulphate and sulphuric acid in the solution in which the said silico-aluminous substance is originally treated, washing the precipitate of aluminium acid sulphate, bringing it into contact with a concentrated solution of hydrochloric acid to produce a solution or suspension having a high concentration of aluminium and passing gaseous hydrogen chloride through the said solution or suspension to precipitate aluminium chloride hexahydrate, and decomposing the latter by heat to produce gaseous hydrogen chloride and the desired metallurgically pure alumina, and recycling the gaseous hydrogen chloride to the chlorination stage.

The solution used for the sulphuric-acid attack is thus formed by recycled solutions. In addition to sulphuric acid, it contains ammonium sulphate and various metallic sulphates, the amount of the metallic sulphates being within the limit of solubility of each of them in the acid solutions of the cycle. The solutions from which the attack solution is obtained will have been recharged with sulphuric acid and/or ammonium sulphate to compensate for the losses of sulphuric acid associated with other metals, for example, the calcium present in the ore, and to compensate for unavoidable losses that occur in the operating the method on an industrial scale.

The following details set forth a practical method of carrying out the invention.

After separation, the residue from the attack is treated to remove the mother liquors, which are of the same composition as the liquid that has been separated from them. For this purpose, use is made of part of the sulphuric acid separated after the major part of the aluminium sulphate has been removed, as will be described hereinafter. The volume of this solution is slightly greater than the volume of the mother liquors that are to be eliminated.

Thus there is obtained a residue impregnated with a sulphuric acid solution containing iron and titanium and little alumina. This residue is washed with water to provide a reside material, which consists mainly of siliceous products, and a dilute sulphuric acid solu containing sulphates of iron, titanium and aluminium.

The liquor resulting from the attack is mixed with the mother liquors removed from the residue from the attack, and subjected, usually at a temperature in the range 100 to 1200C, to vacuum evaporation and then to cooling to below 80"C as described in the specification of our copending Patent Application No. 44670/76 (1 537 904). This provides a precipitate of an aluminium acid sulphate having the formula Al2(SO4)3. 0.5 H2SO4 (11 to 12) H20, which is readily separable from its mother liquor, and a sulphuric acid solution containing, in addition to a portion of the aluminium sulphate, the greater part of the sulphates of iron, titanium and ammonium contained in the liquor after the attack.The aluminium acid sulphate precipitate is brought into solution or into suspension in a concentrated hydrochloric acid solution which is treated with HC1 by introducing hydrogen chloride gas.

The aluminium chloride thus obtained is washed with HC1 and is of high purity: equivalent to that obtained by existing techniques, which involve at least two chloridecrystallization steps. The aluminium chloride is calcined in the manner described in French Patent No. 1,558,347. The mother liquors from the aluminium chloride precipitation may be treated to eliminate sodium from them, and a chlorosulphonic solution is obtained, from which the hydrochloric acid is separated. Finally, a sulphuric-acid solution is obtained and this is recycled to the attack zone.

The mother liquor from the aluminium acid sulphate crystallization, which contains the greater part of the iron sulphates and titanium sulphates dissolved during attack, is used in part for removing the mother liquors which impregnate the attack residue; the rest is mixed with the liquors used for washing this residue. Iron, titanium and other minor impurities are extracted from this liquor in the form of a complex precipitate containing double sulphates of iron and ammonium and of titanium and ammonium. For this purpose, use can be made of the method described and claimed in the specification of our copending Patent Application No. 43331/76 (Serial No. 1,537,904). The residual solution resulting from this treatment is combined with the sulphuric acid solution resulting from the separation of hydrochloric acid; the combination formed by these solutions is recycled to the attack zone.The precipitate containing the double sulphates of iron and ammonium and of titanium and ammonium is calcined to produce ferric oxide, titanium dioxide and gaseous components comprising sulphur oxides and ammonia, which are reintroduced into the cycle in the form of sulphuric acid and ammonium sulphate, further amounts of these compounds being added if necessary.

As will have been seen, the method of the invention is a cyclic method in which, in addition to pure alumina, a mixture of oxides or iron and titanium is isolated, and in which, moreover, the consumptlon of the reagents sulphuric acid, ammonia, ammonium sulphate and hydrochloric acid is extremely low since these reagents are recycled.The main losses of sulphuric acid are due to the various sulphates entrained in the siliceous residue and losses relating to the mechanics of production; the yields of alumina and iron are respectively 96 and 95 % of the aluminium and iron contained in the original ores; the titanium yield can be adjusted as required, the remainder of the titanium remaining insoluble during attack and being contained in the residue; as much as 95 % of the titanium brought into solution during attack can be extracted.

The method of the invention will be better understood by reference to the single figure of the accompanying drawings, which shows in diagrammatic form an example of an arrangement for carrying out the method.

In the drawing, ore and certain recycled solutions are introduced into an attack vessel A.

The slurry obtained is separated at B into a cake S1 and liquor L1. The mother liquors in the cake are removed at C by means of a solution L4, the formation of which will be explained below. The mother liquors L2 extracted in this way are combined with the solution L1. The resultant cake S2 is washed with water at D, and this provides a solution L3 and a mainly siliceous residue SJ, which is disposed of. The combined solutions L1 and L2 are concentrated at E and then cooled to crystallize, at F, an aluminium sulphate having the formula Al2(SO4)3 . 0.5H2SO4 (11 to 12)H20. At G the resulting crystals S4 are separated from a solution L4 containing iron, the titanium brought into solution, and ammonium sulphate.

One part of this solution L4 is used, as stated above, for removing, at C, the mother liquors with which the cake S, was impregnated. The other part is combined with the solution L3 and treated as described below.

The precipitate S4 is washed at H with a recycled sulphuric-acid solution and the resulting liquor L5 is recycled to the attack zone A, and the washed precipitate Sg is dissolved or brought into suspension at I in a concentrated solution of hydrochloric acid; this solution (or this suspension) is kept saturated with hydrochloric acid at J by the injection of hyd rogen chloride gas. In this way crystals 56 of AICl3.6H20 are obtained, and these are washed and calcined at L to obtain the required alumina and moist gaseous hydrogen chloride. The latter is condensed, absorbed and recycled to I in the form of concentrated aqueous solution of hydrochloric acid. The liquor L6, separated from the crystals S6, is a chlorosulphonic solution containing aluminium and possibly sodium.The latter, if present, is extracted at M and for example, in the manner indicated in French Patent No. 1,558,347, to form sodium chloride 57 and a HC1-containing liquor L7. The solution L7 obtained is treated to eliminate gaseous hydrogen chloride, which is recycled to J, and produce sulphuric-acid solution L8, which is used partly for washing the precipitate Sq atH. The remainder is combined with the liquid L5 and, after concentration, with the liquid L9 resulting from separation of the iron and titanium precipitate; this mixture is finally recycled to the attack zone. The liquors L3 and 14 are concentrated at R and are treated at P to precipitate therefrom iron and titanium in the form of a precipitate Sg of complex sulphates or iron, ammonium and titanium.The precipitate Sg is separated at Q from a sulphuric acid solution L9, which is recycled as stated above. The precipitate Sg is decomposed by heat and this permits of separation of ferric oxide and titanium dioxide in the solid conditicn and the gaseous products sulphur dioxide, sulphur trioxide and ammonia, which are recycled after conversion into sulphuric acid and ammonium sulphate.

The following Example is given to illustrate the invention. In the Example, compositions are on a weight basis and the system used is that described above with reference to the drawings.

EXAMPLE A kaolin having the following composition when dry was treated by the method of the invention: A1203 36.7% SiO2 44.9% Foe203 0.94% TiO2 1.83% K2O 0.02% Na2O 0.06% CaO 0.56% various 1.09% loss on burning 13.9% 3675 kg of this undried ore (at 20% humidity) was broken down by pulping in approximately 10.000 kg of a solution containing 53.4% of free sulphuric acid, 7% of ammonium sulphate and 5 % of various metallic sulphates.

After this suspension had been obtained, 5556 kg of the same solution was added, and the mixture was raised to a temperature of 1300C over a period of 3 hours, at A. The solid and liquid constituents of the resultant slurry were separated from each other by filtration.

The cake S l was washed at C with 4425 kg of a recycled sulphuric acid solution containing 46.6 % of free sulphuric acid and 1.1 % of A1203. The cake washed in this way contained only very little alumina. It was washed with water for a second time at D.

The dried residue 53, which contained SiO2 1317 kg A1203 51 kg Fe203 1.5 kg various 252 kg was discarded.

The mother liquors L2, the composition of which approximated to that of the liquor resulting from the attack, were added to this latter liquor.

The washing liquors L3, which had a total weight of 4450 kg and contained A12 3 0.75% Fe203 0.27% TiO2 0.32% (N H4)2SO4 7.06% H2SO4 total 39.2% H2S04 free 30.9% were treated to extract a quantity of iron and titanium corresponding to the quantities introduced during attack.

The liquors L , and L2, which weighed 19.040 kg and contained in total A1203 1211 kg Fe203 55 kg TiO2 62 kg NH3 320 kg SO4H2 free 5959 kg were concentrated at E by subjecting them to effects of a vacuum, to give a free sulphuric acid of concentration of 33%, and they were then cooled at F to a temperature between 80 and 40"C.

In this way an aluminium acid sulphate S4 having the formula Al2(SO4)3 .0.5 H2SO4 11 .5H20 was precipitated, and this was separated from its mother liquors at G. By washing this precipitate Sq at H with a recycled sulphuric acid solution, 8680 kg of a precipitate S5 of this acid sulphate solution was obtained, this precipitate containing only 9 kg of Fe203, 7 kg of Ti02 and less than 1 kg of Na2SO4.

This precipitate Sg was separated from its washing liquors L5 and was dissolved or suspended in a concentrated hydrochloric acid solution at I. The suspension or solution obtained was saturated at J with 1640 kg of gaseous HCI at a temperature of 52"C. In this way a precipitate S6 of aluminium chloride hexahydrate (AlCl3.6H20) was crystallized; this had been separated from the liquorL6 which weighed 12,580 kg and contained: Al203 0.5% Fe203 0.07% TiO2 0.05% NH3 0.05% H2SO4 total 38.2% HCI 13.7% The treated ore contained a sufficiently low quantity of sodium to render it unnecessary to treat the liquor L6 for the purpose of eliminating sodium from it.

The filtrate L6 was heated to regenerate gaseous HCl, which was recycled to J. The quantity of dry gaseous HCI thus recycled was 1640 kg. The quantities of H2SO4 and HCI necessary for compensating losses were introduced from the recycled sulphuric acid into the liquor L6 at this point along the circuit.

The liquor L8 was almost completely free from HCI and had a mass of 11,230 kg and contained: A1203 0.6% Fe203 0.08% TiO2 0.06% NH3 0.06% H2SO4 total 44.0% H2SO4 free 42.1% HCI This liquor was separated into two parts, one of which, approximately 4600 kg in weight, was used for washing the precipitate 54, while the other was mixed with the liquor L5 resulting from this washing operation. These two liquors were together passed to an evaporator K which brough their free H2SO4 concentration to that of the liquor introduced into the attack vessel A.This concentrated liquor weighed 8910 kg and contained: Alas3 0.7% Fe203 0.19% TiO2 0.18% NH3 0.7% H,SO4 free 53.0% H2S04 total 58.7% The diluted hydrochloric acid solution resulting from evaporation at K was used for absorbing the wet hydrogen chloride gas resulting from calcination of the precipitate S6.

During this absorption stage, a 32% HC1 solution was obtained and this was recycled ati for dissolving the precipitate Sg or bringing it into suspension. It had been used previously for washing the precipitate S6. The cake washed in this way weighed 5075 kg and was constituted by 4783 kg of AlCl3.6H20 impregnated with 292 kg of washing liquor. On calcination at L, the precipitate S6 yielded 1000 kg of Al203, allowing for losses due to the mechanics of the calcination process.

Part of the liquor L4 and part of the liquor Lg resulting from the washing of the precipitate 52 with water were treated to eliminate impurities dissolved at various points in the cycle and contained in these liquors L3 and L4, particularly Fe203 and TiO2, and possibly other materials such as MgO and P205.

A treatment for these liquors was described in the specification of our copending Patent Application No. 43331/76. In accordance with the procedure described in that Application, 4970 kg of the liquor L4 and 4450 kg of the liquor L3 were mixed, and 67 kg of recycled ammonium sulphate was added to the mixture. The resultant mixture was concentrated at R by evaporating off 2450 kg of water; this solution was then held at 80"C for three hours at P, and a suspension was thus obtained.At Q, the solid S9 obtained was separated from its mother liquor L 9. This liquor, weighing 6640 kg, contained: A1203 1.17% A Fe203 0.10% TiO2 0.37% NH3 3.27% H2SO4 free 53.9% H2SO4 total 67.3% The solid substance Sg had a weight of 494 kg, and 100 kg of S9 was recycled, as starting material, atR. The remainder, i.e 394 kg contained:: A1203 12 kg Fe203 26 kg TiO2 14 kg NH3 17 kg H2SO4 total 256 kg H20 23 kg various 46 kg (including MgO and P) The solid substance was then decomposed by heat to produce solid substances, particu larly oxides of iron and titanium, which were discarded, and gaseous products, particularly oxides of sulphur and ammonia, which were converted into sulphuric acid and ammonium sulphate for reintroduction into the cycle.

It should be mentioned that the quantity of dry gaseous HC1, separated from L8 i.e. 1640 kg was sufficient to saturate, at J, the solution from which the required hydrated aluminium chloride was precipitated. If it had been necessary to saturate the whole of solutions L1 and L2 to precipitate the same quantity of aluminium chloride, it would have been necessary to use 3200 kg of dry gaseous hydrogen chloride; in that case it would have been necessary to add 2000 kg to this quantity to obtain a chloride and then an alumina of the same purity resulting from a second crystallization of the chloride.These quantities of dry gaseous hydrogen chloride could have been obtained only from the solution obtained by condensing and absorbing the gases emitted during calcination of the chloride at L and by the use of an expensive treatment.

WHAT WE CLAIM IS: 1. A method of treating a silico-aluminous substance containing iron and titanium but less than 0.1 wt% of K20 to produce metallurgically pure alumina as herein defined, that comprises introducing the silico-aluminous substance in the form of fine particles into a solution containing 49 to 59 wt% of free sulphuric acid and 6 to 8 wtWo of ammonium sulphate, together with residual quantities of aluminium sulphate and sulphates of the various metals constituting impurities in the material being treated, heating the resulting suspension at a temperature in the range 125 to 135"C for from 1 to 5 hours, separating the resulting siliceous residue, which is impregnated with liquid, from the remainder of the liquid, concentrating and cooling the said liquid to precipitate an aluminium acid sulphate having the formula Al2(SO4)1.0.5H2SO4 . (11 to 12)H20, removing the mother liquor from the aluminium acid sulphate, using a portion of it to remove from the said siliceous residue the liquid with which the said siliceous residue is impregnated and recombining the resul tant liquid with the liquor from the acid attack, washing the said siliceous residue with water and discarding it, recombining the remainder of the mother liquor from the aluminium acid sulphate with the washings from the said siliceous residue, concentrating the resulting mixed liquor and holding it at 800C for from 2 to 3 hours in order to precipitate a complex

**WARNING** end of DESC field may overlap start of CLMS **.

Claims (6)

**WARNING** start of CLMS field may overlap end of DESC **. A1203 1.17% A Fe203 0.10% TiO2 0.37% NH3 3.27% H2SO4 free 53.9% H2SO4 total 67.3% The solid substance Sg had a weight of 494 kg, and 100 kg of S9 was recycled, as starting material, atR. The remainder, i.e 394 kg contained: A1203 12 kg Fe203 26 kg TiO2 14 kg NH3 17 kg H2SO4 total 256 kg H20 23 kg various 46 kg (including MgO and P) The solid substance was then decomposed by heat to produce solid substances, particu larly oxides of iron and titanium, which were discarded, and gaseous products, particularly oxides of sulphur and ammonia, which were converted into sulphuric acid and ammonium sulphate for reintroduction into the cycle. It should be mentioned that the quantity of dry gaseous HC1, separated from L8 i.e. 1640 kg was sufficient to saturate, at J, the solution from which the required hydrated aluminium chloride was precipitated. If it had been necessary to saturate the whole of solutions L1 and L2 to precipitate the same quantity of aluminium chloride, it would have been necessary to use 3200 kg of dry gaseous hydrogen chloride; in that case it would have been necessary to add 2000 kg to this quantity to obtain a chloride and then an alumina of the same purity resulting from a second crystallization of the chloride.These quantities of dry gaseous hydrogen chloride could have been obtained only from the solution obtained by condensing and absorbing the gases emitted during calcination of the chloride at L and by the use of an expensive treatment. WHAT WE CLAIM IS:

1. A method of treating a silico-aluminous substance containing iron and titanium but less than 0.1 wt% of K20 to produce metallurgically pure alumina as herein defined, that comprises introducing the silico-aluminous substance in the form of fine particles into a solution containing 49 to 59 wt% of free sulphuric acid and 6 to 8 wtWo of ammonium sulphate, together with residual quantities of aluminium sulphate and sulphates of the various metals constituting impurities in the material being treated, heating the resulting suspension at a temperature in the range 125 to 135"C for from 1 to 5 hours, separating the resulting siliceous residue, which is impregnated with liquid, from the remainder of the liquid, concentrating and cooling the said liquid to precipitate an aluminium acid sulphate having the formula Al2(SO4)1.0.5H2SO4 . (11 to 12)H20, removing the mother liquor from the aluminium acid sulphate, using a portion of it to remove from the said siliceous residue the liquid with which the said siliceous residue is impregnated and recombining the resul tant liquid with the liquor from the acid attack, washing the said siliceous residue with water and discarding it, recombining the remainder of the mother liquor from the aluminium acid sulphate with the washings from the said siliceous residue, concentrating the resulting mixed liquor and holding it at 800C for from 2 to 3 hours in order to precipitate a complex

sulphate or iron, titanium and ammonium with other materials, decomposing the complex sulphate to form a solid residue containing principally iron and titanium compounds anc gaseous mixture including ammonia and oxides of sulphur, recycling the gaseous mixture tc form ammonium sulphate and sulphuric acid in the solution in which the said silico aluminous substance is originally treated, washing the precipitate of aluminium acid sul phate, bringing it into contact with a concentrated solution of hydrocholoric acid to produce a solution or suspension having a high concentration of aluminium and passing gaseous hydrogen chloride through the said solution or suspension to precipitate aluminium chloride hexahydrate, decomposing the latter by heat to produce gaseous hydrogen chloride and the desired metallurgically pure alumina, and recycling the gaseous hydrogen chloride to the chlorination stage.

2. A method according to claim 1 in which the solution remaining after precipitation of the aluminium chloride contains sodium chloride, which is extracted from it before any further use of the solution.

3. A method according to claim 1 or 2 in which the aluminium acid sulphate precipitate is washed by part of the solution obtained by elimination of HC1 from the liquor separated from the aluminium chloride, the HCI from that liquor being recycled to a step where HCI is used.

4. A method according to claim 1 carried out in a system substantially as hereinbefore described with reference to the single figure of the accompanying drawings.

5. A method according to claim 1 substantially as hereinbefore described in the foregoing Example.

6. Alumina of metallurgical purity, when prepared by a method according to any one of claims 1-5.