USRE20547E - Method of making crystaiijne alu - Google Patents

Description

Reissued Nov. 2, 1937 .MINA AND A BANE PRODUCT CONTAINING THE Raymond B. Itldgway. Niagara Falls, N. Y., assignor to Norton Company, Worcester, Mass., a corporation of Massachusetts No Drawing. Original No. 2,003,867, dated June 4, 1935,8erlal No. 690,589, September 22, 1933. Application for reissue January 8, 1936, Serial 31 Claims. (C1. 23-142) This invention relates to the electrochemical manufacture of crystalline alumina of required chemical and given physical properties for use in various industrial arts.

In the electrometallurgical production otcrystalline alumina from alumina-containing ores,

such as bauxite, the elimination of the impurities of the ore by reduction with carbon has had serious limitations, from the standpoint of purity and stability of the resultant alumina crystals. Commercial bauxite contains as high as "1% of titanla and 2% of zirconia, as well as silica and alkaline metal oxides. It hasbeen customary to purify the alumina oxide incompletely and to leave contaminating metal oxide impurities in the fused bath to serve as a buffer, which protects the alumina from reduction. In accordance with the prior methods, such as des'cr'lbedin the patent to Saunders No. 1,269,224, bauxite may be greatly enriched'm its alumina content by fushion with carbon and iron in a Higgins type of electric furnace, as shown in the United States Patent No. 775,654, and the purification may be carriedon easily up to about 96 alumina. As an example of a calcined bauxite ore before and after reduction by iuison with carbon, the following analyses are to be noted:

Up to this degree of purity, aluminum carbide or other reduction compounds of' alumina are not formed to any material extent. On the other hand, various impurities present are combined in a slag or glass phase, which surrounds and cements together the crystals of alpha alumina, r and some of this glass appears as an inclusion in the alumina crystals.

An alternative procedure, as employed to reduce the last trace of titania, is-to over-purify the alumina by various methods and then to oxidize the aluminum carbide, or other reduced aluminum compounds thusproduced, by treating the original furnace melt with various solid and gaseous reagents. Also, such an over-purified and reduced mass has been treated after solidification by available means which serve to remove impurities, but such products must thereafter be re-fused in order that the purified crystalline alumina may have the desired physical as well as chemical characteristics. These alternative methods, which involve over-purification and reduction of some of the alumina in the original iurnacing of bauxite, have not produced in practice the desired results, since the oxidizing agents have remained as contaminations in the final product, .or else the reactions have been incomplete. Moreover, those processes which involve an intermediate purification and a subsequent.

' with respect to the carbon reducing agent as to produce a small quantity of hydrolyzable aluminum sulfide in the melt. Thiscauses a complete "removal of the titania and zirconia during the reduction process, but requires the presence of the unstable aluminum carbide in solid solution in the alumina. In order to remove this aluminum carbide, as wellas the droplets of ferroalloy, the fused material has to be cooled rapidly to produce minute crystals of the order of 0.1 mm. and then be broken up by hydrolysis to a very fine subdivision so as to expose the impurities.

Consequently, the resultant disintegrated crystals have been too fine in size to be of direct commercial use, and it has been necessary to remelt the alumina, after the impurities have been separated therefrom, and to recrystallize the product in a slower cooling operation to obtain crystals of commercial sizes.

The final ingot of crystalline alumina, as produced by such prior processes, is a massive solid body which requires expensive crushing and screening operations to obtain grains of the desired sizes. The requirements of the various abrasive and refractory industries call for grain grades, has involved not only a large expense but also a loss of a considerable amount of very fine material which has to be thrown away or else returned to the furnace to be remelted and recrystallized.

An object of this invention is, therefore, toovercome such problems and to provide a single, direct fumacing treatment for bauxite or other impure aluminous ores, which results in the production of coarse sized crystals of alumina of a high degree of purity and chemical stability, which have the physical or structural characteristics useful in the various industries.

A further object is to produce crystalline alumina grains of usable commercial sizes in which each grain is a discrete particle consisting of a single large crystal or aggregates of large crystals, which are substantially free from unstable, reduced inclusions and are not contaminated with other metal oxides.

A further object is to provide a method of directly melting and purifying bauxite which does not require intermediate chemical purification processes or multiple furnacing operations and which will produce alumina substantially free from titanium and zirconium compounds, as well as the alkali'metal and alkaline earth metal impurities, which are normally present in the crude ores. Other objects will .be apparent in the following disclosure.

In accordance with my discovery, I have found that the impurities in bauxite or other aluminaearing materials, such as diaspore, calcined alunite, etc., may be readily removed by a single, direct furnacing operation and that the product may be readily disintegrated into crystal particles of a high purity, above 96 by weight of alumina, the major portion of which are larger than grit size and which may be made large enough to meet the average or maximum requirements of the industry. The purification is achieved by introducing into the original bauxite or the furnace melt a reagent comprising the sulfide of an alkali metal or alkaline earth metal, herein termed an alkaline metal sulfide", such as CaS, which is capable of and proportioned for changing the crystallizing ability of the alumina and reacting with the residual slag product resulting from the reduction of the metal oxide impurities so as to completely free the crystals of alumina from included material, whether oxidized or reduced, and to segregate the residual materials into a matrix phase surrounding crystals of alumina of commercially usable sizes. The matrix phase comprising the water soluble alkaline sulfide may also include a small quantity of the hydrolyzable aluminum sulfide. These sulfides serve as disintegrating agents and cause rapid disintegration of the ingot when treated with a suitable agent, such as water. The required sulfides may be formed by adding iron sulfide to the melt and adjusting the ratio thereof to the amounts of alkaline metal compound and reducing agent added, as explained herein. This chemically unstable matrix is easily converted into a soluble and hydrolyzed form that is readily adapted to separation from the pure crystals of alumina. Since the matrix may be used as a very small proportion of the total melted product, such as 3% or less, it may be discarded without serious economic loss. reagents added to the melt are so proportioned as to. provide an absolute control of the reactability of the matrix and, therefore, its removal.

The

20,547 screening the material to obtain the different For example, I may fuse bauxite. or other suitable crude alumina, in the presence of a reducing agent, such as carbon, which is proportioned to reduce the metal oxide impurities, and with metallic iron, which forms a desired fe'rro-alloy with metals of the oxide impurities, these reagents being proportioned to reduce the refractory oxides, as above explained, and produce alumina of a maximum purity of 96 In orline metal sulfide, but if desired the matrix glass may comprise the water hydrolyzable aluminum sulfide to hasten the decomposition. The alkaline metal sulfide may be formed from its correspending-oxide and the aluminum sulfide from the alumina in the bath by the addition of ferrous sulfide and carbon in suitable proportions. The required quantity of sulfide or sulfides, as herein defined, or reagents capable of developing the same, which are capable of forming said matrix, may be added to the furnace charge either before fusion or at any time up'to that at which the purification has reached the state indicated in the second column of the above table. After the fusion of thefurnace charge, it is slowly cooled in a large mass by allowing the ingot to stand at room temperature and dis- 'sipate its heat after the furnace shell has been removed therefrom and without the aid of a cooling medium, except the water which was employed to cool the iron shell of the Higgins furnace during'the fusion stage, thus forming large crystals.

The presence of an alkaline metal sulfide in proper amount serves: (1) to prevent the formation of aluminum carbide in the "crystalline alumina; (2) to cause the impure metal oxides which normallydissolve in the alumina to separate'into the matrix phase and to provide a low melting matrix of such a character as to permit the free growth of alpha alumina crystals; and (3) to promote the decomposition of the alumina ingot by the presence. of a sufllcient for this purpose, and particularly a water-soluble glass. It is a feature of this process. that the impurities, which are not included in the ferro-alloy at the bottom of the melt, are included in the water-soluble glass so that, when treated with Water, these impurities are segregated. from the crystalline alumina and may be,

freely, washed therefrom. The alkaline earth impurities are converted to sulfides and these,

. as well as that part of the silica, titania and zirextent. 1 The water-solubleglass'phase may be probut small amount of unstable material adapted duced by reactionswithin the melt, rather than by a simple addition of suitable sulfides; and to this end, {I may add to the charge of bauxite a so that there is formed within the alumina melt a glass-like matrix. whichis capable of holding in solution or suspension the unreduced titanium, iron and zirconium compoundsand the alkaline metal impurities originally present in the bauxite.

If desired, the required sulfides may be preformed outside of'the furnace and then added directly to the charge. providing that .sumcient reagent metal sulfide is supplied in addition to take care of the alkaline constituents already present in the bauxite and form alkaline metal sulfide therewith. This reagent metal sulfide is preferably added in amount suillcient to form aluminum sulfide as well as the alkaline metal sulfide.

0f the alkaline metal sulfides which may be employed, I preferably use, because of considerations of low cost and workability in the process, the sulfides of sodium, magnesium and calcium, herein termed alkaline metal sulfides" .and which may be formed by adding to the bauxite charge or to the molten bath the oxides of these respective metals. I may, however, employ the oxides or sulfides of other metals of the first and second periodic groups, and particularly of that group consisting of sodium, potassium, magnesium, calcium, barium, and strontium. These may be added in various forms. For example, sodium may be used in the inexpensive form of soda ash or sodium aluminate. Magnesia may be added as calcined magnesite, and calcium may be used in the. form oi. calcined limestone. Combinations of these materials may be used, such as is found in calcined dolomite containing the oxides of both magnesium and calcium.

For the reagent metal sulfide employed to bring .about the sulfidization of the constituents in the melt which are to be removed from the alumina and to form the water-soluble glass, I select those metal sulfides which can be reduced by the metal of the alkaline metal oxide which is in turn reduced by the carbon during the furnacing operation. That is, if calcium oxide is employed and I the carbon serves to reduce the same to calcium metal, the reagent metal sulfide, such as iron or copper sulfide, should be one which stands below the alkaline metal in the reduction scale and thus be capable of transferring its sulfur content to form calcium sulfide. The choice of these reagent metal sulfides is dependent upon their availability and the permissible constituents of the alloy which is to be formed in the alumina melt. Of these sulfides, I prefer to use iron sulfide, and it may be employed in the form of ferrous sulfide, iron pyrites or pyrrhotite or other suitable carrier of iron. If copper sulfide is employed, and it is to be considered as an equivalent of iron sulfide, then suitable minerals containing the same may be used for the purpose. A

combination of both iron and copper, which is suitable,'is found in chalcop'yrite. The expressions reagent metal sulfide and reducible sulfide as used in the claims are to be interpreted -as limited to either iron sulfide or copper sulfide and their equivalents. As 'a result of the furnace reduction process, the iron sulfide is converted to iron metal which alloys with other reduced met-' als, and the sulfur goes into the glassy matrix.

As specific examples illustrating the application of this invention to bauxite ores, the following procedure may be adopted for treating a bauxite which may be within the composition limits indicated in the following table:

Percent by weight The carbon requirements for the reduction of the impurities in this bauxite are calculated as molecular-equivalents thereof, according to the well-known reactions which involve reducing the oxides to the metals. An ore of the above composition limits will require from 4.5 to 7.0% of carbon for reducing the impurities in the fused ore, depending on the combinations and the 'amount of impurities which may be present.

Carbon must also be provided to reduce the oxides of the alkaline metals. The carbon addition is so adjusted relative to the other reagents that the difiicultly reducible oxides will appear as the air, all oi'.-which is well within the skill 01* one who is familiar, with the operation of a furnace of this type.

Iron pyrites is preferably used as the reagent metal sulfide, and for the alkaline metal oxide I preferably use soda ash, magnesia or lime. Examples of compositions and proportions by weight which are satisfactory for producing a high purity alumina are given in the following tables:

Example A Parts Bauxite 74 Metallic iron 15 Coke 6 Iron pyrites 4 Soda ash 1 Example B Parts Bauxite 75 Metallic iron 15 Coke 5 /2 Iron pyrites 4 Magnesia Example 0 Parts Bauxite 75 Metallic iron 15 Coke 5 Iron pyrites 4 Lime 1 A mixture of the bauxite and reagents in suitable proportions may be chargedinto an opento-pmd, water-cooled electric furnace of the Higgins type. The rate of feed of the charge to the furnace may be suitably adjusted to the power rating of the furnace on the basis of from 0.5

the furnace shell is removed and the ingot is" allowed to cool slowly by radiation at room temperature. The rate of cooling will largely determine the size of the crystal particles but under the conditions specified, the disintegrated particles will be larger than 100 grit size and ordinarily close to the requirements of the industry. A typical example of crystal particle sizes resulting from the use of 1% of lime and 5% of pyrites comprises substantially the following percentages of grit sizes, as measured by the particles held on screens of the meshes per linear inch as indicated:

Screen size 10 24 44 66 100 Finer than 100 Percentage held. 13.5 6.3 39.8 21.9 10.7 7.8

It is feasible to carry on this operation in a tapping furnace only if the scale of operations is particularly large, to provide for slow cooling in the ladle. In that case, the tapped ingot should not be less than several tons in size, so as to be sufilcient to prevent oxidation of the molten material as it flows from the furnace, since the margin of reduction is delicately balanced in the furnace charge. After the melt has been cooled slowly so as to form large crystals, the shell is removed and exposes a well fused ingot. This is broken up into large lumps in the usual manner and separated from the outer layer of incompletely reacted material which forms the crucible for the melt and is called refuse, as well as the metal layer which contains iron alloyed with silicon and titanium. The operation of breaking and sorting the material is carried on while the ingot is still warm so as to avoid any difiiculty with hydrolysis at this stage.

In order to disintegrate the mass, the lumps which havebeen sorted free of refuse and from the layer of reduced ferro-alloy in the bottom of the ingot are placed preferably in a large rotating cylinder or tube with a considerable quantity of water, and the ends of the tube are sealed against'the escape of the hydrogen sulfide gas which is liberated by the hydrolysis of the aluminum sulfide and alkaline earth metal polysulfides. This gas dissolves in the water and aids in carrying into solution the alkaline earth metal sulfides 0r hydrosulfid-es. The tube is preferably provided with bafiles in order to stir the material and shower it into the bath of water therein, thus aiding in breaking up or disintegrating the mass. Owing to the alkaline metal. sulfides going into solution and the aluminum sulfide being converted to the hydroxide and swelling greatly because of the chemical change, the lumps are rapidly and completely broken down to expose the individual crystals or crystal aggregates where they were separated by the matrix. At'the same time, the'stable sulfides are set free and appear in the resultant sludge. Thereafter the suspended impurities are drawn off and the alumina crystals are washed with water.

As the result of this procedure, the material is now found to be in the form of discrete particles of crystalline alumina accompanied by a sludge carrying the refuse, such as the matrix glass and the impurities entrained therein. The aluminum sulfide has hydrolyzed to aluminum hydroxide which swells rapidly and aids in breaking up the mass. The soluble alkaline metal sulfides have gone into solution in the water.

The stable material, such as free sulfur and the to 1.0 kilowatt hour per pound of mixture fed.

unreduced oxides and sulfides of titanium, iron and zirconium which are present in the matrix glass, has been set free and is in suspension in the liquid in a very fine state of subdivision. Any silicon sulfide which formed has volatilized and disappeared from the melt. After this disintegration has been completed, the solution containing the suspended matter is withdrawn, and the alumina crystals are washed with water to remove the adhering sludge. Thecrystalline alumina granules thus produced are ready for use in various industries; but they may be further treated to purify them, such as by passing the material over a magnetic separator to remove an occasional globule of ferro-alloy which may be present, or by treatment with suitable chemical reagents to remove traces of alkali or other objectionable compounds. the disintegration of the ingot is found to be commercially valuable in that it contains a large amount of titanium and zirconium sulfides in a highly concentrated form.

A typical analysis of a product resulting from the furnace melt before it is disintegrated with water is as follows:

analyze as follows:

A typical analysis of the sludge and of the solution resulting from the agitation of the material with water, where the alkaline metal reagent was lime, is as follows:

Sludge Solution Per cent Per cent 25.3 13.8

It is within the scope of this invention to complete the normal purification obtained by a standard carbon reduction process which does not employ any sulfide or alkali. In that case, and just before the shut-down of the furnace run, the required quantities of alkali metal and sulfide reagents are added to the melt to change the nature of the inter-granular glass to a water-soluble compound. The final product is the same as if the addition had been made to the original furnace charge. My invention, therefore, involves the discovery, inter alia, of the fact that a small amount of alkali metal or alkaline earth metal sulfide together with a trace of aluminum sulfide will alter the normal intergranular matrix of an aluminous melt of high purity so that the matrix may be easily removed by water solution and hydrolysis.

The sludge obtained by v The proportions of the ingredients of the furnace charge may be varied, depending upon the glassy matrix which is to be rejected, as well as the consumption of large amounts of energy and,

raw materials in the process. The alkaline metal sulfide is likewise present in a small amount only sufiicient to accomplish its purposes, and particularly to prevent the formation of aluminum carbide and reduce the solubility of the alumina in the glassy matrix, as well as to assist in segregating the titanium, zirconium and other impurities into the liquid phase of slag. It is ordinarily desirable that the alkaline metal sulfide constitute not over 3% of the total mass. The alkaline metal addition may, however, be increased to five parts, with a corresponding increase in the coke addition, without seriously upsetting the necessary reactions. It is, however, preferable to maintain the total sulfide content below 2%, and ordinarily about or below 1% of the total mass; hence the various additions will be soproportioned as to-attain this result. If the alkaline addition is large, a correspondingly large amount of carbon and iron sulfide must-be added to the charge to convert all of the alkali or alkaline earth material to the sulfide. The additions of the reagents should be carefully controlled, since there is a strong tendency for the alkali and alkaline earth metal, which is present in the ordinary aluminous ore or is added to the charge, to react with the more acidic alumina and producealuminates and spinels which contaminate the crystalline alumina. These the conditions required for crystallization of the melt,-and do not go into solution when an attempt is made to disintegrate the glassy matrix with water. Also, the presence of too large an amount of iron sulfide interferes with the reduction reactions. Some sulfur is-lost by direct oxidation; hence, the actual amount of sulfur which is required to be fixed in'the glassy matrix is very small. Only so much iron sulfide should be added as will serve to provide the required amount of total sulfur'in the melt; and this control is quite preferable to prevent the formation of over 2 or 3% of aluminum sulfide.

It is possible to so control the addition of the carbon reducing agent that the resultant crystalline alumina grains derived from the disintegrated product will be completely stable and not require any further treatment to remove impurities therefrom. In the methods employed heretofore, the formation of an unstable aluminum carbide has been unavoidable where a high purification has been accomplished. When more carbon than that required for the reduction of the impurities is added to an aluminous melt, then in a case where iron is present as a lower layer of molten ferro-allo-y, a certain fraction of the excess carbon goes to increase the content of the difilcultly reducible metals, such as aluminum, in that ferro-alloy. Accordingly, a distribution of reduced compounds of aluminum takes place in the furnace whereby such compounds are found in both the slag and the metal. If, however, the sulfide addition is made to the furnace in accordance with this invention, the sulfide sinks into the metal layer and acts to alter this equilibrium in such a direction that the reduced compounds'are .7 solution. Hence, the

are stable under and reentrant dihedral angles, and

,5 converted to sulfides and are transferred from the metal layer to matrix glass.

It is found that the carbide and suboxlde of aluminum,

present in the supernatant slag layer before the addition of the reagent sulfides, form a constituent which reacts with the sulfides or the product of the sulfidization of the metal layer to convert them from the objectionable carbide and suboxide to the removable sulfide. The completion of this adjustmentis facilitated by the presence of appreciable percentages of alkaline earth metals which serve as carriers for the sulfide constituents. It may be considered that the alkaline metal sulfide in the matrix glass acts as a buiier in the melt against the over-reduction of the alumina. This particular buffer material is, however, easily removed from the final product, as compared with other types of materials, such as silica, which have been heretofore suggested for re-oxidization of the over-reduced alumina and have resulted in contaminating the final melt as well as those of the alkali and alkaline earth metals, which are with the reduction product of the oxidizing agent,

such as silicon or ferro-silicon. The matrix glass which is produced in accordance with the present invention is completely extractable by. the water reduction of the alumina can be completely neutralized without interfering with the quality of the purified alumina.

As the alkaline ratio in the water-soluble glass is increased, the crystals of resultant alumina tend to become more sharp and angular and to develop into sliver and plate-like formations. ,Magncsium and sodium in particular tend to produce these platy crystals. Also the larger the percentage of the water-soluble glass in the melt, the greater is the tendency to produce a grain which is punctured with intra-crystal voids and stringers which tend to produce a weak shape. In order to produce grains which tend towards being equi-dimensional in shape or are of a blocky, chunky, cubical or polyhedral type, the alkaline metal addition should be kept below 1% of the total weight of materials used, for the bauxite ore above specified. The ultimate grains are, many of them, of such blocky, having many plane faces, forming both exterior some of the polyhedral type, especially in the smaller sizes, are elongated and someof rhombohedral structure, with both exterior and reentrant angles, and other grains are definitely prismatic, truncated or otherwise. The predominant type is the blocky or chunky type; the others occur mostly in the center of the ingot. All types have reentrant angles, mostly dihedral, but many being saw tooth edges. The reentrant angles increase the cutting efficiency of the grain. Most of the dihedral angles on all but the prismatic types indicate rhombohedral structure. The prisms are mostly trigonal.

The matrix may be considered as made up of hydrolyzable aluminum sulfide, the water-soluble alkaline metal sulfides or polysulfides, together with an excess of free sulfur and the insoluble or stable sulfides of iron, titanium and zirconium and the other entrained solid materials. It is important that the matrix should respond properly to the water-hydrolysis treatment; otherwise, the objectionable impurities cannot be readily removed from the crystalline alumina. A means of chemical control of the composition of the matrix is provided by determining the ratio between the water-hydrolyzable sulfides, the water-soluble chunky form whether the glassy matrix will readily hydrolyze or not. These are such that the stable sulfides should comprise not more than 50% of the total glassy matrix while the water-hydrolyzable and the soluble sulfides comprise not less than 50%. A satisfactory decomposition oi the glass is produced when the three constituents are distributed as follows:

. 7 Per cent by weight Water-hydrolyzable sulfides to 40v Water-soluble sulfides 25 to'lO Stable sulfides 0 to If there is a maximum of not over 2% of total sulfide sulfur content in the alumina melt, then this-distribution assures ready decomposition or disintegration of the mass, although the total percentage of glass is a very small fraction of the total melt.

It is now apparent that ordinarily less than 1% of the original alumina will be converted into the hydrolyzed form and appear as an impurity in the sludge which makes up the rejected portion of the melt. If one attempts'to increase the content of aluminum sulfide in the melt by increasing largely the addition of the reagent iron sulfide and varying the corresponding requirements of the carbonreducing agent, this results in the reduction and waste of a large amount of alumina to take up the sulfide content as aluminum sulfide. This procedure also requires large percentages of coke in order to bring about the necessary reduction reaction to form the sulfides of the glassy matrix. When the concentration of the reducing agent is sufficient to reduce enough alumina to combine with a large amount of sulfide present, and if sufiicient carbon is provided to reduce the difiiculty reducible alkaline metal oxides, whereby they may be converted into soluble sulfides instead of aluminates, the carbon requirement is sufiicient to supply the melt with a certain percentage goes into the alumina crystal and makes it unstable and unsuitable for use as a discrete crystal of alpha alumina. I v

Also, large additions of reducing agents cause an increased-lossof aluminum'by volatilization, by reduction into the ferro-alloy, and by formation of compounds, such as aluminum carbide and sulfide. If the amount of this is appreciable, the entire reaction changesin its nature and the character of the crystallization changes, while the alumina crystals take on inclusions of unstable compounds. It is, therefore, apparent that the practical requirements of this process are attained only when the iron sulfide, the reducing agent and the alkaline metal additions are so carefully limited as to produce the desired watersoluble matrix glass without having anexcess of alkali metal or alkaline earth metal present to produce. aluminate or an excess of aluminum sulfide with its corresponding carbide requirements. It is, therefore, desirable that the percentages of the ingredients be suitably proportioned, within the limitations above indicated, to insure a successful production of not only a' substantially pure alumina particle but also that the ingot of crystalline vmaterialmay be readily disintegrated into the discrete particles of required size.

These percentages as herein specified refer to of aluminum carbide which the percentage distribution of the total sulfide. content or to the amount of sulfur present as a sulfide and not to the amounts of the various 7 metal sulfides used.-

Having thus described the invention, what is claimed as new and desired to secure by Letters Patent is:

1. The process of producing crystalline alumina in large discrete particles by a single furnacing operation from impure aluminous ores containing one or more of the oxides of silicon, titanium and zirconium, comprising the steps of melting and purifying the alumina in an electric furnace at a high temperature by means of a carbons-'- ceous reducing agent and a limited quantity of not over 5% by weight of a, sulfide of a metal of the group consisting of alkali metals and alkaline solidified melt will consist of crystallized alumina held in a matrix of a water disintegratable glass containing the residual unreduced titanium, zirconium and iron impurities, and slowly cooling the melt to produce large crystals of alumina, the major portion of which are over 100 grit size and thereafter disintegrating the matrix by treating the mass with water and separating the alumina crystals from the impurities.

2. The process of producing crystalline alumina in large, discrete particles by a single furnacing operation from impure aluminous ores containing one or more of the oxides of silicon, titanium and zirconium, comprising the steps of melting the alumina with a carbonaceous reducing agent and metallic iron proportioned to produce a purity of over 96.5% alumina in the melt, and in the presence of a limited quantity of the oxide of a metal of the group consisting of the alkali metals andwater disintegratable glass containing the sulfide of said group metal and not over 3% of aluminum sulfide together with residual impurities, slowly cooling the melt so that the major portion of the crystalline alumina will have a grain size larger than grit, and thereafter disintegrating the matrix of the solidified ingot by treatingthe same with water and separating the alumina crystals from the impurities.

3. The method of making crystalline alumina of commercial grit sizes from impure alumina, comprising the steps of fusing the alumina with a reducing agent and in the presence of the sulfide of a metal of the group consisting of alkali metals and alkaline earth metals which is capable-of and is proportioned in amount less than 5% of the total mass for preventing the formation of reduction products of aluminum without y forming an alkaline metal aluminate and causing the impurities to separate from the alumina as a slag phase containing the sulfides and other compounds of the impurity metals, so that the alumina may crystallize freely to a desired crystal size within a melt of purified-alumina.

' 4. The process of producing crystalline alumina in large discrete particles by a single furnacing operation from impure aluminous ores containing one or more of the oxides of silicon, titanium, zirconium, calcium and magnesium, comprising the steps of melting and purifying the alumina in an electric furnace at a high temperature by means of a carbonaceous reducing agent and a limited quantity of the sulfide of a metal of the group consisting of alkali metals and alkaline earth metals which are so proportioned that a matrix is formed oi residual, diificultly reducible and non-reducible impurity constituents in a form lending itself to the complete crystallization of the alumina in large grit sires without forming a stable alkaline aluminate, the total sulfide sulfur oi the matrix constituting less than 3% 'by weight 01 the melt, and slowly cooling the melt to produce large crystals of alumina and thereafter disintegrating the matrix by treating it with water and separating the alumina crystals from the impurities.

5. The method of making granular crystalline alumina from an impure alumina containing one or more 01' the oxides of silicon, titanium, zirconium, calcium and magnesium, comprising the steps of melting the alumina with carbon, iron sulfide and the oxide of a metal oi! the group consisting of alkali metals and alkaline earth metals which are proportioned for reducing all oi-the impurity metal oxides and forming a ferro- -alloy therewith and for producing a matrix, the

sulfide sulfur content oi which constitutes not over 3% of the-total mass, and which contains the sulfide oia metal oi said group in amount sufiicient to prevent the presence or aluminum carblde'and to cause the residual impurity metals to gather largely in the matrix, thereafter cooling v the melt slowly to form an ingot containing large alumina crystals interspersed with veins of the matrix, breaking up the ingot and treating it with water to dissolve the soluble sulfides and hydrolyze any aluminium sulfide present and thereby disintegrating the ingot into discrete crystalline alumina grains of sizes which are directly commercially usable for varied abrasive purposes.

6. The method of making crystalline alumina from bauxite containing silica, titania and iron oxide comprising the steps of melting the bauxite with a limited amount of carbon proportioned to reduce all of the iron oxide and silica to substantial completeness and part oi the titania, and with limited amounts oi carbon, a reducible metal sulfide and the oxide of a metal of the group consisting of the alkali metals and the alkaline alkaline'earth metals present and not over 3%" of the alumina and producing a water disintegratable sulfide matrix carrying substantially all of the'residual, dimcultly reducible and non-rev ducible impurities, slowly cooling the melt and crystallizing the alumina as substantially pure discrete alumina crystals sharply segregated from the matrix, and thereafter treating the solidified mass with water and disintegrating the same.

'7. The method of making crystalline alumina from an impure ore containing titania and zirconia comprising the steps of fusing the ore with metallic iron and limited amounts of carbon, iron sulfide and the oxide of a metal 01' the group consisting of the alkali metals andalkaline earth metals which are just sufilcient to reduce the easily reducible impurities and to convert .to the sulfides all of the alkali metal and alkaline earth metal oxides present and forming a water disintegratable sulfide matrix containing the sulfides of zirconium and titanium and not over 3% 8. The process oi removing traces oi partially with water to disor diillcultly reducible metal oxide impurities from a melt oi alumina formed by the reduction of bauxite with carbonaceous reducing agents comprising the steps of incorporating in the furnace charge an easily reducible metal sulfide and the oxide 01 a metal of the groupconsisting or alkali metals and alkaline earth metals and adjusting the suliur content 01' the mixture so that after fusion and cooling the metal oxide impurities will be separated into a water-soluble or hydrolyzable matrix, the sulfide sulfur content of which is not over 2% by weight oi the total melt.

9. The process of further purii'ying' fused alumina beyond the quality obtainable by carbon reduction in the presence of a ierro-alloy layer whichcomprises the step 01' incorporating in the furnace charge an easily reducible metal sulfide and a compound of a metal of the group consisting of alkali metals and alkaline earth metals together with carbonwhich are capable of and are proportioned for converting the traces of the remaining impurities into a water-soluble and disintegrable glassy matrix containing not more than 3% by weight of aluminum sulfide and a sulfide of said group metal.

10. The method of making crystalline alumina from an impure alumina containing an oxygen compound of a metal of the group consisting of alkali metals and alkaline earth metals comprising the steps of melting the alumina with carbon and'the sulfide of a reagent metal of the group consisting of iron and copper proportioned to form sulfides of all alkali and alkaline earth metals present and not over 3% of aluminum sulfide, while providing a sufficientamount of said sulfides to form a water disintegratable sulfide matrix, slowly cooling the melt and forming discrete alumina crystals segregated from the sulfide matrix, and thereafter treating the solidified mass with water and disintegrating the same.

11. The method of claim 2' in which the furnace charge comprises an oxide of the group of metals consisting of sodium, calcium and magfnesium and ferroussulfide added in such pro-- water, and the total sulfide content is less than I 5% by weight of the melt.

12. The method of purifying an alumina bearing ore containing silica, titania or zirconia comprising the steps of melting it with carbon and metallic iron under conditions of over-purification which serve to reduce substantially, all of the metal oxide impurities and to form aluminum carbide, providing within the melt sufiicient sulfide of a metal of the group consisting of alkali metals and alkaline earth metals, below 5% by weight of the total mass, to form a glassy matrix containing said sulfide which segregates the metal oxide impurities from the alumina, cooling the mass and crystallizing the alumina, and thereafter disintegrating the mass by treatment with water and recovering the alumina crystals free from the matrix.

' 13. The method of claim 2 in which the solidified ingot is crushed and treated with water in a closed chamber to cause the disintegration of the ingot, after which the alumina crystals are separatedfrom the solution and solid residue.

14. The method ofclaim 4 in which the sulfide is a sulfide of a metal of the group consisting of sodium, potassium, calcium, magnesium, barium and strontium.

' charge including impure alumina, carbon, iron, fin

yield a disintegrating product-containing alumina. together with a matrix having from to 70% .15. The method of beneficiating the zirconium and titanium content of bauxite comprising the steps of claim 2 in accordance with which the zirconium and titanium are largely separated into the matrix, after which the ingot is treated with water to dissolve the soluble glass and leave the insoluble titanium and zirconium oxides and sulfides in the sludge which is thereafter separated from the solution and alumina for further treatment. a

16. The method of making crystalline. alumina comprising the steps of electrically fusing .a

sulfur and a metal of the group consisting of alkali metals and alkaline earth metals proportioned .to

by weight of a water soluble sulfide and zero to of a stable sulfide, and in which the total sulfide sulfur contentof the. matrix is not over 2% of the whole melt, thereafter cooling the mass to formcrystalline alumina, and treating the mass with water to disintegrate the same into crystal particles of alumina and a solution and sludge containing the sulfides and other impurities.

1'7. The method of producing crystalline alumina from an impure alumina in which the alumina is melted with reduction agents-which separate the impurities therefrom comprising the steps of incorporating in the melt a compound of a metal of the group consisting of alkali metals and alkaline earth metals and a reagent metal sulfide capable of providing a highly fluid, disintegrable glassy matrix, said ingredients being proportioned to form a sulfide matrix constituting not over 5% by weight of the melt and containing not over 50% by weight of stable sulfides and not less than 50% of water-soluble and hydrolyzable sulfides, and thereafter treating the mass with a reagent capable of disintegrating and separating the alumina crystals from the matrix.

18. The method of claim 2 in which the glassy matrix contains from 25 to 70% by weight of water-soluble sulfides and not over 35% of stable of stable sulfides.

20. The method of claim 17 in which a calcium compound capable of forming calcium sulfide is the alkaline metal reagent.

- 21. The method of claim 17 in which asodium compound capable of forming sodium sulfide is the alkaline metal compound.

' 22. The method of claim 17 in which a, magnesium compound capable of forming magnesium sulfide within the melt is the alkaline metal 7 compound.

23. The method of making an aluminous ingot comprising the steps of fusing bauxite with carbon and iron together with a metal oxide of the group, consisting of sodium, potassium, calcium and magnesium oxides, and with an easily reducible metal sulfide of the group consisting of iron and copper sulfides, in which the ingredients are so proportioned as to produce .a water disintegratable sulfide matrix, the sulfide sulfur content of which is not over 5% by weight of the total melt, cooling thefused mass slowly to form an ingot containing said matrix separated from alumina crystal particles of high purity, the

the reagents are so proportioned that the total pounds of the group consisting of aluminum, ti-

or hydrolysis of the matrix in water.

a line alumina as discrete crystals in commercial I grain sizes loosely cemented with a disintegrable ingot containing crystalline alumina grains cesize and are well formed, polyhedral, non-fracmaior portion of which are larger than 100 grit size and aconsiderable portion is larger than 44 grit size, said ingot being capable of disintegration by water or other hydrolyzing agent or solvent which attacks the matrix.

24. The method according to claim 23 in which content of sulfide sulfur present in the ingot as aluminum sulfideis not over 0.6% by weight.

25. The method ofpuriiying alumina containing metal oxides comprising the steps of fusingit with a reducing: agent and a reagent metal sulfide and crystallizing the material in an ingot in accordance with the steps of claim 2, and thereby providing a decomposable glassy matrix containing a metal sulfide which separates discrete crystals of alumina; and thereafter breaking the ingot into lumps'and treating them in a closed chamber with suflicient water to disintegrate them and to dissolve the hydrogen sulfide formed by the reaction.

26. As a composition of matter, a solidified in. got of fused alumina comprising discrete crystal particles having a content of over 96.5% by weight of alumina which are cemented together by a glassy matrix containing a sulfide of a metal of the group consisting of alkali metals and alkaline earth metals which forms not over 5% by weight of the total mass, together with compounds of titanium and zirconium.

27. A composition of the type covered by claim 26 in which the ingot after disintegration by water comprises crystal particles, the major portion of which are large as 44 grit size.

28. As an article of manufacture, a solidified ingot containing large discrete crystals of alpha alumina of a purity in excess of 96.5% cemented together by a disintegrable glassy matrix containing a large proportion of the sulfide of a metal of the group consisting of calcium, sodium, potassium and magnesiumcombined with comtanium and zirconium, and in which the total sulfide sulfur content is not over 2% by weight of the ingot and the ingot is disintegrable by solution 29. As an article of manufacture, a. solidified ingot containing upwards of of pure crystalmatrix containing the sulfide of a metal of the group consisting of alkali metals and alkalineearth metals associated with compounds of titanium and zirconium and in which the matrix contains from 25 to 70% by weight of alkaline metal sulfide and zero to 35% of stable sulfides, and the total sulfide sulfur content is not over 3% by weight of the whole ingot.

30. As an article of manufacture, a solidified mented togetherwith a water disintegratable matrix containing the sulfides of zirconium, titanium and not over 5% by weight of the sulfide of a metal of the group consisting of alkali metals and alkaline earth metals, in which the major portion of the grains are larger than grit tured, discrete crystals of alumina substantially devoid of inclusions of droplets of metal or a glassy phase and analyzing within the following composition range in percentages by weight: Al:0:-99.64 to 98.9%; TiOz0.2 'to 0.6%; SiOr-lill; FezO:-0.05 to 0.1%; Z1020.05 to 0.2%; CaO0.02 to 0.1%; Mg0--0.04 to 0.1%; and NaaOnil.

31; -A composition of matter comprising cry'stah line alumina grains crystallized from a matrix containing the sulfide of ametal of the group consisting of alkali metals and alkaline earth metals, the major portion of which grains are larger than 100 grit size and are well formed, polyhedral, non-fractured, discrete crystals of alumina having plane faces forming both exterior and re-entrant dihedral angles and which are substantially devoid of inclusions of droplets of metal or a glassy phase and analyzing within the following composition range in percentages by wei ht: A120299.64't0 98.9%; nor-0.2 to 0.6%; SiOr-nfl; Fe2Oa-0.05 to 0.1%; Zr0:0.05 to 0.2%; CaO-0.02 to 0.1%; M800.04 to 0.1%; and NazO-nil; Y

RAYMOND R. RIDGWAY.