CA1172425A - Magnesium aluminum spinels - Google Patents

Abstract

ABSTRACT OF THE DISCLOSURE
Several methods are set forth for preparing polymetallic spinels by coprecipitating two or more metal compounds in a proportion to provide a total of eight positive valences when combined in the oxide form in the spinel crystal lattice. The methods disclosed require coprecipitation of the metals in the hydroxide form or convertible to the hydroxide-oxide form, cal-cining the coprecipitate, and finally sintering the calcined material at about one-half its melting point or greater, thereby forming a spinel which has a density of greater than 50 percent of the theoretical density of spinel crystal. Also disclosed are techniques for preparing spinels having more than two metals incorporated into the spinel lattice, as well as a separate oxide phase associated with the spinel crystallites, and slipcasting compositions.

Description

3L1'7~5 MAGNESIUM ALUMINUM SPINELS

BACKGROUND OF THE I NVENTION
Spinels are well known metal oxides of specif;c structural configuration having a generic formula M~O4 where M represents the same or different metal elements having different valences whose sum o-F
products of the valence times the number of atoms of each element having that valence equal, preferably eight, but may vary up to a few percent excess or defi-ciency of the metal ion relationship from eight.
Exemplary of the common formulae are, e.g., MgAI2O4, and ZnCo2O4 where the sum of the product of the positive elements valence times the number of atoms of each valence equals eight. Exemplary of imbalanced stoichiometry, excess and deficient atom structures, are e 9 , M9o gFe0 1lAl2o4 and Ni0.2 0.79 2 4 respectively.

Most prior art techniques used commercially for preparing ceramic spinels ernploy the fusion C-24,807-B-~ -1-..i

-2~ 2 ~Z ~

technique o-F the metal oxides. This technique is not wholly satisfactory for the preparation of ceramic spinels because the metal atoms do not completely form into the spinel lattice structure; that is, some metal atoms form a segregated oxide phase admixed wi~h the spinel lattice structure and once formed by fusion the crystals are not amenable to shaping by pressure and sintering without the aid of binders which mav be detrimental to acid and/or base resistance and physical properties of the finished product. Organic binders in ceramics made in this way make the body relatively porous when they are removed during or after shaping.
Segregated ceramic binders may weaken the body because they are the site of differential expansion and contraction and/or chemical attack.

The prior art also recognized the phenomena of spinel formation being a physio-chemical reaction based on thermal conditions such that, regardless of the ratio of the metals, some spinel lattice would form at the correct temperature, physical and chemical conditions, albeit those atoms not forming a spinel lattice structure remain as segregated phases of the metal oxides. The spinel shapes which are commercially available usually have been prepared from spinels produced from starting materials containing impurities or one Qr more segregated metal oxides phases and thus are relatively poor with respect to their physical properties, e.g., tensile strength, acid and/or base resistance and porosity.

Numerous patents and scientific literature have been published disclosing different techniques -for preparing spinels (esp. MgAI2O4~. Most procedures C-24,807-B ~ -2-,, ~ ~ 72 ~ ~

employ metal oxides or oxidizable compounds, both of which are converted to a spinel by firing or fusion with or without pressure.

In some patents a magnesium compound and an aluminum compound are mixed to give the requisite molecular const;tution, are wet ground and mixed, and fired at tempera-tures up to 3,000F (ca 1660C) as for example, in U.S. Patent 2,618,566 or shaped before firing into pebbles as in U.S. Patent 2,805,167 Others use pure magnesia and alumina mixtures which are then fired at 2150C and cooled slowly over-night, (e.g. U.S. Patent 3,516,839). Still others mix alumina with magnesium nitrate, dry fire on a schedule to 1400C, and then grind to obtain a powder, (e.g.
U.s. Patent 3,530,209). Another technique follows the fusion route of magnesiwm nitrate hexahydrate and ammonium aluminum sulfate dodecahydrate (both reagent grade) -to 1300C to produce a fine powder, (e.y. U.S.
Paten-t 3,531,}08). A magnesium-salt (MgSO4"7H2O), aluminum-sal-t (Al2(5O4)3"18H2O) mixture, co-crystal has been employed to prepare a powder which is then shaped into ceramic bodies by hot press techniques with or without the use of binders, (e.g. U.S. Patent

3,544,266).

Concomitant with these developments researchers investigated the nature of metal double hydroxides formed by coprecipitation, some of whic.h were shown to convert -to a 5p inel upon calcination.
Early work was performed by Feitnecht and his students who made a series of double hydroxides with Mg/AI
ratios of 2.5 to 1, even employing a reactant range of C-24,807-B~ -3-_4_ ~ 2~5 1.5-4 to 1 Mg/AI, by coprecipitation from magnesium and aluminum chlorides, Helv. Chim Acta 25, 106-31 (1942), 27, 1495-1501 (1944). No change could be detected by x-ray di-ffraction techniques then available for different Mg/AI ratios or a certain degree o~ substi-tution by chloride for hydroxide. A similar double hydroxide~ reported to be a hydrate even after heating to 150C, was reported by Cole and Hueber in "Silicates Industriels" Vol. 11, pp 75-85 (1957). The compound was made by the reaction of NaOH with Al metal or Al2(5O4)3 and MgO or MgSO4 at 65-70C. The product had a Mg/AI ratio of 4/1 even when reactant proportions were varied. However, Mg(OH)2 was observed as a second phase in some cases.

More recently, Bratton in bo-th Journal of The ~merican Ceramic Society, Vol. 52, No. 8 (2969), ancl Ceramic Bulletin, 48, ~8 pp 759-62 (1969) 48, 11, pp 1569-75, reported the coprecipitation of numerous magnesium and aluminium chlorides and oxalates which on heating, drying, calcining or firing, exhibited a spinel x-ray difFraction crystallographic pattern. The coprecipitation product resulted in a magnesium aluminum double hydroxide of composition 2Mg(OH)2l'Al(OH)3, plus a large amount of segregated gibbsite Al(OH)3 phase ~see also U.S. Patent 3,567,472). This is presumably the same product Feitnecht obtained.

Bakker and Lindsay in "Ceramic Bulletin"
Vol. 46, No. 11, pp 1095-1097 (1967) report that a high density spinel body can be made from Mg(OH)2 and Al(OH)3 if 1.5% AIF3 is added as a mineralizer.

C-24,807-B-~ -4-In the works cited above these powders were, in some instances, calcined then fired while in other instances the powders were heated through the calcining range and ultlmately through the firing and even the fusion range. Early work was directed to pre-paxing spinels usable as a decolorant, United States Patents 2,395,931 and 3,413,184 or as antacids, United States Pa-tents 3,323,992 and 3,300,277. In the last case a "highly hydrated mag-nesium aluminate" is claimed as a new composition of matter, the formula of which is Mg~OH)2"2AI(OH)3"XH2O where X = 4 to 8. The material is prepared by the reaction of NaAIO2 (Na2Al2O4), NaOH
and MgCl2 as aqueous solutions at a pH from 8-9. Bratton in United States Patent 3,567,472 also discloses coprecipitation of a magnes-ium and aluminium chloride from a solution having a pH from 9.5 to 10, drying or ~iring to obtain a light-transmitting spinel by adding CaO.
BRIEF DESCRIPTION OF THE INVENTION
According to the present invention, there is provided a coprecipitate comprised of a substantially layered crystallite having the structure dYc Ml aXb = (l+z)MI bXba ~ 2MII dyCd M d c wherein M represents one or more metal cations having valence(s) a, Ml] represents one or more metal cations, at least one of which is different from Ml, having valence(s) c different from a; X and Y
each represent one or more anions having valences b and d, in charge balance with a and c~ respectively, and X and Y are convertible to the oxide on heating; the molecular ratio of _ 5 _ :

`
.
:

~.~72~

MIX to MIIY being (l+z~M X"2MIlY where z equals or is greater than zero but less than 3; and sufficient segregated phases of the formula Mll''O''Y and/or MIIY to provide an overall stoichiometry of M X"2M Y.
The coprecipitate of the present invention can also be defined as comprised of a substantially layered crystalline having the structure M Y
M X = (l+z)M X"2M Y
M Y

wherein M represents a cation of a metal or mixture of cations of me~als having a valence of 1, 2, 4 or 6; M represents a cation of a metal or mixture of cations of metals at least one of which is different rom M~ having a valence of 1, 2, or 3; X and Y are anions having a valence of 1 or 2 selected from the group consist-ing of hydroxyl, halogen, sulfate, formate, hydrogen phosphate, acetate, nitrate, carbonate, bicarbonate or mixtures thereof comprised as haloacetate, hydroxycarbonate, chlorohydroxide; z equals or is greater than zero but less than 3, and sufficient segregated phase of M l''O''Y and/or Ml]Y to produce a stoichiometry of M X"2M Y.
More particularly, the present invention provides a magnesium aluminum coprecipitate comprised of a layered crystalline having the structure Al(OH)3 Mg(OH)2 = (l+z)Mg(OH)2'2AI(OH)3 Al(OH)3 - 5a -~ ~7J~

wherein z = equals or is greater than zero but less than 3, and at least one segregated phase of the formula AIO(OH) and/or ~I(OH)3 wherein the overall stoichiometry of the precipitate is MgAl2(OH)8.
In another aspect, the invention relates to a method of preparing a spinel which comprises mixing a MIX compound and a M Y compound wherein M represents a cation of a metal or mixture of cations of metals having a valence of 1, 2, 4 or 6; ~ll represents a cation of a metal or mixture of cations of metals at least one of which is different from Ml having a valence of 1, 2, or 3; X and Y are anions having a valence of 1 or 2 selected from the group consisting of hydroxyl, halogen, sulfate, formate, hydrogen phosphate, acetate, nitrate, carbonate, bicarbonate or mixtures thereof comprised as haloacetate, hydroxycarbonate, chlorohydroxide together with water to form an aqueous slurry maintained at a pH of from 8 to about 10, washing the mother liquor and precipitate with water or an alkaline solution, separat-ing the solids from the mother liquor-wash liquor~ washing the solids with water, drying and calcining from 400 to 1400C.
Thus, a spinel can be 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 lncluding hydroxycarbonate, chlorohydroxide, the halogenated carboxylates, in a proportion and kind to provide metal atoms of two different valences, albeit they may be the same metal or different metals, to total eight, plus or minus about 10~, positive valences available to combine with four oxygen atoms in the generic stylized formula M304 (or k~2O4). The - 5b -.

' ~1'7Z~;~5 coprecipitation produces, when conduc-ted at pH in the range o~
from 8 to 10 at which coprecipitation occurs, (usually between about 9 and 9.5 for Mg/AI), a product - 5c --6~

having a specific layered crystalline structure which may or may not contain a segregated aluminum hydroxide or oxyhydroxide phase. 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/AI ratio o-f the coprecipitate by the selective dissolution of Al from the coprecipitate. The coprecipi-tate is then tlried and calcined at a temperature of from 400C to 1~0~C thereby forming the crystal lattice of the spinel structure with little or no segregated phases of either metal. The so formed spinel, usually a powder, can be sintered at a temper-ature above about 1500C with or without shaping into a thermally and chemically stable product capable of achieving greater than 99~ of the theoretical densi-ty of a spinel crystal lattice structure. The resulting high density products are resistant to acidic and basic attack and shock including thermal shock.

Thus, it has now been found, for example, if a magnesium compound such as, magnesium hydroxide, or the chloride, hydroxychloride, sulfate, phosphate, acetate, nitrate, halide, carbonate, bicarbonate, and the like, is coprecipitated with an aluminum compound, such as aluminum hydroxide, or the chloride or sulfate, at a pH to coprecipitate the compounds so that at least one of the metals is converted to its respective hydroxide or partial hydroxide during the coprecipi-tation, followed by washing with or without alkalinity before recovering the coprecipitate, there is obtained ~0 a product having the following composi-tion upon drying at approximately 125C -for several hours:

(l+z)MI bXa"2MII dYdc C-24,807-B ~: -6--7~ 5 wherein eacll X and Y is independently selected from the aforementioned anions and at least one X and/or Y is -OH and z represents a number less than 3 and prefer-ably about 1, and where when z is greater than 0 there will be present at least one segregated phase, as for example in the magnesium-aluminum coprecipitate, an alum;num phase of Al(OH)3 and/or AIO(OH), and wherein "a" times the number of at.oms of Ml( ) equals the valence b of X times a, the number of atoms of X, and similarly c times the number of atoms of Mll(d) equals the valence d of Y times c, the number of the atoms of Y, the Mll/MI ratio in the total coprecipitate being maintained at about 2 to 1 respectively, and having a volatile content of about 40% by weight when a Cl atom is present and about 36% by weight when all the X and Y's are -OH moieties, (analysis by thermogravimetric analysis). The exemplified coprecipitate is not a hydrate and individual crystallites have M~l/MI ratios significantly different from those previously reported, 20 for example when Mll is aluminum and Ml is magnesium, as shown by micro-area x-ray fluorescence, electron diffraction and high resolution x-ray diffraction. The dried precipitate is thereafter calcined at a temper-ature of from 400C to 1400C for from typically about 25 4 hours to about 1 hour, respectively. The calcined precipitate has an x-ray diffraction pattern of the.
spinel structure, -For example, MgAI 24 . The so-calcined precipitate can be formed into bricks or other ceramic shapes by pressing at preferably between 1000 to 10,000 psig although higher pressures may be employed and firing the shape at above about 1400C, preferably above 1500C. The shape will densify uni~ormly. Thus, densities may range from as low as 50% to as high as 99% or greater of theoretical density, depending on the following variables:

C-24,807-B-~ -7--8- ~ 2S

1. The chemical composi-tion of the powder;
2. The calcination history of the powder;
3. Powder processing history, i.e. particle size distribution selected for pressing, lubri-cants and binders added to the powder, and - the like;

4. Mode of pressing the powder into a shape and sintering technique used.

In accordance with the present invention a (thermally and chemically stable) spinel can be pre-pared by coprecipitating metal compou*ds (e.g., magnesium and aluminum hydroxides, or chloro hydrox-ides), that is, coprecipitating a metal compound or compounds having an atomic valence of one, two, three, four, or six or convertible thereto on conversion to the oxide, recovering the precipitate as a powder and calcining the powder~ thereby to prepare a spinel suitable for sintering with or without shaping.

Spinels are well known metal oxides having a specific structural configuration and having a generic formula M304 where M is at least two metal atoms Ml and M 1, which may be the same or different metal elements, having different valences whose sum of products of the valence times the number of atoms of each valence equal, preferably eight but may vary up to a few percent excess or deficiency from eight. Exemplary of the common formula are ZnCo204 and MgAI204 where the sum of the product of the valence times the number of atoms equals eight. Exemplary o-f imbalanced stoichi-ometry, excess and deficient atom structures, are e.g.,MgO gFeO l1AI204, and NiO.2 0.79 2 4 C-24,807-B-~ -8-,. . ' ' 9 ~ ~7~Z4~5 In addition to the basic spinel, numerous m;xed spinels were prepared. Mixed spinels can be made in any one of several ways. The preferred way is to add the desired metal a-t the coprecipitation step.
However, this may not always be practical, or the hydroxides may have such a large difference in solu-bility that a coprecipita-te with the desired compo-sition is not formed. The second method of preparation is to mix the separately prepared compounds ;n the desired ratio. This requires only a knowledge of the metal content by, say, x-ray fluorescence. The mixture may be ground intimately if a homogeneous composition (e.g. one mixed phase such as Mgo23CoO27AI133CoO370L~) is desired. It is also to be recognized that when the "mixed spinels" are desired and the third metal is or two or more additional metals are added at the copre-cipitate stage the pH for coprecipitation may have -to be varied, as for example when chromium is added the pH
is adjusted to about 9.7 to insure coprecipitation of all three metals in, for example, a Mg/AI/Cr system.
Alternately, a dry mixture may be mixed poorly, or a gross disparity in the particle size distribution of the starting materials may be introduced, i-f a range of compositions is desired (e.g. Mgx2Col2xAl23yCoy304, where x and y vary from region to region in the mass).
The most preferred way to prepare a range of solid solut`ions within one sample is to add at least one of the metals as the hard burned oxide which limits its reactivity. One should not assume that the same effect will be achieved if the preburned oxide is the spinel component versus it being the additive metal. In general, the higher the preburned component has been calcined, the lower its activity will be toward solid solution formation. In some cases part of the additive C-24,807-B-~ -9-- 1 o ~ L7.'Z9~5 metal may enter the spinel structure and part may form a separate oxide phase. In addition, a doping metal compound may be added to the pre-calcined or post-calcined spinel and may exhibit phase segregation or solid solution formation, depending on its reactivity and that of the spinel phase.

In accordance with the present invention a spinel can be prepared by co-precipitating metal com-pounds, that is the metal sulfates, chlorides, hy-droxides, hydroxychlorides, oxychlorides and the like,or mixtures thereo-F as afore set forth, in a proportion and kind to provide metal atoms oF two different valences, albeit they may be the same metal or different metals, to total eight or about eight positive va ! ences available to combine with oxygen in the generic stylized -formula M304 (or Ma"2Mb~O4). The coprecipitate is preFerably washed with an alkaline solution, dried and calcined at a temperature of from 400C to 1400C, preferably from 1000C to 1200C
thereby forming the crystal lattice of the spinei structure. The so-formed spinel, usually a powder, can be sintered with or wi-thout shaping into a thermally and chemically stable product capable of achieving a density of greater than 99% of the theoretical density of a spinel crystal lattice structure. The resulting high density products have good chemical and mechanical properties.

It is to be further understood that modifica--tions in the stoichiometry may be made so long as it is understood and desired to produce less dense (Final density less than about 90% of theoretical) products and/or separate metal oxide crystal phases admixed with C-24,807-B-~ -10-:~72~2~;i and bordering the spinel crystallites in the body. In fact one can so deviate from the stoichiometry of the normal spinel that there are ob~ained products similar in analysis to those ob-tained by the fusion melt prac-ticed by the industry, i.e., a large proportion o-f segregated phase, ~e.g. high magnesium oxide separate phase) which depending on use, may or may not be detri-m~nta I ~

The modified spinels, in contradistinction to the mixed spinels of the present invention, can be obtained by mixing, (a) during coprecipitation, an - excess of one or more of a metal compound coprecipi-tants, (b) a desired separate phase metal compound with the coprecipitated uncalcined precursor (c) an additive oxide with the calcined intermediate of the present invention prior to sintering, or (d) an additive oxide can be added to the spinel following grinding and sintering, but before shaping, especially i-f simple physical mixtures are desired.

The utility of the products of the present invention permit a wide varia~ion in manufacturiny techniques. For example, ceramic shapes, such as bricks, can be made by casting the calcined spinel powder using an aqueous suspending medium or com-pressing the calcined intermediate under moderate pressure and then sintering, the densification of the spinel occurring during unpressured sintering, or the powder can be subjected to fusion molding of brick or like forms. In addition, a conventional brick can be coated with the calcined intermediate powder and the coating sintered on the surface or a melt of the C-24,807-B-~ -11-sintered spinel can be sprayed on a surface, the sintering, at least-in-part, occurring during the melt-spray technique.

As stated above, modifying metals can be incorporated into the process at various stages with different results. For example, iron oxide may be added at any stage and when added as a part of the coprecipitates it will enter into a spinel lattice since it has the capability of forming both two and three valence atoms which are capable of orientation into the spinel lattice structure form and if the iron is added as exemplified above to the coprecipitates of say Mg and Al, the final spinel will -take the structure exemplified by the formula Mg+a2Fel_aFeb Al~_b4 where a = 0 to 1, b = 0 -to 2 and where, depending on the amount of iron~ the iron may be the principal metal or the modifying metal. If, on the other hand, the iron is added to the already sintered spinel the majority of the iron will be present as a separate phase. In the case of multivalent metals such as iron, a spinel la-ttice such as Fe 2"(Fe 3)2"04 may form as a separate structure and/or become a solid solution type crystal intermingled with the magnesium aluminate spinel to which it has been added. Some variation in the tendency to form a segregated phase is observed when the atmosphere is highly oxidizing vs. when it is inert.

In one embodiment of the present invention 30 sodium aluminate (Na2AI2O4"3H2O) was mixed with mag-C-24,807-B-I' -12-.

.~7~ S

nesium chloride (MgCI2) in the presence of hydrochloric acid (HCI). The d;spersed precipitate was washed, preferably in an alkali solution, filtered and washed again with water, dried and calcined at a temperature of from 900 to 1400C to form fine discrete particles suitable for compression-forming into desired shapes, such as bricks, which can be sintered at above about 1400aC, preferably above about 1500C.

In another embodiment a bulk grade of aluminum hydroxide was dissolved in sodium hydroxide, then filtered and the soluble aluminate employed in the manner of the foregoing description.

In still another embodimen-t aluminum sulfate and magnesium sulfate were employed as the coprecipi-tants employing sodium hydroxide as the source ofalkalinity. The resulting co-precipitate was treated as before.

In like manner, aluminum chloride and mag-nesium chloride were coprecip;tated in -the presence of sodium hydroxide and the precipitate treated as above.

In another embodiment magnesium and aluminum chlorides were reacted with sodium hydroxide and then hydrochloric acid to control the pH of precipitation and the precipitate treated as before.

In yet another embodiment the metal chloride was converted to -the hydroxide, as AICI3 to Al(OH)3, then reacted with the chloride or hydroxy chloride o-F
the other metal and the product treated as above.

C-24,807-B-~ -13-1:~7~;25 More particularly, the present invention is carried out, in a presently preferred manner, by the simultaneous precipitation or coprecipitation of metal compounds which are, or which form on treatment with an alkali wash, separation and heating, the metal hydrox-ides or partial rnetal hydroxides, and then subsequen-tly on heating above about 1400C the metal oxide. The proportion of the metals is such that the sum of -the valences of each metal multiplied by its a~omi.c quantity will total 8 or about 8, i.e. plus or minus ten percent. The spinel structure is identified by this ideal metal valence to the 4 oxygen atoms present as M304 (or Ma"2Mb 04). It is to be understood that a slight deFiciency or excess in total metal valence over 8 may occur with a concomitan-t small change in final product, yet most of the spinel characteristics of the products of this invention remain. Illustrative of the aforedescribed embodiments including the imperfect valence balance are hereafter set forth, with specific reFerence to aluminum and magnesium, it being under-stood that other common metals may be substituted for either or both aluminum or magnesium and still ob-tain the benefits of this invention.

The simultaneous precipitation of metals in the ratio to obtain the spinel structure in accordance with the preferred embod;ment of the present invention results in a precipitate which has an overall stoichi-ometry of MIM2l(0H)8 in more detail depending on the valence, Ml 62MII l(OH)8, Ml 42MII 2(0H)8, Ml 22MII 3(0H)8. The ratio of Ml to Mll is 2 but may vary up to 10~ excess of either. In this latter case it is believed that in some instances the 8 valences C-24,807-B`-~ -14-;

-15- ~72~25 required For the spinel structure and the 2 to 1 metal ratio are fulfilled first and the excess metal forms a separate oxide phase surrounding or occluded within the spinel crystallites.

The coprecipitates tn the above -form, e.~., MIM2l(OH)8 where Ml is a divalent metal atom and Mll is a -tr.ivalent metal atom, is made up of la\~ered crystal-lites with the following composition, as evidenced by x-ray diffraction, electron diffraction, electron microscopy and micro area x-ray fluorescence.

M ~OH)3 = (1~z~M ~OH)2 2(M (O~)3) M ~OH)3 plus separate phases 2MIlO(OH) (also written MIlO3"~2O) and M (OH) to maintain the overall product stoichi-ometry MIM2~(OH)8 when z greater than zero but less than 3. Some of the hydroxide in this structure can be replaced by Cl, Br, nitrate, acetate, sulfate or various other anions and mixtures of anions as discussed previously.

This particular layered structure is in evidence in each of the following preparations made . from the wet state. The crystal structures can be indexed on the basis of a hexagonal unit cell ;n which the a axis is the most sensitive to changes in ca-tion size and the c axis is the most sensitive to changes in anion size. Following calcining of the dry powder the x-ray diFfraction pattern matches that of the spinel plus, provided that if the stoichiometry is not exact, evidence af separate phases of other me-tal oxides.

C-24,807-B-G -15-..

-16~ 7~

Several other techniques can be employed as illustrated below, each -forming the layered s-tructure as the coprecipita-te. The spinel structure on calcining is amenable to low pressure forming and densification on sintering.

Specifically, one can co-precipitate an aluminum magnesium spinel precursor (which after cal-cining forms the spinel) by one of the following tech-niques wherein Ml represents for balanced equations a divalent atom, namely for illustrative purposes only magnesium, and Mll represents a trivalent atom, namely aluminum.

1. MIX ~ MIlY + aqueous alkaline solution T
MIX' + 2MIlY' + alkali or alkaline X, Y in solution .15 for example MgSO4+AI2(5O4)3~8NaOH(aq) T Mg(OH)2"2Al(OH~3+4[Na2sO4]~
or, MgCI2+2AL(OH)3+2NaOH aq T Mg(OH)2"2Al(OH)3+2[Na Cl ], (it is to be understood that the equations given here represent the overall stoichiometry of the reaction and not necessarily the composition of a specific crystal-lite, that is the layered structure, is not here exemplified) which is washed with water or an aqueous alkaline solution (e.g. aqueous caustic), the solid separated and washed again. The product exhibits layered structures as aForedescribed. The product when dried and calcined at between about 400 and 1400C
forms a fi-ne powder which by x-ray diffraction has the spinel structure MIM2lO4.

C-24,~07-B f -16--17~

2. 2AMIlY+MlX + alkaline solution + acidic solution T
MIX'''2MllY'+[A X ], A being alkaline ion, for example 2NaAlO2~MgCI2+NaOH(aq)+HCl(aq) T
(1+z)Mg(OH,CI)2"2Al(OH)3 + 2z(Al(O)OH+Al(OH)3)+[Na Cl ], r followed by washing with or withoLlt added alkalinity, separation, washing the separated prec;pitate and drying.

3. M X 2M Y + aqueous alkaline solution T
M (X)2 2M ~Y)3+[A Y ], for example MgCI2+2AlCI3 + aqueous alkaline solution T
(1+z)Mg(OH,CI)2"2Al(OH)3 + 2z(Al(O)OH+Al(OH)3)+[Na Cl ], followed by treatment of the precipitate as afore-described.

4. MIX~2A''AIY-~HCl T MIX'''2''AIY'+ACl, A being alkaline ion, for example MgCI2+2NaAlO2+HCI T
(1+z)Mg(OH,C1)2"2Al(OH)3 + 2z(Al(O)O~+Al(OH)3)+[Na Cl ], followed by treatment of -the precipitate as afore-described.

Another technique for preparing -the precursor is the comixing of finely divided aqueous slurries:

5. MIX or MIX''nH2O~Mll(OH) mixed as an aqueous slurry will after precipi-tation, recovery, drying and calcining followed by sintering, yield a spinel of the present invention, e.g., MgCI2"2H2O, Mg(OH)CI or Mg(OH)2 mixed with Al(OH)3.

C-24,807-B-~ -17--18- ~ ~7~2~

I-t is also to be understood that the powders produced in accordance with this invention can be pressed onto and into porous surfaces and sintered thereon to give protective spinel coatings or surfaces of similar acid/base resistance.

Examples of the various metals which can be employed in preparing spinels of the present invention are:

+1 +2 +3 +4 +6 Ll Mg Al Ti Mo Fe Fe Mn W
Mn Cr Sn Co Co V
Ni Ga C~l++
Zn++

It is of course to be understood that several metals may be employed to form a single mixed metal spinel as for example MgxNiy'l2(AlaFeb) 4 where x and y represent fractional numbers totalling 1 and a and b represent fractional numbers totalling 1.
In this example of course the Mg and Ni are divalent atoms and Al and Fe are trivalent atoms. In the fore-going example iron may be added in such a way that aspinel of the following structure may be forrned MgxNiyFez~2~AlaFeb~o4 C-24,807-~ ~ -18-~ ;

- 1 9 - ~ .1l'7~2~i where again x+y~z = 1 and a~b = 1 and Mg and Ni and some Fe are the divalent metals and Al and ~he remaining Fe are the trivalent metals.

DETAILED DESCR ! PTION OF THE INVENTION
Example 1 One hundred fifty-nine liters of an aqueous solution of 7.5% by weight Al~(B~3 arl~ 2.5~ hy weight MgSO4 was treated (mixed) with 117.7 liters of B.26%
NaOH containing 15.95% NaCI (a chlorine cell effluent) at a rate to provide a retention time of 28.5 minutes.
Following the treatment of the sulfate solution the overflow containing a precipi-tate a~ 50C was filtered under a six (6) inch mercury vacuum and washed with 3 cake volumes of distilled water. The washed cake was 14.5% solids. This cake was dried to a powder and analyzed -for mole ratio of aluminum to magnesium. The ratio was 2.1 to 1, respectively.

The powder was calcined at 1100C for 3 hours. By x-ray diffraction analysis the calcined powder had acquired a spinel structure. Upon sintering at about 1700C the powder acquired a density of 3.23 g/cc.

The following examples illustrate various modifications in the procedure of Example l employing various magnesium and aluminum compounds:

C-24,807-~ ~ -19--20- ~ ~'7~ %~

Examples _ 3 4 Duration (min) 130 168 350 Reactants:
a) Salt Solution wt. /O Mg Salt (~gCl2) 8.41 2.05 10.18 wt. % Al Salt (AlCl3) - 5-7 wt. %HCl 3.55 2.37 Vol. Liters l9.3 55.15 24.05 b) Alkaline Solution Kind NaAlO2 NaOH NaAlO2 wt. /~aOH 5.53 calc 9.5 5.04 calc wt. /~l(OH)3 6.46 calc - 7.62 calc wt. /~aCl Vol. Liters 33.6 56 49.5 c) Acid Solution Kind - - }ICl wt. % HCl - - 9.77 Vol. Liters - - 8.03 d) Nol Ratio Al/Mg1.62 1.99 2.0 Precipitation @ 50C
pH M pptr. 9.2-9.5 9-9.5 9.2-9.4 Retention time ~min) 22 21 61 Filtration Vacu~m i~ches Hg (abs.) 6 6 6 I.oad Rate gph/ft2/l"cake 59 22 101 Wash rate gph/ft2/l"cake 30 14 68 Wet Cake % solids21.5 16.0 27.2 Dry washed cake wt. % Mg(OH)2 22.55 24 25.0 wt. % Al(OH)3 63.5 72 75.4 Mol Ratio Al/Mg 2.11 2.24 2.26 Density of calcined and sintered product gm/cc - - 3.24 3.50 C-24,807-B`-F -20-, -21- ~ 7 Density Studies The powder o-f Example 2 was calcined at 1000C for approximately four hours and was pressed in a Beckman powder mold under various pressures to pro-duce a 1-1/4 i nch diameter by 1/2 i nch thick -tablet and thereafter sintered at either 1535C or 1400C and the average density thereof determined. The following table sets forth the resu!ts obtained:

~ Sintering Pressure Temperature Density PSI C gm/cc l400 1535 5,000 " 3.29 " 2.21 10,0Q0 " 3. 42 " 2.20 15,000 " 3.42 " 2.11 20,000 " 3 39 " 1.92 The data establishes that a sintering temper-ature of about 1535C or above should be used to achieve the greatest densification and concomittant -therewith a pressure of greater than or equal to about 8000 psig is also advantageous. Sintering below about 1400C results in densification less than about 2/3 theoretical, 3.57 gm/cc being the theoretical density of the MgAI2O4 spinel, based on crystallographic unit cell data for the final product and literature data.
Depending on the chemical composition, calcination history, powder processing methods and pressing and C-24,807-B-~ -21--22~ 25 sintering techniques, sintering densi-ties greater than 9g% of the theoretical values were sometimes obtained.

The tablets pressed at greater -than 5,000 psig and sintered at 1535C were subjected -to contact with caustic beads at 1500C or with 15%
boiling hydrochloric acid. Nei-ther treatment appeared to react or si~nificantlY affect the surface or strength of the tablet thus treated.

The magnesium aluminate spinels of this invention, either the calcined or the powdered or shaped fired spinels, have the ability to combine (even at relatively low temperatures) with other oxides, halides, hydroxides or coprecipitates of aluminum and other metals including, but not limited to, those of the transition metal series to yield products exhibiting altered properties. These properties include sinterability, stability to oxidation or reduction, strength, porosity and catalytic activity.

Examples 5-8 M~AI Spinel Precursor Examples 5 and 6 are precursors that were produced at reaction conditions about identical to Example 4 described above. The reaction slurry was treated with a NaOH solution before fiItering. The precursor had an excellent filter rate and a low cake solids content (18-20% 501 ids). Its Al/Mg mole ratio was 2.01. The precursor sintered to very dense sp;nel after calcining at 1000-1200C.

Example 8 was produced at a higher reaction temperature (60C) and a longer retention time (4 hours) to improve filtration and drying properties.

C-24, 807-B-F -22--23~

The reaction slurry was concentrated by settling, NaOH
treated, and filtered. The filter and dryer capaci-ty were much improved over examples 5 and 6 due to a much higher cake solids content. Although the sintering properties of preliminary precursor Example 7 matched those of Examples 5 and 6, which were calcined at about 1000C and sintered at about 1500C subsequent sinterina of Fxampl~ ~ showed the need -for a higher calcining temperature of ~bout 1200C to obtain high density spinel upon sintering: (at ca. 1500C) unless the precursor was upgraded by size reduction (ball milling or double compaction). The reason advanced was that Example 8 had an excess of aluminum in the precursor phase wh;ch formed a segregated aluminum oxide phase upon calcining. Aluminum oxide inhibits the sintering of the spinel phase as discussed later in Example 13, part 4.

C-24,807-B-~ -23-~ d~
-2L~-Example 5 6 7 _ 8 Duration of run (hrs) 29 28 20 65 Reactants:
a) Salt Solution wt. % Mg C12 10 10 9.94 10 Vol. Gallons 734 703 295 2030 b) Alkaline Solution Kind NaAlO2 w-t. /ONaOH calc. 1.13~1.19 .89-1.15 .87-1.25 .7-1.0 wt. /~aA102 calc. 8.04-8.348.37-8.76 8.71-7.91 8.2-8.7 Vol Gallons 15251416 317 4096 c) Acid Solution Kind HCl wt. % HCl 8.8 lO lO 9.7 Vol. Gallons 299 209 123 679 d) Mol Ratio Al/Mg 2.012.01 2.02-2.07 1.99-2.03 Precipitation @ C 50 50 60 60 pH M pptr. 9.3-9.4-~9.05-9.4* 9.1-9.3* 9-9.4*
Retention time (min) 55-58 69-72 226-237 247-260 Filtration Load Rate gph/ft~/l"cake 43 30-40 58 42 Wash rate gph/ft2/l"cake 24 25-30 56 23 Cake % solids 21.421-22 35.9 35-37 Dry washed cake wt. % Mg~OH)2 26.026.2 - - 26.4 wt. % Al(OH)3 75.172.1 - - 76.0 Density of calcined and sintered product gm/cc 3.44-3.52 3.47-3.53 3.56 3.51 ~' range dur i ng run C - 2 4 , 8 0 7 - B ~ - 2 4 --25- ~ '3 The above data represents that obtained in the laboratory. The batches were large enough to employ a commercial-size filter and the cakes obtained by such use were also analyzed and used in vari OU5 operations described in later examptes. The data for each batch from the Moore -fil-ter cake were: Each slurry was treated with 10% sodium hydroxide and washed with r~w (~Int~rea-ted) water.

Example 5 6 7 Slurry Wash 10% NaOH gal/100 gal 5 5 4.7 ll-,t Water ~cake volumes) 3 3 4 4 Cake-% solids 18-20 18 20xx 35 9 34-36 Dry Solids % Mg(OH)2 26.0 26.2 26.4 Al(OH)3 ' 75.1 72.1 76 Sintered density at 1500 C

of sample calcined at:

1000C (2 samples) 3.46/3.46 3.47/3.48 3.48 3.1 3.3/

` 3.1-3.4 1200C 3.46 3.49 3.56 3.1-3.5 ~This run was settled and decanted before treatment with the caustic wash. xx ranges during run.

C-24,807-B-~ -25--26- ~ ~7 Physical Mixtures with Other Oxides/Halides The spinel powders MgAI2O4 of Examples 5 and

6 were calcined at 1000C for approximately four hours and were -then mixed in various amounts with other metal oxides or fluorides, pressed at 10,000 psig and sintered at 1535C to determine the effect these cornpounds would have on the densification characte,istics of spinel.

In one experiment mixtures of calcined spinel powder and alpha-aluminum oxide were prepared by ba!l-milling. Pellets, 1-1/4 i nch in diameter, weighing approximately 10 gms. each were formed at a pressure of 8,000 psig composed of (1~ pure alpha Al2O3, (2) pure spinel powder (MgAI2O4), as well as -the following mixtures: (3) 90% Al2O3/10% MgAI2O4; (4) 75% Al2O3/25%
MgAI2O~. All pellets were sin-tered -for 2 hrs. at 1535C. The percent volume reduction was as follows:
(1) 11%, (2) 46%, (3) 17%, (4) 22%. As is seen from this data the spinel powder of the present invention can act as a densification aid when mixed with alpha aluminum oxide.

In another example, pellets, 1-1/4 i nch in diameter, weighing approximately 10 gms each, were formed at 8,000 psig using ground powder composed of (1) pure MgO, (2) 90% MgO/10% MgAI2O4 (calcined powder of example 2), (3) 75% MgO/25% MgAI2O4 (calcined powder of example 2). All pellets were sintered for 2 hours at 1535C. Theoretical densities~ measured densities and the percent of the theoretical density obtained are given below. The theoretical density of the composites was calculated from a weighted average of the theo-retical densities of the pure components. MgO and MgAI 24 are almost identical in theoretical density.
-C-24,807-B--~ -26--27~ 25 (a) (b) (c) % MgO 100 90 75 % Spinel MgAI2O4 0 10 25 Theoretical Density (gm/cc) 3.59 3.59 3.59 5 Measured Density (gm/cc)2.41 2.64 2.82 % of theoretical obtained 67% 74% 79%

As is seen from this data the spinel powder of the present invention acts as a densification aid when mixed with magnesium oxide under these conditions.
An alternate way to look at the data is that substan--tial amounts of MgO or Al2O3 will inhibit the densifi-cation of spinel powder. This behavior can be bene--ficial in the manufac-ture o~ catalyst supports where porosity is desirable.

In another example small amounts of lithium fluoride were ground together with the calcined spinel of Examples 5 and 6 and 1-1/4 inch diameter pellets, weighing approximately 10 gms each, were formed at 8,000 psig and fired for 2 hours at 1535C. The measured density obtained, calculated theoretical densities determined as before, and the percent of theoretical density calculated are given below for the compositions indicated. The density used for LiF was 2.635 gm/cc.

(d) (e) (f) 25 % spinel (MgAI2O4) 100 99.5 g9 % LiF 0 .5 1.0 Theoretical density (gm/cc) 3.58 3. 57 3.56 Measured density (gm/cc)3.27 2.732.62 % of theoretical obtained 91% 76% 74%

C-24~8o7-R-F -27-~ ' --28- -~'7~25 As is seen from the data a small amount of LiF greatly inhibits the densification of magnesium aluminate spinel.

In -the three examples discussed above it was shown that the densification proper-ties of the spinel powder of this invention can be altered through the addition of various other oxides or halides. In the case of MgO and Al2O3, the moderating agent remained substantially as a segregated phase, as shown by analyticai-invest;gation. The fate of the LiF was not determined.

Solid Solutions with Other Oxides In addition to physical mixtures, th0 spinel powder of this invention can also form solid solutions with other metal oxides. This ability can be used to alter the characteristics of the resultant system in unique ways. The metal oxides which can be used to form mixed spinel systems include, but are not limited to, members of the transition elements. For example, mixtures of hematite (Fe2O3) and the calcined spinel powder (MgAI2O4) were pressed and sintered at /5,000 psi and 1535C. The resultant products showed a single spinel phase whose cell constants varied in accordance to the composition of the original mixed powders. This relationship holds for the entire series of combina-tions whose end members are magnesium aluminate (MgAI2O4) and magnetite (Fe3O4). The original hematite is incorporated into the spinel lattice invol~ving a reductive alteration of some of the Fe 3 to Fe 2 _ even in air at 1535C, whereas hematite not mixed with the spinel, stays in the trivalent sta-te as Fe2O3 when fired at 1535C in air. A regular progression of densities is noted, as would be expected.

C-24,807-B-~ -28--29- ~1'7Z~2~

As discussed below, -the atomosphere employed plays a role in determining whether a homogeneous phase is observed versus a mixture of phases.

Example 10 Physical mixtures of hematite (a-Fe2O3) and the calcined powder of this invention (MgAI2O4 from Examples 5 and 6) were made by ball-milling the powders together in the following ratios by weight:
90% MgAI2O4/10% Fe2O3; ~0% MgAI2O4/20% Fe2O3, etc., up to 10% MgAI2O4/90% Fe2O3. Ten gram pellets of each of the above compositions were pressed at 5,000 psi and sintered at 1535C under ar~on for 2 hrs. The resultant material was shown to be a sin~le phase spinel of the type MgX2 Fel2x Al23y Fey3O4 by high resolution x-ray diffraction and measurement of the magnetic properties of the samples. The cubic cell parameter determined from the diffraction data was found to vary linearly from /~.08 A for pure MgAI2O4 to /8.40A for pure Fe3O4. In cases where sintering of the above compo-sitions was carried out in air instead of argon a homo-geneous mixed spinel was again observed by x-ray diffraction in cases where the iron oxide content was less than about 4~% by weight, but for higher levels of iron oxide in the starting materials a separa-te Fe2O3 phase was seen in addition to one or more mixed magnesium aluminum iron spinels.

In addition to pressed pellets one inch in diameter, a larger refractory shape (6 x 4 x 1 inches) was pressed and fired at 1535C. No cracking or delami-nation was observed. In this case two magnesiumaluminum iron spinels were observed by x-ray diffraction with different cell constants. This indicates a difference in the magnesium/aluminum/iron ratio.

C-24,807-B-~ -29-3 o ~ 2425 lt is poss;ble to produce a spinel phase incorporating magnesium, aluminum and chromium whose cell constant varies as a function of the chromium content. Sometimes separate hexagonal phase~s) o-f the corundum structure (Al2O3) are also formed, depending on the manner in which the chromium was introduced and the thermal ~reatment which followed. These may be segregated as chromium oxide ~Cr2O3) and aluminum oxide (Al2O3) phases or as solid solu-t;ons of the corundum type (AlxCr2_xO3)~ In general, rapid heating and/or poor mixing increases the tendency to form such segre-gated phases. The surest way to form a homogeneous spinei is to add chromium at the coprecipitation step.
This leads to a precursor hydroxide incorporating chromium. Alternately, a coprecipitated gel of the hydroxides of chromium and aluminum can be prepared and this product mixed, either as dry powder or as a we-t slurry, with the precursor or the calcined spinel powder of this invention.

Example 11 Approximately 18 Ibs. of a coprecipitated magnesium, chromium, aluminum hydroxide were prepared in a manner similar to those outlined in examples 2-4.
Data concerning the formation of a ceramic body from this product as well as that using the post addition method are given under "Examples of Appl;cations".

X-ray diffraction, electron microscopy and micro energy dispersive x-ray -fluorescense indicate a layer hydroxide of magnesium aluminum and chromium, which is less crystalline than that observed for examples 2-4, and a segregated aluminum hydroxide phase(s).

C-24,807-B^F -30 -31- ~ ~t7 2 Example 11 Duration (hrs) 5.5 Reactants:
a) Salt Solution wt. % Mg Salt (MgCl2) 7.42 wt. % Cr Salt (CrCI3) 2.57 Vol. Gallons 36.4 b) Alkaline Solution Kind NaAlO2 wt. /ONaOH calc. 0.95 wt. ~/~aA102 calc. 8.25 Vol Gallons 55.6 c) Acid Solution Kind HCl wt. % HCl 10 Vol. Gallons 3.1 d) Mol Ratio Al/Mg l.99 Precipitation @ 50C
pH (M pptr.) 9.6-9.7 Retention time (min) 61 Filtration Vacuum inches Hg ~abs.) 24 Load Rate gph/ft2/l"cake 2.5 Wash rate gph/ft2/11'cake 21 Cake % solids 20 Density of calcined and sintered product gm/cc @ 1500C 3.29 C 24,807-B- ~ -31-, . .

EXAMPLES OF APPLICATIONS
Refractory Shapes The spinel of the present invention can be used to make a dense single-phase magnesium aluminum oxide refractory shape. For example, 2500 grams of the calcined material is placed in a rectangular die. The die is closed and evacua-ted for nominally 30 minutes. The powder is pressed -to a pressure of 8000 psi using a hydraulic press. Upon - removal from the die, the brick has a green size of 10 about 2.25" x 7" x 8". The brick was sintered at 1535C for 6 hours to obtain a finished product measuring about 1-1/2" x 4-1/2" x 5-1/2" with a density 95% of the theoretical value for a perfect spinel crystal.

i5 Spray Drying lt is possible to avoid evacuation of -the mold by spray drying the calcined powder using estab-lished techniques. For example, 25 Ibs. of the spinel powder of example 2 were spray dried using standard binders, plasticizers and defloculating agen-ts at a commercial facility. This spray dried powder was pressed into brick using conventional, commercial technology at a rate of less than or equal to 30 seconds per brick.

~efractory Shapes with Substitution Two different types of chrome doped spinel bricks were formed using a single action dry pressing mode. The first used a coprecipated chromium aluminum hydroxide added to the spinel precursor oF example 8. Specifically, 500 grams of coprecipitated chromium aluminum hydroxide 30 (Cr/AI Z 1.0) were added to 1500 grams of precursor and dry ball milled for two hours. The product was then C-24, 807-B-f -32--33~ 7~

calcined to 1200C and held at that temperature for two hours. Afterward, the calcined material was remilled for two hours.

A brick shape was produced by placing the powder in a steel mold (coated with oleic acid~, applying vacuum for one hour and pressing at approxi-mately 9700 psig. The chrome-spinel brick was -then sintered to 1535C at a rate of 100C rise per hour.
Holding time at 1535C was four hours. The fired density of this spinel brick was 3.34 gm/cc. From high resolution x-ray diffraction, the approximate compo-sition of the solid solution is MgCrO 2AI1 84 Another type of chrome brick makes use of the coprecipitated magnesium-chromium-alumium precursor hydroxide of example 11.

The hydroxide precursor was first calcined at 950QC and held at -that temperature for six hours. Once cooled, the material was dry milled for one hour and then wet milled for 45 minutes. Additives for wet 20 milling consisted of deionized water, 0.5% polyethylene glycol (on dry weight chrome-spinel basis) having an average molecular weight of about 200, and 4% GELVATOL
resin grade 20-30 (Poly Vinyl Alcohol) made by Monsanto Plastics and Resin Company. After drying the material at 125C for 30 hours, the dried mass was fragmented by hand and ball milled for 1-1/2 hours. This milled chrome-spinel powder was also dry pressed at 9700 Ibs psi and sintered -to 1535C. Holding time at 1535C was six hours. Initially the heating rate was 38C per hour to a set point of L~00C, then changed to 100C per hour. Fixed density of the chromium doped spinel refractory shape was 3.29 grams per cubic centimeter.

C-24,807-B~ 33-34 :l~t7;~5 In both cases discussed above high resolution x-ray diffraction of the final product revealed a single phase magnesium-chromium-aluminum spinel whose lattice constant is larger than that of pure MgAI2O4.
This lattice expansion and the absence of segregated phases indicates substitution of chromium into the spinel crystal lattice. In other cases where poor mixing or rapid heating was employed segregated phases were observed, as discussed previously.

Example 12 ~Mortar/Coating) As an example of another aspect of this invention, a mortar or refractory coating can be macle in the following way. Thirty (30) grams of Finely ground spinel precursor, example 5, was slowly added while stirring, to 70 grams of 15% phosphoric acid (H3PO4). Heat evolution will be noted as the acid base reaction proceeds. This mixture is then heated to /90C for several hours. A grit is then added to produce the desired consistency. This may be, For example, the calcined spinel powder or sin-tered grain of the present invention or another suitable refractory oxide. A typical combination would be 60 grams of calcined spinel powder with 20 grams of the above slurry. Distilled water may be used to adjust the consistency. Upon air drying for several days this mortar has excellent green strength but remains water dispersible. It can be cured to a water impervious form by firing at high temperatures of 500-1000C for several hours.

Example 13 (Catalyst Supports) (1) As an example of a pure spinel ca-talyst support, 50 grams of the loosely packed spinel precursor C-24,807-B-~ -34--35~ 7Z~ ~ ~

o-f the present invention was sintered to 1535C for two hours. The resultant porous form has a total porosi-ty of about 26% by mercury intrusion with approximately 90% of these pores in the 2-10 micron range.
Similarly, sintering of 50 grams of the precursor for -two hours at 1700C yielded a strong porous body of 26%
porosity which also had greater than 9()% of the porosity in th~ 2 to ~ n m i Gron rang~ The ~ime ~nd temperature can therefore be used to acijust the physical strength of the support with only small shifts in the pore size distribution.

(2) Catalyst supports can be made from the spinel precursor of the present invention by slurry or slip casting techniques. For example, 50 grams of the precursor is mixed with 100 grams of water. A small amount of nitric acid (approx. 2 ml) can be added to aid in mixing. The resul-tant "mud" can be place~ in a form or pre-ferably extruded or rolled into the desired shape. The forms are dried at approximately 100C for 20 several hours. The "green" forms are then sin-tered at approximately 1700C to obtain a pure spinel support with /30% porosity by mercury intrustion and the afore-mentioned pore size distribution (2-10 microns).
Alterations in the rate o-f drying, amount of wa-ter used, firing temperature and particle size of the initial powder can be made to change the porosity and pore size distribution.

(3) Adjustments of the total porosity and pore size distribution can be made by incorporating a burn-out agent into the powder prior to sintering. For example, 50 grams of the spinel precursor of the present invention is mixed with 5 grams of METHOCEL

C-24,807-B-~ -35--36~ S

and 150 grams of wa-ter. The resultant paste is shaped and dried for several hours at 100C. The dried forms are sintered at 1535C for two hours. The resultant support has a porosity of approx;mately 32% but with the pore size distr;bution shifted to larger pores than previously cited ~5-20 microns). More specifically, 20% of the pores are now in the 10 to 20 micron range.
Heavier loadinas of METHOCEL . for example up to 40% by weight, can be used to increase to!tal porosity at some sacrifice in strength.

~ 4) The porosity of the support made from the 5p i nel of the current invention may be adjusted by adding a non-sintering ~pretired) grain to the mix.
For example, 100 grams of the calcined precursor is mixed, by ball-milling, with.100 grams of nominally 5 micron alpha alumina ~a-AI2O3.). The resultant physical mix is sintered at 1700C for 2 hours to produce a hard composite support with approximately 40% porosity with the pores being largely in the 2 to 10 micron range.
Similarly, non-sintering grain can be added in the form of hard burned spinel by presintering the spinel of the present invention and grinding to the desired size if a totally spinel system is desired. The addition of conventional binders ~e.g., sodium silicate) may be necessary in these cases to achieve satisfactory pellet strength.

-- C-24,807-B-F -36-. .,

Claims (28)

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

1. A copreciate comprised of a substantially layered crystallite having the structure wherein M1 represents one or more metal cations having valence(s) a, M11 represents one or more metal cations, at least one of which is different from Ml, having valence(s) c different from a;
X and Y each represent one or more anions having valences b and d, in charge balance with a and c, respectively, and X and Y
are convertible to the oxide on heating;
the molecular ratio of M1X to M11Y being (1+z)M1X"2M11Y
where z equals or is greater than zero but less than 3; and sufficient segregated phases of the formula M1"O"Y and/or M Y to provide an overall stoichiometry of M1X"2M11Y.

2. A coprecipitate according to Claim 1 comprised of a substantially layered crystallite having the structure wherein Ml represents a cation of a metal or mixture of cations of metals having a valence of 1, 2, 4 or 6; M11 represents a cation of a metal or mixture of cations of metals at least one of which is different from Ml having a valence of 1, 2, or 3; X and Y are anions having a valence of 1 or 2 selected from the group consist-ing of hydroxyl, halogen, sulfate, formate, hydrogen phosphate, acetate, nitrate, carbonate, bicarbonate or mixtures thereof comprised as haloacetate, hydroxycarbonate, chlorohydroxide; z equals or is greater than zero but less than 3, and sufficient segregated phase of M¦¦"O"Y and/or M¦¦Y to produce a stoichiometry of M¦X"2M¦¦Y.

3. A coprecipitate comprised of a generally laminar structure wherein Ml represents one or more metal cations having valence(s) a, M¦¦ represents one or more of the cations at least one of which is different from Ml having valence(s) c different from a;
X and Y each represent one or more anions having valences b and d, in charge balance with a and c, respectively, and X and Y are convertible to the oxide on heating;
the molecular ratio of M¦X to M¦¦Y being (1+z)M¦X"2M¦¦Y where z equals or is greater than zero but less than 3; and sufficient segregated phases of the formula M¦¦"O"Y and/or M¦¦Y to provide an overall stoichro-metric of M¦X"2M¦¦Y.

4. The coprecipitate of Claim X wherein the metal cations M¦ and M¦¦ are selected from the group consisting of Li, Mg, Fe++' Mn++, Co++, Ni, Ca, Zn, A¦, Fe+++, Cr, Co+++, Ga, Ti, Mn++++, Sn, Mo and W and mixtures thereof.

5. The coprecipitate of Claim 1 wherein the anions X and Y are selected from the group consisting of OH-, I-, Br-, F-, C¦, SO?, C¦(OH)=, H2PO?, HPO?, HCO?, (OHOCO3)-3, O-C(O)CH? its halogenated derivatives, NO?, CO?, HCO3, and mixtures thereof.

C-24,807-B-F - 39 -

6. A magnesium aluminum coprecipitate comprised of a layered crystallite having the structure = (1+Z) Mg(OH)2"2A¦(OH)3 wherein z = equals or is greater than zero but less than 3, and at least one segregated phase of the formula A¦O(OH) and/or A¦(OH)3 wherein the overall stoichiometry of the precipitate is MgA¦2(OH)8.

7. The magnesium"aluminum coprecipitate of Claim 4 wherein z is about 1 and sufficient A¦O(OH)/A¦/OH) is present such that the A¦/Mg ratio in the overall coprecipitate is about 2 to 1, respectively.

8. The coprecipitate of Claim 2 wherein M is a mixture of the metal cations, Li and Mg, in an atomic ratio of about 2 to 9, respectively, M¦¦ is aluminum, X
and Y are each substantially OH , and (Li + Mg)/A¦ is 1 to about 2, respectively.

9. The product of Claim X wherein substantially all X and Y's have been converted to OH's by caustic treatment.

10. The coprecipitate of Claim 2 wherein Ml is a mixture of the metal cations Fe++ and Mg in an atomic ratio of from about 1 to 9 to about 9 to 1, respec-tively, M¦¦ is aluminum, X and Y are substantially each OH, and the Fe-Mg to A¦ ratio is about 1 to 2.

C-24,807-B-F -40-

11. The coprecipitate of Claim 2 wherein M¦ is Mg, M¦¦ is a mixture of the metal cations Cr+++ and Al in a ratio of between 1 to 9 to 9 to l respectively, X and Y are OH and the My to Al-Cr ratio is about l to 2.

12. The coprecipitate of Claim 2 wherein M is a mixture of the metal cations Co++ and Mg in an atomic ratio of from about 1 to 9 to about 9 to l, respectively, M¦ is aluminum, X and Y are each OH, and the Co-Mg to Al ratio is about l to 2.

13. The coprecipitate of Claim 2 wherein M is My, M is a mixture of the metal cations Co+++ and Al in a ratio of between 1 to 9 to 9 to l respectively, X and Y are OH and the Mg to Al-Co ratio is about 1 to 2.

14. The coprecipitate of Claim 2 wherein M is a mixture of the metal cations Fe++ and Mg in an atomic ratio of from about l to 9 to about 9 to l, respectively, M¦¦ is a mixture of the metal cations Fe and aluminum in the atomic ratio of between l to 9 and 9 to l, respectively, X and Y are OH and the Mg-Fe + to Al-Fe ratio is about l to 2.

15. A sinterable spinel comprised of the coprecipitate of Claim 1 which has been heated to between about 400° and about 1400°C.

16. A sinterable spinel comprised of the coprecipitate of Claim l which has been heated to between about 300° and about 1200°C following washing with or without alkaline, recovery of the precipitate and drying prior to heating.

17. A spinel having a density equal to or greater than 90%
of the theoretical density of the spinel comprised of the product of Claim 15, sintered to about 1500°C or greater.

18. A spinel having a density equal to or greater than 90%
of the theoretical density of the spinel comprised of the product of Claim 16, sintered to about 1500°C or greater.

19. A method of preparing a spinel which comprises mixing a MIX compound and a M¦¦Y compound wherein Ml represents a cation of a metal or mixture of cations of metals having a valence of 1, 2, 4 or 6; M¦¦ represents a cation of a metal or mixture of cations of metals at least one of which is different from M1 having a valence of 1, 2, or 3; X and Y are anions having a valence of 1 or 2 selected from the group consisting of hydroxyl, halogen, sulfate, formate, hydrogen phosphate, acetate, nitrate, carbonate, bicarbonate or mixtures thereof comprised as haloacetate, hydroxy-carbonate, chlorohydroxide together with water to form an aqueous slurry maintained at a pH of from 8 to about 10, washing the mother liquor and precipitate with water or an alkaline solution, separat-ing the solids from the mother liquor-wash liquor, washing the solids with water, drying and calcining from 400° to 1400°C.

20. The method of Claim 19 further comprising sintering at a temperature above about 1400°C.

21. A coprecipitate comprised of a substantially layered crystallite having the structure = (1+z)M¦X"2M¦¦Y

wherein Ml represents a cation of a metal or mixture of cations of metals having a valence of 1, 2, 4 or 6; M¦¦
represents a cation of a metal or mixture of cations of metals having a valence o-f 1, 2, or 3; X and Y are anions selected from the group consisting of hydroxyl, halogen, sulfate, hydroxycarbonate, chlorohydroxide, dihydrogen phosphate, hydrogen phosphate, nitrate, carbonate, carboxylates and their halogenated deriva-tives, bicarbonate or mixtures thereof; z equals or is greater than zero but less than 3, and sufficient segregated phase of M¦¦"O"Y and/or M¦¦Y to produce an overall stoichiometry of M¦¦Y and M¦X in the ratio of 1.8 to 1 to 2:2 to 1, prepared by coprecipitating from solution the dissolved M¦X and M¦¦Y at the lowest pH of the solution at which precipitation will occur for the desired components respectively at between about 20°C
and 100°C.

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22. The spinel of Claim 21 prepared by washing tile precipitate with a caustic solution, drying the so washed precipitate and thereafter heating the so washed and dried precipitate to between 400°C and about 1400°C
thereby to convert the precipitate from M¦X"2M¦¦Y to a dry powder having the formula M¦M¦¦204 + (0-20 mole %) segregated phase(s).

23. The densified spinel of Claim 22 prepared by heating the spinel of Claim 15 to above about 1500°C
for a time sufficient to increase the density thereof to from about 90 to 99% of the theoretical density for such spinel.

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24. A method for preparing a coprecipitate comprised of a substantially layered crystalite having the structure = (l+Z)M¦X"2M¦¦Y

wherein Ml represents a cation of a metal or mixture of cations of metals having a valence of 1, 2, 4 or 6; M
represents a cation of a metal or mixture of cations of metals having a valence o-F 1, 2 or 3; X and Y are anions selected from the group consisting of hydroxyl, halogen, sulfate, chlorohydroxide, hydrogen and dihydrogen phosphate, nitrate, carboxylates and their halogenated derivatives, carbonate, bicarbonate or mixtures thereof; z equals or is greater than zero but less than 3, and sufficient segregated phase of M¦¦"0"Y
and/or M¦¦Y to produce an overall stoicheiometry of M¦¦Y
and M¦X in the ratio of 1.8 to 1 to 2:2 to 1, which comprises coprecipitating from solution the dissolved MIX and M¦¦Y at the lowest pH of the solution at which precipitation will occur for all components respec-tively at between about 20°C and 100°C, treating, by washing the mother liquor, the precipitate with water or an alkaline solution, separating the precipitate from its mother liquor, washing the so treated precipitate, and drying the precipitate.

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25. A method for preparing a dry spinel powder capable of densification under heat which comprises calcining the product of Claim 24 at a temperature of from about 300°C to about 1200°C thereby to produce a spinel of the structure M¦M2¦04 o-f a density less than theoretical.

26. A method for preparing a dense spinel which comprises heating the product of Claim 25 to about 1500°C with or without first pressing it.

27. A method for preparing ceramic shapes which comprises compressing the product of Claim 25 under about 5000 to 50,000 psig to the desired shape and heating the so shaped article at above about 1500°C for -from 5 minutes to 100 hours.

28. A method for preparing a catalyst carrier which comprises shaping the product of Claim 25 or Claim 26 with or without binders which may be etchable, combustable, meltable, or vaporizable, said shaping being done under pressure, and firing said shaped article at above about 1500°C, and washing, etching and/or vaporizing said additive if any remains therein, to produce a highly porous shaped article of said spinel, said particles being highly densified and joined to each other.

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