MAGNESIUM ALUMINUM SPINELS

SPINELLES DE MAGNESIUM ET D'ALUMINIUM

English 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.

Claims

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
<IMG>
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
<IMG>
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
-37-

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.
-38-

-39-
3. A coprecipitate comprised of a generally
laminar structure
<IMG>
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 -

-40-
6. A magnesium aluminum coprecipitate comprised
of a layered crystallite having the structure
<IMG> = (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.
-41-

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.
-42-

-43-
21. A coprecipitate comprised of a substantially
layered crystallite having the structure
<IMG> = (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.
C-24,807-B-F -43-

-44-
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.
C 24,807-g-F -44-

-45-
24. A method for preparing a coprecipitate
comprised of a substantially layered crystalite having
the structure
<IMG> = (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.
C-24,807-B-F -45-

-46-
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.
C-24,807-B-F -46-

Description

3L1'7~5
--1--
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-

-12-
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-

-14-
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
EXAMPLE 9
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-
,
. .

-32-
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-
. .,