U.S. patent application number 09/117227 was filed with the patent office on 2001-10-11 for moulded spherical ceramic body, production process and use.
Invention is credited to LUETTE, MARTIN, MOELTGEN, PAUL, WILHELM, PIRMIN.
Application Number | 20010029229 09/117227 |
Document ID | / |
Family ID | 7783580 |
Filed Date | 2001-10-11 |
United States Patent
Application |
20010029229 |
Kind Code |
A1 |
MOELTGEN, PAUL ; et
al. |
October 11, 2001 |
MOULDED SPHERICAL CERAMIC BODY, PRODUCTION PROCESS AND USE
Abstract
The present invention concerns a moulded microcrystalline
spherical Al.sub.2O.sub.3- sintered body, process for its
production as well as its use.
Inventors: |
MOELTGEN, PAUL; (LAUFENBURG,
DE) ; WILHELM, PIRMIN; (BAD SACKINGEN, DE) ;
LUETTE, MARTIN; (MURG, DE) |
Correspondence
Address: |
JERRY COHEN
PERKINS SMITH & COHEN
ONE BEACON STREET
BOSTON
MA
02108
|
Family ID: |
7783580 |
Appl. No.: |
09/117227 |
Filed: |
November 23, 1998 |
PCT Filed: |
January 13, 1997 |
PCT NO: |
PCT/EP97/00126 |
Current U.S.
Class: |
501/127 ;
264/653; 264/662; 423/625; 423/628 |
Current CPC
Class: |
C04B 35/1115 20130101;
C04B 2235/3232 20130101; C04B 2235/94 20130101; C04B 35/62695
20130101; C04B 35/18 20130101; C04B 35/6316 20130101; C04B
2235/3272 20130101; C04B 2235/3275 20130101; C04B 35/622 20130101;
C04B 35/117 20130101; C04B 2235/3244 20130101; C04B 35/478
20130101; C04B 2235/3251 20130101; C04B 2235/3206 20130101; C04B
2235/77 20130101; C04B 2235/96 20130101; B01J 2/16 20130101; C04B
35/6303 20130101; C04B 2235/3224 20130101; C04B 2235/3279 20130101;
C04B 2235/3427 20130101; C04B 2235/3284 20130101; C04B 2235/785
20130101; C04B 2235/3218 20130101; C04B 35/111 20130101; C04B 35/44
20130101; C04B 2235/5445 20130101; C04B 2235/3418 20130101; C04B
2235/5427 20130101; C04B 2235/3239 20130101; C04B 2235/3262
20130101; C04B 35/6263 20130101; C04B 2235/5436 20130101; C04B
2235/5481 20130101; C04B 35/443 20130101; C04B 2235/3241
20130101 |
Class at
Publication: |
501/127 ;
423/625; 423/628; 264/653; 264/662 |
International
Class: |
C04B 035/111 |
Foreign Application Data
Date |
Code |
Application Number |
Jan 25, 1996 |
DE |
196 02 525.7 |
Claims
1. A moulded microcrystalline spherical sintered body based on
.alpha.-Al.sub.2O.sub.3, characterized in that the average grain
size d.sub.50 of the primary crystals is preferably less than 3
.mu.m, the diameters of the moulded sintered bodies is between 0.01
and 10 mm, and the moulded sintered bodies have a hardness of
.gtoreq.16 Gpa (HV.sub.200) and a density of .gtoreq.95% of the
theoretical density TD.
2. A moulded spherical sintered body as defined in claim 1,
characterized in that the average grain size of the primary crystal
size d.sub.50 is .ltoreq.1 .mu.m.
3. A moulded spherical sintered body as defined in claim 1 or claim
2, characterized in that the average grain size of the primary
crystal d.sub.50 is .ltoreq.0.4 .mu.m.
4. A moulded spherical sintered body as defined in one or more of
the claims 1 to 3, characterized in that the hardness
(HV.sub.200).gtoreq.19 Gpa and the density is .gtoreq.98% of the
theoretical density TD.
5. A moulded spherical sintered body as defined in one or more of
the claims 1 to 4, characterized in that the
.alpha.-Al.sub.2O.sub.3 content .gtoreq.99%-wt.
6. A moulded spherical sintered body as defined in one or more of
the claims 1 to 4, characterized in that in addition to
.alpha.-Al.sub.2O.sub.3 they contain one or more constituents from
the group of oxides of elements such as Co, Cr, Fe, Hf. Mg, Mn, Nb,
Ni, rare earths, Si, Ti, V, Zn, and Zr, these amounting to less
than 50%-wt, preferably less than 20%-wt, and in particular less
than 10%-wt, relative to the total quantity of solids.
7. A process for manufacturing moulded microcrystalline spherical
sintered bodies as defined in one or more of the claims 1 to 6,
characterized in that a suspension containing
.alpha.-Al.sub.2O.sub.3 is subjected to fluid-bed spray granulation
and the green bodies so obtained are then sintered at temperatures
between 1200 and 1600.degree. C.
8. A process as defined in claim 7, characterized in that the
solids for manufacturing the suspension are ground down and/or
broken down to an average particle size of .ltoreq.3 .mu.m,
preferably .ltoreq.1 .mu.m, and in particular .ltoreq.0.4
.mu.m.
9. A process as defined in one or more of the claims 7 to 8,
characterized in that the suspension has a solids content of 5 to
70%-wt, preferably 15 - 50%-wt, and the suspension contains 0.5 -
5%-wt organic stabilisers, relative to the solids content, as
auxiliary dispersant.
10. A process as defined in one or more of the claims 7 to 9,
characterized in that the suspension contains 0.5 to 10%-wt of one
or more binders from the group that contains methylcellulose,
dextrin, sugars, starches, alginates, glycols, polyvinylpyrrolidon,
lignin sulphonate, gum arabic, polyvinylalcohol and
polyvinylacetate, relative to the solids content of the
suspension.
11. A process as defined in one or more of the claims 7 to 9,
characterized in that the suspension contains 0.5 to 10%-wt of one
or more binders from the group that contains water glass, silicic
sol, and boehmite sol.
12. A process as defined in one or more of the claims 7 to 11,
characterized in that prior to sintering, the spray granulate is
calcined at temperatures between 300 and 600.degree. C.
13. Use of the moulded microcrystalline spherical sintered body as
defined in one or more of the claims 1 to 12 as grinding balls,
insulating material, filler, catalyst carriers, wear resistant
additives to laminates and lacquers, and for use in ball bearings.
Description
[0001] The present invention relates to a moulded microcrystalline
spherical Al.sub.2O.sub.3 sintered body, a process for
manufacturing this, and the use thereof.
[0002] The processes used to manufacture ceramic balls can be
divided into mechanical, chemical, conventional (fusion)
metallurgy, and powder metallurgy methods.
[0003] The mechanical process are essentially restricted to
achieving the spherical shape by mechanical processing using such
methods as grinding, polishing, or smoothing. One prerequisite is
that an appropriately prepared moulded body be used, this then
being subjected to further processing to form a ball. The
mechanical production of balls frequently requires that a
conventional metallurgy, chemical, or powder metallurgy method be
used beforehand in order to obtain the appropriate moulded body
that is then subjected to further processing.
[0004] The chemical processes are particularly suitable for
obtaining materials that are as pure as possible. One process that
has recently been used more and more frequently is the so-called
sol gel process, In this sol-gel process, so-called colloidal
solutions are formed with suitable solvents, starting from metallic
salts. The solvent is usually water and contains the metal compound
in the form of nano-scale oxides or hydroxides that are present,
dissolved colloidally with the help of appropriate dispersants or
stabilisers. Gelling can be brought about by modifying the pH
value, temperature change, or aging/adding electrolytes. Spherical
gel particles are obtained by dropping the sol into a medium that
promotes formation of the gel, or exposing it to such a medium in
gaseous form. The spherical gel particles are then dried, calcined,
and sintered.
[0005] Sol-gel processes for manufacturing ceramic balls are
described, for example, in GB-A 1 032 105, DE-A 3 035 845, DE-A 2
147 472, DE-A 2 733 384, and DE-B 2 753 503. In most cases, these
processes relate to the production of combustion or fuel particles
based on thorium or uranium. EP-A 0 224 375 describes the
production of transparent spherical microballs based on zirconium
oxide, using the sol-gel method.
[0006] The sol-gel processes are techically costly, and require
relatively costly raw materials; in addition, they are not without
problems from the ecological standpoint because inorganic acids,
such as nitric acid and hydrochloric acid, are frequently used a
stabilisers for the sol; these are then liberated once again as
chlorine or nitrous gases during the calcining or sintering
processes.
[0007] Ceramic balls can be manufactured using conventional
metallurgical methods by dropping the liquid smelt into a cooling
medium, by blowing the smelt with air, or by atomizing the liquid
smelt with an air/water mixture. One elegant process is the
production of spherical ceramic particles using rotating disks, the
smelt being poured onto the rotating disks that then throw off the
still-liquid smelt in the form of droplets. The droplets harden
relatively quickly to form ceramic balls. However, it is difficult
to obtain pure and compact spherical ceramic particles using these
processes, which are particulary well-suited for extracting
metals.
[0008] Powder metallurgy processes have recently become
increasingly important for the production of spherical ceramic
particles. One of the most important processes within this group is
agglomeration. The underlying principle of agglomeration is based
on the clustering of individual powder particles as the result of
systematic movement of a powder bed. In most instances, a binder
must be added to the powder, when either a liquid or solid binder
is selected, depending on the type of powder that is being used.
From the technical standpoint, liquid binders are the most
important; in these, the water and alcohol systems dominate because
they are easier to handle. In the case of processes that use solid
binders, in most instances waxes or stearates are added as agents
that enhance adhesion.
[0009] Air humidity plays an important role in the dry processes,
which work without the addition of adhesion enhancing
additives.
[0010] In the normal course of events, containers or mixers that
can be moved systematically in different ways are uses for powder
agglomeration; several types of movement can also be combined with
each other.
[0011] GB-A 1 344 870 and GB-A 1 344 869 describe the production
processes for moulded spherical ceramic bodies, in which wax and
stearates are used as binders. JP-A 05 137 997 describes the
production of moulded spherical zirconium oxide, aluminum oxide,
and mullite bodies, using water, aqueous solutions of
carboxymethylcellulose, polyvinyl alcohol, and/or
polyethylene-glycol as binder. DE-B 1 229 055 describes the
production of argillaceous-earth balls by rolling activated
argillaceous earth in a cylindrical ball-moulding machine while
simultaneously spraying it with water.
[0012] The demand for low-priced, very pure, wear-resistant ceramic
balls that possess great mechanical strength, to be used, for
example, as grinding bodies, ball bearings, etc., cannot be
satisfied, or can be satisfied to only a limited degree, by using
the processes referred to above.
[0013] DE-A 3 507 376 describes a process and an apparatus for
manufacturing granulates with a very narrow grain-size
distribution, in which the product that is to be granulated is
sprayed into a fluid bed and there applied to appropriate nuclei.
The grain size is adjusted by the strength of the flow of
separating gas of a zigzag separator. Similar processes or
developments of the so-called fluid-bed spray granulation process
are described in DE-A 3 808 277 and DE-A 4 304 405.
[0014] The fluid-bed spray granulation process is usually used for
drying and agglomerating agrochemical substances (fungicides,
insecticides, herbicides, growth regulators, and fertilizers), pest
control agents, pharmacologically effective substances, nutrients,
sweeteners, colouring agents, and inorganic and organic chemicals.
In addition to the active components and thinners, there may also
be inert fillers, dispersants, binders, and/or other additives, for
example, preservatives and colouring agents, in the liquid product
that is to be sprayed in.
[0015] The granulate particles that are obtained by fluid-bed spray
granulation are distinguished by their uniform shape and great
solidity; these characteristics make it simpler to handle, measure
and process the original, finely powdered material, and in some
instances even make these operations possible for the first time.
Because of their microporous structure and the large surface areas
associated therewith, the granulates can be redispersed
spontaneously, which means that the process is predestined for
processing agrochemical substances, pest-control agents, and
pharmacologically effective substances.
[0016] It is the task of the present invention to provide moulded
spherical sintered bodies that do not have the disadvantages found
in the prior art. Surprisingly, it was found during drying trials
with ceramic powder suspensions based on Al.sub.2O.sub.3 that by
using fluid-bed spray granulation, it is possible to obtain
extremely dense green bodies that can be sintered directly to form
a dense ceramic body, without any additional manipulation, such as
compacting. Because of the high basic density of the granulate and
the fineness and sinter activity of the initial powder, for all
practical purposes it is possible to suppress grain growth almost
completely during the sintering process, so that a moulded
microcrystalline spherical sintered ceramic body that is
distinguished by particular toughness and wear resistance
results.
[0017] The object of the present invention are moulded
microcrystalline spherical sintered bodies that are based on
.alpha.-aluminum oxide, the average grain size d.sub.50 of the
primary crystals being preferably smaller than 3 .mu.m, the
diameter of the moulded sintered bodies being between 0.01 and 10
mm, and the moulded sintered bodies being of a hardness of>16
Gpa (HV.sub.200) and a density of>95% of the theoretical density
TD. The moulded microcrystalline spherical sintered bodies
according to the present invention, which have an average grain
size of the primary crystals d.sub.50<1 .mu.m, in particular
d.sub.50<0.4 .mu.m, exhibit particularly good properties. In
addition, the moulded sintered bodies according to the present
invention have hardnesses (HV.sub.200)>19 Gpa and densities of
>98% of the theoretical density TD. It is preferred that the
moulded sintered bodies according to the present invention have an
.alpha.-Al.sub.2O.sub.3 content >99% -wt. In addition to
.alpha.-Al.sub.2O.sub.3, they can also contain one or more
constituents of the oxides of elements such as Co, Cr, Fe, Hf, Mg,
Mn, Nb, Ni, rare earths, Si, Ti, V, Zn, and Zr, these amounting to
less than 50%-wt, preferably less than 20%-wt, and in particular
less than 10%-wt, relative to the total quantity of solids.
[0018] A further object of the present invention is a process for
manufacturing the moulded microcrystalline spherical sintered
bodies, a suspension containing .alpha.-Al.sub.2O.sub.3 being
subjected to fluid-bed spray granulation, the green bodies so
obtained then being sintered at temperatures between 1200 and
1600.degree. C.
[0019] In the process according to the present invention, finely
divided solids are used as the initial substances, and these are
ground down and/or broken up to an average particle size of<3
.mu.m, preferably <1 .mu.m, and especially <0.4 .mu.m, and
used to produce the suspension. Reduction can advantageously be
effected using a vibration mill, an attrition-type mill, or an
agitator-ball mill, or by additional wet grinding to the desired
grain size. It is preferred that the suspension contain 5 to 70%,
preferably 15 to 50% solids, the suspension also containing 0.5 to
5% organic stabilisers, relative to the solids content, as
auxiliary dispersants. It is preferred that the solvent be water.
The use of other solvents such as alcohols, ketones, or other polar
organic fluids is also possible. Very often, however, ecological
and economic factors militate against this.
[0020] The suspension can be stabilized sterically or
electrostatically. In the case of steric stabilisation, all known
auxiliary dispersants can be used. Polyacrylic acids, polyglycol
acids, polymethacrylic acids, organic bases such as triethylamine
or carboxylic acids such as acetic acid or propionic acid are
suitable for this purpose. It is preferred that the suspension
contain between 0.5 and 5%-wt of appropriate organic stabilisers.
In the case of electrostatic stabilisation, volatile inorganic
acids such as nitric acid or hyrochloric acid, as well as ammonia
as a base, can be used to advantage.
[0021] The suspension is stabilised either prior to grinding, or
after grinding, with the help of a disperser; this ensures rapid
and even distribution of the stabiliser. Sintering additives and
binders can be added to the suspension, this being done preferably
prior to, but also during and after stabilisation. All known
sintering aids for Al.sub.2O.sub.3 or its precursors can be used as
sintering additives.
[0022] It is preferred that the suspension according to the present
invention contain 0.5 to 10%-wt of one or a plurality of binders
from the group that includes methylcellulose, dextrin, sugars,
starches, alginates, glycols, polyvinylpyrrolidon, lignin
sulphonate, gum arabic, polyvinylalcohol and polyvinylacetate,
relative to the solids content of the suspension. To equal
advantage, the suspension can contain 0.5 to 10%-wt of one or more
binders from the group that contains water glass, silicic sol, and
boehmite sol.
[0023] Granulation is preferably carried out in air, and can be
initiated in a fluid-bed apparatus that already contains the
starting granulate. However, it is also possible to begin
granulation in an empty apparatus, fluid-bed granulation being
started as spray drying and nuclei being generated in situ.
[0024] The suspension that is to be granulated is introduced into
the fluid bed by way of spray nozzles. The use of binary nozzles is
particularly advantageous. Any gas that is inert under the
prevailing working conditions can be used as the atomising gas. It
is preferred that air be used for Al.sub.2O.sub.3. The quantity of
atomising gas that is used can be varied within a very wide range,
and is generally determined by the size of the apparatus and the
type and quantity of product that is to be sprayed in. The
temperature of the flow of atomising gas or the air entry
temperature can similarly be within a wide range. Generally
speaking, work is carried on at temperatures between 20 and
350.degree. C. The separating-gas temperatures can vary within a
wide range, and here, too, it is preferred that work be done in a
range between 20 and 350.degree. C. The quantity and velocity of
the separating gas is determined by the density and the desired
grain size of the granulate.
[0025] The grain size can be controlled primarily by the gas flow
and velocity of the separator gas. In the case of Al.sub.2O.sub.3,
using the zigzag separator as described in DE-A 3 507 376, it is
possible to select a narrow grain band in the grain size range
between 0.01 and 10 mm with a band width .ltoreq.21 mm.
[0026] The prepared granulate can be sintered directly--or
preferably after a calcining intermediate step at temperatures
between 300 and 600.degree. C. at temperatures between 1200 and
1600.degree. C. Rotating cylindrical kilns, sliding-bat kilns, or
chamber kilns can be used as the sintering kilns. It is
particularly advantageous if the sintering be carried out in a
rotating cylindrical kiln that is heated directly or indirectly, by
which it is possible to obtain high heating rates combined with
short dwell times, since this facilitates the production of dense
sintered bodies without excessively vigorous crystal growth.
[0027] The process according to the present invention makes it
possible to manufacture extremely dense moulded microcrystalline
sintered bodies of great purity that are extremely hard and
resistant to wear, the average primary crystal size of which is
preferably smaller than 1 .mu.m and whose diameter can be selected
to be anywhere between 0.01 and 10 mm.
[0028] Because of these properties, the sintered bodies according
to the present invention are particularly well suited for use as
grinding balls, insulation materials, fillers, for use as
wear-resistant additives for laminates and lacquers, for use in
ball bearings, as catalyst carriers, or the like.
[0029] One object of the present invention is thus the use of the
sintered bodies according to the present invention as grinding
balls, insulation materials, fillers, additives for laminates and
lacquers, for use in ball bearings, as catalyst carriers, or the
like.
[0030] The present invention will be described in greater detail
below on the basis of the following examples, which should not be
considered restrictive as to the present invention.
EXAMPLE 1
[0031] 70 kg .alpha.-Al.sub.2O.sub.3 with an average grain size of
d.sub.501.5 .mu.m, in the form of a 50-% aqueous slip stabilised
with a polyacrylic acid as an auxiliary dispersant, was ground in
an agitator-type ball mill to an average grain size of d.sub.50=0.4
.mu.m. The d.sub.90 value of the suspension was 0.9 .mu.m. The
suspension was diluted with water to a solids content of 30%-wt,
and 10 l of a 10-% aqueous solution of a polyvinylalcohol was added
as a binder (Mowiol 8/88, Hoechst AG, Germany).
[0032] Next, the suspension was processed in a fluid-bed spray
granulator (AGT 150, Glatt, Germany) at an air entry temperature of
95.degree. C., a layer temperature of 45.degree. C., a spray
pressure of 3 bar, and a spray rate of 70 g/min. A fine granulate
fraction with an average grain size of 0.2 mm, which had been
obtained previously by fluid-bed spray granulation by way of in
situ nucleus formation, was used for nucleus formation. Separation
of the desired granulate was effected by a zig-zag separator that
was operated a 9 Nm.sup.3/h air. 70%-wt of the granulate so
obtained had a diameter between 0.8 and 1.2 mm, in approximately
20%-wt the diameter was between 0.3 and 0.8 mm, and in
approximately 10%-wt the granulate had a diameter of .gtoreq.1.2
mm. The residual moisture content of the granulate was less than
1%.
[0033] The granulate was calcined at 500.degree. C. and then
sintered at 1480.degree. C. in a chamber kiln.
[0034] The moulded sintered bodies had a density of 98.3% of the TD
and a hardness of 18.7 Gpa (HV=0.2). The average primary crystal
size was 0.8 .mu.m.
EXAMPLE 2
[0035] As in Example 1, although 2%-wt polyvinylpyrrolidon,
relative to the Al.sub.2O.sub.3 content, was used as binder.
[0036] The moulded sintered bodies had a density of 96.5% of the
TD, and a hardness of 17.6 Gpa (HV 0.2). The average primary
crystal size was 0.8 .mu.m.
EXAMPLE 3
[0037] As in Example 1, the separation of the desired granulate was
effected by a separator chamber tha incorporates a series of
zig-zag separators.. The quantity of air was so adjusted that
98%-wt of the granulate that was removed had a diameter between 0.5
and 0.7 mm. The sintering was carried out directly, without
calcining as an intermediate step, in a rotating cylindrical kiln
at 1480.degree. C.
[0038] The moulded sintered bodies had a density of 98.6% of the
TD, and a hardness of 19.5 Gpa (HV 0.2). The average primary
crystal size was 0.6 .mu.m.
EXAMPLE 4 (use as a grinding body)
[0039] Commercially available .alpha.-Al.sub.2O.sub.3 with an
average grain size d.sub.50 of 1.5 .mu.m was wet ground in an
agitator-type ball mill (Type PMC 25 TEX, Drais) for 8 hours. The
slurry had a solids content of 50%-wt. The grinding was carried out
cyclically, each batch size amounting to 70 kg Al.sub.2O.sub.3. The
grinding body charge in all tested cases was 65%-vol. The grinding
body wear was determined after each grind, by weighing.
[0040] When reading the result obtained with YTZ
(yttrium-stabilized zirconium oxide) grinding bodies, it should be
remembered that the grinding balls are much more costly than the
moulded spherical sintered bodies according to the present
invention so that--given equally good results--grinding costs will
be at least ten times greater compared to the costs associated
width the use of grinding balls according to the present
invention.
1 Commer- Commer- Commercially cially cially Grinding available
available available body Al.sub.2O.sub.3 YTZ Al.sub.2O.sub.3 as in
grinding grinding grinding Example 1 body body body Chemical 95%
Al.sub.2O.sub.3 99.5% Al.sub.2O.sub.3 95% ZrO2 86% Al.sub.2O.sub.3
composition 5% Y2O3 11% [%-wt] SiO2 3% other Ball 1 mm 1 mm 1 mm 1
mm diameter Wear on 3%-wt 20%-wt 5%-wt 7%-wt grinding body Product
0.95 .mu.m 1.18 .mu.m 0.95 .mu.m 1.22 .mu.m fineness .sub.d90
Product 0.46 .mu.m 0.60 .mu.m 0.42 .mu.m 0.63 .mu.m fineness
.sub.d50
* * * * *