Fused alumina-zirconia abrasive material formed by an immersion process

Process for the manufacture of abrasive material in which (1) an abrasive mix is brought to fusion as in an electric arc furnace, (2) a relatively cold substrate is dipped into the molten material whereby a layer of solid abrasive material is quickly frozen (or plated) on the substrate, (3) the plated substrate is withdrawn from the molten material and (4) the solidified abrasive material is broken away from the substrate and collected for further processing to produce abrasive grain.

BACKGROUND OF THE INVENTION 
This invention relates to the manufacture of fused material having a very 
small ultimate crystal size by virtue of a novel method of high speed 
cooling from the molten state. It is intended to apply principally to the 
manufacture of oxidic abrasive materials such as those commonly produced 
by the fusion of chemically purified alumina, or by the fusion and partial 
reduction of bauxite which has been known for many years by the term 
"regular aluminum oxide", as well as to alumina-zirconia fused abrasive 
materials containing amounts of zirconia up to and including the eutectic 
composition, being about 42 percent zirconia by weight. The said 
alumina-zirconia materials may contain varying amounts of other 
constituents commonly associated with bauxite, such as iron oxide, 
titanium oxide, and silicon oxide. The novel process disclosed herein is, 
however, not limited to the compositions set forth above. 
The fusion and reduction of bauxite for abrasive purposes has long been 
known, following the teaching of U.S. Pat. No. 659,926 to Jacobs (Oct. 16, 
1900). The manufacture of alumina-zirconia abrasive materials is likewise 
old, as shown by U.S. Pat. Nos. 1,240,490 and 1,240,491 to Saunders et al 
(both Sept. 18, 1917) as well as by U.S. Pat. No. 3,181,939 to Marshall et 
al. (May 4, 1965). 
It has also been known since the time of U.S. Pat. No. 1,192,709 to Tone 
(July 25, 1916) that pouring molten aluminous abrasive material from the 
furnace into a mold so as to freeze the abrasive material relatively 
quickly will yield a solid abrasive material of small crystal size by 
virtue of which a durable and strong product will result, having utility 
in the production of abrasive grains especially suited for heavy grinding. 
For this reason, much attention has been paid to methods of achieving a 
fine crystal structure. 
More recently, U.S. Pat. No. 3,781,172 to Pett et al. (Dec. 25, 1973) 
describes pouring a molten abrasive composition over relatively cold lumps 
of similar aorasive material to achieve fast cooling and fine crystal 
size. U.S. Pat. No. 3,726,621 to Cichy (Apr. 10, 1973) teaches casting the 
molten abrasive composition into a plurality of steel balls to obtain a 
fast cooling rate and fine crystal size. Canadian Pat. No. 956,122 to 
Scott (Oct. 15, 1974) describes pouring the molten abrasive material into 
a vessel having a plurality of parallel spaced metal plates therein to 
give a fine crystal size. Canadian Pat. No. 924,112 to Shurie (Apr. 10, 
1973) teaches casting the molten material into a plurality of objects 
having a relatively high thermal conductivity. U.S. Pat. No. 3,646,713 to 
Marshall et al. (March 7, 1972) covers an apparatus involving a cool 
rotating casting cylinder and pressure roll to cool and densify the 
abrasive material. 
All of the methods referred to in the proceding paragraph entail pouring 
the melted abrasive material from the furnace into vessels or contrivances 
for subsequent cooling. This requires expensive and complicated furnaces 
capable of pouring the molten material therefrom, usually by tilting the 
furnace. The prior methods also involve the fabrication and maintenance of 
expensive molds or contrivances to handle the poured liquid. The action of 
molten abrasive materials at temperatures usually in excess of 
1800.degree. C. can cause damage to equipment with consequent costly 
maintenance and frequent replacement. The pouring operations also may be 
dangerous to the operating personnel. 
All of these methods also involve a time loss resulting from the 
interruption of the charging and fusing operation by the pouring 
operation. Likewise there is a time lapse between egress of the molten 
material from the furnace and ingress into the receptable. During this 
time, heat is lost from the melt. Consequently, it is necessary in any 
such pouring process, to heat the melt well above its fusion temperature 
to compensate for unavoidable heat losses in pouring and to prevent 
premature solidification. This higher temperature requires a corresponding 
expenditure of extra power in the furnace during melting. Thus, there is a 
waste of energy. 
Another shortcoming in these conventional methods is their relatively high 
demand of operating labor. In such processes there will be, in addition to 
one or more furnace operators, extra personnel to place, assemble, empty, 
clean, and service the receptable equipment. A high production rate 
requires a correspondingly higher requirement of such extra personnel in 
addition to a greater investment in the receptable equipment itself. 
SUMMARY OF THE INVENTION 
The invention which is the subject of this application addresses itself to 
solving or at least ameliorating these problems. Basically the invention 
involves heating an abrasive mixture to a molten condition, immersion (or 
partially dipping) of a cold substrate into the melt so as to cause a thin 
layer or plate of solid abrasive material to freeze with great rapidity on 
the surfaces of the substrate, withdrawal of the plated substrate from the 
melt, and breaking away of the solid abrasive material from the substrate 
as relatively thin plate-like material, and collection of this material 
for further processing into abrasive grain. 
Our invention may be understood by reference to the melting of abrasive 
material in a furnace of the type known as a "Higgins Furnace", described 
in U.S. Pat. No. 775,654 (Nov. 22, 1904). This furnace, widely used in the 
fusion of abrasive materials, is very simple in design and much lower in 
cost than a tilting arc furnace. The Higgins furnace is not capable of 
being tilted but, unlike other processes, this is not necessary in the 
present invention since no tilting, tapping, pouring, or casting of the 
molten abrasive material is involved. 
A simple plate or object of steel (or other suitable heat sink material) 
referred to hereinafter as a "heat sink" is provided with a handle or 
other means of moving and holding the plate so that it may be dipped in 
the melt. The heat sink is then dipped in the melt in the furnace and held 
for a short period of time to allow the abrasive material to solidify on 
it to the desired thickness. It is then removed from the melt together 
with the solid abrasive deposit on it in the form of a uniform plate 
conforming to the surface of the heat sink. The plate remains on the heat 
sink because of its close conformity to the surface but is not otherwise 
adherent thereto. Consequently, a blow with a hammer or the like will 
crack the plate-like deposit and cause it to fall cleanly from the heat 
sink. 
It is believed that the heat sink causes the solid deposit to form because 
it has absorbed sufficient heat to cause the temperature of an immediately 
adjacent layer of the abrasive melt to fall below the freezing point. The 
heat sink thus is heated up to some degree, and it should therefore be 
allowed to cool before it is used again. This cooling may quickly and 
easily be effected by dipping the heat sink in a coolant liquid (usually 
water) and drying whereupon it is ready to be used again in the manner 
described above. 
The heat sink material which serves as the substrate upon which the 
abrasive material is deposited is not limited to any particular geometric 
shape or form, thus rods, balls, tubes, and sheets may be employed in lieu 
of a plate. Although the preferred heat sink material is steel, the 
process may be practiced using other heat sink material. Selection of 
suitable heat sink materials is limited by their ability to withstand the 
process conditions. Clearly, the mass of the heat sink must be 
sufficiently great to permit it to absorb the heat from the melt, without 
itself melting. It is easy to provide a plate of steel, for example, of 
sufficient heat capacity to allow solid abrasive plates of the preferred 
thickness to be obtained in a short time. 
In addition, the heat sink should not be easily frangible and must be able 
to withstand the repeated sharp blows that may be used to dislodge the 
abrasive deposits. Finally, the heat sink material should be able to 
withstand wide temperature variations in the melt, between about 
17.degree. C. and 2000.degree. C. without cracking or breaking. The heat 
sink may be selected from any metal which is non-reactive with the coating 
material. 
The abrasive materials used in the process of the invention are usually in 
a comminuted form and include alumina and alumina-zirconia fused materials 
containing amounts of zirconia up to and including the eutectic 
composition, being about 42% zirconia by weight. Suitable melts may also 
be formed by the fusion of chemically purified alumina or of a product 
made by fusion and partial reduction of bauxite, commonly termed "regular 
aluminum oxide". The oxidic material may contain varying amounts of other 
constituents commonly associated with bauxite, such as the oxides of iron, 
titanium, and silicon. Those skilled in the art will recognize that the 
practice of the instant invention is not limited to the foregoing 
compositions and may be practiced with any chemical composition which is 
ordinarily employed to produce abrasive materials and which may be melted 
in a Higgins furnace. 
The thickness of the abrasive deposit on the heat sink substrate may be 
controlled by adjusting the immersion time of the substrate in the 
abrasive melt. Abrasive deposit thickness on the substrate has been found 
to be proportional to immersion time in the melt and longer immersion 
periods generally result in thicker abrasive deposits. In operation, the 
immersion time required to deposit a given abrasive thickness on a 
particular substrate will vary depending upon the composition of the melt, 
the particular substrate in use and the relative temperatures thereof. The 
melt temperature is not critical and it is sufficient for purposes of the 
invention that the Higgins furnace heat the abrasive mix until it is in 
the molten state. Further heating beyond this point is both unnecessary 
and economically wasteful. This is an important advantage of the present 
invention since the prior art tilting furnaces require that the abrasives 
be superheated (above melting temperature) in order to permit pouring. 
In most instances an abrasive deposit of between about 0.5 and 3.0 
millimeters provides satisfactory abrasive grains from alumina-zirconia 
compositions. Preferably the thickness of the abrasive deposited on the 
heat sink substrate is approximately equivalent to one dimension of the 
abrasive grain size being sought in the final product. 
The process of the invention may be carried out automatically in a conveyor 
system of the type illustrated in FIGS. 1 and 2 or alternatively by means 
of a manual dipping operation, or by other arrangements. 
The instant process presents many advantages over the prior art, including: 
(a) No tapping, pouring, or casting of the molten material is practiced. 
(b) Only a simple relatively inexpensive Higgins furnace or the like is 
required. 
(c) Heat sinks may be simple pieces of ordinary steel. No close fits or 
requirements of shape are necessary. 
(d) Unlike closely fitted parallel steel plate molds, warpage of the heat 
sinks is unimportant since there is no requirement to fit a mold or 
maintain a physical dimension. 
(e) No molds are required. 
(f) There is no dangerous handling of molten abrasive material outside the 
furnace. 
(g) There are no oxidation or other effects resulting from exposure to air 
since the abrasive material is substantially entirely solidified beneath 
the liquid surface. 
(h) Freezing is practically instantaneous, leading to extra fine crystal 
size. 
(i) Energy is conserved since the process is truly continuous, and there is 
no need to provide extra heat as in conventional pouring operations. 
(j) Labor is saved since the process may readily be automated. 
(k) A high yield of useful sizes of abrasive grains will be obtained since 
the thickness of the solidified plates may be controlled to conform with 
sizes desired. 
(l) The process may readily be conducted on a continuous basis. 
The realization of these advantages are objects of this invention. 
In addition it is an object of the present invention to provide abrasive 
material having a very small crystal size, the said material being highly 
uniform and rendered in a plate-like form of uniform controlled thickness 
so as to give a high yield of abrasive grains upon crushing and screening. 
Other objects are to avoid pouring molten abrasive from the furnace, 
provision for the use of inexpensive and simple furnaces, the avoidance of 
critical or close-fitting assemblies, the elimination of molds, the 
protection of the product from air until it is solid, the achievement of 
very fast cooling rates, the conservation of energy, the conservation of 
labor, and the achievement of a continuous process. 
To achieve these objectives we prefer to operate the process essentially as 
described below. It is, however, emphasized that the particular 
embodiments set forth are not exclusive of other arrangements and are but 
two examples and not to be construed in a limiting sense. It should also 
be understood that the hereinafter described apparatus are not the subject 
of this invention.

DESCRIPTION OF THE PREFERRED EMBODIMENT 
Referring to FIG. 1, a Higgins type furnace 1 is supplied with graphite 
electrodes 2 depending into the abrasive melt 3 therein. The abrasive 
material is kept in the molten state by passage of electric current from 
the electrodes through the said abrasive material in a manner well-known 
in the art. Passing overhead, transverse of furnace 1, and between the 
electrodes 2, is a trolley conveyor 4 including beam 5, chain 6, trolleys 
7, and clevis 8 which supports a plurality of heat sinks 9 as shown. 
Movement of the chain, trolley, and clevis assembly transports heat sinks 
9 serially down slope 10 so that the lower portions of heat sinks 9 are 
immersed in the melt. 
Heat sinks 9 remain in the melt for a time established by the linear 
velocity of the chain, whereupon heat sinks 9 are withdrawn from the melt 
by an upward slope of the conveyor 11. The time for which sinks 9 remain 
in the melt is sufficient to plate sinks 9 with the desired thickness of 
solidified abrasive material 9A on the sinks. 
Heat sinks 9 are serially transported so as to impinge upon a roller 12 
which raises the heat sink to an inclined position (Reference sink is 
shown in inclined position 13). As the heat sink rides over roller 12, it 
falls abruptly to position 14 shown in dashed lines, where it forcibly 
strikes an immovable block 15, or the like, thus knocking off the 
loosely-adherent plate-like abrasive material which falls into box 16 or 
alternately into a chute (not shown) for recovery and further processing 
into abrasive grain. 
Heat sink 9, when thus freed of abrasive material 9A, is then transported 
so as to pass over roller 17 after which it resumes its normal depending 
position. At this point the heat sink is still hot. It is then transported 
down slope 19 so that the lower portion of the heat sink is immersed in 
water 20 preferably at a temperature between about 5.degree. C. and 
95.degree. C. in the case of the preferred steel heat sink, contained in 
tank 21 to cool the heat sink as it is moved along. Cooled heat sink 9 is 
raised from the water by slope 22 and further transported to the point of 
beginning. In this diagram, the continuity of the process is indicated by 
dotted line 23. Not shown are conveyor drive, cornering mechanisms, and 
supporting structures, which may be of conventional design. 
Our process employs a Higgins furnace thus obviating the use of an 
expensive and complicated tilting furnace, since it is clear that no 
pouring of the melt is involved. As the conveyor carries the appended heat 
sinks around the circuit, they are automatically dipped into the melt so 
as to acquire a frozen layer of the abrasive material. The heat sinks then 
are withdrawn from the furnace by the forward motion of the conveyor and 
the abrasive material is broken off in plate-like form of controlled 
thickness. The process is therefore readily amenable to automation and 
labor is minimized with a consequent cost saving. Also, it is clear that 
the plate thickness can be controlled for example by adjusting the 
conveyor speed rate and hence the retention time of the heat sink in the 
melt. The final yield of abrasive grain of the desired size after crushing 
is dependent on the initial thickness of the plate. It follows that 
provision of plate of the optimum thickness permits optimization of final 
product yield. 
The heat sinks operate by absorbing heat from the adjacent melt and do not 
require the proximity of other objects or structures. Thus there is no 
requirement for close fitting relations or plurality of parts in the melt. 
This eliminates difficulties associated with such complexity. For example, 
warpage of the heat sinks will not cause difficulty since such a warped 
heat sink is just as effective as an unwarped heat sink. The deposition of 
the plate-like abrasive material conforms to the shape of the heat sink 
even if the heat sink is warped. This is particularly an advantage over 
molds in which parallel plates are arranged. In the latter case, warpage 
of the plates has the effect of closing some of the apertures, and 
increasing others. Difficulties in reassembling the mold elements would 
also be encountered. 
As described above, the heat sinks are cooled in tank 21 containing a 
liquid coolant, preferably water. The preferred coolant, water, may be 
maintained at any level to give the degree of cooling desired. We prefer 
to maintain the water temperature below the boiling point in the range 
5.degree. C. to 95.degree. C. to promote rapid temperature reduction. 
Lower coolant temperatures promote faster cooling thus enabling the 
temperature of the heat sink to be lowered after a short immersion period. 
In most instances the residual surface coolant will evaporate quickly when 
the heat sink is withdrawn from the coolant. Extreme care should be taken 
to see that the surface is dry to avoid the explosive expansion of water 
to steam upon reentry into the melt. When dry, the heat sink may re-enter 
the melt. Although water is the preferred coolant liquid, other coolants 
such as expanding condensed freon or cooled ethyleneglycol-water mixtures 
may be employed to further increase the cooling rate of the heat sink in 
the abrasive melt. As a matter of economy and conservation, the coolant 
liquid may be circulated by means of a pump or other conventional 
apparatus. Alternatively, coolant liquid may be applied by spraying onto 
the heated substrate surface. 
Another illustrative embodiment is shown in FIG. 2. Again, a Higgins 
furnace 1' or the like is used. (Primed numbers are used to refer to 
elements corresponding to those described in FIG. 1.) The abrasive 
material is fused therein by the action of electric current passing 
through melt 3' between electrodes (not shown). 
The apparatus is provided with a base 30 on wheels 31 which permit the said 
apparatus to be located in a desired position in relation to the furnace 
1'. Located on base 30 is a horizontally rotatable upper base 32 supported 
on circularly disposed wheels and track 33 so that the entire 
superincumbent structure is completely rotatable in a horizontal plane. 
A central column 34 has an attached frame 35 to which are secured the lower 
ends of a plurality of air-actuated cylinders 36. These cylinders contain 
pistons (not visible) attached to piston rods 37 extending upward to 
points of flexible attachments 38 to arms 39. Near the outward extremities 
of the arms, heat sinks 9' (as hereinbefore described) are hung by means 
of flexible connectors 41. The opposite extremities of the arms are 
attached to the central column by means of flexible fittings 42. 
In operation, arms 39 are raised by the attached air cylinders using air 
from the compressor 43 to the position shown in solid line so that the 
heat sinks 9', upon rotation of the apparatus, will clear the top of 
furnace 1'. Rotation of the apparatus is effected by motors and controls 
(not shown) so that the rotation is programmably indexed to pause for 
preset angular positions. During these pauses, one of the arms 39 and 
attached heat sinks 9' are located directly above furnace 1', and are 
lowered by the action of the air-actuated cylinders 36 to immerse the 
lower portion of the heat sink in the melt whereupon a layer of abrasive 
material is frozen thereon. Heat sinks 9' are then lifted from the melt 
and the apparatus rotated to the next radial position where the solid 
abrasive material is knocked off the heat sink either manually or by 
automated air-gun (not shown). At the next rotation, the heat sink is 
lowered into a water tank (not shown) to cool said heat sink preparatory 
to re-immersion in the furnace melt. 
The invention will be further illustrated in the following examples. 
EXAMPLE 1 
A mixture of 75 percent by weight of granular regular aluminum oxide (about 
95% Al.sub.2 O.sub.3) and 25 percent by weight of zirconia was brought to 
fusion in an electric arc Higgins furnace. A steel block 
12".times.8".times.11/2", equipped with a steel pipe handle, was dipped 
into the melt for a period of 5 seconds after which it was withdrawn, and 
the plate-like deposit knocked off with a hammer. The plate-like material 
was dense in appearance and about 1/8" in thickness. The heat sink was 
cooled in water and the procedure repeated several times with similar 
results. 
EXAMPLE II 
The preceding trial was repeated except that the steel block heat sink was 
allowed to remain in the melt for 10 seconds before withdrawal. This 
produced a plate of abrasive material about 3/16" in thickness. 
A microscopic examination of this product showed a dense structure which 
was comprised of a primary phase of thin elongated dendritic crystals 
arranged in parallel bundles, oriented generally outward from the cold 
surface, said crystals embedded in a eutectic matrix. The width of these 
primary dendrites (measured transversely to the longitudinal axis of the 
dendrite) was found to vary from about 3 to about 36 micrometers depending 
on the distance from the surface of the heat sink. The average dendritic 
width was about 15 micrometers which is exceptionally small for this 
composition. The same composition cooled by pouring from a furnace onto a 
cold table averaged about 40-50 micrometers on the width of the primary 
dentritic crystals. 
EXAMPLE III 
An abrasive feedstock consisting of granular regular aluminum oxide was 
loaded into a Higgins furnace and heated to the molten state using a 700 
kilowatt electric arc. Steel blocks measuring 12 inches by 8 inches by 
11/2 inches equipped with steel pipe handles were dipped into the melt for 
varying time periods. Each block was withdrawn at the conclusion of the 
immersion time and the abrasive material deposited on the steel dislodged 
by delivering a sharp blow to an uncoated portion of the plate with a 
hammer. The plate thickness was then measured and the following results 
obtained. 
______________________________________ 
AVERAGE 
IMMERSION TIME PLATE THICKNESS 
(IN SECONDS) (IN MILLIMETERS) 
______________________________________ 
1 second 1.67 
5 seconds 2.12 
7 seconds 2.67 
10 seconds 2.89 
15 seconds 3.13 
______________________________________ 
The results indicate that plate thickness is proportional to melt immersion 
time. 
EXAMPLE IV 
Granular baddeleyite (ZrO.sub.2) was added incrementally to the melt of 
Example III to vary the melt composition. Following each incremental 
addition of baddeleyite a steel plate similar to that employed in Example 
I was immersed in the melt for 10 seconds, withdrawn, and the abrasive 
deposit on the plate dislodged by means of a sharp hammer blow to an 
uncoated plate segment. The dislodged abrasive plate was then collected 
and measured. The results are indicated in the following table. 
______________________________________ 
AVERAGE 
SAMPLE PLATE THICKNESS ZrO.sub.2 
NO. (MILLIMETERS) (PERCENTAGE) 
______________________________________ 
1 2.69 1.0 
2 2.54 5.6 
3 2.81 6.3 
4 2.68 8.3 
5 2.97 8.2 
6 3.01 11.0 
7 3.03 12.6 
8 3.43 14.0 
9 3.40 16.5 
10 3.37 20.5 
11 3.23 25.1 
12 3.11 26.6 
13 3.23 30.0 
14 2.93 31.2 
15 2.80 33.7 
16 2.79 39.8 
______________________________________ 
The results indicate that varying abrasive compositions may be successfully 
deposited on a substrate according to the present invention. 
EXAMPLE V 
A mix of 75% by weight of granular regular aluminum oxide and 25% by weight 
of baddeleyite (ZrO.sub.2) was melted in an arc furnace. A heat sink 
comprising a round steel rod 11/8 inch in diameter was immersed for 
varying time periods in said melt. Prior to each immersion, the heat sink 
was cooled to substantially room temperature by submersing in water 
followed by air drying. After each immersion in the alumina-zirconia melt, 
the plate-like solidified deposit on the heat sink was removed by striking 
the heat sink with a hammer. It was found that the average plate thickness 
increased with increasing immersion time as shown by the following Table. 
______________________________________ 
IMMERSION TIME 
AVERAGE PLATE THICKNESS 
(Seconds) (Millimeters) 
______________________________________ 
1 1.0 
2 1.5 
3 1.8 
4 2.1 
5 2.4 
6 2.3 
7 2.5 
8 2.8 
10 3.2 
15 3.7 
______________________________________ 
These data show that the plate thickness can be controlled by proper 
selection of immersion time. 
It is our belief that means other than impact shock, e.g. sonic means, 
thermal shock or the like can be used to remove the plate-like deposit 
from the heat sink. 
While the above descriptions discuss several preferred embodiments for 
practicing our process, it is to be understood that those skilled in the 
art will devise other arrangements for practicing our process. The above 
descriptions therefore are not set forth as a limiting disclosure but only 
to illustrate preferred embodiments of our process. Accordingly, our 
invention is that which is set forth in the following claims.