EP0293834A2

EP0293834A2 - Tool for compacting powders particularly refractory cement

Info

Publication number: EP0293834A2
Application number: EP88108696A
Authority: EP
Inventors: Robert A. Buzzell; Edmund A. Cortellini
Original assignee: Norton Co
Current assignee: Saint Gobain Abrasives Inc
Priority date: 1987-06-05
Filing date: 1988-05-31
Publication date: 1988-12-07
Also published as: KR890000867A; BR8802718A; EP0293834A3; JPS63312803A

Abstract

Compacting of fine, air-absorbing powders such as refractory cements is improved by using a new type of vibratory tool having multiple solid projections (22) extending from a cap section (21) of the tool (10). By this means, need for a separate deairing tool and a separate, time consuming deairing step in the process is avoided, and the amount of dust introduced into the air is reduced.

Description

This invention relates to the field of tools and processes for compacting powders, particularly powdered refractory cements used for constructing heat resistant walls and other structures. Refractory cements comprise powdered refractory materials such as mullite, kyanite, silicon nitride, silicon oxynitride, silicon carbide, fumed silica, alumina, zirconia, magnesia, chromia, etc. In some kinds of cements, the refractory materials are capable of adequate self-bonding by sintering during the heating used to consolidate the cement. In other types, special bonding materials, such as boric acid, calcium carbonate, silicon powder, iron oxide, etc., may be used in addition to the more refractory components. An example of a practical refractory cement composition is given in U. S. Patent 4,476,234. The cements are compacted into a space of the desired shape, defined by a suitable open top mold or other container, and then consolidated into a coherent object by heating the compacted cement shape.
In the prior art, two different types of tools have customarily been required for the compacting of refractory cements: a tool to deair the loose cement and a vibrator to shake the cement particles into a tight packing. The deairing tool commonly has tapered chisel points or forks about 3 cm wide and 13 cm long, extending from a base about 11 cm long and 6.5 cm wide, with the projections on centers about 4 cm apart. The size of the deairing tool varies somewhat with the size of the structure to be made. The vibrator most commonly used has a solid flat square contactor plate about 15 cm on a side and 4 mm thick. The plate is attached to drive means which cause it when actuated to vibrate with an amplitude of about 3 mm at a rate of about 60 vibrations per second . Less often, a vibrator with a perforated plate rather than a solid plate is used. The most often used type of perforated plate has circular perforations about 6 mm in diameter on centers about 25 mm apart. The same drive means are used to actuate a perforated plate into vibrations of the same amplitude and frequency as with a solid plate. A vibrator with a perforated plate is relatively little used, because it normally generates so much dust in the air over the container in which it is used as to cause significant discomfort for the worker(s) using it.
In the prior art, a layer of cement, usually 10-15 cm thick, is placed in a container in which a refractory structure is to be formed. The layer of powder is then deaired by moving the forked deairing tool up and down through all parts of the bed for a total of about two minutes. Generally, the area of a bed covered by one operator is about four to ten times the area of the base of the forking tool used, and the projecting chisel points or forks on the tool are deep enough to penetrate the entire depth of the first powder layer.
After deairing of the layer is complete, the contactor plate of a vibrator as already described is applied to the a portion of the top of the deaired powder bed and set into vibratory motion. An operator moves the actuated contactor plate around over the entire area of the powder layer until a sufficient and reasonably uniform degree of compaction has been achieved throughout the bed; this usually requires about one minute of total contact time, with a single operator again, as with the deairing tool, covering an area about four to ten times the area of the contactor plate of the vibrator.
If a stucture thicker than the first layer of powder is to be made, as it usually is, the surface of the vibrated packed powder bed is then scored with a rake or similar tool to prevent the formation of laminations. If this scoring operation is omitted, a weakness in the structure to be made will normally occur along the zone of the transition between successive layers. This undesirable effect, called "lamination" or some related term in the art, is believed to occur because a thin layer of powder enriched in the coarser particles within the cement tends to accumulate at the top of each layer during the vibration step. If this is not loosened by scoring, so that it can mix readily with some of the finer particle size constituents from the next layer of powder added, a thin highly porous layer is formed in the final structure. Such a high porosity layer is more susceptible to mechanical damage or to penetration by any liquid, such as molten metal, which the eventual refractory structure is to be used to contain.
After the first powder layer has been deaired, vibrated, and scored, more loose powder to form a second layer is added on top of the scored first packed powder layer, and the previous steps of deairing, vibrating, and scoring if needed are repeated until a sufficient thickness of the deaired and compacted powder has been achieved. The compacting process is then complete. It may be a very lengthy process, as the method is often used to build furnace linings or containers that are several square meters or more in base size and have walls more than ten centimeters thick and two meters high or more. Such furnace liners are often used in the heavy metal industries to hold liquid metals for casting or similar purposes.
When the shape and size of the entire desired structure has been built up of properly compacted powder, the powder is converted into the structure by heating according to methods well known in the art. If the refractory structure is built up within removable metal mold walls, as it often is, a blowtorch may be applied around the area of the molds to consolidate the structure; more often the heating equipment of the furnace itself is used for the heating, if the structure is a furnace liner.

SUMMARY OF THE INVENTION

The prior art compacting process can be greatly simplified and speeded by utilizing a vibratory tool having a contactor with solid projections from a solid cap, rather than a flat solid or perforated plate as in the prior art. The cap of the contactor according to this invention will often be referred to hereinafter as a cap plate, because the cap is usually most conveniently constructed in the form of a plate. However, any shape which is solid enough to prevent the motion through it of most of the powder to be compacted, and which, in combination with the projections from it, defines a space in which particles of the powder to be compacted can be set into rapid motion without being able to escape through the top of the space, could serve as an adequate cap. Thus a small section of a large spherical shell, a shallow pyramid, and a wide variety of irregular surfaces could all serve as adequate cap zones. A vibratory tool of this type allows elimination of separate deairing and scoring steps, permits continuous addition of powder to form beds of any thickness, eliminates the need for a separate deairing tool, and produces compacted beds substantially as dense as those obtained by the prior art in significantly less time.
The cap plate of the vibrator contactor according to this invention should be thick enough to provide mechanical ruggedness but not so thick as to require excessive power to drive. Generally steel plates 2-10 mm thick are preferred. The shape of the projections on the contactor does not appear to be particularly important, provided that (1) the projections produce a plurality of vibrating terminal solid surfaces that are small in comparison with the cap plate of the contactor, e.g., usually less than 6 square centimeters, and preferably are 6 to 77 mm distant from the cap plate; (2) the supporting part of the projection is sturdy enough to resist easy breakage when vibrated and to transmit the vibratory motion of the cap with little or no change to the terminal surfaces; and (3) the terminal surfaces on the projections are separated from each other, in a direction parallel to the cap plate, by an average distance that is at least half as great as the average width of the widest part of the projections. A vibrator contactor of this type is applied to the surface of a powder bed with sufficient pressure that both the cap plate and the projections from it are in intimate contact with part of the powder, with a small amplitude vibratory motion imparted to the contactor. After a few seconds of contact with one particular part of the area of the top of the powder layer, the contactor is moved to a different part of the area, until eventually all the area is covered. Additional powder can be added to the part of the bed not covered by the contactor at any given time, in a continuous or interrupted stream, and by properly controlled powder additions and application of the contactor of the vibratory tool according to this invention, an adequately compacted deep bed of powder may be built up to any desired depth within any given area without any interruptions for separate deairing and scoring steps.
While the applicants do not wish to be bound by any particular theory of operation, they believe that the beneficial results of their invention are connected in some way with maintenance of active motion of the powder particles over a depth corresponding to the distance of the most remote projections from the cap of the vibrator contactor. The ends of the projections remote from the cap plate are believed to set the powder particles that they contact most intimately into violent motion, so that powder particles form a loosely packed cloud in the space below the cap plate but between the projections. Interparticle collisions within this zone are believed to help loosen absorbed air from the particles, and the air can flow away through the spaces between the projections and around the edge of the cap plate more easily than in the prior art.
The larger particles in the mixture, which would otherwise tend to accumulate preferentially in the upper part of the vibrating zone, are believed to be returned to the relatively deep zone of vibrating particles between the projections by contact with the cap plate. Whatever the actual mechanism, the formation of layers of powder excessively depleted in the minimum proportion of fine particles needed to form a strong and leak free final structure is avoided by the method of this invention, without needing any separate scoring step.

BRIEF DESCRIPTION OF THE DRAWINGS

Figure 1 is a view of a preferred type of vibratory tool according to this invention. Figure 2 is a side view and Figure 3 is a bottom view of a contactor suitable for use with this vibratory tool. Figure 4 is a side view and Figure 5 a bottom view of another suitable type of vibratory tool.

DESCRIPTION OF THE PREFERRED EMBODIMENTS

For convenience, the projections on the contactor of this invention are usually substantially uniform in size and substantially evenly spaced over the entire surface of the plate opposite the side of attachment to the vibratory drive means. While it is believed that projections of almost any reasonably elongated shape would be satisfactory, it usually most convenient to use projections with simple and easily obtained shapes such as cylinders, cones, pyramids, chisel points (usually flattened somewhat from the shape used in an actual chisel), etc. and to use projections that have their maximum width at the point of contact with the cap plate. The shape of the contactor according to this invention, combining a cap plate with projections, could be obtained by any appropriate means such as casting, but it is generally less expensive and therefore preferable to attach separate projections to a smooth surfaced cap by welding, threaded connectors, or any other suitable method.
The area of the cap plate, the area of powder compacted with a contactor of a given cap plate area, and the amplitude and frequency of vibration of the entire contactor are preferably about the same as in the prior art, with the cap area corresponding to the area of the flat plate contactor used in the prior art. All the steps subsequent to contacting that are required to convert the compacted powder into a useful structure may be performed in the same manner as in the prior art, and the types of powders to which the invention may be applied includes all the range of those used in the prior art.
The projections of the contactor according to this invention are preferably made of metal, with steel usually preferred for economy. When the projections are similar in size and shape, and the tool is to be used with conventional refractory cements, the projections most preferably should be 5-7 mm in maximum width, on centers spaced 20-28 mm apart on a square lattice, and 12-24 mm in length.
The practice of the invention can be further appreciated from the following examples.

Example 1

A conventional refractory cement composition, sold by Norton Company, Worcester, Massachusetts under the trade designation VA-150, was used for this example. This is a fused alumina based dry vibration cement recommended for both acid and basic slag contact. It utilizes a chrome alumina solid solution bond mechanism. A typical chemical analysis of VA-150 cement is 88.0% alumina, 8.0 % chromia, 2.0 % titania, 1.0 % silica, 0.6 % phosphorous pentoxide, and 0.4 % other constituents. Typical physical properties of this cement include a grain size of 6F, a dry rammed density of 2.9 megagrams per cubic meter (Mg/m³), an initial bonding temperature of 870 C, a maturing temperature of 1370 C, and a maximum use temperature of 1815 C.
VA-150 cement was used in this example to fill a square mold having walls about 20 cm high and a base about 30 cm on a side. By the conventional prior art techniques, powder layers about 10 cm high were normally used, so that two such layers were required to fill the mold. For each layer of powder, the deairing step took about two minutes and the vibrating step about one minute. The vibrator used was obtained from Robert Bosch Corp., 50 Bayless Road, Mellville, NY 11746 and is designated by them as Type #06188900008. It produces circular vibrations caused by eccentric weights mounted on the shaft of a three phase squirrel cage motor rotor. The eccentricity was adjusted to give about 235 kilopascals of centrifugal force. A flat square steel contactor plate about 15 cm on each side and about 3 mm thick was used, along with the forking tool, scoring rake, and technique already described above in the Technical Background section. The density obtained for the packed bed was 2.9 Mg/m³.
A vibrating tool according to this invention is shown in Figure 1. It has a drive means 11 as described above, with a shaft 12 and end plate 14. Bolts 13 are used to attach a contactor means 20. A magnified view of the contactor 20 is given in Figures 2 and 3. The cap 21, which is a flat square steel contactor plate 15 cm on each side and about 3 mm thick, has bolt holes 24 for attachment to a vibratory drive. The contactor has cylindrical projections 22, which terminate in flat terminal contact surfaces 23, welded to the cap. When such a tool, with steel projections 6 mm in diameter and 19 mm long on centers 25 mm apart was used, powder could be added continuously to the container while the vibrator was being held against a different part of the accumulating powder layer, and the total time required for the full compacting process to fill the entire mold was only two minutes. The density achieved was equivalent within the precision of measurement to that achieved by the prior art method. It was noticed that considerably less density of dust in the air above the container was generated when using the vibrator according to this invention rather than the vibrator with the prior art plain plate.
An alternative type of contactor, which gave equally good results, is depicted in Figures 4 and 5. This has a cap 41 which is the same shape and material as cap 21, and it has chisel point projections 42 terminating in contactor terminal surfaces 43. The bases of the projections are about 25 by 20 mm and the terminal surfaces 42 are about 2 mm wide and are about 40 mm distant from the cap 41.

Examples 2 and 3

These were the same as Example 1, except that Norton VA-157 and VA-112 cements were used for Examples 2 and 3 respectively. VA-157 is a fused alumina based type cement recommended for acid to neutral applica tions. It utilizes a mullite bond mechanism. A typical chemical analysis for this cement would be 93.5% alumina, 3.9 % silica, 1.9 % titania, 0.2 % ferric oxide, and 0.5 % other constituents. Typical physical properties of the cements include a grain size of 6F, a dry rammed density of 3.0 Mg/m³, an initial bonding temperature of 1200 C, a maturing temperature of 1230 C, and a maximum use temperature of 1820 C.
VA-112 is a fused alumina cement especially designed for low temperature bonding. A typical chemical analysis of this cement would be 91.5% alumina, 4.4 % silica, 1.8 % titania, 0.2 % ferric oxide, and 2.1 % other constituents. Typical physical properties of this cement include a grain size of 6F, a dry rammed density of 2.9 Mg/m³, an initial bonding temperature of 550 C, a maturing temperature of 840 C, and a maximum use temperature of 1705 C.
With both VA-157 and VA-112 type cements, the densities of deaired packed powder obtained were the same within the precision of measurement whether the process according to this invention or the prior art process was used, but considerably less time was required when using the process according to this invention.

Claims

1. A tool for compacting a powder, said tool having a vibratorily drivable contactor for contacting the powder and compacting the same with the vibratory motion thereof, characterized in that said contactor (20, 40) comprises a cap (21, 41) having a substantially smooth surface for contacting the powder on the side opposite its attachment to said tool and a plurality of projections (22, 42) projecting from said surface of said cap.

2. A tool according to claim 1, characterized in that said surface of said cap (21, 41) has an area of at least 25 square centimeters and said projections (22, 42) project from said surface of said cap (21, 41) to respective terminal surfaces (23, 43) of said projections (22, 42) a distance of from about 6 to about 77 mm, an average distance of separation of each of said terminal surfaces (23, 43) from its nearest neighboring terminal surfaces (23, 43) being at least equal to one-half the average maximum width of the projections (22, 42).

3. A tool according to claim 2, characterized in that said vibratory motion has an amplitude of vibration between 1 and 6 mm at a frequency of between 30 and 90 vibrations per second.

4. A tool according to claim 3, characterized in that said cap (21, 41) has an area of not more than 650 square centimeters and said projections (22, 42) have average maximum cross-sectional areas of between about 0.1 and about 15 square centimeters.

5. A tool according to claim 4, characterized in that said cap (21, 41) is a flat plate and has a thickness between 2 and 10 mm, said projections (22, 42) are substantially equal in size and shape, said maximum cross sectional area of said projections (22, 42) is between 0.2 and 6 square centimeters, and the distance of said terminal surfaces from said cap (21, 41) is between 12 and 24 mm.

6. A process for compacting a fine powder in a container with an open top, characterized by contacting a part of the upper surface of a layer of such powder within said container with the tool according to any one of the preceding claims.

7. A process according to claim 6, wherein said powder is a refractory cement.