US1996851A - Method of maturing ceramic articles and apparatus therefor - Google Patents

Description

April 9, 193 R. c. BENNER m- AL METHOD OF MATURING CERAMIC ARTICLES AND APPARATUS THEREFOR Filed Sept. 5, 1952 IIIIIIIJ min-11mm lulu.

I/flflfllllll/fflllll IMQ A LBEZT L. BALL BY H RUSSELL Ho cl-uHS A TTORNEY.

Patented Apr. 9, 1935 UNITED STATES PATENT OFFICE METHOD OF MATURING CEBAIMIC ARTI- CLES AND APPARATUS THEREFOR of Pennsylvania Application September 3, 1932, Serial No. 63 l,644

9 Claims. 01. 51278) This invention relates to the art of ceramics, and particularly to that branch of the artwhich concerns vitrified abrasives. It has to do with exceptionally large ceramic monoliths of dimensions which have never previously been made.

v To illustrate the objects of our invention, we will consider large abrasive r heels such as are used in grinding wood to produce pulp. The use of abrasives for this purpose constitutes the application of the largest abrasive bodies with which we are familiar. The smallest size of stone used commercially for this purpose is about 54 inches in diameter, 21 inches wide and weighs approximately three tons, while the largest stones are about 61 inches in diameter, 54 inches wide and weigh from seven to eight tons.

Until recently, sandstone has constituted the only source of abrasive material for such large bodies. Due to the lack of uniformity of sandstone deposits, some of the larger sized sandstone wheels have been made by cementing together two or more abrasive rings to form a large wheel in order to obtain more uniformity.

Within the past few years, artificial abrasive manufacturers have introduced on the market wheels of suitable dimensions mounted upon iron drum centers, the abrasive being in the form of segments. Other developments along this line have included the type of wheel composed of segments but doing away with the use of a drum, thereby making the wheel more applicable to general use.

There are various objections both to the sandstone and artificial abrasive wheels as at present placed on the market; they are not of uniform composition, and duplicates, therefore, cannot be furnished with certainty; they are rather weak and hence wear down rapidly, requiring frequent dressing to keep them sharp.

The segmental artificial wheel represents a step in advance over the sandstone, due to the possibility of the uniform production of aseries of wheels, one being identical with another. They do not wear out as rapidly, and require much less attention in use. Nevertheless, there are objections to these se mental wheels. For example, segments on rare occasions loosen and fiy out or possibly a portion of abrasive will crack away, leaving a gap in the surface which requires patching. There are also times when the joint cement 1 between segments causes trouble by either wearing away less slowly than the abrasive and pro-' truding above the periphery of the wheel or by wearing away faster than the stone, thus leaving in effect a valley below the surface and causing undue amounts of coarse waste material in the pulp.

Viewed in the light of the above, a ceramic monolith possesses many advantages since such 5 a wheel represents practically the ultimate in uniformity, structural strength, long wear, minimum amount of attention for sharpening, and eliminates pulp waste due to joint trouble. The difficulties involved in making such wheels in extremely large sizes have, however, prevented their use. The largest commercial ceramic wheel which can be made successfully by the usual commercial procedures is a kiln-fired body approximately 48 inches in diameter and 6 inches thick. It will be realized that these dimensions fall far shortof those required for the smallest of commercial pulp grinding wheels.

In the manufacture of large abrasive articles, the size heretofore has been limited by difliculties in drying and handling the unburned article, and in burning out carbonaceous matter which may be contained within the mix. Drying itself presents a dimcult problem, owing to the great weight of the mix and the tendency of the material to shrink or crack during the drying process. Even when drying can be carried out successfully, the unburned wheel is so fragile that with a wheel of massive proportions the material crumbles with the handling required to position the wheel in the kiln for burning. In the fabrication of the usual abrasive wheel a temporarybinder is used, and as such binders are usually carbonaceous, the burning of the resulting carbon from the body of the wheel presents a difllcult problem, especially when the thickness is increased beyond a very few inches. When a clay binder is used, the clay itself may contain a certain amount of carbonaceous matter, and in a thick article this carbonaceous material is not removed by the usual burning procedures.

In carrying out our invention, we have found it possible to overcome these difilculties by forming the article and thereafter effecting either a drying or complete'curing before "the article is removed from the mold. In carrying out this procedure, we have found it possible to consolidate the article without the use of a temporary binder, especially if this consolidation is eifected by a jolting operation. For effective drying, the article may be subjected to reduced pressure, but we preferably employ a forced circulation of air or other gas throughout the pores of the article, as for example, by alternately reducing and increasing the pressure of the atmosphere surrounding \Vthickness. Even in the absence of a temporary binder, this forced circulation of gas through the pores of the article during the final firing operation is of advantage, for the penertation of heated gases into the body of the wheel facilitates uniform heating throughout the mass. If a bond having a maturing temperature of about 1000 degrees centigrade is used, the article can be fired directly within a metal mold in which it is consolidated. The combination of these steps or certain portions of the process as outlined makes possible the production of monolithic abrasive articles of much greater size than has been possible with former methods of manufacture.

The various steps which we employ in manufacturing large ceramic monoliths may be outlined as follows, all or part of which may be used as occasion demands.

1. The ceramic mass, after having been thoroughly mixed, is compacted in a mold by pressing, jolting,.tamping or any combination of these.

2. The drying operation, which is .necessary when a wet mix or plastic temporary binder is used, can be effected by placing the filled mold in the furnace at a low temperature and subjecting the kiln chamber to alternate vacuum and normal pressure.

3. The mass can be matured while being retained in the mold, and if any organic material is present the mass can be subjected to alternate changes in pressureuntil the carbon resulting therefrom has been burned out. This process can also be used in the absence of organic matter to effect more uniform heating.

A general type of apparatus which may be used in carrying out our invention is illustrated in the accompanying drawing in which:

Figure 1 is a vertical section of a mold in which the ceramic mass is compacted and matured;

In Figure 1, mold barrel 1 and inner shell 2v are made of heat resistant metal such as an iron containing 27% chromium or other suitable heat resisting metal. The mold barrel and the inner shell rest upon the base plate 3 which is also made of heat resistant material. The ceramic mass which the mold contains is shown at 4, while 5 represents a heavy follower plate which may be used during the compacting process.

This type of mold should be lined with heavy paper shown at 6 which burns out, thereby facilitating the removal of the matured mass from the mold. The mold also can be lined with sand or some other suitable material which will prevent the wheel from sticking to the mold, facilitate the removal of the wheel, and support the wheel by settling as the metal mold expands (the sand containing no temporary binder to hold it together) This sand is held in place while filling the mold by thin sheet metal cylinders which are removed before the mass is compacted.

Figure 2 shows a split mold composed of 27 per cent chrome iron or other suitable heat resisting metal. The mold consists of an outer shell i and an inner shell 2 whichare fastened to base plate I. The outer and inner shells are fastened at their Joints by means of}, angle irons II and bolts IS. The mold parts are, made of a heat resisting metal such as described above.

Figure 3 shows diagrammatically the essentials of a kiln structure adapted to carrying out our invention. The base 1 represents kiln furniture composed of 27 per cent chrome iron or. other suitable heat resisting metal upon which rest base plate 3 and mold I. The kiln cover 8 is adequately lined with refractory materials (not shown). and is provided with fittings for electrical wires 0 and openings ID at which suction may be applied. We have used electrical energy in maturing the bond when carrying out our process and have selected materials for the equipment and process in accordance with the maximum temperature of 1000 degrees centigrade. Since part of our process often requires this elevated temperature, it is essential that the mold parts i, 2 and 3 (Figure 1) be fabricated from especially heat resistant material such as the alloy mentioned above containing 27 per cent chromium. The heating is produced by means of resistors I 3 which are supplied with electric currents from the leads 9 as indicated in Fig. 3 of the drawing.

We will now describe fully the carrying out of our invention, using, as an illustration only, the making of monolithic abrasive wheels suitable for use in grinding wood to make pulp. The

manufacture of a wheel of this type requires bond, abrasive grain, mixing, compacting, kiln Percent by weight Albany clay 41 Feldspar 30 Flint 10 whiting 3 Magnesi 2 Borax glass"- 15 Total Such a mixture matures at about 1000 degrees centigrade and can be fired in a heat resistant metal mold. The ingredients are powdered and blended by ball milling or other suitable process, and may, for example, constitute 18 per cent by weight of the abrasive wheel mix.

2. We have found that fused alumina can be used to advantage as the abrasive in connection with the above bond, although of course it will be realized that other types of abrasive grain and other bonds can be used. For the purpose in hand we have selected, as an illustrative example, a series of grits which can be used very satis-' factorily with the bond selected, and the sizes and proportions specified will be readily understood by those skilled in this particular art. A mixture of grits which we have found suitable is as follows:

This grit mixture, if proportioned so as to constitute 82 per cent of the total abrasive wheel mix, has been found to yield a mechanically strong abrasive structure, due probably to the bracing eifect of smaller particles filling the voids between larger particles.

3. The bond and abrasive are thoroughl blended by agitation in a mechanical mixer, as for example a mixer of the finger type. The mix is then ready for placing in the mold and compacting.

4. We have found that compacting by iolting is very desirable in carrying out our process, although other forms of fabrication can also be used. After assembling the mold in position upon a. dolting machine, the machine is started and the abrasive bond mixture is added by scattering it reasonably uniformly into the mold until the requisite total has been added. The Jolting action is interrupted at this point in order to position the follower plate I (Figure l) and any other means for applying pressure which may be used, such as by hydraulic pistons. The jolting is then resumed and continued as long as is necessary to yield the desired apparent density of the wheel. The follower plate is removed after the compacting has been completed.

5. The mold, together with its compacted contents, can then be placedin the kiln substantially as shown in Figure 3 and the mechanical connections for energy and control of its input,

pumping and its control mechanisms, etc. are made in preparation for drying and maturing the bond. A typical schedule for conducting the kiln run is most clearly explained by considering separately the changes in temperature and the changes in pressure, although both of these factors are varied simultaneously.

6. A burning schedule in which the temperature is raised at the rate of 15 degrees centigrade per hour to 750 degrees centigrade and held at this temperature from 15 to '20 hours, followed by a further raise of 15 degrees per hour to 1000 degrees centigrade, the temperature being held at this point for 20 hours, is satisfactory. After heating is completed, the article can be cooled at the rate of 5 degrees centigrade per hour.

7. The use of vacuum during the above run is desirable since it facilitates the removal of moisture. By repeatedly changing the pressure or admitting air into the chamber, a fresh supply of air is made available to the innermost parts of the wheel to burn out any carbonaceous mat-' ter present. From room temperature until after the soaking period at 750 degrees centigrade, the kiln can be subjected alternately to 5 minutes of reduced pressure equal to 3 inches of water pressure and 10 minutes of normal atmospheric pressure.

The above process is given mereLv as an illustrative example, and many modifications within the scope of our invention can of course be made.

Referring to the paragraph numbered 1 above, the ceramic bond formula given is one which we have found to mature within temperature limits which can be employed in conjunction with a heat-resistant metal mold. The bond is quite resistant to erosion under normal conditions of use. Many other combinations are known which will fulfill the requirements of maturing at 1000 degrees centigrade and which have the desired water resistance, strength, etc. We have cited the use of raw bond, and while this procedure is entirely practicable, it is in many cases desirable to use calcined, fritted or vitrified and crushed bond. A previously calcined or fritted bond minimizes shrinkage, and calcining also removes the carbonaceous matter which may be contained within the bond.

In paragraph numbered 2, we mention the use of an abrasive grit made by crushing fused alumina. While this material is an excellent abrasive for pulp grinding operations in particular, it is also well suited to a great many metal grinding operations. Other abrasive materials of either artificial or of mineral origin may be processed equally well; for example, silicon carbide, or silica sand. The grit sizes mentioned are not considered to be limiting in any sense, because the intended use of an article being manufactured controls this selection.

The blending of bond and grain has been described in paragraph 3 without the use of water or temporary binder. This is a departure from the usual ceramic practice.

We have found that consolidation by a jolting process is very advantageous in effecting the densecompacting of the material without a temporary binder, for it permits the rearrangement of the particles to give a dense mass. Although the consolidation of the mix without the use of a temporary binder is of advantage, this step is not absolutely necessary, especially with forced circulation during curing. We have conducted many successful applications of our invention using varying amounts of dextrin, lignon or other substances as temporary binders.

Pressing or tamping to compact the ceramic mass as described in the numbered paragraph 4 above can be used for consolidation if desired. It is even possible to produce a bonded article by merely scattering the required weight of mix into the mold, but fairly dense consolidation is usually desirable.

There are various ways for supplying energy to a kiln as well as an infinite number of schedules for temperature and pressure regulation which can be used, although we have found that those mentioned in the numbered paragraphs 5, 6 and '1 above are well suited to the example cited. It is very desirable in the case of large articles to retain the mix in the mold during firing, as this prevents distortion under heat. It is also desirable to produce changes in pressure to the degree required to burn out the mass completely and rapidly, the exact values used depending upon many factors such as the amount of organic or volatile matter in the mix, whether or not a carbonaceous temporary binder has been used, the fineness of the abrasive granules, the porosity of the mass, etc. On occasions, vacuum as high as 28 inches or more of mercury can be utilized. subsequently restoring atmospheric pressure by introducing air or pure oxygen. Other gases such as nitrogen or carbon dioxide can also be used, although in the burning out of carbonaceous matter agas oxidizing to carbon should of course be employed. A forced circulation can also be effected by increasing the pressure of the surrounding atmosphere above that of the usual atmos- I a sufilcient extent, the greater part of the gas within the pores forces itself out of the article, and carries with it the moisture or combustion products which it is desired to remove. By repeating this process at frequent intervals, the same burning eflfect can be attained in a massive article as is obtained in an article of very small cross-section by the ordinary burning processes.

While we have described our invention as relating to articles containing a vitrified bond, the process can also be used in connection with self bonded articles or grains containing a minor proportion of a more fusible constituent than the body of the grain. Certain impure grains can also be bonded by the fusion of the impurities contained therein. It is only necessary that a portion of the material become fused, vitrified or softened so as tobond the formed mass.

Our process is applicable to ceramic articles other than abrasive articles, as for example, refractories, large monolithic blocks, and.- other similar applications.

Having thus described our invention, we claim:

1. The steps in the process of curing a ceramic article which comprise heating a formed ceramic mass in a mold to the temperature of vitrification of a portion of the mass, and repeatedly increasing and decreasing alternately the pressure of the atmosphere to which the article is exposed during heating.

2. The steps in the process of manufacturing monolithic ceramic masses which comprise consolidating the ceramic mix in a mold composed of a highly conducting refractory material which will withstand temperatures in excess of 1000 degreesv centigrade, heating the mix in the mold to approximately 1000 degrees centigrade to effect the curing thereof, and repeatedly decreasing and increasing alternately the pressure of the atmosphere surrounding the mold and in contact with the molded article during the curing of the article.

3. The method of making a ceramic article which comprises forming a mix of granular material and a bond, consolidating the said mix within a mold, heating the consolidated mix while it is retained within the mold, alternately increasing and decreasing in continued succession the pressure of the atmosphere surrounding the said mix during the said heating, and firing the consolidated article to the maturing temperature of the bond.

4. The step in the process of curing a ceramic article which comprises repeatedly decreasing and increasing alternately in a predetermined manner the pressure of the atmosphere surrounding the article during the curing process.

5. The steps in the process of curing a ceramic article which comprise subjecting the article alternately and in continued succession toincreasing and decreasing pressures of a gaseous atmosphere and simultaneously applying heat to the article.

6. The steps in the process of curing a ceramic article containing carbonaceous matter which comprise circulating throughout the pores of the article a gaseous medium which is oxidizing to carbon, the said circulation being effected by exposing the article to the said gaseous medium and repeatedly decreasing and increasing alternately the pressure of the said medium, the article being simultaneously heated to a temperature suflicient to oxidize the carbon contained within the article.

7. A monolithic abrasive article of the character obtainable by molding a mixture of brasive grain and a bond which is more fusible than the grain, and curing the article in the mold by heating it to the temperature of fusion of the bond while subjecting the article being cured to a succession of alternate decreases and increases of atmospheric pressure.

8. The steps in the method of making a ceramically bonded article which comprise mixing refractory particles and a bond which is more fusible than the refractory, consolidating the mixture in a mold, and heating the formed article in the mold to the temperature of maturing of the bond while the pressure of the surrounding atmosphere is alternately decreased and increased repeatedly.

9. The method of making a ceramic article having open pores and consisting of granular refractory particles and a vitrifiable binder therefor which comprises forming a dry mixture of the materials in a mold, and heating the material while supported in the mold sufficiently to vitrify the binder, the pressure of the atmosphere in and around the material within the mold being repeatedly decreased and increased while its temperature is being changed.

RAYMOND C. BENNER. ALBERT L. BALL. HENRY RUSSELL HOUCHINS.

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