US2297169A - Method of particle size grading and product - Google Patents

Description

Sept. 29, 1942. D w 5 2,297,169

METHOD OF PARTICLE SIZE GRADING AND PRODUCT Filed Nov 12, 1941 ATIO 0F SCREEN OPENING |N INCHES TYLER STANDARD SREEN MESH Patented Sept. 29, 1942 l'l'E STTES FATE T OFFCE METHOD PARTICLE SIZE GRADING AND PRODUCT 14 Claims.

This invention relates to the production of massive bodies from granular material, and consists in a mass whose constituent grains are graded in size according to a formula that I have discovered to afford the optimum result and in supplementary methods of procedure, whereby a body of maximum density may be produced. The invention finds practical application in concrete building and in the production of articles of ceramic ware and refractories.

In the accompanying drawing certain data are plotted, and the plotting will be referred to in the ensuing description.

It long has been known in the production of bodies from granular material, that, by sizegrading of the particles, increased density of piling may be gained. Various formulae to determine grading have been published, but the results have been mediocre. Onesuch formula, based on size divisions each 1.414 coarser than the next finer, shows a ratio of 1.20 for the amount on each screen as cdmpared with the next finer (0.8333 for amount on each screen as compared to the next coarser). In concrete building, particularly, the common practice is to accept and use the Portland cement content of the mixture in the condition of granulation that the producer has given to it in placing it on the market, and in such quantity as the concrete mixture requires. This is done in ignorance of the fact that, by bringing the whole of the granular constitutents of the mass (cement included) into conformity to a formula of size grading, superior results are to be attained. In prior practice, furthermore, there has been no recognition of the fact that size grading should, to achieve best results, be carried into the very fine and powder-like portion of the whole-into the portion that will pass through a 325-mesh screen.

To achieve maximum density, the grading should be such that the larger particles stand apart; that, on the average, every particle, tangent necessarily to a larger particle (in this detail the largest particles only are excepted), should be supported elsewhere by particles of smaller size. In a mass so graded, the particles are, geometrically, in an assembly of maximum mechanical instability; and, consequently, in addition to the desired characteristic of maximum density, the so graded mass is (when in slip condition) in a state of maximum fluidity. This further characteristic of maximum fiuidity is of peculiar value in the arts of concrete work, ceramics, and refractories, since but a minimum amount of liquid is required to form the mass into a paste or slip that may be shaped or molded as desired.

I have discovered a formula that, being followed, will afiord in highest degree the characteristics indicated, and as a correlative of ,such formula I have developed an average cumulative plot upon a coordinate chart, as illustrated in the accompanying drawing, indicative of a graded granular mass that is of maximum density, that possesses maximum fiowability, that requires a minimum quantity of water to bring itto the condition of a workable paste, that affords a shaped body of minimum drying shrinkage, a dried body of minimum porosity, and good continuity in the substance of the fired ware.

I found that the percentages on successive screens could be accurately measured to 200- mesh. On finally arriving'at my general formula it became evident that fully as accurate grading is required of the finer sizes as is required for that coarser than ZOO-mesh. This last fact was proven by including from two to six groups of known particle sizes smaller than 325-mesh. Further, I found that, the closer the fractions coarser than ZOO-mesh approached my average curve, with minus ZOO-mesh fractions at a minimum, the closer the mixture approached fiowability, but that satisfactory flowability does not occur unless there be material present of smaller particle size than that which remains one 325-mesh screen, such material of smaller particle size also being 7 preferably graded.

in which S=cumulative volume per cent of any number (n) of fractions that, passing through 0.185-inch openings (4-mesh screen). are arrested on succeeding smaller-meshed screens of tion=0.96. Thus I have discovered that my specific cumulative volume per cent plot matchesthe general mathematical equation y:m in which represents the size (expressed as a function of largest particle size) of screen-mesh and y the percentage (expressed decimally) that passes the screen. It matches, not only in the portion coarser than 200-mesh, but also down to 0.000008-inch size (approximate average size of individual clay particles) which is the smallest size for which comparisons were made. I thus have three points of the specific curve that fit the y:a: curve; at maximum size (point A on the curve), at 200 mesh (point B), andat 0,000008-inch diameter (point C). :m is hence the general equation sought. It is general for any maximum size. Its form has the physical significance that, if the smallest size were infinitely small, the dry porosity of a formed dried mass of such a gradation would be zero, no matter what the size of the maximum particle. This fact may be set forth as follows: This is a particle-size gradation, hence the particles are in contact with one another. Since a uniform law is followed from maximum size'to infinitely small size, there can be no size at which the particles are in different arrangement than at any other size, hence the particles must be uniformly in contact at all sizes. Since the particles are uniformly in contact at all sizes and the smallest is infinitely small, the interstitial space is infinitely small. Geometrically, such a mixture has maximum mechanical instability, and hence exhibits (when in slip form) maximum 'flowability. Further, the respective volumes of material and numbers of particles of the various sizes geometrically insure that size gradation is uniform throughout any fluidized mass having this gradation.

In plotting the :19 equation I lay out the values of x from 0 to 1 algebraically (non-logarithmically) along ten successive logarithmic cycles, on the one-way logarithmic chart, and the values of y from 0 to 1 are ranged uniformly along the non-logarithmic ordinate of the chart. In plotting the curve or graph of the equation. the following values of a: and y are used:

' I also lay out on the abscissa the ratio of the particle diameter d of each fraction of the material to the diameter D of the largest particles,

the values of such ratio beingdistributed logarithmically. The largest particle diameter in this case is 0.1850", which equals the size of the mesh opening of the No. 4 screen of the Tyler screen series. Thus, the value of the ratio d/D for the largest particle or mesh size is etc. Since the largest particle diameter D has unit value (a value of 1), and since 1 on the z axis or abscissa of the chart equals 10 cycles of the logarithmic scale, any value of a: may be computed in accordance with the equation:

:i::1+0.1 (log. d/D) Thus, the value of :1: for the largest particle size or screen:

The value of a: for the next smaller particle or screen size:

' 0.1310 1+0 .1(log =1 +0.1(log .71 =1 +0.1 1.851) =1 +0.1(9.851 10) =1. +.9851 l .9851

The value of a: for the next smaller particle or screen size:

And so it is with each value of x, it being noted that each of such values of a: is a function of the ratio of a particular particle or mesh size to the largest particle or mesh size, and is not limited to any given system of screen sizes.

Each value of y is, of course, equal to the cube of a particular value of 2. That is, when 1:1, 11:1; when :c:.985, 1:.955; when :1:=.9'l0.' y:.913; etc., the values of y being the decimal equivalents of percentages.

In applying my formula (-'-S:y:cumulative. volume per cent finer than any given size) to. the y=x equation, I reduce the percentages expressed in the formula to decimals, that is, to the same sort of units as a: and y in the ZI=$3 equation are expressed. Thus, for the Tyler system of screen sizes In computing in accordance with the formula for the fraction having the. particles of largest size (the 4 mesh Tyler screen), the value of n is 0 and the formula reads:

10.4698 (0)=1 (Note point A on the graph.)

In the case of the 200 mesh screen, where -n=12, the formula reads:

log .96 1.98227= 9.98227- log .96X12=l19.78724120= 1.78724, the number of which: .6127

1.815=.1 8i (Note point C on the graph.)

Thus, it will be seen that my formula, 100S= cumulative volume per cent (expressed decimally in this case), matches the y=ac curve, and, having established this, it will be understood the y=zr equation may be used as the controlling factor in size-grading of materials whose maximum particle size is 0.1850". What particular series of screens is in fact used does not signify, since the value of a: for any particular screen smaller than the maximum mesh size of 0.1850" is a definite function of the ratio of the particular screen size to the largest size; that is, a:=1+0.1 (log d/D).

In case the material to be size graded has a maximum particle size of 0.328" (20 mesh by the Tyler series) the same y=x curve is employed, but the 20 mesh screen is given unit value (as was the 4 mesh screen above) then the values of the ratio of d/D are computed relatively .to size of opening of such 20 mesh screen, and the values of a: for the successive screens are determined in accordance with the equation x=1+0.1 (log (II/D). And so the equation 11 .79 is adaptable to the size-grading of various materials, whatever be the maximum particle size;

To check that other size gradations do not yield the y=at equation, and hence cannot yield the desired results, for which it is the exact, equation, I assumed other values of r at size 0.000008 inch (see accompanying cumulative plot) and, by inserting these in the equation T-r calculated the corresponding valuesof a. Using these assumed values of r and the corresponding derived values of a in the equation for 200 mesh, I found that I had a My curve shows that 18.5 cumulative volumeper cent of a gradation, whose maximum size is 0.185 inch, should be finer than average clay grain size (note point C). It is known that the ultimate clay grain size of refractory clays is commonly of one order. Further, I have found that, using nothing of finer grain size than clay, in preparing masses according to my formulae,

the resulting formed dried mass, made of grains of near zero porosity, has had porosities of near 18.0 per cent. I thus find that the inclusion of suitable percentages of suitably graded sizes of smaller than 0.000008-inch diameter has effect in decrease in the porosity of the formed dried mass. The ratio of average clay grain size to maximum grain size is which expressed as a fraction:

the crusting" during drying of clay containingsuitable amounts of sodium silicate. As drying proceeds, colloidal silica collects near the surface of the clay, with resultant formation of a nonporous layer that retards further drying of the interior of the mass.

Formulae previously devised for the grading of granular masses have been empirical and have not been based on knowledge of what are the best conditions in any given case. On the other hand, y=m defines the limits. Thus, my above formulae produce good flowability, low water content, low drying shrinkage, and low dry porosity, and otherwise improve the formed mass, to such a degree as to result in greatly improved behavior of masses graded in accordance therewith during the various processing steps, and in greatly improved quality of the finished ware, and, specifically, in improved continuity of the structure of the fired ware (absence of voids formed by shrinkage) Tests, consisting of grading of the portion finer than 32'5-mesh, have resulted in checking the above formulae which, as far as I am aware, are different from previous formulae, even in the portion coarser than 325-mesh. The grain sizes in the portions finer than 325-mesh may be determined by elutriation, micro-examination, ultra-centrifuge, ultra-filtration, and by settling tests. In practice, approach to satisfactory grading is indicated by ease of fiow of the wetted mass, low amount of Water required to cause flowing, low porosity of the dried mass after flowing, and (in a ceramic mass) by good continuity in the substance of the fired mass.

I have discovered that, except for surface phenomena, such gradations flow readily. Most of the total particle surface consists of the surface of the particles finer than 325-mesh. Primarily interest accordingly centers in surface effects of the portion finer than 325-mesh (50.4% according to my formula for-4-mesh maximum size) and more particularly in the portion finer than 0.203 micron (18.5% according to said formula).

In certain cases granular material of other gradation values may, by careful preparation, be made to assume a condition of low voids, but the material does not in these cases readily assume such arrangement, and does not afford such continuity of the substance of the fired ceramic ware as is gained in following my invention. On the other hand, gradations made according to my formulae automatically assume such arrangement, without resort to any tamping or arrangement process. For instance, slip-cast wares made by my formulae are uniform throughout as to the distribution of the graded particles. This is, possibly, because of the fact that, for a given weight, the number of spherical particles increases inversely as the cube of their diameter. That is, a sphere of a given diameter has a weight equal to that of eight spheres of half that diameter; the result of this being that there are not enough particles of near any one diameter to permit of their coming into contact,. to cause bridging. Such automatic assumption of particle arrangement is useful in pouring of concrete.

It is possible that in certain masses comprising graded grains, the portion of the mass smaller than a certain grain size does not, in manufacture and in use, shrink much more than does the coarser material. In such case I may prefer to grade according to my formula the flnergrained portion of the whole, up to a given maximum grain size, and to fill more or less completely this graded portion with coarser-grained material. For best results these coarser particles should not be so great aproportion of the total massthat they would make contact one upon another, and thus bridge, and cause discontinuity in the mass. A dry-press raw flint-clay body may be such'a mass, as are most masses of concrete. I may prefer to grind a very fine portion of such a ceramic body, and to mix this with coarser sizes, graded per my formulae, to fluidize the mass with proper agents, and then to use this mass for suitably wetting the coarser-grained raw flint and bringing it to a consistency suitable for dry pressing. I may prefer to use from 60 to 90% by weight of the coarser material.

I have found that wetted masses of certain fine-grained ceramic materials tend to assume a mild set, suflicient to prevent free flowing of the mass. Free flowing is especially necessary in the molding of intricate shapes. My tests indicate that materials that behave in this manner are of near one grain size, and that their grains tend to orient themselves one to another. Such orientation may possibly be caused by forces residing on or in the individual grains, or by some other cause. I have found, however, that if the grains are suitably graded in size according to my formulae, flowing is frequently not appreciably retarded.

An example of one way of obtaining a gradation in accordance with my formulae follows:

It is common knowledge that, on the crushing of homogeneous materials by the usual methods, such as jaw crushers, rolls, dry pans, and similar methods, the majority of the crushed particles form a hump" in the curve indicative of quantity coordinated with particle size, within a size range of approximately 2.5 diameters with the balance of the material extending largely in a smooth rapidly decreasing gradation to smaller sizes.

I find that, if I start with homogeneous material, that is relatively hard and tough and of approximately two-mesh size, and grind it in the manner outlined below to approximately fourmesh size, the resultant product is of smooth gradation, the majority of which extends over a size range of approximately eight diameters, with the balance of the material extending, largely,

in a smooth, rapidly decreasing gradation to smaller sizes.

The method of obtaining such smooth gradatlon consists in grinding in a ball mill containing grinding media of suflicient maximum size to insure ready breaking of the maximum-sized particle of the infed material, together with sufficient water to cause the particles of infed material to adhere to the grinding media, but not to pack. With the maximum-sized grinding media are associated assorted smaller-sized grinding media, but still of such size that they are not crushed by the larger-sized grinding media. For instance, flint pebbles in a silica-lined ball mill may be used, in which the pebbles range from approximately five-inch diameter to approximately two-inch diameter. In case a continuous mill is used, the material, when ground to the desired degree, is forced from the mill by incoming material. If need be, the desired degree of grinding may be attained by passing the material from a first mill to one or more additional mills.

A second stage in the fineness of grind, composed of particles, the majority of which will spread in a smooth gradation over approximately eight additional diameters, can be prepared by using material that has already been ground to approximately twenty mesh, either by ordinary crushing or by the above method of wet grinding, and grinding as above, except that the maximum-sized grinding media are just suflicient to disintegrate the twenty-mesh particles of material. For instance, if flint pebbles are used, the

maximum-sized pebble may be approximately smallest size of the constant curve portion of the grind la'st described above.

Suitable combinations or grinds of the three stages described above yield close approximations to my formulae. A further aid in smoothing out "humps" in the curve indicative of gradation in this three-stage procedure is to grind said suitable combination for a short period of time with grinding media of intermediate size. A further aid in the production of suitable size gradations from clay grogs is the preparation of suitable gradations of unfired clays, slight bonding of these graded grains into dobies, and calcining of the dobies. Such calcined material readily crushes to the original gradation. To insure that the calcine will disintegrate readily, I may prefer to incorporate materials with the clay, such as quartz and kyanite, that expand to friable masses on being flred. 'Ifhis serves to produce the ground material with a minimum of grinding efiort and with a minimum of contamination from the grinding media. Flint clay is responsive to this treatment.

Production of the finer sizes of such size graca tions is aided by grinding together fine-grained materials of different degrees of resistance to abrasion. Instances of this are the grinding together of raw kyanite, heat-softened kyanite, and heat-expanded kyanite; again, raw quartz with heat-expanded silica; and, again, silica or kyanite with clay and diaspore raw or calcined. Although coarse sizes may be similarly benefited, this method is particularly useful in connection with minus 325-mesh particles, as there is apparently no other method of grinding available (on account of insuflicient area of grinding, media) for rapid production of minus 400-mesh sizes, without reducing practically .all of the particles of the mass to these small sizes, thus preventing attainment of the desired size gradation. This method also avoids introduction of excess impurities during such size reduction.

Usual good practice as to mill charge of grinding media and material to be ground applies to this type of grinding. I have found that, if only the particles of the infed material be small enough to be disintegrated by the maximumsized pebbles used as grinding media, the smaller the average pebble-size the finer is the grind that can be produced.

A certain range of colloidal sizes can be at tained in the grinding of raw clay; preferably by grinding the clay to some extent as above. If raw clay be ground together' with harder material, th particles of the harder material will serve in part as grinding media for the clay, while being themselves ground.

Another range of colloidal sizes is attained by the partial hydrolysis of the infed material, occasioned by fine grinding in contact with water. For instance, some colloidal material is produced on the fine grinding of silica, alumina, and the aluminum silicates. In fact, silica and alumina, wet-ground together for suflicient time, produce some hydrous aluminum silicate.

Other means of producing partial hydrolysis are found in the use of means and materials that to some extent attack the particles of the infed material. For instance, on grinding silica and alumina together under elevated steam pressures, hydrous aluminum silicate is the stable phase, and some of it is formed. Furthermore, I find that the treating ef silica, alumina, and aluminosilicates with gaseous fluorine or a gaseous fluorine compound such as hydrofluoric and hydrofiuo-silicic acid gas and of other fluorine compounds such as ammonium fluoride or ammonium bifluoride effects to a degree decomposition of the particle surfaces. Such surface decomposition commonly results in the hydrolysis of one or more of the reaction products, and in the production of some colloidal material. I find that such reactions are aided by being con ducted in conjunction with and during a grinding operation. I may prefer to grind the batch for a time in the presence of these materials, either dry or wet, and at room temperature or at elevated temperatures. In the case of alumina-silicate reactions, removal of either silica or alumina from the reaction allows the decomposition of the sistant to abrasion than is the unfired kyanite.

My invention lies further in the discovery that a graded mass of granular material, preferably graded according to the formula given above,

' may be yet more closely consolidated,-and its density yet more greatly increased, by subjecting it to further particular treatment.

It is common knowledgethat, in clays, exchangeable bases take up definite positions in the crystal lattice of the ultimate clay particles; that in clay suspensions in water, and other suspending media, such exchangeable cations serve as the connections between clay particles; and that, in cases in which such cations bring about coagulation of clay particles, it tends to be an oriented coagulation. Further, it is known that clay particles in such' suspensions can be oriented by passing electric current through the suspensions and by subjecting the suspensions to electric fields.

Reasoning from the above and other known chemical facts, I have concluded that such sus pended .ultimate clay particles, and similar suspensions r' many other kinds of particles, carry, or are capable of carrying, electrical charges. By altering the direction of an "electric current that I may cause to pass through such a suspension, or by altering the direction of lines of force of an electric field to which I may subject such a suspension, it is possible to break down given orientations and form still other orientations in conformity with the altered directions. Still further, since the ultimate particles of such a suspension can carry electric charges, it is possible to orient and alter the positions of the particles relative to each other by subjecting the suspension to any electric, electronic, magnetic field, electro-magnetic induction, or positive or negative ray forces of suflicient magnitude. Similarly, by more or less continuously altering the direction of such-a force, it is possible to keep the ultimate particles in more or less continuous movement relative to one another. By using such forces of sufiicient intensity andby varying the intensity I am able to gain maximum and desirable time of disorientation of ultimate particles and produce other desirable effects.

Multiple forces of the nature specified may be simultaneously applied; they may be applied in I parallel directions or at angles to one another, in the same plane or in different planes, and in changing directions relative to one another. Conventional means may be employed that the dispersions may be penetrated by direct or alternating current, or may be brought within and made subject to magnetic fields, to electro-magnetic induction, or to positive or negative rays. The waves of force may vary, not in direction only, but in intensity as well. In case direct or alternating current is used directly for this purpose, I prefer to use a current of between 22 and 1500 vlolts, and, with amperage suitably small, the current becomes relatively safe to handle.

7 A specific means of accomplishing above rapid change of orientation is to place a mass of the material within an annular coil, so wound that the particles within th mass of material will be rapidly oriented and their orientation altered when an alternating current is passed through the coil. The field intensity is, of course, sulficient to cause orientation.

In concrete and in ceramic masses containing granular and'colloidal material. I may' prefer to cause movement of particles relative to one another, by causing at least some of the particles to vibrate by means of passing high-voltage highfrequency electric current through piezo-electric crystals. the piezo-electric materials being situated either outside of or within the mass being acted upon. I may prefer to use Rochelle salt. quartz, mullite, particles that are largely alumina, magnesium chloride, ammonium chloride, magnesium ammonium chloride, ammonium fluoride, ammonium fiuo-silicate, and other materials, including constituents of the ceramic mass, as the piezo-electric material, and vary the frequency and voltage to suit the material used. I may prefer so to act on ceramic masses while they are under pressure. Crystalline constituents of ceramic masses frequently contain crystal strain, particularly in fired particles. I may vibrate a dispersion by means of piezo-electric vibration of these strained particles.

Piezo-electric excitation may be set up by causing particles of quartz present in a dispersion to vibrate. Particles of quartz are normally present in a concrete mix and in a ceramic casting slip also.

Piezo-electric excitation is commonly carried out by passing the exciting force through the crystal in a given direction. I may prefer to orient piezo-electric crystals in a mass of particles, such as concrete, ceramic materials, and refractory materials by the methods hereof; and, while so oriented, to pass a piezo-electric exciting force through such crystals in the most favorable direction. For instance, I may prefer so to orient particles of quartz, kyam'te, clay, magnesite, chrome, and auxiliary piezo-electric materials, including organic piezoelectric materials, and to subject them to piezo-electric exciting force while so oriented.

In case the two sets of forces interfere, I may prefer to apply them alternately to the piezoelectric particles. In case alternating electrical forces are used for this purpose, I may choose to synchronize these forces, so that one is acting with minimum effect on the particles while the other is acting with maximum effect. For instance, one force might be one fourth of a cycle behind the other.

I may prefer to use this piezo-electric crystal vibration method in the forming of concrete masses and of ceramic masses containing minimum water contents, whether the ceramic mass be shaped by slip casting, by pressing, or by other forming method.

By subjecting the particles of the suspension to such orienting and piezo-electric influences rigidity of a mass may be diminished, facility of flow increased, and other good purposes served.

One specific practical application of the invention is found in decreasing the rigidity of watercontaining masses of concrete, so that they can be flowed to position and form a relatively voidfree finished product. Another application is found in doing the like in the case of a ceramic mass, so that the mass, containing an amount of water less than otherwise is practicable, may still be flowed into a mold, or otherwise shaped. I find a result of disorientation of particles in ceramic and other masses to be low porosity of the formed dried masses.

In particular cases I prefer to keep the particles disoriented only long enough to permit forming (as in a mold) the masses containing them. The particles may then be re-oriented, for the purpose of giving to the molded material a set, so that the molded article can be removed from the mold within a minimum period of time, and so that other useful purposes can be served. The molding may be performed by slip casting, pressing, or by other suitable means.

Referring again to the crystal structure of clay particles, it is commonly known that the molecules of polar liquids, such as water, attach themselves to the ultimate particles of suspensions in definitely oriented positions with reference to the cation bonds between the particles. Reasoning from this and other known chemical facts, such water isf'I believe, little attached to and hence more easily removed from disoriented particles than from oriented material. I, therefore, prefer to alter the orientation of ultimate particles and to disorient them during at least part of the time of the drying of masses containing them. Such treatment serves to permit easier removal of water from the masses.

In ceramic masses, such control of orientation of ultimate particles has the advantage that it permits of wider variety in the applications of a number of ceramic materials. Specifically, it permits the use of certain clays in slip-cast ware that give unusual strength to the ware, and whose particle-size range apparently is unusually great.

I may in particular cases and with good effect disorient the silica particles of silica jellies.

I have described how I size grade according to formula the particles of cements, concrete, and ceramic casting slips to achieve a minimum of voids; how this grading insures automatic arrangement with minimum voids, and howboth by such grading and by methods of polarization and piezo-electric vibration I am enabled to use a minimum of water in the shaping of such masses. The combined result of all these in ceramic ware is a dry porosity of ware which, to my knowledge and belief, is lower than has heretofore been achieved, either by slip casting or by any other method. Size grading according to my formulae has the further advantage in ceramic ware that, with lack of bridging between particles, the raw ware is more rubbery, and, hence, more resistant to injury by thermal shock. I have found this to be a feature particularly valuable in the case of raw wares of high thermal expansion, such as those high in quartz.

This method of particle-size gradation and fiuidizing will be found to be of great utility in the making and placing of concrete. First, in the manufacture of cement, the strength and covering power of the cement can be improved by extending according to my formula the lower limits of particle size. Next, intermediate sizes of aggregate, sand, and fine sand can be used in compliance with my formula to augment and complete present-day gradations. Suitable colloidal material of yet finer sizes than the cement can be introduced according to the'formula, to

fill the interstices between the cement particles.

and the cement can be of such composition that it will liberate a suitable amount of such suitable colloidal material. Such grading of concrete yields a mixture that has a new order of flowability. Also, since concrete commonly contains quartz particles, that can be piezo-electrically excited, such excitation canbe utilized to further fiuidize them.

In the practice of my invention I have produced superior refractory blocks for glass tanks or furnaces. Slip-cast refractory blocks made in accordance with the invention have a more continuous and superior structure.

In accordance with the teachings of the invention' better concrete may be obtained with a smaller proportion of cement, the most expensive ingredient of the concrete mix. Specifically, it appears possible to produce better concrete (than is now used in concrete construction) with mixes that include from 15 to 21 per centpcement, which is from A; to less cement than is used in the usual mixes.

7 particles of less than 0.000008 of an inch The application for these Letters Patent consisted in a continuation in part of application Serial No. 239,184, filed November 5, 1938. And notice is hereby given of my application Serial No. 285,961, filed July 22, 1939, comprising a division of the first filed case.

I claim as my invention:

1. A body of finely divided material of varied and graded particle size and of maximum density, formed of a plurality of grades compounded according to the formula y=x in which y=the part of the whole that will pass through any given screen having segregating action and effect upon'the body, and a=the size of the mesh of that screen, expressed as a function of the ratio of such mesh size to the largest particle size of the material.

2. A dense mass of size-graded particles in which the particles are graded according to the equation y=zr in which i v particular particle size 'maximum particle size) and y is the proportionate part of the whole that is of such particular particle size and smaller.

3. A dense mass of size-graded particles, including small particles that; will pass through a 325-mesh screen, such small particles being of various particle size, graded according to the equation z/=.r in which particular particle size 'r n aximum particle size) and y is the proportionate part of the whole that is of such particular particle size and smaller.

4. A dense mass of size-graded particles, including particles of a diameter less than 0.000008 of an inch, the particles of such small diameters being graded according to the equation Far in which particular particle size 'maximurn particle size) and y is the proportionate part of the whole that is of such particular particle size and smaller.

5. A dense mass of size-graded particles in which the particles are graded according to the equation y=x in which particular particle size Q 'maximum particle size) and y is the proportionate part of the whole that is of such particular particle size and smaller, said mass including substantially 18.5 per cent of in diameter.

I 8. A ceramic slip of size-graded particles in which the particles are graded in accordance with 6. A dense mass of size-graded particles ineluding particles of graded size, from a diameter '7. A dense mass of size-graded particles in which a matrix of finer particles graded in accordance with the equation y=x (in which particular particle size) x 'maximum particle size and y is the proportionate part of the whole that is of such particular particle size and smaller) contains ,from 60 to 95 per cent of coarserparticles.

the equation y=a: in which particular particle size 1 'maximum particle size and y is the proportionate part of the whole that is of such particular particle size and smaller.

9. A dense mass of size-graded particles in which the particles are graded according to the equation 11:09, in which g particular particle size) maximum particle size and y is the proportionate part of the whole that is of such particular particle size and smaller,

said mass including substantially 18.5 per cent.

of particles whose diameters are less than ofthe diameter of the largest particle size.

10. The method of forming particle size gradations that comprises grading fines according to the formula y=x in which y=the part of the whole that will pass through any given screen having segregating action, and .r=the size of the mesh that screen expressed as a function of the ratio of such mesh size to the largest particle size of the fines, and compounding the so-graded fines with a body of coarser particles in such proportion that the coarser particles stand substantially free from contact with each other in the body of the fines.

11. The method herein described that comprises compounding a body of unfired clay particles according to the formula y=w in which y=the part of the whole that will pass through any given screen having segregating action, and ar=the size of mesh of that screen expressed as a function of the ratio of such mesh size to the largest particle size, bonding the so-graded particles into dobies, calcining the dobies, and then crushing the dobies and returning the mass to substantially its original size-graded condition.

12. The method herein described which consists in grinding in common a plurality of solid particulate materials of different degrees of toughness, and compounding the particles of the ground mass according to the formula glen, in which y=the part of the whole that will pass through any given screen having segregating action, and zc=the size of mesh of that screen.

expressed as a function of the ratio of such mesh size to the largest particle size.

13. The method of preparing the finer or colloidal grains of a particulate material that is size graded according to the formula 11:15, in which y=the part of the whole that will pass through any given screen having segregating action, and z=the size of msh of that screen expressed as a function of the ratio of such mesh size to the largest particle size, which method consists in chemically decomposing the surfaces of the coarser particles of the material.

14. The method of preparing the finer or colloidal grains of a particulate material that is size graded according to the formula :19, in which y=the part of the whole that will pass through any given screen having segregating action, and zr=the size of mesh of that screen expressed as a function of the ratio of such mesh size to the largest particle size, which method consists in chemically decomposing the surfaces of the coarser particles of the material while agitating the material and causing such particles to rub against each other.

DONALD W. ROSS.-

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