A Course of Instruction i n Micromeritics J . M . DALLAVALLE Kruse Engineering Company, Newark, New Jersey
M.
UCH LESS is known about particles in the visible stze-range than about the infinitely smaller colloids. This does not mean that colloid theory is more perfect, but rather that the subject of h e particles has received less attention from investigators. As a result, information pertaining to them has never been treated in a systematic manner. Recognition of this need was noted by the Faraday Society in 1936 when it devoted its 64th General Discussion to the subject of disperse systems. The papers presented were outstanding contributions to our knowledge of the behavior of small particles in a gaseous field. The American Society for Testing Materials and the American Chemical Society (especially the former) through their publications and committees have done much to standardize techniques and procedures used in particulate technology. Agricultural physicists and geologists have also pioneered in many important phases of the subject. Obviously the interest shown by such outstanding societies and scientists justifies more extensive study of micromeritics. To anyone who has followed the technical journals during recent years the need for some comprehensive treatment relative to measurement and behavior of particles larger than colloids must be apparent. Furthermore, the number of industries and the variety of products where greater understanding of particle behavior is important are much greater than might be suspected by most physicists and engineers. In agriculture-and in fact throughout Naturesmall particles are responsible for many phenomena necessary to all l i e on this planet. Unfortunately, we have come to regard the fine tangible material everywhere about us as of little consequence, perhaps because there is so much of it. This is not intended to imply that particulate matter transcends in importance the effects of air, light, water, etc., on daily lives, but such matter does enhance those effects with results of inestimable value to man. The following pages stress the need for particular emphasis on the study of small particle behavior and characteristics. It is hardly possible to cover all these properties or even mention all their applications in modern technology, so in order to fix our ideas the objectives of this discussion will be outlined briefly. Although the subject of small particle behavior is admittedly not new, dealing with i t in a systematic man. ner is a recent development. The first step is to give the range of particles considered, since it is important to establish quantitatively the limits of size involved in this discussion. Once such limits are determined, further elaboration or refinement of subject matter is easily accomplished. Next it would be logical to examine static assemblies or
arrangements of particles, both regular and irregular, of the same or diierent size composition. Hoyever, this is a difficult and involved problem which caimot be outlined in a few words. Instead, therefore, we shall discuss the physical and chemical properties of fine particles to show how they are affected by certain arrangements, as in the case of fluid flow through packings, or the absorption of light by a cloud of dust. All this quite naturally includes the applications of fine particle technology to agriculture, industry, hydraulics, and many other fields. Of course, we cannot hope to touch upon all the diverse phases, but a few will suffice to indicate the importance of the subject. Lastly, having shown the place of fine particle technology in engineering progress, it will be very easy to outline a possible and plausible course of instruction in the subject which can readily be set up in colleges and universities. Small particle technology undoubtedly deserves an important place in the curricula of several branches of engineering, since any research and further developments in this field will have far-reaching effects. The term "micromeritics," meaning "science of small parts," is well suited to the general field of fine particle technology. Although this word has been used in a limited way in geology, it is applicable to the entire gamut of particulate properties and is so used in this text. Specifically, the intention is not that it be applied narrowly to particle measurement or to the physical and chemical properties per se, but rather to the phenomena caused by or in any way entered into by fine material above the colloid range.' SIZE-RANGE CONSIDERED
It is only natural to select some definite range of particle size coming within the meaning of fine particles. Since this range is above that generally accepted for colloids, its lower limit is thus readily obtained. The upper limit must be selected arbitrarily. If we agree that any small particle following Stokes' law of falling bodies (with the Milliken-Cunningham correction factor applied) is colloidal in size, then, the lower boundary of fine particles must be that point where the particles just begin to obey Stokes' law without the correction factor mentioned. However, the size-range covered by Stokes' law extends only to particles which are barely visible to the naked eye, and for practical reasons the range must be extended further to include sizes obeying Allen's law, as well as up to and
including a portion of those obeying Newton's law for falling bodies. The latter covers sizes well within the visible range-in fact, all matter beyond the range of Allen's law. In order to fix the size of the largest particle coming within the scope of micromeritics, it seems advisable to follow the procedure of soil physicists and set this limit a t 2 mm. However, the laws of behavior of particles 2 cm. in size are the same as those of 2 mm., except in the case of capillary phenomena; therefore, while the range of particle sizes considered is fixed by dynamic laws, only the lower bound can be considered definite. Fortunately, the upper bound need be no serious deterrent to further discussion; a t the same time it must be clear that the lower bound will depend upon the mass of the particle and the medium in which i t is contained. Hence, there will not be a single limit for all materials, as is the case with the upper bound. We could just as easily set the lower bound at, say, 0.2 p and state that the range under discussion is from 0.2 p to 2000 p. This is undoubtedly simpler, except that with certain substances one can never be quite sure whether particles of 0.2 to 0.5 p should not be regarded as colloids under certain conditions. Because of the peculiar behavior of the finest particles i t seems best to limit their size purely on the basis of Stokes' law. Thus far, in speaking of size the particles have been considered as spheres. Actually, particles are quite irregular and we must either adopt some convention as to the meaning of "size," or else use some statistical measure. Whether we utilize Stokes', Allen's, or Newton's laws of falling bodies to determine diameter, the implication is that the diameter so obtained is that of a sphere of the same density as the particle in question. When it is possible to determine the volume or mass of a particle, the equivalent diameter in tetms of a sphere is readily obtained. The statistical diameter referred to is generally used to determine the "average" size of a group of irregular particles, although it may also be used for single particles. Statistical diameters differ in one important respect from the two previously mentioned: Whereas the diameters determined from laws of falling bodies (or by mass or volume computation) are truly equivalent, statistical diameters are based on two-dimensional measures and assume that all possible orientations of a set of particles occur. This difference may seem of minor importance, but situations arise in practice when it can lead to serious difficulties. As a matter of fact, the methods of measuring particle diameter are legion. We have mentioned only three, but it is easy to cite a dozen more, or conceive of others yet to be applied. The technique of particlesize measurement requires standardization, although almost any method is satisfactory so long as i t is applied uniformly and its limitations are well understood. The term "diameter," when applied to particles in the following paragraphs, is used in a general sense-that is, merely to characterize the size in question. It must be
remembered that irregular particles possess definite volumes and surfaces, and these may be expressed as the cube or square of any previously agreed upon measure of size. But to assign any definite parameter of size, as in the case of a regular geometrical figure, is inconceivable. These points are given to show a basic difficulty in the treatment of micromeritical problems.
Ir~dwtriesor jicldr of AP*licalion
P~~ticirlors
Smoke and industrial
Relation of particle size and composition of soil to plant growth, soil moi.ture, ground water Design of absorption towers, heat exchangers, sand filters, crystal growth, rizing of compounds, solubilities, dust erplorionn, powdered fuel, floa of particulate material, ete. Ore crushing end milling, Rotation, re~aration, siziog. blast furnace practice Classification of sails, soil characteristier in term8 of oartide size. soil moisture relationhips. gro& water, cie. Wafer-holdin.~~. capacity of soils, siltine of streams, erosion Sizing of ceramic material, relation of strength of product to particle sire, product control Size for maximum strength, heat aenerated on wetting, product control Size and shape characteri~ticrin relation to abra. nive and c1eaneing properties Conveying and separation of particles by Buids. packaging (bulk propertied, granulation Control, effect on man
pollution Foods Pigment. Powder mcfallurgy
Taste control. omduet uniformit" Color, covering power Alloys, strength, bearing properties. etc
Soil physics Chemical engineering
Mining and ore bent faction Soil mechanics
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Hydrology Ceramics Cement, concrete, and mortars Abrasives and cleansers
Process engineering
Thus, the following lemma may be drawn with regard to the limits of size-range as given, namely: Methods of particle-size measurement other than those dependent upon application of dynamic laws are subject to arbitrary considerations peculiar to the technique used. PROPERTIES A N D APPLICATIONS
Some concept of the extensive application of micromeritics is shown in Table 1. Only a few of the more important applications are given, but it is evident that the scope of this subject is very broad. If we consider particle size alone-which is the basis of micromeritics-many industries using finely ground materials are involved. One of the foremost problems of micromeritics lies in the standardization of methods for measuring the size of materials and their frequency distributions. Once this is done, the fields encompassed by this science will be greatly increased in number and extent. In many respects fine particles exhibit the properties of fluids. Perhaps no better description of this resemblance can be given than that written by Geoffrey Martin (1928) some years ago: "There is a close resemblance in many ways hetween powders and liquids. Thus very fine powders will pour like liquids through pipes. Dropping a stone into a tank of very fine silica dust will cause the production of ripples just as if the tank held water. Powders can be distilled in a current of air just like liquids. A mixture of powders can thus he separated into simple ones, just as a mixture of different liquids can be dissolved into
simple ones by fractional distillation. Homogeneous powders penetrating far down into the soil to the lower portions resemble homogeneous (or chemically pure) liquids. completely saturated with water. Then, just as an "Thus a fixed volume of air passing at a fixed speed through a homogeneous powder will carry over or evaporate the powder ordinary capillary tube when immersed in water causes at a fined rate, just as a liquid at a fixed temperature will evapo- the water to rise above the water level, so these minute rate at a constant rate. The same volume of air will always capillaries of the soil "draw" the moisture up from one carry with it the same weight of powder (i. e.. be saturated with stage to another'until it reaches the soil surface. it) when passed at the same speed. The higher the speed of Most of us are familiar only with the vertical rise of the air, the larger the weight of powder carried. When the speed liquids in capillaries. But in soils, where the ramificaof the air reaches a certain critical limit the powder will lift and pass over as a whole with the air, thus becoming miscible with tions of minute pore-spaces extend in every direction, it in all proportions, exactly as a liquid will boil at s critical tem- we may have horizontal and oblique as well as vertical nerature. known as its boiline- -mint. Thus the ahenamenan of movements of water. Thus, the familiar irrigation boiling and evaporation in liquids is exactly simulated by pawders, the speed of the air passing through the powder playing the ditch utilizes an important soil property to move water same part as the temperature to which the liquid is subje~ted."~horizontally. Without this property the semiarid re-
The above quotation applies to the, homogeneous particles. But fine particles also exhibit other peculiarities bear in^ no resemblance to fluid. Martin's descrintion is valuable because it reveals a similarity to fluids not often suspected. Fine particles participate in so many natural phenomena that i t is impossible to give them more than passing mention here. For example, to cite the properties of soils composed of fine particles would take many volumes, yet this is a natural starting point for emphasizing those features of micromeritics which have so much influence on our existence. Consider a handful of earth. In i t are unnumbered millions of particles ranging in size from the minutest colloid to others as large as 2 cm. The gradation or size-frequency distribution is not uniform, but this need not trouble us. The composition of this handful of earth is roughly as follows: silica, alumina, lime, organic matter, moisture, and traces of many minerals. Now the very fine alumina is clay of a k i d that may be used for making bricks or coarse ceramic, or combined with silica and lime i t may be used to make cement. The organic material may or may not be particulate, its general function being to furnish food for plant growth. Moisture serves a similar purpose, although i t may also have the more obvious effect of bind in^ the together. There are other important functions of moisture. but these will be discussed later. All this comprises physical data which require no further elaboration, so let us now consider the aggregate of these particles and their capabilities. Clearly, single particles contribute little; together they establish characteristics deserving close attention, primarily because the particles are for the most part loosely bound. Plant roots pass through the matrix of interstices in quest of moisture and food. The growth of these roots causes the soil to expand, thus loosening i t and making it more receptive to moisture. If the soil particles were all large the interstices would not permit retention of moisture and plants would not thrive. Fortunately, in cultivable land there is a high percentage of fines, actually making plant growth possible. Winding through their interstices are millions of small canillan-like ~ a t h extending s in all directions. ~
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MAETIN. "A Treatise on Chemical Engineering," Croshy, Lockwood and Son, London, 1928.
gions of the West could not have been brought under cultivation. All this is due to the aggregation of small particles, and in itself is remarkable. But this moisture which we have discussed is spread over a tremendous s u r f a c e t h e surface of each and every particle. The amount of surface is unbelievably great: Suppose one cubic centimeter of the soil is regarded as an impermeable solid. Its surface area then will be 6 sq. cm. Now cut the cube in half and the surface becomes 8 sq. cm. If i t is quartered i t becomes 12 sq. cm. Proceeding in the same way until the cube is cut into 1,000,000 cubes, each of whose sides is 0.01 cm., we obtain a surface of 600 sq. m. Thus, as the size of the particles is reduced the aggregate surface increases. This large surface in contact with water increases the amount of material dissolved, practically in proportion to the amount of surface touched. In this way mineral food is brought into contact with the roots and absorbed, the rich mineral content of the plants then being passed on to man and all animal life. Little thought is given the fact that soil is a living entity which breathes and functions very much as any plant or animal. It also grows and decays. This is not said merely in an animistic vein of thought; it can readily be proved. The ever-changing pressure of the atmosphere causes air to move upward or downward through the soil. It brings oxygen to help digest the organic material and combine chemically with the inorganic to form new and more stable products. In turn, carbon dioxide is diiused throughout to feed the plants or escape into the atmosphere, and a t the same time moisture is forced to move about. Just how much climate itself is dependent upon this escaping moisture plus the heat retentiveness of the soil cannot be overemphasized. Within the complete analysis of soil behavior and its component fine particles may lie the true cause of droughts and creation of deserts. One other item which may be regarded as coming within the scope of soil physics is ground water. Be-neath the surface of the soil is a vast reservoir of water which gradually finds its way to the surface, the amount being dependent to some extent on the distance from the saturation region to the surface. During rains and melting snows the saturation level rises, and during droughts i t falls. The constant drain of ground water for irrigation or public water supplies in many instances may radically reduce the supply to such an extent that
rainfall is insufficient to maintain normal levels. Indeed, it is not unusual to have ground water levels receding in depth from the surface, and as a result the capillary phenomena bringing water to the surface may disappear entirely. When this takes place we have the beginning of a "dust bowl," so often attributed to lack of rainfall. I t is little realized that rain falling on a dry soil (or aggregate of particles) generates heat. This is a manifestation of surface energy, and obviously its effect on seed germination as well as plant growth is of tremendous importance. The fact that water added to fine particles produces heat regardless of whether or not they are chemically active is therefore of great consequence. If we inquire into the peculiar properties of small particles to hold and distribute moisture, we encounter a most complex problem since the nature of the open areas or pores is difficult to understand. However, it is easily measured and it can be shown that for loosely compacted soils it is never less than 26 per cent of the volume occupied by the particles. If pressure is applied, as in strata at great depths from the surface, this may decrease to as low as 5 per cent, which is precisely the case in oil sands. Thus, if the extent of a well can be measured and the pressure within the stratum determined, no great amount of labor is involved in estimating both rate of yield and amount of oil available. The properties of soils, but briefly outlined here, have been applied by engineers in construction of roads, earth-dams, and large structures on pervious ground. In fact, some of these properties are so highly specialized that a new branch of engineering has developed during recent years, namely, "Soil Mechanics." This engineering branch deals with foundation work and the loads capable of being supported by soils of different composition. There can he no question of the fact that this subject today comprises the most comprehensive experimental data available on the behavior of packings. Basically, however, these aspects of soil mechanics are but a part of micromeritics, depending actually on the properties of small particles in the aggregate. Thus far we have considered the handful of earth under discussion as rather closely compacted. In a dry dispersed state it possesses many other properties. When scattered in air, for example, the finest particles remain suspended for long periods of time and are analogous to dust. Hence we may expect that they will be subject to the action of electric charges and sound waves, and will affect the transmission of light and heat waves.. As a matter of fact, dispersion of fine particles by any means creates an electric charge upon them. This effect is observable in desert sand storms and can actually he measured in many other instances. The nature of the charge taken by the particles is still subject to a great deal of controversy, but if any qnestion of its importance arises we need only cite innumerable disasters which have occurred in flour mills and
elevators, sugar factories, and coal mines. The discharge of such particles ignited dust clouds and caused terrific explosions with the resulting loss of l i e and property. The action of dusts, apart from electrical properties, will be discussed later. The electric charge taken by particles undoubtedly enters into coagulation phenomena. In industry the fact that particles may be charged in different ways is used for separating various components nf heterogeneous materials. The effect of dispersed particles in light transmission is easily observed, and little additional comment is needed. However, few are aware that the blue of the sky and the red at dawn and sunset are actually due to the presence of extremely fine particles suspended in the upper atmosphere. The polarizing action of small particles has barely been appreciated, and should have many practical applications when better understood. As to the effect of particles in suspension on heat transmission, we may recall the use of smudge pots in fruit orchards to blanket them in order to withstand cold and frost. The smallest particles, from 0.05 p to 5 p, are affected by intense sound waves. Those of us who have studied elementary physics are familiar with the Kundt experiment for determining the nodes of stand'mg sound waves in tubes. However, when the material is dispersed (that is, completely suspended in the air column), the particles quickly coalesce and dry out. The potentialities of this phenomena have hardly been explored, and yet it presents many interesting applications. Two other properties of small particles when dispersed may be mentioned. The first of these concerns suspensions of different gravities. The fact that suspensions of any specific gravity up to almost seven times that of water are possible, constitutes a special attribute of small particles. When agitated such suspensions can literally be made to float stone. This property is utilized in the separation of slate from coal and in concentrating ores. In the latter case the ores must be crushed to such a size that a sharp separation is possible. Large particles containing ore and impurities together are difficult to sort. The second noteworthy property of fine suspensions may he termed physiological. Workers exposed daily to heavy concentrations of mineral dusts, especially silica, are known to develop serious lung impairment. The antion of dust on the lung leads to formation of fibrotic tissue, reducing the effectiveness of the organ and shortening the normal span of life. I t has not been possible to elaborate on the properties mentioned so far, nor to add numerous others of almost equal importance, since space is limited. However, it must be apparent that small particles play an important role in our daily lives, and are a subject of special interest. COURSES OF INSTRUCTION
The tabular arrangement in Table 1, giving the
applications of micromeritics, naturally suggests that it has a place in university curricula. In considering such a course there are certain questions to he answered : (a) is a course of study useful; (b) in what fields should it be included; and (c) what should such a course comprise? With regard to the usefulness of micromeritics little need he added to what has been presented. It is definitely valuable, since it covers the characteristics and behavior of small particles and thus explains phenomena peculiar to them. Moreover, any attempt to develop a systematic approach to the subject will in the long run add much to our knowledge of the uses for It is clear that unless such procedures fine are taken, progress will he hit or miss. Even at this time definite recognition is not lackine in reeard to the importance of standardized procedures. The American Society for Testing Materials realized the necessity of standardizing methods for testing fine particle characteristics. But there will be only limited progress unless more universal knowledge of these methods and their underlying theories are available. The second and third questions posed must necessarily be answered together, since the fields of application more or less determine the scope of the course given. In general, as may be seen from the table of applications, there are certain fundamental requirements which apply to almost every field of endeavor outlined, as follows: methods of particle-size measurement, size distribution,. madine of materials, characteristics of pack'mgs, and physical, chemical, and thermodynamic properties. These may be regarded as the foundations on which further specialization can he constructed. For the moment it supplies the tools needed for any field of study engaged in. Micromeritics is very properly within the scope of engineering-chemical, civil, mechanical, agricultural, ceramic, mining, and industrial. A basic course, as already suggested, would be identical for each of these fields, and a complete course suitable for students in such branches of engineering is presented herewith. Included is the estimated number of hours required, the entire course taking one semester. The course in micromeritics, as outlined in Table 2, is intended for juniors or seniors. The most difficult phase probably pertains to analysis of frequency distributions. Fortunately, the necessary mathematics involved is simple and well within the grasp of any student with elementary training in differential and integral calculus. A valuable aspect of treating size frequency distributions, entirely apart from their applications to micromeritics, is that it gives the student valuable knowledge of many practical theories of statistics. The laboratory exercises listed further enhance and accelerate an understanding of sampling procedures and product control which are invaluable engineering tools today. Specifically, the prerequisites for a course in micromeritics are as follows: elementary physics, dif-
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ferential and integral calculus, dynamics, physical chemistry, and chemical engineering. An advanced course in this subject, dealing with special industrial applications such as ceramics, cement manufacture, pigment technology, conveying, etc., is easily developed upon the foundation given. In fact, in each of these specialized engineering fields there is sufficient material available to add another semester's work. Finally, I shall attempt to answer another question which might be uppermost in the mind of the teaching profession, viz.: Is a course in micromeritics worth while adding to engineering curricula? This can TABLE 2
sooaesreo c o v a s e A
m
HOURS 0.
I N 8 m m r m N m MCCRODI~~TXCS
(onesemester)
LI~u"~ Ballistics or dynamic. of particles in Ruidr Measures of particle diameter Statistic. of size disfrihution Sieving and grading of matdals Wckingchaneteri.tics EIeetcical, optical, and sonic propertier Thermodyn.mie erties
j
s,
Puticle size measurement
5
curve fitting
4
Determination of voids Experiments on electrical.. ootical. . . and sonic properties Mearurement of heat flow, heat of wetting, adsorption Determination of sol". bility, dust crpla=ions Experiments on Bow throuzh onelriozs ~ x p e r i m e n ton ~ -infiitration and capillnrity Determination of partid e surface Determination of suspension density Experiments on transport
Grading of materials
3
prop2
Chemical properties
2
Plow through pnckings
3
infiltration and capil1arity
3
Particle surface
3
Muds and &xrries
4
Transport of particles
3
1
I
2 I
be answered positively for those engineering fields already discussed. Not only does such a course reach the very basis of many natural phenomena and explain them, but it helps broaden the student by giving him valuable training of great assistance in obtaining employment. There can be no question hut that the technology of fine particles has barely been touched upon and many excellent opportunities are forthcoming. Many industries today appreciate the importance of particle size in the products they manufacture. The outmoded techniques used are certain to be replaced. Sieve products will no longer be accepted as uniform from batch to hatch, but will require standardization by refined techniques available to those trained in micromeritics. There are also the benefits which will accrue to other sciences with the development of micromeritics into organized study and research. It is indeed a branch of engineering science which offers many opportunities, particularly since i t is new and its horizons unlimited.
