Normal Mix H. COMYN, IRA R. MARCUS, and ROBERT E. MclNTYRE RAYMOND
Agglomerate Mix
Diamond Fuze Laboratories, Ordnance Corps, Washington 25, D. C.
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T H E PROPERTIES O F physical mixtures of dry powders often depend on the thoroughness of blending. Many compositions intended for use in ordnance devices are particularly susceptible to this mixing effect and require special blending techniques for their production. As a result, the study of these mixtures has led to the development of a mixing method which may be applied to other types of compositions. The major part of these investigations has been concerned with the development of reproducible heat and delay powders (2, 7, 77). These are physical mixtures of one or more metal powders and one or more oxidizing agents that will ignite and react a t a predetermined rate when heat or flame is applied. Typical examples are blends of powdered zirconium and barium chromate containing from 20 to 30y0 zirconium. These compositions generate from about 380 to 470 calories per gram and are used for developing heat and time delays in ordnance applications. The combustion of these mixtures is characterized by high reaction temperatures, combustion products that are mainly solids, and the release of very little gas. The fact that no atmospheric oxygen is necessary for combustion makes these powders particularly useful for ordnance devices. The intimacy with which the components are mixed affects the reactions of these metal-oxidant compositions. For example, additional intensive blending of the original mixtures can increase both the burning rates and calorific values. The term, “intimate mixing,” however, cannot be defined precisely because no absolute standard exists.
ordnance compositions can
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Ceramics Sintered ,metal alloys Electronic specialties
I n general, the size of the smallest reproducible sample indicates the intimacy of mixing. A batch of heat powder might be blended satisfactorily for many purposes if all 5-gram portions had identical compositions. This batch would be blended intimately only when extremely small portions had identical compositions. Because of numerous requirements for mixtures with specific and reproducible properties, such as burning rates and calorific values, the agglomerate mixing method was developed to produce a series of different end products. This report summaIizes separate studies rather than one integrated mixing investigation.
mixture was placed in a clear plastic container and tapped gently, visible segregation occurred. Similarly, satisfactory mixtures cannot be made in water containing a dispersing agent which allows the particles to move independently of each other (2). Consider the effect of stirring powdered zirconium and barium chromate together in a dilute solution of Calgon, a commercial dispersing agent. Forces which act on each of the particles as they swirl around in the mixing tank are gravity, buoyant force of the water, centrifugal force, and a drag force opposing relative motion between the particle and the fluid. The drag force may result from the liquid swirling past particles or from particles moving through the liquid. The forces acting on each particle, and the resulting motion, depend on the size and density of the particle as well as the mechanical agitation used. Particles with different sizes and densities will not mix together in a random manner but will separate from each other as they would in elutriators, settling tanks, and centrifugal separators.
Agglomerate Mixing
The agglomerate mixing method is well suited for producing intimate and reproducible mixtures of fine particles. If small portions of the mixture, averaging about 50 microns in size, must be identical, the majority of the component particles must be below 5 or 10 microns in diameter ( 7 , 8 ) . Also, the mixing force must be sufficient to separate these particles and mix them together. However, there is another requirement that is not always obvious: the component particles must not move in relation to their immediate neighbors, once the blending is complete. The importance of component particles maintaining their relative positions is illustrated by the behavior of a mixture of powdered antimony pentoxide and atomized magnesium. These ingredients were blended dry to form a uniform mixture. However, when this
There are three basic requirements for producing agglomerate mixtures F a suspending fluid where particles do not disperse but form agglomerates naturally. This tendency must persist after the mixture i s dried and when the components are being blended ,particles whose size is well below the agglomerate diameter ,a
mechanical method the particles
of
mixing
Mechanical mixing methods discussed in this article include ball mills, colloid mills, and high speed kitchen blenders. Each of these produces liquid shearing forces that break agglomerates into individual particles and allow them to mix together. As soon as the particles move out of the region of fluid shear they VOL. 52,
NO. 12
DECEMBER 1960
995
immediately reagglomerate and act as particles of the mixture, not as individual components. There is no tendency of the components to segregate and the weight ratio of the components in each agglomerate remains constant throughout subsequent handling operations. Methods of Testing Agglomerate Mixtures
Three methods have been used to measure the uniformity of agglomerate mixtures (2) : mixability tests to detect measuresegregation ; calorimetric ments (7, !?) to determine the uniformity of heats evolved by small portions of the mixtures; and measurements of the uniformity of burning times of small quantities of mixtures loaded in bombfuze delay elements. Composition of individual agglomerates was not determined. Mixability Test. .4bout 1 liter of the suspended mixture (solid concentration of about 35 grams per liter) is poured into an Imhoff cone and allowed to settle. Under these conditions any tendency for segregation is exaggerated and usually can be noted visually. If there is no visual indicacion, a siphon is used to separate the suspended mixture into three fractions : the upper 10% of the mixture; the bulk of the mixture; and the lower 107, of the mixture. Small samples from the top and bottom portions may be analyzed and the results expressed as the ratio of these analyses. For example, the mixability of a blend of zirconium and barium chromate poivders may be expressed as the ratio of the percentages of chromate ions in samples taken from the upper and lower portions of the mixture after settling in an Imhoff cone. Calorific Value. The calorific values of the heat mixtures ( 7 > 9 )were measured in a Parr No. 1411 heat powder calorimeter. This is essentially a bomb calorimeter, using water as the calorimetric fluid, which has been developed specifically for measuring the calorific value of metal-oxidant mixtures. The sample is ignited in an inert atmosphere, and at the minimum heat input of 800 calories, accuracy of the instrument is within 10.35Oj, calculated for 957' of observations a t a 90% confidence level. Since the mixtures tested had calorific values near 400 calories per gram, a 2-gram minimum sample was required for accurate measurements. Burning Rate. The burning rates of the metal-oxidant mixtures were compared by determining the burning time of a definite quantity of each mixture loaded into a T2E1 delay element ( 7 7 ) . The T2E1, a standard delay element used in mechanical bomb fuzes for functioning delays in the millisecond range, provided a convenient method for measuring the burning rate of these com-
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Burning rates of metal-oxidant mixtures were compared b y - determining the burning time of a definite quantity of each mixture loaded into a T2E1 delay element
LEAD AZIDE RELAY
positions. In one of the tests discussed below, 90 mg. of a metal-oxidant mixture were pressed at 30,000 p.s.i. in a cylinder having an inside diameter of 0.125 inch. In a second test, the quantity tested \vas 160 mg. The difference in test quantities resulted because these units werc loaded for a specific purpose in each case. not for comparison purposes. The loaded cylindrical subassembly \vas inserted in a heavy metal body which acted as a highly efficien1 heat sink. An 8-megacycle counter chronograph was actuated when the mix was initiated by an M42 primer. 'The mixture burned through to an M6 lead azide relay which detonated and stopped the chronograph. The total elapsed time in milliseconds between the initiation of the mix and the explosion of the relay was taken as the burning time of the test quantity of the mixture. Evaluation of Agglomerate Mixtures
Liquid vehicles used in preparing test compositions include toluene, ethyl acetate, and water. Ball mills, kitchen blenders, and portable colloid mills were used to mix quantities varying 33.3 grams to 20 pounds. Data were collected from performance tests made on mixes prepared a t different time periods for specific uses. i i s a result, only one or two tests were made on many of the mixtures and complete data are not available on all the properties affected by blending. Mixtures Blended in Toluene. A batch of zirconium-barium chromate composition was prepared for loading in 50 i 10 msec. T2E1 delay elements ( 7 7 ) . This mixture contained 21y0 of Foote Mineral Co. Grade 120 A zirconium powder, and 79y0 of reagent grade barium chromate. Eight hundred grams of the dry ingredients were placed in a 1-gallon porcelain jar with 720 ml. of ACS grade toluene and 2400 grams of stainless steel balls, having a diameter of "8 inch. The jar and its contents were rotated for 3 hours at 75 r.p.m. After the mixture was separated from
INDUSTRIAL AND ENGINEERING CHEMISTRY
the balls it was dried at 71 O C. under vacuum. Fifty T2E1 delay elements were loaded and tested using 90 mg. of mixture in each device, and the data illustrate the uniformity of the mixtures. The burning times kvere within 47.14 2.77 msec., as calculated for 95% of all future observations a t a 907' confidencc level. Mixtures Blended in Ethyl Acetate. A number of batches of zirconium, ferric oxide, superfloss mixtures: in the ratios 65 to 25 to 10, were prepared for use as gasless igniters (70). These mixtures were blended in ethyl acetate using the ball mill procedure. Gasless igniters are used to transfer ignition from primers to slow burning delay mixtures and neither their burning rate nor their calorific value generally are considered important. The several thousand performance tests which were made showed that the mixtures were satisfactoiy as igniters, but no measurements were made that could be used as quantitative indica. tion of mixing. However, the fact that the user tests 1vei-e satisfactory indicated that the igniters \vere mixed intimately. In these uses 100 mg. of igniter are pressed at 30,000 p.s.i. in a cylindrical cavity, 0.203 inch in diameter and covered with a metal washer. Under these circumstances, very small volumes of igniter, 3 / 3 2 inch in diarncter and a few thousandths of an inch thick, are exposed to the primer flash, and any inequalities in mixing usually Tvould have resulted in ignition failures. Mixtures Blended in Water. As mentioned above, many dry metaloxidant compositions are ignited readily by friction: sparks, and static electricity. Such mixtures often react exjlosiuel? and the resulting rapid evolution of heat and scattering of molten slag constitutes a dPJnite hazard. Because of these safety problems most of the agglomerate mixtures made in this laboratory have been blended in water and stored under water until used. A number of these mixtures were made to meet specific requirements. Experimental Heat Powder Mixtures. A number of small experimental heat powder mixtures were required for both barium chromate ( Z ) , and mixing
A G G L O M E R A T E MIXING Table I.
Average Burning Times and Calorific Values W e r e Reproducible Between Batches When the Mixture W a s M a d e from the Same Lot o f Zirconium
BaCrOI lot No. Zirconium lot No. Mixability, visual observation Mixability CrOT- rat:o Calories/grarn Burning tirns, msec. Surface area of BaCr04, sq. crn./g.
x
130 144 706-9 706-9 Good Good 1.0041 1.0030 445.6 447.3 38.73 40.61
308 309 706-9 706-9 Poor Poor 1.069
22,800 57,000 3300
investigations. These compositions were prepared by blending 23.33 grams of barium chromate and 10.00 grams of zirconium powders for 20 minutes in a liter of distilled water with either a kitchen blender or a portable colloid mill. Before use, the excess water was decanted and the powder dried at 71 O C. in a vacuum oven. The appearance of the mixtures in the mixability test showed that all except two of the samples were homogeneous. The two poorly mixed compositions had been made of barium chromate with a large particle size (small specific surface area). The homogeneity of 11 or these batches was confirmed by chromate analyses of the top and bottom layers from the mixability tests, and by the reproducibly of burning time tests (Table I). Typical burning times are illustrated by the data for 20 T2E1 delay elements loaded with the mixture shown in column 7, Table I. The burning times were
4600
313 706-9 Good 1.0030 444.5 40.19
314 325 334 706-9 706-9 706-9 Good Good Good 1.0016 0.9955 1.0107 445.4 443.5 446.5 39.50 39.56 39.97
335 313 313 313 706-9 612-3 703-4 705-4 Good Good Good Good 0.9981 1.001 1.003 1.003 444.9 447.1 449.7 449.1 40.15 40.80 45.70 41.60
325 74-103-1 Good 1.001 457.8 38.40
16,600 18,800
within 39.56 f 1.76 msec., calculated for 95% of all future observations at a 90% confidence level. The average burning times and calorific values were reproducible between batches as long as the mixture was made from the same lot of zirconium. The influence of the latter upon these parameters is well known. Zirconium-Barium Chromate-Superfloss Mixtures. A number of 100gram mixtures of zirconium, barium chromate, and superfloss (diatomaceous silica, Johns-Manville) were made in water with a colloid mill for an igniter study. The uniformity of this type of mixture was established by analyzing small samples taken from different portions of two of the dried batches. The chromate contents of 100-mg. samples were determined volumetrically and the total percentage of barium chromate in each sample was calculated using a value of 0.995 for the purity of the barium chromate. Since each of these
Sedirnentalion o f zirconium-barium chromate heat powder in lmhoff cones followed this pattern 1. Agglomerate mixture in diitil!ed water. 2. Incomplete agglomeration because the barium 3. Same heat powder as in No. 1 but in dilute hydrochloric acid. 4. Same chromate is coarse. heat powder as in No. 1 but with Calgon dispersing agent added
batches contained 10% superfloss, the difference between 9070 and the percentage of barium chromate represented the zirconium powder in the mixture. The homogeneity of each mixture was established by determining the percentage of barium chromate in six 100mg. samples. taken from different portions of the blend. In one of the typical mixtures, the percentage of barium chromate was within 24.50 rt 0.3470, calculated for 95y0 of all future observations a t a 90% confidence level. The percentage of barium chromate in a second mixture made in different proportions was 32.28 f 1.18%. Zirconium-Ferric Oxide-Superfloss Mixtures. During a mixing study, three 100-gram batches of zirconium, ferric oxide, and superfloss igniters were blended in water with a portable colloid mill. One batch was blended in distilled water at a p H of 7 ; another at a p H of 3, using HCl; and a third at a p H of 9 using potassium hydroxide. Three tests were run to determine homogeneity of these mixtures: mixability, calorific value, and burning rate. I n the mixability test, the compositions made at pH 7 and 9 appeared to be completely uniform, but the composition blended at a p H of 3 separated into a light and a dark layer in the Imhoff cone. However, no marked difference between these layers was apparent in later tests. As discussed previously, small portions were removed from the top and bottom of the mixtures after they had settled in the cone, and dried under vacuum a t 71' C . Onegram samples were analyzed volumetrically for iron and this was used to calculate the ferric oxide content (Table 11). The latter also summarizes the results of the calorific and burning time tests. Data based on five shots for each test show that the mixture prepared a t a p H of 9 was homogeneous. Compositions blended a t a pH of 3 and 7 do not appear to be as uniform as the other mixture and to have different calorific values and burning times. The data suggest that a small amount of iron oxide may have dissolved a t the lower pH, leading to higher calorific values and faster burning times. VOL. 52, NO. 12
DECEMBER 1960
997
Application of Agglomerate Mixing
Table 11.
Ferric Oxide Content Was Calculated b y Analyzing 1 -Gram Samplesa
9 7 3 pH of mixing slurry Mixability test b E E Appearance 51.5 51.3 51 .O 70FeeOs, bottom layer 51.3 51.3 51.1 % F e ~ 0 3 top , layer Caloridc value 488.3 481.3 491.9 Cal./g. bottom layer 488.0 488.9 492.4 Cal./g. top layer 19.62 0.93 18.88 f 2.86 17.37 i 7.54 Burning timesCin top layer, msec. Burning timesCin bottom layer, 2.43 20.02 19.26 3.22 17.09 =k 3.44 msec. E = excellent. All three mixtures contained 5% superfloss. The difference between 9 ~ 5 7and ~ the ferric oxide percentage shown represents the percentage of zirconium powder in the Burning times of two BO-mg. T w o distinct well-mixed layers of equal volume. mixture. increments in T2E1 delay elements; calculated for 95% of all future observations at a 90% confidence level.
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Factors Affecting Agglomeration
Attractive forces which induce agglomeration of the particles probably are long range London-Van der Walls forces (3, 4 ) . However, factors influencing these forces, such as impurities or chemisorbed surface layers are not sufficiently well defined to permit estimates of agglomerating tendencies in any given situation. In general, any factors that aid in agglomerating particles of a mixture or in preventing their dispersion will improve their blending. Conversely, any factors that tend to keep the particles dispersed will prevent agglomerate mixing. Pure organic nonpolar vehicles have been used for blending a number of inorganic compositions whose particles do not disperse readily in liquids of this type. Inorganic particles often have a chemisorbed moisture layer on their surfaces which prevents them from dispersing in nonpolar solvents. I n fact, these mixtures might be considered as two immiscible phases: one phase being the oil vehicle and the other the moisture layer on the surface of the par ti cles. The agglomeration and deglomeration of particles in water suspensions have been studied as part of mineral flotation investigations (3, 4, 5 ) . These studies showed that a high zeta potential,
caused by the absorption of ions on the particle surfaces, prevented the agglomeration of mineral particles. I n one case, fine alumina clay particles agglotnerated on the surfaces of quartz when the pH of the suspending water was below 9.5. Under these conditions, the quartz had a negative zeta potential and alumina had a positive potential. However, when the pH of the suspension was raised above 9.5, both types of particles acquired a negative zeta potential and repelled each other. As a result, the particles dispersed readily and had no tendency to agglomerate. High zeta potentials caused by the addition of a surface active agent, such as Calgon, to suspensions of zirconium and oxidant powders, cause particles to disperse and prevent agglomeration. However, the zeta potentials of particles may not be the only factor affecting agglomeration. Table 111, shows the zeta potential of zirconium, ferric oxide, and superfloss powders a t a pH of 3, 7, and 9. The most reproducible mixtures were prepared a t a p H of 9, where the particles all had a negative zeta potential and presumably should have repelled each other. I t is possible that the agglomerating tendencies of the iron oxide, which had an extremely low zeta potenrial, were sufficient to envelope the other particles mechanically.
Table Ill. The Most Reproducible Mixtures W e r e Prepared a t a pH of 9, Where the Particles All Had a Negative Zeta Potential and Presumably Should Have Repelled Each Other ( Z e t a potentiai“)
Zirconium Polder
a
Superfloss Powder
+ + + +
Ferric Oxide Powder
Flocculationb -29.0 Flocculation* -27.7 f 0.5 Flocculationb -27.2 - 11.5 + Flocculation? -20.9 -28.0 Rapid flocculation -30.0 -27.0 - Rapid flocculation -32.9 - 30 .O Rapid flocculation -31.8 -36.0 Measured by electrophoresis method ( 6 ) . Flocculation prevented accurate zeta potential
measurements.
998
I N D U S T R I A LA N D ENGINEERINGCHEMISTRY
I n mixing two or more powders by the agglomerate method the problem usually involves the selection of the proper mixing vehicle. Most investigations usually require mixtures composed of definite materials and there is seldom any choice in their selection. However, the possibility of coating powders with a chemisorbed layer of molecular thickness may be advisable if all other methods of inducing agglomeration fail. Mixing vehicles usually are selected on a trial and error basis by shaking in a test tube. When the liquid is well suited for the agglomerate blending of a particular composition, the particles will agglomerate and settle rapidly, and the top inch or tivo or the liquid will be clear in 5 or 10 minutes. When the liquid is not suitable for blending this composition, the particles disperse readily and settle slowly into stratas of the different components; the top section of the liquid may not become completely clear for several days. Alter a mixing vehicle has been selected a 30-gram mixture should be prepared in a liter of the liquid and allowed to settle in an Imhoff cone. If no visible segregation occurs, different portions of the mixture may be removed and dried for either analyses or performance tests. literature Cited (1) Comyn, Raymond H., Couch, M. L., McIntyre, Robert E., “Measurement of Particle Size of the Components of
Gasless Mixtures,” DOFL R e p . TR636, August 28, 1958. (2) Comyn, Raymond, H., Couch, M. L., McIntyre, Robert E., “Specification of Barium Chromate for Use in Gasless Mixtures,” DOFL Rept. TR-635, September 17,1958. ( 3 ) De Bruyn, P. L., “Electrochemical Double Layer,” Physical Chemistry of Flotation, Summer Session, 1957, MIT, Cambridge, Mass. (4) Fuerstenau, D. W., “Electrokinetics and Flotation,” Physical Chemistry of Flotation, Summer Session, 1957, MIT, Cambridge, Mass. (5) Gaudin, A. M., Fuerstenau, D. W., Mining Eng. 7, 66 (January 1955). (6) Kruyt, H. R., “Colloid Science,” Vol. 1, Elsevier Publishing C o , New York, 1952. (7) Marcus, Ira, Standard Calorimetry for Quality Control, Pt. 111, DOFL Rept. TR-576, January 15, 1958. (8) Musgrave, J. R., Harber, H. R., “Turbimetrie Particle Size Analysis,” T h e Eagle-Picher Research Laboratories, Joplin, Mo., 1947. (9) Parr Instrument Co., Moline, Ill., “Instructions for the pio. 1411 Combustion Calorimeter;” Manual 128, October 1958. (10) U. S. Naval Bureau of Ordnance, Naval Ordnance Laboratory, White Oak, Md. Drawing 1170731, March 1955. (11) Yancey, J. D., Jr., Weingrad, R. H . , “Development of T2E1 Pyrotechnic Delay Elements, Pt. I, DOFL Rept. TR-346, June 1, 1956. RECEIVED for review May 3, 1960 ACCEPTED September 28, 1960
