Relative Caking Tendency of Fertilizers J. RICHARD ADAMS
AND
WILLIAM H. ROSS
Bureau of Plant Industry, U. S. Department of Agriculture, Washington, D. C .
dition of the materials a t the time of mixing, (c)-the proportion of each material in the mixture, and ( d ) the method of mixing. Thus, it has long been known that a mixture containing a large proportion of readily soluble salts cakes more readily as a rule than one which is relatively insoluble; that a finely divided mixture tends to cake more than a granular or coarsely crystalline one; and that the tendency of a mixture to cake decreases with increase in the proportion of any conditioning material that may be present. Chemical reactions between the components of a mixture may induce caking-for example, when ammonium sulfate is mixed with superphosphate. It is therefore common practice in the preparation of mixed fertilizers to mix ammonium sulfate and superphosphate long enough in advance of the other components to permit reaction to take place, and then mill and mix with these other components rather than mill and mix all components a t the same time. A study of the literature indicates that little work of a quantitative nature has been done on the caking of fertilizers. Such tests as have been made are more or less of a rule-ofthumb nature. A caking test of this kind consists in storing bags of fertilizers to be tested in a warehouse or other building for a predetermined time and, in cases where caking has occurred, dropping the bags one or more times on a concrete floor from a n arbitrarily chosen height. A comparison can then be made between the different fertilizers by sieving and determining the proportion of the fertilizer that has been broken up by this treatment. This method gives fairly reliable data on the relative caking tendencies of different fertilizer materials and mixtures, but It offers the disadvantages that a considerable quantity of material is required for a test, the necessary time of storage is prolonged, and no provision is usually made for controlling change in temperature or humidity during storage. Therefore, the results from any given test cannot be interpreted in terms of those obtained under other conditions of temperature and humidity. I n the present paper a method and apparatus are described for determining in advance the relative extent to which any material or combination of materials will cake under a given set of conditions. The method as developed offers the advantages that the tests can be made with a comparatively small quantity of material, that the time required for a test is relatively short, that provision is made for controlling temperature, humidity, and moisture changes, and that the results obtained in any given test are strictly comparable with those obtained in any preceding or subsequent test.
The factors that are instrumental in causing caking in fertilizer materials are enumerated, and the influence of these factors in inducing a caked condition in fertilizer mixtures is discussed. A n apparatus has been designed and a procedure developed for measuring the relative caking tendency of fertilizer materials and mixtures. The apparatus, which is in the form of a cylindrical bomb, is capable of exerting a constant and known pressure at any given temperature on any material placed within it, and can be readily disassembled for the removal of the resulting cake. In making a test of the caking tendency of any fertilizer material, a quantity sufficient to give a cake one inch thick is placed in the bomb under a constant pressure of 12 pounds per square inch, and the whole assembly is stored for 7 days at 30' C. (86" F.). A t the expiration of this time the cake is removed from the bomb, and its crushing strength is determined by noting the pressure required to break it in a hydraulic press. The relative crushing strength of cakes of different fertilizers so determined is taken as a measure of their relative caking tendency. Data are given on the relative caking tendency of different fertilizer materials and mixtures as determined by this accelerated test.
NE of the major difficulties encountered in the manu-
0
facture of fertilizers is their tendency to cake or become sticky, A caked or sticky condition may also develop inlcertain fertilizer materials and mixtures after being placed onqthe market. This condition greatly impairs the drillability of a fertilizer and thereby increases the difficulty of bringing about its uniform distribution in the field. The problem of the mechanical condition of fertilizers is therefore of interest both to the producer and the consumer of fertilizers. Different materials become caked from different causes, but the presence of moisture is usually the governing factor (7, IS). Moisture may induce a caked condition (a) by causing the material to set much in the same way as plaster of Paris, ( b ) by increasing the attraction between particles through surface tension, cohesion, and other forces, and (c) by causing the crystals to knit together by solution and recrysfallization as the humidity of the air alternately rises and falls, or as the material undergoes changes in temperature. The rate and extent to which caking takes place in a fertilizer material or in a mixture of materials vary with the moisture content, the size of the particles, the pressure under which the fertilizer is stored, the temperature variations during storage, and the length of storage. The effect of these factors differs with different materials, and the caking of a fertilizer mixture will therefore vary with ( a ) the materials used in the mixture, (b) the mechanical con-
Method APPARATUS. A detailed drawing of the specially designed bomb used in this study is shown in Figure 1. The bomb is made of stainless steel and consists of a cylinder that is divided longitudinally into two parts, a cylindrical ca and cap assembly, and a detachable base. Each end of the cygnder is provided with a male thread, and the base and cap are provided with female threads for attachment t o the cylinder.
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The cap assembly consists of a metallic bellows, a brass head soldered into the open end of the bellows, a s uare shouldered bushing threaded and soldered into the head, an% a pressure gage screwed into the bushing. The square shoulder of the bushing is soldered into a squared hole in the center of the cylindrical cap, and after the gage is screwed home into the bushing, the cap assembly turns as a unit with the turning of the cap. The use of litharge on the pipe threads of the gage joint makes a permanent airtight connection between the inside of the bellows and the pressure gage.
ure Gouge
Meiallic Bellows
Spacer Bars equal& aced o n /Sam. wrcte. Plate ,$' +hk.x 2"d'a.
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pressure recorded on standard gage by combined load of piston and platform of dead-weight tester, lb./sq. in. = inside radius (1 in.) of bomb =
The compressed bellows, when filled with air, is capable of exerting a maximum pressure of about 9 pounds per square inch. Higher pressures were obtained by partially filling the bellows with a liquid such as alcohol. Because of its odor the alcohol also serves to indicate the presence of any leak in the system. PROCEDURE. I n the manipulation of the apparatus for a caking test, the base is screwed on the lower end of the split cylinder and the fertilizer sample is poured into the cylinder. The spoolshaped plunger is dropped on the sample in the cylinder, and the apparatus is given a swirling motion to level the surface of the sample. The cap assembly is then screwed on the top of the cylinder until the bellows makes contact with the upper face of the spool-shaped plunger. Further rotation of the cap causes compression of the bellows, and the rotation is continued until the gage reading corresponds to the pressure under which the fertilizer sample is to be stored. All the joints of the bomb are sealed with a coating of collodion to prevent evaporation of moisture from the sample and the bomb is then held at the desired temperature for a given length of time. During the first day or two the pressure on the fertilizer in the bomb is adjusted at intervals to compensate for settling and compression of the material. On completion of the storage period, the bomb is opened and the fertilizer cake or briquet formed is removed. Figure 2 shows an assembled and a partially dismantled bomb with the cake of fertilizer in place. The fertilizer cake formed in this way is in the shape of a cylinder, 2 inches in diameter, with the top and bottom faces perpendicular to the sides. Some fertilizer materials showed a tendency to stick to the walls of the bomb with such tenacity as to make it difficult to remove the cake without danger of breakage. This difficulty was eliminated by coating the inside of the bomb with the ordinary U. S. P. flexible collodion. Sufficient crystal violet was added to the collodion to give it a deep purple color which served to indicate when the metal was completely covered with the collodion. A filter paper placed between the top of the spool-shaped plunger and the bellows served as a bearing to protect the base of the bellows from frictional wear.
OF CAKING BOMB FIGURE 1. DIAGRAM
Dimensions are in inches.
The pressure gage is calibrated so that i t registers in pounds per square inch the pressure exerted by screwing the cap down on the split cylinder, thereby compressing the bellows against the spool-shaped plunger shown in Figures 1 and 2. The plunger transmits the pressure from the bellows to the material being tested, and its shape ensures that the upper surface of the resulting cake or briquet will be parallel to the lower surface. The use of the plunger also protects the lower end of the bellows from any corrosive action that might result from contact with the material under test. The pressure gage of the cap assembly may be calibrated by any suitable means, such as a Crosby dead-weight pressure gage tester. In making a calibration with this device, the ca and cap assembly are supported in an upright position with the rower end of the bellows in contact with a small piston which rests on the latform of the dead-weight gage tester. The pressure on the gellows is then gradually increased by raising the platform of the dead-weight tester, while at the same time means are provided for preventing upward movement of the cap assembly. As the res sure of the bellows is increased, notations are made of the reazings on the standard gage with the corresponding readings on the gage of the cap assembly. The construction of the dead-weight tester is such that the pressure indicated by the calibrated gage was always five times that of the dead weights used to calibrate it. The value A , in pounds per square inch, of a reading on the pressure gage of the apparatus is then given by the formula
where B = corresponding reading of standard gage of deadweight tester
L
I
FIGURE 2. BOMBASSEMBLY
In opening the bomb a t the completion of a run, it was found convenient to unscrew the base of the bomb slightly before releasing the applied pressure. As soon as the base is lowered, the pressure exerted by the bellows has a tendency to break the cake of fertilizer away from the side walls, thus minimizing danger of breakage in its subsequent removal from the bomb. The crushing strength of the cakes formed in this investigation was measured by the pressure required to break them as determined by the hydraulic press in Figure 3. When a crushing strength test of a cake i s made, it is placed on the movable platform of the hydraulic press and a pilot plate is placed on top of the
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INDUSTRIAL AND ENGINEERING CHEMISTRY
cake as shown. The plate has a smooth lower surface, and its upper surface is provided with a socket that fits a ball rigidly attached to the upper latform of the press. A steadily increasing pressure is developexin the press, and is exerted through the ball and socket at right angles to and with e q u a l intensity over the entire surfaceof thecake. The relative crushing strength of cakes of different fertilizers determined in this way is the measure of their relative caking tendency.
Relative Caking Tendency as Measured b y Cakes of V a r i o u s Thicknesses
The relation found between the thickness of a cake, h, as formed in the bomb, and its crushing strength, when other factors are the same; is shown by the data in Table I and the curves in Figure 4. The results obtained show that, within the range used in the tests, the crushing strength of the cakes decreased with increase in thickness until a constant minimum crushing strength was obtained when h had a value that varied with the different materials from about 1.2 to 1.5 times the diameter, d , of the cake. These values therefore represent the approximate tangents of the angles of shear for the different materials. The force applied to a cake in a crushing strength test may be resolved into two components, one along the shearing face and the other normal to it. If Pt be taken as the shearing stress, P the applied pressure per square inch, and e the angle which the direction of the shearing stress makes with the horizontal face of the cake, then Pt = P sin 8 cos 8
When the ration of h/d for a cake of material is less than the tangent of its angle of shear, a single continuous shear plane cannot lie wholly within the column of material. Under such conditions shear will occur along several discontinuous planes, and the observed crushing strengths will increase with decrease in the value of the ratio h / d . The shearing stress on the plane extending from the edge of the upper surface of the cake to the opposite edge of the lower surface may be calculated from the above equation as follows :
The minimum angle of shear in a cylindrical block of a material under pressure is 45 '. Failuredoes not occur a t this angle with the horizontal h 2r b u t in a plane P,=P( d h l 4r' d h ' + 4r4 = P & Z4r' ) where the shearing resistance where h is the thickness and r is the radius, minus the shearWhen h = r, ing stress equals zero (IO). I n the case of concrete this plane of failand when h = 0.75r, ure makes a n Pit = P ( g i ) = 0.33 P angle with the FIGURE3. HYDRAULIC PRESSWITH horizontal whose BRIQUET IN PLACE tangent is someTherefore, the ratio of these stresses for a given load is 1.2. This ratio, within experimental limits, was found to exist bewhat less than 2. tween observed crushing strengths. Thus, the pressure acTherefore, in making crushing strength tests with a material tually found necessary to crush a cake of potassium chloride of this kind, it is customary t o apply the test on a cylindrical block of which the height is twice the diameter The apparatus shown in Figure 1 is not adapted to the formation of thick cakes of uniform hardness, for the reason that any pressure applied t o the top of a column of powdered or granular material is not uniformly distributed throughout its length (16). This is shown by means of a caking bomb in which the detachable base is replaced by a second cap and cap assembly. With such an arrangement it was found that a pressure of 12 pounds per square inch applied a t the top of a %inch column of finely divided superphosphate in a cylinder 2 inches in diameter registered an initial pressure a t the bottom of the column of only 6.5 pounds. At the end of a 7-day storage period, however, this differential of pressure showed a marked decrease, and the shorter the briquet in the bomb, the more equal became the Crushtna strenqth of briquet Ibs.per sa.tn. pressure a t the top and bottom of the ON CAKINQ FIQTJRE4. EFFECTOF WEIQHTOF MATERIAL briquet.
-)(+
-)
+
.
-
TABLE I. RELATION BETWEEN Wt. of Material Grams
a
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THE
THICKNESS OF BRIQUETS 2 INCHES IN DIAMETER AND THEIRCRUSHING STRENGTH"
(Temperature, 30' C.; particle size, through SO mesh: pressure, 12 pounds per square inch: caking time, 7 days) -Su erphosphate--NHnHsPOa---. --(NHa)rSOa--. --CO(NHs)a,-KCIr-NaXOs-,Heigft of Crushing Height of Crushing Height of Crushing Height of Crushing Height of Crushing Height of Crushing briquet strength briquet strength briquet strength briquet strength briquet strength briquet strength Inches L b . / s q . i n . Inches L b . / s q . in. Inches L b . / s q . in. Inches L b . / s q . in. Inches L b . / s q . in. Inches L b . / s q . in.
Average moisture contents:
superphosphate, 1.1%; X H I H ~ P O 0.2%; ~, (NHdzSOn, 0.1%: CO(XHs)z, 0.1%: KC1, 0.9%; NaNOa, 0.1%.
0.75 inch thick was 1.20 times greater than that required for a 1.0-inch cake. The corresponding value for caked sodium nitrate was 1.19, and for urea 1.33. The maximum shearing stress, Pt, for any given value of P occurs in a plane which makes an angle of 45" with the plane normal to the applied pressure. I n this case Pt = 0.5P. Failure, however, does not take place along this plane but along a plane which makes a n angle of 45" plus one half the angle of friction with the plane normal to the applied pressure. This shows that in crushing strength tests h should be greater than d, but the results given in Table I indicate that for the fertilizer materials used in the tests, h need not exceed 1.5d. A study was also made of the relation between cakes 2 inches thick and 1 inch in diameter, and those 1 inch thick and 2 inches in diameter. The thicker cakes were prepared in the apparatus of Figure 1, with the attachments shown in Figure 5. These consist of a hollow split bushing for reducing the internal diameter of the split cylinder to 1 inch, a piston cap a t each end of the bushing, and an additional cap assembly mounted on the base end of the split cylinder. The pressure applied to the cakes 1 inch thick amounted to 12 pounds per square inch, and to the 2-inch thick cakes, to 48 pounds per square inch. The two sets of cakes thus differed not only in shape but also in the pressure to which they were subjected. All tests were made a t 30" C. The results obtained in crushing strength tests with fertilizer cakes of these dimensions are given in Table 11. These results indicate that the relative crushing strengths of the cakes of different materials used in the test fall in the same order, whether the thickness of the cake is either twice or half its diameter, and either type of cake may be used to determine the relative caking tendencies of fertilizer materials. Direct measurements on the fractured cakes formed in the doublegage apparatus, for which h/d = 2, indicate that the tangent of the angle of shear does not exceed 1.5 and normally approximates 1.2. This supports the conclusion that, for the fertilizer materials used in the tests, h need not exceed 1.5d. Although h should be greater than d, if no other factors are considered, there are several disadvantages to the adoption of a cake of this thickness for crushing strength tests with fertilizer materials and mixtures. PRESSURE GAUGE
A thick cake requires a longer bomb than a thinner one; the time of settling and consequently the time for making a test increases with the thickness of the cake; and a cake that has a thickness equal to or greater than its diameter is less uniform throughout than one in which its thickness is less than its diameter. Since i t has been shown also that the relative caking tendencies of different fertilizers can be determined by means of cakes l inch thick and 2 inches in diameter, as well as by those of opposite dimensions, a cake of this thickness was used in all subsequent work. By noting the weight of material required to give a cake 1 inch thick when the diameter of the bomb was 2 inches, it was simple to duplicate cakes of approximately the same thickness b y using the same weight of material in replicated tests. The apparatus shown in Figure 1 would thus seem to be adapted for measuring the caking tendencies of fertilizers and is more convenient to handle than that shown in Figure 5. It was accordingly used to study the effect of such factors a5 pressure, paiticle size, moisture, and time of storage on the caking of different fertilizer materials and mixtures.
TABLE11. RELATIVE CRVSHING STRENGTHS OF DIFFERENT FERTILIZER CAKES OF 1-INCH AND %INCH DIAMETER Material Calcium metaphosDhate Mbnoammonium phos.phate Potassium chloride Urea Ammonium sulfate Sodium nitrate
Moisture Content
v0
0.23 0.16 0.19 0.35 0.10 0.26
-1-Inch Thickness Inches
.. 2'/a 11,'s 17/, 116/16
l'J/is
Cakes-2-Inoh Crushing Thickstrength ness Lb./sq. in. Inchsa
CakesCrushing strength L b . / r q . in.
None
l l / ~
