Factors Affecting Granulation
of Fertilizer Mixtures AND WILLIAM H.ROSS Fertilizer Research Division, Bureau of Chemistry and Soils, Washington, D. C.
JOHN 0. HARDESTY
from the air a t normal humidities. The general use of materials of this kind in mixed fertilizers would result in an impairment of the physical properties of the mixture unless means were found for improving their drillability without the use of conditioners. Mixtures of good mechanical condition may be prepared without the use of organic conditioners by proper selection of their components and by ammoniating the mixture, b u t the most generally effective method for improving the physical properties of a mixture is a treatment known as granulation. The first work on the granulation of fertilizers was undertaken in the Bureau of Soils in 1922. A number of the synthetic products are readily fusible, and it was found (1, 3,1016) that they can be converted into a granular form by heating the material to a temperature just above the melting point and spraying the molten product into the top of a tower through which a stream of air is passed in an upward direction. The sprayed molten products congeal in their fall through the tower to form spherical particles of uniform size and shape. This method has since been developed commercially for the granulation of urea (8), Calurea (167, CalNitro (7) , and sodium nitrate (4). Granulated materials have become very popular for separate application in the field, but their use in fertilizer mixtures promotes segregation unless all the components of the mixture are made up of particles of the same size. The separate granulation of all the materials in a mixture would add greatly to its cost. A study was accordingly undertaken in this laboratory a number of years ago of methods for the granulation of complete mixtures in one operation (9,10, 13). The Inethod which gives most promise of commercial application is known as that of granulation by rotary drying (2, I S ) . This method is applicable to the granulation of either fusible or infusible materials and mixtures. The process may be carried on simultaneously with ammoniation, and the heat developed by the reaction with ammonia may be used to aid in the granulation of the mixture (5). The steps in the rotary drying process of granulation as developed in this laboratory consist in adjusting the moisture content of the mixture, if necessary, by the addition of water or steam, to the optimum for the mixture to be granulated, raising the temperature of the mass to 60-80' C. by ammoniation or otherwise, rolling the mixture in a drum until granulation occurs, and then drying the granulated product. The present investigation relates to a study of the conditions under which the granulation of mixed fertilizers takes place and of the factors affecting the hardness of the granule.
Fertilizer mixtures of good mechanical condition may be prepared by use of conditioners and by proper selection of their components, but the most generally effective method for improving the mechanical condition of a mixture is granulation. By this treatment the particles of the mixture are agglomerated into granules of uniform size and shape. Granulated materials have become popular for separate application in the field, but the use of such materials in fertilizer mixtures promotes segregation unless all the components of the mixture have particles of the same size. A study has been made of a method for the granulation of a complete mixture in one operation. The method is particularly adapted to the granulation of high-analysis mixtures low in organic matter. The present paper describes the conditions under which a mixture can be converted into granules of uniform size and shape and of sufficient hardness to withstand the handling and support the loads to which they are subjected under commercial conditions.
T
HE organic materials used in mixed fertilizers serve the double purpose of supplying nitrogen and of improving the physical condition of the mixture. I n the early history of the industry the cheapest, source of nitrogenwas the organic ammoniates, and their use in the average mixed fertilizer amounted to about 30 per cent of its total weight. Such a high proportion of organic materials had a marked effect in conditioning the mixture, and little difficulty was experienced in the handling and storage of mixtures of this kind. At present the cost of organic nitrogen is three to four times that of ammonia nitrogen, and two to three times t h a t of nitrate nitrogen. The use of organic nitrogen in fertilizer mixtures is therefore being rapidly replaced by the fixed nitrogen products and other sources of inorganic nitrogen. The fixed nitrogen products have little or no conditioning effect and some have the property of rapidly absorbing moisture
Equipment Figure 1 shows, at right, t h e experimental rotary drum in which the mixtures were rolled or tumbled to bring about granulation. The drum is constructed of Allegheny metal. It is 32 inches in diameter and 12 inches in width, and contains three 2.5-inch fins equally spaced around the inner surface. Ammonia or steam may be added through one of the trunnions supporting the drum. A gas thermometer extends through the other trunnion and dips into the material within the drum. When necessary, external heat was applied to the drum by means of a gas burner. The drum is motor driven at 10 r. p. m. Figure 1 also shows the rotary dryer in which the mixtures were 668
JUNE, 1938
INDUSTRIAL AND ENGINEERIXC CFIEhIISTKI
dried after being granulat.ed. The drying drum is 42 inches in length and 20 inches in diameter, and contains six 2-inch fins equally spaced on the inner surface and extending the entire length of the drum. The drum is motor driven at 6 r. p. m. The dryer is d s o provided with a n electric heater of variable capacity and a fan for blowing the heated air through the appaTatW whereby a wide range of temperature and air flow may be obtained as desired. This Iahoratory equipment was designed to handle mixed fertilizer batches of 50 to 60 pounds. There seems to he no apparent reason, however, why granulation and drying may not be cclrried out continuously in one piece of equipment.
Factors Affecting Granulation
669
granulation of this mixture when it had an initial moisture content of only 5 per cent. The proportion of granules larger than 5 mesh increased as the moisture was increased; a decrease in moisture below this percentage increased the proportion of granules smaller than 30 mesh. "hen this same mixture was ground to pass a 40-mesh screen, a minimum of 9 per cent of mater was required for satisfactory granulation. TIBLC1. SCREEN A N A L Y ~ EOSF GRANULATED FEHTILIZER MIxTonEs
Granulation occurs in the rotary drying process when the attractive forces (cohesion, surface tension, etc.) acting hetween the particles are sufficient to withstand the rollin: treatment to which the granules are subjected. Coarsely crystalline materials will not granulate, but most fertilizer 6 4-8.-7 12 80 18 73 8 1 0 8 12-16-32 56 16 I 27 I 1 70 mixtures are sufficiently plastic in a moistened condition to 9 12.-16-12 6 0 60 50 37 7 6 12-16--12 granulate with little or no grinding. A uniform prodrict can 10 50 8 0 33 1 63 3 I1 8-16-16 9 1 45 0 53 43 3 be obtained, however, only when the average diameter of the 14 8-16-16 16 21 II 11 s 70 49 I9 8-24-8 10 BO 0 51 38 3 8 prepared granules is several times greater t.lian that of the 20 &-l2-10 12 82 4 1 BO 63 22 R-12-20 60 13 70 13 7 8 largest particlesin theoriginal mixture. 24 8-10-16 7 6 12 6 I0 6s 8 Table I gives the screen analyses of a number of granulated 33 8-16-8 76 10 2 I3 42 1 0 0 8.5 6.124 10 I 16 I 75 82 mixtures of different types. All mixtures were passed through a7 1 75 6-12--0 23 6 33 2 58 39 4-8-4 73 2 2 a 40-mesh screen before granulation with the exception of 6 10 88 7 1 8--16-16 .38 a ; 6 "1 15 I1 5 48 and 58. The formulas of the mixtures are given in Table 111. 59 8-ICI-I6 9 4 80 72 7 8 1 74 65 10 I 53 0 6--B-3 40 6 In thcse tests an attempt was m d e to secure a maximum pro1 7s F-S+4 7% I2 6 57 36 1 portion of granules of 5-20 mesh. The maximum propor81 6-8-12 70 15 B 70 23 1 0 83 4 12-8 7% 10 22 0 16 30 38 tion actually obtained was 98 per cent. Table I siiows that the moisture required to give most uniform granulation varieii greatly in different mixtures. Those that consist largely of When sufficient moisture is present, the particles of a mixinorganic materials granulate more readily and with a lower ture increase in size during tlie rolling process by two or more moisture eontent than those relatively high in organic masticking together to form a rounded particle of larger size. terials. Thus, mixtures 33,35, and 39 (Table 1) wliicli are all This continues until the attractive forces which join them toinorRanic and differ only in their content of filler (qnarto sand) gether are offset by the opposing forces which keep them can he granulated with a moisture content of 10 per cent at apart. With each different mixture there is a certain mini75' C. The moisture necessary to produce graniilation must, mum moisture content below which the mixture will not be increased to 25 per cent, however, when the 500 pounds of granulate regardless of the temperature or the time of rolling. sand in mixture 35 is replaced by the same weight of peat t,o A t this point of minimiim moisture content the granules progive mixture 37. duced are small and will no longer increase in size even with While the presence of a soluble salt is not essential to the eontinned rolling. If the moisture is increased a t a given granulation of a mixture, certain readily soluble, easily fusible temperature, the size of the granules will also increase. Mixmaterials, such &s urea and amnionium nitrate, are very efture 6 (Table I) was granulated at 80" C. with a moisture confective in promoting granulation. Thus, mixtures 9 mid 11 which were relatively high in urea and ammonium nitrate, teiit of 12 per cent. The screen analysis of the mixture showed that it contained too high a moisture content to give, at respectively, and low in organic conditioner required moisture contents of only 6 and 9 per the temperature of granulation, a maximum yield of .5cent for satisfactory granulation. IIowever, wlien the 20mesh granules. A larger proportion oi granules of the proportion of urea or desired mesh could have ammonium nitrate was low and the organic conditioner been obtained either by rerelatively high, as in mixducing the moisture slightly tures 8 and 19,it was necesor by granulating at a lower sary to increase the moistemperature. ture content to 15 a.nd 16 The screen analysis of per cent, respectively, in mixture 14 shows the lack order to induce a granulaof uniformity in size of tion of like uniformity. granules that results when The moisture required t,he moisture is not unifor granulation also varies formly distributed tiirougliwith the finenessof the mixout the mixture. When t u r e t o be g r a n u l a t e d . this condition exists, the Before granulation, mixture moisture in certain parts of 58 contained 26 per cent of the mixture may be sufficient to give granules that material coarser than 20 mesh and 53 percent coarser are too large: other pmts of the mixture may be lacking than 40 mesh. The highest proportion of 5-20 mesh in the moisture necessary to manrilleswas obtained in the FIGURE 1. ROTARY DRIX IND DRYEK produce granulation
INDUSTRIAL AND ENGINEERING CHEMISTRY
670 TABLE
Mixture NO.
11. DIsTRIBrnIoN O F h A N T F O O D DIFFERENT SIZEB Mesh Size
Fertilizer Grade
IN
PzOs %
KzO
20.38 20.99 20.73 20.37 31.26 31.51
5.01 5.18 5.14 4.98
% 243
5-20-5
