Preparation of Physiologically Neutral Fertilizer Mixtures Reactions of Monoammonium Phosphate with Limestone and with Dolomite KENNETHC. BEESONAND WILLIAM H. Ross Fertilizer Investigations, Bureau of Chemistry and Soils, Washington,
Monoammonium phosphate reacts with limestone
in all proportions at ordinary temperatures in the presence of moisture to evolve carbon dioxide. Loss of ammonia takes place also when the CaC03 in the mixture exceeds one mole per mole of Pz05. At 90" C. loss of ammonia and carbon dioxide occurs in all proportions of the reacting materials. The rate of the reaction decreases as a rule with increase in the size of the limestone particles. Dicalcium phosphate is formed at 30' C. in monoammonium phosphate-limestone mixtures containing u p to one mole of CaCO, per mole of Pz06, and more or less tricalcium phosphate is formed in mixtures containing in excess of this proportion of CaCOI. Monoammonium phosphate reacts with dolomite in the presence of moisture to evolve carbon dioxide, but no loss of ammonia or formation of tricalcium phosphate occurs in any proportion of the reacting
F"
RTILIZER materials may be classified into three groups according as they exhibit an acid, neutral, or basic influence on the soil. The effect of a fertilizer material on the reaction of the soil may bear no relation to the chemical reaction of the material itself. Thus superphosphate which contains a n acid salt is neutral as regards its influence on most soils. Liming materials, such as calcium hydroxide or carbonate, decrease the acidity of the soil but ammonium hydroxide increases soil acidity by nitrification to nitric acid, All ammonium salts are likewise acid-forming in the soil, but the nitrates of the alkali and alkaline-earth metals have an opposite effect. The potash salts most commonly used in fertilizers have little or no effect on soil reaction. By proper selection of their components it is thus possible to prepare fertilizer mixtures that are essentially neutral as regards their effect on the reaction of the soil. For many years the consumption of the physiologically basic forms of fertilizer nitrogen was more than sufficient to neutralize the effect of the acid-forming ammonium salts. The average fertilizer mixture mas therefore basic in its reaction on the soil (6). As long as the cost of nitrogen in its various forms remained about the same, no added cost was involved in the preparation of nonacid-forming mixtures. Recent developments that have taken place in fertilizer manufacture have changed the situation. Ammonia nitrogen now costs considerably less than nitrate nitrogen or other forms of basic nitrogen with the result that it is largely replacing the latter in the manufacture of mixed fertilizers. The bulk of the complete fertilizers now sold in this country are therefore acid-forming in their influence on the soil. If the use of such fertilizers is to continue, a serious decrease in
D. C.
materials at ordinary temperatures. One-third of Ihe ammonia entering into the reaction is changed into water-insoluble magnesium ammonium phosphate. Formation of some tricalcium phosphate and loss of ammonia may occur when mixtures of three or more moles of CaCO,.MgCO, are heated at 90" C. with jive moles of monoammonium phosphate. The use of dolomite as a neutralizing agent in fertilizer mixtures offers advantages over limestone: ( 1 ) It is much slower in its reaction with the phosphatic components of the mixture: (2) it does not cause loss of ammonia when mixed with monoammonium phosphate at ordinary temperatures; (3) it fixes in nonacid mixtures a portion of the ammonia in the form of a slowly soluble compound, magnesium ammonium phosphate: and (4) it does not cause any appreciable increase at ordinary temperatures in the citrate-insoluble PzO6 in the mixture. the crop-producing capacity of certain soils in the fertilizerconsuming sections of the United States is likely to result (a). Two methods are available for preventing an injurious increase in the acidity of the soil with continued use of acidforming fertilizers. The first consists in treating the soil with suitable separate applications of a liming material. This treatment is now being successfully practiced for many crops, but it is not so effective in the case of other crops, such as cotton, that are sensitive to excessive applications of lime. The second method consists in including a sufficient quantity of a basic or liming material in the fertilizer mixture to give a physiologically neutral product. By using the minimum quantity of liming material required to give a nonacidforming mixture, danger from overliming is eliminated and the expense incident to a separate application of the material is avoided. CLASSIFICATION
O F PHOSPH-4TE M I X T U R E S
The fertilizer mixtures now on the market may be conveniently divided into three groups according as the phosphatic component of the mixture is present as a calcium phosphate, an ammonium phosphate, or both. CALCIUM PHOSPHATE MIXTURES.The chemical reactions that take place in superphosphate mixtures containing limestone or dolomite have been studied by MacIntire and his co-workers (4, 5 ) . Their results show that mixtures of limestone and superphosphate react with evolution of carbon dioxide to change more or less of the monocalcium phosphate in the superphosphate into di- or tricalcium phosphate, or both. The extent to which these reactions take place varies with the proportion of limestone present. Thus a mixture
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INDUSTRIAL AND ENGINEERING CHEMISTRY
containing 8 parts of limestone to 90.5 parts of superphosphate showed no appreciable increase in citrate-insoluble PZOS in 146 days, whereas the citrate-insoluble P20sin a mixture containing 30.5 parts of limestone to 60 of superphosphate increased from 0.09 to 2.81 per cent in the same length of time ( 5 ) . No appreciable increase in citrate-insoluble P20jwas Ouflet In let f o u n d t o take place, however, when t h e l i m e s t o n e was replaced by an equivalent quantity of dolomite of any degree of fineness. CALCIUMPHOSPHATE AND Ahiaiox~rjniPHOSPHATE MIXT U R E s. When superphosphate i s m i x e d with ammonium sulfate, as is now common practice in the preparation of mixed fertilizers, reaction may take place between the ammonium sulfate and the monocalcium phosphate i n t h e s u p e r p h o s phate:
-
FIGURE1. APPARATUS FOR STUDYING REACTIONS WITHOUT APPLIED PRESSURE
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EXPERIMENTAL PROCEDURE MATERIALS.The liming materials used in the tests were calciurh carbonate, limestone, and dolomite. The calcium carbonate was the analytical reagent. The compositions of the limestone and of the dolomite were as follows (in per cent) : VOLA- ACIDTILE XfOISMATT V R F ~ TER
Limestone Dolomite
0.06 0.08
0.14
0.87
INSOL. hfATTER
FesOt
Nil 9.76
Si1 0.55
hlgO CrtO 0.11 55.74 17.10 29.01
COz 43.76 41.87
CaCOa Eaciv.4LENT
99.50 96.22
The different sized particles of limestone and dolomite used in the tests showed little variation in CaC03 equivalent. The purity of the ammonium phosphate was confirmed by analysis and by the pH value (4.38) of its 0.1 M solution. APPARATUS. Fertilizer mixtures as produced in manufacturing plants are frequently stored in large piles, and portions of such mixtures are therefore necessarily subjected to considerable mechanical pressure. In order to simulate commercial conditions, certain of the mixtures used in these tests were subjected to a mechanical pressure corresponding to that developed in mixtures prepared on a factory scale. The apparatus used to give the pressures was described in a previous paper (1). The glass apparatus represented in Figure 1 was used t o study the reactions taking place without applied pressure:
hferz and his co-workers (7) The mixture under observation was placed in glass tube A have shown that calcium sulin turn was placed in the glass-stoppered vessel, B, having fate and monoammonium phosphate are the stable pair of which a gas inlet and an outlet. The annular space between the tube these reciprocal salt pairs. A mixture of monocalcium phos- and the walls of the vessel was filled with a liquid or salt suspenphate and ammonium sulfate containing an excess of the sion having a vapor pressure corresponding to the mixture in former will therefore react in the presence of moisture until the tube A . Duplicates of these two types of reaction vessels were sup- . ammonium sulfate no longer exists in the solid phase. The ported in two oil thermostats of which one was maintained a t average fertilizer mixture contains almost equal molecular 30" and the other at 90" C. A stream of air from which carbon proportions of monocalcium phosphate and ammonium sul- dioxide and any ammonia present were removed was passed fate, and conseuuentlv there a r e IO0 possibilities for an extensive reaction between these t'wo components 90 of mixed fertilizers. Actually the l o w m o i s t u r e c o n t e n t of m o s t -..eo fertilizer mixtures will retard the $ 70 reaction, but more or less mono- -- so ammonium phosphate is no doubt C p so present in all s u p e r p h o s p h a t e 4 mixtures c o n t a i n i n g ammonium F 40 sulfate. I n the ammoniation of double 6 30 superphosphate, e i t h e r mono- or 5 20 -4 diammonium phosphate is formed IO depending on the extent of the am0 moniation (9) : 5 10 15 20 25 30 40 SO 60 5 IO 15 20 25 34 Days Dnys Ca(H,PO& NH, = FIGURE 2. Loss OF CARBONDIOXIDE FROM MIXTURES OF 1 PARTMONOAMMONIUM CaHPOd NHaHzPOa PHOSPHATE AND 0.75 PARTLIMESTONE (OR CaCOJ) OF VARYING PARTICLE SIZE Ca(H2PO& 2a" = ( M o i s t u r e , 10 per c e n t ) CaHPOd (NH4)zHPOn Q
%
'
+
+
+
+
An ammonium phosphate is thus present in all. ammoniated mixtures containing double superphosphate. AMMONIUMPHOSPHATERIIXTURES. Fertilizer mixtures are now being placed on the market in which a n ammonium phosphate is the only source of phosphoric acid. These mixtures are of a concentrated nature and are usually made from materials that are readily soluble in water. The present paper outlines the reactions that take place when a liming material is used in mixtures with monoammonium phosphate. A subsequent paper will describe the corresponding reactions in mixtures containing diammonium phosphate.
over the mixture in each vessel and subsequently passed successively through sulfuric acid and Ascarite to remove any ammonia and carbon dioxide that was evolved from the mixture.
COMPOSITION OF MIXTURES. Pierre (8) has shown that monoammonium phosphate has a physiological acidity equivalent to 107 pounds of CaCOr per unit (20 pounds) of nitrogen. One part of pure monoamnionium phosphate therefore requires 0.65 part of the limestone used in these tests, or 0.68 part of the dolomite to give a physiologically neutral mixture. A fertilizer mixture would ordinarily contain other acid-forming materials in addition to ammonium phosphate. The liming materials used were accordingly increased to 0.75
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part per part of monoammonium phosphate in most of the tests but was varied in some of the tests from 0.25 to 1.3 parts per part of ammonium phosphate. To the mixture thus prepared was added the amount of moisture desired for the particular experiment. An attempt was made to control the moisture content of the mixture by pa.;sing the incoming air stream a t the temperature of the bath through a saturated
mesh size than from those that are somewhat larger or smaller was invariably noted. I n general, however, the carbon dioxide evolved increases with decrease in the size of the limestone particles. The curves obtained when limestone was replaced with dolomite are shown in Figure 3. Mixtures of dolomite and monoammonium phosphate also react to set free carbon dioxide, but the maximum loss of this constituent during the period of the tests was only about threefourths as great as from the limestone mixtures. Here again the maximum loss of carbon dioxide from mixtures containing dolomite of 50- to 60-mesh size was greater than from mixtures containing smaller or larger mesh material. The reduced final reactivity of the liming material with decrease in the size of its particles below DQYS Days FIGURE3. LOSS OF C4RBOX DIOXIDE FROM MIXTURES OF 1 PART MONOAMMONIUM 50- to 60-mesh size may be due PHOSPHATE AND 0.75 PARTDOLOMITE OF VARYINGPARTICLE SIZE to the relatively smaller thick(Moisture, 10 per cent) ness of the moisture laver on the surface of the more finily divided solution of monoammonium phosphate. A saturated solution material as suggested by MacIntire and Shuey ( 5 ) . of the phosphate was also placed in the annular space surEFFECTOF PARTICLE SIZE OF LIMESTONEO N Loss OF rounding the tube containing the mixture. This procedure AMMONIA was reasonably successful for the work a t 30" C., but a t 90" it was found necessary to increase the moisture content of the The loss of ammonia under the same conditions as described incoming air stream by passing it through water a t 90 ", for loss of carbon dioxide from limestone is given in Figure 4. Each run a t 30" C. was continued for approximately These curves show that the rate of loss of ammonia increases 60 days, and a t 90" C. for 15 to 40 days or until the carbon with decrease in particle size as in the case of loss of carbon dioxide set free in 24 hours was less than 0.1 per cent of the dioxide. The maximum loss of ammonia, however, is more totaI originally present. The carbon dioxide liberated was than twice as great a t 90" C. as at 30", whereas the corredetermined a t the end of the first 6 hours and daily there- sponding loss of carbon dioxide is approximately the same at after to the end of the run. It was found that little or no both temperatures. A comparison of the curves in Figure 4 ammonia was evolved during the first part of the run while the heavy evolution of carbon dioxide was taking place. It 20 -.. was determined a t intervals thereafter until the end of the P +? IO run. A sufficient number of duplicate runs was made to ini Io sure that the data given represent the general trend of each %3 reaction. Loss of carbon dioxide and ammonia was found in all cases when moisture was present but in varying amounts, depending upon the particle size of the liming material, the moisture content, the temperature, and the mechanical pressure applied to the mixture.
EFFECTOF PARTICLE SIZE ON Loss
OF
Days
CARBONDIOXIDE
The loss of carbon dioxide that takes place with time from mixtures of 1 part of monoammonium phosphate and 0.75 part of limestone of varying particle size is shown by the curves in Figure 2 when the reacting mixture is maintained at 30" and a t 90" C. The initial moisture content of each of the mixtures used in these tests was 10 per cent, and this remained more or less constant for several days. The changing partial vapor pressure of the mixture as the reaction proceeded made it impractical to maintain a fixed moisture content indefinitely at 90" C., and the samples gradually dried out towards the end of the run. The results shoa that the loss of carbon dioxide in 30 days a t 90" C. is approximately the same as that which takes place in double the time a t 30". With the smaller sized particles of limestone, about half of the carbon dioxide is evolved during the first quarter-day period at 30" C. The curves for particles of 50- to 60-mesh size cross those of 80- to 100-mesh. A greater loss of carbon dioxide from particles of 50- to 60-
FIGURE 4. Loss
OF
AMXONIAFROM MIXTURES
1 PART MONOAMMONIUM PHOSPHATE AND 0.73 PART LIMESTONE OF VARYING PARTICLE SIZE (Moisture, 10 per cent)
OF
with those in Figure 2 shows that little or no ammonia is evolved in any given reaction until the greater part of the maximum loss of carbon dioxide has taken place. At 30" C. the loss of ammonia from limestone mixtures of 30- to 40mesh size, or larger, amounts to less than 1 per cent of the total, There is no appreciable loss of ammonia a t 30" C. from dolomite mixtures of any particle size, but a t 90" a maximum loss of about 20 per cent of the total present may take place from mixtures containing
