Spontaneous Development of Heat in Mixed Fertilizers ,JOHN 0. 11-ARDESTY ,4ND R. 0. E. DAVIS Diuision of Soils, Fertilizers, and Irrigation,
L.. S. Department of Agriculture, Reltsrille. Wld. Excessive development of heat in curing piles of fertilizer containing superphosphate, organic matter, and large amounts of inorganic nitrate is the result of oxidation of organic matter by nitric acid, which is formed in the reaction between the nitrate and free phosphoric acid. The rate of the reaction is accelerated by increases in ( a ) concentration of free phosphoric acid in the liquid phase of the fertilizer, ( b ) size and irisulating properties of the curing pile, (c) amount and mobility of the liquid phase in the mixture, and ( d ) degree of intimacy among the actile ingredients of the mixture. Under certain conditions the heat generated by the oaidation reaction is sufficient to cause spontaneous conibus tion in a 6-14-0 base mixture stored at 30" C. (86" F.). . i t temperatures below 90" C. the reaction is similar for all inorganic nitrates ordinarily used in fertilizers. Above 90' C. the reaction involving ammonium nitrate is many times more rapid than that involving either sodium or potassium nitrates. Neutralization of the free acid in such mixtures prevents these reactions from occurring at ordinary temperatures, but the resulting heat of neutralization and of other curing reactions should riot be allowed to accumulate in the pile. Otherwise the heat of curing confined in the curing pile may be sufficient to cause the hydrolysis of monocalcium phosphate, the production of additional free phosphoric acid, and, consequently, the re-establishment of the oxidation reactions.
A
'
MODERATE temperature rise in the curing pile of many fertilizer mixtures is an indication of proper curing a.hich promotes good physical condition in the final product. The well known exothermic reactions of curing, such as those of neutralization, are ordinarily controlled by regulating the quantity of acidic or basic constitutents of the mixture (4). Hotyever, with 8ome types of mixtures, such as those containing superphosphate, organic matter, and large amounts of inorganic nitrate, the curing pile occasionally develops an excessively high temperature which cannot be accounted for by the ordinary reactions of curing. In a previous paper ( 2 ) the cause of spontaneous combustion in a 140-ton pile of fertilizer base, composed of 400 pounds ammonium nitrate, 1400 pounds superphosphate, and 200 pounds peanut-hull meal per ton, was regarded as the result of heat developed during the oxidation of organic matter by nitric acid, which is formed in the reaction between nitrate and free phosphoric acid. (In the present paper this mixture is referred t o as the standard base mixture.) The rare occurrence of combustion in the curing pile of most fertilizer mixtures indicates that conditions seldom exist which permit the reaction to develop sufficient heat to ignite the combustible material present. However, a prolonged high temperature in the curing pile is frequently obaerved, especially with high-analysis mixtures; this condition may, and often does, result in the loss of available plant food, either by decomposition of nitrogenous compounds to cause loss of nitrogen, or by the reversion of phosphate to less available forms.
The laboratory ignition tests previously described ( 2 ) were conducted at 100' C., since it was found that the reactions causing combustion were accelerated sufficiently at 90 to 100 C. to raise the temperature to the ignition point within a few hours. Tests at this temperature serve to classify mixtures with respect to their combustibility, but they do not indicate the conditions which bring about sucfi high temperatures through the slow, spontaneous development of heat in the mixture during the curing period. The present paper deals with an additional laboratory study of the nature of the chemical reactions involved, the optimum conditions which may exist t o cause excessive heat in the mixture, the effect of concentration of reactants on reaction rates a t temperatures low enough to include those normally found in the fertilizer curing pile, and the effect of neutralizing agents on the progress of such reactions. The data presented should be useful in formulating fertilizers with inorganic nitrates, superphosphate, and organic matter to produce mixtures that are not susceptible to excessive heat accumulation during bulk storage. O
STORAGE C ~ ~ D I T I O N S
The three deciaive conditions that must exist in the occurrence of heat development to cause spontaneous combustion of mixtures stored at normal pile temperatures are ( a ) the presence of a substance that is easily oxidized, ( b ) a sufficient quantity of the mixture t o provide adequate insulation against dissipation of heat, and ( c ) a concentration of reactants, including an oxidizing agent in the mixture sufficient to initiate reaction and to maintain exothermic reactions which produce heat more rapidly than i t ie dissipated. The degree to which these conditions are fulfilled will determine the extent of the temperature rise. It is quite clear that the first two of these conditions are satisfied in a 140ton pile of base mixture such as that mentioned, but the third condition neceafiary to cause spontaneous combustion is not so apparent. All that is known concerning the concentration of reactants in the liquid phase of this mixture which ignited under commercial storage conditions is that the fire occurred in a 65ton lot 7%-hichhad been prepared with green superphosphate; a similar lot prepared with well cured superphosphate, on the other hand, had shown no signs of heat accumulation during storage. This indicates that a high concentration of free phosphoric acid in the mixture stored a t high summer tempgrature was responsible for the initiation of those reactions which raised the temperature of the pile to the ignition point. EXPERIMENTAL CONDITIONS
The observations made under commercial conditions mere confirmed in the laboratory. A 6-kg. lot of the standard base mixture was prepared with well cured superphosphate and divided into two equal portions. One portion was treated with phosphoric acid to give a total free acid content of 6.1670; the other portion was untreated and contained 1.96% free phosphoric acid. Both mixtures contained 5% moisture. They were stored at 30" C. in Dewar flasks equipped with thermocouples attached to a recording pyrometer. On the t w l f t h day of storage the mixture containing a high concentration of free acid began to heat 1298
INDUSTRIAL AND ENGINEERING CHEMISTRY
December, 1946
perature rise durihg storage at 80" C., but no ignition occurred during the 24-hour storage period. The mixtures with free acid concentrations greater than 45%. ignited, and the temperature a t which ignition occurred decreased witb increasing concentration of free acid up to 60%: above this value ignition occurred at 30" C. within 2 hours after mixing. The rates a t which the tem. perature of the different mixtures rose above the storage temperatures are given in Table 11. Loss of water vapor and nitric acid fumes was observed during the development of heat in the mixtures and was particularly evident just prior to ignition. The formation of nitric acid from) inorganic nitrate and phosphoric acid in the r'nix ture may be represented by the following equs tion:
300
Toto1 Free HsPO, Content = 6. I6 010 Total Free Moisture Content * 5.000fo Aqueous H3P04 Concentration = 65.20qo
250
200
u' 0-
c
5 "
150
t
8
100
50
0
1299
+
N H ~ N O I Hap04 +NH4H2P04 5
IO
15 20 25 30 Storage Period Beyond 12 Doys, Hourr
35
45
+ HNOi
(1
MONOCALCIUM PHOSPHATE
Spontaneous Rise in Temperature of Modified Standard Base Mixture Stored a t 30" C.
Figure 1.
Jpontaneously and ignited 44 hours later. Figure 1 shows that the rate of temperature rise increases with increasing temperature of the mass up t o 111 C. Table I shows the hourly rate of temperature change during the spontaneous heating of this mixture. The decreased rate of temperature rise in the range between 42 ' and 58" C. is attributed to the endothermic heat of solution of ammonium nitrate. The decline in temperature from 111' to 62" C. just prior to ignition is attributed to the endothermic hydrolysis of monocalcium phosphate and/or the endothermic decomposition of gaseous nitric acid to form nitrogen oxides, oxygen, and water (8). Considerable liberation of steam from the container was observed during the cooling stage of the reaction; this was followed by an immediate rise in temperature to the ignition point within a few seconds. The second portion of the mixture containing a low concentration of free acid showed no increase in temperature during 4-month storage under the same conditions. O
FREE PHOSPHORIC ACID
With the foregoing indication that the concentration of free phosphoric acid in the mixture is responsible for heat develop ment as a result of exothermic reactions at temperatures as low m 30" C., the effect of different concentrations of free acid on the rate of temperature rise was studied at storage temperatures ranging from 30" to 80" C. Mixtures were prepared consisting of 50 grams of ammonium aitrate, 25 grams of water, 100 grams of peanut-hull meal, and varying amounts of phosphoric acid to provide a series in which the concentrations of free acid ranged from 20 to 70% in steps of 670. (Percentage concentrations of acids in this paper refer to per cent by weight of free phosphoric acid in a solution of water and acid, and not to the percentage of acid in the total liquid phase, which often includes dissolved salts.) The necessary quantity of acid to give the desired concentration was first added to 25 grams of water and 50 grams of ammonium nitrate in a n Erlenmeyer flask and shaken until equilibrium was established a t 30" C. The contents of the flask were then thoroughly mixed with 100 grams of peanut-hull meal and placed in a tightly stoppered 200-cc. Dewar flask, which was equipped with an ironconstantan thermocouple attached to a recording pyrometer. Mixtures representing each concentration of acid were stored in Dewar flasks for 24 hours at constant temperatures of 30°, 40°, 60O, 60", 70 and 80 ' C. The mixtures with acid concentrations of less that 35% showed no development of heat at storage temperatures below 80" C. Those with acid concentrbtions of 35 to 45% showed a slight temO,
A previous paper (8) points out that the initiating agent for the oxidation reactions in the standard base mixture is probably not free acid alone but also the potential acidity represented by the hydrogen in monocalcium phosphate. Elmore and Farr (3' and Hill and Hendricks (6) show that monocalcium phosphate in the presence of moisture hydrolyzes to form dicalcium phosphate and free phosphoric acid according to the following equatioD :
Maximum conversion at any given temperature is obtained when the liquid phase reaches a n acidity sufficient to prevent further kydrolysis. Figure 2, representing results calculated from data given by Elmore and Farr (5), shows the temperature a t which solid mono. calcium phosphate monohydrate and dicalcium phosphate coexist in equilibrium with varying conoentrations of free phos. phoric acid. It indicates that a. saturated aqueous solution of monocalcium phosphate monohydrate a t 30 " C.will hydrolyze ac. cording to Equation 2 until a concentiation of 19% phosphoric acid is present in the liquid phase, whereas a t 100" C. the con centration of acid as H3POI reaches 48Y0. A temperature of 100"C. occurring in the presence of 48% phosphoric acid is far in
TABLE I. RATEOF TEMPERATURE CHANGE, DURING SPONTANEOUS HEATING OF THE STANDARD BASEMIXTURE Storage Period at 30' C. 12 Days plus: Hr. Min.
T ~ of Mixture, a
c.
0
a4
6
37
10
42
~
~
. Temp. Change/Hr.,
c.
+0.6
1.0, 16
43
20
44
26
48
30
62
36
68
40
68
41
72
42
80
0.2
0.2 0.8
0.8 1.2 2.0 4.0
8.0 111
43
31.0 -196.0
43
16
43
26
62
+1368.0
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INDUSTRIAL AND ENGINEERING CHEMISTRY
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TABLE11. EFFECTOF INCREASING COXCENTRATION OF FREE PHOSPHORIC ACID ON DEVELOPMEST OF HEATIN MIXTCRES STORED AT DIFFERESTTEMPERATURES
storageR a t e of Temp. Rise, in C. per Hr., above Storage Temp. a t H3P0, Temp., Concn. of: ' C. 35% 40% 45% 50% 55% 60% 65% 70% 30 None None Kone None 0.1 Igniteda Igniteda Igniteda 40 None None N o n e . N o n e 0.2 Ignited' Igniteda Ignited0 60 None None S o n e 0 . 3 Ignited Igniteda Ignited4 Ignited5 60 None None 0 . 2 Ignited Ignited Igniteda Igniteda Igniteda 70 None 0 . 9 0.4 Ignited Ignited Ignited" Igniteda Igniteda 80 0.4 1 . 2 b 1 . 8 6 Ignited Ignited Igniteda Ignited4 Igniteda a Ignited within 2 hours a f t e r mixing. b Temperature reached a maximum of 110-120° C. a n d then declined with no ignition.
,VOL 38, NO. 12
Loss of water vapor and nitric acid fumes was observed when the heated mixture was removed from the oven for the purpose of cooling it rapidly to prevent ignition. The difference in total PZOScontent of the heated and unheated samples (Table 111) shows that this loss of volatile matter amounted to 6.!)270, or 138.4 pounds per ton of mixture. According to the nitrogen contents of the heated and unheated samples, this loss of volatile matter included a nitrogen loss of 0.08%, or 19.6 pounds of nitrogen per ton of mixture. The remainder of the loss in volatile matter is attributed to moisture and other products of the oxidation of organic material. OXIDATION OF ORGANIC MATTER
The oxidizing action of nitric acid on the same reducing substance under diff went circumstances may produce a variety of chemical reactions and may result in the formation of various decomposition products, depending upon the route of the complete reaction. The reaction between nitric acid and many forms of oxidizable organic matter occurs readily at low temperature to form water, carbon dioxide, and the various oxides of nitrogen. This reaction of nitric acid on carbonaceous material may be represented by the folloR-ing equation: 2HNOa
+ C +COa + Ha0 + KO1 + KO
(3:
During complete combustion in the presence of sufficient carbonaceous material the oxides of nitrogen may react further to form carbon dioxide and elemental nitrogen, as represented by the following equation: concentration of Free H3P04, Percent
2902
Figure 2. Aqueous Concentration of Free HaPo4 in Equilibrium with Mixtures of Monocalcium Phosphate and Dicalcium Phosphate at Different Temperatures
excess of that required to accelerate the reactions which cause excessive development of heat in these mixtures (Table 11). The rate of the reaction is further increased at 100 O C. by an increase in concentration of phosphoric acid as a result of the rapid removal of moisture by evaporation from the reaction zone. At 30" C. none of these conditions exist to accelerate the reaction, and no rise in temperature above that of storage is produced. Table I11 presents a comparison of the extent of hydrolysis of monocalcium phosphate a t 30' and 85 O C. in a mixture containing 1400 pounds of Tvell cured superphosphate, 400 pounds of ammonium nitrate, 200 pounds of peanut-hull meal, and approximately 470 moisture. Phosphate analyses on this mixture a t intervals over a 4-week storage period at 30' C. showed no appreciable change in water-soluble P205 content, but when 200 grams of the mixture were heated to 85 C. in 2 hours and then quickly cooled to prevent ignition, the decrease in water-soluble P205 indicates that about 37% of the monocalcium phosphate originally present was hydrolyzed according to Equation 2. The resulting increase in concentration of free phosphoric acid from 30.3 to 73.570 caused a similar mixture to ignite after 2 hours at 85 C , whereas a t 30' C. there was no rise in temperature during a storage period of 1 month. O
O
TABLE
C
P E R C E N T A G E COJIPOSITION O F
BASERIIXTURE
---f
3COa
252
(4)
In tests with the standard base mixture in which vigorous combustion occurred, it was found that the loss in weight accounted for consumption of all of the organic matter and ammonium nitrate contained in the original mixture. Ignition tests a t 100" C. on the standard base mixture, and on similar mixtures in which the peanut-hull meal was replaced with 200 pounds of different organic materials, showed that the following materials are active participants in the heat-developing reactions which cause spontaneous combustion : peanut-hull meal, cottonseed meal, soybean meal, fish meal, tobacco stems, activated sewage sludge, animal and garbage tankage, lignin (Meadol), cocoa-shell meal, lignite, dried peat from several sources, sawdust, ground cork, ground kraft paper with and without asphalt-laminated sheet, ground burlap, castor pomace, millet seed, oak heartwood, brown rotted wood, cane sugar, dextrose, and cornstarch, Organic materials that did not react under the same conditions were dried blood, egg albumin, casein, pure carbon (lampblack), and cellulose (ground filter paper). Previous experiments (a) showed that as little as 50 pounds of peanut-hull meal per ton of mixture was sufficient t o cause combustion undcr these conditions, Fertilizer practice in the use of organic conditioning agents involves the addition of 100 pounds or more of the organic material per ton of mixture for the purpose of obtaining an appreciable conditioning effect. Therefore, these test3 show that a variation in quantity of organic material, within practical limits, would seem to have little or no effect on the development of heat by these oxidizing reactions.
BEFORE AKD AFTER
STORAGE AT DIFFERENT TEMPERATURES
Phosphoric Oxide ( P Z O S ) ~ WaterCitrateCa(Hd'Oa)2. c. Time Xoisture Nitrogen Total sol., insol. Free H20 CaHPOI 2 8 . 3 2 1 . 1 7 27.15 4.46 30 1 day 3.97 6.70 32.78 None 30 4 weeks 3.71 6.71 33.09 28.44 0.04 1.17 27.27 4.61 85 2 hr.b 3.62 6.16 35.55 24.57 0.04 7 , 2tiC 17.25 10.98 We are indebted t o F. X. Ward of this division for PlOa determinations. A.O.A.C. methods of analysis were used After 2 hours a t 85' C. i t was necessary t o cool t h e mixture rapidly t o prevent ignition. Based on the original free acid plus t h a t calculated from t h e decrease in water-soluble P208 according t o Equation 2.
Storage Temp.,
0
111.
+ 2 x 0 f 3C
HSPOP 1.61 1.61 10.06
Conrn. of HsPOh in Terms of Moisture 28.9 30.3
73.5
December, 1946
1301
oxide 0.1, and cyanogen 0.1 mole per cent. During this slow MEASURING oxidation a t 30 C. a large part of the nitrate nitrogen is reduced to the elemental state. I n the more vigorous reactions, which PeanutConcn. Ratio, occur in the standard base mixture when the heat of reaction is H10, NH4NOaa, HsP04:HaO + Mixture Hull Meal, HsPO4, Grama Grams HsPO4, Wt. % Grams Grams No. conserved by adequate insulation, considerable quantities of 30 20 40 00 1 50 nitrogen oxides are driven from the zone of the reaction. I n the 2 50 30 40 80 43 laboratory tests these highly reactive gases are ordinarily lost 30 60 120 33 3 50 4 50 30 80 100 27 immediately from the small amount of test sample. However, 30 120 240 20 5 50 tests with the sample confined in a tightly stoppered container 4 NHlNOs in s u 5 c i e n t q u a n t i t y to give a constant concentration of salt equipped with a relief valve showed maximum temperatures 16". in aqueous solution. t o 30' higher than those obtained in tests with the same sample in which the container allowed rapid loss of the evolved gases. This increase in maximum temperature would be expected because of the increased pressure of the evolved gases and the proINTIMACY OF MIXING longed contact of nitrogen oxides with organic matter, which reA 6-9-5 grade of fertilizer containing activated sludge 800, amsults in further reduction of the nitrogen oxides. The conditions monium nitrate 200, potassium chloride 164, superphosphate in a tightly closed container are analogous to those existing in the 800, and triple superphosphate 46 pounds per ton was milled to fertilizer curing pile, since any oxidizing gases produced in the pass a 10-mesh screen and divided into equal portions. One porcenter.of the pile are necessarily in contact with additional retion was further ground to pass a 40-mesh sieve, and the other active material during their travel toward the surface. portion remained in its coarse physical state. In ignition tests a t 100 C. the sample passing a 40-mesh sieve ignited and reached COMPARISON OF AMMONIUM NITRATE WITH OTHER NITRATES a temperature of 220' C., whereas the unground sample did not ignite but reached a maximum temperature of 122" C. and then Figure 4 shows the comparative effect of different nitrates on gradually cooled. Similar comparative tests with a modified heat development in mixtures stored a t a temperature of 30 ' C. standard base mixture in which only the organic matter (tankage) The test consisted of placing 147 grams of the sample in a 200-cc. was ground to pass 40 mesh, as compared with the same mixture Dewar flask at 30" C. immediately after mixing. The flask was containing unground tankage, showed that the rate of heating of equipped with a thermocouple attached to a recording pyrometer. the former was more than twice that of the latter. These tests The mixture contained 100 pounds of free phosphoric acid, 400 show that the use of finely divided and bulky organic materials pounds of nitrogen carrier, 200 pounds of peanut-hull meal, and increases the tendency toward spontaneous heating in such mix35 pounds of water per batch of 785 pounds. The remaining 1265 tures. pounds of inactive material required to complete the formula on the ton basis were omitted from the test batches in order to elimiRATE OF REACTION A T 30' C. nate the blanketing effect of dilution on the heat developed in Under the laboratory conditions of measuring the rate of heat such a small test sample. The nitrogen carriers used were amdevelopment, it would be expected that the reactions involved monium, potassium and sodium nitrates, calcium nitrate tetrawould proceed at a lower concentration of phosphoric acid than hydrate, magnesium nitrate hexahydrate, and ferric nitrate conthat indicated by a rise in temperature of the mixture. This was taining approximately 9 molecules of water of crystallization. cofifirmed in tests which determined the rate of the reaction by measuring the volume of gas liberated during storage at 30" C. from mixtures containing ammonium nitrate, peanut-hull meal, and varying concentrations of phosphoric acid. The compositions of the mixtures are given in Table IV. The vaiiation in concentration of acid was obtained by varying the amount of water used in these mixtures; the weight ratio of ammonium nitrate to water is kept constant by a corresponding variation in the quantity of ammonium nitrate used. The reaction was carried out in Erlenmeyer flasks, and the gas evolved was collected over mineral oil for a period of several days. Figure 3 shows the comparative rates of gas evolution corresponding to the percentage concentration of free phosphoric acid present. The dissipation of the heat of reaction was sufficiently rapid to prevent any appreciable rise in temperature of the mixture. The rate of gas evolution decreased with decrease in concentration of the phosphoric acid. At the lower concentrations there was no evolution of gas for a period of 2 to 5 days after mixing, but once the reaction was under way it gained speed rather rapidly. The cause of this initial lag in the reaction rate at low concentrations of phosphoric acid was not determined. The results of the tests show, however, that these oxidation reactions proceed in phosphoric acid concentrations as low as 20%, even though the heat of reaction is dissipated rapidly enough to prevent an appreciable rise in temperature. Qualitative tests showed that the gas liberated during these reactions consisted chiefly of carbon dioxide and nitrogen. QuanTime, Days titative examinatich of the gas with the consolidated mass spectrometer (1) showed that it contained nitrogen 43.5, oxygen 1.7, Figure 3. Rate of Reaction i n Mixtures as Measured argon 0.1, carbon dioxide 42.3, nitrous oxide 12.2, nitrogen diby Volume of Gas Generated
TABLE Iv.
.
INDUSTRIAL AND ENGINEERING CHEMISTRY
COMPOsITlON O F MIXTURESUSED I N RATESOF GASEVOLUTION
O
I N D U S T R I A L A N D E N G I N E E R I N G CHEMISTRY
1302
the mixtures containing other nitrates. After completion of the reaction the organic matter in the mixture containing either sodium or potassium nitrate TTas slightly charred, but that in the mixture containing ammonium nitrate had been converted to ash. This is of practical significance to the fertilizer manufacturer, since fire in storage piles containing ammonium nitrate is consequently more difficult to control than that in piles containing either sodium or potassium nitrate. I n this respect the complete volatility and combustibility of ammonium nitrate a t these temperatures places this fertilizer salt in a class by itself.
P o r k by Weight Phosphoric Acid, H3P0, Nitrogen Corrier Water
350
300
1 I
TotoI
735
Aqueous Concentration of H3P04=74'/o'
I
I
I
I
'
Vol. 38, No. 12
I
1
PREVENTION OF NEAT DEVELOPMENT
I
0
I
I
I
2
4
E
I
I
8 IO Time, Hours
12
14
I6
Figure 4. Comparative Effect of Different Nitrogen Carriers on Spontaneous Heating in Mixtures
'
o n the basis of the reaction of phosphoric acid with nitrate salt (Equation l ) , the amount of phosphoric acid in these mixtures was the limiting factor in the production of nitric acid from the quantity of nitrogen carrier present,. If it is assumed that the phosphate salts produced by the reaction according to Equation 1 are mono-, di-, and tribasic in mixtures containing alkali, alkaline earth, and iron nitrates, respectively, the amount of nitric acid set free by a given quantity of phosphoric acid is three times greater with the nitrate of the trivalent ferric ion, and twice greater with the nitrates of divalent calcium and magnesium ions, than with nitrates of the monovalent sodium, potassium, and ammonium ions. The rate a t which nitric acid is set free is affected by the solubility of the phosphate salt formed during the reaction of phosphoric acid with the different nitrates. For example, the formation of highly insoluble iron phosphate in the mixture containing ferric nitrate would be expected to accelerate the formation of nitric acid, as compared with the slower initial rate of nitric acid formation in the other mixtures containing more soluble phosphates. This accounts for the extremely rapid development of initial heat in the mixture containing ferric nitrate. .Inother factor influencing the temperature rise to some extent in these mixtures is the quantity of heat required to raise the temperature of the free water, which is liberated in varying quantities by the different hydrated salts and by the reaction represented in Equation 3. This formation of free water during the reaction also tends to cause increasing amounts of soluble salts to enter the solution phase of the mixture, and the extremely slow rate of initial temperature rise in the mixture containing ammonium nitrate (Figure 4) is attributed to the high negative heat of solution which this salt possesses. The position of the curves in Figure 4 indicates that the effects of both liberated water and negative heat of solution of the different nitrates on the initial rate of temperature rise are relatively small as compared with the influence of the rate of nitric acid formation. The progress of the highly exothermic reactions represented by Equations 3 and 4, and, consequently, the extent of heat development in the mixture, depends chiefly on the quantity of nitric acid present a t any given stage of the storage period. The greatly accelerated rate of chemical reaction a t 90' t o LOO C., which causes extremely rapid combustion t o occur in the mixture containing a,mmonium nitrate, is not evident in any of O
Results of ignition tests at 100' C. previously reported ( 2 ) showed that spontaneous combustion was prevented in the standard base mixture by the addition of a rapid neutralizing agent such as ammonia, in sufficient quantity to neutralize entirely the potential acidity represented by the hydrogen in the monocalcium phosphate; this prevents the formation of the large quantities of nitric acid according to the reaction of Equation 1. At 30' C., hon-ever, it is necessary to neutralize only the free acid present in the superphosphate, since it was shown (Figure 2) that hydrolysis does not occur a t this temperature to produce free phosphoric acid in concentrations greater than 19%. It was also shown (Figure 3) that the initial reactions occurring a t this temperature and concentration of free acid are exceedingly slow and not likely to provide sufficient heat to cause any appreciable rise in the temperature of the mixture. These observations were confirmed in laboratory tests at 30 O C (86" F.) in which ammonia, hydrated lime, cyanamide, calcium oxide, magnesium oxide, cement flue dust, and dolomitic limestone were added in sufficient quantity to the standard base mixture to neutralize the initial free acid content (6.1GY0 HIPOa) As compared with the rapid development of heat which occurred in the standard base mixture containing no neutralizing agent (Figure l ) , 3-kg. lots of these neutralized mixtures stored in Dewar flasks at 30" C. over a period of 3 months shelved no temperature rise 'above that accounted for by the neutralization reaction. The dissipation of the heat of neutralization during these lahoratory tests with relatively small quantities of mixtures is very rapid as compared with that in large piles under commercial conditions; experience with large piles indicates that the heat of neutralization and other curing reactions, especially in highanalysis mixtures, may be retained in the pile for a period of several months. Under such conditions of prolonged high temperature the hydrolysis of monocalcium phosphate may produce sufficient additional f i ee phosphoric acid to initiate the oxidation reactions that cause excessive heating in the pile. This is demonstrated by laboratory teitq with a 6.8-14-0 base mixture composed of 411 pounds of ammonium nitrate, 235 pounds of peanut-hull meal, 29 pounds of magnesium oxide, and 1325 pounds of superphosphate containing 5.59% free acid. The amount of magnesium oxide in this mixture is sufficient to neutralize the initial free acid present. Immediately after preparation of this mixture, 200 grams were placed in a Dewar flask equipped with a thermocouple attached to a recording pyrometer. The temperature of the surrounding air was kept about 5" below the advancing temperature of the sample to avoid excessive loss of heat During a 13-hour period the temperature of the mixture rose to 60" C. (140" F.) and remained stationary a t 60" C. for 11 hours The temperature of the surrounding air was then raised to 70" C (158' F.). The temperature of the mixture gradually rose above 70" C., and in 27 hours it ignited a t a temperature of 130" C.. reaching a maximum of 279' C. within a few seconds. These results indicate that, even though the initial free acid in such mixtures is neutralized, care must be ex&cised to prevent a prolonged high temperature in the curing pile. This may be accomplished by preneutralization of the superphosphate or b)
.
December, 1946
INDUSTRIAL AND ENGINEERING CHEMISTRY
slow, scatter-type construction of the curing pile to eliminate part of the initial heat. Otherwise the hydrolysis of monocalcium phosphate to form additional free phosphoric acid may nullify the advantage gained by neutralizing the initial free acid, and the oxidation reactions may thus be re-established to cause excessive heat accumulation. CONCLUSIONS
Reactions in fertilizer mixtures, es ecially base mixtures containing relatively green superphospgte, organic matter, and large amounts of inorganic nitrate have been represented by Equations 1 to 4. Reactions represented by Equations 3 and 4 are highly exothermic and cause an excessive rise of temperature, even to the point of combustion, in curing piles containing sufficient nitric acid to react with the organic matter present. The nitric acid in the mixture is formed by the reaction of inorganic nitrates, such as ammonium nitrate, with free phosphoric acid (Equation 1). The rate of heat development increases with increase in concentration of free phosphoric acid and the consequent increase in formation of nitpic acid. Neutralization of the initial free phosphoric acid in the mixture prevents the excessive accumulation of heat in the curing pile by inhibiting the formation of nitric acid. However, the heat of neutralization should be largely dissipated in order to inhibit the hydrolysis of monocalcium phosphate and the formation of additional free phosphoric acid accordin to Equation 2. Otherwise, the reactions of Equations 1, 3, a n 3 4 are likely to be re-estab-
1303
lished. The heat of neutralization may be largely dissipated by preneutralization of the superphosphate or by slow construction of the pile when such mixtures are stored for curing. The rare occurrence of combustion in the curing pile of many potentially combustible mixtures indicates that optimum conditions for the development of these reactions seldom exist. However, in certain mixtures-especially base eoods, such as the standard base mixture used in these tests-it is essential that the content of free phosphoric acid be as low as possible if excessive heat development is to be avoided. The results of the tests indicate that, a t the usual temperatures of the fertilizer curing pile, ammonium nitrate is no more haaardous from the standpoint of heat development than are the other inorganic nitrates. Howeverl if for any reason the temperature of the storage pile reaches the ignition point, the resulting fire is more difficult to control in mixtures containing ammonium nitrate than in those containing any of the other inorganic nitrates tested. LITERATURE CITED
(1) Brewer, A. K.,and Dibeler, V. H., J. Raoearch Natl. Bur. Standards, 35, 125-39 (1945). (2) Davis, R. 0.E., and Hardesty, J. O., IND.ENG.CEEM., 37 59-63 (1945). (3) Elmore, K.L.,and Farr, T. D., Ibid., 32,580-6(1940). (4) Hardesty, J. O.,and Ross, W. E., Ibid., 29,1283-90(1937) (5) Hill, W.L.,and Hendricks, S. B., Ibid., 28,440-7 (1936). PRESENTED before the Division of Fertiliaer Chemistry at the 110th Meetine of the AMERICAN C ~ ~ M I CSOCIETY, AL Chicago, Ill. ,
IMIDE-MODIFIED ALKYD RESINS HOWARD J. WRIGHT’ AND ROBERT N. DUPUIS The Miner Laboratories, Chicago 6, Ill, Replacement of part of the ester linkages in an alkyd eesin with amidic linkages was attempted by introducing part or all of the fatty acid radical as an amide of glyceryl monoamine. The amide appeared to undergo a rearrangement resulting in the formation of an N-substituted imide of the dibasic acid used. These imide-modified alkyds, which can be prepared through substitution of a glycerylamine for part of the glycerol, can be made with very low fatty acid content and low acid numbers, and give hard, fast-drying films. Properties of representative imidemodified alkyds are described, and a Wsible mechanism involved in their preparation is given. Evidence is preeented which indicates that a rearrangement takes place when alkyl fatty amides are heated with phthalic anhydride in the presence of hydroxyl groups. A method of calculation was devised for the prediction of the acid number of an alkyd resin from its formula and for reducing the acid number of an alkyd to any desired value by replacing part of the dibasic acid with an N-hydroxyalkyl imide.
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PREVIOUS report on alkyd resin research (7) showed that
alkyds could be separated into fractions having substantially different properties by a n alcoholic extraction process. The portion insoluble in lower monohydric alcohols is of lesser oil content than the starting material and has superior filmforming properties. The alcohol-soluble fraction is of no utility as a surface coating and acts as a softener of the desirable insoluble part. The unwanted soft material can be reworked into a n ordinary type alkyd, so that, by a cyclic process, a substantial percentage of the insoluble fraction can be continuously produced. Two obvious differences between the two fractions are “oil” oontent and molecular weight; the alcohol-insolubke part has (ess oil and higher molecular weight. It was felt that these differ1 Present address, Department of Chemistry, Northweatern University, Evanston. Ill.
ences were due to uncontrolled migration of fatty acid groupc during resin formation, resulting in unbalanced distribution Assuming that fatty acid migration is responsible for some of the unwanted portion, i t is conceivable that fixation of the fatty acid linkages in a predetermined position might give an improved resin. I n a 40% oil alkyd, for example, only about one sixth of the ester linkages involve fatty acid radicals. Since there is unbalanced distribution probably resulting initially in some small molecules like di- and triglycerides, there must also be large molecules with such low fatty acid content as to lead to premature gelation. Such uncontrolled distribution is believed t o be a serious problem in the ordinary alkyd resin. USE OF AMIDE LINKAGE
If the amide linkage were found to be stable under conditions of resin preparation, some improvement in distribution of fatty acid radicals might be expected if these groups were added as amides of glyceryl a-monoamine (3-amino-1,2-propanediol). The effect of such a n addition was determined. The amides were prepared by dropping one mole of the amine into one mole of fatty acids maintained a t about 220’ C. in an atmosphere of nitrogen. The amides are practically neutral waxy materials with hydroxyl contents corresponding to the expected structure R-CO-NHCHAHOH-CH2OH Alkyds prepared from fatty acid amides of glycerylamine, phthalic anhydride, and glycerol had greatly improved film characteristics in comparison with those of ordinary alkyds, and showed the additional important advantage of retarded gelation during resin cooking, to the extent that resins of low oil content and low acid number could be produced. However, even though the amides were calculated m requiring two acid groups to effect reaction with their two hydroxyl groups, the hyhoxyl numbers of the finished resins were much higher than would be predicted on the basis of the formulation. The degree
