JOHN O. H ARDESTY, J. Y . Y EE,
ANDKATHARINE S. LOVE
D iv is io n o f S o il a n d F e r tiliz e r In v e s tig a tio n s , U. S . D e p a r tm e n t o f A g r ic u ltu r e , B e lts v ille , M d .
The average h y g ro sco p icity o f 1943-44 fertilizer m ix tu res was n o t ap p reciab ly d ifferen t fr o m th a t o f m ix tu res m a d e during, t h e p reced in g sea so n , even th o u g h a m m o n iu m n itrate h a d la rg ely rep laced so d iu m n itr a te as a n itr o g en carrier. E x cep tio n s occu rred in m ix tu r e s c o n ta in in g large am oun ts o f m a te ria ls s u c h as m a n u re s a lts , u rea, or so dium n itr a te in c o m b in a tio n w ith a m m o n iu m n itr a te.
The m o istu r e c o n te n t o f m o s t fe rtiliz er m ix tu res is s u f
ficient to dissolve a ll o f t h e a m m o n iu m n itr a te c o m m o n ly present. T h e h ig h so lu b ility o f a m m o n iu m n itr a te in co n ju n ctio n w ith it s rea c tio n in so lu tio n w ith o th e r c o m m on sa lts, s u c h as p o ta s siu ih ch lo rid e an d p o ta s siu m s u l
fate, greatly d im in is h e s th e u n favorab le in flu en ce w h ic h it w ould o th erw ise ha v e o n t h e hy g ro sco p icity o f m o st m ixtures. M ixed fertilizers t h a t are u n sa tisfa c to ry from the sta n d p o in t o f m o istu r e c o n te n t m a y b e divided in to three c la s se s: (1) th o s e t h a t are u n d esira b ly w et d u e to an
in crease in v o lu m e o f th e liq u id p h ase as a r esu lt o f th e h ig h ly so lu b le sa lts g oin g in to so lu tio n ; (2) th o s e w ith in su ffic ien t in itia l m o istu r e to allow so lu tio n o f h y g ro scop ic so lu b le s a lts, th ereb y p reven tin g c o m p le te cu rin g o f th e m ix tu re; and (3) th o se th a t are hygroscop ic th r o u g h a co m b in a tio n o f in co m p a tib le m a teria ls or th e presence o f large q u a n titie s o f h ygroscopic sa lts in th e solid sta te . A n o p tim u m m o istu r e c o n te n t a t m ix in g tim e is, th e r e fore, h ig h ly im p o r ta n t to th e gen eral p h y sica l co n d itio n o f fertilizer m ix tu res co n ta in in g th e u su a l q u a n titie s o f very so lu b le sa lts. O bservations a t th e factory o n th e ph y sica l c o n d itio n o f m ix tu res w ith resp ect t o m o istu r e ab sorp tion are correlated w ith lab oratory values o n th e e q u ilib riu m m o istu r e c o n te n t o f th e sa m e m ix tu r es in order to fa c ilita te a d a p ta tio n o f th e laboratory procedure in fo reca stin g th e h ygroscopic behavior o f m ix tu r es before th e y are produ ced in large to n n a g e.
E
ARLY in 1943, when it became ap p aren t th a t a considerable tpnnage of amm onium n itra te would become available to the fertilizer industry, this laboratory initiated an investigation on the use of amm onium n itra te in complete fertilizer m ixtures.The first report on these studies (1) presented d a ta on th e hygro
scopic properties of fertilizer m ixtures prepared in th e laboratory according to th e basic form ulation practices of m ost fertilizer manufacturers. I t showed th e relative influence of several im
portant variables, such as th e effect of varying quantities of dif
ferent nitrogen and potash carriers, on m oisture absorption by fertilizers a t different relative hum idities. On th e basis of re
sults obtained, suggestions were offered concerning th e satis
factory form ulation of fertilizers containing ammonium n itrate, but the necessity was recognized for correlating th e laboratory results with those obtained in th e factory and on th e farm.
An additional study was accordingly undertaken of some 450 mixed fertilizer samples, supplied by m anufacturers and b y fer
tilizer control stations and experim ent stations of various states.
These samples were ab o u t equally divided betw een m ixed fer
tilizers th a t we're sold during 1942-43 before solid ammonium ni
trate was available to any appreciable extent, and those sold during 1943-44 when am m onium n itra te was available in quan
tity. Sodium n itrate was utilized extensively during th e former period, b u t little or no sodium n itra te was used in m ixed fertilizers during the second period. A bout 160 of th e samples obtained from fertilizer m anufacturers were accom panied w ith complete information w ith respect to form ula, place, and date of m anu
facture, length of curing period, place of use, and th e observa
tions of those handling th e fertilizer as to its physical condition with respect to m oisture absorption.
The prim ary objects of this investigation were to use th e infor
m ation thus supplied by th e m anufacturers in developing a rapid laboratory procedure for evaluating th e hygroscopic properties of commercial mixed fertilizers, and to utilize this procedure in
determ ining th e effect of varying proportions of ammonium ni
tra te on th e hygroscopic properties of commercial m ixtures.
A study was also m ade of th e solubility of ammonium n itra te in th e liquid phase of typical fertilizer m ixtures and of th e effect of varying th e proportion of this m aterial in th e m ixture on the volume of th e liquid phase.
RATING T H E RELATIVE H Y G RO SC O PICITY
A hygroscopic salt such as ammonium n itra te will absorb mois
tu re in a hum id atm osphere u n til it liquefies and the vapor pres
sure of th e resulting solution reaches equilibrium w ith th a t of th e surrounding air. T he behavior of a m ixed fertilizer th a t consists of a m ixture of hygroscopic and nonhygroscopic m aterials is quite different. If th e m ixture contains only a small proportion of a hygroscopic m aterial, or m ixture of m aterials, it m ay a tta in equilibrium w ith th e surrounding air when th e am ount of mois
tu re absorbed is relatively small. O ther m ixtures m ay absorb much larger quantities of m oisture before reaching equilibrium . Fertilizer m ixtures m ay thus be classified when th eir hygroscopic properties alone are considered, according to th e m axim um quan
tity of m oisture they will absorb under any given set of condi
tions. A convenient classification divides them into th e three groups of nonhygroscopic, hygroscopic, and very hygroscopic m ixtures: A nonhygroscopic m ixture does no t absorb m oisture in excess of th e average m oisture content norm ally occurring in mixed fertilizers a t the prevailing hum idity and tem perature in sections of th e country where fertilizers are m ost commonly used.
A hygroscopic m ixture absorbs m oisture, under th e conditions specified, in excess of the average m oisture content occurring in mixed fertilizers so th a t th e m ixture is susceptible to an unde
sirable dam p condition when the tem perature and relative hum id
ity are above the average. A very hygroscopic m ixture absorbs m oisture, under the conditions specified, to such an extent th a t the m ixture is too dam p for use.
567
568 I N D U S T R I A L A N D E N G I N E E R I N G C S I U M S S
5 10-5-10 Very hygroscopic 3 .4 14.3 38.2 50.8
Ta b l e I I I . Co m p o s i t i o no f Fe r t i l i z e r Mi x t u r e s to interfere seriously w ith th eir drillability in the field.
M ixed fertilizers will absorb or lose m oisture until th ey reach equilibrium w ith th e hum idity of th e air w ith which they are in contact. In th e m ethod developed by Yee and D avis (5) for determ ining th e equilibrium m oisture content of fertilizers, 4- gram sam ples of th e m aterials to te d were exposed for 48 hours to circulating air in each of th ree closed chambers, m aintained a t 30° C. and a t relative hum idities of 59, 65, and 72% . The mois is to have an y value in forecasting th e relative hygroscopic proper
ties of commercial m ixed fertilizers. As th is m ethod offered a rated in th e field as being nonhygroscopic absorbed m oisture d u r
ing th e test to give a m axim um m oisture co n ten t of a b o u t 20%
rium m oisture content m ay b e represented as follows:
Type of Mixture show hygroscopic tendencies, depending on conditions under which th ey are packed and stored. C urve 5 shows th a t th e 10-
Inn», 1945 ^ I N D U S T R I A L A N D E N G I N E E R I N G C H E M I S T R Y 569 on any sam ple in th e set. In m ost cases these values are far above th e average for th e set, as given for each relative hum idity, th is indicates th a t th ey represent exceptional m ixtures. T hus;
th e m ixtures represented by th e m axim um values for In d ian a in 1943-44, for M assachusetts and Texas in 1942—43, and for M ississippi during bo th seasons, are subject to poor condition through excessive m oisture absorption, since they are shown to be very hygroscopic under th e conditions of th e laboratory test.
However, th e m axim um values representing these exceptionally hygroscopic m ixtures are in m ost cases lower for the past season th an for th e preceding season. T his is a fu rth er indication th a t such m ixtures containing am m onium n itra te will give no greater difficulty as a result of m oisture absorption th an those form ulated w ith sodium n itra te before solid ammonium n itra te became avail
able in q u antity.
Ta b l e V. Co m p a r is o n o p Hy g r o s c o p ic it y a t 30°
Me x ed Fe r t i l i z e r s i n 1943—44 a n d 1912-43
C . OF
Region Where
% Moisture in Mixt. a t Equilibrium with:
No. of 59% r.h. 65% r.h. 72% r.h.
Used Year Samples Max. Av. Max. Av. Max. Av.
Ind. 1942-43 17 4.7 2.8 20.7 6 .6 56.2 23.6
1943-44 15 18.6 5.5 30.4 11.8 47.5 28.5
Mass. 1942-43 12 10.7 5.7 35.1 17.6 64.0 29.9
1943-44 12 8.1 4 .7 24.5 13 6 38.7 26.3
Miss. 1942-43 10 16.8 8.5 39.2 26.8 48.5 34.4
1943-44 18 10.6 5.5 32.3 19.4 41.0 27.7
N. J. 1942-43 12 4.7 2.5 10.4 9 .7 33.8 21.9
1943-44 12 7.3 4 .0 16.1 11.0 31 4 24.4
N. C. 1942-43 12 6 .0 4.3 16.2 8.9 23.7 16.1
1943-44 12 5 5 3 .7 13.6 8.6 22.8 14.9
Texas 1942-43 12 5.3 3 .7 41.4 20.9 55.5 35.0
1943-44 _9 7.3 4.7 28.3 17._2 43.8 28.9
Total 1942-43 1943-44 75
78 4.3
4.7 14.2
13.9 2 6.4
25.3
Ta b l eV I.
Figure 1. E q u ilib riu m M o istu re C o n ten t o f F ertilizer a t D ifferen t R ela tiv e H u m id ities
th at used during th e preceding year. Sodium n itra te has been shown to produce som ew hat greater hygroscopicity th a n am monium n itrate (/) on th e basis of equal u n its of nitrogen in the mixture. A sim ilar comparison on th e basis of equal weights of these two nitrogen carriers in th e m ixture (unpublished data) also shows th a t sodium n itra te produces som ew hat greater hy
groscopicity th an amm onium n itrate. Therefore, it would ap
pear from T able IV th a t th e hygroscopicity of 1943-44 fertilizers should not vary greatly from th a t of m ixtures produced in 1942-43
D ata on th e control sta tio n sam ples (T able V) show th a t there has been no general increase in th e hygroscopicity of 1943-44 fertilizers over th a t of th e preceding year, when little solid am
monium n itra te was used. T h e m axim um m oisture value given for each relative hum idity represents th e highest value obtained
T a b l e IV . H y g r o s c o p i c M a t e r i a l s i n M i x e d F e r t i l i z e r s ( i n S h o r t T o n s )
M aterial 1942-43 1943-44«
Sodium nitrate 200,000 None
Ammonium nitrate 45,000 200,000
Urea 30,000 50,000
Sodium chloride6 110,000 120,000
Total 385,000 370,000
Mixed fertilizer 7,577,800 8,000,000«
Lb. m aterial/ton mixt. 102 92
a Based on revised nitrogen supply for 1943-44, Chemicals and Fertiliser Branch, W ar Food Administration.
& From manure salts.
0 Assuming 5% increase over 1942-43.
Mo i s t u r e Ab s o r p t i o nb y 1943-44 Fe r t i l i z e r s a t 30° C .
% Moisture in Mixt. a t Equilibrium with:
No. of 59% r.h. 65% r.h. 72% r.h.
Region Samples Max. Av. Max. Av. Max. Av.
New Eng. 26 8.1 4 .0 26.9 16.7 59.6 35.0
S. Atlantic 54 6.4 4.0 28.3 10.4 42.2 18.1
Mid. Atlantic 35 7.3 3.6 23.4 10.7 55 5 24.1
N. Central 28 18.6 5.1 30.4 10.9 47.5 25.2
S. Central 13 6.9 4.5 29.9 13.4 37.9 22.1
Total 156 4.2 11.9 23.8
T able V I shows the results of m oisture absorption tests on 156 samples representing popular grades of fertilizer used in different sections during 1943-44. The m aximum values a t 65% r.h.
represent m ixtures th a t are likely to absorb m oisture in warm hum id w eather, notw ithstanding th e fact th a t some of them were reported as satisfactory under commercial conditions of storage during th e w inter m onths when, in m ost areas, th e aqueous vapor pressure in th e atm osphere is relatively low.
M IX TU R ES W ITH POTASH CARRIERS
Ma n u r e Sa l t s. T able V II gives form ulation d a ta and results of m oisture absorption tests on typical samples of 1943-44 goods containing increasing quantities of m anure salts and varying am ounts of ammonium n itra te and PiOs. Potassium chloride supplies th e rem ainder of th e potash in all of these mixtures.
Samples 16 and 29 represent m ixtures th a t are entirely satisfac
to ry from th e standpoint of m oisture absorption. While the m ixtures listed in T able V II were reported as satisfactory a t points of m anufacture, th e laboratory tests show th a t samples 15 and 52 are hygroscopic and th a t samples 27, 31, and 50 con
tain nearly th e maximum am ount of incom patible hygroscopic salts th a t can be tolerated.
Ftercent Relative H um idity, 30®C.
S70 I N D U S T R I A L A N D E N G I N E E R I N G C
m.
37, No. 6 to potassium chloride content. A t th e higher hum idities e m oisture absorption increases w ith increase in am o u n t of po as sium chloride p resen t; a t th e low er hum idities th e reverse is tru e. T hus, in these m ixtures containing KC1 in th e solid s ta , th e sa lt p a ir N H 4N 0 3 + KC1 increases th e tendency of th e m ixtu re to absorb m oisture above 67.9% r.h . C onversely, excess potassium chloride exerts a dry in g influence on th e m ixture below 67.9% r.h. W here little or no potassium chloride is p resent, th e amm onium n itra te or its sa lt p a ir w ith some o th er com ponent exerts its influence on th e m oisture-absorbing tendency of th e m ixture.
T he results of th e m oisture abso rp tio n tests on sam ples of com
m ercial m ixtures (T able IX ) show th ere is considerable leew ay w ith respect to th e q u a n tity of am m onium n itra te a n d high- grade potassium chloride th a t m ay be com bined in th e form ula
tion of satisfacto ry m ixtures from th e sta n d p o in t of m o istu re
ab-Po t a s s iu m Su l f a t e. T he extrem ely low equilibrium m oisture content a t 65% relative hum idity of th e m ixtures listed in T able V III as com pared w ith th e average for 156 commercial samples (T able VI) shows th e beneficial effects of potassium sulfate and of sulfate of potash-m agnesia on m oisture absorption by am monium n itra te m ixtures, w ith or w ithout other potash carriers.
T his is especially tru e of sam ple 57, representing a m ixture which contains approxim ately 4 un its of K 20 from 279 pounds of m an
ure salts and 1.5 un its of nitrogen from 90 pounds of ammonium nitrate.
Hi g h- Gr a d e Po t a s s iu m Ch l o r i d e. M erz and co-workers (8) showed th a t, in th e system K +, N H 4+, C l " , N 0 3- , and w ater, w ith KC1 in excess a t th e triple salt point, the relative hum idity of th e air in equilibrium w ith sa tu rated solution is 67.9% a t 30° C.
Figure 2 shows th e effect of increasing am ounts of potassium chloride on m oisture absorption in laboratory preparations of 5-10-1 grades of m ixtures th a t are identical except with respect
Ta b l e V II. In f l u e n c e o f Ma n u r e Sa l t s o n Mo i s t u r e Ab s o r p t i o nb y Mi x e d Fe r t i l i z e r s a t 3 0 ° C .
% M oisture in Mixt.
Lab. Region
Where NH.NO«,
Units
No. Used Grade of N a
16 La. 6-8-4 None*
29 N. Y. 5-10-5 1. 1*
52 Ga. 6-8-«* 2 . 8
31 N. Y. 4-12-4 2 . 0
50 Ga. 5-10-6* 2 .5
27 N. Y. 6- 12-6 1.3«»
15 La. 4-8-8 None*
M anure
Salts* a t Equilibrium 'ith R .H . of:
Lb. Unite 59% 65% 72%
61 100 120 130130 233154
0.7 5 1.25 1.65 1.751.75 2 . 0 0 3.0 0
3 .73 .5 5.6 5.2 6 . 0 4 .9 5 .9
13.49 .5 22.0 19.2 19.7 28 .317.3
22 .9 22.8 30 .4 29.1 26.9 3 1.3 4 2.2
“ Balance of N from (NH^sSOi an d organics unless otherwise indicated.
& Balance of KaO from KC1.
* Contained 90 lb. urea-ammonia liquor.
Represents N H 4NO1 partially from nitrogen solution.
• Manufactured from base goods.
Ta b l e V III. In f l u e n c eo f Po t a s s i u m Su l f a t e o n Mo i s t u r e Ab s o r p t i o nb y Mi x e d Fe r t i l i z e r s a t 3 0 ° C .
P otash Carrier, L b ./T o n
% M oisture in Mixt.
a t E quilibrium with R .H . of:
No. Grade L b./T o n KC1 KiSOi S P M “ 59% 65% 72%
45 5-10-4 140 104 40 4 .5 10.3 17.2
57 4-8-61> 90 Î82 3 .6 9 .3 15.6
59 3-9-9* 16 200 90 3 .9 6 . 2 11 .1
60 3-9-9* 19 '3 8 328 3 .7 5 .6 8 .7
61 3-9-6 17 93 147 3 .4 5 .5 8 . 6
62 3-9-6 19 38 105 73 4 .4 6 .9 8 .9
22 3-9-6 68 69 160 3 .3 7 .8 12 .1
38 3-9-6 80 203 '90 2 .4 4 .6 5 .2
41 3-9-6 87 's é 377 4 .2 8 .7 13.1
“ Sulfate of potash-magnesia.
*> Contained 279 pounds m anure salt«.
* Contains urea-ammonia liquor.
Percent Relative Humidity, 30°C.
F igu re 2. E ffect o f P o ta ssiu m C h loride C o n ten t on M o istu re A b so rp tio n b y M ixed F ertilizers
Before m ixing After m ixin g Shelia after curing
F igu re 3. B ehavior o f G ran u lar A m m o n iu m N itr a te in F er tiliz er M ix tu res
lune. 1943 I N D U S T R I A L A N D E N G I N E E R I N G C H E M I S T R Y S71 sorption. T he high-analysis m ixtures represented by sam ples
96, 97, an d 99 were found to be hygroscopic according to th e labo m oisture absorption. U nder less favorable conditions of pack
age p rotection and clim ate, these m ixtures would be subject to poor physical condition through absorption of m oisture.
Ta b l e I X . In f l u e n c e o p Hi g h- Gr a d e Po t a s s i u m Ch l o r i d e nitrate is not sufficient to produce unsatisfactory m oisture-ab
sorbing properties in th e m ixture except in isolated cases of in
compatibility w ith other m aterials such as m anure salts, urea, or sodium nitrate. comes from th e fact th a t m ost fertilizer m ixtures contain sufficient moisture to dissolve all of th e highly soluble ammonium n itrate rosin, and paraffin. One week after mixing, practically all of the solid ammonium n itra te had disappeared. T he components of
face of the granule during th e m ixing operation. Analysis showed th a t these shells contained less th a n 5% of th e original nitrogen content of the ammonium n itrate. T he m ixture did no t show an increase in m oisture content during th e curing period. Evi
dently enough initial m oisture was available to dissolve all of the ammonium n itrate present. While th e liquid phase of the
fer-tilizer m ixture is satu rated w ith respect to o th er less soluble salts, the highly soluble ammonium n itra te is entirely in solution.
T hus th e hygroscopicity is dependent, in th is case, on the ratio of m oisture absorption from the atm osphere, and the norm al curing reactions do n o t go to completion until th e fertilizer is rem oved for m illing and bagging. This m ay induce secondary setting in th e bag as a result of th e absorption of m oisture and completion of reactions th a t would norm ally occur in the curing pile.
CHANGE IN VOLUME O F F E R T IL IZ E R LIQ UID PHASE W hile th e use of recommended am ounts of amm onium n itrate in fertilizers offers no great difficulty with respect to m oisture ab
sorption, it is frequently observed th a t a m ixture w ith a relatively high m oisture content m ay appear to become more dam p with th e use of increasing am ounts of highly soluble salts such as urea and ammonium nitrate, even though the m oisture content of the m ixture does no t change (#). This change in physical condition of th e m ixture is due to an increase in volume of th e liquid phase of th e fertilizer as a result of th e highly soluble salts going into solu
tion. I t is well known th a t green superphosphate norm ally con
572 I N D U S T R I A L A N D E N G I N E E R I N G C H E'M-fcS-ätR. Y
ryi,
MO. 6 such as urea and ammonium n itrate, m ay produce an undesirably dam p condition of th e m ixture which is no t observed w ith th e use of well-cured superphosphate. T he resulting larger volume of liquid phase appears to possess about th e same w etting powertra te and ammonium sulfate. The potash was supplied by potas
sium chloride. T ests were m ade on four separate m ixtures; one the m oisture content of the original m ixture. Increasing am ounts of ammonium n itrate in th e fertilizer caused an increase in volume of the liquid phase. Assuming this solution to have a w etting power equivalent to th a t of w ater, the apparent increase in mois
tu re content of the m ixture as a result of the use of increasing undesirably w et due to th e increase in volume of th e liquid phase as a result of the" incorporation in th e m ixture of highly soluble salts such as urea and ammonium n itra te: (2) those w ith insuf
ficient m oisture to allow solution of hygroscopic, soluble salts, thereby preventing complete reaction during th e curing process;
and (3) those th a t are hygroscopic through a com bination of in
compatible m aterials or the presence of large am ounts of very hygroscopic salts in th e solid state.
T able X gives results of m oisture absorption experim ents on several different m ixtures th a t were reported by th e m anufacturer as unsatisfactory for bagging and use. T h e first th ree samples listed are typical of th e first class of m ixtures in which th e com
bined contents of m oisture and am m onium n itra te have increased th e volume of liquid phase beyond th e range of a satisfactory dry
condition. T he m oisture absorption d a ta s ow
° Nitrogen derived from nitrogen solution.
b Contained 90 pounds NaNOj.
c Two units nitrogen derived from urea-am m onia liquor.
T he la s t th ree sam ples in T ab le X are rep resen tativ e of th e th ird class of m ixtures th a t are hygroscopic due to a com bination of m aterials incom patible w ith respect to m oisture absorption.
T he 5-10-5 m ixture, m an u factu red in N ew Jersey and containing 95 pounds am m onium n itra te an d 90 pounds of sodium nitrate, had a satisfactory physical condition w hen it was m ixed in Febru
ary ; b u t a fter bagging, it absorbed m oisture to such an extent
" T u B 5 rra 5 “ I N D U S T R I A L A N D E N G I N E E R I N G C H E M I S T R Y 573 factory in more hum id areas, provided it is used in a reasonably
sh o rt tim e a fter th e bags are opened.
H ygroscopicity is only one p roperty of a fertilizer which affects its physical conditions. A nonhygroscopic m ixture, as m easured by th e laboratory test, m ay absorb m oisture to cause secondary curing and caking in th e bag as a resu lt of insufficient m oisture
H ygroscopicity is only one p roperty of a fertilizer which affects its physical conditions. A nonhygroscopic m ixture, as m easured by th e laboratory test, m ay absorb m oisture to cause secondary curing and caking in th e bag as a resu lt of insufficient m oisture