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Effects of anticaking agents on the thermodynamics and kinetics of water sorption by potash fertilizers
POWDER TECHNOLOGY ELSEVIER
Powder Technology 98 ( 1998 ) 79-82
Effects of anticaking agents on the thermodynamics and kinetics of water sorption by potash fertilizers Lee D. Hansen a'*, Frank Hoffmann a'~, Graeme Strathdee h "Department of Chemistr3" and Biochemisto', Brigham Young Universit3', Provo, UT 84602, USA "Potash Corporation ~!fSaskatchewan, Saskatoon, Sask., STK 7C3. Canada Received 29 December 1997; received in revised form 6 March 1998
Abstract
Equilibrium water vapor pressures and kinetics of water sorption on potash fertilizers ( KCI ) were determined by a calorimetric method. Enthalpy changes for water sorption were estimated from the temperature dependence of equilibrium vapor pressures. Treatment of the fertilizers with anticaking agents had no effect on the measured parameters. Anticaking agents apparently prevent cementitious crystals from growing between particles either by preventing contact of surface solution films or by modifying the crystal habit of the cementitious layer, instead of by slowing or diminishing the sorption of water. © ! 998 Elsevier Science S.A. All rights reserved. Keywords: Agglomeration; Caking; Calorimetry; Kinetics; Potash: Thermodynamics; Water
1. Introduction
Sorption and desorption of water by particulate solids often cements the particles together, converting a free flowing material into a solid cake. Hence, the general name 'caking" for this phenomenon. Water-mediated caking occurs by two mechanisms, both involving growth of new crystals between adjacent particles, in one mechanism, exposure of a material to water vapor pressures above the equilibrium value causes partial conversion of an anhydrous material to a hydrate or of a hydrate to a higher hydrate, and growth of crystals of the new hydrate cements the particles together. Decreasing the water partial pressure removes water from the hydrate, but the cake remains because the cementitious hydrate crystals revert to a cementitious layer of the original material. In the case of hygroscopic solids that do not form solid hydrates, exposure to water vapor pressures in excess of the equilibrium vapor pressure of the saturated solution will cause a layer of saturated solution to form on the surface of the particles. Decreasing the water vapor pressure then results in precipitation of a cementitious layer between the particles. Caking is a problem with many, if not most, granular commercial materials. Prevention requires water-vapor impermeable packaging, addition of a drying agent to control the water vapor pressure inside the package, and/or coating of * Corresponding author. Present address: Mettler-Toledo, 35353 Giessen, Germany. 0032-5910/98/$19.00 © 1998 Elsevier Science S.A. All rights reserved. Pil S003 2-59 ! 0( 98 ) 0 0 0 3 7 - 0
the particles with an anticaking agent. The purpose of this study is to determine the effects of anticaking agents on the tt,:rmodynamics and kinetics of water sorption by potash (KCI) fertilizer. Potash fertilizer is generally produced by either mechanical mining of ore followed by primary flotation separation of KC! or by solution mining of evaporite deposits mainly composed of sodium and potassium chlorides, but containing chlorides and sulphates of magnesium and calcium. In solution mining, KCI is separated from the brine by fractional crystallization, and the centrifuged crystals are usually dried alter partial washing or displacement of residual brine. The KCI crystals are thus coated with a mixed layer containing K ÷ , Na +, Mg 2+, Ca 2+. CI-, and SO4 2- ions. Sorption and/ or desorption of water vapor by this layer controls the caking behavior of the product. A variety of amphiphilic organic liquids, typically a mixture of fatty amine and hydrocarbon oils and/or fatty acids, are successful at preventing caking of potash when used as a coating on the particles. The hydrophobic hydrocarbon moiety is thought to slow or prevent access of water vapor to the surface of the crystals. However, hydrocarbon oils alone do not inhibit caking. Long-chain amines, alone or in combination, always contribute positive anticaking properties. The polar head group is thought to function by allowing the anticaking agent to evenly wet and adhere to the ionic crystals. Imaging studies show, however, that the surf'ace is incom-
80
L.D. Hansen et aLI Powder Technology 98 (I 998) 79--82
pletely covered at the usual additive ratio of I to 2 kg tonneof granular product.
2. Materials and methods The materials studied are listed in Table I. Eighteen of the materials are nine pairs of commercial materials from nine sources with one member of each pair treated with anticaking agent (each source used a different agent). Three of the materials are dried brine solids. Synthetic carnalite (potassium magnesium chloride hexahydrate, KMgCI3.6H,O, was also included for comparison. All samples were supplied by member companies of the Saskatchewan Potash Producers Association as a contribution to a joint program on the improvement of fertilizer product quality. All samples as received were stored at room temperature in sealed glass bottles which were only opened to the laboratory atmosphere during transfer of aliquots to the calorimeter ampules. The method for determination of equilibrium water vapor pressures and enthalpy changes and rate constant:~ for water vapor sorption have been described 113]. Briefly, the heat rate ( ~ ) produced by the sample is measured in an isothermal calorimeter as the water vapor pressure in the atmosphere over the sample is continuously scanned. Abrupt changes in (d~b/dp,,o) indicate the appearance of a new phase and occur at the equilibrium vapor pressure of the new phase I I I. The value of (d~/dpn,o) is proportional to the product of the rate constant for water sorption onto the sample, the surface area of the sample accessible to water vapor, and the enthalpy change for sorption of water vapor onto the sample [ I ]. All the samples are granular with similar appearance as to particle size. Surface areas were not measured, but are assumed to be the same between paired samples. Measurements were made at sample temperatures o1" 24.78 and 34.74°C. Water vapor pressure over the sample was continuously scanned from about I kPa to the saturation vapor pressure of pure water at the sample temperature. Water vapor pressure in the calorimeter ampule was changed by scanning the temperature of a vessel of water external to the calorimeter
at a scan rate of either I °C h - ~or 7°C h - ~. A constant flow of nitrogen gas (at approximately ! ml min -~) was used to transport the water vapor to the calorimeter ampule. Measurements at each temperature were made at least in duplicate, most in triplicate, and a few in quadruplicate.
3. Results The raw data from this study are closely similar to, and the results in agreement with, a previous study [2] of water sorption reactions on untreated potash fertilizer. Table 2 gives the values measured for equilibrium water vapor pressures (given both as partial pressures, P~'~,o, and as reiative humidities, hr¢), the rate of change of the measured heat rate with water vapor pressure during the sorption reaction, (d@/dpH,o), and the enthalpy change for sorption of water vapor, AH. The rate of scanning the water vapor pressure made no significant difference, thus showing that the I)ctt,.o values are indeed equilibrium constants. With the exception of sample 10b, two sorption reactions are observed for all samples. Treated sample 10b did not exhibit the first reaction although the untreated sample 10a did. The first reaction exhibits an equilibrium vapor pressure of about 55% relative humidity for the brine samples ( la and I b), of about 61% for both treated and untreated samples, and of about 61% for the KMgCI.a.6H.,O sample. The enthalpy change lbr the first sorption reaction is about 34 kJ m o l ~ for brine samples ( I a and I b). about 42 kJ mol - mfor both treated and untreated samples, and 39 kJ tool-~ for KMgC!~. 6H_,O. For the second reaction, equilibrium vapor pressures are about 8 ! % for brine samples ( I a and I b), about 87% for both treated and untreated samples, and about 84% for KMgCI.~. 6H,O. Enthalpy changes are 32 kJ mol--~ for brine sample l a, 45 kJ mol ~ for brine sample I b. about 42 kJ mol mfor both treated and untreated samples, and 41 kJ m o l ~ lbr KMgCI~.6H,,O. The (dqJ/dp,,o) values vary from source to source, probably because of differences in composition of the surface and particle size, but show no consistent differences between the treated and untreated sam-
Table I Potash fi:rtilizers ( KCI ) and related material.~ Sample number
Ore source, eval~wite layer mined
Separation method
la Ib Ic 2 3 4 5
Esterhazy member Esterhazy member Synthetic canudite Unknown Various Esterhazy member Patience Lake member Patience Lake member Esterhazy member Esterhazy member Patience Lake member Patience Lake member
Evaponrion of high-MgCI, process brine Evaporation ()f residual liquor from l a Crystallization of KMgCI ~. 6H,O Flotation Crystal h.:atitm Flotation in high-MgCI2 process brine Flotation in Iow-MgCI, process brine Flotation in Iow-MgCI, process brine Flotation in high-MgCI2 process brine Flotation in high-MgCI2 process brine Flotation in iow-MgCI2 process brine Crystallization
6
7 8 9 IO
L.D. Hansen et al. / Powder Technology 9811998; 79-82
81
Table 2
Equilibrium vapor pressures ( P~~:o, h~ ). rate constant (dcb/dpH,o). and enthalpy change (AH) for sorption reactions of water vapor on l~)tash fertilizers ( KCI and related materials
Sample no."
Firstreactionh
la
Ph.o (kPa)
h'~ (el,)
(dcb/dpH,o) (/zW kPa - ) )
AH (kJ mnl ))
P)i,o (kPa)
h~ (c/,)
(ddddpH,,)) (/sW kPa ) )
AH ('kJ tool ))
1.74 5:0.04
56+ I 55 +_3 54+ I 53+ I 63 + I 59+0 61 + I 58+11 57 + 2 56 + I 63+1) 61 + 0 635: I 611+. I 58 5:2 58 5:0 62+1 61 5:3 60+ i 6(15:0 685:2 675:11 58+ I 595:1 61 _+ I 58 5: I 625:2 60 5:0 63+ I 58_+0 61 5: I 575:11 63 5:11 59+ I 62+ I 58 5:0 60 + I 575:(I 64 -4- I 60 5:0
247+ 13 292 + 22 192+ 17 155+29 130 + 26 114+ 10 177+5 175+11 147 + 8 135 5:34 191 + 8 131 5: ! t) 182+4 124+8 128 5:18 I I I -4- 16 11111+6 96+27 21)1 5:39 11125: I I 835:8 825:15 204_+63 1975:74 200 5:32 172 + ! 2 208+54 154 + 44 192-+6 203+ 17 162 + 28 1119-+9 243 _+ I I 215+40 1535: II 150 5:16 99 5:26 1045: I 228 5:12
32+7
2.61 +0.08 4.47 + 0.05 2.38+0.03 4.28+0.06 2.83 + 0.03 4.51 +11.112 2.56_+0.01 4.67+1).17 2.56 + 0.02 4.47 + 0.01 2.61 +0.03 4.475:0.01 2.705:0.0h 4.78 +0.05 2.59 5:0.03 4.46 + 0.03 2.63_+0.04 4.99+11.113 2.57+0.03 4.47 + 0.03 2.58 ~0.04 4.35 =:0.03 2.66-+0.05 4.61 +0.03 2.74 5:0.04 4.49 _+0.08 2.78_+0.10 5. ! 3 _+0.06 2.92-+0.113 4.92-+(I.116 2.66 + 0.04 4.71 4-@112 2.81~ _+1).1)3 4.71 +0.08 2.55-+0.07 4.47 + 0.1) I 2.56 5:0.115 4.36+0.04 2.64 5:0.02
85+3 82 + I 77+ ! 78+ I 85 + I 82+ I 83+0 85+3 83 + I 82 5:(1 85+ I 825:11 88_+2 87_+ I 84 5: I 82 _+ I 85_+2 92_+ I 835: I 82 _+ I 83+ I 80+0 86-t-2 84+ I 89 -t- 3 82 5: I 90+3 94 _-4-I 95+ I Oi 4- I 86 _+ I 86-+11 q4 4- I 86-1-2 835:2 82 5:0 83 + 2 80+ I 86 4- 0
266+24 300 + i 5 279+ Ii 240+35 169 + 29 159+ 18 300-t-24 258+21 251 + 13 296 5:42 253+21 181 +85 266+ 16 162+22 21 I -+6 300 _+ 17 277-t- 18 2925:12 319-+20 287 + 33 311 +7 273-+ 18 248-+311 199_+51 231 _+23 250 _+35 195+211 226 + 47 184-+30 228_+58 218 _+74 172_+53 188 _+3q 23(')5:44 199+2 t) 272 5:6 I 231 5:68 249-+48 288 5: I I
32+2
4.45 5:0.03
81 5:0
251 5:45
2.58 5:0.01 4.45 5:0.02
84 + 0 81 5:0
2305:6 232 5:48
2.93+0.15 1.67 4-_0.03 2.87 + 0.03 i .94 + 0.03 3.24 + 0.02
Ib Ic 2a
1.89 4- 0.04
2b
3.15 4- 0.02 1.73 5:0.04 3.05 5:0.04
1.95 4- 0.0 I
3a
3.35 + 0.01 3h
1.93 5:0.03
3.27 5:0.04 1.8(15:0.05 3.14 5:0.03 1.91 + 0.03
4a 4b
3.35 + 0.15
5a
1.85 5:0.04 ~.29 5:0.01
5b
2.10 5:0.06 3.66 + 0.03 .78 5:0.03 3.22 5:O.08 .87 5:0.04 3.14 5:0.03 .92 5:0.06
61t 6b 7a
3.28 5:0.0 I 7h 8;i 8h ()a qh IOa
Second reactionh
.95 5:0.03 3.17_+0.01 .88 5:0.03 3.13 -+0.03 .t)3 _-4-0.01 3.21 5:0.05 .91 5:0.02 3.18 + 0.02 .86 + 0.03 3.13 5:0.02 1.98 5:0.03 3,30 + 0.02
35+3 39 + ! 41 5: I 42 5:4 425: I 411_+2 44 5:5 43_+5 445:3 425:3 465:3 40 5:2 41 5:3 375:2 39 -i-_2 39 _+2 39-+ I 4(1 + 2 39 + I
173 5:6
10b
45+2 41 5:2 43+6 43 + I 435:4 45+6 43 + 3 455:2 43_+2 40+_2 425:2 38 _+3 44-+3 411-+2 44 -+ 2 39 + 2 42-+ I 41 + 2 40 -+ I 42 + I
The lirst row of data for each sample is at 24.78"C. the second low at 34.74"C. Except Ior the materials in the lirst set ( Ia- I c ). the samples labeled ( a ) ;ire untreated. ( b ) samples are treated with an anticaking amine and dust control oil. "See Table I for a description of the samples. h Precision limits are given as the standard deviation of the mean of two to four determinations. pies. T h e a b s e n c e o f a c o n s i s t e n t t e m p e r a t u r e d e p e n d e n c e of
(dq)/dplt,()) i n d i c a t e s t h e a c t i v a t i o n e n e r g y f o r w a t e r s o r p tion is a p p r o x ! m a t e l y zero.
4. Discussion
reaction with KMgCIs. 6H_,O ( sample I c) suggests that the reaction occurring on the fertilizer samples is formation of a hydrated double salt of magnesium. The second reaction appears to be formation of a saturated solution of KCI. Further arguments tbr assignment of these identities to the observed r e a c t i o n s c a n b e f o u n d in Ref. 12 l. T h e m o s t s i g n i f i c a n t f a c t e m e r g i n g f r o m T a b l e 2 is that
The similarity of the thermodynamics of the first sorption reaction on samples l a and lb, and 2 - 1 0 to those of the
( e x c e p t f o r s a m p l e s 10a a n d 10b) t h e r e a r e n o l a r g e d i f f e r e n c e s in either the thermodynamics or kinetics of water sorp-
82
L.D. Hansen el al. /Powder Technology 98 (1998) 79-82
tion between treated and untreated materials. All of the treatments are effective at preventing caking, however. All of the treated samples were free-flowing as received by us, but all of the untreated samples showed poor pourability and some, the presence of weakly cemented, aggregated lumps. Thus, with the possible exception of sample lOb, the mechanism of action of the anticaking agents is not the alteration of either the kinetics or thermodynamics of water sorption as previously supposed. Because the anticaking agents are effective, but do not impede water vapor sorption, they must act by preventing cementation of the particles. The agents could act by preventing contact of saturated solutions on the surfaces of adjacent panicles, the oily film at the surface preventing crystal growth between adjacent particles. The effectiveness of the agents despite incomplete surface coverage may be because they have a higher affinity for a hydrated surface than an anhydrous surface. Another possible mechanism is that anticaking agents may affect the crystal morphology of the cementitious layer. In any case, the data from this study disprove mechanisms requiring retardation of moisture transport and sorption. Although present additives are effective, knowl-
edge of the mechanism of action of anticaking agents should lead to selection and design of even more effective materials for this purpose.
Acknowledgements This work was funded in part by the Saskatchewan Potash Producers Association. LDH expresses appreciation for support from BYU and from CNRS through the Laboratoire de Thermodynamique et Genie Chimique, Universit6 Blaise Pascal, Clermont-Fen'and. FH expresses appreciation for support from the German Academic Exchange Service.
References I I I I.,D. Hansen, J.W. Crawfi)rd. D.R. Keiser. R.W. Wood. Calorimetric method for rapid determination of critical water vapor pressure and kinetics of water sorption on hygroscopic compounds. Int. J. Pharm. 135 (1996) 31--42. J 2 J M.T. Pyne, G. Strathdee, L.D. Hansen, Water vapor sorption by potash fertilizers studied by a kinetic method, Thermochim. Acta 273 ( 1996 ) 277-285.
Powder Technology 98 ( 1998 ) 79-82
Effects of anticaking agents on the thermodynamics and kinetics of water sorption by potash fertilizers Lee D. Hansen a'*, Frank Hoffmann a'~, Graeme Strathdee h "Department of Chemistr3" and Biochemisto', Brigham Young Universit3', Provo, UT 84602, USA "Potash Corporation ~!fSaskatchewan, Saskatoon, Sask., STK 7C3. Canada Received 29 December 1997; received in revised form 6 March 1998
Abstract
Equilibrium water vapor pressures and kinetics of water sorption on potash fertilizers ( KCI ) were determined by a calorimetric method. Enthalpy changes for water sorption were estimated from the temperature dependence of equilibrium vapor pressures. Treatment of the fertilizers with anticaking agents had no effect on the measured parameters. Anticaking agents apparently prevent cementitious crystals from growing between particles either by preventing contact of surface solution films or by modifying the crystal habit of the cementitious layer, instead of by slowing or diminishing the sorption of water. © ! 998 Elsevier Science S.A. All rights reserved. Keywords: Agglomeration; Caking; Calorimetry; Kinetics; Potash: Thermodynamics; Water
1. Introduction
Sorption and desorption of water by particulate solids often cements the particles together, converting a free flowing material into a solid cake. Hence, the general name 'caking" for this phenomenon. Water-mediated caking occurs by two mechanisms, both involving growth of new crystals between adjacent particles, in one mechanism, exposure of a material to water vapor pressures above the equilibrium value causes partial conversion of an anhydrous material to a hydrate or of a hydrate to a higher hydrate, and growth of crystals of the new hydrate cements the particles together. Decreasing the water partial pressure removes water from the hydrate, but the cake remains because the cementitious hydrate crystals revert to a cementitious layer of the original material. In the case of hygroscopic solids that do not form solid hydrates, exposure to water vapor pressures in excess of the equilibrium vapor pressure of the saturated solution will cause a layer of saturated solution to form on the surface of the particles. Decreasing the water vapor pressure then results in precipitation of a cementitious layer between the particles. Caking is a problem with many, if not most, granular commercial materials. Prevention requires water-vapor impermeable packaging, addition of a drying agent to control the water vapor pressure inside the package, and/or coating of * Corresponding author. Present address: Mettler-Toledo, 35353 Giessen, Germany. 0032-5910/98/$19.00 © 1998 Elsevier Science S.A. All rights reserved. Pil S003 2-59 ! 0( 98 ) 0 0 0 3 7 - 0
the particles with an anticaking agent. The purpose of this study is to determine the effects of anticaking agents on the tt,:rmodynamics and kinetics of water sorption by potash (KCI) fertilizer. Potash fertilizer is generally produced by either mechanical mining of ore followed by primary flotation separation of KC! or by solution mining of evaporite deposits mainly composed of sodium and potassium chlorides, but containing chlorides and sulphates of magnesium and calcium. In solution mining, KCI is separated from the brine by fractional crystallization, and the centrifuged crystals are usually dried alter partial washing or displacement of residual brine. The KCI crystals are thus coated with a mixed layer containing K ÷ , Na +, Mg 2+, Ca 2+. CI-, and SO4 2- ions. Sorption and/ or desorption of water vapor by this layer controls the caking behavior of the product. A variety of amphiphilic organic liquids, typically a mixture of fatty amine and hydrocarbon oils and/or fatty acids, are successful at preventing caking of potash when used as a coating on the particles. The hydrophobic hydrocarbon moiety is thought to slow or prevent access of water vapor to the surface of the crystals. However, hydrocarbon oils alone do not inhibit caking. Long-chain amines, alone or in combination, always contribute positive anticaking properties. The polar head group is thought to function by allowing the anticaking agent to evenly wet and adhere to the ionic crystals. Imaging studies show, however, that the surf'ace is incom-
80
L.D. Hansen et aLI Powder Technology 98 (I 998) 79--82
pletely covered at the usual additive ratio of I to 2 kg tonneof granular product.
2. Materials and methods The materials studied are listed in Table I. Eighteen of the materials are nine pairs of commercial materials from nine sources with one member of each pair treated with anticaking agent (each source used a different agent). Three of the materials are dried brine solids. Synthetic carnalite (potassium magnesium chloride hexahydrate, KMgCI3.6H,O, was also included for comparison. All samples were supplied by member companies of the Saskatchewan Potash Producers Association as a contribution to a joint program on the improvement of fertilizer product quality. All samples as received were stored at room temperature in sealed glass bottles which were only opened to the laboratory atmosphere during transfer of aliquots to the calorimeter ampules. The method for determination of equilibrium water vapor pressures and enthalpy changes and rate constant:~ for water vapor sorption have been described 113]. Briefly, the heat rate ( ~ ) produced by the sample is measured in an isothermal calorimeter as the water vapor pressure in the atmosphere over the sample is continuously scanned. Abrupt changes in (d~b/dp,,o) indicate the appearance of a new phase and occur at the equilibrium vapor pressure of the new phase I I I. The value of (d~/dpn,o) is proportional to the product of the rate constant for water sorption onto the sample, the surface area of the sample accessible to water vapor, and the enthalpy change for sorption of water vapor onto the sample [ I ]. All the samples are granular with similar appearance as to particle size. Surface areas were not measured, but are assumed to be the same between paired samples. Measurements were made at sample temperatures o1" 24.78 and 34.74°C. Water vapor pressure over the sample was continuously scanned from about I kPa to the saturation vapor pressure of pure water at the sample temperature. Water vapor pressure in the calorimeter ampule was changed by scanning the temperature of a vessel of water external to the calorimeter
at a scan rate of either I °C h - ~or 7°C h - ~. A constant flow of nitrogen gas (at approximately ! ml min -~) was used to transport the water vapor to the calorimeter ampule. Measurements at each temperature were made at least in duplicate, most in triplicate, and a few in quadruplicate.
3. Results The raw data from this study are closely similar to, and the results in agreement with, a previous study [2] of water sorption reactions on untreated potash fertilizer. Table 2 gives the values measured for equilibrium water vapor pressures (given both as partial pressures, P~'~,o, and as reiative humidities, hr¢), the rate of change of the measured heat rate with water vapor pressure during the sorption reaction, (d@/dpH,o), and the enthalpy change for sorption of water vapor, AH. The rate of scanning the water vapor pressure made no significant difference, thus showing that the I)ctt,.o values are indeed equilibrium constants. With the exception of sample 10b, two sorption reactions are observed for all samples. Treated sample 10b did not exhibit the first reaction although the untreated sample 10a did. The first reaction exhibits an equilibrium vapor pressure of about 55% relative humidity for the brine samples ( la and I b), of about 61% for both treated and untreated samples, and of about 61% for the KMgCI.a.6H.,O sample. The enthalpy change lbr the first sorption reaction is about 34 kJ m o l ~ for brine samples ( I a and I b). about 42 kJ mol - mfor both treated and untreated samples, and 39 kJ tool-~ for KMgC!~. 6H_,O. For the second reaction, equilibrium vapor pressures are about 8 ! % for brine samples ( I a and I b), about 87% for both treated and untreated samples, and about 84% for KMgCI.~. 6H,O. Enthalpy changes are 32 kJ mol--~ for brine sample l a, 45 kJ mol ~ for brine sample I b. about 42 kJ mol mfor both treated and untreated samples, and 41 kJ m o l ~ lbr KMgCI~.6H,,O. The (dqJ/dp,,o) values vary from source to source, probably because of differences in composition of the surface and particle size, but show no consistent differences between the treated and untreated sam-
Table I Potash fi:rtilizers ( KCI ) and related material.~ Sample number
Ore source, eval~wite layer mined
Separation method
la Ib Ic 2 3 4 5
Esterhazy member Esterhazy member Synthetic canudite Unknown Various Esterhazy member Patience Lake member Patience Lake member Esterhazy member Esterhazy member Patience Lake member Patience Lake member
Evaponrion of high-MgCI, process brine Evaporation ()f residual liquor from l a Crystallization of KMgCI ~. 6H,O Flotation Crystal h.:atitm Flotation in high-MgCI2 process brine Flotation in Iow-MgCI, process brine Flotation in Iow-MgCI, process brine Flotation in high-MgCI2 process brine Flotation in high-MgCI2 process brine Flotation in iow-MgCI2 process brine Crystallization
6
7 8 9 IO
L.D. Hansen et al. / Powder Technology 9811998; 79-82
81
Table 2
Equilibrium vapor pressures ( P~~:o, h~ ). rate constant (dcb/dpH,o). and enthalpy change (AH) for sorption reactions of water vapor on l~)tash fertilizers ( KCI and related materials
Sample no."
Firstreactionh
la
Ph.o (kPa)
h'~ (el,)
(dcb/dpH,o) (/zW kPa - ) )
AH (kJ mnl ))
P)i,o (kPa)
h~ (c/,)
(ddddpH,,)) (/sW kPa ) )
AH ('kJ tool ))
1.74 5:0.04
56+ I 55 +_3 54+ I 53+ I 63 + I 59+0 61 + I 58+11 57 + 2 56 + I 63+1) 61 + 0 635: I 611+. I 58 5:2 58 5:0 62+1 61 5:3 60+ i 6(15:0 685:2 675:11 58+ I 595:1 61 _+ I 58 5: I 625:2 60 5:0 63+ I 58_+0 61 5: I 575:11 63 5:11 59+ I 62+ I 58 5:0 60 + I 575:(I 64 -4- I 60 5:0
247+ 13 292 + 22 192+ 17 155+29 130 + 26 114+ 10 177+5 175+11 147 + 8 135 5:34 191 + 8 131 5: ! t) 182+4 124+8 128 5:18 I I I -4- 16 11111+6 96+27 21)1 5:39 11125: I I 835:8 825:15 204_+63 1975:74 200 5:32 172 + ! 2 208+54 154 + 44 192-+6 203+ 17 162 + 28 1119-+9 243 _+ I I 215+40 1535: II 150 5:16 99 5:26 1045: I 228 5:12
32+7
2.61 +0.08 4.47 + 0.05 2.38+0.03 4.28+0.06 2.83 + 0.03 4.51 +11.112 2.56_+0.01 4.67+1).17 2.56 + 0.02 4.47 + 0.01 2.61 +0.03 4.475:0.01 2.705:0.0h 4.78 +0.05 2.59 5:0.03 4.46 + 0.03 2.63_+0.04 4.99+11.113 2.57+0.03 4.47 + 0.03 2.58 ~0.04 4.35 =:0.03 2.66-+0.05 4.61 +0.03 2.74 5:0.04 4.49 _+0.08 2.78_+0.10 5. ! 3 _+0.06 2.92-+0.113 4.92-+(I.116 2.66 + 0.04 4.71 4-@112 2.81~ _+1).1)3 4.71 +0.08 2.55-+0.07 4.47 + 0.1) I 2.56 5:0.115 4.36+0.04 2.64 5:0.02
85+3 82 + I 77+ ! 78+ I 85 + I 82+ I 83+0 85+3 83 + I 82 5:(1 85+ I 825:11 88_+2 87_+ I 84 5: I 82 _+ I 85_+2 92_+ I 835: I 82 _+ I 83+ I 80+0 86-t-2 84+ I 89 -t- 3 82 5: I 90+3 94 _-4-I 95+ I Oi 4- I 86 _+ I 86-+11 q4 4- I 86-1-2 835:2 82 5:0 83 + 2 80+ I 86 4- 0
266+24 300 + i 5 279+ Ii 240+35 169 + 29 159+ 18 300-t-24 258+21 251 + 13 296 5:42 253+21 181 +85 266+ 16 162+22 21 I -+6 300 _+ 17 277-t- 18 2925:12 319-+20 287 + 33 311 +7 273-+ 18 248-+311 199_+51 231 _+23 250 _+35 195+211 226 + 47 184-+30 228_+58 218 _+74 172_+53 188 _+3q 23(')5:44 199+2 t) 272 5:6 I 231 5:68 249-+48 288 5: I I
32+2
4.45 5:0.03
81 5:0
251 5:45
2.58 5:0.01 4.45 5:0.02
84 + 0 81 5:0
2305:6 232 5:48
2.93+0.15 1.67 4-_0.03 2.87 + 0.03 i .94 + 0.03 3.24 + 0.02
Ib Ic 2a
1.89 4- 0.04
2b
3.15 4- 0.02 1.73 5:0.04 3.05 5:0.04
1.95 4- 0.0 I
3a
3.35 + 0.01 3h
1.93 5:0.03
3.27 5:0.04 1.8(15:0.05 3.14 5:0.03 1.91 + 0.03
4a 4b
3.35 + 0.15
5a
1.85 5:0.04 ~.29 5:0.01
5b
2.10 5:0.06 3.66 + 0.03 .78 5:0.03 3.22 5:O.08 .87 5:0.04 3.14 5:0.03 .92 5:0.06
61t 6b 7a
3.28 5:0.0 I 7h 8;i 8h ()a qh IOa
Second reactionh
.95 5:0.03 3.17_+0.01 .88 5:0.03 3.13 -+0.03 .t)3 _-4-0.01 3.21 5:0.05 .91 5:0.02 3.18 + 0.02 .86 + 0.03 3.13 5:0.02 1.98 5:0.03 3,30 + 0.02
35+3 39 + ! 41 5: I 42 5:4 425: I 411_+2 44 5:5 43_+5 445:3 425:3 465:3 40 5:2 41 5:3 375:2 39 -i-_2 39 _+2 39-+ I 4(1 + 2 39 + I
173 5:6
10b
45+2 41 5:2 43+6 43 + I 435:4 45+6 43 + 3 455:2 43_+2 40+_2 425:2 38 _+3 44-+3 411-+2 44 -+ 2 39 + 2 42-+ I 41 + 2 40 -+ I 42 + I
The lirst row of data for each sample is at 24.78"C. the second low at 34.74"C. Except Ior the materials in the lirst set ( Ia- I c ). the samples labeled ( a ) ;ire untreated. ( b ) samples are treated with an anticaking amine and dust control oil. "See Table I for a description of the samples. h Precision limits are given as the standard deviation of the mean of two to four determinations. pies. T h e a b s e n c e o f a c o n s i s t e n t t e m p e r a t u r e d e p e n d e n c e of
(dq)/dplt,()) i n d i c a t e s t h e a c t i v a t i o n e n e r g y f o r w a t e r s o r p tion is a p p r o x ! m a t e l y zero.
4. Discussion
reaction with KMgCIs. 6H_,O ( sample I c) suggests that the reaction occurring on the fertilizer samples is formation of a hydrated double salt of magnesium. The second reaction appears to be formation of a saturated solution of KCI. Further arguments tbr assignment of these identities to the observed r e a c t i o n s c a n b e f o u n d in Ref. 12 l. T h e m o s t s i g n i f i c a n t f a c t e m e r g i n g f r o m T a b l e 2 is that
The similarity of the thermodynamics of the first sorption reaction on samples l a and lb, and 2 - 1 0 to those of the
( e x c e p t f o r s a m p l e s 10a a n d 10b) t h e r e a r e n o l a r g e d i f f e r e n c e s in either the thermodynamics or kinetics of water sorp-
82
L.D. Hansen el al. /Powder Technology 98 (1998) 79-82
tion between treated and untreated materials. All of the treatments are effective at preventing caking, however. All of the treated samples were free-flowing as received by us, but all of the untreated samples showed poor pourability and some, the presence of weakly cemented, aggregated lumps. Thus, with the possible exception of sample lOb, the mechanism of action of the anticaking agents is not the alteration of either the kinetics or thermodynamics of water sorption as previously supposed. Because the anticaking agents are effective, but do not impede water vapor sorption, they must act by preventing cementation of the particles. The agents could act by preventing contact of saturated solutions on the surfaces of adjacent panicles, the oily film at the surface preventing crystal growth between adjacent particles. The effectiveness of the agents despite incomplete surface coverage may be because they have a higher affinity for a hydrated surface than an anhydrous surface. Another possible mechanism is that anticaking agents may affect the crystal morphology of the cementitious layer. In any case, the data from this study disprove mechanisms requiring retardation of moisture transport and sorption. Although present additives are effective, knowl-
edge of the mechanism of action of anticaking agents should lead to selection and design of even more effective materials for this purpose.
Acknowledgements This work was funded in part by the Saskatchewan Potash Producers Association. LDH expresses appreciation for support from BYU and from CNRS through the Laboratoire de Thermodynamique et Genie Chimique, Universit6 Blaise Pascal, Clermont-Fen'and. FH expresses appreciation for support from the German Academic Exchange Service.
References I I I I.,D. Hansen, J.W. Crawfi)rd. D.R. Keiser. R.W. Wood. Calorimetric method for rapid determination of critical water vapor pressure and kinetics of water sorption on hygroscopic compounds. Int. J. Pharm. 135 (1996) 31--42. J 2 J M.T. Pyne, G. Strathdee, L.D. Hansen, Water vapor sorption by potash fertilizers studied by a kinetic method, Thermochim. Acta 273 ( 1996 ) 277-285.