Solid-liquid phase equilibrium for the system ammonium polyphosphate-urea ammonium nitrate-potassium chloride-water at 273.2 K

Xingjuan Liang,Dehua Xu,Zhengjuan Yan*,Jingxu Yang,Xinlong Wang,Zhiye Zhang,Jingli Wu,Honggang Zhen

Ministry of Education Research Center for Comprehensive Utilization and Clean Processing Engineering of Phosphorus Resources,College of Chemical Engineering,Sichuan University,Chengdu 610065,China

Keywords:Water-soluble ammonium polyphosphate Urea ammonium nitrate (UAN)Phase equilibria Solubility Thermodynamics

ABSTRACT Based on the dynamic method,a quaternary system of ammonium polyphosphate(APP)-urea ammonium nitrate (UAN,CO(NH2)2-NH4NO3)-potassium chloride (KCl)-H2O and its subsystems (APP-[CO(NH2)2-NH4NO3]-H2O,KCl-[CO(NH2)2-NH4NO3]-H2O and APP-KCl-H2O) were systematically investigated at the temperature of 273.2 K.Each ternary phase diagram contains one invariant point and three crystallization regions.The crystallization regions are: (1)(NH4)3HP2O7,(NH4)4P2O7 and ((NH4)3HP2O7+(NH4)4P2O7) for APP-[CO(NH2)2-NH4NO3]-H2O diagram;(2) KCl,KNO3 and(KCl+KNO3) for KCl-[CO(NH2)2-NH4NO3]-H2O diagram and (3) (NH4)3HP2O7,KCl and((NH4)3HP2O7+KCl) for APP-KCl-H2O diagram.The quaternary phase diagram of APP-[CO(NH2)2-NH4NO3]-KCl-H2O has no quaternary invariant point but includes four solid phase crystallization regions,i.e.,(NH4)3HP2O7,(NH4)4P2O7,KNO3 and KCl,in which the KNO3 region occupies the largest area.The maximum total nutrient content (N+P2O5+K2O) existing as ionic forms in the APP-[CO(NH2)2-NH4NO3]-H2O,KCl-[CO(NH2)2-NH4NO3]-H2O,APP-KCl-H2O and quaternary systems is 44.70%,32.86%,45.56% and 46.23% (mass),respectively,indicating that the maximum nutrient content can be reached using raw materials of the corresponding systems to prepare liquid fertilizer.In the quaternary system,the content of -N ascends with the increase of the total nutrient content,while the contents of -N and CO(NH2)2-N increase with elevated total N.This work can help optimize the operating parameters for the production,storage and transportation of liquid fertilizers.

1.Introduction

As a significant fertilizer,water-soluble ammonium polyphosphate (APP) is generally used as a basal P source in combination with other nutrients(N,K,Zn,Mg,etc.)during a practical fertilization[1-6].The producing processes of water-soluble APP have two main types of phosphoric acid (PA) ammoniation and ammonium phosphate (AP)-urea condensation [7,8].As early as 1960s,Tennessee Valley Authority (TVA) [9] successfully manufactured the water-soluble APP with appropriate polymerization degree distribution using PA ammoniation method [7].Water-soluble APP had been widely used in developed countries such as Europe and America,and the output of water-soluble APP in the United States exceeded 2 million tons annually [10].The common fertilizer grades for water-soluble APP are 11-37-0 (i.e.,11% (mass) N,37%(mass) P2O5and 0% (mass) K2O in the fertilizer),8-24-0 and 10-34-0.However,the fore mentioned water-soluble APPs are aquiferous,which need to be stored at a relatively low temperature for hydrolysis resistance [11].Moreover,water-contained products could require more input in transportation than corresponding granules,due to solution’s lower nutrient content.Hence,a new kind of solid water-soluble APP has been produced through APurea method in China [8].The grade of this APP is 18-58-0 with 77%of phosphorus as pyrophosphate[12],providing expected prospects for its transportation convenience.

Phase diagram plays an important role in guiding the application of water-soluble APP in agriculture.In the 1950s Slacketal.[13] explored the quaternary systems’ solubility of CO(NH2)2-NH3-H3PO4-KCl-H2O with fixed NH3-H3PO4molar ratios of 1.5,1.6 and 1.7,NH4NO3-NH3-H3PO4-KCl-H2O with fixed NH3-H3PO4molar ratio of 1.7,and [CO(NH2)2-NH4NO3](UAN)-NH3-H3PO4-KCl-H2O with fixed CO(NH2)2-NH4NO3mole ratio of 1.1 or fixed NH3-H3PO4mole ratio of 1.7 at 273.2 K.These quaternary systems contain up to 37% (mass) total plant-nutrient (TPN=N+P2O5+K2O).Subsequently,APP (10-34-0) and APP(11-37-0) were produced by using the superphosphoric acid containing 76% and 78%-80% P2O5,respectively,and the quaternary phase diagrams of the APP(10-34-0)-[CO(NH2)2-NH4NO3](UAN)-KCl-H2O and APP(11-37-0)-UAN-KCl-H2O were measured at 273.2 K,which can reach 45%(mass)[9]and 49%(mass)[11]TPN,respectively.Based on the aforementioned quaternary systems,the equations were established to calculate the change of TPN content under different temperatures.The hard-earned data enlightened the regularity of solubility of polyphosphates under different ammoniation degrees.In addition,some derivative systems have been also exploited.Josephetal.[14] studied the solubility of CO(NH2)2-NH3-H3PO4-H4P3O7-H5P3O10-H2O at 273.2 and 298.2 K.Leonovetal.[15] has tried on the NH4NO3-NH4H2PO4-CO(NH2)2-KCl system and the hygroscopic trend of the quaternary system was obtained.Based on the same reason of producing high-analysis liquid fertilizer,the comprehensive solubility study for phosphoric acid and ammonium thiosulfate in urea ammonium nitrate mixtures were performed [16,17].Kaddamietal.[18,19] measured the ternary system NH4H2PO4-(NH4)2HPO4-H2O to specify a metastable equilibrium of these phosphates at 264.2,266.2,303.2,318.2 and 338.2 K,respectively.

However,as a new type of solid water-soluble APP,APP(18-58-0) is quite different to draw lessons from the above thermodynamic systems just involving liquid APP(10-34-0,11-37-0),which confined the Granular APP’s application.Since the phase equilibrium of APP(18-58-0)-NH4H2PO4-(NH4)2HPO4-H2O system at 298.2 K accomplished in our previous work has opened the prelude of solid-APP dissolution [20],the phase diagram of APP(18-58-0)-[CO(NH2)2-NH4NO3]-KCl-H2O quaternary system comes at the appointed time,herein.This work can help optimize the operating parameters for the production,storage and transportation of APP contained liquid fertilizers.

2.Experimental

2.1.Materials

The experimental materials of water-soluble solid APP(18-58-0)and UAN were provided by Guizhou Chuanheng Chemical Corp.and Jinkai Investment Holding Group.,China with the purity of 98.8% (mass) and 99.6% (mass),respectively.As shown in Fig.1,the P in APP(18-58-0) consists of 23% (mass) of orthophosphate and 77% (mass) of pyrophosphate,which were determined by an ion-chromatography method [21].The contents of ammonium nitrogenand phosphorus pentoxide (P2O5) of APP(18-58-0) are 18.93% (mass) and 58.59% (mass),respectively.For convenience,APP(18-58-0) was referred to as APP thereafter.UAN solution with 32.79% (mass) N consists of 36.11% (mass) CO(NH2)2,45.60% (mass) NH4NO3and 18.25% (mass) H2O.The mass fraction of,nitrate nitrogenand urea nitrogen(CO(NH2)2-N) of UAN are 7.95% (mass),7.99% (mass) and 16.85%(mass),respectively.Analytical pure KCl was purchased from Jinshan Chemical Test,with a purity of 99.5% (mass).All reagents were used without further purification and the water used in this work was deionized water with specific conductivity of 0.055 μS.cm-1.

Fig.1.The polymerization degree distribution of ammonium polyphosphate (a) (P1,orthophosphate;P2,pyrophosphate) and X-ray diffraction pattern of APP (b).

2.2.Apparatus and procedures

The dynamic method was adopted to study the solid-liquid phase equilibrium in this work [22-25].The experimental equipment for the measurement of solubility,was in accordance with the previous work [26].The experiment apparatus of solubility measurement is shown in Fig.2.The solubility of quaternary system(APP-[CO(NH2)2-NH4NO3]-KCl-H2O)was measured as follows:(1) The UAN-water solution of known mass and composition was firstly added to the jacket beaker (250 ml),and was controlled at(273.2±0.1) K by a thermostatic water bath (DFY-10L/20,Gongyi Yuhua Instrument Co.,Ltd,China) by using mercury thermometer to detect the solution’s temperature with uncertainty of ±0.01 K.A magnetic stirrer(B4-1A,Shanghai Sile Instrument Co.,Ltd,China)was used to provide vigorous agitation for homogeneous mixing and quick equilibrium.(2) According to the preset mass fraction of K2O from 0% to 3.10%,a known weigh of KCl was put into the solution and dissolved completely.(3) A known weigh of APP was slowly added into the solution.If the added APP dissolved,a known weighted APP was added again.Until the added APP was not dissolved in the solution with continuous stirring for about 1 h and the new crystal was slowly formed,the solid-liquid phase diagram equilibria were established.

Fig.2.The experiment apparatus of solubility measurement (1.Mercury thermometer;2.Jacketed beaker;3.Magnetic rotor;4.Magnetic stirring apparatus;5.Thermostatic water bath).

As for realizing the mass fraction of K2O is over 3.10%.(1)A certain little amount of APP was first added to the UAN-H2O solution of known composition.(2) Until the added APP fully dissolved,a known weigh of KCl was put into the solution.(3)A small amount of weighed KCl was added again if the added KCl was dissolved.Until the added KCl was not dissolved in the solution with continuous stirring for about 1 h.The solid-liquid phase equilibria were reached.The mixture was kept in the thermostatic water bath without stirring for at least 1 h to settle down solids.The density(ρ) of supernatant with different composition was determinedviaa 10 ml pycnometer with uncertainty of 0.0002 g.cm-3[27,28].All of the additions were weighed by an analytical balance(AL104,Mettler Toledo,USA) with an uncertainty of ± 0.0001 g.Finally,the solid phase was vacuum filtered,freeze-dried for more than 8 h at 253.2 K for X-ray diffraction(XRD,X’Pert PRO,PANalytical B.V.,Netherlands) analysis.

The total addition of UAN,APP and KCl before the last additions of APP or KCl was the solubility of phase equilibrium systems.The content of N,P2O5and K2O in mass fraction in the saturated solution are given as:

wherem=m1+m2+m3+m4;m1,m2,m3,m4represent the masses of APP,UAN,KCl and H2O in the saturated solution,respectively;0.0795,0.0799,0.1685,0.3279,0.1825 are the mass fractions of,CO(NH2)2-N,total N and H2O in UAN,respectively;0.6318,0.1893,0.5859 are the mass fractions of K2O in KCl,NH+4-N,P2O5in APP,respectively.

3.Results and Discussion

3.1.Solid-liquid equilibrium of APP-[CO(NH2)2-NH4NO3]-H2O

The solubility of the ternary system APP-[CO(NH2)2-NH4NO3]-H2O at 273.2 K was measured and the data are presented in Table 1.UAN is one of the important nitrogen sources and the mass ratio of CO(NH2)2and NH4NO3(i.e.,ω(CO(NH2)2)/ω(NH4NO3)) in the UAN solution is fixed at 0.8.Thus,the solute of CO(NH2)2and NH4NO3in UAN solution is discussed.As shown in Fig.3,it can be seen that the isothermal solubility diagram contains one invariant point C1,two single crystallization zones(I1,II1),one eutectic crystallization zone (III1) and two solubility curves (E1C1and C1F1,corresponding to the crystallization of triammonium pyrophosphate((NH4)3HP2O7),ammonium pyrophosphate ((NH4)4P2O7).The invariant point C1is saturated with (NH4)3HP2O7and (NH4)4P2O7,and the composition of the point is ω(CO(NH2)2+NH4NO3) of 64.73%(mass)and ω(APP)of 4.73%(mass).The UAN solution contains not only CO(NH2)2and NH4NO3,but also 18.25%(mass)H2O.Point F1shows the solubility of APP in UAN solution because the UAN solution with 36.11% (mass) CO(NH2)2and 45.60% (mass)NH4NO3would not induce the crystallization at 273.2 K [17].The composition of the point F1is ω(CO(NH2)2+NH4NO3) of 81.53%(mass) and ω(APP) of 0.21% (mass),and the maximum content of CO(NH2)2and NH4NO3in APP-[CO(NH2)2-NH4NO3]-H2O system reached in this point.The illustration in Fig.2 is a partial enlargement of point F1.Because the limitation of the maximum salt content in UAN solution reaches 81.71%,the solubility data of CO(NH2)2+NH4NO3cannot be measured once larger than F1point.The red-dotted line was consequently used to represent the data from this point to U1point.Meanwhile,APP shows no salting-out effect neither on CO(NH2)2nor NH4NO3.

Table 1 The phase equilibria data of the ternary system (APP-[CO(NH2)2-NH4NO3]-H2O) at 273.2 K and atmospheric pressure①

Fig.3.Phase diagram for the ternary system APP-[CO(NH2)2-NH4NO3]-H2O at 273.2 K.W1,H2O;U1,CO(NH2)2+NH4NO3;A1,APP;E1,solubility of APP in water;C1,co-saturated point of(NH4)3HP2O7)and(NH4)4P2O7;F1,solubility of APP in UAN solution;ω1,mass fraction of CO(NH2)2+NH4NO3;ω2,mass fraction of APP.

3.2.Solid-liquid equilibrium of KCl-[CO(NH2)2-NH4NO3]-H2O

According to the experimental results in Table 2,the phase diagram of ternary system KCl-[CO(NH2)2-NH4NO3]-H2O at 273.2 K is shown in Fig.4.The isothermal solubility diagram contains one invariant point C2and two solubility curves (E2C2and C2F2, corresponding to the crystallization of KCl and KNO3,respectively).The invariant point C2is saturated with KCl and KNO3,where the composition is ω(CO(NH2)2+NH4NO3) of 9.42% (mass) and ω(KCl)of 21.28%(mass).The ternary system consists of two crystallization zones (I2,II2),one crystallization coexistence zone (III2) as well.Point F2reflects the solubility of KCl in UAN solution.The composition of the point F2is ω(CO(NH2)2+NH4NO3) of 81.53%(mass) and ω(KCl) of 0.20% (mass) and the maximum content of CO(NH2)2and NH4NO3in KCl-[CO(NH2)2-NH4NO3]-H2O system reached in this point.The solubility above F2point(81.53%(mass))in the phase equilibrium system is not measured due to the influence of water in UAN,hence,the red dotted line represents the solubility of phase equilibrium system ranging from F2to the vertex U1.The metathesis reaction of KNO3is as Eq.(8),in which two compounds exchange components to form two other compounds.

Table 2 The phase equilibria data of the ternary system (KCl-[CO(NH2)2-NH4NO3]-H2O) at 273.2 K and atmospheric pressure①

Fig.4.Phase diagram for the ternary system KCl-[CO(NH2)2-NH4NO3]-H2O at 273.2 K.W2,H2O;U2,CO(NH2)2+NH4NO3;K2,KCl;E2,solubility of KCl in water;C2,co-saturated point of KCl and KNO3;F2,solubility of KCl in UAN solution;ω1,mass fraction of CO(NH2)2+NH4NO3;ω3,mass fraction of KCl.

The substitution reaction occurs between KCl and NH4NO3with the NH4NO3increase in the UAN solution(Eq.(8)).In Fig.4,the cosaturated region of KCl+KNO3is greater than single crystal region of KCl or KNO3,while the areas of KCl and KNO3regions are similar.KCl and KNO3have no salting out effect on CO(NH2)2in this system.

3.3.Solid-liquid equilibrium of APP-KCl-H2O

Phase diagram of the APP-KCl-H2O ternary system at 273.2 K is shown in Fig.5.The corresponding solubility data are listed in Table 3.The isothermal solubility diagram contains one invariant point C3,two solubility curves (E3C3and C3F3,corresponding to the crystallization of(NH4)3HP2O7and KCl,respectively),two crystallization zones (A3E3C3,C3F3K3) and one crystallization coexistence zone (A3C3K3).The invariant point C3is saturated with(NH4)3HP2O7and KCl,and the composition of the point is ω(KCl)of 7.96% (mass) and ω(APP) of 50.68% (mass).The crystal region of (NH4)3HP2O7is much smaller than that of KCl.APP indicates a strong salting-out effect on KCl.

Table 3 The phase equilibria data of the ternary system (APP-KCl-H2O) at 273.2 K and atmospheric pressure①

Fig.5.Phase diagram for the ternary system APP-KCl-H2O at 273.2 K.W3,H2O;K3,KCl;A3,APP;E3,solubility of APP in water;C3,co-saturated point of (NH4)3HP2O7 and KCl;F3,solubility of KCl in water;ω2,mass fraction of APP;ω3,mass fraction of KCl.

3.4.Solubility and phase diagram of quaternary system (APP-[CO(NH2)2-NH4NO3]-KCl-H2O) at 273.2 K

The measured data of the APP-[CO(NH2)2-NH4NO3]-KCl-H2O quaternary system at 273.2 K is listed in Table 4.The isothermal solubility phase diagram was plotted in Fig.6.As shown in Fig.6,the solubility was described as N-P2O5-K2O system so as to facilitate liquid fertilizer (e.g.,APP,UAN and KCl) used in agriculture.The solid lines show the contours of constant TPN in the diagram.For example,contour line labeled 18.72 represents that the TPN along the line is equal to 18.72 at 273.2 K.The broken lines are connected by points with the same co-saturated salt in adjacent crystalline regions,and the corresponding crystallizing phase is divided into four regions.Points A,B(ω(N)=32.73% (mass),ω(P2O5)=0.00% (mass),ω(K2O)=0.13% (mass)),C(ω(N)=0.00%(mass),ω(P2O5)=0.00%(mass),ω(K2O)=13.74%(mass))represent plant nutrient of P2O5,N,K2O,respectively.The TPN reaches the maximum and minimum at point D (ω(N)=11.13% (mass),ω(P2O5)=35.10% (mass),ω(K2O)=0.00% (mass)),and J(ω(N)=6.34%(mass),ω(P2O5)=0.00%(mass),ω(K2O)=6.92%(mass))with values of 46.23% (mass) and 13.26% (mass),respectively.The isothermal solubility diagram consists of three phase boundaries(EF,GF,GHI) and four zones (DEFG,EBF,GFIH,and GHIC,corresponding to (NH4)3HP2O7,(NH4)4P2O7,KNO3,and KCl crystallization,respectively).The contents of each component in the subsystem systems of APP-[CO(NH2)2-NH4NO3]-H2O,KCl-[CO(NH2)2-NH4NO3]-H2O and APP-KCl-H2O were converted into N,P2O5and K2O contents,which can be represented as curves DB,BC and DC in Fig.6,respectively.The curve AC shows that the sub-system contains only P2O5,but there is no component that contains P only in the raw material.Hence that field ADC is an invalid zone where solution fertilizer formulation can impossibly proceed.Since curve BC is the ternary KCl-[CO(NH2)2-NH4NO3]-H2O system without the crystallization of (NH4)3HP2O7or(NH4)4P2O7,the roughly estimated point F (ω(K2O)=28.10%(mass),ω(P2O5)=0.00% (mass),ω(N)=3.03% (mass)) acts as the endpoint of curve GF.

Table 4 Phase equilibrium data and density of the APP-[CO(NH2)2-NH4NO3]-KCl-H2O system at 273.2 K and atmospheric pressure①

Fig.6.Three-component solubility phase diagram for the quaternary system APP-[CO(NH2)2-NH4NO3]-KCl-H2O at 273.2 K.A,P2O5;B,N;C,K2O;D,the point of maximum TPN;E,co-saturated point of (NH4)3HP2O7 and (NH4)4P2O7;F,the crystallization point of KNO3;G,co-saturated point of(NH4)3HP2O7 and KCl;H and I,co-saturated point of KNO3 and KCl;J,the point of minimum TPN.

With respect to the diverse crystallizations generated in the phase diagram of the quaternary systems APP-[CO(NH2)2-NH4NO3]-KCl-H2O and APP(11-37-0)-[CO(NH2)2-NH4NO3]-KCl-H2O(TVA) [11] at 273.2 K,there were five crystallization regions((NH4)5P3O10.2H2O,NH4Cl,NH4NO3,KNO3and KCl) for the later system,but no crystallization of NH4Cl and NH4NO3while emerging the crystal of(NH4)4P2O7for the former system studied within our work.The crystallization region of(NH4)3HP2O7and(NH4)4P2-O7in the left side (AB) of Fig.6 is larger than the corresponding crystallization area of (NH4)5P3O10.2H2O in the phase diagram of APP(11-37-0)-UAN-KCl-H2O.The potential reason is that the content of-N in solid APP(18-58-0) is higher than the one of-N in liquid APP(11-37-0),which results in a more significant ion effect.Finally,all the polyphosphate is easier to salt out in the APP(18-58-0)-[CO(NH2)2-NH4NO3]-KCl-H2O system than the APP(11-37-0)-UAN-KCl-H2O system.Another interesting finding is the crystallization zone of (NH4)4P2O7is located at the lower left corner (B) in Fig.6,which could be attributed to the phosphorous species of the studied APP which contains orthophosphate and pyrophosphate only.The pyrophosphate is composed of(NH4)3HP2O7and (NH4)4P2O7(Fig.1(b)).The pH value increases with enhanced,and the reaction moves to the right direction of Eq.(9).More (NH4)4P2O7is generated in EBF region subsequently.

The XRD patterns shown in Fig.7(a),(b),(c) and (d) well coincide with the four co-saturated regions of KNO3+KCl,(NH4)3HP2-O7+KCl,(NH4)3HP2O7+KNO3,and (NH4)3HP2O7+(NH4)4P2O7,respectively.

Fig.7.X-ray diffraction pattern of invariant point KNO3+KCl(a),(NH4)3HP2O7+KCl(b),(NH4)3HP2O7+KNO3(c)and(NH4)3HP2O7+(NH4)4P2O7(d)of the quaternary system APP-[CO(NH2)2-NH4NO3]-KCl-H2O at 273.2 K.

3.5.Three forms of nitrogen ( -N,-N and CO(NH2)2-N),water content and density of APP-[CO(NH2)2-NH4NO3]-KCl-H2O system at 273.2 K

4.Conclusions

In this work,the solid-liquid phase equilibrium of quaternary system APP-[CO(NH2)2-NH4NO3]-KCl-H2O and its three subsystems (APP-[CO(NH2)2-NH4NO3]-H2O,KCl-[CO(NH2)2-NH4NO3]-H2O and APP-KCl-H2O)at the temperature of 273.2 K were studied by a dynamic method.The ternary systems of APP-[CO(NH2)2-NH4NO3]-H2O,KCl-[CO(NH2)2-NH4NO3]-H2O and APP-KCl-H2O contain two single crystal regions ((NH4)3HP2O7and (NH4)4P2O7;KCl and KNO3;(NH4)3HP2O7and KCl,respectively) and one cosaturated crystal region ((NH4)3HP2O7+(NH4)4P2O7;KCl+KNO3;(NH4)3HP2O7+KCl,respectively).The first two ternary systems have no salting-out effect on CO(NH2)2,while in APP-KCl-H2O system,APP shows a strong salting-out effect on KCl.The phase diagram of quaternary system APP-[CO(NH2)2-NH4NO3]-KCl-H2O is consisted of four solid phase crystallization regions ((NH4)3HP2O7,(NH4)4P2O7,KNO3and KCl),but no quaternary invariant points.The maximum TPN content existing as ionic forms in the ternary and quaternary systems is 32.86%-45.56% (mass) and 46.23% (mass),respectively,indicating that the maximum nutrient content can be reached using raw materials from the corresponding systems to prepare liquid fertilizer.In the quaternary system,the content of-N increases gradually corresponding to the increase of TPN content and the-N and CO(NH2)2-N increases with elevated total N.The phase-equilibrium data obtained in this study provides a theoretical basis for the preparation of liquid fertilizers with different N: P2O5: K2O ratios,especially for a formula involving solid polyphosphates as feedstock.

Data Availability

Data will be made available on request.

Declaration of Competing Interest

The authors declare that they have no known competing financial interests or personal relationships that could have appeared to influence the work reported in this paper.

Acknowledgements

The authors greatly acknowledge the fund supported by the National Natural Science Foundation of China (32172677).