Obtaining Monodisperse Melamine Phosphate Grains by a

Article pubs.acs.org/IECR

Obtaining Monodisperse Melamine Phosphate Grains by a Continuous Reaction Crystallization Process Barbara Cichy* and Ewa Kuzḋ zȧ ł Fertilizers Research Institute, Inorganic Chemistry Division “IChN” Gliwice, Sowińskiego 11, 44-101 Gliwice, Poland ABSTRACT: A continuous MSMPR (mixed-suspension mixed-product removal) crystallizer was used in the process of obtaining melamine phosphate in a precipitation reaction. The effects of the mean residence time of the suspension in the crystallizer and of the concentration of the melamine suspension in water on the efficiency of the process and the quality of the product obtained were studied. Kinetic parameters of the reaction crystallization process under study were estimated. The grain size distribution of the crystals and the specific surface area of the grains were determined. Morphology was defined by means of a shape factor and coefficients of inhomogeneity and variation. Well-formed, homogeneous crystals of similar specific surface areas and practically identical small pore volumes were obtained. This work shows that, within the studied range of solid-phase concentrations and mean residence times, it is possible to obtain a product of monodisperse grains and predicted grain size by controlling the concentration of the melamine suspension and the reaction time during the synthesis process. matrix, including additive FRs, has, in the first place, an effect on the flow properties of the polymer composition and on the mechanical properties of the fire-retarded plastic.10,11 The effect of physical properties on the fire behavior of noncharring polymers was investigated using a one-dimensional pyrolysis model, ThermaKin. This thermokinetic model, which combines the absorption and transfer of thermal energy with Arrhenius kinetics for the decomposition of the polymer, predicts the overall behavior of a pyrolyzing object through mass and energy conservation equations. Recently, the ThermaKin model was effectively utilized as a practical tool for the prediction and extrapolation of the results of standard cone calorimeter test (ISO 5660). The modeling of cone calorimeter behavior as a means of relating the physical properties of a material to its heat release rate (HRR) history in a cone calorimeter was investigated by Patel et al.12 Additive fire retardants, usually added at loadings of 10−70%, have a profound effect on the physical properties, changing the absorption, heattransfer, and decomposition behaviors, as well as inhibiting the gas-phase free-radical processes.12 No literature data describing the effects of the size and shape of MP particles on processability and effectiveness in fire-retarded polymer compositions are available. The optimum size distribution and specific surface area of MP grains that would, at a proper fill ratio, provide optimum flame resistance and the expected performance of a product have not been determined. Continuous reaction crystallization of MP is part of our research. The effects of the solid-phase concentration in water on the nucleation rate and MP particle growth rate and on the specific surface area of the MP particles were determined in our earlier study.9 The kinetics of MP particle precipitation in an aqueous environment resulting from the reaction of melamine

1. INTRODUCTION Melamine phosphate (MP) is a very important compound, widely used to improve the fire retardancy of many polymer materials and polymer coatings. MP is an additive-type flame retardant (FR).1−5 MP is also used as an intermediate for the preparation of melamine polyphosphate (MPP).6,7 Melamine phosphate (MP) is obtained in a reaction between phosphoric acid and melamine at ambient temperature (Figure 1). Solid particles of sparingly water-soluble melamine are

Figure 1. Reaction for the preparation of MP.

dispersed in water, and upon the addition of phosphoric acid, they react fairly rapidly to form a sparingly soluble product (MP) in the form of a solid precipitate. The reaction can be conducted as a periodic2,3,6,8 or continuous9 process. The properties of crystals/grains of a chemical compound depend mainly on the chemical structure of that compound, but the method of its preparation, including manufacturing process variables, has an impact on the structure and fineness of the material. Therefore, the manner of conducting the synthesis reaction has a significant effect on the morphology of the obtained MP grains. The effectiveness of MP as an FR depends not only on its structure and chemical composition but also on its physical properties, such as density, grain size, shape, surface area, and porosity. The morphology of fillers present in the polymer © 2014 American Chemical Society

Received: Revised: Accepted: Published: 6593

December 19, 2013 February 18, 2014 March 29, 2014 March 30, 2014 dx.doi.org/10.1021/ie4043032 | Ind. Eng. Chem. Res. 2014, 53, 6593−6599

Industrial & Engineering Chemistry Research

Article

Figure 2. Scheme of the laboratory MSMPR crystallizer system for the continuous mass crystallization of melamine orthophosphate: (1) melamine slurry tank, (2) melamine slurry pump, (3) melamine slurry buffer tank, (4) phosphoric acid tank, (5) phosphoric acid pump, (6) phosphoric acid buffer tank, (7) MSMPR crystallizer.

rate. The measurements were made at constant temperature of T = 20 °C assuming a 1.1:1 ratio of the reagents (melamine to phosphoric acid). The range of melamine concentrations in water in the study was 5−20% (w/w), and the residence time τ was varied in the range of 900−1800 s. After the crystallization parameters had stabilized, the process was conducted for a period equal to five residence times. The residence time in the reactor was adjusted by varying the flow intensity of the reactant streams entering the reactor, with the flow exiting at a constant predetermined volume of the reaction mass. After that period, the precipitated product (MP) was filtered off on a Büchner funnel and dried in a laboratory drier for 1 h at 105 °C. The effective Reynolds number Rem was calculated as

solid particles dispersed in a water medium with phosphoric acid was investigated by determining the linear rate of crystal nucleation and growth and the particle size distribution of the product at various concentrations of the solid phase (1−10% (w/ w)) under identical mixing conditions (τ =1800 s, T = 40 °C). Investigations of the continuous process of MP preparation were conducted in an MSMPR (mixed-suspension mixedproduct removal) laboratory crystallizer because, as we previously showed,9 the model of the precipitation reaction describes well the nucleation and growth of precipitated MP crystals. We studied the effects of the mean residence time of the suspension in the crystallizer and of the concentration of the melamine suspension in water on the efficiency of the process, the quality of the products obtained, and the kinetics of precipitation conducted at constant temperature (T = 20 °C). The purpose of the investigations presented herein was to determine the effects of process variables on the morphology of MP grains obtained with simultaneous improvement in the efficiency of the synthesis process.

Rem =

nd 2ρ ηs

(1)

and the agitation intensity MI was determined as MI = CRem v

(2)

The values of the constants C and v were adopted from the literature for a two-blade propeller stirrer with an incidence angle of 22.5° (C = 0.985, v = −0.15).9 The viscosity of the liquid phase was determined with a Hoeppler viscometer, and the viscosity of suspensions was determined according to the method of Thomas.13 The density of the product was determined by the pycnometer method using kerosene. The process efficiency determination was based on chemical analyses and calculated as the percentage ratio of the amount of P2O5 in the precipitated product to the amount of P2O5 introduced into the process in the form of phosphoric acid. Particle size analysis of MP was performed on a Coulter LS particle size analyzer. The specific surface area of the MP particles

2. EXPERIMENTAL SECTION MP was synthesized from technical-grade melamine (ZA Puławy) and from technical-grade phosphoric acid (ZCh Alwernia). The synthesis reaction was conducted in a cylindrical glass laboratory reactor of 2.5-dm3 operating capacity and 0.12dm diameter, equipped with a 0.09-dm-diameter propeller stirrer. A schematic diagram of the laboratory MSMPR crystallizer is presented in Figure 2. The speed of the stirrer was constant during all measurements at 5 revolutions per second, which provided stable and sufficient circulation of the suspension. The reactor was continuously fed at a defined flow rate with an aqueous solution of melamine and phosphoric acid, and the effluent was removed at the same flow 6594

dx.doi.org/10.1021/ie4043032 | Ind. Eng. Chem. Res. 2014, 53, 6593−6599


Article

Table 1. Kinetic Parameters of MP Particle Formation no.

melamine concentration in water (%, w/w)

residence time (s)

suspension density, ρ (g/cm3)

suspension viscosity after reaction, ηs (cP)

Reynolds number, Re

population density of particle MP, n0 (m−1 m−3)

nucleation rate, B (s−1 m−3)

linear growth rate, G (m s−1)

process efficiency (%)

1 2 3 4 5 6 7 8 9

5 5 5 10 10 10 20 20 20

900 1800 3600 900 1800 3600 900 1800 3600

1.024 1.039 1.040 1.049 1.055 1.050 1.078 1.110 1.142

2.644 2.710 2.722 4.138 4.038 4.201 4.498 4.540 4.544

15688 15525 15470 10268 10675 10957 9706 9898 10183

7.36 × 1010 1.93 × 1011 5.06 × 1011 2.27 × 1011 1.20 × 1012 3.72 × 1011 3.43 × 1012 2.21 × 1011 3.94 × 1012

1759.82 2027.54 1861.69 4487.40 9944.64 1539.69 43109.94 2732.69 13954.65

2.39 × 10−8 1.05 × 10−8 3.68 × 10−9 1.97 × 10−8 8.28 × 10−9 4.14 × 10−9 1.26 × 10−8 1.24 × 10−8 3.54 × 10−9

82.9 93.5 92.9 86.1 93.0 96.3 88.5 94.1 99.4

3. RESULTS Experiments on the process of continuous preparation of MP were carried out in an MSMPR-type crystallizer, assuming that the reaction proceeds through the stage of fast dissolution of the aqueous suspension of solid melamine in phosphoric acid followed by nucleation and growth of sparingly soluble MP crystals. Experiments were conducted at constant temperature with varying concentrations of melamine suspension and mean residence times in the crystallizer. The effects of the concentration of the MP precipitation and crystallization medium and of the mean residence time of the suspension in the crystallizer on the morphology of the product obtained and on process efficiency were studied. The size distributions of the obtained products were determined; the population density was calculated; and the population density distribution equation was used to determine the values of kinetic parameters of crystal formation, MP crystal nucleation, and growth (Table 1). The relationship between MP crystal size distribution and solid-phase concentration at constant residence time of the suspension in the reactor is shown in Figures 3−5. Morpho-

was determined by means of a Gemini VII analyzer from Micromeritics. Crystal images were obtained by means of a Hitachi SU8010 scanning electron microscope. The kinetic parameters of the continuous reaction crystallization process were calculated with the experimental population density distributions n(L) of the product crystals.9,14 Individual population density values were determined based on mass (or volume) size distribution data according to the equation ni =

mi 3

αρc Li ΔLi

=

Vi αLi 3ΔLi

(3)

Calculations were based on the simplest kinetic model of the crystallizer with an ideally mixed suspension. The population balance can be described by the equation ⎛ L ⎞ n(L) = n0 exp⎜ − ⎟ ⎝ Gτ ⎠

(4)

The calculated values can be shown in the semilogarithmic plot (log n(L), L). From L = 0, the value of the nucleation population density, n0, can be read, and from the slope and the known average residence time, the linear growth rate of crystals, G, can be calculated directly. From the nucleus population density and the linear growth rate, the value of the nucleation rate, B, can be determined as B = n 0G

(5)

To define the size distribution, the coefficients of inhomogeneity (CV, eq 6) and variation (CZ, eq 7) were determined analytically as

(L − L16) CV = 84 2L50

CZ =

σ L50

Figure 3. Particle size distributions of MP obtained from various aqueous melamine suspensions at a residence time of 900 s.

(6)

logical parameters of the obtained crystals are presented in Table 2. Figures 6 and 7 show microscopic images of MP crystals taken

(7)

where L84, L16, and L50 are the crystal sizes for mesh passes corresponding to 16, 84, and 50 wt %, respectively, and σ denotes standard deviation. The shape of grain was determined based on analysis of size distribution, defined as the ratio of the difference between the 90th (L90) and 10th (L10) percentiles to the median k=

L90 − L10 L50

(8)

This factor has been used in a number of research works, including that of Naito et al.15

Figure 4. Particle size distributions of MP obtained from various aqueous melamine suspensions at a residence time of 1800 s. 6595



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residence time of 900 s and a suspension concentration of 5% (w/w). The kinetic data in Table 1 show considerable differences among individual results. It is not possible to find a straightforward relationship between solid-phase concentration and residence time, on one hand, and the obtained values of kinetic parameters on the other hand. This leads to a conclusion that the kinetics of MP crystal precipitation is more complex than assumed in the model, which is also evidenced by the nonlinearity of the MP crystal population density distribution (Figures 8 and 9). The simultaneous effects of the solid-phase concentration and mean residence time of the suspension in the crystallizer on the linear MP crystal growth rate can be given in the form of the correlation

Figure 5. Particle size distributions of MP obtained from various aqueous melamine suspensions at a residence time of 3600 s.

from selected tests, for which the n = f(L) functions were plotted (Figures 8 and 9). The change in the linear growth rate of an MP crystal as a function of variable parameters is illustrated in Figure 10. The shape factors were calculated for all products obtained, and the results are presented in graphical form as a function of solid-phase concentration (Cs) and residence time in Figure 11.

G = 2.44 × 10−8 − 1.99 × 10−10C − 5.27 × 10−12τ

(9)

with a correlation coefficient of R2 = 0.957 and a mean relative error of ±8.3%. The course of this relationship is presented graphically in graph 10. Changing process parameters affects both the size of the crystals discharged from the crystallizer and their homogeneity. The mean grain size falls within the range of 22−62 μm, and the specific surface area varies from 0.3 to 0.7. The smallest grains were obtained at a melamine concentration in water of 10% (w/ w) and a mean residence time of 3600 s, with the largest specific surface area attained under these conditions. The process efficiency achieved with these parameters was 96.3%. The largest grains were obtained at a melamine concentration in water of 20% (w/w) and a mean residence time of 1800 s, with the smallest specific surface area attained under these conditions, along with a process efficiency of 94.1%. Individual MP crystals were usually irregular in shape, so their linear dimensions were not equal in all directions. The diversity of the crystals obtained, in terms of both grain size and specific surface area, does not allow a straightforward relationship between residence time and solid-phase concentration on one hand and morphology of the obtained crystals on the other hand. Thus, a shape factor was determined in accordance with eq 8. The effects of variable parameters of the reaction for obtaining MP on the shape of the crystals obtained can be presented in the form of an empirical correlation, namely

4. DISCUSSION Within the studied range of solid-state concentrations in water and mean residence times, at a constant temperature of 20 °C, the density and viscosity of the reaction suspension increases with increasing concentration and varies slightly with increasing residence time. The agitation intensity in the range studied was found to be practically constant at ca. 0.24. It is apparent that the value of the Reynolds number decreases with increasing concentration and is practically independent of the residence time, which is attributable to the change in viscosity of the reaction system. However, in all experiments within the range studied, the Reynolds number takes on values characteristic of laminar flow. The efficiency of the reaction of producing MP, within the range studied, calculated as the ratio of the amount of phosphates introduced in the process to the amount thereof in the precipitated product, varies within the range of 82.9−99.4%. This depends on both the concentration and the residence time. However, the effect on reaction efficiency becomes more apparent when the residence time of the suspension in the crystallizer is extended. Maximum reaction efficiency was observed with a melamine concentration in water of 20% (w/ w) and a mean residence time of 3600 s. The grains obtained at this concentration are characterized by the lowest coefficient of homogeneity. The lowest reaction efficiency was observed with a

k = 3.94 − 0.023C + 1.12 × 10−4τ − 1.91 × 10−5Cτ (10)

Table 2. Morphological Structure of MP process parameters

crystal product characteristics

no.

melamine concentration in water (%)

residence time (s)

mean grain size (μm)

median grain size (μm)

coefficient of variation, CZ

coefficient of inhomogeneity, CV

D(3,2) (μm)

specific surface area (m2/g)

pore volume (cm3/g)

density of MP, ρ (g/cm3)

1 2 3 4 5 6 7 8 9

5 5 5 10 10 10 20 20 20

900 1800 3600 900 1800 3600 900 1800 3600

45.31 51.47 25.87 34.89 31.57 22.50 25.58 62.00 37.10

26.60 39.04 16.05 20.61 21.35 12.47 16.87 46.00 29.67

1.74 1.10 1.72 1.85 1.54 2.20 1.49 1.08 0.98

0.80 0.74 1.40 1.13 1.27 1.43 1.43 0.66 1.15

6.91 6.96 4.76 5.76 7.38 4.21 7.14 10.04 8.85

0.5670 ± 0.0065 0.3798 ± 0.0075 0.6249 ± 0.0056 0.5228 ± 0.0088 0.4947 ± 0.0057 0.7580 ± 0.0110 0.6417 ± 0.0129 0.3691 ± 0.0103 0.4011 ± 0.0073

0.0011 0.0009 0.0013 0.0013 0.0009 0.0015 0.0013 0.0009 0.0090

1.68 1.69 1.63 1.77 1.77 1.84 1.82 1.72 1.91

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Figure 6. SEM images of MP obtained from a 10% aqueous melamine suspension at a residence time of 3600 s: (a) 500× and (b) 1000× magnification.

Figure 7. SEM images of MP obtained from a 20% aqueous melamine suspension at a residence time of 1800 s: (a) 500× and (b) 1000× magnification.

Figure 8. Population density distribution of MP obtained from a 10% aqueous melamine suspension at a residence time of 3600 s.

Figure 10. Effects of aqueous melamine suspension and mean residence time on the linear growth rate G of MP crystals.

Figure 9. Population density distribution of MP obtained from a 20% aqueous melamine suspension at a residence time of 1800 s.

with a correlation coefficient of R2 = 0.982 and a mean relative error of ±3.7%. The course of this relationship is presented graphically in graph 11. Correlation 9,10 was obtained using nonlinear regression methods based on all experimental data. The shape factor of the highest value was obtained for the grain with the largest specific surface area and smallest size (10% (w/ w), 3600 s). The shape factor of lowest value was obtained at a residence time of 3600 s and a solid-phase concentration of 20%

Figure 11. Effect of aqueous melamine suspension and mean residence time on crystal shape. 6597



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(w/w), which could be the result of crystal breaking during agitation and circulation in the crystallizer occurring at increased viscosity of the system observed under these conditions. However, the majority of crystals were intact, with attrition and breaking of crystals observed in only a few cases. The crystal structure shown in Figure 6 consists of smooth surface grains of various sizes and various shapes. Some crystals show scarce irregularities, which might be caused by long residence times resulting in higher risk of mechanical damage. Well-formed and uniform crystals were obtained at a solid-phase concentration of 20% (w/w) and mean residence time of 1800 s (Figure 7). The lowest coefficient of variation was obtained for crystals of the highest density (20% (w/w), 3600 s), whereas the lowest coefficient of inhomogeneity was obtained for grains of the largest size. The obtained crystals had a low specific surface area at less than 1 m2/g and practically identical small volume of pores. The low values of these parameters have a great impact on MP, because of the tendency to absorb moisture and, hence, to deteriorate its processing properties. By adjusting the process parameters, we were able to control the specific surface area of MP grains without affecting the volume of the pores.

5. CONCLUSIONS The crystals obtained in the study were well-formed and homogeneous and were characterized by a specific surface area that decreased with increasing mean size of the MP grains. The occurrence of a small quantity of crystal agglomerates was detected, which was evidenced by a sparingly low number of particles in the selected range of sizes on size distribution curves. No distinct effects of mechanical attrition or breaking of crystals in the crystallizer were observed, which indicates that favorable process conditions were created in the MSMPR crystallizer for controlled crystal growth. The MP grains differed slightly in the values of homogeneity and inhomogeneity coefficients. The shape of grains determined by the shape factor enabled differences in the shape of crystals obtained under different reaction conditions to be identified. The obtained crystals were characterized by similar specific surface areas and practically identical small volumes of pores. The manner of conducting the MP precipitation reaction in an aqueous environment has an effect on the morphology of the crystals obtained and on the process yield. The values of synthesis process efficiency can be deemed satisfactory. It was found that, within the studied range of solid-phase concentrations and mean residence times, it is possible to obtain a product of monodisperse grains and predicted mean grain size by controlling the concentration of the melamine suspension and the reaction time during the synthesis process.

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NOMENCLATURE α = volumetric shape factor of a crystal ηs = viscosity of the suspension, cP ρ = density of the suspension, kg/m3 ρc = crystal density, kg/m3 σ = standard deviation τ = residence time, s A = Brunauer−Emmett−Teller (BET) specific surface area, m3/g B = nucleation rate, 1/(s m3) Cs = solid-phase concentration, % CV = coefficient of inhomogeneity CZ = coefficient of variation d = stirrer diameter, m FR = flame retardant G = linear growth rate, m/s HRR = heat release rate k = shape factor L = particle size, m Li = mean size of the ith crystal fraction, m L10 = particle size for which Ω(L) = 0.10 L16 = particle size for which Ω(L) = 0.16 L50 = mean particle diameter, μm L84 = particle size for which Ω(L) = 0.84 L90 = particle size for which Ω(L) = 0.90 ΔLi = size range of ith crystal fraction, m MI = mixing intensity mi = mass of the ith crystal fraction, kg MP = melamine phosphate MPP = melamine polyphosphate MSMPR = mixed suspension, mixed product removal n = number of revolutions, s n0 = nucleus population density, 1/(m m3) ni = population density of the ith crystal fraction, 1/(m m3) n(L) = particle population density, 1/(m m3) Re = Reynolds number Rem = equivalent Reynolds number Vi = volume of the ith crystal fraction, m3 Vp = pore volume, cm3/g

REFERENCES

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AUTHOR INFORMATION

Corresponding Author

*Tel.: +48 322313051. E-mail: [email protected]. Notes

The authors declare no competing financial interest.

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ACKNOWLEDGMENTS This study was conducted as part of the statutory activities of the Fertilizers Research Institute, Inorganic Chemistry Division “IChN”, in Gliwice. 6598



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(15) Naito, M.; Hayakawa, O.; Nakahira, K.; Mori, H.; Tsubaki, J. Powder Technol. 1998, 100, 52.

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Obtaining Monodisperse Melamine Phosphate Grains by a

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