Phsophogypsum is a by-product from the processing phosphate rock. Before the use of it in cement industry such as setting regulator is necessary a study of dehydration reaction of phosphogypsum to avoid the false setting during the milling.
The aim is to study the thermal behavior of two different phosphogypsum sources (Spain and Tunisia) under non-isothermal conditions in argon atmosphere by using Thermo-Gravimetriy, Differential Thermal Analysis (TG-DTA) and Differential Scanning Calorimetry (DSC).
DSC experiments were carried out at temperatures ranging from ambient to 350 °C at different heating rates. The temperatures of conversion from gypsum to hemihydrate and anhydrite states and heat of dehydration were determined. Various methods were used to analyze the DSC data for reaction kinetics determination. The activation energy and frequency factor were calculated for dehydration of phosphogypsum. Activation energy values of the main dehydration reaction of phosphogypsum were calculated to be approximately 61–118 kJ/mol.
Los ensayos de DSC se realizaron hasta los 350 °C a diferentes velocidades de calentamiento. La temperatura de conversión del yeso a las formas de hemihidrato y anhidrita y el calor de hidratación fueron determinados.
Las cinéticas de reacción fueron obtenidas analizando los datos de DSC mediante varios métodos. Se calculó la energía de activación y el factor de frecuencia para las reacciones de deshidratación del subproducto. Los valores de energía de activación de las principales reacciones de deshidratación del fosfoyeso fueron obtenidos, aproximadamente 61-118 kJ/mol.
|
Greek symbols |
Subscripts |
Phosphogypsum (PG) is a by-product from the processing phosphate rock by the wet process to obtain acid phosphoric according to Eq. [ Ca5(PO4)3F+5 H2SO4+10 H2O→3H3PO4+5CaSO4∙2H2O+HF [1]
Phosphogypsum consists mainly of calcium sulfate dihydrate with small amount of silica, usually as quartz. Radium and uranium, as well as minor amounts of toxic metals, arsenic, barium, cadmium, chromium, lead, mercury, selenium and silver and phytotoxic fluoride and aluminum are also present in phosphogypsum and its pore water. The concentration of heavy metals and radionucleides depend on the composition of the phosphate rock feed (
For every tone of phosphoric acid produced, about three tones of phosphogypsum are yield. A world PG production is around 200–280 106 t per year (
Nowadays a number of researches are focused on finding new uses of PG: a) agricultural fertilizer or for soil stabilization amendments (
The cement manufactures add between 3 and 6% gypsum depending on its purity to avoid flash (immediate) setting of cement, also affect strength development and volume stability in the cement (
It is well know that during the industrial production of cement hydrated calcium sulfates undergo partial dehydration at 110–130 °C in the cement mill forming hemihydrates CaSO4 0.5H2O and in some cases the total dehydrated, at 170–190 °C, forming anhydrite CaSO4
(
So before to use phosphogypsum such as setting regulator it's necessary to study dehydration reaction of PG in the direction to avoid the false setting by the production of hemihydrate and anhydrite during the milling process. The temperature and the kinetic dehydration of hydrated calcium sulfate could be influenced by different parameter such as origin sample, chemical composition and crystalline structure, (
In this research was study the kinetic characteristics of PG dehydration via differential scanning calorimetry (DSC) in argon atmosphere. The objective of this study is to elucidate the reaction mechanisms and reaction kinetics of the dehydration of PG in a solid-state reaction. A kinetic model was proposed.
The PG samples used in this work came from Fertiberia factory of Bahía of Huelva (Spain) in 2009, named PGS and from Chemical Group (GZT) factory of Gulf of Gabès (Sfax, Tunisia) in 2009, named PGT. In order to obtain a representative sample, the sampling was carried out in situ. 300 kg of each PG sample were mixed and homogenized in a mixer ENRICH, with 200 kg of capacity, then quartered successively up to obtain a representative sample of 1 kg, being subject of our experiments. After filtration and drying at 50 °C during 48 h, the chemical composition of PG, obtained by conventional methods, is listed in
Chemical composition of phosphogypsum samples
Content (wt.%) | PGS | PGT |
---|---|---|
SO3 | 50.3 | 44.7 |
CaO | 34.8 | 30.1 |
SiO2 | 2.4 | 1.4 |
Total P2O5 | 0.9 | 1.2 |
Al2O3 | 0.4 | 0.1 |
Fe2O3 | 0.2 | 0.09 |
Na2O | 0.1 | 0.6 |
K2O | 0.03 | 0.01 |
MgO | 0.04 | 0.02 |
Total F | 3.8 | 4.9 |
Total Radionuclides (Bq/kg) |
2441 | 635 |
LOI | 7.0 | 16.9 |
Total content of radionuclides (238U,234U,235U,226Ra,210Pb,210Po,40K and 232Th) (Tayibi et al. 2011) [
The diffractograms of PG samples were obtained using a X-ray diffractometer (Philips X'Pert PRO MPD) with Kα Cu radiation (40 mA current and 45 kV). The patterns of diffraction were obtained in a 2Θ scanning range from 5° to 80°, with 0.0167° and 0.6 s of scan step and time, respectively.
PG samples were subjected to differential thermal and thermogravimetric analysis (DTA and TGA) in an inert atmosphere (argon). Setaram Sensys Evolution 1500 DTA/TGA analyzer was used to measure and record the sample mass change with temperature over the course of the dehydration reaction. Thermogravimetric curves were obtained at heating rate of 10 °C/min between ambient and 650 °C in argon atmosphere (20 ml/ min) and the sample mass was between 45 and 50 mg.
The kinetic study of the dehydration of PG was performer with Differential Scanning Calorimetry (DSC) analysis. DSC experiments were performed on a Setaram Model mod 3D-EVO. Non-isothermal analysis was carried out at four different heating rates (5, 10, 15, and 20 °C/min) between ambient and 350 °C. Temperature calibration was achieved by using the ICTAC-recommended DSC standards. The precision of reported temperatures was estimated to be ±2 °C. Sample mass was about 60 mg and was placed in a 175 µl Al crucible sealed. All the experiments were conducted in an inert atmosphere, argon with a flow rate of 20 ml/min.
The reproducibility of the experiments is acceptable and the experiments data presented in this paper corresponding to the different operating conditions are the mean values of runs carried out two or three times.
Generally for PG degradation, it is assumed that the rates of conversion are proportional to the concentration of reacted material. The rate of conversion can be expressed by the following basic rate equation [Eq.
Where α is the degree of conversion of reaction, f(α) and
Where is
By combining Eqs. [
Where
By combining the Eqs. [
Friedman analysis (
With
The Flynn-Wall-Ozawa method (
Where
The analysis according to ASTM E698 (
Coats-Redfern method (
The method by Coats-Redfern is one of the most widely used procedures for the determination of the reaction processes. From Eq. [
Algebraic expressions of functions of the most common reaction mechanisms
Mechanism | f(α) | g(α) |
---|---|---|
Autocatalytic | (1- α)n. αm | – |
Avarani-Erofe've (A1.5) | 1.5(1- α) [-ln(1- α)]1/3 | [-ln(1- α)]1/3 |
Avarani-Erofe've (A2) | 2(1- α) [-ln(1- α)]1/2 | [-ln(1- α)]1/2 |
Avarani-Erofe've (An) | n(1- α) [-ln(1- α)](1-1/n) | [-ln(1- α)](1-1/n) |
First-order (F1) | (1- α) | -ln(1- α) |
Second-order (F2) | (1- α)2 | (1- α)-1-1 |
Third-order (F3) | (1- α)3 | [(1- α)-2-1]/2 |
Contracting sphere (R2) | 2(1- α)1/2 | [1- (1-α)1/2] |
Contracting Cylinder (R3) | 3(1- α)2/3 | [1- (1-α)1/3] |
Power law (P2) | 2α1/2 | α1/2 |
Power law (P3) | 3α1/3 | α1/3 |
Power law (P4) | 4α1/4 | α1/4 |
One-dimensional diffusion (D1) | 1/2α | α2 |
Two-dimensional diffusion (D2) | [-ln(1- α)]−1 | [(1- α).ln (1- α)]+α |
Three-dimensional diffusion (D3) | 1.5[1-(1-α)(1/3)]−1(1-α)(2/3) | [1-(1- α)1/3]2 |
Giustling-Brounsthein (D4) | 1.5 [(1-α)(-1/3)-1]−1 | 1-(2α/3)-(1- α)2/3 |
Morphologically, both PG samples were yellowish brown color and relatively soft grains. The particle size of phosphogypsum were D50=53 µm and D50=83 µm for PGS and PGT, respectively. Chemically, the PG mainly consists of SO3, CaO with low contents of SiO2, Fe2O3, Al2O3 and P2O5 as well as traces of Na2O, K2O, TiO2, F and 12–22% ignition loss (LOI). In addition to radionuclides such as 226Ra, 210Pb, 238U and 40K, the chemical analysis of PG is reported in
XRD difractograms of PGT and PGS samples.
The mineralogical composition of the PG depends strongly on its origin, the kind of acid phosphoric process used, environmental conditions of its storage and the age of the studied sample. Generally, PG could be composed by different ratios of three mineralogical phases of calcium sulfate. For example, the presence of these three phases has been described by Ma et al. (2010) (
TG, DTG and DTA curves obtained by heating at 10 °C/min in inert atmosphere (argon): (a) PGS and (b) PGT samples.
The curves show two consecutive and much closed endothermic peaks between 144 °C and 175 °C for PGS sample (
DTA and TGA results for thermal behavior of phosphogypsum
Peak | PGS | PGT | ||||||||
---|---|---|---|---|---|---|---|---|---|---|
|
|
|||||||||
DTA curve | TG curve | DTA curve | TG curve | |||||||
|
|
|
|
|||||||
To (°C) | Te (°C) | Tp (°C) | Interval temperature (°C) | Mass loss (wt, %) | To (°C) | Te (°C) | Tp (°C) | Interval temperature (°C) | Mass loss (wt,%) | |
1 | 133 | 155 | 144 | 119–157 | 4.7 | 143 | 173 | 156 | 143–176 | 12.8 |
2 | 159 | 184 | 176 | 157–197 | 4.7 | 176 | 201 | 191 | 176–233 | 4.0 |
3 | 423 | 453 | 433 | 0 | 412 | 464 | 464 | 0 | ||
Total Mass Loss (40–650 °C) | – | – | – | – | 9.4 | – | – | 16.8 |
(To = initial temperature, Te = final temperature and Tp = maximum temperature peak).
The first endothermic peak observed in the DTA/DTG curves is attributed to the gypsum dehydration reaction and the formation of the hemihydrate, according to the Eq. [ CaSO4 2H2O→CaSO4 0.5H2O+1.5H2O [12] CaSO4 0.5H2O→γ-CaSO4 (or CaSO4III) +0.5H2O [13] CaSO4III→CaSO4II [14]
The difference in DTG and DTA profiles, among two PG, indicates the influence of sample origin, chemical composition and traces on dehydration behavior (
The TG curves analysis indicates that for the PGT sample, the mass loss observed between the ambient temperatures up to 500 °C is ≈17 wt%. This result is in accordance with the mass loss (LOI) obtained by the chemical analysis of the PG sample (
For PGS sample, the total mass loss corresponding to the hydration reaction is 9.4 wt%, being the 4.7 wt% for the first peak and 4.7 wt% for the second peak, which means a relation of ≈1:1. In this case, it should be noted that in the initial PGS sample coexist the dihydrate and hemihydrate phases, which explains the relation of the identified mass loss.
Most literature reported that gypsum dehydration undergoes a two-step process, via hemihydrate (
The dehydration behavior may vary significantly among different gypsum types, such as natural gypsum and many kinds of chemical gypsum. Differences in crystalline characteristics and impurities appear to be the most important factor resulting in discrepancies of dehydration behavior (
The DSC curves obtained during the thermal dehydration at different heating rates (5, 10, 15 and 20 °C/min) up to 350 °C of the: (a) PGS sample and (b) PGT sample.
The CaSO4 2H2O reaction dehydration takes place into steps according to two endothermic peaks. The first peak is observed (depending on the heating rate) at Tp between 142 and 166 °C, for the PGS (
By considering the global dehydration reaction (step 1 and step 2), the
DSC results for the dehydration of phosphogypsum at different heating values
PGS | Peak 1 | Peak 2 | Peak 1 and Peak 2 | |||||||
---|---|---|---|---|---|---|---|---|---|---|
|
|
|
|
|||||||
Heating Rate (°C min−1) | To (°C) | Te (°C) | Tp (°C) | To (°C) | Te (°C) | Tp (°C) | To (°C) | Te (°C) | Tp (°C) | Heat (J/g) |
5 | 123 | 153 | 143 | 153 | 187 | 179 | 120 | 188 | 179 | 249.0 |
10 | 136 | 172 | 156 | 180 | 217 | 203 | 134 | 217 | 201 | 237.3 |
15 | 141 | 179 | 161 | 187 | 222 | 208 | 139 | 228 | 207 | 248.9 |
20 | 147 | 188 | 166 | 194 | 243 | 215 | 187 | 244 | 213 | 235.4 |
Mean | – | – | – | – | – | – | – | – | – | 240.5±7.4 |
|
||||||||||
|
|
|
|
|||||||
|
||||||||||
5 | 139 | 164 | 151 | 169 | 192 | 186 | 139 | 166 | 152 | 509.2 |
10 | 145 | 180 | 158 | 186 | 212 | 202 | 145 | 213 | 159 | 543.2 |
15 | 150 | 188 | 163 | 193 | 217 | 207 | 148 | 221 | 163 | 552.0 |
20 | 150 | 188 | 163 | 193 | 217 | 207 | 151 | 229 | 167 | 536.4 |
Mean | – | – | – | – | – | – | – | – | – | 535.2±18.5 |
(To = initial temperature, Te = final temperature and Tp = maximum temperature peak)
The results of the DSC curves obtained at different heating rates were used to calculate the activation energy of the dehydration reaction for the both PG samples. The activation energy was determined using Flynn-Wall-Ozawa (FWO), Friedman (FR) and ASTM E698 methods.
Firstly, the isoconversional Friedman method was used to calculate the activation energy for different conversion values. The plot of the variation of the
Isoconversional Friedman results for: (a) PGS and (b) PGT samples.
The results of the activation energy for both PG samples are shown in
Activation energies of PGS obtained by FWO, FR and EASTM E698 methods
FWO | FR | ASTM E698 | ||||
---|---|---|---|---|---|---|
|
|
|
||||
α | E (kJ/mol) | α | E (kJ/mol) | Heating Rate (°C/min) | Temp. Max. (°C) | 1000/T (1000/K) |
0.01 | 67.4 | 0.01 | 70.2 | 5 | 453.8 | 2.20 |
0.02 | 68.1 | 0.02 | 71.0 | 10 | 471.3 | 2.13 |
0.05 | 68.6 | 0.05 | 72.8 | 15 | 480.8 | 2.08 |
0.1 | 69.7 | 0.1 | 72.1 | 20 | 488.0 | 2.05 |
0.2 | 68.8 | 0.2 | 65.7 | E (kJ/mol) | 67.9 | |
0.3 | 67.2 | 0.3 | 63.2 | |||
0.4 | 60.4 | 0.4 | 64.5 | |||
0.5 | 59.7 | 0.5 | 62.2 | |||
0.6 | 58.5 | 0.6 | 60.7 | |||
0.7 | 58.6 | 0.7 | 59.6 | |||
0.8 | 57.0 | 0.8 | 57.4 | |||
0.9 | 55.4 | 0.9 | 55.7 | |||
0.95 | 53.8 | 0.95 | 53.1 | |||
0.98 | 52.1 | 0.98 | 54.5 | |||
0.99 | 51.0 | 0.99 | 53.1 | |||
Mean | 61.1 | Mean | 62.4 | |||
Standard deviation | 6.6 | Standard deviation | 6.9 |
Activation energies of PGT obtained by FWO, FR and EASTM E698 methods
FWO | FR | ASTM E698 | ||||
---|---|---|---|---|---|---|
|
|
|
||||
|
E (kJ/mol) |
|
E (kJ/mol) | Heating Rate (°C/min) | Temp. Max. (°C) | 1000/T (1000/K) |
0.01 | 130.1 | 0.01 | 131.5 | 5 | 423.81 | 2.3595 |
0.02 | 127.7 | 0.02 | 113.0 | 10 | 430.78 | 2.3214 |
0.05 | 125.2 | 0.05 | 110.6 | 15 | 434.31 | 2.3025 |
0.1 | 123.5 | 0.1 | 110.0 | 20 | 437.66 | 2.2849 |
0.2 | 120.2 | 0.2 | 113.0 | E (kJ/mol) | 128.4 | |
0.3 | 110.8 | 0.3 | 110.9 | |||
0.4 | 105.5 | 0.4 | 105.5 | |||
0.5 | 99.0 | 0.5 | 100.8 | |||
0.6 | 96.4 | 0.6 | 97.7 | |||
0.7 | 93.2 | 0.7 | 99.3 | |||
0.8 | 90.8 | 0.8 | 99.8 | |||
0.9 | 89.5 | 0.9 | 99.1 | |||
0.99 | 89.0 | 0.99 | 99.7 | |||
Mean | 110.3 | Mean | 107.0 | |||
Standard deviation | 15.8 | Standard deviation | 9.4 |
Secondly, FWO method is an integrated method, which is also independent of the degradation mechanism. Eq. [
FWO plots of: (a) PGS and (b) PGT samples.
The values of activation energy of PGS and PGT are summarized in
Finally, ASTM E698 method based on the assumption that the maximum of the DSC curves of a reaction is reached at the same conversion degree independent of the heating rate. The activation energy was obtained from plot of
Activation energies, conversion factor and order of reaction of PGS and PGT obtained by Coats-Redfern method
Model | PGS | PGT | ||||
---|---|---|---|---|---|---|
|
|
|||||
A (s−1) | E (kJ/mol) | n | A (s−1) | E (kJ/mol) | n | |
Auto catalytic | 1.80×10 | 39.5 | 0.683 | 3.01×103 | 45.1 | 1.11 |
A1.5 | 2.49×102 | 41.1 | 1.5 | 6.58×101 | 33.8 | 1.5 |
A2 | 1.14×101 | 30.3 | 2 | 9.82×10−1 | 19.1 | 2 |
An | 3.20×102 | 41.9 | 1.47 | 8.79×101 | 34.9 | 1.47 |
D1 | 1.24×105 | 68.2 | – | 9.24×103 | 55.9 | – |
D2 | 8.14×106 | 84.8 | – | 9.02×106 | 82.2 | – |
D3 | 1.04×109 | 106.6 | – | 5.64×1010 | 117.9 | – |
D4 | 1.67×107 | 92.4 | – | 7.38×107 | 94.7 | – |
F1 | 1.01×105 | 62.6 | 1 | 2.49×105 | 63.3 | 1 |
F2 | 2.44×1010 | 105.0 | 2 | 1.11×1014 | 132.6 | 2 |
F3 | 5.91×1015 | 147.5 | 3 | 4.97×1022 | 202.0 | 3 |
Fn | 1.55×103 | 48.3 | 0.663 | 1.4×106 | 69.3 | 1.09 |
P1 | 4.15×10−1 | 20.2 | 1 | 5.59×10−4 | -61.0 | 1 |
P2 | 5.36×10−4 | -3.9 | 2 | 9.71×10 −8 | -37.1 | 2 |
P3 | 4.9×105 | -11.9 | 3 | 4.55×10−9 | -47.5 | 3 |
P4 | 1.36×10−5 | -15.9 | 4 | 9.06×10−10 | -52.6 | 4 |
Pn | 1.15×10−1 | 15.4 | 1.11 | 9.32×10−7 | -29.2 | 1.59 |
R2 | 1.02×102 | 41.4 | 2 | 5.9 | 28.6 | 2 |
R3 | 5.38×102 | 48.5 | 3 | 1.09×102 | 40.1 | 3 |
Rn | 5.21×102 | 48.3 | 2.97 | -1.21×105 | 69.3 | 1.5 |
The values of the activation energy are similar when calculated using the FR and FWO methods, while the ones obtained by the ASTM method are higher than the previous ones. Indeed, the ASTM method, using the results of TG curves, provides good kinetics results. However and in this case, it seems that using the results of the DSC curves, the results are very different to the ones obtained by the other calculation methods.
Thus the activation energy of the PG dehydration reaction, calculated from the global reaction (setp 1 and setp 2) varies depending on the calculation methods used between 61 and 63 kJ/mol for PGS sample and 107–118 kJ/mol for PGT samples.
The kinetics equations for PG dehydration is as follows [
The activation energy calculated by means of the Friedman method, according to the conversion degree for the global dehydration reaction of each PG sample.
It is clearly shows that the dehydration reaction takes place via a clearly two differentiated steps.
For PGS sample, the first step of the reaction occurs for 0.02≤α≤0.53, with an average value of activation energy of 68±6 kJ/mol and the second step for 0.53≤α≤0.99 with an average value of activation energy of 51±2 kJ/mol.
For PGT sample, the first step of the reaction occurs for 0.02≤α≤0.74, with an average value of activation energy of 110±6 kJ/mol and the second step for 0.74≤α≤0.99 with an average value of activation energy of 77±2 kJ/mol. In both PG samples, the first step of the reaction, corresponding to the formation of the hemihydrate, contributes most to activation energy of the global reaction of the dehydration than the second step, transformation of hemihydrate to anhydrate.
In the literature, there is a number of studies on kinetics dehydration of gypsum (
Comparing the experimental values of the activation energy obtained in this work to others values reported in the literature, we noted that the PGT is composed exclusively of gypsum and presents an activation energy comparable to that obtained by Elbeyli et al. (2004) (
The differences between these values and the values found in this paper could be attributed to the different origin of the raw material and/or the impurities.
Therefore the obtained results allow to know the phosphogypsum dehydration temperature. This allows to obtain an adequate desing of indrustrial milling system for the cement production.
Before the use of phosphogypsum in the cement production as setting regulator is necessary to do a thermal study to avoid the false setting by the production of hemihydrate and anhydrate phases.
The mineralogical composition of Spanish phosphogypsum PGS was approximately of 64% of CaSO4·2H2O; 33% CaSO4·0.5H2O and 3% of CaSO4. The Tunisia phosphogypsum, PGT is only composed by a 94% of CaSO4·2H2O.
The thermal studies, DTG and DTA, show difference in the dehydration temperature, due to the difference in the origin sample, chemical composition. The dehydration of the PGS sample started at lowest temperature (133 °C) than PGT sample (143 °C).
The kinetics of the thermal dehydration of two PG sources (Spain and Tunisia) was accurately determined through a series of experiments at four heating rates (5, 10, 15 and 20 K/min).
The activation energy was calculated by the isoconversional methods (Friedman, Flyn-Wall-Ozawa and ASTM E986) without previous assumption regarding the conversion fulfilled by the reaction.
Finally, Coats-Redfern method were successfully utilized to predict the reaction mechanism of thermal dehydration of PG. The dehydration reaction model of PGS can be described by “first-order” model, whereas that of PGT by “three-dimensional diffusion” model.
The authors are grateful to the Spanish National R&D&I Plan and FEDER (Project CTQ200802012/PPQ) for the financial support of this study. Dr. I. García-Díaz expresses her gratitude to the Ministry of Economy and Competitiveness for their Postdoctoral Junior Grants (Ref. FPDI-2013-16391) contracts co-financed by the European Social Fund.