Advanced Materials Research
ISSN: 1662-8985, Vols. 361-363, pp 268-274
doi:10.4028/www.scientific.net/AMR.361-363.268
© 2012 Trans Tech Publications, Switzerland
Online: 2011-10-07
Preparation of phosphorus by carbothermal reduction mechanism in
vacuum
Liu Yucheng1, a, Li Qiuxia1,2,b and Liu Yongcheng3,c
1
School of Chemistry and Engieering, Yunnan Normal University, Kunming 650092, China
2
Institue of Vacuum Metallurgy and Material, Kunming University of Science and Technology,
Kunming 650093, China
a
[email protected], [email protected]
Keywords: Fluorapatite; Phosphorus; Vacuum; Carbothermal reduction; Mechanism
Abstract: The purpose of this work was to investigated the carbothermic reaction of fluorapatite
process by the means of thermodynamics analyses, XRD and element analysis, respectively.
Thermodynamic calculations indicated that phosphorus can be prepared by heating the mixture of
Ca5(PO4)3F2 and C at 1173K under the system pressure of 100Pa. CO cannot react with Ca5(PO4)3F2
in the carbothermic reduction process at 973-1873K and 100Pa. Experimental results demonstrated
that phosphorus can be produced by the reaction between Ca5(PO4)3F2 and C, the main reaction phase
is P2(g), CO(g), CaO and CaF2, and with increasing temperature, the greater degree of response. The
best technology conditions, the molar ratio of Ca5(PO4)3F2 to C is 1:7.5 at 1723K for 1h when the
system pressure was about 100Pa. This study to provide experimental evidence for preparation of
phosphorus by carbothermal reaction of fluorapatite in vacuum.
1 Introduction
Phosphorus is an important industrial raw material[1], widely used phosphate, pesticides, food,
medicine, light industry and electronics industry[2], Currently, commonly used for production of
yellow phosphorus to take thermal process, which include thermal electric furnace, blast furnace, kiln
method. Among them, the electric furnace method is the mainstream of yellow phosphorus
production process[3], due to the electric furnace process is a high energy consumption industries,
and the electric furnace to easy corrosion[4]. Generated in the production process furnace gas
contains a lot of dust, gathered in the dust condensation precipitation of phosphorus is difficult to
separate on and greatly reduce the yield of phosphorus[5]. Compared with other methods, the vacuum
technology directly obtained from phosphorus in the phosphate rock, not only shorten technology
process but also in production phosphorus process to reduce the emission of pollutants [6-7].
In research of preparing phosphorus by carbothermal reduction of phosphate ore process, Like
Jiang et al[8] studies suggest that the fluorapatite reduced by carbon without silica at 1400℃; Qiuxia
Li et al[9] study was conducted of direct preparation of phosphorus from phosphate ore by taking
advantage of the peculiarity of vacuum metallurgy; Xiaojun Bao et al[10] proposed on reduction
mechanism of phosphate ore. Firstly, the fluorapatite pyrolysis to CaO, CaF2, P2O5, Then the P2O5
carbothermal reduction into phosphorus, Jinrong Zhang[11] studies the reduction of carbothermal the
process of fluorapatite and propose two mechanisms, one is direct carbothermal reduction of the
fluorapatite to P2, CaF2, CaO, CO, the other is Ca3P2 and CaC2 begin to generate, then Ca3P2 and
CaC2 reduction of the fluorapatite to phosphorus respectively. But above none mentioned methods
systematically studied the method that preparation of phosphorus by carbothermal reduction of
fluorapatite at vacuum condition.
Firstly, in the process of preparation of phosphorus by carbothermal reduction of fluoraptite
method, the reaction of different reaction temperature and molar ratio of Ca5(PO4)3F2 and C on
reaction products are also studied based on thermodynamic analysis. Secondly, by means of XRD
analysis of the phase composition, to systematically study the carbothermal reduction of process. The
main minerals found in the phosphate ore are fluorapatite[12], this used self-made fluoraptite to study
the mechanism of phosphorus.
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2 Experimental
Experiments were carried out in a vacuum furnace(Fig.1), which were designed by National
Engineering Lab for Vacuum Metallurgy of Kunming University of Science and Technology, China.
Self-made fluoraptite (P2O5 43.59%, CaO 52.46%, F 3.95%, mass fraction) was used as raw material,
graphite (C>99.85%, mass fraction) as reductant in experiments.
Fig. 1 Schematic diagram of the vacuum furnace: 1-Vacuum pump, 2-Cooling water inlet,
3-Thermocouple, 4-electrode, 5-Heating jacket, 6-Thermal insulating, 7-Graphite condensing tower
,8-Condenser, 9-Cooling water outlet, 10-Graphite heater , 11-Reaction crucible of high temperature,
12-Vacuum furnace lid.
The mixtures of fluorapatite and graphite with certain molar ratio were milled 3-5 h in
planetary-type ball milling at the rotating speed of 100-400r/min. The mixed powders were
compressedinto blocks by uni-axial pressing in a hardened steel die under 4-6 MPa. Then the blocks
were removed into crucibles and put into vacuum furnace. The pressure is 100Pa when the system
temperatures of 1723, 1623 and 1523K at 1h. The feed was heated to different reaction temperatures
at a heating rate of 10°C/min and the temperature was held for 1h when the system pressure was
100Pa. Then the vacuum furnace power was switched off when products were cooled down to 200°C.
The phosphorus were obtained.
Lattice parameter and phase composition of slag and condensate was investigated by X-ray
diffraction instrument (D/max-3B) using Cu Kα radiationin the range of 5-90 (2θ) with a step of
5()/min. The element content was studied by element analysis instrument.
3 Results and discussion
3.1 Thermodynamic analysis
According to the data and formula in ref, the main reactions may occur as follows [13]:
Ca5(PO4)3F+15C=3P2(g)+15CO(g)+9CaO+CaF2
2Ca5(PO4)3F+24C=3Ca3P2+24CO+CaF2
Ca3P2+6C=3CaC2+P2(g)
2Ca5(PO4)3F + 42C = 9 Ca3P2 + 24CO(g) + CaF2 + 3P2(g)
In addition, In the carbothermal reduction process of fluorapatite may react:
Ca5(PO4)3F + 15CO(g) = 9CaO + CaF2 + 15CO2(g) + 3P2(g)
2Ca5(PO4)3F+5CaC2=3P2(g)+10CO(g)+14CaO+CaF2
2Ca5(PO4)3F+5Ca3P2=8P2(g)+24CaO+CaF2
(1)
(2)
(3)
(4)
(5)
(6)
(7)
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Gibbs free energy of reactions (1)-(4) at 100Pa was calculated and shown in Fig.2, which was based
on the initial reaction temperatures of (1)-(4) at 1320, 1500, 1292 and 1460K, respectively. The
reaction (1) of (the molar ratio of Ca5(PO4)3F to C is 7.5 ) carbothermal reduction of the fluorapatite
to phosphorus, when the temperature is higher than 1320K. The molar ratio is 21 at 1460K
preparation phosphorus, While the molar ratio is 12 were above 1500K generate Ca3P2, Ca3P2 and C
response to generate phosphorus,the initial temperature of 1292K. On the contrary, the molar ratio of
Ca5(PO4)3F to C is 7.5 can be prepared easily because the Gibbs free energy of reaction (1) at lower
temperature.
Fig.2 Relationships between Gibbs free energy and Temperature for Reaction (1)-(4)
The Gibbs free energy of reaction (1) at different pressures is shown in Fig.3, which is based on
the initial reaction temperatures of 1769, 1592, 1447, 1326 and 1224K when the system pressures
are 105, 104, 103, 102 and 10Pa, respectively. It is apparent that the initial reaction temperature of
reaction (1) decreases obviously when the system pressure declines. Therefore, the preparation of
phosphorus in vacuum is easier than that at atmospheric pressure.
Fig.3 Gibbs free energy of reaction (1) at different pressure (p) as function of temperature
Furthermore, Gibbs free energy of reactions (5)-(7) at 100Pa was calculated and shown in
Fig.4, Thermodynamic calculations the reaction (5) indicate that CO cannot react with fluorapatite
in the carbothermal reduction process at 973-1873K in vacuum (100Pa). The results indicate that
the reaction (6)-(7) between fluorapatite with intermediary product CaC2 and Ca3P2 to produce
phosphorus occurred at 1146K and 853K. It shows that Gibbs free energy is negative when the
temperature is higher than 1146K, 853K, respectively, which the reaction can occur according to
the thermodynamics theory. In addition, the reaction (7) can be occurred easily because the Gibbs
free energy is the lowest at higher temperature.
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Fig.4 Relationships between Gibbs free energy and Temperature for Reaction (5)-(7)
3.2 Carbothermal reduction process of slag
Fig.5 XRD patterns of reduction products prepared at 1723K with different molar ratio of Ca5(PO4)3F
to C: (a)1:7.5; (b)1:10; (c)1:12;(d)1:21
Fig.5 showed the XRD patterns of the slags obtained in fluorapatite carbothermic reduction process at
1723K for 1h when the molar ratio of Ca5(PO4)3F to C ranges from 1:7.5 to 1:21. As seen in pattern
(a), Only CaO and Ca5(PO4)3F phase appear when the molar ratio of Ca5(PO4)3F to C is 1:7.5; In
pattern (b), the CaF2 and C is formed when Ca5(PO4)3F peak intensities diminish; In pattern (c)-(d),
the Ca5(PO4)3F diffraction peaks disappears, C increase. When the molar ratio of Ca5(PO4)3F to C is
1:21, C diffraction peaks is strongest. The result indicate that with the increase of C, the fluorapatite
react more completely. The results show that fluorapatite product of the reaction with graphite are:
CaO, CaF2, unreacted fluorapatite and graphite complete. When the condensate collection, the
phosphorus has had the spontaneous combustion, so the phosphorus is not detected. Fluoraptite peak
intensities diminish and C peak intensities increase gradually when the molar ratio of Ca5(PO4)3F to C
increases continually. Diffraction peak according to the relationship between high intensity and
content[14], the phase content increases along with the diffraction peak intensity increases.
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3.3 Condensation products analysis
Fig.6 XRD patterns of condensed products at 1723K with different molar ratio of Ca5(PO4)3F to C:
(a)1:7.5;(b)1:12;(c)1:21
The formation of phase in carbothermal process was studied in temperature at 1723K in vacuum
(100Pa) for 1h. Fig.6 shows XRD patterns of condensed products for different molar ratio of
Ca5(PO4)3F to C is 1:7.5, 1:12, 1:21, respectively. It can be seen from Fig.6, graphite and fluorapatite
in different molar ratio of products obtained under exactly the same phase are CaO, Ca(OH)2, CaF2
and C. There is a small amount condensate of the Ca(OH)2, because the CaO is easy getting to the
water generated Ca(OH)2. Compared with the condensate and reaction phase slag, fluorapatite and
graphite molar ratio in different conditions, obtains the product phase to be completely consistent,
show that in a vacuum carbothermal reduction process of fluorapatite, different graphite addition
amount only to the content of each phase of the reaction have influence, whether to produce new
mutually not have an impact.
3.4 Effects of reaction temperature on reaction products
Fig.7 XRD patterns of products prepared at different temperature: (a) 1523K;(b)1623K;(c)1723K
Fig.7 gives the XRD patterns of the products prepared from fluorapatite at 1523, 1623 and 1723K for
1h when the molar ratio of Ca5(PO4)3F to C is 1:7.5. It is observed that the heating temperature has
great effect on the phase composition of the products. When the temperature rose to 1523K, partial
Ca5(PO4)3F begin to convert to CaF2 and CaO, Only CaO and trace Ca5(PO4)3F were the major phase
components when the treating temperature was 1723K. The results indicate that the higher the
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temperature, the Ca5(PO4)3F easier to be carbothermal reduction. At different temperature only CaO
and CaF2 phase is formed. But phosphorus phase is not detected in experiments owing to the fact that
collection of phosphorus, the phosphorus occurred in spontaneous combustion.
Through above mentioned experiment, Ca5(PO4)3F and C with different molar ratios and the
fluorapatite reaction with graphite began to occur at 1523K, and at different temperature only CaO
and CaF2 phase is formed. it can be inferred that between the fluorapatite and graphite the main
reaction would occurs as follows: 2Ca5(PO4)3F+15C=3P2(g)+15CO(g)+9CaO+CaF2.
3.5 Element analysis
Tab.1 Reduction rate of different molar ratio and temperature
Molar ratio
1523K
1623K
1723K
Raw materials
34.17
34.17
34.17
1:7.5
31.24
20.04
<0.5
1:10
29.27
12.44
<0.5
1:12
21.28
9.77
<0.5
The reduction rate of formula:
)
is preparation of phosphorus rate,
is the weight of the product,
content in the slag,
is P2O5 content in raw materials.
is P2O5
Fig.8 Element analysis of slags products P2O5 content for different molar ratio and temperture
Tab.1 shows element analysis of slags product P2O5 content at 1523, 1623 and 1723K for 1h when the
molar ratio of Ca5(PO4)3F to C is 1:7.5, 1:10, 1:12. It can be seen in Tab.1 that with increasing the
reaction temperature, the fluorapatite in the P2O5 was reduced more easily. The product of reduction
rate listed in Fig.8 as the molar ratio and temperature increase, the reduction rate of P2O5 increase,
when the temperature more than 1723K, the reduction rate of P2O5 is about 99%. Therefore, when at
1723K, the carbothermal reduction more complete.
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4 Conclusions
(1) Based on thermodynamic analyses, the preparation of phosphorus by carbothermal reduction of
fluorapatite in vacuum, the first reaction: 2Ca5(PO4)3F+15C=3P2(g)+15CO(g)+9CaO+CaF2, and CO
cannot react with Ca5(PO4)3F2 in the fluorapatite carbothermal reduction process.
(2) XRD analysis show that no matter a mole of ratio is how many the final product of the reaction are
both CaF2, P2, CO, CaO. Combined with thermodynamics analysis, the reaction occurs mainly:
Ca5(PO4)3F+ 15C=3P2(g)+15CO(g)+9CaO+CaF2.
(3) The element analysis show that as the reaction temperature increases, the fluorapatite in the P2O5
is reduced more easily. The different molar ratio of graphite and fluorapatite results show that the
carbothermal reduction more complete alreadly, when the molar ratio of C to Ca5(PO4)3F is 7.5 at
1723K.
Acknowledgements
This work was financially supported by the Natural Science Foundation of Yunnan Province, China
(2007B045M).
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