Hui Zheng(鄭慧), Meng Yang(楊猛), Cheng-Fa Jiang(江成發(fā)), and Dai-Jun Liu(劉代俊)

Department of Chemical Engineering,Sichuan University,Chengdu 610065,China

Keywords: microwave plasma, phosphorus decomposition, optical emission spectroscopy, reaction mechanism

1. Introduction

Phosphorus is an important raw material for the production of fine phosphorous chemicals and high-quality phosphoric acid. The production source of yellow phosphorous is mainly derived from the decomposition of phosphate rock.Although there are many reserves of phosphate ore resources in the world, there are few rich mines. This requires the development of production technologies that can make good use of low-grade refractory phosphate rock.

At present, the yellow phosphorus production industry mainly adopts the electric furnace method. This method has the advantages of good product quality (purity can reach 99.9%), high labor productivity, and low cost. But it consumes a lot of energy. The operating temperature is around 1600 K.For every 1 t of yellow phosphorus produced,it takes about 10 t of phosphate ore, electricity consumption is about 1.3×104kW·h to 1.5×104kW·h, and slag discharge is 9 t to 11 t. In addition, electric furnace method seriously pollute the environment and is not in line with current green development needs. Therefore,the development of energy-saving and consumption-reducing,clean production technology of yellow phosphorus production is of great significance to the sustainable development.

In recent years,the research and application of microwave plasma technology has shown an obvious upward trend. Microwave plasma technology is to convert microwave energy into the internal energy of gas molecules, so that it can be excited, dissociated, and ionized into active species. Compared with dielectric barrier discharge,direct current arc,and radio frequency plasma,microwave plasma has the advantages of no electrode pollution,high purity,high energy utilization,high electron density, and uniform plasma distribution. This makes it have practical application value and wide application prospects. Microwave plasma can be used in many fields,such as sputtering film formation,[1]plasma chemical vapor deposition (PCVD),[2]plasma polymerization, and initiation polymerization,[3]surface modification of materials,[4]etc.

Spectroscopic diagnosis of microwave plasma characteristics under atmospheric pressure is very important for the understanding of the reaction mechanism. Electron temperature (Te) and electron density (Ne) are important parameters of plasma,and they have an important influence on the application of plasma.[5]Optical emission spectroscopy(OES)was usually used to diagnose plasmas, it can provide a lot of information about plasma species. Using this method,some basic characteristics of the plasma can be obtained by recording the emission intensity of different excited states to obtain the electron temperature,[6,7]or the electron density can be measured by obvious broadening of the spectral line.[8]Because there are many excited atoms and free radicals in the plasma,they directly or indirectly participate in plasma chemical reaction processes and affect the decomposition and formation of substances in the plasma. The plasma species information obtained by OES can be used to explore the chemical reactions occuring in the plasma and help to understand the reaction mechanism.

Up to now,there have been few reports about the decomposition phenomenon of phosphate rock under the action of microwave plasma.In this paper,we mainly study the reaction mechanism of the decomposition of low-grade phosphate rock under the action of atmospheric pressure microwave plasma.In experiment,the phosphate rock and its decomposition products are characterized by x-ray diffraction(XRD)techniques,energy disperse spectroscopy(EDS),and chemical method.At the same time, the plasma emission spectroscopy is used to study the characteristics of atmospheric pressure microwave plasma,to investigate and determine the electron temperature(Te)and electron density(Ne)of the plasma, clarify the composition of free radicals in atmospheric pressure microwave plasma, and obtain the reaction mechanism of the phosphate rock decomposition under the action of atmospheric pressure microwave plasma.

2. Experiment

The phosphate rock is from a factory in Sichuan, China.The minerals are analyzed by ICP-AES and chemical method,volumetric method, and other methods. The main chemical components are shown in Table 1.The powdered activated carbon with analytical grade(AR)is used as the reducing agent.

The self-developed compressed waveguide microwave plasma device is used as the experimental equipment. Its microwave frequency is 2.45 GHz and the output power is adjustable from 0 W to 1300 W.The device is mainly composed of a microwave generator,a reaction chamber,a vacuum system, a cooling system and an air intake control system. The cooling system adopts circulating water cooling, and the intake air flow is controlled by a rotameter, as shown in Fig.1.Multi-channel high-resolution plasma dedicated analysis photoelectric acquisition system spectrometer (AVANTES B.V.AvaSpecULS2048-4-USB2-RM) is used for spectrum measurement. This spectrometer uses a fiber optic detector, the best optical resolution can reach 0.10 nm, and the measurement spectrum band range is 180 nm–710 nm.

Table 1. Chemical composition of raw phosphate rock.

In the experiment, a certain amount of phosphate rock with a particle size of 120 meshes was mixed with activated carbon in a certain proportion, then water was added to mix well (10% of water), and the tablets were dried. Put the compressed mixture into a graphite crucible, and carry out a reduction reaction under the action of microwave plasma.The experiment mainly investigates the influence of different reaction times. The phosphorus content in the product is determined in accordance with GB/T 1871.1-1995 “Determination of phosphorus pentoxide content in phosphate rock and phosphate concentrate-quinoline phosphomolybdate gravimetric method”.The calcium content is determined in accordance with GB/T 1871.4-1995“Determination of calcium oxide content in phosphate rock and phosphate concentrate volumetric method”.

Fig.1. Schematic diagram of the experimental set up.

Turn on successively the water cooling system, the microwave plasma equipment, the gas source, and adjust the power. A certain flow of working gas reaches the nozzle through the plasma resonance cavity. When the microwave power is large enough,the microwave energy at the tip of the nozzle can excite the gas discharge in the area to form a plasma jet. The optical fiber probe of the spectrometer is aimed at the center of the plasma flame to record the plasma spectrum data in time. N2as the working gas and CO as the reducing gas were used in the experiment. The operating parameters for the experiments are given in Table 2. The experimental raw materials and products are tested and characterized by XRD,EDS,chemical analysis,and other technologies.

Table 2. Experimental operating parameters.

3. Results and discussion

3.1. Thermodynamic analysis of phosphate rock decomposition

The change of Gibbs free energy is the only criterion for judging whether a chemical reaction can occur. This reaction can occur only when the Gibbs free energy of the reaction is less than zero. Phosphorus in phosphate rock is in the form of Ca5(PO4)3F,so the process of extracting phosphorus from the phosphate rock is actually the reduction process of Ca5(PO4)3F. When carbon and carbon monoxide are used as reducing agents, the most likely chemical reaction formulas for the phosphate rock decomposition are shown below:[9]

The HSC Chemistry 6.0 software is used to calculate the Gibbs free energy change(?G)of the above reactions with temperature(T). The calculation results are shown in Fig.2.

From Fig.2,it can be seen that when C(s)is used as the reducing agent,the corresponding temperatures when ?G=0 in Eqs.(1)–(3)are 1808 K,1670 K,and 1533 K,respectively.When CO is the reducing agent, the corresponding temperatures when ?G=0 in Eqs. (4)–(6) are 5204 K, 4975 K, and 4653 K, respectively. It can be seen that when carbon and carbon monoxide are used as reducing agents, the minimum temperature of the phosphate rock decomposition is relatively high. If C(g)is used as the reducing agent,the chemical reaction formula of phosphate rock decomposition is[9]

Figure 2(c) shows the Gibbs free energy change(?G, kJ·mol?1) with temperature (T, K) calculated by the HSC Chemistry 6.0 software from Eqs. (7)–(9). From Fig.2(c), it can be seen that when the phosphate rock reacts with C(g),the lowest temperature of the reaction is much less than 100 K,so an attempt can be made to reduce the phosphate rock under the condition of C(g).

Fig.2. Profiles of ?G–T from chemical reaction equations:(a)Eqs.(1)–(3);(b)Eqs.(4)–(6);and(c)Eqs.(7)–(9).

LD Pietanza et al.[10]studied the non-equilibrium plasma kinetics of CO reaction under microwave discharge conditions. It was found that under the conditions of microwave discharge, CO follows a pure vibration mechanism, that is,CO can dissociate directly from CO+M→C+O+M,the ionization energy is 11.128 eV.At the same time, under the collision of electrons, CO ionizes in the following way, namely,e+CO←→e+CO++e, CO++e→C+O. From this we know that C(g) can be obtained from the dissociation of oxygencontaining gas in microwave plasma. Therefore in the present work, CO is applied as the working gas to reduce the phosphate rock under the action of microwave plasma.

3.2. Phosphate rock decomposition

The XRD pattern of the raw phosphorus powder is shown in Fig.3. Figure 4(a) shows the XRD patterns of phosphate rock reduction under microwave plasma at different reaction times. Figure 4(b)shows the intensity changes of the diffraction peaks of Ca2SiO4and CaSiO3at different reaction times.It can be seen from these figures that,when the phosphate rock is reduced under the action of plasma,two diffraction peaks of Ca2SiO4and CaSiO3appear in the XRD pattern of the product,indicating that the phosphate rock reacts with CO to form Ca2SiO4and CaSiO3. As the reaction time increases, the intensity of the CaSiO3diffraction peak first increases and then decreases,while the intensity of the Ca2SiO4diffraction peak basically remains unchanged. It can be known that when the reaction time is 10 min,the output of CaSiO3is the highest.

Fig.3. XRD pattern of raw phosphorus powder.

The EDX spectrum of the phosphate rock decomposed under the action of microwave plasma is shown in Fig.5. P,Ca, Si, and oxygen peaks are clearly identified. The EDX spectrum shows that the P peak is attenuated more than the Ca peak after the phosphate rock reacts for 10 min, indicating that the phosphate rock has decomposed during this process. Therefore, it can be seen from the EDX spectrum that the phosphate rock undergoes a decomposition reaction under the action of microwave plasma.

Fig.4. XRD pattern of microwave plasma dissociated phosphorus at different times.

Fig.5. EDS spectrum of phosphate rock decomposition under microwave plasma: (a) before the phosphate rock decomposes (b) after the phosphate rock is decomposed.

Fig.6. The relationship between the reaction time and the conversion rate of phosphate rock.

Figure 6 shows the relationship between the reaction time and the conversion rate of the phosphate rock. From Fig.6,it can be seen that the phosphate rock has a certain conversion rate under the action of microwave plasma,and as the reaction time increases,the reduction rate of the phosphate rock shows a slow upward trend. According to the experimental results,we can infer that the phosphate rock reduction reaction is completed in a short time under the action of microwave plasma,and a too long reaction time is meaningless to the phosphate rock reduction reaction.

The above results confirmed that the phosphate rock had a decomposition reaction under the action of microwave plasma.The main products are Ca2SiO4and CaSiO3.It can be inferred that certain active particles excited by the microwave plasma promote the decomposition of the phosphate rock.The plasma emission spectroscopy is used to determine the excited atoms and free radicals participating in the decomposition reaction.

3.3. Spectral diagnosis of microwave plasma characteristics

There are many excited atoms and free radicals in the plasma. They participate in the plasma chemical reaction and affect the decomposition and generation of substances in the plasma. At the same time,electrons dominate the plasma ionization and collision between particles. Electron temperature(Te) and electron density (Ne) are the most important physical parameters that characterize the plasma state. Only by spectral diagnosis of atmospheric pressure microwave plasma can we understand the composition of excited atoms and free radicals inside the plasma, calculate the electron temperature and electron density, and infer the surface decomposition reaction mechanism of the phosphate rock under the action of microwave plasma.

In this paper,the electron temperature(Te)of the plasma is measured by the relative intensity of the ion emission line.For the relative intensity ratio of two O II spectral lines, the intensity measurement of spectral lines emitted by the plasma jet,when the assumption of LTE is made,is given by[11]

where λ is the emission spectrum wavelength; h is the Planck constant, 6.626×10?34J·s; c is the speed of light,3×10?8m·s?1; k is the Boltzmann constant, 1.38065×10?23J·K?1; N is the layout density of the ground state energy; Z is the partition function. The subscripts i and k are the main quantum numbers corresponding to the upper and lower levels of the line; Eiis the energy of the corresponding level;giis the statistical weight of the energy level;and Aikis the spontaneous transition probability from level i to level k.Therefore,the electron temperature(Te)can be obtained by using the intensity of two spectral lines with the same ionization state of the same element. The final formula for the electron temperature is given by:

when v is the emission frequency.

Under LTE conditions, the electron density (Ne) can be obtained by using the Saha ionization equation[12]

Here, Z and Z+1 represent two adjacent ionization states of the same element;meis electron mass,9.11×10?31kg;E∞is the ionization energy of Z ionized particles. In this study,the intensity of the two spectral lines O II (301.91 nm) and O II(347.49 nm) were used to calculate the electron temperature,and the intensity of the C I(247.86 nm)and C II(296.62 nm)lines is used to calculate the electron density. The Kurucz database will be used to identify the spectral lines of these elements.[13]

The experiment investigated the influence of CO variable and microwave power on electron temperature and electron density. A typical spectral emission at 1300 W and N2/CO flow rate 3/0.6 L/min is shown in Fig.7.

Fig.7. Typical spectral emission at 1300 W and N2/CO flow rate 3/0.6 L/min: (a)wavelength from 230 nm to 550 nm; (b)wavelength from 413 nm to 424 nm.

Table 3. Species and wavelengths used in measurements.

Fig.8. The variations of electron temperature with different CO flows and different microwave output powers.

According to formulas(10)–(13)and the elemental spectrum data in Table 3,the electron temperature can be obtained under different CO flows and microwave powers, as shown in Fig.8. As the CO flow rate and microwave output power increase, the electron temperature changes significantly. The electron temperature is an important parameter to determine the kinetic energy of electrons in plasma. Figure 8(a) shows that as the CO flow rate increases, the electron temperature shows a downward trend. This is because when the output power is constant, the increase of CO flow requires more energy for the dissociation of CO,while the constant microwave power makes the energy input in the plasma constant,and the kinetic energy of electrons will decrease. Figure 8(b) shows that as the microwave input power increases,the electron temperature increases slightly, but the increase is not significant.The CO flow rate remains unchanged, and the increase of input energy will inevitably cause the increase of the content of excited state particles produced by dissociation. The reason for the insignificant increase is because, in the spectrum obtained according to different output powers,the intensity of the CO+spectrum shows an insignificant increase as the power increases. That is, there is no obvious change in the average kinetic energy of electrons,so that there is no obvious change in the types of excited particles produced by the electron excitation and dissociation processes of the CO molecules.

Fig.9. the variation in the electron temperature with the different CO flows and different microwave output power.

Figure 9 shows the changing trend of electron density under different CO flows and microwave powers. The numerical value of the electron density can indicate the concentration of electrons in the plasma. Figure 9(a)shows that as the CO flow increases,the electron density decreases. The increase of CO flow makes the dissociation of CO require more energies,and the constant microwave power ensures that the input energy remains unchanged,and the energy is not enough to supply the energy required for CO dissociation or recombination, so the electron concentration will decrease with the increase of the CO flow.It can be seen from Fig.9(b)that as the output power increases, the electron density shows a clear upward trend.The flow of CO and nitrogen remains constant, the temperature of electrons changes little,and the average kinetic energy of electrons remains unchanged. The input energy continues to increase,and the dissociation of CO gets a sufficient energy supply. The content of excited particles produced by dissociation is increasing,that is,the electron concentration increases.This is similar to the results reported in the literature.[14]

3.4. The mechanism of the decomposition of phosphate rock under the action of microwave plasma

According to the relationship between the relative intensity of the active particles and the gas production in the spectrum shown in Fig.7, it can be seen that the spectral peaks of C I, C II, and O II are relatively strong. CO can dissociate from CO →C(3P)+O(3P), the dissociation energy of is 11.128 eV. CO molecule has 80 vibrational levels in the ground electronic state, and there are several electronic excited states at the same time, namely three triplet states,a3Π(6.01 eV), a3Σ+(6.86 eV), b3Σ+(10.40 eV) and four singlet states,A1Π(8.03 eV),B1Σ+(10.78 eV),C1Σ+(11.40 eV),E1Σ+(11.52 eV).For C and O atoms,they have four and five electronic energy levels respectively, namely, C(3P), C(1D),C(1S), C(5S0), and O(3P), O(1D), O(1S), O(3S0), O(5S0).The excitation potentials of CO+, C+, and O+are 14.01 eV,11.26 eV,and 13.61 eV,respectively.Their energies have been taken from the Kurucz database. For all the plasma species(CO,C,O,CO+,C+,O+)momentum transfer cross sections(MT),taken mainly from the LXcat database.[15]

Therefore,we can infer the mechanism of phosphate rock decomposition under microwave plasma:.[10,16–20]

(i)CO decomposition

(ii)recombination reaction

(iii)C(g)reacts with phosphate rock

Among them,CO(v)and CO(w)represent CO molecules at v and w energy levels, and they may combine to generate CO2and C.Because the intensity of the O II line in the spectrum is relatively high. The P2produced by the decomposition of phosphate rock may combine with O or O+separated from CO to form phosphorus oxides, which is also the reason for the low conversion rate of phosphate rock.

4. Conclusions

In the present work, we confirm that the decomposition reaction occurs in phosphate rock under the action of microwave plasma. At the same time, plasma optical emission spectroscopy (OES) is used to study the characteristics of atmospheric pressure microwave plasma, and the electron temperature(Te)and electron density(Ne)of the plasma are investigated and measured. The results show that with the increase of CO flow and microwave power, the electron temperature and electron density in the plasma show a decreasing and increasing trend, which has an important relationship with the energy required for CO dissociation and recombination. The experiment also determined the composition of free radicals in atmospheric pressure microwave plasma through the full spectrum was studied by spectroscopy. According to the relationship between the relative intensity of the active particles and the gas production, we obtained the reaction mechanism of phosphate rock decomposition under the action of atmospheric pressure microwave plasma.

It is known from the reaction mechanism that CO decomposes gaseous carbon ions under the action of microwave power,and the presence of gaseous carbon ions promotes the decomposition of phosphate rock. This shows that the application of atmospheric pressure microwave plasma technology to extract phosphorus from phosphate rock has great development potential.