Kinetics of Roasting Decomposition of the Rare Earth Elements by CaO and Coal

The roasting method of magnetic tailing mixed with CaO and coal was used to recycle the rare earth elements (REE) in magnetic tailing. The phase transformation and decomposition process were researched during the roasting processes. The results showed that the decomposition processes of REE in magnetic tailing were divided into two steps. The first step from 380 to 431 ◦C mainly entailed the decomposition of bastnaesite (REFCO3). The second step from 605 to 716 ◦C mainly included the decomposition of monazite (REPO4). The decomposition products were primarily RE2O3, Ce0.75Nd0.25O1.875, CeO2, Ca5F(PO4)3, and CaF2. Adding CaO could reduce the decomposition temperature of REFCO3 and REPO4. Meanwhile, the decomposition effect of CaO on bastnaesite and monazite was significant. Besides, the effects of the roasting time, roasting temperature, and CaO addition level on the decomposition rate were studied. The optimum technological conditions were a roasting time of 60 min; roasting temperature of 750 ◦C; and CaO addition level of 20% (w/w). The maximum decomposition rate of REFCO3 and REPO4 was 99.87%. The roasting time and temperature were the major factors influencing the decomposition rate. The kinetics process of the decomposition of REFCO3 and REPO4 accorded with the interfacial reaction kinetics model. The reaction rate controlling steps were divided into two steps. The first step (at low temperature) was controlled by a chemical reaction with an activation energy of 52.67 kJ/mol. The second step (at high temperature) was controlled by diffusion with an activation energy of 8.5 kJ/mol.


Introduction
Rare earth elements (REE), namely Scandium, Yttrium, and Lanthanides, are irreplaceable strategic resources since they are hailed as "the vitamins of modern industry" and "the treasure house of new materials" [1][2][3].The demand for REE has been increasing dramatically in recent years as they are being exploited in various advanced materials and technologies more frequently, such as catalysts, alloys, magnets, optics, and lasers [4,5].Nevertheless, REE always coexist in nature, and thus, their physical and chemical properties are comparable on account of the high similarity of their atomic structures [6].Typically, bastnaesite (REFCO 3 ), monazite (REPO 4 ), and xenotime are the principal resources of the REE [7,8].The two largest carbonatite-hosted deposits are the Bayan Obo mine in China and Mountain Pass in USA, and both of them are characterized by rich sources of light REE.The Bayan Obo mine, possessing a polymetallic intergrowth ore of various mineral types, is extremely complex [9][10][11].It was originally defined as China's largest iron ore mine in 1927, with almost 1.5 billion tons of reserves [12].As the main contributor to the rare earth industry in China [13], Metals 2017, 7, 213; doi:10.3390/met7060213www.mdpi.com/journal/metals the REE in the Bayan Obo mine was not investigated until decades later.Now, the beneficiation processes of weak magnetic-strong magnetic-flotation are utilized to recover iron resources and REE.The magnetic tailing, which contains a small amount of iron, is subsequently stored in the tailing dam.However, the magnetic tailing not only occupies a large amount of land, but also pollutes the environment.It has become an important factor restricting the sustainability of the Bayan Obo mine [14][15][16].The REE in magnetic tailing are crucial resources.Consequently, it is significant for researchers to investigate the properties of the REE in magnetic tailing [17,18].
According to the studies conducted by researchers from all over the world, methods of decomposition to recover the REE such as NaOH roasting [19,20], oxidation roasting [21,22], tailing re-election [23,24], and NH 4 Cl roasting [25,26], can be used.The technological process of NaOH involves washing Ca with HCl-washing with water-decomposing with NaOH-washing with water-selective dissolving with HCl-mixed RECl 3 solution.Although these processes do not produce emissions of exhaust gas, they do discharge waste water with fluorine and can be used for high grade REE only.The technological process of oxidation roasting involves oxidation roasting-leaching with HCl-decomposing with NaOH-leaching Ce with HCl.Contrarily, this method has lower costs, the product purity of Ce is relatively low, and it discharge waste includes water and gas.The mechanism of NH 4 Cl roasting includes decomposing NH 4 Cl into HCl at a certain temperature, and then the HCl will be used to chloridize the REE.This method employed to recover the REE is ecofriendly with no acid or alkali being produced in the processes.In short, while these exploratory studies have made some progress, there are still some disadvantages, such as a lower decomposition rate and a higher gas pollution.
As a crucial strategic resource, the utilization of the REE in magnetic tailing is urgent.In order to develop cleaner production technics, meanwhile improving the recovery rate of the REE, this paper exploits CaO, being an additive, to promote the decomposition of REFCO 3 and REPO 4 and to simultaneously fix the fluorine released from the decomposition of REFCO 3 .The addition of CaO will accelerate the decomposition of REFCO 3 and REPO 4 because it is an alkaline oxide.Furthermore, the defluorination reaction will proceed at the same time as the decomposition process of REFCO 3 .The fluorine will then be emitted to the atmosphere and cause damage the environment.Therefore, using CaO as an additive can reverse the defluorination reaction to a fluorine-fixing reaction and the fluorine reacts with CaO to form CaF 2 which is left in the slag.Therefore, it will reduce the pollution produced during the process.Besides, if the roasting process is carried out in an oxidation atmosphere, the Ce 2 O 3 will be oxidized to CeO 2 .However, CeO 2 is harder to leach than Ce 2 O 3 during acid leaching.As a result, in this research, coal is used as an additive to prevent Ce 2 O 3 oxidizing to CeO 2 .Eventually, the kinetics of REFCO 3 and REPO 4 decomposition are studied in order to provide theoretical data for their recovery.

Experimental Materials
The magnetic tailing used in this research was provided by a Baotou Iron and Steel Concentrator, located at Baotou in China.The coal used in this work was of industrial quality.The chemical composition of the magnetic tailing and coal are given in Tables 1 and 2, respectively.The chemical composition of the magnetic tailing was analyzed using X-ray Fluorescence (XRF).The chemical composition of the coal was analyzed using an industrial analyzer (TRGF-8000, Tianrun Technology, Hebi, China).The CaO was analytically pure.The particle size distribution was first measured using a laser particle size analyzer (BT9300H, Dandong Bettersize Instrument Co. Ltd., Dandong, China).The results of the particle size analysis are shown in Table 3.According to the result, a particle size smaller than 0.074 mm accounts for 65%.Subsequently, the method of screening is used to obtain the experimental samples smaller than 0.074 mm.Thereafter, accounting for the proportions, the magnetic tailing was mixed with CaO and coal before the further procedures of mixing, briquetting, and roasting.The detailed processes are described below.

Mixing
Based on the experimental protocol, the coal, magnetic tailing, and CaO were separately weighted.Thereafter, they were placed into a mixing tank and were mixed for five hours using the mixer machine.By mixing the three ingredients, the reaction will be more conductive.

Briquetting
After mixing, the mixture was compressed into briquettes in the briquetting machine (769-40C, Shanghai Jingsheng, Shanghai, China).The diameter and thickness of the briquettes were 30 mm and 10 mm, respectively.The aim of briquetting is to ensure that the mixture has a certain strength and has been properly disseminated.This will make it more conducive to the subsequent operation to accelerate the proceed of the reaction.

Roasting
The briquettes were placed in a graphite crucible sealed with a cap.The graphite crucible was then put into the box resistance furnace (SX_12_16, Shenyang Changcheng, Shenyang, China).The sample was roasted under a certain temperature between 600 • C and 800 • C and the REFCO 3 and REPO 4 were gradually decomposed to rare earth oxides (REO).

Analysis
The roasted samples were analyzed using X-ray powder diffraction (XRD) (PANalytical B.V., Almelo, Netherlands), thermal gravimetric and differential scanning calorimetry (TG-DSC) (Netzsch, Sable, Germany), and scanning electron microscope and energy dispersive spectrometry (SEM-EDS) (Carl Zeiss AG, Jena, Germany).Because the REE can be easily dissolved in acid after decomposition, the roasted samples were dissolved in acid and the content of REE in the residue was then measured.The content of REE was measured with Inductively Coupled Plasma-Atomic Emission Specrometry (ICP-AES) (Plasma, Beijing, China).The content of fluorine was measured with the method of chemistry titraten.
The formulas used to calculate decomposition rate of REFCO 3 and REPO 4 and the fixed fluorine rate are as follows: x REFCO 3 and REPO 4 = m 0 w 0 − m 1 w 1 m 0 w 0 × 100 where m 0 is the weight of the magnetic tailing; m 1 is the weight of the roasted sample; w 0 is the content of the REE in the magnetic tailing; w 1 is the content of REE in the roasted sample, which is not decomposed to REO; n 0 is the content of fluorine in the magnetic tailing; and n 1 is the content of fluorine in the roasted sample.

Analytical Facility
The information of the analytical facility used in the experiment is shown in Table 4.

XRD of Roasted Products
The XRD patterns of the unroasted sample and the samples roasted for 60 min at different temperatures are given in Figure 1.As illustrated in Figure 1a-e, the diffraction peaks of REFCO 3 have completely disappeared in all roasted samples.This means that REFCO 3 decomposed completely when the roasting temperature reached 600 • C and higher than 600 • C. Based on the results, it can be inferred that the decomposition temperature of REFCO 3 is lower than 600 • C. A recent study conducted by Bian et al. [27] states that REFCO 3 is first decomposed to rare-earth oxy-fluorine (REOF).At a low temperature, REOF is the major product of the decomposition of REFCO 3 without any additives.However, the diffraction peaks of REOF are not found in Figure 1.Hence, the REOF then reacts with CaO to form RE 2 O 3 and fluorite (CaF 2 ).This allows Reactions (1) and (2) [27] to proceed.Therefore, it can be found that the addition of CaO can cause REFCO 3 to completely decompose to RE 2 O 3 at a low temperature.Meanwhile, the CaO reacts with Fluorine in REFCO 3 to form CaF 2 , and will reduce the pollution caused by fluorine.However, the decomposition temperature of REPO 4 is found to be higher than REFCO 3 .As shown in Figure 1a-e, the diffraction peaks of REPO 4 appear at 600 • C, while disappear in the roasted sample over 650 • C.This indicates that the decomposition temperature of REPO 4 is between 600 • C and 650 • C. Combined with the results of XRD and knowing that there is calcium fluorapatite (Ca 5 F(PO 4 ) 3 ) in the products but no calcium phosphorite (Ca 3 (PO 4 ) 2 ), it can be concluded that CaF 2 contributes to the decomposition reaction of the REPO 4 .The detailed reaction equation is presented as (3) [27].Furthermore, subsequent to the existence of CeO 2 in the roasted sample when the temperature is between 600 • C and 650 • C, there are both CeO 2 and Ce 2 O 3 in the roasted samples at higher temperatures of 700 • C, 750 • C, and 800 • C.This is major a result, demonstrating that the addition of coal to reactants allows CO to be produced.If the coal is not added to the reactants, Ce 2 O 3 will be oxidized to CeO 2 (Reaction (4)).However, CeO 2 is harder to leach than Ce 2 O 3 during acid leaching.As a result, in order to pursue subsequent leaching, adding coal to the experiment will generate a reduction of CO gases.When the roasting temperature increases, some CeO 2 will be reduced to Ce 2 O 3 , and Reaction (6) will then proceed.Simultaneously, some CeO 2 reacts with Nd 2 O 3 to form cerium neodymium oxide (Ce 0.75 Nd 0.25 O 1.875 ), and thus, there shall also be Ce 0.75 Nd 0.25 O 1.875 in the products.The reaction equation is illustrated in Reaction ( 5) [27].From the analysis of XRD, the decomposition products of REFCO 3 and REPO 4 are REO, Ce 0.75 Nd 0.25 O 1.875 , and Ca 5 F(PO 4 ) 3 in general.Based on the XRD analyses, the conversion may follow the following reactions: calcium phosphorite (Ca3(PO4)2), it can be concluded that CaF2 contributes to the decomposition reaction of the REPO4.The detailed reaction equation is presented as (3) [27].Furthermore, subsequent to the existence of CeO2 in the roasted sample when the temperature is between 600 °C and 650 °C, there are both CeO2 and Ce2O3 in the roasted samples at higher temperatures of 700 °C, 750 °C, and 800 °C.This is major a result, demonstrating that the addition of coal to reactants allows CO to be produced.If the coal is not added to the reactants, Ce2O3 will be oxidized to CeO2 (Reaction (4)).However, CeO2 is harder to leach than Ce2O3 during acid leaching.As a result, in order to pursue subsequent leaching, adding coal to the experiment will generate a reduction of CO gases.When the roasting temperature increases, some CeO2 will be reduced to Ce2O3, and Reaction (6) will then proceed.Simultaneously, some CeO2 reacts with Nd2O3 to form cerium neodymium oxide (Ce0.75Nd0.25O1.875),and thus, there shall also be Ce0.75Nd0.25O1.875 in the products.The reaction equation is illustrated in Reaction ( 5) [27].From the analysis of XRD, the decomposition products of REFCO3 and REPO4 are REO, Ce0.75Nd0.25O1.875, and Ca5F(PO4)3 in general.Based on the XRD analyses, the conversion may follow the following reactions:

Analysis of TG-DSC
The roasting of magnetic tailing mixed with CaO and coal (rate of mass:1:0.2:0.04 (w/w/w)) analyzed by TG-DSC at a heating rate of 10 • C min −1 from room temperature to 1100 • C is shown in Figure 2a.The obtained TG-DSC curves imply that the roasting proceeds in two stages.The first stage is from 380 • C to 431 • C, with a clear endothermic peak and a weight-loss peak.According to the XRD patterns, the decomposition temperature of REFCO 3 is lower than 600 • C. Therefore, it can be inferred that the decomposition temperature of REFCO 3 is between 380 • C and 441 • C. To allow a comparison, the TG-DSC curves of the roasting processes of magnetic tailing at a heating rate of 10 • C per min from room temperature to 700 • C (Figure 2b) are studied.Figure 2b demonstrates an endothermic peak and a weight-loss peak between 460 • C and 564 • C. In general, Figure 2 shows that the addition of CaO can reduce the decomposition temperature of REFCO 3 .Conversely, Figure 2b demonstrates another endothermic peak and a weight-loss peak between 605 • C and 716 • C, which indicates the decomposition of REPO 4 .As the decomposition temperature of REPO 4 is higher than 1900 • C [28] without CaO additives, we can conclude that adding CaO can significantly reduce the decomposition temperature of REPO 4 .In conclusion, we can view that the addition of CaO has a positive effect on decomposing the REPO 4 and REFCO 3 , not only reducing the decomposition temperature, but also completely decomposing to REO.

Analysis of TG-DSC
The roasting of magnetic tailing mixed with CaO and coal (rate of mass:1:0.2:0.04 (w/w/w)) analyzed by TG-DSC at a heating rate of 10 °C min −1 from room temperature to 1100 °C is shown in Figure 2a.The obtained TG-DSC curves imply that the roasting proceeds in two stages.The first stage is from 380 °C to 431 °C, with a clear endothermic peak and a weight-loss peak.According to the XRD patterns, the decomposition temperature of REFCO3 is lower than 600 °C.Therefore, it can be inferred that the decomposition temperature of REFCO3 is between 380 °C and 441 °C.To allow a comparison, the TG-DSC curves of the roasting processes of magnetic tailing at a heating rate of 10 °C per min from room temperature to 700 °C (Figure 2b) are studied.Figure 2b demonstrates an endothermic peak and a weight-loss peak between 460 °C and 564 °C.In general, Figure 2 shows that the addition of CaO can reduce the decomposition temperature of REFCO3.Conversely, Figure 2b demonstrates another endothermic peak and a weight-loss peak between 605 °C and 716 °C, which indicates the decomposition of REPO4.As the decomposition temperature of REPO4 is higher than 1900 °C [28] without CaO additives, we can conclude that adding CaO can significantly reduce the decomposition temperature of REPO4.In conclusion, we can view that the addition of CaO has a positive effect on decomposing the REPO4 and REFCO3, not only reducing the decomposition temperature, but also completely decomposing to REO.

Analysis Using SEM-EDS
Figure 3 shows the SEM-EDS results of the examination of the morphology of the unroasted (A) and roasted (B) sample, as well as the analysis of individual REE particles.Spectrum A(a) belongs to a particle consisting of REFCO3, A(b) belongs to mixtures of REFCO3 and REPO4, and A(c) and A(d) belong to mixtures of silica (SiO2) and hematite (Fe2O3), respectively.From the morphology of the unroasted sample, we can see that the surface of the REE is smooth and compact with a bright white color, and the REE exhibit particles with an irregular appearance.The REE, SiO2, and Fe2O3 are coated with each other.Fe2O3 and SiO2 act as substrates and the REE are attached to the surface or collective of the substrate.

Analysis Using SEM-EDS
Figure 3 shows the SEM-EDS results of the examination of the morphology of the unroasted (A) and roasted (B) sample, as well as the analysis of individual REE particles.Spectrum A(a) belongs to a particle consisting of REFCO 3 , A(b) belongs to mixtures of REFCO 3 and REPO 4 , and A(c) and A(d) belong to mixtures of silica (SiO 2 ) and hematite (Fe 2 O 3 ), respectively.From the morphology of the unroasted sample, we can see that the surface of the REE is smooth and compact with a bright white color, and the REE exhibit particles with an irregular appearance.The REE, SiO 2 , and Fe 2 O 3 are coated with each other.Fe 2 O 3 and SiO 2 act as substrates and the REE are attached to the surface or collective of the substrate.The morphology and element composition of selected particles of the roasted sample are shown in Figure 3B.The spectra B(a), B(b), and B(c) belong to REO, REO, and Ca5F(PO4)3, respectively.REFCO3 and REPO4 are surely decomposed into REO (Figure 3B).Compared with the unroasted sample, there are more cracks on the surface of the roasted sample due to the gases generated by the decomposition of REFCO3, and the morphology of REE seems loose and rough.Meanwhile, the roasted REE samples have different colors (e.g., B(a) is bright white, and B(b) is a mixture of white and black).The EDS analysis shows that B(c) is composed of Ca5F(PO4)3 and that B(a) does not contain any Ca5F(PO4)3.Therefore, it seems that B(a) consists of the decomposition product of REFCO3 , while B(b) consists of REFCO3 and REPO4.

Effect of Roasting Time
Figure 4 shows the results of the decomposition rate of REFCO3 and REPO4 with a 20% CaO addition at different roasting times and roasting temperatures ranging from 600 to 800 °C.The decomposition rate REFCO3 and REPO4 increased with the incremental roasting time at all roasting temperatures (Figure 4).However, the growth of the decomposition rate is faster before 60 min and tends to slow down thereafter.Subsequently, when the roasting temperature is lower than 750 °C with a roasting time of less than 30 min, the decomposition rate of REFCO3 and REPO4 increases faster with the incremental roasting temperature.When the roasting time is higher than 40 min, the decomposition rate of REFCO3 and REPO4 rises faster with the incremental roasting temperature below 700 °C and slows down at higher temperatures.Therefore, it can be found that the decomposition rate at 700 °C for 40 min is a special point, and the decomposition rate increases rapidly at this point.This is a result of the decomposition of REPO4.This demonstrates that the REPO4 decomposes slowly before 40 min and needs a long time to decompose.In conclusion, the decomposition rate of REFCO3 and REPO4 reaches its peak after 60 min, providing us with an optimum roasting time of 60 min.The morphology and element composition of selected particles of the roasted sample are shown in Figure 3B.The spectra B(a), B(b), and B(c) belong to REO, REO, and Ca 5 F(PO 4 ) 3 , respectively.REFCO 3 and REPO 4 are surely decomposed into REO (Figure 3B).Compared with the unroasted sample, there are more cracks on the surface of the roasted sample due to the gases generated by the decomposition of REFCO 3 , and the morphology of REE seems loose and rough.Meanwhile, the roasted REE samples have different colors (e.g., B(a) is bright white, and B(b) is a mixture of white and black).The EDS analysis shows that B(c) is composed of Ca 5 F(PO 4 ) 3 and that B(a) does not contain any Ca 5 F(PO 4 ) 3 .Therefore, it seems that B(a) consists of the decomposition product of REFCO 3 , while B(b) consists of REFCO 3 and REPO 4 .

Effect of Roasting Time
Figure 4 shows the results of the decomposition rate of REFCO 3 and REPO 4 with a 20% CaO addition at different roasting times and roasting temperatures ranging from 600 to 800 • C. The decomposition rate of REFCO 3 and REPO 4 increased with the incremental roasting time at all roasting temperatures (Figure 4).However, the growth of the decomposition rate is faster before 60 min and tends to slow down thereafter.Subsequently, when the roasting temperature is lower than 750 • C with a roasting time of less than 30 min, the decomposition rate of REFCO 3 and REPO 4 increases faster with the incremental roasting temperature.When the roasting time is higher than 40 min, the decomposition rate of REFCO 3 and REPO 4 rises faster with the incremental roasting temperature below 700 • C and slows down at higher temperatures.Therefore, it can be found that the decomposition rate at 700 • C for 40 min is a special point, and the decomposition rate increases rapidly at this point.This is a result of the decomposition of REPO 4 .This demonstrates that the REPO 4 decomposes slowly before 40 min and needs a long time to decompose.In conclusion, the decomposition rate of REFCO 3 and REPO 4 reaches its peak after 60 min, providing us with an optimum roasting time of 60 min.

Effect of Roasting Temperature
With the roasting temperature increasing from 600 to 800 °C for 60 min, a series of experiments are carried out at different levels of CaO addition.The results are shown in Figure 5.
It is observed that the roasting temperature is the most significant factor affecting the decomposition rate of REFCO3 and REPO4 (Figure 5).Due to the fact that the decomposition of REFCO3 and REPO4 is an endothermic reaction, the decomposition rate increases with the incremental roasting temperature.When the roasting temperature is 600 °C, the decomposition rates are all under 75% at all levels of CaO addition.When the roasting temperature is 650 °C, the decomposition rate is under 85%.At 700 °C, the decomposition rates are all under 95%.However, when the roasting temperature exceeds 750 °C, the decomposition rate is able to reach a level above 95%.The decomposition rate reaches its maximum of 99.87% with a 20% CaO addition and a roasting temperature of 750 °C.Thus, the optimum roasting temperature is 750 °C.

Effect of CaO Addition
In order to study the effect of the CaO addition level on the decomposition rate of REFCO3 and REPO4, the curve (Figure 6) of the decomposition rate is obtained by adding the CaO addition when the roasting time is 60 min, at a roasting temperature of 750 °C.The decomposition rate does not change significantly with the increased amount of CaO addition (Figure 6).The highest decomposition rate is 99.87%, when the CaO addition level is 20%.The lowest decomposition rate is 96.21%, when the CaO addition level is 15%.It is found that when the CaO addition level increases from 15 to 20%, the decomposition rate only increases by 3.66%.However, 15% is not chosen as the

Effect of Roasting Temperature
With the roasting temperature increasing from 600 to 800 • C for 60 min, a series of experiments are carried out at different levels of CaO addition.The results are shown in Figure 5.
It is observed that the roasting temperature is the most significant factor affecting the decomposition rate of REFCO 3 and REPO 4 (Figure 5).Due to the fact that the decomposition of REFCO 3 and REPO 4 is an endothermic reaction, the decomposition rate increases with the incremental roasting temperature.When the roasting temperature is 600 • C, the decomposition rates are all under 75% at all levels of CaO addition.When the roasting temperature is 650 • C, the decomposition rate is under 85%.At 700 • C, the decomposition rates are all under 95%.However, when the roasting temperature exceeds 750 • C, the decomposition rate is able to reach a level above 95%.The decomposition rate reaches its maximum of 99.87% with a 20% CaO addition and a roasting temperature of 750 • C. Thus, the optimum roasting temperature is 750 • C.

Effect of Roasting Temperature
With the roasting temperature increasing from 600 to 800 °C for 60 min, a series of experiments are carried out at different levels of CaO addition.The results are shown in Figure 5.
It is observed that the roasting temperature is the most significant factor affecting the decomposition rate of REFCO3 and REPO4 (Figure 5).Due to the fact that the decomposition of REFCO3 and REPO4 is an endothermic reaction, the decomposition rate increases with the incremental roasting temperature.When the roasting temperature is 600 °C, the decomposition rates are all under 75% at all levels of CaO addition.When the roasting temperature is 650 °C, the decomposition rate is under 85%.At 700 °C, the decomposition rates are all under 95%.However, when the roasting temperature exceeds 750 °C, the decomposition rate is able to reach a level above 95%.The decomposition rate reaches its maximum of 99.87% with a 20% CaO addition and a roasting temperature of 750 °C.Thus, the optimum roasting temperature is 750 °C.

Effect of CaO Addition
In order to study the effect of the CaO addition level on the decomposition rate of REFCO3 and REPO4, the curve (Figure 6) of the decomposition rate is obtained by adding the CaO addition when the roasting time is 60 min, at a roasting temperature of 750 °C.The decomposition rate does not change significantly with the increased amount of CaO addition (Figure 6).The highest decomposition rate is 99.87%, when the CaO addition level is 20%.The lowest decomposition rate is 96.21%, when the CaO addition level is 15%.It is found that when the CaO addition level increases from 15 to 20%, the decomposition rate only increases by 3.66%.However, 15% is not chosen as the

Effect of CaO Addition
In order to study the effect of the CaO addition level on the decomposition rate of REFCO 3 and REPO 4 , the curve (Figure 6) of the decomposition rate is obtained by adding the CaO addition when the roasting time is 60 min, at a roasting temperature of 750 • C. The decomposition rate does not change significantly with the increased amount of CaO addition (Figure 6).The highest decomposition rate is 99.87%, when the CaO addition level is 20%.The lowest decomposition rate is 96.21%, when the CaO addition level is 15%.It is found that when the CaO addition level increases from 15 to 20%, the decomposition rate only increases by 3.66%.However, 15% is not chosen as the optimum CaO addition level.This is due to the fact that the decomposition of REFCO 3 and REPO 4 is a solid to solid reaction, and the reaction processes lack liquldity.Thus, to ensure that the REFCO 3 and REPO 4 can make contact with enough CaO, the optimum CaO addition level is 20%.optimum CaO addition level.This is due to the fact that the decomposition of REFCO3 and REPO4 is a solid to solid reaction, and the reaction processes lack liquldity.Thus, to ensure that the REFCO3 and REPO4 can make contact with enough CaO, the optimum CaO addition level is 20%.

Analysis of the Effect of Fluorine-Fixing by CaO
Fluorine is the most important strategic resource, but the discharge of fluorine causes serious pollution.Therefore, it is necessary to recover the fluorine resource.The additive CaO can not only decompose the REFCO3 and REPO4, but is also able to bind fluorine.In order to examine the effect of fixing fluorine by CaO, the effect of the roasting temperature on the fix-fluorine rate is studied with a roasting time of 60 min and at a level of 20% of the CaO addition.The results indicate that the fixfluorine rates are all over 93% with no inconsistent change in the fix-fluorine rate.This shows that the effect of CaO on fluorine fixing is very strong.

Kinetics Mode of Mixed Rare Earth Tailing's Decomposition Process
The process of magnetic tailing mixing with CaO and coal involves polyphase reactions of solid to solid.The overall reaction rate is determined by the slowest rate-limiting step in the decomposition processes.
To study the decomposition kinetic process of REFCO3 and REPO4, the magnetic tailing, CaO, and coal are mixed as a proportion of 100:20:4, with the mixture calcined at different temperatures for different durations.The results of the decomposition rate of REFCO3 and REPO4 are shown in Figure 4.The results of Figure 4 are analyzed by Equation (7), which is the equation of the interfacial reaction kinetics model.Although the results of Figure 4 are analyzed by other kinetics models, these models do not produce suitable or ideal results [29].
1 − (1 − X) 1/3 = kt (7) where X is the decomposition rate of rare earth, k represents the rate constants, and t is the reaction time.
The kinetics curves of the decomposition of REFCO3 and REPO4 are shown in Figure 7.This process follows the interfacial reaction kinetics model.According to the experimental results of Figure 7, the kinetic constants are derived from Equation ( 8), also known as the Arrhenius equation, and the results are shown in Figure 8.

RT E
where k is the reaction rate constant, A is the frequency factor, R is the gas constant, T is the temperature, and E is the activation energy.

Analysis of the Effect of Fluorine-Fixing by CaO
Fluorine is the most important strategic resource, but the discharge of fluorine causes serious pollution.Therefore, it is necessary to recover the fluorine resource.The additive CaO can not only decompose the REFCO 3 and REPO 4 , but is also able to bind fluorine.In order to examine the effect of fixing fluorine by CaO, the effect of the roasting temperature on the fix-fluorine rate is studied with a roasting time of 60 min and at a level of 20% of the CaO addition.The results indicate that the fix-fluorine rates are all over 93% with no inconsistent change in the fix-fluorine rate.This shows that the effect of CaO on fluorine fixing is very strong.

Kinetics Mode of Mixed Rare Earth Tailing's Decomposition Process
The process of magnetic tailing mixing with CaO and coal involves polyphase reactions of solid to solid.The overall reaction rate is determined by the slowest rate-limiting step in the decomposition processes.
To study the decomposition kinetic process of REFCO 3 and REPO 4 , the magnetic tailing, CaO, and coal are mixed as a proportion of 100:20:4, with the mixture calcined at different temperatures for different durations.The results of the decomposition rate of REFCO 3 and REPO 4 are shown in Figure 4.The results of Figure 4 are analyzed by Equation (7), which is the equation of the interfacial reaction kinetics model.Although the results of Figure 4 are analyzed by other kinetics models, these models do not produce suitable or ideal results [29].
1 − (1 − X) 1/3 = kt (7) where X is the decomposition rate of rare earth, k represents the rate constants, and t is the reaction time.
The kinetics curves of the decomposition of REFCO 3 and REPO 4 are shown in Figure 7.This process follows the interfacial reaction kinetics model.According to the experimental results of Figure 7, the kinetic constants are derived from Equation (8), also known as the Arrhenius equation, and the results are shown in Figure 8.As shown in Figure 8, the reaction rate controlling steps are divided into two steps.According to the Arrhenius equation (Equation ( 8)), is the slope of the straight line in Figure 8.Hence, the activation energy (E) can be calculated from Figure 8 and Equation (8).As a result, the first region (AC segment in Figure 8) has an activation energy of 8.5 kJ/mol.For the second region (CE segment in Figure 8), the activation energy is 52.67 kJ/mol.The activation energy is the main factor which determines the kinetics restrictive conditions.The reaction rate controlling step is a diffusion control when the activation energy is smaller than 13 kJ/mol.It is a mixing control when the activation energy is 20-34 kJ/mol.The chemical reaction controlled the reaction when the activation energy was bigger than 42 kJ/mol.Therefore, the activation energy implies that the reaction rate of the controlling step is a diffusion control mode at high temperature (AB segment in Figure 8) and a chemical reaction control mode at low temperature (BC segment in Figure 8).

Figure 2 .
Figure 2. TG-DSC curves of the roasting process at a heating rate of 10 °C min −1 (a) magnetic tailing mixed with CaO and coal (b) magnetic tailing.

Figure 2 .
Figure 2. TG-DSC curves of the roasting process at a heating rate of 10 • C min −1 (a) magnetic tailing mixed with CaO and coal (b) magnetic tailing.

Figure 4 .
Figure 4. Effect of roasting time on the decomposition rate of REE at different roasting temperatures.

Figure 5 .
Figure 5.Effect of roasting temperature on the decomposition rate of REE at different levels of CaO addition.

Figure 4 .
Figure 4. Effect of roasting time on the decomposition rate of REE at different roasting temperatures.

Figure 4 .
Figure 4. Effect of roasting time on the decomposition rate of REE at different roasting temperatures.

Figure 5 .
Figure 5.Effect of roasting temperature on the decomposition rate of REE at different levels of CaO addition.

Figure 5 .
Figure 5.Effect of roasting temperature on the decomposition rate of REE at different levels of CaO addition.

Figure 6 .
Figure 6.Effect of CaO addition on the decomposition rate of REE.

Figure 6 .
Figure 6.Effect of CaO addition on the decomposition rate of REE.

Figure 7 .
Figure 7. Plots of at various temperatures.

Figure 7 .
Figure 7. Plots of at various temperatures.

Table 1 .
Chemical composition of the rare earth flotation tailing (%, mass fraction).

Table 2 .
Composition of the coal (%, mass fraction) where Ad = dry basis ash; Vdaf = volatile dry ash-free basis; Fcad = a fixed carbon content of air dry basis; St,d = dry basis total sulfur.

Table 3 .
Particle size distribution of the sample of magnetic tailing (%).

Table 4 .
Information of the analytical facility.