简介概要

An alkaline fusion mechanism for aluminate rare earth phosphor:cation-oxoanion synergies theory

来源期刊：Rare Metals2019年第4期

论文作者：Yi-Fan Liu Shen-Gen Zhang Bo Liu Han-Lin Shen

文章页码：299 - 305

摘要：Waste aluminate rare earth phosphor is an important rare earth elements（REEs） secondary resource,which mainly consists of BaMgAl₁₀O₁₇:Eu²⁺（BAM） and CeMgAl₁₁O₁₉:Tb³⁺（CMAT）. Alkaline fusion process is widely used to recycle REEs from aluminate phosphor, but the related theory remains imperfect. In this paper,a series of alkaline fusion experiments of CMAT were performed to describe the phase change law of CMAT reactions.Based on comprehensive analysis, cation-oxoanion synergies theory（COST） was proposed to explain the aluminate phosphor structure damage. On the mirror plane of aluminate phosphor crystal structure, alkali metal cations（Na⁺, K⁺） would substitute rare earth ions, while free oxoanion(OH^-,CO₃^2-,O₂^2-) can combine with rare earth ions. These two ionic forces ensure that rare earth ions can be substituted by cations. Then, the structure is decomposed. Morphological analysis shows that observable expression of COST can be described by shrinking core model after simplification. Reaction rate constant calculated indicates that the reaction degree is nanometers per second. COST provides a more complete mechanism, and it can help improve rare earth recycling technology furtherly.

稀有金属(英文版) 2019,38(04),299-305

An alkaline fusion mechanism for aluminate rare earth phosphor:cation-oxoanion synergies theory

Yi-Fan Liu Shen-Gen Zhang Bo Liu Han-Lin Shen

Institute for Advanced Materials and Technology, University of Science and Technology Beijing

作者简介：*Shen-Gen Zhang,e-mail:zhangshengen@mater.ustb.edu.cn;

收稿日期：22 February 2017

基金：financially supported by the National Natural Science Foundation of China (Nos. U1360202, 51472030, 51672024 and 515102014);

An alkaline fusion mechanism for aluminate rare earth phosphor:cation-oxoanion synergies theory

Yi-Fan Liu Shen-Gen Zhang Bo Liu Han-Lin Shen

Institute for Advanced Materials and Technology, University of Science and Technology Beijing

Abstract：

Waste aluminate rare earth phosphor is an important rare earth elements(REEs) secondary resource,which mainly consists of BaMgAl₁₀O₁₇:Eu²⁺(BAM) and CeMgAl₁₁O₁₉:Tb³⁺(CMAT). Alkaline fusion process is widely used to recycle REEs from aluminate phosphor, but the related theory remains imperfect. In this paper,a series of alkaline fusion experiments of CMAT were performed to describe the phase change law of CMAT reactions.Based on comprehensive analysis, cation-oxoanion synergies theory(COST) was proposed to explain the aluminate phosphor structure damage. On the mirror plane of aluminate phosphor crystal structure, alkali metal cations(Na⁺, K⁺) would substitute rare earth ions, while free oxoanion(OH^-,CO₃^2-,O₂^2-) can combine with rare earth ions. These two ionic forces ensure that rare earth ions can be substituted by cations. Then, the structure is decomposed. Morphological analysis shows that observable expression of COST can be described by shrinking core model after simplification. Reaction rate constant calculated indicates that the reaction degree is nanometers per second. COST provides a more complete mechanism, and it can help improve rare earth recycling technology furtherly.

Keyword：

Aluminate phosphor; Alkaline fusion; Rare earth; Mechanism;

Received： 22 February 2017

1 Introduction

Rare earth elements (REEs) are very important because of their strategic value [ 1, 2, 3] .China dominates its global supply.A few decades on,China has been producing over90%of the global rare earths [ 4, 5, 6, 7] .Rare earths demand has continued to increase,spurred by growth in clean energy,communication,high-tech products and efficient lighting [ 8, 9, 10] .And also the fact that rare earths provide unique properties from the 4f electrons that cannot be matched by substitutes [ 11] .These cases provoked worry from the world about rare earths supply.Recycling REEs can provide an efficient way to use natural resource sustainably and economically [ 12, 13, 14] .Waste phosphor has a lot of REEs,so it has been a kind of valuable rare earth secondary resources [ 15, 16, 17, 18] .How to recycle REEs from waste phosphor in an efficient and environmental way is worthy of attention [ 5, 19, 20] .

Owing to the outstanding color characteristics and high efficiency,aluminate rare earth phosphor (aluminate phosphor) is widely used in the world [ 21] .Aluminate phosphor mainly consists of blue phosphor (BaMgAl₁₀0₁₇:Eu²⁺,BAM) and green phosphor (CeMgAl₁₁O₁₉:Tb³⁺,CM AT).BAM which can also be calledβ-alumina has a P6₃/mmc space group.A unit cell of BAM contains two spinel blocks[MgAl₁₀O₁₆].And[BaO]mirror plane separated these two spinel blocks [ 22, 23] .The present work generally agrees that Eu atoms are at the mirror plane,and they are at the location of Ba atoms [ 24] .CMAT has a structure type similar to the magnetoplumbite structure,which also consists of two spinel like blocks separated by mirror plane [ 25] .Terbium ions are the activator ions in green phosphor.Large cations,such as Ce³⁺and Tb³⁺,are regarded at the mirror plane.

In waste aluminate phosphor,BAM and CMAT have a rough mass ratio of 1:1.Both BAM and CMAT structures have high bond strength,so they cannot be dissolved in alkali or acid under room temperature [ 23] .Commonly used process for rare earth recycling from aluminate phosphor is alkaline fusion process.Liu et al. [ 26] proposed a new process to recycle waste phosphor,in which BAM and CMAT reacted with caustic soda during alkaline fusion process.The leaching rate of rare earth was improved from 42.0%to 94.6%.Wu et al. [ 27] investigated a Na₂O₂ molten salt calcining process,under which they can recover>99.9%REEs.The alkaline fusion process can effectively recover REEs from waste aluminate phosphor.Understanding alkaline fusion mechanism of aluminate phosphor can benefit a lot for secondary resource recycling,but this mechanism still remains unclear.

A few works about alkaline fusion mechanism of BAM have been reported.Zhang et al. [ 28] proposed a scientific hypothesis of BAM crystal structure disintegration during alkaline fusion process.Under this hypothesis,Na ions would substitute Eu ions in mirror plane,which can provoke more oxygen vacancies as well as interstitial Na ions.Lattice distortion makes BAM unit cell decompose into smaller blocks.This hypothesis emphasized the contribution of cations to BAM structure damage.But this hypothesis cannot explain some experiment results;for example,NaCl cannot react with BAM in the studied temperature range.

Based on the effect of ions,Liu et al. [ 29] discussed the BAM alkaline fusion reaction processes.The phase change law of BAM reaction products was analyzed.Among the six reactants (KOH,NaOH,Ca(OH)2,NaCl,Na₂CO₃ and Na₂0₂),only KOH,NaOH,Na₂CO₃,and Na₂O₂ can make the BAM structure decomposed.Figure 1 shows the reaction flow diagrams.All BAM reactions have similar reaction procedure.The oxoanion theory was proposed to explain these reactions process.It was concluded that the free oxoaion (OH^-,CO₃^2-,O2^2-) is the key group,and Eu and Mg ions were bonded with oxoanion preferentially to ensure that the BAM alkaline fusion reaction can happen.This explanation emphasized the contribution of oxoanion to BAM structure damage.

Fig.1 Flow diagrams of different BAM reactions

These two alkaline fusion theories mentioned above are both based on the analysis of BAM,while the phase change law of CMAT alkaline fusion reactions was rarely reported.And there is no comprehensive theory that can explain the alkaline fusion behavior of aluminate phosphor.In this paper,different alkaline fusion experiments of CMAT were performed separately to describe the phase change law of CMAT reactions.And different alkaline fusion experiments of aluminate phosphor were also performed.X-ray diffraction (XRD) was used to discuss the phase transformation.Scanning electron microscopy (SEM) was applied to do morphological analysis.A simplified shrinking core model was applied to calculate the reaction parameter of aluminate phosphor alkaline fusion.Depending on the analysis,a more complete theory,cation-oxoanion synergies theory (COST),was proposed to explain the alkaline fusion behavior of aluminate phosphor.Meanwhile,the degree of reaction was estimated as nanometer per second.

2 Experimental

2.1 Materials and methods

In this work,pure BAM and CMAT powders were produced by Dalian Luminglight Company,China.The main phase of BAM is Ba_0.95Mg0.912Al_10.0ssO₁₇ (PDF No.84-0818),while the main phase of CMAT is Ce_0.67Tb_0.33Mg Al₁₁O₁₉ (PDF No.36-0073) according to the XRD results.The particle size of BAM and CMAT powders are between3 and 5μm.

CMAT reacted with different reactants (NaCl,Ca(OH)2,KOH,NaOH,Na₂CO₃ and Na₂O₂) separately.In order to ensure that the alkaline fusion reaction can be processed completely,all the reactants are slightly overdosed.The alkaline fusion reaction was carried out at different temperatures for 2 h.Table 1 shows the designed temperature parameters.Some of the alkaline fusion products need to be washed with water for the subsequent analysis.If the products were washed,following the temperature there will be the word"wash"in Table 1.After drying and grinding,all products were analyzed by X-ray diffractometer (XRD,Philips APD-10,Cu Kαradiation) with diffraction angle from 10°to 100°at scanning rate of 10(°)·min^-1.Pure CMAT was also heated at different temperatures,so as to describe the structural change with temperature.

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Table 1 CMAT reaction temperature of different reactants

In order to analyze the alkaline fusion process of aluminate phosphors,pure BAM and CMAT were mixed at a mass ratio of 1:1 to perform the alkaline fusion reaction.The mixture was reacted with NaOH with mass ratio of 1:1at 300,400 and 500℃for 2 h.The reaction products were cleaned with water to avoid the interference of sodium aluminate.After filtration,drying and grinding,the products were made samples for scanning electron microscope(SEM,Quanta 250,USA) analysis.

2.2 A shrinking core model

The shrinking core model assumes that the volume change rate of the unreacted solid is proportional to the surface area of the unreacted core,i.e. [ 30] ,

where k_c is reaction rate constant,t is time,V_c is the volume of unreacted core and S_c is the surface area of unreacted core.

For spherical particles,V=4πr³/3 and S=4πr²,where r is the radius of the sphere.Assuming that x is the conversation rate of solid decomposition reaction at any time(t),and V₀ is the initial volume of the particle,then,

Thereby,Eq.(3) can be obtained from Eq.(2),

where r₀ is the initial radius of the particle and r_c is the radius of unreacted core (Fig.2).

After integration and rearrangement of Eq.(1),the following equation is obtained,

Equation (4) is the expression of the shrinking core model of a spherical nonporous particle with regular surface shape.This equation means that reaction conversation(x) varies regularly with time (t) in the case of a constant reaction rate (k_c).

Fig.2 Particle schematic diagram of a shrinking core model

3 Results and discussion

3.1 CMAT phase analysis

Table 2 lists the main phases of CMAT reaction products.PDF number (detected by JADE software) was used to describe the main phase,which is the number shown in Table 2.Pure CMAT powder contains a portion of Al₂O₃(10-0173) impurities.

Rare earth Ce/Te is always associated with other rare earth elements in natural environment.In Table 2,Tb₃ReO_s (49-0370) and Ce_0.5Zr_0.25Nd_0.25O_1.875 (28-0270)can be detected.Restricted by testing and analysis technology,identifying the associated rare earth is not accurate.So Zr and Nd can be detected during XRD analysis since none of these elements are present in CMAT.These phases are simplified by experimental principle,and Tb₃ReO₈ is regarded as Tb₂0₃,Ce_a5Zr_0.25Nd_a250_1.875 as CeO₂.Magnesium phase is very difficult to be detected in all of these products.It is mainly due to the small amount of magnesium phase and the overlaps of different phases.Based on experimental principle,it is speculated that magnesium exists in the form of magnesium oxide in products.

According to the phase analysis,when CMAT reacted with KOH,NaOH,Na₂CO₃ and Na₂O₂,CMAT structure decomposed and new substances produced.Flow diagrams of phases change were listed to compare these four reactions,as shown in Fig.3.It is obvious that CMAT alkaline fusion reactions have similar processes.Cerium,terbium and magnesium in CMAT preferentially combine with free oxoanion (OH^-,CO₃^2-,O₂^2-),transforming into the stable form:CeO₂,Tb₂O₃ and MgO.The remained aluminate structure continued to break down,eventually existing as aluminate in air.The charge balance of reaction system was maintained by cations (Na⁺,K⁺).

The phase change law of BAM has been discussed in previous work,which was mentioned in Introduction part.Compared the phase change law of CMAT with that of BAM,it is clear that CMAT and BAM can share the similar reaction process or mechanism.Hence,the alkaline fusion hypothesis,which was concluded from BAM alkaline fusion reactions,can be also used to elucidate CMAT reactions.Furthermore,when a new theory of alkaline fusion process was proposed,it could be used to explain the decomposition process of aluminate rare earth phosphor(both BAM and CMAT),as well as other substances with a similar structure.

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Table 2 Main phases of different CMAT reactions

Fig.3 Flow diagrams of different CMAT reactions

3.2 Cation-oxoanion synergies theory

From the experimental results,it is found that NaCl cannot react with both BAM and CMAT in the studied temperature range.KCl also shows the same results.It is clear that alkali metal cations (Na⁺/K⁺) alone cannot change BAM and CMAT structures.The alkaline fusion hypothesis proposed by Zhang et al. [ 28] ,which emphasized the contribution of cations,cannot explain the experimental results.The oxoanion theory proposed by Liu et al. [ 29] explained the reaction process of alkaline fusion,but the contribution of cations was not specified.Obviously,the alkaline fusion theory can be more complete if the theory focuses on the contribution of cations and anions.

Based on the analysis results,a more complete theory,COST was proposed to explain the structural decomposition of aluminate phosphor.This theory can be described as follows.The alkaline flux converted to ionic liquid state at appropriate temperature,where anions and cations are free to move.Alkali metal cations would substitute rare earth(Eu²⁺,Ce³⁺,Tb³⁺) ions on the mirror plane.Free oxoanion (OH^-,CO₃^2-,O₂^2-) can bond with rare earth ions to make them escape from the crystal structure.These two ionic forces ensure that rare earth ions can be substituted.In order to maintain the charge balance,more interstitial cations (Na/K) were added into the reaction system.Crystal structure becomes unstable because of lattice distortion and then breaks down into smaller blocks.The products ultimately exist as their simple oxides.

For better understanding,a diagram illustrating the COST is shown in Fig.4.BAM and CMAT have similar structure,which can be described as consisting of oxygen close-packed spinel blocks of[MgAl₁₀O₁₆],separated by mirror planes.RE-MgAl_nO₁₉ was chosen to represent aluminate rare earth phosphor,where RE means rare earth atom (Eu,Ce,Tb).In Fig.4,cation means Na/K ions,and oxoanion means OH^-,CO₃^2-,O₂²²-.The structure diagramof RE-MgAl₁₁O₁₉ is illustrated as the unit cell parameters of EuMgAl₁1O₁₉.And cation-Al_xO_y,is illustrated as the parameters of NaAlO₂.

On the mirror plane,alkali metal cations would substitute rare earth ions,while free oxoanion can bond with rare earth ions.These two ionic forces ensure that rare earth ions can be substituted.This substitution would cause more oxygen vacancies and interstitial ions,resulting in lattice distortion.Then,the unstable crystal structure breaks down into smaller blocks.When reaction is over,products are RE-O,MgO and aluminate.

Fig.4 Summary diagrams of COST

3.3 Alkaline fusion of aluminate phosphor

Figure 5 shows SEM images of aluminate phosphor (BAM and CMAT mixture) alkaline fusion products at different temperatures.It is observed that particles decomposition increases with temperature.Raw materials,as shown in Fig.5a,are irregular particles with 3-5μm in size.The particle surface is smooth.After alkaline fusion process under 300℃,most particles also have their basic shape,as shown in Fig.5b.But on the surface of particles,a lot of parallel gullies can be found,and it is mainly because of hot alkaline corrosion.When reaction occurs at 400 and500℃,particles decompose completely,and a lot of small agglomerated grains can be observed,as shown in Fig.5c,d.In fact,pure BAM,CMAT as well as waste rare earth phosphor,they all share similar morphological changes during the alkaline fusion process.

Morphological analysis shows that the phosphor particles begin to decompose from the particle surface and finally decompose into smaller particles.This kind of reaction process is consistent with the description of shrinking core model.Assuming that the phosphor particles are uniform spherical and the reaction rate is constant.Then,shrinking core model can be used to describe the reaction behavior of aluminate phosphor.This ideal reaction process is depicted from purely theoretical consideration as shown in Fig.6.

The phosphor particles are regarded as uniform spherical with initial radius (r₀) of 2μm in this study.Depending on experimental results,when the temperature is higher than 350℃,after 1.5 h the reaction is complete.Namely,as for Eq.(4),when t=1.5 h,then x=1.Substituting the parameters into Eq.(4),the reaction rate constant (k_c) is calculated as 3.7×10^-10 m·s^-1.Actually,specific value of reaction rate does not make sense.But the magnitude of this parameter qualitatively indicates that the degree of alkaline fusion is nanometers per second.

As for alkaline fusion process of aluminate phosphor,COST gives a way to explain the alkaline fusion mechanism from a perspective of microscopic crystal structure.Morphological analysis shows that observable expression of COST can be described by shrinlcing core model after simplification.Meanwhile,the degree of reaction is estimated as nanometer per second from a perspective of macro-reaction.

Fig.5 SEM images of aluminate phosphor alkaline fusion (with NaOH) products at different temperatures:a raw material,b 300°C,c 400°C and d 500℃

Fig.6 Schematic diagram of alkaline fusion reaction of aluminate phosphor

4 Conclusion

In this study,a series of alkaline fusion experiments of CMAT were performed separately to describe the phase change law of CM AT reactions.Compared with the phase change law of BAM,it is clear that CMAT and BAM can share the similar reaction process or mechanism.That is to say,the alkaline fusion mechanism,which was concluded from BAM or CMAT reactions,can be used to explain the decomposition process of aluminate rare earth phosphor.A more complete theory,COST,was proposed to explain the crystal structure damage of aluminate phosphor.COST can be described as follows.Both alkali metal cations (Na⁺,K⁺) and free oxoanion (OH^-,CO₃^2-,O₂^2-) have contribution to the decomposition of aluminate phosphors.On the mirror plane,alkali metal cations would substitute rare earth ions,while free oxoanion can bond with rare earth ions.These two ionic forces ensure that rare earth ions can be substituted.This substitution can cause lattice distortion.Then,the unstable crystal structure breaks down into smaller blocks.

Morphological analysis shows that the alkaline fusion behavior of aluminate phosphor could be elucidated by shrinking core model after simplification.Assume that the phosphor particles are uniform spherical and the reaction rate is constant.Reaction rate constant (k_c) is calculated as3.7×10^-1o m·s^-1.This parameter qualitatively indicates that the degree of alkaline fusion is nanometers per second.The estimation of reaction rate is a supplementary explanation of COST from the perspective of macro-reaction.