稀有金属(英文版) 2017,36(10),840-850

Leaching isomorphism rare earths from phosphorite ore by sulfuric acid and phosphoric acid

Hui-Xin Jin Fu-Zhong Wu Xiao-Hao Mao Mei-Long Wang Hong-Yan Xie

College of Materials and Metallurgy, Guizhou University

Guizhou Province Key Laboratory of Metallurgical Engineering and Process Energy Saving

收稿日期：6 May 2017

基金：financially supported by the National Natural Science Foundation of China (Nos. 51364005, 51564003 and 51574094);

Leaching isomorphism rare earths from phosphorite ore by sulfuric acid and phosphoric acid

Hui-Xin Jin Fu-Zhong Wu Xiao-Hao Mao Mei-Long Wang Hong-Yan Xie

College of Materials and Metallurgy, Guizhou University

Guizhou Province Key Laboratory of Metallurgical Engineering and Process Energy Saving

Abstract：

Most of the phosphorite deposits in the world contain isomorphism rare earths(RE) which are considerably difficult to be leached into solution in the wet phosphoric acid process. In this work, a systematic study of leaching RE using sulfuric acid, phosphoric acid, mixed acid and two-step leaching of phosphoric acid and sulfuric acid was performed. The aims are to illuminate the main factors that inhibit RE leaching and to provide insights into the further enrichment of RE in the wet phosphoric acid solution. The results indicate that H₂SO₄ is not an effective acid for leaching isomorphism RE from phosphorite ore.The low RE leaching efficiency attributes to the RE cocrystallized and encapsulated by phosphogypsum(PG) as well as the precipitation of RE by RE sulfates or phosphates. High concentration of H₃PO₄ can enhance the dissolution and diffusion of RE ions. Hence, the optimized leaching mode of improving RE leaching efficiency is to adequately dissolve phosphorite ores in high concentration of H₃PO₄ solution and then add H₂SO₄ to crystallize PG.The effect of co-crystallization or encapsulation of PG on RE can be decreased due to the crystallizing mode of PG in the bulk solution instead of on the interface of solid reactants. RE leaching efficiency can be high up to 65% by the optimized leaching mode.

Keyword：

Phosphorite; Isomorphism; Rare earths; Leaching; Sulfuric acid; Phosphoric acid;

Author： Fu-Zhong Wu,e-mail: gutwfz@163.com;

Received： 6 May 2017

1 Introduction

There are three main existing forms of rare earths (RE) in nature:independent rare earth minerals,weathered crust elution-deposited RE and isomorphism substitution RE,The independent rare earth minerals form the basic mineral compositions of the ore,such as bastnaesite ((La,Ce)FCO₃),monazite ((Ce,La,Y,Th)PO₄) or xenotime(YPO₄) [ 1, 2] .This kind of rare earth minerals usually contain higher RE concentration and serve as the main RE resources to extract RE by metallurgical process.The weathered crust elution-deposited RE is a kind of ion-adsorbed RE and usually adsorbed on the surface or among the particles of some clay and mica minerals [ 3] .This kind of RE can be easily extracted by chemical heaping or spraying leaching.The isomorphism RE is an impurity element usually dispersed in the rock-forming minerals through isomorphism substitutions with the metal elements from the minerals [ 4] .Most of the phosphorite deposits in the world contain RE due to the substitutions of calcium ions by rare earth ions in the lattices of apatite or collophanite [ 5, 6, 7] .This form of RE in phosphorite is very hard to be separated from the ore by beneficiation,which is usually extracted as the by-product in the wet phosphoric acid process.

Phosphorite ore is the main raw material for producing phosphoric acid.At present,more than 90%of phosphoric acid in the world is produced by the wet phosphoric acid process.Hence,the recovery of RE from phosphogypsum(PG) or phosphoric acid solution has been paid wide attentions [ 8] .It has been reported that 70%-85%of RE in phosphorite can be precipitated with dehydrate(CaSO₄·2H₂O) or hemihydrate (CaSO₄·0.5H₂O) [ 9, 10, 11] .However,the quantity of PG produced exceeds the quantity of feeding ore as well as that of acid produced:1-1.5 tons of PG is produced per ton of feed ore,or about 5 tons of PG is produced per ton of acid produced [ 12] ,which leads to a complicated and uneconomical process to recover RE from PG due to the even lower RE concentration in PG than that in phosphorite.Based on this,it is widely expected that RE can be recovered directly from phosphoric acid solution to reduce the cost,and the key problem is how to enhance the leaching efficiency.It was mentioned that during the acidulation of the phosphate rock,the leaching efficiency of RE increased to 56%by introducing aluminum ions,iron ions,silica ions or combinations of aluminum,iron and silica ions into the slurry [ 12, 13] .However,this may lead to some difficulties in the subsequent stages of the phosphoric acid productions.Wang et al. [ 14] reported that the non-ionic surfactant additives led to a significant increase in the RE leaching efficiency up to 75%due to enhanced gypsum crystal growths and less adsorptions of RE.According to the aqueous leaching methods,in recent years there have been a few studies that used organic solvents as the leaching agents.Al-Thyabat et al. [ 15] found that extracting RE in-line using organic solvent (TBP) during phosphorite acidulation reduced RE distributions in PG by approximately 45%.Nevertheless,the high viscosity of organic solvent affects the filtration of PG.

Although there are more researches on the recoveries of RE from the phosphorite ores,limited studies were involved in the mechanism analysis of leaching isomorphism RE in the wet phosphoric acid process.In this work,a systematic study of leaching isomorphism RE from the phosphorite ore was performed by four leaching modes:sulfuric acid leaching,phosphoric acid leaching,mixed acid leaching and two-step leaching of phosphoric acid and sulfuric acid.For each leaching mode,the mechanism of RE leaching was deeply analyzed based on RE leaching efficiency,phase composition of PG and micromorphology of PG.This work provides a fundamental understanding of RE distribution rule in wet phosphoric acid leaching process and provides insights into the further enrichment of RE in the phosphoric acid solutions.

2 Experimental

2.1 Materials

Phosphorite ore was collected from Zhijin phosphorite deposit,which is one of the most representative super largescale isomorphism RE-bearing phosphorite deposits in the southwest of China where the total rare earth oxide reserve is 1.4 million tons and the average concentration is 0.08% [ 16, 17, 18, 19] .The ores were processed by flotation to obtain the concentrate used in this work with the composition of P₂0₅of 32.58 wt%,CaO of 52.20 wt%,Si0₂ of 7.23 wt%,Mg0of 2.27 wt%,Fe₂O₃ of 1.18 wt%and F of 2.36 wt%.The composition of RE in the concentrate is present in Table 1.It can be seen that the total content of RE is up to0.1047 wt%.Y,La,Nd and Ce have higher contents than the rest of the RE.The total content of light rare earths(LREE) is higher than that of heavy rare earths (HREE).X-ray diffraction (XRD) pattern of the concentrate is given in Fig.1.Based on the diffraction patterns,the main mineral phases in the sample are calcium phosphate fluoride(Ca₅(PO₄)3F),silicon oxide (SiO₂) and dolomite(CaCO₃·MgCO₃) with the semi-quantitative phase proportions of 50,5 and 46%,respectively.

  下载原图

Table 1 Content (c) and distribution (D) of RE in concentrate and in PGs obtained using H₂SO₄ leaching

Concentrate is flotation concentrate sample of phosphorite.PGal,PGa2,PGa3 and PGa4 being phosphogypsum samples obtained using H2SO4leaching under stoichiometric ratio of H2SO4 to CaO of 1.0,1.1,1.2 and 1.5,respectively,with leaching period of 120 min

Analytical reagent grade sulfuric acid (H₂SO₄,98%,specific gravity 1.84) and phosphoric acid (H₃PO₄,85%,specific gravity 1.69) were used to prepare the desired concentration acid solutions for the leaching experiments.

2.2 Procedures

Four leaching modes were carried out in the experiments:(a) leaching the concentrate using H₂SO₄ solution,(b) leaching the concentrate using H₃PO₄ solution,(c) leaching the concentrate using the mixed H₂SO₄ and H₃PO₄ acid solution and (d) leaching the concentrate by the two steps of H₃PO₄ and H₂SO₄.It is noted for the mode (d) that H₃PO₄ solution was used firstly to dissolve the concentrate for adequate time aiming to leach RE into solution as much as possible and then add H₂SO₄ to crystallize PG.In this study,the temperature was controlled at 75-80℃,the mass ratio of liquid to solid was 6,the stirring speed was maintained at 300 r·min^-1,and the effects of initial concentrations of the acid solutions and leaching time were investigated.For the leaching experiments,the acid solution with the desired concentration was prepared in advance and heated to 75-80℃with stirring,and then,the ore powder was added into the acid solution to be digested for a required period.In the end of experiment,the slurry was filtered to obtain the solution sample and solid sample which were analyzed by the chemical analysis methods or the advanced instruments.

Fig.1 XRD pattern of concentrate sample

2.3 Analytical methods

The main chemical compositions,P₂O₅ and CaO concentrations in the ores and solid products were analyzed by the national standard GB/T1871.1-1995 and GB/T1871.4-1995,respectively.The rare earth elements were analyzed by inductively coupled plasma-atomic emission spectroscopy (ICP-AES,ICAP6300,Thermo Fisher).The phase compositions were identified by high-temperature X-ray diffractometer (XRD,X'pert PRO MPD,PANalytical).The micromorphology of PG was determined by scanning electron microscopy (SEM,Quanta FEG 250,FEI).

3 Results and discussion

3.1 Leaching results using sulfuric acid

Sulfuric acid is the most important leaching agent to digest phosphorite in the wet phosphoric acid process.So the leaching experiments using sulfuric acid as mode (a) were carried out at first to investigate the RE leaching.As shown in Fig.2,either increasing acid dosage or prolonging leaching time is hard to improve the RE leaching efficiency.Only less than 20%leaching efficiencies were obtained in all cases by varying stoichiometric ratios of H₂SO₄ to CaO and leaching time in this work.So,only sulfuric acid is not an effective reagent for leaching RE from phosphorite ore.Although prolonging leaching time can slowly increase the RE leaching efficiency,the degree of increment is little after 90 min,which indicates that RE leaching tends to reach equilibrium.So 120 min leaching time is sufficient to reach its maximum leaching efficiency.The reasons behind these observations are complicated and probably attributed to the following aspects.

On the one hand,RE is easily to be co-crystallized with PG which leads to a low RE leaching efficiency.As reported by many Refs. [ 10, 15, 20] ,it is believed that due to the similar ionic radius of RE ions (90-130 pm) and calcium ion (126 pm) [ 21] ,RE ions co-crystallized with PG are isomorphism substitutions with Ca²⁺.Furthermore,the LREE,which have ionic radius closer to Ca²⁺,e.g.,La(130 pm),Ce (128 pm) [ 22] and Nd (126 pm) [ 21] ,are more easily co-cry stallized with PG.To further illustrate the effect of co-crystallization,four PG samples were obtained based on the leaching conditions:stoichiometric ratios of H₂SO₄ to CaO from 1.0 to 1.5 with fixed leaching time of 120 min.The distributions of RE in PG were calculated in detail,as shown in Table 1.The distribution ratio of LREE in PG is higher than that of HREE,and the ratio of LREE to HREE increases with the excessive sulfuric acid increasing.When the stoichiometric ratio of H₂SO₄ to CaO is up to 1.2,the amount of LREE in PG is approximately one time higher than that of HREE.Meanwhile,compared with feed,the distribution ratios of three major LREE (La,Ce,Nd) in PGs increase,while that of Y in HREE series decreases,which further reveals the possibility that LREE are more easily co-crystallized with PG.

Fig.2 Effect of stoichiometric ratio of H₂SO₄ to CaO on RE leaching efficiency at 75-80℃and ratio of liquid to solid of 6:1

On the other hand,the encapsulation and passivation of phosphorite particles affect RE leaching efficiency.During the digestion of the phosphorite,excessive sulfuric acid can accelerate the formations of PG on the surfaces of phosphorite particles,which leads to the encapsulation and passivation of phosphorite particles [ 14] .Simultaneously,RE is encapsulated or adsorbed by PG due to slow diffusion rate in comparison with the crystallization rate of PG.In order to further explain the effect of encapsulation or passivation,PGs obtained under stoichiometric ratios of H₂SO₄ to CaO from 1.0 to 1.5 leached for 120 min were analyzed by XRD,as shown in Fig.3.The phase proportions were estimated semi-quantitatively using PANaly tic al X'Pert high-score plus program,and the results are shown in Table 2(a).The results show that besides the main phases of di-hydrate calcium sulfate (CaSO₄·2H₂O) and anhydrous calcium sulfate (CaSO₄),a considerable amount of calcium fluoride phosphate phase (Ca₅(PO₄)3F) is also present.Furthermore,when the stoichiometric ratio of H₂SO₄ to CaO increases,Ca₅(PO₄)3F phase proportion will increase.It is well known that Ca₅(PO₄)3F is the main mineral phase in the phosphorite ore,and more Ca₅(PO₄)3F phase in PG will result in the inadequate digestion or the serious encapsulation of the phosphorite ore.In this work,the dosage of sulfuric acid is enough to meet the adequate digestion of ore.Therefore,the encapsulation is one of the main reasons to inhibit RE leaching.

Fig.3 XRD patterns of PGs using H₂SO₄ leaching

In addition,the solubility of RE sulfates may affect RE leaching efficiency.When phosphorite ores are digested by H₂SO₄ solution,besides co-crystallization and encapsulation,the isomorphism RE in Ca₅(PO₄)3F (as RE phosphates,such as LaPO₄,CePO₄ and YPO₄) may react with H₂SO₄ to produce RE sulfates by the equations:

If rare earth metal ions are precipitated from solution,the solubility of RE sulfates (RE₂(SO₄)3) will be governed by the concentrations of RE ions and sulfate ions as well as the temperature.Reaction (2) will be enhanced by higher concentrations of sulfate ions to precipitate predominantly as their sulfates RE₂(SO₄)3.It was also found that the solubility of RE metal ions decreased with high sulfate ion concentrations in acidic solutions [ 23, 24] .It is well known that the solubility of RE sulfates decreases with the increase in temperature,e.g.,solubility product (K_sp) of La₂(SO₄)3 from 1.27×10^-5 at 20℃to 1.16×10^-7 at80℃,and Y₂(SO₄)₃ from 1.02×10^-2 at 20℃to9.91×10^-5 at 80℃.RE₂(SO₄)3 may be precipitated more or less due to both of excessive sulfuric acid and high temperature at 80℃in these experiments.Nevertheless,it is not a main reason for the low RE leaching efficiency due to the higher solubility of RE₂(SO₄)3 than that of REPO₄.

In a word,the low RE leaching efficiency using sulfuric acid leaching might be attributed to the main effect of the co-crystallization and encapsulation by PG.This result is consistent with the results reported in Refs. [ 10, 14] in which most of the RE (70%-80%) were precipitated by substituting Ca²⁺in PG crystal or encapsulated by PG.

3.2 Leaching results using phosphoric acid

Generally,phosphoric acid is used to pretreat and activate phosphorite in the wet phosphoric acid process so that the negative effects of PG can be decreased.To compare the effects of phosphoric acid and sulfuric acid on RE leaching efficiencies,a series of experiments only using phosphoric acid (the leaching mode (b)) were performed.The concentrations of phosphoric acid solution ranging from 5%to50%(by the weight percentage of P₂O₅) and the leaching time ranging from 30 to 120 min were conducted in experiments.The results are shown in Fig.4.It can be seen that RE leaching efficiency increases significantly by phosphoric acid leaching,which can be up to about 60%by increasing phosphoric acid concentration to 50%of P₂O₅,nearly twice higher than the efficiency of 20%by sulfuric acid leaching.The results also indicate that phosphoric acid,especially high concentrations of phosphoric acid,is more beneficial to RE leaching from the phosphorite ore than sulfuric acid.This may be attributed to the effective dissolution of phosphorite ore by the high concentration of phosphoric acid rather than encapsulation or co-crystallization of PG by sulfuric acid leaching.Commonly,phosphorite ore is dissolved in phosphoric acid solution by the reaction [ 23] :Ca₅(P0₄)₃F+nH₃PO₄=5-Ca(H₂PO₄)2+(n-7)H₃PO₄+HF.The reaction shows that n (n>1) molecules of phosphoric acid are required to dissolve one"molecule"of Ca₅(PO₄)3F.So,the high concentration of phosphoric acid is essential to enhance the dissolution of Ca₅(PO₄)3F,and simultaneously breaks the lattice structure of isomorphism RE within Ca₅(PO₄)₃F lattice to further promote the reaction and diffusion of RE ions.In accordance with RE leaching efficiency of 60%,a very low yield ratio of residue to feed is obtained,as shown in Fig.5,which sufficiently reveals the importance of completely dissolving Ca₅(PO₄)₃F in phosphoric acid solution to improve RE leaching efficiency.

Table 2 Semi-quantitative amount of phase in PG based on XRD pattems  下载原图

PGa1, PGa2, PGa3 and PGa4 being obtained under stoichiometric ratio of H2SO4 to CaO of 1.0, 1.1, 1.2 and 1.5, respectively, and leaching time of 120 min. PGb1, PGb2, PGb3 and PGb4 being obtained under initial P2O5 concentration of 5%, 15%, 25% and 50%, respectively, and leaching time of 120 min. PGc1, PGc2 being obtained under initial P2O5 concentration of 25% and 50%, respectively, stoichiometric ratio of H2SO4 to CaO of 1.0 and leaching time of 60 min. PGd1 and PGd2 being obtained under P2O5 concentration of 25% and 50%, respectively, stoichiometric ratio of H2SO4 to CaO of 1.0, using HgPO4 to dissolve phosphorite for 30 min and then adding H2SO4 to crystallize PG for 30 and 60 min, respectively. PGd3 and PGd4 being obtained under P2O5 concentration of 50%, stoichiometric ratio of H2SO4 to CaO of 1.0, using H3PO4 to dissolve phosphorite for 30 min and then adding H2SO4 to crystallize PG for 90 and 150 min, respectively. Symbol of“-” standing for nothing

Table 2 Semi-quantitative amount of phase in PG based on XRD pattems

Fig.4 Effect of concentration of P₂O₅ on RE leaching efficiency at75-80℃and liquid-to-solid ratio of 6:1

Figure 6 and Table 2(b) present XRD patterns of the residues and the relative proportions of phases,respectively,based on the different concentrations of phosphoric acid with the same leaching time of 120 min.It can be seen that besides incompletely dissolved Ca₅(PO₄)₃F and SiO₂,a new phase,Ca₅(PO₄)₃OH,is detected.This observation is consistent with the results reported by Dorozhkin [ 23] who studied the dissolution kinetics and mechanism of natural fluorapatite under the conditions of the wet-process phosphoric acid process.The fluorapatite is firstly dissolved in a phosphoric acid solution by the following reaction:

Fig.5 Effect of P₂O₅ concentration on the RE leaching efficiency and yield ratio of residue to feed at 75-80℃,liquid-to-solid ratio of6:1 and a leaching period of 120 min

Fig.6 XRD patterns of PGs using H₃PO₄ leaching

The product of Ca₅(PO₄)3OH can continue to react with H₃O⁺or H⁺in terms of reaction conditions.Owing to the similar crystal structures of both Ca₅(PO₄)3F and Ca₅(PO₄)30H,the substitutions of F^-by OH^-will not affect the rare earth ions in isomorphism substitutions with Ca²⁺.So a high proportion of Ca₅(PO₄)₃OH in the residue indicates that still part of RE ions are not dissociated from the fluorapatite crystal lattice,which leads to a low RE leaching efficiency.In contrast,less proportions of Ca₅(PO₄)₃F phase and Ca₅(PO₄)3OH phase were detected in the solid residue obtained under 50%P₂O₅ concentration(the phase compositions of PGb4 shown in Table 2(c)),which shows that a higher concentration of phosphoric acid can continuously strengthen the dissolution of the intermediate product of Ca₅(PO₄)3OH.Thus,the existence of RE in Ca₅(PO₄)3OH will be leached into solution to increase the RE leaching efficiency.

Generally,the dominant species of RE in phosphoric acid are RE phosphate,which has a low solubility,e.g.,the solubility product (K_sp) of LaPO₄ is 4×10^-23 and that of CePO₄ is 1.6×10^-23 at 298 K [ 14] .The solubility of RE metal ions appears to increase with the increase in phosphorous or calcium content in the solution [ 24, 25] .Rare earth metal ions are known to form complex species with phosphate ions,such as , or RE(H₂PO₄) ²⁺.Therefore,the solubility of RE in phosphoric acids can be increased by increasing acid concentrations which will be beneficial to RE leaching from phosphorite ores.

3.3 Leaching results using phosphoric acid and sulfuric acid

Based on the observations above,it can be concluded that phosphoric acid is a more effective acid for leaching RE from phosphorite than sulfuric acid,but in the meantime,a large number of impurities,especially Ca,were also dissolved into the leached product by phosphoric acid.In the wet phosphoric acid process,the phosphorite is digested in a phosphoric acid solution and sulfuric acid was added to precipitate the calcium.Based on this,other leaching modes (c) and (d) as described in Sec.2.2 were carried out to digest phosphorite ores.The main aim was to further investigate the effects of different crystallization modes of PG,concentrations of phosphoric acid solutions and reacting time on RE leaching efficiencies.The comparable results are shown in Figs.7 and 8.Obviously,the leaching mode (d) is more beneficial to increase the RE leaching efficiency than the mode (c).Under the same leachingconditions,the RE leaching efficiency of mode (d) is an average 20%higher than that of the mode (c).Moreover,RE leaching efficiency increases with increasing P₂O₅concentration and prolonging time.In comparison,a low RE leaching efficiency is obtained by leaching mode(c) and similar to that by leaching mode (a),while a high RE leaching efficiency is obtained by leaching mode(d) and similar to that by leaching mode (b).The main reasons may be attributed to the initial crystallization mode of PG and the phase transition of calcium sulfate hydrate.

Fig.7 Effect of P2O5 concentration on the RE leaching efficiency by different leaching modes of (c) mixed acid of H3PO4 and H2SO4 to digest phosphorite for 60 min and (d) H3PO4 to dissolve phosphorite for 30 min and then adding H2SO4 to crystallize PG for 30 min (for all cases,temperature of 75-80℃,stoichiometric ratio of H2SO4 to CaO of 1.0 and liquid-to-solid ratio of 6:1)

Fig.8 Effect of crystallization time on the RE leaching efficiency by leaching mode of (d) P2O5 concentration of 50%,temperature of75-80℃,stoichiometric ratio of H2SO4 to CaO of 1.0,liquid-to-solid ratio of 6:1 (using H3PO4 to dissolve phosphorite for 30 min and then adding H2SO4 to crystallize PG for a required period)

In acid leaching system,the dissolution of fluorapatite and the crystallization of PG may be conducted by the following equations [ 23] :

When the phosphorite particles were added into the mixed H₃PO₄ and H₂SO₄ acid system (the leaching mode (c)),the particles are dissolved rapidly by the acid solution to produce more Ca²⁺on the interface of the solid reactants.Owing to the supersaturated concentrations of Ca²⁺and on the reaction interface of particles,the PG is crystallized simultaneously on the surfaces of phosphorite particles,which form solid solution films to affect the leaching of rare earths as explained in Sect.3.1.So H₂SO₄in the mixed acid system still plays a dominant role to inhibit the leaching of RE.However,for the leaching mode(d),the phosphorite particles are dissolved in H₃PO₄solution for adequate time to make Ca²⁺and RE ions (such as RE³⁺,RE(HPO₄)2,RE(HPO₄)⁺or RE(H₂PO₄)²⁺)produced diffuse into the bulk solution as much as possible,followed by the addition of H₂SO₄ to crystallize PG.In this case,PG can be crystallized in the bulk solution instead of on the interface of solid reactants.Thus,the co-crystallization or encapsulation effect of PG on RE will be decreased.Moreover,the high concentration of H₃PO₄ will delay the crystallization rate of PG and also reduce the RE uptake by PG [ 26] .Hence,based on the leaching modes(c) and (d),the physical models of RE leaching are shown in Fig.9,which provides an easy way to understand the effects of PG crystallization on RE leaching.

Fig.9 Physical models of RE leaching based on leaching modes (c) and (d)(PP,phosphorite particle;PG,phosphogypsum;RI,rare earth ions,such as RE³⁺,RE(

,RE(HPO₄)⁺or RE(H₂PO₄)²⁺)

Fig.10 XRD patterns of PGs using both phosphoric acid and sulfuric acid

Representative PG samples obtained from the leaching modes (c) and (d) were analyzed by XRD (Figs.10,11)and the semi-quantitative phase proportions (Table 2(c,d)).The results show a significant variation of phase compositions in PGs.For the leaching mode (c),the PGs mainly consist of di-hydrate CaSO₄·2H₂O and anhydrous CaSO₄without hemihydrate CaSO₄·0.5H₂O.While for the leaching mode (d),besides the phases of di-hydrate CaSO₄·2H₂O and anhydrous CaSO₄,a large amount of hemihydrate CaSO₄·0.5H₂O is also detected.These observations further reveal the important effect of the crystallizing formation of PG on RE leaching efficiency.According to the phase diagram of CaSO_4-H₃PO₄-H₂O system [ 25, 26] shown in Fig.12,the anhydrous is a more stable phase which can be generated or be transformed from the metastable hemihydrate depending on the temperature and the concentration of phosphoric acid [ 27] .The leaching mode (c) was performed at the temperature of75-80℃.Moreover,the dissolution of ore particles and the crystallization of PG were evolved simultaneously on the reacted interface of ore particles,which easily provide a topochemical higher concentration of phosphoric acid for crystallizing anhydrous.Because of the lower solubility of anhydrous than those of di-hydrate and hemihydrate [ 28, 29] ,the formation of a large amount of anhydrous decreases the diffusion rate of RE,which results in more uptake of RE by PG to decrease the RE leaching efficiency.However,although the leaching mode (d) was also carried out at the high temperature of 75-80℃and the high concentration of phosphoric acid (such as P₂O₅ of 50%),the crystallizing kinetics conditions of PG are different from those in the leaching mode (c).For the leaching mode(d),the phosphorite particles are adequately dissolved in H₃PO₄ solution and then H₂SO₄ was added to crystallize PG in the bulk solution.The dissolution of ore particles and the crystallization of PG can occur independently in the bulk solution.Thus,enough concentrations of Ca²⁺and in bulk solution provide the kinetics conditions in crystallizing hemihydrate [ 30] .The formation of more metastable hemihydrate than the stable anhydrous will reduce the inhibition effect of the PG solid solution film produced on RE diffusion because of the higher solubility of hemihydrate than that of anhydrous,which will be beneficial to improve the RE leaching efficiency.In addition,it can also be seen from Table 2(d) that the percentage of hemihydrate decreases,while that of di-hydrate and anhydrous increases with prolonging the crystallization time.It indicates that part of metastable hemihydrate might be transformed into di-hydrate or anhydrous.Commonly,the transition of calcium sulfate hydrate is accompanied with dehydration,dissolution,recrystallization,etc.In these cases,RE originally encapsulated or adsorbed by PG may be desorbed and transferred into phosphoric acid solution.Hence,RE leaching efficiency can be further improved.Figure 8 shows a little increase in the leaching efficiency from 30 to 90 min.It is a possibility that in this period the crystalline of transformed PG is not perfect with high surface activity to uptake some RE.

Fig.11 XRD patterns of PGs based on leaching mode (d)

Fig.12 Thermodynamic equilibrium phases diagram of CaSO₄-H₃PO₄-H₂O systems (DH,CaSO₄·2H₂O;HH,CaSO₄·0.5H₂O;AH,CaSO₄)

Based on above investigations,it can be observed that using both phosphoric acid and sulfuric acid to leach RE from phosphorite ores,the order of adding acids affects RE leaching efficiency.For example,in leaching mode (d),adding H₃PO₄ firstly to digest phosphorite ores for enough time followed by adding H₂SO₄ to crystallize PG is more beneficial to increase the RE leaching efficiency.Moreover,a high concentration of phosphoric acid can strengthen RE leaching.Nevertheless,the higher concentration of phosphoric acid may affect the crystal morphology of PG as well as be unfavorable for the crystal growth.Generally,the crystal shape and the size of PG are paid a special attention in the wet process of phosphoric acid due to the influence on filtration performance.Therefore,the crystal morphologies of PG samples from leaching mode (d) are observed and the SEM images are shown in Fig.13.The PGd2 obtained at a concentration of50%P₂O₅ presents smaller grain size than the PGd1obtained at a concentration of 25%P₂O₅.Meantime,the crystal shapes of PGd2 are irregular with a lot of fine needles and granular crystals.This indicates that a higher concentration of phosphoric acid is unfavorable for the crystal growth.In addition,with the same concentration of phosphoric acid,prolonging the crystallization time can facilitate the perfection of PG crystals (Fig.13,PGd3 and PGd4),and the crystals gradually grow with large sizes,smooth surfaces,flack and wide needle shapes.Combined with the RE efficiency under the same condition,it can be concluded that slow crystallization rate and enough crystallization time are beneficial to improve the RE leaching efficiency.

Fig.13 SEM images of PGs by leaching mode (d):a PGd1,b PGd2,c PGd3 and d PGd4

4 Conclusion

Sulfuric acid is not an effective acid for leaching isomorphism RE from phosphorite ore.Considerably low RE leaching efficiency (less than 20%) is obtained using sulfuric acid leaching.This attributes to the comprehensive actions of RE co-crystallized and encapsulated.Phosphoric acid,especially a high concentration of phosphoric acid,is beneficial to improve the RE leaching efficiency.Using phosphoric acid leaching,the RE leaching efficiency of60%can be obtained at the high concentration of 50%P₂O₅.A high concentration of phosphoric acid can completely break the lattice structure of fluorapatite crystal to enhance the dissolution and diffusion of RE ions.

By using both phosphoric acid leaching and sulfuric acid leaching,the key to improving the RE leaching efficiency is to dissolve phosphorite particles for adequate time in H₃PO₄ solution to help Ca²⁺and RE ions diffuse into bulk solution as much as possible,and then add H₂SO₄ to crystallize PG in bulk solution instead of on the interface of solid reactants.The effect of co-crystallization or encapsulation of PG on RE can be decreased,and the RE leaching efficiency can be up to 65%under the optimized condition of P₂O₅ concentration of 50%,stoichiometric ratio of H₂SO₄ to CaO of 1.0,temperature of 75-80℃and ratio of liquid to solid of 6:1.

In crystallizing PG,high concentration of phosphoric acid,slow crystallization rate,enough crystallization timeand some transition of hemihydrate into di-hydrate oranhydrous are beneficial to improving the RE leachingefficiency.

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