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Abbreviation (ISO4): Prog Chem      Editor in chief: Jincai ZHAO

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Progress in Chemistry >

2024 , Vol. 36 >Issue 5: 741 - 756

DOI: https://doi.org/10.7536/PC230821

Review

Rare Earth Mineral Processing Technology

  • Meiying Xie 1, 2, 3 ,
  • Fan Yang , 1, 2, 3, 4, * ,
  • Liyan Xue 2, 3 ,
  • Kaixian Wang 2, 3 ,
  • Zhengming Jiang 2, 3 ,
  • Yazhu Li 3
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  • 1 State Key Laboratory of Baiyunobo Rare Earth Resource Researches and Comprehensive Utilization, Baotou Research Institute of Rare Earths, Baotou 014030, China
  • 2 Fujian Province Joint Innovation Key Laboratory of Fuel and Materials in Clean Nuclear Energy System, Fujian Institute of Research on the Structure of Matter, Fuzhou 350002, China
  • 3 Xiamen Key Laboratory of Rare Earth Photoelectric Functional Materials, Xiamen Institute of Rare Earth Materials, Xiamen 361021, China
  • 4 ZhongXi (Chengdu) Rare Earth New Material Technology Co.,Ltd., Chengdu 610042, China
* e-mail:

Received date: 2023-08-25

  Revised date: 2024-01-08

  Online published: 2024-03-15

Supported by

National Key Research and Development Program of China(2022YFB3504302)

Opening Foundation of State Key Laboratory of Baiyunobo Rare Earth Resource Researches and Comprehensive Utilization(2020Z2117)

Science and Technology Service Network Initiative of Fujian(2022T3011)

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Abstract

rare earths have excellent physicochemical properties and have important applications in a wide range of fields,and always been recognized as a key mineral resource by many major countries,including China,the United States,Japan and Australia.However,rare earth raw minerals are diverse,low grade,often closely coexist with gangue minerals of similar nature,and its beneficiation and enrichment relies heavily on the development of the reagents on rare earth mineral processing technology.In This review,the research and development status of mineral-based rare earth ore flotation reagents,including collecting agents(hydroxamic acid,fatty acid,phosphoric acid and others),foaming agents(2#oil and others),modifying agents(depressants,activators)and their flotation mechanism are described.the innovation of chemical mineral processing reagents for ion-adsorption type rare earths ores,including leaching agents,and precipitation regents are summarized.In particular,this review pays special attention to the rare earth flotation collectors,especially for hydroxamic acid which is the current mainstream collector,and this review also provides a more detailed description of the capture mechanism of hydroxamic acid as well as the synthesis route.and at the end of this review,it also points out the future research direction of the reagents on rare earth mineral processing technology should be toward the higher selectivity,higher recovery,and more environmental friendliness.this review provides some useful reference for enterprises or personnel who are engaged in the rare earth mineral processing or related pharmaceuticals。

Contents

1 Introduction

2 Collecting agents on rare earth flotation

2.1 Hydroxamic acid

2.2 Fatty acid

2.3 Phosphoric acid

2.4 Other collectors

3 Foaming agents

4 Modifying agents

4.1 Depressants

4.2 Activators

5 Reagents of chemical beneficiation for Ion-adsorption type rare earth ore

5.1 Leaching agents

5.2 precipitation agents

6 Conclusion and outlook

Key words： rare earth; mineral processing; flotation agents; leaching agents; precipitation agents

Cite this article

Meiying Xie , Fan Yang , Liyan Xue , Kaixian Wang , Zhengming Jiang , Yazhu Li . Rare Earth Mineral Processing Technology[J]. Progress in Chemistry, 2024 , 36(5) : 741 -756 . DOI: 10.7536/PC230821

1 Introduction

Rare earth(RE)contains scandium,yttrium and all the elements in the lanthanide series,a total of 17 elements,with a series of excellent physical and chemical properties,which makes Rare earth have important applications in many civil and military fields such as medical treatment,energy,national defense and so on.Known as"industrial vitamin","universal soil","agricultural hormone"and"war metal",it is recognized as a key mineral resource in the United States,China,Japan,Australia,Canada,the European Union and many other countries or regions[1]。 According to the data of the US Geological Survey,by 2022,the world's proven rare earth oxide(REO)reserves are 120 million tons,mainly distributed in China(36.7%),Vietnam(18.3%),Brazil(17.5%),Russia(17.5%)and India(5.8%)[2]。 At present,the main rare earth mines in the world include:Baiyunebo rare earth mine in China,Maoniuping rare earth mine in Mianning,Zudong rare earth mine in Ganzhou,Jiangxi Province,Mountain Pass Rare Earth mine in the United States,Araxa and Minasu Rare Earth mines in Brazil,Strange Lake Rare Earth ore in Canada,Mount Weld Rare Earth mine in Australia,Zandkops drift rare earth mine in South Africa,etc.in addition,China's Jiangxi,Guangdong,Fujian,Yunnan and other southern provinces have more than 170 high-quality ion-adsorption rare earth mines,which are the main sources of medium and heavy rare earth in the world[3]。
At present,there are more than 250 kinds of rare earth-bearing minerals known,among which bastnaesite（(Ce,La)(CO3)F）,monazite（(Ce,La)PO4）,xenotime（YPO4）,gadolinite（Y2FeBe(SiO4)2O2）,and fergusonite（YNbO4）make up more than 95%of the total mineral-type rare earth ores[4]。 However,mineral type rare earth ores are usually associated with gangue minerals such As quartz,fluorite,barite,feldspar,calcite and other silicate minerals with similar properties.as a result,the grade of rare earth ore is low and the separation is difficult,so it is often necessary to enrich the low-grade rare earth ore to the industrial smelting standard rare earth concentrate through the combined beneficiation process of gravity separation,magnetic separation and flotation[5][6]。 in addition,rare earths in ionic rare earth ores are adsorbed on the surface of other minerals or between crystal layers in the form of ions,and the corresponding rare earth oxides can only be obtained by chemical beneficiation[7]。 the use of rare earth beneficiation reagents is The most critical factor affecting product grade,rare earth recovery,production efficiency,production cost and environmental benefits for both mineral and ionic rare earth ores[8]。
in this paper,the types,action mechanism and research progress of flotation reagents(collectors,frothers,regulators)for mineral type rare earth ores and chemical beneficiation reagents(leaching agents,precipitants)for ionic type rare earth ores are summarized in detail based on the efficient beneficiation and enrichment of rare earth resources.New prospects for the future development of rare earth beneficiation reagents are put forward,in order to provide reference for enterprises or researchers engaged in rare earth beneficiation,rare earth separation and enrichment,or industrial reagent development。

2 Rare earth flotation collector

collector is the key of rare earth flotation technology.Its main function is to change the hydrophilicity and hydrophobicity of the surface of the target mineral,so that the target mineral is easy to adhere to the foam and improve the floatability of the target mineral.According to the different functional groups,it can be divided into hydroximic acid collector,fatty acid collector,phosphonic acid and phosphate collector and other collectors[9,10]。

2.1 Hydroxamic acid collector

hydroxamic acid collector was developed in the 1980s,and it is also the most mainstream rare earth flotation collector at present.hydroxamic acids,also known as hydroxamic acids or hydroxamic acids,contain two tautomers,hydroxamic acid(keto structure,Fig.1A)and hydroxamic acid(alcohol structure,Fig.1b),and are dominated by hydroximic acids,which dissociate to form the same anion。
图1 羟肟酸互变异构体

Fig. 1 Isomers of hydroxamic acid

hydroxamic acid collectors currently used in rare earth flotation mainly include C7-C9 alkyl hydroximic acid,2-hydroxy-3-naphthoic hydroxamic acid(H205),1-hydroxy-2-naphthoic hydroxamic acid(H203),salicylic hydroximic acid(L102),cycloalkyl hydroximate,benzyl hydroximate,octylmalonic dihydroxamate(OMHA),octylamino diisobutyl hydroximate and dihydroxamate.as well as some modified products or mixtures of different hydroximic acids,such as H316(modified product of H205 hydroximic acid),collector P8 or P8-0(main component is hydroxynaphthalene hydroximic acid),LF8#(containing about 98%of hydroxynaphthalene hydroxamic acid),collector 103(main component is salicylhydroxamic acid)and so on[11][12][13][14][15][12,16][17][18]。 Although the hydroximic acid rare earth collector has good rare earth selectivity,it often needs heating flotation,which has a relatively high energy consumption cost,and the cost of hydroximic acid synthesis reagent is relatively high.the partial hydroximic acid structure is shown in Fig.2,and the rare earth flotation performance study of the partial hydroximic acid collector is shown in Table 1。
图2 部分羟肟酸捕收剂结构图

Fig. 2 Structure of some hydroxamic acid collectors

表1 Study on Rare Earth Flotation Performance of Partial Hydroxamic Acid Collector

Table 1 Study of rare earth flotation performance of some hydroxamic acids

Collector Raw material Flotation reagents Concentrate
C7-C9 hydroxamic acid[19] Rare earth ore from Australia with 19.13% rare earth oxide grade (REO). It mainly contains bastnaesite, monazite, limonite, magnetite and hematite. Primary flotation: C7-C9 hydroxamic acid 1 kg/t, NaOH 1 kg/t, water glass 6.4 kg/t, humic acid 0.2 kg/t, sodium fluosilicate 1 kg/t, NaS 0.7 kg/t. After a rough flotation, rare earth concentrate with 30.30% REO and 32.46% recovery was obtained.
H203、H205[20] Rare earth ore from Bao steel processing plant (China) with 11.20% REO. It mainly contains bastnaesite, monazite, limonite, and silicate minerals. Primary flotation: H203 or H205 4.0 kg/t, water glass 4.0 kg/t, XJ-01 foaming agent 0.048 kg/t, temperature 36~38 ℃. After a rough flotation, H203 yields a rare earth concentrate with 31.54% REO and 79.05% recovery; H205 yields a rare earth concentrate with 36.77% REO and 79.40% recovery.
L102[21] Rare earth ore from Bao steel processing plant (China) with 22.80% REO. It mainly contains bastnaesite, monazite, Iron minerals, fluorite, sodium pyroxene, sodium amphibole, and dolomite. Primary flotation: Salicylic acid 2.85 kg/t, water glass 3.30 kg/t, 2# oil 0.076 kg/t, pulp concentration 45~50%, pH=8.5~9.5, temperature 40 ℃. After "one rough and two fine" closed circuit flotation, a rare earth concentrate with 63.50% REO and 56.32% recovery was obtained.
H205、
H316[15]		 Rare earth ore from Bao steel processing plant (China) with 10.10% REO. It mainly contains bastnaesite, monazite, Fluorite, apatite, hematite, pyrite, feldspar, quartz, mica, etc. Bao steel ore dressing plant dilution workshop "one rough two fine" closed-circuit industrial flotation, the daily capacity of 465 t, the colectort was H316, sodium silicate as inhibitors, and H103 used as foaming agent. After "one rough and two fine" flotation, H205 yields a rare earth concentrate with 53.09% REO and 67.11% recovery; H316 yields a rare earth concentrate with 53.31%; REO and 72.98% recovery.
P8[22] Rare earth ore from Baotou (China) with 9.60% REO. It mainly contains fluorite, hematite, bastnaesite, sodium pyroxene, sodium amphibole, monazite and barite. Primary flotation: P8 (the main component is H205) 2.4 kg/t; water glass 5.6 kg/t; turpentine oil 0.36 kg/t; temperature 65 ℃, concentration 30 wt%, pH=8~9. After "one rough, three fine and one sweep" flotation, a rare earth concentrate with 50.3% REO and 78.6% recovery was obtained.

2.1.1 Synthesis of Hydroxamic Acid

A。 Hydroxylamine method
The hydroxylamine method is a method in which carboxylic acid or carboxylic acid derivative R-CO-X(X is OH or OR or Cl)is subjected to an oximation reaction with hydroxylamine under alkaline conditions to generate hydroximic acid,as shown in formula(1).However,because hydroxylamine is easy to absorb water vapor and decompose carbon dioxide at room temperature,its properties are unstable,so hydroxylamine hydrochloride or hydroxylamine sulfate is often used to react with sodium hydroxide in The process of industrial and laboratory synthesis of free hydroxylamine.The common preparation of free hydroxylamine is shown in formulas(2)and(3)。
$\mathrm{RCO}-\mathrm{X}+\mathrm{NH}_{2} \mathrm{OH} \xrightarrow{\text { 碱 }} \mathrm{RCONHOH}+\mathrm{HX}$
$\mathrm{NH}_{2} \mathrm{OH} \cdot \mathrm{HCl}+\mathrm{NaOH} \xrightarrow[40-50^{\circ} \mathrm{C}]{\mathrm{CH}_{3} \mathrm{OH}} \mathrm{NH}_{2} \mathrm{OH}+\mathrm{NaCl}^{-}+\mathrm{H}_{2} \mathrm{O}$
$\mathrm{NH}_{2} \mathrm{OH} \cdot \mathrm{H}_{2} \mathrm{SO}_{4}+2 \mathrm{NaOH} \xrightarrow[40-5 \mathrm{SH}^{\circ} \mathrm{C}]{\mathrm{CH}} \mathrm{NH}_{2} \mathrm{OH}+\mathrm{Na}_{2} \mathrm{SO}_{4}^{-}+2 \mathrm{H}_{2} \mathrm{O}$
According to the reaction kinetics and thermodynamics,the reactivity of carboxylic acid or carboxylic acid derivative with hydroxylamine is:acyl chloride>ester>carboxylic acid.Therefore,in order to improve the yield of hydroximic acid,the raw carboxylic acid can be converted into the corresponding acyl chloride or ester,and then reacted with nucleophilic reagent hydroxylamine.the reaction of carboxylic ester-hydroxylamine method is shown in formula(4)and(5),and the reaction of acyl chloride-hydroxylamine method is shown in formula(6)and(7).Because the acyl chloride-hydroxylamine method has high requirements for equipment and synthesis conditions,the most commonly used method is the carboxylic ester-hydroxylamine method,followed by the acyl chloride-hydroxylamine method。
$\mathrm{RCOOH}+\mathrm{CH}_{3} \mathrm{OH} \xrightarrow[\text { 回流 }]{\text { 浓 } \mathrm{SO}_{4}} \mathrm{RCOOCH}_{3}$
$\mathrm{RCOOCH}_{3}+\mathrm{NH}_{2} \mathrm{OH} \xrightarrow{45-55^{\circ} \mathrm{C}} \mathrm{RCONHOH}+\mathrm{CH}_{3} \mathrm{OH}$
$\mathrm{RCOOC}{{\mathrm{H}}_{\mathrm{3}}}\mathrm{+}\mathrm{N}{{\mathrm{H}}_{\mathrm{2}}}\mathrm{OH}\xrightarrow[\mathrm{45 }\!\!\tilde{\ }\!\!\text{ 55}\ ]{}\mathrm{RCONHOH}\mathrm{+}\mathrm{C}{{\mathrm{H}}_{\mathrm{3}}}\mathrm{OH}$
$\mathrm{RCOCl}+\mathrm{NH}_{2} \mathrm{OH} \xrightarrow[45 \sim 55^{\circ} \mathrm{C}]{\mathrm{NaOH}} \mathrm{RCONHOH}+\mathrm{HCl}$
Carboxyl ester-hydroxylamine method is the most commonly Used method in industry,especially for carboxylic acid raw materials which are prone to esterification.Tang Qing et al.used 2-ethyl-2-hexenoic acid,isooctanoic acid and n-octanoic acid as acid materials,methanol as alcohol source,concentrated sulfuric acid as catalyst,and reflux esterification at 80℃to obtain methyl carboxylate[23]。 Then the molar ratio of methyl ester:hydroxylamine hydrochloride:alkali was fixed at 1∶1.1∶2.2,and the cooled sodium hydroxide solution was dropped into the methanol solution of hydroxylamine hydrochloride at room temperature and stirred for 1 H to obtain the free hydroxylamine solution.Finally,the free hydroxylamine solution was added to the methyl ester,and the reaction was carried out at 50℃for 3.5 H.After acidification and purification,the pure products of 2-ethyl-2-hexenyl hydroximic acid,isooctyl hydroximic acid and n-octyl hydroxamic acid were Obtained.Using benzoic acid and salicylic acid as raw materials,Liu Zien et al.obtained high-purity benzohydroxamic acid and salicylhydroxamic acid products through two synthetic steps of esterification and oximation by carboxylic ester-hydroxylamine method[24]。
The acyl chloride-hydroxylamine method is also one of the main methods for the industrial synthesis of hydroximic acid,which is suitable for long-chain carboxylic acids,cyclic hydrocarbon carboxylic acids and aromatic hydrocarbon carboxylic acids that are not easy to esterify.Common reagents for acyl chlorination of carboxylic acids include phosphorus trichloride（PCl3）,oxalyl chloride（(COCl2)2）,phosphorus oxychloride（POCl3）,phosphorus pentachloride（PCl5）and thionyl chloride（SOCl2）.For example,Huang Linxuan used five-carbon naphthenic acid CnH2n-1COOH（n=7~22）with different carbon chain lengths as acid materials,PCl3as chlorinating agent,and used acyl chloride-hydroxylamine method to synthesize naphthenic hydroxamic acids with different carbon chain lengths and high purity[25][26]。
B。 Other preparation methods
Because the hydroxylamine method needs to consume expensive hydroxylamine as raw material,other synthesis methods have emerged,such as nitro/nitrosoalkane preparation method,amide/secondary amine oxidation preparation method,aldehyde preparation method and microbial preparation method[27,28][29,30][31][32⇓~34]。 However,the amide/secondary amine oxidation method requires high raw materials and low yield;the preparation of hydroximic acid from aldehyde is still in the laboratory stage because aldehyde is not very sensitive to the reaction,and there are many by-products,and the purification of the product is more cumbersome.the nitroalkane of the nitroalkane preparation method can be obtained by the reaction of the low fraction of petroleum and nitric acid in the gas phase,has wide sources,avoids the use of expensive hydroxylamine raw materials,and can greatly reduce the production cost of hydroximic acid.the microbial preparation method has the advantages of mild reaction temperature,acidity and other conditions,moderate reaction time,and can synthesize hydroximic acid products with complex structures.Therefore,the nitroalkane preparation method and the microbial preparation method are the industrial production methods of hydroxamic acid with great potential to replace the hydroxylamine method in the future。

2.1.2 Collection mechanism of hydroximic acid

The O and N in the polar group structure(—CONHOH)of the hydroximic acid collector both contain lone pair electrons and are close to each other,so the metal ions can chelate with the"O,O"in the hydroximic acid to form a five-membered ring chelate(Formula 8),or chelate with the"O,N"in the hydroxamic acid to form a four-membered ring chelate(Formula 9)and be adsorbed on the mineral surface to realize mineral flotation.However,due to the high tension and poor stability of four-membered ring chelates,it is generally believed that five-membered ring chelates are mainly formed.On the whole,the complexing ability of hydroximic acid with alkali and alkaline earth metals such as Na+,Ca2+,Ba2+is the worst,With Fe3+,Al3+,Cu2+,Ti4+,Ta5+,Nb5+and other metal ions have the strongest complexing ability.And intermediate complexe ability with RE3+[35,36]。 Therefore,hydroximic acid has strong collection performance for bastnaesite,hematite,perovskite,ilmenite,niobium-tantalum ore,etc.,and can be separated from fluorite,calcium carbonate and other gangue minerals by flotation.Many practices at home and abroad also show that hydroximic acid is a rare earth mineral collector with high selectivity,high efficiency,low toxicity and excellent performance。
It is generally believed that the collection principle of hydroximic acid collector for rare earth is the result of chemical adsorption and surface reaction.Chemical adsorption involves the chelating reaction between hydroximic acid and metal cations such as RE3+on the surface of mineral lattice,and the adsorption of hydroximic acid on the surface of mineral increases with the increase of temperature in a certain temperature range[37]。 surface chemical reactions include the hydrolysis of cations in the lattice of minerals such as rare earths and the formation of hydroxyl complexes in solution,which are then re-adsorbed or precipitated on the Surface of minerals to provide sites for the adsorption of hydroximic acid[38,39]。

2.2 Fatty acid collector

fatty acid collector is a rare earth flotation collector earlier than hydroximic acid.In the 1950s,oleic acid was successfully used in the flotation of Mountain Pass rare earth ore in the United States.In the early 1960s,China also systematically studied the flotation of rare earth minerals using oleic acid and Oxidized paraffin soap.fatty acid collectors(RCOOH,RCOONa or RCOOK,etc.)Are generally mixtures of C10-C20 mixed saturated and unsaturated carboxylic acids or carboxylates manufactured from natural plant or animal oils,such as oleic acid,which usually refers to mixtures of oleic acid,linoleic acid,linolenic acid and other acids;Tall oil usually refers to a mixture of Fatty acids(oleic acid,linoleic acid,linolenic acid,etc.)and resin acids(abietic acid,neoabietic acid,etc.);oxidized paraffin soap is a mixture of long-chain Fatty acids,alcohols,aldehydes,ketones and unoxidized hydrocarbons。
At present,the main fatty acid collectors used in rare earth mineral flotation are oleic acid,sodium oleate,tall oil,oxidized paraffin soap,bakas nut oil,phthalic acid,naphthenic acid and some oxidized petroleum derivatives[40][22,41][42][43][44][45][46][47]。 However,it is worth noting that fatty acid collectors have good floatability for oxidized ores and silicate minerals such as scheelite,fluorite,barite,apatite,hematite and zircon.the order of collecting ability from strong to weak is fluorite>monazite>barite>calcite>bastnaesite>iron minerals>silicate minerals,so the selectivity of rare earth minerals is not high,and the selective flotation of rare earth minerals often requires heating and adding a large amount of depressants[48][40,49]。 the main components of some fatty acid collectors for rare earth ores are shown in Fig.3,and The rare earth flotation performance of some fatty acid collectors is shown in Table 2。
图3 部分脂肪酸捕收剂主要成分结构

Fig. 3 Structure of the main components of some fatty acid collectors

表2 Study on Flotation Performance of Some Fatty Acid Collectors

Table 2 Study of rare earth flotation performance of some fatty acid collectors

Collector Raw material Flotation reagents Concentrate
sodium oleate [50] Rare earth ore from an Australian processing plant with 1.07% REO. It mainly contains bastnaesite, monazite, quartz, mica, micas, magnetite and hematite. Primary flotation: Sodium oleate 3.0 kg/t, water glass 1.5 kg/t, starch 1.5 kg/t. After "one rough, one sweep and one concentrate" flotation, a rare earth concentrate with REO 2.25% and 63% recovery was obtained.
sodium oleate [51] Rare earth ore from a processing plant in Bayan Obo (China) with 9.35% REO and CaF2 grade 26.49%. It mainly contains bastnaesite, monazite, Fluorite, hematite, magnetite, pyroxene, amphibole, quartz and feldspar. Flotation of rare earths: SR (mixture of hydroxamic acid and Sodium Carbonate) as the collector.
Primary flotation of fluorite: Sodium oleate 0.6 kg/t, acidic water glass 1.4 kg/t, SY (mixture of sodium hexametaphosphate and tannic acid) 1.28 kg/t.		 After “rare earth flotation - fluorite pre-selection - fluorite selection - strong magnetic separation”, it can obtain rare earth concentrate with 50.54% REO and 92.32% recovery; and fluorite concentrate with 95.51% CaF2 grade and 50.98% recovery.
Tarr Oil [42] Rare earth ore from Muntin Pass Rare Earth Mine (USA) with 7.7% REO. It mainly contains bastnaesite, Calcite, barite, quartz, etc. Flotation process at the rare earth ore dressing plant in Muntin Pass, USA, with Tar oil as a collector and lignosulfonic acid as a depressant. Rare earth concentrate with REO 65% and 80% recovery was obtained.
phthalic acid [52] Fluorite concentrate from a processing plant in Bayan Obo (China) with a grade of 86.02% CaF2 and 5.72% REO. t mainly contains fluorite, bastnaesite, Calcite, ilmenite, quartz, etc. Primary flotation: Phthalic acid 2.4 kg/t, water glass 2 kg/t, pH=5. After "one rough and three fine" closed-circuit flotation, the fluorite concentrate with a grade of 95.12% CaF2 and a recovery of 84.05% was obtained.
phthalic acid [53] Baotou mixed rare earth concentrates: bastnaesite (REO 51.18%, purity 79.99%), monazite (REO 12.80%, purity 20.01%), and it also contains fluorite and barite. Primary flotation: Phthalic acid 2.2 kg/t, alum as depressant 4.0 kg/t. After "one rough and two sweeps" bastnaesite concentrate (REO 68.81%, purity 95.04%) and monazite concentrate (REO 58.55%, purity 95.34%) were obtained.
the interaction between fatty acid collectors and the surface of rare earth minerals is considered to be the result of physical adsorption,chemical adsorption and surface chemical reaction[54]。 When the concentration of the collector is low,the fatty acid collector can be partially hydrolyzed in the pulp and other solutions,as shown in formula(10,11),and the dissociated RCOO-can be combined with metal ions to form(RCOO)nM complexes on the mineral surface,as shown in formula(12),and chemical adsorption or surface chemical reaction can be carried out on the mineral surface to realize mineral flotation.When the concentration of the collector is high,the fatty acid collector forms a multi-molecular layer of fatty acid adsorption layer and micelles on the surface of the mineral,and also realizes the flotation of the mineral through physical action[54]。 In addition,the primary and secondary relationship of collection is also related to the types of rare earth minerals.For example,Pavez et al.Studied the surfaces of monazite and bastnaesite before and after sodium oleate flotation by infrared spectroscopy,and found that sodium oleate was mainly chemically adsorbed on the surface of bastnaesite,while physically adsorbed on the surface of monazite[55]。
$\mathrm{RCOONa}\mathrm{+}{{\mathrm{H}}_{\mathrm{2}}}\mathrm{O}\xrightarrow{{}}\mathrm{RCOOH}\mathrm{+}\mathrm{N}{{\mathrm{a}}^{\mathrm{+}}}\mathrm{+}\mathrm{O}{{\mathrm{H}}^{\mathrm{-}}}$
$\mathrm{RCOOH}\xrightarrow{{}}\mathrm{RCO}{{\mathrm{O}}^{-}}\mathrm{+}{{\mathrm{H}}^{\mathrm{+}}}$
$n\mathrm{RCO}{{\mathrm{O}}^{-}}\mathrm{+}{{\mathrm{M}}^{n\mathrm{+}}}\xrightarrow{{}}{{\mathrm{(RCOO)}}_{n}}\mathrm{M}$

2.3 Phosphoric acid collector

Phosphonic acid(—P=O)and phosphonate(—O—P=O)collectors are more effective and less selective than hydroximic acid and fatty acid collectors because the electronegativity of the atoms attached to oxygen in the functional group decreases in the order of N>C>P.However,some literatures have proved that phosphoric acid collectors have certain flotation effect when applied to rare earth minerals。
phosphoric acid collectors currently used in rare earth flotation are:styrene Phosphoric acid,p-toluene phosphonic acid,benzyl phosphonic acid,α-hydroxybenzyl phosphonic acids,(1-hydroxy-1,3-dimethyl)butyl phosphonic acids,P538(monoalkyl phosphate),Flotinor 1682(commercial phosphate esters),Flotinor SM15(commercial phosphate esters),di(2-ethylhexyl)phosphate(DEHPA),dibutyl phosphate(DBP)and tributyl phosphate(TBP),1-hydroxydodecane[46][56][57][58][59]。 in the process of rare earth flotation,in order to improve the selectivity of phosphoric acid collectors,the strategy of combined use of activator and depressant is usually used.the main components of phosphoric acid collectors for some rare earth ores are shown in Fig.4,and the rare earth flotation performance of some phosphoric acid collectors is shown in Table 3。
图4 部分磷酸类捕收剂结构

Fig. 4 Structure of phosphonic acid collectors

表3 Study on Flotation Performance of Some Phosphoric Acid Collectors

Table 3 Study of rare earth flotation performance of some phosphonic acid collectors

Collector Raw material Flotation reagents Concentrate
Styrylphosphonic acid [60] Shandong Huishan (China) rare earth ore with 6.10% REO. It mainly contains bastnaesite, Parisite, hematite, limonite. Primary flotation: Sulfuric acid 0.5 kg/t, kerosene 2.5 kg/t, Styrophylphosphonic acid 1.5 kg/t, pine oil 0.2 kg/t. After "One rough and two fine" open circuit flotation, a rare earth concentrate with 60.13% REO and 48.36% recovery was obtained.
Styrene phosphonic acid, toluene phosphonic acid, alpha-hydroxybenzylphosphonic acid, benzylphosphonic acid, (1-hydroxy-1,3- dimethyl) butylphosphonic acid[46] the same as above Different collectors use different optimal flotation conditions. Rare earth concentrates were obtained: styrylphosphonic acid (REO 41.67%, recovery 50.50%); toluene phosphonic acid (REO 43.23%, recovery 37.29%); α-hydroxybenzylphosphonic acid (REO 55.55%, recovery 66.35%); benzylphosp- honic acid (REO 34.39%, recovery 44.05%); and (1-hydroxy -1,3- dimethyl) butylphosphonic acid (REO 33.77%, recovery 53.69%).
P538[61] Baotou mixed rare earth concentrates:
REO > 68%, bastnaesite 71.67%, Monazite 28.33%.		 Multi-stage flotation, P538 as collector added in batches, rough flotation reagent was: citric acid 200 mg/L, frothing agent MIBC 12 mg/L, pH=5.0. Bastnaesite concentrate with purity of 91% and recovery of 70.18% and monazite concentrate with purity of 61.2% and recovery of 73.18% were obtained.
It is generally believed that the adsorption of phosphoric acid collectors on mineral surfaces is driven by both electrostatic forces and chemical bonds.At pH below the mineral isoelectric point(IEP),the dissociated monovalent collector anion,（R-OP(OH)O−）,can first undergo electrostatic adsorption(Figure 5A)followed by chemisorption(Figure 5B )[62]。 In addition,phosphate collectors can also form bidentate complexes with metal cations(Figure 5C),and bidentate complexes can eventually be converted to dinuclear complexes(Figure 5D)[63,64][64]。 Espiritu et al.showed that the possible adsorption mechanism of F1682 on the surface of rare earth minerals is shown in Fig.5C and Fig.5d through the analysis of mineral Zeta potential and infrared spectrum of phosphonate collector F1682 before and after the flotation test of monazite and bastnaesite[65]。 Fan et al.studied the contact angle,ζpotential,Fourier transform infrared spectroscopy(FTIR)and X-ray photoelectron spectroscopy(XPS)of DEHPA collector on bastnaesite flotation,and speculated that DEHPA may react with Ce(III)atoms at the interface of bastnaesite to form Ce-O-P bonds,and its two 2-ethylhexyl groups attach bubbles outward,resulting in the enrichment of bastnasite flotation,as shown in Figure 6a[58]; By studying the FTIR and XPS changes of HDDPA collector on bastnaesite before and after flotation,it is proposed that Ce(Ⅲ)on the mineral surface interacts with O atoms in the functional group of HDDPA to form a four-membered ring,as shown in Fig.6B[59]。
图5 膦酸捕收剂 (a) 解离并接近矿物表面金属阳离子；(b) 与金属阳离子形成共价键；(c) 双齿配合物；(d) 双核配合物[62⇓⇓~65]

Fig. 5 Phosphonic acid collecto ( (a) sociation and proximity to mineral surface metal cations; (b) Formation of covalent bonds with metal cations; (c) bidentate complex; (d) binuclear complex[62⇓⇓~65]

图6 磷酸类捕收剂与氟碳铈矿表面吸附机理[58,59]

Fig. 6 Mechanism of adsorption of phosphonic acid collectors on the surface of Bastnaesite (a) DEHPA; (b) HDDPA[58,59]

2.4 Other collectors

N-hydroxyphthalimide(F802)has two C=O and one N—O—active groups,which can form a five-membered ring chelate of O=C—N—O—RE—O with rare earth on the lattice surface of rare earth minerals such as bastnaesite,as shown in formula(13)[66]。 In addition,since the solubility of bastnaesite in water is greater than that of monazite,the Ce3+or La3+in the solution and the HCO3-from the hydrolysis of bastnaesite generate CeHCO32+or LaHCO32+to be re-adsorbed on the surface of bastnasite to form active centers,which is more conducive to the adsorption of F802,so F802 can be used for the separation of bastnasite and monazite[43]。 Luojiake and Guyingying et al.Used F802 collector to separate the mixed concentrate of bastnaesite and monazite in Baotou,and the results showed that the effective flotation separation of bastnaesite and monazite could be realized in the pH range of 5~6(near the pKaof F802 )[43][67]。 Under this pH condition,Luo Jiake et al finally obtained a bastnaesite concentrate with a purity of 95.97%and a bastnaesite recovery of 45.14%[67]。
In recent years,some new multifunctional collectors have emerged,such as S-[(2-hydroxyamino)-2-oxoethyl]-N,N-dibutyldithiocarbamate(HABTC),but the separation effect of rare earth flotation is not good[68]。 In order to improve the selectivity of flotation reagents for rare earth minerals and reduce the cost of rare earth flotation,many researchers have used a variety of collector combinations to float rare earth minerals[69⇓-71]。 By comparing the flotation of mixed rare earth ore from Baiyunebo iron tailings with LF-8#collector and N,N,N',N'-tetraoctyl-3-oxyglutaramide(TODGA)combined collector,Zhang Xiaoyan et al showed that TODGA could be chemically adsorbed on the surface of bastnaesite.More hydroximate anions are adsorbed on the surface of bastnaesite,and the floatability of bastnaesite is enhanced.the addition of TODGA can increase the concentrate grade from 54.73%to 56.37%and the recovery from 33.27%to 36.38%[72]。 Xu et al.Compared the flotation of bastnaesite with the combined collector of octyl hydroximic acid(OHA)and sodium oleate(NaOL),and found that the recovery of bastnaesite with the combined collector was 33%higher than that with the single collector of OHA under the same conditions[73]。 Wu et al.Also found that the combined collector of OHA and octylphenol ethoxylate(OP10)could effectively improve the flotation recovery of fine bastnaesite compared with the single collector of OHA,and considered that the synergistic mechanism was that OHA and OP10 were co-adsorbed on the surface of bastnaesite,which enhanced the floatability of rare earth minerals[69]。 Li Shenyi et al.Used the combined collector of salicylic hydroximic acid,sodium oleate and oxidized paraffin soap to carry out the combined process of magnetic separation and flotation for the preconcentration of heavy rare earth ores in Guangshui-Dawu area of Hubei Province,and obtained a rare earth concentrate with a Y2O3grade of 2.32%and a recovery of 40.09%from the rare earth raw ore with a Y2O3grade of 0.101%[74]。
In recent years,Ionic liquids(ionic liquids)have shown good performance in rare earth separation,so some researchers have applied ionic liquids to rare earth flotation.Azizi et al.Used tetrabutylammonium and di(2-ethylhexyl)phosphate ionic liquid（[N4444][DEHP]）as monazite and bastnaesite collectors,and the study showed that the adsorption of[N4444][DEHP]on the surface of monazite and bastnaesite was chemical adsorption,and its collecting power was even stronger than that of hydroximic acid,and its collecting power for typical gangue minerals such as calcite,dolomite and quartz was weak[75⇓~77]。 Li et al.Also obtained that the adsorption of[N2222][EHEHP]on the surface of bastnaesite and hematite was chemical adsorption,but almost no adsorption on the surface of quartz by studying tetraethylammonium and 2-ethylhexyl phosphate mono-2-ethylhexyl ester ionic liquid（[N2222][EHEHP]）as flotation collector[78]。 In addition,Wu et al.Used 1-dodecyl-3-methylimidazolium ionic liquid as collector to recover Y2O3from fluorescent powder,and when the addition of collector and citric acid was 0.3 mg/L and 600 G/t,respectively,and pH=3,the grade of Y2O3could be increased from 30.51%to 56.58%,and the recovery of Y2O3could reach 88.96%[79]。 Therefore,the development of ionic liquid collectors is an important research direction of rare earth flotation collectors in the future。

3 Foaming agent

the addition of frother in flotation can greatly reduce the surface tension of water,promote the pulp to produce a large number of air-water interfaces and produce enough bubbles,so that the hydrophobic mineral particles float to the surface of the pulp with the bubbles.foaming agents can be divided into terpenoid foaming agents according to different functional groups,such as pine oil,terpineol(2#oil),pine needle oil,camphor oil,etc;Fatty alcohol foaming agents,such as methyl isobutyl carbinol(MIBC),2-ethylpentanol,hexanol,butanediol,etc.;ether foaming agents,such as triethoxybutane(butyl Ether oil,No.4 oil,TEB);Ester foaming agents,such as diethyl phthalate,etc。
At present,in the flotation process of rare earth minerals,the main frothers used are 2#oil(terpineol as the main component),SL-301(terpineol as the main component),MIBC,and other new frothers developed,such as FM-132[18][80]。 Terpineol,the main component of 2#oil,is a yellow-brown oily transparent liquid withα-terpenol,β-terpenol andγ-terpenol as the main components,and its structure is shown in Figure 7.Because 2#oil has strong foaming property and can generate foam with uniform size,dense structure and medium viscosity,it is difficult to be replaced,so there is little research on foaming agent in recent years.2#oil is the most important foaming agent for rare earth and other metal minerals flotation。
图7 常见稀土浮选起泡剂结构

Fig. 7 The structure of some rare earth foaming agents (a) main component s of 2# oil; (b) MIBC

the frother not only affects the number,size and lifetime of bubbles in the flotation process,but also affects the contact between mineral particles and the separation of hydrophobic and hydrophilic minerals in the froth layer.the research of Wangsiriguleng shows that Under the condition of keeping the dosage of inhibitor and collector unchanged,adding a certain amount of 2#oil can reduce the grade of tailings and improve the grade and recovery of concentrate.When the dosage of sodium silicate in roughing is 4.0 kg/t,the dosage of SR(hydroximic acid collector)is 1.3 kg/t,and the consumption of 2#oil is 30 G/t.under the condition of 0.38 kg/t sodium silicate and 0.12 kg/t SR,the rare earth concentrate with REO 53.80%and recovery of 90.71%can be obtained[81]。

4 Regulator

regulator is mainly used to change the surface properties of mineral particles,or change the chemical or electrochemical properties of water medium,and then affect the interaction between rare earth minerals or gangue minerals and collectors.It includes pH Regulator,inhibitor,activator,dispersant,flocculant,etc.,among which inhibitor and activator have a greater impact on the flotation index of rare earth minerals。

4.1 Inhibitor

There are many kinds of depressants,including inorganic compounds and organic compounds.Sodium silicate/water glass is the most widely used and studied depressant in the flotation process of rare earth ores[17,18,82]。
Sodium silicate/water glass is alkaline when hydrolyzed in water,as shown in formulas(14)~(16).Silica gel particles and HSiO3-produced by hydrolysis can be preferentially adsorbed on the surface of silicate minerals such as quartz and feldspar to form a relatively hydrophilic layer,so water glass has a good depression effect on silicate minerals[83]。 When the amount of water glass is large,it also has a certain depressing effect on salt minerals such as fluorite,calcite,dolomite and barite,iron-bearing minerals,placer minerals such as zircon and rutile,and rare earth minerals such as bastnaesite and monazite,but the degree of depression is different.the purpose of selective depression of gangue minerals can be achieved by adjusting the amount of water glass[3,84]。 In addition,water glass has a good dispersion effect on the pulp,which can weaken the influence of fine mud and improve the grade of rare earth concentrate。
$\mathrm{N}{{\mathrm{a}}_{\mathrm{2}}}\mathrm{Si}{{\mathrm{O}}_{\mathrm{3}}}\mathrm{+}\mathrm{2}{{\mathrm{H}}_{\mathrm{2}}}\mathrm{O}\underset{{}}{\mathop{\xrightarrow{{}}}}\,\mathrm{2N}{{\mathrm{a}}^{\mathrm{+}}}\mathrm{+}\mathrm{2O}{{\mathrm{H}}^{-}}\mathrm{+}{{\mathrm{H}}_{\mathrm{2}}}\mathrm{Si}{{\mathrm{O}}_{\mathrm{3}}}$
${{\mathrm{H}}_{\mathrm{2}}}\mathrm{Si}{{\mathrm{O}}_{\mathrm{3}}}\underset{{}}{\mathop{\xrightarrow{{}}}}\,{{\mathrm{H}}^{\mathrm{+}}}\mathrm{+}\mathrm{HSiO}_{\mathrm{3}}^{-}{{K}_{\mathrm{1}}}\mathrm{=1}{{\mathrm{0}}^{-\mathrm{9}\mathrm{.43}}}$
$\mathrm{HSiO}_{_{\mathrm{3}}}^{-}\underset{{}}{\mathop{\xrightarrow{{}}}}\,{{\mathrm{H}}^{\mathrm{+}}}\mathrm{+}\mathrm{SiO}_{\mathrm{3}}^{\mathrm{2}-}{{K}_{\mathrm{1}}}\mathrm{=1}{{\mathrm{0}}^{-\mathrm{12}\mathrm{.56}}}$
Wang Liming et al took the iron tailings of a concentrator in Baiyunebo(REO grade 9.35%,CaF2grade 6.49%,main gangue minerals are dolomite,calcite,pyroxene,amphibole,quartz,barite,etc.)As the research object,and carried out the comprehensive recovery of rare earth and fluorite with the process flow of"rare earth flotation-fluorite pre-selection-fluorite cleaning-high intensity magnetic separation".When the dosage of sodium silicate in roughing is 2 500 G/t,the dosage of collector SR(hydroximic acid)is 600 G/t,the dosage of 2#oil is 84 G/T,the pulp temperature is 50℃and the pulp concentration is 40 wt%,the grade of rare earth concentrate in flotation is REO 50.54%and the recovery is 92.32%[51]。 Through single mineral flotation test,artificial mixed ore flotation test,Zeta potential test and FT-IR analysis,Lin Yiming et al revealed that the depression mechanism of sodium silicate on bastnaesite and fluorite in H205 collector system was mainly the chemical adsorption of sodium silicate on the surface of fluorite to form a hydrophilic layer.It hinders the adsorption of H205 on the surface of fluorite,making the fluorite mineral hydrophilic and difficult to float(Fig.8)[85]。
图8 硅酸钠在氟碳铈矿和萤石表面作用机理示意图[85]

Fig. 8 Mechanism of sodium silicate action on the surface of Bastnaesite and fluorite[85]

The main depressing minerals,mechanisms and disadvantages of other rare earth flotation depressants,such as alum,lignosulfonate,sodium fluosilicate and sodium hexametaphosphate,are shown in Table 4。
表4 Some other reported rare earth flotation depressants

Table 4 Other reported flotation depressants of rare earth

depressant Inhibited Mineral Possible Mechanisms of Inhibition disadvantages
Alum [86,87] It has good inhibition effect on barite, fluorite, calcite and other barium calcium salt minerals. And it can also be used to inhibit monazite when separating bastnaesite and monazite. Alum hydrolyzed Al3+ preferentially combines with SO42- on the surface of barite to inhibit barite. Ca2+ on the surface of calcite and fluorite preferentially combines with AlO2- and SO42- hydrolyzed by alum to inhibit calcite and fluorite. Bastnaesite will be partially inhibited when alum used in excess.
Lignosulfonate [86,87] It can effectively inhibit barite and calcite, and there is little difference in the inhibition effect under high temperature conditions. It contains phenolic hydroxyl functional groups, can be selectively adsorbed on the surface of barite and calcite to make the mineral surface hydrophilic. Needs to control the dosage, overuse is not conducive to the flotation of rare earth ores, such as bastnaesite.
Sodium fluorosilicate [86] It can be used to inhibit barite, calcite, fluorite, quartz, feldspar, and other silicates. Hydrolyzed SiF62- continues to be hydrolyzed to SiO2 micelles adsorbed on the mineral surface making the mineral hydrophilic. Hydrolyzed SiF62-continues to be hydrolyzed to HF, which dissolves the dissolved feldspar surface to generate free H2SiO3 micelles, preventing the capture of the trapper. It can preferentially desorb fatty acid-based traps from the surface of vein minerals. It results in a lower pH of the slurry, requiring the addition of more pH adjusters such as sodium carbonate.
Sodium hexametaphosphate [54,57,88] It can be used to inhibit calcite and barite. Hydrolyzed Na4P6O182- forms hydrophilic and stable complexes with Ca2+ on the mineral surface in the slurry. Has a strong inhibition effect on Bastnaesite, so it needs to strictly control the dosage
Sodium sulfide [89] It can be act as inhibitors of zircon and activators of monazite when fatty acid as collector
 Hydrolyzed S2- and HS- ions are adsorbed on the surface of zircon, preventing the adsorption of collectors on zircon or leading to selective resolution of collectors such as oleic acid on the surface of zircon. Easily releases toxic hydrogen sulfide gas in the air.
Citrate [61] It can inhibit fluorite and aluminum silicate minerals such as calcite and mica. And it can also be used to inhibit bastnaesite when separating monazite and bastnaesite in flotation. Citric acid and mineral lattice surface cations have a strong ability to cooperate with the adsorption on the surface of the inhibited minerals, and forming hydrophilic chelates to hinder the adsorption of the collectors on the gangue minerals. Citric acid has a greater ability to dissolve cerium fluorocarbon, and the mineral surface loses more active centers and is subsequently inhibited. High acidity will reduce the pH of the slurry, and dosage also needs to be controlled.
Starch, modified starch [90⇓-92] It can be used to inhibit rutile, ilmenite, zircon, hematite, quartz and so on. Polar functional groups such as hydroxyl groups on the molecular chain are adsorbed on the mineral surface through hydrogen bonding, electrostatic and chelating effects, and are inhibited by the formation of a hydrophilic adsorption layer on the mineral surface. Natural starch has poor water solubility and poor selective adsorption capacity on mineral surfaces. Modified starch has small applicability, and mostly stays in the laboratory research stage.
EDTA[93⇓-95] It is mainly used to inhibit calcium-containing minerals such as fluorite and calcite. Low doses can remove Ca2+ ions from the surface of monazite and realize the activation of monazite, but it also has a certain inhibition effect on monazite in large doses. Complexation with dissolved Ca2+ on the mineral surface hinders the adsorption of collectors such as hydroxamic acid on the surface of minerals such as fluorite. Dosage needs to be controlled and often requires synergistic use with other inhibitors.
CMC[86,96] It can be used to inhibit silicate minerals and minerals containing calcium and magnesium. After hydrolysis, carboxymethyl anions are electrostatically attracted to the cations on the mineral surface, carboxyl groups form a water film with water through hydrogen bonding, and part of the CMC is adsorbed on the mineral surface as negatively charged micelles, which ultimately results in the hydrophilicity of the mineral surface and its inhibition. Requires synergistic use with other inhibitors.
Guar gum [97] Inhibits minerals such as talc and silicates Similar to starch and modified starch inhibition mechanism It is easy to cause the slurry to become viscous, reduce the flotation grade, so it needs to strictly control the dosage.
In addition,the combined use of different depressants can improve the flotation effect of rare earth.Luo Jiake et al.Used sodium silicate and sodium fluosilicate as composite depressants to selectively float rare earth minerals from gangue minerals such as fluorite,barite and calcite,and obtained a concentrate containing more than 95%rare earth minerals[98]。 Ren Jun used water glass and alum as composite depressant,which could not only effectively depress fluorite,but also remove iron minerals,and obtained a rare earth concentrate with REO 62%,rare earth recovery 69.17%,impurity Fe 2.42%and CaO 4.11%[96]。 bastnaesite and fluorite were separated by compound depressant of EDTA and water glass with a ratio of 1∶3,and the final bastnaesite concentrate REO was 67.4%,the recovery was 98.27%,and the separation coefficient of bastnaesite and fluorite was 23.04[99]。

4.2 Activator

Sodium fluosilicate,metal ions such as lead nitrate,lead chloride,cobalt nitrate,ferric chloride and ferrous chloride,and oxalic acid or oxalate are common activators for rare earth minerals[100][11][101][102][103]。 Among them,sodium fluosilicate can be used as a depressant of gangue minerals such as fluorite,and can also be used as an activator of rare earth minerals and is more commonly used[86,104,105]。 It is generally believed that HF,the hydrolysate of sodium fluosilicate,can dissolve sodium silicate on the surface of rare earth minerals while reducing pH,and remove the hydration film on the surface of rare earth minerals,so that rare earth minerals such as bastnaesite are activated and the selective adsorption of collectors such as hydroximic acid on rare earth minerals is improved.Based on the microflotation and surface chemical analysis of Thor Lake rare earth ore,Xia et al.Considered that the hydrolysate of lead nitrate,such as PbOH+,could be adsorbed on the surface of rare earth minerals,and the surface charge of bastnaesite and other rare earth minerals could be converted from negative to positive.The anionic collector is more effectively adsorb on that surface of the mineral;and in addition,the PbOH+can be used as a site activator,and aft being adsorbed on the surface of the mineral,the PbOH+forms a chelate with the collector such as hydroximic acid and the like,thereby playing an activation role[100]。 The activation mechanism of other cobalt salts,iron salts and other metal salts on rare earth minerals is similar。
It should be pointed out that lead salt as an activator causes certain pollution to the environment and water,the price of cobalt salt is high,and the activation of iron salt and oxalate is weak,so sodium fluosilicate is considered to be the most effective,widely studied and applied activator for rare earth minerals。

5 Ion-type rare earth ore chemical beneficiation reagent

rare earth elements in ion adsorption type rare earth ores are mainly adsorbed in kaolinite,feldspar,mica and other minerals in the form of ions,and conventional mineral processing methods can not extract rare earth elements.However,the rare earth cations in the minerals can be leached out in the form of ion exchange by electrolyte solution,and then the rare earth concentrate products can be obtained by chemical beneficiation methods of precipitation,separation and enrichment,washing,drying and ignition.the selection of leaching agent and precipitant is the key factor affecting the chemical beneficiation of ion adsorption type rare earth ore。

5.1 Leaching agent

The study shows that the electrolyte(acid,alkali,salt)solution with appropriate concentration can leach the rare earth cations from the ionic rare earth ore by chemical beneficiation of ion exchange.Table 5 lists the test results of leaching an ionic rare earth ore with some reagents.When the leaching agent is in contact with the ionic rare earth ore,the cation of the leaching agent is exchanged with the RE3+attached to the ionic rare earth ore,as shown in formula(17).The anion of the leaching agent has a complexation reaction with the RE3+in the solution,as shown in formula(18),which not only promotes the reaction direction of formula(18)to the right,but also makes the RE3+not easily adsorbed into the clay mineral 。
$\mathrm{\bar{R}}{{\mathrm{\bar{E}}}^{\mathrm{3+}}}\ \ \mathrm{+}{{\mathrm{M}}^{n\mathrm{+}}}\overset{{}}{\longleftrightarrow}{{\mathrm{\bar{M}}}^{n\mathrm{+}}}\ \ \mathrm{+R}{{\mathrm{E}}^{\mathrm{3+}}}$
$\mathrm{R}{{\mathrm{E}}^{\mathrm{3+}}}\ \ \mathrm{+m}{{\mathrm{A}}^{n\mathrm{-}}}\xrightarrow{{}}\mathrm{REA}_{m}^{\mathrm{3-}mn}\ \ $
In the formula,$\mathrm{\bar{R}}{{\mathrm{\bar{E}}}^{\mathrm{3+}}}$ is RE3+attached to the ionic rare earth ore,RE3+is RE3+dissociated into the solution,Mn+is a monovalent or divalent cation,and A is a monovalent or divalent anion 。
表5 Test Results of Leaching of an Ionic Rare Earth Ore with Partial Reagents[106]

Table 5 leaching results of some Leaching reagents on an ion-adsorption type rare earths ores[106]

Leaching agent Concentration pH Leaching rate of RE /% Leaching agent Concentration pH Leaching rate of RE /%
HCl 2% 0.5 52.92 KCl 1 mol/L 5.4 92.99
H2SO4 2% 0.5 76.09 Ammonium citrate 0.5 mol/L 4.5 95.18
NH4Cl 1 mol/L 5 94.72 Fe2(SO4)3 1% 2.5 70.00
CH3COONH4 1 mol/L 6 94.66 FeSO4 1% 2.5 67.00
NaCl 1 mol/L 5.4 97.53 (NH4)2SO4 2% 4.5 98.50
In the initial stage of industrial exploitation of ion-absorbed rare earth ores,5%~8%NaCl solution is mainly used as leaching agent,but the products of rare earth oxides have the defects of high sodium content,low rare earth grade,and serious harm to the ecological environment caused by soil salinization.Up to now,1%~4%(NH4)2SO4or NH4Cl solution is often used as leaching agent in industry,among which ammonium sulfate has better leaching effect,but its price is higher,and the long-term use of ammonia agents will cause the loss of calcium and magnesium in soil,the damage and death of plant roots,and the serious over-standard of ammonia nitrogen in mining water system[107,108]。
in view of this,In the use of leaching agents,some researchers have developed new ammonia-free leaching agents,such as magnesium sulfate,ferrous sulfate,potassium citrate,magnesium citrate,ammonium citrate and other citrates to solve the problem of ammonia nitrogen pollution[109⇓⇓~112][113][114]。 There are also researchers who pretreat the leaching agent solution by external magnetic field or add sesbania gum,hydroxypropyl methyl cellulose,organic acids and other leaching AIDS to improve the leaching efficiency of rare earth[115][116][117][118]。 In addition,many scholars have used ammonium chloride and ammonium sulfate double salts,magnesium chloride,ammonium chloride and calcium chloride double salts,magnesium sulfate/magnesium chloride and calcium chloride double salts,and magnesium sulfate,ammonium chloride,calcium chloride and other double salts to improve the leaching rate of rare earth and reduce ammonia nitrogen emissions[119][120][121][122]。 Huang Xiaowei et al proposed to use magnesium sulfate or magnesium chloride-based calcium-magnesium double salt instead of ammonium sulfate leaching agent,the cost of which is less than 3/4 of the cost of ammonium sulfate,and the leaching rate of rare earth can reach more than 90%.can solve that problem of ammonia nitrogen pollution,can also make up for the problem of lack of calcium,magnesium and other element in soil elements in the south of China,has bet economic and environmental benefits,and is a composite leaching agent with better application prospect[121]。

5.2 Precipitant

the concentration of rare earth ions in The leaching solution of ionic rare earth ore is usually only 0.5~2 G/L,and it contains a large number of impurity ions such as Al,Fe,Si,etc.,which can not be directly used for subsequent extraction and separation processes,so it needs purification and enrichment treatment such as precipitation。
Oxalic acid(H C O)was the first precipitant used for rare earth precipitation.The solubility product of rare earth oxalate is 10-25~10-29,while the solubility product of common impurity elements such as Fe and Al is 10-6~10-8,so oxalic acid has a high selectivity for rare earth.However,a large amount of impurity ions such as Fe in the mother liquor will consume a large amount of oxalic acid,and the oxalic acid has high cost,strong acidity,low effective utilization rate,and the recovery and treatment of the regenerated liquid is relatively cumbersome,thus limiting its further industrial application 。
Although ammonium bicarbonate has low selectivity to rare earth,it often needs pretreatment to remove most non-rare earth impurities before precipitation,and has the problems of large consumption and ammonia nitrogen water pollution.However,the cost of ammonium bicarbonate is low,and the supernatant after rare earth precipitation can be directly returned to the leaching operation after adjusting the concentration and acidity of the leaching agent,so ammonium bicarbonate is mostly used to precipitate the leaching mother liquor in today's industry。
In order to further break through the disadvantages of ammonium bicarbonate precipitators,researchers have studied the oxides,hydroxides or carbonates of alkali metals and alkaline earth metals,such as NaOH,Na2CO3and Na(HCO3)2,MgO,Mg(OH)2and Mg(HCO3)2,CaO,Ca(OH)2and Ca(HCO )[123⇓~125][126,127][128,129][121][130⇓⇓~133]。 When sodium oxide,hydroxide or carbonate compound is used as precipitant,there are problems of high cost,sodium ion pollution to products and environment.The solubility of MgO/Mg(OH)2is very low,the precipitation time is long and the dosage is large,which affects the purity of rare earth precipitation enrichment.Although Mg(HCO3)2can solve the problem of purity,it is easy to crystallize and difficult to operate in industry.The CaO/Ca(OH)2is a slightly soluble substance with strong alkalinity,fast precipitation and low cost,and because the solubility of calcium sulfate is only about 2.0 G/L at 10℃~60℃,the Ca2+is not easy to precipitate with the SO42-in the leaching solution system such as ammonium sulfate or magnesium sulfate,and has good precipitation effect.Lai et al.Used CaO as a precipitant to enrich rare earth elements in the magnesium salt composite leaching agent system.The results showed that when the rare earth concentration was 0.80 G/L,the Mg2+concentration was 1.0 G/L,pH=9.18,the temperature was 25℃,and the CaO concentration was 0.45 mol/L,the precipitation rate of rare earth was 99.72%,and the purity of rare earth concentrate was 82.21%[130]。 However,in the precipitation process with CaO as precipitant,sulfate will partly enter the rare earth hydroxide lattice,forming part of the basic rare earth sulfate precipitate,and the content of SO3in the precipitate will increase.Therefore,if CaO is used as a rare earth precipitant,it is necessary to use detergents such as sodium hydroxide,sodium succinate and sodium succinate to remove sulfate ions according to the competitive coordination between anions and sulfate ions in order to improve the purity of rare earth precipitation products[130,131]。

6 Conclusion and prospect

rare earth is a dominant strategic mineral resource in China.Although scholars at home and abroad have done a lot of research on rare earth beneficiation reagents and achieved rich research and application results,there are also many drawbacks.Therefore,the development of beneficiation reagents with higher selectivity,higher recovery and greener is an inevitable requirement for the efficient and sustainable development of rare earth resources and cleaner production。
in view of the flotation characteristics of mineral rare earth ores,the future research on flotation reagents should focus on improving the selectivity and recovery of rare earth.Reduce the operation cost and synthesis cost:(1)the current mainstream hydroximic acid rare earth collector has good rare earth selectivity,but it often needs heating flotation,which has high energy consumption cost,and the synthesis cost of hydroximic acid is relatively high.Therefore,the development of new and efficient rare earth collectors and the optimization of collector synthesis routes are still important research directions in the future;(2)Exploring the mixed use of collectors and carrying out the interaction and mechanism research between collectors and frothers or regulators are important research directions to improve the flotation efficiency of rare earth in a short time。
in view of the chemical beneficiation characteristics of ionic rare earth ore,the future research on reagents will focus on improving the recovery rate of rare earth and environmental protection:(1)electrolyte solution with appropriate concentration can exchange rare earth ions from ionic rare earth ore.At present,the mainstream ammonium salt leaching agents have environmental problems such as ammonia nitrogen water pollution and plant root death.the development and screening of green,pollution-free and efficient ammonium-free leaching agents are the key research directions in the future,and the double salt leaching agents such as magnesium sulfate and calcium sulfate have great application prospects;(2)the current mainstream ammonium bicarbonate precipitant has low selectivity for rare earth,complicated operation and ammonia nitrogen water pollution,so the development of green and efficient rare earth enrichment precipitant and the optimization of efficient precipitation process are still important research directions in the future。

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