Cerium extraction from rare earth concentrates by electrochemical and extraction methods for preparation of polishing materials Текст научной статьи по специальности «Химические технологии»

UDC 669.855:[66.087.73+66.061.3]

CERIUM EXTRACTION FROM RARE EARTH CONCENTRATES BY ELECTROCHEMICAL AND EXTRACTION METHODS FOR PREPARATION OF

POLISHING MATERIALS

©2016 O.V.Yurasova ^ T.A. Kharlamova 2), S.A.Vasilenko ^ T.V.Fedulova ^ A.A.Gasanov 1), T.V.Dobrynina 1), O.Yu.Saykina 1), A.F.Alaferdov 2), V.V.Apanasenko1)*

JSC "State Scientific-Research and Design Institute of Rare metals "Giredmet", Grand Tolmachevsky

lane 5, 119017 Moscow, Russia 2LLC "Delfin Aqua", 2nd Yuzhnoportovy proyezd 35, 115088 Moscow, Russia

Tel: +7 (495) 708 4466 1134; E-mail: O.V. [email protected] *

*Work performed under Agreement 14.579.21.0138 at financial support of the Ministry of Education and Science, unique ID of the project RFMEFI57916X0138

Abstract: The Russian rare earth raw metals (REM) are complexmaterials to contain up to 57% of ceriumin the total amount of lanthanides. Authors recommended to separate cerium at the first stages of rare earth concentrate treatment by combined methods of electrochemical oxidation and liquid extraction. The process has been suggested to optimize by using up-to-date equipment, including a diaphragm-type electrolyzer of OXITRON-58L-O2 type and centrifugal type extractor EC-10FA. To organize continuous automated process at chosen equipment, various operation modes of electrochemical oxidation Ce (III) to Ce (IV) and extraction of tetravalent cerium out of mixture of trivalent rare earth materials have been tested. Investigations have been performed over process solutions prepared out of Solikamsk Magnesium Plant concentrates extracted out of loparite. Conditions have been created to achieve oxidation level Ce+3 > 99 % in terms of electric power consumption less than 0.8 kW/h per 1 kg of Ce02 with subsequent cerium extraction out of rare earth concentrate at a cascade of centrifugal extractors to obtain 99.6% of cerium dioxide (from YftEM). Polishing powders have been synthesized on the basis of nanosized cerium oxide to comply with appropriate composition of cerite andpolirite.

Keywords: rare-earth materials; cerium (III) and (IV); oxidation; electrolysis; electrolyzer; extraction; centrifugal extractor; polishing powders

1. Introduction

Materials based on rare earth elements (REE) are widely used in modern areas of science and engineering. Today, it is difficult to find innovation technology that does not use REE as components of application, e.g. development of superconducting materials, special alloys, super magnets, accumulators, catalysts and etc. After the USSR dissolution, Russia lost leadership in the production of rare earth metals (REM), and majority of plants and feedstock sources of REM appeared to beoutside Russia. Today, the world REM

marketis dominated by China which possesses sufficiently rich and easy transformable feedstock of REM. This enables China to fix metal prices and terms of sale.

Nowadays in Russia, under support of the Russian Government the industrial production of REM is revived. Note that mineral feedstock and technogenic waste has been reputed to be a source of REM. Approximate compositions of rare earth concentrates (REC)) and ores for industrial processing are presented in Table 1.

Table 1. Approximate compositions of REC and ores which are promising for development of _industrial processing in Russia_

REE composition in ore,% Loparite Apatite Phosphogypsum Eudialyte Tomtor ore

32.4 0.98 0.52 2.0 13.67

mass.

Ln2O3 X Ln2O3 - 100% in concentrate

La2O3 21.11 27.2 20.86 9.8 23.31

CeO2 57.72 43.55 46.75 26.0 42.68

Pr6On 5.36 5.8 5.15 4.0 4.14

Nd2O3 14.4 14.3 17.31 12.0 16.72

Sm2O3 0.89 1.9 2.38 4.2 2.46

Eu2O3 0.18 0.5 0.63 0.6 0.79

Gd2O3 0.15 1.8 1.80 4.2 1.67

Tb4O7 0.02 0.2 0.07 0.6 -

Dy2O3 0,11 0.7 0.96 3.3 0.83

Ho2O3 0.02 - 0,14 0.6 0.15

Table 1. Approximate compositions of REC and ores perspective for industrial processing in

Russia.

REE composition Loparite Apatite Phosphogypsum Eudialyte Tomtor ore

in ore,%

mass.

Ln2O3 X Ln2O3 - 100% in concentrate

Er2O3 - 0.15 0.27 2.5 0.57

Tm2O3 - - - 0.4 0.06

Yb2O3 - - 0.10 1.8 0.24

Lu2O3 - - - 0.3 -

Y2O3 0.023 3.9 3.56 30.0 6.37

Each source of REM is unique by its composition (REM and impurities content) and structure. Specific feature of domestic feedstock of REM is the predominant content of cerium in it - up to 57% of total amount of rare earth metals. Therefore, in the traditional technological scheme of rare-earth metals production the extraction of cerium from other lanthanides makes up the first stage. The process is based on cerium oxidation in the tetravalent state by chemical (oxidation by O2, ozone, potassium permanganate or hydrogen peroxide) or electrochemical method with subsequent cerium separation from rare earth metals by precipitation, ion exchange or extraction method.

In their previous works, authors [1-3] proved the efficiency of combination of electrochemical oxidation and liquid

extraction for cerium extraction. Benefits of this technology include high productivity, continuous operation, easy automation and achievement of high rates of rare earth metal extraction in the end-product. This method of production was developed by scientific-research institute "Giredmet" in the 1980's of the 20th century and put into practice at industrial enterprises manufacturing rare earth metals. However, due to the collapse of enterprises after the dissolution of the Soviet Union this method has no longer been used in Russia. Nowadays, technological schemes of rather complicated, laborious and high reagent inputs are used in existing domestic processes for cerium extraction from lanthanide mixture [1,2,4,5]. These include: - cerium oxidation using expensive

chemical reagents;

- separation of tetravalent cerium from lanthanides due to pH difference in Ce4+ and REM3+ hydrating;

- cerium purification from REM by multiple reprecipitation or its hydroxide or by extraction.

Extraction purification is carried out in mixing-settling extractors which are traditional in the technologies of REM processing. Such type of apparatus is easy controllable, however, cascade-linked apparatuses require high consumption of technological solutions for charging, including toxic and inflammable organic

2. Exp

solvents that affect process cost and environmental conditions [1,2,6]. Since industry of REM is currently revived in Russia, it is important to renew electrochemical technology of cerium extraction and improve it with advantages of modern equipment. The work analyzes factors capable of optimizing the process of cerium extraction and obtaining cerium oxide with purity of no less than 99.5% through the use of diaphragm-type electrolyzer and centrifugal extractor. Technological parameters and optimal conditions of process have been studied.

Studies have been pursued on a model and afterwards in technological nitrate solutions prepared out of rare earth concentrate of Solikamsk Magnesium Plant. Rare earth concentrate was prepared out of

loparite concentrate containing 293.6 to 345.5 g/l of rare earth oxide (REO). The composition of the prepared rare earth concentrate is shown in Table 2.

Table 2. Composition of prepared rare earth concentrate

Compound Composition, mass.%

La2O3 26.1

CeO2 54.2

Pr6On 5.0

Nd2O3 13.0

Sm2O3 0.97

CaO 0.06

SrO 0.04

Fe2O3 0.001

SiO2 0.02

Cl 0.05

Model solutions were prepared by dissolution of CeO2 in the nitric acidto contain 50-100 g/l of HNO3 and 140-160 g/l of Ce203. In turn, rare earth element solutions were prepared by dissolution of carbonates in the nitric acid. Note that REE ions in solutions were determined by titration method with complexing agent EDTA in the presence of indicator. Cerium ions were detected by

titration using mohr's salt and potassium permanganate solution.

Composition study of aqueous and solid phases has been carried out by inductively coupled plasma atomic emission spectrometry with the help of ICAP 6300 JY-38 spectrometer (Thermo Fisher Scientific) and roentgen-fluorescent method by means of ARL OPTIM'Xspectrometer (Thermo Fisher

h) consumed in oxidation process, defined experimentally and calculated by Faraday's laws of electrolysis, correspondingly. Values of electrical charges were determined according to equations below:

Qt = CCe4+ * "

(2)

Qexp = I * t

Scientific) according to standardized where C, procedures as set forth in "Giredmet". Required concentration of acid in aqueous phase was maintained by adding HNO3. Acid concentration was measured by volume or potentiometric titration. Coulomb efficiency BT was calculated according to the following equation: BT = (ß^Z) * 100% (1)

where Qexp and Qt are electrical charges (A* a

Ce4

is current concentration of Ce

+4

(g/l); V is a volume of treated solution (l); q = 5.224 (g -eqv/ A*h) is electrochemical equivalent; t - time (h).

(3)

where I is current supplied to electrochemical reactor (A), t - time (h). Degree of cerium oxidation was calculated according to the following equation:

= (^r41) * 100% (4)

where CCeQ is total concentration of cerium in the processedsolution (g/l). Studies were conducted in two-ways: - electrochemical oxidation of cerium; -extraction and purification of oxidized cerium by liquid extraction method.

3. Results and discussion

3.1. Electrochemical cerium oxidation

Membrane and diaphragm-type electrolyzers have been considered [7-10] to conduct experiments on electrochemical oxidation of cerium. It should be noted that the diaphragm material is resistant to high temperatures, easily regenerated, adjustable and has no limitations on current density, a diaphragm-type electrolyzer OXITRON-58L-O2 was chosen[11]. The diaphragm-type electrolyzer is manufactured in LLC "Delfin Aqua" and applied for electrochemical synthesis of strong oxidizers - hydrogen peroxide, hypochlorous acid,

peroxydisulphiric acid, etc. Electrochemical setup is shown in Fig. 1. A main part of laboratory setup is an electrochemical reactor MB-26-21-15K shown in Fig. 2. Note that a reactor vessel consists of 4 platinizing titanic anodes and 1 titanic cathode of cylindrical shape while electrode spaces are divided by ceramic diaphragm made of aluminum oxide and zirconium. Two types of circulation are provided in the reactor vessel: external/ internal circulation for anolyte and external one for catholyte. External circulation rate is 30 l/h and the internal one is 100 l/h. Hydrodynamics in reactor provides the mixing of electrolyte in

the whole volume of anode space of reactor and gives equiprobable access of electro-oxidized reagent to the total anode surface. Therefore, in the whole volume of anode space of reactor the same constant concentration of oxidizable cerium is settled. A pressure regulator mounted on the reactor vessel makes it possible to regulate pressures in electrode chambers. Technical features of setup are shown in Table 3. Technological process of electrochemical cerium oxidation initially has been analyzed in model solutions which contained 136 to 150 g/l of Ce+3 and 50 to 120 g/l of HNO3. As a catholyte, aqueous solutions of nitric acid and cerium nitrate are considered. After determination of optimal modes of electrolyzer operation on model solutions the further investigations were conducted in technological solutions of REM concentrates. For this purpose, cathode and anode chambers of reactor were pumped with technological solutions. Then, electrolyte external circulation and anolyte internal circulation preheated up to 50°C was switched on in such a way that circulation rate of anolyte proved to be higher 3 times as compared tothe circulation rate of catholyte.

Fig. 1. Laboratory electrochemical setup

Fig. 2. Electrochemical reactor MB-26-21-15K: 1 - cathode, 2 - anode, 3 - reactor vessel, 4 - plug, 5 -diaphragm, 6 - anode chamber, 7 - cathode chamber, 8 - perfs.

Table 3. Technical features of OXITRON-58L-O2 setup

Parameter

Units of measurement

Value

Feed rate of solution

- in cathode chamber

- in anode chamber Electrode potential in reactor Strength of current in reactor Pressure

- in cathode chamber

- in anode chamber Mass of setup

Dimensions (height*width*depth) Maximal time of operating mode stabilization

l/h

V A MPa

Kg Mm Min

10-40 10-40 3-30 3-30

0-1.0 0-1.0 50

890x480x500 15

The use of pressure regulator made it possible to create a positive pressure of 0.3 atm in the cathode chamber. After that, current was supplied and electrolysis conducted at the constant current density. While in catholyte, a certain quantity of nitric acid was added at regular intervals (~15 min) to maintain its concentration at 50-70 g/l. After completion of operating cycle, the current supply was suspended, pumps switched off and drain

valves open to sample catholyte and anolyte for further analytical control.

Studies have been carried out intechnological solutions to show the possibility of achieving high degree of oxidation Ce+3 (> 99 %) at power consumption less than 0.8 kW-h per 1 kg of Ce02. Kinetics of oxidation process in salt concentrates with £REM and Ce203 ~ 53 % content is given in Table 4.

Table 4. CeO2 concentration vs. electrolysis time

Concentration, g/l

Time, hours -

CeO2_HNO3

35.5 66.88

1

2 63.6 69.98

3 95.1 63.6

4 123.1 56.4

5 142.3 59.9

6 151.1 62.1

7 155.1 66.8

8 155.9 62.4

Note: average indicator BT=76.4%, current density is 2.3 A/dm2, electrolyze time - 8 hours.

Mass balance of Ce+3 oxidation process at consists of 99.68%. Further extraction and

certain operating mode of electrolyzer is shown purification of oxidized cerium from REM

in Table 5. solutions were conducted by means of

Table 5 shows that the level of Ce+3oxidation extraction techniques.

_Table 5. Mass balance of electrolysis process_

Anolyte_^REE, g/l Ce+3 concentration, g/l Ce+4 concentration, g/l Cerium mass, g

Before 300 159.0 0 477

electrolysis

After electrolysis 300_0.5_158.5_477

3.2. Cerium extraction from total amount of REM

Usually, extraction of oxidized cerium from £REM is carried out by means of extraction method in box-type mixing-settling extractor [1,2] which is characterized by long-term conduction process and use of expensive and inflammable reagents. There are no such drawbacks in case of using centrifugal extractors that makesit possible to increase productivity repeatedly and decrease essentially the consumption of very expensive extractants [6]. The use of centrifugal extractors in REM technology has limitations in the equipment which operateswithin the framework of multistage processes. However, modern hardware allows automation control and eliminating these limitations. In this work the application of

centrifugal extractors for obtaining cerium oxide with purity > 99.5 % from solution of REM was considered. As extractant, a well-known reagent in REM processing - tri-n-butylphosphine (TBP) was used. Extraction cascade was formed by series of centrifugal extractors (model EC-10FA, Scygrad Group) [12]. The main technical features of laboratory extractor are shown in Table 6. For tubing connections in cascade REM distribution coefficients D and cerium separation coefficients (( Ce4+ ) in the technological solutions

REE3 +

of REM were determined. Composition of solutions is shown in Table 7.

Table 6. Technical features of laboratory extractor EC-10FA

Parameter

Units of measurement

Value

Capacity

Ratio between densities of initial solutions

Ratio between flow rates of initial solutions

Working volume of mixing chamber Working volume of separation chamber Radius of separation chamber Radius of overflow chamber for heavy phase

Radius of overflow chamber for light phase

Gear- electric motor AAT75-40-3.0-Y3 Installed power Three-phase motor voltage Power network frequency Rotary rotations per minute Dimensions (lengthxwidthxheight) Apparatus mass Apparatus material

l/h

ml ml mm mm

mm

rpm W V Hz rpm mm kg

up to 10 up to 0.95

any

22 32 17 9

adjustable

2750 40 380 50 2620 180x120x240 3.5

polytetrafluorethylene (GOST 10007-80)

*Solution system: 30%TBP in kerosene and 2M HNO3 at the ratio between flow rates of aqueous and organic phases A:O=1:1

Table 7. Composition of technological solutions of REM

Compound Composition, mas.%

CeO2 54.5

La2Ö3 26.1

Pr6Ü11 4.1

Nd2Ö3 14.9

Sm2Ü3 0.86

Gd2Ü3 0.33

EU2Ü3 0.2

Tb4Ü7 0.08

Dy2Ü3 0.04

CaÜ 0.06

Fe2Ü3 0.002

HNÜ3 70-80 g/l

Table 8. Distribution coefficients and REM separation coefficients

REE Dree (ß Ce4+ )

_BttLL_

Ce_10.56_

La_0.029_364.1

Pr_0.087_121.4

Nd_0.099_106.7

Sm_0.148_71.3

Gd_0.201_52.5

Eu 0.415 25.4

To decrease viscosity of TBP, the latter was diluted by kerosene. Extractant was processed by nitric acid solution which came into equilibrium with initial solution at the ratio of aqueous and organic phases A:O=1:1. Laboratory experiments on extraction of REE were conducted in separating funnels. Phase contact time consisted of 5 minutes, which was sufficient for equilibration. Values of D and (fi Ce4+ ) are shown in Table 8.

REE3 +

Saturation of cerium extractant was detected by subsequent contacting of organic phase with aqueous solution at the ratio A:O=1:1. Cerium content in extractant after the second contacting made up 157 g/l. Extractant composition corresponded to 100 % Ce4+. However, there was an inversion (turnover) of aqueous and organic phases due to the similarity of their density values - 1.222 and 1.225 g/cm3 respectively which is not desirable for extraction process. To prevent inversion, an optimal content of REM no

more 300 g/l in the technological solution was selected.

Given the data obtained, the countercurrent mode of extraction for oxidized cerium separation was estimated [13,14], as well as the number of transfer units and ratio of phases in extraction and rinsing parts of cascade calculated. The following parameters were used for calculation: Pce/sm =71; a degree of cerium extraction in the product -99% with content of impurities (La, Pr, Nd, Sm) less than 0.5%.

Pilot study was performed in the laboratory setup and represented a cascade of 18 centrifugal extractors which isidentical to the scheme as described in the work [1,15,16]. Centrifugal extractors were connected with each other by means of tubing that allowed to operate with process solutions at any step of cascade. Trials were carried out to reproduce the process as described in the work [1]; however, the scheme of reactors showed unstable work of cascade and formation of

emulsion in its rinsing part, and as a Therefore, some modifications were done to consequence, low quality of cerium in re- improve the cascade performance - the extractant. number of units was increased up to 20 pieces.

Photo of cascade is shown in Fig.3.

Fig.3. Cascade with centrifugal extractors

After the equilibrium set in a cascade, the test sample of cerium oxide was obtained by oxalate precipitation from cerium re-extractant with subsequent filtration, drying and calcination at a temperature 800-850 0C of the formed sediment in a muffle furnace. Analyses showed that CeO2 content in a

sample obtained made up was 99.6%. Note that a pilot batch of nanosized powders with composition in line with cerite and polirite has been producedout of extracted cerium. The composition of obtained samples has been determined in an accredited laboratory.

4. Conclusion

Analysis has been carried out to obtain cerium oxide with purity 99.5 % and higher by using combination of two methods -electrochemical and extraction in electrochemical reactor MB-26-21-15K and a cascade of centrifugal extractors EC-10FA. The proposed technology is rather promising from industrial application standpoint. Despite the complicated multicomponent composition of initial raw materials, the proposed technological scheme and related equipment provide benefits as follows:

- ensuring a degree of cerium extraction at >99% and power consumption at 0.8 kW-h/kg Ce02;

- reducing multiple consumption of reagents, including those damaging the

environment, and inflammable extractants;

- achieving the purity of Ce02 higher than 99.5% from £REM);

- obtaining polishing cerium-containing nanosized powders.

The main result is that the proposed technology of cerium extraction based on electrochemical and extraction methods can be applied in REM processing out of various cerium-containing raw materials. It is easy to automatically control the proposed technological process and thus allow extracting cerium up to 95% out of REM concentrate that sufficiently reduce processing time and consumption of expensive reagents.

Acknowledgements

Appreciation is made to the Russian Ministry of Education and Science (Agreement №14.579.21.0138, Project ID RFMEFI57916X0138) for financial assistance in the work.

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ИЗВЛЕЧЕНИЕ ЦЕРИЯ ИЗ КОНЦЕНТРАТОВ РЗМ МЕТОДАМИ ЭЛЕКТРОХИМИИ И ЭКСТРАКЦИИ ДЛЯ ИЗГОТОВЛЕНИЯ ПОЛИРУЮЩИХ

МАТЕРИАЛОВ НА ЕГО ОСНОВЕ

©2016 О.В.Юрасова, Т.А.Харламова*, С.А.Василенко, Т.В.Федулова, А.А.Гасанов Т.В.Добрынина, O.Ю.Сайкина, А.Ф.Алафердов*, В.В.Апанасенко

Акционерное общество "Государственный научно-исследовательский и проектный институт редких металлов "Гиредмет" (АО "Гиредмет"), Moсква, Россия; *Общество с ограниченной ответственностью "Делфин Аква" (ООО "Делфин Аква"),

Moсква, Россия

Работа выполнена при финансовой поддержке Министерства образования и науки РФ в рамках соглашения 14.579.21.0138, уникальный идентификатор проекта

RFMEFI57916X0138

Российское редкоземельное (РЗМ) сырье является комплексным и содержит до 57 % церия на фоне суммы лантаноидов. Авторами рекомендовано отделять церий на первых стадиях переработки РЗМ последовательным сочетанием методов электрохимического окисления и жидкостной экстракции. Процесс предложено оптимизировать применением преимуществ современного российского оборудования: электролизера диафрагменного типа ОКСИТРОН-58Л-О и экстракторов центробежного типа модели ЭЦ-10ФА. Для организации непрерывного автоматизированного процесса на выбранном оборудовании разработаны режимы электрохимического окисления Се (III) в Ce (IV) и извлечения четырехвалентного церия из смеси трехвалентных РЗМ экстракцией. Исследования выполнены на технологических растворах, приготовленных из концентрата Соликамского магниевого завода, выделенного из лопарита. Разработаны условия для достижения степени окисления Се+3> 99 % при расходе электроэнергии не выше 0.8 кВт ч на 1 кг СеО2 с последующим выделением церия из концентрата РЗМ на каскаде центробежных экстракторов с получением 99.6 % диоксида церия (от ^РЗМ). Синтезированы полирующие порошки на основе наноразмерного оксида церия состава соответствующего маркам церит и полирит.

Елючевые слова: редкоземельные металлы, церий (III) и (IV), oкисление, электролиз, электролизер, экстракция, центробежные экстракторы, полировальные порошки.

Поступила в редакцию 16.02.2017.