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PRODUCTION OF RARE-EARTH ELEMENT OXIDES BY THERMAL DECOMPOSITION OF
DISPERSED SALTS AND THEIR AQUEOUS SOLUTIONS IN A PLASMA REACTOR
A. Mosse, L. Krasovskaya, I. Dvindenko, A. Gorbunov
To cite this version:
A. Mosse, L. Krasovskaya, I. Dvindenko, A. Gorbunov. PRODUCTION OF RARE-EARTH ELE- MENT OXIDES BY THERMAL DECOMPOSITION OF DISPERSED SALTS AND THEIR AQUE- OUS SOLUTIONS IN A PLASMA REACTOR. Journal de Physique Colloques, 1990, 51 (C5), pp.C5- 83-C5-90. �10.1051/jphyscol:1990511�. �jpa-00230809�
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Colloque C5, supplement au n018, Tome 51, 15 septembre 1990
PRODUCTION OF RARE-EARTH ELEMENT OXIDES BY THERMAL DECOMPOSITION OF DISPERSED SALTS AND THEIR AQUEOUS SOLUTIONS IN A PLASMA REACTOR
A.L. MOSSE, L.I. KRASOVSKAYA, I.A. DVINDENKO and A.V. GORBUNOV
Luikov Heat and Mass Tranfer Institute, BSSR Academy of Sciences, Minsk, U.S.S.R.
Abstract - Production of oxides of rare-earth elements from dispersed aqueous salt solutions in an electric arc plasma reactor has been studied numerically and experimentally. Experimental studies have also been made for oxalates and carbonates.
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INTRODUCTIONPlasmachemical treatment of salts and their aqueous solution is one of the new methods to produce varios-pur~ose metal oxides-Such an approach is especially promising when used in theindustry of rare-eath element (REE).
The latter is characterized by the processes including both separation of raw material by extraction and ion exchange, treatment of the resultant solutions of REE nitrates, sulfates and chlorides, that is their transformation into carbonates or oxalates, dry roasting anb thennal decomposltion in muffle furnaces. This paper is a hed at studylng direct reduction of oxides from REE carbonates, oxalates and nitrate solutions
R
arc plasma reactors.2 - PLASMACHEMICAL PRODUCTION OF REE OXIDES FROM NITRATE SOLUTIONS 2.1 - Numerical Calculations of Interaction of Dispersed Nitrate
REE solutions and an Air Plasma Flow
The basic equations of the mathematical model of the process are the equations of particle number conservation (I ) , of gas (2 ) and of condesed phase (3) continuity, of particle movement (4), of energy balance for dispersed material (5) and for plasma-forming gas (6) :
Article published online by EDP Sciences and available at http://dx.doi.org/10.1051/jphyscol:1990511
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dmP 1
(W p h ) = npa s~,(T -T )+n W -
P P P g P P P
a
[hpr(Tp)-hp(Tp)l ;. (5where CD=the coefficient of the aerodynamic drag of the droplet (particle), d=the particle diameter, g=the acceleration due to gravity, h=the speciflc enthalpy, m=the mass, n =the number of particles per unit volume, Q=the heat flux, S=the surface area, T=the temperature,w=the P
velocity,x=the distance along the axls of a reactor channel,pp=mpfb is the density of the condensed phase. The subscripts are: g, gas, pl, particle; p, totality of the particles, which form condensed phase;
pr, products of evaporation and reactions, W, reactor wall.
Equations (l )
-
(6) are completed with expressions for determini the coefficlents of Intercom onent heat transfer, aerodynarnlc drag3
the droplet, heat flux to ?he reactor wall and the denslty of the gas flow from the droplet surface,with approximate expressions of temperature de endencies of thermophyslcal characteristics of plasma, raw material an$ roducts as well as by e uations, which describe the kinetics of chemgal reactions and some o&er relations required to close the system of equations. Mathematically, the study of the heating and phase changes of particles by the described model is reduced to computer solution of the Cauchy roblem for the system of ordinary differential equations.
The cornputateon algorithm Is given in /l /
The attempt to use such a mathematic& model for studying thermal decomposition of REE nitrates was not a success. Flrst,it was due to the lack of re uireed kinetic data. Second, as in the lanthanum nitrate case, the ava
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lable data /2/ gave contradictory results: for example, with the heat fluxes to a article sufficient for its heating up to the tern eratures exeedlng t%at of thermal stability of lanthanum nitrate, the cafculation degree of dissociation remains to be close to zero Probably, the use of the kinetic relations obtained at heating rate oi 5 W m i n /2/ for plasma tem eratures requires their correction. So, the assumption was made &at the reaction of lanthanum nltrate dls~ociation rooeeds only npon the nitrate aohi~ves 1053 K /3/ and the dissociot&n degree (a) is determined by the amount of heat got I n t oa particle (droplet) ( Q,) and by the termal effect of the reaction at this temperature (mr); a = Qp/AHr.
The following values were taken to be basic In calculations: the initial temperature of particles (droplets) was 298K,the initial particle velocity 4 m/s,temperature of the reactor walls was 5OOK,the pressure In the system of 10' Pa and the solution concentration was 50 mass.%. The initial temperature of plasma was varied between 3500 and 6000K.
Fig. 1 The length -of complete lanthanum nitrate dissociation (a) and gas temper'ature corresponding to the moment of maximum lanthanum nitrate dissociation ( b ) vs initial droplet diameter. Plasma temperature: 1 ,6000;
2, 5500; 3 , 5000; 4 , 4500; 5, 4000; 6 , 3500 K.
Calculations were performed for the dro lets with the initial sizes of
50,100,150, 200, 250 pm for the 0.1 m drameter reactor.
As shown,for raw material and gas flow rates of 10 g/s each, the temperature corres onding to the moment of corn lete nitrate decom osit
B"
onessentially exceejs particle temperature o? 1053 K under dyfferent Table 1 Calculated parameters of plasma treatment of 50 % lanthanum
nitrate solution for raw material and gas flow rates of 20g/s and 10 g/s, respectively
Parameters
Maximum de ee of decomposit%n, X Distance, cm Time, ms
Gas temperature,K Maximum de ee of decomposit%n, %
Distance, cm Time, ms
Gas temperature, K
Particle size, pm
...
5 0 1 1 0 0 1 1 5 0 1 200
Plasma temperature 6000K 100 100 100 100 7.9 35.2 88 213 4.57 21 - 5 56.3 135 2076 181 2 1637 1421
Plasma temperature 5500K
100 100 100 87.6 15 71.2 278 682 9.46 48.7 202 524 1524 1313 1126 If358
5 0 1 1 0 0 1 1 5 0 1 2 0 0
Plasma temperature 5000K 100 90.4 70.6 53.4 25.1 216 365 496 17.7 172 300 415 1246 1058 1058 1058
Plasma temperature 4500K
91 60.2 40.4 2'7.6 67 163 256 350 54.3 139 228 318 1058 1058 1058 1058
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Fig. 2. Lanthanum nitrate dissiclation degree vs specific ener consumption er I kg raw materlal. Inltial boplet sizes: 1, 50; 2, 108 3, 150; 4, 280 pmun.
conditions. This can hardly be considered rational as far as complete use of thermal enerm of a plasma flow Is considered. If the admissible flnal gas tem erature Is taken as 1500 K then the initial plasma temperature s h d be 5000 K and below de eriding on droplet slzes (Fig.
1 b, area between dashed lines). However, l? 1s not always possible to use mlnimum ermlssible plasma tem eratures because of extreme increase of the reactor Pength (Fig. l a). On
P
hls vlew, the reglon of optimum arameters is the one hatched in Fig. fa. It means that at raw material ang
gas flowrates of 10 g/s each, the lnltlal droplet slzes may be varled from 50 to 100 pm, while the plasma temperature may range from 3500 to 5000 K.
Flg. 3 gives the nitrate lanthanum dlssoclation degree vs spesific enerfy consumption for the process realized in O.lm dla reactor at gas and solu Ion flow rates of 10 and 20 g/s,res ect1vely.In addition to specific
R,
energy consumption, to determine opt um rocess conditlons requles regarding for the time of article resldence
&
the reactor necessary for complete nltrate dlssocla?lon and determinl the reactor le th, heat losses to reactor walls wlth a leaving as m? its tem erature %able 1 ).
For example, at 6000 K oom lete dissotkation of 50, 190, 150 and 200 pm particles can be achieved. iowever, taking into account the reactor length and final gas temperature, the re imes marked In Table 1 with asterisks should be considered most efflcienf. Flg. 3 gives the dlstrlbution of the most Important parameters of plasma treatment of the lanthanum nitrate solution along the reactor channel for one of the above o timum conditions: particle size of 50 lnltial plasma temperature of PO00 K and raw materlal consumption o f i 0 g/s. Under thls regime, water is evaporated at a distance of 0.61 cm for 0.99 ms, the gas temperature reduclng to 2106 K. Nitrate dlssotiation starts 1 cm apart, which corresponds to the tlme, 2.2 ms, of artlcle resldence In the reactor.
The adequacy between the thermodynamPca1 ropertles and temperature r es of thermal stability of RFE nitrates
R/
enables one to conslderyhe reported data typical not only for lanthanum nltrate but also for nitrates of other rare-earth elements.Pig. 3.Distribution along the reactor of: 1 , lanthanum nitrate dissociation; 2, particle temperature; 3, gas temperature; 4, particle velocity; 5, gas velosity; 6, heat losses wlth gas; 7, heat ~ O S S ~ S to reactor walls; 8, amount of heat for raw material heating and consitutional and chemical changes. The initial plasma temperature is equal to 5000 K, particle size, to 50 pm.
2.2 - Experimental
Plasma treatment of REE nitrates was studied on an e erimental setup having as a maln unit a vertical water- cooled cylindrlc~ reactor wlth a three -jet mixing chamber
.
A plasma flow was formed due to mging of the jets of three electric -arc d.c. plasma generators spaced 120 apart and normal to the side surface of the mixing chamber. The reactor input power did not exceed 170 kW .The raw material su ply unit was a eumaticS rayer with liquid and air mixed inside it. The material I%w rate tkough the s rayer was regulated by the pressure in the buffer solution tank and by tfte air flow rate. As follows from the calculation of s rayer dispersion at the solution and gas flow rates of 15 g/s and 0.78 g/s respectively, the mean droplet diameter constituted 50 W. The dlameter o?
the maln fraction dro lets between 60 and 80 pm was found ex erimentally us1 the technique o? dro let capture in an Immersion medium F41
?!?he solution included g00 /l mixture of three-valent RFg nltktes. As referred to oxldes, It includes 55% of Ce20,; 25% of La203; 12% of N%O3;
6% of Pr20, and 2% of Sm,Eu,Gd oxldes, in total. Besides the solution contained 30g/l nitric acid, which corresponds to the acid content in liq- uid semi roducts of REE oxide prduction used, for exam le, for
i
synthesizf the polyoxide abrasive material POLYRITE. The reac Ion flow was su li% from the reactor to a bunker to be removed then to a steel grid &?ter. The product output on ihe filter accounted for 60% of the
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theoretically possible amount, that Is the one in case of com lete REE oxide capture on the filter re ardlng for Ce (+3 It Ce (+4) trans&ion.
Table 2 gives the operatag regmes of the setup and the results to estimate the efficiency, of the process. A ood agreement was observed between experlmental data and theoretical reslfits.
Table 2. Main characterlstlcs of ex erimental plasma treatment of the dispersed REE nitrate solu
!
ion2.3- REE Nitrate Thermal Decomposition Products
Fi .4 shows the IR-spectra of the products obtained on the lasma- chemlca!? setup as well as when the inltial REE nitrate so1utfon was evaporated and the dry residue was roasted within an hour in a laboratory furnace. Curve 4 stands for the plasma treatment product with 0.07%
roast- losses. It Is Identical to the spectrum of product formed when a dry residue was roasted in the furnace at 500°C (curve 3) .For the product obtained In run 1 (Table 2 ) with the solution flow rate increased up to 22 g/s and dis ersed- gas pressure several times decreased, i.e. under less essential d?sperslon, the roastlng losses accounted for 15 -3%. The spectrum (curve 5) corres onding to such a product is intermediate between spectra 1 and 2. For op~lmum conditions losses of the products on the filter and in the bunker constitzt","?%. This points both to corn lete nitrate dissociation and to the absence of recomblnatlons interac!ions of REE oxides with nltro en oxides contained In a gas phase. The results
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of the X-ray ghase ana ysls agree with the IR-spectmscopy data and Indicate that t e roduced oxides have a cubic structure.
Statistical Rterpretatlon of the RFE sample observations us- the scann
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electron rnicrosco e Nanolab-7 has shown that the oxide particle pro uced due to plasmacR
emical treatment are close to s her1 a1YS 8
Eones. For one of the t ical samples the shape factor B=4*3.14' pi/Ppl, where Spi is the artic e area and Ppi is the article perimeter, from the minimum 5.005 to the maxlmm 1 -099 am? costltuted 0.859, o:x
aVeratii.We > 180, that is typlcal of the oondltions under study, SeCOMary aerodynamic atomization of solutlon droplets should be realized without essential changing of their temperature /5,6/. Estimations show that 60-80 droplets are atomized into secondary 2.5-4.5 pm droplets whlch Is e uivagnt to oxide particles of 0.5-2.5 p. This
F
witheyectron-microsco e observations , whlch glve the Iollowlng e th wise particle distrlbu!lon (chosen from 487 particles) .Particles less %m-2 p
Reactor iniit,
R~ Nirtate dlssociation
degree (a), %
Plasma temperature,
K Flow rate, g/s
...
Plasma Solution Dlsperser
gas(~JI (Gp)
I
gas G P /GFig. 4. IR-spectra of 1 , dry residue due to evaporation of REE nitrate solution at 120'~; 2,3, products of dry residue roasting within an hover at 300 and 500'~; 4,5 roducts of plasma treatment of the nitrate solution with 0.07 and 15.3
g
roasting losses.in size account for 24.3%, those from 2 to 4 pm, for 36.9%, particles from 4 to 6 pm make U 15.1 % and large articles run 23.7 %. Nofused articles were obsemed, ?!us indicating suci an essential decrease of tie lasma flow tem erature for the time of water evaporation and nftrate dissociatfon that even the finest oxlde particles do not reach the melting point.
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PLASMACmICAL PRODUCTION OF REE OXIDES FROM OXAUTES ANTI CARBONATES Experimental studies were performed on the setup different from the one used for treatkg the solutions by the raw materlal su ply unit.The greater bulk of studies was made for neodymium, cerium and? yttrium oxalates. In a number of e eriments carbonates of the cerium group were studled. The plasma temper,$ure was varled from 4000 to 6000 K, the flow rate ratio ranged between 0.7 and 5. The power supplled to the lasma jet reactor per 1 kg raw materlal, I.e. the power spent to trea! the raw material, was between 0.4 and 4.2 kW@. As found, the degree of dissociation of oxalates and carbonates essentionaly depends on spesific energy consumption which may be considered as a complex characteristic of a production process (Fig. 5).
Studies of the com osition and pro erties of products have shown that the products obtainez at high dissoc?ation degrees of salts com ly with industrial requirements. In experimental production of cerium oxide
,
the content of Fe, Cu, Cr, NI,m
and Ca in products was controlled. Iron did not exceed 0.02 %, the copper content was no more than 0.005 %, theCOLLOQUE DE PHYSIQUE
Pig. 5. Degree of REE oxalates and carbonates dissociation vs specific energy consumption; a, neodymium oxalate; b, cerium oxalate; c, yttrlum oxalate; d, cerium group oxalates
contents of other elements were much below these values. It should be noted, when lasmatmns o erated at 120-1 40 A, iron and copper amounts were lower t k at 200-283 A which is due to less essential electrode erosion In the former case.
Thus, an electric arc plasma may be used not only to produce REE oxides from nitrate solutions, but also to enhance thermal decomposition of REE oxalates and carbonates. Traditlonal furnace roastihg of these salts takes much time, is low-efficient, and not all of the end product meets the requirements to its composition due to nonunlform heating of raw material.
REFERENCES
/l/.Krasovskaya,L.I., Vesti AN BSSR, Ser-Fiz. Energ.Navuk.4 (1987) 45.
/2/.Belov,B.A., Gorozhankin,E.V., Yefremov,V.N., Salnikova,N.S. , Suris,A.L., Zh. Neor
.
Khim. 10 (1985) 2520./W. Glushakova,V.B., ~obmorf lsm of Rare-Earth Element Oxides, Lenlngrd,
1 . - v . 967 _
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/4/.Pazhi,D.G., Galustov,V.S., Liquld Atomizers, Moscow, 197'9.
/S/.Belov,B.A., Gorozhankin,E.V., Suris,A.N., K h h . Visok. merg, 2, (1988) 161.
/6/.Galustov,V.S., Flow Through Sprayers in Power Ehglneering, Moscow,
(1 989)
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