Recovery of rare earth elements adsorbed on clay minerals:I.Desorption mechanism
Georgiana A.Moldoveanu,Vladimiros G.Papangelakis ⁎
Department of Chemical Engineering and Applied Chemistry,University of Toronto,200College Street,Toronto,ON,M5S 3E5,Canada
a b s t r a c t
a r t i c l e i n f o Article history:
Received 30September 2011Accepted 8February 2012
Available online 16February 2012Keywords:
Ion adsorption clays Rare earth leaching Rare earth desorption Lanthanides
Rare earth elements
The ongoing development of new,advanced technologies created increasing demands for rare earth ele-ments (REE)in the international markets,with emphasis on identifying new resources to ensure a
水门桥观后感
dequate supply and access.The prent study investigates the u of clay minerals as a source for extracting rare earth metals by leaching with sulfate and chloride salts.It was found that REE adsorbed on clays can be easily recovered via an ion-exchange mechanism during leaching with monovalent salt solutions under ambient conditions.The leaching ef ficiency of various salts at 0.5M and 25°C was investigated as a function of mono-valent cation type (i.e.Li +,Na +,Cs +and NH 4+
)and salt system (sulfates vs.chlorides).The initial concentra-tion was bad on a 3:1stoichiometric ratio between all trivalent lanthanides in the clay and the exchange
monovalent cation.Leaching ef ficiency (in terms of %REE extracted)decread in the order Cs +>NH 4
+
>-Na +>Li +,from 90%to ~60%,respectively,with sulfates exhibiting ~10%better extraction behavior than chlorides.Differences in rare earth metal desorption capability were explained in terms of differences in cat-ion hydration energies:species with low hydration energy extract to a lesr degree compared to species with high hydration energy (i.e.higher af finity for water).Bad on the findings,(NH 4)2SO 4was identi fied as the lixiviant of choice for further studies.
©2012Elvier B.V.All rights rerved.
1.Introduction and background
Rare earth elements (REE)are mostly associated with the hi-tech industry becau of their various us in high strength permanent magnets,lars,automotive catalytic converters,fiber optics/super-conductors,and electronic devices.Becau of the ongoing develop-ment of new advanced technologies,there is an ever-increasing demand for REE in the international markets,with emphasis on iden-tifying new resources to ensure adequate supply for prent and fu-ture u.World production of REE is dominated by China,the United States and Australia;however,in terms of rerves and re-sources,China dominates the world potential with rerves estimated to be over 90%of the total (Clark and Zheng,1991).
The designation “rare earths ”refers to the elements of the Periodic Table known as “lanthanides ”,further divided as a function of their atomic number into the “cerium group ”(or light REE:La,Ce,Pr,Nd,Pm,Sm,Eu,Gd),and the “yttrium group ”(or heavy REE:Y,Tb,Dy,Ho,Er,Tm,Yb,Lu).The term “rare earth ”is actually a misnomer,be-cau the elements are more abundant in the Earth's crust com-pared to silver,gold or the platinum-group metals and are part of more than 200rock-forming minerals (Cotton,2006),due to the
fact that the size of their trivalent cation is very clo to that of Ca 2+,which they can occasionally replace.They are “rare ”becau they do not naturally occur in metallic form,only as mixed and scattered in minerals and are dif ficult to parate from each other due to very sim-ilar physico-chemical properties.
REE are part of many various rock-forming minerals,but the main commercially signi ficant sources,as reviewed by O'Driscoll (1991)fall into the following categories:
1)Carbonates :Bastnasite,(REE,Ce)(CO 3)F,is a magma-derived fluor-ocarbonate mineral containing 65–75wt.%rare earth oxides (REO).The two major sources in the world for lanthanides are bastnasite deposits at Mountain Pass,California (U.S.A)–devoted solely to REE production,and Bayun-obo,Inner Mongolia (China)–mined primarily for iron ore and REE as by-product.
2)Phosphates :Monazite,(REE)PO 4is a light RE phosphate containing
55–65wt.%REO,associated with granites and beach sands in Aus-tralia,Brazil and India,while Xenotime (Y,REE)PO 4is an yttrium-rich phosphate containing 25–60wt.%Y 2O 3and other heavy REE.Both minerals are recovered as by-products of mining for titani-um,zirconium and tin.
3)Ion-adsorption clays :contain 0.05–0.2wt.%REO adsorbed on the
surface of alumino-silicate minerals (e.g.kaolinite,illite,and smectite).The ion-adsorption clay deposits are the result of in-situ weathering of rare-earth rich host rocks (granitic or igneous),which lead to the formation of alumino-silicate clays.The very fine mineral particles have the capability of adsorbing lanthanide
Hydrometallurgy 117–118(2012)71–78
⁎Corresponding author at:Department of Chemical Engineering and Applied Chem-istry,University of Toronto,200College Street,room WB213,Toronto,ON,M5S 3E5,Canada.Tel.:+14169781093;fax:+14169788605.
E-mail ldoveanu@utoronto.ca (G.A.Moldoveanu),vladimiros.papangelakis@utoronto.ca (V.G.
Papangelakis).
0304-386X/$–e front matter ©2012Elvier B.V.All rights rerved.doi:
10.1016/j.hydromet.2012.02.007
Contents lists available at SciVer ScienceDirect
Hydrometallurgy
j o ur n a l h o m e p a g e :w w w.e l s e v i e r.c o m /l o c a t e /h y d r o me t
ions relead/solubilized during weathering.Sub-tropical climates prent ideal conditions for this lat
eritic process to occur.The best example of this formation process exists in Southern China(lati-tudes24–26°N),where many of such deposits(albeit underdevel-oped)are known to exist,as described by Bao and Zhao(2008).男性不能生育原因
Carbonate and phosphate sources,in spite of being high grade,are associated with elevated recovery costs due to difficulty in mining, paration,and need of aggressive conditions to solubilize the REE. For example,bastnasite is generally leached with concentrated H2SO4or HCl,whereas monazite/xenotime concentrates need to be baked either in98%H2SO4or70%NaOH to render REE soluble (Tran,1991).
Although ion-adsorption clay deposits are substantially lower grade than other types of lanthanide sources,the lower grade is large-ly offt by the easier mining and processing,costs,and the very low content of radioactive elements(normally associated with yttrium). The deposits are mined by open-pit methods and no ore beneficia-tion is required.A simple leach using monovalent salt solutions at ambient temperature can produce a high-grade REO product(Ru'an, 1988;Tran,1991).
Clay minerals are part of the phyllosilicate class,containing lay-ered structures of shared octahedral aluminum and tetrahedral sili-con sheets;water molecules and hydrated cations can move in and out
of the interlayer spaces.Very often,isomorphous substitution of one cation with another(of similar size but with lesr Al3+for Si4+or Mg2+for Al3+)within crystal structures leads to a charge imbalance in silicate clays,which accounts for the permanent negative charge on clay particles,thus the ability of clays to attract cations to the surface.Amphoteric–OH groups at the surface/edge of ,silanol and aluminol groups)may also contribute to sur-face charge(pH-dependent reversible charge).
Bradbury and Baeyens(2002)as well as Piacki and Sverjensky (2008)studied REE speciation/distribution over wide ranges of pH and ionic strength.It was determined that most of the surface-adsorbed lanthanides occur as simple“clay-REE”species derived from straightforward cation-exchange reversible reactions at the per-manent negative charge sites on the clays(physisorption),or as hy-drolyzed“clay-O-REE2+”species derived from permanent complexation reactions at the amphoteric surface hydroxyl groups (chemisorption).While the cation-exchange reactions were found to be pH and temperature-independent,the opposite was true for the chemisorption process,which are endothermic and preferen-tially occur at high pH(Bradbury and Baeyens,2002;Miller et al., 1982and Tertre et al.,2006).
Ion-adsorption clays are leached with concentrated inorganic salt solutions of monovalent
M2SO4and MCl,where M=Na+ and NH4+);during leaching the physisorbed REE are relatively easily and lectively desorbed and substituted on the substrate by the monovalent ions and transfer into solution as soluble sulfates or chlo-rides(Eq.(1)).Solubilized REE are recovered/parated as high purity end-products either by solvent extraction(Li et al.,2006;Preston et al.,1996)or ion-exchange(Ochnkühn-Petropoulou et al.,2002)as individual elements or lectively precipitated with oxalic acid to form oxalates(Eq.(2))that are subquently converted to REO via roasting at900°C(Chi and Xu,1999;Liu et al.,2008).
Clay−Lnþ3MX¼Clay−M3þLnX3ð1Þ2LnX3þ3H2C2O4þ10H2O¼Ln2C2O4
ðÞ3Á10H2Oþ6HXð2Þ
Eq.(1)indicates a3:1stoichiometric ratio between the monova-lent cation and the RE element.
The permanently chemisorbed species can only be liberated via more aggressive and non-lective acid leach and are not the subject of the prent rearch work.
Becau of their abundance in surface layers in nature,high specif-ic surface area for adsorption and relative ea of mining and proces-sing,the clay minerals warrant a detailed study as important so
urces of rare earths.Until prent,extensive rearch has been con-ducted on the uptake/sorption of lanthanides on clay minerals,both from the standpoint of conditions influencing the process and the mechanism.Bruque et al.(1980),Miller et al.(1982,1983),Coppin et al.(2002)investigated the former aspect while Kraepiel et al. (1999),Bradbury and Baeyens(2002),and Tertre et al.(2006)stud-ied the latter.
Despite the clear advantages of processing clay materials as REE sources,virtually no in-depth systematic work has been conducted to rearch the desorption/recovery of lanthanides from clays.The only known investigation on the extraction of REE from kaolinite (Ru'an,1988)mentioned the u of NH4+-bad salts such as chloride, nitrate and sulfate in a percolation leaching process.
The general scope of this paper is twofold:(1)to establish the underlying mechanism of rare earth metal desorption from clay materials and(2)to investigate the desorption capability of various leaching valent metal sulfates/chlorides,where M+=Li,Na,Cs,and NH4)from the standpoint of reaction kinetics and terminal REE recovery levels.A follow-up publication will inves-tigate the influence of various experimental conditions such as lixivi-ant concentration,temperature,pH,and agitation speed on REE extraction.
Thefinal aim of the prent rearch work is identify the optimum extraction conditions in order to acquire knowledge that will enable the development of novel leaching procedures/reagents for the com-mercial production of rare earth oxides from various clay materials.
2.Experimental
2.1.Characterization of solid pha
Two types of natural clays of undisclod origin and known to contain adsorbed REE were employed in the prent work,referred to as YR(yttrium-rich)and YM(yttrium-medium),respectively, depending on their initial yttrium content.X-ray Fluorescence(XRF, Philips PW2404)was employed to determine the overall chemical composition of clays;the samples were analyzed using presd pow-der pellets,with the calibration refined using both Standard Refer-ence Materials(SRMs)and synthetic ones.The composition in terms of adsorbed REE was also determined by aqua regia digestion, following the conventional reflux procedure as described by Nieuwenhuize and Poley-Vos(1991),and Inductively-Coupled Plas-ma(ICP,Optima7300DV)analysis.X-Ray Diffraction(XRD,Phillips PW3719)and Electron-Dispersive Spectroscopy(EDS,JEOL JSM-840)were ud to gain additional qualitative information about the nature of the solid pha.
明镜高悬
2.2.Batch leaching tests
For the preparation of all solutions ud in the prent work,de-ionized water and the following A.agent grade chemicals were ud:M2SO4and MCl,where M=Li,Na,Cs and NH4(Fisher Scientif-ic,min.purity99%).
Measured amounts of dry clay samples and0.5M leaching agent (solids/liquids=1/2by weight)were added to250ml Erlenmeyer flasks plugged with rubber stoppers,at25°C.Theflasks were stirred magnetically to ensure solid suspension for60min.Bruque et al. (1980)found that agitation leaching in excess of1h led to loss of dis-solved REE,likely due to hydrolysis.
At the end of the experiment,the solids were parated by vacu-umfiltration(Whatman541filter paper,22μm pore-size),repeated-ly washed with distilled water,dried in the oven at60°C(overnight), weighted and stored for further analysis.Solution samples collected
72G.A.Moldoveanu,V.G.Papangelakis/Hydrometallurgy117–118(2012)71–78
at regular intervals and afterfiltration were diluted with5%HNO3and subjected to ICP analysis.
3.Results and discussion
臧文仲3.1.Characterization of clays
Qualitative XRD analysis revealed a complex pattern showing il-lite,mica,quartz,and kaolinite as major components.Shaomei (1991)cites similar overall composition of ion-adsorption clay de-posits.EDS patterns indicated Si,Al,Fe,and K,as main elements but failed to indicate any REE(due to their extremely low quantity).De-tailed chemical composition of clays is shown in Table1,and was de-termined by joint ICP and XRF methods with a precision of5.8%;the balance is minor elements(with less than0.1wt.%)and H2O.It indi-cated a content of~0.34%total rare earth oxides(REO)for YR and ~0.40%total REO for YM,respectively,expresd as wt.%;the total rare earth content is equivalent to~2.5mmols REE/100g clay.Rela-tive abundance data of REO in the initial clays(individual REO mass relative to total REO content)(Fig.1),as determined via ICP,indicated Y as major component of YR and La in YM.
3.2.Experimental conditions for batch leaching tests
The majority of studies reporting on the ion exchange process be-tween clay minerals and cations in solution involved the u of Na+, K+or Ca2+,such as in Bruque et al.(1980),Maza-Rodriguez et al. (1992),and Sinitsyn et al.(2000),Coppin et al.(2002).Bad on this information,it was decided to le
ct Li+,Na+,Cs+and NH4+as cations of interest in the prent work,due to the fact that they form soluble sulfates and chlorides under ambient conditions.Ca2+ was discounted becau it forms insoluble Ca-REE double sulfates, as reported by Kul et al.(2008).From the anion perspective,it was decided to lect SO42−and Cl−,as the most common for the vast ma-jority of hydrometallurgical process.Carbonates were eliminated due to the reported insolubility of rare earth carbonates(Chi et al., 2003;de Vasconcellos et al.,2008).
Extensive work on exchange lectivity of lanthanide ions on montmorillonite(Bonnot-Courtois and Jaffrezic-Renault,1982; Maza-Rodriguez et al.,1992)have confirmed the3:1exchange stoi-chiometry between Na+and Ln3+.Bad on the determined total content of REE in clay samples and considering a3:1of M(I):REE(III) stoichiometric ratio for ion exchange,it was decided that a0.5M lixi-viant concentration would provide enough excess over the stoichio-metric requirement for monovalent exchange ions.
Miller et al.(1982),Bradbury and Baeyens(2002)and more re-cently Tertre et al.(2006)studied sorption of lanthanides on clay minerals and reported that the metal exchange RRE(III)displacing M(I)from the clay)are practically pH and temperature-independent(for acidic and near-neutral range under 80°C)whereas surface complexation reactions are endothermic and predo
minant at neutral and alkaline pH.In the prent work, the hypothesis made was that this temperature-independence should hold true for the REE desorption process as well,considering the reversibility of the ion-exchange process and that the clay sur-face charge is always negative,at least in the range20–125°C (Brady et al.,1996).
The OLI software()was employed to simu-late lanthanide speciation and solubility dependence on temperature and pH,both in the sulfate and chloride system,bad on an initial concentration of~3mmols La3+(similar to total REE content in clays);lanthanum was lected as an example of the REE ries be-cau it is the most abundant trivalent lanthanide that is found in major host rocks and exhibits similar physico-chemical properties to all other REE.The OLI suite of Analyzers is a ries of powerful software applications bad on a speciation-bad thermodynamic model that enable reliable estimation of properties and chemical equilibria in concentrated electrolyte solutions of interest to hydro-metallurgical applications.Speciation literature data(Bradbury and Baeyens,2002;Tertre et al.,2006)confirm the OLI model and indi-cate the simple lanthanide trivalent ion(un-complexed Ln3+)as the most important species at acidic and near-neutral pH in the chloride system,while in hydroxide complexes become predomi-nant at basic pH;lanthanide interactions with chlorides are weak, therefore REE complexes with the ligands are not important (Millero,1992;Wo
od,1990).In the sulfate system,the free cation (La3+)and sulfate lanthanide complexes(LaSO4+)are important at acidic pH and near-neutral(Cetiner and Xiong,2008).OLI simula-tions of lanthanum speciation in the range25–100°C indicated very little temperature dependence for main La species in both sys-tems.The natural pH in solution was b7in all risk of major lanthanide loss via hydrolysis,which starts at pH>7(Tertre et al.,2006).
Bad on the literature information and the OLI results,it was decided to conduct the leaching experiments at25°C to minimize lanthanide loss via permanent chemisorption via hydroxide pre-cipitation at higher temperatures(Tertre et al.(2006)).The solu-tion pH was within the acidic-near-neutral range for all the 0.5M sulfate and chloride solutions as shown in Table2.Therefore the predominant species were expected to be free Ln3+and sul-fato complexes in the sulfate system whereas Ln3+in the chloride system,respectively.
3.3.Clay leaching tests
Clay leaching tests were conducted following the procedure de-scribed in the Experimental ction and under conditions lected as explained in the previous ction.Leaching with sulfate salts (Fig.2)indicated extraction efficiencies to vary in the order of Cs2SO4>(NH4)2SO4>Na2SO4>Li2SO
4for both types of clays, with Cs+displacing~90%of total REE from YR(Fig.2-a)and~80% from YM(Fig.2-b);Li+failed to displace more that60%in both cas.The chloride salts(Fig.3)exhibited the same extraction trend, i.e.CsCl>NH4Cl>NaCl>LiCl,although they reached~10%less ulti-mate extraction levels than their sulfate counterparts within the 60min leach time.This is explained in terms of differences in the stability constants for the formation of REE complexes,with stability constants for sulfates of one order of magnitude larger than tho of chlorides(Millero,1992;Wood,1990;De Carvalho and Choppin, 1967),leading thus to stronger complexes.The prent results com-pare with,or are better than,tho reported by Ru'an(1988),who av-eraged total REE extractions of80%during percolation leaching with NH4-bad reagents but did not clearly indicate the initial REE content in clays,nor elaborate on individual element extractions.
Individual lanthanide extractions vary widely from element to el-ement,with Er and Tm exhibiting the lowest extraction from both clays,in both systems,even when leached with Cs-bad lixiviants. Bruque et al.(1980)obrved that the amount of trivalent lanthanide ions adsorbed on montmorillonite was inverly proportional to the ionic radii.Coppin et al.(2002)reported a similar fractionation
Table1
Main chemical composition of clays(as wt.%oxides,for major components). Element YR YM
SiO2(XRF)49.0050.52
Al2O3(ICP+XRF)25.5223.22
K2O(XRF) 3.06 5.20
FeO(ICP+XRF) 1.65 2.19
MgO(XRF)0.460.21
B2O3(XRF)0.380.23
REO total(ICP)0.34±0.020.40±0.0173
G.A.Moldoveanu,V.G.Papangelakis/Hydrometallurgy117–118(2012)71–78
during lective sorption of lanthanides on smectite and kaolinite,with heavy elements (i.e.,higher atomic number:Tb to Lu)being adsorbed stronger that the light ones (i.e.,La to Gd).They related this behavior to the “lanthanide contraction ”in the ionic radii going from light to heavy REE.Bad on the
obrvations,it was inferred that the desorption process must exhibit a similar trend,with heavy REE more dif ficult to extract.Moreover,Olivera-Pastor et al.(1988)and more recently,Bentouhami et al.(2004),reported that hydrolysis tendency in the lanthanide ries at 25°C increas with atomic num-ber and contraction of the atomic radius.They related this to REE ad-sorption on clays in excess of the clay cation exchange capacity value.The same obrvations can also explain the difference in individual extraction levels,with some of the REE (especially the heavier ones)undergoing hydrolysis at pH less than 6,while still adsorbed on the surface of clay,and bonding permanently to the surface via chem-ical reactions,thus becoming unavailable for simple ion-exchange and extraction.
Despite detectable Ce content in both initial clays (i.e.0.008–0.02wt.%),practically no Ce was found in the pregnant leach solutions,regardless of lixiviant type.This fact likely relates to the “negative cerium anomaly ”:contrary to the majority of lanthanide elements which are only trivalent,Ce 3+can be easily oxidized by atmospheric oxygen (O 2)to Ce 4+under alkaline conditions (E 0h ,Ce(IV)/Ce(III)=−1.72V,Bard et al.,1985).The cerium anomaly relates to the decrea in solubility which accompanies the oxida-tion of Ce(III)to Ce(IV);under reducing conditions,Ce 3+is relative-ly soluble while under oxidizing conditions CeO 2precipitates.Conquently,the formation of the minera
l cerianite (CeO 2)facili-tates a natural paration of Ce from the other adsorbed trivalent lanthanides,as described by Bao and Zhao (2008).
By analyzing the comparative leaching of lanthanides from clays,it can be obrved that Cs-bad lixiviants achieved the highest extrac-tion level,and although 0.5M CsCl has half the ion exchange capacity compared to 0.5M Cs 2SO 4,they both achieved comparable extrac-tions.Ammonium-bad lixiviants (either sulfate or chloride)reached similar extraction levels.In addition,0.5M ammonium salt
solutions at 25°C have a pH under 5,which prevents any possible REE hydrolysis.On the other hand,0.5M NaCl achieved rather mod-est extraction levels,below 50%.A two-stage leaching process,where stage-1leach residue (L1-0.5M)was re-pulped in fresh 0.5M NaCl and leached again under the same conditions (L2-0.5M)improved the extraction to 60%.Furthermore,one-stage leaching employing 1M NaCl (L1-1M)achieved about 63%extraction,which is less than the 73%achieved by the 0.5M Na 2SO 4system.Fig.4shows detailed NaCl extraction levels.3.4.Extraction kinetics
The kinetics of REE leach were very fast for both clays,with less than 5min to reach equilibrium valu
花蛤蒸几分钟
es regardless of lixiviant type.Fig.5shows the extraction kinetics of total REE from YM with MCl and M 2SO 4(M =Na,NH 4),respectively.Bonnot-Courtois and Jaffrezic-Renault (1982)studied factors affecting retention of lantha-nide ions on clays and reported fast exchange kinetics as well,while Aja (1998)obrved that Nd adsorption on kaolinite changes very lit-tle after 15-min reaction time.Bruque et al.(1980)found that agitation leaching in excess of 1hour led to loss of dissolved REE,like-ly due to hydrolysis.
3.5.Cation exchange lectivity
Cation exchange at charged clay mineral surfaces is a widely rearched topic,but a uni fied description of the underlying forces that govern lectivity is lacking.One theory argues that lectivity is due to the ionic radius (Jia,1987,and McBride,1980),with smaller cations exhibiting stronger electrostatic interaction with the charged clay adsorbing better.Also,this theory implies that cat-ions with similar radii as lanthanides are more successful in repla-cing them of the clays by ful filling similar steric requirements on the surface.
Table 3lists the ionic radii of lanthanides and cations ud in the prent study as well as their respective hydration enthalpies.It can
be obrved that NH 4+
and Cs +,which demonstrated the highest ex-traction capabilities,have ionic radii much larger than the Ln 3+,while Li +and Na +,despite their similar radii,exhibited rather mediocre leaching/exchange power.
However,a comparison of extraction ef ficiency with hydration en-thalpy (ΔH hyd )reveals an interesting correlation.Teppen and Miller (2005)successfully explained the order Cs +>Rb +>K +>Na +>Li +
10
20
30easy副词
40
50
60W t . %
香橙
REO
Fig.1.Relative content of rare earth oxides (REO)of initial clays.
Table 2
Measured pH for all lixiviant solutions tested (0.5M,25°C).Lixiviant Cs 2SO 4(NH 4)2SO 4Na 2SO 4Li 2SO 4CsCl NH 4Cl NaCl LiCl pH
6.92
5.08
6.75
7.03
6.07
4.71
6.16
6.28
74G.A.Moldoveanu,V.G.Papangelakis /Hydrometallurgy 117–118(2012)71–78
for the lective adsorption of alkali ions on clay minerals by compar-ing hydration energies rather than unhydrated ionic radii,and argu-ing that the more strongly hydrated ion (i.e.with more negative ΔH hyd )is always favored in the solution pha,since it would require a larger amount of energy input to dehydrate and report to the sur-face (at least partial dehydration is required in order to overc
ome the steric hindrance).They de fine cation exchange as a partitioning reaction between the clay surface and aqueous solution:given two cations,the more weakly hydrated (i.e.with less negative ΔH hyd )will tend to partition into the surface clay layer,while the more strongly hydrated ion will tend to report to solution.Spedding et al.(1977)determined that the total amount of ionic hydration increas linearly from La to Lu in the lanthanide ries,due to the contraction of ionic radii,while Jia (1987)and Ikeda et al.(2005)assigned hydra-tion numbers between 8and 9for lanthanides,as oppod to much lower values for the monovalent cations (i.e.~4for Cs and NH 4and ~6for Na and Li,respectively).This is in agreement with the trend in ΔH hyd values in Table 3and rationally explains the alkali lectivity quences.It also explains the lectivity of clays for monovalent
cations over trivalent lanthanides,and thus the ea of REE desorp-tion from clays via ion exchange.3.6.Propod extraction mechanism
Considering the reversibility of the ion-exchange process,a new model for the lanthanide desorption mechanism is propod here,bad on studies conducted on REE adsorption on montmorillonite by Kraepiel et al.(1999)and taking into account the hydration effects described by Teppen and Miller (2005).
According to Kraepiel's model,Fig.6shows a clay particle as having a lamellae structure and substantial porosity,bearing a per-manent negative charge resulting from isomorphic substitution of Si(IV)with M(III)or of Al(III)with M(II)in the allumino-silicate matrix.This permanent negative charge is balanced by the pres-ence of electrostatically-adsorbed counterions (in this ca,triva-lent lanthanides).Cation exchange occurs inside the clay particle at the sites of permanent negative charge and it is considered the predominant sorption mechanism at relatively low
ionic
La
Pr
Nd Sm Eu
Gd
Tb
Dy
Ho
Er
Tm Yb
Lu
Y Total
RE
% E x t r a c t i o n
RE Element
Fig.2.Clay leaching with sulfate-bad lixiviants (25°C,0.5M reagent,1h,S/L =1/2,natural pH b 7):(a)YR;(b)
YM.
10203040506070
酒驾撞车怎么处理
8090La Pr
N d Sm Eu G d
Tb
Dy Ho
Er
Tm Yb Lu Y
Total
RE
% E x t r a c t i o n
Rare Earth Element
Fig.3.Clay leaching with chloride-bad lixiviants (25°C,0.5M reagent,1h,S/L =1/2,natural pH b 6.3):(a)YR;(b)YM.
75
G.A.Moldoveanu,V.G.Papangelakis /Hydrometallurgy 117–118(2012)71–78
