The role of inorganic and organic reagents in the flotation separation of rare-earth ores

International Journal of Mineral Processing, 32 ( 1991 ) 1-22 Elsevier Science Publishers B.V., Amsterdam

The role of inorganic and organic reagents in the flotation separation of rare-earth ores Pradip ~ and D.W. Fuerstenau Department of Materials Science and Mineral Engineering, University of California, Berkeley, CA 94720, USA (Received May 15, 1990; accepted after revision February l, 1991 )

ABSTRACT Pradip and Fuerstenau, D.W., 1991. The role of inorganic and organic reagents in the flotation separation of rare-earth ores. Int. J. Miner. Process., 32: 1-22. The relative adsorption of chemisorbing fatty acid collectors at high temperatures, together with the addition of soda ash modifier and lignin sulfonate depressants, is the key to the industrial processing of carbonatite-type rare-earth ore deposits. A detailed study was made using solution equilibria, electrokinetic measurements, adsorption investigations, microflotation and batch flotation tests to delineate the nature of surface interactions of various reagents with the different mineral constituents of a typical hastnaesite type rare-earth ore. Flotation plant results are interpreted in terms of reagent adsorption mechanisms, and the role of temperature in achieving the desired selectivity has been established. The r~ults also indicate the superiority of alkyl hydroxamate collectors for the beneficiation of these types of rare-earth deposits containing barite and calcite as the typical gangue minerals.

INTRODUCTION

The demand for specific rare-earths such as yttrium, scandium, neodymium, samarium, lanthanum and cerium is expected to increase rapidly in the near future because of recent developments in the use of these elements in high-technology applications including high-intensity supermagnets, lasers and superconducting materials (Vijayan et al., 1989). More than 95% of the world's rare-earth oxides occur in the form of three minerals: monazite and bastnaesite, containing predominantly light rareearths, and xenotime consisting of the heavy rare earths and yttrium (Table 1 ). Monazite is present in the beach sands around the coastal belt of India, Brazil, Australia, South Africa, USSR and the USA. In the majority of the cases, it is produced as a by product during the processing of sands for the recovery of ~Present address: Tata Research Development & Design Centre, l, Mangaidas Road, Pune 411001, India.

0301-7516/91/$03.50

© 1991 - - Elsevier Science Publishers B.V.

3 -

PRADIP

~ , N I ) t ) . W . t:l :I R S I ' E N A I

TABLE I World reser~.es of rare-earth elements m the form of three major mineral sources {after Macke). 198~} Element

Lanthanum Cerium Praseodymium Neodymium Promethium Samarium Europium Gadolinium Terbium Dysprosium Holmium Erbium Thulium Ytterbium Lutetium Yttrium Scandium

Atomic number

Monazite (La,CeJPO4

Bastnaesite (La,Ce)FCO~

57 58 59 60 61 62 63 64 65 66 67 68 69 70 71 39 21

23 } 46 5 93% 19 3 0.1 1.7 O. 16 0.5 0.09 0.13 0.01 0.06 0.006 2 -

32 t 50 4 98.7% 13 0.5 0.1 0.15

India Brazil Australia China US

China US

Major reserves in the world

0.12 0.015 0.1 -

Xenotimc YPO 4

-

1.2 0.01 3.6 1.0 7.5 2.0 6.2 1.27 6.0 0.63 60

10.6%

89.4%

Finland Norway Sweden Malaysia Sri Lanka Thailand

other important minerals like ilmenite, rutile and zircon. The other commercially important mineral source of rare earths, bastnaesite is a rare-earth fluorocarbonate of the cerium subgroup elements. The Mountain Pass ore deposit in California, USA, and the Bautou deposit in China are two of the largest deposits of bastnaesite (Mackey, 1986 ). The purpose of the present paper is to summarize and discuss the results of an investigation on the role of various factors involved in the flotation separation of bastnaesite from the associated gangue minerals in the Mountain Pass ore.

The bastnaesite deposit at Mountain Pass, California The Mountain Pass area of rare-earth mineralization lies along and slightly east of the divide of the Mescal Range in north-eastern San Bernardino County, California, about 60 miles south-west of Las Vegas, Nevada. The rareearth mineral discovery was made in April 1949, and subsequent mapping of

FLOTATION SEPARATION OF RARE-EARTH ORES

3

the area resulted in the discovery of the bastnaesite-bearing Sulfide Queen carbonate ore body in 1950. The Sulfide Queen ore body is about 800 m long, 170 m wide and 100 m deep, indicative of substantial reserves. The detailed mineralogy of the deposit has been excellently reviewed in US Geological Survey Paper 261 (O1son et al., 1954). The ore contains, on an average, 60% carbonates (mainly calcite), 20% barite/celestite, 10% rare-earth fluocarbonates (bastnaesite) and 10% other minor minerals, including silica. Beneficiation of bastnaesite ore at Mountain Pass Under normal operation, the Mountain Pass plant of Molycorp treats 1700 tons per day of the ore producing a flotation concentrate assaying 60% REO at the Mountain Pass plant. During the time period, 1974-1983, the annual production averaged 17,000 tons of rare-earth oxides, and it has been either considerably higher or lower since that time (Molycorp, pers. commun., 1989). The flowsheet of the operation at the Mountain Pass plant of Molycorp involves a combination of froth flotation and leaching steps to achieve the desired beneficiation. The schematic flowsheet that was used before expansion of the plant is shown in Fig. 1 (Anonymous, 1975), and to illustrate procedures used in industrial bastnaesite processing, that flowsheet is presented schematically here. Crude ore, averaging 10% bastnaesite (7% REO ), is first fed to a crushing circuit, which involves two separate systems. In the first, the crude ore goes through a 76× 170 cm jaw crusher to produce a minus 100-mm product. A 16-ram vibrating screen returns the oversize to a 12-cm secondary cone crusher, with the undersize going to storage bins. The alternate crushing circuit utilizes a 120 X 140 cm impact crusher and two radial stackers to build blend piles of plus and minus 13-mm material. The undersize from the crusher circuit is conveyed to the grinding-flotation plant. Grinding is carried out in a 3.7 × 4.9 m ball mill and produces a minus 100-mesh product for flotation. This closed-circuit grinding system is backed up by a 1.8 X 3.0 m rod mill followed by 2.7 × 1.5 m ball mills as standby. The classifiers being used in the plant are 25-cm and 50-cm cyclones. The product, which is 80% minus 100 mesh, undergoes six different conditioning treatments prior to rougher flotation. Conditioning is carded out in 1.8 m diameter by 2.7-m tanks with steam being bubbled through the pulp. In the first stage, the ore slurry is mixed with soda ash (2.5-3.3 kg t - i ), sodium fluosilicate (0.4 kg t - 1) and steam. This step is followed by steam conditioning. The third stage involves the addition of ammonium lignin sulfonate (2.5-3.3 kg t - ~) in the presence of steam. The fourth, fifth and sixth stages condition the pulp with steam, steam and distilled tall oil C-30 (0.3 kg t - ~) and steam, respectively. The pH at this point is around 8.8. The slurry

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(120cmx

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310cm]

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25cm)

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L

I

ROUGHER FLOTATION Z 0

ii

I'TAILINGS (2%REO)

CLEANER-!

_-.-SCAVENGERI

CLEANER-2,3,4

l

FINAL

REGRINDMILL CONCENTRATE

(63 % REO)

Fig. 1. The beneficiationflowsheetfor bastnaesiteore at MountainPass plant. containing 30-35% solids is pumped to the rougher flotation circuit, where rougher flotation is carried out in 12 Galligher flotation cells, each with a capacity of 1.7 m 3. The rougher concentrate, assaying approximately 30% REO, is then transported to cleaner cells operating at 50% solids. Four-stage cleaning yields a final concentrate from the flotation circuit averaging 6063% REO at an overall recovery of 65-70%. The railings are recirculated within the cleaning circuit, except for the tailings from the first cleaner which go through a scavenger flotation circuit. The concentrate after scavenging is reground and sent back to the rougher cells. The tailings from the scavenger circuit are combined with the rougher tailings to yield a flotation plant tailings that average about 1-2% REO. The concentrate is then thickened, filtered, dried and packaged. To further enrich the bastnaesite, the flotation concentrate is leached with 10% hydro-

FLOTATION SEPARATION OF RARE-EARTH ORES

5

chloric acid at pH 1 in 1.8 m diameter× 2.7 m leach tanks. This process dissolves away the residual carbonates and increases the grade to 68-72% REO. After filtration with 1.8 m × 2.7 m EIMCO filters, the product may be sold as enriched bastnaesite. Leached concentrates can be further calcined in roasters to burn off the fluocarbonate content of the rare-earth minerals to produce a plus 90% REO for subsequent processing. The resulting calcine containing 90% REO is leached with HC1. The undissolved cerium is obtained as a filter cake composed of about 70% REO, more than 90% of which is cerium oxide. The separation of the rest of the rare-earth elements from the thickener overflow (assaying about 100 g 1-1 REO) takes place through liquid-liquid ion exchange. A critical study of the plant data and the flowsheet being followed at Mountain Pass indicated that the relatively lower recoveries were essentially tied up with the non-selective nature of the reagents being used in the plant (Pradip, 1981 ). The major gangue minerals to be separated from bastnaesite are barite/celestite and calcite which all have somewhat similar surface properties and all are sparingly-soluble salt minerals. The separation essentially consists of recovering elements of Group IIIB (lanthanides) from a gangue containing Group IIA elements (that is, the alkaline earths, Ca, Sr, Ba, etc. ). The process uses fatty acid collectors, which are known to be nonselective for such separations. High-temperature flotation is, therefore, resorted to in order to achieve semi-selectivity. Chelating reagents having functional groups specific to rare-earth cations as compared to alkaline-earth ions appear to offer an alternative flotation procedure. In our efforts to synthesize more selective reagent combinations for this kind of separation, namely the separation of bastnaesite from alkaline-earth carbonates/sulfates, we have found alkyl hydroxamates to be very promising. The present paper presents some of the salient features of the hydroxamate flotation of bastnaesite ore, as compared to the conventional scheme using fatty acids - lignin sulfonate - soda ash reagents. The role of other auxiliary reagents in achieving the desired selectivity is also delineated through appropriate investigations. MATERIALS AND METHODS

A bastnaesite ore sample (analyzing 24. 1% REO, 15.5% BaO and 11% CaO ) was used for Hallimond tube flotation tests. A hand-picked sample of the mineral (high-grade 'pure' bastnaesite ) analyzing 57.4% REO, 8.8% BaO and 1.6% CaO was used for the adsorption experiments. Barite and calcite mineral samples were obtained through Ward's Natural Science Establishment, New York. The samples were washed with triply distilled water and ground to the desired size fractions. The BET surface area of the ground mineral pow-

ders was found to be 1.73, 2.15, and 2.4 m: g-1 for bastnaesite, barite and calcite, respectively. Potassium octyl hydroxamate ( H X M ) was prepared in the laboratory (Pradip, 1981 ) and characterized through UV, IR and chemical techniques before use. All the other chemicals used were analytical grade reagents. The adsorption of H X M under various conditions was estimated by determining the difference in the concentration of collector in solution before and after the addition of mineral powders of known surface areas. The supernatant was centrifuged and analyzed for hydroxamate using well-known spectrophotometric techniques (Raghavan and Fuerstenau, 1975; Pradip, 1981 ). After conditioning the mineral samples for 15 min at 1% solids ( 4 8 × 6 5 mesh size), they were floated for 2 min with nitrogen gas for bubble generation at a constant flow rate of 60 cc m i n - t in a standard Hallimond tube set up (Pradip, 1981 ). For the bench-scale flotation tests on the Mountain Pass ore (assaying 68% REO), a standard procedure (Pradip, 1981; Fuerstenau et al., 1982) using a 1000-g Denver Cell was followed. The ore was first crushed in a jaw crusher and screened to pass an 8-mesh sieve. One kg of the minus 8-mesh charge was ground for 10 min in a laboratory rod mill with 600 cm 3 of water (60% solids). The size distribution of the flotation feed was 58% minus 325 mesh and 100% minus 48 mesh. The pulp (about 2 1 in volume) was then heated with gas burners while stirring. During conditioning, 25 cm 3 (2.3 kg t - ' ) of a 10% solution of soda ash and 25 cm 3 (2.3 k g t -~ ) of a 10% solution of ORZAN were added. At around 70°C, 15 drops (0.35 kg t -l ) of tall oil (C-30) was added. The pulp was allowed to boil for one minute before transferring it to the Denver laboratory cell for flotation. The grade of concentrate was determined in terms of percentage rare-earth oxide (REO). Distilled tall oil (designated as C-30) collector is essentially a mixture of saturated and unsaturated fatty acids (mostly oleic acid). ORZAN-G, used as a depressant in these experiments, is a m m o n i u m lignin sulfonate obtained from Crown Zellerbach Corporation, USA. Methyl-isobutyl carbinol (MIBC), obtained from Union Carbide Corp., was used as a frother as and when required. Electrophoretic measurements were carried out with a Zeta Meter using a finely ground suspension of each mineral (0.05% solids by weight) in distilled water. RESULTS AND DISCUSSION

Laboratory scale flotation to simulate plant data Using the standard procedure outlined in the foregoing section and starting with a flotation feed having a particle size distribution similar to that in the

FLOTATION

SEPARATION

100

OF RARE-EARTH

i

i

i

7

ORES

i

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REORECOVERY 0-'80

MOUNTAIN PASS BASTNAESITE ORE u..J Q .~">R EO

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PLANT DATA

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ROUGHER CONC.

pH 9

CLEANER-1 CONC.

CLEANER-I CONC.

8.9

8.8

CLEANER-3 CONC,

FINAL CONC.

8.6

Fig. 2. Typical results of a complete batch flotation run on the Mountain Pass ore in a laboratory Denver Cell. Plant data are also shown for comparison. SODA ASH ~- 100 Z u.I r~

ADDITION , K g / t

2

4

G

i

i

i

~ 8o ¢Y w

~ Go .i

W Q ~

20

o u,I 0

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pH Fig. 3. The effect of soda ash addition on the recovery and grade of the concentrate produced in laboratory batch flotation tests on the Mountain Pass bastnaesite ore.

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plant, we were able to reproduce the plant results as well as the major trends observed in the plant operation, during our laboratory tests ( Pradip, 1981 ). For simulating plant performance, the rougher flotation concentrate was refloated (cleaned) in an open circuit three times. The results are compared with the plant data in Fig. 2. The pH during all these tests was controlled through the addition of NazCO3 as in the plant. The effect of soda ash addition on the rougher flotation recovery and grade is illustrated in Fig. 3. Our laboratory experiments clearly demonstrated the role of soda ash addition in achieving proper selectivity. The results of more detailed investigations on this aspect ofbastnaesite flotation are discussed later in this paper.

Flotation with hydroxamate collectors Hydroxamic acid is the name given to N-alkyl derivatives of hydroxylamine represented as: H - - N --OH

R--C :0

They can also be regarded as N-hydroxy derivatives of acid amines or as oximes of carboxylic acids. Hydroxamic acids essentially contain a bidenate group, OH -N - - C

0 -

through which the metal complexation occurs as follows: ~ 0~

N]

+nil +

The stability constants for some metal hydroxamates are available in the literature (Yale, 1943; Schwarzenbach and Schwarzenbach, 1963; Rayaboi et al., 1980). The weakest complexes of hydroxamates are formed with alkalineearth metal cations (Ca 2+, Ba 2+, Sr 2÷ ) and rather strong complexes are formed with the highly charged multivalent rare-earth cations and with the transition elements such as Nb, Ti, V, Mn, Zr, Hf, Ta. The strongest complexes are formed with Fe 3+, Ta 5+ and Nb 5+. It has also been reported that differences in the stability constants of complexes formed with lattice cations of the minerals to be separated in a carbonatite ore (such as lanthanides from alkaline earth-containing gangue minerals) are much greater for hydroxamic acid-based collectors than for carboxylic acids (fatty acids), It is expected, therefore, that alkyl hydroxamates should be more selective in the beneficiation of bastnaesite ore deposits of the type that occur at Mountain Pass, California. The readers are referred to extensive reviews of hydroxamates (Fuerstenau and Pradip, 1984; Pradip, 1987 ) and chelating reagents in general (Somasun-

9

FLOTATIONSEPARATIONOF RARE-EARTHORES

daran and Nagaraj, 1984; Nagaraj, 1987; Pradip, 1988) for more detailed discussion on the topic.

Hallimond tube flotation Microflotation experiments on pure barite and calcite (as the representative gangue minerals) and bastnaesite minerals were performed using potassium octyl hydroxamate reagent in order to evaluate its efficacy as a selective collector for Mountain Pass ore. Figure 4a shows the results of the Hallimond tube flotation experiments with calcite, barite and bastnaesite as a function of hydroxamate concentration. It is evident from the results given in this figure that hydroxamate collectors are indeed specific to the rare-earth mineral. Adsorption isotherms for the same three minerals, as shown in Fig. 4b, indicate the basis of selectivity. Barite is the least responsive to hydroxamate whereas adsorption at the bastnaesite/water interface is the highest. In the concentration range used in this study, the adsorption density at the plateau for bastnaesite is several times ,_1OOz

'

(a) ¢

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Fig. 4. (a) Hallimond tube flotation recoveries as a function of the initial concentration of hydroxamate collector; and (b) adsorption isotherms of hydroxamate as a function of the equilibrium concentration.

l I)

P R A D I P A N D I).W. FI :I::RS I ['N ~\L

larger than that for barite and calcite and corresponds to a mechanism involving multilayer adsorption. On the other hand, on barite and calcite the adsorption density at the plateau corresponds to a horizontal and vertical monolayer, respectively. Relatively weaker interactions of hydroxamates with Bacations on the surface are not sufficient to achieve a close-packed vertical configuration with hydroxamate groups chelated to the surface and their hydrocarbon chains oriented away from the surface, as is possible in the case of calcite (Pradip and Fuerstenau, 1983 ). Hydroxamate adsorption seems to occur through the metal-hydroxy complexes of the constituent metal ions on the mineral surface (Pradip, 1981; Fuerstenau and Pradip, 1984; Fuerstenau et al., 1988; Das and Pradip, 1988 ). The peak of pH 9 is, however, observed for all mineral systems. This peak is attributed to the presence of highly surface active association complexes of neutral and anionic hydroxamate species around pH 9 (pKa of hydroxamate ) which adsorb strongly at the mineral/water interface (Das and Pradip, 1988 ). When the flotation tests were carried out as a function of pH, a characteristic maximum in flotation recovery was observed around pH 9 for all three minerals (Pradip, 1981: Fuerstenau and Pradip, 1984). Flotation recoveries at a hydroxamate concentration of 3X 10 - 4 mol 1-1 are shown in Fig. 5, along with those of a high-grade ore sample (assaying 24% REO ), also from Mountain Pass. At this concentration, barite does not respond at all to hydroxamate flotation whereas bastnaesite exhibits a plateau in the pH range 5-9 and calcite a minimum around pH 8.2. The results from these experiments with pure minerals predict that the most selective flotation separation would occur at this collector concentration and pH 8.2, provided there is complete liberation of the ore at the particle size fractions used for the tests. tO0 PURE BASTNAESITE

uJ

K-OETYLHYDROXAMATE/ ~3xlo-4_M / 48x65 M ES~./

u he" gO

n ~

60

> 0 u.I ~

40

BASTNAESITE/i

2

3

4

5

6

7

8

9

10

11

12

pH Fig. 5. The effect of pH on the Hallimond tube flotation of barite, bastnaesite, calcite and bastnaesite ore in presence of 3 × 10-a M potassium octyl hydroxamate.

FLOTATION

SEPARATION

OF RARE-EARTH

ORES

1 1

Using a sample of bastnaesite ore, the concentrates produced by Hallimond tube flotation analyzed 15.1, 29.6 and 26.7% REO at pH 5.9, 8.1 and 9.7, respectively, confirming the above inference. It should be pointed out that bastnaesite is not liberated completely at 48 × 65 mesh, the size fraction used in these tests (Pradip, 1981; Fuerstenau et al., 1980). Batch flotation tests on Mountain Pass ore

Based on the encouraging results obtained with the Hallimond tube microflotation tests, the Mountain Pass ore assaying 7-8% REO was subjected next to bench-scale batch flotation tests in a standard Denver Laboratory flotation cell. These tests were carried out under conditions essentially identical to those being utilized in the plant, that is, using a reagent scheme consisting of fatty acid as collector (0.35 kg t - ~) soda ash (2.3 kg t - l ) as pH modifier and lignin sulfonate (2.3 kg t - ~) as depressant. Since high-temperature flotation is necessary for this system, both collectors (fatty acids and hydroxamates) were investigated under these conditions and the results are presented in Fig. 6. These results indicate that potassium-octyl hydroxamate is certainly more selective: higher grades at comparable recoveries can be achieved at much lower conditioning temperatures. More detailed optimization with respect to such operating parameters as pH, temperature and the dosage of collector and depressant is necessary in order to achieve optimal flotation performance using hydroxamate collector.

Effect of temperature on hydroxamate adsorption at mineral~water interface Detailed investigations aimed at elucidating the mechanism for the selective flotation of bastnaesite from calcite and barite indicate that the primary Z u.J t~ uJ Q,..

l

100

r

i

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8O

40

iI #

~ Ga

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°C

Fig. 6. The effect of temperature on rougher flotation: recovery and grade of bastnaesite ore sample using fatty acid ( - - ) and hydroxamate collectors ( . . . . ). (Laboratory scale batch flotation tests).

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PRAI)IP 4NI) I).W. Pl [~RSTENAI!

FABLE 2 T h e r m o d y n a m i c parameters for the adsorption of hydroxamatcs Mineral

Barite Calcite Bastnaesite

JG°aos at 298 K ( k J m o l --t)

JHi/ads (kJmol

- 26 -28 - 57

20 45 187

I)

i,~'Oad~ (kJrnot

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reason for selectivity is the selective adsorption of collectors at the different mineral surfaces, especially at elevated temperatures (Pradip and Fuerstenau, 1985). Alkyl hydroxamates, even though more selective than fatty acids at room temperature, become even more selective at higher temperatures. Though hydroxamates chemisorb on all three minerals (bastnaesite, calcite and barite ), they do so with different free energies of adsorption (Pradip and Fuerstenau, 1983 ) and hence have the greatest affinity for bastnaesite. With an increase in temperature, the adsorption (and hence flotation recovery) increases for all three minerals, but again disproportionately for bastnaesite as compared to calcite and barite. In the range of temperatures used for investigations (21 °-61°C), the enthalpies and entropies of adsorption calculated using the Clausius-Clapeyron equation are tabulated along with the standard adsorption free energy for the three minerals in Table 2 (Pradip and Fuerstenau, 1985 ). It is interesting to note that the enthalpies and entropies of adsorption are both positive, indicating endothermic reactions at the surface. Moreover, the extraordinarily high heat of adsorption on bastnaesite is perhaps the reason why elevated temperatures help bastnaesite ore flotation. Both the hydroxamate adsorption and the flotation behaviour of fine particles become more selective at elevated temperatures as a consequence of large enthalpies involved in the adsorption reactions.

Role of auxiliary reagents in bastnaesite flotation The addition of soda ash and lignin sulfonate has been found essential in the flotation of Mountain Pass ore both with fatty acids and hydroxamate flotation reagents as collectors. The role of these reagents was investigated through electrokinetic studies.

Soda ash addition Sodium carbonate addition in the flotation circuit not only increases the alkalinity of the pulp but also controls the carbonate equilibria in solution.

FLOTATION SEPARATIONOF RARE-EARTH ORES

13

Bastnaesite and calcite both have carbonate ions as one of the constituents of their crystal lattice. This indicates that besides controlling the pH, carbonate ions act as potential-determining ions for these minerals. Furthermore, all the major mineral constituents of the ore - barite, celestite, bastnaesite and calcite - are sparingly soluble in water and aqueous carbonate equilibria should play an important role in controlling the adsorption of reagents at the solid/ liquid interface. Calculations of the equilibrium concentration of the various carbonate species in water show that the dominant carbonate species in solution at pH 9 (the pH at which flotation is carried out in the plant) are HCO~- ions. Electrokinetic experiments with pure minerals were conducted in sodium carbonate solutions to study the role of soda ash in controlling the surface charge. In the presence of sodium carbonate, the surface charge on bastnaesite is reversed at around 1 0 - a M Na2CO3 concentrations, with carbonate ions acting as the potential-determining anions in this case. Calcite remains positively charged at pH's in the range 7-11 in the presence of NaNO3 as well as Na2CO3. Even when the concentration of sodium carbonate is as high as 10- 3 M, the surface charge on calcite is not reversed. The results suggest that under flotation conditions, calcite is essentially positively charged. Barite, which is normally positively charged, exhibits some interesting behavior when sodium carbonate is added to the system. Detailed studies were carried out to measure the electrophoretic mobility and equilibrium pH for barite suspensions as a function of sodium carbonate concentration. A sharp change in electrophoretic mobility around 10-3 M Na2CO3 indicates the onset of a surface reaction. Thermodynamic calculations suggest that under these conditions the barite surface is converted to barium carbonate. From bulk solution equilibria, we can write at 20 ° C: BaSOa(s) + HCO~- (aq) =BaCO3(s) + H ÷ (aq) +SO 2- (aq) K = [SO42- ] [H+ ] / [ n c o ~ - ] BaSO4(s) = B a 2÷ +SO42- ; K=8.7 × 10 -11 If sulfate ions are contributed only because of the solubility of barite, we arrive at the following equation describing the BaSO4/BaCO3 equilibrium: pH = 5.72 - log [ HCO~- ] From this equation, it can be shown that in solutions of 10 -3 M NaeCO3, at about pH 9 and above, BaCO3 is the more stable species. Figure 7 presents the electrophoretic mobility curves for BaCO3 and BaSO4 suspensions as a function of pH as regulated through additions of soda ash. These results indicate that at a soda ash concentration of 10- 3 M, the surface is completely transformed to BaCO3 at pH 10. Soda ash thus plays an important role in determining surface properties of barite. It is necessary to recog-

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2

pH

Fig. 7. The electrophoreticmobility of barite and barium carbonate as a function ofpH in presence of sodium carbonate. nize that these conditions where the barium sulfate/barium carbonate surface conversion occurs also corresponds to conditions similar to those encountered in plant practice. Furthermore, the fact that AH ° for this reaction is positive ( + 7.18 kcal m o l - ~) suggests that at higher temperatures the conversion of BaSO4 to BaCO3 at the surface would be favored. Further studies are needed to investigate whether soda ash, in any way, has a detrimental effect on barite depression. Detailed experiments were conducted to delineate the effect of sodium carbonate additions on the electrokinetic behavior of these three minerals and on the equilibrium pH of solutions in contact with the minerals. Figure 8 summarizes the results of these experiments. Carbonate ions cause the zeta potential (electrophoretic mobility) of bastnaesite to become negative in the regular fashion, characteristic of a potential-determining ion. It is clear from Fig. 8 that at approximately 10-3 M sodium carbonate, the barite surface undergoes a sharp charge reversal. If this were used as the only means for the control of barite flotation, one would expect that the adsorption of fatty acids on bastnaesite under these conditions would also be unfavorable since bastnaesite is highly negatively charged. The flotation recovery of bastnaesite is indeed observed to decrease above pH 9,5 at higher dosage of soda ash, as illustrated in Fig. 3. This observation is also consistent with the plant experience at Mountain Pass.

Lignin sulfonate depressant Using the data given in Figs. 7 and 8 one can replot the results as a function of pH in presence of soda ash (see Fig. 9 ). The gangue minerals namely cal-

FLOTATION SEPARATION OF RARE-EARTH ORES ÷4

(a)

I

~

I

I

15

I

+3

q...~ .1~1 + 1

O~

~,

~CALCITE

0 .... BARITE

g -1

uJ

i ~ 1

-2

-3 12

(b)

I

I

]

i

i

i

BASTNAESITE I f

I

iiii 10

Q. X

2

9

7

-400 MESH 0.05 % SOLIDS

S

S

I

L

~

I 0 "G

10 - 5

10 - z

I i0 -3

I

I

I0 -2

10 -1

SODIUM CARBONATE CONCENTRATION,MOLE/LITER

Fig. 8. The electrophoretic mobility (a) and equilibriumpH (b) of calcite, barite and bastnaesite suspensions (0.05% solids) as a function of sodium carbonatein solution. cite and barite are positively charged at the pH of flotation (around pH 9) and hence for flotation with anionic collectors like fatty acids and hydroxamate, an efficient depressant is also required. Amongst a number of inorganic and organic depressants which were investigated on Moufitain Pass ore, oak extract and lignin sulfonate were found to be most efficient (Fuerstenau et al., 1980). It is important to note that lignin sulfonate, again, is an anionic reagent but the surface charge characteristics of the ore are favorable for depressant adsorption, that is, electrostatic interactions favour adsorption of the anionic depressant on gangue minerals, as will be discussed later. Since the lignin sulfonate depressant is presently being used in the plant, its role in bastnaesite ore flotation was investigated in more detail. Lignin sulfonates, a by-product of the sulfite pulping of wood in the paper industry, are formed when the material binding cellulose (called lignin ) is made water soluble through the introduction of sulfonate groups. Lignin is not a well defined

I(~

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I 6

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CALCITE

•

BASTNAESITE

I 7

I 8

i 9

110

1

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pH

Fig. 9. The electrophoretic mobility of the principal constituents of Mountain Pass ore as a function ofpH at 1.0 mM Na2CO3 concentration.

organic compound but made up of several known building blocks (Pradip, 1981). The adsorption of water-soluble lignin sulfonates on various solids such as kaolin and rutile has been reported in the literature, and the results have been interpreted in terms of the contribution of electrostatic and hydrogen bonding forces to the adsorption energy. It is well established (Browning, 1964, 1975 ) that the surface activity of lignin sulfonates is profoundly affected by the hydrogen bonding (phenolic and carboxylic ) groups present in the molecule. The fact that lignin sulfonate adsorbs very poorly on graphite surfaces, which are notably devoid of hydrogen bonding groups but adsorbs quite well upon carbon blacks containing oxygen acid residues lends credence to the hypothesis that adsorption of lignin sulfonate occurs primarily through hydrogen bonding. Furthermore, it is interesting to note that lignin sulfonates are very good depressants precisely because of these polar groups. A naturally hydrophilic surface such as that of silica, remains hydrophilic even when its surface is completely covered with adsorbed lignin sulfonate (Kirchberg, 1968). An electrokinetic study was conducted to investigate the effect of lignin sulfonate on the surface properties of the various constituents of the Mountain Pass ore. Figure 10a shows the electrokinetic behavior of various minerals in the presence of ORZAN-G (the commercial name of this lignin sulfonate). With increasing lignin concentration, calcite and bastnaesite exhibit similar behaviour whereas barite becomes increasingly negatively charged with the mobility (negative zeta potential ) exhibiting a maximum. It is interesting

FLOTATION SEPARATION OF RARE-EARTH ORES +1

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OF ORZAN-G,g/Liter

Fig. l 0. The electrokineticbehaviour of barite, calcite and bastnaesite in the presence of ammonium lignin sulfonate (ORZAN-G): (a) the electrophoreticmobility and (b) the equilibrium pH as a function of ORZAN-G concentration in solution. The curve for equilibrium pH as a function of ORZAN-G concentration with no mineral in suspensioncoincideswith that of barite. The correspondingexperimentalpoints are not shown in the figurefor sake of clarity. to note that all the minerals are negatively charged as a result of the adsorption of a m m o n i u m lignin sulfonate. Figure 10b shows the equilibrium pH of suspensions of the various minerals in the presence of lignin sulfonate. The effect of lignin sulfonate on the solution pH in the presence of barite is the same as that without solids, but in the presence of either bastnaesite or calcite the solution pH is shifted about 2 pH-units higher. This possibly results from both calcite and bastnaesite being basic in nature, that is, having PZC's in the alkaline pH range. Another point to note from this figure is that the addition of lignin sulfonate makes the solutions more acidic, which suggests that higher dosages of lignin sulfonate would require higher consumption of soda ash in the plant at the same pH. To simulate conditions somewhat similar to those in the plant, electrokinetic potentials were determined for three different conditioning procedures: ( 1 ) namely, with lignin and soda ash at room temperature; (2) conditioning with lignin and soda ash at the boiling temperature and then cooling for measurements at room temperature; and (3) lastly conditioning with lignin sulfonate only at high temperature. The results are summarized in Tables 3 and 4. At 10 -4 M Na2CO3 (pH 9.1-9.5) the results show that adsorption occurs on all the minerals (since the zeta potentials are negative in all cases) though barite exhibits special affinity for lignin sulfonate. Heating with Na2CO3 does not significantly affect the results. As a matter of fact, values of the measured zeta potentials are very similar under all three conditions. This result suggests that selectivity in the system is not introduced as a result of selective adsorption of lignin sulfonate at higher temperatures. Lignin sulfonates are selective for barite even at room temperatures under these conditions, perhaps because

][q

PRAI)IPANI,~I).W [:[:IIRSIEN,k[

FABLE 3 Zeta potentials at the mineral/water 10.2 Mineral

Without soda ash pH

Bastnaesite

Barite Calcite

i n t e r l a c e in t h e p r e s e n c e o f 0.01 g 1 ~ l i g n i n s u l l b n a l e at p H 10. I --

7.6 5.9 9.4

zeta potential (mY)

- 21 - 17 - 21

With 10 3 Msoda ash, pH 10.1-10.2 Zeta potential (mV) cold Na2CO 3 cold lignin

N a 2 C O 3 boil lignin boil

cold Na2CO3 l i g n i n boil

- 33 - 40 - 28

- 31 - 27 - 26

- 28 - 39 - 27

TABLE 4 Zeta 9.5

potential at the mineral/water interface in the presence o f 0.01 g 1 ~ lignin sulfonate at p H 9 . 1 -

Mineral

Without soda ash pH

Bastnaesite Barite Calcite

7.6 5.9 9.4

z e t a potential (mV)

-21 - 17 - 21

With 1 0 - 4 M s o d a a s h , p H 9 . 1 - 9 . 5 Zeta potential (mV) cold Na2CO3 c o l d lignin

NazCO3 boil

lignin boil

cold NazCO3 lignin b o i l

-23 - 32 - 21

- 19 - 27 - 23

-23 - 28 - 15

of the highly positive surface of barite at alkaline pH's compared to bastnaesite and calcite. The results given in Table 3 are for a higher dosage of soda ash. An interesting observation is that, contrary to what we expect, anionic lignin sulfonate adsorption increases with increased addition of Na2CO3 (at pH's higher than about 10). Again, heating does not play an important role. These results, however, do explain qualitatively the observation that with increased addition of soda ash, barite is depressed more and hence the grade is better. Also, barite is successfully depressed in the actual flotation circuit. The mechanism of lignin adsorption is not really very clear. There seems to be some specific interactions in addition to electrostatic forces, and a detailed adsorption study is worth pursuing to delineate the mechanism of adsorption in this system.

Role of temperature in bastnaesite flotation The effect of temperature as a parameter in ore flotation has not been systematically studied in detail, although for some ores it has long been realized that selectivity and product grade can be improved by conditioning or con-

FLOTATION SEPARATIONOF RARE-EARTHORES

19

ducting flotation at moderately or substantially elevated temperatures. The flotation of hematite and fluorite at certain plants is carried out at elevated temperatures (Kulkarni and Somasundaran, 1977). In chemisorbing collector systems, where the adsorption of reagent takes place through specific chemical interactions, adsorption is generally enhanced by an increase in the temperature of conditioning. The influence of temperature is, however, more complex in flotation since various other phenomena at the solid/liquid, solid/ gas and liquid/gas interfaces are all affected simultaneously by temperature. It is very clearly established by our investigation (Pradip, 1981 ) that temperature is the key to bastnaesite flotation. Some of the possible factors responsible for selectivity in flotation of Mountain Pass ore are discussed in this section. One effect of heating the pulp is perhaps to clean the surface of the various minerals and thus enhance the selective adsorption of reagent. Flotation experiments were designed to test this hypothesis. Table 5 compares the results of floating the ore with fatty acid collector and lignin sulfonate depressant under three conditions: ( 1 ) room temperature flotation; (2) flotation at room temperature after boiling the pulp without reagents followed by the addition of reagents at room temperature; and (3) the normal practice of conditioning and floating at high temperatures. The results very conclusively demonstrate that conditioning of the ore alone at high temperature does not affect the flotation. The behavior is strikingly similar to that for flotation at room temperature without such treatment and is very different from the third case. Another point to note from the table is that barite is successfully depressed with the combination of reagents presently being used at the Mountain Pass operation, whereas calcite is not. Further, if the effect of temperature is primarily in the conditioning step, then the selectivity is introduced in the system because of the selective adsorption of reagents at higher temperaTABLE5 Effect of high temperature conditioning on the flotation separation of bastnaesite Results

Treatment Heating without reagents to boiling, then addition of reagents at room temperature and flotation

High temperature flotation: heating with reagents to l O0°C and flotation

Cold flotation at room temperature

REO recovery (%)

87.7

86.6

90

Grade (%) REO BaO CaO SrO

11.0 14.5 23.0 2.7

33.4 5.3 21.0 3.1

11.0 15.2 23.0 2.7

2()

PR,M)IP k N I ) l ) . W II, I[!I~,SIEN,\I

tures, compared to nonselective adsorption at room temperature. In the previous section, the role of temperature in the adsorption oflignin sulfonate was discussed. Our results indicate that though lignin sulfonate functions as a selective depressant for barite, it does not become significantly more selective at higher temperatures. This then leaves the only possibility that it is really the selective adsorption of fatty acid collectors (alternatively, even more selective hydroxamate collectors ) which contributes to the selective separation ofbastnaesite at elevated temperatures. Heating perhaps increases the rate of collector uptake by bastnaesite in comparison to barite and calcite similar to what was demonstrated in case of hydroxamate collectors (Pradip and Fuerstenau, 1985 ). Flotation at elevated temperature involves heating the pulp which contains both gangue and valuable minerals in aqueous suspension and obviously this is an energy expensive proposition. As illustrated in Fig. 6, we have investigated the effect of conditioning temperature for both collector systems, namely conventional fatty acids and the proposed hydroxamate collectors, on the flotation of Mountain Pass ore. The auxiliary reagents namely lignin sulfonate and soda ash were also added similar to the plant practice. The results seem to suggest that with fatty acids flotation, optimum temperatures may be around 70°-75 °C only rather than boiling temperatures. There is even a slight drop in concentrate grade observed during rougher flotation above 75 °C. With hydroxamate collectors also, the temperature has been observed to play a beneficial role on the recovery as well as the grade of bastnaesite concentrate. It is important to recognize, however, that hydroxamates are not only more selective than fatty acids at room temperature but remain so even at elevated temperatures (see Fig. 6 ). SUMMARY AND CONCLUSIONS

The flotation of a complex ore containing bastnaesite, barite, calcite and other sparingly soluble alkaline-earth sulfates and carbonates is extremely complex. Such an ore is processed at Mountain Pass, California, using a multistage conditioning procedure involving soda ash, lignin sulfonate and steam with a fatty acid collector. This practice essentially separates carbonates from sulfates. The process uses boiling temperature during conditioning, and this is the key factor to achieve selective separation with fatty acids as collectors. Under flotation conditions, calcite is positively charged, bastnaesite negatively charged, and barite is in the region where its surface is being converted to carbonate. Proper control of sodium carbonate addition is critical in controlling the surface charge of barite and hence its flotation behaviour. Excess carbonate will depress the flotation of bastnaesite. The additions of lignin sulfonate is crucial since it depresses barite quite well. It though adsorbs on all the three minerals, its depressing action on calcite is not significant. High

FLOTATIONSEPARATIONOFRARE-EARTHORES

21

temperature also does not make lignin sulfonate more selective in the depression of calcite and barite. The selectivity in the present system is achieved mainly through the selective adsorption of the collector (fatty acids or hydroxamates) at higher temperatures. Hydroxamate collectors are certainly more selective than fatty acids in separation of bastnaesite (a rare-earth mineral) from associated gangue containing alkaline earth carbonate and sulfate minerals like calcite, barite and celestite. ACKNOWLEDGEMENTS

The authors are grateful to MolyCorp, USA and the National Science Foundation for the support of this research. Pradip acknowledges the University of California for fellowship support during the course of these investigations and the Department of Science and Technology, (DST), India for financial support at Tara Research Development and Design Centre, Pune, where this paper was written. REFERENCES Anonymous, 1975. Mountain Pass operations. Molycorp Overview No. 36, 12 pp. Browning, W.C., 1964. Adsorption of lignin sulfonates. In: Proc. IV Int. Congr. Surface Active Substances, Brussels. Gordon and Breach, London, pp. 141-154. Browning, W.C., 1975. Adsorption of lignin sulfonates. Polymer Symposium, No. 28, 1975, pp. 109. Das, K.K. and Pradip, 1988. Flotation of oxidized chalcopyrite with hydroxamate collectors. In: Y.A. Atria, B.M. Moudgil and S. Chander (Editors), Interfacial Phenomena in Biotechnology and Materials Processing. Elsevier, Amsterdam, pp. 305-316. Fuerstenau, D.W. and Pradip, 1984. Mineral flotation with hydroxamate collectors In: M.J. Jones and R. Oblatt (Editors), Reagents in the Mineral Industry. I.M.M., London, pp. 161168. Fuerstenau, D.W., Pradip and Khan, L.A., 1980. Final Progress Report. Submitted to MolyCorp, USA, 102 pp. Fuerstenau, D.W., Pradip, Khan, L.A. and Raghavan, S., 1982. An alternative reagent scheme for the Mountain Pass rare-earth flotation plant. In: Proc. XIV Int. Miner. Process. Congr., Toronto, IV6: 1-12. Fuerstenau, D.W., Urbina, R.H. and Laskowski, J.S., 1988. Surface properties and flotation behaviour of chrysocolla in the presence of potassium octy! hydroxamate. In: S. Castro and J. Alvarez (Editors), Froth Flotation. Elsevier, Amsterdam, pp. 245-260. Kirchberg, H., Bilsing, U. and Schulze, H., 1968. Freiberg Forschungsh. A, 437: 7-22. Kulkarni, R.D. and Somasundaran, P., 1977. Effect of reagentizing temperature and ionic strength in hematite flotation. Trans. AIME, 262: 120-125. Mackey, T.S., 1986. Recent developments in USA rare-earth technology. In: P. Somasundaran (Editors), Advances in Mineral Processing. AIME, New York, Chapt. 30, pp. 509-533. Nagaraj, D.R., 1987. The chemistry and application of chelating or complexing agents in mineral separations. In: P. Somasundaran and B.M. Moudgil (Editors), Reagents in Mineral Technology. Marcel Dekker, New York, NY, pp. 257-334. Olson, J.C., Shawe, R.D., Pray, L.C. and Sharp, W.N., 1954. US Geol. Surv. Pap. No. 261, 75 PP.

~

PRADIP AND | ) . W . ~:1 :ERSI [iNAI.

Pradip, 1981. The Surface Properties and Flotation of Rare-Earth Minerals. Ph.D. Dissertation, Univ. California, Berkeley, CA, 211 pp. Pradip, 1987. Surface chemistry and applications of alkyl hydroxamatc collectors in mineral flotation. Trans. Indian Inst. Metal., 40 (4): 287-304. Pradip, 1988. Applications of chelating agents in mineral processing. Miner. Metall. Process., 5(2): 80-89. Pradip and Fuerstenau, D.W., 1983. The adsorption of hydroxamate collectors on semi-soluble minerals, part I: adsorption on barite, calcite and bastnaesite. Colloids Surf., 8:103-119. Pradip and Fuerstenau, D.W., 1985. The adsorption of hydroxamate collectors on semi-soluble minerals, part II: effect of temperature on adsorption. Colloids Surf., 15:137-146. Raghavan, S. and Fuerstenau, D.W.. 1975. The adsorption of aqueous octyl hydroxamate on ferric oxide. J. Colloid Interface Sci., 50: 319-330. Rayaboi, V.I., Shenderovich, V.A. and Strizhev, E.F., 1980. Protolytic dissociation ofalkyl hydroxamic acids and their derivatives. Russ. J. Phys. Chem., 54( 5 ): 730-731. Schwarzenbach, G. and Schwarzenbach, K., 1963. Hydroxamatkomplexe I. Die Stabilit~t der Eisen (III)-Komplexe einfacher Hydroxamsauren und der Ferrixoamins B. Helv. Chim. Acta, 46: 1390-1408. Somasundaran, P. and Nagaraj, D.R., 1984. Chemistry and applications of chelating agents in flotation and flocculation. In: M.J. Jones and R. Oblatt (Editors), Reagents in the Minerals Industry. I.M.M., London, pp. 209-219. Vijayan, S., Melnyk, A.J., Singh, R.D. and Nuttall, K., 1989. Rare-earths: their mining, processing and growing industrial usage. Min. Eng., 41: 13-18. Yale, H.L., 1943. The hydroxamic acids. Chem. Rev., 33: 209-256.