Concentration and Separation of Rare Earths from Bastnasite

The separation of pure, individual rare earths from complex ores has presented extractive chemistry with one of its greatest challenges. During the first part of the 20th century, the only technique available for separating rare-earth mixtures was fractional crystallization. In one instance a rare-earth researcher was said to have devoted several years of his life to carrying out 20, 000 successive fractional crystallizations in order to obtain pure praseodymium.

Less tedious column ion-exchange techniques were developed based on pioneering work at the Institute for Atomic Research at Ames, Iowa. Several research organizations extended this work and developed successful ion-exchange processes for producing pure, individual rare-earth compounds. Ion exchange became the standard technique in the rare-earth industry for producing high-purity compounds; however, a desire for increased throughputs and lower processing costs has stimulated interest in other separations techniques to augment ion exchange or, in certain cases, to replace it.

Liquid-liquid ion exchange or solvent extraction techniques possess advantages of high throughput and ease of control and have been exploited to an increasing extent for’metal recovery and separations. Certain of the rare-earth elements have been recovered successfully by solvent extraction, and the technique is potentially capable of economical extension to separation of all of the rare earths. To date, no one solvent system has proven applicable for all rare-earth separation problems, and a variety of techniques must be com¬pared and evaluated for any given situation.

The purpose of this paper is to demonstrate and compare various techniques for recovering and separating rare earths from the mineral bastnasite, with emphasis on solvent extraction. Bastnasite, a fluocarbonate of the light-group rare earths, is an important source of rare-earth elements.

Summary and Conclusions

Techniques are available for recovering and separating all of the light-group rare earths contained in bastnasite. High-purity cerium can be recovered by fractional precipitation, fractional leaching, or solvent extraction. A subsequent solvent extraction operation with a primary amine will produce 95 percent La2O3 at 96 percent recovery. Europium is amenable to recovery by an organophosphate extraction, followed by selective leaching and reductive precipitation. An alternate procedure is to selectively extract the europium with an amine, after which the europium is recovered and purified by a reductive strip. Separation of the remaining rare earths is more difficult, but solvent extraction techniques have been developed which provide separation factors of 3. 8 for praseodymium-neodymium and 10.5 for neodymium-samarium.

Based on the general nature of the separation processes investigated and the large number of extractants and chelating agents available, it is reasonable to assume that existing techniques can be modified to produce pure rare-earth compounds from any source material.

Results and Discussion

Generally, a 60 to 70 percent rare-earth concentrate is prepared from bastnasite ore by crushing and grinding the ore to minus 100 mesh, followed by froth flotation. The resulting concentrate is the feed material for further acid leaching and separation operations. A variety of techniques are available for processing the concentrates.

Reacting the concentrate with NaOH will produce mixed rare-earth hydroxides, which can be heated to oxidize the cerium to the tetravalent state, whereupon the trivalent rare earths can be leached from the cerium with dilute mineral acid. Another method is to roast the concentrate prior to direct acid leaching to remove some fluorine and CO2 as well as to convert some of the cerium to the tetravalent state. Both of the above processes produce an impure cerium product. If a higher quality cerium product is desired, it is frequently advantageous to bring all of the rare-earth values into solution. This can be accomplished by dissolution in strong acid or by fusion with (NH4)2SO4. Direct acid dissolution with sulfuric acid is probably the cheapest method, but fusion with {NH4)2SO4 also converts the rare earths to sulfates with high yield. A roasting step subsequent to the fusion, followed by a water leach, provides rare-earth solutions substantially free of non-rare-earth impurities.

After solubilizing all of the rare earths, the cerium can be recovered in purified form by fractional precipitation or solvent extraction. Either method requires that the cerium be oxidized to the tetravalent state. In one precipitation technique, ozone is used to oxidize the cerium and precipitate it as CeO2. Ozone oxidation-precipitation is capable of producing 98-percent pure CeO2 at 98 per-cent recovery in one step. CeO2 purity can be upgraded to 99.9 per-cent by a secondary ozonation of the redissolved 98-percent pure product. For solvent extraction recovery of cerium from solution, oxidation can be achieved either chemically by reagents, such as sodium bromate or potassium, dichromate, or electrolytically. The latter technique is advantageous in that contaminants or undesirable reduction products are not introduced into the system, Solvent extraction is useful particularly because it is adaptable to continuous, counter-current operation.

One of the most useful extractants for recovering tetravalent cerium from solution is TBP, tri-n-butyl phosphate. This particular extractant is highly selective for tetravalent cerium over the trivalent rare earths, and also has desirable physical properties. The TBP is dissolved in a hydrocarbon diluent, primarily to lower the specific gravity of the organic phase. Ammonium nitrate is added to the feed solution to form the extractable hexanitrato cerate species. Figure 1 shows the effect of organic phase TBP concentration on extraction and product purity when using toluene as the diluent on a separatory funnel basis. It is evident that the highest product purity is obtained at the lowest TBP concentration, although at a sacrifice in recovery.

In practice it would probably be advantageous to operate at 30 percent TBP concentration, or higher, and to increase final product purity by selective scrubbing. It was found that either water or a dilute (5 percent) HNO3 solution was an effective scrubbing agent; however, dilute acid is preferred because less tetravalent cerium is removed from the loaded organic phase. For example, in one system in which the ceric nitrate was 96 percent pure in the organic phase, a scrub with 5 percent HNO3 at a scrub ratio of 2 (2 aqueous to 1 organic) yielded 99.9 percent ceric nitrate at 90+ percent recovery. Higher scrub ratios had little effect on either product purity or recovery. The tetravalent cerium can be recovered from the organic phase by stripping with H2O, H2O2 or dilute H2SO4. The H2SO4 is the most satisfactory stripping agent, and Figure 2 shows that a strip ratio of 1, stripping efficiency increases rapidly up to a strip concentration of 20 percent H2SO4. Using a stripping ratio of 2, on the other hand, cerium recovery was better than 99 percent using 10 percent H2SO4.

To investigate the cerium extraction system further, a continuous extractor was assembled using 6-inch-diameter, unbaffled extraction and scrub columns, and a 2-inch-diameter unbaffled strip column. Using a feed solution containing 50 g/l of total rare earths, 10 percent TBP in benzene, a water scrub column and 10 percent H2SO4 strip solution, resulted in only 55 percent recovery of 99.4- percent pure cerium. This low recovery figure was found to be the result of a large amount of cerium being removed in the scrub column. Reducing the scrub column diameter from 6 to 2 inches increased recovery to 91 percent at 99.5 percent CeO2 purity. Finally, the scrub column was eliminated from the system, and under these conditions cerium recovery was 98.5 percent at 99.5 percent purity. This was accomplished with a piece of equipment having but one stage of extraction and one stage of stripping. If the process were carried out in a few countercurrent stages, both purity and recovery should easily surpass 99.9 percent.

To summarize, three possibilities have been discussed for cerium recovery: (1) Differential leaching of trivalent rare earths from the less soluble tetravalent cerium with acid, (2) dissolution of total rare earths followed by ozone oxidation-precipitation of the cerium, and (3) dissolution of total rare earths followed by extraction of ceric nitrate with TBP. The choice of which process to use depends on the purity and form of the desired end product as well as on the relative process costs.

After-cerium removal, lanthanum is the dominant rare earth remaining in the mixture. One traditional method for lanthanum separation is to take advantage of the high basicity of lanthanum as com¬pared with that of the heavier rare earths. This involves simply raising the pH of the rare-earth solution, usually with ammonia, aqueous ammonia, or sodium hydroxide, thus precipitating the rare earths heavier than lanthanum. Careful control of the pH in this technique will produce lanthanum of better than 90 percent purity, but the yield rarely exceeds 75 percent.

Another precipitation technique is based on the use of ozone to precipitate cerium and the heavier rare earths at an elevated temperature. By this means a filtrate containing 90 percent of the lanthanum at 95 percent purity is obtained. If a higher recovery figure is desired, a multistage solvent extraction system, using a primary amine extractant, should be considered. Primary amines preferentially extract the lighter rare earths from sulfate solutions; however, the selectivity between adjacent elements is not great. Figure 3 shows the relative distribution coefficients of the lighter rare earths in a system using a commercially available primary amine (Primene JM-T) in conjunction with a bastnasite sulfate feed solution. Lanthanum separation in this system will not be efficient even if the cerium is removed in a prior step. The solvent system requires modification to provide higher separation factors, and this is most easily accomplished by addition of a chelating agent such as diethylenetriaminepentaacetic acid (DTPA). The use of DTPA in the system is advantageous because the selectivity sequence of DTPA for the rare earths is opposite to that of the primary amine. Figure 4 shows the addition of DTPA to the system serves to increase the difference between the individual distribution coefficients compared to the results in Figure 3. The beneficial effect is most evident for the pair lanthanum and cerium, where the separation factor is 5.0 at 40 percent chelation with DTPA.

Lanthanum-praseodymium separation factors as high as 20 are obtained. With separation factors of this magnitude, it should be possible to obtain a purified lanthanum fraction from cerium-free bastnasite in a reasonable number of countercurrent stages.

Mixer-settler runs employing 20 stages were made on the cerium-free bastnasite sulfate feed solution, using a primary amine extractant. These operations culminated in the continuous production of 95 percent lanthanum component in the organic phase; at 96 percent recovery. A larger number of stages could be incorporated to increase the recovery, while a higher purity product could be obtained by the addition of a scrubbing section to the countercurrent system.

The remaining solution contains praseodymium, neodymium, samarium, and europium. Recovery of a high-purity europium product is desirable because of its high market value. In one successful commercial operation, europium, along with some heavier rare-earth elements, is initially extracted into an organophosphate solvent in multistage equipment, after which the europium is stripped into an aqueous phase.

The strip solution can be treated by reduction-precipitation techniques, resulting in phosphor-grade europium. Another possibility for concentrating and purifying the europium involves extracting the europium together with some of the other rare earths with an organophosphate or amine organic phase, followed by a reducing strip. In one version of this process the impure europium is essentially quantitatively extracted from a nitrate solution by a quaternary ammonium compound dissolved in an inert diluent. The next step is to contact the loaded organic phase with a strip solution capable of reducing the europium to the divalent state. One such solution containing chromous ion and zinc reduces the europium and strips it into the aqueous phase. This technique is theoretically capable of producing high-purity europium in a multistage system; however, to date we have conducted only single-stage shakeout studies. One foreign company has stated that it plans to use the reductive stripping technique, in conjunction with organophosphate extraction, for producing phosphor-grade europium.

The remaining praseodymium-neodymium-samarium mixture is considerably more difficult to separate. In our laboratory we have conducted shaker-scale studies, but multistage relationships remain to be defined. The most effective results on this mixture were obtain¬ed when using a quaternary ammonium extractant together with DTPA chelating agent and ammonium nitrate salting-out agent. The rare-earth nitrate solution contained 26 percent praseodymium, 70 percent neodymium, and 4 percent samarium. It was decided to investigate techniques for removing the praseodymium first. Probably the most important variable in the system was the equilibrium pH, and a maximum separation was obtained at pH4. Under the test conditions the praseodymium-neodymium separation factor was 5.9 with 37 percent extraction. The extraction figure is too high because the mixture contains only 26 percent praseodymium, but total extraction could be lowered simply by decreasing the phase ratio.

The above separation factor can be used to compute the number of theoretical stages necessary to recover pure praseodymium. For example, to recover 99 percent of the praseodymium at 99 percent purity would require 10 theoretical stages. This is based on a number of assumptions, including a constant distribution coefficient. In practice, it would undoubtedly require many more stages.

The final separation involves the removal of neodymium from samarium. Again, using a quaternary ammonium extractant together with DTPA and ammonium nitrate, we arrive at a neodymium-samarium separation factor of 11, a very high number for rare-earth separation systems. This separation factor corresponds to seven theoretical stages, but again this is subject to the limitations mentioned previously.

Organophosphoric acids are also of general value for separating rare earths. The organophosphoric acids extract by replacing hydrogen ions with rare-earth ions in the extractant structure, with the consequent formation of organic soluble chelates. These chelates are capable of cross-polymerization in the organic phase, and the pH of the system must be carefully controlled to prevent the intermolecular bonding. Studies carried out with extractants, such as di(2-ethyl-hexyl) phosphoric acid, show a definite increase in distribution coefficients with an increase in atomic number for the rare-earth series. Based on this information, techniques have been developed using di(2-ethyl hexyl) phosphoric acid in which praseodymium-neodymium separation factors were high enough to produce pure individual components in a large number of stages. The same general system can be used for fractionating a neodymium-samarium mixture, in which case the separation factor is somewhat more favorable. Solvent extraction procedures of a similar nature are currently receiving industrial attention.

We have demonstrated some potential systems for separating and recovering all of the light-group elements. Extensions of these techniques should also permit the recovery of many of the heavy-group rare-earth elements. All systems are capable of improvement, however, and we are constantly searching for better, more efficient extractants and new and more economical processing techniques.

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By |2019-09-09T14:53:04-04:00August 21st, 2019|Categories: Hydrometallurgy|Tags: |Comments Off on Concentration and Separation of Rare Earths from Bastnasite

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