Separation of rare earth metals

Research Focus

The project will develop novel processes for the selective separation of rare earth metals from leach solutions, preferentially from the mining industry. The work will be performed in cooperation between the Chemistry Department and the Chemical Engineering and Technology Department at KTH and IVL the Swedish Environmental Research Institute. The three techniques that we will investigate are precipitation, selective chromatographic separations and liquid emulsion extraction. The acidic or basic leach solutions will consist of mixtures of different metal ions present at low concentrations. The work will be performed both on an applied level using an engineering approach and on a fundamental level. The goal is to develop processes with a better selectivity and that are more environmentally friendly than currently used processes. At the end of the project the most promising of the novel processes will have been tested in pilot scale at IVL and evaluated based on economic and environmental aspects. The developed techniques will be applicable to extract valuable metals not only from mining leach solutions but also from a wide variety of other waste such as ash, batteries, different electronic products etc. The knowledge gained in the project will thus offer several different industries new economically and environmentally beneficial solutions for improved metal separation and recovery. At the end of the project three PhD students will have graduated with a competence of importance to Sweden and to the Swedish industry. In addition the project will have established a competence hub in the Stockholm region.

Background

Rare earth elements (REE) provide unique spectroscopic and magnetic properties and are needed for a wide variety of products such as catalysts, hybrid vehicles, rechargeable batteries, mobile phones, plasma televisions, disk drives and catalytic converters. The industrial demand for rare earth metals are increasing. Today about 94% of all rare earth elements are produced in China. There is evidence that China will use nearly all of the REE it can produce leaving little for export (Izatt et al., 2010). Based on USGS rare earth deposits survey released on November 16th and open data on China, CIS and India rare earth resources, the rare earth industrial reserves in Brazil rank the first with a proportion of 37%, China is the second with 25%, the third is CIS with 13%, and Vietnam ranks the forth with 10% (Chen, 2011).

The rare earth elements play an important role in many fields of advanced materials science due to their particular spectroscopic and magnetic properties and the industrial demand for them has increased (Abreu and Morais, 2010). The rare earth metals are the fifteen lanthanides (atomic number 57- 71) together with scandium (21) and yttrium (39). They are often found together in nature at low concentrations in various minerals. About 95% of the rare earths occur in only three minerals: bastnäsite, monazite, and xenotime (Gupta, 1992). Due to their chemical similarity they are difficult to separate from each other. On the basis of their separability, they are divided into the ‘‘light rare earth element group’’ (La to Eu) and the ‘‘heavy group’’ (Gd to Lu plus Y). The basicity differences of the REE influence the solubilities of salts, the hydrolysis of ions, and the formation of complex species, and these properties form the basis of separation procedures by fractional precipitation, ion exchange, and solvent extraction (Gupta, 1992). Selective oxidation or reduction can be used to simplify the separation of some of the rare earth elements. All of the REE occur in a trivalent oxidation state, however Ce, Pr, and Tb can also occur in the tetravalent state, and Sm, Eu, and Yb exhibit divalency. In the divalent and tetravalent states the elements exhibit markedly different chemical behaviour compared with that in the trivalent state (Gupta, 1992).

The increasing number of applications of rare earth elements in industry has lead to a growing interest in the exploration and exploitation of new sources and techniques for extraction. Mechanical and pyrometallurgical methods have high-energy demands and consequently high costs. Further dust collecting and/ or gas cleaning systems are required, and they are not as versatile as hydrometallurgical processes. Hydrometallurgical processes generally consist of different steps of pre-treatment, in order to improve metal dissolution rates in the aqueous phase, followed by leaching, concentration and/ or purification and finally recovery of different metals from the leach solution. In the leaching step either an acid or a base can be used depending on the material being treated. The most commonly used hydrometallurgical concentration and purification methods in the mining industry today are precipitation, liquid- liquid extraction (LLE) and ion exchange (IX). For the final recovery step precipitation and electrolysis are often used. Solvent extraction followed by electrowinning has for example become quite popular for copper and nickel recovery in the mining industry. In zinc refineries iron and cadmium are precipitated as residues after which zinc is recovered using electrowinning.

Precipitation

Before 1947 oxidation and reduction together with fractional precipitation techniques were the only available techniques for separating rare earth metals. However, the procedures were relatively inefficient and laborious. Commercial separation of rare earths starting in the 1950s also made use of ion exchange and solvent extraction, which gave a much better purity of the end products obtained (Gupta, 1992). In many commercial processes of today, the rare earths are stripped from loaded solvent extractants using aqueous solutions of inorganic acids. The dissolved rare earths are then precipitated as insoluble oxalates and carbonates (oxide precursors), from which oxides are recovered by calcination (Konishi and Noda, 2001). The stripping and precipitation steps can also be combined (Konishi and Noda, 2001, El-Hefny et al. 2010).

Rare earth metals can be separated from other metals in weakly acidic medium (pH 1-4) by precipitating them as oxalates with oxalic acid, since many other metals (e.g. Fe, Al, Ti, Zr, Nb, and Mo) remain in solution as soluble oxalate complexes. Individual rare earth elements in high purity have also been produced by separation as double nitrates. The separation is more sensitive for the light rare earth elements since the property difference between rare earths decreases as the atomic number increases (Gupta, 1992). One of the most widely used precipitation method to separate rare earth elements (REE) from acidic solutions is by precipitation of sodium double sulphate hydrates (NaRE(SO₄)₂.xH₂O) through the addition of sodium sulphate (Gupta 1992). These salts are only slightly soluble in acidic solutions. Double sulphate precipitation result in separations in two fractions one enriched in the light-REE group rare earths and the other enriched in the heavy-REE group rare earths (Gupta, 1992). The different REE can then be separated from each other by converting the double sulphate into a highly soluble compound, such as RE-hydroxide. Although monazite leach liquor is usually purified by precipitation of REE as sodium double sulphate hydrates the influence of the process variables on the double sulphate precipitation and on its conversion into RE-hydroxide is rarely mentioned in literature (Abreu and Morais, 2010). Abreu and Morais (2010) studied the precipitation of rare earth metals from Monazite sulphuric acid leach solutions in the form of sodium double sulphate hydrates and the subsequent recovery of cerium. The cerium was oxidised from Ce(III) to Ce(IV) by MnO₄^- and then recovered by precipitation of cerium hydroxide. Abreu and Morais (2010) studied the influence of the nature and excess of precipitation agent, temperature, reaction time and pH on the processes. After purification the cerium recovery yield in the form of CeO2 for the overall process was greater than 98% (Abreu and Morais, 2010). Kul et al. (2008) determined the optimum hydrometallurgical parameters to extract the rare earth elements as rare earth double sulfates from a bastnasite ore. The total rare earth double sulphate content was above 90% and the overall recovery of the process was almost 95%. However, further treatment of this product is required to produce more valuable REE products, e.g. by precipitation of cerium hydroxide and solvent extraction for the separation of heavy and light rare earth metals (Kul et al., 2008).

Solvent extraction

In solvent extraction the metal ion solution is mixed with an organic solvent. Extractant chemicals in the organic phase are used to improve the transfer of the metal ions into the organic phase. The technique is well known and widely used. The first studies of solvent extraction of rare earth metals were made already in the thirties (Gupta, 1992). Liquid-liquid extraction is still used to separate rare earth metals from different leach solutions on industrial scale. However, the separation of adjacent rare earth metals by the use of conventional extraction system is still difficult, because such extractive separation processes are based on only the differences in the complex formation ability between the rare earth metals and their extractant (Nishihama, 2003). One method that significantly can improve the separation ability is to convert the metal species by chemical reaction, e.g. a redox reaction, during the actual extraction. Nishihama et al. (2003) studied the solvent extraction of Eu from Sm/Eu/ Gd and of Ce from La/Ce/Pr using ethylenediaminetetraacetic acid (EDTA) as complexing agent and employing a photochemical redox reaction in the aqueous phase. In both cases the redox reaction improved the extractions.

Another limitation in traditional solvent extraction is that large volumes of organic solvents are needed, especially when processing dilute solutions, which is not environmentally friendly. Further the method can be tedious and time consuming in those cases when many steps are needed to reach a sufficient separation. Another drawback is that the method is difficult to automate and that the liquid phases may form emulsions that at times makes it difficult to separate the two phases.

Liquid Emulsion Membranes Extraction

In liquid emulsion membrane (LEM) extraction a stable emulsion between two immiscible phases is first formed, then the emulsion is dispersed into a third continuous phase by agitation. The organic membrane phase consists of an organic solvent that contains an extracting agent and an emulsifier, the internal aqueous phase (droplets) contains a stripping agent and the external continuous phase is the aqueous feed solution containing the species to be extracted. The target species can then be recovered from the aqueous feed into the organic phase and then stripped into aqueous droplets in the emulsion. The emulsion is broken by typically electrostatic coalescence and then the target species can be recovered by for example electrowinning or precipitation. (Hasan et al, 2009)

The LEM process provides a much larger interfacial area than conventional liquid-liquid extraction and it carries out the extraction and stripping operations in one step. It can reduce the amount of expensive extractant about 10 times (Hanapi et al. 2006). The extraction chemistry is essentially the same as in LLE but the transport is governed by kinetic rather than equilibrium parameters (Hachemaoui et al., 2010). Additional attractive features of LEM extraction, compared to LLE, are high selectivity and simple and continuous operation.

The technique is not fully developed and more research is needed before large scale and wide spread application in industry can occur. The main problem associated with the LEM process is emulsion stability. Another problem is osmotic swelling, which occurs when water in the external phase diffuses through the organic membrane phase and swells in the internal aqueous phase. The internal phase is being diluted and the increased volume of the internal phase leads to an increased breakage of the droplets (Hanapi et al., 2006).

Many metal ions have been reported to be successfully extracted by liquid emulsion membrane extraction such as Pd, Cu, Cr(VI), Th, U, Co, In, Ni, Ag and Mo (Hasan et al., 2009, Yadav and Mahajani, 2007). Only a few papers have reported about the extraction of rare earth elements by emulsion liquid membranes (Hasan and El-Reefy, 2009, Kakoi et al. 1997).

Chromatographic separations

Ion exchange has been used since the mid 20^th century to separate rare earth metals from each other and from other metals (Gupta, 1992). Due to the chemical similarities of the rare earth metals complexing agents need to be used to enhance separation factors. Examples of complexing agents are ethylenediaminetetraacetic acid (EDTA) and a-hydroxyisobutyric acid (HIBA) (Pourjavid, 2010).

Adsorption is especially suitable for removal of metals from water at low metal ion concentrations. Molecular recognition technology is a highly selective, non-ion exchange system, using specially designed organic chelators or ligands that are chemically bonded to solid supports such as silica gel or polymer substrates (Izatt et al., 2010). Conventional separation methods such as precipitation, ion exchange, and solvent extraction generally recognize differences between ions based only on a single parameter. Chromatographic separations by molecular recognition systems on the other hand are designed to bind selectively with ions based on several parameters such as size, coordination chemistry, and geometry and thus exhibit high selectivity. Metal separations at mg/L or lower levels that are not possible using traditional technologies can be achieved using these selective chromatographic systems. The technology of molecular recognition is well established and used in both the precious and minor metals industries (Izatt et al., 2010). For example since 1997 Impala Platinum, Ltd. Has used the technology to refine Pd and Tanaka Kikinzoku Kogyo K K has used the technology since the mid 1990s to extract and purify Rh (Izatt et al., 2010). Izatt et al. (2010) suggests that there are attractive possibilities for applying molecular recognition technology to REE recovery from low-level wastes and end-of-life products. Ion imprinted polymers have been investigated for selective separation and pre-concentration of rare earth metals. Adsorbents for e.g. Ce(III) (Zhang et al., 2010), La(III) (Li and Sun, 2007) and Nd(III) (Park and Tavlarides, 2010, Jiajia et al., 2009) have been synthesized.

Li et al. (2011) studied the removal of Pb²⁺, Cu²⁺, Cr³⁺ and Ni²⁺ from their aqueous solutions (20 mg/L) using sulfonic acids modified microporous hypercrosslinked polymers. The study verified the hypothesis that microporous resins should be better adsorbents than mesopourous and macroporous materials due to the comparable size of their pores and metal ions. A good recyclability of the material was also observed which shows potential for industrial applications. Organic microporous polymers have many advantageous features. Their pore structure can be finely tuned, surface functionalities can be introduced by a wide variety of synthetic strategies and most organic polymers are highly stable to air and water moisture (Li et al., 2011). Only few reports have suggested microporous HCP:s as adsorbents for metal ions due to their hydrophobic nature (Li et al., 2011). However, by surface modification of HCP:s the hydrophilic nature of the material can be improved while the micropourous structure is retained. Microporous hypercrosslinked polymers have been used for acidic solutions (Davankov et al., 2009).

Scientific and technical challenges

The project is focused on three different unit operations in different stage of maturity.  Crystallization/precipitation is included because this is an area of particular strength of the applicants, which may lead to novel more competitive solutions when applied to this particular area of industry. Chromatographic processes includes the well established ion exchange methods, but also novel chromatographic media and technologies, e.g. size exclusion principles. Liquid membrane processes are not used much today even though the potential has been recognised for some years. Liquid membrane processes are complex to design and operate but may lead to new, highly selective separations. Challenges to be met in the project include to design and operate processes to the highest possible requirements of sustainability and energy efficiency.

Crystallization/Precipitation

We will study selective precipitation of rare earth metals from leach solutions of different compositions. The goal is to be able to use precipitation alone or in combination with other techniques to be able to separate the different REE from each other. Here we will make use of the small differences in the solubility of the different rare earth metal salts, the hydrolysis of ions and the formation of complex species. Selection of precipitating agents will depend on the specific composition of the leach liquor. Traditionally precipitation has been viewed from a chemical equilibrium standpoint and from an inorganic cation or anion removal standpoint, i.e. the emphasis has been on the solubility rather than on the precipitated compound. We aim to combine chemical speciation considerations with the theory of crystallization to the precipitation systems.

For selective precipitation the challenges on a fundamental level is to determine the thermodynamics and kinetics of the ion speciation in the solution and the solid-liquid equilibriums. These will depend on the solution composition and on the temperature and pressure of the system. We will also determine the nucleation kinetics and the growth mechanism and kinetics of different precipitates. In the domain of thermodynamic stability of a compound, a less stable phase may form due to favourable kinetics. Usually this metastable phase is a precursor phase, which eventually, given the time, should convert to the stable one. Many precipitates formed are often amorphous due to either low temperature or lack of supersaturation control or both. Temperature is an important parameter that will alter both the composition and crystal structure of the precipitates and the growth rate of the crystals. Supersaturation is the most important parameter that controls the properties (particle size, morphology, crystallinity and purity) of a compound produced via aqueous precipitation. The supersaturation can be controlled by pH control, via metal complexation and dissociation, via dilution, via a redox reaction or via a dissolution reaction. Another important parameter that we will study is the pH, which will influence both the solubility and the purity of the precipitates. On an applied level we will also study how the mixing of reagents and agitation influences the process.

Tailored/selective Chromatography

For microporous chromatographic separations we specifically intend to investigate the possibility of using chemically modified microporous hypercrosslinked polymers to separate rare earth metals with high selectivity from different leach liquors. However, for more extreme conditions (pH, temperature) porous inorganic materials might be more appropriate. The distribution of analyte between an aqueous solution and the sorbent is based upon mechanisms such as adsorption, ion exchange, chelation, ion-pair or complex formation and other chemical reactions on or in the sorbents. The challenges on a fundamental level will be to design binding sites that are capable of separating rare earth metals from each other and from other metals. We will investigate the effect of pH and temperature on the adsorption of the target ion and the kinetics of the adsorption. Competitive adsorption will also be investigated in order to determine the selectivity of the resin toward the target ion.

Liquid Membrane Extraction

We will study liquid emulsion membrane extraction of rare earth metals from different leach solutions. The extractant and surfactant concentration, the type of extractant, pH in the external and internal phases and temperature are important parameters that all influences the emulsion stability and recovery and need to be optimized. The diffusion in the emulsion drop, the external mass transfer around the drop and the rate of the formation of complex at the interface will also be addressed and modelled.

Work plan

The overall project idea is a “vertical integration” of competence in basic inorganic chemistry, chemical engineering science, and application near process development. The competence is provided through three main partners and applicants: Division of inorganic chemistry, Department of chemistry, KTH, Divisions of transport phenomena/chemical engineering, Department of chemical engineering and technology and IVL Swedish Environmental Research Institute. The project has engaged three PhD students on 80 % activity during 5 years, and three senior scientists on part time for five years each. There is one senior scientist from each core competence area: chemistry, chemical engineering and pilot plant evaluation. However, each PhD student works with a particular unit operation through all levels of research: (1) solution chemistry and chemical system design, (2) chemical engineering process fundamentals and development, and (3) small scale pilot plant evaluation of the process. This will be strongly supported by an overall leadership of the three applicants, and a day-to-day supervision by the three postdoc level researchers. Initially, the emphasis will be on analysing and designing the chemical systems, this will gradually be overtaken by process evaluation and design. After year three, processes will be scrutinized for pilot plant evaluation. Tentatively, the most promising method within each unit operation will be evaluated in larger scale. Three main work packages are specified below. The time frames are overlapping on purpose. Even though the project will have an overall work plan from chemistry, via chemical engineering to pilot-plant experiments, this is not expected to be a straight sequence with rigid dead lines for each work package. It is rather expected that each PhD project will to a certain extent interchangeably address more fundamental chemistry issues and more process oriented problems especially during the first years, and to some extent even so during the pilot plant experimental phase. During the whole project aspects of importance for industrial feasibility, environmental sustainability and energy efficiency, will be considered.

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