A Short Review on REE Recovery from Ion-Adsorption Clays

Rare Earth Elements (REEs: La-Lu Plus Y and Sc [1]) are known to be enriched in the melt mantle during magmatic differentiation. They can also be enriched in the granitoids through the accumulation of REE-bearing minerals such as apatite, allanite, monazite, titanite and xenotime, since they tend to remain in the melt until the late stages of magmatic differentiation [2]. Rare earth deposits are formed by different geological processes in deep-seated intrusive environment, magmatic–hydrothermal environment, and surface weathering environment. REE resources can be divided into two categories: primary deposits associated with igneous and hydrothermal processes and secondary deposits concentrated by sedimentary processes and weathering. Within these two groups, REE deposits can be further subdivided depending on their genetic, mineralogy and form of occurrence [3]. Two significant types of secondary deposits are the placer and ion-adsorption clays deposits. REEs ion-adsorption type deposits are formed by weathering of igneous rocks (typically granites) that contain particular REEbearing minerals. Due to surface weathering, REE minerals are decomposed, and ionized REEs (REE3+) are absorbed on clay minerals (normally kaolin and smectite groups). Ion-adsorption ores generally contain higher amount of HREEs and lower amounts of Th and U than other REE ores [4].

Promoting of intensive chemical weathering and breakdown of REE-bearing minerals by temperate and tropical climates (based on Koppen-Geiger climate classification [5]) suggest that those regions -with igneous bedrock- are potential zones for ion-adsorption clay REE resources. Southern China has the most developed ion-adsorption type deposits currently [6], however, several projects have been also known in the countries outside China in the recent years [7] such as: The Tantalus REE project in northern Madagascar, The Malawi REE project, The Serra Verde REE project in Brazil [8] as well as some other projects in Myanmar, Vietnam [9] and western Thailand [10].

Mineralogy is a key variable in determining the ease or difficulty, the cost of processing and extraction of REEs. Primary, hard rock deposits with the most abundant REE minerals such as bastnäsite, monazite, xenotime, loparite, parasite, and perhaps apatite is more likely to be economical compared with those deposits with eudialyte, allanite, or zircon [11,12]. Cause the latter group are more refractory and economic processing of REE from these minerals is not currently possible. In ion-adsorption ores, REEs are mostly present in ion-exchangeable phase of weathered granites, therefor, blasting, crushing, grinding, or mineral processing are not needed. REEs are easily extracted by ion exchange using diluted electrolyte solutions such as ammonium sulfate solution at ambient temperature [13-17].

There are different types of leaching methods have been utilized to extract REEs from ion-adsorption clays, such as heap, tank/pool, and in-situ leaching methods [18]. Previously, heap leaching was a commonly operated method which involves excavating minerals, placing them in a mound, and spraying them with solutions [17]. Tank/pool leaching involves placing the minerals into a tank/pool and immersed with solutions [18]. In-situ leach mining is now the dominating technology, given that there is less topsoil removed, the process can be performed on site, and the environmental impacts are reduced [19,20].

The schematic monovalent salt leaching extraction of REE3+ (Figure 1) suggested by recent studies [21] is:

In most cases, the rare earth leach liquor contains a significant amount of some impurities such as (NH4)2SO4, Al3+, Fe3+ and Ca2+, accompanied by a small amount of Fe2+, Pb2+ and Mn2+ [22]. The traditional process of extracting rare earth from such leach liquor is usually treated with chemical precipitation with oxalic acid or ammonium carbonate as precipitant, forming rare earth oxalate or carbonate, followed by washing and filtering and calcining into rare earth oxides [23,24]. However, the rare earth oxides need to be dissolved with hydrochloric acid before being separated by solvent extraction or used as materials for many rare earth users [25]. Although, they have been applied to industry practice, these precipitation technologies are complicated, time-and reagentconsuming, and the serious disadvantage is that these impurity ions cannot be removed, and the product purity is low, in addition, the rare earth yield is also low [26]. Recently, efficient nonprecipitation techniques such as solvent extraction, ion exchange, and liquid membranes has been developed and applied in some mining sites [27].

Simple divalent electrolyte solution such as (NH4)2SO4 are capable to extract the significant amounts of REEs absorbed on clay minerals such as kaolinite, halloysite and montmorillonite at room temperature cause of REEs accumulation in ion-exchangeable phase of ion-adsorption REE deposits. The leachate was then precipitate using oxalic acid or ammonium carbonate followed by a roasting stage. Obtained mixed REEs oxide contains a significant amount of impurities which purify by means of hydrochloric acid dissolution technique before applying further fine separation techniques. New researches are investigating on non-precipitation methods such as solvent extraction, ion-exchange and liquid membranes due to low product purity of precipitation methods.

Research Grant: USM 304.P. Bahan.6050350

Today’s one of the most important aspects of inventory management that has significant role in supply chain operations is the carbon emission of defective products in multi-echelon inventory systems. The decision objective is usually set as the total cost minimization or the total profit maximization without considering carbon emissions in inventory management. However, taking emission reduction into account is likely to change the optimal solution and incurs additional cost. In view of this we include the possible relationship between transportation and storage in the existing model and then investigate the joint impacts of carbon emission. This paper provides a modified integrated inventory model which integrates cost and carbon emissions in transportation and storage. We designed an algorithm to find the optimal inventory strategies in order to minimize carbon emission cost and system cost. Our proposed algorithm may help both the government and the industry to adopt appropriate carbon reduction regulations.

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