**Abstract**

Rare earth elements (REEs) are critical raw materials and are attracting interest because of their applications in novel technologies and green economy. Biohydrometallurgy has been used to extract other base metals; however, bioleaching studies of REE mineral extraction from mineral ores and wastes are yet in their infancy. Mineral ores have been treated with a variety of microorganisms. Phosphate-solubilizing microorganims are particularly relevant in the bioleaching of monazite because transform insoluble phosphate into more soluble form which directly and/or indirectly contributes to their metabolism. The increase of wastes containing REEs turns them into an important alternative source. The application of bioleaching techniques to the treatment of solid wastes might contribute to the conversion towards a more sustainable and environmental friendly economy minimizing the amount of tailings or residues that exert a harmful impact on the environment.

**Keywords:** bioleaching, rare earth elements, recycling, wastes, minerals

### **1. Introduction**

Rare earth elements (REEs) are strategic materials in order to facilitate the transition from current economy based on petroleum to an efficient circular economy based on clean energy. Although often needed in small quantities, these metals are essential to produce a huge number of technologically sophisticated products for electronic, optical and magnetic applications. Among other applications, these elements play a crucial role in the development of clean emerging low-carbon energy technologies [1].

In spite of the archaic term, most of rare earths are common elements and some of them are even more abundant than other metals, such as copper or lead. Though moderately abundant in the Earth's crust, rare earth elements are scarcely concentrated in mineral deposits and this fact complicates its Extractive Metallurgy which is complex and requires economic solutions. The world production of REEs has undergone an exponential growth since its discovery in the 18th century, with a notably increase overtime from 1,000 t in 1930 to 133,600 t in 2010 [2]. The rising REEs production has been the consequence of an escalating demand for REEs as well.

Based on their strong affinity for oxygen, REEs resources are mostly present in oxidic form, mainly as rare earth oxides, phosphates, carbonates and silicates. According to recent estimates, 100 Mt of rare earth oxides are accessible in more than thirty countries all around the world. More than 200 REEs ores have been identified as rock-forming minerals, however, only three are considered mineral ores for economic extraction: bastnasite ((Ce,La)(CO3)F), monazite ((Ce,La,Nd,Th)PO4) and xenotime (YPO4) [3]. Thus, the primary sources of REEs are mineral deposits spread out worldwide, but confined mainly in China, Australia and USA.

Furthermore, REEs are also found in industrial wastes in vast amounts and they have been investigated as potential resources for rare earth metals [4–6]. Among REEs-bearing industrial residues, phosphogypsum, generated during the wet phosphoric acid process from fertilizer production, and red mud residues from the digestion of bauxites in the Bayer are rich in valuable rear earth metals as to be economically treated.

In addition, end-of-life materials can be recycled due to their significant quantities of REE, among them: magnets (38%), lamp phosphors (32%) and metal alloys (13%), retain more than 80% of REE market. Modern fluorescent lamps typically contain more than 20% (w/w) REE (Ce, Eu, La, Tb and Y) [7].

After ore and/or industrial waste concentration processing, rare earth metals are dissolved selectively from raw materials. Actinides, such as uranium and thorium, with similar chemical properties to REEs, are often co-dissolved during hydrometallurgical processes and this could pose a problem. REE primary ores are leached using acid (H2SO4, HCl, HNO3, H3PO4) or alkaline (Na2CO3, NaHCO3) reagents and NaCl or (NH4)2SO4 for REE-ion adsorbed clays [8–10]. Nonetheless, the hydrometallurgical treatment is ore-dependent and has been well established in the case of some REE ores, especially monazite, but is less evident for other key minerals with a very complex mineralogy.

Biohydrometallurgy and more specifically its application to the extraction of metals through bioleaching processes have been successfully practiced at industrial level for the recovery of uranium, copper and gold [11, 12]. Biohydrometallurgical technologies could play a fundamental role for the treatment of REE-bearing materials since they offer an alternative to physico-chemically based methods. Bioleaching is connected to the development of more cost-effective, less energy demanding and less polluting metal extraction processes than pyro- and hydrometallurgical processes and is able to treat low-grade mineral ores and a great variety of residues. These biotechnological processes involve interactions between microorganisms and metal-bearing ores that render valuable metals in solution. REE mobilization from solid matrices has been done with a wide range of microorganisms, both autotrophic and heterotrophic, and using both pure and mixed microbial cultures [13–15].

This chapter provide an insight into the global situation of REEs and the potential application of microorganisms in the extraction of REEs from both REEsbearing minerals and industrial residues.

#### **2. Global situation of REEs: market and technology**

The demand of REEs have increased in the past decade because of their extensive use in several fields related to electronics, in renewable energy capture technologies, biomedical devices, and other industrial products. In 2018, a list of 35 critical minerals, including rare earth elements, was published by the U.S. Department of the Interior and other executive branch agencies [16]. Likewise, the European

#### *Rare Earth Elements Biorecovery from Mineral Ores and Industrial Wastes DOI: http://dx.doi.org/10.5772/intechopen.94594*

Commission developed a critical assessment on non-energy and non-agricultural raw materials in 2017 including heavy rare earth elements, light rare earth elements and platinum group metals [17].

Global mine production was estimated to be 210,000 tons of rare earth oxide (REO) equivalent, which means an 11% increase in comparison with 2018 (**Figure 1a**). China dominates the global production of rare earth minerals, separated compounds and metals. China exports REEs to United States (31%), Japan (27%), the Republic of Korea (11%), the Netherlands (9%) and Germany (6%).

Other countries are making efforts to increase their domestic production of mineral concentrates. For example, United States enhanced the production, all of which was exported, a 44% in 2019 compared with 2018 [18].

Rare earths are relatively abundant in the Earth's crust; however, REEs resources with minable concentrations are less common. Nowadays, about 85o REE deposits have been identified worldwide, which are mainly located in China, Vietnam, Brazil, Russia, India and Australia (**Figure 1b**) [18, 19].

Prices for most rare earth products are increasing compared with those in 2016 reversing the falling trend that began after prices spikes in 2011. Gadolinium, praseodymium and neodymium experienced the greatest increase in the price, while the yttrium and dysprosium prices decreased. The estimated unit value of rare-earth compounds was \$11.60 per kg in 2017 based on information from the U.S. Census Bureau on imports [20].

The estimation of global consumption of rare earth varies significantly due to the limited data transparency and it generally ranges about 140,000 and 170,000 tons of REO equivalent. Furthermore, the global consumption of scandium was estimated in 10–20 tons per year [21].

The amount of specific REEs used strongly depends on the market sector and application. Lanthanum and cerium, and lower amounts of neodymium, are consumed in the catalysts sector. There are different types of permanent magnets but neodymium-iron-boron magnets are fabricated with neodymium and praseodymium, and samarium-cobalt magnets uses samarium and gadolinium. Batteries contain mainly lanthanum, and ceramics, yttrium. Europium, yttrium and terbium are commonly associated with the phosphors sector.

The global growth rate of REEs consumption is expected to be 5–7% per year through 2022. The magnet materials sector would grow more than other sector such as catalysts, ceramic or phosphors. The increasing global demand of REEs as well as the enforcement of environmental and production legislation beyond 2022 lead to higher prices and, consequently, this situation may drive the mining and processing development outside China.

**Figure 1.** *World mine production (a) and reserves (b).*
