Rare-earth mining resources opportunities and environmental challenges in Africa.

      All electronics devices depend on one or more of rare-earth elements. The list contains 17 rare earth elements, all vital to the electronics industry in some form. Yet, despite their name, some rare earth elements are relatively plentiful: cerium is as abundant as copper and the rarest rare-earth element Thulium is 100 times more common than gold. They are regarded as ‘rare’ because deposits of these elements are generally dispersed and not exploitable commercially. They are used for several applications and mostly in alloy. Lanthanum is used in microprocessor, Erbium in memory and Neodynium in magnetic component. As it can be seen in Figure 1, they all show interesting magnetic, optical properties and are used in LED or Laser industry [1]. More than 80% of the supply is from China which leads the market with supremacy [1,2]. However, since 2017, some rare earth elements have been discovered in Africa. A rise in protectionism and boost from the semiconductor/energy industry can lead good opportunities for the African mining plants for taking part of this market.

Figure1: Uses of rare earths with respect Left) to main applications from U.S. Geological Survey Report 2011-5094 and Mineral commodity summaries 2012. Percentages expressed with respect to rare earth oxide. Right) to individual elements, La, Ce, Nd, Pr, and Y represent 97 wt % of all rare earths used in end products for: magnets, phosphors, catalysts, polishing powders, lasers and alloys [1].

 However, this does not mean necessarily good things for the African continent. Do you remember the blood Coltan? Coltan is a short for Columbite-Tantalite which a gem combination of Niobium with Tantalum which are not REE. The biggest supplier is in Africa: Democratic Republic of Congo. Coltan can be used for the so-called acoustic filters. A high-end smartphone must filter, transmit and receive paths for 2G, 3G, and 4G wireless access methods in up to 15 bands, as well as Wi-Fi, Bluetooth and the receive path of GPS receivers. Signals must be isolated from one another without other extraneous signals. To do so, you need many filters and duplexers. Without acoustic filter technology, it would not be impossible. So, it is easy to understand why these 2 elements are essential to the smartphone industry. This had whetted the appetite for mining but during war time and speculative bubble in 2010. That was an explosive situation: illegal exploitation, forced labor & child labor. It had tremendous effect on the DRC society that are still visible nowaday!

Because, mining industry is a much more lucrative business than farming, people are leaving the farms to work in mining despite the tough conditions. International mining conglomerates are identified several promising prospects and the demand is growing. REE business will reach 15B USD in 2025. That is tiny in comparison to the world’s major commodity markets such as iron ore, worth nearer $350B, or copper, worth around $160B [3]. Why, then, are they so important? They have been becoming strategical raw materials. They are so important that a lack of supply may leave governments or organizations vulnerable to disruption of the manufacturing of electronics devices. Also, Green wind energy is highly dependent of REE, mostly Nd, Dy and Pr because of their magnetic properties. The speaker/mic of smartphones need small permanent magnets too. There are currently no competing elements that can be substituted into permanent magnets and deliver the same properties – and this is critical. REE are also central to the whole spectrum of defense technologies that are vital to every military. Without them, countries would be unable to produce much of the military hardware and equipment required for national defense and there are no substitutes.

Here we will explain the REE chemical properties, the mining extraction basics and we will discuss the environmental impact.

Chemical properties:

As mentioned before, a little bit of REE in almost everything can change completely the properties. REE have common properties; they tarnish in the air and are silver-gray glassy color like a metal. The chemical properties of the rare earths are governed by the electron configuration of these elements. In general, most of these elements are trivalent and can bond to 3 other atoms. If we look how the electronic clouds is around the nucleus (made of proton and neutrons), the 4f electrons have lower energies than the outer three valence electrons and the 4f shell is closer the radius, it means 4f electrons are “localized” and part of the ion core. Therefore, they do not directly participate in the bonding with other elements when a compound is formed. That’s why the lanthanides are chemically similar and difficult to separate and why they occur together in various minerals.

Table1: rare earth element properties [3]. FCC: face-centered cubic like. BCC: basic cubic-centered. Promethium is the only radioactive REE! There are two possible sources for natural promethium: rare decays of natural europium and uranium (various isotopes). There appears to be no promethium in Earth’s crust, according to the Los Alamos National Laboratory

From Table 1, it’s worth to notice that the outer or valence electrons for the REEs are the same, 5d6s²; 2 other elements show same properties like scandium (3d4s²) and yttrium (4d5s²), so they belong also to REEs group. There is some variation in the chemical properties of the lanthanides because of the lanthanide contraction and the hybridization, or mixing, of the 4f electrons with the valence electrons. The systematic and smooth decrease from lanthanum to lutetium is known as the lanthanide contraction [4,5]. It is due to the increase in the nuclear charge, which is not completely screened by the additional 4f electron as one goes from one lanthanide to the next. This increased effective charge draws the electrons closer to the nucleus, thus accounting for the smaller radius of the higher-atomic-number lanthanides. The lanthanide contraction also accounts for the decreased basicity (electron donor) from lanthanum to lutetium and is the basis of various separation techniques – one can look at “de Marignac” process. Thanks to this unique type of configuration they can exhibit important magnetic, thermal and optical properties. However, these metals have many similar properties, and that often causes them to be found together in geologic deposits. They are also referred to as “rare earth oxides” because many of them are typically sold as oxide compounds. REEs are classified into 2 groups based on their atomic numbers: Light rare earth elements or LREEs (from La to Eu – also called Ceries) and heavy rare earth elements or HREEs (from Gd-Lu also called the yttrics). HREE have slightly different properties because they have paired electrons in their 4f layer. There are different ores mineral (see Figure 2) from industry productions which include bastnasite [(Ce,La)(CO3)F], monazite [(Ce,La)PO4)],  cerite, (Ce,La,Ca)9(Mg,Fe+3)(SiO4)6(SiO3OH)(OH)3], loparite[(Ce,Na,Ca)(Ti,Nb)O3], xenotime (YPO4), gadolinite [(Ce,La,Nd,Y)2FeBe2Si2O10], euxenite [(Y,Ca,Ce,U,Th)(Nb,Ta,Ti)2O6], apatite [(Ca,REE,Sr,Na,K)3Ca2(PO4)3(F,OH)],  columbite [(Fe, Mn)Nb2O6], tantalite [(Fe, Mn)(Ta, Nb)2O6]  and laterite clays [6]. Other collateral metals often found amongst REE deposits include Niobium, Zirconium, Uranium, Beryllium and Thorium.  The last three radioactive elements belong to the actinides. Actinides have similar properties to lanthanides because of the outer electron shell configurations. As indicated above for the REE, each added electron goes into the 4f orbital. In the actinoid elements, the added electrons also go into an f orbital, in a similar manner but in the 5^th shell instead. Then electrons in the 5f orbitals, being farther from the nucleus, are much less tightly bound than those in 4f orbitals. Because of its position in the 5th shell, this distinguishing electron subshell actually affects weakly the chemical properties of the actinoids and 5f electrons do not contribute significantly to the formation of chemical bonds with other atoms. Therefore, the similarities in the chemical and physical properties between the rare earth elements and the radionuclides, thorium and uranium, means that the rare earth minerals are often associated with radioactivity.

Figure 2: Pictures of the currently exploited rare-earth minerals ore (from Wikipedia and [1]).

REE extraction process

Because of their similar valence properties and affinity with acids, the extraction for rare earth is a complex and multistep process that look unsecure and not environment friendly. Common methods are based: i)on fractional recrystallizations precipitation where the less basic hydroxides of the heavy lanthanides precipitating before those of the lighter ones on gradual addition of alkali ; ii) differences in solubility of salts such as double sulfates/nitrates; and iii) conversion to an oxidation other than 3 like Ce(IV) or Eu(II). Latter process is not applicable to all REE. Methods i) and ii) require much repetition to be efficient. A solvent extraction (with Tri-n-butylphosphate/kerosene) or an ion-exchange technique (with displacement chromatography) can separate uranium and thorium from rare earths [7]. Monazite, REE & thorium bearing phosphate mineral, is one of the major minerals (with bastnaesite) used for the production of rare earth elements.

After extraction, REE production can generally be divided into the following stages:

1. Extraction of REE-containing material such as an ore or a specific waste fraction
2. Concentration of the material (increasing the % content of REE from a very low level to about 60 – 70%) which is done by mechanical, flotation, gravity or magnetic processes. Here it gives you the lowest-value sellable.
3. Purification to produce a REE-containing mixture (usually an acidic solution) pure enough for separation. It produces concentrates that are subsequently leached with aqueous inorganic acids such as HF, HCL, H2SO4 or HNO3. Then a filtration step is done, followed by a counter current decantation.
4. Separation of different REE or REE fractions present by solvent extractants in which the solubility of the REE(III) increases with its atomic weight. Commercially, D2EHPA, HEHEHP, Versatic 10, TBP, and Aliquat 336 have been widely used in rare earth solvent extraction processes. hundreds of stages of mixers and settlers may be assembled together to achieve the necessary separations [8].
5. Refining into a sellable product (REE compounds or metals, either pure or in defined mixtures): the concentrated and pure ore which the highest value.

Mineral can be broken down by either acidic or alkaline attack. In a general way, it is a cycle of several steps: extraction, acidification (also called HF digestion), stripping, precipitation in caustic soda water and then calcination. A common process is based in acidic fluoride (HF) or sulfuric (H2SO4) media at high temperature >200C made on micrometric powder of extract resulting from multiple crushing/grinding processes [9,10]. Indeed, all REE can form triahalides easily due to the high electronegativity of the halogeneous elements, and especially fluor the most electronegative element. Nowaday, sulfuric acid baking is one of the main processing routes for extraction of rare earth elements from monazite. As indicated in Figure 3, H2SO4 reacts with REE and can easily be selective toward Thorium by changing the temperature and the concentration [10,11].

Figure3: Left) Effect of bake temperature on dissolution of major elements after acid baking and leaching of monazite (LRE = La-Nd , HRE = Sm-Lu). Bake conditions: 1700 kg/t acid addition; leach conditions: 0.9 M H2SO4, 40:1 (w/w) liquid to solid ratio, 2 h from [10].Right) Extraction behavior of rare earth metal ions as function of the equilibrium pH of the following aqueous phases: 0.1mM rare earth metal ions, 0.05 M H2SO4-(NH4)2SO4; Ionic Liquid (IL) phases: 10 mM DODGAA in [C8mim][Tf2N] from [11].

In 2016, Bonificio&Clarke could extract a limited list of rare-earth elements using bacteria. The findings suggest that there is an opportunity to harness the diversity of bacterial surface chemistry to separate and recover technologically important rare-earth metals in an environmentally benign manner [12]. The process is based on the biosorption of specific bacteria that will allow them to passively concentrate contaminants (here HREE like Tm, Yb and Lu) onto its cellular structure. Biosorption is a well-known environment-friendly filtering technique for heavy metals [13]. Principles basis is illustrated in Figure 4. A well-known biosorption-based cleanup process of heavy metals is using Chitosan which is similar to our keratin but is a polysaccharide made from the chitin of the shrimps shells.  In its form of aerogel (high porosity/low density), it is yet among the biological adsorbent used for organic pollutants and heavy metals removal [14]. The 2 researchers from Harvard university demonstrate an alternative, biogenic method based on lanthanide adsorption to the bacteria Roseobacter sp. AzwK-3b, immobilized on an assay filter, followed by subsequent desorption as a function of pH. This REE-selective absorption-desorption elution can be explained by the systematic decrease in basicity with increasing atomic mass across REE series and the associated decreasing ionic size across the series, the so-called above-mentioned lanthanide contraction. This method is promising in terms of cost, efficiency and environment. Th and U are not mentioned but biosorption was also studied for this 2 elements with different bacteria (P.aeruginosa and Aspergilus ficuum) that are able to change pH and enhance the chelation of both elements [15]. It is likely that the polysaccharides which are present in cell walls play a key role in the biosorption of the actinoid and REE.

Figure 4: from [13]. The processes for the removal of heavy metal ions by using microorganisms may be divided into four categories: a) adsorption of metal ions on the surface of microbial cells, b) adsorption of metal ions by extracellular biopolymers, such as polysaccharide, c) absorption into microbial cells and d) adsorption by bio-minerals (minerals created by living creatures) such as manganese oxide which are important scavengers for REE.

Rare earth mining: open pit and deep-sea mining

Most Rare-Earth ores are mined by open-pit mines which create a significant amount of waste. There are generally four main operations in a mine that contribute to this load: drilling, blasting, loading and hauling. Open pit mine are generally safer and cheaper than underground mines. However, disadvantages when compared to Underground Mining also exist:

-Very large amounts of waste rock are mined. This creates costs as well as environmental issues with waste rock disposal;

-Major disruption of surface: pit footprint, waste dumps. High visual impact, especially strip mining. After closure, rehabilitation may be difficult, slow & costly;

-Very large volumes of overburden may need to be moved before reaching the orebody (e.g. coal) thus delaying return on capital expenditure;

-Open pits catch rain, making them vulnerable to flooding, which may severely disrupt production;

Pit footprints is destructive for rainforests ecosystem and can impact the life of the people nearby and downstream. It results in significant deforestation through forest clearing. Using satellite data, researchers found that deforestation from mining encompassed 11,670 square kilometers (roughly 4,500 square miles) between 2005 and 2015, an area twice the size of the state of Delaware [16]. Nearly 10 percent of the deforestation in the Brazilian Amazon between 2005 and 2015 was due to mining activities. This study did not take into account the construction of roads which open remote forest areas to transient settlers, land speculators, and small-scale miners. As it can be seen, due to an easy access (open pit and not deep), it attracts a lot of wildcat miners and this flow of newcomers has probably a bigger impact onto the tropical rainforest environment than industrial mining operations. These illegal prospectors clear forest in search of riches. They hunt wildlife, cut trees for building material and fuelwood, and trigger erosion by clearing hillsides and detonating explosives. Miners can also bring diseases to local indigenous populations (where they still exist) and battles over land rights. The biggest REE plants in the world is located in Bayan Obo in China. It generates nearly half of the total REE world supply. As observed from 1990 to 2018, the time-lapse shows obviously environmental damages that can be seen from space. Open pits are 1km deep and the surface is round 50km2 which is almost like the area of Manhattan!

Figure 5: satellite view of the REE mining plant in Bayan-Obo plant in China. Two circular open-pit mines are visible, as well as a number of tailings ponds and tailings piles. Pictures are extracted thanks to the Google Earth Engine timelapse.

Leftover waste from processing the ore is called tailings and is generally in the form of a slurry. This is pumped to a tailings dam or settling pond, where the water is reused or evaporated. Tailings dams are toxic due to the presence of some forms of toxic minerals in the gangue, and often dangerous chemicals which are used in the refining process. Indeed as mentioned before in the refining process part, the slurry can be radioactive due to Thorium or Uranium oxide, it can be also highly acidic due to the HF/H2SO4 leaching/baking process and also flammable due to the presence of organic solvent and kerosene. According to the Chinese Society of Rare Earths, 9,600 to 12,000 cubic meters (340,000 to 420,000 cubic feet) of waste gas—containing dust concentrate, hydrofluoric acid, sulfur dioxide, and sulfuric acid—are released with every ton of rare metals that are mined. Approximately 75 cubic meters (2,600 cubic feet) of acidic wastewater, plus about a ton of radioactive waste residue are also produced If proper environmental protections are not in place, this toxicity can harm the surrounding environment. Two accidents occurred in Brasil in 2 iron ores with the Brumadinho dam disaster occurred on 25 January 2019 and the Mariana dam disaster occurred on 5 November 2015 resulting in flooding 2 villages and around 300 killings. On environment, according to analyses, the mud contains greater than acceptable concentrations of heavy metals, substances harmful to health, such as arsenic, lead and mercury. In the case of Brumadinho, the pollution (from 60 millions tons of spilled mud!) spreads along 650km of rivers, reached a protected natural reserve in the Atlantic ocean and affected 2 countries.

A new type of process is upcoming: deep-sea mining (DSM) where we harvest precious ores from the ocean floor at a depth from 200m and below. Indeed, like Oil and gas drilling, deep-sea mining refers to the discovery and development of oil and gas resources which lie underwater and more accurately under seabeds. In average earth crust contains less than 100ppm of REE. In 2013, deep-sea mud containing over 5000ppm total of REE was discovered near Minamitorishima Island in Japan, a small island of only 1.5km² – it takes only 45min to walk around the island. Yet, it is likely a golden island for Japan! In 2018, a tremendous amount of 16milions tons of REE has been estimated which is almost similar to the total amount of REE in China (18millions) [17]. DSM is more challenging than land-based mining due to the remote and harsher environment. It is only the early stage but much of the innovation concerns overcoming these challenges. Currently, the technology is available but the financial and regulatory uncertainty held the industry back. However, the high growth demand of power electric car with its solid-state batteries and semiconductor bolster the business case for DSM so the long-awaited DSM code chart are due to be finalized soon. What about the environmental cost? Deep-sea refers to area below 200m, a cold and dark environment where pressures can easily exceed 20 times the atmosphere. Yet, this harsh environment contains a vast ecosystem which researchers are still studying. It is the home to crustaceans, sponges, sea cucumbers, starfish, urchins, microbial species, corals, shrimp and various deep-sea fishes. It is a low-energy life which was isolated during millions years if not more. Here, miners can find metal-rich nodules which precipate directly on the fishbones residues and also ores around superheated water that are located along the volcanic ridges.  A unique ecosystem. For example, at 2800m in the Indian Ocean, the scaly-foot snail (Chrysomallon squamiferum) armours its soft interior with an outer iron-sulfides shell [18].  It is the only animal so far that employs a metallic skeleton. Deep-sea animal are endangered because of the threat of mining. Indeed, due to the low-energy environment (cold&dark), all of this ecosystem is more sensitive to disturbances, only negligible migration occurs and life communities presents a slow recovery rate. It is not well studied area and there is a lack of data in terms of biodiversity due to the difficult access but some of the species could have a small distribution and therefore a risk of a global extinction must not be ignored. In 1989 a long-term, large-scale, disturbance and re­colonization experiment abbreviated as DISCOL was started in the tropical southeastern Pacific Ocean to mimic the impact of commercial mining and to achieve a better understanding of the rate, sequence, and direction of benthic community re-establishment after severe anthropogenic disturbance [19]. The removal of manganese nodules will not only destroy the hardbottom fauna. Also, the sediment clouds, created during nodule sampling by the collector vehicle, will impact the surrounding areas. After 26 years, Lledo&al came back and check the biological effects. They found a poor recovery and a significantly lower heterogeneity diversity in disturbed areas and markedly distinct faunal compositions along different disturbance levels [20]. They concluded that the impacts of DSM may be greater than expected and could potentially lead to an irreversible loss of some ecosystem functions, especially in directly disturbed areas.

Conclusion

To conclude, REE are key part of the electronics and the number of African countries that are able to deliver it, is increasing. Currently, it represents a small amount but a highly strategical material: rare earth oxides production worldwide is only worth several billions of dollars, it is yet essential for industries worth trillions. REE have unique chemical properties and a little amount can change significantly the mechanical, magnetic, optical and chemical properties. Refining process of REE from extracted ores is complex chemical process where many toxic chemicals are used and toxic minerals are extracted during the process. From our investigation, we can see obviously that REE mining projects faces environmental hurdles. Several environmental challenges must be taken by the mining industry, especially in countries where water resources are scarce to avoid contaminated groundwater, radioactive tailings and other environmental/health issues. Currently, Thorium and Uranium are seen as dangerous wastes from REE mining. Although, since Fukushima, nuclear plants are focusing on Thorium as a safer&promising alternative fuel than Uranium. So Africa holds promises for REE riches and REE mining futures in Africa is only at the very beginning. With such economic and environmental aspects, it will for sure stimulate much attention and debates in the near future…

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