Environmental pollution and treatment technology

Rare earth products processing and manufacturing industry

1. Introduction

1.1.Defining rare earth element

Rare earth elements consist of a group of 15 metals. In most cases and usage patterns in the modern economy, these 15 elements are oxides. The names of the elements are Cerium, Dysprosium, Erbium, Europium, Gadolinium, Holmium, Lanthanum, Lutetium, Neodymium, Praseodymium, Samarium, Terbium, Thulium, Ytterbium, and Yttrium. The rare earth elements are often informally subdivided into heavy rare earth and light rare earth based on the atomic number of the element. Lanthanum, cerium, praseodymium, neodymium, promethium and samarium with atomic number from 57 to 62 are generally referred to as the light rare earths. Tyyrium, europium, gadolinium, terbium, dysprosium, holmium, erbium, thulium, ytterbium, with atomic numbers 39 and from 63 to 71 are generally considered as heavy rare earths. Although yttrium with an atomic number 39, it is lighter than the light rare earth element, it is included in the heavy rare earth group because of its chemical and physical associations with heavy rare earths in natural deposits (Kanasawa and Kamitani 2006).

1.2.Rare earth distribution in China

There are rare earth reserves in about 34 countries. For China, in the past, it was always claimed that China was abundant in rare earth deposits, and had been the largest producer, consumer, and exporter of rare earth products (Cheng et al. 2010). But thing has been changed with the discovery of new deposits worldwide and the exploitation of China itself through these years. From 1987 to 2010, more than 1.6 million tons of rare earth reserves (count as oxides) were produced. Normally, the recovery rate was about 20%–30%. It can be estimated that about 530–800 t of rare earth resources have been mined. The proved rare earth reserve of China was 43 million tons. So the residue of rare earth reserve should be 3 500 to 3 770 t. If the deposit in Vietnam is included, the proportion of China rare earth reserve will be reduced to 32.72. China has contributed too much rare earth products to the world. But, China was definitely not the only supplier of rare earth products. There are abundant rare earth deposits besides China (Cheng et al. 2010).

1.3.Global rare earth distribution

Rare earth elements are distributed widely in the Earth. In Asia, fourteen countries have rare earth deposits. Japanese companies are constructing joint ventures with five of them, including Vietnam, India, Mongolia, Kazakhstan and Kyrgyzstan; Lynas is establishing a processing factory in Malaysia. In Europe, six countries have been found to have rare earths. In that, Greenland is getting into production; its product will be rare earth concentrate and aim to have 20% of the market in the future. Estonia has a production capacity of 3 000 t oxides and metals with concentrate from Russia. In North America, the United States has just finished its rare earth deposits survey, and its producer MolyCorp is ready to accelerate its rare earth production. For Canada, there are many small scale rare earth reserves with good heavy rare earth elements contents for economic exploitation, and have attracted many investors. One of its mining companies, the Great Western Group is developing its rare earth production in South Africa. In South America, mainly in Brazil, there also have plenty of rare earth reserves. Brazil is one of the oldest countries to produce rare earths. It is said that Brazil has begun to produce rare earths since 1884. In General, There are about 34 countries with rare earth deposits in the world. So rare earth elements are widely distributed in the Earth. It is really not that rare! With time

going, it is believed that more rare earth deposits will be discovered. Based on figure 1, 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% (Cheng et al. 2010)

World mine production of rare earth oxides (REOs) grew rapidly about 7 per cent per year from 1990 through 2006 before decreasing in 2007 owing to worldwide economic conditions, with growth increasing but at a slower pace after 2007 (figure 2). The growth in REO production directly correlates to the growth in REO consumption, which, in turn, has been tied to the general economic growth for the historic uses of REOs such as catalysts for fluid cracking and catalytic converters for automobiles, glass and metallurgical industries, and phosphors and the increase of high-technology uses tied mainly to alternative energy systems such as batteries for hybrid cars and permanent magnet applications for electric motors, stereo speakers, and wind turbine generators (Hedrick and Castor 2006).

Figure 1: Global rare earth element reserve 2010 (Chen et al. 2010).

Figure 2: chart showing world mine production of rare earth oxides from 1990 to 2008. Data are from Hedrick (1996-2009) and Cordier and Hedrick (2010)

2. Technology and processes

2.1.Uses of rare earth element

Rare earths comprise an enormous array of technological applications and are becoming widely used due to their unique catalyst, magnetic, and optical properties. For example, all cell phones and laptops contain rare earths. As developed nations modernize and transition to green economies, rare earth consumption for clean energy applications will become an ever increasing phenomenon and vital to successful conversions from traditional industrial economies. Such modern and clean energy technologies govern today’s societies to the extent experts consider rare earth usage significant economic indicators (Castor and Hedrick 2006).

2.1.1. Commercial application

2.1.1.1. Lathanum

Lanthanum comes from the mineral bastnasite, and is extracted via a method called "solvent extraction." Lanthanum is a strategically important rare earth element due to its activity in catalysts that are critical in petroleum refining. By one estimate, lanthanum "cracking-agents" increase refinery yield by as much as 10%, while reducing overall refinery power consumption (Huang, 2010).

2.1.1.2. Cerium

Cerium is the most abundant of the rare earth elements. Cerium is critical in the manufacture of environmental protection and pollution-control systems, from automobiles to oil refineries. Cerium oxides, and other cerium compounds, go into catalytic converters and larger-scale equipment to reduce the sulfur oxide emissions. Cerium is a diesel fuel additive for micro-filtration of pollutants, and promotes more complete fuel combustion for more energy efficiency (Huang 2010).

2.1.1.3. Neodymium

According to Hedrick and Castor (2006), Neodymium is a critical component of strong permanent magnets. Cell phones, portable CD players, computers and most modern sound systems would not exist in their current form without using neodymium magnets. Neodymium-Iron- Boron (NdFeB) permanent magnets are essential for miniaturizing a variety of technologies. These magnets maximize the power/cost ratio, and are used in a large variety of motors and mechanical systems (Huang, 2010).

2.1.1.4. Europium

Europium offers exceptional properties of photon emission. When it absorbs electrons or UV radiation, the europium atom changes energy levels to create a visible, luminescent emission. This emission creates the perfect red phosphors used in colour televisions and computer screens around the world. Europium is also used in fluorescent lighting, which cuts energy use by 75% compared to incandescent lighting. In the medical field, europium is used to tag complex biochemical agents which help to trace these materials during tissue research (Huang, 2010).

2.1.1.5. Praseodymium

Praseodymium comprises just 4% of the lanthanide content of bastnasite, but is used as a common colouring pigment. Along with neodymium, praseodymium is used to filter certain wavelengths of light. So praseodymium finds specific uses in photographic filters, airport signal lenses, welder's glasses, as well as broad uses in ceramic tile and glass (usually yellow). When used in an alloy,

praseodymium is a component of permanent magnet systems designed for small motors. Praseodymium also has applications in internal combustion engines, as a catalyst for pollution control (Komuro et al. 2006).

2.1.1.6. yttrium

Yttrium is rare in bastnasite, so is usually recovered from even more obscure minerals and ores. Still, almost every vehicle on the road contains yttrium based materials that improve the fuel efficiency of the engine. Another important use of yttrium is in microwave communication devices. Yttrium- Iron-Garnets (YIG) is used as resonators in frequency meters, magnetic field measurement devices, tuneable transistors and Gunn oscillators. Yttrium goes into laser crystals specific to spectral characteristics for high-performance communication systems.

2.1.1.7. Other Rare Earth Element

Most of the remaining lanthanides fall into the group known as the "heavies" and include: Samarium, Gadolinium, Dysprosium, Terbium, Holmium, Erbium, Thulium, Ytterbium, and Lutetium. Samarium has properties of spectral absorption that make it useful in filter glasses that surround neodymium laser rods. Gadolinium offers unique magnetic behaviour. Thus this element is at the heart of magneto-optic recording technology, and other technology used in handling computer data. Dysprosium is a widely used rare earth element that helps to make electronic components smaller and faster. Terbium is used in energy efficient fluorescent lamps. There are various terbium metal alloys that provide metallic films for magnet to optic data recording. Holmium is exceedingly rare and expensive. Hence it has few commercial uses. Erbium has remarkable optical properties that make it essential for use in long-range fibre optic data transmission. Thulium is the rarest of the rare earth elements. Its chemistry is similar to that of Yttrium. Due to its unique photographic properties, Thulium is used in sensitive X-ray phosphors to reduce X-ray exposure (Komura et al. 2006).

2.1.2. Military application

Military applications: rare earth also play a critical role in sophisticated military applications including guidance and control systems, advanced optic technologies; radar and radiation detection equipment and advanced communication systems. Some of the defence related weapons and equipment that contain rare earth are predator’s unmanned aerial vehicles, tomahawk cruise missiles, zumwalt-class destroyers, night vision goggles, smart bombs and sonar transducers.

Table 1: the rare earth elements used for military equipment

2.2.Rare earth element processing cycle

Rare earth production is more involved than the production of well-known base and precious metals. This is due to the complexity of the mineralogy and the fact that one rare earth mineral may contain up to 16 different elements. These elements exist in different distributions specific to each deposit. Generally speaking, the various rare earth elements must be separated from one another in order to be economically saleable.

Figure 3: the general process of rare earth processing

The first step, the mining, most frequently takes the form of open pit mining. However, there are also deposits which would require underground mining, e.g. the Canadian deposit at the Thor Lake. In open pit mining, before reaching the ore rich in the metals to be extracted, the overburden material (soil and vegetation above the bedrock) as well as the waste rock (not ore bearing or having a too low concentration of the ore) need to be removed and are stockpiled. The second step after mining of the crude ore is milling. The ore is crushed and subsequently ground to fine powder in the mill with the aim of creating a high surface which is needed for the further separation. The third step is the separation of the valuable metals from the rest of the ore by physical separation methods. The most commonly used method is flotation, which requires a lot of water and chemicals (flotation agents) as well as a high amount of energy. The input into the flotation is the milled crude ore with usually low concentrations (grades) of REO (often between 1 and 10 %). The product of the flotation is an enriched concentrate with a higher REE-percentage (in the range of 30 – 70 %). The huge waste streams, called tailings, are a mixture of water, process chemicals and finely ground minerals. Usually, the tailings are led to impoundment areas, which can be either artificial reservoirs or even natural water bodies like lakes. They are surrounded by dams. Finally, the concentrate undergoes further processing. It is transported to a refinery which can be off-site. There the REE are further extracted and separated into the different elements as required. This separation of individual REE is particularly difficult due to their chemical similarity (Koyama et al. 2009).

3. Environmental concerns and environmental impacts

3.1.Geochemistry and possible contaminants

Figure 4: the possible contamination of rare earth mining and processing on the environment

3.1.1. tailings

The major short- and long-term risks are constituted by the tailings. The tailings consist of small-size particles with large surfaces, waste water and flotation chemicals. Usually, they remain forever in the impoundment areas where they are exposed to rain water and storm water runoff. During this continuous exposure to water, toxic substances can be washed out. If the ground of the impoundment areas is not leak-proof, there will be steady emissions to groundwater. Another serious risk is the storm water run-off when heavy rain falls occur and the impoundment areas are not able to store the huge amounts of storm water. Then, large amounts of untreated toxic water will pollute surrounding water bodies and soils. The composition of polluting water is site-specific as it depends on the composition of the host minerals and the used flotation agents. However, in most cases, the tailings include radioactive substances, fluorides, sulphides, acids and heavy metals. It is important to note that most of the rare earth deposits contain uranium, thorium and their further decay products. Only very few known deposits are free from radioactive substances. An ecological disaster will occur if the dam collapses and the highly toxic water and sludge flood the surroundings. There are several risks which might cause a dam collapse: the dam might fail due to overtopping from storm water, collapse due to poor construction or burst due to seismic events. These risks require a long-term monitoring as the dams must not only remain stable during the mining operation, but also keep intact over decades and centuries after the closure of the mine (Koyama et al. 2009).

3.1.2. Waste rock stockpiles

A similar risk is given by the waste rock stockpiles. They are also exposed to rain water, and toxic substances such as radioactive substances, fluorides, sulphides, acids and heavy metals will be washed out and spread into water bodies and soil, if no water management and water treatment is installed. In most cases, the potential release is lower than in the tailings, as the rock consists of coarse minerals whereas the tailings consist of finely milled particles.

3.1.3. Open pit

Another environmental risk is the open pit itself, particularly after the closure of the mine. It will be exposed to rain water, which will wash out toxic and radioactive substances as described above for the waste rock stockpiles (Koyama et al. 2009).

3.1.4. Air emissions

Besides the manifold risks due to toxic and radioactive water emissions, the mining and processing also causes serious air emissions if no adequate measures are taken. A main risk factor for the workers and the neighbourhood are wind-blown dust particles containing thorium or other radioactive substance. Further toxic substances in the dust might be heavy metals. The dusts arise from different sources: the mine and the mining operations, the milling, the transportation and storages as well as from the wind-blown dust particles from the waste rock stockpiles or the tailings. The two last-mentioned sources are a long-term risk if no adequate post-operative treatment will be implemented after the closure of the mine.

3.1.5. Land use

Further environmental harm is connected to the land-use. It covers the mine, the storage of the waste rocks, the tailings, the whole infrastructure and the surrounding areas, which are affected by pollution during the mining operation as well as after the mine closure. Another environmental burden is the large water consumption, particularly if the mining is carried out in dry areas.

3.1.6. Climate change

The further refining of the rare earth concentrate is a very energy-intensive process and causes serious air emissions such as SO2, HCl, dust, radioactive substances. If no abatement technologies are installed. Depending on the used energy carriers, high CO2-emissions will arise and contribute to climate change.

3.1.7. Radioactive waste

Furthermore, radioactive waste arises in most cases, as the majority of the rare earths deposits also contain thorium and uranium. The radionuclides are partly separated in the flotation and partly remain in the tailings. The other part enters the further processing with the concentrate and is subsequently separated. A safe disposal is required in all cases.

3.2.The potential risk to human health

3.2.1. Radionuclides

Uranium-238, thorium-232, and their decay products could present a threat to human health and the environment. Radium-226 in the uranium-238 decay chain produces radon-222 gas and bismuth-214, which are dangerous radionuclides. Due to the short half-lives of radon-222 and bismuth-214 from the radioactive decay of radium-226, these isotopes exhibit accelerated decay rates releasing energetic particles and rays in shorter time spans than other radioactive isotopes. This explains why radium-226 is often regulated as opposed to uranium-238 (Long et al. 2010). The energetic particles and rays from radioactive decay could represent a threat to human health and the environment as well. The energetic particles are essentially small fast moving pieces of atoms, while the energetic rays are a form of electromagnetic radiation. The energetic nature of these radioactive by products makes them dangerous. They have the potential to dislodge electrons from important biological molecules including water, protein, and DNA. Ores containing uranium-238 and thorium-232 are very mobile as dust resulting in air and soil contamination, where radon-222 gas is constantly released (Koyama et al. 2009). Radionuclides released into the atmosphere can be carried by wind and travel long distances before settling in soil or water. Uranium is very soluble in water and radium, although less soluble, can also result in groundwater contamination. Thorium is generally insoluble and seldom a groundwater concern (Zakotnik et al. 2010). The radioactive materials reaching the ground can become incorporated by plants, which can then bio accumulate in organisms eating plants, including humans. Various studies have shown low doses of radiation causes humans no harm, but massive amounts of ionizing radiation can cause detrimental health effects. Ionizing radiation from radioactive decay is known to be a human carcinogen. The decay products of radon-222 gas in air represent the greatest risk to developing cancer. The energetic particles can be inhaled and harm lung tissue to the extent cancerous cells can develop. These carcinogenic effects can be observed in all living organisms (cheng et at. 2010).

4. pollution control

4.1.Recycling of rare earth elements

Only a few industrial recycling activities are currently implemented for rare earths. Until now, there has been no large scale recycling of rare earths from magnets, batteries, lighting and catalysts. Principally, the recycling processes for the rare earths are quite complex and extensive if re-use is not possible and a physical and chemical treatment is necessary. Most of the recycling procedures are energy-intensive processes. The main post-consumer activities is the recycling of rare earths from

electric motors and hard disks and other electronic components which will require intensive dismantling. Several constraints for a wider recycling of rare earths were identified: the need for an efficient collection system, the need for sufficiently high prices for primary and secondary rare earths compounds, losses of post-consumer goods by exports in developing countries and the long lifetime of products such as electric motors in vehicles and wind turbines of 10 to 20 years before they could enter the recycling economy (Zhang et al. 2010).

4.1.1. Advantages of recycling

The recycling of rare earths has several advantages in comparison to the use of primary resources. Europe is one of the globally large consumers of rare earths. Increasing amounts of waste from final products containing rare earths are arising in Europe. These valuable resources should be returned to the industrial metabolism by “urban mining”. The dependency on foreign resources will be reduced by supplying the European market with secondary rare earth materials. Apart from a few specialised industries and applications, the know-how in rare earth processing is quite low in Europe. The building up of know-how in recycling will widen the competency of enterprises and scientific institutions in Europe concerning rare earth processing. The processing of secondary rare earths will be free from radioactive impurities. The mining and further processing of primary rare earths is involved in most cases with nuclear radiation coming from radioactive elements of the natural deposits. The recycling requires some energy carriers and chemicals. On the other hand it saves significant amounts of energy, chemicals and emissions in the primary processing chain. It is to be expected that most recycling processes will have a high net-benefit concerning air emissions, groundwater protection, acidification, eutrophication and climate protection (Zhang et al. 2010).

4.2.Steps that should be taken by manufacturers to reduce environmental contamination

Mining of pure monazite minerals is banned due to the high-level radioactive elements and the resulting environmental damage. As for the operation and technological equipment, the facilities for the processing of bastnaesite and mixed minerals are obliged to install a complete treatment system for waste water, waste gas, and solid waste. Regarding ion adsorption deposits, ponding and heap leaching was banned due to massive environmental damage. Instead, the ISL (In-Situ Leaching) method shall be applied. Saponification with ammonia is banned from rare earth refining. Elementary metal refining should not adopt the process of electrolysing metals by their chlorides. (Long et al. 2010). With respect to the electrolysis system when using molten salt fluoride, facilities should be equipped with a treatment system capable of dealing with fluorine-containing waste water and waste gas. Fluor-containing solid waste should be disposed separately and must not be mixed with other industry residues. Requirements for an efficient electricity supply and specifications concerning the maximum energy demand per ton of rare earths produced are also indicated. Regarding the resource aspects, it is also required that the mining-loss rate for mixed rare earth minerals and bastnaesite should not be more than 10 %, while the ore dressing recovery rate of these ores should be not less than 72 %.The ore dressing recovery rate of ion adsorption deposits should not be less than 70 %. The recycling rate of ore dressing waste water of mixed rare earth minerals and bastnaesite should be not less than 85 %, while that of ion adsorption deposits should not drop below 90 %. The rehabilitation of plants and vegetation after mining of ion adsorption deposits should include at least 90 % of the affected area. The yield of refined rare earth metal should be more than 92 % (Rabah, 2008).

5. Economic impacts of rare earth manufacturing

Analysis of the substitution of rare earths and their efficient use the examination of substitutions for scarce REE has shown that there is quite rarely a simple substitution of a REE compound by another compound. In most cases substitution requires a totally new product design. The identified options for

substitution in the case of the major green applications are summarised below: Rare earths are currently used in around 14 % of newly installed wind turbines with a gear-less design and technical advantages in terms of reliability. A supply shortage of rare earths would lead to a shift to alternative turbine types. Further research on a higher reliability of traditional techniques with gears would support this substitution Rare earths are used in permanent motors of hybrid electric vehicles and electric vehicles. Substitutions based on alternative electric motor designs are principally available. However, R&D is required for a higher performance of existing electric motor types and for the realisation of new motor concepts. Most new energy-efficient lighting systems contain rare earths (compact fluorescent lamp, LED, plasma display, LCD display). Substitutions are rare, particularly for compact fluorescent lamps. R&D is required for alternative phosphors with high efficiency and high light quality. Automotive catalysts contain cerium, and catalysts for petroleum cracking and other industrial processes contain lanthanum. Substitutions are rare, and R&D is urgently required for alternative catalysts. Concerning a higher efficiency of the rare earth use, R&D is urgently needed in all fields of application and is also needed on the supply side to enable higher efficiencies in mining, beneficiation and processing. One example for high losses in the production chain is the traditional magnet production in China (Cheng, 2010).

6. Conclusion

As with any mine or refinery, rare earth element production could contaminate the environment if best management practices are not used and the operation is not closely monitored. Federal and state agencies must determine how to best oversee the matter to ensure this operation does not put human health and the environment at risk. Many potential contaminants reside within rare earth element bearing rocks and minerals. The possible contaminants include, but are not limited to, radionuclides, rare earth elements, metals such as barium, beryllium, copper, lead, manganese, and zinc, sulphide minerals, carbonate minerals, and other potential contaminants such as fluorine and asbestos minerals. Mining exposes these possible contaminants, while refining isolates and concentrates the possible contaminants. Rare earth element mining is hardrock mining, so any of the environmental concerns associated with hardrock mining could be a concern with rare earth element production. The possible contaminants cause negative effects towards aquatic and terrestrial organisms in addition to humans. Some of the radionuclides and metals contaminants are even classified as human carcinogens by international and federal health agencies. Others possible contaminants increase the mortality rates of aquatic and terrestrial organisms. Cooperation between all government agencies designed to protect the environment and companies responsible for rare earth element production will prove invaluable in ensuring these operations do not pose a threat to human health and the environment.

7. References

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Komuro, M.; Satsu, Y.; Suzuki, H.: Microstructure and magnetic properties of NdFeB magnets using fluorides nano-coated process, Advanced Research Laboratory, Hitachi Ltd., Japan, Materials Science Forum, Vol. 634-642, pp. 1357-1362, 2010

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Zhang Xuanxu, Yu Danghua, Guo Lianping, Test Study New Process on Recovering Rare Earth by Electrical Reduction – P507 Extraction Separation Method, Journal of Copper Engineering, Vol 1, 1009-3842(2010)-0066-04, 2010.