Rare Earth Elements in agriculture

Rare Earth Elements, REEs, are used in a wide range of applications but there are some which are not so popular such as agriculture and plant growth. In the early 19th century some reports were published on the action of rare earths on plant growth which reported mostly positive effects. In 1972, China started to research in this field and the results were so extraordinary that shortly afterwards the commercial use if rare earths in Chinese agriculture was started.

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Research in China and its application

Today the use of REEs as trace elements in fertilisers in China is common, since in 1972 it started a systematic research including both laboratory and field experiments. Such a big application of REEs requires more information to understand its impact by answering questions such as how extended is this application in China, how the application process is, and which is the influence of REEs on the yield of plant and trees.

Over the last 30 years, the treated area with rare earths constantly increased from a minor area of 1330 ha in 1980 to 16 million ha in 1995 as it can be seen in the following Table. It means that by 1995, the treated surface with fertilisers containing REEs was 160.000 km2, which supposes an area bigger than Greece and almost as big as Tunisia.

The increases of land treated with REE containing fertilisers in China1

Table 1

After research started, different REE containing fertilizers were developed. There are basically three kinds of fertilisers: (1) Nongle, which contains RE chlorides, (2) Changles, which contains RE oxides as well as plant growth elements and (3) MAR, which consists of RE compounds mixed with amino acids. However, Changle is the most widely used of the fertilisers above. Although its composition, shown in the next Table, presents a total amount of rare earth oxides, REO, of 32.19 wt.%, some manufacturers increase it to values above 38 wt.%2.

REO in the composition of Changle in wt.%3Table 2

There are several methods to apply the fertiliser to the plant. For example, it can be used as root fertiliser, which means that it is directly applied to the soil. Besides soil application, other common methods are practised such as seed soaking, foliar spraying and seed dressing. The application stage depends on the growth stage and the specie of the plant; and it has to be continued every year to be effective. In the following appears a summary with some species, the amount of rare earths that has to be applied as well as the method to use it.

Application methods and concentration of rare earths of different crops1 Table 3

To date, an increase in the yield of about 50 plant species such as cereal, sugar or vegetables as well as for more than 20 species trees and pasture grasses have been reported. In addition to the increase of the amount produced during harvest, there are betterments in product quality. In example, sugar content in sugar increases or increased vitamin C content in apples.

Effects of rare earth elements on crops in China1Table 4

If the effects of REEs on agriculture are so amazing, why aren’t they used abroad China?

Research in Western countries and contradictions

Clear understanding of the results reported in China is hard due to the either the access to the source is difficult, or most reports are not available in English, or the papers are often lacking details of experimental method and statistical treatment. Therefore, it is crystal clear that basic and wide research on this field needs to be carried out outside China to verify those results and to provide more information.

From the 1930s to the 1980s, only a few papers on the effects of rare earth elements on plants were published outside China, in which positive effects were the general trend but there were also some authors reporting detrimental results. It was after and during the 1990s when Western countries interest on this field increased again after some Chinese papers became accessible.

Today, studies carried out in different countries such as Australia or Germany show conflicting results with both beneficial and detrimental results. Hence, most of investigations can’t confirm the great effects of rare earths on yields as well as other properties reported in China.

Consequently, there is a need for further well conducted and properly documented experiments on REEs influence on plants. Rigorous statistical treatment of data is also recommended in order to extrapolate and compare the results from different reports. Although there are many questions left unanswered regarding REE effect on plant yield and growth, more detailed reports might open up agricultural use of rare earths to other countries beyond China.

Besides studying the effect on plant growth, there is a need of reports on the effects of rare earths on human consuming rare earth containing diets and their effect on all parts of the food chain as well as on the environment.

References

1. Redling, K., 2006. Rare Earth Elements in Agriculture with Emphasis on Animal Husbandry. PhD Thesis, Ludwig-Maximilians München University.

2. Grirem Advanced Materials, http://www.chinarem.com/eng/english/products/clu.htm, consulted on 28/01/2015.

3. Pang, X., Li, D. C., Peng, A., 2002. Application of Rare-earth elements in the Agriculture of China and its Environmental Behaviour in Soil. Environmental Science and Pollution Research, 9, p143-148.

Municipal Solid Waste. A potential source of Rare Earth Elements?

The main source of Rare Earth Elements, REEs, is natural ore deposits. China, whose export quota restrictions in 2012 triggered the rare earth crisis as explained by Sofia Riaño, has the major REE ore deposits. To become less dependent on China, increasing efforts are being made to find other REE sources such as urban and landfill mining.

Solid residues from Municipal Solid Waste, MSW, could be an unconventional source of REEs and other critical raw materials. Although their recovery seems not to be economically feasible, the potential source of REEs is undeniable.

What is Municipal Solid Waste Incineration?

Incineration is a thermal treatment technology used to reduce the waste volume up to 90% and is one of the widely used technologies for treating municipal solid waste, MSW, prior to disposal at landfills. Most of today’s incineration plants incorporate power generation facilities in order to recover the heat energy produced. Additionally, they include a number of process controls and exhaust gas cleaning measures in order to ensure that the gas emissions meet the standards imposed by regulatory bodies for public health and environmental protection. A schematic diagram of this process can be seen below.

IWMF_shcematic_engFlow chart of a MSWI1

The solid waste, which is continuously fed into the furnace by a crane, is combusted at temperatures above 850 °C for a period of time long enough to ensure complete burning, usually a few seconds. The heat from the combustion is used by a boiler to generate steam that then drives a turbine to generate electricity. The exhaust gas from the boiler is cleaned by advanced pollution control systems to ensure compliance with the environmental standards.

The ash residues from incineration include bottom ash from the furnace and fly ash from the exhaust gas cleaning systems. Both bottom and fly ashes are either used as a fertiliser, reused as construction material or disposed at landfills.

MSW as a source of REEs

The composition of the resultant ashes are different from one plant to another depending on the material burn and hence the concentration of the REEs in the resultant material. Additionally, the amount of bottom ashes is several times higher than the fly ashes, therefore obtaining big differences in terms of annual flows from each source of ashes.

In the following table it can be seen the content of REEs in the bottom ashes from different plants obtained in recent studies2-4 as well as the annual flow of each element just by taking into account the total output of bottom ashes of each plant.

REE mean concentrations and annual flows of the bottom ashes

Table 1

The level of occurring REE oxides used by the industry to determine whether a deposit is economically feasible is at about 0.2 wt.%, while the values obtained in these studies are below 0.03 wt.%. Although deep economic analysis would have to be applied to the REE recovery from this ashes, it indicates that if the recovery of these elements were possible, the potential economic value of REEs in solid waste would be very low. Additionally, the MSWI residues are vitrified, thus requiring highly aggressive chemical treatments to further processing. Although these values are not very favourable, this source of REEs has the advantage of being in a granular form whereas a natural ore deposit requires an expensive and time consuming process to concentrate the ore. However, it is possible to separate the ashes by sieving in order to increase the concentration of certain elements.

The typical cut-off grade for occurring REE oxides is at about 0.2 wt.%, while the values obtained in these studies are below 0.03 wt.%, thus indicating that if the recovery of these elements were possible, the potential economic value of REEs in solid waste would be very low. Additionally, the MSWI residues are vitrified, thus requiring highly aggressive chemical treatments to further processing. Although these values are not very favourable, this source of REEs has the advantage of being in a granular form whereas a natural ore deposit requires an expensive and time consuming process to concentrate the ore. However, it is possible to separate the ashes by sieving in order to increase the concentration of certain elements.

These results are from studies focused on solid waste from households and shops but there are other studies on the composition of the ashes from medical scrap as well as other waste sources such as, food scrap, animal waste, sewage sludge or horticulture waste.

Nowadays, the scenario for recovering REEs from MSW ashes looks not to be economically feasible due to the difficulties to process the ashes and the low concentrations, as well as a lack of incentives and policies that could stimulate recycling of REEs.

However, although it is not possible to do it right now, it is a great challenge for both academia and industry to research further and push the edge of knowledge to either recover them at a reasonable cost or store them properly to avoid its dissipation into the environment.

References

1. Environmental Protection Department. The Government of the Hong Kong, http://www.epd.gov.hk/epd/english/environmentinhk/waste/prob_solutions/WFdev_IWMFtech.html, consulted on 14/01/2015.

2. Morf, L.S., Gloor, R., Haag, O., Haupt, M., Skutan, S., Di Lorenzo, F., Böni, D., 2013. Precious metals and rare earth elements in municipal solid waste – Sources and fate in a Swiss incineration plant. Waste Management, 33, p634–644.

3. Zhang, F., Yamasaki, S., Kimura, K., 2001. Rare earth element content in various waste ashes and the potential risk to Japanese soils. Environment International, 27, p393-398.

4. Funari, V., Bokhari, S.N.H., Meisel, T., Vigliotti, L., Braga, R., 2014. The REE potential in “urban” ore deposits: and evaluation and prospecting tools from Italian municipal solid waste incinerators.  Proceedings of the 1st International Conference on European Rare Earth Resources (ERES), p468-475.