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Residue and Tailings Management
The Mt Weld deposit and its overlying sediments have formed at or near the surface in a strongly oxidising, supergene geochemical environment. Both gangue minerals (waste) and ore minerals are chemically stable and do not undergo any appreciable weathering or alteration when exposed to surface conditions. There are no sulphides or other minerals capable of producing acidic run-off water or otherwise harmful products that occur in the deposit or overburden.
The major rare earth minerals are secondary phosphates with variable calcium contents, which have been precipitated directly from groundwater under near-surface temperature and pressure conditions.
The gangue minerals are dominated by hydrated iron oxides, mostly occurring as very soft, fine grained, porous, friable siltstone. Manganese oxides, secondary silica, secondary calcium-aluminium phosphates and residual apatite and ilmenite are minor components of the ore.
The Mt Weld processing plant produces a residue called tailings. The tailings comprise the rare earth minerals that are not recovered into the rare earth concentrate; the gangue minerals and water. There is no chemical change to the minerals as a result of the processing plant. The majority of the water is recovered or evaporated, with some remaining moisture retained with the solids. An extensive development program over the past 5 years has enabled the recovered water to be retreated and re-used.
In accordance with accepted practice, the processing plant tailings are safely stored in tailings storage facilities (TSFs) within the Mt Weld mining leases. Additional TSFs will be constructed over the life of the mine. The TSFs are engineered impoundment structures that are designed for permanent on-site storage, constructed and operated in accordance with approvals from Department of Mines, Industry Regulation and Safety (DMIRS) and the Department of Water and Environment Regulation (DWER).
The rare earth ores at Mt Weld contain low levels of thorium (Th) and uranium (U) and therefore, there are low levels of radiation present in the tailings. The tailings solids contain approximately 500 ppm ThO2 and 30 ppm U3O8, which is equivalent to a radiation specific activity of 2 Bq/g. This is lower than the ore which contains on an average 750 ppm ThO2 and 30 ppm of U3O8 which is equivalent to a radiation specific activity of 3 Bq/g. The tailings onsite at Mt Weld are classified as benign and non-hazardous.
A Tailings Operation Manual has been developed for the day to day operations of the TSFs which details how to deposit the tailings, inspections and monitoring requirements, and the TSFs are inspected by Lynas plant operators on a daily basis.
A Geotechnical and Management audit is undertaken on an annual basis by an independent third party.
The long-term plan for the TSFs at Mt Weld is documented in the Mine Closure Plan which is approved by the DMIRS and updated on a periodic basis. The key steps are:
- Allow tailings to dry out sufficiently to support construction of the capping layer. This may require some mechanical turning of the tailings.
- The tailings storage facility would be shaped for natural surface drainage and erosion rates similar to those of natural landforms in the area.
- Installation of a drainage layer as the base of the capping system.
- Installation of a protection layer and a topsoil layer. The thicknesses of these materials will depend on the properties of the most suitable materials available at the time of closure, as will the measures to provide erosion resistance.
The capping system will provide the appropriate cover to minimise surface radiation levels. The topsoil layer will promote vegetation growth which will assist with erosion resistance and evapo-transpiration of infiltrated moisture.
The Lynas residue management plan is clear, focused and in line with internationally accepted principles of residue management. The residue management plan approved by the Malaysian regulatory authorities is being implemented under their guidance with the participation of established national and international research organisations. Lynas is confident that with its clear corporate residue management policy, and systematic engagements with all product stakeholders, the best principles in residue management ‘from cradle to grave’ are being realised.
Although Lynas is required to plan and site a permanent disposal facility (PDF) as an additional assurance in dealing with Lynas residues, the success of our commercialisation program to date has reduced the likelihood that a PDF will be required.
LAMP produces two solid gypsum residues as by-products of the treatment of the lanthanide concentrate. These are the Neutralisation Underflow Residue (NUF) and the Water Leached Purification Residue (WLP).
NUF is a Magnesium Rich Gypsum and has been proven to be non-toxic, non-carcinogenic, non-ecotoxic and non-radioactive. Its magnesium rich properties and other chemical properties can be used to condition poor agricultural soil, and to rejuvenate and rehabilitate unproductive and depleted land. NUF is neutralised with lime to produce synthetic gypsum.
The water leach after cracking dissolves the majority of the rare earths and some of the iron as soluble sulphates. Post leaching the liquid is partially neutralised with MgO to facilitate separation of the soluble rare earths sulphates from iron, aluminium and phosphate, which precipitate. The combined insoluble solid residue from the water leach and precipitates from the MgO neutralisation step are what is known as WLP.
The Water Leach Purification (WLP) residue, although classified as a radioactive material, has the same radioactivity level as the feedstock material used in the LAMP process (about 6 Bq/g of Th). This material is classified as very low level radioactive material. The LAMP operation does not enhance or alter the natural radioactivity. By way of example, material with similar levels of radioactivity can be used in roadbase material in the United Kingdom.
Interestingly, WLP is also an iron phosphogypsum with chemical properties that compliment NUF for the same agricultural purposes. The formulation of soil conditioners using WLP, NUF and inert filler materials has been proven through extensive research to be nonradioactive, non-toxic, and non-carcinogenic. The AELB has acknowledged that the formulated soil conditioner is not a radioactive material. During product development and commercialization, Lynas adhered strictly to all the guidelines provided by the regulatory authorities (including the DOE and the AELB) in ensuring that the products developed are both safe for people and safe for the environment.
CondiSoil is a soil ameliorant consisting of Water Leached Purification residue (WLP), Neutralisation Underflow residue (NUF) and Filler material (not scheduled waste or hazardous material).
CondiSoil was developed under the Lynas CondiSoil Project, being part of the Lynas plan for Special Management of its scheduled waste under Regulation 7(1) Environmental Quality (Scheduled Waste) Regulations 2005.
The CondiSoil Project is part of Lynas’s recovery effort to safely commercialise residues generated.
The potential use of Lynas residues as safe commercial products is vast and unlimited. Every year, Lynas invests around 0.5% of its gross income in research to develop new and improved options to reduce and reuse the residues on a commercial scale. In addition, Lynas has allocated another 0.5% of its income to other supporting research and will continue to invest substantially in the coming years. Commercial reuse is the preferred option. Permanent storage is not the preferred option.
ADDITIONAL ASSURANCE FROM A PERMANENT DISPOSAL FACILITY (PDF)
A PDF will always be the last choice in any residue management strategy because it is an unproductive long-term use of the residues and of the land. Lynas subscribes to this strategy but at the same time realizes that an assurance must be given to the public that should the need arise for a PDF, Lynas is prepared for it.
Since the LAMP started its operations under a Temporary Operating License in 2012, Lynas has assured the Malaysian government and the public of the following:
- The WLP residue is being safely stored in a temporary residue storage facility (RSF) within the LAMP. This facility has been designed and constructed and is managed to meet the requirement of a PDF within the LAMP;
- Occupational and health exposures monitored since 2012 have shown that risks to our employees from radiation and chemicals are well within the permissible limits. Lynas undertakes continuous monitoring of the environment both within and outside the plant (up to 20 km). All readings have been within the limits set by the relevant regulatory authorities and there has been no increase in background radiation levels at 1km, 5km, 10km and 20km from the plant; and
- Lynas has complied with all AELB License conditions.
In 2014, the IAEA Report of the International Post-Review Mission on the Radiation Safety Aspects of the Operation of a Rare Earth Processing Facility and Assessment of the Implementation of the 2011 Mission Recommendations on the LAMP concluded that the risk from the LAMP operation is intrinsically low.
All evidence related to the safety, health and environmental status and performance of the LAMP is routinely reported and scrutinized by the relevant authorities. Real-time and online environmental monitoring data are available to the public on the internet and on the Lynas, AELB and DOE websites. All monitoring data has shown that the LAMP operation, including residue management, has had no negative effect on the public and the environment.
The regulator approved Lynas PDF plan can now be substantiated with real information and experiences gathered from the use of the current on-site residue storage facility (RSF) to accurately design and construct a future PDF, if the need arises. However, the preferred option under the Lynas residue management plan is commercial reuse, and not permanent storage.
Lynas’ commitment to safety and the environment includes a security deposit, held by the AELB, as an additional assurance of our commitment to residue management. We have deposited over US$30m with the AELB and will deposit up to US$50m by December 2019, an amount sufficient for a PDF and site remediation, if the need arises.