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Residue and Tailings Management
Click here to view the information that Lynas has prepared on tailings dams in accordance with the Investor Mining & Tailings Safety Initiative.
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 with tailings considered geochemically benign. The majority of the decant water is recovered or evaporated, with some remaining moisture retained with the solids. An extensive development program over the past 7 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 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, commissioned 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 naturally occurring 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. It details the tailings deposition regime, inspections and monitoring requirements. 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 audit report is submitted to DMIRS as an appendix to the annual environmental report.
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 minimum 3-yearly 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 to increase consolidation.
- 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.
TSF3 in foreground. TSF2 and 1 in midground. Evaporation Ponds in background
|TSF3 Scroll tracks as a result from mudfarming. Scrolls direct water to drain towards decant tower.|
Low Ground Pressure Bulldozer levels out scrolls after sufficient drying to maximise consolidation
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.
Lynas Malaysia 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 at Lynas Malaysia process (about 6 Bq/g of Th). This material is classified as very low level radioactive material. Lynas Malaysia’s operations do 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.
Since Lynas Malaysia 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) at Lynas Malaysia. This facility has been designed and constructed and is managed to meet the requirement of a PDF within Lynas Malaysia;
- 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 Lynas Malaysia concluded that the risk from the operation is intrinsically low.
All evidence related to the safety, health and environmental status and performance of Lynas Malaysia 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 Lynas’ operations, including residue management, has had no negative effect on the public and the environment.
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.