Residue and Tailings Management
In line with the circular economy, Lynas’ approach to residues is to reduce, reuse and recycle where ever possible.
In 2019, Lynas completed the Mt Weld Tailings Dam Questionnaire, in response to the Investor Mining & Tailings Safety Initiative. View the information that Lynas provided here.
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 Mt Weld processing plant produces a residue called tailings. The tailings comprise of rare earth minerals that are not recovered into the rare earth concentrate; the gangue minerals, ore minerals and water.
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.
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 has enabled the recovered water to be retreated and re-used.
The processing plant tailings are safely stored in tailings storage facilities (TSFs) within the Mt Weld leases. As needed, 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 very low levels of thorium (Th) and uranium (U) and therefore, there are very 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. Mudfarming is undertaken to minimise water in tailings storage facilities.
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 annual environmental reporting.
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
Lynas Malaysia produces two solid gypsum residues as by-products of the treatment of the lanthanide concentrate. These are:
- Neutralisation Underflow Residue (NUF), a synthetic gypsum, and
- Water Leach Purification (WLP), an iron phosphate
NUF is not toxic to humans, animals, vegetation or aquatic life. Tests carried out by Standard and Industrial Research Institute of Malaysia (SIRIM, 2014 and 2018) and Golders Associates’ report on NUF (2014) confirmed that NUF is not explosive, corrosive, a health hazard, irritant, carcinogenic, ecotoxic or infectious and does not exhibit characteristics of ignitability, reactivity or toxicity.
Gypsum is used in a wide variety of applications, including in the construction industry (cement and plasterboard) and in agriculture (slow release fertiliser and soil conditioning).
WLP is an iron phosphate material that is a very low level Naturally Occurring Radioactive Material (NORM). Lynas Malaysia’s operations do not enhance or alter the natural radioactivity and WLP has the same radioactivity level as the feedstock material used during processing (6 Bq/g). By way of example, material with similar levels of radioactivity can be used in road base material in the United Kingdom.
As announced on 21 May 2019, Lynas will relocate Cracking & Leaching, the first stage of its operations currently located in Malaysia, to Western Australia as part of the Lynas 2025 growth plan. Under Lynas Malaysia’s new licence, the relocation is required to be completed within 4 years. Once the transition to Western Australia is complete, WLP residue will no longer be produced in Malaysia.
Lynas Malaysia was designed from the ground up as a state-of-the-art, environmentally responsible Rare Earths processing facility:
- Environmental monitoring shows no effect on the environment and no increase in background radiation at 1 km, 5km, 10 km and 20 km from the plant
- No public health or environmental incidents since operation began
- The Pakatan Harapan government’s Executive Review Committee report (December 2018) found that Lynas Malaysia is low risk compliant with relevant regulation, and residue storage facilities are operated in a proper manner
- A 2014 report by the International Atomic Energy Agency (IAEA) found that “…risks to members of the public and to the environment associated with the operation of Lynas Advanced Material Plant are intrinsically low”.