The Mt Weld rare earths mine located in the Goldfields Region of Western Australia is one of the highest grade rare earth mines operating in the world. The project is owned and operated by Lynas.
The mine comprises substantial deposits of rare earth elements (REE) including lanthanum (La), cerium (Ce), praseodymium (Pr), neodymium (Nd), samarium (Sm), europium (Eu), gadolinium (Gd), terbium (Td), dysprosium (Dy) and yttrium (Y).
The feasibility study (FS) for the project was completed in March 2005 which estimated the mine development capital cost at £558m (A$1bn).
The REE materials from the mine are transported to the Lynas Advanced Materials Plant (LAMP) in Kuantan, Malaysia.
Although mining at the site commenced in 2007 and the first shipments of concentrate to the processing plant took place in 2011, the LAMP facility in Malaysia commenced commercial operations in 2013.
The plant reached its highest output of 19,737t, including 5,898t of neodymium and praseodymium (NdPr), in 2019.
Location, geology and mineralisation
The Mt Weld rare earths mine is situated approximately 35km southwest of Laverton in the Kalgoorlie Gold Fields District of Western Australia.
The Mt Weld rare earths project comprises four mining tenements including M38/58, M38/59, M38/326 and M38/327, which have long term tenure that can be automatically renewed for 20 year periods.
The deposit lies on top of a two billion year old volcanic plug and an estimated 1.8km of the plug was weathered during this period to form a high grade supergene rare earth oxide (REO) deposit. The REO mineralisation in the deposit ranges from 1% to 5%.
The Mt Weld mine houses Duncan, Crown and Swan deposits. The deposits at Mt Weld are supergene enriched deposits present within the Mt Weld carbonatite regolith, which were formed in the regolith situated above a carbonatite with the concentration of rare earths happening from the removal of calcium carbonate during the weathering process.
The mineralisation at Mt Weld deposit is present in the same weathered carbonatite regolith units with the concentration of niobium, titanium and tantalum happening within the supergene enriched LI and CZ layers while phosphates are concentrated inside the residual AP layer.
Ore reserves at Mt Weld rare earths mine
The proven and probable reserves at the Mt Weld rare earths mine as of June 2019 were estimated to be 19.5Mt with rare earth oxides (TREO) graded at 8.5% containing a total of 1.64Mt of rare earth oxides.
Total measured, indicated and inferred resources stood at stood at 55.2Mt containing 2.98Mt of rare earth oxides.
Mining and ore processing at Mt Weld rare earths mine
The conventional open pit mining method is implemented at the Mt Weld rare earths mine. The ore is initially drilled and blasted and the blasted ore is excavated and loaded on to trucks. The trucks transport the mined ore to the concentration plant located 1.5km away from the mine.
The ore at the concentration plant is crushed before being fed to the ball bill, after which it undergoes flotation. The flotation concentrate is thickened and filtered and the final concentrate is subsequently shipped to the east coast of Malaysia to the Lynas Advanced Materials Plant (LAMP) in Kuantan, where the concentrate is processed.
The concentrate undergoes cracking where the structure of REE mineral is modified to dissolve REE in a weak acid solution. The cracked concentrate is leached to dissolve REE in a solution, which is neutralised for providing rare earth solution as feed to solvent extraction. The REEs are separated using solvent extraction (SX) in two liquid phases using in liquid-liquid extraction method.
Finally, the rare earth elements in the solution undergo precipitation to become solid carbonates or oxalates, while some elements are calcined to the respective oxides.
Mt Weld rare earths mine expansion
The Mt Weld project and Lynas Malaysia plant are being expanded as part of the Lynas NEXT expansion programme.
Lynas was awarded the major project status by the Australian Government in February 2020. The company will build a new rare earths processing plant at Kalgoorlie, which will be used for cracking and leaching the concentrate from Mt Weld mine. The plant will generate approximately 500 jobs during the construction phase and is expected to be operational by 2023.
The third tailings storage facility (TSF) was constructed and commissioned as part of the expansion in 2019. The pilot test work for the expansion of rougher flotation circuit was also completed and StackCell® Flotation technology was installed and commissioned.
The SX circuits were optimised to increase production and one of the four SX trains was converted to a two-stage circuit to produce separated Nd and Pr. The product finishing (PF) circuits were also upgraded to increase the production of separated Nd and Pr.
A new heavy rare-earths facility is also planned to be built in Texas, US, which will process the rare earth materials from the Mt Weld mine.
The contract to supply large rotary kiln and combustion system for baking rare earth elements was awarded to Metso Minerals in July 2020. The value of the contract is £11.7m (€13m). The kiln which will be used in the cracking and leaching plant will be commissioned in 2022.
Lynas was selected by the US Department of Defense (DoD) for the construction of a heavy rare earth separation facility in Texas April 2020. The facility will process the heavy rare earth materials sourced from the Mt Weld mine.
The contractors engaged with the original Mt Weld mine development included Minero Consulting, Abesque Engineering & Construction (now Forge Group), Downer EDI, while the Geovia Surpac software suite was employed for the final ore reserve estimation of the project.