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Rare Earth Minerals Processing
There are two major REE ore minerals from which predominantly LREE can be extracted with relative ease: bastnaesite and monazite.
Bastnaesite, a fluorocarbonate mineral and monazite, a phosphate, contain predominantly either cerium, lanthanum or yttrium.
The world-class rare earth deposit Mountain Pass (California, USA), operated by Molycorp, contains bastnaesite ore as part of a carbonatite body (8-12 % REO) intruded into gneiss.
At the Mountain Pass operation the run of mine material is passed through a cascade of physical processing steps including crushing, screening, grinding and flotation finally producing a bastnaesite pre-concentrate from which products with different REO contents are derived in subsequent thermal and chemical processing steps.
The REE mineral concentrate is digested in hydrochloric acid and the resulting REE laden liquor is passed through a sequence of solvent extraction with several mixer-settler steps. Final products of this element-selective extraction processes are either separated REO or other products following customer requirements.
The Mount Weld (southern Western Australia) REE deposit representing the laterized cap of a large carbonatite shows generally high LREE/HREE ratios with locally enriched HREE and yttrium which are present in secondary phosphates such as crandallite and xenotime.
Physical concentration of the REE minerals is performed at Mount Weld. The run of mine material is subjected to crushing, grinding and flotation to produce a concentrate with a grade of 40% REO. The obtained flotation concentrate is sent to a thickener, the resulting pulp is dewatered by applying a pressure filtration.
The separation into individual REO is carried out at Lynas Advanced Materials Plant in Malaysia. Here the mixed REO concentrate is cracked with sulfuric acid and leached with water. The pH is then reduced by addition of magnesium oxide and precipitated by-products are filtered off. At the separation stage the solution is mixed with hydrochloric acid and extracted in individual extraction stages using organic chemicals to separate the RE chlorides and remove impurities. In the final stage the different extraction liquids are treated with sodium carbonate, neutralized with magnesium oxide and precipitated with sodium carbonate solution or oxalic acid. Precipitated products are filtered and either further treated to obtain RE oxide or carbonate products.
Other major monazite sources are heavy mineral sands from so-called placerdeposits which are usually mined by dredging or scraping, followed by screening and concentration in spiral concentrators. Mineral concentrates are then washed, dried and fed into magnetic separators to remove e.g. ilmenite and other magnetic minerals. In a subsequent step electrically conducting and nonconducting heavy minerals are typically separated in electrostatic plate separators followed by induced roll separators that help to distinguish induced magnetic from non-magnetic minerals (i.e. xenotime and monazite from zircon). The former two minerals can then be separated by wet table or air table taking advantage of their slightly different specific gravities.
The monazite concentrate is typically digested in hot sulfuric acid and partially neutralized. Upon addition of Na2SO4 the lighter REE can be precipitated from the sulfuric solution which holds the remaining heavier REE.
An example of such a placer rutile-zircon-ilmenite deposits is at Eneabba on Australia’s west coast north of Perth, which produced about 2,500 t of monazite annually, with monazite making up 0.5% to 7.0% of the heavy minerals. Present-day production is from India, Malaysia, Sri Lanka, Thailand, and Brazil. In India, monazite production from titania-zircon placers is government controlled, and a domestic plant processes monazite concentrate into REE products.
The highly valuable HREE are currently largely sourced either from xenotime or from ion adsorption clays with the largest deposits and operations located in China, e.g. in the Guangdong, Hunan, Jiangsu and Jiangxi provinces.
Processing of these clays is a fairly simple, low cost operation involving typically ion exchange by washing with (NH4)2SO4, removal of U and Th from the pregnant solution, precipitation of RE carbonates, filtering and calcination to obtain a mixed REO concentrate which can the further be separated and purified into individual REO. Recovery of REE in these operations typically exceeds 90%.
An interesting alternative source of HREE is eudialyte, a common zirconosilicate and principal ore mineral in a number of alkaline igneous REE ore deposits rich in HREE. It can be readily dissolved in different acids, making contained REE amenable to further hydrometallurgical processing.
A principal HREE deposit with above mentioned zirconosilicates is the Norra Kärr deposit in Sweden, which is a peralkaline intrusion hosted rare earth element and zirconium mineralization with predominantly HREE bearing ore minerals such as eudialyte and catapleiite. Processing of the ore involves crushing, grinding and magnetic separation to obtain a REE bearing zirconosilicate (eudialyte) mineral concentrate and feldspar/nepheline and aegirine byproducts. The REE mineral concentrate is then sent to a sulphuric acid digestion followed by subsequent solid/liquid separation and further purification steps to finally obtain an REE carbonate and a zirconium bicarbonate product.
ANZAPLAN is not restricted to the above mentioned REE ore minerals but has a proven history in the development of processing concepts adapted to specific REE mineral assemblages including the concentration and hydrometallurgical REE extraction from less well known REE bearing pay minerals such as apatite, eudialyte, allanite, zircon, euxenite, fergusonite, pyrochlore, florencite and churchite taking full advantage of its close link between analytical and engineering department.