SMART DOG MININGTM

It takes a smart dog to find hidden treasures

Rare Earth Element Mining and Processing

Part 1: Geology of REEs

 Rare Earth Elements (REEs) do not occur as native elemental metals in nature, only as part of a host mineral. Due to this, the recovery of Rare Earth Minerals (REMs) must be done using a complex processing method to chemically break down the minerals containing the REEs.

 While there are more than 200 known REE-bearing minerals, only three are generally considered ores economically feasible for extraction: bastnasite, xenotime, and monazite:

 ·         Bastnasite, the most abundant among the three REE mineral ores, is a carbonate mineral found mainly enriched in Light Rare Earth Elemts (LREEs) (e.g., cerium, lanthanum, and yttrium). Bastnasite is found in vein deposits, contact metamorphic zones, and pegmatites. It forms in carbonate-silicate rocks occurring with and related to alkaline intrusions (e.g., Mountain Pass mine).

·           The two phosphate minerals, xenotime and monazite, can occur together, but crystallize in different temperatures and pressures from an igneous environment. While these minerals can contain any of the REEs (i.e., Heavy Rare Earth Elements (HREEs) or LREEs), enrichment of specific REEs is variable and a function of the temperature and pressure in which they formed. Monazite commonly occurs in placer deposits; xenotime can occur along with monazite, but generally occurs as a more minor constituent of these types of deposits. Deposits of phosphate rare earth ores provide the opportunity to produce co-products of phosphates and REEs. Thorium and uranium may also be found and produced as a co-product, or may represent a significant management challenge. A further description of these two minerals follows:

o   Monazite is generally enriched with the LREEs cerium, lanthanum, and neodymium, but can also contain HREEs, particularly yttrium. The predominance of LREEs is due to the lower crystallization temperature and pressures of this mineral; however, it typically contains more HREEs than bastnasite ore deposits. It occurs in acidic igneous rocks (primarily pegmatites), metamorphic rocks, and some vein deposits. Monazite is resistant to weathering and occurs in many placer deposits as the host rocks are eroded. Thorium may also be associated with monazite in various amounts.

o   Xenotime crystallizes under higher temperatures and pressures; therefore, its crystalline structure more readily accommodates a higher ratio of HREEs (terbium through lutetium, and yttrium) than is commonly found in monazite. It is primarily a yttrium phosphate mineral and occurs as a minor constituent of granitic and gneissic rocks. Although not always present in significant quantities, uranium and thorium can also occur as constituents of xenotime.

·         There are two other important REE-containing minerals:

Euxenite which contains yttrium, erbium, and cerium. It is found mostly in placer deposits in Idaho, and occurs as a tantaloniobates (e.g., minerals where Ta and Nb form the compound) of titanium, rare earths, thorium, and uranium.

o   Allanite is an epidote mineral and contains cerium, lanthanum, and yttrium. It occurs in igneous, metamorphic, and hydrothermal environments and is disseminated in pegmatite or occurs in vein deposits.

These five minerals are considered to represent the principal occurrences and the potentially more significant REE reserves in the United States. However, many other minerals containing REEs do occur, and deposits of these minerals could be found in the United States and prove to be viable for mining. It is also not uncommon for REEs to be produced as a coproduct or byproduct of other mineral production.

 Mineralogy

 Rare-earth minerals occur chiefly in association with highly alkaline (pH Basic) volcanic (igneous) intrusive rocks (plutons) and in placers derived from them.  The rare-earth elements may partially or wholly replace calcium in minerals such as fluorite and apatite.

 Monazite contains rare earths of the cerium and lanthanum subgroups, plus as much as 30% thorium and minor yttrium. It is a yellowish to reddish brown monoclinic mineral having both hardness and a specific gravity of 5. It occurs in commercial concentrations in beach and stream placers and in lesser amounts in veins. It also occurs as accessory minerals in igneous and metamorphic rocks.

 Bastnaesite contains as much as 75% rare-earth oxides of the cerium subgroup. It is a light-yellow to brown hexagonal mineral having a hardness of 4.5 and a specific gravity of 5. It occurs chiefly in carbonatite plutons and subordinately in veins, pegmatites, and skams.

 Xenotime is an yttrium-subgroup phosphate that occurs in igneous and metamorphic rocks, pegmatites, and placers. It is a pale yellow to brownish-green tetragonal mineral having both a hardness and specific gravity of 4.5. Rare-earth elements replace part of the calcium in some apatite, particularly in some carbonatite plugs, alkalic magnetite deposits, and marine collophane deposits.

 Cerite is a brown to gray, calcium, cerium subgroup hydroxyl silicate, having a hardness of 5.5 and a specific gravity of 4.9. It occurs in carbonatites, pegmatites, and skarns. Gadolinite is a ferrous, beryllium, yttrium-subgroup orthosilicate. It is a brown to black monoclinic mineral with a hardness of 6.7 and a specific gravity of 4.4, which occurs chiefly in pegmatites.

 Multiple-oxide minerals occur chiefly as brown to black, heavy (4 to 5.7), hard (4.5 to 6.5), radioactive, metamict minerals in pegmatites, alkalic igneous rocks, and related veins.

and placers. The five commercial minerals are niobate tantalate-titanates. Samarskite, however, lacks titanium, while brannerite contains titanium only. All contain uranium, and all but betafite contain thorium and yttrium-subgroup rare earths. Betafite, brannerite, and euxenitc contain calcium, while brannerite and samarskite contain iron. Euxenite and samarskite contain cerium-subgroup rare earths; fergusonite and samarskite contain erbium.

 Classification of Deposits

 Rare-earth mineral deposits have an stong relation to alkalic igneous rocks, especially carbonatites. Such ores are found in association with a late carbonatite pluton and in the veins that fill the fractures that accompanied its emplacement.  Carbonatite stocks represent excellent exploration targets for large deposits of rare-earth ores in economic concentrations.

 Small quantities of exotic rare-earth minerals occur in igneous pegmatites formed from residual fluids that were derived from nearly complete crystallization of presumably alkalic or subalkalic magmas. Such pegmatitic occurrences are of manly academic interest, unless labor costs are low enough to permit them to be economic.

 Rare-earth elements also occur both as discrete minerals and in apatite, in association with high-temperature, metamorphic, nontitaniferous magnetite deposits. Rare-earth concentrates are logical coproducts derived from the beneficiation of iron ores of this type. Less commonly, rare-earth minerals such as allanite can occur in economic concentrations in skarns.  Economic amounts of monazite, bastnaesite, and xenotime have been found in veins in a few places. .

 Tertiary and Recent beach placers in Brazil, India, Australia, and the United States are major sources of monazite recovered as a coproduct from the mining of magnetite, ilmenite, and rutile sands. Euxenite and brannerite have been mined from recent alluvial placers in south-central Idaho.

 Yttrium occurs in certain marine phosphatic shales such as the Phosphoria Formation of Permian age in Idaho. Although the apatite in these rocks contains only a small amount (as much as 1000 ppm) of yttrium, it might be feasible to extract an yttrium concentrate as a byproduct during the beneficiation of the phosphate rock.

 Carbonatite plutons and ancient, as well as modern, placers should continue to be the principal sources of rare-earth ores.

 Part 2: Mining & Processing

 Mining Methods

 The technology for mining monazite beach placers is similar to that employed for diamonds, gold, or cassiterite. Offshore operations use floating dredges having either suction or bucket elevators, with capacities up to 1200 tph of sand.  Slurry is carried hydraulically from the dredges to the beneficiation plant.

 Subaerial deposit's are worked by draglines or power shovels. However, in places where scattered pockets of ore must be mined selectively, bulldozers and scrapers are used, and the ore is trucked to the concentrator.

 In underdeveloped nations where capital is scarce and labor cheap and abundant, primitive methods dating back to Jason are used, in which hand-dug pits and simple sluice boxes recover black-sand concentrates.

 Conventional open-pit mining methods are employed in the bastnaesite deposit at Mountain Pass, Calif. The ore is drilled, blasted, and loaded into trucks by power shovels, then hauled to the mill.

 Milling Techniques

 Pure monazite contains approximately 70% rare-earth oxides. Standard acceptable grades for monazite-sand concentrates are 55. 60, and 66% rare-earth oxides.

 Primary beneficiation of beach sands is effected either on floating dredges or in land based plants. After oversized particles have been screened, a black-sand concentrate is recovered mechanically using jigs, sluice boxes. shaking tables, and/or spiral concentrators. Through the use of induced-roll electromagnetic separators and high-tension electrostatic roll separators, a monazite concentrate, which may represent only 1 % of the total black sands, can be recovered in the magnetic nonconductive fraction. Flotation cells with oleic acid are used to enrich Indian monazite sand concentrates.

 Pure bastnaesite also contains approximately 70% rare-earth oxides. The crude ore at Mountain Pass, Calif., contains 7 to 10% rare-earth oxides. Following primary and secondary crushing, the ore is passed through a rod mill and a classifier. The slurry is then heated and passed through flotation cells which depress the barite gangue and yield a 63% rare-earth concentrate. This, in turn, is then leached by hydrochloric acid and countercurrent decantation to remove calcite, thereby upgrading the concentrate to 72%. Finally, this concentrate is calcined to remove carbon dioxide from the carbonates, yielding a 92% concentrate of rare earth oxides and fluorides.

 Processing Techniques

 The caustic process and the acid process are two common methods for treating monazite concentrates. In the caustic process, monazite is digested in hot sodium hydroxide, and filtered. The insoluble thorium and rare-earth hydroxides are separated by treatment with weak hydrochloric acid, which dissolves the rare-earth hydroxides and leaves solid thorium hydride. The thorium hydride is then dissolved in nitric acid, and thorium is recovered by solvent extraction.

 In the acid process, monazite is digested in hot sulfuric acid. Rare-earth sulfates are dissolved and removed by filtration. If present (which is common) thorium is then precipitated as a pyrosulfate, leaving the rare-earth ions in solution. Next, the rare-earth elements are precipitated as oxalates or as sodium-rare-earth sulfates. These, in turn, can be roasted to form oxides, which are then dissolved in nitric acid. The rare-earth elements are then separated from each other by solvent extraction.

 Four products may be recovered from the treatment of bastnaesite concentrates: (1) europium oxide, (2) a lanthanum-rich mixture of rare-earth metals, (3) a heavy-subgroup mixture chiefly composed of samarium and gadolinium, and (4) technical-grade cerium. The bastnaesite concentrates are roasted, calcined and leached with hydrochloric acid. Cerium oxide is filtered from the solution. The europium and heavy rare-earth elements are separated from the solution by solvent extraction.  Then the lanthanum-rich product is precipitated. After further solvent extraction, europium sulfate is precipitated, leaving samarium and gadolinium in solution.

 Thorite concentrates are digested in hot nitric acid, and filtered. The thorium is removed from solution by solvent extraction and purified by countercurrent solvent extraction. Such extraction yields a high-quality thorium nitrate from which pure thorium oxide, tetrachloride, or tetrafluoride may be produced.

 Thorium metal may then be made by metallothermic reduction or thermal decomposition of thorium tetrachloride through reduction of thorium halide or oxide with calcium, or by fused-salt electrolysis. Metallic yttrium is produced by direct reduction of yttrium trifluoride.

 Separation and purification of the lanthanide elements is a major problem. Ion exchange is an effective way to separate individual rare-earth elements. EDTA (ethylene diamine tetraacetate) solution is the best elutant for removing the lanthanide elements and yttrium from the resin in that order. Solvent extraction can be used effectively to separate rare-earth subgroups, and to isolate yttrium. Cerium and europium can be separated from other lanthanide elements by valency change reactions. Lanthanide sulfates can be subdivided by adding cold aqueous solutions containing excess alkali sulfates to alkali-rare-earth sulfates. The heavy lanthanides and yttrium remain in solution.  Fractional recrystallization permits the selective separation of lanthanum, praseodymium, and neodymium.

 Rare-earth metals are produced most successfully by electrolysis or by metallothermic reduction of rare-earth halides. Mischmetal is produced by electrolytic fusion of mixed anhydrous rare-earth chlorides. A pilot plant for the production of mischmetal by electrolytic reduction of sulfides and pure rare earths is being constructed.

 

MIke Albrecht, P.E.

o   40+ years’ experience in the mining industry with strong mineral processing experience in Precious metals, copper, industrial minerals, coal, and phosphate

o   Operational experience in precious metals, coal, and phosphate plus in petrochemicals.

o   Extensive experience studies and feasibility in the US and international (United States, Canada, Mexico, Ecuador, Columbia, Venezuela, Chile, China, India, Indonesia, and Greece).