Charley Creek Rare Earth Project — 2013 Technical Report

The Charley Creek Rare Earth Project in the Northern Territory, Australia, is designed to recover rare earth oxides from an alluvial heavy mineral deposit using a combination of wet and dry gravity, magnetic, and electrostatic concentration, followed by a hydrometallurgical refinery.

The proposed treatment process for the Charley Creek rare earth oxides (REOs) is conventional for an alluvial deposit carrying heavy minerals. Concentration plants at the mine site target a REO concentrate grading around 40% total rare earth oxides (TREOs). The mine site plants comprise two wet plants and one dry plant. The first wet plant uses scrubbing to remove coarser waste materials and desliming to remove finer, difficult-to-handle size fractions as fines or slimes. The second wet plant adopts gravity recovery processes using spirals to upgrade the wet concentrate to around 8-12% TREO. The dry plant then utilises various magnetic and electrostatic characteristics of the minerals to produce a clean REO concentrate at about 40% TREO grade. The planned process steps also produce ilmenite and zircon concentrate by-products.

The refinery, located closer to Alice Springs, is designed as a hydrometallurgical plant utilising conventional sulphate roasting and water leaching to solubilise the TREOs, uranium, and thorium. Once leached, a purification stage using precipitation removes thorium, iron, and phosphorous. A uranium recovery step allows the uranium to be removed from the final TREO product. Final TREO recovery as a mixed rare earth carbonate is based on carbonate precipitation.

Critical Data

Overview

The Charley Creek Rare Earth Project entails an alluvial deposit carrying heavy minerals. The mine site hosts two wet concentration plants and one dry separation plant. The first wet plant uses scrubbing trommels and hydrocyclones for size separation, while the second wet plant uses spiral gravity circuits and magnetic separation. The dry plant uses electrostatic and magnetic separation to produce a final REO concentrate. The refinery, located closer to Alice Springs, is a hydrometallurgical operation using sulphate roasting, water leaching, purification, uranium recovery by ion exchange, and carbonate precipitation to produce a mixed rare earth carbonate product. The project was at scoping study level, with further testwork required to confirm projected recoveries.

Key Process Stages

Wet Plant

The first process stage comprises two sets of scrubbing trommels with the coarse +2mm material discharged as waste and the -2mm material sent to further size separation using hydrocyclones. Hydrocycloning is conducted in two stages, with the first and second stage overflows or slimes sent to a slimes thickener, while the first stage underflow is retreated in hydrocyclones and the second stage underflow or sands is sent to a spirals gravity separation plant.

The spirals circuit is made up of a rougher bank followed by a middlings retreatment bank, with first concentrates treated in a cleaner bank of spirals and then retreated in a recleaner bank of spirals. Tailings from rougher and middlings spirals are rejected, while the cleaner and recleaner middlings and tailings are recycled. The final spiral concentrate is at a grade of about 0.9% TREO, containing about 88% of the TREOs, and is sent to the second stage wet separation plant.

The second stage wet concentration plant first screens the concentrates to remove trash. The screen undersize is treated in high energy attritioners to remove iron oxide staining. The attritioned slimes are removed using a bank of hydrocyclones, and the cyclone underflow is treated in a three-stage low intensity magnetic separation step. The non-magnetics from the LIMS is the REO concentrate, while the magnetics streams are sent to a magnetite/ilmenite concentrate dewatering step.

The non-magnetics REO stream is treated using first a hydrocyclone followed by an upflow classifier. The classifier coarse material is sent directly to the final wet plant concentrate containing 61% of the TREOs at 11% TREO, while the fines are retreated in a bank of scavenging spirals. The scavenger spirals tailings are sent to waste, while the concentrate is treated in a wet high intensity magnetic separator. The non-magnetics fraction from the WHIMS is dewatered using hydrocyclones, and the solids are collected as a zircon concentrate. The WHIMS magnetics portion is a further REO stream containing approximately 13% of the TREOs at 9.1% TREO grade. The two final wet products are dewatered using hydrocyclones and the underflow sent to the dry separation plant.

Dry Plant

The dry separation plant dries the wet plant concentrate using a fluidised bed dryer. The dried product is then treated using electrostatic separation in three steps. The first roughing step produces a conductor product separated as ilmenite concentrate and containing about 3.7% of the TREOs. The non-conductor product is retreated in a middlings electrostatic separator, with the non-conductor product comprising a concentrate containing approximately 69% of the REOs at a grade of 16% TREO. The conductor product is combined with the ilmenite concentrate, while the middlings material is retreated in a third stage electrostatic separation step.

The non-conductor product from this third stage is final REO concentrate, while the conductor product joins the ilmenite concentrate. The combined non-conductor REO product is then treated in a two-stage rare earths rolls magnetic separation step. The magnetics from the first stage is a final concentrate grading around 57% TREO with 59% REO recovery. The non-magnetic product is treated in the second stage rolls magnetic separation step to produce a magnetic final product grading approximately 12% TREO at 7.4% recovery. The non-magnetics comprise a zircon concentrate by-product carrying about 3.7% of the REOs at a grade of 1.2% TREO. The two magnetic products are bagged separately and trucked to the refining plant.

Refinery

The refinery first bakes or cures the dry plant concentrate with sulphuric acid. The dried concentrate is mixed with sulphuric acid in a pug mixer. The mixed material is then sulphate roasted at about 250°C in a rotary kiln. Roasting is conducted for about two hours. The kiln discharge drops to a quench tank where water is added. The resultant slurry is pumped to a bank of four water leach vessels, which allows the REOs to solubilise in the water.

The leached slurry is filtered in a plate and frame filter. The filter cake solids are discharged as waste, and the filtrate or REO-bearing liquor discharges to a bank of adjustment vessels. The adjustment involves oxidation-reduction potential control using magnesium hydroxide for pH control and hydrogen peroxide for the ORP control. Ferric sulphate is also added to assist removal of the phosphates. The resultant slurry is then sent to a further purification step using more magnesium sulphate plus recycled seed material to promote precipitation of phosphorous, thorium, and iron. This final slurry is filtered using candle filters, with filter cake discharged to a special repository. The filtrate is then pumped to the uranium recovery circuit, with some of the liquor recycled to the water leach circuit.

The purified liquor is pumped through a bank of three ion exchange resin columns to allow recovery of the solubilised uranium. The liquor is forwarded to the REO carbonate precipitation circuit. The IX columns are eluted using a sulphuric acid eluate. The eluant after elution is forwarded to the uranium recovery circuit. The uranium is recovered by precipitating with hydrogen peroxide using sodium hydroxide as a pH control. The resultant slurry containing the precipitant, uranyl peroxide, is filtered, and the moist filter cake product is packaged for shipment to market. The filtrate is sent to an evaporation pond.

The liquor from the IX circuit contains the REOs, which are then recovered in a carbonate precipitation circuit. Sodium carbonate is used as the precipitant. The final product filter cake is dried using an oil-fired paddle dryer at about 120°C to a moisture content between 1-2%. The final product will be a RE carbonate pentahydrate and will contain around 50% REOs.

Additional Interesting Data and Summary

The project's financial model uses an overall recovery of 60.8% rounded to 61%, while the flowsheet mass balances indicate an overall TREO recovery to market of 60.0%. The target recovery percentages are higher than those achieved to date in AML testwork, where 56% overall TREO recovery was achieved. The AML testwork did not incorporate any recycle streams, and in the opinion of the report authors, an extra 10% recovery could reasonably be achieved from some recycling. Further testwork and circuit refinements will be required to confirm that the targeted overall TREO recoveries can be achieved. Some steps within the uranium recovery circuit, such as product drying, have not been defined and are expected to be included at the next feasibility study stage. The use of classifiers such as screw classifiers should be considered for many of the solids dewatering steps rather than hydrocyclones.

Key Processes

• Wet scrubbing and desliming to remove coarse and fine waste
• Spiral gravity separation for initial upgrading
• Low intensity magnetic separation and wet high intensity magnetic separation for REO concentration
• Electrostatic and rolls magnetic separation in the dry plant
• Sulphate roasting and water leaching to solubilise REOs
• Purification by precipitation to remove phosphorous, thorium, and iron
• Uranium recovery by ion exchange and peroxide precipitation
• Rare earth carbonate precipitation and drying

Source: Charley Creek Rare Earth Project , 2013 Technical Report, December 2013. Project website: Not stated

Project website: Charley Creek Rare Earth Project, 2013 Technical Report