Rare Element Makes Significant Progress with Rare-Earths Metallurgical Testing
1. With vigorous attrition scrubbing, a recovery potential of 90% was achieved in 32 wt-% of the original sample weight while still retaining a grade of 13% rare-earth oxide (REO); the REO resides in the finest fraction (minus 500-mesh) pre-concentrate. 2. Further upgrading of the minus 500-mesh (25(micro)) pre-concentrate by flotation may be able to achieve concentrate grades of 40-50% REO. 3. Hydrochloric acid leaching in an agitation leach system is predicted to recover up to about 85% of the total REO from the original mineralized material. 4. Additional testing is likely to provide further improvements to the processing methods.
Metallurgical testwork is continuing at Mountain States R&D International of
The metallurgical testing is being conducted on oxide samples from an NI 43-101-compliant inferred resource of oxide mineralization consisting of 4.5 million tons averaging 4.3% REO. The oxide resource is part of a larger inferred resource estimated at 9.8 million tons averaging 4.07% REO using a 1.5% REO cutoff grade. The oxide zone of mineralization extends from surface to depths of 300 to 500 feet. Excellent exploration potential for expansion of the oxide zone is being tested by a current program of step-out drilling.
The test program on this oxide mineralization was specifically designed to take advantage of the mode of mineral occurrence of the REE mineralization unique to this mineralized material type. This uniqueness is characterized by the occurrence of fine-grained REE minerals that variably adhere to the surfaces of the coarser gangue minerals. The REE minerals in oxide mineralization from the resource area are nearly all from the bastnasite-group-listed in decreasing order of abundance: synchysite, parisite, and bastnasite. Initially the REE minerals were readily liberated by gentle scrubbing of the oxide mineralized samples that had been crushed to minus 1/4 inch (6 mm). The scrubbed pre-concentrate product consisting of the fine particles less than 500 mesh contained more than 70% of the REO in about 20 wt-% pre-concentrate product of the original sample material and assayed 14% REO.
Most of this news release is derived directly from an MSRDI progress report received in late August, 2009.
Summary In general, the MSRDI tests have shown that the following conclusions apply: 1. The oxidized REE-mineralized material should be initially crushed to minus 1/4". 2. Beneficiation is best accomplished by scrubbing the material with vigorous agitation for an extended period of time. 3. Screened or cycloned minus 500-mesh particles contain up to 90% of the REE values. 4. Sulfuric acid leaching would require extended times and/or the use of an acid roasting circuit to be viable. 5. Concentrated hydrochloric acid leaching at room temperature is preferable to sulfuric acid leaching; it removes the rare-earth oxides within 6 to 24 hours and recovers up to 98% of the values from the treated minus 500-mesh pre-concentrate. 6. Economic evaluations can be made on acid consumption versus weight percent rejection of the gangue based on value of the discarded rare earth oxide content. 7. Heavy media separation recovers some of the larger particles but is minimally efficient and probably not cost effective. 8. Flotation of original oxide mineralized material can recover a substantial amount of the REE values, but the flowsheet would be complicated with recirculation and stages of cleaning; final concentrate grade may approach that of other plants but the recovery may suffer. 9. Flotation of the minus 500-mesh REE-bearing pre-concentrate has the potential for higher concentrate grades with good recovery and removal of more of the acid-consuming gangue minerals.
Effect of Scrubbing
If the rare-earth mineralized sample material is first gently scrubbed in the pre-concentration step, the REO values are enhanced, such that the remaining 22 wt-% of the initial sample contains 70% of the values, with a pre-concentrate grade of 14.25% REO. Subsequent testing showed a recovery potential of 90% of contained REE with vigorous agitation in 32 wt-% of the original material while still retaining a pre-concentrate grade of 13% REO. Because the abundance of rare-earth elements in the mineralized material is quite high, vigorous scrubbing with attendant higher energy costs may be justified to recover the optimum amount of REO values at the expense of somewhat lower rejection of the gangue. In this case, attritioning over extended times generates additional fines in the final product, but the ratio of REO recovery to weight gain is 20% additional recovery to 10% weight gain, which indicates that the vigorous attrition scrubbing is both a liberation technique and a beneficiation procedure.
Figure 1 (see http://files.newswire.ca/675/Rare_Fig1.doc) shows the relationship among the techniques to scrub the original mineralized material. Test results demonstrate that:
1. Scrubbing enriches the minus 500-mesh fraction in REE. 2. Gentle scrubbing produces a higher grade but with reduced recovery. 3. Vigorous agitation results in higher REE recoveries but increases the gangue presence. 4. There is less than one percent difference in pre-concentrate REO grade among the techniques. 5. A projected line connecting the two flotation machine tests shows the estimated weight and recovery at points between 10 and 60 minutes.
Oxide Mineralization Flotation
Flotation recovery of REO values for ores from other rare-earth deposits has been documented and practiced by a few companies in the
Oxide Material Leaching
Sulfuric acid and hydrochloric acid leach tests for rare earths were conducted on both the original oxide mineralized material and the oxide pre-concentrates to determine the preferred extraction method. Variables include acid concentration, leach time, and leach temperature. Sulfuric acid leaching is very slow at lower concentrations, whereas at higher concentrations, there are complications owing to high acid consumption from gangue dissolution, relatively low recovery, and expensive washing to remove the sulfuric acid from the rejected material prior to disposal as tailings.
Hydrochloric acid (HCl) leach tests in a simulated agitation leach system demonstrated possible recoveries up to about 85% of the total REO from the mineralization (90% recovery after beneficiation multiplied by 95% HCl leach recovery). Tests were conducted to determine the preferred extraction method by exploring acid concentrations, time under leach, leach temperature, and type of acid. Testing is continuing because the leaching process is not optimized in terms of reagent consumption.
Hydrochloric acid was chosen to continue the testing. A series of tests varied the leach time from six to forty eight hours to determine the most favorable recovery curve. REO recoveries varied from 95% to 99% in this series of tests. It is apparent that an ambient-temperature concentrated hydrochloric acid leach can deliver excellent recovery in a brief time frame and that acid consumption is moderate over the shorter time. In these tests initial acid addition produced a vigorous reaction, liberating great quantities of gas and creating a froth that was hard to disperse. Acid addition techniques can be devised to mitigate this effect, for example by conducting a preliminary leach using diluted HCl to consume the carbonates followed by full strength acid for recovery purposes.
There are a few techniques available for acid regeneration that MSRDI will present as part of the final flowsheet. The last two leach tests indicated that reducing the concentration of HCl also reduces the leach recovery. Acid regeneration or, more properly, re-concentration of the HCl is critical to driving the reaction and to reducing the acid costs.
A small suite of additional tests should be run to determine the most efficient scrubbing procedure. Minimization of gangue is critical to the hydrometallurgical process in order to control acid consumption for cost reduction.
The application of the flotation alternative to the further up-grading of the minus 500-mesh pre-concentrate obtained from the initial attritioning step may be technically and economically attractive since such a step would reduce the over-all consumption of hydrochloric acid required in the subsequent hydrometallurgical step for recovering the REO values.
Because hydrochloric acid leaching produces the greatest recovery to date from the enhanced minus 500-mesh pre-concentrate, and concentrated HCl is necessary to promote the most efficient recovery, further hydrometallurgical testing will focus on enhancing the leaching process after removing more of the gangue minerals prior to leaching.
Rare Element Resources Ltd (TSX-V:RES) is a publicly traded mineral-resource company focused on rare-earth elements and gold. In addition to the REE exploration and evaluation efforts, the Company and Newmont have entered into the
ON BEHALF OF THE BOARD
SOURCE Rare Element Resources Ltd.