A couple of weeks ago I had the opportunity to visit the mini-pilot plant set up by Quest Rare Minerals Ltd. (TSX:QRM, MKT:QRM) for materials that come from their Strange Lake B-Zone project in northern Quebec. The plant, in Mississauga, Ontario, is hosted and operated by Process Research Ortech Inc, in conjunction with the process metallurgists on staff at Quest. The initial front-end work for the Strange Lake B-Zone flow sheet was completed at the bench scale in 2012 by Hazen Research Inc. at their facilities in Golden, Colorado. The purpose of the mini-pilot plant is to confirm the bench-scale results under continuous processing conditions.
The materials being tested come from two distinct bulk material samples from the deposit; the first is a composite that represents the materials that will be mined during the first 10 years of production at Strange Lake. The second is derived from materials that will come during subsequent production years.
The host rock at Strange Lake is a peralkaline granite; the rare-earth elements (REEs) present in the deposit are primarily found in silicate minerals such as allanite, zircon and gittinsite. Most of the undesirable minerals present are also silicates, such as quartz and feldspar; there are few phosphate minerals present.
The initial Hazen flow sheet which is being tested in the mini-pilot plant utilizes a so-called acid bake water leach (ABWL) process. Material retrieved from the Strange Lake deposit is crushed and ground to particle sizes of 40 µm or less, using a ball mill. It is then blended with sulphuric acid before being heated in a kiln at a temperature somewhere in the range of 150-350 °F (65-175 °C). The small particle size is required to ensure effective reaction kinetics with the acid during the process. This results in a thermal sulphation process and the formation of mixed REE-Nb-Zr sulphates.
Lumps of the calcined or dried materials are broken up before being put into an agitator tank with water, to dissolve the desirable sulphates into water, at room temperature. Approximately 10% of the previously calcined materials dissolves into the water to form a pregnant leach solution (PLS), with the remaining residues being filtered and removed.
Much of what goes on at the Quest mini-pilot plant is understandably proprietary and thus commercially sensitive. As a result no photographs or videos may be taken inside the facility. The company provided the photographs shown below (click on the individual thumbnails to expand). I can confirm that all of the equipment and materials shown in the photographs were present at the facility during the tour.
The proposed flow sheet for Strange Lake is as interesting as it is unusual. At present it does not include a beneficiation stage, which is typically required for rare-earth (and other) projects, for upgrading the initially mined materials into a mineral concentrate, prior to cracking and leaching to form a PLS. The aim of such beneficiation is the reduction in the mass of materials that goes through the subsequent processing steps, thus reducing processing costs. If however the metals of interest can be leached from the minerals with high recovery rates, without pre-concentration, a flow sheet without beneficiation may be considered. The economics of the process will ultimately dictate whether or not beneficiation is required.
Per Peter Cashin, Quest’s President & CEO, Hazen has conducted physical beneficiation tests, including gravity and magnetic separation work. A flotation test program was also completed at the Research & Productivity Council facility in Fredericton, New Brunswick. Initial results from these tests indicated that mass reduction was achievable, but that it was at the expense of subsequent recovery rates. For this reason, to date initial beneficiation steps have not been included in the proposed flow sheet, so that recovery rates can be conserved. The trade off will be that higher processing rates via ABWL will be required, to produce the amounts of PLS required for the subsequent process stages.
Sulphuric acid is consumed at a rate of approximately 180-220 kg / tonne process material. Quest’s mini-pilot plant is currently testing an acid-recovery circuit, as a means of taking unreacted acid, cleaning it up and putting it back into the process. Such circuits are required so that operating costs can be reduced.
To date, the ABWL phase of the mini-pilot plant has been able to extract 85-90% of the REEs, 72-78% of the Zr and 90-95% of the Nb present. This compares to recovery rates of 87-93% of the REEs, 85-90% of the Zr and 93-96% of the Nb during the bench-scale testing by Hazen.
Once the PLS has been produced, it then goes to a solvent-extraction (SX) circuit within the mini-pilot plant, for the extraction of Zr. Quest’s process metallurgists indicated that they plan to optimize each individual SX circuit before moving on to the next. Each initial test is typically run for around four days continuously; once the circuit appears to have been optimizes, the parameters will be tested continuously for at least two weeks.
At present, approximately 100 kg / day of PLS is being processed through the Zr SX circuit, which results in the capability of producing approximately 1-2 kg of ZrO2 / day. ZrO2 has been consistently produced in the mini-pilot plant for some time now. Recoveries of 93-98% of Zr have been achieved in the SX circuit, compared to 96% recoveries at the bench scale.
The next step is for the PLS to be pH adjusted, before passing through the Nb SX circuit. The equipment required for Nb SX testing has recently arrived at the facility and is in the process of being set up. The production of Nb2O5 has previously been achieved at the bench scale, and the process metallurgists will scale up and optimize those processes as appropriate.
During the previous bench-scale work, the remaining PLS from the Nb SX circuit was processed to remove the uranium (U) and thorium (Th) present, before precipitating out a mixed REE oxalate compound. Prior to being dissolved into the PLS, there is approximately 70 ppm U and 300-400 ppm Th present in the feedstocks. Once removed, the grades of Th and U present in the future tailings will be lower, because of the addition of neutralizing materials such as lime.
According to Mr. Cashin, the equipment for testing U & Th removal and for precipitation of the REE oxalates will be ordered soon. He said that the mini-pilot work should be completed in April or May, followed by demonstration-scale piloting work at a rate of 500-1,000 kg / day continuous throughput, for an additional 3-6 months of operation. He indicated that the overall budget for the existing and future process stages of the mini-pilot plant described above, is approximately $1-2M in capital expenditures and $250k / month in operational expenditures. In addition to the aforementioned Nb SX work, U – Th removal and mixed REE oxalate precipitation processes, and scaling up of the existing processes, planned future test work for the materials from Strange Lake include an ongoing bench-scale program to define processes for the separation of individual REEs.
I was impressed with both the Process Research Ortech facilities in Mississauga, and Quest’s mini-pilot plant itself. A methodical approach to the verification of bench-scale test results is vital to the progression of projects like the one at Strange Lake. The combination of the Phase II metallurgical work previously conducted by Hazen in Colorado, and the piloting work in Ontario, should stand Quest in good technical stead as they move their project forward.
My thanks go to Peter Cashin for facilitating my visit to the mini-pilot plant facility in Mississauga.
Disclosure: at the time of writing, Gareth Hatch is neither a shareholder of, nor a consultant to, Quest Rare Minerals Ltd. (Quest). Neither he nor Technology Metals Research, LLC received compensation from Quest or from anyone else, in return for the writing of this article.