DNI Metals Inc. Buckton Deposit PEA, SBH Property, Alberta

This article describes the conceptual bio-heap leach processing route and metal recovery methods proposed for the Buckton Deposit based on a Preliminary Economic Assessment.

Report context

The report, dated January 17, 2014, presents the results of a Preliminary Economic Assessment (PEA) for the Buckton Deposit, part of the SBH Property in Alberta. Hatch Ltd. completed an Initial Metal Economics Evaluation to screen metals for potential economic viability, identifying Cu, Zn, U, Ni, Co, Li, and rare earth elements (including Y) as candidates. Lithium was later omitted due to processing cost constraints identified during the heap leach and processing scoping study.

Processing route

Bio-heap Leaching

The main extraction method is based on a low reagent dosage bio-heap leaching arrangement designed to minimize operating costs while providing reasonably high recoveries. Bio-heap leaching was investigated for the Buckton Deposit through lab-scale test work programs due to its inherently lower capital and operating costs per tonne, typically suited to low grade leach feed.

The savings of a bio-heap leach are mostly attributable to use of a coarse particle size typically of several millimetres, instead of a fine size used in tank leaching. This eliminates the need for fine grinding equipment and reduces power consumption. Coarse particles allow percolation of the leach solution through a stationary heap and collection of a low-solids pregnant leach solution. Bio-heap leaching is typically operated under mildly acidic and oxidizing conditions.

Metal Recovery Section

The metal recovery section is similar to other bio-heap shale processes which use selective base metal precipitation with hydrogen sulfide and uranium recovery using ion exchange. The next steps are neutralization and re-acidification to concentrate rare earth elements and some base metals into a smaller stream. This method was operated for recovery of uranium at the Denison Mine in Elliot Lake, Ontario.

Copper and zinc are recovered from the pregnant leach solution by precipitation with hydrogen sulfide in three agitated tanks in series with solids recycle from the thickener underflow. The discharge from the third precipitation tank is sent to a copper/zinc sulfide thickener, with underflow at approximately 30% to 40% solids by weight sent to a belt filter for washing and dewatering to approximately 25% moisture.

Following copper-zinc recovery, the solution is fed to an ion exchange plant for uranium recovery. Solution is passed through columns filled with ion exchange resin which selectively loads uranium. The loaded resin is washed and eluted with strong sulfuric acid. The uranium eluate is partially neutralized using lime slurry at pH 3 to precipitate calcium and sulfate as gypsum. Uranium is precipitated with hydrogen peroxide, then thickened, washed in a centrifuge, and dried to produce yellowcake.

Neutralization and Re-acidification

To increase grade in the leachate and reject iron, calcium, and sulfate after uranium recovery, most solids are precipitated with lime. This is followed by selective re-dissolution of nickel, cobalt, and rare earths with dilute acid. By controlling re-acidification pH at approximately 4, most iron, gypsum, and thorium remain in solids. Neutralization takes place in four precipitation tanks in series operating at successively higher pHs with air injection for iron oxidation.

The solids from belt filters are re-pulped in three agitated re-acidification tanks operating at approximately 40% solids by weight and pH of about 4. The slurry from the third re-acidification tank is directly filtered on belt filters.

Nickel and Cobalt Recovery

Nickel and cobalt are precipitated from the re-acidified leach solution together by precipitation with hydrogen sulfide, followed by thickening and filtering to produce a mixed nickel and cobalt sulfide concentrate. The precipitation process uses three tanks in series with solids recycle. The underflow, containing approximately 30% to 40% solids by weight, is sent to a belt filter for washing and dewatering to approximately 25% water by weight.

Rare Earth Element Recovery

The overflow from the nickel and cobalt thickener feeds the rare earth element circuit. Oxalic acid is used as a selective precipitation agent for rare earth elements from solution. Precipitation is done in three-in-series agitated tanks with oxalic acid addition. The rare earth oxalate slurry is sent to a thickener, then to a belt filter with two stages of wash water. The rare earth oxalate filter cake feeds a rotary calciner which heats the product to approximately 800°C, converting rare earth oxalates to rare earth oxide and producing a dry product.

Effluent Treatment

Effluent water comes from a bleed stream from the neutralization process at pH of approximately 8.5. Further pH adjustment to 10.5 in a series of two to three tanks with lime addition is anticipated. Treated effluent flows to an effluent pond. Solid residues consist largely of gypsum and various iron precipitates, deposited into a gypsum pond.

Key reported parameters

Technical qualifications

The report states there is no downstream process test work, and the metal recovery process design is based on extrapolation of existing methods applied elsewhere and knowledge of anticipated chemistry. Without metal recovery test work, there is no way to guarantee that the methods proposed will work as intended.

Key risks and uncertainties identified include: the applicability of leaching extractions to full scale bio-heap conditions; extrapolations of acid consumption in a full scale heap leach from limited laboratory data; lack of geo-stability test work for mineralized shale; downstream recovery loss factors of each product, especially rare earth elements; the effect of leachate composition on reagent consumption and product purity; technical feasibility of certain downstream processes on very low grade solution; and ability to meet saleable product purity specifications.

Many assumptions were made in the process design criteria based on Hatch experience from other projects due to limited information collected for the Buckton Deposit.

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*Source: P&E Mining Consultants Inc. Report No. 276, January 17, 2014, DNI Metals Inc. Buckton Deposit PEA, SBH Property, Alberta, Section 17.0 Recovery Methods*