Kuriskova Uranium Project — 2012 Technical Report

The Kuriskova Uranium Project underground processing facility is designed to extract uranium and molybdenum from 600 tpd of ore using carbonate leaching with pressure oxidation.

Report context

This technical report, dated March 2012, describes the recovery methods for the Kuriskova Uranium Project. The processing facility is designed as an underground processing facility (UPF) to treat uranium-bearing ores from the Kuriskova deposit. Metallurgical testwork performed on select ore grade samples informed the process design, which targets production of uranium peroxide yellowcake and a molybdenum sulfide concentrate.

Processing route

Underground Processing Facility

The Kuriskova UPF has been designed to process 600 tpd of uranium bearing ores. Results from metallurgical testwork performed on select ore grade samples from the Kuriskova deposit indicate that a carbonate leach using pressure oxidation is the best alternative available for uranium and molybdenum extraction. The overall process plant and unit operations therein are designed to produce a uranium peroxide yellowcake as well as a molybdenum sulfide concentrate.

The UPF consists of a single stage crushing, the product from which is directed to a crushed ore storage bin. Ore will be drawn from this storage bin into a single stage ball mill grinding circuit. Ground ore will be directed to a conditioning tank which will provide surge capacity ahead of the leach circuit. Slurry from the conditioning tank will be pumped under pressure to the pressure oxidation (POX) circuit. Uranium and molybdenum constituents are extracted using carbonate leach chemistry in conjunction with POX carbonate leaching to increase leach rate. Pregnant solution is separated from the leach residue and precipitated as sodium diuranate (SDU) and is further purified by re-leaching and precipitation of a uranium peroxide as yellowcake. Molybdenum sulfide is precipitated from a bleed stream of the process solution.

All of the tailings generated during the mine life are disposed of as 100 percent paste backfill to the mine or in underground excavations of inert rock. This is the preferred method of tailings disposal because it minimizes the amount of radio-nuclide bearing material that is transported to the surface.

Primary Crushing and Ore Storage

Primary crushing of ore occurs on the crushing level 40 m above the main process plant excavation. A cement-lined raise-bore excavation will be used as a coarse ore bin. Mined ore is fed through a bar grizzly to remove any material greater than 200 mm. Free of boulders, the ore falls onto an apron feeder directly feeding a horizontal shaft impact crusher. Ore draw from the apron feeder into the crusher is regulated to maintain a half meter dead-bed of ore on the pan at all times, preventing ore from hammering the pans. Ore is crushed to a nominal P80 of 12 mm before being discharged directly into the coarse ore bin.

Grinding, Classification, and Thickening

The grinding, classification, and thickening circuit consists of a single ball mill in closed circuit with vibrating screens. Ore reclaimed from the coarse ore bin is fed into the ball mill using a single mill feed conveyor. Carbonate in the form of pulverized soda ash is added to the ore on the belt conveyor prior to the ball mill, allowing carbonate leaching to commence therein. The ball mill operates with a 300 percent circulating load at 75 percent of critical speed.

The ball mill discharges to the vibrating screen pumpbox from which the slurry is pumped to a bank of screens for classification. Grind size classification is designed at a P80 of 75 µm. Oversize material from the vibrating screens is returned to the ball mill feed for additional grinding.

Undersize material from the vibrating screens discharges into the dewatering cyclone feed pumpbox at a slurry density of approximately 20 percent and is dewatered using hydrocyclones working in conjunction with a high rate thickener. Approximately 90 percent of the water along with approximately one-third of the solids reports to cyclone overflow. This slurry is directed to the cyclone overflow clarifier to remove the rest of the solids; the clarified water being recycled back to the grinding circuit. Solids from the clarifier are combined with cyclone underflow, producing slurry with a target density of 40 percent solids to be fed to the leach circuit.

The rationale for using cyclones ahead of the grind thickener was to reduce the size of the thickener. The use of dewatering cyclones before the grind thickener reduced the required thickener diameter from 15 m to 8 m, which is substantially more acceptable given the underground confines within which the mill is positioned.

Major equipment for the Grinding and Classification area includes a crushed ore apron feeder, ball mill feed conveyor, belt weightometer, ball mill (3 m diameter x 4 m long, 375 kW), vibrating screen pumps and pumpbox, vibrating screens, dewatering cyclone pumps and pumpbox, dewatering cyclones, cyclone overflow clarifier, cyclone overflow pump, cyclone underflow pumps, grind thickener, and POX conditioning feed pumps and pumpbox.

Pressure Oxidation and Leaching

Milled slurry is pumped from the grind thickener into the POX conditioning tank, the feed end of which is located on the crusher level. This tank is a steel lined raise-bore excavation which acts as a storage tank. It provides 16 hours of residence and conditioning time between the comminution and leach circuits.

Slurry is pumped from the conditioning tank through a series of three splash tanks. From the splash tanks, the slurry is directed into a pressure leach autoclave where the ore is oxidized and leached. A leach solution concentration of 69 g/L Na2CO3 and 23 g/L NaHCO3 is sufficient to obtain the design extractions; however, process optimization testwork may reduce these concentration requirements. The autoclave has a six chamber design and operates at 200 °C and 2.8 MPa with an oxygen overpressure of 0.7 MPa. The autoclave discharges to a series of three flash tanks before being pumped to the leached slurry belt filter to separate leach residue from the pregnant solution.

The flash and splash tanks are designed to recover heat from the autoclave discharge and preheat the slurry entering the autoclave, respectively. Splash tanks utilize steam from their corresponding flash tanks to heat slurry before the autoclave. Each set of splash and flash tanks operates at different pressures to provide different ranges of steam transfer, thereby allowing increased overall heat recovery. The low temperature splash/flash tanks operate in a vacuum.

Operating Schedule and Availability

The UPF is designed to operate on the basis of three, eight-hour shifts per day, 345 days per year. Process operations crews will work in conjunction with mine operations crews to man-trip to and from the surface facilities so as to efficiently move the labor force in and out of the underground area. Process crews will consistently "hot change" between crews to ensure the process plant has adequate supervision and staffing 24 hours a day.

Overall UPF availability is expected to be 92 percent, or 335.8 operating days per year. This will allow sufficient downtime for scheduled and unscheduled maintenance of process plant equipment. Major scheduled maintenance annually accounts for 20 days of downtime for maintenance of the mill and autoclave. The remaining 9.2 operating days per year show a combination of minor scheduled maintenance and unscheduled maintenance.

Key reported parameters

The following major design criteria were derived using results from metallurgical testwork in conjunction with expert opinion and additional information regarding the equipment components as provided by various vendors.

Technical qualifications

The processing information in this report is based on a pre-feasibility study (PFS) level of design. The process design criteria were derived using results from metallurgical testwork performed on select ore grade samples, combined with expert opinion and vendor equipment information. The report does not provide operational data from a working plant, as the facility is in the design phase. Parameters such as uranium and molybdenum extraction percentages, overall recovery rates, and reagent concentrations are derived from testwork and may be subject to change through process optimization. The grinding circuit ball mill size (3 m diameter x 4 m long, 375 kW) and thickener dimensions (8 m diameter after cyclone installation) are design specifications for the underground facility.

Source: Tetra Tech, March 2012, Kuriskova Uranium Project, Section 17.0 Recovery Methods.