Patterson Lake South Property — 2019 Technical Report

This report documents the proposed uranium recovery process plant design for the Patterson Lake South Property, based on the May 30, 2019 technical report prepared under NI 43-101.

Report context

The technical report dated May 30, 2019 describes the Patterson Lake South Property of Fission Uranium Corp., located in northern Saskatchewan, Canada. The process plant design and costing for the preliminary feasibility study (PFS) were completed by Wood, which has design, construct and commissioning experience on uranium process plants both within the Athabasca Basin and globally. The process route selected for the Project is based on unit processes commonly used effectively in uranium process plants in northern Saskatchewan, while utilizing some new innovations in some of these unit process designs to optimize plant performance.

Processing route

Overview of unit processes

The conceptual mill design will have a nominal feed rate of 350,000 tpa, operate 350 days per year, and be able to produce nominally 15 million lb per year of uranium concentrate. The mill design will have an estimated recovery ranging from 95% to 97%, and is designed to accommodate fluctuations in ore grade that are expected when mining moves from open pit to underground.

The unit processes for uranium recovery are: grinding; acid leaching using hydrogen peroxide as oxidant; CCD (counter current decantation) and clarification; solvent extraction (SX) using strong acid stripping; molybdenum removal from the pregnant aqueous solution; gypsum precipitation; yellowcake precipitation; yellowcake calcining and packaging; tailings neutralization; and effluent treatment with monitoring ponds to confirm quality of effluent discharge.

A zero-based design approach was taken in the mill process design. The design aims to achieve the required throughput with the minimum redundancy, installed equipment, and design allowances. Health, safety, and environmental aspects are not compromised. There are only two instances where circuit design capacity is planned to be more than nominal: grinding capacity has been increased by 20% more than nominal to allow for higher maintenance requirements than the rest of the circuit; the effluent treatment plant has also been designed for more than the nominal flow rate due to the possibility of having periods of excess water from the mines and weather-related surges.

Ore sorting and storage

Ore will be loaded into an ore truck in the open pit. The truck will drive through a radiometric scanner to confirm ore grade and the delivery location of the ore on the ore pad. Different ore grades and types can be stored in different piles.

Grinding

A loader operator will deliver ore to the grizzly hopper. Traffic in the ore storage and reclaim area will be restricted to minimize ore contamination in the site area. A variable speed ore feed conveyor delivers ore from the hopper into the SAG mill at a prescribed rate that will be close to the ground ore feed rate to be fed to leaching. The ore will be weighed on the belt as well as given a gamma radiation scan to check uranium content.

Process water will be added into the SAG mill feed along with the ore to provide the target solids content in the mill. SAG mill discharge reports by gravity to feed the ball mill. The ball mill will also be fed recycled oversize particles from a classification cyclone situated above the ball mill. Process water can be added to the ball mill feed to maintain the target solids composition of slurries in the circuit. Ball mill discharge reports to a pump box that pumps the ore slurry to the classification cyclone or cyclones. The overflow stream of the cyclone is designed to have the target particle size (P80 = 150 µm) as well as the target 50% solids composition for leaching. It will be pumped to twin pulp storage pachucas.

The twin pulp storage pachucas are air agitated and provide surge capacity between the grinding circuit and the leaching circuit and a degree of ore blending. The grinding circuit has tonnage capacity greater than that required for leach feed. This allows the grinding circuit to fill the pulp storage pachucas. When full, the grinding circuit can be shut down to provide short periods (up to 12 hours) of grinding circuit maintenance without disruption of ore slurry feed to leaching.

Leaching

The first leach tank will be fed by a variable speed centrifugal pump to feed the prescribed solids rate to the leaching circuit. The feed solids tonnage combined with the measured grade of the solids gives the target uranium flow rate into the mill. The leaching circuit will be comprised of six mechanically agitated tanks that are connected in series. The flow between tanks will be by gravity. The discharge of each tank will be from a baffled upcomer to ensure minimal solids short circuiting in each tank. The tanks in total provide the target eight hour residence time to oxidize and dissolve the uranium from the ore.

The tanks are heated with steam spargers to the prescribed 50°C leach temperature. Most of the sulphuric acid required will be fed into the first two to three tanks. This is also the case with the sodium chlorate oxidant that will be fed to the leaching tanks. Sulphuric acid will be added to maintain the target minimum 10 g/L to 15 g/L acid content in the final leaching tank discharge. Sodium chlorate will be added to maintain the target 475 meV to 500 meV oxidation reduction potential (ORP).

If low iron ore is being leached, ferric sulphate will be available to provide supplemental ferric iron. Iron in solution is required to provide oxidation of U+4 to U+6 (U+6 is soluble in the acidic solution while U+4 is not). It is expected that 98.3% of uranium in ore will dissolve in the leaching circuit.

Counter current decantation

A variable speed pump will be used to pump slurry from leach tank 6 (the last tank of the leaching train) into a relatively small mix tank. Overflow solution from the CCD 2 will report by gravity into this mix tank as well. The mixed slurry will be pumped to the center well feed of CCD 1 along with a flocculant flow (flocculant enhances settling). The overflow of CCD 1 (pregnant aqueous solution) will report to a pumpbox and will be pumped to feed a pin bed clarifier. Underflow from CCD 1 will be removed by a variable speed pump controlled by the density of the underflow stream as well as the solids load level in the thickener. Underflow will be pumped to a small mix tank where it will be mixed with the gravity overflow of CCD 3. This mixed solution will be pumped to the feed well of CCD 2 where it will be treated with flocculant. In a similar manner, the CCD underflows will be pumped to feed the next CCD, i.e., CCD 3 to feed CCD 4 until underflow of CCD 6 (the final CCD in the train). CCD 6 underflow will be pumped to the tailing reaction tank 1.

Wash water will be fed into the feed mix tank of CCD thickener 6. The wash water is made up of: first priority – a portion of SX raffinate flow (in order to recycle as much acid as possible); second priority – acidized process water. The acid content of the slurry solution passing through the CCD circuit must be maintained to ensure that dissolved uranium is not precipitated in the CCD circuit. Enough wash water will be introduced into CCD thickener mix tank 6 to meet the target uranium concentration in the pregnant aqueous solution that will overflow from CCD thickener 1.

The overflow of CCD thickener 6 flows by gravity to the mix tank feeding CCD thickener 5. The solutions pass from one thickener to the next counter to the direction of the solids slurry. That is, the wash solution passes from CCD thickener 6 to CCD thickener 1, while the solids slurry passes from CCD thickener 1 to CCD thickener 6. Circuit performance is determined by a combination of the concentration of dissolved uranium in the feed solution from leaching, the amount of wash water added to CCD thickener 6 feed, and the underflow slurry densities in the CCD thickeners. Greater than 45% solids are expected in CCD underflows. It is estimated that 99.5% of dissolved uranium will be washed out of the leached residue solids when using a train of six CCD thickeners.

Pregnant solution clarification and storage

The overflow from CCD thickener 1 feeds a reactor clarifier that removes turbidity (fine solids) from the pregnant aqueous solution. The feed to the reactor clarifier will be treated with a small quantity of flocculant to aid settling and clarification. The overflow of the reactor clarifier flows by gravity to the pregnant solution clarifier pump box. From there the solution will be pumped downwards through the set of five pregnant leach solution sand filters. The filtrate flows to the clarified pregnant leach solution tank.

Solvent extraction

The organic in the SX circuit will be made up of three components: a tertiary amine that selectively forms a bond with uranyl sulphate (enough amine will be added into the solution to hold the design g/L U3O8, usually about 6 to 12% amine reagent by volume); isodecanol that will be introduced into the solution to enhance the separation of aqueous and organic after mixing ceases (isodecanol is typically added to about half the volumetric concentration of the amine); and a kerosene-type organic as the main carrier solvent. The design criteria for the organic solvent are 6% amine; 6% isodecanol and 88% kerosene.

There will be four extraction mixer settler units. Clarified pregnant aqueous solution will be pumped from the clarified pregnant leach solution tank into extraction mixer 1, in which it is mixed with organic solution from extraction settler 2. As the organic and the aqueous phases are intimately mixed, the tertiary amine in the organic phase holds onto the uranyl sulphate and removes it (extracts it) from the aqueous phase. After mixing, the mixer discharges the solutions into a settler unit, in which the solution separates into a lower density organic floating on top of the higher density aqueous. A portion of the organic in each extraction settler will be returned to its mixer. This organic recirculation is done to control the volume ratios of organic and aqueous in the mixer; in this case the ratio is 1.5/1 organic/aqueous.

The aqueous that has settled out in extraction settler 1 will be fed to extraction mixer 2 where it will be met with a counter currently moving organic flow from extraction settler 3. In this counter current flow, pregnant aqueous will be fed into extraction mixer 1 and discharges as barren raffinate from extraction settler 4. Conversely, barren organic will be fed into extraction mixer 4 and discharges from extraction settler 1 as loaded organic (high uranium content organic).

Barren raffinate from extraction settler 4 will be pumped to the raffinate tank. Periodically, the organic that accumulates on the raffinate tank surface will be skimmed off to return to the extraction circuit. Much of the raffinate reports to the CCD 6 mix tank where it will be recycled. As much of the raffinate as possible will be recycled to capture the acid that is contained in the raffinate. Recycling of raffinate will however increase the circulating load of contaminant elements. This build up of contaminant levels results in the need to bleed some of the raffinate to the effluent treatment circuit. The raffinate tank can hold about two hours of raffinate generation.

Loaded organic at this point will be expected to contain 99.9% of the uranium that has been fed to SX in the pregnant aqueous solution. The organic will be washed in two scrub mixer settlers with a small flow of acidic water that will be flowing counter current to the organic. The acid solution washes some elements like arsenic from the loaded organic. As well, it washes any small bubbles of aqueous that may have escaped extraction settler 1 with the loaded organic. In both acid scrub mixer settlers, aqueous will be recirculated to obtain the target organic to aqueous ratio in the mixers. Scrubbed loaded organic has a high concentration of uranium and much lower concentrations of contaminating elements. Some elements such as molybdenum can, however, go with the uranium into the loaded organic flow to an extent.

There will be six strip mixer settlers. In stripping, barren aqueous strip solution will be used to strip uranium off the organic. The stripping solution will be a strong acid solution that contains 400 g/L sulphuric acid. Scrubbed loaded organic feeds strip mixer 1 in which it will be mixed with aqueous stripping solution from strip settler 2. The mixed solution separates in strip settler 1. Much of the aqueous in strip settler 1 will be recirculated back to strip mixer 1 to maintain the target organic to aqueous ratio in the mixer. The remainder of the loaded strip solution will be pumped to the loaded strip after settler which allows the remainder of the organic in the loaded strip solution to separate out. From the strip after settler the loaded strip solution will be pumped to the pregnant strip tank. The pregnant strip tank can hold about four hours of pregnant strip as it is generated. The loaded strip will be very concentrated in uranium at 150 g/L U3O8.

Organic from strip settler 1 feeds strip mixer 2 where it will be mixed with aqueous from strip settler 3. This will be a counter current arrangement with the uranium reporting to strip settler 1 aqueous discharge as loaded strip solution, and the barren stripped organic discharging from strip settler 6. The barren strip solution will be fed into strip mixer 6.

Key reported parameters

Technical qualifications

The report states that a high-level economic analysis by RPA showed gold recovery from the Triple R deposit to have limited impact on overall project profitability at current market conditions, and gold recovery was thus excluded from this design. The report notes that should market forces change, gold recovery could be reasonably easily engineered into the existing design and constructed without harming throughput or recovery from the uranium process plant.

Process design has been directed by the metallurgical test program results as well as knowledge from literature, and Wood’s experience with existing successful process methods. The report cautions that if the uranium flow rate is too high, uranium recovery in leaching will be decreased; if too little uranium is fed, the production rate will be low.

Source: Fission Uranium Corp. – Patterson Lake South Property, Technical Report NI 43-101 – May 30, 2019, Section 17: Recovery Methods.