The Panel mill was rehabilitated in 1979. At the outset, problems cropped up with the old mill; specifically, in the pumping system (which had to be upgraded) and with contaminants in the ore. This latter was rectified by closer mining controls. While serious problems were overcome early on, modifications aimed at reducing costs and improving efficiencies are a constant, said Mill Superintendent Bruce Arthur. Currently, mill personnel are examining ways to cut steam costs. Plans are also afoot to boost bioleaching rates underground, which would require modifications to the mill leaching circuit. Most of the controls in the mill and virtually all the monitoring equipment have been updated with a Foxborough 300 computer system. Actual milling is a standard, four- step process that involves ore preparation (crushing and grinding), extraction (leaching), solution recovery (in wash thickeners and drum filters) and purification (through ion exchange and precipitation). It differs from Saskatchewan operations at the purification stage. Rather than an ion exchange process, most if not all the Saskatchewan mills resort to solvent extraction. The Panel mill system begins with — 6-inch material that is crushed underground in a jaw crusher. Approximately 8% of the ore is screened out for use as pebbles in the grinding circuit. According to Arthur, the pebble mill serves two functions: It saves on the cost of grinding balls and reduces liner wear. The pebbles, stored in a pebble bin, are +2 inches to — 3 3/4 inches. The remainder of the ore is crushed in two stages to produce — 3/4-inch material. Screening assures that only — 3/4-inch material is sent to the fine ore bin.
Fine ore, along with water, is fed to Dominion rod mills, which are charged with 3 1/2-inch-diameter steel rods. The discharged, sand-like product is pumped to a cyclone that separates fine material from coarse. The coarse fraction, along with pebbles, goes to the pebble mill. This discharge is sized in the same cyclone and the coarse material reintroduced into the pebble mill for further reduction. The 50%, — 200 mesh fine material from the cyclone is pumped to the concentrator where a cyclone again separates the material. The fine material is fed to Dorr-Oliver neutral thickeners, along with flocculant, which coagulates the material and produces faster settling. The thickener dewaters the ore and produces an underflow stream of about 65% solids. The overflow from the thickener, which is virtually clear water, is sent back to grinding. The thickener underflow is recombined with coarse material from the cyclones, and this stream is filtered on Eimco disc filters for further dewatering to 77% to 78% solids.
The dewatered ore is fed to the leach circuit, which consists of two parallel banks of seven leaching pachuca tanks 22.5 ft in diameter and 50 ft high. These large, air-agitated tanks are heated with live steam to 75 degrees C, and about 75 lb of concentrated sulphuric acid are added per ton of ore treated at the first pachuca of each bank. The slurry flows from pachuca to pachuca until, 36 hours later, it exits the leach circuit. During this time, 96% of the uranium has been extracted from the ore. Before this stream can be further treated, the pH is brought up to two, with the addition of ground limestone at partial neutralization. This protects downstream equipment and enhances the ion exchange (ix) process.
Thickeners and drum filters (in total 16 primary and secondary Eimco drum filters) remove waste ore from the uranium-bearing mother liquor. The partially neutralized slurry is flocculated and fed to the wash thickeners, where the initial solid/liquid separation is performed. The overflow, or unclarified pregnant solution, is sent on for further processing. The underflow is sent to the drum filtration circuit, where the solids are further dewatered and washed with barren solution to displace as much of the mother liquor as possible. Two-stage, counter-current filtration is used. The slurry is filtered on the primary filters, repulped and then filtered on secondary filters. Barren solution is used as wash on the secondary filters. Filtrate from the secondary filters is used as wash on the primary filters. Filtrate from the primary filter is returned to the wash thickeners. The solids discharged from the secondary filters are sent to tailings neutralization and treated with limestone and milk-of-lime to a pH of 8.0 before being pumped to the tailings basin.
The unclarified pregnant solution is clarified in Whitco vacuum leaf precoat filters. The solution is then fed to ix as clarified pregnant solution. ix consists of two sets of adsorption columns, one set of elution columns and a backwash column. Each set contains three columns. Each column contains 450 cu ft of Duolite E-133 Ion Exchange resin. The resin is composed of small, gel-like beads (specifically made for uranium recovery) that have active sites affixed to them. These active sites, depending on conditions, either adsorb or release uranium from or into solution. The clarified pregnant solution passes through the adsorption sets in a downflow fashion. The solution exiting the columns is called barrens. When the resin in the first column becomes fully loaded with uranium, it transfers to the backwash column, where clarified pregnant solution passes through the resin to remove any trapped particles.
In elution, one column will be fully stripped. This is transferred back to the empty adsorption column. The loaded resin is then transferred from backwash to the empty elution column. The elution columns are then stri pped of uranium with 10% sulphuric acid. The stream that exits is concentrated eluate, or conc. el as it’s called. An upgrading from roughly 0.6 g per L U3O8 in the pregnant solution to 24 g per L in the conc. el is achieved. (A Scintrex MU-4 Automated Uranium Analyzer monitors the uranium levels in the IX circuit.) This conc. el is then fed to the precipitation circuit. In the first stage of precipitation, limestone and milk-of- lime are added to the conc. el to remove excess sulphate, iron, thorium and rare earths. The resulting slurry goes through a wash circuit consisting of a thickener and two-stage, counter-current filtration. The solids are returned to leaching to dissolve any precipitated uranium. The solution is then treated in the second stage of precipitation with magnesium hydroxide to form the final yellowcake product. The yellowcake slurry is thickened and filtered. The overflow from the thickener is sent to mine water recovery. (Bacterial leaching from underground is also treated in the mine water recovery). The filtered yellowcake is sent to Nordic mine (a Rio Algom operation) for drying and packing. The dried product contains uranium (65%), iron (1% to 3%), magnesia (1%), sulphate (3% to 4%), thorium ( 1/2%) and trace impurities. The uranium compound is further refined at Eldorado Nuclear’s facility in Blind River, about 40 miles away.
Current mill throughput at Panel is 3,300 tons per day on a 7-day cycle. The mill has 63 employees. Approximately two pounds of U3O8 are produced from a ton of ore. — 30 —
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