Immediately after operations commenced in 1972, Freeport experienced a crusher availability that was less than 50%. It was obvious that a change needed to be made. Primary wet screening was selected to remove the fines which caused plugging of the secondary and tertiary crushers and blinding of the tertiary screens in 1972. Today Freeport’s concentrator is processing 57,000 dmtpd with a wet screening availability of 75 %.
The primary screen feed consists of minus 6 in. (152 mm) ore (80 % passing 40 to 45 mm) and 7.5 % moisture. Ore is delivered to two sets of (2) 8 ft. x 20 ft. (2.4 m x 6.1 m) wet screens and (1) 6 ft. x 16. ft (1.8 m x 4.9 m) wet screen via three parallel reclaim conveyor trains. Two trains each have a 1,200 dmtph capacity, and the capacity of the third original train is 1,000 dmtph.
Many primary screen sizes and types have been tested. Polyurethane screens have provided an acceptable screen efficiency of 75 %, and desirable process variables. The following polyurethane screen panels are used:
- top deck aperture – 26 mm x 105 mm; (5) rows with 50 mm dams ;
- bottom deck aperture – 7 mm x 25 mm; (5) rows with 50 mm dams.
Crusher Slurry Transport Systems
Primary screen undersize slurry gravity flows from the (4) 8 ft. x 20 ft. (2.4 m x 6.1 m) primary screen undersize chute, at a minimum slope of 1¼”/1′ (32 mm/31 cm) through rubber lined pipes, to a rubber lined slurry tank distribution box . The total average slurry flow rate, for four screens is 8,000 gpm (30,200 l/min) which provides an ore flow rate of 1,200 dmtph.
Alternatively, one of the four 8 ft. x 20 ft. (2.4 m x 6.1 m) primary screens was constructed with two underflow pipes. The second gravity flow pipe with a slope of ¾”/1′ (19 mm/31 cm) provides the option to route slurry at 4,000 gpm (15,100 l/min) directly to the original concentrator. By-passing secondary screening is conducted only when the crusher slurry pumps or secondary wet screens are being serviced.
All process variables are controlled with stand alone programmable logic controllers.
The (4) constant speed slurry pumps (two per tank and two in stand-by) are 10 in. x 12 in. (254 mm x 305 mm), and installed with 350 hp motors. Average flow rate per pump is 4,100 gpm (15,500 l/m). At 55 % solids, this provides a line velocity of 15 fps (4.6 m/s). The system requires each pump to produce 120 ft. (37 m) of total dynamic head – 95 ft. (29 m) of static head. Two gasite – white iron – 650 BHN impellers and casings are replaced monthly. An automatic slurry water flush system was installed to purge the slurry pump suction and discharge lines before start up and after shut down of the pumping systems.
Secondary Wet Screening
Secondary screening addressed two main concerns: 1) It provided a safety factor to ensure that particles would gravity flow 1,440 linear ft. (440 m) to the new concentrator facility while maintaining the primary screen oversize moisture to less than 7 %; and 2) It reduced the slurry handling difficulties (tank and line sanding and excessive wear) experienced with the original concentrator.
Slurry is pumped to a secondary wet screen rubber lined distribution box, and distributed to four single deck screens by gravity at a minimum 1″/1′ (25 mm/31 cm) slope. The screens are set at 10° angels to enhance dewatering of the oversize material. The apertures of the poly urethane panels are 2 mm x 16 mm restricted flow with 5 rows of 1 in (25 mm) dams. These panels provide a screen efficiency of 85 %.
Concentrator slurry systems
The original concentrator receives slurry by gravity from the (6 ft x 16 ft (1.8 m x 4.9 m)] primary screen undersize stream and secondary screen collection box, or alternatively from one of the four 8 ft. x 20 ft. (2.4 m x 6.1 m) screens as mentioned above. The new concentrator receives only secondary screen undersize slurry.
Both concentrator facilities have a rubber lined slurry tank distribution box that distributes slurry to (2) urethane lined slurry tanks – four total, two in standby. Each 70,000 gal. (264,950 l) capacity slurry tank is agitated with a rubber lined impeller driven by 100 hp motors. Tank levels are maintained with recirculation lines from the crusher slurry cyclone overflow distribution boxes.
Each tank is retrofited with two slurry constant speed slurry pumps – 10 in. x 12 in. (254 mm x 305 mm); 150 hp; and 8 in. x 10 in. (203 mm x 254 mm); 150 hp and 100 hp. The materials for the concentrator pump components are identical to the crusher slurry pumps.
Pump size and horsepower vary to supply a constant pressure of 10 psi (0.7 kg/sq. cm) to (8) 33 in. (838 mm) inclined slurry cyclones (four in standby) located in separate parts of the concentrator buildings. Cyclones are constructed with natural rubber liners and rubber apex inserts. The average flow to each cyclone is 2,600 gpm (9,800 l/min) at 44 % solids.
Cyclone underflow streams, with a particle size of 80 % passing 5 mm at 64 % solids, report to (7) primary ball mills for conventional milling. The total ore flow rate averages 1,070 dmtph.
Cyclone overflow streams, at 20 % solids with a particle size of 80 % passing 65 mesh report to conventional rougher flotation. Approximately 8 % by weight of the crushing facility’s feed reports directly to cyclone overflow. This material requires no comminution – 270 dmtph of “free” material.
