Pebble Grinding

The operations of Bethlehem Copper are situated in the south-central interior of British Columbia at an elevation of 4900 feet above sea level. Main road and rail transportation systems pass through Ashcroft in the Thompson Valley, which is 28 miles by road from the minesite. The operation was the first low-grade open-pit porphyry copper mine in British Columbia

A cheaper method of grinding was necessary and information was collected to assess the feasibility of pebble grinding, This study showed a higher capital cost, lower operating cost, and substantially increased revenue from copper contained in pebbles consumed. The calculated return on investment was attractive and arrangements were made to pilot the operation at the facility of a mill manufacturer to assure the reliability of the figures being utilized.

Pebbles are conveyed on 18″ conveyors to a 1,000-live-ton cylindrical storage bin. Load level is measured sonically and the signal transmitted to the crusher control room for readout. Pebbles are fed from this bin by vibrating feeders to 18″ conveyors which in turn feed the two pebble mills.

The two 16.5′ x 32′ grate discharge pebble mills are driven by low-speed synchronous motors through air clutches and single helical gearing at 69% of critical speed. Babbitt bearings are hydrodynamically lubricated and the mill journals are lifted by manual pumps for starting.

The pulp discharges through grates with ½” x 1-3/16″ slotted openings to three 16″ x 16″ pumps driven by 300 hp motors. Two pumps operate while the third serves as standby and is linked through ball valves for automatic switching of pumps. Pump lines discharge to D20B cyclopacs with underflow returning to the pebble mills and overflow going to rougher flotation.

In the original installation No. 8 pebble mill was equipped with rubber lining and grates, while No. 9 pebble mill had Ni-Hard throughout.  Cyclone feed pressure ranges from 15 to 20 psi. Overflow density range is 34% to 39% solids, and underflow density range is 68% to 72% solids. The spigot product is diluted with water to 65% solids before it enters the pebble mill. From the beginning a major point of concern has been to get maximum power draw from the pebble mills. Motors are 2,750 hp at 0.8 leading power factor with a 1.15 service factor. A considerable reserve exists in the motors.

Mill specifications called for full motor power draw at 69% of critical speed, 65% pulp solids, 40% pebble charge, and a throughput up to 1,000 tons per hour per mill. According to calculations, power drawn could reach 3,000 hp. The 40% pebble charge was selected as a point on the load power curve with sufficient slope to make control of pebble load simple.


pebble grinding three-stage rod-mill ball-mill grinding




pebble grinding at bethlehem copper corporation ltd

The Granduc ore body is located beneath glacial terrain on the borders of British Columbia and Alaska, some 600 miles from Vancouver and 30 miles from Tidewater at Stewart, B. C. The concentrator and attendant facilities are located in wilderness country at an elevation of 2500 feet. The average annual snowfall is 65 feet. Surface facilities and ore body are linked by a 10 mile railroad haulage tunnel.

Laboratory Testing

Two pilot plant investigations were carried out on samples of ore from the Granduc property. The primary objective was to determine whether the Granduc ores were amenable to some form of autogenous grinding, and if so, to select the most suitable flowsheet. The secondary objective was to confirm in pilot plant continuous flotation operations the metallurgical results obtained in laboratory closed circuit flotation tests.

Three autogenous grinding flowsheets were considered, two of which would have eliminated all steel grinding media, and the third possibly two-thirds of the total steel grinding media requirements.

  1. Primary autogenous grinding of a minus 8 inch primary crusher discharge to produce, (a) a product suitable for second stage pebble milling, or (b) a flotation feed product in a single stage of grinding.
  2. “Intermediate” autogenous grinding in which plus 4 inch rock media is screened from the primary crusher discharge and employed as grinding media to grind a tertiary crusher product to flotation feed size in a single stage.
  3. Rod milling followed by pebble milling of the rod mill discharge to flotation feed size.

Results of Pilot Plant Investigations

Pilot plant operations were therefore limited to investigations on “Intermediate” autogenous grinding and pebble grinding of a rod mill discharge.

The “Intermediate” autogenous grinding investigation initially employed minus 6 plus 4 inch rock media to grind a feed all passing one inch. The minus 6 plus 4 inch grinding media was too small to break the coarser size fractions of the feed and a buildup of this material rapidly choked the mill. The addition of plus 6 inch rock media did not help significantly as this size media broke down rapidly in the mill to minus 4 inches.

Rod mill-pebble mill pilot plant investigations were run on two samples of ore from Granduc. The second investigation was of longer duration and produced the most reliable results. It was found that a rod mill grind to approximately minus 6 mesh produced a feed to the pebble mill which could be ground efficiently with a minus 2-½ plus 1-½ inch pebble. Grinding power requirements totalled 10.7 net kwh/ton of rod mill feed and pebble consumption in the 4 ft diameter pebble mill was 3.1% of the total circuit feed.

It was estimated that the larger diameter pebble mills employed in commercial operations would increase the pebble consumption approximately 100% above that of the pilot plant but that a larger size of pebble, possibly minus 3-½ plus 1-½ inches, could then be employed.

Design of the Fine Crushing Plant

The fine crushing plant includes all operations from the unloading of rail cars of coarse ore into the underground pocket through to delivery of fine ore and rock grinding media to the fine ore and pebble bins. The fine crushing plant is designed to process 700 tons/hr, operating fourteen hours per day, six days per week. A schematic diagram of the fine crushing plant is shown in Figure 1.

Coarse ore from the primary underground crusher is hauled 10 miles in 50 ton side dump cars through the railroad tunnel and dumped into a 3000 ton dump pocket located 700 ft inside the tunnel portal. Twenty car trains are dumped, two cars at a time, tipping to either side, with the air operated mechanism actuated by radio signal by the train engineer.

This unit is a Tyler type F 900, 6 ft by 14 ft, double deck screen rated at 800 tons per hour at 90% efficiency. The top deck is ¾ inch cast manganese steel with four inch square openings; the bottom deck has 6 inch by 1.75 inch slots crosswise to material flow.

The secondary crusher is a seven ft Symons standard heavy duty cone crusher with coarse bowl and medium liner. With a one inch close side setting the rated capacity is 500 tph with 40% plus the close side setting in the crusher discharge.

Design of the Grinding Section

The grinding section occupies one 68 ft bay across the full 240 ft width of the concentrator building, at the level below the foot of the fine ore storage bins. The operating floor is essentially steel grating and occupies the central 160 ft with the foundation floor open for 40 ft at both ends. The regrind mill and associated equipment are installed in the open foundation floor section at the west end and rod mill steel is stored in the east end.

The equipment layout provides for two independent grinding lines, each consisting of a rod mill and two pebble mills one pebble mill to be installed, if needed, at a later date. Each line draws rod mill feed from a pair of fine ore bins but pebble grinding media is supplied by a single conveyor from the pebble storage bin and distributed to the four pebble mills by a system within the grinding bay.

Four variable vibrating feeders withdraw pebbles from the main pebble storage bin to a single conveyor which discharges into a constant volume surge hopper. The overflow from this hopper feeds a short conveyor which discharges into the second surge hopper. Each surge hopper is equipped with two vibrating feeders, one to each pebble mill. The pebble media vibrating feeders are controlled by the power draw on their respective pebble mill motors.

Rod Mills

The two rod mills are Dominion Engineering overflow type, 10’6″ by 16 ft driven at 15 rpm (62.5% critical) through Fawick air clutches by 1000 hp 277 rpm 4000 volt synchronous motors.

Specifications for the rod mills called for each mill to be capable of grinding 4500 tpd of minus 1 inch feed to minus 8 mesh, operating with a 45% volume rod load and 80% solids pulp dilution. The mills were designed to absorb 3.5 net kwh/too of grinding power when operating at the designed tonnage, or 905 hp with zero drive loss.

Pebble Mills

All liners and grates are “Ni-hard.” The feed-head liners are 4.25″ thick and the discharge head back-plates 1.5″ thick. The shell liners are a two piece design with 3″ thick breastplates secured in place by replaceable caps; the overall thickness is 5.5″. All head and shell liners are backed with ¼ inch linatex.

The primary cyclones are 20″ Krebs Model D 20 LB with 5/8 inch thick replaceable full rubber liners with 3″ adjustable air-operated rubber lined apex valves. Two cyclones are provided for each pebble mill and design criteria calls for creating 468 tph of feed at 58% solids to yield 94 tph of overflow at 34% solids and 95% minus 65 mesh and with an anticipated pressure drop of 6 psi.

The primary cyclones are grouped in clusters of four on each side of the central pebble feeding structure, some 20 ft above the operating floor.

Process Control

The pilot plant test work showed that the efficiency of fine grinding of the Granduc ore was closely related to pulp density control in the pebble mills. The Granduc ore pulp has a low viscosity and if the pulp density in the grinding units falls below a certain level, the pulp fails to coat the pebbles effectively. This results in a drop in grinding efficiency and an increase in pebble consumption.

The best correlation with the density in the pebble mills is provided by the primary cyclone underflow density. Control of this density was selected as the key factor in the process control of the two stage grinding circuits.

Each grinding line is provided with eight control loops, three of which are designated “primary loops” and five as “secondary” or “auxiliary loops”. Actually, all loops are essential and of equal importance.

Reclaim of pebbles from the main pebble storage bin is controlled by a ninth loop which maintains a constant weight of pebbles in the surge hoppers supplying pebbles to the individual pebble mills.


pebble grinding fine crushing flow diagram

pebble grinding section


pebble grinding at granduc's tide lake concentrator


By |2019-05-03T15:45:26-04:00April 8th, 2019|Categories: Grinding|Tags: |Comments Off on Pebble Grinding

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