The ore consists of galena and a little pyrite, with quartz and greenstone. The country rock is highly metamorphosed, and the greenstone varies in density and hardness. Some of the greenstones have a specific gravity of about 2.6 and are impregnated with fine crystals of galena. The remainder is dark green, very tough, and heavier, having a specific gravity of 2.8. Most of the quartz is impregnated with galena, and is difficult to separate by gravity methods. However, the galena in the dense greenstone occurs mainly as coarse crystals and the waste can easily be removed by jigging.

Milling

The mine ore is tipped into a crude ore bin, which is built alongside the crushing section, fifty feet from the Lucy Adit Portal.

The bin, which has a total capacity of two hundred tons, is constructed of pitchpine baulks and deal planks and lined with twenty-five pound rails. A mild steel chute at the bottom of the bin forms the throat, and has an aperture measuring two feet by three feet, which dimensions are ample for the usual run of mine ore.

Crushing

Two apron feeders in series deliver to a 30 in. by 16 in. Broadbent Blake type jaw crusher, which reduces the ore to 2 inches in size. This crusher is driven by a 70 h.p., 2,200 volt slip ring induction motor. The discharge from the crusher is taken by a series of conveyor belts to a single-deck Niagara screen fitted with wire cloth of 7/16 in. aperture. The discharge from the screen is conveyed to a 4 ft. Symons cone crusher, set at in., and driven by a 100 h.p., 2,200 volt motor with texrope drive. The discharge is returned to the Niagara screen and the unit is thus in closed circuit.

The undersize from the Niagara screen is conveyed to a fine ore bin, with capacity of 110 tons, at the concentrator house, a distance of 253 ft. from the crushing plant.

All motors for feeders, conveyors and auxiliary plant are on the 440 volt circuit.

All conveyors in the crushing section are standardised with belts 20 in. wide and of canvas construction covered with 1/16 inch rubber on each side. A magnetic pulley is installed ahead of the Symons crusher to remove tramp iron.

In addition to the 200-ton coarse ore bin, there is also a 70-ton reserve bin which by-passes the jaw crusher and delivers directly onto the conveyor feeding the Niagara screen. This bin has a grizzley on top with 2½ in. apertures.

The capacity of the crushing section is well in excess of that of the concentration plant, and is capable of treating up to 40 tons per hour. The daily tonnage which passes through at the present time is about 90 to 100. An eight-hour shift is worked, giving ample time for the two men who staff the section to carry out maintenance and clean up spillage.

Concentration plant

Mill feed from the 110-ton bin is conveyed by two belts in series to a two-deck Niagara screen, 8 ft. long by 28 in. wide, fitted with 6 mm. and 2.5 mm. adamantine screen cloth and operating under sprays of high pressure water. The mill tonnage is calculated from the speed of the first conveyor belt, and an hourly weighing of five feet of the stream of ore.

The oversize from the 6 mm. screen passes directly to two Hartz jigs. The jig screens are of in. punched plate, bedded with in. to ¾ in. galena. The concentrates pass through the screen and drop into the discharge hatch. These concentrates which average around 60 per cent. Pb (chiefly due to chatty ore) are passed to a dewatering plant of the company's own manufacture. The tails, with a value around 0.4 per cent. Pb pass with high pressure water down a 4 in. pipe and are discharged into a in. chipping bin. These jigs treat, on average, 9 per cent. of the total mill feed.

The over 2.5 mm. product is fed to a Pan-American jig, Placer type P.E.2, which was originally designed for gold ores, and has two compartments, 42 in. by 42 in. This jig operates fairly satisfactorily on lead ore, giving a tailings value about 0.5 per cent. Pb. The tails go down a 4 in. steel pipe into a in. chipping bin, and are used for buildings and for making concrete blocks, along with the 3/8 in. chippings. This jig treats on average 5 per cent. of the mill feed. The screen on this jig consists of a in. punched plate, and is bedded with ½ in. to in. galena. The product drops through the screen into the discharge hutch giving a 15 per cent. Pb middling, which goes by way of the Hartz jig launder on to the dewatering screen for ball mill feed.

The under 2.5 mm. product from the head screen flows directly to a four compartment Stokes Hydrosizer which classifies the feed to four tables. The overflow passes directly to a wooden cone box, the sands settle to the bottom of the box and pass to the ball mill feed, and the overflow water goes to a thickener. The first spigot feeds a left-hand Plat-O table made by The Deister Machine Company of Indiana. Since this feed consists of coarse galena with a small quantity of coarse gangue, the concentrate obtained is of very high grade, and a proportion is passed over a stationary screen by high-pressure water into a box as an 84 per cent. Pb product. The output of this high grade concentrate is low, namely 2 tons per 6,000 tons of mill feed. It is despatched in hundredweight bags to the potteries as Potters Lead Ore. The second spigot goes to another Plat-O table, while the third and fourth spigots go to Wilfley-Williams concentrating tables.

The products from the tables are cut by zinc trays, the tails with a value around 0.3 per cent. Pb, going down wooden launders, lined with used rubber belting, to the sump of the tailings pump. The middling product from this section is also delivered to the dewatering plant along with the Hartz and Pan American jig products. The sands from the dewaterer are discharged into the ball mill scoop and the water is piped to a twenty-eight ft. thickener made by R. O. Stokes and Co. Ltd. Concentrates from the second, third and fourth tables, which average 82 per cent. Pb, pass to a collecting box in the filter shed. Boxes are dewatered at the completion of a shift, and the concentrates are measured for the estimation of tonnage. They are afterwards sent to the concentrates storage bin for shipment About 60 per cent, of the production comes from the gravity concentration section and averages 81 to 83 per cent. Pb.

Fine grinding and flotation

The feed to the flotation circuit consists of:—

Of these, all but the thickener underflow are fed to the ball mill together with surplus 6 mm. and 2.5 mm. jig feed for reduction to flotation size.

The ball mill is Hardinge, 6 ft. by 48 in., with texrope drive from a 80 h.p., 2,200 volt motor. The feed averages 94.4 plus 30 mesh, and the discharge averages 88.3 per cent. minus the same mesh. Three-inch carbon steel balls are added as necessary, the consumption being about 2 lb. per ton milled. Liners, which are of chrome steel, are renewed for every 30,000 and 40,000 tons milled.

The ball mill discharges into a Stokes 4 ft. double rake classifier. The weir overflow is pumped into a Denver Conditioner by a Stokes 2½ in. rubber lined centrifugal pump, and the sands discharged from the classifier are pumped back to the Niagara screen by a 3 in. Wilfley pump.

The thickener underflow mentioned above is sent directly to the conditioner, which is manufactured by Denver Equipment Limited, and is primarily a 6 ft. by 6 ft. steel tank with superstructure and mechanism driving an impeller shaft through a stock pipe.

The conditioner slime, carrying 7 to 8 per cent. Pb, is fed to a nine-cell Denver flotation unit, type L.E. 395. Pine oil and cresylic acid are added to the conditioner, while ethyl xanthate and additional frothers are fed to the cells. Generally, the work performed by this unit is very satisfactory. Tails average 0.2 per cent. to 0.3 per cent. Pb., and concentrates 74 to 75 per cent. Pb. This section produces about 40 per cent. of the total production.

Reagent consumption per ton milled is as follows:–

Drawings and Photographs accompanying the article