PICKING TABLES.—In ore dressing it is frequently found to be advantageous to sort the ore before it goes to the fine-crushing machinery which prepares it for the hydraulic machines—picking out the pieces of rich mineral, which do not need to be concentrated, and also the waste or worthless rock. This is often done by hand by men or boys, who go over a lot of ore spread upon a platform, breaking the lumps of ore with spallina hammers to free the pieces of rich mineral, and throw ing them aside into separate piles. A more economical and systematic method is to discharge the coarsely-crushed rock from the rock breaker upon a contin uously moving table, beside which are stationed boys who pick out the pieces of good ore or gangue as they pass.
Picking tables are usually made either in the form of broad, end less belts or circular rotating platforms. In the belt tables the crushed ore is made to drop on it at one end, is carried forward by the travel of the belt, being picked over in the meanwhile, and is discharged into chutes at the other end. The rotary tables generally consist of a large annular platform, covered with punched iron plate and revolving slowly (see Fig. 1). The coarsely-broken rock falls on it from a chute, spreading out, and is carried by the revolution of the table, until, meeting an inclined scraper, the whole is pushed off into the delivery spout. Standing around the table are boys who pick out by sight the large pieces of pure mineral or gangue, which are easily distinguished in the slowly moving layer of ore.
are used for raising crushed ore from breakers or rollers, accord ing to the arrangement of the mill, to the sizing screens. They are usually of the belt-and bucket style, and driven by the upper pulley shaft. The elevator is generally inclined for ward at the top to give a free discharge to the buckets, but in last-running elevators the centrifugal force, as the belt passes over the upper pulley, is sufficient to throw the sand into the chute, even when the belt is hung perpendicularly. An elevator's capacity depends on the size and number of buckets and the speed. Elevators of this kind are encased with wood to prevent loss of spilled material and slopping of water. The lower end, in which the lower pulley is fixed, may be a continuation of the wooden casing, or may be an iron boot with a belt tightener. The belt is always of four or five-ply rubber belting of good quality, and the buckets, of heavy sheet iron or steel, are fastened to the belt by countersunk bolts.
The Link-belt (chain) Elevator consists of a series of iron buckets carried or fixed to an endless chain, instead of to a belt, making a combination of great durability. For ordinary duty a single chain is sufficient, but for great duty and wide buckets two chains side by side are used. The average speed of an elevator is about 200 ft. per minute.
Sand Wheels are a special form of elevators employed at the Lake Superior dressing works for raising the vast quantity of tailings to such height that they can be sluiced to the place of deposit, the contour of the ground on which many of the mills are built being such that this can not be clone naturally. The great wheels used at the Calumet and Reda mills, which will serve as a type of all, are built upon the principle of those used on bicycles, with all spokes in tension. These spokes are in two systems—a set of conically divergent radial spokes for lateral stability and to support the rim, and a set of smaller tangent spokes for driving. The shaft and hub are of gun-metal, cast hollow, in three pieces, and about 23 ft. 8 in. long over all. The greatest diameter of the spider of the hub where the spokes are attached is 7 ft. 4 in. The journals are 22 in. diameter and 40 in. long. The spokes are of forged steel, 3o in. diameter and about 22 ft. long. The rim of the wheel is built up of 18 cast-iron segments bolted together, outside of which is an equal number of gun-iron gear segments. The pitch diameter of this gear is 54 ft., the number of teeth 432, and the face of teeth is 18 in. On either side of this central rim a plate-steel frame is built, carrying internal buckets, making the total width of wheel rim 11 ft. in. The buckets are lined with wood to take the wear. The water and sand are delivered to the wheel by launders, which empty into the buckets on either side. At the top of the wheel on either side is an apron suspended from the roof trusses and extending into the wheel far enough to receive the water and sand delivered by the buckets. whence it is run off through other launders. The wheel is driven by a pinion which meshes into the gear before mentioned. The weight of each wheel com plete, including the wooden lining of the buckets, is about 177 tons, and the weight of con• tabled water and sand about 10 tons. The elevating capacity of this wheel is 30.000,000 gallons of water, containing 2,000 tons of sand, every 24 hours. This amount of sand is nearly sufficient to cover one acre, one foot deep, daily.