The feeding device on a tube mill is used for supplying a continuous stream of pulp to the interior and to provide means whereby pebbles may be taken into the mill.

The usual form of feeding device for pulp now in use is the feed scoop, bolted to the trunnion, which may have as many as three openings. Fig. 41 is a single scoop while Fig. 42 has three openings. This scoop delivers the pulp to a spiral in the trunnion which as the mill' revolves conveys it to the interior of the mill. The ore may be fed through a pipe entering the trunnion with a water-tight joint, as shown in Fig. 43, being the device used at the Waihi Grand Junction mill. In the smaller mills the feed nozzle is 1/4 in. in diameter. A clearance space of 18 in. is left around the feed pipe; hence the pulp discharging into the mill causes a current of air to enter between the two pipes, thus preventing any chance of overflow of pulp at the inlet end.

The Graham feeder, Fig. 44, shows the same idea applied to the pulp feed, with the addition that the pebbles are fed through the same pipe, the pulp being injected with the pebbles.

Usually the discharge trunnion is fitted with a reverse spiral which conveys the pebbles into the mill against the stream of pulp. Pebbles may be fed to the scoop on the feed end of the mill, but the process is slow when feeding by hand and it is impracticable to enclose the feed scoop in a box and depend upon the scoop to pick up the pebbles without abnormal wear on the lip of the scoop or injury to the box. Hand feeding is practical only at the discharge end where, as previously explained, there is a reverse spiral in the trunnion. Various devices have been employed to overcome this difficulty and feed the mill with pebbles by automatic feeders. That shown in Fig. 45, known as the Thomas feeder U. S. Patent No. 1045342 and described as follows, offers a very good solution of the problem :

The above device comprises a chute which leads the pebbles up to a hopper from which they are discharged into the spiral feeder of the mill. A fluted or corrugated roller delivers the pebbles from the chute into the hopper supplying the mill, and it is provided with projecting pins which protrude through corresponding slots in the hinged bottom of the chute and prevent choking or jamming of the pebbles in the chute. Movement of the shaft upon which the roller is mounted is provided by a friction wheel on the shaft moved by a rope from a lever actuated by means of a cam or eccentric on the casing of the spiral intake. The movement of the lever can be regulated and so made to control the movement of the fluted roll governing the feeding of the pebbles.

The pebbles, after entering the hopper, are conveyed through an inclined spout which delivers them directly above the entrance opening of the spiral feeder, into which they fall and are conveyed inside the tube. The movement is so set that the pebbles will be dropped into the intake of the spiral feeder at the proper point of the revolution. By means of this device the chute can be maintained free of pebbles, which will automatically be fed into the tube mill without especial care on the part of the operator. The feeding being continuous, the replacement by new pebbles more nearly approximates the natural rate of wear and avoids the fluctuating efficiency caused by allowing great wear to take place and suddenly replacing it at one time.

At the Anaconda concentrating mill the device, Fig. 46, was successfully adopted for feeding pebbles when it was found impracticable to feed the pebbles in the scoop box. The difficulty was complicated on account of the feed scoop being 7 ft. in diameter in order to lift back into the mill the sand discharged from the Dorr classifier and to retain the necessary grade from the classifier to the tube mill scoop.