Feed may be as coarse as I or 2 in., but 4 or in. is more
economical. Hinged hammer mills are best suited to soft ma terials. Arrastras are nearly obsolete, as also are grinding pans. Roller mills and Chile mills are no longer used to any extent in ore dressing but find some application in commercial grinding. The use of gravity stamps is confined to gold milling. Fine grind ing consumes much power since the power required increases in geometrical ratio with the fineness of the product.
To insure a definite maximum size of particle in crushing operations and also to prepare the ore into a series of products for final separation, ranging from coarse to fine, screens are commonly used for wet work above I or 2 millimetres. They may be made of parallel bars or grates, of plate with holes punched out or of woven wire cloth. Common types have the form of fixed incline screens, of gently sloping revolving cylin drical screens or trommels, of plane surfaces set nearly horizontal and shaken endwise or sidewise, of horizontal or gently sloping plane surfaces in which the whole screen and frame is given a relatively slow bumping motion, of an inclined woven wire screen which is given a very rapid vibration in a direction perpen dicular to the plane of the screen or of an endless travelling belt of woven wire cloth. Fixed inclined screens are used only for coarse work above I or 2 in. and belt screens only for fine work below I mm. while vibrating screens are the favourite screen for work between I mm. and I inches.
are used for wet grading of material below I or 2 mm. which is the practical limit of wet screening. Hydraulic classifiers consist of a trough with a series of pockets. The ore, carried along by water, is successively subjected to a rising cur rent of water in each pocket. The velocities of the currents decrease in succeeding pockets so that the material discharged from the spigots ranges from coarse heavy particles in the first pocket to fine light particles in the overflow of the last pocket. Box classifiers or spitzkasten use no rising currents but increase the size of successive settling chambers so that finer and finer particles settle out from the horizontal carrying current. Mechan ical classifiers employ raking devices which stir up the pulp so that only coarser particles can settle and be raked up an incline and thus removed from the pulp which still contains the finer particles in suspension. Settling tanks and thickeners are large vats in which the water moves extremely slowly so that solids settle out almost completely and are discharged from the bottom while clear water overflows from the top.
Picking is used occasionally to remove either rich material or waste material by hand from a stream of ore on a conveying belt or moving table. The economical minimum limit of hand picking is around 2 inches. Where hammers are used in connection with hand picking to cleave lumps of mixed values and waste, the operation is called cobbing.
serve to disintegrate cer tain types of ores such as nodules of iron oxide in a clay matrix. They agitate the mass of ore and water, thus freeing the nodules from the clay and washing away the clay in the overflow at the lower end of the log washer or in the undersize of the trommel.
are the almost universal machines for separating ore in sizes ranging from II or 2 in. down to A. inch. In the movable sieve type the ore on a screen is stratified by moving the screen up and down in water, while in the fixed-sieve or Harz jig a plunger on one side of the partition causes an alternate up and down current through the ore on a fixed sieve. Heavy minerals settle into the bottom layer of the bed, while lighter minerals are in the top layer. These heavy minerals, if fine enough, pass through the sieve into the hutch below and issue from the spigot outlet at the bottom. If too coarse to pass the sieve the concen trates layer is removed continuously by a device known as an automatic discharge. Fresh water is added continuously. A jig may be used singly, or with two to six cells in series.
These tables take up the work where jigs leave off at about
in. and may handle graded material down as fine as millimetre. The Wilfley table repre sents the original of this type of table and consists of a four-sided plane surface having a slight slope downward. A series of riffles or thin cleats are tacked on a smooth linoleum-topped table and each riffle has its maximum vertical thickness at the right and tapers down to nothing at the left. Ore and water are fed at one end of the upper edge, while wash water is fed along the remainder of the upper edge. A differential reciprocating action moves the ore lengthwise of the table toward the left while the washing action of the water across the table at right angles to the shake causes the ore to spread out in the form of a fan with concen trates nearest the upper or wash water side. The concentrates are finally shaken over the end, while the tailings are washed over the lower edge.