Rod mills are charged initially with a selection of rods of assorted diameters, the proportion of each size being calculated to provide maximum grinding surface and to approximate to a seasoned or equilibrium charge. A seasoned charge will contain rods of varying diameters ranging from fresh replacements to those which have worn down to such a size as to warrant removal. Actual diameters in use range from 25 to 150mm. The smaller the rod the larger is the total surface area and hence the greater is the grinding efficiency. The largest diameter should be no greater than that required to break the largest particle in the feed. A coarse feed or product normally requires larger rods. Generally, rods should be removed when they are worn down to about 25 mm in diameter or less, depending on the application, as small ones tend to bend or break. High carbon steel rods are used as they are hard, and break rather than warp when worn, so do not entangle with other rods. Optimum grinding rates are obtained with new rods when the volume is 35% of that of the shell. Thus reduces to 20-30% with wear and is maintained at this figure by substitution of new rods for worn ones. This proportion means that with normal voidage, about 45% of the mill volume is occupied. Overcharging results in inefficient grinding and increased liner and rod consumption. Rod consumption varies widely with the characteristics of the mill feed, mill speed, rod length, and product size; it is normally in the range 0.1-1.0 kg of steel per tonne of ore for wet grinding, being less for dry grinding. Rod mills are normally run at between 50 and 65% of the critical speed, so that the rods cascade rather than cataract; many operating mills have been sped up to close t0 80% of critical speed without any reports of excessive wear. The feed pulp density is usually between 65 and 85u/o solids by weight, finer feeds requiring lower pulp densities. The grinding action results from line contact of the rods on the ore particles; the rods tumble in essentially a parallel alignment, and also spin, thus acting rather like a series of crushing rolls. The coarse feed tends to spread the rods at the feed end, so producing a wedge- or cone-shaped array. This increases the tendency for grinding to take place preferentially on the larger particles, thereby producing a minimum amount of extremely fine material. This selective grinding gives a product of relatively narrow size range, with little oversize or slimes. Rod mills are therefore suitable for preparation of feed to gravity concentrators, certain flotation processes with slime problems, magnetic cobbing, and ball mills. They are nearly always run in open circuit because of this controlled size reduction.
What is classification?
Classification is a method of separating mixtures of minerals into two or more products on the basis of the velocity with which the grains fall through a fluid medium. In mineral processing, this is usually water, and wet classification is generally applied to mineral particles which are considered too fine to be sorted efficiently by screening. Since the velocity of particles in a fluid medium is dependent not only on the size, but also on the specific gravity and shape of the particles, the principles of classification are important in mineral separations utilising gravity concentrators. Classifiers also strongly influence the performance of grinding circuits.
Advantages of wet grinding
Wet grinding is generally used in mineral processing operations because of the overall economies of operation.
The advantages of wet grinding are:
1. It consumes lower power per ton of product.
2. It has higher capacity per unit mill volume.
3. It makes possible the use of wet screening or classification for close product control.
4. It eliminates the dust problem.
5. It makes possible the use of simple handling and transport methods such as pumps, pipes, and launders.
Trommel screen introduction
One of the oldest screening devices is the trommel or revolving screen, which is a cylindrical screen typically rotating at between 35 and 45% critical speed. Trommels are installed on a small angle to the horizontal or use a series of internal baffles to transport material along the cylinder. Trommels can be made to deliver several sized products by using trommel screens in series from finest to coarsest such as the one shown; or using concentric trommels with the coarsest mesh being innermost. Trommels can handle material from55 mmdown to6 mm, and even smaller sizes can be handled under wet screening conditions. Although trommels are cheaper, vibration-free, and mechanically robust; they typically have lower capacities than vibrating screens since only part of the screen surface is in use at any one time, and they can be more prone to blinding.
