The phenomenon of “surging” in a mill is a subject upon which very little has been written; presumably because it is a condition which cannot be tolerated in mill operation and which must be eliminated by variation of some or many of the physical dimensions or characteristics of the mill or mill charge. The phenomenon known as “surging” consists of a pendulum-like oscillation of the whole charge of the mill, within the mill shell. Thus for one part of the cyclical motion the charge is moving around the centre of the mill in the same direction as the mill shell, and during the second part of the motion, in the opposite direction. As a consequence of this oscillatory motion of the charge, the torque necessary to maintain the mill shell in steady motion fluctuates widely and, in certain circumstances, can make such demands upon the driving motor that the circuit breakers are thrown out. This throwing out of the circuit breakers is, however, a lesser evil since the danger is, in such a case, apparent and must be remedied. A more subtle danger is hidden in fluctuations of torque which are insufficient to bring the circuit protective devices into operation. In ….Read more
In the previous chapter the influence of the various physical quantities defining the mill and mill charge has been studied in connection with the performance of a mill as a device for the creation of new surface in the powder. For some purposes, however, it is also desirable that the product should have a preferred form of size distribution curve. For example, in mineral dressing it is usually necessary that the material below about 200 mesh should be a minimum. To some extent such modifications in the size distribution characteristics can be brought about by changes of the mill dimensions. It must be emphasized, however, that such changes are of limited scope but, even so, are often worth-while.
Effect of Ball Diameter, or Rod Diameter Variation
The first relationship to be considered is the effect of the variation of the ball diameter, or rod diameter, upon the size distribution obtained. In Fig. 6.1 are shown typical size frequency curves obtained by ball milling; these curves being deduced from the data of Coghill and Devaney.
As would be expected, the grinding of a coarse feed with small balls does not give rise to the effective grinding of the coarse particles in the feed ….Read more
The power required to drive a tumbling mill is of interest both to the designer and to the mill operator: to the former as a basis of design for the determination of the necessary size of the elements of the machine; and to the latter because all other factors being equal, the most economical machine is that which demands a minimum power for driving.
The power required to drive a mill depends, to some extent on every one of the physical dimensions defining the mill shell and ball charge and on many of those defining the properties of the powder charge. Thus the number of variables involved is very large. Since even a moderately complete theory for the internal dynamics of the ball mill, in which all these variables are given due importance, has not been propounded, the calculation of the power requirements of a mill, from theoretical considerations, cannot be made. Similarly, owing to the great number of variables, no complete experimental investigation of the power demands of ball, tube and rod mills has been made, the amount of work required for an analysis of all of the variables being prohibitive.
Grinding Mill Power Scale-Up….Read more
In all ore dressing and milling Operations, including flotation, cyanidation, gravity concentration, and amalgamation, the Working Principle is to crush and grind, often with rob mill & ball mills, the ore in order to liberate the minerals. In the chemical and process industries, grinding is an important step in preparing raw materials for subsequent treatment. In present day practice, ore is reduced to a size many times finer than can be obtained with crushers. Over a period of many years various fine grinding machines have been developed and used, but the ball mill has become standard due to its simplicity and low operating cost.
A ball mill efficiently operated performs a wide variety of services. In small milling plants, where simplicity is most essential, it is not economical to use more than single stage crushing, because the Steel-Head Ball or Rod Mill will take up to 2″ feed and grind it to the desired fineness. In larger plants where several stages of coarse and fine crushing are used, it is customary to crush from 1/2″ to as fine as 8 mesh.
Many grinding circuits necessitate regrinding of concentrates or middling products to extremely fine sizes to liberate the closely associated ….Read more
The cost of replacement of the metal worn from the working surfaces of the balls and liners of a mill often represents a considerable fraction of the cost of operation of the plant and so is of some real economical importance. In spite of this importance, however, the amount of information in available on this subject is amazingly small and, unfortunately, is insufficient to form the basis of more than a very tentative analysis of the problem. As far as is known the only attempt to derive a theory for metal wear is that of one of the present authors, Rose, and the relevant parts of this treatment will be outlined briefly here.
The basic idea underlying this treatment is that particles of the material undergoing milling will, upon contact, be driven into the surface of the balls at a rate which depends upon the relative hardnesses of the ball and particle, the quantity of powder present and upon other such factors. It is also assumed that, provided there is relative tangential million between a pair of balls, one or other of the embedded particles will, upon making contact, as in Fig. II. 1a, be torn out. This then leads to the conclusion ….Read more
In the previous chapter, the various “laws” of comminution which have been propounded have been studied in relation to the physical process of size reduction and to the available results of experiments into the fracture of homogeneous and heterogeneous materials by impact and by slow compression. In this study it is seen that no satisfactory “law” of comminution has yet been deduced but that a number of more or less empirical expressions are available for the guidance of the designer of crushing machinery.
The present chapter will be devoted to a study of the process of grinding within a real mill. Since within a “tumbling” type of mill the size reduction can result from direct crushing, from shearing or abrasion between the particles in the feed, between the particles in the feed and the mill bodies or between the particles and the mill lining, the process is much more complicated than in the case of simple “free” crushing conditions considered previously. In addition to this complication, however, the motion of the ball charge itself is extremely complex. Thus the precise mode of transfer of the energy to the crushing surfaces can not be simply represented.
For these reasons, a rigid ….Read more
The problems associated with comminution of solid bodies in numerous and complex. In spite of the large amount of investigation which has been undertaken, the present state of knowledge is such that there is no theory by means of which the behaviour during fracture of even a single particle under the simplest possible mode of loading can be predicted in reasonable detail or with moderate accuracy.
In the absence of an adequate theory applicable to even a simple case, it is clearly unreasonable to hope that the behaviour of a comparatively complicated system, such as a mill, in which numerous particles are involved and the mode of loading gives rise to both impact and abrasive grinding of the particle, can be predicted from theoretical considerations. Because of this absence of adequate theoretical knowledge much of the design of large industrial mills, is based on previous experience with similar mills or is deduced from tests on pilot plant. In spite of the absence of adequate theories, it is intended to give in this chapter a brief survey of the present knowledge of the subject of the fracture of solid particles, since it is believed that such knowledge is desirable if the problems of grinding ….Read more
The motion of the charge, that is the grinding media and the material undergoing grinding, within a mill is of considerable theoretical interest and practical importance, and for these reasons, has been the subject of considerable study by a number of workers, but, even so, no rigid and complete theory, covering all the aspects of the dynamics of the mill charge, has yet been produced. The practical importance of this subject clearly resides in the possibility of the prediction of the grinding behaviour, and other such characteristics, of a mill from the knowledge of the trajectories of the elements of the mill charge. The theoretical interest lies in the study of the dynamics of the system and in the derivation of equations to define the million of the elements of the mill charge in terms of fundamental quantities such as the size and the speed of rotation of the mill. A simple example of the practical importance of this information is in the use of the knowledge of the trajectories followed by the balls in a mill to determine the speed at which the mill must run in order that the descending balls shall fall on the toe of ….Read more
In many industries the final product, or the raw material at some stage of the manufacturing process, is in powdered form and in consequence the rapid and cheap preparation of powdered materials is a matter of considerable economic importance.
In some cases the powdered material may be prepared directly; for example by precipitation from solution, a process which is used in the preparation of certain types of pigments and drugs, or by the vacuum drying of a fine spray of the material, a process which is widely adopted for the preparation of milk powder, soluble coffee extracts and similar products. Such processes are, however, of limited applicability and in by far the greatest number of industrial applications the powdered materials are prepared by the reduction, in some form of mill, of the grain size of the material having an initial size larger than that required in the final product. These processes for the reduction of the particle size of a granular material are known as “milling” or “grinding” and it appears that these names are used interchangeably, there being no accepted technical differentiation between the two.
Why is Ore Grinding Necessary
The reasons for the grinding of industrial materials are numerous but the principal ….Read more