The prospects for the future of size reduction cannot be precisely established. The history and the questions facing those industries that use the technology, however, indicate the areas requiring study. Technology has always stepped up to meet the demand for size reduction. Educational and research facilities have been created to teach and to search for a better understanding of current processes and to develop new, more efficient ones. Although the size-reduction industry faced financial, environmental, and social stumbling blocks in the latter half of the 20th century, the need for continued study never has been more apparent.
Energy
In these early years of the 21st century, there appear to be few limits on the fuels necessary to drive electric generators. The environmental, political, and financial problems associated with the fossil and hydrocarbon fuels used to generate the steam that drives the turbines needed to drive the generators, however, could limit the use of these fuels in the years to come. Hydroturbines seem to be a clean solution—until the magnitude of the construction of dams and large lakes necessary to obtain the necessary water levels is considered. Additionally, the use of nuclear energy is, of course, subject to many environmental and public safety objections.
The availability of the needed energy, then, could be problematic, and reduction and rationing of the fuels or the electricity may prove to be necessary. If this does happen, more and more efficient size-reduction processes will be needed to make use of the available energy.
The production of fines with the current size-reduction processes is inefficient in the use of energy but is economically efficient in processing large continuous flows of heterogeneous materials in circuits that are both easy to operate and easy to control. The efficient continuous flow of material will be required for any new commercial reduction process developed.
Scale-Up
The scale-up to larger equipment can create problems—particularly when the degree of increase for capacity and machine volume are not the same. An example of this is the scale-up of wet-grinding overflow ball mills. The power drawn by ball mills operating at the same: (1) percent of volumetric loading; (2) percent of critical speed; and (3) length of grinding compartment varies as the ratio of the mill diameter is raised to the 2.3 power (D1/D2)23. This formula applies when the power drawn by one mill diameter is known. It keeps the three key variables in the mill power equation at the same level with only the mill diameter and power as variables. The mill volume varies as the diameter squared.
As the mill diameter increases, the volume of the mill available per unit volume of the feed decreases. As ball mill diameter changes for a constant percent of critical speed, mill speed in revolutions per minute changes at inverse ratio of the square root of the mill diameter. The volume of the media in the mill is a function of the ratio of mill diameters squared. As the mill diameter increases, both the mill speed in revolutions per minute and the number of balls per unit of feed decrease, and the number of ore and media contacts decreases. As ball mills become larger in diameter, the largest mills become more inefficient—shown either by a decrease in feed rate needed to produce the specified grind or by a coarser grind at the designed feed rate. These scale-up factors are the same in all tumbling mills—mill power or mill capacity is directly proportional to mill length.
As the diameter of ball mills increase, the retention time of the feed in the mill decreases and the flow rate of the slurry through the mill increases. At this time, when large ball mills become more inefficient, the remedy is to reduce the volume of the ball charge, which increases the mill volume available for the feed. It also decreases the power drawn by the mill, which reduces the capacity of the mill. For each ore and fineness of grind, a method is needed to determine both a minimum retention time required to produce the desired grind and a maximum flow rate of the slurry (that will not disturb the action of the grinding media). Currently, this information can be learned only in the size of the mill to be put in the concentrator. Because of the scale-up factors, minimum retention time and maximum slurry flow rate cannot be determined from small ball mills in a pilot plant.
Simulation, the newest calculation method, uses data from a smaller mill to simulate the performance of a different sized mill. For accuracy, simulation for tumbling mills should take into account the effects of the scale-up factors.