In my opinion, the effect of mill speeds on grinding costs must be studied along with capital investment and dollars gathered together as profits.
Comparing the entire groups of operators with those who have had the opportunity to make slow-speed mill studies, I think you will find the latter small in numbers. Most managers want the equipment worked to its maximum output. There are, however, some installations where plant and mill sizes are such that they can do the job with reduction of mill barrel speeds.
The past and the present installations of the industry are laid out to get the most capacity for the least capital outlay. This is the case even with the plants of Chile Exploration, International Nickel, Morocco, and Anaconda, now under construction or being changed. The industry recognizes that most all equipment it buys today is good and can be depended upon for efficient performance.
Under this scheme of things, I am doubtful that slow-speed ball mill operation will be generally applicable.
To be convinced of possible practical results from the predictions in the conclusions, I think we would have to rely on the analysis of expert cost accountants to furnish the necessary proof figures.
For one, I heartily agree with Mr. Johnson’s opening statement that the effect of mill speeds on grinding costs must be studied along with capital investment and dollars gathered together as profits. It was on this basis and for this reason the paper was written.
Mr. Johnson, on the other hand, takes the position that, on the whole, low speeds are not justified from the economic standpoint, basing his principal reason on the fact that lower mill speeds cut mill capacities and hence reduce the gross income from the product produced. There is no denying this point. It is almost axiomatic. It is for this very reason that the overall advantage of lower mill speeds has been discounted and even overlooked. It was for this reason mainly that the paper was written in the first place.
Decreasing the speed of overflow wet grinding ball mills to about 50 pct to 55 pct of critical ordinarily decreases the cost per ton ground, both in metal wear and in power consumption, with certain limitations. The decrease in metal wear is usually larger than the decrease in required energy; and this enlarges the benefit to be obtained by slower speeds at the present time. In the past decade the cost per pound of grinding media and liners has approximately doubled, while the cost of power has increased only slightly, if at all, so that at present the cost per ton ground in metal wear averages somewhat more than the energy cost. Future cost trends are largely unpredictable.
The object of this paper is to show the economic advantage of operating ball mills at relatively slow speeds. Although many operators know that grinding costs are reduced by operating mills at low speeds, they seldom do so for a variety of reasons. When capital is limited, first cost is of primary importance, and installation of the less costly, high-speed mill is favored. When materials are in short supply, as during a war period, capacity rather than economy is the primary consideration. Existing mills are then speeded up to provide the desired increase in capacity, and new mills are purchased without consideration of maximum grinding efficiency.
If the performance of large mills is indicated
by the performance of laboratory mills, it may be assumed that for high pulp-level operation efficiency is at a maximum in the vicinity of 50 pct of critical speed irrespective of the hardness of the ore. For low pulp-level operation, increased efficiency of grinding with reduced speed may be assumed for soft ores and an indeterminate change in efficiency for hard ores.
Conclusions
Grinding efficiency increases as mill speed decreases within the range of practical operation.
Both power and ball cost per ton of —200- mesh produced decreases with a decrease of mill speed.
A slow speed, high pulp-level mill with sufficient additional volume to equal the capacity of an equivalent higher speed mill will make up the difference in capital cost between the two mills in well under a year’s operating time through the saving in power and ball cost alone.
