The important of crushing your ore and rock fine and properly is often forgotten. The finer you crush, the higher your ball mill tonnage and capacity will be. The effect of ball mill feed size and how it affects circuit throughput can be hard to estimate. Here we described a method of designing a crushing plant using power drawn and power rate to define reduction ratios in each stage of crushing. The plant power and power rates were computed from a Bond calculation as applied to the crushing plant feed and output sizes. A comparison of the low and high energy configurations.
We would design this plant differently today using energy parameters from the pendulum impact tests for calculations. It would only be necessary to use the Bond feed and product size calculation if no pendulum results were available.
Crushing Finer To Reduce Milling Costs
This new high energy or power rate crushing brings a different perspective to comminution flow sheet selection. Generally, up until the early 1960’s the classical flow sheet for a beneficiation plant was primary crushing followed by two stages of cone crushing in closed or open circuit, making feed for rod mills, followed by ball mills. The rod mill was needed to reduce feed size to the ball mill because crushing plant output was normally coarser than 80% passing 10,000 microns. Such feed causes power inefficiency if fed directly to a ball mill. Even though the rod mill could be a relatively inefficient device for both energy and metal consumption, as was evidenced by Bond, it still made the overall circuit energy consumption more efficient.
Under the right operating conditions, high power rate crushing can bring mill feed size down to near 80% passing 7,000 microns and finer, which can be handled more efficiently by ball mills. Based on average field observations, the crushers can do this for less than half the energy and between one-tenth and one-twentieth of the metal consumed in a rod mill.
It is, therefore, feasible to look at designing more efficient single stage ball mill circuits following two stages of fine crushing. The result will be an overall reduction in total applied crushing and milling energy for the same size reduction.
To make the most efficient use of both the crushing and grinding comminution energy, both reductions should be treated as dynamic components of the same system. When the feed to the grinding mills gets coarser and/or harder and the production rate drops, the crushing plant feed rate should be readjusted to a lower level to maximize power rate, which will flow on as a benefit helping to increase the mill output.
The advantages of such schemes will become more obvious when an ore of varying hardness is fed to the crushing and milling systems.
We will consider an ore with a ball mill work index varying between 16 and 13, feeding into a single stage ball mill operation with one million kilowatts per day consumed power. For the particular mill configuration, a performance graph, Fig. (21), has been constructed according to Bond’s methods.
Providing the crushing plant design allows for the machines to be fed continuously and the power on each crushing unit is maximized by adjusting both the feed rate and settings. The power drawn and reduction achieved to the grinding mill feed will be maximized.
The grinding mill output will vary considerably with the Work Index. If the feed size was 13,000 micrometers for the same grind production size, theoretical output from Fig. (21) would change from about 90,000 tons per day on the 13 Work Index down to 65,000 tons per day on the 16 Work Index.
Because of the superior energy efficiency of crushing over milling type processing, when the ore becomes harder in this system significant gains will be made if the feed rate to the crushing plant is reduced to closely match the mill production rate. If we consider the crushing plant runs at an average of 100,000 kilowatt hours per 20-hour day, the available energy for reduction will be:
Soft ore Work Index 13 = 100,000/90,000 = 1.1 kwh/t
Hard ore Work Index 16 = 100,000/65,000 = 1.54 kwh/t
For the purposes of this example, we will hypothesize that the the crushing index of the hard ore with the increased energy input of 1.54 kw/t reduces the ball mill feed size to 6,500 micrometers. As a result, the mill output will increase with this reduced size to approximately 77,000 tons per day. The gain in production compared to the 13,000 micrometer feed will be:
(78,000 – 65,000)/65,000 x 100 = 20%
The theoretical gain will actually be greater because the graph in Fig. (21) is constructed according to the Gates-Gaudin-Schuhmann size distribution used by Bond. We have already shown that this does not apply to crushing processes, which generate increased proportions of fines with higher energy input levels. As a consequence of this, the actual, gain is likely to be closer to 25% and the mill production increased to 65,000 x 1.25 = 81,250 tons per day.
Obviously, this will increase the capacity of the crushing plant and coarsen its reduction, again influencing mill output. Ideally a control system for the whole plant would balance both crushing and milling operations to maximize the benefits described.
Again, we might hypothesize that the crushing and milling output would fluctuate between rates of 78,000 and 90,000 tons per day instead of 65,000 and 90,000 tons per day. The advantages are obvious to all.
- As impact crushing energy is increased, an increasing proportion of finer sizes is produced. In general, the quantity of fines produced is proportional to the applied power rate (KWH/tonne).
- A twin pendulum impact crushing device can generate product size distributions similar to those
produced in commercial cone crushing operations.
- By relating the net energy of pendulum crushing to the total energy in the commercial crusher, a comminution energy efficiency can be obtained for the commercial crusher.
- The breakage size distribution obtained in the pendulum, crushing between two flat vertical plattens, is similar to that produced in a cone crusher. The effects of the circular chamber and the claims by some manufacturers that there is a special and most efficient angle to the horizontal for breakage in a cone crushing chamber are not corroborated by this study of fundamentals.
- It is a claim for some designs of cone crushers that the eccentric throw has some mystical influence on the capacity and size distribution produced by a particular machine. The pendulum tests supported by field observations prove conclusively that reduction and size distribution of products are simply related to the energy applied to the material. For a fixed application of power, increasing eccentric throw will reduce the force available to crush. This lowers the available power rate and, hence, the reduction ratio. The size distribution appears to be independent of the number of impacts causing this energy application.
- The pendulum results prove that the size distribution produced from crushing processes is a natural breakage pattern characteristic of the material and to a lesser extent the mode of breakage.
- The size distribution occurring in crushing processes cannot be described by the Gates-Gaudin-Schuhmann log size versus log cumulative percent passing used by Bond or the Rosin-Rammler relationship. There appears to be little correlation in the finer size ranges of products actually occurring and those predicted by such relationships. New mathematical expressions are needed to define crusher energy size relationships.
- This study of fundamentals does not support the statement of some crusher manufacturers that crushers operate inefficiently at higher energy input levels. The production of coarser sizes will no less efficient, should this be the desired product for a process such as crushed stone production, if power rates greater than that required to maximize that coarse size are applied.
- Crusher power rate, which is influenced by the geometry and eccentric throw of the crushing chamber as well as the feed size and hardness of the processed material, will determine reduction and size distribution in the crusher product. This product is related to the close side setting only if this is considered to control crushing energy, application.
- Automatic setting regulation as applied to cone crushers can increase the average power drawn and maximize power rate along with reduction ratio. The phenomena can be studied with the pendulum impact test apparatus.
- Crusher productivity in quantities of a fixed size distribution is a function of power drawn as opposed to physical machine size. From the results of the pendulum tests, it is possible to visualize a small machine drawing more power outproducing a larger machine drawing less power.
- Energy utilization by crushing and milling machinery is different. As a general conclusion, this is particularly so with rod mills and, to a lesser extent, ball mills. High power rate crushing resulting in increased fines, can bring significant energy conservation to comminution circuits using both forms of breakage. The pendulum test device will allow us to quantify potential gains in the laboratory.
- Crushing machines consume less wear metal than grinding mills. This is especially so in the case of rod mills which compete in the fine crushing area. Because wear metal consumption is related to energy consumed to reduce material to a given size, any energy reduction using high power rate crushing will reduce such metal consumption. The pendulum impact test will give a measure of potential gain from this area in the laboratory.
- Because the pendulum test can be used to predict the size distribution from cone crushers, finite calculations can now be made for the following parameters, some of which have often been decided by “Rules of Thumb”:
(a) The number of crusher reduction stages.
(b) Number of crushing machines and their connected power.
(c) Circuit loadings.
(d) Screen requirements.
(e) Size distribution of plant output.
This methodology will give more accurate results than designs using standardized capacity tables and product size distribution curves.
- The pendulum tester can be used to run tests on drill core sized material so that plant designs can be more accurately formulated from geological exploration data. Pilot plant tests, which are often impractical to run because of the lack of sufficient sample material, are not now a necessity for feasibility and scale-up studies.
- The guestimation type comparisons of flow sheets using conventional crushing and grinding processes versus autogenous milling can now be more accurately made. The high power rate crushing process that has been studied here is not normally part of such engineering evaluations. It can now be made so, quickly and relatively inexpensively, by using the pendulum impact device.
- As a result of these studies, it is postulated that the current method of running crushing plants at maximum capacity through a given screen size will not minimize energy and wear metal costs in beneficiation plants employing downstream grinding mills. Crushing and milling circuit control systems that balance the crushing plant output to the grinding mill capacity will maximize plant capacity at the lowest production cost.
- Properly applied, the technology described will reduce the magnitude of the over-design parameters used in most comminution plant design calculations. This should reduce the capital costs of these plants.