BOND IMPACT CRUSHING WORK INDEX TESTER

Whilst absolute conclusions cannot be drawn in every instance, the following trends and influences are clearly apparent from the discussion. Further studies in many areas are justified and are still the subject of continuing research programs.

1. 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).
2. A twin pendulum impact crushing device can generate product size distributions similar to those produced in commercial cone crushing operations.
3. 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.
4. 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.
5. 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.
6. 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.
7. 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 ore needed to define crusher energy size relationships.
8. 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 be less efficient, should this be the desired product for a process such m crushed stone production, if power rates greater than that required to maximize that coarse size are applied.
9. 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.
10. 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.
11. 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.
12. 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.
13. 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.
14. 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.
15. 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.
16. 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.
17. 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 beneficia-tion 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.
18. 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.