Table of Contents
The design of a pebble crushing circuit as an integral part of a North American style primary autogenous or semi- autogenous grinding circuit presents certain challenges to the process design engineer. Initially, the process requirement for pebble crushing has to be determined as part of:
- the design of a new plant
- the relief of a bottleneck in an existing plant due to new ore characteristics in the mill feed
- the necessity to expand production in cases where grinding equipment has not been correctly sized in the first place, i.e., design throughput has not been achieved
If pilot plant-scale testwork is not possible, the process design engineer will have to rely on certain indicators which, individually or in combination, can be interpreted to predict the necessity of a pebble crushing circuit for achieving production objectives. The usual definition of critical size is that size range, typically 12-50 mm, which can build up in a mill charge due to the inability of or in the absence of a sufficient complement of larger ore particles and/or steel media to crush it.
Rock Quality Designation, or RQD, has been used since the 1960s to provide a measure of the quality of a rock mass. “The total length of intact lengths of drill core, each greater than or equal to 10 cm, divided by the total length of drill core over which RQD is measured with the result expressed as a percentage.”
RQD values are judged along the following guidelines:
RQD >70 The ore or rock type is considered to be competent and amenable to autogenous or semi-autogenous grinding with or without a pebble crusher to deal with critical sizes.
RQD <70 >40 The ore or rock type is considered to be less competent and amenable to semi-autogenous grinding without a pebble crusher.
RQD <40 The ore or rock type is considered to be incompetent and amenable to semi-autogenous grinding provided it is blended with more competent ores or is ground with a substantial ball charge.
Only the smaller pieces of core resulting from close jointing, faulting, weathering or zones of weaker rock, e.g., fault gouge, are discounted. If core is broken by handling or by the drilling process, i.e., the existence of fresh broken surfaces, the broken pieces should be fitted together and considered as one piece provided they form the requisite length.
Determination of the unconfined compressive strength of a particular rock or ore sample is useful in assessing the potential for pebble formation and the probability of employing autogenous grinding and pebble milling.
The pilot plant was run with a primary autogenous mill and a secondary pebble mill. New feed was run-of-mine ore, and pebbles were extracted from the primary mill for media addition to the secondary mill.
Specific power consumptions and operating work indices were recorded as follows:
Effect of Mineralogy
Analysis of Bond work index test results for crushing, rod milling and ball milling will indicate uniformity or otherwise of breakage characteristics in different size ranges. Very often one value is much higher (or lower) than the others and such variations can govern the power distribution between primary and secondary stages of grinding. Also, these observations will give an indication of the potential existence of a critical size relative to autogenous or semi-autogenous grinding.
Various sizes of drill core are available for use in determination of work indices:
Target Operating Work Index
It has been shown (Barratt, 1989) that the operating work index for a single-stage ball milling circuit (SSBM) can be used to estimate overall specific power requirements for a power-efficient SAG or SABC circuit. By definition, single-stage ball milling circuits are designed based on components for secondary/tertiary crushing, rod milling and ball milling using the appropriate Bond work indices. A modified empirical formula can therefore be used to calculate the overall specific power requirement, designated Essbm, following the basic procedure for these components. If the original empirical formula developed by Barratt is used based on the same Bond work indices for crushing, rod milling and ball milling, and the resultant primary mill specific power consumption, ESAG, is deducted from ESSBM, the required specific power consumption for secondary ball milling, EBM, can be calculated, hence:
ESSBM (overall) = ESAG + EBM
Comparison of Bond Work Indices
Although mineralogy plays a part in the values of Bond work index which are determined for an ore, it is important to demonstrate the variation of Bond work index between different stages of comminution. Data in Table 1 illustrates such variation with rock type, degree of oxidation and potassic alteration in a sulphide porphyry copper orebody. It can be seen that in this case:
- the ranges of crushing work index (impact test) can be considerable
- primary ores from a deeper horizon record higher values for crushing work indices compared to secondary ores
- potassic alteration also records higher values for crushing work index.
Grinding Circuit Design
Installation of pebble crushing facilities can be scheduled for initial production or at a later date depending upon the process engineer’s interpretation of ore characteristics and ore production schedules.
It is important in pilot plant testing that the test mill charge level be maintained in the normal industrial-scale range of 25-30 percent by volume to ensure meaningful results. There is the temptation, held by some, to open up the pebble ports to, say, 75-90 mm from 50-65 mm. Also, any increase in the nominal aperture of pebble ports automatically alters the power split between primary and secondary mills and places a heavier load on the secondary mill to the point where:
- the primary mill could be drawing full power with a higher charge density and producing a coarser product
- operation of the secondary mill could be compromised by the coarser feed and production of a coarser product than desired
- feed rate would have to be reduced in order to produce the desired product
- operation of the primary mill would be exacerbated by a further drop in charge level and a limitation placed on power draw.
As discussed previously, the probability of designing a pebble crushing circuit into a plant under these circumstances will have been assessed from a review of:
- Compressive strength tests.
- Comparison of Bond work indices for crushing, rod milling and ball milling.
- Breakage characteristics and process simulation..
The Pebble Crushing Circuit
Once a critical size has been established in a primary mill, an efficient means of extracting it, crushing it and returning the crushed product to the primary mill has to be designed. In some plants, crushed pebbles are advanced in part to secondary grinding.
Pulp Discharger Design: Structurally, pulp dischargers should be bolted individually to the discharge head regardless of whether they are castings or fabricated steel. If these design elements are not followed, the risk of mill shutdowns due to shifting pulp dischargers, lifters and grate sections, etc., and loose or broken bolts is increased.
It is felt that radial pulp lifters would perform better at higher mill speeds because at lower speeds, e.g., 72% C.S., pulp would be lifted but would fall back onto the lower advancing edge of the lifter. With spiral lifters at higher mill speeds, the possibility exists of pulp, having been lifted, falling onto the curved upper surface of the advancing lifter whereas at lower speeds it would miss it.
The Crusher: Selection of pebble crushers has traditionally centred on the Symons short head cone crusher. Recently, Omnicone crushers have been used with success. Particular attention should be paid in the application to:
- the shape and dimensions of pebbles
- the size analysis of crushed pebbles.
It is important to construct a by-pass system, usually through a flop gate and chute, so that pebbles can be recycled directly to the primary mill feed in the event of a maintenance shut down for the crusher. The primary mill would then operate at reduced throughput and circuit availability would be enhanced.