PART I PROJECT DEVELOPMENT
4. GRINDING
Try to avoid locating the office downwind of the ROM or coarse-ore stockpile.
Transfer sizefrom the SAG to the ball mill. Transfer size prediction is some what uncertain but has an important impact on the balance of power between the two stages of the milling circuit. For feasibility studies, a combination of modelling and benchmarking is generally used.
Pebble management. This issue relates to the requirement for scatting, peb- ble recirculation or pebble crushing and is discussed below in section 4.4.
Mill drive arrangement. Mill drives are generally located on the outside of the mill to facilitate maintenance.
Mill discharge arrangement. If the SAG and ball mills feed separate pump systems this is not an issue. If the two stages feed the same pump system, the SAG mill usually drives the discharge arrangement. This is because the SAG mill is usually of larger diameter and may have a requirement for screening of pebbles to protect the mill discharge pumps and cyclones, and to facilitate pebble crushing.
4.2. Feed and product size
The main impact of feed size is on the diameter of the feed spout for the primary mill. Feed spouts are generally selected to be at least three times the diameter of the largest rock in the feed. This is not usually a problem for large throughput facilities where the largest rock may be 250 mm as the mill size will be driven by power draw. However, for small circuits (o250,000 tpa) it may be an issue. A feed size of 100 mm requires a 300 mm feed spout and this can drive the mill diameter and thus mill selection.
Product size is not usually a key driver in the typical gold plant range (63–106mm). However, it becomes more of an issue outside this range. For coarse grind size (4150mm) cyclone selection and operability may be an issue, particularly for small plants. In addition, care must be taken with line sizing to prevent settling, particularly for high specific gravity (SG) sulfides.
For fine product sizes, dilution is necessary to achieve sharp cyclone sep- aration. This affects cost, as it impacts on discharge hopper size, pump size and cyclone numbers.
4.3. Ore material handling properties
High-clay ores present the same problems with material handling in the milling circuit as they do in the crushing circuit. Use of steep-sided feed chutes and incorporation of belt-cleaning facilities is important at the study stage as it affects height and therefore cost.
More competent ores often require higher aspect-ratio mills and this affects the height of the overall grinding circuit. In addition, harder ores often present problems with abrasion. This can be minimized by careful design of chutes, hoppers and pipe systems.
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4.4. Pebble management
More competent ores are likely to generate scats or pebbles. This needs to be accommodated in the study design as it is more expensive to retrofit if al- lowance has not been made upfront. Pebble management can take several forms:
Softer ores will usually produce minor amounts of steel scats and few pebbles. This is accommodated with a chute into a bunker and is cleared on a regular basis.
More competent ores may produce a regular stream of small pebbles. As- saying of these pebbles may indicate whether they are of low grade, in which case they can be treated in the same way as the steel scats. If the pebbles are of high grade they can be recycled directly back to the primary mill. This can be done manually by the use of a loader and emergency hopper. This usually requires a transfer conveyor to convey the pebbles to a small stockpile away from the mill discharge area. Alternatively, a con- veyor system can be used to recirculate the pebbles back to the mill-feed conveyor. If a conveyor system is to be used, it is often wise to make allowance for a future pebble crusher by building in extra height.
If the pebbles are coarse and of significant grade, a pebble crushing circuit is usually required. This will usually include
– a discharge conveyor;
– a primary tramp metal magnet usually suspended over the belt dis- charge;
– a transfer conveyor to feed the pebble bin;
– a secondary belt-magnet for tramp removal;
– a small pebble bin and feeder to ensure the crusher can be choke fed; and – a pebble crusher located over the main mill feed conveyor.
Sizing of the pebble crusher in this application is important, as this duty is onerous. The work index of these pebbles should be tested separately to the ore to ensure that sufficient power is installed on the crusher. There is a trade- off between efficient operation of the SAG mill (with a steady feed of crushed pebbles) and the efficient operation of the pebble crusher (choke fed).
4.5. Power balancing
Determining the transfer size in a two-stage milling circuit can be prob- lematic. As a result, provision is often made for re-direction of some of the cyclone underflow stream to the primary mill-feed chute to increase grinding efficiency. This can impact directly on the cyclone cluster height, as a splitter box is usually required. This can be a modification to the underflow launder, or a separate splitter box.
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4.6. Slurry density, viscosity and specific gravity
Oxide ores can exhibit problems with high viscosity, particularly in the milling area where slurry densities are often higher than in the rest of the gold processing circuit. This affects three areas of design:
The slurry density for milling may need to be reduced to prevent carryover of the ball charge and to improve grinding efficiency.
The slurry density for the cyclone feed may need to be reduced to allow a centrifugal pump to be used in this duty. Centrifugal pumps are usually limited by the yield stress of the slurry.
The cyclone feed density may need to be reduced to provide an efficient cyclone separation, particularly at fine sizes.
Sulfide ores can exhibit high SGs (4–5 for ores with high levels of pyrite and arsenopyrite). Once again, this is particularly important in the milling area where slurry densities are often higher and particle size is coarser than in the rest of the gold processing circuit. Care must be taken with pump and piping system design and with all systems that are impacted by par- ticle SG.
4.7. Ball charging
There are a number of methods for feeding balls into the primary and secondary mills. The method selected often depends on the size of the mill and the cost of labour as ball charging can be labour intensive. A number of methods are listed below:
The simplest method is a manual additiononto the primary mill-feed con- veyor. For small plants this can be as simple as adding balls from a hopper located adjacent to the mill feed conveyor via a manual arc gate. This system can be automated with a ball charger that acts on a timer to reg- ularly dose the mill with balls.
An alternative approach for slightly larger mills is to load balls into the emergency feed hopperwith a front-end loader.
For secondary mills, a direct load-system is required. This normally takes the form of a davit hoist located on the cyclone floor. The hoist is used for both cyclone maintenance and ball charging.
For large mills anautomated system is required. This may include a mag- netic ball loader feeding a kibble/hoist arrangement. This system is designed to minimize the likelihood of personnel being injured. A common crane rail running over the primary and secondary mill feed chutes is required.
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4.8. Presence of gravity circuit
If the gravity circuit is treating the cyclone underflow, then the main impact of the gravity circuit is on the height of the cyclone tower. In this configuration, the gravity circuit is usually fed by means of a take-off from the base of the cyclone underflow launder. This feeds a scalping screen with the oversize feed- ing the ball mill and the undersize forming the feed to the gravity concentrator.
If the gravity circuit is treating cyclone feed, a system for reducing gravity- circuit feed pressure may be required. This may include dummy cyclones or ceramic chokes. This does not apply where the gravity device is pressurized, although the maximum operating pressure may dictate the location of the gravity-separation device.
In either case, the tail from the gravity circuit is usually returned to the mill-discharge hopper to conserve water and the concentrate is gravitated or pumped to the gold room. This desire to gravitate to the gold room influences the gold room location, as the concentrate is dense and requires a steep launder angle to prevent sanding. In addition, pumping of gravity concen- trate is difficult as the material is often coarse and of high SG.
4.9. Spillage handling
Spillage handling in the grinding circuit must deal with two situations:
The routine minor spillage from pumps, hoppers, trommels and cyclones.
This is cleaned up on a daily basis and does not influence the feasibility level of design.
Crash shutdownof the milling circuit that requires dumping of the mill dis- charge hopper. This requirement drives the volume of storage under the milling area and the floor slope in the area. Floor slopes under primary mills are normally 1:12 to 1:15 for smaller plant. However, for larger plants this can be as steep as 1:8 to allow spillage to gather at a common collection point.
For large facilities, manual handling is not feasible and a drive-in sump should be included for spillage handling.