PART I PROJECT DEVELOPMENT
5. GRAVITY CONCENTRATION
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.
Water demand (quality, volume and pressure) Impact on water balance
Security.
These are discussed in detail below.
5.1. Gravity device location
Gravity devices are usually placed in the milling circuit as a primary recovery device or on the leach tail as a scavenging device. The function of the device, whether for free-gold recovery or sulfide recovery, is not part of this discussion.
In the milling circuit the device may be placed in a number of locations:
Under the cyclones to treat cyclone underflow. Normal practice has been to take about 10–25% of the recirculating load and process it through the gravity circuit. Because of the coarse and dense nature of the feed, a scalping screen is normally located above the gravity device. The coarse oversize is discharged into the mill while the undersize feeds the gravity device. This type of approach is common for Knelson and Falcon Super- bowl concentrators.
On top of the mill-discharge hopper if treating cyclone feed. Treating cy- clone feed is still uncommon on any other than small facilities. A bleed from the cyclone feed-line is directed into the gravity device with the tails reporting directly to the mill-discharge hopper and the concentrate ad- vancing to the gold room. As discussed above, for non-pressurized equip- ment such as Knelson or Falcon concentrators, a pressure-reduction system must be included for the bleed. This may be a system of ceramic chokes or a dummy cyclone.
For pressurized devices such as the in-line jig, the device accepts coarser feed and can accept cyclone feed pressure. However, the jig concentrate usually requires secondary processing.
5.2. Product destination
Tails from gravity circuits are normally directed to the mill-discharge hopper to conserve water. This results in a recirculating load of material passing through the cyclone cluster. As a result, the gravity device must normally be located above the mill-discharge hopper (normally in the cyclone tower).
Concentrate from a Knelson or Falcon concentrator or Kelsey jig is rel- atively high grade and is usually fed direct to a hopper in the secure part of the gold room. This may influence the location of the gold room.
Concentrate from an in-line pressure jig is usually coarser and lower grade (up to 20% of the feed mass) and requires secondary processing. The
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secondary processing may take the form of a secondary gravity device lo- cated in the milling area or an intensive leach-reactor, usually located in the gold room. In either case, the pressure in the jig is able to transfer the concentrate to its location.
5.3. Water demand (Quality, volume and pressure)
Most gravity concentrators require water for fluidizing the bed of the cen- trifugal units or filling the hutch of the jig. The centrifugal units have rel- atively fine dispersion holes so the water must be of good quality, adequate pressure, low in solids content and not likely to cause scaling. This normally dictates that the water be supplied from a raw-water source. Centrifugal units should have the water filtered (coarse and fine) and would normally include pressure monitoring and flow control. Water quality and pressure control is also important for the Kelsey jig as the water flow is introduced through small (4–6 mm) spigots.
The water-quality requirement for the in-line jig is somewhat less onerous as it does not have the fine-aperture water-dispersion system. However, water pressure is an important issue.
5.4. Impact on water balance
The majority of water in all gravity devices will report to the gravity-device tailings. In this case it means that the water will report to the mill-discharge hopper. This should be seen as part of the cyclone-feed dilution water. Ide- ally, the water flow should be monitored as part of the overall milling-circuit water-balance so that the cyclone overflow density can be controlled. For a milling circuit with a relatively coarse grind-size (4106mm), a high cyclone overflow density may be desirable to minimize tankage volume downstream.
The gravity-circuit water addition may directly impact on the maximum achievable density and needs to be accounted for.
5.5. Security
Any device producing gravity gold has security implications. The emphasis on security will normally increase inversely with the average wage of the operator. A typical installation will include the following features:
The gravity device will be located in a secure steel cage with a padlocked door.
Concentrate will be discharged through a welded steel pipe directly into the gold room if possible.
Water flow and pressure control is normally located outside the cage.
The control panel for startup/shutdown is normally located outside the cage.
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