Ground slurry from the trash screen enters this EXAMPLE Agitated Leach circuit at a rate of 35.5 cubic meters per hour, and 45% solids. The slurry passes through each of the four tanks, and should remain in each tank for a period of approximately 12 hours. Each tank is 8 meters in diameter by 8.5 meters high, and is designed to hold approximately 350 cubic meters of material. Our EXAMPLE is an Agitated Leach circuit.
Process air is sparged into the bottom of each tank at a rate of 85.5 scfm at 30 psig to provide the oxygen necessary for leaching. A rotameter at each air line gives a local indication of air flow to the tanks. In order for the reaction to occur, it is imperative for the air to be flowing to each leach tank during operation. Manual valves can be used to adjust and shut off the air.
In addition to air, fresh cyanide solution and lime slurry is added to the first leach tank at a rate of 80 and 500 litres per hour respectively. The pH of the slurry should be at least 10.5 upon entering the leaching circuit, as monitored by the pH probe in the cyclone overflow launder. Not only does an alkaline environment enhance the leaching process, but even a neutral one will promote the generation of toxic hydrogen cyanide gas. Lime is added to the first leach tank to maintain the pH level throughout leaching. Lime is also added to the last leach tank in order to increase the pH to 12.5 for filtration requirements. A pH probe, is mounted in the last tank, and an indicating transmitter transmits the instantaneous pH value to a dual pen recorder on the mill control panel. The pH value at the cyclone overflow launder probe is also sent to this recorder.
Each of the four leaching tanks has a 20 HP agitator, which can be controlled locally and stopped at the mill control panel. The control transformer power for each starter is monitored by the PLC; should any of the motors fail on an overload condition, an alarm will sound at the mill control panel annunciator. The agitators will dissolve the oxygen into the slurry solution.
The slurry exits each tank via a ‘riser’; a semi-circular separated portion of the tank with the slurry entrance at the bottom, forcing the slurry to pass the agitator blades prior to exiting. The slurry flows up the riser into a discharge box, where there are two outlets with manual gate valves; the normal outlet to the following leach tank, and a bypass to the next leach tank. In an agitator overload condition, the affected tank must be bypassed until the heaters can be reset and the drive started. If further overloads occur, it may be advised to increase sparge air to the tanks and/or barren solution flow to the trash screen distribution box.
This circuit should be initially started immediately after the Thickener Circuit. As with the thickener rake, once the entire mill is running, the agitators should remain running whether the remainder of the mill is operating or not.
Procedure for Agitated Leaching Circuit Operation & Preparation for its StartUp
All leach tank agitators are run continuously under all but disastrous equipment failure conditions.
The following items must be checked before the equipment in the Leaching Circuit is started.
- The leach tank agitators should be running.
- Check that the leach tank discharge box gate valves are in the appropriate position.
- Check the Filtration Circuit to ensure that it is available to start up.
- Ensure that shut-off valve in the air line to each leach tank is open.
- Ensure that the timers for the lime addition have power and that the shut-off valves in the appropriate lime lines are open.
- Check the level of cyanide solution in the cyanide day tank to ensure there is an adequate amount to start the leach circuit.
Agitated Leaching Circuit Start Up Sequence/Procedure
- Adjust the air rate to each leach tank with flowmeters to 85 scfm.
- Start lime addition timers at the first and last leach tanks, making sure that the lime pipes are not plugged.
- Start cyanide addition to the first leach tank.
Normal Operation of Agitated Leach Circuit
After the Leach Circuit has been brought up to normal operation conditions, the operator must monitor the various process variables and alarms. The operator is also required to sample and analyse process streams and monitor local indicators.
The thickener underflow being pumped to the trash screen, must be diluted with barren solution to 45% solids. Regular hand samples are required to measure the leach feed density and the barren solution flowrate to the trash screen must be adjusted accordingly to maintain the desired density.
Protective alkalinity and free cyanide should be determined in the first leach tank every 2 hours and, if necessary, the appropriate adjustments made to the cyanide feed pump and lime timer.
The pH in the last tank should be monitored regularly to ensure that the desired pH 12.0 is maintained. If the desired pH is not achieved, lime timer must be adjusted accordingly.
The air rate to each leach tank must be checked on a regular basis to ensure that the desired rate of 85 scfm is maintained. A deficiency in air to the leach tanks will result in poor gold and silver dissolution and an excess amount of air is a waste of air and power. The air rate is adjusted with the appropriate flowmeters.
Normal Agitated Leaching Circuit Shutdown Sequence/Procedure of this Counter Current Decantation (CCD) Circuit
- Ensure that the thickener underflow has been put on recycle before shutting down the Leach Circuit.
- Once slurry has stopped flowing from tank to tank, the air to each tank can be shut-off.
- Discontinue cyanide addition to the first leach tank.
- Shut-off lime timers and close appropriate lime shut-off valves.
Leach Circuit Emergency Shutdown Procedure
In the event of an emergency the leach tank agitators can be shutdown by locally mounted STOP switches or from the MCC located in the mill control room.
After the equipment is shut down shut off the cyanide metering pump, lime timer and close the shut off valve in the barren solution line to the trash screen and in the air lines to each leach tank.