In Preparation for Startup, the following items must be checked before the equipment in the filtration circuit is started.
- Check that the filters, repulpers, filtrate pumps, vacuum pumps, flotation blower, and transfer pumps are ready to run.
- Check that the cooling water compressant system is functional. The selected cooling water filter must be clean, and reclaim water must be flowing through the heat exchanger.
- Check that the flocculant mix system has sufficient flocculant for one hour of operation, and is ready for use.
- If the agitators in the filter boot are stopped they should be restarted, and if they trip out on overload because of settled solids the motors should be reset and the boots drained.
- Check that pregnant solution tanks are capable of accepting filtrate and that barren solution is available for wash and repulp.
- Check when material from the last leach tank will be arriving and proceed to start up if sufficient material is available.
- Check the plug valves in the filter distribution box are set for flow to the primary and secondary filters.
Filtration Circuit Start Up Sequence
- Ensure that the barren pump is running, and open barren valves to filter wash and repulp.
- Close vacuum valves at all drum filters.
- Start cooling compressant water system by: starting either circulating pump, and put the sump pump in auto.
- Switch on two of the three vacuum pumps, ensuring that compressant water is flowing at 70 cubic meters per hour.
- Turn on the flotation blower.
- Bring primary filters on line:
* Start drum filters and boot agitators for filters.
* Start filtrate pumps.
* Start flocculant metering pump for feed to the distribution box.
* Turn on spray bar supply (barren solution) and adjust flow to 8.8 cubic meters per hour.
* Start one of the filter feed pumps.
* Open the vacuum valves at the primary filters. Open the bottom butterfly valve wide open to allow full vacuum for maximum solids pickup and open the top valve to the appropriate vacuum required for optimum drying.
* Turn on repulper gland seal water.
* Turn on repulp barren solution spray bar and adjust to 25.7 cubic meters per hour.
* Start repulper.
* Start one of the wash feed pumps.
- The secondary filtration stage must be started as soon as possible to avoid overflowing the wash feed pumpbox.
Bring secondary filters on line:
* Start secondary drum filters and boot agitators.
* Start filtrate pump.
* Turn on spray bar supply (barren and/or reclaim solution) and adjust flow to 8.8 cubic meters per hour.
* Open vacuum valves to vacuum pumps in the same manner as that mentioned for the primary filters.
* Turn on repulper gland seal water.
* Turn on repulp reclaim water spray bar and adjust to 33.3 cubic meters per hour.
* Start the repulper.
Normal Operation of a Merrill Crowe Filtration Circuit
After both filtration stages have been brought up to the normal operating conditions, the various process variables and alarms must be monitored by the operator. The operator is also required to sample and analyse process streams and read local indicators.
Adjust the butterfly valves at each drum filter to give the correct vacuum, 22″ Hg for the bottom valve and a minimum of
5— 7″ Hg for the top valve, as indicated on the vacuum gauges attached to the valves of all drum filters. The top valve should never be closed enough to give a vacuum of less than 5-7″ Hg or the cake may not remain on the filter cloth.
All slurry pumps in the filtration circuit have a backup pump, therefore, in the event of a pump malfunction or failure, the failed pump must be immediately replaced with its appropriate standby pump, associated pump box compartment and gland seal water.
In the event of a vacuum pump failure or malfunction, the failed vacuum pump must be taken out of service and immediately replaced with the standby vacuum pump.
Changes in ore type or other factors may cause difficulty with filtration. The following undesirable conditions may be experienced:
Poor cake pick-up and/or thin cake could be a result of the following major factors:
- LOW PERCENT SOLIDS IN THE FILTER FEED. This is causing settlement in the filter boot or inability to form sufficient cake. Check the percent solids in the feed and if low, correct the fault. This is likely to be a second stage filter problem due to too much repulp water on the first stage filter discharge. Increase vacuum to improve cake pick-up.
- SLIMEY ORE INADEQUATELY FLOCCULATED. Nothing can be done about the ore and so the degree of flocculation must be adjusted to ensure good pick-up. Check the flocculant mix system to ensure that the flocculant is being properly wetted, mixed and aged. If there is no problem with the mix system, increase the flocculant dosage to the drum filters. Also, check the pH of the filter feed slurry. The pH of the slurry directly affects the flocculation (coagulation) of the solid particles. The pH level must be maintained at 12.0. Increase the cycle on the lime timer at the last leach tank or the lime timer at the filter feed pump box if the pH level is below 12.0.
- FILTER DRUM IS ROTATING TOO FAST. This is only likely to be a problem in the early days of plant operation until a good operating speed is established. If, however, a drop in filter tonnage is experienced it may be necessary to adjust the drums variable speed drive.
- FILTER CLOTH IS SCALED. An excessive build up of calcium scale will blind the filter cloth and decrease the vacuum required for cake pick-up. The filter cloth must be acid washed to dissolve the scale. The procedure for acid washing the filter cloth is described in Section 12.0.
A sloppy or excessively wet cake could be the result of the following major factors:
- POOR FLOCCULATION DUE. This may be due to a change in ore type. Follow the instructions described in 2 above.
- INADEQUATE VACUUM. An inadequate vacuum on either the cake pick-up cycle (bottom butterfly valve) or the drying cycle (top butterfly balve) will result in a sloppy cake. The problem is most likely due to inadequate vacuum on the dry cycle and the vacuum is insufficient to draw the filtrate through the cake. The vacuum should be adjusted accordingly.
- TOO MUCH VACUUM. Ironically another cause of sloppy cake in filters using flocculant is caused by excessive vacuum causing premature floe collapse which dramatically decreases filter cake permeability. This could be either a low or high vacuum problem. The vacuum should be adjusted accordingly.
- FILTER DRUM IS ROTATING TOO FAST. Insufficient time is being allowed for cake drying. Slow down the filter drum.
Poor wash efficiencies, resulting in excessive amounts of gold and silver remaining in the filter cake, could be due to the following factors:
- INADEQUATE WASH. Insufficient wash water is being used to displace the pregnant solution. The wash water can be increased on all drum filters.
- SLOPPY CAKE. See the section above.
- FILTER DRUM IS ROTATING TOO FAST. Insufficient time is being allowed for a thorough wash cycle. Slow down the filter drum.
- OVER FLOCCULATION. Too much flocculant may cause an extremely stable floe that has tied up or entrained the pregnant solution in the floe structure. If the floe does not collapse the pregnant solution will not be displaced. The flocculant addition rate should be reduced; however, avoid the problem outlined above as a consequence of inadequate flocculation.
Filtration Circuit Normal Shutdown Procedure
Normally the primary filters are shut down first when flow from the leach area is essentially finished. However if shut down is necessary due to difficulty with another circuit downstream of the filters, the secondary filters can be shut down first.
- Stop filter feed pump.
- Discontinue flocculant addition to filter distribution box.
- Stop filter spray wash.
- Turn off filter drive, leave filter boot agitator running.
- Shut off the repulp spray water.
- Stop the repulper. Shut off repulper gland water.
- Stop wash feed pump. Shut off gland water.
- Repeat the above steps to shut down the secondary filters.
- Stop vacuum pumps, allow vacuum receivers to drain, then stop filtrate pumps.
Emergency Shutdown Procedure of a Filtration Circuit prior to Merrill Crowe
In the event of an emergency, all the equipment in the Filtration Circuit can be shut down by locally mounted STOP switches or from the MCC located in the mill control room.
After all of the equipment is shut down, shut off the flocculant metering pump and close all the shut off valves in the wash and repulp water lines. Also close the valves in the air lines to the filters and the valves in the gland seal water lines to the associated pumps.