Metallurgist & Mineral Processing Equipment in Froth Flotation In June 2013, Metallurgy organized a technical review of its Pyra Metallurgist Operations. This report focuses on the concentrator performance seeking to find the reason for that performance and looks for improvement opportunities mostly on its base metals. Below is a graph of the Metallurgical Performance at Pyra Metallurgist from January 2011 to May 2013.
Around February-March 2012 was the tipping point of Pyra Metallurgist' Metallurgy.
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Pb Conc grade became unstable Cu Conc grade became unstable Zn Conc grade became unstable Cu (Recovery) loses to the Zn Conc started to increase Cu (Recovery) loses to Final Tails started to increase Zn (Recovery) loses to Final Tails started to increase
A Mineralogical study (Diagnostic Metallurgical Assessment) has been ordered on the April and May 2013 monthly composite to assess the metal deportments of each metals in each streams. Until these Mineralogy results arrive (4 weeks from now), plant data and plant operation was analyzed to come up with an early assessment and immediate remedies. Metal Deportment: Where is the Metal Going? Evaluation of operations reports and material balances give an early empirical indication as to where the various base metals reports. Copper: It appears that over the last 18 months, Cu loses to Final Tails have seen a 3% increase while Cu loses to the Zn concentrate currently stand at nearly 20% VS 12% recovering there in 2011 and only 6% pre-2002. Lead: Currently Pb recovery to the Pb concentrate is around 30% with almost 40% of the Pb being recovered to the Zn concentrate and almost 15% recovered to the Cu concentrate while the Pb concentrate assays 35% Pb, 5% Cu, 15% Zn and 13% Fe. In 2002 laboratory float tests indicated Pb could easily upgrade. Example results of concentrate grade and recovery are:
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55% Pb with only 1.5% Cu, 14% Zn and 5% Fe at 37% Recovery
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45% Pb with only 1.2% Cu, 18% Zn and 7% Fe at 55% Recovery
These laboratory tests pointed the need for regrind of the Pb rougher concentrate to maximize Pb VS Zn selective and overall metallurgical performance. The low Pb concentrate grade obtained at Pyra Metallurgists is due to finely interlocked with Zn, as well as pollution/contamination by entrainment of liberated Cu, Fe and gangue material. A Mineralogical study (Diagnostic Metallurgical Assessment) has been ordered to confirm metal deportment to the Pb concentrate. The photo to the right shows a Pb rougher concentrate containing much gangue, pyrite and sphalerite. Zinc: Zn concentrate grade has fallen around 1% to 53% while it recovery 2% to 88%. Currently a typical Zn concentrate assays 53% Zn, 2% Cu, 2% Pb and 6% Fe.
In 2007 mineralogy done on the Zn concentrate indicated final concentrate grade could be improved up to 59% Zn by rejecting its contaminants (Cu, Pb, Fe, gangue) via regrinding and dilution (column cell) cleaning in the Cu and Zn circuits. Metal Deportment: Why is the Metal Going There? A Mineralogical study (Diagnostic Metallurgical Assessment) has been ordered on the April and May 2013 monthly composite to assess the metal deportments of each metals in each streams. Froth Handling: An early diagnostic of the Pyra Metallurgist Plant done via simple visual assessment by walking and talking and observing the flotation circuit bring the conclusion that the current lower Cu and Zn recoveries are caused, partly at least, by material handling issues caused by scaling/plugging of the concentrate launders and pipe system. In the Cu and Pb circuit particularly, only 30% of the flotation cells are "pulling their weights" and producing concentrate/mass recoveries. This appears to be a phenomenon forced by the operators not wanting overflow their launders with excess froth. Froth Levels: The pulp levels on all cells appears high (shallow froth thicknesses). This unstable froth which constantly produces a low density "run-away" or wet, watery undrained rougher concentrate rich in entrained "other materials" and containing most of the frother (added to flotation feed) itself. **
causes
Operating at shallow froth bed causes much of the gangue rich "water" or slurry to poor over the lips of the flotation cell. This creates a rougher concentrate that is hard to clean for it was produced un-selectively, is of low grade and has a high concentration in chemical frother. Here to the right, is a graph displaying the how froth depth affects water recovery to the concentrates. The more is recovered with shallow froth concentrate grade drops as more water is recovered in it. Overall poor flotation will be seen shallow froth conditions occur in all roughing, scavenging and cleaning stages. Such shallow froth depth causes flooding, not floating of metals.
while selectivity
** After interviewing maintenance and operating staff and reviewing emails, it was confirmed that in early 2012 saw an plant-wide initiative of increasing pulp levels on most flotation cells.
Reagent Usage: Tracking data from January 2011 to date reveals a general drop in metallurgical results synchronized with a drop/change in all key flotation reagents at the Pyra Metallurgist concentrator. Below is are graphs tracking/displaying the monthly usage of only 4 of the most important reagents used in flotation. It is important to note how reagent usage became relatively erratic in 2012 and indicate the results unpredictability.***
MBS pH modifier: The excessive reduction of MBS will allow the flotation of Fe & Zn in the Cu and Pb Circuit. -23% MBS
Collectors: Adjusting collectors will cause a increase/decrease in selectivity (recovery) for all metals. -9% Xanthate / -28% Aerofloat / +30% Aerophine
CuSO4: Affects Zn recovery and selectivity of Zn over Fe thereby allowing Fe to float in the Zn Conc. -27% CuSO4
MIBC Frother: Increasing MIBC typically increases recoveries and reduces concentrate grades. +21% MIBC
ZnSO4: Is best known to prevent activation of Zn in Cu or Pb circuit. -100% ZnSO4 A full reagent usage review should be done for evaluating usage and purpose of each reagent at each addition point. As a minimum Pyra Metallurgist should likely return to the usage it was successful with in the past.
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Case is point for re-evaluation: Here below is a passage from the supplier's textbook on the usage of Aerophine 3418A
Currently Pyra Metallurgist does not use an auxiliary collector for its Pb flotation which will typically cause recovery issues. It also uses MIBC in its Pb cleaners which will typically cause selectivity issues. MIBC frother counts 9 addition points across all circuits. This often indicate and under-collected flotation system in which a froth bed is difficult to form/maintain where metal recovery is being chased chemically instead of mechanically using air. Choosing and balancing froth flotation reagents must consider the plant configuration, the ore mineralogy, gangue associations, the froth persistence, froth liquid drainage, frother/collector interaction, pH, kinetics and all of their possible downstream effects. *** The correct froth bed leads to a better grade of recovered metal, improves recovery kinetics and gives operators better control of the flotation circuit. First Things First:
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It necessary for Pyra Metallurgist to insure the basic mechanics of its concentrator are in place and properly functioning to allow metallurgy to happen. Insure froth from each/all flotation cells can be easily carried/flow to the next stage. Rougher concentrates needs to flow and be pumped to the cleaner stages with ease. Each rougher, each scavenger and cleaner cell must float "some metal". No cell should be left idle. This will require re-piping of most launders directly to its destination pump box where-ever possible. Start with the Cu Cleaners, Cu Roughers + Zn Cleaners, Zn Roughers, then Pb Cleaners, Pb Roughers
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Insure each streams assigned to regrind is fully reground for full mineral liberation. Regrind all Zn Rougher conc and stop the intermittent partial by-passed of this stream. Send the Zn 2nd cleaner tails to the Zn 1st cleaner feed directly. This will reduce the load on the Zn regrind cyclone overflow line and might solve the problem of intermittent need for by-pass. Failing this, either install the pump on the Zn regrind cyclone overflow or re-pipe the cyclone overflow into a large pipe to handle the full stream. Properly regrinding/liberating Zn from diluents like Fe will reduce lime consumption is the cleaners and while improving grade. A reduced lime usage will result in reduced pipe de-scaling agent.
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Eliminate Zn conc contamination by Pb circuit spills. Currently the Pb Rougher conc pump is located on the bottom floor in the heart of your Zinc circuit. That pump and all Pb streams should be located and insulated to within the Zinc circuit.
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Cu and Pb rougher cells have an unstable float and unselectively float coarse particles. Change the OutoTec FloatForce agitator back to the standard stator/rotors you have had for years before.
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The OSA is un-reliable Put a task force on the Peri OSA calibration. At time of writing, out of 13 streams, only 4 or 5 streams currently bring value to the operation. Calibrate the OSA to read all streams naturally VS calibrating the plant to be read by the OSA. Insure all reject stream go to the current place.
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Improve Regrind Efficiency The Cu and Zn regrinds are currently operated at 20% and 30% solids. To minimize grinding media wear and the minimize metal loses to over-grinding and these should be at +50% solids. Smaller diameter apexes should be tested on the regrind cyclones to bring the apex density up. Currently 2.5" apexes are used while 1.25" are likely more appropriate.
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Reinstate more appropriate deeper froth bed levels on all flotation cells. Air pushes metal loaded bubbles upwards while water (dirty gangue loaded pulp) washes/drains down in the froth zone. Too deep a froth causes lose in recovery, to shallow a froth causes poor grade, much to low a froth causes drops in both grade and recovery.
More About Froth Beb Depth/Level and Frother Usage The froth bed level and the frother plays a critical role in the flotation of precious and base metals. Its main function is to stabilize the bubbles that transport the hydrophilic value minerals to the surface froth zone for exist of the cell over the lip. The correct froth bed leads to a better grade of recovered metal, improves recovery kinetics and gives operators better control of the flotation circuit. What is unique about frother is its ability to strengthen the mineral loaded bubbles while allowing them to coalesce without rupturing, hence forming the froth zone. Good frother and froth bed levels allow for sufficient froth liquid drainage so that entrained gangue minerals can be washed out and away. Correct froth usage provides good froth mobility to transport the mineral particles to the cell's lip and then into the launder to the next stage. The froth zone must not collapse before the value metals can be recovered, yet must be sufficiently transient to allow its bubbles to break down and re-form in the next stage of flotation. The perfect frother and frother addition will of the wanted minerals, but not so stable that it
cause the bubbles to be stable enough to carry the weight survives beyond the launder and pump box.
Choosing and balancing froth flotation reagents must consider the plant configuration, the ore mineralogy, gangue associations, the froth persistence, froth liquid drainage, frother/collector interaction, pH, kinetics and all of their possible downstream effects.
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Increase Cleaning Capacity
Re-commission the Cu column cells as a high grade producing unit before mechanical cleaner circuit. Pipe them according to the attached flowsheet. Pending performance review of this new flowsheet, prepare to re-commission the previously called Cu 3rd cleaner. A retention time tracer test would confirm necessary flotation time needed to maximize metallurgy.
Re-commission the Zn column cells as a high grade producing unit before each 1st cleaner. Pipe them according to the attached flowsheet. Pending performance review of this new flowsheet, prepare to re-commission the previously called Zn 3rd cleaner. A retention time tracer test would confirm necessary flotation time needed to maximize metallurgy.
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Pb Metallurgy
Do a 2 Lab scale test on 25kg of ore samples at the opposite ends of the Cu/Pb ratio spectrum (hi/low) to better understand the metallurgical performance of these materials and how to improve it (a quote for work is pending). Do a Mineralogical study (Diagnostic Metallurgical Assessment) on all you concentrates, feed and tails to confirm the regrind needs of you Pb conc and the optimization of all other streams. These results, will justify the capital expenditure for a small Pb Regrind and small Pb column cell according to this flowsheet.
Other Opportunities
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Zn Thickener Overflow The Zn thickeners are currently operated in series. Performance might be enhanced and Zn loses to tails reduced if the 2 thickeners were operated in parallel.
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Training It appears the mill operation personal very much wants training in the technicals of metallurgy. How to better relate theory and practice in the operating environment is an everyday need. Operators crave technical guidance and support.
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Air, Frother and Pulp Levels Air flow to each and all float cell (rotor shaft) should be routinely cleaned and never be allowed to restrict how much air can be added to its respective flotation cell. No Air, No Recovery! A higher pulp level is not an adequate nor equivalent replacement for lack of air on a cell. Frother is not an adequate nor equivalent replacement for lack of air on a cell. Although essential to "froth flotation" it is only a complement to air.
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Steel Loading Best practice is for the primary ball mill and all regrind mills to receive fresh grinding media daily. This would insure a more consistent grind size to flotation.
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Grind Targets Best practice would be for the assay lab to perform daily sieve analysis on flotation feed, Cu and Zn regrind cyclone overflows and well as Cu, Pb, Zn final concentrates. This data would give mill operators vital data for their work.
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Staffing It would be greatly valuable to have solid night-shift supervision compared to non currently. Pyra Metallurgist would greatly benefit from having a dedicated Flotation Operator for its Cu/Pb/Zn circuit.
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5 x 7 milling Operations Pyra Metallurgist was once a 4000 TPD operation. It now runs nearly half that rate with most of its equipment still in place. Economic studies and trials should be undertaken to run only 4 or 5 days per week while carefully evaluate the impacts on the underground operations such as backfill.
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