Grindability variations. Grindability variations are probably the most important factor to consider in setting design factors for flotation capacity (Weiss 1985). This is particularly so for plants with SAG mills in the grinding circuit ahead of flotation. Variations in grindability.can cause changes to plant feed rate, flotation circuit feed size or both. In most cases, grinding circuit throughput is the constraint on production and the flotation circuit must be able to handle what it gets. Plant throughput variability must therefore be accounted for in determining equipment volume and capacity. Of particular importance are the grindability values in the initial year or,two of operation as these are what will be experienced before any significant changes or additions to the plant can be made. Often the ore from open pit mines increases in hardness with depth, resulting in lower throughput and/or coarser grind as the operation progresses. Conversely, initial throughput rate can be significantly higher than the overall average. Design flotation volume must provide the required retention time for what will be fed to the plant in significant amounts. There will be extreme situations where retention will be reduced by excessive flow rates and where the amount of this ore does not justify the extra capital to install full retention capacity. The amount of variation that can be covered has to be waded off against the higher capital and operating cost of installing extra cell volume.
In some instances the flotation circuit capacity may be limited by the need to make a specific separation or meet a required product specification. This should not be a factor at the design stage, unless cell requirements get very excessive at high throughputs or there are constraints on total cell volume. The upstream grinding equipment can influence control over throughput rates. If the grinding circuit contains a fixed speed SAG mill, the mill must be fed at its full grinding capacity in order to protect the mill from grindouts. With a variable speed drive on a SAG mill, grinding rate can be limited if necessary to control extreme flow rates to the downstream process. For rod mill-ball mill or single stage ball mill circuits, feed rate can also be adjusted somewhat im cpcndcnlly of rinnability. In Ikk cases, the allowances for MANY uanatioak can be smaller than for fixed speed SAG circuits.
The possibility of a coarser grind as a result of harder ore or higher throughput has to be considered for its effect on the flotation circuit. Expected particle size affects slopes on launders as well as particle suspension in the cells. Particularly when the grinding circuit includes a large SAG mill and has a relatively coarse product to flotation, such as is common in porphyry copper circuits, it is often useful to include some form of safety screen ahead of the flotation feed distributor. This can prevent buildup of coarse particles in the cells that would otherwise result from cyclone blockages in the grinding circuit. An alternate approach is positive and reliable control of cyclones to prevent blockage of the underflows that would send coarse particles to the overflow.
Feed grades and variations. Variations in feed grades will have been considered at the test stage when selecting the target retention times. While feed grades won't necessarily change rougher-scavenger cell volumes, they do impact on concentrate handling considerations. If extreme variations in grade are expected, concentrate launder capacity must be adequate to handle the extreme flow, particularly in the first few cells. At very high grades froth carrying capacity may be exceeded, which indicates that more cell froth area is required. Extra launders or launder lip length may be required to reduce froth travel distance to the launder. This can be determined from the specific scaled up flotation rate to estimate expected concentrate volumes at each cell. Launder or piping size must allow for the maximum expected concentrate flow.
The expected range of feed grades has a significant effect on total cell volume at the cleaning stages. While cleaner volume may not allow for the absolute maximum feed grade expected, it has to be able to handle what is expected to occur for a significant fraction of the plant feed, especially in the first year or two of operation. Pump capacity is also determined by the maximum concentrate production rate allowed for in the design.
Concentrate grades. When considering how much of the variation in grindability and feed grade to allow for in setting maximum design throughput rates, the specific application has to be kept in sight. What is the impact of reduced concentrate grade or recovery if retention time is reduced? How acceptable is it to make a less than optimum product in consideration of product sales? How is separation of multiple products affected if cleaning capacity is exceeded? What effect does excessive flotation time have on product grade or on premature recovery of next-stage product in sequential flotation of multiple product ores? Many of the answers to these questions relate to flotation kinetics and have to be evaluated on an economic basis to see what the best tradeoffs are on a case-by-case basis.
Air Holdup. An allowance for the volume of the cell taken up by air in the pulp has to be added to the cell volume. For mechanical cells this is typically 15% so the design factor will be divided by 0.85. For tank-type cells the holdup is likely to be 10% or less. If cell requirements have been produced by the use of a simulator, determine how much, if any, allowance for air holdup has been included in the simulation.
Rheology. Presence in the ore of certain minerals that increase pulp viscosity may affect cell volume requirements. As an example, clay minerals can have a significant impact on suitable pulp density to be able to make a clean separation. This should have been evaluated at the ore testing stage.
If there is a significant presence of such interfering material in the ore, it may be useful to treat the slimes fraction separately from the coarser fraction. In this case, the slimes are treated at a lower density to achieve lower viscosity in the pulp and reduce entrainment of gangue slimes in the concentrate. The lower density may also reduce overall reagent consumption. At the lower density cell volume has to be increased to provide the required retention time. The coarser fraction on the other hand can be treated at a higher density, reducing the total cell volume for that fraction.
The coarse and fine fractions might also have different flotation rates, which would change I get retention times for each stream. Depending on the particular cell manufacturer, the impeller/ ator configuration may be different for coarse and fine materials.
Recycle streams. Where recycle streams are required, they must be included in the total,flow for calculation of cell volume. Routing of recycle flows will have been established at the testing stage of flowshect development. Return point(s) should provide an escape route for impurities or middlings that could otherwise keep building up in the circuit, until overloading of the system results in loss of clean product along with the unwanted or untreatable fraction. Recycle streams often have a different density than the main stream and the dilution (usual) must be taken into account when calculating total slurry flow volume.
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