Table of Contents
The trend in both the metallic and non-metallic mining industry is toward processing increasingly higher tonnages of lower grade ores, thereby resulting in a growing interest in progressively larger volume individual machines in the flotation circuit. Because of the relatively high cost of large flotation machine development and evaluation, it is important to establish a rational basis by which the existing smaller flotation machine experience can be extrapolated to establish the configuration of the next larger family of flotation machines with a minimum of development modification in the field.
Hydrodynamic “Scale-Up” Considerations
In this type of flotation cell, air is induced (path A) into the fluid from the top of the cell due to the mechanical action of a rotating impeller which also serves to circulate the liquid, or pulp, from the bottom of the cell (Path B) .
The consequence of these dual flow paths (A and B) is the mixing of the two fluid phases and one solid phase in the highly agitated region (1) where physical contact between the air, now sheared into a large population of very fine bubbles, and the solid specie, to be floated, is accomplished.
Following separation, the flotable specie-air bubble matrix enters the skim (froth) region where concentration is accomplished by liquid drainage and froth collapse, and where concentrate removal from the flotation cell is finally performed.
In operation, rotation of the impeller generates a vortex in the pulp which extends from the interior midpoint of the standpipe, through the rotor, down to the top region of the draft tube. The resulting vacuum, generated in the core of this vortex, induces air through the air inlet duct into the interior of the rotor. Air flowing between the rotor vanes is mixed with pulp which is simultaneously circulated, by the rotor, from the bottom of the vessel through the draft tube to the rotor.
Mechanism Development Program
Specific mechanism hydrodynamic (i.e., water only) performance data, representing the parameters discussed in the previous section, was developed for all “mechanical induced air” (MIA) flotation machine sizes ranging from 500 cu ft (14.15 m³) down to 1 cu ft (0.028 m³). Analysis of these data showed that air transfer, liquid circulation, power, rotor diameter, rotor speed, and submergence can be related in the following functional manner:
Closed form empirical equations quantitatively representing these functional relationships served to guide the rotor size determination in scaling up to the 1000 cu ft (28.3 m³) flotation machine. However, while these empirical relations provide a degree of confidence in determining the rotor size required to accomplish a designated air transfer and liquid circulation objective, for the 1000 cu ft (28.3 m³) flotation cell, it does not quantify that objective.
Design analysis, using the empirical equations identified above, indicated that these hydro- dynamic objectives could be accomplished by a 30 inch (76.2 cm) diameter rotor operating at 185 rpm with 12 inch (30.5 cm) submergence.
Finally, the air transfer (430 cu ft/min) (12.2 m³/min) translated, for the anticipated 1000 cu ft (28.3 m³) cell configuration, to an escape velocity into the froth bed of 3.5 ft/min (1.07 m/min) which compares favorable with the 2.5 – 3.8 ft/min (0.071 – 1.16 m/min) range common in smaller “MIA” flotation machine practice.
Prototype Flotation Cell
The two principal geometric-considerations governing the configuration of the flotation cell are (1) the distance between the disperser and the cell wall and (2) the distance between the bottom of the rotor and the cell floor.
The disperser to cell wall distance influences the hydrodynamic parameters, already discussed, in the manner shown by the experimental data. A reduction in this distance results in a reduction in both air transfer and liquid circulation; however, too great a distance results in pulp short-circuiting through the cell and also an excessive floor area penalty. The typical range for the “disperser to cell wall distance” is between 1.4 and 1.8 times the rotor diameter, and this consideration governs cell, width.
The location of the rotor relative to the floor of the cell is influenced by the size consist of the solids in the pulp, being somewhat lower for the coarser particle application. The typical range for this parameter, in “MIA” flotation machines, is between 1 and 2 times the rotor diameter, and this consideration primarily influences cell depth.
The 164 (1000 cu ft) (28.3 m³) prototype flotation cell was hydrodynamically tested with water only, and the air transfer and liquid circulation performance levels established in the mechanism test program were confirmed.
The solids suspension test consisted of the progressive, addition of very coarse (+48M) solids to the prototype unit during operation, and noting the resulting change in mechanism air transfer and power draw. As expected, increased pulp density is accompanied by a reduction in air transfer and an increase in power draw.
In the fall of 1976, a joint research program between Kennecott Copper Corporation-Chino Mines Division and WEMCO was commissioned to provide operational data under conditions normally en-countered in an operating plant.
The beneficiation duty selected for this evaluation was in the middlings circuit, and the installation was accomplished by replacing a 72 ft (21.95 m) three-cell row of air agitated cells with the single 164 flotation machine 10 ft (3.05 m) in length.
A single machine test is, of course, not a proper basis for metallurgical evaluation because of the liklihood of (1) short circuiting and (2) crowding of fine fractions in the froth thereby precluding coarse particle recovery. It was generally felt, however, that it would be valuable to monitor the metallurgical “performance” of the single prototype machine even though the resulting measured performance is likely to be lower than a comparable machine row.
Solids concentration in the feed varied between 30% and 33% (wt), during the test program, with the representative (avg) particle size distribution.
Pulp residence time tests, conducted using a NaCl tracer, indicated significant short circuiting for this single machine test. The “effective residence time” of 7.92 minutes, as determined by pulp (electrical) conductance change following tracer addition, represented a 54% factor of the “calculated residence time” as determined by cell contained volume and pulp flow rate. This result is not atypical for isolated single machine residence time tracer tests, but does support the contention that metallurgical recovery is less than that expected for a flotation machine row operating at comparable pulp flow conditions.