POWER INPUT IN CONVENTIONAL MECHANICAL FLOTATION CELLS

J.H. IMMINK

SYNOPSIS Power input in flotation slurries is analysed using the equation for power consumption of the flotation rotor. The effect of slurry density, rotor speed, diameter and the power number are discussed and their critical interaction with bubble surface area rate, superficial gas velocity, solid suspension and froth-slurry interface stability are described. It is concluded that a balance should be found in the quest for increased power to ensure optimum metallurgical performance.

Figure 1 : Stable flotation froth interface

INTRODUCTION The view during the early nineties was that electrical power would become a serious cost in large beneficiation plants. It was, therefore, a BATEMAN objective in flotation design to conserve power and the CSIRO of Australia was commissioned to jointly develop and patent the BATEMAN rotor and stator. The power conservation journey progressed steadily until it was dealt a blow in the year 2000 when the P9M programme of AMIRA showed that a correlation existed between power input in flotation slurries and recovery of certain flotation species. This lead to a drive by flotation machine customers and suppliers to increase electrical motor power consumption in Rougher flotation to 3kW/m3 and to 2kW/m3 in Cleaner flotation. It can be argued that electrical motor power criteria for flotation metallurgical performance is flawed because power consumption could be increased by lowering the efficiency of the drive train and it is, therefore, important to maintain a honest, positive approach to materialise improved performance.

ROTOR POWER (P) The power consumed by a rotating rotor in slurry can be represented by the following equation:

P
Where: Dp Np N Da

Dp x Np x N3 x Da5 / 216 000 Effective aerated volume

= = = =

Equation 1 : Power consumption by a flotation mechanism

This power P in kW is regarded as the real energy transferred into the slurry by the flotation mechanism and it is clear from the equation that the only differentiating factor between different rotor systems is the power number (Np).

A FEW POWER RELATED PROCESS CONSIDERATIONS 1. Suspension with stable froth slurry interface The suspension of solids in the slurry is directly dependent on the pumping rate through the mechanism in relation to the suction area (Figure 2).

Figure 2 : Diagrammatic representation of the suction area below a BATEMAN rotor after shut down.

The danger lies in increasing the rotor power through diameter enlargement to the point where the froth slurry interface is destabilised (Figure 3).

Figure 3 : The BATEMAN pumping objective Suspension and fluidisation of coarse material with stable froth-slurry interface

Pump rate (aerated) = N x Da3 x Relative pump number x Power ratio

Equation 2 : Pump rate for an aerated flotation mechanism

The challenge of balancing the pumping rate (Equation 2) with a stable froth-slurry interface is difficult when high specific gravity, relative large particles are present in the flotation feed as shown in Table 1. The deposition velocities that need to be overcome were also calculated for different size BATEMAN flotation machines.

Size

Deposition velocity m/s

Relative pumping rate m3/min

Suction diameter

Suction 400mm area lift speed m2 m/s

BQR BQR BQR BQR

1300 700 500 200

5.4 5.1 4.8 4.5

245.6 178.6 143.8 88.7

3.0 2.6 2.4 1.9

0.7 0.5 0.5 0.3

5.7 5.5 5.2 5.1

Table 1 : Comparison of pump rates for different BATEMAN flotation mechanisms for d80 = 350 micron particles with a SG of 4,8 t/m3

2. Superficial gas velocity (Jg) The correct superficial gas velocity need to be maintained in order to sustain a stable froth transfer and subsequent mass pull and recovery. Since the pumping rate through the rotor determines the air flow rate (cm3/s) it plays an important role in the optimisation of Jg. Froth manipulation by surface area (Figure 4) adjustment gives the operator the option to alter the superficial gas velocity as shown in Figure 5.

Figure 4 : Froth crowding to optimise superficial gas velocity

Figure 5 : Adjustment of the BATEMAN mechanism for froth manipulation

Jg = (gas flow rate per second)/(surface area)

Equation 3 : Superficial gas velocity (Jg)

Froth manipulation is feasible only when the froth-slurry interface is stable and the air flow rate is within acceptable ranges.

3. Bubble surface area rate (Sb) The flotation rate achieved is partly dependant on the Bubble Surface Area Rate (Bubble Area flux) as described in the P9L AMIRA research programme (Equation 4). K Where K = P = Rf = = P x Sb x Rf

Flotation rate constant Ore characteristics in terms of flotability Froth recovery factor

Equation 4 : Relationship between bubble area flux and the flotation rate

Sb is dependent on the air shear (Figure 6) at the rotor tips and is determined according 0,1 cm diameter bubbles and Jg.

Figure 6 : Bubble generation at rotor shear surfaces

FINDING THE BALANCE IN DESIGN 1. Rotor speed It is critical to run the rotor at the correct tip speed in spite of the temptation to run it faster to achieve higher power measurements due to the N3 relationship. On the operational side it should be kept in mind that cavitation at the rotor tips can induce excessive rotor wear

2. Pulp density The misconception was that the feed pulp density was outside the control of the flotation machine designer. This seems to be true only for traditional flotation where the feed particle size was 80% minus 75 micron at 28% feed solids. Modern flotation plants, however, often encounters two or more phased solids size distributions where only the finer fractions contain the valued minerals. In these applications, a density gradient exists within the flotation machine. The extent of the gradient differs by application and extensive sampling campaigns have shown variation of up to 15% from feed pulp density. The effect of apparent slurry density on power input is linear but extensive. 3. Rotor diameter The rotor diameter has a major effect on power input (Da5) and also influences critical flotation parameters as follows (Equation 5): Rotor tip speed = x Da x N where Da N = = Diameter of the rotor (m) Revolutions per second

Equation 5 : Tip speed

The major effects of tip speed and pump rate on wear, suspension, froth-slurry interface stability, superficial gas rate and bubble area flex have been discussed but can not be over emphasised. A perfect balance need to be reached since modifications is costly, time consuming and disastrous to metallurgical performance. In the BATEMAN flotation machine this balance is made easier by the hooded rotor that enforces a quiescent zone directly above the stator. 4. Power number Although the power number (Np) has a linear relationship to power input, it is a relative large number that can be changed by altering the direct environment at the tip and around the rotor. Therefore, it is a dimensionless number that is dependent on the geometry of the rotor and its surroundings including the stator and fluid environment (Figure 7).
Figure 7 : Hooded rotor-stator geometry (BATEMAN)

The Power number can be represented by the following equation:

Equation 6 : The power number for flotation mechanisms

Since the Power Number for the BATEMAN mechanism can vary between 5 and 15, it has a large effect on the power consumption of the mechanism. 5. Rotor The configuration of shear surfaces in the rotor has an influence on the bubble formation capability of the rotor and the power number. In the BATEMAN rotor this is optimised by the open topped rotor (Figure 8).

Figure 8 : The BATEMAN open topped rotor

6. Stator The gap between the stator blades and the rotor shear surfaces influences bubble size, wear and the power requirement. In the BATEMAN mechanism this is optimised by introducing baffling in critical pumping and bubble generation areas only. Although reduced baffling conserve power, it is a saving of power that is not utilised in the flotation process at all.

Figure 9 : Unnecessary baffling in flotation

CONCLUSION In the process engineers quest for designs to improve recoveries that pay the bills, it is of crucial importance not to forget partners and colleagues in operations and maintenance. It is, therefore, an equal challenge to: Improve ease of maintenance Reduce wear surfaces Increase wear resistance of materials Improve after sales and value add services

This represents more balances to be maintained that can only be achieved through co-operation between technology suppliers and mineral producers. Gone are the days when flotation machine electrical motors have been selected to only ensure start up under load!

J.H. Immink Pr.Eng B.Eng Metallurgy B.Eng (Hon) Extractive Metallurgy Member S.A.I.M.M. , ECSA