1. COLUMN FLOTATION CELLS

2. Key Features-1 There is no mechanical agitation/shear
The cell is relatively tall and narrow
Gas bubbles are generated by sparing
Froths are deeper, and wash water is applied to the surface of the froth
Industrial columns are 6-14 m in height and m diameter
There are both rectangular and circular columns.
Wash water is commonly added via pans, perforated with mm diameter holes, located cm above the froth

3. Key Features-2 Wash Water
The key objective of wash water is to minimize recovery by entrainment
Wash water replaces feed water that would have otherwise reported to the concentrate
Decrease rate of bubble coalescence
Hence, a column froth is usually very stable even when deep
An industrial column froth is typically about m deep
The wash water stream/droplets must be large enough to penetrate the top layer of the froth
If it is too light, the water may bypass the froth directly into the concentrate
For heavy froth, wash water can be given via perforated pipes immersed below the top of the froth

4. Key Features-3 Superficial Gas Velocity (Jg, cm/s) Bias (JB, cm/s)
Jg is calculated by dividing the total flowrate by the column cross sectional area
The range of Jg in industrial columns is between 1 and 2 cm/s (at the top of the column)
Jg is about half at the bottom of the column due to 1 atm static head.
Bias (JB, cm/s)
The difference between wash water flow rate and concentrate water flow rate
Wwash > Wconc Bias is positive, bias is negative when the reverse occurs
A common approach is to operate in the range of zero bias (or a bit lower or higher)

5. Key Features-4 Carrying Rate (Ca, tph/m2)
The concentrate solids flux is referred to as the carrying rate
Industrial columns operate between 1 and 3 tph/m2 depending on wash water addition and particle size
Smaller particles will result in lower carrying rates
The maximum carrying rate is referred as the Carrying Capacity (CM)
Gas Holdup and Bubble Size (, %)
The volume of the pulp zone occupied by gas bubbles
Gas holdup is a function of both gas rate and bubble size
Average bubble size can be inferred from the gas rate and gas holdup
Smaller gas bubbles are generally preferred
However, in primary cleaning with high kinetics, very small gas bubbles negatively affect the froth mobility and undesirable

6. Key Features-4 Bubble Generation Jet Sparging (SlamJet)
The most common method is jet sparging of air through orifices
Air pressure is between psig
High shear exists between gas-slurry interface results in generation of air bubbles
Bubble diameter is affected by jet velocity; the higher the jet velocity the smaller the bubbles
A large column uses spargers
Microcel Technology (Cavitation Tube)
Portion of underflow is pumped through static mixers back into the pulp
Air is injected at the static mixer and the resultant high shear generates very small gas bubbles
Bubble generation is external to the column
A typical column will have several static mixers fed by a common pump

7. Cavitation Tube
Slam Jet

8. Pilot Testing and Scale-up-1
The ideal approach for design and sizing of columns is via pilot testing
However, if;
The column application is for final cleaning
The head grade is low, and hence the amount of rougher concentrate is consequently low,
Kinetic data obtained from bench mechanical flotation cells can be employed
It is difficult to apply the bench data directly for column sizing
The selectivity obtained from bench data can be used to design the number of cleaning stages
Modelling tools should be used for the design

9. Pilot Testing and Scale-up-2
Apply first-order kinetic model to particle collection
Select froth zone recoveries appropriate for the size of the column and froth mobility
Particle retention time can be significantly different from that of water, especially for the coarse particles
Therefore, the retention time of the particles should be calculated and applied
Entrainment is calculated based on water recovery to the concentrate
For columns operating with a bias close to zero, the effect of entrainment is usually zero

10. Pilot Testing and Scale-up-3
Froth Recovery
A key factor in column design is the froth recovery
Rf controls the extent of re-circulation within the column
Rf of pilot column is higher than the industrial column, due to the stability imparted by the walls in a pilot column
Therefore, in sizing the large columns from pilot data the retention time will be longer for two reasons;
short-circuiting; fluid flow in large columns is closer to perfect mixing than a plug flow. Longer retention time is required for large column
Lower froth recovery; large columns will have lower froth recovery (i.e. higher internal circulating load), it is necessary to have a higher pulp zone recovery to attain the same overall recovery as in pilot column

11. Pilot Testing and Scale-up-4
Carrying Capacity (CM)
CM for a large column can be substantially lower than a pilot column (about 50%)
The value of operating bias is important for Ca.
10 cm Diameter Column (Cu Rgh Conc)

12. Pilot Testing and Scale-up-5
Bias (JB)
Operation of a column at zero or slightly positive bias maximize the concentrate grade
Further increase in bias results in minimal grade increase substantial recovery loss

13. Column Height
The height of the column is generally determined by the required retention time
Short circuiting and froth dropback should be taken into account
The plant layout is also important and the height of the column should fit to the plant height
If the feed grade is high and flotation kinetic is very fast, tall column is not suitable;
The froth become fully loaded with solids in short retention time
Any further retention time is ineffective as the bubbles in the pulp will be fully loaded before reaching the froth zone

14. Circuit Examples (Porphyry Copper Ores)

15. Circuit Examples (Copper & Zinc Ores)

16. Circuit Examples (Zinc Flotation, Agnico Eagle’s Laronde)

17. Circuit Examples (Zinc Flotation (Les Mines Selbaie)

18. Design Criteria and Column Sizing

19. Design Criteria and Column Sizing
Rougher-Scavenger Columns
Sizing is based on two criteria: residence time and carrying capacity (tph/m2)
Typical scale up factors are 3 to 5 times of bench scale test
Carrying capacity 1-3 tph/m2

20. Example
A cleaner circuit consisting of column cells is going to be designed based on the following data. Calculate the required cell volume and dimensions of the cells. Explain the criteria of your selection. The specific gravity of the ore is 4 g/cm3. The required flotation time from batch sale kinetic test is determined as 5 min for cell 1 and 7 min for cell2.
Feed 1
30 tph
30% w/w
Conc 1
10 tph
Feed 2
20 tph
20% w/w
Conc 2
5 tph
Tail
Cell 1
Cell 2

21. Solution Cell 1 (Option 3) Cell 1 Cell 1 (Option 2)
Height (-1 m froth)=7 m
Diameter= 2.5 m
Ca (tph/m2) = 2.04
Residence time (hr)=0.44
Cell 1 (Option 3)
Height (-1 m froth)=13 m
Diameter= 1.8 m
Ca (tph/m2) = 3.93
Residence time (hr)=0.43
Flotation time (hr)= 5 min*5/60=0.42 hr
Vol. Feed Rate = 77.5 m3/h
Effect. Cell Vol.=77.5*0.42=32.29 m3
Req. Cell Volume = 32.29/0.85=38 m3
Column Cell
Height (-1 m froth)=7 m
Diameter= 2.6 m
Ca (tph/m2) = 10 tph/5.3 m2 = 1.88