Question 4 of 7: Column Flotation

Question 4: Column Flotation (20%)

Question text not reproduced: the examination questions are © Engineers and Geoscientists BC. Open the official past paper (linked at the top of this page) to read the question, then follow the worked solution below.

a) Design differences from conventional flotation machines

A flotation column is a tall (typically 9–14 m), unagitated vessel in which slurry is fed part-way up the column and flows counter-currently against a rising swarm of fine air bubbles generated at spargers near the base – there is no mechanical impeller, and particle–bubble contact happens purely through this counter-current motion in a relatively quiescent collection zone. A deep froth zone at the top receives a continuous downward flow of wash water added at or near the froth surface. A conventional (mechanical) cell, by contrast, is a shallow, agitated tank in which an impeller both suspends solids and shears/disperses coarse air bubbles, with a comparatively shallow, unwashed froth. The column's principal design features are therefore: no moving parts in the pulp zone, a long collection zone (giving bubbles and particles far more contact time and multiple opportunities to attach), and engineered wash-water addition specifically to clean the froth as it rises.

b) Advantages and disadvantages

Advantages: the combination of a long quiescent collection zone and washed froth gives markedly better selectivity for fine particles (where mechanical cells lose recovery to poor bubble–particle contact) and much lower gangue entrainment, so a single column cleaning stage can often replace several conventional cleaner/re-cleaner stages while raising concentrate grade; no impeller means lower power draw, no wear parts to maintain, and a smaller plant footprint per unit of cleaning duty. Disadvantages: the same quiescent design that helps fines handles coarse or dense particles poorly (they can settle and "sand up" the base of the column); level and, especially, bias control require more instrumentation and more disciplined operation than a mechanical cell; and spargers are prone to plugging or scaling, needing routine cleaning/replacement. Columns are consequently a poor choice for a coarse, fast-floating rougher duty and a very good choice for a fine, high-value cleaning duty.

c) Typical flowsheet placement

Columns are used almost exclusively in cleaning duty – most often as a final cleaner or re-cleaner stage downstream of conventional rougher/scavenger cells, where their low-entrainment, high-selectivity froth produces a final shippable concentrate in fewer stages than an all-mechanical cleaner train. They also appear as a first-cleaner stage feeding a conventional cleaner-scavenger circuit that recovers value from the column's own tailings (see part e).

d) Bias, gas hold-up and superficial velocity

Bias is the net downward flow of liquid through the froth – wash water added at the top minus the water that reports with the concentrate – and it is kept positive (net downward) by design, because a positive bias physically washes entrained gangue back down into the pulp zone rather than letting it ride out with the concentrate. Gas hold-up is the fraction of the column's internal volume occupied by air bubbles at any instant; it governs both bubble surface area available for particle attachment and the residence time of both phases. Superficial velocity is a volumetric flow rate divided by the column's empty cross-sectional area (e.g. J_g for gas, J_l for slurry/tailings) – it standardises flow rates independent of column diameter and is the primary design/scale-up variable for matching bubble-swarm hydrodynamics between a test column and a full-size unit.

e) Two-stage column–conventional cell circuit

The conventional cleaner-scavenger cell's function in this arrangement is to recover the residual value still leaving in the column tailings before that material is finally rejected – mechanical cells are well suited to this scavenging duty because they can be sized/agitated aggressively to chase the last increment of recovery from a lean stream, a duty the quiescent column itself is not designed for. The scavenger's own concentrate is recycled back to the head of the column train rather than shipped directly, so that any coarse or poorly-selective material it recovers gets a further pass through the column's fine-selectivity cleaning action.

f) Mechanisms for introducing air

Common column air-introduction mechanisms include: porous/sintered spargers (ceramic or polymer, mounted internally near the column base, generating very fine bubbles but prone to plugging/scaling); perforated rubber membrane spargers (external, self-cleaning by membrane flexing, widely used in North American practice); static in-line mixers that entrain and shear air into the slurry stream before it enters the column; and external recycle-slurry eductors/venturi spargers, which use a side-stream of pressurised tailings or process water to draw in and disperse air, avoiding internal moving parts or fine-mesh elements that could block.

g) Typical froth depth

Column froth depths are substantially deeper than a conventional cell's – typically about 0.5–1.5 m (commonly around 1 m), compared with roughly 0.1–0.3 m in a mechanical cell. The much deeper froth gives wash water time to percolate down through it and displace entrained gangue before the froth overflows, which is the mechanism that gives columns their characteristic low-entrainment, high-grade concentrate.