Physical Aspects of Flotation

Contents Particle shape & texture Bubble formation Bubble/particle interactions Mixing Kinetics Effects of bubble size and particle size Froth drainage

Particle Shape & Texture (1) Particles are formed by breakage The breakage is random so that the resulting particles are of complex shape and structure Particles after crushing are angular & elongated Broken shapes have advantage for flotation - they have sharp edges and points – thus more likely to penetrate a liquid film around a bubble

Particle Shape & Texture (2) The particle texture is classified based on mineral association and liberation This classification useful for processing as nature of locked minerals will affect flotation behaviour But number of particle types increases quite rapidly with number of phases 4 phases – 11 particle types 5 phases – 16 particle types Particle structure important for 2 main reasons: It determines whether or not the particle will float After separation it determines the contribution of impurities to concentrate or losses of values to tailing

Particle Shape & Texture (3) Particle structure important for 2 main reasons: It determines whether or not the particle will float After separation it determines the contribution of impurities to concentrate or losses of values to tailing Must have hydrophobic mineral on surface for flotation How much needed depends on many factors – particle size, reagent conditions, cell agitation, etc. There is evidence that quite a low surface exposure can induce flotation in some situations Increasing the extent of the surface exposure increases the effectiveness of the flotation

Particle Shape & Texture (4)

Bubble Formation (1) The flotation process requires a continuous stream of small bubbles They must be stable enough to give plenty of opportunity for particle bubble collisions The froth phase which overflows the cell lip should then break down for easy transport Sizes of a few mm have proved the best in general. Smaller sizes have advantages for very small particles.

Bubble Formation (2) Bubbles can be generated in liquids by forcing gas through some orifice.  Single bubbles in water vary in shape, depending on size Small spheres (1 – 2mm) Oblate spheroids (6 – 40mm) Large spherical caps (>88mm) They rise in the water, expand as hydrostatic head falls and break immediately at the surface.

Bubble Formation (3) A stream of bubbles in water are unstable and rapidly coalesce, grow, rise to the surface where they break Lowering the surface tension means smaller bubbles Frothers lower surface tension and also give higher viscosity to the liquid films – this increases the stability When the rate of generation of bubbles exceeds the rate of breakage at the surface a froth layer forms

Bubble Formation (4) Smaller bubbles are also generated by shearing larger bubbles to beak them up. This is one aspect of the rotor mixing mechanisms in a flotation cell. Induced air systems suck air into the rotor vortex and break up the bubbles in the high shear rotor region. This air supply is less controllable than a forced air system

Bubble Formation (5) The use of frothers is essential for the operation of a flotation system When frothers is added to a cell bubbles of about 2mm are generated and stable froths are formed In the early cells where particle loading is high froths are stable. In later cells, as reagents deplete and particle loading falls, the bubble size increases through coalescence and the stability of the froths falls significantly.

Anglo Platinum Bubble Sizer (APBS) Collection tube 1st ball valve 2nd ball valve Battery Camera Light Collection chamber

Images of Air Bubbles

Bubble-Particle Interactions (1) For particle capture by a bubble we need Collision between a particle & a bubble The fluid film between them must thin down Then rupture of film to give a gas-solid interface  In a conventional cell, collection occurs by different mechanisms in different parts of the cell Near the impellor collisions are by a shearing mechanism. Velocity gradients high. Bubbles will be at center of opposing streams of liquid bearing particles In quieter regions of cell the relative motion between particles and bubbles is due to gravity with bubbles rising at a terminal velocity appropriate to their size

Bubble-Particle Interactions (2)

Induction Time Induction Time:The time required to penetrate the water film around the bubble. Induction time decreases with increasing particle hydrophobicity If induction time is higher than the contact time, particle will not attach

Probability of Collision Flow stream lines deflect the particles and reduce the probability of collision Probability of collision is the actual number of particle collide relative to the number in the path of the bubble. The probability of collision is small (one in hundred) Large number of bubbles is required to increase probability of collision

Attachment Hydrophobic particle breaks the water film and attach Hydrophilic particles slides

Contact Time Contact time is the time takes a particle to slide around a bubble. Contact time of a particle depends on its weight The speed of the larger particle is higher and does not have enough time to attach

Bubble-Particle Interactions (3) Theoretical attempts to describe capture Calculate path of particle as it passes a bubble Particles within a distance R of the centre line of the bubble will touch the bubble as it passes The efficiency of capture E is then the ratio of the area of the collision tube to the bubble cross sectional area E = (πR2)/(πrb2)   or E = (dp/db)N where N is about 2. Thus capture rate increases strongly with increasing particle size or with decreasing bubble size.

Bubble-Particle Interactions (4)

Bubble-Particle Interactions (5) This is a highly idealised situation. The problem of calculating from first principles the rate of collision of particles and bubbles in a turbulent stirred vessel is still too difficult to be solved. The increase in capture as particle size increases is counteracted at large sizes by detachment mechanisms. Particles are too large to be firmly held by the bubbles and can be broken away by the agitation To break attachment, work must be done to overcome the surface tension forces. This work is supplied by the turbulent eddies in the agitated vessel.

Mixing (1) Mixing in a cell is designed to do many things: Suspend the mineral particles Distribute the flotation chemicals evenly Provide high shear conditions near the air sparge so that small bubbles are generated Promote particle bubble contacts Maintain a quieter region at top of cell where separation can occur between froth and pulp Mixing efficiency (cost) is also important

Mixing (2) The time particles spend in the cell is also very important - there must be time for capture In a cell of volume V (m3) with an exit flow of v (m3/s), the mean residence time  in the cell is given by  = V/v (s). On average particle will spend time  in the cell The most efficient cell would have all particles spending the same time  in the cell, eg like a pipe In practice there will be a spread of times defined by the residence time distribution.

Mixing (3) A single cell has a wide range of residence times Thus flotation cells are connected together in banks to get satisfactory overall efficiency Countercurrent operation is the most efficient giving maximum oportunity for purifying concentrate and exhausting tailings Achieved by arranging cells in banks, then banks in countercurrent arrangements The flotation column, where bubbles rise from the bottom to the top, and particles fall from near the top, gives maximum oportunities for capture

Kinetics (1) Contact angle and hydrophobicity are equilibrium concepts But industial flotation involves rate processes - the faster the process the cheaper the operation It is commonly accepted that flotation can be simply represented by first order kinetics:   dc/dt = -kt where c = floatable material left in batch cell t = time k = flotation rate constant

Kinetics (2) Then ∫(dc/c) = -∫kdt and c = coe-kt where co is the initial concentration of flotable material in a batch cell   The problem is that k is only constant for that set of conditions. It is affected by many factors such as the chemical conditions, the particle size, the bubble size and air rate, the agitation, etc, etc.

Froth Drainage (1) Bubbles loaded with mineral rise in pulp, enter froth layer and rise through the froth to the top where they finally overflow the cell lip Bubbles entering the base of the froth will entrain liquid containing hydrophilic gangue material To produce a high concentrate grade this entrained material must drain from froth before discharge The concentrate purity increases with height in the froth. It is important to have effective drainage of the froth to achieve good enrichment.

Froth Drainage (2) Particles in froth affect froth stability and drainage Particle size and hydrophobicity are both important Hydrophilic particles may act as a buffer between opposing air-liquid interfaces thus reducing drainage Rough hydrophobic particles and smoothe spheres of low contact angle can form stable contact lines with the surfaces of the films, and thus stabilise the films Orthorhombic particles like galena, will cause rupture as soon as they bridge both film surfaces Smoothe spheres at high contact angle, will destabilise a film when both surfaces are pierced

Froth Drainage (3)

Froth Drainage (4) Possible methods of froth stabilisation:   Froth heavily mineralised with fine particles. The films are prevented from collapsing by the high concentrations of hydrophobic particles hydrophillic particles Froth film with a large hydrophobic particle of roughly spherical shape, with a low contact angle.