SURFACE CHEMISTRY BASIS OF FLOTATION

INTERFACES IN FLOTATION SYSTEM The surface energy of the interfaces mineral/water, mineral/air and water/air is important for; Thermodynamic analysis of the attachment between mineral particles and air bubbles The action of flotation reagents

The elemental particles existing in the surface possess a greater amount of potential energy than those inside the bulk phase. This excess potential energy is called surface energy (ergs/cm2). Surface tension is an equivalent concept applied to liquids and possesses dimension of force over length (dynes/cm).

The Change in the Density of Free Energy in the Surface Layer Phase 1 Phase 2 f1 f2 h1 h2 Specific surface energy Energy Density Interface Boundry

Surface Energy (ergs/cm2) In principle there is a definite correlation between the solubility of minerals and their surface energies. Surface Energy (ergs/cm2) Gypsum 39 (highest solubility) Quartz 780 Calcite 78 Topaz 1080 Flourite 146 Corundum 1550 (lowest solubility) Apatite 186 Feldspar 358

Surface Tension of a Liquid ΔX g RX = Wrev =  (2LX) from which it follows that

Surface Tension of Solutions Surface tension of water is 72.7 dynes/cm. Reagents that markedly lower the surface tension of water (fatty acids and alcohols) contain a polar hydrophilic group and a non-polar hydrophobic group. polar Non-polar Heteropolar reagents adsorbed preferentially at the interface and presenting the non-polar end to the gas phase, lowers the surface tension. The opposite effect is observed when the reagent is an ionic salt; the surface tension usually increases over the value of pure water.

Mineral surfaces (1) Flotation requires the aggregation of mineral particles and bubbles in an aqueous medium. This attachment is governed by whether or not the mineral surface is wetted by water. if wetted then hydrophillic if not wetted then hydrophobic The degree of wetting is measured by the contact angle .

Mineral Surfaces (2) LG LIQUID GAS  SL SG SOLID

Mineral surfaces (3) If complete wetting then contact angle  = 0 deg If no wetting then contact angle  = 180 deg The angle  (between 0 and 180 deg) gives a measure of the hydrophobicity of the surface Teflon exhibits the max measured contact angle of 108 deg Most minerals are hydrophillic. They must be specially treated to make them hydrophobic. A few minerals – coal, graphite, sulphur, molybdenite, talc are naturally hydrophobic.  

Mineral surfaces (4) The three-phase equilibrium at the gas-solid-liquid contact is given by Young’s equation: SG = SL + LGcos where SG, SL & LG are surface tensions of solid/gas, solid/liquid and liquid/gas respectively  

Mineral Surfaces (5) G1= lg+ sl G2= sg gas water gas solid solid 1) Before contact 2) After contact G1= lg+ sl G2= sg G= G2-G1 The free energy change per unit area corresponding to the attachment process (displacement of water by air) is given by Dupre’s equation: G = SG - ( SL + LG) For a gas bubble to attach to a solid surface; G < 0 for θ>0°  

Mineral Surfaces (6) Then expressing this in terms of contact angle   G = LG(cos - 1) Hence the attachment process will be spontaneous for all finite contact angles, ie lower energy For a gas bubble to attach to a solid surface; G < 0 for θ>0°

Mineral Surfaces (7) The energy involved in the adhesion – or otherwise - of water to the surface has been shown to have three components:   Ionization energy – charge effects (can be large) Hydrogen bonding – water system (can be large) Dispersion forces – induced dipole attraction (generally minor)

Surface Charge (1) Charge separation between the solid and the liquid can occur at the surface. The solid surface can become charged either positively or negatively. Charges can be generated in several ways: Specific chemical interactions Preferential dissolution of ions Lattice substitution

Surface Charge (2) Specific chemical interactions – a chemical reaction occurs. When this occurs with some solute the process is called chemisorption. A common example on oxides and silicates is the formation of surface acid groups that respond to changes in pH.

Silica surface

Surface Charge (3) As pH is changed the surface charge can be changed from positive (in strong acid) to negative as the pH is dropped. Hydrogen ion, in this case, is the potential determining ion (pdi), because the surface charge is determined by its activity in the bulk phase. In between the surface is uncharged. This condition is termed the point-of-zero-charge of charge (pzc) or isoelectric point (iep) For quartz this value is pH 1.8

Table of pzc of oxides Oxide Type pzc(pH) Examples pzc (pH) M2O > 11.0 Ag2O, 11.2 MO 8.5 - 12.5 MgO, 12.4 NiO, 10.4 CuO, 9.5 HgO, 7.3 M2O3 6.5 - 10.4 Al2O3, 9.1 Fe2O3, 8.5 Cr2O3, 7.0 MO2 0 - 7.5 UO2, 6.0 SnO2, 4.7 TiO2, 4.7 SiO2, 1.8 M2O5, MO3 > 0.5 WO3, 0.3

Surface Charge (4) Preferential dissolution of ions For simple univalent ionic solids there must be equal numbers of positive and negative ions on a cleavage plane. Depending on the relative hydration energies of the ions, one may hydrate to a greater extent – ie preferentially dissolve. This process will leave an excess of the opposite ion on the surface and a surface charge is established.

Surface Charge (5) AgI  Ag+ + I-   AgI  Ag+ + I- With excess iodide ions the particles are negatively charged and with excess silver ions the surface is positively charged. The silver and iodide ions are the potential-determining ions.

Surface Charge (6)  If NaI is added to the water saturated with AgI then the iodide ions determine the surface charge, but the Na ions have no effect. But if AgNO3 is added then Ag+ will be the potential determining ion, not nitrate. Finally, if KCl is added it will not affect the surface charge, but will affect ionic strength. They are called indifferent ions.

Surface Charge (7) Lattice substitution Atoms in the surface lattice can substitute giving rise to a surface potential. The replacement of aluminium for silicon in clays, for example, is responsible for the differences in the electrical properties of the surfaces and the edges of these mineral plates. When Al3+ replaces Si4+ a negative charge forms in the clay lattice and K+ enters between the layers to neutralise the excess charge. Such defect structures can affect behaviour.

Electrical Double Layer (1) Most solids possess a surface charge in aqueous solutions. With a surface charge the solid surface acquires an electrical potential with respect to the solution. The surface charge is balanced by an equal and oposite charge distributed in the aqueous phase. These charges – the surface charge and the solution charge are referred to as the electrical double layer.

Representation of double layer

Electrical Double Layer (2) Potential determining ions may be ions from the solid lattice; hydrogen or hydroxyl ions; collector ions forming insoluble salts on the surface; complexing ions. Counter ions have no special affinity with the surface. They are adsorbed by electrical attraction, eg Cl- ions or collector ions at low concentration.

Electrical Double Layer (3) Overall neutrality thus charge density in diffuse layer d must equal charge density on surface s   s = -d  The potential difference between surface and bulk solution is the double layer potential 0. Difficult to measure 0 but electrokinetic effects permit another potential – termed the zeta potential  - to be measured. This is the potential difference between the shear plane and the bulk solution.

Electrical Double Layer (4) Zeta potential is measured in several ways Movement of the particles Electrophoresis Sedimentation potential Movement of the fluid Streaming potential Electroosmosis

Surface Charge Electrophoresis - + - + - + + - Negative electrode Pozitive electrode - + - + + - Direction gives the sign; Velocity gives magnitude

Electrical Double Layer (5) · Electrophoresis: Determine the migration velocity of the particles when placed in a potential gradient. z = 4  (/D) (V/E) x 9 x 104, where  = zeta potential, volts m = solution viscosity, poise D = dielectric constant V = particle velocity, cm/sec E = potential gradient, volt/cm

Electrical Double Layer (6) · Streaming potential: Potential generated when bulk solution is forced through packed bed of particles.    = 84.85 x 107 ( Es) / (D p) , where  = zeta potential, volts m  = solution viscosity, poise D = dielectric constant  = conductivity, mho / cm Es = streaming potential, volt p = pressure drop, cm Hg

Effects of Surface Charge (1) If particles very small surface charges lead to the ‘stabilization’ of the suspension. Stabilization is the reverse of aggregation - all the particles of a particular mineral will have similar charges which will repell one another. If surface charge is reduced to the pzc then there is no electrical repulsion and particles can collide. At very close distances they attract one another due to dispersion forces (or Van der Waals forces) and aggregates or flocs are formed. Such effects are only important for very fine particles – below about 5 microns.

Effects of Surface Charge (2) Small particle effects particularly important for sedimentation and clarification , but they also have some importance with fine particle flotation:   ·     For flotation of fine particles – which is generally difficult – it can help to aggregate the particles to larger sizes before flotation ·     For supression of gangue minerals it is an advantage to have well stabilised particles so that they do not stick to floating particles but instead drain easily from the froth.

Effects of Surface Charge (3) But the most important effect of surface charge is to provide a basis for strong Coulombic attractions with polar flotation reagents and ionic species in the solution. The charges form the basis for hydrogen bonding and ionic adsorption at the mineral surface.   Adsorption mechanisms correlate well with zeta potential. Physical adsorption affects magnitude of zeta potential. A change in sign of the zeta potential with a significant concentration of adsorbate indicates chemisorption.

Adsorption Mechanism and Zeta-Potential No reagent Zeta potential (mV) pH Physisorption Chemisorption

Zeta-potential and Flotation of Geothite (FeO(OH)

Effects of Surface Charge (4) Some other complicating factors: The influence of other inorganic ions in solution can be very complex - either activating or depressing the flotation depending on pH or Eh. Interactions between hydrocarbon chains. They tend to clump together (hydrophobic) as the concentration increases. These clumps - ‘micelles’ or ‘hemi-micelles’ - form in solution or on the surface. Micellisation takes reagent from solution and lowers the efficiency of the collectors. Influence of neutral organic molecules - can co-adsorbe with the chains of ionic collectors and improve the effectiveness of the original collector.