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Theory

Selective Adhesion

Froth flotation depends on the selective adhesion of air bubbles to mineral surfaces in a mineral/water slurry. The air bubbles will attach to more hydrophobic particles. The attachment of the bubbles to the surface is determined by the interfacial energies between the solid, liquid, and gas phases. This is determined by the Young-Dupré Equation:

$\gamma _{lv}{\text{cos}}\theta =(\gamma _{sv}-\gamma _{sl})$

where:

• γ_lv is the surface energy of the liquid/vapor interface
• γ_sv is the surface energy of the solid/vapor interface
• γ_sl is the surface energy of the solid/liquid interface,
• θ is the contact angle, the angle formed at the junction between vapor, solid, and liquid phases.

Minerals targeted for separation may be chemically surface-modified with collectors so that they are more hydrophobic. Collectors are a type of surfactant that increase the natural hydrophobicity of the surface, increasing the separability of the hydrophobic and hydrophilic particles. Collectors either chemically bond via chemisorption to the mineral or adsorb onto the surface via physisorption.

Different types of collectors, or surfactants, used in froth flotation.

IMFs and Surface Forces in Bubble-Particle Interactions

Collision

The collision rates for fine particles (50 - 80 μm) can be accurately modeled, but there is no current theory that accurately models bubble-particle collision for particles as large as 300 μm, which are commonly used in flotation processes.

For fine particles, the Stokes equation underestimates collision probability while the potential equation based on Surface Charge overestimates collision probability so an intermediate equation is used.

It is important to know the collision rates in the system since this step precedes the adsorption where a three phase system is formed.

Adsorption (Attachment)

The effectiveness of a medium to adsorb to a particle is influenced by the relationship between the surfaces both materials. There are multiple factors that affect the efficiency of adsorption in chemical, thermodynamic, and physical domains. These factors can range from surface energy and polarity to the shape, size, and roughness of the particle. In froth flotation, adsorption is a strong consequence of surface energy, since the small particles have a high surface area to size ratio, resulting in higher energy surfaces to form attractions with adsorbates. The air bubbles must selectively adhere to the desired minerals to elevate them to the surface of the slurry while wetting the other minerals and leaving them in the aqueous slurry medium.

Particles that can be easily wetted by water are called hydrophilic, while particles that are not easily wetted by water are called hydrophobic. Hydrophobic particles have a tendency to form a separate phase in aqueous media. In froth flotation the effectiveness of an air bubble to adhere to a particle is based on how hydrophobic the particle is. Hydrophobic particles have an affinity to air bubbles, leading to adsorption. The bubble-particle combinations are elevated to the froth zone driven by buoyancy forces.

The attachment of the bubbles to the particles is determined by the interfacial energies of between the solid, liquid, and vapor phases, as modeled by the Young/Dupre Equation. The interfacial energies can be based on the natural structure of the materials, or the addition of chemical treatments can improve energy compatibility.

Collectors are the main additives used to improve particle surfaces. They function as surfactants to selectively isolate and aid adsorption between the particles of interest and bubbles rising through the slurry. Common collectors used in flotation are anionic sulfur ligands, which have a bifunctional structure with an ionic portion which shares attraction with metals, and a hydrophobic portion such as a long hydrocarbon tail. These collectors coat a particle’s surface with a monolayer of non-polar substance to aid separation from the aqueous phase by decreasing the adsorbed particle solubility in water. The adsorbed ligands can form micelles around the particles and form small-particle colloids improving stability and phase separation further.

Desorption (Detachment)

The adsorption of particles to bubbles is essential to separating the minerals from the slurry, but the minerals must be purified from the additives used in separation, such as the collectors, frothers, and modifiers. The product of the cleaning, or desorption process, is known as the cleaner concentrate. The detachment of a particle and bubble requires adsorption bond cleavage driven by shear forces. Depending on the flotation cell type, shear forces are applied by a variety of mechanical systems. Among the most common are impellers and mixers. Some systems combine the functionalities of these components by placing them at key locations where they can take part in multiple froth flotation mechanisms. Cleaning cells also take advantage of gravitational forces to improve separation efficiency.

Performance Calculations

Relevant Equations

A common quantity used to describe the collection efficiency of a froth flotation process is flotation recovery ($R$). This quantity incorporates the probabilities of collision and attachment of particles to gas flotation bubbles.
$R={\frac {N_{c}}{\left({\tfrac {\pi }{4}}\right)\left(d_{p}+d_{b}\right)^{2}Hc}}$
where:

• $N_{c}=PN_{c}^{i}$, which is the product of the probability of the particle being collected ($P$) and the number of possible particle collisions ($N_{c}^{i}$)
• $d_{p}$ is particle diameter
• $d_{b}$ is bubble diameter
• $H$ is a specified height within the flotation which the recovery was calculated
• $c$ is the particle concentration

The following, are several additional mathematical methods often used to evaluate the effectiveness of froth flotation processes. These equations are more simple than the calculation for flotation recovery, as they are based solely on the amounts of inputs and outputs of the processes.

For the following equations:

• $F$ is the weight percent of feed
• $C$ is the weight percent concentrate
• $T$ is the weight percent of tailings
• $c$,$t$, and $f$ are the Metallurgical assays of the concentrate, tailings, and feed, respectively

Ratio of feed weight to concentrate weight ${\tfrac {F}{C}}$ (unitless)
${\frac {F}{C}}={\frac {c-t}{f-t}}$

Percent of metal recovered ($\mathrm {X} _{R}$) in wt%
$\mathrm {X} _{R}=100\left({\frac {c}{f}}\right)\left({\frac {f-t}{c-t}}\right)$

Percent of metal lost ($\mathrm {X} _{L}$) in wt%
$\mathrm {X} _{L}=100-\mathrm {X} _{R}$

Percent of weight recovered $\left(\mathrm {X} _{W}\right)$ in wt%
$\mathrm {X} _{W}=100\left({\frac {C}{F}}\right)=100{\frac {f-t}{c-t}}$

Grade-Recovery Curves

Grade-recovery curves are useful tools in weighing the trade-off of producing a high grade of concentrate while maintaining as low of a recovery rate as possible, two important aspects of froth flotation. These curves are developed empirically based on the individual froth flotation process of a particular plant. As the curves are shifted in the positive x-direction (to the right) and the positive y-direction (upward) the performance of the froth flotation process is regarded as improving. A disadvantage to these curves is that they can only compare the grade-recovery relations of a specific feed grade and feed rate. If a company has a variance of feed grades and rates used (an extremely common occurrence) in their froth flotation process, grade-recovery curves for every pairing of feed grade and recovery rate would have to be constructed in order to provide meaningful information to the plant.

Example grade-recovery relationships seen in froth flotation. Shifts in the curves represent changes in flotation effectiveness.