11.3 Colloids in mining and mineral processing

Colloids play a crucial role in mineral processing, from extraction to purification. Their unique properties impact mineral separation, surface modification, and reagent distribution. Understanding colloid behavior is key to optimizing processing techniques and improving efficiency.

Colloids can enhance selectivity in processes like flotation and flocculation, but may also interfere by causing slime coatings. Their stability, governed by attractive and repulsive forces, affects their interactions with minerals and reagents, influencing separation effectiveness.

Colloids in mineral processing

• Colloids play a crucial role in various stages of mineral processing, from extraction to separation and purification
• Understanding the behavior and properties of colloids is essential for optimizing mineral processing techniques and improving efficiency
• Colloids can have both beneficial and detrimental effects on mineral processing, depending on their stability and interactions with minerals and reagents

Role of colloids

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• Colloids can enhance the selectivity and efficiency of mineral separation processes, such as flotation and flocculation
• They can modify the surface properties of minerals, making them more amenable to specific processing techniques
• Colloids can also act as carriers for reagents, such as collectors and depressants, improving their distribution and effectiveness
• However, colloids can also interfere with mineral processing by causing slime coatings, increasing reagent consumption, and reducing the quality of concentrates

Colloid properties

• Colloids are characterized by their small size (1-1000 nm), large surface area-to-volume ratio, and unique surface properties
• The stability of colloids is governed by the balance between attractive (van der Waals) and repulsive (electrostatic and steric) forces
• Colloid properties, such as size, shape, and surface charge, can significantly influence their behavior in mineral processing systems
• The hydrophobicity or hydrophilicity of colloids can affect their interactions with minerals and reagents, as well as their response to separation processes

Stability of colloids

• Colloid stability is a critical factor in mineral processing, as it determines the effectiveness of separation and purification techniques
• Stable colloids can remain dispersed in the processing medium, while unstable colloids tend to aggregate or settle, leading to operational issues
• Factors affecting colloid stability include pH, ionic strength, temperature, and the presence of surface-active agents (surfactants and polymers)
• Strategies for controlling colloid stability include the use of dispersants, flocculants, and surface modification techniques (such as coating or adsorption of specific reagents)

Flotation process

• Flotation is a widely used separation technique in mineral processing, based on the selective attachment of hydrophobic particles to air bubbles
• It exploits the differences in surface properties between valuable minerals and gangue (unwanted) materials, allowing for their efficient separation
• The success of flotation depends on the proper selection and use of reagents, as well as the optimization of various process parameters

Principles of flotation

• Flotation relies on the creation of a hydrophobic surface on the target mineral particles, which enables their attachment to air bubbles
• The hydrophobized particles are carried to the surface by the rising bubbles, forming a stable froth layer that can be collected as a concentrate
• Gangue materials, which remain hydrophilic, do not attach to the bubbles and remain in the pulp, allowing for their separation from the valuable minerals
• The selectivity of flotation is achieved through the use of specific reagents that adsorb onto the mineral surfaces, altering their hydrophobicity

Flotation reagents

• Collectors are surfactants that selectively adsorb onto the surface of target minerals, rendering them hydrophobic and promoting their flotation
• Frothers are reagents that stabilize the air bubbles and create a stable froth layer, facilitating the collection of hydrophobic particles
• Regulators, such as activators, depressants, and pH modifiers, are used to control the flotation behavior of specific minerals and improve selectivity
• The choice of flotation reagents depends on the mineralogy of the ore, the desired recovery and grade, and the environmental and economic constraints

Factors affecting flotation

• Particle size: Flotation is most effective for particles in the size range of 10-200 μm, as smaller particles may not have sufficient momentum to attach to bubbles, while larger particles may detach due to gravitational forces
• Pulp chemistry: The pH, Eh (redox potential), and ionic strength of the pulp can significantly influence the adsorption of reagents and the flotation behavior of minerals
• Mineralogy: The surface properties, liberation, and associations of minerals in the ore affect their response to flotation reagents and the overall separation efficiency
• Operational parameters: Factors such as air flow rate, impeller speed, pulp density, and residence time can impact the bubble size distribution, particle-bubble interactions, and the recovery and grade of the concentrate

Flotation of sulfide minerals

• Sulfide minerals, such as chalcopyrite (CuFeS₂), galena (PbS), and sphalerite (ZnS), are commonly processed using flotation
• The natural hydrophobicity of sulfide minerals allows for their flotation with relatively simple collector systems, such as xanthates and dithiophosphates
• The selectivity between different sulfide minerals can be achieved by exploiting their differences in surface chemistry and using specific depressants and activators
• Challenges in sulfide mineral flotation include the presence of slime coatings, the oxidation of mineral surfaces, and the activation of unwanted minerals (e.g., pyrite)

Flotation of non-sulfide minerals

• Non-sulfide minerals, such as oxides, silicates, and carbonates, often require more complex flotation strategies due to their hydrophilic nature
• The flotation of these minerals typically involves the use of specialized collectors (e.g., fatty acids, amines, and sulfonates) that can adsorb onto their surfaces and impart hydrophobicity
• The selectivity in non-sulfide mineral flotation is often achieved through the use of depressants, which prevent the flotation of gangue minerals
• Examples of non-sulfide minerals processed using flotation include hematite (Fe₂O₃), cassiterite (SnO₂), and apatite (Ca₅(PO₄)₃(F,Cl,OH))

Flocculation in mineral processing

• Flocculation is a process in which fine particles aggregate into larger flocs, facilitating their separation from the liquid phase
• In mineral processing, flocculation is used for the settling and dewatering of fine particles, as well as for the removal of impurities and the recovery of valuable minerals
• Flocculants are polymeric reagents that bridge between particles, promoting their aggregation and settling

Principles of flocculation

• Flocculation occurs when polymeric flocculants adsorb onto the surfaces of multiple particles, creating bridges that bind the particles together
• The adsorption of flocculants is driven by a combination of electrostatic interactions, hydrogen bonding, and van der Waals forces
• As particles aggregate into larger flocs, their settling velocity increases, allowing for more efficient solid-liquid separation
• The effectiveness of flocculation depends on factors such as the type and dosage of flocculant, the particle size and surface chemistry, and the solution conditions (pH, ionic strength)

Flocculation mechanisms

• Charge neutralization: Flocculants with opposite charges to the particles can adsorb onto their surfaces, reducing the electrostatic repulsion and promoting aggregation
• Bridging: Long-chain polymeric flocculants can adsorb onto multiple particles simultaneously, creating bridges that link the particles together
• Sweep flocculation: In the presence of high flocculant dosages, particles can be entrapped within the precipitating flocculant, leading to rapid settling
• Patching: Flocculants with high charge density can adsorb onto particles in a non-uniform manner, creating patches of opposite charge that attract other particles

Flocculant types and properties

• Natural flocculants: Derived from natural sources, such as starch, guar gum, and chitosan, these flocculants are biodegradable and have low toxicity but may have limited effectiveness
• Synthetic flocculants: Produced through polymerization reactions, these flocculants (e.g., polyacrylamide, polyethylene oxide) offer high flocculation efficiency and can be tailored to specific applications
• Cationic flocculants: Positively charged flocculants that are effective for the flocculation of negatively charged particles (e.g., clays, silicates)
• Anionic flocculants: Negatively charged flocculants that are suitable for the flocculation of positively charged particles (e.g., metal oxides, carbonates)
• Non-ionic flocculants: Uncharged flocculants that can adsorb onto particles through hydrogen bonding and van der Waals forces, providing stability in high salinity conditions

Factors affecting flocculation

• Flocculant type and molecular weight: The choice of flocculant depends on the particle surface chemistry and the desired flocculation mechanism, while higher molecular weight flocculants generally provide better bridging and floc strength
• Flocculant dosage: Insufficient dosage may lead to incomplete flocculation, while excessive dosage can cause particle restabilization and hinder settling
• Solution pH and ionic strength: These factors influence the surface charge of particles and the conformation of flocculants, affecting the adsorption and bridging processes
• Particle size and solid concentration: Finer particles require higher flocculant dosages due to their larger surface area, while higher solid concentrations can promote particle collisions and floc formation

Applications of flocculation

• Thickening and clarification: Flocculation is used to increase the settling rate of fine particles, enabling the production of clear overflow and concentrated underflow in thickeners and clarifiers
• Filtration: Flocculated particles form more permeable filter cakes, improving the efficiency of filtration processes such as vacuum and pressure filtration
• Tailings management: Flocculation is employed to enhance the settling and consolidation of tailings, reducing the volume of waste and minimizing the environmental impact of mineral processing operations
• Water recycling: Flocculation can remove suspended solids and impurities from process water, enabling its reuse in the mineral processing circuit, thus reducing fresh water consumption and effluent discharge

Dispersion in mineral processing

• Dispersion is the process of breaking down aggregates and maintaining particles in a stable, suspended state in a liquid medium
• In mineral processing, dispersion is essential for achieving efficient grinding, classification, and separation, as well as for preventing the agglomeration of particles during downstream processing
• Dispersants are chemical additives that adsorb onto particle surfaces, providing electrostatic or steric stabilization and preventing particle aggregation

Principles of dispersion

• Dispersants adsorb onto particle surfaces, creating a barrier that prevents the particles from coming into close contact and aggregating
• Electrostatic dispersion: Dispersants with charged functional groups (e.g., carboxylates, sulfonates) can adsorb onto particles, increasing their surface charge and electrostatic repulsion
• Steric dispersion: Non-ionic dispersants (e.g., polyethylene oxide) adsorb onto particles and create a physical barrier that prevents particle-particle interactions
• The effectiveness of dispersion depends on factors such as the dispersant type and dosage, particle size and surface chemistry, and the solution conditions (pH, ionic strength)

Dispersant types and properties

• Anionic dispersants: Negatively charged dispersants (e.g., sodium silicates, sodium polyphosphates) that are effective for the dispersion of positively charged particles
• Cationic dispersants: Positively charged dispersants (e.g., quaternary ammonium compounds) that are suitable for the dispersion of negatively charged particles
• Non-ionic dispersants: Uncharged dispersants (e.g., polyethylene oxide, polypropylene glycol) that provide steric stabilization and are less sensitive to changes in pH and ionic strength
• Low-molecular-weight dispersants: Smaller dispersant molecules that can adsorb onto particle surfaces more easily but may provide less steric stabilization
• High-molecular-weight dispersants: Larger dispersant molecules that create a thicker adsorbed layer and provide better steric stabilization but may be more sensitive to shear forces

Factors affecting dispersion

• Particle size and surface area: Finer particles require higher dispersant dosages due to their larger surface area and higher surface energy
• Surface chemistry: The presence of specific functional groups (e.g., hydroxyl, carboxyl) on particle surfaces can influence the adsorption of dispersants and the overall dispersion stability
• Solution pH: The pH affects the surface charge of particles and the ionization of dispersants, impacting the adsorption and electrostatic stabilization
• Ionic strength: High ionic strength can compress the electrical double layer around particles, reducing the electrostatic repulsion and potentially leading to aggregation
• Temperature: Higher temperatures can increase the thermal motion of particles and the desorption of dispersants, potentially destabilizing the dispersion

Applications of dispersion

• Grinding: Dispersants are used to prevent the agglomeration of fine particles during grinding, improving the efficiency of size reduction and reducing energy consumption
• Classification: Stable dispersions enable more accurate particle size classification in hydrocyclones and classifiers, as particles settle according to their individual sizes rather than as aggregates
• Froth flotation: Dispersants help maintain particles in suspension, preventing slime coatings and improving the selectivity of the flotation process
• Slurry transport: Dispersed slurries have lower viscosity and better flow properties, reducing pumping costs and minimizing pipeline wear
• Wet magnetic separation: Dispersants prevent the agglomeration of magnetic particles, enabling their efficient separation from non-magnetic gangue minerals

Rheology of mineral suspensions

• Rheology is the study of the flow and deformation behavior of materials, including mineral suspensions
• Understanding the rheological properties of mineral suspensions is crucial for optimizing various mineral processing operations, such as grinding, classification, flotation, and slurry transport
• The rheology of mineral suspensions is influenced by factors such as particle size, shape, and concentration, as well as the chemistry of the suspending medium and the presence of additives

Rheological properties

• Viscosity: A measure of a fluid's resistance to flow, expressed as the ratio of shear stress to shear rate. Mineral suspensions often exhibit non-Newtonian behavior, where viscosity varies with shear rate
• Yield stress: The minimum stress required to initiate flow in a suspension. Suspensions with high yield stress require more energy to pump and can be prone to settling and blockages
• Thixotropy: A time-dependent decrease in viscosity under constant shear, followed by a gradual recovery when the shear is removed. Thixotropic suspensions can be challenging to handle and transport
• Rheopexy: A time-dependent increase in viscosity under constant shear, followed by a gradual recovery when the shear is removed. Rheopectic suspensions are less common but can occur in some mineral systems
• Viscoelasticity: The exhibition of both viscous (fluid-like) and elastic (solid-like) behavior. Viscoelastic suspensions can store and release energy during deformation, affecting their flow and settling properties

Factors affecting rheology

• Particle size and distribution: Finer particles generally increase the viscosity and yield stress of suspensions due to their higher surface area and inter-particle interactions
• Particle shape: Elongated or platy particles can increase the viscosity and yield stress compared to spherical particles, as they have a higher aspect ratio and can form more entangled structures
• Solids concentration: Higher solids concentrations lead to increased viscosity and yield stress due to greater particle-particle interactions and reduced free space for flow
• pH and ionic strength: These factors influence the surface charge and electrostatic interactions between particles, affecting the suspension stability and rheology
• Temperature: Higher temperatures typically reduce the viscosity of the suspending medium and can promote particle dispersion, while lower temperatures can increase viscosity and lead to aggregation

Rheology modifiers

• Dispersants: These additives adsorb onto particle surfaces and provide electrostatic or steric stabilization, reducing the viscosity and yield stress of suspensions
• Thickeners: Additives that increase the viscosity of the suspending medium, such as natural gums (e.g., guar gum) or synthetic polymers (e.g., polyacrylamide), can be used to improve suspension stability and reduce settling
• Rheology control agents: Specialized additives, such as low-molecular-weight polymers or surfactants, can be used to modify the rheological properties of suspensions for specific applications
• pH modifiers: Adjusting the pH of the suspension can alter the surface charge of particles and the effectiveness of rheology modifiers, allowing for the optimization of rheological properties

Rheology in mineral processing

• Grinding: The rheology of the slurry in grinding mills affects the particle size reduction efficiency, the wear of grinding media, and the energy consumption of the process
• Classification: The rheological properties of the feed slurry influence the separation efficiency in hydrocyclones and classifiers, as well as the capacity and performance of the equipment
• Flotation: The rheology of the pulp affects the bubble-particle interactions, the froth stability, and the overall recovery and grade of the concentrate
• Thickening and filtration: The rheology of the feed slurry determines the settling rate, the clarity of the overflow, and the filtration rate and cake moisture content
• Slurry transport: The rheological properties of the slurry dictate the pumping requirements, the pipeline pressure drop, and the risk of settling and blockages

Adsorption at mineral interfaces

• Adsorption is the accumulation of molecules or ions (adsorbates) at the interface between a solid surface (adsorbent) and a liquid or gas phase
• In mineral processing, adsorption plays a crucial role in the interaction between minerals and reagents, such as collectors, depressants, and modifiers, which alter the surface properties