Leaching in Extraction Processes

Leaching and Extraction

• Leaching is the extraction of a soluble component from a solid material using a selective liquid solvent.
• Extraction is the separation of one or more components from a mixture by contacting it with another phase (solid or liquid). This includes liquid-liquid extraction (solvent extraction) and solid-liquid extraction (leaching).
• Other extraction methods include crystallization and supercritical fluid extraction.

Major Steps in Extraction Process

• Bring the feed and solvent into close contact by dispersing one phase into the other as droplets.
• Separate the extract and raffinate phases based on density differences.
• Recover the solute from the extract phase in a pure form (by distillation, evaporation, crystallization, etc.).
• Recover the solvent from each phase, typically by distillation.

Conditions Where Extraction is Preferred Over Distillation

• Removing inorganic substances from organic or aqueous solutions.
• Removing contaminants present in low concentrations.
• Recovering heat-sensitive materials which extraction may be less expensive than vacuum distillation.
• Separating close-melting or close-boiling liquids.

Application of Leaching and Extraction in Food Industry

• Separating sugar from sugar beets using hot water.
• Extracting oils from peanuts, soybeans, sunflower seeds, cotton seeds, and halibut livers.
• Supercritical extraction of caffeine from coffee.
• Extracting fish oil from waste fish using organic solvents.

Example of Leaching

• Steeping a tea bag in hot water to extract the tea. The hot water leaches the soluble components from the tea leaves to the water, resulting in the familiar tea beverage.

Process/Stages Involved in Leaching

• Dissolution of the solute in a selective solvent.
• Diffusion of the solute through the pores of the particles.
• Transfer of the solute into the bulk solvent.

Factors Influencing Leaching Rate

• Particle size (smaller size = higher surface area = faster rate)
• Solvent (solubility of the solute in the solvent)
• Agitation (increases mass transfer)
• Temperature (higher temperature = faster rate, but can affect solubility).

Key Points to Remember

• Smaller particle size leads to higher extraction rate.
• Higher solute concentration can result in higher viscosity and thus lower extraction rate.
• Diffusivity improves with temperature as viscosity decreases.
• Agitation increases mass transfer rate from the particle surface and prevents sedimentation of fine solids.

Classification of Leaching

• Fixed bed
• Moving bed
• Agitated

Leaching Equipments

• Leaching methods are categorized by the method of contact:
  • Rotation (batch process)
  • Percolation.
• Materials that disintegrate during leaching are treated using different equipment types than particulate solids.

Rotating Extractor (Batch Process)

• A cylindrical vessel that rotates horizontally.
• Has a hatch for loading and unloading solids.
• Has a drain line for extracting and draining off the extractant.
• Equipped with baffle plates for efficient extraction.

Leaching Equipments (Based on Percolation)

• Continuous percolator
• Batch percolator
• Single and multi-deck rack classifiers
• Bucket elevator contractors
• Horizontal basket percolators
• Belt percolators

Bollman Extractor

• Primarily used for recovering additional oil from residues after mechanical pressing of solids.
• Consists of a vertical chamber with perforated baskets attached to a chain conveyor.
• Solids and solvent flow concurrently to maximize oil extraction.

Rotocel Extractor

• A horizontal basket divided into compartments with a permeable floor.
• The basket rotates slowly, allowing liquids to permeate.
• Solids are added to each compartment at the feed point.

Kennedy Extractor

• Originally used for leaching tannins but now for oilseeds.
• Uses a series of tubs that are pushed by paddles, with the solvent flowing in a counter-current direction.
• Perforations in the paddles allow solids to drain between stages.

Leaching Equipment (Based on Dispersion of Solids)

• Continuous dispersed leaching
• Vertical plate extractor
• Gravity sedimentation tanks
• Batch stirred tanks
• Screw conveyor extractor

Single Stage Batch Extraction

• A single process of contacting feed with solvent involving mixing, settling, and separation of the extract and raffinate.

Continuous Counter Current Extraction

• A continuous process in which feed and solvent flow in opposite directions through a series of stages.

Equilibrium Stages for Leaching

• Plotting equilibrium data (weight fraction) on a rectangular diagram for components (solute, inert solid, solvent).
• Calculating solute concentration in slurry using formula: N = (kg B) / (kg A + kg C).

Equilibrium Diagrams for Leaching

• Underflow: the settled solid leaving a stage contains some liquid that dissolved solute, the concentration of which is equal to the concentration of solute in the liquid slurry.
• Overflow: the liquid leaving the stage that has solute. This is indicated by an xy plot on a 45° line.
• Formulas for composition of solute A in liquid fraction (overflow and underflow): XA = (kg of A) / (kg of A + kg of C), and YA = (kg of A) / (kg of A + kg of C).

Single Stage Leaching

• M = total flow rate (A +C) / hour
• X_AM and N_AMare the coordinates of point M.
• Balance on component C is not needed.
• Straight lines: L₁MV₁ and L₀MV₂

Material Balance for Single Stage Leaching

• L0 + V2 = L1 + V1 = M
• L0Y_AO+ V₂X_A2 = L₁Y_A1+V₁X_A1 = MX_AM
• B = N₀L₀ = N₁L₁ = N_MM

Multiple Stage Counter Current Leaching

• Mass balances can be written around any stage or multiple
• Diagram: Flow of light stream is represented by V, and heavy stream flow by L
• Concentrations in light phase by x and heavy phase by y

Material Balance (Multiple Stage Leaching)

• Eliminating V_n+1 between the equations
• Formula to find Xn+1

Stage Efficiency

• Measure of deviation from theoretical equilibrium conditions; expressed as Murphee efficiency.
• Formula: η_M = (X_o - X) / (X_o - X*)

Washing

• Similar to extraction, but water is typically the product and solvent
• Multiple washings: impurities in x amount of water are added to the wash water (xw + yxw), impacting the impurity concentration in the resultant washing solution.

Numerical Examples (1-4)

• Detailed calculations for specific scenarios, including varying percentages of solute and solvent, and determining quantities and compositions of overflow and underflow in single and multiple stage leaching operations.

Liquid-Liquid Extraction (LLE)

• Extraction between two immiscible liquid phases.
• Solute transferred from one liquid phase to another.
• Desired solute is in the initial feed.
• Residual feed solution (raffinate) contains little solute.

Equilibrium in LLE

• Equilibrium is when the chemical potential of the extractable solute is the same in each phase.
• This leads to the definition of a distribution coefficient (K); C₁/C₂ = K, where C1 and C2 are the solute's equilibrium concentrations in the two phases.
• Distribution coefficient is a measure of the solute's preference for the solvent.

Equilateral Triangular/Ternary Phase Diagram

• Diagram for LLE, showing regions of single-phase and two-phase mixtures in ternary mixtures: solute, carrier, and solvent.
• Key Points: Vertices represent pure components; Each base line shows the binary composition; Solute, carrier, and solvent are indicated by the vertices.

Regions in Ternary Diagram

• Single liquid region (all components are dissolved in a single phase).
• Two liquid phase region (mixture splits into two immiscible liquid phases).
• Separation region boundaries (miscibility boundaries/liquid-liquid equilibrium lines).

Water-EG-Furfural Phase Diagram

• Water, ethylene glycol, and furfural are liquid compounds.
• Water-EG & furfural-EG are completely miscible.
• Furfural-water is partially miscible.
• At plait point, the two liquid phases have identical compositions.
• Furfural can be used as a solvent to remove ethylene glycol.

A Single Stage Extraction Example

• Example calculations for extracting a substance from a mixture using a solvent (e.g., the extraction of oil from soybean using hexane).

Extraction Equipments

• Equipment for mixing phases during extraction (e.g., agitated columns, mixer-settlers, spray extraction towers, packed extraction towers, centrifugal extractors).

Agitated Columns (Scheibel and Karr)

• Scheibel Tower: Agitators on a central shaft, wire mesh pickings for coalescence.
• Karr Reciprocating Plate Tower: Perforated trays for mixing and separation.

Mixer-Settlers

• Device for contacting phases and mechanically separating them, allowing for efficient mass transfer.

Spray Extraction Towers

• Heavy phase enters at the top, forms continuous phase.
• Light phase is sprayed in fine droplets (upward).
• Light phase coalesces, flows out at the top.

Parameters of LLE

• Solvent selection
• Operating conditions
• Mode of operation
• Extractor type
• Design criteria

Special Extraction Techniques

• Recovery of sensitive biological products using only aqueous phases.
• Example: protein separation using two-aqueous phases (e.g., polyethylene glycol, dextran, phosphate salts).
• Supercritical fluid extraction (using a solvent held above its critical temperature and pressure).
• Examples of a supercritical fluid solvent are carbon dioxide (CO₂), used in decaffeinating coffee.

Advantages and Disadvantages of SCFE using CO₂

• Advantages: Moderate temperature, non-toxic/non-flammable, very volatile solvent, good mass transfer, selective dissolution.
• Disadvantages: Limited solvation ability, high operational pressure, challenging for continuous operation.

This quiz explores the concept of leaching in various extraction processes. Participants will learn about the primary purpose of leaching, common solvents used, and different leaching methods. Test your knowledge on the key aspects of leaching and its applications in material science.