13.2 Extraction and leaching

Extraction and leaching are key separation techniques in thermodynamics. They involve transferring solutes between phases, like pulling caffeine from coffee beans or metals from ore. These methods are vital in industries from food to metallurgy.

Understanding extraction processes helps us grasp how molecules move between phases. We'll look at different setups, like cross-current and counter-current, and important factors like distribution coefficients and solvent choice. This knowledge is crucial for designing efficient separation systems.

Extraction Processes

Liquid-Liquid Extraction and Solid-Liquid Extraction

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• Liquid-liquid extraction involves transferring a solute from one liquid phase to another immiscible or partially miscible liquid phase
  • Commonly used in chemical processing, metallurgy, and nuclear reprocessing
  • Relies on the difference in solubility of the solute between the two liquid phases (aqueous and organic solvents)
• Solid-liquid extraction involves the separation of a solute from a solid matrix using a liquid solvent
  • The solvent dissolves the desired solute, leaving behind the insoluble solid residue
  • Widely applied in the food industry for extracting oils, flavors, and bioactive compounds from plant materials (coffee, tea, spices)

Leaching and Soxhlet Extraction

• Leaching is a specific type of solid-liquid extraction where a solvent is used to extract a soluble component from a solid mixture
  • The solvent percolates through the solid, dissolving the desired solute and leaving behind the insoluble residue
  • Used in hydrometallurgy to extract metals from ores (copper, gold, uranium) and in the food industry to extract sugar from sugar beets or oil from seeds
• Soxhlet extraction is a continuous solid-liquid extraction method that uses a specialized apparatus to repeatedly extract a solute from a solid sample
  • The solvent is heated, vaporized, and condensed, percolating through the solid sample in a thimble and extracting the solute
  • Commonly used for extracting lipids from food samples and pesticides or pollutants from soil samples

Extraction Configurations

Cross-Current and Counter-Current Extraction

• Cross-current extraction involves contacting the solvent and the feed material in a single stage, with the solvent flowing perpendicular to the feed
  • The solvent and feed are mixed, allowed to reach equilibrium, and then separated
  • Simpler to operate but less efficient than counter-current extraction
• Counter-current extraction involves contacting the solvent and the feed material in multiple stages, with the solvent flowing in the opposite direction to the feed
  • The most concentrated solvent contacts the most depleted feed, while the least concentrated solvent contacts the most concentrated feed
  • Achieves a higher extraction efficiency and requires less solvent than cross-current extraction (used in liquid-liquid extraction columns and leaching processes)

Extraction Parameters

Distribution Coefficient and Partition Coefficient

• The distribution coefficient ($K_D$) is the ratio of the concentration of a solute in the extract phase to its concentration in the raffinate phase at equilibrium
  • Represents the effectiveness of the extraction process and the affinity of the solute for the extract phase
  • A higher $K_D$ indicates a more favorable extraction, requiring fewer stages or less solvent to achieve the desired separation
• The partition coefficient ($K_P$) is similar to the distribution coefficient but specifically refers to the ratio of the solute concentration in two immiscible liquid phases at equilibrium
  • Used in liquid-liquid extraction to describe the partitioning of the solute between the aqueous and organic phases
  • Depends on factors such as temperature, pH, and the presence of other solutes or additives

Solvent Selection

• Solvent selection is crucial for optimizing extraction efficiency, selectivity, and downstream processing
  • The solvent should have a high affinity for the desired solute, low affinity for unwanted components, and be immiscible with the feed phase
  • Other factors to consider include solvent toxicity, flammability, environmental impact, and cost
• Common solvents used in extraction processes include:
  • Hydrocarbons (hexane, heptane) for extracting oils and lipids
  • Alcohols (ethanol, methanol) for extracting polar compounds and natural products
  • Chlorinated solvents (dichloromethane, chloroform) for extracting pesticides and pollutants
  • Supercritical fluids (carbon dioxide) for extracting high-value compounds with minimal solvent residues