LIQUID-SOLID LEACHING PROCESS

Leaching Processes

•Use to separate the desired solute constituent or remove an undesired solute component from the solid phase. •The solid is contacted with a liquid phase in which the two phases are in intimate contact. •This allow the solute diffuses from the solid to the liquid phase resulting separation.

Leaching Process for Biological Substances •Leaching of sugar from sugar beets with hot water. •Production of vegetable oil, where organic solvent, e.g. hexane, acetone and ether, are used to extract oil from peanuts, soybens, sunflower seeds, cotton seeds, and halibut liver. •Pharmaceutical products are obtained by leaching plant roots, leaves, and stems. •Production of soluble instant coffee where ground roasted coffee is leached with fresh water. •Soluble tea is produced by water leaching of tea leaves.

Leaching Process for Inorganic and Organic Materials •Extensively used in the metals processing industries. •Leaching is used to remove metals as soluble salt. •Copper salts are dissolved or leached from its ore by sulfuric acid or ammoniacal solutions. •Gold is leached from its ore using aqueous sodium cyanide solution. •Cobalt and nickel salts are leached using sulfuric acid-ammonia-oxygen mixtures.

Preparation of Solid Leaching •Crushing and grinding of solid to allow soluble portions are made more accessible to the solvent thus increasing rate of leaching. •Simple washing can be applied if the soluble substance is widely distributed throughout the whole solid. •For biological material, i.e. sugar beet, they are cut into wedge-shape slices for leaching. •For soybeans and many vegetable seeds they are ruptures by rolling and flaking in such a way that the vegetable oil is easily accessible to the solvent. •For pharmaceutical product e.g. leaves, stems and roots, drying of the material before extraction help to rupture the cell wall and hence solvent can directly dissolved the solute.

Equilibrium Relations and Single-Stage Leaching

Assumptions: 1. Solute-free solid is insoluble in the solvent. 2. Solvent present in sufficient amount so all solute can be completely leached out

from solid. 3. Equilibrium is reached when solute is dissolved. 4. No absorption of solute by the solid. 5. Solution in liquid phase leaving the stage = solution that remains with the solid

matrix.

UNDERFLOW/SLURRY STREAM – the settled solid leaving a stage that contains some liquid containing dissolved solid. OVERFLOW STREAM – Concentration in the liquid solution accompanying the slurry stream.

Single-Stage Leaching Process

flow

Material balance

COUNTERCURRENT MULTISTAGE LEACHING

However, the Ln (the liquid retained in the underflow) will vary and the slope of (12.10-3) will vary from stage to stage.

Variable Underflow in Countercurrent Multistage Leaching

Estimation of the operating point Δ,

Assignment