Removal of materials by dissolving them away from solids is called leaching. The chemical process industries use leaching but the process is usually called extraction, and organic solvents are often used. Basic concept. The theory and practice of leaching are well-developed because for many years leaching has been used to separate metals from their ores and to extract sugar from sugar beets. Environmental engineers have become concerned with leaching more recently because of the multitude of dumps and landfills that contain hazardous and toxic wastes. Sometimes the natural breakdown of a toxic chemical results in another chemical that is even more toxic. Rain that passes through these materials enters ground water, lakes, streams, wells, ponds, and the like.

Although many toxic materials have low solubility in water, the concentrations that are deemed hazardous are also very low. Furthermore, many toxic compounds are accumulated by living cells and can be more concentrated inside than outside a cell. This is why long-term exposure is a serious problem; encountering a low concentration of a toxic material a few times may not be dangerous, but having it in your drinking water day after day and year after year can be deadly.

Heap leaching is a countercurrent process where the solid is in a stationary heap and the solvent percolates through the solid. An example is a dump or landfill. Click to see figure. This leaching is essentially countercurrent. In industrial leaching, solvent and solid are mixed, allowed to approach equilibrium, and the two phases are separated. Liquid and solids move countercurrently to the adjacent stages. The solvent phase, called the extract, becomes more concentrated as it contacts in stagewise fashion the increasingly solute-rich solid. The raffinate becomes less concentrated in soluble material as it moves toward the fresh solvent phase.

The main theory of leaching neglects mechanisms for holding the material on the solid. Although adsorption and ion exchange can bind materials tightly to solids, we will simplify the analysis and consider only dissolving a soluble constituent away from an insoluble solid. An example is removing salt from sand by extraction with water. Batch calculation

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School of Engineering, Rensselaer Polytechnic Institute, Troy, NY 12180