What is leaching?

The process of removing substances from solids by dissolution is called leaching.

Leaching is used in chemical processing industry, but this process is usually called extraction and organic solvents are often used. For many years, leaching has been used to separate metals from ores and extract sugar from sugar beet, so its theory and practice have been highly developed. Recently, environmental engineers have paid more attention to leaching because garbage dumps and landfills contain harmful and toxic wastes. The natural decomposition of toxic chemicals sometimes produces more toxic chemicals. Rainwater flows between these substances and will flow into groundwater, lakes, rivers, wells, ponds and so on.

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The process of removing substances by dissolving them from solids is called leaching. (This is the concept of leaching)

Leaching is used in chemical processing industry, but this process is usually called extraction, and organic solvents are often used. The theory and practice of leaching are quite mature, because leaching has been used for many years to separate metals from ores and extract sugar from beets. Recently, environmental engineers began to pay attention to leaching, because a large number of garbage dumps and landfills contain dangerous and toxic waste. Sometimes the natural decomposition of a toxic chemical will produce another more toxic chemical. Rainwater passing through these substances enters groundwater, lakes, streams, wells, ponds, etc.

Although the solubility of many toxic substances in water is very low, the concentration considered harmful is also very low. In addition, many toxic compounds are accumulated by living cells and are more concentrated inside the cells than outside the cells. This is why long-term exposure is a serious problem; Exposure to low concentrations of toxic substances several times may not be dangerous, but exposure to toxic substances in your drinking water day after day and year after year may be fatal.

Heap leaching is a countercurrent process, in which the solid is in a static heap and the solvent permeates from the solid. A garbage dump or landfill is an example. In industrial leaching, solvent and solid are mixed to make them close to equilibrium, and then the two phases are separated. Liquid and solid move to adjacent stages in countercurrent. The solvent phase, called extract, becomes more concentrated as it comes into contact with solids with increasing solute content. When the raffinate flows to the fresh solvent phase, the concentration of soluble substances decreases.

The main leaching theory ignores the mechanism of keeping substances on solids. Although adsorption and ion exchange can closely combine substances with solids, we will simplify the analysis and only consider soluble solids. One example is extracting salt from sand with water.