Difference Between Leaching and Extraction

Leaching vs Extraction
 

The difference between leaching and extraction can be explained in terms of the chemical principles used in these two processes. Both leaching and extraction refer to the isolation of one or several compounds from a mixture that they are originally present in. When a solid mixture is brought in contact with a solvent in order to separate the components that are soluble, the process is called leaching. When compounds in a mixture, in one chemical phase, are being separated out to another, it is referred to as extraction.

What is Leaching?

Leaching is a process to separate the components from a solid mixture by bringing that mixture in contact with a liquid solvent in which these components are soluble. There are three important factors that are required for leaching to occur. They are a compound mixture, a solute, and a solvent. When a liquid or solvent is applied or brought in contact with a compound mixture, the components which are soluble in the solvent starts to dissolve while other components remain in a slurry. These components that dissolve are called ‘solutes.’ Therefore, upon the application of the solvent in excess, the solutes can be removed from the initial compound mixture. Even though it is expected only for the solutes to be present in the solvent, it only happens under ideal conditions. Therefore, the solvent usually contains other impurities from the slurry. Leaching is a type of ‘solid-liquid’ extraction.

This method is commonly used in industries when solid materials are to be separated from a solid mixture. Some common examples include separation of sugar from sugar beet with hot water, separation of metal from metal ore using acid, etc. In nature, it is through leaching that heavy metals and other soil contaminants enter the ground water ways.

What is Extraction?

Extraction is also a process to separate components from a compound mixture, but here, the compounds in one chemical phase are being separated out to another phase. Usually extraction takes place between two immiscible solvents, which is explicitly known as ‘solvent-solvent’ extraction. A compound mixture can be separated into components among two immiscible solvents depending on the affinities of the various components to each solvent used. The affinity mentioned above is usually due to the polarity of the compounds and the respective solvents. Some common solvent systems used are water: ethyl acetate, water: methylene chloride, water/methanol mixture: methylene chloride, water/methanol mixture: ethyl acetate, etc.

This technique is often used under technical chemical laboratory conditions where organic compounds are generated or which, as a part of a mixture, needs to be separated out. Hence, extraction into organic solvents are performed. The process of extraction of a given compound in one phase to another phase is governed by the “Partition Theory.” Once a compound or several compounds have been separated out from their initial mixture into a second solvent, the compounds can be isolated through the evaporation of the excess solvent. An instrument called the ‘rotary evaporator’ is used for this purpose.

There are also other types of extraction such as solid phase extraction. Some modern variations include super critical carbon dioxide extraction, ultrasonic extraction, microwave-assisted extraction, etc.

What is the difference between Leaching and Extraction?

• Definition of Leaching and Extraction:

• Leaching is the process where a solid material in a mixture is separated out by dissolving it in a suitable solvent.

• In extraction, a given compound is separated from one chemical phase to another due to the polarity differences.

• Chemical Principle:

• Leaching occurs via a concentration gradient for soluble components.

• Extraction is governed by the Partition theory.

• Application:

• Leaching, which is simpler in approach, is commonly applied at industrial scale.

• Extraction is often used in laboratory level.

Images Courtesy:

1. Iron leaching via Wikicommons (Public Domain)
2. Separatory funnel by PRHaney (CC BY-SA 3.0)