Leaching

What is Leaching?

Leaching is the liberation of metals from ores by chemical dissolution, and it forms the basis of most hydrometallurgical extraction processes. The main aim of leaching is getting the maximum amount of the valuable metal. This unit operation is an established and relatively successful method of metal extraction, especially when treating high grade ores.

In geology, leaching results in the loss of soluble compounds and colloids by percolating moisture from the top layer of soil. The materials lost are transported downward (eluviated) and in a lower layer are normally redeposited (illuviated). This transportation results in a thick, compact lower layer and a brittle and free top layer. With the volume of moisture, high temperatures, and the removal of protecting plants, the rate of leaching increases.

Table of Contents

• Leaching Of Soil
• Types of Leaching
• Advantages and Disadvantages of Leaching
• Frequently Asked Questions on Leaching

Leaching of Soil

Leaching occurs as excess water removes water-soluble nutrients out of the soil, by runoff or drainage. Leaching is an environmental problem for agricultural professionals whether chemical-heavy fertilisers or chemicals are swept out and find their way into water bodies.

Leaching is closely related to solvent extraction, in which a soluble substance is dissolved from one liquid by a second liquid immiscible with the first. Both leaching and solvent extraction are often called extraction. Leaching is also known as solid-liquid extraction, lixiviation, washing etc.

Leaching is a mass transfer process which takes place through the extraction of a substance from a solid material that has come into contact with the liquid. Leaching is the normal mechanism by which water-soluble compounds from soil or waste are washed out. Such leached-out contaminants affect surface and subsurface water contamination.

Types of Leaching

The mechanism by which components of a solid material are released into a touching water phase is leaching. Although some species may be of greater environmental significance than others, the leaching mechanism is indiscriminate in such a manner that all components (e.g., major or minor components of the matrix as well as inorganic , organic and radionuclide contaminants) are emitted under a general collection of chemical phenomena that may include mineral dissolution, desorption and complexation, and mass transport processes.

1. Cyanide Leaching

A cyanide solution, or lixiviant, is percolated into ore deposited in vats, columns or heaps during the cyanide leaching process. The cyanide dissolves gold, which is then extracted from the heap or columns. It is then removed by adsorption of carbon or resins from the pregnant leaching solution.

2. Ammonia Leaching

In hydrometallurgical processes, ammonia and ammonium salts have been recognised as efficient leaching agents due to low toxicity and expense, fast recovery and elevated selective metal recovery. New research studies on the major benefits of leaching by these agents and the removal of acid leaching-related issues have resulted in a new worldwide approach to this process.

3. Alkali Leaching

Generally, alkaline leaching works more selectively. In addition, it is considered cost-effective, quick and fast to operate and handle the extraction of zinc from oxidised zinc ores or waste, it can hardly be leached out in alkaline media for impurities such as Fe, Cu, Cd, Co , Ni, etc. and its lower energy usage in the electrowinning process compared to acidic electrowinning process

4. Acid Leaching

As the first stage of extraction of base metals, acid leaching is often used, especially in the case of copper, for which mineral acids such as HCl, H₂SO₄ or HNO₃ are used in conjunction with H₂O₂, a powerful oxidant that enhances the acid’s leaching efficiency. For the recovery of copper from PCB, the mixture of H₂SO₄ as a leaching agent and H₂O₂ as an oxidising agent is widely used.

Advantages and Disadvantages of Leaching

A leaching chamber is constructed of lightweight material that can be transported comfortably to the trench that has been excavated. As used in traditional trench systems, no additional perforated pipe or geotextile fabric is needed.

However, when leaching eliminates too much nitrate from the soil, the pH decreases too much and the soil becomes over-acidic. Soil acidification in itself has many harmful effects, including changes in soil microbe species, pollution of surface water and dwindling earthworm populations.

Leaching of nutrients is of major environmental concern as high concentration of some ions in the drinking water is harmful to human health. Phosphorus (P) in soils is important because adequate availability of this nutrient is required for plant growth and crop production.