[Chemistry Class Notes] Leaching Process Pdf for Exam

The process of leaching is used to extract the substances from the solids. This process is carried out when the given substance is allowed to dissolve in a liquid. It is carried out either through a natural process or industrially.

The leaching process shows the release of both the organic as well as the inorganic radionuclides or contaminants from a solid-state to a liquid state when they get influenced by different processes like mineral dissolution, complexation, and desorption.

The process of leaching is known to be a universal process in which water tends to leach the material components that come in contact with it. This can be its surface or its interior depending on how porous the material is.

The ore of the given metal can be concentrated using this process when a chemical reaction is caused with the help of a reagent that would eventually lead to the ore getting dissolved and the impurities undissolved. 

Leaching Process Examples

The leaching process example includes the leaching of bauxite or Al₂O₃ . 2H₂O with the concentrated and heated sodium hydroxide. The concentrated NaOH here tends to dissolve the aluminium present in the given bauxite, while on the other hand, the impurities like SiO₂, TiO₂, and Fe₂O₃ do not get dissolved. The chemical reaction of this leaching process is given as follows.

[ Al_{2}O_{3} . 2H_{2}O + 2NaOH rightarrow 2NaAlO_{2} = 3H_{2}O ]

Another example of the process of leaching is leaching of the noble metals like silver and gold in the presence of the dilute aqueous solutions of either potassium cyanide or sodium cyanide in the presence of air.  The chemical reaction for this process of leaching for silver is given below.

[ Ag_{2}S + 4 NaCN rightarrow 2NaAg(CN)_{2} + Na_{2}S ]

The leaching process also causes the loss of nutrients that are present in the soil because of heavy rainfalls. 

Advantages and Disadvantages of Leaching Process

Advantages of Leaching are as Follows.

1. The leaching process is easier when it comes for the execution.

2. It is not a harmful process in comparison to the other pyrometallurgical methods.

3. It does not lead to any sorts of gaseous pollutants.

Disadvantages of Leaching are as Follows.

1. The residual liquid waste that is generated from the leaching process is highly acidic in nature.

2. The effluent of the leaching process is toxic.

3. The efficiency of the leaching process is entirely dependent on temperature.

Types of Leaching Processes in Metallurgy

The different kinds of leaching processes which are used in industrial purposes for metallurgy are given below.

1. Heap Leaching: It refers to a process which extracts uranium, copper and many other precious metals from their ores.

2. In-situ Leaching: It is a process which recovers uranium and copper.

3. Tank Leaching and Vat Leaching: They are the processes in which the ores are to be placed in vats or large tanks that consist of the leaching solutions. These processes are used for the extraction of the metals from their ores.

Leaching Process

Leaching is the process of releasing solid material constituents into a contacting water phase. Although some species may pose a greater environmental risk than others, the leaching process is indiscriminate, meaning that all constituents (e.g., major and minor matrix components, as well as inorganic, organic, and radionuclide contaminants) are released through a common set of chemical processes that include mineral dissolution, desorption and complexation, and mass transport processes. These events, in turn, are influenced by a variety of factors that can affect the rate and degree of leaching. These are some of the aspects to consider:

• Chemical and physical responses within the body

• external stressors imposed by the environment

• Physical deterioration of the solid matrix as a result of erosion or cracking, as well as

• The leaching process itself causes a loss of matrix elements.

The partitioning of pollutants between a solid and liquid phase (e.g., assuming local equilibrium) is combined with the mass transport of aqueous or dissolved elements in the leaching process. The sum of diffusion, impeded diffusion, tortuosity effects, and effective surface area effects through the material’s pore structure to the environment is known as mass transport. Solution pH, redox, the presence of dissolved organic matter, and biological activity are all important chemical parameters that influence a constituent’s liquid-solid partitioning (LSP).

Physical properties including relative hydraulic conductivity, porosity, and fill geometry all play a role in influencing how quickly constituents move through a solid and into a passing liquid phase.

The process is universal in nature, since any substance exposed to water may leach components from its surface or interior, depending on the porosity of the material

Process of Leaching

The solvent comes into touch with the solid matrix for the first time. A solvent is a liquid that dissolves a material or solute. A solvent dissolves a solute, which is an element. As a result, the solvent will turn into a liquid, while the solute will be the component extracted from the solid matrix. Furthermore, in our tea example, the solute would be the extracted green tea, and the solvent would be hot water.

There are many different types of leaching processes, which are mainly categorized based on the reagents utilized, Such as Cyanide leaching (e.g. gold ore),Ammonia leaching (e.g. crushed ore),Alkali leaching (e.g. bauxite ore),Acid leaching (e.g. sulfide ore)

The reagents needed are determined by the ores or pretreated material being processed. Oxide or sulphide is a common feed for leaching.