Leaching Process: Definition and Explanation with Diagrams

Leaching Process

Edited By Shivani Poonia | Updated on Jul 02, 2025 06:09 PM IST

Leaching can be defined as the process of extracting a given soluble substance by washing or dissolving it with a liquid solvent. In the activity of metallurgy, disengagement in an aqueous solution is used for the extraction of metals from mineral materials. This way, it abandons the unwanted insoluble components. This treatment works very well on the back of its being an economical method of processing low-grade ores at a large scale.

This Story also Contains

2. Kinds of Leaching Processes
3. Relevance and Applications
Some Solved Examples
Summary

Leaching Process

Solvent: A liquid containing chemicals like acids or bases prepared to dissolve the target metals.
Ore: Naturally occurring solid material that can be used to derive a metal or valuable mineral.
Concentrate A material that has been subjected to processing in a way that leads to an increase in the concentration of the wanted metal.
In this method, the powdered ore is treated with a suitable chemical reagent which dissolves the ore while impurities remain insoluble in that reagent.
For example:
Bauxite is separated from Fe₂O₃, SiO₂, and TiO₂, with the help of NaOH in which Al₂O₃ gets dissolved while the rest are insoluble.
For example: Al₂O₃( s)+2NaOH(aq)+3H₂O(l)→2Na[Al(OH)₄](aq)
The sodium aluminate present in the solution is neutralized by passing CO₂ gas and hydrated Al₂O₃ is precipitated. At this stage, a small amount of freshly prepared sample of hydrated Al₂O₃ is added to the solution. This is called seeding. It induces the precipitation.
2Na[Al(OH)₄](aq)+CO₂( g)→Al₂O₃⋅xH₂O(s)+2NaHCO₃(aq)
Sodium silicate remains in the solution and hydrated alumina is filtered, dried, and heated to give back pure Al₂O₃.
Al₂O₃⋅xH₂O(s)→1470 K Al₂O₃( s)+xH₂O(g)

2. Kinds of Leaching Processes

Heap Leaching

Heap leaching is a common practice for the extraction of metals such as gold, copper, and uranium. The heaps are normally placed in huge heaps and a solution for leach is sprayed over the heap to dissolve metals, which get collected at the bottom.

In-situ Leaching

In-situ leaching involves directly injecting the leaching solution into an ore deposit by boring holes. The leachate then percolates through the ore deposit, hence, dissolving metals that get pumped and passed to the surface for recovery. This method is to a larger degree less disruptive to the environment compared to conventional mining.

Tank Leaching

Tank leaching is carried out in large tanks filled with crushed ore and the leaching solution. Such a procedure allows far greater control of leaching conditions than other procedures and is used for more complex ores.

Vat Leaching

Following is the process: Ore used to be placed in large vats or containers, and it came into direct contact with the leaching solution that had flooded them. This process is normally used in the case of high-grade ores, which require intensive processing.

3. Relevance and Applications

Industrial Applications

This process of leaching is used in vast amounts in the mining industry for the extraction of noble metals – such as gold and silver along with base metals like copper and iron. Leaching is comparatively a cheap and efficient technique that suits the extraction of noble metals; leaching becomes very necessary when the ore to be treated is of low grade. Leaching is also used in the extraction of heavy metals from electronic wastes by leachates in the recycling industry for resource management and pollution control.

Academic Research

The study of leaching processes today within the halls of academia is directed toward the objective of reducing the environmental impact as well as developing new methods that can effectively improve extraction. Optimally, sure the kind of research today must be all about optimization for leaching conditions, understanding of the underlying chemical reactions, and figuring out green solvents that could replace the hazardous chemicals.

Environmental Impact

Essentially, leaching processes would harbor very ill-fated hazards to the environment if good work practices are not maintained. Leaching is one more hazard of the chemicals involved throughout the whole process. Cyanide in leaching gold requires careful handling and disposal to avoid any water source pollution. In fact, studies take the lead in identifying suitable application methods of leaching agents to promote safe practices and proper handling of wastes to reduce these impacts.

Some Solved Examples

Example 1

Question: Which one of the following benefaction processes is used for the mineral Al₂O₃⋅2H₂O?

Froth floatation
Leaching
Liquation
Magnetic separation

Solution: Leaching is done to concentrate ore. In this method, the powdered ore is treated with a suitable chemical reagent that dissolves the ore while impurities remain insoluble in that reagent. For example, Bauxite is separated from Fe₂O₃,SiO₂, and TiO₂ with the help of NaOH, in which Al₂O₃ gets dissolved while the rest are insoluble. The chemical reactions involved are:

Al₂O₃(s)+2NaOH(aq)+3H₂O(l)→2Na[Al(OH)₄]

The sodium aluminate present in the solution is neutralized by passing CO₂gas, and hydrated Al₂O₃ is precipitated. This process is known as seeding. The reaction is:

2Na[Al(OH)₄](aq)+CO₂(g)→Al₂O₃⋅xH₂O(s)+2NaHCO₃(aq)

Hence, the correct answer is option (2).

Example 2

Question: In the isolation of which one of the following metals from their ores, the use of cyanide salt is not commonly involved?

1)Zinc

2)Gold

3)Silver

4) (correct)Copper

Solution:

Silver and Gold are directly reached out using NaCN.

NaCN is used as a depressant in a mixture of ZnS and PbS.

Cyanide salt is not involved in the extraction of copper.

Hence, the answer is the option (4).

Example 3

Question: Match List - I with List - II.

List - I (A) Concentration of gold ore
(B) Leaching of alumina
(C) Froth stabilizer
(D) Blister copper

List - II (I) Aniline
(II) NaOH
(III) SO₂
(IV) NaCN

Choose the correct answer from the options given below.

(A) - (IV), (B) - (III), (C) - (II), (D) - (I)
(A) - (IV), (B) - (II), (C) - (I), (D) - (III)
(A) - (III), (B) - (II), (C) - (I), (D) - (IV)
(A) - (II), (B) - (IV), (C) - (III), (D) - (I)

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Solution:

(A) NaCN is used for the concentration of gold ore. (IV)
(B) Leaching of alumina is done by NaOH (II)
(C) Froth stabiliser →rightarrow→ Aniline (I)
(D) Blister copper →rightarrow→ due to the evolution of SO₂ (III)

Hence, the correct answer is option (2).

Summary

Leaching is one of the basic chemical processes by which alternatives are made in the extraction of appreciated metals from various materials. The process is certainly applicable at the industrial and environmental levels, with its greatest utility found where knowledge of heap, in-situ, tank, and vat leaching routines is applied to mining, recycling, and academic work. The dissolution of metals in the process makes it a process of metal recovery, with metals getting dissolved and then refined with a solvent. This method has, however, been widely applied in the recovery of metals from low-grade ores and electronic wastes.

Frequently Asked Questions (FAQs)

1. What are some alternatives to cyanide leaching for gold extraction?

Alternatives to cyanide leaching include thiosulfate leaching, thiourea leaching, and halide leaching. These methods aim to reduce environmental and safety concerns associated with cyanide while maintaining extraction efficiency.

2. How does bacterial leaching differ from chemical leaching?

Bacterial leaching, also known as bioleaching, uses microorganisms to extract metals from ores. These bacteria oxidize sulfide minerals, releasing metals into solution. Chemical leaching, on the other hand, uses chemical reagents to dissolve metals directly.

3. How does pressure leaching differ from atmospheric leaching?

Pressure leaching is conducted in sealed vessels under high pressure and temperature, which can significantly increase reaction rates and metal solubility. Atmospheric leaching occurs at normal pressure and typically lower temperatures, resulting in slower but less energy-intensive processes.

4. How is copper extracted using the leaching process?

Copper is often extracted by heap leaching using sulfuric acid as the lixiviant. The acid dissolves copper minerals, forming a copper sulfate solution. This solution is then processed using solvent extraction and electrowinning to recover pure copper.

5. What is the role of oxidation in leaching processes?

Oxidation plays a crucial role in many leaching processes by converting insoluble metal compounds into soluble forms. For example, in gold leaching, oxygen is necessary to form the soluble gold-cyanide complex.

6. What are the main advantages of leaching over pyrometallurgical processes?

The main advantages of leaching over pyrometallurgical processes include lower energy consumption, reduced air pollution, the ability to process low-grade ores, and the potential for in-situ extraction without mining.

7. What is heap leaching, and how does it work?

Heap leaching is a method where crushed ore is piled in heaps on an impermeable pad, and a lixiviant is sprayed over the top. As the solution percolates through the heap, it dissolves the metal, which is then collected at the bottom for further processing.

8. What is the role of lixiviants in the leaching process?

Lixiviants are the chemical solutions used to dissolve the desired metal from the ore. They play a crucial role in selectively extracting the target metal while leaving behind unwanted materials. The choice of lixiviant depends on the metal and ore composition.

9. Can you explain the concept of in-situ leaching?

In-situ leaching is a method where the lixiviant is injected directly into the ore body underground, dissolving the metal without the need for traditional mining. The metal-rich solution is then pumped to the surface for processing, minimizing environmental disturbance.

10. Why is cyanide commonly used in gold leaching?

Cyanide is used in gold leaching because it forms stable, water-soluble complexes with gold, allowing for efficient extraction. It is highly selective for gold and silver, leaving behind most other metals, which makes it effective for processing low-grade ores.

11. What is leaching in the context of metal extraction?

Leaching is a process used to extract metals from their ores by dissolving them in a suitable solvent. The metal-containing solution is then separated from the remaining solid material, allowing for the recovery of the desired metal.

12. Why is leaching considered a hydrometallurgical process?

Leaching is considered a hydrometallurgical process because it uses aqueous solutions to extract metals from ores. The term "hydro" refers to water, and "metallurgy" is the science of extracting metals from their ores.

13. How does the leaching process differ from smelting?

Leaching is a chemical process that uses solvents to dissolve and extract metals at relatively low temperatures, while smelting is a high-temperature process that melts the ore to separate the metal. Leaching is typically more selective and can be used for lower-grade ores.

14. What factors affect the efficiency of the leaching process?

The efficiency of leaching is affected by factors such as particle size of the ore, temperature, pressure, concentration of the lixiviant, agitation, and contact time between the ore and the lixiviant.

15. How does particle size of the ore impact leaching efficiency?

Smaller particle sizes generally increase leaching efficiency by providing a larger surface area for the lixiviant to contact the ore. This allows for faster and more complete dissolution of the desired metal.

16. What is the significance of pH in leaching processes?

pH is crucial in leaching as it affects the solubility of metals and the stability of complexes formed during the process. Different metals require different pH conditions for optimal extraction, and pH control is essential for selective leaching.

17. How does temperature affect the leaching process?

Higher temperatures generally increase the rate of leaching by accelerating chemical reactions and improving the solubility of metals. However, excessive temperatures can lead to unwanted side reactions or decomposition of the lixiviant.

18. How does the presence of gangue minerals affect leaching efficiency?

Gangue minerals can negatively impact leaching efficiency by consuming reagents, interfering with metal dissolution, or forming precipitates that coat ore particles. Understanding gangue mineralogy is crucial for optimizing leaching processes.

19. How does the concept of "liberation" relate to leaching efficiency?

Liberation refers to the degree to which the desired mineral is physically separated from gangue minerals. Better liberation, typically achieved through finer grinding, exposes more of the target mineral to the lixiviant, improving leaching efficiency.

20. How does the presence of clay minerals affect leaching operations?

Clay minerals can negatively impact leaching by absorbing reagents, increasing solution viscosity, and causing permeability issues in heap leaching. They may also form slimes that interfere with solid-liquid separation after leaching.

21. What is meant by "refractory ores" in the context of leaching?

Refractory ores are those that resist standard leaching processes, often due to the metal being finely disseminated in the ore matrix or associated with minerals that prevent its dissolution. These ores typically require pre-treatment before effective leaching.

22. How does the concept of "kinetics" apply to leaching processes?

Kinetics in leaching refers to the rate at which the metal is dissolved from the ore. Understanding leaching kinetics is crucial for optimizing process parameters such as residence time, reagent concentration, and temperature to achieve maximum metal recovery.

23. What are the environmental concerns associated with leaching processes?

Environmental concerns include potential contamination of groundwater and soil from leaching solutions, generation of acid mine drainage, and the use of toxic chemicals like cyanide. Proper containment, treatment of waste solutions, and use of more environmentally friendly reagents are important considerations.

24. What is the role of oxidation-reduction potential (ORP) in leaching processes?

ORP is a measure of the solution's ability to oxidize or reduce species. In leaching, controlling ORP is crucial for maintaining the right chemical environment for metal dissolution and preventing unwanted reactions or precipitations.

25. How does the presence of organic matter in ores affect leaching processes?

Organic matter can interfere with leaching by consuming reagents, forming complexes with metals, or coating mineral surfaces. In gold leaching, it can lead to preg-robbing. Pre-treatment methods like roasting may be necessary to remove organic matter before leaching.

26. What is the significance of "recovery" versus "grade" in leaching processes?

Recovery refers to the percentage of metal extracted from the ore, while grade refers to the concentration of metal in the pregnant solution. Balancing these factors is crucial, as high recovery might come at the cost of lower grade, affecting downstream processing efficiency.

27. What is the role of surfactants in some leaching processes?

Surfactants can be added to leaching solutions to improve wetting of ore particles, reduce surface tension, and enhance the contact between the lixiviant and the mineral surface. This can lead to improved leaching kinetics and overall metal recovery.

28. How does the concept of "leach residue" relate to environmental management in mining?

Leach residue, or tailings, is the solid material left after leaching. Proper management of leach residues is crucial for environmental protection, as they may contain residual metals, acids, or other potentially harmful substances. Neutralization, stabilization, and proper disposal are important considerations.

29. How does the presence of soluble salts in ores affect leaching processes?

Soluble salts can impact leaching by altering solution chemistry, affecting pH, and potentially forming precipitates that hinder metal extraction. Pre-washing of ores to remove soluble salts may be necessary in some cases to optimize leaching efficiency.

30. What is the significance of "acid consumption" in leaching processes?

Acid consumption refers to the amount of acid used up by reactions with gangue minerals rather than the target metal. High acid consumption can make a process uneconomical and may require alternative leaching methods or pre-treatment of the ore.

31. How does the concept of "leachability" differ from "solubility" in the context of metal extraction?

Leachability refers to how easily a metal can be extracted from its ore under specific leaching conditions, while solubility is the maximum amount of a substance that can dissolve in a given solvent. A metal may be soluble but not easily leachable due to mineralogical constraints.

32. How does the concept of "leach kinetics" relate to the design of industrial leaching circuits?

Understanding leach kinetics helps in designing the size and number of leaching tanks, determining residence times, and optimizing reagent addition. It ensures that the leaching circuit is designed to achieve maximum metal extraction within practical operational constraints.

33. What types of ores are most suitable for leaching processes?

Leaching is particularly suitable for low-grade ores, oxidized ores, and ores containing metals that form soluble compounds easily. It's commonly used for copper, gold, uranium, and some rare earth elements.

34. What is the purpose of agitation in leaching processes?

Agitation in leaching processes helps to increase the contact between the ore particles and the lixiviant, promotes uniform mixing, prevents settling of solids, and enhances mass transfer. This leads to improved leaching efficiency and faster extraction rates.

35. What is meant by "preg-robbing" in gold leaching, and how can it be mitigated?

"Preg-robbing" occurs when certain carbonaceous materials in the ore adsorb dissolved gold complexes, reducing recovery. It can be mitigated by adding activated carbon to the leach solution (carbon-in-leach process) or by pre-treating the ore to deactivate the preg-robbing components.

36. What is the difference between percolation leaching and agitation leaching?

Percolation leaching involves the slow passage of a lixiviant through a stationary bed of ore (e.g., heap leaching). Agitation leaching involves mixing finely ground ore with the lixiviant in tanks or vats, providing more intense contact between the ore and solution.

37. How does the leaching of oxide ores differ from sulfide ores?

Oxide ores are generally more amenable to leaching as they dissolve more readily in acidic or alkaline solutions. Sulfide ores often require oxidation (either chemical or bacterial) to convert the sulfides to more soluble forms before effective leaching can occur.

38. What is the role of complexing agents in leaching processes?

Complexing agents form stable, soluble complexes with metal ions, enhancing their solubility and facilitating their extraction. For example, cyanide acts as a complexing agent in gold leaching, forming the soluble [Au(CN)2]- complex.

39. What is the purpose of "washing" or "rinsing" in leaching processes?

Washing or rinsing in leaching processes is done to recover residual dissolved metal from the leached ore (tailings). This step helps maximize overall metal recovery and reduces metal loss in the discarded solids.

40. How does the leaching of uranium differ from other metals?

Uranium leaching often uses alkaline solutions (e.g., sodium carbonate) rather than acidic ones. This is because uranium forms stable carbonate complexes in alkaline conditions, allowing for selective extraction from ores containing acid-consuming gangue minerals.

41. What is the significance of "pregnant solution" in leaching processes?

The "pregnant solution" is the metal-rich solution produced after leaching. It contains the dissolved target metal and is the intermediate product that undergoes further processing (e.g., solvent extraction, precipitation) to recover the pure metal.

42. How does the concept of "selectivity" apply to leaching processes?

Selectivity in leaching refers to the ability of the process to dissolve the desired metal while minimizing the dissolution of unwanted elements. High selectivity is crucial for producing a clean pregnant solution and reducing impurities in the final product.

43. How does the leaching of rare earth elements differ from base metals?

Rare earth element leaching often requires more aggressive conditions due to the stability of their minerals. It may involve the use of strong acids, high temperatures, or multiple leaching stages. The separation of individual rare earth elements is also more complex than for base metals.

44. How does the concept of "passivation" impact leaching efficiency?

Passivation occurs when a protective layer forms on the mineral surface, preventing further reaction with the lixiviant. This can significantly reduce leaching efficiency. Understanding and preventing passivation is crucial for maintaining high metal extraction rates.

45. What is the principle behind "sequential leaching" and when is it applied?

Sequential leaching involves using different lixiviants or conditions in stages to selectively extract different metals from complex ores. It's applied when ores contain multiple valuable metals that require different extraction conditions, allowing for more efficient overall recovery.

46. What is the role of "conditioning" in some leaching processes?

Conditioning involves pre-treating the ore or adjusting the leaching solution to optimize extraction. This may include pH adjustment, addition of oxidants, or pre-leaching to remove interfering substances, all aimed at improving the efficiency of the main leaching stage.

47. How does the presence of clay minerals affect solution viscosity in leaching processes?

Clay minerals can increase solution viscosity, which can lead to poor mixing, reduced percolation rates in heap leaching, and difficulties in solid-liquid separation. This can significantly impact the efficiency of leaching operations and downstream processing.

48. What is the principle behind "resin-in-pulp" leaching?

Resin-in-pulp leaching involves adding ion exchange resins directly to the leaching pulp to adsorb dissolved metals as they are leached. This can improve overall recovery by capturing metals immediately upon dissolution, reducing losses and simplifying the process flow.

49. What is the significance of "preg-borrowing" in gold leaching processes?

"Preg-borrowing" occurs when gold adsorbed onto carbonaceous material in the ore is re-released into solution later in the leaching process. While less detrimental than preg-robbing, it can complicate process control and metal accounting in gold leaching operations.

50. How does the concept of "heap porosity" affect the efficiency of heap leaching operations?

Heap porosity influences the flow of leaching solution through the ore pile. Good porosity ensures even distribution of the lixiviant, promoting contact with ore particles and efficient metal extraction. Poor porosity can lead to channeling, reduced contact time, and lower overall recovery.