Leaching is a method by which inorganic, organic, or radionuclides diffuse from the solid phase into the aqueous phase by mineral dissolution, desorption, and complexation processes.
What is leaching?
It is a fundamental extraction operation in hydrometallurgical processing that involves transferring a metal of interest from naturally occurring minerals into an aqueous solution. In simple words, it entails selectively dissolving precious minerals by subjecting the ore to an active chemical solution known as a leach solution.
It is one of the most useful in the metallurgical industry for extracting metals from Al, Ni, Co, Mn, and Zn ores. In addition, it’s used to make tea and coffee, extract many pharmaceuticals from plant roots and leaves, extract sugar from sugar beets using hot water, extract oil from oil seeds using organic solvents, and remove tannin from different tree barks by leaching with water. The leaching procedure is used to remove chemicals from solids. This occurs when the given substance is allowed to dissolve in a liquid. It is done either naturally or in an industrial setting.
Different solvents, or lixiviants, can be employed in the leaching process in metallurgy depending on the target metal and its original matrix. The metals are removed from the initial matrix by the lixiviant. Lixiviants can either be basic or acidic. Lixiviants differ in terms of pH, redox potential, and chelating agents, which can speed up or improve the selectivity of the metal’s dissolution.
Sulfates can be leached with either water or sulfuric acid, whereas oxides must be removed using a sulfuric acid or sodium carbonate solvent. Sodium hydroxide is utilized for oxides, while ammonium hydroxide is used for native ores, carbonates, and sulfides. The precious metals are dissolved in cyanide solutions, while certain chlorides are dissolved in sodium chloride solutions. The leach solvent must always be strong, readily available, inexpensive, and preferably selective for the values present.
The process of releasing solid material constituents into a contacting fluid phase is known as leaching. It is indifferent to all constituents—such as major and minor matrix components, as well as inorganic, organic, and radionuclide contaminant. They are released by means of a standard series of chemical processes that include mineral dissolution, desorption, complexation, and mass transport processes.
A soluble chemical is extracted from a solid matrix by this process. The solid matrix can be a body of ore, a mineral deposit, a formation that holds water, or a landfill. Metals like copper, or non-metals, like arsenic, can be soluble substances.
A solvent, such as water, acid, or air, is passed through the solid matrix to achieve leaching. The soluble material is dissolved and removed by the solvent. It is frequently used to purify wastewater, recover metals from ores and minerals, clean up landfills and water-bearing formations, and eliminate contaminants.
Types of leaching
Physical leaching is a simple procedure for solute dissolution in a solvent. In this method, the solvent is frequently made up of water, alcohol, and other organic solvents.
Microorganisms are used in the bioleaching procedure to dissolve insoluble metal forms. This technique employs a diverse range of microorganisms from several taxonomic groups. Initially, this method was primarily used to recover base metals from ores. This method has been used successfully to dissolve a variety of precious metals, including U, Ni, Co, Cu, and Zn.
Bioleaching provides various advantages over traditional chemical leaching methods.
- These include cost-effective in-situ leaching,
- Little energy usage,
- No need for harmful chemical compounds,
- Less secondary waste formation.
- Unique microbe strains are required for specific metals
- Conditions must be maintained for proper microbial activity
- The recovery period can be lengthy.
Chemical leaching is a process that involves the reaction of a solid with a liquid. When compared to pyrometallurgical processes, chemical leaching has shown to be more advantageous for separating metallic components from sludges because it emits no harmful gaseous compounds, does not produce particulate matter, and uses much less energy because it only uses solvents and acids. Additionally, it has much higher recovery rates and is simple to execute. Therefore, It is a well-established hydrometallurgical method that is thought to be both environmentally and economically effective for recovering metals from sludge.
For some heavy metals, such as Zn derived from low-grade oxidized ores or waste with contaminants like Cu, Cd, Ni, and Co, the alkaline leaching process is recognized as economically viable and simple to use. Alkaline leaching may be used to selectively separate and recover particular metals from industrial sludges that have been polluted with multiple metals.
Thiosulfate solutions can be used as an alternative to cyanide-based leaching solutions, which are known to be extremely hazardous and have the potential to have negative effects on the environment. Au and Ag, are abundant in the waste products produced by the cell phone industry and can be recovered with great efficiency using thiosulfate leaching.
In contrast to other anionic complexes, thiourea is a sulfur-based complexing agent that can generate soluble, cationic complexes with specific metals. This complex has a leaching rate of up to 99%, which makes it easily soluble in the solvent media.
Thus, thioureation is among the most popular leaching methods for extracting precious metals from industrial waste. Thiourea is an inexpensive, non-toxic technique that additionally provides quick leaching and few co-ion interference challenges.
Halogenic leaching agents fulfill the requirements for being the best leaching agents for metal recovery since they are selective towards target metals, economically viable, and don’t interfere with downstream processes. Au removal in a halide media is possible with high solubility, enhanced release rates, and high redox potentials.
Despite concerns about toxicity, using cyanide-based solvents is an established technique that has been successfully utilized to recover valuable metals like Au and Ag from raw ores and industrial wastes.
In this process, Cyanide ions serve as a leaching agent, whereas oxygen serves as an oxidant.
This procedure involves treating aqueous ammonia under appropriate pressure with crushed copper, nickel, and cobalt ore. As a result, certain metals are selectively leached by ammonia, creating soluble complexes of those metals.
Different methods of the leaching process
In situ leaching
The term “solution mining” also applies to this process, which involves circulating a solvent over and through the ore bed to allow minerals to be percolated and leached in place at a mine. When leaching low-grade copper, this method is used. In these experiments, the solvent or reagent is continually fed through a set of pipes that have been bored into the ore, and the resulting solution is released through a different set of pipes. Alternately, the solvent or reagent can be intermittently introduced to the ore bed and removed through the same well. This method avoids crushing and grinding the ore.
Crushed, agglomerated, and charged ore is placed in a container with an impermeable coating on top. After that, the lixiviant is irrigated on top of the heap, diffused through it, and then collected and pumped out for additional processing. This technique is typically applied to low-grade ores which cannot be treated economically by conventional methods. Heap leaching is an effective method for obtaining uranium, copper, nickel, silver, gold, and silver. Since it can be used to recover precious and base metals from waste management sites or abandoned scrap metal, heap leaching technology is crucial for the future of metal recovery.
Heap and in-situ leaching characteristics are combined in dump leaching. Depending on the dump location, an impermeable layer may or may not be employed in a dump leach. The ore is dumped to allow similar processing to heap leaching, but the geological conditions of the area allow a valley or pit to act as the sump.
Vat leaching, also referred to as “agitated tank” leaching, is a process in which the lixiviant interacts with the metal material in sizable vats or tanks that can be stirred to speed up the reaction kinetics. Crushing and grinding are frequently used to reduce the size of the metal-containing material before leaching, such as concentrate, ore, leftovers, or slag. Agitators are frequently used in vats to maintain solids suspended in the vats and increase solid-liquid interaction.
Autoclave reactors are used for reactions at higher temperatures, which might increase the rate of the reaction. Similarly, autoclaved allow the use of gaseous reagents in the system. Usually, ores and concentrates which are not suitable for atmospheric leachings, such as sulfides, are subjected to this aggressive form of leaching.
The ore is placed in a tank and treated with a leaching chemical throughout this procedure. The valuable metals are dissolved by the agent, collected, and then purified.
Separation of leach liquors
Almost all hydrometallurgical processes include the leaching of particles in order to dissolve valuable components. Before generating the final product, solid-liquid separation is frequently performed. In this stage, the leach liquor solution is separated from the solid residues using one or more methods. Common methods of separation of leach liquors involve:
This process is based on the density difference between metallic ore and contaminants. When the ore is processed with a stream of running water, lighter impurities are washed away, leaving heavier ore particles behind.
The separation of a suspension into a solid filter cake and a liquid filtrate by passing it through a permeable filtering material is known as filtration. The qualities of the suspension (e.g., size distribution, concentration), filtering materials (for example, the width and shape of pores), as well as the forces, applied to the suspension are important factors to be considered during this process.
In leaching operations, thickening is the separation of a slurry or solid-liquid mixture into a dense slurry containing the majority of the solids and an overflow of liquor. The solids in a suspension settle in a tank due to gravity and produce a thick pulp.
The pulp and clear liquid at the tank’s top can be removed continuously or occasionally.
In this method of solid-liquid separation, the leachate obtained after leaching is neutralized, and the neutralized slurry is separated and removed by adding flocculants
(A flocculant is a chemical that can be added to water to help colloids and other suspended particles combine to generate heavier particles)
Factors affecting the leaching process
Size of the inert solid
If the inert solid is finely ground and employed in the process as flakes, the solute will have no problem diffusing through the solid and becoming dissolved in the liquid. When trying to diffuse through the interior of a solid to dissolve in the majority of the liquid, a soluble solute will encounter significant resistance if the inert particles are coarse in character.
Rates of diffusion
The rate of diffusion of reactants or products can influence the rate of a leaching mechanism. When a species in the solution phase diffuses slowly to and from the mineral surface, enhancing the degree of agitation of the solution, it increases the frequency at which the species diffuses.
The porosity of the inert solid
Compared to a solid with very little porosity or one that is non-porous, an inert solid that is very porous offers less barrier for the solute present inside the solid to move through and dissolve in the liquid. In non-porous materials, the solute on the surface dissolves first, and the solute inside the solid must diffuse through the layers of resistance the solid provides to reach the liquid. It slows down the entire process.
In general, agitating a solid-liquid solution causes the solid-liquid mass transfer coefficient to rise as the agitation level rises, although caution must be taken to prevent the inert solid from disintegrating. Agitation will not significantly affect the leaching process if the majority of the mass transfer resistance is located inside the solid.
If an insoluble reaction product is generated during leaching, its rate will be determined by the nature of the product. The rate of leaching will be considerably reduced if it forms a non-porous layer. If the solid product is porous, the rate will be affected only slightly or not at all.
As the temperature rises, the solute becomes more soluble in the solvent. With an increase in temperature, the solute’s mobility or diffusivity in the solvent increases. Increased temperature causes the viscosity of the liquid to decrease, which increases the solute’s mobility through the liquid or solvent.
When it comes to dissolving the solute, the liquid utilized should be exceedingly selective. The liquid’s composition should be such that it does not react with or dissolve the carrier solid. To allow for relatively simple diffusion of the solute, it should have a low viscosity.
The concentration of the leaching agent
The rate of leaching increases as the concentration of the leaching agent increases.
Advantages of the leaching process
- The method is simple to carry out, and it aids in the greater extraction of metals and minerals from ore.
- There is less energy needed.
- It has a lower environmental impact than other mining technologies.
- The leaching chamber is easily transportable to the excavated trench.
Disadvantages of the leaching process
- The leaching process results in leftover liquid waste that is extremely acidic in composition.
- This process causes soil acidification, which has several negative impacts.
- If improper procedures are taken, water could get contaminated.
- It can be a very slow procedure, consuming extra time.
- There are numerous dangerous waste products produced, all of which need to be treated carefully.
- The temperature has a direct impact on how effectively the leaching process works.