Solid Liquid Extraction (Leaching)

Ola Akinsunmade

Chemical Engineer

What is Solid-Liquid Extraction?

Solid-Liquid Extraction (SLE), otherwise known as Leaching, is a solvent extraction separation technique which involves the dissolution of a solute attached to an insoluble solid phase via an extraction solvent.

Leaching operations typically occur in the following three stages [1]:

Dissolution of solute (soluble constituents)
Separation of solution from solid residue
Washing of solid residue

Just as is the case in Liquid-Liquid Extraction (LLE), separation is driven mainly by solute solubility differences between the carrier and solvent phases whilst extraction is driven by phase immiscibility. It is commonly used in the extraction of oils from seeds and leaching of metal ores.

Factors Affecting Leaching Extraction Rate

Factors that impact the efficiency of a leaching process include:

Particle size
Solvent
Temperature
Agitation

Particle size: Smaller particle size increases interfacial area resulting in a higher rate of mass transfer. Mass transfer is impeded in the leaching of very fine particles if some form of liquid agitation is not introduced due to high settling velocities.
Solvent: An ideal solvent will have a high saturation limit, high selectivity for the desired solute and a low viscosity to allow for free circulation.
Temperature: In most cases, a higher extraction temperature increases the solute solubility in the solvent.
Agitation: As is in the case of LLE, agitation increases eddy diffusion and mass transfer across phases. Agitation is also vital in the leaching of fine particles, preventing sedimentation and creating an effective use of the interfacial area [1].

Solid-Liquid Extraction Equipment

Below are a few examples of equipment used industrially for solid-liquid extraction:

Bollman-type Extractor
Batch Tank
Dorr Classifier
Kennedy Extractor
Bonotto Extractor
De Smet Belt Extractor
Rotocel Extractor
Pachuca Tanks
Continuously Stirred Tank Reactors (CSTR)
Autoclaves

Equipment used in solid-liquid extraction is primarily dependent on the nature (granular or cellular) and texture (coarse or fine) of the solid phase in the process. The texture of the solid phase becomes a particularly important design consideration when selecting the ideal solvent contacting method.

Finer solid particles offer a higher resistance to solvent percolation due to their higher settling velocities. As a result, solid-liquid extraction is performed in a dispersed solids leaching system often requiring some form of agitation to prevent sedimentation.

In general, industrial leaching systems are distinguished and classified by the following operating design choices [2]:

Operating Cycle (batch, continuous, multi batch)
Direction of Streams (concurrent, countercurrent, hybrid)
Staging (single-stage, multistage, differential stage)
Contacting Method (sprayed percolation, immersed percolation, solids dispersion)

Here’s a video of rake classifier in operation:

Classifications of Common Leaching Equipment and Applications

	Operating Cycle	Direction of Streams	Staging	Method of Contacting	Application Examples
Bollman-type Extractor	Continuous	Hybrid	Multistage	Sprayed Percolation	Extraction of soybean oil [1]
Batch Tank	Batch	N/A	Single-stage	Sprayed Percolation	Oxide ore processing (prior to 1980)[2]
Dorr Classifier	Continuous	Hybrid	Single-stage	Percolation	Wastewater processing, Leaching of metal ores
Kennedy Extractor	Continuous	Countercurrent	Multistage	Immersed Percolation	Extraction of flaked oil seeds [1]
Bonotto Extractor	Continuous	Countercurrent	Multistage	Solids Dispersion	Extraction of seed oils
De Smet Belt Extractor	Continuous	Countercurrent	Multistage	Spray Percolation	Extraction of seed oils
Rotocel Extractor	Continuous	Hybrid	Multistage (compartmented)	Sprayed Percolation	Extraction of soybean oil [1]
Pachuca Extractor	Batch	N/A	Single-stage	Solids Dispersion	Mineral processing (prior 1960)[2]
Autoclaves	Continuous/Batch	N/A	Single/Multistage (compartmented)	Solids Dispersion	Sulfuric acid leaching of cobalt and nickel from laterite ores [2]

Equilibrium in a Solid-Liquid Extraction

In a typical continuous single or multistage leaching process, the stream containing the solid phase is called the underflow whilst the solute-rich stream is referred to as the overflow stream.

Equilibrium in a solid-liquid stream is established when the concentration of solute in the underflow and overflow streams leaving a stage are equal.

This is with the assumption that the solute is not adsorbed to the leached solid and is completely dissolved and distributed in both streams.

Where:

V1 = Overflow Solution Flow

V2 = Solvent Feed Flow

L0 = Feed Solid/Slurry Flow

L1 = Underflow Solution Flow

xA,1 = Solute Composition in Overflow

xA,0 = Solute Composition in Solvent Feed

yA,0 = Solute Composition in Feed Slurry

yA,1 = Solute Composition in Underflow

The equilibrium concentration of the solute in the overflow and underflow stream is represented on the x-y plot as a 45 degree line.

The equilibrium plot is typically represented by plotting the ratio of the entrained solids in a stream/solution (N) with respect to the solute and solvent mass versus the solute equilibrium concentration (x, y).

References

D. G. Peacock and J. F. Richardson, “Leaching,” in Coulson & Richardson Chemical Engineering, Oxfors: Butterworth-Heinemann, 2012.
J. M. Coulson and J. F. Richardson, “Liquid Liquid Extraction,” in Particle Technology and separation processes, Oxford: Pergamon Press, 1998.
D. Michaud, “Rake classifier,” Mineral Processing & Metallurgy, 13-May-2021. [Online]. Available: https://www.911metallurgist.com/blog/rake-classifier. [Accessed: 04-May-2022].

One Response

Joellen Alicea says:

December 16, 2023 at 7:30 pm

Hi theengineersperspectives.com admin, Your posts are always well-formatted and easy to read.

Reply