My previous article was about Liquid-Liquid Extraction. Let's check another side of Extraction, the second type of extraction which is Solid-Liquid Extraction.
Solid-Liquid Extraction is also one of the most important mass transfer operations. Unlike Liquid-Liquid extraction, Solid Liquid is a mass Transfer from solid to Liquid phase.
Solid-liquid extraction has its application in many fields such as Chemical, Petroleum, Food, Pharmaceutical, etc. For example, coffee making also works on the principles of Solid-Liquid Extraction.
What to Know How?
Read the full Post to learn that also much more.
What is Solid-Liquid Extraction
Solid-Liquid Extraction is a solid-liquid contact mass transfer operation in which solute particles are transferred from solid to liquid.
Solid-liquid extraction is also called Leaching.
Solid-liquid extraction works on the principle of difference in solubility of specified solids in liquids.
The liquid used for solid-liquid extraction is called the solvent. The Solid which carries solute particles is called an insoluble solid.
Step Involved in Solid-Liquid Extraction
Solid-liquid extraction there are steps
- Size reduction of solids
- Mixing of solids with solvent Liquid
- Overflow and underflow separation
- Solvent recovery from overflow and underflow
Method of Operation of Solid-Liquid Extraction
First of all, solids are crushed and pulverised to make them of the desired size. As the size of solid particles reduces, their surface area increases which make contact with liquid easier and more amount of mass transfer takes place.
Then pulverised solid is mixed with liquid solvent in a tank with an agitator. Agitation takes place inside the tank continuously to mix solid thoroughly with liquid and mass transfer from solid to liquid takes place.
After that clear liquid is taken off from the top of the mixture and slurry is taken out from the bottom.
Clear liquid taken off from the top is called overflow which contains solvent with solute transferred from solid.
From the bottom, the slurry taken out is called underflow. Underflow is consists of an insoluble solid with a solute that is not dissolved in a solvent and a small amount of solvent.
Recovery of solute from overflow and also the separation of solvent from underflow takes place.
Solvent recovered from overflow and underflow is mixed and recycled back to mixing tank.
Factors Affect Solid-Liquid Extraction Operation
- Solid: Solid used for solid-liquid extraction can be porous or nonporous. The solute may be distributed on a solid surface or inside pores of solid. Recovery of solute from the solid surface is easier than from pores of solid. Pores of solid create another mass transfer resistance for the solvent to go in and for the solute to come out. Solid particle's size also plays an important role in the operation. Recovery of solute from small size solid particles are easier and more effective than bigger particle sizes.
- Solvent: The solvent used for operation is also an important parameter to take into consideration in operation. The solvent which contains desirable properties which are listed below is more preferred.
- Temperature: Temperature is also an important parameter in solid-liquid extraction because the solubility of solid particles is also dependent on temperature.
- Mixing: Mixing of solid particles and liquid solvent decides the effectiveness of contact which is also important for parameter solid-liquid extraction. More thorough mixing confirms more contact between solid and liquid and this results in higher mass transfer.
Desired Solvent Characteristics for Solid-Liquid Extraction
- The solubility of the solute in solvent should be higher.
- The solute dissolving capacity of solvent should be high.
- The solubility of insoluble solid should be very low or zero (Ideally).
- The solvent should be less viscous.
- Recovery of solvent from overflow and underflow should be easy.
- The solvent should be non-toxic.
- The solvent should be non-flammable
- The solvent should be cheap and easily available.
Operation of Solid-Liquid Extraction
Solid-liquid extraction can be used batch-wise, semi-batchwise and also continuous operations.
Also, solid-liquid extraction can be operated at a steady or unsteady state.
Classification of Solid-Liquid Extraction
There are major two types of Solid-Liquid Extraction processes based on resistance
- Diffusional Extraction: In this type of extraction, mass transfer resistance inside a solid is higher. So diffusion of a solvent through pores of solid controls overall mass transfer operation. For example, Sugar recovery from sugar beet is done by using hot water as solvent. Hot water diffusion through sugar beet pores determines the sugar recovery from it.
- Washing Extraction: In this type of extraction, mass transfer resistance of liquid film outside solid surface is higher which determines the rate of mass transfer. The diffusion of solute from the liquid film is higher which makes the process such as washing. In washing extraction solvent amount in clear liquid is the same as inside slurry.
Uses of Solid-Liquid Extraction
- Extraction of soluble coffee from coffee beans by using water.
- Extraction of fish oil from fish by using hexane as solvent.
- Extraction of sugar from sugar beet by using hot water.
- Recovery of vegetable oil from oilseed by using hexane as solvent.
- Recovery of flowery odour from flowers by using ethanol as solvent.
- Recovery of vanilla from vanilla beans by using the ethanol-water solution.
- Phosphoric acid production and recovery from phosphate rocks by Sulphuric acid
- Gold recovery from its ore by sodium cyanide.
- Copper recovery from its ore by Sulphuric acid.
- Separation of lignin from wood chips by using sodium hydroxide or sulphide or sulphate solutions.
References
- Principles of Mass Transfer and Separation Processes by Binay K. Dutta
- Mass Transfer: Theory and Practice by N. Anantharaman and K. M. Meera Sheriff Begum