Mass Transfer Design CCB2053

Dr Sintayehu Mekuria Hailegiorgis Department of Chemical Engineering

CCB2053

Solid-liquid Extraction (Leaching)

Lesson outline

• Introduction

• Mode of leaching operations and equipment for leaching

• Working principles of solid-liquid processes

• solid-liquid equilibrium

Lesson outcome

At the end of the session, the students are able to:

Discuss the principles of solid-liquid processes and basic design considerations.

Able to estimate the exit stream amounts and compositions of single stage solid-liquid extractor.

Recap of pervious lesson

CCB2053

Determination of multiple stages required for a desired separation using

• Material balance and graphical method

Countercurrent Multiple-Contact Stages extraction processes

Introduction • Solid-liquid separation/Leaching: A process that involves treatment of a

finely divided solid with a liquid that dissolves out and remove a solute contained in the solid.

• Biological and food industries

sugar from sugar beet; hot water solvent

vegetable oils from nuts and seeds; organic solvents (hexane, ether)

Pharmaceutical products; water/organic solvents

• Solute(s) from solid medium diffuses into the liquid (solvent) upon intimate contact with the liquid (solvent)

Application

• Fluid is used to extract out a solute from a solid.

• Inorganic and organic materials

Metal processing soluble salts

Copper salts; sulphuric acids /ammoniacal solvents

Gold; sodium cyanide solvent

If solvent is water also called as washing

• Solids must be prepared for extraction/leaching Grinding/crushing

metals, inorganic materials

• Minimizing diffusion surface Cut/chop – food Drying – pharmaceutical, food Rolling/flaking – food

Raw material preparation for leaching processes

Overall Process

• Bulk solvent solution to solid surface • Solvent diffuses into solid • Solute dissolves into solvent • Solute diffuses to surface • Solute transferred to bulk solution

Overall Process • Key leaching processes Rate of mass transfer, specifically diffusion • Dissolution rate of mass transfer from solid to

solvent controls • For pure solid or very rapid solid diffusion,

)( AASLA cck

AN

−=

mass transfer coefficient

Concentration Saturation solubility of solid

particles surface area

Kg mol of A dissolving to the solution

6.1

• From material balance, the rate of accumulation of A in the solution is equal to the rate of A that dissolves from the inert solid (B), thus;

)( AASLAA ccAkN

dtVdC

−==

• Integrating from t=o to t=t and from CA=CAo to CA=CA

∫∫ ==

−

t

tLC

CAAS

A dtV

AkCC

dCA

Ao 0

• Solving;

tV

Ak

AoAS

AASL

eCCCC

−

=−−

6.2

6.3

6.4

Mode of Leaching Operations

• Batch operations • Continuous stage operations

– steady state – unsteady state

Equipment Types Fixed bed leaching

Solvent

Solute solution

Solute Solid Bed

Moving bed leaching Bucket type

Moving bed leaching screw conveyor

Agitated Solid leaching Countercurrent contactor-agitator settler

(Overflow) (Underflow)

Equilibrium Relations

Solute free solids – insoluble in solvent

Sufficient solvent to dissolve all solute – in first stage

No adsorption of solute by solid

Some liquid solution will be retain in the solid – slurry stream

• To analyze single stage or multiple stage leaching processes, material balance or an operating line equation that relates the equilibrium between the two streams are needed.

• For equilibrium analysis, the following assumptions are made:

• Consequently, Solute concentration at overflow equals that at underflow x-y plot has a 45o equilibrium line

• Experimental equilibrium data showing the variation of the amount and composition of the solution retained in the solid as a function of the solute composition must be obtained.

• equilibrium data can be plotted on the rectangular diagram as weight fraction (wt) for the three components, ie., solute (A), inert of leached solid (B) and solvent (C)

• The two phases are the overflow liquid phase and the underflow slurry phase.

• Another convenient method of plotting the equilibrium data which is similar to enthalpy-concentration method of distillation processes can be used.

• With the three basic components – solute (A), inert solid (B) and solvent (C);

• Let N be concentration of inert solid, B,

solution kgsolid kg

kg kg kg

=+

=CA

BN 6.5

Where: N = 0 in the overflow and N = varies in the underflow

Similarly, the composition of solute A in the liquid overflow and underflow will be expressed as

solution kgsolute kg

kg kg kg

=+

=CA

AxA

solution kgsolute kg

kg kg kg

=+

=CA

AyA

In overflow

In underflow

6.7

6.6

Equilibrium Diagram • When solute A is infinitely soluble in solvent C,

the upper curve of N versus yA for the slurry underflow that represents the separated solid under experimental conditions is similar to the actual stage processes as shown in the Figure.

• the bottom layer of N verses xA , where N=0 on the axis, represents the overflow liquid composition where all the solid has been removed.

• In such cases the tie line are vertical, and lies on x-y diagram (45o line). The equilibrium line for the two phases coincides with the yA = xA on the 45o line.

• Example: in the system soybean oil (A) –soybean inert

solid meal (B)-hexane solvent.

Equilibrium Relations

• During the leaching process : if there is no insufficient contact time, so

that all the solute is not dissolved ;

adsorption of A on the solid will occur or solute soluble on B.

• In such a case, the equilibrium diagram is as shown in the figure and the tie lines are not vertical.

Next lesson

• Single stage leaching process • Multiple stage leaching processes