Separation Process ICDB2013

Dr Chew Thiam Leng

Chapter –Solid-Liquid

Leaching

Lesson outcome

• Introduction

• Mode of leaching operations and equipment for leaching

• Working principles of solid-liquid processes

• solid-liquid equilibrium

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

Discuss the principles of solid-liquid processesand basic design considerations.

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

Lesson outcome

Recap of pervious lesson

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 involvestreatment of a finely divided solid with a liquid that dissolves outand 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.

• 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

• Solute transferred to bulk solution

• Solvent diffuses into solid

• Solute dissolves into solvent

• Solute diffuses to surface

Solute

Solid

Solvent

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

A

N

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 thesolution is equal to the rate of A that dissolves from the inertsolid (B), thus;

)( AASLAA ccAkN

dt

VdC

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

t

t

LC

CAAS

A dtV

Ak

CC

dCA

Ao 0

• Solving;

tV

Ak

AoAS

AAS

L

eCC

CC

6.2

6.3

6.4

Mode of Leaching Operations

Batch operations

Continuous stage operations

• steady state

• unsteady state

Equipment for Leaching

Fixed bed leaching

Solvent

Solute solution

Solute

Solid

Bed

Movable

Cover

Movable

Bottom

Equipment for Leaching

Moving bed leaching

Bucket type

Equipment for Leaching

Moving bed leaching

screw conveyor

Equipment for Leaching

Agitated Solid leaching

Countercurrent contactor-agitator settler

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 theequilibrium 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 theamount and composition of the solution retained in the solidas 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 or leached solid (B) and solvent (C)

• The two phases are theoverflow liquid phase andthe underflow slurry phase.

• Another convenient methodof plotting the equilibriumdata which is similar toenthalpy-concentrationmethod of distillationprocesses can be used.

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

• Let N be concentration ofinert solid, B,

solution kg

solid 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 overflowand underflow will be expressed as

solution kg

solute kg

kg kg

kg

CA

AxA

solution kg

solute kg

kg kg

kg

CA

AyA

In overflow

In underflow

6.7

6.6

Equilibrium Diagram

• When solute A is infinitely solublein solvent C, the upper curve of Nversus yA for the slurry underflowthat represents the separated solidunder experimental conditions issimilar to the actual stage processesas shown in the Figure.

• the bottom layer of N verses xA ,where N=0 on the axis, representsthe overflow liquid compositionwhere all the solid has beenremoved.

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

Equilibrium Relations

During the leaching process :

if there is no insufficient contacttime, so that all the solute is notdissolved ;

adsorption of A on the solid willoccur or solute soluble on B.

In such a case, the equilibriumdiagram is as shown in thefigure and the tie lines are notvertical.

Next lesson

Single stage leaching process

Multiple stage leaching processes