cdb2013 leaching lecture 1
DESCRIPTION
Separation Process LeachingTRANSCRIPT
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