Chap 5 Single-multiple Stage Leaching

8/10/2019 Chap 5 Single-multiple Stage Leaching

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CHAPTER 5


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The student should be able to :

Explain the definition and application ofcontinuous countercurrent leaching

Understand the equilibrium relations in leaching

Solve for single stage leaching: Solve for Multi-stage countercurrent system:

Identify the equipments used in leaching process


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What is Leaching

Leachingis the preferential solution of one or

more compounds from a solid mixture by

contact with a liquid solvent.

The desired component diffuses into the

solvent from its natural solid form.

The solvent partially dissolves the solid

material so that the desired solute can be

carried away.

Explain the definition and application of continuous countercurrent leaching


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PROCESSTO CONTACT A SOLID WITH A LIQUID

PHASE.

SIMILARTO EXTRACTION BECAUSE OF TWOIMMISCIBLE PHASES

SIMILARTO ABSORPTION BECAUSETWO

PHASES ARE NORMALLY PRESENT

DISSIMILARTO OTHERS BECAUSE ONLY

EQUILIBRIUM IN LIQUID PHASES IS CONSIDERED

Leaching Processes



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1. The solvent is transferred from

the bulk solution to the surface ofthe solid.

2. The solvent penetrates or diffusesinto the solid (intraparticle

diffusion).3. The solute dissolves from the

solid into the solvent.

4. The solute diffuses through the

mixture to the surface of the solid(intraparticle diffusion).

5. The solute is transferred to thebulk solution.

Generally there

are five rate stepsin the leaching

process:



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Typical users include

a) The metals industry for removing mineral from

ores (acid solvents).

Leaching is used to remove the metals from

their ores, which contains many undesirable

constituents, as solute salts.

A good example is gold leaching, gold isleached from its ore using an aqueous sodium

cyanide solution.


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b) the vegetable oil industry for removing oil fromsoybeans, peanuts, sunflower seeds, cotton seeds,etc. (hexane, acetone, ether or similar organicsolvents)

c) the pharmaceutical industry, many differentpharmaceutical products are obtained by leachingplant roots, leaves, and stems.

d) For the production of soluble instant coffee,

ground roasted coffee is leached with fresh water.Soluble tea is produced by water leaching of tealeaves.

Typical users include



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Assumption in leaching

1) there is sufficient solvent present so that all the

solute in the entering solid can be dissolved into the

liquidequilibrium is reached.2) When the solute is dissolved. Hence, all the solute is

completely dissolved in the first stage. There usually

sufficient time for this to occur in the first stage.

3) the solid is insoluble, and no adsorption will happenfor the solute in the solid, meaning that the solution

in the liquid phase leaving a stage is the same as

the solution remaining with the solid

Understand the equilibrium relations in leaching

EQUILIBRIUM RELATIONS and Single-

Stage Leaching


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EQUILIBRIUM RELATIONS AND SINGLE-STAGE

LEACHING

To analyze single-stageand countercurrent-stage

leaching,

An operating-line equation or material-balance

relation and the equilibrium relations between the twostreams are needed

).(

).(

).(

6912

5912

4912

1100

11112200

1120

MNLNLNB

MxVxLyVxLy

MVLVL

M

AMAAAA

Refer p.811

B= kg solid N= kg solid/kg solution solution=A+C


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Equilibrium diagrams for leaching

Since we have three components:

1) solute (A), inert or

2) leached solid (B), and3) solvent (C),

A rectangular diagram is used to showthe equilibrium data.


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The following figure shows a single-stage leaching

process where V is kg/hof overflow solution with

compositionxAand L is the kg/h of liquid in the slurry solution

with composition yAbased on a given flow rate B kg/h of dry

solute-free solid.

Single-Stage Leaching


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EQUILIBRIUM DIAGRAMS

Solute and Solvent are Infinitely

Soluble

Some of the solid is soluble in thesolvent or is in suspension

Tie lines show unequal distribution

between phases.

Variable underflow

Treybal, Mass Transfer Operations, 2ndEd.

McGraw-Hill, 1968, p. 657


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EQUILIBRIUM DIAGRAMS

Solute has limited solubilityin Solvent

xSrepresents the strongestclear solution that can be

obtained. Mixture right of PS forms

clear solution (S) and slurry(U) based on mix point M

Mixture left of PS formsclear overflow liquid




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COUNTER-CURRENT LEACHING

EQUILIBRIUM DIAGRAMS No adsorption of Solute

Constant Underflow

Solid is not soluble in the

solvent.




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SINGLE STAGE MASS BALANCES

SOLID FEED, L0 LEACHED SOLID, L1B = SOLID B = SOLID

N0, yA0 N1 , yA1

SOLVENT FEED EXTRACT

V2, xA2 V1 , xA1

).(

).(

).(

6912

5912

4912

1100

11112200

1120

MNLNLNB

MxVxLyVxLy

MVLVL

M

AMAAAA


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EXTRACT SOLVENT FEED

SOLID FEED, Lo LEACHED SOLID, L1

OVERFLOW STREAM

UNDERFLOW STREAM


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GRAPHICAL SOLUTION

EQUILIBRIUM LINE INDICATES THESOLUTE CONCENTRATION IN THESOLVENT IS THE SAME IN BOTHTHE SOLID UNDERFLOW AND

LIQUID OVERFLOW.

20

2200

20

VL

xVyLx

MB

VLBN

AAAM

M

Refer 12.9-6


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LESS THAN EQUILIBRIUM STAGEPARTIAL APPROACH TOEQUILIBRIUM MAY RESULTFROM INCOMPLETE

SEPARATION OF PHASESOR SHORT RESIDENCETIME.

EQUATION FOREFFICICIENCY IS:

L1

V2

10

10

AA

AALEACH

yy

yy

'


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Refer p.808

Fresh solvent enter the first stage thickenerThe clear, settled liquid leaves and flows from stage to stage.

The feed solids enter the last stage, and contacted with solvent from

previous stage and then enter the settler slowly rotating rake moves the

solid to the bottom discharge solid with some liquid are pumped as a

slurry to the next tank.


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Sanitary landfill is the cheapest satisfactory means of disposal,but only if suitable land is within economic range of the source

of the wastes; typically, collection and transportation account for75 percent of the total cost of solid waste management. In amodern landfill, refuse is spread in thin layers, each of which iscompacted by a bulldozer before the next is spread. When about3 m (about 10 ft) of refuse has been laid down, it is covered by athin layer of clean earth, which also is compacted. Pollution of

surface and groundwater is minimized by lining and contouringthe fill, compacting and planting the cover, selecting proper soil,diverting upland drainage, and placing wastes in sites not subjectto flooding or high groundwater levels. Gases are generated inlandfills through anaerobic decomposition of organic solid

waste. If a significant amount of methane is present, it may be

explosive; proper venting eliminates this problem.


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Chap 5 Single-multiple Stage Leaching

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