Perry’s Chemical Engineers Handbook 8th Edition
7th Edition
Section 18
Section 18
Pages 18-59 to 18-66
Pages 18-55 to 18-59
Perry’s Chemical Engineers Handbook 8th Edition
7th Edition
Section 18
Section 18
Pages 18-59 to 18-66
Pages 18-55 to 18-59
Leaching (Lexiviation orSolidLiquid Extraction) The dissolving, by a liquid solvent, of soluble material from its mixture with an insoluble solid. Examples are washing of soluble salt from the surface of an insoluble precipitate, extraction of sugar from sugar beet, extraction of tannic acid from bark, extraction of alginic acids from seaweed
SELECTION OR DESIGN OF A LEACHING PROCESS 1. Process and Operating Conditions - The major
parameters that must be fixed or identified are the solvent to be used, the temperature, the terminal stream compositions and quantities, leaching cycle (batch or continuous), contact method, and specific extractor choice. 2. Temperature - The temperature of the extraction
should be chosen for the best balance of solubility, solvent-vapor pressure, solute diffusivity, solvent selectivity, and sensitivity of product.
3. Choice of Solvent - The solvent selected will offer
the best balance of a number of desirable characteristics: high saturation limit and selectivity for the solute to be extracted, capability to produce extracted material of quality unimpaired by the solvent, chemical stability under process conditions, low viscosity, low vapor pressure, low toxicity and flammability, low density, low surface tension, ease and economy of recovery from the extract stream, and price.
4. Terminal Stream Compositions and Quantities - These
are basically linked to an arbitrary given: the production capacity of the leaching plant (rate of extract production or rate of raw-material purification by extraction). 5. Leaching Cycle and Contact Method - As is true
generally, the choice between continuous and intermittent operation is largely a matter of the size and nature of the process of which the extraction is a part.
6. Type of Reactor - The specific type of reactor that is
most compatible (or least incompatible) with the chosen combination of the preceding parameters seldom is clearly and unequivocally perceived without difficulty, if at all. 7. Extractor-Sizing Calculations - For any given
throughput rate (which fixes the cross-sectional area and/or the number of extractors), the size of the units boils down to the number of stages required, actual or equivalent.
Composition Diagrams
In its elemental form, a leaching system consists of three components: inert, insoluble solids; a single non-adsorbed solute, which may be liquid or solid; and a single solvent. Thus, it is a ternary system, albeit an unusual one, as already mentioned, by virtue of the total mutual “insolubility” of two of the phases and the simple nature of equilibrium.
Right Triangle Diagram
Modified Ponchon Savarit Diagram
VARIABLE UNDERFLOW Here, or the weight of the retained solution varies from stage to stage. The number of Ideal Stages may be determined graphically. 1. Using Rectangular Diagram or Coordinates (McCabe-Smith Method) Coordinates chosen are: (B = solids; A= solute; S = solvent) Y
X
B
, mass ratio of solids to solution
A S
A
, concentration of solute in solution
To plot underflow curve, an experimental data of R vs X must be known where:
R and
mass solution retained mass solids
X = concentration of solution
Note that Y
1
Y
X
2.Triangular Diagram Procedure: 1. Plot X and Y coordinates of known streams, usually,
V b , V a , L
a
2. Determine J point by setting up over-all balance, V a
L a V b
Lb V b La
Y J Y L a
J
Y V b
Y J
3. Determine point Lb along underflow curve by extending line
4. Determine operating point P from a balance around stages (1) to (n): L a
V n 1
L n
L n
V a
1
V a
L
a
P
V b
L b
P
V n 1
V n
L
n
V a
La
P
5. Determine Theoretical N following Ponchon-Savarit method
Constant Underflow Since is constant, the slope of the operating line is constant. With a linear operating line and in leaching, the equilibrium curve is always linear the number of ideal stages may be determined using the ABSORPTION FACTOR METHOD or the TILLERTOUR Equation. log N
log
*
y b
y b
y a y b
y a y a
*
*
*
where:
*
y b
y
*
xb
x
Since x a is not known i.e, the fresh feed does not contain any retained solution, to apply the equation, we omit the first stage and just apply it to the (N-1) stages, thus log N
1
log
where:
' * ' ya xa ya
and
* y b y b
' * y a ' y b y a * ' * y b y a
' y a
' y a
is determined from the
log N 1
log
x1 y 2 x N y N
1
x1 x N y 2 y N
1
Assumptions - Solid B is insoluble in solvent - No solid B in overflow - Steady state operation - Solid B in feed = Solid B in any underflow The characteristic of the adhering solution is the same as that of the strong solution leaving a particular stage. X1=Y1 X2=Y2 XN=YN
1. Constant Solvent Retention solvent
If solid ratio is constant, concentrations are expressed in mass solute mass solvent Retention = mass solvent retained/mass solid B L1’ = L2’ = L3’ =……= LN’ = L’ V2’ = V3’ = V4’ =…..= VN+1’ = V’ y2 = L’/V’ ( x1 – xN ) + yN+1
2. Constant Solution Retention If solution ratio is constant, concentrations are expressed in solid
masssolute masssolution
Retention = mass solution retained/mass solid B L1 = L2 = L3 =……= LN = L V2 = V3 = V4 =…..= VN+1 = V
y2 = L/V ( x1 – xN ) + yN+1
