Leaching

SOLID – LIQUID EXTRACTION

Perry’s Chemical Engineering Handbook 7th EDITION

8th EDITION

Section 18

Section 18

Pages 18-59 to 18-66

Pages 18-55 to 18-59

LEACHING A unit operation in which a soluble component is removed from an inert solid by a solvent which preferentially dissolves the soluble matter.

LEACHING 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 acid from seaweed

CONSTANT UNDERFLOW Assumptions:

• • • •

Solid B is insoluble in solvent No solid B in overflow Steady state operation Solid B in feed = solid B in any underflow

CONSTANT UNDERFLOW The characteristics of the adhering solution leaving a particular stage. y1 = x 1 y2 = x 2 yN = xN

CONSTANT SOLVENT UNDERFLOW If solvent / solid ratio is constant, concentrations are expresses in mass solute / mass solvent Retention = mass solvent retained mass solid B L1’ = L2’ = L3’ = LN’ = L’ V2’ = V3’ = VN’ = VN + 1’ = V’ y2 = (L’/V’) (x1 – xN) + yN + 1

CONSTANT SOLUTION UNDERFLOW If solution / solid ratio is constant, concentrations are expresses in mass solute / mass solution Retention = mass solution mass solid B L1 = L 2 = L 3 = L N = L V2 = V3 = VN = VN + 1 = V y2 = (L/V) (x1 – xN) + yN + 1

NUMBER OF STAGES

LEACHING Which of the following operations does not involve leaching? a. dissolves gold from ore b. dissolving pharmaceutical products from bark or roots c. dissolving sugar from the cells of the beet d. removing nicotine from its water solution by kerosene

LEACHING The rate of leaching increases with increasing a. temperature b. viscosity of solvent c. pressure d. size of the solid

LEACHING Tea percolation employs a. liquid-liquid extraction b. leaching c. absorption d. adsorption

LEACHING The major problem in leaching is to promote diffusion of the solute out of the solid and into the liquid. The most effective way of doing this is a. to reduce the solid to smallest size feasible b. to use the countercurrent operation c. to use cross flow operation d. none of these

PROBLEM SOLVING

PROBLEM NO. 1 Powdered limestone (CaCO3) containing 10000 ppm NaOH is to be washed in a two step continuous counter current washing system to remove most of the NaOH. Two cells and 100000 L of water per 10 MT/day of limestone fed are used. The slurry discharged and removed from the underflow contains 0.091 MT of water per MT CaCO3. Assuming complete mixing and washing, the NaOH content of the washed and dried limestone in ppm is a. 0.6 b. 0.5 c. 0.8 d. 0.4

PROBLEM NO. 2 A counter current multiple contact extraction system is to treat 50 tons/hr of wet beets with fresh water as the solvent. The beets have the following analysis: Components Mass fraction water 0.48 pulp 0.40 sugar 0.12 The strong solution leaving the system is to contain 0.15 mass fraction sugar, 97% of the sugar in the sliced beets is to be recovered. Determine the number of extraction cells required, assuming equilibrium between the underflow and overflow in each cell. If each ton of dry pulp retains 3 tons of water a. 16 b. 14 c. 15 d. 17

PROBLEM NO. 3 A multiple contact extraction system is to treat 1.25 tons (2500 lbs) per hour of dry black ash containing 40% Na2CO3 and 60% insoluble matter with 30 gpm of water. If 5% of the Na2CO3 remains unextracted, calculate the number of ideal stages by the absorption factor method. The mass ratio of insoluble matter to solvent in the underflow from the stage is 1:2. The number of ideal stages required is a. 3 b. 2 c. 4 d. 5

PROBLEM NO. 4 Roasted copper containing the ore as CuSO4 is to be extracted in a countercurrent stage extractor. Each hour, a charge consisting of 10 tons gangue, 1.2 tons CuSO4 and 0.5 ton water is to be treated. The strong solution produced is to consist of 90% water and 10% CuSO4 by weight. The recovery of CuSO4 is to be 98% of that in the ore. Pure water is to be used as fresh solvent. After each stage, one ton of inert gangue retained 2 tons water plus the copper sulfate dissolved in that water. Equilibrium is attained in each stage. The number of stages required is a. 12 b. 10 c. 8 d. 14

PROBLEM NO. 5 Tung meal containing 55% oil is to be extracted at a rate of 4000 kg/hr using n-hexane containing 5% wt oil as solvent. A counter current multiple stage extraction system is to be used. The meal retains 2 kg solvent per kg of oil free meal while the residual charge contains 0.11 kg per oil free meal while the product is composed of 15 weight percent of oil. The theoretical number of stages is a. 4 b. 3 c. 5 d. 6

SEATWORK The countercurrent washing operation uses Dorr thickeners for the recovery of the rich solution. The slurry from the agitators, with the copper in solution as copper sulfate, is fed to the thickeners at the rate of 300 tons/hr. According to operating records, the underflow from each thickener retains 1.22 tons of solution per ton of gangue, and the streams have the following compositions in mass percent. Determine the number of theoretical stages and the quantity of wash water used. Feed to Thickener

Strong Solution

Underflow Leaving System

CuSO4

6.10%

6.69%

1.0%

Gangue

14.92%

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Water

78.98%

93.91%

99%

ASSIGNMENT Problem No. 2 Unit Operations by Brown page 294 Problem No. 23.2 Unit Operations, 6th ed by McCabe page 770 Five solid-liquid extraction equipment