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Mass Transfer (ChE - 392) 4-Leaching (Solid-Liquid Extraction) Saeed GUL, Dr.Techn, M.Sc. Engg. Associate Professor

Department of Chemical Engineering, University of Engineering & Technology Peshawar, PAKISTAN

Introduction Leaching is concerned with the extraction of a soluble constituent from a solid by means of a solvent The desired component diffuses into the solvent from its natural solid form

Typical users include:  the metals industry for removing mineral from ores (acid solvents)  the sugar industry for removing sugar from beets (water is solvent)  the oilseeds industry for removing oil from soybeans, etc. (hexane or similar organic solvents) 6 May 2016

Dr. Saeed GUL, Department of Chemical Engineering, UET Peshawar, Pakistan

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General Principles Generally, the process can be considered in three parts: 1. The change of phase of the solute as it dissolves in the solvent 2. Solute diffusion through the solvent in the pores of the solid to the outside of the particle 3. The transfer of the solute from the solution in contact with the particles to the main bulk of the solution

Any one of these three processes may be responsible for limiting the extraction rate, though the first process usually occurs so rapidly that it has a negligible effect on the overall rate 6 May 2016

Dr. Saeed GUL, Department of Chemical Engineering, UET Peshawar, Pakistan

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General Principles If the solute is uniformly dispersed in the solid: The material near the surface will be dissolved first, leaving a porous structure in the solid residue. The solvent will then have to penetrate this outer layer before it can reach further solute, and the process will become progressively more difficult and the extraction rate will fall. If the solute forms a very high proportion of the solid: The porous structure may break down almost immediately to give a fine deposit of insoluble residue, and access of solvent to the solute will not be impeded.

6 May 2016

Dr. Saeed GUL, Department of Chemical Engineering, UET Peshawar, Pakistan

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General Principles If the soluble material is distributed in small isolated pockets in a material which is impermeable to the solvent: Such as gold dispersed in rock, for example. In such cases the material is crushed so that all the soluble material is exposed to the solvent. If the solid has a cellular structure: The extraction rate will generally be comparatively low because the cell walls provide an additional resistance

6 May 2016

Dr. Saeed GUL, Department of Chemical Engineering, UET Peshawar, Pakistan

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Factors influencing the rate of extraction There are four important factors to be considered: 1. Particle Size I. Smaller II. Larger III. Same particle size range 2. Solvent I. Selective II. Viscosity 3. Temperature I. Solubility II. Diffusion Coefficient 4. Agitation of the Fluid I. Increases eddy diffusion II. Prevent Sedimentation 6 May 2016

Dr. Saeed GUL, Department of Chemical Engineering, UET Peshawar, Pakistan

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Mass Transfer in Leaching Operations Mass transfer rates within the porous residue are difficult to assess because it is impossible to define the shape of the channels through which transfer must take place. It is possible, however, to obtain an approximate indication of the rate of transfer from the particles to the bulk of the liquid.

Using the concept of a thin film as providing the resistance to transfer, the equation for mass transfer may be written as:

6 May 2016

Dr. Saeed GUL, Department of Chemical Engineering, UET Peshawar, Pakistan

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Mass Transfer in Leaching Operations For a batch process in which V, the total volume of solution, is assumed to remain constant, then:

The time t taken for the concentration of the solution to rise from its initial value c0 to a value c is found by integration, on the assumption that both b and A remain constant. Rearranging:

6 May 2016

Dr. Saeed GUL, Department of Chemical Engineering, UET Peshawar, Pakistan

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Mass Transfer in Leaching Operations If pure solvent is used initially, c0 = 0, and:

which shows that the solution approaches a saturated condition exponentially. In most cases the interfacial area will tend to increase during the extraction and, when the soluble material forms a very high proportion of the total solid, complete disintegration of the particles may occur. Although this results in an increase in the interfacial area, the rate of extraction will probably be reduced because the free flow of the solvent will be impeded and the effective value of b will be increased. 6 May 2016

Dr. Saeed GUL, Department of Chemical Engineering, UET Peshawar, Pakistan

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Example 10.1

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Dr. Saeed GUL, Department of Chemical Engineering, UET Peshawar, Pakistan

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Example 10.1

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Dr. Saeed GUL, Department of Chemical Engineering, UET Peshawar, Pakistan

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Equipment for Leaching Leaching by percolation through stationary solid beds Moving-bed leaching Dispersed-solid leaching The selection of the equipment for an extraction process is influenced by the factors which are responsible for limiting the extraction rate. Thus, if the diffusion of the solute through the porous structure of the residual solids is the controlling factor, the material should be of small size so that the distance the solute has to travel is small. On the other hand, if diffusion of the solute from the surface of the particles to the bulk of the solution is the controlling factor, a high degree of agitation of the fluid is required. 6 May 2016

Dr. Saeed GUL, Department of Chemical Engineering, UET Peshawar, Pakistan

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Equipment for Leaching Processes involved Three distinct processes are usually involved in leaching operations: 1. Dissolving the soluble constituent. 2. Separating the solution, so formed, from the insoluble solid residue. 3. Washing the solid residue in order to free it of unwanted soluble matter or to obtain as much of the soluble material as possible as the product. The type of equipment employed depends on the nature of the solid — whether it is granular or cellular and whether it is coarse or fine. The normal distinction between coarse and fine solids is that the former have sufficiently large settling velocities for them to be readily separable from the liquid, whereas the latter can be maintained in suspension with the aid of only a small amount of agitation 6 May 2016

Dr. Saeed GUL, Department of Chemical Engineering, UET Peshawar, Pakistan

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Equipment for Leaching Extraction from cellular materials With seeds such as soya beans, containing only about 15 per cent of oil, solvent extraction is often used because mechanical methods are not very efficient. Light petroleum fractions are generally used as solvents. Trichlorethylene has been used where fire risks are serious, and acetone or ether where the material is very wet

6 May 2016

Dr. Saeed GUL, Department of Chemical Engineering, UET Peshawar, Pakistan

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Extraction from cellular materials Bollmann Extractor  Continuous moving bed extractor Series of perforated baskets  widely used with seeds which do not disintegrate on extraction  A typical extractor moves at about 1 revolution per hour  Each basket containing some 350 kg of seeds.  Generally, about equal masses of seeds and solvent are used.  The final solution, known as miscella, contains about 25 per cent of oil by mass 6 May 2016

Dr. Saeed GUL, Department of Chemical Engineering, UET Peshawar, Pakistan

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Equipment for Leaching

6 May 2016

Dr. Saeed GUL, Department of Chemical Engineering, UET Peshawar, Pakistan

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Equipment for Leaching Leaching of coarse solids

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Dr. Saeed GUL, Department of Chemical Engineering, UET Peshawar, Pakistan

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Equipment for Leaching Leaching of fine solids Whereas coarse solids may be leached by causing the solvent to pass through a bed of the material, fine solids offer too high a resistance to flow. Particles of less than about 200-mesh (0.075 mm) may be maintained in suspension with only a small amount of agitation, and as the total surface area is large, an adequate extraction can be effected in a reasonable time. Because of the low settling velocity of the particles and their large surface, the subsequent separation and washing operations are more difficult for fine materials than with coarse solids.

6 May 2016

Dr. Saeed GUL, Department of Chemical Engineering, UET Peshawar, Pakistan

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Equipment for Leaching Leaching of fine solids

6 May 2016

Dr. Saeed GUL, Department of Chemical Engineering, UET Peshawar, Pakistan

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Leaching Operations

Figure 1: Single-stage leaching unit

Figure 2: Multistage cross-flow leaching unit

Figure 3: Multistage countercurrent leaching unit 6 May 2016

Dr. Saeed GUL, Department of Chemical Engineering, UET Peshawar, Pakistan

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Material Balance-Countercurrent Process

 The stages are numbered in the direction of flow of the solid.  The light phase is the liquid that overflows from stage to stage in a direction opposite to that of the flow of the solid, dissolving solute as it moves from stage N to stage 1.  The heavy phase is the solid flowing from stage 1 to stage N.  Exhausted solids leave stage N, while concentrated solution overflows leave from stage 1.  For purposes of analysis, it is customary to assume that the solute free solid is insoluble in the solvent so that the flow rate of this solid is constant throughout the process unit 6 May 2016

Dr. Saeed GUL, Department of Chemical Engineering, UET Peshawar, Pakistan

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Design and Predictive Equations

The solute /solvent equilibrium and process throughput determine the cross-sectional area and the number of theoretical and /or actual stages required to achieve the desired separation.

The equation for the operating line is obtained by writing a material balance. From Figure above

6 May 2016

Dr. Saeed GUL, Department of Chemical Engineering, UET Peshawar, Pakistan

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Design and Predictive Equations

If the density and viscosity of the solution change considerably with solute concentration, the solids from the lower stages might retain more liquid than those in the higher stages. The slope of the operating line then varies from stage to stage. 6 May 2016

Dr. Saeed GUL, Department of Chemical Engineering, UET Peshawar, Pakistan

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Design and Predictive Equations

If, however, the mass of the solution retained by the solid is independent of concentration, LN is constant, and the operating line is straight. These two mentioned conditions describe variable and constant overflow, respectively. It is usually assumed that the inerts are constant from stage to stage and insoluble in the solvent. Since no inerts are usually present in the extract (overflow) solution and the solution retained by the inerts is approximately constant, both the underflow LN and overflow VN are constant, and the equation for the operating line approaches a straight line. 6 May 2016

Dr. Saeed GUL, Department of Chemical Engineering, UET Peshawar, Pakistan

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Design and Predictive Equations Since the equilibrium line is also straight, the number of stages can be shown to be (with reference to Fig. 12.12)

The above equation should not be used for the entire extraction cascade if Lo differs from L1, L2, . . . , LN (i.e., the underflows vary within the system). For this case, the compositions of all the streams entering and leaving the first stage should first be calculated before applying this equation to the remaining cascade. 6 May 2016

Dr. Saeed GUL, Department of Chemical Engineering, UET Peshawar, Pakistan

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Single stage problems EXAMPLE Calculate the grams of water that need to be added to 40 g of sand

containing 9.1 g salt to obtain a 17 % salt solution. If the salt solution is to be reduced to 0.015, calculate the amount of salt that must be removed (“leached”) from the solution. SOLUTION: Set V to be the grams of water required. The describing equation is:

Let Z be equal to the final amount of salt in the sand–water–salt solution. The describing equation is

The amount of salt removed is therefore 9.1 2 0.61 ¼ 8.49 g. 6 May 2016

Dr. Saeed GUL, Department of Chemical Engineering, UET Peshawar, Pakistan

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Single stage problems A sand–salt mixture containing 20.4% salt enters a solid–liquid extraction at a rate of 2500 lb /h. Calculate the hourly rate of fresh water that must be added for 99% of the salt to be “leached” from the sand–salt mixture if the discharge salt–water solution contains 0.153 (mass) fraction salt.

SOLUTION: Let W equal to the hourly rate of water. The describing equation from a mass balance is

Also note that the feed consists of 510 lb salt and 1990 lb sand. On discharge, the sand contains only 5.1 lb salt. The discharge water solution consists of the 2820 lb water plus 504.9 lb salt. 6 May 2016

Dr. Saeed GUL, Department of Chemical Engineering, UET Peshawar, Pakistan

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Compositions in Ternary Diagrams

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Dr. Saeed GUL, Department of Chemical Engineering, UET Peshawar, Pakistan

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Triangular Grid Method

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Dr. Saeed GUL, Department of Chemical Engineering, UET Peshawar, Pakistan

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Representation of a three-component system on a right-angled triangular diagram

Representation of underflow stream

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Dr. Saeed GUL, Department of Chemical Engineering, UET Peshawar, Pakistan

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Graphical method of solution with a triangular diagram

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Dr. Saeed GUL, Department of Chemical Engineering, UET Peshawar, Pakistan

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Number of Stages for Countercurrent Washing by Graphical Methods The separation of Soluble constituents from a solid by extraction with a solvent may be considered to consist of two steps: 1. Contacting the solid with liquid phase and 2. Separation of the liquid phase from the solid In actual operation it is impossible to completely separate the liquid phase from the solid. Consequently, the streams resulting from the 2nd step will consist of:  A liquid phase stream (solution) which during normal operation does not contain any solid. This stream will be called overflow  A slurry consisting of the solid plus adhering solution

6 May 2016

Dr. Saeed GUL, Department of Chemical Engineering, UET Peshawar, Pakistan

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It is convenient to use the concept of a stage in performing the calculations. A stage consist of two steps: Contacting solid with liquid and Separation of the overflow from the underflow

In Solid-Liquid extraction , an ideal stage is defend as : A stage in which the solution leaving in the overflow is of the same composition as the solution retained by the solid in the underflow

For the purpose of calculations, the solid-liquid extraction system may be considered to consist of three components: 1. The solute component (A) 2. The inert solid component (B) 3. The solvent component (S) 6 May 2016

Dr. Saeed GUL, Department of Chemical Engineering, UET Peshawar, Pakistan

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The calculations for leaching system may be based on the material balance and the ideal stage concept. Both mathematical and graphical methods of solution may be used. The graphical solution possesses advantage in presenting a generalized treatment of the more complex cases and in permitting a better visualization of what is occurring in the process, although it may be inconvenient to use if a large number of stages are involved but typically in most of the cases the number of stages used is not large

6 May 2016

Dr. Saeed GUL, Department of Chemical Engineering, UET Peshawar, Pakistan

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Rectangular Diagram

Mole fraction of Component S, xs

S

1

0.8

0.6 0.2

0.4

0.4 0.6

0.2 0.8

B

100 % B 6 May 2016

100 % S

0.2

0.4

0.6

0.8

A

100 % A

Mole fraction of Component A, xA Dr. Saeed GUL, Department of Chemical Engineering, UET Peshawar, Pakistan

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Single Stage Extraction system Overflow Product

V1, y1 Feed

Solvent Feed

Single Stage Extraction System

V2, y2 Underflow Product

L0, x0

L1, x1

Overall Material Balance L0 + V2 = L1 + V1

The Material Balance for components A, B and S: L0(xA)0 + V2(yA)2 = L1(xA)1 + V1(yA)1 L0(xB)0 + V2(yB)2 = L1(xB)1 + V1(yB)1 L0(xS)0 + V2(yS)2 = L1(xS)1 + V1(yS)1 6 May 2016

Dr. Saeed GUL, Department of Chemical Engineering, UET Peshawar, Pakistan

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Counter Current Multistage Extraction System V1

L0

V2 Stage 1 L1

V3 Stage 2 L2

Vj+1

Vj

Lj -1

Stage j

Lj

Vn

Ln-1

Vn+1 Stage n Ln

Overall Material Balance L0 + Vn+1 = Ln + V1 The Material Balance for components A, B and S: L0(xA)0 + Vn+1(yA)n+1 = Ln(xA)n + V1(yA)1 L0(xB)0 + Vn+1(yB)n+1 = Ln(xB)n + V1(yB)1 L0(xS)0 + Vn+1(yS)n+1 = Ln(xS)n + V1(yS)1 6 May 2016

Dr. Saeed GUL, Department of Chemical Engineering, UET Peshawar, Pakistan

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Dr. Saeed GUL, Department of Chemical Engineering, UET Peshawar, Pakistan

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Dr. Saeed GUL, Department of Chemical Engineering, UET Peshawar, Pakistan

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Dr. Saeed GUL, Department of Chemical Engineering, UET Peshawar, Pakistan

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Dr. Saeed GUL, Department of Chemical Engineering, UET Peshawar, Pakistan

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