12.8. INTRODUCTION AND EQUIPMENT FOR LIQUID–SOLID LEACHING

12.8A. Leaching Processes

1. Introduction

Many biological and inorganic and organic substances occur in a mixture of different components in a solid. In order to separate the desired solute constituent or remove an undesirable solute component from the solid phase, the solid is contacted with a liquid phase. The two phases are in intimate contact and the solute or solutes can diffuse from the solid to the liquid phase, resulting in a separation of the components originally in the solid. This separation process is called liquidsolid leaching or simply leaching. The term extraction is also used to describe this separation process, although it also refers to liquid–liquid extraction. In leaching, when an undesirable component is removed from a solid with water, the process is called washing.

2. Leaching processes for biological substances

In the biological and food processing industries, many products are separated from their original natural structure by liquid–solid leaching. An important process, for example, is the leaching of sugar from sugar beets with hot water. In the production of vegetable oils, organic solvents such as hexane, acetone, and ether are used to extract the oil from peanuts, soybeans, flax seeds, castor beans, sunflower seeds, cotton seeds, tung meal, and halibut livers. In the pharmaceutical industry, many different pharmaceutical products are obtained by leaching plant roots, leaves, and stems. For the production of soluble “instant” coffee, ground roasted coffee is leached with fresh water. Soluble tea is produced by water leaching of tea leaves. Tannin is removed from tree barks by leaching with water.

3. Leaching processes for inorganic and organic materials

Leaching processes are used extensively in the metals processing industries. The useful metals usually occur in mixtures with very large amounts of undesirable constituents, and leaching is used to remove the metals as soluble salts. Copper salts are dissolved or leached from ground ores containing other minerals by sulfuric acid or ammoniacal solutions. Cobalt and nickel salts are leached from their ores by sulfuric acid–ammonia–oxygen mixtures. Gold is leached from its ore using an aqueous sodium cyanide solution. Sodium hydroxide is leached from a slurry of calcium carbonate and sodium hydroxide prepared by reacting Na2CO3 with Ca(OH)2.

12.8B. Preparation of Solids for Leaching

1. Inorganic and organic materials

The method of preparation of the solid depends to a large extent upon the proportion of the soluble constituent present, its distribution throughout the original solid, the nature of the solid—that is, whether it is composed of plant cells or whether the soluble material is completely surrounded by a matrix of insoluble matter—and the original particle size.

If the soluble material is surrounded by a matrix of insoluble matter, the solvent must diffuse inside to contact and dissolve the soluble material and then diffuse out. This occurs in many hydrometallurgical processes where metal salts are leached from mineral ores. In these cases crushing and grinding of the ores is used to increase the rate of leaching, since the soluble portions are made more accessible to the solvent. If the soluble substance is in solid solution in the solid or is widely distributed throughout the whole solid, the solvent leaching action may form small channels. The passage of additional solvent is then made easier, and grinding to very small sizes may not be needed. Grinding of the particles is not necessary if the soluble material is dissolved in solution adhering to the solid. Then simple washing can be used, as in washing of chemical precipitates.

2. Animal and vegetable materials

Biological materials are cellular in structure and the soluble constituents are generally found inside the cells. The rate of leaching may be comparatively slow because the cell walls provide another resistance to diffusion. However, to grind the biological material sufficiently small to expose the contents of individual cells is impractical. Sugar beets are cut into thin, wedge-shaped slices for leaching so that the distance required for the water solvent to diffuse in order to reach individual cells is reduced. The cells of the sugar beet are kept essentially intact so that sugar will diffuse through the semipermeable cell walls, while the undesirable albuminous and colloidal components cannot pass through the walls.

For the leaching of pharmaceutical products from leaves, stems, and roots, drying of the material before extraction helps rupture the cell walls. Thus, the solvent can directly dissolve the solute. The cell walls of soybeans and many vegetable seeds are largely ruptured when the original materials are reduced in size to about 0.1 mm to 0.5 mm by rolling or flaking. Cells are smaller in size, but the walls are ruptured and the vegetable oil is easily accessible to the solvent.

12.8C. Rates of Leaching

1. Introduction and general steps

In the leaching of soluble materials from inside a particle by means of a solvent, the following general steps can occur in the overall process. The solvent must be transferred from the bulk solvent solution to the surface of the solid. Next, the solvent must penetrate or diffuse into the solid. The solute dissolves into the solvent. The solute then diffuses through the solid solvent mixture to the surface of the particle. Finally, the solute is transferred to the bulk solution. The many different phenomena encountered make it virtually impracticable if not impossible to apply any one theory to the leaching action.

In general, the rate of transfer of the solvent from the bulk solution to the solid surface is quite rapid, while the rate of transfer of the solvent into the solid may be somewhat rapid or slow. These are not, in many cases, the rate-limiting steps in the overall leaching process. This solvent transfer usually occurs initially, when the particles are first contacted with the solvent. The dissolving of the solute into the solvent inside the solid may be a simple physical dissolution process or an actual chemical reaction that frees the solute for dissolution. Our knowledge of the dissolution process is limited and the mechanism may be different for each solid (K1).

The rate of diffusion of the solute through the solid and solvent to the surface of the solid is often the controlling resistance in the overall leaching process and may depend on a number of different factors. If the solid is made up of an inert porous solid structure, with the solute and solvent in the pores in the solid, the diffusion through the porous solid can be described by an effective diffusivity. The void fraction and tortuosity are needed. This is described in Section 6.5C for diffusion in porous solids.

In biological or natural substances, additional complexity occurs because of the cells present. In the leaching of thin sugar beet slices, about one-fifth of the cells are ruptured in the slicing of the beets. The leaching of the sugar is then similar to a washing process (Y1). In the remaining cells, sugar must diffuse out through the cell walls. The net result of the two transfer processes does not follow the simple diffusion law with a constant effective diffusivity.

With soybeans, whole beans cannot be leached effectively. The rolling and flaking of the soybeans ruptures cell walls so that the solvent can more easily penetrate by capillary action. The rate of diffusion of the soybean oil solute from the soybean flakes does not permit simple interpretation. A method for designing large-scale extractors involves using small-scale laboratory experiments (O2) with flakes.

The resistance to mass transfer of the solute from the solid surface to the bulk solvent is in general quite small compared to the resistance to diffusion within the solid (O1) itself. This has been found for leaching soybeans, where the degree of agitation of the external solvent has no appreciable effect on the extraction rate (O3, Y1).

2. Rate of leaching when dissolving a solid

When a material is being dissolved from the solid to the solvent solution, however, the rate of mass transfer from the solid surface to the liquid is the controlling factor. There is essentially no resistance in the solid phase if it is a pure material. The equation for this can be derived as follows for a batch system. The following can also be used for the case when diffusion in the solid is very rapid compared to diffusion from the particle.

The rate of mass transfer of solute A being dissolved to the solution of volume V m3 is

Equation 12.8-1


where is kg mol of A dissolving to the solution/s. A is surface area of particles in m2, kL is a mass-transfer coefficient in m/s, cAS is the saturation solubility of the solid solute A in the solution in kg mol/m3, and cA is the concentration of A in the solution at time t sec in kg mol/m3. By a material balance, the rate of accumulation of A in the solution is equal to Eq. (12.8-1) times the area A:

Equation 12.8-2


Integrating from t = 0 and cA = cA0 to t = t and cA = cA,

Equation 12.8-3


Equation 12.8-4


The solution approaches a saturated condition exponentially. Often the interfacial area A will increase during the extraction if the external surface becomes very irregular. If the soluble material forms a high proportion of the total solid, disintegration of the particles may occur. If the solid is completely dissolving, the interfacial area changes markedly. Also, the mass-transfer coefficient may then change.

If the particles are very small, the mass-transfer coefficient to the particle in an agitated system can be predicted by using equations given in Section 7.4. For larger particles, which are usually present in leaching, equations for predicting the mass-transfer coefficient kL in agitated mixing vessels are given in Section 7.4 and reference (B1).

3. Rate of leaching when diffusion in solid controls

In the case where unsteady-state diffusion in the solid is the controlling resistance in the leaching of the solute by an external solvent, the following approximations can be used. If the average diffusivity DAeff of solute A is approximately constant, then for extraction in a batch process, unsteady-state mass-transfer equations can be used, as discussed in Section 7.1. If the particle is approximately spherical, Fig. 5.3-13 can be used.

EXAMPLE 12.8-1. Prediction of Time for Batch Leaching

Particles having an average diameter of approximately 2.0 mm are leached in a batch-type apparatus with a large volume of solvent. The concentration of the solute A in the solvent is kept approximately constant. A time of 3.11 h is needed to leach 80% of the available solute from the solid. Assuming that diffusion in the solid is controlling and the effective diffusivity is constant, calculate the time of leaching if the particle size is reduced to 1.5 mm.

Solution: For 80% extraction, the fraction unextracted Es is 0.20. Using Fig. 5.3-13 for a sphere, for Es = 0.20, a value of DAeff t/a2 = 0.112 is obtained, where DAeff is the effective diffusivity in mm2/s, t is time in s, and a is radius in mm. For the same fraction Es, the value of DAeff t/a2 is constant for a different size. Hence,

Equation 12.8-5


where t2 is time for leaching with a particle size a2. Substituting into Eq. (12.8-5),



4. Methods of operation in leaching

There are a number of general methods of operation in the leaching of solids. The operations can be carried out under batch or unsteady-state conditions as well as under continuous or steady-state conditions. Both continuous and stagewise types of equipment are used in steady or unsteady-state operation.

In unsteady-state leaching, a method commonly used in the mineral industries is in-place leaching, where the solvent is allowed to percolate through the actual ore body. In other cases the leach liquor is pumped over a pile of crushed ore and collected at the ground level as it drains from the heap. Copper is leached by sulfuric acid solutions from sulfide ores in this manner.

Crushed solids are often leached by percolation through stationary solid beds in a vessel with a perforated bottom to permit drainage of the solvent. The solids should not be too fine, or a high resistance to flow will be encountered. Sometimes a number of tanks are used in series, called an extraction battery, and fresh solvent is fed to the solid that is most nearly extracted. The tanks can be open tanks or closed tanks called diffusers. The solvent flows through the tanks in series, being withdrawn from the freshly charged tank. This simulates a continuous countercurrent stage operation. As a tank is completely leached, a fresh charge is added to the tank at the other end. Multiple piping is used so that tanks do not have to be moved for countercurrent operation. This is often called the Shanks system. It is used widely in leaching sodium nitrate from ore, recovering tannins from barks and woods, in the mineral industries, in the sugar industry, and in other processes.

In some processes the crushed solid particles are moved continuously by bucket-type conveyors or a screw conveyor. The solvent flows countercurrent to the moving bed.

Finely ground solids may be leached in agitated vessels or in thickeners. The process can be unsteady-state batch or the vessels can be arranged in a series to obtain a countercurrent stage process.

12.8D. Types of Equipment for Leaching

1. Fixed-bed leaching

Fixed-bed leaching is used in the beet sugar industry and is also used for the extraction of tanning extracts from tanbark, for the extraction of pharmaceuticals from barks and seeds, and in other processes. In Fig. 12.8-1 a typical sugar beet diffuser or extractor is shown. The cover is removable so that sugar beet slices called cossettes can be dumped into the bed. Heated water at 344 K (71°C) to 350 K (77°C) flows into the bed to leach out the sugar. The leached sugar solution flows out the bottom onto the next tank in series. Countercurrent operation is used in the Shanks system. The top and bottom covers are removable so that the leached beets can be removed and a fresh charge added. About 95% of the sugar in the beets is leached to yield an outlet solution from the system of about 12 wt %.

Figure 12.8-1. Typical fixed-bed apparatus for sugar beet leaching.


2. Moving-bed leaching

There are a number of devices for stagewise countercurrent leaching where the bed or stage moves instead of being stationary. These are used widely in extracting oil from vegetable seeds such as cottonseeds, peanuts, and soybeans. The seeds are usually dehulled first, sometimes precooked, often partially dried, and rolled or flaked. Sometimes preliminary removal of oil is accomplished by expression. The solvents are usually petroleum products, such as hexane. The final solvent–vegetable solution, called miscella, may contain some finely divided solids.

In Fig. 12.8-2a an enclosed moving-bed bucket elevator device is shown. This is called the Bollman extractor. Dry flakes or solids are added at the upper right side to a perforated basket or bucket. As the buckets on the right side descend, they are leached by a dilute solution of oil in solvent called half miscella. This liquid percolates downward through the moving buckets and is collected at the bottom as the strong solution or full miscella. The buckets moving upward on the left are leached countercurrently by fresh solvent sprayed on the top bucket. The wet flakes are dumped as shown and removed continuously.

The Hildebrandt extractor in Fig. 12.8-2b consists of three screw conveyors arranged in a U shape. The solids are charged at the top right, conveyed downward, across the bottom, and then up the other leg. The solvent flows countercurrently.

3. Agitated solid leaching

When the solid can be ground fine to about 200 mesh (0.074 mm), it can be kept in suspension by small amounts of agitation. Continuous countercurrent leaching can be accomplished by placing a number of agitators in series, with settling tanks or thickeners between each agitator.

Sometimes the thickeners themselves are used as combination contactor-agitators and settlers, as shown in Fig. 12.8-3. In this countercurrent-stage system, fresh solvent enters the first stage thickener as shown. The clear, settled liquid leaves and flows from stage to stage. The feed solids enter the last stage, where they are contacted with solvent from the previous stage and then enter the settler. The slowly rotating rake moves the solids to the bottom discharge. The solids together with some liquid are pumped as a slurry to the next tank. If the contact is insufficient, a mixer can be installed between the settlers.

Figure 12.8-3. Countercurrent leaching using thickeners.


Figure 12.8-2. Equipment for moving-bed leaching: (a) Bollman bucket-type extractor, (b) Hildebrandt screw-conveyor extractor.


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