Leaching and washing of leached solids from the percolation tanks by the crosscurrent methods will inevitably result in weak solutions of the solute. The strongest solution will result if a countercurrent scheme is used. Here the final withdrawn solution is taken from contact with the freshest solid and the fresh solvent is added to solid from which most of the solute has already been leached .
In this case the arrangement is given below
Method of operation
Initially tank 6 is empty. Tanks 1-5 are filled with solid. Tank 5 most recently and tank 1 for the longest time. Tank 1-5 are also filled with leach liquid, and the most concentrated is tank 5 since it is in contact with the fresh solid. Fresh solvent has just been added to tank 1 Withdraw the concentrated solution from tank 5 ,transfer the liquid from tank 4 to 5 ,from3-4 , from 2-3, and from 1-2 Discard the spent from tank 1. transfer the liquid from tank 5 -6, from 4-5, from 3-4, and from 2-3 . Add fresh solvent to tank 2 Continue the operation in the same manner as before
After several cycles have been run through in this same manner ,the concentrations of solution and in the solid in each tank approach very closely the values obtaining in a truly continuous counter current multistage leaching. It can be operated with a no of 6 to 16 tanks They need not to be arranged in a circle but can be placed in a row, called extraction battery. The tanks can be placed at progressively decreasing levels, so that the liquid can flow from one to the other by gravity with a minimum pumping.
Industrial applications
Metallurgical industries Recovery of tannins from tree barks and woods Leaching sodium nitrate from Chilean nitrate bearing rock (caliche)
2.
Percolation in closed vessels
When the pressure drop for liquid flow of is too high for gravity flow ,closed vessel must be used and the liquid is pumped through the bed of the solid. Such vessels are sometimes called diffusers. Closed tanks are needed to prevent the evaporation of solvent.
In leaching sugar from sugar beet slices a diffuser of the above type is used. They are arranged in a battery containing upto 16 vessels, and the beets are leached with hot water in a counter current stream as in the case of a shank’s
system. Heaters are placed between them to make the temperature at the range of 70-78 0C. In this manner 95-98% of sugar in beets containing about 18% beets can be leached to form a solution of 12% concentration .
3.
Filter –Press Leaching
Finely divided solids ,too fine for treatment by percolation tanks can be filtered and leached in the filter press by pumping the solvent through the press cake. This is a common practice in washing mother liquor from precipitate which have been filtered
4.
Agitated Vessels
Channeling of the solvent in percolation or filter press leaching of fixed beds with its incomplete leaching can be avoided by stirring the liquid and solid in leaching vessels.. In such case closed cylindrical vessels are arranged vertically(fig a) and are filtered with power driven paddles or stirrers on vertical shafts as well as false bottoms for drainage of the leached solution at the end of the operation. In others the vessels are horizontal with the stirrups arranged on horizontal shafts.(fig b).
Contd.
In some cases a horizontal drum s the extraction vessel and the solid and the liquid are tumbled about inside by the rotation of the drum on rollers(fig c). These devices are operated in batch wise fashion and provide a single leaching stage. They can be used singly but are frequently used in batteries arranged for counter current leaching.
Finely divided solids can be suspended in leaching solvents by agitation PACHUCA TANK is an example of agitated vessel which is normally employed in metallurgical industries. Construction material : wood ,metal or concrete and often lined with lead The bubbles of air rising through the central tube cause the upward flow of the liquid and suspended solid in the tube and consequently vertical circulation of the tank contents
After the leaching has bees completed the suspended solids are allowed to settle and the clear supernatant liquid is decanted by siphoning over the top of the tank or by withdrawal through discharge pipes. Agitation and settling is necessary if amount of solution retained is considerable(counter current operation prefered).
Batch settling
Particles settle down through liquid at a rate dependent upon particle size , relative density and viscosity. For concentrated slurries behaviour is different(mutual interference happens). The demarcation between clear liquid and settled solids is more prominent for concentrated solutions.
Zone A –clear liquid at top Zone B –concentration of solids is uniform and equals to initial value W 0 Zone D- compression zone Rate of settling is usually followed by plotting the height of line of demarcation between A and B against time.
Batch settling is effected by
Flocculation Stirring Concentration Initial height of settling.
Flocculation
If finely divided solids particles are similarly electrically charged they repel each other and remain dispersed. Electrolyte is added to neutralize the charge and aggregates are formed. Electrolyte is the flocculating agent.
Stirring
Very slow stirring(eddy currents are not formed)floc structure is altered –solid concentration in zone B is no longer uniform. Zone D may not be clearly defined.The ultimate height of the settled slurry is only a fraction of that obtained without stirring
Concentration
The rate of settling decrease with increased initial concentration of the solids owing to the increase of the effective density and viscosity of the medium. The effect of concentration on settling rate is illustrated:
Height
Settling curves for same slurry begun at different initial heights have been plotted. The initial settling rate is independent of height provided some minimum value is exceeded. Time for settling is directly proportional to initial height Z0. Settling curves for deep tanks can be effectively predicted by this method.
Percolation vs. Agitation
Solid in the form of large lumps:
Crush to coarse lumps and leach by percolation
Grind it fine and leach by agitation and settling
Fine grinding is costly but provides more rapid and possibly more thorough leaching. Coarsely ground particles have leach more slowly and less thoroughly. But require less washing. For more fibrous solids, leaching in an agitated vessel is more effective than by percolation because large static liquid holdup.
