207 



The space of which fig. la represents the longitudinal and fig. ih 

 the cross section is divided into two parts by a plane LN {ov FG, 



, ~Fi T\ 



==i5l .- fi.-- 

 I li n 



e,v-. 



--G 



ia. lb. 



respectively). In the upper space the mass to be lixiviated moves 

 from the left to the right (we call this direction positive) in the 

 lower space the lixiviating liquid moves in the opposite direction. 

 The wall LJSF (or FG, respectively) which separates the two liquid 

 layers corresponds in reality either with a boundary layer between 

 two layers of liquid or with an actually existing permeable wall. 

 The dimensions of the whole have been chosen in such a manner 

 that the size of the surface LXFG is equal to the surface of the 

 actual dividing layer between the two volumes of liquid, and also 

 that the volumes of the upper and lower space are in the proportion 

 of V : V. (Hence, the volume of the insoluble matter R present in 

 the mass has not been taken into consideration). From this arrange- 

 ment, it follows that the velocity of motion of the mass to be 

 lixiviated and that of the lixiviating liquid are equal '). 



II. Semi-continuous lixiviation according to the counter-current principle. 



Let us imagine, as indicated in fig. la, the whole space to be 

 divided, by planes perpendicularly to the longitudinal direction, into 

 a finite number {n) of equal parts whose length is A/. 



In each of the thus formed divisions the liquid is homogeneous 

 in regard to the concentration. A small quantity of liquid which 

 flows from one division into another will at once mix homogene- 

 ously with the liquid contained therein. The condition in the whole 

 system is stationary; hence, the concentration in each division is 

 constant and is indicated bj the. letters inscribed. 



Let us imagine the process to be in operation. The mass is moved 



di 

 along with a velocity -- , the lixiviating liquid has the velocity 



dl 

 — — . Let us consider the divisions in which the concentration is 

 dt 



6, or a^, respectively. 



The amounts {ih) of the substance <S which, in the small period oi 



1) See note 1 p. 210. 



