CHAPTER XIV 

 The Defecation of Cane Juice 



By defecation* is understood the process by means of which a clear negotiable 

 juice is obtained by the combined action of heat, lime, settling and decanta- 

 tion. This simple process is that which is used in the manufacture of 96° 

 test sugars, and in combination with sulphurous and phosphoric acids, in the 

 manufacture of plantation white and yellow grocer\' sugars. 



The Mechanism of Settling. — The general law under which bodies fall 



through a resistant medirnn was first given by Stokes^ : v = ~ — 



r2 g 



i 

 Z 



y. •■;;-. ^ 



where r is the velocity of the falling body, d-^^ and ^2 are the densities respect* 

 ively of the falling body, and of the resistant medium, r is the 

 radius of the body, g is the acceleration due to gra\'ity, and fx 

 is the \'iscosity of the resistant medium. In a cane juice i/j and 

 r will not suffer much change from juice to juice; d^ will vary 

 . between the hmits i • 05 and i • oS. and of the change in /z nothing 

 is known except that it will fall rapidly ^vith rise in temperature, 

 and that otherwise it wiU not var\^ greatly. However, d^ will 

 always be much larger than d.^, so that change in the latter 

 will not affect the value of d-^^ — d.2 enough to be of importance in 

 design. It follows then from these considerations that the settling 

 of one cane juice should be tj'pical of all, and that setthng should, 

 over a great part of the process, take place at a uniform rate. 



If the settling of a suspension such as almnina hydroxide, of 

 concentration about • 2 gram per 100 c.c. be studied in a tall, 

 narrow tube, there will be seen a short preliminan,' phase lasting 

 about two minutes, during which no individual particles can be 

 recognised, and over which no settling occurs. After settling has 

 begun, the system soon resolves itself into five zones. The 

 uppermost zone, i {Fig. 154) is quite clear : next in order is 

 zone 2, characterized by the presence of isolated falling particles 



which m.av be called stragglers. 



-^(r'■;■r^. 



These stragglers terminate in a zone 3, about 0-5 cm. deep, 

 in which the particles have only a vertical downward movement. 

 Below this is zone 4, in which at an early stage of settling is ^'^^- ^54 

 contained the great proportion of the suspended matter. In 

 this zone there can be recognised a continuous downward and upward stream 

 of particles, the boundaries of which are the contingent stufaces of zones 3 

 and 5. Zone 5 consists of those particles that have come to rest on the 

 bottom of the tube, or later on the top of the column being built up. 



The upward and downward stream of particles in zone 4 will be seen to 

 be continuous ; as upward-mo%-ing particles approach zone 3 they turn 



269 



