CHAPTER XVII 

 FILTRATION 



THE importance of filtration in a raw sugar factory depends on the class 

 of sugar made. When 96 test crystals form the output, nitration is usually 

 confined to the scums formed on defecation, and it is only exceptionally that 

 the juices themselves are filtered. When, however, white sugars are made 

 by a combined defecation and sulphitation process, filtration is of importance 

 since the appearance of the sugar largely determines its market value, and 

 bright sugars can only be made from a transparent juice free from suspended 

 matter ; in fact, this feature is of equal importance with the colour of the 

 syrups. In the carbonation process, also, filtration becomes of importance 

 because of the very large quantity of material that has to be filtered. 



Routines followed in Defecation Processes. The heated and limed juice 

 is allowed to settle in tanks, whence the clear juice is decanted, leaving 

 from 10 to 15 per cent, of the whole volume of the juice as a mud. The mud 

 may be : 



(a) Pumped direct to the presses, where it may or may not be washed. 



(b) The mud is run to resettling tanks, diluted with water, blown up with 

 live steam and sent to the presses. 



(c) The method in b may be systematized so as to economize water by 

 using the filtrate from the presses to dilute the original mud, which, after 

 blowing up, is allowed to settle. The clear dilute juice drawn off is sent to 

 the evaporators, the mud being then diluted with water, blown up and sent 

 to the presses. 



(d) When the mud is washed in the presses the dilute juice may be 

 economically used to dilute the original mud. 



(e) After one pressing the mud may be dropped from the presses unwashed, 

 pugged in a mixer with water, and pressed a second time. As in the other 

 routines, economy may be effected by using the second filtrate to dilute the 

 original mud. 



(/) In combination with any of the above schemes the decanted juice 

 may be filtered through leaf filters, through sand, " excelsior," bagasse or 

 other similar material. 



(g) The whole juice may be filtered en masse through plate and frame 

 presses. 



Of all these schemes decantation combined with scum filtration in plate 

 and frame presses with washing in the press combined with the systematized 

 use of the dilute washings is to be preferred. With certain juices washing 

 of the scums is a very slow process, and in such cases double filtration is 

 preferable. 



The filtration of the juice in bulk can only be satisfactorily performed 



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