300 CHAPTER XVII 



with juices of high purity, and even then it is probable that more lime than 

 necessary to effect defecation must be used. 



Filtration of the clear decanted juice is supererogatory when making 

 96° test sugars since the small amount of suspended matter present in well 

 defecated juice is immaterial with this type of sugar. 



Routines followed in Carbonation Processes. — No choice exists here. The 

 whole mass of the juice is filtered in plate and frame presses after both 

 first and second carbonation. The filtration is followed by washing as 

 these materials offer no obstacle thereto. 



Routines followed in Sulphitation Processes. — Any of the schemes men- 

 tioned under defecation may be followed. As brightness is now of value, 

 the filtration of the decanted juice occupies a position of importance. In 

 place of this operation it is, however, more common and rational to filter the 

 syrup after it leaves the evaporator and to thus remove also in one operation 

 the material separated on concentration. 



Treatment of Scums before Filtration. — The rate of filtration of scums is 

 much greater when they are hmed to distinct alkalinity as indicated by 

 phenolphthalein than when they are neutral. Also the rate is increased 

 when the scums are actually boiled for a very short time, and both these 

 treatments are common. The admixture of the very alkaline filtrate with 

 the bulk of neutral juice is, however, not advisable. Means to eliminate 

 this trouble and yet to employ a distinct alkalinity are indicated in the chapter 

 on Defecation. A third treatment which is used to a certain extent is the 

 admixture of kieselguhr or diatomaceous earth with the scums. Although 

 the advantageous action of this material is easily demonstrable, it is not 

 extensively used in the cane sugar industry, the effects not being commensu- 

 rate with the expense of obtaining the material, except in special cases. 



Filtering Media. — In nearly every case the medium through which 

 filtration takes place is a stout, closely woven cotton cloth. This material 

 is used to form the filtering surface in bag filters, leaf filters and in presses. 

 Very lately, however, a woven metallic cloth has been put into use. Other 

 materials that are employed in special forms of filters are coke, gravel, sand, 

 sawdust, wood shavings and bagasse. All of these materials can, however, 

 only be used to remove a very small quantity of suspended matter from 

 juices that are very nearly clear. 



Principles involved in Filtration. — ^The variables to be considered in filtra- 

 tion are the pressure under which filtration occurs, the thickness of the cake 

 through which the filtrate has to pass, the viscosity of the liquid and the 

 size of the solid particles. When the last two factors were constant Almy 



and Lewis^ found that with the pressure also constant R = -^ where R 



is the rate of flow, V is the volume of the filtrate, and i^ is a constant ; 

 evidently at any moment V is proportional to the thickness of the cake. 



With pressure varying they found that the relation R = — r^r^held. 



They point out that filtration may be considered as a flow through a 



■n-r* P 

 rapillarv tube, the equation for which is C = —^r ^^'^^'"^ ^ ^^ ^^^ radius 



