294 CHAPTER XVI 



in the presence of tin or of zinc. About 1904 stable hydrosulphites were 

 manufactured, the calcium salt being sold under the name of " Redos " 

 and the sodium salt as " Blankit." These react under the equation, 

 Na 2 S 2 4i -f- -f H 2 = 2NaHS0 3 . In the cane sugar industry they have 

 been chiefly employed in the decolor ization of syrups in white sugar manu- 

 facture. As the bleached material colours again on exposure to the air, they 

 are used in the vacuum pan shortly before striking. The quantity required 

 to obtain the maximum effect varies with different juices. With those that 

 the writer has had to deal, it is about one Ib. per ton of sugar, though the 

 makers state that considerably less is usually required. The claim that so 

 small a quantity can materially affect the viscosity is unworthy of con- 

 sideration. 



Phosphoric Acid and Phosphates. Phosphoric acid is employed as a 

 defecant either as the free acid or as a soluble phosphate. Their action 

 depends on their property of forming a bulky precipitate with lime, which 

 on its formation entangles and carries down colloid matter. This action 

 occurs to a certain extent in lime defecation due to the presence of phosphates 

 in juices. To obtain the maximum effect the lime and acid should be present 

 in the proportions to form the tri-basic salt. The quantity used is generally 

 about 5 Ibs. per 1,000 gallons of juice. Apart from this action, phosphoric 

 acid is a very weak acid, and it hence forms a convenient agent for obtaining 

 an acid reaction when using an acid thin- juice process, and it is this function 

 which is employed in Demerara and in Mauritius. These two effects may 

 of course be combined in the same factory, though not in the same operation. 

 The descriptions extant of the routines adopted in Java in white sugar 

 manufacture do not indicate that phosphoric acid is used there in an acid 

 thin-juice process, though there would be advantages in doing so, reserving 

 sulphurous acid for the final decolorization of the syrup. Sodium phosphate 

 in the form of the di-basic salt has also been used as a means of removing 

 small traces of iron salts from syrups immediately before boiling to grain. 



The use of phosphates as a defecant seems to be first mentioned in patent 

 13634, 1851, granted to Oxland and Oxland, and again in one issued to Col- 

 lette (i of 1854). Their introduction into the cane sugar industry is due to 

 Ehrmann in Mauritius about 1860. 



Alumina. Salts of aluminium in the presence of alkalies afford a very 

 bulky precipitate of the hydroxide which at the moment of its formation 

 carries down much colloid and colouring matter. Such a reaction was used 

 in the cane sugar industry at least 150 years ago, and is described in the 

 Marquis of Cazaud's treatise of date 1770. The property is also included as a 

 claim in Howard's patent (3754, 1813), and the alumina so prepared was for 

 long known as " Howard's finings." Proposals for its use in one or another 

 way are still occasionally made. 



Tannin. Although tannin (and tannic acid) is one of the bodies most de- 

 sirable to remove from juices, the bulky precipitate that is formed by the 

 action of lime, etc., has led to the idea that plant extracts containing tannin 

 afforded a purification of the cane juices to which they were added. The 

 idea has been put into practice from very early days, and is still employed 

 by the ryots of British India in their domestic processes. Its use forms the 

 subjects of patents issued to Stokes (5555, 1827) and to Watson (7124, 1836). 



