EVAPORATION 373 



Scale in Evaporators. — The concentration of the juice which obtains in the 

 evaporators results in certain of the non-sugars becoming insoluble, and 

 being deposited as " scale " on the heating surfaces of the evaporators. In 

 addition to the scale formed from bodies originally in solution, there is that 

 caused by the introduction of suspended matter due to inef&cient defecation. 

 The latter deposit is foimd mostly in the first ceU, and the former in the last 

 cell, where the concentration of the juice is greatest. The scales which are 

 found in cane sugar houses fall into three classes — siHcate, phosphate, and sul- 

 phate scales, the two former being the most frequent. The quantity of scale 

 formed is also a function of the lime employed, which may introduce sihca. 

 The use or non-use of phosphoric acid and sulphur will affect the quantity of 

 the phosphates and sulphates in the juice. In the absence of the use of these 

 agents the greater portion of the phosphoric acid is precipitated and is found 

 in the piess cake. Its maximum precipitation, however, depends (c/. Chapter 

 XIII) on the use of an excess of lime when simultaneously lime salts enter 

 into solution. 



Sulphates are frequently absent from the deposit of scale, but may occur in 

 certain juices in vet}' large quantity. The cause of this appearance is obscure ; 

 it is, however, certainly to be correlated with variety of cane. 



It is evident that, while a deposit of scale due to suspended matter may be 

 eliminated by careful work, that due to the deposit of dissolved matter is 

 obHgatory. It may be controlled by the use of selected limestone, and by using 

 no more lime than necessary to protect the juice from inversion. The presence 

 of sulphates in the juice is the most troublesome factor. Peck^^ advises the 

 use of sodium carbonate in the clarification, to precipitate the lime as carbonate 

 and to substitute sodium sulphate for calcium sulphate. 



The prevention of the deposit scale has been attempted by placing in the 

 tubes rods or chains, en wliich it was intended that the scale should deposit, 

 and which by tapping continuously against the walls of the tubes would prevent 

 the scale adhering. Rapid circulation is also believed to prevent the adherence, 

 and the system of reversing circulation used in the LiUie horizontal film 

 evaporators is claimed also to keep the surfaces free ; but no mechanical 

 means can alter the solubihty of the substances causing scale, so that the most 

 these schemes can do is to remove the scale from one part of the sugar house 

 to another. 



Recognising the unavoidabiUty of scale, the means for its removal remain 

 to be considered. The agents most often employed are caustic soda or car- 

 bonate of soda followed bj^ hydrochloric acid. The strength of these solutions 

 is from i per cent, to 2 per cent., smd apparatus are boiled out periodically. 

 The time required varies with the deposit of scale, but generally four hours' 

 boiling under atmospheric presstire with each reagent is sufficient to maintain 

 a reasonable ef&ciency in the apparatus, if done once a week. 



The experiments of Peck and of Thurlow^^ indicate that generally sodiimi 

 carbonate is as efficient in combination with acids as is caustic soda, and the 

 use of the former is essential when removing a calcium sulphate scale, which 

 has to be converted into carbonate before it can be attacked by acid. \^ ith 

 a silica scale the use of caustic soda in indicated, and with a phosphate scale 

 acid alone is enough, provided the scale is not protected by a layer of fats or 

 grease. It would appear not unreasonable to use a mixture of carbonate and 

 caustic soda. The WTiter's experience, however, is that all solution methods 

 are inferior to mechanical ones as regards cost, speed, and efficiency. Small 

 compressed-air motors operating wire brushes, and specially designed for 



