26 THECUBAREVIEW 



The ordinary grade of calcium carbonate is one of the most common as well as one of the 

 most widely distributed minerals found on the earth's surface. It occurs in large masses in the 

 form of marble, limestone and chalk and often in large deposits where great islands are built 

 up from these deposits of the calcareous bodies of countless coral insects. In fact the numerous 

 deposits found in the interior of continents are often of organic origin. These vast deposits 

 are the mineral remains of animal life, such as crinoids, corals antl molusks during bj-gone 

 ages when the land was submerged. 



In addition to the above deposits limestone of sedimentary origin often occurs. The 

 limestone found in these deposits often appears as stalagmites, stalactites, pillars and in various 

 other forms. These are from waters bearing lime in the hydroxide or other forms which come 

 in contact with waters laden with carbon dioxide, the carbonate form or limestone resulting, 

 and since tliis is pi-actically insoluble in water it is deposited as mentioned above. The following 

 equation shows how limestone is formed in such cases: 



Calcium hydroxide Ca (0H)2 plus carbon dioxide CO2 — •> 

 Calcium carbonate or limestone CaCOs plus water HoO. 



Since calcium does not occur in the pure state, it is obvious that lunestone is not formed in 

 nature directly from the elements themselves. Instead combinations of these elements unite, 

 forming calcium carbonate according to the above equation. In fact, this union is taking 

 place constantly in various parts of the earth's surface and in many instances it gives rise to 

 the deposits where the rocks are widely quarried for economic purposes. This same change is 

 constantly taking place where the lime in the oxide and hydroxide form used on the field and 

 in the factory is left to stand in the open air or in contact with water laden with carbon dioxide. 

 Likewise, a similar chemical change is constantly taking place only to more limited extent 

 when the lime (calcium oxide and calcium hydroxide) is applied to cane juices heavily laden 

 with impurities. When lime is applied in the clarification of juices it first combines with the 

 acids which are neutrahzed. A portion then unites with various impurities and changes them 

 from light flocculent substances to a heavier form. This gives rise to the heavy precipitate 

 found in the lower portion of the settUng tanks and defecators. 



Manufacture of Lime. 



In the manufacture of lime from calcium carbonate the reverse process takes place to that 

 of forming the rock. Just as heat is evolved during the union of certain chemical compounds 

 heat is also required in breaking up the compounds. In the case of the reducing of calcium 

 carbonate to the oxide form intense heat is required to bring about the change. The following 

 formula indicates the change which takes place: 



Calcium carbonate CaCOs in the presence of a high temperature — > 

 Calcium oxide or quicklime (CaO) plus carbon dioxide CO2. 



Where it is desired to employ carbon-dioxide gas in the clarification of juices the lime is 

 always burned at the factory. This system is universally employed at the beet-sugar factories 

 and it is used to some extent nowadays in connection with cane-sugar manufacture where im- 

 proved systems of clarification with carbon-dioxide gas are employed in the manufacture of 

 high-grade plantation sugar. 



Limekiln 



The kQn or furnace in which lime is burned consists essentially of a suitable structure where 

 alternate layers of the lime rock and combustible material are deposited and where suitable 

 conditions obtain for biirning the lime. The modern types of kilns used in preparing lime for 

 sugar factories usually consist of cylindrical structures of masonry work or iron which are lined 

 with fire-resisting brick. Figure 1 is a common type of kiln found in American factories. The 

 so-called continuous kilns are the most satisfactory . These are so arranged that they may be 

 filled with new rock and combustible material and the finished lime removed at the same 

 time. This permits the continuous production of carbon d,ioxide as weU as lime and produces 

 a high-grade finished product when properly operated. Plate X shows a view of a smaU kiln 



