922.] The Need for TiiME : How to Meet It. 



871 



way out with the bulk of the material i thus taking up valuable 

 space which might be better occupied by stone particles still 

 requiring reduction. Mills of this type can be employed when 

 a very fine product is aimed at, such as would leave 5 per cent, 

 residue on a 60 screen or finer. 



A grinding plant to deliver 50 tons of ground stone a day, 

 and consisting of machines similar to those described, would 

 require approximately 100 h.p. to operate it, and the actual 

 quarry and mill costs involved in producing the ground material 

 should not amount to more than about 8s. per ton. To this 

 must be added sums representing overhead charges, including 

 interest on capital and maintenance of plant. 



The qualities of stone suitable for agricultural purposes, the 

 degree of fineness of grinding, the chemical composition, etc., 

 have all been dealt with in the previous article, to which 

 reference should be made. Following this brief sketch of the 

 means available for increasing the supplies of ground limestone, 

 an outline may now be given of the plant and processes used 

 in preparing quicklime, devoting particular attention to kilns 

 and methods most likely to be of service to agriculturists who 

 recognise the necessity of cheap lime, and are prepared to take 

 the necessary steps to burn it for themselves. 



Lime Burning. — The very ancient craft of lime burning con« 

 sists in heating any fairly pure limestone or chalk (CaCO§) to 

 a temperature high enough to drive off the carbonic acid gas 

 (CO 2), wiiich is combined with it, leaving behind the lime or 

 calcium oxide (CaO). It is one of the very simplest of com- 

 mercial chemical processes, but it is not, for all that, one w^hich 

 can be uniformly carried out w-ithout both theoretical know- 

 ledge and experience. 



Before describing some of the types of kiln in w^hich the 

 burning is done, it w^ill be well to look at the theoretical con- 

 siderations which are involved. Carbon dioxide begins to be 

 driven off from limestone at a temperature of about 600° C, 

 but it is necessary to raise the temperature to somewhere 

 between 900° C. and 1,200° C. in order to make sure that the 

 burning is complete. Even so, it is necessary that the stone 

 should remain at this temperature for an appreciable time, and 

 not simply be hurried through the hottest part of the kiln. The 

 effect of either too low a temperature, or a too rapid passage 

 through the reaction zone (as that part of the kiln is called 

 where the highest temperature is attained) , is the production of 

 lime in which there is a large amount of " core," composed of 



