1922.] The Need for Lime : IIow to Meet It. 875 



give a maximum yield of lime per ton of fuel burned. In order 

 to avoid this loss and to obtain certain other advantages in the 

 way of more direct control over the temperature, many kilns 

 have been built, in which by one means or another, the stone 

 is pre-heated by the waste gases before the fuel is mixed with it. 



R'tng -Fired Kilns. — One of the most thermally efficient, and 

 perhaps most effective w^ays of pre-heating the stone alone, 

 consists in building a kiln in such a way that the pre-heating 

 section is of pronouncedly smaller diameter than the reaction 

 zone, the step in size being made as sudden as possible, with 

 the result that the stone, as it descends from the small part 

 into the larger, leaves a considerable space between itself and 

 the increased diameter of the kiln v/all, into which it is possible 

 to introduce the fuel, as required, by means of sloping passages 

 in the kiln wall, fitted wdth fire doors. The re-distribution of 

 the lime blocks, as they tumble down from above is relied upon 

 to produce a sufficiently even mixture of stone and fuel. The 

 distribution is, however, not usually as uniform in such kilns 

 as when the coal and lime are fed together into the top of the 

 kiln. There is therefore a definite limit to the ratio, diameter 

 to depth of the reaction zone in kilns of this type. 



In the case of each kiln so far described, the product as it 

 comes through the eye is a mixture of lime and ashes. When 

 the lime is exclusively for agricultural use, there is little dis- 

 advantage in this, and in any case a separation of the small 

 lime and ashes can always be effected by handling it w-ith forks 

 when loading. 



Separate Furnace Kihis. — In this kiln the lime is delivered in 

 a purer condition than in the kilns described above, and is quite 

 free from ash. This is accomplished by arranging that the 

 fuel itself never comes in contact with the stone. It is burnt 

 in furnaces built in the kiln wall, but out of the line of direct 

 descent of the stone. By this means the gases from the 

 furnaces containing the w^hole of the heat generated by the 

 fuel (except a small proportion lost by radiation from .the 

 furnace crown and walls) enter the kiln through aperatures in 

 the side at the bottom of the " reaction zone "; here they 

 mingle with the stone and pass up through the kiln, pre- 

 heating the descending charge in the ordinary way. The 

 regenerative principle can only be applied, however, if fans 

 are used to draw the air for the furnace through the cooling 

 lime. This system has the advantage that any type of fuel 

 which may be available in the district can be used, and the 



