41 



ducts of combustion are introduced) into the cylinder at its 

 lower end, and drawn through it, and escape up a stack set at 

 the upper end of the dryer. 



The dryer abovfe described is the simplest, and is most com- 

 monly used. For handling the small percentages of water con- 

 tained in most cement materials it is very efficient, but for deal- 

 ing with high percentages of water, such as are encountered 

 when marl is to be used in a dry process, it seems probable that 

 dlouble-heating dryers will be found more economical. This 

 type is exemplified by the Ruggles-Coles dryer, in 

 which a double cylinder is employed. The wet raw 

 material is fed into the space between the inner and outer 

 cylinders, while the heated gases pass first through the inner 

 cylinder, and then, in a reverse direction, through the space be- 

 t \\een the inner and outer cylinders. This double-heating type 

 of dlryer is employed in almost all of the slag cement plants in 

 the United States, and is also in use in several Portland cement 

 plants. 



When vertical kilns were in use, drying floors and drying 

 tunnels were extensively used, but at present they can be found 

 in only a few places, being everywhere else supplanted by the 

 rotary dryers. 



The cost of drying will depend 011 the cost of fuel, the per- 

 centage of water in the wet material and the type of dryer, 

 iwen under the most unfavorable conditions five pounds of 

 water can be expected to be evaporated per pound of coal used, 

 while a good dryer will usually evaporate seven or eight pounds 

 of water per pound of coal. 



GRINDING AND MIXING DRY METHODS. Part at least of 

 the grinding is usually accomplished before the drying, but for 

 convenience the subjects have been separated in the present 

 paper. Usually the limestone: is sent through a crusher at the 

 quarry or mill before being dried, and occasionally the raw ma- 

 terial is further reduced in a Williams mill, etc., before drying, 

 hut the principal part of the reduction always takes place after 

 the material has been dried. 



After the two raw materials have been separately driedi they 

 may be mixed immediately, or each may be further reduced sep- 

 arately before mixing. Automatic mixers, of which many 

 types are on the market, give a mixture in proportions deter- 

 mined upon from analysis of the materials. 



