416 CEMENTS, LIMES, AND PLASTERS. 



by a blast of air supplied through tuyeres, and a pressure about 10 to 

 20 Ibs. above that of the atmosphere is maintained in the furnace, 

 whereby the materials are melted, the molten cement being afterward 

 drawn off, cooled, and pulverized. The carbon dioxide derived from the 

 carbonate of lime is reduced to carbonic oxide by the incandescent 

 fuel, and in this atmosphere any oxide of iron in the raw materials is 

 said to be reduced to metallic iron, which sinks and can thus be sepa- 

 rated from the molten cement, whereby a superior product is obtained. 

 The carbonate of lime may be preliminarily calcined and the carbon 

 dioxide introduced together with air into the calcining furnace, where it 

 is reduced and then again burned to carbon dioxide. The increased pres- 

 sure is maintained either by arranging the height of the kiln so that the 

 combustion gases formed in the lower part are prevented from escaping 

 freely by the height of the mass of materials above or by a throttle- 

 valve arranged in the outlet at the top of the kiln. 



Von Forell has taken out foreign patents on processes of quite similar 

 type. 



(4) Wet Methods of Preparation. 



Wet methods of preparing Portland-cement mixtures date back 

 to the time when millstones and similar crude grinding contrivances 

 were in use. With such imperfect machinery it was almost impossible 

 to grind dry materials fine enough to give a good Portland-cement mix- 

 ture. The advent of good grinding machinery has practically driven 

 out wet methods of manufacture in this country, except in dealing 

 with materials such as marls, which naturally carry a large percentage of 

 water. Two plants in the United States do, it is true, deliberately 

 add water to a limestone-clay mixture; but the effect of this practice 

 on the cost sheets of these remarkable plants is not encouraging. 



In preparing cement mixtures from marl and clay, a few plants dry 

 both materials before mixing. It seems probable that this practice 

 will spread, for the wet method of mixture is inherently expensive. 

 At present, however, almost all marl-plants use wet methods of mixing, 

 and it is therefore necessary to give some space to a discussion of such 

 methods. 



Certain points regarding the location, physical condition, and chem- 

 ical composition of the marls and clays used in such mixtures have impor- 

 tant effects upon the cost of the wet process. As regards location con- 

 sidered on a large scale, it must be borne in mind that marl deposits 

 of workable size occur only in the Northern States and in Canada. In 

 consequence the climate is unfavorable to continuous working through- 

 out the year, for the marl is usually covered with water, and in winter 



