418 



CEMENTS, LIMES, AND PLASTERS. 



A Bonnot 22' X 5' tube mill used at a marl-plant on not very wet 

 slurry ground about 20 to 30 barrels per hour, taking 30 H.P. in doing 

 so. This slurry had not been previously treated except by passing it 

 through a stone separator, so that the total power for grinding raw 

 material at this plant was from 1 H.P. to !$ H.P. per barrel cement. 



At another plant a Bonnot 16-foot tube mill ground 12 barrels per 

 hour raw wet mix, taking 15 to 20 H.P. in doing so, the marl having 

 previously been passed through a stone separator and pug-mill and the 

 clay through dry-pans. 



Other plants report slightly different results with wet-tube mills. 

 To sum up, from all data it seems that the preparation of a wet mix 

 in tube mills will usually require from 1 H.P. hour to 2 H.P. hours 

 per barrel cement. The power and product, however, will vary greatly 

 with the percentage of water in the mix, as well as with the hardness 

 of the particular marl and clay employed. 



The highest power consumption per barrel was shown by a plant 

 which required 3.9 H.P. hours per barrel for preparing its raw mate- 

 rials for the kiln. The marl used at this plant is unusually hard, and 

 the mixture is made with less water than usual. This gives a fairly 

 high kiln efficiency (100 barrels per day per kiln with a fuel consump- 

 tion of 160 Ibs. coal per barrel), but it largely increases the work to 

 be done by the grinding machinery on the raw side. Several of the 

 grinding-mills used at this plant are, in addition, very inefficient types, 

 and to this combination of unfavorable conditions is to be ascribed 

 the high-power consumption on the raw side of the plant. 



All of the tanks containing slurry must be provided with some appli- 

 ance for agitating the mixture or otherwise the heavier portion would 

 settle at the bottom of the tanks, leaving fairly clear water above. Three 

 different methods of agitating are in use at various marl-plants : 



1. A vertical central shaft equipped with long arms or paddles; 



2. A horizontal shaft crossing the tank a little above its bottom 



and fitted with screw blades; 



3. The injection at intervals of jets of compressed air. 



Any of these three devices gives fairly good results, but none of them 

 seems entirely satisfactory to the managers of the plants in which they 

 are installed. The first two use an unexpectedly large amount of power. 



It may be of interest, for comparison with the above description 

 of the wet process with rotary kilns, to insert a description of the semi- 

 wet process as carried on a few years ago at the dome-kiln plant of the 

 Empire Portland Cement Company of Warners, N. Y. The plant has 

 been remodeled since that date, but the processes formerly followed 



