462 CEMENTS, LIMES, AND PLASTERS. 



in certain parts of Europe and are comparatively inexpensive and of 

 extraordinary hardness. It is the combination of these qualities that 

 makes them preferable to the steel balls, even to the extent of building 

 the mill four times as large as would be required with the steel balls. 

 These flint balls are sold in four different sizes. The mill is originally 

 charged with all four sizes in a set proportion, but replacements are 

 always made with the largest size, as in practice the smaller pebbles 

 seek the outlet end, and replacements are made at the inlet end. 



"On average Portland-cement clinker the pebbles wear at the rate 

 of about 1 Ib. to 30 barrels of product. 



"Little or no harm follows the introduction into the mill of sub- 

 stances foreign to the intended supply, such as steel or iron. When 

 such substances reach the interior of the mill they either become grind- 

 ing mediums and remain in the mill to accomplish that work, or else 

 they are ground by the action of the mill and pass out with the product. 

 This result is so unlike that following the introduction by accident of 

 a piece of hard material into crushers, rollers, buhrstones or like ma- 

 chines, where the grinding surface is of metal, emery or dressed stone, 

 that this one fact recommends it promptly to the observing purchaser. 



"Two sizes of the Davidsen tube milt are made: 



"No. 16. Requires 60 H.P. and a floor space of 36 feet by 13 feet, 

 and turns 25 revolutions per minute. 



"No. 12. Requires 27 H.P. and a floor space of 29 feet by 11 feet, 

 and turns 27 revolutions per minute. 



"The tube mill is a pulverizer, not a coarse grinder. 



"The ' ' grindability' of materials, even of the same class, varies so 

 widely that it is impossible to give more than a general idea of the ca- 

 pacity of the tube mill. As an index, however, it may be stated that 

 the No. 12 tube mill will pulverize from 3500 to 6500 Ibs. of Port- 

 land-cement clinker per hour, 95 per cent of which will pass 10,000 meshes 

 per square inch, such clinker having been preliminarily ground to pass 

 No. 20 or No. 30 sieve. 



"The No. 16 has about two and one quarter times the capacity of 

 the No. 12." 



Krupp tube mill. The Krupp tube mill is described by its agents 

 as follows: 



"The Krupp tube mill is a cylinder or drum 5 feet in internal diam- 

 eter and 22 feet long between the heads of the mill. This mill is so 

 designed as to admit either silex or iron lining being installed. 



"The feeding mechanism, the most accessible and most easily ad- 

 justed feeder on the market, is so designed that in making a change 



