330 



NATURE 



[June 15, 1916 



hardening it looked like Portland stone ; but though 

 the composition was similar to irliat of modern Port- 

 land cement, the mixed material was only lightly 

 calcined. Portland cement is manufactured in Eng- 

 land, chiefly about the Thames and Medway, Rugby, 

 Leamington, Cambridge, Hull, and the north-east 

 coast, and also in South Wales. 



The preparation and mixing of the raw material 

 before burning is effected by the dry process or the 

 wet process. The method known as the semi-wet 

 process is practically the same as the wet process, 

 using less water. 



In the dry process the raw material is stored under 

 cover before being crushed, so that the exact amount 

 of moisture may be ascertained and allowed for when 

 mixing lime with the shale or clay. After preliminary 

 crushing in gyratory or jaw-crushers, the raw mate- 



to burn the slurry than the dry powder in the dry 

 process. 



At the Aberthaw cement works are beds of hard 

 crystalline limestone interstratified with beds of shale, 

 all the necessary materials thus occurring together on 

 the spot. The quarrying is done by means of a steam 

 navvy, aided by a small amount of powder to shake 

 the face of the quarry. 



The crushed material is ground in vertical mills 

 (chiefly in America), or in horizontal mills (mostly in 

 Europe). Horizontal mills are generally installed in 

 pairs, a ball-mill for preliminary grinding, and a tube- 

 mill as a finishing mill. The tube-mill is much longer 

 than the ball-mill, and contains flint pebbles of various 

 sizes instead of steel balls. The ground material from 

 the ball-mill passes through sieves to reach the tube- 

 mill, the portion retained by the sieves being auto- 



FlG. I. — Raw-mill grinding-housejat Aberthaw during construction, showing arrangement of ball and tube-mills. 



rials are dried, then weighed, and delivered to the mills 

 in definite proportions. After grinding to an extremely 

 fine powder the mixture is fed into the kiln for burn- 

 ing. In the wet process the material is often delivered 

 in the correct proportions from the quarry into crushers 

 or wash-mills. 



On the Thames and Medway, the raw material, con- 

 sisting of soft chalk and river mud, is washed through 

 fine-meshed sieves, and the " slurry " is then pumped 

 or elevated to the kiln. At Aberthaw, the raw mate- 

 rial, consisting of hard limestone and shale, is crushed 

 in jaw-crushers and delivered to the wet mills for 

 grinding with water to a fine slurry. 



In the wet process less power is required to grind 

 hard material, and the slurry is easily dealt with by 

 means of pumps ; but more fuel is needed in the kiln 



NO. 2433, VOL. 97] 



matically returned to the ball-mill for further grinding. 

 In the wet process similar mills are employed with 

 only coarse sieves or screens, as otherwise thev would 

 tend to get choked. 



At Aberthaw, after leaving the mills, the slurry falls 

 into a trough, and by means of a special conveyer is 

 delivered to two slurry pumps, which deliver the 

 slurry into one of two large storage tanks. The 

 chemist takes half-hourly samples from each mill, and 

 hourly samples from the large storage tanks while 

 being filled. The mixture in the tanks is thus kept 

 practically constant, and is continually agitated. 



From the storage tanks the slurry is delivered to 

 the feeding apparatus of the nearly horizontal rotary 

 kilns. Dried, finely-powdered coal-dust is blown into 

 the outlet end of the kiln, and ignites 8 to lo ft. from 



