206 



It is a mistake to suppose that the spacing of the screen bars 

 is a measure of the average or largest particles which will come 

 through; the pieces of stone in the disintegration chamber are 

 being driven round with a very high cn^cumferential speed, and 

 only when they are very much smaller than the width of the 

 slots between the bars do they find their way through the screen. 

 For this reason it will seldom be necessary to space the bars 

 nearer than J in., and for grinding chalk f to J in. wdll be 

 suitable. The harder and more friable limestones will require the 

 closest spacing of screens in order to give a satisfactory propor- 

 tion of very fine dust. 



When the mill has become much worn, care should be taken 

 when renewing old screen bars that no ridge exists between the 

 old and the new ones, since any unevenness in the periphery 

 of the grinding chamber will cause a totally disproportionate 

 quantity of large particles to go through. 



The best results are obtained w^hen the mill is run right up to 

 the safe working maximum speed as stated by the maaufac- 

 turers : loss of speed means not only smaller output, but a less 

 finely ground product. 



All varieties of limestone, if freshly quarried, i.e., not much 

 exposed to the weather after being broken, will be dry enough 

 to grind without any preparation. This is, however, far from 

 being the case with chalk. Soft chalk will need to be dried before 

 it is passed through the mill, oi- it viill " pug " and form a 

 paste which will effectively block up the screen and stop the 

 machine. 



It is not easy to devise means for artificially drying challi. 

 Undoubtedly the best course to adopt, where covered storage 

 space is available, is to quarry the chalk and leave it stacked 

 under cover for several months to dry before it is ground. 

 Where there is no such storage space, a drying fioor is probably 

 the simplest means, but experience shows that it will take nearly 

 1 cwt. of coal to dry a ton of chalk sufficiently to enable it to be 

 ground. Some of the harder chalks will give much less trouble 

 than soft chalk, such as that of the Xorth or South Downs. 



In any c;iven instance it is easy to ascertain whether the raw 

 material wdll require to be- dried, for if on taking a fair sample 

 and drying it out completely it is found to lose more than r2 

 to 14 per cent, of its weight, it will not satisfactorily go through 

 one of these small portable uiills without being dried beforehand. 



