MINERAL INDUSTRIES FULLERS EARTH. 3I 



The driers employed are for the most part rotary cyhnders. 

 Those in use in the Florida mines are from 40 to 60 feet in length 

 and about 6 feet in diameter. When in operation they rotate 

 slowly, the earth being moved along by means of flanges attached 

 to the inside of the cylinder. These cylinders are heated to a 

 moderate heat by petroleum burners, the heat being applied either 

 at the end where the wet earth enters, or at the opposite end from 

 which the diy earth escapes. Overheating is not feared in these 

 plants, as the earth is used for filtering mineral oils. When the 

 earth is to be used for edible oils precautions are taken to avoid 

 overheating, as driving off the combined water is supposed to be 

 injurious. To guard against overheating especially constructed 

 rotary cylinders are used, or the earth is run into brick form and 

 is dried in tunnel driers through which hot air is forced. Although 

 the English fullers earth is injured by driving off the combined 

 water, it has been found that some at least of the American 

 earths bleach fully as well after the combined water is removed, 

 and it is probable that these precautions against overheating the 

 earth for edible oils are in some cases at least unnecessary. 



GRINDING AND BOLTING. 



In grinding the fullers earth a variety of mills are in use. 

 After grinding, the earth is bolted. That intended for refining 

 petroleum is bolted to a definite size and is placed on the market 

 graded as 15 to 30 mesh, 30 to 60 mesh, 60 to 80 mesh. The 

 coarser sizes are in most demand, there being as a rule no market 

 for material passing 90 mesh, which is not infrequently a total 

 loss, being thrown into the dump. For the edible oils it is said 

 that the earth should be ground to pass 100 mesh, but that there 

 should not be an excess of exceedingly fine material which if pres- 

 ent will clog the pores of the coarser material and prevent success- 

 ful filtering. It is apparent that the different fullers earths differ 

 in the degree of fineness to which they can be successfully ground. 

 While the English earths are ground to a 120 mesh without having 

 an excess of very fine particles, many of the American earths can- 

 not be ground finer than 100 mesh for edible oils. It is true also 

 that the mill employed must be adapted to the particular earth 

 for which it is used. 



