Industrial Minerals Treatment Methods - Metal Consumption in Hammer Mills at Norris Dam (T. P. 824, with discussion)

The construction of Norris Dam, built by the Tennessee Valley Authority on the Clinch River, a tributary of the Tennessee River, involved the production of coarse and fine aggregate for approximately 1,000,000 cu. yd. of concrete. A hammer-mill plant for the manufacture of sand from quarried rock was indicated as the most economical method of providing this material. The abrasive character of the feed as compared to the average hammer-mill feed made it necessary to control operations rather closely, and this resulted in the accumulation of many data hearing on production and maintenance. This paper will deal only with three specific sets of data, accompanied by such general information as may be necessary to explain the conditions under which they were obtained and followed by discussions on the relation of silica content to hammer wear, comparative costs of slugger and stirrup hammers, and comparative costs of tool-steel and manganese-steel grate bars. Coarse Aggregate Aggregate for the concrete poured at Norris Dam came from a dolomite quarry within 2000 ft. of the west abutment of the dam. Geologically this rock is known as a Knox dolomite. A typical chemical analysis is given in Table 1. Table 1.—Typical Chemical Analysis Per Cent H2O, 110° F............................................. 0.02 lgnition loss.............................................. 43.56 MgO.................................................... 19.19 CaO ................................................... 29.07 Fe²O³.................................................... 0.39 Al2O³ .................................................. 1.94 SiO2..................................................... 5.75 SO2...................................................... Tr. Rock was carried by 12-yd, dump trucks to a 42-in. gyratory crusher set at 6 1/2in. The product of the primary crusher was taken by a belt conveyor to a double.-deck vibrating screen followed by a 5 1/2-ft. cone