Page 379 - Chemical Process Equipment - Selection and Design

342 DISINTEGRATION, AGGLOMERATION, AND SIZE SEPARATION OF PARTICULATE SOLIDS
TABLE 12.2. Typical Values of the Work Index 4 kWh/ton, principal equipment for cryogenic processing when products of
of Eq. (12.3) 50-100 mesh are adequate. Scrap automobiles and rubber tires are
chilled with liquid nitrogen and are thereby made highly brittle to
Work Work
Index Index facilitate grinding. Nitrogen consumption runs about 0.25 kg/kg
Material w; Material w; steel and up to 0.65 kg/kg rubber [Biddulph, Chem. Eng., (11 Feb.
1980)l.
All materials tested 13.81 Kyanite 18.87 This equipment is particularly suited to crushing of soft, friable
Andesite 22.13 Lead ore 11.40 materials to cube-shaped products with small proportions of fines.
Barite 6.24 Lead-zinc ore 11.35 For fibrous materials, the screen is provided with cutting edges.
Basalt 20.41 Limestone 11.61 Some data are in Table 12.7(c). A list of materials that are handled
Bauxite 9.45 Limestone for cement 10.18
Cement clinker 13.49 Manganese ore 12.46 in hammer mills is in Table 12.5(a), and other products are referred
Cement raw material 10.57 Magnesite, dead burned 16.80 to in Table 12.10.
Chrome ore 9.60 Mica 134.50 Tumbling mills consist of vessels rotating about the horizontal
Clay 7.10 Molybdenum 12.97 and charged with a mass of relatively small elements that tumble
Clay, calcined 1.43 Nickel ore I I .88 and crush the process material as they fall. Their function may be to
Coal 11.37 Oil shale 18.10 mix as well as grind, in batch or continuous operation, in open or
Coke 20.70 Phosphate fertilizer 13.03 closed circuit. Figure 12.4(d) shows a closed circuit arrangement
Coke, fluid petroleum 38.60 Phosphate rock 10.13 with a ball mill. The crushing elements most commonly are steel
Coke, petroleum 73.80 Potash ore 8.88
Copper ore 13.13 Potash salt 8.23 balls of several sizes, or ceramic pebbles, or rods the length of the
Coral 10.16 Pumice 11.93 shell, or a range of sizes of the process material that is thus made to
Diorite 19.40 Pyrite ore 8.90 grind itself. In processing of minerals, tumbling mills often operate
Dolomite 11.31 Pyrrhotite ore 9.57 wet with slurries of about 80% solids, just thin enough to permit
Emery 58.18 Quartzite 12.18 flow in and out of the equipment.
Feldspar 11.67 Quartz 12.77 The mode in which the material grinds itself is called auio-
Ferro-chrome 8.87 Rutile ore 12.12 genous grinding. Such operation can achieve size reduction from
Ferro-manganese 7.77 Sandstone 11.53 25 cm to 0.1 mm in one step. Autogeneous mills operate at 80-85%
Ferro-silicon 12.83 Shale 16.40 of the critical speed, which is the speed at which the grinding media
Flint 26.16 Silica 13.53 are thrown to the wall and cling to it. They are desirable for mineral
Fluorspar 9.76 Silica sand 16.46
Gabbro 18.45 Silicon carbide 26.17 treatment since they release the mineral content without
Galena 10.19 Silver ore 17.30 overgrinding which could complicate a subsequent flotation process,
Garnet 12.37 Sinter 8.77 for instance. Materials for which the process is used are friable and
Glass 3.08 Slag 15.76 grainy, such as silica rock, asbestos, basic slag, bauxite, cement
Gneiss 20.13 Slag, iron blast furnace 12.16 clinker, dolomite, ferrosilicon, limestone, specular hematite, and
Gold ore 14.83 Slate 13.83 taconite. In comparison with ball milling, steel consumption is
Granite 14.39 Sodium silicate 13.00 largely eliminated but energy costs are greater by between 25 and
Graphite 45.03 Spodumene ore 13.70 100% because of lower impacting forces with low density materials.
Gravel 25.17 Syenite 14.90
Gypsum rock 8.16 Tile 15.53 Rod mills [Fig. 12.5(f)] are capable of taking feed as large as
Ilmenite 13.11 Tin ore 10.81 50 mm and reducing it to 300 mesh, but ordinarily the cutoff point is
Iron ore 15.44 Titanium ore I I .a8 larger. The performance data of Table 12.6(e) shows a product
Hematite 12.68 Trap rock 21.10 range from 8 to 65 mesh. Rods in use range from 25-150mmdia;
Hematite-Specular 15.40 Uranium ore 17.93 smaller ones tend to bend and break. The ratio of rod length to
Oolitic 1 1.33 Zinc ore 12.42 vessel diameter is kept in the range of 1.4-1.6. Ratios below 1.25
Limanite 8.45 tend to result in tangling. Maximum usable rod length is about 6 m;
Magnetite 10.21 above this they tend to bend. About 45% of the bulk volume of the
Taconite 14.87
mill is occupied by rods. Rotation is at 50-65% of critical speed.
Rod consumption normally is in the range of 0.1-1.0 kg steel/ton of
[F.C. Bond, Bri. Chem. Eng. 6, 378-385, 543-548 (1961)l.
ore for wet grinding, and about 10-20% less for dry grinding.
Because the coarse feed tends to spread the rods at the feed end,
grinding takes place preferentially on the large particles and results
in a product of relatively narrow size range. Accordingly, rod mills
are nearly always run in open circuit.
Ball mills serve as a final stage of comminution. Balls have a
EXAMPLE 12.2 greater ratio of surface area to weight than rods so they are better
Power Requirement for Grinding suited to fine grinding. The length to diameter ratio ranges from less
Cement clinker is to be reduced from an initial dao = 1500 pm to a than 1 to about 1.5. Rotation speed is greater than that of rod mills,
final d,, of 75 pm. From Table 12.2 the work index is Wi = 13.49. being 70-80% of critical. Mills that are subjected to vibration can
Substituting into Eq. (12.3), operate above the critical speed. The bulk volume of balls is about
50% of the mill volume.
W = 10(13.49)(1/fi- 1/m) = 12.1 kW/(ton/hr). The Denver ball mills for which operating data are shown in
Table 12.6(a) normally are charged with equal weights of 2-, 3-, and
According to Table 12.7(b), a 3 ft x 24 in. ball mill requires 10 HP 4411. balls; or for finer grinding, with equal weights of 1.5, 2-, and
for a rate of 0.5 tons/hr, or 14.9 kW/(ton/hr), a rough check of the 3-in. balls. Figure 12.5(d) is of the widely used conical shape of mill
result from the equation. in which a range of sizes of balls group themselves axially during
operation. The balls range from 5in. down, the large ones for
crushing the large lumps and the small ones acting on the small