Grinding media

Oxidized castor oils are excellent nonmigrating, nonvolatile plasticizers (qv) for ceUulosic resins, poly(vinyl butyral), polyamides, shellac, and natural and synthetic mbber (see Rubber, natural). The high viscosity products are also used as tackifiers in gasket compounds and adhesives (qv) because of good oil and solvent resistance. They also serve as excellent pigment grinding media and as a base for inks (qv), lubricating oils, and hydrauHc oils (62). 

Raw material for dry process plants is ground in closed-circuit ball mills with air separators, which may be set for any desired fineness. Drying is usually carried out in separate units, but waste heat can be utilized directiy in the mill by coupling the raw mill to the kiln. Autogenous mills, which operate without grinding media are not widely used. For suspension preheater-type kilns, a roUer mill utilizes the exit gas from the preheater to dry the material in suspension in the mill. 

Rod and ball mills are cylindrical vessels into which the ore, water, and steel balls or steel rods, called grinding media are charged. The vessels rotate on horizontal axes to cascade the ore and grinding media and thus grind the ore. Typical hall-mill sizes range from 2.5 m in diameter and 3 m in length, driven by a 187-kW motor, up to ca 5 m in diameter and 7 m in length with a 3000-kW drive. 

The hardness of a mineral as measured by the Mohs scale is a criterion of its resistance to crushing [Fahrenwald, Trans. Am. In.st. Min. Metall. Pet. Eng., 112, 88 (1934)]. It is a fairly good indication of the abrasive character of the mineral, a factor that determines the wear on the grinding media. Arranged in increasing order or hardness, the Mohs scale is as fohows 1, talc 2, gypsum 3, calcite 4, fluoride 5, apatite 6, feldspar 7, quartz 8, topaz 9, corundum and 10, diamond. 

Capacities of pebble mills are generally 30 to 50 percent of the capacity of the same size of ball miU with steel grinding media and liners this depends directly on the density of the media. 

Operation Cascading and cataracting are the terms applied to the motion of grinding media. The former applies to the rolling of baUs or pebbles from top to bottom of the heap, and the latter refers to the throwing of the baUs through the air to the toe of the heap. 

The amount of material in a mill can be expressed conveniently as the ratio of its volume to that of the voids in the ball load. This is known as the material-to-void ratio. If the solid material and its suspending medium (water, air, etc.) just fill the ball voids, the ratio is 1, for example. Grinding-media loads vary from 20 to 50 percent in practice, and ratios are usually near 1. 

Performance The grinding-media diameter should preferably be 10 times that of the feed and should not exceed 100 times the feed diameter. To obtain improved efficiency when reducing size by several orders of magnitude, several stages should be used with different media diameters. As fine grinding proceeds, rheological factors alter the charge ratio, and power requirements may increase. 

A variety of grinding media are available, as shown in Table 20-18. Size availability varies, ranging from 1.3 cm (V2 in) down to 325 mesh (44 lm). 

Grinding-media wear in phosphate ball-mill grinding systems ranges from 5 to 25 g/Mg (0.05 to 0.20 Ib/ton) ground Dall-mill huers show an average consumption of 2.5 to 100 g Mg (0.01 to 0.05 Ib/ton) ground. 

Frequently ball and tube mills are combined into a single machine consisting of two or three compartments, separated by perforated-steel diaphragms and charged with grinding media of different size. Rod mills are hardly ever used in cement plants. The compartments of a tube mill may be combined in various circiiit arrangements with classifiers, as shown in Fig. 20-59. 

Improved grinding media and linings, roller mills, high-efficiency classifiers, wet process slurry Dewatering with filter presses... 

For many years, the standard method for poUshing glass in the laboratory has been by rubbing the glass over a sheet of brown paper covered with a mixture of flom emery and a solution of camphor in turpentine. C. V. Boys (1927) has stated that he can find no reason other than tradition for the use of the solution of camphor in turpentine rather than pure turpentine for lubricating grinding media. 

With their rest potential much higher than that of the steel medium listed in Table 1.6, sulphide minerals act as a cathode while the steel medium acts as an anode. During grinding, minerals and grinding media (steel) come in repeated contact with each other, and galvanic current flows between the two surfaces of the sulphide minerals may be altered. 

In the case of mineral-mineral interactions, a mineral with higher potential acts as a cathode, while a mineral with lower potential acts as an anode. For a multiple mineral/grinding media(steel)system. The galvanic interactions become more complex than the two-electrode systems. The galvanic reactions among multielectrode systems are also governed by the mixed potential principle as shown in an example of polarization curves involving pyrite, pyrrhotite and mild steel in Fig. 1.9 (Pozzo and Iwasaki, 1987). 

Nakazawa and Iwasaki (1985) and Pozzo, Malicsi and Iwasaki (1988) investigated a pyrite-pyrrhotite contact and a pyrite-pyrrhotite-grinding media contact on flotation, respectively. They found that the floatability of pyrrhotite increased in the presence of pyrite, whereas it decreased in the presence of both pyrite and grinding media (mild steel). Similarly, a galvanic contact between nickel arsenide and pyrrhotite decreased the floatability of pyrrhotite (Nakazawa and Iwasaki, 1986). 

Cases et al. (1990, 1991) studied the influence of grinding media on the adsorption of xanthate on galena and pyrite using XPS and FTIR. They found that only lead xanthate, either monocoordinated or in the three dimensional form... 

---