Ball milling is a size reduction technique that uses media in a rotating cylindrical chamber to mill materials to a fine powder. As the chamber rotates, the media is lifted up on the rising side and then cascades down from near the top of the chamber. With this motion, the particles in between the media and chamber walls are reduced in size by both impact and abrasion. In ball milling, the desired particle size is achieved by controlling the time, applied energy, and the size and density of the grinding media. The optimal milling occurs at a critical speed. Ball mills can operate in either a wet or dry state. While milling without any added liquid is commonplace, adding water or other liquids can produce the finest particles and provide a ready-to-use dispersion at the same time.
Grinding media comes in many shapes and types with each having its own specific properties and advantages. Key properties of grinding media include composition, hardness, size and density. Some common types include alumina, stainless steel, yttria stabilized zirconia and sand. Ball milling will result in a ball curve particle size distribution with one or more peaks. Screening may be required to remove over or undersized materials.
The 911MPEPB500 Planetary Ball Mills are used for fine grinding of soft, hard to brittle or fibrous materials. Dry and wet grindings are possible. They support the daily sample preparation for laboratory- and development usage.
Planetary Ball Mills consist of several cylindrical grinding jars (positioned on the sun wheel as shown on the figure) which are filled with loose grinding balls. Two superimposed rotational movements move the grinding jars:
Like in a planetary system the grinding jar rotates on a orbit around the centre. This rotational movement is the self-rotation of the grinding container superimposed. The resulting centrifugal and acting acceleration forces lead to strong grinding effects. Furthermore there are forces working according to the coriolis acceleration. The result is an intensive grinding effect between the grinding balls and the sample.
Depending on the speed ratio different movement patters of the grinding balls / media can be achieved. It can be achieved that the grinding media are crossing the grinding jar and loosen from the wall. At hitting the wall of the grinding jar the sample will be stressed. At a different motion pattern the grinding balls roll over the sample and stress the ground material.
The selection of the right grinding jar and the correct filling level has a big impact on the grinding result. According to the application you have to select the correct material and amount/volume for the grinding jar and the grinding balls.
A jar filling should consist of about 1/3 sample and 1/3 ball charge. The remaining third is the free jar volume that is necessary for the movement of the balls. The following table provides recommendations.
Planetary ball mill is a very often used machine for mechanical alloying, especially in Europe. Because very small amount of powder (for example, as little as a few grammes), is required, the machine is suitable for research purposes in the laboratory. A typical planetary ball mill consists of one turn disc (sometimes called turn table) and two or four bowls. The turn disc rotates in one direction while the bowls rotate in the opposite direction. The centrifugal forces created by the rotation of the Mechanical Alloying.
A short milling duration of only 30 to 60 min. In cases where relatively high temperature is necessary to promote reaction rate, even this may be an added advantage to the process. In addition, the planetary ball mill may be modified by incorporating temperature control elements.
Two types of bowls are commercially available: steel including hardened chrome steel, stainless CrNi-steel and hardmetal tungsten carbide (WC+Co) and ceramic bowls including sintered corundum (Al2O3), agate (SiO2) and zirconium oxide (ZrO2). They generally are available in three different sizes of 80, 250 and 500ml. For high energy mechanical alloying, however, steel bowls are recommended since ceramic bowls can cause contamination due to minute chipped off or fractured particles from the brittle surfaces of the milling bowl and balls. Generally, bowls and balls of the same material are employed in the mechanical alloying process to avoid the possibility of cross contamination from different materials.
Based on powder particle size and impact energy required, balls with size of 10 to 30 mm are normally used. If the size of the balls is too small, impact energy may be too low for alloying to take place. In order to increase impact energy without increasing the rotational speed, balls with high density such as tungsten balls may be employed. Table 2.1 gives the recommended number of balls per bowl to be applied.
Table 2.2 gives a summary of abrasion properties and densities for the selection of bowl and ball materials. It can be seen that the oxide materials show the lowest density while tungsten carbide, the highest density. Hence, at the same rotational speed and ball size, the oxide ball with the lowest density will generate the lowest collision energy.
Another popular mill for conducting MA experiments is the planetary ball mill (referred to as Pulverisette) in which a few hundred grams of the powder can be milled at the same time (Fig. 4.4). These are manufactured by Fritsch GmbH (Industriestrae 8. D-55743 Idar-Oberstein, Germany; +49-6784-70 146 www.FRITSCH.de) and marketed by Gilson Co. in the United States and Canada (P.O. Box 200, Lewis Center, OH 43085-0677, USA, Tel: 1-800-444-1508 or 740-548-7298; www.globalgilson.com). The planetary ball mill owes its name to the planet-like movement of its vials. These are arranged on a rotating support disk, and a special drive mechanism causes them to rotate around their own axes. The centrifugal force produced by the vials rotating around their own axes and that produced by the rotating support disk both act on the vial contents, consisting of the material to be ground and the grinding balls. Since the vials and the supporting disk rotate in opposite directions, the centrifugal forces alternately act in like and opposite directions. This causes the grinding balls to run down the inside wall of the vialthe friction effect, followed by the material being ground and the grinding balls lifting off and traveling freely through the inner chamber of the vial and colliding with the opposing inside wallthe impact effect. The grinding balls impacting with each other intensify the impact effect considerably.
The grinding balls in the planetary mills acquire much higher impact energy than is possible with simple pure gravity or centrifugal mills. The impact energy acquired depends on the speed of the planetary mill and can reach about 20 times the earths acceleration. As the speed is reduced, the grinding balls lose the impact energy, and when the energy is sufficiently low there is no grinding involved; only mixing occurs in the sample.
Even though the disk and the vial rotation speeds could not be independently controlled in the early versions, it is possible to do so in the modern versions of the Fritsch planetary ball mills. In a single mill one can have either two (Pulverisette 5 or 7) or four (Pulverisette 5) milling stations. Recently, a single-station mill was also developed (Pulverisette 6). Three different sizes of containers, with capacities of 80. 250, and 500 ml. are available. Grinding vials and balls are available in eight different materials agate, silicon nitride, sintered corundum, zirconia, chrome steel, Cr-Ni steel, tungsten carbide, and plastic polyamide. Even though the linear velocity of the balls in this type of mill is higher than that in the SPEX mills, the frequency of impacts is much less than in the SPEX mills. Hence, in comparison to SPEX mills, Fritsch Pulverisette can be considered as lower energy mills.
Some high-energy planetary ball mills have been developed by Russian scientists, and these have been designated as AGO mills, such as AGO-2U and AGO-2M. The high energy of these mills is derived from the very high rotation speeds that are achievable. For example, Salimon et al. used their planetary ball mill at a rotation speed of 1235 rpm corresponding to the mill energy intensity of 50 W/g. It has been reported that some of these mills can be used at rotation speeds greater than 2000 rpm.
A recent development in the design of the Fritsch mills has been the incorporation of a gas pressure and temperature measuring system (GTM) for in situ data acquisition during milling. Generally, the occurrence of phase changes in the milled powder is interpreted or inferred by analyzing the powder constitution after milling has been stopped. Sometimes a small quantity of the powder is removed from the charge in the mill and analyzed to obtain information on the progress of alloying and/or phase transformations. This method could lead to some errors because the state of the powder during milling could be different from what it is after the milling has been stopped. To overcome this difficulty, Fritsch GmbH developed the GTM system to enable the operator to obtain data during milling.
The basic idea of this measuring system is the quick and continuous determination of temperature and pressure during the milling process. The temperature measured corresponds to the total temperature rise in the system due to the combination of grinding, impact, and phase transformation processes. Since the heat capacity of the container and the grinding medium is much higher than the mass of the powder, it is necessary to have a sensitive temperature measurement in order to derive meaningful information. Accordingly, a continuous and sensitive measurement of gas pressure inside the milling container is carried out to measure very quickly and detect small temperature changes. The measured gas pressure includes not only information about the temperature increase due to friction, impact forces, and phase transformations, but also the interaction of gases with the fresh surfaces formed during the milling operation (adsorption and desorption of gases). The continual and highly sensitive measurement of the gas pressure within the milling container facilitates detection of abrupt and minute changes in the reactions occurring inside the vial. The pressure could be measured in the range of 0-700 kPa, with a resolution of 0.175 kPa, which translates to a temperature resolution of 0.025 K.
Bachin et al carried out MA of dispersion-strengthened, nickel-base superalloys in a centrifugal planetary ball mill. The mechanics of this mill are characterized by the rotational speed of the plate p, that of the container relative to the plate v, the mass of the charge, the size of the ball, the ball to powder ratio and the radius of the container. A schematic of the planetary ball mill is shown in Fig.2.4. Figure 2.5 shows a laboratory planetary mill.
diameters (0.5 to 2.5 m) to achieve high energy by rotating it just below the critical speeds c (up to 0.9 c ). Even though the time required to accomplish MA by these mills is longer compared to attritor mills, the overall economics are favourable.
As far as the grinding media are concerned, common practice is to use hardened high carbon-high chromium steel balls (4 to 12 mm diameter), normally specified for ball bearings. Stainless steel balls have also been used. When it is necessary to minimize iron contamination in the charge, balls of tungsten carbide have also been used. When necessary, the balls have been coated with the necessary oxide that was to be dispersed in the composition to be mechanically alloyed.
Wet grinding is a nonlinear process and hence, estimation of breakage rates based on the linear model cannot be strictly used. Also the use of a functional form for breakage rates in an estimation scheme can produce artificially unrealistic values. In this study a technique for estimation of breakage rates for all size classes is investigated without imposing any functional form. A method known as the G-H scheme is used for estimation. A natural spline-interpolation technique is used to generate short time grinding data based on real experimental data obtained for relatively long grinding time intervals. Using natural spline techniques combined with the G-H scheme, acceleration or deceleration of breakage rates of all individual size classes in wet grinding is determined when the grinding environment is varied.