Ball milling has been used to produce fine particles from a coarse feed for an extended period of time. Traditional powder metallurgy uses various types of ball mills to produce fine material powders by the pulverization of the starting materials. However, in a traditional ball mill , the energy exchange between the tumbling balls themselves and the powder particles tends to be chaotic.
Chaotic ball motion and insufficient and uncontrolled grinding of the powders characterize this process. In order to obtain a homogeneous and reproducible product, good control of the milling process, in particular control of the ball movement, is essential. For this goal, we report on the development and the characterization of a ball mill for use in mechanical alloying. In this design, a magnetic field is introduced to the ball mill. Various modes of a controllable and reproducible operation may be obtained through the adjustment of the applied magnetic field. The improved ball mill used here has a cylindrical chamber 12.5 cm in diameter and 8.75 cm wide. It is shown that the control of the ball motion during the milling of limestone leads to a reduction in grinding energy of 40% and a more homogeneous product.
BALL MILL INTRODUCTION
The mechanical alloying process was developed over thirty years ago for the production of Oxide Dispersion Superalloys [2]. Many types of milling devices are used for mechanical alloying [3]. Each type has limitations in terms of the specific needs of the mechanical alloying process. These limitations became more obvious when it was found that the product quality depends on the type of the milling device and the energy consumed in the grinding process [3,4].
In the size reduction process of hard rock, a specific design of liner and lifter is used to control the grinding media movement and improve the comminution performance. In the mechanical alloying process, the liner and lifter are not commonly applied. This paper describes the design and characterization of an improved ball mill in which the ball movement can be controlled by using external
magnets. This improved ball mill provides the impact energy of the attritor mill and the powder mixing of the rotating mill [3] and has been tested in terms of particle size reduction and energy consumed in the grinding of limestone.
DESCRIPTION OF THE IMPROVED BALL MILL
The improved ball mill shown in the exploded view of Figure 1 has a cylindrical chamber 12.5 cm in diameter and 8.75 cm wide made from paramagnetic austenitic stainless steel and is connected to a motor through a bearing block, pulley and a belt. The motor speed is variable. A number of hardened ferromagnetic stainless steel balls, 1.25 cm in diameter, are used as grinding media. An adjustable magnet holder is connected to the base plate. Standoff adjusting screws are fixed on the magnet holder to permit the adjustment
of the distance between the outer periphery of the mill and the magnet positions. The face of the mill is made of Plexiglas® to permit the monitoring of the ball movement while adjusting the magnetic field for each mode of operation of the improved ball mill. The critical speed of the mill was found to be 126 RPM.
BALL MILL MODES OF OPERATION
The above picture shows the possible modes of operation of the improved ball mill. For each mode of operation, the ball movements during the milling process are confined to the vertical plane by the mill chamber walls and by the applied external magnetic field. The strength and direction of the magnetic field are adjusted externally by repositioning the magnets. The trajectories of the balls are controllable and reproducible for each mode of operation in this design. The general patterns of the ball movement are a function of the magnet positions and the rotational speed of the mill. In Case A (Figure 2) the mill operates in the conventional mode of operation; no magnetic field is introduced to the mill. After a number of revolutions, the trajectories of the individual balls are quite chaotic. In Case B (Figure 2) the introduction of a magnetic field to the ball mill now controls the trajectories of the balls.
By moving the magnet away from or nearer to the mill, different attractive forces can be generated. This dramatically affects the ball movement
pattern. In Case B (Figure 2) the two magnets positioned at the bottom of the mill result in a characteristic change in ball movement and split the balls into two groups with the ball rotation in each group being in opposite directions. Case C (Figure 2) can be obtained by reducing the mill rotational speed or by reducing the strength of the magnetic field; the balls both rotate and oscillate around the
equilibrium position at the bottom.
Further reduction of the magnetic field strength creates the ball movement characteristic of Case D (Figure 2). The cluster-like assembly creates a sliding movement that rotates and oscillates up and down the arc of the cylindrical chamber and generates a shearing action between the balls and the inner mill wall. Case E (Figure 2) shows an arrangement of three magnets configured to produce a closed geometry magnetic field. It causes the ball trajectories to be more uniform and makes the balls fall as projectiles. The falling balls hit the balls rotating in the opposite direction at the bottom; consequently, strong impact collisions occur and a shearing action is created between the colliding balls. The unique feature of this design of ball mill is that the specific ball movement pattern in every case of operation is well defined and highly reproducible. This contrasts with the chaotic and unpredictable ball movement characteristic of other ball milling devices.
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The development and characterization of a ball mill for mechanical alloying
Ball Milling equipment Abstract
Ball milling has been used to produce fine particles from a coarse feed for an extended period of time. Traditional powder metallurgy uses various types of ball mills to produce fine material powders by the pulverization of the starting materials. However, in a traditional ball mill , the energy exchange between the tumbling balls themselves and the powder particles tends to be chaotic.
Chaotic ball motion and insufficient and uncontrolled grinding of the powders characterize this process. In order to obtain a homogeneous and reproducible product, good control of the milling process, in particular control of the ball movement, is essential. For this goal, we report on the development and the characterization of a ball mill for use in mechanical alloying. In this design, a magnetic field is introduced to the ball mill. Various modes of a controllable and reproducible operation may be obtained through the adjustment of the applied magnetic field. The improved ball mill used here has a cylindrical chamber 12.5 cm in diameter and 8.75 cm wide. It is shown that the control of the ball motion during the milling of limestone leads to a reduction in grinding energy of 40% and a more homogeneous product.
BALL MILL INTRODUCTION
The mechanical alloying process was developed over thirty years ago for the production of Oxide Dispersion Superalloys [2]. Many types of milling devices are used for mechanical alloying [3]. Each type has limitations in terms of the specific needs of the mechanical alloying process. These limitations became more obvious when it was found that the product quality depends on the type of the milling device and the energy consumed in the grinding process [3,4].
In the size reduction process of hard rock, a specific design of liner and lifter is used to control the grinding media movement and improve the comminution performance. In the mechanical alloying process, the liner and lifter are not commonly applied. This paper describes the design and characterization of an improved ball mill in which the ball movement can be controlled by using external
magnets. This improved ball mill provides the impact energy of the attritor mill and the powder mixing of the rotating mill [3] and has been tested in terms of particle size reduction and energy consumed in the grinding of limestone.
DESCRIPTION OF THE IMPROVED BALL MILL
The improved ball mill shown in the exploded view of Figure 1 has a cylindrical chamber 12.5 cm in diameter and 8.75 cm wide made from paramagnetic austenitic stainless steel and is connected to a motor through a bearing block, pulley and a belt. The motor speed is variable. A number of hardened ferromagnetic stainless steel balls, 1.25 cm in diameter, are used as grinding media. An adjustable magnet holder is connected to the base plate. Standoff adjusting screws are fixed on the magnet holder to permit the adjustment
of the distance between the outer periphery of the mill and the magnet positions. The face of the mill is made of Plexiglas® to permit the monitoring of the ball movement while adjusting the magnetic field for each mode of operation of the improved ball mill. The critical speed of the mill was found to be 126 RPM.
BALL MILL MODES OF OPERATION
The above picture shows the possible modes of operation of the improved ball mill. For each mode of operation, the ball movements during the milling process are confined to the vertical plane by the mill chamber walls and by the applied external magnetic field. The strength and direction of the magnetic field are adjusted externally by repositioning the magnets. The trajectories of the balls are controllable and reproducible for each mode of operation in this design. The general patterns of the ball movement are a function of the magnet positions and the rotational speed of the mill. In Case A (Figure 2) the mill operates in the conventional mode of operation; no magnetic field is introduced to the mill. After a number of revolutions, the trajectories of the individual balls are quite chaotic. In Case B (Figure 2) the introduction of a magnetic field to the ball mill now controls the trajectories of the balls.
By moving the magnet away from or nearer to the mill, different attractive forces can be generated. This dramatically affects the ball movement
pattern. In Case B (Figure 2) the two magnets positioned at the bottom of the mill result in a characteristic change in ball movement and split the balls into two groups with the ball rotation in each group being in opposite directions. Case C (Figure 2) can be obtained by reducing the mill rotational speed or by reducing the strength of the magnetic field; the balls both rotate and oscillate around the
equilibrium position at the bottom.
Further reduction of the magnetic field strength creates the ball movement characteristic of Case D (Figure 2). The cluster-like assembly creates a sliding movement that rotates and oscillates up and down the arc of the cylindrical chamber and generates a shearing action between the balls and the inner mill wall. Case E (Figure 2) shows an arrangement of three magnets configured to produce a closed geometry magnetic field. It causes the ball trajectories to be more uniform and makes the balls fall as projectiles. The falling balls hit the balls rotating in the opposite direction at the bottom; consequently, strong impact collisions occur and a shearing action is created between the colliding balls. The unique feature of this design of ball mill is that the specific ball movement pattern in every case of operation is well defined and highly reproducible. This contrasts with the chaotic and unpredictable ball movement characteristic of other ball milling devices.