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A ball mill, sometimes referred to as rock tumbler (see below), is a type of mill widely used to grind and blend materials, such as minerals, salts, pyrotechnics, etc. Ball mills are popular due to their simple construction, being easy to operate and overall satisfactory performance.
Ball mills consist of a container, usually cylindrical, filled with grinding balls, which is connected to an electric motor, which spins the container. The container is made of metal or plastic. The grinding media (balls) are often made of lead-antimony alloy, or tungsten carbide. Steel is usually avoided, since it will spark.
There are several types of ball mill:
- Horizontal: the drum sits on freely rotating bodies and is rotated with a transmission belt or by a rotating body directly connected from the motor. This type is also known as rock tumbler.
- Vertical: Also known as planetary ball mill. The mill drum has the appearance of a large jar. The drum is positioned on a platform eccentrically or rotates off-center, known as sun wheel. The rotation of the platform causes the balls to move in a circular pattern inside their drums, increasing the grinding movement. This type of mill is used for obtaining very fine powders.
- Magnetic: This type of mill is rarely encountered, due to its price and they tend to be used mostly for polishing objects, usually jewelry, rather than grinding materials. They work by spinning magnetic stainless steel pellets or pins with a rotating magnetic field.
By turning on the motor, the drum rotates. Inside the drum, the grinding balls are lifted up on the rising side of the drum wall as it rotates and then drop down on to the feed from near the top of the drum chamber. In doing so, the solid particles in between the balls and ground by the impact and friction, reducing their size.
Ball mills have several advantages over most type of mills, such as:
- Low cost of installation and grinding medium
- Simple construction and maintenance
- Low power usage
- Suitable for both batch and continuous operation
- Applicable for materials of all degrees of hardness.
Ball mills of all shapes and sizes can be purchased online or from various companies. Price depends on the brand, age and complexity.
DIY ball mill
First you will need a cylindrical container, made of either steel or thick plastic. The container should be rigid or slightly flexible, but not brittle. Inside the container, steel or lead-antimony balls can be used as grinding bodies. To rotate the cylinder, there are two ways:
- The bottle container is connected to the motor via a transmission belt, while the container rests on free spinning wheels or cylinders. This method is easy to employ, due to its simple construction.
- The drum rests it on two cylinders, where one rotates freely, while the other is connected directly to a motor. The rotation of the motor cylinder rotates the mill drum.
Planetary ball mill
A strong lidded container is placed on a rotating platform, which is designed to rotate off-center. Alumina or lead-antimony balls are used as grinding medium.
Magnetic ball mill
Similar in construction with a magnetic stirrer, this can be done by adding small beads made of magnetic materials, such as steel, nickel or magnetic stainless steel in a large strong lidded container. Beneath the container, a magnet glued to an electric motor rotates, which causes the beads to spin. Due to the strong friction caused by the large number of beads, you will need strong magnets and electric motors.
- Make metal powders
- Grind cleaving materials (a pain in the ass to grind with only a mortar and pestle)
- Clean dirty materials using abrasion (planetary ball mill)
Ball mills may build up static charge, more so if the drum is made of plastic. This makes grinding flammable or pyrotechnic materials, such as metal powders, organic powders or black powder, extremely dangerous as air-powder mixtures are explosive, while in the case of black powder a singe spark can cause detonation of the powder. Ball mills in general are not used for grinding black powder, powder mills are instead employed.
Grinding minerals, such as rocks will produce plenty of airborne fine particles, such as silica, which, if inhaled, may irritate the lungs and cause respiratory problems.