be ground. Hardness: The grinding media needs to be durable enough to grind the material, but, where possible, not so tough that it also wears down the tumbler. Composition: Various grinding applications have special requirements. Some of these requirements are based on some of the grinding media being in the finished product, while others are based on how the media will react with the material being ground. Where the color of the finished product is important, the color and material of the grinding media must be considered. Where low contamination is important, the grinding media may be selected for ease of separation from the finished product (e.g., steel dust produced from stainless steel media can be magnetically separated from non-ferrous products). An alternative to separation is to use media of the same material as the product being ground. Flammable products have a tendency to become explosive in powder form. Steel media may spark, becoming an ignition source for these products. Either wet-grinding, or non-sparking media such as ceramic or lead must be selected. Some media, such as iron, may react with corrosive materials. For this reason, stainless steel, ceramic, and flint grinding media may each be used when corrosive substances are present during grinding. The grinding chamber can also be filled with an inert shield gas that does not react with the material being ground, to prevent oxidation or explosive reactions that could occur with ambient air inside the mill. Advantages of the ball mill Ball milling boasts several advantages over other systems: the cost of installation and grinding medium is low; the capacity and fineness can be adjusted by adjusting the diameter of the ball; it is suitable for both batch and continuous operation; it is suitable for open and closed-circuit grinding; it is applicable for materials of all degrees of hardness. Varieties Aside from common ball mills there is a second type of ball mill called a planetary ball mill. Planetary ball mills are smaller than common ball mills and mainly used in laboratories for grinding sample material down to very small sizes. A planetary ball mill consists of at least one grinding jar which is arranged eccentrically on a so-called sun wheel. The direction of movement of the sun wheel is opposite to that of the grinding jars (ratio: 1:−2 or 1:−1). The grinding balls in the grinding jars are subjected to superimposed rotational movements, the so-called