Dipl Ing Th Fahrland Dr Ing K H Zysk Loesche GmbH iour of the vertical roller and ball mill systems means that particles remain in a ball mill system for about 20 to 30 min utes before they leave the classifier as product They are impacted repeatedly during that time with the result that some particles are ground more than necessary By con trast the limited amount of grinding for each particle in the VRM system before classification