Impact of grinder configurations on grinding rate, particle size, and trace element contamination of plant samples

Samples of varying abrasiveness, including rice hulls, rice straw, wheat grain, orange leaves, and filter paper, were ground to pass a 0.5-mm screen in two cyclone mills (Tecator Cyclotec model 1093 and Newport Scientific model 6200) with original and modified internal components. The ground samples were then digested in nitric acid and analyzed for 12 elements by Inductive Coupled Plasma (ICP) spectroscopy. Analysis of aluminum (Al), copper (Cu), iron (Fe), and zinc (Zn) showed that the amount of metal contamination from each mill was related to the abrasiveness of the plant material and the metal composition of the internal components of the mill. Least contamination was achieved using the Newport Scientific 6260 mill fitted with a stainless steel impeller and an abrasive steel strap with industrial diamonds set in pure nickel. For abrasive samples such as rice hulls a stainless steel impeller was about six times more durable than an aluminum impeller. The Newport mill ground samples in less time and reduced plant dry matter to finer particles, but impeller wear caused more variation in the distribution of particle size, than the Cyclotec mill.