Crushing performance and contamination tdals of a tungsten carbide ring mill compared to agate grinding

Crushing and contamination trials have been carried out on a new tungsten carbide ring mill，compared to material ground in a automated agate pestle and mortar grinding system．Granite and reagent grade quartz sand were crushed in the ring mill for times ranging between 30 and 120 seconds per sample．Agate-system samples were ground for 60 mimtes．Grain size analysis of the resulting pulps shows that grain size distributions of the material crushed in the ring mill compare very favourably with those from the agate mortar．Mill times of only 60 seconds produce pulp with grain size distributions equivalent to 60 minutes grinding in the agate system．Extending the crushing time to 90 seconds lowers median grain size to 4 microns，compared to 6-10 microns in the agate ground samples．Increasing the mill time to 120 seconds does not significantly reduce median grain size，and 90 second mill time is thus optimal for the sample weights used．
Granite and quartz sand pulps were analysed by XRF to test for contamination from the ring mill．Results show that crushing times of 60-20 seconds produce only 15-20 ppm Co，and inferred W contamination of 190-200 ppm．No other contaminants can be identified from the XRF data．Estimation of maximum contamination by proportion between measured Co contents and assayed abundances in the ring mill tungsten carbide show that contamination of all other elements is trivial．This suggests that powders crushed with equipment such as that utilised here can be safely used for analysis of low-level trace elements（e.g．Nb，Ta）by more precise methods such as ICP/MS，without materially affecting geologic intelpretations．Variations in element abundances between the splits crushed for different times，and between beads prepared from the same split，indicate that chemical variability introduced by subsample heterogeneity is a far more serious problem，being several orders of magnitude greater than possible contamination from the mill．