Comparison of One and Two Stage High Pressure Rolls Grinding followed by Ball Milling with Conventional Rod and Ball Milling

Abstract

With the development of the autogenous wear surface, the high pressure roller press (HPR) is receiving interest from the mineral industry. Previous work by the Coleraine Laboratory had indicated energy saving using a single stage of HPR compared to rod milling and an energy saving using 2 stage HPR compared to single stage HPR. The purpose of this study was to compare three flowsheets using the same feed and producing a final size of about 85 percent passing 270 mesh. The flowsheets were as follows: (1) Rod mill followed by wet magnetic separation with ball mill grinding of the magnetic concentrate; (2) HPR closed with a 3 mesh screen followed by wet magnetic separation of the screen undersize and ball milling of the magnetic concentrate; (3) HPR closed with a 3 mesh screen with the screen undersize being upgraded by dry magnetic separation followed by open circuit HPR on the magnetic concentrate with wet magnetic separation of the HPR product and ball milling of the wet magnetic concentrate. The test work indicated that the single stage HPR required 27.25 kWh/It of new feed compared to 34.0 kWh/It for the rod mill to produce an 85 percent passing 270 mesh ball mill discharge. The two stage HPR flowsheet was even more energy efficient, requiring only 23.29 kWh/It of new feed. With the single stage HPR, the energy savings appears to be only in the coarse grinding. The ball mill grindability, as measured by the operating work index, was essentially the same for the rod mill and HPR magnetic concentrates. The two stage HPR resulted in energy savings in both the coarse grinding and in the ball milling. The operating work index for the ball mill portion only was 23.5 kWh/It of ball mill feed for the two stage flowsheet compared to about 29 kWh/It for the other two flow sheets. Davis tube tests on the ball mill discharges suggest that there is a liberation benefit associated with the two stage HPR flowsheet, but not with the single stage HPR flowsheet.