Predicting the size distribution in the product and the power requirements of a pilot scale Vertimill INTRODUCTION • Vale, the second largest mining company in the world, has instigated a detailed study for the utilization of Vertimills as an alternative to conventional tube ball mills in pellet feed grinding applications. INTRODUCTION • Three samples were prepared with mixtures of different feeds. Normally, the grinding circuits in pelletizing plant receive the product from different mines with different properties in size distributions, hardness, density etc. Then, the preparation of distinct mixtures for testing is important. COMMINUTION MODEL dm (t ) i 1 i S m (t ) b S m (t ) i i ij j j dt j 1 COMMINUTION MODEL P Si S H E i SE 2 ln E 1 ln x 2 ln x S1 xi 1 xi 1 1 . Bi , j x x j j SAMPLES 100 % Passing 80 60 40 Mix2 Mix 3 20 Mix 4 0 0.01 0.10 1.00 Size (mm) 10.00 VERTIMILL PILOT TESTS • The Vertimill was operated with screw speed of 87 rpm in direct circuit configuration closed with a Derrick Vibrating Screen. Samples of new feed, mill discharge and screen oversize and undersize were collected during the tests for solids density and size distribution analysis. BALL SIZE DISTRIBUTIONS Sample Mix 2 Mix 3 and 4 Ball size (mm) Weight (%) Weight (%) 25 30.7 - 19 34.6 40.7 15 29.7 - 12 5 42.5 9 - 16.8 Total 100 100 LAB TESTS • Each batch grinding test is carried out in a diferent time interval in dry basis. An additional grinding test is carried out with water at the desired solids concentration. LAB TESTS • The tests are designed to reach the desired product size distribution specified as a P80 value. In order to achieve this, grinding times are calculated assuming first order breakage and the previous test result at a shorter grinding time. LAB TESTS Batch Tube Mill Test Mill Diameter (mm) 254 Mill Length (mm) 254 Ball Charge Level - J (%) 40 Voids - U (%) 100 Critical Speed (%) 70 Charge Lifters Yes, 8 PARAMETERS • The grinding parameters were determined using a specialized optimization software developed by Mineral Technologies International, Inc. – MTII called BatchMillTM. The software interpolates the size distribution of each milling time in order to determine the model parameters PARAMETERS Selection Function Sample S1E S1E* t/kWh t/kWh 1 2 Breakage Function Error NSR Mix 2 3.784 5.108 0.115 -0.435 1.508 2.348 0.414 0.277 Mix 3 9.623 Mix 4 3.857 5.207 0.009 -0.401 2.307 2.661 0.837 0.459 12.99 0.626 -0.312 2.500 5.549 0.635 0.230 1 The parameter S1E was multiplied by 1.35 to correct for the higher efficiency of the Vertimill with respect to the standard ball mill. SIMULATIONS • Data from mass balance of each test was used to perform simulations using the HerbstFuesternau ball mill model implemented in the Modsim plant-wide Simulator. The feed and product size distributions of the mill were considered to check the accuracy of the model. MIX 2 Simulation Cumulative % smaller than 100 80 60 Feed 40 Product Herbst-Fuerstenau 20 0 0.010 0.100 Particle size, mm 1.000 MIX 3 Simulation Cumulative % smaller than 100 80 60 Feed 40 Product Herbst-Fuerstenau 20 0 0.010 0.100 Particle size, mm 1.000 MIX 4 Simulation Cumulative % smaller than 100 80 60 Feed 40 Product Herbst-Fuerstenau 20 0 0.010 0.100 Particle size, mm 1.000 CONCLUSIONS • As a preliminary conclusion it is very likely that the grinding mechanisms that prevail in a Vertimill are the very same grinding mechanisms that occur in the conventional ball mill. • The difference between these two types of mills is basically their energy efficiency. CONCLUSIONS • It is recommendable to validate this methodology with coarser ore samples because it is possible that other factors may influence the product size distribution and the net power drawn in the Vertimill, as for example the settling velocity in the classification and the grinding zones of the Vertimill. QUESTIONS?