The characterization of structural changes in hematite ground in a confined particle bed using Rietveld analysis

Abstract

The interparticle breakage of fine feed fraction of hematite concentrate was investigated by stressing two particle beds with a pressure between 255 and 1000 MPa. The experiments were conducted in such a way that the wall friction effects during compression were eliminated. The effects of interparticle breakage in a confined bed on the structural changes of hematite concentrate were studied using a combination analysis of XRD line broadening, BET and particle size measurements. The specific energy comminution was estimated using loading and de-loading hysteresis curves. It was found that energy absorption by the particle bed varies between 6 and 31 J/g depending on the applied pressures and bed heights. The experiments indicated that energy absorption was a major factor for the interparticle breakage of hematite. In addition, it was revealed that an increasing bed height brought about a higher stiffness and hence reduced energy absorption and subsequently declined the surface area, solid content as well as structural changes. The linear energy–force relationship stands well even if the particle-bed heights are changed. The maximum BET surface area was measured about 1.4 m 2/g after energy absorption of 31 J/g by the particle bed. Structural changes were followed by XRD line broadening analysis using Rietveld refinement and Warren–Averbach approach. It was found that the intensity and the broadening of XRD diffraction patterns decreased and increased, respectively, by increasing energy absorption with a first approximation. With increasing absorbed energy by the bed up to 15 J/g the degree of amorphization increased sharply and afterwards continued to change slightly. The maximum X-ray amorphization was calculated at maximum energy absorption, accounting for 31%. The volume and surface weighted crystallite sizes reduced to about 108 and 53 nm, respectively, after releasing 31 J/g specific grinding energy. For the same energy, the root mean square strain (RMSS), < ε L 2 = 10 nm > 1/2, and maximum lattice strain, e, increased to 9.4 × 10 − 4 and 4.1 × 10 − 3 respectively. The comparison of bed grinding with tumbling milling revealed that the grinding in tumbling mill needs much more energy to induce the same structural changes as in bed grinding. The results obtained from the two methods are discussed and compared in details.