**5. Summary and Conclusions**

The breakage of beds made up of narrow sizes of iron ore concentrates in confined conditions was studied in great detail. Stress versus relative density curves exhibited deformations that were purely inelastic for pressures of up to about 180 MPa. Up to this value, breakage of the top size fraction increased proportionally with input energy, reaching saturation at about 2 kWh/t. The saturation corresponded to proportions remaining in the top size that were as low as 10% for the 150–125 μm size range and as low as half for the 53–45 μm size range, depending on material.

At higher pressures, breakage of the top size fraction reached a maximum, whereas generation of additional fines from breakage of the progeny from the initially broken particles occurred. In this interval, progressively larger elastic deformations were observed, besides a more modest increase in specific surface area, reaching a point of full bed saturation (compaction) at specific energies beyond about 6 kWh/t, beyond which no increase in BSA occurred. This was evident from the achievement of the maximum increase in BSA under such conditions. Results also show the reasonable difference between the energy utilization calculated on the basis of the input energy and the inelastic energy (elastic energy subtracted from the input energy), even showing that a part of the inelastic energy may be used in dissipative processes such as friction, plastic deformation, and particle packing and not contributing to new surface area generation.

A pressing parameter κ was then proposed to quantify the propensity of a material contained in a bed to reduce in size when loaded under confined conditions. It correlated well with the increase in the Blaine specific surface area for beds with different initial size distributions, including narrow sizes and wide size ranges (multiple pressings) as well as different final pressures. It also captured the bed saturation condition in the maximum achievable value of κ observed in the experiments, which varied from about 0.10 to 013 for the materials studied.

A comparison of stressing at progressively higher pressures and multiple pressings at an intermediate pressure (160 MPa) showed that it is possible to enhance breakage of the top size material, as well as reach higher values of energy utilization, when multiple pressing stages are used. These results provide the technical justification and motivation for the application of multiple stages of pressing in HPGRs.

**Author Contributions:** Conceptualization, T.M.C., G.B., and L.M.T.; methodology, T.M.C. and G.B.; formal analysis, T.M.C. and L.M.T.; investigation, T.M.C., G.B., and L.M.T.; data curation, T.M.C. and L.M.T.; writing—original draft preparation, T.M.C. and L.M.T.; writing—review and editing, T.M.C., G.B., and L.M.T.; supervision, L.M.T. All authors have read and agreed to the published version of the manuscript.

**Funding:** This research was funded by Vale S.A. The authors would also like to thank the Brazilian Agencies CNPq (grant number 310293/2017-0) and FAPERJ (grant number E-26/202.574/2019).

**Acknowledgments:** The authors would like to thank Vale S.A. for financial and technical support for the research.

**Conflicts of Interest:** The authors declare no conflict of interest.
