Towards an understanding of the correlation between the physicochemical and thermal properties of ground rice husks and particle size

The comminution of rice husk has been commonly used to convert it into biogenic silica, bio-oil, biogas, and bioenergy. The composition and structure of biomass are anisotropic, e.g., the surface of the rice husk has a higher silica content than the bulk. It is possible to expect that the physicochemical properties of grinding and sieving rice husks will be distinct due to the difference between the mechanical properties of inorganic and organic matter. A sample of rice husk was ground in a pin mill and sieved. The anisotropic distribution of the inorganic matter was measured on the cross-section of rice husk by SEM-EDX. The chemical composition and textural properties of the rice husk ash were determined by X-ray fluorescence and nitrogen adsorption of each granulometric fraction. Thermochemical, proximate, and ultimate properties of non-calcined grinding rice husks were measured through thermal analysis. Pin, hammer, and knife mills were used to process the rice husks and to determine the effect of the mill type on the comminution operation. Silica is anisotropically distributed in the rice husk but mainly concentrates on the surface. So, The granulometric fractions have very different physicochemical and thermal properties; those with smaller diameters have a higher inorganic matter content; specifically, the pan fraction presents twice the ash content as the non-grinded rice husk. Interestingly, through multivariate analysis, it is possible to classify the fractions into homogeneous groups according to their physicochemical properties for use in technological applications such as pellet fabrication.