Classification Effect in a Roll Mill

Abstract

The classification effect in a roll nip of a three-roll mill was investigated with a simulation model and experiment. In the experiment, fly ash was used as pigment and linseed oil varnish as vehicle. Premixed mill base of density, ρ₀ which was supplied between feed and center rolls was scraped off from apron roll at n times by sampling weights of 1/n of total weight. The densities, ρ_c and particle size distributions of these samples were measured. The experiment was carried out with various particle weight fraction, φ_w0, particle size level and viscosities of vehicle.
The following results were found from the simulation and experiment. At the beginning, samples of low density, having fine particles were scraped and coming to the end, sample density increased and particle size became larger. The degree of classification is represented with the sample density, ρ_ci scraped off at the i-th time as follows,
S=n∑i=1/ρ_ci/ρ₀-1//n
The values of S can be correlated with the ratio of 50% particle diameter by weight to mean minimum nip clearance. It was found that the values of S increase with increasing this ratio. Moreover, the values of S/φ_w0 was well correlated with this ratio independent on φ_w0, particle size level and viscosity of vehicle, when the particle size distributions of the mill base can be represented with Rosin-Rammler equation and the constant in regard to sharpness of distribution in this equation is almost equivalent for all mill base. Experimental values of S/φ_w0 were smaller than simulated values which were estimated by assuming that particles were rigid sphere and the scraping efficiency was 100%. Thus it is considered that a dispersion of particle agglomerate and a lowering of scraping efficiency occured in practice. By the simulation with the consideration of these effects, the change of ρ_c can be predicted.