Abstract
In industrial sodium chloride crystallizers, the oscillation phenomena of the crystal size distribution is known even under constant operational conditions. Crystal hardness is also considered to oscillate, accompanied by crystal size distribution oscillation. In order to study the correlation between crystal hardness and operational factors, the hardness of crystals produced under the constant operational condition was measured using an industrial crystallizer with an external heat exchanger. Crystal hardness was found to change with the change in crystal size distribution. Also, crystal hardness has a strong correlation with the change in slurry concentration, and crystal became harder when the slurry concentration was low and the average particle size of salt nears its maximum. Furthermore, crystal hardness is also connected with the nucleation rate and average values of crystal growth rate, which are calculated by Toyokura's model. The smaller the nucleation rate, and the higher the crystal growth rate, the harder the crystal. We examined crystal hardness from the aspect of crystal internal structure, and observed that crystal hardness was related to the crystal void fraction. The void fraction of crystals increased when the particle size of salt increased, and the fraction began to increase more steeply when the particle size reached 800-900 μm. This may be explained by a change in crystal surface conditions, as observed using electron microscopic photographs.
