Characterization of caking and cake strength in a potash bed
When a water soluble granular fertilizer, such as potash, is wetted and then dried during storage and transportation processes, clumps or cakes often form in the material even when the maximum moisture content is less than 1% by mass. In order to avoid or decrease these occurred cakes, it is essential to characterize cake strength and to explore the process of cake formulation or caking through theoretical/numerical analysis. In this thesis, both experimental measurements of cake strength and theoretical/numerical simulations for recrystallization near a contact point are used to investigate the relationship between the caking process and the cake strength for important factors such as initial moisture content and drying time. In this research, a centrifugal loading method has been developed to determine cake strength in a caked ring specimen of potash fertilizer where internal tensile stresses dominate. Research on fracture mechanics states that brittle materials, such as caked potash, fail at randomly positioned fracture surfaces in tension – so the centrifuge test method is well suited to provide good data. A two-dimensional plane stress analysis was used to determine the area-averaged tensile stress at the speed of the centrifuge when each specimen fractures. Repeated tests and uncertainty calculations give data with a narrow range of uncertainty. The centrifuge test facility was used for a series of tests in which the initial moisture content, drying time, particle size and chemical composition (i.e. magnesium content) of the samples were varied. For particle sizes in the range from 0.85 to 3.35 mm, experimental data show that the cake strength increased linearly with initial moisture content for each drying method and particle size, and decreased with increasing particle size for each initial moisture content and drying method. As well, it was also found that cake strength will increase essentially linearly with magnesium content from 0.02% to 0.1% for samples with the same initial moisture content, particle size and drying method. All data show that potash samples tend to form a stronger cake with a slower drying process. A theoretical/numerical model is presented in this thesis to simulate ion diffusion and crystallization near one contact point between two potash (KCl) particles during a typical drying process. The effects of three independent factors are investigated: initial moisture content; evaporation rate; and degree of supersaturation on the surface surrounding the contact point. The numerical results show that the mass of crystal deposition near the contact point will increase with increased initial moisture content and decreased evaporation rate. These numerical predictions for recrystallization near the contact point are consistent with the experimental data for the cake strength of test samples of particle beds. With variations in the solid crystal surface degree of supersaturation near the contact point, simulations showed up to 5 times the increase in the crystal mass deposition near the contact point. This prediction of increased roughness is consistent with another experimental investigation which showed that the surface roughness of NaCl and KCl surfaces increased by a factor of five after one wetting and drying process.
contact region, ion diffusion, recrystallization, moving boundary, tensile stress, cake strength, caking, centrifuge
Master of Science (M.Sc.)