Percolation Segregation and Flowability Measurement of Urea under Different Relative Humidity Conditions

Abstract

Dry blended fertilizers are known to segregate. Furthermore, researchers have documented that the size of blended fertilizers is the most dominant physical property contributing towards segregation. Additionally, it is known that flowability is also affected by the size and moisture content of blended fertilizers. Therefore, segregation and flowability of binary size mixtures were studied at three different equilibrium relative humidity conditions 40%, 50%, and 60% with the goal to evaluate the feasibility to mitigate segregation using moisture content. To that end, binary size mixtures were prepared using coarse and fine size urea of size ratio 2.0 and 1.7 mixed in weight proportions 33:67 and 50:50, respectively (commonly found in 10-10-10 blends). Urea is the most hygroscopic and expensive component of the blended fertilizers. Percolation segregation was quantified using the Primary Segregation Shear Cell (PSSC-II). Based on experimental results using the PSSC-II, the segregated fines mass, normalized segregation rate (NSR), and segregation rate (SR) of fines for binary urea mixtures were higher at equilibrium relative humidity of 40% vs. 50% and 60%. The NSR is defined as the amount of fines percolated from the total initial fines in the binary mixture based on the total time of PSSC-II operation (kg/kg-h). For size ratios 2.0 and 1.7, only 2.8% and 7.0% decrease in NSRs were recorded for the increase in relative humidity by 10 points (from 40% to 50%), respectively, whereas 36.0% and 45.0% decrease in NSRs were recorded for increase in relative humidity by 20 points (from 40% to 60%), respectively (P<0.5). Additionally, the flowability of binary size mixtures was quantified using a true Cubical Triaxial Tester (CTT). For size ratios 2.0 and 1.7, angle of internal friction increased from 31.3° to 35.9° to 39.0° and 27.4° to 32.0° to 36.0° when relative humidity increased from 40% to 50% to 60%, respectively. The angle of internal friction values were significantly different (P<0.05) but cohesion values, at different relative humidity conditions were not significantly different (P>0.05). Based on experimental results, relative humidity, if implemented carefully, could be used as a tool to mitigate segregation in blended fertilizers.

Keywords

• cubical triaxial tester
• primary segregation shear cell
• segregation rate
• normalized segregation rate
• angle of internal friction, and cohesion