Publication Details
Abstract
This study quantifies how the amount of thermally activated dolomite added to a urea melt and the temperature at which that dolomite is calcined jointly govern the properties of the resulting granules. Local dolomite ( 27.52 %, 20.24 %, 43.21 %) was calcined at , , , , and 900 °C and introduced into molten urea at ten levels from 0.5 to 5.0 wt %, giving a two-factor full factorial design of 60 compositions plus a dolomite-free control. Dissolution time, melt crystallisation temperature, hygroscopic point, porosity and crushing strength were measured for every composition. Dolomite raised the dissolution time in all 60 cases, from 93.8 s for the control to 94.28–207.62 s, raised the crushing strength to 2.56–3.46 MPa and lowered the porosity below the control value of 5.75 % throughout. Each response was described by an additive linear model in the two factors (–; the interaction term was insignificant, –). Grouping the model residuals by calcination temperature revealed that three independent responses reach an extremum at exactly 700 °C and change sign between and 800 °C, coinciding with the calculated appearance of free and marking the transition of dolomite from a comparatively inert filler to a reactive additive. Four criteria complete activation of the magnesium component, limitation of free , at least 45.5 % total nitrogen and at least 2.5 MPa crushing strength identify a single regime: 2 g of dolomite calcined at 700 °C per 98 g of urea, a product that dissolves 1.38 times more slowly than unmodified urea, is 12.6 % stronger, 9.9 % less porous and contains 45.72 % N and 0.55 % entirely in the active form, with only 0.10 % free . Compared with a urea–formaldehyde system studied under the same criteria it is 6.7 % stronger, carries 4.5 absolute per cent more nitrogen and requires no imported formalin.
This study quantifies how the amount of thermally activated dolomite added to a urea melt and the temperature at which that dolomite is calcined jointly govern the properties of the resulting granules. Local dolomite ( 27.52 %, 20.24 %, 43.21 %) was calcined at , , , , and 900 °C and introduced into molten urea at ten levels from 0.5 to 5.0 wt %, giving a two-factor full factorial design of 60 compositions plus a dolomite-free control. Dissolution time, melt crystallisation temperature, hygroscopic point, porosity and crushing strength were measured for every composition. Dolomite raised the dissolution time in all 60 cases, from 93.8 s for the control to 94.28–207.62 s, raised the crushing strength to 2.56–3.46 MPa and lowered the porosity below the control value of 5.75 % throughout. Each response was described by an additive linear model in the two factors (–; the interaction term was insignificant, –). Grouping the model residuals by calcination temperature revealed that three independent responses reach an extremum at exactly 700 °C and change sign between and 800 °C, coinciding with the calculated appearance of free and marking the transition of dolomite from a comparatively inert filler to a reactive additive. Four criteria complete activation of the magnesium component, limitation of free , at least 45.5 % total nitrogen and at least 2.5 MPa crushing strength identify a single regime: 2 g of dolomite calcined at 700 °C per 98 g of urea, a product that dissolves 1.38 times more slowly than unmodified urea, is 12.6 % stronger, 9.9 % less porous and contains 45.72 % N and 0.55 % entirely in the active form, with only 0.10 % free . Compared with a urea–formaldehyde system studied under the same criteria it is 6.7 % stronger, carries 4.5 absolute per cent more nitrogen and requires no imported formalin.