Scale-Up of Agrochemical Urea-Gypsum Cocrystal Synthesis Using Thermally Controlled Mechanochemistry

Atom- and energy-efficient synthesis of a crystalline calcium urea sulfate ([Ca(urea)4]SO4) cocrystal was explored using thermally controlled mechanochemical methods with calcium sulfate compounds containing various amounts of crystalline water (CaSO4·xH2O, x = 0, 0.5, 2). Small-scale (200 mg) experiments in a shaker mill were first performed, and the progress was monitored by in situ Raman spectroscopy and in situ synchrotron powder X-ray diffraction. Time-resolved spectroscopy data revealed that the presence of water in the reagents' crystalline structure was essential to the reaction and largely determined the observed reactivity of different calcium sulfate forms. Reactions at elevated temperatures were shown to proceed significantly faster on all synthetic scales, while changes in rheology caused by adding external water hindered the reaction progress. The average yield of a 21 mm horizontal twin-screw extruder experiment was ∼5.5 g/min of extrusion (∼330 g/h). Energy consumption during the milling reactions required to achieve complete conversion ranged from 7.6 W h/g at 70 °C for a mixer mill to 3.0 W h/g at a 50 g scale and 4.0 W h/g at a 100 g scale for a planetary mill or 4.0 W h/g at both 70 °C and RT for a twin-screw extruder, showing a significant improvement in energy efficiency at large-scale production. The obtained crystalline cocrystal exhibited a significantly lower solubility in aqueous solutions, nearly 20 times lower per molar basis compared to that of urea. Furthermore, reactive nitrogen emissions in air at 90% relative humidity, measured as NH3, showed slow and nearly linear nitrogen loss for the cocrystal over 90 days, while the same level of emissions was achieved with urea after 1–2 weeks, showing the potential of this cocrystal material as a large-scale nitrogen-efficient fertilizer.