Abstract
In this work, a new slow-release fertilizer (SRF) of calcium magnesium ammonium polyphosphate (CMAPP) is constructed using unpurified wet-process phosphoric acid (WPA), discarded phosphorus tailings and urea. The molecular weight distribution (MWD), polymerization degree (PD), morphology, crystal phases and chemical environment of phosphorus (P) of CMAPP are characterized systematcially to identify its structure, speculate the polymerization mechanism and release characteristics. The resulting MWD and hierarchical PD of CMAPP are sensitive to calcination temperature and time. The optimal polymerization process is calcination at 225 °C for 10 min. During the polymerization, pyrophosphate is formed by the condensation of urea-phosphate intermediate and HRPO4−, then the -OH in pyrophosphate further condense with other -OH to form longer chain polyphosphates. The optimal CMAPP is an amorphous porous-structured aggregates with PD¯ of 2.87. The leaching and release experiment show that CMAPP not only ensure the continuous supply of orthophosphate, but also alleviate the nutrients leaching by introducing of calcium (Ca) and magnesium (Mg) for anchoring. This SRF for Ca, Mg and P nutrients derived from discarded phosphorus tailings and unpurified WPA might be a cost-effective candidate to improve agronomic benefits.
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