The present invention discloses a method for preparing a slow-release
phosphate fertilizer from ammoniated
slag produced by monoammonium
phosphate, which specifically includes the following steps: step 1: adding
phosphoric acid or
phosphate-containing salts to the ammonated
slag, adjusting the mixture of
citric acid-soluble
phosphorus: water-soluble
phosphorus to 0.5-1.5:1, controlling the ratio of the
mass of
sulfur,
magnesium, iron, and
calcium in the mixture to the
mass of P2O5 in the mixture to 0.1-0.3:1, controlling the pH of the mixture to 1-4, and then adding
urea to adjust the
molar ratio of P2O5:N to 2-3:1, and heating to obtain a mixed
slurry after the
urea is dissolved. The present invention converts part of the orthophosphate in the ammonated
slag into
ammonium polyphosphate by controlling the
polymerization conditions, while ensuring that the proportion of effective
phosphorus is as high as 97% to 100%, and the
ammonium polyphosphate chelates the medium and trace elements in the ammonated slag during the
polymerization process. The
polyphosphate in the prepared polymorphic
phosphate fertilizer containing medium and trace elements has the characteristics of long-term slow release and high
utilization rate, and the chelated medium and trace elements are more easily absorbed by plants, which greatly improves the
fertilizer effect of the ammonated slag and realizes the added value of industrial-grade
ammonium polyphosphate.