Mechanochemical synthesis of kaolin–KH 2PO 4 and kaolin–NH 4H 2PO 4 complexes for application as slow release fertilizer

Abstract

A milling process to reduce kaolin to amorphous phase in the presence of KH 2PO 4 or NH 4H 2PO 4 and allow mechanochemical (MC) reaction for incorporation of KH 2PO 4 and NH 4H 2PO 4 into the kaolin structure was investigated in this work. Mixtures of kaolin and KH 2PO 4 and NH 4H 2PO 4 in separate systems were prepared by milling in a planetary ball mill. Tests with kaolin contents ranging from 25 to 75 wt.% and mill rotational speeds from 200 to 700 rpm were performed to evaluate incorporation of KH 2PO 4 and NH 4H 2PO 4 and release of K +, NH 4 + and PO 4 3− ions into solution. Analyses by XRD, DTA and ion chromatography indicated that the MC process was successfully applied to incorporate both KH 2PO 4 and NH 4H 2PO 4 into the amorphous kaolin structure. Release of K + and PO 4 3− ions from the system (kaolin–KH 2PO 4) when dispersed in water for 24 h reached only up to 10%. Under similar conditions for the system (kaolin–NH 4H 2PO 4), release of NH 4 + and PO 4 3− ions reached between 25 and 40%. These results indicated that the MC process can be developed to allow amorphous kaolin to act as a carrier of K +, NH 4 + and PO 4 3− nutrients to be released slowly for use as fertilizer.