Revisió bibliogràfica de cristal·litzadors d’estruvita i realització d’algunes primeres proves experimentals

Abstract

The aim of this TFG is to implement a laboratory-scale experimental procedure to obtain struvite. The work is made of two parts: a literature review and experimental synthesis of struvite. The first part of the work deals with a bibliographic review about struvite precipitation. The second part deals with the experimental synthesis of struvite based on a scientific article from the bibliographic review. The results of the article have been successfully reproduced. The bibliographic review indicates that currently fluidized-bed reactors and stirred reactors are the most used. However, other sort of reactors are also used, e.g. ion exchange reactor or electrochemical precipitation reactor. According to the material readily available in the research laboratory to perform the experiments, a discontinuous stirred reactor has been chosen for struvite precipitation. The experiments in this kind of reactors are simpler to model and easier to operate than fluidizedbed reactors. Many researchers have performed struvite precipitation experiments in discontinuous stirred reactors. Not all of them are performed in the same operating conditions. After reading carefully several articles, the following paper has been chosen as reference model: Lee et al. (2016). This article claims a high recovery of phosphorus (> 90 %) and provides a quite well defined methodology and experimental data to be replicated. White solid crystals are collected and observed at microscopy. Nevertheless, the X-ray diffraction analysis (XRD) shows that the crystals collected are amorphous. Two key concepts justify this fact: the ageing and the decomposition of struvite. These concepts are not included in the methodology of most of the articles consulted, but they are of paramount importance. The ageing allows the crystals growth, meanwhile the crystals formed could decompose to other products, depending on the drying process.A cyclic feedback between experiments and bibliographic review provides the answer to the unexpected amorphous XRD results. Drying the solid struvite at an oven at 50 ºC under vacuum decomposes the struvite (partially or totally) to amorphous magnesium phosphate. In order to verify this assumption, two experiments more are performed drying the solid at ambient temperature and keeping it in a desiccator with silica gel. Unfortunately, XRD results has not arrived in time to add them to this report