Dissolution kinetics model of a by-product rich in magnesium oxide suitable for struvite precipitation

Abstract

Phosphorus is a limited resource which availability is gradually depleting. Due to the shortage of phosphorus within the European Union (EU) and the EU's dependence on phosphorus imports, phosphorus has been classified by the EU as a critical raw material. Accordingly, the EU is seeking technologies and alternatives to recover phosphorus. One of the most promising possibilities under study is the recovery of phosphorus present in wastewater through the precipitation of struvite. However, the problem of struvite precipitation is the cost associated with the addition of magnesium and an alkaline reagent. The price of magnesium sources (e.g. magnesium chloride, magnesium hydroxide) is relatively high, thereby imposing economic constraints on the process economic feasibility. Moreover, magnesium also classifies as an essencial raw material for the EU. As a result, cheaper magnesium sources are being sought to carry out struvite precipitation. One of the promising new sources is the utilization of a by-product rich in magnesium oxide (named LG-MgO) from the calcination of natural magnesite. It has already been demonstrated that this by-product can precipitate phosphorus as struvite with a high efficiency (60% - 90%). The problem of LG-MgO utilization lies in the limited knowledge of its dissolution behaviour, with a lack of understanding regarding the release of magnesium and hydroxide ions from this by-product. Therefore, this study develops a dissolution kinetic study for the LG-MgO in both neutral and acidic media, with the aim of helping optimize the LG-MgO dosage and usage. Two kinetic models have been proposed, allowing the calculation of the amount of magnesium dissolved under a wide range of operational conditions. This research provides valuable information for the utilization of this industrial by-product for struvite precipitations by reducing the need for raw materials for struvite precipitation and taking a first step into circular economy.