The solubilities of phosphate and sulfate salts in supercritical water

Inorganic compounds are regularly present in aqueous streams. To understand their influence and behavior on these streams at supercritical conditions, little to no property data is available, which can be used as starting point for further research or application design. Since inorganic compounds tend to precipitate at these conditions, scaling, blocking and erosion can occur as a consequence. Furthermore, a separation of (precious) compounds from the bulk stream due to the precipitation is possible. Here, phosphate compounds are regarded as interesting for further investigation since resources are assumed to be depleted in future. As phosphate is present in many waste streams, these could be used as sources for recoverable phosphate. Resulting from these facts and options, a proper understanding and knowledge of these systems is important for later industrial applications. Therefore, the authors have investigated the behavior of salts (e.g. NaCl, NaNO(3) and MgCl(2)) in supercritical water in previous works.

To extend this knowledge, the solubilities of the sulfate salts MgSO(4) and CaSO(4) in a range of 18.8-23.2 MPa and 655-675 K as well as of the phosphate salts Na(2)HPO(4), NaH(2)PO(4) and CaHPO(4) in a range of 20.5-24.2 MPa and 665-690 K were investigated in this work with a continuous flow method in continuation of former work of the authors. The solubilities were compared with existing data available from open literature. A quantitative correlation on base of a phase equilibrium between the present phases was used to describe the behavior and to compare it with previous results. For the investigated calcium salts, CaSO(4) and CaHPO(4), it was found that a significant solubility decrease already happens at subcritical conditions resulting in precipitation in unwanted locations. For the remaining compounds, a parallel hydrolysis reaction was found as could be seen from a change in pH in the effluent stream. (C) 2010 Elsevier B.V. All rights reserved.