Influence of calcining parameters of FePO₄ and CaO compositions on the solubility of phosphorus compounds

Abstract

Purpose

To determine the influence of alkali components (especially Ca compounds) on the hematite phase formation during thermal processing of sewage sludge and the observation of transformations proceeding at different ratios of Ca to FePO₄.

Design/methodology/approach

The model compositions were heated to temperatures of 600°C and 950°C, then calcined within 3 hours without mixing. Cooled products were subjected chemical analysis regarding the content of phosphorus that was soluble in cold water, 0.4 per cent HCl, 2 percent citric acid and aqua regia as well as crystallographic identification.

Findings

On calcining at 600°C, CaO reacts with FePO₄, forming hydroxylapatite. The hematite phase which is insoluble in mineral acids crystallizes above 600°C. Further increasing the calcining temperature leads to transformations resulting in Ca₉Fe(PO₄)₇ and hematite formation. Molar ratios of CaO to FePO₄ · 2H₂O of 1.0 and 2.0 lead to hematite phase formation and increase the availability of phosphorus compounds (characterized by phosphate solubility in 0.4 percent HCl and 2 percent citric acid) in compositions after calcining.

Practical implications

The addition of CaO to sewage sludge before the combustion process should be beneficial from the point of view of further extraction of phosphorus compounds from the ash obtained. Suitable CaO addition favours iron binding into the hematite phase and raises the solubility of the phosphorus compounds in the ashes formed.

Originality/value

This paper is a new source of information which complements existing knowledge about phosphorus recovery from sewage sludge.

Citation

Wzorek, Z., Gorazda, K., Kulczycka, J. and Rzepecki, T. (2005), "Influence of calcining parameters of FePO₄ and CaO compositions on the solubility of phosphorus compounds", Management of Environmental Quality, Vol. 16 No. 6, pp. 627-638. https://doi.org/10.1108/14777830510623691