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This paper sets out to observe the changes occurring during thermal processing of Na₂CO₃ and FePO₄ · 2H₂O in control trials, then analyses the influence on availability of phosphorus compounds in comparison with pure FePO₄ · 2H₂O.

Control trials of Na₂CO₃/FePO₄ · 2H₂O with molar ratios in the range 0.5‐5.0 were heated to a temperature of 850°C, then calcined within 3 hours without mixing. Cooled products were submitted for chemical analysis regarding the content of phosphorus soluble in cold water, 0.4 percent HCl, 2 percent citric acid and aqua regia as well as crystallographic identification.

The addition of Na₂CO₃ significantly increases the availability of phosphorus compounds from FePO₄ · 2H₂O. The total amount of phosphorus in control trials after the calcining process is soluble in cold water, citric acid and HCl. The molar ratios used affect the type of crystalline phase formation only in the case of a molar ratio near to 1.0. Near‐perfect phosphorus availability in hematite phase formation was observed.

These preliminary results confirm the necessity for further investigation of the addition of Na₂CO₃ into industrial sewage sludge before the calcining process. The results may allow for a decrease in energy use during the combustion process and may create the possibility of recovering phosphorus compounds from ash using only cold water instead of mineral acids during the extraction process.

This paper is a new source of information about phosphorus recovery from sewage sludge.

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.

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.

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.

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.

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