Monitoring phosphate reactions in real time with Raman spectroscopy

Significance of the Topic

This summary explores how Raman spectroscopy can transform phosphate fertilizer production by enabling non-invasive, real-time monitoring of key reaction species. Conventional methods such as titration and gravimetric analysis are slow, laborious, and rely on hazardous reagents, leading to delayed process feedback. Integrating Raman spectroscopy addresses these challenges and supports tighter process control and improved product quality.

Objectives and Study Overview

The primary goal of the study was to evaluate the feasibility of using Raman spectroscopy to track the speciation of phosphate and sulfate during a model fertilizer production reaction. Dicalcium phosphate (DCP) was reacted under acidic conditions, and spectral changes were correlated with pH adjustments to simulate key process steps.

Methodology

• A 500 mg sample of DCP was dissolved in 10 mL of 0.5 mol/L HCl to generate phosphoric acid species.
• Sulfuric acid was added to introduce sulfate ions and mimic industrial sulfate levels.
• The mixture was titrated with 1 mol/L NaOH in 0.25 mL increments while continuously recording Raman spectra at 1064 nm (30 s integration).
• pH was monitored in real time using a Metrohm 913 meter to correlate spectral shifts with protonation and precipitation events.

Instrumentation

• i-Raman NxG 785H Raman spectrometer equipped with a fiber-optic probe (100–2800 cm⁻¹ range).
• Metrohm 913 pH meter with Electrode Plus for continuous acidity measurement.
• Standard laboratory glassware for titration and sample handling.

Main Results and Discussion

Raman analysis identified characteristic peaks for H₃PO₄ (889 and 1189 cm⁻¹) and H₂PO₄⁻ (1076 cm⁻¹) upon dissolution of DCP. Addition of sulfuric acid introduced a sulfate peak at 983 cm⁻¹, while shifts in phosphate peaks indicated dynamic protonation changes. Incremental NaOH titration triggered precipitation, evidenced by peak shifts (889 → 879 cm⁻¹) and intensity changes. Sulfate removal exceeded expectations, implying gypsum and mixed calcium phosphate–sulfate phases. Spectra of the recovered solid confirmed a dominant 1001 cm⁻¹ band, consistent with a mixture of gypsum, brushite, and ardealite.

Benefits and Practical Applications

Raman spectroscopy offers several advantages:

• Real-time, reagent-free detection of multiple ionic species in one measurement.
• High sensitivity to protonation state and precipitation events.
• Potential for inline process monitoring, reducing downtime and waste.
• Improved quality control by tracking reaction progress and product composition continuously.

Future Trends and Opportunities

Advancements in probe design, deeper integration with process analytical technology (PAT), and machine-learning-driven spectral interpretation promise to expand the applicability of Raman monitoring. Inline fiber-optic probes and robust chemometric models could enable full-scale implementation in fertilizer plants, further enhancing efficiency and sustainability.

Conclusion

Raman spectroscopy demonstrated clear capability to monitor phosphate protonation, sulfate concentration, and precipitation dynamics in a simulated fertilizer production process. Its non-destructive, real-time nature supports enhanced process control, faster optimization, and improved product quality in phosphate fertilizer manufacturing.

References

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2. US EPA Office of Air and Radiation. AP-42, Fifth Edition, Volume I, Chapter 8.9: Phosphoric Acid. 2020.
3. Metrohm AG. Determination of Total Phosphate in Phosphoric Acid and Phosphate Fertilizers with Thermometric Titration; Application Bulletin AB-314.
4. Barua R., Daly-Seiler C. S., Chenreghanianzabi Y., et al. Comparing Physicochemical Properties of Dicalcium Phosphate Dihydrate and Polymeric DCPD Cement Particles. J Biomed Mater Res. 2021;109(10):1644–1655.
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