Eco-Efficient Utilization of Calcite-Rich Phosphate Waste for Feed-Grade Phosphate Production and Integrated CO₂ Sequestration

Phosphate mining activities generate enormous quantities of calcite-rich residues whose management poses both environmental and economic challenges, largely due to their low reactivity and limited industrial value. In this work, a sustainable valorization route is proposed to transform this carbonate waste into feed-grade calcium phosphates while simultaneously capturing carbon dioxide in a stable mineral form. The process is based on a two-step chemical pathway integrating alkaline conversion and phosphate synthesis.

In the first stage, the calcite-dominant residue was subjected to alkaline activation using sodium hydroxide (NaOH), leading to the in situ generation of calcium hydroxide [Ca(OH)₂] and sodium carbonate (Na₂CO₃). The reaction parameters were systematically optimized through a statistical design of experiments combining Plackett–Burman screening with a central composite design. Among the tested factors, NaOH concentration, solid-to-liquid ratio, and reaction time were identified as the most influential variables. The optimized conditions (7.95 mol L⁻¹ NaOH, 75 °C, S/L = 4.28 g per 50 mL, and 26 min) resulted in a conversion yield of 88%, confirming the spontaneous and exothermic character of the process (ΔG ≈ −58 kJ mol⁻¹; ΔH ≈ −59 kJ mol⁻¹). Sodium carbonate was recovered by crystallization from both the filtrate and washing effluents, achieving an average recovery of 86%, equivalent to the sequestration of approximately 0.33 t of CO₂ per ton of calcite treated.

The second step involved the neutralization of the Ca(OH)₂-rich product with phosphoric acid, enabling the controlled precipitation of monocalcium phosphate (MCP) or dicalcium phosphate (DCP) depending on the targeted Ca/P molar ratio. X-ray diffraction (XRD), X-ray fluorescence (XRF), and scanning electron microscopy coupled with energy-dispersive spectroscopy (SEM-EDS) confirmed the formation of well-crystallized, pure calcium phosphates exhibiting high solubility in 2% citric acid (>90%), in accordance with international feed additive standards.