Synthesis of high acid-resistant ultramarine blue pigment through coal gangue, industrial zeolite waste and corn straw waste recycling

The massive stacking of the coal gangue (CG) in the coal mining process, discarded industrial zeolite waste (IZW) and agricultural corn straw (CS) has caused serious environmental pollution and resource waste. To achieve the recycling of solid waste, an economical method for synthesizing ultramarine blue pigment using a two-step calcination process of the CG/IZW/Na₂CO₃/S/CS with the mass rates of 1.50: 0.50: 2.50: 3.50: 1.00 (the first stage at 400°C for 0.50 h and the second stage at 900°C for 2.00 h) is proposed in this paper. The structure and composition of the synthesis ultramarine blue pigment were characterized by XRD, FT-IR, Raman, as well as SEM technologies, and results showed it had a sodalite structure containing S₃⁻ and S₂⁻ radicals. Furthermore, SiO₂ (1.20 mL of tetraethyl orthosilicate (TEOS) as the precursor and 4.50 mL of NH₃•H₂O as the catalyst) coated the synthesis ultramarine blue pigment (1.00 g) was successfully synthesized by sol-gel technique to improve the acid resistance of the pigment (pH=2.50-3.00). This new method of preparing ultramarine blue pigments not only achieves resource reuse at a low cost but also improves the acid rain resistance of the pigments.