Experimental Study of Salicylic Acid as a Calcium Sulfate Dihydrate Scale Inhibitor, Analyzed from Surface Properties and Crystal Growth

Static and dynamic experiments were carried out to study the antiscale performance of salicylic acid (SA) to calcium sulfate dihydrate (CSD) scale. The CSD scale formed in the reuse of processing of wastewater of phosphorite flotation. The scale surface physicochemical properties have been investigated by means of scanning electron microscopy (SEM), transmission electron microscopy (TEM), X-ray diffraction (XRD), Fourier transform infrared spectroscopy (FTIR), ζ potential, and contact angle measurements. The antiscale mechanisms of SA to CSD were studied from surface properties and crystal growth. The results showed that the scale inhibition efficiency of SA to CSD reached 98.9% (6 mg/L) under static conditions. Under dynamic conditions, the faster the fluid velocity, the less CSD formed. The order of scaling capability on different material surfaces was 20#CS > 304SS > PC > PE. The growth of CSD was inhibited by SA resulting in the surface becoming porous. The deprotonated SA could easily interact with the Ca²⁺ to make the CSD surface potential negative. The wettability properties of the CSD are greatly improved when the contact angle is reduced. The surface tension values of CSD without and with 6 mg/L SA are 19.06 and 240.69 mN/m², respectively. SA as a scale inhibitor can significantly inhibit crystallization of CSD.