Extended agitation facilitates pelleting coagulation for enhanced solid-liquid separation of highly concentrated waste drilling fluid from shale oil fields

In this study, high-intensity extended mechanical agitation is applied before pelleting coagulation, and polyaluminum chloride (PAC) and cationic polyacrylamide (PAM) are employed to achieve floc densification. The results demonstrated that pelleting coagulation (PC) with extended agitation significantly improved pollutant removal efficiency, achieving 98.32 % COD, 99.59 % color, and 99.96 % turbidity removal under optimal conditions (extended agitation intensity and duration = 1600 rpm and 90 min; PAC = 12.8 g/L; PAM = 250 mg/L). The process reduced the primary particle size (d₅₀ from 35.23 μm to 16.56 μm) and promoted the formation of dense, spherical “pelleting flocs” with low porosity. These flocs exhibited a higher fractal dimension (1.728) than those obtained via conventional coagulation (1.353), ultimately enhancing settling velocity to 31.83 mm/s. Floc strength analysis confirmed strong resistance to shear-induced breakage. Extended agitation fragmented large aggregates into uniform primary particles, and facilitated cluster-based accumulation under controlled hydraulic conditions. Overall, PC with extended agitation is a promising solution for sludge volume reduction and enhanced water recovery in water-scarce shale oil regions.