Low-grade and high-reserve scheelite, which is associated with calcite, has similar surface properties that cause difficulty in separation. In this study, sulfomethylated kraft lignin (SMKL) was used as a novel eco-friendly inhibitor for the flotation separation of scheelite and calcite. The flotation test results showed that 60 mg/L SMKL had a significant influence on depressing calcite flotation, while it had a slight effect on scheelite flotation. Furthermore, it enhanced the WO₃ grade of the concentrate in the artificial mixed ore to 62.02% with a recovery rate of 80.37%. The contact angle and zeta potential showed that SMKL could effectively decrease the surface floatability of calcite and caused the negative shift of minerals’ surface potential. XPS and DFT calculations revealed that the sulfonic acid group of SMKL had an electron-donating ability and was easily adsorbed on the positively charged surface of calcite, which hindered the adsorption of sodium oleate on calcite. SMKL could separate calcium-bearing minerals with a high efficiency and selectivity, providing a new method for industrial production.

Efficiently discovering the interaction of the collector oleate and siderite is of great significance for understanding the inherent function of siderite weakening hematite reverse flotation. For this purpose, investigation of the adsorption behavior of oleate on siderite surface was performed by density functional theory (DFT) calculations associating with atomic force microscopy (AFM) imaging. The siderite crystal geometry was computationally optimized via convergence tests. Calculated results of the interaction energy and the Mulliken population verified that the collector oleate adsorbed on siderite surface and the covalent bond was established as a result of electrons transferring from O1 atoms (in oleate molecule) to Fe1 atoms (in siderite lattice). Therefore, valence-electrons’ configurations of Fe1 and O1 changed into 3d^6.514s^0.37 and 2s^1.832p^4.73 from 3d^6.214s^0.31 and 2s^1.83p^4.88 correspondingly. Siderite surfaces with or without oleate functioned were examined with the aid of AFM imaging in PeakForce Tapping mode, and the functioned siderite surface was found to be covered by vesicular membrane matters with the average roughness of 16.4 nm assuring the oleate adsorption. These results contributed to comprehending the interaction of oleate and siderite.

For effective utilization of the residues, calcium sulfate whiskers were prepared from semi-dry desulfurization residues by hydrothermal synthesis reactions. Aiming at collecting the products with a long length and large aspect ratio, the additives, including sodium oleate, sodium dodecyl benzene sulfonate (SDBS), and sodium citrate, were added to control the growth of the crystal. Compared with no additives, whiskers with relatively a longer length and larger aspect ratio could be obtained in the presence of sodium oleate or SDBS. The sodium citrate made the whiskers thicker and shorter. Then, the effects of the additives on crystal growth and the morphology of the hydrothermal products were investigated with the aid of X-ray diffraction (XRD) patterns and molecular dynamics simulations. According to the results of XRD, the diffraction intensity of the crystal face (400) increased under the influence of sodium oleate, promoting crystal growth along the c-axis. The molecular structures of the corresponding faces were built based on the strong peaks shown in the XRD patterns. The atomic distribution on the computed crystal faces was presented. The interaction energies on different faces were calculated to illustrate the different adsorption configurations of the additives. Among the calculated faces, the interaction energies on (400) were both most negative for sodium oleate and SDBS. Therefore, sodium oleate and SDBS preferred to adsorb on (400). As a result, the growth of (400) was thus inhibited and the crystal grew along the c-axis. Compared with sodium oleate and SDBS, the lowest adsorption energy of sodium citrate on the face (310) indicated that sodium citrate has no positive effect on the directional growth of the crystal parallel to c-axis. The molecular simulation results were virtually identical to the crystal faces analysis results.

Collectors commonly have synergetic effects in ores flotation. In this work, a waste engine oil (WEO) was introduced as a collector to an ilmenite flotation system with sodium oleate (NaOL). The results show that the floatability of ilmenite was significantly improved by using WEO and NaOL as a combined collector. The recovery of ilmenite was enhanced from 71.26% (only NaOL) to 93.89% (WEO/NaOL combined collector) at the pH of 6.72. The optimum molar ratio of NaOL to WEO was about 2.08 to 1. The WEO and NaOL also have synergetic effects for the collection of ilmenite, because to obtain the ilmenite recovery of 53.96%, the dosage of 45 mg/L NaOL is equal to 38.56 mg/L WEO/NaOL combined collector (30 mg/L NaOL + 8.56 mg/L WEO). In other words, 15 mg/L of NaOL can be replaced by 8.56 mg/L of WEO. It is an effective way to reduce the dosage of the collector and reuse WEO. Therefore, it is a highly valuable and environmentally friendly approach for WEO reuse. WEO mainly consists of oxygen functional groups, aromatics, and long-chain hydrocarbons, especially for the RCONH₂ and RCOOH, thereby forming a strong interaction on the ilmenite surface. The adsorption mechanism of waste engine oil and sodium oleate on the ilmenite surface is mainly contributed by chemical adsorption. Therefore, WEO exhibits superior synergistic power with NaOL as a combined collector. Herein, this work provided an effective collector for ilmenite flotation and a feasible approach for reducing NaOL dosage and recycling WEO.