The effect of saline water on the critical degree of coal surface oxidation for coal flotation

In this work, the effects of water with different salinities on the flotation of oxidized coals were studied and the underlying mechanisms were revealed. Flotation tests were conducted using coal samples with different degrees of surface oxidation and in water of different salinities. A critical degree of coal surface oxidation, above which the coal particles cannot be recovered by true flotation, was observed in flotation with all types of water. It is interesting to find that the critical degree of surface oxidation increased with water salinity, suggesting that coal flotation in saline water had a better tolerance for surface oxidation. The effects of water salinity on the flotation of oxidized coals were studied by investigating the change of coal surface properties and froth stability. On one hand, the surface hydrophobicity of oxidized coals decreased in saline water, which was unfavorable for coal flotation. On the other hand, the compression of the electric double layer in saline water could increase the flotation efficiency by reducing the electrostatic repulsion between particle and bubble. In addition, smaller bubbles and a higher stability froth were generated in saline water, which could also increase the flotation efficiency

Predicting the degree of surface oxidation on fine coals by measuring the oxygen transfer rate in coal suspensions

Coal surface oxidation plays a dominant role in differential coal flotation and the utilization of coal products. However, a robust and reliable tool to determine coal surface oxidation in coal preparation plants is not currently available. In this study, a novel technique was developed to determine the degree of coal surface oxidation by measuring the adsorption rate of oxygen on coal surfaces after understanding the nature of oxygen transfer in water and oxygen adsorption on oxidized and un-oxidized coal surfaces. In this study, coal samples with different extents of surface oxidation were prepared and the degree of coal surface oxidation was quantified by X-ray photoelectron spectroscopy (XPS) as the percentage of oxidized carbon. Oxygen was purged into the coal suspension at a constant flow rate and the change of dissolved oxygen (DO) concentration was monitored. It was found that the DO concentration increased with oxygen purging time and the rate of increase was dependent on the degree of coal surface oxidation. A faster increase in DO concentration was observed for more oxidized coals, which is related to a slower adsorption of oxygen on oxidized coal surfaces and, therefore, more dissolved oxygen remained in the suspension. The kinetics of the change of DO concentration was calculated using the oxygen transfer equation, based on which the rate of oxygen adsorption on coal surfaces was obtained. A linear relationship was found between the oxygen adsorption rate and the degree of coal surface oxidation. This technique may be implemented in coal preparation plants as a daily tool to closely monitor the coal oxidation status due to its simplicity and accuracy

Effect of saline water on the synergistic interaction between diesel and Triton X-100 in the flotation of oxidized coal

In this study, the effect of medium saline water on the synergistic interaction between diesel and Triton X-100 in the flotation of oxidized coal was investigated. The results showed that the flotation yield of oxidized coal in saline water was higher than that in de-ionized (DI) water due to the promotion of diesel adsorption, which was attributed to the screening of electrostatic repulsion between diesel droplets and coal particles in saline water. Meanwhile, the flotation of oxidized coal could be significantly improved when Triton X-100 was added with diesel as a composite collector, and less Triton X-100 was required in saline water than that in DI water to achieve the same true flotation yield, indicating that saline water could increase the effectiveness of Triton X-100 in improving oxidized coal flotation. A mechanism study revealed that Triton X-100 was able to promote diesel adsorption on oxidized coal through emulsification, thus increasing the surface hydrophobicity of oxidized coal through hydrogen bonding between the headgroups of Triton X-100 and the oxygenated groups on coal surfaces. The non-ionic characteristic of Triton X-100 ensured its capability of enhancing oxidized coal flotation in both DI water and saline water

The adsorption behavior of surfactants on mineral surfaces in the presence of electrolytes - A critical review

How saline water affects the adsorption of surfactants on mineral surfaces and the subsequent flotation behavior is still unknown despite the widespread application of saline water in mineral flotation. However, in the field of surfactants, the effect of electrolytes on the adsorption of surfactants on solids has been well studied. This paper presents a comprehensive review of surfactant adsorption on solids in the presence of electrolytes based on the previous research findings. The review shows that the presence of electrolytes significantly influences the solubility and aggregation of surfactants in the liquid phase either through electrostatic interactions or "salting-in" and "salting-out" effects. In the process of surfactant adsorption on solids, electrolytes influence the adsorption density of ionic surfactants and non-ionic surfactants by affecting the surfactant-surfactant interactions and surfactant-solid interactions. Once surfactants adsorb on solids, electrolytes modify the structure and morphology of adsorbed surfactant layers and even induce the structural transition. This paper provides a thorough understanding of surfactant adsorption on solids in the presence of electrolytes and guides the application of surfactants in mineral flotation using saline water

The role of calcium and carbonate ions in the separation of pyrite and talc

Carboxymethyl cellulose (CMC), a negatively charged polymer, is commonly used as a depressant of talc. However, the separation of pyrite and talc can be deteriorated by the presence of calcium ions, which can facilitate the absorption of CMC onto both talc surfaces and pyrite. In this work, carbonate ions were introduced to solve this problem, and the effects of calcium and carbonate ions on the adsorption of CMC were also studied. Micro-flotation results revealed that CO . not only eliminated the inhibition of pyrite by Ca and CMC, but also had no influence on the depression of talc at pH 6–10. The surface-specific study confirmed that the adsorption of Ca onto talc basal planes at pH 8.5 was negligible and could be improved by the presence of CO . The atomic force microscopy analysis showed the correlation between the coverage of CMC onto talc surfaces in the presence and absence of CO . A mechanism for the adsorption of CMC onto talc surfaces was proposed based on the morphological differences in the presence and absence of CO

Magnetic Zr-Based Metal-Organic Frameworks: A Highly Efficient Au (III) Trapper for Gold Recycling

In this work, the magnetic Zr-based MOF composites with excellent retrievability were prepared using Fe3O4@SiO2 as the core and UiO–66–NH2 as the shell. Fe3O4@SiO2 core could introduce mesopores and result in capillary condensation in MOF composites, which aggravated with the dosage of Fe3O4@SiO2. The as-synthesized MOF composites could be rapidly retrieved from aqueous solution via magnetic separation in 10 seconds. pH imposed an important effect on Au (III) adsorption by governing the ion exchange and electrostatic interaction between Au (III) anions and adsorbents, and the optimal adsorption happened at pH 7. The adsorption process fitted well with the pseudo-second order kinetics model and Langmuir adsorption model. The maximum adsorption capacity of Au (III) by FSUN–10 and FSUN–50 at 298 K were determined to be 611.18 mg∙g−1 and 463.85 mg∙g−1, respectively. Additionally, Au (III) uptakes increased with temperature. Beyond experiments, the adsorption mechanisms were thoroughly studied through systematic characterization, molecular dynamics simulation (MDS) and density functional theory (DFT) study. It was verified that Au (III) was adsorbed via coordination to hydroxyl and amino groups and was reduced to Au (I) and Au (0) by amino groups. The diffusion coefficient of Au (III) along UiO–66–NH2 was calculated to be 5.8 × 10−5 cm2∙s−1. Moreover, the magnetic Zr-based MOF composites exhibit great industrial value in gold recycling with high adsorption selectivity and good recyclability

Selective and efficient adsorption of Au (III) in aqueous solution by Zr-based metal-organic frameworks (MOFs): an unconventional way for gold recycling

Recycling precious metals from secondary resources is of great environmental and economic significance. In this study, the Zr-based MOFs UiO-66-NH2 was synthesized and used to adsorb Au (III) in aqueous solution. The ultrafine particle size (∼50 nm), excellent crystallinity and huge specific surface area (1039.2 m2 ·g−1) were verified by transmission electron microscope (TEM), powder X-ray diffraction (PXRD) and surface area analysis. About 50 % Au (III) was adsorbed within 6 min and the maximum adsorption capacity at 298 K reached up to 650 mg·g−1, showing superiority to traditional adsorbents. The general order kinetics model and Liu equation were suitable to describe the adsorption process, which was spontaneous, endothermic and driven by the increasing system entropy. Electrostatic attraction between -NH3+ and Au (III) anions and inner complexation to Zr-OH played a vital role in adsorption. Au (Ⅲ) was reduced to Au° by amino groups via redox reaction certified by X-ray photoelectron spectroscopy (XPS), PXRD and high-resolution transmission electron microscopy (HRTEM) analysis. Moreover, UiO-66-NH2 displayed high selectivity, robust stability and excellent reusability, making it an ideal candidate for gold recycling in industrial practice

Beneficial effects and mechanism of lead ion on wolframite flotation

In this study the effects and mechanism of lead ions influence on wolframite flotation with benzohydroxamic acid (BHA) were studied through micro-flotation, adsorption experiments, zeta potential measurements, logarithmic concentration diagram, and X-ray photoelectron spectroscopy. It was observed that lead ions could significantly enhance the recovery of wolframite in flotation and adsorption density of collector BHA onto the wolframite surface. The results showed that Pb existed in the forms of lead ion, monohydric lead, and lead hydroxide at the water-wolframite interface respectively, at three pH ranges. They increased the zeta potential of wolframite. However, the zeta potential of wolframite was still negative, resulting in repulsive electrostatic force to anionic collector BHA. Combining with XPS spectra, it revealed the chemisorption of BHA onto the wolframite surface. In addition, PbO or Pb(OH)2 was observed on the wolframite surface due to the reaction between lead ions and wolframite. These reaction products increased the adsorption site of BHA on the wolframite surface because Pb-hydroxamate was found on the wolframite surface

Effects of sodium salts on the sulfidation of lead smelting slag

The effects of sodium salts on the sulfidation behavior, phase transformation, ZnS particle growth, and zinc floatability of lead smelting slag (LSS) were investigated by roasting experiments and flotation tests. The roasting results indicated that sodium salt additives could enhance the reactivity of zinc sulfidation and the percentage of liquid phase and thus promoted the sulfidation of LSS and the growth of ZnS particles. However, with the increase in temperature, the positive effects were reduced because the reactivity of the sulfidation and the percentage of liquid phase were not the determining factors at high temperatures. The effect of NaCO was stronger than NaSO, while NaSO was stronger than NaCl on the sulfidation of zinc and the growth of ZnS particles. The addition of NaCO or NaSO favored the conversion of marmatite to wurtzite, which could be evaporated at temperatures above 1000\ua0°C. Flotation test results revealed that sodium salt additives had different influences on the zinc grade and recovery at different temperatures. NaCO was the best additive in roasting for increase in zinc grade, due to the formation of metallic iron rather than iron sulfides