Effects of nanobubbles on fine coal column flotation and underlying mechanisms have been investigated under different operating parameters including frother dosage, collector dosage, feed flow rate, air flow rate and wash water flow rate with a Chinese sub-bituminous coal sample. Experimental results show that the use of nanobubbles significantly improved the column flotation efficiency with fine coal particles. The collector dosage was reduced by almost one half in the presence of nanobubbles as a result of nanobubbles playing a role as a secondary collector and a similar reduction in the frother dosage was observed due to distinctly different mechanisms of nanobubble generation as compared to traditional bubble formation methods. Nanobubbles generated based on the hydrodynamic cavitation mechanism also reduced required air dosage and improved carrying capacity. The aggregation effect of fine coal particles caused by the preferential formation of nanobubbles on the surface of hydrophobic coal particles in the flotation system improves the rate of mineralization of bubbles. The flotation efficiency enhancement by nanobubbles is a result of multiple benefits to flotation processes brought by nanobubbles that form directly on the surface of coal particles.

This paper studied the macerals enrichment of the low-rank coal which produced by Shigetai Coal Mine, Shenhua Group. The coal sample fed to the Falcon were the product of the rod mill after 10 minutes grinding. Three factors were carried out, rotational frequency, feed rate and water pressure. The orthogonal experiment was designed to investigate the interaction between the three factors and a model was established between the factors and response values. Taking the vitrinite recovery rate as the response value, there was an interaction between the rotational frequency and the water pressure, moreover, the effect of the rotational frequency on the vitrinite recovery rate was greater than the water pressure, while, the feed rate on the vitrinite recovery rate was not significant. Taking the vitrinite enrichment ratio as the evaluation index, the Quadratic model was recommended, there was an interaction between the rotational frequency and the feed rate, moreover, the rotational frequency had better response to the vitrinite enrichment ratio than the water pressure and the feed rate. In order to optimize the experimental conditions, and the vitrinite enrichment ratio was set to 1.2, the vitrinite recovery rate was the larger the better, the optimal separation conditions obtained were the rotational frequency 30Hz, the water pressure 0.03MPa, and the feed rate 15ml/s in the laboratory.

The enhanced gravity concentrator with water medium has been used for the low-rank coal maceral enrichment and achieved a favorable effect. This study utilized the magnetite powder as the separation medium in the enhanced gravity separation test. The enhanced gravity separation with different medium was contrasted, the heavy medium separation possessed a more excellent separation mechanism and better effect, and the vitrinite content and recovery of concentrates was increased to 80.40 vol.% and 54.85%. In addition, the results of float–sink test and enhanced gravity separation with heavy medium were compared under the same separation density, the difference of vitrinite content and between the two methods was below 5%, and the vitrinite recovery of enhanced gravity separation was increased to 77.9% with the separation medium of 1.40 g/cm³. The density difference of macerals was proved by the EDS and XRD measurement, the more quantity of polycondensation aromatic rings and tighter microcrystalline structure leads to the higher density of inertinite.

The spatial distribution of fine high-sulfur lean coal under different backwash water pressures and rotational frequencies in enhanced gravity field using the Falcon concentrator was investigated. It deals with the distribution characteristics of products, including yield, ash content, sulfur content, and pyritic sulfur content, in overflow, upper groove, and lower groove from the top to the bottom in space. Backwash water pressure had coarse adjustment while rotational frequency had fine adjustment on the Falcon concentration process; meanwhile, the desulfurization and de-ashing results were discussed by combining them. The extensive migration characteristic of pyritic sulfur in space was also studied.

In this study, the rotary triboelectrification separation technology is applied to the separation of macerals for low rank coal, which has not been studied by previous researchers. The effects of air humidity and temperature on permittivity and specific-charge were tested to ensure that vitrinite and inertinite were effectively separated under obvious electrical differences. The best separation index of macerals was determined by single factor test, including airflow rate, feed rate, friction wheel speed, and plate voltage. The test results of specific-charge and permittivity showed that the difference is most reasonable when the air humidity is 20% and the temperature is 40°C. Single factor test showed that the optimal conditions are the feed rate of 1.5 g/s, the friction wheel speed of 3000 r/min, the plate voltage of 20 kV and the air flow rate of 6 m/s. Finally, this study obtained the best maceral separation indexes are vitrinite content of 72.03 vol%, vitrinite recovery of 67.86% and benefication ratio of 1.33. Comparison with other maceral separation technologies, the rotary triboelectrification technology is a clean dry and economical process technology, it has inherent advantages in the separation of macerals for low rank coal.

Pulverized coal is separated by rotary triboelectric separation to confirm the best charger rotary speed, separation voltage, and airflow velocity. On this basis, pulverized coal surface is modified by studying four modifiers to explore the effect of pulverized coal surface modification on permittivity, specific-charge, and separation indices, including kerosene, diesel, acetic acid, and sodium stearate. Finally, separation indices of triboelectric separation are compared before and after modification. Experimental results show that kerosene has the best modification effect for pulverized coal. Under the best conditions after modification, ash deash rate is increased by 1.87%, desulfurization rate is increased by 0.44%, clean coal recovery is increased by 6.07%, and ash content is reduced by 1.95%. After the modification of pulverized coal, the specific-charge and permittivity of clean coal and gangue minerals form a greater difference, which indirectly increases the trajectory difference of different minerals in the process of rotary triboelectric separation, achieving better separation of clean coal and gangue minerals.