Calcium atoms are often replaced by rare earth elements (REEs) in the lattice of fluorapatite (Ca₁₀F₂(PO₄)₆), making the phosphate ore an important potential rare earth resource. In this paper, the electronic properties of REEs (La, Ce, Nd and Y) bearing fluorapatite crystals have been investigated by density functional theory. Results of calculation indicated that the existence of REEs increased the cell parameters of fluorapatite in varying degrees. The REEs substitution made the Fermi level of fluorapatite to move to higher energy levels, making it easier to accept electrons. Except for Y, all the other REEs (La, Ce and Nd) showed that the electronic state mainly exists in the valence band. The Fermi level of REEs were mainly contributed by La5d, Ce4f, Nd4f and Y4d, respectively. The Mulliken values of REE–F and REE–O bonds in REEs-bearing fluorapatites were larger than those of Ca–F and Ca–O bonds in the perfect crystal, and the values of Y–F and Y–O bonds were the largest. The results of interaction between fluorapatite and oleic acid by frontier molecular orbital analysis suggested that the substitution of REEs can improve the reactivity of fluorapatite with oleic acid.

The float-sink test is a commonly used technology for the study of coal washability, which determines optimal separation density for coal washing based on the desired sulfur and ash yield of the cleaned coal. In this study, the float-sink test is adopted for a high-sulfur Late Permian coal from Hongfa coalmine (No.26), southwestern Guizhou, China, to investigate its washability, and to analyze the organic affinities and distribution behaviors of some toxic and valuable trace elements. Results show that the coal is difficult to separate in terms of desulfurization. A cleaned coal could theoretically be obtained with a yield of 75.50%, sulfur 2.50%, and ash yield 11.33% when the separation density is 1.57 g/cm³. Trace elements’ distribution behaviors during the gravity separation were evaluated by correlation analysis and calculation. It was found that Cs, Ga, Ta, Th, Rb, Sb, Nb, Hf, Ba, Pb, In, Cu, and Zr are of significant inorganic affinity; while Sn, Co, Re, U, Mo, V, Cr, Ni, and Be are of relatively strong organic affinity. LREE (Light rare earth elements), however, seem to have weaker organic affinity than HREE (Heavy rare earth elements), which can probably be attributed to lanthanide contraction. When the separation density is 1.60 g/cm³, a large proportion of Sn, Be, Cr, U, V, Mo, Ni, Cd, Pb, and Cu migrate to the cleaned coal, but most of Mn, Sb and Th stay in the gangue. Coal preparation provides alternativity for either toxic elements removal or valuable elements preconcentration in addition to desulfurization and deashing. The enrichment of trace elements in the cleaned coal depends on the predetermined separation density which will influence the yields and ash yields of the cleaned coal.

The level of radionuclides is an important index for the preparation of building materials from industrial solid waste. In order to investigate the radiological hazard of five kinds of typical general industrial solid wastes in Guizhou, China, including fly ash (FA), red mud (RM), phosphorus slag (PS), phosphogypsum (PG), and electrolytic manganese residue (EMR), the radiation intensity and associated radiological impact were studied. The results show that concentrations of ²³⁸U, ²³⁵U, ²³²Th, ²²⁶Ra, ²¹⁰Pb, and ⁴⁰K for different samples vary widely. The concentration of ²³⁸U was both positively correlated with ²³⁵U and ²²⁶Ra, and the uranium contents in the measured samples were all of natural origin. The radiation levels of PG, EMR, EMR-Na (EMR activated by NaOH), and EMR-Ca (EMR activated by Ca(OH)₂) were all lower than the Chinese and the world’s recommended highest levels for materials allowed to be directly used as building materials. The values of the internal and external illumination index (I_Ra and I_γ, respectively) for FA and RM were higher (I_Ra > 1.0 and I_γ > 1.3 for FA, I_Ra > 2.0 and I_γ > 2.0 for RM). The radium equivalent activity (Ra_eq), indoor and outdoor absorbed dose (D_in and D_out, respectively), and corresponding annual effective dose rate (E_in and E_out) of RM, PS, and FA were higher than the recommended limit values (i.e., 370 Bq/kg, 84 nGy/h, 59 nGy/h, 0.4 mSv/y, and 0.07 mSv/y, respectively), resulting from the higher relative contribution of ²²⁶Ra and ²³²Th. The portion of RM, FA, and PS in building materials should be less than 75.44%, 29.72%, and 66.01%, respectively. This study provides quantitative analysis for the safe utilization of FA, RM, PS, PG, and EMR in Guizhou building materials.

Phosphate ore is an important strategic mineral resource. The efficient utilization of phosphate resources faces challenges such as low grade of raw ore and difficulty in discharging gangue minerals. One of the key problems to be solved urgently in the reverse flotation of phosphate ore is the effective depression of apatite. However, research on the influence mechanism of acid depressants on the surface properties and adsorption characteristics of apatite is still insufficient. In this study, the influence of different depressants (such as sulfuric acid, phosphoric acid or mixed acid of sulfur acid and phosphorus acid) on the flotation separation performance of an artificial mixture of apatite and dolomite (gangue mineral) was investigated through laboratory flotation tests. On this basis, with the addition of different depressants, the contact angle, zeta potential, XPS and TOC were used to investigate the surface wettability, surface charge, surface species and the adsorption characteristics of the collector (sodium oleate) on the apatite surface, and, accordingly, the inhibiting mechanism was discussed. The results show that, when mixed acid of sulfur acid and phosphorus acid is used as a depressant, a concentrate with a P₂O₅ grade of 33.53% and a recovery of 88.92% can be obtained, and the parameters are better than when using phosphoric acid with a P₂O₅ grade of 30.15% and a recovery of 80.12% or sulfuric acid with a P₂O₅ grade of 30.12% and a recovery of 80.58%. Our analysis shows that the mixed acid has the best inhibiting effect on apatite, which is mainly due to the following: (a) after adding the mixed acid, chemicals such as CaSO₄, CaHPO₄/Ca(H₂PO₄)₂ are generated on the surface of apatite, resulting in a significant reduction in the contact angle and stronger surface hydrophilicity; (b) the mixed acid reduces the zeta potential of apatite, produces new species and weakens the non-selective adsorption of negatively charged oleate on the surface of apatite, thus preventing the apatite from floating.

Pyrite is a mineral often associated with coal in coal seams and is a major source of sulfur in coal. Coal–pyrite is widely distributed, easily available, low-cost, and non-toxic, and has high light absorption coefficient. So, it shows potential for various applications. In this paper, the density-functional theory (DFT + U) is used to construct coal–pyrite with carbon doped in the sulfur and iron vacancies of pyrite. The effects of different carbon defects, different carbon doping concentrations, and different doping distributions in the same concentration on the electronic structure and optical properties of coal–pyrite were studied. The results show that the absorption coefficient and reflectivity of coal–pyrite, when its carbon atom substitutes the iron and sulfur atoms in the sulfur and iron vacancies, are significantly higher than those of the perfect pyrite, indicating that coal–pyrite has potential for application in the field of photovoltaic materials. When carbon is doped in the sulfur vacancy, this impurity state reduces the width of the forbidden band; with the increase in the doping concentration, the width of the forbidden band decreases and the visible-light absorption coefficient increases. The distribution of carbon impurities impacts the band gap but has almost no effect on the light absorption coefficient, complex dielectric function, and reflectivity, indicating that the application of coal–pyrite to photovoltaic materials should mainly consider the carbon doping concentration instead of the distribution of carbon impurities. The research results provide a theoretical reference for the application of coal–pyrite in the field of photoelectric materials.