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Lead (Pb) ions are chronically detected in soil, underground and natural water, there is a need for low-cost in situ remediation techniques. A novel mineral based magnetic adsorbent, consisting of natural vanadium, titanium-bearing magnetite particles coated by humic acid (abbr. VTM-HA), was developed for removing Pb(II) from wastewater. In this study, adsorption-desorption characteristics and mechanisms of Pb (II) on the VTM-HA adsorbent were investigated. The regeneration and reuse feasibility of the VTM-HA adsorbent were also conducted. The adsorption tests further verified that Pb(II) was removed rapidly and efficiently by the VTM-HA adsorbent. Moreover, Pb (II) adsorbed on the VTM-HA adsorbent could be easily eluted using a small amount of acidic eluent, and the desorption of Pb (II) could reach 99.3 wt% in 0.1 M HNO3 (pH = 1). After five adsorption-desorption regeneration cycles, the magnetism and Pb(II) adsorption capacity of the regenerated VTM-HA adsorbent almost kept unchanged. The results indicated that the VTM-HA adsorbent had excellent adsorption capacity and regenerative ability, which could be used as ideal adsorbents for removing Pb (II) from acid wastewater in commercial application. The mechanisms of Pb (II) adsorption and desorption were explored by FTIR, XPS analysis and DFT calculation.

Carbonization of pure MnO2 by CH4-H2 gas mixture with various CH4/(CH4 + H2) molar ratios in the temperature range of 900–1250 °C had been performed in our previous work. This study further investigated the carbonization and alloying process of MnO2 by CH4-H2 gas mixture in the presence of Fe2O3 at 900–1300 °C using XRD, optical microstructure, SEM-EDS and phase diagram analyses. Carbonization test of pure Fe2O3 was also conducted to understand the effect of Fe2O3 on the carbonization of MnO2 more explicitly. It was found that Fe2O3 could be carbonized to Fe3C at 900–1150 °C. The Fe2O3 briquettes began to melt at 1100 °C and a large amount of Fe-C melts appeared as the temperature went up to 1150 °C. When the MnO2 + 25 wt%Fe2O3 briquettes were reduced at 900–1000 °C in CH4-H2 gas mixture, the Fe2O3 was preferentially carbonized to Fe-C phase while the MnO2 was only reduced to MnO. The formation of Fe-C phase was beneficial to the carbonization of MnO since the carbonization temperature of MnO can decrease from 1100 °C to 1000 °C. The carbonization degree of MnO was dramatically accelerated over 1100 °C due to the transformation of Fe-C phase from solid state to melts. Morphology analysis indicated that the MnO core was wrapped by the shell of Fe-C melts. The alloying process of Mn-Fe-C was conducted through the elements migration of Fe and Mn between the core and shell. At 1300 °C, the Mn-Fe-C alloy turned to melt and aggregated since the highest melting temperature of the Mn-C-25 wt% Fe system was only 1274 °C. Finally, a ferromanganese crude alloy with 68–72 wt% Mn, 20–25 wt% Fe and 6.0–8.0 wt% C was obtained when the MnO2 + 25 wt% Fe2O3 briquettes were carbonized by CH4-H2 gas mixture at 1300 °C for 60 min.

Phase transformation of MnO 2 and Fe 2 O 3 roasted in air atmosphere at temperatures of 500–1400 °C for synthesizing manganese ferrite (Mn x Fe 3 − x O 4 ) has been reported. In current work, the morphologies and properties of the Mn x Fe 3 − x O 4 products with various x values during the synthesis process were characterized by XRD, XPS, SEM-EDS, AFM, VSM and Vickers-type microhardness analyses to further understand the formation mechanism of Mn x Fe 3 − x O 4 from MnO 2 and Fe 2 O 3 by solid-state reaction. Results showed that the x value in the Mn x Fe 3 − x O 4 gradually increased to 1 with the extension of roasting time from 30 min to 120 min at 1300 °C. Morphology measurements indicated the Mn x Fe 3 − x O 4 particles presented polyhedron structure and the shape of polyhedron particles gradually became regular with the increasing of x value. In addition, it's interesting to find that the polyhedron particles had multi-layer stacking structure and the thickness of each layer was about 5 nm. The saturation magnetization of the Mn x Fe 3 − x O 4 samples increased with the rising of x value. A polyhedral MnFe 2 O 4 (x = 1) product with saturation magnetization of 78.5 emu/g was obtained when the MnO 2 and Fe 2 O 3 briquettes with molar ratio of 1:1 were roasted at 1300 °C for 120 min. Moreover, the Hv hardness of the Mn x Fe 3 − x O 4 increased from 154.8 kg/mm 2 to 339.4 kg/mm 2 with elevating the temperature from 1000 °C to 1300 °C.

An investigation on the behavior of SnO 2 in the tin-bearing iron concentrates during the reduction sintering process (RSP) has been conducted by the authors' group. It was found that only about 30 wt.% of tin was removed under the optimal conditions. However, the underlying reason why it was so difficult to remove tin by the RSP was not understood. In this study, the phase evolution of tin oxide, iron oxide and calcium oxide under simulative sintering conditions was investigated by varying the gas compositions, sintering temperature and basicity (CaO/SiO 2 ). The results indicated that the Ca-Sn oxides were prior formed at 900 °C–1000 °C in the reduction atmosphere with CO / (CO + CO 2 ) = 10–40 vol.%, while the phases of Fe-Sn spinel (Fe 3 − x Sn x O 4 ) and Ca-Fe-Sn oxides were easily generated as the temperature increased to 1100 °C–1200 °C. The phases of Fe 2 − 4x/3 Sn x O 3 and Ca-Fe-Sn-Si oxides began to generate at 1100 °C in an oxidative atmosphere with O 2 /(O 2 + CO 2 ) = 25–100 vol.%. In addition, increasing alkalinity accelerated the formation of Ca-Sn oxides under reduction atmosphere, which brought about an adverse effect on the tin removal in the sintering process.

High-purity Na 2 SnO 3 product has been successfully prepared from SnO 2 and Na 2 CO 3 via a novel process developed by the authors' group. This study proposed a function mechanism of CO-CO 2 atmosphere on the formation of Na 2 SnO 3 in the roasting process using SEM-EDS, micro-region XRD and FTIR analyses. The results showed that under 5 vol%–20 vol% CO/(CO + CO 2 ) atmosphere, the main reactions in the SnO 2 -Na 2 CO 3 system included Na 2 CO 3 = Na 2 O + CO 2 and Na 2 O + SnO 2 = Na 2 SnO 3 . The reactions were affected by the CO 2 partial pressure as well as by the CO content in the CO-CO 2 atmosphere. The formation degree of Na 2 SnO 3 increased with the decrease of CO 2 partial pressure. When the CO content varied in the range of 5 vol%–20 vol%, the formation degree of Na 2 SnO 3 was promoted. There was a lot of bridging oxygen (SnO 2 ⋯ O – ) on the SnO 2 surface, while in the roasting process, the CO gas molecules were firstly adsorbed on the SnO 2 surface and then combined with the bridging oxygen so that some oxygen vacancies were formed, and the reaction was expressed as SnO 2 ⋯ O – + CO = SnO 2 + CO 2 + e. These vacancies were replenished by the active oxygen anions coming from the Na 2 O. This process accelerated the formation of Na 2 SnO 3 .

A selective reduction volatilization (SRV) process developed by the authors could remove excessive tin from the complex tin-bearing iron concentrates. But it was especially noteworthy that the volatilization ratio of Sn and the residual tin content were obviously influenced by the variation of roasting parameters during the SRV process. Hence, the existence form of residual tin in the roasted products and the possible reactions between SnO2 and Fe3O4 need to be further determined. In the present study, the effect of roasting parameters on the reduction of SnO2 in the tin-bearing iron concentrates was first investigated. Then, samples with diverse cassiterite/magnetite (C/M) mass ratios were used to determine the effect of magnetite on phase transformation of SnO2 by using XRD, SEM–EDS, etc. The results indicated that magnetite markedly affected the tin volatilization and phase transformation of SnO2 under CO–CO2 atmospheres. Fe–Sn spinel was easily formed during the reduction process, and higher roasting temperatures promoted the formation of Fe–Sn spinel, which had an adverse effect on the Sn volatilization ratio.

MHA (modified humic acid based) binder has been found as an effective substitution for bentonite in the pelletization of specularite concentrates. This study aimed to clarify the roasting behavior of specularite pellets with MHA binder under different oxygen atmospheres. The thermal behavior of the MHA binder was initially determined by TG-DSC, TG-FTIR and MRU gas analysis. The reducing gaseous products, such as CO and H2, affected the oxidation and consolidation of the specularite pellets with MHA binder due to the pyrolysis of organic matter in the MHA binder heated in air. The primary phase compositions of the specularite pellets with MHA binder roasted in N2 and 10% oxygen atmospheres were composed of hematite and magnetite, whereas there was only hematite in the pellet roasted in a 20% oxygen atmosphere. The images by optical microscopy and SEM further indicated that the oxygen content of the roasting atmosphere markedly influenced the microstructure feature of the specularite pellets with MHA binder.