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Calcium stannate (Ca2SnO4) is a common dielectric ceramic that is generally prepared by a high-temperature solid-state method in an air atmosphere with a roasting temperature of more than 1300 °C. In this study, Ca2SnO4 was much more easily synthesized in a CO-CO2 atmosphere at a relatively low temperature of less than 1000 °C for 30 min. We comparatively investigated the formation behavior and microwave dielectric properties of Ca2SnO4 synthesized from tin dioxide (SnO2) and calcium carbonate (CaCO3) by a solid-state method in air and CO-CO2 atmospheres using X-ray diffraction, scanning electron microscopy, inductively coupled plasma atomic emission spectroscopy, and vector network analysis. The formation behavior of calcium stannate indicated that the reactions between SnO2 and CaCO3 in CO-CO2 and air atmospheres were both controlled by three-dimensional diffusion. In a CO-CO2 atmosphere, the reaction had a higher reaction rate constant (k) and lower apparent activation energy (E). In addition, Ca2SnO4 was synthesized by a low-temperature solid-state method in a CO-CO2 atmosphere. The results indicated that Ca2SnO4 ceramics had a higher dielectric constant and lower dielectric tangent loss than those synthesized by the high-temperature solid-state method in air. However, the synthesis temperature and time were reduced by more than 300 °C and 8 h, respectively.

Sodium stannate (Na2SnO3) has been successfully prepared by a novel process of roasting cassiterite concentrates and sodium carbonate (Na2CO3) under CO&ndash, CO2 atmosphere, namely soda roasting-leaching process. However, more than 22 wt. % tin of the cassiterite was not converted into Na2SnO3 and entered the leach residues. Quartz (SiO2) is the predominant gangue in the cassiterite, and phase evolution of SnO2&ndash, SiO2&ndash, Na2CO3 system roasted under CO&ndash, CO2 atmosphere was still uncertain. In this study, the effect of SiO2 in cassiterite concentrates on preparation of Na2SnO3 was clarified. The results indicated that Na8SnSi6O18 was inevitably formed when cassiterite and Na2CO3 were roasted above 775 &deg, C under CO&ndash, CO2 atmosphere via the reaction of SnO2 + 6SiO2 + 4Na2CO3 = Na8SnSi6O18 + 4CO2, and formation of Na8SnSi6O18 would be increased with increasing roasting temperature and Si/Sn mole fraction. In addition, it was found that Na8SnSi6O18 was insoluble in the leachate at pH value range of 1&ndash, 14, which, therefore, was enriched in the leach residues. The silicon content of the cassiterite concentrates should be controlled as lower as possible to obtain a higher conversion ratio of Na2SnO3.

Tin-, iron-bearing tailing is a typically hazardous solid waste in China, which contains plenty of valuable tin, iron elements and is not utilized effectively. In this study, a multistage utilization process was put forward to get the utmost out of the valuable elements (tin and iron) from the tailings, and a gradient-recovery method with three procedures was demonstrated: (1) An activated roasting followed by magnetic separation process was conducted under CO-CO2 atmosphere, tin and iron were efficiently separated during magnetic separation process, and 90.8 wt% iron was enriched in magnetic materials while tin entered into non-magnetic materials; (2) The tin-enriched non-magnetic materials were briquetted with CaCl2 and anthracite and roasted, then tin-rich dusts were collected during the chloridizing roasting process; (3) The roasted briquettes were infiltrated in melting NaNO3 to prepare NaNO3/C-PCMs by a infiltration method. Three kinds of products were obtained from the tailings by the novel process: magnetic concentrates containing 64.53 wt.% TFe, tin-rich dusts containg 52.4 wt.% TSn and NaNO3/C-PCMs for high temperature heat storage. Such a comprehensive and clean utilization method for tin-, iron-bearing tailings produced no secondary hazardous solid wastes, and had great potential for practical application.

Part of tin in the magnetite-type tin-bearing ores existed as isomorphous Sn4+ in the lattice of magnetite (Fe3-xSnxO4, X = 0–1.0), which was impossible to separate and recover by physical beneficiation methods In this study, a calcified roasting process under CO-CO2 atmosphere was applied to separate tin from the magnetite-type tin-bearing tailings, and the results indicated that the isomorphous Sn4+ in the magnetite was removed effectively. Then, Fe2.6Sn0.4O4 was synthesized to investigate the interface reaction mechanisms between CaO and Fe3-xSnxO4. The results indicated that the calcified roasting process can be divided into three stages: the ionic migration of Sn4+ on the interface between CaO and Fe3-xSnxO4 and formation of calcium stannates (Ca2SnO4/CaSnO3), diffusion of Sn4+ through the product layer (CaSnO4/Ca2SnO4), the oxidation of FeO to Fe3O4 by CO2. These findings can provide theoretical support for the utilization of magnetite-type tin-bearing tailings by the calcified roasting process.

High temperature solid state is reported the most common synthesis technique for Ca2SnO4 ceramics, nevertheless it requires high thermal treatment temperature and long synthesis time. Our previous studies indicated that Ca2SnO4 was much easier to form if SnO2 and CaCO3 were roasted under CO-CO2 atmosphere at relatively low temperatures. In this study, the reaction mechanism between SnO2 and CaCO3 under 15 vol% CO/(CO+CO2) atmosphere and air atmosphere were firstly investigated by using XRD, SEM, XPS, etc. In addition, the Eu-doped Ca2SnO4 was synthesized by low-temperature solid state reaction under CO-CO2 atmosphere. The results indicated that the crystal structure and photoluminescence properties of the products were similar to those of samples synthesized under air atmosphere, whereas the synthetic temperature was reduced by more than 300 °C.

Chemical vapor transport (CVT) process is a simple and efficient method for synthesizing nano-SnO products, and previous references reveal that the CVT process is characterized as high temperature and long synthesis time. Our previous studies indicated that SnO2 was easily reduced to solid-state SnO and then volatilized as gaseous SnO under certain CO-CO2 atmosphere, so that a novel CVT process under CO-CO2 atmosphere was proposed in the present work. The nano-SnOs synthesized by the CVT under 15 vol% CO/(CO + CO2) atmosphere were characterized using XRD, SEM, TEM, XPS, etc. The results showed that tetragonal structure nano-SnO diskette and sphere with uniform granularity of 50–100 nm were obtained with high purity. During the CVT process, CO-CO2 mixed gases could reduce SnO2 to SnO and act as a CVT medium to take away the newly formed gaseous SnO, resulting in the decrease of the partial pressure of SnO. In addition, the nano-SnOs could enhance the SCR deNOx activity of activated carbon (AC), and both the reaction temperature window of AC and the NO conversion were increased if the AC was coated by nano-SnOs. Keywords: Nano-SnO, Chemical vapor transport, Reduction volatilization, SCR activity