Extraction of W, V, and As from spent SCR catalyst by alkali pressure leaching and the pressure leaching mechanism.

Spent selective catalytic reduction (SCR) catalysts are environmentally hazardous and resource-enriching. In this work, V, W, and As in a spent SCR catalyst was extracted by alkali pressure leaching. Results showed that the V, W, and As were loaded on the anatase TiO2 crystal grains as amorphous oxides. The optimum pressure leaching conditions were NaOH concentration of 20 wt%, reaction temperature of 180 °C, reaction time of 120 min, L/S of 10 mL/g, and stirring speed of 300 rpm. The leaching efficiency of W, V, and As reached 98.83%, 100%, and 100%, respectively. The experiment revealed the preferential leaching of V and As rather than W, and the leaching mechanisms of V, W, and As were studied through experiment and density functional theory (DFT). The leaching kinetics of W conformed to a variant of the shrinking core model and the leaching process of W is controlled by both chemical reactions and diffusion processes. During the leaching process, Na2Ti2O4(OH)2 product powder layer was generated, which affects the mass transfer of W. The destruction of the TiO2 skeleton in the spent SCR catalyst is essential for adequate W extraction, especially for the extraction of W embedded in the TiO2 lattice. The DFT simulation result indicated that the V and As loaded onto the TiO2 support are easier to absorb hydroxide ions rather than W, and the leaching reaction energy of V and As was lower than W, As, and V has leaching priority over the leaching of W. Furthermore, an anatase TiO2 photocatalyst with the {001} crystal surface exposed was successfully prepared from the alkali pressure leaching residue. This work provides theoretical support for the metal leaching and utilization of spent SCR catalysts via alkali pressure leaching.

Novel Synergistic Process of Impurities Extraction and Phophogypsum Crystallization Control in Wet-Process Phosphoric Acid.

Phosphogypsum, as a byproduct of wet-process phosphoric acid reaction, has caused many environmental pollution problems. To improve the property and purity of phosphogypsum in the wet-process phosphoric acid process, a liquid-solid-liquid three-phase acid hydrolysis synergistic extraction reaction system was established by adding a certain amount of extractant in the actual production process. In order to study the extraction effect and residue of impurities in the reaction system, the phase, morphology, and impurity occurrences of phosphogypsum were systematically analyzed. The results showed that when the reaction time was 7 h, the reaction temperature was 80 °C, the reaction speed was 200 r/min, the volume ratio of the extractant to diluent (dilution ratio) was 1:4 and the volume ratio of the oil phase/aqueous phase (O/A ratio) was 1:1, P2O5 conversion was the highest in phosphate rock, and the residual P2O5 content in phosphogypsum was as low as 0.36%. The morphology of the phosphogypsum crystal was uniform and coarse long strip. The main forms of residual impurities were silicate, aluminum fluoride with crystal water, aluminate, phosphate, and fluoride. Meanwhile, the residual amount of main impurities in phosphogypsum was significantly reduced. Through this novel method, the property of phosphogypsum can be improved through the generation process and is greatly beneficial for its utilization and the recycling development of the wet-process phosphoric acid industry.

Eco-friendly recycling of silicon-rich lye: Synthesis of hierarchically structured calcium silicate hydrate and its application for phosphorus removal.

Silicon-rich lye (SRL), a byproduct generated from pre-treatment of coal-based solid waste (CSW), was considered as a preponderant silicon source to prepare hierarchically nanostructured calcium silicate hydrate (C-S-H). Through the novel mild-causticization synthesis strategy, C-S-H was prepared under optimal caustic process conditions at time of 3 h, temperature of 80 °C, Ca/Si of 1.25:1, and active CaO to obtain a conversion rate of Si up to 97.33 % during the high-value utilization of SRL. The synthesized C-S-H possesses abundant mesoporous structure and massive exchangeable active sites, whose formation is advanced through an appropriate elevation regulation of caustic temperature and time. The silicate chain depolymerization occurs to C-S-H prepared in the highly alkaline system at higher caustic temperature, longer caustic period, especially at existence of massive sodium ions, but it presents higher polymerization degree at more aluminum co-existing. The adsorption capacity up to 119.27 mg/g for C-S-H presents a valid removal performance toward phosphorus in the wastewater than massive present reports. The removal mechanism of phosphorus can be identified as the surface chemisorption and formation of calcium phosphate co-precipitation. This study can provide considerable and potential guidance to the coordinated disposal between industrial solid wastes and wastewater purification.

Study on the correlation between Fe/Ti forms and reaction activity in high-alumina coal fly ash.

High-alumina coal fly ash (HAFA) is an important aluminum and silicon resource. In the process of preparing aluminum-silicon materials from HAFA, the influence of impurity elements on its performance must be considered. In this work, the occurrence state of impurities in HAFA, micro morphology, and the bond energy of different impurity coordination were studied. Sulfuric acid leaching method and density functional theory were used to study the leaching behavior of impurities to verify the difficulty of removing different impurity elements. The results show that iron existed in the form of magnetic particles (34.78%), amorphous phase (49.24%), and crystalline phase (15.96%) in HAFA. Titanium mainly existed in amorphous phase (29.34%) and crystalline phase (69.4%). In sulfuric acid leaching, titanium was more difficult to leach, and the content of TiO2 decreased from 2.30% to 2.25%, whereas that of Fe2O3 decreased from 1.50% to 0.86%. The actual leaching behavior of impurity elements was consistent with the simulation results, with more energy required to remove Ti than Fe. These studies of impurity elements in HAFA will provide theoretical support for the preparation of aluminum-silicon materials.

Mechanisms of mechanochemical activation during comprehensive utilization of high-alumina coal fly ash.

High-alumina coal fly ash (HAFA) is an important aluminum and silicon resource. In terms of its comprehensive utilization, how to efficiently separate aluminum and silicon must be resolved. In this work, a mechanochemical activation method in the desilication process is adopted and investigated. Desilication ratio, efficient desilicated ratio, microscopic appearance, and coordination structure change in aluminum-silicon atoms are investigated during mechanochemical activation process. Results indicate that the Al-O-Si coordination structure in HAFA can be transformed from Q4 (3Al) and Q4 (2Al) to reactive Q2 (1Al) structure, and the radio of Q2 (1Al) with high activity increased from 6.93% to 14.47%, which can improve HAFA reaction activity obviously. During the desilication process, the aluminum-silicon mass ratio (A/S) can be elevated from 1.23 to 2.56 after the process optimization, which provides a high-quality raw material for further high-valued utilization of HAFA.

Effects of Impurities on CaSO4 Crystallization in the Ca(H2PO4)2-H2SO4-H3PO4-H2O System.

Wet-process phosphoric acid is a fundamental process in the fertilizer industry. The influence of impurities on crystallization kinetics of CaSO4 was investigated in the Ca(H2PO4)2-H2SO4-H3PO4-H2O system using a mixed suspension mixed product removal crystallizer. Effects of Si, Al, Fe, Mg, K, and Na on crystal morphology and structure were examined in the highly acidic system through scanning electron microscopy and high-resolution transmission electron microscopy. Results show that the increase of Mg, K, and Na content facilitates crystal growth. Si, Al, and Fe are beneficial to CaSO4 crystal growth at a certain concentration range. Impurities also affect the crystal morphology, and the addition of Si, Fe, and Na promotes the formation of needle-like crystals compared with other impurities. X-ray diffraction results show that the preferred crystal growth direction is (020), and the interplanar spacing of the crystals is affected by the element radius of the impurities.

Non-isothermal thermal decomposition reaction kinetics of dimethylhexane-1,6-dicarbamate (HDC).

The thermal decomposition behavior and reaction kinetics of dimethylhexane-1,6-dicarbamate (HDC) were investigated by TG-DTG, DSC and IR techniques. It is shown that the decomposition process can be divided into 3 main stages and the first stage is the thermal decomposition reaction for HDC to dimethylhexane-1,6-diisocyanate (HDI). The main gaseous products of the decomposition are CO(2) and CH(3)OH. The kinetic parameters of the reaction for HDC to HDI are Ea=119.51 kJ mol(-1), lg(A/s(-1))=14.82, respectively. The kinetic equation is d(α)/d(t) = 10(15.12)(1-α)(3/2)e(-1.4375×10(4)/T).