Distribution, occurrence, and leachability of typical heavy metals in coal gasification slag.

Coal gasification slag (CGS) contains variable amounts of heavy metals, which can negatively impact the environment. The mineral composition, element distribution, occurrence, and leaching characteristics of heavy metals in coal gasification coarse slag (CGCS) and coal gasification fine slag (CGFS) are studied to explain the leaching behavior of heavy metals in CGS. The movable components of heavy metals in CGFS (0.06 %-63.03 %) are significantly higher than those in CGCS (0 %-18.72 %). Leaching Environmental Assessment Framework 1313 data shows that heavy metals Zn, Cr, Cd, As, Pb, Ni, and Cu exhibit high leaching rates at low pH conditions, with Zn leaching concentrations as high as 2.11 mg/L at pH 2. Zn, Cr, and As exhibit obvious amphoteric leaching characteristics, and the leaching concentration of As at high pH (1.34 mg/L) even exceeds that at low pH (1.31 mg/L). Except for Cu, all heavy metals in CGS exceed the class III groundwater standard in some cases. Therefore, evaluation is needed before resource utilization of CGS due to potential leaching of some heavy metals.

Removal of nitrides and fluorides from secondary aluminum dross by catalytic hydrolysis and its mechanism.

Secondary aluminum dross (SAD) refers to hazardous waste from secondary aluminum refinement. It contains a large amount of aluminum nitride and fluorides that cause serious environmental pollution for direct discharge and hinder the resource utilization of SAD. However, it is difficult to remove nitride and fluoride simultaneously for their complicated phases. In this paper, the catalytic hydrolysis of SAD using NaOH as a catalyst to remove nitrides and fluorides synchronously was investigated systemically through single factor and response surface experiments. In addition, the chemical speciation and transformation of nitrides and fluorides were analyzed systematically. The catalytic hydrolysis removal mechanism was summarized. The optimal conditions for catalytic hydrolysis were established as follows: reaction temperature 96.60 °C; reaction time 2.85 h; liquid-solid ratio 9.28 mL/g and catalyst addition 12.62 wt %; and removal efficiency of nitrides and fluorides reached 99.03% and 81.93%, respectively. The mechanism of nitrides removal was that aluminum nitride was hydrolyzed to Al(OH)3 and NH3. NaOH reacting with Al(OH)3 covering on the surface of AlN and the rapid escape of NH3 promoted the hydrolysis of AlN under the catalysis of NaOH. The mechanism of fluorides removal was that the encapsulated fluoride particles were opened by catalytic hydrolysis to be dissolved in the solution. In this research, nitrides and fluorides were removed efficiently and synchronously. The hydrolysis residues can be used to prepare polyaluminum chloride (PAC) and ceramic materials. The hydrolysate can be prepared NH3·H2O by evaporative in alkaline solution. Then the solution without NH4 + was prepared Al(OH)3 by precipitation of adjusting pH value using HCl. And the remained liquid after removing NaAlO2 was used to prepare refining agent by evaporative crystallization. The work in this paper was beneficial for the utilization of SAD.

Recovery of tungsten and titanium from spent SCR catalyst by sulfuric acid leaching process.

The widespread use of selective catalytic reduction (SCR) catalysts has resulted in a large accumulation of spent SCR catalysts. These spent catalysts present a significant risk of environmental hazards and potential for resource recovery. This paper presents a feasible process, which works using atmospheric pressure leaching, of tungsten and titanium recovery from spent SCR catalysts. In this new method, titanium and tungsten are simultaneously leached with sulfuric acid as the leaching agent. After hydrolysis and calcination, titanium-tungsten powder with low impurity and reconstructed pore properties was obtained. The optimal conditions for the leaching of Ti and W were as follows: temperature, 150 °C; reaction time, 60 min; H2SO4 concentration, 80 %; mass ratio of H2SO4/TiO2, 3:1; and diluted H2SO4 concentration, 20 % after reaction. With these optimum conditions, the leaching efficiency of Ti and W were found to be 95.92 % and 93.83 %, respectively. The ion speciation and reaction mechanism of W were studied by Raman spectroscopy, FTIR, and UV-vis. The formation of heteropolytungstate with a Keggin structure is essential for the synergistic leaching of Ti and W, as the heteropolytungstate can be stably dissolved in the acid solution. During the hydrolysis process, heteropolytungstate gradually decomposed into Ti4+ and WO42- due to the formation of insoluble Ti(OH)4 from Ti4+ in the solution. This study demonstrated an effective method for synergistic recovery of titanium and tungsten from the spent SCR catalyst.