Bing Peng

Modeling and optimization of lime-based stabilization in high alkaline arsenic-bearing sludges with a central composite design

ABSTRACT This study focuses on the modeling and optimization of lime-based stabilization in high alkaline arsenic-bearing sludges (HAABS) and describes the relationship between the arsenic leachate concentration (ALC) and stabilization parameters to develop a prediction model for obtaining the optimal process parameters and conditions. A central composite design (CCD) along with response surface methodology (RSM) was conducted to model and investigate the stabilization process with three independent variables: the Ca/As mole ratio, reaction time and liquid/solid ratio, along with their interactions. The obvious characteristic changes of the HAABS before and after stabilization were verified by X-ray diffraction (XRD), scanning electron microscopy (SEM), particle size distribution (PSD) and the community bureau of reference (BCR) sequential extraction procedure. A prediction model Y(ALC) with a statistically significant P-value <0.01 and high correlation coefficient R2 = 93.22% was obtained. The optimal parameters were successfully predicted by the model for the minimum ALC of 0.312 mg/L, which was validated with the experimental result (0.306 mg/L). The XRD, SEM and PSD results indicated that crystal calcium arsenate Ca5(AsO4)3OH and Ca4(OH)2(AsO4)2·4H2O formation played an important role in minimizing the ALC. The BCR sequential extraction results demonstrated that the treated HAABS were stable in a weak acidic environment for a short time but posed a potential environmental risk after a long time. The results clearly confirm that the proposed three-factor CCD is an effective approach for modeling the stabilization of HAABS. However, further solidification technology is suggested for use after lime-based stabilization treatment of arsenic-bearing sludges.

Selective reduction process of zinc ferrite and its application in treatment of zinc leaching residues

Abstract The traditional zinc hydro-metallurgy generates a large amount of zinc ferrite residue rich in valuable metals. The separation of iron is crucial for resource recycling of valuable metals in zinc ferrite residue. A novel selective reduction roasting–leaching process was proposed to separate zinc and iron from zinc leaching residue which contains zinc ferrite. The thermodynamic analysis was employed to determine the predominant range of Fe 3 O 4 and ZnO during reduction roasting process of zinc ferrite. Based on the result of thermodynamic calculation, we found that V (CO)/ V (CO+CO 2 ) ratio is a key factor determining the phase composition in the reduction roasting product of zinc ferrite. In the range of V (CO)/ V (CO+CO 2 ) ratio between 2.68% and 36.18%, zinc ferrite is preferentially decomposed into Fe 3 O 4 and ZnO. Based on thermogravimetric (TG) analysis, the optimal conditions for reduction roasting of zinc ferrite are determined as follows: temperature 700–750 °C, volume fraction of CO 6% and V (CO)/ V (CO+CO 2 ) ratio 30%. Based on the above results, zinc leaching residue rich in zinc ferrite was roasted and the roasted product was leached by acid solution. It is found that zinc extraction rate in zinc leaching residue reaches up to 70% and iron extraction rate is only 18.4%. The result indicates that zinc and iron can be effectively separated from zinc leaching residue.

Structural and Genetic Diversity of Hexavalent Chromium-Resistant Bacteria in Contaminated Soil

ABSTRACT Culture-independent and -dependent methods were used to evaluate the genetic diversity of hexavalent chromium-resistant bacteria in contaminated soils. Denaturing gradient gel electrophoresis (DGGE) showed that hexavalent chromium-resistant bacteria in contaminated soils had significant structural diversity that was not related to the concentration of hexavalent chromium, while the diversity of soil bacterial communities was correlated with soil pH. Twenty-two chromium-resistant bacterial strains were isolated from contaminated soil, including members of Brevundimonas, Micrococcus, Exiguobacterium, Alcaligenes and Pannonibacter. Repetitive sequence-based polymerase chain reaction showed that the BOX- primer produced the more complex banding patterns than the ERIC- and REP- primers, indicating a high degree of genotypic diversity. These bacterial strains varied in ability to reduce hexavalent chromium. Pannonibacter reduced 300 mg·L−1 of hexavalent chromium in solution within 24 h. X-ray photoelectron spectra (XPS) analysis revealed that Cr(OH)3, Cr2O3, and CrCl3 composed approximately 83.51% of the total chromium in the reduction product.

Cascade sulfidation and separation of copper and arsenic from acidic wastewater via gas-liquid reaction

Copper and arsenic in acidic wastewater were separated by cascade sulfidation followed by replacement of arsenic in the precipitates by copper in the solution which was realized by recycling precipitates obtained in the first stage into the initial solution. The effects of reaction time, temperature and H2S dosage on copper and arsenic removal efficiencies as well as the effects of solid-to-liquid ratio, time and temperature on the replacement of arsenic by copper were investigated. With 20 mmol/L H2S at 50 °C within 0.5 min, more than 80% copper and nearly 20% arsenic were precipitated. The separation efficiencies of copper and arsenic were higher than 99% by the replacement reaction between arsenic and copper ions when solid-to-liquid ratio was more than 10% at 20 °C within 10 min. CuS was the main phases in precipitate in which copper content was 63.38% in mass fraction.

Recovery of valuable metals from zinc leaching residue by sulfate roasting and water leaching

Abstract Zinc leaching residue (ZLR), produced from traditional zinc hydrometallurgy process, is not only a hazardous waste but also a potential valuable solid. The combination of sulfate roasting and water leaching was employed to recover the valuable metals from ZLR. The ZLR was initially roasted with ferric sulfate at 640 °C for 1 h with ferric sulfate/zinc ferrite mole ratio of 1.2. In this process, the valuable metals were efficiently transformed into water soluble sulfate, while iron remains as ferric oxide. Thereafter, water leaching was conducted to extract the valuable metals sulfate for recovery. The recovery rates of zinc, manganese, copper, cadmium and iron were 92.4%, 93.3%, 99.3%, 91.4% and 1.1%, respectively. A leaching toxicity test for ZLR was performed after water leaching. The results indicated that the final residue was effectively detoxified and all of the heavy metal leaching concentrations were under the allowable limit.

Physicochemical properties of arsenic-bearing lime–ferrate sludge and its leaching behaviors

Abstract Physicochemical properties and leaching behaviors of two typical arsenic-bearing lime–ferrate sludges (ABLFS), waste acid residue (WAR) and calcium arsenate residue (CAR), are comprehensively described. The chemical composition, morphological features, phase composition and arsenic occurrence state of WAR and CAR are analyzed by ICP–AES, SEM–EDS, XRD, XPS and chemical phase analysis. The toxicity leaching test and three-stage BCR sequential extraction procedure are utilized to investigate arsenic leaching behaviors. The results show that the contents of arsenic in WAR and CAR are 2.5% and 21.2% and mainly present in the phases of arsenate and arsenic oxides dispersed uniformly or agglomerated in amorphous particles. The leaching concentrations of arsenic excess 119 and 1063 times of TCLP standard regulatory level with leaching rates of 47.66% and 50.15% for WAR and CAR, respectively. About 90% of extracted arsenic is in the form of acid soluble and reducible, which is the reason of high arsenic leaching toxicity and environmental activity of ABLFS. This research provides comprehensive information on harmless disposal of ABLFS from industrial wastewater treatment of lime–ferrate process.

Leaching kinetics modelling of reductively roasted zinc calcine

ABSTRACT The kinetics of the acid leaching of reductively roasted zinc calcine was studied on sample particles with a fine-grained sediment structure. This structure has a porous surface during acid leaching, and the leaching behaviours of roasted zinc calcines are complicated. The leaching of iron is controlled by reactions at the solid–liquid interface with an active energy of 51.40 kJ mol−1, the kinetics model is described by The extraction of zinc could be expressed as with an active energy of 10.01 kJ mol−1 and this process is controlled by diffusion through the porous structure. Temperature is a key factor for the iron extraction, while acidity particle size may play an important role in the leaching of zinc. It is concluded that the selective leaching of zinc is favoured at a high acidity above 90 g L−1 and a low temperature.

Reductive roasting and ammonia leaching of high iron-bearing zinc calcines

ABSTRACT The separation of iron and zinc is a considerable challenge in the zinc smelting processes. A reduction roasting-ammonia leaching process is proposed in this paper. In roasting experiments, the apparent active energy of the reductive roasting of zinc ferrite is 31-35kJmol−1, and the diffusion of the reactant is the rate-determining step. Zinc ferrite was well decomposed into ZnO and Fe3O4 in reductive roasting. However, byproducts including FeO and Fe0.85−xZn xO were also detected. To achieve a high zinc recovery and to avoid the leaching of FeO, ZnO was selectively extracted via ammonia leaching, while magnetite and ferrous oxide were left in residues and recovered through magnetic separation (MS). The optimal operations of the proposed process were 750°C for 45min under a CO concentration of 8% and a CO intensity of 20%, and leached at 30°C for 1 h with a solid/liquid ratio of 1:7 and a stir speed of 200 revmin−1. A total of 88.56% of the zinc was extracted, and just 1.2% of the iron was extracted. The recovery and grade of iron reached 86.46 and 46.5%, respectively, after MS. This reduction roasting-ammonia leaching process is a favourable option for efficient zinc recovery from high iron-bearing zinc ores.

Iron extraction from lead slag by bath smelting

Abstract A new bath smelting process was proposed to recover iron for solid wastes reduction. 99.79% of iron metallization, 99.61% of iron recovery, pig iron with 93.58% Fe, 0.021% S, 0.11% P, 1.38% C, 0.22% Si, 0.01% Pb and 0.031% Zn were achieved after the wastes were smelted at 1575 °C for 20 min under C/Fe molar ratio of 1.6 and basicity of 1.2. The produced pig iron could be used in steel-making. This study provides a way for recycling iron from smelting slag and hydrometallurgical residue.