Determination of the Optimum Conditions for the Leaching of Lead from Zinc Plant Residues in NaCl–H₂SO₄–Ca(OH)₂ Media by the Taguchi Method

This research is part of a continuing effort to reduce environmental conflicts and occupational hazards of lead-bearing zinc plant residues (ZPRs), and to break through this problem and recover lead of the wastes. The residue with an assay of 14.4% Pb was used in chloride leaching for lead recovery, and sulfate was controlled in the leaching stage by the addition of Ca­(OH)₂. In this paper, the effects of influential factors on extraction efficiency of Pb from ZPRs were investigated. Taguchi’s method based on orthogonal array design (OAD) has been used to arrange the experimental runs in order to maximize lead extraction from a ZPR. Orthogonal array (OA) L₈(2⁷) consisting of seven parameters, each with two levels, was employed to evaluate the effects of NaCl concentration (<i>C</i> = 300 and 400 g/L), stirring speed (<i>R</i> = 500 and 700 rpm), reaction temperature (<i>T</i> = 55 and 65 ◦C), reaction time (<i>t</i> = 1 and 8 h), liquid-to-solid ratio (L/S = 6 and 20), acidic pH (pH_a= 2 and 3.5), and neutral pH (pH_b= 4 and 5.5) on lead extraction percent. Statistical analysis, ANOVA, was also employed to determine the relationship between experimental conditions and yield levels. The results showed that the pulp density, and NaCl concentration were significant parameters, and increasing pulp density reduced leaching efficiency of lead. However, increasing NaCl concentration promoted the extraction of lead. The obtained optimum conditions from this study were <i>C</i>₂, 400 g/L; <i>R</i>₂, 700 rpm; <i>T</i>₁, 55 °C; <i>t</i>₁, 1 h; (L/S)₂, 20; (pH_a)₂, 3.5; and (pH_b)₁, 4. But only two significant factors (<i>C</i>₂, 400 g/L; and (L/S)₂, 20) were used to estimate the performance at the optimum conditions. The calculated leaching percent (85.91%) was in reasonable agreement with the experimental results in optimum conditions

Horizontally rotating disc recirculated photoreactor with TiO₂-P25 nanoparticles immobilized onto a HDPE plate for photocatalytic removal of <i>p</i>-nitrophenol

<p>In this study, a horizontally rotating disc recirculated (HRDR) photoreactor equipped with two UV lamps (6 W) was designed and fabricated for photocatalytic removal of <i>p</i>-nitrophenol (PNP). Photocatalyst (TiO₂) nanoparticles were immobilized onto a high-density polyethylene (HDPE) disc, and PNP containing solution was allowed to flow (flow rate of 310 mL min^–1) in radial direction along the surface of the rotating disc illuminated with UV light. The efficiency of direct photolysis and photocatalysis and the effect of rotating speed on the removal of PNP were studied in the HRDR photoreactor. It was found that TiO₂-P25 nanoparticles are needed for the effective removal of PNP and there was an optimum rotating speed (450 rpm) for the efficient performance of the HRDR photoreactor. Then effects of operational variables on the removal efficiency were optimized using response surface methodology. The results showed that the predicted values of removal efficiency are consistent with experimental results with an <i>R</i>₂ of 0.9656. Optimization results showed that maximum removal percent (82.6%) was achieved in the HRDR photoreactor at the optimum operational conditions. Finally, the reusability of the HRDR photoreactor was evaluated and the results showed high reusability and stability without any significant decrease in the photocatalytic removal efficiency.</p

Study of the Effect of Additives on the Photocatalytic Degradation of a Triphenylmethane Dye in the Presence of Immobilized TiO₂/NiO Nanoparticles: Artificial Neural Network Modeling

In the present work, TiO₂/NiO coupled nanoparticles were prepared from a powder mixture of the corresponding component solid oxides by using an impregnation technique. Then, the prepared TiO₂/NiO nanoparticles were immobilized on glass plate and used as a fixed-bed photocatalytic system for photodegradation of Acid Fuchsin (AF), as a triphenylmethane dye pollutant. The effects of nature and concentration of various additives included inorganic oxidants (such as HSO₅^–, IO₄^–, ClO₃^–, S₂O₈^2–, H₂O₂, and BrO₃^–), inorganic anions (such as CH₃COO^–, CO₃^2–, NO₃^–, Cl^–, H₂PO₄^–, and SO₄^2–), and transition-metal ions (such as Co²⁺, Zn²⁺, Fe²⁺, Cu²⁺, Ni²⁺, and Mn²⁺) on photocatalytic degradation of AF, were investigated. It was found that the nature and concentration of studied additives significantly affected the photocatalytic degradation of dye pollutant in fixed-bed systems. The transition-metal ions and inorganic oxidants have a positive effect on the photocatalytic degradation rate of AF dye, whereas inorganic anions have a negative effect. An artificial neural network (ANN) model was designed for modeling of the photocatalytic degradation rate of AF dye. The results showed that the predicted data from designed ANN model were in good agreement with the experimental data. Designed ANN provides a reliable method for modeling the photocatalytic activity of immobilized TiO₂/NiO nanoparticles in the presence of various additives