An alternative approach to recovering valuable metals from zinc phosphating sludge

Reduction-Magnetic Separation of Pickling Sludge by Biomass Pyrolysis Reducing Gas

The neutralization process of carbon steel pickling wastewater produces a large amount of steel hydrochloric acid pickling sludge (SHPS), and improper treatment of this sludge poses a serious threat to the environment. Considering that SHPS contains a large amount of iron oxide and given the huge demand for iron concentrate in China's ironmaking industry, refining iron oxide in SHPS into iron concentrate will have great environmental and economic benefits. This paper proposes a new method that uses biomass (corncob) to replace conventional coal-based reductants for the recovery of iron components in SHPS to simultaneously utilize two kinds of solid waste resources. Factors that affect the iron recovery rate and iron grade of SHPS, such as the reaction temperature, corncob dosage, residence time, and magnetic field strength, were studied using a fixed bed and a magnetic separator. These studies were combined with thermodynamic analysis, thermogravimetric analysis, X-ray diffraction, inductively coupled plasma-mass spectrometry, gas chromatography, etc. The results showed that when the reaction temperature was 680 °C, the corncob dosage was 5%, the residence time was 20 min, and the magnetic field strength was 200 mT, the recovery rate of iron reached 91.83%, and the iron grade of the recovered products was 67.72%, meeting the level I requirements in GB/T 32545-2016. Based on this result, a process involving SHPS reduction roasting with corncob pyrolysis reducing gas-magnetic separation was established to recover iron from SHPS. This process not only effectively utilizes the iron oxide in SHPS by converting it into iron concentrate powder for the ironmaking industry but also proves that the pyrolysis gas of corncob has good reduction ability.

Synthesis of porous iron hydroxy phosphate from phosphate residue and its application as a Fenton-like catalyst for dye degradation

Phosphate residue is a kind of hazardous solid waste and if not properly disposed of, could cause serious environmental contaminations. The abundant iron salt available in phosphate residue can be used to prepare photo-Fenton catalytic reagent for wastewater treatment. In this study, the phosphate residue was effectively purified by a hydrothermal recrystallization method, reaching an iron phosphate purity of 94.2%. The particles of iron phosphate were further processed with ball milling with their average size reduced from 19.4 to 1.6 μm. By hydrothermal crystallization of iron phosphate and thermal decomposition of oxalate precursor, porous iron hydroxy phosphate was prepared. The modified porous iron hydroxy phosphate (m-PIHP) of higher surface area with iron oxalate on its surface can degrade 98.87% of Rhodamine B in 15 min. Cyclic experiment showed that the catalyst still had a good catalytic activity after six cycles (>40%). The X-ray photoelectron spectroscopy results showed that the iron oxalate complex on the catalyst surface decomposed to produce ferrous ions and accelerated the rate of •OH production. The current work demonstrated that the m-PIHP synthesized from phosphate residue and modified with iron oxalate can be used as an effective dye wastewater treatment agent.

Synthesis of nitrogen-doped carbon nanotubes-FePO4 composite from phosphate residue and its application as effective Fenton-like catalyst for dye degradation

Phosphate residue is regarded as a hazardous waste, which could potentially create significant environmental and health problems if it is not properly treated and disposed of. In this study, nitrogen-doped carbon nanotubes-FePO₄ (NCNTs-FePO₄) composite was successfully synthesized from phosphate residue, and its application as an effective catalyst was explored. Firstly, an effective method was developed to recover FePO₄ from phosphate residue, achieving an impressive FePO₄ mass recovery rate of 98.14%. Then, the NCNTs-FePO₄ catalyst was synthesized from the recovered FePO₄ by two main reactions, including surface modification and chemical vapor deposition. Finally, the synthesized NCNTs-FePO₄ was applied to photo-degrade 15 mg/L Rhodamine B (RhB) in a Fenton-like system. The results showed that 98.9% of RhB could be degraded in 60 min, closely following the pseudo-first-order kinetics model. It was found that even after six consecutive cycles, NCNTs-FePO₄ still retained a high catalytic capacity (>50%). Moreover, •OH radicals participating in the RhB degradation process were evidenced using quenching experiments and electron paramagnetic resonance analysis, and a rational mechanism was proposed. It was demonstrated that the materials synthesized from hazardous phosphate residue can be used as an effective catalyst for dye removal.

Influence of processing time on the phase, microstructure and electrochemical properties of hopeite coating on stainless steel by chemical conversion method

Distinct nanoscale structures of hopeite coating on stainless steel are found which may have potential significance for biomedical applications.

Role of sodium ions in the vitrification process: glass matrix modification, slag structure depolymerization, and influence of metal immobilization

This study investigates the role of Na ions, a common flux, in the vitrification process. Artificial glass systems composed of Al2O3, CaO, and SiO2 with various Na concentrations were melted at 1450 degrees C. The specimens were cooled by air cooling and water quenching and the metal mobility was evaluated using a sequential extraction procedure. The X-ray diffraction analysis and scanning electron microscopy observations showed that Na ions governed the air-cooled slag's structure. Na ions initially depolymerized CaSiO3-linked chains into CaSiO3 chains, and further cut them into shorter and nonuniform ones, making the slag structure amorphous. With even more Na ions, CaSiO3 chains were divided into single SiO4 tetrahedrons and formed Na-related crystals (Na2Ca3Si2O8 and NaAlSiO4). The phase distributions of Al, Cr, Cu Mn, and Ni showed that Na has a positive effect on the immobilization of heavy metals at suitable concentrations, but a negative effect when in excess amounts. Implications: Vitrification has been widely used to treat hazardous materials. The Na-bearing additives were often used as a flux to improve the melting process. This study described the role of Na played in the vitrification process. The Na ions acted as glass modifier and depolymerize the chain structure of slag. With adequate addition amount of Na ions, the immobilization of heavy metals was improved. The results provided much information about the crystalline phase variation, metal mobility, and surface characteristics while Na serves as a flux.

Recovery of valuable metals from electroplating sludge with reducing additives via vitrification

In this study, vitrification was applied to treat Ni-Cu electroplating sludge. The sludge was mixed with additives (limestone:cullet = 4:6) and then heated to 1450 °C. The cooled product could be separated into slag and ingot. An atomic absorption spectrometer was used to determine the metal levels of specimens and toxicity characteristic leaching procedure (TCLP) tests, whereas the crystalline and surface characteristics were examined using quantitative X-ray diffraction (XRD) analysis and scanning electron microscopy, respectively. With a glassy structure, the slag was mainly composed of Ca, Si, and Mg. The TCLP results of slags met the Taiwan regulated standards, suggesting that slag can be used for recycling purposes. With the aid of additives, the crystalline phase of slag was transformed form CaMgSiO4 into CsSiO3. The ingots were mainly composed of Ni (563,000-693,800 mg/kg), Cu (79,900-87,400 mg/kg), and Fe (35,000-43,600 mg/kg) (target metals) due the gravity separation during vitrification. At appropriate additives/sludge ratios (>0.2), >95% of target metals gathered in the ingot as a recoverable form (Ni-Fe alloy). The high Ni level of slag suggests that the ingot can be used as the raw materials for smelters or the additives for steel making. Therefore, the vitrification approach of this study is a promising technology to recover valuable metals from Ni-Cu electroplating sludge.

Evaluation of effect of reducing additives during vitrification via simulation and experiment

This study investigates how reducing additives governed the vitrification of prepared specimens. In the experiments, pure CaO/CaCO3 and SiO2 served as the major components of glassy matrix (basicity = mass ratio of CaO/SiO2 = 2/3) with doping of hazardous metals (Cr Cu, and Ni). The substitution ratio of CaCO3 for CaO was used as an operating parameter. The specimens were vitrified at 1400 degrees C and a sequential extraction protocol was used to determine the phase distribution of Cr, Cu, and Ni. The volume fractions of crystalline and amorphous phases were measured using semiquantitative x-ray diffraction (XRD) analysis. A commercial software package (HSC Chemistry 6.0) was used to simulate the experiment to acquire additional information. The simulation results showed the addition of CaCO3 generated CO and CO2 at high temperature. This reducing atmosphere might enhance Cu and Ni to be easily separated from slags and elevated the levels of Cu and Ni in ingots. At higher CaCO3 mol(%), the polymerization of silicate (from sorosilicate to inosilicate) in slag rose and the CaSiO3 amount increased. In addition, the immobilization of metals and the acid resistance of slags were improved. The results indicate that CaCO3 addition is favorable for increasing the metal level in ingots and the metal encapsulation in slag in vitrification.