Metal-bearing Waste Research Articles

Energy Evaluation and Mathematical Modeling of Pellet Production from Metal-Bearing Waste with a Focus on Alternative Applications of Reducing Agents

The authors of this study focused on the energy and material assessment of processes for processing pellets from metal-bearing waste, specifically Fe concentrate. A mathematical model was created for process evaluation, with which thermotechnical calculations of parameters in the processing of metallized pellets were carried out. Thermodynamic calculations were performed to determine the enthalpy of the charge in individual devices (drying chamber, rotary kiln, cooler). For the reduction of Fe oxides, carbon from coke (with Fe oxide reductions of 50%, 61%, and 92%) and lignite (with Fe oxide reductions of 69% and 92%) were considered as part of the pellets. The degree of reduction of iron oxides was a determining parameter, and the consumption of the reducing agent corresponded to the direct reduction of Fe oxides by carbon with a coefficient of 1.5. Another determining parameter was the input and output temperature in individual devices. For a more precise description of the processes in individual devices, calculations were carried out zonally. The results of the calculations are analyses and recommendations for feasible alternatives for the reducing agent and associated processes.

Recovering Co3O4 from waste choline chloride/ethylene glycol electrolyte containing Co(II) ions along with solvent regeneration

Efficiently recovering critical metals from secondary resources is crucial to decreasing the environmental impact of mineral-resource mining and reducing production costs and process energy consumption. Herein, we propose a simple and comprehensive approach to recycle Co3O4 from waste choline chloride/ethylene glycol (named Ethaline) electrolyte containing Co(II) ions along with solvent regeneration. Through the rational coordination adjustment using water, [CoCl4]2− complex in the Ethaline-based electrolyte is transformed to [CoCl2(H2O)4]0 and [CoCl2(H2O)5]+ to facilitate precipitation separation. Adding oxalic acid (OA) as a precipitator can reach 97.1% cobalt recovery efficiency with a high value-added Co3O4 rod-like structure obtained by calcining the oxalate precipitate. In addition, the water-containing solvent can be regenerated via reduced pressure distillation with a high recovery efficiency of up to 98.0%. FT-IR and 1H NMR characterizations verify that the regenerated Ethaline exhibits good structural stability. Impressively, the proposed recovery path shows high recovery efficiencies for both cobalt and Ethaline in various practical Co(II) ion-containing electrolytes. This study establishes a novel technical pathway for the resource utilization of metal-bearing waste Ethaline electrolytes, which shows an excellent prospect in solvent recycling after metal processing.

Towards source reduction and green sustainability of metal-bearing waste streams: The electrochemical processes

With the increase in globalization and industrialization, the concomitant heavy metal pollution and scarcity of critical metals have been widely reported around the world. It is highly desirable to develop technologies that minimize pollution during the production and recovery of these metals from end-of-life waste streams. Due to the characteristic redox nature of metals and their oxides, sulfides, alloys, and hydroxides, electrochemical processes that utilize electrons as the oxidant/reductant rather than exogenous chemicals have been successfully employed for the source reduction and sustainable restoration of metal-bearing wastes. The efficient and selective electrochemical treatment of metals-bearing wastes typically involves adsorption, oxidative leaching, reductive deposition, integrated membrane separation, biological and photochemical reactions. In this review, we systematically summarize the recent progress toward the development of electrochemical processes for the environmental remediation of metal-bearing waste and unconventional natural resources. In addition, the current challenges and future perspectives, including the detailed mechanism and practical applications.

Closing the Loop: Key Role of Iron in Metal-Bearing Waste Recycling

AbstractThe role of iron in metal-bearing waste bioleaching was studied. Four various types of waste (printed circuit boards (PCBs), Ni-Cd batteries, alkaline batteries and Li-ion batteries) were treated by bioleaching using the acidophilic bacteria A. ferrooxidans and A. thiooxidans (separately or in mixture). Role of main leaching agents (Fe3+ions or sulphuric acid) was simulated in abiotic experiments. Results showed that oxidation abilities of Fe3+ions were crucial for recovery of Cu and Zn from PCBs, with the efficiencies of 88% and 100%, respectively. To recover 68% of Ni from PCBs, and 55% and 100% of Ni and Cd, respectively, from Ni-Cd batteries both oxidation action and hydrolysis of Fe3+were required. The importance of Fe2+ions as a reducing agent was showed in bioleaching of Co from Li-ion batteries and Mn from alkaline batteries. The efficiency of the processes has increased by 70% and 40% in Co and Mn bioleaching, respectively, in the presence of Fe2+ions. Based on the results we suggest the integrated biometallurgical model of metal-bearing waste recycling in the effort to develop zero-waste and less energy-dependent technologies.

Nano-indentation on nickel aluminate spinel and the influence of acid and alkaline attacks on the spinel surface

Nickel aluminate spinel phase was successfully sintered from the thermal reactions between nickel oxide and kaolinite or γ-alumina precursor, to simulate the stabilization mechanism of hazardous metal-bearing waste by ceramic matrix. The product phases were then tested using nano-indentation to obtain their nanohardness and Young's modulus, as a means to evaluate the properties of the product after incorporating the waste material. The results indicate the beneficial effect of forming aluminate spinel phase in the system due to its superior mechanical properties. A higher sintering temperature was found to enhance crystal growth in the spinel phase, together with its nanohardness and modulus. The minimum temperature for fabricating nickel aluminate spinel with a surface property comparable to ceramic materials was found to be 1200°C, although the initiation of the spinel phase can be achieved at a lower temperature. Nano-indentation experiments performed on the spinel-containing samples leached by strong acid and alkaline solutions reveal the superiority of nickel aluminate spinel in resisting acid and alkaline attack and also suggest a reliable mechanism for hosting hazardous nickel in the crystal structure.

Phase transformation and its role in stabilizing simulated lead-laden sludge in aluminum-rich ceramics

This study investigated the mechanisms of stabilizing lead-laden sludge by blending it into the production process of aluminum-rich ceramics, and quantitatively evaluated the prolonged leachability of the product phases. Sintering experiments were performed using powder mixtures of lead oxide and γ-alumina with different Pb/Al molar ratios within the temperature range of 600–1000 °C. By mixing lead oxide with γ-alumina at a Pb/Al molar ratio of 0.5, the formation of PbAl 2O 4 is initiated at 700 °C, but an effective formation was observed when the temperature was above 750 °C for a 3-h sintering time. The formation and decomposition of the intermediate phase, Pb 9Al 8O 21, was detected in this system within the temperature range of 800–900 °C. When the lead oxide and γ-alumina mixture was sintered with a Pb/Al molar ratio of 1:12, the PbAl 12O 19 phase was found at 950 °C and effectively formed at 1000 °C. In this system, an intermediate phase Pb 3(CO 3) 2(OH) 2 was observed at the temperature range of 700–950 °C. Over longer leaching periods, both PbAl 2O 4 and PbAl 12O 19 were superior to lead oxide in immobilizing lead. Comparing the leaching results of PbAl 2O 4 and PbAl 12O 19 demonstrated the higher intrinsic resistance of PbAl 12O 19 against acid attack. To reduce metal mobility, this study demonstrated a preferred mechanism of stabilizing lead in the aluminate structures by adding metal-bearing waste sludge to the ceramic processing of aluminum-rich products.

Immobilisation of heavy metal in cement-based solidification/stabilisation: A review

Heavy metal-bearing waste usually needs solidification/stabilization (s/s) prior to landfill to lower the leaching rate. Cement is the most adaptable binder currently available for the immobilisation of heavy metals. The selection of cements and operating parameters depends upon an understanding of chemistry of the system. This paper discusses interactions of heavy metals and cement phases in the solidification/stabilisation process. It provides a clarification of heavy metal effects on cement hydration. According to the decomposition rate of minerals, heavy metals accelerate the hydration of tricalcium silicate (C3S) and Portland cement, although they retard the precipitation of portlandite due to the reduction of pH resulted from hydrolyses of heavy metal ions. The chemical mechanism relevant to the accelerating effect of heavy metals is considered to be H+ attacks on cement phases and the precipitation of calcium heavy metal double hydroxides, which consumes calcium ions and then promotes the decomposition of C3S. In this work, molecular models of calcium silicate hydrate gel are presented based on the examination of 29Si solid-state magic angle spinning/nuclear magnetic resonance (MAS/NMR). This paper also reviews immobilisation mechanisms of heavy metals in hydrated cement matrices, focusing on the sorption, precipitation and chemical incorporation of cement hydration products. It is concluded that further research on the phase development during cement hydration in the presence of heavy metals and thermodynamic modelling is needed to improve effectiveness of cement-based s/s and extend this waste management technique.

Investigating solid waste production and associated management practices in private dental units

In the municipality of Thessaloniki in 2006 mercury-bearing dental wastes were not managed properly by 80% of dentists and metal-bearing waste was handled in accordance with internationally established best management practices by less than 50% of dentists. Those results were documented through a biennial field-based research study that took place in private dental units within the Thessaloniki Urban Area. For quantifying the waste produced, structured questionnaires were used and interviews with dentists were performed. In the present work, results of this survey are presented; critical parameters and factors affecting the quantity and quality of the dental waste stream are reported together with the analysis and classification of dominant conditions and needs of the dental sector in the waste management field.

Mobility of heavy metals in self-burning waste heaps of the zinc smelting plant in Belovo (Kemerovo Region, Russia)

Approximately 70,000 tons of slags, containing a high concentration of fine-grained carbon and heavy metals were accumulated in the town of Belovo (Kemerovo region, Russia) as a result of Zn-smelting between 1950 and 1994. Spontaneous ignition of fine-grained carbonaceous material and the subsequent burning of the waste heaps produced a zoning pattern which is characterized by three types of altered waste material: burned (or pyrometamorphic) waste, metamorphosed waste, and weakly altered waste. The self-combustion process is the reason for a redistribution of various elements. Leaching tests were carried out to understand the behavior of soluble (extracted by distilled water) and mobile (extracted by 1N HCl) metal forms; water analyses were performed using atomic absorption spectroscopy. The mobile metal species were accumulated close to the combustion center, in the burned and metamorphosed waste. In contrast to this, the soluble species were precipitated further away from the combustion center, on the surface of the waste heaps near active fumaroles, which provide evidence for ongoing combustion in the interior parts of the heaps. These observations are confirmed by mineralogical studies using routine mineralogical techniques: elementary copper and newly formed zincite and willemite are found in the burned and thermally metamorphosed waste, whereas soluble sulfates of Cu, Zn and Fe were deposited in the weakly altered zones near the surface of the waste heaps. The relative ability of the studied elements to migrate, determined as the proportion of the mobile or soluble to total metal concentrations, decreases in the order Cd>Zn>Cu>Pb>Fe for both the soluble and mobile forms. Primary metal-bearing waste material appears to be more resistant to weathering than the secondary phases formed as a result of combustion. Every hour, drainage waters remove about 3 kg Zn and 2 kg Cu from the waste heaps.

Removal of H2S by Metal Ferrites Produced in the Purification of Metal-Bearing Waste Water. Study of the Reaction Mechanism

Waste water polluted with heavy metals can besuccessfully purified by precipitation of the metals from analkaline solution containing iron (II), giving rise to aferrite sludge. The solid metal ferrites obtained in thismanner can be used to remove hydrogen sulphide from a gasstream. Based on a Taguchi experimental design, ferrite solidparticle and pore size, and the temperature resulting inmaximum retention of H2S by the solid were optimised.Under the optimum conditions, predicted by the method, eachgram of ferrite was able to retain 0.274 g H2S.In addition, a ferrite containing a known lead concentration,obtained by the precipitation method under optimal conditionsof pH, temperature and Fe/Pb ratio, was used to study theexothermic H2S retention reaction. The chemical reactionoccurring between the ferrite and the H2S wasinvestigated by characterisation of the compounds before(Pb0.04FeII 0.96 FeIII 2O4·nH2O solids composed ofPbxFe3-xO4, magnetite Fe3O4 andhydrated lead oxide PbOn·H2O) and after (PbS, PbSO4, S, FeS2 and α-FeO(OH)) the retention process.

The implications of the Basel Convention for developing countries: the case of trade in non-ferrous metal-bearing waste

Abstract The decision to introduce a ban on trade in hazardous waste with secondary values between Annex VII countries (the OECD, EU and Liechtenstein) and Annex VIII countries (other signatories) could have important implications for developing countries. The specific case of non-ferrous waste has been chosen for analysis since it is an important component of waste trade flows and since a number of reports have been written which argue that the ban may have adverse economic and environmental effects for a number of developing countries. The study examines some of the relevant trade data as well as the underlying economic factors behind the trade. In particular, the generation of non-ferrous metal (NFM)-bearing waste, the use of non-ferrous metals in manufacturing sectors, and the production of secondary non-ferrous metals in the two areas will be examined. The main findings of the paper are that: metal-bearing solid waste generation amongst developing countries is lower in absolute terms, but the rate of growth is much higher than amongst OECD countries; many developing countries are exhibiting much faster rates of growth in demand for non-ferrous metal than OECD countries due to shifts in the sectoral composition of their economies; international trade in NFM-bearing waste and scrap is relatively significant for some metals, but those types of waste most likely to be affected by the ban do not appear to be important parts of this trade; and, imports of non-ferrous metal waste and scrap help to explain the production of secondary metals, thus indicating that at least some of the trade in waste is motivated by demand for the waste for reclamation. The report concludes that while the ban is far from being an ideal environmental policy instrument, it must be understood as a reflection of both market failures (in the market for hazardous waste treatment and reclamation) and policy failures (in previous efforts to control adverse effects from the trade). If appropriate criteria are applied (mainly waste classifications and Annex classifications) then the environmental implications of the ban may be positive and the negative economic consequences quite small. However, given the rate of growth in generation in many of the Annex VIII countries, such benefits are likely to be slight relative to the problem of the treatment and reclamation of waste generated domestically. Thus, it is argued that if the ban is to be introduced, the dormant `positive' measures in the Convention related to financial and technical assistance need to be revived.