An overview of recovery of metals from slags

Various slags are produced as by-products in metallurgical processes or as residues in incineration processes. According to the origins and the characteristics, the main slags can be classified into three categories, namely ferrous slag, non-ferrous slag and incineration slag. This paper analysed and summarised the generation, characteristics and application of various slags, and discussed the potential effects of the slags on the environment. On this basis, a review of a number of methods for recovery of metals from the slags was made. It can be seen that a large amount of slags is produced each year. They usually contain a quantity of valuable metals except for blast furnace slag and they are actually a secondary resource of metals. By applying mineral processing technologies, such as crushing, grinding, magnetic separation, eddy current separation, flotation and so on, leaching or roasting, it is possible to recover metals such as Fe, Cr, Cu, Al, Pb, Zn, Co, Ni, Nb, Ta, Au, and Ag etc. from the slags. Recovery of metals from the slags and utilisation of the slags are important not only for saving metal resources, but also for protecting the environment.     

Energy from gasification of solid wastes

Gasification technology is by no means new: in the 1850s, most of the city of London was illuminated by "town gas" produced from the gasification of coal. Nowadays, gasification is the main technology for biomass conversion to energy and an attractive alternative for the thermal treatment of solid waste. The number of different uses of gas shows the flexibility of gasification and therefore allows it to be integrated with several industrial processes, as well as power generation systems. The use of a waste-biomass energy production system in a rural community is very interesting too. This paper describes the current state of gasification technology, energy recovery systems, pre-treatments and prospective in syngas use with particular attention to the different process cycles and environmental impacts of solid wastes gasification.     

Potential useful products from solid wastes

Wastes have been aptly defined as “items i.e. resources, that have been discarded because their possessors no longer have an apparent use for them”. Accordingly, “wastes” have a significance only in relation to the items and those who have discarded them. The discarded items now are resources awaiting reclamation. Reclamation usually involves either salvage or conversion—or in modern terminology, “reuse” or “recycling”. Reclamation for reuse consists in refurbishing or other upgrading without significantly altering original form and composition. Examples of wastes amenable to reuse are containers (bottles, etc.), cartons and repairable tires. With “recycling” (i.e. conservation), the discarded items are processed such that they become raw material i.e. resources in the manufacture of “new” products. The variety of processes is wide, ranging from simply physical (grinding) through thermal (melting, gasification, combustion), to biological (composting, biogasification, hydrolysis, microbial protein production). In the paper, reuse and recycling (conversion) are evaluated in terms of advantages and disadvantages (limitations) and their respective technologies are described and discussed in detail.     

A pilot plant two-phase anaerobic digestion system for bioenergy recovery from swine wastes and garbage

A pilot plant bioenergy recovery system from swine waste and garbage was constructed. A series of experiments was performed using swine feces (SF); a mixture of swine feces and urine (MSFU); a mixture of swine feces, urine and garbage (MSFUG); garbage and a mixture of urine and garbage (AUG). The system performed well for treating the source materials at a high organic loading rate (OLR) and short hydraulic retention time (HRT). In particular, the biogas production for the MSFUG was the highest, accounting for approximately 865-930Lkg(-1)-VS added at the OLR of 5.0-5.3kg-VSm(-3)day(-1) and the HRT of 9 days. The removal of VS was 67-75%, and that of COD was 73-74%. Therefore, co-digestion is a promising method for the recovery of bioenergy from swine waste and garbage. Furthermore, the results obtained from this study provide fundamental information for scaling up a high-performance anaerobic system in the future.     

Thermal and hydrometallurgical recovery methods of heavy metals from municipal solid waste fly ash

Heavy metals in fly ash from municipal solid waste incinerators are present in high concentrations. Therefore fly ash must be treated as a hazardous material. On the other hand, it may be a potential source of heavy metals. Zinc, lead, cadmium, and copper can be relatively easily removed during the thermal treatment of fly ash, e.g. in the form of chlorides. In return, wet extraction methods could provide promising results for these elements including chromium and nickel. The aim of this study was to investigate and compare thermal and hydrometallurgical treatment of municipal solid waste fly ash. Thermal treatment of fly ash was performed in a rotary reactor at temperatures between 950 and 1050 °C and in a muffle oven at temperatures from 500 to 1200 °C. The removal more than 90% was reached by easy volatile heavy metals such as cadmium and lead and also by copper, however at higher temperature in the muffle oven. The alkaline (sodium hydroxide) and acid (sulphuric acid) leaching of the fly ash was carried out while the influence of temperature, time, concentration, and liquid/solid ratio were investigated. The combination of alkaline-acidic leaching enhanced the removal of, namely, zinc, chromium and nickel.     

火电厂节水技术综述

对火电厂用水、废水的产生情况进行了研究,提出相应的废水处理及再利用对策,并介绍了国内外火电厂的节水技术方案和措施,可为火电厂的节水工作提供借鉴.     

An overview on olive mill wastes and their valorisation methods

Olive mill wastes represent an important environmental problem in Mediterranean areas where they are generated in huge quantities in short periods of time. Their high phenol, lipid and organic acid concentrations turn them into phytotoxic materials, but these wastes also contain valuable resources such as a large proportion of organic matter and a wide range of nutrients that could be recycled. In this article, recent research studies for the valorisation of olive mill wastes performed by several authors were reviewed: second oil extraction, combustion, gasification, anaerobic digestion, composting and solid fermentation are some of the methods proposed. Special attention was paid to the new solid waste generated during the extraction of olive oil by the two-phase system. The peculiar physicochemical properties of the new solid waste, called two-phase olive mill waste, caused specific management problems in the olive mills that have led to the adaptation and transformation of the traditional valorisation strategies. The selection of the most suitable or appropriate valorisation strategy will depend on the social, agricultural or industrial environment of the olive mill. Although some methods are strongly consolidated in this sector, other options, more respectful with the environment, should also be considered.     

High-temperature thermal destruction of poultry derived wastes for energy recovery in Australia

The high-temperature thermal destruction of poultry derived wastes (e.g., manure and bedding) for energy recovery is viable in Australia when considering resource availability and equivalent commercial-scale experience in the UK. In this work, we identified and examined the opportunities and risks associated with common thermal destruction techniques, including: volume of waste, costs, technological risks and environmental impacts. Typical poultry waste streams were characterised based on compositional analysis, thermodynamic equilibrium modelling and non-isothermal thermogravimetric analysis coupled with mass spectrometry (TG–MS). Poultry waste is highly variable but otherwise comparable with other biomass fuels. The major technical and operating challenges are associated with this variability in terms of: moisture content, presence of inorganic species and type of litter. This variability is subject to a range of parameters including: type and age of bird, and geographical and seasonal inconsistencies. There are environmental and health considerations associated with combustion and gasification due to the formation of: NO_X, SO_X, H₂S and HCl gas. Mitigation of these emissions is achievable through correct plant design and operation, however, with significant economic penalty. Based on our analysis and literature data, we present cost estimates for generic poultry-waste-fired power plants with throughputs of 2 and 8 tonnes/h.     

Energy recovery by incineration of solid wastes in the Federal Republic of Germany

Incineration of solid waste with recovery of energy is practiced in communities serving over 30% of the population of the F.R.G. Although solid waste has an energy content corresponding to only 2% of the total energy demand, in individual cases, up to 9% of the electrical requirements (e.g. Munich) are supplied by waste incineration. The basic problems of energy recovery via incineration are reviewed, and a number of practical suggestions are made for the planning and implementation of such schemes.     

Characterization of char derived from various types of solid wastes from the standpoint of fuel recovery and pretreatment before landfilling

Carbonization is a kind of pyrolysis process to produce char from organic materials under an inert atmosphere. In this work, chars derived from various solid wastes were characterized from the standpoint of fuel recovery and pretreatment of waste before landfilling. Sixteen kinds of municipal and industrial solid wastes such as residential combustible wastes, non-combustible wastes, bulky wastes, construction and demolition wastes, auto shredder residue, and sludges were carbonized at 500 degrees C for 1h under nitrogen atmosphere. In order to evaluate the quality of char as fuel, proximate analysis and heating value were examined. The composition of raw waste had a significant influence on the quality of produced char. The higher the ratio of woody biomass in waste, the higher heating value of char produced. Moreover, an equation to estimate heating value of char was developed by using the weight fraction of fixed carbon and volatile matter in char. De-ashing and chlorine removal were performed to improve the quality of char. The pulverization and sieving method seems to be effective for separation of incombustibles such as metal rather than ash. Most char met a 0.5 wt% chlorine criterion for utilization as fuel in a shaft blast furnace after it was subjected to repeated water-washing. Carbonization could remove a considerable amount of organic matter from raw waste. In addition, the leaching of heavy metals such as chrome, cadmium, and lead appears to be significantly suppressed by carbonization regardless of the type of raw waste. From these results, carbonization could be considered as a pretreatment method for waste before landfilling, as well as for fuel recovery.     

红雁池电厂冲灰回水系统低温阻垢处理

国电新疆红雁池发电有限公司冲灰水闭路循环系统运行一年多,回水系统结垢严重,电除尘器下的冲灰器50 mm进水管因结垢几乎全部堵死,严重威胁发电厂的安全生产.针对新疆地区冬季环境温度在0～-25 ℃的特殊情况,对冲灰水低温阻垢方法进行了探讨,采用分段联合阻垢方案解决了低温下回水管路的结垢问题.     

An overview of municipal solid waste management in China

Municipal solid waste management (MSWM) in China warrants particular attention as China has become the largest MSW generator in the world and the total amount of MSW it produces continues to increase. In recent years, central and local governments have made great efforts to improve MSWM in China. New regulations and policies have been issued, urban infrastructure has been improved, and commercialization and international cooperation have been encouraged. Considering these developments, an overview is necessary to analyze the current state as well as new opportunities and challenges regarding MSWM in China. This paper shows that since the late 1990s, the amount of MSW collected has been largely decoupled from economic growth and incineration has become an increasingly widespread treatment method for MSW. We identify and discuss four major challenges and barriers related to China’s MSWM, and propose an integrated management framework to improve the overall eco-efficiency of MSWM.     

碳热还原电厂粉煤灰冶炼硅铝铁合金

电厂粉煤灰中主要含有SiO2和Al2O3,且含有一定量的残炭。采用粉煤灰和铝土矿等为主要原料进行了高温下制备硅铝铁合金研究,结果表明,利用电厂粉煤灰和铝土矿为主要原料,配加赤铁矿、硅石和添加剂CaF2,在1 880℃时经碳热还原可以制备硅铝铁合金,提高粉煤灰的利用价值,减少其环境污染。在1 750℃时,还原反应进行不完全,得不到硅铝铁合金。在1 880℃时还原出的合金主要成分为Si,Al和Fe,其平均含量分别为27.69%,8.75%和58.81%,合金的主要物相为Al0.3Fe3Si0.7,SiC,FeSi和FeC。     

Investigation on thermal dechlorination and catalytic pyrolysis in a continuous process for liquid fuel recovery from mixed plastic wastes

A continuous system (feeding rate >1 kg/h) consisting of thermal dechlorination pre-treatment and catalytic pyrolysis with Fe-restructured clay (Fe-RC) catalyst was developed for feedstock recycling of PVC-containing mixed plastic waste. The vented screw conveyor which was specially designed for continuous dechlorination was able to achieve dechlorination efficiency of over 90 % with a feedstock retention time longer than 35.5 min. The chlorine content of the pyrolytic oil obtained after dechlorination was in the range of 6.08–39.50 ppm, which meet the specification for reclamation pyrolytic oil in Japan. Fe-RC was found to significantly improve the yield of pyrolytic oil (achieved to 83.73 wt%) at the optimized pyrolysis temperature of 450 °C and catalyst dosage of 60 g. With the optimized parameters, Fe-RC showed high selectivity for the C₉–C₁₂ and C₁₃–C₁₉ oil fraction, which are the major constituents of kerosene and diesel fuel, demonstrating that this catalyst can be applied in the pyrolysis of mixed plastic wastes for the production of kerosene and diesel fuel. Overall, the continuous process exhibited high stability and consistently high-oil yield upon reaching steady state, indicating its potential up-scaling application in the industry.     

电厂燃煤过程中重金属污染及其控制技术研究

综述了燃煤重金属污染问题的严重性和污染途径，对燃煤重金属的种类和危害进行了论述，并重点阐述了国内外采取的控制措施，同时还指出了燃煤重金属污染研究中存在的问题及今后的研究方向。     

Feasibility study for thermal treatment of solid tire wastes in Bangladesh by using pyrolysis technology

In this study on the basis of lab data and available resources in Bangladesh, feasibility study has been carried out for pyrolysis process converting solid tire wastes into pyrolysis oils, solid char and gases. The process considered for detailed analysis was fixed-bed fire-tube heating pyrolysis reactor system. The comparative techno-economic assessment was carried out in US$ for three different sizes plants: medium commercial scale (144 tons/day), small commercial scale (36 tons/day), pilot scale (3.6 tons/day). The assessment showed that medium commercial scale plant was economically feasible, with the lowest unit production cost than small commercial and pilot scale plants for the production of crude pyrolysis oil that could be used as boiler fuel oil and for the production of upgraded liquid-products.     

火电厂NOx污染排放控制方法探讨

NOx的排放控制是火电厂未来环境保护的重要任务。从控制燃烧过程中NOx的生成以及在燃烧后脱除烟气中NOx两个方面探讨了减少NOx排放的有效技术。     

Cogeneration from thermal treatment of selected municipal solid wastes. A stoichiometric model building for the case study on Palermo

This paper aims to calculate the energetic and environmental effects of an integrated solid waste management system in Palermo, Italy. In particular, the thermal treatment of Municipal Solid Waste (MSW) with energy recovery is assessed. The current characterization at the local scale is taken into account. Two different options of collection are taken into account: (1) unselected wastes; and (2) sorted collection, according to the current Italian regulation. Combustion process is analyzed and the following features are calculated: (1) stoichiometric content of air and air excess; and (2) temperature and enthalpy of flue gases. Energy recovery is performed in the hypothesis of Hirn cycle both with steam condensation to produce only power, and with bleeding cycle for the combined production of power and thermal energy. Total electric efficiency is assumed as representative index of the technological level of the assessed plant. Results show that the thermal treatment of selected MSW, associated with a cogenerative recovery of energy, represents a relevant sustainable strategy of waste valorization as an alternative to fossil fuels.