28-Nov-2009
The ongoing boom in the iron and steel industry combined with scarcity in raw material supply, their availability has caused a dramatic increase of prices for raw materials and steel roducts. The whole chain of production from front-end side of iron making up to the finishing stage of steel products has to be observed and optimized continuously by introducing highly efficient new technologies, tools, and emission and residue free, environment-friendly processes for production of high value steel products. Among the various innovative iron making processes like Midrex, COREX®, FINEX®, Finmet, Fastmet, Romelt, Primus etc.
ZEWA (Zero Waste) is one such process. Here we are going to discuss in brief about the background and some features of ZEWA process.ZEWA (Zero Waste) is a new metallurgical process which converts blends of industrial waste materials and residues into hot metal and mineral products such as Hydraulic Binders for cement production, metallurgical powders for desulphurization practices and materials suitable for road construction. ZEWA (Zero Waste) process involves high temperature smelting reduction (SR) operations which are carried out in a specially designed, electrically operated reactor.
A demonstration plant with all necessary auxiliary facilities was erected at the Vitkovice Steel Works in the Czech Republic. An earlier report says that it took eleven test campaigns to prove the technical and economical feasibility of the process with respect to the generation of hot metal and useful mineral products from the residues of carbon and stainless steel production.
ZEWA (Zero Waste) Process - A Background
The basis for the development of the ZEWA (Zero Waste) process was laid at the Central Recherche Metallurgique (Center for Metallurgical Research) in Belgium where a laboratory scale smelting reduction process was developed to convert various residues from Steel Plants into valuable metallic and mineral products. Pilot plant tests using a hollow electrode for the pneumatic injection of residue materials into a furnace were then conducted by CRM, MSFOS (Sweden) and FEhS (German Research Institute) in the framework of the IBPM (Internal By-product Melting) project.
In 2000, Voest Alpine (VAI) teamed up with CRM and a large consortium of partners as part of a multi-national project team supported by the European Union. A process concept based on the previously tested pneumatic injection of residue materials into a furnace via an injection lance was chosen. The partnership within this so-called “Fifth Framework Programme” is as follows:
=> From the steel industry; CRM (project coordinator), VAI, ARCELOR, and Vitkovice Research team.
=> From the cement industry; LAFARGE
=> From the car dismantling industry; the Belgian SME Comestsambre.
=> From the coal industry; the ICPC (Institute for Chemical Processing of Coal, Poland)
This project works with a goal of developing a viable technology for the so-called ZEWA (from Zero-Waste) process and to test it on a demonstration scale. The main task to be carried out was thus the design and erection of a dedicated pilot plant, the performance of the pilot test campaigns and the final evaluation of the ZEWA as the basis for commercialization.
The Principal of ZEWA (Zero Waste) Process
The ZEWA (Zero Waste) process is based on smelting reduction of suitable blends of basic and acidic residue materials from industrial production as follows:
=> From the steel industry basic steel making slags and dusts, and silica containing residues from scrap handling (mixtures of glass and plastics) in EAF plants;
=> Complementary acidic residues from another industrial sectors, such as fly ash from coal-fired power plants, automotive shredder residues (ASR) or bottom ash from urban incinerators (BI ash).
The main products are the refined slag (or mineral product) with targeted chemical composition, and hot metal or metal product to be recycled for steel production. For the smelting reduction (SR) process carbon based reductant (coke, anthracite, coal etc.) are added to the blend of residue materials. Depending on the raw material and the mineral product, small quantities of stronger reductants like ferro-silicon, or additives such as lime or bauxite, may also be added when necessary. Process dust with high zinc content is recovered in an off gas filtering unit. Targeted mineral products are a Portland clinker substitute for use in cement production and metallurgical powders for use in secondary steel refining units.
ZEWA (Zero Waste) - The Process Technology
The smelting reduction in ZEWA or Zero Waste process is done in an electrically heated ladle-type furnace which is equipped to allow for top charging of liquid steel making slag and coarse solid materials into the foamy slag bath, and also for the deep pneumatic injection of powdery materials into the hot metal with a lance. Coarse solid materials like solid slags are charged by gravity, via a fibro-feeder or by a chute. Injected powdery materials are mainly steel making dusts, fly ash and reductants. Other features of the ladle include bottom stirring (to enhance mixing and reaction kinetics) and post combustion in the upper part of the slag bath (to recover chemical heat through the partial combustion of the CO from the reduction reactions). The ZEWA (Zero Waste) process technology is quite flexible with regards to the input materials, firstly because it allows for the charging of the liquid slag, and secondly because it can cope with highly variable charging ratios of liquid slag, coarse solids and powdery materials. Dry dusts can be directly used as a raw material. Sludges and other residue materials require only drying and micro granulation. No pelletization or briquetting is necessary, thus reducing the material pre-treatment costs substantially. Because of the filtering effect of thick foamy slag bath, ZEWA technology is also very efficient in terms of lowering dust emissions. Moreover, due to the low thermal losses by radiation from the arc and the metal bath, ZEWA process is very effective in lowering the energy consumption too.
