UTILISATION OF EAF DUST AS RAW MATERIAL FOR STEELMAKING OPERATIONS Below brief report is prepared to emphasize the importance of EAF dust in terms of environmental, economical, waste recovery and metallic source concerns. General waste treatment processes, trends, alternatives, potentials and main technological layout and principles of Rotary Hearth Furnace application specifically designed to process waste mill scale and EAF dust. EAF DUST, FORMATIONS, SPECIFICATIONS POTENTIALS AND OTHERS; The production of steel in electric arc furnace (EAF) generates a by-product called EAF dusts. This dusts called as EAFD is collected in bag house and amounts to approximately % 2 of the steel produced. These steelmaking flue dusts are classified in most industrialized countries as hazardous residues according to the US EPA classification, dated-back 1980 because the heavy metals contained in them, tend to leach under slightly acidic rainfall conditions.it is estimated that the world-wide total production of EAF dust could be as high as several million tons, all of which must be treated, recycled or land-filled. EAFD can vary greatly in composition of the metallic charge, the furnace additivies used and the type of steel produced. Below two tables can be considered as a general composition of EAF dust for scrap based and DRI based Steelmaking operations. Figure1, EAF dust samples from Scrap based EAF operations. Figure2 ; EAF dust sample from DRI based EAF operations. It should be noted that main components of EAFD changes a lot depending on many parameters, the following graphs shows variations in percentages in EAF shops at USA. 1
Figure 3; ZnO variations at Scrap based EAFDust formations. Figure 4; PbO variations at Scrap based EAF dust formations Figure 5 ; CdO variations at Scrap based EAF Dust formations. 2
Figure 6 ; FeO variations at Scrap based EAF Dust formations. Figure 7; Another EAFD example showing heavy metal contents. Due to its high levels of zinc, which is an essential nutrient for plant growth, steel industrywastes can include lead, arsenic, cadmium, chromium, nickel and dioxin, among other toxic substances. Electric arc furnace (EAF) dust with high chloride content increases the threat of dioxin emissions and the high chloride content reduces the value of recycled zinc oxide produced by EAF dust recycling plants. Moreover because EAF dust includes dioxin, plants in Japan that recover Zinc from EAF dust are designated as special facilities under the law concerning special measures against Dioxins. But at the same time contained zinc species can be used as a source to obtain valuable byproducts. These considerations had led to the treatment of these dusts and thus reducing their environmental impact. There are two possibilities to manage these EAF dusts: hydrometallurgical and pyrometallurgical processes. Existing treatment processes are industrially viable only when the zinc content is sufficiently high. They consisted in the extraction of nonferrous metals, i.e. zinc, in order to recycle the residue in the steelmaking industry; however, the hydrometallurgical processes are regarded as more eco-friendly for treating materials having a relatively low zinc content, A third option that can be considered for these dusts is their inactivation, i.e. their stabilization and solidification prior to permanent disposal.several factors are essential to the decision of selecting either alternatives. The main factors are economic ( treatment and transportation costs), environmental ( regulations and conservation) long term liability and concerns over public 3
opinion. Almost % 60 of total worlwide EAF Dust recycled in different High Temperature Metals Recovery ( HTMR) facility ; % 35 utilize a Landfill, solidification, cement additives, brick making material ; and % 5 employ other recycling techniques, several employ more than one disposal solution. Generally if economical considerations does not justify any recycling process, the best option is to convert EAFD to non hazardous form by stabilisation / soldification procedure before permanent disposal. However, steel mini-mills are becoming increasingly reluctant to landfill EAF dust because of the potential for future cleanup liabilities. STABILISATION / SOLIDIFICATION TREATMENT; i. Solidification/ Stabilisation ( S/S) is a widely used treatment forthe management and disposal of broad range of contaminated materials and wastes particularly those contaminated with substances classified as hazardous. This process protects human health and the environment by immobilizing contaminants within the treated material, preventing them from migrating to plants, animals and humans. ii. Leachability of hazardous/ toxic elements can be decreased by embedding it in the mixture for the production of concrete and Portland cement. iii. In that process EAFD as a granular waste that can be readily mixed with cement based binders and water to produce a building like material. iv. But the presence of a considerable amount of Zinc in the waste has to be considered as a major concern for the effectiveness of the Stabilisation / Solidification process, since the compound is well known retarding agent for cement hydration.moreover, it lowers the strength of the dust and cement mixtures. This is due to the reaction of the lime with Zinc Oxide which forms the low solubility Zinconates. v. Up to certain percentages addition of EAFD to portland cement does not cause any loss of strength but the time dependence of the strength development of these mixtures depends on ZnO content and the kind of Iron oxide in the dust. vi. The setting process will be delayed considerably as the waste -to-cement ratio vii. increased irrespective of the cement based binding system. Waste addition above the optimum level product can not ensure necessary integrity of the waste form. viii. The main problem forthe environmental disposal of the EAFD is the high leachability of the various toxic metals that contains and mainly of Pb. If the Pb leachability problem could be solved EAFD can be utilised as raw material in the production of concrete based building materials like bricks, tiles etc. There are ways to prevent leachability of Pb, but in industrial scale, that possibility needs to be strictly controlled, otherwise, direct contact of Leached Pb with human body is not avoidable. ix. One of the most efficient way to prevent leachability or to increase time to leach of hazardous elements in EAFD before landfill or to convert to building materials is the compaction. Compaction decreases diffusibility of these elements. Certain percentages of water and high compaction pressure gives best results. But in any way the waste material can not be used as building material if it contains considerable molybdenum and cadmium concentrations. x. Simple mixing may not be enough to prevent leaching of hazardous elements, compaction under pressure of this kind of waste material as treatment prior to land disposal in a sustainable landfill for a long time is recommended. 4
xi. In any way all EAFD treatment process other than recycling to recover Zinc and Fe, needs big care to prevent leaching of heavy metals at land fill or any concrete product in which waste is imbedded. PYROMETALLURGICAL SEPERATION PROCESSES; A method for the separation and recovery of metals selected from the group consisting of iron, cadmium, zinc, and lead, from raw material comprising a mixture of metals, which comprises the steps of heating the raw material to a temperature sufficient to substantially vaporize cadmium, zinc, and lead, and insufficient to substantially vaporize iron; separating secondary dust and vapors produced during the first step from the residual sinter mass, which mass comprises iron; slurrying the secondary dust in an aqueous solution of ammonia ammonium carbonate to dissolve zinc and cadmium; separating a zinc/cadmium bearing leach liquor from substantially insoluble lead containing particles by filtration; treating the zinc/cadmium bearing leach liquor to recover cadmium by adding metallic zinc to the leachate to produce a cadmium containing cement; separating the cement from the leach liquor; and removing ammonia from the leach liquor to precipitate basic zinc carbonate. As known very well, Steel melts at approx 1480 C but zinc boils at 907 C, Volatile impurities vaporize, oxidize and form dust (Zn, Pb, Cd, etc) The main pyrometallurgical technique is Waelz process, which represents more than % 80 of the recycling capacity. The other techniques are ; Rotary Hearth ( Kobelco, Inmetco ) Multiple Hearth ( Paul Wurth PRIMUS) Shaft Furnace ( Cupola Oven, IS Zinc Smelter, Mitsui) Melting Cyclone ( VAI; HRD) Plasma Furnace ( Mintek) Electrothermal ( Toho) Advantages of High temperature process is Zn and Fe is highly or completely recovered,disadvantages are High capital and operating costs, air emissions, Hydro- metallurgical Leaching in NH4 Cl ( Ezinex, HST) Leaching in H2SO4 ( Recupac, Hydromet, ZincOx) Leaching in NaOH ( Zimaval) Advantages of Hydro-metallurgical process are Lower capital & operating costs,halides can be removed ( washing ) but Disadvantages are Low Zn recovery (zinc ferrite cannot be easily leached), Fe not recovered, it lost the value, significant waste disposal,waste water issues. 5
Figure 8 ; EAFD treatment Techniques. WAELZ PROCESS; Most zinc oxide (ZnO) is made from zinc metal derived from ore or scrap zinc metal resources. Zinc metal is an internationally traded commodity, where prices are set by the London Metal Exchange and at any point in time will reflect supply and demand balances. ZnO is also found in a wide range of chemicals and pharmaceuticals used for medical and cosmetic purposes such as wound healing and sunscreen. Emerging ZnO markets include ITO replacement for displays and photovoltaic panels, as a semiconductor for making inexpensive transistors, and as applications in thin-film batteries and spintronics. Before entering the description of the Waelz technology and its applications on the treatment of EAFD, a short definition is required for clarifying the difference to other techniques. Waelz Technology is volatilization of non ferrous metals like Zn, Pb, Cd, etc out of an oxidized solid mixture by means of reduction by coke breeze in a rotary kiln without generating a liquid slag. Waelz Technology is in use since many years, originally introduced for the enrichment of low grade Zinc ores, further adopted on the re processing of the neutral leaching residues of Zn Smelter residues and for the past 30 years successfully applied on the treatment of EAF dust. The Waelz Kiln process is a well established technology that uses a Rotary Kiln to treat steel mill dust. Dust containing Zinc Oxide and a carbon source, such as coke, are charged into the Rotary Kiln and heated by combustion heat. Compared to the RHF process, the rotary kiln has lower productivity because of a lower operating temperature and less contact between dust and coal. The temperature in the kiln is generally below 1200 C. Because of lower temperature and inferior dust and coal contact the Waelz process achieves lower iron metallization and less dezincifications than RHF. Therefore Iron product can not be used as a metallic at EAF and it must be disposed of in special landfills. 6
Figure 9 ; Waelz process flow chart. Figure10 ; Typical input and out put composition of Waelz process. EAF DUST TREATMENT AT ROTARY HEARTH FURNACE ; The accumulation of EAF dust and its disposal have become a serious issue world wide. EAF dust contains valuable metal resources such as Iron, Zinc, Lead, and other elements.effective recovery of these metals would contribute to the development of a sustainable society. 7
Our purpose in that Project is to produce DRI by using EAF dust and Steel plants Mill Scale through coal based Rotary Hearth Furnace technology. Main criterias in our decision making are as follows. i. Effectively recover all valuable metals resources such as Iron,Zinc and others. ii. To convert EAFD to valuable products as raw material for steelmaking and Zinc smelting operations. All above mentioned recycling, stabilisation, recovery and land filling practices have some detrimental effects to the environment or not enough to treat hazardous wastes without any leaching effect in the long term. iii. By considering wastes as a new source to create value addition to the losses of steelmaking operations we projected to use a technology which is defined as Direct Reduction of metallic oxides through Rotary Hearth Furnace ( RHF ), brief introduction about that process is given as below. iv. Trend in steelmaking world is to create alternative metallic sources to existing scrap or iron ore based process routes, DRI production is one of them as can be seen below. For steelmakers to pay effort to recover all metallic wastes into usable form in steelmaking vessels is getting more and more crucial. Figure 11 ; Iron and Steelmaking metallic sources. v. Resource recovery is an innovative industry and a natural trend, all countries has to switched from waste disposal and treatment to resource recovery, life cycle assesment, clean production and environment friendly products. RHF process is not a simple disposal or recovery process, is a state of art technology to recover metallic values in Mill Scale and EAF dust. vi. Many economies are developing vigorously the resource recovery, regeneration, rcycle and reuse of waste. Besides establishing laws and regulations, they promote Technologies for resource recovery. To pursue sustainable development, countries of the world have embarked on the R& D of resource regeneration Technologies in hopes to ensure sustainable use of resources in the earth. 8
Figure 12; Trends in Iron and Steelmaking raw material sources and demand. Above table shows the increasing demand and the way to meet that demand by available sources and DRI production. The characteristics of the RHF process versus the Rotary Kiln process are as follows; i. RHF operates at higher temperatures, over 1300 C, ii. RHF achieves higher metallization and dezincification ( more than % 95 ) because of higher metallization and recovery of zinc in DRI. iii. Fines generation is lower because the agglomerated raw materials ( iron and coal ) do not roll, but stationary on the rotary hearth. iv. Zinc,lead and other volatile substances are vaporized and reoxidized in the flue gas. The vaporized substances in the flue gas are collected in a bag filter and the Zincis recovered as Zinc oxide. The sensible heat in flue gas is recovered by combustion air by using a heat exchanger. v. The Zinc content of the recovered flue dust is higher because the amount of dust generated is lower and the dust can be seperated in the flue gas system. vi. The amount of Dioxin in the DRI is lower because the dioxin in the EAF dust is broken down under high temperatures. Moreover for controlling the dioxin emission the adsorption technique by activated carbon or lignite coke is used. vii. The amount of Dioxin in the flue gas is lower, because the hot flue gas from the RHF is cooled rapidly in the flue gas system to prevent de novo formation. viii. ix. DRI produced in RHF will be melted into steelmaking vessels. Main components of EAF Dust namely Zinc and Fe, will be converted to valuable products and totaly recycled without giving any detrimental impact to the environment. Total recovery of Zinc and Fe at any other recycling process is not possible.typical flue gas composition is given below and shows that all paramaters are well below the limits indicated by regulations. x. Main process flow chart of RHF is given as below. 9
Figure 13; RHF type EAF dust treatment to produce DRI and Zinc oxide. Figure 14; Pictural material flow at RHF type DRI production by using EAF Dust. Figure 15; Typical Flue gas composition at RHF. CONCLUSION; From the viewpoint of ecology, low carbon requirements and resource recovery are natural trends in the future, and the prospect of the resource recovery industry is very promising. However, to understand the property of wastes and value added technologiesis the 10
prerequisite to resource recovery in order to maximize the advantagesof resource use, to minimize impacts on environment, and so to achieve sustainable development of industries. Waste treatment has been expanded from source control in the past into resource reuse and sustainable development following the rise of the environmental awareness across the globe. Due to the overuse in the past few decades, resources on the earth are drying out, whether coal, petroleum or natural gas. Based on the current consumption rate, the remaining petroleum and major metals will be used up within the next five decades. For this reason, waste reuse and development has become the mainstream in the future. Therefore, countries in the world have gradually made the effective use of resource the new direction of environmental policies and the focus of environmental management. From this point of view, EAF dust issue is considered as a resource reuse opportunity and Rotary Hearth Furnace technology is choosed to convert all components of EAF dust into value and resource for other industries such as steelmaking and Zinc smelting. Existing waste EAF dust treatment practices namely to use it in the feed material composition of cement Rotary Kilns and as raw material in the production of concrete based building materials like bricks, tiles etc, can not prevent leachability of heavy metals such as Cd,Co, Pb, Dioxin and others, but by using RHF technology all valuable resources can be used within the limits of environmental regulations as explained above. As the products of RHF process; Zinc oxide will be used to feed Smelters to produce metallic Zinc and DRI will be used as raw material for existing Steelmaking operations to produce steel. 11