ENVIRONMENTAL FRIENDLY RECYCLING OF EAF STEEL MILL DUST - GSD S STATE-OF-THE-ART WAELZ PROCESS

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1 ENVIRONMENTAL FRIENDLY RECYCLING OF STEEL MILL DUST - GSD S STATE-OF-THE-ART WAELZ PROCESS BY DR. JUERGEN RUETTEN 1 ), RON CRITTENDON 2 ) SYNOPSIS: The Pyro-metallurgical Waelz process is the most often applied technology for the recycling of dusty steel mill residues globally, especially for and Zn enriched BOF dust. The target of the recycling process is to make the metals, which are contained in the residues, available for re-use. The recycling of dust closes the material loop between the steel and the zinc industry. More than 60 % of the produced zinc metal is used for corrosion protection on steel plates or structures. The steel at the end of its use is recycled as scrap in electric arc or induction furnaces for producing new steel. During this process the zinc coating is going to the dust. Nowadays still more than 50% of the generated dust is lost by sending it to landfill, which means an annual loss of more than 900,000 t of zinc. The paper explains the raw materials and their careful preparation, the reducing agents, the consumption figures and the different products. It focuses on the state-of-the-art techniques for environmental protection. The process can be operated free of wastewater, with extremely low diffuse dust emission and with very low atmospheric emission. The release of heavy metals or dioxin is well controlled by appropriate techniques. The Waelz process as it is applied by GSD is listed in the BREF-Notes of the European Community. Keywords: Waelz process, pyro-metallurgy, steel mill residues, dust, BREF-Notes, dioxin reduction 1 ) Executive Vice President Technology, Global Steel Dust Ltd., Lausanne / Switzerland (presenter) 2 ) Executive Vice President Global Operation, Global Steel Dust Ltd. & General Manager, Global Steel Dust (Thailand) Ltd., Bangkok / Thailand 1

2 1. Introduction In May 2012 we presented a paper on the SEAISI 2012 conference, which gives a detailed overview of the current situation in regard of global dust recycling and the applied technologies [1]. That allows us to keep this introduction short. Since many years the Waelz technology is in use for the volatilization of non-ferrous metals like Zn, Pb and Cd out of an oxidized solid mixture. Originally, it was introduced for the enrichment of low-grade zinc ores, further adopted on the re-processing of the neutral leaching residues of Zn-smelters and for the past 30 years successfully applied on the treatment of dust. Table 1 History of the Waelz Process 1881 Proposal for Zn volatilization by George Drue 1913 First patent on the process in Germany 1925 First Waelz plant put in operation for low grade ore enrichment (Lünen / Germany) 1940 s Application on Zn/Fe residues of Zn smelters 1970 s First dust co-treatment in Duisburg 1980 s New Waelz kilns for dust in USA, Europe and Japan 2012 More than 80 % of dust globally treated in Waelz kilns 2. The Standard Waelz Technology A Waelz plant consists normally of the units Raw Material Preparation, Rotary Kiln Line and Flue-gas Treatment. In the Raw Material Unit the solid feeding materials are properly prepared to guarantee a steady and homogeneous feed to the Waelz kiln. The solid feeding materials are the dust from the different steel mills, the reducing agent coke breeze and the slag forming agent sand or lime. The different kinds of dust are mixed with reduction coke and lime in an intensive mixer, where water is added for agglomerating, producing selfreducing pellets (SRP). 1 3 Coke fines 2 Water dry dust PELLETIZER MIXER pellets pellets sand Coke breeze SILO TRUCK TIP TRUCK Figure 1- Raw Material Unit 2

3 In the Kiln Line the feed is transformed into the 2 products Waelz Zinc Oxide (WZO), which accumulates the non-ferrous components of the feed, mainly Zn of 54 to 60 % concentration, and Waelz Iron Product (WIP), which is rich in iron and slagging components and used in the cement industry as iron additive or for road construction. The Kiln Line consists of the solid material dosing / feeding part, the rotary Waelz kiln with dust settling chamber and the slag granulation section. In the dosing the different solid feed components (SRP, coke, recovered coke and eventually sand or limestone) are fed in set ratios by weighing devices. The Waelz kiln is operated in a counter-current way: the solid material is advancing from the feeding side by the rotation of the kiln and its slope while the air is sucked in at the slag outlet side. The solid charge is first dried then heated up until the reaction starts. The maximum temperature of the solid achieves about 1200 C. Normally the process does not require additional heating (auto-thermal). Air is entering the kiln at ambient temperature, the gas leaves the kiln at 700 to 800 C. pellets pellets sand Coke breeze dust.1 Sand.2 Return Coke.3 Coke.4 Air COOLER BAG HOUSE DRAFT FAN STACK To Washing plant Water WAELZ KILN Air SETTLING CHAMBER Waelz Oxide Crude Return Coke Slag Figure 2- Waelz Plant The off-gas treatment starts already in the dust settling chamber, where the carry-over from the feed is separated in order to achieve a high zinc concentration in the product. In this chamber water can be injected for initial cooling. The main cooling is achieved by the addition of ambient air. In some plants tubular coolers are installed for reducing the off-gas volume. The crude Waelz oxide powder is separated in bag house filters; electrostatic precipitators are not more used due to low de-dusting efficiency. Waelz plants sell the crude Waelz Zinc Oxide directly to Zinc smelters or other chemical plants. In a few plants an additional Washing Plant is installed for the de-halogenation of the crude oxide. 3

4 Table 2: Input / Output Composition of Waelz plants (typical range) dust WIP (Slag) WZO Washed WZO Zn [%] Pb [%] Cd [%] < Cl [%] F [%] Na 2 O [%] K 2 O [%] FeO [%] SiO 2 [%] ) / ) CaO [%] ) / ) Hg [ppm] 1-5 < C [%] S total [%] ) acid ²) basic 3. Recent Process Improvements In the past 15 years a process of gradual improvements of the Waelz process started mainly in Europe driven by tighter environmental legislation and increasing operation costs. Economy, Throughput The standard size / capacity of a Waelz plant in the 1990s was in the order of 60 to 80 thousand tons annual dust throughput. A significant improvement was the introduction of self-reducing pellets as feed material. Instead of pelletizing the dust at the steel mills with water to mm agglomerates and feeding these pellets separately to the coke breeze to the rotary kiln, dusty dust is mixed in an intensive mixer with 10% fine coke and water to form micro-pellets (1-6 mm) which contain theoretically sufficient reducing carbon. This gave the name self-reducing pellet (abbreviated SRP). The advantage is that the reducing carbon is present as fine particles close to the fine particle of metal oxides in the SRP, which increases the reaction velocity and reduces the necessity of excess coke feed. Consequently, the specific coke consumption could be reduced and the flue-gas volume was reduced, which was used to increase the throughput for a given plant. The standard improvement was 20-30% reduction in coke consumption and % increase of throughput. These improvements could be topped by the intervention of the SDHL Waelz process [2] in Germany in 1998, which unfortunately generates a non-leaching resistant slag (see below). Table 3: Improvement in Specific Coke Consumption 4

5 Standard SRP SDHL [kg coke /DMT.D ] Economy, Reducing Agent Even the intervention of SRP improved the coke consumption the variable costs for reducing agent are still the most important block (30-40%) in a Waelz plant. Based on the origin of the process the material of first choice was coke breeze, which is the fine fraction (0-12 mm) of metallurgical coke. It s cheaper than the lumpy metallurgical coke, but the prices increased significantly over the time. Two alternative materials could be found: fine anthracite coal (6-13 mm) with low ash content and PET coke, which is a by-product of the petrochemical industry. The use of the later material is only possible when integrated in the SRP. PET coke sometimes contains a high sulfur content of 6-9%, which limits its application. But not for the Waelz process: up to 75% of the sulfur intake (no matter whether it comes with the dust or the reducing agent) is bound in the slag, mainly as CaS. A part of the volatized sulfur is used for the fuming of lead as PbS, which is later oxidized to PbSO 4 and captured with the WZO in the bag house filter. The remaining part is leaving the rotary kiln with the flue-gas as SO 2. But this SO 2 is not emitted to the atmosphere because the fine particles of ZnO are an excellent adsorbent for SO 2 (and H 2 S) and ZnO represents 75% of material in the WZO layer on the surface of the filter bags. Finally, the SO 2 ends as ZnSO 4 in the WZO. Ecology, Atmospheric Emission In recent years also the off-gas treatment of Waelz plant has been steadily improved. The simple installation of evaporation cooling unit and hot electrostatic precipitator has been replaced by sophisticated cooling equipment with bag house filter and 2-stage dioxin / mercury removal equipment [3, 4]. The state-of-the-art Waelz plant fulfills the strictest requirements of the German TA-Luft. Consequently, it is notified as Best Available Technology and listed in the BREF-Notes of the European Community. The following table shows typical emission figures: Table 4: Atmospheric Emission of a Waelz Plant Item State-of-the-Art TA-Luft Dust < 4 mg/m³ (5 mg/m³) SO 2 < 50 mg/m³ (350 mg/m³) NO x < 10/50 mg/m³ (350 mg/m³) HCl < 5 mg/m³ (30 mg/m³) HF < 1 mg/m³ (3 mg/m³) Hg < 20 µg/m³ (50 µg/m³) PCDD/F < 0.1 ng/m³ (0.4 / 0.1 ng/m³) Ecology, Impact on Water 5

6 A further ecological aspect is the impact on the water. A Waelz plant without washing unit operates free of wastewater. It consumes industrial water for the pelletizing unit (SRP) and for evaporation cooling. The shower water and the laundry water are used as grey water with the industrial water. Consequently, only some sewage water from the toilets is released. The acidic run Waelz plants generate a very stable slag, which is suitable for road construction application. The US plants run without the addition of any fluxes (lime/sand) in a neutral to slightly basic mode. In both cases (acidic and neutral ), the remaining heavy metals are embedded in a silicate matrix. It is recurrently checked for its leaching behavior (TCLP-test). Typical results are shown in the following table. For some elements the limits of the German drinking water regulation are given for comparison. Table 5: Leaching of WIP (Slag) Item Typical German Drinking Water Reg. ph CSB < 15 mg/l Cl - < 10 mg/l 2- SO 4 < 50 mg/l Pb < 0.05 mg/l 0.04 mg/l Zn < 0.05 mg/l Cr < 0.05 mg/l 0.05 mg/l Cd < mg/l mg/l Hg < mg/l mg/l In addition, the acidic Waelz slag passes the eco-toxidity test of UN-H14. Therefore, it can be applied without restriction in road construction, normally as drainage layer and for leveling. Several Waelz plants apply the so-called basic process, in which significant addition of lime to the feed increases the basicity (ratio of CaO to SiO 2 ) of the WIP to 3.0 or more. This leads to an excess of lime in the slag with the consequence that the ph value of the leaching test increases to 11 and lead is leached out. That restricts the application of the slag to construction material in landfills with leachate treatment facility. 4. GSD s Layout In its newly designed Waelz plants GSD combines the experiences and the process knowledge of the best American plant according to the Best Demonstrated Technology as shown in the following photo with the Best Available Technique of European plants, which is also applied in Taiwan. 6

7 Figure 3 American Best Demonstrated Technology Waelz Plant The highlights are: Dry WIP (slag) cooling in a rotary cooling drum, using the latent heat for pre-heating of the process air; Effective sealing of rotating cooling drum and Waelz kiln for avoiding any diffuse emission of process gas and zinc fumes; Special, CFD optimized Dust Settling Chamber for improved separation of impure carry-over, optimized CO post-combustion and effective evaporation cooling by twophase water injection; SRP production of all dust in intensive mixer with recycling of DSC carry-over to the mixer for homogeneous feed preparation and improved WZO quality; Avoiding of diffuse dust emission by closing and exhausting of all handling buildings for raw and intermediate materials and final products, all external belt conveyors in closed pipes and exhausted; Working areas of DSC and gas treatment also enclosed and exhausted for avoiding diffuse dust emission; 7

8 Two-stage bag house filter with special surface treated fabrics and low air-to-cloth-ratio for effective filtration and integrated dioxin and mercury adsorption on AC/HOK; Extensive application of process control techniques and process visualization for online process management; Wastewater free process, expediting of WZO in big-bags or silo trucks; Application of neutral Waelz operation mode for minimizing flux consumption and producing WIP with high iron content and low leachability of heavy metals. The result of our efforts will be an excellent performance as shown in the following table and an environmental friendly operation. Spec. Consumption [1/DMT.D ] Table 6 - Performance Data of Waelz Process Standard GSD Coke Breeze kg kg Anthracite Coal (Alternative to Coke) PET-Coke ( kg) 100 kg Fluxes (Lime/Sand) kg 0 20 kg Electricity kwh < 150 kwh Natural Gas (or other Combustible) kwh kwh Spec. Production WZO * ) kg kg [1/DMT.D ] WIP (slag) * ) kg kg Zinc Yield % % Zinc Content in WZO % % * ) Depending on Zn content in dust Sources: [1] Juergen Ruetten, Ron Crittendon, Sustainable Recycling of Steel Mill Dust Waelz Process vs. other Technologies, SEAISI conference, Bali, May 2012 [2] Klaus Mager etal., Recovery of Zinc Oxide from Secondary Raw Materials New Developments of the Waelz Process, 4 th Int l Symposium on Recycling of Metals and Engineered Materials, edited by D.L. Steward, R.L. Stephens and J.C. Daley, TMS 2000, p [3] Juergen Ruetten, Waste Gas Cleaning for the Processing of Zinc-Containing Residues in a Waelz Rotary Kiln Process, Pollutant Control Symposium, Alicante, September

9 [4] Klaus Mager, Urban Meurer, Juergen Wirling, Minimizing Dioxin and Furan Emissions during Zinc Dust Recycle by the Waelz Process, JOM, August 2003, p