Zinc-coated galvanized steel scrap is the most widely used feedstock for Electric Arc Furnace (EAF) steel production. World crude steel production reached 1,66 billion metric tons for the year 2014, up by 3.9% compared to 2013 (World Steel Association). Around 26% of crude steel is produced using the EAF technology.
During the EAF process, the high temperatures required to melt the feed material produce a zinc byproduct (EAF dust) that leaves the furnace along with the off-gases. The gases are filtered and the EAF dust is collected in bag houses.
The EAF dust waste steam is considered a hazardous material due to the potential for the heavy metals to leach into the ground, contaminating ground water and sewage systems.
The EAF dust generated by steel mills can be recycled in an economically viable way using the Waelz Kiln technology. The recycling process is part of the larger steel industry loop that allows the reuse of two important natural resources - zinc and iron.
Energy efficient and clean, the Waelz Kiln technology is especially suited to the increasing demands of environmental laws being implemented globally.
(1) EAF steel dust is transported by road or rail to a steel dust recycling plant.
(2) The feed materials are provided in either dry compact-moist or pelletized form. The compact and pelletized moist materials, coarse fluxes and coke are stored in boxes while the dry dust is transported by pneumatic conveying systems to raw material silos.
(3) Typically 55 meters long with a four-meter diameter, the Waelz Kiln is slightly inclined 2-3% and has an approximate rotary speed of 1.0 rpm. As the moist material moves through the rotary kiln, it is dried and preheated by the kiln gas, flowing in a counter-current pattern. In the reaction zone, reduction of the metal oxides occurs around 1200°C as the zinc is vaporized as metal vapor into the gas cleaning system. Zinc is then re-oxidized in the kiln atmosphere that is operated with excess air (process air is injected from the kiln end). Chlorine and alkalis volatilize jointly with the heavy metals and the dust-laden off-gas is treated in a downstream off-gas system.
(4B) In the first stage, coarse particles mechanically entrained are separated in a dust settling chamber and returned to the kiln. The hot, dust-laden off-gas is cooled and Waelz Zinc Oxide is separated in a high efficiency collector.
(4A) The co-product, Waelz Iron Product, can be used directly in road construction, civil construction (backfilling), cement manufacturing or can be reprocessed to add more value to the iron content which is currently under research and development.
(5) The dust-free off-gas is then treated, filtered and discharged.
(6) Waelz Zinc Oxide, the main product of the process, is delivered to zinc smelters for zinc production.