Fluidized bed treatment of EAF dust

Details Fluidized bed treatment of EAF dust Fluidized bed treatment of EAF dust

1998-11-16

2001-04-24

Andrews, Melvyn (Department: 1742)

Specialized metallurgical processes, compositions for use therei

Processes

Electrothermic processes

C075S961000

Reexamination Certificate

active

06221124

ABSTRACT:

BACKGROUND TO THE INVENTION
This invention relates to the treatment of electric arc furnace (EAF) dust.
In general, for every ton of liquid steel that is produced in mini mills, i.e. steel mills with a capacity of the order of one million tons of liquid steel per annum, approximately 15 kg of EAF dust is produced. It is estimated that in excess of three million tons of EAF dust is produced worldwide each year by carbon steel manufacturers. The EAF dust which is produced is designated by certain authorities, such as the Enviromnmental Protection Agency in the USA, as a hazardous waste material inter alia because of high zinc content. As a result it is necessary to treat the dust rather than merely dump it,
Traditionally, EAF dust is transported to a central treatment facility. One traditional treatment technique is the Waelz Kiln process operated by Horsehead Industries in the USA. However, treatment of the EAF dust in this way presents several serious problems, including the fact that the actual cost of EAF dust treatment is extremely high. This is at least partly because of the high capital cost of treatment facilities operating the process. The high capital cost means that facilities operating the process tend to have high capacities and furthermore that a single, central facility is usually provided to serve steel producers spread out over a large area. Transportation of the dust from the steel mill, often over large distances, contributes to the overall high cost of dust treatment using the process. Added to this, the hazardous nature of the dust, together with its small particle size, typically less than one micron, means that the actual transportation thereof from the steel mill to a treatment facility requires special handling measures, again increasing the overall costs of treatment.
A typical EAF dust may include the following constituents by weight:
ZnO
27%
PbO
2%
Fe
2
O
3
44%
C
6%
Halides
2 to 4%
Cd
220 ppm
A zinc oxide content of this magnitude can be attributable to the smelting of large amounts of galvanised scrap in the steel mill.
An objective of the present invention is to provide a method whereby EAF dust can be treated on site in an economical manner. Given the high Zn and Fe content of the typical EAF dust, another objective of the present invention is to provide for economical recovery thereof.
SUMMARY OF THE INVENTION
According to one aspect of the present invention there is provided a method of treating EAF dust, the method comprising the steps of preheating and decontaminating the dust under conditions which oxidise the magnetite content of the dust to hematite, and thereafter introducing the preheated and decontaminated dust into a fluidised bed reactor in which hematite is reduced, by means of a hot reducing gas generated by reforming natural gas in a non-catalytic plasma-arc heating process, to produce an iron-rich material suitable for recycling to the EAF.
In addition, the method may comprise the step of recovering ZnO fume (i.e. powder) from the fluidised bed reactor, typically in a bag-house/filter system located downstream of the fluidised bed reactor.
Typically, the reducing fluidised bed reactor is operated at a temperature of the order of 800° to 1000° C., preferably 850° to 1000° C., most preferably about 950° C. The hot reducing gas typically comprises a mixture of H
2
and CO. In the preferred method the EAF dust is initially preheated in an oxidising fluidised bed reactor, typically to a temperature of the order of 1000° C., by heat derived from off-gases produced in the reducing fluidised bed reactor.
These gases may, after passing through a water scrubber, be heated in the presence of air before being introduced into the oxidising fluidised bed reactor.
Contaminants, such as halid es and Cd, may be removed from the oxidising fluidised bed reactor through a water scrubber whereafter they may be disposed of.


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Steel Times, “Fluidized Bed Treatment of Steelplant Dust” Mar. 1992, Redhill, Surrey, GB.
Erzmetall, “Process Selection Factors for the Retreatment of Zinc-Bearing Scrap and Residues,” Feb. 1991, Weinheim, DE.

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