MXPA98009916A - Method of treatment through fluidized bed of arco electric oven powder - Google Patents

Abstract

The invention is concerned with a method for treating electric arc furnace powder (EAF). In the method the powder is pre-heated and decontaminated under conditions that oxidize the magnetite content of the powder to hematite. After this, the pre-heated and decontaminated powder is introduced to a fluidized bed reactor in which the hematite is reduced, by means of a hot reducing gas generated by natural reforming gas in a plasma-arc heating process not catalytic, to produce a material rich in iron suitable for recycling to the EAF and a high-grade zinc oxide product

Description

METHOD OF TREATMENT THROUGH FLUIDIZED BED OF ELECTRIC ARC OVEN DUST BACKGROUND OF THE INVENTION This invention is concerned with the treatment of electric arc furnace powder (EAF). In general, for each ton of liquid steel that is produced in mini-factories, that is, steel mills with a capacity of the order of one million tons of liquid steel per year, approximately 15 kg of EAF powder are produced. It is estimated that more than three million tons of EAF powder are produced globally each year by the carbon steel manufacturers. The EAF powder, which is produced, is designated by certain authorities, such as the Environmental Protection Agency in the United States of America, as a hazardous waste material inter alia, due to the high zinc content. As a result, it is necessary to treat the powder instead of just discarding it. Traditionally, the EAF powder is transported to a central treatment facility. A traditional treatment technique is the Kiln Kiln process operated by Horsehead Industries in the United States of America. However, treating EAF powder in this manner presents several serious problems, which include the fact that the actual cost of treating EAF powder REF: 28972 is extremely high. This is at least partially due to the high capital cost of the treatment facilities that put the process into operation. The high cost of capital means that the facilities that put the process into operation tend to have high capacities and also that a single central facility is usually provided to service steel producers dispersed over a large area. Frequently, the transportation of the powder from the steel mill, over long distances, contributes to the high global cost of dust treatment when using the process. In addition to this, the hazardous nature of the dust, together with its small particle size, usually less than one miera, means that the actual transportation of the same from the steel mill to a treatment facility requires special handling measures, which increases again the global treatment costs. A typical EAF powder may include the following constituents by weight: ZnO 27% PbO 2% Fe203 44% c 6% Halides 2 to 4% Cd 220 ppm A zinc oxide content of this magnitude may be attributable to large melting amounts of scrap or galvanized waste in the steel mill. An object of the present invention is to provide a method by which the EAF powder can be treated at the site in an economical manner. Given the high zinc and iron content of the typical EAF powder, another objective of the present invention is to provide economic recovery thereof.

BRIEF DESCRIPTION OF THE INVENTION In accordance with one aspect of the present invention, there is provided a method for treating the powder of EAF, the method comprises the steps of preheating and decontaminating the powder under conditions that oxidize the magnetite content of the powder to hematite and after that introduce the pre-heated and decontaminated powder to a fluidized bed reactor in which the hematite is reduced, by means of a hot reducing gas, generated by reforming the natural gas in a non-catalytic plasma-arc heating process, to produce a material rich in iron, suitable for recycling to the EAF. In addition, the method may comprise the step of recovering ZnO fumes (ie, dust) from the fluidized bed reactor, typically in a bag filter bag system located downstream of the fluidized bed reactor. Typically, the reducing fluidized bed reactor is operated at a temperature of the order of 800 ° to 1000 ° C, preferably 850 ° to 1000 ° C more preferably 950 ° C. The hot reducing gas normally comprises a mixture of H2 and CO. In the preferred method, the EAF powder is initially pre-heated in an oxidant fluidized bed reactor, usually at a temperature of the order of 1000 ° C, by the heat derived from release gases produced in the reducing fluidized bed reactor. These gases can, after passing through a water scrubber, be heated in the presence of air before being introduced to the oxidant fluidized bed reactor. Contaminants, such as halides and Cd, can be removed from the oxidant fluidized bed reactor through a water scrubber after which they can be discarded.

BRIEF DESCRIPTION OF THE DRAWING The invention will now be described in more detail by way of example only with reference to the accompanying flow chart (Fig. 1) illustrating the steps of an EAF powder treatment method according to the invention.

Description of a preferred embodiment In the first step in the method represented by the accompanying flowchart, the EAF powder produced from an EAF steel plant is introduced into a first fluidized bed reactor 10, as indicated by the number 12. In reactor 10, the EAF powder is pre-heated by the release gases produced in the second fluidized bed reactor, as described hereinafter. The release gases are passed through a burner 14 fed with air 16 before being introduced to the fluidized bed reactor 10 at a temperature sufficient to pre-heat the EAF powder to a temperature of the order of 1000 ° C. Under the oxidizing and high temperature conditions prevailing in the reactor 10, the magnetite content of the EAF powder is oxidized to hematite by the following reaction: 2Fe30 + ΔI 02? 3Fe203 The gases produced in the reactor 10 are extracted through a water scrubber 18 in which the important contaminants normally present in the EAF powder, such as halides and Cd, are separated. The rest of the gas is expelled into the atmosphere through a chimney as indicated by the arrow 20. Then, the pre-heated and decontaminated powder is introduced into a second fluidized-bed reactor 22 as indicated by arrow 24. The second fluidized bed reactor is fed with hot reducing gas along the line 26. The hot reducing gas produced by reforming natural gas 28, together with steam, in a noncatalytic plasma-arc reformer 30, energized independently, it comprises a mixture of H2 and CO, usually around -75% H2 and 25% CO. The non-catalytic plasma-arc reformer 30 is put into operation at a temperature sufficient for the production of a hot reducing gas, which normally has a temperature of about 950 ° C, but generally in the range of 800 ° C to 1000. ° C and preferably in the range of 850 ° to 1000 ° C. In practice, the non-catalytic plasma-arc reforming process carried out in the reformer 30 makes use of a plasma heating torch incorporating an anode and cathode, which is connected to the reformer's reaction chamber by means of a sliding valve. The natural gas (CH4) is reformed stoichiometrically together with the steam at the very high temperatures generated by the torch, usually of the order of 15000 ° C, in the absence of catalyst according to the following reaction: CH4 + H20? CO + 3H2 In reactor 22, the hematite is reduced to a product 32 rich in iron, that is, direct reduced iron (DRI) according to the following reaction: 2Fe203 + 3H2 + 3C0? 4Fe + 3H20 + 3C02 The DRI is suitable for recycling to the EAF of the steel plant. Thus, the iron content of the EAF is usefully recovered. The zinc oxide (ZnO) and lead oxide (PbO) in the EAF dust are reduced by H2 and CO to metal but, importantly, under the operating conditions prevailing in reactor 22, zinc and lead are in vapor phase and are transported by the downstream fluidizing gas to a cooler 34. At a temperature of about 800 ° C and below, the zinc reacts again with the oxygen present to form zinc oxide (ZnO). It is considered undesirable that the last reaction be carried out in reactor 22, because this would lead to the possibility of mixing between the iron-rich material and the ZnO smoke which could in turn result in low recoveries of the ZnO fumes and that the iron is contaminated with ZnO.

Cooling in cooler 34 at temperatures of 800 ° C and below results in the oxidation of zinc and lead to form zinc oxide rich fumes (ZnO) that are recovered in a bag filter system 36. Smoke 38 It is a powder rich in high grade zinc oxide, with a low halide content. These fumes are suitable for sale to producers of primary electrolytic zinc. The slurry gases produced by the reactor 22 are washed in a water scrubber 40 and thereafter heated in the burner 14, as described above, for the purposes of preheating and decontaminating the raw EAF powder under the oxidizing conditions. in reactor 10. It will be noted that the raw EAF powder is effectively converted to produce two useful products, ie, a smoke rich in marketable zinc oxide (ZnO), and an iron-rich material which can be used in the EAF steel plant, with little or no waste. In addition to the fact that the EAF powder is converted to produce useful products there is the additional advantage that the process of the invention can be carried out economically in situ in the steel plant itself.

EXAMPLE The following example describes a laboratory-scale test carried out at Procedyne Corporation of New Jersey, USA using a 60 cm Inconel fluidized bed reactor of 15 cm (6 inches) in diameter. The EAF powder used as feedstock is provided by North Star Steel of Michigan, USA. The product samples are analyzed by Mintek of South Africa. The overall objective of the test was to convert the EAF powder to a high-grade zinc oxide product and iron-rich material without generating substantial waste products. A specific objective of the test was to produce a high-grade dehalogenated zinc oxide product with a zinc oxide content of greater than 80% by weight and a material rich in metallized iron at a level of at least 90% by weight. The reducing fluidized bed reactor produces a material having the following properties:% ZnO (by weight) - 88% Fe203 (by weight) - 7.8% PbO (by weight) - 0.05 It will be seen that the specific objective of an oxide content Zinc greater than 80% is obtained successfully. The product of the process can therefore be considered as a super high grade zinc oxide product.

The metallization, this is the production of metallic iron, at 98% levels is also obtained in the test. In addition, dehalogenation, that is, extraction of chlorine, is obtained at levels greater than 90%. The test results clearly indicate that the EAF powder can be successfully converted to produce a high grade zinc oxide product suitable for sale, for example for zinc smelters for the production of zinc metal and iron-rich product, appropriate for recycling to the EAF itself or to other steel producers. It is believed that the success of the method of the invention is largely attributable to the use of the hot reducing gas, generated by natural reforming gas in a noncatalytic plasma-arc reformer in the reducing fluidized bed reactor. It is noted that, in relation to this date, the best method known to the applicant to carry out the aforementioned invention, is that which is clear from the present description of the invention. Having described the invention as above, property is claimed as contained in the following:

Claims (10)

1. Claims 1. A method of EAF powder treatment (electric arc furnace), the method is characterized in that it comprises the steps of preheating and decontaminating the powder under conditions that oxidize the magnetite content of the powder to hematite and after that , introducing the pre-heated and decontaminated powder into a fluidized bed reactor in which the hematite is reduced, by means of a hot reducing gas, generated by natural reforming gas in a non-catalytic plasma-arc heating process to produce a material rich in iron suitable for recycling to the EAF (electric arc furnace). The method according to claim 1, characterized in that it also comprises the step of recovering ZnO-rich fumes from the product of the fluidized-bed reactor. The method according to claim 1 or claim 2, characterized in that the reducing fluidized bed reactor is operated at a temperature in the range of 800 ° to 1000 ° C. 4. The method according to claim 3, characterized in that the reducing fluidized bed reactor is operated at a temperature in the range of 800 ° to 1000 ° C. 5. The method according to claim 4, characterized in that the reducing fluidized bed reactor is operated at a temperature of approximately 950"C. The method according to any of the preceding claims, characterized in that the hot reducing gas comprises a mixture of H2 and CO. The method according to any of the preceding claims, characterized in that the EAF powder (electric arc furnace) is initially pre-heated in an oxidant fluidized bed reactor, by means of the heat derived from release gases produced in the reducing fluidized-bed reactor 8. The method according to claim 7, characterized in that the EAF powder is pre-heated to a temperature of the order of 1000 ° C. according to claim 7 or claim 8, characterized in that the release gases, after passing through a scrubber of a They are heated in the presence of air before being introduced to the oxidant fluidized bed reactor. The method according to any of the preceding claims, characterized in that the contaminants, such as halides and cadmium are separated from the oxidant fluidized bed reactor by means of a water scrubber and thereafter discarded.