MXPA97008321A - Process to reduce powders in electric steel structures and installation for implement - Google Patents

Abstract

The present invention relates to a process for processing powders of electric steel structures and carrying zinc, high iron content, oxidized materials in a low frequency induction furnace of the coreless type, resulting in: - reduction of oxide contents from iron to cast iron, -concentration of non-ferrous metal oxides in vapors recovered from the process, -formation of slag suitable to be disposed in normal tanks. The bath of molten iron, maintained at high temperature and circulated quickly by the induced currents which flow through it, constitute the "reduction section" of the furnace, within which the reduction of metal oxides which constitute the Feeding: Zn, Pb, Cd, Fe. The coating layer of the bath, occupied by the feed supply (granulated powders of steel structures, granular carbon, slag forming agents) constitutes the oxidation section of the furnace within which it burns coal producing CO, and vapors of Zn and other metals from the supply of food leaving the bath, are oxidized again and return to the vapors. An appropriate vapor collection unit collects powders and recoveries of concentrated Zn, Pb, Cd more than twice as high levels relative to their initial concentrations at the start of the process.

Description

PROCESS TO REDUCE DUSTS IN ELECTRIC STEEL STRUCTURES AND INSTALLATION TO IMPLEMENT IT DESCRIPTION OF THE INVENTION It is well known that the melting of iron fragments in an electric furnace causes 10-20 kg of powder to be formed per ton of steel produced. This lightweight, fine and easily dispersible powder (referred to as "EAF") contains, as its oxides, 20-25% Fe, 18-25% Zn, 2-4% Pb, and still other impurities more or less dangerous. Considering the size of the steel structures present, the amount of powder produced is extremely large and, due to health, environmental and economic reasons, a process is urgently required to recover and render valuable metals inert. The first solution tried was the recycling of the powders in the same furnace which produces them. By this form, iron can be recovered and oxides of non-ferrous metals can be gradually concentrated in the vapors, but great difficulties were experienced in the steel structures very soon, in the furnace that loads and collects the vapors, the energy consumptions were be large, and the capacity of the furnaces decreased, in this way it is contemplated that the method is not desirable for economic and, above all, environmental reasons. At present, several thermal processes for environmental reduction are used to process the EAF powders, and are carried out in rotary kilns, forge ovens, plasma ovens and flame kilns. The most widely known and widespread process is the Waelz-Berzelius process in which the powders are mixed with fine carbon, gypsum and silica and the resulting mixture is fed to a large rotary kiln. The reduction takes place in that portion of the furnace in which temperatures of 1200 ° C are reached: Zn and Pb change to vapors and are recovered as powders in units of filtrations of vapors and condensers, while the iron oxide, reacting with the Slag forming agents produce slag which can not always be disposed of as an inert material. In the oxides of Waelz the Zn, Pb reaches levels of concentration of 60% and, respectively 10%: therefore, this material is suitable for recovery of both metals. The HTR process, conceptually similar to the previous one, achieves a certain energy saving by feeding the dust to that portion of the furnace in which temperatures of 1400 ° C are reached: it does not reduce FeO and change to slag.

The Plasmadust process (Swedish) uses a non-transferred arc plasma, which causes a gas to flow through the discharge of an electric arc and keeps burning between two electrodes installed inside the nozzles of a metallurgical furnace fed in the top with coke flowing from the top down. Coal-granulated powders and fluxes are injected into the flame of the plasma torch. The oxides of iron are reduced to molten iron, Zn and Pb are reduced and volatilized, and are collected in an external condenser. The lime and silica react with other powdered components, changing them to slag. At present, this process is mainly used to process powders from electric ovens used to produce stainless steel. Also the process of Inmetco (USA) is preferably used to process the stainless steel powders that produce electric ovens. A rotary table oven is used inside which is subjected to a first reduction to a mixture of powders with coal and coke fine, granulated: the Zn and Pb are concentrated in the vapors of this oven. The pre-reduced granules which contain all the iron, together with fragments and oxide layers of the lamination, are charged to an electric submerged arc furnace to obtain an iron alloy with Cr, Ni and Mo contained in the original powders. The St. Joe reactor is a vertical steel furnace with a water cover subdivided into two stages. The burner, fed with coke powder, is fed with air enriched with oxygen, in this way a flame is generated particularly at high temperature (approximately at 2000 ° C). The metallurgical load is injected pneumatically to the flame reduction region. The Zn, Pb and Cd, are reduced, evaporated and collected in a bag or bag filter. The slag is granulated with high iron content and can be sold to cement industries (in order to add iron to the cement mixture), or it is used as raw material for blast furnaces. The Tetronics process by British Steel Co. uses a transferred arc plasma furnace. A plasma torch is installed on top of the furnace and, in order to distribute the energy to the bath, it can rotate with a variable inclination towards the vertical axis. The vapors of the stainless steel mixed with 28% anthracite are continuously fed to the furnace (with a feed rate of 500 kg / h) at a constant temperature of approximately 2000 ° C. The Cr, Ni, Mo is recovered as iron alloys. The exhaust gases are removed from the dust inside the bag filters. Also the EAF powders containing 18% Zn are tested, obtaining ZnO concentrations up to 60%.

Also the Kaldo process for Boliden developed for lead-bearing waste, and the process for forming slag fumes, very well known for recovery of Zn and Pb from silica from wind kiln slag, can be taken into account as represents a possible solution for the processing of EAF powders. In the vertical electrothermal furnace developed by St. Joe Minerals, studied for zinc minerals, EAF powders can be charged and sintered by mixing them with the residue of calcined sphalerites. The resulting sintered product then flows through a rotary preheating furnace and then enters the electrothermal furnace from the top. The coke feeds the furnace with energy and creates a conductive region by electrical energy supplied by means of graphite electrodes. Under the operating conditions of the furnace, the Zn is volatilized. The vapors are sent to bubble through a cold Zn bath that acts as a condenser. The feed must not contain less than 40% Zn. The amount of EAF powders which can be processed in this process is limited by the large amounts of impurities it contains. Finally, the Michigan Technological University tried to add to the dome furnace, rusted waste granules along with cast iron and fragments. Operating at 1510-1538 ° C, the slag is formed and reduced in iron and recovered in the liquid state, while Zn and Pb are volatilized and recovered as unpurified zinc oxide. It could be considered that the granulated oxides represent only 5% of the load fed to the dome furnace. The facilities for carrying out the aforementioned process require high investment costs and the resulting financial expenses are such that they can be borne exclusively by the primary steel mills or consortiums. The metals produced must be reprocessed in order to exploit them at a commercial level. Only some of the above processes produce adequate slag to be disposed in normal tanks. The purposes of the present invention are to provide a direct and simple process for: recovering, in the metal form, the iron contained in the EAF powder; - separating and concentrating, without loss, the oxides of Zn and Pb and other impurities, whether metallic or non-metallic (Cd, F, Cl and others); - Remove all other components of the powder, send them to form a suitable slag to be disposed in normal tanks, that is, with non-toxic or non-harmful character.

According to the present invention, in order to achieve the above purposes, an installation which is suitable for small-medium production speeds and possibly known to those skilled in the steelmaking industry should be used. The energy and operation costs must be limited and, on the other hand, competitive with those processes currently used for EAF powder processing. In order to achieve such purposes, the present invention proposes a process for processing powders of electric steel structures and materials that carry zinc, highly oxidized iron, mainly in order to recover the iron and zinc thereof, characterized in that the powders are fed to an induction furnace only partially filled with a load of metal or alloy, such as, usually, molten iron, to be melted, such as a load which is in the molten state as a bath under turbulent conditions within the oven due to the effect of the induced currents, with the powders that in this way make contact with the free surface of the turbulent foundry bath inside the furnace, with zinc and iron oxides reduction reactions contained in the powders that take place consequently within from the oven. The present invention is characterized first of all by the means selected to carry out the reduction / oxidation reaction necessary to rationally process EAF powders and oxidized materials containing zinc and high iron content., which are strongly penalized if the classic processes of zinc production should be followed. The low frequency induction furnace of the coreless type, which is selected according to the present invention, has been known for many years in the metallurgy of non-ferrous metals and secondary steels as a fast and effective casting medium. According to the known technique, such a furnace is used only as a foundry medium in which the load in all known cases is constituted by coalesced masses of powder or fragments that provide high yields of metals, variable according to the metal or alloy to be melted For example, in the case of the production of molten iron, a charge can be taken into account as standard, as follows: Coalesced fragments or masses of clean molten iron powder 95% Master alloys 2-3% Slag forming agents 1-2% According to the prior art, only the low frequency induction furnace is used as a foundry means to prepare liquid metal for melting.

Otherwise, the present invention uses, for the first time, the low-frequency induction furnace not as a melting medium, but as an apparatus for simultaneously carrying out the reduction and oxidation reactions and therefore proposes to be charged , for example, of powder granules that typically have the following composition: ZnO 16-24% FeO 18-20% MnO 2-4% PbO 4-6% CaO 6-8% Si? 2 4-5% S 0.5- 1% F 0.5-1% Cl 0.5-2% where all metals are in their oxidized forms. This surprising application according to the present invention is made possible by the characterization aspect that the furnace, before initiating loading of the powder granules, is filled to approximately half the height typically with molten iron, which constitutes the "heel" for the reaction. The high intensity flow of induced currents heats the bath to 1450-1500 ° C and keeps it vigorously stirred. Now, the granules, preferably dried and preheated, are charged to the kiln mixed with 14% granular carbon, with small amounts of aggregate slag-forming agents. The reduction of the oxidized compounds to the Zn metal takes place in the region of contact between the surface of the molten iron bath and the hottest layer of the lower part of charged granules. The rapid and continuous renewal of the cast iron layer which moistens the ZnO and FeZn? 4 contained in the granules, provokes the reaction with the carbon contained in the molten iron: ZnO + C (Fe)? Zn + Co O) for proceed with a considerably high speed. In turn, the bath, due to its contact with the carbon granules contained in the powder, re-establishes the original level C of molten iron. The Zn produced by the reaction (1), due to the high temperature of the reaction zone, vaporizes and rises through the powder oxide layers, effectively reducing the iron oxides: FeO + Zn? Fe + Zn (2) The process takes place in two stages and in two different zones of the oven: I I - a reductive stage, in which the active agent is coal contained in the molten iron, which is kept moving rapidly by the inductive phenomenon (addressing). The area of interest is the bath / pellet contact region, which is the hottest region in the furnace and a large amount of energy must be supplied; - an oxidative stage, which takes place inside the powders, in which the mixed coal is burnt producing Co and generating the necessary energy to maintain the high temperature value and allow the oxides of iron to oxidize the vapors of Zn that rise from the bottom. During the course of the process, the quality of the slag formed must be carefully monitored, and, if necessary, modified possibly with an adequate flow to flow. Also the volatilization of Pb must be adequately assisted, adding small amounts of CaCl 2 in order to cause the low boiling of PbCl 2 to be formed by the reaction of calcium chloride added with PbO. A further ot of the present invention is an installation for implementing the process described above, the installation comprising an induction furnace for such use as provided by the process according to the same invention.

An installation according to the present invention is described schematically with reference to the figure of the accompanying drawings. According to such figure, with (10) a low-frequency induction furnace of the non-core type is shown, within which, with (11), a molten cast iron charge is shown schematically and, as will be seen, only Partially fills the oven. Such a load is consequently presented as a molten bath under turbulent conditions within the furnace due to the effect of the induced high current currents typical of the induction furnace. Powders of electric steel structures, rich in zinc oxide and iron, are fed to the furnace (10) through the inlet (12), and they are then forced to flow along an inclined drum (13), in countercurrent with respect to the flow of hot air coming out of the oven (10). The heat of reaction generated inside the furnace is used in this way to dry and preheat the powders as they flow into the furnace. The non-ferrous metal oxides come out of the furnace introduced by the hot air stream rich in CO. The reaction of CO with the entrained air: CO + 1/2 02? C02 (3) takes place during the passage of the vapors through the inclined drum (13), along which the flow of wet powder granules flows downwards, in countercurrent with respect to the vapors. After they exit the drum (13), the exhaust gases undergo a first cooling by the addition of air in (14) and enter a dust extractor (15) in which the thicker and heavier components are removed. The complete dust removal takes place inside a bag filter (16) of the "jet impulse" type, installed upstream of a chimney (1 7). Non-limiting examples of a process according to the present invention are now reported.

EXAMPLE 1 Processing an EAF powder with the normal composition (for example: a powder similar to one described in the following Example 2) according to the present invention, the following is obtained: 25-27% Unpurified cast iron 34-36% Powders recovered from the vapors (50- 60% of Zn) 34-36% Slag in the composition of which, according to the international release test, falls within the limits of Table "A" of the test.

Inside a low frequency Induction furnace of the non-core type with a capacity of 900 I, a t / h of granulated EAF powder can be processed, with the following consumptions: Electric power 1000 kW / h Coal 130 kg CaCl 2 4 kg CaF2 4 kg 02 130 m3 CH4 20 Nm3 Material 1 refractory 6 kg The above consumptions include all subsidiary operations (vapor filtration, granulation air, and the like).

Example 2 They are charged to a low frequency furnace of the non-core type with a power of 150 kW and with a capacity of 750 kg of cast iron, with a refractory alumina magnesite jacket, 350 kg of cast iron to be the heel of the reaction. The bath is heated to 1450-1500 ° C and then the powder feed is started, consisting of 150 kg of preheated dry granules, and 20 kg of carbon granules. The average composition of granulated powders is: Zn 21.5%; Pb 5.6%; Fe 27.4%; Mn 0.8%; C 2.85%; Ca 3.30%; S 0.45%.

In order to make the slag flow, 0.1 kg of CaCl2 and 0.3 kg of CaF2 are gradually added through the duration of the test. In order to save electricity, 20 m3 of oxygen is injected. Within one hour after the start of the test, the addition of the filler is completed and the reaction has proceeded to its completion, a fluid slag of the acidic type having a glassy appearance being formed. The complete analysis of the slag is as follows: MgO 1 .66%; CaO 14.43%; ZnO 1.98%; FeO 12.27%; PbO 0.05%; AI2O3 7.74%; Yes? According to the release test, this slag meets the quality requirements of Table "A". The following products are obtained from processing 150 kg of powder: 38.5 kg of cast iron with 0.92% of Mn, 3.6% of C 52.5 kg of oxides with 58.3% of Zn, 15.3% of Pb, 0.44% of Fe 53.0 kg of slag with 1.59% Zn, 0.05% Pb, 9.54% Fe. Totalizing, the following aspects of the invention are valuable, being highlighted: 1) The choice of the induction furnace, which, while it is Well known as a melting furnace, it is used here to carry out the reduction / oxidation reactions. 2) The intense agitation induced in the cast iron bath, in such a way that the reduction of ZnO is favored by the mixed carbon ZnO + C (Fe)? Zn + CO by the continuous renewal of the surface, caused by the intense induced currents that flow through the bath. 3) The reduction of FeO by vapors of Zn emitted from the preceding reaction zone, multiplies the effect of reduction of FeO of the coal contained in the load. 4) The maximum energy exploitation of the available elements reduces the consumption of the process.

Claims (10)

1 . A process for processing powders of electric steel structures and materials containing high content of iron, oxidized zinc mainly for the purpose of recovering iron and zinc from them, characterized in that the powders are fed to an induction furnace only partially filled with a load of metal or alloy, such as, usually, cast iron, to be melted, with such a charge that is contained within the furnace in the molten state as well as a bath under turbulent conditions due to the effect of the induced currents, with the powders that make contact in this way with the free surface of the turbulent molten bath inside the furnace, with reduction reactions of zinc and iron oxides contained in the powders consequently taking place inside the furnace.

2. The process according to claim 1, characterized in that the charge is constituted by cast iron and the zinc oxide contained in the powder reacts with the carbon contained in the cast iron according to the following reaction: ZnO + C (Fe )? Zn + CO (1).

3. The process according to claim 1, characterized in that the powders are mixed with carbon, preferably in granular form.

4. The process in accordance with the claims 2 and 3, characterized in that the cast iron bath keeps its carbon level constant by being in contact with the coal mixed with the powders.

5. The process according to claim 2, characterized in that the zinc metal produced in the reaction (1) vaporizes and reacts with the iron oxide contained in the powders according to the reaction: FeO + Zn? Fe + Zn (2)

6. The process according to claim 1, characterized in that the powders are fed to the furnace after they are dried and preheated.

7. The process according to claim 1, characterized in that the iron and zinc are commonly recovered as cast iron and zinc oxide, respectively.

8. The use of an induction furnace for the process according to one or more of the preceding claims.

9. The installation for processing the powders from electric steel structures and materials containing zinc and high oxidized iron contents mainly for the purpose of recovering iron and zinc from them, characterized in that it comprises an induction furnace in which they are The powders are fed to the free surface of a molten iron bath with which the furnace is partially filled.

10. The installation according to claim 9, characterized in that it comprises an inclined drum in order to cause the powders to be fed to the furnace to flow in countercurrent with respect to the hot air leaving the furnace, a dust extractor to cause the sediments to settle. heavier components, a filter for dust collection in which the materials to be recovered are separated. SUMMARY OF THE INVENTION The present invention relates to a process for processing powders of electric steel structures and carrying zinc, high iron content, oxidized materials in a low frequency induction furnace of the coreless type, resulting in: - reduction of contents of iron oxide to molten iron, - concentration of non-ferrous metal oxides in vapors recovered from the process, - formation of slag suitable to be disposed in normal tanks. The molten iron bath, maintained at high temperature and rapidly circulated by the induced currents which flow through it, constitutes the "reduction section" of the furnace, within which the reduction of the metal oxides which constitute Feeding: Zn, Pb, Cd, Fe. The coating layer of the bath, occupied by the feed supply (granulated powders of steel structures, granular carbon, slag forming agents) constitutes the oxidation section of the furnace inside which coal is burnt producing CO, and vapors of Zn and other metals from the feed supply leaving the bath, are oxidized again and return to the vapors . An appropriate vapor collection unit collects the powders and recovers oxides of concentrated Zn, Pb, Cd more than twice as high levels relative to their initial concentrations at the beginning of the process.