EP2569455A1

EP2569455A1 - Chemical pretreatment and preheating of steel scrap

Info

Publication number: EP2569455A1
Application number: EP10720547A
Authority: EP
Inventors: Hans-Bernd Pillkahn
Original assignee: Proassort GmbH
Current assignee: PROASSORT GMBH
Priority date: 2010-05-10
Filing date: 2010-05-10
Publication date: 2013-03-20
Also published as: WO2011141036A1

Abstract

The invention relates to a method for reprocessing steel scrap, wherein the steel scrap is preheated with the aid of exhaust gases arising in a melting furnace. In order to prevent dioxins, furans, or other problematic organic substances from forming in the exhaust gases during the preheating of the steel scrap, the steel scrap is pretreated before the preheating by introducing the steel scrap into an alkaline and/or acidic solution. At the same time, the components of the steel scrap responsible for the formation of dioxins and furans in the exhaust gas are removed.

Description

CHEMICAL PRE-TREATMENT AND PREHEATING OF STEEL SCRAP

The invention relates to a method for processing steel scrap with metallic coatings and / or organic coatings for steel and cast iron production.

The steel and cast iron production takes place today primarily by means of blow molding processes, in particular the LD process, via cupola furnaces and via electrical steel processes. The trend is towards electrical steel production with electric arc furnaces and induction shaft furnaces, which can also be driven, and in particular with 100% scrap iron. In this process, electrical energy is used to melt the steel scrap used. In electric arc furnaces, the heat is generated by arcs between one or more electrodes and the feedstock. Both AC electric arc furnaces and DC electric arc furnaces are known. Alternatively, the heat can be generated by induction as in shaft furnaces. Also known or under development are natural gas-fired furnaces for smelting steel secondary raw materials (Schratten), so-called Primary Energy Melter (PEM).

In the electric steel process, steel or cast iron is melted from steel scrap and possibly other feedstocks such as sponge iron and alloying elements. Above the liquid steel, a slag layer is formed in which unwanted components are collected for the steel. At the same time, the slag layer closes the melt upwards and thus protects it from the atmosphere.

CONFIRMATION COPY In all of the aforementioned methods of steel and cast iron production, the high calorific waste gas can be used to preheat scrap. In the LD process, which today according to the state of the art melts between 20% and 30% of the amount of crude steel via scrap additive, the aim of scrap preheating is to increase the scrap utilization rate and to reduce CO ₂ emissions through reduced use of pig iron. Also in electric steel production, the exhaust gases can be used to preheat the steel scrap to be melted in the crucible. For this purpose, various constructions, including so-called finger shaft ovens, are known for the electric arc furnaces, in which the scrap is fed to the electric furnace through a shaft, wherein the exhaust gases produced in the electric furnace are passed in countercurrent through the shaft to preheat the steel scrap. The required energy for melting the scrap, which must be supplied by means of an arc or by induction, decreases accordingly. Both from a business as well as from an environmental point of view, a scrap preheating by means of exhaust gas would therefore be advantageous.

However, it has proved to be problematical in this context that, in the case of scrap preheating, organic compounds in the exhaust gas which are harmful to the environment occur to a greater extent. This applies particularly to dioxins and furans, in particular polychlorinated dibenzo-p-dioxins and polychlorinated dibenzofurans (PCDD, PCDF). The reason for the increased occurrence of these substances in the scrap preheating is the cooling of the exhaust gas to a temperature at which the organic components are no longer destroyed.

In order to comply with the statutory exhaust emission limits, an afterburning of the exhaust gases is therefore necessary. For CO afterburning, for example, a heating in the presence of oxygen to at least 600 ° C, for the destruction of other volatile organic substances heating to at least 750 ° C at a residence time of 1, 5 s necessary. Dioxins and furans decompose only at temperatures above 850 ° C, wherein also a residence time of at least 1, 5 s is necessary. Subsequently, the exhaust gas must be cooled as quickly as possible to a temperature of below 250 ° C, otherwise it would lead to a de novo Synthesis of dioxins and furans in a temperature window between about 300 and 800 ° C can come. Overall, the fact that a separate Abgasnachverbrennung is necessary when carrying out a scrap preheating in which the already cooled exhaust gases must be heated to a temperature> 850 ° C, the benefits of scrap preheating in terms of energy balance partially canceled. The energy savings by the Schrottvorwärmung is the exhaust gas afterburning, the z. B. can be performed by supplying natural gas in the exhaust stream, neutralized to a large extent. Although there is a certain energy savings, but this is hardly relevant from a business point of view, which is why currently operated in Germany only one electric arc furnace of a total of 15 producing under scrap preheating with Abgasnachverbrennung.

Overall, therefore, the task is to provide a method for processing steel scrap available, which allows the Schrottvorwärmung without entailing an increased formation of dioxins and furans, which must be eliminated by a complex post-combustion.

This object is achieved by a method for processing steel scrap with the following steps:

Introducing the steel scrap into an alkaline and / or acid solution,

Separating the solution,

Preheating the steel scrap by means of exhaust gases,

- introducing the steel scrap into a melting furnace and

- removing the resulting exhaust gases and using the exhaust gases to preheat further steel scrap. In fact, it has been found that, in the case of an upstream treatment of the steel scrap in acidic or alkaline solution, at the same time harmful deposits and impurities of the steel scrap are removed, which are to a great extent responsible for the formation of dioxins and furans. Such impurities are in particular oils, greases, lacquers, rubber and plastic coatings. These organic deposits can be almost completely removed by the pre-treatment.

The process according to the invention is particularly advantageous when the work-up concerns metal-coated steel scrap, in particular with zinc or tin or hybrid-coated steel scrap. In this case, during the treatment with the alkaline / acidic solution, in addition to the cleaning of organic surface coatings, an at least partial detachment of the applied metal layer takes place. If necessary, this can be recycled directly from the solution, which further increases the overall cost-effectiveness of the process. In principle, however, the method according to the invention can be used for any types of metal-coated and uncoated blades, for example both for production-fresh scraps and for old scraps. However, the material flow should be directed before reprocessing in the sense of treatable Schratten and possibly sorted.

Although individual steps of the method according to the invention from the prior art are already known, but new is the overall process. Of particular importance is the inventive idea that with the removal of the metal layer on the steel scrap not only an already valuable raw material is detached from the steel scrap and recovered, but at the same time the energy balance of steel production is significantly improved, on the one hand a scrap preheating with the aid of exhaust gases is possible, on the other hand, however, can be dispensed with a complex Abgasnachverbrennung to destroy pollutants. Another advantage is that considerably less time is required to produce the molten steel due to the preheating of the scrap, resulting in shorter tapping intervals (tp-to-tap times). This time saving is used to effectively reduce today's performance-enhancing addition of fuels such as natural gas and / or coal and oxygen for the actual smelting process. This results in a significant potential for reducing the CO2 emission of electrical steel production even at the level of electrical steel production itself.

Due to the fact that steel scrap is increasingly metal-coated steel scrap, the process is particularly economical because the released coating can be recovered in metallic form or otherwise used. Overall, by scrap preheating energy savings and a furnace performance increase of up to 25% can be achieved. The energy savings for recovering the metallic recyclables is significantly higher in the proposed method compared to conventional methods.

It makes sense, before the introduction of steel scrap into the alkaline / acidic solution, a mechanical pretreatment and processing of the scrap is carried out in order to increase the contact surfaces to the solution. The mechanical pretreatment can in particular consist in a comminution of the parts, for example by shredding. Other types of roughening and deformation are also suitable. In this way, the surface of the steel scrap is increased and the number of attack points for the solution, which are not yet coated with a passivation layer, increased. From the impurities created by the pretreatment, the detachment of the coatings can continue. In addition, the individual scrap parts are brought to a manageable size. By using mechanically pretreated scrap with bulk density that can be moved and specified within narrow limits, it is possible to noticeably increase the furnace output of an electric furnace. Ovens with such a scrap supply can be operated continuously, in contrast to today's batch batch process.

The detachment of the organic surface coverings and, where appropriate, the metal layer from the steel scrap can be carried out both in alkaline and in acidic solution, where appropriate, a sequentially switched detachment in both media would be conceivable. Detachment in acidic solution has the advantage that it can be carried out faster and at a lower temperature, however, certain amounts of iron also go into solution, which may in some cases have to be separated off during the recovery of the coating metal. However, it has been shown that the presence of oils and fats in acidic solutions has a strong inhibitory action against iron dissolution. In this respect, the inventive partial process steps in the replacement of organic and metallic surface coatings complement each other. It has also been shown that in the presence of a leading amount of zinc in the acidic electrolyte zinc removal accelerated by the scrap takes place and the iron dissolution is prevented. On the other hand, iron is not attacked by an alkaline solution, but alkaline treatment must be carried out at elevated temperature for a prolonged period of time.

Steel is increasingly protected against corrosion by a zinc coating. Already more than half of world production of zinc is used industrially for this purpose. Accordingly, the invention relates in particular to the workup of galvanized steel scrap. The energy savings in the smelting process and the concomitant recovery of the metallic raw materials, especially the zinc, from the surface of the steel scrap are two resource efficiency technologies brought together by the invention.

The alkaline solution for pretreating the steel scrap is typically a solution of sodium or potassium hydroxide, with sodium hydroxide being preferred for reasons of cost. The amount of sodium or potassium hydroxide is advantageously at least 15 wt .-% in order to ensure a sufficiently rapid detachment of a metal layer. It makes sense, moreover, to heat the alkaline solution to a temperature higher than room temperature, for example. Of at least 75 ° C, advantageously at least 85 ° C. In addition, a relative movement of the solution to the steel scrap, for example by stirring or similar measures should be brought about.

In order to further increase the dissolution rate of the metal coating, the detachment of the metal coating can take place as galvanic corrosion, that is, the metal coating serves as the anode at which the oxidation into ions of the metal takes place, while a second metal stable in the galvanic solution acts as the cathode at which hydrogen is evolved. These may be metals having a comparatively low hydrogen overvoltage as disclosed in US 5,302,260, for example, high surface area nickel- and cobalt-based materials, e.g. B. Raney nickel, Raney cobalt, nickel molybdate, nickel sulfides, nickel cobalt thiospinels, etc., or metals with relatively high hydrogen overvoltage, for example, a steel alloy as proposed by EP 0 996 774 B1. Other metals that can be used are platinum, palladium, gold or silver. Reason for the acceleration of the metal layer dissolution in the presence of another metal is the high overvoltage of hydrogen evolution z. As to zinc surfaces and the resulting inhibition of the process. For this reason, it makes sense to bring the metal coating to be dissolved in electrically conductive contact with materials on which the hydrogen evolution can proceed uninhibited and which are not attacked by the alkaline solution used to dissolve the metal coating. Another possibility for accelerating the dissolution of a zinc coating is the addition of iron sulfide, as proposed by EP 1 626 098 A2.

An acceleration of the dissolution of the metal coating can also be done by applying an external voltage to the anode and the cathode. On the one hand in this way, although the energy consumption is increased, on the other hand, especially in cases where the steel scrap is in the form of bundles with hard to reach surfaces, the dissolution rate can be brought to an acceptable level.

A further possibility for increasing the dissolution rate is proposed by EP 0 479 326 B1, namely the addition of an oxidizing agent, in particular sodium nitrate.

Galvanized steel scrap may also be, prior to incorporation into the alkaline solution, at a relatively high temperature above the melting point of zinc of 419 ° C, for example at a temperature of at least 500 or 600 ° C δ

to be brought. The period of time over which the steel scrap is kept at this temperature should preferably be about 10 to 15 minutes. The heating causes the zinc from the zinc coating in the steel and iron diffuses out of the steel into the zinc coating, whereby the electrical contact between the metals increases at the surface, so that the galvanic corrosion resistance of the steel scrap increases.

The period of immersion of galvanized steel scrap in the alkaline solution is typically a maximum of two hours, for example 30 to 60 minutes, but may be shorter if additional measures are used to speed up the dissolution.

When the dissolution of a zinc coating is carried out in acidic solution, a sulfuric acid solution is typically used therefor, which typically contains 100 to 300 g / l, preferably 150 to 250 g / l of sulfuric acid. In comparison to the basic treatment, significantly lower temperatures and a shorter time suffice, for example 20 to 60 ° C., in particular about 40 ° C. At this temperature and using a 200 g / l H ₂ SO ₄ solution, it takes about 3 minutes to contact with deicing steel scrap. Although a certain disadvantage of acid dezincification is that a small proportion of the iron from the steel scrap also goes into solution, at the indicated temperatures and times, however, the undesired iron entry is kept comparatively low. Moreover, the prior art discloses ways of removing the iron from a solution, for example precipitation in the form of the hydroxide, in the form of jarosite, goethite or hematite. The corresponding methods are z. B. from Ullmann's Encyclopedia of Industrial Chemistry, 4th ed., Volume 24, p 602 et seq. Known.

In order to increase the accessible surface of the steel scrap, it is useful to pretreat it mechanically before immersion in the alkaline solution, in particular to crush, roughen and / or deform. In this way it is ensured that there are not too large steel sheets in the solution, in which the detachment of the coatings would take too long a period of time. In addition, the Roughening or deformation that new areas of a metal coating are exposed to the attack of the alkaline solution, which are not yet covered by a passivation layer. From there, the further dissolution process can continue. In addition, a sorting of the resulting steel scrap in the sense of effective material flow management, which is upstream of the process described here, is also sensible. The use of mechanically pretreated steel scrap also enables the continuous charging of the melting furnace.

Before the metal is recovered from the solution by electrolysis, further purification steps are usually required, which may include several steps. This may initially be a Fes liquid separation, followed by precipitation of the iron and removal of impurities with more noble metals, for example cadmium, by a so-called cementation with zinc dust. In addition, chemical precipitation methods can be used, for example the precipitation of nickel with dimethylglyoxime or of cobalt with α-nitroso-β-naphthol. When cementing with zinc dust different activators can be used, for example, arsenic, antimony or copper sulfate. Cementing can also be done in several stages to remove different metal contaminants.

The metal removed from the steel scrap, in particular the zinc, is recovered in a conventional manner by means of electrolysis.

In order to achieve as complete as possible separation of the impurities of the steel to be processed in the form of paints, fats, rubber, plastics, etc., the steel scrap before or after introduction into the alkaline or acidic solution or after separation of this solution additionally with a paint stripper in Be brought in contact. Paint strippers with which organic contaminants such as paints can be removed are known in principle from the prior art. In addition to alkaline paint strippers are usually organic solvents such as aliphatic or aromatic hydrocarbons, chlorinated hydrocarbons, alcohols, glycol ethers, dicarboxylic acid esters, acetone, etc. is particularly common Methylene chloride used for the removal of paints from metal surfaces. Other known paint strippers are phenols, formic acid and hydrofluoric acid esters, benzyl formate, N-methylpyrrolidone together with an alkanolamine or hydrogen peroxide. The effect can be optimized by adding cosolvents, emulsifiers, evaporation retardants, wetting agents, activators and thickeners.

While pretreatment in an alkaline solution will often be sufficient to remove varnishes, oils, etc., this may be the case in acid pretreatment. not the case. The additional treatment with a paint remover thus has special importance in the pretreatment in acid, especially sulfuric acid solution. Possibly. However, the treatment may also be combined with an alkaline solution and the treatment with an acidic solution, for example in such a way that the organic impurities are first removed from the metals with an alkaline solution, while the detachment of the metal layer takes place essentially during the subsequent acid treatment ,

In general, the method according to the invention in connection with an electric furnace, in particular an electric arc furnace, z. B. so-called. UHP furnaces (Ultra High Power) are used, although in principle, other electric furnace such as induction furnaces are conceivable. In particular, it may be a shaft furnace or finger shaft furnace, wherein the shaft serves to be filled on the one hand with the steel scrap and on the other hand to pass the exhaust gases along the steel scrap in order to preheat this to an elevated temperature. However, it is also conceivable to use the invention in connection with other furnaces, which are operated for example with natural gas.

The exhaust gas flow usually has a temperature in a height of about 1000 ° C. Thus, the steel scrap can already be preheated to a significantly elevated temperature, the z. B. between 300 and 1000 ° C, in particular at about 700 ° C. Accordingly, the time required to completely melt the steel scrap by means of arcs or induction is significantly reduced. As less time is needed for the melting process In addition, the overall process within the furnace will be faster, so that tapping may occur at shorter intervals. Overall, therefore, the efficiency of a melting furnace operated according to the invention increases not only in terms of energy, but also in terms of productivity.

The operating time gained can be used in electric furnace so that when used with comparable performance of the prior art today and the melting performance to achieve short Abstichintervalle increasing additives such as natural gas, oxygen and coal no longer need to be used in today's scale. This reduces carbon dioxide emissions at the smelting stage and contributes significantly to the environmental compatibility of the steel and cast iron manufacturing process.

Optionally, the waste heat remaining after scrap preheating can be used to heat the alkaline or acidic baths used to strip the coatings.

Due to the preliminary removal of zinc contained in the steel scrap, the processing of the dusts, which inevitably occur in the steel production, can be carried out without problems. The amount of dust is significantly reduced by the pretreatment. The filter dusts of electric steelworks contain valuable raw materials in the form of metal oxides, in particular iron oxide, chromium oxide and nickel oxide. Zinc oxide virtually no longer occurs in the process according to the invention, since the zinc was removed in the preliminary de-zincing step. Possible reprocessing processes of the dusts are reintroduction into the metallurgical cycle, magnetic separation processes or pyrometallurgical processes.

The invention is explained schematically in FIG. 1 by means of a dezincification. The galvanized steel scrap is brought into the Entzinkungsbad and there freed of its zinc layer. Subsequently, the clean and descaled steel scrap is preheated in a shaft in countercurrent through the hot exhaust gases of the electric furnace, before the preheated steel scrap finally enters the actual electric furnace in which the steel production takes place. In contrast, the solution used in the dezincing bath to remove the zinc layer from the steel scrap is fed to zinc recycling.

Claims

claims

1. Method for processing steel scrap with the following steps:

Introducing the steel scrap into an alkaline and / or acid solution,

Separating the solution,

Preheating the steel scrap by means of exhaust gases,

- introducing the steel scrap into a melting furnace and

- removing the resulting exhaust gases and using the exhaust gases to preheat further steel scrap.

2. The method according to claim 1, characterized in that the steel scrap is metal-coated and in contact with the alkaline and / or acidic solution, an at least partial detachment of the metal coating takes place.

3. The method according to claim 2, characterized in that the metal coating is a zinc or tin layer or a hybrid metal coating.

4. The method according to any one of claims 1 to 3, characterized in that the resulting exhaust gases are not subjected to Abgasnachverbrennung.

5. The method according to any one of claims 1 to 4, characterized in that the alkaline solution is a sodium hydroxide or potassium hydroxide solution.

6. The method according to claim 5, characterized in that the treatment with the alkaline solution at a temperature of at least 75 ° C, in particular at least 85 ° C, takes place.

7. The method according to any one of claims 2 to 6, characterized in that the detachment of the metal coating takes place as galvanic corrosion, wherein the metal coating serves as an anode and a second, stable in the alkaline solution metal as the cathode.

8. The method according to claim 7, characterized in that an external voltage is applied to the anode and the cathode.

9. The method according to any one of claims 1 to 8, characterized in that the acidic solution is a sulfuric acid solution.

10. The method according to claim 9, characterized in that the sulfuric acid solution contains 100 to 300 g / l, preferably 150 to 250 g / l, in particular about 200 g / l H ₂ S0.

11. The method according to claim 9 or 10, characterized in that the treatment with the acidic solution at a temperature of 20 to 60 ° C, in particular about 40 ° C.

12. The method according to any one of claims 2 to 11, characterized in that the metal-coated steel scrap before the In contact with the alkaline or acidic solution mechanically pretreated, in particular crushed, roughened and / or deformed.

13. The method according to any one of claims 2 to 12, characterized in that the detached metal coating is recovered from the solution, in particular by means of electrolysis.

14. The method according to any one of claims 1 to 13, characterized in that the melting furnace is an electric furnace, in particular a shaft furnace or finger shaft furnace.

15. The method according to any one of claims 1 to 14, characterized in that the steel scrap to a temperature of 300 to

1000 ° C, in particular about 700 ° C, is preheated.