HK1203570A1

HK1203570A1 - Pyrometallurgical treatment of slags

Info

Publication number: HK1203570A1
Application number: HK15104105.7A
Authority: HK
Inventors: ‧德桑德爾; P‧德桑德尔
Original assignee: 庞滕诺萨股份公司
Priority date: 2012-04-17
Filing date: 2013-04-10
Publication date: 2015-10-30
Also published as: EP2839044A1; CN104379780A; RU2014145782A; ITAN20120043A1; WO2013156832A1

Abstract

There is described a method for treating slag produced by metallurgical processes, in particular the waelz process, aimed to recover non-ferrous metals such as lead and zinc contained in such slag in the form of oxides. The method provides for bringing the slag to melting in a static tank furnace (2.7) heated by combustion, keeping the molten slag in a sufficiently reducing environment so as to reduce such oxides to metal. The metals evaporate from the molten mass, are oxidized again in an oxidizing atmosphere above the mass itself, removed and collected. As regards the molten mass thus purified, it is cooled and prepared for a commercial reuse thereof. The addition of scorifiers is provided, apt to decrease the melting point of the mass to be treated, if excessive for a combustion furnace. An advantage of the invention is to obtain the purification of slag containing metals such as lead and zinc up to make it a reusable product, thus obtaining reasonably low energy consumptions. A further advantage of some variants is the use, as scorifier, of exhaust foundry sands or scrap from other processes that would otherwise be unusable.

Description

Pyrometallurgical treatment of slags

Description

The object of the invention is a pyrometallurgical treatment for treating slag from metallurgical processes.

More specifically, the object of the invention is a process for the purification of slags containing non-ferrous metals such as lead, zinc and cadmium or their oxides or chemical compounds thereof.

Even more particularly, the invention relates to the recovery of lead, zinc and any cadmium contained in the slag produced by Waelz treatment.

Electric arc steelmaking cupola furnaces are currently used to produce steel products (rolled sheet, profiled bar, steel bars for reinforced concrete, etc.) whose feed is iron-containing waste material that, depending on the source, may be plated or coated with zinc and lead based products.

During the melting of the scrap, the zinc and lead compounds leave the charge, leaving the blast furnace as very fine dust, which mainly contains iron, zinc and lead oxides, with lesser amounts of other oxides and chlorides, including cadmium compounds. This powdery residue is referred to as steel mill flue gas, or "Electric arc furnace Dust" (hereinafter referred to as EAF Dust).

This EAF powder was classified as a special hazardous waste that was delivered to the dumping process. From the eighties of the last century, the treatment of them by the so-called Waelz process has undergone a very rapid development. This enables recovery of significant amounts (up to 35% by weight) of zinc and lead contained therein. In europe, about 80% of the EAF dust is currently treated by the Waelz process, which amounts to over one million tons per year.

The Waelz process is a metallurgical technique aimed at recovering non-ferrous metals, especially zinc and lead, from natural raw materials or from other wastes and residues of iron-making, steel-making or metallurgical operations, such as, inter alia, EAF powders. The raw material mixed with the reducing agent (for example, coke breeze, anthracite, petroleum coke) and the basic ratio modifier (basic ratio corrector) constitutes a whole charge which is continuously fed into a converter (generally 50-70 m long and about 3-4 m in diameter) and heat-treated in the solid state in the furnace at a temperature exceeding 1000 ℃.

Such processes typically occur as "alkaline processes" in which the material is in a solid state ("alkaline processes" or "acidic processes" refer to processes performed in an alkaline or acidic environment, respectively). The addition of an alkaline ratio modifier, containing calcium oxide (CaO), is usually done in an amount that not only ensures the alkaline process but also raises the melting point of the treated charge to a level that prevents localized melting or converter lock.

The reduction of non-ferrous metal oxides by the carbon obtained by burning the carbon and the high operating temperatures (slightly above 1000 ℃) enable the reduction of the oxides to metals and their sublimates; immediately after leaving the charge, these metals recombine to oxides and form dust, which the combustion gases leave the converter with, and are collected in suitable equipment, to give a commercially available, useful product, internationally referred to as "Waelz oxide".

Zinc, lead and other compounds are thus extracted from the charge, but this extraction is not complete because the process temperature is not high enough to cause all such metal oxides in the same particle to migrate toward the surface of the EAF dust particle.

At the end of this treatment, the charge leaves the converter as a granular mass, constituting a so-called "Waelz slag" whose non-ferrous metal content is markedly reduced but is not sufficient. In particular lead, the content of which is generally higher than 0.5%; this is a limit to the reuse of slag that must be properly dumped, as will be seen below.

Having to do a dumping disposal is a serious drawback because it is difficult to find a suitable and approved location (and must be rather close to the plant where the Waelz process is performed). The operational capacity of a plant performing Waelz processing is largely limited by the dump capacity it has.

In the european union, any substance for production use or as a component of a compound with a yield equal to or higher than 1 ton/year cannot be manufactured or marketed if the provisions of REACH (chemical registration evaluation approval) rules are not met.

The european union manages and enforces the terms of the REACH rule by the ECHA (european chemical authority).

In 11 months 2010, ECHA registered Waelz slag with a lead content below 0.5% as a product, and thus was available for more different uses.

If Waelz slag could be analyzed to be free of polluting elements, its use could be greatly expanded, as a road base material, as a raw material for concrete plants or steel plants, or as a substitute for inert substances in concrete mixes.

The main object of the present invention is to provide a process for the treatment of slags still containing moderate amounts of zinc, lead and optionally cadmium, which enables sufficient purification of said slags, so that they can be classified as products rather than wastes, thus obtaining a reasonably low energy cost.

Further objects of the invention are, at least according to some variants of the process:

-performing Waelz slag treatment to reduce its lead content to less than 0.5%;

-also using process additives comprising slag from other processes;

-obtaining a product that can be used at least partially as an inert in road foundations or concrete mixes or similar applications.

These and other objects, which will become better apparent hereinafter, are achieved by a pyrometallurgical process and corresponding process equipment according to the independent claims, the nature of which will become more clearly apparent from the description of the preferred embodiments and by way of non-limiting example in the accompanying drawings, in which:

figure 1 shows the layout of a known Waelz process plant;

figure 2 shows the same Waelz device, but equipped with a Waelz processing device according to the invention;

FIG. 3 shows CaO, SiO in the form of a ternary phase diagram₂The possible chemical-physical state of FeO in the Waelz blast furnace charge and the possible chemical-physical state of the Waelz slag during the treatment according to the invention;

FIG. 4 shows, in phase diagram form, the melting temperature of the Waelz slag as a function of the lime (CaO) and the silicon oxide (SiO) present in the slag charge to be treated₂) And the ratio therebetween.

In the following, for the sake of brevity, the term "non-ferrous metals" is used to denote the metals that are the object of the extraction process according to the invention, such as zinc, lead and optionally cadmium, or also other metals that can be extracted by the process according to the invention.

The term "material" means a material from which the non-ferrous metal can be extracted using the process according to the invention, typically slag from a previous metallurgical treatment.

The term "charge" refers to a material consisting of the material and additives required for the process.

Of course, for those materials having a composition that can be treated according to the invention without the addition of additives, the charge composition is the same as the material composition.

According to the invention, a material containing oxides therein from which non-ferrous metals, in particular lead and zinc, can be extracted is melted by combustion heating in a static bath furnace maintained in a reducing environment and at a sufficient temperature to reduce at least a portion of the non-ferrous metal oxides to metal and then to allow the non-ferrous metals to escape by evaporation.

According to a preferred variant, the addition of additives to the mass tends to form a charge whose melting point is low enough to be more easily melted by the heat of combustion.

Also according to the invention, if the material to be treated does not itself contain a sufficient amount of reducing substances to reduce substantially all the amount of non-ferrous metal oxides contained therein to metal, at least the additional amount required to achieve this purpose is added.

The metal escaping from the molten charge by evaporation is then reformed again into the powdered oxide in an environment that remains sufficiently rich in oxygen above the liquid phase, that is, controlled in quantity of oxygen beyond that required for the combustion of the fuel, and then the powdered oxide is removed and collected according to known methods and means, forming a commercially viable product entirely similar to Waelz oxide.

If the material to be treated contains chlorine in any form in addition to the non-ferrous metals, chlorides can also be formed; they are subjected to the same treatment as the oxides and have the same purpose of use, and they are not mentioned again in view of their similarity to the oxides for the purposes of the present invention.

The purified molten charge and slag can thus be tapped continuously or intermittently, left to cool, and prepared for reuse.

In fact, by means of said process it is possible to eliminate most of the zinc and substantially all of the lead contained in the molten charge, or at least to reduce its content to a value well below that at which ECHA is set to classify this material as a product of 0.5%.

The treatment temperature chosen for the molten charge is preferably just sufficient to reduce these non-ferrous metal oxides to metal and only slightly above the melting temperature.

Additives that tend to lower the melting point of the charge (or "slagging agents") are also referred to hereinafter collectively as "low melting additives" or "low melting slagging agents".

The above reducing substances, low-melting slagging agents, and any other additives required or suitable for the proper functioning of the process according to the invention can be added to the charge, wholly or partly, before introduction into the blast furnace, or in the same blast furnace.

Preferably, a methane burner is used for the combustion; even more preferably, an oxygen-fuel type burner (a burner in which the combustible is pure oxygen rather than air) is used.

In addition to being able to reach higher temperatures, oxy-fuel burners also have a reduced sensible heat dispersion of the combustion gases, since they have a significantly reduced mass if compared to the combustion gases obtained when using air as the combustible.

The burners can be arranged in such a way as to be set according to the liquid quality, with the double benefit of reducing the energy consumption, since the heat exchange efficiency is increased and the intense remixing that occurs increases the reaction kinetics.

Methane or carbon delivery nozzles (methane or carbon producer-slag nozzles) may also be provided below the slag; the fuel thus introduced also functions effectively to create the reducing environment required in the molten charge.

The invention is described in more detail below with reference to Waelz slag processing, which can be considered the most advantageous and therefore the most preferred application.

However, the known Waelz apparatus and process will first be described in detail.

Referring to fig. 1, a slag handling plant 1 according to the Waelz process comprises a Waelz converter 1.1, a feed loading platform 1.2(EAF dust, carbon, alkaline modifier), an air inlet 1.3, a Waelz slag outlet 1.4, a powdered oxide outlet 1.5, and a collection unit 1.6 of the dust, a product known on the market as Waelz oxide.

The evolution of the Waelz process can be seen in FIG. 3 for CaO-FeO-SiO₂Ternary phase diagram of the system.

The dashed area (denoted by W) represents the operating area of the Waelz furnace, with predetermined CaO and SiO₂The iron oxides present at higher valence levels in the flue gas are gradually converted to other species of lower valence by reduction of carbon, until a small fraction of finely dispersed metallic iron is included (equal to 2 in the example of figure 3). Immediately after the reduction step of the iron oxides (endothermic reaction) is completed, the temperature in the furnace rises, so that the reduction reaction of zinc and lead oxides and their elimination from the material become the predominant reaction. Blowing air at the furnace tail partially reoxidizes the iron, recovers most of the energy, shifts the slag composition to the right of the zone and thus to a higher FeO value, and shows the word "wustite" near the ternary phase diagram zone, which has a low melting point. It is absolutely necessary to avoid melting of the slag into the furnace, which would otherwise lock the furnace operation causing it to shut down quickly, so that the operating temperature is kept low to reduce the lead and zinc abatement efficacy.

But the temperature can be raised to melting by melting the Waelz slag in a bath furnace, thereby increasing the kinetics of the reaction leading to lead elimination and at the same time to significant reduction of zinc.

To prevent excessive temperature levels from being reached and to maintain them in the range of not more than 1300 ℃, it is preferred according to the invention to modify the slag composition leaving the Waelz furnace by suitably adding a slag-forming agent liable to lower the melting point of the charge obtained, so as to make it easier to melt in the bath furnace fed with fuel and to reduce energy costs.

The addition of a low-melting slagging agent makes it possible to limit the treatment range of the charge obtained from the Waelz slag to CaO-FeO-SiO in FIG. 3₂Within the dotted line region indicated by F in the ternary phase diagram of the system (wherein the low-melting slagging agent is SiO)₂) Without significantly altering the properties of the Waelz slag, thereby maintaining the characteristics defined according to REACH.

It has been found that region F in the phase diagram of FIG. 3 is very close to but below that corresponding to CaO/SiO₂A line with a ratio of 1; i.e. in the presence of CaO/SiO in an ensured alkaline process₂In the region of > 1.

For example, if the charge composition is represented by point P of region F, then SiO₂The percentage is 22 percent, the percentage is 27 percent, CaO/SiO₂The ratio is about 1.2.

For example, FIG. 4 shows melting points of the charge as a function of CaO and SiO in the charge₂Curve (patterrn) of the ratio of contents in which the total content of calcium oxide in the Waelz slag is predetermined by increasing SiO₂In such an amount that the ratio is reduced.

It has been found that by combustion heating, absolutely feasible melting temperatures can be achieved in the bath furnace.

But for the purposes of the present invention, the values are not limited; only adding silicon oxide (SiO)₂) Or other substances with similar effects, it is preferred in any case to ensure an alkaline process.

If the temperature profile is as shown in fig. 4, operating in a zone with the lowest possible melting temperature, but at the same time favouring the alkaline process treatment, it means that a good process control is possible if the addition of additives capable of changing the chemical composition is considered to be a suitable behavior; it is also desirable that the molten charge have excellent homogeneity.

In view of the above considerations, such careful control is only possible in crucible furnaces.

As mentioned, it is considered that said region F corresponds very closely to CaO/SiO₂The line with a ratio of 1 clearly makes it virtually impossible to achieve process control in a converter which is likewise advantageous for the alkaline process. In fact, the response time from the introduction of any additive into the inlet to the later time when only verifiable results are obtained at the outlet is too long.

On the other hand, in bath furnaces, the process profile can be monitored simply in real time, where appropriate, and a map for immediate inspection is also optionally possible.

Moreover, the bath furnace ensures homogeneity of the molten charge, which homogeneity can also be assisted by the possible intense mixing obtained by suitably arranged burners as described above (which is not possible in a converter).

If desired, the low melting slagging agent comprises silicon oxide (SiO)₂) It may comprise or consist of siliceous sand or preferably spent foundry sand. In this latter case, there is an advantage of recovering casting waste which is not useful per se, in addition to the advantage of recovering Waelz slag. Glass waste may also be used for this purpose.

Alternatively or additionally, it may also be advantageous to use clays and/or bentonite as low-melting slagging agents, since they are predominantly composed of silicon oxide (SiO)₂) And alumina (aluminum oxide Al)₂O₃) Composition is carried out; the latter can affect the melting temperature, not actually by lowering its minimum value but rather expanding the low melting region denoted by F in the ternary phase diagram of FIG. 3; more precisely, the presence of a medium amount (2 ÷ 4%) of alumina in the charge ensures that the charge, compared to the absence of the same alumina (effect simplified in a qualitative manner by the dotted line in fig. 4), has a broader CaO and low-boiling substance (such as SiO)₂) The weight ratio remains within the low melting point region within the range of variation.

This simplifies the process management, allows less accurate dosing of the low-boiling slagging agent, or allows no addition at all. In fact, in some cases, the presence of the above-mentioned amount of alumina in the charge (either because it is already present in the treated slag or because it is added as a slagging agent) makes it possible to consider the amount of low-boiling substances already present in the slag as being sufficient, at least to a moderate amount if no alumina is present.

When alumina is added, bauxite may be used as a slagging agent in addition to the clay and/or bentonite.

Of course, the reduction and evaporation of non-ferrous metals (in particular Zn and Pb) is only possible in a reducing environment, which is very advantageous for the purposes of the process according to the invention, since Waelz slag always contains significant residues of unburned carbon.

Otherwise, or when treating other kinds of slag, a necessary amount of a reducing agent such as carbon must be added.

The metals are thus reduced and evaporated and, once they reach the surface from the liquid phase, they are subjected to an oxidizing environment due to the presence of oxygen, thus being converted again into oxides in powder form, which arrive, together with the dust coming from the Waelz furnace, at the dust treatment unit of the plant.

At the same time, the molten material is tapped continuously or periodically and, after cooling, a subsequent size classification operation for the respective application can be carried out.

The advantage of this process is that chemically stable compounds such as calcium silicate and calcium iron silicate are formed by neutralisation of the calcium oxide (CaO) present in the slag with silicon oxide and iron (at least always present in Waelz slag), and the resulting material is a stable ceramic compound which can be used at least as an inert in road foundations or concrete mixes.

Another advantage of the present invention is that since the process envisages the use of an energy source such as methane instead of precious electric energy for melting the charge, its required energy consumption is sufficiently limited and economically sustainable. And if oxy-fuel burners as described above are used, the energy consumption is lower.

Fig. 2 shows the main components of a plant for Waelz slag treatment using the process according to the invention.

Reference numeral 2 denotes an overall apparatus. Reference numeral 2.7 denotes a bath furnace with combustion heating. Reference numeral 2.1 denotes a loading section of any additives, such as slag formers and reducing agents apt to lower the melting point of the charge, the slag inlet 1.4 to be treated coinciding with the Waelz slag outlet 1.4 from which the slag can advantageously be taken directly.

The furnace 2.7 is shown in open (uncovered) form, which is only for the sake of simplicity shown, and it is evident that it is closed at the top by a dome, the evaporated metal being subjected to an oxidizing environment generated by a suitable excess of combustion air or by a suitable introduction of an oxidizing agent in the burner, being converted into oxides and carried along by the combustion fumes along the duct 2.3 to the collection unit 1.6. The collecting unit 1.6 may advantageously be an existing unit of the Waelz device 1. Arrows 2.2 indicate fuel and combustible inlets.

The purified material may simply be tapped continuously through conduit 2.5, preferably at the surface of the molten charge; periodic tapping of the material from the furnace floor via the pipe 2.4 can also be carried out for maintenance operations or other operations.

Once cooled, the purified material can be conveyed to the granulator 2.6, from where it exits in a more suitable size as a reusable product 2.7.

In summary, it is possible according to the invention to make the Waelz slag handling plant 2 advantageously an accessory part of the Waelz plant 1, as an integral part thereof, for the final step of EAF casting dust handling.

The apparatus for treating slag other than Waelz slag according to the present invention has the same structure as described above.

The nature and amount of the additive according to the invention and the process temperature, which varies according to the slag composition to be treated, remain within the range of Waelz slag.

The values provided in the present description are therefore non-limiting and are only examples of particular cases, demonstrating the feasibility, simplicity and low cost of the process, while it is within the abilities of one skilled in the art to apply the present description to each particular case.

Claims

1. A method of treating slag produced by a steel making and iron making process to reduce the content of non-ferrous metals such as lead and zinc contained therein in the form of oxides, characterized by:

-assessing whether the slag has a composition that can be melted by combustion heating in a static bath furnace (2.7),

-if not, adding a sufficient amount of additional slagging agent to lower its melting point to a temperature that can be reached by said static bath furnace (2.7);

-assessing whether the slag itself contains sufficient reducing substances to enable, after melting, the reduction of at least a part of the oxides of the non-ferrous metals contained in the slag to metal,

-if not, adding a sufficient amount of an additional reducing substance to reduce at least a portion of the non-ferrous metal oxide to metal;

-feeding a charge comprising the slag, the additional amount of slagging agent and an additional amount of reducing substances into the static bath furnace (2.7);

-adding all or part of the additional slagging agent and/or reducing substance before the slag is introduced into the static bath furnace (2.7) and/or adding the additional slagging agent and/or reducing substance into the same furnace;

-melting said charge in said static bath furnace (2.7), reducing at least part of the oxides of said non-ferrous metals to metals and evaporating with a reducing environment;

-ensuring a sufficient oxidation temperature at the dome of the static bath furnace (2.7) to re-oxidize the portion of non-ferrous metal that has just evaporated;

-removing the oxides just formed from the dome of the static bath furnace (2.7) by known means;

-withdrawing the molten charge thus purified from the static bath furnace (2.7) in a continuous or discontinuous manner.

2. A method of treating slag according to the preceding claim, wherein the charge material is maintained at a temperature slightly above the melting temperature.

3. Method of treating slag according to the preceding claim, characterized in that the slagging agent comprises an amount (2 ÷ 4%) of alumina (Al) suitable for expanding the low melting region of the charge₂O₃)。

4. The method of treating slag according to the preceding claim, wherein the slag comprises aluminum oxide (Al)₂O₃) The slagging agent consists of bauxite.

5. A method of treating slag according to any one of the preceding claims wherein the slagging agent comprises a low boiling slagging agent, i.e. a slagging agent which tends to lower the melting point of the charge.

6. The method of treating slag according to the preceding claim, wherein the low-boiling slagging agent comprises silicon oxide (SiO)₂)。

7. The method of treating slag according to the preceding claim, wherein the slag contains silicon oxide (SiO)₂) The low-boiling slagging agent comprises siliceous sand.

8. The method of treating slag according to claim 6, wherein the slag contains silicon oxide (SiO)₂) The low boiling slagging agent of (a) comprises spent foundry sand.

9. The method of treating slag according to at least claim 5, wherein the low-boiling slagging agent comprises clay.

10. The method of processing slag according to any one of the preceding claims,

the slagging agent is added in an amount sufficient to ensure that the treatment is in any case an alkaline process.

11. The method of processing slag according to any one of the preceding claims,

-arranging burners oriented towards the molten charge for heating in the static bath furnace (2.7),

-mixing of the molten charge is achieved by the action of the flame on the surface of the molten charge.

12. Method for treatment of slag according to any of the previous claims, characterized by raising the combustion temperature and minimizing the sensible heat loss of the combustion fumes and using oxygen instead of air as oxidizer.

13. The method of processing slag according to any one of the preceding claims, wherein a sufficient amount of fuel is introduced below the slag to at least facilitate:

-providing thermal energy for maintaining the molten state,

-forming a reducing environment.

14. Method for treatment of slag according to any of the preceding claims, characterized in that an oxidizing environment is created at the dome of the static bath furnace (2.7) by a suitable excess of combustion air in the burners and/or by specifically introducing an oxidizing agent.

15. A method of treating slag according to any one of the preceding claims, wherein the purified molten material is cooled and granulated according to the most suitable size by known methods.

16. The method of processing slag according to any one of the preceding claims,

-the slag to be treated contains lead,

-adding a sufficient amount of said slagging agent and reducing substances to reduce the lead content present in the molten charge to below 0.5%.

17. The method of processing slag according to any one of the preceding claims, wherein the slag is waelz slag.

18. A method for treating EAF dust according to the Waelz process, characterized in that the method further comprises treating Waelz slag as claimed in one or more of the preceding claims.

19. Slag treatment plant (2) susceptible of implementing the method according to one or more of the preceding claims, characterized in that it comprises a static bath furnace (2.7) closed at the top with a dome and heated by burner combustion.

20. The slag processing apparatus of claim 19, wherein the burner is a methane burner.

21. The slag processing apparatus of claim 19, wherein the burner is of oxy-fuel type.

22. Slag processing apparatus according to at least claim 19, wherein the burners are arranged to be directed at the molten charge to cause mixing thereof.

23. A slag treatment plant according to any of the preceding claims, starting from claim 19, characterized in that the static bath furnace (2.7) is equipped with means for providing oxygen in excess of the amount required for combustion of the fuel, so as to ensure oxidation of non-ferrous metals escaping from the molten charge by evaporation.

24. A slag treatment plant according to any one of the preceding claims, starting with claim 19, wherein the plant includes a nozzle below the slag for ease of introduction of fuel into the molten material.

25. A slag processing plant according to any of the preceding claims, starting from claim 19, characterized in that the plant comprises an additive loading section (2.1).

26. A slag processing plant according to any of the preceding claims, starting from claim 19, characterized in that the plant comprises a granulator (2.6).

27. The slag processing apparatus according to any one of the preceding claims, beginning with claim 19,

-the slag to be treated is a Waelz slag taken directly from a Waelz converter (1.1),

-the unit (1.6) for collecting the non-ferrous metal oxides is the same as the unit (1.6) for collecting oxides of the Waelz slag processing plant (1).

28. Waelz plant (1), characterized in that it further comprises a slag handling plant according to any of the preceding claims, starting from claim 19.