D E S C RI P T I O N

The object of the present finding is a pyrometallurgical treatment aimed to treat slag resulting from metallurgical processes.

More in particular, the object of the present invention is a treatment for purifying slag containing non-ferrous metals such as lead, zinc and cadmium or oxides thereof or chemical compounds thereof.

Even more in particular, the present invention relates to the recovery of lead, zinc, and any cadmium contained in the slag generated by a waelz treatment.

Electric arc steel and iron furnaces are used for producing steel and iron products (laminates, section bars, rods for reinforced concrete,...) which are fed with ferrous scrap which, according to the source, may be galvanised or coated with zinc- and lead-based products.

During the melting of the scrap, the zinc and lead compounds leave the charge, exiting the furnace in the form of very fine dust, mainly consisting of iron, zinc and lead oxides, with smaller amounts of other oxides and chlorides, among which cadmium compounds. Such powdery residues are known as steel plant fumes or by the English term Electric Arc Furnace Dust (hereinafter referred to as EAF dusts).

Such EAF powders are classified as special hazardous waste and were once delivered to dump. Starting from the Eighties of the past century, their treatment - through a process known as waelz process has experienced a very quick growth. This has allowed the recovery of the zinc and lead contained therein in significant amounts (up to 35% by weight). In Europe, about 80% of EAF dusts are currently treated with the waelz process, corresponding to over one million tons a year.

The waelz is a metallurgical technology aimed to recover non-ferrous metals, especially zinc and lead, from natural raw materials or from scrap and residues of other iron and steel or metallurgical operations, such as in particular EAF powders. The raw materials, admixed with reducing agents (such as coke dust, anthracite, petroleum coke) and basic ratio correctors, constituting as a whole the charge, continuously feed a rotary furnace (generally 50 - 70 metre long and with a diameter of about 3 - 4 metres) where they are thermally treated at the solid state at a temperature that exceeds 1000 °C.

Such process generally takes place as a "basic process" and with material at the solid state (by "basic process" or "acid process" it is meant a process carried out in a basic or acid environment, respectively). Generally, the basic ratio correctors, among which calcium oxide (CaO), are added in such amounts as to not only ensure the basic process but also raise the melting point of the charge treated to such levels as to prevent the local melting thereof, or the furnace would lock.

The reducing action of carbon towards non-ferrous metal oxides along with the relatively high operating temperature (a little more than 1000 °C), obtained by burning the carbon, allows the reduction of the oxides into metals and their sublimation; as soon as they exit the charge, such metals recombine into oxides and form a dust that, carried over by the combustion gases, exits from the furnace to be collected into suitable equipment, obtaining a commercially valid product internationally known as "waelz oxide".

Zinc, lead as well as other compounds are thus extracted from the charge but the extraction is not total also because the process temperature is not sufficiently high to allow all that part of such metal oxides that is inside the same particles to migrate towards the surface of the EAF dust particles. At the end of the treatment, the charge exits from the furnace in the form of a granular mass and constitutes the so-called "waelz slag" which is significantly but not sufficiently free from the non-ferrous metal content. Lead, in particular, is generally present in amounts higher than 0.5 %; a limit value, as will soon be seen, for reusing the slag which must therefore be disposed of in suitable dumps. The need of disposal into said dumps is a serious drawback since it is difficult to find suitable and authorised sites (which among the other things must be reasonably close to the factory where the waelz process is active). Substantially, the operating capabilities of a factory for waelz treatment are conditioned by the accommodation capacity of the dumps it has access to.

In the European Community, any substance produced to be used as is or as a compound component, in amounts equal to or higher than 1 t/year, cannot be manufactured or put on the market if not in compliance with the provisions of the REACH (Registration Evaluation Authorization CHemicals) rules.

Through the ECHA (European Chemical Agency), the European Union manages and enforces the provisions of the REACH rules.

In November 2010, the ECHA registered the waelz slag having lead contents below 0.5% as a product and thus with different and larger use purposes.

The possibilities of analysing the waelz slag free from polluting elements could greatly increase having the possibility to range from uses as road bed material, raw material for concrete plants or iron and steel works, or as a replacement of inerts in concrete mixes.

The main object of the present finding is to provide a treatment process for slag still containing moderate amounts of zinc, lead and optionally cadmium, capable of purifying the same slag to a sufficient point to allow the classification thereof as a product and not as waste, thus obtaining reasonably low energy costs.

Further objects of the present finding, at least according to some variants of said process, are:

- carrying out the waelz slag treatment reducing the lead content thereof to less than 0.5 %; - also using process additives consisting of slag from other processes;

- obtaining a product at least partly usable as inert in road beds or concrete mixes or similar applications.

These and other objects, which shall clearly appear hereinafter, are achieved with the pyrometallurgical process and corresponding process apparatus according to the independent patent claims, the features whereof shall appear more clearly from the description of preferred embodiments thereof and illustrated, by way of a non-limiting example, in the annexed drawings, wherein:

- figure 1 shows the layout of a known waelz process plant;

- figure 2 shows the same waelz plant but equipped with the waelz treatment plants according to the invention;

- figure 3 shows, in a ternary diagram CaO, Si0 ₂ , FeO the possible chemical- physical states of the charge in a waelz furnace and of the waelz slag during the treatment thereof according to the invention;

- figure 4 shows, in a diagram, the pattern of the melting temperatures of waelz slag as a function of the ratio between lime (CaO) and silica (Si0 ₂ ) present in the slag charge to be treated.

Hereinafter, for shortness, the term "non-ferrous metals" will be used to indicate metals such as zinc, lead and optionally cadmium too, object of the extraction process according to the present invention, or also other metals capable of being extracted according to the same process.

By the term "mass" it is meant the material, generally slag from previous metallurgical treatments, from which said non-ferrous metals are to be extracted with the process according to the invention.

By the term "charge" it is meant the material consisting of said mass with the addition of the additives needed for said process.

Of course, for those masses that have such a composition as to be treatable according to the invention without the need of adding additives, the charge composition coincides with that of the mass.

According to the invention, a mass from which said non-ferrous metals are to be extracted, contained therein in the form of oxides (in particular lead and zinc) is brought to melting through heating by combustion in a static tank furnace where it is kept in a reducing environment and at a sufficient temperature to obtain the reduction to metal of at least a part of said non-ferrous metal oxides, with the consequent escape of said non-ferrous metals by evaporation.

According to a preferred variant, additives are added to the mass, apt to form a charge the melting point whereof is sufficiently low to make the same melting easier through heating by combustion.

Again according to the invention, if the mass to be treated does not per se includes reducing substances in a sufficient amount to reduce to metal substantially all the amount of oxides of the non-ferrous metals contained therein, at least the additional amount thereof needed to achieve such object is added.

Said metals escaped by evaporation from the molten charge are then recombined again into powdery oxides above the liquid phase in an environment kept sufficiently rich in oxygen, the oxygen being administered in excess with respect to the amount required for the fuel combustion, and they are then removed and collected according to known methods and means and form a product with commercial possibilities totally similar to the waelz oxide.

If chlorine was present in the mass to be treated in any form thereof, in addition to said non-ferrous metals, chlorides may also form; they undergo the same treatment and the same use purpose as said oxides and shall not be mentioned again anymore considering them similar to oxides for the purposes of the invention.

The molten charge and the slag thus purified are continuously or intermittently tapped off, left to cool and prepared for a reuse thereof.

In fact, the elimination of most of the zinc present in the molten charge and substantially of all the lead is obtained with such process, or at least the content thereof is reduced to values well below 0.5% that the ECHA has set as a limit to classify such material as a product. Preferably, the temperature of the treatment chosen for the molten charge is just that sufficient to reduce to metal such non-ferrous metal oxides and is substantially a little higher than the melting temperature.

Hereinafter, the additives (or "scorifiers") apt to decrease the melting point of the charge shall also be, generically, called "low melting additives" or "low melting scorifiers".

The above reducing substances, the low melting scorifiers and any other additives suitable or needed for a correct execution of the process according to the invention may be added to the mass wholly or partly before the introduction into the furnace, or in the same furnace.

Preferably, methane burners are used for the combustion; even more preferably, the burners are of the oxy-fuel type (burners where the combustible is pure oxygen instead of air).

Besides allowing higher temperatures to be reached, the oxy-fuel burners have reduced sensitive heat dispersions through the combustion gases since these have a significantly lower mass if compared with combustion gases obtained using air as combustible.

The burners may be arranged in such a way as to be directed on the liquid mass, with the dual advantage of reducing energy consumptions, since the heat exchange efficiency is improved, and of producing a strong remixing that increases the reaction kinetics.

Methane or carbon fed under-slag nozzles may also be provided; the fuel thus introduced also validly contributes to form the reducing environment required in the molten charge.

The invention shall be described in more detail hereinafter with reference to the treatment of waelz slag, this being deemed the most advantageous and therefore preferred application.

However, it is first suitable to go in some detail about the known waelz plant and process.

With reference to fig. 1, a slag treatment plant 1 according to the waelz process comprises the waelz rotary furnace 1.1, the loading station 1.2 of the incoming materials (EAF dusts, carbon, basic correctors), the air inlet 1.3, the waelz slag outlet 1.4, the powdery oxide outlet 1.5 and the collection unit 1.6 of said dusts, a product known on the market as waelz oxide.

The waelz process development is visible in the ternary diagram of the CaO - FeO - Si0 ₂ system of fig. 3.

The dashed area (indicated with W) represents the working field of the waelz furnace where, with a predetermined ratio between CaO and Si0 ₂ (equal to 2 in the example of fig. 3), by the effect of the reducing action of carbon, the iron oxides present in the fumes at the higher level of valence are progressively transformed into others at a lower valence up to having also finely dispersed metal iron in a small part. Once the iron oxide reduction step has ended (endothermic reaction), the temperature in the furnace increases so the reduction of the zinc and lead oxides and their elimination from the mass becomes prevalent. Air is blown in the end part of the furnace which partly re-oxidises the iron, with an important energy recovery, moving the slag composition to the right of the area, thus towards higher FeO values, in the proximity of the area of the ternary diagram that shows the wording wustite which has low melting points. The melting of the slag into the furnace, however, must be absolutely avoided or it would lock the operation of the same furnace for a quick closing thereof, thus the operating temperatures are kept relatively low reducing the efficacy of the lead and zinc elimination.

However, by melting the waelz slag in a tank furnace it is possible to increase the temperature thereof up to melting, with the consequent increase of the kinetics of the reactions that lead to the elimination of lead and to the drastic concurrent reduction of zinc.

To prevent reaching too high temperature levels and remain in a range not higher than 1300 °C, according to the invention it is preferred to correct the slag composition exiting from the waelz furnace with suitable additions of scorifiers apt to lower the melting point of the charge thus obtained so that bringing it to melting in a fuel-fed tank furnace is easier and energetically less expensive. The addition of the low melting scorifier allows the treatment range of the charge obtained from the waelz slag to be brought in the dashed area indicated with F in the ternary diagram of the CaO - FeO - Si0 ₂ system of fig. 3 (where the low melting scorifier is Si0 ₂ ) without substantially changing the nature of the same waelz slag, so as to keep the features as per REACH definition.

It can be seen that such area F, in the diagram of fig. 3, is very close but however below the line corresponding to a CaO/Si0 ₂ = 1 ratio; i.e. in a zone where CaO/Si0 ₂ > 1 which ensures a basic process.

By way of example, if the charge composition is represented by point P of area F, the Si0 ₂ percentage is 22%, that of CaO is 27% and the CaO/Si0 ₂ ratio is about 1.2.

By way of example, fig. 4 shows the pattern of the melting point of the charge as the ratio between the amounts of CaO and Si0 ₂ present in the charge varies, where with a predetermined total amount of calcium oxide present in the waelz slag, a reduction of such ratio is obtained by increasing the amount of Si0 ₂ . It can be seen that melting temperatures can be reached which are absolutely feasible in a tank furnace with combustion heating.

However, for the purposes of the invention, said values are not binding; it is only preferred that also with the addition of silicon oxide Si0 ₂ or of other substances having a similar effect, a basic process is in any case ensured.

If the temperature pattern is as shown in fig. 4, operating in a zone with the lowest possible melting temperature but compatible with such preference of carrying out the basic process treatment implies a good process control if it is deemed appropriate to add additives that alter the chemical composition; a good uniformity of the molten charge is also required.

Due to the following considerations, such careful control is only possible in a crucible furnace.

Considering, as said, that said area F is very close to the line which corresponds to a ratio CaO/Si0 ₂ = 1, it is clear that the process control, again imposing such preference for the basic process, would be virtually impossible in a rotary furnace. In fact, the response times therein between the introduction of any additives to the inlet and results only later verifiable at the outlet would be too long.

On the other ^" hand, in a tank furnace, the process pattern is easily monitored in real time, drawings for an immediate check being optionally possible, if deemed appropriate.

Moreover, a tank furnace ensures uniformity of the molten charge and said possible strong mixing that may be obtained with the burners suitably arranged as mentioned above may also help in obtaining said uniformity (which is impossible in a rotary furnace).

If it is desired that the low melting scorifier added contains silicon oxide Si0 ₂ , it may comprise or consist of siliceous sands or preferably, exhaust foundry sands. In this latter case, there is the further advantage of also recovering the foundry scrap, per se unusable, besides the waelz slag. Glass scraps may also be suitable for the purpose.

As an alternative or in addition, clay and/or bentonite may also advantageously be used as low melting scorifiers, as they are substantially composed of silica (Si0 ₂ ) and alumina (aluminium oxide; A1 ₂ 0 ₃ ); the latter has the capability of affecting the melting temperature not really by reducing the minimum value thereof but enlarging the low melting area indicated with F in the ternary diagram of fig. 3; more exactly, the presence of alumina in moderate amounts (2 ÷ 4 %) in the charge ensures the stay of the charge in the low melting area for wider variations of the weight ratio between CaO and low boiling substance (e.g. Si0 ₂ ) than in the absence of the same alumina (an effect simplified in a qualitative manner by the dashed line of fig. 4).

This simplifies the process management allowing a less accurate dosing of the low boiling scorifier to be added or allowing it not to be added at all. In some cases, in fact, the presence of alumina in the charge in the amounts indicated above (or because already present in the treated slag or because added as scorifier) may make the amount of low boiling substances already present in the slag be deemed sufficient, amounts that otherwise, in the absence of alumina, should be added at least in a moderate amount.

Bauxite may be used as scorifier for the addition of alumina, besides said clay and/or bentonite.

Of course, the reduction and evaporation of the non-ferrous metals (in particular Zn and Pb) is only possible in a reducing environment, a condition ensured by the fact that the waelz slag, very advantageously for the purposes of the process according to the present invention, always contains significant residues of unburnt carbon.

Otherwise, or for treating other types of slag, nothing prevents adding the necessary amount of reducing agents such as carbon.

The metals thus reduced and evaporated, as soon as they surface from the liquid phase, encounter an oxidising environment due to the presence of oxygen, are again transformed into oxides in powdery form and are carried over to the dust treatment unit of the plant along with the dusts coming from the waelz furnace. At the same time there is a continuous or periodical tapping of the molten mass which, after cooling, can be subjected to subsequent grain size classification operations specific for each use.

An advantage of the process is that the calcium oxide CaO present in the slag is neutralised forming, with silica and iron (at least in the waelz slag, always present), chemically stable compounds such as calcium silicate and calcium iron silicate so that the resulting material is a stable ceramic compound usable at least as inert for road beds or concrete mixes.

Another advantage of the invention is that it requires an energy consumption sufficiently limited to be economically sustainable due to the fact that the process is so conceived as to use, in order to bring the charge to melting, an energy source such as methane instead of precious electric energy. Moreover, if oxy-fuel burners are used, as already said, energy consumptions are even lower. Fig. 2 shows, in the essential components thereof, a plant that implements the process according to the invention applied to the waelz slag treatment.

Reference numeral 2 indicates the plant as a whole. Reference numeral 2.7 indicates the tank furnace with combustion heating. Reference numeral 2.1 indicates the loading section of any additives (such as scorifiers apt to lower the melting point of the charge and reducing agents) whereas inlet 1.4 of the slag to be treated coincides with the outlet of the waelz slag 1.4 from which said slag can advantageously be directly taken.

Furnace 2.7 is shown uncovered only for schematic simplicity but it is clear that it is closed at the top by a dome wherein the evaporated metals, encountering an oxidising environment generated by a suitable excess of the comburent air of the burners or by the administration of an oxidising agent suitably introduced, are transformed into oxides and are carried over by the combustion fumes along conduit 2.3 which takes them to the collecting unit 1.6. The latter can advantageously be the existing unit of the waelz plant 1. Arrow 2.2 indicates the inlet of fuel and combustible.

The purified mass can be easily tapped continuously through conduit 2.5, preferably at the surface of the molten charge; the possibility of periodically tapping said material also from the furnace bottom through conduit 2.4 is also provided for maintenance operations or others.

Once cooled, the purified mass is fed to a granulator 2.6 from which it exits as reusable product 2.7 in a more suitable size.

In conclusion, advantageously, the waelz slag treatment plant 2 according to the invention can constitute an appendix of the waelz plant 1 of which it becomes an integral part where the final step of the EAF foundry dust treatment is carried out.

Plants according to the invention for treating slag other than waelz slag have a structure equivalent to that just described.

The nature and the amounts of the additives according to the invention as well as the process temperatures of course change according to the composition of the slag to be treated, also remaining within the scope of waelz slag. The values provided in this description therefore are non-limiting but only exemplary of particular cases, proving the feasibility, simplicity and inexpensiveness of the process, while the application of the teachings of this description to each particular case is within the capacity of the man skilled in the art.