Applicant: DANIELI & C. Officine Meccaniche S.p.A., Via Nazionale 41, 33042 Buttrio (UD), Italy

TECHNICAL FIELD

The present invention refers to a process and a plant for the treatment of slag from steelmaking processes, in particular of slag coming from the production of steel by means of electric arc furnaces (EAF), so as to recover metals, ferroalloys and other materials from said slag that can be reused in the steelmaking process itself or in other product sectors.

BACKGROUND OF THE INVENTION

The production of steel through melting in an electric furnace (EAF) produces slags containing high concentrations of iron oxides (Fe _x O _y ) that currently have a low possibility of reuse and valorization, these slags are generically referred to as black slag. Black slag therefore corresponds to the secondary product that is produced when the steel scraps, or the direct reduced iron, are melted to produce new steel, and consists of the oxides of the unwanted elements, such as silicon and phosphorus, or of the inevitable oxidation of iron and of the other alloying metals of steel, such as manganese in addition to the added slag formers, such as lime (CaO) and magnesia (MgO).

An electric arc furnace is a furnace that heats the material by means of an electric arc, combined with the action of the chemical energy provided by the use of oxygen and fuel. The use of the electric arc furnaces allows producing steel starting from a raw material composed of up to 100% metal scraps, which allows to reduce the energy required for the production of steel compared to the primary production of steel from ferrous minerals. As mentioned, the electric arc furnace is an equipment capable of melting different charge mixes, just like the scrap, but also direct reduced iron (DRT) and hot briquetted direct reduced iron (High Briquetted Iron, HBI).

The steel industry considers the slag remaining from this smelting process a by-product that can be disposed of in part by using it as a building material (for example as a thickness in road construction), considering that the harmonized European standards require mechanical strength and stability, compliance with hygiene, health and environmental requirements and safety in the use of these materials.

An example of the composition of the black slag is approximately: 47.7 % by weight of CaO, 19.6 % by weight of Fe _x O _y , 13.6 % by weight of SiCh, 5.8 % by weight of AI2O3, 5.3 % by weight of MnO, 3 % by weight of MgO, 0.96 % by weight of C CE, 0.5 % by weight of P2O5 e 0.46 % by weight of TiCh. About 120 kg of black slag are produced per ton of steel tapped and therefore a steel mill with a productivity of 1 Mt/year also generates 120,000 t/year of black slag containing about 30,000 t of iron oxide and therefore 21,000 t of potentially recoverable metallic Iron. As seen, the black slag can be used for road construction but this is not enough for the reuse of all the amount generated and there is a risk of having to landfill, for a fee, a material that still contains valuable components.

Other types of oxides, in addition to iron oxide, may occasionally be present in the slag (such as Ba and V oxides) depending on the composition of the scrap and of the additives. Ba, Cr and V can be sometimes responsible for elution problems that occur during the analysis of the slags: in fact, any calcium silicates present in the slag are soluble in water and responsible for the elution phenomenon. However, for the final use, the slag must be inert, that is, it must not give rise to the elution of Ba, Cr or V beyond the limits.

The recovery of the metals from the black slag through reduction with Carbon of the slags (formula 1) is not applied at an industrial level as it requires a high energy requirement:

This is an endothermic reaction and high CAPEX (contraction from CAPital Expenditure) and OPEX (contraction from Operational Expenditure are needed without a convenient return on investment.

This process is certainly efficient in the metal recovery but, because of the endothermicity of the reactions, it is only convenient for the treatment of waste materials coming from highly alloyed steel, because the recovery of Iron alone would not make the economic sustainability sufficient. The following documents deal with the treatment of slag from metallurgical or steelmaking processes: US 2008/0156144 Al, DE 2 307 237 Al, US 2006/0196308 Al, US 5 882 375 A, EP 3 375 764 Bl, US 6,241,797 Bl, and WO 2017/100808 Al, but still have drawbacks in the management of slag treatment within the steelmaking procedure.

DISCLOSURE OF THE INVENTION

The invention aims to overcome the aforementioned drawbacks and to propose a process and a plant for valorizing the black slag, avoiding treating the slag only as waste or as a material of little value, but vice versa in order to be able to treat it as a real by-product of high added value. A further object of the invention is the recovery of Iron and possibly of other alloying metals of steel from the slag. Another object of the invention is to propose a process and a relative plant for treating the slag that transforms it into an inert slag easily usable in the cement industry. A further object of the invention is to propose a process and plant to reduce the storage of slag produced between the various heats (melts) of an EAF. Another object of the invention is to reduce the heat input during slag treatment as much as possible.

Further objects or advantages of the invention will become apparent from the following disclosure.

In a first aspect of the invention, the object is achieved by means of a process for the treatment of slag coming from the production of steel by means of electric arc furnaces, defined as starting slag, in particular black slag, comprising the following steps:

(i) mixing said starting slag, comprising iron oxides (Fe _x O _y ) and preferably other oxides of alloying metals of steel, in particular chromium oxide (Cr _x O _y ), in particular in liquid form, with aluminium and/or silicon scrap;

(ii) metallothermic reaction between said iron oxides, and preferably said other oxides of alloying metals of steel, in particular chromium oxide (Cr _x O _y ), with said aluminium and/or silicon scrap with the consequent production of

(a) Iron, and preferably of other alloying metals of steel, and

(b) aluminium (AhO 3) and/or silicon (SiCh) oxide in admixture with the other oxides pre-existing in the slag;

(iii) separation (a) of the Iron, and preferably of the other alloying metals of steel, and (P) of the treated slag with reduced iron oxide content, and preferably with reduced content of oxides of alloying metals of steel present in the slag, which incorporates said aluminium (AI2O3) and/or silicon (SiCh) oxides formed in step (ii); wherein the reaction of step (ii) occurs with still-liquid slag directly coming from the electric arc furnace, due to the exothermicity of the reaction itself and the heat provided by the stillliquid slag.

With the above process, the alteration of the chemical composition of the slag, in particular of the black slag coming from electric arc furnaces producing steel, is obtained.

The aforementioned exothermicity of the metallothermic reaction and the heat of the liquid slag ensure a high-temperature reaction, thus avoiding the need to have to provide additional external heat not derived from the same processes involved. It is therefore a passive slag treatment process; different from an active process, which would require in effect the additional active input of heat from outside. The slag produced with this process is similar to the slag of the blast furnace and to the clinker and is an excellent material for the production of cement. Advantageously, the process according to the invention is carried out in an online manner, that is, it is in shadow to the production of steel, according to the following mode,

- it comprises before step (i) the production of steel with the simultaneous production of slag by means of an electric arc furnace wherein, in the terminal part of the steel production process, the slag is extracted from the furnace and at this point directly subjected, in a still liquid form, to treatment according to steps (i) to (iii) at the same time as the end of the steel production cycle and during a subsequent new steel production cycle, after the steel produced in the previous cycle has been tapped, and

- wherein steps (i), (ii), (iii) are repeated with the parallel production of a new steel production heat and slag.

Iron, and preferably other alloying metals of steel obtained by the slag treatment can be added to the new or subsequent steelmaking heat, or are put to other uses.

Thus, steps (i), (ii), and (iii) of slag treatment preferably begin and end between the start of one deslagging and the next deslagging in parallel with the smelting process that takes place in the electric arc furnace. To allow for this management, steps (i), (ii) and (iii) as a whole preferably last between 30 min and 90 min.

In a preferred embodiment of the invention, step (i) occurs directly after deslagging, in particular preferably starts within 2 min of deslagging, and/or concurrently therewith, by joining the aluminium and/or silicon directly to the slag stream cascading out of the deslagging port forming part of the electric arc furnace. This makes optimal use of the latent heat of the slag to make the chemical reaction with the aluminium and/or silicon take place, without additional heat inputs.

In an embodiment of the invention, step (ii) takes place in a reactor without any energy input devices, i.e., without any additional heating devices, such as heating tubes in the reactor wall or electrodes inserted into the reactor. Step (i) also advantageously takes place without any preheating of the mixed components.

Thus, the slag treatment process according to the invention is a process that preferably takes place tap-to-tap. This means that each time the electric arc furnace (EAF) deslags to then tap steel from the melt, that amount of slag (as mentioned the so-called black slag) is processed on its own, avoiding the accumulation of it to cool and/or to add it to other amounts of slag from previous castings or other plant areas (e.g., white slag from ladle furnaces (LF)).

Indeed, in a preferred embodiment of the invention, said starting slag in step (i) is not mixed with slag from other processes and/or additional aluminium or silicon oxides. This allows for faster treatment of the still hot slag and avoids the need to have additional dispensers for the addition of these additional materials. The slag obtained after treatment is directly usable in the cement industry or road construction.

Steps (i), (ii) and (iii), advantageously, take place in air and not in a controlled and inert atmosphere, resulting in plant simplification.

Conversely, in the state of the art, producers of steel in EAF tend to accumulate as much slag to be treated by different heats, so as to optimize costs since they use a special active furnace to carry out slag treatment, with consequent aggravation of CAPEX and OPEX. The invention, on the other hand, favours the aforementioned process in an "online" mode, so as not to require "other" energy inputs, since the heat needed for the reactions is already present in the slag. In addition, the use of passive reactors allows the slag to be treated in the shadow of production with the elimination of the special storage area for the discarded slag awaiting treatment in an active furnace, which then becomes superfluous in a process and plant according to the invention, while avoiding various environmental problems related to storage. The following table 1 reports examples of the composition of the slag from an electric arc furnace (not usable as such in cement plants) and of a clinker. Advantageously, the composition of the slags is approximated to that of the clinker to adapt it to the use in the cement industry:

Table 1

Metallothermic processes, such as aluminothermic or siliconthermic ones, allow the reduction of iron oxides to metallic Iron, simultaneously producing stable oxides (SiCh, AI2O3):

3 FeO + 2 Al - 3 Fe + AI2O3 + energy (2)

2 FeO + Si - 2 Fe + SiO2 + energy (3)

Metals and relative oxides suitable for applying the metallothermy method are derived from Ellingham diagrams. The Ellingham diagram is a Cartesian graph that relates the free energy and the temperature of the reactions forming the metal oxides starting from the elements that make them up.

Metallothermy is a metallurgical process that makes use of aluminium, or of another metal having high chemical affinity with oxygen, such as silicon, to reduce metal oxides and the reaction heat is such as to cause the reduced metal to melt. The energy produced can then be recovered/used (e.g. for steam production).

The recovery of the ferrous part of the slag and of other alloying metals of steel with the generation of a high-value alloy simultaneously allows the production of a high-value slag through a process that is self-sustaining from an energy point of view, possibly with the energy recovery of the exothermic reaction. This process reduces the content of iron oxide and of alloying metals of steel in the slag while producing at the same time aluminium and/or silicon oxides with a composition comparable to that of the clinker.

The metallothermic process can be carried out in a dedicated container called a metallothermic furnace, where the liquid slag can be fed together with the reducing reagents, but advantageously takes place in a reactor without heating devices. In the presence of liquid slag as a starting material, a source of energy is not necessary, which is instead required in the case of feeding solid slag, for triggering the exothermic reaction. If, however, a reaction in a metallothermic furnace is desired, such as in the presence of also high Carbon inputs, heating in the metallothermic furnace can take place: with resistors, with electric arc, plasma torch, etc. In a preferential embodiment, in said step (i) it is also provided for feeding Carbon and in step (ii) the added Carbon reacts with said iron oxides, and preferably with said other oxides of alloying metals of steel, forming CO and Iron, and preferably other alloying metals of steel. It is partly solubilised in liquid Iron. The addition of Carbon, causing an endothermic reaction (1), serves to manage the temperature of the system The residual Carbon dissolves in the liquid Iron producing steel. In an alternative embodiment, residual Carbon dissolves in liquid Iron producing pig iron. The energy balance of the above reactions (2) and (3) depends on the consumption of Al and/or Si (exothermic reaction with metal oxides, such as FeO) and on the consumption of Carbon (endothermic reaction with metal oxides, such as FeO). The addition of Carbon also serves to adjust the liquidus point of the liquid metal. The liquidus is the geometric point in a phase diagram indicating the temperature above which the liquid phase exists and below which the solidification process begins with the coexistence of melt and crystals.

Advantageously, out of a total amount of additives composed of aluminium and/or silicon scrap and of Carbon, 50.0 - 99.9 % by weight is formed by the aluminium and/or silicon scrap. It is understood that Carbon does not necessarily have to be present, but its presence can be adjusted based on needs. The additional amounts of Carbon are determined by the person skilled in the art through his general knowledge.

The process according to the invention is particularly advantageous when it produces highly alloyed Iron alloys, a case that takes place when the starting slag comprises oxides of alloying metals of steel, such as for example chromium oxides, and when in step (ii) also the oxides of said alloying metals are reduced by means of aluminium and/or silicon generating the relative alloying metals of steel, which together with Iron form a relative ferroalloy.

The process according to the invention makes it possible to recover also the waste from photovoltaic cells by being able to use the silicon contained therein in the metallothermic process. In a preferred embodiment of the invention, said silicon scrap may therefore come from scrapping photovoltaic cells.

Preferably, the mixture formed in step (ii) between the starting slag and the aluminium and/or silicon scrap, and optionally the Carbon, comprises:

(I) a base mixture consisting of:

(a) 90 - 96 % by weight of slag, preferably comprising 15 - 40 % Fe _x O _y ; 10 - 20 % SiCh; 3 - 9 % AI2O3; 35 - 55 % CaO; and 2 - 10 % MgO and other metal oxides in concentration dependent on the composition of the steel in production;

(P) 4 - 10 % by weight of aluminium and/or silicon scrap; and optionally also

(II) Carbon in an amount such that out of the total amount composed of aluminium and/or silicon scrap and of Carbon, 50.0 - 99.9 % by weight is formed by the aluminium and/or silicon scrap.

Advantageously, the slag produced by the metallothermic process is characterized by a reduced iron oxide content, and preferably a reduced content of oxides of alloying metals of steel, formed in step (ii) comprises, in weight percentages:

43 - 57 % CaO;

10 - 20 % SiO ₂ ;

17 - 27 % AI2O3;

1 - 4 % MgO;

< 5 % of iron oxides and oxides of alloying metals of steel in case of treating the slag of low alloy steels or > 5% of iron oxides and oxides of alloying metals of steel in case of treating the slag of high alloy steels.

Furthermore, advantageously, said Iron (or steel) and preferably said alloying metals of steel forming an alloy produced in step (ii) comprise:

88 - 94 % Fe;

4 - 6 % Si; 0 - 3.5 % C; and

2 - 4 % other metals, such as Mn, V, Cr.

In an embodiment of the invention, said Iron is a cast iron produced in step (ii) that comprises 88 - 94 % Fe, 4 - 6 % Si, C in amounts between > 2 % and < 3.5 %, and 2 - 4 % other metals, such as Mn, V, Cr.

In a particularly preferred embodiment of the invention, the slag is used in the production of cement.

A second aspect of the invention concerns a plant for slag treatment comprising:

(a) an electric arc furnace for the production of steel and black slag with at least one deslagging door;

(b) a metallothermic reactor without heating devices, suitable for receiving said slag and for the separate extraction of liquid metal and resulting slag;

(c) first conveying means to transfer said slag to said metallothermic reactor without heating devices, wherein the first conveying means advantageously are said deslagging door forming part of the electric arc furnace;

(d) upstream of said metallothermic reactor without heating devices second conveying means for transporting and introducing aluminium and/or silicon scrap, and optionally Carbon, into said metallothermic reactor without heating devices;

(e) a slag composition analysis system associated with a control system configured to dose the respective necessary quantities of aluminium and/or silicon scrap and optionally of Carbon through said second conveying means to said metallothermic reactor without heating devices; and

(f) a plant for collecting fumes generated by the partial reduction of the oxides contained in the slag with Carbon and any splashes or metals sublimated by the metallothermic reactor without heating devices.

Advantageously, the aluminium and/or silicon scrap fed in step (i) is shredded. The plant according to the invention therefore preferably comprises a shredding device upstream of said tank container.

It is also conceivable to store the slag produced by an electric arc furnace and use it later after cooling thereof. In this regard, it should be heated again to melt it, for example, by means of an energy feed system without the use of fossil fuels, such as for example electrical resistors, plasma torch, electric arc, etc. that could be provided in the plant in this regard.

A third aspect of the invention concerns a mixture for the recovery of Iron and preferably of other alloying metals of steel from slag coming from the production of steel by means of electric arc furnaces, said mixture being suitable for use in the process according to the invention and comprising:

(I) a base mixture consisting of:

(a) 90 - 96 % by weight of slag, preferably comprising 15 - 40 % Fe _x O _y ; 10 - 20 % SiCh; 3 - 9 % AI2O3; 35 - 55 % CaO; and 2 - 10 % MgO and other metal oxides in concentration dependent on the composition of the steel in production;

(P) 4 - 10 % by weight of aluminium and/or silicon scrap; and optionally further

(II) carbon in an amount such that out of a total amount composed of aluminium and/or silicon scrap and of Carbon, 50.0 - 99.9 % by weight is formed by the aluminium and/or silicon scrap.

The features and advantages described for one aspect of the invention may be transferred mutatis mutandis to the other aspects of the invention.

The industrial applicability is obvious since it is possible to recover useful metals in order to produce a highly alloyed ferroalloy and a slag usable in cement production and this with low energy demand, given the exothermicity of the process according to the invention. Thanks to the exothermicity of the process according to the invention, the process is also economically viable for low alloyed iron alloys.

Of course, in preferred embodiments of the process according to the invention, the energy produced in step (iii) may be used for other purposes within the steel mill comprising a plant according to the invention.

Not producing CO2, if not at low flow rates of CO2 in the case of Carbon feed and low energy requirements, the process according to the invention also has a very low environmental impact. The invention therefore achieves the object of presenting a black slag recycling process, which allows to obtain ferroalloys, i.e. steel (-200 kg/t of black slag) and material for the production of cement (~800kg/t of black slag). This new slag treatment route, with the connected production of ferroalloys, uses recycled metals (such as Al, Si) as reagents, does not develop CO2 (except in the case of partial feed of carbon) and does not require energy since the metallothermic reaction is exothermic. It is a circular economy that sees the slag as a secondary product from steel production (regarding it as a by-product) and aluminium and/or silicon scraps entering into the cycle and that sees materials immediately usable in various market sectors exiting the cycle.

The advantages deriving from the invention are therefore the recovery of metals, the production of highly valorized materials for the cement industry, no consumption of natural gas and low/no consumption of electrical energy to carry out the reaction, but the use of energy produced by the process itself possibly recovered. The process is applicable, in particular, to low-C steel and alloy steels.

By providing in a preferred embodiment of the invention for the parallel execution of steel and black slag production in an electric arc furnace and passive treatment of the black slag, and by repeating these processes essentially continuously, not only the need for external energy input is avoided, but also the storage of the black slag produced while awaiting subsequent cumulative treatment in a dedicated active furnace.

The objects and advantages will be further highlighted in the disclosure of preferred examples of embodiments of the invention given by way of non-limiting example only.

Variant and further features of the invention are the subject matter of the dependent claims. The description of the preferred examples of execution of the process, of the plant and of the mixture according to the invention is given by way of example and not of limitation with reference to the attached drawing. In particular, unless otherwise specified, the number, shape, size and materials of the plant and of the individual components may vary, and equivalent elements may be applied without deviating from the inventive concept.

DESCRIPTION OF PREFERRED EMBODIMENT EXAMPLES

Fig. 1 represents a block diagram schematically illustrating the process according to the invention.

In figure 1, it can be noted a metallothermic furnace or reactor 2 which is fed (arrows 10) with different starting materials: slag from an EAF, aluminium and/or silicon scrap and optionally Carbon, to create an optimal mixture for a metallothermic reaction. Downstream of the occurred reaction inside the metallothermic furnace 2, the following products 6 can be extracted (arrow 12): ferroalloys, the treated slag and any low carbon monoxide flows that are conveyed (arrow 16) in the fume collection and treatment plant 18 for the oxidation thereof to CO2. The treated slag is delivered (arrow 14) to the cement industry 8. The scheme also describes a plant according to the invention, wherein the arrows 10, 12 and 14 indicate conveying means and the starting reagents 4 from feeding means of sources of aluminium and/or silicon scrap, slag (in this case an electric arc furnace) and optionally Carbon. On the other hand, metallothermic furnace feed analysis systems and Al and/or Si scrap shredding systems, which can be provided in the plant for the purposes illustrated above, are not represented. As a metallothermic reactor, it is possible to use the same type of containers used industrially for the production of ferroalloys via the metallothermic route. The following table 2 provides an example of thermodynamic equilibrium mass balance for the process according to the invention:

Table 2 The efficiency of the process can be noted: the iron oxide contained in the slag has been almost completely extracted, i.e. reduced to metallic Iron, while the increase in aluminium oxide produced by the metallothermic reaction with Al can be noted.

In the example, the addition of Carbon with aluminium scraps was calculated so as to produce liquid steel with a lower melting temperature than liquid Fe. Summarizing, it can be said that the invention, applicable to the recovery from EAF of slags of low Carbon steel or of alloy steel, allows the recovery of metals (Fe, Cr, Si, ...) thanks to an exothermic process that uses Al (or Si) scraps for the reduction of the metal oxides.

This approach according to the invention allows a wide process flexibility depending on the expected products, which are: Iron alloys, containing Si, Cr, C etc. with a high market value and slag for the cement industry.