Process and plant for SLAG treatment

Abstract

A process for treating slag coming from steel production by means of an electric arc furnace (1), or an oxygen converter, and defined as starting slag, the process comprising the following steps: (i) mixing said starting slag in liquid form, comprising iron oxides (FexOy) with Aluminum and / or Silicon and / or Ferrosilicon scrap; (ii) metallothermic reaction between said iron oxides and said Aluminum and / or Silicon and / or Ferrosilicon scrap resulting in the production of (a) Iron, and (b) aluminum oxide (AI2O3) and / or silicon oxide (SiO2); (iii) separating the Iron and the treated slag, with reduced iron oxide content, said treated slag incorporating said aluminum oxides (AI2O3) and / or silicon oxides (SiCfe) formed in step (ii); wherein said mixing occurs gradually, at least partially by directly feeding said Aluminum and / or Silicon and / or Ferrosilicon scrap into a flow of the starting slag in liquid form during a pouring step from a first container to a second container; wherein said second container receives both the starting slag flow and the Aluminum and / or Silicon and / or Ferrosilicon scrap added thereto and acts as a metallothermic reactor; and wherein the Aluminum and / or Silicon and / or Ferrosilicon scrap is fed in a controlled manner by means of a dosing device (6).

Description

[0001] PROCESS AND PLANT FOR SLAG TREATMENT

[0002] ***********

[0003] Field of the invention

[0004] The present invention relates to a process and a plant for treating slag resulting from steelmaking processes, in particular black slag from the production of steel by electric arc furnaces (EAF) or by oxygen converters, so as to recover, from said slag, silica-based minerals, metals, ferroalloys, and other materials which can be reused in the steelmaking process itself or in other product industries.

[0005] Background art

[0006] Steel production by melting in an electric arc furnace (EAF) produces slag containing high concentrations of iron oxides (FexOy), which currently has a low potential for reuse and valorization.

[0007] This slag is generally referred to as black slag. Black slag therefore corresponds to the by-product which is produced when steel scrap, or direct reduced iron, is melted to produce new steel, and consists of the oxides of undesirable elements, such as silicon and phosphorus, or by the inevitable oxidation of iron and other steel alloying metals, such as manganese in addition to added slag formers, such as lime (CaO) and magnesia (MgO).

[0008] An electric arc furnace is a furnace which heats and melts material by means of an electric arc, combined with the action of chemical energy provided by the use of oxygen and fuel. Using electric arc furnaces allows for the production of steel starting from a raw material consisting of up to 100% scrap and / or metal pellets, HBI, cast iron. This allows reducing the energy required for producing steel with respect to the primary production of steel from ferrous minerals. As mentioned, the electric arc furnace is an equipment capable of melting different charge mixes, precisely like scrap, but also direct reduced iron (DRI) and high briquetted iron (HBI) and cast iron.

[0009] The converter instead uses oxygen lances to chemically decarburize cast iron and obtain steel.

[0010] The steel industry considers the slag remaining from these melting processes a byproduct which can only be partially disposed of, by using it as a building material (for example as thickness in the construction of roads), considering that, for example, the European harmonized standards require mechanical strength and stability, compliance with hygiene, health and environmental requirements and safety in the use of these materials.

[0011] Since about 120 kg of black slag are produced per ton of steel tapped from the electric furnace, a steelworks with a productivity of 1 Mt / year also generates 120,000 t / year of black slag which contains about 30,0001 of iron oxide, and therefore 21 ,000 t of potentially recoverable metallic Iron. The same specific amount of slag is also produced by the BOF. As seen, black slag can be used for road construction, but this is not sufficient to reuse the entire amount generated and there is a risk of having to dispose of a material which still contains valuable components in a landfill, for a fee.

[0012] Furthermore, the known system for recovering metals from black slag through the reduction of slag with Carbon (formula 1) is not applied at the industrial level, since it entails a high energy requirement:

[0013] FeO + C -> Fe + CO (g).

[0014] It is an endothermic reaction, and high CAPEX and OPEX are required, without a favorable return on investment.

[0015] This process is certainly efficient in the recovery of metals, but, due to the endothermicity of the reactions, it is cost-effective only for treating waste materials originating from highly alloyed steel, because the sole recovery of Iron would not render the economic sustainability of the operation sufficient.

[0016] Recently, a process for treating black slag, and generally slag with a high concentration of Iron oxide, has been developed, which includes the following steps:

[0017] (i) mixing black slag in liquid form, comprising iron oxides (FexOy) and preferably other oxides of steel alloying metals, in particular chromium oxide (CrxOy), with Aluminum and / or Silicon scrap;

[0018] (ii) metallothermic reaction between said iron oxides, and preferably said other oxides of steel alloying metals, in particular chromium oxide (CrxOy), with said Aluminum and / or Silicon scrap resulting in the production of

[0019] (a) Iron, and preferably other steel alloying metals, and

[0020] (b) aluminum oxide (AI2O3) and / or silicon oxide (SiC>2) mixed with other preexisting oxides in the slag; (iii) separating Iron, and preferably the other oxides of steel alloying metals, and producing the treated slag, with reduced content of iron oxide and preferably with reduced content of oxides of steel alloying metals present in the slag, which incorporates said aluminum oxides (AI2O3) and / or silicon oxides (SiCfe) formed in step (ii).

[0021] This treatment process is disclosed in IT 2022 0002 5869 A1 .

[0022] With this process, using the principle of metallothermy, the slag is treated with aluminum and / or silicon scrap to transform the iron oxides and any oxides of other steel alloying metals respectively into Iron and ferroalloys, at the same time converting the aluminum and silicon into respective oxides. The metals of the steel are thus recovered from the starting slag, and there is obtained a slag being utilizable, preferably but not only, even in the production of cement.

[0023] However, since this process is a recent concept, it requires further developments compared to the base version. In particular, the effect of the metallothermic reaction requires maximization so as to fully take advantage of the features of the elements involved and the parameters thereof.

[0024] Therefore, the need is felt to provide a process and a plant for treating slag which allow overcoming the aforesaid drawback.

[0025] Summary of the invention

[0026] It is an object of the present invention to provide a process for treating slag which allows the still-liquid slag, coming from the production of steel by electric arc furnaces or by oxygen converters, to be more, substantially completely, involved in a metallothermic reaction, with a substantial total recovery of Iron and ferroalloys and a substantial total conversion of aluminum and silicon into the respective oxides thereof, by optimizing the feeding of liquid slag and scrap.

[0027] It is a further object of the present invention to optimize such a process for treating slag, allowing the recovery results to be maximized without requiring the use of energy or materials in an excessive manner.

[0028] It is another object of the present invention to provide a plant for treating slag which allows the aforesaid process to be carried out.

[0029] The present invention achieves at least one of such objects and other objects which will be apparent in light of the present description, by a process for treating slag, conning from steel production by an electric arc furnace or by oxygen converters and defined as starting slag, the process comprising the following steps:

[0030] (i) mixing said starting slag in liquid form, comprising iron oxides (FexOy) and preferably other oxides of steel alloying metals, in particular chromium oxide (CrxOy), with Aluminum and / or Silicon and / or Ferrosilicon scrap;

[0031] (ii) metallothermic reaction between said iron oxides, and preferably said other oxides of steel alloying metals, in particular chromium oxide (CrxOy), with said Aluminum and / or Silicon and / or Ferrosilicon scrap resulting in the production of

[0032] (a) Iron, and preferably other steel alloying metals, and

[0033] (b) aluminum oxide (AI2O3) and / or silicon oxide (SiCh) possibly mixed with other pre-existing and / or added oxides in the slag;

[0034] (iii) separating Iron, and preferably the other oxides of steel alloying metals, and the treated slag, with reduced content of iron oxide and preferably with reduced content of oxides of steel alloying metals present in the slag, said treated slag incorporating said aluminum oxides (AI2O3) and / or silicon oxides (S1O2) formed in step (ii); wherein said mixing occurs in a continuous or discontinuous manner, at least partially by directly feeding said Aluminum and / or Silicon and / or Ferrosilicon scrap into a flow of the starting slag in liquid form during a pouring step from a first container to a second container, thus before entering the second container; wherein said second container receives both the starting slag flow and the Aluminum and / or Silicon and / or Ferrosilicon scrap, previously added directly into said slag flow outside said second container, and acts as a metallothermic reactor; and wherein the Aluminum and / or Silicon and / or Ferrosilicon scrap is fed in a controlled manner by means of a dosing device to satisfy the following ratio: for each amount x in kg of starting slag, the Aluminum and / or Silicon and / or Ferrosilicon scrap is fed in an amount in kg from 0.05x to 0.15x.

[0035] Preferably, together with said scrap, a controlled feeding of any other components (oxides) added for conditioning the starting slag and / or carbon can be provided.

[0036] A further aspect of the invention relates to a plant for treating slag, suitable for carrying out the aforesaid process, comprising:

[0037] - a first container adapted to contain said starting slag in liquid form; - a second container adapted to receive both a flow of the starting slag in liquid form, poured from said first container, and the Aluminum and / or Silicon and / or Ferrosilicon scrap previously added thereto during the pouring from said first container, acting as a metallothermic reactor; wherein a dosing device is provided for dosing the amount of Aluminum and / or Silicon and / or Ferrosilicon scrap to be fed; wherein said dosing device has at least one outlet section proximal to a pouring opening of said first container, when the latter is in a pouring position, whereby the Aluminum and / or Silicon and / or Ferrosilicon scrap is intercepted by the flow of the starting slag exiting from said first container.

[0038] Advantageously, the solution of the present invention, in the several variants thereof, provides for a controlled feeding of Aluminum and / or Silicon and / or Ferrosilicon scrap directly into the liquid slag flow exiting from the pouring opening of said first container, thus in a zone upstream of said second container, which ensures an optimal mixing of said scrap with the black slag in liquid form. Said pouring or slagging opening corresponds to the slagging door in the case of an electric arc furnace and to the upper opening in the case of a converter, and it is generally used, in a known manner, on the one hand for slagging and on the other for tapping steel, by tilting the furnace or the converter itself as a function of the process step.

[0039] This controlled feeding of the scrap in the flow of liquid slag, before the mixture comprising scrap and liquid slag enters into the second container, allows for a greater, substantially complete, involvement of the black slag in the metallothermic reaction and therefore an optimal recovery of Iron and ferroalloys and a substantial total conversion of aluminum and silicon into the respective oxides thereof, as well as a total mixing with the possible oxides added for conditioning the starting slag and / or with the possible carbon.

[0040] In the present description, the term black slag means both the slag resulting from the melting process in an electric furnace, and the slag resulting from production in an oxygen converter. Metallothermy is a metallurgical method which makes use of aluminum, or of other metal having high chemical affinity with oxygen, such as silicon, to reduce metal oxides, and the reaction heat is such as to melt the reduced metal.

[0041] The metallothermic processes, such as aluminothermic or silicothermic ones, for example, allow for the reduction of iron oxides to metallic Iron, simultaneously producing stable oxides (SiO2, AI2O3):

[0042] 3 FeO + 2 Al -> 3 Fe + AI2O3 + energy (1 )

[0043] 2 FeO + Si -> 2 Fe + SiO2 + energy (2)

[0044] Metals and oxides thereof suitable for applying the metallothermic method are derived from thermodynamic equilibrium diagrams (such as Ellingham’s diagram).

[0045] The recovery of the ferrous part of the slag and of other steel alloying metals, with the generation of a high-value alloy, simultaneously allows for the production of a high-value slag by means of a process which is self-sustaining from an energy point of view, by virtue of the energy recovery of the exothermic reaction.

[0046] This process reduces the content of iron oxide and of steel alloying metals in the slag, simultaneously producing aluminum and / or silicon oxides with a composition comparable to that of clinker.

[0047] The metallothermic process can be carried out in a dedicated container, referred to as a metallothermic reactor, where the liquid black slag can be fed together with the reducing reagents. However, in the case of the invention it is important that the mixing of liquid slag and reducing reagents occurs, at least partially or completely, upstream of said reactor. In the presence of liquid slag as starting material an energy source is not necessary, which instead is required in the case of solid slag feed, for the initiation of the exothermic reaction.

[0048] If, however, a reaction in a metallothermic reactor were desired, for example also in the presence of high Carbon additions, the heating in the metallothermic reactor can occur by means of: electric resistors, electric arc, gas burners, plasma torch, etc.

[0049] In fact, in a possible process variant it is possible in the mixing step (i) to also feed Carbon, to be repeated also after a dosed quantity of coal can possibly be added for temperature control. In step (ii) of the metallothermic reaction, the added Carbon reacts (endothermic reaction) with the iron oxides, and preferably with other oxides of steel alloying metals, forming CO and Iron, and preferably other steel alloying metals. In part, the Carbon dissolves in liquid Iron. The addition of Carbon, causing an endothermic reaction, serves for managing the system temperature. The residual Carbon dissolves in the liquid Iron, producing steel. The energy balance of the reactions (1) and (2) above depends on the consumption of Al and / or Si and / or Ferrosilicon (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 for the adjustment of the liquidus point of the liquid metal.

[0050] The process according to the invention is particularly advantageous when producing highly alloyed Iron alloys, a case which occurs when the starting slag comprises oxides of steel alloying metals, for example chromium oxides, and when in step (ii) the oxides of said steel alloying metals are also reduced by aluminum and / or silicon, generating the corresponding steel alloying metals, which, together with Iron, form a corresponding ferroalloy.

[0051] The process according to the invention also allows for the recovery of photovoltaic cell waste since it can use the silicon contained in said cells in the metallothermic process. In a preferred variant of the invention, said silicon scrap can derive from scrapping photovoltaic cells.

[0052] The advantages of the invention further comprise:

[0053] - an alteration of the chemical composition of black slag, which allows the slag not to be considered waste or a low-value material, but to be considered a high value- added by-product;

[0054] - a treatment of slag which transforms it into an inert slag, similar to blast-furnace slag and clinker, directly usable for producing cement;

[0055] - the possible elimination, or otherwise the reduction, of the storage of slag produced between the various batches of an EAF / converter;

[0056] - the possible elimination of the heat input during the slag treatment;

[0057] - a very low environmental impact since CO2 is not produced, except for low flow rates of CO2 in the event of Carbon feed.

[0058] The invention is particularly applicable to the recovery of slag from steel with a low Carbon content, or from alloy steel, allowing for the recovery of metals (Fe, Cr, Si, etc.) by virtue of an exothermic process which uses Al and / or Si and / or Fe-Si scrap for the reduction of metal oxides.

[0059] Further features and advantages of the invention will become more apparent in light of the detailed description of exemplary but non-exclusive embodiments.

[0060] The dependent claims describe particular embodiments of the invention.

[0061] Brief description of the drawings

[0062] In the description of the invention, reference is made to the accompanying drawings, given by way of explanation and not of limitation, in which:

[0063] Figure 1 depicts a block diagram showing an example of the process according to the prior art;

[0064] Figure 2 diagrammatically depicts a side view of a first embodiment of a plant according to the invention;

[0065] Figure 3 depicts a first variant of the feeding trend of Aluminum and / or Silicon and / or Ferrosilicon scrap with respect to the trend of the slag flow exiting from a slag door; Figure 4 depicts a second variant of the feeding trend of Aluminum and / or Silicon and / or Ferrosilicon scrap with respect to the trend of the slag flow exiting from a slag door;

[0066] Figure 5 diagrammatically depicts a side view of a second embodiment of a plant according to the invention;

[0067] Figure 6 diagrammatically depicts a side view of a third embodiment of a plant according to the invention.

[0068] The same reference numerals and letters in the Figures identify the same elements or components.

[0069] Description of exemplary embodiments of the invention

[0070] In all the embodiments of the invention, the black slag treatment process, said black slag coming from steel production by means of an electric arc furnace 1 or by means of oxygen converters and defined as starting slag, comprises the following steps:

[0071] (i) mixing the starting slag in liquid form, comprising iron oxides (FexOy) and preferably other oxides of steel alloying metals, in particular chromium oxide (CrxOy), with Aluminum and / or Silicon and / or Ferrosilicon scrap and possibly with further oxides added to condition the starting slag and / or with carbon; (ii) metallothermic reaction between said iron oxides, and preferably said other oxides of steel alloying metals, in particular chromium oxide (CrxOy), with said Aluminum and / or Silicon and / or Ferrosilicon scrap resulting in the production of

[0072] (a) Iron, and preferably other steel alloying metals, and

[0073] (b) aluminum oxide (AI2O3) and / or silicon oxide (SiO2) possibly mixed with other pre-existing and / or added oxides in the slag;

[0074] (iii) separating by gravity Iron, and preferably the other steel alloying metals, and the treated slag, with reduced content of iron oxide and preferably with reduced content of oxides of steel alloying metals present in the slag, incorporating said aluminum oxides (AI2O3) and / or silicon oxides (SiCh) formed in step (ii).

[0075] Advantageously, the mixing of the starting slag in liquid form with the Aluminum and / or Silicon and / or Ferrosilicon scrap, and possibly with said further conditioning oxides and / or carbon, occurs gradually, in a continuous or discontinuous manner, at least partially by a direct feed of the scrap and of other reagents (such as Ferrosilicon), and of possible further conditioning oxides and / or of possible carbon, in a flow of the starting slag in liquid form during a pouring step from a first container to a second container. Therefore, said scrap feed occurs directly in the flow of the liquid slag exiting from a pouring opening of the first container, therefore occurring outside the second container, i.e., before the mixture comprising liquid slag and scrap enters said second container, as shown in Figures 2 and 5-6.

[0076] Therefore, said second container receives both the flow of the starting slag and the Aluminum and / or Silicon and / or Ferrosilicon scrap (and said possible further conditioning oxides and / or carbon) previously added directly into said flow and acts as a metallothermic reactor.

[0077] Advantageously, the Aluminum and / or Silicon and / or Ferrosilicon scrap, the possible further conditioning oxides and / or the possible carbon are fed in a controlled manner by means of a dosing device to satisfy the following ratio: for each amount x in kg of starting slag, the Aluminum and / or Silicon and / or Ferrosilicon scrap, together with the possible further conditioning oxides and / or possible carbon, is fed in an amount in kg from 0.05x to 0.15x. In a first embodiment of the process of the invention, diagrammatically shown in the Figure 2, said first container is the electric arc furnace 1 and said second container is a slag pot 3.

[0078] The mixing occurs gradually during a slagging step of the electric arc furnace 1 , at least partially by means of a feed of at least said Aluminum and / or Silicon or Ferrosilicon scrap directly into a flow of the starting slag exiting from at least one slagging opening 2, such as the slag door of the electric arc furnace 1 , for example exiting from a single slag door 2. Therefore said mixing occurs outside the slag pot 3, i.e. , before the mixture comprising liquid slag and scrap enters said slag pot 3.

[0079] The slag pot 3 is arranged below the slagging opening 2, and receives both the starting slag flow and the Aluminum and / or Silicon and / or Ferrosilicon scrap previously added thereto, and acts as a metallothermic reactor or furnace.

[0080] Therefore, the slag pot 3 receives liquid starting slag and scrap already mixed with each other.

[0081] In a second embodiment of the process of the invention, diagrammatically shown in Figure 5, said first container is a slag pot 3 which receives the flow of the starting slag in liquid form from at least one slagging opening 2 of the electric arc furnace 1 . In this case the mixing occurs gradually during a step of pouring the starting slag in liquid form from said slag pot 3 into a second container 3’, for example a further slag pot or ladle, at least partially by feeding at least said Aluminum and / or Silicon and / or Ferrosilicon scrap directly into the flow of the starting slag poured from the slag pot 3.

[0082] Therefore, said mixing occurs outside the second container 3’, i.e., before the mixture comprising liquid slag and scrap enters said second container 3’.

[0083] Before said mixing, the slag pot 3 moves from a first position, or slagging position, in which said slag pot receives the flow of the starting slag in liquid form from at least one slagging opening 2, to a second position in which said slag pot is emptied, for example by being tilted, pouring said starting slag in liquid form into the second container 3’. This allows for greater process flexibility, possibly giving more time for the completion of the metallothermic reaction and leaving the area proximal to the electric arc furnace free. Alternatively, the slag pot 3 can be provided with suitable openings or tapping holes, radially positioned, on the side surface thereof, and / or on the bottom of the same slag pot.

[0084] The second container 3’ is arranged below said second position and receives both the flow of the starting slag and the Aluminum and / or Silicon and / or Ferrosilicon scrap, previously added therein, and acts as a reactor or metallothemnic furnace. Therefore, the second container 3’ receives liquid starting slag and scrap already mixed together.

[0085] In a third embodiment of the process of the invention, diagrammatically shown in Figure 6, said first container is an oxygen converter 11 and said second container is a slag pot 3.

[0086] The mixing occurs gradually during a slagging step of the converter 11 , at least partially by a feed of at least said Aluminum and / or Silicon or Ferrosilicon scrap directly into a flow of the starting slag exiting from at least one slagging opening 2, for example exiting from a single slagging opening 2, which is defined by the upper opening of said converter. Therefore said mixing occurs outside the slag pot 3, i.e., before the mixture comprising liquid slag and scrap enters said slag pot 3.

[0087] The slag pot 3 is arranged below the slagging opening 2, and receives both the starting slag flow and the Aluminum and / or Silicon and / or Ferrosilicon scrap, previously added thereto, and acts as a metallothermic reactor or furnace.

[0088] Therefore, the slag pot 3 receives liquid starting slag and scrap already mixed with each other.

[0089] In all these embodiments of the process of the invention, once the metallothermic reaction in the second container has been completed, it is possible to pour a flow of the treated slag, which occupies the upper part of the second container, being lighter than the recovered Iron, and preferably than the other recovered steel alloying metals, which occupy the part proximal to the bottom of the second container.

[0090] Preferably, a coolant, for example water, is added, by addition means 8, to the flow of treated slag which is poured from the second container to obtain treated slag in granular form. For example, water can be sprayed, preferably by means of a plurality of nozzles, directly onto the treated slag flow, during the pouring from said second container. The use of other fluids additional to or alternative to water is not excluded. In a first variant of the process of the invention, the Aluminum and / or Silicon and / or Ferrosilicon scrap, together with any additional conditioning oxides, is continuously fed directly into the continuous flow of the starting slag poured in a cascade from the at least one pouring or slagging opening 2 or from the slag pot 3, respecting said ratio. The sensitive heat of the slag is thus used in an optimal manner to cause the chemical reaction to occur with the Aluminum and / or the Silicon and / or the Ferrosilicon and optionally with further conditioning oxides and with the addition of carbon, without additional thermal inputs.

[0091] In particular, as shown in Figure 3, the Aluminum and / or Silicon and / or Ferrosilicon scrap is fed continuously following a substantially linear trend, for example fed in a substantially directly proportional manner to the amount of starting slag.

[0092] In a second variant of the process of the invention, the Aluminum and / or Silicon and / or Ferrosilicon scrap, together with any further conditioning oxides and possibly carbon, is fed discontinuously directly into the continuous flow of the starting slag exiting from the at least one pouring or slagging opening 2, or poured from the slag pot 3, respecting said ratio.

[0093] As shown in Figure 4, the Aluminum and / or Silicon and / or Ferrosilicon scrap is fed discontinuously, following a stepwise pattern.

[0094] For example, after slagging 10 kg of starting slag, from 0.5 to 1.5 kg of Aluminum and / or Silicon and / or Ferrosilicon scrap and any further conditioning oxides and / or carbon are fed, always directly into the continuous flow of the starting slag upstream of the second container.

[0095] Preferably, a detection of the weight of the second container, which acts as a metallothermic reactor, is provided by weight sensors 4. For example, one or more weight sensors 4 can be arranged on the outer lower surface of the second container (3 in Figure 2; 3’ in Figure 5).

[0096] Load cells can be used as weight sensors, e.g., pneumatic load cells, hydraulic load cells, strain gauge load cells or capacitive load cells.

[0097] In a preferred variant of the process, a detection of the weight of the empty second container is provided, and as the weight of the second container increases during the slagging step or during the pouring from the slag pot 3, the weight of the second container being continuously detected by the weight sensors 4, the Aluminum scrap and / or Silicon scrap and / or Ferrosilicon scrap and possibly Carbon and any further conditioning oxides are fed, as a function of said ratio, by means of the dosing device 6, in turn controlled by a control system 7 which receives the weight data detected by said weight sensors 4.

[0098] In a further variant of the process of the invention, before the mixing step (i), a predetermined feeding of the Aluminum and / or Silicon and / or Ferrosilicon scrap and of any Carbon and of any further conditioning oxides into the empty second container is provided, so as to form a bed of said scrap and of said any further conditioning oxides and / or carbon for a mixing with the initial part of starting slag poured into the second container.

[0099] In all variants of the process of the invention, the metallothermic reaction (step ii) advantageously occurs with the slag being still liquid, directly or indirectly coming from the electric arc furnace or from the converter, by virtue of the exothermicity of the reaction itself and of the heat supplied by the still-liquid slag.

[0100] The aforesaid exothermicity of the metallothermic reaction and the heat of the liquid slag ensure a reaction at high temperatures, therefore they avoid the need to add external heat not derived from the processes involved. It is therefore a slag treatment process of the passive type. It is not an active process which would require the active supply of heat from the outside.

[0101] The step (ii) occurs in a simple container, such as a slag pot or ladle, which acts as a passive reactor, i.e. , lacking heating devices, and, in any case, without heat inputs additional to those of the exothermic metallothermic reaction and of the heat of the liquid slag. The mixing step (i) advantageously occurs even without any preheating of the mixed components.

[0102] Preferably, the process according to the invention comprises, before mixing the starting slag with the Aluminum and / or Silicon and / or Ferrosilicon scrap, a steel production step with the simultaneous production of slag which occurs in an electric arc furnace 1 or in an oxygen converter (BOF). At the end of the steel production process, the slag is extracted from the furnace or from the converter and directly subjected, in still liquid form, to the process according to steps (i) to (iii) simultaneously with a new steel production cycle in the electric arc furnace or in the converter after extracting the steel produced in the previous cycle. Therefore, steps (i), (ii), (iii) are repeated with the simultaneous production of a new batch of steel and slag.

[0103] Therefore, steps (i), (ii) and (iii) of the slag treatment process preferably start and end between the start of a slagging and the next slagging. Within this time interval, between one slagging and the next, a new melting process is carried out. In order to allow this management, steps (i), (ii) and (iii) as a whole preferably have a duration between 30 min and 90 min.

[0104] Therefore, the slag treatment process according to the invention is a process which preferably occurs tap-to-tap. This means that each time that the electric furnace (EAF) or the converter slags and then taps the molten steel, the black slag is processed per se, avoiding the accumulation of said black slag for a cooling thereof and / or for adding thereto other amounts of slag from preceding pours or from other parts of the plant (e.g., white slag from ladle furnaces). In fact, said starting slag in step (i) is not mixed with slag from other processes. This allows for a faster treatment of the still-hot slag and avoids further dispensers for the addition of these further materials.

[0105] Steps (i), (ii) and (iii) advantageously occur in air and not in controlled and inert atmosphere.

[0106] Preferably, the Aluminum and / or Silicon and / or Ferrosilicon scrap fed in step (i) is previously shredded.

[0107] By way of mere example, the new slag formed in step (ii) by the process of the invention, characterized by a reduced iron oxide content, and preferably by a reduced content of oxides of steel alloying metals, comprises, in percentages by weight:

[0108] 40 - 57 % CaO;

[0109] 15 - 30 % SiO2;

[0110] 20 - 30 % AI2O3;

[0111] 1 - 4 % MgO;

[0112] < 2 % of iron oxides.

[0113] Furthermore, advantageously, said Iron (or steel) and, preferably, said steel alloying metals forming an alloy, produced in step (ii), comprise:

[0114] 96 - 99 % Fe; 0 - 4 % Si;

[0115] 0 - 3.5 % C; and

[0116] 0 - 4 % other metals, such as Mn, V, Cr.

[0117] A plant for treating slag, suitable for carrying out the above-described process, is described below. Such a plant, in all the embodiments thereof, comprises:

[0118] - a first container adapted to contain said starting slag in liquid form;

[0119] - a second container adapted to receive both a flow of the starting slag in liquid form, poured from said first container, and the Aluminum and / or Silicon and / or Ferrosilicon scrap, and the possible Carbon and / or the possible further conditioning oxides, previously added to said starting slag during the pouring from said first container, said second container acting as a metallothermic reactor, and also adapted to separate, preferably by gravity, the iron, and preferably other steel alloying metals, in liquid phase from the treated slag which incorporates said aluminum oxides (AI2O3) and / or silicon oxides (SiCfe).

[0120] A dosing device 6 is provided for dosing the amount of Aluminum and / or Silicon and / or Ferrosilicon scrap, and possibly Carbon and the further conditioning oxides, to be fed, in particular configured to feed these elements in a controlled manner, in an amount in kg from 0.05x to 0.15x for each amount x in kg of starting slag.

[0121] Advantageously, said dosing device 6 has at least one outlet section proximal to a pouring opening of said first container, when said first container is in a pouring position, whereby the Aluminum and / or Silicon and / or Ferrosilicon scrap is intercepted by the flow of the starting liquid slag exiting from said first container, allowing the liquid slag and scrap to be mixed before entering into the second container.

[0122] In particular, the outlet section of the dosing device 6 is proximal to said pouring opening of the first container, when said first container is in pouring position, and distal from said second container.

[0123] Preferably, the distance between the outlet section of the dosing device 6 and the closest point of the pouring opening with respect to said outlet section, when the first container is in a pouring position, is in a range from 0.3 to 2 meters. This distance, which corresponds to the minimum distance between the outlet section of the dosing device 6 and the pouring opening of the first container in the pouring position, is important to better ensure the correct feeding of the scrap and compliance with the operating conditions prescribed by the process.

[0124] In a first embodiment of the plant of the invention, diagrammatically shown in Figure

[0125] 2, said first container is an electric arc furnace 1 , which produces steel and said starting slag, and is provided with at least one pouring or slagging opening 2, and said second container is a slag pot 3.

[0126] Said at least one slagging opening or slag door 2 is adapted to directly feed the flow of the starting slag into said slag pot 3.

[0127] In a second embodiment of the plant of the invention, diagrammatically shown in Figure 5, said first container is a slag pot 3 adapted to move from a first position or slagging position, proximal to the electric arc furnace 1 (or the oxygen converter), and in which said slag pot can receive the flow of the starting slag in liquid form from at least one slagging opening or slag door 2 of said electric arc furnace 1 , to a second position, distal from the electric arc furnace, in which said slag pot can pour, for example being tiltable, in a pouring position said starting slag in liquid form into the second container 3’.

[0128] In a third embodiment of the plant of the invention, diagrammatically shown in Figure 6, said first container is an oxygen converter 11 , which produces steel and said starting slag and is provided with at least one pouring or slagging opening 2, and said second container is a slag pot 3.

[0129] Said at least one pouring or slagging opening 2, which is the upper opening of the converter, is adapted to directly feed the flow of the starting slag into said slag pot

[0130] 3.

[0131] Preferably, in all the embodiments, the second container is adapted to be tilted to pour, once the metallothermic reaction is completed, a flow of the treated slag. Coolant addition means 8 can be provided, for adding a coolant, e.g., water, to said treated slag flow 9, so as to obtain a treated slag in granular form.

[0132] Alternatively, however, it is not excluded that a tapping is carried out through openings or holes arranged at different heights along the wall of the second container, which correspond to the different heights reachable by the residual contents of the metallothermic reaction, said residual contents being uniformly distributed in height based on their own density. Advantageously, the dosing device 6 has at least one outlet section proximal to the at least one slagging opening 2, or proximal to the second position of the slag pot 3, so that the Aluminum and / or Silicon and / or Ferrosilicon scrap and the possible further conditioning oxides and / or the possible carbon are intercepted by the flow of the starting slag exiting from said at least one slagging opening 2, or poured from the slag pot 3 in said second position.

[0133] Such a dosing device 6 is configured to feed the Aluminum and / or Silicon and / or Ferrosilicon scrap, and, as said, possibly Carbon and further conditioning oxides, continuously or discontinuously, as shown in Figures 3 and 4, for example.

[0134] In a preferred variant of the plant of the invention, the following can advantageously be provided:

[0135] - weight sensors 4 to detect weight data of the second container (3 in Figure 2; 3’ in Figure 5);

[0136] - a control system 7 adapted to receive the weight data, detected by said weight sensors 4, and accordingly control the dosing device 6.

[0137] Preferably, the dosing device 6 comprises at least one rotary valve or at least one feeding means which can work under pressure or at atmospheric pressure.

[0138] Optionally, at least one buffer, or hopper, 5 is provided for feeding the Aluminum and / or Silicon and / or Ferrosilicon scrap and any carbon upstream of the dosing device 6, and connected to the latter. A further buffer or hopper can also be provided for feeding said further conditioning oxides, upstream of the dosing device 6 and connected to the latter.

[0139] Preferably, the plant according to the invention comprises a shredding device (not shown) upstream of said at least one buffer 5 and of the possible further buffer, for shredding said Aluminum and / or Silicon and / or Ferrosilicon scrap and said possible further conditioning oxides, reducing the size thereof to make them react more quickly.

[0140] In a further variant of the plant of the invention, an analysis system for the chemical composition of the slag can be provided, associated with the control system 7 configured to dose the respective necessary amounts of Aluminum and / or Silicon and / or Ferrosilicon scrap and optionally of Carbon and of the further conditioning oxides. In the case of also adding Carbon in the mixing step (i), resulting in the production of carbon monoxide in the gaseous phase, it is preferable to also provide a collection system for the fumes generated by the partial reduction of the oxides contained in the slag with the Carbon, as well as for any splashes or metals sublimated from the container 3, which acts as a metallothermic furnace.

Claims

CLAIMS1 . A process for treating slag conning from steel production by means of an electric arc furnace (1), or an oxygen converter, and defined as starting slag, the process comprising the following steps:(i) mixing said starting slag in liquid form, comprising iron oxides (FexOy) and preferably other oxides of steel alloying metals, in particular chromium oxide (CrxOy), with Aluminum and / or Silicon and / or Ferrosilicon scrap;(ii) metallothermic reaction between said iron oxides, and preferably said other oxides of steel alloying metals, in particular chromium oxide (CrxOy), with said Aluminum and / or Silicon and / or Ferrosilicon scrap resulting in the production of(a) Iron, and preferably other steel alloying metals, and(b) aluminum oxide (AI2O3) and / or silicon oxide (SiCh) possibly mixed with other pre-existing and / or added oxides in the slag;(iii) separating Iron, and preferably the other oxides of steel alloying metals, and the treated slag, with reduced content of iron oxide and preferably with reduced content of oxides of steel alloying metals present in the slag, said treated slag incorporating said aluminum oxides (AI2O3) and / or silicon oxides (SiCk) formed in step (ii); wherein said mixing occurs continuously or discontinuously, at least partially by directly feeding said Aluminum and / or Silicon and / or Ferrosilicon scrap into a flow of the starting slag in liquid form during a pouring step from a first container to a second container, thus before entering the second container; wherein said second container receives both the starting slag flow and the Aluminum and / or Silicon and / or Ferrosilicon scrap, previously added directly into said starting slag flow outside said second container, and acts as a metallothermic reactor; and wherein the Aluminum and / or Silicon and / or Ferrosilicon scrap is fed in a controlled manner by means of a dosing device (6) to satisfy the following ratio: for each amount x in kg of starting slag, the Aluminum and / or Silicon and / or Ferrosilicon scrap is fed in an amount in kg from 0.05x to 0.15x.

2. A process according to claim 1 , wherein said first container is the electric arc furnace (1), or said oxygen converter, and said second container is a slag pot (3), and wherein said mixing occurs during a slagging step of the electric arc furnace (1)or of said converter, at least partially by feeding said Aluminum and / or Silicon and / or Ferrosilicon scrap directly into a starting slag flow exiting from at least one slagging opening (2) of said electric arc furnace or said converter.

3. A process according to claim 1 , wherein said first container is a slag pot (3) receiving the flow of starting slag in liquid form from said electric arc furnace (1) or said converter, and wherein said mixing occurs during a step of pouring the starting slag in liquid form from said slag pot (3) into the second container (3’), at least partially by feeding said Aluminum and / or Silicon and / or Ferrosilicon scrap directly into the flow of starting slag poured from said slag pot (3); preferably wherein, before said mixing, the slag pot (3) moves from a first position, or slagging position, in which it receives the flow of starting slag in liquid form from said electric arc furnace (1 ) or said converter, to a second position in which it pours said starting slag in liquid form into the second container (3’).

4. A process according to any one of the preceding claims, wherein, once the metallothermic reaction in the second container has been completed, said second container is tilted to pour a treated slag flow while a coolant is added to said treated slag flow to obtain treated slag in granular form; and wherein Iron and preferably the other steel alloying metals remain in said second container, being heavier than said treated slag.

5. A process according to any one of the preceding claims, wherein the Aluminum and / or Silicon and / or Ferrosilicon scrap is continuously fed directly into the continuous starting slag flow, satisfying said ratio, preferably fed substantially directly proportionally to the amount of starting slag; or wherein the Aluminum and / or Silicon and / or Ferrosilicon scrap is discontinuously fed directly into the continuous starting slag flow, satisfying said ratio; preferably wherein after slagging 10 kg of starting slag, 0.5 to 1.5 kg of Aluminum and / or Silicon and / or Ferrosilicon scrap is fed.

6. A process according to any one of the preceding claims, wherein a detection of the weight of the second container is provided by weight sensors (4), and wherein said dosing device (6) is controlled by a control system (7) which receives as input weight data detected by said weight sensors (4).

7. A process according to claim 6, wherein a detection of the weight of the second container when empty is included, and as the weight of the second container increases during the pouring step from the first container to the second container, being continuously detected by said weight sensors (4), the Aluminum and / or Silicon and / or Ferrosilicon scrap is fed as a function of said ratio by said dosing device (6) controlled by said control system (7).

8. A process according to claim 6, wherein, before step (i), a predetermined feeding of the Aluminum and / or Silicon and / or Ferrosilicon scrap into the second container, when empty, is provided so as to form a bed of said Aluminum and / or Silicon and / or Ferrosilicon scrap to be mixed with the initial part of starting slag poured into said second container.

9. A process according to any one of the preceding claims, wherein in step (i) the mixing of said starting slag in liquid form is also carried out with further slag conditioning oxides and / or with carbon.

10. A process according to any one of the preceding claims, wherein said mixing occurs in a zone upstream of said second container, whereby a mixture comprising said starting slag in liquid form and said scrap is produced before it enters into said second container; preferably wherein said second container is a slag pot or a ladle acting as a passive reactor.

11. A slag treatment plant, suitable for carrying out the process according to any one of the preceding claims, comprising- a first container adapted to contain said starting slag in liquid form;- a second container adapted to receive both a flow of the starting slag in liquid form, poured from said first container, and the Aluminum and / or Silicon and / or Ferrosilicon scrap previously added thereto during the pouring from said first container, acting as a metallothermic reactor; wherein a dosing device (6) is provided for dosing the amount of Aluminum and / or Silicon and / or Ferrosilicon scrap to be fed; wherein said dosing device (6) has at least one outlet section proximal to a pouring opening of said first container, when the latter is in a pouring position, whereby theAluminum and / or Silicon and / or Ferrosilicon scrap is intercepted by the flow of the starting slag exiting from said first container.

12. A plant according to claim 11 , wherein said first container is an electric arc furnace (1) or an oxygen converter for producing steel and said starting slag, provided with at least one pouring opening (2), and said second container is a slag pot (3), and wherein said at least one pouring opening (2) is adapted to feed said starting slag flow directly into said slag pot (3); or wherein said first container is a slag pot (3) adapted to move from a first position or slagging position, in which it can receive the flow of starting slag in liquid form from an electric arc furnace (1 ) or an oxygen converter, to a second position in which it can pour said starting slag in liquid form into the second container (3’).

13. A plant according to claim 11 or 12, wherein said second container is adapted to be tilted to pour, once the metal lothermic reaction has been completed, a flow of treated slag (9); and wherein there are provided coolant addition means (8) for adding a coolant to said flow of treated slag (9) so as to obtain treated slag in granular form.

14. A plant according to any one of claims 11 to 13, wherein the outlet section of the dosing device (6) is distal from said second container.

15. A plant according to any one of claims 11 to 14, wherein the distance between the outlet section of the dosing device (6) and the closest point of the pouring opening with respect to said outlet section, when the first container is in a pouring position, is in a range from 0.3 to 2 meters.

16. A plant according to any one of claims 11 to 15, wherein there are provided- weight sensors (4) for detecting weight data of the second container;- a control system (7) adapted to receive the weight data, detected by said weight sensors (4), and to control said dosing device (6) to satisfy the following ratio: for each amount x in kg of starting slag, the Aluminum and / or Silicon and / or Ferrosilicon scrap is fed in an amount in kg from 0.05x to 0.15x.

17. A plant according to any one of claims 11 to 16, wherein said dosing device (6) comprises at least one rotary valve or at least one feeding means adapted to work under pressure or at atmospheric pressure.

18. A plant according to any one of claims 11 to 17, wherein at least one Aluminum and / or Silicon and / or Ferrosilicon scrap buffer (5) is provided upstream of said dosing device (6) and connected to the latter.

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