FIELD OF THE INVENTION

The present invention concerns a method for melting minerals containing iron and titanium, such as for example but not only, titanomagnetites or titanohematites, for the most part consisting of iron oxides and titanium oxides and also containing vanadium oxides.

In particular, the present invention can be used in the iron and steel field for the optimized production, starting from said minerals containing iron, titanium and vanadium oxides, of liquid cast iron with high vanadium content, and slag with high titanium oxide content.

BACKGROUND OF THE INVENTION

Industrial processes are known, for example from US 3.929.461, for the recovery of titanium and vanadium from ferrous materials which contain them.

Indeed it is known that titanium and iron can be present, together with vanadium, in different mineral lodes, in the form of oxides. Minerals mainly composed of iron oxides and titanium oxides and also containing vanadium oxides, can include, for example titanomagnetites, if the iron is present in the form of magnetite (FE ₃ O ₄ ), and titanohematites, If the iron is present in the form of hematite (FE ₂ O ₃ ).

The need to use these minerals containing iron, titanium and vanadium as raw material to obtain cast iron containing vanadium and slag containing titanium oxide is also known.

It is also known that a liquid cast iron can be used for the extraction of vanadium, and that a liquid slag, derived from the melting of said minerals, can be used for the extraction of titanium oxide.

To this purpose, techniques are also known for melting said minerals which use blast furnaces, or submerged arc electric furnaces (SAF) or open hath furnaces, in which the mineral containing iron and titanium is melted under reducing conditions by adding coke.

These known techniques can provide that the minerals are fed into the melting furnace as they are after extraction, or pre-redueed, typically by means of firing in rotary kilns.

Using these techniques it is possible to obtain liquid cast iron containing part of the vanadium possibly present in the original mineral, and an essentially liquid slag containing part of the titanium oxide present in the original mineral. Subsequent workings can provide to obtain the vanadium from the cast iron, while the titanium oxide can be obtained from the slag.

When, for example, the original mineral is ilmenite - a titanohematite - it is possible to obtain, using known techniques, a slag with a high titanium oxide content, that is, greater than 80%.

Known techniques for melting titanohematites have the disadvantage that they operate under process conditions that do not allow to maximize the recovery of vanadium in the liquid cast iron. Indeed, given the extremely high concentration of titanium oxide, the electric conductivity of the slag must be limited when operating in only moderate reducing conditions, so as to obtain a concentration of iron oxide in slag greater than 5% in weight. In practice, therefore, the recovery yield of the vanadium, that is, the ratio between the vanadium contained in the slag and the vanadium initially present in the mineral, using these known techniques, is less than 75%.

The melting furnaces normally used for this type of melting can be electric furnaces of the two types indicated above, i.e. submerged electric arc and open bath furnaces, generally of the fixed type, that is, not tiltable, with walls completely covered with refractory material.

Melting techniques of titanomagnetites are also known which provide to use blast furnaces or electric furnaces, fixed, that is not tiltable, with submerged arc or open bath, for the production of liquid cast iron, that is subsequently treated to recover the vanadium therefrom, and slag which is not treated, downstream, for the recovery of titanium oxide. Indeed the purpose of these known techniques is not to maximize the content of titanium oxide in slag, which is normally less than 40% in weight, but to reduce the vanadium oxide.

For this reason, the melting processes according to the known techniques as above provide to use materials, such as limestone and dolomite, that dilute the slag, reducing the concentration of titanium oxide and therefore also viscosity. In this way a sufficiently fluid slag is obtained to allow to operate at temperatures less than 1,500°C, and, since generally the titanomagnetites used for these processes have a vanadium oxide content greater than 1% in weight, it is possible to obtain a recovery yield of vanadium higher than 75%.

One disadvantage of known melting techniques, that provide melting processes carried out in non-tiltable electric furnaces according to an open bath process, is that they have little flexibility in adjusting the quantity of slag contained in the furnace. The slag (like liquid east iron) is made to exit through through holes made at a predefined height on the walls of the shell of the furnace, which makes it impossible to maximize the emptying of the slag without risking making it exit together with the cast iron, given that the tapping hole must necessarily be made at a certain height from the separation interface between cast iron and slag.

On the other hand, from the process of melting minerals containing iron, titanium and vanadium in order to simultaneously obtain slag with a high content of titanium oxide and liquid cast iron with a high content of vanadium, it is required that, at the end of melting, as much slag as possible is tapped. This necessity is due above all to the fact that the slag itself constitutes a product that can be treated downstream, and moreover has the purpose of preventing the excessive permanence of the titanium oxide produced inside the furnace under reducing conditions (high temperature and high content of coke in the bath), needed to obtain cast iron with a high content of vanadium, so as to prevent the excessive formation of titanium carbides, that can cause an excessive increase in the viscosity of the slag.

It is also known that the high percentage of oxides contained in the minerals (excluding iron oxide) entails a considerable production of slag, that can also be, indicativcly, in the order of about 0.5 tons for each ton of liquid cast iron produced. This therefore entails the need to control the level of slag and its discharge.

Another disadvantage of the state of the art of tapping using lateral channels on a fixed furnace is that it does not not allow an efficient and easy control of the thickness of the slag inside the melting furnace. This clashes with the need to have, in the melting furnace, a controlled thickness of slag so as to obtain both an adequate speed of reducing the vanadium oxide present in the slag by the coke contained in the liquid cast iron, and to facilitate the melting in the liquid cast iron of the coke fed together with the mineral.

It is therefore a disadvantage of the known techniques described above that they have reduced efficiency and little operating flexibility, connected above all to the static configuration of the melting furnace.

This disadvantage is particularly severe where it is intended to maximize the selective recovery of vanadium and at the same time maximize the concentration of titanium oxide in slag using an initial material (a mineral as it is or pre- reduced) for which regulating the quantity of slag inside the furnace is particularly important.

To maximize the selective recovery of vanadium oxide by reduction reactions it is necessary to operate, especially for minerals containing a low percentage of vanadium oxide (for example less than 1% in weight), at a temperature higher than 1,450°C and with a carburized metal bath, that is, with a cast iron with a concentration of carbon higher than 3% in weight.

It is also necessary, in order to maximize the productivity of the furnace, to minimize the quantity of slag inside the melting furnace in such a way that the mineral fed (as it is or pre-reduced) easily comes into contact with the liquid cast iron that contains the coke needed for the reduction process.

Minimizing the content of slag inside the melting furnace also answers the need to minimize the average time the titanium oxide stays in the melting furnace itself, and therefore minimize the formation of titanium carbides which tend to form because of the high concentration of titanium oxide and of the particularly intense reducing conditions. In this way it is intended to limit the viscosity of the slag, in order to improve the functioning of the furnace and the efficiency of reducing the vanadium oxide, whose reduction speed depends not only on the temperature at which the melting process is conducted, but also on the viscosity of the slag.

One disadvantage of the melting in a blast furnace of titanomagnetites is due to obstructions due to the formation of titanium carbides.

Melting in a submerged arc electric furnace would have the disadvantage of not allowing to maximize the recovery of the vanadium in the liquid cast iron, given the low processing temperature that slows down the chemical reduction kinetics, and does not allow a sufficient fluidity of the slag, which is provided with high viscosity associated to the high content of titanium oxide.

Melting in a non-tiltable electric furnace with an open bath poses the problem of lack of flexibility in regulation and in particular has the disadvantage of not allowing to minimize the head of slag inside the furnace. Regulating the head of slag in the reduction process of titanomagnetites is necessary in order to maximize the reduction of vanadium oxide and at the same time to minimize the time the slag remains inside the furnace.

One purpose of the present invention is to perfect a method that, by melting minerals containing iron, titanium and vanadium, such as titanomagnetites, allows to simultaneously obtain both a liquid cast iron with a vanadium content higher than 75% of the vanadium initially contained in the minerals, and also a slag with a titanium oxide content higher than 40% in weight.

Another purpose of the present invention is to achieve a method for melting minerals containing iron, titanium and vanadium that guarantees operating flexibility, high productivity, efficiency and control of the melting process steps.

Another purpose of the present invention is to perfect a method for melting minerals containing iron, titanium and vanadium that allows to contain and optimize both the energy consumption and the time and costs of simultaneously obtaining the maximum degree of reduction of vanadium oxide and the maximum concentration of titanium oxide in slag, so as to render as efficient as possible the subsequent processes of extracting the vanadium from the cast iron and titanium oxide from the slag.

The Applicant has devised, tested and embodied the present invention to overcome the shortcomings of the state of the art and to obtain these and other purposes and advantages.

SUMMARY OF THE INVENTION

The present invention is set forth and characterized in the independent claims, while the dependent claims describe other characteristics of the invention or variants to the main inventive idea.

In accordance with the above purposes, a method for melting minerals containing iron, titanium and vanadium is carried out in a melting furnace provided with a containing body developing around a central axis tiltable by rotation on a tilting axis transverse with respect to said central axis.

The method provides first of all to feed said minerals, possibly with the addition of auxiliary materials such as reducing and/or deslagging agents, at an increasing delivery rate, into the containing body in order to reach a steady-state delivery, and to supply electric energy at an increasing rate in order to reach a steady-state power.

The steady-state delivery rate depends on the productivity to be achieved, on the overall time of the process needed to bring one cast to an end, that is, the time between the final tapping of one cast and that of the subsequent cast, and possible transitory steps in which the feed rate of the mineral can be less than the steady- state delivery rate.

According to the present invention, the steady-state power must be such as to maintain a steady-state temperature equal to or higher than 1,450°C, needed to maximize the recovery yield of the vanadium and to limit the viscosity of the slag, due to the high concentration of titanium oxide that it contains.

The steady-state power depends, according to an inverse proportional ratio, on a specific delivery of the mineral which is fed to the melting furnace; the specific delivery is calculated as ratio between feed delivery and electric power supplied at the same time.

According to some aspects of the present invention, the specific delivery of mineral is comprised between about 10 kg/(min*MW) and about 35 kg/(min*MW), in particular between 15 kg/(min*MW) and about 30 kg/(min*MW).

The specific delivery depends on the characteristics of the mineral fed, in particular on the degree of metalization, intended as a ponderal ratio between metallic iron and total iron, on the ponderal percentage content of carbon, on the temperature of feed, and on the overall ponderal percentage content of oxides, excluding iron oxides.

An increase in the degree of metalization, of the carbon content, of the temperature, each equal to the other factors, corresponds to a reduction in the electric power supplied, while an increase in the content of oxides determines a corresponding greater need for supplied power.

Together with the pre-reduced mineral, auxiliary materials are fed to the furnace, such as reducing materials and slag-forming materials.

Coke is preferably used as a reducing material of a quality suitable to the metallurgical processes, metallurgical coke for example.

The slag-forming materials are used to correct the composition of the slag with the aim of improving its physical properties, in particular reducing the viscosity.

For example, limestone, dolomite and materials containing aluminum oxide (A1203) and/or calcium fluoride (CaF2) can be used as slag-forming materials.

The specific consumptions of these auxiliary materials, measured as mass of auxiliary material fed during the course of one cast for each unit of mass of mineral fed during a cast, depend on the chemical composition of the mineral used, in particular on the ponderal ratio between titanium oxide and total oxides, excluding iron oxides.

According to the present invention, to obtain the required degree of reduction of vanadium and slag with a ponderal content of titanium oxide higher than 40%, the specific consumption of reducing materials is comprised between about 15kg/kg and 40 kg/kg, in particular between about 20kg/kg and 35 kg/kg, while the specific consumption of slag-forming materials is less than 40 kg/kg, in particular less than 35 kg/kg.

It can also be provided that, during the entire process, in order to render the degree of agitation of the liquid bath more efficient and therefore to improve the contact between slag and cast iron in order to promote the reduction process, it is possible to use one or more porous plates, installed on the bottom of the shell, fed with inert gas (for example nitrogen, argon).

The steady-state temperature is maintained, in a first melting step, for a desired melting period in order to melt the minerals containing iron, titanium and vanadium and, as products of this melting, liquid cast iron containing vanadium and liquid slag containing titanium oxide.

During steady-state operating conditions, the correct balance of electric power and delivery of mineral and auxiliary materials allows to obtain a recovery of vanadium in the cast iron equal at least to 75% of the total vanadium contained in the mineral and a recovery of titanium in the slag equal to at least 90% of the total titanium contained in the mineral.

In particular, the present invention allows to obtain slag with a minimum content of titanium oxide greater than or equal to 40%, directly proportional to the ratio between the ponderal percentage of titanium oxide present in the mineral fed, and the overall ponderal percentage of oxides, excluding iron oxides, contained in the mineral fed,

During the melting period, it is preferable to periodically monitor the temperature of the bath and the chemical composition of liquid cast iron and liquid slag in order to control that the reduction process is being carried out efficiently.

The desired melting period continues until a determinate maximum quantity of liquid slag is reached.

The maximum quantity of liquid slag can be defined as the maximum volume of slag that can be physically contained inside the furnace, such that, if this value is exceeded, it causes the spontaneous and therefore undesired spillage of the liquid slag from the containing body of the melting furnace through a lateral deslagging aperture.

In a variant solution, the maximum quantity of liquid slag can be defined by a maximum thickness of slag such that, if exceeded, it would cause difficulties in reducing further material fed, because of the lack of contact with the liquid cast iron. The maximum thickness of slag depends on the productivity required and on the degree of reduction of vanadium oxide required.

Due to the sizes of the melting furnace, the maximum thickness of slag is, according to the present invention, less than 650 mm.

The method therefore provides to detect the chemical composition of the liquid cast iron and the liquid slag thus obtained in order to verify that the liquid cast iron contains at least 75% of the vanadium initially contained in the charge mineral and that the liquid slag contains at least 90% of the titanium oxide initially contained in the charge mineral, and at least 40% of titanium oxide in weight.

If there is a positive result of the verification, the method according to the invention provides to discharge the liquid slag separately, by tilting the containing body in the first direction, and the liquid cast iron, by tilting the containing body in a second direction, opposite the first direction.

The method can also provide, if the result of the verification of the chemical composition is negative, that the process time includes, following the first melting period, a possible period of maintaining the temperature without the addition of mineral in order to complete the reduction process, and then, after a further verification of the chemical composition of both the liquid cast iron and the liquid slag, to discharge the liquid slag separately, by tilting the containing body in the first direction, and the liquid cast iron, by tilting the containing body in a second direction, opposite the first direction,

In variant solutions of the present invention, when the desired melting period has passed, if there is a need to reduce the volume of slag in order to obtain the desired process conditions, the feed of the mineral and the auxiliary materials into the containing body is interrupted and the chemical composition of the slag and the cast iron is detected and verified. After this detection and verification, and after a possible subsequent period of maintaining the temperature with supply of electric power and possible feeding of only auxiliary materials, in order to obtain the degree of reduction required if this has not been reached during the melting period, a partial discharge of liquid slag is carried out, by tilting the containing body in a first direction.

At the end of said possible partial discharge of the slag, the melting method then provides a second melting step, performed in the same way as the first melting step as described above, until the maximum quantity of liquid cast iron that can be contained in the furnace without determining said spontaneous spillage of slag is obtained. The method then provides to verify the chemical composition of the liquid cast iron and of the liquid slag thus obtained and, after a possible period of maintaining the temperature without adding mineral in order to complete the reduction process, to discharge the liquid slag separately, by tilting the containing body in the first direction, and the liquid cast iron, by tilting the containing body in a second direction, opposite the first direction.

In particular, the liquid cast iron is tapped when it contains at least 75% of the vanadium initially contained in the charge mineral and the liquid slag is discharged when it contains at least 90% of the titanium oxide initially contained in the charge mineral.

Thanks to the partial discharge of the liquid slag during the process it is possible to control with considerable flexibility the quantity contained in the containing body, and to selectively control the kinetics of the reactions which occur between minerals, liquid slag and liquid cast iron, and to obtain the desired characteristics for both products of the melting.

In this way, it is possible to selectively and separately discharge the liquid products of the melting of the minerals containing iron, titanium and vanadium, that is, liquid slag with a high vanadium content and liquid cast iron with a high titanium oxide content.

This way of working also allows to control, during the melting, the quantities of both components present inside the containing body.

According to variant aspects of the present invention, before the maximum quantity of liquid cast iron allowed by the geometry of the containing body is reached, and during the second maintenance period, the melting method provides at least one further partial discharge of liquid slag, to maintain the level of the latter less than 650 mm, depending on the geometry of the containing body and to the type of minerals of iron, titanium and vanadium treated.

BRIEF DESCRIPTION OF THE DRAWINGS

These and other characteristics of the present invention will become apparent from the following description of some forms of embodiment, given as a non- restrictive example with reference to the attached drawings wherein:

- fig. 1 is a plan view of a melting apparatus according to the present invention;

- fig. 2 is a lateral view in section of the apparatus in fig. 1, in different steps of the corresponding melting method;

- fig. 3 is a three-dimensional view from below of one part of the apparatus in fig. l ;

- fig. 4 is a lateral sectioned view of a component of the apparatus in fig. 1 ;

- fig. 5 is an example diagram of the steps of a method according to the present invention.

In the following description, the same reference numbers indicate identical parts of the apparatus for melting minerals containing iron, titanium and vanadium according to the present invention, also in different forms of embodiment. It is understood that elements and characteristics of one form of embodiment can be conveniently incorporated into other forms of embodiment without further clari fications. DETAILED DESCRIPTION OF SOME FORMS OF EMBODIMENT We shall now refer in detail to the various forms of embodiment of the present invention, of which one or more examples are shown in the attached drawing. Each example is supplied by way of illustration of the invention and shall not be understood as a limitation thereof. For example, the characteristics shown or described insomuch as they are part of one form of embodiment can be adopted on, or in association with, other forms of embodiment to produce another form of embodiment. It is understood that the present invention shall include all such modifications and variants.

With reference to figs. 1 and 2, an apparatus 10 according to the present invention is usable for melting minerals containing iron, titanium and vanadium, to produce liquid cast iron G and liquid slag S.

In particular the minerals can contain vanadium oxide V ₂ O ₅ to an amount more than 0.5% in weight and have a ponderal ratio between titanium oxides and other oxides - excluding iron oxides - higher than 50%.

In the following description reference will be made, merely by way of non- restrictive example, to minerals containing iron, titanium and vanadium belonging to the family of titanomagnetites.

The apparatus 10 comprises at least an electric arc melting furnace 1 1, which can conventionally include a containing body, or shell 12, the perimeter walls 13 of which delimit a melting chamber 14.

In some forms of embodiment, a protective covering 15 can be associated to the perimeter walls 13 of the shell 12, and can include one or more covering layers, for insulating purposes or for removing the heat, for example layers of refractory covering 15a, located inside the melting chamber as a shield for the perimeter walls 13 from the heat generated inside the melting chamber 14.

Some forms of embodiment, which can be combined with all the forms of embodiment described here, can provide a protective covering 15 that includes one or more layers of cooling panels with bundles of tubes 15b to allow the cooling, by removal of heat by means of a heat-carrying fluid, of the perimeter walls 13 and possibly also of the layers of refractory covering 15a.

The shell 12 can develop around a central axis X and can have, for example, a substantially cylindrical shape or defined by a solid of revolution. The shell 12 can include a first deslagging aperture 17, from which the liquid slag S that is generated as a product of the melting of the titanomagnetites can be discharged, and a second tapping aperture 18, from which the liquid cast iron G derived from said melting can be discharged.

The deslagging aperture 17 and the tapping aperture 18 are made on opposite sides of the shell 12 with respect to the central axis X.

It can be provided that the tapping aperture 18 is made on one side of the shell 12 and that in correspondence thereof the melting furnace 1 1 includes a tapping spout 19, which acts as a vehicle for discharging the cast iron G from the melting chamber 14 toward a first container 20 below the shell 12.

It can also be provided that the tapping aperture 18 is made on the bottom of the shell 12, in which case the tapping spout 19 is not provided.

Below the deslagging aperture 17 there can be a second container 21, able to receive the liquid slag S from the melting furnace 1 1.

Figs. I and 2 are used to describe forms of embodiment in which the melting furnace 1 1 is the type functioning on alternate current and includes three electrodes 16 positioned symmetrically at the center of the melting chamber 14.

However, the present invention can also be applied to melting furnaces on continuous current with one or more pairs of electrodes.

The melting furnace 1 1 can include a support structure 22, visible in figs. 1 and 3, on which the shell 12 rests. The support structure 22 can have an annular portion 22a (circular in fig. 1 and rectangular or square in fig. 3) that delimits a housing 22b inside which the shell 12 is at least partly housed.

The support structure 22 can also include, in its lower part, at least a pair of curved appendixes 23 which act as guide member and define a tilting axis B of the shell 12, transverse, for example orthogonal, with respect to the central axis X.

The curved appendixes 23 allow the shell 12 to rotate with respect to the tilting axis B when the support structure 22 is tilted in a known way by movement means (not shown in the drawings). Linear actuators can be included between the movement means, configured to thrust upward or pull downward a lateral portion of the support structure 22 located in proximity to one of the deslagging 17 or tapping 18 apertures, or electric or hydraulic motors or screw or rack actuators acting directly on the curved appendixes 23.

The tilting of the shell 12 can occur, along a vertical plane containing the central axis X, both in a first direction toward the deslagging aperture 17 and in a second direction, toward the tapping aperture 18.

In possible implementations, the vertical plane along which the shell 12 is tilted passes through the center line of the deslagging aperture 17 and/or through the center line of the tapping aperture 18.

The tilting can be symmetrical or asymmetrical with respect to a condition in which the central axis X is substantially vertical.

The tilting of the shell 12 in the first d irection is defined by a deslagging angle a measured between the tilted position assumed each time by the central axis X and the position of said axis X in the vertical condition.

The tilting of the shell 12 in the second direction is defined by a tapping angle β measured between the tilted position assumed each time by the central axis X and the position of said axis X in the vertical condition.

In possible forms of embodiment, the deslagging angle a can be comprised between 0° and 15°.

In some forms of embodiment, which can be combined with all the forms of embodiment described here, the tapping angle β, which depends on the type of furnace 11 and on the geometry of the shell 12, can be comprised between 0° and 40°.

For example, when tapping from the bottom of the shell 12, the tapping angle β is comprised between 0° and 25° while, when tapping through the tapping spout 19, the angle can be comprised between 0° and 40°.

The method for melting the titanomagnetites carried out by the apparatus 10 in the melting furnace 1 1, if the slag has high electric conductivity, is advantageously the exposed arc type which, as can be seen in fig. 2, provides that the electrodes 16 are raised above the liquid slag S and therefore they set off the electric arc outside the liquid bath consisting of liquid slag S and liquid cast iron G below.

According to the invention, it is possible to feed the melting furnace 1 1 with pre-reduced titanomagnetites with a degree of metalization above 90%, in order to contain the specific energy consumption of the melting process. The method according to the invention provides to simultaneously obtain, from the melting, a liquid cast iron G, usable for the extraction of vanadium and containing, for example, more than 0.5% in weight of vanadium, and a liquid slag S for the extraction of titanium, with, for example, a concentration of TiO ₂ above 40% in weight.

The percentages indicated above are given merely by way of example of possible products of melting titanomagnetites performed according to the present invention.

In particular, by means of said method it is possible to obtain a liquid cast iron G containing at least 75%, preferably at least 80%, of the vanadium initially contained in the mineral of iron, titanium and vanadium, and a liquid slag I, containing at least 90%, more preferably at least 95% of the titanium oxide initially contained in the mineral provided.

Let XTi02 s be the minimum percentage of titanium oxide obtainable in the slag, let XTiO2_m be the ponderal percentage of titanium oxide present in the mineral fed, and let Xox ni be the overall ponderal percentage of oxides, excluding iron oxides, contained in the mineral fed, then the minimum ponderal percentage of titanium oxide X ΠΟ2 can be calculated with the following formula; Χ Π02 s = q*(XTi02_m)/(Xox_m), where q is a coefficient that substantially depends on the quantity of deslagging materials fed.

First of all the melting must be carried out at a high processing temperature, at least 1,450°C, maintained during a whole process period, except for initial and final transitory periods, and possible intermediate transitory periods. The process period can be comprised between 60 mins and 130 mins, for example 94 rnins, depending on the specific requirements linked to the composition of the titanomagnetites to be melted and to the productivity required.

The high temperature is needed to prevent excessive viscosity of the slag, and hence to keep the slag S liquid. The viscosity of the slag renders its discharge from the melting furnace 1 1 problematic, and also slows the kinetics of the process of reduction of the vanadium oxide.

The high viscosity may be due to the high concentration of titanium oxide in the slag. The high concentration of titanium oxide in slag may be due to the minimization of the addition of deslaggers (limestone, dolomite), needed to obtain a liquid slag S with a high TiO ₂ content.

Moreover, the high temperature cited above allows to obtain an adequate recovery of the vanadium in the liquid cast iron G, especially in cases where the concentration of V ₂ O ₅ in the initial titanomagnetite is low, that is, indicatively, less than 1% in weight.

However, the thermal stress of the perimeter walls 13 of the shell 12 is problematic, linked to the high process temperatures.

This stress can also be caused by the simultaneous presence inside the melting chamber 14 of both an electric arc which can be outside the liquid bath, that is, above the liquid slag S, and also the liquid slag S with a high titanium oxide content and therefore with high electric conductivity.

This problem can require the melting method to include a preparatory step, for preparing the melting furnace 1 1 , in which the protective covering 15 is applied to the shell 12.

Fig. 4 is used to describe example forms of embodiment in which the protective covering 15 can include a conventional refractory covering 15a' on the bottom of the shell 12 and a thermally conductive covering 15a" on the lateral perimeter walls 13 of the shell 12.

In possible solutions, the conventional refractory covering 15a', and possibly the thermally conductive covering 15a", can be replaced by a refractory covering suitably studied for the specific requirements connected to the melting process of minerals of iron, titanium and vanadium, in particular to resist the corrosion linked to the high concentration of titanium oxide in slag.

The thermally conductive covering 15a" can be partial and not affect the top part of the perimeter walls 13 of the shell 12, where one or more layers of cooling panels with bundles of tubes 15b can be provided, positioned inside the perimeter walls 13 themselves.

In possible implementations, bundles of tubes 15b or other type of temperature cooling or conditioning devices can be positioned outside the shell 12 and can surround the perimeter walls 13 thereof, in correspondence to the interface zone between liquid cast iron G and liquid slag S.

The thermally conductive covering 15a" and the bundles of tubes 15b can cause the solidification of a portion, located peripherally, of the liquid slag S, which solidification protects the thermally conductive covering 15a" against wear.

At the temperatures (higher than 1,450°C) at which the titanomagnetites are melted according to the method in question, the kinetics of formation of the titanium carbides is high, since it increases with the increase in temperature. The titanium carbides, if present in the slag above a certain concentration, can render the slag excessively viscous, thus neutralizing the effect on this of the temperature itself, and leading to the same disadvantages described above with regard to viscosity.

According to the invention, to conduct a process that simultaneously allows an efficient reduction of the vanadium in the liquid cast iron G and a high concentration of TiO ₂ in the liquid slag S, it is necessary to adopt temperatures higher than 1,450°C, which represent favorable conditions for the formation of titanium carbides. To limit the problems indicated above, it is therefore necessary to minimize, compatibly with the kinetic needs of the process, the time the slag remains inside the furnace.

For this reason, the melting method according to the present invention provides to use a tiltable melting furnace 1 1 as described above and tilting around a tilting axis B.

In the method according to the invention, the melting furnace 1 1 is advantageous compared with fixed electric furnaces, since the process conducted with the tiltable melting furnace 1 1 simultaneously provides the recovery of liquid slag S with a high content of titanium oxide and an efficient reduction of vanadium in the liquid cast iron G, when necessary, by intervening on the liquid slag S.

The tiltable melting furnace 1 1, by means of a suitable inclination (fig. 2), allows to discharge the desired quantity of liquid slag S, avoiding the risk that it might be cast together with the liquid cast iron G, and in general to easily keep under control the head of liquid slag S by managing the times and tilting modes of the melting furnace 1 1.

On the basis of the above, after the preparatory step of covering the shell 12, the method for melting titanomagnetites provides that, if in the shell 12 there is no liquid bottom, or "liquid pool" H, a preliminary step in which a limited quantity is introduced, a few tons for example, of solid metal material, for example scrap iron, in the melting chamber 14.

A preliminary melting of the solid metal material is then carried out, by means of the electric arc, to obtain said liquid pool H, consisting at least of liquid metal. If the melting furnace 1 1 has already carried out operating cycles, the liquid pool H can consist of liquid cast iron G and liquid slag S remaining from a previous melting and suitably maintained inside the melting chamber 14.

The melting furnace 1 1 containing the liquid pool H is ready for the subsequent step of melting proper, that provides an initial step during which electric energy is supplied at a reduced power, comprised between 50% and 80% of the power supplied in the subsequent melting step, and the feed of the solid- state titanomagnetites is started inside the melting chamber of the melting furnace 1 1.

During the initial step, together with the titanomagnetites, or after them, one or more reducing agents, for example coke, can be introduced into the melting furnace 1 1.

According to the invention, it is advantageous to maintain desirably reducing conditions inside the melting chamber 14,; to this purpose, particular strategies can be used to prevent the entrance of air inside it. For example, a careful and hermetic closing of the possible gaps between components of the furnace can be provided, by adding sealing refractory material.

In possible implementations, at the same time as the titanomagnetites or after them, possible deslagging agents can be introduced into the melting chamber 14, for example, limestone and/or dolomite and/or materials containing oxides such as A12O3 and CaF2.

Before being inserted in the melting furnace 1 1, the titanomagnetites are subjected to chemical analysis to identify the content of each component involved (iron, vanadium, titanium oxide), thus determining an initial known quantity thereof.

In a subsequent transitory step, the supply of electric energy is increased until a steady-state power is reached, proportional to the productivity required and to the type of mineral, as will be clear from the detailed description reported hereafter; the delivery of solid material is also increased, that is, titanomagnetites and/or reducing and/or deslagging agents, fed to the melting furnace 1 1 until the steady-state temperature of more than 1,450°C is reached.

The steady-state temperature is maintained for a minimum melting period of about 30 minutes, for example comprised between 30 minutes and 130 minutes. The time profiles for supplying electric power and feeding the melting furnace 1 1 with pre-reduced mineral depend on the chemistry of the latter, which determines the energy needed for melting. The profiles have already been studied, preliminarily, for a particular type of pre-reduced mineral, and in relation to a determinate required productivity.

After said melting period, the melting method can include an intermediate step during which samples of liquid slag S and liquid cast iron G are taken, and the chemical composition of the liquid slag S and the liquid cast iron G is then verified.

At the end of the melting period, moreover, it is possibly provided to interrupt the feed of titanomagnetites and possible auxiliary materials such as reducing and/or deslagging agents.

In some cases, the melting period can define the time needed to obtain the maximum quantity of liquid cast iron permitted by the geometry of the shell 12.

In other forms of embodiment, the melting period can be followed by a period of maintenance of the maximum temperature for about 5 minutes, for example comprised from 2 to 10 minutes, during which the electric energy is supplied without mineral feed.

After, or in association with, the verification of the chemical composition, the intermediate step can also provide the partial discharge of a part considered excess of the liquid slag S. This can be done through the deslagging aperture 17 after the tilting of the shell 12 by a desired deslagging angle a.

The purpose of the partial discharge is to limit the thickness of liquid slag S inside the melting furnace 1 1 and reduce the volume thereof so as to obtain desired process conditions. Depending on the specific needs or conditions that can occur on each occasion, the partial discharge can allow to discharge substantially any quantity of liquid slag S, even up to 90% of the total liquid slag S present in the melting chamber 14.

The invention provides as a variant that the tilting of the shell 12, both by the deslagging angle a and the tapping angle β, has a precision less than 0.5°, advantageously 0.1°.

Thanks to the precision of the tilting of the shell 12, it is possible to obtain a precise discharge of the liquid slag S even in the order of some tens of kilograms, which confers extreme flexibility and operating precision on the melting method.

It is obvious that obtaining a constant optimal value of the level of the liquid slag S guarantees a better quality of the result.

It can also be provided, after said partial discharge, that the melting method can include a second melting step, with its own second melting period, with a minimum duration of about 30 minutes, for example comprised between 30 minutes and 70 minutes.

According to a variant, the melting method provides to alternate more than two melting steps, each with its own melting period, separated by a series of partial discharges of the liquid slag S , also of different quantities.

At any moment during the process time, it is possible to control the temperature and to detect and verify the chemical composition of the liquid cast iron G and the liquid slag S.

On the basis of the results of this detection and verification, when the liquid cast iron G contains at least 75%, for example 80% of the known initial quantity of vanadium and the liquid slag S contains at least 80%, for example 90-95%, of the known initial quantity of Ti0 ₂ , and in particular contains a ponderal percentage of TiO ₂ equal to at least 40%, the supply of electric energy is interrupted and the liquid slag S and the liquid cast iron G are separately discharged.

This separate discharge occurs by means of a first progressive tilting of the shell 12 of the melting furnace 1 1 until a desired deslagging angle a is reached, which can be for example about 8°, and a subsequent second progressive tilting of the shell 12 until a tapping angle β is reached, on the opposite side with respect to the first tilting.

By way of example, in the case of tapping from the bottom of the shell 12, the tapping angle β can be about 14°, while in the case of tapping with a tapping spout 19 the tapping angle β can be about 35°.

We shall now describe an applicative example of the method according to the present invention.

In this example, a productive target is considered that requires to feed on average, during a period of process time (between two successive castings) of about 120 minutes, an average delivery of about 73 ton h of mineral. Supposing thai, within the process time, the feed time of the material is equal to about 94 mins, the average delivery of mineral is 93 ton/h. Bearing in mind an initial transitory step in which the delivery is less than the steady-state delivery, the steady-state delivery Qmax may be about 96 ton/h.

An example of pre-rcdueed mineral containing iron, titanium and vanadium, usable in a melting process according to the present invention, can have the following chemical composition, expressed in ponderal terms:

Fe = 63%, FeO = 6.1% (degree of metalization 93%), C = 2.5%, CaO = 1%, MgO = 2.7%o, SiO ₂ = 4.2%, A12O3 = 4.0%, Ti02 - 15.1%, V205 = 0.7%.

The mineral, a titanomagnetite, is fed at ambient temperature, indicatively less than 40°C.

The productivity target is such that the furnace must be fed with an average delivery of titanomagnetites equal to 73 t/h (referred to the average casting time, that includes the time in which the material is not fed).

From the above, it can be preferable to opt for a melting furnace 11 in which the internal diameter of the shell 12 is sufficient to allow a big contact surface between liquid cast iron G and slag S. It is therefore suggested to use a melting furnace 1 1 with an internal diameter of the shell 12 equal to about 7 m. Moreover, the shell 12 should be such that it can produce and contain, at every easting, a total quantity of liquid cast iron G equal to 100 tons.

The chemical composition reported above for the titanomagnetite given by way of example is such that, for every ton of material fed, a quantity of liquid cast iron G of around 0.69 tons is produced and a quantity of liquid slag S of around 0.30 tons.

Therefore, during the course of a casting, about 145 tons of titanomagnetite must be fed, and the process time is therefore equal to about 120 mins.

Considering for the liquid slag a density of about 2.2 ton/m3, the volume of slag generated in one casting is equal to 13.6 m3.

A diameter of liquid bath is defined, that is, the part of the melting chamber 14 able to contain the liquid cast iron G and the liquid slag S, which is determined by the internal diameter of the shell 12, net of the thickness of the walls of protective covering 15 and the layer of solid slag adhering to the protective covering 15.

The latter layer is present in the hypothesis that a refractory covering 15a thermally conductive is adopted, and with external cooling of the shell 12 by means of bundles of tubes 15b.

If we hypothesize a thickness of the protective covering 15 equal to about 0.55 m, and a thickness of solid slag equal to about 0.33 m, compared to a diameter of the shell 12 equal to 7 m, the diameter of the liquid bath is about 5.3 m, and the surface of the liquid bath 22 is 2 m.

The head of liquid slag S generated during the process time is quantifiable at about 620 mm.

Therefore, if all the liquid slag S generated during the melting remained contained in the furnace, and considering that at the start of easting the thickness of slag must be at least 200 mm so as to submerge the electric arc therein as much as possible, the final thickness of the slag would be about 820 mm. This thickness would make the reduction process extremely difficult, especially during the advanced phase of the process. For this reason, given the chemistry of the material fed and the sizes of the melting furnace 1 1, it is advisable to opt for a process that, at the end of the melting period, comprises an intermediate discharge of liquid slag S.

A single-phase process, without partial discharge, would be possible by increasing the diameter of the melting furnace 1 1, in order to limit the head of liquid slag S; but this, as well as a greater cost of the apparatus, would mean greater heat dispersion and therefore less energy efficiency of the process.

Therefore, having decided to perform a process with intermediate discharge of the liquid slag S, it is established to make the discharge after having fed half of the total material, during said melting period. The thickness of liquid slag S expected at the beginning of partial discharge is therefore equal to about 510 mm.

With reference to fig. 5, an example process comprises the following steps: Minutes 0-7: initial step of preparing the melting furnace 1 1 (control of functionality of apparatuses and components of the melting furnace 1 1); Minutes 7- 1 5: initial transitory step, with simultaneous feed o material at reduced delivery (50-70 t/h) and supply of reduced electric power (35-55 MW), in order to bring the liquid bath to the required process temperature (>1,450°C). At the same time, the reducing agent and the slag-forming materials can be fed, together with the mineral or through alternative entry points.

For example, with reference to the mineral specified above and also supposing that it is characterized by a ponderal ratio between titanium oxide and total oxides (excluding iron oxides) equal to 0.53, the specific consumption (ratio between the kg of material fed during one casting and the kg of mineral fed during one casting) needed to obtain the required degree of reduction of vanadium and slag with a high content of titanium oxide (>40%) is in the following range: coke 20-34 kg/kg, slag- forming materials less than 35 kg/kg.

Minutes 15-54: melting step, with maximum supply of electric power (65-75 MW) and maximum delivery of mineral (85-105 t h).

For example, with reference to a pre-reduced mineral characterized by metalization 93%, carbon 2%, total oxides excluding iron oxides 28%, ambient temperature (5-40°C), the specific delivery of mineral Msp fed to the furnace during the melting step is comprised in the range 20-25 kg/(min*MW). This range determines the maximum steady-state electric power to be supplied in the melting step. The steady-state power being defined Pmax, its value is obtained from the following formula: Pmax=k*Qmax/Msp, For example, with the values of Qmax and Msp as described above, Pmax equals about 70 MW.

With a mineral having a different chemical composition and/or temperature from that considered, the ratio changes between electric power and delivery of mineral during the maintenance step. For different values of said characteristics, the value of the specific delivery of mineral Msp identified above can be corrected by multiplying it by a coefficient k, function of the metalization of the mineral, the percentage content of carbon, the percentage content of oxides, excluding iron oxides, and the feed temperature of the mineral.

During the melting, the reducing agent and slag-forming materials can be fed at the same time as the mineral or through alternative entry points. A few minutes before stopping the feed of the mineral, the chemical analysis of the slag and liquid cast iron is carried out. Minutes 54-57: maintenance step, with possible maintenance or increase of the temperature by supplying electric energy without feeding the mineral (but with possible addition of reducing agent and/or slag-forming materials), in order to complete (based on the chemical analysis of the two steps) the reduction of vanadium before the liquid slag S is discharged.

Minutes 57-61 : partial discharge of liquid slag S by tilting the shell 12 of the melting furnace 1 1. Controlling the quantity of liquid slag S discharged can be carried out by means of visual inspection of the head of liquid slag S inside the shell 12, or by measuring the quantity of liquid slag S discharged in a suitable receptacle, or by a system of weighing the melting furnace 11. The end is to bring the head of liquid slag S in the shell 12 to values comprised between 150 mm and 250 mm.

Minutes 61-66: as minutes 7-15.

Minutes 66-107: as minutes 15-54.

Minutes 1 07- 1 13 : as minutes 54-57.

Minutes 1 13-120: final deslagging by tilting the furnace and subsequent tapping of the liquid cast iron by tilting in the opposite direction.

Tt is clear that modifications and/or additions of parts may be made to the apparatus 10 and method for melting minerals containing iron, titanium and vanadium as described heretofore, without departing from the field and scope of the present invention.

It is also clear that, although the present invention has been described with reference to some specific examples, a person of skill in the art shall certainly be able to achieve many other equivalent forms of apparatus and method, having the characteristics as set forth in the claims and hence all coming within the field of protection defined thereby.