WO2016120529A1

WO2016120529A1 - Method for producing titanium oxide-containing slag and pig iron from ilmenite and a plant

Info

Publication number: WO2016120529A1
Application number: PCT/FI2016/050053
Authority: WO
Inventors: Helge Krogerus; Pasi MÄKELÄ; Jarmo Saarenmaa; Sauli PISILÄ; Petri PALOVAARA
Original assignee: Outotec (Finland) Oy
Priority date: 2015-01-30
Filing date: 2016-01-28
Publication date: 2016-08-04
Also published as: EA201791434A1; CA2974263A1; BR112017015641A2; CN107208184A; ZA201704911B; FI20155066A

Abstract

The invention relates to a method for producing titanium oxide-containing slag and pig iron from ilmenite. The method is characterized in that it comprises the steps: a) pre-reducing ilmenite in the presence of reductant in a prereduction reactor to metallize at least part of iron in the ilmenite; b) optionally removing unreacted reductant and gangue from the pre-reduced ilmenite; and c) smelting the pre-reduced ilmenite in an electric arc furnace to separate titanium oxide into a molten slag phase and pig iron into a molten metal phase. The invention also relates to a plant for titanium oxide-containing slag and pig iron production from ilmenite and to titanium oxide-containing slag and pig iron.

Description

METHOD FOR PRODUCING TITANIUM OXIDE-CONTAINING SLAG AND PIG IRON FROM ILMENITE AND A PLANT

TECHNICAL FIELD

The present disclosure relates to a method and a plant for producing titanium oxide-containing slag and pig iron by ilmenite smelting.

BACKGROUND ART

Ilmenite smelting in electric arc furnaces is the main process used in producing titanium oxide- containing slag. The nominal composition of ilmenite is FeTi0₃ or FeO.Ti0₂. In the process, ilmenite is smelted by using electrical energy. A mixture of il- menite and carbonaceous reductant, such as coke or coal, is continuously fed through a furnace roof into a molten bath.

During smelting, the iron oxides present in the ilmenite are reduced to metallic form by the car- bon present in the reductant. This produces the tita^¬ nium oxide-containing slag with high titanium oxide content and low iron oxide content. Metallic iron is collected as pig iron from the bottom of the furnace. Titanium oxide-containing slag is typically sold to pigment producers for downstream treatment where the slag is processed into pure Ti0₂ pigment.

The two basic reactions in ilmenite smelting are the following:

Reduction of FeO from the slag:

FeO + C → Fe + CO (1)

Partial reduction of Ti0₂ in the slag: Ti0₂ + ½C→ TiOi.s + ½CO (2)

Reaction (1) proceeds further to the right than reaction (2) . Reduction reactions that take place during the smelting are highly endothermic. Therefore the smelting process is very energy intensive. When using direct smelting of ilmenite, ashes from the re- ductant as well as gangue and other contaminants in the ore go to the titanium oxide-containing slag, depressing its Ti0₂ content and potentially limiting its processing alternatives.

US 3765868 A discloses a method for selec^¬ tively recovering metallic iron and titanium oxide values from ilmenite ores. The method comprises reduc- ing ilmenite ore in particular form by heating in the presence of an iron oxide-reducing agent and separat^¬ ing the reduction product into magnetic and non^¬ magnetic fractions. The magnetic fraction in the par^¬ ticulate form is subjected to electrical arc smelting with the addition of carbon by heating to a temperature above about 1730 °C. The resulting molten iron and fluid slag layers are separated. The fluid slag comprises at least about 93.5% Ti0₂ and not more than 6.3% FeO.

US 6306195 Bl discloses a process for prepa^¬ ration of high grade synthetic rutile from ilmenite with pig iron as a by-product. The process comprises subjecting ilmenite to reduction with coal, cooling and removing unreacted coal to obtain a product having 80-95% metallization, smelting the metallized ilmenite mixed with less than 10% carbon (w/w) in a transferred arc plasma using arc current under flow of inert gas for a fixed time, and separating the metal as pig iron and Ti0₂ as slag.

WO 2006048283 Al discloses a process and a plant for producing titania slag from ilmenite. Granu^¬ lar ilmenite is partially reduced in a reduction reac^¬ tor at a temperature of at least 900 °C. The pre- reduced ilmenite is transferred into an electric fur- nace, wherein the inlet temperature of the ilmenite entering the furnace is at least 550 °C, and molten in the presence of a reducing agent, thereby forming liq^¬ uid pig iron and titania slag.

US 8088195 B2 discloses a method for manufac^¬ turing titanium oxide containing slag from a material including titanium oxide and iron oxide. The method includes heating a raw material mixture including ti^¬ tanium oxide, iron oxide, and carbonaceous reductant in a reducing furnace, reducing the iron oxide in the mixture to form reduced iron, feeding the resultant mixture to heating melting furnace to melt the reduced iron and separate the reduced iron from a titanium ox^¬ ide containing slag, and discharging the titanium oxide containing slag out of the furnace. The raw mate^¬ rial mixture is agglomerated to enhance heat transfer efficiency in rotary hearth furnaces, which are used as a reducing furnace and a heating melting furnace.

SUMMARY

The purpose of the present disclosure is to provide a new method and a plant for production of ti^¬ tanium oxide-containing slag and pig iron by ilmenite smelting .

The method according to the present disclo- sure is characterized by what is presented in claim 1.

The plant according to the present disclosure is characterized by what is presented in claim 23.

The titanium oxide-containing slag according to the present disclosure is characterized by what is presented 29.

The pig iron according to the present disclo^¬ sure is characterized by what is presented in claim 30. The method and the plant according to the present disclosure can offer at least one of the fol^¬ lowing advantages over prior art:

The electricity consumption in the titanium oxide-containing slag and pig iron production process can be reduced.

The quality of titanium oxide-containing slag can be improved. Alternatively, lower-quality carbona^¬ ceous reductant can be used without compromising the quality of titanium oxide-containing slag. Further, the amount of reductant can be reduced during smelt^¬ ing .

The amount of carbon in the pig iron can be reduced by using the method according to the present disclosure.

The gas formation during smelting of pre- reduced ilmenite is smaller than when non-pre-reduced ilmenite is used. This allows the construction of a smaller gas-handling system in the electric arc fur- nace .

If the pre-reduced ilmenite that is fed to the electric arc furnace is pre-heated, the energy consumption can be further decreased and the capacity of the electric arc furnace can be increased.

BRIEF DESCRIPTION OF THE DRAWINGS

The accompanying drawings, which are included to provide a further understanding of the invention and constitute a part of this specification, illus- trate embodiments of the invention and together with the description help to explain the principles of the invention. In the drawings:

Fig. 1 is a flow chart illustration of the method according to the present disclosure.

Fig. 2 is a schematic presentation of a plant according to the present disclosure. DETAILED DESCRIPTION

In one aspect, a method for producing tita^¬ nium oxide-containing slag and pig iron from ilmenite is disclosed. The method comprises the steps of pre- reducing ilmenite in the presence of reductant in a pre-reduction reactor to metallize at least part of iron in the ilmenite; and smelting the pre-reduced il^¬ menite in an electric arc furnace to separate titanium oxide into a molten slag phase and pig iron into a molten metal phase. The method according to the pre^¬ sent disclosure may comprise an additional step of op^¬ tionally removing unreacted reductant and gangue from the pre-reduced ilmenite after the pre-reduction step.

Alternatively, the method comprises the steps of a) pre-reducing ilmenite in the presence of carbonaceous reductant in a pre-reduction reactor to metallize at least part of iron in the ilmenite; b) optionally removing unreacted carbonaceous reductant and gangue from the pre-reduced ilmenite; and c) smelting the pre-reduced ilmenite in an electric arc furnace to separate titanium oxide into a molten slag phase and pig iron into a molten metal phase.

The titanium oxide-containing slag contains mainly titanium oxide. By titanium oxide is herein meant all forms of titanium oxide present in the il^¬ menite or in the titanium oxide-containing slag. The majority of the titanium oxide is Ti0₂, but also, for example, Ti₂0₃ can be present, albeit the proportion of other titanium oxides is lower than that of Ti0₂. The slag additionally contains non-reduced iron oxides (mainly FeO) in the range of several percentages. The presence of iron oxides is important for the fluidity of the slag. Although their removal might be desired from purity point of view, iron is typically not com- pletely reduced during smelting and the remaining ox^¬ ides are separated into the slag. In one embodiment, the titanium oxide-containing slag contains less than 12 wt%, preferably less than 10 wt%, more preferably approximately 7 wt% iron oxides.

The titanium oxide-containing slag additionally contains various impurities in amounts that de- pend on the raw material and reductant properties as well as on the method steps. Typical impurities are aluminium, magnesium, silicon, chromium and sulfur, all of which are present in amounts of approximately 1-2 wt% or lower.

Pig iron contains mainly metallic iron, alt^¬ hough some carbon and trace amounts of other elements are present. In one embodiment, the pig iron contains less than 1 wt% carbon, preferably less than 0.8 wt%, more preferably less than 0.6 wt%, most preferably less than 0.4 wt% carbon.

By ilmenite is herein meant the titanium ox^¬ ide- and iron oxide-containing mineral feed material for titanium oxide-containing slag and pig iron pro- duction. Ilmenite can be concentrated by any means known in the art, such as flotation concentration. In cases where the gangue content of the raw material is low enough, no concentration is required and raw ilmenite can thus be used. The ilmenite can contain, for example, approximately 45 wt%, 55 wt% or 60 wt% tita^¬ nium oxide. Additionally, the ilmenite can contain, for example, approximately 30 wt%, 35 wt% or 38 wt% iron oxides . In step a) of the method, the ilmenite is pre-reduced. In this step, a part of the iron oxides in the ilmenite is reduced to metallic iron. In one embodiment, the reductant in step a) is a carbonaceous reductant, such as anthracite, coke, coal, char, char- coal, carbon monoxide-containing gas, or a combination thereof. A carbonaceous reductant is used for the re^¬ action which follows the formula FeO + C → Fe + CO. Different carbonaceous reductants vary in their reac^¬ tivity, purity, texture and price, which are all pa^¬ rameters that the skilled person can take into account when selecting a suitable reductant for a given appli- cation. For example, anthracite, coke, coal, char, charcoal, carbon monoxide-containing gas or a combination thereof can be used as the carbonaceous reduct^¬ ant. In one embodiment, anthracite, coke, coal, carbon monoxide-containing gas, or a combination thereof, is used as the carbonaceous reductant in step a) . The grain size of the reductant is preferably 0.5 - 20 mm. This size range optimizes the reduction rate without extensive loss of reductant into the gas phase.

In one embodiment, the reductant in step a) is molecular hydrogen-containing gas, hydrocarbon- containing gas or a combination thereof. A hydrocarbon-containing gas may be, for example, natural gas. In an embodiment, a combination of a carbonaceous re^¬ ductant and another type of reductant is used. For ex- ample, a combination of molecular hydrogen and carbon monoxide gases may be used.

The pre-reduction can take place in a rotary kiln or in a fluidized bed reactor. Typical for these methods is that fossil fuels are used for the heating required by the process. In one embodiment, a rotary kiln or a fluidized bed reactor is used as the pre^¬ reduction reactor in step a) . The ilmenite can be introduced into the pre-reduction reactor in an agglom- erated form, such as pellets, briquettes or nodules. It is also possible to introduce the ilmenite into the pre-reduction reactor in a non-agglomerated form.

Typically, the pre-reduction takes place in a reducing atmosphere. Hot metallic iron very easily re-oxidized to iron oxides and this needs to be avoid^¬ ed by a protective atmosphere, such as nitrogen or ar^¬ gon. Further, if, for example, carbon monoxide is used as the carbonaceous reductant, safety reasons require strictly containing the reductant gas.

In the current method, the metallization de^¬ gree of iron after the pre-reduction is at least 50%. By metallization degree is herein meant the proportion of total iron in the ilmenite that is in the metallic form. In other words, the metallization degree is calculated as (Fe_met/Fe_totai ) 100%. Preferably, the metal^¬ lization degree after pre-reduction is, for example 70% or 80%. It is possible to reach metallization de^¬ grees up to 90% and over, for example 93%. However, since the titanium oxide-containing slag and pig iron are heated in the electric arc furnace for separation, it might be advantageous to use the smelting step for finalizing the reduction of iron oxides in the ilmenite .

In one embodiment, at least 50 wt%, prefera^¬ bly at least 70 wt%, more preferably at least 85 wt% of the iron of the ilmenite is metallized in step a) .

As a side process, also the reduction of Ti^¬ tanium from Ti0₂ to Ti₂0₃ takes place. The progression of this reaction might in some situations limit the possibility of increasing the metallization degree of iron .

In step b) of the method, unreacted carbon and gangue are removed from the pre-reduced ilmenite. This step is optional and it is therefore possible to perform the method according to the present disclosure without the removal of carbon and gangue. The degree of removal depends, for example, on the gangue content of the ilmenite used in the method. The carbonaceous reductant typically contains at least trace amounts of impurities, which are also at least partly removed in this step. This might allow the production of purer titanium oxide-containing slag and/or pig iron. Therefore, it might be possible to use lower-purity carbo- naceous reductant in the pre-reduction without adverse effects on the purity of the final products.

In one embodiment, magnetic separation is used in step b) to remove unreacted reductant and gangue . In one embodiment, magnetic separation is used in step b) to remove unreacted carbonaceous reductant and gangue. In order for the magnetic separation to be feasible, the pre-reduced ilmenite needs to cool or be cooled below its Curie temperature. In this case, mag- netic separation is feasible at temperatures of 700 °C and below.

In one embodiment, unreacted reductant and gangue are removed at a temperature of 700 °C or low^¬ er. In one embodiment, unreacted carbonaceous reduct- ant and gangue are removed at a temperature of 700 °C or lower. When other methods than magnetic separation, based on, for example, density or grain size of the pre-reduced ilmenite are used, it might be advanta^¬ geous to keep the temperature of the pre-reduced il- menite higher than 700 °C to reduce energy consumption in step c) of the method. Thus, in some embodiments, it is possible to perform step b) of the method at a temperature above 700 °C. For example, the temperature at step b) can be 800-1,000 °C.

If it is necessary to cool (either actively or passively) the pre-reduced ilmenite after step a) , it is possible to re-heat it before step c) . This might increase the throughput of the smelting and fur^¬ ther reduce its energy consumption. This preheating can be done at least partly with recycled heat from the process, for example from steps a) or c) . In one embodiment, the pre-reduced ilmenite is preheated be^¬ fore step c) . The reductant can be preheated to, for example 600 °C

To improve the efficiency of the process, the unreacted carbonaceous reductant can be circulated back to the pre-reduction system. This way, heat con- tained in the reductant as well as material use effi^¬ ciency can be improved. In one embodiment, unreacted carbonaceous reductant separated from the pre-reduced ilmenite is circulated back to the pre-reduction reac- tor .

In one embodiment, unreacted reductant sepa^¬ rated from the pre-reduced ilmenite is circulated to the pre-reduction reactor and/or to the electric arc furnace. The reductant may be circulated to the elec- trie arc furnace. The reductant may be carbonaceous reductant. The separation and circulation of reductant to the electric arc furnace allows regulating the amount of reductant carried into the smelting step. Since it is possible to circulate the unreacted re- ductant to the pre-reduction reactor in addition or alternatively to circulating the reductant to the electric arc furnace, the material flow of the unre^¬ acted reductant may be adjusted in a way best suited to the process. In other words, the proportion of un- reacted reductant fed to the pre-reduction reactor may vary between 0 and 100%. The proportion of unreacted reductant fed to the electric arc furnace may vary be^¬ tween 0 and 100%. The unreacted reductant and gangue may, alternatively or in addition to circulating to the pre-reduction reactor or to the electric arc furnace, be removed from the process.

Further, it is possible to regulate the ra^¬ tio between pre-reduced ilmenite and reductant by sep^¬ arating the unreacted reductant from only a part of the pre-reduced ilmenite. In other words, both pre- reduced ilmenite from which unreacted reductant and gangue has been removed and unseparated pre-reduced ilmenite may be fed to the electric arc furnace, i.e. step c) of the process. It is also possible to add raw ilmenite into the electric arc furnace 2. Thus, the circulated unreacted reductant may be used to achieve the correct FeO content for smelting. The unreacted reductant circulated to the pre-reduction reactor and/or to the electric arc furnace may be heated or cooled depending on the process specifics. In step c) of the method, the pre-reduced ilmenite is smelted in an electric arc furnace. In this step, titanium oxide and metallic iron are sepa^¬ rated into a molten slag phase and metal phase, re^¬ spectively. It is to be emphasized that this step may contain further reduction of iron oxides present in the pre-reduced ilmenite. However, this is not neces^¬ sary and embodiments can be envisaged, wherein step c) is used only to separate metallic iron and titanium oxide-containing slag.

The bottom metal contains substantially only metallic iron so that iron oxides remain in the slag. Therefore, the iron oxide content of the slag phase depends on the metallization degree of iron present in the ilmenite. This can be adjusted by the amount of carbonaceous reductant in the smelting process. To re^¬ duce the iron oxide content of the slag, the amount of reductant relative to the ilmenite fed to the electric arc furnace is increased. More of the iron oxides are reduced to metallic iron and consequently separated to the molten metal phase. In one embodiment, reductant is added in the electric arc furnace in step c) . In one embodiment, carbonaceous reductant is added in the electric arc furnace in step c) .

In the opposing situation, less of the carbo- naceous reductant relative to the pre-reduced ilmenite feed is introduced into the electric arc furnace. This might be necessary, if the fluidity of the slag de^¬ creases, for example. Sometimes, it is possible not to add any carbonaceous reductant into the electric arc furnace. This might a relevant alternative especially if the metallization degree of the pre-reduced ilmen^¬ ite is in the range of 80-90 %. In one embodiment, no reductant is added in the electric arc furnace in step c) . In one embodiment, no carbonaceous reductant is added in the electric arc furnace in step c) . In other words, smelting is performed in the absence of added reductant.

If sufficient slag iron oxide concentration is not achieved in this manner, it is possible to sup^¬ plement the pre-reduced ilmenite with raw ilmenite that has not been pre-reduced. In one embodiment, raw ilmenite or ilmenite without pre-reduction is added in the electric arc furnace in step c) . The addition of raw ilmenite could be used as an alternative, for ex^¬ ample, if process step b) of separating the pre- reduced ilmenite from the unreacted carbonaceous re- ductant is to be omitted and the pre-reduced ilmenite contains substantial amounts of the reductant.

Typically, the ilmenite is agglomerated be^¬ fore being charged into the electric arc furnace in order to increase its particle size. This needs to be done if a substantial portion of the ilmenite parti^¬ cles would be blown away before being incorporated in^¬ to the molten slag. After pre-reduction, however, it might not be necessary to agglomerate the ilmenite. This is due to higher specific gravity of the pre- reduced ilmenite compared to non-pre-reduced ilmenite. In one embodiment, the pre-reduced ilmenite used in step c) is non-agglomerated.

In one embodiment, step c) is performed in a direct current electric arc furnace.

A direct current electric arc furnace (DC furnace) differs in its construction and using parame^¬ ters from an alternating current electric arc furnace (AC furnace) . For example, finer material can be fed into a DC furnace. It offers more flexible adjustment possibilities for adjusting slag basicity. Further, the surface reduction of oxides is more efficient in a DC furnace. This is due to the higher surface tempera^¬ ture of the molten material than in an AC furnace. Further, the volatile components are easier to trap into the gas phase in a DC furnace. The consumption of the electrode is reduced in a DC furnace, as there is only one electrode, in contrast to the three elec^¬ trodes of an AC furnace. Typically the investment for a DC furnace is also smaller than that of an AC fur^¬ nace .

In one embodiment, the temperature of the molten slag phase in the direct current electric arc furnace is 1,450-1,850 °C, preferably 1,550-1,750 °C, more preferably 1,680-1,720 °C, most preferably ap- proximately 1, 700 °C; and the temperature of the mol^¬ ten metal phase is 40-200 °C, preferably 80-150 °C lower than the temperature of the molten slag phase.

In one embodiment, step c) is performed in an alternating current electric arc furnace.

In one embodiment, the melting temperature in the alternating current electric arc furnace is 1,450-1,850 °C, preferably 1,530-1,750 °C, more pref^¬ erably 1,570-1,700 °C, most preferably 1,650-1,680 °C .

After ilmenite has been pre-reduced, the metallized iron very easily re-oxidizes as long as the pre-reduced ilmenite remains hot. Therefore, it has to be protected from oxidizing substances, such as oxygen in the atmosphere. It is possible to cool down the pre-reduced ilmenite and store it in, for example bar^¬ rels, bags or silos. The storage may be air-tight and optionally comprise protective gas. In one embodiment, the pre-reduced ilmenite is stored after step a) , or optionally after step b) , before feeding into step c) .

It is possible, however, to omit the storage step. If kept under protective gas, such as nitrogen or argon, the pre-reduced ilmenite can be directly brought to the electric arc furnace. The process may contain a separation of unreacted reductant (step b) ) , or smelting (step c) ) can take place directly after pre-reduction step (step a) ) without step b) in be^¬ tween .

By a direct feeding is herein meant a pro^¬ cess in which the pre-reduced ilmenite is not stored in intermediate storage or moved to a storage contain- er outside the process flow. It is possible that the pre-reduced ilmenite remains in a bin or other storage vessel, if the storage vessel is coupled with the pro^¬ cess. The coupling can be in the form of conveyer belts, feeding tubes etc. Even if the pre-reduced il- menite is fed directly from step a) or step b) to step c) , it might be necessary to keep a pool of pre- reduced ilmenite within the process to level the speed differences between process steps. In one embodiment, the pre-reduced ilmenite is fed directly from step a) , or optionally from step b) , to step c) .

In the reduction of iron through a carbonaceous reductant, carbon monoxide is formed. It can re^¬ act with atmospheric oxygen in an explosive manner and therefore, process steps where carbon monoxide is formed, need to be protected. In one embodiment, the gas atmosphere of the electric arc furnace of step c) is separated from the gas atmosphere of step a) and/or step b) . In some embodiments, wherein, if step b) is performed, the gas atmosphere of step a) is separated from the gas atmosphere of step b) . The pressure and gas composition of steps a) , b) and c) are adjustable independently of each other. In step c) , the pressure is kept below ambient pressure to avoid gas leakage from the electric arc furnace.

The separation of gas atmospheres in differ^¬ ent process steps can be brought about by a gas seal. Many types of gas seals are known in the art and it is within the knowledge of the skilled person to choose an appropriate arrangement. The electric arc furnace can be configured to function batch-wise or continuously. By a batch- wise configuration is meant operation in which the feed (pre-reduced ilmenite and optionally reductant and/or raw ilmenite) is introduced in to the electric arc furnace in discrete steps. If the process is con^¬ tinuous, the feed material is introduced in to the electric arc furnace substantially continuously. Even a continuous process can have interruptions due to, for example, measurement, monitoring or maintenance. By a semi-continuous process is herein meant a process in which feeding takes place during extended periods, but there are intended breaks of potentially variable length. In all these process types, monitoring of slag and metal depth is possible, and also sampling to as- certain the product properties can be performed.

The continuous, semi-continuous or batch- wise feeding described above can be implemented either directly after step a) or with the optional step b) in between. In one embodiment, the pre-reduced ilmenite is fed continuously or semi-continuously from step a) , or optionally from step b) , to step c) .

The products of the method according to the present disclosure are titanium oxide-containing slag and pig iron. In one aspect, titanium oxide-containing slag is disclosed. It is characterized in that it is produced through a method according to the present disclosure. Especially, the titanium oxide-containing slag might contain Ti₂0₃ in addition to Ti0₂. The tita- nium oxide-containing slag might further contain Ti₃0₅. The different titanium oxide compounds present the two oxidation states of titanium, namely Ti⁴⁺ and Ti³⁺. Their proportions vary depending on the process spe^¬ cifics, especially the FeO content of the slag.

In one aspect, pig iron is disclosed. It is characterized in that it is produced through a method according to the present disclosure. Especially, pig iron of low carbon content is produced. The carbon content of the pig iron produced by the method accord^¬ ing to the present disclosure can contain, for exam- pie, less than 1 wt% carbon, preferably less than 0.8 wt%, more preferably less than 0.6 wt%, most prefera^¬ bly less than 0.4 wt% carbon. It is possible to pro^¬ duce pig iron containing, for example, 0.25 wt% car^¬ bon .

In another aspect, a plant for producing titanium oxide-containing slag and pig iron from ilmen- ite by the method according to the present disclosure is disclosed. The plant comprises

- a pre-reduction reactor for pre-reducing ilmen- ite ;

- optionally a separation station for removing unreacted carbon and gangue from the pre-reduced il- menite; and

- an electric arc furnace for separating titanium oxide into a molten slag phase and pig iron into a molten metal phase. The plant is characterized in that

- the pre-reduction reactor is operationally connected to the separation station and/or to the elec- trie arc furnace for passing the pre-reduced ilmenite to the separation station and/or to the electric arc furnace ;

- the optional separation station is operationally connected to the pre-reduction station for receiving the pre-reduced ilmenite, and to the electric arc fur^¬ nace for passing the pre-reduced ilmenite from which unreacted carbon and gangue have removed to the elec^¬ tric arc furnace; and

- the electric arc furnace is operationally con^¬ nected to the pre-reduction reactor and/or to the sep- aration station for receiving the pre-reduced ilmen- ite, from which optionally unreacted carbon and gangue have been removed.

The operational connection between the pre^¬ reduction reactor and the optional separation station, the pre-reduction reactor and the electric arc furnace, and the optional separation station and the electric arc furnace can be organized with methods known in the art. Each of the operational connections can be independently from each other, for example, a conveyor belt or a tube. The operational connection can comprise storage bins or silos for adjusting the rate of material moving to the next process step. The operational connections can be computer-controlled.

The plant may comprise an operational con- nection between the separation station and the prereduction reactor for circulating unreacted reductant from the separation station to the pre-reduction reactor. The plant may comprise an operational connection between the separation station and the electric arc furnace for circulating unreacted reductant from the separation station to the electric arc furnace. The plant may comprise an operational connection leading from the separation station to the outside of the pro^¬ cess for leading unreacted reductant away from the process. The operational connections for transporting the unreacted reductant may comprise storage arrange^¬ ments. The operational connections leading from the separation station to the pre-reduction reactor and to the electric arc furnace may have a shared storage ar- rangement .

The operational connections may comprise de^¬ vices for heating or cooling the material transported within the plant. Thus, the plant according to the present disclosure may further comprise means for heating or cooling the separated unreacted reductant and gangue . The plant according to the present disclo- sure may further comprise means for heating the pre- reduced ilmenite before feeding into the electric arc furnace. The plant may further comprise means for heating the raw ilmenite before feeding to the pre^¬ reduction reactor.

In one embodiment, the plant according to the present disclosure contains means for feeding the pre-reduced ilmenite the electric arc furnace continu^¬ ously or semi-continuously .

In one embodiment, the gas atmosphere of the electric arc furnace is separated from the gas atmos^¬ phere of the pre-reduction station and/or the separation station.

In one embodiment, the pre-reduction reactor is a rotary kiln or a fluidized bed reactor.

In one embodiment, the electric arc furnace is an alternating current electric arc furnace.

In one embodiment, the electric arc furnace is a direct current electric arc furnace. The embodiments of the invention described hereinbefore may be used in any combination with each other. Several of the embodiments may be combined to^¬ gether to form a further embodiment of the invention. A method, a substance or a device, to which the inven- tion is related, may comprise at least one of the em^¬ bodiments of the invention described hereinbefore.

EXAMPLES

Reference will now be made in detail to the embodiments of the present invention, an example of which is illustrated in the accompanying drawings. Figure 1 illustrates a method according to one embodiment for producing titanium oxide-containing slag and pig iron through ilmenite smelting. This ex- emplary embodiment begins by pre-reducing ilmenite in the presence of reductant in a pre-reduction reactor to metallize at least part of the iron in the ilmenite (step a) ) . Without limiting the current disclosure to any specific theory, in this step, carbon of the re- ductant reacts with the iron oxides present in the il^¬ menite producing metallic iron and carbon monoxide. Step a) takes place in a pre-reduction reactor.

In step b) of the method, unreacted carbon and gangue are removed from the pre-reduced ilmenite. This step is optional and it is within the knowledge of the skilled person to evaluate in each application if the step is necessary.

In step c) of the method, the pre-reduced il^¬ menite is smelted in an electric arc furnace to sepa- rate titanium oxide into a molten slag phase and pig iron into a molten metal phase. Each of the phases can be removed from the electric arc furnace separately, as is known in the art. The method thus produces tita^¬ nium oxide-containing slag on the one hand, and pig iron on the other.

Fig. 2 is a schematic presentation of a plant for producing titanium oxide-containing slag and pig iron according to the present disclosure. The black arrows indicate the direction of the process flow and the boxes indicate equipment present in the plant. Op^¬ tional equipment and method steps are depicted with dashed outline. The plant 3 for producing titanium ox^¬ ide-containing slag and pig iron from ilmenite com- prises numerous structural details relating to process monitoring and safety, for example. All such equipment is omitted from the figure for clarity. In the embodiment of Fig. 2, the plant 3 com^¬ prises firstly a pre-reduction reactor 1. The material components, i.e. the ilmenite concentrate and/or raw ilmenite, the reductant and possible other additives, are brought in to the pre-reduction reactor 1 typical^¬ ly from a storage arrangement 6. In this embodiment, there are three storage arrangements 6, from which ma^¬ terial can be fed into the process. Each storage ar^¬ rangement 6 can be for one material component or for a mixture of two or more material components. The number of the storage arrangements 6 depends on the process specifics. There could be, for example, two or four storage arrangements 6 for feeding the material into the pre-reduction reactor 1. The reductant in a stor- age arrangement 6 may be carbonaceous reductant, such as coke, or other type of reductant, such as molecular hydrogen .

The proportions of the ilmenite concentrate and/or raw ilmenite, the reductant and possible other additives, which are introduced into the pre-reduction reactor 1 are adjusted either already in or before the storage arrangement 6 or during the loading of the pre-reduction reactor 1. The storage arrangements 6 are typically bins or silos with means for controlling the amount of material released for transport further into the process. There can be also mixing devices (not shown) for preparing the material components to be fed in the pre-reduction reactor 1. The release of the material can be computer controlled. The computer- controlled material release can be arranged to respond to process parameters measured later in the process. To this end, the plant 3 according to the present dis^¬ closure comprises a device for receiving at least one process parameter signal and regulating the material release in response to this signal. For example, the pre-reduction degree of the pre-reduced ilmenite can be measured and this information can be used to regu- late the amounts of different material components fed into the pre-reduction reactor 1. Optionally, the pro^¬ portions of the material can still be adjusted during the processing in the pre-reduction reactor 1. The pre-reduction reactor 1 is, for example, a rotary kiln .

The pre-reduction (step a) of the method ac^¬ cording to the present disclosure) is performed in the pre-reduction reactor 1. After this, the pre-reduced ilmenite is transported to either separation station 4 for performing the optional step b) of the method ac^¬ cording to the present disclosure, or to the electric arc furnace 2 for performing the step c) of the method according to the present disclosure. The transport takes place through operational connections 5. Option^¬ ally, there are storage arrangements 6 between the pre-reduction station 1 and the separation station 4 and/or the pre-reduction reactor 1 and the electric arc furnace 2. The storage arrangements 6 are used to regulate the rate of pre-reduced ilmenite going to the next process step. As above, the storage arrangements 6 can be bins or silos and they can be computer con^¬ trolled. It is possible to add further material compo^¬ nents to the pre-reduced ilmenite at the storage ar- rangement 6 or elsewhere along the operational connec^¬ tion 5 to adjust the composition of the pre-reduced ilmenite going to step b) or step c) . For example, raw ilmenite or carbonaceous reductant can be added before step c) .

In case the method comprises step b) , the pre-reduced ilmenite from which unreacted reductant and gangue have been removed is transported from the separation station 4 to the electric arc furnace 2 by an operational connection 5. Also this operational connection, which is, for example, a conveyor, can comprise a storage arrangement 6. The unreacted re^¬ ductant can be recycled back into the pre-reduction reactor 1 for being utilized further (the process arrow is dashed to indicate the optional nature of this alternative) . It is possible that the recycling in^¬ volves purification steps to remove gangue . Means for performing such steps are omitted from the figure for clarity .

The unreacted reductant can be circulated in^¬ to the electric arc furnace 2 for being utilized as reductant in smelting (the process arrow is dashed to indicate the optional nature of this alternative) . It is possible that the circulation involves purification steps to remove gangue. Means for performing such steps are omitted from the figure for clarity. Espe^¬ cially if unreacted reductant is circulated to the electric arc furnace 2, the need for adding further reductant during smelting may decrease. It is possible that the need for adding further reductant is omitted completely. To adjust the process parameters during smelting, it is alternatively possible to add circu- lated reductant and a further reductant, such as an^¬ thracite in the electric arc furnace 2.

Gas seals 7 are indicated along the opera^¬ tional connections 5 in the plant 3 according to the present disclosure. The gas seals 7 are used for sepa- rating the gas atmospheres of individual process steps from each other. Although only one gas seal 7 is presented for each operational connection 5, their number and positioning can vary. In Fig. 2, they are located before each next process step, but the location can be adjusted according to the design specifics of the plant 3. Further, it is possible to have a gas seal 7 both right after the previous process step and before the next process step. In this case, it is possible to regulate the gas atmosphere during the transport of the material and/or pre-reduced ilmenite independently of the gas atmospheres of the process steps. The pre-reduced ilmenite is transported to the electric arc furnace 2 from the pre-reduction sta^¬ tion 1 or from the separation station 4. The operational connection 5 along which the transport takes place is optionally equipped with one or more storage arrangement 6. In Fig. 2, the electric arc furnace 2 is depicted as having a molten slag phase (horizontal hatching) and a metal phase (diagonal hatching) . The tapping positions of slag and metal phases are indi- cated with horizontal bars on the left and right sides of the electric arc furnace 2, respectively. The type of the electric arc furnace 2 is not specified in Fig. 2. It could be a DC furnace, for example, in which case the electric arc furnace 2 would typically have one electrode.

The pre-reduced ilmenite, with optional addi^¬ tives, is smelted in the electric arc furnace, the ti^¬ tanium oxide-containing slag and pig iron are separat^¬ ed and collected. The electrode, and electricity cir- cuiting, gas outlets and inlets, cooling arrangement, gas scrubbers, as well as all other design specifics of the electric arc furnace 2 have been omitted for clarity as their design is obvious for the skilled person. Also freeze lining of solidified titanium ox- ide-containing slag and/or metal phase within the electric arc furnace 2 has been omitted. The freeze lining "insulates" the molten slag from the refractory lining of the electric arc furnace, thus protecting the refractory lining from the corrosive effects of the molten slag. The presence of the freeze lining can be beneficial for both the durability of the electric arc furnace refractory lining as well for the purity of the final product. The description below discloses some embodi^¬ ments of the invention in such a detail that a person skilled in the art is able to utilize the invention based on the disclosure. Not all steps of the embodi^¬ ments are discussed in detail, as many of the steps will be obvious for the person skilled in the art based on this specification.

EXAMPLE 1 .

Ilmenite containing 48 wt% Ti0₂ equivalents and 48 wt% iron oxides (FeO and Fe₂0₃) (batch 1) or 54 wt% Ti0₂ equivalents and 40 wt~6 iron oxides (batch 2) was pre-reduced in a fluidized bed reactor. The pre-reduction was performed in a tempera^¬ ture of approximately 950 °C and in a closed gas envi^¬ ronment. Coke was used as carbonaceous reductant. The process typically produces iron metallization degree of 75-85 %, but metallization degree up to 95 % is at^¬ tainable. The metallization degree of the pre-reduced ilmenite varied between 14 % and 58%.

Ti0₂ equivalents (Ti0₂ ^ekv) were calculated by analyzing the total Ti content of the sample and pre^¬ senting its amount as if only Ti0₂ were present. In other words, the Ti content of samples containing var^¬ iable proportions of Ti⁴⁺ and Ti³⁺ can be presented in a comparable manner.

In this case, the pre-reduced ilmenite was stored in a separate silo and charged to the electric arc furnace through vibrating feeders. However, in one alternative, the pre-reduced ilmenite is fed into the electric arc furnace through a specific feeding silo. Alternatively direct feeding through an expansion in the feeding tube can be used. In these cases, a timed double-seal arrangement is used to keep the gas atmos^¬ phere of the electric arc furnace separate from the atmosphere of the previous step.

The non-reacted coke was separated from the pre-reduced ilmenite through magnetic separation be^¬ fore smelting. Alternatively, density- or grain size- based methods could be used. The recovered coke was recycled back into the pre-reduction process. The mag^¬ netic separation took place at a temperature of 700 °C or below.

The pre-reduced ilmenite was smelted in an alternating current electric arc furnace using crushed coke as a carbonaceous reductant. As comparison, raw ilmenite was used. Coke was added based on the FeO content of the titanium oxide-containing slag, which was determined during smelting. The amount of coke added to the electric arc furnace varied between 1.5 and 6.7 kg per each 100 kg of pre-reduced ilmenite. Coke of different grain sizes can be used and select^¬ ing a suitable grain size depends on the electric arc furnace, feed composition and other process parameters and can be determined by a skilled person. The smelt^¬ ing temperature varied between 1,530-1,579 °C and 1,650-1,680 °C. With the higher temperature range, a higher iron reduction level was achieved.

The resulting titanium oxide-containing slag contained 58.3-75.4 Ti0₂ ^ekv and 5-10 wt% FeO. Addition^¬ ally, the slag contained 1.6-2.7 wt% Si0₂, 2.5-3.3 wt% A1₂0₃, 0.07-0.09 wt% carbon, 0.02-0.05 wt% sulfur and 0.4-1.9 wt% CaO.

The resulting pig iron contained 0.54-0.98 wt% carbon. The concentration of sulfur, titanium, manganese and phosphorus remained low and partly below detection limits.

Compared to non-reduced ilmenite, the elec- trical energy consumption of the smelting was reduced. The calculated electric energy consumption for the pre-reduced ilmenite varied between 610-1,075 kWh/t slag. Without pre-reduction, the corresponding values would have been 808-1,529 kWh/t slag. Thus, the calcu- lated energy consumption was reduced by approximately 25-30 %. EXAMPLE 2

Ilmenite containing 59.8 wt% Ti0₂ and 34.8 wt% iron oxides (FeO and Fe₂0₃) was used for producing pre-reduced ilmenite in a rotary kiln with coal as carbonaceous reductant at a temperature of 1,100-1,150 °C. The pre-reduced ilmenite was then smelted in a di^¬ rect current electric arc furnace for producing tita^¬ nium oxide-containing slag and pig iron.

Two batches of pre-reduced ilmenite were used in the experiment, one with metallization degree of 85.7 % (Batch A) and another one with metallization degree of 58.2 % (Batch B) . Batch A was fed to the electric arc furnace as a 50 %/50 % mixture with raw ilmenite, whereas Batch B was fed to the electric arc furnace as such. The results were compared to a pro^¬ cess using raw ilmenite only. The pre-reduced ilmenite was stored in barrels and charged to the electric arc furnace through vibrating feeders. However, direct feeding through a silo or an expansion in a feeding tube, equipped with a timed double-seal arrangement is an alternative.

The pre-reduced ilmenite was smelted in an direct current electric arc furnace using anthracite as a carbonaceous reductant. The amount of reductant was varied and was 3.5 or 5.7 wt% of total feed (i.e 96.5 or 94.6 wt% of ilmenite and 3.5 or 5.7 wt% of an^¬ thracite) . The amount of anthracite added to the elec^¬ tric arc furnace was thus 3.7 or 5.7 kg per each 100 kg of ilmenite.

The ratio between the power of the electric arc furnace and feed rate was adjusted so that the slag temperature remained at approximately 1,680-1,700 °C, depending on the slag FeO content. All in all the temperature varied between 1,630-1,730 °C. A slag tem- perature of 1690-1720 °C provided titanium oxide- containing slag with 7-8 wt% FeO. The target was to keep slag fluidity at a high enough level. At the same time, a so-called freeze lining of solid slag was formed along the electric arc furnace wall. This pre^¬ vented the dissolution of the electric arc furnace lining into the tapped slag. The molten metal tempera- ture was approximately 1,560-1,580 °C. For example, the molten metal temperature was 1,550 °C.

The electric arc furnace operated with a slight vacuum (0.1 mbar) generated by an off gas fan. The combustible components of the electric arc furnace gas were burned in a combustion chamber.

When Batch B ilmenite was smelted, and 3.4 % carbon relative to the weight of ilmenite was added to the electric arc furnace, the resulting titanium ox^¬ ide-containing slag contained 8.5-9.7 wt% FeO. When the carbon feed ratio was increased to 3.7 wt%, slag containing 5.4-6 wt% FeO was produced. During the ex^¬ periment, the FeO content of the slag varied between 3 and 12 wt% and was typically 5-10 wt%. The amount of carbon was calculated as carbon available for reduc- tion (Cfix) . It was calculated by subtracting the ash, volatiles, sulfur and phosphorus content (wt%) of the carbonaceous reductant from the mass of the carbona^¬ ceous reductant fed into the process.

The bulk of the slag contained titanium oxide (in the form of Ti0₂ and Ti₂0₃) . The Ti0₂ content of the slag varied between 76.9 and 88.4 wt% calculated as Ti0₂ ^ekv. The slag additionally contained 0.15-0.5 wt% sulfur (average 0.10 wt%) , while the phosphorus con^¬ tent remained below the detection limit of 0.05 wt%.

The resulting pig iron contained 0.25 wt% carbon, which is a very low value compared to process^¬ es utilizing non-pre-reduced ilmenite. The metal con^¬ tained 0.17-0.3 wt% sulfur, averaging 0.20 wt%. The phosphorus content ranged between 0.07 and 0.4 wt% with an average of 0.10 wt%. Compared to non-reduced ilmenite, the elec^¬ trical energy consumption of the smelting was reduced as depicted in Table 1.

Table 1: Energy consumption for steady-state smelting process (kWh/t ilmenite)

Pilot scale Industrial scale proj ection*

Raw ilmenite 2, 654 1, 531

Mixture** 2, 359 1, 361

Pre-reduced il^¬ 1,031 1, 171

menite (Batch B)

*thermal efficiency of 78 % is assumed

** 50 %/ 50 % of raw ilmenite and Batch A The calculated electric energy consumption for the industrial-scale projection for the mixture of Batch A pre-reduced ilmenite and raw ilmenite was thus 88 % of the raw ilmenite. When only pre-reduced ilmen^¬ ite (Batch B) was used, the energy consumption was 76 % of the raw ilmenite.

It is obvious to a person skilled in the art that with the advancement of technology, the basic idea of the invention may be implemented in various ways. The invention and its embodiments are thus not limited to the examples described above; instead they may vary within the scope of the claims.

Claims

1. A method for producing titanium oxide- containing slag and pig iron from ilmenite, c h a r a c t e r i z e d in that the method comprises the steps:

a) pre-reducing ilmenite in the presence of reduct- ant in a pre-reduction reactor (1) to metallize at least part of iron in the ilmenite;

b) optionally removing unreacted reductant and gangue from the pre-reduced ilmenite; and

c) smelting the pre-reduced ilmenite in an electric arc furnace (2) to separate titanium oxide into a mol^¬ ten slag phase and pig iron into a molten metal phase.

2. The method according to claim 1, wherein a rotary kiln or a fluidized bed reactor is used as the pre-reduction reactor (1) in step a) .

3. The method according to claim 1 or 2, wherein the reductant in step a) is a carbonaceous re^¬ ductant, such as anthracite, coke, coal, char, char- coal, carbon monoxide-containing gas, or a combination thereof .

4. The method according to claim 1 or 2, wherein the reductant in step a) is molecular hydro^¬ gen-containing gas, hydrocarbon-containing gas or a combination thereof.

5. The method according to any of the preced^¬ ing claims, wherein at least 50 wt%, preferably at least 70 wt%, more preferably at least 85 wt% of the iron of the ilmenite is metallized in step a) .

6. The method according to any of the preced^¬ ing claims, wherein magnetic separation is used in step b) to remove unreacted reductant and gangue.

7. The method according to claim 6, wherein unreacted reductant and gangue are removed at a tem- perature of 700 °C or lower.

8. The method according to any of the preced^¬ ing claims, wherein unreacted reductant separated from the pre-reduced ilmenite is circulated to the pre^¬ reduction reactor (1) and/or to the electric arc furnace (2 ) .

9. The method according to any of the preced- ing claims, wherein the pre-reduced ilmenite used in step c) is non-agglomerated.

10. The method according to any of the pre^¬ ceding claims, wherein the pre-reduced ilmenite is preheated before step c) .

11. The method according to any of the pre^¬ ceding claims, wherein no reductant is added in the electric arc furnace (2) in step c) .

12. The method according to any of claims 1- 10, wherein reductant is added in the electric arc furnace (2) in step c) .

13. The method according to any of the pre^¬ ceding claims, wherein raw ilmenite or ilmenite with^¬ out pre-reduction is added in the electric arc furnace (2 ) in step c) .

14. The method according to any of the pre^¬ ceding claims, wherein step c) is performed in a di^¬ rect current electric arc furnace (2) .

15. The method according to claim 14, wherein the temperature of the molten slag phase in the direct current electric arc furnace (2) is 1,450-1,850 °C, preferably 1,550-1,750 °C, more preferably 1,680-1,720 °C, most preferably approximately 1,700 °C; and the temperature of the molten metal phase is 40-200 °C, preferably 80-150 °C lower than the temperature of the molten slag phase.

16. The method according to any of claims 1- 13, wherein step c) is performed in an alternating current electric arc furnace (2) .

17. The method according to claim 16, wherein the melting temperature in the alternating current electric arc furnace (2) is 1,450-1,850 °C, preferably 1,530-1,750 °C, more preferably 1,570-1,700 °C, most preferably 1, 650-1, 680 °C .

18. The method according to any of the pre^¬ ceding claims, wherein the pre-reduced ilmenite is fed continuously or semi-continuously from step a) , or op^¬ tionally from step b) , to step c) .

19. The method according to any of the pre^¬ ceding claims, wherein the pre-reduced ilmenite is fed directly from step a) , or optionally from step b) , to step c) .

20. The method according to claim 19, wherein the gas atmosphere of the electric arc furnace (2) of step c) is separated from the gas atmosphere of step a) and/or step b) .

21. The method according to any of claims 1-

18, wherein the pre-reduced ilmenite is stored after step a) , or optionally after step b) , before feeding into step c) .

22. The method according to any of the pre- ceding claims, wherein the titanium oxide-containing slag contains less than 12 wt%, preferably less than 10 wt%, more preferably approximately 7 wt~6 iron ox ides .

23. The method according to any of the pre- ceding claims, wherein the pig iron contains less than

1 wt% carbon, preferably less than 0.8 wt%, more pref^¬ erably less than 0.6 wt%, most preferably less than 0.4 wt% carbon.

24. A plant (3) for producing titanium oxide- containing slag and pig iron from ilmenite by the method according to any of the preceding claims, the plant comprising

a pre-reduction reactor (1) for pre-reducing ilmenite ;

- optionally a separation station (4) for removing unreacted reductantand gangue from the pre-reduced ilmenite; and an electric arc furnace (2) for separating tita^¬ nium oxide into a molten slag phase and pig iron into a molten metal phase; c h a r a c t e r i z e d in that the pre-reduction reactor (1) is operationally connected to the separation station (4) and/or to the electric arc furnace (2) for passing the pre-reduced ilmenite to the separation station (4) and/or to the electric arc furnace (2);

the optional separation station (4) is opera- tionally connected to the pre-reduction station (1) for receiving the pre-reduced ilmenite, and to the electric arc furnace (2) for passing the pre-reduced ilmenite from which unreacted carbon and gangue have removed to the electric arc furnace (2); and

- the electric arc furnace (2) is operationally connected to the pre-reduction station (1) and/or to the separation station (4) for receiving the pre- reduced ilmenite, from which optionally unreacted car^¬ bon and gangue have been removed.

25. The plant (3) according to claim 24, wherein the plant (3) contains means for feeding the pre-reduced ilmenite the electric arc furnace (2) con^¬ tinuously or semi-continuously .

26. The plant (3) according to claim 24 or 25, wherein the gas atmosphere of the electric arc furnace (2) is separated from the gas atmosphere of the pre-reduction station (1) and/or the separation station ( 4 ) .

27. The plant (3) according to any of claims 24-26, wherein the pre-reduction reactor (1) is a rotary kiln or a fluidized bed reactor.

28. The plant (3) according to any of claims 24-27, wherein the electric arc furnace (2) is an al^¬ ternating current electric arc furnace.

29. The plant (3) according to any of claims

24-27, wherein the electric arc furnace (2) is a di^¬ rect current electric arc furnace.

30. Titanium oxide-containing slag, c h a r a c t e r i z e d in that it is produced through a meth^¬ od according to any of claims 1-23.

31. Pig iron, c h a r a c t e r i z e d in that it is produced through a method according to any of claims 1-23.