Field of the invention

This invention relates to the production of ferrotitanium, and more particularly to the production of ferrotitanium by aluminothermic reduction. In an advantageous aspect, the invention provides ferrotitanium having a higher titanium content than normally achievable by aluminothermic processes including ilmenite.

Background of the invention

Ferrotitanium is an alloy of iron and titanium that is employed in steel making as a cleansing agent and grain refiner. The titanium is highly reactive with oxygen, sulphur, nitrogen and carbon, with which it forms insoluble compounds that migrate to the slag. The alloy is especially valuable as a deoxidant and as an agent for achieving finer grain structure.

There are two broad processes by which ferrotitanium is usually produced i.e. an induction melting process and an aluminothermic process. In the former of these processes, scrap Fe metal and scrap Ti metal are melted together in a furnace (typically an induction furnace but plasma arc and electroslag furnaces are also employed) to generate an FeTi product with approximately 70% Ti (the eutectic), conventionally represented as FeTi70. This ferroalloy has a melting point of about 1100°C. The 70% grade is targeted firstly because it is the lowest melting point FeTi alloy, which minimises the cost of energy in production, but more particularly as the low melting point is lower than the melting points of the steels and stainless steels where it is used as a deoxidant and grain refiner, which means the alloy will rapidly melt in the steel process. While this is the favoured ferrotitanium product in the developed world, it is rapidly getting priced out of the market as the price of Ti scrap soars. As the demand for Ti metal climbs (mainly driven by aerospace demands), the scrap generated becomes more valuable as it is a key part of the production of Ti metal through recycling. Ferrotitanium producers then have to compete with Ti metal producers for the scrap - and the price rises further. The second process, which predates the induction process just described, is the aluminothermic production of ferrotitanium, which results in a ferroalloy of approximately 30% Ti content, conventionally represented as FeTi30. In this process ilmenite is mixed with powdered aluminium metal, preheated (typically to 400-600°C) and then ignited. The following exothermic (almost explosive) reactions take place:

Ti0 ₂ + /sAI = Ti + ² /3AI ₂ 03

Fe ₂ 0 ₃ + 2AI = 2Fe + AI ₂ 0 ₃ (1) FeO + ² /sAI = Fe + ¹ /3AI ₂ 0 ₃

These reactions result in a molten bath of FeTi with a slag layer floating on top. The phases are separated to recover the ferrotitanium alloy.

The amounts of the feed components (Ti0 ₂ and iron oxide derived from the ilmenite) are adjusted to achieve a composition of approximately 30% Ti, which equates to TiFe ₂ . The adjustment can be by addition of separate rutile mineral to achieve a higher Ti0 ₂ content in the ferrotitanium. Typically, ferrotitanium produced by this process will contain 4-6% Al (as metal), but Al content can be as high as 15% depending on the conditions. The Al ₂ 0 ₃ reports to the slag, which is separated from the melt in order to recover ferrotitanium alloy.

It has also been proposed, for example, in Chinese patent applications 101457270 and 101967531, to employ a feedstock that comprises rutile and a ferrous substance, which can be iron ore concentrate. In another variation, Sokolov et al ("Aluminothermic studies of a liquid partial reduced ilmenite", Minerals Engineering 21 (2008), ps 143- 149), propose a two stage process involving a preliminary reduction of iron in the ilmenite with lime and anthracite. This produces a first stage melt in which the molten metallic iron and oxide phases separate. The slag produced contains about 90% Ti0 ₂ and is thus a partially reduced ilmenite. This fluid slag is then poured into a reactor already containing aluminium and iron ore for the aluminothermic reaction. To obtain higher Ti content ferrotitanium alloys generally requires higher grade T1O ₂ feedstocks and higher ratios of Al metal, though the reaction slows considerably and is often speeded up by the addition of Ba0 ₂ + Al or NaCI0 ₄ or KCIO ₄ + Al, which also provides additional heat. The highest grade from the aluminothermic process of about 45% Ti seems to correspond to TiFe in stoichiometry.

The aluminothermic process is cheaper to operate than the induction melting process but does produce a lower grade product with a significant level of aluminium impurity. Consumption of aluminium is a significant cost. Moreover, the product has a higher melting point and so dissolves more slowly in the steel and stainless steel melts where it is used. This process dominates the production of ferrotitanium in China but is increasingly employed in Europe as Ti scrap prices rise.

It is an object of the invention to provide an improved aluminothermic process for the production of ferrotitanium that at least in part addresses the above discussed relative disadvantages of this process.

Reference to any prior art in the specification is not, and should not be taken as, an acknowledgment or any form of suggestion that this prior art forms part of the common general knowledge in Australia or any other jurisdiction or that this prior art could reasonably be expected to be ascertained, understood and regarded as relevant by a person skilled in the art.

Summary of the invention

The essential concept of the invention is to employ the metallised iron in titanium oxide matrix material known as reduced ilmenite as the primary feedstock to an aluminothermic reduction process for producing ferrotitanium. Reduced ilmenite is a grey-coloured particulate mineral having a similar granular nature to the parent ilmenite from which it is made. It is typically derived by reduction of the iron component in ilmenite (usually around 40%), and is essentially a matrix of substantially metallic iron (for example 90-97% metallised) in titanium dioxide, with minor levels of impurity oxides - typically 2-5% but can be higher depending on the quality of the ilmenite source. It may be represented as FeM i02. Some coalescence of the iron occurs in the reduction process so that the metallic iron comprises microparticles of metallic iron of 1-5micron diameter. The titanium dioxide particles are arpund 150 micron in diameter but may range from 50 to 350 microns in diameter. One known source of reduced ilmenite is as an intermediate in kiln processes for converting ilmenite to synthetic rutile, for example by carbothermic reduction (the Becher process) or by hydrogen reduction. Reduced ilmenite is already employed as a welding electrode material, especially in China.

In accordance with the invention, there is provided a process for producing ferrotitanium by aluminothermic reduction, including reacting reduced ilmenite comprising a solid matrix of substantially metallic iron microparticles in titanium dioxide with aluminium at a temperature whereby titanium dioxide constituents of the reduced ilmenite are exothermically reduced to titanium metal to provide a reaction melt product also containing aluminium oxide and metallic iron from the reduced ilmenite, and recovering titanium and iron form the reaction melt product as an alloy comprising ferrotitanium.

The aluminothermic process for reduction of ferrotitanium has traditionally been viewed as a multi-component reduction process according to the equations (1) set out above, i.e. reduction of both titanium dioxide and iron oxides (primarily Fe2C ^« 3 and FeO) to the metals as the aluminium forms aluminium oxide. The concept of the invention is to appreciate that reduced ilmenite can be employed as the feedstock to simplify the reduction process, in turn thereby increasing the proportion of titanium in the alloy and lowering both the consumption of aluminium and the proportion that remains in the alloy as an impurity. The saving of aluminium is an immediate cost benefit.

It is thought that in the reactions summarised by equations (1) above, an equilibrium establishes between the amount of titanium reduced and the amount of aluminium oxide present: the more Al ₂ 0 ₃ the less Ti and vice versa. By lessening the amount of metal oxide, specifically iron oxides, that have to be reduced, not only is less aluminium required as noted earlier but there will be less AI2O3 present. It is thought that this may shift the equilibrium in favour of increased titanium reduction because of the lower oxidising potential of the system, and therefore a higher titanium content will be achieved in the ferrotitanium.

More particularly, the ferrotitanium produced by the aforedescribed process of the invention preferably has a titanium content greater than 30%, preferably greater than 40%, most preferably greater than 45% and most advantageously of the order of 50%. The ferrotitanium thus produced is of a superior grade in terms of Ti content and is obtained from its feedstock with lower aluminium consumption.

The aluminium metal content of the ferrotitanium produced by the process of the invention is preferably less than 10%.

Preferably, the aforesaid temperature at which the reduced ilmenite is reacted with the aluminium is in the range 1500°C - 1700°C, preferably around 1600°C. In the absence of the highly exothermic aluminium-iron oxide reaction of equations (1) above, other heating regimes are required. One method is to preheat the reduced ilmenite to an intermediate temperature (e.g. in the range 400-1200°C), at which the reduced ilmenite may remain solid, and then to provide any additional heat necessary through electrical energy to maintain the required metal and slag fluidity. This electrical energy may melt the reduced ilmenite. The direct application of electrical energy may be in an induction, arc or other suitable electrical furnace to which the aluminium is delivered and in which the aluminothermic reaction takes place. In an alternative heating regime, the reduced ilmenite is heated from room temperature to the reaction temperature in a single operation, e.g. an induction or arc furnace.

It will be appreciated that, in the process of the invention, the aluminium is being employed primarily as a reductant as, in contrast the conventional aluminothermic process, it supplies only a small amount of the required heat (with the balance having to be supplied externally).

Advantageously, at least 10% of the titanium in the reduced ilmenite is trivalent titanium, Ti ³⁺ or lower titanium oxidation state. With the aforementioned typical derivation of reduced ilmenite by reduction of the iron oxide component in ilmenite to produce an intermediate in the conversion of the ilmenite to synthetic rutile, the intermediate reduced ilmenite is usually around 15% Ti ³⁺ , and the balance primarily Ti ⁴⁺ . For the purposes of the process of the present invention, reduced ilmenite with a Ti ³⁺ content greater than 15% up to 20% or even 30% is advantageous as this further reduces the aluminium required for the aluminothermic reduction of the titanium species to Ti metal in the ferrotitanium alloy.

The aluminothermic reduction process may typically be a batch process. The feed components (reduced ilmenite, optionally some rutile and aluminium powder or ingots) are charged to a suitable furnace and heated to the reaction temperature. The reaction, which may be vigorous rather than almost explosive as in the conventional aluminothermic process, is allowed to run its course, resulting in a molten bath of FeTi with a slag layer floating on the top. When starting with reduced ilmenite, the slag might be described as more a crust on the surface of the metal rather than a true molten slag - mainly because the melting point of AI2O3 is much higher than the system is likely to reach. If necessary, suitable additives (such as CaO) can be added to lower the melting point of the slag to keep it fluid and to facilitate the separation of the ferrotitanium from the slag.

There are various known ways of separating the slag from FeTi and thereby recovering titanium and iron for the reaction metal product as ferrotitanium. These include but are not restricted to:

• pouring off the slag by tilting the vessel with some manual "scraping" of the surface to remove all slag, followed by pouring the FeTi into a refractory or sand lined container to allow it to cool - the resulting block of FeTi is then broken up and crushed to the final size required by the steelmakers.

• tapping of the metal phase from the furnace into a refractory or slag container with tapping being terminated when slag appears in the tap hole, or alternatively the slag diverted to another container for cooling and disposal - the slag and metal phases generally have quite distinct colours making identification of the change relatively simple.

As used herein, except where the context requires otherwise, the term "comprise" and variations of the term, such as "comprising", "comprises" and "comprised", are not intended to exclude further additives, components, integers or steps.

Examples

Table 1 sets out predicted parameters for four aluminothermic reaction scenarios, numbered 1 to 4. Scenarios 1 and 2 are embodiments of the process of the invention employing reduced ilmenite (Rl) feedstock while scenarios 3 and 4 are conventional processes employing ilmenite as the feedstock. Assumptions made in determining the scenarios included the following:

1. System Heat losses = 15% of total heat of products.

2. Metal Recoveries to FeTi

• Ti Recovery 65% · Fe Recovery (in ilmenite) 65% (same as Ti)

• Fe Recovery from metallic iron in Rl 100%

• Sulphur recovery 100% Fe Recovery (FeS) 100% Mn Recovery 100% · Mn Recovery (MnS) 100%

Si Recovery 100%

3. Theoretical Al requirements = 120% of the stoichiometric requirements for the above metal recoveries.

4. Lime (CaO) addition to achieve 1 :1 molar ratio CaO to Al ₂ 0 ₃ . 5. TP → Tr reduction in Reduced llmenite: 15.9%

6. The ilmenite used in scenarios 3 and 4 is the same as used to produce the reduced ilmenite of scenarios 1 and 2.

In scenario 1 , the charge containing reduced ilmenite, aluminium and calcium oxide is preheated to 1650°C by electrical preheating. In scenario 2, the charge is heated by melting in an arc or induction furnace: it will be seen that with all other conditions the same, both scenarios obtain 43.3% titanium and have a residual aluminium content of only 6.6%. Aluminium consumption is the same. However it is observed that even if the Rl is preheated to 1650°C (not practical) there is still not enough heat generated to increase the product temperature >1600°C, which is considered the minimum temperature to melt the slag. The aluminothermic reaction would most likely commence during the preheating. Scenario 2 is thus preferred.

For comparison, scenarios 3 and 4 are conventional processes employing an ilmenite feed. Less separate heating energy is required to achieve the necessary 1600°C product temperature (a preheat temperature of 1074°C and 600°C respectively) particularly in scenario 4 where additional Fe ₂ 0 ₃ is added as haematite to provide energy for the iron oxide-aluminium reactions. Scenario 3 obtains a higher grade product (43.6% Ti) but with much higher aluminium content (13.3%); scenario 4 has the traditional combination of low Ti (29.6%) and high A1 (12.6%). It should be noted that the Ti grade in FeTi is driven by the Ti recovery: the examples above use 65% which is near the upper limits in published literature. The recovery would be affected by the quantity of slag made and whether it melts during the aluminothermic process. In a practical environment, the ilmenite scenarios would probably have lower recoveries and would lower the Ti content of the allow. Table 1

Scenarios

Units 1 2 3 4

Rl Rl llmenite llmenite

Charge Preheat Temp °C 1650 25 1074 600

Rl/llmenite kg/t FeTi 1628 1628 1847 1253

Al Addition kgA FeTi 394 394 667 631

CaO Addition kg/t FeTi 350 350 563 383

Electrical Energy (preheating) kWh t FeTi 1054 838 248

Electrical Energy (melting) kWh/t FeTi 1180

Fe ₂ 0 ₃ Addition kg/t FeTi 452

Product Temperature °C 1430 1600 1600 1600

Slag Make kg/t FeTi 1372 1372 2073 1721

FeTi Composition

Ti % 43.3 43.3 43.6 29.6

Fe % 47.6 47.6 40.6 56.1

Mn % 1.7 1.7 1.7 1.2

Si % 0.8 0.8 0.8 0.5

S % 0.1 0.1 0.0 0

Al % 6.6 6.6 13.3 12.6