Technical Field The present invention relates to a process for producing titanium slag from ilmenite, and to a corresponding plant.

Ilmenite, which contains titanium dioxide and iron oxides, is one of the most important starting materials for recovering metallic titanium and titanium com- pounds, such as titanium dioxide used for pigment production. Separating the iron from the coal used for reducing the ilmenite usually is effected by electric smelting of ilmenite in a metallurgical furnace, the iron oxides being reduced to metallic iron, which is precipitated from the slag containing titanium dioxide. A major disadvantage of this process is the very high demand of electrical energy, which is about 2200 kWh per ton of titanium slag and represents the majority of the production costs.

Up to now, titanium slag facilities are only economically viable in countries with low power costs, preferably in countries where hydro power is available, such as Canada or Norway, or where power is generated from low cost coal, such as South Africa. Increasing power costs and restriction of power supply for power intensive industries resulted in a negative impact on the economic situation of the slag producers. Currently, about half of the world's titanium slag production uses power generated from coal, whereas the other half uses hydro power. Furnaces of various designs are in operation, such as rectangular or round shape, prebaked or self-baking electrodes, hollow or solid electrodes, feeding of materials through the hollow electrode or through the furnace roof, AC or DC smelting power. The majority of the plants operate according to the conventional smelting technology by feeding cold fresh ilmenite to the smelter. It has been proposed to produce titanium slag on the basis of pre-reduced ilmenite, wherein the ilmenite and a solid reductant, such as coal or char, is introduced into a rotary kiln serving as reduction reactor. The hot off gas of the rotary kiln is directed into an after burning chamber in which the carbon monox- ide and hydrogen contained in the off gas is burned and then the off gas having a temperature of 900 to 1000°C is transferred to a waste heat boiler to generate steam. Due to the high temperatures generated in the after burning chamber it is necessary to inject water to avoid the formation of accretions on the walls of the after burning chamber. The equipment costs are quite high and the energy efficiency is not satisfying.

The present applicant has proposed in document WO 2006/048283 A1 a process for producing titanium slag from ilmenite, wherein granular ilmenite first is partially reduced with a reducing agent in a reduction reactor, then the hot mate- rial having an inlet temperature of 500 to 900°C is transferred into an electrical furnace and molten there in the presence of a reducing agent to form liquid pig iron and titanium slag. The reduction reactor comprises a circulating fluidized- bed into which the ore is introduced after having passed several preheating stages and a carbonization reactor. From the fluidized bed reduction reactor, a mixture of partially reduced ilmenite and char is withdrawn at a temperature of about 1000°C and cooled to about 700°C before it is charged to a magnetic separator, where a fraction rich in titanium dioxide and metallic iron is separated as magnetic product from a non-magnetic fraction. The magnetic fraction is charged into an electric smelting furnace operated at about 1 700°C and produc- ing titanium slag with 75 to 90 wt.-% titanium dioxide and liquid pig iron with more than 94 wt.-% metallic iron. The off gas from the electric furnace contains more than 90 vol.-% carbon monoxide and, after dedusting, is burned in an after burning chamber. The hot flue gas is supplied to a gas heater for heating the fluidizing gas to be introduced into the reducing reactor. While the system dis- closed in document WO 2006/048283 A1 already provides for a substantial reduction of energy consumption in the production of titanium slag, there is the potential of further improvement.

Description of the Invention

It is thus the object of the present invention to further reduce the energy consumption in the production of titanium slag. In addition, CO ₂ emission shall be reduced. According to the present invention, there is provided a process comprising the features of claim 1 . For producing titanium slag from ilmenite, granular ilmenite is partially reduced with a reducing agent in a reduction reactor at a temperature of at least 900°C, in particular 1000-1 150°C. The partially reduced hot ilmenite then is transferred to an electric furnace, where it is smelted in the presence of a reducing agent to form liquid pig iron and titanium slag, which is withdrawn from the electric furnace. Within the context of the present application the terms electric furnace, electric reduction furnace, and smelter are used synonymously to describe the same element. According to the invention, the off gas of the reduction reactor is introduced into a waste heat boiler. Preferably, there is a direct connection between the waste heat boiler and the reduction reactor.

Contrary to the prior art processes, the off gas of the reduction reactor is not supplied to an after burning chamber, in which the carbon monoxide and hydrogen contained in the off gas is burned and then the off gas having a temperature of 900 to 1 100°C is transferred to a waste heat boiler to generate steam. Rather, the invention proposes to abandon the after burning chamber and to connect the waste heat boiler to the reduction reactor. Thereby, equipment costs can be considerably reduced. Further, the water injection necessary in the after burning chamber can be dispensed with as the respective cooling effect is not neces- sary. In a preferred embodiment of the present invention, the reduction reactor is a rotary kiln, to which in particular coal and/or char are added as a solid reductant. In an alternative embodiment, the reduction reactor may comprise a circulating fluidized bed as described in WO 2006/048283 A1 wherein a carbon or hydrogen containing gas is used as reductant.

In order to even further increase the energy efficiency of the process, the off gas of the electric furnace is also introduced into the waste heat boiler.

Preferably, the off gas of the electric furnace is cooled and/or cleaned prior to the introduction into the waste heat boiler.

In particular when coal and/or char is used as reducing agent, it has been shown to be advantageous to subject the partially reduced ilmenite to magnetic separation before charging the material into the electric furnace, in order to separate the magnetic fraction including titanium dioxide and iron oxides from a non-magnetic fraction substantially containing ash and, if used as reducing agent, surplus char. Only the magnetic fraction obtained by the magnetic sepa- ration is transferred into the electric furnace. In this case, the temperature of the partially reduced material used during the magnetic separation preferably is about 700°C. In accordance with the invention, the magnetic fraction subsequently is transferred into the electric smelting furnace without cooling or heating. The energy required for heating the material supplied to the electric furnace to the operating temperature in the furnace on the other hand thus is minimized without a substantial re-oxidation of the partially reduced material before introduction into the electric furnace.

The energy efficiency of the process is even further increased if the surplus solid fuel (char) from the reduction reactor and/or carbon containing waste fines separated by magnetic separation are introduced into the waste heat boiler and burned therein. Thereby, the calorific value of these materials can also be used in the process. A plant in accordance with the invention, which is suitable for carrying out the process described above, comprises the features of claim 7. In particular, the plant comprises a reduction reactor for the partial reduction of granular ilmenite, a magnetic separator for separating the reduced ilmenite from a non-magnetic fraction by magnetic separation, and an electric furnace for smelting the ilmenite in the presence of a reducing agent to produce titanium slag and pig iron. A waste heat boiler is connected to the reduction reactor.

Preferably, the waste heat boiler comprises a radiation section and a convection section, wherein the radiation section is connected to the reduction reactor.

In a particularly preferred embodiment, additional burners are provided in a side wall of the waste heat boiler to burn the solid carbon containing residues introduced into the waste heat boiler. Preferably, the reduction reactor is a rotary kiln. In an alternative embodiment, the reduction reactor comprises a circulating fluidized bed.

Downstream the reduction reactor a cooler may be provided. Preferably, the cooler is a rotary cooler, at which indirect cooling takes place by heat exchange with water to cool the solid material leaving the reduction reactor at a temperature of about 1000°C to a temperature of about 700°C prior to charging it to the magnetic separator. In the waste heat boiler, steam is produced as generally known in the art. Preferably, a turbine generator is provided downstream the waste heat boiler for electric power generation.

It is within the present invention to provide a return conduit for transferring off gas from the electric furnace into the waste heat boiler, so that the heat of this off gas can also be used in the process.

The invention will now be described in more detail on the basis of a preferred embodiment and the accompanying drawing. All features described and/or illustrated form the subject-matter of the present invention per se or in any combination, independent of their inclusion in the claims or their back reference.

Brief Description of the Drawing

Fig. 1 is a flow sheet of a plant according to the present invention,

Fig. 2 is a diagram comparing the capacity, CO ₂ emission and power consumption of plants according to the prior art and plants comprising features of the present invention,

Fig. 3 shows the possible annual saving of power costs if the present invention is included in an existing titanium slag smelter.

Description of the Preferred Embodiments

In the process for producing titanium slag from ilmenite as shown in Fig. 1 , a mixture of ilmenite and coal and/or char is fed from bins 1 , 2, 3 onto a roller feeder 4 or equivalent feeding device and supplied to a gas-tight double pendulum valve 5 and from there to the entry section of a reduction reactor 6, in particular a rotary kiln. The rotary kiln preferably is inclined at 1 to 3% to assist the movement of the solid material through the reactor. Process air is introduced into the rotary kiln through shell air fans 7. Additional coal for temperature control and/or sulphur, if required for partial manganese removal, and air are injected counter-currently through lance 8. In the reduction reactor 6 the ilmenite is partially reduced by the reducing agents, in particular by coal and char, at a temperature of 1000 to 1 150°C, in particular about 1 100 °C, to a degree of metallization of about 70%, based on its iron content. From the reduction reactor 6, a mixture of partially reduced ilmenite and surplus char with a temperature of about 1 100°C is continuously withdrawn via chute 9 into a rotary cooler 10 where it is cooled indirectly by water to a temperature of about 700°C. Similar to the rotary kiln, the rotary cooler 10 is slightly inclined to assist the movement of the material flow. At the outlet end of the rotary cooler 10 bigger ore lumps are removed, e.g. by means of a screen, and crushed and separately treated to recover the titanium and iron.

The remaining material is charged to a magnetic separator 1 1 at a temperature of about 700°C, where magnetic material (titanium dioxide and metallic iron) is sepa- rated from non-magnetics by hot magnetic separation. The non-magnetics comprise in particular ash and surplus char not used for the reduction in the reduction reactor 6.

The magnetic fraction (reduced ilmenite) then is charged into an electric reduction furnace 12 (smelter) operated at about 1700°C to produce titanium slag with 75 to 90 wt.-% titanium dioxide and liquid pig iron comprising more than 94 wt.-% metallic iron.

The off gas of the reduction reactor 6 is introduced into a waste heat boiler 20 having a radiation section 21 and a convection section 22. The reduction reactor 6 is directly connected to the radiation section 21 of the waste heat boiler. Additional solid fuel, in particular surplus char recovered by the magnetic separator 1 1 and air also introduced into the waste heat boiler 20 to promote the combustion. The off gas from the electric reduction furnace 12 contains a substantial amount, often more than 90 vol.-% of carbon monoxide and, after dedusting, is preferably recycled to the waste heat boiler 20 through a return conduit 23.

Additional burners 24 may be installed in the side wall of the waste heat boiler 20 to burn the carbon containing material. An emergency flap 25 is activated at pro- cess upset conditions. Dust settling at the bottom of the waste heat boiler 20 is withdrawn and recycled to the reduction reactor 6 through line 26. The off gas is withdrawn from the waste heat boiler 20 through outlet 27 and cleaned in an electrostatic precipitator (ESP) 28 before it is discharged through stack 29. The remaining dust may be recycled to the reduction reactor 7 or returned to the mine as waste.

The heat produced in the waste heat boiler 20 is used to produce high pressure steam in a steam drum 30 at a pressure of 40 to 60 bar and a temperature of 350 to 600°C, preferably 400 to 540°C. The steam is further heated by passing it through the convection section 22 of the waste heat boiler 20 and fed to a turbine generator 31 to produce electric energy. The expanded steam is condensed in a condenser 32 associated with a cooling tower 33. The water produced thereby is deaerated in a deaerator 34 prior to being supplied to a feeding tank 35 for use in the steam production. Additional raw water may be added after having been de- mineralized in a demineralizer 36. The water then is passed through the convection section 22 of the waste heat boiler 20 to preheat it prior to its introduction into steam drum 30.

By using preheated, reduced ilmenite as well as the heat from the reduction reactor 6 and the electric reduction furnace 12 the energy consumption of the electric furnace 12 can be reduced by about 60% as compared with conventional smelting of raw ilmenite. In addition, it is possible to substantially increase the capacity of the electric furnace 12 and to improve the product quality by reducing the ash amount introduced into the smelter.

Examples

Fig. 2 shows the influence of using pre-reduced ilmenite and the use of latent heat from the various plant devices on the capacity, the CO ₂ emission and the energy consumption of a plant for producing titanium slag.

In Fig. 2, option A refers to the standard process wherein the ilmenite is introduced into the smelter at room temperature (25°C) and with zero metallization. In option B the degree of metallization is 70% but still the ilmenite is fed at a temperature of 25°C, while in option C the temperature has additionally been raised to 650°C. Option D refers to a concept similar to option C wherein the heat from the kiln off gas is used in a waste heat boiler. In option E, additionally surplus char withdrawn from the rotary kiln is burned in the waste heat boiler, while in option F also the off gas from the electric furnace 20 is returned and utilized in the waste heat boiler.

The size of the electric reduction furnace (smelter) and the power of the respective electric transformers are equal for all options A to F. The greatest impact on the energy reduction is provided by options C (use of reduced ilmenite with 70% metallization and a feed temperature of 650°C) and D (additional use of latent heat from the off gas of the rotary kiln for power production). Option E additionally includes the combustion of surplus char of the kiln discharge, and option F also the combustion of cooled and cleaned carbon monoxide (CO) and hydrogen (H ₂ ) containing off gas of the electric furnace. Further energetic optimization would be possible by utilizing the sensible heat of these off gases leaving the electric furnace at a temperature of about. 1400 - 1500°C in a separate waste heat boiler.

Basic option A corresponds to the currently most common conventional produc- tion of titanium slag. Feeding pre-reduced and preheated ilmenite in accordance with option C reduces the consumption of electrical energy at the electrodes of the electric reduction furnace to about 58% of the value required in option A. Due to this the capacity of the electric reduction furnace can be increased by about 73%. By additional energetic utilization of the off gases and the surplus char of the rotary kiln as well as the off gas of the electric furnace for power production the total energy consumption of the electric furnace per ton titanium slag can be further reduced to about 39% of the value in option A. This already considers the electrical energy required for the pre-reduction in the rotary kiln and the power generation. The values determined for options A to F are evident from Table 1 . The calculations were based on an electric smelting furnace with an annual capacity of 250.000 tons for cold feed as in option A. For a feed of preheated (650°C) and pre-reduced ilmenite with 70% metallization the capacity of a same sized electrical smelting furnace is increased to 432.000 t/a. Table 1: Energy consumption of the electric smelting furnace for the production of titanium slag

In this example, option D provides for a power reduction of 1 134 kWh per ton titanium slag. Based on a price of 1 ,0 US ct /kWh this results in a cost reduction of 1 1 ,34 USD per ton of slag. Based on a more realistic price of 5,0 US ct/kWh this results in a cost reduction by 56,70 USD per ton or about 24,5 million USD per year. Fig. 3 shows the possible annual savings in options C to F based on different energy prices.

Another important advantage of the combination of the rotary kiln with the elec- trie smelting furnace is the reduction of the CO2 emission, which results for option F in only 69% of CO2 emission in option A. This reduces the CO2 emission by about. 1000 kg per ton of titanium slag. Option B provides a moderate reduction of CO2 emission only due to the additional emission of CO2 from the reduction plant. This basically compensates the reduced CO2 emission achieved by the reduced energy requirements of the electric smelter. The real reduction of CO2 is achieved by feeding heated ilmenite and by using the latent heat for power generation. Table 2 shows the calculated CO2 emission for the options A to F. Table 2: CO _∑ emission when producing titanium siag

With the example above about 0,4 tons of pig iron are simultaneously produced per ton titanium slag. One should thus also consider the respective CO2 emission of an external pig iron production. Standard values are about 1500 kg CO2 per ton of pig iron. This further reduces the CO2 emission by about 600 kg CO2 to about 1700 kg CO2 per ton of titanium slag. List of Reference Numerals

1 feed bin (ilmenite)

2 feed bin (coal)

3 feed bin (char)

4 feeder

5 double pendulum valve

6 reduction reactor

7 shell air fan

8 lance

9 chute

10 cooler

1 1 magnetic separator

12 electric reduction furnace (smelter)

20 waste heat boiler

21 radiation section

22 convection section

23 return conduit

24 burner

25 emergency flap

26 line

27 outlet

28 electrostatic precipitator

29 stack

30 steam drum

31 turbine generator

32 condenser

33 cooling tower

34 deaerator

35 feeding tank

36 demineralizer