The present invention relates to a process for the heat treatment of solids containing titanium, in which fine-grained solids are heated to a temperature of 700 to 1000 ⁰ C in a reactor with circulating fluidized bed and are discharged from the reactor together with waste gases into a downstream separator, in which the solids are separated from the waste gases and are recirculated to the reactor at least partly and/or phase by phase. Furthermore, the invention relates to a corresponding plant.

Such processes and plants are used for instance for the magnetizing calcination of ilmenite (X*Tiθ ₂ Y*FeOZ*Fe ₂ O ₃ ). In the past, a reactor with stationary fluidized bed was used for the magnetizing calcination of ilmenite, which reactor has, however, only a small control range and a low reaction density. In addition, when using a reactor with stationary fluidized bed, only a comparatively low flow rate is possible with respect to the container volume. The temperature and retention time control also frequently is unfavorable in the case of such reactors with stationary fluidized bed.

Therefore, it is also known to effect a magnetizing calcination of ilmenite under oxidizing conditions in reactors with a circulating fluidized bed. For this purpose, hot gas is introduced into the reactor through a tuyere bottom (gas distributor) for fluidizing the solids. This hot gas is mostly generated in an external burner, in which e.g. natural gas or fuel oil and ambient air are burnt. The solids discharged from the reactor together with the waste gases are separated from the waste gases in a separator and at least partly recirculated to the reactor. For controlling the recirculation of solids from the separator into the reactor a so-called "L-valve" is used, which can be controlled by supplying gas.

Before the further processing of the ilmenite calcined in the reactor, the same must be cooled. To this end it is known, for instance, to use a fluidized-bed cooler, in which the product heat is dissipated. In these known processes and

plants it is, however, possible that changes in the magnetizingly calcined ilmenite occur during the cooling period, so that the positive magnetic properties achieved beforehand are deteriorated again.

Therefore, it is the object of the present invention to provide a process as mentioned above, in which ilmenites can be treated that are not suited for magnetizing calcination under oxidizing conditions, wherein changes of the product after the heat treatment are largely avoided.

In accordance with the invention, this object substantially is solved in that the ilmenite is calcined under reducing conditions.

To improve the product quality and to avoid changes of the product after the heat treatment downstream of the reactor and/or the separator preferably a cooling apparatus, which allows for a relatively rapid cooling of the product. This may be effected by indirect cooling in an irrigation or shower cooler or by direct cooling by means of water injection. Preferably, an injection cooler is provided, in which the solids are cooled to below 250 ⁰ C by injecting a coolant and possibly are cooled further in another cooler, for instance a fluidized-bed cooler, downstream of the injection cooler, fluidizing gas being introduced into the injection cooler with such a gas velocity that the Particle-Froude-Number in the fluidized bed is between 0.01 and 10, in particular between 0.1 and 1. Preferably, the Particle-Froude-Number at the bottom of the fluidized-bed cooler is between 0.1 and 0.25, in particular about 0.17. At the top of the fluidized-bed Gooier, the Particle-Froude-Number preferably is between 0.35 and 0.55, in particular about 0.47.

The Particle-Froude-Numbers are each defined by the following equation:

Fr _P =

with

u = effective velocity of the gas flow in m/s Pt = effective density of the fluidizing gas in kg/m ³ p _s = density of a solid particle in kg/m ³ (apparent density) dp = mean diameter in m of the particles of the reactor inventory (or the secondary agglomerates formed) during operation of the reactor g = gravitational constant in m/s ² .

When using this equation it should be considered that dp does not designate the mean diameter (d_50) of the material used, but the mean diameter of the reactor inventory formed during operation of the reactor, which can differ significantly from the mean diameter of the material used (primary particles).

In this process in accordance with the invention, the product withdrawn from the reactor or the separator first of all is cooled very much in the cooling apparatus to e.g. about 100 to 200 ⁰ C within a very short time. Changes in the magnetizingly calcined ilmenite during the cooling period largely can be avoided in this way. Due to the rapid cooling, a particularly high product quality of the magnetizingly calcined ilmenite thus can be achieved. This high product quality ensures a high degree of separation during a subsequent magnetic separation. Due to the large temperature range during cooling it is necessary to not only take care of changes in the product, but to also properly adjust the quantity and velocity of the gas introduced for fluidization into the injection cooler, which is preferably used as the cooling apparatus, so that the fluidized bed does not expand too much when the injected coolant is evaporated. In accordance with the invention, the gas velocity of the fluidizing gas in the injection cooler therefore is chosen such that a comparatively dense fluidized bed is obtained. The fluidized bed is denser at the bottom of the injection cooler than at the top of the injection cooler, as the coolant injected is evaporated there. In the fluidized-bed cooler downstream of the injection cooler, the product heat no longer usable in the process is dissipated.

Preferably, water is injected into the injection cooler as coolant. The gas content in the fluidized bed of the injection cooler then can include between 50 and 70%, in particular about 60 % steam.

With the process in accordance with the invention, all kinds of ores containing titanium, in particular those which additionally contain iron oxides, can be heat- treated effectively. In particular, the process is suited for the magnetizing calcination of ilmenite. The mean particle size (d_50) of the solids supplied to the reactor preferably is between 75 and 350 μm, in particular between about 100 and 150 um. The maximum grain size of the solids supplied to the reactor is about 2 mm, preferably less than 250 μm. The grain size of the ilmenite magnetizingly calcined in the reactor preferably lies in the same ranges indicated above.

The generation of the amount of heat necessary for the operation of the reactor can be effected in any way known to the expert for this purpose. In accordance with a preferred embodiment of the invention it is provided to cover the required amount of heat by supplying hot gases or preheated solids. It is preferred to introduce a gaseous reducing agent, e.g. molecular hydrogen, gas mixtures containing molecular hydrogen, carbon monoxide or gas mixtures containing carbon monoxide, e.g. reformer gas, into the reactor. It is also possible to produce the above mentioned reducing gas in the reactor, e.g. by the unterstoichiometrical burning of fuels such as natural gas, oil or coal.

In the process in accordance with the invention, a particularly high product quality can be achieved when the retention time of the solids in the reactor is between 10 and 50 minutes, in particular about 30 minutes. The Particle- Froude-Number in the reactor can lie in a range from about 0.3 to 30, in particular between 0.5 and 15.

The energy consumption of the process can be reduced in that in the separator at least part of the waste gas of the reactor is separated from solids and supplied to a preheating stage upstream of the reactor. The preheating stage can, for instance, comprise a heat exchanger such as a Venturi preheater, and a separator such as a cyclone or the like. In this way, the solids supplied to the reactor are dried and preheated, whereby the heat treatment in the reactor is facilitated. A multi-stage preheating of solids is also possible, the waste gas of the reactor being cooled step by step.

In accordance with a development of this invention it is provided that the waste gases of the reactor together with the e.g. steam-loaded waste gases of the injection cooler are cleaned in a waste gas cleaning stage downstream of the preheating stage. The gases then can possibly be recirculated to the process.

In accordance with a preferred embodiment of the invention it is provided that the recirculation of solids from the separator into the reactor is effected in a self- regulating manner. In this way, an intensive internal and external re-mixing of the solids treated in the reactor can be effected, so that a uniform temperature and reaction profile is achieved in the reactor.

A plant in accordance with the invention, which is in particular suited for performing the process described above, includes a reactor with circulating fluidized bed, downstream of which a separator is provided. Downstream of the reactor and/or the separator there is furthermore provided a cooling apparatus formed as an injection cooler and downstream of the same a separate fluidized- bed cooler. In the injection cooler, the product can be cooled quickly, i.e. within few seconds, to temperatures between e.g. 100 and 200 ⁰ C by injecting for instance water. This rapid first cooling is decisive for the product quality, as for instance during the magnetizing calcination of ilmenite changes in the product are possible during a too long cooling time. The final cooling of the product then

is effected in the separate fluidized-bed cooler, which is provided downstream of the injection cooler.

Preferably, cooling coils are provided in the fluidized-bed cooler, through which a coolant is passed countercurrently. These cooling coils can for instance be combined to form cooling bundles.

In the fluidized-bed cooler, the product heat no longer usable in the process can be dissipated particularly effectively, when the same has two or more chambers through whose bottom fluidizing gas is introduced by means of a blower. The fluidizing gas on the one hand is used for cooling the product and at the same time effects an intensive intermixing of the solids to be cooled.

For adjusting the temperatures necessary for the heat treatment of the solids, the reactor preferably has a separate upstream burning chamber, in which the hot gas is produced. It, however, is equally possible to provide a lance assembly and/or bottom tuyeres opening into the same, e.g. disposed laterally, which are connected with a supply conduit for especially gaseous reducing agents, so that the fuel is burnt inside the reactor.

In accordance with a preferred embodiment of the invention, a self-regulating U- shaped seal is provided between the reactor and the separator, by means of which the supply of solids from the separator into the reactor is controlled. An expensive control system, for instance by using an L-valve known from the prior art, thus can be omitted.

To decrease the energy consumption of the plant, a preheating stage can be provided upstream of the reactor, in which the solids are dried and preheated. The preheating stage includes e.g. a drier, which is connected with the waste gas conduit of the separator downstream of the reactor, so that the heat present in the waste gas stream can be utilized for predrying/ preheating the solids.

Further developments, advantages and possible applications of the invention can also be taken from the following description of an embodiment and the drawing. All features described and/or illustrated form the subject-matter of the invention per se or in any combination, independent of their inclusion in the claims or their back-reference.

The only Figure shows a process diagram of a process and a plant in accordance with an embodiment of the present invention.

In the process as shown in the Figure, which is suited in particular for the magnetizing calcination of solids containing titanium, such as ilmenite, moist solids are introduced into a preheating stage via a screw conveyor 1. This preheating stage comprises a Venturi drier 2, in which the raw material is suspended, dried and preheated, and a separator 3, e.g. a cyclone, downstream of the Venturi drier 2. The solids separated from waste gases in the separator 3 are charged into a reactor 4.

The reactor 4 constitutes a fluidized-bed reactor with a circulating fluidized bed. For fluidizing the solids, bottom tuyeres are provided in the reactor 4, through which gas, e.g. air or mixtures with air, is introduced by means of a blower 5. Via lateral lances 6, natural gas is supplied to the reactor 4, which is burnt inside the reactor together with the fluidizing gas. Alternatively or in addition, fuel can be introduced into the reactor 4 via a conduit 7 by means of bottom tuyeres. In this combustion the gas composition, e.g. the content of CO and CO ₂ , is adjusted such that the ilmenite is magnetically calcined or roasted. The reducing potential of the gas depends on the relation CO:CO ₂ .

In the fluidized-bed reactor 4, the solids are carried upwards by the fluidizing gas. Part of the solids separate out in the reactor and are thereby recirculated to the circulating fluidized bed, in order to be carried upwards again by the fluidizing gas. Together with a waste gas stream from the reactor 4, the other part of the solids is discharged upwards through a conduit 8 and in a

downstream separator 9, for instance a cyclone, separated from the gas stream for the most part. Through a conduit 10, the solids from the separator 9 are recirculated to the fluidized-bed reactor 4. By means of this intensive internal and external remixing a particularly uniform temperature and reaction profile is achieved inside the fluidized-bed reactor 4.

The control of the amount of solids recirculated from the separator 9 into the fluidizedbed reactor 4 is effected via a self-regulating U-shaped seal 11 which is provided in the conduit 10. As a result, a control and regulating unit for metering the amount of solids recirculated into the fluidized-bed reactor 4 can be omitted.

The gas stream separated from the solids in the separator 9 is supplied to the Venturi drier 2, so that the heat content of the gas stream leaving the separator 9 is utilized for drying and preheating the solids.

From the fluidized-bed reactor 4 and/or the separator 9 hot solids are withdrawn and supplied to an injection cooler 13 via conduits 12a and 12b, respectively. In the injection cooler 13, the hot solids are fluidized in a stationary fluidized bed. For this purpose, air is introduced into the injection cooler 13 as fluidizing air via a blower 14. The gas velocity of the fluidizing gas is chosen such that the fluidization in the injection cooler 13 is low, so that the stationary fluidized bed expands only little. At the same time, water is injected into the injection cooler 13 as coolant via conduit 15. The water is evaporated in the injection cooler 13, so that the stationary fluidized bed in the upper region of the injection cooler 13 expands more than in the bottom region of the injection cooler. By injecting water, the hot product is quickly cooled to temperatures of e.g. below 200 ⁰ C.

Downstream of the injection cooler 13 a separate fluidized-bed cooler 16 is provided, in which the product heat no longer usable in the process is dissipated. In the illustrated embodiment, the fluidized-bed cooler has two chambers 16a and 16b, into which e.g. water is countercurrently introduced as coolant through schematically illustrated cooling coils 17, whereby the product

is further cooled to the temperature necessary for the further processing, such as the magnetic separation. Via a blower 18, air is introduced into the two chambers i6a and 16b of the fluidized-bed cooler 16, in order to fluidize and cool the product. The cooled product then is supplied to the further processing via a conduit 19.

Via conduits, the separator 3 of the preheating stage, the injection cooler 13 as well as the fluidized-bed cooler 16 are connected with a waste gas cleaning stage 20, which for instance has a bag filter. In this waste gas cleaning stage 20, the gas streams partly containing solids and/or steam are cleaned. Via conduit 21 , the solids can be charged from the waste gas cleaning stage 20 into the fluidized-bed cooler 16.

Example (magnetizing calcination of ilmenite)

In a plant for the magnetizing calcination of ilmenite as shown in the Figure, 43 t/h of moist ilmenite from a storage tank are charged via the screw conveyor 1 into the Venturi drier 2. In the Venturi drier 2, the moist ilmenite was suspended, dried and preheated by hot waste gases from the separator 9. In the cyclone 3, which is provided downstream of the Venturi drier 2, the dried and preheated ilmenite was separated from the gas stream and introduced into the reactor 4 with circulating fluidized bed.

The waste gas of the cyclone 3 was supplied to the waste gas cleaning stage 20, partly recirculated and oxidized and supplied to a chimney. The dry ilmenite dust separated in the waste gas cleaning stage 20 was passed through a conduit directly into the product stream 19.

Via the blower 5, 46.000 Nm ³ /h of gas, consisting of 15.000 Nm ³ /h air and 31.000 Nm ³ /h recirculated waste gas, were introduced into the fluidized-bed reactor 4 for fluidization. At the same time, about 1.800 Nm ^3/ h of natural gas were supplied to the reactor 4 via the lateral lances 6 as well as conduit 7 and

were burnt in the fluidized bed together with air. The reducing gas obtained leads to a temperature of the ilmenite charged into the fiuidized-bed reactor 4 of about 800°C, the Particle-Froude-Number in the reactor being about 1.2. A magnetizing calcination of the ilmenite was achieved with retention times of the solids between 10 and 50 minutes.

Together with the waste gases of the reactor 4, the solids were transported into the separator 9, separated there and for the most part recirculated to the reactor 4 through conduit 10. An amount of ilmenite product, which corresponds to the amount charged into the reactor 4, was supplied to the injection cooler 13 through conduits 12a and 12b, respectively. The average particle size (d_50) both of the ilmenite charged into the reactor 4 and of the calcined ilmenite was about 100 to 150 μm with a maximum grain size of about 250 μm.

The injection cooler 13 was operated as stationary fiuidized bed by supplying about 6300 Nm ³ /h of recircuiated waste gas as fluidizing gas into the injection cooler 13 via the blower 14. At the same time, about 8 m'lh of water were introduced into the injection cooler 13 via conduit 15, so that the hot ilmenite was cooled to about 150 ⁰ C within few seconds. Due to the evaporating water, the steam was about 60% of the total amount of gas in the fluidized bed of the injection cooler 13. The gas velocity of the fluidizing gas introduced via the blower 14 was chosen such that the Particle-Froude-Number at the bottom of the injection cooler 13 was about 0.17 and at the top of the injection cooler about 0.47.

The final cooling of the product was effected in the two chambers 16a and 16b of the fluidized-bed cooler 16. For fluidization, about 6000 Nm ³ /h of recirculated waste gas were introduced into the fluidized-bed cooler 16 via the blower 18. At the same time, cooling water was countercurrently passed through the chambers 16a and 16b via conduit 17. In the chambers 16a and 16b, the conduit 17 had cooling bundles.

In this way, a reducing magnetizing calcination of ilmenite could be effected, and due to the rapid cooling no changes were detected during the cooling period, so that the calcined ilmenite had a high product quality.

List of Reference Numerals

1 screw conveyor

2 Venturi drier

3 cyclone

4 fluidized-bed reactor

5 blower

6 lance

7 conduit

8 conduit

9 separator

10 conduit

11 U-shaped seal

12a, 12b conduit

13 injection cooler

14 blower

15 conduit

16 fluidized-bed cooler

16a, 16b chamber

17 conduit

18 blower

19 conduit

20 waste gas cleaning stage

21 conduit