Process and Plant for the Heat Treatment of Fine-Grained Mineral Solids

This invention relates to a process for the heat treatment of fine-grained mineral solids, in particular for calcining clay or clay-like substances or gypsum, and to a plant for performing this process.

Calcining fine-grained mineral solids, such as clay, conventionally is effected in rotary kilns or multiple-hearth furnaces. This ensures the maintenance of a low temperature with a residence time necessary for the treatment in this process.

U.S. patent 4,948,362 for instance describes a process for calcining clay, in which kaolin clay is treated in a multiple-hearth calcining furnace by means of a hot calcining gas to increase gloss and minimize abrasiveness. In an electro- static precipitator, the calcined clay powder is separated from the waste gas of the calcining furnace and processed to obtain the desired product.

There are also known processes which allow to avoid a movable plant equipment, such as a rotary kiln or rotating scrapers in multiple-hearth furnaces, and to reduce the residence time. The same include flash reactors and fluidized-bed technologies.

From U.S. patent 6,168,424 a plant for the heat treatment of suspended mineral solids, in particular clay, is known, in which the solids are supplied to a flash reactor upon preheating in a plurality of preheating stages. In the flash reactor, the solids are calcined in a heat treatment conduit by means of hot gases, which are generated in a combustion chamber. The calcined product is then cooled to the desired product temperature in a plurality of cooling stages.

In the paper "Properties of Flash-Calcined Kaolinite" in "Clays and Clay Minerals", Vol. 33, No. 3, 258-260, 1985, D. Bridson, T.W. Davies and DP. Harrison also describe the use of flash calcination for the treatment of kaolin. In this process, the solids are heated very quickly, maintained at the temperature for a short period and then quickly cooled again. The kaolin was flash-calcined for 0.2 to 2 seconds at temperatures between 900 and 1250 ⁰ C. It was recognized, however, that despite a sufficient high temperature only a partial dehydroxyla- tion is effected, since this short treatment time is not sufficient to achieve an equilibrium.

In flash reactors, the residence time is very short, which is compensated by an elevated treatment temperature in the reactor. In the case of temperature- sensitive substances, such as clay or gypsum, maximum temperatures must be observed, which involve the risk of the material being sintered when they are exceeded. Moreover, clay in particular involves the risk that the pozzolanic reactivity gets lost at excessive temperatures. Pozzolans are silicatic and alu- mosilicatic substances which react hydraulically with calcium hydroxide (lime hydrate) and water and form calcium silicate hydrates and calcium aluminahy- drates. These crystals also are obtained as a result of the hardening (hydration) of cement and lead to e.g. the strength and structural density of concrete. For kaolinitic clay, a temperature of 800 ⁰ C therefore should rather not be permanently exceeded. At such temperatures, the desired material properties can, however, not be achieved due to the short residence time in the flash reactor.

From DE 102 60 741 A1 , there is known a process for the heat treatment of gypsum, in which the solids are heated to a temperature of about 750 ⁰ C in an annular fluidized-bed reactor with recirculation cyclone and calcined to anhydrite. By means of the annular fluidized bed a sufficiently long solids residence time is achieved and at the same time a good mass and heat transfer.

DE 25 24 540 C2 describes a process for calcining filter-moist aluminum hydroxide, in which the aluminum hydroxide is charged to a fluidized-bed reactor supplied with fluidizing air, in which a temperature of 1100 ⁰ C is obtained by two- stage combustion, and calcined. Upon separation of the gas, the solids dis- charged from the fluidized-bed reactor are supplied to a residence time reactor, in which the solids in turn are maintained in a slight turbulent movement at a temperature of 1100 ⁰ C by adding gas with a low velocity. A partial stream of the solids is recirculated to the fluidized-bed reactor via a conduit. The residence time in the reactor system is divided between fluidized-bed reactor and resi- dence time reactor in a ratio of 1 :3.3.

It is the object of the invention to propose an energy-efficient configuration to ensure the desired particle properties in particular when calcining clay or clay- like substances or gypsum.

For the solution of this object by a process in accordance with the present invention, the solids are passed through a flash reactor, in which they are contacted with hot gases at a temperature of 450 to 1500 ⁰ C, preferably 500 to 890°C, and subsequently are passed through a residence time reactor at a temperature of 500 to 890 ⁰ C, from which they are withdrawn after a residence time of 1 to 600 minutes, preferably between 1 and 60 minutes when using a reactor with stationary fluidized bed, and between 10 and 600 minutes when the same is configured as rotary kiln, and possibly are supplied to a further treatment stage.

The flash reactor provides for a fast performance of the first treatment step. Due to thorough mixing of the particles, the heat and mass transfer is substantially improved, so that chemical reactions proceed very much faster than in a revolving-tube or multiple-hearth calcining furnace. Subsequently, a sufficient residence time is ensured by the residence time reactor to provide the desired

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material properties by observing the specified maximum temperature. This provides a more economic design of the process and of the plant used therefor.

Due to the thorough mixing in the flash reactor, it is possible without any risk to briefly expose the material to be calcined to a temperature distinctly higher than the usually admissible calcining temperature. The temperature of the hot gas can lie more than 200 ⁰ C above the average temperature in the flash reactor.

This is possible because the contact with the hot gas only is very short and a fast dissipation of heat is possible. Hence, there is no negative change of mate- rial.

In accordance with a preferred aspect of the invention, the residence time of the solids in the flash reactor is between 0.5 and 20 seconds, preferably between one and ten seconds, and in particular between two and eight seconds. In de- pendence on the treated materials and the desired material properties as well as the configuration of the flash reactor, the gas velocities and hence the residence times of the solids can be determined. Even with a minimum residence time in the residence time reactor of only one minute, there is obtained a very short treatment time in the flash reactor as compared to the residence time reactor of preferably smaller than 1 :6 and in particular smaller than 1 :7.5. With a longer residence time in the residence time reactor, this ratio correspondingly is reduced down to 1 :1200.

In particular when calcining clay or clay-like substances, the temperature in the flash reactor in accordance with the invention is about 550 to 850 ⁰ C, preferably 600 to 750 ⁰ C, and particularly preferably between 650 and 700 ⁰ C.

The temperature in the flash reactor can be achieved both by an external combustion, e.g. in an upstream combustion chamber, and by an internal combus- tion in the flash reactor. Hot waste gases from other process steps or other

plants can also be used. Internal combustion is preferred in particular at higher process temperatures above 700 ⁰ C.

In accordance with a development of the invention it is possible to charge the flash reactor with cold or hot pyrolysis and/or gasification products or products from substoichiometric combustions (e.g. CO-containing gases) and perform a further combustion in the flash reactor. There can, however, also be used special fuels with a low burning temperature, e.g. propane.

The internal combustion in the flash reactor can be controlled e.g. by the residence time, the size of the flash reactor or the construction, e.g. as tube or as cyclone. A complete internal combustion is preferred, but it is also possible to provide an afterburning chamber after the flash reactor, in order to ensure a complete combustion of the fuel.

When calcining gypsum, the temperature in the flash reactor is about 540 to 880 ⁰ C, but when supplying hot gases it preferably is about 650 to 850 ⁰ C and particularly preferably between 700 and 75O ⁰ C, in the case of an internal combustion preferably between 740 and 850 ⁰ C, particularly preferably about 750 to 800 ⁰ C.

In accordance with a development of the invention, the heat treatment in the residence time reactor is effected by means of hot gases, wherein the residence time of the gases in the residence time reactor preferably is between 0.1 and 10 seconds. In this way, the temperature in the residence time reactor can be adjusted very accurately. In a residence time reactor which constitutes a rotary kiln, the residence time of the solids preferably is 20 to 300 min, and in a reactor formed as fluidized bed it preferably is 1 to 30 min.

In the calcination of clay or clay-like substances in accordance with the invention, the temperature in the residence time reactor is about 550 to 850°C, preferably about 600 to 750 ⁰ C, and particularly preferably about 650 to 700 ⁰ C, whereby an impairment of the pozzolanic reactivity is reliably prevented.

In the case of the heat treatment of gypsum, however, the temperature in the residence time reactor in accordance with the invention is slightly higher, namely about 540 to 880 ⁰ C, preferably about 550 to 850 ⁰ C, and particularly preferably about 700 to 800 ⁰ C. At the higher process temperatures, however, an internal combustion likewise is possible here.

Delivery in the flash reactor, which in a wider sense is an entrained-bed reactor, is effected by a gas stream which entrains the solids. Preferably, a hot gas stream is supplied. In accordance with a preferred aspect of the invention, the Particle-Froude-Number in the flash reactor lies between 40 and 300, preferably between 60 and 200, whereby it is ensured that the solid particles pass through very quickly and hence with corresponding short residence times. The Particle- Froude-Numbers each are defined by the following equation:

wherein u = effective velocity of the gas flow in m/s

P _s = density of a solid particle in kg/m ³ pf = effective density of the fluidizing gas in kg/m -. ³ 3 dp = mean diameter in m of the particles of the reactor inventory (or of the particles formed) during operation of the reactor g = gravitational constant in m/s ² .

When using this equation it should be considered that d _p does not designate the grain size (dso) of the material supplied to the reactor, but the mean diameter of the reactor inventory formed during operation of the reactor, which can differ significantly in both directions from the mean diameter of the material used (primary particles). From very fine-grained material with a mean diameter of 3 to 10 μm, particles (secondary particles) with a grain size of 20 to 30 μm are formed for instance before introduction into the plant or the flash reactor or during the heat treatment. On the other hand, some materials or secondary particles formed are disintegrated during the heat treatment or as a result of the mechanical load in the gas flow.

In accordance with the invention, the efficiency of the process is increased in that the solids are preheated before introduction into the flash reactor. For preheating, waste gases from the flash reactor preferably are used completely or in part. During preheating, dusts usually are obtained, which can directly be supplied to the flash reactor or the residence time reactor.

In accordance with a development of the invention, the waste gas of the residence time reactor is recirculated to the flash reactor, in order to increase the yield of the process. The dust-laden waste gas first can roughly be cleaned, e.g. by means of a cyclone, and the dust separated can be supplied to the cooling means. For an optimum utilization of the heat contained in the waste gas, recirculation to a preheating stage is effected in accordance with the invention.

The hot solids from the residence time reactor subsequently are cooled directly or indirectly, and the heat preferably is used for heating the combustion gas for the flash reactor or the upstream combustion chamber. The heat produced in a possibly present afterburning chamber can also be used in the process, e.g. for preheating the gas or the solids.

This invention also extends to a plant for the heat treatment of fine-grained mineral solids, in particular for calcining clay and gypsum, which is suitable for performing the process described above. In accordance with the invention, the plant comprises a flash reactor, through which the solids are passed at a tem- perature of 450 to 1500 ⁰ C, preferably 500 to 890 ⁰ C, and a residence time reactor, through which the solids subsequently are passed at a temperature of 500 to 89O ⁰ C.

In accordance with one aspect of the invention, the residence time reactor is a rotary kiln. In accordance with another preferred aspect of the invention, the residence time reactor includes a gas-solids suspension, e.g. a stationary fluid- ized bed, or a conveying section.

In accordance with a development of the invention, a cooling system is arranged behind the residence time reactor, comprising direct and/or indirect cooling stages, in particular cooling cyclones and/or fluidized-bed coolers. In a direct cooling stage, the cooling medium directly gets in contact with the product to be cooled. Even during the cooling process, desired reactions such as product refinements still can be performed. In addition, the cooling effect of direct cool- ing stages is particularly good. In indirect cooling stages, cooling is effected by means of a cooling medium flowing through a cooling coil.

For adjusting the necessary process temperatures in the flash reactor, a combustion chamber with supply conduits for fuel, oxygen and/or heated gas, pref- erably air, is provided upstream of the same, whose waste gas is introduced into the flash reactor as hot conveying gas. The combustion chamber can, however, also be omitted, when the reactor temperature can be chosen high enough for an ignition and stable combustion (internal combustion in the flash reactor).

In accordance with a development of the invention, at least one preheating stage for preheating the solids is provided before the flash reactor.

For separating the solid particles from the gas stream, a separator, in particular a cyclone separator, is provided downstream of the reactor in accordance with the invention.

Further features, advantages and possible applications of the invention can also be taken from the following description of embodiments 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.

In the drawing:

Fig. 1 shows a basic flow diagram of the process of the invention,

Fig. 2 shows an aspect of the process for calcining clay, and

Fig. 3 shows an aspect of the process for calcining gypsum.

Figure 1 schematically shows a plant for performing the process of the invention.

Via a supply conduit 1 , the solids to be treated, such as clay or gypsum, are supplied to a preheating stage 2 and heated to a temperature of about 300 ⁰ C. Via a waste gas conduit, the waste gas is supplied to a non-illustrated dust separator or other parts of the plant. The solids then are heated to a temperature of 300 to 500 ⁰ C in a second preheating stage 4, before they are supplied to a flash reactor 5. In the flash reactor 5, which for instance is an entrained-bed

reactor with a height of about 30 m, the solids are calcined with hot gases, which are generated in a combustion chamber 6, at a temperature of 600 to 850 ⁰ C, in particular 650 to 700 ⁰ C (clay) or 700 to 750°C (gypsum). Into the flash reactor 5, such a volume flow of hot gases is introduced that a Particle-Froude- Number of 40 to 300, in particular about 60 to 200 is obtained and the solids are conveyed through the flash reactor 5 very quickly. In accordance with the invention, a residence time of preferably two to eight seconds is provided. Depending on the material and the desired heat treatment, the residence time of the solids in the flash reactor can, however, also lie between 0.5 and 20 seconds.

The solids discharged from the flash reactor 5 together with the hot conveying gas are separated from the conveying gas in a non-illustrated separator, in particular a cyclone, and supplied to a residence time reactor 7 configured as rotary kiln or stationary fluidized bed, in which the solids are subjected to a heat treatment depending on their composition (result of the flash calcination) and the desired product properties for 1 to 600 minutes, preferably for 1 to 30 minutes when the residence time reactor 7 includes a stationary fluidized bed, and for 10 to 600 minutes when the residence time reactor 7 is configured as a rotary kiln.

In accordance with the invention, the temperature in the residence time reactor 7 is about 550 to 850 ⁰ C, and for the calcination of clay preferably about 650 to 700 ⁰ C, whereas for the calcination of gypsum it preferably is about 700 to 750 ⁰ C. The temperature in the residence time reactor 7 is controlled by the supply air, which is supplied via a conduit 8. The residence time of the gases in the residence time reactor 7 is between 1 and 10 seconds, so that the temperature can be adjusted and adapted to the desired product properties very accurately. In addition, fuel can be supplied to the residence time reactor 7 for an internal combustion. The dust-laden waste gas from the residence time reactor 7 is recirculated to the second preheating stage 4 via a return conduit 9. In the process, the dust-laden waste gas also can roughly be dedusted.

The solids are withdrawn from the residence time reactor 7 and supplied to a first cooling stage 10, in which the product is cooled in one or more stages in counterflow with the combustion air, wherein a direct or indirect cooling can be performed. Via conduit 11 , the air heated in this way is supplied as combustion air to the combustion chamber 6, in which fuel supplied via a fuel conduit 12 is burnt and thereby heats the combustion air, which subsequently is supplied to the flash reactor 5. Part of the preheated air can also be used for fluidizing the residence time reactor.

Subsequently, the product can further be cooled with air in a second cooling stage 13 and then be supplied to a fluidized-bed cooler 14, in which the solids are cooled with air and/or cooling water to the desired product temperature, e.g. about 50 to 60 ⁰ C.

Example 1 (calcination of clay)

A plant for producing 1300 t of calcined clay per day, which is schematically shown in Fig. 2, is operated with natural gas which has a net calorific value (NCV) of 50000 kJ/kg.

With a moisture of 7%, the clay-like starting material rich in kaolin is preheated to a temperature of 500 ⁰ C in two successive preheating stages, which consist of Venturi preheaters 2a, 4a and cyclone separators 2b, 4b, and charged to the flash reactor 5. The same is operated at 650 to 700 ⁰ C and with a residence time of 5 seconds. The residence time reactor 7 is configured as a stationary fluidized-bed reactor and operated at 630 to 680 ⁰ C. There is desired a Particle- Froude-Number of 3, which in operation lies in the range from 2 to 4 due to the variation of particle size. The residence time is 13 to 22 min, preferably 16 to 20 min.

The hot gas for adjusting the necessary process temperature in the flash reactor 5 is generated in a combustion chamber 6. For providing 77000 Nm ³ /h of hot gas at a temperature of 1000 ⁰ C ₁ 1600 kg/h of natural gas are required. The combustion air is preheated to a temperature of 340 ⁰ C by cooling the product leaving the residence time reactor 7 with a temperature of 65O ⁰ C and supplied to the combustion in the combustion chamber 6. In the process, the product is cooled from 650 ⁰ C to about 15O ⁰ C and finally is cooled to the desired final temperature of 55°C in a fluidized bed cooler 14.

Example 2 (calcination of gypsum)

A plant for producing 700 t of calcined gypsum per day, which is schematically shown in Fig. 3, is operated with lignite which has a net calorific value (NCV) of 22100 kJ/kg.

With a moisture of 8%, the starting material is preheated to a temperature of 320 ⁰ C in two successive preheating stages, which consist of Venturi preheaters 2a, 4a and cyclone separators 2b, 4b, and precalcined; additional heat is supplied to the Venturi 4a by supplying a hot gas of 1050 ⁰ C to the Venturi 4a, which is generated in a combustion chamber 15 with 0.5 t/h of lignite and 7500 Nm ³ /h of air. The preheated and precalcined solids are charged to the flash reactor 5. The same is operated at 700 to 750 ⁰ C and with a residence time of 10 seconds. The residence time reactor 7 is configured as a stationary fluidized-bed reactor and operated at 700 ⁰ C. There is desired a Particle-Froude-Number of 3, which in operation lies in the range from 2 to 4 due to the variation of particle size. The residence time is 15 to 25 min, preferably 18 to 22 min.

The hot gas for adjusting the necessary process temperature in the flash reactor

5 is generated in a combustion chamber 6. For generating 27000 NnrϊVh of hot gas at a temperature of 1050 ⁰ C, 1.5 t/h of lignite are required. The required

combustion air of 26300 Nm ³ /h is preheated to a temperature of 250 ⁰ C by cooling the product leaving the residence time reactor 7 with a temperature of 700 ⁰ C and supplied to the combustion in the combustion chamber 6. In the process, the product is cooled from 700°C to about 250 ⁰ C and finally is cooled with cool- ing water to the desired final temperature of 60 ⁰ C in a fluidized bed cooler 14.

List of Reference Numerals

1 supply conduit

2 first preheating stage

2a Venturi preheater

2b cyclone separator

3 waste gas conduit

4 second preheating stage

4a Venturi preheater

4b cyclone separator

5 flash reactor

6 combustion chamber

7 residence time reactor

8 air conduit

9 return conduit

10 first cooling stage

11 combustion air conduit

12 fuel conduit

13 second cooling stage

14 fluidized-bed cooler

15 combustion chamber