BE1027979A1 - Process for heat treatment and color optimization of natural clays - Google Patents

Abstract

The present invention relates to a device for the thermal treatment of mineral solids, the device having at least one preheater and an entrained flow reactor 10, a separation device 20 being arranged at the outlet of the entrained flow reactor 10, characterized in that a thermal treatment zone at the outlet of the gas flow of the separation device 20 30 is arranged, wherein the output of the thermal treatment zone 30 is connected to the input for the gas flow of the preheater.

Description

Method for heat treatment and color optimization of natural clays The invention relates to a device for the production of color-optimized cement clinker from starting materials which contain natural clays.

Cement is a hydraulically hardening building material that consists of a mixture of finely ground, non-metallic-inorganic components. It is usually produced by grinding the burnt cement clinker together with gypsum and, if necessary, other Supplementary Cementing Materials (SCM).

The main raw material for clinker production is limestone, which is mined in quarries, pre-crushed in crushers and transported to the cement works. After grinding and drying, it is mixed with other ground components such as sand, clay or iron ore to form a raw meal. This raw meal is burned to clinker in a rotary kiln at temperatures above about 1,450 ° C and then cooled in a cooler to a temperature of preferably below 200 ° C. The resulting granules are then ground in a mill, for example a ball or roller mill, together with plaster of paris or anhydrite to form cement.

Due to the enormous growth in cement demand in developing countries, the share of cement production in total anthropogenic CO2 emissions has risen steadily and is estimated by some sources to be around 10% or around 6% of total anthropogenic greenhouse gases. This happened despite the important improvements in production efficiency and the efforts of the cement industry to reduce emissions since the 1970s (Karen L. Scrivener, Vanderley M. John, Ellis M. Gartner; Eco-efficient cements: Potential conomically viable solutions for a low-CO2 cement -based materials industry, United Nations Environment Program (2017)). Since more than half of the CO2 emissions in clinker production are caused by the raw material limestone, reducing the clinker content (clinker factor) by replacing another component can make a significant contribution to reducing these emissions.

For example, calcined clay or naturally tempered pozzolans have been proposed as cement substitutes or SCMs. Calcined clay is a suitable, naturally occurring clay that has been thermally activated at a suitable temperature so that it acquires pozzolanic properties. Clays suitable for this generally contain clay minerals in the form of 1-layer and / or 2-layer sheet silicates, for example kaolinite, illite or montmorillonite. These clays can also contain accompanying minerals, such as quartz, feldspars, calcite, dolomite, but also metal oxides and hydroxides or especially iron hydroxides. Naturally occurring clays are usually rich in iron and / or contain other coloring metals, so that, for example, conventional calcination results in a reddish discoloration of the product. Although this color is not relevant for the strength and other building material properties, it is classified as undesirable by plant operators and building material customers. According to the current state of affairs, the acceptance of a building material by end users, i.e. the market potential of the calcined clays and thus the potential for possible CO »2 savings, depends largely on their color.

The calcination of fine-grain mineral solids, such as clay, is conventionally carried out in rotary kilns or multi-tier grate ovens. This ensures that a low temperature is maintained with a residence time necessary for the treatment in this process. For example, US Pat. No. 4,948,362 A describes a method for calcining clay in which kaolin clay is treated in a deck roasting furnace with the aid of a hot calcination gas in order to increase the gloss and minimize the abrasiveness. The calcined clay powder is separated from the exhaust gas of the calcining furnace in an electrostatic filter and further processed in order to obtain the desired product.

Against the background of the global reduction in CO2 emissions, substances that contribute to the development of strength in cement and / or concrete and the production of which less CO2 is released than in the cement clinker burning process are increasingly coming into focus. Natural clays and / or zeolites that are subjected to thermal treatment (calcination) seem to have promising potential in this regard. DE 10 2008 020 600 A1 discloses a method for calcining clay or gypsum, the solids being passed through a flash reactor in which they are brought into contact with hot gases at a temperature of 450 to 1,500 ° C . The solids are then passed through a residence time reactor at a temperature of 500 to 850 ° C. and then optionally fed to a further treatment stage.

From DE 10 2008 031 165 A1 it is known to use the plant for producing the cement itself for the production of calcined clay, at least two preheating strands being provided, one of which is used to preheat the clay and the other to heat the clinker raw material. Hot gases are generated in a combustion chamber, which are used to calcine the clay and which are fed through the preheating stages in countercurrent to the solids. The clay used in this process, however, has a high kaolinite content of more than 40% by weight and is very expensive, so that no economically marketable clinker substitute can be made from it.

DE 690 10 646 T2 relates to ceramic microspheres made from bauxite and the use of these microspheres as reinforcement materials and functional fillers. The bauxite proposed as the source of the microspheres comprises 55 to 63% aluminum oxide and 7 to 13% silicon dioxide, the silicon dioxide being essentially present as kaolinite. Usually the mineralogy comprises 30 to 50% gipsite with 15 to 45% boehmite, 16 to 27% kaolinite with less than 0.2% quartz and 6 to 10% oxides of iron and 3 to 5% titanium oxides. On a laboratory scale, calcination takes place at around 900 ° C to drive off water. The furnace is then heated to around 1,300 ° C before the material is cooled and then quickly brought to a sintering temperature between 1,300 ° C and 1,600 ° C and fired. In US 3,941,872 A, clay is heat-treated first under reducing and then under oxidizing conditions in order to produce calcined clay with a desired brightness.

DE 102011 014498A1 discloses a method for producing a clinker substitute for use in cement production, wherein a clay comminuted to a grain size of <2 mm is initially at a temperature of 600 to

1,000 ° C is calcined. With a subsequent reducing treatment

Temperatures of 600 to 1,000 ° C with a CO-containing gas changes the color of the red, calcined clay to gray calcined clay. In WO 2012/082683 A1, the task was to produce synthetic pozzolans with the desired color properties, in particular a light shade of gray. As a solution, it is stated that a raw material which is suitable for the formation of an amorphous aluminum silicate is heated up to an activation temperature at which the raw material is converted into synthetic pozzolan. The synthetic pozzolan is then cooled from the activation temperature to a temperature at which it is color-stable. This cooling process takes place at least in part under reducing conditions. The result is a pozzolan with the desired shade of gray.

A method for heat treatment of natural clays and / or zeolites is known from DE 10 2014 116 373 A1, trivalent iron being at least partially converted into bivalent iron and / or bivalent iron present in the starting material remaining in this valence level.

A method for producing synthetic pozzolan is known from US Pat.

It is therefore customary to add reducing gas components, for example carbon monoxide (CO), hydrogen (Hz), carbon and / or / or hydrocarbons, to optimize the color during the thermal treatment, for example during the calcination. These have a reduced effect on the metal oxide compounds in the clays and at least partially reduce them, so that an undesired red coloration of the product is reduced or prevented and / or a gray coloration of the product is achieved.

In order to enable effective and more far-reaching color optimization, it is necessary that gas components with a reducing effect are also present at the end of the thermal treatment process step, for example calcination and color optimization. These reducing gas components are therefore present in the exhaust gas of the system. However, these cannot simply be released into the environment. The exhaust gas from the system must therefore be post-treated. This usually takes place in an afterburning, the energy generated thereby, for example in heat exchangers, is recovered. In addition, the incineration must be carried out at sufficiently high temperatures and for a sufficiently long time 5 to ensure that no harmful substances are released into the environment. For this purpose, it is usually necessary to add more fuel. However, this process is complex and the energy generated can only be used to a limited extent with reasonable effort.

The object of the invention is to provide a device for the thermal treatment of clays with color optimization, in which a safe removal of harmful components in the exhaust gas of the process stage with color optimization, for example carbon monoxide or hydrocarbons, takes place and at the same time the energy generated is introduced into the overall process as efficiently as possible .

This object is achieved by the device with the features specified in claim 1 and by the method with the features specified in claim 7. Advantageous further developments result from the subclaims, the following description and the drawings.

The device according to the invention is used for the thermal treatment of mineral solids, in particular natural clays. In particular, these are solids which contain iron, manganese, chromium or other coloring metals that can be present as independent mineral phases and / or embedded in mineral phases of the clay. The device has at least one preheater and an entrained flow reactor. The entrained flow reactor is operated with an atmosphere that has a reducing effect on coloring metal oxides in order to optimize the color of the raw materials used in the thermal treatment. For example and preferably, the entrained flow reactor is designed for the use of hydrogen, carbon monoxide, hydrocarbons, natural gas, oil or coal. A separation device is arranged at the outlet of the entrained flow reactor, in which the solid is largely separated from the gas phase.

The entrained flow reactor is used to optimize the color, in which the thermal treatment, in particular the thermal activation of the mineral solid, in particular the clay, can preferably also take place. The color optimization can also follow the thermal treatment.

In order to process natural clays into a product that is as colorless as possible, the entrained flow reactor, for example, has an atmosphere that has a reducing effect on coloring metal oxides. In the context of the invention, “colorless product” is understood to mean that the product has no color, for example a red color, but rather has a white or gray appearance. In the context of the invention, “reducing atmosphere” is understood to mean that it has gas or fuel constituents which are reducing, in particular and preferably the gas or fuel constituents are not completely burned out, for example hydrocarbons, carbon monoxide, hydrogen or oil or coal but contains little or no oxygen in the gas. These can already be contained in the gas phase that is fed to the entrained flow reactor, added to the gas phase in a targeted manner or generated by a deliberately incomplete combustion of natural gas, oil, coal, biomass or other fuels, for example in the burner or in the entrained flow reactor. As a result, the gas stream that leaves the downstream separation device contains these reduced compounds, that is to say hydrogen, carbon monoxide and / or hydrocarbons, in concentrations that are considered to be environmentally harmful. In addition to the reducing compounds (reducing constituents), the gas flow naturally contains inert components, in particular nitrogen (N2) and carbon dioxide (CO2).

Therefore, a thermal treatment zone is arranged at the outlet of the gas flow of the separation device, in which, for example and in particular, the incompletely burned-out components are almost completely burned out. The outlet of the thermal treatment zone is connected to the inlet for the gas flow of the preheater. In the thermal treatment zone, of course, only the reducing constituents are oxidized, the inert constituents (components) remain unchanged.

The advantage of arranging the thermal treatment zone at this point is that the gases have a comparatively high temperature at this point in time, for example 800 ° C. or more. Usually, however, the gases do not exceed a temperature of 1200 ° C at this point in time. Thus, the gases have roughly the temperature required for the safe combustion of these substances, which should not be released into the environment. This saves the fact that the exhaust gas flow has to be reheated to such a high temperature after dedusting and that this heat then has to be recovered again. At the same time, the heat energy gained is obtained and used at this high temperature level and transferred directly to the flow of the raw material in the preheater.

Clays which contain clay minerals in the form of 1-layer and / or 2-layer sheet silicates, for example kaolinite, illite or montmorillonite, are preferred. These clays can also contain accompanying minerals, such as quartz, feldspars, calcite, dolomite, but also metal oxides and hydroxides or especially iron hydroxides. In a further embodiment of the invention, the device additionally has, for example, a drying unit which first dries the raw material if raw material is also to be processed which has too high a moisture content. Furthermore, the device can additionally have, for example, a comminution device, for example a mill, in order to comminute raw material further, provided that it is not already delivered in the required fineness or can tend to aggregate in storage. Furthermore, the device can additionally have a cooling device which cools the product emerging from the entrained flow reactor. For example, this cooling can preferably be carried out in two stages. For example and preferably, the cooling can take place, at least partially, under inert conditions.

In a further embodiment of the invention, the device is part of a plant for the production of cement clinker. The plant for the production of cement clinker can additionally have a drying unit, for example, which first dries the raw material, provided that raw material is also to be processed which has too high a moisture content. The plant can also be used for the production of

Cement clinker additionally have, for example, a comminution device, for example a mill, in order to comminute raw material further, unless it is already delivered in the required fineness or can tend to aggregate in storage. Furthermore, the plant for the production of cement clinker can additionally have a cooling device which cools the product emerging from the entrained flow reactor. For example, this cooling is carried out in two or more stages, with the cooling taking place at least in the first cooling stage under inert or reduced conditions, that is to say without or negligibly small oxygen content in the gas surrounded by the product.

In a further embodiment of the invention, the thermal treatment zone is designed to be cylindrical. In a further embodiment of the invention, the thermal treatment zone is arranged in the form of a tube and perpendicularly above the separation device. In order to achieve the necessary residence time in the thermal treatment zone and to achieve reliable combustion of the reduced compounds, the thermal treatment zone has a length of, for example, 5 m to 50 m, preferably 10 m to 40 m. At the usual gas flow rates for such systems, a dwell time of a few seconds is made possible. This results in a safe combustion of the reduced compounds. In a further embodiment of the invention, the thermal treatment zone has at least one first feed device for a fuel. The type of first feed device depends on the type of fuel. Solid fuels, liquid fuels and gaseous fuels can be used as fuel. An example of a gaseous fuel is natural gas, an example of a liquid fuel is oil, and an example of a solid fuel is pulverized coal. The thermal treatment zone can have a plurality of first feed devices for a fuel. This enables better equalization. In a further embodiment of the invention, the thermal treatment zone has at least one second supply device for oxygen. The oxygen can be supplied in the form of pure oxygen, but this is usually avoided for reasons of cost. The oxygen can be supplied in the form of air. Exhaust gas can also be used as a further oxygen source, which, although it has a reduced oxygen content, is already heated. The thermal treatment zone can have a plurality of second supply devices for oxygen. This enables better equalization.

In a further embodiment of the invention, the second supply device is designed for supplying oxygen under increased pressure. This results in better mixing in the interior of the thermal treatment zone.

In a further embodiment of the invention, the thermal treatment zone has a second supply device with which the oxygen or the air is supplied to the thermal treatment zone under increased pressure compared to the internal pressure of the thermal treatment zone so as to allow the supplied oxygen to be mixed with the gas within the thermal Ensure treatment zone. The increased pressure can be generated, for example and in particular, by means of a fan or compressor. Air can also be provided in the form of compressed air.

The thermal treatment zone particularly preferably has both a first feed device for a fuel and a second feed device for oxygen. This makes it possible to optimally set the temperature conditions for safe and reliable combustion of the reduced gases. Furthermore, it is possible, for example, to use ambient air at ambient temperature, the energy required for heating being provided by the fuel.

In a further embodiment of the invention, the preheater is designed as a cyclone preheater, in particular as an at least two-stage cyclone preheater.

In a further aspect, the invention relates to a method for the thermal treatment of mineral material, in particular for the production of naturally tempered pozzolans, the method having the following steps: a) preheating a mineral material in a preheater,

b) Thermal treatment of the mineral material in an entrained flow reactor in a reducing atmosphere, c) Separation of the solid-gas mixture which comes from the entrained flow reactor in a separation device. According to the invention, the method additionally has the following steps: d) oxidizing the reducing constituents of the gas coming from the separation device in a thermal treatment zone, e) feeding the gas emerging from the thermal treatment zone into the preheater.

The advantage of the method according to the invention is that the energy obtained through the oxidation of the reducing constituents of the gas stream is released directly and immediately in the preheater to the raw material and is thus used entirely in the process.

The reducing atmosphere in step b) has, for example and preferably, carbon monoxide (CO), hydrogen (H2), carbon and / or hydrocarbons as the reducing component. The atmosphere usually also contains inert gas, in particular nitrogen (N2) and carbon dioxide (CO2). The reducing components serve in particular to reduce coloring metal compounds in the mineral solid to a low oxidation level and thus to optimize the coloring. The separation of the solid-gas mixture that comes from the entrained flow reactor is preferably carried out in a separation device which is designed as a cyclone separator. In a further embodiment of the invention, the oxidation in step d) takes place at a temperature of 700 ° C. to 1,000 ° C. and over a period of 1 s to 10 s.

In a further embodiment of the invention, a fuel is fed to the gas in step d). The type of fuel can be different. Solid fuels, liquid fuels and gaseous fuels can be used as fuel.

An example of a gaseous fuel is natural gas, an example of a liquid fuel is oil, and an example of a solid fuel is pulverized coal. In a further embodiment of the invention, oxygen is fed to the gas in step d). The oxygen can be supplied in the form of pure oxygen, but this is usually avoided for reasons of cost. The oxygen can be supplied in the form of air. Exhaust gas can also be used as a further source of oxygen, although it has a reduced oxygen content, but is already heated. This gas supply can take place under increased pressure.

The device according to the invention is explained in more detail below using an exemplary embodiment shown in the drawings. 1 shows a schematic view of the device. In FIG. 1, the device is shown by way of example. The device has an entrained flow reactor 10, a separation device 20, a thermal treatment zone 30, a first preheating cyclone 40, a second preheating cyclone 50 and a cooler 60.

At the beginning, the raw material 100 is added. This can, for example, have been dried and ground beforehand before it is introduced into the device here. The raw material 100 is mixed with the slightly cooled gas stream 220 which comes from the first preheating cyclone 40. In the first preheating stage 110, the gas flow transfers the heat to the raw material. In the second preheating cyclone 50, the gas flow is separated from the solids. The aforementioned raw material 120 is then mixed with the fuel gases 210 which come from the thermal treatment zone. In the second preheating stage 130, the gas flow gives off its heat to the raw material. The solid is then again separated from the gas flow in the first preheating cyclone 40. The heated raw material 140 is fed to the entrained flow reactor 10. The thermal conversion of the raw material into the product and the color optimization of the product take place in the entrained flow reactor 10. For color optimization, for example, carbon, hydrogen or natural gas is supplied via the reducing agent supply 300. Then in the

Separation device 20, which is also designed as a cyclone, separates the solids from the gas flow. The hot product 150 is fed into the cooler 60, cooled there and can be removed as product 160. The cooler 60 can, for example, be multi-stage, in particular two-stage, for example the cooler 60 can be or have a fluidized bed cooler, a moving bed cooler, a cooling screw, a cyclone cooler, a fluidized bed cooler, a drum cooler. The gas flow 200 is fed to the entrained flow reactor 10. For example, a burner can provide the necessary temperature here. This burner can be operated, for example, in such a way that it generates carbon monoxide (CO), for example, and thus introduces reducing constituents into the gas flow. Likewise, in this way, for example, hydrocarbons or hydrogen, for example as unconverted fuel gases, can be introduced. Alternatively or additionally, a further burner can be arranged. The gas flow carries the solids through the entrained flow reactor 10 and is separated from the solids in the separation device 20. In the example shown, the gas flow passes from the separation device 20 directly and immediately into the thermal treatment zone 30, which is arranged vertically above the separation device 20. In the example shown, the thermal treatment zone 30 has a first feed for fuel 310 and a second feed for oxygen 320. This results in complete combustion of the reducing constituents contained in the gas stream, for example hydrogen (H2), carbon monoxide (CO) or hydrocarbons. At the same time, the gas flow is preferably heated by the released combustion energy, or heat losses are compensated for, for example by releasing it to the environment or by supplying further, particularly cold components, for example air. The gas flow emerges as hot fuel gas 210 and is mixed with the preheated raw material 120 and passed into the second preheating stage 130. The gas stream is then separated from the solids in the first preheating cyclone 40 and the slightly cooled gas stream 220 is mixed with the raw material 100 and fed into the first preheating stage 110, in which the residual heat of the gas stream is transferred to the solids. The solids are then separated from the gas flow in the second preheating cyclone

50. From the second preheating cyclone 50 third cooled gas stream 230 from. Reference number

10 entrained flow reactor 20 separation device 30 thermal treatment zone 40 first preheating cyclone 50 second preheating cyclone

60 cooler 100 raw material 110 first preheating stage 120 preheated raw material

130 second preheating stage 140 heated raw material 150 hot product 160 product 200 gas flow

210 fuel gases 220 slightly cooled gas flow 230 cooled gas flow 300 reducing agent feed 310 fuel

320 oxygen

Claims (10)

Claims 1. Device for the thermal treatment of mineral solids, the device having at least one preheater and an entrained flow reactor (10), a separating device (20) being arranged at the outlet of the entrained flow reactor (10), characterized in that at the outlet of the gas flow of the separating device ( 20) a thermal treatment zone (30) is arranged, the outlet of the thermal treatment zone (30) being connected to the inlet for the gas flow of the preheater. 2. Device according to claim 1, characterized in that the thermal treatment zone (30) is cylindrical. 3. Device according to one of the preceding claims, characterized in that the thermal treatment zone (30) is arranged in the form of a tube perpendicularly above the separation device (20). 4. Device according to one of the preceding claims, characterized in that the thermal treatment zone (30) has at least one first feed device for a fuel (310). 5. Device according to one of the preceding claims, characterized in that the thermal treatment zone (30) has at least one second supply device for oxygen (320). 6. Device according to one of the preceding claims, characterized in that the preheater is designed as a cyclone preheater (40, 50), in particular as an at least two-stage cyclone preheater (40, 50). 7. A method for the thermal treatment of mineral material, the method comprising the following steps: a) preheating a mineral material in a preheater, b) thermal treatment of the mineral material in an entrained flow reactor (10) in a reducing atmosphere, c) Separation of the solid-gas mixture which comes from the entrained flow reactor (10) in a separation device (20), characterized in that the method additionally has the following steps: d) Oxidation of the reducing constituents of the from the separation device (20) coming gas in a thermal treatment zone (30), e) feeding the gas emerging from the thermal treatment zone (30) into the preheater. 8. The method according to claim 7, characterized in that the oxidation in step d) takes place at a temperature of 700 ° C to 1,000 ° C and over a period of 1 s to 10 s. 9. The method according to any one of claims 7 to 8, characterized in that in step d) a fuel (310) is fed to the gas. 10. The method according to any one of claims 7 to 9, characterized in that in step d) oxygen is supplied to the (320) gas.