WO2000033947A1

WO2000033947A1 - Method for reducing the concentration of pcdd and pcdf in the flow of exhaust gas in a sintering plant and sintering plant for carrying out said method

Info

Publication number: WO2000033947A1
Application number: PCT/EP1999/009510
Authority: WO
Inventors: Reinhard Holste; Wolfgang SCHÜTTENHELM; Rüdiger WEMHÖNER; Klaus Kersting
Original assignee: Bbp Environment Gmbh
Priority date: 1998-12-07
Filing date: 1999-12-06
Publication date: 2000-06-15
Also published as: KR20010086081A; EP1148930A1; AU1862600A; DE19856260C1

Abstract

The invention relates to a method for reducing the concentration of PCDD and PCDF in the flow of exhaust gas in a sintering plant, especially an iron ore sintering plant. According to said method, the initial PCDD/PCDF concentration in the exhaust gas is reduced adsorbently by adding an adsorption means (AK) and the charged adsorption means is deposited, together with the dust from the sintering process, in a filter (7) and is fed back into said sintering process. The aim of the invention is to reduce the emission of fine dust with the sintering exhaust gas behind the filter. To this end, the fine grained or dust-shaped solid materials (F), which are deposited in the filter, are subjected to a grain coarsening (14) using at least one aggregate (Z) before said solid materials are fed back into the sintering process (1). The aggregate is preferably added to the exhaust gas stream before the filter. The invention also relates to a sintering plant which is configured accordingly.

Description

description

Process for reducing the concentration of PCDD and PCDF in the exhaust gas stream of a sintering plant and sintering plant for carrying out the

Procedure

The invention relates to a method for reducing the concentration of PCDD and PCDF in the exhaust gas stream of a sintering plant, in particular iron ore sintering plant, in which the PCDD / PCDF initial concentration in the exhaust gas is reduced by adding an adsorbent and the loaded adsorbent together with the dust the sintering process is separated in a filter and returned to the sintering process.

In the following description and in the claims, PCDD is understood to mean polychlorinated dibenzo-dioxins and PCDF polychlorinated dibenzo-furans.

From DE 196 51 822 A1 a generic method is known, in which the loaded adsorbent is returned together with the dust from the sintering process directly into the sintering process. With the adsorptive reduction, which is preferably designed as an entrained flow adsorption process, the initial PCDD / PCDF concentration is reduced to 1/5 - 1/20. In a subsequent process step, the PCDD and PCDF concentration of the exhaust gas pre-cleaned by the adsorption is reduced to a value below the legally prescribed limit value by catalytic conversion.

Electrofilters are usually used to clean the sintered exhaust gas from dust. In the usual temperature range of the sintered exhaust gas of 90-150 ° C, in particular 130 ° C, the electrical resistance of the sintered dust is at largest and therefore the dust most difficult to separate under these boundary conditions in the electrostatic filter. To improve dust separation, S0 ₃ conditioning of the exhaust gas upstream of the electrostatic precipitator is carried out in some sintering plants. The feedstock required for this in the form of pressure-liquefied S0 ₂ is expensive and, from the point of view of occupational safety, questionable because of its toxicity. S0 ₃ conditioning of the exhaust gas upstream of the electrostatic precipitator should therefore be avoided.

The dust separated from the sintered exhaust gas in the electrostatic filter essentially consists of iron oxide, potassium, sulfate, chloride, lead and carbon. The content of Si0 ₂ and silicates and phosphates is extremely low. The maximum temperature in the sinter bed, which is built up from a fine ore fuel mixture, is determined by the melting temperature of the iron ore used and, depending on the ore, is between 1300 ° C and 1480 ° C. The maximum possible temperature is only in a small layer thickness of the sintered bed. In a first approximation, the maximum temperature zone on the sintered belt moves linearly over the length of the sintered belt from the surface of the bed on the sintered belt to the underside of the sintered bed. At these temperatures, alkali and heavy metal chlorides evaporate, mainly potassium chloride and lead chloride. An integration of these substances in the sintered product does not take place because of the low Si0 ₂ - or silicate and phosphate content. Due to the steep temperature gradients across the height of the sintered layer, the vaporous alkali and heavy metal chlorides cool down quickly. This leads to the formation of a highly reactive salt melt, which, depending on the alkali content, binds S0 ₃ from the flue gas as sulfate. After the molten salt has solidified, mineral transformation occurs in some cases by gaseous HCI being expelled from the alkali and heavy metal chlorides through the stored sulfate. The HCl released can then partly react to iron chloride or also partly catalytically react to the existing iron oxides and copper oxides of the sintered bed to form elemental chlorine. Furthermore, the alkali and heavy metal chlorides are sulfated directly with the sulfur dioxide and the oxygen of the sintered exhaust gas in the gas phase at approx. 600 to 700 ° C. Elemental chlorine is released. The SO ₂ required for the reaction is typically contained in several hundred mg / m ³ . The mechanism shown should be one of the main contributions to dioxin / furan formation in the sinter exhaust gases. The chlorine formed during the processes described above in turn reacts with the S0 ₂ and water vapor contained in the exhaust gas to form HCl and S0 ₃ and makes the sulfates available for integration into the salt melt via this reaction path. Due to the maximum temperature zone migrating from top to bottom over the length of the sintering belt, the alkali and heavy metal chlorides evaporate and condense several times until they are discharged from the bed with the sintered exhaust gas and separated in dust form in the electrostatic filter. From the electrostatic precipitator, they are fed back into the sintering process via the provided dust recirculation, ie a significant proportion of the dust separated in the electrostatic precipitator results from a circulation enrichment of potassium and lead. A major sink for these substances has so far been the emission to the environment via the residual dust in the sintered exhaust gas after the electrostatic precipitator. When returning the electrostatic filter dust to the sintering process, e.g. B. 20 to 30% of the dust separated in the filter as potassium and lead chloride.

It is the object of the present invention to provide a generic method in which the fine dust emission with the sintered exhaust gas after the filter is significantly reduced.

This object is achieved in that the fine-grained or dust-like solids separated in the filter are subjected to a coarsening process together with at least one additive before being returned to the sintering process.

Due to the procedure according to the invention, the potassium and lead components which are responsible for the fine dust particles after the filter are incorporated in the slag formed with the additive. S0 ₃ conditioning is not absolutely necessary.

The coarse coarsening can be achieved by a process selected from the group of briquetting, press granulation, pelleting, extrusion.

In addition to the aggregate, an auxiliary liquid, preferably water, may be required for the coarsening. In order to achieve a particularly high incorporation rate of the compounds responsible for the fine dust fractions in the slag, at least one additive is preferably selected from the group: silicon dioxide, silicates, in particular calcium silicates, phosphates, in particular calcium phosphates, and mixtures thereof.

At the same time, the aggregate should be able to form a liquid slag even at relatively low temperatures, preferably at a temperature lower than or equal to 1200 ° C., but at least at a temperature lower than the maximum temperature in the sintering bed.

In the simplest and preferred case, the additive consists of clay minerals, in particular those clay minerals with a high Si0 ₂ - and / or high calcium silicate content. In this way, the absolute amount of aggregate can be kept low.

To improve the inclusion of slag and the flow of slag, it can also be expedient that a mixture of clay minerals and phosphates, preferably calcium phosphates, possibly with the addition of lime, is used as the additive.

It may also be expedient for a coarsening process to add a substance to lower the melting temperature, preferably borax or a carbon-containing substance.

It is possible that the aggregate is added to the solids separated in the filter.

It has proven expedient that when the additive is added to the solids separated in the filter, the additive is added in an amount of 10-50% by weight, preferably 20-30% by weight.

Since when the solids separated in the filter and coarsened in their grain size are returned to the sintering process, the dust load on the sintered exhaust gas in front of the filter is reduced, and thus also the amount of dust separated from the exhaust gas, this is particularly important when adding activated coke as an adsorbent to the dioxin / Furan adsorption section in front of the filter Limited amount of coke for safety reasons and therefore less than with untreated dust recirculation. However, in order to achieve maximum adsorption of the dioxins / furans, it is desirable to add as much activated coke to the exhaust gas as possible. It is therefore particularly preferred to add the additive to the exhaust gas stream before or after adding the adsorbent upstream of the filter. This increases the proportion of inert material and the amount of activated coke can be maximized without any safety concerns. (The quantity ratio of aggregate to the deposited non-aggregates should essentially correspond to the weight ranges given above). At the same time, the additive also acts as an adsorbent, albeit with a lower effectiveness for the dioxins / furans compared to the activated coke, and thereby enables a higher purification of the sintered gas from dioxins and furans, so that the further reduction in PCDD- / PCDF concentration can be avoided by catalytic conversion.

The grain coarsening is carried out in such a way that it leads to a grain size which essentially corresponds to the grain size sought in the sintering process. When using an extrusion system with mixing and kneading extenders, in which the mixture of separated solids and aggregate is ejected via an extrusion die, z. B. pellets of about 2 to 10 mm in diameter and 5 - 30 mm in length, in particular 5 mm in diameter and 10 mm in length, and then fed to the sintering plant.

During the return, it is possible for the returned grains or bodies to be mixed uniformly into the sintering mixture via conveying and / or mixing devices which already exist for the sintered material. However, it is preferred that the returned grains are introduced into the lower layer of the sintered bed, since the most favorable temperature conditions for the inclusion of the potassium and lead chlorides in the slag formed with the aid of the tensile impact material are given there.

As in the prior art, an adsorbent selected from the group: activated coke, preferably hearth furnace coke, activated carbon, zeolites, aluminum oxide, silica gel, diatomaceous earth, clay, layered silicates, diatomaceous earth or mixtures thereof can be used as the adsorbent. Finally, the invention is also directed to a sintering system with a sintering belt, an exhaust pipe, an adsorber connected in the exhaust pipe and a filter downstream of the adsorber, and a feedback device for returning solids separated in the filter to the sintering belt.

In this sintering plant, it is provided according to the invention that the return device has a device for coarsening the separated solids together with an additive.

The invention will now be explained with reference to the accompanying figures. It shows:

Fig. 1 shows a system in which the aggregate is mixed with the solids separated from the filter, and

Fig. 2 shows a system in which the aggregate is fed to the exhaust gas stream upstream of the filter.

In the system shown in FIG. 1, sintered material S in the form of a fine ore fuel mixture is fed to a sintering belt 1. To start the sintering process, an ignition furnace 2 is assigned to the feed end of the sintering belt 1. Air L is supplied to the top of the sintering belt and the exhaust gas A is collected below the belt in an exhaust pipe 3. Activated coke AK is dosed as adsorbent from a storage silo 4 and mixed with the exhaust gas by a blowing device 5. The adsorbent is loaded with the PCDD and PCDF from the exhaust gas in a reaction section 6 downstream of the blowing device 5 and separated from the exhaust gas in a subsequent electrostatic filter 7. The cleaned exhaust gas is fed to a chimney 9 via a blower 8. If the PCDD and PCDF concentration behind electrostatic filter 7 is still above the legally prescribed values, a burner 10 for increasing the temperature and a downstream oxidation catalytic converter 11 can optionally be provided.

The solids F separated in the electrostatic filter 7 are fed to a dust silo 12. Furthermore, a silo 13 for aggregate Z is provided. Solids F and Z are metered out of the silos 12 and 13 into a mixer / extruder 14 fed for coarser coarsening. Water is fed to the mixer / extruder 14 as an auxiliary fluid for the coarsening process via line 15.

The grains or bodies K produced in the mixer / extruder 14 are guided to the feed side of the sintered belt by means of suitable conveying means and, together with the sintered material S, are fed to the belt.

As mentioned in the introduction to the description, the task can preferably not be carried out in admixture, but the grains K are first placed on the sintering belt according to FIG. 1 and then covered with the sintered material S so that they lie in a lower layer of the sintered bed come.

In the embodiment according to FIG. 2, the additive (s) Z is added from a silo 13 ', from which the additive is blown into the entrained flow reaction path 6 upstream of the electrostatic filter 7 via a suitable blowing device 15 in the flow direction of the exhaust gas A behind the blowing device 5. 2 also indicates that the blowing device 15 'for the aggregate can also be in front of the blowing device 5 for the adsorbent AK.

In the introduction to the description, it was already explained that in the course of the process according to FIG. 2, by blowing in the additives or additives into the entrained flow reaction path, the inert component in the entrained flow path 6 and in the e-filter 7 is increased, as a result of which the necessary for the separation of PCDD and PCDF Amount of adsorbent, in particular activated coke, can be increased. 2, it can be assumed that oxidation catalysis according to the E filter is no longer absolutely necessary.

Systems are described in connection with FIGS. 1 and 2, in which the adsorptive reduction in the content of PCDD and PCDF takes place in an entrained flow reaction path 6. Under certain circumstances, it is also conceivable to provide a reaction bed instead of an entrained-flow reaction path. Bed material and material from the filter and aggregate and water are then fed to the device for coarsening. It is also under 00/33947

8th

Under some circumstances conceivable to use a hot gas fabric filter instead of an electrostatic filter.

In the two embodiments according to FIGS. 1 and 2, the solids F, as shown in dashed lines in FIG. 1, could additionally be subjected to leaching in a leaching container 16 in order to increase the effectiveness of the process before the coarsening. The leaching is preferably carried out with water supplied at 17, by slurrying the fine-grained solids F brought in by the electrostatic filter 7 in a ratio of 1 to 1.5 to 1 to 10, preferably 1 to 2, with water. Here, the readily water-soluble alkali chlorides, in particular potassium chloride, are preferably converted into the liquid phase. After one of the leaching 16 downstream solid / liquid separation 18, the z. B. can be designed as a vacuum belt filter, the chloride-depleted filter cake FK can be transferred to a correspondingly designed storage container 12 'and then mixed and extruded in the mixer / extruder 14 with the additive Z. The chloride-containing liquid phase (waste water) originating from the solid / liquid separation 18 can either be fed via line 19 to another waste water stream present at the site of the installation (e.g. waste water from a blast furnace gas cleaning system) and further treated there or via a special one for this wastewater to be installed cleaning system. The cleaned, but still chloride-containing wastewater is discharged into a receiving water.

(2 sheets of drawings)

Claims

claims

1. A method for reducing the concentration of PCDD and PCDF in the exhaust gas stream of a sintering plant, in particular iron ore sintering plant, in which the initial PCDD / PCDF concentration in the exhaust gas is reduced by adsorbing by adding an adsorbent and the loaded adsorbent together with the dust from the sintering process deposited in a filter and returned to the sintering process, characterized in that the fine-grained or dusty solids separated in the filter are subjected to a coarsening process together with at least one additive before being returned to the sintering process.

2. The method according to claim 1, characterized in that a method selected from the group: briquetting, press granulation, pelleting, extrusion is used for the coarsening.

3. The method according to claim 1 or 2, characterized in that an auxiliary liquid, preferably water, is used for the coarsening in addition to the aggregate.

4. The method according to at least one of claims 1 to 3, characterized in that at least one additive selected from the group: silicon dioxide, silicates, in particular calcium silicates, phosphates, in particular calcium phosphates, and mixtures thereof are used.

5. The method according to at least one of claims 1 to 4, characterized in that clay minerals, in particular those clay minerals with a high Si0 ₂ - and / or high calcium silicate content, are used as the additive.

6. The method according to at least one of claims 1 to 5, characterized in that a mixture of clay minerals and phosphates, preferably calcium phosphate, possibly with the addition of lime is used as the additive.

7. The method according to at least one of claims 1 to 6, characterized in that a material for lowering the melting temperature is added to the coarsening, preferably borax or a carbon-containing substance.

8. The method according to at least one of claims 1 to 7, characterized in that the additive is added to the solids separated in the filter.

9. The method according to claim 8, characterized in that when the additive is added to the solids deposited in the filter, based on the amount of solids deposited in the filter, the additive in an amount of 10-50% by weight, preferably 20-30%. -%, is added.

10. The method according to at least one of claims 1 to 8, characterized in that the additive is added to the exhaust gas stream before or after addition of the adsorbent before the filter.

11. The method according to at least one of claims 1 to 10, characterized in that the coarsening is carried out in such a way that it leads to a grain size which essentially corresponds to the grain size sought in the sintering process.

12. The method according to at least one of claims 1 to 11, characterized in that the returned grains or bodies are mixed uniformly into the sintered mixture via conveying and / or mixing devices for the sintered material.

13. The method according to at least one of claims 1 to 11, characterized in that the returned grains are introduced into the lower layer of the sintered bed.

14. The method according to at least one of claims 1 to 13, characterized in that an adsorbent selected from the group: activated coke, preferably hearth furnace coke, activated carbon, zeolites, aluminum oxide, silica gel, diatomaceous earth, layered silicates, diatomaceous earth or mixtures thereof are used.

15. The method according to at least one of claims 1 to 14, characterized in that the exhaust gas emerging from the filter for further lowering the PCDD and PCDF concentration is passed over a catalyst for the catalytic conversion of PCDD and PCDF.

16. The method according to at least one of claims 1 to 15, characterized in that that the adsorptive reduction is carried out in the entrained flow process or in the bed process.

17. The method according to at least one of claims 1 to 16, characterized in that an electrostatic filter or a hot-tissue filter is used as the filter.

18. The method according to at least one of claims 1 to 17, characterized in that the fine-grained or dust-like solids deposited in the filter are subjected to leaching before the coarsening.

19. Sintering plant with a sintering belt (1), an exhaust pipe (3), an adsorber (5; AK) switched on in the exhaust pipe and a filter (7) connected downstream of the adsorber (5) and a feedback device for the return of solids separated in the filter onto the sintered belt (1), characterized in that the return device has a device (14) for coarsening the separated solids (F) together with an additive (Z).