FR2684564A3 - Process and plant for filtering dust-laden gaseous waste - Google Patents

Abstract

Process for filtering dust-laden gaseous waste, of the type employing a bag filter (5, 6, 7, 8, 9) through which the said gaseous waste passes, according to which an operation of unclogging the said filter (5, 6, 7, 8, 9) is performed at regular intervals, characterised in that, after each unclogging operation the deposition of a layer of a pulverulent material (25) is performed on the said filter, the material having the property of forming on the said filter a layer which can be easily detached during the following unclogging stage. Another subject of the invention is a filtration plant for making use of this process. The invention applies especially to the purification of roasting gases from a plant for agglomerating iron ore.

Description

METHOD AND PLANT FOR FILTERING RELEASES
GASEOUS LOADS OF DUST
The invention relates to the field of filtration of fumes charged with dust produced by an industrial installation, such as an installation for agglomeration of iron ore.

 The iron ore agglomeration installations are intended to condition it in the form of cooked pellets intended to supply blast furnaces.

These installations are notable emitters of atmospheric pollutants (dust, SO2, NOx, CO, heavy metals, acid aerosols, organic compounds). In particular, the dust emitted represents up to 40% of all that which is rejected by an integrated steel plant. It is therefore very important to strive to reduce the quantity of polluting discharges emitted by these installations.

 The modifications to the characteristics of the agglomeration process itself which have been tested so far have only resulted in modest improvements from the point of view of gaseous releases and acid aerosols. In addition, these modifications prove in certain cases detrimental to the quality of the agglomerates produced.

 Currently, the solution adopted is to use an agglomerate production method optimized from the point of view of the quality of the material, and to purify the cooking fumes from the installation by means of cyclones and electrofilters, seeking to obtain the best possible performance for these devices. We thus manage to discharge into the atmosphere fumes containing only about 100 to 200 mg / m3N of dust, 800 to 2300 mg / m3N of SO2, 500 to 1500 mg / m3N of NOx. These performances are still unsatisfactory. In particular, if the agglomeration installations were to one day be classified as combustion installations, they could not meet the European and national standards in force and to come.

These provide (European Directive of 02.21.1984) for maximum emissions of 50 mg / m3N for dust, 250 mg / m3N (400 in 1995) for S02 and 800 mg / m3N (400 in 1995) for NOx.

 Bag filters are known to be very effective means of filtering gaseous emissions in other industrial fields (for example, thermal power plants, steelworks, foundries, the food industry, the pharmaceutical industry), and could provide a satisfactory solution to the problem of eliminating dust and acid and organic aerosols. Attempts to use such filters on agglomeration plants which have been made so far, however, have not been convincing. First, the high temperature and acidity of the fumes made them too aggressive with respect to the filtering materials conventionally used (textiles, felt). Then the development of materials adapted to these difficult conditions (glass fibers, PTFE) did not solve all the problems. Indeed, the nature of the materials to be filtered leads to rapid clogging of the filters, because very fine dust, coming from a vapor phase and consisting largely of organic compounds and alkali chlorides, have a strong tendency to agglomerate and adhere strongly to the filter bags. The filter unclogging operations are thus incomplete, and very quickly the filters eventually become clogged and no longer function under satisfactory conditions.

 The object of the invention is to allow the use of bag filters in installations emitting dust which have a strong tendency to agglomerate and which are unsuitable for unclogging the filters, in particular in ore agglomeration installations. of iron.

 To this end, the subject of the invention is a process for filtering gaseous discharges laden with dust, of the type using a bag filter through which said gaseous discharges pass, according to which a cleaning operation of said filter is periodically carried out, characterized in that, after each unclogging operation, a layer of pulverulent material is deposited on said filter, having the property of forming an easily detachable layer on said filter during the next unclogging phase.

 The invention also relates to a filtration installation for the implementation of this process.

 As will be understood, the invention consists in depositing on the filter bags, after unclogging and at the start of each filtration period, a film of a powdery material, such as slaked lime, which adheres sufficiently little on the sleeve walls to be easily expelled during the unclogging operation. During this operation, the pulverulent material drives in its fall the dust which is deposited on it during the filtration period. It is thus possible to ensure at each operation a suitable unclogging of the filter, the performances of which thus acquire an excellent stability over time which allows the use of the installation as continuous as possible.

 Remember that a bag filter is a device in which the dusty gas stream passes through a fabric. A dust cake is formed on this fabric, which then provides most of the particle separation mechanism from the gas flow. As the filtration operation progresses, the deposit of dust increases in thickness and the space available for the passage of gases decreases, which increases their pressure drop. When this reaches a level deemed incompatible with the proper functioning of the installation, the filter is unclogged so that it finds a satisfactory pressure drop. This unclogging can be carried out in a purely mechanical manner by shaking the support of the fabric, or in a pneumatic manner by circulation through the fabric of a gas against the flow of the filtered gas, or by sudden blowing of compressed air. on the fabric, the latter being the most effective. In all cases, the deposits which were on the filters fall into the lower part of the enclosure which contains the installation, from where they are periodically removed.

The invention will be better understood on reading the following description, which refers to the appended figures
- Figure 1 which shows schematically sectional view of an installation for filtering dusty gaseous discharges according to the invention
- Figure 2 which schematically shows a filter during the filtration operation
- Figure 3 which shows schematically the same filter during the unclogging operation
- Figure 4 which shows the evolution as a function of time of the pressure drop of the gases as they pass through the filter when using a bag filter under usual conditions
- Figure 5 which shows this same evolution when using the method according to the invention.

 The filtration installation shown in FIG. 1 is associated, for example, with an agglomeration chain of iron ore, not shown. The fumes from the cooking of the agglomerate are collected at the level of the wind boxes and circulate in a pipe 1 to which the installation is connected. This comprises an enclosure 2, in the substantially frustoconical lower part 3 from which the gas to be treated reaches laterally. The enclosure 2 comprises a substantially horizontal partition 4 into which vertical filters 5, 6, 7, 8, 9 are inserted and which defines in the enclosure 2 an upstream compartment 10 and a downstream compartment 11. To pass from a compartment at the other, the gases must pass through the filters 5, 6, 7, 8, 9. The upstream compartment comprises, at its lower end, an airlock 12 which allows the periodic evacuation of the dust 13 which accumulates in the bottom of compartment 10 under the effect of successive unclogging of filters 5, 6, 7, 8, 9. The downstream compartment 11 of enclosure 2 comprises a ramp 14 of nozzles capable of blowing compressed air, taken from a tank 15, through the filters 5, 6, 7, 8, 9 in the opposite direction to the path of the gases to be filtered, to ensure the cleaning of the filters 5, 6, 7, 8, 9. On the upper part of the downstream compartment 11 is connected a gas evacuation pipe 16, connected to a suction installation ion not shown ensuring the extraction of gases from the wind boxes and their circulation in the filtration installation. The arrows reflect the course of the combustion gases through the various elements of the installation.

 Figure 2 shows in more detail one of the filters, such as the filter 6. This figure shows the partition 4 and the ramp 14 of Figure 1. The filter 6 comprises a metal basket 17 inserted in an orifice 18 formed in the partition 4. This basket is formed by a three-dimensional network of intertwined wires. It is generally cylindrical or parallelepipedic. A filter cloth 19, made of a material such as a textile, a polymer or a combination of these materials, is fixed to the partition 4, and completely surrounds the basket 17 on its free faces. The installation also includes a venturi 20 inserted inside the orifice 18 of the partition 4, and facing a nozzle 21 formed in the ramp 14. FIG. 2 represents the filter during a filtration phase, where the gas to be filtered flows from the downstream compartment 10 to the upstream compartment 11 as indicated by the arrows. The gas stream presses the fabric 19 against the basket 17, and the dust is deposited on the fabric 19 to gradually form a cake 22 whose thickness increases as the filtration operation progresses.

 Figure 3 shows the same filter 6 during the unclogging phase. Compressed air is injected inside the fabric 19, from the nozzle 21 of the ramp 14. Thanks in particular to the venturi 20, this air causes a strong increase in the pressure inside the fabric 19, of which it causes swelling. The detachment of the cake 22 occurs abruptly when the swelling of the fabric 19 stops, and the pieces 23 of the cake 22 fall into the lower part of the enclosure 2. After which the fabric 19 regains its initial shape.

 It is the usually deficient course of this unclogging operation which has dissuaded, until now, users of agglomeration chains of iron ore from applying bag filters to the dedusting of the combustion gases from their installations. In fact, dust tends to agglomerate under the effect of various phenomena, such as the humidity of the medium and the chemical reactions between the dust itself, between the dust and the gases, or even between the dust and the materials of some filters. The effect of these unclogging imperfections is that after this, the pressure drops, which had decreased at the end of the unclogging, rise until quickly reaching a level such that the filter can no longer treat the gases with a sufficient flow and must be unclogged again. As this unclogging must, in certain cases, be carried out outside a filtration period, this means that the aspiration of the gases must be completely stopped, or they must be diverted to another filtration installation operating in parallel with the unclogged one. . Thus, in FIG. 4, it can be seen that after each unclogging, the pressure drop in the filters (for a filtration speed of approximately 50 m / h, 770 g / m2 woven glass sleeves and a dust content of treated gases of the order of 500 mg / m3N) is established at approximately 180 mm of water column (NB: the brief drops to O mm apparent in FIG. 4 are only due to the stopping of the filtration during unclogging), and then takes only 5 minutes to reach the value of 200 mm of water column. This value is taken as the acceptable upper limit for the pressure drop in the installation considered because a value of 250 mm would correspond to an irreversible clogging requiring replacement of the filter. The unclogging must therefore occur every 5 minutes, which is hardly compatible with an industrial operation of the installation.

In order to overcome this drawback, in accordance with the invention, there are added to the installation which has just been described, members making it possible to introduce a pulverulent material, such as slaked lime, into the gas stream.
Ca (OH) 2, immediately after unclogging the filters. These members, shown diagrammatically in FIG. 1, include in particular a hopper 24 containing the lime 25 and a valve 26 controlling the admission of the lime 25 in the pipe 1 for supplying the gases to the filtration installation. The lime is introduced after each unclogging operation, in an amount of the order of 10% of the total amount of dust incident on the filter. After this introduction of lime, the filtration of the gases is continued normally, until the next unclogging which will be followed by a new introduction of lime. FIG. 5 shows the results obtained on the same installation as above, treating the same gases in which, for a brief period following each unclogging, particles of slaked lime are added so as to form on the filter a prelayer of thickness of the range from 10 to 100 µm. It can be seen that, under these conditions, after each unclogging the pressure drop in the filter is lowered to approximately 120 mm of water column. Then, the increase in this pressure drop is very gradual, since the 200 mm of water column that one seeks not to exceed is only reached after practically ~ 4 hours of operation of the installation (at compare to the 5 minutes observed in walking without injecting lime). Under these conditions, the use of bag filters on agglomeration facilities for iron ore becomes perfectly viable.

 It is thus possible to limit the quantity of dust released to the atmosphere with the filtered gases to less than 5 mg / m3N, which is much lower than the legal requirements present and foreseeable for these releases. In addition, slight decreases in acid discharges (HF and HC1) and volatile organic discharges are observed, probably due to their partial capture by the lime of the precoat. However, care must be taken to ensure that this reaction only concerns a relatively limited thickness of the lime deposit, so that it retains its ability to be easily detached from the filter.

 Of course, the invention is not limited to the example which has just been described. In particular, other pulverulent materials than slaked lime can be used to coat the filters with a pre-coat.

The main thing is that they have the same unclogging facilitation properties as slaked lime. Likewise, other modes of unclogging the filters than that described in the example can be used. Finally, it is possible to envisage applying the invention to installations other than an agglomeration of iron ore, insofar as it is desired to facilitate the unclogging of bag filters which ensure the purification of the dusty gases which they emit. .

Claims (6)

 1) A method of filtering gaseous discharges loaded with dust, of the type using a bag filter through which said gaseous discharges pass, according to which a unclogging operation of said filter is carried out periodically, characterized in that, after each unclogging operation, is carried out on said filter the deposition of a layer of a pulverulent material having the property of forming on said filter a layer easily removable during the next unclogging phase.  2) Method according to claim 1, characterized in that said pulverulent material is slaked lime.  3) Method according to claim 1 or 2, characterized in that said unclogging is performed by blowing compressed air through the filter sleeves.  4) Method according to one of claims 1 to 3, characterized in that said gaseous discharges are cooking fumes from an installation of agglomeration of iron ore.  5) Installation for filtering gaseous discharges laden with dust, of the type comprising at least one bag filter (5,6,7,8,9), means (1,2,4) for passing said filter (5, 6,7,8,9) by said gaseous discharges, and means (14, 20) for unclogging said filter, characterized in that it also comprises means (24, 26) for mixing with said gaseous discharges a pulverulent material ( 25) intended to coat the surface of the sleeves of said filter (5,6,7,8,9).  6) Installation according to claim 5, characterized in that said unclogging means of said filter are constituted by means of blowing compressed air through the sleeves of said filter.