LU88441A1 - Process for the treatment of a mixture of water vapors and air polluted by sulfur gases - Google Patents

Description

PROCESS FOR THE TREATMENT OF A MIXTURE OF WATER VAPORS AND AIR POLLUTED BY SULFUR GASES.

The present invention relates to a process for the treatment of a mixture of water and air vapors, polluted by sulphurous gases which is formed during the production of granulation from blast furnace slag. The invention also relates to an installation for implementing this method.

Several processes and installations are currently known for the granulation of blast furnace slag. It is for example frequent to inject a powerful jet of water using a spray head in a flow of molten slag. Advantageous embodiments of such spray heads are described for example in European patent application EP 0 082 279.

A poorly controlled problem in this type of installation is constituted by the water vapors contaminated inter alia by sulphide gases including hydrogen sulphide H2S and sulfur dioxide SO2, generated in large quantities and at essentially variable flow rates during injection of water into the molten slag.

After granulation, the slag can be dehydrated for example in a rotary cylinder delimited on the outside by a filtering surface. This process and the corresponding installation are described in United States patent US-4,205,855 to which the reader would refer for further detailed explanations. During this dehydration of the slag, a mixture of water and air vapors is also formed, polluted by sulfur gases.

Sometimes the polluted vapors produced during granulation are discharged without treatment into the atmosphere, which naturally constitutes a nuisance for the inhabitants around the factories, given the particularly unpleasant odor and the toxicity of H2S.

Some installations are equipped with a closed condensation tower which is located above the granulation tank in which the slag is cooled. In this condensation tower, sprinklers spray water on the rising hot gases and vapors by convection. The sprinkler water from the sprinklers and the condensates are collected by troughs located below the sprinklers. However, these installations are not entirely satisfactory. Since the slag flow rates and therefore the rising steam and gas flows are variable as a function of time, it is indeed difficult to create and maintain a stable and sufficient depression by condensation of the water vapors in the zone of sprinklers. At certain times, there is an overpressure in the condensation area of the tower, which prevents the vapors from going up to the nozzles. Sufficient condensation of water vapors and efficient removal of sulfur gases is no longer guaranteed. Polluted vapors can therefore escape uncontrollably into the open air.

German patent application P 3511958.6, filed April 2, 1985, describes a blast furnace slag granulation plant which uses a closed gas circulation system. The vapors generated during the injection of water into the molten slag are entrained by a jet of water directly into the granulation tank filled with water to be partially condensed there. The residual vapors and gases emanating from the granulation tank are sprayed with water, using nozzles installed in a condenser located above the water reserve. The rising vapors and the water spray therefore constitute two counter-current flows. It is claimed that the residual gases and vapors which are not eliminated by this washing are returned to the spray head by an internal circulation, which would be created in the installation. The German document does not mention how an overpressure in the installation is avoided, which generates an uncontrolled leakage of polluted gases to the outside. This system can also not be used to equip an existing installation. The granulation installation must in fact be constructed so as to constitute a perfectly gas-tight system.

In addition to the specific disadvantages mentioned, these methods and installations fail to remove hydrogen sulphide in an effective manner. This gas is special in the sense that the human nose detects it at minute concentrations which are barely measurable and which are far below its toxicity threshold. Since hydrogen sulphide is characterized by a particularly unpleasant odor of rotten eggs, it presents a strong nuisance for people living and working near installations generating hydrogen sulphide H2S. The concentration limits imposed by legislation are therefore very severe.

One solution for removing hydrogen sulfide from a gas mixture is to pass a polluted gas mixture over an iron wool filter. The hydrogen sulfide is then removed by conversion to iron sulfide. This process is especially suitable for small installations and cannot be applied to the mixture of air and vapors, formed during the production of blast furnace slag aggregate because, on the one hand, the vapor content of water is very high which would risk making the iron wool filter ineffective, and on the other hand, the 'gas flow rates to be treated are very high which requires huge filters and expensive to maintain.

The aim of the present invention is to provide an improved process for the treatment of a mixture of air and vapors, formed during the production of blast furnace slag granulate and polluted by sulfur gases, which makes it possible to treat 'an efficient way of highly variable pollutant vapor flow rates, and effectively remove sulfur dioxide and hydrogen sulfide.

In accordance with the present invention, this object is achieved essentially by the fact that said gas mixture is first subjected to a gas washing in order to partially condense it and extract part of the pollutants therefrom, which is collected and that the non-condensed and undissolved part of said mixture is released near a flow of molten slag.

The mixture of vapors and air polluted by sulphide gases, which is formed during the granulation of blast furnace slag, is first subjected to a conventional gas washing consisting in spraying an aqueous solution on the mixture gas which condenses a large part of the condensable gases and eliminates most of the sulfur dioxide by dissolving in water. Gas washing is only ineffective in removing hydrogen sulfide because it, unlike sulfur dioxide, is not very soluble in water. According to the literature, we manage to dissolve in cold water T 22.8 g / 100 cm of sulfur dioxide while we can only dissolve 437 cm3 / 100 cm3 of hydrogen sulfide gas, which which corresponds to 0.7 g / 100 cm under the same conditions. Hydrogen sulfide is removed by channeling and releasing the uncondensed and undissolved part of the gas mixture above the stream of molten slag. Hydrogen sulfide is oxidized to sulfur dioxide, because near the flow of molten slag the temperatures are high and the air is rich in oxygen. The sulfur dioxide thus formed is entrained in the granulation plant and is then removed via the gas washing.

According to a first advantageous embodiment, said mixture is first channeled in an upward flow, and then the mixture flows in a downward flow in an enclosure maintained in depression where a solution is sprayed, in parallel flow alkaline aqueous in said downflow which depollutes and partially condenses said mixture. The non-condensed gases are removed by a forced and adjustable current so as to create and maintain a vacuum inside said enclosure.

The mixture of vapors and air polluted by sulphurous gases, which is formed during the granulation of blast furnace slag, is channeled in an upward flow. This upward movement is partially a convection movement, reinforced however by an extraction of the non-condensed treated gases from the installation.

An adjustable dynamic vacuum is created inside said enclosure by the controlled evacuation of non-condensed gases from the lower part of the latter towards the outside of the tower. The condensation of water vapors by sprinkling water also contributes to maintaining said enclosure under vacuum. This depression creates in said enclosure a downward flow of the mixture of vapors and gases. The condensation of the vapors results from cooling by spraying cold water into the vapors. The elimination of sulfur-containing gases, mainly of sulfur dioxide SO2, comes mainly from a passage in solution and incidentally from an absorption, a precipitation and / or redox reactions in the finely pulverized alkaline aqueous solution. It will be noted that the water is sprayed into the mixture of vapors and gases descending inside said enclosure. Consequently, a forced and parallel flow of irrigation water and gases and vapors is established between the open upper end and the closed lower end of the enclosure. The advantage is that the irrigation water condenses the vapors, reacts with the sulphurous gases and at the same time entrains the non-condensed gases towards the bottom of the enclosure where these are evacuated. A possible accumulation of air and uncondensed vapors leading to a backflow which prevents polluted vapors from reaching the condensation zone is not possible. An annoying phenomenon, often encountered in the state of the art, has thus been effectively eliminated.

The present invention also describes a device for the treatment of a mixture of polluted water and air vapors which is formed during the production of blast furnace slag aggregate in a granulation installation, characterized by a tower closed above the granulation installation, intended to capture said mixture of vapors and gases by nozzles arranged inside said closed tower - these nozzles being connected to a pipe for dispensing a washing solution gases - by at least one sensor located below said nozzles, by at least one conduit for washing solution and condensate connected to said at least one sensor, by at least one conduit for suction of uncondensed gases emerging in said tower, by at least one variable flow extraction pump connected to said suction duct and intended to evacuate said gases outside said tower and to maintain a vacuum in said tower, by at least one conduit conveying and releasing said gases discharged near a channel leading a flow of molten slag towards a granulation head. The installation implementing the present invention will be able to equip most of the existing blast furnace slag granulation installations, and this at a relatively low cost. Unlike the installation described in German patent application P 3511958.6, the granulation installation must not be gas-tight.

Advantageously, the device also comprises at least one oblong enclosure closed at its lower end and open at its upper end, arranged vertically inside said closed tower.

Preferably, the variable flow pump used is a fan.

It will be appreciated that the pump can be connected to a probe for adjusting the flow rate thereof as a function of the vacuum inside said enclosure.

The exhaust gas flow is adjusted so as to maintain the tower in vacuum, which prevents polluted vapors from escaping uncontrollably from the installation at any time. However, during the granulation of blast furnace slag, the slag flow rates and, consequently, the flow rates of vapors and sulfur gases formed, are very variable. The depollution and condensation installation must therefore be able to control very variable flow rates of polluted vapors and air. This flexibility with regard to the variations in the flow rates of polluted vapors and air to be treated is ensured by varying the flow rate of the current - of extraction * of the partially depolluted air from said enclosure. Other particularities and characteristics will emerge from the detailed description of an advantageous embodiment, presented below, by way of illustration, with reference to the appended drawing. In this drawing: FIG. 1 schematically represents a preferred embodiment of an installation for treating polluted vapors formed during the granulation of blast furnace slag illustrating this invention.

In Figure 1, we see a granulation installation 8 which is generally located in a granulation building (not shown). A powerful jet of water is injected using a spray head 14 into a flow of molten slag conveyed by a channel 10 of a blast furnace or a slag pocket in the installation 8. This jet causes the formation of slag granulate which · falls into a cooling basin 18. From this basin 18, said aggregate is continuously discharged to a dewatering installation (not shown) as described, for example, in US Pat. No. 4,204,855.

The vapors and gases formed during the sudden cooling of the molten slag by the jet of water, as well as the gases and vapors which emanate from the basin 18 follow an upward movement and are captured by a closed tower 20 situated above the basin 18. In this tower 20 is an oblong cylindrical or prismatic enclosure 34. This enclosure 34 is closed at its lower base by a funnel-shaped bottom 22 and open at its upper base. In FIG. 1, it can be seen that the enclosure 34 is installed in the tower 20 so as to define a chimney 21, formed by a restriction in the cross section of the tower 20. This chimney channels the gaseous mixture, formed of vapors, of heated gas and air, towards the inlet of the enclosure 34. The enclosure 34 is maintained in depression with respect to the external atmosphere by a continuous evacuation of gases treated via a conduit 24 located in the lower part of the enclosure 34. To this end, the conduit 24 is connected to a pump or to a fan 26 which maintains a downward flow 23 of the gas mixture to be treated in the enclosure 34. The gases evacuated via the conduit 24 are channeled and released near the channel 10 conveying the slag to the granulation head 14. The hydrogen sulfide which has not been removed during the washing of the gases in tower 20 is oxidized to sulfur dioxide. The gases conveyed to the channel are entrained by the flow of slag towards tower 20 to be reprocessed there.

In the enclosure 34 are installed sprinklers 30. These nozzles 30, which are preferably installed on several superimposed rows in the upper part of the enclosure 34, are connected to a supply line 63 of cold alkaline water. They allow this cold alkaline water to be sprayed into the downward flow 23 of the gas mixture. The condensation of the vapors is obtained by intimate contact between the sprayed cold water and the vapors. Spraying promotes contact between the two phases, which has a beneficial effect on the course of these depollution reactions. It will be noted that the sprayed water jets from the nozzles have the secondary effect of reinforcing the dynamics of the downward flow of the gas mixture to be treated in the enclosure 34 and of promoting an aspiration of the non-condensed gases and of the vapors at the level of the open upper end of the enclosure 34. This effect is moreover reinforced by the arrangement of the nozzles 30 in superimposed rows.

In the enclosure 34, the spray water and the condensate are collected by the funnel-shaped bottom 22 and are evacuated by an evacuation pipe 58.

In the lower part of the enclosure 34, the gas phase therefore essentially contains air which has undergone a first washing.

A probe 66, installed in the lower part of the enclosure 34, measures the vacuum inside it. This probe 66 can be used to adjust the flow rate of the fan 26 which evacuates the partially purified gas, so as to maintain a stable vacuum in the enclosure 34.

The water vapors and the air coming from the aggregate dewatering installation (not shown) can advantageously be purified by the same installation described above. For this purpose, the dehydration installation is provided with a hood which captures the mixture of vapors and ascending air by - convection. This mixture is conveyed from the dehydration installation to the tower 20 via a chimney 38 possibly provided with a valve 36. This valve 36 makes it possible to control at any time the flow rate of the mixture of air and vapors brought into the tower 20 The invention is not however limited to the embodiments which have been described and represented above, but it can be advantageously applied in all slag granulation and dehydration installations in which there is release of sulfur vapors and gases. Therefore, other known gas washing methods can be used.

Claims (6)

KEVEiJNUlCATlUflS 1. Process for the treatment of a mixture of water vapors and air polluted by sulphurous gases and formed during the production of blast furnace slag aggregate in a granulation installation, characterized in that first subjecting said gas mixture to a gas wash in order to partially condense it and extract part of the pollutants therefrom, in that it collects and releases the non-condensed and undissolved part of said mixture near a stream of molten slag. 2. Method for the treatment of a mixture of water vapors and polluted air according to claim 1, characterized in that the said mixture is channeled first in an upward flow and then the mixture flows in a downward flow (23) in an enclosure (34) maintained in depression where one sprays, in a parallel flow, an aqueous alkaline solution in said downward flow which depollutes and partially condenses said mixture and in that one collects and evacuates the non-condensed gases outside said installation in a forced and adjustable current so as to create and maintain a vacuum inside said enclosure. 3. Device for the treatment of a mixture of water, air and sulfur gas vapors, generated during the production of blast furnace slag aggregate in a granulation installation, characterized by a closed tower (20) located above the granulation installation (8), intended to capture said mixture of vapors and gases, by nozzles (30) arranged inside said closed tower (20), these nozzles (30) being connected to a distribution pipe (63) of a gas washing solution, by at least one sensor located below said nozzles, by at least one flow conduit (58) of the washing solution and the connected condensate to said at least one sensor, by at least one suction duct (24) for the non-condensed gases emerging in said tower (20), by at least one extraction pump (26) with variable flow rate connected to said suction duct ( 24) intended to evacuate said gases outside said tower and to maintain a e depression in said tower, by at least one conduit conveying and releasing said gases evacuated near a channel leading a flow of molten slag towards a granulation head. 4. Device according to claim 3 characterized by at least one oblong enclosure (34) closed at its lower end (22) and open at its upper end, arranged vertically inside said closed tower (20), by at least one flow conduit (58) for washing water and condensate opening into the lower end (22) of the enclosure (34), through at least one suction conduit (24) for non-condensed gases opening into the lower part of said enclosure (34). 5. Device according to claim 3 or 4, characterized in that the variable flow pump used is a fan (26) 6. Device according to any one of claims 3 to 5, characterized in that the pump (26) is connected to a probe (66) for adjusting the flow rate of the pump (26) as a function of the vacuum inside said enclosure (34).