LU88703A1 - Method for desulfurization of a mixture of water vapor and polluted air formed during the granulation and dehydration of blast furnace slag - Google Patents

Description

PROCESS FOR DESULFURIZING A MIXTURE OF VAPORS OF WATER AND POLLUTED AIR. FORMED DURING GRANULATION AND DEHYDRATION OF HAUT FOURNEAU DAIRY

The present invention relates to a process for desulfurization of a mixture of water vapors and polluted air which is formed during the production of granulation of 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 known to inject a powerful jet of water using a spray head into 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 water vapors contaminated, among other things, by sulfur gases, including hydrogen sulfide H2S and sulfur dioxide SO2 generated in large quantities and at essentially variable flow rates during granulation by injecting 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 US Pat. No. 4,205,855 to which the reader will refer for further detailed explanations. During this dehydration of the slag, a mixture of water vapors and polluted air is also formed.

Sometimes the polluted vapors produced during granulation are discharged without treatment into the atmosphere, which naturally constitutes a nuisance for the inhabitants of the surroundings of the factories, given the particularly unpleasant odor 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 flows and consequently the rising vapor 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 DE-A 35 119 58, filed on 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 water jet 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 indeed be constructed so as to constitute a perfectly gas-tight system.

Patent application EP 0 573 769 proposes a process and an installation for desulphurization in which the polluted mixture is channeled in a downward flow in an enclosure situated in a condensation tower. This enclosure is kept under vacuum and an alkaline aqueous solution is sprayed therein in a parallel flow in said downflow which cleans the air and condenses the vapors and the non-condensed gases are discharged outside of said installation in a forced and adjustable current so as to create and maintain a vacuum inside said enclosure.

The object of the present invention is to provide an improved desulphurization process for a mixture of vapors and polluted air generated during the granulation and dehydration of blast furnace slag.

According to one aspect of the present invention, this object is achieved by a process for the desulphurization of a mixture of water vapors and air polluted by sulphurous gases and formed during the production of blast furnace slag granulate in a granulation installation, characterized in that said mixture is brought into contact with a first solution which is maintained at a pH of between 10 and 12.5 to remove the sulfur compounds from said mixture.

One of the advantages of this process is that it is effective. It effectively eliminates the vast majority of sulfur contaminants from the mixture of vapors and polluted air formed during the granulation of blast furnace slag while consuming little basic reagent.

Another advantage of the process is that it is economical. The use of washing solutions at a pH between 10 and 12.5 makes it possible to reduce the amount of basic reagent, generally sodium hydroxide, used.

In fact, it has been observed that the elimination of sulfur compounds is favored by a high pH depending on the reactions: H2S + 2 NaOH -► Na2S + 2H20 S02 + H20 —► H2S03 H2S03 + 1/2 02 -> H2S04 H2S04 + 2 NaOH -> Na2S04 + H20

However, when the pH is very high, the following reaction also takes place and consumes a non-negligible part of the base used.

C02 + 2 NaOH - »Na2C03 + H20 According to the results of the experiments carried out, it has been observed that when the pH of the washing solution is higher than pH = 13, approximately 100% of the H2S are eliminated from the mixture of vapor and d polluted air, but almost all of the carbon dioxide CO2 in the air is also removed. On the other hand, at a pH below 12.5, only 20% of the CO2 contained in the air is eliminated while approximately 100% of the H2S is eliminated.

At first glance, the CO2 concentrations in the air which are around 500 ppm seem negligible. However, as the quantity of gas treated in such processes (of the order of magnitude of 100,000 Nm ^ / h) is very large, the overall quantity of CO2 passing through the system is appreciable. If we manage to control the elimination of CO2, we manage to save an appreciable amount of basic reagent because we must not forget that the concentrations of CO2 in the air are comparable to those of hydrogen sulfide and SO2 in the mixture to be treated. A reduction in the elimination of CO2 makes it possible to minimize the amount of basic reagent used for the treatment of the polluted mixture.

The results of the CO2 absorption tests of H2S and SO2 respectively at different pHs are shown in Table 1 below.

Table 1: percentage of CO2 of H2S and SO2 respectively removed from a gas mixture at different pH values.

It has also been found that the absorption percentages of the gases are strongly influenced by the way in which the two phases are brought into contact. If the contact between the two phases is very intimate, absorption rates comparable to those of lower pH values are observed. Thus, it has been found that it is possible to work at lower pHs, of the order of magnitude of pH 10 when using an unstructured packed column for carrying out the gas washing and that one has to work at pH equal to 12-12.5 if the gases are washed by spraying a liquid into a gas flow.

According to an advantageous embodiment, the mixture is subjected to a prewash using a second solution which comprises the first used washing solution, the pH of which has been adjusted to a value between 7.5 and 9.

If the second washing solution is recycled after use as the first washing solution, the following reactions take place at pH = 7.5 to 9:

A significant part of the removal of H2S from the mixture is thus eliminated from the gas phase without the intervention of the basic reagent. This recycling step of the second washing solution in a prewash step therefore saves sodium hydroxide which is preferably used as a basic reagent. The process is therefore more economical since part of the hydrogen sulfide is removed without the intervention of the basic reagent.

At pH = 7.5 and 9, the majority of hydrogen sulfide molecules in solution are in the form of HS 'ions. A release of hydrogen sulfide gas from the liquid phase is not to be feared at such pH values.

According to another advantageous embodiment, the mixture of vapor and polluted air is first sprayed with a third washing solution maintained at a temperature close to the boiling temperature of water in order to de-sand the mixture and removing the solid particles from said mixture before subjecting it to desulfurization. If the mixture to be treated contains a high concentration of solid particles, there is a risk that deposits will form at certain points in the installation which would, in the long run, adversely affect the proper functioning of the installation.

To carry out this desanding, water or an aqueous solution is used, the temperature of which is close to the boiling point, ie close to 100 ° C. to avoid, as far as possible, condensation of the vapors. of water contained in the mixture to be desulfurized. This step, the desanding, removes all solid particles from the mixture of vapor and polluted air.

It is preferable to use water from the granulation water circuit to carry out the desanding. An advantage linked to the use of granulation water is that this water is available in large quantities and that it is at the desired temperature when the granulation installation is operating in regime. There will therefore be no need for a separate heating system to heat the solution used for the sand removal. A cooling tower is also not necessary since the solution used for the desanding of the mixture of steam and polluted air is returned to the granulation water circuit after use.

If an installation as described in US Pat. No. 4,205,855 is used to dehydrate the granulated slag, it is not even necessary to filter the granulation water before its use as a solution for de-blasting the mixture of steam and air polluted because the dehydration device permanently filters the granulation water and the solids are evacuated together with the granulated slag.

The washing step using the first solution is preferably carried out using a packed column which makes it possible to increase the efficiency of the contact between the gas phase and the liquid phase and thus to promote desulfurization. Such a column comprises a filling material such as, for example, plastic rings, and the gas and the washing solution are brought into contact in countercurrents. The washing solution is sprinkled on the rings and flows slowly down the column. The mixture to be treated traverses the column in reverse and the contaminants are removed very effectively.

According to another advantageous embodiment, the used alkaline solutions are poured into the granulation and cooling water circuit of the slag. As these solutions have a high pH, they will help to increase the pH of the granulation water. The formation of gaseous hydrogen sulfide inside the granulation basin is thus disadvantaged because the balance between the different gaseous and ionic forms of hydrogen sulfide is shifted towards the ionic forms (HS 'or S2') which remain in solution.

If necessary, the solutions can be oxidized, for example using hydrogen peroxide treatment. Sulfur-containing compounds are oxidized to sulphate before pouring the solutions into the granulation water circuit. As the granulation water is rich in lime, the majority of the sulfate will then be eliminated from the cooling water circuit in the form of gypsum with the dehydrated granulate.

The present invention also describes a device for the treatment of a mixture of water vapors and polluted air, which is formed during the production of blast furnace slag aggregate in a granulation installation, comprising: - a first enclosure inside which are installed nozzles for spraying a washing solution whose pH is about 7.5-9 on said mixture, - a second enclosure in communication with said first enclosure in which the vapors are brought into contact with a washing solution whose pH is between 10 and 12.5, and - a pipe for discharging said mixture thus purified.

According to an advantageous embodiment, the device also comprises a third enclosure communicating with said first enclosure, in which are arranged nozzles for spraying a third washing solution in a parallel flow on the descending gases to deblast said mixture, the temperature of said first washing solution being close to the boiling temperature of water.

This coarse gas purification is carried out by spraying an aqueous solution maintained at a temperature between 85 and 95 ° C.

I "in the current which drives the vapor current towards the lower part of said third enclosure. The desanding solution is recovered from the bottom of said third enclosure and is evacuated therefrom.

Said third enclosure is connected to a first enclosure in which the vapor mixture is subjected to a first stage of desulfurization. An aqueous solution of pH between 7.5 and 9 is sprayed into the mixture of vapors.

The pretreated vapor mixture is then evacuated to a second enclosure where the final desulfurization takes place. In this second enclosure, the residual mixture of vapor and contaminated air is brought into contact with the solution which is at a pH between 10 and 12.5. In this step, the rest of the sulfur compounds are removed and the purified vapors and air are released to the atmosphere.

The acid sulfur pollutants contained in the mixture will thus be effectively eliminated because the polluted mixture and the alkaline solutions are mixed in a turbulent manner, which promotes exchanges between the two phases.

This desulphurization installation will be able to equip most of the existing blast furnace slag granulation installations, and this at a relatively moderate cost. In fact, unlike the installation described in German patent application DE-A 3511958, the granulation installation must not be gas-tight.

The desulfurization plant is capable of controlling very variable flow rates of vapors and polluted air.

Other features and characteristics will emerge from the figure representing an advantageous embodiment, presented below, by way of illustration.

Figure 1 schematically shows a preferred embodiment of an installation for treating polluted vapors formed during the granulation of blast furnace slag illustrating this invention.

The invention is however not 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 vapors and gases sulfides.

Claims (8)

1. Process for the desulphurization of a mixture of water vapors and air polluted by sulphurous gases and formed during the production of blast furnace slag granulate in a granulation installation, characterized in that said mixture is placed in contact with a first solution which is maintained at a pH of between 10 and 12.5 to remove the sulfur compounds from said mixture. 2. Method for the desulfurization of a mixture of water vapors and polluted air according to claim 1, characterized in that the mixture is subjected to a prewash using a second solution which comprises the first solution used washing machine, the pH of which has been adjusted to between 7.5 and 9. 3. Method for the treatment of a mixture of water vapors and polluted air according to claim 1 or 2, characterized in that the mixture is subjected to a desanding by spraying it with a third washing solution which comprises water used for slag granulation. 4. Method for the treatment of a mixture of water vapors and polluted air according to any one of claims 1 to 3, characterized in that the mixture of steam and polluted air is first sprayed d a third washing solution maintained at a temperature close to the temperature, boiling the water in order to de-bead the mixture and to remove the solid particles from said mixture before subjecting it to desulfurization. 5. Method for treating a mixture of water vapors and polluted air according to claim 4, characterized in that the temperature of the third solution is maintained at a temperature between 85 ° C and 95 ° C so as to minimize the condensation of water vapors in this step. 6. Method for the treatment of a mixture of water vapors and polluted air according to any one of claims 3 to 5, characterized in that water is used from the granulation water circuit for remove the sand. 7. Method for the treatment of a mixture of water vapors and polluted air according to any one of claims 1 to 6, characterized in that one or more of the solutions used are poured into the water circuit of granulation and cooling of the slag. 8. A method for treating a mixture of water vapors and polluted air according to claim 7, characterized in that the solutions are oxidized by a suitable oxidant before discharging them into the granulation water circuit. Legend to FIG. 1 10 supply line for the vapor / air mixture 12 de-sanding enclosure 14 desander washer with parallel jets 16 supply line for the washer 14 18 de-watering solution discharge line 20 line 22 primary washer 24 main washer 26 packed column 27 nozzles 30 evacuation line of the vapor / desulphurized air mixture 32 NaOH supply line 34 water supply line 36 pump 38 washing solution supply line 40 capture device 42 line of the washing solution