GB1589829A - Process for removing sulphur dioxide and solids from the exhaust gas of a sintering process - Google Patents

Description

(54) PROCESS FOR REMOVING SULPHUR DIOXIDE AND SOLIDS FROM THE EXHAUST GAS OF A SINTERING PROCESS (71) We, METALLGESELLSCHAFT AKTIENGESELLSCHAFT, a body corporate organized under the laws of the German Federal Republic, of 6000 Frankfurt (Main) 1, Reuterweg 14, German Federal Republic, do hereby declare the invention for which we pray that a patent may be granted to us, and the method by which it is to be performed, to be particularly described in and by the following state ment:- This invention relates to a process for removing sulphur dioxide and solid pollutants from the exhaust gas from an iron oresintering machine, in which a first exhaust gas partial stream having a low SO2 content is sucked into a first gas-collecting manifold from wind boxes disposed under the first part of the sintering machine, substantially all polluting dust is removed from said first partial stream in a gas purifier, a second partial stream having a higher SO2 content is sucked into a second gas-collecting manifold from wind boxes disposed under a succeeding part of the sintering machine, and substantially all polluting dust and substantially all SO2 are removed from said second partial stream in a second gas purifier and scrubber.

The exhaust gases from iron ore-sintering plants contain dust and gaseous pollutants, including sulphur dioxide, which is particularly undesirable ecologically, as well as vaporous pollutants produced by the evaporation of non-ferrous metals. Whereas polluting dusts are usually removed from the exhaust gas by electrostatic gas purifiers, these purifiers cannot remove sulphur dioxide and other gaseous pollutants. Vaporous pollutants are condensed only in part in the electrostatic gas purifier. For this reason, these pollutants, particularly sulphur dioxide, must be removed in a separate gas purifier, which is preferably in the form of a scrubber. In most cases, the exhaust gases have only a very low content of pollutants and the removal thereof involves very high costs because exhaust gases at a very high rate must be purified.

In order to decrease the costs involved in the scrubbing of the gas, it is usual to feed the scrubber only with a partial stream of exhaust gas and to control said partial stream so that it contains a major part of the pollutants which become available.

In conjunction with the sintering of iron ores, it has been proposed to withdraw exhaust gas containing at least 16% oxygen from the rear part of the sintering zone (i.e.

where sintering has been completed or substantially completed), to conduct said exhaust gas as sintering air through the forward part of the sintering zone and to feed a scrubber with at least a partial stream of the exhaust gases thus produced in the forward part of the sintering zone. As a result, a large part of the gaseous pollutants are enriched in a smaller gas volume and can be scrubbed therefrom. Gas hoods must be provided over the sintering machine for that process. It has also been proposed to subject an exhaust gas partial system which contains a major part of the gaseous pollutants which became available to a mechanical preliminary dust-collecting treatment and then to feed it to a scrubber whereafter the scrubbed partial stream can be combined with the remaining partial stream of exhaust gas and fed to an electrostatic gas purifier.

In another process previously proposed in Specification No. 1,485,259 for sintering iron ores a first exhaust gas partial stream having a low content of gaseous or vaporous pollutants is sucked into a first gas-collecting manifold through transfer or connecting conduits from wind boxes disposed under the first and last parts of the sintering machine and is treated in a first electrostatic gas purifier to remove substantially all dustlike pollutants and is then fed to a chimney, a second exhaust gas partial stream having a higher content of gaseous or vaporous pollutants is sucked in to a second gascollecting manifold through transfer or connecting conduits from wind boxes disposed at least under the intervening part of the sintering machine and is treated m a second electrostatic gas purifier to remove substantially all dust-like pollutants and is then treated in a scrubber to remove substantially all gaseous and vaporus pollutants and is subsequently fed into a chimney, and the connections of the wind boxes and connecting conduits to the first and second gascollecting manifolds are controlled in such a manner that the first partial stream has a temperature and a gas volume flow rate which are favourable for the purification in the first electrostatic gas purifier and that the second partial stream has a temperature and a gas volume flow rate which are favourable for the purification in the second electrostatic gas purifier and said second partial stream contains a predominant part of the gaseous or vaporous pollutants.

This latter process requires a control of the connection and disconnection of wind boxes to and from the first and second gascollecting manifolds and an independent control for maintaining at a desired location the burn-through point of the mixture being sintered.

Various methods of controlling the speed of travel of sintering conveyors so as to maintain the burn-through point at the desired location have been described by Wendeborn and Cappel in "Sintern von Eisenerzen", Verlag Stahleisen m.b.H., Dusseldorf, 1973, on pages 251-273. A control depending upon the exhaust gas temperature pattern at the end of the sintering conveyor depends, eg. on the sealing of the sintenng machine at the rear end. A higher air inleak rate at that point may simulate an exhaust gas temperature peak if the last suction box or suction box section is very short so that the temperature depends mainly on the rate at which air is leaking in from the rear end. On the other hand, the location of the temperature peak cannot be reliably determined when the peak is not very pronounced. Besides, the desired location of the temperature peak can be adjusted only within narrow limits by the control unless the temperature-sensing points are shifted because the control is based on the assumption that the exhaust gas temperature varies in accordance with a parabola although this assumption is permissable only near the peak. In the sintering of various ores, the temperature curve is flat because the permeability of the lower zone to air is subjected to highly different influences depending upon the retention of heat. In those cases the exhaust gas temperature peak cannot be used as a definite indication of the completion of the sintering process.

It has been attempted to avoid these disadvantages by the use of the temperature of all the exhaust gases in the gas-collecting manifold before the entrance to the electrostatic precipitator as a controlling variable. As the sintering process approaches its end and as the sinter is cooled on the machine when the process has been completed, more sensible heat is transferred from the sinter to the exhaust gas. A constant exhaust gas temperature indicates that the incandescent zone extends to a constant point in the charge. For this reason the exhaust gas temperature is a suitable indication of the progress of the process. On the other hand, such a control system can be used only to control the location of the burn-through point rather than to control the inclusion of sulphur dioxide in an exhaust gas partial stream.

It is an object of the invention to remove sulphur dioxide and solid pollutants from the exhaust gas in such a manner that a larger part of the sulphur dioxide which has been evolved is included in one partial stream and the latter is scrubbed to remove sulphur dioxide so that the automatic control expenditure required to ensure the inclusion of a major part of the sulphur dioxide even in the case of fluctuations of the operating conditions of the sintering machine is minimized and said control is simplified.

According to the present invention there is provided a process for removing sulphur dioxide and solid pollutants from the exhaust gas of an iron ore-sintering machine, wherein a first exhaust gas partial stream having a low content of sulphur dioxide is sucked into a first gas-collecting manifold from wind boxes disposed under a first part of the sintering machine, substantially all polluting dust being removed from said first partial stream in a gas purifier, and a second partial stream having a higher content of sulphur dioxide is sucked into a second gas-collecting manifold from wind boxes disposed under a following art of the sintering machine, substantially an polluting dust and substantially alp the sulphur dioxide being removed from said second partial stream in a second gas purifier and scrubber, and the exhaust gas temperature of the second partial stream is determined before the removal of the dust and sulphur dioxide and is compared with an empirically determined, desired value of said temperature, which desired value corresponds to a desired sulphur dioxide content of the second partial stream and to an empirically determined, desired location of the burn-through point of the mixture being sintered, the speed of travel of the sintering machine being increased when said exhaust gas temperature exceeds said desired value, and being decreased when said exhaust gas temperature is lower than said desired value, until said exhaust gas temperature again equals said desired value.

For a given mixture to be sintered, the curve representing the sulphur dioxide content of the exhaust gases from the several wind boxes from the point of ignition of the charge to the point at which the sintering process is completed is determined for operating conditions under which the burnthrough point of the mixture being sintered is located at the desired point of the sintering machine. Those wind boxes in which exhaust gases having a high sulphur dioxide content become available are connected to the second gas-collecting manifold. The number and locations of the wind boxes are selected so that the desired volumetric rate of sulphur dioxide, i.e. more than 50% of the entire sulphur dioxide, is included. Consideration is also given to the desire to minimize the volumetric rate of the second partial stream and/or to provide for favourable conditions for the removal of the sold pollutants in both partial streams. Most of the wind boxes connected to the second gas-collecting manifold are generally disposed in the second half of the length of the sintering machine from the point of ignition of the charge to the point at which the sintering process is completed, preferably in the last one-third of the length of the sintering machine. The exhaust gas temperature of the second partial stream thus controlled is determined, this temperature being the empirically determined, desired value of the exhaust gas temperature. The desired value need not be determined on the sintering machine but may be determined by suitable experiments. During operation, the exhaust gas temperature of the second partial stream is constantly or periodically measured and the exhaust gas temperature is determined and compared with the desired value and the speed of travel of the sintering machine is correspondingly changed when a deviation in the compared values occurs. The speed of travel may be varied in steps in order to avoid overshooting.

Before the sulphur dioxide is scrubbed from the second partial stream, part or substantially all of the dust removed from said partial stream so that contamination of the scrubbing fluid with dust is reduced or substantially avoided. The dust content may be removed in mechanical separators, such as cyclones, or in electrostatic separators.

Statements relating to the length of the sintering machine refer always to the sintering zone, i.e. to the entire length of a machine which discharges hot sintered material into a separate cooler shortly behind the burn-through point and to the sintering zone of a machine on which the sintered material is also cooled.

The exhaust gases from the wind boxes disposed under the last part of the sintering machine are preferably sucked into the first gas-collecting manifold. The "last part" is a length portion which corresponds to the length of one to six standard wind boxes such as are used with the respective sintering machine. The number of wind boxes depends upon the amount of sulphur dioxide which is to be included. By this means, the temperature of the first partial stream in the first gas-collecting manifold can be increased and the exhaust gas rate of the second partial stream can be decreased be cause the exhaust gas temperature and the exhaust gas rate in the last wind boxes are much higher than in the preceding wind boxes. Furthermore, a leak of air into the second exhaust gas partial stream is avoided in a non-cooling sintering machine.

The exhaust gas temperature of the second partial stream is conveniently measured in the second gas-collecting manifold, thus enabling an exact measurement of temperature at a single measuring point to be obtained rather than at a plurality of measuring points in the wind boxes or in the connecting conduits from the wind boxes to the gas-collecting manifold.

In order to enable the invention to be more readily understood, reference will now be made to the accompanying drawing, which illustrates an embodiment thereof, and which is a graph showing the exhaust gas temperature of a partial stream and the sulphur dioxide content diagrammatically and by way of example thereof in relation to the wind boxes of a sintering machine.

Referring to the drawing, there are shown the last wind boxes 18 to 22 of a sintering machine on which subsequent cooling of the sinter is not effected, the wind boxes being aligned along the x-axis. The last two wind boxes 21 and 22 are shorter than the preceding standard wind boxes. Curve A represents the proportion of sulphur dioxide included in the second partial stream, based on the total rate at which sulphur dioxide becomes available in the axhaust gas of the sintering machine. Curve B shows the exhaust gas temperature in the second partial stream (rather than the temperatures in the several wind boxes). Point C of curve B represents the empirically determined, desired temperature value which corresponds to the desired proportion of sulphur dioxide included in the second partial stream and the desired location of the burnthrough point, which is disposed over the wind box 20 in this case. The speed of travel of the sintering machine is controlled so that the exhaust gas temperature in the second partial stream agrees with the desired value C.

An advantage of the present process is that the inclusion of the desired proportion of sulphur dioxide and the desired location of the burn-through point of the mixture being sintered can be controlled in dependence upon a single measured variable. At the same time, other gaseous or vaporous pollutants may be substantially included in the exhaust gas partial stream which is fed to the scrubber because the formation of such pollutants substantially accompanies the formation of sulphur dioxide. Furthermore, no windboxes are connected to or disconnected from the two gas-collecting man ifolds as a result of the single temperature measurement, but the desired amount of sulphur dioxide in the partial stream and the position of the burn-through point are kept at the desired values by adjusting the speed of the machine as a result of this one measurement.

WHAT WE CLAIM IS: 1. A process for removing sulphur dioxide and solid pollutants from the exhaust gas of an iron ore-sintering machine, wherein a first exhaust gas partial stream having a low content of sulphur dioxide is sucked into a first gas-collecting manifold from wind boxes disposed under the first part of the sintering machine, substantially all polluting dust being removed from said first partial stream in a gas purifier, and a second partial stream having a higher content of sulphur dioxide is sucked into a second gascollecting manifold from wind boxes disposed under a following part of the sintering machine, substantially all polluting dust and substantially all the sulphur dioxide being removed from said second partial stream in a second gas purifier and scrubber, and the exhaust gas temperature of the second partial stream is determined before the removal of the dust and sulphur dioxide and is compared with an empirically determined, desired value of said temperature, which desired value corresponds to a desired sulphur dioxide content of the second partial stream and to an empirically determined, desired location of the burn-through point of the mixture being sintered, the speed of travel of the sintering machine being increased when said exhaust gas temperature exceeds said desired value, and being decreased when said exhaust gas temperature is lower than said desired value, until said exhaust gas temperature again equals desired value.

2. A process as claimed in Claim 1, wherein the exhaust gases from the wind boxes disposed under the last part of the sintering machine are sucked into the first gas-collecting manifold.

3. A process as claimed in Claim 1 or 2 wherein the exhaust gas temperature of the second partial stream is measured in the second gas-collecting manifold.

4. A process for removing sulphur dioxide and solid pollutants from the exhaust gases of an iron ore-sintering machine substantially as herein before described with reference to the accompanying drawing.

**WARNING** end of DESC field may overlap start of CLMS **.

Claims (4)

**WARNING** start of CLMS field may overlap end of DESC **. the exhaust gas partial stream which is fed to the scrubber because the formation of such pollutants substantially accompanies the formation of sulphur dioxide. Furthermore, no windboxes are connected to or disconnected from the two gas-collecting man ifolds as a result of the single temperature measurement, but the desired amount of sulphur dioxide in the partial stream and the position of the burn-through point are kept at the desired values by adjusting the speed of the machine as a result of this one measurement. WHAT WE CLAIM IS:

1. A process for removing sulphur dioxide and solid pollutants from the exhaust gas of an iron ore-sintering machine, wherein a first exhaust gas partial stream having a low content of sulphur dioxide is sucked into a first gas-collecting manifold from wind boxes disposed under the first part of the sintering machine, substantially all polluting dust being removed from said first partial stream in a gas purifier, and a second partial stream having a higher content of sulphur dioxide is sucked into a second gascollecting manifold from wind boxes disposed under a following part of the sintering machine, substantially all polluting dust and substantially all the sulphur dioxide being removed from said second partial stream in a second gas purifier and scrubber, and the exhaust gas temperature of the second partial stream is determined before the removal of the dust and sulphur dioxide and is compared with an empirically determined, desired value of said temperature, which desired value corresponds to a desired sulphur dioxide content of the second partial stream and to an empirically determined, desired location of the burn-through point of the mixture being sintered, the speed of travel of the sintering machine being increased when said exhaust gas temperature exceeds said desired value, and being decreased when said exhaust gas temperature is lower than said desired value, until said exhaust gas temperature again equals desired value.

2. A process as claimed in Claim 1, wherein the exhaust gases from the wind boxes disposed under the last part of the sintering machine are sucked into the first gas-collecting manifold.

3. A process as claimed in Claim 1 or 2 wherein the exhaust gas temperature of the second partial stream is measured in the second gas-collecting manifold.

4. A process for removing sulphur dioxide and solid pollutants from the exhaust gases of an iron ore-sintering machine substantially as herein before described with reference to the accompanying drawing.