METHOD TO CONTROL SLAG FOAMING IN A SMELTING PROCESS

Abstract

A method to control slag foaming in a smelting process in a vessel for smelting an iron-containing feed material including the steps of: measuring vibration of the metallurgical vessel with an accelerometer at one or more positions on the vessel, comparing values derived from accelerometer data with a threshold value which indicates the onset of a slag foaming incident, and adjusting the smelting process if the value derived from the accelerometer data passes a predefined alarm value, wherein the smelting process is adjusted by adjusting the amounts of the gaseous and/or the solid components injected in the smelting process.

Claims

1. A method to control slag foaming in a smelting process in a vessel for smelting an iron-containing feed material comprising the steps of: measuring vibration of the metallurgical vessel with an accelerometer at one or more positions on the vessel to obtain accelerometer data, comparing values derived from the accelerometer data with a threshold value which indicates the onset of a slag foaming incident, and adjusting the smelting process if the value derived from the accelerometer data passes a predefined alarm value, wherein the smelting process is adjusted by adjusting the amounts of the gaseous and/or the solid components injected in the smelting process.

2. The method according to claim 1, wherein said adjusting the smelting process comprises adjusting the amount of oxygen injected in the smelting process.

3. The method according to claim 1, wherein said adjusting step comprises adjusting the amount of coal injected in the smelting process.

4. The method according to claim 1, wherein said adjusting step comprises adjusting the amount of iron-containing feed material injected in the smelting process.

5. The method according to claim 1, wherein said adjusting step comprises adjusting the amount of lime injected in the smelting process.

6. The method according to claim 1, wherein the basicity of the slag is monitored and the amount of lime injected in the smelting process is adjusted to keep the basicity of the slag in a predefined range or restore the basicity of the slag to within the predefined range.

7. The method according to claim 2, wherein the adjusting step comprises adjusting the amounts of gaseous and solid components by reducing the amounts of gaseous and solid components when the value derived from the accelerometer data is in the alarm range.

8. The method according to claim 7, wherein the method further comprises draining slag from the vessel.

9. The method according to claim 1, wherein the amount of oxygen injected in the smelting process is adjusted to a predefined excess of CO gas.

10. The method according to claim 1, wherein the adjusting step comprises the adjustment of the gaseous and solid components injected in the smelting process which is started with the adjustment of the amount of oxygen injected in the smelting process followed by the adjustment of solid components in the following order: coal, iron-containing feed material and lime.

11. The method according to claim 1, wherein the adjusting step comprises the adjustment of the amounts of the gaseous and the solid components injected in the smelting process when the value derived from the accelerometer data comes at the right side of the threshold value by increasing the amounts of the gaseous and the solid components injected in the smelting process.

12. The method according to claim 1, wherein the vibration of the metallurgical vessel is measured with the one or more accelerometers for predefined periods of time at predefined time intervals.

13. The method according to claim 12, wherein a relevant frequency range is determined from which the accelerometer data is going to be processed.

14. The method according to claim 12, wherein the method comprises the steps of: converting the obtained accelerometer data set from a time domain into a frequency domain, integrating the data set in the frequency domain to a frequency/velocity data set, determining the peak velocity value and the variation of the peak velocity value over time, comparing the determined peak velocity value with a determined threshold value which correlates with a slag foaming event, and said adjusting the amounts of the gaseous and/or the solid components injected in the smelting process when the determined maximum velocity value corresponds to the threshold value.

15. The method according to claim 14, wherein a moving average of peak velocity values is determined and compared with the threshold value.

16. The method according to claim 14, wherein a relevant frequency range is determined from which the accelerometer data is going to be processed.

Description

BRIEF DESCRIPTION OF THE DRAWINGS

[0039] The invention will be further explained on hand of the example shown in the drawing, in which:

[0040] FIG. 1 shows schematically a cross-section through an installation with a smelting reduction vessel and a smelt cyclone;

[0041] FIG. 2 shows converted accelerometer data of the installation under normal operation conditions, and

[0042] FIG. 3 shows converted accelerometer data of the installation at the time of a slag foaming event.

DETAILED DESCRIPTION OF THE DRAWINGS

[0043] In FIG. 1 a smelting reduction installation 1 is shown with a smelting reduction vessel 2, a smelt cyclone 3 and an off-gas duct connecting part 4. The smelting cyclone comprises a vertical cylindrical chamber 5 provided with tuyeres 6 for injecting solid metalliferous feed materials and tuyeres 7 for injecting oxygen-containing gas into the chamber. The metalliferous feed materials are partly reduced and smelted before these arrive in the smelting reduction vessel 2. The smelting reduction vessel 2 defines a smelting chamber 8 and includes lances 9 for injecting solid feed materials and lances 10 for injecting oxygen-containing gas into the smelting chamber 8 and is adapted to contain a bath of molten metal and slag.

[0044] The smelting reduction vessel 2 includes a forehearth 11 connected to the smelting chamber 8 via a connection that allows continuous metal product 12 outflow from the vessel. The forehearth 11 operates as a molten metal-filled siphon seal, which allows the molten metal level in the smelting chamber 8 to be known and controlled to within a small tolerance. Molten slag 13 produced in the process is discharged from the smelting chamber 8 through a slag tap hole 14.

[0045] The level of the slag 13 under stable operation conditions is about as indicated in the figure. The slag 13 will splash around at about this level and is stable in height and in heat transfer to the copper cooled panels provided in the wall of the smelting reduction vessel.

[0046] With a slag foaming event the slag level will raise considerable and when the level of the oxygen lances 10 is reached the slag will become even more gaseous and will raise further in an accelerated manner. The foaming slag will reach the smelt cyclone 3 and beyond and at the same time could push the molten metal out of the smelting chamber 8 and out of the forehearth 11. With that the maximum possible damage is done and a time consuming and costly clean-up of the installation will be necessary.

[0047] FIG. 2 shows converted accelerometer data of the installation under stable operation conditions. Accelerometers are placed on the outside of the installation, typically on the smelting reduction vessel 2 such that acceleration in various directions can be measured. An accelerometer is configured to measure accelerations for a period of time, after which the accumulated data is converted into the frequency domain using the Fast Fourier Transform. Since the installation or the smelting reduction vessel is vibrating at relatively low frequencies and with low speeds the converted data are integrated to view the vibrations in velocity rather than as acceleration.

[0048] With the present installation useful results were obtained by measuring the acceleration every minute for a period of 0.5 seconds over a frequency range of 1-4000 Hz and converting the data to a spectrum of velocity (mm/s) and frequency. By looking at a single spectrum there is in most cases not a clear pattern visible, but by plotting a number of successively measured spectra in the same graph a clear pattern becomes visible showing a peak velocity value around 45 Hz.

[0049] FIG. 3 shows converted accelerometer data of the installation at the time of a slag foaming event. It can be seen that the peak values around 45 Hz have decreased significantly which as it turned out can be used as a good indication of an impending foaming event.

[0050] At the lower frequency a large peak is shown which has to with the limitation of accelerometers at lower frequencies which is about 5 Hz and lower. For that reason all data below 5 Hz should not be used. It can further be seen that values above about 140 Hz do not show significant changes over time and for that reason should also not to be taken into account.

[0051] Good results have been obtained by only using data in a frequency range which is relevant for the method, which in this case is the range of 5-100 Hz. In order to damp the effect of sudden large deviations in peak velocity values a moving average of peak velocity values is determined and compared with the threshold value. A further correction is to apply an exponential smoothing to damp the peaks.

[0052] As an example: for the method an algorithm for a prediction model was build which comprises the following:

1. the installation should be in production mode for at least 15 minutes.
2. sample accelerometer data every minute in measurement periods of 0.5 seconds,
3. take the sum of all peaks between 5-100 Hz within one measurement period.
4. take the maximum peak in the 5-100 Hz range and divide the maximum peak by the sum of all peaks. This gives the contribution of the maximum peak with respect to the others.
5. Apply exponential smoothing with =0,2 to damp the peaks
6. Take the 5 minute moving average of the calculated contribution factor to smooth the curve.
7. If there are 3 contribution factors within subsequent 5 minutes below 0.045 (threshold value), give an alarm.
Testing the prediction model based on the above algorithm on the historical data gave an alarm 20 minutes in advance before a slag foaming event and no alarms during stable production.

[0053] Instead of the above measurement periods of 0.5 sec every minute other measurements periods at different time intervals could be used which will also provide good results. For instance, with the given setup of the smelting reduction installation good results can be obtained if accelerometer data is sampled in successive sets of 0.1-2 seconds at an interval of 0.2-2 minutes.