US3341323A - Blast furnace control method - Google Patents

Description

Sept. 12, 1967 K H. GEE

BLAST FURNACE- CONTROL METHOD Filed March 31, 1964 5 TONE C/IHRGE l/V L55 JUL PHI/R IRO/V TEMPERATURE 5 Sheets-Sheet 1 IZOOO loaoo CAST I l l l l IZHM 3 6 INVENTOR fIenncf/I b. Gee

Sept. 12, 1967 K. H. GEE 3,341,323

BLAST FURNACE CONTROL METHOD Filed March 31 1964 3 Sheets-Sheet 2 .STO/VE 45,7 5 000 Aeo E ll l ' INVENTOR Aennefh Gee United States Patent Ofifice 3,341,323 BLAST FURNACE CONTROL METHOD Kenneth H. Gee, Bethlehem, Pa., assignor, by mesne assignments, to Bethlehem Steel Corporation, a corporation of Delaware Filed Mar. 31, 1964, Ser. No. 356,112 2 Claims. (Cl. 75-41) This invention relates to an improved method of controlling the operation of a metallurgical blast furnace and more particularly to a method of controlling the operation of an ironmaking blast furnace based upon variations in slag basicity which occur from cast to cast.

Control of iron quality by blast furnace operators has been largely dependent upon interpretation of the appearance of slag, iron and tuyeres and upon analysis of silicon and sulfur in the iron. Such methods of control are unreliable.

In a copending application, Ser. No. 213,820, filed July 31, 1962, and now abandoned, there is described a method of blast furnace control using iron temperature, as determined during a cast, as a basis for adjustment of the thermal balance of the furnace to maintain the furnace temperature within proper range.

It is an object of this invention to provide a method of controlling the operation of a blast furnace, which, together with the iron temperature method of control described in the above copending application, will promote more consistent operation than has heretofore been possible.

It is another object to control the operation of a blast furnace when it is performing within the desired iron temperature range by varying slag basicity to meet the desired iron sulfur specification flux charged to the furnace.

It is still another object of the invention to provide a method of controlling the basicity of blast furnace slag in order to improve control over the composition of the iron.

The objects and advantages of this invention will be more clearly understood from the following description with reference to the accompanying drawings in which below-average and then subsequent above-average slag basicity determinations for a series of casts.

FIGURE 2 shows the application of the invention in the operation of a furnace which required a decrease and then an increase in stone charge to compensate for initial above-average and then sub-sequent below-average slag basicity determinations for a series of casts.

the application of the invention in furnace which required an increase in stone charge to compensate for a gradually decreasing slag basicity which had the effect of causing an undesirably high iron sulfur.

One of the main functions of the blast furnace is to reduce the sulfur in iron. The reduction of sulfur is accomplished by flux materials. The lime and magnesia in the flux combine with the gan gue in the ore burden and the ash in the coke to form slag. Blast furnace slag composition must be controlled for good furnace operation. Adjustment of slag composition is normally accomplished by increasing or decreasing stone in the furnace charge.

Slag composition is commonly referred to in terms of slag basicity which may be expressed in terms of a basicity ratio. A common ratio and the one discussed hereinafter is the B/S ratio [(CaO+MgO)+SiO Other basicity ratios could be used as well.

The ratio reflects the basicity of slag and increases as 3,341,323 Patented Sept. 12, 1967 the basicity rises and decreases as slag basicity falls. To increase the amount of sulfur removed from the iron the ratio must be raised by increasing the flux charge (CaO-f-MgO). If more sulfur can be tolerated in the iron the ratio may be reduced by decreasing the flux charge. The amount of sulfur that has to be removed from iron varies from one operation to another and may dictate somewhat different basicity ratios at various operations. Within a given operation however, sulfur removal from iron is controlled in the manner described above, i.e. by increasing or decreasing the slag basicity ratio.

Flux is commonly charged as limestone or dolomite, generally referred to as stone, but it could also be charged through flux incorporated in sinter. As mentioned above, the lime and magnesia in the flux combine with the gangue in the ore burden and the ash in the coke to form slag. The gangue and ash generally have little chemical variation within a plant practice. Consequently, changing the flux charged to the furnace alters the slag basicity ratio in a predictable fashion.

Any satisfactory analytical method may be used for slag analysis, e.g. the quench method of analysis, which involves determining the liquidus temperature of a small sample. Slag basicities are normally reported to the furnace operator within two hours after cast, along with iron silicon and sulfur.

Variations in slag basicity occur during both flush and cast. Cast slag has proved less variable than flush slag and is preferred for slag sampling. A standard time for sampling has been adopted, i.e. upon raising the second gate in the slag runner.

Decisions to alter the stone charge are based upon slag basicity, iron sulfur analysis, and the immersion thermocouple readings of iron temperatures described in the above mentioned copending application. A slag basicity control range for each furnace is established from data obtained from a given set of operating conditions. It is that range of slag basicities which will produce iron sulfur within acceptable limits for a furnace operating within the desired iron temperaturegange. For example, a blast furnace was found to provide maximum production with minimum fuel costs when its maximum iron temperatures were kept within the range of 2720-2760 F. For this range of iron temperature, the furnace operator was able to meet the desired specifications on iron sulfur, .035% average sulfur with no casts exceeding 0.050%, by adjusting slag basicity between 1.45 8/5 and 1.55 B/S by varying the amount of flux charged to the furnace.

In carrying out this invention, corrective stone changes are made when slag basicity falls outside the control range for two successive casts provided the iron temperature curves for those casts are normal. For any particular furnace there are periods when, for a given set of operating conditions, the furnace is said to be operating smoothly or in balance. Throughout these periods a minimum of adjustment of the thermal balance is required for operations, and iron production is high. Iron temperatures smooth operation, when the furnace is neither heating up or cooling down, will result in normal curves of similar appearance. Stone changes are restricted to these periods of normal iron temperature.

In the accompanying figures, operating data are shown for maximum temperature of iron, iron sulfur analysis, slag composition in terms of the B/S ratio,

and the pounds of stone per charge. The maximum temperature of iron is a convenient guide and shows in a manner the thermal state of the furnace in these illustrations, and additional temperature data used by the operator, such as temperature at appearance of slag, are not shown although developed during these periods. These additional data would normally be shown in discussing the moves made to adjust the thermal state of the furnace, as fully described in the aforementioned copending application.

FIGURE 1 shows the above mentioned operating data for a series of seven casts on a particular furnace. At the time of the casts the furnace was operating in a desired iron temperature range of 27102760 F. and the slag basicity control range had been established between 1.50 and 1.56 in order to produce iron having a sulfur analysis of under .035 As mentioned above, the slag basicity control range is established by measuring the basicity of the cast slag during a period when the furnace is operating smoothly in the desired iron temperature range and producing iron having a sulfur analysis at the maximum specified amount or less.

The iron sulfur analysis for casts 1 and 2 was undesirably above the specified maximum of 035%. The slag basicity ratios for casts 1 and 2 were well below the established range of between 1.50 B/S and 1.56 B/S while the iron temperatures for these casts were within the desired iron temperature range. Accordingly, the furnace operator increased the amount of stone on the furnace by 300 pounds per charge after cast 2 in an effort to reduce the iron sulfur. Iron temperature, iron sulfur and slag basicity were all within specified limits for cast 3. For casts 4 and 5 iron temperatures were satisfactory and iron sulfur was well within limits, but drifting lower. However, slag basicity had risen beyond the desired control range during cast 4 and continued even higher for east 5. In accordance with the method of the invention, the furnace operator, observing that the slag basicity was above the control range for two successive casts, reduced the stone charge 300 pounds. Iron temperature was within limits for east 6, sulfur was below the maximum allowable but slightly higher than the previous cast, and slag basicity was still beyond the desired control range but coming down. By cast 7, iron temperature, iron sulfur and slag basicity were all within desired limits.

FIGURE 2 shows operating data for a series of eight casts on another furnace. This furnace, at the time, was operating within a desired temperature range of between 2710 and 2760 F., with a slag basicity range established between 1.50 B/S and 1.56 B/S to produce iron having a sulfur analysis below .030%. After the furnace operator observed that slag basicity exceeded the control range for two successive casts, 2 and 3, and that iron temperatures were within specified limits, he decreased the amount of stone in the charge by 300 pounds. During casts 4 and 5 slag basicity was within the control range but drifting downwardly, iron sulfur remained below 030% and iron temperatures varied only slightly. During casts 6 and 7 the downward trend of the slag basicity ratio continued and iron sulfur approached its upper limit of .030%. With iron temperatures within the desired operating ranges for these casts, the furnace operator knew that slag basicity was the contributing cause to the increasing iron sulfur. Accordingly, the stone charge was increased 300 pounds. During cast 8 slag basicity increased sharply from the prior casts and fell within the control range. Iron sulfur was satisfactory and iron temperatures were within prescribed limits. Control factors fell within specified limits for subsequent casts from this furnace.

FIGURE 3 shows a series of casts which again illustrate the value of the method of this invention in controlling iron sulfur by slag basicity when iron temperatures are under control. The desired iron temperature range for the furnace from which these data were obtained was between 2700 and 2750 F. It had been established that for this furnace to produce iron bearing a sulfur analysis of under 040%, slag basicity had to fall within the range of 1.50 B/S to 1.56 B/S, or, in another form, slag basicity could not deviate more than $0.03

from the desired level of 1.53 B/S. For cast 1 iron temperatures, iron sulfur and slag basicity were all within desired limits. For cast 2 the iron sulfur was at the upper limit of 040%, and slag basicity had fallen to 1.49 B/ S, more than 0.03 below the desired level of 1.53 B/ S. The iron temperatures for cast 3 were satisfactory but iron sulfur rose sharply to 050% and slag basicity for the second successive cast was more than 0.03 below the desired level of 1.53 B/S. Consequently, the furnace operator increased the stone charge by 300 pounds to correct for the slag basicity deficiency. Slag basicity and iron sulfur for east 4 were both outside specified limits but iron temperatures, though drifting lower, were still satisfactory. The stone change made after cast 3 had not had time to affect furnace operation so no changes in operation were made at this time. At cast 5 slag basicity was at the desired level of 1.53 B/S, iron sulfur had dropped below the maximum specified and iron temperatures were at 2740 F., well within limits. The 300 pound stone change the furnace operator made after cast 3 had produced the desired results. Casts 6 and 7 found iron sulfur, slag basicity and iron temperatures within specified ranges.

The data shown in the figures and described above were obtained from furnaces having a hearth diameter of 26' and an iron pr-oduction'of between 2000 and 2200 tons per day. Stone changes for these furnaces were made in increments of 300 pounds per charge. Naturally, it will be understood that the method of this invention can be practiced in any size furnace regardless of the amount of the stone charge. The normal or standard stone change must be sufiiciently large to be measured accurately by the furnace weighing system and should also be large enough to cause a detectable change in the chemistry of the slag. On the other hand, a normal stone change should not be so large as to cause the slag composition to move from the lower to the upper edge of the control range. The 300 pounds stone change made in the above described examples met these requirements.

The slag basicity control aim is determined by the basicity which provides the desired iron sulfur when iron temperature is under control. Maintaining the furnace temperature and basicity of the slag within proper range contributes to smoother furnace operation and results in improved quality iron and increased tonnage.

It must be emphasized that for most satisfactory performance the slag basicity method of controlling a blast furnace should be utilized only when a furnace is operating smoothly and the iron temperatures, are within the established range of satisfactory performance. The slag basicity control range or desired slag basicity is established from operating data as that range of slag basicities or that desired slag basicity which will produce iron sulfur within acceptable limits for furnaces; it is determined from experience during a period when the furnace is operating smoothly.

Although certain novel features of my invention has been shown and described, it will be understood that changes and modifications can be made in the procedure without departing from the spirit of the invention or the scope of the appended claims.

I claim:

1. The method of controlling the operation of a blast furnace to produce molten iron, having a sulphur content within a desired control range, from a burden comprising coke, ore and flux, which comprises:

(A) obtaining a slag basicity control range by (1) taking samples of slag from a plurality of casts from said furnace when operating during periods when the iron temperatures 'as measured during casting fall within a desired temperature range and when producing iron having a sulphur content within said desired control range,

(2) analyzing said slag samples to obtain the range of slag basicities thereof,

(B) obtaining the basicity of slag cast from said furnace during subsequent casts by (1) taking samples of slag from consecutive subsequent casts, (2) analyzing said slag samples to obtain the slag basicities thereof, and

(C) changing said burden when the iron temperatures as measured during subsequent casts fall within said temperature range by increasing the flux charge when the slag basicities of at least two of said consecutive subsequent casts fall below said control range and by decreasing the flux charge when the slag basicities of at least two of said consecutive subsequent casts fall above said control range.

2. The method of controlling the operati-on of a blast furnace to produce molten iron, having a sulphur content within a desired control range, from a burden comprising coke, ore and flux which comprises:

(A) obtaining a slag basicity control range by (1) taking samples of slag from a plurality of casts from said furnace when operating during periods when the iron temperatures as measured during casting fall within a desired temperature range and when producing iron having a sulphur content Within said desired control range,

(2) analyzing said slag samples to obtain the range of slag basicities thereof,

(B) obtaining the basicity of slag cast from said furnace during subsequent casts by (1) taking samples of slag from two consecutive subsequent casts, (2) analyzing said slag samples to obtain the slag basi-cities thereof, and (C) changing said burden when the iron temperatures as measured during subsequent casts fall within said temperature range by increasing the flux charge when the slag basicities of said two consecutive subsequent casts fall below said control range and by decreasing the flux charge when the slag basicities of said two consecutive subsequent casts fall above said control range.

References Cited UNITED STATES PATENTS 2,208,245 7/ 1940 Boynton 75-41 2,832,682 4/1958 Reygagne 75-41 2,918,365 12/1959 Kanamori et al 75-41 3,030,150 2/1962 Reed 75-41 OTHER REFERENCES Bray-Ferrous Production Metallurgy, John Wiley & Sons, 1942, pages 164, 165.

The Making, Shaping and Treating of Steel, 7th ed.,

25 1957, page 258.

DAVID L. RECK, Primary Examiner. H. W. TARRING, Assistant Examiner.

Claims (1)

1. THE METHOD OF CONTROLLING THE OPERATION OF A BLAST FURNACE TO PRODUCE MOLTEN IRON, HAVING A SULPHUR CONTENT WITHIN A DESIRED CONTROL RANGE, FROM A BURDEN COMPRISING COKE, ORE AND FLUX, WHICH COMPRISES: (A) OBTAINING A SLAG BASICITY CONTROL RANGE BY (1) TAKING SAMPLES OF SLAG FROM A PLURALITY OF CASTS FROM SAID FURNACE WHEN OPERATING DURING PERIODS WHEN THE IRON TEMPERATURES AS MEASURED DURING CASTING FALL WITHIN A DESIRED TEMPERATURE RANGE AND WHEN PRODUCING IRON HAVING A SULPHUR CONTENT WITHIN SAID DESIRED CONTROL RANGE, (2) ANALYZING SAID SLAG SAMPLES TO OBTAIN THE RANGE OF SLAG BASICITIES THEREOF, (B) OBTAINING THE BASICITY OF SLAG CAST FROM SAID FURNACE DURING SUBSEQUENT CASTS BY (1) TAKING SAMPLES OF SLAG FROM CONSECUTIVE SUBSEQUENT CASTS, (2) ANALYZING SAID SLAG SAMPLES TO OBTAIN THE SLAG BASICITIES THEREOF, AND (C) CHANGING SAID BURDEN WHEN THE IRON TEMPERATURES AS MEASURED DURING SUBSEQUENT CASTS FALL WITHIN SAID TEMPERATURE RANGE BY INCREASING THE FLUX CHARGE WHEN THE SLAG BASICITIES OF AT LEAST TWO OF SAID CONSECUTIVE SUBSEQUENT CASTS FALL BELOW SAID CONTROL RANGE AND BY DECREASING THE FLUX CHARGE WHEN THE SLAG BASICITIES OF AT LEAST TWO OF SAID CONSECUTIVE SUBSEQUENT CASTS FALL ABOVE SAID CONTROL RANGE.

Images