b Column 1 - Difference between top gas 808 lb., and bosh gas, 532 lb. CO2 removed between various planes.

d O2 removal between various planes; 122 pounds deducted from oxygen removal from between planes 3 and 4 for limestone calcination.

Column 1 shows the quantity of O2 being removed per minute by the gases at each plane; this is compared with bosh gas. Column 2 gives the quantity of 02 being removed per minute from the burden, the amount in the bosh gas, or that entering by way of the air has been substracted. Column 3 indicates the O2 that is being removed from the burden per minute between plane 1 and the stock line, plane 3, and 1, 4, and 3, and between the tuyere level and the elevation of plane 4. The results include O2 which is being removed from the limestone in the form of carbon dioxide.

Results of analyses of limestone removed from the hole at plane 4 have shown that the stone is calcined by the time it reaches this level, and knowing the temperature and pressure at which limestone is calcined it can safely be said that calcination has taken place between planes 3 and 4. This is borne out by the results shown in column 3: 217 pounds of O2 is removed between planes 3 and 4 out of a total of 276. Reference again to the balance sheet of materials shows that 122 pounds of O were charged with the limestone; this is obviously being removed between planes 3 and 4, and it has therefore been substracted from the results shown in column 4. Column 4 therefore shows how much 0 is being removed from the oxides in the burden. From this the percentage and cumulative percentage have been calculated. The cumulative percentage, or the percentage reduction which has been affected by the time the ore has reached the various planes, has been plotted in Figure 10. The results show the following:

Six per cent of the O2 of the ore burden has been removed between the stock line and plane 1; 12.4 per cent between planes 1 and 3; 61.6 per cent between planes 3 and 4; and 19.5 per cent between plane 4 and the tuyere level. Reference to column 6 shows that 80.5 per cent of the O2 of the ore should be removed by the time the burden reaches plane 4, which is 19 feet 3 inches above the tuyere level. Results of stock sampling show that the iron in three samples taken at plane 4 was in the following form: 84.3 per cent Fe,. 5.2 per cent FeO, and 10.4 per cent Fe2O3. In other words, there were 95.52 parts of Fe and 4.38 parts of 02. In the original ore, charged as Fe2O3, there were 95.52 parts of Fe and 30.5 parts of 02. At hole 4 the relation of O2 to that in the original ore was 4.38/30.5 or 14.4 per cent, which means that 85.6 per cent of the O2 of the ore as charged had been removed by the time the ore had reached plane 4. calculated result from gas analyses gives 80.5 per cent.

CAUSES OF NON-UNIFORMITY IN GAS COMPOSITION

The

The non-uniform composition of the gas, as shown by the samples obtained on the various planes, is due to a combination of several causes: (1) A difference in porosity in the stack column; (2) segregation of iron oxides in the outer portion of the column; (3) unequal stack flow; and (4) unequal gas flow.

In practice, the charge dumped from the bell to the stock line takes a shape similar to that of a V. The coke is probably distributed fairly uniformly over the area of the stock line, while the iron ore falls nearer the wall of the furnace. Large pieces of ore and coke may tend to roll to the center of the V. The method of charging employed causes two conditions (1) a more porous and open charge in the center of the shaft, and (2) a segregation of iron oxide near the walls of the furnace. It would seem advisable to keep the charge a little more dense on the walls, thus reducing gas flow and protecting the wall. The more porous center leaves, however, a path of lower resistance through which a freer passage of gases may take place, thus causing non-uniformity in the gas flow. method of charging, which allows the greater part of the ore to fall near the wall, will result in gases of low CO2 content emerging from the center of the

shaft.

The

On the hearth a void is created at the nose of each tuyere. As the carbon reaching the hearth is changed from the solid to the gaseous phase in a restricted zone of combustion, the effect of the restricted combustion zones is probably transmitted to the stock column in a manner which accentuates unequal stock flow.

The foregoing results show that there is a marked non-uniformity of gas composition across any plane above plane 4, which is 20 feet above the tuyere level, and this is due to a combination of the effect of the four conditions enumerated above.

It is obvious that better practice and a lower coke consumption would result if more intimate contact between gas and solid could be obtained.

CONCLUSIONS

This investigation of a 300-ton furnace in operation provos the following:

1. The oxygen of the blast has been consumed, in the process of combustion, at a point 27 inches above the center line of the tuyeres, and the penetration of the combustion zone in a vertical direction is equivalent to that in a horizontal direction at the tuyere level.

2. The composition of the gas across a plane 20 feet above the tuyere level is constant. The excess oxygen there in is due to reduction taking place in the bosh.

3. As the composition of the gas at plane 4 is constant, the abnormal gas composition shown in the center of the hearth near the tuyere level is a local condition. This is due to a combination of three factors, these being, in order of importance: (a) restricted circulation of gases in the center of the hearth area: (b) so-called direct reduction; and (c) formation of cyanides.

4. The uniformity of gas composition at plane 4, 20 feet above tuyere level, does not indicate uniformity of flow.

5. Analyses of samples at planes 3, 2, and 1, which were approximately 41, 53, and 63 feet above the tuyere level, show unequal gas composition across the planes. It is pointed out that this is due to the effect of four factors: (a) a difference in porosity in the stock column; (b) segregation of iron oxides in the outer part of the column; (c) unequal stock flow; and (d) unequal gas flow.

6. The results indicate that better practice with lower coke consumption might be obtained if operation could be so maintained that the gas composition on any plane above No. 4 would be uniform in CO2 content, and also so maintained that the CO2 content of the gases would increase with distance from the hearth level. This condition will exist if the materials are so arranged in the stack that the composition of the charge is uniform throughout the column. It then follows that the flow of gas and stock in the column must be maintained uniformly.

Acknowledgments

Acknowledgment is made to the officials and men of the Central Iron and Coal Company at Holt, Alabama, who permitted and helped in this investigation, to D. A. Lyon, chief metallurgist of the Bureau of Mines, under whose direction the investigation was conducted, and to A. C. Fieldner, supervising chemist and superintendent of the Pittsburgh Experiment Station.- Reports of Investigations, Bureau of Mines, Department of Commerce.

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