US2379423A - Reduction of iron ores and enhancement of gases - Google Patents

Description

. Patented July 3, 1945 REDUCTION OF IRON ORES AND ENHANCEMENT OF GASES Arthur T. Cape, Santa Cruz, OallfgHerman A. Brassert, Washington, Coma, and Llewellyn H. Thomas, Columbus, Ohio Serial No. 455,690

No Drawing. Application August 29, 1942,

. 2 Claims. (Cl.'l5-35) This invention relates, as indicated, to the reduction of iron ores, but has reference more particularly to the accomplishment of such reduction by means of gases relatively high in hydrogen.

It has heretofore been proposed to reduce iron ore by means of substantially pure hydrogen gas at relatively low temperatures, 1. e., from about 900 F. to about 1500" F. This method of reduc-- tion, requiring low temperatures, is highly advantageous in that it involves low heat consumption, the heat losses from the reductionunit are small, the use of special refractories and heat resistant alloys is obviated, and the apparatus required is of simple construction,

It is extremely difficult, however, to obtain pure hydrogen in the quantities necessary for reduction in a commercially feasible scale,'and sucha gas, moreover, is expensive.

We have discovered that instead of using hydrogen, we can use for reduction purposes other readily available, inexpensive gases, containing relatively large percentages of hydrogen, such for example as coke oven gas, retort coal gas, oil gas, etc. The advantages of such a gas, particularly coke ,oven gas, aside from itsrelatively low cost and availability in commercially desirable quantities, is that the reaction of 'the hydrogen content of the gas, in the reducing process, is not inhibited unless the carbonmonoxideis present in amounts greater than a predetermined maximum, as will be presently explained. The other major constituent of the gas, namely, methane, does not decompose at the temperature at which the reduction takes place, i. e., fromabout 900 F. to about 1350 F. and therefore does not .precipitate large quantities of carbon. Carbon monoxide, particularly at the low reduction temperatures stated above, decomposes to form carbon dioxide and carbon, until some equilibrium value is reached. When this occurs and large quantities of carbon dioxide are present, this means that an oxidizing and not a. reducing-gas is formed, the oxygen tension being increased to a point at which reduction will not occur. Theoreticallv, the carbon content by volume must be kept below a value of 7%, which actually means that the combined volumes of carbon :monoxide and carbon dioxide must be ,kept below this value. The rate of decomposition at these reducingtem- 'peratures is low, and .quite possibly, a larger volume of carbon monoxide may be permitted, the permissible volume of this gas being determined experimentally for any specific case. Coke oven gas, moreover, contains hydrogen sulphide in smal1 amounts,'the presence of which in the gas is detrimental because 'the sulphur is absorbed by the iron during the process of reduction. We have found, however, that we can'utilize these gases, particularly coke oven gas, for'reduction purposes, by observing certain precautions in the selection and/or treatmentoi the gas. More specifically, where coke oven gas is used, it is essential that the combinedfpercentages by volume of the carbon monoxide and carbon dioxide contents of the gas be less than about 15%, and that the ratios of hydrogen to hydrogen sulphide be above the values set forth in Table A below, at the reaction temperatures indicated.

I TAeLs A r I Values for the ratios of Hz to H28 in equilibrium s with Fe and FeS Temperature T.

TABLE B Equilibrium ratios of H20 to H2 at various temperatures in equilibrium with iron and iron oiide Temperature F. EL /m The gas, as thus preheated, is preferably passed,

in countercurrent relation to the ore, through av multiple hearth furnace, in the manner described of a higher B. t. u. value gas.

in the copending application of Cape, Foerster and Griswold, Serial No. 449,088.

The coke oven gas may be handled in either of two ways.

The first method involves the steps of (a) desulphurizing the gas, (b) preheating the gas, (c) effecting the reduction of the ore. (d) cooling the spent gas to remove water therefrom, and (e) utilizing the spent gas, which has a smaller volume, but has a higher B. t. u. value per cubic foot of gas than the original coke oven gas, for other operations or uses.

The second method involves the steps of (a) desulphurizing the gas, (b) mixing it with a. portion of the spent cooled gasfrom the reduction operation, (c) preheating the mixed gases to the desired temperature for reducing purposes, (d) effecting the reduction of the ore, and (e) utilizing the spent gas, which has a smaller volume, but has a higher B. t. u. value per cubic foot of gas than the original coke oven gas, for other operations or uses. This second method has the advantage that smaller quantities of fresh or raw gas are used for the reduction, necessitating the desulphurization of smaller amounts of coke oven gas.

In the second method which involves the recirculating of a portion of the spent cooled gas it is obvious that if the whole of the spent gas were recirculated even after cooling, the reaction would rapidly cease and virtually no reduction would be eflected. Furthermore, the selection of the quantity of the gas to be recirculated is limited by two factors: (1) the decrease in the hydrogen content of the reducing gas to a point at which the reaction would he too slow and (2) the increase in the quantity of carbon monoxide and carbon dioxide. The most feasible procedure is to use equal volumes of coke oven and spent gas, but the greatest amount of recirculated gas which can be used with coke oven gas of the composition stated below is determined primarily by the increase in the.

amounts of carbon monoxid and carbon dioxide which are present.

The following are examples of specific pro,- cedures involving the use of coke oven and recirculated gas in the reduction of magnetite ores.

EXAMPLE I Percent Hydrogen Methane Carbon monoxide plus carbon dioxide Remainder, (largely nitrogen).

The B. t. u. value. of the spent gas would be approximately 569 per cubic foot considering only the hydrogenand methane content thereof.

This means that for every 2000 pounds of iron produced, 59,000 cubic feet of coke oven gas would have to be supplied, but there would always be available for other uses 42,000 cubic feet The composition of the-final reducing gas would be I Percent Hydrogen 37.9 Methane 42.0

In the above example, the volume 1:. of recirculated gas is represented by the following equation:

in which E represents the fraction of the hydrogen in the mixed gas which is used for reduction, ,a is the fraction of hydrogen, by volume, in the coke oven gas, and m is the minimum fraction of hydrogen which must be present in the spent gas.

EXAMPLE II Percent Hydrogen 41 Methane 40.5

The B. t. u. value of the spent gas is 537 per cubic foot. For every ton of iron, 91,500 cubic feet of coke oven gas is required, but 74,400 cubic feet of spent gas is available for other purposes. The

composition of the reducing gas will be Percent Hydrogen 46.0 Methane 1 36.75

In order to heat the total volume, i. e., of 183,000 cubic feet of coke oven and spent gas, to 1170" R, which is a temperature required to heat the ore and supply the heat for the reaction, 5,155,000 B. t. u. are required. The spent gas emerges at a temperature of 967 F., and contains 4,123,000 B. t. u. This sensible heat may be utilized, by heat exchange, to preheat the incoming reducing gas to nearly the desired reaction temperature, the remainder of the heat required being supplied by the usual methods.

Instead of using unreformed coke oven gas, coke oven gas which has been desulphurized and then reformed with water vapor or air may be utilized, the reaction in such case being carried only to the formation of carbon dioxide, the carbon dioxide being then removed, and the remaining gas utilizedin the reduction process.

The utilization of the spent gas for purposes other than reduction makes it possible to employ large volumes of coke oven gas without materially destroying its thermal value. Actually, owing to the increase in the methane content, after the water vapor has been precipitated out, the thermal value is increased per unit of volume. Large volumes of coke oven gas permit the supplying of heat to the reduction reaction, to the heating up of the ore, and to the balancing of the heat losses of the reduction unit by the sensible heat of the gas. The process is thus most eflicient and owing to desulphurization of the gas, its value has been markedly increased over and above the increase in its thermal content.

There is naturally imposed a limit to the extent to which the hydrogen reaction will proceed,

perature, perhaps 50% 'of the hydrogen can be oxidized to water vapor. The limit in this case, however, is the temperature of the decomposition of methane, and this decomposition may become 7 rapid at 1500 F.

If the intention is to conduct the reduction so that the gas at its outgoing is in equilibrium with FeO, then substantial advantage is obtained by passing the gas transversely through the ore, as described in BrassertPatent No. 2,316,664 so that the fresh gas completes the reduction under the will be increased in terms of the methane content for the coke oven gas composition in the above examples, to 36.83% and decreased by the hydrogen falling to 46.4%. It will be seen from Example I, that in the case in which 2.27 volumes of spent gas are recirculated for every volume of fresh gas, the composition of the spent gas will Percent Hydrogen 32.8

Methane 46.2,

If considerations involvingv carbon monoxide and carbon dioxide, as hereinbefore described, than larger amounts of gas may be recirculated with corresponding increases in the methane content of the spent gas.

We claim:-

1. In a method of reducing iron oxide ores, the steps which consist in bringing the ore into contact with a reducing gas at a temperature of from about 900 F. to about 1350 F., said reducing gas consisting principally of hydrogen and methane, but containing minor amounts of carbon monoxide, carbon dioxide, hydrogen sulphide and water vapor, the sum '0f the percentages of carbon monoxide and carbon dioxide being less than about 15%, removing at least a portion of the 'spent gas, cooling said portion, and mixing the latterwith fresh reducing gas, the amount of gas thus recirculated being limited so as not to increase the percentage of carbon monoxide and carbon dioxide in the reduction gas beyond said 15%. 1

2. In a method of reducing iron oxide ores, the steps which consist in bringing the ore into contact with coke oven gas at a temperature 01' from about 900 F. to about 1350 F., the sum of the percentages of carbon monoxide and carbon dioxide in said gas being less than about 15 removing at least a portion of the spent gas, cooling said portion, and mixing the latter with fresh coke oven gas, the amount of gas thus recirculated being limited so as not to increase the percentage of carbon monoxide and carbon dioxide in the reduction gas beyond said 15%.

ARTHUR T. CAPE. HERMAN A. BRASSERT. HEWELLYN H. THOMAS.