FROM THE FITZGERALD & BENNIE LABORATORIES.In the numerous experiments that have been made with the object of producing steel direct from iron ore, one of the most serious problems was that of heating the furnace charge to the necessary temperature. In the blast furnace this is readily accomplished by mixing with the ore an excess of carbon, so that a part acts as reducing agent and a part as fuel, which gives the necessary heat by combustion in the blast. It is possible to make a mixture of ore and carbon where the latter is only present in the proportions necessary for reduction, to place this mixture in a suitable container and then to heat the latter by means of the combustion of fuel, so as to obtain an iron having the desired percentage of carbon; but such a process is not commercially feasible. In order to produce iron from the ore in a commercial manner it appears to be necessary to generate the heat in the charge itself.
The production of electric energy at a relatively low price and the consequent developments in the design of electric furnace led to the hope that the problem of the direct production of steel might be solved. Theoretically, it should be possible to place in an electric furnace a mixture of ore with only that amount of carbon necessary for reducing and for the proper carbon content in the metal to turn out the desired grade of steel. Experiment, however, has shown that this is not commercially feasible, because of technical difficulties which quickly appear when the attempt is made.
Successful experiments with the "Lash process" in the case of the open-hearth furnace on a commercial scale have indicated the possibility of its application to an electric furnace process.
In this process finely divided ore is intimately mixed with carbon, a certain quantity of finely divided carboniferous iron, such as cast iron borings or granulated pig iron, sawdust and fluxes suitable for the ore under treatment. The mixture is then ready for charging, either loose or in briquetted form, and in general it may be said that it must simply be heated to a sufficiently high temperature in order to obtain the steel.
The working of the process can perhaps be best explained by comparison with the open-hearth "ore process," which consists in first forming a bath of molten pig iron and then adding thereto a sufficient amount of ore (iron oxide) to reduce the carbon content of the metal to the desired amount. The oxygen of the ore combines with a certain amount of the carbon in the pig iron, forming carbon monoxide gas, and the iron of the ore is set free to mix with the bath of molten metal. Following this method we may work up together a mixture of approximately 75% pig iron and 25% ore. But in the Lash process these proportions are very different, for instead of treating a large excess of pig iron with a relatively small amount of ore, we can use a large proportion of ore with a smaller proportion of carboniferous iron.
Turning now to a more detailed examination of the Lash process, a typical mixture has the following percentage composition :
|Iron Ore|| 54 |
|Cast Iron Borings or Granulated Pig Iron||27|
| Sawdust ||4|
All the constituents of this composition are in a fine state of division intimately mixed, and when heated to a sufficiently high temperature the chemical reaction between the carbonaceous material and the ore proceeds readily, the iron oxide being reduced by the carbon, forming carbon monoxide gas and metallic iron. The function of the sawdust in the mixture is to make it porous, since at an early stage of the heating the sawdust carbonizes and leaves the mass in a porous state, permitting the easy escape of the gases formed during the reaction.
If we imagine the Lash mixture given above, heated to a high temperature, it is easy to see that the reactions which occur are similar to those found in the open-hearth furnace using the ore process. The cast iron borings correspond to the molten bath of pig iron and react with the ore, but the latter being in large excess it is necessary to supply a certain amount of free carbon in the form of coke, to supply sufficient carbon for the complete reduction of ore.
It is perfectly obvious where the great economy of the Lash process is found, since the proportion of ore to pig iron is very much greater than in the ordinary open-hearth process, where a large amount of pig iron and a relatively small amount of ore, or a mixture of pig iron and scrap, must be used. Put briefly, the Lash process is a method whereby a large amount of relatively cheap iron in the form of ore may be used to replace more expensive pig iron or scrap.
Now, in using the Lash mixture in the open-hearth furnace it is necessary to have a bath of molten metal, for were it to be charged into an empty open-hearth furnace it would not be practicable to heat it to the reacting temperature without losing carbon in the mixture by combustion. If, however, the charge is put into an electric furnace no difficulty of this kind is met with, since the gas in an electric furnace is neutral in contradistinction to the oxidizing atmosphere of an open-hearth furnace.
Experiments using the Lash mixture have been made in the electric furnace on a small scale, and the results have been very successful. In these experiments no bath of pig iron was used, but the mixture was heated alone in an electric furnace. It was found that the yield of metal amounted to 98 per cent of the metallic content of the mixture. Using electric energy as a source of heat is in general more expensive than heating by fuel, in spite of the fact that the heat generated is used much more efficiently in the electric than in fuel furnaces; but the economy in the electric furnace is found in the saving of pig iron, which must be used in the open-hearth process.
Thus, the average cost of the Lash process in the open-hearth furnace for Canada to produce 100 tons of ingots is as follows:
|124 Tons Mixture||$6.09 || $755.16 ||60% metallic||74.4 tons|
|32 Tons Pig||$22.00||$704.00||95%||30.4|
|2 Tons Ore||$5.00||$704.00||60%||1.20|
|1/2 Tons Ferro||$62.00||$704.00||Total contents||106.00|
|Total Cost Charge|| $1,500.16 |
|Credit 2 Tons Scrap @ $18.00||$36.00|
|Cost per Ton|| $14.64 |
|Cost Conversion||$ 4.00|
If, on the other hand, the electric furnace is used, we would have the following figures:
|174.7 Tons Mixture||$6.09||$1,063.92||60% metallic || 104.8 tons |
| 2 Tons Ore ||$5.00||$10.00||60% metallic || 1.2 tons |
| 0.5 Tons Ferro ||$62.00||$31.00||Total contents|| 106.0 tons |
|Total Cost Charge|| $ 1,104.92 |
|Credit 2 Tons Scrap @ $18.00|| $ 36.00 |
|Total|| $ 1,068.92 |
Cost per ton - $11.69
Allowable cost of conversion in electric furnace to make total cost same as open hearth furnace - 7.95
Now, the experimental work carried out in the electric furnace for the reduction of iron ore indicates that the cost of conversion should be as low or less than $7.95 per ton and arrangements have therefore been made to test various electric furnaces, with the object of finding out those forms to which the Lash process is applicable.
Recapitulating, the electric furnace cannot be used at the present time for the direct production of steel, owing to certain technical difficulties, but by applying the Lash process for the production of steel in the open-hearth furnace, with suitable modifications for the electric furnace, it is believed that the problem can be solved. Therefore, in all cases where for whatever reason the use of the electric rather than fuel furnace is desirable, steel may be produced directly from the ore in the former.
Image Credit: Photographic Collection