Non-technical chats on iron and steel, and their application to modern industry

Since the beginnings of iron manufacture, charcoal has been a favorite fuel. Though during the past two centuries coke has grown to be the standard, with anthracite and some few bituminous coals finding use in certain favored localities, charcoal may be considered the fuel which developed the iron industry, at least until recent years.

Charcoal

As most of us know, charcoal is completely charred wood, usually hard wood, though sometimes resinous or other soft woods are used. Of well-dried timber more than 50 per cent by weight is moisture. This and certain other constituents are driven off by heat in the absence of air, which process is usually called “destructive distillation.”

By primitive methods a considerable part of the wood was completely burned and wasted during the production of charcoal. Stacked in piles or long rows the cut wood was well covered with earth, except for a small opening at the top through which the fire was lighted down a center cavity left to the bottom of the pile. The air coming in through the opening at the top was sufficient to keep the wood smoldering. After a period, which had been shown by experience to give the best results, the opening was closed and the fire smothered.

Brick ovens of the beehive shape were built at a later date where considerable charcoal was to be made. These were operated on much the same general principle as the meillers or earth-covered piles, described above. The fire was lighted at the bottom of the central cavity of the corded wood, the only air at first coming from the top, though later in the process a little was admitted through holes in the walls. After about ten days, when gas ceased to come off, the kiln was tightly closed for a period of twenty days more for the fire to die out and the charcoal to cool.

By both of these processes valuable constituents were burned or driven off by the heat and lost. These were mainly methyl alcohol, acetic acid, and wood tar.

Modern industry so emphatically disapproves of any waste of materials that apparatus has been devised to produce charcoal which allows of recovery of the by-products at the same time. In northern Michigan, which is practically the only district in the United States in which the charcoal industry as an industry still survives, long steel tubes or retorts are built with brick fire-boxes under each end, much as a stationary boiler is set. Into these retorts are run steel cars loaded with the wood. The retorts being closed, the heat drives or distills off the moisture and gaseous compounds through pipes connecting them with condensing apparatus. After about twenty hours the wood has been charred, the doors of the kilns are suddenly opened and the cars are rushed into other and similar retorts for cooling, while fresh loads of wood replace them in the first.

As may be surmised vast quantities of wood and of wood-producing land are required for extensive charcoal manufacture, and this is the most serious problem for the manufacturer of charcoal. Several square miles of timber land must be cut over each year and the wood efficiently transported in order to operate a large plant profitably.

Pig iron as a by-product is a rather novel idea, but that is practically what the charcoal pig iron produced in our Lake Superior region is. Several companies operate wood distillation plants for the production of methyl alcohol, acetic acid, acetate of lime, etc., and use their charcoal in the manufacture of charcoal pig iron from the ores so close at hand.

The very low sulphur content and the small amount of ash have been the great advantages possessed by charcoal over other solid fuels. Resulting characteristics made charcoal pig iron a former favorite for manufacture of certain articles such as chilled car wheels, etc., and it, therefore, brought a higher price than coke pig iron. During recent years, however, by careful selection of coal and improvements in the coking process the sulphur and ash of coke have been so reduced that charcoal has not so great an advantage as formerly. Charcoal iron to-day brings only about $1.50 per ton more than coke iron; whereas, the differential a few years ago was as great as $5.00 or $6.00 per ton.

Charcoal is quite fragile and structurally weak, so much so that blast furnaces for its use cannot be built higher than sixty feet; whereas, the great strength of coke allows them to be built to exceed one hundred feet in height with correspondingly increased output. What this means may be realized by every one conversant with the demands of modern industry.

Coke

As charcoal is completely charred wood, so coke for analogy’s sake may be said to be completely charred coal, practically always of the bituminous type. By “baking” bituminous coal at a cherry-red heat, its volatile constituents are driven off as the well-known “coal-gas” of almost every small town, and a strong, brittle and porous material or coke residue is left. If the baking is done without any admission of air to the retort, practically none of the coal burns and the “cake” or coke which is left contains the ash of the original coal and what is known as the “fixed carbon,” i.e., carbon which cannot be distilled or driven off by heat alone, though it would burn were air admitted.

The gases or volatile constituents which are given off consist mainly of moisture and a mixture of gaseous chemical compounds, which are known as “hydro-carbons.” These contain that part of the carbon of the original coal which does not remain as “fixed carbon” in the coke.

Just why some coals will coke while others of apparently the same composition as shown by the chemist’s analyses, will not, but instead of the hard brittle mass will leave a heap of brown or black powder, is not as yet definitely known. It is easy enough for chemists to determine with accuracy the amounts of hydrogen, nitrogen, oxygen, carbon, sulphur, and other elements; but it is a difficult and perhaps an impossible matter to determine just how these elements are “hitched up” in the very complex mineral, coal,—one of the most complex substances which we know.

Various theories have been advanced in the attempt to explain the coking quality. A bulletin of the United States Geological Survey claims that the relative percentages of hydrogen and oxygen in the coal determines it; others have held that it depends upon the compounds of a tarry or asphaltic nature present. The fact remains that some coals coke without trouble, while others do not coke at all. As yet the only real way to tell whether a new variety of coal will or will not coke is to try it.

Since 1713, when Abraham Darby in England succeeded in introducing it as a substitute for the fast disappearing charcoal for use in blast furnaces, coke has become the standard fuel. It is very strong and will bear up under the great weight of iron ore and limestone with which the furnace is charged. So furnaces for use with coke may be built much larger than those in which charcoal is to be the fuel. The porous nature of coke allows it to burn rapidly with intense heat, so that the output of an iron works is greatly increased through its use—a very desirable thing in these days of big things. It has its disadvantages, of course, mainly high sulphur, a deleterious substance for which molten iron, unfortunately, has a voracious appetite, and a rather high percentage of ash which must be fluxed out. But all in all, it is a very desirable fuel for blast furnace and other metallurgical purposes, as is shown by the fact that it is used in the production of about ninety-nine per cent of all iron and steel now made.

What is known as the Appalachian coal region produces coal for more than seventy-five per cent of the coke made in the United States. This region includes the strip of territory extending from Western Pennsylvania and Ohio down to Tennessee, Georgia, and Alabama. The famous Connellsville district is a part of this region.

Illinois and Indiana have a great deal of coal, which, however, has rather indifferent coking qualities. Almost constant experimentation has been carried on in the attempt to induce these semi-coking coals to coke. The best that has so far developed is the use of a considerable percentage of them in admixture with coals of good coking qualities. Such mixtures yield quite satisfactory coke.

The Beehive Oven Process

In the old days there was no desire or incentive to avoid waste of coal resources. If during the coking process some air got into the oven and part of the coal was burned, or if all of the gas given off was wasted, it did not matter. There was plenty more of coal and the thing desired was to get the requisite coke in the quickest and cheapest way.

In Western Pennsylvania, Ohio, and Virginia, were great beds of high grade coking coal. In this region and particularly around Pittsburg, numerous blast furnaces and steel mills grew up. The coke for these was made in the most convenient way—in the wasteful beehive ovens.

As the name signifies, these ovens or retorts are brick chambers shaped like beehives. In the larger plants they are built either in single rows against long hills or in double rows back to back. Over the tops of the ovens in each row runs a car called a charging “lorry.” Coal is poured from the bottom of this through a hole in the top of each oven while it is still hot from the preceding charge. No air gets in except that admitted through the hole in the oven top and a small slit left over the one side door, through which the coke is drawn when the coking process is finished. The heat of the oven starts the distillation of the moisture and the volatile compounds which escape through the hole in the oven top. The small amount of air admitted burns a little of the coal and gas and raises the temperature of the oven to that required for coking.

After 48 or 72 hours a spray of water is thrown in over the glowing coal to quench the fire. The partially cooled coke is drawn through the open door, sorted and loaded into cars for shipment.

Though this method of coking is a very wasteful one, it yet produces the larger quantity of the coke made in the United States. However, conditions are rapidly changing and it will not be many years before the much less wasteful “by-product” process gains the ascendency. By 1914 it had already come to produce about twenty-five per cent of the total coke made here, and since that date the percentage has been rapidly increasing.

By this system of coking a greater yield of coke is obtained and most of the by-products are saved. The value of the latter depends largely, of course, upon local conditions, such as transportation, costs of the material, cost of labor, and available market for the coke oven gas. They are usually figured as having a value of $1.50 per ton of coal coked, equivalent to a total of $71,000,000 per year for the coal coked in the United States.

The ovens and apparatus required are considerably more expensive, but, since this industry has developed in this country during the last twenty-two years to a point where one-quarter of all of the coke manufactured is made by the by-product process, there can be no doubt that it is a profitable proposition and that eventually the wasteful beehive ovens will be a thing of the past.

Practically all of the types of by-product coke ovens in use have been developed in Germany or Belgium, where circumstances forced earlier conservation of resources than in this country. The three best known types are the Semet-Solvay, the Otto Hoffman, and the Koppers—the latter a recent arrival. They differ mainly in details of construction and operation.

In a general way a “battery” of coke ovens consists of from 40 to 80 long narrow brick-walled chambers placed closely side by side with heating flues or “checker-work” between them. The fire for the baking process is in these flues, which are interconnected, and the heat developed is sufficient to drive off the moisture and volatile substances of the coal in the narrow chambers just on the other side of the brick walls. Charging is done by a “lorry” as in the beehive process. After from seventeen to twenty-four hours at a red heat, the coke is “pushed” from the ovens, one after another, by an electric ram which enters at one end. The 30 × 7 × 1½ foot block of glowing coke emerges from the other end, where, breaking under its own weight into good-sized pieces, it falls into a steel car on a track just beneath. A spray of water quenches it and it is taken to the storage bins to be sorted.

Rich coal-gas is the main by-product. That which comes off during the first seven hours is the richest and has the greatest illuminating or “candle” power. After washing free from dust, tar, ammonia, etc., the gas is usually run into holders or tanks from which it is distributed for use for illuminating or for heating purposes. That which comes off during the latter part of the coking period has much less of those constituents which give illuminating value. It has good heat value, however, and as fuel is required for keeping the ovens up to the coking temperature, this poorer gas from the coking chambers is switched into and burns in the flues between the coking chambers as mentioned.

Thus the larger part of the gas is sold to customers, usually in the city near which the ovens have been located, and the poorer part is utilized in heating the ovens and the steam boilers which run the plant.

The coal tar, which the German chemists have made so famous through its manufacture into the almost endless variety of beautiful dyes, is another of the by-products which is recovered by this, but burned or lost in the beehive oven process. From a long main over the tops of the ovens which connects the gas pipes, the tar flows along with the gas to the scrubbing and gas cleaning plant, where by rather intricate operations it is freed from other substances.

In this country much of the tar is used for building purposes, etc., and some as fuel, but not much has been made into the chemical products for which Germany is so famous. For a long time a few dyes and other chemical compounds have been made here from coal tar. Since the early days of the war in Europe and the cessation of imports of such materials on this account, there has come about considerable expansion in their manufacture here; but it is doubtful if the time is yet ripe for a wholesale entry into the manufacture of these coal tar “derivatives,” especially the very extensive variety of dyestuffs.

Naphthalene and benzol from which many other chemical compounds as well as munitions of war can be made, are among the by-products.

Most of the ammonia which the corner drug store sells, comes from the by-product manufacture of coke. The largest part of the ammonia which is produced in the process, however, is manufactured into sulphate of ammonia, a well-known fertilizer.

Coal

Anthracite or hard coal has been used in certain districts in the United States, especially in New Jersey and eastern Pennsylvania. It is not an ideal fuel as it is too solid to burn rapidly, spalls or cracks under heat and interferes with the blast. Since 1860 when coke became available here much less coal has been used, though some is yet used in admixture with coke. Some bituminous coals which contained little tarry matter also have been used in this way.

Limestone, the rock which is ordinarily used for fluxing purposes, needs no introduction to any of us. As the marble of statuary, the material of which oyster and other sea shells and the white tombstones of our cemeteries are composed, it is well known. Any of these varieties of the material may be used for fluxing purposes, but usually it is limestone which is quarried for the purpose or obtained as chippings or spalls from building blocks.

The active agent, which produces the chemical or fluxing action in the blast furnaces, is carbonate of calcium (lime) of which limestone contains about 98 per cent. Dolomite is a mixture of carbonates of lime and magnesium, about 53 per cent of the former and 45 per cent of the latter, and is sometimes used in place of limestone. Fluor spar, a rock composed of calcium and fluorine, is used in small quantities in some of the metallurgical processes. It is a very powerful flux.