184 



KNOWLEDGE, 



[OCTOBEE 1, 1892. 



showed that in this process of reduction the element carbon 

 abstracts tlie oxygen from tlie ore, forming carbonic acid. It 

 was pointed out, also, that in such reducing processes the 

 carbon plays a two-fold part, for while one portion unites 

 with the oxygen of the air, and in this process generates 

 heat, the high temperature thus brought about enables 

 the remainder of the carbon to abstract the oxygen from 

 the ore, forming more carbonic acid, and leaving the 

 metal. It is merely a matter of arrangement and of dis- 

 position of the parts of apparatus whether one portion of 

 carbon is used as the heating, and a separate portion as 

 the reducing agent, or whether the two processes are 

 carried on simultaneously in the same parcel of the 

 material. The former condition obtains, for example, 

 when a powdered metallic oxide mixed with finely-divided 

 charcoal is placed in a glass or porcelain tube, which is 

 heated in a charcoal furnace. The second condition would 

 obtain when the common hiBmatite iron ore (an oxide) 

 was mixed with charcoal m the old-fashioned iron furnace 

 of Spain, and the charcoal having been set alight, and the 

 heat having been urged by a blast of air, the iron was 

 reduced to the metallic state by the action of the carbon. 



In any process of iron smelting in which an oxide of 

 iron is reduced by some foriu of carbon the initial and the 

 final states may be represented thus : — 



Oxide of iron + carbon ^ metallic iron -I- carbonic acid. 

 The symbol = must here be interpreted "may be caused 

 to yield." But as our acquaintance with a chemical 

 process becomes more intimate we generally find that a 

 knowledge of the initial and final states of matter is not 

 sutlicient either for a proper understanding of what goes 

 on, or, in many cases of manufacturing processes, for regu- 

 lating the process so as to obtain a good " yield " of the 

 valuable product. For this it is often necessary to learn 

 tlie steps between the initial and final stages, even if the 

 "intermediate" bodies formed have but a transitory 

 existence. This is especially the case in the reduction of 

 iron ore in the blast furnace. The modern improvements 

 which enable manufacturers to keep pace with the demand 

 for iron roads and iron ships are based upon the conditions 

 of formation and decomposition of the lower oxide of 

 carbon, carbon monoxide gas, which is formed either in 

 the incomplete combustion of carbon, or when heated car- 

 bon acts upon carbonic acid. Carbon monoxide gas readily 

 takes fire in presence of oxygen, burning with formation of 

 carbonic acid. In so doing the carbon takes up a second 

 dose of oxygen, being combined in carbonic acid with just 

 twice as much oxygen as in the lower oxide. Carbon 

 monoxide also acts as a reducing agent, the gas at a high 

 temperature extracting the oxygen from metallic ores. 



If, in any metallurgical process a portion of the coal, coke, 

 or charcoal used is only burnt to carbon monoxide, then, 

 in respect of that portion, we only get a part of the heating 

 effect of the fuel, and half the reducing effect of the 

 reducing agent. True, the hot carbon monoxide issuing 

 from the apparatus burns in the air, forming the final 

 product, carbonic acid, and producing more heat ; but we 

 want that heating effect in the apparatus, not outside it, 

 and we wish, if it be possible, that the extra dose of 

 oxygen required for complete combustion of the monoxide 

 should come from the ore and not from the atmosphere. 



In the blast furnace the materials put in at the top are 

 oxide of iron, coke {i.e., cltarrcd coal), and limestone. The 

 lime of the limestone combining with siliceous matter 

 mixed with the ore removes these impurities in the form 

 of a light fusible slag, which floats upon the top of the 

 liquid iron collected at the bottom of the furnace. The air 

 is supplied in a forced blast through tubes called twyeres, 

 placed near the bottom of the furnace. The furnace is 



always kept full, or nearly full, and we may consider the 

 conditions at any part of the furnace as remaining con- 

 stant, as, when once lighted, the furnace is kept always in 

 operation till it is "blown out," after perhaps twelve or 

 fourteen years' work. The furnaces used at the beginning 

 of the century were about forty-eight feet high. Those 

 used now ai-e eighty feet. It was found that by giving 

 more time to the ascending current of carbon monoxide a 

 greater proportion of ore was reduced. A great deal of 

 the gas in the smaller furnace escaped without, so to speak, 

 having the chance of becoming fully oxidized. 



A still more important improvement was the introduction 

 of the hot liliist. It appears paradoxical that one ton of 

 coke burnt to heat the air before entering the furnace 

 should serve three tons of coke in the furnace itself; yet 

 so it is. The heating (with hot blast) being partly 

 done from outside, it is not necessary to put so much coke 

 into the furnace ; consequently, there is room for more of 

 the ore. The power of the ore and limestone to intercept 

 heat is double that of the coke which has been replaced, 

 and there is a greater surface of the ore to act on the 

 diminished and slower current of carbon monoxide. 

 Hence, greater economy in the production of pig-iron was 

 effected by the use of the hot blast, and by raisiug the 

 height of the furnace to eighty feet. Some furnaces 

 were built of one hundred and three feet, but it was 

 found that no further economy was effected. The 

 present approved dimensions of furnace and temperature 

 of blast appear to give a better yield of metal than if the 

 furnace be larger and the blast hotter, and a better yield 

 than if the furnace be smaller and the blast colder. 

 The reason is as follows: — We have together in the 

 furnace, carbon monoxide and iron ore, carbon dioxide 

 and coke. If the heat contained in the carbon monoxide 

 is more completely intercepted before passing out of the 

 furnace, then, as we have seen, more of the ore is reduced 

 and more of the monoxide is burnt to dioxide. But, on the 

 other hand, if the temperature of the carbon monoxide be 

 still further increased by the use of a still hotter blast, and 

 if the heat of the up-flowing carbon monoxide be yet more 

 completely intercepted before the gas issues at the mouth 

 of the furnace, then we find that the temperature of the 

 other materials is raised so far that the incandescent coke 

 reduces the carbonic acid to carbon monoxide as fast as the 

 ore oxidizes the monoxide to carbon dioxide. Consequently 

 we have two opposite reactions which balance one another 

 under certain conditions. When the temperature rises so 

 much that the reduction of carbon dioxide is more rapid 

 than the oxidation of carbon monoxide, it is obvious that 

 we have passed the point where the furnace works most 

 economically, for the reduction of the carbonic acid is 

 accompanied by an absorption of heat. It appears there- 

 fore, as the result of practical experience, that we must 

 always be content to only half burn in the blast furnace a 

 large proportion of the fuel, sending out a niixtnir of carbon 

 monoxide and of carbon dioxide from the mouth of the 

 furnace. 



One of the first great improvements m blast furnace 

 practice was that of heating the blast. A more recent 

 improvement has been to utilize outside the furnace the 

 burning of that proportion of the carbon monoxide the 

 presence of which in the escaping gases is, as we have 

 shown, a necessity. In the modern furnaces the mixed 

 carbon monoxide and carbon dioxide are drawn oft' near 

 the top of the furnace, instead of being allowed to come in 

 contact with the air. The hot gases are only admitted to 

 the presence of oxygen when they have been brought by 

 pipes to the furnace where the air for the blast is heated. 

 The combustion of the carbon monoxide not only heats 



