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sion of the operations, called medium cleaning and fine cleaning, with apparatus protected by patents. The sludge of dust and water was removed by the application of centrifugal force, separating gas and liquid in specially designed fans or washers (Theissen, Brassert, etc.)- The disposal of the water created a problem, as contamination of rivers is against the law in most industrial countries. In deposit ponds the settling of the impurities was incomplete and its removal a tedious manual operation ; and the Dorr thickener, developed in ore- concentration districts, was adopted, as assuring continuous service automatically by means of a special mud pump requiring little atten- tion. A drawback of wet systems was that the sensible heat of the blast-furnace gas was absorbed by the cleaning water and lost be- yond recovery. On the other hand, it permitted the installation of smaller gas-piping and dispensed with the insulation of the lines against heat loss, thus saving appreciable capital outlay in the case of long-distance distribution. Another drawback of the wet method was that recovery of the dust required driving off the water from the heavy mud in any briquetting or concentrating process attempted.

(2) The dry methods of gas-cleaning had their advocates where every little economy was watched, such as retaining the sensible heat of the gas and saving the expense of water- handling in keeping the dust dry. To separate the dust out of the hot gases, filtration appeared to be the best process. With mechanical filtration, finely woven cloth or asbestos-fabric bags or slag-wool layers let the gas pass at low velocity but retained the solid dust, which was removed periodi- cally by return currents of clean gas. The principle was adopted in the Beth-Halberg system in Europe and the Kling-Weidlein appara- tus in the United States.

Through the researches of Dr. F. G. Cottrell, in the United States, electricity promised to serve as a filtering medium in the so-called electric precipitation process. By creating high-tension discharge currents in the flow of the gas the solid particles became separated from the gas. At the end of the decade 1910-20 high potential electric deposition had its noteworthy applications among non-ferrous blast- furnace installations. The utilization of the thermal value of blast- furnace gas was sometimes credited against the conversion cost of ore, plus limestone, plus coke, into pig-iron. A ton of iron blown, re- quiring 2,200 Ib. of coke of about 80% pure carbon (equal to 1,700 lb.), produced an amount of gas, expressed in cub. ft., equal to 90 to 100 times that weight, or some 160,000 cub. ft., averaging about 100 B.T.U. per cub. ft. In assuming a value of 2 cents or id. per 1,000 cub. ft., the gas would represent an asset of some $3.20, or 133. to 145., per ton, a figure indicating the great importance of the by- product gas in the cost-sheet of the plant. The gross valuation must be diminished by such factors as the cost of cleaning, but a net surplus of about $l per ton per day was not uncommonly left after allowing for material and conversion outlays of the plant.

The chief ways in which blast-furnace gases were utilized were as follows:

(a) Cowper or Hot-Blast Stoves. The absence of dust in the gas provided for rational stove design, as the complicating side issues of clogged-up passes and slagged-up checker holes disappeared, as well as the periodic waste of cooling, cleaning and warming-up of each unit. The clean gas meant a reduction of the area of heating surfaces and brick volume expressed by fewer stoves per blast furnace three to four per furnace against four to five 10 years earlier. Then began a systematic study of the heat-transmission phenomena within the mass of checker work, a study which in 1920 was not yet completed. Close observation and scientific research, coupled with improved combustion methods regulating more closely and more positively the two elements of combustion gas and air, promised at that time to lead to a further reduction of the number of stoves, and also to higher blast temperatures and less gas consumption.

(b) Boilers. Clean blast-furnace gas allowed for advantageous use in connexion with steam boilers; first, through more efficient com- bustion, in effect less gas per pound of steam produced; second, higher ratings of boilers, in effect more steam per unit of boiler evaporating surface or fewer boilers for a given plant capacity; and, third, quick adaptation to any load required, in effect flexibility or ease of operation. Many efficient burners were invented and some were installed on a large scale.

(c) Metallurgical Furnaces. The removal of flue-dust made possible a wider distribution of the blast-furnace gas, and in Europe use was made of the surplus gas with success in all kinds of furnaces. The low calorific value coupled with the small amount of air required for complete combustion opened fields where so-called mellow heating flames are demanded, such as core drying, mould drying, annealing, roasting and ore concentrating.

(d) Gas Engines. The principle that clean gas was indispensable for internal-combustion engines was long recognized, but its practical application did not occur until after 1910. Also a cool gas was re-

. garded as essential to secure adequate volumetric efficiency of each cylinder. Among gas engines the four-cycle type outranked con- siderably the two-cycle type. Devices for close regulation were developed on the principle of qualitative-quantitative mixture. The built-up cylinder seemed to win greater favour than the one- piece casting. The safety of operation reached a parity with that of steam-engines or turbines, the gas being clean. The exhaust heat of the engines, representing some 40% of the energy, was utilized to generate steam, and 70 % was thus recovered in some instances.

Of all the various uses made of the gaseous by-products of the blast furnace only the heating of the hot-blast stoves was universally applied. All experts agreed that 30 % to 40 % of the gases are best employed for that purpose. The surplus of 60 % to 65 % was utilized for the other purposes already mentioned. In the utilization to produce blast pressure and to develop power, the battle for suprem- acy between the gas-engine using blast-furnace gas and the steam boiler using the gas to supply energy to engine or turbine remained undecided. Thermal efficiencies were not the only issues at stake. In Europe the gas-engine had the firmer standing, while in America the boiler seemed to be the more in favour. Even for generating the blast pressures, the competition between gas-engine-driven air com- pressors, steam-engine blowing engines and turbo-blowers had gone on without absolutely proving the superiority of any one combina- tion. Varying economic conditions in each country and different local considerations, as well as the purely technical aspects of the problem, were deciding factors. Europe, with its skilled workmen and more stabilized market conditions, presented a background different from that of America with its fluid trade conditions and its unsettled, unskilled labour.

The fact that the dust in blast-furnace gas is made up of coke, ore and flux additions, combined with the fact that the cleaning plants provide for collecting it, led to the reintroduction of the material into the furnaces. By previous nodulization, as in rotary kilns, or briquetting under presses with or without binding agent, the flue- dust became available for use. Numerous processes were developed, of which the Dwight-Lloyd sintering system gave good results, judging from the number of installations in America.

Plant Layout and Size. A single blast furnace built alone on a site, no matter how well chosen, proved not to be a logical industrial enterprise. The number of such plants existing was the result of competition, of fluctuating market conditions, and constituted an economic waste, speaking generally. With combined units the ac- cessory equipment became cheaper in installation cost and in terms of iron output and more efficient in operation, through flexibility and insurance against breakdown. Three to six furnaces grouped in well-laid-out plants were established as an economic whole. To avoid the loss of the sensible heat of the molten pig-iron and to refine the metal without cooling, steel-works were logically joined to blast-furnace plants. The two separate departments were thus combined in one industrial unit, with the added advantage that the surplus of power available at the furnaces could be absorbed in the rolling-mills.

Electric Pig-Iron Furnaces. Tests at Trollhattan, Sweden, made on a cooperative basis by steel interests and the Government, were conclusive only for high-grade pig-iron similar to the Swedish char- coal pig-iron. Since 1918 the Domnarfret works in Sweden had operated several shaft-type furnaces (with gas circulation using 60 % to 62 % of iron ore and charcoal as a reducing agent). Mixtures of charcoal and coke up to 50 % coke were found satisfactory. Per ton of pig-iron produced, 3,400 lb. of ore (containing 61-5% Fe), 120 lb. of lime and 740 lb. of charcoal were charged; 15,000 cub. ft. of gas at 240 B.T.U. per cub. ft. were captured at the top; 2,150 kilowatt-hours was the electric energy consumption per 2,000 lb. of pig-iron. The problem of using electric current for supplying heat in the blast-furnace reactions had particular interest for the eastern Pyrenees in France, British Columbia, Brazil, Italy, as well as Sweden and Norway, where fuel is scarce and low-priced electricity might be made available.

The Steel Plant. The usefulness of mixers as an important adj unct of the steel-making plant was universally recognized, as numerous installations attest. Their field was established in equalizing quali- tatively the successive outgivings of the blast furnaces and in de- sulphurizing the molten metal. To accelerate the removal of sul- phur, less than 0-5 % of manganese proved most helpful. The shape of the mixer that gave best results was the simple cylinder rotating on its axis. The most popular size proved to be 1 ,000 to 1 ,400 tons' containing capacity. Simple oil or gas burners without regenerating chamber in the United States, with pre-heating checkers sometimes in Europe, completed the equipment. In Germany a 2,ooo-ton- capacity mixer was reported built, but only after considerable dis- cussion as to its size. The mixer was r c4uCDurse, brought into being for receiving metal from the blast furnace and delivering to the ladle for transport to the steel plant as needed. Slag that floats on the top of the bath must be skimmed off from time to time.

Converter Plants. No noteworthy development took place in the acid operating (Bessemer) converter or in the basic operating (Thomas) converter for making steel. The 20- to 25-ton-capacity vessel remained nearly universal. A 4O-ton size was proposed in 1918 by a Belgian engineer. As between Europe and the United States, the hydraulic tilting mechanism of the former did not give way to the electric drive of the latter, nor did the gas-engine blowing units succumb to the turbo-blowers of American practice.

Open-Hearth Plants. Without radical change in type, sizes of open-hearth furnaces increased up to and above loo tons' capacity, but the tendency was toward fully controllable sizes. The practice in the United States settled to 80 to loo tons and in Europe 40 to 50 tons. Volumes of checker chamber increased to get better so-called flywheel effect. Greater attention was paid to port and head con- struction to lengthen life, and to a reinforced roof. Reversing valves