v. To avoid as much as possible, waste of heat by the

escaping gases.

A. FOR RAPIDITY OF MELTING.

A. i.—_Enlarged Grate Area._—In the older methods of working, there was a general tendency to employ a furnace of standard size, and improvements in the economy of the process were in the direction of reducing the fuel bill as much as possible for the given size of furnace. This was effected by keeping the grate area fairly small.

In modern practice, economical working still involves having the ratio of size of hearth to size of fire-box as large as possible, but instead of reducing the dimensions of the fire-grate to suit the hearth, a large grate is built to commence with, and the hearth is constructed of such a size as will utilise all the heat available. From this principle of burning a large quantity of fuel and melting with it as much charge as possible, the efficient and economical working of large furnaces has been developed.

A grate area of about 28 square feet is now regarded as the minimum for economical work at modern smelters, and fire-boxes up to 128 square feet in area are usual in practice.

In small fire-boxes, only small quantities of fuel can be burned at once, and in consequence, fresh firing is continually required, which interferes greatly with the work of the furnace and decreases the rapidity of heating. Each addition of cold fuel has a cooling effect on the fire and furnace gases, the temperature in the hearth being found to drop for a period of five or ten minutes by as much as 100° C., the flame becoming smoky, red, and cold. A similar time is required for the original temperature to be attained once more. Cold air is also admitted every time the fire-box doors are opened for charging.

The advantages of large grate area therefore include:—

(_a_) Much less cooling of the furnace by frequent additions of fuel. (_b_) Higher temperatures, owing to the increased calorific intensity of large quantities of fuel burned at once. (_c_) Less blanketing of the fire by fuel additions. (_d_) Less chance of the whole of the grate area being clinkered up at once, and in consequence, less likelihood of interference with the rapid combustion of the fuel.

The most rapid and economical smelting at the present day requires that at least 0·7 lb. of coal be burned per minute per square foot of hearth area.

A. ii.—_Draft._—The charge in a reverberatory furnace hearth is melted chiefly by the heat from the hot gases passing over it, and in giving up their heat to the charge, the gases become cooled down. The heating of the charge is made continuous by the continual addition of fresh fuel in the fire-box, and by the drawing of the flames over the hearth by means of flues situated at the other end of the furnace and leading to the stack. The flues and stack must be large enough to cause sufficient draft through the furnace for the heated gases to be drawn over the charge with sufficient rapidity, and much unsuccessful work has been due to the fact that these requirements have not been fulfilled. There should be a suction equivalent to at least 1 inch to 1·5 inches water pressure up the stack, this being readily measured by water-manometers—a feature of modern working.

Reverberatories may be worked either by forced or natural draft, the latter being usually preferred, though it necessitates a large stack and spacious flues.

Forced draft by fan or blower under the fire-grate has been in use at several smelters, the ashpit then being closed. It was at one time adopted at Anaconda, but was given up later. The use of forced draft has the advantage that leakages of cold air into the furnace are to a large extent prevented, hot gases tending to be forced out rather than cold air drawn in, but the objections to its use include the facts that—

(_a_) Special power and machinery are required. (_b_) The intense action near the fire-bars produces, from the ash of the coal, a massive clinker in a semi-fused condition, difficult to deal with. (_c_) It is stated by smelters to have a cooling action near the fire-bridge.

A. iii.—_Firing and Grating._—This question is closely connected with the dimensions of the grate, since the use of a small fire-box necessitates methods of firing and grating which are not conducive to the most rapid and efficient combustion of the fuel. In addition to the cooling action of frequent fresh fuel charges in the small fireplace, attendant disadvantages include the closing up of the spaces in the grate by which air enters for burning the fuel, and the consequent necessity for frequent grating with small beds of fuel, which entails numerous objections.

The addition of fresh coal to the fire causes the production of large quantities of volatile hydrocarbons which require an increased air supply for proper combustion, and this air admission is just prevented by the blanketing action of the fresh fuel added. This is indicated by the red smoky flame, and means waste and cooling. The difficulty is overcome by the arranging of a series of air-holes at the fire-box end of the furnace, near the fire-bridge, and by the opening of these directly after firing, the volatiles are immediately burnt up. This is an important feature in successful working, and with a large fire-grate and this air-admission, the effect of adding even 1½ tons of fuel on to the fire at once causes little difference in the furnace temperature. The flame is observed through a window let into the off-take flue, which allows of the changes in appearance being noted by the fireman on the fire-box platform.

The fire is kept moderately shallow, to allow of rapid burning of the fuel, though deep enough to keep up the enormous body of heat necessary in the furnace.

B. THE PREVENTION OF HEAT LOSSES.

B. i.—_Avoiding Leakage of Cold Air._—The admission of cold air was the cause of much waste in the older processes of working. Each time the doors were opened, either at the fire-box, or during charging on to the hearth, large quantities of cold air were admitted; air entered through the working door whilst slag was skimmed off, whilst matte was being tapped, and whilst the furnace hearth was being clayed; all of which operations occupied considerable time. The doors were opened during the levelling down of the fresh charges, and at later periods when the charge was stirred and the half-fused masses sticking to the bottom were worked up.

In modern practice, an essential feature of working is to keep all the doors closed as much as possible, and, as will be indicated shortly, every means is taken to eliminate the heat losses from the causes just referred to. Air leakage is also occasioned by bad grating, which causes the formation of channels in a few parts of the bed of fuel, admitting excess of air at these places, instead of causing it to come regularly through the bed in all parts. Channelling is now checked by the drop of suction-pressure in the flues, as registered by the manometer.

B. ii.—_Prevention of Radiation through Walls and Roof._—Such heat losses are now minimised by thickening these parts, and blanketing the outside of the roof with sand, keeping the construction together by very heavy bracing.

B. iii.—_Prevention of Cooling of the Hearth on Withdrawal and on Charging._—By far the most important cause of heat losses in working was occasioned by the withdrawal of the whole of the melted products, the charging of fresh cold ores, and the efficiency of the furnace was very greatly reduced in consequence. In the older methods, fully three-quarters of the time and fuel, and almost all the labour, were spent in manipulating the charges and bringing them up to the point of fusion, the actual smelting operation being responsible for but a small proportion. The withdrawal of the hot slag and matte abstracts much of the heat of the furnace, and the cold charge which is fed in, not only cools the furnace hearth on which it rests, but being a poor conductor, prevents the heat from again penetrating through it to the hearth and to the undermost portion of the charge. It has been estimated through the use of pyrometers, that the temperature in the furnace after such withdrawal and recharging may drop to less than 700° C.—a dull red heat—and there is no way under such circumstances of heating up the hearth again, except by conduction through the charge. Some hours’ hard firing were thus required to bring the furnace to the desired temperature again, after which it was necessary to re-open the working doors, in order to stir the materials so as to prevent the half-fused masses, still lying on the hearth, from sticking to it. This also occasioned delay in the operations, and caused much waste of fuel, heat, and labour.

B. iv.—_Utilising the Heat of Melted Charges for the Heating of Fresh Additions._—All the above difficulties, and many others, have been overcome by maintaining a deep pool of hot molten matte in the furnace, and by feeding hot charges upon this matte layer. These are two of the most vital and successful changes introduced into modern reverberatory practice, and will be reviewed in detail subsequently.

B. v.—_Utilising the Heat of the Escaping Gases as much as possible._—Improvements in this direction have been brought about—

(_a_) By constructing the furnace of as great a length as will allow of maintaining the charge in a sufficiently fluid state to permit of its being tapped from the furthermost end of the furnace. (_b_) By using the still hot escaping gases under boilers.

=Modern Reverberatory Practice.=—The requirements for the successful operation of the reverberatory furnace, and the methods for ensuring its efficient working which have just been reviewed, involve the application of the following principles, which are the essential factors in modern reverberatory smelting practice:—

1. The grade of the furnace products is controlled at the roasters. 2. The melting must be as rapid as possible. 3. The employment of very large furnaces. 4. The use of a heated matte-pool in the furnace. 5. The charging of hot calcines. 6. The regulation of the furnace working by draft pressures. 7. The continuous working of the furnace. 8. Modified constructional details.

1. _Control of Furnace Products at the Roasters._—This feature has already been indicated in dealing with roasting practice. The importance of this system in the economy and efficiency of the furnace working is very marked.

(_a_) The roasting plant affords the most ready means of control over the desired sulphur elimination, this being its sole function. The modern roaster is so designed as to allow of almost perfect regulation in this respect, since amount of feed and rate of passage of the sulphides through the furnace are under perfect control.

(_b_) The work of the reverberatory is thus confined to one object only, that of rapid melting down, to which the foreman can give his sole attention free from the necessity of manipulating the grade of the matte at the same time.

In modern work it is usual to pass the whole of the charge (concentrates as well as flux) intended for the reverberatories, through the roasting plant. The advantages of such procedure are—

(i.) The flux is preheated at little extra expense, there being usually plenty of heat to spare for this, and the roaster capacity is not unduly decreased. (ii.) Intimate mixing of the charge is assured, and this greatly facilitates the fusion and reaction. (iii.) More rapid and thorough roasting is effected, since the presence of the inert flux prevents clotting or undue sintering of the sulphides in the roaster. (iv.) The charge is found to be in a much better condition, both physically and chemically, for successful reverberatory smelting.

Lime in the roaster charge appears to assist the thoroughness of the roast, whilst an incipient slag formation is commenced owing to the juxtaposition of basic oxides and silica, in the hotter parts of the roaster furnace.

2. _Rapidity of melting_ is an indispensable feature of modern work. The conditions necessary for rapid melting have been reviewed above.

3. _Use of Large Furnaces._—Reverberatory furnaces appear to have replaced the blast furnace in Great Britain somewhere about 1700, and by 1854 they were in general use in this country. At this period the usual dimensions were, for the hearth 13 feet by 9 feet, with a fire-box 4 feet by 4 feet, the furnace having a capacity of 12 tons per twenty-four hours. In Great Britain the size increased very slowly, and it was in the United States of America that the important increase in dimensions and in enormous outputs were developed. The work was commenced systematically in about 1878 by Richard Pearse (a Swansea-trained metallurgist) at the Argo Smelter in Colorado. Table V. indicates the gradual improvements in practice resulting from these developments (see also Fig. 23, p. 90).

TABLE V.—DEVELOPMENT IN SIZE OF THE REVERBERATORY FURNACE.

─────┬─────────────┬────────────────┬─────┬───────────┬────────────── │ Fire─box │ Hearth │ │ │ Tons Ore per Year.│ Dimensions. │ Dimensions. │Stack│ Capacity. │ Ton Coal. ─────┼─────────────┼────────────────┼─────┼───────────┼────────────── 1878,│4' 6" × 5' │ 9' 8" × 15' │2' 9"│12 tons. │2·4 tons. 1882,│4' 6" × 5' │10' 4" × 17' 10"│2' 9"│17 " │2·43 " 1887,│4' 6" × 5' 6"│12' 8" × 21' 2"│3' 0"│24 " │2·67 " 1891,│4' 6" × 6' │14' 2" × 24' 4"│3' 0"│28 " │2·8 " 1893,│5' × 6' 6"│16' × 30' │3' 6"│35 " (43)‡│2·7 " (3·3) ‡ 1894,│5' × 6' 6"│16' × 35' │4' 0"│ (50)‡│ (3·7) ‡ 1903,│5' 6" × 10' │20' × 50' │5' 5"│ (70)‡│ (3·1) ‡ 1910,│8' × 16' │19' × 116' │ .. │ (275)‡│ (4·66)‡ ─────┴─────────────┴────────────────┴─────┴───────────┴────────────── ‡ The charges of calcines were fed whilst still red hot.

This practice has been continued in modern smelter work, the developments being in the direction of attempting to melt the largest possible quantity of charge in one furnace as rapidly as possible. This has been found to depend upon the rapidity with which the fuel is burned, and the enlarging of the fire-box had a specially important influence in effecting this rapidity of combustion.

Then, with the size of grate fixed and the most efficient burning of the fuel arranged for, the capacity of the furnace depends simply on increasing the area of the hearth to as great an extent as the heat generated is capable of maintaining at the desired temperature.

The breadth of the furnace is however, limited by—

(_a_) The span of arch which can be supported in the construction. (_b_) The length of the tools which can be conveniently managed.

The maximum width so far found satisfactory is about 19 feet, so that this dimension being fixed, the furnace capacity is enlarged by increasing the length, and this is limited only by the distance from the fire-box to which the flame can maintain the temperature necessary for keeping the charge in a state of perfect fluidity. For many years the length was regarded as limited to 50 feet, smelting about 2·7 to 3·0 tons of charge per ton of coal, but E. P. Mathewson, at Anaconda, finding the escaping gases still very hot, gradually increased the length of the hearth, first to 60 feet, then to 80 feet, and finally up to 116 feet, when the furnace smelted 4·83 to 5·0 tons of charge per ton of coal. The gases then left the furnace at a temperature of about 950° C., and contained sufficient heat to fire two Stirling boilers, each of 375 H.P. Every furnace thus provided about 600 H.P. from this waste heat, and the gases finally escaped at a temperature of 320° C.

The capacity of these large furnaces is about 270 to 300 tons of charge per day, and in addition to the economy and efficiency resulting from the treatment of such large quantities of material at once, there are the further great advantages in that—

(_a_) Settling of matte and slag is much more perfect when such large quantities of fluid material are stored. (_b_) Tapping of matte and slag is easier and more efficiently conducted.

About 110 feet appears to be the practicable maximum for furnace length, and reverberatories of this size are being constructed wherever circumstances permit, several new smelters having erected such furnaces—there are eight at Anaconda, Mont.; two at Garfield, Utah; five at Tooele, Utah; four at Cananea, etc. The length of the hearth is naturally dependent upon the character of the fuel, particularly the length of flame given out on burning. Bituminous fat coals are the most suitable for this purpose, and in localities where such fuel is not available, the use of liquid fuel has now been successfully adopted.

4. _Maintaining a Heated Matte Pool in the Furnace._—This is probably the most important and beneficial advance made in reverberatory practice.

In certain stages of the old Welsh process, a store of matte was retained in the furnace after skimming off the slag, but the object was to collect a sufficiently large quantity of matte in the furnace for convenient tapping out.

The modern practice has several objects and possesses enormous advantages—

(i.) It assists efficient settling. (ii.) It conserves the heat inside the furnace. (iii.) It presents a highly heated surface for the fresh charge to fall upon, and thus greatly increases the rapidity of melting, by ensuring that the charge is heated both from above and from below. (iv.) It prevents the sticking of half-fused charges to the furnace bottom, the removal of which masses would necessitate much labour, and occasion cooling of the furnace by the opening of working doors. (v.) It preserves the furnace bottom.

Liquid matte has practically no action on the siliceous material of the hearth, and so presents an inert mass between the bottom and the charge. This charge consists of calcines (mainly oxides of iron), which would, during the process of melting down, slag with and corrode the furnace hearth were it not protected by the matte layer.

(vi.) It allows of continuous charging and withdrawal of materials, and of continued high temperature in the furnace, thus protecting the furnace lining from much wear and tear. Nothing damages furnace linings more than exposure to changes of temperature, on account of the continual expansion and contraction of the brickwork and the low thermal conductivity of the silica. Furnace linings wear out much more from such action than from long exposure to continued high temperature.

(vii.) There is effected an enormous saving of time, fuel, and labour by maintaining a constant high temperature, instead of having to heat the furnace up again after each tapping and charging, as was the case with the older methods of working.

(viii.) The levelling of the charges in the furnace is greatly facilitated. The charges would otherwise pile up under the charging hoppers, and form heaps which are not only difficult to melt down, but which tend to stick to the furnace bottom, requiring time and arduous labour for their removal. In modern practice, charges in quantities of 10 to 15 tons at a time maybe dropped in, these merely spread themselves out on the bath of molten material and float down in a thin stream towards the skimming door at the end, and they generally melt and disappear when half-way down the furnace.

By this means, the working doors at the side need practically never be opened for manipulating the fresh charges.

5. _The Charging of Hot Calcines._—This improvement was also introduced by Pearse, and possesses very many advantages; he was able to increase the furnace output by 23 per cent. with the aid of this device.

Instead of allowing the materials from the roasters to cool down, they are taken straight from the roaster bins to the hoppers which feed the reverberatory furnace, where they retain much of their heat until charged into the furnace, being then still red hot as a rule. Much time and fuel is thus saved owing to the charge requiring less heating up, and the cooling action of charging is diminished.

A charge of 15 tons is completely melted within an hour.

6. _Regulation of Furnace by Draft Pressure._—It has already been pointed out that rapid combustion of fuel, and consequently rapid melting, is greatly assisted by good draft through the furnace. In modern practice, where the factors, such as charge composition, nature of fuel, and furnace proportions, have been satisfactorily arranged for independently, the actual working of the furnace is regulated by the draft pressures. These are registered automatically by water-manometers arranged at various points. One usually communicates with the furnace, above the fire-bridge; another is connected to the down-take flues. The indications of these instruments enable a record to be kept of the various operations, and of the charging of the furnace, as well as of the condition of the fire. The usual draft pressure worked with corresponds to about 0·8 inch of water, registered above the fire-bridge.

On opening the hopper for charging, the pressure drops almost to zero; the opening of any doors causes a reduction in pressure; the charging of coal is also rendered noticeable by a drop in the record. Reduction of pressure also indicates “airing” of the furnace by an excess of air entering through channels in the bed of coal; draft-pressure thus acting as a check on the firing and also on the grating, since the formation of excessive clinker in the fire-box is indicated by an increase in the pressure.

Corresponding to such record over an 8-hour shift, as shown on fig. 24, Offerhaus noted the following furnace manipulations, illustrating how accurately the operations are checked by this method:—

a.m. 7.00–7.14 Skimming (coal charged during this period). 7.16–7.16½ Side door opened. 7.28–7.31 Coal charged. 7.52–7.57 _Charged._ 8.05–8.15 Tapped. 8.15 Coal charged. 8.40 Coal charged. 8.54–8.59 Grating. 9.05 Side door opened. _Charged._ 9.27 Coal charged. 9.49 Coal charged. 10.07 _Charged._ 10.25 Coal charged. 10.41 Coal charged. 10.45–10.58 Skimming. 11.04 Coal charged. 11.16 _Charged._ 11.16–11.35 Some grating. 11.36 Coal charged. 12.03 p.m. Coal charged. 12.04 _Charged._ 12.37–12.48½ Tapped, 1½ ladles (about 11 tons). 12.45 Coal charged. 1.00 _Charged._ 1.11–1.45 Grating. 1.26 Coal charged. 1.44 _Charged._ 1.51 Coal charged. 2.18 Coal charged.

Total charges during shift, 16 coal, 7 calcines.

The draft record is placed close to the charging platform, in order to be in a convenient position for the guidance of the workmen. The draft in the main flues is 1·7 to 1·8 inches water pressure; this is similarly recorded in the foreman’s office.

7. _Continuous Working of the Furnace._—The continuous working of the furnace is a most important factor in modern practice, and is naturally inseparably bound up with the principle of maintaining the heated matte-pool in the furnace, which allows of the continuous charging of hot “calcines,” and the continuous or regular withdrawal of slag and of matte when required.

The matte (which can be efficiently settled, owing to the prevailing high temperature and the large mass of heated material in the furnace) is stored there until required at the converters, when the desired quantities are tapped out. The slag which is produced by the smelting action gradually accumulates, and at regular intervals most of it is run out (rather than skimmed). This usually takes place every four hours. The slag accumulates until it reaches a level some 3 or 4 inches above the skimming plate at the end of the furnace, and the quantity which is run out at each “skimming” amounts to some 60 or 80 tons, the contents of the furnace being lowered to such an extent that a fresh accumulation of material may proceed during the next four hours. No pulling of the slag is required as in the older methods of working, since the material is so very hot and fluid that it simply pours out of the furnace, and twenty minutes usually suffices for the whole of the 60 or 80 tons to run off, the rabble being used chiefly to regulate and control the stream, and to keep back siliceous crusts or floaters. The slag is run out until the matte is seen underneath, on flapping back a thin layer, or until the level of the skimming plate is reached, and its removal is such a short and simple operation that there is very little interference with the regular and continuous running of the furnace. Similarly, the tapping of as much as 50 to 100 tons of matte from the store of 250 tons of hot fluid material has little influence on the continuous working. Charging of coal and calcines is performed at regular intervals, and the charges of 15 tons of “calcines” fed in at a time, readily melt down and settle. Practically the only interference with continuous running is the necessity for claying and repairing, and the use of the matte pool on the hearth has lessened the frequency for this to a large extent, the hearth bottom itself being protected from corrosion, owing to the sulphides exerting no action upon it, whilst the oxides in the charge which would be capable of attacking the siliceous bottom are slagged off before they get an opportunity of reaching it. The hearth bottom, if properly put in, is practically permanent.

The portion of the furnace most subject to corrosion is at the slag line, where deep channels are gradually cut out. Every four to six weeks the furnace is tapped dry, repaired, and fettled, as much as 20 tons of fettling sand being often required for this purpose. The sand is thrown in and patted into place by long rabbles, the operations occupying about eighteen hours. Every nine months or so the furnace is repaired more fully, 20 or 30 feet of brickwork near the fire-bridge being taken down, and the great cavities in the side walls repaired by masons, using silica bricks. The employment of higher temperatures in modern work allows of more siliceous slags being produced, which lessens the tendency to the eating away of the walls.

The feeding of siliceous copper ores through a series of small hoppers situated in the roof, near to the walls, has lately been introduced with a view to protecting the furnace sides from the corrosive action of the slag, and to exposing a suitable siliceous flux to this material. This appears to have fulfilled its purpose to some extent, but various difficulties have been encountered in practice, especially the tendency for the cold added material to form floaters, which require limestone additions in order that they may be fluxed off; and the cooling effects and leakages through the openings have also given trouble.

8. _Modified Constructional Details._—In addition to the increased size of fire-box, hearth, and flues, and to the necessity for very heavy staying in order to keep the enormous arch in permanent shape, which are characteristic of modern practice, the construction of modern furnaces involves the building of a suitable hearth to carry the heavy burden of hot and fluid matte which is stored in the furnace.

It was formerly considered correct practice, in the smaller types of furnace, to construct the hearth over a vault, in order to keep the underside cool and thus prevent the corrosion and eating away of the siliceous bottom by the oxidised charges, during the process of melting down. In modern practice it is absolutely essential to work with a perfectly solid structure.

(_a_) Because the hearth must be kept as hot as possible, so as to ensure rapid melting of the charge and maintain the products in a perfectly fluid condition. Any circumstance tending to cool the hearth is rigorously avoided, this being the contrary of the older practice. The protective influence of the heated matte-pool in modern work preserves the bed from the corroding effects of fresh oxidised charges, and in consequence, the maximum degree of heat can with safety be maintained on the furnace hearth.

(_b_) The enormous weight of charge and the heavy arch and walls demand the strongest possible foundations and support.

In building modern reverberatories, the foundation for the hearth is constructed of solid masonry or brickwork, or as at Anaconda, of a solid bed of slag, some 24 inches in depth, run in from an adjacent furnace. The I-beams used for carrying the bracing are erected in a surrounding trench, and a further quantity of slag (4 feet thick by 2 feet deep) is run in, thus yielding a perfectly rigid and impervious foundation (Fig. 26). On the top of this slag-foundation is built a layer, 12 inches thick, of silica bricks, and upon this, the actual working bottom of the furnace is constructed.

This bottom is now put in also in a manner different to the older practice, and excellent results have accrued from the change.

The old method of constructing sand bottoms consisted of putting in the beds of sand, layer by layer, and thoroughly fritting each one before the addition of the next: in modern practice, it is found that proper consolidation is not attained with beds of the enormous area now employed, when the bottom is constructed in such layers.

The present method of working the reverberatory furnace is not to drop the charge on to the sand hearth at all, but into the deep pool of matte, and the sand-hearth is regarded more as a convenient foundation for the support of this liquid working-bed, on account of its constituting a cheap non-conducting and fire-proof material which is unaffected by the materials resting upon it. It was found, however, on commencing this matte-pool practice, that the older method of putting in the bottom in successive sand layers was not suitable for this work; after a little wear, the beds became raised in layers, this being especially the case if any holes happened to be eaten through in places. Moreover, the large weight of matte tended to find its way down between the layers and raise them up bodily, or else it worked down at the edges of the hearth and side walls, and either broke out underneath the former or through the latter. When it was ascertained that liquid matte itself had no corrosive action on the siliceous hearth if the latter be kept constantly covered, and that the causes of breakouts were principally due to mechanical weaknesses, it required only improvements in design and construction in order to avoid them. This is now attained by constructing the bed in a compact and perfectly massive form, and is best accomplished by putting in the whole layer of 26 inches of sand at once, and firing as hard as it is possible for the brickwork to stand. The method has met with exceptional success in practice, rigid and impervious hearths are obtained; it being found that less than 1 inch has worn off the bed after two years’ working.

=Large Reverberatory Furnaces: Details of Construction.=—The large furnaces at Anaconda were the first of the modern type to be constructed, they have met with enormous success in practice and constitute the standard form. Similar furnaces are now in operation or under construction at many of the large modern camps, and are of similar design and construction.

The _hearth_ is 102 to 116 feet long by 19 feet wide.

_Grate_, 16 feet by 8 feet = 128 square feet grate area.

Ratio of hearth to grate area is 16 : 1.

_Distance_ from hearth to level of fire-bridge, 26 inches; hearth to crown of arch, 6 feet 5 inches. _Walls_ are 26 inches thick. _Roof_ is 15 inches thick (except for 4 to 6 feet over the fire-bridge, where it is 20 inches). The bracing of the furnace is necessarily particularly strong (see Fig. 29). Lined inside with silica brick, said to be the finest in the world. The bed is of the finest Dillon sand (97·5 per cent. silica), ground to pass ¼-inch mesh; the bed has a slope of 8 inches towards the tap-holes, of which there are two. During the construction of the large furnace there are left in the roof ten expansion openings of 3 inches each, which by the time the furnace has attained its working temperature, become closed up (see Fig. 30). The conker plate which runs through the fire-bridge is 14 to 15 feet long, and is made thicker near the furnace side, where it is 3 inches thick. The air space through the plate is 2 feet 3 inches by 9 inches, and serves the purpose of keeping the fire-bridge cool; air passes through it continuously, and if the plate shows signs of becoming hot, a blast of cold high-pressure air is sent through it. Still further heating of the plate and signs of red heat are an indication that the 2 feet of silica of the fire-bridge wall are being burnt through.

=Working of the Reverberatory Plant at Anaconda.=—The plant consists of eight large furnaces, built parallel to one another, seven being usually at work whilst the eighth is undergoing repair. Each furnace treats 300 tons of hot calcines and flue-dust daily.

_Charging._—The furnaces are charged every 65 to 70 minutes with 15-ton charges, and as soon as one charge is melted, another is added; with average running, 150 charges are worked in the seven furnaces daily. The charge train, consisting of an engine and three cars, each of which carries 5 tons of charge, travels from the roasters and enters the reverberatory building by an overhead track running above the charge bins of the furnaces. It discharges through hoppers into the bins which extend across the entire width of the hearth. Bins were formerly arranged at intervals all the way down the furnace, but now only the two bins nearest to the fire-bridge are employed. Into the back bin, 10 tons of charge are placed, and into the other, 5 tons. Each of these bins discharges through two hopper discharge openings, feeding the furnace through holes in the roof (Figs. 29, 30), which are closed, when not in use, by round firebrick tiles 20 inches in diameter and 2½ inches thick; these are moved in and out of position by means of levers operated from the fire-box platform.

The temperature maintained in the furnace is high, approximating to 1,500° C., and just previous to dropping in a fresh charge, a workman, by means of a rabble, feels about the hearth below the charging hopper in order to ensure that all of the previous charge has been melted, and that none of it is sticking to the furnace hearth. By employing only the comparatively small quantities of 15 tons, this sticking is avoided, since such charges are not heavy enough to sink unmelted through the 8 inches of slag and 8 inches of matte in the furnace. The former practice of feeding charges amounting to 45 tons through hoppers situated all the way along the furnace had given serious trouble in that respect, and had consequently to be discarded. When the examination of the hearth is completed, the time occupied being very short, the side door is closed, and sealed with sand; the covers to the holes in the roof are now withdrawn, the gates closing the hoppers pulled back, and first the 5-ton, then the 10-ton charge is dropped into the furnace. The whole operation, including the preliminary opening of the door to test the furnace bottom, occupies five minutes.

Very little hand labour is required round these enormous furnaces, except for the grating of the fires, for the charging of coal and calcines every hour by the operation of levers from the fire-box platform, for the skimming of slag at intervals of four hours, and for the tapping of matte when required. The whole of this work is conducted by the skimmer and two helpers to each furnace, one of the men also looking after the boilers.

As soon as the charge has been dropped on to the pool of molten material, the mass appears to spread out over the surface and float towards the skimming door, in a thin slow-moving stream which disappears when about half-way down, being usually melted within one hour. The former 40-ton charges required as much as eight hours for melting.

Owing to the great heating effect of the large bath of hot material below, and of the intense flame above, there is but little cooling action on adding the fresh charge; whilst with this length of furnace, practically all the dust is settled, and very little is carried into the flues.

_Coaling._—The quantity of coal employed amounts to 20 to 25 per cent. of the charge, or about 50 to 60 tons per day per furnace, 1 ton of coal smelting rather less than 5 tons of calcines.

Coal is charged every 40 minutes in quantities of 1½ tons at a time, from bins which extend across the entire width of the fireplace, feeding through four hoppers into openings 1 foot square in the roof of the fire-box, and the withdrawing of the gates is operated by means of levers at the platform. Over the fire-bridge are two rows of air-holes used for regulating the length and character of the flame in the furnace; the flame, however, plays a subordinate part in the smelting reactions. The coal employed is from Diamondsville, Wyoming, and gives a flame 125 feet in length, the appearance of which is gauged through the window fixed in the off-take flue, this being visible from the fire-box platform. The coal is run-of-mine quality, and considerable slack is used. It possesses a high calorific power and a large proportion of volatile constituents, but clinkers rather badly, and a clinker grate is worked with.

_Grating._—The fire rests upon 3-inch round bars placed at 4½ to 6-inch centres, and is maintained at a depth of about 27 inches. Grating requires to be conducted at fairly frequent intervals, usually twice per shift, in order to keep the fire free and to prevent channelling, which is indicated on the draft gauge by a drop from 0·75 inch to 0·50 inch, due to airing. It serves further to prevent clinkering, which, when taking place in the fire, causes a rise of from 0·75 up to 1·0 inch on the gauge. The operation of grating usually occupies about half-an-hour; the work is arduous, and the heat to which the workman is exposed is itself very trying.

_Coke Recovery._—A constant stream of half-burnt fuel and ashes falls through the bars, and during the clinkering operations large quantities are dropped. The material all falls down a bank inclined at 45°, into a channel where it is met by a stream of water which washes it along launders and through a grizzle, to a settling tank. The settled products are subsequently jigged, the recovered coke being washed over the tail-board to a trommel, and by this means 10 per cent. of the fuel charged into the furnace is recovered in a useful form. This coke is used up as a constituent of the briquettes.

TABLE VI.—DAILY REPORT—REVERBERATORY FURNACES. AUGUST 17TH, 1908 (GOOD DAY).

─────┬─────┬──────┬───────────────────────────────────────────────── │ │ │ Charge. │ │ ├──────┬─────────┬─────────┬─────────┬──────┬───── Furn-│ │ Total│ │ │ Blast │ │ │ ace │ │ Smel-│ Cal- │Macdougal│ Furnace │ Main │ │Resi- No. │ Coal│ ted │cines │Flue-Dust│Flue-Dust│Flue-Dust│Extras│dues ─────┼─────┼──────┼──────┼─────────┼─────────┼─────────┼──────┼───── │ Tons│ Tons │ Tons │ Tons │ Tons │ Tons │ Tons │ Tons │ │ │ │ │ │ │ Fine │ │ │ │ │ │ │ │ lime │ │ │ │ │ │ │ │ rock.│ 1 │ 60·6│ 288·8│ 279·2│ .. │ 8·9 │ .. │ 0·7 │ .. 2 │ 57·2│ 277·7│ 262·7│ .. │ 2·9 │ 11·8 │ 0·3 │ .. 3 │ 64·1│ 286·7│ 253·2│ 12·0 │ 8·9 │ 11·8 │ 0·8 │ .. 4 │ 60·5│ 278·7│ 264·7│ .. │ 2·6 │ 3·9 │ 0·2 │ 7·3 5 │ 57·3│ 245·9│ 221·7│ 12·0 │ 11·2 │ .. │ 1·0 │ .. 6 │ 57·3│ 273·1│ 264·4│ .. │ 7·9 │ .. │ 0·8 │ .. 7 │ .. │ .. │ .. │ .. │ .. │ .. │ .. │ .. 8 │ 57·4│ 278·7│ 266·8│ 11·9 │ .. │ .. │ .. │ .. ─────┼─────┼──────┼──────┼─────────┼─────────┼─────────┼──────┼───── Total│414·4│1929·6│1812·7│ 35·9 │ 42·4 │ 27·5 │ 3·8 │ 7·3 ─────┴─────┴──────┴──────┴─────────┴─────────┴─────────┴──────┴─────

─────┬────────┬───────┬───────────────────────┬──────┬───────┬─────── │ │ Cost │ Delays. │ │ │Ladles │ │of Coal├─────┬────────┬────────┤ │ │ of Furn-│Copper │ per │Wait-│ Waiting│ │ │ │ Matte ace │Material│ Ton │ ing │ for │ Miscel-│ Total│Boilers│ in No. │Smelted │ of │ for │Calcines│laneous │Delays│Working│Furnace │per Ton │ Metal │Coal │ │ │ │ │at End │of Coal │Melted │ │ │ │ │ │of Day. ─────┼────────┼───────┼─────┼────────┼────────┼──────┼───────┼─────── │ Tons │ $. │Hours│ Hours │ Hours │ Hours│ Hours │ 1 │ 4·77 │ 0·95 │ ─┐ ┌─ │ 24 │ 10 2 │ 4·85 │ 0·94 │ │ │ │ 24 │ 10 3 │ 4·47 │ 1·02 │ │ │ │ 24 │ 10 4 │ 4·61 │ 0·99 │ ├────── No delays. ─────┤ │ 24 │ 10 5 │ 4·29 │ 1·06 │ │ │ │ 24 │ 10 6 │ 4·77 │ 0·95 │ │ │ │ 24 │ 10 7 │ .. │ .. │ │ │ │ .. │ .. 8 │ 4·85 │ 0·94 │ ─┘ └─ │ 24 │ 10 ─────┼────────┼───────┼─────┬────────┬────────┬──────┼───────┼─────── Total│ .. │ .. │ .. │ .. │ .. │ .. │ 168 │ 70 ─────┴────────┴───────┴─────┴────────┴────────┴──────┴───────┴───────

Draft, 1·7 inches. Number of furnaces running, 7·00 All furnaces working slow. Number of charges, 140 Furnace No. 5, one bad charge. Ladles matte tapped, 34 Cupriferous material smelted per furnace, 275·6 tons.

──────────────────────────────────────────────────────────────────── DAILY REPORT—REVERBERATORY FURNACES. AUGUST 19TH, 1908. ─────┬─────┬──────┬───────────────────────────────────────────────── │ │ │ Charge. │ │ ├──────┬─────────┬─────────┬─────────┬──────┬───── Furn-│ │ Total│ │ │ Blast │ │ │ ace │ │ Smel-│ Cal- │Macdougal│ Furnace │ Main │ │Resi- No. │ Coal│ ted │cines │Flue-Dust│Flue-Dust│Flue-Dust│Extras│dues ─────┼─────┼──────┼──────┼─────────┼─────────┼─────────┼──────┼───── │ Tons│ Tons │ Tons │ Tons │ Tons │ Tons │ Tons │ Tons 1 │ 55·4│ 143·0│ 143·0│ .. │ .. │ .. │ .. │ .. 2 │ 55·4│ 246·4│ 240·1│ .. │ .. │ .. │ .. │ 6·3 3 │ 62·1│ 250·7│ 236·9│ .. │ .. │ 13·8 │ .. │ .. 4 │ 58·9│ 262·7│ 262·9│ .. │ .. │ .. │ .. │ .. 5 │ 62·2│ 247·8│ 247·8│ .. │ .. │ .. │ .. │ .. 6 │ 59·1│ 241·9│ 241·9│ .. │ .. │ .. │ .. │ .. 7 │ .. │ .. │ .. │ .. │ .. │ .. │ .. │ .. 8 │ 55·1│ 252·9│ 252·9│ .. │ .. │ .. │ .. │ .. ───────┼─────┼──────┼──────┼─────────┼─────────┼─────────┼──────┼──── Total, │408·2│1645·6│1625·5│ .. │ .. │ 13·8 │ .. │ 6·3 ───────┴─────┴──────┴──────┴─────────┴─────────┴─────────┴──────┴────

───────────────────────────────────────────────────────────────────── DAILY REPORT—REVERBERATORY FURNACES. AUGUST 19TH, 1908. ─────┬────────┬───────┬───────────────────────┬──────┬───────┬─────── │ │ Cost │ Delays. │ │ │Ladles │ │of Coal├─────┬────────┬────────┤ │ │ of Furn-│Copper │ per │Wait-│ Waiting│ │ │ │ Matte ace │Material│ Ton │ ing │ for │ Miscel-│ Total│Boilers│ in No. │Smelted │ of │ for │Calcines│laneous │Delays│Working│Furnace │per Ton │ Metal │Coal │ │ │ │ │at End │of Coal │Melted │ │ │ │ │ │of Day. ─────┼────────┼───────┼─────┼────────┼────────┼──────┼───────┼─────── 1 │ 2·58 │ 1·76 │ .. │ .. │ 8·00 │ 8·00 │ 22 │ 2 2 │ 4·45 │ 1·02 │ .. │ .. │ .. │ .. │ 24 │ 8 3 │ 4·04 │ 1·13 │ .. │ .. │ .. │ .. │ 24 │ 6 4 │ 4·46 │ 1·02 │ .. │ .. │ .. │ .. │ 24 │ 6 5 │ 3·98 │ 1·14 │ .. │ .. │ .. │ .. │ 24 │ 8 6 │ 4·09 │ 1·11 │ .. │ .. │ .. │ .. │ 24 │ 8 7 │ .. │ .. │ .. │ .. │ .. │ .. │ .. │ .. 8 │ 4·59 │ 1·19 │ .. │ .. │ .. │ .. │ 24 │ 8 ───────┼────────┼───────┼─────┼────────┼────────┼──────┼───────┼──── Total,│ .. │ .. │ .. │ .. │ 8·00 │ 8·00 │ 166 │ 46 ───────┴────────┴───────┴─────┴────────┴────────┴──────┴───────┴──── Draft, 1·7 inches. Furnace No. 1 delayed 8 hours tapping and claying. Furnace No. 7 down for repairs. Bad coal on all furnaces. Number of furnaces running, 6·67 Number of charges, 118 Ladles matte tapped, 47 Cupriferous material smelted per furnace, 246·7 tons.

_Tapping the Furnace._—Matte is usually withdrawn from these large stores upon such occasions as it is required for the converters, though sometimes when the supply has got ahead of the converters’ demands, the matte is tapped and run outside the reverberatory building, being cast into large matte-beds. The tap-holes are situated between the second and third doors, and between the fourth and fifth; and each consists essentially of a copper plate 2 inches thick and 25 inches square, which at first stands back 9 inches from the outside of the wall. Through this plate a 1-inch hole has been drilled. The tapping bar is maintained inserted up this hole, being passed through the conical clay plug which closes it. At the back of the plate is 21 inches of lining material through which the tapping-hole passes. When the copper plate shows signs of a red heat, it is an indication of the lining tending to burn through; this part of the furnace is then cooled, the plate taken out, a 9-inch layer of sand is rammed into position, and the plate is thus moved forward a corresponding distance. Such a tap-hole plate lasts for about five months.

The reverberatories are usually not tapped until they contain about 250 tons of matte. The operation of tapping is performed by withdrawing the rod by means of a wedge and ring, when the matte flows along the launders leading to the ladles for the converters; two ladles of about 8 tons capacity each are usually filled at once, each ladleful being sampled at the runner. The tap-hole is then stopped with a cone of clay, and the tapping-rod driven through it again.

Typical daily reports of the furnaces are appended in Tables VI. and VII., and a monthly report on Table VIII.

TABLE VII.—FROM DAILY ASSAY REPORT—REVERBERATORY FURNACES. AUGUST 19, 1908.

─────────────────┬─────────────────────────────── │ Per Cent Copper in Slag. Furnace Number. ├──────────┬──────────┬───────── │ Shift 1. │ Shift 2. │ Shift 3. ─────────────────┼──────────┼──────────┼───────── 1 │ 0·30 │ 0·30 │ 0·30 2 │ 0·30 │ 0·35 │ 0·25 3 │ 0·30 │ 0·30 │ 0·35 4 │ 0·45 │ 0·30 │ 0·25 5 │ 0·30 │ 0·40 │ 0·35 6 │ 0·30 │ 0·20 │ 0·20 7 │ .. │ .. │ .. 8 │ 0·35 │ 0·25 │ 0·30 ─────────────────┼──────────┼──────────┼───────── Average in slag, │ 0·35 │ 0·30 │ 0·30 ─────────────────┴──────────┴──────────┴───────── ┌─ │ SiO_{2}, 29·5 per cent. │ FeO, 37·3 " Composition of calcines ──┤ S, 7·7 " │ CaO, 2·7 " │ Copper, 8·6 " └─ ┌─ │ SiO_{2}, 39·4 per cent. Composition of slag, ──┤ FeO, 40·7 " │ CaO, 4·3 " └─ Copper in matte, 38·6 "

TABLE VIII.—MONTHLY REPORT—REVERBERATORY FURNACES. TOTAL CHARGE—ALL FURNACES.

───────────────────────┬───────┬────────────────┬──────────────── │Charge.│ SiO_{2}. │ FeO. ───────────────────────┼───────┼─────────┬──────┼─────────┬────── │ Tons. │Per cent.│ Tons.│Per cent.│ Tons. Calcines and lime rock │ 50,054│ 27·20 │13,616│ 39·40 │19,721 M‘Dougal flue-dust, │ 977│ 30·50 │ 298│ 21·90 │ 214 Blast flue-dust, │ 1,639│ 35·90 │ 588│ 22·00 │ 361 Converter flue-dust, │ 132│ 1·90 │ 2│ 6·60 │ 9 Main flue-dust, │ 1,034│ 30·2 │ 312│ 17·80 │ 184 ───────────────────────┼───────┼─────────┼──────┼─────────┼────── Total, │ 53,836│ .. │14,816│ .. │20,489 ───────────────────────┼───────┼─────────┼──────┼─────────┼────── Matte to converter, │ 10,950│ .. │ .. │ 36·70 │ 4,019 Matte chips to B.F., │ 74│ 8·20 │ 6│ 38·10 │ 28 Slag chips to B.F., │ 609│ 39·50 │ 241│ 37·40 │ 228 ───────────────────────┼───────┼─────────┼──────┼─────────┼────── Deduct from above │ │ │ │ │ total, │ 11,633│ .. │ 247│ .. │ 4,275 ───────────────────────┼───────┼─────────┼──────┼─────────┼────── Leaves for slag, │ .. │ .. │14,569│ .. │16,214 ───────────────────────┴───────┴─────────┴──────┴─────────┴──────

───────────────────────┬────────────┬─────────────┬───────────────── │ Lime. │ Sulphur. │ Copper. ───────────────────────┼──────┬─────┼──────┬──────┼───────┬───────── │ Per │Tons.│ Per │ Tons.│ Per │ Lbs. │ cent.│ │ cent.│ │ cent.│ Calcines and lime rock,│ 2·30 │1,150│ 8·40 │ 4,205│ 8·266 │8,274,799 M‘Dougal flue-dust, │ 1·30 │ 13│14·00 │ 137│ 7·884 │ 152,295 Blast flue-dust, │ 4·30 │ 70│ 6·70 │ 110│ 5·698 │ 186,782 Converter flue-dust, │ .. │ .. │12·10 │ 16│68·743 │ 181,482 Main flue-dust, │ 2·10 │ 22│ 8·80 │ 90│ 7·128 │ 147,405 ───────────────────────┼──────┼─────┼──────┼──────┼───────┼───────── Total, │ .. │1,256│ .. │ 4,558│ 8·305 │8,942,763 ───────────────────────┼──────┼─────┼──────┼──────┼───────┼───────── Matte to converter, │ .. │ .. │26·40 │ 2,891│38·209 │8,367,872 Matte chips to B.F., │ 0·20 │ .. │21·80 │ 16│32·811 │ 48,560 Slag chips to B.F., │ 2·30 │ 14│ 2·20 │ 13│35·597 │ 43,357 ───────────────────────┼──────┼─────┼──────┼──────┼───────┼───────── Deduct from above │ │ │ │ │ │ total, │ .. │ 14│ .. │ 2,920│ .. │8,459,789 ───────────────────────┼──────┼─────┼──────┼──────┼───────┼───────── Leaves for slag, │ .. │1,242│ .. │ .. │ .. │ .. ───────────────────────┴──────┴─────┴──────┴──────┴───────┴─────────

Analysis. ┌─────────┴──────────┐ _Slag Calculation_─ │Calculated. Actual.│ SiO_{2} in slag, 14,569 ÷ 38,538 37·8 37·1 FeO " 16,214 ÷ 38,538 42·1 43·2 CaO " 1,242 ÷ 38,538 3·2 2·8 ────── ────── ────── ───── 32,025 at 83·17 = 38,538 83·10 83·10 ═══════════════ ══════ ═════

=Fuels for Reverberatory Furnace Work.=—The chief requirements of the fuel for good reverberatory work will now be apparent, particularly with regard to length of flame. This depends to a large extent upon the proportion of volatile hydrocarbons, but also on the conditions under which they are given off. For instance, a coal which rapidly parts with its hydrocarbons and leaves in the grate a dense layer of slow-burning coke would be unsuitable for reverberatory work, though some caking is necessary in order that the fuel should not burn away too rapidly, as it should yield a good bed of the required depth.

The great success of large reverberatory furnaces worked under suitable conditions, has had the tendency to tempt smelters in different parts of the world to erect furnaces of similar size independently of the character of the available fuel, and in several cases results have been unsatisfactory, at least in the earlier stages.

These preliminary failures have, however, served the purpose of developing the adaptation of other fuels for this work, and from the employment of oil for the purpose, important extensions in practice will undoubtedly develop in the future of reverberatory furnace working.

The device of using pulverised coal as a fuel has attracted attention at several smelters where the local coal as mined was proved to be unsuitable for use. In practice, however, the method has, up to the present, given unsatisfactory results, for although a longer flame and higher temperature have been obtained in the furnace, difficulties in working have arisen which appear to bar its use. One of the chief drawbacks has been due to the fine ash from the fuel, which is deposited in the flues in large quantities and even causes considerable slagging in them, impeding the working of the furnace and preventing the recovery of heat from the furnace gases. Further difficulty, though not quite so serious, was caused by the dust being blown upon the charge and tending to settle upon it; forming a non-conducting blanket which retarded the melting of the material by the flames. The method does not appear at present to offer much promise of extended application to copper smelting.

_Oil Fuel in Reverberatory Practice._—The successful application of oil as a fuel marks a useful advance in reverberatory practice, particularly in connection with the working of large furnaces.

On several of the smaller plants, oil fuel has been in use with considerable success for some time, but within recent years the building of large-sized furnaces without having at hand suitable coal resources has led to attempts to employ oil in its place, and the preliminary difficulties appear to have been to a large extent successfully overcome. The work at the Cananea Smelter with oil fuel, and the discussion on Ricketts’ first report of his experience, afford valuable indications of the possibilities of this method. Working on charges consisting to a large extent of flue-dust, several thousand tons of material have been smelted in furnaces yielding 245 tons daily output, at a cost which compares very favourably with that of ordinary practice. This success is particularly noteworthy in view of certain features in the preliminary system of working which will doubtless be altered at no very distant date, and of the fact that flue-dust is sometimes a difficult material to melt in a reverberatory furnace, even when good coal is available as a fuel.

The chief difficulties in working appear to have been largely in connection with the regulation of the flame and the management of the oil-burners. In endeavouring to obtain the requisite high temperature over the entire length of the furnace-hearth, an intense local action was caused near the place where the oil in the form of a spray entered the furnace, resulting in the burning out of the roof-arch on several occasions. These difficulties will doubtless be overcome with further experience in the design and management of the burners constructed for this class of work.

At Cananea, four oil burners of the Shelby type are employed on each furnace, and this form is stated to project the flame further into the furnace, and to prevent its impinging on the roof, more successfully than the other types tried. The waste heat fires three Stirling boilers of 664 H.P. Less than one barrel (42 gallons, or 310 lbs.) of oil is consumed per dry ton of charge, and of this quantity 0·43 barrel is chargeable to steam-raising under the boilers. The manner of working the charges, and the furnace construction in other respects, follow very closely the methods of operation already described.

=Costs of Oil-fired Reverberatory Working.=—Ricketts has contributed a useful analysis of the costs of reverberatory work using oil as fuel, under the conditions prevailing at Cananea, Mexico. He noted that the use of too much oil should be avoided. This precaution led to a decrease in the amount of repairs necessary. 550 barrels of oil were required to get the furnace into fairly good condition, and 8 barrels per furnace per hour to keep it going well. It is hoped ultimately to reduce the oil consumption to 0·8 barrel gross per ton of charge.

Analysis of Oil-fired Reverberatory Furnace Costs—Cananea—February to July, 1911, inclusive.

Furnace Days, 312·5.

TONNAGE CHARGED— Dry Tons. Per cent. of Total. Flue-dust, 21,019 34·99 Calcines, 35,533 59·15 Ores, 3,040 5·06 Limestone, 479 0·80 —————— —————— 60,071 100·00 ====== ======

DISTRIBUTION OF COSTS— Amount. Per Dry Ton. Operating expenses, $111,687·17 $1·8593 Slag and matte expense, 5,111·07 0·0851 Boiler-house, 11,468·77 0·1909 General expense, 4,218·58 0·0702 Cost of flux, 817·46 0·0136 ————————- ——————- $133,303·05 $2·2192 Steam credit, 48,861·86 0·8134 ————————- ——————- Operating cost, $84,441·19 $1·4057 =========== =======

ANALYSIS OF COSTS—

(1) _Operating_— Amount. Per Dry Ton. Labour, $17,829·42 $0·2968 Power, 592·36 0·0099 Fuel oil, 88,028·99 1·4654 Coal, 243·61 0·0041 Water, 91·68 0·0015 Transportation, 380·45 0·0063 Sundries, 315·64 0·0053 Flux, 817·46 0·0136 ————————- ——————- $108,299·61 $1·8029 =========== ======= (2) _Repairs_— Labour, $11,063·93 $0·1842 Supplies, 12,425·30 0·2068 Shop expense, 1,514·21 0·0252 ———————— ——————- $25,003·44 $0·4162 ———————— ——————- Total, $133,303·05 $2·2191 Steam credit, 48,861·86 0·8134 ————————- ——————- Net total, $84,441·19 $1·4057 =========== =======

_Gaseous Fuel._—The proposal to employ gaseous fuel in copper smelting dates from the introduction of this method of furnace-firing by Siemens 50 years ago. It is, however, not in general use, although at several smelters gas-firing is employed in furnaces for the refining of the metal.

The chief difficulties have been in connection with the control of the flame, burning-out of the roof having been a not infrequent occurrence when employing gaseous fuel, and the method has been tried and given up at the Great Falls Smelter in Montana, and at several other works.

The practical difficulties ought not, however, to be insuperable should gas-firing be otherwise found most practicable for the particular conditions at the smelter, although there appear to be certain physical characteristics of such flames which may be responsible for some of the difficulties met with in employing this type of fuel for the working of very large reverberatory furnaces.

=The Condition of the Charge for Good Reverberatory Work.=—The considerations which decide the advisability or otherwise of installing at a smelter, any particular types of furnace, whether reverberatory or blast furnace or both, cover a very wide field, and will be more apparent when blast-furnace practice has been reviewed in detail. It is clear that the blast furnace is unsuited for the direct smelting of fine materials as such, and that the reverberatory form of furnace is best fitted for their treatment when large quantities of this material require to be dealt with. Actual practice has shown, however, that the reverberatory does not give equally satisfactory results on all classes of fines, and that there are certain physical and chemical conditions of the charge which appear to be necessary for the most successful and rapid smelting. When such conditions are not adhered to, less satisfactory working has resulted. Recent experience has, to some extent, defined more clearly the nature of these requirements, and has indicated the procedure which is necessary in order to avoid an undue supply of the less suitable material for the reverberatory charge.

It is usual to smelt in the reverberatory furnaces, where such are available, the greater portion of the dust which accumulates in very large quantities in the flues at the smelter. The reverberatory is the only type of furnace in which such material could be treated directly, under the present conditions of working. In practice, however, it has been found in several instances, though not universally, that such dust is considerably more difficult to treat in the furnace, and entails considerably more expense in smelting than does the ordinary roasted concentrate. It is estimated by Ricketts that this extra cost is practically equivalent to the expense of roasting an equal weight of concentrate.

Flue-dust, as a rule, consists mainly of material in a minute state of division, in which condition, as is well known, a much higher temperature is required for its fusion than if it were in the form of coarser particles. This is largely due to the poor conductivity for heat which generally characterises such dust, and to the insulation by the air envelopes surrounding the individual grains, which thus prevents the heat passing from particle to particle, and retards their clotting, even when the prevailing temperature would otherwise be sufficient to cause fusion. The particles of flue-dust moreover, have been blown from the surface of the charge, especially in the blast-furnace process, and are thus rapidly and often almost completely oxidised in passing through the oxidising atmosphere which prevails above the charge and in the flues. Such oxides clot only with the greatest difficulty, and are characterised by comparative infusibility and poor conducting power, and hence are found to melt with considerable difficulty when treated in the reverberatory furnace.[10]

Roasted fine concentrate, on the other hand, constitutes an ideal material for the reverberatory furnace charge, and the system of passing both the concentrate and the flux through the roasters has been shown to possess numerous advantages. In addition to the thorough mixing and the preheating of the furnace charge, it was found that its chemical and physical conditions were particularly well suited for the subsequent reverberatory furnace treatment. The particles of concentrate, being gradually heated and constantly stirred in the presence of the small proportion of flux usually required, roast well, and lose the desired quantity of sulphur without an undue amount of preliminary clotting which would otherwise interfere with the operation, whilst any residual sulphide in the product is uniformly distributed through the roasted charge. In addition, at the higher temperatures which prevail in the later stages of the roasting process when almost as much sulphur as was desired has been driven off, the materials are raised to a point approaching incipient fusion and slagging. The heat in the reverberatory furnace is sufficient to complete this effect, and enable the necessary chemical combinations and physical separations to be readily accomplished.

The roasted concentrate should therefore form the main proportion of the reverberatory charge, working in with it, in moderate quantities, such flue-dust as is made at the smelter. Of this flue-dust, it is naturally desirable to produce as small an amount as possible, not only on account of the difficulties in subsequent treatment, but also on account of the actual losses in the economy of the furnace processes and the cost of rehandling, etc. In modern smelting, naturally, every effort is made to reduce the quantity of dust to the lowest practicable limit.

The greater portion of the dust results from the treatment of unsuitably fine material in the blast furnace, and by decreasing the quantity of this constituent the flue-dust problem will be largely overcome. The smelting of fine concentrate in the blast furnace has up to the present been considered judicious where circumstances have rendered imperative the addition of sulphides to the charge irrespective of their physical condition (either to act as a base for the matte, or on account of their fuel values), though naturally the proportion of fines has been kept as low as possible.

The recent developments in sintering processes, however, suggest the possibility of the future successful treatment, after preliminary agglomeration, of fine concentrate in the blast furnace, and if it be found possible to conduct the sintering by utilising the heat of oxidisation of the more free sulphur atom of the pyrites, and thus leave the bulk of the iron-sulphide fuel values in the sintered product, as suggested by Peters, the difficulties in connection with excessive flue-dust production from the above causes will be largely overcome, and the reverberatories will thus be relieved of this difficult constituent of their charge.

It therefore appears desirable, when circumstances permit, either to agglomerate fine concentrates and then treat them in the blast furnace, or else to roast them and smelt the product in the reverberatories.

So far as present experience has gone, it appears that—other circumstances being equally favourable—the correct scheme of treatment depends almost entirely upon the composition of the concentrate, there being for each process a particular class of fines for which it is best suited. The sintering process deals most satisfactorily with one class of concentrate, whilst the roasting process seems more particularly suited for a different type of material.

Thus the higher the iron and sulphur values, and the lower the silica content, the more successful, cheap, and efficient is the roasting process—the Anaconda material for example roasts well, requires practically no external fuel or heating, and with the added flux, works very successfully in the reverberatories.

As the silica content increases, however, and the iron and sulphur contents diminish, there is a consequent decrease in the natural fuel values of the material, and as a result, the roasting is neither so efficient nor so cheaply operated, owing to the need of external fuel for giving the required roasting temperatures. On the other hand, it appears to be just this class of material which is best suited for blast-roasting.

It is found in actual working practice that material which does not contain a certain proportion of silica does not work well in the blast-roasting or sintering processes, the resulting product being found to be more irregular in composition and more difficult to operate in the sintering plant. It would therefore appear that a certain class of fine concentrate higher in silica and lower in iron and sulphur contents, which is not quite so suitable for ordinary roasting (owing to the necessity for external heating, due to lower fuel values) is eminently suited for blast roasting or sintering processes, yielding lump products very suitable for subsequent blast-furnace treatment.

The reverberatory furnace thus deals most successfully with fine table concentrates high in iron and sulphur, moderately low in silica; roasted, with its required flux, to the necessary extent, and then charged whilst still red hot into the furnaces. To relieve the reverberatories of the greater bulk of the blast-furnace flue-dust, which it treats with more difficulty, fine concentrates, as such, require to be kept out of the blast-furnace charge, either by subjecting the more siliceous material to a preparatory sintering process, or by reserving the highly pyritic variety for roasting and subsequent reverberatory treatment.

References.

Peters, E. D., “Principles of Copper Smelting.” Offerhaus, C., “Modern Reverberatory Smelting of Copper Ores.” _Eng. and Min. Journ._, June 13, 1908, pp. 1189–1193; June 20, 1908, pp. 1234–1236. Ricketts, L. D., “Experiments in Reverberatory Practice at Cananea, Mexico,” and discussion, _Trans. Inst. Min. and Met._, vol. xix., 1909–10, pp. 147–185. Ricketts, L. D., “Developments of Cananea Practice.” _Engineering and Mining Journal_, Oct. 7th, 1911, p. 693.

LECTURE VI.

BLAST-FURNACE PRACTICE.

Functions of the Furnace — As Melting Agent—Reduction Smelting — Oxidation in the Furnace — The Pyritic Principle — Features of Modern Practice: Water-Jacketing, Increase in Furnace Size, External Settling — Constructional Details of the Furnace.

=The Functions of the Blast Furnace.=—The functions of the blast furnace may be considered from three points of view:—

=1. As a Melting Agent.= =2. As a Reducing Medium.= =3. As an Oxidising Medium.=

In modern copper smelting practice, the blast furnace is under ordinary circumstances never employed in the capacity of a reducing medium, but is used for a variety of work in which its operations range from those of a melting furnace to those more particularly of an oxidising medium, as its oxidising functions are becoming developed to a gradually increasing extent.

In the older processes of copper smelting, when working on oxidised charges, the melting and reducing functions of the furnace were exercised simultaneously; when, at a later stage, sulphides were smelted in the charge, the directly reducing function was utilised to a very much smaller extent. In the reducing atmosphere then maintained inside the furnace, the sulphides liquated and melted down without causing much concentration of the copper in the product, elimination of sulphur being effected mainly by the direct action of heat on the pyritic constituents of the charge, and by the interactions between the sulphides and the oxidised compounds of copper present.

When, however, increasing quantities of sulphide ore became available, modifications in blast-furnace smelting practice were introduced with a view to increasing the concentration of the copper, this being attempted either by preparatory roasting or by the addition of oxidised cupriferous materials to the charge, sulphur being thus eliminated and some concentration resulting in consequence. In such work the furnace chiefly exercised its melting function, allowing, as in the case of reverberatory working, of the formation and thorough fusion of sulphide matte and silicate slag from the mixture of oxides and sulphides in the charge. In the latest developments of practice, the oxidation has been carried out to a continually increasing extent by the air blast at the tuyeres of the furnace.

1. =The Melting Functions of the Blast Furnace.=—The blast furnace is under ordinary circumstances, usually regarded as the cheapest of melting agents. Compared with the reverberatory, the heat in the blast furnace is utilised more efficiently. Reverberatory working involves the passing of a flame over the surface of the charge, and the transference of this heat through the mass depends upon the conducting power of the material itself, which is, however, usually poor. Although the modern reverberatory practice of melting thin layers of preheated charge both from above and from below has greatly increased the efficiency of the furnace in this respect, the closer contact of charge and fuel in the blast furnace allows of a more thorough communication of the heat.

The principal features of blast-furnace working which tend to make it the cheaper and more efficient agent for the treatment of cupriferous materials—with the exception of fines—are those of construction, working, and fuel economy.

(_a_) The construction of the furnace is comparatively simple, and it is not excessively expensive to erect; furnaces and accessory plant can be purchased complete and easily set up and taken down again when required. (_b_) The furnace is elastic in its operation, especially where the supply of material varies from time to time, involving changes in the composition of the charge. (_c_) The furnace is readily started, shut down, and restarted at will, and without much difficulty or additional expense. (_d_) The operation and smelting are rapid and cheap, the capacity can be made enormously large; all classes of material—except fines—such as ores, slags, and residues, which accumulate to a considerable extent round a smelter, can be conveniently dealt with directly, whilst fines can now, where necessary, often be prepared into a suitable form for blast-furnace treatment. (_e_) The heat is more efficiently communicated to the individual parts of the charge, in consequence of the more intimate contact of charge and fuel. (_f_) The fuel consumption is low, the natural fuel values of the iron and sulphur on the charge can be utilised, and the degree of oxidation (and consequent concentration) can be controlled in the furnace operation. (_g_) The furnace works continuously (in modern practice the reverberatory furnace is also continuous in its action).

Owing to the great elasticity in blast-furnace operation, and its capability of dealing with practically every class of copper-bearing material in lump form, modern practice is of the most diverse character.

2. =The Blast Furnace as a Reducing Medium.=—In modern smelting practice, with but a few exceptional instances, a distinctly reducing atmosphere is avoided as far as possible. This arises largely from the fact that the material available in modern work usually demands oxidation in order that satisfactory concentration may be effected.

In the early days of copper smelting, however, the reducing action was the chief function which was exercised, mainly because at that time oxidised ores constituted an important part of the charge, and a reducing action was required to obtain marketable products from such material. At a later stage in the development of blast-furnace practice, the sulphide ores which became available were roasted, and the resulting oxidised products were subjected to reduction smelting, in order to extract the metal. On such oxidised charges, blast furnaces were almost universally employed, using carbonaceous fuel either in the form of coke or charcoal, this material fulfilling the double purpose of fuel and reducing agent, the excess carbon causing the reduction of the metal from the oxidised ore.

This operation was known commonly as “black-copper smelting.” At the present time such oxidised ores are rarely met with in sufficient quantity by themselves to be worked by this method, which involves also very serious losses in operation. Further, such oxidised materials are in many cases valuable for smelting along with sulphide charges, greatly assisting the concentration, and it is usually advantageous to employ them in this manner.

The losses and difficulties in “black-copper smelting” are, however, of interest in so far as they apply to certain analogous problems in modern work. These difficulties in reduction smelting arose largely from three causes:—

(_a_) Losses of copper in the slag. (_b_) Simultaneous reduction of iron with the copper. (_c_) Chilling in the furnace hearth.

(_a_) In the case of reduction smelting where sulphides are not present in any appreciable quantity, the losses of copper may be either

(i.) As silicate, or (ii.) As metal.

(i.) Sulphur is the natural protector of the copper in the furnace charge, as, owing to their powerful affinity, a fusible, fluid and dense product is formed, which is very slightly soluble in slag; and on this account, a ready separation of the copper from the earthy materials can be effected. So long as sulphur is present in moderate quantity there is little chance of copper entering the slag as silicate.

In reduction smelting, however, and especially in black-copper smelting where sulphur is lacking, such losses are liable to occur, since copper oxide is itself strongly basic, and readily fluxes off with silica at high temperatures, yielding silicates. These products are less dense, and are markedly soluble in the other silicates which constitute the slag; moreover, the copper oxides themselves are likewise partly soluble in, and are readily carried in suspension by, the silicate slags.

In order to prevent such losses as much as possible, the reducing conditions in the furnace must be increased by the employment of more coke, so as to ensure the reduction of the copper oxides and silicates. These reducing conditions must not, however, be too drastic, especially if the temperature of working be high, on account of the great tendency to cause (_b_) a reduction of metallic iron, which results in the formation of bears and scaffolds, with their attendant difficulties of removal and their interference with working.

Between these opposing causes of loss and difficulty, a careful balance has to be observed in the smelting operations. (In modern practice, losses of copper as silicate and oxide, for reasons such as those detailed above, occur to a marked extent in those operations where the sulphur is present in small proportions only, and particularly where the reactions are intensely oxidising, as in the furnace-refining operations and the later stages in the converter process. The slags in such cases usually carry considerable quantities of copper in the form of silicate and oxide, not infrequently to the extent of 20 to 30 per cent., or even more. The quantity of this slag is, however, kept as small as possible, and copper in the material is readily recovered by the addition of these slags to the blast-furnace charge.)

(ii.) Losses of copper as metal also, were formerly serious in black-copper smelting, the metallic copper held in suspension in the slag being indeed the chief source of loss in this method. The efficient separation of copper from slag, especially in the small quantities formerly operated, was therefore of importance. Satisfactory settling was, however, difficult of application, since the behaviour of metallic copper is very different from that of sulphides. It is much less fusible, much less fluid, and the small globules, as reduced, do not readily coalesce, whilst the high temperatures favourable to good fluidity of the products and to good settling, promote copper losses from the other causes noted above.

Moreover, the high melting point of the metal and its great conductivity added to the difficulties in providing suitable arrangements for settling, since the copper not only tended to chill readily in any external settler, but it was also very liable to do so in the crucible of the ordinary form of water-jacketed blast furnace, such masses being exceedingly difficult to remove, whilst the working of the furnace was necessarily much interfered with.

In order to conduct the necessary internal settling, the older type of blast furnace was required, in which water-jacketing near the hearth was dispensed with, a large crucible bottom of non-conducting brasque or brickwork being employed instead. Such a form of furnace is not adapted to the modern methods of smelting where enormous capacity and output are essential, whilst such a system of working interferes with the rapid and continuous smelting of large quantities, to a greater extent than if the whole of the molten products are run out of the furnace continuously and the settling performed in an external vessel.

3. =The Blast Furnace as an Oxidising Medium: Sulphide Ores in the Blast Furnace.=—In modern blast-furnace practice, the oxidising function of the furnace is the principal feature of working. Sulphide ores now constitute the chief source of copper, and the smelting operations involve the oxidation of the accompanying constituents and the elimination of the resulting oxidised products.

Such ores when smelted in the blast furnace with carbonaceous fuel, and under the reducing conditions characteristic of the older methods of working, would yield a product showing low concentration of the copper, since the reducing conditions would largely retard the oxidation of sulphur which is an essential for the enrichment of the matte. Except for the sulphur eliminated from the pyritic constituents by the direct action of heat, and a certain quantity by the interactions with oxides as already indicated, the loss of sulphur would be slight. The furnace under such circumstances would thus tend mainly to exercise its melting function, and the result of such working would be the melting down and subsequent separation of the sulphides and slag, with even less tendency to concentration than occurs in the reverberatory furnace, where the atmosphere is less distinctly reducing.

The modern method of smelting sulphide ores being essentially an oxidising process, it is necessary that oxygen be added to the charge with the object of promoting the elimination of the sulphur and iron, and the consequent concentration of the copper.

This oxygen may be added in one of three ways:—

A. Addition of oxygen to the charge previous to the blast furnace smelting operation (_Roasting_).

B. Addition of oxygen to the charge during the smelting operation itself.