Chapter C: THE MACDOUGAL TYPE.—The most important form of modern roaster (2)

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occupied being very short, the side door is closed, and sealed with sand; the covers to the holes in the roof are now withdrawn, the gates closing the hoppers pulled back, and first the 5-ton, then the 10-ton charge is dropped into the furnace.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.

i. By adding oxidised materials to the charge
(_Blast-furnace smelting with carbonaceous fuel_).
ii. By using the air blast of the furnace for
oxidising the iron and sulphur, thus at the
same time utilising these elements as fuel
and proportionately diminishing the amount of
carbonaceous fuel required (_The pyritic principle
of smelting_).

A. _Roasting practice_ has already been discussed, and the reasons for avoiding the operation where practicable, on account of the expenses of an extra process, the losses involved, the fineness of the product, and the loss of fuel values, have been indicated (Lecture IV., pp. 66–80).

B. i. _Addition of Oxidised Charges in the Blast Furnace._—The tendency for oxidised cupriferous materials to interact with sulphides finds useful application in copper smelting, since it assists the concentration of the copper in the resulting mattes. The principal reactions involved in this method are—

2CuO + Cu_{2}S ➡ 4Cu + SO_{2}
2Cu_{2}O + Cu_{2}S ➡ 6Cu + SO_{2}
CuSO_{4} + Cu_{2}S ➡ 3Cu + 2SO_{2}

whereby copper is produced and sulphur is eliminated as SO_{2}. The liberated copper interacts with the excess of iron sulphide usually present in the furnace charge, and enters the matte as sulphide, whilst the iron which is thus set free is oxidised and carried into the slag as silicate, the ultimate reactions being indicated approximately by the equation—

2Cu + FeS + _x_FeS ➡ Cu_{2}S . _x_FeS (matte) + Fe
(oxidised and enters slag).

Copper silicates readily interact with iron sulphides in the charge, producing copper sulphides and iron silicates, thus—

Cu_{2}O . _x_SiO_{2} + FeS ➡ Cu_{2}S (enters matte)
+ FeO . _x_SiO_{2} (enters slag).

6(CuO . _x_SiO_{2}) + 4FeS ➡ 3Cu_{2}S (enters matte)
+ 4(FeO . _x_SiO_{2}) + 2_x_SiO_{2}(enters slag) + SO_{2}.

All the above reactions lead to an enrichment of the matte in copper contents, and at the same time, to the transference of iron from the matte to the slag, and although the conditions in the more reducing atmosphere of the coke-fed blast furnace are not so favourable to the fullest operation of these reactions as are the more neutral conditions of the reverberatory, the addition of oxidised materials constitutes a valuable means of increasing the concentration in this method of smelting.

The blast furnace is thus also particularly suited for the recovering of the copper from the oxidised residues, such as converter slags and scrap, “calcine-barrings,” and the like, which accumulate in very considerable quantities at a smelter, and which by reason of their carrying much copper as oxide or silicate, not only add their quota of copper to the products, but materially assist the concentration and the furnace operation generally.

B. ii. _The Pyritic Principle in Blast-Furnace Smelting._—This is the most important principle introduced into modern blast-furnace smelting practice.

It has been evolved by the application of the results of experiments conducted from two different points of view—one series mainly on a laboratory scale, the other from actual industrial practice.

Starting from theoretical considerations, John Holway demonstrated by experiment that the heat of oxidation of the iron and the sulphur of pyritic copper ores was so great as to make their smelting a self-supporting operation under suitable conditions. On the other hand, within comparatively recent years, smeltermen as a result of working practice, have found that an increase of sulphides on the furnace charge has led to less and less carbonaceous fuel being necessary for the smelting operations, providing that the conditions in the blast furnace be sufficiently oxidising.

In utilising these results for general blast-furnace practice, the extended and successful application of this pyritic principle has led to marked advance in modern working.

The results obtained in a series of trials at the Keswick smelter, California, are typical of such experiments on a practical scale, and in spite of the two anomalous instances, the general effects of the increase of sulphides in the charge are strongly marked (see Table IX., p. 120).

TABLE IX.—EFFECT ON COKE CONSUMPTION OF INCREASED SULPHUR IN THE FURNACE CHARGE (Keswick Smelter, Cal.).

┌────────────────────┬───────────────────┐
│ Sulphur in Charge. │ Coke Consumption. │
├────────────────────┼───────────────────┤
│ 6·8 per cent. │ 15·7 per cent. │
│ 7·7 " │ 16·3 " │
│ 13·6 " │ 10·2 " │
│ 17·0 " │ 7·7 " │
│ 19·5 " │ 8·5 " │
│ 22·8 " │ 7·1 " │
│ 24·5 " │ 6·8 " │
└────────────────────┴───────────────────┘

Recent practice at Anaconda affords another instance of the utilisation of the pyritic principle. A large quantity of the ore available (known as second-class ore) requires wet dressing before it can be treated most profitably at the furnaces, and the operation thus produces considerable quantities of sulphide concentrate, of which a moderate proportion is coarse—well suited for blast-furnace treatment. The charge if submitted to reduction smelting with carbonaceous fuel, would yield a matte too low in copper contents for immediate converter treatment, since there is not available a sufficient supply of oxidised cupriferous material to effect a high enough concentration for the direct production of a converter-grade matte. Instead of roasting so as to reduce the sulphur contents to the required degree, and then smelting with the usual amount of carbonaceous fuel, the pyritic principle has been utilised to the fullest possible extent, by smelting the raw charge containing as much of the coarse concentrate as is available, with a strongly oxidising blast, thus effecting the desired concentration, and occasioning the use of a lower coke proportion than would otherwise have been necessary. By gradually increasing the sulphide on the charge until the sulphur proportion reached 8 to 9 per cent., the coke consumption was reduced to about 11 to 12 per cent. During the past two or three years the advantages of introducing more and more sulphide have become so apparent, that increasing quantities of ⅜ inch concentrates are being included in the charge, and although such material is exceedingly difficult to deal with in the blast furnace, the advantages arising from its use outweighs the trouble it causes in actual working. By this further increase of the sulphur proportion, from the former 8 to 9 per cent. up to 11 to 12 per cent., the coke consumption has been steadily reduced until it now amounts to about 9 per cent. only.

The fuel value of the iron and sulphur is augmented at a rate much greater than their actual increase in numerical proportion would suggest, on account of the much higher calorific intensity of large and massive quantities of fuel burned at once than that resulting from smaller amounts disseminated throughout a mass of inert material such as gangue.

The practical application of the pyritic principle to blast-furnace practice thus involves the employment of the furnace as a medium for conducting the required oxidation of the charge, as a result of which, the heat of this combustion proportionately reduces the amount of carbonaceous fuel required for the smelting and separation of the products, whilst at the same time the desired concentration is also effected. The basis of such working is, therefore, the powerful oxidising action within the furnace itself, and the fullest utilisation of the heat resulting from this oxidation of the sulphides.

In order to supply the heat necessary for the reactions and fusions of smelting, a definite quantity of fuel is essential in the furnace. In those cases where the proportions of sulphide are not sufficient to supply the required amount, a supplementary quantity of coke fuel becomes requisite.

The extent to which coke is necessary for the smelting operations decides whether the process may be termed “true pyritic” or “partial pyritic” smelting. In the former case, the coke allowance may be reduced to such small proportions that its influence in the smelting zone of the furnace is practically negligible.

In partial pyritic smelting, coke is necessary to the extent of supplementing the heat derived from the sulphide fuel, and the proportion employed in modern work is reduced to the lowest possible quantity. Not only is economy in coke allowance one of the chief essentials in furnace management, but the presence of a larger amount than is absolutely necessary decreases the efficiency of the smelting operations, since, owing to its reducing action and its consumption of the oxygen in the air blast which is to be utilised for the combustion of the iron and sulphur, the concentration of the copper in the resulting matte would be decreased.

The extent to which the pyritic principle may be operated in actual working depends in the first instance upon the nature of the charge itself, especially upon the relative proportions of copper, iron, and sulphur, and on the quantity of gangue. Since these vary in the ore supply of different localities, the extent to which the principle may be applied and the coke consumption be reduced, will be subject to alteration accordingly.

Thus in the case of an ore which contains such proportions of these constituents as would on simple melting yield a matte of converter grade, the pyritic effect in the furnace would necessarily be very small, and the smelting would be almost entirely a melting operation requiring from 10 to 15 per cent. of coke on the charge, even though the sulphur contents of the charge be high. Ores and charges of such a composition are, however, rarely met with in modern practice, the ratio of copper to iron sulphides usually being low.

On the other hand, in the case of an ore consisting largely of iron sulphides with but little copper—_i.e._, a massive low-grade pyritic ore—the pyritic effect in the furnace might reach a maximum, and the coke required on the charge be reducible to very small proportions. Such material is well suited for true pyritic smelting.

Hence modern practice ranges from the true pyritic smelting, where pyritic fuel is principally employed, through varying degrees of partial pyritic smelting, where the pyritic fuel is supplemented to the required degree by coke, to reduction smelting, relying mainly on carbonaceous fuel for the necessary heat supply.

In all cases, the object of the operation is to oxidise inside the furnace so much sulphur and iron as is necessary to yield a matte product of converter grade, utilising the natural sulphide fuel values of the material so as to reduce to the lowest possible proportion the quantity of coke required.

=Features of Modern Practice.=—Apart from the applications of pyritic smelting, which will be considered separately, three features of great importance have been introduced into modern blast-furnace working. These involve:—

A. The practice of water-jacketing the furnace.
B. The development in the size of the furnace.
C. The practice of external settling.