Table VII — From Daily Assay Report — Reverberatory Furnaces. August 19, 1908
| Furnace Number. | Per Cent Copper in Slag. | ||
|---|---|---|---|
| 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 |
| Composition of calcines | SiO2, | 29·5 | per cent. | |
| FeO, | 37·3 | ″ | ||
| S, | 7·7 | ″ | ||
| CaO, | 2·7 | ″ | ||
| Copper, | 8·6 | ″ | ||
| Composition of slag, | SiO2, | 39·4 | per cent. | |
| FeO, | 40·7 | ″ | ||
| CaO, | 4·3 | ″ | ||
| Copper in matte, | 38·6 | ″ |
TABLE VIII.—Monthly Report—Reverberatory Furnaces.
Total Charge—All Furnaces.
| Charge. | SiO2. | 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 cent. | Tons. | Per cent. | Tons. | Per cent. | Lbs. | |
| 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. | ||
| SiO2 | 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.