Semipyritic smelting is so called because, to a certain extent, the sulphur in the ore is utilized as fuel; this means that the coke in the charge can be kept much lower than in a real reducing fusion. It depends on the fact that the copper will go with what sulphur is left, even though a portion of the sulphur is burned out by the blast. Furnaces operating on such a basis commonly have a very large volume of air blown in and there is no necessity for a high ore column, the flames often playing entirely through the charge.
In the semipyritic type of sulphide reduction, there is the greatest development of sulphide copper smelting. This constitutes a very great use for the blast furnace, and at one plant smelting lump copper ores in this way we have the largest blast furnace ever constructed, which will treat some 3,000 tons of charge in 24 hours.
Semipyritic smelting is the great blast-furnace process in the western States and likely will continue long to be so, in so far as lump sulphide-copper ore is available. It used to be of relatively more importance before the development of reverberatories. The blowing of fine particles of the charge out of the furnace to make flue dust always was troublesome and now is taken care of entirely by smelting concentrates and fine materials in reverberatory furnaces.
Fig. 34. Copper Smeltery at Cannen, Mexico Coutrsey of the "Mining World".
A large plant for this sort of smelting, with all the supplementary facilities for handling the ores, flue dust, fumes, and gases, the converting of the matte, and the necessary power production, constitutes quite a metallurgical community. Such plants are situated at Anaconda, and Great Falls, Montana; Garfield, Utah; Kennett, California; Clarksdale and Douglas, Arizona; and Cananea, Mexico.
A typical one is seen in the illustration, Fig. 34, which shows the Cananea smeltery from the hills above the plant. From a distance the conspicuous objects are the ore bins, and bedding plant, the mills, the huge stacks, the enormous flues, and the multitudinous buildings, each for a specific operation in the plant.
Pyritic smelting utilizes the sulphur in the ore as almost the entire source for the smelting operation. It is found that with a sulphide ore and siliceous gangue there is available from the burning of the sulphur and from the formation of ferrosilicate just about enough heat to accomplish the smelting operation successfully. On account of this close heat margin it is seldom attempted to run without a slight addition of coke, although such has been done when occasion demanded it. Other than in the use of this internal fuel, the chemistry of the process is exactly the same as in any sulphide smelting. Matte of course is the product of the fusion.
Small plants have been operated on this principle in Montana, and at Leadville, Colorado, but the greatest plant for this type of smelting has been and still is in operation at Mt. Lyell, Tasmania.
With the exploitation of the low-grade porphyry and copper mines of the western States and the production of enormous quantities of fine sulphide-copper concentrates, the mechanical multihearth roasting furnace has found a wide field of application. The outlet for these concentrates is by means of reverberatory smelting, and their sulphur content first must be lowered considerably in order to bring the concentration of the copper in the matte high enough for Bessemerizing in converters.
The mechanical roaster for this fine sulphide ore has several hearths; the ore is fed in at the top, dry, and most of the sulphur is burned out by the current of air over the hearths as the ore descends from one to the other on being worked across each hearth by the rabbles. The calcines preferably will be trammed hot to the reverberatory.
There are several excellent mechanical roasters on the market, differing somewhat in manner of drying the ore, in the construction of the hearths, in the means for cooling and rotating the rabble arms, and in the facilities for admitting the air to carry out the oxidation.
Fig. 35. Multihearth Mechanical Roasting Furuace Courtesy of Wedge Mechanical Furnace Companv.
Mechanical roasters of the general type illustrated in Fig. 35 have been brought to a stage of very high tonnage production and of extremely cheap treatment cost per ton. Furnaces are run to treat as much as 100 tons in 24 hours on one set of 20-foot hearths, while the total cost of operation will probably be less than 25 cents a ton. The perfection of this type of furnace has strengthened the development and stability of reverberatory smelting enormously.
Present Development Copper reverberatory smelting, in its most essential features as practiced today, is one of the most interesting technical developments of the generation; from little furnaces treating hardly more than 20 or 30 tons a day in 1880, their size has grown until now they are built 25 feet wide and 140 feet long, and will smelt up to 800 tons in 24 hours.
The most remarkable advance has been in fuel economy and in general furnace efficiency. Particularly, it has come to be appreciated that the secret of rapid smelting is a sufficient excess of hearth temperature over the formation temperature of the slag. From the old coal-burning fire box there has been developed gradually the use of gas, oil, and powdered coal as far superior means of producing the enormously long flame required to heat the furnace properly.
In the mechanical construction of the furnace, reverberatories have been greatly improved, while the physical structure is now made of a size which a very few years ago could have been proven impossible.
The copper reverberatory is essentially a huge heated receptacle in which the charge is melted down. It is on this basis that its attainments have been so remarkable. The furnace is depended upon now to oxidize considerable of the sulphur of the charge, while the matte and slag produced hardly differ from those produced in the copper blast furnace.
The general proportions of a modern reverberatory furnace fired with pulverized coal are seen in Fig. 36. The furnace is essentially an extremely long but rather broad and thin melting box. From the burners at the firing end the hottest part of the flame spreads out and covers the bath up to the section where the roof is lowered; the picture indicates that this is about at that point where the matte is tapped off. Immediately above this hottest section are the hoppers for letting in the charge. The charge has a long way to travel and abundant opportunity to separate into matte and slag before it reaches the skimming door situated at the opposite end. Slag will be skimmed near the end farthest from the burners, while the matte commonly is tapped off at about one-third of the total distance from the burners, and is sluiced directly into the converters, or is handled in ladles. It is noticed that the furnace is built massively and is held together thoroughly with a great number of steel I-beams placed entirely about the furnace walls.
Reverberatorics usually are equipped with waste-heat boilers for recovering as much heat as possible from the exit gases. Converting Copper Matto.
Copper matte produced either in blast furnaces or in reverberatories is poured into large receptacles to be blown to what is known as blister copper. A few years ago these steel-bound receptacles were lined exclusively with siliceous material, and, in burning out the sulphur and the iron from the matte, this siliceous material was depended upon for making slag with the iron oxide produced. Metallurgists recognized the disadvantage of this consumption of the lining of the furnace and of the frequent renewals, and it is due to the efforts of two eminent metallurgists, Smith and Pearce, that, in 1904, converting was accomplished successfully in converters lined with basic material which would withstand indefinite operation.
Fig. 37. View of Two of the Great Falls Conveners at Anaconda.
We now are able to maintain the integrity of a converter lining for many months. The siliceous material required for the formation of the iron slag is added as necessary during the conversion of the matte to metal.
The size of the converters has increased continuously until they now are built 20 feet or so across. These enormous barrel- or pear-shaped steel monsters are tipped back and forth by hydraulic or electric power for charging, blowing, and pouring. A picture of some of these largest pear-shaped converters is seen in Fig. 37. The nearest one is upright and the matte is being blown as is seen by the light at the mouth of the converter. The second converter apparently is red hot inside and may be just pouring the metal or may be in some other stage of the process which requires tipping over so that the contents will run out. As the converter is in this position, the tuyeres and the air box are fully exposed high in the air; when tipped back in blowing position, they evidently are so placed that the air will squirt through the bath of red-hot matte inside.
The principle of the chemical change in the converter is that, when air is blown through the metal, the sulphur and iron are oxidized and practically are removed before the copper itself is attacked. Some of the sulphur and all of the iron will be oxidized to leave what is known as white metal, a practically pure sulphide of copper, after which the blowing will be continued until this all is changed to metal by the complete elimination of the sulphur as sulphur dioxide.
Converters are a necessary accessory in all large sulphide smelting establishments, and change the matte into metal at a cost of a small fraction of a cent per pound.