Of the several methods of reducing copper ores outlined below there are now only two of minor importance - strong reduction, and pyritic smelting for sulphide ores. All the others at present are operating on very extensive scales. The old Welsh processes of roastings and reductions are of only theoretical and historical interest.
(B) Sulphide ores smelted in blast furnace to matte; matte converted to blister.
(1) Strong reduction for low-sulphur ores.
(2) Semipyritic smelting for medium-sulphur ores.
(3) Pyritic smelting for high-sulphur silicious ores.
(A) Reverberatory smelting to bullion and slag.
(B) Mechanical roasting; then reverberatory smelting to matte.
(B) Sulphate ore leached with acid solution, and copper precipitated as cathode metal by electricity.
Smelting oxidized copper ores to black copper, as the metal thus recovered is called, is one of the simplest of all smelting operations. The ore with flux and coke is charged into a suitable cold blast furnace; the metal is collected in the crucible until enough is present to tap out into the ingot molds.
Fig. 32 shows in perspective and section a round copper blast furnace; the larger rectangular furnaces are built with close adherence to the proportions as seen in the round type. The interior is rather short and wide - not much height is required to effect reduction of the metal; water jackets inclose the smelting zone and the charge column up to the feed floor. The metal collects inside until enough has accumulated to tap out into slabs or bars; the slag likewise is tapped intermittently from a level slightly above the metal tap. The gases will go to waste directly, or after simple elimination of the coarser dust.
Fig. 32. Round Copper Blast Furnace in Full View and Section Courtesy of Colorado Iron Work; Denver, Colorado.
This type of smelting is apt to be found on frontiers where the surface ores are awaiting this easy treatment. The process had a very noteworthy run on the surface ores in Arizona before the enormous development of the deeper sulphide ores which are being mined today. A furnace doing this identical work can be seen in operation almost any day in the city of Chicago.
The most remarkable case of oxide smelting in blast furnaces is to be found at Katanga, in Central Africa. Huge rectangular furnaces made in the United States are run there by United States men on the largest oxide deposits (excepting the sulphates of Chili) which always have come within the range of blast furnaces.
An oxide blast furnace necessarily is limited by the generally meager extent of such ores; from the fact that it is difficult to make clean slags economically, small mines will prefer to turn over their ores to large sulphide smelteries where they can be treated equally well with the regular sulphide ores.
The smelting of coarse sulphide of copper ores is a type of reduction which has developed in different chemical aspects, as well as in furnace capacity. As indicated in the outline, some occasions have required the development of a smelting which much resembles the strongly reducing conditions of lead and iron smelting. This is to conserve the sulphur and to assemble all of the copper in a matte or artificially enriched sulphide compound of iron and copper. Sulphur and copper have a remarkable affinity for each other, and, with conditions at all reducing, they will go through the furnace unchanged and come out together as matte for further treatment to make blister copper.
A few occurrences of this type of smelting have been found noteworthy but there is little use for the process at the present time. The most conspicuous example is at Mansfield in Germany.
Fig. 33 shows a copper-sulphide furnace which illustrates well the features as now in use in this very important method of copper reduction. The crucible is made very shallow and is set on jacks so that the bottom of the furnace not only is kept cool but is removable easily. The smelting zone is water-jacketed thoroughly and, from the picture, it is seen that not only is the breast well water cooled but the matte spout is likewise. The tuyeres are arranged closely together along both sides of the furnace, and it will be observed from the picture that the water overflow from each jacket comes out to the drain pipe entirely exposed so that the attendant can see always exactly how much water is going through any single section. The top of the furnace, which of course will be above the feed floor as the furnace is arranged in the building, is made of brick, and the gases leave through a large steel flue. The opening at the side allows of charging the full length of the furnace.
Sulphide furnaces of course are all charged mechanically, the cars usually dumping the material in along the side of the furnace. A further consequence of the large tonnage is that slag and matte are run off continuously into a forehearth or settle where the separation of matte and slag takes place. We know that in connection with oxide-copper furnaces internal crucibles are necessary, because of the easy chilling of metallic copper; with matting furnaces, however, the fiery matte is just as apt to cause trouble by eating through its container, and preferably is gotten out of the furnace as quickly as possible. From the matte settler the slag will be run off as overflow into slag pots, while the matte will be tapped out from a lower level and taken to the converters.