The mineral processing which is the subject article is far more complex for it deals with at least twenty five end products, coming from only three different metallurgical concentrates which may, in some cases, originate in a single mine. I will anticipate immediately objections that no one mine contains recoverable quantities of all of these elements, nor does any ore, smelter, or refinery produce all the products listed, or for that matter, does any one company produce all the products that are extracted and recovered during the treatment of ores mined for the recovery of copper, lead, and zinc. Excluded from consideration are materials that are removed from ores during the mining and milling operations such as molybdenum, vanadium, rhenium, and so forth, for the removal of these from the lead, zinc, or copper concentrate leads to simplification of metallurgy rather than complexity. However, in smelting copper, lead, and zinc concentrates the following elements and (or) compounds of these elements are recovered as shown on Fig. 1: copper, lead, zinc, silver, gold, platinum, palladium, arsenic, antimony, bismuth, sulfur, selenium, tellurium, nickel, cobalt, indium, thallium, tin, cadmium, germanium, and mercury. While recovering twenty-one elements from three different smelting circuits is, in itself, fairly complex, most of these elements must be circulated and, frequently, between two or more circuits before they are concentrated enough to make their extraction possible and, more difficult still, their recovery as an element or compound, profitable.
In addition to the twenty one recoverable elements there are at least nine other elements which must be separated, usually in the form of a discard slag, at one stage or another of the smelting procedure, and which can in some cases lead to disaster if mishandled. These are iron, calcium, silicon, manganese, magnesium, aluminum, chlorine, fluorine, and oxygen. In addition, many other elements in the periodic table are at times found in the concentrates and fluxes used, and usually find their way into the discard slag. At present, the writer knows of no special procedures necessitated by their presence or of any commercial procedures resulting in their recovery. However, it is highly probable that such procedures; do exist and, almost certain, more will be developed in the future. Through all steps in the smelting and refining processes a detailed knowledge of the many reactions with carbon, oxygen and hydrogen are absolutely essential.
Thus, in the smelting of copper, lead, and zinc, which are so completely interwoven that they can seldom be considered separately, we are dealing with no less than thirty-two elements and a great number of compounds.
The tracing of these elements through the complicated metallurgical circuits and particularly the diversion and losses of recoverable elements (all too often unnecessary) demand the use of the most advanced techniques in analytical chemistry. Frequently, improvements in the recovery of by-product elements are primarily due to improvements in analytical methods.
The steps through which copper, lead and zinc concentrates must pass to effect the basic recovery of the essential elements are not particularly complex. Fig. 2 shews the basic steps in copper smelting and refining, Fig. 3 the same for lead and Figs. 4 and 5 for zinc. These are all the steps that would be required if the concentrates of copper, lead, and zinc were pure sulfides or even if the pure sulfides were mixed with gangue containing at least some of the unrecoverable elements previously listed. It is the presence of the seventeen additional elements which must be removed to achieve a commercially acceptable product that makes the metallurgy of copper, lead, and zinc the most complex in the world.
Why copper, lead and zinc? Why mix the three together? Each smelting circuit by itself would be much simpler. The fact is that many copper concentrates contain lead, antimony, and bismuth that must be recovered in a lead smelting circuit and most copper concentrates contain impurities that require electrolytic refining, the by-products of which, containing lead, antimony, and bismuth, are shipped to a lead smelter or refinery. There is at least one copper concentrate high enough in zinc to be tied into a zinc recovery circuit. There is probably no lead concentrate so free of copper, that it is not tied into a copper recovery circuit and many, if not most, lead concentrates are rich enough in zinc to make it desirable to tie in the lead smelting process to a zinc recovery process by way of a Slag Fuming or a Waelz Kiln step. The smelting of most zinc concentrates produces leady by-products and residues which are most economically smelted in lead circuits for the recovery of lead and copper together with other values.
Thus, there is a very considerable circulation between lead and copper circuits and between lead and zinc circuits and there is at least one case where there is a direct circulation between copper and zinc circuits. Some smelting circuits are notable for handling particularly clean concentrates of copper, lead or zinc and do not receive by-products from other smelters. Nevertheless, from most circuits with which I am familiar, occasional batches of byproduct material in which other elements are concentrated (usually by circulation) must ultimately be shipped at least from the refinery circuit to prevent a build-up of objectionable, although sometimes valuable, impurities.
A series of figures illustrate the intricate procedures made necessary by these “impurity elements”. On the figures the elements underscored with a solid line are moving in the direction we would prefer to have them go, while those underscored with a broken line are moving in the wrong direction.
THE COPPER CIRCUIT
Let us start with the copper mineral processing circuit and explore the paths through which the by-product elements must flow to effect their exclusion from the commercially pure copper, and recovery in a marketable form. Frequently, especially in plants installed some years ago, a drying, preheating and/or roasting step is inserted before the smelting operation, as shown in Fig. 6. In such plants the gases from the roasters carry the sulfur as SO2 and may be conveyed to a sulfuric acid plant. (The term “gas” is used to indicate a true gaseous phase like SO2; “fume” to indicate a solid or liquid phase suspended in a gas stream by condensation or precipitation by chemical reaction from a gaseous phase as, for example, As203 or ZnO; and “dust” a solid phase that has been mechanically carried into the flue system by the velocity of the gas stream. Flue dust, when it can be substantially segregated from fume, is normally returned to the charge and will be ignored in this paper. Unfortunately, it is easier to separate “dust” and “fume” by definition than it is in practice.) To continue with the roasting of copper concentrates, the fume concentrates the arsenic as the trioxide to form a feed to the arsenic recovery plant where the arsenic is refumed and caught in kitchens. Of course, copper, lead, antimony, and so forth, are also found in the original fume and they, in turn, are concentrated in the residue from the arsenic plant and are returned to the copper roasters or shipped to a lead plant, depending upon the relative concentration of copper, lead, and antimony. Thus, you will note there is a dual circulation of material to and from the byproduct recovery plant and you will find this pattern to be characteristic of most by-product recovery systems.
In most plants, the reverberatory smelting step, Fig. 7, does not concentrate by-products for the gases are vented, the flue dust is usually circulated back to the charge and the slag, except in one case where it is fumed for zinc recovery, is discarded or sold for some direct use where metallurgical recovery of the elements in the slag is not attempted.
Perhaps this is a good place to point out that the indication of the desired and undesired direction of flow is not always indisputable. Most copper men will insist that even though antimony is a valuable element, the best place for it in the copper circuit is in the discard slag. If the Slag is to be dezinced, zinc and lead will be desired in the slag and antimony will not.
In copper converting, Fig. 8, the complexity of the copper smelting operation first becomes evident. Of the impurities encountered in the matte, sulfur reports in the gas phase as S02 and is recoverable as sulfuric acid. The large quantity of iron present in the matte is oxidized and slagged with silica in which form it is returned to the reverberatory furnace and joins the slag discard from the system. Lead, antimony, arsenic, bismuth, selenium, tellurium, zinc, cadmium, and thallium report in the fume and are shipped to a lead smelter. This is the major outlet for recovered lead, antimony, bismuth, and thallium from the copper smelter.
These same elements report to a greater or lesser degree in the converter slag and are circulated back to the reverberatory smelting furnace where they divide between the matte and slag and build-up in the matte until the distribution level between matte and slag balances the plant intake.
The elements, not circulating in the converter slag and not removed from the system in the fume, report in the blister copper from which they must be subsequently separated. They are (Fig. 9) silver, gold, platinum, palladium, nickel, selenium, and tellurium. Of course, minor amounts of lead, antimony, bismuth, arsenic, sulfur, and iron are also in the blister copper but, as they are separated from the copper, they are merely shuttled back to their point of principal separation in the smelting steps. As a matter of fact, the degree to which the copper is blown in the converter or in the subsequent anode furnace is largely dictated by the economics of the removal of these elements at this vs some subsequent stage for, if the lead, arsenic and antimony do not demand heroic measures the blowing (oxidation) is merely enough to insure an acceptable low level of sulfur when reduction of the excess oxygen in the metal (by poling) is completed. It is the other elements, silver, gold, platinum, palladium, selenium, tellurium, and nickel in the copper together with bismuth that necessitate the electrolytic process which adds materially to the complexity and cost of copper refining.
Fig. 10 shows that instead of casting final shapes, as has been indicated in Fig. 2, anodes must be cast and in order to cast anodes the copper must be fire-refined to control oxygen and gas evolving elements which would spoil the set. This operation is almost identical with the operation to produce finished copper if there were no impurities present. These anodes, Fig. 11, must be electrolyzed and the cathodes produced must be melted and cast into the final shape. Both the electrolyte and anode slimes produced by this operation must be treated for copper and impurity recovery and control involving a series of steps so complex that they are difficult to represent on a flowsheet. In addition, it is impractical to dissolve all the anode in the electrolytic process; consequently, from 12 to 20 pet of the anode must be returned to the anode furnace and remelted and recast into anodes. Worse still, all the copper as it dissolves from the anode is not plated directly on the cathode for the efficiency of copper dissolution at the anode usually substantially exceeds the efficiency of copper deposition on the cathode. To prevent the build-up of copper in the electrolyte, either of two processes may be employed. Cells having insoluble lead anodes may be inserted in the electrolyte circulation circuit, or copper sulfate may be removed from the electrolyte, as such, by evaporation and crystallization and the equivalent amount of new acid added to the circuit. In addition, nickel sulfate builds up in the electrolyte and this necessitates at least periodic evaporation and crystallization steps to effect its recovery, for this torturous path is the usual procedure by which nickel is recovered from copper, lead and zinc circuits. Fortunately, in removing the nickel, in addition to removing arsenic and antimony, one can also remove excessive amounts of other soluble sulfates, such as those of sodium, magnesium, calcium, zinc, and so forth, which, while not necessarily present in the anodes, do build up in the copper electrolyte. Organic matter in the electrolyte is also destroyed or rendered innocuous by this operation.
It should be noted that the copper refining steps so far reported, while made necessary by the presence of impurities in the blister copper, have not accounted for the recovery of any of these impurities except nickel. All steps outlined are for the basic purpose of recovering copper in a marketable form. During electrolysis the impurities divide between—remaining in solution in the electrolyte and—precipitating in the anode slimes or mud. In fact, some elements, if not all, do both; but, fortunately, most of the elements do substantially one or the other, that is, report principally in the electrolyte or remain substantially in the anode slimes. The two elements which report substantially in the electrolyte are nickel and arsenic, for they will build up in some cases to over 10 g per liter if an outlet for them is not provided in the electrolytic refining procedure.