In a broad sense, sulfide copper pyrometallurgy is a batch sequence in three separate vessels, i.e., reverberatory furnace to yield “matte copper,” a converter to supply “blister copper,” and another reverberatory furnace to supply fire-refined copper. Matte smelting is relatively unchanged since it turned common apply some 50 to 60 years in the past. It is now serving wide selection of smelter feed materials, however many concentrates ore not immediately amenable to the method. There may be an extra or deficiency of essential matte-forming, parts, so adjustments have to be made to the furnace charge. When smelter feed carries a enough extra of sulfur, byproduct manufacturing of elemental sulfur or H2SO4, may be included into the metallurgical flowsheet. At the other extreme, there arc plentiful copper assets which lack adequate sulfur and different parts to maintain the complete chemistry involved in matte smelting. This necessitates adding sulfur to make the product fit the process, and often none is recovered. From the standpoint of pollution alone, improvement of a smelting method for low-sulfur concentrates not requiring the addition of sulfur can be a big achieve.
The Twin Cities Metallurgy Analysis Middle has been engaged in a research to uncover a more direct pyrometallurgical process for profitable copper from sulfides. The initial goal is focus containing primarily chalcocite (Cu2S), and the objective is a smelting method not requiring the addition of supplemental sulfur. Improvement work, nonetheless in progress, factors to a new process through which the following sequence of occasions happen desulfurization, metallization, smelting. It is contemplated that the first two can be carried out sequentially in a 2-stage fluid-bed operation. The third might be carried out in any furnace from which both slag and metallic might be tapped. The metallic, just like blister copper, requires refining to take away small percentages of sulfur and iron but this is readily completed by well-established industrial procedures.
Uncooked Materials and Process: The research has been carried out on concentrates (from a large, integrated mining operation) which have been primarily all minus 100-mesh, with 90 % finer than 400-mesh. Roughly 90 % of the copper worth occurred primarily as chalcocite (Cu2S), with minor quantities of cuprite (Cu2O), covellite (CuS), chalcopyrite (CuFeS2) and bornite (Cu5FeS4). The remaining 10 % was present as native copper. The most important gangue mineral was quartz but there have been vital portions of kaolinite and sericite as properly. Desk 1 presents chemical and sizing analyses of two samples that are attribute of current mill follow.
Roasting was carried out in a fluid-bed reactor (figure 1) which is principally a 4-in.-diameter sort 316 chrome steel column 9 ft lengthy encompassed by an 18-in. square flue. The flue was low-thermal-conductivity brick supported by a metal body. The base of the flue was open, and the top cowl was related to an exhaust stack system. The reactor was heated by the combustion of pure fuel in four flame nozzles in a ring on the base. Temperature was measured by 5 thermocouples situated along the vertical axis. An automated temperature controller was related to the No. 3 thermocouple situated 24 inches above the base of the reactor tube. Fluidizing air, after passing by means of a moisture lure, was regulated by flowmeter and then entered the reactor by means of a one-inch ceramic ball examine valve under the base plate. An overflow discharge pipe, hooked up to the reactor base plate and extending into the column, established fluid-bed depth which might be various from 12 to 52 inches. An air-lock table feeder was used for bottom-feeding. Solids to be reacted have been entrained with air and carried by means of the ball verify valve and into the fluid column. New feed thus pressured an equal quantity of roasted materials into the overflow pipe.
Preliminary desulfurization exams with uncooked concentrates proved unsuccessful because average air velocities brought about excessive dust losses; low air velocities allowed the particles to stick to one another, defluidize, and create a fused mattress.
In either occasion desulfurization was very poor. Pelletizing the focus solved these issues by permitting higher volumes of fluidizing air.
Pellets ranging in measurement from 35-mesh to 10-mesh have been prepared utilizing both bentonite or sulfite liquor as a binder. As a result of of the small dimensions, the standard criteria for evaluating unfired pellets (e.g., inexperienced and dry compression strengths, drop quantity) have been meaningless. An abrasion and degradation check was developed using a 1-¼-in. diameter cold tube as a check vessel during which small portions of dried mini-pellets have been fluidized for 30 minutes at 150 % mattress enlargement, and dust losses have been decided. The info introduced in determine 2 present that deeper beds have been a think about preserving integrity of the pellets.
With the expertise gained in some preliminary experiments, it was attainable to proceed to round-the-clock operation of the Four-in, reactor on mini-pellets. Measured quantities of chalcocite flotation concentrates, binder and water have been blended in a 36-in.-diameter mix-muller and pelietized in a 36-in.-diameter stepped disk. Most of the desulfurization roasts have been made on dry, minus 10- plus 20-mesh pellets bonded with one to two % bentonite.
Results: A quantity of continuous runs have been made with variations in temperature, feed price and bentonite content material of the mini-pellets. The campaigns ran from one to 5 days, however the system reached equilibrium within a number of hours, so in the lengthier campaigns, it was attainable to finish a number of individual checks. Some of the info have been gathered in table 2.
An endeavor to realize more complete elimination of sulfur by 2-stage roasting had the other impact. Typical knowledge are the results of exams 1 and 2 (table 2) the place the primary stage was carried out at either 400° or 600° C and the second at 800 or 900° C. The operation was clean mechanically, but the sulfur content material of the merchandise proved solely that standard and primary copper sulfates shaped at decrease temperatures have been extraordinarily troublesome to decompose, even at 900° C. Examination of typical merchandise by X-ray disclosed solely tenorite (CuO) and dolerophanite (CuO·CuSO4). The microscope revealed small quantities of copper sulfide, either chalcocite (Cu2S) or digenite (Cu1.8S), and cuprite (Cu2O) as properly. Chemical evaluation confirmed that soluble sulfates of copper have been current in pellets calcined at 900° C. This was contrary to the findings of Wadsworth et al who found solely water-insoluble CuO·CuSO4 above 850° C; nevertheless, their mean retention time extended beyond 12 hours as compared with often less than 4 in the present research. All succeeding campaigns have been single-stage exams carried out at 900° – 925° C. Sulfur analyses properly underneath 1.0 % have been achieved.
In regards to the main working variables, temperature and residence time are most essential. Residence time was a perform of three elements: feed fee, air movement and bed peak. The typical retention ranged from one to 6 hours, however these occasions have been strictly operating estimates. The fluid-bed system is analogous to a stirred-tank reactor; some incoming grains exit virtually immediately, and others stay for many hours. Feed fee and bed peak exercised probably the most direct affect on time. Three kilos per hour was established as a very good workable feed price but, on reflection, it’s possible that 4 or five might have been managed. Equally, the 12-in. overflow did not use sufficient of the reactor, but 27-in. and 41-in. overflows have been satisfactory.
Air flows decide the bed enlargement, so the dilution issue additionally affects residence time. In the early runs there was a bent to function with considerable extra air, however it was steadily brought right down to about half the original amount. A terrific excess of oxygen to dilute the SO2 evolving from the solids was not essential to insure continued desulfurization; nevertheless, a sure air volume was required to dissipate response heat and inhibit agglomeration and attainable defluidization.
There was a marked high quality distinction between one % and two % bentonite mini-pellets, which was mirrored in the mud load. The lost two campaigns of desk 2, checks 14 and 15, have been fairly comparable in respects aside from bentonite content material. Based mostly on product weights the mud in teat 14 comprised 27.5 %, whereas that of check 15 was only eight.0 %. The dust could possibly be recycled to the pelletlzer circuit with recent focus. These knowledge level out the essentiality of feeding structurally sound, dry chalcocite pellets. Surviving copper oxide pellets have been robust, porous and easily reducible in the following process step.
Metallization of Oxidized Pellets
Raw Supplies and Process: Following the oxidizing roast the pellets have been decreased in the fluid-bed reactor using the bottom-feed procedure described earlier. A number of additions have been accomplished beforehand, together with a posh fuel manifold and a 10,000-ft³ strain vessel to retailer decreasing gases. Thereafter air, nitrogen or decreasing fuel might be metered to the reactor. The reductant was 15 % hydrogen in nitrogen, a business heat-treating mixture termed “formed gas” which was neither explosive nor toxic. Both fluidization and discount have been effected by this mixture.
Two composite samples of roasted focus pellets, one containing less than 1.0 % sulfur, the opposite slightly extra, have been the feed for metallization. The chemical and physical properties of these pellets seem in desk three.
The reactor was preheated with a mattress of the low-sulfur pellets fluidized in air to preserve nitrogen and shaped fuel. When the reactor attained 750° C at No. three
thermocouple, a 15-minute move of nitrogen flushed the system. Reduction was then initiated by switching to shaped fuel. Concurrently, roasted pellets have been backside fed to the reactor by introducing them into the fluidizing fuel stream from the air-lock desk feeder. Discharge was by means of a 36-inch standpipe, the pipe being dumped by way of a valve each 20 minutes. The recent pellets have been protected from oxidation by cooling beneath nitrogen and sampled hourly. The shaped fuel at room temperature was metered to the reactor at a fee of 5 scfm for all exams; this is 4 occasions the amount theoretically required to metallize the copper contained within the pellets on the highest feed price used.
Outcomes: Reduction of roasted pellets was an easy, trouble-free operation; no operational difficulties have been experienced during three checks of 12- to 24-hour period. Mud (7 % or much less) was recovered by asbestos luggage, however was not recycled. Metallization was near 90 % at 800°, 850° and 900° C, and the typical outcomes appear in desk Four.
Reduction of CuO to metallic isn’t temperature dependent within the 800° – 900° C vary, so precise control was not required. Feed rates have been different from 3 lb/hr to 7 lb/hr and there was a slight, however not necessarily vital, lower in metallization on the 7 lb/hr degree. Retention time was estimated, from a mean reactor mattress weight of 10.3 lb, to be 3.Four, 2.1, and 1.5 hr, respectively, at feed charges of three, 5, and 7 lb/hr.
Determine 3 is a refined section of a metallized copper pellet. The silicate bond, shaped in the course of the oxidizing roast between grains of gangue material (mild gray), provides the pellet the required power to face up to the fluid-bed reduction with small mud losses. The metallic particles (white) are additionally bonded to the pellet structure, but because the brittle gangue mineral predominates, the pellets are usually not malleable. Some of the copper has coalesced (giant white grains) and taken on a rounded appearance. This facilitates copper separation from the slag during smelting by growing the dimensions of the globules.
Two elements have to be thought-about in smelting these pellets: (1) They include up to one % sulfur which can enter the metallic or type a small quantity of matte, and (2) not all of the copper oxide is decreased, so small quantities of Cu2O stay in the sample. It’s useful to course of the metallized pellets underneath barely decreasing circumstances, in order that what takes place is actually melting. If the environment is uncontrolled, some copper will reoxidize and then react with the sulfide as follows:
2Cu2O + Cu2S → 6Cu + SO2
2CuO + Cu2S → 4Cu + SO2
Ordinarily, the prevalence of these reactions can be desirable, but within the small laboratory crucibles the persistence of SO2 evolution deterred a sharp slag-copper separation.
Raw Materials and Process: Metallized pellets from the fluidized reduction have been the feed materials for smelting. They analyzed 34.6 % complete Cu, 30.7 % metallic Cu, 5.2 % Fe, zero.67 % S, 10.3 % Al2O3, and 38.4 % SiO2. The fluxes used have been burnt lime, limestone, and dolomite (table 5).
The preliminary smelting checks have been carried out in a muffle furnace, however the outcomes have been unsatisfactory; the copper content of the slag usually exceeded several-fold the goal degree of 1.zero % copper. Bigger batches (500 g) in fireclay crucibles have been smelted successfully in a 20-kw induction furnace, the crucial factor being the mildly decreasing circumstances offered by the recent graphite susceptor within the induction furnace. Cooled crucibles containing the smelted charge have been weighed and broken, and the contents have been separated into slag and copper buttons. Recoveries have been calculated from analyses and weights of the feed and slags.
Results: Desk 6 presents some of the info from smelting metallized pellets in the induction furnace at 1300° C. Observe the very excessive restoration of copper. The requirements for passable operation included the following: (1) Slag basicity of 0.5 or much less, (2) well-metallized pellets, (three) a weakly decreasing environment above the bathtub and (4) formation of a small quantity (less than 7 %) of matte. The final merchandise was extra of an effect than a trigger and was a immediate sign that atmospheric circumstances had been sufficiently decreasing to inhibit oxidation.
Determine Four exhibits some typical products of the processing up so far. The roasted pellets are the A pile and those in B have been lowered to metallic. The black-and-white print doesn’t reveal the change in colour but does show the pellets endure little degradation from the remedy. C is a sectioned crucible after smelting, which illustrates the sharp copper-slag separation attainable beneath favorable circumstances.
The copper recovered from smelting was contaminated with sulfur and iron and had a composition roughly the identical as blister copper (table 7, Head Sample). Hearth-refining of such material is a routine business operation. A number of checks have been carried out along these strains to point out only that the product would respond to standard refining procedures. Desk 7 summarizes the info.
The top pattern was obtained by melting down a 10-lb lot of copper buttons from earlier smelting exams and sampling the bathtub. This was completed in an induction-heated silicon carbide crucible. Twenty grams of SiO2 have been added, and blowing was initiated by inserting an iron pipe into the molten tub and metering compressed air to the pipe via rubber tubing. The first air price resulted in an excessively vigorous agitation of the bathtub. After about an hour the liquid degree was restored with new feed, another 20 g of SiO2 was added, the air fee was significantly diminished, and blowing continued for 1-½ hours.
The final analysis signifies sulfur and iron had been lowered to passable ranges and the metallic was ready for poling. Some deoxidation of the metallic with hydrogen was carried out however was not carried to completion. Nevertheless, the few refining checks demonstrated how readily each sulfur and iron might be eradicated.
A new course of for recovering copper from chalcocite concentrate consists of roasting in air (to about zero.5 % S) followed by gaseous discount to metallic. These operations, carried out in two states of fluid-bed processing, have been successful solely when preceded by pelletizing. The metallized materials was smelted with 10 % CaO to yield about 97 % restoration of Cu in a product just like blister copper. Blowing the latter with air to simulate refining brought the sulfur and iron contents right down to very low values.
Though the analysis continues to be underway, the above seems to be a extra direct method for profitable copper from chalcocite than the traditional matte-smelting and converting route. Moreover it is completed without the extra sulfur required for matte smelting.