Metamorphosis of the Copper Reverberatory Furnace: Oxygen Sprinkle Smelting
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Metamorphosis of the Copper Reverberatory Furnace: Oxygen Sprinkle Smelting Paul Etienne Queneau R. Schuhmann, Jr. Thayer School of Engineering School of Materials Engineering Dartmouth College Purdue University Hanover, New Hampshire West Lafayette, Indiana A new process transforms existing reverberatory furnaces into oxygen flash smelting units. A new process, "oxygen sprinkle smelting," has been de The typical reverberatory furnace is obsolete in today's vised which transforms existing reverberatory furnaces into world because of its profligate fossil-fuel consumption - it oxygen flash smelting units. The thrifty concept can greatly suffers seriously from inefficiency in heat transfer and as improve the performance of old-fashioned smelters in respect a chemical reactor, and hence has inherent environmental to productivity, operating costs, energy conservation, and woes. Supplying the heat for smelting by burning fossil fuels, environmental protection. This novel oxygen technology without fully using the calorific power of the sulfide concen requires substantial capital outlay - mainly for concentrate trates, is now hard to justify, even with concentrate roasting drying and oxygen generation - but resulting major enhance and care in air preheating, oxygen enrichment, waste heat ment of plant metallurgy and economy makes it attrl!ctive in recovery as steam, and other measures of energy manage view of alternatives reportedly involving much greater ex ment. A concomitant. intractable aspect of the reverberatory penditures, e.g., billions of dollars/ Modem practice and is the cost of cleaning the enormous volume of its sulfurous, technological innovation which will be described can rejuven often arsenical, dusty exhaust gas. ate outdated plants and allow time for development of the Our conviction that the conventional furnace is super Q-SOxygen Process2 and of other processes able to reap the annuated and should be retired led us paradoxically to the full advantages of oxygen technology.3 These superior, more at-first startling thought of furnace metamorphosis by sophisticated, continuous operations - ideally producing oxygen technology. This would conserve financial resources: high-grade matte or crude metal and clean slag in a single the capital investment which the furnace and its infrastruc sealed reactor - can then enjoy the concentrate driers and ture represent. From this initial perception, our thinking oxygen supply which will be awaiting them! proceeded to the concept of oxygen sprinkle smelting, and COPPER CONCENTRATE + FLUX + OXYGEN COPPER CONCENTRATE + COAL + OXYGEN CONVERTER SLAG ALTERNATE MAnE Figure 1. Oxygen sprinkle smelting furnace (converted conventional copper reverberatory furnace). 12 JOURNAL OF METALS • December, 1979 further, to the design, manufacture, and testing of a full of oxygen sprinkle smelting to local smelting practices, is scale, prototype oxygen sprinkler burner in collaboration achieved by mixing minor amounts of coal with the concen with Professor Horst Richter of Dartmouth College. Its de trates fed to the sprinkler burners. This use of coal is energy sign demanded, under conditions of limited headroom, excel efficient and does not add to the nitrogen burden of the off lence in gas-liquid-solid contact of reactants and in particu gases. As one illustration of coal use, the heat and material late dispersion over a wide area. balances for the concentrate composition, heat-loss rate, and Figure 1 shows a reverberatory furnace transformed into air infiltration rate specified in Table I were calculated,for a an oxygen sprinkle smelting furnace by closing all unneces smelting rate of 1,500 tons concentrate per day and a matte sary openings and installing three sprinkler burners. The grade of 50% Cu. A thermally balanced operation is obtained burners are designed to accomplish several important func by adding to the concentrates 45 tons per day of coal (3% of tions within the constraints of the existing structure. The the concentrates) with a heating value of 12,000 Btu/lb most critical are the intimate mixing of finely divided con The resulting heat balance, summarized in Table II, shows centrates with oxygen-rich gas for flash smelting purposes, that about 70% of the required heat input is supplied by oxi and uniform sprinkling of particulates over most of the bath dation of iron sulfides and the balance by coal. Oxygen con surface. Provision is made for the admixture of a small per sumption is 0.2 Ib/lb of concentrate, slightly below that for centage of pulverized coal with the solids feed to one or more autogenous smelting to a 64% Cu matte (Table I), and ex burners. The slag is cleaned prior to discharge by drenching ing process into useful practice will not be a "free lunch," but it with a shower of low-grade matte produced by flash melt the difficulties certainly can be overcome. ing of the copper concentrate. The process is, of course, favored by reverberatories with As the above description indicates, the sprinkler burner sprung-arch basic roofs, since less induced air is needed for installation goes far beyond flash oxidation of sulfides. It is for smelting and reduction of c,?pper-, nickel-,andcobalt directed specifically at optimization of interphase heat and containing materials. At this point, we believe that a fixed mass transfer and control of heat and mass distribution refractory enclosure equipped withan oxygen sprinkler burn throughout the furnace volume. Accomplishment of these er capability is a versatile chemical reactor applicable to functions can transform the notoriously sluggish reverbera many other fields beyond smelting of chalcopyrite and pent tory furnace into a relatively efficient and flexible chemical landite: it is a new unit process. reactor. For the conversion of an existing copper reverberatory fur nace to an oxygen sprinkle smelting unit, the expected per formance can be estimated with a high degree of confidence Table I: Heat Balances for Autogenous from available data on the furnace operation and feed ma Oxygen Sprinkle Smelting terials. The initial examples here are based on a wet-charge reverberatory furnace operation, as documented by Kellogg and Henderson! wherein 1,040 dry tons of copper concen Matte grade (% Cu) 55 60 65 70 trate (29.5% Cu, 26% Fe, 31% S, 8% SiO.) are treated per day Amt. of 98% 0., to obtain a 35% Cu matte and a 0.46% Cu slag, both at Ib/lb concentrate 0.18 0.20 0.22 0.25 2200°F. Furnace off-gas (23OO°F) contains 1% SO •. The heat SO. in flue gas (vol. %) 38 40 42 44 loss rate by conduction, convection, and radiation to the surroundings is 518,000 Btu/min.; the same heat-loss rate Heat Output, Btu/lb concentrate will be assumed after the furnace is converted to oxygen Sensible heat in gas 301 315 328 343 sprinkle smelting. Sensible heat in matte 204 179 155 136 Another significant item of heat consumption remains to be Sensible heat in slag 265 313 360 392 estimated, that due to air infiltration. For instance, 10,000 Heat loss to surroundings 186 186 186 186 scfm of cold air infiltration may be an operable level in a Total 956 993 1,029 1,057 suspended basic roof reverberatory furnace adapted to oxygen sprinkle smelting. Most of the infiltrated oxygen serves to Heat Input, Btu/lb concentrate reduce the consumption of commercial oxygen, but the re Oxidation of iron sulfides 842 949 1,046 1,122 mainder and the nitrogen consume heat to the extent of some 400,000 Btu/ min. Thus the total furnace heat require Heat deficiency, Btu/lb concentrate ment (heat loss + air infiltration) to be supplied by the 114 44 -17 -65 process comes to about 900,000 Btu/ min. Trial heat balances based on the above data, assuming the original smelting rate of 1,040 tons concentrate per day, show that the heat input from exothermic smelting reactions Table II: Heat Balance for Oxygen Sprinkle Smelting is insufficient to supply both the sensible heat in smelting with Coal Addition· products and the furnace heat requirements. This is true even if enough oxygen is supplied to smelt the concentrate· to 75% Cu matte by oxidizing nearly all the iron sulfide in the con Heat output, centrate. For autogenous oxygen sprinkle smelting under the Btu/lb concentrate given furnace conditions, higher concentrate tonnages must Sensible heat in gas 387 be smelted. Table I summarizes the heat balance calculations Sensible heat in matte 234 for smelting at the rate of 2,000 tons concentrate per day. Sensible heat in slag 208 The heat balances for the four assumed grades of matte indi Heat loss to surroundings 249 cate that the process is thermally balanced and autogenous Total 1,078 for production of a matte grade of about 64% Cu, correspond Heat input, ing to a consumption of 0.22 Ib tonnage oxygen (98%) per Btu/lb concentrate Ib of concentrate. In this case, furnace exhaust gas contains Oxidation of iron sulfides 723 about 42% SO. and is exhausted at the rate of about 16,000 Combustion of coal 355 scfm. The latter is about one-third of the off-gas flow rate of the fossil-fuel-fired reverberatory furnace, even when the Total 1,078 conventional practice smelts only half the tonnage of the oxygen sprinkle smelting operation. • Calculated for 1,500 tons concentrate per day, matte grade of 50% Cu, for the concentrate Wide choice of operating conditions, for ready adaptation composition, heat-loss rate, and air infiltration rate specified in Table I. JOURNAL OF METALS • December, 1979 13 Let us revert to the metallurgical engineer's "bottom line": converters are employed. The smelting conditions in respect making money by increasing productivity, and saving metal, to analyses of concentrates, flux, and slag, and furnace heat energy, and the environment. It must be emphasized that loss rate and product temperatures are assumed the same as converting existing reverberatories into oxygen sprinkle in Table I.