Extractive Metallurgy of Copper Part 9 potx

CHAPTER 14 Capture and Fixation of Sulfur About 85% of the world’s primary copper originates in sulfide minerals. Sulfur is, therefore, evolved by most copper extraction processes. The most common form of evolved sulfur is SO2 gas from smelting and converting. SO2 is harmful to fauna and flora. It must be prevented from reaching the environment. Regulations for ground level SO2 concentrations around copper smelters are presented in Table 14.1. Other regulations such as maximum total SO2 emission (tonnes per year), percent SO1 capture and SO2-in-gas concentration at point-of-emission also apply in certain locations. In the past, SO2 from smelting and converting was vented directly to the atmosphere. This practice is now prohibited in most of the world so most smelters capture a large fraction of their SOz. It is almost always made into sulfuric acid, occasionally liquid SO2 or gypsum. Copper smelters typically produce 2.5 - 4.0 tonnes of sulfuric acid per tonne of product copper depending on the SKU ratio of their feed materials. This chapter describes: (a) offgases from smelting and converting (b) manufacture of sulfuric acid from smelter gases (c) future developments in sulfur capture. 14.1 Offgases From Smelting and Converting Processes Table 14.2 characterizes the offgases from smelting and converting processes. SOz strengths in smelting furnace gases vary from about 70 volume% in Inco flash furnace gases to 1 volume% in reverberatory furnace gases. The SO2 strengths in converter gases vary from about 40% in flash converter gases to 8 to 12 volume% in Peirce-Smith converter gases. 217 2 18 Extractive Metallurgy of Copper Table 14.1. Standards for maximum SO2 concentration at ground level outside the perimeters of copper smelters. Maximum SOz + SO, concentration Country Time period (parts per million) U.S.A. Yearly mean (EPA, 2001) daily mean 3-hour mean Ontario, Canada Yearly mean (st. Eloi et ai., 1989) daily mean I-hour mean 0.03 0.14 0.5 0.02 0. IO recommendation 0.25 0.5 hour average 0.3 (regulation) The offgases from most smelting and converting hrnaces are treated for SO2 removal in sulfuric acid plants. The exception is offgas from reverberatory furnaces. It is too dilute in SO2 for economic sulfuric acid manufacture. This is the main reason reverberatory furnaces continue to be shut down. The offgases from electric slag cleaning furnaces, anode furnaces and gas collection hoods around the smelter are dilute in SOz, <0.1%. These gases are usually vented to atmosphere. In densely populated areas, they may be scrubbed with basic solutions before being vented (Inami et al., 1990; Shibata and Oda, 1990; Tomita et al., 1990). 14.1.1 Surfur capture eflciencies Table 14.3 shows the S capture efficiencies of 4 modem smelters. Gaseous emissions of S compounds are I 1% of the S entering the smelter. 14.2 Sulfuric Acid Manufacture (Table 14.4) Fig. 14.1 outlines the steps for producing sulfuric acid from SO2-bearing smelter offgas. The stcps are: (a) cooling and cleaning the gas Table 14.2. Characteristics of offsases from smelting and converting processes. The data are for offgascs as they enter the gas-handling system. SO2 concentration Temperature Dust loading Furnace (volume%) (“C) (kgiNm’) Destination lnco flash furnace 50-75 1270-1 300 0.2-0.25 H2S04 occasionally liquid SO2 plant Outokumpu flash furnace 25-50 1270-1350 0.1-0.25 HZSO4 plant, occasionally liquid SO2 plant Outokumpu flash converter 35-40 1290 0.2 H2S04 plant Outokumpu direct-to-copper 43 1320-1400 0.2 HzS04 plant Mitsubishi smelting furnace 30-35 1240- 1250 0.07 HzSO4, occasionally liquid SO2 plant Mitsubishi converting furnace 25-30 1230-1250 0.1 H2S04. occasionally liquid SO2 plant Noranda process 15-25 1200-1240 0.015-0.02 H2S04 plant Teniente furnace 12-25 1220-1250 H2S04 plant Isasmelt furnace Electric furnace Reverberatory furnace Peirce-Smith converter Hoboken converter Electric slag cleaning furnaces Anode furnaces 20-25 1 150-1220 -0.01 H2S04 plant r? 1 1250 -0.03 Vented to atmosphere (made into gypsum in one In 7 sa a $ 2-5 400-800 H~SOJ or liquid SO2 plant or vented to atmosphere plant, scrubbed with flotation tailings in another) 8-15 1200 12 1200 0. I 800 10.1 1000 H2S04 plant or vented to atmosphere HzS04 plant Vented to atmosphere (occasionally scrubbed with basic solution) s 9 Vented to atmosphere (occasionally scrubbed with 5 basic solution) Y Gas collection hoods around the smelter <0. 1 50 Vented to atmosphere (occasionally scrubbed with N \D basic solution) - 220 Extractive Metallurgy ojCopper Table 14.3. Distribution of sulfur in four copper smelters. Toyo, Japan Timmins, Canada Tamano, Japan Norddeutsche, (Inami et ai., (Newman et aL, (Shibata and Germany 1990) 1993) Oda, 1990) (Willbrandt, 1993) Outokumpu Mitsubishi Outokumpu flash Outokumpu flash flash furnace smelting/ furnace furnace Peirce-Smith converting Peirce-Smith Peirce-Smith converters converters converters 96.6 Percent of incoming S in: Sulfuric acid 95 96 96.2 Gypsum 2.1 1 .o Slag 1.2 1.4 1.2 1.2 Dust 0.2 2.0 (to Zn plant) Other 1 .o 0.3 Neutralized liquid effluent 0.6 1.8 0.8 Gaseous emissions 0.2 1 .o 0.1 0.8 (0.6*; 0.4') * from dryer, anode furnace and vcntilation stacks from acid plant tail gas (b) drying the gas with 93% H2S04-7% H20 sulfuric acid (c) catalytically oxidizing the gas's SO2 to SO3 (d) absorbing this so3 into 98% H2S04-2% HzO sulfuric acid. The strengthened acid from step (d) is then blended with diluted acid from step (b) and sent to market or used for internal leach operations, Chapter 17. The acid plant tail gas is cleaned of its acid mist and discharged to the atmosphere. Tail gases typically contain less than 0.5% of the S entering the gas treatment system. Several smelters scrub the remaining SOz, SO3 and HzS04 mist with Ca/Na carbonate hydroxide solutions before releasing the gas to atmosphere (Bhappu et al. 1993; Chatwin and Kikumoto, 1981; Inami et al., 1990; Shibata and Oda, 1990; Tomita et al. 1990). Basic aluminum sulfate solution is also used (Oshima et al., 1997). The following sections describe the principal sulfuric acid production steps and their purposes. Capture and Fixation ofsulfur 221 Cool gases to 300°C for entry into electrostatic precipitators. Recover heat in waste heat boilers. Drop out dust. Clean gas, recover dust. Absorb CIZ, FZ and SOa. Remove dust. Precipitate and absorb vapors, e.g. AS&, Condense water vapor. Remove acid mist and final traces of dust. Remove moisture to avoid H2S04 condensation and corrosion in downstream equipment. Prepare for SOs absorption Smelting and converting 1250°C, 518% SO2 Gas cooling and dust removal 300°C Electrostatic precipitation of dust 300°C Gas scrubbing and cooling 35°C - 40°C mist precipitation 35°C - 40°C 'i %HzS04 5-7% HzO Air for SOz oxidation (if necessary) 93%HzS04-7%Hz0 Gas drying with 93% diluted 93% HzS04 HzS04 to blending with 98+ Oz/S02 ratio - 1:1, 0% HzO 410°C after heat exchange oxidation of SO2 to SO3 -200°C (after heat exchange) e 98%HzS04-2%HzO Create HzS04 by absorbing SO, into into -98% HzS04 98+%HzS04 to dilution and market -98% H2S04-2%H20 solution Tail gas (-80T) to stack or scrubbing with basic solution Fig. 14.1. Flowsheet for producing sulfuric acid from smelting and converting gases. 222 Extractive Metallurgy of Copper 14.3 Smelter Offgas Treatment 14.3. I Gas cooling and heat recovery The first step in smelter offgas treatment is cooling the gas in preparation for electrostatic precipitation of its dust. Electrostatic precipitators operate at about 300°C. Above this temperature their steel structures begin to weaken. Below this temperature there is a danger of corrosion by condensation of sulfuric acid from SO3 and H20(g) in the offgas. Gas cooling is usually done in waste heat boilers, Fig. 14.2 - which not only cool the gas but also recover the heat in a useful form - steam (Peippo, et al., 1999). The boilers consist of: (a) a radiation section in which the heat in the gas is transferred to pressurized water flowing through 4 cm diameter tubes in the roof and walls of a large (e.g. 25 m long x 15 m high x 5 m wide) rectangular chamber (b) a convection section (e.g. 20 m long x 10 m high x 3 m wide) in which heat is transferred to pressurized water flowing through 4 cm diameter steel tubes suspended in the path of the gas. The product of the boiler is a water/steam mixture. The water is separated by gravity and re-circulated to the boiler. The steam is superheated above its dew point and used for generating electricity. It is also used without superheating for concentrate drying and for various heating duties around the smelter and refinery. Dust falls out of waste heat boiler gases due to its low velocity in the large boiler chambers. It is collected and usually recycled to the smelting furnace for Cu recovery. It is occasionally treated hydrometallurgically (Chadwick, 1992). This avoids impurity recycle to the smelting furnace and allows the furnace to smelt more concentrate (Davenport et al., 2001). An alternative method of cooling smelter gas is to pass it through sprays of water. Spray cooling avoids the investment in waste heat recovery equipment but it wastes the heat in the gases. It is used primarily for Teniente, Inco, Noranda and Peirce-Smith gascs. 14.3.2 Electrostatic precipitation of dust After cooling, the furnace gases are passed through electrostatic precipitators (Parker, 1997, Conde et a/., 1999, Ryan et a/., 1999) for more dust removal. The dust particles are caught by (i) charging them in the corona of a high voltage Capture and Fixation of Surfur 223 a Fig. 14.2. Waste heat boiler for the Ronnsktir flash fkrnace (Peippo et al., 1999). Note, left to right, (i) flash furnace gas offtake; (ii) radiation section with tubes in the walls; (iii) suspended tube baffles in the radiation section to evenly distribute gas flow; (iv) convection section with hanging tubes. Note also water tank above radiation section and dust collection conveyors below the radiation and convection sections. electric field; (ii) catching them on a charged plate or wire; (iii) collecting them by neutralizing the charge and shaking the wires or plates. The precipitators remove 99+% of the dust from their incoming gas (Conde et al., 1999). The dust is usually re-smelted to recover its Cu. About 70% of the dust is recovered in the cooling system and 30% in the electrostatic precipitators. 14.3.3 Water quenching and cooling After electrostatic precipitation, the gas is quenched with water in an open or venturi tower. This quenching: (a) removes the remaining dust from the gas (to 1 or 2 mg/Nm3 of gas) to (b) absorbs C12, F2, SO3 and vapor impurities (e.g. AS&). avoid fouling of downstream acid plant catalyst 224 Extractive Metallurgy of Copper The gas is then cooled further (to 35 or 40°C) by direct contact with cool water in a packed tower or by indirect contact with cool water in a heat exchanger. The gas leaves the cooling section through electrostatic mist precipitators to eliminate fine droplets of liquid remaining in the gas after quenching and cooling. Mist precipitators operate similarly to the electrostatic precipitators described in Section 14.3.2. They must, however, be: (a) constructed of acid-resistant materials such as fiber-reinforced plastic, alloy steels or lead (b) periodically turned off and flushed with fresh water to remove collected solids. 14.3.4 The quenching liquid, ‘acidplant blowdown ’ The water from quenching and direct-contact cooling is passed through water- cooled heat exchangers and used again for quenching/cooling. It becomes acidic (from SO3 absorption) and impure (from dust and vapor absorption). A bleed stream of this impure solution (‘acid plant blowdown’) is continuously withdrawn and replaced with fresh water. The amount of bleed and water replacement is controlled to keep the H2S04 content of the cooling water below about 10% - to avoid corrosion. The quantity of bleed depends on the amount of SO3 in the offgas as it enters the water-quench system. Several smelters have found that SO3 formation is inhibited by recycling some cooled offgas to the entrance of the waste heat boiler. This has the effect of slowing SO2 + SO3 oxidation and decreasing ‘blowdown’ production rate. The ‘acid plant blowdown’ stream is acidic and impure. It is neutralized and either stored or treated for metal recovery (Terayama et al., 1981; Inami et a1.,1990; Trickett 1991, Newman et al., 1999). Fig. 14.3 shows the Toyo smelter’s flowsheet for ‘blowdown’ treatment. 14.4 Gas Drying The next step in offgas treatment is H20(g) removal (drying). It is done to prevent unintentional H2S04 formation and corrosion in downstream ducts, heat exchangers and catalyst beds. The H20 is removed by contacting it with 93% H2S04-7% H20 (occasionally 96 or 98%) acid. H20 reacts strongly with HzS04 molecules to form hydrated acid molecules. Capture and Fixation of Sulfur 225 CaCO, + Acid plant blowdown from H2S04 plant Gypsum CaS04.2H20 Gypsum plant Sulfidization plant CuS (to smelter) (selective precipitation) AsZOS (to arsenic plant) NaHS Arsenates and hydroxides Water purification Ca(OH)2 FeS04 + + O2 + I- plant + Water discard Fig. 14.3. Acid plant 'blowdown' treatment system at Toyo smelter (Inami, et al., 1990). The plant treats 300 m3 of blowdown per day. The blowdown analysis is: Item Concentration, kg/m3 cu 0.5 - 1 As 2-5 Zn 0.5 - 2 HzS04 80 - 150 CI 1-5 The contacting is done in a counter-current packed tower filled with 5 to 10 cm ceramic 'saddles', Fig. 14.4. The sulfuric acid flows down over the 'saddles'. The gas is drawn up by the main acid plant blowers. The liquid product of gas drying is slightly diluted 93% H2S04 acid. It is strengthened with the 98+% acid produced by subsequent SO3 absorption (Section 14.5.2). Most of the strengthened acid is recycled to the absorption tower. A portion is sent to storage and then to market. The gas product of the drying tower contains typically 50-100 milligrams H20/Nm3 of offgas. It also contains small droplets of 'acid mist' which it picks up during its passage up the drying tower. This misr is removed by passing the dry gas through stainless steel or fiber mist eliminator pads or candles. 226 Extractive Metallurgy of Copper > Slightly diluted 93% H2S04 to strong acid circuit and/or market Gas outlet Mist eliminator Acid distributor Ceramic saddles Ceramic packing ,>>>>>>> Cool acid to tower (45°C) 1 acid cooling Fig. 14.4. Drying tower and associated acid circulation and cooling equipment. Acid is pumped around the tubes of the acid-water heat exchanger to the top of the tower where it is distributed over the packing. It then flows by gravity downward through the packing and returns to the pump tank. The mist eliminator in the top of the tower is a mesh ‘pad’. In most SO3 absorption towers this ‘pad’ is usually replaced with multiple candle type mist eliminators. 14.4. I Main acidplant blowers The now-dried gas is drawn into the main acid plant blowers - which push it on to SO2 -+ SO3 conversion and acidmaking. Two centrifugal blowers, typically 3000 kW, are used. They move 100 to 200 thousand Nm3 of gas per hour. The gas handling system is under a slight vacuum before the blowers (typically -0.07 atmospheres gage at the smelting furnace) and under pressure (0.3 to 0.5 atmospheres gage) after. [...]... ( 199 4) Extractive Metallurgy o Copper, 3rd Edition, f Elsevier Science Press, New York, NY, 298 299 Chadwick, J ( 199 2) Magma from the ashes Mining Magazine, 167(4), 221 237 Chatwin, T.D and Kikumoto, N ( 198 1) Surfur Dioxide Control in Pyrometallurgy TMS, Warrendale, PA Conde, C.C, Taylor, B and Sarma, S ( 199 9) Philippines Associated Smelting electrostatic precipitator upgrade In Copper 99 -Cobre 99 ... Overall conversion of SO2 is approximately: [12% SO, (in initial gas) - 0.025% SO2 (in final gas)] 12% SO2 (in initial gas) x 100 = 99 .8% 238 Extractive Metallurgy of Copper 100 Q 99 .5 - 2 99 - $ 98 .5 - ' c c 0 r 8 98 To final absorption Equilibrium - 97 .5 - c - 5 e 97 a" 96 .5 - From intermediate absorption and reheat heat exchangers 96 400 410 420 430 440 450 460 Temperature ("C) Fig 14 .9 Equilibrium... Sulphur 98 Preprints - Volume 2, British Sulphur, London, UK, 123 145 246 Extractive Metallurgy of Copper Ross, K.G ( 199 1) Sulphuric acid market review In Smelter Off-gas Handling and Acid Plants, notes from Canadian Institute of Mining and Metallurgy professional enhancement short course, ed Ozberk, E and Newman, C.J., Ottawa, Canada, August 199 1 Ryan, P., Smith, N., Corsi, C and Whiteus, T ( 199 9) Agglomeration... ( 199 9) Recent operation and environmental control in the Kennecott smelter In Copper 99 -Cobre 99 Proceedings o f the Fourth International Conference, Vol V Smelting Operations and Advances, ed George, D.B., Chen, W.J., Mackey, P.J and Weddick, A.J., TMS, Warrendale, PA, 29 45 Newman, C.J., MacFarlane, G., and Molnar, K.E ( 199 3) Increased productivity from Kidd Creek Copper operations In Extractive Metallurgy. .. Suenaga, C., Okura, T and Yasuda, Y ( 199 0) 20 years of operation of flash furnaces at Saganoseki smelter and refinery Paper presented at the Sixth International Flash Smelting Congress, Brazil, October 14- 19, 199 0 Trickett, A.A ( 199 1) Acid plant design and operations 2 In Smelter Off-gas Handling and Acid Plunts, notes from Canadian Institute of Mining and Metallurgy professional enhancement short course,... Holubec, 1 and Tan, C.G., Metallurgical Society of CIM, Montreal, Canada Parker, K.R (1 99 7) Applied Electrostatic Precipitation, Chapman and Hall, London, England Peippo, R., IIolopainen, H and Nokclaincn, J ( 199 9) Copper smelter waste heat boiler technology for the next millennium In Copper 99 -Cobre 99 Proceedings of the Fourth International Conference, Vol V Smelting Operations and Advances, ed George,... Friedman, L.J ( 199 9) Analysis of recent advances in sulfuric acid plant systems and designs (contact area) In Copper 99 -Cobre 99 Proceedings ofthe Fourth International Conference, Vol V Smelting Operations and Advances, ed George, D.B., Chen, W.J., Mackey, P.J and Weddick, A.J., TMS, Warrendale, PA, 95 117 Guenkel, A.A and Cameron, G.M (2000) Packed towers in sulfuric acid plants - review of current industry... 3 100 (rnax) 12 >I3 29 17 (max) 13 11.1 3766 11.05 11.88 3283 2400 190 0 2614 2130 98 .5 98 ,70 98 .5 98 .5 11 11 236 Extractive Metallurgy o Copper f Table 14.5 Physical and operating of two single absorption sulfuric acid manufacturing plants, 2001 Design of the Mt Isa plant is discussed by Daum, 2000 Smelter Start-up date Manufacturer Gas source Single or double absorption number of catalyst beds intermediate... 10 8 8 8.5 8.5 0.76 0.81 0 .99 1.12 0 .99 0 .94 0 .94 0 .94 0.8 0.87 0 .91 0.87 1.02 Monsanto LP 120 BASF+0. 19 m Cs ring type catalysts BASF+O. 19 m Cs ring type catalysts bed 2 Monsanto LP 120 BASF ring type BASF ring type bed 3 Monsanto LP 110 BASF ring type BASF ring type bed 4 Monsanto LP 1 10 BASF Cs ring type BASF ring type Startup date Single or double absorption number of catalyst beds intermediate... Ojima, Y ( 199 0) Clean and high productive operation at the Sumitomo Toyo smelter Paper presented at the Sixth International Flash Smelting Congress, Brazil, October 14- 19, 199 0 Jensen-Holm H ( 198 6) The Vanadium catalyzed sulfur dioxide oxidation process Haldor Topsoe N S , Denmark King, M.J ( 199 9) Control and Optimization of Metallurgical Sulfuric Acid Plants Ph.D Dissertation, University of Arizona . et al. 199 3; Chatwin and Kikumoto, 198 1; Inami et al., 199 0; Shibata and Oda, 199 0; Tomita et al. 199 0). Basic aluminum sulfate solution is also used (Oshima et al., 199 7). The following. precipitation of dust After cooling, the furnace gases are passed through electrostatic precipitators (Parker, 199 7, Conde et a/., 199 9, Ryan et a/., 199 9) for more dust removal. The dust particles. al., 198 1; Inami et a1., 199 0; Trickett 199 1, Newman et al., 199 9). Fig. 14.3 shows the Toyo smelter’s flowsheet for ‘blowdown’ treatment. 14.4 Gas Drying The next step in offgas treatment