The Effect of Deposit Temperature on the Catalytic SO 2 -to-SO 3 Conversion in a Copper Flash Smelting Heat Recovery Boi
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DUCTION
FLASH smelting is a widely used method to extract primary copper[1] and nickel[2] from sulfide ores through exothermic combustion reactions. The process is continuous and nearly autothermal, making it very cost-effective and environmentally sustainable.[3] However, due to the presence of large amounts of sulfur-containing species in the feedstock, the process gas in a copper flash smelting heat recovery boiler typically contains around 40 vol pct SO2, which can be oxidized to sulfur trioxide (SO3) during the process. In addition to the high SO2 concentration in the flue gas, roughly 5 vol pct H2O is also present, which, when reacting with SO3, enables sulfuric acid (H2SO4) formation under suitable conditions. The factor that most influences the amounts of SO2 and H2O is whether the heat balance is controlled by oxygen enrichment or burning fuel. The H2SO4 formation originates most likely from the presence of catalytic species in the flue dust, promoting the oxidation of SO2 to SO3, which, when the temperature decreases, will then further react with water vapor to form H2SO4,[4] either through a reaction involving an adduct (Eq. [1])[5,6] or a water dimer (Eq. [2]).[7] The flue dust particles are partly sintered due to the highly
JUHO LEHMUSTO, EMIL VAINIO, and TOR LAURE´N are with the Laboratory of Inorganic Chemistry, Johan Gadolin Process Chemistry Centre, Abo Akademi University, Piispankatu 8, 20500 Turku, Finland. Contact e-mail: juho.lehmusto@abo.fi MARI LINDGREN is with the Outotec Research Center, Kuparitie 10, P.O. Box 69, FI-28101 Pori, Finland. Manuscript submitted March 31, 2017.
METALLURGICAL AND MATERIALS TRANSACTIONS B
exothermic sulfation reactions taking place in the process gas.[8] The sintered particles might adhere to heat-transfer surfaces in the heat recovery area, resulting in deposits decreasing the heat-transfer efficiency and blocking the gas flow path.[9] Inside the deposit, at temperatures below the dew point temperature, the H2SO4 will condense at the deposit/heat-transfer interface, resulting in corrosion and severe material degradation.[10,11] In fact, a strong indication of sulfuric acid-induced corrosion was observed in full-scale measurements carried out in an operating copper flash smelter plant.[11] Interestingly, there are less corrosion-related problems in nickel flash smelters compared to copper flash smelters. Although the reason for this ambiguous, it might be due to differences in process conditions, in feed impurities, or in lower deposit build-up rate. SO3 H2 OðgÞ þ H2 OðgÞ ! H2 SO4 ðgÞ þ H2 OðgÞ ½1
SO3 þ H2 OðgÞ H2 OðgÞ ! H2 SO4 ðgÞ þ H2 OðgÞ ½2 There is a notable temperature gradient within the deposit, ranging from roughly 1173 K (900 °C) of the outermost part of the deposit to around 553 K (280 °C) in the deposit/heat-transfer interface.[12] Therefore, the SO2-to-SO3 conversion occurs over a range of temperatures, as the deposit is constantly flushed by SO2-rich process gas. This temperature gradient effect has been previously addressed with the focus on high-temperat
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