ulfide minerals, which humans extract and use. These uses generally lead to the formation of so If we wish to prevent this SOz from getting into the atmosphere, we can use any of the methods described in this chapter, all of which have the effect of capturing the sulfur dioxide in the form of CasO4 2H20 that will then be returned to the earth, normally in a landfill. Most often the overall reaction will be CaCO3+So2+0.502--->CaSo4+C02 (112) (limestone) In this reaction one kind of widely available rock(limestone) is mined and used to produce another rock(anhydrite or, with 2H20, gypsum), which we put back into the ground, and to release carbon dioxide to the atmosphere We are concerned about adding to the CO2 in the atmosphere but not nearly as much as we are about adding an equivalent amount of SOz. Although Eq (11.2) ppears simple, the details of carrying it out on a large scale are complex, as discussed in this chapter In natural gas most of the sulfur is in the form of H2 S, which is easily separated from the other constituents of the gas. In oil (liquid petroleum)and also in oil shales and tar sands, the sulfur is chemically combined with the hydrocarbon compounds; normally it cannot be removed without breaking chemical bonds. In oils the sulfur is concentrated in the higher-boil ing fraction of the oil, so the same crude oil can yield a low-sulfur gasoline(average 0.03%S)and a high-sulfur heavy fuel oil(e.g, 0.5 percent to I percent S). In coal much of the sulfur is also in the form of chemically bound sulfur, but some coals have a large fraction of their sulfur in the form of small (typically 100 u) crystals of iron pyrite("fools gold, "FeS2). When the fuel is burned, almost all of the sulfur in the fuel, whether chemically bound or pyritic, is converted to sulfur dioxide(so) and carried along with stack gas. Some small fraction is captured in the ash, and some is converted to SO3. Mixtures of So and SO are sometimes called SOx to remind us that some of the sulfur is in the form of SO. Usually the SO3 is negligible, and we speak of these streams as if the only ulfur oxide they contained was so2 The other important source of soz attributable to humans is the processing of sulfur-bearing ores The principal copper ore of the world is chalcopyrite, CuFeS2. The basic scheme for obtaining copper from it is the overall high-temperature smelting reaction, CuFeS2+5/2 0, Cu+FeO+2SO in which the iron is converted to a molten oxide that will float on the molten copper( with a silica flux) and thus be separated from it. The sulfur is converted to gaseous SO2. The principal ores of lead, zinc, and nickel are also sulfides, whose processing is similar to Eq(11. 3) Because the SOz liberated in the preceding process has been widely recognized as an air pollutant for many years, considerable effort has been devoted to finding other ways to process these ores that do not produce SO2. There has been some success in developing processes that treat these ores by aqueous chemistry without producing any SO2 at all. Currently such processes are economical for partly oxidized copper oxide ores containing smaller amounts of sulfur. However, for ores like chalcopyrite, the processes have not proven economical and most of these ores are currently smelted with air or oxygen The sulfur-containing gas streams most often dealt with in industry belong to three categories--reduced sulfur, concentrated SO2 streams, and dilute SO2 streams --each with its own control method. as discussed in this chapter 11.3 The Removal of Reduced Sulfur Compounds from petroleum and Natural Gas As discussed, we can convert sulfur in organic compounds to various forms by oxidation or reduction. Here we discuss the technology for removing sulfur from gas streams when the sulfur is present in reduced form. These gas streams occur in many natural gas deposits and in many by-product gases produced in oil refining and in the fuel gases produced by coal gasification This is a large liquid flow rate. To make the system practical, one must find a solvent that can absorb much more H2s than can the water. Fortunately, for many of the gases of air pollution and industrial interest, we can do that. H2S, SO, SO, NO2, HCl, and COz are acid gases, which form acids by dissolving in water. For H2S the process H2S (gas)+H2S( dissolved in water)+*H*+ HS If we can add something to the scrubbing solution that will consume either the h or the HS, then more H,s can dissolve in the water. and much less water is needed. For acid gase s, the obvious l1-311－3 sulfide minerals, which humans extract and use. These uses generally lead to the formation of SO2. If we wish to prevent this SO2 from getting into the atmosphere, we can use any of the methods described in this chapter, all of which have the effect of capturing the sulfur dioxide in the form of CaSO4 . 2H20 that will then be returned to the earth, normally in a landfill. Most often the overall reaction will be CaCO3 + SO2 + 0.502 ----> CaSO4 + CO2 (11.2) (limestone) In this reaction one kind of widely available rock (limestone) is mined and used to produce another rock (anhydrite or, with 2H20, gypsum), which we put back into the ground, and to release carbon dioxide to the atmosphere. We are concerned about adding to the CO2 in the atmosphere, but not nearly as much as we are about adding an equivalent amount of SO2. Although Eq. (11.2) appears simple, the details of carrying it out on a large scale are complex, as discussed in this chapter. In natural gas most of the sulfur is in the form of H2 S, which is easily separated from the other constituents of the gas. In oil (liquid petroleum) and also in oil shales and tar sands, the sulfur is chemically combined with the hydrocarbon compounds; normally it cannot be removed without breaking chemical bonds. In oils the sulfur is concentrated in the higher-boiling fraction of the oil, so the same crude oil can yield a low-sulfur gasoline (average 0.03% S) and a high-sulfur heavy fuel oil (e.g., 0.5 percent to 1 percent S). In coal much of the sulfur is also in the form of chemically bound sulfur, but some coals have a large fraction of their sulfur in the form of small (typically 100 μ) crystals of iron pyrite ("fools gold," FeS2). When the fuel is burned, almost all of the sulfur in the fuel, whether chemically bound or pyritic, is converted to sulfur dioxide (SO2) and carried along with stack gas. Some small fraction is captured in the ash, and some is converted to SO3. Mixtures of SO2 and SO3 are sometimes called SOx to remind us that some of the sulfur is in the form of SO3. Usually the SO3 is negligible, and we speak of these streams as if the only sulfur oxide they contained was SO2. The other important source of SO2 attributable to humans is the processing of sulfur-bearing ores. The principal copper ore of the world is chalcopyrite, CuFeS2. The basic scheme for obtaining copper from it is the overall high-temperature smelting reaction, CuFeS2 + 5/2 O2 → Cu + FeO + 2SO2 (11.3) in which the iron is converted to a molten oxide that will float on the molten copper (with a silica flux) and thus be separated from it. The sulfur is converted to gaseous SO2. The principal ores of lead, zinc, and nickel are also sulfides, whose processing is similar to Eq. (11.3). Because the SO2 liberated in the preceding process has been widely recognized as an air pollutant for many years, considerable effort has been devoted to finding other ways to process these ores that do not produce SO2. There has been some success in developing processes that treat these ores by aqueous chemistry without producing any SO2 at all. Currently such processes are economical for partly oxidized copper oxide ores containing smaller amounts of sulfur. However, for ores like chalcopyrite, the processes have not proven economical and most of these ores are currently smelted with air or oxygen. The sulfur-containing gas streams most often dealt with in industry belong to three categories--reduced sulfur, concentrated SO2 streams, and dilute SO2 streams --each with its own control method, as discussed in this chapter. 11.3 The Removal of Reduced Sulfur Compounds from Petroleum and Natural Gas Streams As discussed, we can convert sulfur in organic compounds to various forms by oxidation or reduction. Here we discuss the technology for removing sulfur from gas streams when the sulfur is present in reduced form. These gas streams occur in many natural gas deposits and in many by-product gases produced in oil refining and in the fuel gases produced by coal gasification. This is a large liquid flow rate. To make the system practical, one must find a solvent that can absorb much more H2S than can the water. Fortunately, for many of the gases of air pollution and industrial interest, we can do that. H2S, SO2, SO3, NO2, HCI, and CO2 are acid gases, which form acids by dissolving in water. For H2S the process is H2S (gas) ↔ H2S (dissolved in water) ↔ H+ + HS- (11.4) If we can add something to the scrubbing solution that will consume either the H+ or the HS- , then more H2S can dissolve in the water, and much less water is needed. For acid gases, the obvious