Sulfur oxides from smelting

Despite the clear evidence of a subpopulation of individuals sensitive to near-ambient peak levels of sulfur dioxide, the two-decade-old NAAQS for sulfur dioxide has not been changed nor has a short-term peak standard been instituted. A considerably greater amount of attention has been placed on the contribution of airborne particulates, particularly those associated with sulfur oxides, to adverse health effects. The carbon-, mineral-, and heavy metal-based particles produced during fossil fuel combustion and smelting promote the conversion of sulfur dioxide to sulfuric acid. Recognition of sulfur dioxide-particle interactions comes as a result of findings garnered from a number of animal studies and the characterization of sulfuric acid, ammonium sulfate, and ammonium bisulfate associated with atmospheric particles. The importance of the coexistence of sulfur oxides and particulate matter is reflected in the... 

Ammonia is used in scrubbers to neutralize sulfur oxides in their removal from stack gases in electric power generation and other furnace operations such as smelting. It is also used to improve the efficiency of electrostatic precipitators in the removal of particulate matter. 

Chemical recovery ia sodium-based sulfite pulpiag is more complicated, and a large number of processes have been proposed. The most common process iavolves liquor iaciaeration under reduciag conditions to give a smelt, which is dissolved to produce a kraft-type green liquor. Sulfide is stripped from the liquor as H2S after the pH is lowered by CO2. The H2S is oxidized to sulfur ia a separate stream by reaction with SO2, and the sulfur is subsequendy burned to reform SO2. Alternatively, ia a pyrolysis process such as SCA-Bidemd, the H2S gas is burned direcdy to SO2. A rather novel approach is the Sonoco process, ia which alumina is added to the spent liquors which are then burned ia a kiln to form sodium aluminate. In anther method, used particulady ia neutral sulfite semichemical processes, fluidized-bed combustion is employed to give a mixture of sodium carbonate and sodium sulfate, which can be sold to kraft mills as makeup chemical. 

Roasting. Copper and lead sulfides are direcdy smelted but not zinc sulfide. However, theoretical calculations are encouraging (20) and, if an efficient means of condensing zinc rapidly from 1600 K in the presence of carbon dioxide, sulfur dioxide, and steam can be devised, the process may be feasible. The reaction of zinc vapor to yield zinc oxide or zinc sulfide presents the main difficulty. 

The isolation of zinc, over 90% of which is from sulfide ores, depends on conventional physical concentration of the ore by sedimentation or flotation techniques. This is followed by roasting to produce the oxides the SO2 which is generated is used to produce sulfuric acid. The ZnO is then either treated electrolytically or smelted with coke. In the former case the zinc is leached from the crude ZnO with dil H2SO4, at which point cadmium is precipitated by the addition of zinc dust. The ZnS04 solution is then electrolysed and the metal deposited — in a state of 99.95% purity — on to aluminium cathodes. 

The S02 from the Wellman-Lord process or any other concentrated S02 stream (e.g. gases from copper smelting) can be oxidized to S03 to produce sulfuric acid as discussed in Chapter 6. 

The molten lead flows to the bottom of the smelting furnace (which is usually made of clay) and from there, through a channel and an opening at the bottom of the furnace, collecting in a lead reservoir. Part of the sulfur dioxide formed during the reaction is further oxidized by still more air oxygen into sulfur trioxide ... 

Acid rain is caused primarily by sulfur dioxide emissions from burning fossil fuels such as coal, oil, and natural gas. Sulfur is an impurity in these fuels for example, coal typically contains 2-3% by weight sulfur.1M Other sources of sulfur include the industrial smelting of metal sulfide ores to produce the elemental metal and, in some parts of the world, volcanic eruptions. When fossils fuels are burned, sulfur is oxidized to sulfur dioxide (SO2) and trace amounts of sulfur trioxide (SC>3)J21 The release of sulfur dioxide and sulfur trioxide emissions to the atmosphere is the major source of acid rain. These gases combine with oxygen and water vapor to form a fine mist of sulfuric acid that settles on land, on vegetation, and in the ocean. 

Hybinette A process for extracting nickel from sulfide ores. The nickel ore that occurs in Canada is a mixture of the sulfides of nickel, copper, and iron. Several methods have been used to separate these metals. In the Hybinette process, the ore is first smelted in a blast furnace, yielding a nickel-copper matte (i.e., a mixture of their lower sulfides). This is roasted to remove sulfur and leached with dilute sulfuric acid to remove copper. The resulting crude nickel oxide is used as the anode of an electrochemical cell. The nickel deposits on the cathode, which is contained in a cloth bag. Precious metals collect in the anode slime. The process was invented by N. V Hybinette in 1904 and operated at the Kristiansand refinery, Norway, from 1910. 

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