Sulfuric acid production from sulfide ores

Elemental sulfur, also called brimstone," is the preferred raw material for sulfuric acid production whenever it is available at a reasonable cost. Elemental sulfur may be mined by the well-known Frasch process or recovered from volcanic ores, sour" natural gas, or oil. It is also possible and sometimes commercially feasible to produce elemental sulfur from pyrites, other sulfide ores, and coal. 

The principal raw material source for sulfuric acid production is SO obtained on burning sulfur. In addition, the main minerals of many metals, for example, zinc, lead, copper and silver, are sulfides. A significant quantity of SOj is also obtained as a byproduct from the metallurgical treatment of these ores. Partly this is a result of laws restricting SO emissions, but partly it is also due to the desire to improve process economics. Sulfur dioxide is oxidized to the trioxide SOj in the contact process at 450°C. A catalyst is used, which has rnain component. SOj is absorbed... 

The production of sulfur and sulfur dioxide serves mainly for the purpose of sulfuric acid production. Sulfur is obtained from the desulfurization of natural gas or fuels, from sulfidic ores (e.g., FeS2, CU2S, or ZnS), or from natural reservoirs of elemental sulfur. The production of sulfur from natural reservoirs is carried out by either direct mining or extraction of sulfur from the ground by hot water injection (Frasch process). The further processing of elemental sulfur to sulfuric acid is treated as a process example in this book (Section 6.3). 

In mineral technology, sulfur dioxide and sulfites are used as flotation depressants for sulfide ores. In electrowinning of copper from leach solutions from ores containing iron, sulfur dioxide prereduces ferric to ferrous ions to improve current efficiency and copper cathode quaHty. Sulfur dioxide also initiates precipitation of metallic selenium from selenous acid, a by-product of copper metallurgy (326). 

Sulfide Ores ores. In the Zairian ores, cobalt sulfide as carroUite is mixed with chalcopyrite and chalcocite [21112-20-9]. For processing, the ore is finely ground and the sulfides are separated by flotation (qv) using frothers. The resulting products are leached with dilute sulfuric acid to give a copper—cobalt concentrate that is then used as a charge in an electrolytic cell to remove the copper. Because the electrolyte becomes enriched with cobalt, solution from the copper circuit is added to maintain a desirable copper concentration level. After several more steps to remove copper, iron, and aluminum, the solution is treated with milk of lime to precipitate the cobalt as the hydroxide. 

The most efficient processes in Table I are for steel and alumintim, mainly because these metals are produced in large amounts, and much technological development has been lavished on them. Magnesium and titanium require chloride intermediates, decreasing their efficiencies of production lead, copper, and nickel require extra processing to remove unwanted impurities. Sulfide ores produce sulfur dioxide (SO2), a pollutant, which must be removed from smokestack gases. For example, in copper production the removal of SO, and its conversion to sulfuric acid adds up to 8(10) JA g of additional process energy consumption. In aluminum production disposal of waste ciyolite must be controlled because of possible fiuoride contamination. 

When the sulfide ore carroUite, CuS C02S3, is the starting material, first sulfides are separated by flotation with frothers. Various flotation processes are applied. The products are then treated with dilute sulfuric acid producing a solution known as copper-cobalt concentrate. This solution is then electrolyzed to remove copper. After the removal of copper, the solution is treated with calcium hydroxide to precipitate cobalt as hydroxide. Cobalt hydroxide is filtered out and separated from other impurities. Pure cobalt hydroxide then is dissolved in sulfuric acid and the solution is again electrolyzed. Electrolysis deposits metallic cobalt on the cathode. 

Selenium and tellurium are much less abundant than sulfur and no ores are rich in these elements. They are recovered from the anode slime deposited in the electrolytic purification of copper (having been present as impurities in the copper sulfide ores), as by-products in other sulfide ore processing, and in sulfuric acid manufacture. 

The decomposition of the lower sulfides of the heavy metals and the recovery of the metal as soluble salts and of sulfur in the elemental form have been demonstrated for pyrite, pyrrhotite, chalcopyrite, sphalerite, galena, molybdenite, and associated metals such as nickel and cobalt. Pyrite and chalcopyrite are higher sulfides and to be amenable to this treatment have to be thermally decomposed at 600-650 C prior to leaching. The reactions with nitric acid are exothermic, and are carried out below 1 atm and at around 100°C. In addition to the sulfides, this technique has been applied successfully to the extraction of nonferrous metals from partly oxidized sulfide ores, fayalite slags, copper scrap, and other intermediate products, such as residue from electrolytic zinc plats. 

Nonferrous ores occur mainly in the form of pyrites. The large emission factors associated with nonferrous metal production derive from the fact that sulfur contained in the ores escapes mostly as S02 in spite of control measures. The most significant contribution to S02 emissions from industrial processes lies in the manufacture of sulfuric acid. The conversion of pulp to paper leads to emissions of H2S and organic sulfides, but their magnitude is comparatively small. The combustion of natural gas, which is another important source of energy, causes negligible sulfur emissions so that it is not even listed in Table 10-8. This fuel has a low sulfur content to begin with, and almost all of it is removed before use. 

The price of crude oil can materially affect the transportation/shipment costs of raw sulfur to the manufacturing plants which, in turn, inaeases the cost of manufacturing sulfuric acid from solid/molten sulfur compared to by-product acid from ferrous/non-ferrous metallurgical plants based on sulfide ores. 

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