Iron-sulfur minerals, sulfuric acid

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). 

Although ilmenites and leucoxenes can be used in the chloride process, ores having higher Ti02 contents, eg, mineral mtile, which is not readily attacked by sulfuric acid, are preferred in order to minimise loss of chlorine in iron chloride by-product. 

More recendy, the molten caustic leaching (MCL) process developed by TRW, Inc. has received attention (28,31,32). This process is illustrated in Eigure 6. A coal is fed to a rotary kiln to convert both the mineral matter and the sulfur into water- or acid-soluble compounds. The coal cake discharged from the kiln is washed first with water and then with dilute sulfuric acid solution countercurrendy. The efduent is treated with lime to precipitate out calcium sulfate, iron hydroxide, and sodium—iron hydroxy sulfate. The MCL process can typically produce ultraclean coal having 0.4 to 0.7% sulfur, 0.1 to 0.65% ash, and 25.5 to 14.8 MJ/kg (6100—3500 kcal/kg) from a high sulfur, ie, 4 wt % sulfur and ca 11 wt % ash, coal. The moisture content of the product coal varies from 10 to 50%. 

It can be seen, therefore, that ferrous iron and chalcopyrite oxidation are acid-consuming reactions, while pyrite oxidation and iron hydrolysis are acid-producing reactions. Thus, whether the overall reaction in a dump is acid producing or acid-consuming depends on the relative proportions of chalcopyrite and pyrite and on the pH conditions. In practice, sulfuric acid additions to the leach solution applied to the dump are usually required to overcome the acid consuming reactions of the gangue minerals and to keep the pH in a suitable range, typically 2 to 2.4, to optimize bacterial activity and minimize iron hydrolysis. 

In most uranium ores the element is present in several, usually many diverse minerals. Some of these dissolve in sulfuric acid solutions under mild conditions, while others may require more aggressive conditions. Thus, while it may be comfortable to recover 90-95% of the uranium present, it may be tough or impractical to win the balance amount of a few percent economically. Some of the most difficult uranium minerals to leach are those of the multiple oxide variety, most commonly brannerite and davidite. These usually have U(IV) as well as U(VI), together with a number of other elements such as titanium, iron, vanadium, thorium, and rare earths. To extract uranium from these sources is not as easy as other relatively simpler commonly occurring sources. 

HAL [Hot acid leaching] A process for purifying silica sand or zircon by leaching out surface iron compounds with hot sulfuric acid. Derived from an earlier process, invented in 1955 by British Industrial Sand, in which silica sand was treated with hot, gaseous hydrogen chloride. The process for cleaning zircon sand was developed jointly by Hepworth Minerals Chemicals, UK, and Metallurgical Services Pty, Australia, in 1991. 

Acid leaching is usually carried out using sulfuric acid under pressure to dissolve the majority of the iron minerals and to release the cobalt and nickel. If this is carried out at 150-250°C, then the iron(III) is precipitated as haematite or jarosite, reducing the amount of iron in the leachate. The... 

Although Johann Heinnch Pott knew that it is not a lead mineral, he confused it with graphite, Reissbley, and believed that it contained lime, iron, and sulfuric acid (50). 

Society of Edinburgh that lie had discovered soda in several varieties of Scottish whinstone and m lava from Mt, Aetna (25). He used the term whinstone to include basalt, trap, and certain kinds of porphyry, wacke, and other argillaceous stones. When he analyzed a specimen which had been broken from one of the famous basaltic columns of Staffa, he found that the sum of the earths, silica, and iron never amounted to more than 94 per cent. Suspecting the presence of an alkali, he heated the pulverized mineral with pure sulfuric acid and extracted a salt which he identified as sodium sulfate (25). He proved, moreover, that the sodium compounds had not been dissolved from his glass apparatus. Dr. Kennedy also found 4 per cent of soda in a specimen of lava brought to him by Sir James Hall and Dr. James Home from the famous current of Mt. Aetna which in 1669 had destroyed part of the town of Catania. He published these analyses in 1800 in Nicholsons Journal (25). 

The reddish brown calx dissolved in sulfuric acid to give a yellow solution which became purple when reduced with zinc, tin, or iron, and when the pulverized mineral was fused with powdered charcoal, a purple slag was formed. 

In 1771 G. W. Scheele investigated a green variety of fluorspar from Garpenberg and a white one from Gislof in Scania. He found that the green specimen contained a trace of iron but that the white one did not. When he heated the pulverized mineral with oil of vitriol [sulfuric acid], he noticed that the inner surface of the glass retort became corroded,... 

Pyrite is the most common mineral among sulfides. It occurs not only as a major mineral of sulfide ore deposits of base metals, such as Cu, Pb, Zn, in vein-t) e, massive-replacement t) e, kuroko-t5q3e deposits, etc., but also sporadically as an accessory mineral in volcanic, sedimentary, and metamorphic rocks. It also occurs as a precipitate in hot springs, and it may be formed by bacterial action. Pyrite itself is not an ore of Fe, though it contains iron, and at best may have economic value as an ore to obtain sulfuric acid. However, due to its occurrence in and... 

Minerals of economic importance within sedimentary formations include, hut are not limited to fluorite, barite, phosphorite, and oolitic hematite. Fluorite is utili/ed us a flux in steelniakiitg and when of high quality as lenses and prisms in the optical industry. Barite is an essential mineral used m gas- and oil-well drilling. Phosphorite, a product of chemical precipitation from seawater, when ircaled with sulfuric acid, produces superphosphate fertilizer, (.killtic hematite deposits of extensive size are important sources of iron ore. 

A very different conceptual model is based on the assumption that the early Martian atmosphere was dominated by acidic volatiles from volcanic activity, which leads directly to sulfuric acid weathering (Fig. 5.15). This model leads to the dominance of sulfate minerals and leaves open the possibility of both ferrous and ferric iron sulfates. In addition to atmospheric acidification,... 

Both Eqs. 3 and 4 show that hydrogen ion concentration should affect the rate of hemicellulose hydrolysis, but the neutralization capacity of biomass is not always taken into consideration in models reported in the literature. Neutralization is caused by basic minerals containing potassium, sodium, calcium, iron, and other cations present in biomass reacting with sulfuric acid and reducing available hydrogen ions stoichiometrically (17) ... 

A major contribution of this paper was pointing out the importance of bioturbation and bioirrigation on chemical processes associated with carbonate dissolution. In the movement of sulfidic sediment from depth to near the interface by biological processes, oxidation of the sediment produces sulfuric acid which ends up titrating alkalinity, lowering pH, and thus lowers saturation state (e.g., Berner and Westrich, 1985). Actually this process is very complex, involving many reactive intermediate compounds such as sulfite, thiosulfate, polythionates, etc. Aller and Rude (1988) demonstrated an additional complication to this process. Mn oxides may oxidize iron sulfides by a bacterial pathway that causes the saturation state of the solution to rise with respect to carbonate minerals, rather than decrease as is the case when oxidation takes place with oxygen. 

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