A history of the changing fortunes of pyrite I
Pyrite has been known since antiquity. Mentioned by the Greek writer Theophrastus (ca. 372-287 BC), it was named for the Greek word for fire "pyr," as it produces sparks when struck with another stone. Gold and pyrite, very similar in appearance, are often hard to distinguish visually. Pyrite was used for ornaments and jewelry in ancient Greek and Roman civilizations. The Incas even used polished pyrite slabs as mirrors. Being cubic in atomic structure pyrite usually forms attractive cubic crystals. It also exists in a rhombic phase called marcasite.
Pyrite is the most common sulphide mineral and is widely associated with other metal sulphide deposits. Containing about 53 per cent sulphur, pyrite was once of great strategic importance as a source of elemental sulphur for gunpowder manufacturing and of sulphur dioxide for sulphuric acid production. In the 16th century, heap leaching of copper-containing pyrite was practiced in the Harz Mountains area in Germany and the Río Tinto mines in Spain. Here, pyrite was piled and left for months in the open air, where the action of rain and air brought about oxidation and the dissolution of copper. A solution containing copper sulphate was drained from the heap and collected in a basin. Metallic copper was precipitated from this solution by scrap iron — a process that became known as “cementation,” ostensibly from the Spanish word for precipitation, “cementación.”
The importance of pyrite can be judged from Johann Friedrich Henckel’s (1679-1744) book on the mineralogy of sulphide minerals, Pyritologia, oder Kiess-Historie. Published in Leipzig in 1725, the book was translated into English in 1757 and French in 1760. In 1907, the French chemist P. Turchot published a comprehensive book on pyrite. However, when other sources of cheap elemental sulphur became available, pyrite not only lost its importance but also became a nuisance to the metallurgical industry.
Pyrite for gun powder manufacture
Gunpowder, also called black powder, was the only explosive available for centuries before the invention of dynamite. It consisted of, approximately, 75% saltpetre (potassium nitrate), 15% charcoal, and 10% sulphur. The sulphur used in gunpowder had to be of high quality. Limited amounts of sulphur were mined from a deposit in Sicily that comprised a mixture of sulphur, gypsum, limestone and clay that typically contained about 25% sulphur. Sulphur melted from the excavated material was purified by sublimation. Major quantities of sulphur were, however, obtained by heating pyrite in absence of air. Agricola (1494-1555), in his De Re Metallica, described the recovery of sulphur from brimstone and pyrite.
The process was also described by Diderot in his Encyclopaedia (1751-1772). Pyrite lumps were stacked on logs that were ignited. The resultant sulphur vapours were condensed on the roof of the chamber and collected in pots below. Sulphur in the pots was then refined by heating and the sublimed crystals are collected in a cone constructed in the roof of the furnace. So important was this operation that the distinguished French chemist Antoine Lavoisier (1743-1794) was in charge of the black powder factory in Paris. After Lavoisier was executed during the French Revolution, his student, Eleuthére Irénée du Pont de Nemours, migrated to America and, in 1802, established a plant near Wilmington, Delaware, to produce black powder based on the French technology.
Pyrite for sulphuric acid manufacture
The second important role of pyrite was in the manufacture of sulphuric acid, the backbone of the chemical industry. The major industrial demand for sulphuric acid was for the Leblanc process of making sodium carbonate (developed c.1790)1, for treating phosphate rock to manufacture superphosphate and for the pickling of steel. Sulphuric acid was usually produced from brimstone found in Sicily. When the Neapolitan Government granted a French firm the monopoly over brimstone trade in 1838, the firm raised its price from £5 to £14 per tonne. Sulphuric acid manufacturers, therefore, turned to pyrite as a source of sulphur dioxide, obtained by the following reaction:
2 FeS2 + 11/2 O2 Æ 4 SO2 + Fe2O3
In 1817, the Swedish chemist Jöns Jacob Berzelius (1779-1848), observing the formation of a red deposit in a lead chamber acid plant using pyrite from the Falun mine, discovered selenium. Lead chambers were also deteriorated when pryrite containing traces of mercury was roasted because volatilized mercury attacked the chambers’ lead lining. Mercury and selenium are also harmful because they poison the catalyst used in the contact process, which had replaced the chamber process.
The manufacture of phosphatic fertilizers, organic dyestuffs, nitro compounds for explosives; the pickling of steel; the refining petroleum products; and many other applications led to increased demand for sulphuric acid. Sulphur dioxide was also in demand in the pulp and paper industry.
A variety of roasters were specially developed for roasting pyrite for acid manufacture. The first multiple hearth roaster was designed by the British inventor Alexander Parkes (1813-1890), which was later bettered by the American engineer, John Brown Francis Herreshoff (1851-1932). Multi-stage roasters was later replaced by the more efficient fluidized bed reactors invented in Germany in 1922 by Fritz Winkler (1888-1950).
While sulphur dioxide can be readily converted to sulphur trioxide and then to sulphuric acid (H2SO4), iron oxide (called cinder) cannot be used directly for manufacturing iron because of the presence of impurities. Also, some pyrite contained appreciable amounts of copper and zinc and it was desirable to recover these values. In 1844 in England, W. Longmaid patented a method of purifying cinder and simultaneously recovering the nonferrous metals present in it. The method was first applied by William Henderson of Scotland in 1859. Lower copper content in pyrite, especially since World War I, compelled the extraction of further products from pyrite. In the Longmaid-Henderson process, pyrite cinder was roasted with sodium chloride and then leached with water to recover nonferrous metal chlorides. The technology was adapted in Germany for over a century at the Duisburger Kupferhütte plant in Duisburg from 1876 to 1982.
In this plant, pyrite was imported from all over the world and sold to acid manufacturers in Germany on the agreement that the iron oxide (cinder) resulting from roasting would be shipped back to Duisburg for further treatment to recover nonferrous metals, precious metals and metallic iron. The pyrite cinder is mixed with common salt (NaCl) and heated continuously in a multiple hearth furnace at 800 degrees Celsius to transform nonferrous metals into water-soluble chlorides. Each batch requires about two days for leaching in vats. Copper is precipitated from the solution in two steps:
Cu + CuCl2 Æ Cu2Cl2(ppt)
Cu2+ + Fe Æ Cu + Fe2+
Copper for the first step is obtained from the second step. Interestingly, the blue copper chloride solution turns pink after reduction and precipitation of cuprous chloride due to the presence of cobalt ions in the remaining solution. The residual copper is colourless. The recovered cuprous chloride is treated with calcium hydroxide to precipitate copper (I) oxide, which is reduced with coal in a furnace to black copper:
Cu2Cl2 + Ca(OH)2 Æ Cu2O + CaCl2 + H2O
2Cu2O + C Æ 4Cu + CO2
The black copper was cast into anodes and refined electrolytically, with precious metals being collected in the anodic slimes. The solution obtained after cementation is evaporated under vacuum to recover Na2SO4.10H2O.
In 1975, the production of black copper was abandoned in favour of a hydro-electrowinning process in which cuprous oxide was leached in recycle acid and the copper sulphate solution obtained was electrolyzed to produce copper cathodes.
The residue, called “purple ore,” a high-grade iron ore (61 to 63% Fe), is sintered and delivered to the blast furnace. Because lead and silver form chlorides during roasting that are insoluble in the leaching step, they remained in the purple ore. When the sintered purple ore was charged in the blast furnace, lead-silver alloy was formed. Being insoluble in iron and having a higher density, it settled to the bottom of the hearth. An opening was let into the furnace below the iron notch to tap the lead-silver alloy once a week.
Duisburger had an excellent research laboratory staffed by highly skilled chemists and engineers who devised a complex and ingenious process.
Smelting of pyrite concentrates
Pyrite smelting was developed to melt massive sulphide ore to form matte and to recover the excess sulphur in the elemental form. Pyrite smelting was first successfully operated in 1928 by Orkla Grube in Norway. Similar operations existed in Sweden, Portugal, Spain and Russia. Pyrite containing about two per cent copper is mixed with coke, quartz, and limestone and heated in a blast furnace. In the upper part of the furnace, one atom of sulphur in pyrite is distilled as elemental sulphur. In the oxidizing zone, iron sulphide formed is oxidized to ferrous oxide and sulphur dioxide. In the middle part of the furnace, the reduction zone, sulphur dioxide is reduced by coke to elemental sulphur, which is volatilized as vapour. The reactions are as follows:
Upper zone: FeS2 Æ FeS + 1/2 S2
Oxidation zone: FeS + 3⁄2 O2 Æ FeO + SO2
Middle zone: SO2 + C Æ CO2 + 1⁄2 S2
Carbon disulphide and carbon oxysulphide are formed in the furnace. They are converted to elemental sulphur on catalytic beds. The matte produced contains six to eight per cent copper and is usually re-smelted with coke, silica, and limestone to 40% Cu. The average analysis of the ore treated at Orkla Grube, and the composition of slag, matte and the exit gases from the furnace are given in the table. Ores containing arsenic are difficult to treat by this method because sulphur vapour containing a large percentage of arsenic starts to condense at 350 degrees Celsius, but arsenic-free vapour condenses at a much lower temperature. Further, liquid sulphur containing arsenic is viscous and difficult to handle. Passing the gases over catalyst beds for converting carbon disulphide and carbon oxysulphide to sulphur is therefore not successful.
Sodium chloride was treated with concentrated sulphuric acid to form sodium sulphate, which was then reduced with carbon to form sodium sulphide, which, in turn was reacted with limestone to form sodium carbonate and calcium sulphide. The Leblanc process was supplanted by the Solvay process in 1863.