Pyrite: vitriol and sulphuric acid

Sentences without a label are documented facts; the others are marked "probable" or "uncertain".

At Brosso pyrite, an iron sulphide, was first a waste product, then the raw material of green vitriol and finally of sulphuric acid. This page follows the mineral from the factories of the 18th century to the 1862 report by the engineer Zampari and to Montecatini, and explains the chemistry behind it.

From discarded pyrite to green vitriol

In the oldest period haematite was extracted at Brosso and worked with the low fire "in the Brosso style" probable. As the oligist ran out, Chiappino writes, pyrite, which had previously ended up on the waste heap, became the main ore: by roasting and leaching it became iron sulphate, the "green vitriol" used for dyes probable. The ISPRA page confirms that between the 18th and 19th centuries the exploitation of pyrite took hold, once discarded, from which vitriol is obtained. Novarese reports that according to Sclopis the mine changed in the second half of the 18th century from an iron mine to a pyrite mine, with a green vitriol factory probable. According to the ecomuseum pyrite already appears in the annals in 1715, when the factory is built in the Bore area probable.

Sources: ISPRA, page on the Brosso museum; Chiappino 2010, pp. 25-26; Novarese 1901, p. 75; Brosso ecomuseum page, «Cronologia delle miniere»

The factories and the privileges

In 1769 Count Valperga and Francesco Chiumino found a company that gives a boost to the new factories probable. In June 1781 King Victor Amadeus III grants Chiumino for fifteen years the privilege of making vitriol in the valleys of "Brozzo", Chy and Pont; the day is the 26th for the ecomuseum and the 16th for Chiappino, who quotes the text of the decree: "vitriol, oil, spirit, aqua fortis, rock alum, sulphur" uncertain. In 1790, after the death of Count Valperga, Chiumino cedes his rights to the creditors, who keep the mines and the factory until about 1824 probable.

According to the ecomuseum, in 1826 Carlo Ballauri buys the mines and the Bore factory, and in 1839 Luigi Michele Ballauri hands everything over to the Sclopis brothers, who go on making iron sulphate with a few hundred tonnes of pyrite a year probable. Towards the middle of the 19th century the vitriol factory is no longer competitive, and in 1872 it is closed and demolished probable.

Sources: Chiappino 2010, p. 26; Chiappino 1996, p. 46; Brosso ecomuseum page, «Cronologia delle miniere»; ISPRA, page on the Brosso museum

The chemistry: from sulphide to sulphate and to acid

Pyrite is an iron disulphide, FeS₂. In contact with air and water it oxidises and forms ferrous sulphate and sulphuric acid; in simplified form: 2 FeS₂ + 7 O₂ + 2 H₂O → 2 FeSO₄ + 2 H₂SO₄. This is what the vitriol factories were after: the pyrite was roasted in special furnaces, then washed (leaching), and the sulphate, which dissolves in water, passed into the "mother water", which was concentrated until the green vitriol, FeSO₄·7H₂O, crystallised. In 1862 Zampari counts the cost of fuel for both stages: L. 0.36 per tonne of pyrite for roasting and L. 1.80 for concentrating the mother water.

For sulphuric acid the pyrite is roasted until sulphur dioxide, SO₂, is obtained, which oxidises to sulphur trioxide, SO₃; this with water gives H₂SO₄. The pyrite of Brosso was suitable: Jervis says it is generally free of arsenic probable and Zampari describes the compact pyrite as "free of arsenic" and with 45-50% sulphur; arsenic would have made the acid impure. The same oxidation happens on its own in abandoned tunnels: in 1996, in the Salvere complex, Chiappino found two small pools of reddish water with a pH below 3, due to the oxidation of the pyrite.

Sources: Chiappino 2010, pp. 25-26; Zampari 1862, pp. 1-2; Jervis 1873, p. 118; Chiappino 1996, pp. 70-71

The Sclopis and sulphuric acid

According to Chiappino the Sclopis sulphuric acid factory in Turin was founded in 1812 by Vittorio Sclopis probable. The brothers Ignazio, Giuseppe and Camillo buy the Brosso mines in three deeds, in 1839, 1844 and 1849 probable. Towards the middle of the 19th century the family begins to use the pyrite to produce sulphuric acid in Turin probable: from 1855 according to the ecomuseum, instead of natural sulphur probable. Pyrite replaced sulphur also because the King of Naples had given the firm Taix e C. the exclusive right to the Sicilian sulphur mines, making prices rise probable. In 1858 the factory no longer depends on foreign supplies and pyrite production grows a great deal probable.

In 1873 Jervis writes that the Sclopis extract about 10,000 tonnes of pyrite a year. According to Sclopis, as summed up by Novarese, only in the 19th century did Brosso become a pyrite mine in the proper sense: the ore was sold to the sulphuric acid factories of northern Italy or exported without processing it on the spot probable. The engineer Vittorio Sclopis united the Montefiorito and Baio concessions into a single one of 900 hectares probable; by the end of the 19th century production had grown more than tenfold compared with the age of vitriol probable.

Sources: Chiappino 1996, p. 47; Chiappino 2010, p. 26; ISPRA, page on the Brosso museum; Brosso ecomuseum page, «Cronologia delle miniere»; Jervis 1873, p. 118; Novarese 1901, p. 75

The Zampari report of 1862

On 28 December 1862 Francesco Zampari, assistant of the Mines, signs in Turin a report on the "Miniera di pirite di ferro di Brozzo dei Fratelli Sclopis", the Sclopis brothers' iron pyrite mine of Brozzo. The mine yields 2,200 tonnes of pyrite a year. About 1,000 tonnes, the "less compact" pyrite, are roasted on the spot for iron sulphate: about 200 kilos per tonne are obtained, sold at 90 lire a tonne. The other 1,200 tonnes of compact pyrite go unprocessed to the sulphuric acid factories of Turin.

Zampari looks above all at the costs. A tonne of pyrite costs L. 4.75 to dig, L. 7.50 to transport from the mine to Ivrea and L. 4.10 from Ivrea to Turin: transport weighs more than digging. He proposes a cart road from the mine to Caglia, which would bring the cost of transport to Ivrea down to L. 3.50, and imagines selling 1,000 tonnes a month to the Casa Daniel of Marseilles, by train to Genoa and then by ship: the value of the mine would double. In his accounts the net income is L. 64,960 a year.

Sources: Zampari 1862, pp. 1-3

Pyrite in the 20th century

The last known step of the Sclopis is a works at Valcava for the mechanical preparation of the ores and the enrichment of the pyrites poor in sulphur probable; from 1907, Chiappino writes, a cableway carries the concentrate to the station of Montalto Dora probable. In 1931 the mines pass to Montecatini, the Italian mining and chemical company, which extracts pyrite there until it runs out in the 1960s probable. According to the last director, the engineer Bonino, from the 18th century onwards Brosso was worked only for pyrite and the oligist was left in place probable.

Traversella too, after the beginning of the 20th century, extracts pyrite and chalcopyrite, used above all to produce sulphuric acid. In the silo built by FIAT around 1940 pyrite and chalcopyrite are recovered by flotation probable.

Sources: Chiappino 2010, p. 26; Vescovo 2002, pp. 79-80; Costa et al. 2019, p. 2; Museo della Miniera website, «Il Museo»

What remains

Among the remains of Brosso Chiappino lists the Bore vitriol works and the conical furnaces for roasting pyrite probable. On the 1924 plan of the workings that she uses in her thesis there are two conical furnaces for pyrite, marked with the numbers 7 and 8, near the 560-metre contour; the same map shows the Sclopis villa with the Bore vitriol works and two cubic furnaces. What was roasted in the kilns of the Strada delle Vote, on the other hand, is disputed: the page "Who thinks otherwise" tells the story.

Sources: Chiappino 2010, p. 29; Chiappino 1996, pp. 64-65, fig. 10.1

For the class

Calculations to try with Zampari's figures. If each tonne of roasted pyrite yields about 200 kilos of iron sulphate sold at 90 lire a tonne, how much is the sulphate obtained from the year's 1,000 tonnes worth? (About 200 tonnes, 18,000 lire.) How much did it cost to bring a tonne of pyrite to Turin compared with digging it? (L. 11.60 for transport against L. 4.75 for digging: about two and a half times as much.) Finally, with the atomic masses of S, Fe, O and H, calculate the percentage of sulphur in pure pyrite and compare it with the 45-50% of the compact pyrite of Brosso.

Sources: Zampari 1862, pp. 1-3