MINIERE DI GORNO-ONETA - LAVERIA DI RISO
flotation plant (Pb, Zn) / Gorno / 1914 - 1982 / 2009, 2010
The Riso flotation plant is one of the few remaining evidences of the ancient lead and zinc mining activity in the Bergamasque Alps, initiated already during the Roman Age. Its construction (1911-14) was promoted by the English Crown Spelter which, together with the Belgian Vieille Montagne, owned most of the mines in the upper Seriana valley at that time. The purpose of this flotation plant was to treat poor zinc/lead sulfurs such as galena and sphalerite through a process of enrichment, thus leading to an increase of concentration from 18-20% to 30-31%. At its inauguration, the plant had a capacity of 120 t/d. In 1922 the activities of the English Crown Spelter were taken over by the Vieille Montagne. The latter promoted a general refurbishment of the flotation plant in 1925-27, which led to a complete reconstruction as well as to an increase of daily capacity up to 180 t. To ensure a better connection with the upper mining sites an aerial ropeway was installed, while a couple of inclined planes provided with funiculars served the narrow gauge railway coming from the lower mines.
After a decade of constant intensification of mining production at Gorno (+35,61% in 1935), in 1940-41 the Vieille Montagne was forced to leave the driving due to technical and financial troubles arisen with the German invasion of Belgium. The Italian Ministry of Corporations thus assigned the mining concessions and related facilities (included the Riso flotation plant) to AMMI-Azienda Minerali Metallici Italiani, a public-private enterprise founded in 1936 and focused on nonferrous mining and metallurgy. In 1946 AMMI transferred all its activities in the Bergamo mining district to its subsidiary SAPEZ-Società per Azioni Piombo e Zinco. Under SAPEZ a general restructuring took place, which foresaw amongst other things the concentration of the whole mineral processing in the Riso flotation plant. After a significative improvement – which extended the range of processed minerals towards lead sulfurs and carbonates and zinc sulfurs, carbonates and silicates, – the flotation plant reached a maximum capacity of 700 t/d. The mining and processing activities developed quite regularly until the 1970s, when AMMI was taken over at first by EGAM and, in 1977, by ENI group. Although initially ENI promoted new underground researches and thus prospected possibile future developments for the Gorno mining complex, the assignment of this to its subsidiary SAMIM-Società Azionaria Minerario-Metallurgica (1981) revealed soon the intention of disengaging from the lead and zinc mining activity, no more considered as a strategic business. Accordingly, the flotation plant and all the rest were progressively stopped in the course of 1982.