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From 1924 to 1926, the technique of enrichment at Maiern was modernized, thanks to the introduction of the flotation process, a procedure for separating lead and zinc, still used today. The Maiern plant remained in operation until 1979. Over the decades, some mechanical parts have been improved, but the essential principle is still that of 1925.
It works like this: in the crushing plant, the highest building of all, raw ore is transported by vibration towards a double-jawed crusher which reduces it to the size of gravel. As this process is ‘dry’, it is not only extremely noisy but it also produces great quantities of dust. Mine trucks are used to move the crushed mineral through an intermediate depot to two smaller depots, where it is then carried by hoppers to the mills. After the Second World War, trucks on rails were replaced by conveyor belts. Crushing is the first true innovation of the enrichment process.
The ore mills are drum-shaped, and can crush raw mineral to a finess of 0.1 or 0.2 mm, and thus extract lead and zinc in the form of very small grains. As water is also pumped into the mills, the resulting product is a slurry, consisting of water and tiny grains of metal-bearing minerals, including silver, lead and zinc.
The slurry reaches the flotation unit, where the true process of separation occurs. The minerals are separated out in two rows of flotation tanks, the upper one being used for skimming lead and the lower one for zinc.
The principle of flotation exploits the fact that the addition of certain chemicals makes the surface of the mineral particles in a slurry either hydrophobic (that is, they repel water) or hydrophilic (they attract water). Rotating blades continually move the slurry around in the tanks. Air is also pumped in through the bottom of the tanks and rises as bubbles. Regular, uniformly sized bubbles are made to form by the addition of foaming agents (e.g., pine oil). If a xanthate (also called collector) is added, the lead grains become hydrophobic, whereas the addition of sodium cyanide makes other metal-bearing grains, like zinc, become hydrophilic. The lead particles, being hydrophobic, stick to the air bubbles and rise to the surface. Foam enriched in particles of lead is then regularly skimmed off the top of the slurry by rotating blades.
The remaining slurry, wich contains not only grains of mineral but also blende, flows into larger tanks, where mainly copper sulphate is added. The zinc particles react in a hydrophobic fashion, which means that, in the second group of tanks, they rise to the surface and are skimmed off. The correct degree of acidity (pH) of the slurry is essential for good flotation of zinc and lead particles. The pH is regulated by careful addition of lime and soda, and must be about 7 for lead and about 11 for zinc.
Even this modern separation process does not give pure lead or zinc, but only enriched concentrates. However, compared with the preceding methods of enrichment, the essential metal contents are higher. The percentage of lead in the concentrate is on average 65.5% and that of zinc 51.5%.
When flotation is over, a filtering plant removes the liquid from the two concentrates in slowly revolving, drum-shaped filters. The product is now ready for smelting.
The lead and zinc concentrates, looking like moist sand, used to leave the enrichment section and were taken by truck to industrial foundries in northern Italy or abroad. During smelting, silver could still be extracted from the lead concentrate (about 1 kg per ton) and cadmium from the zinc concentrate (4-5 kg per ton).

Franz Wild (born in 1925) worked for the mine for 35 years, until 1983, almost exclusively in the enrichment plant, where he became foreman in 1951. As he recalls, from 1948 until about 1960, the deafening machinery was in operation throughout the year, without notable interruptions. Work even continued on Saturdays and Sundays, in three shifts. Some miners never took even one day off. There were no ear-plugs then. Some departments of the plant produced great quantities of dust, and the chemicals used in the flotation section damaged workers’ health. Like most miners, Franz Wild too suffers from silicosis.