Heat generation in a production scale sulphide heap leach operation

M.Sc. Antti Arpalahti, Professor Mari Lundström

A heap leaching study was conducted with a pyrrhotite rich pentlandite black schist ore originating from Talvivaara, Finland. The ore contains about 20 wt.-% metal sulphides, with pyrite and pyrrhotite as the dominant sulphide species. Valuable metals for the operation are present in pyrrhotite (nickel), pentlandite, violarite and sphalerite. The ore was crushed to 80 % below 8 mm size, agglomerated and stacked to two 8.5 m tall heaps with 350 m width and 132 m top length. The first heap was constructed in autumn 2015 and the second during winter 2016. Irrigation with 5 to 7 L/m2h was applied from the top of the heap and air feed (ca. 0.06 m3/ton ore/hour) applied with two level piping inside the heap. Heat generation was estimated from the heat transferred to the solution across the heaps. Optical cables were installed inside the heap sections and temperature read by the Distributed Temperature Sensing (DTS) method. DTS method was shown work in a heap with temperatures up to 160 °C. The heat generation rates were observed up to 10 W/ton ore, representing oxygen (in air) efficiencies from 10 to 14 %. The heap section constructed during winter experienced a lag in heat generation while the one constructed in autumn heated up in two to three days after irrigation and aeration were started.