Vanadium Mining in Finland
The Otanmäki mine produced 30 million tonnes of ore in a 32 year life and produced both magnetite and ilmenite concentrates and V2O5 via roast/leach of magnetite. The mine also sold leached iron pellets for steel mill feed. The Mustavaara mine produced 13.4 million tonnes of ore and produced magnetite concentrate with V2O5 produced via roast/leach of concentrate. Both of these mines were world class producers of vanadium operated by Finnish company Rautaruukki Oy now a major Finnish and European steelmaker and the mines closed in the mid 1980’s due to low commodity prices and exhaustion of shallow resources. Vulcan’s tenure encompasses or is adjacent to these mines.
The Otanmäki process is the name given to the process created at the Otanmaki Mine for processing of magnetite concentrate by Rautaruuki Oy. The following description is an abbreviated extract from Rautaruuki.
Vanadium is contained in the magnetite in ‘solid solution’ as vanadium spinel which means that trivalent vanadium has partly replaced the trivalent iron. In the roasting process, when magnetite oxidises into hematite, vanadium becomes pentavalent through oxidation. Consequently the vanadium is forced out of the hematite lattice and seeks its way out of the grain. The process is facilitated by providing a receptive reagent outside the hematite grain. This reagent in the Otanmäki process is soda, which reacts with the vanadium to form sodium vanadate. Sodium vanadate is easily soluble in water.
Run of mine ore is crushed, ground and fed through magnetic separators to create a magnetite concentrate. Magnetite concentrate is dehydrated, mixed with soda and pelletised by drumming with a small amount of water.
The pellets are fed into sintering furnaces (roasters), which are of the shaft type. Gas effluent passes through a dust separator and the dust is returned to the pelletising process. Furnace temperature and the magnetite’s oxidation temperature jointly determine the reaction zone temperature. For a good yield from the reaction the temperature must be high enough, but not too high, because the pellets may get soft and start agglomerating before melting.
After sintering the pellets are fed into a leaching system consisting of steel tanks connected in series where the sodium vanadate is dissolved in water. The system is based on the counter-flow principle so that the solution becomes gradually concentrated as it flows through the tanks.
The solution is at its richest at the point where it meets the pellet flow from the sintering furnace. In the final leaching stage the pellets are washed with clean water and then stockpiled in the open. The Otanmäki pellets were used at the Raahe Steel Works as concentrate feed for the blast furnaces. Only a small quantity of Mustavaara pellets were used at Raahe as cooling material in the converters as the high titanium content makes them less suitable for blast-furnace use.
Precipitation to separate the vanadium is effected from a hot solution by means of ammonia or ammonia salt and sulphuric acid. The precipitate thus obtained is ammonium polyvanadate. It is separated from the liquid in a filter press, washed and dried.
Filter cake is charged into a reverberatory smelting furnace. When heated, the ammonium polyvanadate dissolves, liberating first the water, then the ammonia, leaving pure V2O5 characterised by a relatively low melting point, about 700°C. The fused vanadium pentoxide is poured onto a water-cooled rotating plate to form a thin layer which breaks up into small flakes. When cooled a little, they are packed into sheet-metal drums.