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Metallurgy and Ore Beneficiation

Unlike the more typical magnetite deposits where the mineralisation is finely disseminated in siliceous bedding planes and commonly referred to as taconite deposits, the magnetite at Buena Vista is an intrusion into a volcanic andesite and as such is coarser in association with the siliceous host rock. Liberation of the magnetite occurs at a coarser size range, which permits the rejection of waste materials at coarser sizes.  The prime benefit of this type of ore as against taconite ore is that less comminution energy is required to achieve a marketable concentrate grade.


Conventional processing is proposed incorporating coarse and fine crushing followed by two stages of wet grinding; each followed by wet LIMS. This is sufficient to achieve concentrates typically in the range 66% to 69% Fe with low levels of silica, typically 1.5–4.5% and alumina at typically less than 1%. In addition the ore at Buena Vista has negligible sulphur content (0.003% S) and phosphorous (0.003% P), the latter, which is liberated on grinding and is readily removed into the non-magnetic waste at the magnetic separation stages.


Base Case Concentrate grades

Fe SiO2 Al2O3 TiO2 V
67.5% 3.3% 1.1% 0.23% 0.27%
0.003% 0.003% 0.03% 0.30% 0.30%

Levels of titanium and vanadium are low in the ore, the former occurring as non-magnetic sphene that predominantly reports to waste on magnetic separation, the latter is intimately associated as ex-solved solution within the magnetite and follows the magnetite through to concentrate. This association is not untypical of magnetite and the vanadium is usually recovered on smelting but with no bonus payment to the seller. Typical levels in the Buena Vista concentrate are 0.2% TiO2 and 0.27% V.


In the case of the low grade ore where the silica levels are higher than average for the deposit an appreciable amount of the silica and alumina can be rejected after fine crushing enabling this low grade ore to be readily pre-concentrated up to average grade prior to grinding.  This reduces tonnage to the mill and more importantly removing the harder gangue elements results in a saving in the grinding power requirements.


It is proposed for Buena Vista that the initial process plant design will be directed to the treatment of medium grade ore assaying in the range 24–27% Fe where pre-concentration has limited advantage. After year 7 the ore will be predominantly low grade typically in the range 16–19.5% Fe and it is proposed to install additional crushing capacity and a dry LIMS pre-concentration stage between crushing and grinding to pre-concentrate this ore to a comparable grade of the medium grade ore.


The plant design will be based on modular parallel units through to final concentrate, which has the advantage of minimising the risk of zero production thereby ensuring a continuous output of concentrate.