GENERAL45823 - Laserfiche WebLink

Home Browse Search

1 - ;~ ~ Hibbing Minerals Fax• 18-262-7328 :Jan 29 '95 2332 ~:~;'"•`rP:02:-:_-.,_< : _ ' ., ~ IIIIIIIIIilllllllll . .~ ... . Drainage Quality from Cresson Waste: Rock: Summary ~ :: Kim Iapakko 1716. Ashland Avenue ' St: 1?au1; Minnesota 55104 . PreYious geologic stadY ~Bg~~ ~.~: Pyrite and tarhonare mineral cotitenr of tack in the Cresson pit area was fairly uniform after: its deposition.. Subsequent exposure to atmospheric .. oxygen and water resulted in oxidauon,:.which proceeded from the surface in a downward direction This weathering created a compositional variation with depth. With respect to the proposed mine, this variation can be sir~lified into two zottes: 1) an oxidized zone in which mosf of the pyrite has.bcea oxidized and much of that which remains'is present as inclusions and 2)i a transitional zone.. of higher Pyrite cotttenf.. Most of the rock to be ucavated ftvm the Cresson pit fits the oxidized zone description. That is, the sulfur convent of most of the rock is ,low, as exemplified by the median total sulfur content of 0.11. (Sec. figure 1 and the' accgmpatrying.disaussion in I'Charaaerizatitin of Waste Rock from the Proposed Cresson Pit".) The sulfide sulfur data presented is figure 1B suggest a maximum pyrite content of about 5% for rthe transitional zone. Figure 2 iadicates..thar the sulfide sulfur wntetu is..fairiY ~lY coti'elated with the .total sulfur content: ate.virtualty do calcium/magaesium carbonate minerals is these zones. The carbonates. Illy present wet+e dissolved due to contact with the acid generated as a result of the pyrite ion. Due.to their low tarbonate content. these rocks have little potential to neutralize acid: . I They potartial of.the Cresson pit rock to genetate.acid generally increases witir the solid-phase . sulfur content. Acid is geueiaud as a tesnlt of pyrite oxidation, the rate of which is proportional .to the exposed pyrite sutfaoe area...$iace the acid neutralization potential of the rock is fairly uniformly low. the potential to generate acid increases continuously as the exposed pyrite surface area ~~~^~. The rate ofpyrite oxidation is roughly proportional to the sulfate.concenlration iti the. drainage, while. the available pyrite surface area can be represented by the total sulfur content. (See addendtmm for' additional eiplanation.) The .variation of the pyrite oxidation rate with exposed.pyrite surface area is expressed `in figure 3 (attached), which plots the sulfate conae~ation3n.humidity.cell drainage at.week2l vs total solid phase sulfur content. The resultant increase in acid generation (equivaleotly;.decrease in drainage pH) with total sulfur • .content is prtsented in figure 4. i Thelines included on figures 3 and 4 rieptesem visual "best fits' of the data. It should be noted that the three; samples with sulfur contents of .0.929ti, 0.92% and 2.04% produced sulfate concentrations about 'an .order of magnitude higtier than would be predicted based on the remaining samples. Indeed,a change is scale,was required to include the sulfate co~~~+++*ations oa the same graph (figure 3): These three satrrples getutated. drainage pH values below 2.5, at . leasC 0.75 units' lower than those of the remaining samples" at wcek 21 (figure 4). The elevated . sulfate release a~ depressed drainage pH_ittdicate. bacterially catalyzed ferric iron oxidation of the .pyrite present in these. samples. flevated ENIF vahies and iron concenaadons support this hypothesis. - . I j ' ~~ ~ ~ i :~ ~.. - ~.