Dewatering: Thickening, Filtering, CCD, Water Treatment & Tailings Disposal

Dewatering: Thickening, Filtering, CCD, Water Treatment & Tailings Disposal 2017-03-23T09:42:05+00:00
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Reductive Conversion of Jarosite to Melanterite under Partial Saturation (4 replies)

Marshal Dienes
1 year ago
Marshal Dienes 1 year ago

Does anyone have information (based on field data, preferably, but a well-documented experiment also would be helpful) they can share for a reaction path under partially-saturated conditions for a conversion of Jarosite to Melanterite (or another hydration state of ferrous sulfate)? A sample reaction would be:

Jarosite-K + 15 H2O + 6 H+ + SO42- + 3 e- ==> K+ + 3 Melanterite

Actual textural evidence would be the best, of course. I am specifically interested in the reductive dissolution of Jarosite, rather than the possible routes of Pyrite oxidation to ferrous or ferric depending on spatially ranging oxidation state locally.

Maya Rothman
1 year ago
Maya Rothman 1 year ago

I did a lot of experimental work on Jarosite solid solutions and end member phases and one thing is certain, it has a very stable structure( TOT stacking down the c-axis that includes a Kagome sub-lattice Fe(OH)2). Amongst other things I have tried previously to convert Jarosite to schwertmannite (experimental) but never achieved this. Thermodynamically I would suggest the reaction would be more favourable as melanterite dissolution and Jarosite precipitation (log K for Jarosite is around -11 but the dissolution reaction is incongruent above circa pH 3.5-4 compared to melanterite log K which is about -2.5) . pH will be critical for these types of reactions involving ferric sulphates - do you know the native pH that these mineral assemblages are in equilibrium with? From previous experience I would definitely recommend any lab study be done on natural specimens rather than synthetic analouges - Jarosite can be made easily in the lab but it never forms stoichiometrically pure end member phases - whereas field sample often are (which I would suggest is down to thestructural / geochemical stability of end member phases).

Marshal Dienes
1 year ago
Marshal Dienes 1 year ago

This is very helpful. I appreciate your emphasis on the mineral structure as important to understanding. For many reasons, we completely agree with you that field materials are essential to understanding; not only are there the stoichiometric considerations, but the field problem that gives rise to the question is certainly an open system, to both energy and mass, so we must be careful with simplifications.

The pH is a matter we are trying to suss out. We have a good handle on the effluent pH over time scales of a few decades and we have recent pore-water samples in-situ, for a system that is quite well oxygenated. There remains a substantial acid-neutralization capacity in the silicate phases, as well as abundant remaining pyrite. Although this is kinetically constrained, the solution chemistry clearly shows that the ANC is available. If we leave the system open to O2, then there is no reason to expect that the system behaviour will much change, and jarosite will be stable. We have good data (form various scales and sorts of measurements/calculations) on the oxidation rate of the rock-pile materials, and if the O2 flux can be controlled, our expectation is that we will evolve (probably quite quickly) from an oxic to an anoxic environment, and it is this transition in which we are interested looking forward.

Maya Rothman
1 year ago
Maya Rothman 1 year ago

Sounds like the projects been running for a while, do you have any old samples that have been sealed with zero headspace for a couple of years? That you could possibly compare mineralogy with a fresh sample by XRD. Alternatively you could try mimicking the processes of the dump turning anoxic by sparging a sample with N2/H2 gas and keeping it in a sealed chamber for a couple of weeks / months (depending on project timescales) and again compare and contrast mineralogical changes with fresh oxic samples by XRD.

Marshal Dienes
1 year ago
Marshal Dienes 1 year ago

Alas, no sealed, old samples - I agree that would be a neat experiment (and we may try that). I am hesitant to use H2 (because I don't want to introduce a new electron donor that is not present), but sparging with N2 could certainly be done. We may have to skunk-works that, but no matter what happens, the waste-rock piles will be there for X Millions of years, and the owners will be involved with closure management "in perpetuity.". Whatever this may mean.

Excellent suggestions

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