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incongruent dissolution of bornite

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Hi

 

Please help - this is urgent

 

How do I force all Cu2++ from kinetic bornite dissolution to form kinetic CuS (covellite - ALTHOUGH IT MAY BE UNDERSATURATED) and Cu+ must go into solution.

The covellite must then dissolute kinetically into Cu++ and SO4--.

I want Covellite also as kinetic mineral so I dont want to swap Cu++ with covellite in the basis pane.

 

While I am writing here I am thinking prehaps to specify two new (artificial) minerals in the database - one containing the Cu+ part of the bornite and the other one the Cu++ part and then give them seperate kinetic rates. But now I think this will not work because I want to state a kinetic rate for the production of the Cu++ from Bornite dissolution and a separte rate for Cu++ production from the formed covellite.

 

Regards

Johan

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Hi Johan,

 

I think what you'll need to do is modify your thermodynamic dataset to include a fictitious "CuS(aq)" species. A reaction like Covellite = CuS(aq) (which replaces the current reaction for Covellite, Covellite + H+ = Cu++ + HS-) with an arbitrarily low stability (logK = -9, perhaps) will ensure that this aqueous "species" will only exist in very small quantities. You'll need to add CuS(aq) to the database under the redox species section by summing a few reactions. There are a few ways of setting this up and I believe any will work, so here's an example:

 

Covellite + H+ = Cu++ + HS- (logK 25 = -22.8280)

HS- + 2 O2(aq) = SO4-- + H+ (logK 25 = 138.3443)

CuS(aq) = Covellite (logK 25 = 9.0000)

 

-> CuS(aq) + 2 O2(aq) = Cu++ + SO4-- (logK 25 = 124.5160)

 

Now you should have three copper redox states (Cu+, Cu++, and CuS(aq)). Decouple each of these redox couples, and specify two (balanced of course) kinetic rate laws.

 

1) Bornite -> Covellite + Cu+

2) Covellite -> Cu++ + SO4--

 

If you want to force Covellite to form despite being undersaturated, you'll need to set order2 to 0 when you're defining variables

for your kinetic rate law (order 2 is the power of the (1-Q/K term)). This should force Covellite to form until all of the Bornite is consumed. Obviously you should be careful when ignoring the thermodynamic limitations to a reaction.

 

See the example in Geochemical and Biogeochemical Reaction Modeling Section 17.5 about creating "fake" redox species. The GWB

Reaction Modeling Guide (4.2.2 and 4.4.2) discusses nonlinear rate laws and the "order" terms.

 

Hope this helps,

 

Brian Farrell

Aqueous Solutions

Makers of The Geochemist's Workbench

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Hi Brian

 

Thanks for the detailed reply! That is why I am using GWB.

 

Anyway, I will try your recommondation and will have a look at the examples.

 

I will let you know whether I came right.

 

Thanks for your help!

 

Regards

Johan

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In the end I let bornite dissolution generate the superficial CuS(aq) specie (instead of Cu++) in the database and just let CuS(aq) decompose (using redox kinetic)according to the kinetic rate of covellite.

It is a shortcut and I hope it is "legal".

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Glad you made some progress. I suppose what determines the quality of a model is how useful it is to you. Does this satisfy most of what you were trying to accomplish? Do you have experimental data that you are trying to fit?

 

Brian

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Hi Brian

 

I have a column test with Cu material (mostly bornite, very little chalcopyrite) I want to model. I have tried the modelling but it still doesnt yield satisfactory results.

Usually I have no problem to simulate columns with pyrite but this one is very different and I want to figure out why the SO4 production is so different. There was some lag time (8weeks) before SO4 was suddenly produced (at a significant rate) which I found very suprising. I thought it might have something to do with the mineral kinetics but I am going to send the sample for SEM to see if it could recognise any secondary mineral phases - I think this might help with understanding the results. Has anyone else experience with columns tests on Cu material?

 

Kind Regards

Johan

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