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Kinetic run: forcing a mineral to precipitate, even if it is undersaturated


joelbrugger
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Dear group,

 

I'm trying to model the kinetics of replacement of calaverite (AuTe2) by gold (Au), in order to fit experimental data.

 

A simple model with a simple kinetics low for calaverite works rather well if I suppress Au(OH)(aq) from the run. If I use Au(OH)(aq), then the model predicts that my experimental solution is always undersaturated with respect to native Au, and hence calaverite dissolves, but no gold is formed. There is little justification for suppressing Au(OH)(aq) - it is well characterized at 300C and Steffanson and Seward (2003) suggest that the properties are good to ~200C (I'm working at 220C).

 

It seems that my problem could be solved if I could 1) force gold to precipitate, even in an undersaturated solution (Q/K = 1e-5), and 2) prevent the formed Au to dissolve once it has formed.

 

I've tried a negative reaction rate for Au (negative is precipitation, right?), in conjuction with the nucleus keyword, but had no luck - always zero mole of Au in the system.

 

Two example runs are visible at

http://gallery.me.com/minmetsol/100108.

 

I'm using react v. 7.0.3, and the script with kinetic laws for both Au and Calaverite is below. Let me know if you want the database and conf file to have a play.

 

Thanks for your help,

 

JOEL

 

 

 

React> show

Reaction runs from 0 years to 2.5 days

Temperature is 220 C

Thermo dataset: ..\thermo_MMS_11May18.dat

Working directory: z:\gwb_work\tellurium

Options: Debye-Huckel

 

Basis is:

H2O .015 free kg

Na+ .1054 molal

Cl- .01 molal

H2TeO3(aq) 1e-8 molal

O2(aq) 8.45e-5 mol

Au+ 1e-8 molal

HPO4-- .1998 molal

H+ charge balance

Reactants:

10 mg of Calaverite by kinetic rate law

Rate constant = 1.8e-9 mol/cm2 sec

Specific surface area = 51.06 cm2/g

React 0 mol of O2(aq)

Au by kinetic rate law

Rate constant = -3e-9 mol/cm2 sec

Specific surface area = 100 cm2/g

Nucleus density = 200 cm2/cm3

Critical saturation index = 1e-8

 

Other "show" options: type "show show"

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