Newton-Raphson didn't converge any more (on a former working rea file)

[Frank Bok]

Dea Jia, dear Brian,

I tried to re-run one of my older react input files (simple acid mine drainage calculation, just adding O2 to pyrite). This input file did work since 2017 (see images attached) and I didn't do any changes, but now I get an error message:

Quote

Newton-Raphson didn't converge after 999 iterations, max residual =           1, Xi = 0.0000
Largest residual(s):
                       Resid     Resid/Totmol   Cbasis
---------------------------------------------------------
 Cl-                2.612e+201            1    -nan(ind)
 Na+                4.801e+200            1    -nan(ind)
---------------------------------------------------------

Is there a way to fix this calculation?

Many thanks in advance and best regards,
Frank

React-24.rea

[Brian Farrell]

Hi Frank,

This example has run in earlier releases, but it did not converge very well. I think the initial system is not especially well posed. If you wanted to simulate titration of table salt into pure water, then you could set negligibly small but nonzero concentrations for Na+ and Cl- in the Basis. It appears you’re trying to do that for O2(aq), but I don’t think that’s a good idea when redox reactions are considered. It’d be better to set a more precise constraint on the oxidation state, like an f O2(g) or Eh value (or in an oxidizing fluid, a measured free O2(aq) concentration), that is consistent with pyrite stability. Within the installed examples there is a similar script in which the coexistence of pyrite and hematite sets the oxidation state. In your installation directory, go to Script > GBRM3 scripts > Ch16_pyrite.rea to see the example, which is discussed in Chapter 16 of the Geochemical and Biogeochemical Reaction Modeling text, Third Edition.

Hope this helps,

Brian Farrell
Aqueous Solutions
 

[Frank Bok]

Hi Brian,

thank you for the answer, I will take a closer look on the script in Craigs book!

The idea behind my calculation was actually to add oxygen to a reduced system to oxidize the pyrite. I will find another solution for this scenario.

Many thanks and best regards,
Frank