Help with modeling mine tailings dissolution

[Karen HE]

I'm would really appreciate some help with some modeling I am trying to do using React. I have a 'solid' made up of known amounts of minerals including pyrite and sphalerite, and I'd like to react this 60 times with 200 mL of 6 uM CaSO4 having a pH of 5.5 (i.e., adding 200 mL of this solution 60 times to my 'solid'). I would like to remove the solutions generated each time I react the solution and be able to plot their compositions. I would like to do this to compare the calculated solution compositions to measured leachate compositions.

 

From my reading of the manual and 'Geochemical Reaction Modeling' I think the best way to go about this is defining a pore fluid in initial equilibrium with my 'solid'. pick up the reacted fluid (as the pore water) and then flush 200 mL of my 6 uM CaSO4 solution through 60 times. I have attached an input file I have developed, but which I have had serious problems with in running in React. If I can be put in the right direction as to how to fix this to get it to run, that would be great.

 

Also, I would like to model the sorption of Zn++ onto any Fe(OH)3(ppd) that forms. I think this should be done using the surface_data = FeOH.dat, etc. file. I have added this in my input file, but am not sure if it is appropriate.

 

Finally, I would like to measure Cd in the solution, but this is neither in the basis nor, I presume, specified as a compenent in any of the minerals that are known to contain it (sphalerite and wurtzite). Is there any way to modify the database to account for Cd?

 

Thank you,

Karen H-E

KHE_react_inputfile_5Oct2010.doc

[Tom Meuzelaar]

Hi Karen:

 

Your model approach should work (this type of problem is more ideally suited for 1D reactive transport, ie. a column model), however, I can give you a couple of suggestions that might advance the model along:

 

• Your initial fluid is both in equilibrium with atmospheric oxygen and reduced sulfides, hence it will not equilibrate. Since this is your oxgyenated titration fluid, it does not need to be in equilibrium with the sulfides. I would just define the fluid composition sans minerals, speciate it, and pick it up as reactant
  
• If you use the flush method, each successive iteration will completely equilibrate with your sulfide minerals. Does this mimic your experimental conditions, or is it worth considering kinetic dissolution rates for your sulfides?
  
• You might consider fixing the fugacity of oxygen in the titrant, since it sounds like oxygen is buffered in your experiment
  
• In your Basis, when defining the sulfide+fluid setup, you'll want to do this with minimal initial oxygen. You could even constrain the oxygen concentration via equilibrium with one of the sulfides
  

 

I hope that helps. You'll find cadmium in the extended LLNL database (thermo.com.v8.r6+.dat). Zn sorption should work as long you have physical ferrihydrite mass present in your system. I'd add these latter elements in one by one- first make sure the dissolution model works, so that it's easier to troubleshoot if your model doesn't converge.

 

Regards,

 

Tom Meuzelaar

RockWare, Inc.

[Karen HE]

Thanks, Tom. That is very helpful - I'll give all your suggestions a try.

 

Karen H-E