I want to predict the salt precipitation in Ca-Mg-Na-K/Cl-H2O system.
In fact, I have a carnallite sample (containing 20 g CaCl2, 270 g NaCl, 180 g KCl and 140 g NaCl) and after complete dissolution in sufficient water, I want to evaporate it in 20 stage at different temperature (50, 70 and 100 C). However, how to predict the composition of solution and crystallized salt at each stage by PHREEQC?
Hi Sadegh,
Assuming that you get complete dissolution of your carnallite sample you can create a solution (lets make it Solution 1) with the chemistry of the carnalite sample. You can calculate the molar concentration of each element (Ca, Cl, Na, K), set the temperature at 25 degree C and run that solution with your database of choice.
Check the potential error in the ion balance and the ionic strength of the solution.
Considering that your goal is to precipitate salts, re-run your model with the Pitzer database and again check the ion balance and ionic strength.
Look at saturation indexes and note if anything is supersaturated.
If you are happy with the results, you can then do several runs of evaporation.
In PHREEQC, you can remove water in several ways, one of the easiest is by using REACTION as it allow you to add or subtract elements and phases in several steps.
Here we will remove water, the following step remove 90% of the water, in 5 steps, at 25 degrees. Please note that we are removing water, and therefore the system does not contain 1L of water anymore. If this solution was to be used for other calculations afterwards it would require to be put back to 1L
SOLUTION 1
temp 25
[...]
REACTION 1
H2O -1 # this works as Formula of the phase (H2O) and stoechiometric factor (-1)
50 moles in 5 steps
END
Once that is done, you can run the model and check that the SI as the reaction progress, pick the relevant minerals out of the evaporitic suite and modify your model as follow (example using calcite, halite, CO2 and O2):
SOLUTION 1
temp 25
[...]
REACTION 1
H2O -1
50 moles in 5 steps
EQUILIBRIUM_PHASES 1
O2(g) -0.67 10 (works as PHASE, gas partial pressure, initial mass)
CO2(g) -3.5 10
Calcite 0 0 (works as PHASE, SI, Initial Mass)
Halite 0 0
END
You should replace calcite and halite by the minerals that are supersaturated in your solution.
Once happy with this you can run it at various temperature, and degree of evaporation.
Let me know if it is clear!
Cheers,
Julien
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