Halite/Gypsum Precipitation

[sttr6]

Hi
I have 2 questions:
1) I want to calculate the amount of halite precipitation in a certain period of time from a solution. I tried to use "rates" block but I don't know which equations should I use and the right coding. I tried to use Calcite example in the user guide but I got confused. I appreciate if anyone can help me.

2) I used Equilibrium Phases to calculate the amount of precipitation of Gypsum. Ca in the solution is 0.003 mole but phreeqc calculated d_gypsum=10.01 mole. How is that possible ?

Thanks

[dlparkhurst]

Here is an example of kinetic dissolution of halite using a common rate equation. However, the rate of dissolution (rate constant k) was arbitrarily adjusted to reach equilibrium within 1 day. The rate is not consistent with any experimental data.

As for the d_gypsum, I cannot say. Perhaps you had 10 moles of anhydrite that converted to gypsum? I don't know. Perhaps example 2 in the manual would be helpful.


SOLUTION
END
RATES
Halite
10 k = 20/86400  # 1 per day
20 rate = k * (1 - SR("Halite"))
30 moles = rate * TIME
40 SAVE moles
END
USER_GRAPH 1
    -headings               Days SI(Halite) TOT(Na)
    -axis_titles            "Days" "SI(Halite)" "Moles dissolved"
    -initial_solutions      false
    -connect_simulations    true
    -plot_concentration_vs  x
  -start
10 GRAPH_X TOTAL_TIME/86400
20 GRAPH_Y SI("Halite")
30 GRAPH_SY TOT("Na")
  -end
    -active                 true
USE solution 1
KINETICS
Halite
  -m 10
  -steps 86400 in 10 steps
END