Silica precipitation via CO2 acidification of a sodium silicate solution

[RenAmb]

How can I know when SiO2(a) will precipitate vs NaHCO3 and Na2CO3 when I inject CO2 in a solution of sodium silicate with a initial pH of 13 up to a pH wanted (for example 7) using PHREEQC ? How do I model the injection of 100 % CO2 at 200 ml/min ?

[dlparkhurst]

You can study this script, look up the keyword data blocks, and experiment with changing the input. I used pitzer.dat, which had the carbonate minerals.

Code: [Select]

SOLUTION
-units mol/kgw
pH 13 charge
Na 2
Si 1
END
USE solution 1
REACTION
CO2 1
3 in 60 steps
EQUILIBRIUM_PHASES
SiO2(a) 0 0
Natron 0 0
Nahcolite 0 0
USER_GRAPH 1
    -headings               rxn SI(SiO2(a)) SI(Natron) Si(Nahcolite) pH
    -axis_titles            "CO2 added, moles" "Saturation index" "pH"
    -initial_solutions      false
    -connect_simulations    true
    -plot_concentration_vs  x
  -start
10 GRAPH_X RXN
20 GRAPH_Y SI("SiO2(a)"), SI("Natron"), SI("Nahcolite")
30 GRAPH_SY -LA("H+")
  -end
    -active                 true
END