Calculation of precipitated solid volume / Alkaline Reagent Consumption

[Onur]

Hello, I will compare the precipitated materials (sludge) produced by the neutralization of mine water using different alkaline reagents (NaOH,CaOH2, MgCO3 etc.) for my MSc project.
Is there any way to calculate the volume of precipitated materials such as ferrihydrite, calcite etc. on PHREEQC?
Secondly, what would be the best way to compare the alkaline agent consumption to reach a specific pH? When I include the CO2 (-3.5) in the EQUILIBRIUM_PHASE, I cannot obtain pH values above 8 (since PHREEQC calculates the ultimate equilibrium). For example: what is the best way to calculate Ca(OH)2 amount required to obtain pH 9.5 solution? Should I compare them without adding CO2 as an EQUILIBRIUM_PHASE?

[dlparkhurst]

There is a Basic function PHASE_VM which returns the molar volume of a phase, for example PHASE_VM("Calcite") returns the molar volume of calcite in cm^3/mol. However, most of the databases do not have the molar volumes defined. The identifier -Vm needs to be defined in PHASES for each mineral of interest. phreeqc.dat probably has the most definitions, but you can add definitions yourself by adding PHASES definitions with -Vm in the input file if necessary.

Here is a code snippet that would calculate the total volume of minerals that are defined in EQUILIBRIUM_PHASES

Code: [Select]

100 REM equilibrium_phases ======================
 110 e = SYS("equi", count, equi$, type$, moles)
 120 FOR i = 1 TO count
 130 min_vol = EQUI(equi$(i)) * PHASE_VM(equi$(i))
 140 min_vol_tot = min_vol_tot + min_vol
 150 NEXT i

If you include CO2(g) in equilibrium phases, the solubility of CO2 will increase, possibly to many moles, as the pH is set to higher and higher values. PHREEQC may fail to converge if the solubility is too great (and you provide a large number of moles in the EQUILIBRIUM_PHASES definitions). For starters, I would probably leave CO2(g) out of the calculation, a closed system. You can consider the open system after the first pass.

[Onur]

Hello Sir, thanks for your answer. It is really helpful.
How should I include the open system factor into the model? Also, I am trying to model a treatment plant working with a pH 9.5-10 using Ca(OH)2, and the reactors are open to the atmosphere (In the first stage pCO2 reaches -3.3 with the help of aeration, before adding the alkaline). What would be the best way to simulate this process in PHREEQC? When I use an open system (-3.5 CO2, -0.67 O2), pH does not rise above 8.02. Thanks again for your time and consideration.

[dlparkhurst]

Sounds like you want a two-step simulation, where first your influent solution is aerated to pCO2 -3.3, followed by addition of alkali in a closed system. In the addition of alkali, you probably reach some point intermediate between an open and a closed system with respect to CO2(g). It is unlikely you would attain both a pH of 10 and atmospheric pCO2. To do so would require much more alkali (cost prohibitive) and would dissolve a large amount of CO2. It is most likely your plant would operate much closer to a closed system than an open system.

Here is a simulation with an arbitrary Ca-Na-HCO3 solution. You can compare the mole transfers between a closed and an open system.

Code: [Select]

PHASES
Fix_H+
H+ = H+
log_k 0.0
SOLUTION 1
pH 7.5
Alkalinity 3
Ca  1 Calcite 0.0 10
Na  1 charge
END
USE solution 1
EQUILIBRIUM_PHASES 1
Fix_H+  -10 Ca(OH)2 10
#CO2(g) -3.4
END

If you have the data, for a given addition of alkali, you could estimate the amount of CO2(g) that dissolves to arrive at a measured pH.

Code: [Select]

PHASES
Fix_H+
H+ = H+
log_k 0.0
SOLUTION 1
pH 7.5
Alkalinity 3
Ca  1 Calcite 0.0 10
Na  1 charge
END
USE solution 1
REACTION 1
Ca(OH)2 1
0.0015 mole
EQUILIBRIUM_PHASES 1
Fix_H+  -10 CO2 10
END


[Onur]

It worked perfectly well, thank you so much, sir. To calculate the amount of precipitated materials (say calcite) and the required alkaline amount, what would be the most realistic approach? (I know that the sludge is containing calcite)

To find the required amount of alkaline material to increase the pH 10 (with Fix pH 10 command) without adding calcite in the EQUILIBRIUM_PHASES. Then use this amount in the REACTION block and add the calcite in the EQUILIBRIUM_PHASES (results in a different pH [9.39] because of calcite precipitation). 

Or directly calculating the required alkaline with the Fix pH command while calcite is in the EQUILIBRIUM_PHASES?
Thanks again for your time.

[dlparkhurst]

I think I would make the calculation in two steps: (1) raise the pH to 10, and (2) react to equilibrium with calcite. Reality may be that some calcite precipitates as the pH is raised, resulting in a greater addition of alkali than the two-step calculation.

Here is an example  script. You may want to add another step that aerates the first solution to achieve a pCO2 of -3.4.

Code: [Select]

PHASES
Fix_H+
H+ = H+
log_k 0.0
SOLUTION 1
pH 7.5
Alkalinity 3
Ca  1 Calcite 0.0 10
Na  1 charge
END
#USE solution 1
#EQUILIBRIUM_PHASES 1
#CO2(g) -3.4 10
#SAVE solution 1
#END
USE solution 1
EQUILIBRIUM_PHASES 1
Fix_H+  -10 Ca(OH)2 10
SAVE solution 2
END
USE solution 2
EQUILIBRIUM_PHASES 2
Calcite 0 0
#CO2(g) -3.4 10
END


[Onur]

Thank you so much sir.

[bobolovac]

Thx