Tailing dissolution: AsH3 and H2S

[nesa]

Hi there. I will be dissolving the tailing containing sulfides and amongst them arsenopyrite using H2SO4 in O2 atmosphere under pressure in autoclave. My concern is an evolution of H2S gas and in particular arsine gas. I have a couple of questions:

1.  O2 atmosphere under various pressures in closed reactor can be modeled using e.g. -pressure 6 (6 bar of O2) in SOLUTION block and in GAS_PHASE block, O2(g) 1, -fixed_volume, where -volume 0.75 corresponds to headspace volume in reactor. Is it correct way?

2. I need to monitor H2S and AsH3 gasses. As for H2S, on different temperatures defined in REACTION_TEMPERATURE, in phreeqc output, H2S is present as aqueous species but SI("H2S(g)") is below 0 on some temperatures, which is fine telling me there is some H2S in solution but there is no outgassing, but on some temperatures SI("H2S(g)")>0. Is there any way to calculate what would be an increase in pressure in headspace of reactor (cumulative pressure from O2 and H2S)?

3. even if SI("H2S(g)") is below 0 in pressure reactor on some temperatures, my concern is what happens when reactor lid is opened. My question is on how to model pressure drop from e.g. 6 bars in reactor to 1 bar when reactor lid is opened. Can that be done: after reaction at 6 bar, SAVE SOLUTION 2, then USE SOLUTION 2, same GAS_PHASE block as before and REACTION_TEMPERATURE 25 (when is cooled) and REACTION_PRESSURE, 1? Would that be the correct way to show potential outgassing of H2S seen in the increase in SI("H2S(g)") in solution when lid is opened?

4. Using llnl.dat database, in arseopyrite dissolution block there is AsH3 as a product, I guess as a solution species only? However, in calculation, arsenopyrite is completely dissolved, but in distribution of species AsH3 or As(-3) is always 0, but As(3) has some values. Does it mean that phreeqc firstly calculates AsH3 species during arsenopyrite dissolution and then adjust redox and turn As(-3) into As(3)? How can I track that?

5. arsine gas may evolve, but in llnl.dat or other databases I could not find something like AsH3(g), only AsH3  species. Is there any way on how to add a new phase AsH3(g) gas?

My apologies if any of these questions have already been posted somewhere on the forum but I couldn't find it.

Regards

[dlparkhurst]

That's too many questions. I prefer specific questions about specific scripts and the way PHREEQC does calculations. I'll answer the first.

1.  O2 atmosphere under various pressures in closed reactor can be modeled using e.g. -pressure 6 (6 bar of O2) in SOLUTION block and in GAS_PHASE block, O2(g) 1, -fixed_volume, where -volume 0.75 corresponds to headspace volume in reactor. Is it correct way?

I'll accept that the pressure is 6 bar if you say so, but is the gas pure O2, or air with 78% nitrogen?

"O2(g) 1" in the GAS_PHASE input means 1 atm of O2(g). PHREEQC will do an initial calculation to calculate the number of moles of O2(g) at the temperature and volume (0.75 L) specified in GAS_PHASE, such that the O2(g) partial pressure is 1 atm. This calculation occurs before any reactions with the solution. When you calculate equilibrium between the gas phase and a solution, presumably some O2(g) will react and the partial pressure in the GAS_PHASE following the reaction will be less than 1 atm. Of course temperature changes will also affect the partial pressures and equilibrium with the solution.

[nesa]

Thank you for the answer. As for the first question, I tried to model the sulfides dissolution under constant pressure of 6 bar of pure oxygen. A headspace of autoclave is 0.75L occupied by gas therefore I put -volume 0.75. What I would like to model is that the autoclave headspace of 0.75L is constantly under that pressure.

Code: [Select]

SOLUTION 1
    temp      25
    pH        7
    pe        8
    redox     pe
    units     mmol/kgw
    density   1
    -water    0.5 # kg
    #-pressure 6

EQUILIBRIUM_PHASES 1
    Arsenopyrite 0 0.000044
    Chalcopyrite 0 0.000178
    Galena    0 0.000413
    Pyrrhotite 0 0.068854
    Sphalerite 0 0.000447

GAS_PHASE 1
    #-fixed_volume
    -fixed_pressure
    -pressure 6
    -volume         0.75
    -temperature    20.0
    O2(g)           1
    H2S(g)          0

REACTION_PRESSURE 1
    6
   
REACTION_TEMPERATURE 1
    20  30  40  50  60  70  80 
    90  100  110  120  130  140  150  160 
    170  180  190  200

SAVE SOLUTION 1
END

# ------------------------
# Added 0.5 mol of H2SO4

USE SOLUTION 1
USE EQUILIBRIUM_PHASES 1
USE REACTION_PRESSURE 1
USE REACTION_TEMPERATURE 1
REACTION 1
    H2SO4     1
    0.5 moles in 1 steps

USER_GRAPH 1 H2S species concentration in pregnant solution
    -headings               ?? 0.5_mol_H2SO4
    -axis_titles            "Temperature (C)" "H2S species in pregnant solution" ""
    -chart_title            "Effect of H2SO4 addition on digestion"
    -axis_scale x_axis      20 200 auto auto
    -initial_solutions      false
    -connect_simulations    false
    -plot_concentration_vs  x
  -start
10 graph_x TC
20 graph_y mol("H2S")
  -end
    -active                 true

USER_GRAPH 2 H2S(g) species concentration in pregnant solution
    -headings               ?? 0.5_mol_H2SO4
    -axis_titles            "Temperature (C)" "Saturation index H2S(g) species in pregnant solution" "Partial pressure H2S(g) species in pregnant solution"
    -chart_title            "Effect of H2SO4 addition on digestion"
    -axis_scale x_axis      20 200 auto auto
    -initial_solutions      false
    -connect_simulations    false
    -plot_concentration_vs  x
  -start
10 graph_x TC
20 graph_y SI("H2S(g)")
30 graph_sy PR_P("H2S(g)")
  -end
    -active                 true


USER_GRAPH 3 AsH3 species concentration in pregnant solution
    -headings               ?? 0.5_mol_H2SO4
    -axis_titles            "Temperature (C)" "tot(As(-3)) species in pregnant solution" "totmole(As(-3)) species in pregnant solution"
    -chart_title            "Effect of H2SO4 addition on digestion"
    -axis_scale x_axis      20 200 auto auto
    -axis_scale y_axis      auto auto auto auto log
    -axis_scale sy_axis     auto auto auto auto log
    -initial_solutions      false
    -connect_simulations    false
    -plot_concentration_vs  x
  -start
10 graph_x TC
20 graph_y tot("As(-3)")
30 graph_sy totmole("As(-3)")
  -end
    -active                 true
END
For the constant pressure of 6 bar, I am not sure if I need that pressure to specify in solution block with -pressure 6, or in gas_phase block with -pressure 6 or in REACTION_PRESSURE? What I noticed pe is always low. Could you please tell me how to do that?

Regards