Processes > Dissolution and precipitation

Burkeite Scale


Hello all,

In the Kraft recovery process, the Burkeite scale (2Na2.SO4.Na2CO3) precipitates during evaporation.
However, by adding fatty acids (Oleic Acid) and Resin (Abietic Acid), the scale reduces to 40% of the initial amount.
The pH is adjusted with NaOH pellets to 12 and the temperature is 100 Celsius.
My question is that is there a way to model the two processes in PHREEQC?
a) without fatty acid and resin
b) with fatty acid and resin

I have put together a script that simulates evaporation followed by adding an ion-exchanger to represent a "resin". The simulation is based on no information other than the scale is assumed to be burkeite.

First, you must consider which database you are going to use. pitzer.dat is best for concentrated solutions, but it has relatively few elements (although all majors are included). It will also be difficult to add dissolved ligands, given the large set of interaction coefficients that would be needed to add a new ligand.

Ion-association models are poorer at high ionic strength, but adding ligands would be easier. You will get quite different results for phreeqc.dat, minteq.v4.dat, sit.dat, and llnl.dat. The latter two did not produce burkeite precipitation for the attached simulation.

The current script assumes that all solutions are maintained at pH 12 by addition of NaOH. If only some of the solutions are adjusted, you will need to modify the script. Note that adding NaOH adds Na+ to the solution and increases the potential for scale formation.

Burkeite is defined in pitzer.dat, llnl.dat, and sit.dat; however, none of the databases have any temperature dependence for the log K of burkeite. Either a delta H or an analytical expression will be required to do calculations at 100 C.

The initial solution I defined was essentially at equilibrium with halite, with a defined amount of carbonate/bicarbonate and sulfate. Approximately half of the water in the initial solution is removed by a REACTION to simulate evaporation. As the solution is evaporated, halite should precipitate, but was not included in equilibrium phases. In general, other minerals may be supersaturated and should precipitate in addition, or, thermodynamically, in preference to burkeite.

After the evaporation addition of a "resin" is simulated by adding an exchanger. Here, it is assumed that Na+ is sequestered by the exchanger to limit the precipitation of burkeite. In this case, the added exchanger is potassium dominated. The log Ks for KX and NaX make Na preferred on the exchanger relative to K, so when the exchanger is added, Na is removed from solution and some of the burkeite that accumulated during evaporation dissolves. The amount that dissolves depends on the log Ks of the exchanger (0 for Na and -1 for K are assumed in the example) and the amount of exchanger that is added (4 mol reduces the scale  by about 50% in this example).

You may need to use a ligand instead, or in addition to the resin. You can use EDTA as an example, found in llnl.dat). For exchange or a ligand, you need to consider the initial state (KX for the exchanger in this example), and all ions that might compete for exchange or ligand sites.

--- Code: ---PHASES
    H+ = H+
    log_k     0
    Na6CO3(SO4)2 = CO3-2 + 6Na+ + 2SO4-2
    log_k     -0.772
    -Vm       152 cm3/mol
    X             X-           
X- = X-
    log_k     0
Na+ + X- = NaX
    log_k     0
K+ + X- = KX
    log_k     -1
    temp      25
    pH        12.0 
    units     mol/kgw
    C(4)      .1
    Na        1 charge
    Cl        6
    S(6)      .1
    H2O        -1
    27.75 moles in 20 steps
    Burkeite  0 0
    Fix_H+    -12 NaOH      10
    -headings               rxn Burkeite,moles SI(Burkeite)
    -axis_titles            "Evaporation factor" "Moles" "SI"
    -initial_solutions      false
    -connect_simulations    true
    -plot_concentration_vs  x
10 GRAPH_X 1/TOT("water")
20 GRAPH_Y EQUI("Burkeite")
30 GRAPH_SY SI("Burkeite")
SAVE solution 2
SAVE equilibrium_phases 2
TITLE Add "resin"
USE solution 2
USE equilibrium_phases 2
KX 3

--- End code ---


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