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Calcium Carbonate & Dicalcium Phosphate - CO2 Problem

  1. May 27, 2014 #1
    Hello everyone,
    I would really appreciate your help on figuring out the issue of undesirable CO2 production in this situation.

    1. The problem statement, all variables and given/known data
    In a mixture of CaCO3 + CaHPO4; I am observing undesirable CO2 production under conditions of 20-25°C (room temp) and an undesirable excess of CO2 production under conditions of 50+°C. The time of exposure to these temperatures is a repetitive 6 hour oscillation (to 50°C) and an 18 hour de-oscillation (to 25°C).
    High moisture content in the environment is a factor, but is outside the immediate physical contact with the material. Without moisture in the environment though, CO2 production still does occur.

    1. I am trying to understand why the CO2 production is occuring and the equation responsible.
    2. I am trying to understand possible solutions for stopping the CO2 producing reaction.
    3. Does the high moisture content move the equation towards accelerating CO2 producing?

    2. Relevant equations

    1. H3PO4 + CaCO3 → CaHPO4 + CO2 + H20 (not the exact equation, but the most informative)
    2. CaCO3 + CaHPO4 + Heat → (CO2 producing equation)?
    3. CaCO3 + CaHPO4 + Heat + H20 → (CO2 producing equation)?
    4. CaCO3 releases CO2 in interactions with acids - and CaHPO4 is an acid.

    5. 2(CaHPO4) + CaCO3300-900°C C2P2O7 + CaCO3 + H20 → further → 2(C2P2O7) + CaCO3>900°C Ca3(PO4)2 + CO2 (how does this compare to a repetitive exposure to 50°C)

    3. The attempt at a solution
    Well...these are my thoughts
    1. CaCO3 (thermal decomposition) → CaO + CO2
    CaCO3 degrades towards CO2 production as a byproduct under high heat conditions (around 800°C), though in my problem I am experiencing CO2 production at a much lower temp. (≈50°C)
    2. Would simply cooling down the mixture below room temp stop the CO2 producing reaction? Would a desiccant be effective in reducing the CO2 production by reducing the moisture content in an enclosed heated situation?
    3. Is there a material that can be included within the solution to prevent this reaction from occurring?

    Any and all help will be appreciated. Thanks for your time.
  2. jcsd
  3. May 27, 2014 #2


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    Staff: Mentor

    Why not just

    CaCO3 + 2CaHPO4 -> Ca3(PO4)2 + H2O + CO2
  4. May 27, 2014 #3

    I figured that both might interact and form CO2 regardless of heat - but wasn't exactly sure of the equation or the legitimacy. Thank you for that.

    My goal is to try to keep the reaction to the left as to limit the production of CO2 - if I added a desiccant to limit H20 presence, would this push the equation more-so to the left therefore limiting CO2 production? Would that have any bearing on preventing H20 production, or would it just be an after-the-fact solution of removing H20?

    Would you personally have any suggestions on how to keep the reaction from occuring if it is at all possible? Any kind of help I'd be thankful for.
  5. May 27, 2014 #4


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    Staff: Mentor

    If the Ca3(PO4)2 is thermodynamically favored there is not much you can do. Keeping the mixture dry will probably slow the process, but I doubt it will stop it completely. Besides, removing water will shift the equilibrium to the right, which is not what you want.

    I am afraid I can't help you much - as far as I am aware calcium phosphate systems are quite complicated, with many possible variants of composition, including basic phosphates (hydroxyapatite). That means it not easy to predict how will the system evolve.
    Last edited: May 27, 2014
  6. May 27, 2014 #5
    Would the equation that you posted remove the need for thermal breakdown for production into CO2? Just the presence of the two ingredients together would incur CO2 production? Thanks
  7. May 27, 2014 #6
    Thanks for your response Borek. Off the top of your head, would decreasing the temperature be the simplest thing to do to limit this reaction?

    Or would a humectant help slow the reaction?
    Last edited: May 27, 2014
  8. May 27, 2014 #7


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    Staff: Mentor

    I don't think you need thermal breakdown for the reaction to occur. It will be slow, as its a solid phase reaction, but IMHO it will still happen just because the substances are mixed.

    Both keeping it cold and keeping it dry should slow down the reaction.
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