Zimmer slipstream magnesium hydroxide recovery demonstration. Volume I of II. Final report, April 1, 1993--May 31, 1995 Page: 30 of 132
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The double precipitation is followed by mechanical separation of the two compounds.
Hydroclones are used to separate relatively small magnesium hydroxide crystals (<10m) from the
larger gypsum crystals (>50m). The overflow from the hydroclones, rich in magnesium
hydroxide, is thickened and stored in an agitated tank where it can be delivered for acid
neutralization via an on-site tanker.
The gypsum by-product, underflow from the separation process, is sent back to the FGD
thickener. In the thickener it settles and exits the scrubbing system via the dewatering
equipment.
B. Magnesium hydroxide Recovery Process Chemistry
The magnesium hydroxide recovery facility uses TOF liquor as feed. This thickener overflow
contains dissolved magnesium. This magnesium is in the form of MgSO3, Mg(HSO3)2, MgCl2
and MgSO4.
In the oxidizer the sulfites are oxidized to sulfates by the following reactions:
1
1) MgSO3 + 2O -+ MgSO4
2) Mg(HSO3 I +02 -> MgSO4 + H2SO4
The oxidized solution is pumped to the crystallization tank where it is reacted with lime slurry.
The following reactions occur in the crystallization tank:
3) MgSO4 + Ca(OH)2 + 2H20 -+ Mg(OH)2 + CaSO4 * 2H20
4) MgCl2 + Ca(OH)2 -+ Mg(OH)2 + CaCl2
5) H2SO4 + Ca(OH)2 -+ CaSO4 * 2H20
Excess gypsum and liquor (containing CaCl2) are added back to the FGD thickener centerwell.22
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Zimmer slipstream magnesium hydroxide recovery demonstration. Volume I of II. Final report, April 1, 1993--May 31, 1995, report, March 1, 1998; Columbus, Ohio. (https://digital.library.unt.edu/ark:/67531/metadc693041/m1/30/: accessed July 16, 2024), University of North Texas Libraries, UNT Digital Library, https://digital.library.unt.edu; crediting UNT Libraries Government Documents Department.