Process for converting FGD gypsum to ammonium sulfate and calcium carbonate

A continuous countercurrent flow process for converting FGD gypsum to ammonium sulfate and calcium carbonate comprising the following steps:

• filtering an aqueous slurry of FGD gypsum to result in a FGD gypsum liquid filtrate and a FGD gypsum filter cake;
• washing the FGD gypsum filter cake with water and disposing a wash water filtrate;
• combining the washed FGD gypsum filter cake with an ammonium carbonate and an ammonium sulfate solution in a first reactor to chemically react and form an ammonium sulfate solution containing a precipitated calcium carbonate and a FGD gypsum remnant and an ammonium carbonate remnant;
• filtering the precipitated calcium carbonate from the ammonium sulfate solution to result in a product ammonium sulfate solution filtrate and an initial product calcium carbonate filter cake;
• employing a part of the product ammonium sulfate solution filtrate to retain as product ammonium sulfate solution and a remaining part to recycle back into the first reactor;
• combining the initial product calcium carbonate filter cake with a portion of the ammonium carbonate solution in a second reactor to chemically react and form an initial product ammonium sulfate solution;
• filtering the initial product ammonium sulfate solution to result in an initial product ammonium sulfate solution filtrate and a final product calcium carbonate cake and retaining the final product calcium carbonate cake;
• washing the final product calcium carbonate cake with unreacted ammonium carbonate solution, then with water to result in an ammonium carbonate wash filtrate containing ammonium carbonate and calcium sulfate, and a clean final product calcium carbonate cake;
• adding the ammonium carbonate wash filtrate to the second reactor;
• employing part of the initial product ammonium sulfate solution filtrate to recycle to the first reactor and part of the initial product ammonium sulfate solution filtrate to recycle to the second reactor;
• employing the ammonium carbonate wash filtrate as the absorbing liquid in an absorber tower and contacting the ammonium carbonate wash filtrate with ammonia and carbon dioxide to produce ammonium carbonate, and recycling the ammonium carbonate as a process reactant.

A continuous countercurrent flow process for converting FGD gypsum to ammonium sulfate and calcium carbonate comprising the following steps:

• filtering an aqueous slurry of FGD gypsum on a continuous filter having two zones, zone A and zone B, where in zone A water is washed over the FGD gypsum to result in a FGD gypsum waste filtrate and a FGD gypsum filter cake and in zone B, the FGD gypsum filter cake is washed with an ammonium sulfate solution containing a residual ammonium carbonate to result in a purer ammonium sulfate solution as process product and a reacted FGD gypsum filter cake containing calcium carbonate;
• combining the reacted FGD gypsum filter cake with an ammonium carbonate solution and an ammonium sulfate solution in a first reactor to chemically react and form an ammonium sulfate solution containing a precipitated calcium carbonate and a FGD gypsum remnant and an ammonium carbonate remnant;
• separating the precipitated calcium carbonate from the ammonium sulfate solution to result in a product ammonium sulfate solution residue and an initial product calcium carbonate cake;
• employing a part of the product ammonium sulfate solution residue to retain as product ammonium sulfate solution and a remaining part to recycle back into the first reactor;
• combining the initial product calcium carbonate cake with a portion of the ammonium carbonate solution in a second reactor to chemically react and form an initial product ammonium sulfate solution;
• filtering the initial product ammonium sulfate solution on a continuous filter having three zones, zone A, zone B and zone C, wherein the initial product ammonium sulfate solution is filtered in zone A to result in an initial product ammonium sulfate solution filtrate and a final product calcium carbonate filter cake containing a second FGD gypsum remnant and in zone B the final product calcium carbonate filter cake is washed with a second ammonium carbonate solution to remove the second FGD gypsum remnant, resulting in an ammonium carbonate filtrate that is recycled to the second reactor, and in zone C the final product calcium carbonate filter cake is washed with water to result in an ammonium carbonate wash filtrate and the final product calcium carbonate cake is retained as process product;
• recycling the ammonium carbonate wash filtrate to the second reactor;
• and
• employing part of the initial product ammonium sulfate solution filtrate to recycle to the first reactor and part of the initial product ammonium sulfate solution filtrate to recycle to the second reactor.