Disclosed herein are systems and methods from processing flue gas desulfurization (FGD) gypsum feedstock and ash feedstocks, either separately or together. FGD gypsum conversion comprises reacting FGD gypsum (e.g. calcium sulfate) feedstock, in either batch or continuous mode, with ammonium carbonate reagent to produce commercial products wherein the commercial products comprise ammonium sulfate and calcium carbonate. Ash conversion comprises a leach process followed by a precipitation process to selectively precipitate components at predetermined pHs resulting in metal hydroxides which may be optionally converted to oxides or carbonates. The processes may be controlled by use of one or more processors.

To provide a method for producing a calcium carbonate block for medical use which is useful as a bone substitute or a bone substitute raw material needed in medical care, which is a method for producing a calcium carbonate block that satisfies the following desired properties: 1) the calcium carbonate block has excellent mechanical strength; 2) the calcium carbonate block can be produced by a simplified production method; 3) the calcium carbonate block contains no impurity; and 4) the calcium carbonate block has high reactivity. A method for producing a calcium carbonate block, comprising a step of shaping a water-containing calcium hydroxide block and a carbonation step of immersing the calcium hydroxide block in a carbonate ion-containing aqueous solution.

Techniques for growing crystalline calcium carbonate solids such that the crystalline calcium carbonate solids include a volume of 0.0005 mm.sup.3 to 5 mm.sup.3, include a slaker to react quicklime (CaO) and a low carbonate content fluid to yield a slurry of primarily slaked lime (Ca(OH).sub.2); a fluidized-bed reactive crystallizer that encloses a solid bed mass and includes an input for a slurry of primarily slaked lime, an input for an alkaline solution and carbonate, and an output for crystalline calcium carbonate solids that include particles and an alkaline carbonate solution; a dewatering apparatus that includes an input coupled to the crystallizer and an output to discharge a plurality of separate streams that each include a portion of the crystalline calcium carbonate solids and alkaline carbonate solution; and a seed transfer apparatus to deliver seed material into the crystallizer to maintain a consistent mass of seed material.

There is provided a method for producing calcium carbonate by utilizing seawater and calcinated shells, and calcium carbonate and a calcium agent produced thereby. The method for producing calcium carbonate includes: eluting calcium by mixing calcinated shells, seawater, and sugar; and generating calcium carbonate by injecting carbon dioxide into the calcium eluate generated in the eluting calcium. The calcium agent includes vaterite-type calcium carbonate.

Provided herein are methods and systems to form calcium carbonate comprising vaterite, comprising dissolving lime in an aqueous base solution under one or more precipitation conditions to produce a precipitation material comprising calcium carbonate and a supernatant solution, wherein the calcium carbonate comprises vaterite.

Provided herein are methods and systems to form calcium carbonate comprising vaterite, comprising dissolving limestone in an aqueous base solution under one or more precipitation conditions to produce a precipitation material comprising calcium carbonate and a supernatant solution, wherein the calcium carbonate comprises vaterite.

A method for sequestering CO.sub.2 by creating precipitated calcium carbonates including Calcite, Dolomite, Vaterite and Struvite; (1) Utilizing a mutually beneficial bacterial/algal colony that can fix CO.sub.2 as Calcite, Dolomite, Vaterite and Struvite (2) providing sunlight, water, CO.sub.2 from either the air or industrial waste streams; and (3) assisting microbial/algal induced carbonate precipitation of Calcite, Dolomite, Vaterite and Struvite, thereby sequestering most of the CO.sub.2 introduced in step (2). In addition, chlorine, sulfur, H.sub.2S, NOx and toxic heavy metals will be fixed into the Calcite, Dolomite, Vaterite and/or Struvite matrix, rendering them environmentally harmless.

Provided herein are methods and systems to form calcium carbonate comprising vaterite, comprising dissolving lime in an aqueous base solution under one or more precipitation conditions to produce a precipitation material comprising calcium carbonate and a supernatant solution, wherein the calcium carbonate comprises vaterite.

Methods of producing solid CO.sub.2 sequestering carbonate materials are provided. Aspects of the methods include introducing a divalent cation source into a flowing aqueous liquid (e.g., a bicarbonate rich product containing liquid) under conditions sufficient such that a non-slurry solid phase CO.sub.2 sequestering carbonate material is produced. Also provided are systems configured for carrying out the methods.

Disclosed herein are system and methods for producing a high purity ammonium sulfate product as well as either a lower or a high purity calcium carbonate product by reacting flue gas desulfurization (FGD) gypsum feedstock in batch or continuous mode using synthesized ammonium carbonate from ammonia and carbon dioxide gases. The high purity calcium carbonate is obtained by using a precipitation catalyst, filtering out the impurities, then precipitating a high purity calcium carbonate. Alternatively, the lower purity calcium carbonate may be dissolved in acid, impurities filtered out, then a high purity calcium carbonate is precipitated out using a soluble carbonate salt.