Two stages extraction method for synthesizing precipitated calcium carbonate

A method comprising producing precipitated calcium carbonate from a starting waste material comprising calcium and silica, wherein the starting waste material is selected from the group consisting of oil shale ash, coal ash and waste cement, the method comprising continuously performing the following steps:

• (a) a first calcium extraction step for extracting the starting waste material with a first ammonium-based aqueous solution to yield a first suspension comprising a first calcium rich eluate and a first residual waste material, and separately recovering the first calcium rich eluate and the first residual waste material from the first suspension;
• (b) a first carbonating step for carbonating the first calcium rich eluate recovered in the first calcium extraction step to yield a second suspension comprising a first calcium carbonate precipitate and a first regenerated ammonium-based aqueous solution, and separately recovering the first calcium carbon precipitate and the first regenerated ammonium-based aqueous solution from the second suspension;
• (c) a second calcium extraction step for extracting the first residual waste material recovered from the first calcium extraction step with the first regenerated ammonium-based aqueous solution to yield a third suspension comprising a second calcium rich eluate and a second residual waste material, and separately recovering the second calcium rich eluate and the second residual waste material from the third suspension;
• (d) a second carbonating step for carbonating the second calcium rich eluate from the second calcium extraction step to yield a fourth suspension comprising a second calcium carbonate precipitate and a second regenerated ammonium-based aqueous solution, and separately recovering the second calcium carbon precipitate and the second regenerated ammonium-based aqueous solution from the fourth suspension;
• and
• (e) a recycling step for recycling each of the first and second regenerated ammonium-based aqueous solutions recovered after the respective first and second carbonating steps for use in the first and second calcium extraction steps;
• wherein steps (a)-(e) are performed under conditions such that each of the first and second calcium carbonate precipitates recovered after the respective first and second carbonating steps has a calcium carbonate concentration of greater than 95 w/w % and the second residual waste material recovered after the second calcium extraction step has a higher concentration of silica than the starting waste material.