Method of extracting and carbonating calcium from alkaline industrial waste or by-product materials

A process for producing calcium carbonate from a calcium-containing alkaline slag material, the process comprising:

• extracting the alkaline slag material in a series of extraction steps, including at least 2 extractions, using extraction solvent(s) containing salt in an aqueous solution, whereby a calcium-containing filtrate and a residual slag is formed in each extraction step,
• carrying out a step of extractive grinding simultaneously with a first extraction step of the series of extraction steps,
• separating the residual slag from the calcium-containing filtrate after each extraction step,
• carrying each residual slag to a following extraction in the series of extraction steps, to be used as raw material in said following extraction, and discarding the residual slag separated from the last extraction of the series of extraction steps,
• carrying each calcium-containing filtrate to a previous extraction in the series of extraction steps, to be used as an extraction solvent in said previous extraction, and
• carrying a first calcium-containing filtrate, separated from a first extraction step, to a carbonating step,
• carbonating calcium as calcium carbonate from the first calcium-containing filtrate, the first calcium-containing filtrate also used as a carbonation solvent, and using a carbonation gas, whereby calcium carbonate precipitates,
• separating and recovering the calcium carbonate from remaining carbonation solvent, and
• recycling the remaining carbonation solvent to the last extraction step in the series of extraction steps, to be used as extraction solvent.