Wasted gypsum boards are crushed and calcined to gypsum granular solid, and the gypsum granular solid is mixed with water to form gypsum slurry. Gypsum particles are deposited from the gypsum slurry in a crystallization tank. Heated steam is blown into the gypsum slurry at a height from surface of the gypsum slurry and down to upper ? of the gypsum slurry to heat the gypsum slurry and to eliminate foam on the gypsum slurry.

The invention relates to a process for extracting metals and salts from titanium-bearing minerals such as perovskite. More particularly, although not exclusively, the invention relates to extracting titanium dioxide and optionally other compounds from melter slag derived from an iron-making process.

The invention relates to a process for extracting metals and salts from titanium-bearing minerals such as perovskite. More particularly, although not exclusively, the invention relates to extracting titanium dioxide and optionally other compounds from melter slag derived from an iron-making process.

A process for converting gypsum into precipitated calcium carbonate including reacting a mixture comprising gypsum and a seed, a mineral acid, or both with at least one carbonate source, whereby precipitated calcium carbonate is produced in the form of calcite and/or aragonite directly without conversion from a vaterite polymorph. Also, a process for converting gypsum into precipitated calcium carbonate including providing a mixture comprising i) gypsum ii) a seed, a mineral acid, or both iii) at least one additive selected from the group consisting of ammonium sulfate, an organic acid, or an iron material, and reacting the mixture with at least one carbonate source to produce precipitated calcium carbonate in the form of vaterite. The precipitated calcium carbonates having desired and unique compositions, polymorph and crystal size characteristics formed by these processes.

A process for converting gypsum into precipitated calcium carbonate including reacting a mixture comprising gypsum and a seed, a mineral acid, or both with at least one carbonate source, whereby precipitated calcium carbonate is produced in the form of calcite and/or aragonite directly without conversion from a vaterite polymorph. Also, a process for converting gypsum into precipitated calcium carbonate including providing a mixture comprising i) gypsum ii) a seed, a mineral acid, or both iii) at least one additive selected from the group consisting of ammonium sulfate, an organic acid, or an iron material, and reacting the mixture with at least one carbonate source to produce precipitated calcium carbonate in the form of vaterite. The precipitated calcium carbonates having desired and unique composition, polymorph and crystal size characteristics formed by these processes.

Methods and systems that remove impurities from phosphogypsum (PG), including from radium and heavy metal salts, and produce gypsum binders and products. In one embodiment, PG is reacted with a chloride solution in an acidic environment under mechanical manipulation and/or heat followed by galvanic and/or zeolite absorption removal of impurities.

A method for complex treatment of phosphogypsum comprising crushing of phosphogypsum and its washing with a solution of sulphuric acid, concentration 2-15%, and stirring at 50-80 C., wherein the resultant mixture is separated into a liquid fraction and a sediment containing mostly calcium sulphate, which is characterized in that metals are precipitated from the liquid fraction, mainly lanthanides, phosphates and sulphates of metals soluble in diluted sulphuric acid, and the sediment containing mostly calcium sulphate is converted in the presence of ammonia liquor and carbon dioxide to ammonium sulphate and calcium carbonate, wherein filtered and dried sediment of calcium carbonate is dissolved in a 15-30% solution of nitric acid while stirring continuously, and then the resultant CO.sub.2 is recirculated and used in the conversion of the first phase of the sediment, and the resultant mixture is separated into a solution of calcium nitrate with dissolved metals and a sediment of fluorides and silicates with metal precipitate.

Processes for regenerating alkali process streams are disclosed herein, including streams containing sodium hydroxide, magnesium hydroxide, and combinations thereof. Systems for regenerating alkali process streams are disclosed herein, including streams containing sodium hydroxide, magnesium hydroxide, and combinations thereof.

Magnesium oxide produced by a process is used as a neutralizing agent for preliminary neutralization treatment of a leached slurry obtained by leaching a nickel oxide ore at a high temperature and pressure with sulfuric acid added. A neutralizing agent is added to a leachate, obtained by leaching a nickel oxide ore, to separate impurities, and a sulfurizing agent is added to the resulting neutralized solution to obtain nickel and cobalt sulfides, followed by separating the sulfurized solution; discharge waste water, obtained by adding a neutralizing agent to the sulfurized solution to separate aluminum and manganese, is concentrated to precipitate and separate calcium contained in the discharge waste water as calcium sulfate; the resulting solution is concentrated to precipitate magnesium in the solution as magnesium sulfate; the magnesium sulfate is roasted with a reducing agent to obtain magnesium oxide and a sulfurous gas; and the magnesium oxide is washed.

Reclaiming useful products for commercial use and/or to be reused in an exhaust gas clean-up system. This is accomplished by subjecting the power plant exhaust gas to the various cleaning fluids followed by reclaiming the useful products from the exiting cleaning liquid. As the exhaust gas passes through a medium of water and calcium carbonate in a wet scrubber, the exiting cleaning fluid contains various calcium salts. Likewise, as the exhaust gas passes through a medium of calcium hydroxide and water other calcium salts are produced. By passing these respective calcium salts through solid-liquid centrifuge separators and other separating stirred tanks, various useful products are reclaimed and reused in the process or commercially sold. Some useful products are carbon dioxide, calcium sulfate, calcium carbonate, and mercuric salts.