The present invention aims to provide techniques for preparing complexes of calcium phosphate particles and a fiber. According to the present invention, complexes of calcium phosphate particles and a fiber are provided. According to the present invention, calcium phosphate-fiber complexes in which titanium is retained can be further obtained.

Disclosed herein are system and methods to effectively leach coal ash with hydrochloric acid and separate an insoluble silica product and then selectively precipitate, from the leachate, a number to value-added, strategic, marketable products using a hydroxide reagent. The resulting precipitated products include iron, aluminum, magnesium, calcium, and a mixture of rare earth elements and transition metals. These can be separated as hydroxides or converted to oxides or carbonates. Using hydrochloric acid for leaching and converting the chloride to sodium chloride in the final step results in practically no waste for this process. The silica can be further purified using sodium hydroxide fusion or caustic leach methods and some minor streams from this process are recycled to minimize any waste stream. These systems and methods can be applied to a number of other industrial waste products such as red mud from the aluminum process, slag from steel furnaces, mine tailings, and other metal-bearing waste streams.

Aspects of the invention include methods of removing carbon dioxide (CO.sub.2) from a CO.sub.2 containing gas. In some instances, the methods include contacting CO.sub.2 containing gas with a bicarbonate buffered aqueous medium under conditions sufficient to produce a bicarbonate rich product. Where desired, the resultant bicarbonate rich product or a component thereof may then be stored or further processed, e.g., combined with a divalent alkaline earth metal cation, under conditions sufficient to produce a solid carbonate composition. Aspects of the invention further include systems for practicing the methods, as well as products produced by the methods.

A mineral composition is shown which contains a mixed solid phase of synthetic calcium and magnesium carbonates, formed of a crystallized calcic portion and a plate-like crystallized magnesian portion. The crystals of the calcic portion and those of the magnesian portion are aggregated in the form of composite aggregates. These aggregates themselves are at least partly agglomerated in the form of agglomerates. The calcic portion has at least one carbonate such as calcite or the mixtures of calcite and aragonite. The magnesian portion contains hydromagnesite in plate-like form. The aggregates are further formed of a calcic core on which hydromagnesite plates are aggregated. The mineral composition has defined thermal conductivity properties making it suitable for use as a building material.

The invention provides a method for the production of a composite comprising microfibrillated cellulose (MFC) and precipitated calcium carbonate. The method is characterized in that MFC is added to a suspension of calcium hydroxide during carbonation, whereby calcium carbonate is precipitated onto fibers or fibrils of the MFC in a controlled manner. By adding microfibrillated cellulose to the calcium suspension during the carbonation, the brightness and the strength of the MFC/PCC-composite is enhanced. Moreover, the inventive method facilitates the distribution of calcium dioxide and MFC in the suspension and thus gives rise to a more homogenous product.

The present invention is directed to a process for producing precipitated calcium carbonate with improved resistance to structural breakdown, wherein the milk of lime is carbonated in the presence of at least one gas other than carbon dioxide, or the carbonation is carried out in the presence of a static gas bubble comminution unit as well as to precipitated calcium carbonate obtained by such a process.

The invention relates to a particularly energy efficient, two-step method and to a system for the continuous or semicontinuous production of crystalline calcium carbonate (precipitated calcium carbonate, PCC) by reacting calcium hydroxide with CO.sub.2, the calcium hydroxide being lime milk. In the first step of the germination, the CO.sub.2-source is exclusively flue gas having a CO.sub.2-content of between 4-25% <sb/><sb/>. In the second step, the complete conversion of the lime milk reacted in the first step to a maximum of 90%, preferably between 10-90%, is carried out exclusively using a rich gas which comprises 30-99% CO.sub.2, preferably using biogas.

In a preferred embodiment, the invention relates to a process of sequestering carbon dioxide. The process comprises the steps of: (a) reacting a metal silicate with a caustic alkali-metal hydroxide to produce a hydroxide of the metal formerly contained in the silicate; (b) reacting carbon dioxide with at least one of a caustic alkali-metal hydroxide and an alkali-metal silicate to produce at least one of an alkali-metal carbonate and an alkali-metal bicarbonate; and (c) reacting the metal hydroxide product of step (a) with at least one of the alkali-metal carbonate and the alkali-metal bicarbonate produced in step (b) to produce a carbonate of the metal formerly contained in the metal silicate of step (a).

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.

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.