Preparation Of Α-calcium Sulfate Hemihydrate Research Articles

Preparation of α-calcium sulfate hemihydrate with high mechanical strength from natural fibrous gypsum in KCl-mediated glycerol-water solution

Natural fibrous gypsum is a kind of high-quality gypsum resources, whose high-value added utilization pathway is to be transformed to α-calcium sulfate hemihydrate (α-HH), a cementitious material that can react with water to form pastes with mechanical strength. In this work, α-HH was prepared from natural fibrous gypsum in KCl mediated glycerol-water solution under mild condition. KCl played a simultaneous function on regulating the phase-transition kinetics, morphology and comprehensive strength of α-HH. Addition of low concentration of KCl prominently retarded the nucleation and growth rate of α-HH and decreased its aspect ratio due to the formation of syngenite on the side faces. While further increase of KCl concentration accelerated the nucleation and growth rate and decreased the crystal sizes. In the presence of 0.04 M KCl, the prepared α-HH presented a hexagonal prism shape with a length / width of 257.69–579.71 μm / 13.13–36.61 μm, and its dry comprehensive strength had reached 38.1 MPa that was comparable to those prepared in commercial, chloride and nitrate system. Altogether, the foreign ion-free glycerol-water solution contributed to transforming natural fibrous gypsum to α-HH with high purity and high mechanical strength.

Experimental Study and Mechanism Analysis of Preparation of α-Calcium Sulfate Hemihydrate from FGD Gypsum with Dynamic Method.

Flue-gas desulphurization (FGD) gypsum is a highly prevalent industrial by-product worldwide, which can be an excellent alternative to natural gypsum due to its high content of CaSO4·2H2O. The preparation of α-calcium sulfate hemihydrate is a high-value pathway for the efficient use of FGD gypsum. Here, a dynamic method, or an improved autoclaved process, was used to produce α-calcium sulfate hemihydrate from FGD gypsum. In this process, the attachment water of the mixture of FGD gypsum and crystal modifiers was approximately 18%, and the pH value was approximately 6.0. The mixture did not need to be pressed into bricks or made into slurry, and it was directly sent into the autoclave reactor for reaction. It was successfully applied to the practical production and application of FGD gypsum, citric acid gypsum and phosphogypsum. In this work, the compositions and morphology of the product at different stages of the reaction were examined and compared. In particular, single-crystal diffraction was used to produce the crystal structure of CaSO4·0.5H2O, and the results were as follows: a = 13.550(3); b = 13.855(3); c = 12.658(3); β = 117.79(3)°; space group C2. The preferential growth along the c-axis and the interaction mechanism between the carboxylate groups and the crystal were discussed throughout the analysis of the crystal structure.

Effect of maleic acid and pH on the preparation of α-calcium sulfate hemihydrate from phosphogypsum in Mg(NO3)2 solution

Effect of maleic acid and pH on the preparation of α-calcium sulfate hemihydrate from phosphogypsum in Mg(NO3)2 solution

Preparation of α-calcium sulfate hemihydrate from industrial by-product gypsum: A review

Preparation of α-calcium sulfate hemihydrate from industrial by-product gypsum: A review

Effects of phosphorus impurities on the preparation of α-calcium sulfate hemihydrate from waste phosphogypsum with the salt solution method under atmospheric pressure

Impurity is an important factor that determines the crystallization process of α-calcium sulfate hemihydrate (α-CSH), but the effects of phosphorus impurities in waste phosphogypsum on the α-CSH prepared using the salt solution method are still not well known.

Facile Preparation of α-Calcium Sulfate Hemihydrate with Low Aspect Ratio Using High-Gravity Reactive Precipitation Combined with a Salt Solution Method at Atmospheric Pressure

As one of the important bone regeneration materials, α-calcium sulfate hemihydrate (α-CSH) has attracted widespread attention. In this study, a facile method was developed to prepare α-CSH with low aspect ratio via high-gravity continuously reactive precipitation of a calcium sulfate dihydrate (CSD) precursor, using ethanol as a morphology modifier in a rotating packed bed (RPB) reactor, combined with CaCl2 salt solution method under atmospheric pressure with the addition of citric acid with a low concentration (0.06 wt %). The precipitation time of CSD powders in the RPB was dramatically shortened from 30 min to 1 s, and the precursor exhibited more regular short rod shapes and much smaller size, compared to that obtained in a conventional stirring tank reactor (STR). Furthermore, calcium sulfate bone materials with high stability and good mechanical property were prepared by mixing α-CSH and normal saline with a liquid–solid ratio of 0.6. The RPB products had a higher compressive strength (2–3 times) th...

Effect of Butane Diacid on Crystal Morphology and Reaction Process of α-Calcium Sulfate Hemihydrate in Preparation from Flue Gas Desulphurization Gypsum

Modifying crystal morphology was the key technology of the preparation of α-calcium sulfate hemihydrate from flue gas desulphurization gypsum using the hydrothermal method under atmospheric pressure. For exploring the mechanism of butane diacid effect on crystal morphology of α-calcium sulfate hemihydrate in preparation from flue gas desulphurization gypsum, the crystal morphology, convert ratio, and reaction process of α-calcium sulfate hemihydrate were studied by scanning electron microscope observation, and crystal water testing. The results showed, firstly, crystal modifier made the growth rates of crystal in all directions equal and made the crystal into hexagonal short column, by selectivity adsorbing on (111) crystal plane of α-calcium sulfate hemihydrate to form buffer film layer which hindered combining and growing of crystallization unit on the crystal plane of c-axis direction. Secondly, butane diacid was a good crystal modifier. When adding butane diacid of optimal amount of 0.02%~0.05%, the crystal morphology of α-calcium sulfate hemihydrate was satisfactory, the crystal length-diameter ratio was 1:1, but the convert ratio was only 91%~93%.

Effect of Organic Diacid Carbon Chain Length on Crystal Morphology of α-Calcium Sulfate Hemihydrate in Preparation from Flue Gas Desulphurization Gypsum

Modifying crystal morphology was the key technology of the preparation of α-calcium sulfate hemihydrate from flue gas desulphurization gypsum using the hydrothermal method under atmospheric pressure. For exploring crystal modifier, the crystal morphology of α-calcium sulfate hemihydrate in preparation from flue gas desulphurization gypsum with various organic diacid was studied, by polarizing microscope profile observation. The results showed, When the space between two carboxyls was triplicate length of C-C, the organic diacid had the best effect of modifying crystal morphology.

Pilot scale preparation of α-calcium sulfate hemihydrate from FGD gypsum in Ca–K–Mg aqueous solution under atmospheric pressure

Preparation of α-calcium sulfate hemihydrate (α-HH) from flue gas desulfurization (FGD) gypsum is one of the most promising alternatives and of great value in effectively utilizing the FGD gypsum. Based upon a salt solution method and the optimum operating conditions obtained in the laboratory scale experiments, pilot scale tests have been conducted to confirm the feasibility of the process. The pilot experiments were carried out in a Ca–K–Mg chloride solution medium at 94±2°C under atmospheric pressure on a 500–1000kg/batch setup. The crystal water content, solid phases, crystal morphology, strength and chemical composition of the products were examined and compared. It was found that short prism-shaped α-HH was successfully prepared within 5–6h, with the purity of α-HH up to 95%, the 2h bending strength of 6.8–7.3MPa, 2h compressive strength of 12.2–23.3MPa and dry compressive strength of 29.6–37.9MPa. It can be concluded that the process is highly feasible in the pilot scale, with high quality of the products. This research can provide necessary and useful information for the industrial production in the future.

α-calcium sulfate hemihydrate preparation from FGD gypsum in recycling mixed salt solutions

A pilot scale preparation of α-calcium sulfate hemihydrate (α-HH) from flue gas desulfurization (FGD) gypsum was conducted for the first time in mixed salt solutions under atmospheric pressure. The effects of salt solution recycling on the transition of α-HH from FGD gypsum were investigated by a comparison with that in laboratory scale, focusing on the dehydration rate of FGD gypsum, the crystal morphology, chemical composition and mechanical strength of the products. α-HH could be transformed from FGD gypsum in 3.5–6.0 h even though the salt solution was recycled seven times in laboratory tests and eight times in pilot tests. The product obtained in pilot scale has a purity of 95% and the α-HH crystals are mostly a prismatic morphology, with volume weighted mean diameter (D[3,4]) in the range of 52.9–84.0 μm, 2 h bending strength of 5.6–6.2 MPa and 3 d compressive strength of 15.7–25.3 MPa. The salt solution recycling achieves 56% recovery of salts in pilot scale and puts almost no negative impact on the product quality, which can be one of the best strategies to reduce the production cost. It is very likely that the process of salt solution recycling can be adopted in industry.

The Effect of Process Conditions on the Preparation of α-Calcium Sulfate Hemihydrate from FGD Gypsum Using the Hydrothermal Method under Atmospheric Pressure

This paper studies the laws of crystal growth, percent conversion and the rate of reaction of α-calcium sulfate hemihydrate from FGD gypsum under different conditions using the hydrothermal method under atmospheric pressure. The crystal morphology was observed by using SEM, polarizing microscope profile, and percent conversion and the rate of reaction were obtained by assaying crystal water content and calculating. The results showed, (1) with the increase of reaction temperature, the dehydration rate increased and the formed α-calcium sulfate hemihydrate crystal had a larger particle size; (2) with the increase of salt concentration or slurry concentration, the formed α-calcium sulfate hemihydrate crystal was smaller, percent conversion and the rate of reaction was nearly unchanged; (3) with the increase of pH value of solution, the rate of reaction increased and percent conversion was nearly unchanged, and with pH value ranging from 5 to 7 the formed α-calcium sulfate hemihydrate crystal was crassitude. In conclusion, the perfect technological parameters were as follows: reaction temperature ranging from 95°C to 100°C, salt concentration ranging from 15% to 20%, slurry concentration ranging from 15% to 20%, pH value ranging from 5 to 7, and reaction time not exceeding 90min.

Influence of Seed Crystal and Modifier on the Morphology of α-calcium Sulfate Hemihydrate Prepared by Salt Solution Method in Pilot Scale

Preparation of α-calcium sulfate hemihydrate (α-HH) from flue gas desulfurization (FGD) gypsum is of great value in effectively utilizing the FGD gypsum. This paper focuses on the effect of seed crystal and some modifiers on the morphology of α-HH on a 500-1000 kg/batch setup based on a salt solution method established on the previous work. The pilot tests were carried out in a Ca-Mg-K chloride solution medium at 94 oC (±2 oC) under atmospheric pressure. The results show that a small amount of seeds is advantageous to prepare short-prism α-HH but results in needle-like and spherical crystals in the case of more than 1% addition. The well-shaped α-HH crystals can also be obtained by adding appropriate amounts of crystal modifiers like Fe3+ and the mixed salts of Al3+ and organics.

Preparation of α-calcium sulfate hemihydrate from FGD gypsum in K, Mg-containing concentrated CaCl 2 solution under mild conditions

To prepare α-calcium sulfate hemihydrate (α-HH) from flue gas desulfurization (FGD) byproduct under atmospheric pressure, the dehydration of FGD gypsum in a concentrated Ca and Mg chlorides solution medium at 95 °C was investigated, and the effects of K + ions were focused on. The crystal water content variation, material phases, and morphological characteristics of the dehydration products were examined and compared. The solubilities of α-HH in the mixed salt solutions with different K + ion concentrations were also determined. It was found that with low concentration of K + ions in the mixed salt solution (i.e. 0.001–0.035 M), the dehydration rate was enhanced, while with higher K + ion concentration (i.e. 0.087–0.263 M), this process was retarded. α-HH was the dehydration product when K + ion concentration was at appropriate level, while anhydrite (AH) occurred when K + ion concentration was above 0.173 M. K + ions were assumed to behave as a capping agent to control the morphology of the α-HH, resulting in crystals of lower aspect ratio. FGD gypsum can be successfully transformed into α-HH in concentrated Ca and Mg chloride solution under mild conditions, and K + ions play an important role in the process.

Preparation of α-Calcium Sulfate Hemihydrate by Reaction of Sulfuric Acid with Lime

Direct preparation of α-calcium sulfate hemihydrate out of sulfuric acid solution by reaction with lime under atmospheric pressure conditions is systematically investigated. Two preparation methods are investigated: one involving addition of dry lime to sulfuric acid solution (standard procedure) and the other involving addition of sulfuric acid solution to slaked lime (reverse procedure). On the basis of the standard procedure, the operating window is first determined, followed by thorough examination of preparation conditions such as acidity, retention time and CaO/H2SO4 molar ratio. Dihydrate was found to form as intermediate phase that converts to hemihydrate under all tested conditions. The typical morphology of α-hemihydrate crystals produced by the standard procedure is needle-shaped with 50−100 μm length and 1−3 μm thickness. However, when the reverse procedure was used, a different growth mechanism apparently applied yielding shorter (∼60 μm) but thicker (5−10 μm) column-shaped crystals within otherwise the same operating window.