Bassanite Research Articles

PRODUCT CHARACTERISTICS IN SIMULTANEOUS CRYSTALLIZATION OF NaCl AND CaSO4 FROM AQUEOUS SOLUTION WITH SEEDING

Zero liquid discharge strategies for industrial wastewater treatment have become prominent in recent years. When evaporative simultaneous crystallization is applied, knowledge about the particle crystallization mechanisms and morphology are important to ease solids downstream handling and for recovery of their valuable components. In this work, batch simultaneous crystallization of sodium chloride (NaCl) and calcium sulphate hemihydrate (CaSO4.0.5H2O) from aqueous solution was studied. It was found that CaSO4.0.5H2O is not an effective substrate for NaCl heterogeneous nucleation, but agglomerates with NaCl particles instead. CaSO4.0.5H2O particles on the surface of NaCl crystals sterically hamper agglomeration of NaCl particles with each other and, for a sufficiently high CaSO4.0.5H2O seed load, cover the NaCl crystals so that NaCl supersaturation rises, inducing NaCl primary nucleation. Simultaneous crystallization of CaSO4.0.5H2O and NaCl yields a satisfactory product for downstream handling even for high CaSO4.0.5H2O content in the crystallizer. However, seeding with CaSO4.0.5H2O is not recommended as it reduces the mean size and increases product size dispersion. NaCl seeding may be considered if it is desirable to separate CaSO4.0.5H2O from NaCl downstream the crystallizer by size classification, as it favors CaSO4.0.5H2O to build up in product sizes of 200 μm and below.

Effect of Doping with Sodium and Potassium on the Phase Formation in the Synthesis of Calcium Sulfate

Materials based on calcium sulfate in which Na+ or K+ cations were substituted to a concentration of 5, 10, or 20 mol % for Ca2+ cations were synthesized and studied. The introduction of Na+ produced materials with the structure of calcium sulfate hemihydrate and dihydrate, and the incorporation of potassium yielded a mixture of calcium sulfates with various contents of chemically bound water and sulfates in which potassium cations formed compounds with different Ca : K ratios. The doping of calcium sulfate with sodium and potassium cations increased the solubility of materials by a factor of 1‒2 and 3‒4, respectively. The developed materials are promising in medical applications for filling bone tissue defects and also for using as a cement matrix for targeted drug delivery.

Crystal structure formation of hemihydrate calcium sulfate whiskers (HH-CSWs) prepared using FGD gypsum

Hemihydrate calcium sulfate whiskers (HH-CSWs) were hydrothermally synthesized in a sulfuric acid solution using different concentrations of CuCl2 (0, 0.5, 1.0, 1.5, and 2.0 wt%). The morphologies, phase structures, and structural formations of the hydrothermal products were analyzed. It was found that the synthesized HH-CSW samples were mixtures of crystals with different structures. Further, as the CuCl2 concentration was increased, the primary crystal structure of the samples changed from monoclinic to hexagonal and then back to monoclinic. The sample synthesized using 1.0 wt% CuCl2 primarily consisted of crystals with a hexagonal structure. Scanning electron microscopy imaging confirmed that the samples consisted of crystals of different morphologies (i.e., those with hexagonal and monoclinic structures). The effect of the CuCl2 concentration on the crystal structure transition is discussed.

FABRICATION OF POROUS CARBONATE APATITE BASED ON THE DISSOLUTION-PRECIPITATION OF CALCIUM SULFATE HEMIHYDRATE

Background : Carbonate apatite (C-Ap) has been used as a bone replacement material because it has osteoconductive properties. The pores contained in C-Ap are useful so that cells can diffuse during new bone formation. C alcium sulfate hemihydrate (CaSO 4 .1/2H 2 O) precursor has been used as it possesses Ca 2+ ions, while polymethylmetacrylate (PMMA) is chosen because it is easily burned out at low temperatures. O bjectives: To fabricate porous carbonate apatite based on a calcium sulfate hemihydrate precursor using the dissolution-precipitation method and identify its phases by X-ray diffraction. Methods : The pores were made by crushing polymethyl methacrylate (PMMA) granules and sieving them to sizes of 300–500 µm. Specimens of porous carbonate apatite were prepared from a mixed calcium sulfate hemihydrate precursor with different quantities of porogen: 0 wt% PMMA (n = 60 ), 25 wt% PMMA ( n = 60 ), and 50 wt% PMMA ( n = 60 ). Calcium sulfate hemihydrate containing PMMA was mixed with distilled water at a water: powder ratio of 0.5 ml: 1g. Calcium sulfate dihydrate specimens were placed in an oven at 700˚C for 4 h to burn out the PMMA, resulting in porous calcium sulfate anhydrate. This was then immersed in phosphate mixed solution (Na 3 PO 4 ) and carbonate solution (Na 2 CO 3 ) for 1, 3, and 5 days to obtain carbonate apatite through a precipitation-dissolution process. An X-ray diffraction technique (X-Ray Diffractometer, PANalytical XPert PRO) was used to analyze each group of specimens to identify the phase formation of porous carbonate apatite. Results : The X-ray diffraction pattern showed peaks of carbonate apatite in all specimens with 0, 25, and 50 wt% PMMA in 1, 3, and 5 days of immersion, except in the 0 wt% specimens at 1 day. The carbonate apatite diffraction peaks were at 2 q = 25.9 °, 29.2 °, 32 °, and 34 °. The earliest complete formation of the carbonate apatite phase was identified from the preparation of porous anhydrous CaSO 4 with porogen of 50 wt% PMMA immersed for 1 day. Conclusion : Porous carbonate apatite can be produced from precursors of calcium sulfate hemihydrate and porogen of PMMA via a precipitation-dissolution method.

Behaviour of calcined gypsum-based materials during setting

The behaviour at early age of several composed gypsum-based materials was investigated. The binary binder, composed of calcium sulphate hemihydrate and slaked lime, several ternary binders with different types of pozzolans were prepared, and their behaviour during setting was studied. Unrestrained volume changes of calcium sulphate-based pasts were measured by displacement of liquid, in which the specimens were submerged. The hydration heat of composed binders was measured by isothermal calorimetry. The behaviour of composed materials with different types of pozzolans was compared with the behaviour of calcined gypsum.

Microemulsion Synthesis of Nanosized Calcium Sulfate Hemihydrate and Its Morphology Control by Different Surfactants

Calcium sulfate hemihydrate with different structures and morphologies has a broad scope of applications. Herein, we reported a strategy for nanosized calcium sulfate through room-temperature microemulsion by using calcium carbonate and sulfuric acid. After characterizations, it was indicated that the morphology of calcium sulfate products fabricated could be changed and controlled using different surfactants TritonX-114, SDBS, and CTAB by the microemulsion method. The method demonstrated in this work would be a benefit for the synthesis of nanomaterials with special structures in the future.

Repair of rabbit articular cartilage defect by a novel injectable bioactive glass scaffold

Objective By comparing different repairing effects of different methods on articular cartilage defects in rabbit model, the ability of a new bioactive glass scaffold to repair cartilage defect was studied. Methods We prepared the PSC/CS bone cement by mixing the bioactive glass (BG) powder composed of 10.8%P2O5-54.2%SiO2-35%CaO (PSC), chitosan solution (CS) and calcium sulfate hemihydrate (CSH), then the bone cement was sized by a mold to form cylindrical scaffold. In vivo experiment, 18 male rabbits were divided into three groups randomly, including blank group, BG group and TGF-β1 group, which was added TGF-β1 into BG scaffold. Both knees of each rabbit were made cartilage defect for the same group, and no intervention was applied in B group, then implanted scaffolds into defects in both experimental groups. At 6 and 12 weeks after surgery, observed the macroscopic growth, histologic staining and collagen II immunohistochemistry (IHC), and the International Cartilage Repair Society (ICRS) and Wakitani score were used to analyze the experimental results quantitatively. Results At 6 weeks after surgery, there was no obvious difference between blank group and BG groups, but the macroscopic result of TGF-β1 group was better than the other two groups and its ICRS score 4.67±0.52 points was statistically higher than BG group 2.83±0.75 points (t=-2.817, P=0.015). As to the comparison of histologic staining and Wakitani socre among three groups, no statistical difference was observed (blank group=13.67±0.52, BG group=13.83±0.41, TGF-β1 group=13.33±1.03). At 12 weeks after surgery, there was still no obvious difference between blank and BG groups, while the results of macroscopic observation and ICRS score in TGF-β1 group were significantly higher than them 9.01±0.63 points (blank group vs TGF-β1 group: t=-2.289, P=0.022; BG group vs TGF-β1 group: t=-2.326, P=0.020). More importantly, much deeper positive staining were observed in TGF-β1 group, and the Wakitani score was higher than the other two groups (blank group=9.83±1.33, BG group=9.51±1.05, TGF-β1 group=6.50±1.38, blank group vs TGF-β1 group: t=-2.771, P=0.007; BG group vs TGF-β1 group: t=-2.756, P=0.006). By comparing the degree of histologic staining and Col II expression with normal cartilage, the regenerated tissue in TGF-β1 group was similar. Conclusion Single PSC/CS scaffold doesn’t possess excellent ability to repair cartilage defect. When TGF-β1 was added into PSC/CS bioactive glass, the scaffold was able to promote cartilage defect repair, and the regenerated tissue was similar to normal cartilage. Key words: Tissue engineering; Glass; Tissue scaffolds; Chondrocytes

A novel synthetic approach to low-crystallinity calcium deficient hydroxyapatite

Abstract Synthetic calcium deficient hydroxyapatite (CDHA, with the formula Ca9(HPO4)(PO4)5OH) is a promising material which could be potentially used in biomedicine. The aim of this study was to develop a simple and cost-effective synthesis route for the preparation of low crystalline CDHA granules. A method based on an incessant dissolution-precipitation reaction eventually resulting in a solid and shaped material consisting of the desired phase was utilized. The calcium sulphate hemihydrate was selected as the starting material. The synthesis products obtained were characterized by Fourier transformed infrared spectroscopy (FTIR), X-ray diffraction analysis (XRD) and scanning electron microscopy (SEM). The monophasic calcium deficient hydroxyapatite granules of about 300–350 μm in diameter were synthesized by three slightly different preparative approaches at 70 °C temperature and compared. Independently of the composition of the raw granules synthesized ones were composed of low crystalline an apatitic phase. The granules were calcium deficient as contained HPO42− group in their structure. It was demonstrated that the addition of (NH4)H2PO4 or (NH4)2HPO4 into the raw granules could promote the growth of the CDHA crystallites. SEM revealed that the microstructure of synthesized materials was influenced by the chemical composition of the raw granules. Chemical composition and presence of no an X-ray amorphous phase in obtained samples was confirmed by reheating the obtained granules at 800 °C to get monophasic β-TCP. Our work provides a novel and simple approach to produce CDHA granules for various applications.

Dehydration Pathways of Gypsum and the Rehydration Mechanism of Soluble Anhydrite γ-CaSO4.

The dehydration products of gypsum under different temperature and water vapor pressure were investigated by thermodynamic theory. Additionally, the rehydration mechanism of soluble anhydrite was also studied by Monte Carlo (MC) simulations. The thermodynamic calculation results reveal that the dehydration mechanism of gypsum significantly depended on ambient temperature and water vapor pressure. In the high-temperature and low water vapor pressure region, gypsum dehydrates to form γ-CaSO4 in a single-step process (CaSO4·2H2O → γ-CaSO4); with increasing water vapor pressure, gypsum undergoes the CaSO4·2H2O → γ-CaSO4 → β-CaSO4·0.5H2O reaction path and as water vapor pressure increases further, the occurrence of a two-step conversion path CaSO4·2H2O → β-CaSO4·0.5H2O → γ-CaSO4 was observed. It was also found that gypsum is stable in the low-temperature and high water vapor pressure region and does not dehydrate to form any calcium sulfate hemihydrate. Finally, the rehydration mechanism of soluble anhydrite was studied by MC simulations. The simulation results are in agreement with the experimental data and support the finding that γ-CaSO4 rehydration forms CaSO4·0.67H2O in high relative humidity. Another important result revealed by the MC simulation is that γ-CaSO4 has an extraordinary ability to capture water molecules from an extremely dry atmosphere, which is very useful in some fields, such as in drying processes and even for extracting liquid water from extremely dry atmosphere.

Ultrasonic Irradiation during the Calcium Sulfate Hemihydrate to Dihydrate Transformation Process

AbstractThe ultrasound‐assisted transformation of calcium sulfate hemihydrate (CSH) to calcium sulfate dihydrate (CSD) was studied using three ultrasonic power settings. In addition to continuously measuring the variation in the conductivity value with time, the process was followed by X‐ray diffraction and scanning electron microscopy (SEM) analysis. Thermogravimetric analysis, Fourier transform infrared, mass spectrometry, and filtration analyses helped to determine the physicochemical properties of the as‐obtained crystals. The morphological changes in the surface morphology were observed by SEM. The experimental results indicate that ultrasonic irradiation and its intensity have a significant effect on the crystal size, morphology, adsorption, and filtration characteristics. Mandelic acid in combination with ultrasonic irradiation can be used as an accelerator for the transformation process from CSH to CSD.

A kinetic and mechanistic study into the transformation of calcium sulfate hemihydrate to dihydrate.

The conversion of CaSO4·0.5H2O to CaSO4·2H2O is of great importance industrially, being the reaction behind plasterboard production and the setting of medical plasters. A detailed kinetic and mechanistic study of this process was conducted using time-resolved synchrotron X-ray diffraction in this work. The CaSO4·2H2O product is very similar regardless of whether the α- or β-form of CaSO4·0.5H2O is used as the starting material, but the reaction process is very different. The induction time is usually shorter for α-CaSO4·0.5H2O than β-CaSO4·0.5H2O, and a greater conversion percentage is observed with the former (although in neither case does the reaction proceed to 100% completion). The temperature of the system, widely used in industry as an indirect measure of the extent of the hydration process, is found to be a poor proxy for this, with the maximum temperature reached well before the reaction is complete. The Avrami-Erofe'ev and Gualtieri models could both be fitted to the experimental data, with the fits being substantially closer in the case of α-CaSO4·0.5H2O. The rate of reaction in the Avrami model tends to increase with the amount of gypsum seeds added to accelerate the process, and the importance of nucleation declines. The Gualtieri analysis suggested that the rate of nucleation increases substantially with the amount of seeds added, while there are less distinct changes in the rate of crystal growth. At low seed concentrations (<0.5% w/w) the rate of crystal growth is greater than the rate of nucleation, but at concentrations above 0.5% w/w nucleation is faster. These findings represent the first synchrotron study of the conversion of CaSO4·0.5H2O to CaSO4·2H2O, and will be of importance to gypsum producers globally.

Bassanite Whiskers: Effects of Cationic Polyacrylamide on Hydrothermal Formation of Ultralong α‐CaSO4·0.5H2O Whiskers (Crystal Research and Technology 4/2019)

Crystal Research and TechnologyVolume 54, Issue 4 1970016 Cover PictureFree Access Bassanite Whiskers: Effects of Cationic Polyacrylamide on Hydrothermal Formation of Ultralong α-CaSO4·0.5H2O Whiskers (Crystal Research and Technology 4/2019) First published: 08 April 2019 https://doi.org/10.1002/crat.201970016AboutPDF ToolsRequest permissionExport citationAdd to favoritesTrack citation ShareShare Give accessShare full text accessShare full-text accessPlease review our Terms and Conditions of Use and check box below to share full-text version of article.I have read and accept the Wiley Online Library Terms and Conditions of UseShareable LinkUse the link below to share a full-text version of this article with your friends and colleagues. Learn more.Copy URL Share a linkShare onFacebookTwitterLinkedInRedditWechat Graphical Abstract The cover shows a scanning electron microscopy image of hydrothermally grown ultralong calcium sulfate hemihydrate (α-CaSO4·0.5H2O) whiskers using cationic polyacrylamide as the morphology modifier. The hydrothermal formation mechanisms of α-CaSO4·0.5H2O involved the initial nucleation of bassanite nanocrystals/nanorods and their subsequent self-assembly into long whiskers through oriented attachment, which was directed by the cationic polyacrylamide molecules. This is reported by Jing Wang, Lan Xiang and co-workers in article number 1800224. Volume54, Issue4April 20191970016 RelatedInformation

Product characteristics in simultaneous crystallization of NaCl and CaSO4 from aqueous solution under different evaporation rates

Knowledge about simultaneous crystallization in evaporative processes for water reuse is important for the promotion of particles with desirable characteristics. In this work, simultaneous crystallization of sodium chloride and calcium sulfate hemihydrate (HH) from aqueous solutions in unseeded batch mode is addressed. Both the crystallization mechanisms and the resulting particle morphological features have been identified. NaCl particles are generated by heterogeneous primary nucleation in solution, followed by growth and NaCl agglomeration. Calcium sulfate hemihydrate (HH) particles are also formed by heterogeneous primary nucleation in solution followed by crystal growth. HH crystals agglomerate with NaCl in all particle size ranges. Product particles are thus agglomerates with multimodal population densities in submillimetric size ranges, with HH widely dispersed throughout the size span of the particulate product. In comparison with NaCl binary solutions, simultaneous crystallization promotes the formation of larger, less polydisperse NaCl particles. Low supersaturations (low evaporation rates) inhibit HH agglomeration with NaCl. The submillimetric sized particles formed during simultaneous crystallization of NaCl and HH indicate ease in solid-liquid downstream separation.

Effects of AlCl3 on the Crystal Morphology of Calcium Sulfate Whisker Prepared from FGD Gypsum

Utilizing the purified flue gas desulfurization (FGD) gypsum as raw material and AlCl3 as crystal modifier to prepare calcium sulfate whiskers (CSWs) by hydrothermal method, the effects of the AlCl3 concentration and the pH of reaction slurry on the crystal morphology of CSW were discussed. The results show that the AlCl3 concentration and the pH of reaction slurry had a significant effect on the crystal morphology of CSWs. CSWs grown in the AlCl3-H2O slurry had diameters ranging from 2 to 6 um and aspect ratio above 80 μm, and their phase structure was calcium sulfate hemihydrate (HH).

Enhanced strength, in vitro bone cell differentiation and mineralization of injectable bone cement reinforced with multiferroic particles

Self-setting calcium sulphate hemihydrate (CSH) composites were prepared by reinforcing with BiFeO3 (BF, a multiferroic material) with an aim to induce in situ charge generating capacity and increase its strength. CSH-BF composites (0–15 wt% BF), prepared by ball-milling, were evaluated in terms of setting and injectability, compressive strength, in vitro degradation, electrical/magnetic properties. Compressive strength of CSH-BF composites increased with BF concentration and up to 3-fold increase was observed compared to pure-CSH. Injectability and setting time of these composites were comparable to that of standard bone cements. In vitro degradation studies revealed ~27–33% degradation with good apatite precipitation on these composites. Remanant polarization of these composites varied between 0.056 and 0.251 μC/cm2. The composites with BF ≤ 5 wt% exhibited 200–375% increase in the cell viability with 200mT magnetic treatment. Osteoblast differentiation experiments showed high and rapid production of Alkaline Phosphatase followed by accelerated bone mineralization on the composites compared to pure-CSH. Electrical stimuli (remanant polarization) and its coupling during magnetic field treatment are responsible for enhanced bone-cell differentiation and mineralization on CSH-BF composites. Our results show that BF reinforcement of CSH not only improves its strength but also impart charge generating capacity, which can be effectively utilized to accelerate tissue healing or regeneration.

Design of novel organic-inorganic composite bone cements with high compressive strength, in vitro bioactivity and cytocompatibility.

In this work, novel bioactive organic-inorganic composite bone cements consisting of tricalcium silicate (C3 S), sodium alginate (SA), and calcium sulfate hemihydrate (CS) were successfully fabricated for the first time via a special method designing material composition and internal structure simultaneously, which was intended to enhance mechanical performance by combining progressive hydration process of C3 S with distinctive gelation capacity of SA and further improve degradability and self-setting properties with the addition of CS. Depending on the synergistic combination of hydration and gelation, the C3 S/SA/CS composite cements (45/45/10 wt %) obtained extremely higher compressive strength up to 92.41 MPa as compared with each single component. The reinforcing mechanisms involving interfacial interaction and interior microstructure were proposed to explain this enhancement phenomenon. Additionally, the final setting time could be reduced from 68 min to 21 min with the increasing CS content. The composite cements possessed good apatite mineralization ability in simulated body fluid solution and moderate degradation rate in phosphate buffer solution. What's more, the composite cements exhibited excellent cytocompatibility and increased proliferation of rat bone-marrow stem cells. This study could provide guidelines for the preparation of bioactive composite cements with enhanced mechanical performance, which may be suitable for load-bearing bone repair. © 2019 Wiley Periodicals, Inc. J Biomed Mater Res Part B: Appl Biomater 107B: 2365-2377, 2019.

Modification of fly ash produced in the process of burning fossil fuels - granulation method

The low grain size of fly ashes from the combustion of fossil fuels creates a number of ecological problems both during storage and transport, and often makes their re-use more difficult. One way to avoid this problem is to compact these materials in the granulation process. The granulation process generally requires the use of different binders, which are divided into matrix, film, and chemical type binders. Depending on the chemical composition of the fly ash and the type of the binder used, the obtained aggregate forms (granulates) find numerous applications, e.g. in the production of adhesives, zeolites and/or concrete, in manufacture of building materials, and in aggregates for road works or for ground leveling. In many applications, especially in road engineering works or in ground leveling, the basic requirements, next to the compressive strength of the granules obtained, are the leachability of sulfate ions and the pH-value of the eluates. The paper presents a research study investigating the granulation process of fly ashes produced by the combustion of hard and brown coals burned in both pulverized and fluidized furnaces, with the application of the following binders: water, calcium sulfate hemihydrate, and mixture of calcium sulfate hemihydrate and calcium hydroxide. For the resulting granulates, tests were conducted to determine compressive strength, SO2 sorption capacity, the pH-value, and the content of sulfate ions in the eluate before and after sorption tests. The obtained results show that the granulation process mostly decreases leachability of sulfate ions and the pH-value of the eluates while simultaneously providing adequate compressive strength of the obtained compacted forms (granulates).

Calcium sulfate-based bioactive cement for periodontal regeneration: An In Vitro study.

Various types of osteoconductive graft materials are used for the management of alveolar bone defects arising out of periodontal disease. Inorganic, self-setting, bioactive bone cements are suggested to be most appropriate because they can conformally fill the bone defect and resorb progressively along with the regeneration of the host site. A new calcium sulfate-based bioactive bone cement (BioCaS) is developed, having simplicity and effectiveness for bone grafting applications. The response of primary human periodontal ligament (hPDL) cells to this material is investigated through in vitro cell culture model so as to qualify it for the repair of periodontal infrabony defects. The BioCaS was designed as powder-liquid combination with in-house synthesized high purity calcium sulfate hemihydrate incorporating hydrogen orthophosphate ions. hPDL cells were isolated, cultured and characterized using optimized primary cell culture techniques. The cytotoxicity and cytocompatibility of the BioCaS samples were evaluated using the hPDL cells, with hydroxyapatite ceramic material as control. Osteogenic differentiation of the hPDL cells in presence of BioCaS was also evaluated using Alizarin red staining, Alizarin red assay, Von Kossa staining and Masson's trichrome staining. The primary cell culture techniques yielded a healthy population of periodontal ligament cells, with fibroblast morphology and characteristic marker expressions. The hPDL cells exhibited good viability, adhesion and spreading to the BioCaS cement in comparison to sintered hydroxyapatite. In addition, the cells differentiated to osteogenic lineage in the presence of the BioCaS cement, without extraneous osteogenic supplements, confirming the inherent bioactivity of the cement. The new BioCaS cement is a potential candidate for the repair of periodontal defects.

Preparation, Characterization, and Formation Mechanism of Calcium Sulfate Hemihydrate Whiskers

Calcium sulfate hemihydrate whiskers were synthesized successfully via one-step hydrothermal crystallization method using phosphogypsum at 130 °C for 240 min with an initial slurry mass fraction of 2.5wt%. The phase compositions, microstructures, thermal properties and molecular structures of asprepared samples were analyzed by XRD, ESEM, EDS, TG-DTA, and FT-IR. The influence of raw materials’ ball-milling time on the morphologies of whiskers was investigated. The effects of impurities on crystallization morphologies and length to diameter ratio (L/D) of calcium sulfate hemihydrate whiskers were studied. The results indicated that the calcium sulfate dihydrate crystalline could be translated directly into fibrous calcium sulfate hemihydrate whiskers. It was beneficial to form fine fiber structure when the ball-milling time of the raw material was 15 min. Aspect ratio of calcium sulfate hemihydrate whiskers decreased with increasing content of impurities. Moreover, the relative growth mechanism of whisker crystals via one-step hydrothermal crystallization method was discussed in detail.

Phosphoric acid purification sludge: Potential in heavy metals and rare earth elements

The present study was carried out to show the potential in heavy metals (HM) and the rare earth elements (REE) which presents the residues of phosphoric-acid(PA) purification. Three different cadmiferous solid residues (according to the nature of the purification process of the PA: BG, BC and BS) were collected from an industrial site located in the south of Tunisia.The mineralogical study showed the predominance of anhydrite, accompanied by quartz, malladrite; calcium sulfate hemihydrate and fluorapophyllite. The microanalysis showed (i) the association of cadmium and zinc, (ii) as well as the presence of associated REEs. The chemical analysis showed that (i) the calcium sulfate concentrations are majority in samples BS, BG and BC (44, 34 and 44%, respectively), (ii) significant concentrations of phosphoric acid (28, 18 and 21% P2O5, respectively), (iii) the HM: Cd, Zn, Cr, Ni, V, Cu, Pb, Co, Mo, Mn and U have proportion in the order of 0.1%. The concentrations of Cd, Zn and Cr are respectively in the order of: 230, 149 and 189 mg/kg for BS, 346, 243 and 153 mg/kg for BG and 183, 129 and 440 mg/kg for BC and (iv) the REEs: La, Ce, Nd, Eu, Y et Yb present considerable mass percentages able to reach 0.2%.A series of extraction tests was led on the cadmiferous sludges to evaluate the rates of HM (Cd, Zn) and REE dissolution, using two solvents (deionized water (DW) and aqueous sodium based alkaline metal solution). The results showed that the dissolution rates of Cd and Zn are respectively in the order of (12–29% and 41–45% for DW; 67–86% and 83–93% for Na2SO4 solution). The extractability of HM and REE is strongly influenced by pH, solvent nature and mineral load in the cadmiferous sludges. The water-soluble metals represent a significant mobile fraction, making the toxic elements more sensitive to mobilization processes, such as leaching and erosion. Whereas, the metals extractable by the Na2SO4 solution represent a very important exchangeable and “co-crystallization” fraction, which reflects the bioavailability of these metals.