Solubility Of Calcium Sulfate Research Articles

Two steps leaching process for recovery of rare earths from moroccan phosphogypsum

Phosphogypsum (PG), a primary by-product of the wet process phosphoric acid production, poses significant challenges to the fertilizer industry due to its voluminous generation; for each ton of phosphoric acid (P2O5), 5 tons of phosphogypsum are generated. Traditional management practices involving landfill storage or sea discharge are associated with environmental and health risks. As a result, there is an urgent need for innovative management strategies. One promising avenue is the valorization of valuable elements present in PG, notably rare earths elements (REEs). Although previous research has delved into REEs leaching from PG, fully understanding the mechanisms remains a complex endeavor due to the intricacy of the matrix and the variability in impurities concentrations, including that of rare earths. To address this knowledge gap, this study presents a robust two-step hydrometallurgical process that explores the relationship between REEs concentration and their leaching efficiency. Initial experiments focused on optimizing REEs leaching from Moroccan PG, using three agents: nitric acid, hydrochloric acid, and sulfuric acid, under systematic conditions. Subsequent experiments investigated the correlation between REEs concentrations and their leaching efficiency from PG. Three PG samples with deliberate REEs concentration were prepared and subjected to leaching under previously determined optimal conditions. Both nitric and hydrochloric acids demonstrated high leaching efficiencies with results reaching up to 60%. Intriguingly, when the concentration of REEs in PG was adjusted, the leaching efficiency surged to 76% under analogous conditions. This underlines a strong correlation between REEs leaching efficiency and their concentration in PG, with calcium sulfate solubility, emerging as a pivotal factor influencing both variables. Data from these systematic leaching experiments were further utilized for kinetics investigations, revealing that the diffusion of REEs through the product layer dominates as the rate-controlling step. In conclusion, this research bridges critical gaps in understanding the correlation between the concentration of REEs and their leaching from PG and offers insights propelling REEs recovery for potential widespread applications.

CALCIUM SULFATE SCALE IN THE PETROLEUM INDUSTRY

Oilfield scale is defined as the precipitation of hard, adherent deposits of inorganic solid originating from aqueous media. This constitutes sulfate and carbonate of the alkaline earth metals calcium, barium and strontium and complex salts of iron. Generally, the process of the scale deposition occurs when the product solubility of a compound considered is exceeded. The formation of scale, such as calcium sulfate, has long recognised as one of the serious problems in oil and gas production leading to reduced production rates as flow becomes restricted. Calcium sulfate scale found in the oilfield is in the form of gypsum (CaSO4, 2H2,0) which is the most stable form at temperatures of 40 °C or less at atmosphere pressure. Above this temperature, anhydrite (CaSO4,) may be found, although hemihydrate (CaSO. 1/2H,O) may form under certain conditions. The reaction for precipitation of calcium sulfate is as follows: Ca+2 (aq) + so4-2 (aq) = CaSO4 (solid) The solubility of calcium sulfate in distilled water is 2080 mg/l at 25 "C. Calcium sulfate scale arises from several causes, such as temperature, dissolved salts, pressure, and time. The main points of this paper are focused on nomenclature, chemical structure, the occurrence of calcium sulfate scale, example of calcium sulfate scale in the petroleum industry, and calculation of calcium sulfate solubility in brine.

Characterisation of Purified Gypsum and Insoluble Impurities Obtained from Phosphogypsum Waste

In this study, the chemical and phase composition of two samples of phosphogypsum from the waste dumps of the Industry of ChemicalProducts “Elixir – Prahovo” (Serbia) were examined, as well as the possibility of recrystallization of gypsum from an aqueoussuspension of phosphogypsum. The negative effect of higher temperatures on the solubility of calcium sulfate (13.08 mmol/dm3 at95°C vs. 15.43 mmol/dm3 at 40°C) was utilized. In several repeated cycles, calcium sulfate component was progressively dissolvedin water at room temperature and then precipitated at 100°C, using the same liquid phase throughout the experiment. Therefore,phosphogypsum was separated into recrystallized (purified) gypsum, insoluble residue and supernatant, and the mass balance forthe experiment was calculated. Elemental, XRD and SEM-EDS analyses were performed on raw phosphogypsum, purified gypsumand insoluble residue. The whiteness of raw phosphogypsum and purified gypsum were determined and compared. The main objectiveof the study was to investigate the nature of insoluble impurities, in order to define and optimize the methods for their removalduring a potential industrial processing of phosphogypsum.

Influence of Corona Discharge on CaSO4 Solubility of Silicone Rubber Insulator Surface

Some studies have shown that the operating state of silicone rubber insulators is not only affected by surface corona discharge, but also different due to the surface covered with different pollutants, but few studies have linked the two. In this paper, by designing the corona discharge test of silicone rubber insulator, the output of nitric acid from its surface product was calculated, and the concentration of nitric acid under different air humidity was obtained. The solubility test of slightly dissolved salt calcium sulfate in nitric acid solution at 293.15K and 333.15K was designed to obtain the solubility of calcium sulfate at different concentrations of nitric acid at two temperatures. Finally, a quantitative analysis of the solubility of calcium sulfate on the surface of insulator FXBW-10/70 in nitric acid solution shows that the surface contamination of insulator with good hydrophobicity may be completely dissolved by nitric acid solution, but the surface of insulator with loss of hydrophobicity cannot be completely dissolved by nitric acid. This study provides a theoretical basis for the dissolution characteristics of silicon rubber insulator corona discharge on its surface soluble salt composition and provides a new idea for the influence of insulator corona discharge on its electrical properties.

Model development for estimating calcium sulfate dihydrate, hemihydrate, and anhydrite solubilities in multicomponent acid and salt containing aqueous solutions over wide temperature ranges

Thermodynamic solubility of calcium sulfate in multicomponent electrolyte aqueous solutions is of significant importance in many disciplines in science and engineering. A knowledge of environmental chemistry and aqueous solubility of calcium sulfate and several of its hydrates; anhydrite (CaSO4), hemihydrate (CaSO4.0.5H2O), and dihydrate (CaSO4.2H2O); in acid and salt-containing media is of huge practical and theoretical importance. In this study, a solubility calculation model was developed based on coupling of Least Squares Support Vector Machine and Coupled Simulated Annealing (CSA-LSSVM) to model the solubility of various forms of calcium sulfate in aqueous solutions over wide temperature ranges. Comparing the model results to 3079 experimental data gathered from the literature yields a correlation coefficient of 0.9897 and RMSE of 0.0068. The model is able to present accurate solubility results for calcium sulfate dihydrate (CaSO4.2H2O) in aqueous solutions containing H2SO4, ZnSO4, CuSO4, NiSO4, MnSO4, H3PO4, NaCl, CaCl2, MgCl2, KCl, FeCl2, Na2SO4, HNO3, Ca(NO3)2, HCl, AlCl3, Ca(HCO3)2, NaNO3, LiNO3, Li2SO4, MgSO4, Al2(SO4)3, NaClO4, NaOH, KOH at temperatures up to 100 °C, calcium sulfate hemihydrate (CaSO4.0.5H2O) in solutions containing NaCl, CaCl2, MgCl2, KCl, FeCl2, HCl at temperatures up to 125 °C, and calcium sulfate anhydrite (CaSO4) in solutions containing H2SO4, NaCl, CaCl2, MgCl2, Na2SO4, HNO3, HCl, NaNO3, LiNO3, NiSO4, MgSO4, Al2(SO4)3, Mg(NO3)2 at temperatures up to 350 °C. Comparison of the CSA-LSSVM model results to those of Pitzer ion interaction model as well as BR-ANN model in binary, ternary, and multicomponent mixtures reveals the superiority of the CSA-LSSVM model developed in this study in all cases.

Effect of ammonium sulfate and phosphogypsum application on nutrients dynamics and acidity of black soil

The problem of phosphogypsum accumulation in dumps of chemical plants has been an urgent problem for several decades. The ecological situation is aggravated by the fact that more and more areas are allocated for its conservation. A negative point in the application of phosphogypsum is the intake of radionuclides and fluorine into the soil and plants, small particles could be dispersed to the atmosphere by wind. But given the presence of macro-, mezo- and microelements in it and the high price of mineral fertilizers, it is now considered as a good fertilizer and ameliorants, especially for alkaline soils. The goal of the research was to study the effectiveness of phosphogypsum application (from Sumykhimprom) and ammonium sulfate in increasing doses of nitrogen 50‒150 on the dynamics of nitrogen, phosphorus, potassium, calcium and hydrolytic acidity of typical middle loam black soil. An increase in nitrogen led to growing the content of hydrolyzed, nitrate and ammonium forms of nitrogen in the soil. The maximum availability of N-NO3 in the soil is characteristic for the first period of sampling, in the tillering stage. At this period, the maximum difference is observed between the control and fertilized variants of the experiment. The application of phosphogypsum with N150 almost threefold increased the content of nitrates in the soil. Variants with lower doses of nitrogen also affect the accumulation of nitrates in layers 0‒20 and 20‒40 cm. After harvesting, an insignificant difference was found between the control and fertilized variants (except for N150) with a general decrease in the level of nitrate availability to 0.1‒0.2 mg/100 gm of soil. The impact of fertilizers was less on the content of labile phosphorus and exchangeable potassium. A year after fertilization, a significant increase in the value of hydrolytic acidity is observed in the fertilized variants.
 It is especially noticeable at a dosage with nitrogen of 120‒125. In these variants, the hydrolytic acidity in both the arable and subsoil layers exceeds 4 mmol+/100 gm of soil. Changes in the content of water-soluble calcium are insignificant which can be explained by the fact that the solubility of calcium sulfate is not high enough and it takes more time for calcium to appear in an ionic form.

Single-Stage Stimulation of Anhydrite-Rich Carbonate Rocks Using Chelating Agent: An Experimental and Modeling Investigation

SummaryCalcium sulfate (anhydrite) exists in most of the carbonate reservoirs, and its content can reach more than 20%. The high content of anhydrite affects the efficiency of the acidizing process because of the low solubility of calcium sulfate in different acids. Carbonate stimulation, matrix acidizing, or acid fracturing is carried out mostly using hydrochloric acid (HCl)-based fluids. The solubility of calcium sulfate in HCl or HCl-based fluids and organic acids is very low.In this study, a new formulation was developed to stimulate carbonate rocks with high anhydrite content. A formulation was developed to dissolve both anhydrite and carbonate at the same time. Anhydrite dissolution was achieved by converting anhydrite to calcite using the newly developed formulation. This treatment can be conducted in a single stage, and the formulation consists of a high pH chelating agent such as ethylenediaminetetraacetic acid (EDTA) in addition to potassium carbonate as a converter. Coreflooding experiments were conducted for carbonate rocks with varying anhydrite content. The effluent samples of the coreflooding experiments were analyzed for cations and anions concentration using inductively coupled (IC) and inductively coupled plasma (ICP) measurements. Computed tomography scans were conducted to show the wormholes generated in the samples. Rock mineralogical analysis using X-ray diffraction, thin section, and the QEMSCAN® automated mineralogy solution (FEI Company, Hillsboro, Oregon, USA) was conducted as well. The reaction kinetics were investigated using a rotating disk apparatus (RDA) using a disk that consists of almost 24% anhydrite and 71% dolomite. Also, a two-scale continuum model was built to simulate the reaction between the new formulation and carbonate rocks. The model could capture the experimental outcomes such as the number of pore volumes (PV) to break through and the shape of the wormholes. This provided confidence in the reaction parameters obtained because the model could reproduce the experimental outcomes.Coreflooding experiments showed that the new formulations were very effective in stimulating carbonate rocks with high anhydrite content in a single stage. The effluent analysis showed high sulfate concentration that indicates the dissolution of anhydrite compared to conventional acid treatments. Reaction kinetics results showed that the new formulation increased the reaction rate with anhydrite through a mass transfer reaction regime. The model reproduced the wormholing behavior obtained from the experiments, indicating that the branched wormhole was due to the anhydrite conversion.

Correction of flashover voltage of catenary insulators based on analysis of soluble salt compositions in saline‐alkali region

AbstractCalcium sulfate and other slightly soluble salts in the natural pollution of catenary insulators could result in differences in flashover voltages between artificially and naturally polluted insulators at the same salt deposition density. Based on the insulator pollution characteristics in saline‐alkali areas and the saturated water absorption of polluted insulators, the equivalent salt deposit density and nonsoluble deposit density of insulators are corrected according to the influence of other coexisting soluble salts on the solubility of calcium sulfate. The correction formula of the insulator flashover voltage is proposed, which considers the equivalent salt deposit density, nonsoluble deposit density and pollution components. The results from the correction formula are compared with the experimental data from the artificial pollution test of the ceramic cap and pin insulator. The results show that salt‐alkali areas are more easy to flashover than other areas. The remaining pollutants can change the conductivity of the pollutant layer by inhibiting or promoting the dissolution of calcium sulfate. Moreover, its effect on the flashover voltage cannot be ignored. The error between the calculated value based on the correction formula and the experimental data is below 6%. A pollution flashover warning mechanism based on relevant meteorological data is proposed. The results can be used as an important reference for the operation and maintenance of catenary insulators in saline‐alkali areas. © 2020 Institute of Electrical Engineers of Japan. Published by John Wiley & Sons, Inc.

Thermodynamic Modeling of Calcium Sulfate Hydrates in the CaSO4-H2O System from 273.15 to 473.15 K with Extension to 548.15 K.

Calcium sulfate is one of the most common inorganic salts with a high scaling potential. The solubility of calcium sulfate was modeled with the Pitzer equation at a temperature range from 273.15 to 473.15 K from published solubility data, which was critically evaluated. Only two Pitzer parameters, β(1) and β(2), with simple temperature dependency are required to model the solubility with excellent extrapolating capabilities up to 548.15 K. The stable temperature range for gypsum is 273.15–315.95 K, whereas above 315.95 K the stable phase is anhydrite. Hemihydrate is in the metastable phase in the whole temperature range, and the obtained metastable invariant temperature from gypsum to hemihydrate is 374.55 K. The obtained enthalpy and entropy changes at 298.15 K for the solubility reactions are in good agreement with literature values yielding solubility products of 2.40 × 10–05, 3.22 × 10–05, and 8.75 × 10–05 for gypsum, anhydrite, and hemihydrate, respectively. The obtained Pitzer model for the CaSO4–H2O system is capable of predicting the independent activity and osmotic coefficient data with experimental accuracy. The mean absolute average error of activity coefficient data at 298.15 K is less than 2.2%. Our model predicts the osmotic coefficient on the ice curve within 1.5% maximum error.

Thermodynamic Modeling of CaSO4–(NH4)2SO4–NH3–H2O Quaternary System with Asymmetric E-NRTL Model

Ammonium sulfate [(NH4)2SO4], an undesired byproduct in the caprolactam process, can be converted into recyclable NH3 by the substitution reaction with lime milk to reduce the ammonia consumption and produce high-quality gypsum. However, the thermodynamic mechanism of this process has not been clarified, which results in the unpredictable crystal phase of calcium sulfate products. In this work, a thermodynamic model for the CaSO4–(NH4)2SO4–NH3–H2O quaternary system with the asymmetric E-NRTL activity coefficient model was developed to understand the thermodynamic behavior. The modeling works involved the data fitting of binary solubility, osmotic coefficient, enthalpy of solution, and heat capacity data as well as ternary solubility data. It was shown that the model could predict the solubility of ammonia and calcium sulfate in aqueous ammonium sulfate solution with temperature ranging from 0 to 160 °C and pressure up to 1.0 MPa. Furthermore, the constructed model for the quaternary system was proved to b...

Insights into water-mediated ion clustering in aqueous CaSO4 solutions: pre-nucleation cluster characteristics studied by ab initio calculations and molecular dynamics simulations.

The molecular structure of growth units building crystals is a fundamental issue in the crystallization processes from aqueous solutions. In this work, a systematic investigation of pre-nucleation clusters and their hydration characteristics in aqueous CaSO4 solutions was performed using ab initio calculations and molecular dynamics (MD) simulations. The results of ab initio calculations and MD simulations indicate that the dominant species in aqueous CaSO4 solutions are monodentate ion-associated structures. Compared with charged ion clusters, neutral clusters are more likely to be present in an aqueous CaSO4 solution. Neutral (CaSO4)m clusters are probably the growth units involved in the pre-nucleation or crystallization processes. Meanwhile, hydration behavior around ion associated species in aqueous CaSO4 solutions plays an important role in related phase/polymorphism selections. Upon ion clustering, the residence of some water molecules around Ca2+ in ion-associated species is weakened while that of some bridging waters is enhanced due to dual interaction by Ca2+ and SO42-. Some phase/polymorphism selections can be achieved in aqueous CaSO4 solutions by controlling the hydration around pre-nucleation clusters. Moreover, the association trend between calcium and sulfate is found to be relatively strong, which hints at the low solubility of calcium sulfate in water.

Mixed Anion Exchange Resins for Tunable Control of Sulfate–Chloride Selectivity for Sustainable Membrane Pretreatment

The recovery of water during inland desalination processes using reverse osmosis, or RO, is often limited by the solubility of calcium sulfate, or CaSO4. Reducing or eliminating the presence of sulfate from the feedwater will allow the process to be operated at higher recoveries and produce less waste brine. Ion exchange may be used as a pretreatment method for the control and removal of sulfate using a hybrid ion exchange-reverse osmosis (HIX-RO) process. However, for viable application, it is imperative that the process is self-regenerating; that is, the use of external regenerants has to be avoided altogether. Various properties of the ion-exchange resin control the overall selectivity toward sulfate including the resin matrix and basicity of the functional group. Finer control over selectivity is also possible through mixing of characteristically different ion exchange resins. This study focused on how mixing anion exchange resins with different matrix and functional groups may optimize sulfate/chlori...

Thermodynamic modelling of a membrane distillation crystallisation process for the treatment of mining wastewater.

Membrane distillation (MD) could be applicable in zero liquid discharge applications. This is due to the fact that MD is applicable at high salinity ranges which are generally outside the scope of reverse osmosis (RO) applications, although this requires proper management of precipitating salts to avoid membrane fouling. One way of managing these salts is with MD crystallisation (MDC). This paper focuses on the applicability of MDC for the treatment of mining wastewater by thermodynamically modelling the aqueous chemistry of the process at different temperatures. The paper is based on the typical brine generated from an RO process in the South African coal mining industry and investigates the effect water recovery and operating temperature have on the salts that are predicted to crystallise out, the sequence in which they will crystallise out and purities as a function of the water recovery. The study confirmed the efficacy of using thermodynamic modelling as a tool for investigating and predicting the crystallisation aspects of the MDC process. The key finding from this work was that, for an MDC process, a purer product can be obtained at higher operating temperatures and recoveries because of the inverse solubility of calcium sulphate.

Model study of the constituents of wall painting degradation patinas: The effect of the treatment with chelating agents on the solubility of the calcium salts

A model study on the application of chelating solutions on superficial calcium degradation patinas of wall paintings is presented. For this purpose the solubility of calcium sulfate, carbonate and oxalate in aqueous solutions of the Ca2+ chelators EDTA and citrate, was evaluated. Both the obtained solutions and their insoluble materials were analyzed by several analytical techniques. These studies revealed that the treatment of solid samples containing calcium sulfate and carbonate as the models of painting patinas resulted in higher solubilities of calcium sulfate and carbonate over that of oxalate. Moreover, our investigations confirmed the higher capacity of EDTA to chelate Ca2+ compared to that of citrate. All these results were interpreted and discussed on the basis of speciation models, solubility products of the salts and formation constants of the calcium complexes in solution. Finally, we report the characterization of a sodium calcium double citrate salt formed as an unexpected product in the treatment of the calcium sulfate with citrate. Overall our results suggest that the low solubility of calcium oxalate prevents its dissolution upon treatments with chelators, and that the capacity of citrate to dissolve the calcium salts is lower than that of EDTA irrespective of the duration of treatment.

Geotechnical Characteristics of Anhydrite/Gypsum Transformation in the Middle Miocene Evaporites, Red Sea Coast, Egypt

The Middle Miocene evaporites of Abu Dabbab Formation, north of Quseir City along the Red Sea coast of Egypt are composed mainly of thinly laminated, bedded and nodular anhydrite and microcrystalline gypsum especially in its lowermost part. Highly cracked inter-bedded clay, dolomite and stromatolite layers were recorded within the evaporite succession. The cracks in the inter-bedded clays are filled with evaporite materials indicating arid climatic conditions during exposure episodes. Swelling mechanisms in clay-evaporites include mechanical swelling due to hydration of clay minerals and transformation of the anhydrite into gypsum. Field observations as well as powder x-ray diffraction investigations have shown that anhydrite/gypsum modal ratio decreases gradually downward indicating that gypsum has transformed to anhydrite by solar heating. The clay minerals of the inter-bedded clay layers are dominated by smectite (46 %), illite–smectite mixed-layer (18 %), chlorite (13 %), palygorskite (9‘%), kaolinite (8 %), and illite (6 %). The liquid limit of the Abu Dabbab Formation (anhydrite and inter-bedded clays) is higher than 65 %, so considered as of very high swelling capabilities. Also, its swelling percentage was found to be (11–14 %), (28–35 %) and (58–65 %) for gypsum, clay and anhydrite, respectively. The swelling pressure was found to be (1.4–1.5 kg/cm2), (2.3–3.1 kg/cm2) and (4.7–5.1 kg/cm2) for gypsum, anhydrite and inter-bedded clay. The swelling pressure of gypsum is moderate and for both anhydrite and clay is high. The highly swelling capability of anhydrite and the inter-bedded clays causes geotechnical problems when got into contact with water. Alternating volume change due to phase transformation and solubility of calcium sulfate adds to the severity of problems associated with the host expansive clay strata.

Prevention of calcium sulfate formation in seawater desalination by ion exchange

In mechanical vapor compression (MVC) desalination, high operating temperatures (>120°C) allow for high heat transfer coefficients and reduced compressor size, both of which lower total capital costs. To prevent formation of calcium sulfate scale on high-temperature desalination heat exchangers, the inverse solubility of calcium sulfate requires high sulfate removal. To selectively remove sulfate from seawater, a weak-base anion-exchange resin (Relite MG 1/P) was used. In synthetic seawater, the resin shows high sulfate selectivity. Conveniently, the exhausted resin can be regenerated with concentrated acidified (pH 4) blowdown brine from the desalination heat exchanger. Using the same exhaustion and regeneration times, high desalination concentration factors increase chloride concentrations in the blowdown brine, which allows for slower regenerant flow rates thereby increasing regeneration efficiency and sulfate removal. Using a high concentration factor helps regeneration; however, this must be balanced...

Phase Stability and Inhibition of Calcium Sulfate in the System NaCl/Monoethylene Glycol/H2O

Summary Calcium sulfate is one of the major mineral scales in oil and gas production. Hemihydrate (CaSO4·0.5H2O) and anhydrite (CaSO4) are the predominant sulfate scales formed at high temperature, while gypsum (CaSO4·2H2O) scale may form at low temperatures (<~45°C). However, it has been shown in this study that anhydrite can form at low temperature in the presence of excess amounts of monoethylene glycol (MEG), and this may occur during offshore production with long tie-backs. The prediction and prevention of calcium sulfate scales requires knowledge of the phase behavior of the three major phases of calcium sulfate. The phase behavior of different calcium sulfate phases is related to the supersaturation state, temperature, and fugacity of water. In this study, the effect of a common hydrate inhibitor, MEG, on calcium sulfate solubility and phase behavior was investigated. This study was run with NaCl/CaSO4/MEG/H2O solutions at 0–6 molality (M) NaCl and 0–95 wt% MEG at 4–70°C. Three approaches were taken to determine the kinetics of calcium sulfate phase transition at various temperatures, ionic strengths, and MEG concentrations: (1) dissolution of gypsum, (2) dissolution of anhydrite, and (3) nucleation and precipitation of calcium sulfate by mixing calcium- and sulfate-containing solutions. The effect of scale inhibitors on phase transition was also evaluated. Phase transition of gypsum to anhydrite was observed in the presence of high concentrations of NaCl and MEG, regardless of the experimental approach. The transition boundary of temperature and concentrations of NaCl and MEG can be estimated from solubility of calcium sulfate and the fugacity of water. The inhibition mechanism of hexamethylene diamine tetra (methylene phosphonic acid) (HDTMP), one of the most effective inhibitors for calcium sulfate scale, was also tested by investigating the kinetics of precipitation and inhibition of calcium sulfate.

Analysis of Calcium Sulphate Filler and its Application in Papermaking Process as Functional Filler

Calcium sulphate is rich in natural resources and by-product of many industrial processes. However, most calcium sulphate product was only used with low added value or treated as waste, harmful to environment. If the calcium sulphate can be used as papermaking filler, very high added value can be gained and also the environment problem can be solved. In this paper calcium sulphate from by product of industry was characterized by X-ray diffractometer (XRD) and scanning electron micrograph (SEM), and then used as papermaking filler. The influence of calcium sulphate filler on pulp suspension and paper properties was studies. The results showed that the calcium sulphate fillers were composed of three crystal forms, dihydrate, hemihydrate, anhydrite, and it was needle or fiber shape, hollow in the middle; calcium sulphate showed high brightness and high conductivity in water, compared with talc or ground calcium carbonate(GCC) filler. When calcium sulphate filler was used in papermaking process the filler retention was very small because of its high solubility in water. Calcination at high temperature can lower the solubility of calcium sulphate and slightly increase the retention, but the retention was still quite small. The future work should focus on controlling the solubility of calcium sulphate.

Solubility phenomena involving CaSO4 in hydrometallurgical processes concerning heavy metals

Deposit of CaSO4 in hydrometallurgical processes involving heavy metals usually decreases production quality and harms the production process. To avoid its formation at false time and sites, a sound understanding of the solubility behavior of CaSO4 in the quaternary systems CaSO4 + H2SO4 + (heavy metal)SO4 + H2O over large ranges of temperature and concentration is a prerequisite. Up to now, although a large amount of solubility data has been reported in these systems, little information is available on the solubility of anhydrite, especially at salt concentrations near saturation points. In this paper, we introduce the effect of CaSO4 in hydrometallurgical processes involving heavy metals, review published solubility data of calcium sulfate and its hydrate in relevant systems, and report our newly determined solubility results involving anhydrite in the quaternary systems CaSO4 + H2SO4 + MSO4 + H2O (M = Cu,Zn,Mn) at 348.1 and 363.1 K. Based on the newly obtained data some application examples were given. On account of the complexity of the solubility phase diagram of these quaternary systems, where the calcium sulfate solubility is a function of its crystal type, temperature, compositions of sulfuric acid, and heavy metal sulfate, a complete solubility phase diagram is not available until some basic data measurement, for instance, water activity at temperatures higher than 298 K, and final modeling has been finished.

Transformation of Flue-Gas-Desulfurization Gypsum to (α-Hemihydrated Gypsum in Salt Solution at Atmospheric Pressure

Direct phase transformation of flue gas desulfurization gypsum in hot salt solution at atmospheric pressure was investigated. The effects of temperature, salt species, salt concentration, solids content, pH and modifier were examined. The crystals obtained under different conditions and solubility of calcium sulfate in contact with solid gypsum were also determined. α-Calcium sulfate hemihydrate crystals of stubby columnar shape and regular pentahedral sides were obtained under the following conditions: salt concentration 20%–30%, operation temperature 95–100 °C, solids mass content in the slurry 10%–30% and neutral pH. Thermodynamic analysis revealed that phase transformation of calcium sulfate dihydrate to α-calcium sulfate hemihydrate occurs because of the difference in solubilities between the two solid gypsum phases in this system.