Precipitation Of Calcium Sulfate Dihydrate Research Articles

Staged purification of phosphogypsum using pH-dependent separation process.

Phosphogypsum (PG) is an industrial by-product of the transformation of phosphate rocks. For decades, PG has been a source of environmental concern due to the massive amount produced thus far, i.e., 7 billion tons, with a current production rate of 200-280 million tons per year. Phosphate minerals contain various impurities that precipitate and concentrate within PG. These impurities hinder PG usability in various sectors. This paper aims to purify PG using an innovative process based on staged valorization of PG. Initially, PG dissociation by ethylenediaminetetraacetic acid (EDTA) was optimized. After screening of different parameters and monitoring the ionic conductivity of solutions, it was disclosed that a pH-dependent solubilization process in the presence of EDTA resulted in high solubility of PG, up to 11.82g/100mL at pH > 11. Subsequently, a recovery of the purified PG by selective precipitation of calcium sulfate dihydrate (CSD) from obtained filtrate through pH adjustment to 3.5 were investigated. An abatement of 99.34% Cr, 97.15% Cd, 95.73% P2O5, 92.75% Cu, 92.38% Al2O3, 91.16% Ni, 74.58% Zn, 72.75% F, 61.43% MgO, 58.8% Fe2O3, 56.97% K2O, and 55.41% Ba was achieved. The process relied on the variation of EDTA chelation properties towards monovalent, divalent, and trivalent cations at different pHs. According to the findings of this study, a staged purification process in the presence of EDTA is an effective method for removing impurities from the industrial PG.

Development of a novel three‐dimensional printing technology for the application of “raised” surface features

AbstractA simple procedure to ink‐jet print raised images using two water‐soluble inorganic inks is reported and it has the potential to be utilised in domestic and commercial environments. The advantages of such a procedure lies in the ability to print moulded objects, Braille type and to engineer special gonio‐specific effects that may have value in the security printing area. The study focuses on printing gypsum through the ready precipitation of calcium sulphate dihydrate by co‐jetting calcium chloride and ammonium sulphate solutions. The results in this preliminary study are encouraging and offer a potential method for durable surface structuring of material surfaces with haptic and visual effects for both the blind and the sighted.

Polycarboxylate superplasticizer modified by phosphate ester in side chain and its basic properties in gypsum plaster

In this study, 2-hydroxylethyl methacrylate phosphate was introduced as side chain to modify polycarboxylate superplasticizers (PCEs), and its performance in gypsum paste was systematically investigated. The results show that the introduction of phosphate ester can increase the water reduction ratio from 24% to 58%, and also extend the final setting time, showing excellent dispersing ability and retarding effect. The excellent dispersion was due to the stronger adsorption capacity of phosphate ester leading to high adsorption amount and high absolute value of zeta potential. In addition, the negative effect of the high adsorption amount on the crystal precipitation of calcium sulfate dihydrate is considered to be an important reason for retarding effect.

Zwitterionic coating on thin-film composite membranes to delay gypsum scaling in reverse osmosis

Precipitation of calcium sulfate dihydrate (i.e., gypsum) on the membrane active layer negatively impacts the efficiency of reverse osmosis (RO) systems by increasing overall operation and maintenance costs. The interfacial free energy between RO membranes and scalants is expected to play a paramount role in crystal nucleation and adsorption. In this work, we modified the surface of a thin-film composite RO membrane with a zwitterionic polymer brush via atom transfer radical polymerization (ATRP) to impart superhydrophilicity for enhanced resistance to gypsum scaling. The zwitterionic polymer coating was optimized and a highly hydrophilic membrane surface displaying a water contact angle of <20° was achieved. Characteristics of both the control and the zwitterion-modified RO membranes such as surface roughness, chemical composition, and surface energy components, were investigated to elucidate the factors controlling gypsum scaling behavior. Results from bench-scale RO scaling experiments indicate that membranes modified with the zwitterionic brush effectively delay gypsum surface nucleation and crystal adsorption relative to the control membrane. At the end of the RO scaling experiments with only-heterogeneous gypsum nucleation, the zwitterion-modified membrane exhibited a flux decline of 23.7% (relative to the initial water flux), much smaller than a 49.5% flux decline of the control RO membrane. We further verified that a precipitous flux decline by deposition of gypsum crystals in the combined homogeneous and heterogeneous nucleation pathway was about 1.6 times delayed for the zwitterion-modified membrane compared to the control RO membrane. Our results highlight that developing a more hydrophilic membrane surface is crucial to resisting gypsum scaling in RO systems.

Precipitation of calcium sulfate dihydrate in the presence of fulvic acid and magnesium ion

The present work focused on the reduction of the calcium and sulfate ions in a membrane permeate generated from a newly developed two-stage of tight ultrafiltration (TUF) for recovering humic substance from leachate nanofiltration (NF) concentrate by a precipitation process of calcium sulfate dihydrate (gypsum). The operating parameters (temperature and pH), mechanism of precipitating behaviors, and kinetics of this process in the presence of fulvic acid (FA) and magnesium ion simultaneously were investigated. It was found that Ca2+ removal increased with increasing temperature. The decline in pH, especially towards low pH range of 2–3 around the point of zero charge of gypsum, showed a significant effect on gypsum precipitation. Adsorption of FA onto gypsum surface had an important effect on the precipitating behavior of gypsum whose mechanism might be mainly weak interactions of hydrogen bonding and hydrophobic effect at low pH, while electrostatic interactions became dominant at neutral pH. In contrast, retardation effect of Mg2+ ion on gypsum precipitation was negligible at pH 2.5. The soluble calcium removal during precipitation process corresponded with first order rate equation. The results indicated that the optimal operating parameters were found at 70 °C and around pH 2.5, which can be applied to the scale-up and industrial design of the precipitation process of gypsum for removing the calcium and sulfate ions in the presence of FA and magnesium ion.

Effect of calcium, sulphate and gypsum on copper-activated and non-activated sphalerite surface properties

This work presents results of an experimental study on the depression of sphalerite by the precipitation of calcium sulphate dihydrate (gypsum, CaSO4·2H2O), and the role of the addition of sodium carbonate (Na2CO3) in restoring hydrophobicity. Zeta potential, microflotation, contact angle measurement and scanning electron microscopy were used. The zeta potential results show that sphalerite and gypsum were negatively charged over the pH range studied (5–10), and therefore no interaction between the two species was observed; however, when sphalerite was activated with 5mg/L of Cu at pH 9, the mineral developed a positive charge, thus enabling the electrostatic interaction with gypsum particles which resulted in gypsum covering a significant fraction of the mineral surface. The contact angle measurements show that the simultaneous presence of Ca2+ and SO42- ions decreased the hydrophobicity that the mineral had developed due to copper activation in the absence of these two species (19.6° and 28.7°, respectively). The presence of 10mg/L of CaSO4·2H2O in equilibrium with the saturated solution (0.0205mol/L CaSO4), further decreased the contact angle to 9.7°. In turn, the addition of 1g/L of Na2CO3 and 15min of conditioning substantially restored the hydrophobicity of the sphalerite by a mechanism consisting of gypsum dissolution and calcium consumption via calcium carbonate precipitation, which disperses in the bulk solution. This last practice resulted in a contact angle of 23.4°.

Investigations on the evaluation of polymeric calcium sulfate dihydrate (gypsum) scale inhibitors in the presence of phosphonates

Abstract The effects of polymers with different architecture, phosphonates, and polymer/phosphonate blends on precipitation of calcium sulfate dihydrate (gypsum) precipitation are reported in this paper. It has been found that gypsum inhibition by polymers strongly depends on polymer architecture. Among the various phosphonates (i.e., aminotris(methylene phosphonic acid), AMP; 1-hydroxyethylidine 1,1-diphosphonic acid, HEDP; 2-hydroxyphosphono acetic acid, HPA; and 2-phosphonobutane 1,2,4-tricarboxylic acid, PBTC); tested AMP shows the best inhibition for gypsum precipitation. It has also been observed that presence of PBTC exhibits synergistic effect on the performance of polymers containing carboxyl group. Under the experimental conditions used, no synergistic effects were observed with polymer/phosphonate (AMP, HEDP, HPA) blends.

Investigations on the evaluation of polymeric calcium sulfate dihydrate (gypsum) scale inhibitors in the presence of phosphonates

Abstract The effects of polymers with different architecture, phosphonates, and polymer/phosphonate blends on precipitation of calcium sulfate dihydrate (gypsum) precipitation are reported in this paper. It has been found that gypsum inhibition by polymers strongly depends on polymer architecture. Among the various phosphonates (i.e., aminotris(methylene phosphonic acid), AMP; 1-hydroxyethylidine 1,1-diphosphonic acid, HEDP; 2-hydroxyphosphono acetic acid, HPA; and 2-phosphonobutane 1,2,4-tricarboxylic acid, PBTC); tested AMP shows the best inhibition for gypsum precipitation. It has also been observed that presence of PBTC exhibits synergistic effect on the performance of polymers containing carboxyl group. Under the experimental conditions used, no synergistic effects were observed with polymer/phosphonate (AMP, HEDP, HPA) blends.

Influence of some phosphates and polyphosphate on the precipitation of calcium sulfate dihydrate in sodium chloride solutions

The inhibiting effect of phosphates and polyphosphates on the precipitation rate of calcium sulfate dihydrate (gypsum) in sodium chloride solutions has been studied using constant composition technique in which the super-saturation and ionic strength were maintained constant at 25°C. The presence of phosphoric acid H3PO4, orthophosphate NaH2PO4, metaphosphate Na2HPO4, pyrophosphate PYP and hexametaphosphate HMP, even at relatively low concentration 10−6 M, markedly reduced the rates of precipitation. The action of additives can be interpreted in terms of adsorption, following the Langmuir isotherm, of additives at the active crystal growth sites. Dependence of the degree of inhibition with the change in driving force has been observed. The order of the degree of inhibition on the rate of precipitation of gypsum for various additives studied is as follows: H3PO4 > NaH2PO4 > Na2HPO4> HMP > PYP.

Influence of additives on the precipitation of gypsum in sodium chloride solutions

The kinetic of precipitation of calcium sulfate dihydrate (gypsum) in sodium chloride solutions was investigated at various supersaturation from 1.05–2.49, pH = 8.50, 25°C and ionic strength 0.5 M. The rate of precipitation showed a quadratic dependence upon the relative supersaturation which suggests a surface-controlled growth mechanism. Influence of a number of amino acids on the rate of reaction has been studied at several additive concentrations. The addition of amino acids even at relatively low concentration (10 −6 mol 1 −1) markedly retard the rates of precipitation. Moreover, the effect was enhanced as the relative supersaturations decreased. Additionally the synergistic effect of one amino acid and other inhibitors was studied. The retardation effect of these additives hasbeen attributed to the blocking of active sites by adsorption of these additives at the crystal surfaces. Inhibition of crystallization of gypsum by amino acids can be interpreted in terms of a Langmuir adsorption isotherm. This research will help to understand the usefulness of these compounds for industrial applications such as the control of scale.

Continuous precipitation of calcium sulfate dihydrate from waste sulfuric acid and lime

AbstractPrecipitation of calcium sulfate dihydrate, gypsum, from (i) a pure sulfuric acid/lime suspension and (ii) a waste sulfuric acid/lime suspension in a continuous pilot plant in the temperature range from 40 °C to 80 °C was studied. It was observed that in the case of waste sulfuric acid with a high content of Mg2+ and Fe2+ ions, several hours after the beginning of the precipitation, partial dissolution of the product and modification of the crystals from needle‐ and plate‐like to agglomerated structures occurred. It is suggested that the secondary changes occur due to the increased concentration of Mg2+ and Fe2+ ions in the reactor. Below 60 °C, and above 70 °C plate‐like and needle‐like single crystals respectively were formed. The mean size (d50) of the crystals was found to increase with increasing temperature and decrease with the initial concentration of lime. Gypsum produced between 70 °C and 80 °C is suitable for further processing for construction plaster. In a precipitation process with pure sulfuric acid only single perfect needle‐like crystals occurred. Copyright © 2005 Society of Chemical Industry

Polyelectrolytes inhibition effect on crystallization of gypsum

AbstractThe spontaneous precipitation of calcium sulfate dihydrate (gypsum) at 30 °C was investigated in the presence of polyelectrolytes. The degree of inhibition is measured as the reduction in initial crystallization rate, determined by conductivity of the crystallizing solution. The inhibition increases with acid content of the copolymer. Changing the butyl ester to a hydrophilic ethylene oxide resulted in higher inhibition efficiency. The inhibition efficiency is believed to arise from the closer chain packing on the crystal surface. (© 2004 WILEY‐VCH Verlag GmbH &amp; Co. KGaA, Weinheim)

Kinetics of the precipitation of calcium sulfate dihydrate in a desalination unit

The kinetics of calcium sulfate dihydrate spontaneous precipitation in supersaturation solutions for temperatures ranging between 20 and 70°C was investigated The quartz microbalance technique (QMC) was used. From measurements of the induction period for gypsum nucleation, interfacial tension was found to be 48.5 and 46.2 mJ.m 2 at 20 and 70°C, respectively. The dependence of the induction period on temperature determined the activation energy, which ranged from 71 to 51 kJ.mol −1 for the supersaturation range between 2.5 and 11. From the dependence of the induction period on temperature and supersaturation, it was possible to distinguish between the homogeneous and heterogeneous nucleation mechanisms. It was also observed that the induction period depends highly on the lattice cation/anion molar ratio. This dependence is more important for the lower supersaturations. In all cases, the DRX technique showed that the formed solid phase was solely the calcium sulfate dihydrate. Finally, this work demonstrates that the QMC is a very attractive instrument to identify precipitation mechanisms and to measure the amount of the deposited crystals.

Parametric study on the production of phosphoric acid by the dihydrate process

In this work a computer program has been developed to simulate a three-CSTR pilot plant leaching process of phosphate rock with sulfuric acid for the production of phosphoric acid and precipitation of calcium sulfate dihydrate as a byproduct. The simulation model has been examined with real experimental data obtained from the Jordan Phosphate Mines Company (JPMC), a phosphoric acid pilot plant at Rusaifa, Jordan. The predicted results are in very good agreement with the experimental data with a relative absolute error of less than 3.5%. A parametric study has been made to find the optimum operating conditions of the pilot plant for a given phosphate rock feed flow rate, chemical composition, and particle size distribution. The effect of varying reactor(s) temperature, sulfuric acid feed rate, agitator–impeller speed, ratio of slurry recycle to feed rate, and ratio of return acid to feed rate have been investigated. A reactor temperature of 80°C, slurry recycle to feed ratio of 80, and return acid to feed ratio of 2.5 have been found to give best results. The optimum conditions for sulfuric acid feed rate and agitator speed are determined only from power limitations and economics of the plant itself.

Role of Temperature in the Spontaneous Precipitation of Calcium Sulfate Dihydrate

The spontaneous precipitation of calcium sulfate in supersaturated solutions over the temperature range between 25.0 and 80.0 °C was investigated by monitoring the solution specific conductivity during desupersaturation. From measurements of the induction times preceding the onset of precipitation the surface energy of the forming solid, identifield as gypsum, was found between ca. 12 and 25 mJ m-2 for the temperature range between 80.0 and 25.0 °C, respectively. Kinetics analysis showed that over 50 °C it is possible that anhydrous calcium sulfate is forming as a transient phase converting into the more stable calcium sulfate dihydrate. The linear dependence of the rates of precipitation on the relative solution supersaturation suggested a mechanism according to which the growth units are integrated into the active sites of the supercritical nuclei by surface diffusion. According to the morphological examination of the crystals it is possible that crystal growth occurs by the advancement of steps.

Kinetics of calcium sulfate formation in aqueous media: effect of organophosphorus compounds

The kinetics of spontaneous precipitation of calcium sulfate dihydrate at pH 7.0, 30°C were investigated in the presence of organophosphorus compounds. The measurement of the kinetics parameters was done at conditions of constant supersaturation probing the conductivity of the supersaturated solutions. Aqueous solutions of 1-hydroxy-ethylidene-1,1-diphosphonic acid (EHDP), nitrilotrimethylene phosphonic acid (NTMP) and N,N,N′,N′-ethylene diaminotetramethylene phosphonic acid (ENTMP) were included in unstable supersaturated solutions of calcium sulfate. NTMP and ENTMP at concentration levels between 0.1 and 1.0 μM increased drastically the induction times preceding precipitation while the rates of precipitation were reduced by 90% at concentrations below 1μM. EHDP was effective in reducing the precipitation rates but at considerably higher concentrations (70% reduction at 0.1 μM). Examination of the morphology of the precipitated crystals suggested that the kinetic results obtained could be attributed to the adsorption of the organophosphorus compounds onto different gypsum crystal faces. EHDP did not inhibit the [0 2 0] face resulting in elongated prismatic crystals, while NTMP and ENTMP favored the development of [0 2 0] resulting in shorter and broader crystallites.

Spontaneous precipitation of calcium sulphate dihydrate in the presence of some metal ions

The precipitation of calcium sulphate dihydrate from aqueous solutions has been studied at super-saturations sufficient for spontaneous precipitation but low enough to enable highly reproducible experiments to be carried out with respect to induction periods and rates of precipitation. The induction periods and rates of precipitation are both strongly dependent on degree of saturation. Over a range of supersaturation, 1.5–4.1, the precipitation reaction appears to be controlled by a surface polynucleation process. The influence of some metal ions on the precipitation process have been studied at several additive concentrations. The reduction in precipitation rate can be interpreted in terms of a Langmuir-type adsorption insotherm.

Crystallization of gypsum from batch chemical neutralizations

The sulphuric acid-calcium hydroxide-water system similar to those which occur in batch neutralization processes in industry, has been studied. The rate of precipitation of calcium sulphate dihydrate during batch neutralization of sulphuric acid solutions with lime, in the temperature range of 10 to 60°C and acid concentrations from 0.03M to 0.125M, was studied. On the basis of the theoretical correlation between the individual parameters and the experimental data, a mathematical-statistical model has been developed. With the aid of this model it is possible to observe and predict changes in the supersaturation/concentration of calcium sulphate with time.

Precipitation of calcium sulfate dihydrate at constant calcium activity

The formation of calcium sulfate is important in a number of environmental geological, industrial and energy production processes. At constant calcium activity at 25°C, the kinetics of precipitation were found to be independent of pH, and an apparent order of reaction of n = 4, significantly different from the order of 2 found by seeded growth experiments, suggested a polynucleation mechanism. The electrophoretic mobility of the calcium sulfate dihydrate particles showed a marked dependence on the solution pH, which may affect the crystal growth of the initially formed calcium sulfate dihydrate crystallites by altering the Ca 2+/ SO 2- 4 ratio on their surface.

Studies of the effects of electrolytes on the rates of precipitation of calcium sulphate dihydrate using an ion‐selective electrode

AbstractA calcium ion selective electrode has been used to follow the acceleration of the rate of precipitation of calcium sulphate dihydrate by inorganic electrolytes at 15, 25 and 35°C. At zero and low added electrolyte concentrations, the rate of precipitation is strictly proportional to the square of the supersaturation, has an activation energy 75 kJ mol−1in agreement with previous studies and is indicative of a surface diffusion controlled process. However, with electrolytes containing sulphate ions in excess of 0.05 mol dm−3marked departures from a square law are observed; the rate is best represented as the sum of a square and first order term in supersaturation, the first order term becoming dominant at concentration ofc.0.1 mol dm−3. Under these conditions the rate has an activation energy (29 kJ mol−1) which is close to that for the volume diffusion of ions in aqueous solutions and indicates that the surface process has been accelerated to such an extent that the diffusion of ions from solution to the crystal face has become the slowest and, hence, the rate controlling step. It appears that the electrolytes accelerate the surface process by increasing the rates of dehydration of the calcium and sulphate ions at the interface.