Precipitation Of Calcium Sulfate Research Articles

Novel Non-Phosphorous Antiscalants for Inhibition of Calcium Phosphate and Calcium Sulfate Precipitation

A copolymer of acrylic acid (AA)/allylpolyethoxy carboxylate (APEC) was synthesized, characterized and examined as a non-phosphorous antiscalant for industrial cooling-water. Its ability to prevent calcium phosphate and sulfate precipitation was compared with inhibitory power of acrylic acid (AA)/allylpolyethoxy sulfate (APES) and polyamino polyether methylenephosphonate (PAPEMP). Results indicate that the presence of these antiscalants does reduce the extend of calcium phosphate and sulfate precepitation and AA/APEC exhibits the best performance with 99.59% calcium-phosphate inhibition when the dosage of AA/APEC was 6 mg/L and 82.88% calcium-sulfate inhibition at the dosage of 3 mg/L AA/APEC. AA/APEC has also been proven to be an effective inhibitor for calcium phosphate and calcium sulfate even at elevated solution temperature, pH and Ca2+concentration.

Effect of bacteria and salinity on calcium sulfate precipitation

Membrane and thermal desalination are plagued by composite fouling because of inorganic, biological and organic constituents. Interactive effects in composite fouling, though of paramount practical importance, are mostly ignored. This paper presents interactive effects of biological matter, Pseudomonas Fluorescence (PF), on precipitation of calcium sulfate, a sparingly soluble salt and an inorganic foulant. Experiments were conducted in isothermal (25°C) batch tests. The effect of PF and its concentration on CaSO4 precipitation and deposit structure at varying salinities were determined. The Pitzer model, suitable for high salinities, is used in calculation of instantaneous CaSO4's ion activity products. The Scanning Electron Microscopy (SEM) and X-Ray Diffraction (XRD) show that salinity and concentration levels alter the crystal structure but not the crystal phase (gypsum). Under the influence of PF, crystallization of CaSO4 did not occur and sulfur levels significantly increased; it can be surmised that microbial activity of PF reduced the sulfate ions to elemental sulfur and altered precipitation of CaSO4. This does not suggest that biological fouling or presence of bacteria generally inhibits precipitation fouling of calcium sulfate; it underlines that the generally ignored interactive effects are of significant importance and should be considered as corner stone in advancement of fouling research.

Quantitative Evaluation of Calcium Sulfate Precipitation Kinetics in the Presence and Absence of Scale Inhibitors

Summary In this study, the nucleation kinetics of calcium sulfate scale formation in the presence and absence of scale inhibitors has been studied. The nucleation kinetics of calcium sulfates in 0–3.2 M NaCl solution was measured from 0 to 200°C at various supersaturation conditions [saturation index (SI) = 0–1.3]. The phase behavior of these various calcium sulfate phases was monitored by X-ray diffraction (XRD). The inhibition study was performed by evaluating the inhibition efficiency of calcium sulfate precipitation at the same supersaturation and temperature ranges as those of the nucleation study. Several polyphosphonate, polymeric, and environmentally friendly inhibitors were found to be effective inhibitors. The study has shown that calcium sulfate scales are very difficult to inhibit at SI > 1. Calcium sulfate scale is inhibited most effectively by hexamethylenediamine tetra(methylene phosphonic acid) (HDTMP) if the saturation index is less than approximately 1.0. In addition, a semiquantitative model to predict precipitation kinetics of calcium sulfate as a function of temperature, pH, SI, and HDTMP concentration is established with experimental results from this study. The effect of methanol and methylene glycol (MEG) on calcium sulfate precipitation kinetics and inhibition is also presented.

Experimental and Theoretical Study of Calcium Sulphate Precipitation in Porous Media Using Glass Micromodel

Mixing of two incompatible waters in water injection projects is usually associated with mineral scale formation and deposition in porous media. Deposition process dramatically affects the performance of water injection scenarios by reduction of porosity and mainly permeability of the rock. In this study, a series of experiments has been conducted to investigate the effect of different parameters on the gradual process of Calcium Sulphate precipitation. These include temperature, concentration of mixing brines, pressure, and flow rate. Due to the visual nature of the glass micromodel, a glass sandstone pattern with water-wet characteristics was used as porous medium to easily observe the scaling formation and distribution. In addition, tracing the movement of the solid particles is highly facilitated in this newly suggested experimental setup. The captured photos in microscopy scanning show that the deposition is initiated from the walls of the pores and throats and extend toward the middle space of porous medium and solid crystals look like chicken roost. For better understanding of the effect of any aforementioned parameter, the related permeability reduction curve versus injected pore volume of the brine solutions was plotted. The results indicated that as the temperature, brine concentration, and flow rate increase the scaling tendency increases as well. The pressure has a minor role on the process development. Deposition of CaSO4 manifests a functional form of permeability reduction due to the effect of different parameters. Therefore, an exponential functionality (correlation) was proposed which incorporates all physical parameters that affect the behavior of the system in dimensionless form. Reynolds number, scaling index, and deviation from equilibrium conditions are the backbone of this correlation. The adjustable exponents of the equation was determined and optimized by means of Genetic Algorithm optimization scheme. This meaningful correlation can also predict the core extracted data with reasonable accuracy.

Calcium sulphate scaling in membrane distillation process

AbstractFormation of precipitates containing CaSO4 during membrane distillation, applied to the concentration of aqueous salt solutions, is discussed in this paper. It was found that the concentration of SO42− ions in such solutions should not exceed 600 mg L−1 when they are subjected to concentration. However, concentration of sulphates at the level of 800 mg L−1 in the feed is permissible provided that the excess of CaSO4 is removed in a crystallizer. Crystallisation of salts, mainly CaSO4 · 2H2O, on the surface and inside the membrane was observed at higher feed concentrations, causing damage of the module. Precipitation of calcium sulphate was also observed during the production of demineralised water when high values of the water recovery coefficient (above 90 %) were used. In this case, the formed precipitate also contained CaCO3, the co-precipitation of which significantly changed the properties of the scaling layer. The precipitate containing both CaSO4 and CaCO3 was formed mainly on the membrane surface and it could easily be removed by rinsing the module with a HCl solution.

Utilisation of synthetic soda-ash industry by-products

A method of soda-ash industry waste utilisation was analysed in this study. Two waste streams, distiller waste and sludge, were considered. The method described is based on calcium sulphate precipitation. The aims of this study were to find the optimal precipitation parameters and to perform simple economic analysis of the process. Except for calculating the direct (running) costs, the authors applied a simple method for environmental cost estimation. The optimal precipitation conditions were obtained by using the simplex optimisation method. Based on optimal conditions of calcium sulphate precipitation, the material balance was calculated. The provided economic analysis takes into account both the investment costs and the running costs. For the Polish case study, this technology can generate 135,000€ profit per year. The overall effect (direct plus environmental) is estimated to be 363,000€ per year.

Effect of scalant and antiscalant concentrations on fouling in a solar desalination unit

In the present work, experiments were carried out to investigate the effect of scalant and antiscalant concentrations on fouling in a desalination unit using the solar multiple condensation evaporation cycle principle. Synthetic solutions equivalent to those of feed water obtained after different condensation–evaporation cycles were used. Kinetic and thermodynamic analyses were established to explain the gypsum precipitation with varied feed water concentrations in salts. It was shown that, the saturation level and therefore calcium sulphate precipitation rate increased with operating time. The solubility of CaSO 4·2H 2O increased with increasing the ionic strength and the saturated solutions kept metastable equilibria for saturation level of ∼2.5. To prevent gypsum formation, different concentrations of poly(acrylate) were used. This antiscalant strongly affected the precipitation rate, the texture and the morphology of gypsum.

Calcium sulfate precipitation in the presence of water-soluble polymers

The effect of four different polymers on the precipitation of calcium sulfate was investigated in the present work. The degree of inhibition was estimated from measurements of the calcium ion activity and from specific solution conductivity measurements in the supersaturated solutions during the course of the precipitation process. The effects of polyacrylic acid (PAA, three different polymers with average molecular weight 2000, 50,000, and 240,000, respectively) and of a co-polymer of PAA with polystyrene sulfonic acid (PSA, average molecular weight <20,000) were investigated with respect to their effect on the kinetics of spontaneous precipitation of calcium sulfate salts. The results of the kinetics experiments suggested that the spontaneous precipitation from supersaturated calcium sulfate solutions at 25 °C yielded exclusively calcium sulfate dihydrate (gypsum) both in the absence and in the presence of the polymeric additives. The induction times, preceding the formation of the solid increased in all cases in the presence of the polymeric additives. Polymer concentrations as low as 2.0 ppm increased induction time from practically zero to 10 min. The rates of precipitation were reduced according to the solutions content in the polymers added and precipitation was completely suppressed in the presence of 6.0 ppm of the polymers tested, depending on their molecular weight. The lower the molecular weight of PAA, the more efficient was the threshold inhibition and the stronger the reduction of the rates of spontaneous precipitation. PSA yielded the poorest inhibition efficiency in comparison with the PAA, possibly because of the relatively lower affinity of the sulfonate groups for the calcium ions of the surface of the solid forming. The kinetics results analysis assuming Langmuir-type adsorption of the polymeric molecules on the growing supercritical gypsum nuclei showed different affinity for the polymers tested in agreement with the respective inhibition efficiency, in the order: PAA1>PAA2>PSA>PAA3. The presence of the polymers in the supersaturated solutions resulted in modification of the precipitated gypsum crystals morphology.

Laboratory Evaluation of In-Situ Gelled Acids for Carbonate Reservoirs

SummaryThree different commercial formulations (A, B, and C) of in-situ gelled acids are compared in detail at temperatures up to 150°F and acid concentrations from 5 to 20 wt% HCl. In-situ gelled acids are said to work by a gelation mechanism that occurs at the rock surface as the acid is spent. These acids contain a polymer, a crosslinker, and a breaker, in addition to other additives. Detailed viscosity measurements of each in-situ gelled acid were made with live, partially neutralized, and spent acid. A new experimental procedure was developed to partially neutralize the in-situ gelled acid with calcium carbonate, and then the apparent viscosity was measured as a function of shear rate in the range 1 to 3,000 s−1. The pH values of these samples varied from 0 to 6. Relative reaction rates with reservoir rock of the three in-situ gelled acids were compared at 100°F. Coreflood experiments were conducted with small acid volumes, so that permeability could be measured before acid breakthrough occurred.In-situ gelled acids all retarded the reaction of acid with reservoir rock, primarily as a result of the polymer present in the acid formulae.From viscosity measurements, live in-situ gelled acids A and B behaved more like gelled acids. Their viscosity in live acid was significantly higher than that of Acid C. In spent acid, the viscosity of Acid C was higher than that of Acids A or B. Acid C was most effective at initial HCl concentrations of 5 and 10 wt% at 100 and 150°F. Acids A and B were effective only at an initial HCl concentration of 10 wt% and 100°F.Coreflood studies showed that the polymer and crosslinker component of in-situ gelled acids irreversibly reduced the permeability of carbonate reservoir rock.As with any spent acids, mixing spent in-situ gelled acids with seawater resulted in calcium sulfate precipitation for all three of the acid systems.

Utilization of distiller waste from ammonia-soda processing

The purpose of the research was to examine the utilization of basic waste of soda ash industry: distiller waste. The aim of the process described is to obtain calcium sulphate in the form of gypsum by means of calcium ions contained in distiller waste liquid. Another raw material is sludge, a suspension which is created during the process of brine evaporation in the salt processing industry. The process of calcium sulphate precipitation is based on calcium ions contained in distiller waste liquid and sulphate ions contained in sludges. (1) CaCl 2 + Na 2 SO 4 + 2H 2 O→ CaSO 4 · 2H 2 O↓+ 2NaCl Results of research have shown that the production of gypsum based on soda ash waste materials is possible. The new products achieve the standards necessary for production of building-quality gypsum. The additional advantage of the described method is obtaining not only gypsum sediment, but also sodium chloride solution (semi-brine). Soda ash factories may also benefit from the introduction of the technology (sale of gypsum, sale of semi-brine, release from environment pollution fees).

Assessment of the effect of velocity and residence time in CaSO 4 precipitating flow reaction

The effect of flow hydrodynamics on kinetics of precipitation of sparingly soluble salts that cause fouling of process equipment has not been addressed previously. Precipitation of sparingly salts from solutions is usually considered to be due to crystallization on the surface. Precipitation in the bulk or in the boundary layer and its subsequent deposition has been traditionally neglected and to our knowledge, no previous research work has focused on this important area. Computational fluid dynamics was used as a tool to examine the effect of flow on and the location within the flow domain where calcium sulphate precipitation occurs. Initially, isothermal conditions were used to isolate the effect of flow; non-isothermal conditions were examined for turbulent flows usually encountered in heat exchange systems. Precipitation kinetics was modelled using a simple second-order reaction usually used in fouling of calcium sulphate. In laminar flow, the characteristics of the velocity distribution led to the emergence of radial concentration gradients and as a result a radial diffusive flux was induced. Similar behaviour was evident under turbulent conditions but not to the same extent. In turbulent flow, particles were mainly produced in the viscous sub-layer rather than in the turbulent bulk. The reduced velocity within the laminar flow or turbulent viscous sub-layer increased the residence time within these regions, enabling precipitation to take place and more particles to form. The generated particles could deposit by particulate fouling in addition to the fact that consumption of ionic species by precipitation in the boundary layer would induce ionic concentration gradient resulting in diffusion of ions to the walls and further crystallization fouling on the walls. Deposit layer formed by crystallization on the surface has a different structure than that formed by particulate fouling. The effect of velocity and residence time on fouling, its mechanisms, and deposit structure should not be overlooked.

A Model of Membrane Fouling by Salt Precipitation from Multicomponent Ionic Mixtures in Crossflow Nanofiltration

A coupled model of concentration polarization, ion transport in membrane pores, and fouling by salt precipitation is used to predict the permeate flux decline due to scaling during crossflow nanofiltration of a multicomponent ionic mixture. The model considers a fouling layer buildup due to salt precipitation once the solubility product of the sparingly soluble salt in an ionic mixture is exceeded. The precipitated salt deposits on the membrane surface, and reduces the permeate flux through the membrane. The primary novelty of the presented methodology is its ability to predict the local fouling behavior at different axial positions in a crossflow filtration channel. Using the model, we assess the fouling behavior of a ternary mixture of Na2SO4 and CaSO4 for various feed concentrations, pertinent membrane properties, and operating conditions to predict the axial location in a crossflow filtration channel where scaling due to calcium sulfate precipitation will initiate. Simulations for a four-component mixture (Na2SO4/CaCl2) are also performed to depict the fouling behavior in a complex ionic mixture closely resembling feed waters used in membrane treatment operations.

ミルクオリゴ糖の糖修飾基情報から生理活性素材を創る！ 糖質化学分野における新マテリアルサイエンスの創成

It is well known that the carbohydrate moieties of glycoproteins or glycolipids, which exist on mammalian cell surfaces, are modified by sulfate or phosphate, etc, in a few cases. These modifiers are recognised as possible receptors in intercellular communications. On the other hand, mammalian milk or colostrum contains many kinds of free oligosaccharides other than lactose in common. These oligosaccharides are also suggested to have biological roles such as inhibition of the adhesion of pathogenic microorganisms to the intestinal tract of infant or brain development - stimulation in infants. Recently, a few milk oligosaccharides have also been shown to be modified and the biological significance of these saccharides are assumed as follows: 1) Sialyllactose lactone: Some proportion of Neu5Gc(α2-3)Gal(β1-4)Glc was found to occur as free lactones between the carboxyl group of Neu5Gc and OH of Gal or Neu5Gc itself (sialyllactose lactone) in ovine colostrum. The sialyl oligosaccharides in milk or colostrum may attach to bacterial toxin and viruses, such as influenza virus in the intestine, thus protecting the infant. The lactone can be assumed to be resistant to virus sialidase. When the sialic acid is liberated from sialyl oligosaccharides by virus sialidase, they would lose their protective effect but the lactone should retain this effect because of its resistance to the action of virus sialidase. 2) 4-O-acetylated sialyllactose: Neu5Ac(α2-3)Gal(β1-4) Glc, whose Neu5Ac was O -acetylated at OH-4, was identified in the milk of echidna, one of the monotremes. Although this saccharide was assumed to be a possible inhibitor for the adhesion of viruses or bacteria whose receptors are O -acetylated Neu5Ac, the exact function of this saccharide is still unknown. 3) sulfated oligosaccharides: Gal(β1-4)Glc-6'-O-sulphate (lactose 6'-O -sulfate) and Neu5Ac(α2-3)Gal(β1-4)Glc-6'-O-sulfate (N-acetylneuraminyllactose 6'-O-sulfate) were identified in rat milk, whereas Gal(β1-4)Glc-3'-O-sulfate (lactose 3'-O-sulfate) was found in dog milk. The presence of N-acetylneuraminyllactose 6'-O -sulfate is suggested in human milk, too, and, in addition, this milk contained oligosaccharides whose GlcNAc residues were replaced by sulfate at OH-6 position. These compounds may permit the simultaneous delivery of two essential nutrients, sulfate and calcium, in early life, avoiding the precipitation of insoluble calcium sulfate in milk. 4) phosphorylated oligosaccharides: Bovine or mare colostrum contained phosphorylated oligosaccha-rides including Neu5Ac(α2-6)Gal(β1-4)GlcNAc-a1-phosphate (cow), Neu5Ac(α2-6)Gal(β1-4)GlcNAc-6-phosphate (cow) and Gal(β1-4)GlcNAc-α1-phophate (horse). These oligosaccharides may also permit the simultaneous delivery of phosphate and calcium, avoiding the precipitation of calcium phosphate. The existence of these modified oligosaccharides in milk may offer valuable in-formation for manufacturing bio-functional materials in the food industry.

Modification of the rheological properties of concentrated slurries by control of mineral–solution interfacial chemistry

The rheological properties of a chalcopyrite slurry have been measured as a function of different chemical conditioning treatments. The magnitude of the yield stress ( τ B) was reduced when either hydrochloric, nitric or sulfuric acid was used to decrease the pH. These results were consistent with an increase in repulsive electrostatic forces between particles. Surface analysis of the particles in the slurry revealed that reducing the pH removed an overlayer of insoluble calcium sulfate. This left the underlying heavily oxidised chalcopyrite surfaces covered with metal hydroxide species. For hydrochloric and nitric acids, the viscosity ( η B) of the slurry was also reduced with decreasing pH — behaviour which was attributed to changes in the surface chemistry of the particles with pH. However, for sulfuric acid, η B increased with decreasing pH. It is proposed that precipitation of insoluble calcium sulfate increased the solid content of the slurry as the pH was reduced, resulting in a significant increase in viscosity. Addition of phosphates to the slurry also reduced τ B and η B. Adsorbed phosphate apparently produces an enhanced repulsive force between particles due to the presence of long-range electrostatic and, possibly, short-range steric interactions. Surface analysis showed that the phosphates dispersed insoluble calcium sulfate from the surface of the chalcopyrite particles. Additional phosphates adsorbed onto the chalcopyrite surfaces. The rheological parameters determined were used to calculate the energy required to pump the slurry through a pipeline. The calculations indicated that the use of either acids or phosphates permits the solid content of the slurry to be increased by over 10 wt.% for the same pumping energy input.

Self inhibition of aluminum leaching from coal fly ash by sulfuric acid

The coal fly ash (CFA) that is produced by coal fired power plants in Israel is alkaline and contains aluminum that can be leached by different acids. In this work, the mechanism of aluminum leaching from CFA by sulfuric acid is considered. It is shown that higher CFA content, which indicates higher solid to liquid ratio in the leaching suspension, decreases the fraction of leached aluminum by means of sulfuric acid. This behavior constitutes a new unexplained phenomenon, which could not be explained by analysis of the mass action law of the dissolution reactions, but rather by mass transfer considerations. It is shown that the leaching process involves a self inhibition mechanism due to the precipitation of calcium sulfate on the surface and within the CFA particles. The effects of CFA content, acid concentration, temperature, and pre-leaching conditioning, upon leaching rates and yields, were tested. Increasing the acid concentration produces two opposing effects simultaneously. An increase in concentration of the hydronium ion enhances the dissolution of aluminum, whereas the increase in concentration of the sulfate and the dissolved calcium ions intensifies the formation of calcium sulfate precipitates. These precipitates hinder mass transfer across the surface of the CFA particles, and in this sense they generate a self inhibition effect. Conditioning of the CFA with hydrochloric acid at pH 4 removes 65% of the calcium. Consequently, the conditioned CFA can be leached more efficiently with sulfuric acid. This higher leachability is linked to the reduction in calcium sulfate precipitation on the CFA.

Sequence stratigraphy of the Middle Devonian Winnipegosis carbonate-prairie evaporite transition, southern Elk Point Basin

The Middle Devonian Winnipegosis reefs in the southern Saskatchewan portion of the Elk Point Basin contain extensive vadose diagenetic features, such as dissolution breccia, and cavities/caves filled by microbialite, pisolite and anhydrite. Basinal facies adjacent to reef buildups are characterized by the Ratner laminite, which consists of three brining-upward successions of laterally continuous, laminated dolomite and anhydrite. The basal cycle starts with a relatively thick unit of anhydrite-free, millimeter-scale dololaminite, changing upward into interlaminated carbonate and anhydrite, and ending with enterolithic, nodular to mosaic anhydrite. Subsequent cycles generally lack the dololaminite of the basal cycle. The Ratner laminite grades upward into the bedded to massive mosaic anhydrite of the Whitkow Member (lower Prairie Evaporite) in areas adjacent to reefs. Deposition of the Ratner laminite and the Whitkow Anhydrite is interpreted as genetically related to the vadose diagenetic processes, when the Elk Point Basin became restricted. The carbonate laminite in the basal Ratner was accumulated when seepage of fresh marine water through the barrier kept pace with the rate of evaporation, preventing a complete drawdown and desiccation of the basin. Precipitation of the laminated carbonate was stimulated by vadose diagenesis of the carbonate buildups and by microbial activity. Each Ratner brining-upward succession represents a progressive drawdown when the rate of basin brine evaporation exceeded seepage of marine water into the basin. Marine water seeping through the reefs was enriched in calcium cations by Mg++−Ca++ exchange with the limestone (dolomitization) and by dissolution of reef rocks, and was responsible for the precipitation of calcium sulphate in areas adjacent to the carbonate reefs through brine mixing processes. During vadose diagenesis of the Winnipegosis reefs and deposition of the Ratner laminite and Whitkow Anhydrite, brine level in the barred Elk Point Basin was controlled by the rate of seepage of marine water through the barrier, which in turn was controlled by eustatic sea level changes in the open ocean. The basal anhydrite-free dololaminite of the Ratner represents a Falling Stage System Tract when evaporative drawdown was largely compensated by seepage of marine water into the basin. The interlaminated carbonate and anhydrite of the middle and upper Ratner are interpreted as a Lowstand System Tract associated with evaporative drawdown and increased cyanobacterial activity under near-desiccation conditions when evaporation exceeded seepage. The Whitkow Anhydrite represents a Transgressive System Tract deposited during sea level rise in the open ocean that led to an increased rate of seepage, higher basin brine level and diminished microbial influence on bedding structures.

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.

Mechanism and kinetics of aluminum and iron leaching from coal fly ash by sulfuric acid

The mechanism and kinetics of leaching of aluminum and iron from Coal Fly Ash (CFA), by sulfuric acid, involves a rate controlling step of mass transfer. It is shown that, in the leaching process, particles follow the shrinking core model with respect to formation of unreacted core that is encapsulated by a leached, porous, layer. Formation of diffusion resistant calcium sulfate precipitates, within the pores and on the surface, induces a self-inhibition effect to mass transfer at the leaching sites. The removal of calcium from the ash, by a preleaching conditioning step, diminishes the self-inhibition effect, and enhances the kinetics of subsequent leaching with sulfuric acid. However, no selective removal of calcium is possible as all metals are dissolved from the same active compounds. The major self-inhibition effect is shown to result from in situ precipitation of calcium sulfate, where dissolved calcium reacts with sulfate ions at the active leaching sites. A self-inhibition model for the description of the leaching kinetics is developed. The model incorporates the buildup of resistance to mass transfer, by diffusion, due to increased pore length, and formation of calcium sulfate barriers as precipitates. The model verifies that the rate of leaching is an exponentially decreasing function of concentration of dissolved aluminum, and consequently the latter follows a logarithmic relation with time. This, experimentally established, logarithmic relation, is characterized by two kinetic parameters. The proportional increase of these parameters with the CFA content was established both experimentally and theoretically. Finally, the model facilitates prediction of the leaching kinetics, in the range of CFA content studied, once it is calibrated with appropriate sample data.

Osteogenesis promoted by calcium phosphate N, N-dicarboxymethyl chitosan

The effects of N, N-dicarboxymethyl chitosan (DCMC) on the precipitation of insoluble calcium salts, namely phosphate, sulfate, oxalate, carbonate, bicarbonate and fluoride, and magnesium salts, namely phosphate and carbonate, were studied. Results indicated that the chelating ability of DCMC interfered effectively with the well-known physico-chemical behaviour of magnesium and calcium salts. Dicarboxymethyl chitosan formed self-sustaining gels upon mixing with calcium acetate, as a consequence of calcium chelation. DCMC mixed with calcium acetate and with disodium hydrogen phosphate in appropriate ratios (molar ratio Ca/DCMC close to 2.4) yielded a clear solution, from which, after dialysis and freeze-drying, an amorphous material was isolated containing an inorganic component about one half its weight. This compound was used for the treatment of bone lesions in experimental surgery and in dentistry. Bone tissue regeneration was promoted in sheep, leading to complete healing of otherwise non-healing surgical defects. Radiographic evidence of bone regeneration was observed in human patients undergoing apicectomies and avulsions. The DCMC–CaP chelate favoured osteogenesis while promoting bone mineralization.

The molybdate/ascorbic acid blue method for the phosphorus determination in very dilute and colored extracts by segmented flow analysis

Abstract A segmented flow analysis method for the determination of trace levels of orthophosphate is described. The method was based on the world wide accepted procedure of Murphy and Riley, using ascorbic acid as reductant, allowing the use of the common automated systems with minor adaptations. A segmented flow system was built using a single 700 mm long dialysis path. The dialyzer was used to overcome the interferences due to organic and colloidal matter, often present in waste waters and 0.01 mol/L CaCl2 or similar extracts of soils. The loss of sensitivity was minimized by the optimization of the dialysis. The detection limit was 3 μg/L. To prevent calcium sulphate precipitation, the use of H2SO4 to acidify the medium and the use of wetting agents with sulphate compounds was also prevented. No coating on the coil or on the tubing of the system was observed, decreasing the need of a frequent maintenance procedure.