Crystallization Of Sodium Sulfate Research Articles

Deterioration Evolutive Process of Cement-Based Materials under Sulphate Physical Crystallization Attack

The influencing factors of sodium sulfate solution physical crystallization attack on cement mortars were investigated through analyzing the changes of macroscopical capability and microstructure of cement mortars in attack process. Results indicate that with partial soaking time increasing, more and more sulfate sodium crystals grow on cement mortars surface, and cement mortars surface is denuded gradually. Compressive and flexural strength increase first and then decrease in the deterioration process. In the attack process, porosity of cement mortars decreases first and then increases. Lots of sodium sulfate crystals accumulate in pores of cement mortars and crystallization pressure is on the rise, resulting in physical crystallization attack taking place.

Nonprocess-element control in the liquor cycle using an ash leaching system

To lengthen run time between washes and reduce thermal cycling of the recovery boiler, the Evadale, Texas, mill has been attempting to reduce the levels of nonprocess elements (NPEs) in the liquor cycle. The mill has periodically purged electrostatic precipitator ash in an effort to reduce and control potassium (K) and chloride (Cl) concentrations. Additional efforts to lower chloride concentrations involved changing the makeup chemical from diaphragm- to membrane-grade caustic and improving the operation of the chlorine dioxide generator spent acid salt filter. The change in makeup chemical NPE content successfully reduced the amount of boiler plugging due to sticky ash formation, but did not provide the desired amount of time between boiler washes. In an effort to further reduce the amounts of potassium and chloride in the liquor cycle, an ash leaching system was installed. The leaching system partially dissolves precipitator ash in hot water. The potassium and chloride are selectively leached into the liquid phase and remain in solution. Sodium sulfate crystals are separated from the leaching slurry using a decanting centrifuge and returned to the liquor cycle via a mix tank. The potassium- and chloride-rich liquid is sewered. Since the implementation of these nonprocess-element reduction methods, the chloride content of the incoming electrostatic precipitator ash has been reduced by roughly 55% and the potassium content by approximately 50%. Approximately 80% of the soda entering the ash leaching process is recovered into the liquor cycle.

Nondestructive Testing Evaluation of Drying Process in Flooded Full-Scale Masonry Walls

After an investigation on the most recent floods occurred in Italy that damaged the Cultural Heritage masonry buildings, an experimental research started on-site on full-scale masonry models exposed to the environmental agents in Milan. The masonry materials used for the full-scale models were largely investigated in the past and the models were subjected to decay caused by the capillary rise and by the crystallization of sodium sulfate coming from the foundations. These walls can actually simulate the state of naturally contaminated walls before a flood and represent a construction where the main parameters are known. A flood has been simulated by adding water for several days to the walls of the full-scale models previously contaminated by salts, then the walls were left to naturally dry. The objective is to check the effectiveness of nondestructive (ND) techniques in detecting the presence of water and the drying process and also the influence of surface treatments presence. Radar tests, thermography tests, sonic tests, as well as the minor destructive powder drilling tests were applied successfully to evaluate the moisture distribution in the masonry after flooding and during natural drying.

Suppression of salt weathering of porous limestone by borax-induced promotion of sodium and magnesium sulphate crystallization

Abstract The effects of borax on the crystallization of sodium and magnesium sulphate, two of the most damaging salts affecting porous stones, have been studied. Borax promotes the crystallization of mirabilite and inhibits epsomite crystallization in open glass beakers. The additive is preferentially adsorbed onto {140} mirabilite and {111} epsomite faces, thus acting as an effective habit modifier. In contrast, in the presence of a calcitic support (either Iceland spar single crystals or a porous limestone – a biocalcarenite) crystallization is promoted in the presence of borax, irrespective of the salt tested. Apparently, this is due to a high stereochemical affinity between borate molecules adsorbed (and/or co-precipitated) onto calcite, and mirabilite and epsomite crystals. Salt weathering tests using a biocalcarenite show a significant damage reduction upon borax addition to the saline solutions. Borax promotes the crystallization of both mirabilite and epsomite within the pores of the stone, reducing its porosity. Crystallization promotion favours nucleation at a low supersaturation, thereby resulting in very low crystallization pressure and minimal damage. Application of borax to porous limestones affected by mirabilite and/or epsomite crystallization could be a new means of suppressing salt weathering.

Nanofiltration as energy-efficient solution for sulfate waste in vacuum salt production

In vacuum salt production sulfate is an important impurity, but it is also used to remove other cationic impurities from the raw brine. Removal of excess sulfate is currently done by purging salt crystallizer mother liquor from the brine plant, or crystallizing sodium sulfate through evaporative or cooling crystallization. The use of nanofiltration for the production of brines supersaturated in sodium sulfate is a technically feasible and attractive alternative. Crystallization of sodium sulfate in the membrane modules can be avoided by the presence of a primary nucleation inhibitor in the feed to the nanofiltration unit. Sodium sulfate crystallization can be induced in a separate crystallizer outside of the membrane unit by addition of crystal seeds. NF270 is a suitable membrane for this application and is preferred over Desal DK, Desal DL, TS-80 and MPF-34. Chloride and bromide retentions are negative and they are a function of the difference in sulfate concentration between concentrate and permeate. Carbonate retentions are linearly dependent on sulfate retentions. Calcium retention for NF270 is in excess of 95% and proven to be stable during 1200 h of continuous on-site pilot plant operation. The same applies for the potassium retention albeit at a lower level of 10%.

Sulfate Crystallization Attack on Cement-Based Materials

The physical crystallization action of sulfate in cement based material, which often occurs by dry-wet cycle, capillary rising and evaporating action, can damage seriously cement-based materials as compared to chemical corrosion by sulfate. The deterioration mechanism of cement-based materials by sulfate crystallization attack, experimental investigations of the process of sodium sulfate crystallization and the factors affecting the process, and the invading track of salt solution by capillary effect are presented in this paper. Results show that the crystal type and crystallization velocity of sodium sulfate is influenced greatly by relative humidity and ambient temperature. There is a close relationship between invading depth and the porosity of concrete. Addition of mineral admixture to concrete can significantly reduce the invading depth of sodium sulfate solution and thereby enhance the resistance of concrete to sulfate crystallization attack.The profile of invading track of salt solution by capillary effect is similar to the shape of concave parabola.

Modification of the porous network by salt crystallization in experimentally weathered sedimentary stones

The aim of this study is to understand how the porous network of a stone is modified by the crystallization of sodium sulphate. Samples of five different stones have been experimentally weathered and evaluated thanks to mercury porosimetry in three different states: fresh, weathered by salt and weathered cleaned from the salts. Optical and electronic microscopy observations have also been led to complete these measurements. The results show that porosity and the general aspect of the porous network remain quite similar after weathering. Nevertheless, crystals tend to grow on all the grains regardless of the size of the related voids (pores or cracks), and these crystallizations seem to be harmful for the stone: a lot of voids of different entry sizes (from 10 nm to 20 μm) have been affected by the accelerated ageing tests. This study confirms that generally stones with a high amount of small pores (up to several μm) are the most susceptible to suffer from salt damage. Nevertheless, the influence of a few other features (high porosity, pore shape and pre-existent intragranular cracks) on the long-term behaviour of the stones suffering from salt decay is discussed.

Crystal size distribution (CSD) of batch salting-out crystallization process for sodium sulfate

The crystal size distribution (CSD) of sodium sulfate from the batch crystallization of sodium sulfate from concentrated sulfuric acid (spent acid) by salting-out with methanol was analyzed. The effects of amount of methanol added, the agitation rate, temperature and residence time (sampling time) on CSD were investigated. The mean crystal size and the coefficient of variation (c.v.) were the criteria used and the Rosin–Rammler distribution model was assumed. The mean crystal size varied from 0.03 to 0.07 mm while the range of c.v. was 5–45%, though most of the runs gave a c.v. of 20% or less, thus a size independent growth rate could be assumed. Larger mean crystal sizes were obtained for the following conditions: small amount of added methanol (low level of supersaturation), low temperature, low agitation rates and longer residence time. Reduction in coefficient of variation was enhanced by the same factors. A 2 4 factorial design analysis confirmed these findings.

The role of saline solution properties on porous limestone salt weathering by magnesium and sodium sulfates

Saline solution properties, viscosity in particular, are shown to be critical in salt weathering associated with sodium and magnesium sulfate crystallization in porous limestone. The crystallization of sodium and magnesium sulfate within a porous limestone has been studied at a macro- and microscale using different techniques, including mercury intrusion porosimetry, environmental scanning microscopy and X-ray computed tomography. Such analysis enabled the visualization of the crystallization process in situ, and at high magnification, yielding critical information as to where and how salts crystallize. Sodium sulfate decahydrate (mirabilite) tends to crystallize in large pores as euhedral micron-sized crystals formed at low supersaturation near to the surface of the stone. In contrast, magnesium sulfate heptahydrate (epsomite) tends to precipitate as anhedral wax-like aggregates formed at high supersaturation and distributed homogeneously throughout the stone pore system filling large and small pores. While the former crystallization behavior resulted in scale formation, the latter led to crack development throughout the bulk stone. Ultimately, the contrasting weathering behavior of the two sulfates is explained by considering differences in flow dynamics of solutions within porous materials that are mainly connected with the higher viscosity of magnesium sulfate saturated solution (7.27 cP) when compared with sodium sulfate saturated solution (1.83 cP). On the basis of such results, new ways to tackle salt weathering, particularly in the field of cultural heritage conservation, are discussed.

Sodium Sulfate Crystallization in the Presence of Phosphonates: Implications in Ornamental Stone Conservation

Phosphonates were tested as potential crystallization inhibitors for sodium sulfate, one of the most damaging soluble salts in historic building and sculpture. Although mirabilite (Na2SO4·10H2O) crystallization is promoted in the presence of 1-hydroxyethylidene-1,1-diphosphonic acid (HEDP), aminotris(methylenephosphonic acid) (ATMP), and diethylenetriaminepentakis(methylphosphonic acid) (DTPMP) at salt solution natural pH (6.4), crystallization is inhibited at moderately alkaline pH (8−8.5). Molecular modeling of additive adsorption on specific mirabilite (hkl) faces, in addition to ESEM observations, allowed us to identify how phosphonates inhibit mirabilite crystallization. ATMP and DTPMP display the best stereochemical matching with mirabilite {100} surfaces, which explains why they are the most effective inhibitors. Inhibition of sodium sulfate crystallization in the presence of phosphonates may promote the growth of efflorescence on porous ornamental stone, rather than damaging subefflorescence. The ...

Membrane distillation crystallization of concentrated salts—flux and crystal formation

Membrane distillation crystallization (MDC) can be used to process highly concentrated aqueous solutions. In MDC, membrane distillation (MD) is used to recover water and to generate the desired supersaturation in the crystallizer where product crystals can be precipitated. This paper discusses factors influencing flux at close to saturation and the formation of salt crystals. The flux behaviour was investigated using two aqueous salt solutions of sodium sulfate (Na 2SO 4) and sodium chloride (NaCl) which have different solubility–temperature coefficients. For both salts it was observed that MD was operable at high concentrations at feed temperatures of 50 and 60 °C with fluxes up to 20 L m −2 h −1. It was found that both concentration and temperature polarization influenced the performance of MD. When operated in batch concentration mode without the crystallizer the flux gradually declined due to vapour pressure suppression and concentration polarization, up to a critical degree of saturation. The flux data could be predicted by heat and mass transfer modeling or by a simple empirical relationship involving the overall vapour pressure driving force and the degree of saturation. Beyond the critical degree of saturation, rapid flux decline was observed due to crystal deposition and scale formation on the membrane which reduced the membrane permeability. The Na 2SO 4 solution was able to operate at slightly higher degrees of saturation, which may be because the negative solubility–temperature coefficient favours solubility in the polarization layer. In MDC the temperature and saturation level both in the MD and the crystallizer are critical operating parameters. Anhydrous sodium sulfate crystals can be produced by means of MDC with a relatively narrow crystal size distribution and average size of 60–80 μm.

Intrinsic factors determining the physical behaviour and durability of the Miocene sandstones used to build the Zaghouan-Carthage aqueduct (Tunis)

The present paper addresses the relationship between the intrinsic factors, physical behaviour and durability of Miocene Age sandstone used to build Tunisian Heritage Monuments, and more specifically the Zaghouan-Carthage aqueduct. A petrography study (optical microscopy and mercury intrusion porosi metry) was conducted to characterize the porous system of the rock. Stone hydric behaviour was also determined by finding vacuum saturation, desorption, capillary and water vapor transmission. Finally, mechanical strength (compressive strength, abrasion resistance) and durability (via accelerated sodium sulfate crystallization ageing) were also found. The results obtained were indicative of good hydric performance due to the macroporous nature of the stone and the connectivity of its porous system. This rock was also found to have very low mechanical strength due to its scant lithification, making it particularly susceptible to salt weathering. It was also observed to be highly resistant to chemical alteration, given the absence of chemically unstable minerals in its composition. The durability of the material was consequently found to depend directly on the presence or absence of salts in the monument.

How does sodium sulfate crystallize? Implications for the decay and testing of building materials

The fundamental behavior of sodium sulfate crystallization and induced decay in concrete and other building materials is still poorly understood, resulting in some misinterpretation and controversy. We experimentally show that under real world conditions, both thenardite (Na2SO4) and mirabilite (Na2SO4·10H2O) precipitate directly from a saturated sodium sulfate solution at room temperature (20°C). With decreasing relative humidity (RH) and increasing evaporation rate, the relative proportion of thenardite increases, with thenardite being the most abundant phase when precipitation occurs at low RH in a porous material. However, thenardite is not expected to crystallize from a solution at T<32.4°C under equilibrium conditions. Non-equilibrium crystallization of thenardite at temperatures below 32.4°C occurs due to heterogeneous nucleation on a defect-rich support (i.e., most porous materials). Anhydrous sodium sulfate precipitation is promoted in micropores due to water activity reduction. Fast evaporation (due to low RH conditions) and the high degree of solution supersaturation reached in micropores before thenardite precipitation result in high crystallization pressure generation and greater damage to porous materials than mirabilite, which crystallizes at lower supersaturation ratios and generally as efflorescence. Data from the environmental scanning electron microscope (ESEM) show no hydration phenomena following wetting of thenardite; instead, thenardite dissolution occurs, followed by thenardite plus mirabilite crystallization upon drying. These results offer new insight into how damage is caused by sodium sulfate in natural geological, archaeological, construction and engineering contexts. They also help explain some of the controversial results of various commonly used sodium sulfate crystallization tests.

Drowning-out crystallisation of sodium sulphate using aqueous two-phase systems

A novel method to obtain crystals of pure, anhydrous salt, using aqueous two-phase systems was studied. A concentrated salt solution is mixed with polyethylene glycol (PEG), upon which three phases are formed: salt crystals, a PEG-rich liquid and a salt-rich liquid. After removal of the solid salt, a two-phase system is obtained. Both liquid phases are recycled, allowing the design of a continuous process, which could be exploited industrially. The phase diagram of the system water–Na 2SO 4–PEG 3350 at 28°C was used. Several process alternatives are proposed and their economic potential is discussed. The process steps needed to produce sodium sulphate crystals include mixing, crystallisation, settling and, optionally, evaporation of water. The yield of sodium sulphate increases dramatically if an evaporation step is used.

Salt weathering: influence of evaporation rate, supersaturation and crystallization pattern

Micro- and macroscale experiments which document the dynamics of salt damage to porous stone have yielded data which expose weaknesses in earlier interpretations. Previously unexplained differences are found in crystal morphology, crystallization patterns, kinetics and substrate damage when comparing the growth of mirabilite (Na2SO4. 10H2O) and thenardite (Na2SO4) versus halite (NaCl). The crystallization pattern of sodium sulphate was strongly affected by relative humidity (RH), while a lesser RH effect was observed for sodium chloride. Macroscale experiments confirmed that mirabilite (crystallizing at RH > 50 per cent) and thenardite (crystallizing at RH < 50 per cent) tend to form subflorescence in highly localized areas under conditions of constant RH and temperature. This crystallization pattern was more damaging than that of halite, since halite tended to grow as efflorescence or by filling the smallest pores of the stone in a homogeneous fashion, a result which contradicts Wellman and Wilson's theoretical model of salt damage. Low RH promoted rapid evaporation of saline solutions and higher supersaturation levels, resulting in the greatest damage to the stone in the case of both sodium sulphate and sodium chloride crystallization. At any particular crystallization condition, sodium chloride tended to reach lower supersaturation levels (resulting in the crystallization of isometric crystals) and created negligible damage, while sodium sulphate reached higher supersaturation ratios (resulting in non-equilibrium crystal shapes), resulting in significant damage. ESEM showed no damage from sodium sulphate due to hydration. Instead, after water condensation on thenardite crystals, rapid dissolution followed by precipitation of mirabilite took place, resulting in stone damage by means of crystallization pressure generation. It is concluded that salt damage due to crystallization pressure appears to be largely a function of solution supersaturation ratio and location of crystallization. These key factors are related to solution properties and evaporation rates, which are constrained by solution composition, environmental conditions, substrate properties, and salt crystallization growth patterns. When combined with a critical review of salt damage literature, these experiments allow the development of a model which explains variations in damage related to combinations of different salts, substrates and environmental conditions.

Durability of two water repelents applied to granite

The durability of two water-repellents for granitic stonework was determined. Weathered and sound samples of granites widely used in building construction and restoration in Galicia (NW Spain) were treated with water repellents of known efficacy, and then subjected to two artificial weathering tests: prolonged exposure to UV light, and sodium sulphate crystallization cycles. In both tests, but especially in the salt crystallization cycles, both treatments rapidly lost their water-repellency. Furthermore, the hydrophobic layer of water repellent impeded salt mobility, favouring fissuration parallel to the treated surface, which was eventually shed in the form of a plaque.

SODIUM SULPHATE FROM BITTERN BY METHANOL

Sodium sulphate is produced mainly from land ores by conventional crystallization methods of evaporation and dehydration; these sources are fast being exhausted and new processes should be found which do not use the conventional sources. Seawater bittern produced as by-product from solar halite plants contains about 6% by weight of sodium sulphate, the recovery of which by conventional methods, is made difficult by the presence of other salts. Methanol has the property of markedly depressing the solubility of certain inorganic salts while hardly affecting the solubilities of others, because of this property, it can be used to promote various crystallization processes and some double decomposition reactions between inorganic salts. This paper examines one example of the second type of application in which magnesium sulphate and sodium chloride present in the bittern interact in the presence of methanol resulting in the formation and selective crystallization of anhydrous sodium sulphate. Experimental work is described which follows the progress of the reaction under wide range of conditions some given above 94% recovery of high purity sodium sulphate with a crystallizer reactor residence time of 20 minutes. SEM photographs reveal rhombic shapes crystals of size varying between about 1 to 5 μrn. The results presented give promise of forming the basis of a commercially viable process for the fabrication of sodium sulphate.

Radiation and electric field induced effects on the order-disorder phase in lithium sodium sulphate crystals

The temperature dependence of specific heat at constant pressure, Cp(T), has been measured for lithium sodium sulphate, LiNaSo 4 crystals, at different γ-radiation doses and external bias electric field (E b ), in the temperature range 300-900 K. A nonlinear dependence of transition temperature, T 1 and a remarkable change in the thermodynamic parameters, were obtained as the effect of both electric field and γ-radiation. The effect of γ-radiation doses on the phase transition in LiNaSO 4 crystals was explained as due to an internal bias field, E b, originating from the interaction of polar defects with the order parameter of the host lattice. The internal bias field effect on the behaviour of Cp(T) in LiNaSO 4 crystals was similar to that of the external electric field (E).

Crystallization of sodium sulphate in polymer impregnated plasters

The common occurrence and harmful effects of sodium sulphate make it a major cause of deterioration in plasters. The investigation conducted showed that a plaster composed of eminently hydraulic lime impregnated with water-repellent solutions (a perfluoropolyether or an oligomeric alkylalkoxysiloxane) or admixed with an acrylic latex is significantly more durable than plain plaster. In particular, the different modes of salt crystallization are discussed in relation to the microstructural properties conferred by the polymers, and to the thermohygrometric conditions of the system.

Evaluation of water repellent treatments applied to stones used in andalusian cathedrals. II. Salt crystallization test

In a previous paper we have studied the changes that have ocurred in the properties related to water access and movement in the stone after the application of water repellent treatments. In this work we compare the weathering resistance of treated and untreated samples by means of sodium sulphate crystallization test. After finishing the test (75 cycles) properties related to water have been measured again to know if the treatments have undergone any kind of deterioration and lose their water repellent characteristics.