Continuous salt precipitation and separation from supercritical water. Part 2. Type 2 salts and mixtures of two salts

Using a continuously operated laboratory plant for the catalytic hydrothermal gasification of biomass featuring a supercritical water salt separator we investigated the separation performance of three different binary type 2 salt-water mixtures and three ternary salt-water mixtures that consisted either of two type 1 salts or two type 2 salts dissolved in water. It turned out that a concentrated salt brine could not be recovered at the salt separator for the binary type 2 salt-water mixtures of Na₂CO₃, Na₂SO₄, and K₂SO₄. These salts precipitate as solids from supercritical water and thus lead to salt deposits inside the salt separator vessel.The ternary mixtures of two type 1 salts dissolved in water (KH₂PO₄-K₂HPO₄-H₂O and three different mixtures of NaNO₃-K₂CO₃-water) exhibited a separation performance similar to the binary solutions of type 1 salts that were discussed in Part 1 of this article. However, the mixtures showed separation performances that were different from the corresponding single salt solutions.It was also possible to recover a concentrated brine when feeding solutions containing the two type 2 salts Na₂CO₃ and K₂SO₄. For these mixtures a certain amount of the type 1 salt K₂CO₃ might form in supercritical water leading to salt separation efficiencies up to 95% for these mixtures.     

The solubilities of phosphate and sulfate salts in supercritical water

Inorganic compounds are regularly present in aqueous streams. To understand their influence and behavior on these streams at supercritical conditions, little to no property data is available, which can be used as starting point for further research or application design. Since inorganic compounds tend to precipitate at these conditions, scaling, blocking and erosion can occur as a consequence. Furthermore, a separation of (precious) compounds from the bulk stream due to the precipitation is possible. Here, phosphate compounds are regarded as interesting for further investigation since resources are assumed to be depleted in future. As phosphate is present in many waste streams, these could be used as sources for recoverable phosphate. Resulting from these facts and options, a proper understanding and knowledge of these systems is important for later industrial applications. Therefore, the authors have investigated the behavior of salts (e.g. NaCl, NaNO₃ and MgCl₂) in supercritical water in previous works.To extend this knowledge, the solubilities of the sulfate salts MgSO₄ and CaSO₄ in a range of 18.8-23.2 MPa and 655-675 K as well as of the phosphate salts Na₂HPO₄, NaH₂PO₄ and CaHPO₄ in a range of 20.5-24.2 MPa and 665-690 K were investigated in this work with a continuous flow method in continuation of former work of the authors. The solubilities were compared with existing data available from open literature. A quantitative correlation on base of a phase equilibrium between the present phases was used to describe the behavior and to compare it with previous results. For the investigated calcium salts, CaSO₄ and CaHPO₄, it was found that a significant solubility decrease already happens at subcritical conditions resulting in precipitation in unwanted locations. For the remaining compounds, a parallel hydrolysis reaction was found as could be seen from a change in pH in the effluent stream.     

Continuous hydrothermal gasification of glycerol mixtures: Effect of glycerol and its degradation products on the continuous salt separation and the enhancing effect of K3PO4 on the glycerol degradation

At the Paul Scherrer Institut (PSI) a continuous process for the catalytic hydrothermal gasification of wet biomass to synthetic natural gas (SNG) has been developed. The catalytic reactor is operated at temperatures of 400–450 °C and pressures of 25–30 MPa. Salts contained in the biomass and released during the liquefaction step are continuously withdrawn in the supercritical salt separation step and recovered as a concentrated brine upstream of the catalytic reactor. The recovered salts may be recycled as valuable nutrients or fertilizers after a certain work-up.Salt management was identified as critical issue in many different hydrothermal processes such as supercritical water oxidation (SCWO) and in catalyzed or non-catalyzed gasification technologies in near- and supercritical water. In this article we focus on the influence of organics, in this case glycerol and its hydrothermal degradation products, on the continuous salt separation performance. In the presence of organics higher temperatures are needed in the salt separator for an efficient salt separation and recovery due to a higher overall fluid density in the presence of glycerol compared to the density of pure water at the same conditions. Increasing temperatures in the salt separator lead to an increased degradation and, in particular, gasification of the glycerol. The salt studied, i.e. K₃PO₄, catalyzed the gasification of the glycerol to CO, H₂, CO₂, and CH₄ as well as the water gas shift reaction. Due to the increased glycerol gasification at 460 °C in the salt separator, the fluid mixture density was lowered to similar values of pure water under the same conditions. Hence, at the fluid temperature of 460 °C in the salt separator the same salt separation performance was observed for water–K₃PO₄ and for an aqueous mixture of 20 wt.% glycerol with K₃PO₄.     

Rapid Molten Salt Synthesis of Isotropic Negative Thermal Expansion ScF3

With a simple ReO₃‐type structure, ScF₃ exhibits a rare property of isotropic negative thermal expansion over a large temperature range. In this study, a rapid and low‐temperature synthesis route has been developed to prepare pure phase of ScF₃ in which NaNO₃ or KNO₃ as reaction media and Sc(NO₃)₃ and NH₄HF₂ as precursors (i.e., 30 min and 310°C). The sample of ScF₃ has relatively regular morphology and shows high crystallinity as well as single‐crystalline nature. The type of molten salts has obvious impact on morphologies of the particles. Substituting NaNO₃ with KNO₃, cubes of ScF₃ turns to be nanosticks. The thermal stability of the as‐prepared ScF₃ was investigated by thermal analysis. Molten salts play a significant role in eliminating the nonstoichiometric impurity of ScF_2.76 which is a common impurity during the conventional chemical reaction. This study reveals that molten salt is in favor of preparing those fluorides and relatives which is inert with moisture.     

Mechanochemical synthesis of kaolin-KH2PO4 and kaolin-NH4H2PO4 complexes for application as slow release fertilizer

A milling process to reduce kaolin to amorphous phase in the presence of KH₂PO₄ or NH₄H₂PO₄ and allow mechanochemical (MC) reaction for incorporation of KH₂PO₄ and NH₄H₂PO₄ into the kaolin structure was investigated in this work. Mixtures of kaolin and KH₂PO₄ and NH₄H₂PO₄ in separate systems were prepared by milling in a planetary ball mill. Tests with kaolin contents ranging from 25 to 75 wt.% and mill rotational speeds from 200 to 700 rpm were performed to evaluate incorporation of KH₂PO₄ and NH₄H₂PO₄ and release of K⁺, NH₄⁺ and PO₄³⁻ ions into solution. Analyses by XRD, DTA and ion chromatography indicated that the MC process was successfully applied to incorporate both KH₂PO₄ and NH₄H₂PO₄ into the amorphous kaolin structure. Release of K⁺ and PO₄³⁻ ions from the system (kaolin-KH₂PO₄) when dispersed in water for 24 h reached only up to 10%. Under similar conditions for the system (kaolin-NH₄H₂PO₄), release of NH₄⁺ and PO₄³⁻ ions reached between 25 and 40%. These results indicated that the MC process can be developed to allow amorphous kaolin to act as a carrier of K⁺, NH₄⁺ and PO₄³⁻ nutrients to be released slowly for use as fertilizer.     

Electrochemical behaviour of a vanadium anode in phosphoric acid and phosphate solutions

Anodic polarisation of a vanadium electrode has been studied in H₃PO₄ solutions and some phosphate solutions: LiH₂PO₄, NaH₂PO₄, KH₂PO₄ and NH₄H₂PO₄. The anodic behaviour of a vanadium electrode showed similarities in weak concentrated H₃PO₄, in LiH₂PO₄ and NaH₂PO₄ solutions: the polarisation curve exhibited a current peak followed by current oscillations and then a current plateau. Concentrated H₃PO₄, 1 M KH₂PO₄ and NH₄H₂PO₄ solutions involved vanadium passivation with a very slight current density plateau. Yellow compound identified to VOPO₄·2H₂O was obtained after controlled potential oxidation of vanadium in 5-10 M H₃PO₄. Green products were obtained in 1 M phosphate solutions and in 1-3 M H₃PO₄ on vanadium anode after controlled potential electrolysis. All these vanadophosphate compounds contained the monovalent cation which was present in the solution.     

Separation performance of a nanofiltration membrane influenced by species and concentration of ions

Nanofiltration (NF), which has been largely developed over the past decade, is a promising technology for the treatment of organic and inorganic pollutants in surface and ground waters. The ESNA 1 membrane from the Nitto Denko Corporation of Japan is made of aromatic polyamide, which provides salt rejection from 50% to 90%. In this paper permeation experiments of aqueous solutions of five chlorides (NH₄Cl, NaCl, KCl, MgCl₂ and CaCl₂), three nitrates (NaNO₃, Mg(NO₃)₂ and Ca(NO₃)₂), and three sulfates (NH₄)₂SO₄, Na₂SO₄ and MgSO₄) were carried out. The effects of species and concentration of salts on the separation performance of the ESNA 1 membrane were investigated. The experimental results showed that the rejection to most salts by the ESNA 1 membrane decreased with the growth of the concentration. Then, the reflection coefficient and solute permeability of ESNA 1 membrane were calculated by the Spiegler-Kedem equation from experimental data. The reflection coefficients of the ESNA 1 membrane to salts are all above 0.95. The salt permeabilities, except for magnesium and calcium salts, increased with the growth of concentration. The sequence of rejection to anions by the ESNA 1 membrane is R(SO²⁻₄) > R(Cl⁻) > R(NO⁻₃) at the same concentration which ranges from 10 mol/m³ to 100 mol/m³. The sequence of rejection to anions by the ESNA 1 membrane can be written as follows: R(Na⁺) > R(K⁺) > R(Mg²⁺) > R(Ca²⁺) at 10 mol/m³ concentration and R(Mg²⁺) > R(Ca²⁺) > R(Na⁺) > R(K⁺) at 100 mol/m³ concentration.     

Oxidative desulfurization of dibenzothiophene based on molecular oxygen and iron phthalocyanine

Direct oxidation of dibenzothiophene (DBT) based on molecular oxygen and iron tetranitrophthalocyanine (FePc(NO₂)₄) catalyst was performed in hydrocarbon solvent under water-free condition for deep desulfurization. Conversion of DBT in decalin reached 98.7 wt.% at 100 °C and 0.3 MPa of initial pressure with 1 wt.% of FePc(NO₂)₄ over the whole solution for 2 h. In addition to FePc(NO₂)₄, another two catalysts, FePc(NO₂)₃NH₂ and FePc(NH₂)₄, were synthesized to investigate the effect of substituents of iron phthalocyanines on their catalytic activities. The results show that the catalytic activity of these phthalocyanines decreases in the order of FePc(NO₂)₄ > FePc(NO₂)₃NH₂ > FePc(NH₂)₄, indicating that the electron-donating group has negative effect on the catalytic properties. Activity of FePc(NO₂)₄ was kept unchanged after 5 runs of oxidation; whereas, activity of FePc(NH₂)₄ decreased because of its decomposition. Moreover, FePc(NO₂)₃NH₂ was supported on a polyacrylic cationic exchange resin and its activity was remarkably enhanced to the level of FePc(NO₂)₄. Oxidative desulfurization of a model fuel, 500 μg/g solution of DBT in decalin, was performed based on the catalytic oxidation using molecular oxygen and FePc(NO₂)₄ catalyst. The lowest sulfur content in the model fuel could be decreased to less than 4 μg/g after the treatment of this oxidation and a combined adsorption.     

Effect of nitrogen sources on γ-linolenic acid accumulation in Spirulina platensis

The effect of ammonium phosphate on the growth, lipid content, and γ-linolenic acid accumulation was determined in the cyanobacterium Spirulina platensis. After 14 d on media containing 0.041 g N/L, γ-linolenic acid concentration of cultured cells increased up to 35.3±0.13%, w/w. In treatments containing NaNO₃, NH₄NO₃, and NH₄Cl, γ-linolenic acid concentration increased up to 31.2±0.23%, w/w. After the same period, lipid content in the dry biomass was 12.2±0.03%, w/w, with (NH₄)₂HPO₄ compared to about 14.1±0.12%, w/w, in treatments with NaNO₃. When (NH₄)₂HPO₄ concentration in the medium was increased to 0.082 g N/L, 30.8±0.28%, w/w, γ-linolenic acid had formed after 10 d and the lipid percentage in the dry cell mass was 16.7±0.16%, w/w. However, in treatments with NaNO₃, NH₄NO₃, or NH₄Cl, γ-linolenic acid concentration increased up to 30.6±0.23%, w/w, and the lipid content was found to be 18.0±0.17 to 18.9±0.03%, w/w. These data showed that (NH₄)₂HPO₄ is a suitable source of nitrogen for growth of S. platensis, with increased accumulation of γ-linolenic acid at lower N concentration.     

Effect of various environmental conditions on the swelling property of PAAm/PAAcK superabsorbent hydrogel prepared by ionizing radiation

Polyacrylamide/potassium polyacrylate (PAAm/PAAcK) superabsorbent hydrogels were prepared by using ionizing radiation for possible use in agricultural purposes. The influence of environmental conditions such as water quality, fertilizer salts, soil pH, and surrounding temperature on PAAm/PAAcK water absorbency and retention was investigated. The water absorbency apparently decreased with an increase in the valence and ionic strength of the salt solutions. The swelling of PAAm/PAAcK immersed in the solutions containing fertilizer of different nitrogen sources followed the order: Urea > NH₄NO₃ > (NH₄)₂SO₄. The PAAm/PAAcK water absorbency in solutions containing different types of phosphate sources was in the order: H₃PO₄ > KH₂PO₄ > K₂HPO₄. However, the water absorbency of PAAm/PAAcK in the solution of different types of potassium salts followed the order: KCl > K₂SO₄ > K₂HPO₄. PAAm/PAAcK hydrogel is fully swollen at pH 6. The relation between the ability of PAAm/PAAcK to retain water against time at different temperatures was studied. As the environmental temperature increases, the water retention of PAAm/PAAcK decreases. Potential application of PAAm/PAAcK in agriculture was evaluated. © 2006 Wiley Periodicals, Inc. J Appl Polym Sci 101: 3955–3962, 2006     

Synthesis of silicon-substituted hydroxyapatite by a hydrothermal method with two different phosphorous sources

Silicon-substituted hydroxyapatite (Si-HA) with up to 1.8 wt% Si content was prepared successfully by a hydrothermal method, using Ca(NO₃)₂, (NH₄)₃PO₄ or (NH₄)₂HPO₄ and Si(OCH₂CH₃)₄ (TEOS) as starting materials. Silicon has been incorporated in hydroxyapatite (HA) lattice by partially replacing phosphate (PO₄³⁻) groups with silicate (SiO₄⁴⁻) groups resulting in Si-HA described as Ca₁₀(PO₄)_6−x(SiO₄)_x(OH)_2−x. X-ray diffraction (XRD), Fourier transform IR spectroscopy (FTIR), inductively coupled plasma AES (ICP-AES) and scanning electron microscopy (SEM) techniques reveal that the substitution of phosphate groups by silicate groups causes some OH⁻ loss to maintain the charge balance and changes the lattice parameters of HA. The crystal shape of Si-HA has not altered compared to silicon-free reference hydroxyapatite but Si-incorporation reduces the size of Si-HA crystallites. Based on in vitro tests, soaking the specimens in simulated body fluid (SBF), and MTT assays by human osteoblast-like cells, Si-substituted hydroxyapatite is more bioactive than pure hydroxyapatite.     

Effects of electrolytes on the reduction of 2,4‐dinitrotoluene by zero‐valent iron

BACKGROUND: Dinitrotoluenes (DNTs) are environmentally persistent, making the remediation of contaminated streams and groundwater difficult. Zero‐valent iron (Fe⁰) can be used as an electron source for the reduction of recalcitrant DNTs in waste‐water and thus enhance their biodegradability. However, little is known about the qualitative effects of major anions and cations present in waste‐water on the reduction of DNTs by Fe⁰. RESULTS: The presence of Na₂SO₄ and NaCl at levels between 0.25 and 2 mmol L^?1 was observed to enhance the reactivity of Fe⁰ towards 2,4‐DNT. The positive effect of K₂SO₄ is stronger than that of Na₂SO₄ at the same level (1 mmol L^?1). Varying (NH₄)₂SO₄ from 0.1 to 1.0 mmol L^?1 improved the efficiency of 2,4‐DNT degradation by Fe⁰. The effects of varying NaNO₃ and NaNO₂ from 0 mmol L^?1 to 4.7 mmol L^?1 and 0 mmol L^?1 to 5.8 mmol L^?1, respectively, were also investigated. Both NaNO₃ and NaNO₂ at low concentration improved the efficiency of 2,4‐DNT degradation by Fe⁰, however, at high concentration, inhibiting effects appeared. CONCLUSION: SO₄^2?, Cl^?, Na⁺, K⁺ and NH₄⁺ notably enhanced 2,4‐DNT reduction by Fe⁰ at the tested concentrations. The positive effect of K⁺, Cl^? was relatively stronger than that of Na⁺ and sulfate (SO₄^2?). However, the effect of NH₄⁺ was relatively weaker at concentrations greater than 1.0 mmol L^?1. The presence of low concentrations of NO₃^? and NO₂^? promoted 2,4‐DNT reduction by Fe⁰ and inhibited the reaction. The results suggest that 2,4‐DNT reduction by Fe⁰ can be controlled by the ions composition of the waste‐water. Copyright © 2010 Society of Chemical Industry     

X-ray investigation of sintered cadmium doped hydroxyapatites

Pure and cadmium (Cd) doped hydroxyapatites (HA, Ca₁₀(PO₄)₆(OH)₂) were synthesized by a precipitation method from aqueous solutions of Ca(NO₃)₂4·H₂O for the former and Cd(NO₃)₂4·H₂O for the latter, by using (NH₄)₂HPO₄ as the phosphate source, while pH was kept in the range of 11–12. The effect of incorporation of Cd²⁺ ions into the structure of HA was investigated after the air sintering at 1100 °C for 1 h. The results indicate that Cd²⁺ addition into HA yields nearly fully densified products with respect to pure stoichiometric HA. The XRD patterns showed that Cd doping increases the crystallinity of HA. The 2, 4.4, and 8.8 mol% Cd doped HAs had calcium oxide (CaO) impurity phase in their lattice. The CaO phase in the HA structure gradually disappeared with increasing Cd amount, and was replaced with cadmium oxide (CdO) in the CdHA doped with 11 mol% Cd. Cd²⁺ ion incorporation decreased the a- and c-axis lattice constants and unit cell volume of HA.     

Dissolution of serpentine using recyclable ammonium salts for CO2 mineral carbonation

Low efficiency of mineral dissolution and unrecyclable use of additives are two barriers for the development of CO₂ mineral carbonation. A new pH-swing CO₂ mineralisation process using recyclable ammonium salts is proposed to overcome these barriers. This paper presents the studies of mineral dissolution with ammonium salts. In this study, dissolution of a serpentine sample was performed using a series of aqueous solutions, which include (NH₄)₂SO₄, NH₄Cl, NH₄HSO₄ and H₂SO₄. NH₄HSO₄ is the most efficient in terms of extracting Mg extraction from the serpentine sample. At 100 °C 1.4 M NH₄HSO₄ extracted 100% of Mg from serpentine in 3 h, as well as 98% of Fe and 17.6% of Si. The rate limiting mechanism of serpentine dissolution with NH₄HSO₄ is a chemical reaction with product layer diffusion control and the activation energy of this dissolution was 40.9 kJ mol⁻¹.     

Preparation of hydroxyapatite coating on smooth implant surface by electrodeposition

A novel process for the deposition of a hydroxyapatite (HA) coating on a smooth implant surface has been developed. Specimens were firstly subjected to electrodeposition at −1.8 V (versus Ag/AgCl) in a mixed solution of 0.042 M Ca(NO₃)₂·4H₂O and 0.025 M NH₄H₂PO₄ at 85 °C for 5 s, and then post-treated in 1 M NaOH solution for 30 min. The experimental results showed the specimens prepared by the designed process to have better adhesion properties than those prepared by the traditional electrodeposition process.     

Uptake and apparent utilization of urea and ammonium nitrate in wheat seedlings

Intact wheat (Triticum aestivum cv. Quern) seedlings that were grown in presence or absence of NH₄NO₃ were exposed to solutions containing CO(NH₂)₂, NH₄NO₃, CO(NH₂)₂ + NH₄NO₃, CO(NH₂)₂ + KNO₃ and CO(NH₂)₂ + (NH₄)₂SO₄ for consecutive periods of 3, 3, 6, 12 and 24h and N uptake determined by solution depletion measurements. Differences in ethanol-soluble N and ethanol-insoluble N content of roots and shoots of control (zero time) seedlings and seedlings exposed to CO(NH₂)₂, NH₄NO₃ and CO(NH₂)₂ + NH₄NO₃ for 48 h were used to characterize N utilization during/following uptake.Regardless of initial N status, uptake of N from CO(NH₂)₂ was less than one-third of that from NH₄NO₃. Relative absorption of the CO(NH₂)₂ and NH₄NO₃ was not substantially altered by acidity control of the uptake solutions. There was a reciprocal antagonism between uptake of CO(NH₂)₂ and uptake of NH₄NO₃. Whereas CO(NH₂)₂ inhibited NH₄ absorption in each set of seedlings, it decreased NO₃ uptake only in seedlings that had been pretreated with N. Simultaneous presence of KNO₃ enhanced CO(NH₂)₂ uptake but presence of (NH₄)₂SO₄ decreased it to the same extent as NH₄NO₃. All absorption processes involving CO(NH₂)₂ and NH₄ were substantially restricted by pretreatment of the seedlings with NH₄NO₃. The results suggest that apparent utilization of ambient N was dependent on initial N status of the seedlings and on the nature of the N species to which they were exposed.     

Electrocopolymerization of aniline and ortho-phenylenediamine via facile negative shift of polyaniline redox peaks

Oxidative electropolymerization of aniline (Ani) in phosphoric acid (H₃PO₄) on composite 2B pencil graphite was accomplished using selected inorganic salts as supporting electrolytes. These salts determined the degree of conductivity of polyaniline (PAni) formed. The conductivity was in the order of CaCl₂ > KCl > ZnCl₂ > ZnSO₄ > Ca₃(PO₄)₂. The three pairs of redox peaks in the voltammogram of PAni formed in the presence of 0.06 M Ca₃(PO₄)₂ and 0.2 M ZnSO₄ have shifted 300 mV to the negative potential. The shifting of peaks is strongly influenced by type of anions' presence in the salts. However, the nature of the available cations had no significant effect. The negative shifts of redox peaks were exploited to facilitate the electrocopolymerization of Ani and ortho-phenylenediamine (oPD). The formation of the poly(Ani-co-oPD) was confirmed by the FTIR spectra.     

Synthesis and characterization of ZnxMg1 − xGa2O4:Co fabricated by low-temperature burning method

A series of Zn_xMg_1 − xGa₂O₄:Co²⁺ spinels (x = 0, 0.25, 0.5, 0.75, and 1.0) was successfully produced through low-temperature burning method by using Mg(NO₃)₂·4H₂O, Zn(NO₃)₂·6H₂O, Ga(NO₃)₃·6H₂O, CO(NH₂)₂, NH₄NO₃, and Co(NO₃)₂·6H₂O as raw materials. The product was characterized by X-ray diffraction, transmission electron microscopy, and photoluminescence spectroscopy. The product was not merely a simple mixture of MgGa₂O₄ and ZnGa₂O₄; rather, it formed a solid solution. The lattice constant of Zn_xMg_1 − xGa₂O₄:Co²⁺ (0 ≤ x ≤ 1.0) crystals has a good linear relationship with the doping density, x. The synthesized products have high crystallinities with neat arrays. Based on an analysis of the form and position of the emission spectrum, the strong emission peak around the visible region (670 nm) can be attributed to the energy level transition [⁴T₁(⁴P) → ⁴A₂(⁴F)] of Co²⁺ in the tetrahedron. The weak emission peak in the near-infrared region can be attributed to the energy level transition [⁴T₁(⁴P) → ⁴T₂(⁴F)] of Co²⁺ in the tetrahedron.     

Synthesis of FePO4·xH2O for fabricating submicrometer structured LiFePO4/C by a co-precipitation method

Amorphous hydrated iron (III) phosphate has been synthesized by a coordinate precipitation method from equimolecular Fe(NO₃)₃ and (NH₄)₂HPO₄ solutions at an elevated temperature. Hydrated iron (III) phosphate samples and the corresponding LiFePO₄/C products were characterized by XRD and SEM. The electrochemical behavior was studied by cyclic voltammetry (CV) and electrochemical impedance spectroscopy (EIS). The LiFePO₄/C fabricated from as-synthesized FePO₄ delivered discharge capacities of 162.5, 147.3, 133.0, 114.7, 97.2, 91.3 and 88.5 mAh g⁻¹ at rates of 0.1C, 0.2C, 0.5C, 1C, 2C, 3C and 4C with satisfactory capacity retention, respectively.     

Dimensional stability under wet curing of mortars containing high amounts of nitrates and phosphates

Investigations were carried out in order to solidify in cement some aqueous streams resulting from nuclear decommissioning processes and characterized by a high salinity (300 g/L), as well as important concentrations of nitrate (150-210 g/L) and phosphate ions (0-50 g/L). Special attention was paid to the influence of these compounds on the dimensional variations under wet curing of simulated solidified waste forms. The length changes of mortars containing nitrate salts only (KNO₃, NaNO₃) were shown to be governed by a concentration effect which involved osmosis: the higher their concentration in the mixing solution, the higher the swelling. The expansion of mortars containing high amounts of phosphates (≥ 30 g/L in the mixing solution) was preceded by a shrinkage which increased with the phosphate concentration, and which could be suppressed by seeding the cement used with hydroxyapatite crystals. This transitory shrinkage was attributed to the conversion into hydroxyapatite of a precursor readily precipitated in the cement paste after mixing.