Precipitation Rate of Calcium Sulphate Crystals

Results of experiments with continuous neutralisation of sulphuric acid with milk of lime have been used for the calculation of the nucleation and growth rates of calcium sulphate dihydrate. The treatment is based on experimental values of the mass of deposited crystals and of the supersaturation measured for varying experimental conditions. The data so obtained are compared with values published in the literature.     

Precipitation of Strontium Sulphate

Precipitation of unseeded supersaturated aqueous solutions of SrSO₄ and growth of SrSO₄ crystals at the size of approximately 20 μm were studied. Within the examined range of supersaturations, i.e. S_a < 16, the nucleation in aqueous solutions is controlled by the heterogeneous nucleation mechanism. The induction period of precipitation depends on the rates of both the nucleation and growth of the formed nuclei. The growth of SrSO₄ crystals in supersaturated solutions at S ⪅ 5.6 is of the 2nd order with respect to the concentration and at temperatures 25 ÷ 45°C the process is controlled by the surface reaction mechanism. The number of crystals in the precipitated slurry is decreasing rapidly as a result of the Ostwald maturing.     

Semi-Batch Precipitation of Calcium Oxalate Monohydrate

Calcium oxalate monohydrate (COM) formation on the semi-batch precipitation of CaCl₂ and Na₂C₂O₄ solutions in the stoichiometric ratio carried out at pH = 6 and 37 or 70°C, respectively, was studied. When a certain level of supersaturation in the system is reached, individual COM crystals of a rather uniform size are formed. These crystals then grown and form ‘loose’ agglomerates that later develop into compact and spherical particles. The particle size distribution (PSD) rapidly shifts during early stages of precipitation towards larger sizes as a result of crystal growth and agglomeration. Later the PSD reaches a shape and position on the size axis that remain virtually constant with progressing precipitation. COM agglomerates consisting of mainly intergrown crystals are formed by mechanism of primary and secondary agglomeration. The primary agglomeration can constitute an important factor in urolithiasis.     

Batch Precipitation of Potash Alum Crystals by Mixing Aqueous Solutions of the Component Salts

The effects of temperature, supersaturation, excess of reacting component salts and method of precipitation on the batchwise precipitation of potash alum crystals, by mixing aqueous solutions of potassium sulphate and aluminium sulphate, have been investigated. From the primary characteristics of the process (crystal size, size distribution and yield) relative crystal growth rates and relative nucleation rates are calculated and interrelation of these rates with the corresponding supersaturation leads to a value of the nucleation order. It is concluded that secondary nucleation plays an important role in the batch precipitation of potash alum crystals.     

Growth Kinetics of Adenine Sulphate Crystals

The kinetics of growth and dissolution of adenine sulphate in 1.5 mol · 1⁻¹ H₂SO₄ at different super- and undersaturations has been studied. The solubility of adenine sulphate in H₂SO₄ was determined over the temperature range 20–50 °C. The growth rate of the (100) face is by about 25% higher than that of (111) and the crystal habit is changed by a high supersaturation almost to a bipyramide. The surface diffusion was determined as a prevailing processes and the BaS model gives the best fit with experimental data.     

Growth and Kinetic Measurements of Triglycine Sulphate Crystals

Synthesis and two-step recrystallisation have been used to produce TGS basic substance. Kinetic measurements have been performed for (110)-type face on a large scale of supersaturations up to about 3%. Evidences of impurity influence for small supersaturation and the “coverage” effect has been estimated.     

Nucleation Studies in Supersaturated Triglycine Sulphate Solutions

Rates of nucleation of supersaturated aqueous solutions of Triglycine sulphate are measured by the induction period over the temperature range 30–50 °C. The effects of temperature and supersaturation on the induction period are reported. The interfacial tension, energy of formation and radius of the critical nuclei of the crystal are calculated in accordance with classical nucleation theory. Nucleation rate increases with increase in supersaturation and temperature, while interfacial tension decreases with increase in temperature.     

On Isotropic Points in Potassium Ammonium Sulphate Crystals

Crystallography Reports - An effect of uniaxial pressure on the spectral and temperature dependences of birefringence of K1.75[NH4]0.25SO4 single crystals has been studied. Different changes in the...     

Precipitation of magnesium hydroxide

Mg(OH)₂ was precipitated from MgCl₂ and NaOH solutions in a continuous agitated reactor and the effects of the solutions concentration, residence time of suspension in the reactor and stoichiometric ratio of the reacting components on the properties of Mg(OH)₂ were studied. The properties of the resulting suspension were evaluated on the basis of its filtration resistance α as determined by a filtration test. The value of α is inversely proportional to mean residence time of the suspension in the reactor and it depends, at the same time, on the ratio of concentration of the precipitated solutions γ = [MgCl₂]/[NaOH]. A critical value of γ can be assessed for a continuous reactor and when this value is exceeded a comparatively well filtering suspension with α ≦ 10¹² m · kg⁻¹ is always obtained. The relationship between α and the parameters of the precipitation process was expressed quantitatively within the framework of the exiting theories.     

Effect of Selected Parameters on Crystallization of Copper Sulphate Pentahydrate

Different batch cooling modes (quick and slow cooling with constant cooling rate, program-med cooling with nearly constant supersaturation) of copper sulphate aqueous solutions have been studied in order to find best conditions for the investigation of the effect of additives on crystallization. Three types of additives (solvents, ionic substances and surfactants) have been used and their effect on crystal size, habit and yield was studied.     

Some Factors Influencing the Rate of Heterogeneous Nucleation of Strontium Sulphate

The effect of the agitation intensity on nucleation, agglomeration, ripening, and recrystallization of SrSO₄ crystals prepared by batch precipitation of equimolar solutions of SrCl₂ and Na₂SO₄ (0.01, 0.015, and 0.02 M) at 25 °C was investigated. The dependence of the SrSO₄ crystals shape and number on the time of the suspension ageing was also studied.     

Influence of the pH-Value on the Metastable Zone Width of Potassium Sulphate

Aqueous solution of the potassium sulphate with addition of sulphuric acid or potassium hydroxide has been chosen for the metastable zone width measurements in order to check how the solution structure affects the nucleation. Hydration analysis of corresponding sub-systems indicated involved structure changes. The solubility of K₂SO₄ increases with addition of H₂SO₄ and decreases with addition of KOH. The hydration analysis has shown that low acid concentrations have an inexpressive effect on the structure of solution. Much distinct effect exhibit low concentrations of the hydroxide. The metastable zone width decreases with increasing the value of pH. This decrease is less perceptible in the acidic region but more distinct in the alkaline region. The same holds for the critical nucleus size L* and the specific surface energy σ*. The behaviour quoted above supports the conception that the pH affects the solution structure and that the latter has a significant effect on the nucleation.     

Precipitation of Catalyst Precursors Theoretical Fundamentals

Very high supersaturations, low stability of highly supersaturated solutions, marked secondary changes in the solid phase formed, chemical changes during the ageing of precipitates — this are reasons why our contemporary knowledge on precipitation is far more limited and is also inconsistent compared with mass crystallization of readily soluble substances. In view of this situation, it seems to be highly desirable to show the general fundamentals which could lead us to systematic understanding of the various cases apparently unrelated with each other.     

Effect of Doping and Irradiation on Optical Parameters of Triglycine Sulphate Single Crystals

Crystals of triglycine sulphate (TGS) doped with orthonitroaniline (ONA) irradiated with different doses of gamma-radiation were used to investigate gamma-radiation effect on electrical and optical parameters. The absorption coefficient and the reflectance were measured and hence the extinction coefficient, the optical refractive index and the dielectric constants (e_r′, e_r″) of unirradiated and irradiated TGS crystals were calculated. Values of the allowed indirect optical energy gap E_g^opt. of TGS doped with ONA were calculated as a function of gamma-dose. Value of E_g^opt. decreases from 4.72 eV to 4.25 eV with increasing gamma-doses from 0 up to 3 Mrad. The values of the static dielectric constant e_r(0) and the effective electrical conductivity sigma₀ at room temperature and their dependence on gamma-dose were also calculated.     

Precipitation of Alkaline-Earth Metal Silicate Hydrates from Aqueous Solution: Ionic Equilibria,Crystalline Phases and Precipitation Mechanisms

The precipitations of magnesium, calcium, strontium, barium and zinc silicate hydrates from aqueous solutions and suspensions at ambient temperature to 200 °C are surveyed. The relevant ionic equilibria (silicate and polysilicate anion formation, hydroxocation formation, alkaline-earth metal silicate hydrate and hydroxide precipitation from supersaturated solution) that may influence these precipitations are examined. - The microcrystalline and crystalline phases precipitated in systems of different cation/silicate compositions and temperatures are tabulated. Precipitation mechanisms are analysed.     

Precipitation of Stoichiometric Hydroxyapatite by a Continuous Method

In this paper we present the precipitation of hydroxyapatite (HA), Ca₅(OH)(PO₄)₃, from highly concentrated CaCl₂ and K₂HPO₄ solutions, carried out by a continuous method in a MSMPR reactor. The procedure consists of adding the reagents in a ratio Ca to P equal to 1.67, maintaining a temperature of 85 °C, inert N2 atmosphere inside the reactor, and monitoring and adjusting automatically the pH by means of a pH‐stat system (pH = 9.0 ∓ 0.1). Under these conditions HA with a Ca to P ratio equal or close to the stoichiometric composition (Ca/P=1.667), with a high yield (up to 99%) and a high production rate (up to 1.17 g/l.min) is obtained at steady state. The CSD, morphology, crystallinity of the precipitates and impurities present fit the requirement for its biomedical applications.     

Precipitation of supersaturated chromium solutions in gallium arsenide

Transmission electron microscopy has been used to investigate precipitation processes in gallium arsenide doped with chromium and Cr₂O₃. Whatever the doping technique, at concentrations of chromium 4 × 10¹⁶ cm⁻³ the solid solution was observed to contain precipitates which grew, at least partially, due to the migration of substitutional atoms and their transformation into the precipitates. The process is accompanied by generation of vacancies from dislocation loops.     

EPR and Optical Absorption Spectra of Cr3+ Ions in Lithium Sulphate Single Crystals

Optical absorption and EPR spectra of Li₂SO₄ · H₂O crystals doped with Cr³⁺ are studied at liquid nitrogen temperature. The bands are found in absorption spectra with maxima about 17000, 23 800 and 37 200 cm⁻¹, assigned to the ⁴A₂ → ⁴T₂, ⁴A₂ → ⁴T₁ and ⁴A₂ → ⁴T₁ (⁴P) transitions, respectively. The crystal field theory parameters were determined and appeared to be as follows: Dq = 1700 cm⁻¹, B = 667 cm⁻¹, C = 3002 cm⁻¹. The lines resulting from Cr³⁺ ions are found in EPR spectra. All lines are doublets, which is indicative of presence of two magnetically unequivalent centre positions, and have the hyperfine structure resulting from interaction of the unpaired electron spin with Cr⁵³ isotope nucleus. Centres are oriented in such a way, that z-axes, corresponding to two centre positions, are situated at both sides of a-axis at an angle of about 3°. Spin Hamiltonian parameters were found as follows: g_x = 1.985, g_y = 1.984, g_z = 1.988, D = 0.130 cm⁻¹, E = 0.016 cm⁻¹, |A| = 17.8 · 10⁻⁴ cm⁻¹.