Formation and filtration characteristics of solids generated in a high level liquid waste treatment process

The solids formation behavior in a simulated high level liquid waste (HLLW) was experimentally examined, when the simulated HLLW was treated in the ordinary way of actual HLLW treatment process. Solids formation conditions and mechanism were closely discussed. The solids formation during a concentration step can be explained by considering the formation of zirconium phosphate, phosphomolybdic acid and precipitation of strontium and barium nitrates and their solubilities. For the solids formation during the denitration step, at least four courses were observed; formation of an undissolved material by a chemical reaction with each other of solute elements (zirconium, molybdenum, tellurium) precipitation by reduction (platinum group metals) formation of hydroxide or carbonate compounds (chromium, neodymium, iron nickel, strontium, barium) and a physical adsorption to stable solid such as zirconium molybdate (nickel, strontium, barium).     

Removal of nitric acid from a simulated high level liquid waste by a safe chemical denitration

The heat and off-gas generation behavior was experimentally examined during a safe chemical denitration, pre- and mild-denitration, of simulated HLLW with a nitric acid concentration of 2 to 7.5 M. The maximum heat and off-gas generation were no more than 100 cal/s·1 and about 0.8 l/min, respectively. The solution temperature does not reach boiling temperature and no solution was squirted out from the denitration vessel. The pre-and mild-denitration technique could be considered as one of safe methods for removing nitric acid from the HLLW with various nitric acid concentrations. The pre- and mild-denitration also has an advantage to improve the filtration characteristics of precipitates produced by the denitration of simulated HLLW. The denitration of HLLW with 7.5M nitric acid concentration induced formation of “very easy-to-filter” solid. Moreover, a good filter cake washing is possible.     

Filtration characteristics of slurry formed by denitration of simulated high-level liquid waste

Precipitate formation behavior in high-level liquid waste (HLLW) and its filtration characteristics were examined experimentally, using a simulated HLLW. The amount of precipitate formed by denitration became minimum, only at about 5% of Mo, Zr, Te and Ru, if the simulated HLLW was pre-heated until the total heat input exceeded 7.9·10⁶ J/I HLLW before denitration or denitrated with the total heat input of more than 1.1·10⁷ J/I HLLW. Under these conditions, a needle-shaped precipitate with 0.51.0 m diameter and 35 m length was formed. This precipitate can be separated easily by vacuum filtration. While, fine particles of about 0.1 m diameter were precipitated during denitration, if the simulated HLLW was denitrated under the conditions the amount of newly formed precipitate was not minimum. It was difficult to separate the fine particles by vacuum filtration.     

Solid formation behavior during the conditioning of simulated high level liquid waste for transuranic elements extraction

The acidity change and solid formation in a simulated high level liquid waste (HLLW) containing precipitate were experimentally examined, when the acidity was reduced from 2M to 0.5M by denitration or simple dilution. The acidity of the simulated HLLW containing precipitate could be adjusted from 2M to around 0.5M by means of denitration or dilution, as well as the case of simulated HLLW without precipitate. The precipitation fractions of Zr, Mo and Te during denitration decreased with increasing amount of the precipitate already contained in the simulated HLLW. The amount of solid formed in the dilute simulated HLLW also decreased with increasing amount of precipitate in the simulated HLLW. Two process flow sheets for preparing HLLW for transuranic elements extraction were developed. One was a denitration process and the other a dilution process.     

Influence of urea on initiation and termination of reaction between nitric acid and formic acid

The influence of urea on initiation and termination of the reaction between nitric and formic acids was experimentally examined. The urea injection can terminate the denitration reaction in 2 to 10M salt-free nitric acid solutions and the simulated high level liquid wastes (HLLWs) with a nitric acid concentration of 2 to 6M. An excess of urea can interrupt the initiation of denitration in both simulated HLLW and salt-free nitric acid solutions. The initiation and termination of denitration are in relation with nitrous acid formation and decomposition. Urea reacts with nitrous acid easily in the denitrating solution and decomposes nitrous acid. As the urea concentration increases in the solution, the continuance of denitration become impossible because the decomposition rate of nitrous acid exceeds the generation rate. In addition, the nitrous acid concentration can not be high enough to initiate the denitration in the solution with an excess of urea because nitrous acid is decomposed by urea.     

Precipitation of Zirconium and Molybdenum in Simulated High-level Liquid Waste Concentration and Denitration Process

This paper focuses on the precipitation behaviors of zirconium and molybdenum during concentration and denitration by using simulated HLLW. The dependences of precipitates on final acidity of nitric acid, concentration factors and initial mass ratio of Mo/Zr were investigated. As a result, the presence of zirconium molybdate hydrate (ZMH) is confirmed in all experiments by the analysis of X-ray diffraction (XRD), but mass ratio of Mo/Zr in ZMH shows large deviations from the precipitate yield of Mo/Zr in solution. This is considered as a result of the generation of amorphous precipitate of Mo.     

Selective separation behavior of silica gel for zirconium in simulated high level radioactive liquid waste

Zirconium in simulated high level radioactive liquid waste (HLLW) was selectively adsorbed and separated by self-made high adsorption activity silica gel. The selective adsorption mechanism was analyzed according to the structure character of self-made silica gel and performance of zirconium in acid simulated HLLW. The results show that the adsorption selectivity of self-made silica gel for zirconium is strong, because zirconium has higher positive charge and zirconium ion hydrolyzes easily. Distribution coefficient of self-made silica gels for zirconium is 53.5 ml/g. There are 6.5 (OH)/nm² on the surface on self-made silica gels which provide more adsorption activity places, thus self-made silica gels have higher adsorption capacity for zirconium (31.4 mg/g). The elution rate of the adsorption of zirconium on self-made silica gel by 0.2 mol/l H₂C₂O₄ is more than 99%. The solubility of the self-made silica gel in nitric acid is low, the chemical stability of self-made silica gel is very strong.     

Local properties of cake in cross-flow microfiltration of submicron particles

The local properties of filter cakes, such as porosity and specific filtration resistance, in cross-flow microfiltration of submicron particles are studied based on an analysis of force. The packing of particles in a filter cake can be divided into two modes. When the solid compressive pressure is smaller than the critical value, there exists an equilibrium distance between neighbouring particles due to the electrostatic repulsive force, and the local cake porosity can be estimated by using the cell model proposed in this study. When the solid compressive pressure is greater than the critical value, the compressive force can overcome the repulsive barrier, the particles then come into contact with neighbours, and the power-type empirical relationship between cake porosity and solid compressive pressure can be employed to estimate the local cake porosity. It can be found that the half of the cake near the filter membrane has a compact structure, and a high filtration resistance within the operating conditions of this study. On the other hand, the portion of cake near the cake surface has a high porosity due to the separation of particles. By using this model, the effect of electrolyte concentration on cake properties can be analyzed, and the estimated values of average porosity and average specific filtration resistance under various electrolyte concentrations, cross-flow velocities, and filtration pressures agree fairly well with the experimental data.     

Characterization of the precipitates formed during the denitration of simulated HRLW

The denitration of several chemical compositions of simulated HRLW (highly radioactive liquid waste) was performed using formic acid as reducing agent. Precipitates formed during the denitration of simulated HRLW were analyzed using X-ray diffraction and⁵⁷Fe Mössbauer spectroscopy. Goethite and amorphous fraction were the principal phases in these precipitates. It was found that the chemical composition of simulated HRLW and the experimental conditions of denitration have more influence on the crystallinity and the particle size than on the phase composition of the precipitates. Thermal treatment of denitrated precipitates caused the solid state transformation of goethite+amorphous fraction into hematite. The values of hyperfine magnetic field (HMF) of hematite were decreased, thus indicating the substitution of Fe³⁺ ions with other metal cations.     

Crossflow microfiltration of mono-dispersed deformable particle suspension

Crossflow microfiltration of mono-dispersed deformable particles of Saccharomyces cerevisiae and Ca-alginate, and rigid PMMA particles was conducted to compare the structure of the flux-limiting layer. The effects of particle deformation due to the frictional drag and mass of the cake, and the area contact among particles on the reduction of porosity were examined to determine how these variations lead to an increase in filtration resistance. The dynamic analysis proposed by Lu and Hwang (AIChE J. 41 (1995) 1443–1455) was modified to examine cake formation during crossflow filtration of deformable particles by taking the transient effect of cake compression and the effect of the area contact between particles into consideration. In situ measurement of filter cake thickness using the infrared reflection method was applied to verify the theoretical results. Both experimental and simulated results showed that the cake formed by deformable particles exhibits a rapid increase in flow resistance or a decrease in local porosity and a high resistant limiting layer is formed next to the filter medium during filtration due to the deformation of particles.     

Modeling of Compressible Cake Filtration

The transport of suspended solid particles in a liquid through porous media has importance from the viewpoint of engineering practice and industrial applications. Deposition of solid particles on a filter cloth or on a pervious porous medium forms the filter cakes. Following a literature survey, a governing equation for the cake thickness is obtained by considering an instantaneous material balance. In addition to the conservation of mass equations for the liquid, and for suspended and captured solid particles, functional relations among porosity, permeability, and pressure are obtained from literature and solved simultaneously. Later, numerical solutions for cake porosity, pore pressure, cake permeability, velocity of solid particles, concentration of suspended solid particles, and net rate of deposition are obtained. At each instant of time, the porosity decreases throughout the cake from the surface to the filter septum where it has the smallest value. As the cake thickness increases, the trends in pressure variation are similar to data obtained by other researchers. This comparison shows the validity of the theory and the associated solution presented. A sensitivity analysis shows higher pressure values at the filter septum for a less pervious membrane. Finally, a reduction in compressibility parameter provides a thicker cake, causes more particles to be captured inside the cake, and reduces the volumetric filtrate rate. The increase of solid velocity with the reduction in compressibility parameter shows that more rigid cakes compress less.     

Pressure and concentration profiles in filter cake consisting of core/shell latex particle

Poly(styrene-co-acrylic acid) latex particles with different acrylic acid contents have been synthesized and used for filtration studies. Effective pressure and dry matter concentrations were measured at different positions in the filter cakes during the filtration processes, and dry matter concentration was not found to change significantly with effective pressure. Nevertheless, the local dry matter concentration did increase with time for latex particles containing 1 and 3%, w/w acrylic acid, which indicate that filter cake comprising latex particles with a high acrylic acid content will creep during the filtration stage. The filter cakes were examined using stepped-pressure filtration experiments as well, and an almost instantaneous deformation of the filter cake was observed after the pressure step. Furthermore, a minor deformation was observed over the following 2 h for latex particles both containing and not containing acrylic acid. This is thought to be due to the rearrangement of particles in the filter cake.     

Design parameters for rotating cylindrical filtration

Rotating cylindrical filtration displays significantly reduced plugging of filter pores and build-up of a cake layer, but the number and range of parameters that can be adjusted complicates the design of these devices. Twelve individual parameters were investigated experimentally by measuring the build-up of particles on the rotating cylindrical filter after a fixed time of operation. The build-up of particles on the filter depends on the rotational speed, the radial filtrate flow, the particle size and the gap width. Other parameters, such as suspension concentration and total flow rate are less important. Of the four mechanisms present in rotating filters to reduce pore plugging and cake build-up, axial shear, rotational shear, centrifugal sedimentation and vortical motion, the evidence suggests rotational shear is the dominant mechanism, although the other mechanisms still play minor roles. The ratio of the shear force acting parallel to the filter surface on a particle to the Stokes drag acting normal to the filter surface on the particle due to the difference between particle motion and filtrate flow can be used as a non-dimensional parameter that predicts the degree of particle build-up on the filter surface for a wide variety of filtration conditions.     

Effect of backwash on the performance of submerged membrane filtration

Particles with a mean diameter of 5 μm were filtered by a ceramic tubular membrane to study the effects of backwash on the performance of submerged membrane filtration. A periodic backwash can completely remove the formed cake, diminishing a part of membrane internal fouling, and, therefore, recover the filtration flux. In a membrane-blocking/cake formation comparable filtration system, the filtration resistance due to membrane-internal fouling is over twice as high as that due to cake formation. The irreversible filtration resistance increases progressively during operation, and it can be regressed to a power-type empirical relationship. Filtration period data were analyzed using blocking models. Membrane blocking occurs in the early filtration periods and is followed by cake filtration. The filtration flux can be simulated by employing blocking models and empirical equations for filtration resistance. The backwash effectiveness was examined by comparing filtrate productivity and washing efficiency. The calculated results of productivity under various backwash durations agree well with experimental data. An increase in backwash flux or duration leads to higher productivity, when the duration is shorter than 2 min; however, the productivity may be decreased with an increase of backwash duration due to the back pumping of more filtrate. A longer filtration time in each cycle results in higher backwash efficiency since a formed cake may efficiently prevent further membrane pore clogging and is more easily removed by a backwash. The optimal backwash conditions can be determined appropriately by the proposed method, with respect to both backwash efficiency and filtrate productivity.     

Solid formation in simulated high level liquid waste of relatively low nitric acid concentration

Solid formation in a simulated high level liquid waste (HLLW) was experimentally examined at 2M and 0.5M nitric acid concentrations. The precipitation studies were conducted by refluxing the simulated HLLW around 100°C. Zr, Mo, Te and Ru were major precipitation elements in both 2M and 0.5M HNO₃ solutions. The amount of precipitate in 2M HNO₃ solution decreased with decreasing Zr concentration and no precipitation was found in the solution without Zr. Only about 10% of Zr, Mo and Te were precipitated, if the Mo/Zr ratio in the 0.5M HNO₃ solution was kept below 0.5. Complete removal of Zr and Mo was the most effective way to prevent solid formation in the solution with 2M and 0.5M HNO₃ concentrations.     

Modeling of Fouling of Crossflow Microfiltration Membranes

Abstract

Steady-state and transient models are reviewed for predicting flux decline for crossflow microfiltration under conditions in which both external cake buildup and internal membrane fouling are contributing factors. Experimental work is not covered in the scope of this review, although reference is made to a few recent studies which have compared experimental measurements with theory. The steady-state cake thickness and permeate flux are governed by the concentration polarization layer adjacent to the cake of rejected particles which forms on the membrane surface. Depending on the characteristic particle size and the tangential shear rate, Brownian diffusion, shear-induced diffusion, or inertial lift is considered to be the dominant mechanism for particle back-transport in the polarization layer. For typical shear rates, Brownian diffusion is important for submicron particles, inertial lift is important for particles larger than approximately ten microns, and shear-induced diffusion is dominant for intermediate-sized particles. For short times, it is shown that the transient flux decline due to cake buildup is closely approximated by deadend batch filtration theory, independent of the tangential shear rate. For long times, however, the steady or quasi-steady flux increases with shear rate, because the tangential flow sweeps particles toward the filter exit and reduces cake buildup.     

Reaction of calcium and phosphate ions with titanium,zirconium, niobium,and tantalum

To understand the bone formation ability of constituent metal elements of new titanium alloys, titanium, zirconium, niobium, and tantalum, these metals were immersed in various electrolytes containing calcium and/or phosphate ions and characterized using X‐ray photoelectron spectroscopy. In addition, cathodic polarization of the metals in the electrolytes was performed to evaluate the stability of the surface oxide films on the metals in the electrolyte. The calcium phosphate layer formed on Ti in electrolytes containing calcium and phosphate ions is relatively protective against mass transfer throughout the layer. However, the zirconium phosphate layer formed on Zr is much more protective and stable than that on Ti. Therefore, calcium ions were not incorporated. Nb and Ta formed calcium phosphate, but the amount was smaller than that in Ti, because phosphates formed on Nb and Ta are somewhat protective and the incorporation of the calcium ion is inhibited. Titanium played the most important role in forming calcium phosphate, while zirconium inhibited the formation of calcium phosphate on titanium alloys. The control of bone formation is feasible by the design of titanium alloys. Copyright © 2015 John Wiley & Sons, Ltd.     

Reduction of Breakdown Pressure by Filter Cake Removal Using Thermochemical Fluids and Solvents: Experimental and Numerical Studies

The process of well cleanup involves the removal of an impermeable layer of filter cake from the face of the formation. The inefficient removal of the filter cake imposes difficulty on fracturing operations. Filter cake’s impermeable features increase the required pressure to fracture the formation. In this study, a novel method is introduced to reduce the required breakdown pressure to fracture the formation containing the water-based drilling fluid filter cake. The breakdown pressure was tested for five samples of similar properties using different solutions. A simulated borehole was drilled in the core samples. An impermeable filter cake using barite-weighted drilling fluid was built on the face of the drilled hole of each sample. The breakdown pressure for the virgin sample without damage (filter cake) was 6.9 MPa. The breakdown pressure increased to 26.7 MPa after the formation of an impermeable filter cake. Partial removal of filter cake by chelating agent reduced the breakdown pressure to 17.9 MPa. Complete dissolution of the filter cake with chelating agents resulted in the breakdown pressure approximately equivalent to the virgin rock breakdown pressure, i.e., 6.8 MPa. The combined thermochemical and chelating agent solution removed the filter cake and reduced the breakdown pressure to 3.8 MPa. Post-treatment analysis was carried out using nuclear magnetic resonance (NMR) and scratch test. NMR showed the pore size redistributions with good communication between different pores after the thermochemical removal of filter cake. At the same time, there was no communication between the different pores due to permeability impairment after filter cake formation. The diffusion coupling through NMR scans confirmed the higher interconnectivity between different pores systems after the combined thermochemical and chelating agent treatment. Compressive strength was measured from the scratch test, confirming that filter cake formation caused added strength to the rock that impacts the rock breakdown pressure. The average compressive strength of the original specimen was 44.5 MPa that increased to 73.5 MPa after the formation of filter cake. When the filter cake was partially removed, the strength was reduced to 61.7 MPa. Complete removal with chelating agents removed the extra strength that was added due to the filter cake presence. Thermochemical and chelating agents resulted in a significantly lower compressive strength of 25.3 MPa. A numerical model was created to observe the reduction in breakdown pressure due to the thermochemical treatment of the filter cake. The result presented in this study showed the engineering applications of thermochemical treatment for filter cake removal.     

A novel approach for modeling concentration polarization in crossflow membrane filtration based on the equivalence of osmotic pressure model and filtration theory

A theoretical model for prediction of permeate flux during crossflow membrane filtration of rigid hard spherical solute particles is developed. The model utilizes the equivalence of the hydrodynamic and thermodynamic principles governing the equilibrium in a concentration polarization layer. A combination of the two approaches yields an analytical expression for the permeate flux. The model predicts the local variation of permeate flux in a filtration channel, as well as provides a simple expression for the channel-averaged flux. A criterion for the formation of a filter cake is presented and is used to predict the downstream position in the filtration channel where cake layer build-up initiates. The predictions of permeate flux using the model compare remarkably well with a detailed numerical solution of the convective diffusion equation coupled with the osmotic pressure model. Based on the model, a novel graphical technique for prediction of the local permeate flux in a crossflow filtration channel has also been presented.     

Synthesis of mono and diesters using eco-friendly solid acid catalyst – zirconium titanium phosphate

Abstract

Amorphous zirconium titanium phosphate (ZTPA), an advanced inorganic ion exchanger of the class of tetravalent bimetallic acid (TBMA) salt, has been synthesized by sol gel route. ZTPA has been characterized by inductively coupled plasma atomic emission spectrometer (ICP-AES), Thermal analysis, Fourier transform infrared spectrometer (FTIR), and X-ray diffraction (XRD). The acid sites present in ZTPA indicate good potential to be explored as solid acid catalyst. The catalytic characteristics of ZTPA have been evaluated by determining surface area and surface acidity. The catalytic performance of ZTPA as a solid acid catalyst has been explored by studying esterification as a model reaction wherein mono and diesters have been synthesized. Crystalline zirconium titanium phosphate as well as amorphous and crystalline phases of zirconium phosphate and titanium phosphate have also been synthesized, characterized, and their catalytic performance investigated for comparative studies. TBMA salt exhibits enhanced catalytic activity compared to its single salt counterparts and amorphous phases exhibit higher catalytic activity compared to their crystalline phases.