Optimization of the wash liquor flow rate to improve washing of pre-deliquored filter cakes

Filter cake washing is a common way of improving the quality in purity of particulate products in solid-liquid separation processes. In industrial applications, this process step is usually treated as a black box where optimization is performed through trial and error. Decisive progress has been made in academia by the development of physical models to describe the washing phenomena inside a porous structure such as filter cakes. Although some models have led to a better understanding of the process, experimental efforts are still inevitable for determining optimization strategies. This contribution is an empirical approach to investigate the influence of important process parameters of washing pre-deliquored cakes with the focus on the wash liquor and filtrate flow rate. In washing a pre-deliquored cake, low moisture content at the time of wash liquor addition can be very disadvantageous for the wash efficiency due to inhomogeneous flow. This can be the result of a poor re-wetting process while washing at low wash liquor flow rates. An increase of the flow rate was observed to be very advantageous to set higher moisture content of the filter cake and therefore enforce a homogeneous distribution of the wash liquor. A homogeneous distribution of the wash liquor inside the cake is generally required for good washing.     

A WASHING THEORY FOR UNSATURATED FILTER CAKES USING THE CAPILLARY MODEL

A rigorous washing theory is developed using the area averaging technique for unsaturated filter cakes. Limiting cases of the resulting equations are investigated. The assumptions used in the modelling are verified.

The mass transfer parameters, which are evaluated by the application of the moment analysis, are shown to be directly proportional with the volumetric flow rate of the wash liquor. Experimental results also indicate that, for a given wash liquor volumetric flow rate, the increase in the cake thickness causes an increase in the mass transfer parameters.

It should be noted that the proposed model is applicable for large particle sizes. As particle size becomes finer and the size distribution becomes wider, the model should be modified by including dead void spaces containing pockets of the mother liquor in the filter cake     

Calcium‐Ion Removal from Peat by a Mechanical Filter Cake Washing Process

This contribution aims to bridge two different fields of science, viz., geoecology and mechanical process engineering. The study reports on the application of mechanical washing processes especially filter cake washing, on calcium‐ion removal from peat, which is a natural material that is used in different fields of application such as agriculture, medicine, cosmetics, etc. The interesting properties of peat such as its porous structure and the sorption behavior influence the distribution of liquid inside the bulk and the liquid flow behavior through the porous structure. Experimental results are obtained from filtration tests using differential gas pressure in a filter nutsche. The washing efficiency is determined for different pressures and specific amounts of applied wash liquor. It is found that the water repellent phenomena that occurs when peat has been dried, affects the washing efficiency in a very negative way. The results obtained are compared to the conventional filter cake washing process.     

滤饼过滤中滤饼的洗涤及应用

对置换洗涤过程、机理、洗涤效率、洗涤滤液中溶质质量分数的变化进行了分析;导出洗涤比R为变数时,多级逆流置换洗涤的计算方程;通过对滤饼洗涤试验得出:滤饼中溶质的去除与洗涤液用量有关与其是否含溶质基本无关;同时对柠檬酸生产中的石膏滤饼的洗涤试验表明:置换洗涤效率E在R>1时,E随着R的增加而减小;对石膏滤饼多级逆流置换洗涤结果的试验值与计算值十分接近,该计算方程可用于指导工业生产。     

OPTIMIZATION OF THE LOCATIONS OF SIDE STREAMS IN A FILTER CAKE WASHING PROCESS

A macroscopic model was developed to perform optimization studies for countercurrent, non-equilibrium, displacement filter cake washing on a belt filter with incoming side streams. Each washing stage was assumed to contain a number of perfect mixing cells connected in series. The optimum locations of the side streams, which yielded the minimum exit solute concentration for the washed cake, were evaluated. Three cases were considered: Incoming side stream added to the wash liquor flow, incoming side stream added to the cake flow and incoming side streams added to both flows simultaneously. A parameter study was performed systematically to determine the effect of washing parameters on the optimum side stream locations.     

Axial dispersion models for displacement washing of packed beds of wood pulp fibres

Axial dispersion models were developed for displacement washing of black liquor from packed beds of wood pulp fibres using pure wash water. For high initial bed liquor concentrations, the simple axially dispersed plug flow model was satisfactory for describing the experimental displacement washing breakthrough curves. For low initial bed liquor concentrations, the model was modified by the addition of a source term to account for the release of adsorbed and absorbed solutes which are transferred from the fibres to the wash fluid, and which become significant at low concentrations. Equilibrium between the fibres and the surrounding liquor was assumed. The highest Peclet numbers corresponding to the highest displacement washing efficiencies were obtained at high bed heights and low bed consistencies, for any superficial velocity.     

Fourier series solution for multistage countercurrent cake washing and segregated wash effluent circulation

A Fourier series solution method was developed for solving a multistage countercurrent cake washing problem, so that the solute concentration gradient inside a cake between washing stages was preserved. The cake washing process was described by the advection-dispersion equation. The solution method was employed to explore the multistage countercurrent cake washing in a segregated and a non-segregated wash effluent circulation. The segregated wash effluent circulation gave better solute recovery than the non-segregated wash effluent circulation especially when the wash ratio was around one.     

Improved process to prepare high-purity anhydrous potassium fluoride from wet process phosphoric acid

The production of potassium fluoride (KF) through recovering fluorine from wet process phosphoric acid (WPA) is very attractive as it not only makes a profitable use of the fluorine resource but also purifies WPA to some extent. The current route to prepare KF from WPA was assessed and significantly improved. Both the purity and yield of the KF product are increased with the upgraded process. Key improvements include (1) the recovery of the KF wrapped in the silicon dioxide filter cake with controlled amount of water and (2) the removal of unreacted KOH and meta aluminate impurities in the KF mother liquor with controlled amount of fluosilicic acid solution. The suitable dosage of the wash water and fluosilicic acid solution were recommended, which can be practically controlled by measuring the density and pH of KF mother liquor, respectively. Repeated experiments have verified that the improved process, compared to the current one, successfully increases the purity of KF product from 96 to 99% and the yield from 78 to 93%.     

Some Observations on Colour Fastness to Washing with Particular Reference to Disperse Dyes on Polyester

In recent years there has been a trend away from the traditional soap-based washing tests towards methods based on commercial-type washing powders. Experience with tests based on washing powders, and the relationship between these tests and the traditional soap-based tests, are described. It is shown that the degree of staining of adjacent fabrics, particularly nylon, increases as the detergent concentration in the wash liquor is decreased. Post-stentering and the quality of fabrics used as adjacents in the wash test can have a considerable influence on the results.     

On the Relation between Filtrate Flux and Particle Concentration in Batch Cross flow Micro filtration

Abstract

The relation between the filtrate flux and particle concentration in batch cross flow micro filtration is investigated using a model based on classical filtration theory and the Kern–-Seaton theory of surface fouling. The model, which includes the effects of cake compressibility but not of membrane fouling, is solved for both laminar and turbulent tangential flows. It is found that the sole effect of cake compressibility is to reduce the flux without altering the general shape of the flux versus concentration curve. Fluxes which increase with increasing concentration are shown to be a result of enhanced cake removal due to the increased wall shear stress brought about by increased suspension viscosity. A sigmoidal relation between flux and concentration is reproduced by the model only if there is a reduction in the cake removal rate as the tangential flow regime changes from turbulent to laminar.     

The influence of temporal cake moisture content on a discontinuous washing process in the centrifugal field

In solid/liquid separation processes, filter cake washing is an essential step in improving the quality of particulate products by elimination of impurities. During cake washing and dewatering, the cake saturation changes depending on the flow conditions and it cannot always be measured and controlled accurately. This article deals with investigations on the influence of the initial and temporal cake saturation on washing PVC and silica sand particles in the centrifugal field. It was found, that high initial saturation levels and high maximum saturations during the washing process had a positive impact in inducing a high hydrostatic pressure for advective flow and enabling a homogeneous distribution of the wash water inside the cake. This was achieved by increasing the wash water flux and/or decreasing the g‐factor. A good method to obtain low final impurity quantity is the combination of washing at a low g‐factor and dewatering at a high one. © 2009 American Institute of Chemical Engineers AIChE J, 2009     

The effects of amine additives and flow rate on the performance of mixed-bed ion exchange at ultralow concentrations

Experimental data were obtained to evaluate the effects of amine additives for pH control of solution and the volumetric flow rate of feed solution on the performance of mixed-bed ion exchange for the removal of ionic impurities in solution. The experiments were performed under various temperatures and cation resin ratios by using a continuous column system with NaCl solution. The breakthrough curves of ions, plotted as the ratio of the effluent to influent concentration versus run time or treated solution volume, give detailed results about the effects of the existence of the pH controller, such as ammonia and morpholine, and the variable flow rate on the fate of each ion in the units. The experimental results show that the morpholine breakthrough occurs earlier than the ammonia breakthrough and that the effect of ammonia on both sodium and chloride exchange rates is more significant than that of morpholine. The addition of ammonia in solution results in the decrease of cation resin capacity for the sodium removal much more than the addition of morpholine. The step changes in the flow rate affect significantly the shapes of sodium and chloride breakthrough curves. The effluent concentrations of sodium and chloride change according to the flow rate. However, the effect increases with decreasing operation capacity of cation resin, while it becomes serious around the breakthrough time of chloride and negligible after the time.     

硝酸分解钠硼解石制备硼酸的工艺研究

以玻利维亚钠硼解石矿为原料,采用硝酸酸解硼矿制备硼酸。考察了钠硼解石的洗矿和酸解工艺,结果表明：在洗矿工艺中,控制液固体积质量比为6 mL/g和洗矿时间为20~30 min,氯离子的脱除率达到92.17%;在酸解工艺中,控制硝酸用量为理论用量的90%、液固质量比为5、反应温度为60 ℃和反应时间为40~60 min,硼的浸出率达到98%以上。在上述工艺条件下进行硼酸母液循环研究,结果显示,随着循环次数的增加,虽然母液中的硝酸钙和硝酸钠不断积累,其质量浓度不断增加,但硼酸的质量浓度保持稳定。合适的循环次数有利于硼的回收和母液中硝酸盐浓度的提高,使母液可以用于生产硝酸钠和碳酸钙等附加值高的副产品。     

Filterkuchenwäsche makroporöser Kieselgelpartikel

Displacement washing of filter cakes containing porous particles is a challenging task, because mother liquor is contained in the inner pores of the particles as well as in the bulk material. Filter cakes consisting of silica gel particles with pore diameters of 100 nm were cleaned by displacement washing. The total load of the filter cake can be reduced to 1 %, though, the inner pores contain about 50 % of the mother liquor. Calculations were performed to estimate the diffusion time out of a single pore, which is useful to choose the washing apparatus most suitable.     

Filter Cake Washing of Mesoporous Particles

Particle synthesis in liquid phase usually requires a subsequent washing step in order to remove undesirable impurities such as unreacted educts, byproducts, solvents and salts. In particular, the washing of mesoporous particles is a challenging task due to large specific surface areas and void volumes. The filter cake washing of mesoporous SiO₂ particles contaminated either with an adsorbing fluorescent dye or sodium sulfate is presented. The effects of wash liquid mass flow rate, mean particle size, and mean pore size on the wash efficiency were investigated. Especially the interaction between impurities and the solid surface, and thereby the initial impurity distribution within the filter cake, led to distinctive washing behaviors.     

Durchströmungswäsche gesättigter und untersättigter Filterkuchen

In large-scale processes displacement washing is a suitable method for the purification of a filter cake. Through pre-dewatering prior to the wash step undesirable mixing of suspension and wash liquid is prevented. The effect of the undersaturated cake on the washing process is investigated in a laboratory nutsch filter. Dewatering kinetics before the wash step and rewetting kinetics at the beginning of the wash step show that rewetting is significantly faster. The initial saturation of S = 1 is no longer reached. Instead an equilibrium saturation S < 1 appears. The pre-dewatering step shows no negative effect on the three areas of the washing process. However, it increases the process time.     

Radiotracer detergency testing in a horizontal-axis washing machine

Proposed regulations by the U.S. Department of Energy have spurred development of energy-efficient washing machines that utilize less water and operate with lower energy requirements than conventional machines. As a result, major changes in washing machine design are required. Among expected changes are increased use of a horizontal-axis wash tub, an increase in fabric-to-wash liquor ratio, greater surfactant concentration in the wash water, and reduced average washing temperatures. As a result, surfactants used in future detergent formulations will be required to clean effectively in this new regime while producing minimal foam. Detergency test methods utilizing radiotracer techniques have been developed to study the detergency process in energy-efficient washing machines. Detergency and redeposition of radiolabeled oily soils can be determined in a full-size horizontal-axis washing machine through scintillation counting of wash and rinse water samples. Measurements can be made after each wash process step and combined to determine total cycle detergency. This is a distinct advantage over conventional reflectance detergency methods where only total detergency at the end of the entire washing and rinsing process can be conveniently measured. Also, in contrast to indirect reflectance methods, measurements of absolute soil removal are obtained with the radiotracer method. In this study, soil redeposition was determined by measuring residual radioactivity on fabric swatches and then performing a material balance on the oily soil.     

Use of Ultrasound in the Washing Process of Titania Pigment Production: Water Saving and Process Optimization

Washing of metatitanic acid (H₂TiO₃) consumes a large amount of water during the production of titanium dioxide (TiO₂) by the sulfate process. In this work, ultrasound technology is used for the pretreatment of a metatitanic acid slurry before washing. The water consumptions of washing process are compared in the absence and presence of ultrasound treatment. The effects of wash liquor properties, ultrasound time, and ultrasound power on removal of iron impurities are investigated. The washed product was characterized by X-ray diffraction, scanning electron microscopy, and particle size distribution. The effects of ultrasound on the crystal structure of TiO₂ and the mechanism of water saving are discussed. The results show that better removal of iron impurities from metatitanic acid is achieved using ultrasound. The saving wash liquor when using ultrasound was 93 mL for washing a 100 mL metatitanic acid slurry when using a pH 2 wash liquor, ultrasonic time of 30 min, and 185 W ultrasonic power. The average particle size was reduced from 2.11 to 1.02 µm, which improves washing performance. The crystal lattice parameters of TiO₂ exhibited no effect on rutile type or quality of product.     

A cake-washing model with an overall cation transfer in kraft pulp washing

Cake washing of freely mobile ions was described by an advection–dispersion equation combined with the Donnan equilibrium and an overall ion transfer model between the external liquid phase and the fiber phase of kraft pulp. The ion exchange rate coefficients between the phases were determined from the earlier published leaching test results for an oxygen delignified Scandinavian hardwood kraft pulp. The time constant of the first order process model was about 19 s for calcium and manganese and about 6 s for sodium and magnesium. The calculated results for various elements during the acid displacement were compared with the published displacement experiments. Calculated concentrations and pH profiles in wash effluent and cake were shown. Moreover, changes in pH values and redistribution of ions between the external phase and the fiber phase in the cake during the acid displacement were also demonstrated.