Agglomeration Of Fines Research Articles

Characterization and modeling of a sub-micron milling process limited by agglomeration phenomena

The objective of this work was to characterize a sub-micron grinding process in a wet ball mill including the agglomeration of fines inside the grinding chamber. To do so a population balance model has been developed. A Dynomill ball mill was used and the grinding medium consisted of zirconium oxide beads. The product under investigation was poorly water-soluble. The particle size distribution of the initial powder ranged from 1 to 100um. Laser diffraction was used to analyze the particle size distribution.During grinding the average particle diameter of a particulate product is reduced to a minimum value. These grinding experiments showed that for a specific product this minimum value is a fixed constant within the range of tested operating conditions. This minimum average particle diameter could be influenced by modification of the particle surface properties with surface active agents.The minimum mass mean particle diameter is a result of grinding and simultaneous agglomeration of broken particles. To build a representative model, a dynamic population balance model was developed. The crystallite size as measured with X-Ray diffraction was chosen as the minimum achievable particle size by grinding. Simulations showed that the applied population balance model is an adequate tool to forecast the evolution of the particle size reduction process and the time required to reach the final product specifications.The product could be defined as “very brittle” using the approach from Kwade et al. (1996b) and Stadler et al. (1990). The shear induced by the translation movements of the grinding medium beads was thus enough to break the particles.The median particle size of the broken particles (x50) was plotted as function of the number of stress events (SN) in the mill. This number of stress events was calculated for one initial product particle. That is the representative number of stresses that needs to be applied on an initial particle to break it into fractions with the desired fine particle size. SN is proportional to the number of media contacts and their frequency, and the probability that a media contact leads to particle breakage. That frequency of media contact is defined as the angular rotation speed (ω) of the mill corrected by an efficiency factor (γF). The stress number SN is characteristic of the ground product.The efficiency factor γF describes the efficiency with which the impellor transfers its energy to the grinding medium. γF was a function of the mill design and more specifically the impellor shape. Factor γF is taken as the surface of the impellor in contact with the grinding media.This approach leads to a general representation of the grinding profile of the studied product in a stirred media mill.

Role of hydrophobicity and surface tension on shear flocculation and oil agglomeration of magnesite

This paper describes the role of hydrophobicity and surface tension on the shear flocculation and oil agglomeration of magnesite mineral. The experimental results have demonstrated that these processes were closely correlated with the particle hydrophobicity; however, they were not lowered by increasing the surface charge due to sodium oleate adsorption. Also, higher degrees of hydrophobicity were required to achieve the maximum aggregation in the oil agglomeration of magnesite fines in comparison to its shear flocculation. On the other hand, these aggregation processes decreased depending on the decrease in the contact angle with decreasing surface tension. Eventually the shear flocculation and oil agglomeration of fine particles in the suspension did not take place below a particular value of surface tension, corresponding the critical surface tension of wetting ( γ c) and the critical solution surface tension ( γ c-a) values, respectively.

Water release from fractionated stock suspension. Part 2. Effects of consistency, flocculants, shear, and order of mixing

The rate of gravity drainage from a papermaking stock suspension is highly dependent on the initial consistency of the stock, and on the presence of cellulosic fines. In this study, we fitted the results of testing to a model based on different linear contributions to drainage resistance from fibers and each type of fines. Deviations from the model at relatively high consistency were tentatively attributed to flocculation phenomena. Substantially higher rates of dewatering were achieved by selectively treating the fines, the fibers, or the combined furnish with cationic polyelectrolytes. Results were consistent with several mechanisms, which might act in parallel during ordinary papermaking. Attachment of cellulosic fines to fiber surfaces can prevent those fines from migrating to choke points within a wet web. Agglomeration of fines to each other can reduce their effective surface area. Flocculation of the fibers can make the fiber mat less uniform, thus providing larger channels for water to flow from the mat.

Sintering behaviours of chromite fines and the consolidation mechanism

A study on sintering of imported chromite fines using sinter pot (diameter 150 mm, depth 600 mm) was conducted. The effect of sintering parameters including coke breeze size and dosage, sinter mix moisture, bed depth, ignition duration and suction, and sintering suction on the sintering performance of chromite fines was investigated. It is shown that normal sintering process is suitable for agglomeration of chromite fines. And the sintering of chromite fines is characterized by wider combustion zone (75.00 mm), higher sintering temperature (1450 °C–1500 °C) and longer cooling time when compared with iron ores. Furthermore, the consolidation mechanism of chromite sinter was determined by using Leica DMRXP microscope equipped with Leica Qwin image processing and analysis application, scanning electron microscope (SEM) and energy dispersive spectrometer (EDS). It is revealed that the consolidation of chromite sinter mainly depended on about 20% liquid phase of olivine along with solid-state reactions and recrystallization of Cr 2O 3 contributing to sinter strength to some extent, which is quite different from the sintering of iron ores with about 35%–40% liquid phase.

Experimental Validation of Calcium Phosphate Precipitation Modeling in a Pellet Reactor

Calcium phosphate precipitation is studied in this article. The P-recovery process is carried out in a fluidized sand bed, the so-called pellet reactor which presents major advantages from the hydrodynamical viewpoint. The associated chemistry is yet relatively complex, due to pH gradient along the column and to the residence time of the various precipitates. The experimental observations showed three different phenomena: first, an agglomeration of fines around the sand grains is observed, second, a stagnation of fines in the bed occurs while a significant amount of fines also leaves the bed with the liquid effluent. The purpose of this work is to validate the thermodynamical model developed in our previous works on a semi-industrial sized pilot. Additional experimental runs carried out for various operating conditions showed the robustness of the model. These results open some interesting perspectives for the determination of optimized operating conditions at industrial scale.

Coal recovery from fines cleaning wastes by agglomeration with colza oil: a contribution to the environment and energy preservation

The objectives of this work were to recover high-calorific value/low-ash content coals from coal fines cleaning wastes by agglomeration with colza oil and to decrease the risk of spontaneous combustion of the dump where the coal wastes are disposed of. Three Spanish coal fines wastes from different coal cleaning plants were agglomerated with refined colza oil over a wide range of oil concentration. The results were evaluated by the percentages of organic matter recovery and ash rejection as well as the ash content of the recovered coal and the amount of organic matter remaining in the dump after oil agglomeration. It was concluded that the agglomeration of coal fines wastes with colza oil allows the recovery of significant amounts of energy that is currently wasted and reduces the risk of spontaneous combustion of coal dumps as well as the amount of coal fines wastes to be handled and disposed of.

97/03521 New applications of roller presses in coal-related technologies

The roller press was invented around the middle of the 19th century. Its objective was to develop an economic method for the agglomeration of coal fines. The first successfully operating machine was constructed by the Belgian Louiseau and was installed in a US coal briquetting plant in the late 1870s; therefore this type of equipment was later also referred to as {open_quotes}Belgian Press{close_quotes}. Until the middle of this century many manufacturers produced roller presses and a large number of plants was built, particularly in Europe, to produce mostly egg-shaped briquettes from hard coal fines with coal tar pitch or bitumen as a binder. These briquettes were used as fuel for home and industrial heating. After other, cleaner sources of energy, such as gas, oil, and electricity, became affordable, coal briquetting lost its importance. A small number of almost historic plants is still operating but construction of new ones is scarce. Only a few manufacturers of roller presses are still offering such equipment, mostly for other applications. Recently, a renewed interest in roller presses has developed in connection with innovative coal technologies. These include: Briquetting of non-coking coal for the generation of {open_quotes}form-coke{close_quotes}; production of smoke-less coal briquettes; briquetting of coalmore » for gasification or liquefication techniques; size enlargement by briquetting of fine, upgraded coals, e.g. dried sub-bituminous coal or products from advanced coal cleaning processes, to improve their transport and handling properties; agglomeration of by-products from flue gas desulfurization; and, last but not least, briquetting of coal ashes to allow disposal or beneficial use. In the following such new applications of roller presses for coal related technologies will be reviewed and features of modern machines which are required for their successful operations will be described.« less

Collection efficiencies and particle size distributions from sampling cyclones — comparison of recent theories with experimental data

AbstractThe grade collection efficiencies and particle size distributions from Stairmand‐type cyclones are compared with predictions from four theories, viz. the models of Leith and Licht (1972), Dietz (1981) Mothes and Loffler (1988) and Li and Wang (1989). It is shown that the theory of Mothes and Loffler, which recogizes different flow regions within a cyclone, coupled with a realistic estimate of the particles turbulent diffusivity, is capable of providing good estimates of both the grade‐efficiency curves and particle size distributions of the cyclone catch and/or outlet dusts. The lack of adequate theories or empirical formulae to estimate the effective turbulent diffusivity under cyclone flow seems the major factor hindering the use of the Mothes and Loffler theory for predictive purposes. High loadings and agglomeration of fines, if present, are responsible for strong departures from theoretical predictions.

The modelling of oil agglomeration of coal fines

Coal-oil agglomeration is an emerging process in the treatment of coal fines. In this paper a mathematical mode for the agglomeration growth is constructed. In the model the rate at which individual coal particles get coated with oil, and the rate at which they consequently agglomerate with other coated particles are considered as two separate steps in the agglomeration process. Parameters characterising the process are introduced and the prediction of the model for the resultant growth in agglomerate size is discussed.

Recent trends in iron and steelmaking technology

Iron and steelmaking technology, including the preparation of raw materials and utilisation of wastes, has undergone marked changes over the last five decades. The hand mining of ores/minerals has almost completely been replaced by mechanised mining. The role of mineral beneficiation and sizing has therefore increased. The current emphasis is on maximising the use of mined materials through multiple beneficiation stages and agglomeration of fines.

Acceleration of breakage rates of anthracite during grinding in a ball mill

Abstract The breakage of anthracite in tumbling ball mills exhibits an unusual behavior of acceleration at fine levels of grinding. This phenomenon was discovered by using the selection function and the primary breakage distribution parameters to predict the size distribution of anthracite using the kinetics of grinding approach. The results show that while the predicted and experimental size distributions are in close agreement up to 6 min grinding time, beyond this the predicted values fall behind the experimental in both dry and wet grinding conditions but particularly in dry grinding. Unlike anthracite, bituminous coals show hindered breakage at longer grind times due to extensive agglomeration of fines. This behavior of anthracite is examined in this paper.

Extending Present Coal Reserves with Self-Bursting Coal Pellets: ABSTRACT

A new scheme is proposed which would extend our present reserves of coal by making use of what is now a waste product. End_Page 519------------------------------ Studies show that up to 25% of the volume of coal is lost as fine coal particles in the coal mining and handling process. These fines, considered to be nontransportable and useless may be concentrated as thick deposits' in tailings ponds. Actually, if the fines can be transported (as in the case of local slurry pipelines directly to the user), they are preferred, for most coal-burning electrical power plants inject the coal into their boilers as dust (-200 mesh). This allows for greater burning efficiency. Unfortunately, since fines are rarely available, utility companies must grind all coal received. Such grinding is very expensive, frequently costing as much as the coal itself. At present, the concept of agglomeration of coal fines into coherent pellets strong and stable enough to allow transportation is being considered, and several pilot-scale operations have been undertaken. These have shown that while pelletization can be done with relative ease, the economics of the process is at best marginal. The binders considered to date include bentonite, various oils and asphalts, and organic waste. Obviously a binder which burns is preferable to one which contributes to the ash. Unfortunately, in each case, the coal pellets must be reground to dust to be injected into the furnace. If, however, a binder is employed which is both combustible and contains a small amount of water, the expensive grinding stage can be eliminated. When a pellet held together by a water-soluble polymer or other water-based binder is introduced into a hot environment (a pre-heating chamber or the boiler itself), the vapor contained in the binder vaporizes and undergoes a rapid volume increase, causing a dramatic pressure increase inside the pellet. Meanwhile, the tensile strength of the pellet is being lowered by the degradation of the binder. Once the internal pressure exceeds the ability of the pellet to contain it, the pellet bursts. This explosion reduces the pellet once again to dust, since any pellet fragments would likewise burst. No grinding is necessary, and the economic pict re of the process improves dramatically. Application of this process finds breadth when one considers the potential sources of coal fines. As environmental regulations tighten, coal-cleaning standards rise. To effectively remove organic sulfur from coal, the coal must first be crushed, and conceivably large supplies of coal dust would be available. Pelletizing might also find application between the end of a slurry pipeline and the ultimate user. Another potential source is in-situ comminution of thin, deep coal seams, which reduces the coal to small fragments before pumping it to the surface. To be sure, coal agglomeration will find widespread use in the near future, and the use of a water-based binder will make the process economically feasible. Indeed, the self-bursting concept may revolutionize the burning industry. End_of_Article - Last_Page 520------------

Theory of Clarifier Operation. IV. Orthokinetic Flocculation in Concentrated Slurries

Abstract The operation of a continuous-flow sludge blanket clarifier is simulated by use of the continuity equations. The result of including an orthokinetic flocculation term due to the small-scale turbulence generated by the falling particles is examined and compared with the results obtained when only a “catch-up” flocculation mechanism is included. Turbulence flocculation permits the agglomeration of fines and results in an increase in clarifier efficiency. The effects of hydraulic loading and influent solids volume fraction are studied.

Agglomeration of fines and dust when rocks are wetted

Agglomeration of fines and dust when rocks are wetted

The sintering process

The sintering process has a special importance for three technical disciplines: powder metallurgy, ceramics, and the agglomeration of ore fines (pelletising). All these methods have been operated empirically for some time, without obtaining a deeper insight into the nature of the process. Powder metallurgy and ceramics have also developed independently to a large extent; powder metallurgy Itself can be said to have been developed systematically since about 1900.