Solid Liquid Separation: Equipment Selection and Process Design
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Tarleton , Richard J. An innovative computer programme for analysing filtration data and filter calculation.
Liquid-Solid Separation Testing | Mining | SGS Peru
The application of mechatronic principles in pressure filtration and its impact on filter simulation, Filtration. Tarleton , R. Measuring concentration and pressure profiles in dead-end filtration. Johansson , H.
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Manufacturer warranty may not apply Learn more about Amazon Global Store. No customer reviews. Share your thoughts with other customers. Write a customer review. Discover the best of shopping and entertainment with Amazon Prime. Prime members enjoy FREE Delivery on millions of eligible domestic and international items, in addition to exclusive access to movies, TV shows, and more. The products are cost effective, but have to be used at relatively high dose levels, some also, as part of their mode of action, precipitate and add solids to the system, thus adding to the volume of solids to be disposed of.
They are particularly effective in dilute solids systems, e. A further specialised group of products are the polyaluminium chlorides PAC. These are particularly effective inorganic coagulants and are generally used in similar situations to the more traditional products described above. In solution they form a net structure that assists in solids capture and they have a tendency to form larger floes than aluminium sulphate, the closest comparison from the standard inorganic coagulants Malhotra, In addition PAC tends to be more dose efficient than aluminium sulphate.
Due to the normal overall negative charge of most systems these organic coagulants are usually highly cationic in nature. There are a number of different chemical types but most common are based on either quatemised polyamines or polyDADMAC poly diallyl dimethyl ammonium chloride. Rather than reducing the overall negative charge as occurs with the inorganic flocculants, it is believed that these products work by a charge patch mechanism.
Computer based selection of solid/liquid separation equipment
This is shown diagrammatically in Figure 2. Organic coagulants are useful in applications similar to those described in Section 2. They tend not to be effective in heavy metals removal but can be useful in removal of colour humates from drinking water Greville, and some textile effluents. Organic coagulants are also, at times, used in combination with high molecular weight flocculants. In particular, the addition of a coagulant improves filtrate and centrate clarities, when the flocculant treatment alone is unable to attain acceptable levels.
A further common combination treatment is the pressure belt filtration of mineral slurries, where the coagulant is added after an anionic flocculant. This is in contrast to coagulants previously discussed that affect each particle independently of the others.
This is the most likely mechanism occurring when treating organic materials such as sewage sludge or paper effluent with cationic flocculants. It is likely that non-ionic products such as polyacrylamide and polyethylene oxide adsorb via this mechanism. Whilst the individual hydrogen bond may be quite weak the overall adsorption can be strong due to the relatively high number of bonds that can be formed.
This is the main reason why it is possible to flocculate mineral particles that carry an overall negative charge with anionic flocculants. Each of these modes of adsorption is illustrated in Figure 2. As time progresses and greater adsorption occurs then the particles are drawn closer and closer together forming a floe. The size of the floe formed is dependent upon the particle size of the material being treated and the characteristics of the flocculant applied.
The effect of the flocculant characteristics is described in more detail below. In the vast majority of cases flocculation is an almost instantaneous effect, the limiting factor tends to be the rate at which the substrate which at times can be viscous and the polymer solution also often viscous mix together homogeneously. It should also be noted that continued mixing after floe formation will cause a gradual and permanent breakdown of the floe. It is unlikely that the original substrate particle size will be reached but such breakdown can result in the requirement for higher flocculant doses than would otherwise be required.
By and large they have been superseded by synthetic products, as described below but there are still niche markets where the fact that they are natural products can have benefit, for example in treating materials that may be included in animal feedstuffs. Significant changes to the products have taken place over this period of time, probably the most noticeable being that of increasing molecular weight Mohammed et al, Such polymers lead to commercially desirable dose efficient dewatering compared to lower molecular weight analogues Weir et al, The initial products probably had molecular weights in the region of one million, whilst today many of the products used probably have molecular weights of 25 million or more.
In fact it is very difficult to actually state molecular weight with any accuracy as it defies even the most sophisticated of analytical instruments to obtain a true measure. The poly aery lamide group of reagents have considerable widespread use in industries ranging from drinking water treatment to sewage treatment and from paper making to copper extraction. The secret to their success is their versatility, and this is mainly due to the fact that it is possible to copolymerise acrylamide, which is nonionic, with ionic monomers whether anionic or cationic in any ratio to give products tailored to specific substrates.
There are other polymer types, such as polyethylene oxide, that are used and that have niche applications but it is the polyacrylamides that currently predominate. The variables that can be controlled by the polymer chemist are ionicity, molecular weight and structure and these are described in further detail below. When considering possible flocculant choice then it is the chemical characteristics of the substrate that dictates what ionic character is likely to be the optimum. Perhaps surprisingly, the chemical constituents that affect optimum ionicity are not always the solid particles i.
This can be illustrated in its simplest form by evaluating products on a kaolinite slurry. Due to the complex nature of suspensions it is difficult to provide hard and fast rules in terms of optimum ionic content for specific substrates, however there is some general guidance that can be given. This is presented in Figure 2. The molecular weight of products can be modified by the polymer technologist in a variety of ways. All of these can affect the average molecular weight and the molecular weight distribution of the polymer produced.
The average molecular weight can range from a few thousand up to 25 million or more, but it is only products that have an average molecular weight of at least one million that will act as flocculants. However, what will change is the optimum molecular weight. A general approach is that the more rigorous and shear intensive the separation process used then the higher the molecular weight and the higher degree of structure is required.
There are two main areas where surfactants do find application.
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Firstly as dewatering aids, mainly in vacuum filtration and in batch pressure filtration where their effectiveness is derived from an ability to lower the surface tension of the liquor and thus enhance the ease of water removal Besra et al, Details of the mechanism involved has been published by Pearse and Allen There has been little development in this area in recent times and the sulphosuccinates still prevail in this application. The other area of use is that of foam control. Foam can be an environmental nuisance as it can become airborne, it can also trap solid fines.
In some cases it is the fines themselves that stabilise the foam. A variety of antifoams and defoamers are available to help prevent foams forming or destabilise foams once they have formed. It should be noted that most of the test methods described are batch tests and these are being used to simulate what, in most cases, are continuous processes.
If this is not possible then careful thought needs to be given to experimental design. If several different batches of substrate have to be used then it is recommended that products are directly compared at similar doses on the same batch of substrate, rather than using a different batch for each product. The number of "control" tests required also needs some careful consideration. If it is rapid then it may be necessary to use different batches of sample since, in this case, the ageing effects could be more significant than any batch-to-batch variation.
One particular problem area is that of liquor re-circulation. In closed circuits the liquor being returned can contain some of the pre-treatment chemicals that are under evaluation, so it can prove to be very difficult to obtain samples that are truly untreated. It is also desirable to take the sample from a fast flowing vertically downward or upward flow pipe as opposed to a horizontal pipe or open channel. This is due to the increased possibility of sedimentation or stratifying in a horizontal pipe, particularly in a slow flowing region.
A typical example would be that if there is insufficient mixing during sub sampling then there is likely to be a gradual increase in suspended solids content due to settlement, particularly of coarse material. Temperature can affect the speed of the separation process itself, the product of choice and the final results obtained in terms of cake solids, etc.
Where the substrate is treated at elevated temperatures it is recommended that, as well as conducting the test work at the appropriate temperature, the substrate is not allowed to cool between sampling and testing.