Filter aids admix

The resistivity of the cake, however, very often increases with time. Under the pressure of filtration or the friction of the fluid that continues to pass through the cake, the filtered particles continue to compact and reduce the area available for flow. Section 7.S.4.2 also describes the empirical methods that are used to characterize the compressibility of the cake. It is significant that the particles produced by precipitation from the brine are often highly compressible. At the same time, they have a tendency to become lodged in the pores of the filter medium. Filtration of these particles can become very difficult. The usual solution to this problem is the use of a filter aid. The filter aid is applied to the surface of the filter medium before introducing the brine. This is the precoating operation. Filter aids are selected for their desirable characteristics, and the precoat material protects the pores from penetration by filtered solids while at the same time offering little resistance to the flow. Compression of the precipitates still occurs, and so filter aid ( admix ) is also added in small quantities to the brine to improve the characteristics of the cake. 

Filter aids as well as flocculants are employed to improve the filtration characteristics of hard-to-filter suspensions. A filter aid is a finely divided solid material, consisting of hard, strong particles that are, en masse, incompressible. The most common filter aids are applied as an admix to the suspension. These include diatomaceous earth, expanded perlite, Solkafloc, fly ash, or carbon. Filter aids build up a porous, permeable, and rigid lattice structure that retains solid particles and allows the liquid to pass through. These materials are applied in small quantities in clarification or in cases where compressible solids have the potential to foul the filter medium. 

The ability of an admix to be retained on the filter medium depends on both the suspension s concentration and the filtration rate during this initial precoat stage. The same relationships for porosity and the specific resistance of the cake as functions of suspension concentration and filtration rate apply equally to filter aid applications. 

When DE is used as an admix to produce a less resistant cake, selection of the type and amounts of filter aids depends on laboratory tests with a constant pressure filter cell. A graph illustrating the effect of filter aid addition on the average flow rate is shown in Figure 22.15 (Tiller 1978). Overdose of Alter aid resulted in a decrease of filtrate rate. 

Filtration aids are employed to enhance filtration characteristics, particularly for hard-to-filter suspensions. These are normally applied as an admix to the suspensions. The role of the filter aid is to built up a porous, permeable and rigid lattice structure that assists in retaining solid particles while allowing liquid to flow through. 

Filter aids may be applied to the filtration operation in two ways. The first way is to precoat the filter medium using a precoat filter aid. The precoat is to behave as the actual filter medium. The function of such an application is to prevent the filter medium from clogging or fouling as well as to facilitate the removal of the formed cake at the end of filtration. The second way is to pretreat the suspension using a filter aid powder with a coarser size distribution prior to the filtration process. Such material is called body aid or admix. The functions of body aid are to increase the porosity of filter cake and to decrease its compressibility, resulting in a decrease in the cake resistance and in turn an increase in the filtration rate. 

The immobilized carbamates (40 pmol) were transferred to a sealable 96-well Weflon plate, and admixed with 10 pmol each of various primary or secondary amines dissolved in 400 iL of anhydrous toluene. After sealing, the plate was irradiated in a multimode microwave instrument, first generating a ramp to reach 130 °C within 45 min and then holding this temperature for an additional 15 min. After cooling, the resins were filtered with the aid of a liquid handler and the filtrates were concentrated to obtain the desired substituted ureas in good purity and reasonable yields. Anilines reacted rather sluggishly and 2-substituted benzyl carbamates afforded somewhat inferior results. 

Isolation. The fresh crushed potato sprouts are allowed to remain for 4S hours with twice their weight of 2% acetic acid. The acid solution is separated with the aid of a centrifuge, filtered with a little celite, and rendered distinctly alkaline by the addition of ammonia. After 24 hours the precipitate is separated by filtration, washed, and dried. The admixed solanidine is separated by exhaustive ether extraction and may be obtained in impure form by evaporation of the ether. The ether-insoluble portion is extracted with hot 80% ethanol and, upon concentrating and cooling the extract, the crude solanine separates. It may be recrystallized from 80% ethanol or from dioxane (39, 56). As thus purified it melts at 285° and has [a]D — 42 (dilute hydrochloric acid), —56.5° (pyridine), —60° (pyridine) (31, 62). 

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