Precoat quantity

Manufacturers of TLC materials and accessories are well prepared to satisfy the needs for professionally performed PLC. High-quality precoated preparative plates are available from a number of eommercial sources. Alternatively, less expensive or specialty preparative plates ean be homemade in the laboratory, and loose sorbents and coating devices ean be purehased for this purpose. More-or-less-automated devices can also be purehased for band application of higher quantities of sample solutions to preparative layers. At least for some users, sophisticated densitometric and other instrumental techniques are available as nondestructive tools for preliminary detention and identification of separated compounds in order to enhance the effieiency of their isolation. The only aid still missing, and maybe the most important of all, is a comprehensive monograph on PLC that might encourage and instruct many potential users on how to fully benefit from this very versatile, efficient, relatively inexpensive, and rather easy to use isolation and purification technique. This book was planned to fill that void. 

The material to be filtered is fed into the vessel under pressure, and separation takes place with the solids being deposited on the leaf surface, and the liquid passing through the drainage system and out of the filter. Cycle times are determined by pressure, cake capacity or batch quantity. Where particularly fine solids must be removed, a layer of precoat material may be deposited on the leaves prior to filtration, using diatomaceous earth, Perlite, or other suitable precoat materials. 

Crack and joint sealants are applied only on cracks and joints. Seal coats include hot asphalt-rubber sprayed on the surface followed by precoated aggregate. Interlayers are the application of seal coats followed by either a standard overlay or an asphaltic-rubber overlay. Asphalt-rubber, when blended with an aggregate hot mix at about 9 to 10 percent by weight, serves as a binder in the thin overlay applied to the road surface. The hot mix binder holds the greatest potential for using large quantities of scrap rubber because of the thickness and quantity of the overlay. 

Table 11.3. Precoat filtration quantity and quality of adjuvants required to treat different products (Paetzold, 1993)...

A two-layer precoat must be prepared on the filter rack prior to starting filtration. The second layer activates the filtration cycle. The first, mechanical, layer is made using a coarse adjuvant (permeability above 1 Darcy), with the possible addition of 10% of a cellulose-based product. The quantities... 

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. 

Coating speeds in excess of 305 m/min can be attained. The latex coating can be dried in forced-air or radiant-heat ovens (240,241). Multiple coats are applied, particularly in paper coating, to reduce pinholing (242). A precoat is often used on porous substances to reduce the quantity of the more expensive VDC copolymer latex needed for covering (243). The properties of a typical coating are listed in Table 23. 

Just as surfactants may determine the mobility behavior of environmental chemicals in the soil, surfactant transport may also be influenced by other accompanying substances. For example, surfactants adsorbed on layer silicates may be exchanged by ionic pesticides, as illustrated in Fig. 18 for a clay mineral precoated with DTAB [20]. The preferentially adsorbed organic dication paraquat (PQ) displaces the single charged DTAB (the substance quantities are given in equivalent quantities (eq) to clarify the comparison). For each equilibrium concentration, the associated adsorbed amounts of PQ... 

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