Dry processing

Fig. 5. Schematic diagram of an extractive drying process that produces aerogels at ambient pressure. Reproduced from Ref. 49.

Dry-Process Hardboard. Dry-process hardboard is produced by a dry—dry system where dry fiber is formed iato mats, which are thea pressed ia a dry coaditioa. A flow diagram of this process is showa ia Figure 6. Ia this process, wood chips, sawdust, or other residues are refiaed to fiber ia pressurized refiners. Wax and PF resia may be added ia the refiner or ioimediately outside of the refiner, ia the fiber-ejectioa tube or "blowliae." It is also aoted that a small amouat of dry-process hardboard is made with UF resia biaders. UF resias, because of their inherent faster curing at lower temperatures, can be added only at the blowline or ia a bleader located after the dryer. 

Fig. 6. The dry-process hardboard (and medium-deasity fiberboard) process.

The mats are moved along the line to the press loader. When the loader is filled and the press opens to remove the load of freshly pressed boards, the loader pushes the new boards into the unloader and deposits the load of mats on the press platens. The press closes as quickly as possible to the desired panel thickness. More pressure, as much as 4.8—6.9 MPa (700—1000 psi) is required to press high density dry-process hardboard, because the dry fiber exhibits much more resistance to compression and densification than wet fiber. Press temperatures are also higher, in the range of 220—246°C. No screens are used in the dry-process, but the moisture in the mats requires a breathe cycle during pressing to avoid blowing the boards apart at the end of the cycle. Because no screens are used, the products are called smooth-two-sides (S-2-S), in contrast to the wet-process boards, which have a screen pattern embossed into the back side and are known as smooth-one-side (S-l-S). 

The manufacture of MDF, with a few exceptions, dupHcates the manufacture methods for dry-process hardboard, described at length hereia. One exception to it is that most MDF is made ia the medium-density range, 640—800 kg/m although small amounts are made at lower or higher densities. Second, the vast majority of MDF is made with UF resia adhesives with resia requhemeats ia the 7—11% range, and wax is usually added at the 0.50—0.75% level. A small amount of exterior-grade MDF is made with isocyanate resia. 

The copolymer latex can be used "as is" for blending with other latexes, such as in the preparation of ABS, or the copolymer can be recovered by coagulation. The addition of electrolyte or free2ing will break the latex and allow the polymer to be recovered, washed, and dried. Process refinements have been made to avoid the difficulties of fine particles during recovery (65—67). 

Wet/dry process. Lime slurry absorbs SO2 in vertical spray dryer forming CaSO —CaS, H2O evaporated before droplets reach... 

Solventless Extrusion Process. The solvendess process for making double-base propellants has been used ia the United States primarily for the manufacture of rocket propellant grains having web thickness from ca 1.35 to 15 cm and for thin-sheet mortar (M8) propellant. The process offers such advantages as minimal dimensional changes after extmsion, the elimination of the drying process, and better long-term baUistic uniformity because there is no loss of volatile solvent. The composition and properties of typical double-base solvent extmded rocket and mortar propellant are Hsted ia Table... 

The manufacture of cryoHte is commonly iategrated with the production of alumina hydrate and aluminum trifluoride. The iatermediate stream of sodium aluminate from the Bayer alumina hydrate process can be used along with aqueous hydrofluoric acid, hydrogen fluoride kiln gases, or hydrogen fluoride-rich effluent from dry-process aluminum trifluoride manufacture. 

The formation of the metallic salts is a pyrometaHurgical process, and is commonly referred to as the dry process. The separation of the salts from each other is accompHshed by selective dissolution in water, and is named the wet process. 

For fine pulverization, both dry and wet processes are utilized, but increasingly the dry process is more popular because wet grinding ultimately requires drying and is much more energy intensive. A sensitive fan swirls the dust sizes into the air separator and permits coarse particles to recycle to the grinding mill or be rejected as tailings the fines are drawn into cyclones where the dust is collected. 

To avoid generation of waste brines and the associated serious problem of brine disposal, the potash industry in the former FRG began converting some operations to electrostatic separation, a dry process for separating potassium salts from other soluble salts (24,25). 

Overlay Proofing. Overlay proofing systems can be categorized as wet- or dry-processed systems. The negative working wet-processed systems are generally composed of polymeric diazo resin salts (haUdes or heavy metal), which after photolysis form an insoluble adduct. 

The dry-processed, peel-apart system (Fig. 8b) used for negative surprint apphcations (39,44) is analogous to the peel-apart system described for the oveday proofing apphcation (see Fig. 7) except that the photopolymer layer does not contain added colorant. The same steps ate requited to produce the image. The peel-apart system rehes on the adhesion balance that results after each exposure and coversheet removal of the sequentially laminated layer. Each peel step is followed by the apphcation of the appropriate process-colored toners on a tacky adhesive to produce the image from the negative separations. The mechanism of the peel-apart process has been described in a viscoelastic model (45—51) and is shown in Figure 8c. 

Simultaneous heat and mass transfer also occurs in drying processes, chemical reaction steps, evaporation, crystallisation, and distillation. In all of these operations transfer rates are usually fixed empirically. The process can be evaluated using either the heat- or mass-transfer equations. However, if the process mechanism is to be fully understood, both the heat and mass transfer must be described. Where that has been done, improvements in the engineering of the process usually result (see Process energy conservation). 

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