Cooling thickening

When cooked, fresh pastes are light to dark buff in appearance, depending upon the degree of conversion. As the pastes cool to room temperature they turn opaque, becoming nearly white. Low conversion white dextrins have little viscosity stability, and when cooled, thicken or set-back to form soft gels. By contrast, high conversion white dextrins cook-up to form pastes that congeal less, are more fluid, and exhibit better viscosity stability. 

Agar occurs as a cell-wall constituent of the red marine algae Rho ophyceae, from which it is extracted by hot water, and marketed as a dry powder, flakes, or strips. It dissolves in hot water and sets on cooling to a jelly at a concentration as low as 0-5%. Its chief uses are as a solid medium for cultivating micro-organisms, as a thickener, emulsion stabilizer in the food industry and as a laxative. 

In the post-dispersion process, the soHd phenoHc resin is added to a mixture of water, cosolvent, and dispersant at high shear mixing, possibly with heating. The cosolvent, frequently an alcohol or glycol ether, and heat soften the resin and permit small particles to form. On cooling, the resin particles, stabilized by dispersant and perhaps thickener, harden and resist settling and agglomeration. Both resole and novolak resins have been made by this process (25). 

For products intended to remain stable dispersions for an extended period, a particle size of 2 p.m or less is desirable. A thickening agent is usuaUy added after the reaction has been completed and the mixture is cooled in order to prevent settling and agglomeration. Examples of thickeners are guar gum, xanthan gum, and hydroxyethylceUulose. The final products are generaUy between 40 and 50% soUds, with a viscosity of 1500 5000 mPa-s(=cP). 

Lateritic Ores. The process used at the Nicaro plant in Cuba requires that the dried ore be roasted in a reducing atmosphere of carbon monoxide at 760°C for 90 minutes. The reduced ore is cooled and discharged into an ammoniacal leaching solution. Nickel and cobalt are held in solution until the soflds are precipitated. The solution is then thickened, filtered, and steam heated to eliminate the ammonia. Nickel and cobalt are precipitated from solution as carbonates and sulfates. This method (8) has several disadvantages (/) a relatively high reduction temperature and a long reaction time (2) formation of nickel oxides (J) a low recovery of nickel and the contamination of nickel with cobalt and (4) low cobalt recovery. Modifications to this process have been proposed but all include the undesirable high 760°C reduction temperature (9). 

This type of equipment can also be used for apphcations in which the only heat removed is that required for adiabatic cooling of the incoming feed solution. When this is done and the fines-destriiction feature is to be employed, a stream of hqiiid must be withdrawn from the settling zone of the ciystaUizer and the fine ciystals must be separated or destroyed by some means other than heat addition—for example, either dUiition or thickening and plwsical separation. 

In some crystalhzation apphcations it is desirable to increase the solids content of the shiny within the body above the natural consis-tencw, which is that developed by equilibrium cooling of the incoming feed solution to the final temperature. This can be done by withdrawing a stream of mother liquor from the baffle zone, thereby thickening the shiny within the growing zone of the crystallizer. This mother liquor is also available for removal of fine ciystals for size control of the product. 

For many bonding applications a variety of adhesives can perform adequately. Hot melt adhesives are normally chosen where process speed is critical. Since hot melts have no carrier vehicle (solvent or water), and thicken rapidly as they cool, they are limited in their ability to (1) penetrate low porosity substrates or wet out very rough surfaces (2) cut through or imbibe surface contaminants and (3) wet out high thermal conductivity substrates (e.g. metals). Nonetheless, hot melts are increasingly the adhesive of choice in automated production environments because of their fast set speed. 

In a 5-I. round-bottom flask, fitted with an efficient mechanical stirrer and surrounded by an ice-salt mixture, is placed a solution of 121 g. (0.5 mole) of pure dry finely-powdered benzoylperoxide (m.p. 104°) (Note 1) in about 1.51. of dry toluene. The temperature should be below —5°. A solution of sodium ethylate, prepared by dissolving 23 g. (1 mole) of sodium in 500 cc. of absolute alcohol, and cooled to o°, is introduced from a separatory funnel while the mixture is vigorously stirred. This operation should not require more than eight or ten minutes. The liquid whitens and thickens considerably on account of the formation of sodium benzoylperoxide. 

Wunderlich30 and Zubov33 suppose that ECC under high pressures occur as a result of an isothermal thickening of folded-chain lamellae. However, this contradicts the later data of Wunderlich and of Japanese authors31 who have shown that folded-chain crystals (FCC) are formed after ECC, when the melt is cooled. According to Kawai22, crystallization under hydrostatic compression can he considered as a variant of the bicomponent crystallization. 

The seal is made by cutting 1-cm diameter tubing to give one piece with a square end. Near the end of the other piece a spindle is drawn. The shoulder of the spindle is heated in a small hot flame so that it thickens, and then, with a fairly cool flame, the spindle beyond the shoulder is drawn out to a fine capillary about 1 mm diameter. This is heated in a very cool flame and bent first one way and then the other, as m Figure 47, II. The end is finally sealed off. The tip thus prepared is inserted into the square end of the other piece of tubing, as in Figure 47, /, and a joint is made by directing a small flame at... 

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