Thixotropic slurries

Dense solids may be difficult to remove completely from the bottom of crystallizer, complicating transfer to filter. Thixotropic slurries are very difficult to transfer. 

In a thixotropic slurry, the slurry will, when left undisturbed, develop a three-dimensional network structure where particles attach loosely to each other in an extremely weak gel. This holds the solid particles suspended indefinitely. The shear stress required to cause the thixotropic slurry to flow can then be broken... 

The structure can then be broken up by stirring, so that the slurry can be refluidized to a free-flowing state. The structural shear stress of the thixotropic slurry decreases with time during agitation according to the following expression ... 

Solids concentrations can vary from a few percent to well over 50% in a typical stirred tank. Solids concentration, particle shape, and the viscosity of the suspending phase are the main factors affecting the rheology and settling characteristics of the slurry. Cubic- and spherical-shaped solids tend to form Newtonian slurries, while needle-, oblong-, and plate-shaped solids form thixotropic slurries. Such slurries exhibit yield stresses even at quite low solids concentrations. This can lead to the development of caverns, as shown in Section 9.4. Proper design can usually overcome these stagnation problems. 

For thixotropic slurries, methods are used to predict start-up pressure after a shutdown of the pipeline, and the time required to clean the conduit of gelled material before resuming pumping. 

The start-up pressures for thixotropic slurries may be quite high, particularly when these slurries coagulate into gels inside the pipeline. During an initial of period of time, gels must be expelled from the pipeline. Crude oils, fuel oils, bentonite clay, and drilling... 

As more and more of the filtrate is removed, the slurry graduaUy thickens and may become thixotropic. The soHds content of the thickened slurry may be higher than that obtained with conventional pressure filtration, by as much as 10 or 20%. A range of velocity gradients from 70 to 500 L/s has been suggested as necessary to prevent cake formation and to keep the thickening slurry ia a fluid state (27). 

Extraction of Bertrandite. Bertrandite-containing tuff from the Spor Mountain deposits is wet milled to provide a thixotropic, pumpable slurry of below 840 p.m (—20 mesh) particles. This slurry is leached with sulfuric acid at temperatures near the boiling point. The resulting beryUium sulfate [13510-49-1] solution is separated from unreacted soflds by countercurrent decantation thickener operations. The solution contains 0.4—0.7 g/L Be, 4.7 g/L Al, 3—5 g/L Mg, and 1.5 g/L Fe, plus minor impurities including uranium [7440-61-1/, rare earths, zirconium [7440-67-7] titanium [7440-32-6] and zinc [7440-66-6]. Water conservation practices are essential in semiarid Utah, so the wash water introduced in the countercurrent decantation separation of beryUium solutions from soflds is utilized in the wet milling operation. 

The turbo-tray dryer can handle materials from thick slurries [1 million (N s)/m (100,000 cP) and over] to fine powders. It is not suitable for fibrous materials which mat or for doughy or tacky materials. Thin slurries can often be handled by recycle of dry product. Filter-press cakes are granulated before feeding. Thixotropic materials are red directly from a rotary filter by scoring the cake as it leaves the drum. Pastes can be extruded onto the top shelf and subjected to a hot blast of air to make them firm and free-ffowing after one revolution. 

Examples of industrial chemical waste are given in Table 13.1. Common industrial sources of pollution are given in Table 13.2. Since waste can result in pollution, the two terms are often used synonymously. Wastes can be in the form of solid, liquid or gas, or any combination, e.g. solid waste comprises liquid slurries, sludges, thixotropic solids and solids of varying particle sizes. Typical examples are given in Table 13.3. 

Let us see how thixotropic behavior relates to the phenomena of coagulation. The data shown in Figure 4.14a were obtained for a 7% slurry of carbon black in water. Curve 1 shows the results obtained immediately after the dispersion was prepared in a high-speed blender. After additional mild agitation, the results shown in curve 2 are obtained these are independent of further agitation. With shorter periods of mild agitation, a family of curves lying between 1 and 2 would be obtained. 

Flow and self-leveling characteristics of these products are governed by the rheological behavior of the slurry-like materials. At the low water-cement ratios required to ensure proper suspension of the solids, most self-leveling compositions are characterized by a yield stress and thixotropic behavior [75], To obtain selfleveling properties, the yield stress has to be reduced and this is achieved by the... 

Naphthali-Sandholm method, 404 dgorithm flowsketch, 411 Nitric acid reactor, 576 Nitrogen fixation, 574,578,588 Nitrotoluene isomers separation, 544 Noncatalytic reactions with solids, 595 Non-Newtonian liquids, 100, 103-109 Bineham. 104.105.107-109 dilatant, 103, 104 laminar flow, 108,109 pressure drop in lines, 106, 109 pseudoplaslic, 103, 104 rheopectic, 104,105 slurries, 71 thixotropic, 104-106 viscoelastic, 105, 106 Notation, 672 NPSH, pumps, 133,146 centrifugal pumps, 146 positive displacement pumps, 134, 135 various pumps, 144 NRTL equation, 475... 

---