Rake

Other designs of kilns use static shells rather than rotating shells and rely on mechanical rakes to move solid material through the reactor. 

Hearth incinerators. This type of incinerator is designed primarily to incinerate solid waste. Solids are moved through the combustion chamber mechanically using a rake. 

A close inspection under normal illumination reveals many indications of the condition of the painting and previous repairs. Also, because oil paints become more transparent with age, pentimenti, which originally would have been invisible after the overpainting, can be observed. Raking light illumination is very useful to determine the extent of cracking, distortions of the support, delaminations of the paint layers, etc. This stage of the examination is often done in close cooperation with styHstic experts. Thus, obvious problematic areas can be identified before the other tests are started. 

In the case of thickeners, the process of compaction of the flocculated material is important. The floes settle to the bottom and gradually coalesce under the weight of the material on top of them. As the bed of flocculated material compacts, water is released. Usually the bed is slowly stirred with a rotating rake to release trapped water. The concentrated slurry, called the underflow, is pumped out the bottom. Compaction can often be promoted by mixing coarse material with the substrate because it creates channels for the upward flow of water as it falls through the bed of flocculated material. The amount of compaction is critical in terms of calculating the size of the thickener needed for a particular operation. The process of compaction has been extensively reviewed in the Hterature (41,42). 

The seaweed is harvested by raking and hand-gathering. Mechanized harvesting or use of divers has met with limited success. The collected seaweed is dried mechanically in many areas and shipped to the processing plants. 

Sodium sihcate (41°Bh, 1 3.22 ratio Na20 Si02) is added in the milling operation to disperse the slime, mosdy kaolin. Dispersion also aids the grinding process. The rod mill serves to grind the ore to 0.833 mm (—20 mesh) or to the point where mica, quart2, feldspar, and iron minerals are Hberated. Cyclones, or rake, hydrauhc, or other types of classifiers, are used after grinding to produce coarse and fine mica fractions that are treated separately. 

Fig. 6. Schematic of a mechanical classifier with submerged spiral rake where (-) is the pool level and slope = 1 4 to 1 3 (27).

The common types of dryers are rotary, hearth, flash (spray), and fluidized beds (10). Hot gases are used invariably to remove moisture. The gas flow can be either cocurrent or countercurrent to the flow of soHds, the former tends to be more efficient. In the hearths, the gas flow is countercurrent as the soHds are raked down from one hearth to the next below. Flash dryers are very rapid because the soHds are exposed only briefly to the hot gases. Fluidized-bed dryers, which use hot gases to suspend the soHds, are rapid and efficient, but require elaborate dust coHection systems. These are preferred when fine soHds are involved, and are used commonly for drying fine coal. Indirect-fired dryers are used when the soHds are heat sensitive or combustible. 

Fig. 38. Various mixer types for viscous pastes (a) the change-can mixer, (b) rake mixer, (c) double-rake mixer, and (d) sigma-blade kneader.

Circular raking mechanisms are sometimes also used ia square basias with horizontal flow across the basia. Such designs have to iacorporate supplementary rake arms that reach iato the corners of the square vessel (Fig. 3). 

The center-drive mechanism and feed launder are usually supported by a walkway that extends across one-half or the whole diameter of the basin. Devices having drive mechanisms and rakes supported by a tmss across the diameter of the thickener are referred to as bridge machines. The bridge thickeners usually do not exceed 25—45 m in diameter. In thickeners with larger diameters, the drive mechanism is supported by a central column or pier and the rates are driven and supported by a drive cage. The sediment is discharged into an aimular trench around the bottom of the column. 

The rake arms are driven by fixed connections or dragged by cables or chains suspended from a drive arm that is rigidly coimected to the drive... 

The traction thickener includes a traction mechanism where the movement of the rake is suppHed by a single long arm pivoted around the center column and driven by a troUey that moves on a peripheral rail around the basin. Such units have diameters of 60—130 m. 

Fig. 5. Settling-pool classifier showing the rake and the pattern of rake motion. Incline is from 14—20° (25—35 cm, /m).

Burning Pyrites. The burning of pyrite is considerably more difficult to control than the burning of sulfur, although many of the difficulties have been overcome ia mechanical pyrite burners. The pyrite is burned on multiple trays which are subject to mechanical raking. The theoretical maximum SO2 content is 16.2 wt %, and levels of 10—14 wt % are generally attained. As much as 13 wt % of the sulfur content of the pyrite can be converted to sulfur trioxide ia these burners. In most appHcations, the separation of dust is necessary when sulfur dioxide is made from pyrite. Several methods can be employed for this, but for many purposes the use of water-spray towers is the most satisfactory. The latter method also removes some of the sulfur... 

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