Drum thickeners

Machinery selection is done according to technological and economic requirements. 

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On leaving the dispersed air flotation cell, the de-inked stock is further diluted before passing through another fine screening process to remove small solids. The materials are then pumped to a drum thickener prior to he dispersion stage. 

Following de-inking, the stock is pumped to a second drum thickener from which, at around 6% concentration, it is available to be blended with other constituents, if required, ahead of the papermaking process. 

Ammonium sulfate is produced as a caprolactam by-product from the petrochemical industry, as a coke by-product, and synthetically through reaction of ammonia with sulfuric acid. Only the third process is covered in our discussion. The reaction between anunonia and sulfuric acid produces an ammonium sulfate solution that is continuously circulated through an evaporator to thicken the solution and to produce ammonium sulfate crystals. The crystals are separated from the liquor in a centrifuge, and the liquor is returned to the evaporator. The crystals are fed either to a fluidized bed or to a rotary drum dryer and are screened before bagging or bulk loading. 

Rotary Drum Vacuum Filter The underflow from the thickener is the feed to this filter. The filter concentrates the solids (predominately calcium sulfite), which then leave the filter in a cake form and are transferred to a cake repulp tank where they can be reslurried. The liquid generated from the filtering process (filtrate) is collected and pumped to the lime reactor vessel. 

Sedimentation tests are of value particularly for rapid evaluation of the effects of aging, flocculants, vibration, and any other variables that conceivably could affect a rate of filtration. The results may suggest what kinds of equipment to exclude from further consideration and what kind is likely to be worth investigating. For instance, if sedimentation is very rapid, vertical leaves are excluded, and top feed drums or horizontal belts are indicated or it may be indicated that the slurry should be preconcentrated in a thickener before going to filtration. If the settling is very slow, the use of filter aids may be required, etc. Figure 11.1 illustrates typical sedimentation behavior. Figure 11.2 summarizes an experimental routine. 

In the process to produce alumina (Fig. 1), bauxite is crushed and wet ground to 100-mesh, dissolved under pressure and heated in digesters with concentrated spent caustic soda solution from a previous cycle and sufficient lime and soda ash. Sodium aluminate is formed, and the dissolved silica is precipitated as sodium aluminum silicate. The undissolved residue (red mud) is separated from the alumina solution by filtration and washing and sent to recovery. Thickeners and Kelly or drum filters are used. The filtered solution of sodium aluminate is hydrolyzed to precipitate aluminum hydroxide by cooling. The precipitate is filtered from the liquor, washed, and heated to 980°C in a rotary kiln to calcine the aluminum hydroxide. 

Decrease agitation intensity thickeners. decrease temperature. Decrease mixer impeller or drum rotation speed or fluid-bed gas velocity. 

To obtain a formula for sizing a rotary-drum filter, the mechanism of liquid flow through a porous medium must be considered. As the filter drum rotates through the slurry tank, a porous solid deposits on the surface of the dmm, increasing the resistance to liquid flow. The surface of the filter cake is at atmospheric pressure. If it is assumed that the pressure downstream of the filter medium is constant (created by a vacmun punp), then the pressme drop across the filter cake and medium is constant. As the filter cake thickens, the liquid flow rate decreases because of the increasing resistance to flow. 

Performance often is improved by appropriate pretreatment of the slurry with flocculants or other means. An operating practice that is receiving increased acceptance is the delaying of cake deposition by some mechanical means such as scraping, brushing, severe agitation, or vibration. In these ways most of the filtrate is expelled before the bulk of the cake is deposited. Moreover, when the cake is finally deposited from a thickened slurry, it does so with an open structure that allows rapid filtration. A similar factor is operative in belt or top feed drum filters in which the coarse particles drop out first and thus form the desirable open structure. A review of such methods of enhancement of filtration rates is by Svarovsky (1981). 

Rate of growth (high deformability) Decrease binder viscosity. Increase agitation intensity. Increase particle density. Increase rate of nuclei formation, collision frequency, and residence time, as above for low-deformability systems. Decrease binder concentration or change binder. Decrease any diluents and polymers that act as thickeners. Decrease operating temperature for systems with simultaneous drying. (Otherwise increase temperature. Increase mixer impeller or drum rotation speed or fluid-bed gas velocity. 

In the lower part of each vessel is the calaiidria or steam drum formed by two plates fitted with vertical copper tubes, through which the juice can circulate from the lower part of the vessel to rise just above the top of the calandria, and as it boils and thickens the vapour passes into the calandria of the next vessel. The juice in this makes it act as a surface condenser and assist the boiling in the first vessel, while the vapour of the second vessel enters the calandria of the third vessel, which is connected to the vacuum pump. The steam supplied to the first vessel thus brings about an evaporation nearly three times what it would do in a single vessel, and secures the economy in fuel which has made the... 

The Husch-tvpe flare is a sealed, metal container (powder canister) 3/4 full of thickened fuel with a 1/8 to 3/16th-inch hole in the bottom. The container is placed with cap end down in half of a 55-gallon drum 3/4 full of thickened fuel. A reflector assembly made from 24-inch culvert should extend about 24 inches above the top of the drum half rim (figure below). When the fuel in the drum half is ignited, the heat from the burning fuel produces vapor in the powder canister, this vapor is... 

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