Thickener, Dorr

FIG. 18 100 Ultrahigh-rate thickener. Dorr-Oliver EIMCO.)... 

FIG. 18-105 E-Duc Feed dilution system installed on a 122-m diameter thickener. Dorr-Oliver ElMCO.)... 

Dormicum Dormodor Dormonoct Dorr thickeners Dortmund... 

The Dorr agitator (Coulson and Richardson, loc. cit.) consolidates in one unit the principles of the thickener and the Pachuca tank. Resembling a rake-equipped thickener, it differs in that the rake is driven by a hollow shaft through which the solids-liquid suspension is lifted and circulated by an air stream. The rake moves the pulp to the center, where it can be entrained by the air stream. The unit may be operated batchwise or continuously. 

This stream cools the vapors and scrubs the remaining catalyst out of the cracked products. Most of the slurry of catalyst in heavy oil withdrawn from the bottom of the tower is recirculated to the top of the disc-and-donut section, while a small portion is withdrawn for recovery of catalyst. The latter stream (slurry return) usually amounts to 3 to 10% of the volume of fresh feed to the reactor. Catalyst concentration in the slurry can be decreased by increasing the rate of withdrawal, and is usually maintained below 0.5 lb./gallon to avoid erosion of slurry pumps and valves. The slurry-return stream may be pumped to a separate settler (e.g., a Dorr thickener or a simple cone-bottom tank) or the settler may be incorporated in the bottom of the fractionating tower (25). About 70% of the heavy oil is removed from the settler as a clarified oil containing less than 0.01 lb. catalyst/gallon. The sludge is diluted with fresh feed and pumped to the reactor to return the catalyst to the system. 

Dorr Thickener.—The tank and the distinctive Dorr mechanism assisting in remvoing the settled solids is shown in Figs. 11 and 12. The essential mechanism is plow blades set at an angle on arms secured to a slowly revolving shaft... 

Fig. 12.—Dorr thickener with double raking device.

With 8 parts of liquid to one of solids the settling area required by Dorr thickeners ranges from 5 to 25 sq. ft. per ton of solids. 

Uses of Dorr and Other Thickeners.—Outside of the separation use to which these devices can be put is their employment in what is termed counter current washing/ To show the extractive and recovery possibilities a hypothetical computation based on three tanks will be made. It is assumed for convenience that the tanks hold 110 tons of liquid and solid in the proportion by weight of 10 parts of liquid to one of solids. It is desired to save the liquid which is valuable and reject the solids with the minimum of valuable liquid. Let it be further assumed that the solids will settle to a ratio of liquid to solids of 1 1 in each tank. 

Continuous Counter-current Decantation.—This system is being quite generally used in the chemical world to replace the older intermittent agitation and decantation washing systems as well as more modern filter installations. It has as its basis the application of the Dorr thickener to remove suspended solids contin-... 

The Dorr classifier and the Dorr thickener it will be noted are designed to handle two different types of material. The classifier handles quick-settling sandy material which does not require long-washing periods while the thickener is best adapted to handling slow-settling slimes which require longer periods of detention in the washes. 

FIG. 18-101 Deep Cone paste thickener. (Courfesj/o/Dorr-O/ioer E/MCOJ... 

Thickeners and Centrifitgals. The simplest form of thickener is an ordinary settling tank arranged for the decantation of the clear liquid after settling. Nonmechanical, continuous settling tanks have conical bottoms from which thickened slurries high in solids are continuously discharged. The most widely used thickener is the mechanical type, of, which the Dorr is typical. Cost estimates for continuous thickeners are shown in Fig. 6-29. 

Neutral slurry is classified in hydrocyclones (3500/12°). Underflow fiom the hydrocyclones is pumped to acid leaching, and the overflow to five Dorr thickeners, each with a diameter of 9.5 meters. Magnafloc 338 is used for this process (0.017 kg per toime of cathode zinc). Underflow from each Dorr thickener is pumped to low acid leaching, and after classification and washing, to Larox filters. The filter cake with a maximum moisture content of 17 % is fed to the Waelz kilns. 

Figure 1.3 Photograph of a basin thickener installation (Dorr-Oliver Eimco).

Figure 1.8 Schematic of an E-Cat high-rate clarifier (sometimes referred to as a thickener clarifier) showing the internal circulation of liquors. Inset photograph with permission from Dorr-Oliver Eimco.

In the USA several companies market more efficient and compact thickeners, fuUy equipped with instrumentation and sensing devices. Some claims are made for improved equipment design but the higher thickening rates are mostly due to better use and control of flocculant addition. One example is the Dorr-Oliver Hi-rate Thickening System which uses a Fitch feed well (ref. 19, Figure 5.24). This is a new type of feed well consisting of three horizontal... 

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