Thickening continuous settling

Borax decahydrate is produced from borate ores, primarily colemanite and also from dry lake brines. When produced from its ore, the ore is crushed and then blended with B2O3. The blend is mixed with hot recycle liquor in a dissolving plant. Rock and clay particles from the liquor are removed over vibrating screens. The hquor is then fed to thickeners for settling of insolubles after which the underflow mud is washed with water. The strong liquor of borax hydrates is then pumped into continuous vacuum crystallizers for the separation of the pentahydrate first and then the decahydrate. The products are dried in rotary or fluid bed driers. 

The simplest technological procedures is thickening by settling employing the sedimentation properties of suspensions. Thickeners are designed for either discontinuous or continuous operation. 

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. 

Continuous Settling Hindm ed Systems/Thickening, This process is usually conducted in a raked cylindrical tank with a sloping conical base leading to a central outlet for the shidge, an overflow launder around flie per hery of flie tank for the clear liquid and a central feed point. 

Sedimentation equipment can be divided into batch-operated settling tanks and continuously operated thickeners or clarifiers. The operation of the former is simple. Whereas use has diminished, these are employed when small quantities of Hquids are to be treated, for example in the cleaning and reclamation of lubricating oil (see Recycling, oil). Most sedimentation processes are operated in continuous units. 

The primary function of a continuous thickener is to concentrate sus-penaed solids by gravity settling so that a steady-state material balance is achieved, solids being withdrawn continuously in the underflow at the rate they are supphed in the feed. Normally, an inventory of pulp is maintained in order to achieve the desired concentration. This volume will vary somewhat as operating conditions change on occasion, this inventoiy can be used for storage of sohds when reed and underflow rates are reduced or temporarily suspended. 

The thickener is the industrial unit in which the concentration of a suspension is increased by sedimentation, with the formation of a clear liquid. In most cases, the concentration of the suspension is high and hindered settling takes place. Thickeners may operate as batch or continuous units, and consist of tanks from which the clear liquid is taken off at the top and the thickened liquor at the bottom. 

Cold-Water Process. The cold-water bitumen separation process has been developed to the point of small-scale continuous pilot plants. The process uses a combination of cold water and solvent. The first step usually involves disintegration of the tar sand charge, which is mixed with water, diluent, and reagents. The diluent may be a petroleum distillate fraction such as kerosene and is added in a ca 1 1 weight ratio to the bitumen in the feed. The pH is maintained at 9-9.5 by addition of wetting agents and ca 0.77 kg of soda ash per ton of tar sand. The effluent is mixed with more water, and in a raked classifier the sand is settled from the bulk of the remaining mixture. The water and oil overflow the classifier and are passed to thickeners, where the oil is concentrated. Clay in the tar sand feed forms emulsions that are hard to break and are wasted with the underflow from the thickeners. 

Most of the available information on settling concerns small laboratory units. In practice, settling basins and thickeners are of large size and are kept in continuous operation. Comings (1940) presented an excellent practical discussion of these and other factors affecting their operation. However, it remained for Kammermeyer (1941) to evaluate the importance of these factors quantitatively. 

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