Thickeners

The practice of thickening or clarification is usually carried out in decantation tanks employing gravity settling or sedimentation. These tanks are fitted with mechanical scrapers to collect and move the settled solids to the point of discharge, the clear overflow being collected and removed by means of peripheral launders.

These tanks, equipped with mechanical scraping devices and launders are usually termed thickeners in the metallurgical field and clarifiers in the sewage and water purification fields. Thickeners, equipped with the patented spiral scrapers and angle rakes are a distinct improvement over all types of mechanical raking devices now in use.

Specifications and thickener rake design

All Thickeners have the same general specifications, varying only as to type of machine. The specifications given for one will cover all.

Thickener Sizes and Superstructure

Thickeners up to 45′ are all furnished with the low head type superstructure, consisting of heavy steel beams, rigidly and laterally braced, and arranged for mounting on either steel, concrete, or wooden tanks, with superstructure extended for mounting diaphragm pumps if specified. On larger units, beams are arranged for field bolting or welding as required.

Large size Thickeners, above 45′ diameter, use the bridge-type superstructure. The trusses are connected by steel floor beams and crossbraced with angles all joined by heavy bolts, although field welding can be arranged. This type superstructure is usually carried on piers outside the tank rather than on the tank sides.

Thickener Rakes and Supports

Metallurgically, and mechanically, Rakes are correctly designed. From a metallurgical point, the exclusive design of the Spiral Rake imparts a continuous turning motion which conveys the thickened pulp from the outside of the tank to the center cone. The more quickly this settled material is raked to the discharge cone, the less chance there is for overload.

Mechanically, the patented Rakes are strongly braced to withstand the most severe over-load. The rakes are fabricated of rolled steel sections, to the correct curvature to transport the material to the center. The rakes are carried on heavy channel sections. On larger units these rakes are braced to the arms with angles and tied by means of tie rods. Angle rakes are used at points beyond the radius of the spiral. The whole rake assembly is supported on a steel spider attached to the shaft and is built of welded sections. The whole assembly is braced to the shaft by means of tie rods so that the rakes and rake arms form two trusses: one in the horizontal and one in the vertical plane. The use of a spider, and built up sections, eliminates excessive weight and assures a stronger unit.

Major Factors Influencing Thickener Design

• Process requirements for the overflow liquor quality and underflow slurry density. These determine the mechanism design.
• The quality of solids to be handled. Usually expressed as area per unit weight of dry solids per day. High rates usually require a combination of a stronger thickener mechanism and a lifting device.
• The amount of material larger than 250 micron (+60 mesh) in the feed. This affects tank bottom slope, drive and strength of mechanism. It may also require a rake lifting device.
• Specific gravity of the solids. The greater the specific gravity the more likely a stronger drive and mechanism will be required.
• Feed, overflow, and underflow systems capable of handling additional material when other thickeners are out of service.
• Feed and underflow material settling characteristics that may require special rake construction such as blades located a distance below the rake arms on posts or spikes on the blades to cut into packed solids.
• Scale build up tendency of feed slurry may require special arms and drive.
• An operating requirement to accumulate solids for defined periods of time will require a special mechanism design, as it is not a normal operating procedure.
• Froth control or removal may require sprays, froth baffles or skimmers.
• Slurry temperature, vapors, gases, etc. may require covered and/or insulated tanks with attendant seals.
• Soil conditions and ground water elevation affect foundation design and may determine tank and mechanism type.
• Climatic conditions may require special considerations, such as enclosures around the drive and instrumentation.

3′ to 20′ Spiral Rake Thickener

Thickener Designed with Beam + Metal Tank

5′ to 20′ Diameter Thickener

Thickener Designed with Beam+ Wooden Tank

22′ to 50′ Diameter Thickener

Thickener Designed with Beam + Steel Tank

Thickener Designed with Beam + Wood Tank

50′ to 80′ Diameter Thickener

Thickener Designed with Truss + Steel or Wood Tank

Thickener Designed with Truss + Concrete Tank

Thickener Types

Thickener options have grown in the last 10—15 years for two main reasons:

• Ability to produce non-Newtonian underflows
• Development of rakeless thickeners

Thickener options can conveniently be divided into four major types:

• Conventional or high-rate type
• Rakeless ultra-high-rate type
• High-density type
• Deep-cone type

To illustrate the differences between these thickener types. Figure 18.5 depicts underflow produced by the thickener compared with the relative yield stress of the material. High-rate thickeners produce the lowest density and yield stress (<20 Pa). The highest yield stress underflow is produced by deep-cone thickeners and can reach over 150 Pa.

Source: This article is a reproduction of an excerpt of “In the Public Domain” documents held in 911Metallurgy Corp’s private library.