Thickening or dewatering may be defined as the removal of a portion of the liquid from a pulp or slime made up of a mixture of finely divided solids and liquids. The early methods of thickening employed plain, flat-bottomed tanks into which the pulp was fed until the tank was full. The solids were then allowed to settle as long as required, the top liquid was decanted, the settled solids were discharged, and the operation was repeated. Such settling was usually carried out in a number of tanks so that a regular cycle of filling, settling, and discharging could be maintained. Later it was found that feeding the tank behind a baffle allowed some decantation of clear liquid while still feeding, and this also was introduced.
Attempts to make thickening continuous, by using hopper-bottom tanks or cones, were not entirely successful. In these tanks the feed ran in continuously, settled solids were drawn off through a spigot and solution overflowed continuously at the top of the tank. The chief drawbacks to this method were the multiplicity of units required and the fact that settled solids hung up on the sloping sides which made it extremely difficult to obtain anything approaching a discharge of uniform density.
The Workings of a Thickener
The invention of the Dorr thickener made possible the continuous dewatering of a dilute pulp whereby a regular discharge of a thick pulp of uniform density took place concurrently with an overflow of clarified solution.
Scraper blades or rakes, driven by a suitable mechanism, rotating slowly over the bottom of the tank, which usually slopes gently toward the center, move the material settled on the bottom to a central opening or discharge. The rakes revolve at a speed sufficient to move the material as fast as it settles without enough agitation to interfere with settlement.
Dorr thickeners are used in the metallurgical field to thicken prior to agitation and filtration, in the countercurrent washing of cyanide slime, for thickening ahead of flotation, for thickening concentrates, and for dewatering tailing to recover the water for reuse in the mill.
The standard construction of Dorr thickener mechanisms is of iron and steel. The tanks are usually made of steel or wood for medium-sized machines, but in the larger sizes they are often constructed of concrete or earth or a combination of these materials. Various types of Dorr thickeners, which it is unnecessary to describe here, are available for specific uses, including constructions to resist corrosion. Power requirements are low, about ½ hp. for thickeners less than 50 ft. in diameter. Attendance and repairs are also low.
Figures 27 and 28 illustrate the development of the original central-shaft type of thickener from one using superstructure to the present beam type of support whereby headroom is saved and an improved mechanism running in oil is made conveniently accessible to the operator.
The Dorr tray thickener (Fig. 28) has been developed to meet the definite demand for large settling area in limited space. Each compartment increases the capacity approximately to the same degree as an additional thickener unit, of the same size, without increasing the floor space required. The tanks are divided into two or more settling compartments by means of steel trays or diaphragms suspended from the sides. The mechanism is made up of a central vertical shaft, driven by worm gear and with radial arms attached above each try. These arms carry plow blades set at an angle, and as the mechanism slowly revolves, they move the settled material to a discharge opening at or near center.
Several types of tray thickeners have been developed, classified according to the arrangement of feed and discharge in the various compartments. Probably the most generally applicable is the balanced-type tray thickener. In this each compartment has a separate feed and overflow, but the settled solids from all compartments are brought together and are discharged through a central outlet from the bottom compartment. For good dewatering, you always need a pressure filter after thickening.
The Dorr traction thickener is the type most frequently used for heavy- duty work with large tonnages. The machine is so called because the thickening mechanism is driven by means of a traction wheel which travels around the periphery of the tank on a rail. The mechanism, which is slowly carried around the tank as the traction wheel travels, consists of a truss to which are attached the raking blades which sweep the floor of the tank. This truss is supported at the center of the tank by a column.
Power is brought in through a conduit which comes up through this column and is carried out along the truss to the drive unit which operates the traction wheel (see Fig. 26).
The Dorr torq thickener is a new development in the unit thickener field. The raking mechanism is supported from a stationary central pier on which the driving motor and gear reduction are mounted. The feed may be introduced at the center through a launder suspended above the top of the liquid leavel or by means of a siphon feed through the hollow central pier. In addition to compact strength provided with the central pier construction, the machine is characterized by a new rake-lifting feature whereby when overloaded the rakes raise backward and upward at an angle to clear the overload and at the same time maintain a full raking load until the overload is removed and the rakes assume their normal position.
The Dorr washing thickener is of the multiple-tray type and is, in effect, a complete countercurrent decantation plant in a single compact unit. It is adapted to the needs of the small chemical and metallurgical plants where a relatively small tonnage of finely divided solids must be washed free from a solution in a minimum space. Four to five stages of washing are usually provided in separate tray compartments.
Multiple-stage washing is effected by operating the tray compartments in series rather than in parallel. Feed enters, and strong solution leaves the uppermost compartment. Washed solids are discharged from the bottom compartment. Solids, after each successive settlement, gravitate from one compartment to another through sludge seals in each tray. Wash water, introduced in the bottom compartment, rises successively through the compartments in a direction counter current to that of the sludge. One tall thickener thus does the work of several relatively shallow thickeners of the same diameter. Floor space is conserved, heat insulation is simple and effective, and power is reduced. Furthermore, all control of the washing operation is centered at a single point.
Cyanidation and concentration of gold and silver ores
Factors affecting Thickening rates
As discussed in Chap. II, a number of chemical and physical factors affect the settling rate of ore particles suspended in water or cyanide solution. The use of lime and caustic starch has been mentioned. It was found that aeration and the presence of sulphates aided pulp settlement:
Laboratory settling tests on the feed to the decantation thickeners have shown that sulphates—whether added in solid form, as, for example, (NH4)2SO4, or present in the barren solution, as CaSO4—increase the free settling rate about 25 per cent over that obtained with water made alkaline with lime. Laboratory tests and plant operation have also established that a well-aerated pulp settles better than one in which aeration is incomplete. This probably is the reason why thickening rates in the cyanide plant are sometimes appreciably reduced when an ore is being milled which has partially oxidized in the mine and consequently contains appreciable amounts of reducing salts.
The use of a comparatively new flocculating agent C.M.C. Carboxel, or sodium carboxymethylcellulose, is described by E. E. Brown, mill superintendent of the Chesterville Mines, Ltd. A water-soluble “H.V.” grade used in the amount of 0.01 lb. per ton of ore increased the settling rate in the thickeners by 8 to 10 per cent.