# Dewatering

## Hydrostatic Bearing Type Center Mechanism

Development of the Hydrostatic Bearing Mechanism evolved from the caisson thickener design. Conventional thickener mechanisms are driven by gearing which rides on ball or roller type anti-friction bearings. It would not be feasible to use this method of support with a very large diameter bearing, especially with high torque, vertical and unbalanced loads. Additional stresses are produced in large bearings by temperature changes in machine tolerances. These stresses make proper design and fabrication costly and operating difficult. It is also important to note with large thickeners that the loads increase by an exponential function as the diameter increases.

The basic principle involved in the hydrostatic bearing design is simple. The main gear is attached to a movable continuous runner that passes over a series of stationary pads. An oil film, capable of supporting the load, is maintained between the two surfaces. A separate hydrostatic journal bearing system is incorporated to stabilize the runner in a horizontal direction. The motion between the two surfaces is slow due to the speed required for thickener operation and no dynamic force is present to sustain the loads. For this reason, an external source is used to maintain the pressure. The term hydrostatic is used because ….Read more

## Batch Thickening Theory

Kynch showed that a constant concentration layer propogates upward at constant rate in batch thickening tests under the assumption that settling velocity is a function of only the concentration, as described earlier under Continuous Thickening Theory. His conclusion can be expressed as:

Where Ci is the concentration of solids in the layer, and Vi is the settling velocity of the layer. VRi is the rate of upward propogation of the constant concentration layer.

The following equations are obtained by material balance:

Where tu is the settling time, Hu is the interface height at tu, and Hi is the intercept of the tangent to the settling curve with the interface height axis. The relationship of the above equations is illustrated in Figure 6.

The average concentration, Cu, of the slurry bed at the settling time tu is expressed as:

Note that Equation (21) for batch thickening has the same form as Equation (10) for continuous thickening where CL corresponds to Ci and Cu corresponds to Ca.

Talmage and Fitch obtained the unit area from batch ….Read more

## Selecting Thickener Depth Determination

Where there is little significant effect of compression factors, the bed depth can be selected based upon operating characteristics of the thickener installation. In this case, information must be available (or assumed) concerning the normal fluctuations in feed rate and concentration. Also, sludge storage requirements should be considered.

For illustrative purposes, the following example of thickener operation is given: A thickener is being fed at an average rate of 1000 kg of solids per hour, and the thickener has 72 m² of surface area. It is expected that the feed rate could increase by as much as 20% for up to 10 hours and decrease by as much as 30% for up to 20 hours. The unit area for this thickener is 3.0 m²/mtpd, the average underflow concentration is 400 g/l and the limiting concentration is found by calculation or graphical methods to be approximately 200 g/l.

In this case, a minimum desirable sludge depth is set at 0.5 meters on the sidewall of the thickener, because no significant change in underflow concentration occurs with normal fluctuations in bed depth. Then, considering the low feed rate situation, the expected bed drop would be:

## Flocculation

The flocculation behavior of coal, quartz and clay was studied by measuring the turbidity of the supernatant liquid after one hour of settling. Figure 14 is a plot of turbidity and adsorption density against equilibrium concentration. The turbidity of the supernatant liquid decreases until a minimum is reached and tends to rise thereafter. The results are consistent with the bridging theory which stipulates that the polymer molecule attaches itself to one or more adsorption sites; the remainder of the molecule extending into the solution, free to interact with vacant sites on another particle. As the polymer concentration increases, fewer sites are available for adsorption and the extended segment is less able to adsorb on the surface of another particle. The flocculation, then, deteriorates because of the formation of a protective coating of polymer on the particle surface adding its own charge to that of the particle. Further addition can in fact lead to stabilization of the suspension. It can be seen from the figure that the optimum dosage for coal using polymer A is reached at less than one-sixth coverage of the surface and that the performance of the polymer deteriorates once the optimum dosage is exceeded. The flocculation performance ….Read more

## Thickener Process Control Strategy

The objectives of thickener control are varied, but in most instances, the objective is to dewater the slurry, reclaim the clear overflow for reuse in the process, and produce a thickened or more dense underflow, which can either be rejected, in the case of tailings, or used as feed for subsequent unit operations.

The general Process Control Strategy for a thickener are shown in Fig. 13. By knowing the feed density, feed flow rate, underflow density, and underflow flow rate, the solids in, solids out, and solids inventory in the thickener can be maintained by adjusting the pump speed. Overrides, or limits, to the controlled variables are underflow density, drive torque, and solids inventory level. If the flow out is less than the flow in, the pump speed will not increase unless the density is such that the drive-torque upper limit or the maximum solids inventory has been reached. If the flow out is greater than the flow in and the outflow density is lower than desired, the pump speed will be decreased, provided the drive-torque limit is within a safe operating range and the solids inventory is below the upper limit. The polymer feed rate will be controlled by using a ….Read more

## Recover Concentrate from Thickener Overflow

SRL Pump More Than Pays For Itself in Recovery Of Flotation Concentrate From Thickener Overflow

Problem: Froth building up on surface of concentrate thickener tends to carry mineral values to waste.

Solution: A small combination sump and overflow box was installed at the base of the thickener tank and a 2″ x 2″ SRL Pump was connected to the conical bottom of the sump.

To recapture the floating concentrate the thickener overflow flows to this combination sump and overflow box. As the thickener overflow flows down the pipe to the sump, the flocs of concentrate lose their entrapped air and the concentrates sink very rapidly to the bottom of the sump. The overflow from the sump goes directly to waste.

The underflow from the sump is pumped by a 2″ x 2″ SRL Pump to the concentrate pump which feeds the thickener. This procedure also saves having to add additional water at this point.

The savings in concentrates recovered at this plant have more than paid for the pump and its total operation without considering the savings effected by using reclaimed water for conveying the concentrates to the thickener.

“SUB-A” FLOTATION RECLAIMS ZINC FROM CONCENTRATE THICKENER OVERFLOW

## CONTROL DENSITY OF THICKENER DISCHARGE

DIAPHRAGM PUMP WITH AUTOMATIC STROKE ADJUSTMENT ACCURATELY CONTROLS DENSITY OF THICKENER DISCHARGE is through his aggressive leadership of our Research and Development Department that many new and improved machines have been developed.

Exacting control of thickener underflow density is a critical point in many operations. Normally it is desirable to maintain the highest possible underflow density to provide maximum removal of fluids. This is essential for efficiency in counter-current washing systems, cement plants, and in processes involving filtering and drying.

The Adjustable Stroke Diaphragm Pump with Automatic Stroke Adjustment provides a positive method of accurately controlling thickener underflow density. The unit further answers the requirement for full automation of modern plants.

A gamma-ray device is installed on the thickener underflow line. The device transmits a signal to a proportional controller. The controller is connected to the automatic stroke actuator on the Adjustable Stroke Diaphragm Pump. Depending upon the signal received, the actuator either increases or decreases the length of the pumping stroke or leaves it unchanged.

The pump remains in continuous operation. Pump capacity is, of course, directly proportional to the length of stroke. Adjustment of stroke length is the only effective method of varying pump capacity. ….Read more

## SAMPLING THICKENER PULP AT VARYING LEVELS

An important potash producer in Europe uses this simple, useful unit for sampling the pulp in their thickeners. It is particularly applicable to any potash operation and its low cost will appeal to many other plants where thickener density must be sampled.

The unit is operated manually and the depth from which the sample is taken is shown on a depth indicator. Suction on the discharge line draws thickener contents into the tube.

When the sampling pipe is depressed into the thickened pulp zone the thickened pulp is immediately visible in the glass and the depth of the top level of this zone is read from the quadrant type depth indicator.

SAMPLING THICKENER PULP AT VARYING LEVELS

## Deslimer

The deslimer is provided with a central feed box, having a baffled bottom to break the velocity of the inflowing pulp. The bottom, or discharge chamber, has a clear-water inlet, which directs, the flow upward along the axis of the deslimer. The spigot is inserted in a movable holder, attached to the bottom of the discharge chamber, permitting an instant change in the diameter of opening without wetting the operator. Between the feed box and discharge chamber is a valve with sides parallel to the tank and suspended on the vertical center, and which can be raised or lowered by means of a hand wheel on the suspension rod. This produces a narrow channel between the valve and tank through which the hydraulic water rises and the sands fall.

The feed entering the deslimer is checked by the baffle, causing the sands to settle through a narrow, rising current of water surrounding the valve. This current increases in velocity until the discharge chamber is reached, by which time the sands are thoroughly deslimed. A hydraulic pressure of 5 or 6 lb. per square inch is maintained in the discharge chamber. The width ….Read more

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