Sedimentation Testing

Introduction

Mass and energy transfer processes are amongst the major cost drivers of most processing plants thus it is imperative that they be optimized if the plant is to operate economically. Effective optimization of these processes is derived from accurate rheology data on the plant’s slurry hence effectually determining the design criteria for the mass and energy transfer.

Typically, rheology investigations are carried out during the final stages of metallurgical test work, which is too late to set the design criteria for the pilot plant. This leads to increased test work cost, and sometimes lowers confidence in the quality of the data produced. In addition, the engineering and feasibility studies are often delayed because of the lack of pertinent rheology data.

This document serves to outline sedimentation process tests under dynamic and static conditions.

Sedimentation

Sedimentation is defined as a process of separating specifically heavier, suspended matter by gravity settling. Solid matter settles on the bottom of the vessel and the liquid above it is poured off. The primary purpose of the process is to produce a clarified liquid. Figure 1 illustrates the definition.

 

 Sedimentation process

 

 Figure 1: Sedimentation process

 

 

 

Sedimentation Zones

Sedimentation processes can be described as either being under;

  • Static batch settling conditions or
  • Dynamic settling conditions.

Under static batch settling conditions, four sedimentation zones are present namely;

  1. A clear water zone
  2. An initial slurry concentration zone
  3. A transition zone and
  4. A compressions zone.

Thickener Design

A design method for thickeners and clarifiers known as the batch flux curve technique was developed over 20 years ago, but still is not well-known in design and operating circles.

Equations forming the basis of design

The following assumptions will be used for the design;

  • Separation is by gravity settling due to density differentials and
  • The thickener being designed is circular.

Flow scheme for thickenersThe mass (Eq. 1) and material balances (Eq. 2) that form the foundation for the technique (see Figure 1) are:

Qf  =  Qu  +  Qe     (1)

Qf xf =  Qu xU  +  Qe xe        (2)

Under normal conditions, no solids are desired in the effluent:

 xe = 0             (3) 

Figure 2: Flow scheme for thickeners

The feed flowrate (Qf) and concentration (xf) are normally design parameters. This leaves is with the underflow pumping rate (Qu) (sometimes called blowdown) as the only parameter required to achieve a desired underflow effluent concentration (xu) to determine. Since the underflow is normally pumped, this variable can be controlled during operation. Also, the pump must be sized to adequately handle the largest expected loadings.

Types of Setting

Four regimes of settling are recognized as aforementioned. Figure 3 merges the respective sedimentation zones to the settling rate graph. 

Settling regimes

Figure 3: Settling regimes

Discrete settling occurs when particles are independent of each other, with no inter-particle forces. Examples of this regime are removing sand and sub stones from water. In this regime a force balance using Stokes' law is appropriate. In flocculent settling the particles begin to influence each other, occasionally sticking together to form flocs. Design in the flocculent regime may use quiescent settling tests, or batch-flux design to improve the slurry product.

In hindered settling, particles are so close they have a major effect on each other. This is the regime in which batch-flux analysis applies.

The compression regime is not well studied, and since it normally operates only at the very bottom cone of the thickener, it goes beyond conventional design requirements.

Theory of Batch Flux

The development of the technique began with Kynch, who modeled the settling of particles in water. This was followed by work of Yoshioka and Dick, and Ewing, which formed the current technique. Since the surface area of the settling unit is the main variable, the mass flow through the system is considered a flux, mass per unit area per time. This allows system properties to be described in a general way for any size unit. In summary, the technique looks at steady-state conditions as solids move downward in a thickener at a single flux rate. The settling rate is a function of the feed solids concentration. This settling flux is combined with the flux caused by Qu (drawing product off the bottom), and balanced with the water rising out of the thickener. If the flux in (xf Qf/A) exceeds the flux out (xu Qu /A), solids will be carried out in the effluent.

In the batch flux method, a graphical technique represents the settling flux and the underflow flux as functions of solids concentration. The settling properties are plotted as a curve. The applied flux and underflow pumping rate are presented as an operating line where the applied flux is the y-intercept of the operating line, and the pumping rate is the slope of the operating line.

Design Parameters and Objective

In settling, cross-sectional surface area is the key parameter. The depth of the tank is not a factor. The depths used in practice are typically 2.5 to 5 m, which allow for inlet and outlet turbulence, and solids storage in the bottom of the unit. This dependence on area holds for discrete settling when Stokes' law applies, as well as for hindered settling, when the batch flux analysis applies

The objective is to;

  • determine the area of a thickener,
  • determine the optimum underflow pumping rate and,
  • the corresponding underflow concentration

Determining the Batch Flux Curve

Test work setupCalculation of the curve requires several settling tests of thickener feed at various concentrations. The goal is to measure the unhindered settling velocity at several concentrations. Each test (i) produces a settling velocity (Vi) and a solids concentration (xi). One large sample can be diluted or concentrated to make several samples of different concentrations. In diluting, use the supernatant as a dilutant rather than adding water. This keeps the solution chemistry consistent.

The following apparatus will be required for the test;

  • graduated cylinder or long glass tube,
  • a stopwatch and in some cases
  • wire stirrer operating at 1 rpm (for slow settling materials such as bioprocess solutions to minimize wall effects).

Figure 4: Illustrates the test work setup.

Sedimentation Processes

  • Static batch settling conditions
  • Dynamic settling conditions

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