Thickening

Solid Liquid Separation by Flocculation

When is use of a flocculant justified in terms of overall process economy?

  1. when it results in process improvement, such as producing a clear liquor for electrolysis, precipitation, ion exchange, or solvent extraction;
  2. when it results in satisfying requirements of pollution abatement ordinances;
  3. when it results In increased recovery of values that would otherwise be lost; or
  4. when it results in considerable savings in capital expenditure due to use of smaller equipment.

Flocculation is a process wherein individual particles are united into more or less tightly bound agglomerates or flocs, thereby increasing effective particle size of solids suspended in a liquid.

Degree of flocculation of a suspension of finely divided solids in a liquid is controlled by a combination of probability of collision between particles and probability of adhesion after the collision has occurred. Probability of collision can be increased commercially through use of a paddle-type flocculator, combination flocculator, and clarifier or flocculating-type feedwell. Probability of adhesion usually can be increased by addition of a reagent known as a flocculant.

Reagents act as flocculants through one or a combination of three possible mechanisms. The first is electrolytic neutralization of intermolecular repulsive force due to Zeta potential. This neutralization enables Van der Waal’s cohesive

Thickener Design Underflow Pump Arrangement and Automatic Controls

A number of unique thickener mechanism designs with the operating results achieved in various installations are discussed. These unusual designs include new types of raking arms, lifting devices, and feed-wells employed to obtain higher underflow densities and minimum operator control or improved overflow clarity.

Thickening

The conventional raking arms consist of a relatively simple truss with the blades attached to the bottom chords. This design provides proper concentration of many ‘normal’ slurries. A modification of this design as shown in the center is being used with excellent results on large thickener basins where it is desirable to have a

thickener-design-types-of-raking-arms

double slope to minimize overall depth of the tank while providing a steeper slope near the center to promote easier movement of coarser solids settling in this area. This configuration with Thixo Arms in the center area is used on a 325-ft diam tailings thickener at a large copper concentrator in the Southwest. The blades near the center are extended on posts so that the truss members do not move through the highest density pulp. This reduces torque load on the mechanism and promotes thickening action in this area by the well known picket

Polyelectrolytic Bridge Flocculation in Dewatering and Processing of Clay Slurries

Flocculation, based on the bridging mechanism, using organic polymers has been utilized to overcome such problem situations. This method of flocculation offers solutions not possible by previous methods. Normally these flocculents are used to assist dewatering. In this case not only is dewatering promoted, but also further processing is assisted by a change in the dilatant character of the filter cake that is formed. Discharge of filter cake is easier, extrusion of filter cake on to conveyor dryer is possible or improved and heat transfer and drying rates of the extruded filter cake are substantially higher.

Process

To understand the methods and problems of dewatering and subsequent processing of kaolin slurries, let us look at a typical flowsheet for a kaolin product.

flocculation flowsheet

Following removal of grit from the mine crude slurry and blending of crudes as required, the next step towards preparation of the desired product is to fractionate the crude clay slurry into portions that represent specific particle size distributions. The majority of kaolin products are predicated upon these specific particle size distributions which are essential to imparting specific properties to the material. The particle size fractionation is carried out on

Chemical Aspects of Thickening and Clarification

Chemistry plays a more important role in milling operations than is generally recognized, particularly in fluid-solid separation processes. Chemical aspects are also very important insofar as plant water quality is concerned, and in the manner in which this water quality effects plant design and operation.

One aspect of the use of mining chemicals which has received scant recognition is the realization that in many cases, the equipment in which the chemical is to be used must be designed specifically for use with the chemical, or the equipment, or its method of operation must be modified in order to obtain optimum results. This is especially true of equipment such as filters, thickeners, clarifiers, reagent mixers, reagent feeders and distribution systems, pumps, etc.

thickening flowsheet

Based on the flowsheet, the reasons for these modifications are the following:

  1. flocculent added at Drag Classifier A could decrease the load going to Thickener D and perhaps improve the centrifuging rates in Centrifuge Z.
  2. in order to take full advantage of flocculated condition of the thickener underflow.
  3. it would be desirable to install a filter surge feed tank which would feed the filters by gravity and provide a means for the

Solid Liquid Suspensions

In the field of minerals processing the concentration of valuable minerals normally takes place in suspensions of ore particles in liquids. The fluid resistance offered by Newtonian liquids to the movement of free settling spheres for the limiting cases of creep flow and turbulent flow is well described by Stokes’ and Newton’s Laws. However, these equations are not applicable to non-Newtonian fluids, since the relationship between applied shearing stress and resulting rate of shear is a non-linear function, and more than one viscosity parameter is necessary to describe their rheological properties.

The apparatus for this investigation consists of a transparent column fitted with an electronic sensing device which determines the velocity of metallic balls passing through a solid-liquid opaque suspension. A suspension was formed by allowing water of constant head to flow through a bed of fine glass beads contained in the column. The degree of dilation of the suspension was accurately controlled. The suspension density was measured hydrostatically.

  1. The concentration of solids.
  2. The size of the glass beads.
  3. The density of the glass beads.

Suspensions are classified as

a) Static suspension, those composed of spherical solids of identical diameter and densities and which remain in fixed position with respect to a fixed

Thickener Design and Theory Problems

This paper will review thickening theory, from the study but of the practicing engineer, with particular emphasis on areas of uncertainty or disagreement. It will attempt to outline broadly what we know and what we don’t know. This should serve two purposes. First, it will warn the process engineer where he might get into trouble. At least it will alert him to the risk involved in uncritically accepting any of the recommended design procedures. Second, it will identify for researchers some areas in which further work might be fruitful.

“Thickening” means different things to different people. For this paper we define it to mean sedimentation behavior characterized by line settling. Solids subside with a clear line of demarcation between settling solids and supernatant. Although the supernatant will usually be clear, in some cases it may be turbid.

Thickening Regimes

Coe and Clevenger believed that throughout “free settling” the settling velocity u was a function of C only. In mathematical terms, u = u(C). About compression they made the following statement:

“The water liberated by compression finds its way out of zone D” (their compression zone) “through tubes or channels which form drainage systems upwards through the zone.”

Thus they conceived channeling to

Hydra-Sludge Removal System for Mine Drainage and Coal Preparation Plant Sludge

The environmental demands on the mining industry coupled with rising production costs have created a need for a low cost sludge removal system. The HYDRA system meets these demands by offering a unique means of removing settled solids from the bottom of a clarifier or a thickener. This underflow system combines experience and proven concepts to form a low cost sludge removal system.

The operation of the HYDRA system can be compared to a conventional rake-type clarifier or thickener. The sludge in a conventional thickener flows from the ends of the rakes through the center column ports or windows and then through the suction line to the underflow pump. In this arrangement the pumps are generally located at the periphery of the thickener.

The HYDRA system utilizes this same flow principal. The pump creates a low pressure area. This low pressure combined with the positive head supplied, by the weight of the slurry and liquid moves the sludge through the orifices, segments, valves, header and into the pump inlet. As each segment of the suction network is activated, the pump suction is evenly distributed over a fixed area.

The typical HYDRA rake system components include inlet orifices spaced on centers which are determined

Lamella Thickeners

Stringent environmental regulations, which have limited the use of sludge ponds, coupled with the economic incentive to clean more coal or to add fine coal cleaning additions to existing preparation plants have increased the use of static thickeners in preparation plants. The Lamella Gravity Settler, sometimes designated Lamella Thickener, is a shallow depth sedimentation device which has been applied in a variety of industrial process and wastewater control projects in place of the conventional raked thickeners.

Theory and Design

The Lamella Thickener is an inclined, shallow depth sedimentation device. It performs the same function as a conventional Thickener, but it occupies only a fraction of the space.

For an initial understanding of the Lamella Thickener, it is best to restrict the initial discussion to suspensions in which the particles exhibit “free settling.” This phenomenon occurs when the concentration of particles is low enough that the individual particles or flocs settle independently of one another and follow Stokes’ Law. At higher concentrations, the settling particles interfere with each other, and “hindered settling, ” which is characterized by a clearly defined interface between the suspension and the clarified liquid, is encountered.

The shallow depth sedimentation device is impractical since it is difficult to remove

High Capacity Thickener

Conventional practice is to add flocculant as a dilute solution to the feed slurry in a launder or feed pipe, possibly using staged addition so as to improve the floc growth. Mixing in launders is not necessarily optimum, and with deep launder pulp depths lower layers of slurry do not readily rise to the surface and become contacted with reagent. Baffles, ridges, and other static mixing devices, might be utilized in a launder to improve this mixing action, but these can create excessive shear or simply not provide uniform mixing conditions. To offset this, additional reagent must be added beyond that which would be predicted from a simple batch test.

The transition from feed launder to feedwell often presents several problems. Feedwells may be attached to the thickener rakes and if a lifting device is employed, sufficient clearance must be allowed between the bottom of the launder and the top of the feedwell in its raised position. This means that the entering slurry must not only be slowed down in its horizontal path, but lowered to the feedwell surface without increasing its velocity. This requires some sort of restriction, such as a valve, which in turn produces the shear which may

Earth Bottom Thickener Design & Construction

The essential elements of thickener basin construction consist of:

  • Laboratory testing and selection of soil materials for thickener bottom and determination of required thickness of treated blanket.
  • Rough grading of site to bottom slopes of thickener basin. If imported soil materials are to be used, establish rough grade at bottom slopes minus 150 to 450 mm to allow for placement of imported materials.
  • Excavation for thickener access tunnel and center pier foundation or caisson foundation.
  • Form and place concrete for tunnel and pier foundation.
  • Backfill over tunnel and compact to specified density.

The perimeter wall (or ringwall) is designed as a simple retaining wall to withstand hydraulic pressure exerted by the slurry and to retain the earth embankment. Wall height is determined by thickener diameter and the compound slopes of the rake arms. Figure 1 is a section through a wall showing a typical relationship of footing, bottom seal layer and outside embankment.

Soil materials selected for seal layer construction will ideally contain 10 to 20% clay, 25 to 40% passing 74 microns and not more than 10% plus 9 mm.

Laboratory testwork on submitted soil samples will develop the following data:

  • Optimum moisture content for maximum compacted density.
  • Permeability coefficient for cores

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