## AMIT 145: Lesson 6 Dewatering & Clarification

### Introduction

Dewatering involves a separation between solids and water for one of the following purposes:

- To prepare feed to a downstream process that requires higher solids concentration;
- To reduce the volume of slurry being fed to downstream processes;
- To achieve a product having a moisture content that is equal to or less than a given target value.

Clarification is the process of treating waste slurry to provide water of sufficient quality to be recycled for use in the preparation plant.

### Dewatering Equipment

### Filtration

Filters are used when it is desired to maximize the recovery of particles having a size less than 325 mesh.

Typical vacuum disk filters are relatively low cost and achieve high particle recovery. However, product moisture values are generally greater than 25%.

To achieve both high particle recovery values and acceptable product moisture values, high pressure filters are used. Two problems:

- High capital cost, i.e., >$500,000 each;
- Semi-batch operation and thus two units are required for continuous operation.

### Filter Product Moisture Factors

- Cake Thickness
- Pressure Drop across cake
- Drying Time
- Volume of Air pulled through cake
- Viscosity of Liquid
- Surface Tension of Liquid
- Filter Media
- Size Distribution of solids
- Permeability of Cake
- Specific gravity of dry solids
- Inherent Moisture of solids
- Surface Properties of solids
- Type of Filter
- Homogeneity of cake formation
- Interfacial tension of solids and liquid
- Temperature of gas and liquids

*(cfm/ft ^{2})(P/W)(td)*

where *cfm* of air per filter area in ft^{2},

*W* is the weight of the dry filter cake per filter revolution,

*P* the total pressure drop, and

*td* the filtration time per cycle.

As such, production moisture decreases with:

- An increase in P and td.
- A decrease in the filter cake weight W

### Filter Design

The required area and number of filters needed is a direct function of the required product moisture and the required amount of filtrate needed to be separated from the solid.

The filtrate, Q, can be assumed using the D’Arcy equation:

*Q = AΔp/U(R _{m}+R_{c})*

ΔP = the pressure drop across the cake or driving pressure,

U – the filtrate viscocity,

A = the filter area,

R_{m} and R_{c} = membrane and cake resistance

### Clarification (Thickeners)

The principle objectives of a thickener are to:

- Clarify water for reuse in the plant;
- Thickening of the solids.

To design a thickener we must know:

- Total feed volumetric and mass flow;
- Identity of unwanted solids in the overflow stream;
- Desired concentration of solids in the underflow stream.

The design parameters to be determined include:

- Cross-sectional area;
- Total depth.

There are three modes of sedimentation in a thickener:

*Clarification*in which solids settle either individually or are collected into separate floccules, each of which then settles at its own characteristic settling rate, closely related to Stoke’s law;*Zone**settling*in which particles cohere into a structure such that all in a given neighborhood subside at the same rate, but the structure does not lend mechanical support;*Compression*in which the structure is capable of mechanical support.

### Particle Growth

Particle aggregation is a key component to consider in the design and operation of a thickener.

Particle enlargement affects:

- Clarity of the overflow stream;
- Settling rates (reduces thickener size);
- Underflow solids concentration.

Particle enlargement typically involves the addition of:

- pH modifiers;
- Adjusts surface charge
- Coagulants;
- Neutralizes surface charge.
- Flocculants.
- Aggregates particles
- Cationic = added first to initiate particle aggregation.
- Anionic = high molecular weight added last to bridge particles for finalize aggregation.

### Thickener Design

### Clarification Zone Design

Clarification zone requirements are commonly expressed in terms of overflow velocity, *v _{o}*, and,

*t*, retention time.

These parameters relate to pool area and depth according to the following expression:

*v _{o} = Q/A t = V/Q = Ah/Q = h/vO*

A = settling area of the clarifier in m2,

h = depth of the clarification zone in meters,

V = clarification zone volume in m^{3},

Q = the volume of the overflow per unit time in m^{3}/h.

Residence time should be sufficient to allow the aggregates to settle through the clarification zone.

Typical settling rates are 4 to 6 inches/minutes and may go up to 8 to 10 inches/minute for undersized thickeners.

### Thickener Area Design

Coe and Clevinger provided an expression that relates the settling rates of the particle aggregates and the required area of the thickener when the medium is water:

*A = 1.333 (F-D)/V*

A = the area in ft2/tph of dry solids per 24 hours (unit area),

F&D = the liquid-to-solid weight ratio in feed and underflow, respectively,

V = aggregate settling velocity in feet/hr.

The relationship between area and depth is most important for the following reasons:

- Tank volume must provide sufficient residence time considering both operational efficiency and mechanical design;
- Thickener efficiency decreases as the ratio between depth and diameter decreases.

As such, a number of batch tests are run with vayinf F values and thus different V.

A plot of unit area A versus F establishes the maximum value of A which must be used for design purposes.