Thickening & Sedimentation Processes in Mineral Processing PDF

# 15.2 Thickening ## 15.2.1 Introduction Thickener is a process of removing water from a slurry to increase the solid content. This is done by allowing the solid particles in the slurry to settle to the bottom of a tank. The process is crucial in mineral processing. ## 15.2.2 Basic Thickener Oper...

# 15.2 Thickening ## 15.2.1 Introduction Thickener is a process of removing water from a slurry to increase the solid content. This is done by allowing the solid particles in the slurry to settle to the bottom of a tank. The process is crucial in mineral processing. ## 15.2.2 Basic Thickener Operation Thickener consists of a tank with a central feedwell and a peripheral launder for overflow. 1. The slurry is fed into the thickener through the central feedwell. 2. The solid particles settle to the bottom of the tank, forming a thickened underflow. 3. The clarified liquid, which is free of solids, overflows the launder. 4. The thickened underflow is withdrawn from the tank at the bottom. ### Factors Affecting Thickener Operation: - **Particle size and density** - **Feed solids concentration** - **Flocculant dosage** - **Tank diameter and depth** - **Rake speed** - **Underflow discharge rate** - **Temperature** - **Slurry viscosity** ## 15.2.3 Thickener Types There are four types of thickeners: - **Conventional:** These are the most common type of thickener. They have a more or less open tank design. - **High rate:** This type of thickener is optimized for flocculation and designed for high throughput. - **High density:** This type of thickener is designed for slurries with a high concentration of solids. - **Paste:** This type of thickener is designed for slurries with an extremely high concentration of solids. The conventional thickener is the most widely used type. It consists of a cylindrical tank with a diameter ranging from 2 to 200 m and a depth of 1-7 m. The clarified liquid overflows a peripheral launder, and the thickened pulp is withdrawn from an outlet at the center. ## 15.2.4 Thickener Operation and Control The two primary functions of the thickener are: - **Production of a clarified overflow:** This is achieved by ensuring that the upward velocity of the liquid is lower than the settling velocity of the slowest settling particle. - **Production of a thickened underflow:** This is achieved by controlling the residence time of the particles in the thickener. The solids concentration in a thickener varies from that of the clear overflow to that of the thickened underflow being discharged. When materials settle with a definite interface between the suspension and the clear liquid, as is the case with most flocculated mineral pulps, the solids-handling capacity determines the surface area. **Control of the thickener includes:** - **Monitoring the bed level:** This is crucial in monitoring the efficiency of the thickener. - **Controlling the feed mass flow rate:** This is important to control the clarity of the overflow water. - **Flocculant Dosage Rate:** Keeping the dosage to a minimum while maintaining target performance is important. ## 15.2.5 Thickener Sizing Thickener sizing methods include: - **Experience:** This is used if there is no sample, or to prepare a rough first draft. - **Cylinder (or jar) settling tests:** These are used to prepare a rough first draft for sizing equipment for budget purposes at the beginning of the project. - **Pilot plant testwork:** This is the most reliable method. Pilot plant units are small-scale versions of commercial thickeners. **Batch settling tests** are the most commonly used. There are two well-known methods to design thickeners based on the settling curve: 1. **The Coe & Clevenger (1916) method** 2. **The Talmage and Fitch (1955) method** ## 15.2.6 Other Gravity Sedimentation Devices Other gravity sedimentation devices include: - **Tray thickener:** This type of thickener saves space and is often used for slurries that settle quickly. - **Lamella thickener:** This type of thickener minimizes the distance for particle sedimentation. - **Hydrocyclone:** This device utilizes centrifugal force for separation. ## 15.3 Centrifugal Sedimentation Centrifugal separation increases the settling rates of particles by applying centrifugal force. It can separate emulsions which are not normally stable in a gravity field. **Types of centrifugal separation:** - **Hydrocyclones**: These devices are simpler and cheaper than centrifuges but have limitations in terms of solids concentration and particle size. - **Centrifuges**: These devices are more costly and complex than hydrocyclones but offer higher clarifying power. ## 15.4 Filtration Filtration separates solids from liquid by passing a slurry through a porous medium. **Steps in filtration:** - **Cake formation:** The solids build-up on the filter. - **Moisture reduction:** Water is removed from the cake. - **Cake washing:** This is optional and helps remove impurities. - **Cake discharge:** The cake is removed from the filter. - **Medium washing:** The filter medium is cleaned. **Filter types:** - **Vacuum filters:** Commonly used in mineral processing and can be batch or continuous. - **Pressure filters:** These are used to handle higher flow rates and achieve lower cake moisture. ## 15.4.1 Brief Theory Filtration is governed by the Darcy and Poiseuille’s Law: (Equation: 1/A dV/dt = ΔP/(µ αV)) ## 15.4.2 The Filter Medium The filter medium provides support for the filter cake and should be chosen for resistance against blinding and corrosion and offering minimum resistance to filtrate. ## 15.4.3 Filtration Tests Filtration tests are crucial to determine the performance of a filter before scaling up. ## 15.4.4 Types of Filter Filter types include graimetric dewatering, cake filters (pressure and vacuum), and tube press. ## 15.5 Other Solids Separation Devices Other solids separation devices include flotation, screening and magnetic separation. ## 15.6 Solid-Liquid Separation System Design and Operation The design and operation of a solid-liquid separation system depends on the specific application. Considerations to be made include the feed characteristics, desired product quality, and economic factors.