Advantages Of Disc Screen In Processing Difficult-To-Screen Materials And Its Coal Industry Application

Introduction

In today’s mining, mineral processing and other industries, the screening and classification of materials are one of the core links. Traditional screening equipment often has problems such as low screening efficiency, easy clogging of screens and severe equipment wear when dealing with difficult-to-screen materials with high moisture content, high viscosity or a large amount of fine mud. For example, in the coal industry, raw coal often contains a large amount of clay and moisture. During the screening process, it is very likely to adhere to the screen mesh, causing the screen holes to be blocked. In severe cases, it may even force the production line to stop for cleaning. Different from the traditional vibrating screen that relies on the excitation of vibrating motors, the disc screen achieves the classification of materials by having multiple rollers with screen discs rotate in the same direction or at different speeds, driving the materials to roll, collide and move forward on the screen surface.

Typical difficult-to-screen materials in the coal industry

Raw coal with high moisture and high viscosity

With the improvement of coal mining mechanization and the wide application of measures such as hydraulic coal mining and water spraying for dust prevention, the external moisture content of raw coal has significantly increased. In many mining areas, the moisture content of raw coal is as high as 7% to 14%. Meanwhile, the content of pulverized coal (fine particles) in raw coal has also increased due to mechanized mining. The combination of moisture and fine particles forms typical moist fine-grained viscous materials, which are prone to accumulate and adhere to the screen surface, blocking the screen holes.

High-concentration water-coal slurry

In the field of coal chemical industry (such as Texaco gasification process), high-concentration water-coal slurry is a material that is recognized worldwide as difficult to screen. Its concentration requirement is greater than 60%, viscosity as high as 2000 to 3000cp, and it contains extremely fine particles (such as 99.5% over 40 mesh). The purpose of screening is to remove the large particles produced during the pulping process to prevent them from wearing out pumps, nozzles and clogging valves, ensuring the stable operation of the gasification system.

Other types

According to the characteristics of the materials, difficult-to-screen materials also include thermally viscous materials, liquid-solid mixed materials, and stone materials with strong viscosity mud, etc. However, in the coal industry, the first two are the most common and difficult to handle.

Why can disc screen effectively handle difficult-to-screen materials

Roller structure and motion mode

The core working component of disc screen is multiple cylindrical rollers arranged in parallel. When the rollers rotate at a certain speed, the material is conveyed forward in the gaps (sieve holes) between the rollers. At the same time, through the rotation of the rollers, the material is continuously turned, stirred and rubbed, which can effectively break the sticky aggregates formed by the high moisture and fine particle content of the raw coal. Reduce the accumulation and adhesion of materials on the screen surface.

A highly adaptable material conveying and screening mechanism

The roller clearance of the disc screen serves as the screening channel. Under the push of the rollers, the material passes through the screen surface in a laminar flow manner, avoiding the material splashing and intensified adhesion caused by high-speed vibration. The smooth or specially coated surface of the rollers can reduce the adhesion of the slurry. Meanwhile, through the continuous rotation of the rollers, large particles (impurities to be removed) are conveyed forward, while fine particles that meet the requirements fall through the gaps, achieving efficient separation of solid-liquid mixed materials.

The universal advantage over other difficult-to-screen materials

Dynamic self-cleaning ability

The continuous rotation of the roller can achieve self-cleaning through the friction and collision between the material and the roller surface, reducing the adhesion of sticky materials.

Anti-clogging design

Compared with the static screen surface of fixed screens or vibrating screens, the roller clearance is less likely to be jammed by fibrous and sticky lumps of materials, and is especially suitable for screening stone materials containing sticky mud.

Mild handling characteristics

For thermally viscous materials, the slow conveying and low-impact effect of the rollers can prevent the intensification of material adhesion caused by overheating or squeezing, ensuring the stability of the screening process.

The influence of material properties on the screening effect

The moisture content of the material

There are two types of moisture in materials. The moisture contained on the surface of the material is called external moisture. The underground water spraying for dust removal in raw coal mining is external moisture. Another type exists in the pores of the material, that is, the internal moisture. During the screening process, external moisture has a relatively significant impact on the screening.

The mud content of the material

If the material contains clay-like substances that are prone to clumping, even if the moisture content of the material is relatively low, the screening process will be more difficult. This is because clay-like substances tend to stick together and clog the screen holes, resulting in a decrease in screening efficiency

Particle shape

The screening of particles of different shapes varies significantly. Spherical or nearly spherical particles have good fluidity, are easy to sieve, and have high screening efficiency. Flat, needle-like or irregularly shaped particles are prone to clogging and bridging, hindering the passage of materials. For instance, flaky particles are difficult to pass through traditional vibrating screens, tend to accumulate and get stuck in holes, reducing efficiency and increasing costs. The rotation of the disc screen roller can cause stronger disturbance and flipping of irregular particles, promoting their change of posture, reducing retention, and improving the screening problem.

Particle density

It usually refers to the bulk density of the material. Under natural packing conditions, it is the mass of the material per unit volume, including the internal pores of the particles and the voids between the particles. It is greatly affected by the shape of the particles, particle size distribution, surface friction coefficient and humidity. During the screening process, the particle density will directly affect the movement state of the material on the screen surface and the stratification effect.

The specific application of disc screen in the coal industry

Coal conveying system of thermal power plants

In the coal conveying system of thermal power plants, the disc screen is usually installed before the coal crusher to pre-screen the incoming coal. Its function is to separate qualified coal particles smaller than the set particle size (such as 30mm) and directly enter the next stage, and send large pieces of coal larger than the set particle size to the coal crusher for crushing.

Raw coal preparation workshop of the coal preparation plant

In coal preparation plants, disc screens are used for the pre-screening, desliming (removing fine coal slime) or classification of raw coal. Especially when dealing with raw coal with high ash content, high moisture content and easy to muddle, its anti-clogging characteristics stand out. We once had a coal preparation plant project where we replaced the original fixed bar screen with disc screen, effectively solving the problem of material blockage for the client and significantly improving the screening efficiency.