Coal washer also named coal washing machine, coal washing equipment is a key equipment used in coal processing plants.
Coal Washing Description
Coal with particle size larger than 1mm is usually separated from waste material using a dense medium separation process. This process takes advantage of the density differences between the coal (typically RD 1.30 – 1.50) and the gangue materials (RD > 1.75).
A stable medium of a known Relative Density is made up, and in the beneficiation process the coal floats on top of the medium whilst the gangue sinks to the bottom. This gravity process is often sped up by utilising Dense Medium Cyclones.
Magnetite (Fe3O4) is the preferred ferromagnetic mineral used to create the suspension required in the beneficiation process. There are a number of reasons for this:
1. Magnetite has a RD >4.9, and can be used to create stable suspensions from RD1.30 – 1.80 which is the preferred operating range for the majority of Coal Beneficiation Plants.
2. Magnetite is magnetic and hence can be recovered by Magnetic Separators and re-used in the process
3. Magnetite is relatively inexpensive when compared to other high density materials such as Ferro Silicon (FeSi)
Coal washer overview
Coal washer is an important washing machine in coal washing process. We know, when the raw coals are mined from underground, they need to pass some coal working procedure so that coal can be used in our modern life.
Coal washing is an essential pre-process of coal cooking. Coal can divide into raw coal and clean coal. Raw coal is generally used as fuel for energy, clean coal or fine coal is used for coal cooking.
Clean coal should be washed with water to sulfur, to impurities and other industrial processes, in order to achieve the criteria used coke. So, coal washer also called coal washing machine plays an important role in this coal processing.
The Coal Washing Process– a typical Dense Medium Cyclone Plant
Dry Magnetite powder is mixed with water and pumped into the magnetite circuit. A sump containing the suspension at the required density is known as the Correct Medium Sump.
Raw Coal from the Mine is typically crushed to -50mm and is then delivered to a desliming screen where the fine coal (-1.4mm typ.) is rinsed off and is sent to the fine coal and ultrafine coal circuits.
The 50 x 1.4mm is mixed together with the magnetite suspension from the Correct Medium Sump, and is pumped to the Dense Medium Cyclone(s) for separation.
Once separated, the Product and the Reject report to Drain & Rinse screens in order to recover the magnetite. On the first part of the screen, magnetite is drained and reports back to the Correct Medium sump for re-use in the circuit. On the second part of the screen, magnetite adhering to the coal particles is rinsed off with water from the process. This rinsed medium reports to a Dilute Medium sump from where it is pumped to Magnetic Separators that recover the magnetite and bleed fine coal out of the system.
Concentrated magnetite from the Magnetic Separators (RD>2.0 typ.) usually reports to an Overdense Sump from where it is distributed into the required circuit.
Installation and debugging of sand washer
1. Motor should outfit rain shelter to protect equipment from creep age.
2. Sand washer should be installed on the back of vibrating screen. The rotary direction must strictly according to the requirement.
3. Check every connective bolt has been fastened before trial run.
4. Check every movable part has been lubricated well before trial run.
5. Drive belt pulley run by man before trial run. Make sure there are not any blocks in the process of working, and then user can carry on blank test. During two hours there are not any striking, pulley swinging and bearing loosen occur and the bearing temperature rise cannot exceed 35 centigrade. If the above trouble occurs, please stop machine immediately and check it.
6. The machine should be installed on the reinforced concrete. Beside this, machine user need to build a concrete sedimentation basin to deal with the waste water. The waste water can be reused after precipitation. These techniques not only meet the environmental requirement but also reduce the sand’s cost.
7. Because this machine’s working place and conductor materials are contacted with water, so users must guarantee the lead is insulated.
Magnetite losses are usually expressed in kilograms of magnetite per tonne raw coal processed (kg/t ROM), although it is becoming more common to express losses in terms of kilograms of magnetite per dry coarse coal feed (kg/t dccf).
Modern coal plants typically have consumption in the order of 0.5kg/t ROM.
Losses of magnetite occur in four main areas:
With Product Coal
With Reject material
Magnetic Separator Effluent
Leaks & Spillages
As magnetite costs can run into the million of dollars per annum, it is important that the Coal Process Engineer pays attention to this area of the process. Martin & Robson, as a reputable magnetite supplier, ensures great attention is paid during magnetite processing.