Coal with particle size bigger than 1mm is usually separated from waste materials utilizing a dense medium separation process. This process takes benefit from the density differences between the coal (usually RD 1.30 ?¡ìC 1.50) and the gangue materials (RD > 1.75).
A reliable medium of the known Relative Density is made up, and in the beneficiation procedure the coal floats on top of the medium whilst the gangue sinks towards the bottom. This gravity process is often sped up by utilising Dense Medium Cyclones.
Magnetite(Fe3O4) is the preferred ferromagnetic mineral accustomed to produce the suspension needed in the beneficiation process. There are a variety of causes of this:
- Magnetite has a RD >4.9, and can be employed to create stable suspensions from RD1.30 – 1.80 which is the preferred operating range for the majority of Coal Beneficiation Plants.
- Magnetite is magnetic and hence could be recovered by Magnetic Separators and re-used in the process
- Magnetite is comparatively inexpensive in comparison with other high density supplies such as Ferro Silicon (FeSi)
The Coal Washing Process?¡ìC an average Dense Medium Cyclone Plant
Dry Magnetite powder is combined with water and pumped into the magnetite circuit. A sump containing the suspension in the required density is known as the Correct Medium Sump.
Raw Coal from the Mine is usually crushed to -50mm and it is then sent to a deszenith screen where the fine coal (-1.4mm typ.) is rinsed off and it 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 towards the Dense Medium Cyclone(s) for separation.
Once separated, the merchandise and also the Reject are accountable to Drain & Rinse screens to be able to recover the magnetite. On the first part of the screen, magnetite is drained and reports back to the right Medium sump for re-use in the circuit. About the second area of the screen, magnetite sticking with the coal particles is rinsed off with water in the process. This rinsed medium reports to some Dilute Medium sump where it is pumped to Magnetic Separators that recover the magnetite and bleed fine coal from the system.
Concentrated magnetite in the Magnetic Separators (RD>2.0 typ.) usually reports for an Overdense Sump from where it is distributed in to the required circuit.
Magnetite losses are usually expressed in kilograms of magnetite per tonne raw coal processed (kg/t ROM), although it has become more prevalent to express losses when it comes to 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 encounter the million of dollars per annum, it is important the Coal Process Engineer gives creedence to this area of the procedure. Martin & Robson, like a reputable magnetite supplier, ensures great attention pays during magnetite processing.