We recommend that you begin by screening the iron ore to separate fine particles below the crusher’s CSS before the crushing stage. A static screen is used to divert the fine particles for crushing. This step prevents overloading the crusher and increases its efficiency.
Fine particles make the crusher less efficient and increase the power draft for crushing such small particles which increases the kWhr per ton crushed. Screening and diverting fine particles significantly increases the performance of the crusher.
2. Crushing and stock piling
This step involves breaking up the iron ore to obtain an F 80 grind size before it can be ground. Once the ore has been crushed, it is diverted to a stockpile. The aim of this step is to ensure a smooth crushing process and minimise downtime in the event of a breakdown or to allow planned maintenance in the crushing system, without stopping the grinding and other further steps. If the crusher breaks down, the performance of the entire factory may be affected.
3. Feeding the crusher (Reclaim)
The surge pile allows a constant supply of material to be fed into the mill, which ensures consistent results and stability in the ore recovery process.
The primary grinding mill grinds the material before it undergoes spiral gravity separation. Efficient grinding means that there will be minimal recirculation material from a cyclone cluster or screening after milling, which decreases the kWhr per ton ground.
This step involves the material being retained inside the mill for a specific time (with or without grinding balls) which is calculated by applying Bond’s work index to the process calculation. This index takes into account the material ore hardness and feed size, mill speed, material recirculation, desired fineness, etc. When the material exits the mill, the iron ore is classified using cyclones or vibrating screens that separate fine and large particles. The large particles are diverted back into the crusher and the fine particles will undergo a spiral gravity separation process.
5. Spiral gravity separation
The gravity separation feed range is approximately 30% solid concentration. Process water is used to assist the separation process. The heaviest particles are separated from finer particles by centrifugal force. A collector at the bottom of each spiral collects the heavy concentrate (so-called hematite concentrate) and eliminates the lighter unwanted particles (middlings and tails). This step is repeated two or three times, depending on the desired liberation and iron concentration.
6. Magnetic separation
The concentrate collected from the middlings and tails after spiral gravity separation has a lower iron concentration (magnetite) so it must be further concentrated to achieve the customer’s 65% Fe concentration. It is immediately directed to a magnetic separator circuit. Magnetised drums collect magnetic material, so it can be further concentrated (magnetite iron minerals content). The lighter particles that were separated during the spiral gravity separation step are rolled into balls (combined with bentonite or a similar binding agent) and cooked in an induction oven. This process is known as pelletisation. Rejected non-magnetised material is diverted to the residue disposal area (tailing).
Depending on the hardness and size of particles and the crushing results, there may be several crushing, grinding and mineral separation stages.
The best advice that we at Multotec can give you is to use products specially adapted to iron processing and also get to know your machines. Our team of experts can help you with any questions you may have about cyclones, spirals, drums and samplers, and provide support during the stages of iron ore processing.
Click on image to view the Iron Ore Flow Sheet.