What is concentrator?
An iron ore concentrator is a facility that processes raw iron ore to increase its iron content and remove impurities such as silica, alumina, and other non-ferrous materials. The goal is to produce a concentrate that can be used in steelmaking.
The concentrator plant design for ArcelorMittal Liberia’s 15 Mtpa project is optimized to produce a wet product using staged magnetic and gravity separation processes. The design allows for future expansion to process hard transition ore, with minimal modifications needed to convert to a -1.0 mm coarse concentrate product. The plant includes a coarse tailing dewatering system for dry stacking, reducing the tailings storage facility footprint and cost. The overall design enables the staged addition of 5 Mtpa processing modules, with a gradual ramp-up to full capacity of 15 Mtpa.
The Process plant operates at 85.5% of total hours per year. This allows planned maintenance for 10% of the yearly hours and 5 % for breakdowns. The 10% planned maintenance hours is made up of: 8 hrs / week. 16 hrs / month – which combined with the weekly 8hr will give 24 hr once a month. 96 hrs/ yr – which combined with the above weekly and fort nightly shut will give 120 hrs (i.e. 5 day shut once a year). 172 hrs/yr ad hoc. The above maintenance hours total 876 hrs/yr (10% of hours in a year). On top of this, we have allowed 5% for breakdowns. Which means the plant is running 7490hrs /yr (85.5% of available hours). Raw ore stockpile to process plant: 22.68 Mtpa (wet). Operating at 7490 hrs/yr (85.5% of total hours per year. 10% planned maintenance and 5% breakdowns gives 7490 hr running per year see below for breakdown). Gives 3028 tph (wet). Process plant to product stockpile: 15 Mtpa (wet). Operating at 7490 hrs/yr. Gives 2003 tph (wet). Course Rejects 4.23 Mtpa (wet). Operating at 7490 hrs/yr. Gives 564 tph (wet) (assuming 8% moisture is achievable). Fine Rejects 3.73 Mtpa (wet). Operating at 7490 hrs/yr. Gives 499 tph (wet) (assuming 15% moisture is achievable). Total Rejects — 7.96 Mtpa (wet). Operating at 7490 hrs/yr. Gives 1062 tph (wet).
Concentrator Process description.
The concentrator plant has been broken down into the following areas: SAG milling (Area 3410) , Concentration (Areas 3420, 3430). Concentrate dewatering (Area 3440) Tailings disposal (Area 3450) , Ancillary areas (Areas 3460, 3920) . The plant is designed for three process lines, 5 Mtpa product each with potential sprint capacity, except for combined circuits such as thickening and conveying systems. The description below generally follows an individual 5 Mtpa process line.
SAG milling: In the milling area the crushed ore will be reduced in size by the SAG mills. Fresh feed to the plant will be fed into the mill via a SAG mill feed conveyor. The mill product will then pass across the mill trommel, with any oversize being discharged into a tramp bin. The material that passes through the trommel will then be sized on three SAG mill double deck screens, where the top deck oversize is recycled back to the SAG mill feed conveyor for regrinding, the middle deck product chute will be modified to tie into the oversize chute and report back to the SAG mill, and the undersize is passed on to the concentration area. Due to the friable nature of the oxide and transition ores, which results in a non-standard raw ore size distribution feed to the SAG mills, a state-of-the-art SAG mill monitoring system has been added to the instrumentation design to improve the mill operation, to lower potential damage to the mill liners, and to maintain a more consistent volume and product size distribution out of the SAG mill.
Concentration: The concentration section of the process plant uses both gravimetric and magnetic separation equipment to upgrade the ore to a suitable product grade. 1 .1 mm x 100 micrometre gravity circuit design The SAG mill screen undersize at -1 mm is pumped to the bank of de-sliming cyclones. Cyclone underflow product flows to the gravity circuit, while the overflow reports to the fines magnetic circuit. The gravity circuit feed will be split to feed four multi-deck vibrating screens into coarse and fine feed streams. These feed streams are fed to the REFLUX classifier units. The REFLUX classifiers utilize a laminar shear process for certain size ranges, which is why there will be “coarse” and “fine” units. Elutriation water is added to the classifiers to create an internal “heavy liquid” separation, with lighter ores (mainly silica) carried with the main water flow out of the top of the classifier while the denser iron concentrate is discharged from the bottom of the classifier. The REFLUX classifier concentrate from both the coarse and fine REFLUX classifiers is collected in the coarse concentrate launders and sent to the pan filters for dewatering, while the tailings from both the coarse and fine classifiers are collected in the coarse tail launder and sent to the coarse tailings dewatering area of the concentrator plant. -100 micrometre fines magnetic circuit design The -100 micrometre fines from the de-sliming cyclones is sent to a distributor, which feeds the double drum LIMS. The first LIMS drum removes the bulk of the non-magnetic material while the second drum re-cleans the first drum magnetics to produce a final quality concentrate. In addition, the LIMS removes any highly magnetic material (magnetite) to protect the MIMS. The LIMS concentrate is collected in a common pumpbox with the REMS concentrate, which pumps the LIMS and REMS combined concentrates to the concentrate thickener (previously called the “clarifier” for the pan filters filtrate in the original AMEC design). The LIMS tails flows by gravity to the hydro-separator (one hydro-separator for all three lines) to remove fine silicates from the MIMS feed, as well as to thicken the feed to the MIMS. The hydro-separator adds elutriation water to control the fluid rise rate inside the hydro-separator thus making a size split at the desired micron size. Hydro-separator overflow is sent to the tailings thickener. The hydroseparator underflow is pumped to the REMS feed distributor, with a single pump feeding each REMS. Each REMS circuit consists three single drum REMS units. The REMS concentrate is combined with the LIMS concentrate and is pumped to the concentrate thickener, while the REMS tailings report by gravity to the tailings thickener.
Concentrate dewatering
Coarse concentrate (-1 mm x 100 micrometre) from the REFLUX classifiers reports to the coarse concentrate collection launder and split to feed two pan filters to dewater the concentrate. The pan filters remove excess water from the final product before it is stockpiled for transport. This is achieved through a vacuum being applied to the “filter cake” before it is scraped off the filter for collection onto the concentrate collection conveyor. The pan filter filtrate is collected in a sump at each filter and pumped to the concentrate thickener. The -100 micrometre magnetic concentrate from all process lines will be collected in the concentrate thickener along with the filtrate from the pan and ceramic filters. Concentrate thickener overflow will be collected in a sump and pumped to the process water tank. Concentrate thickener underflow is pumped (one operating, one standby) to two slurry storage tanks for surge storage prior to filtering at the ceramic filters. It is expected that the ceramic filters will produce a lower moisture content in the filter cake versus including the fines concentrate with the pan filters. There will be a total of six ceramic filters for the three process lines. The “filter cake” will be collected onto the concentrate collection conveyor and transferred to the concentrate stockyard for storage. The filtrate is returned to the concentrate thickener. An acid cleaning system is included for periodic cleaning of the ceramic discs. The combined pan and ceramic filter moisture content is expected to be equal to, or less than, 8.0%.
Tailings disposal
The final stage of the plant is the handling, disposal and storage of the concentrator plant tailings. The coarse tailings stream from the REFLUX classifiers will be collected in a sump and pumped to dewatering cyclones. The cyclone underflow is then screened to create a coarse tailings “sand” product. Two dewatering screens per line will be required. The dewatered coarse tailings will be conveyed to a surge bin to allow for mine haulage truck disposal of the tailings. Cyclone overflow flows by gravity to the tailings thickener. The dewatering screen underflow is collected in a sump and pumped to the tailings thickener, with the thickener underflow being pumped to the tailings storage facility, and the thickener overflow being reused in the process.
