The rare-earth roll, generating magnetic field intensities, is very effective for concentrating or removing weakly magnetic minerals from a dry process stream. The rare-earth roll magnetic separator is designed to provide peak separation efficiency and is typically used when a high purity product is required. The roll is constructed of thin neodymium-boron-iron permanent magnetic discs sandwiched with thin steel pole pieces. Roll diameters typically range from 3 to 4 inches, although machines as large as 12-inches in diameter have been built and tested. The steel poles are saturated with magnetic flux and generate a magnetic field in excess of 21,000 gauss. The magnetic roll is configured as a head pulley in the separator. A thin belt, usually 3 to 20 mils in thickness, runs around this magnetic head pulley and conveys feed material to the magnetic field. When feed enters the magnetic field, the non-magnetic particles are discharged from the roll in their natural trajectory. The paramagnetic, or feebly magnetic, particles are attracted to the roll and are deflected out of the non-magnetic particle stream.
A splitter arrangement is used to segregate the two streams. This separator has a roll width up to 60 inches. The schematic diagram shown in Figure 7 illustrates the magnetic circuit arrangement for a typical rare-earth roll magnetic separator.
The rare-earth roll magnetic separator can effectively treat a wide variety of industrial minerals resulting in high purity products. In fact, it is the separator of choice for upgrading the raw materials for glass production such as silica, quartzite, feldspar, and fluorspar.
The roll separator is capable of processing 100 kg/hr/cm of roll width of 20 by 200 mesh material resulting in capacities up to 10 TPH on a 1.5-meter wide separator. Typically iron levels are reduced to 0.02 to 0.05 percent. This separator is also used in many specialty and value-added type applications, such as high-purity quartz, as well as many ceramic feedstocks such as alumina, kyanite, mullite, and zircon.
It is typically the case that a double-stage separation is required with the magnetic cleaning of industrial minerals. The non-magnetic product from the first stage separation is repassed to a second stage to further remove any residual iron-bearing components. Generally, between 60 and 75 percent of the magnetics removed in a two-stage separation are removed in the first separation stage. Table I provides a listing of various industrial applications currently using rare-earth roll separators.
With any type of rotating separator, the magnetic attractive force is opposed by centrifugal force. The primary variables affecting separation efficiency are the magnetic field strength, feed rate, linear speed of the separator surface, and particle mass. An effective separation requires an equilibrium among these variables. In assessing the feed rate, a balance must be struck between an economic feed rate, product specifications, and recovery. As the feed rate increases, the burden depth on the separator surface increases resulting in a loss of efficiency. The increase in burden depth can be offset by increasing the drum speed, resulting in an improved collection of magnetic particles. A practical limit exists, however, due to the centrifugal force acting on the particles. The centrifugal force exerted by the drum or roll surface is the critical factor affecting separation. Beyond the critical speed, the centrifugal force overcomes the magnetic attractive force and the separation efficiency deteriorates.
Particle size also affects separation efficiency. Coarse particles provide a high burden depth on the separator surface and respond with a relatively high magnetic attractive force. Coarse particles typically provide high unit capacities with high separation efficiencies. Fine particles demonstrate a lower magnetic attractive force. As a consequence, lower burden depths must be maintained resulting in lower process capacities.
Locked particles consisting of a magnetic portion and a non-magnetic portion are usually collected with a magnetic separator. In some applications the collection of these particles can be problematic. A locked particle reporting to a magnetic concentrate will in many cases account for any trace contamination, such as the case of silica in hematite or ilmenite concentrates.
The low-grade siliceous chromite ore from Ghutrigaon, Odisha, India, containing ~16% Cr2O3, with Cr/Fe ratio of 1.97 and ~55% of SiO2, does not find any use in metallurgical industry and hence considered as waste. Mineralogical investigation indicates the presence of chromite and quartz as major minerals with minor fuchsite and kaolinite. The beneficiation studies reveal that the product can be enriched to a Cr/Fe ratio of 3.35 and 3.02 by gravity concentration (wet shaking table) and wet high intensitymagnetic separation, respectively. Tiny Cr-grains within quartz and fine silica dusts within chromite inhibit liberation of chromite resulting in poor response to physical beneficiation. As an alternative, processing of ore through pyro-metallurgical route was evaluated. Chromite fines mixed with carbon and lime in the form of pellets/granules was charged to a plasma reactor. In about ten minutes, the metal globules/prills were separated from the slag in 1:6 ratio. The metal, examined through XRD and optical microscope, was found to be ferrochrome alloy. In situ EDAX analysis indicated the metal to have 61.51% Cr, 26.52% Fe and 13.1% C with minor silica (2.42%), and the slag was composed of Ca2Al2SiO7 which revealed that both metal and slag so obtained could suitably be used in different industries.
Cicek T, Cocen I, and Birlik M, Applicability of multi-gravity separation to Kop chromite concentration plant, in Mineral Processing on the Verge of the 21th Century, Balkema, Rotterdam. Proceedings of 8th International Mineral Processing Symposium, Antalya (2000), p 87.
Meegoda JN, Hu Z, and Kamolpornwijit W, Conversion of Chromium Ore Processing Residue to Chrome Steel. Final Report, New Jersey Department of Environmental Protection, New Jersey Institute of Technology (2007), p 19.
Murthy I N, Babu A N, and Rao J B, High Carbon Ferro Chrome SlagAlternative Mould Material for Foundry Industry, in International Conference on Solid Waste Management, 5IconSWM, Procedia Environmental Sciences 35 (2016) p 597.
The authors wish to thank the Council of Scientific & Industrial Research (CSIR) for the financial support to one of the authors (AKD) in the form of JRF[09/1036(0014)/2019-EMR I]. The authors would like to thank The Director of Institute of Minerals and Materials Technology (IMMT)-CSIR for providing necessary facilities to carry out the various experiments. The authors would also like to acknowledge Ravenshaw University for providing necessary laboratory facilities for carrying out the work.
Das, A.K., Khaoash, S., Das, S.P. et al. Processing of Low-Grade Chromite Ore for Ferroalloy Production: A Case Study from Ghutrigaon, Odisha, India. Trans Indian Inst Met 73, 23092320 (2020). https://doi.org/10.1007/s12666-020-02032-5Get in Touch with Mechanic