Characterisation of Bangladesh ilmenite in the process of its upgradation into synthetic rutile

Abstract

Heavy minerals found in the beach sands of Bangladesh are one of the most important mineral deposits that have, so far, been identified in Bangladesh. Ilmenite is the most abundant mineral in these deposits and constitutes about 25 per cent of all the heavy minerals. It has been estimated that about one million tons of ilmenite can be separated from the beach sands of Bangladesh. In 1977, a pllot planl was set up at Cox's Bazar by the Bangladesh Atomic Energy Commission to separate the indlvidual heavy mmerals from these deposits. Unfortunately, however, the ilmenite produced at Cox's Bazar pilot plant contains less than 40 percent TiD, together with a large amount of iron in the form of oxides, silica, magnesium and unaeeept1ble amounts of manganese and chromium. Ilmenite containing less than 55 pereent titanium di-oxide is not considered suitable for industrial use. As a result, this valuable mmeral separated from the beach sands of Bangladesh, which is used in Ihe mnnufaeture of paints, welding electrodes, ceramics, etc. could not find any market in Bangladesh or elsewhere. In view of rapid depletion of known reserves of natural rutile (TiD,) efforts are being made throughoutlhe world to transform ilmenite into rutile through thermo-chemical treatment. As a result of these investigations, a number of eommereial processes for the produetion of rutile from ilmenite have been developed. This rutile is known as synthetic rutile. No efforts arc known 10 have been made to convert Bangladesh ilmenite to synthetic rulile. It was, therefore, considered necessary to explore the possibility of converting Bangladesh ilmenite to synthetic rutile througb thermo--chemical treatment. It is believed that the successful conversion of Bangladesh llmem1Cinto synthetic rutile will make litis valuable mineral of Bangladesh useful at home and abroad, enhance the confidence in our ability 10use cur own natural resources and also the preslige of the nation. In this dissertation mvestigations were carried out 10charactenze Bangladesh ilmenite in the process of its up-gradation into synthetic rutile. Raw sand containing the heavy minerals was collected from the back dune surfuce layers ofhcavy mineral deposits at Teknaf village area (Lengurbil deposit) from points located 2-3 Km away from the water line and 5-6 meter above mean sea leveL The area selected fur the study falls between the latitude 20'52'7" to 20'52'21"N and longitude 92'16'10" to 92"15'58"E at Teknaf. lbe heavy mineral proportions VllJ of the concentrates were determined to be 89-92% by point counting grams using both reflected and transmitted light microscopy. Among the minerals found in the Tdmafbeach sands ilmenite is the most important One It constitutes around 24% percent of heavy mineral tonnage, the other important constituent minerals of the heavy minerals being rutile and zircon. Factory grade llmenite was produced at thc Beach Sand Mineral Exploitation Centre, Cox's Baulr. The techniques of seperation are based on the difference in physical properties hke density, electrical and magnetic properties, The reduction-leach routc of up gradation of ilmenite was followed in this investigation. The sample of ilmenite was analyzed chemically by standard methods of wet chemical analysis in the as received condition, after oxidation and reduction and also after magnetic and sizc fractionation. X-ray diffraction analysis was performed to identify the phases present in the various fractious. Thermo-gravimetric analysis of the magnctic fractions was carried out to detect the presence of water in the sample. Combined water in ilmenite is an indicator of tile extent of alteration, The effect of oxidation on the extent of reduction and also on subsequent leaching of Bangladesh ilmenite has been investigated. The effects of time and temperature on the oxidation ofllmenite have been followed by chemical analysis, X-ray diffraction and optical microscopy. A few samples of ilmenite wete also observed under a scanning electron microscope. The sample of ilmenite both oxidized and as received was reduced with charcoal in the temperature range of800"C to llOO'C for treatment tlme ofup-to 5 hr, The magnctic fractions and the size fractions were also reduced under identical conditions. The extent of reduction was followed by chemical analysis and X-ray diffractlOn analysis. The reduced samples werc also observed under optical and scanning electron microscope. The reduced samples were leached in hydrochloric acid. An attempt has been made to understand the kincties and mechanisms ofleaching process. llemenite sample under invcstigallon contained 25.32% FeD, 32.53% Fe,O, and 39.96% TiO,. From the sieve analysis it was found that more than %%ofthe ilmenite belongs to the US sieve no, 100, 140 and lOO.Thesphericity of all three samples was found he in a narrow range, 0.822-0,843. However, finer size fractions were found to have a slightly hlgher sphericity than the coarser one. Magnetic fractionation showed that ahont 96% ofTeknaf ilmenite belongs to the fractions separated by a current of 0.10 - 0,30 amperes and relativc sp,gr, range of 4, 1-4.5 in comparison of synthetic rutile sp.gr. 4.7 X-ray diffraction patterns of different magnetic fractionated as received ilmenite shows the presence of pscudorutile phase and TGA report indicate the presence of bonnd water. " 1. .- Detailed investigatIon on the X-ray pattern and TGA report led to the conclusion Teknaf ilmenite is in a partially altered stale bUInol hydrated enough. Samples of ilmenite have been oxidized in the tcrnp1:rature range of 750-950'C for a period of up--to 2 he Scanning electron microscopic studies revealed tbe presence of cracks in the o:<idizoo samples. Chemical analysIs results have shown that the oxidation of the as received ilmenite in normal furnace atmosphere at 950°C for I hr, converts all but 0.57 percent FeO. The samples of ilmenite were reduced by charcoal in thetemperaturc lllDge 800"C to 1100"C for treatment lIme of up-to 5 hr. The optimum conditions for reductions were found to be l050"C and 4 he.The maximum extent of reduction was found to be 87.61 %. It was also identified the optimum ilmenite coke ratio for reduction is I: L Finer feactions ofTeknafilmemte contain higher percentage ofiron oxide. They actually get reduced at a faster rate and the redueed samples of finer fraction contain a lower amount of iron in the mode form. So adaptation of separate reduction procedure for each of the size fractions is not necessary. Ilmenite fractionated at lower current contains higher percentAge of iron oxide. Fractions separated at lower currents are reduced to a higher elctent than those separated at higher currents and the reduced samples separated at lower currents aetually contain lower amount of iron in the oxide form. Thus adaptation of separate reduction procedure for each of the fraction will not be helpful to enhance the reduction process. Reduction with charcoal under optimum condition showed that the maximum metallic iron content of the sample reduced after oxidation were higher (39.36%) than in the samples reduced (38.36%) in the as received condition. Reduction of ilmenite m the oxidized condition was found 10 proceed in two stages. The first stage is the reformation of ilmenite according to the either or both of the follo"1ng reactions: The second stage is progressive reduction of ilmenite 10 metallic iron and rutile according to following reaction: On the other hand, the reduction of as received Bangladesh ilmemte involves (i) conversion of pseudorutile into rutile and ilmenite and (ii) change of ilmenite into metallic iron and rutile, The reduced ilmenite samples were leached m 5 10 15 pereent hydrochloric acid solution in temperature range of 30 10 85°C for pcnods of up 10 2 hr. Attempts werc madc 10 identify the effects of oxidation on the extent of leaching. Oxidation prior 10 reduClion of Teknaf ilmenite w.lS found 10 inerease the rate of leaching. The iron removal from leached ilmenite after reduction in the oxidized condition is more than from ilmenite leached after reduction in the as received condition, The kinetic data of leaching of ilmenite reduced after oxidation was found to foHow ftrst order reaction modcl i.c. G (a) = -In (l-a) up-to an a value of 0.5 (up.to 50 percent reaction) and then ehange to spherical model i.e.,G (a) =[1-{ l-a)tll] = (kit) t On the other hand, leaching of ilmenite reduced WIthout OXIdIzing was found to f?llow the spherical model i.e., G (a) = [1_(I.a)'IlJ = (klr)! .al-through thc leaching process. Oxidation of ilmenite prior to reduction was also found to have decreased the activatIon energy of leaching from 23 kJlmol, found for samples leached after reduction in the as received condItion, WIthout oxidizing, to 16 kJ/moL for sample leached after oxidation and reduction, The X-ray diffraction pattern of the leached sample did not show the presence of metallic iron or llmemte, the maximum intensity peak was that of rutile. TiO, content of synthetic rutile prepared from Teknafilmenite is very much compareab[e with commercially available s)nthetic rutile. The values for the other constituent of iron oxide are weH comparable, The variations may be due to the difference in the raw materials used, In general it can be stated that S)nthelle rutile prepared from Teknaf ilmemte by this oxidation-reduction-leach route is equally good if not better than the commercially available sample, So synthetic rutile obtained m these studies contains 91.50 perccnt TiO" which is very much compareable in chemical constitution with the commercially available synthetic rutile.