Beneficiation Studies On Molybdenite Ore From Harur Area, Tamil Nadu

Abstract

Molybdenum is a versatile alloying and refractory metal of strategic importance. In India almost all molybdenite is obtained currently as st byproduct from copper and uranium deposits. The present Indian production is only about 66 tonnes while the demand is over 1000 tomes. It thus becomes important to explore newer deposits of molybdenum and develop efficient beneficiation strategies to recover the metal. A potential source of primary rnolybdenite has been found in the Ham area of Dharmapuri district in Tamil Nadu. In the present investigation, taking into consideration the importance of the problem beneficiation studies have been undertaken on core - drill molybdenite ore samples from the Hanu area. Initially, detailed characterization of the molybdenite ore has been carried out with respect to its mineralogy and chemical composition. The X-ray diffkactogam of the ore has also been recorded These studies have indicated that quartz and sericite are the major minerals followed by carbonates and feldspar. Among the sulphide minerals, pyrite is dominant with trace amounts of galena, sphalerite, chalcopyrite and molybdenite. The molybdenum content in the ore sample has been determined to be about 0.1%. Liberation studies have revealed that molybdenum is more or less uniformly distributed in the different size fractions from 1.7 mm down below to 75 microns. However, mineralogical analysis of the different size fractions has indicated that about 85% of the molybdenite is liberated. from associated silicate and carbonates below 2 12 microns. The results of the flotation kinetic tests have shown that percentage recovery and grade of molybdenum foLZow an inverse relationship with time. Studies carried out to evaluate different reagent - combinations, reveal that optimum values of grade and recovery could be achieved using kerosene and M1IBC. Further, these studies have shown that flotation time of 4 minutes is adequate, as beyond this time the grade of molybdenum is found to deteriorate. Detailed mesh-of-grind studies have indicated that 20 minutes of grinding yields optimum values of molybdenum and lead in the float product and this time was fixed for all subsequent experiments. Preliminary experiments conducted to evaluate djfferent depressants for galena have shown that satisfactory results are obtained only with sodium dichromate. Optimisation studies performed to arrive at the dosage of sodium dichromate have shown that the addition of 0.25kg/t of sodium dichromate gives optimum results with respect to overall recovery and grade of molybdenum, copper and lead in the float and tailing fractions. In a similar manner, the optimum dosage of kerosene (collector for molybdenite) has been arrived at to be 0.8kg/t taking a holistic view of the grades and recoveries of the different metal values of interest. Based on the studies carried out with respect to optimization of the mesh-of-grind, depressant and collector concentrations, it became apparent that a grade of about 0.8% Mo only could be achieved with over 80% recovery. The redeeming feature however was that over 90% of the feed could be rejected at the rougher flotation stage itself with a negligence loss of about 0.02% Mo in the tailings. It was thus considered logical to further upgrade the Mo content by regrinding. An extensive flotation campaign was therefore mounted, adopting the following strategies: (1) Ist stage regrinding and one cleaning (2) IInd stage regrinding and three cleanings (3) IIIrd stage regrinding and two cleanings Based on the first stage regrinding tests it was found that by regrinding to 100% passing 106 microns, the Mo assay could be enhanced to over 32% from a initial value of about 0.1% in the feed. The enrichment ratio at this stage corresponds to about 340.After the second stage regrinding followed by three stages of cleaning, close to 75% recovery of Mo with an assay value of about 52% could be achieved, further enhancing the enrichment ratio to about 540. A few experiments were conducted using sodium cyanide during second stage regrinding tests and it was found that copper assay could be brought down to 0.3% from about 0.8%, without affecting the grade of lead and molybdenum. The results of the third stage regrinding tests have highlighted that a final concentrate assaying about 55% molybdenum with over 70% recovery could be obtained. It is worthy to mention that the concentrate so produced meets the specification of metallurgical grade molybdenite. The overall enrichment ratio obtained, commencing from a feed assaying 0.1% Mo, corresponds to about 580. In order to explore the efficacy of organic depressants, which are more acceptable on environmental grounds, detailed flotation studies were carried out using dextrin with particular emphasis on galena depression. A series of experiments was carried out in an identical manner to those conducted using sodium dichromate. Here again, three stages of regrinding/cleanings test where carried out, akin to those performed using sodium dichromate. A dextrin concentration of 0.01 kg/t was found to be an optimum value from the point of grade, wt. % recovery and distribution of molybdenum, lead and copper in the cleaner concentrate. It is noteworthy that after second stage regrinding followed by four stages of cleaning, the molybdenum assay value has significantly improved from 16% to about 53% with over 70% recovery. Another notable feature is that copper content in the concentrate is only about 0.2% even without the addition of sodium cyanide. The results of the third stage regrinding/cleaning tests have shown marginal improvement in the molybdenum assay values without affecting the grade of copper and lead. On a comparative basis, it can be inferred that dextrin holds promise to be used as a potential substitute for the inorganic depressants such as sodium dichromate and sodium cyanide. On the basis of the extensive investigations carried out on the beneficiation of molybdenite ore, a comprehensive flowsheet has been developed.