Beneficiation of Low Grade Limestone from Madukkarai, Coimbatore District, Tamil Nadu, India

A low grade Limestone from ACC mines, Madukkarai, Coimbatore district, Tamil Nadu, India, was subjected to beneficiation by cationic reverse flotation process with the aim of producing cement grade and metallurgical grade concentrates. The low grade limestone analyzed 43% CaO, 76%TC, 18% SiO2, 1.3% MgO, 1.30% Fe2O3, 2.50% Al2O3, 0.33% alkalis and 36.00% LOI. It contained mainly calcite and quartz which were mutually intergrown with fair degree of liberation at 65 mesh size. Reverse cationic flotation was preferred to direct soap flotation, as practiced in beneficiation plant at Madukkarai. Inverse flotation studies were carried out to float siliceous impurities using cationic collectors varying collector type, collector dosage, mesh of grind and pulp density. Cement grade composite concentrate ( non-float and slimes) assaying 9.51% AI, 90% Total carbonates at wt.% yield of 88 by a process comprising of grinding to MOG D80 400 microns, desliming/ screening over 400 mesh, rougher conditioning with 0.4kg/t SOKEM565C for 2minutes at 50%S, rougher flotation for 4 minutes, at pH 8 and 28% S. The float sand fraction assaying 80.30% AI, 18% total carbonates may be used as eco sand. On the contrary, premium metallurgical grade concentrate assaying 54.64% CaO, 0.6% SiO2, 0.7% MgO, 1.4% Fe2O3, 0.7% Al2O3, 41.0% LOI with weight% yield of 55.2, could be produced at 20% solids, MOG D80 400 microns, and pH 8 with 1.0 kg /t of SOKEM 565 C. The evolved nil waste process is stable, selective, and easily adaptable in the existing anionic soap direct flotation plant at ACC Madukkarai cement works, yielding valuable products.

Madukkarai is located 2.50 km away from Madukkarai Cement Works, which lies 10 km from Coimbatore.
Geographically the mining lease area (ML3) fall between the latitude 10° 55' to 10° 56' and longitude 76° 56' to 76° 59'. The topography is gently undulated and surrounded by Calc -granulites hills. Country rock is garnetiferous sillimanite schist in most of the places, at places the limestone also occurs in association with charnockite and calc gneiss. Limestone bands of varying width are well exposed in roadside west block with intervening calc granulite bands. The limestone is greyish white and light to dark grey in colour, crystalline and coarse to fine grained in nature. It is generally observed that at the contact zone with calc granulite, the limestone is pink in colour. The limestone commonly shows inclusion of diopside, biotite, muscovite and graphite. Limestone bands are separated by calc granulite and at places there are thin lenses of calc granulite within the limestone band itself. Intrusions of pegmatite and occasional thin quartz veins are common within limestone. Occurrence of clay within the limestone is commonly observed. Since, 1965, ACC is mining low grade limestone by opencast mining method and is partly beneficiating to sweeter grade limestone for blending and using it with raw mix. It has a 4000 tpd cement manufacturing based on semi-wet process. Concentrate produced in the flotation plant assaying 83% total carbonate is the feed to the cement kiln. Limestone up gradation process comprised of multistage crushing, grinding and flotation. The flotation plant is operated in two parallel lines at a rated capacity of 60-65 tph per line. Limestone received in the cement works from Madukkarai mine and +15mm fraction from Walayar mine in 1:1 ratio is mixed, crushed and screened at site in jaw and impact crushers to all -15mm size. This blend assaying 76-77% Total Carbonates forms the feed to two ball mills operated in close circuit with 350mm hydrocyclones. Overflow from the hydrocyclone is deslimed in a cluster of 100mm hydrocyclones. The overflow joins the concentrate thickener whereas the underflow constitutes the feed to flotation. Flotation is carried in two parallel batteries, one of Dorr-Oliver make and other of Outokumpu make equipped with automatic level controller. Process flowsheet is given in figure 1. Each flotation battery has 12 cells. First 8 cells produce concentrate and last 4 cells are used as scavenger cells. The scavenger concentrate is fed back to the conditioner, whereas the scavenger tails forms the final rejects. The concentrate joins the thickener. Thickener underflow assays around 83% Total Carbonates and is the feed to cement kiln. The reject assays around 25-30% Total Carbonates. Anionic collector used in the flotation is a mixture of soap, resin and caustic soda. The reagents consumption is 1 kg/t of ROM and is partly added in the conditioner and remaining in the various flotation cells.

Experimental
Material and Methods; Lime stone samples of 200 kgs was collected from Lime stone mining area of ACC mines, Madukkarai, Coimbatore district, Tamil Nadu, India. The flotation regents were collected from M/s Somu organics Ltd., Bangalore. The as received sample was stage crushed to -10 mesh using primary lab jaw crusher [150 x225mm -25 mm set], lab roll crusher [200mm x 150mm] 300 mmx600mm 16 mesh screen. The crushed sample was subjected to standard feed preparation by adopting sampling procedures. The sample was ground at 67% S in 175mm x 350 mm rod mill 5 kg rod charge -10 Nos. of 40mm, 25mm and 20mm dia., varying grinding time. The ground pulp was subjected to froth flotation using D12 Denver type MPE lab sub aeration flotation machine. The feed and products after dewatering followed by drying were weighed, sampled and subjected to characterization studies. MOG, Kinetics, Choice of collector, Collector dosage and% solids were varied.

Results and Disucussions
Characterization studies; The whitish gray coloured limestone sample had bulk density of 1.81t/m 3 and 35 0 angle of repose. The work index of the sample was found to be 11.5 KWh/short ton. The sample contained fine grained calcite intimately associated with minor amounts of fine grained aggregates of quartz, iron oxides, clay and trace amounts of feldspar. The sample was siliceous low grade granular limestone with fair degree of liberation at -65 mesh size. The sample analyzed 43% CaO, 18%SiO 2 , 1.23% MgO, 1.30% Fe 2 O 3 , 1.50% Al 2 O 3 , 0.33 alkalis 76% total carbonates and 36% LOI. The diagnostic amenability test on -65 mesh sample involving sink and float test at 2.8 specific gravity were conducted and observed 5% acid insolubles in sink and slimes assayed 10% acid insoluble.
-16 mesh samples were ground in rod mill for varying time from 5 to 15 minutes and samples were subjected to size analysis. The data is given in Table 1. The grindability data indicated that the sample was medium soft in nature Effect of mesh of grind [MOG]: Inverse flotation tests were conducted varying mesh of grinding time 5'/10'/15' with respective D 80 400/200/150 microns respectively at natural pH of 8, with 1 Kg/t anionic collector SOKEM 565 C. The results have been tabulated Table -2        Effect of pulp density on flotation: Flotation tests were conducted varying% of solids from 20/47. Increase in% of solids though increases yield, but reduces selectivity. Tests were conducted by varying pulp density 19/33/47% S. The results are given in Table-4. Incidentally similar results were obtained by Rao et.al(2009) by working on flotation of low grade limestone samples of Andrapradesh. This may be attributed to better dispersion of air bubbles in the pulp and better dropping of entrapped silica in the froth. But to obtain low silica metallurgical grade concentrate 20% solids seems to be optimum. Shandilya and Jha (2012) while working in ACC Madukkarai flotation plant opined that 20-24% S was found optimum for maximum selectivity, total carbonate recovery and productivity. Fig. 4[a]. Effect of%Solids on Wt.% yield. Fig. 4[b]. Effect of%Solids on% AI assay.
Collector dosage variation: Tests were conducted at D 80 size of 400 microns by varying collector SOKEM 565C dosage from 0.4 to 1.2 kg/t. The results are shown in Table 5. Result indicated that the increase in collector dosage decreased Wt% yield and % AI grade (except at 1.2kg/t dosage) with a best result at 1.0 kg/t. Rao et. al (2009) obtained optimum results at 0.6kg/t SOKEM 565C for cement grade concentrate. Rachappa Kadli et. al. (2015) produced metallurgical grade concentrate with 1.0 kg/t SOKEM 565 C. Hence, to produce cement grade concentrate 0.4 kg/t of collector is sufficient to float less gangue while to produced metallurgical grade concentrate with low silica 1.0 kg/t of collector may be required to remove siliceous gangue which is logical.  Final test for cement grade concentrate production; The test comprised of grinding the sample to MOG D 80 400 microns, desliming/ screening over 400 mesh, rougher conditioning with 0.4kg/t SOKEM565C for 2minutes at 50%S, rougher flotation for 4 minutes at pH 8 and 28%S. The non-float and slimes constituted the final concentrate. The test was carried out to simulate the industrial condition. The results are given in Table 7. The results indicate that a composite of slime and deslimed non float yielded a cement grade assaying 9.51% AI, 90% Total carbonates at wt% yield of 88. The concentrate size was coarse [D 80 0.3mm] w.r.t. ACC concentrate [D 80 0.2mm]. The float sand fraction assaying 80.30% AI, 18% total carbonates may be used as eco sand. Incidentally, Shandilya (2012) recommended the flotation rejects as ACC eco sand for plastering and concrete works. The above desliming-inverse flotation nil waste process appears to be stable, easily adaptable at site,producing raw materials for civil construction like ACC Eco sand as an alternative to river sand and cement grade limestone.