The paper deals with the washing studies conducted with a limestone sample received from Rourkela Steel Plant with a view to reduce the insolubles. The sample analysed 14.88% insolubles and it was required to reduce the same to below 12% level. As the sample contained some amount of clay both in the fines and coating the lumps, it was decided to conduct washing after crushing the sample to the desired size. Bench scale washing studies indicated that, bulk of the clayey matter could be removed. As the insolubles content of the coarser lumps was quite low, pilot plant scale washing studies were conducted both with -50 mm and -25 mm material separately. Results indicated that in both the cases, the insolubles could be reduced to below 12% level with a marginal loss of CaO values

Kunwar, R K

Prasad Rao, P D

English

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PROCEEDINGS : PROF-97©NML. JAMSHEDPUR; pp.141-148Washing studies with a limestone sample forreduction of insolubles - A case studyP. D. PRASAD RAO and R. K. KUNWARNational Metallurgical Laboratory. Jamshedpur • 831007.ABSTRACTThe paper deals with the washing studies conducted with a limestonesample received from Rourkela Steel Plant with a view to reduce theinsolubles . The sample analysed 14.88% insolubles and it was required toreduce the same to below 12% level . As the sample contained some amountof clay both in the fines and coating the lumps , it was decided to conductwashing after crushing the sample to the desired size . Bench scale washingstudies indicated that, bulk of the clayey matter could be removed. As theinsolubles content of the coarser lumps was quite low, pilot plant scalewashing studies were conducted both with -50 mm and -25 mm materialseparately. Results indicated that in both the cases , the insolubles could bereduced to below 12% level with a marginal loss of CaO values.INTRODUCTIONSteel and cement industries are the largest consumers of limestone. It is usedas flux by the steel industry to remove the nonmetallic constituents as slag whileit is the basic raw material for the cement industry. India has been blessed with vastdeposits of limestone , distributed all over the country.Limestone beds are covered by a thick crust of clay and /or rock . During mining,the over burdens removed before winning the limestone . Yet some part of the claygets mixed up with the ore resulting in increase of the insoluble content . In addi-tion, some of the rock beds associated with the deposit also get mixed up in thesame process . Yet in some cases , clay would be found filling all the crevices.cavities and the fissures in the limestone beds. Apart from all these extraneousconstituents , some minerals like quartz, mica, feldspars etc., that are present in thedeposit also add to the insoluble content of the sample . Clayey matter may be easilyremoved by scrubbing and wet screening , whereas the inherent mineral constitu-ents may be removed only after flotation of the limestone . The present sample isassociated with clay cgntaminated during mining and hence beneficiated by scrub-bing and wet screening . NML has conducted studies on various limestone samples141P. D. PRASADA RA (1 and R. K. KU VARfor the reduction of insolubles and silica contents by flotation.Rourkela Steel plant gets lime stone from the quarries at Purnapani. Afterremoving the thick crust of clayey overburden , limestone is mined and crushed to80 min size and is screened into the following sizesI. -80 min +25 mm2. -25mm + 6mm and3. -6mmThe -80+25 mm lumps are directly used in the blast furnaces. The -25 + 6 mmportion after crushing is used in the sinter plant. All the -6 mm fines are rejecteddue to their high insoluble content. At present the -25 mm fraction has been foundto be generally high in insolubles due to the presence of the extraneous clay. It isreported that during mining, some amount of clay always gets admixed with lime-stone. This contamination causes a duel problem viz., increase in the insolublecontent and also reducing the handling convenience to a great extent, specially inthe monsoon season. With a view to removing the extraneous clay and reducing theinsolubles, NML has undertaken studies for washing of the limestone.SAMPLEAbout 17 tonnes of run of mine limestone sample was received at the MineralBenef iciation Pilot Plant of NML from Purnapani limestone quarries of RourkelaSteel Plant. The sample contained lumps upto 200 mm(8") with considerableamount of fines and clay. Occasionally quartzite lumps of various sizes were alsoobserved in the sample. The sample was crushed to 80 mm size in a jaw crusherafter hand cobbling the larger lumps. The crushed ore was further sampled intodifferent lots for preparation of head sample, screen analysis, batch and large scalewashing studies. Complete chemical analysis of the sample is given in Table 1.Table 1 : Complete analysis of sample as receivedConstituent % AssayCaO 40.16MgO 4.16SiO, 9.40Al,O, 7.55Fe2O, 2.40SO1 0.045LOI 36.50P 0.025Insolubles 14.88142P. D. PRASADA RAO and R. K. KUNttARChemical analysis indicated that the sample contained 40.16% CaO and 4.16%MgO with 14.88%• insolublcs.Size distribution of the crushed ore with CaO and insoluble contents in therespective fractions are recorded in the Table 2.Table 2 : Screen analysis and chemical analysis of the sample% Assay % DistSize (mm) Wt % CaO Insol CaO Insol-80+50 41.8 43.20 10.48 45.0 29.8-50+25 30.2 42.20 11.60 31.8 23.8-25+6 17.1 36.55 18.32 15.6 21.3-6 10.9 28.30 34.00 7.6 25.1Head (Cale.) 100.0 40.13 14.72 100.0 100.0The results indicated that -80+50 nom and -50+25 mm lumps contained lessthan 12% insolubles, and may he acceptable for use in the blast furnaces. The-25+6 mm lumps analyzed 36 .55% CaOand 18.32% insoluhles and need washing.The -6 mm fines which contained bulk of the clayey material assayed 34%insolubles.BATCH SCALE WASHING STUDIESThe crushed sample was sized to -80+50 mm , -50+25 mm , -25+6 mm and-6 mm. Each of the sized products ( upto +6 mm ) were scrubbed separately withenough water in a concrete mixer for twenty minutes. The turbid water was re-moved and lumps were washed again to ensure the complete removal of claysticking to the lumps . The -6 min fines were deslimed in a spiral classifier for theseparation of sand and clay. Results of the batch scale washing test are recordedin Table I.Table 3 : Bench scale washing studiesr/( Assay % DistProduct Wt % CaO Insol CaO Insol-80+50 mm 39.8 43.57 9.60 43.2 25.8-50+25 min 28.4 42.65 10.30 30.2 19.8-25+6 mm 15.1 40.80 13.10 15.4 13.4-6 mm Sand 8 .5 35.23 19.20 7.5 11.0Slimes 8.2 18.30 54.13 3.7 30.0Head (Cale.) 100.0 40.11 14.79 100.0 100.0143P. D. PRASADA RAO and R. K. KUNWARTest results indicated that -80+50 mm lumps assayed 9.6% and 10.3%insolubles respectively, i.e., about I % less than the feed. Insoluble content of the-25+6 mm lumps which originally 18.32%, was reduced to 13.1%. The classifiersand contained 192% insoluhles and the combined slime assayed 54.13%insoluhles having a distribution of 30% of the total insolubles. From the resultsit may be concluded that by washing 30% of the total insolubles could be removed.The rest 70% of the insoluhles were inherent to the ore. The slime constituting8.2% of the feed assayed 18.3% CaO having a distribution of 3.7% CaO in it.Overall test results are given in Table 4.Table 4 : Overall results of the washing studies% Assav % Dist.Product Size Wt % CaO insol CaO Insol-80+50 mm 39.8 43.57 9.60 43.2 25.8-80+25 rnm 68.2 43.00 9.90 73.4 45.6-80+6 mm 82.3 42.60 10.50 88.8 59.0-80+ Sand 11.8 35.23 19.20 7.5 11.0Slimes 8.2 18.30 54.13 3.7 30.0It may be seen that the combined washed -50+25 mm lumps represented 68.2%of the feed, assaying 43.0% CaO and 9.9% insolubles. But when the -25+6 mmlumps were also mixed with the -80+50 mm lumps. the bulk represented 82.3%of the feed assaying 42.6% CaO and 10.5 insoluhles. The total washed samplerepresented 91.8% of the feed, assaying 41.9% CaO and 11.3% insolubles. Thegrade of the combined product was within the specification.PILOT PLANT SCALE WASHING STUDIESBatch scale washing studies, indicated that -80+50 mm and -50+25 mm lumpsdid not show much improvement after washing. However, two pilot plant scaletests were conducted with crushed ore samples prepared earlier. In the first test-80+50 mm lumps were screened out before washing whereas in the second test-80+25 mm lumps were removed and the rest of the material was subjected towashing.The crushed ore was fed from the bin through a constant weight feedometer intoa gravel washer provided with blades extending upto half the length . Wash waterwas added at the feed end. The gravel washer discharge was fed into a double deckvibrating screen provided with 25 mm and 6 mm screens . Water was sprayed onthe screen to wash the clayey material adhering to the lumps. The screen oversizewas collected separately and the under size i.e ., -6 mm tines were fed into a spiral144P. D. PRASAD.4 RAG and R. K. KUNW4Rclassifier . The classifier overflow was collected in settling tanks and the sand wascollected after an additional wash water spray inside the classifier itself. Flowsheetof the washing set up is indicated in Fig. 1.IFeeder1< - WaterScrubber1< WaterVibratingScreeLumps FinesIClassifierISand SlimesFig. I : Schematic flowsheet for the washing of limestone.Test No. 1In this test -80+ 50 nun lumps were screened out before washing . Only the -50mm material was subjected to washing under the following conditions:Feed rateWater1.5 tonnes per hour10 G.P.M. in the washer10 G.P.M. on the screenThe test results are recorded in Table 5.Table 5 : Results of pilot plant scale washing studies(%) Assay (%) DistributionProduct Size Wt % CaO Insol CaO Insol-80+50 mm 41.8 43.00 10.60 45.0 30.4-50+25 mm 27.7 42.10 10.60 29.2 20.2-25+6 mm 16.3 39.50 13.20 16.1 14.9-6 mm Sand 7.5 35.20 19.40 6.6 10.0Slimes 6 .7 18.80 53.20 3.1 24.5Head (Cale.) 100.0 39.97 14.54 100.0 100.0145P. D. PRASADA RAO and R . K. KUNIV4RResults indicated that -50+25 mm lumps and -25+6 mm lumps representing27.7% and 16.3% by weight of the feed assayed 42.1% CaO and 10.6% insolubleand 39.5% CaO and 13.2(/ insoluble respectively. These results were comparableto those obtained during bench scale studies. The -6 mm classifier sand represent-ing 7.5c/c by weight of the feed contained 35.2% CaO and 19.4% insoluble. When-80+50 nom unwashed lumps and -50+25 mm washed lumps were combined, thebulk represented 69.5%% of the feed, assaying 42.64% CaO and 10.6% insoluble.If the -25+6 mm lumps were also mixed, the combined product representing85.8% by weight and assayed 42.04% CaO and I l. l% insoluble . The classifieroverflow (slime) representing 6.7% by weight of the feed assayed 18.8% CaO532% insoluble with a distribution of 3.1% of the total CaO and 24.5% of the totalinsoluhles present in the ore. Overall test results are recorded in Table 6.Table 6: Overall pilot plant washing studiesProduct Size Wt 'I CaOAssayInsol% DistCaO Insol-80+50 mm 41.8 43.00 10.60 45.0 30.4-80+25 mm 69.5 42.64 10.60 74.2 50.6-80+6 mm 85.8 42.04 11.10 90.3 65.5-80 mm + Sand 93.3 41.49 11.76 96.9 75.5Slimes 6.7 18.80 53.20 3.1 24.5Head (Cale.) 100.0 39.97 14.54 100.0 100.0Test No. 2In this case all the +25 mm lumps were screened out before washing and onlythe -25 nom fraction was %%ashed in the gravel washer under the conditions as statedearlier. Test results are recorded in Table 7.Table 7: Pilot plant washing studies with 25 nnn_feedProduct Size Wt '/ CaO% AssayInsol CaO% Dist.Insol-80+50 nun 70.1 42.70 10.50 74.5 51.7-25+6 mm 16.5 39.50 12.20 16.2 14.1-6 mm Sand 7.4 35.10 20.10 6.5 10.5Slimes 6.0 19.00 56.10 2.8 23.7Head (Cale.) 100.0 40.18 14.23 100.0 100.0Results in Table 7 indicated that the dry screened -80+25 mm lumps represent-ing 70. I % of the feed assayed 42.7% CaO and 10.5% insoluble . This portion may146P. D. PRASADA RAO and R. K. KUNIIARbe used in the blast furnace . The -25+6 mm washed lumps representing 16.5% byweight assayed 39.5% CaO and 12.2% insoluble. This product may be used in thesinter plant. If the classifier sand assaying 35.1% CaO and 20.1% insoluble wasmixed with -25+6 mm washed lumps, the combined product would assay 38.13%CaO. There was a rejection of 23.7% insoluble in the slime with a loss of only 2.8%CaO. Overall results are given in Table 8.Table 8 : Overall results of washing studies with -25 mm size% Assay % Dist.Product Size Wt % CaO Insol CaO Insol-80+25 mm 70.1 42.70 10.50 74.5 51.7-80+6 mm 86.6 42.10 10.82 90.7 55.8-80 mm + Sand 94.0 41.54 11.55 97.2 -Slimes 6.0 19.00 56.30 2.8 23.7Results indicated that the unwashed -80+25 mm lumps and -25+6 mm washedlumps when combined , yielded a product assaying 42.1% CaO and 10.82% in-soluble . When the classifier sand also was combined the bulk represented 94% ofthe teed assaying 41.54% CaO and 11.55 % insoluble.CONCLUSIONSAt the instance of M/s . Rourkela Steel Plant, washing studies were undertaken toreduce the insoluble content of the lime stone sample from Purnapani to below 12%.The sample analyzed 40.16% CaO and 14.88% insoluble. Clayey matter associatedwith the sample indicated the possibility of improving the insoluble content bywashing.The sample after crushing to 80 mm was sized on 50, 25 and 6 mm screens.Chemical analysis of the individual sized fractions indicated that the insolublecontent varied from 10.48% in the +50 mm lumps to 34% in the -6 mm fines.Washing studies conducted with the sized portions indicated the improvement inthe insoluble content varying from 9.6% in the +50 mm lumps to 19.2% in -6 minsand. The washing slimes analyzed 54.13'f insoluble.It may be seen that the +50 mm lumps showed marginal improvement in theinsoluble content after washing. As this portion of the sample may be directly feedto the blast furnace, pilot plant scale washing studies were conducted with -50 mmand -25 mm material only, bypassing the respective lumps from the circuit.Washing studies with -50 mm feed indicated the rejection of 6.7% of the feedas slimes assaying 53.2% insoluble. The insoluble content of the various washed147P. 1). PRASADA RAO and R. K. KUNWI'ARand unwashed products ranged from 10 . 6% to 19.4 %. The combined washedmaterial assayed 41.497 CaO and 11 . 76% insoluble.Similar studies with 25 mm portion, has eliminated 6.0% of the feed as slimeassaying 56.3% insoluble. The unwashed +25 mm fraction assayed 42.7% CaOand 10.5% insoluble. After washing, the insoluble content was found to be 12.2%in the +6 mm portion and 20.1 % in the -6 mm sand. With the removal of the slimes,the overall grade of the sample has improve to 41.54% CaO and 11.55% insoluble.From the results it may he concluded that the insoluble content of the samplecould he reduced to the desired level by scrubbing followed by wet screening i.e.,washing however, as the +25 in portion constitutes the major portion of the crushedore, with low insoluble content, this may be taken directly to the plant feed. Only-25 m portion needs washing to reduce the insoluble content. After washing, thehandling of the ore particularly in the monsoon season improves along with theadvantage of reduction in the insoluble content.ACKNOWLF.DGENIENTSThe authors place on record their sincere thanks to Prof. P. Ramachandra Raofor his kind permission to present this paper at the seminar. They are also thankfulto their other colleagues who have helped them in conducting the large scaleexperimental studies.REFERENCI,SI I I Indian Minerals Year book , (1992), Indian Bureau of Mines publication.121 Monograph on ores and minerals, NML Publication131 R.K. Kunwar, P.D. Prasad Rao. S.K. Banerjee and G. P. Mathur, (1972), Washingstudies with a lime stone sample from Purnapani - Hindustan Steel Ltd., NML lR No.692/72.148

Washing studies with a limestone sample for reduction of insolubles: a case study

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Washing studies with a limestone sample for reduction of insolubles: a case study

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