(7) GYPSUM AND SELENITE Gypsum and Selenite have the same chemical Similar tests conducted at 10 mm size yielded a composition but differ in crystallisation habit. Selenite product analysing 45.65% SO3 and 0.31% insolubles is usually crystalline and the association of the with 97.7% SO3 distribution in it. Soaking and impurities is superficial which are eliminated by washing at -10 mesh size produced a concentrate washing. Gypsum is fine grained in nature and is assaying 46.10% SO3 but the recovery was 91.6% SO3 beneficiated by flotation only. Both the minerals only. Further improvement may be achieved if the find extensive use in the fertilizer industry and a sample is ground further before washing and desliming. minor quantity in the manufacture of plaster of paris and cement industry. Sample No. 3 The sample was received from M/s. Bikaner Beneficiation studies at N.M.L. Gypsums Ltd., for batch and pilot plant scale investi- A Selenites gations. Complete chemical analysis of the sample was as follows : Two selenite samples from Rajasthan were received from M/s. Bikaner Gypsums Ltd., Calcutta, Constituent Assay % for the reduction of the insolubles in the samples. S0, 45.16 Sample No. 1 CaO 31.71 The sample was composed of crystalline selenite Fe,O, 0.32 Al.0, 0.08 which was usually white and transparent. Occa- Si0, 0.97 sionally grey colour crystals were also observed and Insolubles 1.06 had a fine coating of clay. The sample was to be Moisture 0.12 upgraded for the manufacture of plaster of paris. Combined H_0 19.50 Complete chemical analysis of the sample was as Co" 0.29 LOl 20.86 follows : Mgo Nil NaCl Trace Constituent Assay a/a Soaking and washing tests with the ROM sample SO1 45.20 CaO 31.80 reduced the insolubles to 0.73% with 68% insoluble H , 0 20.60 distribution in it. Similar tests conducted at 19 mm, Co, 0.48 10 mesh and 20 mesh sizes respectively yielded si 0.. 0.44 concentrates analysing 0.64% in-sol. with 61% A1 ,0 ,1 0. 85 distribution, 0.64% in-sol. with 54.3% distribution N a Cl 0.50 and 0.4% in-sol. with 20.9% distribution in it Fe,0 , 0 . 15 L.O.I. 22.50 respectively. As the results warranted for washing Alkalies & MgO Trace at finer size, 19 mm washed lump were ground to 10 mesh and 20 mesh and washed again where in the Soaking of the sample in selenite saturated water concentrates analysed 0.54% in-sol. and 0.41% and washing with ROM sample, increased the grade in-sol, with in-sol. distribution of 44.5% and to 45.45% SO, with 99.2% SO3 distribution. 27.2% respectively. Based on the batch scale results 333 it was concluded and pilot plant scale tests were distribution in it. Soaking, scrubbing and washing conducted with two stage washing i.e. washing tests with ROM sample yielded a concentrate assaying the ore after crushing to 19 mm size followed by 45.77% SO, and 0.6% in sol. with 91.8% SO3 distri- regrinding to 20 mesh and washing again. butic n. Similar test with 19 mm crushed sample prcduced a cercentrate analysing 46.16% SO, In the pilot plant run the ROM ore was crushed to and 0.32% in-sol. with 90.1% SO5 distribution in it. 19 mm size, scrubbed in a gravel washer and then the slimes were separated in a rake classifier. The Continuous semi-pilot plant scale tests with classifier underflow which contained the lumps was 19 mm crushed ore incorporating scrubbing, and ground in a continuous ball mill to 20 mesh size and washing produced a washed selenite concentrate washed in a spiral classifier where in the selenite assaying 45.5% SO4 with 87.6% SO3 distribution sand and slimes were separated. The slimes i.e. in it. classifier overflow was further treated in a hydro- classifier to recover some more sand which was B Gypsum combined with the classifier sand. The combined Six different samples of gypsum from different sand product obtained from spiral classifier and hydrc- areas in Rajasthan and having different analysis classifier analysed 0.41% insolubles with 29.6% were received from M/ s. Bikaner Gypsums Ltd. in-sol. distribution. The product is quite suitable In the first batch four samples were there and two for the plaster of paris industry. different samples subsequently. Sample No. 2 Batch No. 1 A low grade selenite sample drawn from Thobo Four samples were received in this batch from area in Baramer Dist., Rajasthan was received from Jamsar area and their complete chemical analysis the Director State Mining Fr Geology for beneficia- was so follows : tion studies. The final concentrate should be suitable for the manufacture of surgical grade of plaster of paris. Complete chemical analysis of the sample Constituent Assay % was as follows : Sample No. 1 2 3 4 SO:, 32.84 37.15 38.55 36.56 Constituent Assay % CaO 28.83 29.19 32.73 29.78 Combined H •.0 14.83 16.40 17.66 16.70 SO3 27.54 Free H2O 0.68 0.45 0.46 0.50 CaO 28.02 CO2 2.43 3.11 3.98 3.17 MgO 2.34 Sio. 14.37 8.98 4.32 8.95 H3O 13.17 A1,0, 3.84 2.34 0.85 1.85 SiO., 12.28 Fe=O5 1.08 0.66 0.63 0.86 A120. 3.69 M90 0.80 0.89 0.82 1.03 CO. 4.74 NaCl 0.01 0.01 0.01 0.01 Alkalies 1.33 Insolubles 19.23 1 1 .99 5.23 11.37 Examination of the sample indicated the presence Microscopic examination of the samples indicated of large amount of clay which was actually responsible the presence of crystalline aggregates of gypsum. for the poor grade of the selenite. The gangue was ccmpcsed of quartz followed by Dry screening tests with the ROM sample as well as hornblende, calcite, clay and occassionally occurring 19 mm crushed sample after removing the -10 mesh garnet, magnetite and celestite. Gypsum was fines respectively analysed 30.77% SO3 with 96.5% liberated frcm the gangue below 14 mesh size. SO, distribution and 33.7% SO3 with 90.3% SO3 Heavy media separation tests to separate the distribution in them. silicates as heavy portion conducted with the Washing tests with ROM sample yielded a con- -10 mesh sized portions indicated that upto centrate analysing 45.36% SO3 with 97.2% SO -48 mesh size, there were no separate silicates and the grade of the products was always over 40% SO3. Mineralogical examination of the sample indicated Sink product obtained in +65 and +100 mesh that crystalline aggregates of gypsum were associated portions was very little (less than 0.5%) in each with quartz and calcite followed by minor amounts of portion. Float portions obtained from 100 mesh micas, hornblende chlorite, tourmalene, garnet and and below sizes analysed were of poorer grade. clay minerals. Quartz was liberated at 65 mesh size It was concluded from the overall test results that by and the test of the gangue at 100 mesh size. discarding the -150 mesh fines from the 10 mesh ground product would produce 87% gypsum grade Chemical analysis of the sieve products revealed with recoveries ranging from 85-90% SO3. that the product coarser to 100 mesh analysed 41.3% SO3 with 79.57% SO3 distribution in it. Portions finer Hydro-classification of the -10 mesh sample of that 100 mesh analysed about 26% SO8 with 20.5% the four samples yielded sand portions analysing SO3 distribution in it, indicating the seggregation of 38.0% SO1 with 81.3% SO3 distribution, 40.76% SO3 the silicates insolubles in the -100 mesh portions. with 85.5% SO3 distribution, 40.01% SO3 with 80.6% distribution and 40.37% SO3 with 82.2% SO1 Rougher flotation with -150 mesh portion from distribution. If the coarser sand portion was only the sample with 0.5 kg/tonne of sod. silicate followed taken, the analysis would be 40-43% SO3 with by two cleanings produced a concentrate assaying 40 to 46% SO3 distribution in it. Hence hydro- 43.62% SO3 with 54.9% distribution (w.r.t. -150 classification method was considered helpless for the mesh portion of the r.o.m.) in it. Rougher flotation tests production of fertilizer grade of gypsum concentrate. with 1.5 kg/tonne NaOH, 1.0 kg/tonne of sod. silicate, 1.25 kg/tonne of sod. oleate and 0.05 kg/tonne of Flotation tests with the -150 mesh fines obtained pine oil, followed by four cleanings produced a con- from -10 mesh crushed product with 0.25 kg/tonne centrate 44.85% SO3, 0.54% insol. with 87.3% SO3 of sod. oleate produced concentrates analysing distribution in it. 45.51%, 45.55%, 45.81% and 45.06% SO.1 with Sample No. 6 recoveries 70.1%, 83.1%, 85.5% and 66.3% res- pectively for samples No. 1, No. 2, No. 3 Ft No. 4. The sample was of amorphous nature, containing almost all fines and occasional lumps upto 80 mm Flotation test conducted with -10 mesh sample in size. Complete chemical analysis of the sample was ground to 100 mesh using amines to float the deslimed as follows: silicates and leaving gypsum in the tailing produced a gypsum concentrate analysing 44.57% SO3 with Constituent Assay % 85.6% SO3 distribution in it.