Evaluation for the beneficiability of silica sands from Cherthala area of Alappuzha district, Kerala, India.

Transcription

1 Indian Journal of Geo Marine Sciences Vol. 46 (08), August 2017, pp Evaluation for the beneficiability of silica sands from Cherthala area of Alappuzha district, Kerala, India. P. Raghavan, S. Ramaswamy, Sathy Chandrasekhar & M. Sundararajan* CSIR- National Institute for Interdisciplinary Science and Technology, Trivandrum , Kerala, India. [*E.Mail: Received 12 March 2014 ; revised 11 November 2016 In the present study, representative raw sand samples from Panambukattuveli Mines and Hyma Mines situated in the coastal tract of Vembanattu backwaters in Kerala state of India were selected. Since both the samples are found to have similar chemical properties, they were blended in equal weight basis for preparing the representative feed sample. Blended sand (feed) sample analysed 96.5% SiO 2 and ancillary impurity minerals were assayed as 0.327% Fe 2 O 3, 0.50% TiO 2 and 2.101% of Al 2 O 3. XRD pattern showed the presence of high percentage of silica. Beneficiation of the sand samples by employing techniques such as screening, attrition and wet high intensity magnetic separation is found to produces special grade glass making sand as the principal product. Optional principal product produced during the process is Grade I glass sand and foundry sand. By products of beneficiation are coarse grit of above 600 m which can find use as landfill, a heavy mineral concentrate which can be added to the second stage feed stock of a beach sand mineral processing plant and finally a fine sand of below 180 m which after milling can be use for investment casting. Thus almost all the fractions can be made use of. [Key words: Silica sand; Beneficiation; XRD; Magnetic separation.] Introduction Silica sands, perhaps, has got the most diversified use among all the non-metallic minerals. This is because of its common occurrence around the world, distinctive physical characteristics such as hardness, chemical and heat resistance as well as low price. Silica bearing rocks and minerals such as quartz, quartzite, silica sand together with other varieties of silica like agate, amethyst, jasper, flint etc. are used in a host of industries such as glass, ceramics, foundries, ferro-alloys, abrasives, refractories, ornamentation etc. Impurities usually present in the silica sand are free and coated iron oxides, clay, titania and smaller amounts of sodium, potassium and calcium minerals. The iron, being the most detrimental impurity, can be reduced by a number of physical, physico-chemical or chemical methods, the most appropriate method depends on the mineralogical forms and distribution of iron in the ore 1,2,3,4,5,6,7,8. Upgrading of silica sand requires partial removal of iron, and other minerals which are detrimental to its end use. While much of the liberated impurities can be reduced or removed by physical operations such as size separation (screening), gravity separation (spiral concentration), magnetic separation etc., some times, physicochemical or even chemical methods are to be adopted for the effective removal of iron which may be in intimate association with the mineral. The State of Kerala is endowed with fairly good quality of silica sand and the majority of the deposits are situated in the Panavally, Pallipuram, Thycattussery and Kokkathamangalam areas of Chertala. The estimated reserves in this area are about *Corresponding Author

2 INDIAN J. MAR. SCI., VOL. 46, NO. 08, AUGUST million tons. Though open cast sand mining is carried out at this area for many decades, so far no concentrated effort is made for processing and value adding the same within Kerala. Again, though many processing plants are set up in Gujarat, Uttar Pradesh (UP) and a number of other states based on the studies in Indian Bureau of Mines and National Metallurgical Laboratory, no such effort has been done for silica sands of Chertala for developing a suitable flow sheet for the processing and value addition of the silica sands of Cherthala. The aim of this investigation is to adopt simple, cost effective as well as environment-friendly processes and operations for value addition of the sand so that even a small/medium level entrepreneur can set up a beneficiation plant without much capital investment. For the same, it was also decided to employ sieving (screening) and other physical operations as far as possible without going for chemical processing. Materials and Methods The raw silica sand samples were collected from 2 different locations (Panambukkattuveli and Hyma mines) situated in Pallipuram in Cherthala Area. Samples were drawn both from heaps (dumps) and the ground by digging up to a depth of about one metre after removing the surface grits and vegetation. About 100 Kg. of material was collected from each location. Each sample was prepared by thoroughly blending the material by centre displacement method so as to obtain apparently a uniform material. The 100 kgs of the sand is heaped at one spot and then the entire material is heaped on a second spot by shovelling (thus displacing the centre). This is repeated for 10 times (five heaps at each spot). Since both the samples are found to have similar chemical properties, they were blended in equal weight basis for preparing the representative feed sample for the investigation. Standard wet chemical methods supported by instrumental analysis were adopted for determining the chemical constituents. The samples were analyzed for silica, alumina, loss on ignition and oxides of iron, titanium, calcium, sodium and potassium. Silica was estimated gravimetrically by volatilizing with hydrofluoric acid, alumina by complexometric (indirect EDTA) titration, iron and titanium by spectrophotometry and calcium, sodium and potassium by flame photometry. X-ray diffraction (XRD) of the powder sample provides one of the easiest and semi quantitative methods of identifying the minerals present in clays. The powder XRD patterns were obtained on a diffractometer (Philips analytical) using Cu K α radiation operating at 40KV and 20mA on a diffraction range (2θ). Result and Discussion A small amount of sample is drawn from the blended bulk for different characterization for determining their chemical constituents, mineral contents and physical properties. Chemical analysis was done initially to select the sample(s) based on their silica content before starting the beneficiation experiments. The representative feed sample was further subjected to detailed characterization. Again at each stage of processing, the products were analyzed for the chemical constituents so that the extent of value addition or suitability of the beneficiation product with respect to standard specifications could be evaluated. Properties like particle size distribution and chemical assay of the sand are important for the glass making whereas AFS, clay content and acid demand value are also of relevance in foundry application. The data for chemical characterisation is presented in Table 1. Though the silica values are above 95%, aluminium (corresponding to clay), iron and titania values of both the samples were found to be above the specified limits besides the presence of small amounts of sodium, potassium and calcium minerals. Loss of ignition (LOI) is uniformly less. These samples cannot directly be used for any value added application. Both the samples (Panambukattuveli and Hyma Mines) contained lower percentage of silica and, in turn, higher quantities of impurity minerals. Since the silica assays are almost similar, they were blended in equal weight ratio (50:50) to prepare the bulk sample for the investigation. All the screening s, attrition and magnetic separation were done on this blended sample.

3 1598 RAGHAVAN et al.: EVALUATION FOR THE BENEFICIABILITY OF SILICA SANDS FROM CHERTHALA Table 1. Indian Standard specifications for glass making sands 2 nd revision [IS 488: 1980] (i) Requirement for glass making sand Sl No. (ii) Size grading Characteristic (% by mass) Retained on 1 mm IS sieve - Nil Retained on 600 micron IS sieve, % by mass, Max Special Grade Passing through 600 micron IS sieve, but retained on 300 micron IS sieve, % by mass, Max Passing through 300 micron IS sieve, but retained on 125 micron IS sieve, % by mass, Min Passing through 125 micron IS sieve, % by mass, Max The XRD pattern showed the presence of large amount of highly crystalline quartz form of silica (Fig. 1). Q- Quartz REQUIREMENT Grade I Grade II Grade III 1 Loss on ignition, Max Silica (as SiO 2 ), Min Iron Oxide (as Fe 2 O 3 ), Max Aluminium Oxide (as Al 2 O 3 ), * * * * Max. 5 Titanium Dioxide ( as TiO 2 ), * Max. 6 Manganese Oxide (as MnO) 7 Copper Oxide (as CuO) 8 Chromium Trioxide (as Cr 2 O 3 ) * These requirements shall be as agreed to between the purchaser and the supplier Magnetic separation s for the recovery of the heavy minerals and the fine sands were done using a blend of the -180 m fractions from both the feed samples.the loss in weight corresponded to the ignition loss of the material. Relevant data of chemical analyses are given in Tables 2 to 8 for various operations. The blended bulk sample was characterized by XRD to identify the mineral constituents in the feed sample. Figure 1 XRD pattern of Blended raw silica sand The chemical assay of the sieve fractions 125, +90μm and -90μm gives high percentages of impurity minerals containing Al, Fe, Ti and Ca with subsequent decrease in silica (Table 2).

4 INDIAN J. MAR. SCI., VOL. 46, NO. 08, AUGUST Table 2 Chemical analysis of individual raw silica sand samples % wt. Sample SiO 2 Al 2 O 3 Fe 2 O 3 TiO 2 Na 2 O K 2 O CaO LOI Panambukkattuveli Hyma mines Average Table 3. Dry sieving and fractional chemical analysis of blended raw sand samples (Panambukkattuveli + Hyma mines) =50:50 Sieve Size, m %Wt. SiO2 Fe2O3 TiO2 Al2O3 CaO Na2O K2O LOI -1440, , , , , , , , Analysis of the sample has revealed that iron oxides are present both in free form and as well as stains mainly on the surface of the silica sand grains 9. In foundry applications, angularity of the sand is very important and has to match with the specifications given as per Bureau of Indian Standards (BIS). The angularity of the sand grains value has been found to be 1.4 for the desired fraction of Cherthala sand while specification gives a range 1.4 to 1. Bureau of Indian Standards (BIS) has put forth specifications for four grades of silica sand for glass making (Table 1). Since, Cherthala sand contains relatively less amount of impurities; it was decided to upgrade the same to the highest level, i.e., special grade sand for glass making. This requires sand with size fraction in between 600 and 125 m and also with minimum 99% silica with maximum permissible level of 0.02% Fe 2 O 3 and 0.1% TiO 2. The dry sieving of the blended bulk sand sample was carried out in order to understand the fractional mass distribution as well as fractional mineral distribution by chemical analysis. Results (Table 3) showed that about 3.1% is above 710 m in size. The weight fraction below 125 m is only about 0.8%, which contains a good percentage of black heavy minerals (>20% TiO 2 ). About 80% of the material is in the size range 500, +180 m which is, incidentally, within the desired fractions for glass making sand. The fractional chemical analysis revealed that the silica assay remains more or less constant at about % up to 180 m, but drastically reduces below this size towards lower values. Ancillary mineral impurities such as heavy minerals (TiO 2 ), clay/ sillimanite (Al 2 O 3 ) and salts of calcium and sodium etc, on the other hand, showed an increase towards finer fractions. The reduction of percentage of silica in 180, +125 m and below is primarily due to the increased presence of clay and heavy minerals in these fractions.

5 1600 RAGHAVAN et al.: EVALUATION FOR THE BENEFICIABILITY OF SILICA SANDS FROM CHERTHALA Table 4 Wet sieve analysis of feed sample and chemical characterization of wet-sieved desired fractions (WSDF) Sieve Size, m %Wt. SiO 2 Fe 2 O 3 TiO 2 Al 2 O 3 LOI , , * Weighted average assay of -600,+125 m Total (92.3) * -300,+180 = 60.7% and 180,+125 = 12.7% by wt Table 5 Wet (sub-)sieving of -300,+125 m fraction and its chemical characterisation Seive size m %Wt. -300,+ 125 W.r.t. Original feed SiO 2 Fe 2 O 3 TiO2 Al 2 O 3 LOI -300, , Wet sieving Wet sieving was carried out in tune with the requirements for glass making sand and hence the sieves selected were 600, 300 and 125 m. The data is given in Table 4. The fractions above 600 m, which is about 6% of the feed sample, is the coarse grits and is taken separately since it is not suitable for glass making. Similarly the fraction below 125 m (~1.7% of feed sample) which contains a lot of heavy minerals is also kept aside since the same is also unsuitable for glass making. The weight fraction of 600, +125 m cut is about 92.3%, which, as per the size specification, is the Desired Fraction (DF). We shall denote it as WSDF since it is wet-sieved. In WSDF, while -600, +300 m is 20.5%, the other fraction (-300, +125 m) is 79.5% which satisfies the IS specifications. Though the mass proportion as per the specification is satisfied, the chemical analysis showed that the iron and titania contents in 300, +125 m fraction is above the specifications even for Grade I. Hence -300, +125 portions are again subjected to intra-sieve analysis so as to get two fractions. This showed (Table 5) that 2 nd fraction ( 180, +125 m) contains most of the impurities. While 300, +180 fraction analysed 0.03% of Fe 2 O 3, the iron content in 180, +125 m is 0.479%, which is more than 16 times of the former one. The titania was also found higher than the specified limit. In order to reduce the influence of iron and titania in WSDF, it was decided to eliminate 180, +125 m fraction. After the elimination, the mass distribution changed to 23.7 and 76.3% for the respective fractions. It could be seen that even after the removal of 180, +125 m fraction, the mass proportion for glass making sand is maintained within specified limits. This fraction (-180, +125) constitutes about 12.7% by weight of the feed sample. BIS specification shows that ( 600, +300) m fraction [A] should not be more than 50% and ( 300, +125) m fraction [B] should be 50% in minimum.thus the wet

6 INDIAN J. MAR. SCI., VOL. 46, NO. 08, AUGUST Table 6 Chemical characterisation of Wet - sieved modified desired fractions (WSMDF) Sieve Size, m %Wt -600,+180 feed w.r.t Original ( % w/w) SiO 2 Fe 2 O 3 TiO 2 Al 2 O 3 LOI -600, , Combined Table. 7 Influence of solid loading on attrition, impurity liberation and removal Solid loading, Non-magnetic product Wt% %SiO 2 %Fe 2 O 3 %TiO Cooling water getting heated up Could not subject to attrition due to infinite viscosity Fixed conditions: Media Kg., Size-1-1.3mm fused ceramic beads; Attrition time 10minutes, agitator rpm-1150 Magnetic separation : Magnetic field (equivalent current in Amp.) 3.2 Medium : Iron balls (diameter of 6.3 mm) -1.2 Kg. (Total slurry volume 250 ml) sieved modified desired fractions (WSMDF) for glass making now constitutes only 600,+300 and 300,+180 microns. These fractions were in the weight ratio of 1:3.2 and are found to constitute about 79.6% of the total feed sample. Analysis of WSMDF sample shows the presence of 99.1% SiO 2 and 0.028% Fe 2 O 3 and 0.090% TiO 2 (Table 6) in the sample. Though the same satisfies the specifications for Grade 1 sand (Minimum 99%SiO 2 and maximum 0.04% Fe 2 O 3 and 0.1% TiO 2 ), it failsto satisfy the minimum quality required for Special grade sand. Hence WSMDF was subjected to further processing. Since the combined iron was found to be mainly as coating on the surface of the sand grains, it was decided to go for attrition followed by magnetic separation. Attrition s carried out in Netzsch mill which is a vertical cylindrical Attritor fitted with a specially designed agitator. The mill is charged with the attrition medium, viz., fused ceramic beads of mm size and later on the material is added to the mill in slurry form. The agitator is switched on and is brought to the required rpm. The mill has got external cooling system for removing the heat generated during the attrition process. After the completion of the set time, the total charge is taken out and the attrition media is separated from the slurry. During the experiments, the agitator rpm is kept at 1150 so as to generate a small vortex at the centre of the mill jar. Two variables were tried: slurry solid content and the attrition time. Solid loading was varied from 10 to 55%.

7 1602 RAGHAVAN et al.: EVALUATION FOR THE BENEFICIABILITY OF SILICA SANDS FROM CHERTHALA Table 8 Influence of attrition time on impurity liberation and removal Attrition time, minutes Non-magnetic product %SiO 2 %Fe 2 O 3 %TiO Fixed conditions: Media Kg., Size-1-1.3mm fused ceramic beads; % Solids for attrition 45; agitator rpm-1150 Magnetic separation : Magnetic field (equivalent current in Amp.) 3.2 Medium : Iron balls (diameter of 6.3 mm) -1.2 Kg. (Total slurry volume 250 ml) Towards 50%, externally circulated cooling water of the attritor was getting heated up and at 60% the contents did not move at all due to infinitely increased viscosity condition. Hence the optimum solid concentration has been taken as 45%. Data is presented in Table 7. Table 8 show the pattern of the liberation and removal of iron minerals with respect to time of attrition. It could be seen that about 15 minutes is enough to get an iron value below 0.02% which was the specification for the special grade glass sand. The attrition media is removed by sieving the product slurry using a 600 m sieve. No sand was found along with the recovered media. Again the whole slurry was passed through a 125 m sieve so as to remove the liberated iron impurities along with some over-ground (fine) sand. The attrition resulted in size reduction of sand and grinding medium. The particle size distribution before and after attrition is presented in Table 9. It could be seen that about 4.2% of 180, +125 m fraction is generated which is favourable with respect to the specification. The mass distribution between desired fractions after attrition is 22.9% of -600, +300 and 77.1% of 180,+125 m which is well with in the scope of the specifications.attrition operation generates a sand fraction of about 3.1% below 125 m. This is considered to be a loss since fraction below this size cannot be used for glass making. This is equivalent to 2.5% with respect to the original feed sample. This includes the liberated iron and titanium minerals which is, relatively, a very small quantity. Incidentally, this is the only solid waste generated in the entire beneficiation flow sheet. It is possible to recover the fine sands from this waste by carrying out a cyclone classification whereby the liberated iron oxides may get separated and removed through the overflow. However, this operation is not included in the final selected flow sheet. The grinding media loss has been estimated to be about 1.5%. A Wet High Intensity Magnetic Separator (WHIMS) which can generate an optimum magnetic field of 1.8 Tesla (18000 Gauss) at a maximum input current of 3.2 Amps. was used for magnetic separation. The magnetic separation, which is integral with the attrition, was carried out using a Carpco machine which is operated on batch basis. The canister (separating chamber) of the magnet is filled with the medium which, in the present case, is iron balls of about 6.3 mm in diameter (small size) and the magnetic field is generated by switching on the current. The dry sample is spread on the top of the medium and the water is added manually and uniformly on the surface so that it wets the material and takes it through the voids among the balls. The water equivalent to 25% slurry is added initially. The liberated magnetic particles are caught by the balls and the sand slurry flows through the medium down to the collection pot unaffected by the field. Plain water is added into canister over the balls as wash water in order to dislodge the sand particles entrapped among the balls. The washings are collected along with the non-magnetics, i.e., sand. After the product sand is collected, the magnetic field is removed by switching off the current and the canister is flushed with water in order to dislodge the magnetics (iron impurities) from the medium. Both the materials are dried and weighed. While WSMDF after attrition followed by magnetic separation at optimised conditions is found to satisfy all the specifications for special grade glass making that without attrition and magnetic separation satisfies the specifications for Grade I sand. Salient results of s such as LOI and other chemical assays and size grading are given in Table 9 in comparison with BIS specifications.

8 INDIAN J. MAR. SCI., VOL. 46, NO. 08, AUGUST Table 9 Particle size distribution before and after attrition Sieve Size, m Before attrition %Wt. After attrition %Wt W.r.t. feed sample Before attrition After attrition -600, , , +125 Nil Nil discard ed The BIS specifications for grain fineness grading and other properties for standard silica sand for raw material ing in foundries is given in Table 11 and Table 12 give the comparative figures for WSMDF before attrition and magnetic separation. It could be seen that the size range is very similar to that for glass making except for more fractional specifications in between the upper and lower sizes. WSMDF is having slight mis-matching with the specifications with respect to individual mass percentages. However, the same satisfies with the three consecutive fractions and hence has been taken as such for foundry applications also (without subjecting to attrition and magnetic separation). WSMDF was subjected to different s such as clay content, hydrochloric acid demand value, angularity (Grain shape) etc. to check its suitability for using the same as raw material ing in foundries. It has been found (Table 12) that the same satisfies all the requirements for the use as foundry sand. The aim of processing the fraction below -180 m which essentially contains fine sand and heavy minerals is to recover these constituents so as to convert them to useful products. Since the heavy minerals are mostly magnetic and sand is almost nonmagnetic, it was decided to employ magnetic separation as a technique to separate these minerals each other Accordingly, -180 m fractions from both the feed samples were blended in equal proportion to prepare feed material for the magnetic separation to recover the heavy minerals and the fine sand. This material constitutes 14.4% of the total feed sample and analysed, 79.5% SiO 2, 3.2% TiO 2 and 4.2% Fe 2 O 3, 6.1% Al 2 O 3. A sieving at 45 m was effected for removing most of the clay particles (slimes) and has helped in concentrating the clean sand and the heavy phases. The fine fraction after the clay removal assayed 88.3% SiO 2, 4.85% TiO 2 and 2.17% Fe 2 O 3, 1.1% Al 2 O 3 and weighed 13.5% with respect to he original feed sample. Carpco WHIMS, which was used for the magnetic separation for the special grade glass sand, is used for these s also. The s were conducted exactly in the same manner as the magnetic separation after attrition for the special grade glass making sand. While the magnetics (heavy minerals) constitutes 18%, the non-magnetics (fine silica sand) constitute 82% by weight of the feed to the magnetic separator. The heavy mineral concentrate (I) assayed 18.6% TiO 2 while the non mag (I) analysed 2.04% TiO 2. The chemical assays of both the products are presented in Table 13. The magnetics from the 1 st stage of separation is again subjected to a 2 nd stage magnetic separation in order to further upgrade the heavy minerals. The was conducted at identical conditions and the analytical results showed that the heavy mineral concentrate (II) contains 39.2% TiO 2 with 22% silica. Relevant chemical analysis of the final heavy mineral concentrate and the fine sand I and II as well as that for the combined sand are given Table 13. The 2 stage magnetic separation of the fine sand fraction below 180 m resulted in a heavy mineral concentrate with +39.2% TiO 2. The mass of final heavies is 6% and, that of the combined (fine) sand is 94%. These are 0.86 and 13.54% with respect to the original feed material. The final heavy mineral concentrate can be added to the 2 nd stage feed stock (after the preliminary screening and spiral concentrations at the collection/dredging site) of a beach sand mineral separation plant. The sands fraction from 1 st and 2 nd stage magnetic separations taken together assayed 94.3% silica with 2.8% titania and 0.09 of iron oxide. This sand after grinding can find use as silica flour for investment casting.the beneficiation gives the principal product as Special Grade glass making sand. Two optional principal products are Grade I glass making sand and sand for raw material ing in foundry. Three by products are also being produced viz., oversize grits, +600 m (By product I), a heavy mineral concentrate

9 1604 RAGHAVAN et al.: EVALUATION FOR THE BENEFICIABILITY OF SILICA SANDS FROM CHERTHALA Table 10 Comparison of WSMDF before and after attrition and magnetic separation with respect to Grade I and Special Grade glass making sands (i) Chemical Sl No. Characteristics (% by mass) WSMDF Before attrition and magnetic separation (Grade I glass sand or foundry sand) WSMDF After attrition and magnetic separation (Spl. Grade glass sand-principal product) IS Specifications for Grade I glass sand IS Specifi- Cations for Special Grade glass sand 1 Loss on ignition, Max. 2 Silica (as SiO 2 ), Min Iron Oxide (as Fe 2 O 3 ), Max. 4 Aluminium Oxide (as * * Al 2 O 3 ), Max. 5 Titanium Dioxide ( as TiO 2 ), Max. 6 Manganese Oxide (as MnO) To pass the To pass the 7 Copper Oxide (as To pass To pass CuO) the the 8 Chromium Trioxide To pass To pass (as Cr 2 O 3 ) the the (ii) Size grading (Table continued) IS Specifications Grade I Spl. Grade WSMDF after attrition and Magnetic separation Before Attrition and Magnetic separation Retained on 1 mm IS sieve Nil Nil Nil Nil Retained on 600 micron IS Nil Nil sieve, % by mass, Max. Passing through 600 micron IS sieve, but retained on 300 micron IS sieve, % by mass, Max Passing through 300 micron IS sieve, but retained on 125 micron IS sieve, % by mass, Min. Passing through 125 micron IS sieve, % by mass, Max Nil Nil (By product II) and a fine sand fraction (-180 m, By product III ). While the main and optional principal products can find use for designated applications, the by product I can be used as a land fill or for building works. By product II can be added to the feed stock of beach sand processing unit and finally, by product III, after pulverisation, can find use in investment casting as silica flour. Based on the laboratory processing studies conducted on the blended raw sand sample collected from Panambukattuveli and Hyma mines of Cherthala area of Kerala, an integrated zero wastebased beneficiation flow sheet suggested is as follows.

10 INDIAN J. MAR. SCI., VOL. 46, NO. 08, AUGUST Table 11 Indian Standard specifications for standard silica sand for raw material ing in foundries [IS 3018: 1977] Table 12 Tests for the suitability of the WSMDF before attrition and magnetic separation for raw material ing in foundries (i) Grain Fineness Grading Sl. IS Sieve, Percent Retained No Microns , Max to to to to , Max , Max. 8 Pan 0.1, Max. Sl. No. Particulars 1 Grain size grading m -600, +425 m -425, m -300, m -212, +150 m -150, m Sample (WSMDF before attrition and magnetic separation) Wt. % As per IS Specifications Wt. % 1.0, max , max. Note : Further, three consecutive fractions when added up should be within the range specified.for example, -600, +212 should be between 59 to 84% and -425,+150 should be between 65 to 95% (ii) Clay content - 0.2% (Max) (iii) Chemical behaviour The sand shall be almost neutral. The hydrochloric acid demand value shall not exceed 8 mg/100 g of sand (iv) Grain shape grains shall be round as far as possible having no compounded grains and fissures (cracks) and the Degree of Angularity shall be 1.4 to 1.5 The sand collected from the two mines is blended in equal weight basis and is first wet-screened using a single deck vibrating screen having a 600 m screen. The oversize grits is collected separately to be used as a land fill while the undersize slurry is pumped to a second stage double deck vibrating screen fitted with300 and 180 m screens. The fraction below 180 m along with most of the water is collected separately and sent to the by product processing circuit. The (-600, +180 m) fraction slurry after adjusting the pulp density is sent to attrition mills. The attrited sand slurry is screened using a vibrating screen fitted with 600 m screens in order to remove the attrition media. The sand slurry, again, is passed through 125 m vibrating screen so as to remove the slimes which contains a major portion of the liberated iron and some broken attrition media as well as overmilled silica sand particles. After adjusting the pulp density, the desired fraction (-600, +125 m) was pumped in to wet magnetic separator to remove the remaining iron impurities. Three consecutive fractions -600, m -425, +150 m Clay content, % Acid Demand value, mg/100 g of sand 4 Grain Shape (Degree of Angularity) The non-magnetic slurry containing the desired silica sand faction is collected and pumped to the hydrocyclones for first stage de-waters and finally to a spiral classifier for 2 nd stage and final dewatering to obtain the special grade glass making sand as the principal product. While the sand without attrition and magnetic separation can form optional principal products such as glass making sand Grade I or Sand for raw material ing in foundries, the coarse grits from the 1 st screen can form one of the byproducts which can be used for land filling. The -180 m fraction (slurry) is deslimed to remove the clayey material and thickened to remove excess water and is processed using 2-stage magnetic separation so as to separate and concentrate the heavy minerals which forms another by product. The fine sands from both the magnetic separators which are the non-magnetic fractions is combined to form the 3 rd by-product which is essentially in the size range m. This, after milling, can be used for investment casting as silica flour.

11 1606 RAGHAVAN et al.: EVALUATION FOR THE BENEFICIABILITY OF SILICA SANDS FROM CHERTHALA Table 13 Percent mass distribution and chemical assay of products of separation (2 stage magnetic separation for byproducts) 1 st Stage separation Assays Particulars Wt% SiO 2 TiO 2 Fe 2 O 3 Feed I Mags I (Heavies I) Nonmags I (Fine sands I) nd Stage separation (Note: Mags I of the 1 st stage becomes the feed II to the 2 nd stage) Feed II W.r.t. Feed I Feed II Mags II (Heavies II) Nonmags II (Fine sands II) Combined fine sand Conclusions The laboratory studies have shown that the main impurities present in the samples are iron and titania minerals. Results of beneficiation studies reveal that while wet sieving/screening removes majority of these contaminating minerals to give product sand suitable for Grade I glass making and also for raw material ing in foundries (optional principal products), attrition and magnetic separation is required to upgrade the same to special grade glass making sand. The effort to value-add and recover all the fractions of beneficiation resulted in three by products, viz., coarse grits, (+600 m, by product I), a heavy mineral concentrate (by product II) and a fine sand fraction (-180 m, by product III ). Based on the laboratory study, a zero waste based integrated commercial beneficiation flow sheet has been designed for a plant which can process 147 Tons per day of raw sand to get tons of special grade glass making sand and three other by products. Flow sheet is designed to find commercial use for most of the fractions and constituents of beneficiation. Technical feasibility is well proven since all the operations are known and established. Acknowledgement The project team is grateful to the sponsors, Department of Industries and Commerce (DIC), Govt. Of Kerala for financial support for this study. Authors thank the Director, CSIR-NIIST, Trivandrum for his support for execution of the work. References 1. Taxiarchaou, M., Panias, D., Douni, I., Paspaliaris, I., Kontopoulos, A., Removal of iron from silica sand by leaching with oxalic acid, Hydrometallurgy, 46, pp Farmer, A.D., Collings, A.F., Jameson, G.J., The application of power ultrasound to the surface cleaning of silica and heavy mineral sands, Ultrasonics Sonochemistry, 7, pp Ay, N., Arica, E, Refining Istanbul s silica sand, August. 4. Indian Bureau of Mines, Nagpur,, 1993, Quartz and Silica Sand; Bulletin No Bureau of Indian Standards, IS specification for standard silica sand for raw material ing in foundries, IS: , Indian Standard Institution, New Delhi. 6. Bureau of Indian Standards, IS specification for glass making sands, IS: , Indian Standard Institution, New Delhi. 7. William D. Baasel. Preliminary Chemical Engineering Plant Design, 1977, Elsevier, New York. 8. Wills, B. A Mineral processing Technology. Pergamon Press, New York. 9. Directorate of Mining and Geology, Kerala; Report on silica sand.