Lithium based Borate Fusion of Gypsum/High Sulfate Samples: A New Approach.

Transcription

1 868 Lithium based Borate Fusion of Gypsum/High Sulfate Samples: A New Approach. Abstract Donald J. Broton Construction Technology Laboratories, Inc Skokie, Illinois Manufacturing portland cement requires chemical analysis of raw materials, clinker, and cement. Producing gypsum calibration standards by pressed powders or glass beads using sodium tetraborate creates additional calibration curves which must be maintained by drift correction or re-calibration due to instrumental changes. Sodium tetraborate disks are also hygroscopic and cannot be stored effectively. Currently many cement plants use lithium tetraborate fusions for raw materials/clinker/cement analysis but SO, loss is generally too high to accurately determine SO, in gypsum, insufflation dust, etc. and disks routinely devitrifjr causing cracking. Fine grinding of samples, controlling fusion time and temperature results in pristine disks and calibration curves with acceptable standard error of estimates. Background Portland cement (PC) clinker is manufactured by combining finely ground limestone and clay in closely controlled composition and burning in a rotating kiln at about 1475 C. Accurate elemental analysis of these materials and others which include many alternate sources of raw materials and industrial by-products such as slag, fly ash, etc. is necessary to insure the proper composition of the final product. Gypsum is interground with the burned clinker at appropriate levels to produce the final product - portland cement. Each of the materials must be analyzed for a minimum of eight elements, Si, Al, Fe, Ca, Mg, Na, K. S. At the cement plant, timely analyses are also necessary to insure a consistent product. Fine grinding of samples ( in a ring and puck type mill for about 4 minutes to obtain 90% ~10 micrometer material) followed by pressing into briquettes has been the method of choice as a hundred or so samples per day may be analyzed. Therefore the two most important factors for analysis of raw materials and finished product are speed and nccul ucy. Fusion techniques for geological materials have been established by many authors including Baker (1982) Kocman (1984) Tertian (1982) and Claisse(l994). For the fusion technique to be acceptable for cement manufacturing plants it must be completed in about the same time as pressed powders. Virgin earthen materials analysis is well suited for the briquetting type of sample preparation. However, with the addition of the many alternate sources of materials used to formulate kiln feed, particle size effects, preferred orientation and segregation, as well as matrix effects play a key role and accurate analyses are no longer easily obtained by briquetting without creating new calibration curves. Fly ash, and quartz type materials do not bind well. Significant amounts of binder are

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3 869 necessary for these material types creating a more dilute specimen and again another calibration set. Another problem associated with pressed powders is created with inverted x-ray tube geometry. Loose powders from the samples can easily deposit on the tube creating scatter and decreased intensities. Fusion into a glass bead eliminates particle size effects, minimizes matrix effects and does not dust the tube. Thus, by,msing the sample into a flat, homogeneous glass bead many inherent x-ray problems are solved or minimized. This type of specimen preparation technique is very well suited for x-ray analysis. Fusion, however, creates another problem. Volatile elements such as alkalies and sulfur may be lost during the high temperature fusion (about 1OOO C) modifying the flux/sample ratio and creating a most undesirable effect - an inaccurate result. Baker (1982) found that sulfur can only be retained in its highest oxidation state- as SO4, (sulfate). Although many types of fluxes were tested for sulfate retention Baker found combinations of borate and nitrate retained sulfur quantitatively in the fusion process. A highly oxidizing flux in an oxidizing environment (a muffle furnace) was used to insure the sulfur stayed in the sulfate form. The combinations of fluxes were either Li,B,07 or NaZB407 combined with LiN03, NaN03 or CeNO, at 80%/20% by weight. The addition of LiNO, increased the alkalinity and oxidizing power of the flux and shifted the composition of the flux from lithium tetraborate towards the eutectic, located near lithium metaborate. Kocman also proposed using sodium tetraborate at 12: 1 flux/sample ratio fused for C for the analysis of gypsum and found that time and temperature were also critical to retain sulfate sulfur. However, introducing sodium into the fluxing process is not suitable for cement plants as sodium must be analyzed. Flame photometry or atomic absorption could be used in addition but are time consuming methods. Therefore cement plants could fully utilize the fusion technique for manufacturing process control if the following parameters were met: 1) Rapid specimen preparation 2) Retain sulfate sulfur 3) Calibrate for a wide range of materials using a single flux and optimized fusion parameters Experimental Procedure The tision technique is performed by igniting the specimen at 950 C in a mutile furnace for an hour and cooling in a dessicator. Five grams (ignited basis) of 67% lithium tetraborate/33% lithium metaborate is placed into a 95%platinum/5% gold crucible. One gram of sample is then placed into the crucible and the crucible is then placed into the fusion apparatus. The sample must be placed directly on top of the flux to minimize

4 870 contact with the walls and bottom of the crucible. No mixing is necessary and furthermore is detrimental to the procedure. The minimum loss on fusion with intermixing was 0.8%. A Claisse Fluxy was used for the fusion process and the following parameters were programmed in the instrument. Step FO Fl F2 F3 F4 F5 F6 F7 F8 F9 Speed Gas Time(s) Propylene was used as the fuel gas. The regulator pressure was set at 6 psi. Temperature was determined by introducing a Type S thermocouple into the fusion mixture in a stationary crucible at fusion temperature. The setting at steps F4-F7 gave 1OOo C+2O C molten flux temperature with our unit. Temperature measurements touching the tip of the thermocouple to the crucible showed temperatures in excess of 1100 C. ACS reagent grade CaS04 was used for trials, and loss on ignition was measured by completing the fusion process and weighing the glass bead and platinum crucible on an analytical balance. The tare weight of the crucible was removed from the final weight and the loss on fusion was calculated. The loss of HBr anti-wetting agent could not be corrected for and its use was eliminated from the measuring process. The platinum molds were polished using one micrometer diamond paste before the start of the investigation. This high polish eliminated the need for HBr anti-wetting agent. RESULTS and DISCUSSION Claisse lists lithium tetraborate flux (M.P. 940 C) as the one flux of choice for cements, raw feeds and silicate rocks. Portland cement (PC) is a basic material with CaO combined as calcium silicates at about 65%. Lithium tetraborate flux does dissolve cements and clinkers(the precursor to PC). Shortcomings of this flux include long fusion times for high Si and Al geological samples and sulfate sulfur is proportionally lost. Experience has shown that recrystallization is frequent for silicate rocks (containing <80% Si+Al) using this flux. Decreasing the fusion time from 10 minutes to just under 3 minutes for cement samples indicated more sulfate was retained. The newly fused specimens were analyzed against standards prepared by fusing the normal ten minutes and showed the amount of SO3 increased about 0.2%. Sulfate loss during the fusion process is known in the cement industry and has been moderately controlled by keeping time and temperature consistent; thus proportional amounts of SOS lost during standards preparation will reflect similar results for samples. The amount of sulfate in pc is relatively small, only about 3%, and if the conditions of time and temperature for fusion were consistent sulfur as sulfate curves yielded standard error of estimates (SEE) of about 0.05 in the pc calibration range using NIST Standard Reference Materials. The calibration curves for sulfate also were consistent producing larger SEE s, for calibration curves that included high purity

5 871 gypsum. Sulfate loss was calculated by loss on fusion at 1.6% as SO, using the minimum time (170 seconds) at 1000 C necessary for complete fusion of reagent grade CaSO,. Unfortunately as the amount of SiO, in gypsum or other raw materials increased the chances of the disc cracking also increased using the shortened fusion time. The sample would have microscopic inclusions of un-fused or re-crystallized materials thus creating a weak plane in which the disc would crack. Processing samples (post ignition at 950 C) to pass a No 200 US Standard Sieve dramatically helped in securing a completely fused bead but as the amount of Al+Si in other raw materials reached 80% fusion time had to be increased a few minutes. Keeping time consistent and minimized as well as retaining SO, were the primary goals so another approach was necessary. Fusion time was minimized by altering the type of flux to obtain a mixture of tetraborate and metaborate. Increasing the amount of metaborate to tet/met easily dissolved silicates but unfortunately the high Ca of limestone s devitrified the disc creating a milky white appearance and caused cracks. Moving away from the 50/50 towards 67/33(M.P. 875) does complete the fusion of limestone s as well as fly ash, quartz, gypsum, and clay and appears to retain SO, quantitatively. The time necessary to reduce SO:, loss to less than 0.01% was 128 seconds. Each of the DOMTAR gypsum standards Gyp A, Gyp B, Gyp C, Gyp D, FGD-1 and FGD 2 were used to determine loss on fusion. The gypsum varied in SiOz content up to about 11% on an ignited basis. The tet-met mix therefore appears better suited for the portland cement industry as the analyses can yield curves which do not appear susceptible to inaccuracies due to SO, volatilization under these conditions. One calibration curve can be generated - not one for each type of material analyzed as many cement plants do now for pressed powders. If the SEE need be decreased, (accuracy improved), a simple separation of similar matching standards from one wide range calibration is easily accomplished. X-ray instrument manufacturer s software packages and today s modern computers have made data collection and analysis effortless. Anti-wetting (or releasing) agents are normally introduced onto the mixture prior to fusion. Many types are available; however if the molds are kept polished and handled carefully anti-wetting agents can be minimized or eliminated when fusing basic materials Iron ores or silicates still require releasing agents. One drop of a 50/50 solution of deionized water and a saturated solution of lithium bromide (filtered through a #41 Whatman paper) was used for the calibration samples. Baker found that the addition of LiBr also lowered the melting point of the flux increasing the fluidity of the molten material during pouring into the molds. Gypsum using a 5: 1 sample/flux ratio did not require any releasing agent but for consistency of sample preparation throughout all types of geological samples normally used in cement manufacturing one drop was used for calibration samples. Evaluating the loss on fusion of the SOS required the elimination of the anti-wetting agent. Pure, clear

6 872 flux beads for use as an analytical blank were easily fused at 67/33 tet/met along with pure lithium tetraborate under the same fusion conditions. CONCLUSIONS Lithium tetraborate/lithium metaborate flux can be used for fusion of geological materials i.e.gypsum, limestone, clay and the manufactured product portland cement clinker, and cement. Loss on fusion should be measured using reagent grade CaS04 under defined conditions of time and temperature for any instrument used to produce glass beads. Time and temperature must be kept minimized or sulfate will be lost. The success rate for producing strain-free glass beads with CaS04 as well as a wide variety of geological materials used for portland cement production was 100% when the lithium tetraborate/metaborate mixture was used. ACKNOWLEDGMENTS I thank Scott Nettles who faithfully carried out many of the trial fusions necessary for this work. His attention to detail and creativity helped in securing a technique viable for gypsum analysis using a lithium based flux. REFERENCES Baker, J.W., 1982, Volatilization of sulfur in fusion techniques for preparation of discs for x-ray analysis, Adv. X-Rav Anal., 25:9 1. Pella, P.A., 1978, Effect of gas burner conditions on lithium tetraborate fusion preparations for x-ray fluorescence analysis, Anal Chem.,SO: Tertian, R. and Claisse, F., 1982, Principles of Quantitative X-ray Fluorescence Analysis, Heyden London Couture, R.A. 1989, An improved fusion technique for major-element rock analysis by XRF, Adv. X-ray Anal, 233:238. Kocman, V., Rapid multielement analysis of gypsum and gypsum products by X- ray fluorescence spectrometry. The chemistry and technology qf gypsm. American Society Testing Materials Special Technical Publication 86 1 Claise F., 1994 Glass discs and solutions by borate fusion for users of Claisse fluxers, Claisse Fluxv Instruction Manual 5:22