Anal. Chem. 1087, 59, 513-519
individual pyrolysis products.
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(12) Tagakl, H.; Washlda, N.; Aklmoto, H.; Okuda, M. Anal. Chem. 1981, 53, 175-179. (13) Sarkanen, K. V.; Ludwig, C. H. Lignins; Wiley: New York, 1971; pp
. - .-.
ACKNOWLEDGMENT
isle
We thank N. M. M. Nibbering for his valuable comments on the paper.
(15) (14) Adler, Freudenberg, E. wood K.sei. Holzfofschung,977, 1964,,1, 18,169-2,8, 3-9. (16) klmz, H. H. Angew. Chem., Int. Ed. En@. 1974. 73,313-321. (17) Kratzl, K.; Czepel, H.; Gratzl, J. Holz Roh.-Werksf. 1985, 23, 237-240. (18) Bracewell, J. M.; Robertson. G. W.; Williams, 8. L. J . Anal. Appl. W O / ~1880, S &2, 53-62. (19) Schenck, P. A.; de Leeuw, J. W.; Viets. T. C.; Haverkamp, J. In Petroleum Gmchmistry and Exploration of Europe; Brooks, J., Ed.; Blackwell Scientlfic Publlcatlon: Oxford; 1983; pp 267-274. (20) Martin. F.; SaizJimenez. C.; Gonralez-Vila, F. J. Holzfwschung 1979, 33,210-212. (21) SalzJlmenez, C.; de Leeuw, J. W. Ora. Geochem. 1984, 6 , 417-423. (22) Obst, J. R. J . WoodChem. Techno/. 1083, 3 , 377-397. (23) Boon, J. J.; Pouwels, A. 0.; Eijkel, G. B. Trans. Biochem. SOC.1988, 15. 170-174. (24) Genult, W.; Chen HaNeng; Boerboom, A. J. H.; Los, J. Int. J . Mess Spectrom. Ion Phys. 1983, 51, 207-213. (25) Ejorkman, A. Sven. Papperstidn. 1956, 59, 477-485. Schweers. W. Holzfwschung 1973, 2 7 , 224-229. (26) Falx, 0.; (27) Cramers, C. A.; Scherpenzeel, G. J.; P. A,. Leclercq, P. A. J . Chromatogr. 1981, 203, 207-216.
Registry No. MWL, 8068-00-6.
LITERATURE CITED Irwin, W. J. Analyticel Pyrolysis, a Comprehensive Guide; Marcel Dekker: New York, 1982 pp 1-578. Meuzelaar, H. L. C.; Haverkamp, J.; Hileman, F. D. pvrolvsls Mass Spectrometry of Recent and Fossil Meterlels; Elsevier: Amsterdam, 1982 pp 1-293. Boon, J. J.; Tom, A.; Brandt, B.; Eijkel. 0 . B.; Kistemaker, P. G.; Notten, F. J. W.; Mikx, F. H. M. Anal. Chim. Acta 1984. 183, 193-205. Meuzelaar, H. L. C.; Wlndlg, W.; Harper, A. M.; Huff, S. M.; McCiennen, W. H. Richards, J. M. Science 1984, 226, 268-274. Vallls, L. V.; MacFie, H. J.; Gutteridge, C. S. Anal. Chem. 1985, 5 7 , 704-709. Meuzelaar, H. L. C.; Kistemaker, P. 0.; Eshuls, W.; Boerboom, A. J. H. In Advances In Mass Spectrometry;Daley, N. R., Ed.; Heyden: London, 1978; Vol. 78, pp 1452-1456. Meuzelaar H. L. C.; Huff, S. M. J . Anal. Appl. Pyrolysls 1981, 3 , 111-129. Whitehouse, M. J.; Boon, J. J.; Bracewell, J. M.; Gutteridge, C. S.; Pldduck, A. J.; Puckey, D. J. J . Anal. Appl. Pvrolysls 1985, 8 , . . . 515-533. Genuit, W.; Boon, J. J. J . Anal. Appl. Pyrolysls 1985, 8 , 25-40. Geltman, S. Phys. Rev. 1956, 702, 171-179. Washida, N.; Aklmoto, H.; Tagaki, H.; Okuda, M. Anat. Chem. 1978, 50, 910-915.
RECEIVED for review May 16,1986. Accepted September 30, 1986. This work is part of the research program of the Dutch Foundation for Fundamental Research on Matter (FOM) and was made possible by financial support from the Dutch Foundation for Technical Research (STW).
Quantitative Analysis of Quartz and Cristobalite in Bentonite Clay Based Products by X-ray Diffraction J. Robert Carter,* Mark T. Hatcher, and Larry Di Carlo NL ChernicalslNL Industries, Inc., P.O. Box 700, Hightstown, New Jersey 08520
Quartz and cristobalite in beneficiated bentonite clay and organoclay materlals have been quantitatively determlned by X-ray diffraction. An internal standard method and a ma88 absorption coefflclent correction method were employed to compensate for sample absorption effects. The lower limit of quantltatlon for quartz and crlstobalite In these types of materials is estimated to be 0.01 wt % and 0.03 wt %, respectively. Absolute errors at the 95 % confklence level are estimated to be f0.20% quartz and f0.35% cristobalite for the Internal standard method and f0.12% quartz and f0.18 % cristoballte for the mass absorption coefficient ooctectlon methad. A procedure Is also described that utilizes the ma88 absorption coefflclent measurement determlned by X-ray fluorescence to correct quartz dltfractlon line Intensities In bentonite clays contalnlng high levels of lmpurltles. These methods were successfully employed to monitor the quartz balance of a clay degangulng process In a plant environment and to quantlfy quartz and crlstoballte In commercially available organoclay rheological additives.
With the advent of “right to know” legislation, there is a need to monitor various impurities normally associated with clay and organoclay rheological additives. In particular, there has been considerable interest in monitoring the levels of total quartz and cristobalite in clay-based products since a potential 0003-2700/87/0359-0513$01.50/0
hazard may exist from exposure to dusts containing respirable crystalline silica (I). Numerous methods of analyzing quartz have been reported in the literature, for instance, infrared spectroscopy (2),wet chemical analysis ( 3 , 4 ) ,and differential thermal analysis (5). Each of these methods suffers from drawbacks, such as nonspecificity, the lack of sensitivity at low levels, or poor reproducibility. Since X-ray diffraction is highly specific and can readily distinguish between silica polymorphs, it was decided to use this technique to quantify both cristobalite and quartz. This article describes XRD methods for analyzing cristobalite and f or quartz in crude bentonite clay, refined bentonite clay, and commercial organoclay rheological additive products. The developed standard version of the XRD method illustrates the use of external standard calibration curves and two different techniques in correcting diffraction intensities for matrix effects. Corrections are necessary since variations in chemical composition do occur that can significantly affect the strength of the quartz and cristobalite diffraction intensities. The techniques consist of determining the mass absorption coefficient of each sample with a separate X-ray fluorescence measurement or employing LiF as an internal standard. Special cases where the intensity correction is unnecessary are also discussed. Clays are natural products and typically contain feldspar, illite, gypsum, zeolite, and calcite in addition to quartz and cristobalite. When present in significant amounts, some of these minerals can contribute errors in the analysis for quartz 0 1987 American Chemical Society
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and cristobalite because of spectral line interferences. Fortunately, these interfering minerals are essentially removed in the early stages of commercial clay refinement and only affect the analytical results when they are concentrated in the clay sample. An illustration of a correction for illite is described in the text. Of a more paramount concern is the inability to properly evaluate the degree of crystallographic integrity of the natural quartz and cristobalite with respect to the standard materials used for comparison. Effects on the diffraction intensities due to particle size, degree of crystallinity, interstitial impurities, lattice imperfections, and the presence of amorphous silica are very difficult to determine. Perhaps the best approach is to select standard materials that have diffraction profiles which are similar to the material being analyzed. The reader should be aware of these potential sources of error and recognize that t h e magnitude of the estimated errors in this paper relate to the calibration standards. EXPERIMENTAL SECTION Instrumentation. The XRD analysis was performed with a
Philips APD 3600 automatic powder diffraction system. The system consists of a vertical diffractometer equipped with a Theta compensating divergent slit and an automatic sample changer capable of holding 35 samples. The X-ray generator is a Philips XRD 3000 model which powers a long fine focus copper target diffraction tube. The detection system uses a graphite crystal monochromator with a scintillation counter and associated NIMS electronics. Additional accessories include a Model 4010-1 graphics display terminal and a Model 4631 hard copy unit both by Tektronix. The system is controlled by a Data General Nova 4 s computer. The X-ray fluorescence equipment employed in determining mass absorption coefficients consists of an Edax Exam I1 energy dispersive system utilizing a pulsed rhodium target X-ray tube. The grinding and mixing equipment used in sample preparation included a Spex catalog no. 8510 shatterbox grinder/blender with Spex catalog no. 8507 hardened steel grinding containers and a Spex catalog no. 8000 mixer/mill. Standard Materials. The standard quartz material used was Min-U-Sil5, which can be obtained from Pennsylvania Glass Sand Corp. Berkeley Springs, WV. The standard cristobalite material was purchased from the Materials Research Laboratory, Pennsylvania State University, State College, PA. The bentonite clay base stock was prepared in the laboratory from bentonite clay stock obtained from the Baroid Division of NL Industries' Colony, Wyoming, bentonite mine. Supercentrifuged Wyoming bentonite clay was prepared in the laboratory. Sample Preparation. Three types of samples were analyzed: the raw bentonite clay and associated products from various stages of the cleanup process (type A); commercial organoclay products made from beneficiated bentonite clay (type B); and the purified bentonite clay where the major amounts of inorganic gangue have been removed via a commercial wet cleanup process (type C). The organoclay products (type B) consist of bentonite clay that has been chemically treated with quaternary ammonium chloride salts such that the quaternary ammonium ions occupy cation exchange positions on the surfaces of the clay platelets. A number of such products are commercially available including dimethyl dihydrogenated tallow (2M2HT) ammonium clay, benzyl methyl dihydrogenated tallow (BM2HT) ammonium clay, benzyl dimethyl hydrogenated tallow (BPMHT) ammonium clay, trimethyl hydrogenated tallow (3MHT) ammonium clay, and methyl trihydrogenated tallow (M3HT) ammonium clay. Type A and type C samples were obtained in water slurry form. A representative portion equivalent to approximately 10 g of dried solids was transferred into a disposable aluminum dish and dried overnight at 110 OC in air. The dried clay was completely removed from the aluminum dish, broken into pieces small enough to fit the entire sample into a grinding container, and then ground in the shatterbox grinder/blender. Grinding the freshly dried clay in this fashion for 3-min intervals will reduce the flakes to a homogeneous powder with a fine consistency ready for analysis.
1987
Two or three 3-min grinding intervals are usually sufficient. A 5-min cooling interval between grinds was employed to minimize the possibility of clay decomposition. The hardened steel grinding containers were cleaned thoroughly between samples with scouring powder and water. They were immediately rinsed with acetone after cleaning to facilitate drying and to retard oxidation of the iron. The lack of iron oxide contamination is important, especially in samples that will be analyzed with no absorption corrections. If the clay is not dried prior to grinding, it will produce a waxy agglomerate instead of a fine powder when ground. The clay samples have to be kept desiccated whenever they are not being used because they are hygroscopic; i.e., they can pick up as much as 8% water depending on relative humidity. Water absorption occurs between the clay platelet faces. The water forms hydrated ions with the inorganic cations that are present and causes the clay to swell. This swelling is a result of an increase in the size of the basal spacing and causes a shift in the peak position of the (001) reflection near the quartz peak. This shift results in changes in the curvature of the background under the quartz peak and becomes a potential source of error when determining the net integrated intensity. The type B samples were obtained as dry powders and contain between 25 and 50 wt 70organic matter. In general commercial organoclays will pass through a 325-mesh screen and the powders can be used directly for analysis. If agglomerates are present, the sample can be broken up in a few minutes by use of a mixer/mill with plastic mixing balls and containers. The organoclays are hydrophobic and generally contain less than 2% adsorbed water and need not be desiccated. X-ray fluorescence (XRF) liquid sample cups are filled with fine powder samples or standards and covered with 0.25 mil thick Mylar when measuring mass absorption coefficients. The filled cups are inverted and gently but firmly tapped against a clean flat surface to remove excess air from the powder-Mylar interface to present a uniform surface for analysis. When powders are prepared for diffraction scans, a 1in. X 3 in. microscope slide is placed against the front surface of a specimen holder and then attached securely with tape. The powder samples and standards are then firmly packed into the holder cavity by using another glass slide and hand pressure. Duplicate packings of each sample are prepared and placed into the sample changer. Standard Preparation. Individual sets of standards were prepared to cover the ranges of crystalline silica typically found in all three types of samples. Type A samples were analyzed only for quartz in order to evaluate the efficiency of a commercial deganguing process. The range for type A standards was 16-52 wt % quartz. Type B standard samples contained 0-4 wt % quartz and 0-13 wt '7" cristobalite. Type C standard samples contained 0-2.5 wt % of both quartz and cristobalite. Appropriate additions of cristobalite and/or quartz (Min-U-Si1 5) were admixed with clay and organoclay base stocks having less than 0.1% crystalline silica contents. The clay-base stocks were prepared by repeatedly centrifuging aqueous suspensions of bentonite clay mined at Colony, WY, removing the heavy fraction and drying the purified suspension. This procedure was repeated until peaks from all other gangue constituents were absent and only traces of quartz and/or cristobalite could be detected in diffraction scans of the purified clay. The concentrations of quartz and cristobalite in the clay base stock were then determined by a standard additions extrapolation. Appropriate organoclay bases were made from this material. All additions were weighed on an analytical balance capable of reading to 0.1 mg. The standards were mixed for 15 min in polystyrene vials with several acrylic mixing balls in a mixer/mill. When the internal standard method was employed, 10% by weight of LiF was added to portions of the previously prepared standard sets and similarly mixed for 15 min. When the internal standard method was not used, intensity reference standards were employed to compensate for long-term instrumental drift and slight changes in goniometer alignment, thus eliminating the need for frequent recalibration. Two intensity reference standards of quartz were employed, a standard for high levels of quartz (smaller counting time interval-larger scanning range) and a standard for low levels of quartz (larger counting time interval-smaller scanning range).
ANALYTICAL CHEMISTRY, VOL. 59, NO. 3, FEBRUARY 1, 1987
For type A samples an Arkansas stone sample supplied by Philips was used. For type B and type C samples a 15 wt % quartz standard was prepared by mixing 15 wt % of 5-pm Min-U-Sil with cellulose powder and pressing a thin sheet approximately 2 mm thick at 20 tons/in.2. An intensity reference standard was also prepared for cristobalite by hot pressing a similar sheet from a 15 wt 70mixture of cristobalite in Lucite powder. Pieces of these sheets were cut to size and permanently set into specimen holders with epoxy. These standards were lightly spray coated with clear acrylic (Krylon) to protect the surfaces. Type A, B, and C samples can vary significantly in chemical composition. The variance in composition influences the diffraction intensities. Corrections for attenuation effects therefore must be applied to the measured diffraction intensities. To illustrate the effect of FepO3 on the diffracted intensities from quartz, four samples were prepared that contained 3% quartz and varying amounts of pigmentary grade Fez03 (0.5 pm average particle size). The balance of these test samples was beneficiated Wyoming bentonite clay stock which contained 0.03% quartz. The Fe203content in the clay stock is known to be on the order of 3%. The mass absorption coefficient was determined experimentally by the XRF Compton peak method for each sample and the quartz contents were determined with and without intensity corrections. Data Collection. Scanning of the diffraction samples begins once the X-ray generator has been allowed to equilibrate for at least 30 min at power settings of 45 kV and 35 mA. Scanning and initial data processing conditions are selected for batch mode analysis. For type B and type C samples the strongest line of quartz (d = 3.34 A) is step scanned from 25.9' (28) to 27.2' in 0.02' increments for 15 s per increment. Likewise, the strongest line of cristobalite (d = 4.05 A) is scanned from 21.1' (28) to 22.6' at the same rate. When LiF is used as an internal standard, the d(lll)line of LiF is also scanned at the same rate from 38' (28) to 39.5' (28). If no internal standard is used, the appropriate intensity reference standard is scanned once for every four specimens under the same conditions as the analyte. Type A samples, which typically are high in quartz, are scanned over a larger 28 range (25.5-27.5') to accommodate broader peaks due to higher intensities. Type A samples are also scanned at 12'/min from 5 to 50' (28) to determine if interferences are present. When illite is observed to be present in type A samples, the illite d(ool,peak is scanned over the 28 range of 7.5-9.5' using the same scanning conditions that were used for scanning the quartz peak. The mass absorption coefficient value is determined in air at power settings of 45 kV and 32 pA. A total of 400000 counts are collected for each specimen at an energy of 18.9 keV (Rh, peak) using a window width of 17 channels with an energy increment of 20 eV per channel and no background correction. The total time in seconds necessary to accumulate 400000 counts is recorded. Data Processing. The raw data from a diffraction scan is smoothed by a five-point weighted smoothing technique used in Philips' software and then displayed as a trace on the graphics terminal. Visual estimates of the background positions on each side of the peak are located by using a movable cmor and marked. Hard copies of the labeled scan and the raw data file are made. With the marked values in the trace as a guide, the first three background values on each side of the peak are chosen in the raw data file. The first and last data points, along with the increment size, determine the number of data points that define each individual peak. The intensity of each data point defining the peak is summed up to determine the gross peak intensity. The average values for the first three and the last three entries are considered to represent the background intensity on each side of the peak. A straight line interpolation between these points is generated and the area under the interpolated background subtracted from the gross peak intensity to obtain a net integrated peak intensity. The analyte intensity ratio of the sample peak to the reference or internal standard peak is then used to calculate the analyte concentration.
RESULTS AND DISCUSSION In principle, the concentration of any particular crystalline phase in a powder mixture is proportional to the net integrated
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intensity (peak minus background) of any diffraction line (d(hk0) of that phase provided that (1) the particle size of the powder mixture is fine enough (ideally
