Energy & Fuels 1990,4,11-14
11
Chemical Beneficiation of Shale Kerogen? John D. McCollum* and W. F. Wolff Research and Development Department, Amoco Oil Company, Naperville, Illinois 60566 Received April 27, 1989. Revised Manuscript Received August 11, 1989 ~~
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By aqueous caustic digestion of oil shale at 150-165 "C followed by acid extraction, as much as 95% of the mineral matter can be removed from raw shale without significantly altering the kerogen liquefaction reactivity from that in its virgin state. This paper describes bench-scale results of beneficiation of the Green River shale using this caustic-acid method and some properties of the concentrated kerogen.
Introduction Oil shales with 10-30% organic matter are dilute hydrocarbon sources compared with most coal or tar sands fossil fuel reserves. For characterization of chemical properties and for testing diagenetic-catagenetic reaction paths, shales have generally been concentrated by HC1-HF demineralization.' For pyrolytic oil production, partial mineral removal by physical beneficiatiod and by carbonate removal with carbonic3 or dilute mineral acids4 has been examined. For Green River shales from the Piceance Basin in Colorado, the latter processes give beneficiated shales with Fischer assay oil yields in the range 40-60 gal/ton compared with 15-30 gal/ton for feed shales. Even 60 gal/ton material contains over 50% mineral and is a dilute source compared with typical run-of-mine coals. To prepare substantial quantities of highly concentrated kerogen for compositional and reactivity assessments, we explored alternate routes to HF demineralization to avoid complications attending toxicity and corrosive reactivity of that reagent. One route, digestion of feed shale with concentrated aqueous caustic at 150-170 "C followed by extraction with mineral acid, proved capable of producing kerogens with less than 10% mineral ash. The first step of this base-acid (BA) demineralization procedure resembles the Baeyer process for recovery of alumina5 and potentially could be applied not just to laboratory preparations but also to large-scale chemical beneficiation of shales. An initial report of this work has appeared in the patent literature.6 This paper describes the chemistry of the BA process and some properties of the concentrates produced. During this exploratory study, mineral separation methods related to BA treatment appeared. A molten caustic treatment of oil shale was described by Meyer's group at TRW;' the process employs much higher temperature (ca. 300 "C) in the caustic step, but some chemistry is likely common with our process. More recently, caustic-acid demineralization of c0al~9~ and coal ashlo has been described also with similar chemistry to the shale processing. Results Figure 1 sketches the base-acid sequence employed for this work. With Green River shale, digestion of a rich shale (44 gal/ton, 72% ash) with caustic above 150 "C removes about one-third of the silicon leaving a base residue of about 65% ash. Treatment of the residue with a mineral Presented at the Symposium on Advances in Oil Shale Chemistry, 193rd National Meeting of the American Chemical Society, Denver, CO, April 5-10, 1987.
Table I. Effect of Base Digestion Variables upon Ash Content of Base-Acid Concentrate wt % ashn in BA shale grade, concen% org NaOH, wt temp, "C time, h % in H20 carbon trate 7.0 150 4 50 13.0 150 2 14.7 7.0 50 150 4 7.0 70 5.5 170 4 70 15.4 7.0 70 12.2 7.0 170 16 14.4 22.0 150 4 50 170 16 85 22.0 9.2 150 4 25 26.5 24.2 150 4 24.2 50 9.4 150 4 70 7.4 24.2 a Ash figures correspond to products from treatment of the digestate with 1 M HCl for 4 h at room temperature.
acid then removes most of the remaining mineral matter. The resulting BA kerogen has an ash content comparable to HC1-HF demineralization. A fairly complex chemistry is reflected in dependence of demineralization upon process variables in the beneficiation. Base Digestion. Three shales, two rich (22-24% carbon) and one lean (11% C), were used for this work. To examine reaction variables, an autoclave was used to maintain a constant caustic concentration and to permit good recovery of products. Runs at higher caustic concentration for preparing kerogen stocks were done at atmospheric pressure (Experimental Section). In shales of varying richness (or organic carbon), effects of digestion conditions on the final acid-extracted product ash, a measure of demineralization effectiveness, are shown in Table I. For base digestion under comparable conditions (1) Durand, B.; Nicaise, Cr. Kerogen; Durand, B., Ed.; Editions Techamp: Paris, 1980; p 35. (2) Daltas, R. S.; Salotti, C. A. 16th Oil Shale Symposium Proceedings, Colorado School of Mines and Laramie Energy Technology Center: Golden, CO, 1983; pp 417-425. (3) Vandergrift, G. F.; Winans, R. E.; Horowitz, E. P. Fuel 1980,59, 634. (4) Rajeshwar,T.; Mraz, T.; Rosenwold, R.; DuBow, J. Fuel 1984,63, 920.
(5) Maczura, G.; Goodbody, K. C. Kirk-Othmer Encyclopedia of Chemical Technology, 3rd ed.; Wiley: New York, 1979; Vol. 2, p 218. (6) McCollum, J. D.; Wolff, W. F. US.4,584,088, 1986, US.4,668,380, 1987. (7) Meyers, R. A.; Hart,W. D. U S . 4,545,891, 1986. (8) Chi, C. Y.; Markuszewski, R.; Wheelock, T. D. Prepr. Pap.-Am. Chem. Soc., Diu. Fuel Chem. 1985, 30(2), 40. Chi, C. Y.; Fan, C. W.; Markuszewski, R.; Wheelock, T. D. ACS Symp. Ser. 1986, No. 319,30-41. (9) Yang, R. T.; Doc, S. K.; Tsai, B. M. C. Fuel 1985, 64, 735. (10) Waugh, A. B.; Bowling, K. M. Fuel Process Technol. 1984,9,217.
0887-0624/90/2504-0011$02.50/0 0 1990 American Chemical Society
McCollum and Wolff
12 Energy & Fuels, Vol. 4, No. 1, 1990 -I SiO;. Raw shale
l50'
C o g . ...
Table 111. Recovery of Low Ash Kerogen Concentrate
c SI(OH)A,Ar3,Ca*?Mg *2,Fe +3..
\
digestion
zg$eash Mineral acid Base-Acid Concentrate 10% ash
-
Figure 1. Base-acid demineralization scheme with Green River shale. Table 11. Effect of Acid Extraction Variables on Base-Acid Concentrate Ash wt % ash" in BA
concn, mol/L 0.1 0.1 1.0 12.0
temp, "C 80 22 22
22
concentrate 66 23 21
59
OAsh values taken for shale (22% organic carbon) digested 16 h at 170 O C with 70 wt % NaOH (aqueous) and treated with HCI at conditions in this table.
(150 "C, 4 h), product ash is not strongly dependent upon shale grade, ash decreasing from 13 to 9% as grade rises from 7 to 24% organic carbon. Increasing caustic concentration gives a lower ash product at 150 "C, e.g., from 27 to 7% ash as NaOH increases 25-70%, although at higher temperature (170 "C) an ash minimum was found near 85% NaOH.6 Ash level also decreases slowly with digestion time, from 15 to 13% for 2-4 h at 150 "C, 50% NaOH, or from 15 to 12% for 4-16 h at 170 "C, 70% NaOH, for lean shale. Digestions at 125 "C required long times to give any ash reduction and were not further considered. Autoclave runs were all well stirred, but stirring rate was not explicitly examined. Again, earlier work6 indicated stirring produced much lower ash materials. Although generally a caustic to shale ratio of 1.5 was used, we found that aside from NaOH consumed dissolving silica and converting alkaline carbonates to silicates, 80% of the caustic could be recovered. This confirmed early scouting experiments that caustic usage was about 0.8 g/g of shale for mahogany zone shales. Accordingly, runs with caustic/shale ratio of 1.0 proved as effective as those with 1.5 ratio. Acid Extraction. Any dilute mineral acid serves to decompose and dissolve inorganic components of the base-digest shale. However, the salts must be easily soluble to leave a low ash kerogen product. Thus, dilute sulfuric, nitric, hydrochloric, and sulfurous acids all break up the aluminosilicates, but sulfuric acid leaves calcium sulfate in the kerogen and is less suitable than the latter three. Table I1 shows for HCl that, to obtain minimum ash concentration in product, elevated temperature and high acid concentration must be avoided. As noted below, excessive ash in these concentrates results not from incomplete dissolution of shale mineral but from silica gel arising by polymerization of silicic acid, the product of acid decompositions of aluminosilicates. The excess SiO, can be removed by a second base wash (50% NaOH at about 50 O C ) to give again a low-ash concentrate. Kerogen Product Yield. Recovery of organic matter by the BA process is quite high. Table I11 shows yields both for autoclave and for atmospheric pressure preparative ("oven") runs. With care, 95% of the organic matter was recovered showing that high yields from BA demineralization can be expected. The principal loss was in the
shale grade, % C preparative method temp, "C time, h NaOH concn, % extraction acid product yield: w t ?h ash free oxide ash, wt %
24.2 autoclave 4 50 HzS03, 1.0 M
22.0 oven 165 16 70-90 HzS03, 1.0 M
22.0 oven 165 16 70-90 HCl, 1.0 M
95.5 f 1.8
78.1 f 2.8
80.1 f 5.6
16.2 f 2.4
8.2 f 1.6
8.0 f 1.2
150
" Values and mean and standard deviation of four to eight runs each. filtration and washing step following base digestion. The higher temperature runs directed toward bench-scale preparation of low-ash concentrate for characterization work gave a more intractable digestion residue, a caked solid at room temperature, which led to greater losses. Yields were still fair although the large deviations reflect less precise operations. There is little difference between yields using HCl or H2S03in extraction step. Surprisingly, despite an appreciable carboxylic acid content, not much organic matter is lost in the caustic treatment. This may be due in part to a high average molecular weight of the acid; indeed much is bound to the insoluble kerogen network (bonding to the mineral matrix is disrupted in the base treatment). It may also reflect the very high ionic strength of concentrated caustic which depresses the solubility or "salts out" long-chain aliphatic acids.
Discussion Mineral Changes Attending Base-Acid Beneficiation. Qualitatively the mineral profile of shales used in this work was common to Green River shales.'l Table IV includes X-ray diffraction indication of mineral types for a rich mahogany zone 22% organic carbon material. Quantitative estimates by infrared12 show siliceous and carbonate contents typical of mahogany zone material although the quartzlfeldspar ratio is on the high side. Pyrite was also present but not quantified in this analysis. Removal of silcon by base digestion results in conversion of aluminosilicates to sodalitic framework minerals davyne and cancrinite, and tobermorites (calcium silicates) observed in XRD and IR.13 This is also reflected in decrease of the Si/Al ratio from 4.1 in raw shale to about 2.7 in the base-treated residue. Instability of siliceous lattices with high aluminum content to acid treatment is well-kno~n,l~ and removal of most aluminum and more basic cations as soluble salts leaves the BA concentrate. As described above, the lattice silicon also dissolves as silicic acid. Polymerization of is not well-defined.15 It is accelerated by high silicic acid concentration, high temperature, and high acid concentration. Accordingly, varying amounts of silica can be left in the BA ash unless careful control of these variables is exercised and practiced. Aside from silica, the ash in the BA kerogen concentate shows some residual (unconverted) quartz and silicate although quantitative XRD characterizations are difficult because of a high background from XRD-amorphous (11)Hendrickson, T.A. Synthetic Fuels Data Handbook; Cameron Inc.: Denver* 1975;PP 39-42. (12) Snyder, T. W.; Painter, P. C.; Cronauer, D. C. h e 1 1983,62,1205. (13)Farmer, V. C., Ed. Infrared Spectra of Minerals; Mineralogical Society: London, 1974;pp 371-372,4550. (14)Uytterhoven, J. B. Sixth International Zeolite Conference Proceedings;Butterworth*: London, 1984; p 51. (15) Iler, R.K.Chemistry of Silica; Wiley Interscience: New York,
1979.
Chemical Beneficiation of Shale Kerogen
Energy & Fuels, Vol. 4, No. 1, 1990 13
Table IV. Mineral Components: Concentrates from Mahogany Zone Shale
calcite, dolomite feldspar, Na, K, Ca Ca, Mg silicate quartz clay, zeolite silica gel cancrinite tobermorite
shale, 22% org C XRD IR,' % major 19 minor 14 major intermed
digestion residue XRD
minor major minor
26 12
(variable) major minor
'Shale products from the oven procedure with HCl extraction in Table 111. Table V. Inorganic Elemental Analyses' wt %
Si Ca A1 Mg Fe K Ti As Rb Ni Mo
feed shale (22% C) 12.7 7.3 3.1 2.8 1.7 1.0 0.1 0.09 0.08
