Distribution of Trace Metals During Oil Shale Retorting Arun D. Shendrikar' and Gerald B. Faudel' Tosco Corp., 18200 West Highway 72, Golden, Colo. 80401
Three samples of raw oil shale from the Green River formation of Western Colorado were retorted under conditions simulating a potentially commercial process. The raw shales and all resulting retort products were analyzed for various trace metals in an attempt to ascertain migratory patterns and resulting distributions through the retort system. Some evidence of low-level fluoride, boron, and copper partitioning to the water fraction was found. Similar behavior of arsenic and zinc partitioning to the shale oil product was observed with the remainder of the metals investigated predominantly retained in the spent shale fraction. Environmental regulatory agencies are concerned with the mobility of trace metals in coal during its burning for electrical power generation and for coal gasification processes (2-3). Therefore, we in the oil shale industry were anxious to investigate the behavior of trace metals during retorting of oil shale, so that effective control measures could be implemented if necessary. Previous studies have been conducted to determine individual elemental concentrations and some group concentrations on various samples of either raw shale or the pyrolyzed solid residue (4-8). None, however, has been concerned with the possible migration of the trace metals from an initial raw shale sample through a simulation of a potentially commercial process. This investigation was initiated to quantify the distribution of trace metals during retorting of oil shale using the Fischer assay method (9).Fischer assay is a laboratory method for retorting oil shale, and it produces a product slate similar to the TOSCO I1 process (9, 10). The Fischer assay of oil shale produces four products: spent shale, oil, water, and gas. The raw shale to be retorted and Fischer assay products were examined for trace metals of interest. Analytical data were tabulated for 15 elements as percent recovery of each element and percent distribution in the various Fischer assay products.
Experimenta 1 Fischer Assay Procedure. Fischer assay is a laboratory method that has been used to estimate oil yields for economic evaluation of oil shale reserves for nearly the last three decades ( 2 1). At Tosco Corp. this method has been routinely utilized to produce total material balances (9). In the standard Fischer assay, a known sample weight (=IO0 g) of minus 65 mesh oil shale is placed into a 7-oz aluminum can, together with three aluminum heat transfer disks placed at regular intervals in the sample. The container is then placed in a steel retort, and thermocouples are inserted both in the sample and on the periphery 01 the retort. The sealed retort is placed in a retort heater connecting a tared adapter and centrifuge tube along with a condenser. The system is purged three times with nitrogen to ensure an inert atmosphere. The condenser system is maintained a t 32 O F . The retort is heated following a predetermined temperature-time profile and then held a t 932 OF for 20 min. Subsequently, the retort is allowed to cool, and the apparatus is disassembled. The volume of water in the receiver is recorded, and a sample of the oil is
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Present address, Labs., Inc., 1437 Sadlier Circle W. Drive, Indianapolis, Ind. 46239.
332 Environmental Science 8 Technology
removed for specific gravity determination. The spent shale is removed from the retort and weighed. The weight of product oil is obtained by subtracting the weight of water (assuming unit specific gravity) from the total weight of liquid product. The measurement of specific gravity permits calculations of oil yields in gallons per ton of raw shale retorted. In these experiments the product gas was also collected for analysis. The weight of gas produced was calculated from gas chromatographic (GC) analysis. Sample Description. Three samples of raw shale were tested: Tosco-1-164,Tosco-2-164, and Tosco-3-164. The raw shale samples were composites of representative Mahogany zone oil shale core taken from the Colony mine site in the Piceance Creek Basin of Western Colorado. Weight percent distributions of Fischer assay products for the composite samples are given in Table I. The water fract,ion
Table 1. Percent Distribution of Oil Shale Samples in Terms of Various Fischer Assay Products SamDle number Product ( w i % )
Spent shale Oil
Water Gas
T~SCO 1-1 - 64
TOSCO-9- 164
Tosco-3-164
82.98 12.62 1.38 3.02
82.62 13.18 1.20 3.00
83.20 12.82 1.10 2.88
100.00
100.00
100.00
Table II. Sample Preparation Methods Element
Antimony Arsenic Beryllium Boron Cadmium Chromium Cobalt Copper Fluoride Lead Manganese Mercury Molybdenum Nickel Selenium Vanadium Zinc
Sample preparation methoda
Combustion and total absorption of all products into acid Sample digestion in mixture on nitric acid and sulfuric acid ( 78) Low-temperature ashing (LTA) Samples fused at 900-950 O C with carbonate salts of sodium and potassium, fusion method Low-temperature ashing (LTA) High-temperature ashing (HTA) Low-temperature ashing (LTA) Low-temperature ashing (LTA) Samples fused at 900-950 O C with carbonate salts of sodium and potassium, fusion method Low-temperature ashing (LTA) Low-temperature ashing (LTA) Nitric:sulfuric acid sample digestion ( 75) Low-temperature ashing (LTA) High-temperature ashing (HTA) Low-temperature ashing (LTA) Low-temperature ashing (LTA) Low-temperature ashing (LTA)
The numbers in parentheses indicate reference numbers.
0013-936X/78/0912-0332$01 .OO/O
@ 1978 American Chemical Society
from this retorting process was only about 1.2 wt % of the total products. The small sample size restricted quantitative analysis to only a few elements in the water product. Data for trace metals in retort water have been incorporated from previous analyses (12). The gas fraction of the Fischer assay product slate accounted for about 3 wt % of the products. Analysis of the gas for trace metals was limited to arsenic and mercury. Sample Preparation. A variety of sample preparation approaches were attempted. Special efforts were directed to analyze as many elements as possible with a single sample preparation technique. Table I1 summarizes the techniques finally selected for each of the elements included in this study. Water samples were analyzed without additional treatment. Gases from a typical Fischer assay were absorbed in 100 mL of 1:l nitric acid:water mixture that was subsequently boiled down, cooled, and diluted to a known volume. Trace metal determinations in gas fractions were carried out using this resulting solution. Only arsenic and mercury could be determined in this fraction. The concentrations of the other elements were below detection limits. Analytical Approach. Acidic sample solutions of all Fischer assay products and raw shale were used for the determination of antimony, beryllium, cadmium, chromium, cobalt, copper, lead, manganese, nickel, vanadium, and zinc by using an atomic absorption technique (13) (Perkin-Elmer Model 305B). Mercury determinations used a flameless atomic absorption method (14, 15). Boron (16) and molybdenum (I 7) contents were determined by well-established colorimetric methods. Fluoride analyses were by specific ion-electrode. The Gutzeit method (18)was used for arsenic analyses. Quantitative catalytic reduction of methylene blue (19) by selenium sulfide was utilized for determination of selenium. Verification of analytical methods was made with a National Bureau of Standards sample No. 1632, “Trace Elements in Coal”, which was analyzed concurrently. A comparison showed that the methodology used in these experiments gave acceptable determinations.
Results and Discussion The antimony concentrations in all the three samples of raw shale, spent shale, oil, and water were less than 4 ppm. These low concentrations did not allow a material balance for this element to be made.
Table 111. Trace Metal Balances-Oil Element
Arsenic Beryllium
Boron Cadmium Chromium Cobalt Copper Fluoride Lead Manganese Molybdenum Nickel Selenium Vanadium Zinc
Raw shale
60.0 1.o 63.3 1.25 41.7 6.5 47.5 1162. 29.3 230.
30.0 23.9 14.6 57.1 65.0
Atomic absorption techniques for arsenic determinations could not be made without interference. Variable results were obtained when evolved arsine gas from a sample solution was trapped in a silver diethyldithiocarbamate (20) solution. The Gutzeit method (18) gave good, reproducible results and was used in this study. Tables 111-V show the material balances for all elements examined, including arsenic. Only about 90% of the arsenic could be accounted for. One explanation for arsenic losses could be the formation of gaseous arsine, since retorting is done under reducing conditions. Attempts were made to identify arsenic in the retort gas, resulting in arsenic concentrations of 0.5 ppm by weight of product gas. This low concentration could not account for the missing 10%. The distribution of beryllium and boron after oil shale retorting is also shown. Two of the three samples analyzed gave a good boron material balance. Only 70% of the boron could be accounted for in the third sample. A specific ion-electrode was used for fluoride determinations of the samples of raw shale, spent shale, oil, and water. All sample digests were diluted 1:lO with total ionic strength activity buffer solution (available from Orion Research, Stock No. 94-09-09) and measured against standards. The mercury material balances were variable, ranging from 58 to 196% recovery of the mercury present in raw shale. For this reason mercury data will not be reported in this paper. Good elemental balances for chromium, cobalt, copper, lead, manganese, nickel, vanadium, and zinc were obtained. The major portion of these trace metals was retained in the spent shale. Sample solutions prepared for chromium and nickel determinations by low-temperature ashing (LTA) gave nonreproducible results. Sample solutions prepared by high-temperature ashing (HTA) were used to obtain the material balances reported.
Conclusions This program examined the trace metal distribution during retorting of oil shale using the Fischer assay process. The majority of the trace metals are retained by the spent shale. Fluoride (3-4 ppm), boron, and copper (0.2-0.5 ppm) were detected in the water fraction. Chromium, cobalt, manganese, molybdenum, nickel, selenium, and zinc were also found in the retort water fraction a t concentrations
