Chemical Geology, 25 (1979) 333--338
© Elsevier Scientific Publishing Company, Amsterdam -- Printed in The Netherlands
VATERITE FORMATION DURING COAL LIQUEFACTION
R O Y J. B R U N S O N *
and J O S E P H J. C H A B A C K
Exxon Research and Engineering Company, Baytown, TX 77520 (U.S.A.) Thermodynamics Research Center, Texas Engineering Experiment Station, Texas A & M University, College Station, TX 77843 (U.S.A.) (Received April 27, 1978; accepted for publication February 22, 1979)
ABSTRACT Brunson, R.J. and Chaback, J.J., 1979. Vaterite formation during coal liquefaction. Chem. Geol., 25: 333--338. The vaterite form of calcium carbonate seldom occurs in geological formations or during chemical processing. However, vaterite crystals form during the liquefaction of a Wyoming subbituminous coal. Gravitational separation of the coal showed vaterite to form because of the presence of some other mineral in the coal matrix. Model compound studies with calcium acetate identified silica and possibly iron pyrite to be minerals found in coal which seed vaterite crystals. INTRODUCTION
The stable forms of calcium carbonate are calcite and aragonite. Vaterite is a metastable crystal form identified by Vater (1893--1901). Vaterite can be produced in the laboratory by rapidly mixing 1 M solutions of CaC12 and Na2 CO3 (Turnbull, 1973). In addition, Kitano and Hood (1965) found that certain organic compounds can promote vaterite formation. Documentation of occurrences in nature, however, are scattered. Identification of natural samples include mineral deposits in Northern Ireland (McConnell, 1960) and gastropod egg shells (Hall et al, 1971). Vaterite was produced in boiler scale during small-scale experiments at the Fuel Research Station of the Department of Scientific and Industrial Research, London (Aubrey, 1954). No mechanism was determined for the unusual crystal form occurrence but it was postulated that something in the feed water repressed the crystallization of the more stable calcite. The most notable differences in the composition of the water which led to vaterite formation were increased amounts of Pb and reduced amounts of St, Mg and Si. Vaterite is also formed during coal processing. Low-rank coals contain organic O in the form of phenols and humic acids. Durin8 coal aging Ca diffuses into * To w h o m correspondence should be addressed. Present address: Kraft Inc., 801 Waukegan Road, Glenview, IL 60025, U.S.A.
334 the coal pores and forms calcium salts with organic oxygen compounds. Upon heating, the calcium salts decompose to form calcium carbonate. Under certain conditions the calcium carbonate forms as vaterite. COAL LIQUEFACTION TECHNIQUES Liquefaction is a process of thermal cracking and hydrogenation of coal to produce a clean burning liquid fuel. Coal is finely ground and mixed with a coal derived solvent to form a slurry. The slurry is heated to about 450°C at a reaction pressure of 10 MPa. Coal partially dissolves in the solvent and is converted to hydrocarbon fuels and chemical gases. Two types of coal were liquefied in this study. Botl~ were low-rank coals from the western U.S.A. One of these was a subbituminous coal from Campbell County, Wyoming. The second was a lignite from Mercer County, North Dakota. Both types of coal produced calcium carbonate when heated to liquefaction temperatures. During this study, coal was liquefied in two types of reactors (Furlong et al., 1976). The first type of reactor was a tubular flow reactor. The flow reactor allowed growth of calcium carbonate particles. Once vaterite formation was substantiated in a flow reactor, a short study was initiated in the tube autoclave reactor to identify key elements of the mechanism of formation. Tube autoclaves are batch reactors which make possible a large number of tests with various coal samples and model compounds. Reaction products from both reactors were washed with toluene and dried to obtain solids for analyses. The presence of carbonates was determined by acid evolution. The crystal form was identified by X-ray diffraction. Calcium-carbonate-rich samples were also examined with the aid of an electron microscope with energy dispersive X-ray elemental analysis. FLOW REACTOR DEPOSITS In situ calcium carbonate formation in a flow reactor is evidenced by wall scale and aggregates which grow within the reaction medium. These deposits tend to accumulate and interrupt reactor operation. A random sample of wall scale was determined to consist of 3 wt.% combustible material and 82 wt.% calcium carbonate. The remainder of the wall scale was primarily iron sulfide produced by partial corrosion of the reactor wall. The observed crystalline forms of calcium carbonate produced in coal liquefaction have been calcite and vaterite. The predominant crystal form depends on the coal being fed to the flow reactor. Fig. 1 shows a face on picture of scale recovered after processing North Dakota Lignite. X-ray diffraction confirms that the visible platelets are calcite. On the other hand, liquefaction of Wyoming coal from Campbell C o u n t y produced the spherulites in Fig. 2. By X-ray analyses, the spherulites were identified as vaterite and the angular crystals in Fig. 2 found to be iron sulfide. An enlargement of a portion of the sur-
face in Fig. 2 is shown in Fig. 3. The worm-like protrusions are postulated to be growth sites which were active at the time the reaction was halted. MECHANISM OF VATERITE FORMATION
Vaterite formation can be caused by organic (Kitano and Hood, 1965) or mineral matter (Aubrey, 1954). Thus, vaterite formation in Wyoming coal could be due to organic or inorganic constituents. One way to vary the mineral content without altering the organic matter is to split a single coal sample by flotation in liquids of successively increasing specific gravity. Table I shows how the mineral matter for a Campbell County, Wyoming coal sample is concentrated in the heavier specific gravity fractions. Despite the general trend for mineral matter and especially pyritic-S there is no trend in the concentration of Ca in the three splits. The uniform Ca distribution is due to its incorporation with organic acid groups. Upon liquefaction in a tube autoclave, the mineral rich 1.5 float produced the most vaterite. The lightest sample, the 1.3 float, produced primarily calcite. The 1.4 float portion, which represents about half of the original coal, produced the usual split of calcite and vaterite. Therefore, some constituent of the mineral matter is responsible for vaterite formation. Further insights into crystallization mechanisms were derived from model com-
Fig. 1. Face on picture of calcite wall scale, produced during liquefaction of North Dakota Lignite, at 1000 magnification.
Fig. 2. Face on picture of vaterite wall scale, produced during liquefaction of Wyoming subbituminous coal, at 200 magnification.
Fig. 3. Growth sites for vaterite wall scale at 5000 magnification.
337 TABLE I Mineral distribution and resultant CaCO 3 crystal form for Campbell County, Wyoming subbituminous coal Float specific gravity
Calcium content (wt.%)
Ash Pyritic (wt.%) sulfur
CaCO 3 crystal form
1.3 1.4 1.5
1.2 1.3 1.2
5.4 5.5 8.3
calcite mixed vaterite
0.06 0.08 0.11
pounds studies, since removal of individual minerals from the coal is not possible. Calcium acetate thermally decomposes to form calcium carbonate (Ardagh, et al., 1924). At liquefaction conditions calcium acetate produces calcite. The addition of Wyoming coal changes the decompositon product to vaterite. Further, vaterite forms even if the Ca originally in the coal is either acid-extracted or precipitated as unreactive calcium sulfate prior to the test. This reinforces the suggestion made above that a substance in the coal is responsible for vaterite formation. Silica is a predominant coal constituent which could seed the vaterite crystal form. Calcium acetate which is mixed with colloidal silica in an acidic aqueous media prior to subjection to liquefaction conditions does indeed produce vaterite. However, silica in other forms such as bentonite (high-silica) and kaolinite (aluminum silicate) clays and various grades of quatz failed to produce vaterite. The crystals shown in Fig. 2 suggest that some form of iron sulfide plays a role in vaterite formation. As a test, calcium acetate was heated with various mixtures of FeS and FeS~. Some vaterite was formed in the cases when FeS2 was present, b u t calcite was formed exclusively when only FeS was added. Therefore, FeS2 or some intermediate as it is reduced to FeS is also capable of seeding vaterite. Therefore, Ca, which is exchanged into coal pores during the aging process, is interspersed with coal minerals. Upon heating, the Ca salts d e c o m p o s e to form calcium carbonate. Silica, iron pyrite, and possibly other coal minerals influence the calcium carbonate to form in the less stable crystal pattern called vaterite. ACKNOWLEDGEMENT
The authors acknowledge partial financial support by the Electric Power Research Institute (Agreement No. RP778--1) and the U.S. Energy Research and Development Administration [Contract No. E (49--18) - - 2 3 5 3 ] .
338 REFERENCES Ardagh, E.G.R., Barbour, A.D., McClelland, G.E. and McBride, E.W., 1924. Distillationof acetate of lime. Ind. Eng. Chem. 16: 1133--1138. Aubrey, K.V., 1954. Occurence of ~-CaCO 3 in boiler scale. Nature London, 174: 81. Furlong, L.E., Effron, E., Vernon, L.W. and Wilson, E.L., 1976. The Exxon donor solvent process. Chem. Eng. Progr., 72 (8): 69--75. Hall, A. and Taylor, J.D., 1971. The occurrence, of vaterite in gastropod egg shells.Mineral. Mag., 38: 521--522. Kitano, Y. and Hood, D.W., 1965. The influence of organic material on the polymorphic crystallization of calcium carbonate. Geochim. Cosmochim. Acta, 29: 29--41. McConnell, J.D.C., 1960. Vaterite from Ballycraigy, L a m e Northern Ireland. Mineral. Mag., 32: 535--544. Turnbull, A.G., 1973. A thermochemical study of vaterite. Geochim. Cosmochim. Acta, 37: 1593--1601. Vater, H., 1893--1901. A series of papers in Zeitschrift f'~r Kristallographie und Mineralogie (in German).