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Hardness of fine ground coals and mineral residues to predict slurry erosion
SUMMARY
general mathematical description of mechanis+s of slur?y qosion is hampered by lack of a quantitatiue definition of hardness that describes the same property of. both metal or.ceramic surfaces and erosive particles. The exception is the Mohs hardness scale, defined by a comparison of simple scratch tests. This test cannot be applied to the solidsin coal slurries, which are mixtures of different particles with a wide uariation in relative hardness_ Two simple devices were built to measure an equivalent Mohs hardness for fine powders and to demonstrate its correlation with established hardness scales. These tests can be applied to the suspended solids in coal liquefaction feed and discharge streams, in coal-oil fuel slurries, and in both raw and steam-dried coal-water slurries for pipeline transportation and process feed, to define their relative erosiveness. The values can be compared with the hardness, defined by the same method and on the same scale, for candidate metals and ceramics for slurry valves and pumps, to predict rates 0 f erosion. Precise,
and prediction
INTRODUCTION
Studies on the erosion of materials of construction by abrasive slurries involved in coal liquefaction processes are limited to narrow ranges of conditions, with resulting data not clearly comparable. This is due largely to the lack of any universal standard of ‘hardness’ that applies equally to both brittle and ductile materials and that makes no. distinction between which is scratching or eroding which. The Mohs scale, defined by.simply scratching a series of reference minerals with each other, 0032-5910/85/$3.30
can be extended to finely ground materials, which are often composed of different cornponents’whose hardness alone may vary by over an order of magnitude. This study reports two designs for simple devices to measure Mohs hardness and to define the relative abrasiveness or abrasion/erosion resistance of any material, whether slurries or the materials selected to contain them. Consider the situation of a coal slurry flowing through a steel valve, in particle sizes from 300 ,um (0.12 in or 50 mesh) to 100% 75 nm (0.003 in or 200 mesh) for some processes_ A large fraction of the particles, ranging from under 10% for typical raw lignites to substantially higher for solid residue of the H-coal or SRC processes, may have an intrinsic hardness exceeding that of the hardest metals. For instance, pyrite, a major mineral component in many coals, may have a Snoop hardness up to 1840, approaching the 2000 reported for silicon carbide and exceeding the 1635 for fused alumina, both among the hardest materials discussed for use as trims for let-down valves in coal slurry applications. Even a 670 Knoop steel, intended for abrasion resistance, is less hard than half of the minerals found in coal. It is therefore no surprise that these slurry streams are so effective in grinding up the hardest available valve trims.
HARDNESS
-
A PROBLEM
OF DEFINITION
‘Hardness’ is defined as a measure of the damage that one material can inflict on another by sliding contact or impact. Such damage is a result of-this property of both materials. Its magnitude and units must be defined independently of any geometrical or environmental factors. It must be the same
0 Elsevier Sequoia/Printed
in The Netherlands
76 property, with the same units, measured in the same way, for both materials. That is, any empirical, predictive function relating rates of surface damage to this property N of both materials, S = f(H,/H,), must be equally valid no matter which of the materials is inflicting abrasive damage on the other_ Figure 1 illustrates the problem of defining and measuring a universal hardness in view of the microscopic mechanisms of surface erosion. The Mohs hardness, defined by a series of arbit.rarily selected, pure minerals, is the only definition to date that meets the above requirement of reciprocity_ Only microscopic examination reveals differences in the mechanisms of material removal along the scratch. MICROSCOPIC
Fig. 1. Abrasive hardness definition.
Relative hardness experiments have typically used the ‘sandblaster’ approach in which a high-velocity air jet, bearing abrasive particles, impinges on a smooth surface. The rate of surface loss is a function of the controlled variables, impact velocity Vi and impact angle Qi, and of the relative ‘ductility’ or ‘brittleness’ of the materials. These properties, along with compressive strength, shear strength and various friability indices, overlap in part the definition of ‘hardness’ used here but are in no way synonymous with it. Some studies [l, 2, 3, 43 have shown that maximum rates of surface removal occur at high impact
angles for ‘brittle’ surfaces and at low angles for ‘ductile’ surfaces. In practical slurry erosion applications, the angle and velocity of impacting particles are highly variable in turbulent flow through complex channel geometries_ Thus, an objective of the work reported here is to define an empirical measure of ‘hardness’ or relative erodability/erosiveness that can be applied both to surfaces and fine particles, independent of velocities or impingement angles, with the universality and reciprocity inherent in the Mohs scale of macroscopic hardness.
REVIEW
OF HARDNESS
DATA
Initial selection of standard minerals for the Mohs hardness comparison was fairly and attempts to interpolate arbitrary, between whole integers revealed major inconsistencies in these increments near the upper end of the scale, initially defined as 10.0 for diamond_ The upper end has been redefined (by Ridgeway et al_ [5]), calling the hardness of diamond 15.0 and establishing standards in the 8 - 15 range, which applies to most abrasives, intentional and otherwise, and surfaces designed to resist them. In Table 1, this scale is compared with the less relevant, penetration-based definitions of hardness, developed for ductile surfaces. In an effort to interpolate accurately the hardness of coals between whole integers on the Mohs scale, Haywood [S] measured the load required to make a scratch 100 pm wide on the surface of polished specimens of assorted coals, relative to that of a particular soft coal. This scale, still arbitrary, gives values of 1.92 and 5.71 for calcite and pyrite, respectively, compared with 3.0 and 6 to 6.5 on the Mohs scale. Thus, in Table 1, Haywood’s coal data were increased by 3/1.92 to roughly approximate Mohs values for coal. The Spencer-Bierbaum Microcharacter test [S] involves a diamond tool pressed into a polished surface with constant pressure. The surface is drawn under the point, producing a scratch, whose width is measured under a microscope_ The hardness is simply reported as 10 OOO/(scratch width)*_ This provides a quantitative, interpdlative measure of scratch resistivity, equally applicable to ductile or brittle materials.
:
~~. _ :
TABLE 1 Comparisonof
.,.: hardnessscales
..
1
Material
Original Mohs references Gypsum Caicite Apatite Orthoclase Quartz (crystalline) Topaz Corundum (AiaOa) Extended Mohs references [ 51 Garnet Zirconia, tantalum carbide Fused alumina‘ ‘i’ungsten carbide Silicon carbide Boron carbide Diamond Metals [S] Aluminum Copper Iron Lead Manganese Silver Tin Zinc Chromium Stainless steels [7] AISI-304 AISI-316 ‘Cast Ni steels for abrasion resistance’ ‘Advanced coating materials’ [ I] BN (Cubic)
-.
,_.I
._ :-
: ..
.~. -.-
-;
‘.
1.
:
.7_7: .-
.:
LMOhsscak
2 3 5 6 7 8 9 10 11 12 12 13 14 15
._--
’ __.
Penetration tests Knoop [315]*
35.: 90 100-160 .45@ - 690 650-930
1000 - 1500 1420 2050
- 2000 - 2750
1635 2000 2230 (6000
- 10000)
[1]
2 - 2.9 2.5 - 3.0 4-5 1.5 5.0 2.6 1.7 2.5 9.0 1602 160% 350 - 670+ 3700 3700 3350 2800 2500 1800
B4C
TiBZ TiC SIC WC Other minerals found in coaI Shales lllite Muscovite Kaolins Kaolinite Sulfides Marcasite, F&p Pyrite, Fe& Carbonates, incl. calcite Rhodochrosite, MnCOa Siderite, FeC03 Smithsonite, ZnCOa Ankerite, (Ca, Mg, Fe) CO3 Dolomite, (Ca, Mg) COs Rock saIt, NaCI Albite, NaAl SisOs Anorthite, CaAlaSiaOs Magnetite, Fes04
.,
2 - 2.5
- 4000
- 3000 - 3000 - 2200
19 - 34 40 - 85
2 - 2.5 316 - 6.5
38
760 - 1650 1000 - 1840
3 - 5.5
2 6 - 6.5 6 - 6.5 5.5 - 6.5
240 370 500 350 480 20 1682 150 480
-
370 440 660 490 575 50
(?) - 740
(Continued
overleaf)
78 TABLE
1 (continued) Mohs
Material
Penetration
scale
Knoop Other minerals found in coal Hematite, Fez03 Kyenite, AlaSi Lepidocrocite, FeaOx-Ha0 Tourmaline (borosilicates) Zircon (ZrSiOJ) Coals Bituminous As function 92% 80% 65% SO% 75% 70% 65% Composite
and boghead of % carbon
[S] [3 ]
5.5 - 6.5 4-7 5 - 5.5 7 - 7.5 7.5
750 500 150 1190 1110
2.5
?
bituminous
U.K. coals [S] Two U.K. anthracites *Data reported of accuracy.
directly
-
40-60 22 26 31-36 29-34 23 14 14 -
range
Miscellaneous
and lower-rank
U.S.
and
0.5
1100 2150 465
1480 1510
29 30
27 60
- 1.7
2.7%
[S] in Knoop
tests
13, 5]*
units
or approximated
Other established measurements are the Brine& Rockwell, Vickers, and Knoop tests [3, 91, which measure the penetration of a ball or point pressed into the surface. These measure its resistance to plastic deformation, and not to abrasion or erosion. All of these tests share the disadvantage that they only apply to homogenous surfaces. The problem of comparing published hardness data on candidate metals for valve design is compounded by metallurgical tradition that uses severai variations of the Brinell and Rockwell scales, rarely the Vickers Diamond Pyramid scale and almost never the Snoop or Mohs scales. The stainless steel data in Table 1 were found in Brinell hardness units in one reference [7] and converted to the Vickers Diamond Pyramid scale, which is ‘close to’ the Knoop scale [7] _ Similarly, the data for the ‘Advanced Coating Materials in Table 1 was reported as Vickers hardness but is included here as a probable approximation to their Knoop hardness. For crushed coal, standard procedures include the Hardgrove Grindability Index (ASTM D409-45) and tumbler test (ASTM D441-45) for friability [lo] _ These are functions of the strength and shatter resistance of coals and are measures of what various
from
Brinell,
Rockwell,
or Vickers
units
with
some
loss
surfaces of processing equipment can do to the coal. The interest of this study, on the other hand, is in what the coal can do to the equipment. The variation in hardness of single grams in coal components in Table 1 can best be appreciated by an attempted correlation of the Mohs and Knoop hardnesses in Fig. 2. The heavy black lines, comparing the reference minerals for both scales, show a consistent correlation up to Mohs 8. Beyond that, in the range of the Ridgeway-Ballard-Bailey extension, there is a disturbing discontinuity. The limited data for coals in Table 1 tells us that the hardness of whole coal, within a broad experimental scatter, is lower by nearly an order of magnitude than that of most of its mineral components, which, in turn, exceed the range of most steels.
HARDNESS
OF
COALS
AND
THEIR
MINERAL
RESIDUES
For this study, simple experiments were undertaken to determine the Mohs hardness of some coals and their mineral residues_ For fine numerical resolution of scratch data, one must establish more reference surfaces than
data from Table 1
Hardness
Extended Mohs scale 0
Fig. 2. Comparison scales.
TABLE
Reference minerals from alternative source [ 161 of uncertainty
of Mohs and
Knoop
hardness
reflects the unavoiaably-~~jective--iuatio~ of some of the-scra~h.~atterns.iFor-in~ce, the al&hum and. lucitk ‘-plastic both scratched gy@tim but not -calcite and were both scratched by calcite, establishing them in the range of-2.to 3. Within this range, alumi-num made no more -thah the most margiriai mark on lucite, while’ lucite made conspicuous gouges in aluminum. This defines lucite as harder than aluminum. Using such interpretations, one can assume reasonable approximations for the hardness of secondary reference surfaces, such aS 2.3 f 0.2 for aluminum and 2.7 k-O.2 for lucite. Similar interpolative techniques define published fractional values for minerals, in some cases’to the nearest 0.25 unit [11-J_
2
Scratch tests for calibration Secondary
of secondary
reference standards
reference surface
l+imary reference mineral
Mohs hardness
Talc Gypsum
1.0 2.0 22.5 3.0 3.1 3.1 4.0 3.8 4.4 5.0 4.6 5.1 5.3 6.0 6.0 5.8 6.1 6.1 5.5 6.5 7.0 8.0 9.0 8.7
Aluminum ‘Lucite’ clear acrylic plastic Calcite ‘Transite’, asbestos lab. bench Carbon steel Stainless steel (316 Window glass
Fluorite
or 304)
Apatite*
‘Tile grade l’, brown, coarse glaze Inconel ‘Tile grade 4’, brown, glossy glaze
‘Tile grade 6’, white glaze ‘Tile grade 2’, tan glaze Diorite ‘Tile grade 3’, cream glaze with blue spots ‘Tile grade 5’, solid gray with blue and-brown
Refractory *Suitable
Orthoclase* Albite*
specks
Quartz Topaz Corundum
brick, 99% alumina apatite and orthoclase
specimens
not available_ Alternate,
albite, hardness from literature
2.5 - 2.9 - 3.6 - 3.9 - 4.5 - 4.9 - 5.2 - 5.3 - 6.2 -
6.5 6.8 6.8 6.8 6.8 6.8
- 9.3
[12].
For primary standards, no suitable orthoclase was available and albite was used as its equivalent. One source [ 121, however, reports the hardness of albite as variable, 6.0 - 6.5. The only available specimen of apatite was a crumbly aggregate of tiny crystals that could inflict faint scratches on glass, for example, but was useless as a reference surface. Therefore, the secondary reference surfaces between 4 and 6 in Fig. 3 were ranked mainly by their hardness relative to each other. For application of this methodology, any convenient material can serve as a secondary standard if its Mohs hardness proves to be in the desired range. More attention is needed, however, in obtaining primary mineral standards as per Fig. 1, which should be relatively pure specimens of as large crystals as possible solidly fused together. Equipped now with smooth surfaces of hardness known to within 0.5 Mohs unit, scratch tests were done for a variety of coals and mineral residues_ These are identified in Table 3. Results of scratch tests are shown in Table 4. Among strongly stratified lignites, as with many rocks, apparent hardness varied, depending on whether the scratch was on a cleavage plane or crossing the cut end of laminae. At the lower extremes of their hardness, lignites tended to crumble rather than scratch. As expected, slags and ashes were harder than whole coal. It is of interest that
the gasifier slags were consistently harder than slags formed in combustion processes_ This is attributed to more extreme mineral modification in the reducing atmosphere of the University of North Dakota Energy Research Center’s slagging gasifier at 1000 - 1500 “C. The very hard slag 5 contains roughly 2% chrome, leached out of the gasifier’s refractory lining by the caustic, silicious slag, while SEM studies of slag 6 have revealed substantial deposits of coarse, nearly pure alumina crystals (Mohs 9+)_ Given a finely ground mixture of known composition, it is possible that the effective hardness should be related to each component by something like H = x,H, + xb Hb + __ . , etc., where x is the weight fraction of each mineral_ However, due to differences in crystalline form and friability of different minerals, it is probable that they will have widely different particle size distributions within that of the mixture, so that the frequency of impacts and mass of impacting particles will not be in proportion to the weight fractions_ Such inhomogeneity also applies to composite materials for erosion-resistant surfaces_ It is reported that cemented carbides represent the state-of-the-art materials for valve trim [lo] and that their failure is usually by preferential erosion of the relatively soft binder phase. Thus, if hardness values are to be defined for finely ground materials, they must be based
l/4” FROM DRILL COLLAR SECTION MATCHING ROTATING
OF
POWER
Wf
SET
HAND
SCREW
SQUARE
HOLE TO SAMPLE
ROTATING
SAMPLE
CHAMBER
BEARINGS STATIONARY GRANULAR
TEST
MATERIA
PLUNGER
SUPPORT W/
“0”
RING
PLUNGER TOOTHED
CHARACTERISTIC SCRATCH PATTERN (For real. physical appearance see A on Fig. 5.)
Fig. 3. Model I abrasion test device, for coarser granular solids.
SEAL
_.
~.
.:
‘-.
TABLE 3 Coals and mineral residues tested
Designation
Description or origin
Lignite 1 Lignite 2 Lignite 3 Bituminous Anthracite
Indian Head mine, ND (Typ. 10% Ash) Baukol-Noonau, ND Freedom mine, Beulah, ND Colorado Carbon County, PA
Liquid residue 1 Liquid residue 2
Solid residue of continuous liquefaction process (29.8% aah) Same, after 10 h at 500 “F to carbonize or oxidize residual tars (45.7%
Coke
Petroleum
Ash1 Ash2
Low-temperature Iaborator?; ashes, minimum mineral alteration Absaloka subbit. (air 750 =C) Decker subbit. (carbon tetrafluoride, 300 “C)
Ash3 Ash4 Ash5
Residues subjected to oxidizing atmosphere during combustion Sarpy Creek subbit. baghouse ash ‘Bottom slag’ Indian Head lignite (typical) ‘Bottom slag’ Arapahoe subbit. (rare, glossy)
Slag
Slag Slag Slag Slag
ash)
coke, ash free
Residues subjected to reducing atmosphere during gasification, all from Indian Head lignite Normal production slag, water quenched Normal wall deposit from gasifier ‘Stalactite’ from taphole, high alkali and sulfides ‘Stalactite’ from taphole, black, g1oss.y ‘Stalactite’ from taphole, pink bubbIy mass containing 2% chrome from damaged refractory_ High alumina
1 2 3 4 5
Slag 6
TABLE
.-
Slow-cooled, coarse crystalIine slag from hearth, high alumina
4
Results of manual scratch tests (big lumps) of assorted coals and residues Test sample*
Mohs hardness range Definite
Ambiguous**
Raw coals Lignite 1 Lignite 2 Lignite 3 Bituminous Anthracite
2.3 1.5 2.3 2.7 3.1
0.5 - 1.8 2.0 - 2.7
Petroleum
coke
3.4 - 3-9
Oxidation Ash4 Ash5
residues
Reduction Slag I Slag 2 Slag 4 Slag 5
residues
-
2.6 1.8 2.7 3.0 3.9
3.6 - 4.2 4.6 - 5.4 5.8 3.7 5.4 8.5
-
6.5 4.4 5.9 9.5
3.6 - 4.7
2.8 - 4.4 5.4 - 6.6
*Identified in Table 3. **Broader range covera non-reciprocal or otherwise inconsistent data.
upon scratch tests of the materials as they are encountered in real-world applications, representing an effective hardness for any combination of particle components. Thus, a test is needed whereby a smooth surface is scratched by a sample of fine powder. For a valid Mohs hardness, it must scratch the next softest material but not the next hardest material_ One method for measuring the abrasive effect of a granular material on a smooth surface [13] invoives a plunger pressing the abradent down a tube against a point on a turntable of the surface to be eroded. Another method is to operate tumbling mills with balls or rods made of different materials, comparing their relative resistance with abrasion under the same conditions [ 51. The much simpler devices of Figs. 3 and 4 were built to provide interpolative measures of Mohs -hardness of granular materials. Whether the granular material makes a mark or not after some standard contact time will locate its Mohs hardness relative to the references.
82
RUBBER SAMPLE
WHEEL GRINDS INTO REFERENCE
IRCULATION TEFLON BUSHING REFERENCE SURFACE, I
,-----,.,=A END
‘“0”
SEMI-SCHEMATIC, ASSEMBLY DETAILS
l/4” SHAFT F-ROM HAND POWER I DRILL
RINGS’ SIDE
VIEW NOT COMPLETE
VIEW
lllllllllllllllllln~
CHARACTERISTIC SCRATCH PATTERN (For real, physical appearance see B on Fig. 5.)
Fig. 4. lIode1 II abrasion test device, for fine granular solids.
Conversely, any flat spot of a test material greater than 50 mm in diameter can be evaluated with a series of granular reference materials_ Both devices are pocket-sized and need only an electric drill as a power source. The coarse abrasion test device, Model I, in Fig. 3 combines the pressure applied to the plunger and centrifugal force to push the ground abradent into the reference surface around t.he periphery of the rotor, leaving a circular pattern of fine scratches (mainly on a ductile, metallic surface) or pits (on brittle surfaces) if it. is harder than the reference material_ The ‘0’ ring protects the surface from direct damage by the steel rotor. The limitation of this device is that fine solids, less than about --2O mesh (-0.08 mm), fail to transmit motion from the rotor to the particles direct-ly against the test surface. Therefore, the i\lodel II device of Fig. 4 was built for finer powders_ Typical scratch patterns for both devices are shown in Fig. 5, using materials giving a high HP/H, for purposes of illustration. Figure 6 shows a magnified view of the scratch pattern left by a material of roughly H, = 3.5 + 0.3 on a mineral specimen conveniently composed of both calcite and fluorite_ The roughly vertical scratches appear on the calcite only, stopping abruptly at the edge of the fluorite_ Horizontal lines are due to polishing of the surface before the test. Unlike the conspicuous surface damage of Fig. 5, Fig. 6 represents a case of minimally detectable scratches, as expected from the HP/H, = 1.0 + 0.2. One published source [l]
suggests that erosion is substantial and less dependent upon hardness at HP JH, above 1.3 to 1.7, while erosion is insignificant at Hp/Hs below 0.7 to 1.1, which applies to Fig. 6. Results of tests with the Model I device are given in Fig. 7. These and most following tests used 5 s of moving contact time, on the assumption that if the abradent is decisively harder than the surface, this will still amount to thousands of potential replications of a single scratch as shown in Fig_ 1. For marginal cases (surface equal to or slightly harder than abradent), longer contact time did not inflict scratches when the first 5 s inflicted none.
Fig. 5. Typical scratch patterns made by abrasion testers. Surface: Aluminum (Mohs hardness: 2.3 + 0.2). A, Circular pattern of Model I tester with -20
mesh (9 - 0.85 mm) quartz (Mohs hardness: 7.0); B, smudge pattern of Model II tester with -100 mesh (9 - 0.15 mm) petroleum coke (Mohs hardness: 6.5 A 0.5). Axis of rotation vertical_ (Small parallel smudge caused by shift of plate_) Grinding time in both cases: approx. 5 s. Hp/Hs = 2.9 -I 0.1 in both cases.
83
This also applied to the Model II device. In Fig. ‘I, note that specific numbers are not assigned to the secondary reference surfaces. This is to avoid any unjustified delusion of precision in interpolation between the established primary standards. For example, in the interval between Mohs 4 and 6, six secondary standards are merely arranged in sequence and
assumed to be roughly evenly distributed. With the Model I test device, the wide clearance (more than 0.5 mm) simply prevented most tiny, natural mineral grains from being ground into the test surface. One of the observed distinctions between kinds of reference surfaces was that abrasive powders tend to make scratches on ductile (metallic) surfaces, for the full length of their contact, while leaving a pattern of chipped pits in brittle surfaces_ As an example, using the Model I or II tester, some ashes or slags leave a pattern of fine but definite scratches on inconel (Mohs 5+) but not on Tile grade 1 or window glass (Mohs 5-), even though the glass and tile can be easily scratched by a large, hand-held bar of inconel. A probable theoretical explanation of this lies in the essentially liquid molecular arrangement of vitreous or glassy solids, which have a prestressed surface analogous to the surface film of conventionally defined liquids. In the case of the above inconel-tile experiment, a substantial normal stress was concentrated at the single point of contact to puncture the surface film. Distributed fine particles, on the other hand, find only a perfect surface with no cleavage planes or grain boundaries at which to initate cracks. This suggests that a source of error, ignored by the original
Fig. 6. Typical scratch pattern on composition primary reference surface by Model I tester. Light phase: calcite; Mohs hardness: 3.0. Dark phase: fluorite; lMohs hardness: 4.0. Horizontal lines left from polishing of surface. Diameter of view shown: approx. 3 mm, located along circilar pattern as shown in Fig. 5(A).
MOHS HARDNESS
TEST SAMPLE 1
2
3
5
Fig. 7. Results of coarse abrasion tests with Model I test device-
6
7
hand, represent abnormal, high-temperature residues partially depleted of alkalis and silicon, the latter tending to volatilize as its unstable monoxide, SiO, in wet, reducing atmospheres at temperatures above 1000 "C. The most common coal minerals are of the kaolin and shale groups, with Mohs hardness in the range of 2.0 to 2.5, dispersed throughout coal. Isolating these ash blends in unaltered form is virtually impossible. As to particle size, coal minerals can be extremely fine. In Beulah, North Dakota, lignite, for instance [14], all of the identified mineral species occur in the 5 to 40 pm range, except for some aggregates of soft clays up to 400 pm_ In the various ashes and slags reported here, all of these components, unfortunately, have undergone major modification, converting relatively soft kaolins, carbonates, and organically bound alkalis and calcium to much harder minerals. Therefore, predictions of the abrasiveness of coal residue slurries based on data from ashes or slags will lead to high estimates of erosion rates. While uncertainty in interpolation of abrasion patterns increases with decreasing particle size, such data nevertheless come closer to describing the behavior of the very fine, unmodified coal minerals in liquefaction processes. Figure 9 shows data from the Model II abrasion tester for very fine solids. In addition to the actual liquefaction residue, the low-temperature ashes represent coal minerals as close as possible to original particles, not fused or modified by high temperatures- Fly ash and finely ground gasifier slag (both under 0.15 mm) are both included to determine whether the effects of fusion can be detected in fine particles. Figure 10 shows a magnified view of a scratch pattern on
definition of Mohs hardness, is the pressure applied at the point(s) of contact for scratch tests. While various glazed, ceramic tiles represent a handy range of hardness (Mohs 5 to 7), according to manual scratch tests, they tend to give erroneously high results when subjected to fine particulate abrasion_ With metals, on the other hand, fine particulate samples of slightly lower hardness often produce detectable brightening or dulling of the surface, yet. with no visible scratches or other mechanical modification of surfaces features, even when viewed under high magnification. These anomalies account for the width of the bars in this form of data reporting. To measure relat.ive abrasiveness in this size range of -100 mesh (0.15 mm), the rubber tire of the Model II tester grinds the sample directly against the reference surface. Figure 8 shows tests run with -20 mesh (0.85 mm) and -40 mesh (0.425 mm) materials. Additional ambiguities appear due to either a polishing or dulling action on reference surfaces, without leaving any scratches or other distortion of surface features visible through a microscope_ For esample, raw Indian Head lignite inflicts a slight polishing/brightening and very few microscopic scratches on ductile carbon steel (3.5 - 4.0), but makes no mark on either brittle calcite (3.0) or lucite plastic (2.7). Thus, for a material with a wide range of components, we can detect the effects of the hardest minority of its components. Slag 2 was a deposit from high on the walls of the gasifier, consisting of unaltered grains of coal minerals and coke, cemented together by a deposition of volatile mineral components, which appear to consist mainly of H,Si04 and Na,S. Slags 5 and 6, on the other
Fig. 8. Results
of fine abrasion
tests with
LModel II device.
-,
Definite;
n
, ambiguous.
85 TEST SAIIPLE
1
2
3
IIOHSHARDNESS 4
a
6
i
SLAG1
I
Fig. 9. Results of very fine abrasion tests with Model
Fig. 10. Scratch pattern by Model II tester for North Dakota lignite on carbon steel. Surface: carbon steel; Mohs hardness: 3.5 + 0.3. Abradent: Indian Head lignite; -100 mesh (-0.15 mm); Mohs hardness: 2.0 to 3.5.
polished carbon steel, representing the upper limit of abrasiveness possible with -0.15 mm Indian Head (North Dakota) lignite. We see here a fine haze of vertical scratches, whose length represents the width of a narrower version of the smudge (B) in Fig. 5, which was just visible to the naked eye. The horizontal lines represent the initial, coarse polishing of the surface and the dark spots are corrosion pits. The two tracks of heavier scratches, punctuated by pits, are anomalies representing a few larger and/or harder particles. While not identified, these could well have consisted of pyrite (H = 6 to 6.5), which commonly occurs in lignites in relatively large crystals that would resist grinding, compared with the soft, friable lignite. Figure 11 shows several such pyrite crystals, which are often big enough to be seen with the naked eye. Note the regular spacing of pits along the
~ II test device_ m,
Definite;
L-_l,
ambiguous.
Fig_ 11. Typical pyrite crystals foL:Qd in lignites. Magnification: 1230 times, by scannhlg electron microscope. Longest edge of biggest crystals: roughly 0.007 mm (similar crystals often found exceeding 1.0 mm).
vertical track near the right edge of Fig_ 10, with ‘strides’ of roughly 0.2 mm, which suggests rolling motion of an octahedral crystal similar to those of Fig. 11. The great majority of patterns observed in this study, however, for HP/H, = 1.0, consisted of the haze of scratches as in the center of Fig_ 10, with no interesting anomalies. To be valid, data from manual scratch tests and the two particulate hardness tester concepts demonstrated here must show a reasonable comparability. In general, the three methods appear equivalent, to within the inherent resolution of the Mohs hardness scale. The most notable exceptions are for the thermally modified ashes and slags in the +4 Mohs hardness range. The most significant materials are the liquefaction residues and the minimally modified Ashes 1 and 2, which best ZipprGxkEite the form of cod minerals
present in commercial liquefaction processes. The discrepancy seen in the harder ashes and slags (Mohs 4 to 7) is attributed to the problem, discussed above, of glassy surfaces on some of the tiles used as secondary standards. The broad span of Model II data for Lignite 1 appears due to the release of more abrasive components in grinding from 0.55 to 0.15 mm.
DISCUSSION
Consider as a comparison several designs for some novel slurry valve concept for an application involving slurries of 50 to 70 wt.% solids, of fine coal or its mineral residues, through pressure drops approaching 14 MPa (2000 psig) at 370 “C (700 “F). To build and test a series of such devices under working conditions will be extremely costly. Clearly, there is a great economic stimulus to develop such designs as far as possible by low-cost simulation, applying principles of dimensional analysis to predict erosion rates. It is probable that there exists some generalized equation to predict erosion rates, that might be of the following form: s =
HP j F, X”‘~“C(VD~//I)~ i 5
)
where S is some/any quantitative measure of rate of surface loss due to erosion of the containing surface at the point of interest; HS, HP are the hardnesses of the surface and of the eroding particles, respectively, which must be in the same units; X is the weight or volume fraction of solids in the slurry; d is the average or effective particle size; (VDp/p) is the Reynolds number for the slurry flow at the point of interest, with either estimated or measured properties for the slurry, and D = the effective hydraulic diameter; j, m, n and h are empirical exponents; and C is some empirical constant, with consistent units, as needed to make the equation balance. Next, let us suppose, for example, that we are considering and seeking to optimize some valve design with trim made from fused alumina (H, = 12) passing a liquefaction residue of approximately HP = 3, from Fig. 9. Then the hardness term in eqn. (1) will be 0.25’. To study a number of design variations cheaply,
at an accelerated rate, they could be made from aluminum (H, = 2.3 2 0.2) and simulate the slurry with finely ground quartz (HP = 8) in some aqueous, oil or glycol carrier to approximate actual liquid properties, but at ambient temperatures. The hardness term then becomes 3_5j_ The rate of erosion is thus (3.5/0.25)’ or 14’ times as fast as it would be under real-world conditions and at a fraction of the cost per test fixture and test loop operation_ It becomes apparent that such an approach can offer a tremendous’ yield of research data for very modest cost. Alternatively, suppose that the valve is already in service, with a plug and seat made of inconel (H, = 5.5 f 0.3). It is proposed that the valve trim be replaced by some customfabricated, fused boron carbide parts (E& = 14, from Table 1). As a quick approximation, the above equation, once confirmed, would tell us that the rate of erosion and frequency of shut-down and replacement would be reduced by (5_5/14)’ or O-39’, which provides a tradeoff against the increased cost. There are some troublesome uncertainties that arise when we consider the anomalous results shown in Figs. 8 and 9. Whole, raw lignites show a Mohs hardness range from around 3.0 down to less than 1.0, although they have high ash contents. The ash components including materials with Mohs hardness ranging up to 7.5, per Table 1, which accounts for the scattered, occasional scratches on surfaces substantially harder than the coal itself, as in Fig. 10. it might be assumed that the long-term abrasion/erosion rate will be in proportion to a hardness factor of the form
[W,,,Xa
+H,.,-&
+H,.c&
+ ---YHsI’
(2)
If we assume, for example, that 2.0 is the long-term, effective hardness of only the carbonaceous portion of a lignite, and that it has 10% ash of an average hardness of 5.0, then its true, long-term hardness would be (2 X 0.9 + 5 X 0.1) = 2.3, which is fairly reassuring in that the additional 0.3 is within the range of uncertainty_ The full sensitivity of erosion rates to uncertainty in assigning Mohs hardness values to whole slurries cannot be predicted until complete predictive equations are developed. The approach presented here predicts only the erosion rate component caused by and in
-.
~..~ ::
:.
:
: ...87-
-. some_ proportion to the. relative hardness of particles and surfaces. It does not predict the additional ..erosion caused by cavitation in applications involving -flashing liquids, which may account for as much_ damage td high pressure let-down valves as does slurry erosion alone_ Cavitation is a totally different basic mechanism and beyond the immediate scope of this study. Another limitation of this coldsimulation approach is that the hardness of many materials may decrease substantially with increasing temperature. It is reported, for instance, that the Rockwell hardness of some high-chrome steels may decrease by 60% at temperatures beyond 1000 “F [15]. Therefore, a complete prediction of hightemperature erosion phenomena will require some data on temperature dependence of surface hardness. The methodology in this study was restricted to very short grinding times, about 5 s, to avoid deviating too far from the yesor-no, single-scratch method established to define the Mohs hardness scale. It is nevertheless fairly obvious that even the hardest materials are worn down by the extended action of far softer materials, such as tool steel cutting edges, rocks subject to water erosion, or, more to the point, the hardest materials of construction subjected to high velocity slurries of ‘soft’ ground coal. The method prescribed here extends the qualitative, yes-or-no data defining the Mohs hardness scale to predict not whether erosion will occur, as it will in any case, but at what rate.
REFERENCES. 1
Materials S&r fion for High-Pressure Values in Coal Likuefactiqn S&ter&
SAND.SO-1526C or CONF-810417-1, DOE Contract No..DEAC04-76-DP00789. 2 L. K. Ives, J. P. Young and -A. W. Ruff, Particle Erosion.Measurements on Metals at Elevated Temperatures, National Bureau of Standards,
Special Publication 468, April 1977. L. G. Austin, Mechanical and Comminufive Properties of Coal, F’KOC. Nat. Sci. Foundn. Conf. on Materials Problems and Research Opportunities in Coal Conversion, April 16 - 18, 1974. 4 J. G. A. Bitter, We&, 6 (1963) 169. 5 R. R. Ridgeway; A_ H.,Ballard and B. L. Bailey, 3
Hardness
Values
for
Electrochemicai
Products,
Electrochem. Sot., May 1933. 6 A. F- Taggart (ea.), Handbook of Mineral Dressing, Wiley, New York, 1967 (10th printing)_ Steels, International Nickel Co., 7 Nickel Alloy Development and Research Division, New York, 1949. 8 H. Haywood, J_ Insf. FueZs, 9 (1935) 94. 9 R. Hultgren, Fundamentals of Physical MetaZZurgy. Prentice-Hall, Engelwood Cliffs, 1952. 10 American Society for Testing and Materials, Gaseous Fuels; Coal and Coke; Aimospheric Analysis, Procedure ASTM D409-45, Part 26, 1978. of Miner11 P. Frazer, Tables for the Determination aIs, Lippincot, Philadelphia, 4th edn, 1897. Guide to Rocks and Miner12 Simon and Schusfer’s als, Simon and Schuster, New York, 1977. IO+ (1982) 13 A_ Misra and I. Finnie, Trans. ASME, 14
15
ACKNOWLEDGEMENTS
This study was made possible by the U.S. Department of Energy, through Contract No. DE-FC21-83FE60181.
s_. Randich, Let-do+
16
94. S. A. Benson, C. Zygarlicke and F. R. Karner, The Occurrence and Disfribufion of Inorganic Constituents in Lignite from the Be&ah Mine, North Dakota. Rocky Mountain Coal Sync-
posium, Bismarck, ND, October 2 - 4,1984. W_ L. Silence, Sliding Wear in Coal Gasification, VoL II of Materials Problems and Research Opportunities in Coal Conversion, National Science Foundation and Office of Coai Research, Ohio State, Columbus, April 16 - 18, 1974 (Workshop Director, R. W. Staehle). American Society for Metals, Metals Handbook, 1948.
general mathematical description of mechanis+s of slur?y qosion is hampered by lack of a quantitatiue definition of hardness that describes the same property of. both metal or.ceramic surfaces and erosive particles. The exception is the Mohs hardness scale, defined by a comparison of simple scratch tests. This test cannot be applied to the solidsin coal slurries, which are mixtures of different particles with a wide uariation in relative hardness_ Two simple devices were built to measure an equivalent Mohs hardness for fine powders and to demonstrate its correlation with established hardness scales. These tests can be applied to the suspended solids in coal liquefaction feed and discharge streams, in coal-oil fuel slurries, and in both raw and steam-dried coal-water slurries for pipeline transportation and process feed, to define their relative erosiveness. The values can be compared with the hardness, defined by the same method and on the same scale, for candidate metals and ceramics for slurry valves and pumps, to predict rates 0 f erosion. Precise,
and prediction
INTRODUCTION
Studies on the erosion of materials of construction by abrasive slurries involved in coal liquefaction processes are limited to narrow ranges of conditions, with resulting data not clearly comparable. This is due largely to the lack of any universal standard of ‘hardness’ that applies equally to both brittle and ductile materials and that makes no. distinction between which is scratching or eroding which. The Mohs scale, defined by.simply scratching a series of reference minerals with each other, 0032-5910/85/$3.30
can be extended to finely ground materials, which are often composed of different cornponents’whose hardness alone may vary by over an order of magnitude. This study reports two designs for simple devices to measure Mohs hardness and to define the relative abrasiveness or abrasion/erosion resistance of any material, whether slurries or the materials selected to contain them. Consider the situation of a coal slurry flowing through a steel valve, in particle sizes from 300 ,um (0.12 in or 50 mesh) to 100% 75 nm (0.003 in or 200 mesh) for some processes_ A large fraction of the particles, ranging from under 10% for typical raw lignites to substantially higher for solid residue of the H-coal or SRC processes, may have an intrinsic hardness exceeding that of the hardest metals. For instance, pyrite, a major mineral component in many coals, may have a Snoop hardness up to 1840, approaching the 2000 reported for silicon carbide and exceeding the 1635 for fused alumina, both among the hardest materials discussed for use as trims for let-down valves in coal slurry applications. Even a 670 Knoop steel, intended for abrasion resistance, is less hard than half of the minerals found in coal. It is therefore no surprise that these slurry streams are so effective in grinding up the hardest available valve trims.
HARDNESS
-
A PROBLEM
OF DEFINITION
‘Hardness’ is defined as a measure of the damage that one material can inflict on another by sliding contact or impact. Such damage is a result of-this property of both materials. Its magnitude and units must be defined independently of any geometrical or environmental factors. It must be the same
0 Elsevier Sequoia/Printed
in The Netherlands
76 property, with the same units, measured in the same way, for both materials. That is, any empirical, predictive function relating rates of surface damage to this property N of both materials, S = f(H,/H,), must be equally valid no matter which of the materials is inflicting abrasive damage on the other_ Figure 1 illustrates the problem of defining and measuring a universal hardness in view of the microscopic mechanisms of surface erosion. The Mohs hardness, defined by a series of arbit.rarily selected, pure minerals, is the only definition to date that meets the above requirement of reciprocity_ Only microscopic examination reveals differences in the mechanisms of material removal along the scratch. MICROSCOPIC
Fig. 1. Abrasive hardness definition.
Relative hardness experiments have typically used the ‘sandblaster’ approach in which a high-velocity air jet, bearing abrasive particles, impinges on a smooth surface. The rate of surface loss is a function of the controlled variables, impact velocity Vi and impact angle Qi, and of the relative ‘ductility’ or ‘brittleness’ of the materials. These properties, along with compressive strength, shear strength and various friability indices, overlap in part the definition of ‘hardness’ used here but are in no way synonymous with it. Some studies [l, 2, 3, 43 have shown that maximum rates of surface removal occur at high impact
angles for ‘brittle’ surfaces and at low angles for ‘ductile’ surfaces. In practical slurry erosion applications, the angle and velocity of impacting particles are highly variable in turbulent flow through complex channel geometries_ Thus, an objective of the work reported here is to define an empirical measure of ‘hardness’ or relative erodability/erosiveness that can be applied both to surfaces and fine particles, independent of velocities or impingement angles, with the universality and reciprocity inherent in the Mohs scale of macroscopic hardness.
REVIEW
OF HARDNESS
DATA
Initial selection of standard minerals for the Mohs hardness comparison was fairly and attempts to interpolate arbitrary, between whole integers revealed major inconsistencies in these increments near the upper end of the scale, initially defined as 10.0 for diamond_ The upper end has been redefined (by Ridgeway et al_ [5]), calling the hardness of diamond 15.0 and establishing standards in the 8 - 15 range, which applies to most abrasives, intentional and otherwise, and surfaces designed to resist them. In Table 1, this scale is compared with the less relevant, penetration-based definitions of hardness, developed for ductile surfaces. In an effort to interpolate accurately the hardness of coals between whole integers on the Mohs scale, Haywood [S] measured the load required to make a scratch 100 pm wide on the surface of polished specimens of assorted coals, relative to that of a particular soft coal. This scale, still arbitrary, gives values of 1.92 and 5.71 for calcite and pyrite, respectively, compared with 3.0 and 6 to 6.5 on the Mohs scale. Thus, in Table 1, Haywood’s coal data were increased by 3/1.92 to roughly approximate Mohs values for coal. The Spencer-Bierbaum Microcharacter test [S] involves a diamond tool pressed into a polished surface with constant pressure. The surface is drawn under the point, producing a scratch, whose width is measured under a microscope_ The hardness is simply reported as 10 OOO/(scratch width)*_ This provides a quantitative, interpdlative measure of scratch resistivity, equally applicable to ductile or brittle materials.
:
~~. _ :
TABLE 1 Comparisonof
.,.: hardnessscales
..
1
Material
Original Mohs references Gypsum Caicite Apatite Orthoclase Quartz (crystalline) Topaz Corundum (AiaOa) Extended Mohs references [ 51 Garnet Zirconia, tantalum carbide Fused alumina‘ ‘i’ungsten carbide Silicon carbide Boron carbide Diamond Metals [S] Aluminum Copper Iron Lead Manganese Silver Tin Zinc Chromium Stainless steels [7] AISI-304 AISI-316 ‘Cast Ni steels for abrasion resistance’ ‘Advanced coating materials’ [ I] BN (Cubic)
-.
,_.I
._ :-
: ..
.~. -.-
-;
‘.
1.
:
.7_7: .-
.:
LMOhsscak
2 3 5 6 7 8 9 10 11 12 12 13 14 15
._--
’ __.
Penetration tests Knoop [315]*
35.: 90 100-160 .45@ - 690 650-930
1000 - 1500 1420 2050
- 2000 - 2750
1635 2000 2230 (6000
- 10000)
[1]
2 - 2.9 2.5 - 3.0 4-5 1.5 5.0 2.6 1.7 2.5 9.0 1602 160% 350 - 670+ 3700 3700 3350 2800 2500 1800
B4C
TiBZ TiC SIC WC Other minerals found in coaI Shales lllite Muscovite Kaolins Kaolinite Sulfides Marcasite, F&p Pyrite, Fe& Carbonates, incl. calcite Rhodochrosite, MnCOa Siderite, FeC03 Smithsonite, ZnCOa Ankerite, (Ca, Mg, Fe) CO3 Dolomite, (Ca, Mg) COs Rock saIt, NaCI Albite, NaAl SisOs Anorthite, CaAlaSiaOs Magnetite, Fes04
.,
2 - 2.5
- 4000
- 3000 - 3000 - 2200
19 - 34 40 - 85
2 - 2.5 316 - 6.5
38
760 - 1650 1000 - 1840
3 - 5.5
2 6 - 6.5 6 - 6.5 5.5 - 6.5
240 370 500 350 480 20 1682 150 480
-
370 440 660 490 575 50
(?) - 740
(Continued
overleaf)
78 TABLE
1 (continued) Mohs
Material
Penetration
scale
Knoop Other minerals found in coal Hematite, Fez03 Kyenite, AlaSi Lepidocrocite, FeaOx-Ha0 Tourmaline (borosilicates) Zircon (ZrSiOJ) Coals Bituminous As function 92% 80% 65% SO% 75% 70% 65% Composite
and boghead of % carbon
[S] [3 ]
5.5 - 6.5 4-7 5 - 5.5 7 - 7.5 7.5
750 500 150 1190 1110
2.5
?
bituminous
U.K. coals [S] Two U.K. anthracites *Data reported of accuracy.
directly
-
40-60 22 26 31-36 29-34 23 14 14 -
range
Miscellaneous
and lower-rank
U.S.
and
0.5
1100 2150 465
1480 1510
29 30
27 60
- 1.7
2.7%
[S] in Knoop
tests
13, 5]*
units
or approximated
Other established measurements are the Brine& Rockwell, Vickers, and Knoop tests [3, 91, which measure the penetration of a ball or point pressed into the surface. These measure its resistance to plastic deformation, and not to abrasion or erosion. All of these tests share the disadvantage that they only apply to homogenous surfaces. The problem of comparing published hardness data on candidate metals for valve design is compounded by metallurgical tradition that uses severai variations of the Brinell and Rockwell scales, rarely the Vickers Diamond Pyramid scale and almost never the Snoop or Mohs scales. The stainless steel data in Table 1 were found in Brinell hardness units in one reference [7] and converted to the Vickers Diamond Pyramid scale, which is ‘close to’ the Knoop scale [7] _ Similarly, the data for the ‘Advanced Coating Materials in Table 1 was reported as Vickers hardness but is included here as a probable approximation to their Knoop hardness. For crushed coal, standard procedures include the Hardgrove Grindability Index (ASTM D409-45) and tumbler test (ASTM D441-45) for friability [lo] _ These are functions of the strength and shatter resistance of coals and are measures of what various
from
Brinell,
Rockwell,
or Vickers
units
with
some
loss
surfaces of processing equipment can do to the coal. The interest of this study, on the other hand, is in what the coal can do to the equipment. The variation in hardness of single grams in coal components in Table 1 can best be appreciated by an attempted correlation of the Mohs and Knoop hardnesses in Fig. 2. The heavy black lines, comparing the reference minerals for both scales, show a consistent correlation up to Mohs 8. Beyond that, in the range of the Ridgeway-Ballard-Bailey extension, there is a disturbing discontinuity. The limited data for coals in Table 1 tells us that the hardness of whole coal, within a broad experimental scatter, is lower by nearly an order of magnitude than that of most of its mineral components, which, in turn, exceed the range of most steels.
HARDNESS
OF
COALS
AND
THEIR
MINERAL
RESIDUES
For this study, simple experiments were undertaken to determine the Mohs hardness of some coals and their mineral residues_ For fine numerical resolution of scratch data, one must establish more reference surfaces than
data from Table 1
Hardness
Extended Mohs scale 0
Fig. 2. Comparison scales.
TABLE
Reference minerals from alternative source [ 161 of uncertainty
of Mohs and
Knoop
hardness
reflects the unavoiaably-~~jective--iuatio~ of some of the-scra~h.~atterns.iFor-in~ce, the al&hum and. lucitk ‘-plastic both scratched gy@tim but not -calcite and were both scratched by calcite, establishing them in the range of-2.to 3. Within this range, alumi-num made no more -thah the most margiriai mark on lucite, while’ lucite made conspicuous gouges in aluminum. This defines lucite as harder than aluminum. Using such interpretations, one can assume reasonable approximations for the hardness of secondary reference surfaces, such aS 2.3 f 0.2 for aluminum and 2.7 k-O.2 for lucite. Similar interpolative techniques define published fractional values for minerals, in some cases’to the nearest 0.25 unit [11-J_
2
Scratch tests for calibration Secondary
of secondary
reference standards
reference surface
l+imary reference mineral
Mohs hardness
Talc Gypsum
1.0 2.0 22.5 3.0 3.1 3.1 4.0 3.8 4.4 5.0 4.6 5.1 5.3 6.0 6.0 5.8 6.1 6.1 5.5 6.5 7.0 8.0 9.0 8.7
Aluminum ‘Lucite’ clear acrylic plastic Calcite ‘Transite’, asbestos lab. bench Carbon steel Stainless steel (316 Window glass
Fluorite
or 304)
Apatite*
‘Tile grade l’, brown, coarse glaze Inconel ‘Tile grade 4’, brown, glossy glaze
‘Tile grade 6’, white glaze ‘Tile grade 2’, tan glaze Diorite ‘Tile grade 3’, cream glaze with blue spots ‘Tile grade 5’, solid gray with blue and-brown
Refractory *Suitable
Orthoclase* Albite*
specks
Quartz Topaz Corundum
brick, 99% alumina apatite and orthoclase
specimens
not available_ Alternate,
albite, hardness from literature
2.5 - 2.9 - 3.6 - 3.9 - 4.5 - 4.9 - 5.2 - 5.3 - 6.2 -
6.5 6.8 6.8 6.8 6.8 6.8
- 9.3
[12].
For primary standards, no suitable orthoclase was available and albite was used as its equivalent. One source [ 121, however, reports the hardness of albite as variable, 6.0 - 6.5. The only available specimen of apatite was a crumbly aggregate of tiny crystals that could inflict faint scratches on glass, for example, but was useless as a reference surface. Therefore, the secondary reference surfaces between 4 and 6 in Fig. 3 were ranked mainly by their hardness relative to each other. For application of this methodology, any convenient material can serve as a secondary standard if its Mohs hardness proves to be in the desired range. More attention is needed, however, in obtaining primary mineral standards as per Fig. 1, which should be relatively pure specimens of as large crystals as possible solidly fused together. Equipped now with smooth surfaces of hardness known to within 0.5 Mohs unit, scratch tests were done for a variety of coals and mineral residues_ These are identified in Table 3. Results of scratch tests are shown in Table 4. Among strongly stratified lignites, as with many rocks, apparent hardness varied, depending on whether the scratch was on a cleavage plane or crossing the cut end of laminae. At the lower extremes of their hardness, lignites tended to crumble rather than scratch. As expected, slags and ashes were harder than whole coal. It is of interest that
the gasifier slags were consistently harder than slags formed in combustion processes_ This is attributed to more extreme mineral modification in the reducing atmosphere of the University of North Dakota Energy Research Center’s slagging gasifier at 1000 - 1500 “C. The very hard slag 5 contains roughly 2% chrome, leached out of the gasifier’s refractory lining by the caustic, silicious slag, while SEM studies of slag 6 have revealed substantial deposits of coarse, nearly pure alumina crystals (Mohs 9+)_ Given a finely ground mixture of known composition, it is possible that the effective hardness should be related to each component by something like H = x,H, + xb Hb + __ . , etc., where x is the weight fraction of each mineral_ However, due to differences in crystalline form and friability of different minerals, it is probable that they will have widely different particle size distributions within that of the mixture, so that the frequency of impacts and mass of impacting particles will not be in proportion to the weight fractions_ Such inhomogeneity also applies to composite materials for erosion-resistant surfaces_ It is reported that cemented carbides represent the state-of-the-art materials for valve trim [lo] and that their failure is usually by preferential erosion of the relatively soft binder phase. Thus, if hardness values are to be defined for finely ground materials, they must be based
l/4” FROM DRILL COLLAR SECTION MATCHING ROTATING
OF
POWER
Wf
SET
HAND
SCREW
SQUARE
HOLE TO SAMPLE
ROTATING
SAMPLE
CHAMBER
BEARINGS STATIONARY GRANULAR
TEST
MATERIA
PLUNGER
SUPPORT W/
“0”
RING
PLUNGER TOOTHED
CHARACTERISTIC SCRATCH PATTERN (For real. physical appearance see A on Fig. 5.)
Fig. 3. Model I abrasion test device, for coarser granular solids.
SEAL
_.
~.
.:
‘-.
TABLE 3 Coals and mineral residues tested
Designation
Description or origin
Lignite 1 Lignite 2 Lignite 3 Bituminous Anthracite
Indian Head mine, ND (Typ. 10% Ash) Baukol-Noonau, ND Freedom mine, Beulah, ND Colorado Carbon County, PA
Liquid residue 1 Liquid residue 2
Solid residue of continuous liquefaction process (29.8% aah) Same, after 10 h at 500 “F to carbonize or oxidize residual tars (45.7%
Coke
Petroleum
Ash1 Ash2
Low-temperature Iaborator?; ashes, minimum mineral alteration Absaloka subbit. (air 750 =C) Decker subbit. (carbon tetrafluoride, 300 “C)
Ash3 Ash4 Ash5
Residues subjected to oxidizing atmosphere during combustion Sarpy Creek subbit. baghouse ash ‘Bottom slag’ Indian Head lignite (typical) ‘Bottom slag’ Arapahoe subbit. (rare, glossy)
Slag
Slag Slag Slag Slag
ash)
coke, ash free
Residues subjected to reducing atmosphere during gasification, all from Indian Head lignite Normal production slag, water quenched Normal wall deposit from gasifier ‘Stalactite’ from taphole, high alkali and sulfides ‘Stalactite’ from taphole, black, g1oss.y ‘Stalactite’ from taphole, pink bubbIy mass containing 2% chrome from damaged refractory_ High alumina
1 2 3 4 5
Slag 6
TABLE
.-
Slow-cooled, coarse crystalIine slag from hearth, high alumina
4
Results of manual scratch tests (big lumps) of assorted coals and residues Test sample*
Mohs hardness range Definite
Ambiguous**
Raw coals Lignite 1 Lignite 2 Lignite 3 Bituminous Anthracite
2.3 1.5 2.3 2.7 3.1
0.5 - 1.8 2.0 - 2.7
Petroleum
coke
3.4 - 3-9
Oxidation Ash4 Ash5
residues
Reduction Slag I Slag 2 Slag 4 Slag 5
residues
-
2.6 1.8 2.7 3.0 3.9
3.6 - 4.2 4.6 - 5.4 5.8 3.7 5.4 8.5
-
6.5 4.4 5.9 9.5
3.6 - 4.7
2.8 - 4.4 5.4 - 6.6
*Identified in Table 3. **Broader range covera non-reciprocal or otherwise inconsistent data.
upon scratch tests of the materials as they are encountered in real-world applications, representing an effective hardness for any combination of particle components. Thus, a test is needed whereby a smooth surface is scratched by a sample of fine powder. For a valid Mohs hardness, it must scratch the next softest material but not the next hardest material_ One method for measuring the abrasive effect of a granular material on a smooth surface [13] invoives a plunger pressing the abradent down a tube against a point on a turntable of the surface to be eroded. Another method is to operate tumbling mills with balls or rods made of different materials, comparing their relative resistance with abrasion under the same conditions [ 51. The much simpler devices of Figs. 3 and 4 were built to provide interpolative measures of Mohs -hardness of granular materials. Whether the granular material makes a mark or not after some standard contact time will locate its Mohs hardness relative to the references.
82
RUBBER SAMPLE
WHEEL GRINDS INTO REFERENCE
IRCULATION TEFLON BUSHING REFERENCE SURFACE, I
,-----,.,=A END
‘“0”
SEMI-SCHEMATIC, ASSEMBLY DETAILS
l/4” SHAFT F-ROM HAND POWER I DRILL
RINGS’ SIDE
VIEW NOT COMPLETE
VIEW
lllllllllllllllllln~
CHARACTERISTIC SCRATCH PATTERN (For real, physical appearance see B on Fig. 5.)
Fig. 4. lIode1 II abrasion test device, for fine granular solids.
Conversely, any flat spot of a test material greater than 50 mm in diameter can be evaluated with a series of granular reference materials_ Both devices are pocket-sized and need only an electric drill as a power source. The coarse abrasion test device, Model I, in Fig. 3 combines the pressure applied to the plunger and centrifugal force to push the ground abradent into the reference surface around t.he periphery of the rotor, leaving a circular pattern of fine scratches (mainly on a ductile, metallic surface) or pits (on brittle surfaces) if it. is harder than the reference material_ The ‘0’ ring protects the surface from direct damage by the steel rotor. The limitation of this device is that fine solids, less than about --2O mesh (-0.08 mm), fail to transmit motion from the rotor to the particles direct-ly against the test surface. Therefore, the i\lodel II device of Fig. 4 was built for finer powders_ Typical scratch patterns for both devices are shown in Fig. 5, using materials giving a high HP/H, for purposes of illustration. Figure 6 shows a magnified view of the scratch pattern left by a material of roughly H, = 3.5 + 0.3 on a mineral specimen conveniently composed of both calcite and fluorite_ The roughly vertical scratches appear on the calcite only, stopping abruptly at the edge of the fluorite_ Horizontal lines are due to polishing of the surface before the test. Unlike the conspicuous surface damage of Fig. 5, Fig. 6 represents a case of minimally detectable scratches, as expected from the HP/H, = 1.0 + 0.2. One published source [l]
suggests that erosion is substantial and less dependent upon hardness at HP JH, above 1.3 to 1.7, while erosion is insignificant at Hp/Hs below 0.7 to 1.1, which applies to Fig. 6. Results of tests with the Model I device are given in Fig. 7. These and most following tests used 5 s of moving contact time, on the assumption that if the abradent is decisively harder than the surface, this will still amount to thousands of potential replications of a single scratch as shown in Fig_ 1. For marginal cases (surface equal to or slightly harder than abradent), longer contact time did not inflict scratches when the first 5 s inflicted none.
Fig. 5. Typical scratch patterns made by abrasion testers. Surface: Aluminum (Mohs hardness: 2.3 + 0.2). A, Circular pattern of Model I tester with -20
mesh (9 - 0.85 mm) quartz (Mohs hardness: 7.0); B, smudge pattern of Model II tester with -100 mesh (9 - 0.15 mm) petroleum coke (Mohs hardness: 6.5 A 0.5). Axis of rotation vertical_ (Small parallel smudge caused by shift of plate_) Grinding time in both cases: approx. 5 s. Hp/Hs = 2.9 -I 0.1 in both cases.
83
This also applied to the Model II device. In Fig. ‘I, note that specific numbers are not assigned to the secondary reference surfaces. This is to avoid any unjustified delusion of precision in interpolation between the established primary standards. For example, in the interval between Mohs 4 and 6, six secondary standards are merely arranged in sequence and
assumed to be roughly evenly distributed. With the Model I test device, the wide clearance (more than 0.5 mm) simply prevented most tiny, natural mineral grains from being ground into the test surface. One of the observed distinctions between kinds of reference surfaces was that abrasive powders tend to make scratches on ductile (metallic) surfaces, for the full length of their contact, while leaving a pattern of chipped pits in brittle surfaces_ As an example, using the Model I or II tester, some ashes or slags leave a pattern of fine but definite scratches on inconel (Mohs 5+) but not on Tile grade 1 or window glass (Mohs 5-), even though the glass and tile can be easily scratched by a large, hand-held bar of inconel. A probable theoretical explanation of this lies in the essentially liquid molecular arrangement of vitreous or glassy solids, which have a prestressed surface analogous to the surface film of conventionally defined liquids. In the case of the above inconel-tile experiment, a substantial normal stress was concentrated at the single point of contact to puncture the surface film. Distributed fine particles, on the other hand, find only a perfect surface with no cleavage planes or grain boundaries at which to initate cracks. This suggests that a source of error, ignored by the original
Fig. 6. Typical scratch pattern on composition primary reference surface by Model I tester. Light phase: calcite; Mohs hardness: 3.0. Dark phase: fluorite; lMohs hardness: 4.0. Horizontal lines left from polishing of surface. Diameter of view shown: approx. 3 mm, located along circilar pattern as shown in Fig. 5(A).
MOHS HARDNESS
TEST SAMPLE 1
2
3
5
Fig. 7. Results of coarse abrasion tests with Model I test device-
6
7
hand, represent abnormal, high-temperature residues partially depleted of alkalis and silicon, the latter tending to volatilize as its unstable monoxide, SiO, in wet, reducing atmospheres at temperatures above 1000 "C. The most common coal minerals are of the kaolin and shale groups, with Mohs hardness in the range of 2.0 to 2.5, dispersed throughout coal. Isolating these ash blends in unaltered form is virtually impossible. As to particle size, coal minerals can be extremely fine. In Beulah, North Dakota, lignite, for instance [14], all of the identified mineral species occur in the 5 to 40 pm range, except for some aggregates of soft clays up to 400 pm_ In the various ashes and slags reported here, all of these components, unfortunately, have undergone major modification, converting relatively soft kaolins, carbonates, and organically bound alkalis and calcium to much harder minerals. Therefore, predictions of the abrasiveness of coal residue slurries based on data from ashes or slags will lead to high estimates of erosion rates. While uncertainty in interpolation of abrasion patterns increases with decreasing particle size, such data nevertheless come closer to describing the behavior of the very fine, unmodified coal minerals in liquefaction processes. Figure 9 shows data from the Model II abrasion tester for very fine solids. In addition to the actual liquefaction residue, the low-temperature ashes represent coal minerals as close as possible to original particles, not fused or modified by high temperatures- Fly ash and finely ground gasifier slag (both under 0.15 mm) are both included to determine whether the effects of fusion can be detected in fine particles. Figure 10 shows a magnified view of a scratch pattern on
definition of Mohs hardness, is the pressure applied at the point(s) of contact for scratch tests. While various glazed, ceramic tiles represent a handy range of hardness (Mohs 5 to 7), according to manual scratch tests, they tend to give erroneously high results when subjected to fine particulate abrasion_ With metals, on the other hand, fine particulate samples of slightly lower hardness often produce detectable brightening or dulling of the surface, yet. with no visible scratches or other mechanical modification of surfaces features, even when viewed under high magnification. These anomalies account for the width of the bars in this form of data reporting. To measure relat.ive abrasiveness in this size range of -100 mesh (0.15 mm), the rubber tire of the Model II tester grinds the sample directly against the reference surface. Figure 8 shows tests run with -20 mesh (0.85 mm) and -40 mesh (0.425 mm) materials. Additional ambiguities appear due to either a polishing or dulling action on reference surfaces, without leaving any scratches or other distortion of surface features visible through a microscope_ For esample, raw Indian Head lignite inflicts a slight polishing/brightening and very few microscopic scratches on ductile carbon steel (3.5 - 4.0), but makes no mark on either brittle calcite (3.0) or lucite plastic (2.7). Thus, for a material with a wide range of components, we can detect the effects of the hardest minority of its components. Slag 2 was a deposit from high on the walls of the gasifier, consisting of unaltered grains of coal minerals and coke, cemented together by a deposition of volatile mineral components, which appear to consist mainly of H,Si04 and Na,S. Slags 5 and 6, on the other
Fig. 8. Results
of fine abrasion
tests with
LModel II device.
-,
Definite;
n
, ambiguous.
85 TEST SAIIPLE
1
2
3
IIOHSHARDNESS 4
a
6
i
SLAG1
I
Fig. 9. Results of very fine abrasion tests with Model
Fig. 10. Scratch pattern by Model II tester for North Dakota lignite on carbon steel. Surface: carbon steel; Mohs hardness: 3.5 + 0.3. Abradent: Indian Head lignite; -100 mesh (-0.15 mm); Mohs hardness: 2.0 to 3.5.
polished carbon steel, representing the upper limit of abrasiveness possible with -0.15 mm Indian Head (North Dakota) lignite. We see here a fine haze of vertical scratches, whose length represents the width of a narrower version of the smudge (B) in Fig. 5, which was just visible to the naked eye. The horizontal lines represent the initial, coarse polishing of the surface and the dark spots are corrosion pits. The two tracks of heavier scratches, punctuated by pits, are anomalies representing a few larger and/or harder particles. While not identified, these could well have consisted of pyrite (H = 6 to 6.5), which commonly occurs in lignites in relatively large crystals that would resist grinding, compared with the soft, friable lignite. Figure 11 shows several such pyrite crystals, which are often big enough to be seen with the naked eye. Note the regular spacing of pits along the
~ II test device_ m,
Definite;
L-_l,
ambiguous.
Fig_ 11. Typical pyrite crystals foL:Qd in lignites. Magnification: 1230 times, by scannhlg electron microscope. Longest edge of biggest crystals: roughly 0.007 mm (similar crystals often found exceeding 1.0 mm).
vertical track near the right edge of Fig_ 10, with ‘strides’ of roughly 0.2 mm, which suggests rolling motion of an octahedral crystal similar to those of Fig. 11. The great majority of patterns observed in this study, however, for HP/H, = 1.0, consisted of the haze of scratches as in the center of Fig_ 10, with no interesting anomalies. To be valid, data from manual scratch tests and the two particulate hardness tester concepts demonstrated here must show a reasonable comparability. In general, the three methods appear equivalent, to within the inherent resolution of the Mohs hardness scale. The most notable exceptions are for the thermally modified ashes and slags in the +4 Mohs hardness range. The most significant materials are the liquefaction residues and the minimally modified Ashes 1 and 2, which best ZipprGxkEite the form of cod minerals
present in commercial liquefaction processes. The discrepancy seen in the harder ashes and slags (Mohs 4 to 7) is attributed to the problem, discussed above, of glassy surfaces on some of the tiles used as secondary standards. The broad span of Model II data for Lignite 1 appears due to the release of more abrasive components in grinding from 0.55 to 0.15 mm.
DISCUSSION
Consider as a comparison several designs for some novel slurry valve concept for an application involving slurries of 50 to 70 wt.% solids, of fine coal or its mineral residues, through pressure drops approaching 14 MPa (2000 psig) at 370 “C (700 “F). To build and test a series of such devices under working conditions will be extremely costly. Clearly, there is a great economic stimulus to develop such designs as far as possible by low-cost simulation, applying principles of dimensional analysis to predict erosion rates. It is probable that there exists some generalized equation to predict erosion rates, that might be of the following form: s =
HP j F, X”‘~“C(VD~//I)~ i 5
)
where S is some/any quantitative measure of rate of surface loss due to erosion of the containing surface at the point of interest; HS, HP are the hardnesses of the surface and of the eroding particles, respectively, which must be in the same units; X is the weight or volume fraction of solids in the slurry; d is the average or effective particle size; (VDp/p) is the Reynolds number for the slurry flow at the point of interest, with either estimated or measured properties for the slurry, and D = the effective hydraulic diameter; j, m, n and h are empirical exponents; and C is some empirical constant, with consistent units, as needed to make the equation balance. Next, let us suppose, for example, that we are considering and seeking to optimize some valve design with trim made from fused alumina (H, = 12) passing a liquefaction residue of approximately HP = 3, from Fig. 9. Then the hardness term in eqn. (1) will be 0.25’. To study a number of design variations cheaply,
at an accelerated rate, they could be made from aluminum (H, = 2.3 2 0.2) and simulate the slurry with finely ground quartz (HP = 8) in some aqueous, oil or glycol carrier to approximate actual liquid properties, but at ambient temperatures. The hardness term then becomes 3_5j_ The rate of erosion is thus (3.5/0.25)’ or 14’ times as fast as it would be under real-world conditions and at a fraction of the cost per test fixture and test loop operation_ It becomes apparent that such an approach can offer a tremendous’ yield of research data for very modest cost. Alternatively, suppose that the valve is already in service, with a plug and seat made of inconel (H, = 5.5 f 0.3). It is proposed that the valve trim be replaced by some customfabricated, fused boron carbide parts (E& = 14, from Table 1). As a quick approximation, the above equation, once confirmed, would tell us that the rate of erosion and frequency of shut-down and replacement would be reduced by (5_5/14)’ or O-39’, which provides a tradeoff against the increased cost. There are some troublesome uncertainties that arise when we consider the anomalous results shown in Figs. 8 and 9. Whole, raw lignites show a Mohs hardness range from around 3.0 down to less than 1.0, although they have high ash contents. The ash components including materials with Mohs hardness ranging up to 7.5, per Table 1, which accounts for the scattered, occasional scratches on surfaces substantially harder than the coal itself, as in Fig. 10. it might be assumed that the long-term abrasion/erosion rate will be in proportion to a hardness factor of the form
[W,,,Xa
+H,.,-&
+H,.c&
+ ---YHsI’
(2)
If we assume, for example, that 2.0 is the long-term, effective hardness of only the carbonaceous portion of a lignite, and that it has 10% ash of an average hardness of 5.0, then its true, long-term hardness would be (2 X 0.9 + 5 X 0.1) = 2.3, which is fairly reassuring in that the additional 0.3 is within the range of uncertainty_ The full sensitivity of erosion rates to uncertainty in assigning Mohs hardness values to whole slurries cannot be predicted until complete predictive equations are developed. The approach presented here predicts only the erosion rate component caused by and in
-.
~..~ ::
:.
:
: ...87-
-. some_ proportion to the. relative hardness of particles and surfaces. It does not predict the additional ..erosion caused by cavitation in applications involving -flashing liquids, which may account for as much_ damage td high pressure let-down valves as does slurry erosion alone_ Cavitation is a totally different basic mechanism and beyond the immediate scope of this study. Another limitation of this coldsimulation approach is that the hardness of many materials may decrease substantially with increasing temperature. It is reported, for instance, that the Rockwell hardness of some high-chrome steels may decrease by 60% at temperatures beyond 1000 “F [15]. Therefore, a complete prediction of hightemperature erosion phenomena will require some data on temperature dependence of surface hardness. The methodology in this study was restricted to very short grinding times, about 5 s, to avoid deviating too far from the yesor-no, single-scratch method established to define the Mohs hardness scale. It is nevertheless fairly obvious that even the hardest materials are worn down by the extended action of far softer materials, such as tool steel cutting edges, rocks subject to water erosion, or, more to the point, the hardest materials of construction subjected to high velocity slurries of ‘soft’ ground coal. The method prescribed here extends the qualitative, yes-or-no data defining the Mohs hardness scale to predict not whether erosion will occur, as it will in any case, but at what rate.
REFERENCES. 1
Materials S&r fion for High-Pressure Values in Coal Likuefactiqn S&ter&
SAND.SO-1526C or CONF-810417-1, DOE Contract No..DEAC04-76-DP00789. 2 L. K. Ives, J. P. Young and -A. W. Ruff, Particle Erosion.Measurements on Metals at Elevated Temperatures, National Bureau of Standards,
Special Publication 468, April 1977. L. G. Austin, Mechanical and Comminufive Properties of Coal, F’KOC. Nat. Sci. Foundn. Conf. on Materials Problems and Research Opportunities in Coal Conversion, April 16 - 18, 1974. 4 J. G. A. Bitter, We&, 6 (1963) 169. 5 R. R. Ridgeway; A_ H.,Ballard and B. L. Bailey, 3
Hardness
Values
for
Electrochemicai
Products,
Electrochem. Sot., May 1933. 6 A. F- Taggart (ea.), Handbook of Mineral Dressing, Wiley, New York, 1967 (10th printing)_ Steels, International Nickel Co., 7 Nickel Alloy Development and Research Division, New York, 1949. 8 H. Haywood, J_ Insf. FueZs, 9 (1935) 94. 9 R. Hultgren, Fundamentals of Physical MetaZZurgy. Prentice-Hall, Engelwood Cliffs, 1952. 10 American Society for Testing and Materials, Gaseous Fuels; Coal and Coke; Aimospheric Analysis, Procedure ASTM D409-45, Part 26, 1978. of Miner11 P. Frazer, Tables for the Determination aIs, Lippincot, Philadelphia, 4th edn, 1897. Guide to Rocks and Miner12 Simon and Schusfer’s als, Simon and Schuster, New York, 1977. IO+ (1982) 13 A_ Misra and I. Finnie, Trans. ASME, 14
15
ACKNOWLEDGEMENTS
This study was made possible by the U.S. Department of Energy, through Contract No. DE-FC21-83FE60181.
s_. Randich, Let-do+
16
94. S. A. Benson, C. Zygarlicke and F. R. Karner, The Occurrence and Disfribufion of Inorganic Constituents in Lignite from the Be&ah Mine, North Dakota. Rocky Mountain Coal Sync-
posium, Bismarck, ND, October 2 - 4,1984. W_ L. Silence, Sliding Wear in Coal Gasification, VoL II of Materials Problems and Research Opportunities in Coal Conversion, National Science Foundation and Office of Coai Research, Ohio State, Columbus, April 16 - 18, 1974 (Workshop Director, R. W. Staehle). American Society for Metals, Metals Handbook, 1948.