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Influence of lipidic coating on the sedimentation of metallic powders
Influence of Lipidic Coating on the Sedimentation of Metallic Powders B. PRUNET-FOCH,* F. LEGAY-DESESQUELLES,* N. LOCQUET, t A N D M. VIGNES-ADLER *'1 *Laboratoire d'A#rothermique du CNRS, 4ter route des gardes, 92190 Meudon, France, and tSollac-C.E.D., BP 2, 60160 Montataire, France Received September 4, 1990; accepted January 31, 1991 Sedimentation tests have been used to study the influence of a lipidic coating, e.g., fatty acids and glycerol derivatives with octadecyl chains, on suspensions of micrometric iron-carbonyl or steel particles dispersed in cyclohexane. By variation of the lipid concentration and saturation degree, the aggregative properties of the suspension could be changed in a continuous way. The final sediment height follows a saturation-like behavior with two plateaus as the lipid concentration is varied. The transient settling behavior shows an unexpected stabilization of the iron-carbonyl suspension occurring at an intermediate fatty acid concentration ranging between the two previous plateaus; at large concentrations of highly unsaturated fatty acids, discontinuity layers which are correlated to the formation of iron soaps and which cannot be predicted by a classical Kynch-type theory are obtained. © 1991AcademicPress,Inc. INTRODUCTION
The behavior of small solid particles suspended in a liquid is a central concern for a number of industrial applications ( 1 ). In the chemical and petroleum industries, sedimentation, wherein particles fall under the action of gravity, is commonly used as a way of separating particles from fluid. In the steel rolling industry, finely divided submicrometric particles torn from the strip may remain in the lubricating liquid. Because their presence, especially in agglomerates, is a real drawback for the quality of the final product (2, 3), minimization of the particle agglomerates is necessary. It requires a better understanding of the interactions between the steel particles and the various constituents of the oil-in-water emulsion: water, mineral oil, lipids of various origins, surfactants, various additives designed to avoid corrosion, oxidation, and excessive wear (4). The effect of water in such colloidal suspensions has been studied by several authors (5-7); water induces special reactions and behavior and its influence is a whole topic To whom correspondence should be addressed.
in itself. Thus, it was eliminated from the present study and our research has focused on the interactions between metallic particles suspended in mixtures of apolar oil and lipidic substances. Numerous experimental and theoretical investigations of sedimentation have been conducted (8-14). Nevertheless the relation between the macroscopic properties of a sedimenting suspension and the local microphysical particle interactions in the presence of liquid molecules is far from well understood. Since the pioneer work of Verwey and de Boer (15), which emphasized the role of amphiphilic molecules on solid particles interactions, and except for some related studies (5, 6), not much progress has been made in this area. Our study is devoted to filling this gap. Two types of metallic powders (iron-carbonyl and steel) were immersed in an apolar medium where the lipid concentration was gradually increased. Various fatty acids and their glycerol derivatives were used as additives; they can adsorb easily on the metallic surfaces. In the development ofsteric repulsion between solid particles, this adsorption lowers 524
0021-9797/91 $3.00 Copyright© 1991by AcademicPress,Inc. All rightsof reproductionin any formreserved.
Journal of Colloidand InterfaceScience, Vol. 145,No, 2, September!991
525
SEDIMENTATION OF METALLIC POWDERS
the influence of the van der Waals-London forces and strongly modifies the settling experiments. This modification depends on the solid material, on the nature of the lipid, and mainly on its concentration. Attention is focused on the surfactant efficiency in particle dispersion which drastically influences the sedimentation of metallic particles. A relationship is found between the stabilization of the suspension, the final sediment height, and the concentration in fatty acids. This relationship resembles a saturation curve where pronounced changes occur at a critical concentration of additives: before reaching the plateau value, the settling rate sometimes exhibits a minimum which corresponds to better stabilization of the suspension. Above the critical concentration, no important effect is found in the settling process except for the appearance in some supernatants of gradated colored
zones which coincide with the presence of iron soaps chemically detected. EXPERIMENTAL
Materials Powders. Two different powders were used. Their characteristics are reported in Table 1: iron-carbonyl powder from Poudmet Co. (ref. 430 TC) and steel powder, obtained from the sludges accumulated on the magnetic filters of the industrial rolling mill. The sludges were washed in a Soxhlet extractor using trichlorethylene several times, and then acetone, as solvents for about l month (7). The SEM pictures of these two powders are given in Fig. 1. Iron-carbonyl particles are rather spherical and monodisperse (t~mean = 4.5 ~tm) whereas steel particles are of irregular shape and size (t~mean
TABLEI Cha~ctefisticsofthePowde~ Chemical components (wt %)
Powder
Density (g/cm ~)
Solid volume fraction
Surface mean diameter (,am)
Specific area (m2/g)
Fe
C
S
N2
Mn
Steel Iron-carbonyl
5.36 7.8
0.033 0.088
5.5 4.5
25.21 0.435
74.7 97.2
1.37 0.94
0.05 0
0.16 1.1
0.29 0.03
Discrete diameter histogram Iron-carbonyl
Steel
(`am)
Number fraction
(tam)
Number fraction
1 2 3 3.5 4 4.5 5 5.5 6.3 7 8 10
0.045 0.026 0.067 0.086 0.083 0.077 0.07 0.063 0.084 0.058 0.062 0.028
1 2 3 4 6 8 12 16 24 32 64 128 192
0.072 0.049 0.056 0.050 0.063 0.061 0.087 0.081 0.093 0.045 0.022 0.091 0.091
Journal of Colloid and Interface Science. Vol. 145, No. 2, September 1991
526
PRUNET-FOCH
_-_ 5.5 #m). Actually, due to their history, the steel particles are highly oxydized and show a rather low density.
Liquids. The liquids were mixtures of - - a mineral fraction: hydrocarbons as dimethylnaphthalene ( D M N ) C12H12, mixture of isomers from Aldrich, or extra pure (>99%) cyclohexane C6H12 from Sigma. - - a n organic fraction: saturated or unsaturated fatty acids and derivated monoglycerides with hydrocarbon chains of various lengths. All liquids were of pure quality (9899%) from Sigma. They are listed in Table II. Concentrations of the organic fraction in the mineral ranged from 10 -4 to 5 moles/liter with D M N and from 10 -5 to 10 -1 mole/liter with cyclohexane. At low concentrations, surface tensions are nearly unchanged; for example, for concentrations below 5 X 10 -2 mole/liter, the surface tension of the mixture is lowered by less than 3%.
Methods Experiments were conducted using two kinds of borosilicate test tubes: 140 m m long with a 11.6-mm inner diameter for iron-carbonyl and 120 m m long with a 9-mm inner diameter for steel. The tubes were carefully cleaned by degreasing with acetone, rinsing with water, and washing in fresh sulfochromic acid. Then, 10 ± 5 X 10 -4 g of iron-carbonyl and 1.5 +__ 5 X 10-4 g of steel were weighted. These masses of powder correspond to solid volume fractions of 0.088 for iron-carbonyl and 0.033 for steel, i.e., near dilute suspensions according to the usual classification (16). The tubes were filled with the selected mixture up to a constant height H0 and well sealed; then they were strongly shaken by hand until a homogeneous suspension was obtained. Ultrasonic stirring was not used because it sometimes induces formation of indestructible agglomerates. The importance of an identical stirring process has been demonstrated by Michaels and Bolger (16) who studied the effect of various stirring Journal of Colloid and Interface Science. Vol. 145, No. 2, September | 991
ET AL.
processes on aggregate size, interface sharpness, and settling rate. Then, the initially well-mixed suspension was held vertically and separated into three regions (Fig. 2): a layer of more or less clarified fluid referred to as the "supernatant," a "falling" or "clarification" zone where the suspension presently settles, and a "sediment" layer often named the "compression" or "consolidating" zone where the solid volume fraction gradually increases as the liquid present between the particles is slowly squeezed out by the weight of solids. The aspect and extension of each zone, the nature (sharp or diffuse) of the interfaces where discontinuities in the solid volume fraction occur, are strongly dependent on the suspending liquid, on the additive concentration, and on the solid particles. A videorecording of the tubes was made during sedimentation and, from the measurements, results were obtained by computer. Heights of sediment H were plotted against time for each run. Moreover, the final equilibrium height Hs of the sediment was measured; it can be reached after a time ranging from some minutes to several days. It is a relevant parameter to indicate the degree of aggregation between particles and the state of particle packing: the larger and the more irregular the agglomerates are, the larger Hs ( 13, 17). From knowledge ofHs, we deduced the corresponding final solid volume concentration @~= solid volume/total volume at H~. The initial settling rate ( d H / d t ) o has been obtained by a polynomial approximation from the sedimentation curves H vs t. In dividing it by Ap • g/#, the linear dependence on the liquid viscosity and on the buoyancy forces has been eliminated and the aggregation-disaggregation phenomenon has been emphasized. Indeed, quantities
A-
IdH/dtlo
( Ap. g/~)
are proportional to the square of the mean hydrodynamic diameter of the aggregates and
SEDIMENTATION OF METALLIC POWDERS
527
FIG. 1. SEM pictures of the powders: (a) iron-carbonyl,(b) steel. they are independent of the tube dimensions. In order to save steel powder, thinner tubes were used for these tests; however, the ratio
between the particles and the tube diameters are very small ( ~ 1 0 -4) and we actually checked that the experimental results are inJournal of Colloid and Interface Science. Vol. 145, No. 2, September 1991
528
PRUNET-FOCH ET AL. TABLE 1I Characteristics of Liquids Molecular Density weight(g) (g/cm~)
Product C12HI2 C6HI2
156.23 84.16
Monocaprylin (Cs:0) Monostearin (C~a:0) Monoolein (C,a: 1) Monolinolein (C~8:2) Monolinolenin (C18:3)
218.3 358.6 356.5 354.5 352.5
Stearic acid Oleic acid Linoleic acid Linolenic acid
284.5 282.47 280.46 278.44
1.01 0.78
0.94 0.89 0.90 0.92
Viscosity (cP) 4.2 at 22°C 1.02 at 17°C
39 at 20°C
dependent of the tube diameter for the same initial height and the same solid volume fraction. To eliminate the tube length dependence, the final solid volume fraction ~bshas been used instead of H~. After some experimental runs, the supernatants were analyzed by IRFT (infrared Fourier's transform) spectroscopy; the quantity of free acid still present in the supernatant after adsorption on the particles and the quantity of iron soaps formed during the settling were systematically measured ( 18, 19).
FIG. 3. Sedimentation of iron-carbonyl powder in solutions of oleic acid in DMN at t = 420 min. Concentrations (moles per liter) in the tubes are respectively from left to right (1) pure DMN, (2) 3 × 10-4, (3) 4.6 X 10 -3, (4) 7.5 X 10 -3 , (5) 7.9 X 10 -3 , (6) 1.2 × 10 -2 , (7) 1.9 × 10 -2, (8) 1.9 × 10 -2, ( 9 ) 4 . 2 × 10 -2, (10) 0.1, (11) 1.4, (12) 5.0, (13) pure oleic acid.
ofoleic acid from Merck and DMN in various proportions. A photograph of the tubes in their final stage is given in Fig. 3. The supernatant is clear for low (<3 × 10 -4 mole/liter) and high ( >/4.2 × 10 -2 mole / liter) concentrations of oleic acid in DMN. For concentrations ranging from 5 × 10-3 to 2 × 10 -2 mole/liter, the color of the supernatant changes and colored zones form
RESULTS
Results with Dimethylnaphthalene A few experiments have been made with iron-carbonyl powder sedimenting in mixtures
supernatant falling zone
H(t
sediment
FIG. 2. Different zones in a settling tube. Journal of Colloid and Interface Science.
Vol. 145,No. 2, September1991
FIG. 4. Discontinuity layers observed after settling of iron-carbonyl powder in solutions of oleic acid in DMN at t = 522 rain. Tubes 3, 4, 5, 6 of Fig. 3.
529
SEDIMENTATION OF METALLIC P O W D E R S
behavior resembles a saturation curve; it has a clearly physicochemical origin. The purity of the liquids used in these experiments was not very high and the present experiment could not be easily interpreted. Hence, experiments have been reproduced with extra pure cyclohexane and lipids from Sigma, whose impurities content, lower than 50 ppm, was checked by gas chromatography.
Hs
a.u.
colou red zones
in
supernatants
I
i
i
1
lff 4
10-2
I C (rnol/I 1
FIG. 5. Influence of the oleic acid concentration in D M N on the final height Hs of iron-carbonyl powder. (. - - . ) Pure DMN; *, solution.
from dark brown at the bottom near the sediment to light brown for the highest one (Fig. 4). These colored layers maintain themselves for 4-5 h or more and then they disappear. They appear again when the tubes are shaken and left to settle. The origin of these discontinuities is discussed under Discussion. The final sediment height Hs is plotted against the concentration c of oleic acid in DMN in Fig. 5; above c = 4.10 -3 mole/liter, Hs is reduced to about 75% of its value in pure DMN or in lower concentration mixtures. Its
i
H
"
i
Results with Cyclohexane As a general result, the presence of any lipid has a strong influence on the sedimentation experiments; the effect is more important with iron-carbonyl powder than with steel. Some adsorption resulting from particleliquid interactions may occur during the time interval between the preparation of the suspension and the actual start of the sedimentation run. This interval, which has been called "waiting time," has been systematically studied for each experiment. Its influence is weak except with stearic acid. Indeed stearic acid, as all solid lipids (5), is sensitive to temperature changes and recrystallization may occur at room temperature.
i
i
a.u. 10
!1 -
.,:,..
~,, "~,,",~o ~.-+'~-v
0
,
~'a,
I 500
o ~,.~___" ~ o -
,
I 1000
i
t(s)
FIG. 6. Sedimentation height o f i r o n - c a r b o n y l p o w d e r vs t i m e in solutions o f l i n o l e n i c acid in cyclohexane (c in moles per liter). +, pure cyclohe×ane; e , c = 3.24 × 10-4; II, c = 1.04 × 10-3; V, c = 3.76 × 10-3; &, c = 3.68 × 10-2; ©, c = 1.17 × l 0 -1. Journal of Colloid and Interface Science. Vol. 145,No. 2, September 1991
530
PRUNET-FOCH
Each tube has been shaken twice and the sedimentation height has been recorded and plotted versus time. For a given height the difference in time between the two experiments is less than 10%. The second sedimentation conducted 3 h after the first one was selected for presentation of the results. We are reporting first on the influence of the lipid concentration, the more important parameter, and then on the role of the lipid characteristics: length of the hydrocarbonated chain, its saturation degree, and the nature of the head (fatty acid or monoglyceride ). Special emphasis is given to the influence of the fatty acids. Influence of lipid concentration. Solutions in cyclohexane of various fatty acids with 18 carbon atoms and zero, one, two, or three double bonds were used at concentrations ranging from 10 -5 to 10 -I mole/liter. Examples of sedimentation curves are given in Figs. 6 and 7. The sediment height decreases linearly with time as long as it is not sensitive to the consolidation of the sediment due to the finite size of the tube (bottom effect). Except for steel powder at very low ( - 10-5 mole / liter) concentrations in oleic acid where it exi
ET
AL.
ceeds its value in pure cyclohexane, and in the case o f c - 10-3 mole/liter with iron-carbonyl which corresponds to the actual minimum observed, the initial slope of H(t) decreases with increasing concentrations, showing the stabilizing effect of the fatty acids on the suspension. The final height Hs, the final solid volume fraction ~bs, and the initial normalized settling velocity A have been systematically plotted against acid concentration in Figs. 8 and 9. Their variations are much larger for the ironcarbonyl than for the steel powder. Indeed, with the iron-carbonyl powder, A decreases from 36 #m 2 in pure cyclohexane and low acid concentration to 3.5 ~m 2 for large acid concentrations passing through a minimum 2.3 ~tm2 at intermediate concentrations while it decreases monotonically from 15 to 3 #m 2 with the steel powder. ~s increases from 0.31 to 0.62 with the iron-carbonyl and from 0.14 to 0.19 with the steel powder. The evolutions ofHs ( or ~bs)and A obviously resemble a saturation effect where pronounced decreases in H~ and A (and increase in ~s) begin simultaneously at a critical concentration in fatty acids. This critical concentration cc is
|
i
I
!
I
l
H
a.u. 10
0
i
500
t
1000
t(sl
lOG. 7. Sedimentationheight of steel powdervs time in solutionsof oleicacid in cyclohexane(c in moles p e r l i t e r ) . + , p u r e c y c l o h e x a n e ; O, c = 1.23 X 1 0 - s ; V, c = 1.71 × 1 0 - s ; A, c = 1.07 X 1 0 - 2 ; O , c = 1.1 × 10 -t " Journal of Colloid and Interface Science. V o L
145, N o . 2, S e p t e m b e r 1991
SEDIMENTATION OF METALLIC POWDERS
Hs
(O ........
/
t
, I
(iii) ........
.......
,
,
3~
~,
t
I.
,
I-
"--..-
i
21-
y/
-
I
(i)
(ii) ........
'
"
. . . . ,,-,,-- ,+,--1.6 I+tearicacid
I
L__
,
<2<.
~
t
,
',
~s
Hs
~
4
'r
+3
-~'--
2
10-.t
10-4
|
. . . .
~ . "
~
-"~
=
I i
•
l
Q )s
4
t
linoleic acid I1
-I.4
%_
•
•
•
•
.3
10-3'
10-2
: "~+
,
I
I I
2~,10 ~- , . . . . . . . t , .......... 10-5 10-4 10-3 f
C
I
I
•
+..m , ~ ...... 10-2 C
I
" '"'"
........
t I
v- . . . .
•
f ~-
I
I I
2
(ii)
I ................
-]5
-12
Hs
,
t
AF--_k 10-5
(iii)
..................
--I.4
--.~,
531
v &/
'
.......
~
: .......
~)s
/ w
Y I
'
• A-
A ~ A - - A - - A
,
t
T
1
t
I0-11
2,,10-121 t 10- 5
t I i ,h.o,
[ i
I0-I"
2),10-I',
. . . . . . . . . . . . . . . . . . . . .
10-4 I
Cc
10-3
t
10-2
i I
C
Cs
......... 10-5
10-4
10-3
t
I
I
Cc
Cs
10-2
C
FIG. 8. Influenceof fatty acid concentration on settling behavior of iron-carbonyl powder in cyclohexane (A, Hs in arbitrary units; ~7, @s;L A in square meters; c moles per liter).
10 -4 mole/liter for iron-carbonyl and 10 -3 mole/liter for steel; these values are identical for the four fatty acids. Above 3 × 10-3 m o l e / liter for iron-carbonyl and 2 × 10 -2 m o l e / liter for steel, Hs, @s, and A are rather stabilized on a plateau, with the four additives. This p h e n o m e n o n will be referred to below as the "saturation-like effect," and the concentration corresponding to the beginning of the plateau will be called the "saturation concentration" cs. However, with iron-carbonyl powder suspended in a solution of linolenic acid in cyclohexane, gelation effects were observed at high concentrations; the suspension became a stiffpaste, and the final volume still increased above the plateau value while the settling rate decreased. Another surprising behavior is observed slightly above the critical concentration: with iron-carbonyl powder, the A curves exhibit a more or less pronounced m i n i m u m before in-
creasing to the plateau; with steel powder, this m i n i m u m does not exist, and the transition is only more or less sharp according to the number of double-bonds of the acid. Moreover, with the steel powder, stratified layers of gradated colors appear in the supernatant of unsaturated fatty acid-cyclohexane solutions, as in D M N experiments. These layers are stable for 4-5 h or more; they can disappear by hand shaking and appear again later. Their existence coincides with the presence of iron soaps detected and dosed by 1RFT analysis. In Tables Illa and IIIb, some experimental results obtained with fatty acid-cyclohexane solutions are summarized.
Influence of the lipid nature: chain length. Comparison has been carried out between mono-caprilin (C8:0) and monostearin (C18:0) at concentrations respectively equal to 1.25 × 10-3 and 1.02 × 10 -3 mole/liter. These concentrations correspond to values located in two different zones of the curves: just above Journal o f Colloid and Interface Science, Vol. 145, No. 2, S e p t e m b e r 1991
532
PRUNET-FOCH ET AL. (0
Hs
(iii)
.................
i
•
I
(ii)
.........
Hs
~ ........
. . . . . . .
3
~ i l
.
.
.
.
.
.
.
.
i
2
- - I.....
........
.1
I
I
I
I
ii~
] i i ~-,~,1
--
I
...........
,,.,~
10"4
, , ........
10-3 I
, ...... ,
10.121
.................
C
I0-5
I0-4
10-21 L
b
.................
3
I
.........
I m~----~l
i ~
! ........
Hs
i
.................
. . ~ ~ .
u ~ m ~ -c~ - - -LD-- -- -- - O ~ -
- - o .....
2
A ~ (m 2) 10-'1
"
I
~'='~=-1--
I
I
I i
i I
I
I
I
1042 10 - 5
........
, I0 -4
I
I
........
, 10 -3 l
Cc
I
........
o.o--N-i
i ........ 10-2i I
2
"-"
I I
[
-
__i
•
, c
""~1
Cs
. . . . . . .
~
10-4
. . . . . . . .
~
10"3 I
3
_o__"
.2
•~
1
i
I
I
I 10"'1 10-5
~s
I
I
_.
........
10-21 C I r .........
I
1
,. ~].I i
l J J
.........
I0"3 i J .........
i
Hs
m~.o . . . .
'l-X, ]
10-11[
10-5
J 3
l
....
10421
, f,,i .....
.
] stearic acid ]
J . . . . . . . .
, [ , ,,,,,,t 10-21 C i
Cc
Cs
FIG. 9. Influence of fatty acid concentration on settling behavior of steel powder in cyclohexane( t H~ in arbitrary units; [], 4>~;m, A in square meters; c in moles per liter).
the critical concentration with steel and near the saturation concentration with iron-carbonyl. In both cases the presence of monoglyceride induces a significant decrease in A correlated to an increase in ~b~(or a decrease in Hs), but results (Table IV) show that the chain length is not a crucial parameter for settling, although the longer the chain is, the smaller H~.
Influence of the lipid nature: the saturation degree of fatty acids and glycerol derivatives (Figs. 10 and I1). The influence oflipids depends on their saturation degree and on the head nature; it is mainly noticeable in the range of concentrations corresponding to the transition observed in Figs. 8 and 9, i.e., between Cc and c~. With the iron-carbonyl powder, at concentrations equal to l0 -3 mole/liter, Hs is reduced by a factor of 1.5 and A by a factor of l0 with respect to pure cyclohexane values by any solute presence, except when the n u m b e r o f d o u Journal o f Colloid and lnterface Science, Vol. 145, No. 2, September 1991
ble bonds is 3. Then, surprisingly, as Hs is slightly lower with the glycerol head, the settling rate is three times larger. Actually, it corresponds to the m i n i m u m observed with the acid in Fig. 8. With the steel powder, at c = 10 -3 m o l e / liter, Hs is only slightly influenced ( < 10%) by the presence of the acids whatever n; the effect on Hs is more important with the glycerol derivatives and it reaches 20% for zero, one, or two double bonds. There is no influence for three double bonds. A is roughly divided by 1.5-2, indicating a rather low stabilization of the suspension. DISCUSSION The settling behavior of the present suspensions results from the balance between the particle collisions which create the aggregates, the van der Waals forces, and possibly the magnetic forces which tend to tighten the ag-
SEDIMENTATION
533
OF METALLIC POWDERS
TABLE Ilia Iron-Carbonyl a Supernatant
Acid
Stearic
Oleic
Linoleic
Linolenic
Concentration (mole/liter)
H, (a.u.)
Aspect
Iron soaps (mole/liter)
Colored layers
Acid used (mole/liter)
1.02 x 10 -5 2.14 X 10 -4
3.3 3.0
L L
---
No No
---
3.92 X 10 -4
2.7
3.1
1.17 X 10 -3 1.15 X 10 -2
2.1 1.7
4.9 5.3
L
--
No
--
L L
No No
No No
-3.1 X 10 -3
1.04 X 10 -~
1.7
4.3
L
No
No
4.96 X 10 -2
1.23 0.96 4.57 0.98 1.07
10 -5 10 -4 10 -4 10 .3 10 2
3.6 3.2 2.5 1.9 1.8
31 19 2.75 4.5 3.8
L L L L L
-No --No
No No No No No
----1 × 10 -3
1.1 × 10 -~
1.7
5.3
L
No
No
3 X 10 -2
10 -5 10 .4 10 .3 10 -2
3 3 2 1.8
20 11 5.2 5.4
L L L L
No No No No
No No No No
--2.6 X 10 -4 6 X 10 -3
1.16 × 10 ~
1.8
3.5
L
No
No
6.4 X 10 -2
3.8 3.24 1.3 3.76
10 -5 10 -4 10 -s 10 -3
3.2 2.9 2.5 1.9
29 12 3 6.7
L L L L
--No No
No No No No
-----
5.8 × 10 -2 1.17 × 10 -~
3.2 3.7
2.5 2.5
L C
No 3.2 × 10 -2
No No
---
9.8 3.19 1.26 0.98
× x X × ×
X × × ×
X × × ×
A (#m2)
31 12
" R e s u l t s o f s e d i m e n t a t i o n o f p o w d e r s in solutions o f fatty acids in c y c l o h e x a n e (for acid c o n c e n t r a t i o n below 10 -3 mole/liter, I R F T s p e c t r o m e t r i c m e a s u r e m e n t s are not sensitive enough). L, limpid; C, colored.
gregates, the solvation forces and the steric repulsion which tend to stabilize the suspension, eventually Brownian motion, and mainly the hydrodynamic stresses which tend to rupture the agregates. It should be noted that the influence of magnetic forces was checked by repeating one experiment before and after an in situ demagnetization of the powders. With the iron-carbonyl powder there was absolutely no difference. With the steel powder, the settling velocity was slightly larger (<15%) before demagnetization, showing that the magnetic forces favor aggregation. Moreover, from (10) it is known that Brownian motion is only significant for particles that are so small that sed-
imentation due to gravity is negligibly slow; it is obviously not the case here except for the very small particles ( < 1 ~tm). Both effects will be neglected. If it were assumed that no aggregates formed during sedimentation, the hydrodynamic contribution could be appreciated by comparing the normalized settling velocity (curves A vs c in Figs. 8 and 9) to the relation derived by Batchelor (20), AB - - = 1 -- 6.55q~, Ast
[1]
valid for dilute suspensions of monodisperse, rigid, and spherical particles sedimenting in a Newtonian fluid at very small particle ReynJournal of Colloid and Interface Science, Vol. 145, No. 2, September 1991
PRUNET-FOCH ET AL.
534
TABLE Illb Steel a Supernatant Concentration (mole/liter)
Acid
H0 (a.u.)
A (~m)~
Stearic
2.14 × 1.17 × 4.09 × 1.15 X 1.04 X
10-4 10-3 10-3 10-2 10-~
2.75 2.7 2.4 2.2 2
9.2 5.8 4.6 4.1 3.7
Oleic
0.96 0.98 1.71 5.09 1.1
10-4
12 7.5 5.5 3.8 2.8
Iron soaps (mole/liter)
Colored layers
Acid used (mole/liter)
No No No No No
No No -No No
----0.45 X 10-2
L L L CO O ++
No No -9.2 X 10-3 2.3 X 1 0 -2
No No -Yes Yes
---2.94 × 10-2 7.3 X 10 -2
Aspect
L L L L L
× × × X X
10-3 10-2 10-~
2.5 2.5 2.5 1.7 1.8
Linoleic
9.8 × 1.26 X 4.11 × 5.42 X 1.16 X
10-5 10-3 10-3 10-2 10-1
2.9 2.6 2.4 1.95 2.1
7.6 7 4.7 3.6 2.4
L L L C C
---5.5 X 10-2 1.7 × 10-2
---Yes No
---1.76 X 10-2 5.44 X 10-2
Linolenic
3.8 × 3.24 × 3.76 × 3.68 X 1.17 X
10-5 10-4 10-3 10-2 10-~
2.9 2.6 2.4 1.9 2.1
9.2 9.8 5.1 3.3 --
L L L O C
----
----
----
3.3 × 10-2
Yes
1.05 × 10-~
10 -3
a Results of sedimentation of powders in solutions of fatty acids in cyclohexane (for acid concentration below 10-3 mole/liter, IRFT spectrometric measurements are not sensitive enough). L, limpid; C, colored; O, opaque; O ++, quite opaque.
o l d s n u m b e r s . I n t h i s r e l a t i o n , ~b is t h e i n i t i a l s o l i d v o l u m e f r a c t i o n a n d Ast = V M ( A p • g / u ) Vst = 2( A p • g i l l ) R 2 i s t h e S t o k e s
where
trans-
W i t h t h e steel p o w d e r t h e u s e o f B a t c h e l o r ' s r e l a t i o n is q u e s t i o n a b l e s i n c e t h e p a r t i c l e s a r e q u i t e p o l y d i s p e r s e a n d far f r o m s p h e r i c a l .
lational v e l o c i t y o f a single r i g i d s p h e r e o f ra-
N e v e r t h e l e s s let u s n o t e t h a t f o r t h e steel p o w -
dius R falling t h r o u g h an u n b o u n d e d
d e r w i t h q~ -- 0 . 0 3 3 a n d R -- 2.75 ~tm,
quies-
cent N e w t o n i a n fluid o f d y n a m i c viscosity # at l o w R e y n o l d s n u m b e r ( h e r e R e -
10-4).
For the iron-carbonyl powder whose granu l o m e t r i c analysis gives a rather perfect Gauss i a n d i s t r i b u t i o n , t h e s u s p e n s i o n c a n b e reg a r d e d as m o n o d i s p e r s e w i t h q~ = 0 . 0 8 8 a n d R = 2.25 u m ; o n e o b t a i n s
As~ = 1.68 # m 2
and
AB = 1.32 # m 2.
T h e s e t h e o r e t i c a l v a l u e s a r e b e l o w t h e exp e r i m e n t a l data for b o t h p o w d e r s (Figs. 8 a n d 9 ) . It s h o w s t h a t t h e o n l y c o n s i d e r a t i o n o f t h e h y d r o d y n a m i c f o r c e s is n o t sufficient t o p r e d i c t t h e a v e r a g e s p e e d o f fall o f t h e p a r t i c l e s : t h e contribution o f the intermolecular a n d surface
2
Ast = ~ R
2
= 1.125 # m 2
and
AR = 0 . 4 7 /.tm 2. Journal of Colloid and InterfaceScience, Vol. 145, No. 2, September 1991
f o r c e s is far f r o m negligible. T h e s e s u r f a c e f o r c e s a r e t h e l o n g - r a n g e attractive van der Waals interactions, and possibly t h e " s o l v a t i o n " ( o r "structural") f o r c e s
535
SEDIMENTATION OF METALLIC POWDERS TABLE IV Influence of the Chain Length
Powder
Monoglyeerides
//m (a.u.)
A (~m z)
Iron carbonyl
Without C8:0 Cjs:0
3.3 2.5 2.1
36.0 4.7 4.3
Steel
Without C8:0 C]8:0
2.7 2.3 2.15
12.0 8.1 7.0
created by the liquid density oscillations at some solid-liquid interface. These "structural forces" were shown to occur in apolar liquids such as cyclohexane with fairly rigid and spherical molecules. The reordering of the molecules at the solid contact surface, at least on mica, gives rise between immersed particles to solvation forces which oscillate with distance between attraction and repulsion over three or four diameters (21 ). The balance between the attractive van der Waals forces, the structural forces, and the steric repulsions depends on the lipid concentration and limits the size and the number of aggregates. The advantage of the sedimentation tests, for our purpose, is extreme sensitivity to the particle interactions; hence they provide information on the adsorption of the lipids which induces more or less aggregation.
other because of the van der Waals forces as long as these forces are not exceeded by the repulsive solvation ones: it is even plausible that the cyclohexane molecules are expelled from the metallic surface at the contact point near two neighboring particles, reducing considerably their mutual distance. Then the particles form very strongly aggregated clusters which cannot be disrupted by the hydrodynamic stresses during their settling; the size of the aggregates is then mainly limited by the collision probability with other particles or aggregates. It results in the formation of strong and irregular aggregates which do not pack properly and leave many particle-free spaces even if the interparticle distances are very small inside the aggregates. The loose packing obtained explains the large Hs observed. (ii) At large acid concentrations, c > 3 X 10 -3 mole/liter, the three variables Hs, ~bs, and A reach their saturation value; A - 3 to
Hs a.u. 3
®
Iron-Carbonyl Powder
0
In Fig. 8, three main features can be distinguished: (i) At low acid concentrations, c < 10 -4 mole/liter, the final values Hs (or 4~) and the initial settling rate A remain near their values in pure cyclohexane and they are independent of the solute concentration, A - 30 pm 2; hence A/AB _~ 64 and the equivalent hydrodynamic diameter of the aggregates can be roughly estimated to eight times the mean diameter of noninteracting particles. When the like metallic particles are immersed in pure cyclohexane, they attract each
1
2
n
3
A
(mZ)
®
I0-11
c~
10-12
0
2
n
3
FIG. 10. Influence of the number n of double bonds on the sedimentation ofiron-carbonyl powder in solutions (c = 10 -3 mole/liter) in cyclohexane of fatty acids (O) and monoglycerides (11). ( . - - - ) Pure cyclohexane. Journal of Colloid and lnterJi~'e Science, Vol. 145, No. 2, September 1991
536
PRUNET-FOCH HS
a.u,
3
o
®
o
2
1
1
0
2
n
3
A
(m 2)
I0-11
-
-
10-12
0
2
n
3
FIG. 1 1. Influence of the n u m b e r n of double bonds on the sedimentation of steel powder in solutions (c = 10 -3 mole/liter) in cyclohexane of fatty acids (IS]) and monoglycerides (11). (. - - . ) Pure cyclohexane.
5 m m 2, A/AB ~ 10, and hence the aggregates are mainly doublets or triplets. The final solid volume fraction reaches 0.62, which is very near the theoretical value 0.64 for random, close-packed, monodisperse spheres (26). Indeed, addition of fatty acids strongly modifies the previous system: the way they adsorb onto the metallic surface deserves some comment. We have not measured the adhesion energies of the various constituents on the iron-carbonyl surface; nevertheless, it is likely that the anionic carboxylate head of the fatty acids can adsorb quite easily on the positive sites of the particles and can expell the cyclohexane molecules (15). However, the diameter of C O O - is about 0.23 nm (22) while the diameter of the hydrocarbon tail is 0.4 nm: these dimensions are much smaller than the diameter of the cyclohexane which is about 0.57 nm (21 ). Actually, the specific surface of the present iron-carbonyl is 0.435 m2/g. To ensure the closest packing of the stearic acid, Journal of Colloid and Interface Science, Vol. 145, No. 2, September 1991
ET AL.
for example, on the whole metallic surface, assuming that each molecule of acid occupies a surface equal to 0.16 nm 2 and the acid has been totally adsorbed, requires a minimal concentration equal to 3 X 10 -3 mole/liter. As can be observed in Fig. 8, such a concentration corresponds to the beginning of the saturation plateau. Beyond this concentration, the solid surface is totally saturated with the carboxylate heads while the hydrocarbon tails prevent a too near approach of the solid particles because of steric repulsion and can interpenetrate each other, creating a weak interaction and increasing A. Now the net van der Waals forces between two spherical particles decrease as d -2 where d is the interparticle distance (21 ). The aggregates are then very easily ruptured by hydrodynamic stresses. Particles are dispersed and settle independently at a rate slightly higher than that deduced by [1] which proves that the particles are weakly interacting through the adsorbed chains. In addition, the adsorbed lipidic layer permits easier sliding of the particles against each other because a liquid film surrounds them. They can reach closer packing under gravitational forces; Hs is minimum ( and 4~s is maximum). In the case of linolenic acid, the behavior of Hs, 4~s, and A at large acid concentrations is quite different due to the observed gelation effects. The three double bounds of this acid probably permit a chemical reaction in the presence of iron. (iii) At intermediate values, 10-4 ~< c ~< 3 X 10 -3 mole/liter, ~b~ and H~ are smoothly varying between their two extrema values, but A presents an anomalous minimum which shows an unexpected stabilization of the suspension. Indeed A decreases to 2.3 ~tm2; hence A/AB - 5 and the aggregates are only doublets when they exist. The origin of this minimum in A, obvious in three of the four A vs c curves of Fig. 8, denotes some stabilization process which slows considerably the settling of the iron-carbonyl suspension. At such concentrations, the me-
SEDIMENTATION OF METALLIC POWDERS
537
tallic surface is clearly not saturated with fatty rical considerations (specific surface) as for the acids; in other words, it is not totally depleted iron-carbonyl; of cyclohexane molecules. The length of the fatty acid molecules with 18 carbons is typi- - t h e different shapes of the two types of cally 2.6 nm which represents about four mo- powder are probably much more influential; lecular diameters of cyclohexane. The coex- iron-carbonyl particles are nicely spherical, ofistence of acid and cyclohexane molecules ad- fering a large contact surface for two particles, sorbed at the solid surface ensures a large and then enhancing the lipid adsorption while interparticle distance while the solvation forces steel particles look rather like sharp needles due to the cyclohexane are still operative and and are strongly anisotropic. The aggregates prevent the interpenetration of the hydrocar- which are formed with steel particles are much bon chains: as a result the stability of the sus- looser and small scale effects such as those due pension is enhanced. With the linoleic acid, to solvation are not amplified. The irregular the minimum is not displayed but it may exist shape of the steel particles can also explain the between c = 3 × 10 4 and 10 -3 mole/liter; very low final solid volume fraction which does with the linolenic acid, it really exists just be- not exceed 0.17 (against 0.6 for iron-carbonyl ) low ¢ = 10 -3 mole/liter. The shift in the acid due to the very loose packing of these sharp concentration corresponding to the maximum particles. stability which is observed when the number of double bonds increases is probably due to The nature (acid or glycerol) of the head a change in their adhesion energies. seems to be an unimportant factor for the ironcarbonyl at c = 10 -3 mole/liter except for n Steel Powder = 3 (Fig. 10). Actually, for the linolenic acid, A is near its minimum and Hs has not yet Similar results were obtained with the steel reached its saturation value. We have not suspensions; a saturation-like effect was obstudied extensively monoglyceride behavior; served with other limits for the concentration: however, this discrepancy may be attributed Cc ~ 10 -3 mole/liter and q _-__2 X 10 -2 m o l e / to different values of q and Cc due to the difliter (Fig. 9). These values, quite different ferent adhesion energies of the carboxyl and from those found with iron-carbonyl, are in glycerol heads with the solid. This should also close agreement with the specific surface of be true for the differences observed in Fig. 11 the steel powder. Indeed, as its specific surface with the steel powder. is 25 m2/g, the minimal concentration reBefore concluding this paper we would like quired to coat the whole metallic surface is to comment briefly on the colored discontiequal to 3 X 10 -2 mole/liter with the same nuity layers which were observed within steel assumptions as previously considered for ironsuspensions in unsaturated fatty acid-cyclocarbonyl powder. hexane solutions at concentrations larger than It corresponds again to the beginning of the 5 X 10 2 mole/liter and with iron-carbonyl saturation plateau. However, A does not show suspensions in technical oleic a c i d - D M N soany minimum at intermediate concentrations. lutions at concentrations between 5 X 10 3 All the curves in Fig. 9 exhibit smooth and and 2 × 10 -2 mole/liter; their appearance was regular variations between the two plateaus. always correlated to the presence of iron soaps There might be two reasons: detected by IRFT analysis. In both experi- - t h e large variety of constituents entering ments, it should be some c o m m o n and still the steel composition may influence to some unidentified substance catalyzing the formaextent the lipid adsorption; however, this effect tion of iron soaps. Their production increases should not be drastic since the saturation con- with the unsaturation degree of the lipids (Tacentration can be predicted only on geomet- bles Ilia and IIIb). Journal of Colloid and Interface Science. Vol. 145, No. 2, September 1991
538
PRUNET-FOCH ET AL.
This correlation between the colored discontinuity layers and the actual presence of synthetized iron soaps has been confirmed by a simple test: when a small quantity of synthetized iron stearate powder was suspended in pure cyclohexane, layers ofgradated colors from the dark bottom to the light top were observed in the tube. These discontinuities were first explained in a purely hydrodynamic context by Kynch (8) who recognized that the settling rate at any point in a column of suspension is a function only of the concentration at that point. Depending on the initial values of ~b, kinematic waves of constant particle concentration emanate from the bottom and propagate upward through the suspension, producing solid volume fraction discontinuities which can be stable under stated conditions. On the basis of Kynch's work, numerous articles (23-26) have appeared in recent years in order to account for the sediment compressibility and for the Brownian motion which promotes diffusion mainly in the sediment. As far as we know no one has ever related these discontinuities to some physicochemical process. The present experimental conditions are such that Kynch's type predictions can reasonably be applied. Actually, discontinuities should not arise for the solid volume fractions studied and they indeed do not exist with ironcarbonyl powder. However, we have obtained them with steel and high acid concentration. They were induced by the presence of iron soaps while the kinematic conditions have not been a priori drastically modified compared to that prevailing for the low acid concentrations: same initial solid volume fraction, same diameter range of particles, and so on, and even without a drastic change in A. It is plausible that the presence of iron soaps has totally modified the various fluxes inside the sediment. This modification could be precise if we knew whether the soaps are crystallized inside the liquid and form a suspension or whether they cover the iron particle surface, hence creating some kinds of bridges between the particles. This was beyond the scope of this study. Journal of Colloidand InterfaceScience, Vol. 145,No. 2, September1991
CONCLUSION
This study has shown how much the macroscopic properties o f a sedimenting suspension are related to the local microphysical interactions between the different particles. Indeed, sedimentation tests appeared as a very sensitive tool in analyzing the role of the lipid fraction in oil solutions. When fatty acids, even in small amounts, adsorb at the particles surface, they hinder their aggregation, enhance suspension stabilization, and promote a closer packing of the sediment, to an extent which depends on the acid concentration. By variation of the lipid concentration and saturation degree, the aggregative properties of the suspension could be changed "continuously" from a state of strong aggregation toward a state of very weak or weak aggregation. With the iron-carbonyl powder the closest packing of the particles (0.62) could be achieved almost as soon as there is enough acid to saturate their surface. Moreover, it seems that we could visualize the effect of the solvation forces which enhance considerably the stabilization of the suspension at some intermediate concentrations. With the steel powder, the variety of constituents and the heterogeneity of the particles smooth out the smallest scale effects. However, the saturation-like effect still exists and it shows definitely the strong correlation between the lipid adsorption and the sedimentation. The closest packing is far from being obtained. It seems also that the production of iron soaps at high acid concentration modifies drastically the kinematic behavior of the suspension and generates unexpected solid volume fraction discontinuities. REFERENCES 1. Novotny, V., Colloids Surf. 24, 361 (1987). 2. Vucich, M. G., and Vitellas, M. X., Iron Steel Eng. 29, (Dec. 1976). 3, Shuck, R. R., Nat. Coil. Coat. Ass. (1978). 4. Modolo, R., Jacquet, D., de Werbier, P., and Hocquaux, H., Usinor-Sacilor CCR, R1 88619 (1988). 5. Rowell, R. L., Vasconcellos, S. R., Sala, R. S., and Farinato, R. S., Ind. Eng. Chem. Process Dev. 20, 283 ( 1981 ).
SEDIMENTATION OF METALLIC POWDERS 6. Malbrel, C. A., and Somasundaran, P., J. Colloid Interface Sci. 133, 404 (1989). 7. Legay-D6sesquelles, F., Prunet-Foch, B., and VignesAdler, M., J. Mat. Sci. 26, 361 (1991). 8. Kynch, G. J., Trans. Faraday Soc. 48, 166 (1952). 9. Fitch, B., AIChE J. 25, 913 (1979). 10. Davis, R. H., and Acrivos, A., Annu. Rev. FluidMech. 17, 91 (1985). 11. Batchelor, B. K., and Janse Rensburg, R. W., J. Fluid Mech. 166, 379 (1986). 12. Aidi, M., Feuitlebois, F., Lasek, A., Anthore, R., Petipas, C., and Auvray, X., Rev. Phys. Appl. 24, 1077 (1989). 13. Vargha-Butler, E. I., Moy, E., and Neumann, A. W., Colloids Surf 24, 315 (1987). 14. Davis, R. H., and Hassen, M. A., Z FluidMech. 196, 107 (1988). 15. Verwey, E. J. W., and de Boer, J. H., Rec. Tray. Chim. 57, 383 (1938).
539
16. Michaels, A. S., and Bolger, J. C., I & EC Fundam. 1, 24 (1962). 17. Tiller, F. M., and Khatib, Z., J. Colloid Interface Sci. 100, 55 (1984). 18. Locquet, N., Sollac Recherche C. E. D. 18704-CD (1989). 19. Locquet, N., Sollac Recherche C. E. D. 18848.89 CEDNTi (1989). 20. Batchelor, G. K., J. FluidMech. 52, 245 (1972). 21. Israelachvili, J. N., "Intermolecular and Surface Forces." Academic Press London, 1985. 22. Pauling, L., "The Nature of the Chemical Bond." Cornell Univ. Press, New York, 1980. 23. Wallis, G. B., "One-Dimensional Two-Phase Flow." McGraw-Hill, New York, 1969. 24. Tiller, F. M., AIChE J. 27, 823 ( 1981 ). 25. Fitch, B., ALChEJ. 29, 940 (1983). 26. Davis, K. E., and Russel, W. B., Phys. Fluids A 1, 82 (1989).
Journalof ColloidandInterfaceScience,Vol. 145,No. 2, September1991
The behavior of small solid particles suspended in a liquid is a central concern for a number of industrial applications ( 1 ). In the chemical and petroleum industries, sedimentation, wherein particles fall under the action of gravity, is commonly used as a way of separating particles from fluid. In the steel rolling industry, finely divided submicrometric particles torn from the strip may remain in the lubricating liquid. Because their presence, especially in agglomerates, is a real drawback for the quality of the final product (2, 3), minimization of the particle agglomerates is necessary. It requires a better understanding of the interactions between the steel particles and the various constituents of the oil-in-water emulsion: water, mineral oil, lipids of various origins, surfactants, various additives designed to avoid corrosion, oxidation, and excessive wear (4). The effect of water in such colloidal suspensions has been studied by several authors (5-7); water induces special reactions and behavior and its influence is a whole topic To whom correspondence should be addressed.
in itself. Thus, it was eliminated from the present study and our research has focused on the interactions between metallic particles suspended in mixtures of apolar oil and lipidic substances. Numerous experimental and theoretical investigations of sedimentation have been conducted (8-14). Nevertheless the relation between the macroscopic properties of a sedimenting suspension and the local microphysical particle interactions in the presence of liquid molecules is far from well understood. Since the pioneer work of Verwey and de Boer (15), which emphasized the role of amphiphilic molecules on solid particles interactions, and except for some related studies (5, 6), not much progress has been made in this area. Our study is devoted to filling this gap. Two types of metallic powders (iron-carbonyl and steel) were immersed in an apolar medium where the lipid concentration was gradually increased. Various fatty acids and their glycerol derivatives were used as additives; they can adsorb easily on the metallic surfaces. In the development ofsteric repulsion between solid particles, this adsorption lowers 524
0021-9797/91 $3.00 Copyright© 1991by AcademicPress,Inc. All rightsof reproductionin any formreserved.
Journal of Colloidand InterfaceScience, Vol. 145,No, 2, September!991
525
SEDIMENTATION OF METALLIC POWDERS
the influence of the van der Waals-London forces and strongly modifies the settling experiments. This modification depends on the solid material, on the nature of the lipid, and mainly on its concentration. Attention is focused on the surfactant efficiency in particle dispersion which drastically influences the sedimentation of metallic particles. A relationship is found between the stabilization of the suspension, the final sediment height, and the concentration in fatty acids. This relationship resembles a saturation curve where pronounced changes occur at a critical concentration of additives: before reaching the plateau value, the settling rate sometimes exhibits a minimum which corresponds to better stabilization of the suspension. Above the critical concentration, no important effect is found in the settling process except for the appearance in some supernatants of gradated colored
zones which coincide with the presence of iron soaps chemically detected. EXPERIMENTAL
Materials Powders. Two different powders were used. Their characteristics are reported in Table 1: iron-carbonyl powder from Poudmet Co. (ref. 430 TC) and steel powder, obtained from the sludges accumulated on the magnetic filters of the industrial rolling mill. The sludges were washed in a Soxhlet extractor using trichlorethylene several times, and then acetone, as solvents for about l month (7). The SEM pictures of these two powders are given in Fig. 1. Iron-carbonyl particles are rather spherical and monodisperse (t~mean = 4.5 ~tm) whereas steel particles are of irregular shape and size (t~mean
TABLEI Cha~ctefisticsofthePowde~ Chemical components (wt %)
Powder
Density (g/cm ~)
Solid volume fraction
Surface mean diameter (,am)
Specific area (m2/g)
Fe
C
S
N2
Mn
Steel Iron-carbonyl
5.36 7.8
0.033 0.088
5.5 4.5
25.21 0.435
74.7 97.2
1.37 0.94
0.05 0
0.16 1.1
0.29 0.03
Discrete diameter histogram Iron-carbonyl
Steel
(`am)
Number fraction
(tam)
Number fraction
1 2 3 3.5 4 4.5 5 5.5 6.3 7 8 10
0.045 0.026 0.067 0.086 0.083 0.077 0.07 0.063 0.084 0.058 0.062 0.028
1 2 3 4 6 8 12 16 24 32 64 128 192
0.072 0.049 0.056 0.050 0.063 0.061 0.087 0.081 0.093 0.045 0.022 0.091 0.091
Journal of Colloid and Interface Science. Vol. 145, No. 2, September 1991
526
PRUNET-FOCH
_-_ 5.5 #m). Actually, due to their history, the steel particles are highly oxydized and show a rather low density.
Liquids. The liquids were mixtures of - - a mineral fraction: hydrocarbons as dimethylnaphthalene ( D M N ) C12H12, mixture of isomers from Aldrich, or extra pure (>99%) cyclohexane C6H12 from Sigma. - - a n organic fraction: saturated or unsaturated fatty acids and derivated monoglycerides with hydrocarbon chains of various lengths. All liquids were of pure quality (9899%) from Sigma. They are listed in Table II. Concentrations of the organic fraction in the mineral ranged from 10 -4 to 5 moles/liter with D M N and from 10 -5 to 10 -1 mole/liter with cyclohexane. At low concentrations, surface tensions are nearly unchanged; for example, for concentrations below 5 X 10 -2 mole/liter, the surface tension of the mixture is lowered by less than 3%.
Methods Experiments were conducted using two kinds of borosilicate test tubes: 140 m m long with a 11.6-mm inner diameter for iron-carbonyl and 120 m m long with a 9-mm inner diameter for steel. The tubes were carefully cleaned by degreasing with acetone, rinsing with water, and washing in fresh sulfochromic acid. Then, 10 ± 5 X 10 -4 g of iron-carbonyl and 1.5 +__ 5 X 10-4 g of steel were weighted. These masses of powder correspond to solid volume fractions of 0.088 for iron-carbonyl and 0.033 for steel, i.e., near dilute suspensions according to the usual classification (16). The tubes were filled with the selected mixture up to a constant height H0 and well sealed; then they were strongly shaken by hand until a homogeneous suspension was obtained. Ultrasonic stirring was not used because it sometimes induces formation of indestructible agglomerates. The importance of an identical stirring process has been demonstrated by Michaels and Bolger (16) who studied the effect of various stirring Journal of Colloid and Interface Science. Vol. 145, No. 2, September | 991
ET AL.
processes on aggregate size, interface sharpness, and settling rate. Then, the initially well-mixed suspension was held vertically and separated into three regions (Fig. 2): a layer of more or less clarified fluid referred to as the "supernatant," a "falling" or "clarification" zone where the suspension presently settles, and a "sediment" layer often named the "compression" or "consolidating" zone where the solid volume fraction gradually increases as the liquid present between the particles is slowly squeezed out by the weight of solids. The aspect and extension of each zone, the nature (sharp or diffuse) of the interfaces where discontinuities in the solid volume fraction occur, are strongly dependent on the suspending liquid, on the additive concentration, and on the solid particles. A videorecording of the tubes was made during sedimentation and, from the measurements, results were obtained by computer. Heights of sediment H were plotted against time for each run. Moreover, the final equilibrium height Hs of the sediment was measured; it can be reached after a time ranging from some minutes to several days. It is a relevant parameter to indicate the degree of aggregation between particles and the state of particle packing: the larger and the more irregular the agglomerates are, the larger Hs ( 13, 17). From knowledge ofHs, we deduced the corresponding final solid volume concentration @~= solid volume/total volume at H~. The initial settling rate ( d H / d t ) o has been obtained by a polynomial approximation from the sedimentation curves H vs t. In dividing it by Ap • g/#, the linear dependence on the liquid viscosity and on the buoyancy forces has been eliminated and the aggregation-disaggregation phenomenon has been emphasized. Indeed, quantities
A-
IdH/dtlo
( Ap. g/~)
are proportional to the square of the mean hydrodynamic diameter of the aggregates and
SEDIMENTATION OF METALLIC POWDERS
527
FIG. 1. SEM pictures of the powders: (a) iron-carbonyl,(b) steel. they are independent of the tube dimensions. In order to save steel powder, thinner tubes were used for these tests; however, the ratio
between the particles and the tube diameters are very small ( ~ 1 0 -4) and we actually checked that the experimental results are inJournal of Colloid and Interface Science. Vol. 145, No. 2, September 1991
528
PRUNET-FOCH ET AL. TABLE 1I Characteristics of Liquids Molecular Density weight(g) (g/cm~)
Product C12HI2 C6HI2
156.23 84.16
Monocaprylin (Cs:0) Monostearin (C~a:0) Monoolein (C,a: 1) Monolinolein (C~8:2) Monolinolenin (C18:3)
218.3 358.6 356.5 354.5 352.5
Stearic acid Oleic acid Linoleic acid Linolenic acid
284.5 282.47 280.46 278.44
1.01 0.78
0.94 0.89 0.90 0.92
Viscosity (cP) 4.2 at 22°C 1.02 at 17°C
39 at 20°C
dependent of the tube diameter for the same initial height and the same solid volume fraction. To eliminate the tube length dependence, the final solid volume fraction ~bshas been used instead of H~. After some experimental runs, the supernatants were analyzed by IRFT (infrared Fourier's transform) spectroscopy; the quantity of free acid still present in the supernatant after adsorption on the particles and the quantity of iron soaps formed during the settling were systematically measured ( 18, 19).
FIG. 3. Sedimentation of iron-carbonyl powder in solutions of oleic acid in DMN at t = 420 min. Concentrations (moles per liter) in the tubes are respectively from left to right (1) pure DMN, (2) 3 × 10-4, (3) 4.6 X 10 -3, (4) 7.5 X 10 -3 , (5) 7.9 X 10 -3 , (6) 1.2 × 10 -2 , (7) 1.9 × 10 -2, (8) 1.9 × 10 -2, ( 9 ) 4 . 2 × 10 -2, (10) 0.1, (11) 1.4, (12) 5.0, (13) pure oleic acid.
ofoleic acid from Merck and DMN in various proportions. A photograph of the tubes in their final stage is given in Fig. 3. The supernatant is clear for low (<3 × 10 -4 mole/liter) and high ( >/4.2 × 10 -2 mole / liter) concentrations of oleic acid in DMN. For concentrations ranging from 5 × 10-3 to 2 × 10 -2 mole/liter, the color of the supernatant changes and colored zones form
RESULTS
Results with Dimethylnaphthalene A few experiments have been made with iron-carbonyl powder sedimenting in mixtures
supernatant falling zone
H(t
sediment
FIG. 2. Different zones in a settling tube. Journal of Colloid and Interface Science.
Vol. 145,No. 2, September1991
FIG. 4. Discontinuity layers observed after settling of iron-carbonyl powder in solutions of oleic acid in DMN at t = 522 rain. Tubes 3, 4, 5, 6 of Fig. 3.
529
SEDIMENTATION OF METALLIC P O W D E R S
behavior resembles a saturation curve; it has a clearly physicochemical origin. The purity of the liquids used in these experiments was not very high and the present experiment could not be easily interpreted. Hence, experiments have been reproduced with extra pure cyclohexane and lipids from Sigma, whose impurities content, lower than 50 ppm, was checked by gas chromatography.
Hs
a.u.
colou red zones
in
supernatants
I
i
i
1
lff 4
10-2
I C (rnol/I 1
FIG. 5. Influence of the oleic acid concentration in D M N on the final height Hs of iron-carbonyl powder. (. - - . ) Pure DMN; *, solution.
from dark brown at the bottom near the sediment to light brown for the highest one (Fig. 4). These colored layers maintain themselves for 4-5 h or more and then they disappear. They appear again when the tubes are shaken and left to settle. The origin of these discontinuities is discussed under Discussion. The final sediment height Hs is plotted against the concentration c of oleic acid in DMN in Fig. 5; above c = 4.10 -3 mole/liter, Hs is reduced to about 75% of its value in pure DMN or in lower concentration mixtures. Its
i
H
"
i
Results with Cyclohexane As a general result, the presence of any lipid has a strong influence on the sedimentation experiments; the effect is more important with iron-carbonyl powder than with steel. Some adsorption resulting from particleliquid interactions may occur during the time interval between the preparation of the suspension and the actual start of the sedimentation run. This interval, which has been called "waiting time," has been systematically studied for each experiment. Its influence is weak except with stearic acid. Indeed stearic acid, as all solid lipids (5), is sensitive to temperature changes and recrystallization may occur at room temperature.
i
i
a.u. 10
!1 -
.,:,..
~,, "~,,",~o ~.-+'~-v
0
,
~'a,
I 500
o ~,.~___" ~ o -
,
I 1000
i
t(s)
FIG. 6. Sedimentation height o f i r o n - c a r b o n y l p o w d e r vs t i m e in solutions o f l i n o l e n i c acid in cyclohexane (c in moles per liter). +, pure cyclohe×ane; e , c = 3.24 × 10-4; II, c = 1.04 × 10-3; V, c = 3.76 × 10-3; &, c = 3.68 × 10-2; ©, c = 1.17 × l 0 -1. Journal of Colloid and Interface Science. Vol. 145,No. 2, September 1991
530
PRUNET-FOCH
Each tube has been shaken twice and the sedimentation height has been recorded and plotted versus time. For a given height the difference in time between the two experiments is less than 10%. The second sedimentation conducted 3 h after the first one was selected for presentation of the results. We are reporting first on the influence of the lipid concentration, the more important parameter, and then on the role of the lipid characteristics: length of the hydrocarbonated chain, its saturation degree, and the nature of the head (fatty acid or monoglyceride ). Special emphasis is given to the influence of the fatty acids. Influence of lipid concentration. Solutions in cyclohexane of various fatty acids with 18 carbon atoms and zero, one, two, or three double bonds were used at concentrations ranging from 10 -5 to 10 -I mole/liter. Examples of sedimentation curves are given in Figs. 6 and 7. The sediment height decreases linearly with time as long as it is not sensitive to the consolidation of the sediment due to the finite size of the tube (bottom effect). Except for steel powder at very low ( - 10-5 mole / liter) concentrations in oleic acid where it exi
ET
AL.
ceeds its value in pure cyclohexane, and in the case o f c - 10-3 mole/liter with iron-carbonyl which corresponds to the actual minimum observed, the initial slope of H(t) decreases with increasing concentrations, showing the stabilizing effect of the fatty acids on the suspension. The final height Hs, the final solid volume fraction ~bs, and the initial normalized settling velocity A have been systematically plotted against acid concentration in Figs. 8 and 9. Their variations are much larger for the ironcarbonyl than for the steel powder. Indeed, with the iron-carbonyl powder, A decreases from 36 #m 2 in pure cyclohexane and low acid concentration to 3.5 ~m 2 for large acid concentrations passing through a minimum 2.3 ~tm2 at intermediate concentrations while it decreases monotonically from 15 to 3 #m 2 with the steel powder. ~s increases from 0.31 to 0.62 with the iron-carbonyl and from 0.14 to 0.19 with the steel powder. The evolutions ofHs ( or ~bs)and A obviously resemble a saturation effect where pronounced decreases in H~ and A (and increase in ~s) begin simultaneously at a critical concentration in fatty acids. This critical concentration cc is
|
i
I
!
I
l
H
a.u. 10
0
i
500
t
1000
t(sl
lOG. 7. Sedimentationheight of steel powdervs time in solutionsof oleicacid in cyclohexane(c in moles p e r l i t e r ) . + , p u r e c y c l o h e x a n e ; O, c = 1.23 X 1 0 - s ; V, c = 1.71 × 1 0 - s ; A, c = 1.07 X 1 0 - 2 ; O , c = 1.1 × 10 -t " Journal of Colloid and Interface Science. V o L
145, N o . 2, S e p t e m b e r 1991
SEDIMENTATION OF METALLIC POWDERS
Hs
(O ........
/
t
, I
(iii) ........
.......
,
,
3~
~,
t
I.
,
I-
"--..-
i
21-
y/
-
I
(i)
(ii) ........
'
"
. . . . ,,-,,-- ,+,--1.6 I+tearicacid
I
L__
,
<2<.
~
t
,
',
~s
Hs
~
4
'r
+3
-~'--
2
10-.t
10-4
|
. . . .
~ . "
~
-"~
=
I i
•
l
Q )s
4
t
linoleic acid I1
-I.4
%_
•
•
•
•
.3
10-3'
10-2
: "~+
,
I
I I
2~,10 ~- , . . . . . . . t , .......... 10-5 10-4 10-3 f
C
I
I
•
+..m , ~ ...... 10-2 C
I
" '"'"
........
t I
v- . . . .
•
f ~-
I
I I
2
(ii)
I ................
-]5
-12
Hs
,
t
AF--_k 10-5
(iii)
..................
--I.4
--.~,
531
v &/
'
.......
~
: .......
~)s
/ w
Y I
'
• A-
A ~ A - - A - - A
,
t
T
1
t
I0-11
2,,10-121 t 10- 5
t I i ,h.o,
[ i
I0-I"
2),10-I',
. . . . . . . . . . . . . . . . . . . . .
10-4 I
Cc
10-3
t
10-2
i I
C
Cs
......... 10-5
10-4
10-3
t
I
I
Cc
Cs
10-2
C
FIG. 8. Influenceof fatty acid concentration on settling behavior of iron-carbonyl powder in cyclohexane (A, Hs in arbitrary units; ~7, @s;L A in square meters; c moles per liter).
10 -4 mole/liter for iron-carbonyl and 10 -3 mole/liter for steel; these values are identical for the four fatty acids. Above 3 × 10-3 m o l e / liter for iron-carbonyl and 2 × 10 -2 m o l e / liter for steel, Hs, @s, and A are rather stabilized on a plateau, with the four additives. This p h e n o m e n o n will be referred to below as the "saturation-like effect," and the concentration corresponding to the beginning of the plateau will be called the "saturation concentration" cs. However, with iron-carbonyl powder suspended in a solution of linolenic acid in cyclohexane, gelation effects were observed at high concentrations; the suspension became a stiffpaste, and the final volume still increased above the plateau value while the settling rate decreased. Another surprising behavior is observed slightly above the critical concentration: with iron-carbonyl powder, the A curves exhibit a more or less pronounced m i n i m u m before in-
creasing to the plateau; with steel powder, this m i n i m u m does not exist, and the transition is only more or less sharp according to the number of double-bonds of the acid. Moreover, with the steel powder, stratified layers of gradated colors appear in the supernatant of unsaturated fatty acid-cyclohexane solutions, as in D M N experiments. These layers are stable for 4-5 h or more; they can disappear by hand shaking and appear again later. Their existence coincides with the presence of iron soaps detected and dosed by 1RFT analysis. In Tables Illa and IIIb, some experimental results obtained with fatty acid-cyclohexane solutions are summarized.
Influence of the lipid nature: chain length. Comparison has been carried out between mono-caprilin (C8:0) and monostearin (C18:0) at concentrations respectively equal to 1.25 × 10-3 and 1.02 × 10 -3 mole/liter. These concentrations correspond to values located in two different zones of the curves: just above Journal o f Colloid and Interface Science, Vol. 145, No. 2, S e p t e m b e r 1991
532
PRUNET-FOCH ET AL. (0
Hs
(iii)
.................
i
•
I
(ii)
.........
Hs
~ ........
. . . . . . .
3
~ i l
.
.
.
.
.
.
.
.
i
2
- - I.....
........
.1
I
I
I
I
ii~
] i i ~-,~,1
--
I
...........
,,.,~
10"4
, , ........
10-3 I
, ...... ,
10.121
.................
C
I0-5
I0-4
10-21 L
b
.................
3
I
.........
I m~----~l
i ~
! ........
Hs
i
.................
. . ~ ~ .
u ~ m ~ -c~ - - -LD-- -- -- - O ~ -
- - o .....
2
A ~ (m 2) 10-'1
"
I
~'='~=-1--
I
I
I i
i I
I
I
I
1042 10 - 5
........
, I0 -4
I
I
........
, 10 -3 l
Cc
I
........
o.o--N-i
i ........ 10-2i I
2
"-"
I I
[
-
__i
•
, c
""~1
Cs
. . . . . . .
~
10-4
. . . . . . . .
~
10"3 I
3
_o__"
.2
•~
1
i
I
I
I 10"'1 10-5
~s
I
I
_.
........
10-21 C I r .........
I
1
,. ~].I i
l J J
.........
I0"3 i J .........
i
Hs
m~.o . . . .
'l-X, ]
10-11[
10-5
J 3
l
....
10421
, f,,i .....
.
] stearic acid ]
J . . . . . . . .
, [ , ,,,,,,t 10-21 C i
Cc
Cs
FIG. 9. Influence of fatty acid concentration on settling behavior of steel powder in cyclohexane( t H~ in arbitrary units; [], 4>~;m, A in square meters; c in moles per liter).
the critical concentration with steel and near the saturation concentration with iron-carbonyl. In both cases the presence of monoglyceride induces a significant decrease in A correlated to an increase in ~b~(or a decrease in Hs), but results (Table IV) show that the chain length is not a crucial parameter for settling, although the longer the chain is, the smaller H~.
Influence of the lipid nature: the saturation degree of fatty acids and glycerol derivatives (Figs. 10 and I1). The influence oflipids depends on their saturation degree and on the head nature; it is mainly noticeable in the range of concentrations corresponding to the transition observed in Figs. 8 and 9, i.e., between Cc and c~. With the iron-carbonyl powder, at concentrations equal to l0 -3 mole/liter, Hs is reduced by a factor of 1.5 and A by a factor of l0 with respect to pure cyclohexane values by any solute presence, except when the n u m b e r o f d o u Journal o f Colloid and lnterface Science, Vol. 145, No. 2, September 1991
ble bonds is 3. Then, surprisingly, as Hs is slightly lower with the glycerol head, the settling rate is three times larger. Actually, it corresponds to the m i n i m u m observed with the acid in Fig. 8. With the steel powder, at c = 10 -3 m o l e / liter, Hs is only slightly influenced ( < 10%) by the presence of the acids whatever n; the effect on Hs is more important with the glycerol derivatives and it reaches 20% for zero, one, or two double bonds. There is no influence for three double bonds. A is roughly divided by 1.5-2, indicating a rather low stabilization of the suspension. DISCUSSION The settling behavior of the present suspensions results from the balance between the particle collisions which create the aggregates, the van der Waals forces, and possibly the magnetic forces which tend to tighten the ag-
SEDIMENTATION
533
OF METALLIC POWDERS
TABLE Ilia Iron-Carbonyl a Supernatant
Acid
Stearic
Oleic
Linoleic
Linolenic
Concentration (mole/liter)
H, (a.u.)
Aspect
Iron soaps (mole/liter)
Colored layers
Acid used (mole/liter)
1.02 x 10 -5 2.14 X 10 -4
3.3 3.0
L L
---
No No
---
3.92 X 10 -4
2.7
3.1
1.17 X 10 -3 1.15 X 10 -2
2.1 1.7
4.9 5.3
L
--
No
--
L L
No No
No No
-3.1 X 10 -3
1.04 X 10 -~
1.7
4.3
L
No
No
4.96 X 10 -2
1.23 0.96 4.57 0.98 1.07
10 -5 10 -4 10 -4 10 .3 10 2
3.6 3.2 2.5 1.9 1.8
31 19 2.75 4.5 3.8
L L L L L
-No --No
No No No No No
----1 × 10 -3
1.1 × 10 -~
1.7
5.3
L
No
No
3 X 10 -2
10 -5 10 .4 10 .3 10 -2
3 3 2 1.8
20 11 5.2 5.4
L L L L
No No No No
No No No No
--2.6 X 10 -4 6 X 10 -3
1.16 × 10 ~
1.8
3.5
L
No
No
6.4 X 10 -2
3.8 3.24 1.3 3.76
10 -5 10 -4 10 -s 10 -3
3.2 2.9 2.5 1.9
29 12 3 6.7
L L L L
--No No
No No No No
-----
5.8 × 10 -2 1.17 × 10 -~
3.2 3.7
2.5 2.5
L C
No 3.2 × 10 -2
No No
---
9.8 3.19 1.26 0.98
× x X × ×
X × × ×
X × × ×
A (#m2)
31 12
" R e s u l t s o f s e d i m e n t a t i o n o f p o w d e r s in solutions o f fatty acids in c y c l o h e x a n e (for acid c o n c e n t r a t i o n below 10 -3 mole/liter, I R F T s p e c t r o m e t r i c m e a s u r e m e n t s are not sensitive enough). L, limpid; C, colored.
gregates, the solvation forces and the steric repulsion which tend to stabilize the suspension, eventually Brownian motion, and mainly the hydrodynamic stresses which tend to rupture the agregates. It should be noted that the influence of magnetic forces was checked by repeating one experiment before and after an in situ demagnetization of the powders. With the iron-carbonyl powder there was absolutely no difference. With the steel powder, the settling velocity was slightly larger (<15%) before demagnetization, showing that the magnetic forces favor aggregation. Moreover, from (10) it is known that Brownian motion is only significant for particles that are so small that sed-
imentation due to gravity is negligibly slow; it is obviously not the case here except for the very small particles ( < 1 ~tm). Both effects will be neglected. If it were assumed that no aggregates formed during sedimentation, the hydrodynamic contribution could be appreciated by comparing the normalized settling velocity (curves A vs c in Figs. 8 and 9) to the relation derived by Batchelor (20), AB - - = 1 -- 6.55q~, Ast
[1]
valid for dilute suspensions of monodisperse, rigid, and spherical particles sedimenting in a Newtonian fluid at very small particle ReynJournal of Colloid and Interface Science, Vol. 145, No. 2, September 1991
PRUNET-FOCH ET AL.
534
TABLE Illb Steel a Supernatant Concentration (mole/liter)
Acid
H0 (a.u.)
A (~m)~
Stearic
2.14 × 1.17 × 4.09 × 1.15 X 1.04 X
10-4 10-3 10-3 10-2 10-~
2.75 2.7 2.4 2.2 2
9.2 5.8 4.6 4.1 3.7
Oleic
0.96 0.98 1.71 5.09 1.1
10-4
12 7.5 5.5 3.8 2.8
Iron soaps (mole/liter)
Colored layers
Acid used (mole/liter)
No No No No No
No No -No No
----0.45 X 10-2
L L L CO O ++
No No -9.2 X 10-3 2.3 X 1 0 -2
No No -Yes Yes
---2.94 × 10-2 7.3 X 10 -2
Aspect
L L L L L
× × × X X
10-3 10-2 10-~
2.5 2.5 2.5 1.7 1.8
Linoleic
9.8 × 1.26 X 4.11 × 5.42 X 1.16 X
10-5 10-3 10-3 10-2 10-1
2.9 2.6 2.4 1.95 2.1
7.6 7 4.7 3.6 2.4
L L L C C
---5.5 X 10-2 1.7 × 10-2
---Yes No
---1.76 X 10-2 5.44 X 10-2
Linolenic
3.8 × 3.24 × 3.76 × 3.68 X 1.17 X
10-5 10-4 10-3 10-2 10-~
2.9 2.6 2.4 1.9 2.1
9.2 9.8 5.1 3.3 --
L L L O C
----
----
----
3.3 × 10-2
Yes
1.05 × 10-~
10 -3
a Results of sedimentation of powders in solutions of fatty acids in cyclohexane (for acid concentration below 10-3 mole/liter, IRFT spectrometric measurements are not sensitive enough). L, limpid; C, colored; O, opaque; O ++, quite opaque.
o l d s n u m b e r s . I n t h i s r e l a t i o n , ~b is t h e i n i t i a l s o l i d v o l u m e f r a c t i o n a n d Ast = V M ( A p • g / u ) Vst = 2( A p • g i l l ) R 2 i s t h e S t o k e s
where
trans-
W i t h t h e steel p o w d e r t h e u s e o f B a t c h e l o r ' s r e l a t i o n is q u e s t i o n a b l e s i n c e t h e p a r t i c l e s a r e q u i t e p o l y d i s p e r s e a n d far f r o m s p h e r i c a l .
lational v e l o c i t y o f a single r i g i d s p h e r e o f ra-
N e v e r t h e l e s s let u s n o t e t h a t f o r t h e steel p o w -
dius R falling t h r o u g h an u n b o u n d e d
d e r w i t h q~ -- 0 . 0 3 3 a n d R -- 2.75 ~tm,
quies-
cent N e w t o n i a n fluid o f d y n a m i c viscosity # at l o w R e y n o l d s n u m b e r ( h e r e R e -
10-4).
For the iron-carbonyl powder whose granu l o m e t r i c analysis gives a rather perfect Gauss i a n d i s t r i b u t i o n , t h e s u s p e n s i o n c a n b e reg a r d e d as m o n o d i s p e r s e w i t h q~ = 0 . 0 8 8 a n d R = 2.25 u m ; o n e o b t a i n s
As~ = 1.68 # m 2
and
AB = 1.32 # m 2.
T h e s e t h e o r e t i c a l v a l u e s a r e b e l o w t h e exp e r i m e n t a l data for b o t h p o w d e r s (Figs. 8 a n d 9 ) . It s h o w s t h a t t h e o n l y c o n s i d e r a t i o n o f t h e h y d r o d y n a m i c f o r c e s is n o t sufficient t o p r e d i c t t h e a v e r a g e s p e e d o f fall o f t h e p a r t i c l e s : t h e contribution o f the intermolecular a n d surface
2
Ast = ~ R
2
= 1.125 # m 2
and
AR = 0 . 4 7 /.tm 2. Journal of Colloid and InterfaceScience, Vol. 145, No. 2, September 1991
f o r c e s is far f r o m negligible. T h e s e s u r f a c e f o r c e s a r e t h e l o n g - r a n g e attractive van der Waals interactions, and possibly t h e " s o l v a t i o n " ( o r "structural") f o r c e s
535
SEDIMENTATION OF METALLIC POWDERS TABLE IV Influence of the Chain Length
Powder
Monoglyeerides
//m (a.u.)
A (~m z)
Iron carbonyl
Without C8:0 Cjs:0
3.3 2.5 2.1
36.0 4.7 4.3
Steel
Without C8:0 C]8:0
2.7 2.3 2.15
12.0 8.1 7.0
created by the liquid density oscillations at some solid-liquid interface. These "structural forces" were shown to occur in apolar liquids such as cyclohexane with fairly rigid and spherical molecules. The reordering of the molecules at the solid contact surface, at least on mica, gives rise between immersed particles to solvation forces which oscillate with distance between attraction and repulsion over three or four diameters (21 ). The balance between the attractive van der Waals forces, the structural forces, and the steric repulsions depends on the lipid concentration and limits the size and the number of aggregates. The advantage of the sedimentation tests, for our purpose, is extreme sensitivity to the particle interactions; hence they provide information on the adsorption of the lipids which induces more or less aggregation.
other because of the van der Waals forces as long as these forces are not exceeded by the repulsive solvation ones: it is even plausible that the cyclohexane molecules are expelled from the metallic surface at the contact point near two neighboring particles, reducing considerably their mutual distance. Then the particles form very strongly aggregated clusters which cannot be disrupted by the hydrodynamic stresses during their settling; the size of the aggregates is then mainly limited by the collision probability with other particles or aggregates. It results in the formation of strong and irregular aggregates which do not pack properly and leave many particle-free spaces even if the interparticle distances are very small inside the aggregates. The loose packing obtained explains the large Hs observed. (ii) At large acid concentrations, c > 3 X 10 -3 mole/liter, the three variables Hs, ~bs, and A reach their saturation value; A - 3 to
Hs a.u. 3
®
Iron-Carbonyl Powder
0
In Fig. 8, three main features can be distinguished: (i) At low acid concentrations, c < 10 -4 mole/liter, the final values Hs (or 4~) and the initial settling rate A remain near their values in pure cyclohexane and they are independent of the solute concentration, A - 30 pm 2; hence A/AB _~ 64 and the equivalent hydrodynamic diameter of the aggregates can be roughly estimated to eight times the mean diameter of noninteracting particles. When the like metallic particles are immersed in pure cyclohexane, they attract each
1
2
n
3
A
(mZ)
®
I0-11
c~
10-12
0
2
n
3
FIG. 10. Influence of the number n of double bonds on the sedimentation ofiron-carbonyl powder in solutions (c = 10 -3 mole/liter) in cyclohexane of fatty acids (O) and monoglycerides (11). ( . - - - ) Pure cyclohexane. Journal of Colloid and lnterJi~'e Science, Vol. 145, No. 2, September 1991
536
PRUNET-FOCH HS
a.u,
3
o
®
o
2
1
1
0
2
n
3
A
(m 2)
I0-11
-
-
10-12
0
2
n
3
FIG. 1 1. Influence of the n u m b e r n of double bonds on the sedimentation of steel powder in solutions (c = 10 -3 mole/liter) in cyclohexane of fatty acids (IS]) and monoglycerides (11). (. - - . ) Pure cyclohexane.
5 m m 2, A/AB ~ 10, and hence the aggregates are mainly doublets or triplets. The final solid volume fraction reaches 0.62, which is very near the theoretical value 0.64 for random, close-packed, monodisperse spheres (26). Indeed, addition of fatty acids strongly modifies the previous system: the way they adsorb onto the metallic surface deserves some comment. We have not measured the adhesion energies of the various constituents on the iron-carbonyl surface; nevertheless, it is likely that the anionic carboxylate head of the fatty acids can adsorb quite easily on the positive sites of the particles and can expell the cyclohexane molecules (15). However, the diameter of C O O - is about 0.23 nm (22) while the diameter of the hydrocarbon tail is 0.4 nm: these dimensions are much smaller than the diameter of the cyclohexane which is about 0.57 nm (21 ). Actually, the specific surface of the present iron-carbonyl is 0.435 m2/g. To ensure the closest packing of the stearic acid, Journal of Colloid and Interface Science, Vol. 145, No. 2, September 1991
ET AL.
for example, on the whole metallic surface, assuming that each molecule of acid occupies a surface equal to 0.16 nm 2 and the acid has been totally adsorbed, requires a minimal concentration equal to 3 X 10 -3 mole/liter. As can be observed in Fig. 8, such a concentration corresponds to the beginning of the saturation plateau. Beyond this concentration, the solid surface is totally saturated with the carboxylate heads while the hydrocarbon tails prevent a too near approach of the solid particles because of steric repulsion and can interpenetrate each other, creating a weak interaction and increasing A. Now the net van der Waals forces between two spherical particles decrease as d -2 where d is the interparticle distance (21 ). The aggregates are then very easily ruptured by hydrodynamic stresses. Particles are dispersed and settle independently at a rate slightly higher than that deduced by [1] which proves that the particles are weakly interacting through the adsorbed chains. In addition, the adsorbed lipidic layer permits easier sliding of the particles against each other because a liquid film surrounds them. They can reach closer packing under gravitational forces; Hs is minimum ( and 4~s is maximum). In the case of linolenic acid, the behavior of Hs, 4~s, and A at large acid concentrations is quite different due to the observed gelation effects. The three double bounds of this acid probably permit a chemical reaction in the presence of iron. (iii) At intermediate values, 10-4 ~< c ~< 3 X 10 -3 mole/liter, ~b~ and H~ are smoothly varying between their two extrema values, but A presents an anomalous minimum which shows an unexpected stabilization of the suspension. Indeed A decreases to 2.3 ~tm2; hence A/AB - 5 and the aggregates are only doublets when they exist. The origin of this minimum in A, obvious in three of the four A vs c curves of Fig. 8, denotes some stabilization process which slows considerably the settling of the iron-carbonyl suspension. At such concentrations, the me-
SEDIMENTATION OF METALLIC POWDERS
537
tallic surface is clearly not saturated with fatty rical considerations (specific surface) as for the acids; in other words, it is not totally depleted iron-carbonyl; of cyclohexane molecules. The length of the fatty acid molecules with 18 carbons is typi- - t h e different shapes of the two types of cally 2.6 nm which represents about four mo- powder are probably much more influential; lecular diameters of cyclohexane. The coex- iron-carbonyl particles are nicely spherical, ofistence of acid and cyclohexane molecules ad- fering a large contact surface for two particles, sorbed at the solid surface ensures a large and then enhancing the lipid adsorption while interparticle distance while the solvation forces steel particles look rather like sharp needles due to the cyclohexane are still operative and and are strongly anisotropic. The aggregates prevent the interpenetration of the hydrocar- which are formed with steel particles are much bon chains: as a result the stability of the sus- looser and small scale effects such as those due pension is enhanced. With the linoleic acid, to solvation are not amplified. The irregular the minimum is not displayed but it may exist shape of the steel particles can also explain the between c = 3 × 10 4 and 10 -3 mole/liter; very low final solid volume fraction which does with the linolenic acid, it really exists just be- not exceed 0.17 (against 0.6 for iron-carbonyl ) low ¢ = 10 -3 mole/liter. The shift in the acid due to the very loose packing of these sharp concentration corresponding to the maximum particles. stability which is observed when the number of double bonds increases is probably due to The nature (acid or glycerol) of the head a change in their adhesion energies. seems to be an unimportant factor for the ironcarbonyl at c = 10 -3 mole/liter except for n Steel Powder = 3 (Fig. 10). Actually, for the linolenic acid, A is near its minimum and Hs has not yet Similar results were obtained with the steel reached its saturation value. We have not suspensions; a saturation-like effect was obstudied extensively monoglyceride behavior; served with other limits for the concentration: however, this discrepancy may be attributed Cc ~ 10 -3 mole/liter and q _-__2 X 10 -2 m o l e / to different values of q and Cc due to the difliter (Fig. 9). These values, quite different ferent adhesion energies of the carboxyl and from those found with iron-carbonyl, are in glycerol heads with the solid. This should also close agreement with the specific surface of be true for the differences observed in Fig. 11 the steel powder. Indeed, as its specific surface with the steel powder. is 25 m2/g, the minimal concentration reBefore concluding this paper we would like quired to coat the whole metallic surface is to comment briefly on the colored discontiequal to 3 X 10 -2 mole/liter with the same nuity layers which were observed within steel assumptions as previously considered for ironsuspensions in unsaturated fatty acid-cyclocarbonyl powder. hexane solutions at concentrations larger than It corresponds again to the beginning of the 5 X 10 2 mole/liter and with iron-carbonyl saturation plateau. However, A does not show suspensions in technical oleic a c i d - D M N soany minimum at intermediate concentrations. lutions at concentrations between 5 X 10 3 All the curves in Fig. 9 exhibit smooth and and 2 × 10 -2 mole/liter; their appearance was regular variations between the two plateaus. always correlated to the presence of iron soaps There might be two reasons: detected by IRFT analysis. In both experi- - t h e large variety of constituents entering ments, it should be some c o m m o n and still the steel composition may influence to some unidentified substance catalyzing the formaextent the lipid adsorption; however, this effect tion of iron soaps. Their production increases should not be drastic since the saturation con- with the unsaturation degree of the lipids (Tacentration can be predicted only on geomet- bles Ilia and IIIb). Journal of Colloid and Interface Science. Vol. 145, No. 2, September 1991
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PRUNET-FOCH ET AL.
This correlation between the colored discontinuity layers and the actual presence of synthetized iron soaps has been confirmed by a simple test: when a small quantity of synthetized iron stearate powder was suspended in pure cyclohexane, layers ofgradated colors from the dark bottom to the light top were observed in the tube. These discontinuities were first explained in a purely hydrodynamic context by Kynch (8) who recognized that the settling rate at any point in a column of suspension is a function only of the concentration at that point. Depending on the initial values of ~b, kinematic waves of constant particle concentration emanate from the bottom and propagate upward through the suspension, producing solid volume fraction discontinuities which can be stable under stated conditions. On the basis of Kynch's work, numerous articles (23-26) have appeared in recent years in order to account for the sediment compressibility and for the Brownian motion which promotes diffusion mainly in the sediment. As far as we know no one has ever related these discontinuities to some physicochemical process. The present experimental conditions are such that Kynch's type predictions can reasonably be applied. Actually, discontinuities should not arise for the solid volume fractions studied and they indeed do not exist with ironcarbonyl powder. However, we have obtained them with steel and high acid concentration. They were induced by the presence of iron soaps while the kinematic conditions have not been a priori drastically modified compared to that prevailing for the low acid concentrations: same initial solid volume fraction, same diameter range of particles, and so on, and even without a drastic change in A. It is plausible that the presence of iron soaps has totally modified the various fluxes inside the sediment. This modification could be precise if we knew whether the soaps are crystallized inside the liquid and form a suspension or whether they cover the iron particle surface, hence creating some kinds of bridges between the particles. This was beyond the scope of this study. Journal of Colloidand InterfaceScience, Vol. 145,No. 2, September1991
CONCLUSION
This study has shown how much the macroscopic properties o f a sedimenting suspension are related to the local microphysical interactions between the different particles. Indeed, sedimentation tests appeared as a very sensitive tool in analyzing the role of the lipid fraction in oil solutions. When fatty acids, even in small amounts, adsorb at the particles surface, they hinder their aggregation, enhance suspension stabilization, and promote a closer packing of the sediment, to an extent which depends on the acid concentration. By variation of the lipid concentration and saturation degree, the aggregative properties of the suspension could be changed "continuously" from a state of strong aggregation toward a state of very weak or weak aggregation. With the iron-carbonyl powder the closest packing of the particles (0.62) could be achieved almost as soon as there is enough acid to saturate their surface. Moreover, it seems that we could visualize the effect of the solvation forces which enhance considerably the stabilization of the suspension at some intermediate concentrations. With the steel powder, the variety of constituents and the heterogeneity of the particles smooth out the smallest scale effects. However, the saturation-like effect still exists and it shows definitely the strong correlation between the lipid adsorption and the sedimentation. The closest packing is far from being obtained. It seems also that the production of iron soaps at high acid concentration modifies drastically the kinematic behavior of the suspension and generates unexpected solid volume fraction discontinuities. REFERENCES 1. Novotny, V., Colloids Surf. 24, 361 (1987). 2. Vucich, M. G., and Vitellas, M. X., Iron Steel Eng. 29, (Dec. 1976). 3, Shuck, R. R., Nat. Coil. Coat. Ass. (1978). 4. Modolo, R., Jacquet, D., de Werbier, P., and Hocquaux, H., Usinor-Sacilor CCR, R1 88619 (1988). 5. Rowell, R. L., Vasconcellos, S. R., Sala, R. S., and Farinato, R. S., Ind. Eng. Chem. Process Dev. 20, 283 ( 1981 ).
SEDIMENTATION OF METALLIC POWDERS 6. Malbrel, C. A., and Somasundaran, P., J. Colloid Interface Sci. 133, 404 (1989). 7. Legay-D6sesquelles, F., Prunet-Foch, B., and VignesAdler, M., J. Mat. Sci. 26, 361 (1991). 8. Kynch, G. J., Trans. Faraday Soc. 48, 166 (1952). 9. Fitch, B., AIChE J. 25, 913 (1979). 10. Davis, R. H., and Acrivos, A., Annu. Rev. FluidMech. 17, 91 (1985). 11. Batchelor, B. K., and Janse Rensburg, R. W., J. Fluid Mech. 166, 379 (1986). 12. Aidi, M., Feuitlebois, F., Lasek, A., Anthore, R., Petipas, C., and Auvray, X., Rev. Phys. Appl. 24, 1077 (1989). 13. Vargha-Butler, E. I., Moy, E., and Neumann, A. W., Colloids Surf 24, 315 (1987). 14. Davis, R. H., and Hassen, M. A., Z FluidMech. 196, 107 (1988). 15. Verwey, E. J. W., and de Boer, J. H., Rec. Tray. Chim. 57, 383 (1938).
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16. Michaels, A. S., and Bolger, J. C., I & EC Fundam. 1, 24 (1962). 17. Tiller, F. M., and Khatib, Z., J. Colloid Interface Sci. 100, 55 (1984). 18. Locquet, N., Sollac Recherche C. E. D. 18704-CD (1989). 19. Locquet, N., Sollac Recherche C. E. D. 18848.89 CEDNTi (1989). 20. Batchelor, G. K., J. FluidMech. 52, 245 (1972). 21. Israelachvili, J. N., "Intermolecular and Surface Forces." Academic Press London, 1985. 22. Pauling, L., "The Nature of the Chemical Bond." Cornell Univ. Press, New York, 1980. 23. Wallis, G. B., "One-Dimensional Two-Phase Flow." McGraw-Hill, New York, 1969. 24. Tiller, F. M., AIChE J. 27, 823 ( 1981 ). 25. Fitch, B., ALChEJ. 29, 940 (1983). 26. Davis, K. E., and Russel, W. B., Phys. Fluids A 1, 82 (1989).
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