Control of binder content in carbon article manufacture

A method of preparing carbon articles including the steps of mixing binder and carbon aggregate, forming the resulting mixture into an article, measuring the slump of the article at least to hundredths of an inch following the forming and prior to baking, and, if necessary, changing the relative proportions of binder and carbon aggregate in subsequent executions of the mixing step to yield a target amount of slump.

BACKGROUND OF THE INVENTION 
The present invention relates to the manufacture of carbon articles from 
mixtures of binder and carbon aggregate. 
It has been shown in studies on baked carbon articles used, e.g. in the 
aluminum industry as electrodes, that properties of interest, for example, 
baked apparent density, move through e.g. maximum values as binder content 
is increased from zero relative to the carbonaceous aggregate content. 
See, for instance, the article entitled "Dependence of the Density and 
Other Properties of Bonded Carbons on the Binder Proportion in the Green 
Mix", by J. Okada and Y. Takeuchi, in Proceedings of the Fourth Conference 
on Carbon, (New York: Pergamon Press, 1960). The optimum amount of binder 
will, however, vary depending on such things as the microstructure of the 
carbon aggregate, the particle size distribution of the carbon aggregate, 
etc. See, for instance, the text of the paper entitled "Optimum Adjustment 
of Pitch Content in the Fabrication of Prebaked Anodes Moulded with a 
Press in the Aluminum Industry", by M. Jarry and J. Pinoir, reproduced in 
Proceedings of the Third Czechoslovakian Aluminum Symposium, held in 
Banska Bystrica on Sept. 21, 1976. 
SUMMARY OF THE INVENTION 
It is an object of the present invention to provide for improved control of 
the amount of pitch used in the manufacture of carbon articles from 
pitch/carbon aggregate mixtures. 
Another object of the present invention is to provide a production line 
technique for sensing whether the binder content is optimum, without 
actually having to run, for example, the volume and weight tests needed to 
determine baked apparent density. 
These as well as other objects which will become apparent in the discussion 
that follows are achieved, according to the present invention, by 
providing a method of preparing carbon articles including the steps of 
mixing binder and carbon aggregate, forming the resulting mixture into an 
article, measuring the slump of the article at least to hundredths of an 
inch following the forming and prior to baking, and, if necessary, 
changing the relative proportions of binder and carbon aggregate in 
subsequent executions of the mixing step to yield a target amount of 
slump. 
The term "slump" refers to the amount a dimension has changed. For 
instance, if a dimension is initially set by compaction of the mixture in 
a mold and the resulting article is removed from the mold and set aside, 
its initially set dimensions will change due to the weight of the higher 
placed parts of the article pressing on the lower placed parts. This 
change of dimension is referred to as slump. 
In using the present invention, however, slump does not have to be 
expressed directly. It can be expressed indirectly, for instance, by just 
giving the final dimension and not subtracting off the original dimension. 
Or, it can just be expressed as a voltage read from a linear variable 
differential transformer, it being then just a matter of having a 
calibration relationship between the voltage and the desired property, 
e.g. baked apparent density.

DESCRIPTION OF THE PREFERRED EMBODIMENTS 
Referring first to FIGS. 1 and 2, there is illustrated a carbon article 
that can be manufactured using the present invention. This particular 
carbon article is an anode which may be used for producing aluminum metal 
in a process based on the electrolysis of alumina dissolved in a molten 
cryolite-based solvent. The anode 3 has on its upper face 4 protrusions 1 
which are useful for the spacing of such anodes from one another during 
their baking in a furnace. Additionally provided in the upper face are 
stub holes 2. These stub holes are later provided with metal stubs for the 
conduction of electrical current from a current source into the anode. 
Such anodes are typically manufactured, as shown in FIG. 4A, by mixing a 
selected combination of coarse, intermediate and fine aggregate with 
binder in a mixer and forming the resulting mixture using a hydraulic 
press in molds containing the mixture. In the mixer, which may e.g. be of 
the Baker Perkins Koneader continuous type, the aggregate and binder are 
kneaded until the aggregate is well wetted by the binder. Temperature at 
mixing may be about 150.degree. C., while forming will be at e.g. 
130.degree. C. Typical size distributions for the coarse, intermediate and 
fine fractions are given in Table I of U.S. Pat. No. 3,855,086, issued 
Dec. 17, 1974, in the names of W. C. Sleppy and R. J. Campbell, for 
"Carbon Anode Protection in Aluminum Smelting Cells". Exemplary 
proportions of coarse, intermediate and fine fractions are given below 
with respect to FIGS. 5 and 6. The pitch will, for example, have a 
softening point of 110.degree. C. as determined by ASTM Method D2319. 
Examples of suitable carbon aggregate are delayed coke and fluid coke. 
These are calcined, in keeping with aluminum industry practice, to drive 
off essentially all volatiles. Examples of suitable binders are coal tar 
pitch and petroleum pitch. 
As illustrated in FIG. 4B, the formed carbon anodes 3 are transported on a 
conveyor system away from the forming operation. Their temperature cools 
sufficiently that the pitch viscosity increases to such an extent that the 
formed anodes may be eventually lifted by a crane and set down into a ring 
furnace for baking. 
Their situation in a ring furnace is illustrated in FIG. 3 where it will be 
noted that the anodes were first tilted on their ends before being lifted 
down into the furnace so that protrusions 1 space adjacent anodes from one 
another. The details of the operation of ring furnaces are familiar to 
those skilled in the art and are described, for instance, in U.S. Pat. No. 
3,975,149, issued Aug. 17, 1976, in the names of B. J. Racunas and R. 
Kastelic, for "Ring Furnace". As illustrated in FIG. 3, fluid coke 
particles 5 are provided in the furnace pit 6 around and between the 
anodes and in the stub holes 2 for the purpose of guarding against 
collapse of the stub holes during baking and to protect against 
air-burning of the anodes while they are at high temperature. 
According to the present invention, anodes leaving the forming operation 
are moved past a dimension sensing device for measuring slump. This 
dimension sensing device is advantageously constructed on the basis of 
linear variable differential transformers (LVDT's), one on each side of 
the anode as it moves off of conveyor system 7, as illustrated in FIG. 4B. 
This placement, one on each side of the anodes, is so that the 
measurements will not be influenced by the particular placement of the 
anodes, left and right in FIG. 4B, i.e. in the direction of LVDT armature 
movement. The armatures of the LVDT's are activated using sensing rollers 
8 mounted on shafts 8a of linear ball bushings, e.g. SUPER ball bushings 
TWN-8-BG of Thomson Industries, Manhasset, New York. In addition to the 
use of two LVDT's, it is advantageous to first carry out a positioning of 
the anodes before they pass between the sensing rollers 8. This 
positioning of an anode is effected by holding a drop fence 9, in the form 
of a metal plate, in the way of the anode by pushing the fence up between 
the rollers of the conveyor system. The powered rollers 14 cause the anode 
to adjust itself flat against the drop fence. Pallet loading ram 10 the 
pushes the anode onto pallet conveyor 11. An additional ram 13 is located 
opposite push-off ram 10 and serves to reject unacceptably wide anodes by 
returning them to conveyor system 7 for subsequent comminution and use as 
part of the charge to the mixer. 
The armature of each LVDT is spring biased so as to lie in its innermost 
position. Each armature is moved some measurable distance when an anode is 
pushed past and onto the pallet conveyor. Each LVDT is connected to a 
demodulator which produces a voltage output linearly proportional to the 
movement its armature undergoes. 
The voltage outputs of the LVDT's are added at the demodulator, and the 
resultant sum is used as the input for the deflection of a recorder. The 
chart resulting from the recorder thus gives a plot of anode width (sum of 
deflection undergone by the armatures of each LVDT) versus position along 
the anode for each anode, the conveyor system and recorder chart both 
running at constant speed. The width itself is taken as the measurement of 
slump, although it would be possible to subtract the inner width of the 
die. It does not matter, as long as one calibrates appropriate to the 
particular measurement used. In general, the maximum slump occurs halfway 
along the length of an anode, and this maximum value is the one used. 
A listing of examples of suitable LVDT equipment is given in Table I. In 
addition to items mentioned below, shielded coaxial cable may be necessary 
when the demodulator and/or recorder is placed at a position remote from 
the LVDT location. 
Table I 
______________________________________ 
Instrument Manufacturer Part No. 
______________________________________ 
Demodulator 
Automatic Timing 
6101F-1X 
& Controls Co. 
King of Prussia, PA 
LVDT " 6234A02B01PX 
______________________________________ 
Besides this automated dimension measuring technique, it will, of course, 
be recognized that it is possible to manually use calipers to achieve the 
same measurement. Other means of automatic measurement might also be 
employed, such as non-contacting devices, e.g. laser type. 
FIGS. 5 and 6 are the results of two separate tests in which anode slump 
was measured as a function of pitch content. The asterisks are single data 
points while the numeral "2" represents two data points at the same 
location. The tests differ only in the relative amounts of coarse, 
intermediate and fine fractions. The test of FIG. 5 uses a standard 
aggregate size distribution, while that of FIG. 6 is for a finer 
aggregate. The vertical axis on both graphs gives the maximum anode width 
as a measure of slump, and it will be seen that the onset 12 of major 
slump occurs at a lower pitch value for the coarser aggregate of FIG. 5. 
The aggregate for the tests of FIGS. 5 and 6 was delayed coke, and the 
pitch was coal tar pitch. 
The weight and temperature of the anode affects the relationship between 
the measured size of the anode and its binder content. The slope of the 
curve in FIGS. 5 and 6 holds true for large anodes weighing on the order 
of 1,000 lbs. or more. Smaller anodes, weighing 500 lbs. or less appear to 
be more affected by entrapped air in the green mix which expands when mold 
pressure is released than by slumping. Since the amount of entrapped air 
in the anode is inversely proportional to the amount of binder, a plot of 
anode dimension versus binder content can show a downward slope. As anode 
weight is increased, slumping becomes more pronounced and tends to obscure 
the effect of air expansion. Larger anodes tend to cool more slowly than 
smaller anodes and are frequently measured when they are above the 
softening point of the binder, thus increasing the tendency to slump. For 
larger anodes, one obtains an upward slope in a plot of anode dimension 
versus binder content as shown in FIGS. 5 and 6. 
It has also been discovered that the pitch value for the onset of major 
slump coincides, for practical purposes, with the pitch value for maximum 
baked apparent density in a plot of baked apparent density versus pitch 
content, as shown in FIG. 7. 
What this means, for practical purposes, is that, for a given aggregate 
sizing, one need simply watch for the onset of major slump using the width 
measuring apparatus in order to know that the correct amount of pitch is 
being used in order to get maximum baked apparent density. What is 
involved here is a type of feedback system where one strives to put in as 
much pitch as the given aggregate will take. When the width measurements 
indicate that slump is becoming major, then subsequent batches of pitch 
and aggregate will be compounded with a somewhat reduced amount of pitch. 
Such adjustments can be made manually or by machines controlled by the 
output of the LVDT's. 
Of course it is not necessary according to the present invention that one 
operate right at the onset 12, the point of inflection, in FIGS. 5 and 6. 
For example, it may be desired to operate somewhat into the region of 
major slump in order to better obtain the advantages of maximized pitch 
content in the anode. Thus in the case of FIG. 6, 18% pitch might be used, 
and the anode width to be looked for would be 27.060 inches. As the amount 
of pitch increases, a problem of sticking together of anodes in the ring 
furnace can arise, but this is overcome by provision of the protrusions 1. 
Pitch which exudes during baking then only causes bonding between adjacent 
anodes at the locations of the protrustions 1, and the anodes are later 
easily broken apart because of the limited extent of the bonding, it being 
limited only to the location of the protrusions 1. 
An advantage of the present invention is that it allows one to observe the 
occurrence of unexpected changes in the aggregate being fed to the mixing 
operation. For example, if one were using the aggregate of FIG. 6 at an 
18% pitch content and the aggregate size distribution would change due, 
for example, to a blockage of the fines input door, so that the size 
distribution of the aggregate would then change to that of FIG. 5, then 
one would begin to notice that the anode was slumping more and giving a 
width measurement now at 27.090 instead of 27.050. The process operators 
are then able to note the change in the process conditions and take 
appropriate countermeasures. For instance, if an aggregate of a new size 
distribution is in fact being supplied and will be supplied for some time 
to come, then the countermeasure would be to decrease the pitch content to 
perhaps 17.40% in order to operate on the same part of the curve of FIG. 5 
as was being used for the aggregate of FIG. 6. If the cause was a blockage 
of the fines feed door, then the countermeasure would be to unblock the 
door. 
Another advantage of the present invention is that slump measurement of the 
unbaked, or "green", anodes allows one to predict when baking of an anode 
is going to result in an unsatisfactory product. Consequently, a green 
anode showing an unsatisfactory slump can be called out, comminuted and 
fed back as part of the charge in the mixer. 
It has been found that the amount of slump is a function of the time which 
has elapsed since the forming operation. For example, if the anodes come 
out of the forming operation and then move along a conveyor at some fixed 
speed, then the slump noticed will be a function of how far down the 
conveyor the width measurements are made. This is illustrated in FIG. 8. 
Thus in the practice of the present invention one must always measure 
anodes which have set the same amount of time flollowing forming. Or else, 
one must compensate when measurements at different times are to be 
compared. It will be apparent, of course, that the slump will approach a 
terminal value as the temperature of the anode decreases to room 
temperature, since then the viscosity of the pitch will be so high that 
the slump stops increasing. 
Measurements were also made for slump using the lengths of the anodes as 
well as the widths and while as shown in FIG. 9 the dimensional changes 
are different depending on whether width or length is being measured, 
there is an approximately parallel, linear functional relationship between 
width and length measurements so that either could equally well be used in 
the present invention. Notice that in this figure the analysis of slump is 
presented in terms of the difference between the slumped dimension and the 
original dimension (original dimension equals the inner die dimension). 
For the width measurements, also checked was the effect of how far up the 
sides of the anodes the measurements were made. As illustrated in the 
sketch in FIG. 10, measurements were made at the bottom, center and top at 
the sides of the anode, i.e. the distances respectively 4, 10 and 14 
inches up the sides. The results are plotted in this FIG. 10, and it will 
be seen that discrimination is better when the measurements are taken low 
on the anode. 
Percentages herein are on a weight basis unless stated otherwise. The 
binder percentage is calculated with respect to the total weight of binder 
and aggregate. 
It will be understood that the above description of the present invention 
is susceptible to various modifications, changes and adaptations, and the 
same are intended to be comprehended within the meaning and range of 
equivalents of the appended claims.