Prediction of bulk density of particulates with a correlation based on moisture content

A method for predicting the bulk density of coal after it is dropped from a known height into a container from measurements of bulk density and moisture content of the coal before it is dropped into the container. The method includes obtaining a plurality of samples of coal, measuring the bulk density of each coal sample by directing nuclear radiation through the coal, subsequently dropping each sample from said known height in a test facility and measuring the bulk density of each coal sample after drop, measuring the moisture content of each coal sample, determining a correlation between the bulk density of said coal samples after drop with the bulk density determined before drop and the moisture content, and predicting the bulk density of coal after drop into a container from subsequent measurement of the bulk density and moisture content and the previously determined correlation from test facility measurements.

TECHNICAL FIELD 
This invention relates to prediction of the bulk density of a plurality of 
particulates after the particulates are dropped from a known height, and 
particularly to predicting the bulk density of said particulates from a 
correlation based on bulk density and moisture content of the particulates 
measured prior to drop, particularly while the particulates are on a 
traveling conveyor belt. 
BACKGROUND ART 
It is important to control the bulk density of coal charged into ovens for 
the production of metallurgical coke used in the iron blast furnace. Bulk 
density affects proper heating of the coal to produce coke, the level of 
coal in the oven, the pressure on the walls of by-product coke ovens 
during the coking process, and the strength of the coke produced. To 
control the bulk density of a coal blend being prepared for charging into 
a coke oven, common practice is to measure the bulk density of a coal 
sample taken from coal on a conveyor belt as it travels from a hammermill, 
or some other machine designed to pulverize the coal, to coal bunkers for 
storage prior to charging. The bulk density of the coal is adjusted based 
on a measured value by adding diesel grade oil, similar oils or other 
substances in varying amounts to the coal to obtain a desired bulk density 
value when the coal is subsequently dropped from a known height by a larry 
car as it is charged into the coke oven. In a manual system, bulk density 
measurements are made regularly by taking a sample of the coal from the 
conveyor belt and pouring the sample from a known height into a box of 
known volume, and then weighing the coal to arrive at a bulk density value 
in pounds per cubic foot. The flow rate of oil being added to the coal is 
adjusted manually as required to obtain a predicted post-drop bulk density 
that is equal to the desired value when the coal is charged into the coke 
oven. 
There are two systems available for automatic control of the bulk density 
of coal in preparation for the production of metallurgical coke, namely 
weigh belts and nuclear gamma ray units. The weigh-belt system is 
relatively expensive to operate and will not be discussed further. With 
the gamma-ray system, the bulk density of the coal on the conveyor belt is 
measured as the coal leaves a pulverizer or coal mixer. In both systems 
the amount of oil or other substance needed to adjust the bulk density is 
automatically regulated to attain the desired density value. The nuclear 
gamma-ray measurement system, while complex is capable of performing with 
an accuracy of plus or minus one pound per cubic foot (i.e. plus or minus 
16 kilograms per cubic meter). However, the gamma-ray system is reportedly 
influenced by a number of factors as described in The Making, Shaping & 
Treating of Steel, 10th Edition, pages 146-148. Some of the most important 
factors reported in the reference are: (1) the depth of the coal on the 
belt as it passes under the radioactive source; (2) changes in the 
radiation-absorption coefficient of the coal; (3) dust or other material 
in the signal path; (4) temperature of the detector; (5) thickness and 
tension of the belt; and (6) size consistency, moisture content and 
temperature of the coal. While moisture content has been generally 
recognized as a factor in the measurement of bulk density using gamma 
radiation, it generally is believed that the effect is small based on the 
difference in the absorption coefficient of gamma radiation by water and 
coal. 
U.S. Pat. No. 3,678,268 to Reim et al, discloses a gamma radiation bulk 
density gauge for measuring the bulk density of coal on a conveyor belt 
and a system for controlling the bulk density by varying water and oil 
addition rates based on the bulk density measurements. The rate of water 
addition is controlled to bring the rate of oil addition within a desired 
range. Prior to leveling the coal on a second conveyor and measuring the 
bulk density, the coal is dropped from a first conveyor onto the second 
conveyor. The height of the first conveyor is adjusted to make the drop 
substantially the same as the drop the coal undergoes in the coke oven. 
Where the conveyor system cannot be adjusted to simulate the drop into the 
oven, alternate devices are described for simulating the drop, e.g. the 
sled and weights shown in FIGS. 6-8 and the paddle wheel assembly of FIG. 
9 of the reference. This reference does not disclose prediction of the 
post-drop coal bulk density by the measurement of the coal bulk density 
and moisture content prior to drop, for example while the coal is on a 
conveyor belt, nor the correlation of such measured bulk density and 
moisture content with the predicted post-drop bulk density. R. H. Lux and 
A. D. Strauss, 1996 Ironmaking Conference Proceedings, pages 515-520 also 
describe another automated bulk density control system in which gamma 
radiation bulk density gauges are used. 
A gauge for continuously measuring the moisture content of coal of varying 
thickness on a conveyor belt using microwaves is disclosed in U.S. Pat. 
No. 5,333,493 to Cutmore. A gauge that measures both moisture content and 
bulk density of coal is described in U.S. Pat. No. 4,766,319. This gauge 
uses a low activity neutron radiation source to measure moisture and a low 
activity gamma radiation source to measure bulk density. A correction 
signal is applied to the density measurement based on the moisture content 
to account for gamma radiation produced by a thermal neutron capture 
reaction which occurs when neutron radiation is used to measure moisture 
content of the coal. The reference does not suggest prediction of 
post-drop bulk density based on a correlation of bulk density and moisture 
content measurements made while the particulates are at rest before drop, 
particularly while the particulates are at rest on a conveyor belt. 
Other miscellaneous references are: U.S. Pat. Nos. 4,304,636; 4,450,046; 
4,506,541 and 5,435,541. 
DISCLOSURE OF INVENTION 
According to this invention, a method is provided for predicting the bulk 
density of particulate material, after the particulate material is dropped 
from a known height, as a function of the measured bulk density and 
moisture of the particulate material prior to drop. Preferably the bulk 
density before drop is measured by nuclear radiation, most preferably 
gamma radiation, directed from a source through the particulates. The 
amount of radiation passing through the particulates is detected at a 
location opposite the source as a measure of the bulk density. Preferably 
the moisture content is measured by directing microwaves through the 
particulates and determining the amount of microwaves passing 
therethrough. In a preferred form the bulk density and moisture contents 
are measured continuously or periodically at spaced time intervals. Most 
preferably the bulk density and moisture measurements are made on 
particulate material being conveyed on a traveling conveyor belt. The 
method also includes controlling the bulk density of the particulates by 
adding at least one substance, preferably a liquid substance, to the 
particulates to adjust the predicted post-drop bulk density to a desired 
value based on the bulk density measurement and moisture measurement of 
the particulates prior to drop, particularly while the particulates are on 
a traveling conveyor belt.

MODES FOR CARRYING OUT THE INVENTION 
Referring to FIG. 1, large coal particles are transported on conveyor belt 
10 to a pulverizer or hammermill 12 where the coal is crushed to smaller 
size suitable for charging into an oven (not shown) for producing 
metallurgical coke. A feed system is provided to add oil or another 
substance to the coal as it is crushed in the hammermill so that the oil 
or other substance becomes well mixed with the coal. The feed system 
includes pipe 14, flow control valve 16 and programmable controller 18 and 
a source of oil or other substance (not shown). The amount of oil or other 
substance added is dependent upon the deviation of a predicted post-drop 
coal density and a desired value. The feed system may be placed at other 
locations, for example, subsequent to the hammermill, to add the oil while 
the coal is on one of the conveyor belts. Crushed coal mixed with oil is 
transported from the hammermill by conveyor 20 in a direction shown by 
arrow 21 and dropped onto conveyor 22. An automatic sampling device is 
schematically illustrated at 24 for obtaining samples of the coal as it is 
dropped from conveyor 20 to conveyor 22. The height of the drop of coal 
from conveyor 20 to conveyor 22 in our system is not adjustable and does 
not correspond to the distance of the drop of the coal into the coke 
ovens. For purposes of the claims the phrase "measurement prior to drop" 
includes measurements made on the particulates under various conditions, 
for example, when the particulates are in a container, or on a traveling 
conveyor belt, and regardless of prior handling of the particulates, such 
as a drop from one conveyor to another. A first coal-leveling device of 
conventional known design is illustrated at 26 for leveling the coal on 
the belt to an approximate desired height. A second coal-leveling device 
of conventional known design is illustrated at 28 for leveling the coal on 
the belt to the final desired height. The leveling devices assure that the 
coal on the conveyor belt is of a known, uniform depth prior to its 
passing before the bulk density and moisture measurement gauges. A nuclear 
gamma radiation gauge is provided for measuring the bulk density of the 
coal on the belt and includes a gamma radiation source 30 and a gamma 
radiation detector 32. An electrical signal is transmitted by line 34 from 
detector 32 to programmable controller 18 indicating the amount of gamma 
radiation that has passed through the coal as a measure of the bulk 
density of the coal on the belt. A microwave moisture gauge is provided to 
measure the moisture of the coal on the belt and includes a microwave 
source 36 and a microwave detector 38. An example of a suitable microwave 
moisture measurement device is described in U.S. Pat. No. 5,333,493 to 
Cutmore, the specification of which is incorporated herein and made a part 
hereof. An electrical signal is transmitted by line 40 from detector 38 to 
programmable controller 18 indicating the phase shift and attenuation 
changes of the microwaves that have passed through the coal and therefore 
the amount of moisture in the coal. Programmable controller 18 calculates 
the predicted post-drop bulk density based on the bulk density and 
moisture content of the coal on conveyor 22 and determines the oil flow 
adjustment required to bring the predicted bulk density to the desired 
value. A signal based on these calculations is transmitted from 
programmable controller 18 on line 42 to valve 16 to adjust the oil flow 
to the proper rate to obtain the desired post-drop bulk density. 
A series of tests were run in which samples of coal were obtained using the 
automatic sampling device 24. Nuclear gamma radiation instrument readings 
were obtained at the same time as the sample was being taken. The actual 
weighed post-drop bulk density was determined on each sample using a 
modified test based on Procedure A of ASTM Method 291-86. The ASTM method 
is modified in that the size of the container is 2/3 of a cubic foot 
versus a one cubic foot container in the ASTM test. The apparent coal bulk 
density determined using the modified ASTM test is converted to a 
post-drop coal bulk density corresponding to the bulk density determined 
by Procedure B of ASTM Method 291-86 based on a relationship developed 
from a plurality of tests carried out before the present invention. The 
relationship between the apparent coal bulk density determined by the 
modified ASTM test and the Procedure B test is shown in FIG. 4. The actual 
coal moisture was measured by placing a split of the coal sample from the 
drop test into an oven for at least four hours. The sample weight loss 
after drying was determined to indicate actual moisture content. From over 
250 such samples and tests a correlation was found between the post-drop 
bulk density, i.e. the bulk density found from the modified ASTM tests as 
converted to the bulk density of ASTM test Procedure B, and bulk density 
and moisture measurements made while the coal was on the conveyor. The 
basic correlation found on our tests was as follows: 
EQU BD.sub.AD =A+BD.sub.BD .times.B+% M.times.C 
Where 
BD.sub.AD =Post-drop Bulk Density 
BD.sub.BD =Bulk Density Mesurement before drop (i.e. on the conveyor belt) 
M=Moisture in wt. % 
A=a first constant 
B=a second constant 
C=a third constant 
Actual values of the constants will depend on the particular conditions 
under which the tests are performed. We determined the following values 
for the constants under our test conditions: 
A=21.23 
B=0.58 
C=-0.18 
The effect of the correlation on the variability of the predicted post-drop 
bulk density is illustrated by comparing the graphs in FIG. 2 and FIG. 3. 
FIG. 2 shows actual post-drop bulk density versus the coal bulk density 
measured on the belt. FIG. 3 shows a substantial reduction of variability 
in a graph of actual post-drop bulk density versus predicted post-drop 
bulk density based on measured bulk density and moisture content while the 
coal is on a conveyor belt. 
We have also found that a separate correlation equation may provide the 
best prediction of post-drop bulk density for the coal being conveyed to 
different coal bunkers. For example, for the coal being conveyed to the 
No. 5 Bunker at the Clairton, Pa. Plant of the assignee, a non-linear 
correlation equation was found to provide the best fit, as follows: 
EQU BD.sub.AD =30.79+0.38(BD.sub.BD)-0.0086% M-0.07184(% M-% M.sub.AV).sup.2 
Where % M.sub.AV =Average Percent Moisture on all tests. 
The data for coal being conveyed to the No. 7 Bunker at the Clairton Plant 
indicated that a linear equation provided the best fit, as follows: 
EQU BD=33.93+0.31(BD.sub.BD)-0.1806(% M) 
It should also be noted that the addition of waste substances, such as tar, 
to the coal has been found to affect the measurements to some extent. The 
affect of tar did not appear to be consistent but did influence the 
correlation on some occasions. 
Thus, the specific equation and the form of the correlation equation may 
vary depending on the bulk density measurement instrument used, the type 
and thickness of the conveyor belt, the height of coal on the belt after 
passing through the leveling devices, the angle of the nuclear source and 
detector with respect to the coal, the strength of the nuclear source and 
other conditions. The actual form of correlation applicable to any given 
test situation may be determined by known mathematical correlation 
techniques. 
Industrial Applicability 
As a result of the invention it is possible to decrease the variability of 
the bulk density of coal being charged into a coke oven and to increase 
the aim bulk density without problems attendant with prior control 
methods. Thus, the productivity of the coke ovens may be increased, more 
uniform heating and more stable operation of the coke ovens can be 
obtained, and the quality of the coke produced is enhanced.