Continuous quality control of mined hard and soft coals

A method is provided for determining the shale content of mined coal by monitoring the thorium content of the coal. Thorium content and ash content are shown to be related whereby a direct reading of the thorium will be indicative of the shale content of the coal and the ash content of the coal. The method utilizes the natural radiation of thorium to provide the continuous or selective control of mined coals.

This invention relates to a method for determining the quality of mined 
coals. More particularly, this invention relates to a method for 
determining the quality of coals by using the natural radiation of 
thorium. 
As liquid hydrocarbons have become less available and consequently more 
expensive, increasing attention has turned to solid hydrocarbons such as 
coal for energy supplies. As the demand for coal has increased, it has 
become necessary to increase production to meet this demand. Coal as mined 
is of many different qualities or ranks. Many methods are available for 
judging the quality of coal. Among them there are carbon and oxygen 
content, nitrogen content, moisture, volatile matter, ash, sulfer, etc. 
One of the most common quality checks is the ash content of coals as 
mined. This check is currently carried out according to a method similar 
to that found in ASTM D3174-73. Briefly, the method comprises weighing the 
residue remaining after burning the coal under rigidly controlled 
conditions of sample weight, temperature, time and atmosphere. 
It is readily apparent that when determining the quality of large amounts 
of coal, that such a method, while valid, is cumbersome, slow and relies 
upon a limited number of samples to determine the quality of any 
particular coal. For example, to achieve a statistically significant 
sample of 10 tons of coal, many samples must be ashed using the procedure 
described in order to determine the quality of the coal. These methods, 
while they have served in the past, become less and less efficient as the 
amount of mined coal increases. In addition, these samples are normally 
taken after the coal has been removed from the mine and stockpiled, thus 
allowing for the possibility of surface samples reflecting only the later 
mined coal with the coal at the bottom of the stockpile going untested and 
perhaps falling outside of the quality limits determined by the tests. 
It would therefore be of great benefit to provide an easy, efficient and 
accurate method for the determination of the ash content and hence the 
quality of mined coals. Such a method would ideally allow both spot and 
continuous checks and provide a valid relationship to the ash content of 
the coals. 
It is therefore an object of the present invention to provide a method for 
determining the ash content of mined coal. Other objects will become 
apparent to those skilled in this art as the description proceeds. 
It has now been discovered according to the present invention that when the 
shale content of a coal has been determined, the ash content can be 
directly correlated thereto. In the utilization of the instant invention 
the shale is determined using spectroanalysis techniques and utilizing the 
gamma ray emissions of thorium contained in the shale. 
All coals contain shale to varying extents and all shales are naturally 
radioactive to some extent. It is well known that all shales contain both 
uranium and thorium. However, the amount of uranium in shale is no 
indication of the amount of shale since uranium can be transferred from 
place to place by water when in its oxidized state, whereas thorium is not 
water soluble and remains as originally laid down in the shale thus giving 
an accurate reading as to the shale present. 
It has also been known that radioactivity can be used in mining for various 
purposes. For example, U.S. Pat. No. 3,019,338 relates to a method for 
controlling the position of a mining machine relative to clay and shale 
zones overlying and underlining coal deposits. The patent provides means 
such that the mining machine will irradiate the formation together with a 
detector for measuring radiation scattered by the formations together with 
suitable electronic apparatus for converting the output of the detector 
into a warning or signal for the operator when the machine is about to 
enter a hard zone. 
U.S. Pat. No. 2,487,058 teaches that the natural radiation of subterranean 
formations can be monitored and that the gamma radiation monitored can be 
used to differentiate between different rock materials. 
U.S. Pat. Nos. 2,897,368 and 3,105,149 show devices which separate the 
radioactive emissions from elements such as thorium, uranium, and 
potassium so that each is determined individually. These references relate 
to geophysical prospecting devices such as can be used from airplanes. 
None of these references, however, provide a method for the quality control 
of mined coal as provided in the instant invention. Most references relate 
only to mining techniques or finding of large formations. 
In carrying out the process of the instant invention, a calibration of the 
instrument measuring the radioactivity of thorium must be made. This 
calibration can be done by any means well known to those skilled in the 
art. However, for the purposes of this example, a sample of coal will be 
obtained and the ash content determined according to the method of ASTM 
D3174-73. The ash content will then be related to the thorium content of 
the sample and a correlation drawn therebetween. In addition, the amount 
of radioactivity found in the thorium sample will be simultaneously 
determined. 
Since, as set forth above, thorium is essentially constant throughout shale 
deposits in any given deposit of coal, as opposed to uranium which can 
vary widely, once the background count of thorium in a particular deposit 
has been calibrated, and the relationship of thorium to ash in the deposit 
has been determined, a direct reading of background count will thereafter 
allow direct correlation to thorium and hence to ash.

Calibration of the detection instrument can be easily accomplished by 
taking a statistically significant number of samples, usually from 25 to 
50, of coal from the deposit with a known but variable ash content as 
determined by the ASTM method referenced above. The thorium content as 
counts per minute per unit weight are measured utilizing laboratory 
apparatus. A plot is then made comparing ash vs the thorium count based on 
the samples tested to determine the relationship which is essentially 
linear. Thereafter, using a large detector in the same configuration as to 
be used in the plant mine slurry line, belt conveyor, or wherever the 
detector is utilized, the thorium count can be determined on a few large 
samples of coal of varying ash content. During this calibration it should 
be insured that the large samples are measured in the first laboratory set 
as well. The large sample measurement will provide the working calibration 
curve, comparing thorium counts on an unknown sample and automatically 
setting forth the ash content. Referring to the calculated figure, it can 
be seen in FIG. 1 for example, that 900 thorium counts per minute would 
roughly correspond to an ash content of about 19 percent while a thorium 
count of about 200 per minute would indicate an ash content of about 2 
percent. The actual numbers developed will be highly dependant on crystal 
size, crystal type, distance from the sample and other variables. 
While the instant invention is applicable, of course, to coals already 
mined and in storage, it is believed that the primary use will be found 
during the mining operation itself. For example, in many modern 
operations, such as long wall mining, coal is removed on conveyor belts. A 
determination of ash quality of coal can be easily made simply by 
installing a radiation detector at an appropriate distance over the 
conveyer belt such that when the ash content changes, appropriate routing 
of the coal can be made. These checks can, of course, be made 
intermittently or continuously as desired. Should coal be removed by a 
slurry in pipelines, the instant method is applicable when the density of 
the coal in the slurry is taken into account. Should gondola cars or the 
like be used, again the background radiation will be effective in 
determining the ash content of the coals. 
Usually the detector instrument will comprise a gamma ray spectrometer 
which uses an energy proportional detector in the sensing probe. Thorium 
has a 2.6 MEV gamma ray which is very penetrating and completely separable 
from potassium and uranium and is essentially unaffected by uranium 
radiation. Since the total thorium radiation is reasonably low compared to 
surrounding elements, longer counts can be used to increase the 
statistical accuracy of the system if necessary. In addition, tradeoffs 
can be made, such that by increasing the crystal size counting time can be 
reduced to the desired period. Usually such detectors will comprise sodium 
iodide or cesium iodide crystals which allow the detection of thorium 
radiation. Many such systems are well-known and are commercially 
available, among them a TN1705 analyzer, manufactured and sold by Tracor 
Northern Corporation. However any instrument which performs a similar 
function can of course be used. 
By directly linking to the detecting instrument, variations of thorium 
count rate level can be used to shift coal from one area to another 
depending upon the amount of ash in a given amount of coal. It will be 
readily apparent that the present invention thus provides a simple, 
effective and inexpensive method for determining the ash content of mined 
coals. While in practice it will likely be necessary to recalibrate the 
detection instruments at intervals, such a method will greatly decrease 
the amount of laboratory determinations which are currently carried out. 
It will thus be apparent that a large advance in the art has been obtained 
in that much manual labor and uncertainty has been eliminated from the 
determination of ash content of coals. 
While certain embodiments and details have been shown for the purpose of 
illustrating this invention, it will be apparent to those skilled in this 
art that various changes and modifications may be made herein without 
departing from the spirit or the scope of the invention.