Process for the beneficiation of carbonous materials with the aid of ultrasound

A process for reducing the sulfur and ash content of coal and the like by treatment in an aqueous slurry with ultrasound followed by subsequent separating steps.

BACKGROUND 
Coal as a fuel is an abundant resource of energy comprising mostly carbon, 
and small percentages of hydrogen, sulfur and ash. When coal is burned to 
produce energy, the presence of the sulfur and ash is generally 
undesirable. The ash enters the atmosphere as small particles 
(particulates) and the sulfur as noxious sulfur oxide gases. Sulfur is 
present in coal in three principal forms: pyritic sulfur (a combination of 
iron and sulfur); sulfate sulfur, generally in very small quantities, say 
0.5 to 0.1 percent by weight; and organic sulfur, that is chemically 
combined sulfur within the coal structure. 
Pyritic sulfur can, to a large extent, be washed out of coal by 
conventional coal washing methods. These methods are not, however, 
suitably efficient on a large scale and at best only a small portion of 
the mined coal can be sufficiently up-graded by washing alone. 
Sulfate sulfur can easily be separated from coal by dissolving it in water. 
For example, it may boiled out of the coal matrix by elevated temperature 
processes which have already been developed. 
At the present time there appears to be no commercial process for removing 
organic sulfur from coal. Removal of organic sulfur requires drastic 
chemical treatment causing the breaking of bonds between the sulfur and 
the carbon within the structure of the coal molecule. Where the sulfur 
content of coal is very near the permissible level as designated by 
government anti-pollution laws and regulations, it still may not be 
possible to economically upgrade the coal by removal of organic sulfur. 
Thus, it becomes necessary to treat exhaust gases with expensive scrubbers 
which use large quantities of chemicals and which can create additional 
pollution problems. 
Processes have been conceived and to some extent developed for removal of a 
portion of the organic sulfur coal. At this time, they require very 
expensive treatment facilities utilizing high pressures say up to 500 to 
1,000 psi, and temperatures up to 600.degree. F. (about 400.degree. C.). 
Clearly, from an engineering and processing point of view, it does not 
make sense to treat coal in order to reduce the initial sulfur content of 
the coal from say 1.5% to a 0.6 to 0.8% level by use of these processes. 
Summarizing the numerous processes which have been proposed for upgrading 
coal to remove various forms of sulfur, the following have been 
considered: (1) Oxidation of sulfur in the coal in an aqueous medium to 
form soluble sulfates; (2) reduction of the sulfur to elemental sulfur in 
which form it can be vaporized or removed by organic solvents; (3) 
reaction with hydrogen to form gaseous hydrogen sulfide; (4) vapor 
deposition selectively on the pyritic form of sulfur followed by magnetic 
separation of the pyrites; (5) oxidation of the sulfur with nitric oxide 
vapors to form gaseous sulfur oxides; (6) leaching with a sodium and 
calcium oxide lixiviant; and (7) leaching with aqueous ferric sulfate. 
The applicant's process disclosed herein has the potential for providing a 
commercial process for removal of the three basic forms of sulfur from 
coal and coal-like materials. At the same time, the process reduces the 
amount of ash within the coal or coal-like material. The process involves 
the use of atmospheric pressures and low temperatures (temperatures near 
room temperature) and may be practiced with rugged processing equipment. 
BRIEF DESCRIPTION OF THE INVENTION 
Briefly according to this invention, there is provided a method of treating 
coal and coal-like materials to reduce the sulfur content. The method 
comprises the first step of crushing and sizing the coal to a more or less 
uniform size. Particular size to be selected depends upon the type of coal 
and the amount of sulfur that must be removed and of course the type of 
sulfur within the coal itself. Certain coals have been found to respond to 
treatment very well if crushed to pass one-quarter inch mesh screen. It 
should be understood that the process described herein can be used for the 
treatment of residue from coal washing processes sometimes referred to as 
pond coal, in which case the starting material is already very fine, say 
minus 28 mesh Tyler. In this instance, it is not necessary to crush and 
size the coal starting material. The second step comprises combining the 
coal with water in a bath to form a slurry. A third step comprises 
applying ultrasound to the slurry. This may be done in either of two ways. 
The slurry may be dumped into a large tank to which ultrasound is applied 
for some relatively long period of time followed by draining the tank. On 
the other hand, the slurry may be continuously pumped through an 
ultrasound cell where it is resident in the cell for only a relatively 
short period of time. A fourth step comprises removing the coal from the 
water and washing the coal to recover a coal with a reduced sulfur and ash 
content. According to preferred methods, a small amount of oil is added to 
the slurry. The oil appears to aid in the displacement of organic sulfur 
from the coal structure via the action of ultrasound. The oil added to the 
slurry is preferably added in an amount between a stoichiometric ratio of 
sulfur to oil of 1:1 and 1:5. A further preferred embodiment involves the 
addition of sodium chloride to the slurry. 
It is preferable that the applied frequency of the ultrasound be between 
about 20 and 40 kilocycles per second and that the temperature of the 
slurry be maintained less than about 75.degree. C. 
DESCRIPTION OF THE PREFERRED EMBODIMENTS 
Further features and other objects and advantages of this invention will 
become clear from a study of the following examples.

EXAMPLE 1 
A specimen of low sulfur metallurgical quality coal having a raw sulfur 
content of 0.89% by weight was crushed and sized to pass one-quarter inch 
mesh screen and to rest upon a 100 mesh screen. 
A portion of the specimen was treated in a salt solution under heat and 
pressure (15 psi) in a process described generally in my earlier patent 
application, now U.S. Pat. No. 4,127,390. 
Another portion of the specimen was treated in a salt solution with 
ultrasonic vibration. The solution comprised 500 ml of water with 13 grams 
of sodium chloride and 7 grams of sodium carbonate added thereto. The 
slurry comprised 100 grams of coal and 500 ml of salt solution. In the 
case of the specimen treated by ultrasound, the slurry was subjected to 
ultrasonic vibrations of frequency 20 kHz for a period of 30 minutes. The 
power applied to the ultrasound generated was 220 watts (0.7 
watts/cm.sup.2). 
In each instance, the fine coal was separated from the solution and washed 
and in each instance chemically analyzed. The sulfur content was reduced 
from 0.89 to 0.65 percent by treatment in the salt solution under heat and 
pressure, as expected from my prior work. The sulfur content of the 
portion of the specimen treated in the salt solution with ultrasonic 
vibration applied thereto was reduced from 0.89 to 0.58 percent. 
Hence, the new process described herein was at least as effective as my 
earlier patented process and has the advantage that pressure vessels are 
not required for the process. To be sure, means for generating ultrasound 
are required. Each process has its relative advantages. 
EXAMPLE II 
A specimen of low sulfur coal from Kentucky was sized and slurried and 
treated with ultrasonic vibration substantially as was the specimen of 
Example I. Another portion of the same specimen was treated with a saline 
solution of hydrogen peroxide as generally described in my earlier 
application, now U.S. Pat. No. 4,183,730. 
The following table sets forth the characteristics of the specimen, both 
before and after treatment by both processes. 
______________________________________ 
Saline Solution 
Ultrasound 
With Hydrogen 
Raw Coal Treatment Peroxide 
______________________________________ 
Ash 14.93% 6.21% 7.07% 
Sulfur 1.14% 0.80% 0.91% 
BTU/ 11,606 12,005 12,731 
Pound 
______________________________________ 
It should be noted that all analyses presented in this patent specification 
are based upon dry specimens. 
The specimen treated by ultrasound was treated in a slurry comprising 20 
grams of salt per 1 liter of water. The specimen treated in the saline 
solution with hydrogen peroxide comprised 200 grams of coal combined with 
400 milliliters of a 6 percent solution of hydrogen peroxide and 40 grams 
of salt. In both cases, the coal was floated and separated from other 
residue. The new process disclosed herein was at least as effective at 
sulfur reduction, if not more so, than the process requiring the use of 
hydrogen peroxide. 
EXAMPLE III 
A specimen of high sulfur subbituminous coal from Illinois was sized into 
two fractions. One portion of the specimen was crushed to pass a 
five-eighth inch mesh screen and the other was crushed to pass a 
one-eighth inch mesh screen. The specimens were both treated in a saline 
solution substantially as described in Example 1. The following table sets 
forth characteristics of the raw coal compared with the specimens treated 
with ultrasonic vibration in a saline solution. 
______________________________________ 
Raw Coal Minus 5/8 Ins. 
Minus 1/8 Ins. 
______________________________________ 
Ash 31.01% 5.12% 3.96% 
Sulfur 5.33% 3.00% 2.59% 
BTU/pound 
9,328 11,909 11,843 
______________________________________ 
The saline solutions comprise 40 grams of salt per 500 ml of water to which 
was added 200 grams of sized coal. 
The specimen treated with ultrasonic vibration was washed and the coal 
floated from the residue. The power applied to the ultrasonic vibrator was 
about 220 watts. This example establishes that the smaller particle size 
coal had a greater ash and sulfur reduction. The raw coal for this example 
was typically analyzed for type of sulfur as follows: pyritic sulfur 
2.73%; sulfate sulfur 0.40%; organic sulfur 2.06% for a total of 5.19%. 
On the basis of this analysis, it may be concluded that the process 
described with reference to this example does not easily remove organic 
sulfur. It was conceived that the ultrasound might be more effective if a 
hydrocarbon substance were provided to replace the sulfur within the 
structure of the coal as it is broken free by the ultrasonic action in the 
presence of salt. 
EXAMPLE IV 
An apparatus for continuously treating a coal slurry was set up to pump the 
slurry from one tank through an ultrasonic processing cell to a second 
tank. The cell was equipped with a booster horn capable of transmitting 
industrial power level vibrations into the cell. The slurry of coal from 
Example III (minus one-eighth inch) was made up as follows: 4 pounds of 
coal; 5 gallons of water; 20 grams of salt; 20 grams of sodium carbonate; 
vegetable oil present in a stoichiometric 1 to 1 ratio to organic sulfur 
present in the coal. 
The slurry was pumped through the ultrasonic cell at the rate of 
three-eighths gallon per minute. After treatment, the coal was cleaned 
with hot tap water and the sample floated in a froth flotation cell to 
separate the coal from the liquid and gangue in the process slurry. The 
coal after treatment analyzed: 
______________________________________ 
Ash 4.07% 
Sulfur 0.122% 
BTU/pound 
19,483. 
______________________________________ 
EXAMPLE V 
A sample of Pittsburgh seam coal residue from a coal washing process, so 
called pond coal, being a very fine material (minus 200 mesh) was 
processed substantially as described in Example IV. The coal was also 
processed with the addition of vegetable oil. The results of processing 
are set forth in the following table. 
______________________________________ 
Treated In Brine 
Raw Pond Treated In Slurry With Vegetable 
Coal Brine Slurry Oil Added 
______________________________________ 
Ash 38.27% 4.10% 4.07% 
Sulfur 
1.42% 1.07% 0.125% 
BTU/ 8,598 14,065 15,503 
pound 
______________________________________ 
The salt concentration for the specimen treated in brine only was 20 grams 
of salt per 100 grams of coal in 15 liters of water. The salt 
concentration for the specimen treated in brine with addition of vegetable 
oil was 15 grams of salt per 200 grams of coal in 15 liters of water. 
Another specimen of the pond coal was simply floated in a froth flotation 
cell. No significant reduction of sulfur was demonstrated. Furthermore, 
ash reduction was not as effective. Results of mere floating the coal are 
set forth in the following table. 
______________________________________ 
Ash 5.03% 
Sulfur 1.22% 
BTU/pound 
12,705. 
______________________________________ 
A specimen of the coal described in Example IV was slurried and treated 
with vegetable and ultrasound only. At this point, the treated coal 
analyzed as follows: Ash-4.11%; Sulfur-0.96%; BTU/pound-11,140. 
This test established that satisfactory results may be obtained without the 
use of sodium chloride in the water used to slurry the coal prior to 
ultrasonic treatment. In some instances, the addition of salt to the 
solution used to form the coal slurry may be detrimental. It is believed 
that the chlorine content of the coal may build up as chlorine replaces 
organic sulfur. 
The treated slurry of this example was then mixed with distilled water plus 
a coal depressant. Tiny solids coagulated on the top of the mixture and 
were skimmed off the top and chemically analyzed. The skimmings analyzed 
3.31% by weight elemental sulfur. The point here is that the tendency for 
the coal to float after ultrasound treatment and the tendency of minuscule 
elemental sulfur particles to form (not even visible with the naked eye) 
can result in elemental sulfur reconcentrating with the coal. It is 
preferable to keep the coal particles sufficiently large so that they may 
be depressed (caused to sink) and to thereby enable the elemental sulfur 
to be washed away or skimmed off. 
Another specimen of the coal treated as described in this example (Example 
IV-A) was mixed with sodium chloride in a 3% solution of hydrogen 
peroxide. This was done because the mixing of the elemental sulfur with 
the coal was apparent. The sulfur content of the washed coal (washed 
subsequent to treatment with sodium chloride and hydrogen peroxide 
solution) was remarkably low, that is, 0.0007% by weight. The point here 
is that the ultrasound treatment frees elemental sulfur but a careful 
unmixing of the elemental sulfur and coal is required. Described in this 
paragraph is a chemical unmixing which results in a washing liquor 
analyzing 0.06% sulfur and having a pH of 1.8. Obviously, this washing 
liquor itself comprises a disposal problem and hence physical separation 
techniques for separating the elemental sulfur and coal are preferred. 
EXAMPLE VI 
A composite sample of an Ohio coal crushed to all pass 100 mesh Tyler was 
estimated to have the following properties. 
______________________________________ 
Ash 12% 
Sulfur 2.2% 
BTU/pound 
11,000 
______________________________________ 
Because this was a composite sample, the values given are only approximate. 
The composition was treated substantially as described with reference to 
Example V but with no addition of vegetable oil. The results of treatment 
were as follows. 
______________________________________ 
Ash 4.86% 
Sulfur 0.90% 
BTU/pound 
13,690. 
______________________________________ 
EXAMPLE VII 
A particularly difficult to treat Ohio coal (subbituminous) has the 
following characteristics. 
______________________________________ 
Ash 15.71% 
Sulfur 4.84% 
BTU/pound 
9,166 
______________________________________ 
Treatment with brine and ultrasound produced a coal product having the 
following characteristics. 
______________________________________ 
Ash 4.8% 
Sulfur 3.53% 
BTU/pound 
10,385 
______________________________________ 
Treatment with oil and ultrasound (that is, no salt added) produced a coal 
product having the following characteristics. 
______________________________________ 
Ash 5.46% 
Sulfur 3.82% 
BTU/pound 
10,526 
______________________________________ 
This example establishes that the degree with which the process disclosed 
herein is effective for removing sulfur depends upon the characteristics 
of the coal itself. 
EXAMPLE VIII 
A larger particle size subbituminous coal was treated with brine in a 
vessel with applied ultrasound. The particular coal was of relatively 
large particular size, one and three-quarter inches and down. The 
characteristics of the coal before and after treatment are set forth in 
the following table. 
______________________________________ 
Raw Treated 
______________________________________ 
Volatile matter 29.88% 27.65% 
Fixed carbon 55.89% 65.66% 
Ash 14.23% 6.69% 
100.00% 100.00% 
Sulfur 7.75% 2.55% 
BTU/pound 9,161 10,149 
______________________________________ 
It has been known that coal containing sulfur as pyrites can be nicely 
upgraded by "floating" fine coal to separate ash and pyritic sulfur. 
Floating is a type of washing process. Washing techniques do not 
concentrate sulfur in the coal recovered because while ash containing no 
sulfur is removed, part of the sulfur containing pyrites are also removed. 
Of course, the organic sulfur prevails and cannot be removed by washing. 
Coal is normally floated at some specific gravity, say within the range of 
1.1 to 1.7. In this instance, a large portion of the ash and pyrite sinks. 
When a fine coal slurry is treated ultrasonically, as described herein, the 
flotation process is enhanced. More coal appears to float even in plain 
water than with conventional floating techniques. More coal can therefore 
be recovered. Difficult to float coals tend to coagulate on the top of 
water after ultrasound treatment. 
The used washing water left over from the process disclosed herein need not 
be extensively treated with neutralizer as with other desulfurization 
processes, for the reason that the amount of sulfur converted to sulfuric 
acid is much less. The elemental sulfur and inorganic matter removed from 
the coal can be removed from the water by conventional methods of 
coagulation and filtration. 
After application of ultrasound to the coal slurry according to this 
invention, elemental sulfur and pyrites are often present in very fine 
particular size making the separation of the sulfur and pyrites from the 
coal a process requiring careful attention. A first step should comprise 
separating the coarser coal in a deep tank, hydrocyclone, screen or 
whatever available equipment. Coarser coal at this point will sink to the 
bottom of a deep tank. (This is the least expensive method of removing the 
coal from the liquor.) Liquor may be decanted from the top of the vessel 
and coal slurry pumped from the bottom of the vessel to a second tank. A 
second step will involve rinsing the coal with clear water. It has been 
found that the microscopic pyrites and sulfur particles will readily float 
in the rinse water and can be skimmed from the top of the tank in which 
the coal is being rinsed. Surface wetting agents may be employed for the 
purpose of preferentially wetting the coal surfaces. These agents tend to 
depress the coal and enhance the sulfur extraction because the sulfur will 
float much better. A number of products are available as wetting agents 
and include the following sold by trademark or trade name: Aero Depressant 
633; Aerosol MA; Triton X-100; and Santomerse S. These agents would 
typically be added in an amount of about 1/2 pound or more per ton of 
coal. 
The applicant does not wish to be tied down to any specific mechanism for 
explaining the effect of ultrasonic vibration upon the coal slurry to aid 
in the removal of sulfur. However, the following thoughts may be 
pertinent. Ultrasonic treatment of various liquids and solids has been 
known for some time to promote chemical changes. Numerous frequency ranges 
of ultrasonic vibration have been experimented with. There has been found 
a phenomenom known as cavitation which is induced in liquids and slurries 
by ultrasonic vibration. Cavitation is the formation of partial vacuums 
within the liquid. Ultrasonically induced cavitation appears to promote 
chemical changes of substances within the liquid. Agitation itself 
provided by ultrasound may produce physical and chemical changes within 
the liquid to which the sound is applied. For ultrasonic treatment, when 
water is used as treatment medium, cavitation and agitation may both be 
involved. Most such applications require frequency ranges of 20 to 40 
kilocycles per second. Cavitation effects may be most pronounced by using 
either magnetostrictive or ceramic sources for generation of ultrasonic 
waves. 
Of course temperature affects the speed and frequency of ultrasonic waves 
within a given medium. Generally at about a temperature of 73.degree. C. 
cavitation and frequency of ultrasonic waves within water begins to 
deteriorate. It is therefore desirable to maintain the maximum 
temperature. The slurry is used in this process below 75.degree. C. 
The applicant hypothesizes that the coal molecules which are very large 
chain hydrocarbons connected in many ways to both organic and inorganic 
elements can be disturbed by ultrasound. Following this reasoning, one may 
conclude that upon breaking apart the molecular chains of the hydrocarbon 
structure some loose ends will remain actively seeking to form or reform. 
Thus if a sulfur atom tied to a hydrocarbon molecule of the coal is 
removed, it will leave behind an active site seeking to replace the "lost" 
sulfur atom. By having present in the slurry a vegetable oil (which is a 
somewhat reactive oil) the active site can be satisfied by the oil rather 
than by recombination with the sulfur molecule. This may be the basis for 
explanation of the excellent result of the process as exemplified in 
Examples IV and V. 
Oils that were used in Examples IV and V were vegetable oils which are 
members of a group of semi-reactive oils known as fixed oils--fatty 
substances of vegetable and animal organisms--containing esters of fatty 
acids. It is expected that volatile or essential oils--odorous principals 
of vegetable organisms--containing terpenes and related camphors would 
also be effective. Further, it is believed that mineral oils derived from 
petroleum and its products would be effective. 
Where the product of the process according to this invention is very fine 
coal, say 100 to 400 mesh Tyler, there exists at least two methods of 
utilizing the processed coal. It may be mixed with fuel oil and the fuel 
oil and coal mixture processed through oil burners to thus reduce the 
total amount of oil required in a given application. In this case, the oil 
may be floated on a tank over which the fine coal has been caused to 
coagulate and float. The coal will move into the oil and be carried away 
from the tank by the oil. In another embodiment, the oil and water may be 
vigorously stirred together and then the oil and coal mixture allowed to 
rise and float over the top of the water prior to separation. 
Where the fine coal is to be used with a stoker, it must be pelletized 
prior to use, for example as taught in my U.S. patent application Ser. No. 
23,744, filed Mar. 26, 1979. 
The examples herein illustrate the usefulness of adding small amounts of 
certain chemical agents such as salt, sodium carbonate, and hydrogen 
peroxide to the coal slurry prior to the ultrasound treatment (see also my 
U.S. Pat. Nos. 4,183,730 and 4,127,390). Other agents may be added to the 
slurry, for example, small amounts of semi-reactive oil as explained 
herein. Certain of the agents can be profitably added together to the coal 
slurry, for example salt and semi-reactive oil. It has also been 
discovered that sodium hydroxide is an excellent agent to add to the coal 
slurry prior to treatment of the coal slurry with ultrasound. However, oil 
may not also be added to the slurry when sodium hydroxide is added. 
Otherwise, the oil and sodium hydroxide will react to form a soap. 
Extremely pure coal (very low in sulfur) can be obtained using a process 
described herein with sodium hydroxide as an agent in the slurry in at 
least a stoichiometric 1 to 1 ratio of sodium hydroxide to the organic 
sulfur in the coal present in the slurry. 
Having thus described my invention with the detail and particularly 
required by the Patent Laws, what is desired protected by Letters Patent 
is set forth in the following claims.