Coal cleaning by gaseous carbon dioxide conditioning and froth flotation

A process for froth flotation of coal using gaseous carbon dioxide includes a preconditioning treatment of the coal with gaseous carbon dioxide followed by froth flotation, preferably also using gaseous carbon dioxide. The pretreatment causes the coal to show improved results in that less reagent promoter and frother are required, and the flotation time is reduced. The process is particularly useful for producing "super" clean coal.

BACKGROUND OF THE INVENTION 
1. Field of the Invention 
This invention relates to coal flotation, and more particularly to coal 
froth flotation utilizing gaseous carbon dioxide for production of clean 
coal concentrates. 
2. Description of the Prior Art 
Froth flotation is a physicochemical separation process that depends on the 
attachment of air bubbles to hydrophobic particles. Other (hydrophilic) 
particles are wetted by the aqueous phase and will not attach to the air 
bubbles. Thus the separation of coal particles from gangue minerals by 
froth flotation occurs, for example, as dispersed air bubbles pass through 
a suspension of coal particles (-28 mesh). The bubble/particle aggregates 
of coal float to the surface and may be collected as a clean coal 
concentrate separated thereby from the wetted gangue particles. 
Generally this process involves the use of suitable reagents (neutral 
molecular oils) to enhance the hydrophobic character of coal particles 
while the gangue mineral particles remain hydrophilic. These neutral 
molecular oils such as kerosene or fuel oil are called promoters and are 
used to enhance the attachment of air bubbles at the coal surface by 
forming a thin oil coating over the coal particles to be floated. Further, 
a frother is added to establish a stable froth phase to hold the 
bubble/particle aggregate. In coal flotation, frothers such as methyl 
isobutyl carbinol, terpinol, cresols, and polyglycols are frequently used. 
The choice of frother and oil depends on the desired level of selectivity 
with respect to ash and sulfur. 
Although the foregoing known processes are successful for conventional coal 
flotation, it is more difficult to produce "super-clean" coal by flotation 
of finely ground coal (-400 mesh). The promoter which is used to increase 
hydrophobicity inadvertently adsorbs on the gangue minerals, and these 
gangue minerals subsequently migrate to the concentrate, thus decreasing 
the quality of the product. Also, reagent consumption is high because fine 
coal, due to its high surface area, adsorbs significant amounts of 
promoter and frother. Finally, the rate of fine coal flotation by existing 
flotation techniques is very slow. As a result, the production of 
super-clean coal by conventional froth flotation methods has had limited 
success. 
A super-clean coal product is particularly desirable in the production of 
coal/water fuel. Coal/water fuel contains roughly 70% of the super-clean 
coal and is stabilized by the addition of various chemical additives so 
that it can be pumped, stored and used much like oil for which it is 
intended as a substitute. 
In regard to the utilization of carbon dioxide in coal cleaning processes, 
some work has been done. For example, U.S. Pat. No. 4,522,628 to Savins 
discloses a method for removing ash from coal using liquid carbon dioxide 
under pressure in order to fracture and crush coal, not for flotation. In 
Savins, after comminution at high pressure and elevated temperature with 
liquid carbon dioxide, conventional flotation is used for coal recovery. 
Santhanam, in U.S. Pat. No. 4,206,610, also uses liquid carbon dioxide. 
The carbon dioxide is used in the Santhanam reference as a liquid to 
replace water as a medium to transport coal from mine to a remote 
processing plant. However, liquid carbon dioxide processes (transportation 
and cleaning) have inherent problems which relate to the chemical and 
physical properties of carbon dioxide (the need to keep it under positive 
pressure, etc.). 
Other references treating coal for various purposes include U.S. Pat. No. 
3,998,604 to Hinkley which discloses the use of acids for grinding of 
coal. Hinkley uses acid treatment (not gas) during grinding. This is a 
type of leaching reaction. Carbon dioxide is mentioned in passing as a 
companion to carbonic acid for the sole purpose of grinding and making an 
acid slurry. Subsequently, the ground coal is floated and whether the coal 
"floats" or "sinks" is dependent on the flotation reagents, not the gas 
used. 
Steam and gaseous carbon dioxide have also been used in a high temperature 
coal process. Robinson et al, in U.S. Pat. No. 4,053,285, discloses a 
steam/carbon dioxide process for chemically treating (using hydrogen) 
previously prepared coal to lower the sulfur content. At high temperature 
and pressure, sulfur in the coal reacts with carbon dioxide and steam and 
is reduced to a sulfide. Flotation is not involved in this prior art 
reference. 
No process is known which utilizes gaseous carbon dioxide to improve the 
hydrophobic character of the coal surface. That is, no process is known 
which uses gaseous carbon dioxide as a surface active reagent in the froth 
flotation cleaning of coal. 
SUMMARY OF THE DISCLOSURE 
The aforementioned prior art problems are obviated by utilizing gaseous 
carbon dioxide for froth flotation of coal to produce, particularly, super 
clean (ash free) coal concentrates. A higher rate of recovery, by a factor 
or two, has been demonstrated for a western, high-volatile bituminous 
coal. Other coal types are also amenable to the process of this invention. 
In this discovery, carbon dioxide is used for gas phase conditioning of a 
coal/water suspension which, for example, may be pressure filtration of 
the slurry or pressurized mixing in a stirred tank reactor, prior to 
flotation of the material in a flotation cell, also using carbon dioxide.

DETAILED DESCRIPTION OF THE PREFERRED EMBODIMENT(S) 
Laboratory experiments were conducted to establish the efficacy of this 
invention. 
Flotation Experiments 
Experimental results are reported for a western, high-volatile bituminous 
coal (UP&L) obtained from Utah Power and Light Company, Salt Lake City, 
Utah, containing 6-7% ash. Further details are given in Table I. 
In Table I, column 1 indicates the origin of the coal sample. Column 2 
indicates the ash content in percent, and column 3 the volatile matter. 
Column 4 represents the fixed carbon. 
TABLE I 
______________________________________ 
Volatile Fixed 
Coal type Ash % Matter, % Carbon, % 
______________________________________ 
Western Coal (UP&L) 
7.67 47.46 44.87 
Eastern Coal (Elkhorn) 
6.38 35.43 58.19 
______________________________________ 
The western, UP&L coal, as received, was wet ground in a conventional ball 
mill for a specified time at 50% solids as is well known in the art. 
Flotation experiments were conducted using a 2-liter, Galigher flotation 
machine at 8.3% solids with addition of commercial methyl isobutyl 
carbinol (MIBC) as a frother and a kerosene as collector as is well known 
in the art. Flotation products were analyzed for ash by standard 
thermogravimetric technique. All carbon dioxide flotation experiments were 
compared to air flotation experiments under the same experimental 
conditions (pH, temperature, and gas flowrate of 6.5 standard liters per 
min.). 
In one series of experiments, shown in FIG. 1, batches of freshly ground 
coal slurry were first treated or conditioned using carbon dioxide at the 
indicated pressure. Conditioning in this case involved pressure 
filtration. The coal was then repulped in the flotation cell with carbon 
dioxide saturated water and floated with staged additions of promoter and 
frother. FIG. 1 is a plot of clean coal yield as a function carbon dioxide 
conditioning pressure and demonstrates the significant improvement in coal 
recovery by conditioning with carbon dioxide. The effect is observed both 
in the absence and presence of promoter. For example, with 1.5 g/kg 
kerosene promoter, the percent of coal recovered in the clean coal 
concentrate increases from 39 percent for conditioning at a low carbon 
dioxide pressure (0.004 psia) to almost 90 percent for conditioning at 500 
psia. 
In another series of experiments, UP&L coal was ground to 81 percent 
passing 38 microns. The flotation yield of this material for carbon 
dioxide pretreatment (33.4 psia) was compared to that for air pretreatment 
(33.4 psia) at three different levels of kerosene promoter addition. At 
0.9 g/kg, the ash content of carbon dioxide treated coal was 2.3% by 
weight at 87.7% yield (i.e., weight percent recovered), where for 
air-treated coal, the ash content was 2.8% at 63.3% yield. FIG. 2 
demonstrates again the improved separation that can be achieved with 
carbon dioxide conditioning pretreatment. 
As can be seen from FIG. 3, for UP&L coal, under the experimental 
conditions indicated, the rate of flotation of fine coal (100% passing 38 
microns) with carbon dioxide pretreatment is much faster than that with 
air or nitrogen. For example, with the addition of 0.48 g/kg methyl 
isobutyl carbinol and 0.72 g/kg of kerosene promoter, 84.0% of the coal 
can be recovered in the concentrate in four minutes as compared to 45.0% 
and 54.0% with air and nitrogen, respectively. 
These results presented in FIG. 1-3 for UP&L coal show the significant 
improvement of clean coal yield with carbon dioxide conditioning, i.e., 
pretreatment. 
Also of importance is the quality of the clean coal product, i.e., its ash 
content. In this regard, the effectiveness of the carbon dioxide flotation 
process was evaluated with respect to ash content of the clean coal 
product. In addition to bench scale flotation tests with the western UP&L 
coal, an eastern coal (Elkhorn) was also studied. The proximate analyses 
for both coal samples are given in Table I. The western coal, as received, 
was wet-ground in a conventional ball mill for a specified time at 50% 
solids. The eastern (Elkhorn) coal was crushed and pulverized to -28 mesh 
and then wet-ground under conditions similar to those for the western 
coal. The freshly ground coal slurries were filtered (at 33.4 psia) using 
carbon dioxide in one case and air in another case. 
After filtration, the filter cake was kept under carbon dioxide gas for 
thirty minutes. After thirty minutes, the filter cake was repulped in the 
flotation cell with a saturated aqueous solution of the respective gas 
phase. 
Flotation experiments were conducted, again using a Galigher flotation cell 
at a gas flowrate of 5 standard liters/min and at 900 rpm. 
Commercial-grade MIBC and kerosene were used as frother and promoter 
respectively. As before, stage additions of frother and promoter were 
implemented throughout the flotation experiment. The first stage of 
flotation was conducted for ten minutes after an initial addition of 1.5 
g/kg promoter and 0.1 g/kg frother. After five minutes during the first 
stage flotation, an additional 0.05 g/kg frother was added to maintain the 
froth. In subsequent cleaning stages, 0.05 g/kg frother and 0.25 g/kg 
promoter were added per stage. As-received western coal was floated under 
similar conditions. However, in the subsequent cleaning stages only 0.05 
g/kg frother was added per stage. Reagent levels depend upon the type of 
coal being floated. 
Products from the flotation experiments were analyzed, and from these data, 
ash/yield curves were prepared as shown in FIGS. 4 through 6. In FIGS. 4 
through 6, the dotted lines indicate the intrinsic ash level which is a 
measurement of ultimate ash level that might be achieved in the clean coal 
product as determined by acid leaching. FIG. 4 refers only to UP&L coal 
and shows yield versus percent ash for two different particle size 
distributions. FIG. 4 demonstrates that, even for carbon dioxide 
flotation, liberation must be achieved in order to reduce the ash content 
of the clean coal product. Notice that an excellent clean coal product can 
be made containing 1.5 percent ash at a yield of at least 60 percent. 
FIG. 5 refers to UP&L coal and FIG. 6 relates to Elkhorn coal. In each 
Figure, the percent ash was measured against yield comparing air to carbon 
dioxide. From these graphs it is evident that the carbon dioxide treatment 
provides for improved separation efficiency for both the eastern and 
western coal samples as evidenced by the ash content and yield. For 
example, in the case of UP&L coal with air pretreatment, it will be 
impossible to produce a clean product containing 1.5 percent ash at a 
yield even of 40 percent. Whereas with carbon dioxide pretreatment, such 
as product can be made at a yield of at least 60 percent. Similarly, in 
the case of the Elkhorn coal, at a yield of about 75 percent, air 
pretreatment will result a clean coal product containing 3.1 percent ash 
whereas the carbon dioxide pretreatment will result in a clean coal 
product containing 2.3 percent ash. 
Surface Chemistry 
Specific interactions of carbon dioxide at the coal surface may account for 
the improved separation efficiency. To test this proposition, the affinity 
of carbon dioxide for a coal surface was measured by bubble attachment 
with a contact-angle goniometer. Experimental results indicated that the 
equilibrium contact angles are slightly larger for carbon dioxide 
(45-48.degree.) than for nitrogen and air (38-40.degree.). More 
significantly, however, the bubble attachment time for carbon dioxide was 
decreased by a factor of five. Results of this experiment are presented in 
Table II. 
The hydrophobic character of Midcontinent coal for different gas-phase 
treatments at pH 5.0.+-.0.2 are presented below in Table II. Column 1 
indicates gas phase. Column 2 indicates attachment time as measured in 
milliseconds. Column 3 indicates contact angles as measured in degrees. 
TABLE II 
______________________________________ 
1 2 3 
GAS BUBBLE ATTACHMENT CONTACT ANGLES, 
PHASE TIME, ms degrees 
______________________________________ 
Carbon 20 45-48 
Dioxide 
Nitrogen 
80-90 35-40 
Air 100-110 38-40 
______________________________________ 
Thus, although the exact method by which this invention operates is not 
known, it is believed that the selectivity of the flotation separation is 
due to the specific adsorption of carbon dioxide at the coal surface which 
enhances its hydrophobicity as reflected by the results presented in Table 
II. 
In a typical coal preparation plant utilizing the process of this 
invention, and with reference to FIG. 7, feed coal 10 is reduced in size 
by conventional methods such as wet or dry grinding as represented at 12. 
Size reduction, as is well known, is essential for ash reduction. Coal 10, 
after the size reduction step, should be less than 300 microns in size. 
The extent of size reduction will be dependent on the coal type and the 
desired level of ash. An average size of 10 to 20 microns is typically 
preferred for the production of super clean coal. The coal may then be 
slurried with water, if necessary, to the desired percent coal. A typical 
slurry for carbon dioxide pretreatment might be about 50 percent coal by 
weight. 
The coal/water slurry is then passed to a pressure vessel for the carbon 
dioxide gas treatment or preconditioning as shown at block 14. The process 
can be carried out either batch wise or continuous. In a batch process, 
the slurry would be charged (pumped) to the vessel which would 
subsequently be pressurized with the carbon dioxide gas or perhaps a more 
economical mixture of carbon dioxide and air. In a continuous process, the 
slurry would be pumped to the vessel which is already pressurized. The 
vessel atmosphere would then be equilibrated as is known in the art. The 
conditioning step may be carried out in any suitable pressure vessel such 
as an autoclave, pressurized stirred tank or by pressure filtration. The 
specific pressure depends on the type of coal and must be determined by 
experiment as was discussed previously in reference to FIG. 1. Generally, 
the pressure required to achieve this effect would not exceed about 100 
psia, and a pressure of about 50 psia is believed suitable. The time for 
the conditioning step is expected to be five to fifteen minutes, but again 
this time period can be refined by experiment with the particular type of 
coal in a manner similar to that discussed previously in regard to the 
experimental results. 
Following conditioning, step 14, the carbon dioxide treated product is 
combined with additional water, if needed, shown at 15, and transferred to 
a flotation cell or cells as represented by 16. The slurry is preferably 5 
to 10 percent coal which is established as a compromise between capacity 
and separation efficiency. In flotation cell(s) 16, conventional frother 
and collectors, indicated on the diagram as reagents, are added as needed 
following standard known procedures. The carbon dioxide used in the froth 
flotation step of this invention supplements the preconditioning step. 
This is, it is important for maximizing the benefits of the 
preconditioning step to use carbon dioxide at this step. Thus, carbon 
dioxide saturation, using gaseous carbon dioxide or even dry ice, may be 
preferred, but is not required. Air could be substituted. Also, carbon 
dioxide gas is also preferably added during the flotation step itself. The 
carbon dioxide the flotation step is used at conventional flow rates as is 
well known in the art. Carbon dioxide may also be used in the form of dry 
ice in the flotation step or be used to carbonate the flotation reagents 
prior to use. Clean coal shown at 18 floats and is recovered by standard 
methods. 
There are many variations which may be practiced within the scope of this 
invention. The process is applicable to various grades and types of coal. 
For example, this process may also be used for oxidized coal. 
The process of this invention has many advantages. Chiefly, this process 
permits an enhanced degree of coal cleaning; that is, "super" clean coal, 
new product with good market potential, may be produced by this method. 
Further, the rate at which the clean coal is produced is increased, 
thereby increasing the efficiency of the process. Lastly, reagent demand 
can be reduced. 
Having now illustrated and described the invention, it is not intended that 
such description limit the invention, but that the invention be limited 
only by a reasonable interpretation of the appended claims.