Coal conditioning process

A process for improving the efficiency of coal dewatering and deashing uses complexing agents such as sodium gluconate in alkaline solution to form soluble compounds with cations such as Al.sup.+3, Fe.sup.+3, Mg.sup.+2, and Ca.sup.+2. The formation of such compounds reduces the adsorption and/or desorbs clay slimes from the coal surface.

BACKGROUND OF THE INVENTION 
I. Field of the Invention 
Many commercially important coal measures contain substantial proportions 
of ash. Almost all uses of coal can be accomplished more efficiently by 
reducing the moisture and ash content of the coal. Especially where the 
coal is used for generation of heat or electrical power, the moisture 
content should be reduced to the lowest practical level, and the higher 
heating value should be as high as possible. Reductions in moisture and 
ash content produce corresponding increases in the higher heating value of 
the coal. 
My invention relates to a novel chemical coal conditioning process which 
uses complexing agents such as sodium gluconate in alkaline solution to 
form soluble compounds with cations such as Al.sup.+3, Fe.sup.+3, 
Mg.sup.+2, and Ca.sup.+2. I have found that the formation of such 
compounds reduces the adsorption and/or desorbs clay slimes from the coal 
surface. In a preferred embodiment of my invention, deashing and 
dewatering is further enhanced by addition of surfactants such as those 
disclosed in my co-pending application, Ser. No. 8-047787 filed Apr. 15, 
1993, the disclosure of which is incorporated herein by reference. 
II. Description of the Prior Art 
Some information is available concerning the chemistry of clay/coal 
interaction. In Burdon, R. G.; Booth, R. W. and Mishra, S. K., "Factors 
Influencing the Selection of Process for the Benificiation of Fine Coal", 
Proceedings of the 7th International Coal Preparation Congress, Sydney, 
N.S.W., Australia, Paper E.1 (1976), for example, a theoretical discussion 
of the mechanisms of clay adsorption on the organic constituents of coal 
is presented, and it is hypothesized that Fe.sup.+3 and Al.sup.+3 cations 
play a role in promoting such adsorption. Eamer, B. J., "Surface Chemical 
Treatment of Fine Coal With Slime Problem", M. P. Appl. Sc. Thesis, 
Western Australian Institute of Technology (1981) and Mishra, S. K. and 
Eamer, B. T., "Effect of Clay Slimes On Flotation Behavior of Coal", Paper 
presented at the Fine Particle Society Conference (1985) include similar 
discussions. 
Conventional dewatering and deashing processes may use a rotary drum vacuum 
filter or comparable device to remove ash and water from slurries of fine 
coal. A variety of chemical pretreatments have been disclosed to improve 
the performance of these process steps. U.S. Pat. No. 5,192,338 (Waugh), 
for example, discloses a treating process using serial treatment with 
aqueous solutions of citric acid and glycerol at elevated temperature for 
a period of 30 to 45 minutes to beneficiate coal. Mention is made of the 
use of "organic complexing agents capable of complexing with metal 
cations" (col. 3, lines 38-39 and 51-56), but the only reagents disclosed 
in that category are ethylene diamine tetraacetic acid and its disodium 
salt; 8-hydroxyquinoline and mercaptoethanol. 
U.S. Pat. No. 5,089,142 discloses using sodium hexametaphosphate to control 
slime formation in centrifugal dewatering processes. 
Some investigators, as in U.S. Pat. No. 4,231,868 (Wang, et. al.), have 
suggested adding sulfosuccinate surfactants to slurries of fine coal in 
order to improve dewatering efficiency. A similar approach was suggested 
in U.S. Pat. No. 4,985,162 (Cole). Other researchers also have suggested 
using sulfur and nitrogen-based compounds, as in U.S. Pat. No. 4,897,201 
(Yamomoto, et. al.). The introduction of sulfur-containing surfactants 
into the coal/water slurry, however, requires application rates 
substantially higher than those used in my invention. Such surfactants 
also cause undesirable foam formation downstream of the filtration 
operation that interferes with subsequent coal processing. Therefore, 
additional anti-foaming agents may be required. The addition of sulfur to 
the processed coal is also undesirable from an air pollution standpoint. 
Still other investigators have suggested the addition of other reagents to 
the coal/water slurry. A quaternary amine surfactant was suggested in U.S. 
Pat. No. 4,892,663 (Keys). U.S. Pat. No. 4,290,897 discloses the addition 
of organopolysiloxanes and U.S. Pat. No. 4,447,344 (Roe) discloses a 
variety of ethoxylated alcohols. 
None of these references, alone or in combination, suggests that the 
effectiveness of the coal cleaning processes can be improved by using any 
complexing agents in basic solution, or by following such treatment with 
the addition of surfactants such as sodium laureth-13 carboxylate salt in 
aqueous solution in the form of a foam. 
SUMMARY OF THE INVENTION 
Improving the effectiveness of specific gravity separation of clay slimes 
from coal appears to depend upon reducing or preventing the adsorption of 
the clay slimes onto the organic matrix of the coal itself, and/or 
desorbing the clay slimes from the coal surface. I have found that the 
extent to which clay slimes adsorb onto the coal particles can be reduced 
and/or desorbed by treatment with certain complexing agents; most 
preferably, with sodium gluconate at low concentration in basic solution. 
Such pretreatment can be followed advantageously with the addition of a 
foam made from aqueous solutions of surfactants, preferably sodium laureth 
13 carboxylate, to the filter cake which is formed during a subsequent 
dewatering operation. The combination has been found to provide more 
effective deashing and dewatering than is obtainable by the use of either 
an organic complexing agent alone, or by means of a surfactant alone. 
Accordingly, one object of my invention is to provide a coal dewatering and 
deashing process that uses sodium gluconate or related molecules in a 
basic solution substantially at room temperature to react with various 
cations, forming water-soluble complexes that prevent the adsorption 
and/or desorb clay slimes from the organic coal matrix, thereby improving 
the efficiency of the coal cleaning process. 
Another object of my invention is to provide a coal dewatering and deashing 
process that avoids the use of sulfur-containing surfactants by applying a 
foamed chemical dewatering agent such as sodium laureth-13 carboxylate 
salt directly to the permeable cake following pretreatment with a 
complexing agent in a basic solution, to achieve significant ash and 
moisture content reductions.

DETAILED DESCRIPTION OF THE INVENTION 
In the process of my invention, coal is washed in a conventional coal 
washing facility, which typically segregates the coal based on particle 
size. Fine coals are separated from coarse coals and continue through the 
washing process. Fine coal of -28 mesh typically is further washed using a 
froth flotation step followed by vacuum filtration of the overhead from 
the froth filtration operation. 
My improvements comprise: (1) the addition to the wash water of a basic 
(alkaline) aqueous solution of a complexing agent capable of complexing 
with the cations that are naturally present; and (2) the application to 
the surface of a permeable cake of a dewatering foam made from aqueous 
solutions of carboxylic acid or carboxylate salts according to the process 
of my copending application Ser. No. 8-047787 filed Apr. 15, 1993, the 
disclosure of which is incorporated herein by reference. Complexing agent 
addition alone improves the results of conventional deashing and 
dewatering operations; still further improvement is obtained by complexing 
agent addition followed by the use of a dewatering foam. 
More specifically, I have found that the following complexing agents can be 
used to improve the efficiency of dewatering, ash removal and increase the 
yield of clean coal: gluconic acid, glucaric acid, gulonic acid, 
glucoheptonic acid and glucuronic acid as well as their sodium and 
potassium salt forms. It should be understood that other additional 
reagents could be added to promote the deashing of the coal. Such other 
promoter reagents include: ethylene diamine tetraacetic acid (EDTA), 
pyrophosphate, hexameta phosphate, hydroxy ethylidene diphosphonic acid, 
amino methylene tri phosphonic acid, phosphonobutane tricarboxylic acid, 
hexa methylene diamine tetra phosphonic acid, polyacrylic acids, 
polymethacrylate, acrylate-acrylamide copolymer and maleic anhydride 
copolymer. 
The concentration of complexing agent following its addition to the wash 
water should be between about 50 and 5000 lbs of active complexing agent 
per one million lbs of wash water. The most preferred complexing agent 
concentration corresponds to a loading of approximately 500 lbs of active 
complexing agent per one million lbs of wash water. The amount of 
complexing agent required depends upon the specific complexing agent used 
the coal chemistry, the hardness of the wash water, and the solution pH. 
However, the specific application can be optimized by using simple 
laboratory testing procedures. 
At the time the complexing agent is added, the pH of the wash water should 
be adjusted to between about 7.0 and 12.0, and most preferably about 9.0 
by addition of suitable bases (e.g., NaOH or KOH). 
The complexing reactions will be sufficiently complete within about 5 
minutes from the time the coal contacts the wash water. Reaction times can 
be further shortened by altering concentration and/or pH. 
In the preferred embodiment of my invention, the next processing step is 
continued washing and separation of coal particles based on particle size. 
Finer particles (minus 1/4 inch) are separated from coarse particles for 
further processing and/or dewatering. Typically, coal particles ranging in 
size from minus 1/4 inch to plus 28 mesh are next dewatered using modified 
screen bowl centrifuges. The minus 28 mesh coal is further washed using a 
froth flotation cell. The froth removed from the top of the froth 
flotation cell is then dewatered via rotary drum vacuum filtration. The 
effectiveness of dewatering using centrifugal dryers and rotary vacuum 
systems can be enhanced by using the dewatering foam technique described 
in my copending application Ser. No. 8-047787 filed Apr. 15, 1993. Any of 
the dewatering foams disclosed in that application can be used in the 
process of this invention following the complexing and froth flotation 
steps. 
Laboratory tests were performed to determine quantitatively the performance 
of certain specific embodiments of my invention. The examples are solely 
illustrative and do not restrict the scope of my invention. 
EXAMPLE 1 
About 2750 ml. of aqueous solution containing approximately 1000 ppm of 50 
wt % gluconic acid was prepared. Its pH was raised to about 9.0 by adding 
NaOH. Approximately 250 grams of -28 mesh bituminous Elkhorn no. 2 coal 
was added. The slurry was mixed and aerated for 5 minutes. The duration of 
the mixing was selected to duplicate the residence time associated with 
actual froth flotation cell used at coal processing facilities. After 5 
minutes, about 2 ml. of kerosene was added to the slurry and the froth was 
removed using a collection header attached to a vacuum system. The 
collected froth was dewatered using a standard vacuum filtration system. 
A blank was run under the foregoing conditions without gluconic acid. The 
results of these test were: 
______________________________________ 
% ash dry basis 
______________________________________ 
Blank 9.36 
Treated 6.34 
______________________________________ 
Thus, the washed coal that was exposed to gluconic acid treatment had about 
56% less ash. 
EXAMPLE 2 
Two blanks and a treated sample were prepared using the procedure of 
Example 1. Residue from the bottom was collected and analyzed for ash 
content. The results were: 
______________________________________ 
% ash dry basis 
______________________________________ 
Blank 1 24.86 
Treated 35.54 
Blank 2 25.71 
______________________________________ 
The increased ash content of the residue further demonstrates that more of 
the organic matrix of the coal was liberated from the ash, thereby 
decreasing the ash content and increasing the yield of the washed coal. 
EXAMPLE 3 
A blank and a treated sample were prepared as described in Example 1. The 
collected coal removed as froth was then dewatered using a Buchner funnel 
system controlled at a 15" Hg vacuum for 1 minute. Approximately 0.45 lbs 
of foamed dewatering agent (specifically, sodium laureth 13 carboxylate) 
per ton of dry coal was applied as a foam at a 10:1 expansion ratio to 
both the blank and the gluconic acid-treated sample using the process 
described in my copending application Ser. No. 8-047787. The results were: 
______________________________________ 
% moisture 
______________________________________ 
Blank 35.3 
Blank with dewatering foam 
30.1 
Treated sample 32.5 
Treated sample with dewatering foam 
18.6 
______________________________________ 
Thus, the addition of pretreatment with gluconic acid complexing agent to 
the process of my copending application Serial No. 8-047787 produces a 
further 43% improvement in dewatering efficiency. 
It will be apparent to those of ordinary skill in the art that changes and 
modifications could be made while remaining within the scope of my 
invention. For example, other foamed dewatering agents could be used, as 
disclosed in my copending application Ser. No. 9-047787 It is my 
intention, therefore, to cover all such equivalent processes, and to limit 
my invention only as specifically set forth in the following claims.