Method to minimize viscosity and improve stability of coal-water fuels

A coal aqueous slurry having a lower viscosity and resistance to thickening with time is provided. The coal aqueous slurry includes water, particulate coal comprising carbonaceous material and water-soluble minerals, a surfactant present in the coal aqueous slurry in an amount sufficient to disperse the particulate coal in the water and a chelating agent. The included chelating agent is capable of forming a chelation complex with the metal ions formed by the water-soluble minerals dissolving into the water. The presence of the chelating, or complexing, agent reduces coagulation of the coal particles, because the chelating agent complexes soluble mineral matter and thereby prevents collapse of the protective water layer surrounding the coal particles. The low viscosity of the coal aqueous slurry is maintained as a function of time by the chelating agent since, as the mineral matter present on the surface of the coal slowly dissolves into the aqueous medium, the chelating agent present in the slurry continues to form complexes therewith. This prevents the newly dissolved mineral matter from disturbing or collapsing the coal particles' protective water layer and therefore prevents a coagulation of the particulate coal during storage of the coal aqueous slurry.

The present invention is related to coal-water fuels and, more 
particularly, is directed to a coal-water fuel not only having low 
viscosity, but also having improved resistance to thickening during 
extended periods of storage. Also encompassed by the present invention is 
a method of forming a coal-water fuel having low viscosity which is 
resistant to thickening and a method to minimize the viscosity and improve 
the stability of coal-water fuels. 
It has been widely recognized that coal-water fuels, being a dispersion of 
coal and water having a sufficient amount of coal therein to be suitable 
for use as a combustion source, have inherent problems which include 
unwanted sedimentation, high viscosity and can exhibit the undesirable 
characteristic of increasing viscosity during storage. The instability 
with time is, of course, of significant importance if the coal-water fuel 
is to be transported or stored for extended periods and maintain favorable 
combustion characteristics. 
These disadvantages have prevented extensive use of the world's abundant 
supply of coal in coal-water slurries useful as a fuel. 
To be commercially acceptable on a large scale for combustion, coal-water 
fuels should be storable and easily transported to combustion sites. The 
feasibility of long-term storage, as well as the cost of such storage, and 
transportation of coal-water fuel is directly related to the resistance to 
flow of the coal-water fuel, as well as its ability to maintain a uniform 
composition with consistent rheological properties over a period of time. 
Many additives have been utilized in coal-water fuels in an attempt to form 
stable dispersions of coal in water. While many of these have achieved 
some success in obtaining coal-water fuels with low viscosity and high 
shear stability upon mixing, these properties are often not always 
consistent with time. 
It has been found that coal-water fuels may thicken during extended periods 
under shear, hence, during long periods of high shear pumping, thickening 
of the slurry causes significant problems. 
In addition, coal-water fuels will thicken without the application of shear 
forces during storage with such thickening being a function of time. For 
example, many coal-water slurries thicken to a nonfluid state after only a 
few days of storage and are thereafter impossible to pump. 
Well known is the fact that coals contain a variety of mineral matter and 
such mineral matter leaches into the water in which the coal is dispersed 
over extended periods of contact therebetween. 
Effective dispersion or suspension of coal in water in coal-water fuels 
generally requires that the coal particles be completely surrounded and 
protected from contacting one another and researchers have tried many 
dispersing agents and surfactants to achieve such dispersions. 
However, because of the enormous surface area presented by fine particles 
in the coal-water suspension, there is a large exposure of mineral matter 
present on the surface of the coal. When the mineral matter dissolves into 
the aqueous media, flocculation of the coal particles is promoted through 
collapsing of the protective water layer between the particles. 
As an example, it is well known that alumina Al.sup.+3 Ferric Fe.sup.+3 and 
calcium Ca.sup.+2 can coagulate coal in settling ponds. 
The kinetics of the solution of the mineral matter is slow, hence, there is 
a continuing change in the concentration of metal ions over an extended 
period of time which can cause a continual aggregation of the coal. 
SUMMARY OF THE INVENTION 
In accordance with the present invention, a coal aqueous slurry having 
lower viscosity and resistance to thickening with time consists 
essentially of water, particulate coal comprising carbonaceous material 
and water-soluble minerals, a surfactant present in the coal-aqueous 
slurry in an amount sufficient to disperse the particulate coal in water, 
and a chelating agent capable of forming a chelation complex with metal 
ions formed by the water-soluble minerals dissolving into the water. 
The presence of the chelating, or complexing, agent reduces coagulation of 
the coal particles because the chelating agent complexes soluble mineral 
matter, thereby preventing collapse of the protective water layer 
surrounding the coal particles. 
More particularly, in accordance with the present invention, the chelating 
agent may comprise an aminocarboxylic acid, or a hydroxycarboxylic acid, 
which is capable of forming complexes with Al.sup.+3, Fe.sup.+3, Mg.sup.+2 
and Ca.sup.+2 ions. 
Preventing flocculation of the coal particles lowers the viscosity of the 
slurry. In addition, this low viscosity is maintained as a function of 
time since as the mineral matter present on the surface of the coal slowly 
dissolves into the aqueous media, the chelating agent present in the 
slurry continues to form complexes therewith, thereby preventing the newly 
dissolved mineral matter from disturbing or collapsing the coal particles' 
protective water layer and causing a coagulation of the particulate coal. 
It has been found that a chelating agent may be selected from the group 
consisting of ethylenediaminetetraacetic acid, 
hydroxyethylethylenediaminetriacetic acid, nitrilotriacetic acid, 
N-dihydroxyethylglycine and ethylenebis(hydroxyphenylglycine). 
In addition, it has been unexpectedly discovered that the use of a 
chelating agent, such as ethylenediaminetetraacetic acid, reduces the 
amount of surfactant that otherwise may have to be used if the chelating 
agent were not present. 
If a polymeric stabilizer is incorporated into the coal-aqueous slurry, the 
presence of the complexing, or chelating, agent prevents cross-linking of 
the polymeric stabilizer and, thereby, maintains slurry fluidity through 
extended periods of storage. 
It has been found that in accordance with the present invention that the 
addition of a chelating agent is effective in preventing the thickening of 
coal slurries over a period of time when the coal slurry includes a first 
surfactant having a hydrophilic portion comprising a relatively large 
number of ethyleneoxide units, and a second surfactant having a 
hydrophilic portion comprising of a relatively small number of 
ethyleneoxide units compared to the first surfactant. In this type of 
slurry, the first and second surfactants are present in the coal-aqueous 
slurry that disperse the particulate coal in the water. 
The first surfactant may have a hydrophilic portion comprising between 
about 40 and 150 ethyleneoxide units and the second surfactant may have a 
hydrophilic portion comprising between about 4 and 40 ethyleneoxide units, 
with the first surfactant having a large number of ethyleneoxide units 
relative to the second surfactant. 
Other chelating agents such as one selected from the group consisting of 
tartaric acid, citric acid, gluconic acid, and 5-sulfosalicyclic acid, may 
be used in the present invention. 
A method in accordance with the present invention for forming a coal-water 
fuel includes the steps of admixing particulate coal including 
carbonaceous material and water-soluble minerals with ingredients 
consisting essentially of water, a surfactant for dispersing the 
particulate coal in the water, and a chelating agent capable of forming a 
chelation complex with the metal ions formed by the water-soluble minerals 
dissolving in the water. 
Also included in the present invention is a method for improving stability 
of coal-water fuels against thickening with time. This method includes the 
step of admixing a coal-aqueous slurry comprising particulate coal 
including carbonaceous material and water-soluble minerals, water and a 
surfactant with a chelating agent capable of forming a chelation complex 
with metal ions formed by the water-soluble minerals dissolving in the 
water over a period of time. 
DETAILED DESCRIPTION 
The thickening of coal-water fuels over a period of time is caused, to a 
large extent, by the solvation of mineral matter present on the surface of 
the coal which causes flocculation of the coal particles and increases the 
viscosity of the coal-water fuel. 
The present invention utilizes a chelating agent to form complexes with the 
metal ions formed. 
Within the meaning of this invention, a chelating agent is a compound 
containing donor atoms that can combine by coordinate bonding with a metal 
atom to form a cyclic structure, called a chelation complex, or chelate. 
Many chelating agents are known, and a number of them including 
aminocarboxylic and hydroxycarboxylic are shown in the hereinafter-recited 
examples to be effective in forming metal ion complexes in coal-water 
fuels to reduce viscosity and prevent thickening with time thereof in 
accordance with the present invention. 
Preferably, the method in accordance with the present invention utilizes an 
ethylenediaminetetraacetic (EDTA) and/or citric acid. 
Heretofore, simple salts such as sodium chloride and potassium chloride 
have been incorporated in various coal-water formulations in order to 
reduce slurry viscosity. However, they are ineffective in preventing 
continual increasing of the slurry viscosity because they are unable to 
form complexes with the dissolving mineral matter. 
It is believed that this invention is the first discovery of the use of 
chelating agents in coal-water fuels in order to provide an instant 
reduction of viscosity and a continuing way of preventing solvated 
minerals in the coal particles from causing flocculation of the coal, 
which results in viscosity increases of the coal-water fuel over extended 
periods of time. 
A number of surfactants may be utilized to disperse or suspend the coal 
particles in water, a particularly useful dispersant is polyalkyleneoxide 
nonionic surfactant such as commercially available glycol ethers of the 
general formula: 
EQU R--O--(CH.sub.2 CH.sub.2 O).sub.n --CH.sub.2 --CH.sub.2 --OH 
wherein R is substituted or unsubstituted alkyl of from 1 to 18 carbon 
atoms, substituted or unsubstituted aryl, or an amino group and n is an 
integer from about 10 to about 100. 
Of particular advantage is the utilization of a blend of nonylphenoxy 
polyethyleneoxide surfactants as described in U.S. Pat. Ser. No. 515,829, 
Filed July 21, 1983 and which is totally incorporated herein by specific 
reference thereto. 
The preferred surfactant is a blend of nonylphenoxy polyethyleneoxide 
surfactants having about 100 ehtylene units and a nonylphenoxy 
polyethyleneoxide surfactant having about 10 ethyleneoxide units. 
In the past, the type of coal which could be successfully formulated into 
coal-water fuels was limited in part because of the problems which result 
from the presence of mineral matter and the subsequent solvation thereof 
into the water. 
In fact, many investigators have found it necessary to provide a complex 
coal preparation process by which much of the mineral matter is removed. 
It has now been found that the addition of a small amount of chelating 
agent such as EDTA complexes soluble minerals such as aluminum, iron and 
calcium, do prevent thickening of the slurry with time. 
Surprisingly, however, the addition of a small amount of chelating agent 
also significantly reduces the viscosity of the resulting coal-water fuel 
and can significantly reduce the amount of surfactant necessary to 
disperse, or suspend the coal in the water. 
It has also been found that the presence of a complexing agent can prevent 
cross-linking of polymeric stabilizers added to the coal-water fuel to 
further maintain slurry fluidity during extend periods of storage. 
A wide variety of suitable coals may be used in the present invention, such 
as anthracite, high and low volatile bituminous, sub-bituminous, mine 
tailings and fines. 
The following are specific examples and preferred embodiments of the 
present invention. There is no intention that the claims be limited 
thereto, since many variations are within the skill of the art. 
For instance, while the chelating agent may be used in conjunction with one 
or more surfactants in a coal fuel, two surfactants were used in many of 
the following examples for illustrative purposes only.

EXAMPLE 1 
Water was mixed with two surfactants, namely, a nonylphenoxy 
polyethyleneoxide having about 100 ethylene units (molecular weight about 
4680) (IGE CO990, GAF Corporation, New York) and a nonylphenoxy 
polyethyleneoxide surfactant having about 10 ethyleneoxide units 
(molecular weight about 682) (IGE CO660, GAF Corporation, New York) 
along with an anti-foam agent such as Foamaster and Ucarcide until all the 
material is well dispersed. Coal ground to approximately 80% less than 200 
mesh was then added to the solution and the mixture was agitated at 1900 
RPM for 15 minutes. 
To this slurry, a polymeric stabilizer such as xanthan gum was added and 
the mixture was agitated for another 15 minutes at 1900 RPM. The 
stabilizer may comprise from about 0.01 to about 0.1 percent by weight of 
the coal-water fuel, although 0.075 percent by weight is specifically 
recited in this example. Weight percentages of the resulting coal-water 
fuel are shown in Table 1. 
TABLE 1 
______________________________________ 
Component Weight Wt. % 
______________________________________ 
Coal 350 g 70.0 
CO990 (surfactant) 2.0 0.4 
CO660 (surfactant) 0.5 0.1 
Foamaster (antifoam) 
0.5 0.1 
Ucarcide (preservative) 
1.0 0.2 
Xanthan Gum (121/2%) 
2.88 0.075 
(stabilizer) 
Water 143.12 29.1 
______________________________________ 
Viscosities were measured using a Haake RV-2 Rotational Viscometer, using 
an M500 measuring head and an MV-III rotor at a shear rate of 110 
sec.sup.-1. 
Slurries were then prepared as hereinabove-described except that a 
viscosity modifier was incorporated into the slurry. Table 2 shows the 
weight percentage of viscosity modifier added and the resulting viscosity 
of the coal-water slurry. Without the addition of a viscosity modifier, 
the slurry exhibited a viscosity of about 1124 cp (row one). 
With the addition of 0.15 wt.% EDTA or 0.20 wt.% citric acid as chelating 
agents, the viscosity of the slurry was reduced to 806 cp and 901 cp 
respectively. This significant reduction in viscosity did not occur when 
non-chelating modifiers such as NaCl or CaCl.sub.2 were incorporated into 
the slurry, in fact, the viscosity of the slurry increased as a result of 
incorporating NaCl or CaCl.sub.2. 
TABLE 2 
______________________________________ 
Coal wt. % 
Viscosity Viscosity cp 
Solids 
(10% Ash) 
Modifier wt. % 
(110 Sec) Content wt. % 
______________________________________ 
70 -- 1124 70.75 
70 .15 EDTA 806 70.90 
70 .15 NaCl 1134 70.90 
70 .15 CaCl.sub.2 
1246 70.90 
70 .20 Citric Acid 
901 71.33 
______________________________________ 
The viscosity is, of course, dependent on the solids content, with the 
viscosity being higher as the solids content increases. Bearing this in 
mind, it can be easily seen from Table 2 that the addition of 0.15 weight 
percent EDTA still reduces the viscosity of the coal-water fuel (without 
the addition of EDTA) from 1124 cp to 806 cp. That is, a reduction of 
about 28%, while the solids content is actually increased from 70.75 wt.% 
to 70.90 wt.%. 
The addition of 0.20 weight percent citric acid also significantly reduces 
the viscosity of the resulting coal-water fuel to 901 cp while the solids 
content increased from 70.75 wt.% to 71.93 wt.% due to water loss during 
slurry preparation. 
The weight percent of the surfactants utilized in the herein presented 
Example may vary in accordance with the ranges taught in U.S. patent 
application Ser. No. 515,829, Filed July 21, 1983, namely, the CO990 may 
be present in an amount from about 0.1 percent to about 3.0 percent by 
weight of the coal-water fuel and the CO660 may be present in an amount 
from about 0.01 to about 1.5 percent by weight of the coal-water fuel. 
EXAMPLE 2 
To determine the effectiveness of EDTA in decreasing the viscosity of 
coal-water fuels as a function of the amount of EDTA added, various 
slurries were made as hereinbefore set forth in Example 1 except varying 
the EDTA content from about 0.05 weight percent to about 0.20 weight 
percent. The results of viscosity measurements on these slurries are shown 
in Table 3. 
TABLE 3 
______________________________________ 
Coal wt. % Viscosity cp 
Solids 
(10% Ash) 
EDTA wt. % (110 Sec) Content wt. % 
______________________________________ 
70 -- 1186 71.16 
70 .05 1021 71.17 
70 .10 983 71.19 
70 .15 896 71.20 
70 .20 911 71.22 
______________________________________ 
It is apparent from FIG. 2 that as little as 0.05 weight percent EDTA 
significantly reduces the viscosity of the resulting coal-water fuel. 
EXAMPLE 3 
The presence of complexing agent also significantly reduces the amount of 
surfactant necessary to suspend or to disperse the coal in the water. 
Slurries were prepared as hereinbefore set forth in Example 1 with a 
reduced total amount of surfactant as shown in Table 4. 
TABLE 4 
______________________________________ 
Coal wt. % 
Surfactant Stabiliz- 
Viscosity cp 
(6% Ash) 
wt. % EDTA wt. % er wt. % 
(110 sec.sup.-1) 
______________________________________ 
70 .55* -- .075 1802 
70 .40** .10 .075 1599 
______________________________________ 
*CO990 0.40%; CO660 0.15% 
**CO990 0.30%; CO660 0.10% 
As can be seen in Table 4, the use of 0.01 percent EDTA reduces the total 
amount of surfactant necessary to suspend 70% coal in water from about 
0.55 weight percent to about 0.40 weight percent. 
The following example demonstrates the important feature of the present 
invention in improving stability of coal-water fuels against its viscosity 
increasing with time. 
EXAMPLE 4 
Water was mixed with a nonylphenoxy polyethyleneoxide having about 100 
ethylene units (molecular weight about 4680) (IGE CO990, GAF 
Corporation, New York) along with an anti-foam agent such as Foamaster and 
Ucarcide until all the material is well dispersed. Coal ground to 
approximately 80% less than 200 mesh was then added to the solution and 
the mixture was agitated at 1900 RPM for 15 minutes. 
To this slurry, a polymeric stabilizer such as xanthan gum (Flocon 4800C) 
was added and the mixture was agitated for another 15 minutes at 1900 RPM. 
The stabilizer may comprise from about 0.01 to about 0.1 percent by weight 
of the coal-water fuel, although 0.075 percent by weight is specifically 
recited in this Example. Weight percentages of the resulting coal-water 
fuel (Slurry A) are shown in Table 5. 
TABLE 5 
______________________________________ 
(Slurry A) 
Component Wt. % 
______________________________________ 
Coal (10% Ash) 69.0 
CO990 (surfactant) 0.8 
Foamaster (anti-foam) 0.1 
Ucarcide (preservative) 
0.1 
Xanthan Gum (121/2%) (stabilizer) 
0.075 
Water 29.9 
______________________________________ 
A second coal-water fuel (Slurry B) was then made in accordance with the 
method hereinabove-described for Slurry A except a chelating agent was 
incorporated into the coal water fuel to form Slurry B, weight percentages 
being shown in Table 6. 
TABLE 6 
______________________________________ 
(Slurry B) 
Component Wt. % 
______________________________________ 
Coal (10% Ash) 68 
CO990 (surfactant) 0.8 
Foamaster (anti-foam) 0.1 
Ucarcide (preservative) 
0.1 
Xanthan Gum (121/2%) (stabilizer) 
0.075 
EDTA (chelating agent) 
0.19 
Water 30.7 
______________________________________ 
Slurry A and B were allowed to stand and were observed over a period 
sufficient to determine the time necessary for Slurry A to thicken from a 
fluid state to a non-fluid state. As shown in Table 7, Slurry A (without a 
chelating agent) thickened from a fluid to a non-fluid in seven days while 
Slurry B (with EDTA chelating agent) did not exhibit any thickening. 
TABLE 7 
______________________________________ 
Chelating Appearance 
Slurry Agent 1 Day 2 Days 7 Days 
______________________________________ 
A No Fluid Thick Fluid 
Non-Fluid 
B Yes Fluid Fluid Fluid 
______________________________________ 
EXAMPLE 5 
Coal-water fuels may be made in accordance with Examples 1-4 herein except 
that instead of EDTA or citric acid as a chelating agent, tartaric acid, 
gluconic acid or 5-sulfosalicylic acid may be used as chelating agents 
with similar results as shown in Examples 1-4. 
Although there has been hereinabove-described a specific coal-water fuel, 
its method of manufacture and method for improving the stability of 
coal-water fuels against thickening with time and reducing the viscosity 
of the resulting slurry, all in accordance with the invention for the 
purposes of illustrating the manner in which the invention may be used to 
advantage, it would be appreciated that the invention is not limited 
thereto. Accordingly, any and all modifications, variations or equivalent 
arrangements which may occur to those skilled in the art should be 
considered to be within the scope of the invention as defined in the 
appended claims.