UNITED STATES BUREAU OF MINES

UNITED STATES DEPARTMENT OF THE INTERIOR

RI 9585 REPORT OF INVESTIGATIONS/1995

Gel Sealants for the Mitigation of Spontaneous

Heatings in Coal Mines
U.S. Department of the Interior

Mission Statement

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also has a major responsibility for American Indian reservation

communities and for people who live in island territories under U.S.

administration.
Report of Investigations 9585

Gel Sealants for the Mitigation of Spontaneous

Heatings in Coal Mines

By Yael Miron 

UNITED STATES DEPARTMENT OF THE INTERIOR

Bruce Babbitt, Secretary

BUREAU OF MINES

Rhea Lydia Graham, Director
International Standard Serial Number

ISSN 1066-5552
CONTENTS

Page

Abstract ........................................................................................... 1

Introduction ........................................................................................ 2

Sealants ........................................................................................... 4

Bentonite-water mixtures ........................................................................... 4

Silicate gels ...................................................................................... 4

Guar gum gels ................................................................................... 6

Preparation methods of guar gum gels ................................................................... 6

Electrolytes ...................................................................................... 6

Ingredient proportions and ambient conditions ......................................................... 6

Inhibition of bacteria and mold formation ............................................................. 6

Optimal gel mixtures .............................................................................. 7

Effects of coal, rock dust, and mine floor materials ...................................................... 7

Mixing procedures ................................................................................ 7

Batch scale and mine tests ............................................................................. 7

Gels in tubs ...................................................................................... 7

Gels in the mine floor ............................................................................. 8

Continuous mixing preparation ........................................................................ 8

Eductors ........................................................................................ 8

Gunite machine .................................................................................. 9

Commercial mixing units ........................................................................... 9

Additional modifications ........................................................................... 9

Cost of gel ......................................................................................... 9

Summary and recommendations . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 10

Acknowledgments . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 10

References . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 10

ILLUSTRATIONS

1. Freshly prepared gel being poured into a jar containing rocks and coal dust . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 12

2. Appearance of gel after 1 week in a jar . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 12

3. Two plastic tubs with coal pieces and rock dust placed next to the coal rib while dry ingredients with

ethylene glycol are being mixed prior to the addition of water . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 13

4. Mixing of all ingredients with a wooden paddle to form a gel . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 13

5. Freshly prepared gel is poured into a tub . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 14

6. Gel in tub in underground mine . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 14

7. Freshly prepared gel in pit in mine floor . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 15

8. Appearance of gel after 3 months in pit in mine floor . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 15

9. Small eductor arrangement . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 16

10. Large eductor arrangement . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 16

TABLES

1. Experimental silica gel formulations ................................................................ 5

2. Guar gum gel formulations ....................................................................... 7

3. Cost of ingredients to prepare guar gum gel .......................................................... 9
UNIT OF MEASURE ABBREVIATIONS USED IN THIS REPORT

Metric Units

cm centimeter m meter

g gram mL milliliter

kg kilogram EC degree Celsius

L liter

U.S. Customary Units

gal gallon pct percent

lb pound s second
 Chemical engineer, Pittsaburgh Research Center, U.S. Bureau of Mines, Pittsburgh, PA 1 1

GEL SEALANTS FOR THE MITIGATION OF SPONTANEOUS HEATINGS 

IN COAL MINES

By Yael Miron1

ABSTRACT

Coal mines that undergo self-heatings, and specifically mines that sustain both floor heave and self-heatings,

would benefit from the use of sealants to minimize air ingress to susceptible areas. The U.S. Bureau of Mines

(USBM) evaluated three sealant systems for this purpose, bentonite-water mixtures, silicate gel, and guar gum

gel. The guar gum-based gel was the only sealant that possessed the desired properties of elasticity, ease of

preparation, adaptability to the mine, and a long life. In addition, since the gel consists mostly of water, it could

quench or cool nearby heatings. Optimal formulations were determined in laboratory tests, and then tested in

the USBM Safety Research Coal Mine. Two formulations resulted in stable gels that lasted for at least 12 months

in the mine. Semicontinuous preparation of gel using an eductor was evaluated and appears feasible. Although

guar gum gel is expensive, selective use to prevent heatings in specific areas would be cost-effective, both in

terms of productivity and enhanced mine safety.
2

INTRODUCTION

Fires in underground coal mines can result in loss of life and The Warrior Basin coalbeds contain large amounts of highproperty. Between 1978 and 1988, approximately 15 pct of quality methane. Methane drainage ahead of mining is

underground coal mine fires in the United States were practiced by the mines in the area, including the No. 4 and No.

attributed to spontaneous combustion (1). These fires usually 5 Mines (7). This practice helps to reduce methane emissions 2

occur in worked out or gob areas of the mine, and are difficult at the face by as much as 60 to 70 pct. However, high

to detect. In general, low-rank coals are more susceptible to ventilation rates are prevalent throughout many of the mine

self-heating and the resultant spontaneous combustion, than areas because of the methane hazard and the depth of the

high-rank coals. However, heatings can occur in mines of mines. These high ventilation rates produce high differential

high-rank coal due to the effects of other parameters such as pressures across stoppings and seals, resulting in air leakage

mining and geological conditions and ventilation rates, among and thereby providing ample air for the oxidation of both pyrite

others. and adjacent coal. In large part, the leakage occurs at the

One such mine, mining a high-rank coal, is the Jim Walter junction of the stopping with the strata and through cracks in

Resources (JWR), Inc., No. 5 Mine located in the Black the strata. To eliminate leakage, the use of balancing chambers

Warrior Coal Basin in north-central Alabama. This mine is the is recommended for the short term; however, for the long term,

farthest southward into the basin of all four company mines injection of cement or backfill material is preferable (8).

that extract the Blue Creek Coalbed of the Mary Lee Coal

Group. The mine began to experience floor heave in 1981.

The area of the mine that experienced the floor heave is in a

zone characterized as the "expanded Blue Creek Seam" where

the coalbed consists of upper and lower benches. In this area,

the lower bench has a highly variable strata consisting of

interspersed thin layers of coal, shale, and fire clay (2). The

mine, which produces high-grade bituminous coal, is over 600

m deep. At this depth, the rock is under considerable stress

from the weight of the overlying rock.

Severe floor heave caused production delays and required

extensive and expensive ground control measures. New pillar

design improved ground control, but did not eliminate the

heave problem completely (3-4). Then in November 1986, a

heating was discovered in the floor of the mine. Since then equipment are associated with such procedures (9).

many other heatings have occurred (5). All of the heatings

have been located in a section of the mine in the vicinity of a problems as the No. 5 Mine, numerous mines encounter at least

fault line, where an "expanded Blue Creek Seam" exists a few of these problems. Additionally, some floor upheaval is

beneath the floor. Analysis of samples from the area beneath common to many mines. However, when floor heave is

the floor showed the presence of varying amounts of finely extensive and frequent, floor bolts and stiff concrete and grout

disseminated pyrite (6). Such pyrite is highly reactive and is will not hold up, but instead will crack, break, and become

easily oxidized when the ground heave exposes the material

beneath the floor to air and moisture. This oxidation is highly

exothermic in nature and, as a result, the pyrite self-heats. The

higher temperatures, in turn, accelerate the oxidation of

additional pyrite and of the carbonaceous matter in contact with

the pyrite (6). The end result is a heating, which may progress

to a fire if not detected in time. Two heatings have occurred

during 1994-95 in the floor of the adjacent No. 4 Mine. These

heatings did not occur in the expanded seam area, but did

involve pyrite.

Italic numbers in parenetheses refer to items in the list of references at the 3

end of this report.

To eliminate or reduce the rate of pyrite and coal oxidation

in the JWR No. 5 Mine, or in other mines facing similar

problems, access of air to areas enriched with pyrite or to areas

containing coal that is susceptible to spontaneous combustion,

should be minimized. Control of mine floor heave depends on

the cause and type of floor heave. Wuest (9) describes physical

characteristics and mining conditions that contribute to floor

heave, as well as procedures for control of heave. The latter

include methods of mine maintenance, supplemental support,

and mine design and layout. Supplemental support methods include the installation of floor bolts or concrete linings or the

injection of polyurethane. These are extreme measures that,

depending on the size of the affected area, can become

impractical. In addition, high costs of material, labor, and

Although few mines suffer from such an assortment of

useless. Even backfill material will be shifted away from the

area if the magnitude of upheaval is large. In such cases, the

only practical choice may be to block access of air to specific,

critical areas, and use sealants that are elastic in nature.

A review of the literature regarding the use of sealants in

mines for the purpose of reducing occurrences of spontaneous

combustion indicates the use of clay mixtures, various cements,

and grouting material. However, in most of these cases, floor

upheaval is not cited as a problem; rather, it is air leakage

around seals and through cracks in the strata that is of major

concern.
3

Bentonite clay has been used by various mines as a grouting pressurized air. However, it is not known if this method was

material. Bentonite is a montmorillonite-type clay formed by actually used by the mine. Good results were also reported for

the alteration of volcanic ash. It is highly colloidal and can the use of a mixture of tar and burnt oil to prevent spontaneous

absorb large amounts of water. Absorption of water is combustion of coal in quarry benches in India (16). However,

accompanied by a large increase in volume. Bentonite grouting

was used at the Agecroft Colliery, in the Doe Seam, in

Lancashire, United Kingdom (10). This deep mine (640 to 670

m) had to change its working horizon due to a particular pct of finely dispersed rubber in water solutions were evaluated

geological condition including a steep gradient and the in some mines. Neoprene latex emulsions were sprayed onto

appearance of very hard sandstone immediately above the coal mine surfaces in eastern Europe (14, 17). Initially, results

seam. As a result, much coal was left in the roof, and several

heatings took place. A permanent grout pumping operation

was established in the mine to afford continuous pumping into

the gob and, in combination with continuous vigilance, the

problem was contained. This mine did not sustain floor

upheaval occurrences.

The use of a sealant consisting of a mixture of Portland

cement, limestone, exfoliated vermiculite, and other ingredients, was reported some years ago for a mine in eastern

Ohio (11). This mine, one of the mines located near the edge

of the Pittsburgh No. 8 coal basin, had pyrite and a clayey shale

in the roof. Both weathered badly, especially in the moist air of

summer, and caused roof falls. Fire clay (a clay predominantly

composed of kaolinite, high alumina, or silica that forms the

seat earth of many coalbeds), present in the floor strata, also

weathered at a high rate and, via its action on the pillars,

contributed to further roof falls. The sealant, developed in the

United Kingdom, was used successfully to seal stoppings and

overcasts, as well as for fireproofing and construction.

Mixtures of clay with other ingredients were tried in coal mines

in various other countries. In the former Soviet Union (12),

slurries of clay with sodium silicate were used. In Japan (13),

clay suspensions and clay-cement mixtures were tried, but the

results were not good due to formation of cracks in the sealants.

Finally, suspensions of clay with aqueous solutions of calcium

chloride (CaCl ) were used in eastern European mines that were 2

experiencing heatings and fires due to spontaneous

combustion, with very good results reported (14).

Asphalt-based sealants were tested in some mines. Excellent

results were cited for an asphalt solution that was sprayed on

various surfaces inside a mine (13). An asphalt emulsion

consisting of 40 pct water and 60 pct semisolid oil asphalt was should be easy to prepare and cost-effective. These sealants

applied onto samples of shale from the roof of a coal mine that would also be effective in mines with a spontaneous heating

experienced roof deterioration (15). Pyrite and other minerals problem, but without floor heave. 

in the roof oxidized, and the resultant roof fall caused

spontaneous combustion. The tests in the laboratory with the

emulsion and roof material gave good results, and the mine

planned to apply the emulsion to the roof with a spray gun and

asphalt- and tar-based coatings or sealants will ignite from

contact with fires, so their safe use is questionable.

Sealants based on latex preparations consisting of 60 to 70

seemed good, but on drying the sealants shrank and did not

perform well.

Finally, sealants based on gel solutions of water glass or

other materials are also reported in the literature, usually with

good results. Gel solutions of water glass with ammonium

sulfate and water were injected into gob areas in Russian coal

mines (18). A water glass solution was used in the laboratory

to coat coal samples from a mine and was found capable of

retarding oxygen adsorption by the treated samples (19). A

pillar in the mine was then infused with the solution and the

rate of oxygen adsorption by the pillar was likewise lowered

(20). A gel-like solution, able to adhere to the roof, rib, and

floor in a mine, was prepared from a commercial mixture of

"Light Water TM" with other additives (12). The elastic gel

was applied to a thickness of approximately 1 cm to the ribs

and roof of Northern Coal Co.'s Rienaw No. 2 Mine in

Colorado, with the intent of preventing spontaneous

combustion after closing the mine. However, results were not

reported.

The cited references indicate that sealants were generally

individually and uniquely devised and prepared to fit a specific

mine's conditions and needs. The choice of a sealant depended

on various factors such as cost-effectiveness, availability of

materials, and ease of preparation, in addition to its suitability

to the need at hand. 

The U.S. Bureau of Mines (USBM) examined several

materials for sealing specific areas in a heaving mine floor,

such as near stoppings and in longwall entries. Such sealants

should be elastic in nature, have a long shelf life of at least a

few months, or for the period needed to afford safe mining, and

be stable in a mine atmosphere. In addition, such sealants

Three sealant systems were chosen for study. They were

bentonite-water mixtures, sodium silicate-based gels, and guar

gum-based gels. The work done with these sealants is

described in the following sections.
4

SEALANTS

BENTONITE-WATER MIXTURES SILICATE GELS

The bentonite used in the tests, Volclay bentonite PLS-50, A technical sodium silicate solution, or water glass

was obtained from American Colloid Co. This sodium [Na SiO @(H O) ], of 40E to 42E Bé was used in the preparation of

bentonite is chemically and polymerically treated. In its final the gels.

form it is free-flowing and has a high swelling potential with Aqueous solutions of the alkali silicates hydrolyze extensively,

water. It swells up to 13.8 times its volume in water at full as follows:

saturation, and in the process it absorbs nearly five times its

weight in water. On drying it shrinks to its original volume. O H Na i aOH.

Swelling is reversible and can be done an infinite number of

times, if the water used is fairly pure. When mixed with 7 to 10

parts water, gelatinous or creamlike pastes form. The bentonite

can be mixed with water with or without mechanical devices,

such as pumps or mixing paddles. When mixing devices are

not used, the Volclay bentonite is added slowly to a measured

quantity of water and allowed to swell and form a gel.

However, clay-bentonite water mixtures are not gels in the

strict sense of the word. They are suspensions of very fine particles but do not possess the molecular bonds that form in

proper gel structures.

Mixtures of bentonite with water, varying in concentration

from 4 to 8 pct of bentonite, were prepared and placed in either

empty glass trays or were poured into glass trays lined with

coal lumps or with coal lumps covered with thin layers of rock

dust. The glass trays were left open to the ambient atmosphere.

All the mixtures in the trays dried and cracks formed in them

within a few days. Rewetting the dried bentonite layers by

spraying them with water or by adding water to the trays, did

not result in renewed gels or creamy pastes of the original

quality. Distilled water was used in all the preparation steps.

However, electrolytes must have leached from the coal and

rock dust into the water, and these dissolved electrolytes

affected the ability of the bentonite to form gels. In the

presence of electrolytes the bentonite clay forms flocs, which

settle. The higher the valency of the cations of the electrolytes,

the stronger the effect. Repeated tests with various

concentrations of bentonite gave similar results. To slow down

the vaporization of the water, hydrated calcium chloride

(CaCl2 2 @XH O) flakes were sprinkled on top of freshly prepared

bentonite layers, after the layers were poured on the coal pieces

in the trays. The flakes dissolved in the paste, but probably

remained concentrated at the surface portions of the layers.

Calcium chloride is a highly hygroscopic salt and has been used

for the sorption of water vapor from air (21). The procedure

did slow down the dehydration process, but did not prevent it.

Based on these initial results, no additional tests were conducted with the bentonite mixtures.

2 3 2n

3

2Na Si 2 3 2 225 + O 6 S O + 2N

If the silicate solutions are acidified, mononuclear orthosilicic acid

[Si(OH) ] is liberated. It exists in equ 4 ilibrium with its conjugate

bases, SiO(OH) and SiO (OH) ( 3 2 2 22). With time, the silicic acid -1 -2

condenses to polymeric aggregates of polysilicic acid, as follows

(23):

Initially, the aggregates remain dispersed as a colloid, but finally

they set to a gel, with complete immobilization of the fluid. The

addition of electrolytes, provided the solution is not too dilute,

accelerates the gelling time, which depends on the hydrogen ion

concentration (pH) of the mixture. Silica gels rich in water are

transparent, soft, and fairly elastic. Freshly prepared gels may

contain over 200 moles of water per mole of SiO (24). 2

Various electrolytes were evaluated as gelling agents to accelerate the gelling time. They included ammonium sulfate

[(NH ) SO ], ammonium chloride (NH Cl), monoammonium or- 42 4 4

thophosphate (NH H PO ; MAP), sodium tetraborate deca 42 4 hydrate

(Na B O 24 7 2 @10H O; borax), sodium nitrate (NaNO ), and calcium 3

chloride dihydrate (CaCl@2H O), among others. Concentrations of 2 2

ingredients used in the preparation of silica gels are shown in table

1. Good gels that were soft, relatively elastic, and structurally stable formed with ammonium sulfate, borax, and MAP. With some

of the other salts either slurries formed, which rapidly precipitated,

or the solutions remained transparent and fluid. Gelation time

depended on the relative proportions of the various ingredients and

varied from 1 to about 3 min. The gels prepared with the borax

took much more time to gel. When small amounts of ethylene

glycol (CH OHCH OH) were used as an additional solvent, 2 2

gelation times were between 10 and 15 min. Another

BaumNe (BNe) gravity scale for liquids heavier than water is defined as 3

follows: Degree BaumNe = 145 -

A specific gravity, G, of 1 at 60 EF (16 EC) [relative to the specific gravity of

water at 60 EF (16 EC) corresponds to 0.0E BNe.
5

Table 1.—Experimental silica gel formulations

Electrolyte

Electrolyte

weight, mL glass, time,

g mL s

Water, Water Gelation

Comments

O ................... (NH )42 4 S 1.0 23 5.0 ~40 No mixing.

(NH ) SO . . . . . . . . . . . . . . . . . . . 42 4 1.0 25 3-4 60 None.

(NH ) SO . . . . . . . . . . . . . . . . . . . 42 4 1.0 26 3.0 65 Do.

NH NO . . . . . . . . . . . . . . . . . . . . . 4 3 1.0 27 3.0 60 Ammonia smell.

NH NO . . . . . . . . . . . . . . . . . . . . . 4 3 1.3 27 3.0 50 Do.

NH Cl . . . . . . . . . . . . . . . . . . . . . . . 4 1.0 25 3.0 NAp Slurry formed; ammonia smell.

NH Cl . . . . . . . . . . . . . . . . . . . . . . . 4 1.4 27 3.3 NAp Do.

NH H PO . . . . . . . . . . . . . . . . . . . 42 4 1.0 25 3.0 20 None.

NH H PO . . . . . . . . . . . . . . . . . . . 42 4 1.0 27 2.7-3.0 130 Do.

NH H PO . . . . . . . . . . . . . . . . . . . 42 4 1.0 30 2.5 160 Soft gel.

NH H PO . . . . . . . . . . . . . . . . . . . 42 4 1.0 30 3.0 180 Do.

NH H PO . . . . . . . . . . . . . . . . . . . 42 4 1.0 26 3.3 NA Mixed slowly; gelled slowly.

NH H PO . . . . . . . . . . . . . . . . . . . 42 4 1.3 27 3.3 110 None.

Na PO @12H O . . . . . . . . . . . . . . . 34 2 1.0 25 3.0 NAp No gelling; pH = 12.

Na PO @12H O . . . . . . . . . . . . . . . 34 2 1.3 27 3.3 NAp Do.

NaNO ...................... 3 1.0 25 3.0 NAp No gelling.

CaCl @2H O . . . . . . . . . . . . . . . . . . 2 2 1.0 25 3.0 NAp Slurry formed; pH = 8-9.

CaCl @2H O . . . . . . . . . . . . . . . . . . 2 2 0.5 25 3.0 NAp Do.

Na B O @10H O . . . . . . . . . . . . . . . 24 7 2 1.0 25 3.3 NA Slow gelling; pH = 10.

Na B O @10H O . . . . . . . . . . . . . . . 24 7 2 1.4 27 3.3 1,500 None.

Na B O @10H O . . . . . . . . . . . . . . . 24 7 2 1.3 29 3.3 3,600 Do.

Na B O @10H O . . . . . . . . . . . . . . . 24 7 2 1.0 27 4.0 Nap No gelling.

Na B O @10H O + NH H PO . . . . 24 7 2 42 4 0.5 27 3.3 NAp Slurry formed; ammonia smell. 90.8 

Do. Same as above.

NA Not available.

NAp Not applicable.

alcohol methanol (CH OH), has been gelation times of certain shrinkage occurs and cracks form in the specimens during the 3

silicic acid solutions by 50 pct or more (25). drying stage. Various drying procedures have been devised to

With time, the gels lost water, shrank, and eventually dried

out and crumbled into a powder. Some gels were kept in a

humidity chamber, but these also lost water, although at a

slower rate than the gels that were kept open in the laboratory

air. The partially dried gels and the more extensively dried gels

did not retain good structural cohesion and were not deemed

suitable for sealing purposes in a mine. coated with the water glass and left to dry in an open tray in the

The tendency for slowly drying bodies to crack has been laboratory. In about a week, a transparent hard layer of glass

noted for clay (26) and for silica gels (27). A microscopic formed on the coal pieces and completely sealed them. The

observation of the dehydration process undergone by samples

of sodium silicate gels (28) has shown that, after an induction

period, cracks appear at random sites and that subsequent cracks

originate on the surfaces of the initial cracks. This process is

followed by the formation of aggregates of cracks, which then

coalesce and eventually cover the entire surface. Finally, this

dehydration front moves into the sample.

Silica and alumina gels are used in industry as precursors in

the preparation of glass by pyrolysis of the gels. Often,

eliminate cracks and shrinkage; however, good results are not

always obtained. For use as sealants in a mine, extensive

drying, shrinking, or excessive cracking is not tolerable.

Plain water glass will dry in air into a transparent hard glass.

It will retain some of the water, with the amount depending on

the thickness of the layer formed (29). Chunks of coal were

glass layers retained their integrity. Another property of

silicates that is of interest to a mine that contains reactive

pyrites, is the ability of the silicate to form insoluble products

with ferrous and ferric ions. Such insoluble silicates will retard

the oxidation of the pyrite. Thus, the application of plain water

glass in a coal mine that contains pyrites merits additional

attention. Another attractive feature of inorganic silicates is

their nontoxic and fire-resistant nature. Drying time of the

water glass is long. While drying, it can seep into the ground
6

and away from the area where it is needed. Special application

procedures will have to be developed to contain the solution.

GUAR GUM GELS

Various materials, both natural and manmade, can form gels.

These gels consist of polymers immersed in a liquid medium.

The liquid prevents the polymer network from collapsing while

the network holds the liquid in place. Cellulose compounds,

starches, saccharides (sugars), and proteins are among the

natural gel-formers, whereas a polymer such as polyacrylamide

is an example of a manmade gel-former.

The gel chosen for this study as a sealant was guar gum.

Guar gum belongs to a group of hemicellulose compounds

(sugars that contain cis-1,2-diols such as mannose and

galactose) that include substances found in fruits and in the

endosperm of seeds of various plants, such as quince, flax, and

psyllium, and in locust bean and locust kernel. Guar gum is a

complex carbohydrate polymer of galactose and mannose (30),

or galactomannan. A monomer unit is as follows: although

these galactomannans can differ widely in their 

galactos/mannose compsition ratios. Guar gum has many uses

in the food, paper, cosmetic, and pharmaceutical industries, as

well as in mining and in the production of explosives (31).

Many derivatives of guar gum are available commercially.

Hydroxyalkylated gums are soluble in water-miscible solvents

such as glycols, and are used in the production of various

explosives. Other hydroxyalkylated gums are compatible with

calcium salt solutions and are used in the preparation of aqueous

slurry explosives and in oil well fracturing.

Two guar gums, Jaguar 2332 and Jaguar 2379, were

obtained for initial tests. These two are both self-crosslinking 4

blends of guar with a crosslinker and pH buffers, and were

chosen for ease of preparation.

PREPARATION METHODS OF GUAR GUM GELS

ELECTROLYTES INHIBITION OF BACTERIA AND

Initial mixtures of the two guar gums with water only,

produced viscous liquids that did not gel. Electrolytes were Guar gums are sugars and, therefore, are easily attacked by

required to change the pH of the solutions so that they would bacteria and mold. The presence of electrolytes does not deter

gel. Electrolytes tested were sodium chloride (NaCl), lithium or prevent spoilage. Small bacteria/mold colonies of varied

chloride (LiCl), potassium chloride (KCl), CaCl2 2 @XH O, barium colors were observed on the surfaces of the gels within a few

chloride dihydrate (BaCl @2H O), borax, sodium bicarbonate days or within a week or two of preparation time. If left 2 2

(NaHCO ), MAP, and calcium sulfate (CaSO ). 3 4 Good gels undisturbed, the colonies grew in size; with time they affected

formed with hydrated calcium chloride, with MAP, or with a the gel structure and eventually destroyed it.

mixture of the two. Three inhibitors were evaluated for the gels. They were

INGREDIENT PROPORTIONS AND commercial preparations (recommended by the producers of

AMBIENT CONDITIONS the guar gum), Dowicil 75 and Ottasept. Of the three

Mixtures were prepared at various ratios of guar gum, any kind, over periods of time as long as a year, and was used

electrolyte, and water in order to identify optimal mixtures. in all subsequent gel preparations.

Properties observed were gelation times, weight loss, syneresis

(expulsion of liquid from the gel network due to induced

contraction, brought about by additional bond formation after initial

gelling, or by breakdown of the gel), and shrinkage. Adhesion to

the walls of the containers was also checked at intervals.

The effects of high relative humidity on the properties of these

gels were monitored by placing duplicate samples of the gels in

high-humidity chambers.

MOLD FORMATION

sodium benzoate (NaC H O ), a preservative, and two 752

5 6

compounds, only Ottasept prevented formation of colonies of

Hi-Tek Polymers, Inc., Jefferson, KY. Now Rhone-Poulenc, Louisville, 4

KY 40299.

Dowicil 75; Dow Chemical USA, Western Division, Pittsburg, CA 94565. 5

Ottasept technical; Ferro Corp., Ferro Chemicals Group, Bedford 6

Chemical Division, Bedford, OH 44145-4494. Ottasept has since been sold

to Nipa Hardwicke, Inc., Wilmington, DE, and is now sold under the trade

name Nipacide-PXR.
7

Table 2.—Guar gum gel formulations

(weight in g)

Ingredients Formulation 1 Formulation 2

Water ....................... 1,000 950

Ethylene glycol . . . . . . . . . . . . . . . . — 50

Jaguar 2379 . . . . . . . . . . . . . . . . . . 23 23

CaCl @2H O . . . . . . . . . . . . . . . . . . 2 2

1 20 20

Ottasept technical (Nipacide-PXR) . 2 2

 CaCl H on, the weight is modified When 1

2 2 @ O is used in the preparati 

accordingly.

OPTIMAL GEL MIXTURES EFFECTS OF COAL, ROCK DUST,

Two formulations gave overall satisfactory results. Thereafter, these were used in small laboratory-size preparations as Once formulations 1 and 2 were selected, their performance

well as in batch preparations for both laboratory and mine when in contact with coal, rock dust, or other materials likely

applications. These formulations are shown in table 2. to be found in mine floor strata, was of paramount importance.

Both formulations resulted in stable gels, which lasted for

long periods of time. Ethylene glycol was used in formulation MIXING PROCEDURES

2 to improve gel strength. Sugars and polyols are known to

modify gelation properties of gelatin gels (32) and of When water was the only solvent, the dry ingredients were

polysaccharide gels such as carrageenan (33), and to increase

gel strength in each case. Carrageenan is similar to guar gum;

it is characterized by an alternating disaccharide unit of two

types of galactose. Thus, it is highly probable that the same

effect of increasing gel strength and improving quality, will be

noted for guar gum gels, as well.

It is important to note that both formulations consist mostly

of water. The concentration of water is 96 and 91 pct by

weight in the two formulations, respectively. In case of a

heating nearby, they can also act as coolants, to quench or

otherwise extinguish a self-heating. In addition, the CaCl salt 2

can act as an adsorbent for water present as moisture in the

mine air, and aid the quenching. Calcium chloride is used in

some underground coal mines as an inhibitor for self-heatings;

thus, it can also act in that capacity. As for the guar gum, it is

present in small amounts, and although it is a fuel, will not

contribute much to the fuel load.

AND MINE FLOOR MATERIALS

Thus, gel mixtures were prepared and poured into trays and

larger containers lined with coal pieces and covered with rock

dust, and into trays and larger containers lined with various

rock material and/or clay, to simulate mine strata. These gels

were observed for periods as long as 2 to 3 months. When the

gels contained Ottasept they performed well, and were not

affected by the coal, rock dust, or other materials. That is to

say, they retained all their desirable characteristics of elasticity,

stability and adherence to the walls. Figure 1 shows a freshly

prepared gel being poured into a large glass jar containing

rocks and rock dust; figure 2 shows the appearance of the gel

after 1 week in the jar.

weighed separately and then mixed together in a container.

Water was then added to the dry powder at a fast rate while

mixing with a glass rod when small quantities were prepared in

a beaker, or with a wooden mixing paddle when larger

quantities were prepared. Mixing continued until all the

powder was dispersed in the water, and the gel started to form.

By that time, the powder usually had dissolved.

When both water and ethylene glycol were used, the dry

powders were weighed and mixed together in the same way.

Ethylene glycol was then added to the powder mix, and the

powder was dispersed in the small amount of ethylene glycol

by mixing. Water was then added to the glycol-powder

combination. This procedure was helpful in the preparation of

the larger batches for the mine. For these latter tests, the

glycol-powder combination was prepared in the laboratory and

then taken to the mine. The addition of water and the final

mixing was conducted in the mine.

BATCH SCALE AND MINE TESTS

GELS IN TUBS mine (Pittsburgh Coalbed), and rock dust was sprinkled on top

The next step was to observe the performance of the gels in Initially, the preparation of large batches, 30 L each, was

a mine environment. Large plastic tubs (76- by 61- by 15-cm practiced in the laboratory. The rate of addition of water to the

deep) were placed in selected areas of the USBM Safety glycol-powder mixture, and the rate of mixing was important

Research Coal Mine, so that the gels would be exposed to for obtaining good gels. Both wooden paddles and a handheld

various ventilation flows and mine conditions. The tubs were electric mixer were evaluated. Better gels were obtained with

lined with coal chunks from the the wooden paddle. When

of the coal chunks.
8

the preparation procedure was established, gels were prepared GELS IN THE MINE FLOOR

in the mine and poured into the tubs. Figures 3 to 6 depict

some of the steps in the preparation of the gels in the mine. While the gels were aging in the tubs, three pits were dug in

Four tubs were filled with gel and the gel was observed for a the mine floor, two in one area in May 1991, and one in another

year. During this time, temperatures as low as 4 EC and as high area in July 1991. During that time of year, both relative

as 21 EC were recorded; likewise, relative humidities ranged humidity and temperature in the mine were relatively high. The

from 15 to 100 pct, but they predominated on the high side. size of the first two pits was 91- by 46- by 23-cm deep; the

Low relative humidities were noted only during late October third pit was 76- by 38- by 15-cm deep. All of the pits had

and early November 1991. The high relative humidity slowed slightly sloping sides. Coal pieces and some rock dust lined the

the evaporation of the water from the gels and was beneficial to bottom of the third pit. Gel batches of 30 L each were prepared

gel stability. using the formulation with ethylene glycol. Weighing and

During the test period, the gels lost some water and their mixing of the dry powders were done in the laboratory. Mixing

height in the tubs was lowered by about 2 to 3 cm. However, with the fluids was done in the mine at the site of the pits.

throughout that time they continued to adhere to the sides of the These gels were observed for a year. They, like the other gels

tubs, and bacterial colonies did not form. Mine dust, both coal in the laboratory or the tubs, lost water over time and receded

and rock dust, settled on the surface of the gels in the tubs and to a lower level by about 2 to 3 cm. However, throughout this

may have helped to retard the loss of water to a small extent. period, the gels adhered to the side walls and appeared to form

Within 3 or 4 months, the surface layers of the gel also changed a good seal, as seen in figures 7 and 8.

texture. A thickened and somewhat crinkled skin of gel that included the accumulated dust, formed on top.

CONTINUOUS MIXING PREPARATION

The batch preparation of gels is easy and fast. For sealing suffice for a chosen time period of water flow. This mixture of

small areas, such as near stoppings, this is a satisfactory the powders was manually fed through the funnel into the water

method. For sealing larger areas in a mine, a method for stream, and the resultant gel was directed into various

continuous mixing of the ingredients is required. Two methods containers, placed in the mine, for observation.

were appraised for the continuous preparation and application Initial tests indicated that neither the granular nor the flake-type

of gels in the mine: (1) the use of an eductor for mixing the calcium chloride that are available commercially, dissolved or

powder with water and (2) the use of a gunite machine for mixed well with the water. The granular calcium chloride was then

mixing and application purposes. ground to a finer size prior to being mixed with the guar gum. This

EDUCTORS good gels.

An eductor is a device that utilizes the cavitation produced is quite common. These mixing units contain intersecting

at a pipe constriction, or venturi, through which water is channels with frequent change of direction. They accelerate

pumped at a high velocity, for mixing purposes. When a mass transfer and produce dispersions due in part to their large

powder is fed into the water stream, it will be carried by the interfacial area. Pressure drop in these units is low. Static

stream out of the eductor, and at the same time will be mixed mixing elements are available in different sizes and designs; a

with the water, especially if it is water-soluble. Both small and few of these were used in the flow tests with the eductors. The

large eductors were used in the tests. The eductors were static mixing elements with a more open design did not get

equipped with a variety of components, such as a funnel to feed clogged and assisted in the mixing, while the ones with a more

the powder, piping to hold a static mixer at the output of the convoluted design, or twist ratio, did get clogged with gel.

eductor, and a hose to direct the flow of the final mixture to the This method of continuous mixing and application of a gel

desired location. Valving was also utilized to control the rate appears feasible. Additional testing is required to refine the

of water flow. Figures 9 and 10 show the arrangements of the design. Bins or containers to hold the dry ingredients, in

small and large eductors, respectively. conjunction with a feeding screw, or two feeding screws, one

The amounts of guar gum, calcium chloride, and inhibitor for guar plus inhibitor and one for calcium chloride, are needed

needed per unit time for a specific water flow rate, were for a more automated mixing process.

calculated, and a mixture of the powders was then prepared to

powder mixture did mix well with the water and formed relatively

The use of static mixing elements to process viscous fluids
9

GUNITE MACHINE highly automatic. The choice of equipment depends on the

A gunite machine is used to pneumatically apply a Portland eration, the degree of flexibility required, and the cost.

cement mortar to various areas in the mine. The cement and Simplified systems, with some degree of manual feeding in a

sand are mixed, wetted with water, and applied to roof, roads, batch fashion, can reduce the cost, but can result in poor

ribs, and other mine areas as a sealing agent to prevent mixtures. Polymer feed systems, some of which could be

deterioration. Since the gunite machine is present in many applied to gel preparation, are described elsewhere (34). Some

mines and essentially performs the same steps that are required

to prepare a gel, it was tested as a means for gel preparation and

application. A small gunite machine at the USBM's Lake Lynn

Laboratory near Fairchance, PA, was cleaned and used. Results

were not satisfactory. A specified ratio of water to solids is

required to form a gel; this ratio could not be obtained with the

gunite machine. In addition, the mixing mode of the water

stream with the mixed powder of guar gum and calcium

chloride was not conducive to gel formation. It is possible that

the gunite machine could be modified to permit the desired

ratio of water to solids and to afford the needed mixing mode.

COMMERCIAL MIXING UNITS

Polymeric flocculating agents are used in water or waste

water treatment plants to enhance settling, to aid filtration or to

accelerate solids dewatering. These polymers are available in

three basic types: dry powder or granules, emulsion, and neat

solution. Equipment for wetting, diluting, mixing, and

blending the different types of polymers with the water varies

from the more simple to the

type of operation, continuous or infrequent, the size of the opof these systems should be evaluated and may prove highly

effective for the preparation and application of gels in a mine.

ADDITIONAL MODIFICATIONS

Slight modifications in gelling time can simplify continuous

mixing procedures. The effects of temperature on gelling time

were evaluated by using both cold and warm water in the

preparation of the gels. Water temperatures as low as 10 EC

and as high as 70 EC were utilized. Gelling time was shortened

by the warm water and was slightly extended by the cold water.

In either case, the change was less than a minute and was not

enough to affect the gelling time significantly. Reducing the

quantities of guar gum and/or CaCl by very small amounts (1 2

to 3 pct) will increase the gelling time without affecting the

overall characteristics and performance of the resultant gels.

Longer gelling times will permit the use of longer mixing

sections in the eductors. The resultant gels will have good

consistency and will not clog the mixing elements. Changes in

the pH values of the gels may also affect gelling times, and are

worthy of consideration.

COST OF GEL

The 1995 cost of ingredients to make 100 gal (378.5 L) of heatings in specific areas, it would be cost-effective. The cost

guar gum gel is shown in table 3. for a mine will depend on the area to be covered and the depth

The guar gum gel is costly; both the guar gum and the of the gel needed to seal each specific location.

inhibitor are expensive. However, if used selectively to prevent

Table 3.—Cost of ingredients to prepare guar gum gel 

[Cost basis: 100 gal (378.5 L) gel]

Ingredients Amount Cost Total 

 cost, $ lb (kg) $/lb ($/kg) Based on—

Guar gum, Jaguar 2379 ..................... 19.2 (8.71) 1.10 (2.43) 50-lb bag 21.11 

CaCl @H O flakes .......................... 2 2 16.7 (7.58) 0.23 (0.51) 50-lb bag 3.84 

Ottasept (Nipacide-PXR) . . . . . . . . . . . . . . . . . . . . 1.7 (0.77) 8.05 (17.75) 50-kg drum 13.44 

Total cost ($) per 100 gal (378.5 L) .............................................................. 38.39 

NOTE.—Calculations were made using U.S. customary units, which are standard in the industry.
10

SUMMARY AND RECOMMENDATIONS

A guar gum-based gel was developed for use as a sealant in bags. The guar gum and inhibitor can be kept together in one

coalmines that undergo self-heatings, and especially in mines bag; the calcium chloride flakes or granules can be stored in

with both floor upheaval and self-heatings. The gel is easily another bag. For immediate use, all three ingredients can be

prepared, in batch form, from a few ingredients. It is elastic in placed in one bag. The preparation of the gel in a continuous

nature and has a useful life in excess of a year. The gel is to be mode requires additional development. The use of the eductors

used in susceptible areas in the mine to minimize air ingress for mixing and applying the gels is feasible, but needs

and thereby prevent, or at least slow down, self-heatings. For additional modifications. The use of commercial units is even

mines that undergo self-heatings and minimal upheaval, only more attractive. A variety of mixing units that will be able to

small, specific areas will require elastic sealants. These mines prepare and apply the gels are available. The optimal ones will

can prepare the gel in batch mode, mix them manually, and have to be found by trial. The gel is costly and is

pour directly from the container onto the areas that require recommended for use as a sealant only in specific areas in

sealing. The dry ingredients for a batch can be preweighted which rigid sealants would not suffice due to cracking.

and kept in plastic bags, such as "zip-lock"

ACKNOWLEDGMENTS

The author acknowledges John Leong and Richard Pro, preparation of mine sites for the various mine tests, and for

physical science technicians, for their assistance in the mine collection of daily mine data. Special thanks are due to Charles

tests; Kenneth Mura, physical science technician and Michael P. Lazzara, supervisory research chemist, for his encouraging

Miller, physical science technician aide, for their help in the support and guidance of this project. All are with the USBM

assembly and testing of the eductor systems; and Paul Stefko Pittsburgh Research Center, Pittsburgh, PA.

and Paul Kolarosky, miner mechanic foremen, and Frank

Karnack, miner mechanic, for the

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12

 Figure 1.—Freshly prepared gel being poured into a jar containing rocks and coal dust.

 Figure 2.—Appearance of gel after 1 week in a jar.
13

Figure 3.—Two plastic tubs with coal pieces and rock dust are being placed next to the coal rib while dry ingredients with ehtylene

glycol are being mixed prior to the addition of water.

 Figure 4.—Mixing of all the ingredients with a wooden paddle to form a gel.
14

Figure 5.—Freshly prepared gel is poured into a tub.

 Figure 6.—Gel in tub in underground mine.
15

Figure 7.—Freshly prepared gel in pit in mine floor.

 Figure 8.—Appearance of the gel after 3 months in pit in mine floor.
16

Figure 9.—Small eductor arrangement.

Figure 10.— Large eductor arrangement.