Coal-water fuel production

A process and apparatus for producing coal-water fuel comprises crushing and primary grinding of the coal to form a liberated granular material, multiple stage froth flotation of the granulated coal to reduce its ash content and pyritic sulfur content. The resulting product from the froth flotation operations is dewatered to yield a product having a selected solids content. A stable slurry is then produced from the product by establishing a selected particle size distribution. Refuse accumulated from the various previous steps is collected, dewatered and the water is clarified for use in the process steps.

BACKGROUND OF THE INVENTION 
The present invention relates to a method and apparatus for producing 
coal-water fuel (CWF) on a commercial scale which uses a unique 
application of conventional, commercially available equipment. 
Individual unit operations in the invention include coal crushing, rod 
milling, sieve bend screening, froth flotation, vacuum filtration, refuse 
dewatering, and ball milling. These have been practiced in the coal 
preparation and minerals beneficiation industries for many years. The 
invention also uses a reverse flotation operation. 
The size reduction unit operations; crushing, rod milling, and ball 
milling, are common in mineral processing plants, e.g. copper and 
molybdenum ore concentration operations. Rod and ball milling are not 
found in conventional coal beneficiation operations. Current practice is 
to avoid the production of fine coal, primarily because of the 
inefficiency of conventional fine coal cleaning operations. 
In conventional coal froth flotation, chemical reagents are added to the 
pulverized coal-water mixture to permit air bubbles to selectively attach 
to coal particles, causing them to rise to the surface. The particles of 
mineral matter remain on the bottom of the flotation cell. For reverse 
flotation, a different chemical reagent package provides for depression 
(sinking) of the coal particles and selective attachment of air bubbles to 
particles of liberated pyrite. Thus, pyrite, which is the principal 
sulfur-containing mineral associated with coal, rises to the surface and 
can be skimmed off, resulting in a reduction of the sulfur content of the 
feed stream. 
Froth flotation is a commercially proven technique for reducing the ash 
content of the feed coal. In most conventional coal flotation 
applications, only ten to twenty percent of the total plant feed is passed 
through the flotation circuit. In the present invention the entire feed 
stream may be directed to the flotation circuit depending on coal 
characteristics. Separation of the flotation feed into coarse and fine 
streams (split feed) has been demonstrated to improve the performance of 
the flotation circuit. Several commercial operations do practice split 
feed flotation, but this is not common. Separate flotation of coarse coal 
was first performed about 1960 at the pipeline plant of Hanna Coal Company 
in Cadiz, Ohio. The Kerr McGee Company has also installed split feed 
flotation for processing 28 mesh.times.0 raw coal in their newest 1200 TPH 
preparation plant. Multiple stage or "rougher-cleaner" flotation has been 
practiced in the coal industry for over 20 years. The first 
rougher-cleaner circuits in the coal industry in the U.S. were designed 
and installed in 1963 at three plants of Bethlehem Mines Corporation in 
Washington County, Pa. The rougher-cleaner flotation circuits were desiged 
for 60 TPH of 28 mesh.times.0 coal. 
The reverse flotation process has been tested at both the laboratory and 
pilot plant levels (12 TPH coal feed) on a number of Pennsylvania and West 
Virginia coals. These tests indicate that 70% to 90% of the pyritic sulfur 
could be removed by reverse flotation. Much of the early work, beginning 
in the late 1960's, was supported by the U.S. Bureau of Mines. Technical 
details of the process are available in the literature. Several individual 
companies continued this work in privately sponsored research programs. 
The use of a vacuum disc filter for dewatering of fine particles is common 
practice in both coal beneficiation and minerals processing plants. The 
present invention, however, requires more sophisticated control than 
commonly found in existing coal cleaning plants. However, such 
sophisticated control is standard practice in iron ore benefication 
systems where filter cake moisture is a crucial parameter in the 
subsequent pelletizing operation. 
The final stage in the CWF production process of the present invention, 
high density ball milling, has been demonstrated at a pilot scale. A 50 to 
100 TPD continuous pilot plant located at Kennedy Van Saun Corporation in 
Danville, Pa., has been in operation since February 1982. The coal-water 
fuel technology is covered in U.S. Pat. Nos. 4,282,006 and 4,441,887 to 
Funk. 
SUMMARY OF THE INVENTION 
The present invention is drawn to a method and apparatus for the production 
of coal-water fuel (CWF) on a commercial scale and using a unique 
combination of unit operations which, in and of themselves, are 
conventional. As noted above, the individual unit operations include coal 
crushing, rod milling, sieve bend screening, froth flotation, vacuum 
filtration, refuse dewatering, and ball milling, as well as reverse 
flotation operation. The operations include the production of fine 
particles through staged size reduction in the rod and ball mill circuits 
which is not a common practice in the coal industry. Neither is the 
complexity in scope of the froth flotation circuit found in this industry. 
According to the invention the beneficiation circuit is also positioned 
between the size reduction devices. 
Integration of a coal beneficiation circuit in the inventive process 
provides the capability of reducing the ash and sulfur content of the raw 
coal. This capability expands the potential supply of acceptable raw coal 
feedstocks and provides for the possibility of supplying various quality 
fuels to meet specific customer requirements. The process extends the 
commonly accepted limitations of conventional coal beneficiation 
operations. This is possible because the fine grinding required for CWF 
production also results in liberation of undesirable mineral matter and 
pyritic sulfur from the raw coal. Production of a coal-water fuel also 
eliminates the need for thermal drying of the ground coal and the 
subsequent handling and storage problems associated with fine, dry coal. 
Accordingly an object of the present invention is to provide a method and 
arrangement of existing apparatus for producing a coal-water fuel 
comprising a crushing and primary grinding step and equipment for 
liberating undesirable components of the coal, a conventional froth 
flotation step and equipment for pyrite removal, a dewatering step and 
equipment for concentrating the solids content, a slurry preparation step 
and equipment for controlling particle size distribution and a refuse 
dewatering and water clarification step and equipment. While the 
individual function circuits remain constant in the various embodiments of 
the invention, individual items of the equipment can be substituted. Thus 
in an operating plant, parallel equipment would be installed and process 
piping arranged so that individual units could be by-passed in the event 
of equipment failure or for alternative product preparation. 
The various features of novelty which characterize the invention are 
pointed out with particularity in the claims annexed to and forming a part 
of this disclosure. For a better understanding of the invention, its 
operating advantages and specific objects attained by its uses, reference 
is made to the accompanying drawings and descriptive matter in which 
preferred embodiments of the invention are illustrated.

DESCRIPTION OF THE PREFERRED EMBODIMENT 
Referring to the drawings, the invention embodied in FIG. 1 is an apparatus 
and process for producing coal-water fuel. 
The invention includes six functional circuits. These are the crushing and 
primary grinding circuit generally designated 2, a froth flotation circuit 
for ash and pyritic sulfur reduction designated 4, a product dewatering 
circuit designated 6 for establishing a selected solids content, a slurry 
preparation circuit designated 8 for producing a desirable particle size 
distribution in the fuel, and a refuse dewatering and water clarification 
circuit designated 10 for treating refuse from one or more of the other 
functional circuits and for clarifying water from those circuits and for 
reuse in the CWF production process. 
FIG. 2 shows another embodiment of the invention with a crushing circuit 
22, a froth flotation circuit 24, a reverse flotation circuit 26, a 
dewatering circuit 28, a slurry preparation circuit 30 and a refuse 
treatment circuit 32. 
The separate functional circuits of the invention will now be described 
individually with reference to FIGS. 1 and 2. 
Crushing and Primary Grinding 
Raw coal arriving at a plant is sampled and stored in separate piles (not 
shown) if desired. Coal would be moved from the piles to one of several 
raw coal storage bins one of which is shown at 202 in FIG. 2. Separate 
feeders on each bin would permit blending of coals ahead of the process to 
meet specific feed requirements. 
Initial size reduction of the nominal 3 to 5 inches.times.0, 10-20% ash raw 
coal will be accomplished using a impact type crusher 104 in FIG. 1 or 204 
in FIG. 2. Several crushers of this type are commercially available. 
Hammermill and cage mill designs are potentially attractive alternatives. 
The crushers 104 or 204 are sized and operated to produce a 3/4 
inch.times.0 product for subsequent processing. An overall reduction ratio 
of approximately 6:1 is required. Staging of the crushers may be necessary 
to achieve this reduction ratio. The maximum particle size of the crushed 
product may be adjusted to meet the specific needs of the particular coal 
being processed. 
In FIG. 1 the crushed coal flows by gravity to the primary wet grinding 
operation at 106. This wet grinding operation serves several important 
functions: (1) it ensures a consistent coal particle size distribution to 
downstream processes independent of the raw coal size distribution, (2) it 
creates highly active, freshly ground coal surface sites for subsequent 
froth flotation processing, (3) it inhibits surface oxidation of the newly 
produced active coal sites, and (4) it acts as an efficient 
wetting/mixing/conditioning device. 
Conventional wet ball mills (206 in FIG. 2) or rod mills are potential 
alternatives for this unit operation. Both of these devices are capable of 
producing a 28 mesh.times.0 product from the 3/4 inch.times.0 feed, 
corresponding to a reduction ratio of 33:1. Either mill would be operated 
at approximately 50% solids. The mill operating conditions and final 
product size distribution will be determined by the characteristics of the 
coal being processed. 
Two different grinding circuit designs have been considered. The first 
(FIG. 2) is a conventional closed circuit wet ball milling process. In 
this mode of operation, the mill product is pumped to a hydrocyclone 
classifier 208. Underflow from the cyclone, containing oversized coal and 
fine pyrite particles (cyclone separation is based on particle mass), is 
passed over a sieve bend 210. The sieve bend overproduct is returned to 
the mill 206 for regrinding, and the pyrite enriched underproduct is 
directed to a refuse thickener 284 in circuit 32. Cyclone overflow is 
directed to the froth flotation circuit 24. This type of circuit may be 
useful for coals containing a relatively high amount of coarse pyrite 
contamination. 
Open circuit rod milling is a second alternative (not shown). The rod mill 
alternative would be expected to provide a narrower size distribution, 
i.e. fewer ultrafine particles, while still producing the minus 28 mesh 
product. Reducing the amount of ultrafine particles should improve the 
performance of the froth flotation circuit. 
The minus 28 mesh product from the grinding circuit 22 may be directed to 
either the beneficiation circuit 24, 26 or possibly to a vacuum filtration 
system 229 for dewatering in circuit 28 as feed to the slurry preparation 
circuit 30. The latter option will be used if the coal is of sufficient 
quality to meet customer specifications without further ash or sulfur 
reduction. This option may be used if a pre-cleaned coal is chosen for 
feed to the process. 
Conventional Froth Flotation 
The performance of the flotation process is dependent to some extent on the 
distribution of particle sizes present in the feed, as are all 
beneficiation techniques. Flotation kinetics and optimal cell operating 
conditions are particle size dependent. Therefore, close control of 
particle size may be required to improve selectivity and, hence, ash 
rejection and coal recovery. The flotation feed may be split into coarse 
and fine fractions depending on the characteristics of the coal being 
processed. This choice involves determination of the feed size 
distribution, to predict the mass flows to each circuit, and analysis of 
the flotation behavior of individual size fractions. Note that the 
grinding mill may be controlled to adjust the product size distribution. 
Referring to FIG. 2, the grinding circuit product can be classified using a 
two-stage sieve bend arrangement 212. Provided the grinding mill has been 
adjusted to produce a consistent minus 28 mesh product, the first stage 
sieve bend is designed to separate the feed stream into 28.times.48 mesh 
at 214 (or 28.times.65) and minus 48 mesh (or minus 65 mesh) products at 
216. The actual size differentiation will be determined by the 
characteristics of the coal being processed. Screening inefficiencies will 
result in some carry-over of fine material with the sieve bend 
overproduct. This overproduct can be passed over a second sieve bend (not 
shown, but again designed for a 48 or 65 mesh cut) to improve removal of 
the fine material. Water sprays are needed on this second sieve bend to 
improve screening efficiency. The coarse and fine fractions are collected 
in separate sumps for pumping to the appropriate multistage flotation 
circuit. 
Multistage flotation involves retreating the froth product for further ash 
and sulfur reduction. Typically, at least one flotation stage 218-223 
would be necessary. The actual number of stages required depends on the 
measured froth product quality and charactistics of the coal being 
processed. Generally, each successive stage is operated to provide an 
increasingly higher quality product. Physically, the stages are located at 
different levels in the plant so that the froth product from one stage may 
be gravity fed to the next. Note that no recycling of the high ash, high 
sulfur tailings products is intended. These products are directed to the 
refuse dewatering and water clarification circuits 32 (dash lines). 
Each stage consists of one or more individual flotation cells. The froth 
from each cell may be collected separately so that product quality can be 
closely monitored and controlled. If necessry, the froth may be sprayed 
with water to remove any loosely held middlings particles. Flotation 
reagents will be added directly to the flotation cells or to a 
conditioning tank ahead of the cells. An alcohol or glycol frother, such 
as methyl amyl alcohol (or methyl isobutyl carbinol), will be used to 
produce a selective, stable froth. If necessary, fuel oil (No. 2 or No. 6) 
or alternative flotation promoters will be added to improve coal recovery. 
The actual reagent package required will be coal specific and must be 
identified by laboratory research for each application of the process. 
Separation of the grinding circuit product into coarse and fine streams may 
or may not be required, depending on the charactistics of the coal being 
processed. 
Following size classification of the grinding circuit product, the solids 
content of the coarse fraction is too high for effective flotation since 
most of the water passes through the sieve bend with the fine particles. 
Plant recirculating water (dot-dash lines), from the water clarification 
operation 32, is added to the coarse coal sump to dilute the feed to the 
rougher flotation unit to approximately 10% solids. Low reagent dosages 
(0.1 to 0.5 pounds of reagent/ton of coal) and relatively mild aeration 
(0.05 to 0.20 cfm per cubic foot of cell volume) are used in the first two 
rougher cells 218, to increase selectivity. The froth products from these 
cells may or may not require retreatment in the cleaner stage 219. 
Additionally, chemical reagents may be added to the remainder of the 
rougher cells to float as much material as possible. These froth products 
may be directed to coarse-cleaner flotation 219. The quality of the froth 
product from the last rougher cell may be substantially lower than that 
from previous cells. This low grade middlings product may be passed over a 
sieve bend, (not shown in the figure) with the overproduct returned to the 
crushing circuit 22 for regrinding and the underproduct directed to the 
refuse dewatering circuit 32. 
The appropriate froths from the rougher stage 218 may be fed to the cleaner 
stage 219. Water is added to the rougher cell froth launders 218 to dilute 
the feed to the cleaner stage 219 to approximately 10% solids by weight. 
The purpose of this stage is to produce a final clean coal product in 
terms of ash and sulfur content and carbon yield. Pyrite depressing 
reagents, such as CaO, KMnO.sub.4, or K.sub.2 Cr.sub.2 O.sub.7, may be 
added to the flotation cells to improve sulfur reduction. The coarse 
cleaner froth products flow to the vacuum filter feed sump 228. As in the 
rougher stage, the froth from the last cell may need to be screened and 
returned to the grinding mill 206. The cleaner tailings are directed to 
the refuse dewatering circuit 32. 
The underflow from the classification of the grinding circuit product on 
line 216 (typically 48 mesh.times.0 or 65 mesh.times.0) flows directly to 
the fine coal rougher flotation feed sump. The feed solids content of the 
fine coal rougher circuit 221 is less than that of the coarse rougher 218, 
probably on the order of 5 to 7% solids. This lower solids content is a 
result of most of the water from the grinding circuit product (at 50% 
solids) passing through the sieve bend 212 with the fine coal. It would be 
impractical to include a dewatering device at this location in the 
process. Therefore, the fine coal rougher flotation unit 221 must handle 
all of this water. The dilute feed is beneficial to flotation performance, 
but may increase the size or number of flotation cells required. 
If necessary, the froth products from all of the fine rougher cells 221 may 
be cleaned at 222 and then recleaned at 223 at about 10% solids by weight 
to remove ash and as much sulfur as possible. The actual number of 
flotation stages will be dependent on the characteristics of the coal 
being processed. The sulfur reduction at this point will essentially be 
limited to particle sizes between 48 and 150 mesh. The tailings from all 
of the stages are directed to the refuse disposal and water treatment 
circuit 32 for dewatering and water clarification. 
Multiple stage flotation of the fine coal produces an acceptable clean coal 
product in all sizes in terms of ash content at maximum carbon recovery. 
Some minus 100 mesh pyritic sulfur may be present in the final froth 
product leaving the recleaner 223. This product can then be directed to 
the fine pyrite flotation circuit 26. 
Pyrite (or Reverse) Flotation 
The reverse flotation circuit 26 is operated to reject fine pyritic sulfur 
and maximize fine coal recovery. reverse flotation is not applicable for 
ash reduction, nor is it efficient for separation of plus 100 mesh pyrite. 
Consequently, the reverse flotation circuit must be preceded by 
conventional coal flotation in circuit 24. 
The inventive process includes a two-stage 226, 227 reverse flotation 
circuit for reducing the sulfur content of the fine coal froth. This froth 
product, at 20% to 25% solids by weight, must be conditioned to prepare 
the particle surfaces for coal depression and pyrite flotation. 
Approximately 0.4 to 0.7 pounds of depressant reagent/ton of coal and 0.4 
to 0.7 pounds of pyrite flotation reagent/ton of coal are added to a 
conditioning tank 224 (dotted line). The actual reagents and reagent 
quantities used are characteristic of the particular coal being processed. 
Additionally, the tank contents must be adjusted to a pH of 4. This acidic 
condition helps to remove certain chemical groups from the pyrite particle 
surfaces rendering them more hydrophobic. Dilution to 15% to 20% solids 
may be required prior to feeding this conditioned slurry to the rougher 
reverse flotation unit 226. 
Previous experience has indicated that the rougher reverse flotation stage 
226 produces a high sulfur froth and a corresponding low sulfur clean coal 
tailings product. The reverse flotation rougher tails are directed to the 
clean coal dewatering circuit 28. However, the froth from the last few 
cells in the rougher unit 226 may contain excessive amounts of carbon. To 
recover this carbon, the rougher froth will be retreated in a cleaner 
stage 227. 
The high sulfur froth product from the reverse flotation cleaner stage 227 
may be passed over a sieve bend to (not shown) remove coarse coal/pyrite 
particles containing a significant amount of carbon. The overproduct would 
then be directed to the crushing circuit 22 for regrinding and liberation 
of the pyrite particles. The sieve bend underproduct would flow to the 
refuse disposal and water treatment circuit 32 for dewatering and water 
clarification. 
The reverse flotation cleaner tails may be considered a coal middlings 
product which can be returned to the reverse flotation rougher feed 226. 
This product could also be sent back to the crushing circuit 22 for 
regrinding. 
Clean Coal Dewatering 
The clean coal dewatering circuit 28 must be designed to provide a closely 
controlled, high solids content feed to the slurry preparation circuit 30. 
The approximately 25% solids feed to the disc filter 229 must be dewatered 
to approximately 75% to 78% solids. This feed is comprised of the 
conventional coal flotation froth products and the reverse flotation 
tailing products. Should the beneficiation circuits 24 and 26 be 
by-passed, the crushing circuit 22 product will be sent directly to the 
disc filter feed sump 228. 
The filter feed sump 228 serves as a storage and mix tank for the filter 
feed. Laboratory experience indicates that the froth from the flotation 
circuit 24 products should break up fairly easily under mild agitation. A 
consistent filter feed at maximum solids concentration aids filter 
performance. 
To maintain optimum filter performance, a filter vacuum must be maintained 
at a constant, high level. Dual-stage vacuum pumps are required to 
maintain vacuum with ground coals of varying filter cake porosity. A 
second means of maintaining a high vacuum is to ensure that the filter tub 
remains full. Filter rotation speed and, hence, production of filter cake, 
is controlled to match the tonnage of clean coal product from the froth 
flotation curcuits. However, coal flotation products would be pumped to 
the filter at a rate higher than the operating filter capacity so that a 
steady overflow back to the filter feed sump 228 is provided. This 
overflow results in a constant flotation product level in the filter tub 
of the vacuum disk filter 229. A snap blow feature should be included for 
a good cake discharge. 
The importance of the dewatering circuit 28 to coal-water fuel production 
cannot be overemphasized. The solids content of the dewatered product must 
be kept as high as possible to provide some degree of flexibility in the 
subsequent slurry production circuit 30. 
Coal-Water Slurry Preparation 
The slurry preparation circuit 30 consists of a second grinding step at 230 
to produce the optimal particle size distribution. Slurry rheology is 
controlled in two sets of high sheet mixer tanks 232 in series; the first 
for viscosity control, the second for controlling slurry stability. Note 
that the slurry preparation system 30, like the flotation systems 24 and 
26, includes the necessary chemical handling, storage, and metering 
equipment (not shown). 
The dewatered, clean coal filter cake from the vacuum disc filter 229 falls 
directly onto a belt conveyor through plastic lined chutes. Conveyor belt 
scales are used to provide an accurate measurement of the feed rate to the 
grinding mill 230. The cake drops into a ball mill screw feeder where a 
portion of the chemical dispersant reagent, pH adjustment chemicals, and 
any required dilution water will also be added (dotted line). The regrind 
ball mill 230 is operated in a high solids mode (70% to 78% solids by 
weight). Consistent control of the product particle size distribution is 
achieved by controlling the viscosity of the mill coal-water slurry via 
the addition of chemical based disperants. The regrind ball mill operating 
variables, including ball size distribution, ball charge loading, and mill 
speed, are chosen to maximize product throughput and minimize power 
consumption. Since the ball mill is a very efficient mixer, the need for 
sophisticated solids takedown mechanical mixers is eliminated. 
For some coals, high solids ball milling may not efficiently produce a 
sufficient amount of very fine particles to maintain the correct 
rhelogical and stability properties of the slurry. To correct this 
situation, a portion of the vacuum filter cake may be directed instead to 
a stirred ball mill (not shown). Enough water would have to be added to 
dilute the stirred ball mill feed to 50% to 60% solids. The product from 
this ultrafine grinding device would then be added to the ball mill feed 
to provide the needed amount of fine particles. It is important to note 
that the solids content in the ball mill 230 must be maintained at a high 
level, about 70% solids. If the amount of stirred ball mill product is 
sufficient to significantly reduce the overall slurry solids content in 
the ball mill, it may be necessary to return this product back to the disc 
filter 229 rather than feed it directly to the ball mill 230. 
The semi-finished coal-water slurry from the mill will have a viscosity 
ranging from 1500 to 4000 centipoise and a solids content of 70% to 75%. 
This slurry is pumped to a viscosity process blend tank (not shown) 
equipped with low speed, high efficiency impeller mixers where the 
remaining chemical based dispersants are added to lower the slurry 
viscosity to approximately 500 to 2000 centipoise. 
The product from the viscosity process blend tank (not shown) is pumped to 
a high frequency vibrating screen 231 for removal of oversize material 
(+48 mesh particles). The amount of oversize material is projected to be 
less than 3% by weight and will be recycled to the ball mill 230 feed for 
additional grinding. This vibrating screen is an external classifier which 
forms a closed grinding circuit to provide control of the maximum particle 
size. 
The vibrating screen 231 underflow will flow by gravity to a stabilizer 
process blend tank 232 for final slurry preparation. A chemical stabilizer 
to inhibit particle settling and caustic chemical for pH adjustment may be 
added to the blend tank to obtain final product quality. All of the 
chemicals used in slurry preparation are commercially available, 
environmentally acceptable, and can be readily obtained from existing 
chemical suppliers. 
The final coal-water fuel (CWF) product at 240, is pumped from the 
stabilizer process blend tank to storage tanks. These storage tanks are 
insulated and equipped with mixers to ensure product homogeneity. The 
product can be transferred from the storage tanks for shipment by tanker 
truck, rail, or barge. 
Refuse Dewatering and Water Clarification 
The refuse dewatering and water clarification circuit 32 is designed to 
prepare the plant refuse for environmentally acceptable disposal and to 
provide clean process water for reuse in the plant. Any contamination of 
recirculating water can adversely affect product quality. Therefore, 
proper performance of this system must be assured to maintain overall 
process performance and product quality. 
All of the process rejects, including tailings from the conventional froth 
flotation circuits 24, froth from the rougher reverse flotation operation 
26, and filtrate from the vacuum disc filter 229, flow by gravity to a 
static thickener 284. The thickener provides a fairly quiescent 
environment in which solid particles may settle out, leaving a clarified 
water layer. This thickener overflow is returned to the plant water supply 
system for recycling to the process. Fresh makeup water must also be added 
to the water supply system. 
Thickener underflow, at approximately 25% to 30% solids by weight, is 
pumped to a belt filter press 286 for further dewatering. The belt filter 
press was selected because of its ability to handle ultra fines and clay 
slimes. The belt press filtrate is returned to the static thickener 284. 
The dewatered refuse filter cake, 60% to 80% solids by weight, is 
transferred to an external storage pile by a belt conveyor for subsequent 
landfill disposal. 
To protect the CWF production process against shutdown in the event of an 
upset in any portion of the refuse dewatering and water clarification 
circuit, several ponds will be constructed at the plant site. The ponds 
also provide storage for excess plant water, supply process makeup water 
on a continuing or intermittent basis, and provide a receiving basin for 
thickener drainage during scheduled and unscheduled plant shutdowns. 
The inventive process offers several advantages for both slurry production 
and coal beneficiation. These advantages may be broadly categorized as 
resulting from the modular design or attributed to operating flexibility. 
The invention evolved through consideration of distinct unit operations to 
address specific functional needs such as ash and pyritic sulfur 
liberation, ash reduction, pyritic sulfur removal, dewatering, and slurry 
preparation. This approach resulted in a modular structure which should 
permit: 
Optimal management of individual unit operations. 
Sampling between modules to pinpoint sources of specific performance 
problems. 
Addition of parallel units to increase plant capacity. 
Substitution of advanced unit operations as they are developed (coal 
beneficiation in particular). 
The invention has been designed to permit a high degree of operating 
flexibility to respond to variations in feed coal quality and customer 
product specifications. A few examples of flexibility include: 
Control of the feed size distribution to the initial milling operation to 
respond to variation in the coal breakage parameters. 
Control of the size distribution to the beneficiation process dependent on 
the degree of grinding required for ash and pyrite liberation. 
Ability to by-pass the benefication circuit completely if the raw coal 
quality satisfies the customer's specifications. This would also allow the 
refuse filtration system to be shut down. 
Ability to consolidate the split feed froth flotation operation. 
Ability to by-pass the sulfur reduction portion of the beneficiation 
circuit for low sulfur, high ash feed coals. 
The inventive CWF production process is unique in that it specifically 
addresses the problem of fine particle pyritic sulfur removal. 
Test Results 
The feasibility of the basic concept has been demonstrated in the 
laboratory using several coals. The coal samples were first ground in a 
laboratory batch rod mill, cleaned by multiple stage froth flotation with 
a laboratory flotation machine, dewatered by vacuum filtration, and ground 
in a laboratory batch ball mill. Representative test results for two coal 
seams, the Upper Freeport seam coal and the Pittsburgh seam coal, are 
presented here. The individual operations will be discussed sequentially. 
Rod Milling 
The degree of primary grinding required for liberation of the mineral 
matter and pyritic sulfur contaminants from the coal matrix is dependent 
on the nature and distribution of these contaminants. The contaminants in 
the Pittsburgh seam coal are more finely disseminated than those 
characteristic of the Upper Freeport seam coal. Therefore, the Pittsburgh 
seam coal must be ground finer to attain the desired liberation. The size 
distribution of the final coal-water fuel product represents a limit to 
the amount of grinding permitted at this stage. Flotation feed particle 
size distributions produced by rod milling in the laboratory are presented 
in FIG. 3 and FIG. 4. 
Froth Flotation 
The froth flotation circuits examined included multiple stage coal 
flotation and reverse flotation. Significant reductions in the ash and 
sulfur contents of the feed were achieved with high recoveries of the 
combustible material as shown in the following table. 
TABLE 
______________________________________ 
FROTH FLOTATION CIRCUIT PERFORMANCE 
UPPER 
PITTSBURGH FREEPORT 
PERCENT SEAM COAL SEAM COAL 
______________________________________ 
ASH IN FEED 6.08 9.17 
SULFUR IN FEED 1.32 1.74 
ASH IN PRODUCT 3.45 5.16 
SULFUR IN PRODUCT 
1.07 0.76 
BTU RECOVERY 85 92 
______________________________________ 
Vacuum Filtration 
The froth products from the flotation testing were dewatered using a filter 
leaf test kit. A range of filtration cycle characteristics and vacuum 
pressures were investigated. The tests indicated that dewatered cake 
solids contents ranging from 68% to 77% solids could be obtained from the 
fine particle flotation products. 
CWF Preparation 
The dewatered froth products were mixed with chemical reagents and ground 
in a laboratory batch ball mill to produce stable coal water slurries. The 
slurry chracteristics are indicated in the following table: 
______________________________________ 
PITTSBURGH 
UPPER FREEPORT 
SEAM COAL SEAM COAL 
______________________________________ 
Solids Content-% 
70.6 73.4 
Slurry Viscosity - C.sub.p at 
870 850 
100 reciprocal sec. 
______________________________________ 
The particle size distributions of the slurries are presented in FIGS. 5 
and 6. 
While specific embodiments of the invention have been shown and described 
in detail to illustrate the application of the principles of the 
invention, it will be understood that the invention may be embodied 
otherwise without departing from such principles.