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In 1848 a Patent was granted to William Easby for a METHOD
OF CONVERTING FINE COAL INTO SOLID LUMPS. In his application Easby
made only one claim, "The formation of small particles of any
variety of coal into solid lumps by pressure". In an equally
brief description of the process he mentions, "The utility
and advantage of the discovery are that by this process an article
of small value and almost worthless can be converted into a valuable
article of fuel for steamers, forges, culinary and other purposes
thus saving what is now lost."
Easby in his few words had patented the entire coal briquetting
industry and had as well stated the rationale for its existence.
Almost 50 years later economic pressure joined forces with
technological progress to give substance to Easby’s vision.
The coal briquetting process as it ultimately evolved in the United
States consisted of first drying the coal, then crushing and screening
it; mixing the dry coal with about 6% molten asphalt binder, briquetting
this mixture in roll type briquette machines and finally cooling the
briquettes on a conveyor before loading them into cars or diverting
them to stockpile. Over 6 million tons of coal briquettes were produced
annually in the United States before the process was doomed by cheap
oil and gas just after World War II.
The briquettes made by this process were used primarily for domestic
heating and many attempts were made to eliminate the asphalt binder,
as the smoke from the binder was the major objection to the product.
The briquetting of coal today is of more than historic interest.
Coal is briquetted as an initial step in the production of activated
carbon. There is growing interest in briquetting coal to reclaim
stockpiles of abandoned screenings. Coal that has been crushed for
conveying by pipe line or crushed for cleaning to remove sulfur
and ash cannot readily be shipped without reagglomerating it to
a larger size. Coal smaller than one-quarter inch cannot be used
without agglomeration in some of the processes for synthetic fuels.
Briquetting is used in the production of form coke and has advantages
in the production of metallurgical grade coke as well.
Coal, which launched a major briquetting industry in the first
half of this century, may well come full circle in the second.
Many other materials are briquetted with binders. Iron and
chromite ore are briquetted with a binder consisting of lime and
molasses. Fluorite also is briquetted with lime and molasses binder
as well as with sodium silicate. Portland cement is used as a binder
for bauxite and the lignosulfonate binders which are a residue of
the paper industry are used for copper ores and for magnesite. In
glass batch mixes, water and soda ash form a binder for the silica
sand.
Binders are divided by their function into matrix type binders,
film type and chemical binders. Some examples of each of these are
listed below:
MATRIX TYPE FILM TYPE CHEMICAL BINDERS
Coal Tar Pitch Water
CA(OH)2 + Molasses Petroleum Asphalt Sodium Silicate
Sodium Silicate + C02 Portland Cement Lignosulfonates
2 Component Plastics such as the epoxies
Matrix type binders embed the particles into a substantially continuous
binder phase. The properties of the briquettes, therefore, are largely
determined by the properties of the binder.
Film type binders are like glues and usually depend upon the evaporation
of water or some solvent to develop their strength.
Solvent type binders are sometimes used, even though the material
can be briquetted with pressure alone, as lower pressures can be
employed and briquettes with a more porous structure can be made this
way.
Chemical binders can be either film or matrix type. The chemical
binders used for foundry sands are good examples of the film type
binders.
Briquettes made with binders are usually pressed at low pressure.
When briquettes are made without binders, however, the success of
the process depends upon crushing or plastically deforming the particles
to bring them closely together. It is not surprising that many crystalline
organic components can be briquetted with pressure alone. The forces
that bind these crystals together are neither strong nor specific,
so it is necessary only to squeeze the individual crystals into
close contact.
It is more surprising that typical ionic solids, with their
strong well-ordered crystal lattices, are so easily formed into
satisfactory briquettes. Sodium chloride, for example, is extensively
briquetted for recharging Zeolite-type water softeners. Potassium
chloride is briquetted and compacted for the production of granular
fertilizer. Calcium oxide is also extensively briquetted, but at
much higher pressures.
Hot briquetting is generally understood to mean the briquetting
of materials that have been heated to temperatures above ambient
where plasticity has developed or become imminent. It is not surprising
that materials heated to such temperatures should form better briquettes.
Temperatures well below these, however, may also be beneficial.
This seems to be particularly true of the ionic compounds. Sodium
and potassium chlorides and cyanides, for example, form better briquettes
when they are heated to 100-200° C. Temperature affects a number
of properties of dispersed solids and is an important variable in
the briquetting process.
Compacting and Granulating Systems
When specifications call for an agglomerated product smaller than
8mm, it is generally made by first compacting the original material
then crushing and screening it to recover the fraction having the
desired mesh size.
The initial compacts can be conventional briquettes, stick-shaped
briquettes or continuous flat or corrugated sheets. A particularly
attractive flake-type granulated product is obtained when the thickness
of the compacted sheet is substantially less than the mesh size
of the product. Granulation then is required in only two directions.
Fertilizers and other products are compacted and granulated to
prevent them from caking in bags or containers. Many pharmaceutical
products, such as aspirin and antibiotics, are compacted in roll
type machines and then granulated to produce free flowing, granular
material from which tablets are made.
Continuous bars and strips can also be rolled from particulate
materials with roll type briquette machines. If the rolls are arranged
one above the other, as in a rolling mill, the strips will issue
from the machine horizontally, where they can be fed into a roller
hearth, or other furnace, for subsequent processing. Portland cement,
sponge iron, coke, metals and alloys, sintered ores, dusts from
steel making and other metallurgical processes, and many other products
can be made or processed in this way. Dust losses can be reduced
and thermal efficiencies can possibly be increased. No application for this
technique outside the metals industry appears to be practiced commercially
at this time, but with our growing concern about ecology and the
conservation of energy and materials, a fresh look at established
industrial processes does not seem to be amiss.
Roll type briquette machines apply pressures to particles
by squeezing them between two rolls rotating in opposite directions.
Cavities or indentations cut into the surfaces of the rolls form
the briquettes.
Characteristics of Roll Type machines
In early briquette machines, the axes of the rolls were always horizontal
and their centers were fixed in the frame. A simple feed box or
hopper mounted above the rolls held the material to be briquetted
and it flowed from there into the rolls by gravity.
Modern briquette machines usually have only one roll in a fixed position
in the frame. The other roll is moveable, but is restrained by hydraulic
cylinders. The rolls may be arranged horizontally or side by side
in the frame, or they may be arranged vertically or one above the
other as in rolling mills. The rolls additionally may be located
symmetrically between the bearings or they may be mounted outside
the bearings at the end of cantilevered shafts. Each of these four
arrangements has certain unique properties. Other features of the
machines as well can be varied to satisfy special process conditions.
Six characteristics in all determine the behavior of roll type briquette
machines.
Briquette machine rolls are classified according to their
construction as integral, solid or segmented. Integral rolls, as
the name implies, are made integral with the shafts. These rolls
usually have a band of stainless steel or some corrosion or abrasion
resistant material welded to their circumference or working face.
Since they have no joints or mating surfaces, integral rolls are
frequently used for briquetting food or pharmaceutical products
where cleanliness is of primary concern. Integral rolls can easily
be steam heated or water cooled. They are not generally suitable
for abrasive materials.
Solid rolls or tires are the most commonly used briquetting rolls
and consist of replaceable rings keyed or shrink fitted to the shafts.
The rolls are made from a variety of abrasion and corrosion resistant
materials. Unlike integral rolls, which require some compromise
in materials of construction, solid rolls and shafts can each be
made from the most suitable material.
Segmented rolls are made in a series of sections or segments which
are mechanically clamped to the shafts. The advantages of segmented
rolls are obvious to anyone who has ever changed conventional rolls,
so rolls of this type have been the subject of continuing investigation
since the beginning of the briquetting industry. Segmented rolls
are recommended for briquetting hot or abrasive materials and are
made from materials suitable for such applications.
The mechanical construction of the rolls determines such important
characteristics as reliability, ease of maintenance and cost of
operation. The effect that the rolls will have upon materials passing
through them, however, depends on their geometry.
In most briquette machines, the moveable roll is pressed against
a fixed roll by hydraulic cylinders. Stops located between the bearing
blocks prevent the rolls from coming in contact with each other.
Material passing between the rolls attempts to spread them apart.
The hydraulic cylinders resist this effort until the force exerted
by the material exceeds that exerted by the cylinders. The moveable
roll is then displaced and in turn displaces the pistons in the
hydraulic cylinders until both efforts become equal. The oil displaced
by the pistons is stored under pressure in a gas filled accumulator.
It returns from there as needed to push the moveable roll back against
the stops.
The hydraulic system acts like a spring. The initial force holding
the rolls together can be adjusted by the pressure of the oil in
the cylinders. The incremental force necessary to displace the moveable
roll is also adjustable by the volume of the gas in the accumulator.
The success of the modern roll type briquette machine is due in
no small part to this ability of the hydraulic system to match the
slope of the force-displacement curve of the moveable roll to the
requirements of the briquetting process.
When roll type briquette machines were limited to compacting
materials, which were mixed with binders, the simple gravity type
feeder was usually adequate. Briquetting in this case is primarily
a forming or molding process and little change in the density of
the product occurs as it passes through the rolls. The pressure
required for such applications is low and the virtue of simplicity
frequently outweighs the advantages possible from more sophisticated
control. Gravity type feeders consequently are still used for some
purposes.
For dry or finely divided materials, screw or auger type feeders
are commonly used. These feeders in addition to controlling the
mass of material passing between the rolls, frequently have important
secondary effects. They may precompress the material before it reaches
the rolls. They may crush the particles to achieve a more favorable
size consistency. There is speculation that the mobility of the particles
in the feed screw allows the crystal axes to align themselves more
favorably so as to produce better quality briquettes. Heating of the
particles in the screw feeder may also have a significant effect.
Whatever the mechanisms may be, briquettes of better quality frequently
can be made by using a screw feeder.
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