Note : Les descriptions sont présentées dans la langue officielle dans laquelle elles ont été soumises.
CA 0223470~ l998-04-09
W O 97/13827 PCT/GB96/02479
BRIOUETTING OF MINERAL FINES
This invention relates to the briquetting of mineral fines
such as coal fines.
A variety of techniques are known for producing agglomerated
structures such as briquettes or tablets from coal fines. The
most popular method is to form ovoid shapes by a cold roll-press
operation. In this instance, the coal fines mixed with a binder
are squeezed, at pressures of up to about 2.1 x 106 kg/m2) (3000
psi), between two metal rolls each having half-ovoid depressions.
The briquettes so formed fall from the press on to a conveyor
belt ~or transfer to storage and subsequent packaging.
Several types of binder have been used in this process with
varying degrees of success. For example, sodium silicate gives
moderately strong briquettes, but has poor water resistance and
because of increased ash content reduces the fuel value of the
coal. Binders such as bitumen and sulphite lye result in the
emission of fumes and smoke from the burning briquette which is
clearly undesirable.
The use of ester-cured alkaline phenol-formaldehyde resole
resin is known to give moderately strong briquettes with good
burning properties and US Patent No. 4802890 is an example.
However, a green strength additive, commonly a starch, must be
used to provide very early strength so that the freshly formed
ovoid can fall intact to the conveyer belt. Starch is
susceptible to bacterial/fungal attack and therefore must be used
with a biocide or fungicide. Furthermore, the inclusion of
starch can detract from the strength achievable from the phenolic
resin. From the mechanics of the operation, this system is also
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complex because o~ the need ~or ~our components.
There is there~ore a need ~or a binder ~or coal ~ines which
substantially avoids these disadvantages. In addition the
problem is not con~ined to coal ~ines since there is also a need
to agglomerate ~ines o~ a variety o~ minerals whether in the ~orm
o~ particles or ~ibres.
According to the invention in one aspect an ambient
temperature curing binder, ~or agglomerating mineral ~ines,
comprises an alkaline resole phenol-~ormaldehyde resin, an ester
bo ~L ~ ~ <~7471-i~C~
co-reactant and a~ oxyanion to enhance gree~ strength.
Also according to the invention there is provided a method
o~ agglomerating a mineral ~ine in which the ~ine is bound into
larger agglomerates by a binder comprising an alkaline phenol-
~ormaldehyde, an ester co-reactant and a~ oxyanion to enhance
green strength.
Thus, we have found that the inclusion o~ certain oxyanions,
borate in partlcular, as a third component rapidly increases the
viscosity o~ the resin-ester mixture and thus enhances the green
strength o~ the system. This permits the briquette to ~all
intact onto a conveyer belt. The rapid increase in viscosity is
believed to result ~rom a decrease in pH o~ the resin coupled
with ionic crosslinking between the methylol groups on the
phenolic resin and the oxyanion. For example, the addition o~
3~ by weight o~ sodium tetraborate to the resin can give as much
as a ~our-~old increase in viscosity.
The binder may be supplied as a briquettlng binder system
~or binding mineral ~ines at ambient temperatures comprising as
separate components:
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(a) an alkaline resole phenol-~ormaldehyde resin;
(b) an ester co-reactant, and
(c) a~ oxyanion to enhance green strength o~ the resulting
binder, to be mixed together with the mineral ~ines to
~orm a mixture to be briquetted.
This system may have an indication that the resulting binder is
use~ul ~or briquetting mineral ~ines such as coal. It may be
supplied with the components in separate containers which may be
o~ a relative size such that ~ ~f the contents o~ the
containers with the ~ines will provide a resulting mixture where
the components are in the desired relative proportions.
Unlike prior systems which include an organic material like
starch, the briquettes resulting ~rom the invention are not
susceptible to mould and bacterial growth. There~ore, they do
not need to include a bactericide or ~ungicide yet still remain
bright and clear o~ mould on storage.
The alkaline resole phenol-~ormaldehyde resin may, ~or
example, be one prepared by reacting a monohydric phenol such as
phenol or a cresol, or a dihydric phenol such as resorcinol, with
formaldehyde under alkaline conditions. The molar ratio o~ the
monohydric or dihydric phenol to ~ormaldehyde can be ~rom 1:1 to
1:3, but the pre~erred range is 1:1.6 to 1:2Ø To this reactant
mixture is added alkali as a solid or an aqueous solution.
Hydroxides o~ calcium, sodium and potassium may be used, but the
latter is pre~erred. The amount o~ solid alkali added can be
~rom 8 to 18~, more pre~erably 12 to 18~, by weight of the resin,
though the most pre~erred range is 11 to 15~ by weight o~ the
resin.
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The ester co-reactant (curing agent) may, for example, be
the acetic acid esters of ethylene glycol, propyl-ene glycol,
butylene glycol and glycerol, lactones such as propriolactone and
gammabutyrolactone, and carbonate esters such as propylene
carbonate and blends of ethylene/propylene carbonates. Mixtures
o~ these es~ers may also be used.-
~u1~a~1~ oxyanions ~rc bor~t~, ~lum~-nate~ and stannate,~
Z~ltho~lg~ borate is preferred. Also the oxyanion can be present
in the ~orm of a salt such as the potassium or sodium salt.
O The amount o~ alkaline phenol-formaldehyde resin to be added
to the fines such as coal will normally be 1 to 6~ by weight of
the fines with the quantity of ester curing agent being 15 to 25~
by weight based on the weight o~ resin. The oxyanion, which can
conveniently be added as a 5~ aqueous solution, will usually be
added in an amount of 1 to 6~ by weight based on the fines.
These amounts are generally higher than the amounts which would
be needed ~o~ larger sized particles than fines.
The preferred order o~ addltion to the mineral fines is to
add the oxyanion first, then the ester and ~inally the resin,
allowing time between each addition to disperse the component in
the mixture.
The binding will take place at ambient temperatures and so
no separate heating of the components is required Thus, in the
case of the briquetting of coal fines little or no change is
required to the cold roll-press operation other than the use o~
a binder according to the invention in place o~ traditional
binders such as sodium silicate and normally the coal fines will
be in the form o~ a damp mixture to avoid dust problems.
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W O 97113827 PCT/GB96/02479
Besides coal fines the agglomeration process of the invention can
be used with other types of fines such as carbon and graphite,
and with other minerals such as quartz, calcium silicate and
alumino-silicate. Mixtures of fines such as mixtures of various
carbon based fines and silica fines are also possible.
The coal and other fines will normally have a maximum size
of such that they will pass a mesh of 5 millimetre square
aperture, and preferably a mesh of 3 millimetre square aperture.
The fines will therefore contain a range of particle sizes up to
the maximum noted above. More preferably the fines are of a
maximum size range o~ ~rom 150 to 200 mesh.
In some circumstances it may also be desirable to add a
silane to assist in bonding between the binder and the mineral
and a particular example is quartz. The silane, if present, is
preferably added in an amount of from 0.1 to 1.0% by weight of
the resin.
The invention can be illustrated by the ~ollowing Example.
A resole phenol-formaldehyde resin~:~s prepared by reacting,
under mild alkaline conditions, 450 g of phenol and 270 g of 91%
paraformaldehyde in the presence of 351 g of water for 15 minutes
at 100 C. After cooling to 80 C, 93 g of potassium hydroxide
(45% w/w a~ueous solution) was added and the reaction continued
at 80 C until the viscosity had increased to 17A (The Paint
Research Association Bubble Tube ~ 25 C). After cooling the
reaction mass, 325 g of potassium hydroxide (45% w/w aqueous
solution) were added. Finally, 4 g of gamma-aminopropyltriethoxy
silane were added. The ~inished resin had a viscosity of 350
centipoise (Brookfield ERV-8 @ 20 C/Spindle 4/loo rpm) and a
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W O 97/13827 PCT/GB96/02479
solids content of 53~ (3h ~ 120C)
A coal ~ines mixture comprising 70~ anthracite, 20
petrocoke and 10~ bituminous with an overall moisture content of
8.5~ were mixed with 3~ by weight of a 5~ aqueous solution of
sodium tetraborate followed by 1~ by weight of an ester
comprising equal parts o~ triacetin and gamma-butyrolactone. The
resin as prepared above was then added at 4~ by weight based on
the ~ines and dispersed for 1 minute.
This mixture was formed into briquettes by the roll-press
technique, the ovoids so formed remaining intact on falling from
the press Green strength measurement within 30 seconds from
forming gave values of about 2.46 x 104 to 3.16 x 104 kg/m2 (35
to 45 psi) After 24 hours the briquettes had good scratch
hardness, the compression strength had increased to about 9.84
x 104 to 1.12 x 105 kg/m2 (140 to 160 psi) and immersion tests
showed them to be water resistant. Burn tests showed the
briquettes to have excellent hot strength and to evolve very
little smoke or ~umes.
The above briquetting exercise was repeated with the 5
aqueous solution of sodium tetraborate replaced, on a weight-
weight basis, by 3~ of water alone. Over 70~ of the ovoids
disintegrated on falling ~rom the roll-press, showing little
evidence of any measurable green strength.
