[0001] The present invention relates to a composition comprising an aqueous suspension of
pet-coke and an stabilizer.
[0002] Pet-coke is a high heat value product with low content of ashes, and a great interest
exists hence in converting said pet-coke into a fluid product by dispersing it in
liquids, in particular in water.
[0003] The problem to be faced with pet-coke is that it gives rise to completely unstable
suspensions in water.
[0004] After a short rest time indeed an aqueous suspension of pet-coke gives rise to a
lower layer of highly packed pet-coke, and to an upper layer of water.
[0005] Homogenizing again the two resulting phases is very difficult.
[0006] The use of traditional stabilizers of polysaccharide type (xanthan gums) is not very
efficient from the point of view, and also because it induces considerable increases
in viscosity.
[0007] The use of bentonite to stabilize coal-water mixtures is known from US 4 505 716.
[0008] It has been surprisingly found, and this is the object of the present invention,
that the aqueous suspensions of pet-coke can be stabilized by adding to them bentonite
in amounts comprised within the range of from 0.2 to 3% by weight relatively to the
weight of the suspension.
[0009] As for the concentration of pet-coke in water, it is usually comprised within the
range of from 50% to 80% by weight.
[0010] The composition contains, in addition to the aqueous suspension of pet-coke and to
the bentonite, normal fluidifiers, in particular those obtained by the sulphonation
and salifying of coal tar, and possible condensation with formaldehyde, or the non-surfactant
anionic polyelectrolytes, in particular those having alkyl-substituted polynuclear
aromatic groups and having a molecular weight higher than 300 and preferably comprised
within the range of from 500 to 3000.
[0011] Among these latter, it is worth while mentioning the salts of monovalent cations
of polymerized alkyl naphthalene-sulphonic acids, and among these the compounds known
under the trade names DAXAD 15 (W.R. Grace), DAXAD 19 (W.R. Grace) and Reoplast 203
(Fratelli Lamberti).
[0012] As for the granulometry of pet-coke in the aqueous suspension, it is such as to have
particles of size lower than 300
11m, and preferably the suspension is formed by two groups of particles of different
average size, the first group of particles having an average size between 210 11m
and 60 11m, whilst the par- tides of the second group have an average size comprised
between 1/6 and 1/20 of the average size of the particles of the first group.
[0013] The particles of the first group are at least 50% and up to 80% by weight of the
total of the particles.
[0014] As for the preparation of the suspension of pet-coke containing the stabilizer according
to the present invention two routes can be followed: dry process and wet process.
[0015] In the dry process, the bentonite is added to the fluidifier-containing pet-coke
in the step of pet-coke grinding, or in the step of mixing with water and additives.
[0016] In the wet process, in case of a continuous-type granulometry of the particles of
pet-coke, the process is carried out as described above, with the exception of the
fact that the grinding is carried out under wet and not dry conditions.
[0017] On the contrary, in case mixtures of pet-coke with two different granulometric fractions
are used, as preferable, the bentonite can be added either during the wet grinding
step in which the coarser fractions are produced, or, preferably, during the wet grinding
during which the finer (micronized) fraction is produced.
[0018] In order to obtain the coarser fraction, a rod mill is generally used, whilst to
obtain the finer fraction a ball micronizer is used.
[0019] The end particle sizes of the two pet-coke fractions are those as above mentioned.
[0020] The two fractions mixed with each other give rise to the aqueous suspensions of pet-coke.
[0021] The wet milling to obtain the coarser fraction is preferably carried out by mixing
the raw pet-coke to be ground with the wet-micronized fraction, continuining the grinding,
until the size desired for the coarse fraction is reached. This is the grinding process
adopted in Examples 2 and 3.
[0022] Some Examples having the purpose of better illustrating the invention shall be now
given, it being intended that the same invention is not limited by them or to them.
Example 1
Dry Process: Addition of bentonite in the mixing step
[0023] The pet-coke having the following characteristics has been used:
volatile matter 12.6% by weight
sulphur 4.63% by weight
ashes 0.4 : 0.5% by weight
gross heat value, referred to
dry pet-coke 2035 J/Kg (8520 Kcal/Kg)
grindability index 22.3 Hardgrove
[0024] A slurry has been prepared with the coarse fraction and the micronized fraction,
in the weight ratio of 65/35 at a concentration of pet-coke of 72.0% by weight, and
with an amount of DAXAD 15 of 0.5%. Bentonite has been metered into it in the amounts
of respectively 0%, 1.0%, 1.5%, 2.0%, 2.5%.
[0025] The samples stored inside closed containers have shown a good stability already at
the bentonite concentration of 1 %, but the optimum concentration value has resulted
to be 1.5%.
[0026] Larger amounts of bentonite confer a too pseudoplastic character to the product.
[0027] The values of stability, as % after 48 hours, for the five cases, are respectively
91.0; 96.0; 98.5; 99; 99.5.
[0028] The rheological characteristics obtained are the following:
wherein π (P) is the viscosity in Poises or in 10-
1 Pascal.s computated as the shear stress/shear rate regression line between 70 and
130 s
-1 of shear rate, K and n are related with each other by the relationship τ=KÝn wherein
τ is the shear stress,Ý is the shear rate, K is the consistency index and n is the
index describing the newtonian character of the suspension.
Determination of the Static Stabilitv
[0029] The determination of the static stability is carried out by pouring the coal suspension
into a 170x24 mm plexiglas cylinder and keeping it resting for a determined time at
constant temperature.
[0030] After such time, the cylinder is introduced in a freezer to cause the suspension
to solidify.
[0031] The 150-mm high core is thex extracted from the cylinder and from its ends two samples
of about 15 mm of height are drawn.
[0032] The two samples are then analyzed for the determination of the content of solids.
[0033] The % ratio of the solids content found in the top sample to that of the bottom sample
allows to calculate the stability.
[0034] The static stability is expressed by a number comprised between 0 and 100.
Example 2
Wet Process: Addition of bentonite durina the end rod millina step
[0035] The pet-coke used was that of Example 1.
[0036] The wet-micronization has been carried out at a concentration of 50% of solids, with
an amount of DAXAD 15 corresponding to the concentration of 0.5%, as referred to the
end rod milling.
[0037] The coarse/fine ratio in the end rod milling has been 65/35.
[0038] At the beginning of the wet rod milling, the following amounts of bentonite have
been respectively added:
0%, 0.2%, 0.4%, 0.5%, 0.6%.
[0039] The end concentration of pet-coke has been of 71% by weight.
[0040] The results obtained are reported in the following Table.
Example 3
Wet Process: Addition of bentonite during the micronization step
[0041] The pet-coke used was that of Example 1.
[0042] Wet-micronizations have been carried out at a concentration of 50% of solids, with
the addition of 0, 0.1%, 0.2%, 0.3%, 0.5%, 0.7%, 0.9% of bentonite, referred to the
overall micronized matter.
[0043] The whole amount of fluidifier additive DAXAD 15 has been added in the micronization
step, so as to obtain in the end rod milling a concentration of 0.5%.
[0044] Using a coarse/fine ratio of 65/35 in the end rod milling, the bentonite amounts
above listed correspond to 0%, 0.05%, 0.10%, 0.15%, 0.25%. 0.35%. 0.45% in the end
product.
[0045] The concentration of pet-coke in the end product has resulted around 71 %.
[0046] In the following Tables, the rheological characterizations are reported of the product
from the micronization step, and of the product from the rod-milling step, and the
stability thereof.
[0047] From the results obtained, it can be observed how, to the purpose of minimizing the
concentration to be added, the use of bentonite in the micronization step is more
convenient. The optimum concentration of bentonite to be added is of 0.2-0.4%, referred
to the end rod milling.
[0048] In the Figure the stability (ordinate, as %) obtained by adding bentonite (abscissae,
as %) in the rod-milling step (white circles) and in the micronization step (black
circles) is comparatively shown.
[0049] It can be observed that better stabilities and with lower concentrations are obtained
by adding bentonite in the micronization step.