EP0200074B1

EP0200074B1 - Stabilizer for aqueous suspensions

Info

Publication number: EP0200074B1
Application number: EP86105103A
Authority: EP
Inventors: Vincenzo Lagana'; Carlo Piccinini; Elio Donati
Original assignee: SnamProgetti SpA
Current assignee: SnamProgetti SpA
Priority date: 1985-04-23
Filing date: 1986-04-14
Publication date: 1990-02-07
Anticipated expiration: 2006-04-14
Also published as: SU1731065A3; DE3668908D1; GR860933B; IT8520456A0; AU587610B2; FI861554A0; FI861554A; EP0200074A2; NO861561L; EP0200074A3; ZA862533B; DD244561A5; IT1187660B; IN167190B; JPH068420B2; JPS61247794A; AU5599086A; ES8802578A1; ES554668A0; BR8601992A

Description

The present invention relates to a composition comprising an aqueous suspension of pet-coke and an stabilizer.
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.
The problem to be faced with pet-coke is that it gives rise to completely unstable suspensions in water.
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.
Homogenizing again the two resulting phases is very difficult.
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.
The use of bentonite to stabilize coal-water mixtures is known from US 4 505 716.
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.
As for the concentration of pet-coke in water, it is usually comprised within the range of from 50% to 80% by weight.
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.
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).
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.
The particles of the first group are at least 50% and up to 80% by weight of the total of the particles.
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.
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.
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.
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.
In order to obtain the coarser fraction, a rod mill is generally used, whilst to obtain the finer fraction a ball micronizer is used.
The end particle sizes of the two pet-coke fractions are those as above mentioned.
The two fractions mixed with each other give rise to the aqueous suspensions of pet-coke.
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.
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

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

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%.
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%.
Larger amounts of bentonite confer a too pseudoplastic character to the product.
The values of stability, as % after 48 hours, for the five cases, are respectively 91.0; 96.0; 98.5; 99; 99.5.
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

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.
After such time, the cylinder is introduced in a freezer to cause the suspension to solidify.
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.
The two samples are then analyzed for the determination of the content of solids.
The % ratio of the solids content found in the top sample to that of the bottom sample allows to calculate the stability.
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

The pet-coke used was that of Example 1.
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.
The coarse/fine ratio in the end rod milling has been 65/35.
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%.

The end concentration of pet-coke has been of 71% by weight.
The results obtained are reported in the following Table.

Example 3

Wet Process: Addition of bentonite during the micronization step

The pet-coke used was that of Example 1.
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.
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%.
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.
The concentration of pet-coke in the end product has resulted around 71 %.
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.
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.
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.
It can be observed that better stabilities and with lower concentrations are obtained by adding bentonite in the micronization step.

Claims

1. A composition comprising an aqueous suspension of pet-coke and a stabilizer characterized in that the stabilizer is bentonite present in amounts comprised within the range of from 0,2 to 3% by weight relatively to the weight of the suspension and that the particles of pet-coke aqueous suspension are of a size lower than 300 11m.

2. Process for making the composition of claim 1, characterized in that the bentonite is added to pet-coke during its dry or wet milling.