STABILIZER FOR AQUEOUS SUSPENSIONS

Abstract: Aqueous suspensions of pet-coke are stabilized by means of the addition of bentonite in a percentage com­ prised within the range of from 0.2% to 3% by weight relatively to the weight of the suspension.

Description: The present Invention relates to a stabilizer for aqueous suspensions. More particularly, the present invention relates to a stabilizer for aqueous suspensions of pet-coke. In the present disclosure, reference shall be made to this latter case, which is the most interesting to the Applicant, it being intended that the stabilizer accord­ ing to the present invention can be used in a wide range of suspensions of solids in water. 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 dis­ persing 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 phases results very dif­ ficult. The use of traditional stabilizers of polysaccharide type (xanthan gums) results not much efficacious from the view point of stability, and on the other hand induces considerable increases in viscosity. It has been surprisingly found, and this is the object of the present invention, that the aqueous suspen­ sions 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 suspension contains moreover normal fluidifiers, in particular those obtained by the sulphonation and salifying of coal tar, and possible condensation with formaldehyde, or the non-surfactant anionic polyelectro­ lytes, in particular those having alkyl-substituted poly­ nuclear aromatic groups and having a molecular weight higher than 300 and preferably comprised within the range of from 500 to 3000. Among these latter, mentioning is worth while the salts of monovalent cations of polymerized alkyl naphtha­ lene-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 suspens¦on, it is such as to have particles of size lower than 300 µm, 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 µm and 60 µm, whilst the particles 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 inven­ tion 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 addi­ tives. 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 prefera­ ble, the bentonite can be added either during the wet grinding step in which the coarser fractions are pro­ duced, 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 frac­ tions 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 illus­ trating the invention shall be now given, it being in­ tended 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: 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 concen­ tration of 1%, but the optimum concentration value has resulted to be 1.5%. Larger amounts of bentonite confer a too pseudoplas­ tic 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 η (P) is the viscosity in Poises 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 = K n wherein τ is the shear stress, 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 Stability The determination of the static stability is carried out by pouring the coal suspension into a 170×24 mm plexiglas cylinder and keeping it resting for a deter­ mined 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 determina­ tion 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 com­ prised between 0 and 100. Example 2 Wet Process: Addition of bentonite during the end rod milling 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 refer­ red 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 follow­ ing 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 con­ centration 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 character­ izations are reported of the product from the microniza­ tion 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 to 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. Stabilizer for aqueous suspensions of pet-coke, characterized in that it is bentonite. 2. Stabilizer according to claim 1, wherein the bentonite is in a percentage comprised within the range of from 0.2% to 3% by weight relatively to the weight of the suspension. 3. Stabilizer according to the above claims, charac­ terized in that it is added to the pet-coke during its dry or wet milling.

Inventors: DONATI, ELIO, LAGANA', VINCENZO, PICCININI, CARLO
Applicants: SNAMPROGETTI S.P.A.
Application year: 1986
Main classification: C10L1
IPC codes: B01F17, C10L1, C10K1