EP0188157A1 - Process for the preparation of aqueous solid-fuel slurries by realizing an optimum granulometric distribution - Google Patents

Abstract

Raw coal ground to a particle size range of 0-15 mm feeds a mill B1 in the presence of water. The ground product is screened through a screen C and the undersize material forms the coarse fraction of the final mix. The oversize material employed to constitute the middle and fine fractions is ground in B2 and screened and then feeds a first hydrocyclone HC1 whose overflow (5) feeds a second hydrocyclone HC2. The fines recovered in the underflow (4) from HC1 and in the overflow (7) from HC2 constitute a second constituent of the final mix. The excess of the coarse material originating from the underflow of HC2 is ground in B3 into ultrafine particles which form a third constituent of the final mix. Fuel suspensions with a solids content of more than 45%, suitable for conveying and use. <IMAGE>

Description

The present invention relates to the preparation of aqueous suspensions of solid fuels with a high solids content by achieving the optimum particle size distribution.

The use of coal as an industrial energy source had shrunk considerably with the development of inexpensive petroleum products. The return to this source of energy widely available in the world appeared as the solution for the survival of the activity beyond the oil shocks and in the more or less long term of the exhaustion of the low price oil sources.

This return of coal and more generally of solid fuels, remained handicapped by the difficulties of handling and implementation of this material, which is not very conducive as it stands to simple modern reliable solutions, which can be automated under good safety conditions and with little difficulty. 'environmental impact.

One of the most suitable routes for the use of coal in a modern form of presentation is represented by coal-water mixtures with a high solids content, but used by their liquid presentation as petroleum products.

Numerous researches carried out in recent years in this field have shown that one of the conditions which must be met in order to obtain such mixtures is a good particle size distribution of solid particles. The optimal distribution must have the consequence of filling as completely as possible the particle size voids left by a particle size section given by grains of smaller size.

Indeed, the particle size distribution curve resulting from a grinding carried out using a grinder is a bell curve in which the major part of the ground materials is found around medium. of.the medium size. When two components are each mixed from a mill with particular characteristics, the particle size distribution curve has two peaks and constitutes a curve called bimodal. Such processes do not allow the above-mentioned condition to be perfectly satisfied.

dans laquelle D_µi est la taille médiane de la tranche granulométrique considérée en um, D la taille des plus petites particules en um, D_L The work, the results of which were the subject of American patent n ° 4,282,006, has shown that the optimal distribution can consist of a mixture of particles of size less than 1180 μm containing at least 5% by weight of particles less than 3 um, whose particle size distribution conforms to Alfred's formula according to the equation:

in which D _µi is the median size of the particle size range considered in µm, D the size of the smallest particles in µm, D _L

the size of the largest particles, n an exponent whose value varies from 0.2 to 1.0.

The particle size distribution curve according to this formula is an exponential curve with increasing values with the size of particles having no peaks or no pronounced peaks.

According to the known methods for obtaining such mixtures, it is possible to mix several constituents each coming from a different mode of grinding, sometimes by proceeding to the clipping of certain fractions. Such a method necessarily leads to the elimination and reprocessing in several stages of certain fractions and does not seem economical.

Another method is to perform high-density grinding in one step through periodic sampling to check

the conformity of their particle size distribution to that of the Alfred curve. This method has the drawback of high energy consumption, since the comminution is not carried out according to the conventional reduction law and a large part of the energy supplied is dissipated as heat. Although close to the ideal curve, the distribution however needs to be corrected on the coarse side by removing particles larger than 200 μm for example and on the fine side by adding very fine particles obtained separately using another means of grinding. Furthermore, it is impossible according to this method to add to it an ash removal technique without diluting the suspension, in which case the initial benefit of operation in a single step is lost.

The present invention relates to a process for the preparation of suspensions with a high solids content according to the particle size distribution according to the Alfred equation and is based on the use of hydrocyclones as a means of classification.

As with all means of classification, a hydrocyclone is characterized by its cut-off section assigned a partition coefficient indicating the percentage of this section listening to itself through the underflow. By For example, a hydrocyclone designated by d ₉₀ whose cut-off section is 70 μm separates in which 90% of the section of approximately 90 μm flows underwater and 10% of the same section overflow.

While a sieve makes clean slices, the hydrocyclones have imperfections in the classification they operate and it is from these imperfections that advantage is taken in the context of the present invention to constitute an ideal distribution with a minimum of energy expenditure and the greatest simplicity and efficiency.

The method according to the invention is characterized in that a hydrocyclone is supplied with crushed particles the size of the largest particles of which is less than X μm, X being at most equal to 1180 μm, said hydrocyclone operating a separation in which 90 % of the cut-off section varying between 1/4 and 1/2 of X flows under the overflow and 10% of the same section overflow, the overflow supplying a second hydrocyclone having the same cutting characteristics as the first, the overflow of the second hydrocyclone being combined with the underflow of the first and forming a constituent of the final mixture, the underflow of the second hydrocyclone being ground in a fine mill into a mixture of particles whose size for 80% of the mixture measures between 1/10 and 1 / 20 of X and which forms another constituent of the final mixture, the two constituents being poured into a mixing tank in which the solids content is at least 45%.

One of the important characteristics of the method according to the invention is based on the use of comminution means such that they correspond to the manufacture of the particle size sections sought in the best conditions of efficiency, while the particle size sections not retained in a given step are reprocessed in the next step to produce a new particle size range according to the needs of the particle size distribution of the final product.

Thus, according to a preferred embodiment, the raw fuel is first crushed in a grinder of the bar crusher type producing relatively coarse grains which, screened on a screen, leave passers-by constituting the relatively coarse fraction of the final product and rejects which are subjected to finer grinding in a ball type grinder whose screened grinding products leave passers-by intended to constitute the medium and fine slices of the final product, said pass fraction feeding the first, then the second hydrocyclone.

Refusals of the first grinding of the bar mill type circulate in a closed circuit between the ball type mill and a screen of the same characteristics as the screen used to screen the product from the first bar type mill.

The passers-by resulting from the screening of products from the first grinding can either be poured directly into the mixing tank as final component, or feed the first, then the second hydrocyclone together with the passers-by resulting from the screening of products from the second type of grinder. with balls. ,

The second hydrocyclone underflow is ground into ultra-fine particles in an attrition mill.

The product from the ball mill is characterized by a unimodal distribution centered on the dimensions corresponding to the average slices of the final product. However, the proportion of these tranches greatly exceeds the requirements. It is the combined set of hydrocyclones which makes it possible to adjust the extraction of these slices as required while clipping the overabundant medium slices which are evacuated at the level of the underflow of the second hydrocyclone. The fine regrinding of this excess in the attritor mill finally makes it possible to produce the complement of fines necessary for the final distribution.

According to an embodiment particularly suitable for the treatment of coal, the raw coal must have a particle size of 0 to 15 mm. The grinding reducing the coal to this particle size can be carried out by various means akin to grinding by means of a bar crusher. It may result from a previous treatment or be carried out in the context of the installation for producing the desired particle size according to the invention in which it precedes the group of hydrocyclones. In the latter case, the raw fuel in particles from 0 to 15 mm is ground in a bar crusher in water, the grinding product is sorted on a sieve with a mesh opening of 200 µm, the refusals of the sieve feeding a ball mill in water, the product of this last grinding is sieved on a sieve with a mesh opening of 200 µm and the rejects recirculated in a closed circuit to the ball mill, the pass fractions of the two sieves feed a first, then a second hydrocyclone.

The screen in this case is common for products from bar and ball mills, the recovery of large particles being entrusted to the first hydrocyclone, in the underside of which these large particles are discharged together with the large particles resulting from grinding by means of the grinder with balls.

As a variant, the product of the first bar crusher is classified into two fractions, each of which constitutes 45 to 55% of the ground product, the passing fraction is poured directly into the mixing tank final as a final constituent, while the rejects are subjected to grinding in the ball mill to be reduced entirely to the particle size imposed by the size of the screen openings, .and the product classified on a sieve whose passers-by feed a first , then a second hydrocyclone.

According to a particular embodiment, the first hydrocyclone operates a separation in which 90% of the cutting section of 70 μm flows in underflow and 10% of this same section flowing in overflow of this first hydrocyclone feed the second hydrocyclone operating a separation in which 90% of the 70 μm cutting slice flows underwater to feed an attritor mill in which the solids are ground to the maximum size of 15 μm, while the underwater product of the first hydrocyclone is combined with the overflow product from the second hydrocyclone to be mixed in the final mixing tank with the product leaving the attritor mill.

• la figure 1 présente un schéma du procédé
• la figure 2 un diagramme de répartition granulométrique du mélange final, et

Other features of the invention will become apparent in the light of the description of an embodiment of the method presented by way of nonlimiting example with the aid of drawings, of which

• Figure 1 shows a diagram of the process
• FIG. 2, a particle size distribution diagram of the final mixture, and

le.tableau 1 presenting the distribution of the relative weights of the grain size ranges.

The invention is illustrated by a process applied to an American bituminous coal of the Freeport type. This raw coal ground to a particle size of 0 to 15 mm feeds a bar mill B ₁ in the presence of water producing relatively coarse grains, 80% of which measures less than 800 μm.

The ground product is led by 1 on a screen C consisting of a woven fabric passing approximately 54% of the ground product of particle size less than 200 μm. This fraction is completely recovered without further treatment to constitute the coarse fraction of the final product. The rejection of screening is then used to constitute the medium and fine slices of the final product. To this end, this refusal is first brought down entirely to a dimension of less than 200 μm using a ball mill B ₂ in the presence of water, the treated product circulating by means of the pipe 2 in closed circuit between the mill B ₂ and the screen C.

Screen C is common to crushers B ₁ and B ₂ .

The product from the grinder B ₂ is characterized by a distribution unimodal centered on the dimensions corresponding to the average slices of the final product. However, the proportion of these tranches far exceeds needs. It is the combined set of hydrocyclones HC, and HC ₂ which makes it possible to adjust the extraction of these slices according to the need, while clipping the overabundant medium slices which are evacuated at the level of the underflow of the hydrocyclone HC ₂ .

All the passers-by of the screen C feeds by, 3 the hydrocyclone NC ₁ , whose cutting section is 70 μm and the partition coefficient 90%. Fines pass into underflow 4 in decreasing quantity with their particle size. The large ones pass into the overflow 5 in decreasing quantity for slices of decreasing particle size. This is used to separate by the overflow certain sections which would be overabundant in the final particle size composition according to the needs of the ideal curve.

It is the aim of the second hydrocyclone HC ₂ with the same cutting characteristics as the first and fed by the overflow 5 to recover these large overabundants in the underflow 6 so as to subsequently subject them to fine grinding in the mill B ₃ working by attrition . As the fines are not in excess relative to the final composition, they are recovered by the underflow 4 of the first hydrocyclone HC and, finally, in the overflow 7 of the second hydrocyclone HC ₂ . The two streams 4 and 7 are combined at 8, drained in a drum filter F ₁ and poured into a final mixing tank M as the final constituent of the mixture.

The excess of large recovered in 6 in the underflow of the first hydrocyclone HC ₁ feed by 6 a drum filter F ₂ , then are reground in ultra-fine of which 80% measures at most 15 µm in the attritor mill B ₃ and the product crushed is poured by means of line 9 into the mixing tank M, in which the water content is adjusted to produce a suspension of 50% or more solids.

At several points along the route, DC ash removal steps can be introduced.

• rayures ascendantes: les passants sur crible C en provenance direct du broyeur à barres B₁
• rayures descendantes: les passants sur le crible C en provenance du broyeur à boulets B₂
• marques rondes: les fines récupérées dans la surverse 7 du second hydrocyclone H^C ₂
• les croix: les grosses des surverses des hydrocyclones HC₁ et HC₂
• laissé en blanc: les ultra-fines en 9 en provenance du broyeur B₃.

This exemplary embodiment illustrates the application of Alfred's formula for the following values: Q _L = 200 μm, D _s = 0.2 μm, n = 0.37, to which correspond Table 1 of the various intermediate particle size distributions and final, as well as. the diagram showing in percentage by weight the various particle size sections constituting the final mixture. The source of the fractions of these slices is indicated as follows:

• ascending stripes: passers-by on a screen C coming directly from the bar crusher B ₁
• descending stripes: passers-by on the screen C coming from the ball mill B ₂
• round marks: fines recovered in overflow 7 of the second hydrocyclone H ^C ₂
• the crosses: the large overflows of the hydrocyclones HC ₁ and HC ₂
• left blank: ultra-fine at 9 from mill B ₃ .

The result shows good agreement with the theoretical optimal curve. Settings in the case of a. industrial installation allow techniques to be adjusted to feed variations and to improve particle size distribution, in particular by adjusting the grinder-screen assemblies and the operating characteristics of hydrocyclones, such as operating pressure and openings outlet nozzles.

The present process is applicable to all fuels capable of being reduced to fine particles by conventional means of comminution, such as coals, lignites, coke of coal, petroleum, hard asphalts, etc.

The final mixture can be packaged in a stirring tank into which the fluidity and stability reagents are introduced, as well as a possible supply of water to constitute the fluid mixture ready for transport and use. The additives used are those adapted to the basic solid product. This mixing can also be carried out in a finishing ball mill operating at high solids density.

When the water content of the final product resulting from the process exceeds the fixed limits, the concentration is adjusted by proceeding in the grinder by attrition to a high density grinding in solids which makes it possible to adjust the granulometry to the optimal granulometry of the overall product, but which requires at this point the addition of a surfactant which can range up to 1% by weight of the solid product.

The process according to the invention does not postulate any particular condition with regard to the process water. The only condition to be observed concerns the production of a fluid mixture and its compatibility with the additives. If necessary and during filtration, washing with clear water can be carried out to correct the salinity and to remove products present initially, such as flotation reagents. Many variants within the reach of ordinary skill in the art can be made when carrying out the method according to the invention, which is essentially characterized by a succession of comminution and classification steps leading to reducing all of the initial product to the desired particle size distribution without surpluses or contributions, the adjustment of the particle size constituents being effected only by adjustments to the means used.

Claims (10)

1. Process for the preparation of aqueous suspensions of fuels with a high solids content and containing particles of size less than 1180 μm and at least 5% by weight of particles less than 3 u, the particle size distribution of which conforms to the formula Alfred according to the equation:

in which D _µi is the median size of the particle size range considered in µm, D the size of the smallest particles in µm, D _L the size of the largest particles, n an exponent whose value varies from 0.2 to 1.0 , characterized in that a hydrocyclone is supplied with crushed particles, the size of the largest particles of which is less than X μm, X being at most equal to 1180 μm, said hydrocyclone operating in a separation in which 90% of the cutting edge varying between 1/4 and 1/2 of X flows in underflow and 10% of the same slice in overflow, the overflow feeding a second hydrocyclone having the same cutting characteristics as the first, the overflow of the second hydrocyclone being combined with the underflow of the first and forming a first constituent of the final mixture, the underflow of the second hydrocyclone being ground in a fine mill into a mixture of particles whose size for 80% measures between 1/10 and 1/20 of X and which forms a second constituent of the final mixture, the two constituents being poured into a mixing tank in which the solid content is at least 45%. 2. Method according to claim 1, characterized in that the raw fuel is first crushed in a grinder of the type of bar crusher producing relatively coarse grains which, screened on a screen leave passers-by constituting the relatively coarse fraction of the final product and rejects which are subjected to finer grinding in a ball type grinder whose screened grinding products leave passers-by intended to constitute the medium and fine slices of the final product, said pass fraction feeding the first, then the second hydrocyclone . 3. Method according to claim 1 or 2, characterized in that the refusals of the first grinding of the type of bar crusher circulate in a closed circuit between the grinder of the ball type and a screen of the same characteristics as the screen used to screen the product from the first bar type crusher. 4. Method according to one of claims 1 to 3, characterized in that the passers-by resulting from the screening of products from the first grinder of the bar type and the passers-by resulting from the screening of products resulting from the screening of products from the second grinder of the ball type supply the first, then the second hydrocyclone. 5. Method according to one of claims 1 to 3, characterized in that the passers-by resulting from the screening of products of the first grinding are poured directly into the mixing tank as final constituent. 6. Method according to one of claims 1 to 5, characterized in that the head of the second hydrocyclone is ground in a mill by attrition. 7. Method according to one of claims 1 to 6, characterized in that the raw fuel in particles from 0 to 15 mm is ground in a first bar crusher in water, the grinding product is sorted on a sieve whose the mesh opening is 200 μm, the rejects of the sieve feeding a second ball mill in water, the grinding product is sieved on a sieve whose mesh opening is 200 μm and the refusals recirculated in circuit closed towards the ball mill, the fractions passing from the two sieves feed a first, then a second hydrocyclone. 8. Method according to one of claims 1 to 7, characterized in that the product of the first mill is classified into two fractions, each of which constitutes 45 to 55% of the ground product, the pass fraction is directly poured into the final mixing tank as a final constituent, while the rejects are subjected to grinding in the ball mill and the classified grinding product on a sieve whose passers-by feed a first, then a second hydrocyclone. 9. Method according to one of claims 1 to 8, characterized in that any product resulting from a previous treatment and having a particle size distribution close to that produced by a grinder of the bar crusher type can replace the product resulting from the first bar crusher. 10. Method according to one of claims 1 to 9, characterized in that the first hydrocyclone operates a separation in which 90% of the cutting section of 70 µm flows underflow and 10% of this same section flowing in overflow of this first hydrocyclone feed the second hydrocyclone operating a separation in which 90% of the cutting section of 70 μm flows in underflow to supply a attritor mill in which the solids are ground to the maximum size of 15 μm, while that the underflow product of the first hydrocyclone is combined with the overflow product of the second hydrocyclone to be mixed in the final mixing tank with the product leaving the attritor mill.

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