EP0848742B1

EP0848742B1 - Process for the preparation of coal aqueous suspensions and new anionic dispersing agents contained therein

Info

Publication number: EP0848742B1
Application number: EP96923980A
Authority: EP
Inventors: Riaz Ahmed Khan; Paolo Mariotti; Luciano Navarini; Roberto Gilli; Paul Adam Konowicz
Original assignee: Fbc Future Biologic Chemicals Srl
Current assignee: Fbc Future Biologic Chemicals Srl
Priority date: 1995-07-20
Filing date: 1996-07-09
Publication date: 1999-10-20
Anticipated expiration: 2016-07-09
Also published as: IT1275577B; AU6419096A; WO1997004046A1; EP0848742A1; DE69604814D1; CN1140752A; ITMI951568A1; ITMI951568A0

Description

FIELD OF THE INVENTION

The present invention concerns a process for the preparation of coal aqueous suspensions having high fluidity and stability and high coal loading, containing new anionic dispersing agents, advantageously useful in coal transportation through piping, even to long distances, and in direct use as fuel.

PRIOR ART DISCLOSURE

Nowadays coal represents a valid source of energy alternative to petroleum; in fact it is widely available, being present in huge deposits equally distributed in the different parts of the world, and it is a potentially inexhaustible natural resource.

However, unlike petroleum, coal is present in nature in solid state and therefore it involves transport, preservation and use processes more onerous, expensive and problematic than the ones required by liquid hydrocarbons.

In fact, coal cannot be pumped in piping in solid form; in order to overcome such a problem, coal is usually finely grinded and dispersed in water in the form of powder, thus obtaining aqueous suspensions advantageously conveyable in piping and transportable even to long distances.

However, such a method turns out to be advantageous only if coal water slurries have high coal concentrations, at least higher than 60% by weight with respect to the total suspension. In fact, in these conditions the transport is carried out with acceptable efficiency levels and the transported suspension may be directly used as fuel in the place of destination.

To the contrary, when coal loading in the suspensions is not high enough, the transportation efficiency is reduced and dewatering processes become expensive and laborious in order to reduce moisture levels before combustion. Therefore, in this case, commercial advantages of coal transportation by the formation of coal water slurries are lost. The increase in coal loading however determines an increase in suspension viscosity and a subsequent loss of fluidity, which render coal transportation along piping impossible and avoid its final use.

When coal concentration in aqueous suspensions exceeds 30% by weight, it is ordinarily notable a drastic increase in the viscosity of aqueous suspensions themselves, if no dispersing agent is added. However, this critical value of coal concentration can vary, being dependent on the kind and the size of the coal particles used. In fact, with high coal loadings, the supension viscosity is mainly determined by coal particles size, by their distribution and by the optional presence of suitable chemical dispersing agents in the suspension itself. The reduction in size of the coal particles and the optimization of the distribution of their size improve the suspension viscosity; however, such processes are quite expensive.

The addition of suitable dispersing agents is therefore necessary in order to improve the viscosity of coal water slurries at industrially acceptable costs.

The term "dispersing agent" indicates a compound able to act at the interface between the liquid phase (for instance water) and the solid phase (for instance coal particles), preventing the agglomeration of the solid particles and allowing the obtainment of a stabilized dispersion state, capable of flowing.

Many processes for the preparation of coal suspensions in a water carrier, as well as advantages offered by transportation and use of coal aqueous suspensions having high contents of solids are well known in the state of the art.

US Patent no. 4,477,259 describes a process for the preparation of coal aqueous slurries having low viscosity and containing from 60 to 82% by volume of coal, wherein an amount of 12-50% by weight of coal is constituted by particles with granulometry ranging from 0.5 to 40 µm, while an amount of 50-98% of coal particles have an average size higher than 40 µm. Moreover, such suspensions are characterized by the addition of 0.01-4% by weight of a dispersing agent of different nature, both inorganic (such as NaOH) and organic (of anionic, cationic, not ionic or amphoteric type; preferably anionic, such as polyacrylate); furthermore, said dispersing agent can be a polyelectrolyte (such as condensed naphthalenesulfonic acid salt) or an inorganic electrolyte.

The above suspensions have the great disadvantage of requiring a specific distribution of coal particles, selected from two different size ranges, which causes additional working costs and times.

European Patent Application EP 0 050 412 A2 describes a process for the preparation of coal water suspensions in the presence of salts of organosulfonates with alkaline earth metals, and particularly calcium lignin-sulfonate. However, even in this case, coal particles must present a specific granulometric distribution, which is necessary to obtain suitable levels of fluidity and stability of the suspension: 10-30% by weight of the suspension must consist of particles having sizes of less than 10 µm, while the remaining coal content must consist of particles having sizes ranging from 20 to 200 µm. Furthermore, the anionic dispersing agents used (lignin-sulfonate derivatives) derive from sources of exhaustible and not renewable raw materials, with more and more difficult perspectives of reperibility in the future.

US Patent no. 4,330,301 uses, as dispersing agents, products of sulphonation of polycyclic aromatic compounds or the condensation products of the latter with formaldehyde, and particularly the condensation products of the formaldehyde with naphthalenesulphonates. However, such stabilizing agents have the disadvantage of being produced by industrial processes with high environmental impact, requiring the use of toxic reagents, such as for instance formaldehyde, and leading to the formation of by-products of difficult and expensive draining. Such disadvantages take place both in the synthesis and in the final use of the product in which they are contained. Such drawbacks are moreover particularly evident in the light of the increasing sensitiveness to environmental protection and of the increasing demand of ecocompatible products, having a reduced environmental impact.

Among the above mentioned anionic dispersing agents, polyelectrolytes of lignin-sulfonate kind, condensation products of formaldehyde with naphthalenesulfonates and polyacrylates have the further disadvantage of being effective only when used with particular kinds of coal; moreover, they do not show the ideal chemical characteristics for final combustion (G. Bognolo, "Coal-Water Slurries", Industrial application of surfactants; The proceedings of a symposium organized by the North West Region of the Industrial Division of the Royal Society of Chemistry, University of Salford, 15-17th April 1986, Special Publications; no. 59, editor D. R. Karsa, pag. 235-249). Such technical drawbacks could be partially eliminated by using some non-ionic dispersing agents, such as ethylene oxide derivatives; however such products, having non-ionic nature, are able to act only through stabilizing effects of steric and non electrostatic kind. Therefore, when steric stabilization is not sufficient, for instance in the case of oxidized coals, it is necessary to modify the structure of these products by the presence of specific chemical groups; this inevitably implies a remarkable increase in costs.

In the UK patent application no. GB 2 099 451, the salts of polymerized alkyl naphthalenesulfonic acids with monovalent cations are used as dispersing agents, optionally in association with stabilizers of polysaccharidic nature, in order to obtain a suspension having good characteristics of stability and fluidity, even after long periods of time. However, the presence of two separate groups of coal particles, having different granulometries, is required; the described aqueous coal suspension comprises 40-60% by weight of coal particles with an average size of 60-210 µm, while the particles of the second group must have an average size of from 1/6 to 1/20 of the average size of the particles of the first group. Coal's grinding at two different granulometry levels implies an unavoidable increase of working costs and times.

SUMMARY OF THE INVENTION

The Applicant has now unexpectedly found a process for the preparation of coal aqueous suspensions having high fluidity and stability and high coal content, comprising the following steps:

A) admixing:

coal having a particle size not higher than 400 µm, in a quantity ranging from 50 to 80% by weight with respect to the total suspension;
at least an anionic dispersing agent, essentially consisting of starch sulfate, optionally esterified and/or mixed with aryl acids, arylaliphatic acids, the corresponding salts or the corresponding anhydrides, said dispersing agent being optionally salified;
water;

B) stirring the mixture obtained in step (A).

The process of the invention allows the obtainment of coal aqueous suspensions having low viscosity and high coal content, containing the above-mentioned anionic dispersing agents.

It is another object according to the present invention a new class of anionic dispersing agents consisting of starch sulfate, mixed and/or esterified with aryl acids or arylalkyl acids, the corresponding salts and the corresponding anhydrides, said anionic dispersing agents being optionally salified with cations of alkali metals, alkaline-earth metals or ammonium cations.

DESCRIPTION OF THE DRAWINGS

Figure 1 shows the rheologic behaviour of the coal aqueous suspensions of the invention, containing coal with an ash content lower than 10% by weight.

Figure 2 shows the rheologic behaviour of the coal aqueous suspensions of the invention, containing coal with an ash content higher than 10% by weight.

Figure 3 shows the rheologic behaviour of the coal aqueous suspensions of the invention, containing coal with an ash content lower than 10% by weight.

DETAILED DESCRIPTION OF THE INVENTION

The characteristics and the advantages of the process for the preparation of coal aqueous suspensions, of the thus obtained coal aqueous suspensions and of the new class of anionic dispersing agents, according to the present invention, will be better illustrated in the following detailed description.

The Applicant has surprisingly found that the disadvantages of the processes known in the state of the art can be overcome by a new process for the preparation of aqueous suspensions having high fluidity and stability and high coal loading, comprising the mixing of water and coal together with a new class of anionic dispersing agents, essentially consisting of starch sulfate, optionally esterified and/or mixed with aromatic acids, the corresponding salts or the corresponding anhydrides.

In the present description, the term "starch" means a polysaccharide constituted by unities of D-(+)-glucose linked by α-glucoside bonds, both the α-(1→4) bonds, typical of the amylose, and the α-(1→6) bonds, characteristic of the amylopectin.

The term "dispersing agent" means a compound able to disperse, fluidify and stabilize coal suspensions in water.

The coal aqueous suspensions according to the present invention contain coal in a quantity ranging from 50 to 80% by weight with respect to the weight of the final suspension, and preferably from 55 to 75%. Coal particle size is lower than 400 µm; therefore, said suspensions do not require either particular granulometry distributions of coal particles or the presence of several groups having different particle sizes.

Coal powder, having the above-mentioned granulometry, can be prepared according to different processes known in the state of the art, such as dry grinding or wet grinding, i.e. in presence of water, and preferably by wet grinding.

According to the present invention, each kind of coal, having different origins, can be advantageously used, such as anthracite, semianthracite, bituminous and subbituminous coal, lignite and coke. The process of the present invention allows to obtain fluidified coal water suspensions, even with coals having an ash content lower than 10% by weight, which are considered particularly hard by the skilled-man in the art.

The word "ash" indicates impurities not made of coal, such as inorganic sulphur, various metallic sulphides, clay and other earth particles.

The anionic dispersing agents of the invention allow to obtain aqueous suspensions having desired rheologic properties and stability, even using kinds of coal having a very low ash content, for instance of 4% by weight. which are considered particularly difficult to pump. To the contrary, this is not possible using the dispersing agents up to now known in the state of the art, which often need further addition of stabilizing agents in order to obtain the desired stability and fluidity, when coals with very low ash contents are used.

The coal aqueous suspensions of the invention can further contain inerts accompanying the coal, or said inerts can be previously separated from coal, by procedures known in the state of the art.

In the preparation of the above mentioned suspension, any kind of water can be advantageously used, independently of the degree of hardness; even waste-waters coming from industrial processes can be advantageously used.

Said coal aqueous suspensions contain at least an anionic dispersing agent according to the present invention, in a quantity ranging from 0.01 to 5% by weight, with respect to the weight of the final aqueous suspension.

Such dispersing agents have an average molecular weight higher than 1,000 dalton, preferably ranging from 1,000 to 2,000,000 dalton, and more preferably from 2,000 to 1,500,000 dalton.

Said anionic dispersing agents essentially consist of starch sulfate, wherein sulfate groups may vary from 5% to 61% by weight of the total agent; the degree of substitution (DS) ranges from 0.05 to 3, and preferably from 0.1 to 2.8.

The phrase "degree of substitution (DS)" indicates the number of functionalized hydroxyls per glucoside monomer of starch. Said anionic dispersing agents may be moreover esterified and/or mixed with organic acids, the corresponding salts or the corresponding anhydrides, as it will be subsequently pointed out in detail.

Said anionic dispersing agents may be moreover salified with alkaline metal cations, preferably Na⁺ and K⁺, with alkaline-earth metal cations, preferably Ca²⁺ and Mg²⁺, or with ammonium cations, preferably NH₄ ⁺.

In the process according to the present invention, said anionic dispersing agents can be optionally used in mixture with other known anionic dispersing agents, such as lignin-sulfonates and their salts, condensation products of naphthalenesulfonates with formaldehyde and their salts, polyacrylates and their salts, polystyrene sulfonates and their salts.

The coal aqueous suspensions of the invention can be prepared according to the methodologies known in the state of the art; the addition order of the various ingredients for the preparation of the coal aqueous suspension does not turn out to be a critical factor. The anionic dispersing agents according to the present invention may be added to coal aqueous suspension both in solid form and in the form of aqueous solution, and preferably in the form of aqueous solution. Furthermore, said dispersing agents can be added to water before the dispersion of coal particles, or can be added to the coal suspension in water; preferably, they are added before the dispersion of coal particles.

The stirring in step (B) can be carried out preferably at a slippage gradient of at least 100 s^-1.

A further object of the present invention is to provide coal aqueous suspensions, containing at least one of the above mentioned anionic dispersing agents, which can be advantageously obtained by the above-described process.

Such suspensions are characterized by high levels of fluidity and stability, both static and dynamic, and they are able to maintain high levels of homogeneity, even after long periods of time.

Such properties allow the use of the suspensions of the invention in coal transportation along suitable piping systems and their direct use as fuel. In fact said aqueous suspensions show very good characteristics of combustion.

Moreover the aqueous suspensions of the invention, as it has been already formerly pointed out, have the great advantage of allowing the use of any kind of coal.

The rheologic behavior of the coal aqueous suspensions comprising a coal having an ash content lower than 10% and at least an anionic dispersing agent of the invention consisting of starch sulfate, under the same coal concentration and coal particle size, can vary from dilatant (characterized by an increase in the viscosity with an increase in the slippage gradient) to pseudoplastic (characterized by a decrease in the viscosity with an increase in the slippage gradient) with slippage threshold, depending on the degree of substitution of the starch with sulfate groups.

In particular, the behaviour of the suspensions is dilatant for degrees of substitution lower than 0.7, while it is pseudoplastic for degrees of substitution higher than 1.4, as it is evidenced in Fig. 1. The rheologic behaviour of coal aqueous suspensions comprising a coal with an ash content higher than 10% and at least one anionic dispersing agent of the invention. under the same coal concentration and coal particle size, is of pseudoplastic kind with slippage threshold and it does not depend on the degree of substitution of the starch with sulfate groups, as it is evidenced in Fig. 2.

Finally, the rheologic behaviour of coal aqueous suspensions comprising a coal with an ash content lower than 10% and at least an anionic dispersing agent of the invention in mixture and/or esterified with aromatic acids. their salts or anhydrides, preferably sodium benzoate or sodium pnthalate, under the same coal concentration and coal particle size, can vary from dilatant to pseudoplastic with slippage threshold, therein included the newtonian kind, as it is reported in Fig. 3.

Therefore, from the data reported in the figures, it is evident that the use of the anionic dispersing agents of the invention allows the treatment of any kind of coal, even with ash contents lower than 10%, which are considered of very difficult employment in the state of the art. Surprisingly, by using the above mentioned dispersing agents, it is possible to obtain rheologic behaviours of coal aqueous suspensions which are considered as ideal, such as the newtonian and the slightly pseudoplastic ones. The obtainment of such rheologic properties, using coals having low ashes content, is not usually obtainable with the dispersing additives known in the state of the art, which need the further addition of stabilizing agents. such as clay or polysaccharides, in order to obtain suspensions with good characteristics of stability and fluidity. even after long periods of time.

A further object of the present invention is constituted by new anionic dispersing agents, able to disperse, fluidify and stabilize coal aqueous suspensions, as it has been previously described. The anionic dispersing agents according to the present invention may be easily obtained, according to processes known in the state of the art. starting from starch, a raw material which is largely available on the market at low costs; moreover they have the great advantage of being ecocompatible. Said dispersing agents may be prepared from starch by a sulfation reaction, which may be carried out both in homogeneous systems, aqueous or non-aqueous, and in heterogeneous systems, aqueous or non-aqueous, and furthermore in solid state.

Preferably, the sulfation reaction is carried out in non-aqueous systems, homogeneous or heterogeneous, using chlorosulfonic acid and formamide, according to the processes known in the state of the art (F. Schierbaum and K. Kordel "Reaction of Starch with the Chlorosulfonic Acid-Formamide Reagent", Carbohydrate Sulfates, ACS Symposium Series no. 77, 1978, Ed. R. G. Schweiger, pages 173-192). In fact, the use of the chlorosulfonic acid/formamide system as sulfating reagent renders such a reaction particularly advantageous, since formamide is able to act as swelling and solubilizing agent for the starch, as complexing agent with chlorosulfonic acid, as buffer agent during the reaction and finally as solubilizing agent for the reaction products.

The above-described sulfation reaction allows to obtain, working at suitable temperatures and at suitable molar ratios of reagents, starch sulfate with different degrees of substitution, ranging from high values (DS>1.4) to low values (DS<0.7). Furthermore, said anionic dispersing agents can contain up to 35% by weight of reaction secondary products, such as for instance starch, oligosaccharides and oligosaccharide sulfates.

According to a particular embodiment of the present invention, in said anionic dispersing agents, the free hydroxyls of the starch are esterified with acyl residues of aryl acids or arylaliphatic acids; the number of hydroxyls esterified with said acyl residues per glucoside monomer (i.e. the degree of substitution) preferably ranges from 0.001 to 1, and more preferably from 0.01 to 0.8. Said radicals are acyl radicals of mono, bi or tricarboxylic acids, containing up to 20 carbon atoms, and preferably up to 9 carbon atoms, aryl or arylalyphatic, saturated or unsaturated, optionally substituted on the aromatic ring and/or on the alkyl chains with alkyl radicals C₁-C₉, linear or branched.

Such radicals are preferably the acyl radicals of benzoic, phenylacetic, toluic, phthalic, isophthalic, terephthalic, naphthoic, diphenic, naphthalic or cinnamic acid; more preferably, said acyl radicals derive from benzoic or phthalic acid.

Said acyl radicals can be bound to the free hydroxyls of the starch by an esterification reaction, according to one of the procedures known in the state of the art, prior to starch sulfation.

Such esterification reaction is carried out preferably in Schotten-Baumann conditions, modified in organic solvents; in this case the esterification reaction and the subsequent sulfation reaction can be carried out one pot, in the same reaction solvent and in the same container or reactor, without isolating intermediate products. Such process presents therefore the advantage of reducing working times and costs.

The esterification may be carried out even using activated forms of the above-mentioned acids, known in the state of the art, such as the corresponding anhydrides, preferably benzoic anhydride or phthalic anhydride, the corresponding esters or the corresponding acyl chlorides.

According to a further aspect of the present invention, the anionic dispersing agents, consisting of starch sulfate as formerly described, are mixed with aromatic compounds, contained in concentrations ranging from 20 to 80% by weight with respect to the final weight of the dispersing agent, and preferably from 30 to 60%.

Said aromatic compounds are mono, di or tricarboxylic acids, containing up to 20 carbon atoms, and preferably up to 9 carbon atoms, aryl or arylaliphatic, saturated or unsaturated, optionally substituted on the aromatic ring and/or on the alkyl chains with alkyl radicals C₁-C₉ linear or branched, the corresponding anhydrides, both pure and mixed, and the corresponding salts with alkaline metal, alkaline-earth metal or ammonium cations.

Said aromatic compounds are preferably selected from the group consisting of benzoic acid, phthalic acid, naphthoic acid, alkylbenzoic acids comprising an alkyl chain C₁-C₉, linear or branched, saturated or unsaturated, alkylphthalic acids comprising an alkyl chain C₁-C₉, linear or branched, saturated or unsaturated, alkylnaphthoic acids comprising an alkyl chain C₁-C₉, linear or branched, saturated or unsaturated, the corresponding salts with alkaline metal, alkaline-earth metal or ammonium cations, and the corresponding anhydrides. More preferably, said aromatic compounds are selected from the group consisting of benzoic, phthalic and phenylacetic acid, the corresponding salts with alkaline metal, alkaline-earth metals or ammonium cations, and the corresponding anhydrides.

The above-mentioned alkaline metal cations are preferably sodium or potassium ions; the above-mentioned alkaline-earth metal cations are preferably calcium or magnesium ions; finally the above-mentioned ammonium cation is preferably NH₄ ⁺.

The process for the preparation of the anionic dispersing agents of the invention consists in admixing starch sulfate, as described above, with said aromatic compounds. The use of said anionic dispersing agents in mixture with aromatic compounds, not chemically bound, turns out to be more profitable from the economical point of view with respect to the use of the dispersing agents of the invention wherein the hydroxyl groups of the starch are esterified with said aromatic acyl radicals.

The following examples are reported for illustrative purposes.

EXAMPLE 1 Preparation of an anionic dispersing agent of the invention, consisting of starch sulfate with a degree of substitution DS=0.57.

25 ml of formamide were introduced into a 500 ml three-necked jacketed flask, provided with a mechanical stirrer having teflon blades, dripping funnel and thermometer, connected to a thermocryostat, and 18.6 ml of chlorosulfonic acid were dropped, in a period of about 30 minutes, under constant stirring, at a temperature lower than 10 °C. To the thus obtained mixture, 200 ml of formamide and 25 g of "Starch soluble" (Aldrich) starch, having a moisture content of 10% by weight, were slowly added under constant stirring.

The temperature was slowly raised to 20 °C. under constant stirring, and after 30 minutes it was raised to 50 °C. After 60 minutes, the product was recovered by precipitation from MeOH. The precipitate was filtered, washed with MeOH (250 ml) and dried in vacuum stove, at the temperature of 50 °C, yielding 31.9 g of starch sulfate with a degree of substitution of 0.57.

The physico-chemical characteristics of the obtained starch sulfate were the following:
Molecular weight: 75,000 dalton.
FT-IR spectrum: two strong bands, not present in the spectrum of the starting starch, were detectable; said bands were centred at about 1259 cm^-1 and 826 cm^-1 and were attributable respectively to S=0 stretching and to S-O-C symmetric stretching. Such data confirmed that sulfation of starch hydroxyls had taken place.
¹³C-NMR Spectrum: in addition to starch typical signals. further peaks were detectable at 80.00 ppm, 70.53 ppm and 68.56 ppm, attributable to the substitution in the different accessible positions of the glucosidic unity and mainly in position C6.
Colorimetric analysis: this analysis revealed a content of sulfate groups of 25% w/w, corresponding to a DS of 0.57.

EXAMPLE 2 Preparation of an anionic dispersing agent of the invention, consisting of starch sulfate with DS = 2.14, salified with sodium.

50 ml of formamide were introduced into a 500 ml three-necked jacketed flask, provided with a mechanical stirrer having teflon blades, dropping funnel and thermometer, connected to a thermocryostat, and 38.0 ml of chlorosulfonic acid were dropped, during about 30 minutes, under constant stirring, at a temperature lower than 10 °C.

To the thus obtained mixture, 200 ml of formamide and 25 g of "Starch soluble" (Aldrich) starch, having a moisture content of 10% by weight, were slowly added, under constant stirring.

After 30 minutes at 20 °C, the temperature was raised to 50 °C, under constant stirring. After 60 minutes, the temperature was then raised to 70 °C and the mixture was maintained under stirring for 30 minutes. The reaction product was recovered by precipitation from MeOH (1 liter). The precipitate was filtered, washed with methanol (250 ml) and solubilized in water.

The aqueous solution was neutralized with NaOH (0.1 M solution). The product was recovered by precipitation from isopropyl alcohol (800 ml); the precipitate was filtered, washed with MeOH (250 ml) and finally dried in a vacuum stove, at the temperature of 50 °C, yielding 49.2 g of starch sulfate, with a degree of substitution of 2.14, salified with sodium. The physico-chemical characteristics of the obtained starch sulfate salified with sodium were the following:
Molecular weight: 22,000 dalton.

FT-IR spectrum: two strong bands, not present in the spectrum of the starting starch, were detectable; said bands were centred at about 1259 cm^-1 and 826 cm^-1, attributable respectively to S=0 stretching and to S-O-C symmetric stretching. Such data confirmed that occurred sulfation of starch hydroxyls had taken place.
¹³C-NMR Spectrum: in addition to starch typical signals, further peaks were detectable at 77.05 ppm, 69.84 ppm and 67.00 ppm, attributable to the substitution in the different accessible positions of the glucoside unity and mainly in position C6.
Colorimetric analysis: this analysis revealed a content of sulfate groups of 54% w/w, corresponding to a DS of 2.14.

EXAMPLE 3 Preparation of an anionic dispersing agent of the invention, consisting of starch sulfate with DS = 0.67 and benzoylated with DS = 0.1.

25 g of "Starch soluble" (Aldrich) starch, having a moisture content of 10% by weight, were introduced into a 500 ml three-necked jacketed flask, provided with a mechanical stirrer having teflon blades, dropping funnel and thermometer, connected to a thermocryostat, and 50 ml of formamide and 5.55 g of NaOH in the form of tablets were added.

The thus obtained suspension was maintained under constant stirring, at a temperature of 5 °C, and 16.12 ml of benzoyl chloride were dropped, during about 30 minutes, under constant stirring. The mixture was maintained at 5 °C for 60 minutes; then the temperature was raised to 25 °C and the mixture was still maintained under stirring for 16 hours.

After cooling of the mixture at 5 °C, 13.85 ml of chlorosulfonic acid was dropped, during about 30 minutes, under constant stirring. The temperature was raised to 50 °C and the mixture was maintained under stirring for 4 hours. The product, recovered by precipitation from isopropylic alcohol (200 ml), was finally filtered and dried in a vacuum stove, at the temperature of 50 °C, yielding 100.0 g of starch sulfate with DS of 0.67 and benzoilated with DS of 0.1. The physico-chemical characteristics of the obtained sulfated and benzoilated starch were the following:
Molecular weight: 48,000 dalton.
FT-IR spectrum: two strong bands, not present in the spectrum of the starting starch, were detectable; said bands were centred at about 1259 cm^-1 and 826 cm^-1, attributable respectively to S=0 stretching and to S-O-C symmetric stretching. Such data confirmed that sulfation of starch hydroxys had taken place.

Moreover, one band at about 1730 cm^-1, not present in the starch spectrum, corresponding to the esteric stretching C=0 was detected, in confirmation of the occured benzoilation.
UV Spectrum: it confirmed the presence of aromatic groups.
¹H-NMR Spectrum: a system of peaks at about 8.00 ppm, attributable to the aromatic protons was detectable. According to such a spectrum, the DS relative to the benzoilation was 0.1.
Colorimetric analysis: this analysis revealed a content of sulfate groups of 27.8% w/w, corresponding to a DS of 0.67.

EXAMPLE 4 Preparation of an anionic dispersing agent of the invention, consisting of starch sulfate with DS = 0.5 and benzoylated with DS = 0.05.

20 g of "Starch soluble" (Aldrich) kind starch, having a moisture content of 10% by weight, were introduced into a 500 ml three-necked jacketed flask, provided with a mechanical stirrer having teflon blades, dropping funnel and thermometer, connected to a thermocryostat, and 50 ml of formamide and 4.44 g of NaOH in the form of tablets were added.

The thus obtained suspension was maintained under constant stirring, at a temperature of 5 °C, and 12.9 ml of benzoyl chloride were dropped, during about 30 minutes, under constant stirring. The mixture was maintained at 5 °C for 60 minutes, then the temperature was raised to 25 °C and the mixture was still maintained under stirring for 16 hours.

After cooling of the mixture at 5 °C, 11.08 ml of chlorosulfonic acid were dropped, in a period of about 30 minutes, under constant stirring. The temperature was raised to 50 °C and the mixture was maintained under stirring for 4 hours. The product recovered by precipitation from isopropylic alcohol (200 ml) was finally filtered and dried in a vacuum stove, at the temperature of 50 °C, yielding 100.0 g of starch sulfate with DS of 0.5 and benzoilated with DS of 0.05. The physico-chemical characteristics of the obtained sulfated and benzoilated starch were the following:
Molecular weight: 8,300 dalton.
FT-IR spectrum: two strong bands, not present in the spectrum of the starting starch. were detectable; said bands were centred at about 1259 cm^-1 and 826 cm^-1, attributable respectively to S=0 stretching and to S-O-C symmetric stretching. Such data confirmed that sulfation of starch hydroxyls had taken place.

Moreover, one band at about 1730 cm^-1, not present in the starch spectrum, corresponding to the esteric stretching C=0 was detectable, in confirmation of the occured benzoilation.
UV Spectrum: it confirmed the presence of aromatic groups.
¹H-NMR Spectrum: a system of peaks at about 8.00 ppm, attributable to the aromatic protons was detectable. According to such a spectrum, the DS relative to the benzoilation was 0.05.
Colorimetric analysis: this analysis revealed a content of sulfate groups of 21.8% w/w, corresponding to a DS of 0.5.

EXAMPLE 5 Preparation of an anionic dispersing agent of the invention, consisting of starch sulfate with DS = 0.35 and benzoylated with DS = 0.08, salified with sodium.

10 g of "Starch soluble" (Aldrich) starch, having a moisture content of 10% by weight, were introduced into a 500 ml three-necked jacketed flask, provided with a mechanical stirrer having teflon blades, dropping funnel and thermometer, connected to a thermocryostat, and 40 ml of formamide and 2.2 g of NaOH in the form of tablets were added.

The thus obtained suspension was maintained under constant stirring, at a temperature of 5 °C, and then 6.45 ml of benzoyl chloride were dropped, in a period of about 30 minutes, under constant stirring. The mixture was maintained at 5 °C for 30 minutes; after raising the temperature to 25 °C, the mixture was still maintained under stirring for 16 hours.

After cooling the mixture at 5 °C, 7.4 ml of chlorosulfonic acid were dropped, during about 30 minutes, under constant stirring. The temperature was raised to 50 °C and the reaction mixture was maintained under stirring for 2 hours. The product was recovered by precipitation from isopropylic alcohol (200 ml) and the supernatant liquor was decanted. The so recovered product was dried in a vacuum stove, at the temperature of 50 °C, yielding 20.0 g of starch sulfate with DS of 0.35 and benzoilated with DS of 0.08. Such a product was finally solubilized in 300 ml of water, neutralized with a solution of NaOH (0.1 M) and then placed in dialysis against water. The product salified with sodium was recovered by lyophilization.

The physico-chemical characteristics of the obtained sulfated and benzoilated starch, salified with sodium. were the following:
Molecular weight: 95,000 dalton.
FT-IR spectrum: two strong bands, not present in the spectrum of the starting starch, were detectable; said bands were centred at about 1259 cm^-1 and 826 cm^-1, attributable respectively to S=0 stretching and to S-O-C symmetric stretching. Such data confirmed that sulfation of starch hydroxyls had taken place.

Moreover, one band at about 1730 cm^-1, not present in the starch spectrum, corresponding to the esteric stretching C=0 was detected, in confirmation of the occured benzoilation.
UV Spectrum: it confirmed the presence of aromatic groups.
¹H-NMR Spectrum: a system of peaks at about 8.00 ppm, attributable to the aromatic protons was detectable. According to such a spectrum, the DS relative to the benzoilation was 0.08.
Colorimetric analysis: this analysis revealed a content of sulfate groups of 21.8% w/w, corresponding to a DS of 0.5.

EXAMPLE 6 Preparation of an anionic dispersing agent of the invention, consisting of starch sulfate with DS = 0.57 and esterified with phthalic anhydride with DS = 0.38.

65 ml of formamide were introduced into a 500 ml three-necked jacketed flask. provided with a mechanical stirrer having teflon blades, dropping funnel and thermometer, connected to a thermocryostat, and they were heated at a temperature of 60 °C. After the addition of 10 g of "Starch soluble" (Aldrich) starch, having a moisture content of 10% by weight, the mixture was maintained under constant stirring for 30 minutes. 4.5 ml of pyridine and 4.13 g of phthalic anhydride were then added and the mixture was maintained at 60 °C, under constant stirring, for 120 minutes. After lowering the temperature to 20 °C, the mixture was maintained under stirring for 96 hours; after cooling at a temperature lower than 10 °C, 8.5 ml of chlorosulfonic acid were then dropped into the mixture, during about 30 minutes, under constant stirring.

After slowly raising the temperature to 50 °C, the mixture was maintained under constant stirring for 120 minutes. The reaction product was then precipitated from MeOH (400 ml), filtered and dried in a vacuum stove, at the temperature of 50 °C, yielding 18.83 g of starch sulfated with DS of 0.57 and phthaloilated with DS of 0.38. The physico-chemical characteristics of the obtained sulfated and phthaloilated starch, salified with sodium ions, were the following:
Molecular weight: 80,000 dalton.
FT-IR spectrum: two strong bands, not present in the spectrum of the starting starch, were detectable; said bands were centred at about 1259 cm^-1 and 826 cm^-1, attributable respectively to S=0 stretching and to S-O-C symmetric stretching. Such data confirmed thta sulfation of starch hydroxyls had taken place.

Moreover, one band at about 1730 cm^-1, not present in the starch spectrum. corresponding to the esteric stretching C=0 was detected, in confirmation of the occured phthaloilation.
UV Spectrum: it confirmed the presence of aromatic groups.
¹H-NMR Spectrum: a system of peaks at about 8.00 ppm, attributable to the aromatic protons, was detectable. According to such a spectrum. the DS relative to the phthaloilation was 0.38.
Colorimetric analysis: this analysis revealed a content of sulfate groups of 25% w/w, corresponding to a DS of 0.57.

EXAMPLES 7-10 Preparation of the anionic dispersing agents of the invention, consisting of starch sulfate with a degree of substitution DS = 0.57, in mixture with sodium phthalate in quantities respectively of 75, 50, 37.5 and 30% by weight, with respect to the weight of the final dispersing agent.

Example 7: 0.1 g of product obtained as described in the Example 1 were mixed with 0.3 g of sodium phthalate.

Example 8: 0.3 g of product obtained as described in the Example 1 were mixed with 0.3 g of sodium phthalate.

Example 9: 0.5 g of product obtained as described in the Example 1 were mixed with 0.3 g of sodium phthalate.

Example 10: 0.7 g of product obtained as described in the Example 1 were mixed with 0.3 g of sodium phthalate.

EXAMPLE 11 Preparation and characterization of coal aqueous suspensions containing the anionic dispersing agents according to the present invention.

68 g of coal of the Kopperston low ash content kind, characterized by a content of volatile matter of 31.9% by weight, fixed carbon of 63.99% by weight, sulfur content of 0.79% by weight, granulometry lower than 250 µm, moisture content of 0.76% by weight and ash content of 4.11% by weight, were introduced into a 250 ml becker and water was added to a total weight of 100 g; to the thus obtained suspension, were finally added 0.7 g of:

an anionic dispersing agent according to the present invention in the solid form, obtained as described in the Examples 1-10 reported above;
a dispersing agent available on the market, in particular:
1) DAXAD 19®, naphthalenesulfonate condensed with formaldehyde, by the Hampshire Chemical Corporation, Kentucky, USA;
2) LOMAR PL®, naphthalenesulfonate condensed with formaldehyde, by the Diamond Shamrock Corporation. New Jersey, USA;
3) TAMOL-N®, naphthalenesulfonate condensed with formaldehyde, by the BASF A. G., Germany;
4) DAXAD 30®, sodium polymethacrylate, by the Hampshire Chemical Corporation, Kentucky, USA;
5) DAXAD 31®, copolymer of sodium maleate and diisobutylene, by the Hampshire Chemical Corporation, Kentucky, USA;
6) VERSA TL 70®, alkylbenzene sulfonate, by the National Starch and Chemical Corporation, New Jersey, USA.

The thus obtained mixtures were stirred at 500 rpm, at room temperature for 30 minutes, using a mechanical stirrer having a 6 blades screw stirring bar.

In order to verify if the coal aqueous suspensions obtained as described above had also advantageous rheologic characteristics, measures of viscosity were carried out at various velocity gradients, and flux curves, using a Rheometrics RFS 8500® rotational rheometer. In particular, said coal aqueous suspensions were introduced into the rheometer slot, thermostated at 25 °C, and viscosity was measured for velocity gradients γ ˙ ranging from 0.05 to 1,000 sec^-1.

Furthermore, flux curves (rheograms) reported in Figures 1-3 were recorded in order to assess the rheologic behaviour of the suspensions under examination. Moreover, the stability of the above-mentioned coal aqueous suspensions was tested after 10 days from their preparation, according to the following procedure:

an aliquot of each suspension, obtained as described above, was poured into a plexiglass cylinder having the inner diameter of 2.1 cm and length cf 18 cm, to a height of 15 cm, and the cylinders were hermetically closed at the two endings.

After a settling period of 10 days at room temperature, in vertical position, the cylinders were placed into a freezer, always in vertical position, and left therein until the complete freezing of the suspensions (generally for one night). The freezed suspensions were then extracted from the cilinders and two sections with thickness of 15 mm were cut from the two endings of each of them with an hacksaw. The solid percentage in each of said sections was determined by weighing, after drying. The suspensions stability was finally expressed as the ratio between the solid percentage of the upper section (at the top) and the solid percentage of the lower section (at the end), multiplied by a factor of 100.

The obtained results are hereinafter reported in Tables 1 and 2. In particular, Table 1 shows the values of viscosity η of the suspensions, measured at the constant value of velocity gradient γ ˙ of 10 s^-1, the rheologic behavior and the percentage stability.

As regards stability, the value 100% corresponds to the maximum obtainable stability, while the value 0% corresponds to the minimum obtainable stability.

Viscosity, rheologic behaviour and stability of the coal aqueos suspensions according to the present invention.
Dispersing agent	Viscosity η Pa^•s (γ ˙=10s^-1)	Rheologic behaviour	Stability %
Example 1	0.67	Slightly dilatant	67
Example 2	0.97	Pseudoplastic	75
Example 3	0.10	Dilatant	65
Example 4	0.10	Dilatant	65
Example 5	0.15	Dilatant	67
Example 6	0.25	Dilatant	70
DAXAD 19®	0.10	Dilatant	3
LOMAR PL®	0.34	Dilatant	5
TAMOL-N®	0.10	Dilatant	3
DAXAD 30®	not fluidified	n.d.	n.d.
DAXAD 31®	not fluidified	n.d.	n.d.
VERSA TL 70®	not fluidified	n.d.	n.d.

The obtained results show that, under the same operative conditions, the anionic dispersing agents according to the present invention are able to provide coal aqueous suspensions with rheologic properties equal or even better than those obtainable with the dispersing agents known in the state of the art, as well as with remarkably superior levels of stability.

Table 2 shows viscosity values η of the suspensions, measured at a velocity gradient γ ˙ of 100 s^-1, and their rheologic behaviour. Table 2 Viscosity and rheologic behaviour of the coal aqueous suspensions according to the present invention.

Dispersing agent	Viscosity η Pa^•s(γ ˙=100s^-1)	Rheologic behaviour
Example 7	0.42	Highly Pseudoplastic
Example 8	0.46	Newtonian
Example 9	0.68	Slightly Pseudoplastic
Example 10	1.20	Slightly Pseudoplastic

The above experimental results confirm the high fluidity and stability of coal aqueous suspensions according to the present invention, showing optimal rheologic characteristics even without the addition of further stabilizing additives; said characteristics remain unaltered even after long preservation periods.

EXAMPLE 12 Preparation and characterization of coal aqueous suspensions containing the anionic dispersing agents according to the present invention.

68 g of coal of the Russian fossil kind, characterized by a content in volatile matter of 24.32% by weight, fixed carbon of 60.4% by weight, sulfur content of 0.49% by weight, granulometry lower than 250 µm, moisture content of 5.16% by weight and ash content of 15.28% by weight, were introduced into a 250 ml becker and water was added to a total weight of 100 g; 0.7 g of an anionic dispersing agent according to the present invention in the solid form, obtained as described in Examples 1 and 2, were added to the suspension.

The thus obtained mixtures were stirred at 500 rpm, at room temperature, for 30 minutes, using a mechanical stirrer having a 6 blades screw stirring bar.

In order to verify if the coal aqueous suspensions obtained as formerly described had also advantageous rheologic characteristics, measures of viscosity were carried out at different velocity gradients, and flux curves, using a Rheometrics RFS 8,500® rotational rheometer. In particular, said coal aqueous suspensions were introduced into the rheometer slot, thermostated at 25 °C, and viscosity was measured for velocity gradients γ ˙ ranging from 0.05 to 1,000 sec^-1.

Furthermore, flux curves (rheograms) reported in Figure 2 were measured, in order to define the rheologic behaviour of the suspensions under examination.

Claims

A process for the preparation of coal aqueous suspensions having high fluidity and stability, comprising the following steps:

A) admixing:

coal having a particle size not higher than 400 µm, in a quantity ranging from 50 to 80% by weight with respect to the total suspension;

at least an anionic dispersing agent, essentially consisting of starch sulfate, optionally esterified and/or mixed with aryl acids, arylaliphatic acids, the corresponding salts or the corresponding anhydrides, said dispersing agent being optionally salified;

water;

B) stirring of the mixture obtained in step (A).
The process according to claim 1, characterized in that coal is contained in a quantity ranging from 55 to 75% by weight.
The process according to claim 1, characterized in that said anionic dispersing agent is contained in a quantity ranging from 0.01 to 5% by weight with respect to the aqueous suspension.
The process according to claim 1, characterized in that said anionic dispersing agent has an average molecular weight higher than 1,000 dalton.
The process according to claim 4, characterized in that said anionic dispersing agent has an average molecular weight ranging from 1,000 to 2,000,000 dalton.
The process according to claim 5, characterized in that said anionic dispersing agent has an average molecular weight ranging from 2,000 to 1,500,000 dalton.
The process according to claim 1, characterized in that said starch sulfate has a degree of substitution of the sulfate groups ranging from 0.05 to 3.
The process according to claim 7, characterized in that said degree of substitution of the sulfate groups is ranging from 0.1 to 2.8.
The process according to claim 1, characterized in that said anionic dispersing agent is salified with alkali metal cations, alkaline-earth metal cations or ammonium cations.
The process according to claim 9, characterized in that said alkali metal is Na or K, said alkaline-earth metal is Ca or Mg, and said ammonium cation is NH₄ ⁺.
The process according to claim 1, characterized in that, in step (B), said stirring is carried out at a slippage gradient of at least 100 s^-1.
A coal aqueous suspension, having high fluidity and stability, comprising:

coal having a particle size not higher than 400 µm, in a quantity ranging from 50 to 80% by weight with respect to the total suspension;

at least an anionic dispersing agent, essentially consisting of starch sulfate, optionally esterified and/or mixed with aryl acids, arylaliphatic acids, the corresponding salts or the corresponding anhydrides, said dispersing agent being optionally salified.
The coal aqueous suspension according to claim 12, characterized in that said anionic dispersing agent is contained in amounts ranging from 0.01 to 5% by weight with respect to the total suspension.
The coal aqueous suspension according to claim 12, characterized in that said anionic dispersing agent has an average molecular weight higher than 1,000.
The coal aqueous suspension according to claim 14, characterized in that said anionic dispersing agent has an average molecular weight ranging from 2,000 to 1,500,000.
The coal aqueous suspension according to claim 12, characterized in that coal is contained in a quantity ranging from 55 to 75% by weight.
The coal aqueous suspension according to claim 12, characterized in that said starch sulfate has a degree of substitution of the sulfate groups ranging from 0.05 to 3.
The coal aqueous suspension according to claim 12, characterized in that said anionic dispersing agent is salified with alkaline metal cations, alkaline-earth metal cations or ammonium cations.
An anionic dispersing agent consisting of starch sulfate, wherein the free hydroxyl groups of starch are esterified by the acyl radicals of aryl or arylaliphatic acids, optionally salified.
The anionic dispersing agent according to claim 19, characterized in having an average molecular weight higher than 1,000 dalton.
The anionic dispersing agent according to claim 20, characterized in that said average molecular weight is ranging from 2,000 to 1,500,000 dalton.
The anionic dispersing agent according to claim 19, characterized in that said starch sulfate has a degree of substitution of the sulfate groups ranging from 0.05 to 3.
The anionic dispersing agent according to claim 19, characterized in being salified with alkaline metal cations, alkaline-earth metal cations or ammonium cations.
The anionic dispersing agent according to claim 19, characterized in that said free hydroxyl groups of starch are esterified by said acyl residues with a degree of substitution ranging from 0.001 to 1.
The anionic dispersing agent according to claim 24, characterized in that said degree of substitution ranges from 0.01 to 0.8.
The anionic dispersing agent according to claim 19, characterized in that said acyl radicals are the acyl radicals of mono, bi or tricarboxylic acids, containing up to 20 carbon atoms, aryl or arylaliphatic, saturated or unsaturated, optionally substituted on the aromatic ring and/or on the alkyl chains with alkyl radicals C₁-C₉, linear or branched.
The anionic dispersing agent according to claim 26, characterized in that said radicals are preferably the acyl radicals of the acids selected from the group consisting of benzoic, phenylacetic, toluic, phthalic, isophthalic, terephthalic, naphthoic, diphenic, naphthalic and cinnamic acids.
The anionic dispersing agent according to claim 27, characterized in that said acids are benzoic or phthalic acids.
An anionic dispersing agent consisting of starch sulfate mixed with aryl acids, arylaliphatic acids, the corresponding salts or the corresponding anhydrides, optionally salified.
The anionic dispersing agent according to claim 29, characterized in having an average molecular weight higher than 1,000 dalton.
The anionic dispersing agent according to claim 30, characterized in that said average molecular weight ranges from 2,000 to 1,500.000 dalton.
The anionic dispersing agent according to claim 29, characterized in that said starch sulfate has a degree of substitution of the sulfate groups ranging from 0.05 to 3.
The anionic dispersing agent according to claim 29, characterized in being salified with alkaline metal cations. alkaline-earth metal cations or ammonium cations.
The anionic dispersing agent according to claim 29, characterized in that said aryl acids, arylaliphatic acids, their salts or anhydrides are contained in a quantity ranging from 20 to 80% by weight.
The anionic dispersing agent according to claim 34, characterized in that said aryl acids, arylaliphatic acids, their salts or anhydrides, are contained in a quantity ranging from 30 to 60% by weight.
The anionic dispersing agent according to claim 29, characterized in that said aryl acids, arylaliphatic acids, their salts or anhydrides are mono, bi or tricarboxylic acids, containing up to 20 carbon atoms, saturated or unsaturated, optionally substituted on the aromatic ring and/or on the alkyl chains with alkyl radicals C₁-C₉, linear or branched.
The anionic dispersing agent according to claim 36, characterized in that said aryl acids, arylaliphatic acids, their salts or anhydrides are selected from the grcup consisting of benzoic, phthalic and naphthoic acids, alkylbenzoic acids comprising an alkyl chain C₁-C₉, linear or branched, saturated or unsaturated, alkylphtalic acids comprising an alkyl chain C₁-C₉, linear or branched, saturated or unsaturated, alkylnaphthoic acids comprising an alkyl chain C₁-C₉, linear or branched, saturated or unsaturated.
The anionic dispersing agent according to claim 37, characterized in that said aryl acids, arylaliphatic acids, their salts or anhydrides are selected from the group consisting of benzoic, phthalic and phenylacetic acid, benzoic and phthalic anhydrides, sodium benzoate and sodium phthalate.