GB2230207A - Dispersants for coal water mixtures - Google Patents

Abstract

A dispersant composition for coal in water comprises an ionic dispersant selected from the alkali, alkaline earth, organic amine or the ammonium salts of naphthalene or lignin sulphonic acids or mixtures thereof, and at least one organic dispersant selected from a methacrylic or acrylic acid water soluble polymer optionally co-polymerised with fumaric acid, itaconic acid, citraconic acid, hydroxy-sulphonate and phosphonate functional monomers, hydrocarbon monomers, vinyl esters, vinyl ethers, methacrylate and acrylate esters, amides and nitriles or compositions thereof.

Description

Dispersants for Coal Water Mixtures This invention relates to dispersant compositions for the production of coal water mixtures, particularly where the coal used is a low grade feedstock of high mineral matter such as high ash washery fines, and other low grade coals obtained during coal beneficiation processes.

The objective of developing coal water mixture technology is to reduce the dependency on oil as an energy source. The utilisation of low grade coal feedstocks for coal water mixture preparation offers the opportunity to produce a more cost-effective, alternative fuel.

Suspensions of coal particles in water have been studied for many years as an alternative fuel for oil in steam raising and process heating applications, presently using heavy fuel oil.

Coal water mixtures can be transported, stored, pumped, atomised and burned in a similar manner to oil. To meet these requirements they should have a high solids loading of coal (within the range 50-80% by weight) and be stable ie. no solids should settle with minimal or no agitation over long periods of time. At the same time, the slurry viscosity should be less than 1000 centipoises (cps) at a shear rate of 100 sec-l measured with a Haake viscometer, and have suitable rheological properties - ideally pseudoplastic.

Without suitable dispersing aids above a concentration of 55% solids, the viscosity of the resulting coal water mixture is too high for utilisation as an alternative fuel to oil. Lower viscosities can be obtained by reducing solids content, but to the detriment of the combustion characteristics of the coal water mixture.

Numerous prior specifications propose the use of anionic, non-ionic and other dispersants, either alone or in combination, for use in coal water mixtures primarily containing high grade coal (low mineral matter). Little attention has been paid to low grade coals (high mineral matter) as feedstocks for coal water mixtures, as conventional dispersants do not give the desired rheological properties at the high solids content required. For example, naphthalene sulphonates and lignosulphonates (effective dispersants for coal in water) have limited applications with low grade coals as at solids loadings approaching 70 per cent by weight, the coal water mixture is too high in viscosity to allow easy handling and efficient atomisation. This has been attributed to the presence of swelling clays that are liberated during the fine grinding stage in the coal water mixture preparation.

Attention has been paid to froth flotation concentrates.

Typically the particle sizes of flotation concentrates are in the range 0-0.5mm, containing generally between 10 and 20% of non-combustible minerals depending on the source of coal. In some cases they may contain in excess of 25%. Essentially the non-combustible materials comprise clays, other fine ground clastic sediments and crystalline materials such as pyrite and carbonate minerals.

GB2178057 discloses a coal in water slurry comprising 50-90t by weight of coal solids including a surfactant of the general formula R-O(-CxH2xO-)n'H wherein R is a straight or branched chain alkyl group or an alkaryl group, x is an integer from 2-6, and n has a value from 1-200. Although such a surfactant is proposed as being suitable for any grade of coal or anthracite, particular attention was paid to coal particles recovered from froth flotation procedures. However, such coal water mixtures have not been introduced commercially as uneconomical amounts of additive were required to produce high solids dispersions at low viscosity. Furthermore, the coal water mixtures disclosed in this specification were not intended for use in oil retrofit applications where the fuel is to be atomised.The particle size distribution is much coarser than would be required for efficient burn-out.

The present invention provides a dispersant composition for use in the preparation of coal water mixtures which are of relatively low viscosity and high solids content, particularly wherein the coal is of a low grade containing a substantial amount of non-combustible materials.

Accordingly, in general, the present invention provides a dispersant composition for coal in water comprising an anionic dispersant selected from the alkali, alkaline earth, organic amine or preferably the ammonium salts of naphthalene or lignin sulphonic acids or mixtures thereof, and at least one organic dispersant selected from a methacrylic or acrylic acid water soluble polymer optionally co-polymerised with fumaric acid, itaconic acid, citraconic acid, hydroxy-sulphonate and phosphonate functional monomers, hydrocarbon monomers, vinyl esters, vinyl ethers, methacrylate and acrylate esters, amides and nitriles or compositions thereof. Preferably, the organic polymer is employed in the form of its alkali, alkaline earth, organic amine or most preferably, its ammonium salt.

Alkali metal salts of the dispersants of the invention are not recommended for use with any coal, as an increase in the alkali metal oxide content of the ash residue results in a lower melting point and increases the potential for deleterious fouling of heat exchange surfaces, refractories and the like.

As mentioned above, the invention has particular application to coal particles which are the product of froth flotation processes, although it is not limited to such application.

The fine coal recovered by froth flotation is created by many different processes in mining, including cutting the coal, transporting and various coal preparation processes, so that there is no single characteristic peak in its size distribution. The size distribution will vary according to the type of coal and its mining and subsequent processing, and accordingly, it is not expected to be the same from different coal preparation plants, but this is not critical in the present invention.

A particular advantage of froth floated fines in the present invention is that there is a reduced grinding requirement compared to graded coals, but at the same time conventional coal water mixture technology is not effective in producing coal water mixtures with acceptable rheological properties from low grade coal feedstock. A further problem with froth floated fines is that chemicals used in coal preparation, for example frothers and collectors, may interfere with the action of previously known and recommended dispersants for coal water mixtures.

Coal water mixture technology offers the possibility to incorporate additives to give secondary benefits. The inclusion of other additives such as combustion catalysts or other materials to limit the levels of gaseous pollutants Sox in flue gases, for example nickel stearate or Prussian blue, may be beneficial as combustion catalysts and/or the addition of materials such as calcium oxide, limestone, magnesium carbonate and similar compounds for controlling the emission of sulphur compounds on combustion.

To achieve stable, high solids coal water mixtures, the particle size of the coal needs to be controlled, as this determines the rheology of the dispersion. The aim is to produce a broad particle size distribution with a top size of 250-300pm (for combustion considerations) with a lower limit of less than lpm. Such a broad particle size distribution will impart low viscosity, and stability through colloidal interactivity.

Generally, additional stabilisers such as guar gum or polysaccharides are not required.

The acrylic polymers used in the invention may be made by the normal free radical methods well known in the art.

Polymerisation may be conducted in a solution, emulsion or suspension process; typically aqueous solution polymerisation would be employed. Initiation may be achieved by any of the standard free radical initiators (typically azo or peroxygen compounds), by the use of W irradiation in the presence of a suitable UV-sensitive material, or by the use of ionising radiation. Molecular weight and polydispersity may be controlled by the use of chain-transferring additives, for example secondary alcohols and thiols.

The acrylic polymers used in the invention are primarily composed of residues derived from acrylic or methacrylic acid.

Optionally functional groups derived from additional monomers such as maleic acid, fumaric acid, citraconic acid, crotonic acid, hydroxy- and sulphonate- functional monomers, vinyl esters, vinyl ethers, acrylate esters, methacrylate esters, amides and nitriles may be present.

According to the practice of the invention, a predetermined amount between 0% and 25% by weight of flotation concentrate is ground in water optionally with some or all of the dispersant composition, until about 50% of the charge has a particle size of -2.5pm. This can be conveniently carried out in a stirred bead mill. The resultant ultrafine slurry is mixed with the bulk flotation concentrate and remaining dispersants, and ground for a short period conveniently in a ball mill to form the desired coal water mixture.

Generally, the amount of dispersants required is between 0.2 and 2.0% of which in preferred aspects of the invention, at least 50% by weight of the total dispersant is an anionic sulphonate and 10-50% of the total dispersant is at least one other organic dispersant as herein defined.

The preferred anionic sulphonates are alkali, alkaline earth, organic amine or most preferably ammonium naphthalene sulphonates which are commercially available from several sources. Alternative anionic sulphonates are alkali, alkaline earth, organic amine or preferably ammonium lignin sulphonates, also commercially available from several sources.

The invention is further illustrated by reference to the following Examples.

Example 1 Acrylic acid homopolymer Acrylic acid (400g) and a solution of ammonium persulphate (20g) in water (100g) were added dropwise during three hours to boiling water (300g) which was mechanically agitated. The reaction temperature was kept at 98-1040C throughout the addition and for 30 minutes thereafter. The resulting polymer solution was cooled and neutralised with aqueous ammonia to yield a 45% solution with a viscosity of 850cps (Brookfield LVT).

Examples 2-10 To demonstrate the effects of the dispersants used in the preparation of coal water mixtures, a standard preparation technique was adopted. The coal used was a froth flotation fines filter cake from the Rufford Colliery which had a maximum particle size of less than lmm and a solids content of approximately 75% of which the ash content was 15.5%.

Ultrafine material was produced by grinding 10% of the filter cake charge in a stirred bead mill in the presence of the dispersant composition according to the invention, until a product with a median size of 2.Siim was produced. The maximum particle size was 20ism. This was mixed with the remaining filter cake charge and water as necessary, and ground in a ball mill to produce the desired coal water mixture having a maximum particle size of 300cm.

Viscosity measurements were made on a Brookfield viscometer (No.3 spindle at 60rpm). A viscosity of 1300cps measured using this instrument was judged to be an acceptable limit for handling and atomisation of the coal water mixture.

Coal water mixtures were prepared as discussed above incorporating variable quantities of dispersant compositions according to the invention.

The effects of the dispersants in providing coal water mixtures containing high solids contents and acceptable rheology for their use in coal transportation or direct combustion are demonstrated in Table 1.

Table 1

Dispersant Compositions Final Visc.

Ex. Naphthalene rLigno- 6770 211A Solids cp Sulphonate* Sulphonate* % 2 0.75 - 0.40 - 68.7 1180 3 0.75 - 0.45 - 69.5 1070 4 0.60 - 0.40 - 69.1 1280 5 1 0.75 - - 0.60 68.8 1140 6 0.75 - - 0.45 68.8 1340 7 0.75 1 - 0.75 - 68.1 1060 8 0.60 0.15 0.40 - 69.1 1240 9 0.75 1 - 0.55 - 70.7 1260 10 0.75 1 - 0.20 - 67.4 1140 * - Ammonium salts 6770 - Bevaloid 6770AN 211A - Bevaloid 211A Comparative Example 11 A coal water mixture was prepared according to the method disclosed for Examples 2-10 with the addition of 0.75% ammonium naphthalene sulphonate as sole dispersant. The resultant mixture had a final solids content of only 63% and a viscosity in excess of 2000cps measured on the Brookfield viscometer.

The mixture of Example 11 had a very high viscosity which made it unsuitable for pumping and atomisation prior to combustion.

In contrast the products of Examples 2-10 had much lower viscosities and higher solids contents and were all suitable for such applications.

Claims (16)

1. A dispersant composition for coal in water characterised by an anionic dispersant selected from the alkali, alkaline earth, organic amine or the ammonium salts of naphthalene or lignin sulphonic acids or mixtures thereof, and at least one organic dispersant selected fran a methacrylic or acrylic acid water soluble polymer optionally co-polymerised with fumaric acid, itaconic, citraconic acid, hydroxy-sulphonate and phosphonate functional monomers, hydrocarbon monomers, vinyl esters, vinyl ethers, methacrylate and acrylate esters, amides and nitriles or compositions thereof.

2. A dispersant composition according to Claim 1 characterised in that the organic polymer is employed in the form of its alkali, alkaline earth or organic amine.

3. A dispersant composition according to Claim 1 characterised in that the organic polymer is employed in the form of its ammonium salt.

4. A dispersant composition according to Claim 1 characterised in that the acrylic polymers are composed of residues derived from acrylic or methacrylic acid.

5. A dispersant composition according to Claim 4 characterised in that functional groups derived from additional monomers are present.

6. A dispersant composition according to Claim 5 characterised in that the additional monomers are selected from maleic acid, fumaric acid, citraconic acid, crotonic acid, hydroxy- and sulphonate- functional monomers, vinyl esters, vinyl ethers, acrylate esters, methacrylate esters, amides and nitriles.

7. A dispersant composition according to Claim 1 characterised in that at least 50% by weight of the dispersant composition is an anionic sulphonate and 10-50% is at least one other organic dispersant.

8. A dispersant composition according to Claim 7 characterised in that the anionic sulphonate is selected from alkali, alkaline earth, organic amine or ammonium naphthalene sulphonates.

9. A dispersant composition according to Claim 7 characterised in that the anionic sulphonate is selected from alkali, alkaline earth, organic amine or ammonium lignin sulphonates.

10. A coal water mixture is characterised by finely divided coal in water together with a dispersant composition comprising an anionic dispersant selected fran the alkali, alkaline earth, organic amine or the ammonium salts of naphthalene or lignin sulphonic acids or mixtures thereof, and at least one organic dispersant selected from a methacrylic or acrylic acid water soluble polymer optionally co-polymerised with fumaric acid, itaconic, citraconic acid, hydroxy-sulphonate and phosphonate functional monomers, hydrocarbon monomers, vinyl esters, vinyl ethers, methacrylate and acrylate esters, amides and nitriles or compositions thereof.

11. A coal water mixture according to Claim 10 characterised in that the coal is in the form of a froth flotation filter cake.

12. A coal water mixture according to Claim 11 characterised in that the maximum particle size of the filter cake is less than Imn and has a solids content of approximately 75% of which the ash content is 15.5%.

13. A method of making the coal water mixture claimed in Claim 12 characterised by grinding 10% of the filter cake in the presence of the dispersant composition until a product with a median size of 2.5um is produced, the maximum particle size being 20um, mixing the product with the remaining filter cake and water, and grinding the same to produce a coal water mixture having a maximum particle size of 300um.

14. A dispersant composition substantially as hereinbefore described with reference to the Example 1.

15. A coal water mixture substantially as hereinbefore described with reference to any one of the Examples 2 to 10 and Table 1.

16. A method of making a coal water mixture substantially as hereinbefore described with reference to Examples 2 to 10.