The production of fuel products from fuel powders using binders such as coal tar pitch gives a material with a smooth surface finish and good abrasion resistance. This is achieved by using powdered fuel ground to a very fine particle size, typically 100% less than 500 microns. To achieve this degree of fineness it is necessary to dry the fuel to less than 4% moisture content. The use of an industrial drier and grinder typically causes the powder to reach 70-90°C and produces a powder of the desired moisture content and particle size. Once the binder is added to the powdered fuel the mix is compacted in moulds under pressure.

Typically this equipment will be a roll press where, once formed, the briquettes are emptied from the mould and air cooled or oven treated to de-smoke (devolatilise) the product for domestic use. Once empty, the moulds are cycled ready for another set of briquettes to be prepared.

Whilst this is a highly productive process it suffers a serious disadvantage. The coal tar pitch contains a high proportion of polyaromatic hydrocarbons which are carried on dust particles from the product and represent a serious health hazard.

It is known that aqueous alkaline phenol-formaldehyde resins can be hardened at room temperature by the addition of ester hardeners and are, therefore, useful binders of sand in refractory applications such as the production of foundry moulds and cores (US patents 4,426,467, 4,468,359 and 4,474,904). In foundry practice it is essential to control the temperature of the sand aggregate as the workable life of the binder and mixture is shortened as the temperature of the sand is increased.

Sand temperatures greater than 40°C would render the binder and sand mix unworkable due to the rapid reaction rate of the binder. It is therefore necessary to ensure the sand is cooled before binder addition.

This type of process is described in EP-A-0241156 which uses an aqueous alkaline phenol-formaldehyde resin cured with an ester curing agent to agglomerate wet coal fines followed by the drying and curing of the agglomerates. The particle size of the coal fines is said to be 95% greater than 500 microns and it would not be possible to produce a much finer grind at the moisture content of the coal required for the process to apply. It is not possible, therefore, to generate the same degree of fine surface finish as with a hot process. At the same time, the ester cured phenol-formaldehyde resin takes some time, between 30 and 45 minutes to start to develop its strength. The process requires as an essential feature the use of heat to dry the briquette and cure the resin. Thus, there is no real energy saving from using dry coal fines at the outset.

Meanwhile, whilst the briquettes are drying and curing they will be vulnerable to damage during handling and transportation. This problem can be so serious that the product can disintegrate on first impact after release from the mould and is, therefore, a cause of serious production losses. The use of a thickening agent to impart green strength to the formed briquette can drarnatically improve the durability of the briquette during this period but no mention is made in EP-A-0241156 of this improvement.

It is desirable, therefore, to have a highly productive process which can be used with hot, powdered fuel to benefit from the improved surface which a fine ground powder can give whilst replacing the health risk from coal tar pitch. The main problem in using an ester cured phenol- formaldehyde resin is that a mix of the components typically employed for foundry moulds and those described in EP-A-0241156 cures much too rapidly at temperatures of 70-90°C typically found in fuel discharged from a drier/grinder. By a combination of changes to the normal processes and components we have discovered a briquetting process which overcomes these obstacles such that we are able to mass produce a product with a fine surface finish and good abrasion resistance.

Accordingly, the invention provides a method of briquetting hot, dry finely powdered fuel comprising the steps of preparing a mixture of finely powdered fuel, a water-diluted alkaline phenol-formaldehyde resin, a curing agent for the resin which is at least one ester and a water-soluble thickening agent, shaping the mixture into briquettes and then allowing the briquettes to cool.

The method is applicable to the production of briquettes using finely powdered coal, such as anthracite, bituminous coal, coking coal or petroleum coke. The fuel powder is preferably produced by grinding lump and/or particulate fuel to a particle size of about 500 microns or less.

Most preferably, the fuel powder comprises a particle size 100% of which is less than 500 microns. The fuel powder is dry, that is to say it has moisture content not greater than 4% by weight, preferably less than 3.5 % boy weight and, more preferably, it has a moisture content of less than 3% by weight.

The temperature of the powdered fuel will preferably be within the range of from 50 to 90°C.

The method of the invention makes use of a water-diluted alkaline phenol-formaldehyde resin. Such alkaline phenol-formaldehyde resins are typically prepared by reacting together a monohydric phenolic compound, such as phenol itself or a cresol, or a dihydric phenolic compound, such as resorcinol, with formaldehyde at a molar ratio of phenolic compound to formaldehyde in the range of from 1: 1 to 1: 3 in the presence of an alkali, such as sodium hydroxide or potassium hydroxide. The amount of alkali used will typically be such that the molar ratio of alkali to phenolic compound will be in the range of from 0.05: 1 to 1.2: 1. The alkali phenol- formaldehyde resin will be diluted prior to use, i. e., prior to being mixed with the hot, dry powdered fuel. Preferably, the resin is diiuted such that the resin solids comprise from 10 to 30% by weight of the diluted resin.

The amount of diluted resin mixed with the dry powdered fuel will typically be in the range of from 8 to 15% by weight based on the weight of the dry powdered fuel.

The curing agent, for the alkali phenol-formaidehyde resin is at least one ester. Preferably, the ester curing agent is an ester selected from esters of polyhydric alcohols, carbonate esters, lactones, monomethyl or monoethyl esters of aliphatic carboxylic acids, dimethyl or diethyl esters of aliphatic dicarboxylic acids and mixtures of two or more of such esters.

As examples of esters of polyhydric alcools, there can be mentioned glycol esters, such as glycol acetates, and glycerol esters, such as glycerol acetates. A preferred embodiment of the invention in this respect lies in the use, as curing agent, of triacetin. A monomethyl or monoethyl ester of an aliphatic carboxylic acid or a dimethyl or diethyl ester of an diphatic dicarboxylic dicarboxylic acid, which may be used as the curing agent in the present invention will, typically, be one with a relatively low reactivity to prevent any significant degree of precure of the binder in the mixture of hot, dry fuel powder, resin, ester and thickening agent before this is pressed into a mould. If precure does occur the ultimate strength of the formed briquettes is compromised. Examples of such methyl or ethyl esters having a relatively low reactivity include, but are not limited to, monomethyl or monoethyl esters of aliphatic carboxylic acids, such as acetic acid, proprionic acid, lactic acid, stearic acid and oleic acid and dimethyl or diethyl esters of aliphatic dicarboxylic acids, such as oxalic acid, malonic acid, succinic acid, glutaric acid, adipic acid and maleic acid.

Only one type of ester may be used or, alternatively, a blend of two or more of the above-identified esters may be used. The alkaline hydrolysis rates of, and hence the binder reactivity and rate of hardening achieved by, the above-identified methyl and/or ethyl esters is slower than the rates of other esters, such as glycol acetates, glycerol acetates, carbonate esters and lactones, which are conventionally used in the production of foundry moulds and cores using sand as the particuiate material to be bound together. However, an ester belonging to this group of more reactive esters, for example, triacetin, can be used in the present invention to good effect with the water-diluted resins as described above.

The ester curing agent will, typically, be used in an amount of from 0.5 to 1.5% by weight based on the weight of the dry powdered fuel.

The present invention also makes use of a thickening agent.

Preferably, the thickening agent is used in an amount of from 0.1 to 1.0% by weight based on the weight of the dry powdered fuel. The preferred thickening agent will be water-activatable or water-swellable and, typically, will be a natural product or a derivative of a natural product, such as starch or a starch derivative, cellulose or a cellulose derivative or a natural gum, such as gum arabic or guar gum.

We have found that dilution of the resin with water followed by mixing and dispersing throughout the hot, finely powdered fuel before addition of the ester hardener enhances the working life of the fuel mix. This, therefore, constitutes a preferred embodiment of the invention. The resin dilution improves the distribution of resin in the fuel mix and extends the working time. The effect is to allow complete mixing and sufficient time for the mix to be transported to a roll press via a distribution pan. At the same temperature, a heated, wet fuel of equivalent total moisture has a short working life when mixed with an undiluted resin and hardener. The undiluted resin is not dispersed throughout the fuel mix efficiently and the reactivity of the undiluted resin in contact with ester is rapid leading to premature polymerisation before the mix reaches the roll press.

The following examples illustrate the principes and benefits of the invention.

In these Examples the resin used was an alkaline phenol-formaldehyde resin having a formaldehyde: phenol mole ratio of 1.9: 1 and using a sodium hydroxide: phenol mole ration of 0.6: 1. The resin had a viscosity (at 25°C) of 250-400 cP, a specific gravity (at 25°C) of and a solids content (3 hours @ 100°C) of 37-41%.

Laboratory Examples The effect of using wet fuel versus dry fuel and a water diluted resin Example 1 (wet fuel) A sample of anthracite was heated to 90°C in an oven. On removal from the oven the moisture content of the anthracite was measured at 2.6%. Using a Kenwood laboratory mixer and bowl, 1 kg of the hot, dried anthracite was mixed with 3g of guar gum powder (thickener) and 1 OOg water. After 30 seconds mixing, 35g of resin and 5.6g of triacetin were pre-mixed and added to the wet anthracite. After commencement of mixing a 100g sample was removed every 30 seconds whilst continuing mixing. Each sample was placed into a 2"compression tube and compacted using a hydraulic press at a fixed pressure. The compressive strength of each specimen, except the 2, was measured after 20 minutes from the time of release from the press to give a plot of compressive strength versus mixing time. The compression strength of the 2nd specimen was measured after 24 hours.

Example 2 (Dry fuel and a water diluted resin) A sample of anthracite was heated to 90°C in an oven. On removal from the oven the moisture content of the anthracite was measured at 2.6%. Using a Kenwood laboratory mixer and bowl, 1kg of the hot, dried anthracite was mixed with 3g of guar gum powder (thickener). The resin, 35g, was diluted with 100g of water and then charged to the dry anthracite. After 30 seconds mixing 5.6g of triacetin was added. After commencement of mixing a 100g sample was removed every 30 seconds whilst continuing mixing. Each sample was placed in a 2"compression tube and compacted using a hydraulic press at a fixed pressure. The compressive strength of each specimen, except the 2r'd, was measured after 20 minutes from the time of release from the press to give a plot of compressive strength versus mixing time. The compression strength of the 2nd specimen was measured after 24 hours.

Example 1 Example 2 Specimen Mix Time 20 min compressive strength kN 1 30 sec 3.80 3.22 3 1 min 30 sec 2.56 3.23 5 2 min 30 sec 2.10 3.04 6 3 min 1.90 2.67 24 hr compressive strength kN 2 1 min 6.09 9.74 The benefit of diluting the resin before addition to the heated anthracite is seen by the strengths achieved over the 3 minute mixing period in example 2. By comparison, in example 1 in which an undiluted resin is used, there is a rapid reduction in compressive strength in test pieces prepared during the same mixing time.

The effect of curing rate of various esters Example 3 A 1 Og amount of resin was weighed into a boiling tube and conditioned in a water bath to 60°C. A 1.5g amount of ester was pre- weighed into a 3m ! pipette. The ester was added to the top of the hot resin, a timer started and the resin and ester immediately mixed continuously with a thermometer. The mixture temperature was maintained at 60°C during the experiment. The time taken from addition of the ester to the gel point of the mixture was recorde.

No. Ester Gel time at 60 y-butyrolactone 9 sec 2 triacetin 19 sec 3 methyl acetate 30 sec 4 dibasic ester* 58 sec 5 ethyl acetate 158 sec *"dibasic ester"-a blend of 62% dimethyl glutarate, 23% dimethyl succinate and 15% dimethyl adipate.

It can be seen that the cure time of methyl and ethyl esters are well in excess of those of typical lactones and polyol acetates.

The effect of using esters of different cure times on compressive strengths of fuel products Example 4 (Triacetin) A 1 OOg sample of anthracite with measured moisture content of 2% was weighed into a Kenwood laboratory mixer bowl. The bowl, containing the anthracite, along with a 2"compression tube was preheated to 60°C in an oven. 60g of water was added to the hot anthracite and mixed in the Kenwood Chef for 30 seconds. 3g of guar gum powder (thickener) then was added to the mixture and mixed for 30 seconds. 50g of resin and 6g of triacetin were premixed in a cup for 15 seconds until they were homogenous, and then added to the hot mixture whilst continuing to be mixed. A timer was started immediately and the mixture continuously mixed. A 1 00g sample was removed every 60 seconds whilst continuing mixing. Each sample was placed into a 2" compression tube and compacted using a hydraulic press at a fixed pressure. The compressive strength of each specimen, except the 4Ih, 6th and 8th, was measured after 20 minutes from the time of release from the press to give a plot of compressive strength versus mixing time. The compressive strength of the 4"specimen was measured after 1 hour.

Example 5 (Dibasic ester) A 100g sample of anthracite with measured moisture content of 2% was weighed into a Kenwood laboratory mixer bowl. The bowl, containing the anthracite, along with a 2"compression tube was preheated to 60°C in an oven. 3g of guar gum powder (thickener) was added to the hot anthracite and mixed for 30 seconds. 50g of resin and 60g of water were premixed and added to the mixture, the mixture was mixed for a further 30 seconds. 6g of dibasic ester (as in Example 3 above) was added to the mixture via a syringe during continued mixing and the timer started. As with Example 4, a 100g sample was removed every 60 seconds whilst continuing mixing. Each sample was placed into a 2"compression tube and compacted using a hydraulic press at a fixed pressure. The compressive strength of each specimen, except the 4 h, 6, h and 8xh, was measured after 20 minutes from the time of release from the press to give a plot of compressive strength versus mixing time. The compressive strength of the 42"specimen was measured after 1 hour.

Example 4 Example 5 Specimen Mix Time 20 min compressive strength kN 1 1 min 0.88 * 1.89 2 2 min 0.59* 1.73 3 3 min 0.38 1.73 5 5 min 0.25* 1.20** 7 7 min 0.21 * 1. 11 ** 9 9 min 0.17* 0.93* 1 hr compressive strength kN 4 4 min 0.48* 2.08 *Top surface of briquette crumbled on release from tube.

** Slight chipping to top surface of briquette on release from tube.

The effect of using a slow reacting dibasic ester in combination with a diluted resin is illustrated when comparing Examples 4 and 5. In Example 5 the mix life is extended resulting in stronger specimens produced over the duration of the mix.