GB2223505A - Briquetting process - Google Patents

Abstract

Fine coal is briquetted using a binder and up to 2% by weight of the mix of a dessicant or drying agent. The briquettes show an improved initial green strength and shatter index combined with a good final strength. Preferred desiccants are anhydrous calcium chloride and magnesium chloride.

Description

Briquetting Process The present invention concerns a briquetting process. More especially it concerns such a process which gives improved green strengths in "cold" briquetting, that is, briquetting at temperatures of up to 1000C.

Our UK published Patent Application No 2,187,754 discusses a variety of cold coal briquetting processes and discloses a useful process utilising a combination of molasses and inorganic hardening agent as binder.

Our co-pending application No. 8721757 discloses avariation of the process of GB 2,187,754 in its use of concentrated sugar solutions. The disclosures of both these applications are incorporated herein by reference.

We have found that a great many cold briquetting processes, in which a water-containing binder is used, while apparently producing satisfactory briquettes of adequate final strength, and even acceptable to good burning properties, result in green briquettes of inadequate initial strength. That is to say, the briquettes, when emerging from the briquette forming step, have a low crushing strength and are easily broken. In practice on the industrial scale, mechanical handling of the green briquettes results in undesirably high breakages and loss of complete briquettes, requiring recycle of broken briquette pieces or the acceptance of less desirable and less valuable product.

Our British Patent Application No. 8725252 describes a process for cold briquetting a mixture of fine coal with a binder wherein the briquetting mix contains up to 2% by weight of the mix of a cement.

Although initial green strength of the briquettes is improved particularly at elevated temperatures in some instances the necessary level of cement addition results in a reduced final cured strength and impaired combustion characteristics.

It has now been discovered that the incorporation of minor quantities of a desiccant or drying agent into a water-containing mixture of coal and binder yields worthwhile improvements in initial green strength.

Magnesium or calcium chlorides, in particular, have been found to increase green strength without reducing final cured strength and in some instances where a subsequent drying or curing operation is involved to also increase the rate at which the briquettes develop strength.

The present invention provides a process for the cold briquetting of fine coal, comprising mixing fine coal with a binder to produce a water-containing briquetting mix, and characterised by the presence in the briquetting mix, preferably in an amount of up to 2% by weight of the mix of a desiccant, and briquetting the mix whereby the initial green strength of the resulting briquettes is improved, as also may be the rate at which crushing strength develops.

Although the process is nominally "cold", it is found that on the industrial scale the mix enters and leaves the briquette formation steps at a temperature above ambient, for example of the order of 60or. For example, it may be necessary to dry the fine coal by heating, before briquetting, and this mix retains sensible heat, or, depending upon the briquette formation step, friction and/or compression may give rise to heating of the briquette during formation. It has been found that the present invention is applicable with regard to initial green strength at both ambient and elevated briquetting temperatures.

Desirably, the binder is a combination of molasses or a sugar solution and an inorganic hardening agent and we refer to our prior applications mentioned above for examples of such binders. The invention however is applicable to any water-based binder which produces briquettes with unsatisfactory green strengths.

The desiccant used is preferably solid inorganic dessicant e.g. anhydrous calcium chloride, which exhibits good results in tests and is readily available at low price, but other desiccants may be used. The desiccant may be pre-mixed with dry coal, either alone, or in combination with other hardening agents, prior to adding water-containing binders. But preferably the desiccant is added to the coal at the same time as or after the coal is mixed with binder and optionally other hardening agents. Specific desiccants and coal/binder mixes should be routinely tested to ensure that they are satisfactory and yield briquettes of the desired appearance and properties. Preferably, the desiccant is used in an amount of up to 1% by weight of the mix.Suitable desiccants include anhydrous calcium chloride, magnesium chloride, silica gel and activated alumina Preferred coals are high rank non-caking coals, especially those having low smoke emissions such as anthracite. Desirably the coal is of a particle size mainly below 3mm, and anthracite duff is especially suitable. The invention is also applicable to coals for power stations or steam raising and to coal blends containing caking coal components and/or treated coals, e.g. by mild oxidation or pyrolysis. The coal may be crushed or be the direct product of coal cutting.

The briquetting step of the present invention includes all methods of forming agglomerates from fine coal, and these agglomerates may be of any size or shape according to market requirements. There may be mentioned forming agglomerates by extrusion, ringrollor roll-pressing, die-pressing, rotary table pressing and pelletising, e.g. on a disc pelletiser.

The process preferably includes a hardening stage to permit the green briquettes to gain additional strength, depending upon the binder used. Hereinafter, reference will be made for convenience only to binders comprising molasses and an inorganic hardening agent.

The green briquettes harden over a period of 1 to 3 days at ambient temperature to give adequate crushing strengths, but the briquettes tend to have inadequate water resistance. However, the briquettes may be bagged in impervious sacks and allowed to further harden during storage. Preferably, however, a hot curing step is included to speed up the hardening stage and to make the briquettes water resistant, and this may be carried out at temperatures of the order of 200 to 3000C for up to an hour. Hot curing may be conveniently carried out by passing the briquettes on a conveyor through an oven, in an atmosphere which may contain nitrogen, carbon dioxide, water vapour and/or oxygen. It is to be noted that the hot curing step does not correspond to a carbonisation step, and thus not only are there energy savings, but the solid yield on a dry basis is very high.Additionally, there is a reduced risk of pollution.

The water resistance and/or appearance of the briquettes may be improved by an additional treatment with a spray or bath of a waterproofing agent. Several agents are known to have been proposed including, for example, aluminium acetate.

The present invention also provides briquettes formed using a process as described above.

The present invention will now be described by way of example only.

Example 1 A fine anthracite, from South Wales, was prepared by crushing and screening to minus 3mm. The anthracite was admixed with 10% of molasses, 1% iron ore and 1% phosphoric acid by weight of the final composition.

The mixture was briquetted in a pilot plant roll press, for comparison with the same mixture together with additions of 1.0% anhydrous magnesium chloride and 1.0% anhydrous calcium chloride.

Briquette testing was carried out under laboratory conditions at ambient temperatures of 200C. Crushing strengths according to standard tests were established for the green briquettes, hot briquettes and briquettes in which the final strength had developed.

The results obtained are presented below: Additive Green Strength Hot Crushing Final Strength Strength (kg) Product Crushing Shatter (%) ~~~~~~~~~~~~ Crushing 5 5 mins 10 mins Strength (kg) kg +1 1/4" +7/8" (+32mm) (+22mm) ~~~~~~ ~~~~~~~ ~~~~~~~ None 2.7 53 59 2.4 11.7 110 Magnesium chloride (anhydrous) 3.6 69 76 6.6 30.1 115 Calcium chloride (anhydrous) 8.7 83 86 7.4 15.8 105

Claims (12)

1. CLAIMS 1. A process for the cold briquetting of fine coal comprising mixing fine coal with a binder to produce a water-containing briquetting mix and briquetting the mix, characterised by the presence in the briquetting mix of a desiccant or drying agent in an amount such that the initial green strength of the resulting briquettes is improved.
2. 2. A process according to claim 1, wherein the mixture is briquetted at an elevated temperature.
3. 3. A process according to claim 2, wherein the mixture is briquetted at a temperature above 50 C.
4. 4. A process according to any preceding claim, wherein the binder comprises molasses or a sugar solution and an inorganic hardening agent.
5. 5. A process according to any preceding claim, wherein the binder comprises molasses in an amount of from 5 15% by weight of the mix together with a hardening agent selected from iron oxide (or iron ore), calcium carbonate (or limestone) calcium phosphate (or phosphate rock) and aluminium oxide (or bauxite) optionally together with an acid.
6. 6. A process according to any preceding claim, wherein the desiccant is anhydrous calcium chloride or magnesium chloride.
7. 7. A process according to any preceding claim, wherein the desiccant is mixed with dry coal before addition of the binder.
8. 8. A process according to any of claims 1 to 6, wherein the desiccant is mixed with the coal simultaneously with or after addition of the binder.
9. 9. A process according to any preceding claim, wherein the desiccant is present in an amount of up to 2% by weight of the mix.
10. 10. A process according to any preceding claim, wherein the coal is a high rank non-caking coal.
11. 11. A process according to claim 10, wherein the coal is anthracite duff.
12. 12. A process for the cold briquetting of fine coal substantially as hereinbefore described with reference to the Example.