Briquetting process

Description

[0001] 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 100°C.

[0002] 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 87 21757 discloses a variation 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.

[0003] 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.

[0004] It has now been discovered that the incorporation of minor quantities of a cement into a water-containing mixture of coal and binder, especially under the normal operating temperatures of industrial processing, yields worthwhile improvements in initial green strength. Ordinary Portland Cement ("OPC") has been proposed as a binder for solid fuel briquettes, but has required the addition of 5% or more by weight of the briquette mix of the cement, resulting in high total ash contents for the briquettes. We believe that for a variety of reasons, cement-bound briquettes have not been commercially successful. Our present invention surprisingly requires only a limited quantity of cement, below that which would act as a successful binder, in combination with another binder, in order to improve initial green strength.

[0005] 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 of up to 2% by weight of the mix of a cement, and briquetting the mix, whereby the initial green strength of the resulting briquettes is improved.

[0006] 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 60°C. 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 offers particular advantages as regards initial green strength if the briquetting step is carried out at temperatures above 50°C.

[0007] Desirably, the binder is a combination of molasses and an inorganic hardening agent, or a sugar or starch solution and an inorganic hardening agent, and we refer to our prior applications mentioned above for examples of such binders. Other binders may, however, be used if these are water-based and produce briquettes which have unsatisfactory green strength.

[0008] The cement used is preferably OPC, which exhibits good results in tests and is readily available at low price, but other cements such as special portland cements, and high alumina cement may be used. Alternatively, Sorrel cement (which is based on magnesium oxide and magnesium chloride) may be used. It is recommended that the cement be pre-mixed with dry coal, either alone, or in combination with other hardening agents, prior to adding water-containing binders. Alternatively, the cement may be added to wet coal if it is combined with dry binders such as starch, either with or without hardening agents. Specific cements and coal/binder mixes should be routinely tested to ensure that they are satisfactory and yield briquettes of the desired appearance and properties. Cement setting and hardening may be adjusted in known manner by mixing appropriate cements and/or by the incorporation of accelerators or retarders. Preferably, the cement is used in an amount of up to 1%, more preferably, in an amount of up to 0.5%, by weight of the mix. Good results in preliminary tests have been achieved with amounts of cement of approximately 0.5% by weight.

[0009] 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, eg by mild oxidation or pyrolysis. The coal may be crushed or be the direct product of coal cutting.

[0010] 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, ringroll - or roll-pressing, die-pressing, rotary table pressing and pelletising, eg on a disc pelletiser.

[0011] 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 300°C 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.

[0012] The water resistance of the briquettes may be improved by an additional treatment with a spray or bath of a water proofing agent. Several agents are known to have been proposed including, for example, aluminium acetate.

[0013] The present invention also provides briquettes formed using a process as described above.

[0014] The present invention will now be described by way of example only.

EXAMPLE 1

[0015] A fine anthracite, from South Wales, was prepared by crushing and screening to minus 3mm. The anthracite was admixed with 8% of mollasses, 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 0.25% and 0.5% of ordinary portland cement.

[0016] Briquette testing was carried out under laboratory conditions at ambient temperatures of 20°C, and also after heating to a commercial operating temperature of 60°C. Crushing strengths according to standard tests were established for the green briquettes and also green briquette shatter indices for the various briquettes, the latter being a measure of the size of broken briquette pieces by measuring the quantity by weight retained on screens of the specified operative. The results obtained are presented below:

Cement Addition (% by wt)	Briquette green strength(kg)		Briquette shatter index(%)
		temperature	Ambient
	Ambient	60°C	+ 38 mm	+ 22mm
Nil	5.4	6.0	60	68
0.25	6.3	8.3	85	90
0.5	7.5	14.8	94	95

[0017] It is readily seen that the addition of quite small amounts of cement have a significant effect on green strength and shatter index.

EXAMPLE 2

[0018] A briquette manufacturing plant was continuously operated using anthracite which was crushed and screened to minus 3mm. It was then mixed with molasses, iron ore and phosphoric acid as in Example 1 and the mixture was further mixed with 1% by weight of Portland cement. The resulting mixture was briquetted in a roll press and hot cured. It was noted that there was a reduction in breakage of the briquettes through mechanical degradation, and this reduction resulted in an average increase in product yield of 13%.

EXAMPLE 3

[0019] Example 1 was repeated except that 0.5% by weight of the final composition of xanthan gum was added. The addition of the gum had only a marginal effect upon the green crushing strength but produced a significant improvement on green shatter strength, thereby imparting improved mechanical handling characteristics.

EXAMPLE 4

[0020] Example 3 was repeated except that 0.5% by weight of the final composition of guar gum was added. Again the addition of guar gum had only a marginal effect upon the green crushing strength but produced a significant improvement in green shatter strength, thereby imparting improved mechanical harding characteristics.

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 up to 2% by weight of the mix of a cement whereby the initial green strength of the resulting briquettes is improved.

2. A process according to claim 1, wherein the mixture is briquetted at an elevated temperature e.g. a temperature above 50°C.

3. A process according to claim 1 or 2, wherein the binder comprises molasses, a sugar or a starch solution and an inorganic hardening agent.

4. A process according to any preceding claim, when 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.

5. A process according to any preceding claim, wherein the cement is an ordinary Portland cement.

6. A process according to any preceding claim, wherein the cement is mixed with dry coal before addition of the binder.

7. A process according to any of claims 1 to 5, wherein the cement is mixed with the coal during or after addition of the binder.

8. A process according to any preceding claim, wherein the cement is present in an amount of up to 0.5% by weight of the mix.

9. A process according to any preceding claim, wherein the briquetting mix further contains a gum, in an amount of up to 1% by weight of the mix.

10. A process according to claim 9, wherein the gum is zanthan gum or guar gum.

11. A process according to any preceding claim, wherein the coal is anthracite duff or another high rank non-caking coal.