GB2155491A

GB2155491A - Method of making coal briquettes and briquettes so made

Info

Publication number: GB2155491A
Application number: GB08506479A
Authority: GB
Inventors: Patrick John Tiddy
Original assignee: Individual
Current assignee: Individual
Priority date: 1984-03-13
Filing date: 1985-03-13
Publication date: 1985-09-25
Also published as: GB8506479D0; GB8406524D0

Abstract

A known way of making coal briquettes includes the steps of mixing coal powder with a carbon- based binder and shaping the mixture, heating the shaped mixture to carbonise the binder and then applying water to prevent the briquette so formed from igniting. In order to increase the cohesion of the briquette during burning and thus reduce the loss of unburned carbon in the ash-pan, an aqueous solution of a salt such as sodium silicate and/or sodium borate is applied to the briquette, instead of water, and is absorbed by voids in the briquette.

Description

SPECIFICATION Method of making coal briquettes and briquettes so made This invention relates to a method of making coal briquettes, particularly briquettes formed from anthracite dust, and to briquettes made by such a method.

Various techniques are known for making briquettes from coal dust. It is the usual practice to incorporate a binding agent with the coal dust to form a paste which is moulded or otherwise shaped to briquettes.

Various types of binding agents have been proposed in the past and the details of the process depend on the type of binding agent.

The present invention relates to a process employing a carbon-based binder, such as bitumen or coal tar pitch, in which the briquettes are heated to carbonise the binder and to render the product smokeless. The conventional practice with such a process is to cool the briquettes by spraying with or immersion in water after carbonising to prevent the finished product from igniting.

According to one aspect of the present invention there is provided a method of forming a coal briquette comprising the steps of: shaping a paste comprising coal powder and a carbon-based binder, such as bitumen or coaltar pitch, into a briquette; heating the green briquettes (i.e. the shaped material) to carbonise or oxidise the binder; and then applying an aqueous salt solution, such as a sodium salt solution of sodium silicate and/or sodium borate, to the hot briquette. The sodium salt can be in a dilute solution and typically contains 2 to 4% by weight of sodium salt depending on the porosity of the briquettes to be treated. The solution serves the purpose of cooling the hot briquettes for subsequent handling but more particularly sodium salt is absorbed into the slightly porous structure of the briquette which has been formed by the carbonisation of the binder.Typically it is desired that the amount of sodium salt absorbed in the briquette is substantially one half percent by weight of the total solids in the briquette but, more generally, it may be between 0.25% and 1% by weight of the total weight of the briquette.

The amount of binder, for reasons of economy, is kept as low as possible consistent with providing adequate binding. It would in general be less than 8% of the weigth of the coal powder in the case of a bitumen binder, and substantially 6 to 6.5% in the case of a coal-tar pitch binder.

The invention has particular application to the manufacture of smokeless fuel briquettes and, for this reason, the coal is preferably anthracite or other low volatile coal. The "desmoking", by oxidation or carbonisation, of the shaped briquette may be effected in an oven, typically at a temperature of the order of 300"C.

With the process described above the sodium salt enters into the voids in the finished de-smoked shapes and especially in the case where sodium silicate is used strengthens the structure of the briquettes. This is particularly important when the briquettes are subsequently being burned. The sodium salt increases the cohesion of the briquette during burning and thus gives a structure which remains intact as combustion takes place. This is highly desirable in promoting efficient combustion. The sodium salt acts as a fluxing agent causing the resultant ash substance after combustion to densify. This facilitates subsequent removal and disposal of the ash.

The strenghtened structure of the briquette during combustion results in more complete combustion and reduces the undergrate carbon loss. The amount of carbon passing through the grate with the ash, on combustion of briquettes made by the above technique has been found to be very substantially less than the amount with conventional briquettes using only a bitumen or coal-tar pitch binder. Hence the total volume and weight of ash products is reduced.

The sodium salt, especially in the case where sodium silicate is used also aqs as a supplementary binder improving the strength of the briquette for handling.

The invention furthermore includes within its scope a coal briquette made by the abovedescribed method.

The following is a description of one example of the invention and of modifications thereto: Anthracite coal dust was mixed with a bitumen binder, the amount of bitumen being about 5% of the total weight, and the mixture was heated to soften the pitch, mixed and pressed into shaped briquettes. The coal dust employed in this example was anthracite dust such as would pass a 1 /8th inch sieve and comprised the finer materials obtained when washed anthracite from a mine is screened.

The shaped briquettes were de-smoked by oxidation or low temperature carbonisation by heating them in an oven at about 300"C to remove volatile constituents from the bitumen leaving the bitumen residue as a carbon binder; the removal of the volatile constitutents is to improve the smokeless nature of the subsequent feul. The briquettes were discharged from the oven into a dilute aqueous solution of sodium silicate which served to cool the briquette for subsequent handling.

The sodium silicate in the solution however enters into the voids in the briquettes. In this particular example, the sodium silicate solution contained 3.5% by weight of sodium silicate and the amount of sodium silicate absorbed in the briquettes amounted to about one half percent by weight of the solids in the briquettes. More generally, the strength of solution may be chosen, typically in the range of 2 to 4% by weight, in accordance with the porosity of the briquettes to give the desired amount of sodium silicate in the finished product.

It was found that the briquettes so produced had greater structural strength and remained intact for longer during combustion than similar briquettes employing only bitumen as the binder. This resulted in more complete combustion. The resultant ash was denser and contained less carbon than similar briquettes which had not been treated with sodium silicate. In particular, the pick-up of 1/2% sodium silicate into the briquette has been found to produce significant improvement in "undergrate carbon loss", reducing this by between one half and two thirds.

The example described above may be modified in a number of ways.

For example, an aqueous solution of sodium borate, or an aqueous solution of sodium silicate and sodium borate, may be directly substituted for the aqueous solution of sodium silicate. Also the use of certain poly-phosphate salts is contemplated to increase the cohesion of the briquettes as they burn.

A binder of coal-tar pitch may be used instead of bitumen, in which case the amount of coal-tar pitch is preferably about 6 to 6.5% of the total weight of the coal powder and pitch mixture.

Claims

1. A method of forming a coal briquette comprising the steps of: shaping a paste comprising coal powder and a carbon-based binder into a briquette; heating the shaped material to carbonise the binder; and the applying an aqueous salt solution to the hot briquette to cool the briquette and also to penetrate the briquette and increase the cohesion of the briquette upon subsequent burning of the briquette.

2. A method of forming a coal briquette comprising the steps of: shaping a paste comprising coal powder and a carbon-based binder into a briquette; heating the shaped material to carbonise the binder; and then applying an aqueous solution of sodium salt to the hot briquette.

3. A method as claimed in Claim 1 or 2, wherein the salt is sodium silicate and/or sodium borate.

4. A method as claimed in Claim 2 or 3 wherein the amount of sodium salt absorbed in thebriquette is between 0.25% and 1% by weight of the total solids in the briquette.

5. A method as claimed in Claim 4 wherein the amount of sodium salt absorbed in the briquettes is substantially one half percent by weight of the total solids in the briquette.

6. A method as claimed in any of Claims 2 to 5 wherein the sodium salt solution contains 2 to 4% by weight of sodium silicate and/or sodium borate.

7. A method as claimed in any of the preceding claims wherein the amount of binder is less than 8% of the weight of coal powder but is sufficient to bind the coal powder.

8. A method as claimed in any preceding claim, wherein the carbon-based binder is bitumen.

9. A method as claimed in any of Claims 1 to 7, wherein the carbon-based binder is coal-tar pitch.

10. A method as claimed in any of the preceding claims wherein the heating of the shaped briquette is effected in an oven.

11. A method as claimed in any of the preceding claims wherein the coal is anthracite.

1 2. A method of forming a coal briquette substantially as hereinbefore described with reference to the foregoing example.

1 3. A coal briquette made by the method of any of the preceding claims.