GB2129714A

GB2129714A - Method of and apparatus for preparing very fine coal

Info

Publication number: GB2129714A
Application number: GB8330114A
Authority: GB
Inventors: Siegfried Heintges; Wolfgang Epper
Original assignee: Kloeckner Humboldt Deutz AG
Current assignee: Kloeckner Humboldt Deutz AG
Priority date: 1982-11-13
Filing date: 1983-11-11
Publication date: 1984-05-23
Also published as: AU2106183A; DE3242058A1; GB8330114D0

Abstract

A method of preparing very fine grain material, such as coal, comprises separating the solid fraction below a grain size of 0.06 mm and thereafter floating it. The invention also includes apparatus suitable for carrying out the method.

Description

SPECIFICATION Method of and apparatus for preparing very fine coal The invention relates to a method of preparing very fine coal containing ash, the preparation of which has proved problematical in the past.

It is usual to prepare the very fine coal by flotation and then to dewater the flotation concentrate by vacuum filtration. Special particles with very small grain sizes are not, however, separated sufficiently selectively by the usual flotation. In particular, the particularly fine fraction below about 0.025 mm still has a relatively high ash content even after the flotation. Since, in the case of very fine coal, the proportion below about 0.025 mm is not inconsiderable and in addition has a tendency to rise, an undesirably high ash content of the flotation concentrate results with the usual methods of preparation unless the preparation of the fractions below about 0.025 mm is renounced. This is predominantly the case today and is associated with a corresponding loss of pure coal.

The invention seeks to provide a method of preparation and a preparation plant for the preparation of very fine coal which are in a position to prepare the very fine coal with a greater selectivity than was hitherto possible and in particular to float the very finest fraction with low ash contents in the concentrate.

According to a first aspect of the invention, there is provided a method of preparing very fine grain material wherein a solid fraction having a grain size below 0.06 mm is separated and then floated.

Preferably the separated and floated fraction has a grain size below 0.025 mm.

As a result of a separation of the finest fraction, e.g. the fraction with a grain size below o.06 mm, particularly below 0.025 mm, and a subsequent special flotation, it is surprisingly possible to carry out a selective preparation of this finest fraction. In this case, it is a particular advantage that the wash water circuit is freed of the finest fractions and no disturbing concentration of the finest fractions takes place in the wash water.

The said separation of the finest fraction may be effected in a solid-jacket screen centrifuge - prefer ably in the form of overflow centrifugate. As has been found, solid-jacket screen centrifuges are parti cularly well suited to carry out the separation of the solid fraction, which is obtained in the overflow centrifugate. The overflow centrifugate is then furth er prepared by the following flotation.

The said separated finest fraction may be sub jected to a pneumatic flotation without an agitator.

Pneumatic flotation without an agitator has surpris ingly proved suitable for the reflotation of the finest fractions with the above-mentioned grain sizes because in it there are particularly favourable condi tions for the accumulation of the finest particles on the air bubbles. Thus, in conjunction with the separation by the solid-jacket screen centrifuge, an ideal combination for the preparation of the very fine coal is produced.

The pneumatic flotation may introduce the solids into flotation cells from above and allow them to rise in a large calmed zone. This type of pneumatic flotation has proved particularly suitable for solving the problems mentioned at the beginning. Other types of pneumatic flotation may, however, also be used, for example with an expansion aeration, a nozzle aeration from below etc.

According to a second aspect of the invention, there is provided an apparatus for preparing fine grain material comprising means for separating the solid fraction below 0.06 mm and means for floating the separated fraction. This apparatus may comprise a solid-jacket screen centrifuge and a pneumatic flotation plant without agitator - connected to one another. Thus the method according to the invention - particularly the separation of the finest fractions can be carried out with known plant components which surprisingly, supplementing one another in their mode of operation, lead to the required success, hitherto unachieved.

The solid-jacket screen centrifuge may comprise a screen with a gauge of 0.3 mm. Such a solid-jacket screen centrifuge has proved particularly suitable for the separation of the finest components because the undersize of such a solid-jacket screen centrifuge can easily be supplied to the feed again, without an appreciable additional circuit loading resulting, while the finest fractions collect in the overflow centrifugate and can then be further treated.

The flotation plant may be constructed in the form of a free-jet flotation plant wherein the air - entrained by the free jet - is introduced into the flotation cells.

This type of aeration has proved particularly favourable for the success of the method according to the invention. In particular, the required selectivity results during the flotation of the finest fractions, so that, altogether, a reduction in the ash content of the flotation concentrate by 8% to 7% results, as thorough experiments have shown.

The flotation plant may comprise a plurality of flotation cells with connection pipelines for the repeated feed, known per se, of the solid fraction.

thus the selectivity of the flotation with the reduction in the ash content is achieved in the desired manner.

The invention will now be described in greater detail, by way of example, with reference to drawings, in which: Figure lisa flow sheet of the preparation of very fine coal according to the invention; and Figure 2 shows a pneumatic flotation cell with the upper portion in section.

In Figure 1, 1 designates the solid-jacket screen centrifuge with a feed 1' via which the flotation concentrate from the normal fine-coal flotation is delivered. The overflow centrifugate emerges from the solid-jacket screen centrifuge 1 at 2', the other two arrows 2" and 2"' indicating the emergence of the undersize and of the dry solid material. The overflow centrifugate first enters the reservoir 2 and passes via the pipeline 3' to the centrifugal pump 3 which conveys the overflow centrifugate to the flotation plant which comprises pneumatic flotation cells 4,7 and 10.In the flotation plant, to which the overflow centrifugate is supplied via the pipeline 4', and which also comprises measuring and control equipment, the flotation concentrate emerges from the flotation cells 4,7 and 10 via the pipeline 5', 8' and 11'. The sediment is taken away through the pipeline 6', 9' and 12'. The flotation reagent is supplied to the flotation plant through the pipeline 13', namely 0 to 500 g per tonne of throughput.

Disposed between the flotation cells 4 and 7 is a centrifugal pump 6 and between the flotation cells 7 and 10 a centrifugal pump 9 which introduce the energy for the free jet so that the flotation plant comprises three centrifugal pumps, three flotation cells, three concentrate offtakes and three sediment offtakes - that is to say three flotation units. The number three is only to be understood by way of example; the number of units in the flotation plant may also be greater or smaller. Three flotation cells have, however, proved particularly favourable for solving the problem.

In Figure 2, 20 designates a flotation cell which is preferably round in construction. It comprises a free-jet supply pipe 21 and an encasing tube 22 round the free-jet 23. The stripping off of the concentrate is effected by the foam skimmer 31 which conveys the concentrate into an inclined discharge channel 24. From there the concentrate is carried away via the discharge socket 25. At the side, the flotation cell has a level control 27 which is not shown in detail, and an outlet 28. The bottom of the flotation cell 20 is constructed in the form of a cone 29 at the lower tip of which a discharge valve 30 is disposed. Either a flotation cell of the type shown or any other pneumatic flotation cells may be used which have an equally great selectivity for coal particles below 0.06 mm, particularly below 0.025 mm.

The invention is intended, in particular, for the preparation of very fine coal. Without departing from the scope of the invention, however, other very fine grain material can be prepared such as ores or fibre suspensions.

Claims

1. A method of preparing very fine grain material wherein a solid fraction having a grain size below 0.06 mm, is separated and then floated.

2. A method as claimed in claim 1, wherein the separated and floated fraction has a grain size below 0.025mm.

3. A method as claimed in claim 1 or 2, wherein the said separation of the solid fraction is carried out in a solid-jacket screen centrifuge.

4. A method as claimed in claim 1, 2 or 3, wherein the separated material is in the form of an overflow centrifugate.

5. A method as claimed in any one of claims 1 or 4, wherein the said separated solid fraction is subjected to a pneumatic flotation without an agitator.

6. A method as claimed in claim 5, wherein the pneumatic flotation of the solids introduces the solids into flotation cells from above and allows them to rise to a large calmed zone.

7. An apparatus for preparing fine grain material comprising means for separating the solid fraction below 0.06 mm and means for floating the separated fraction.

8. An apparatus as claimed in claim 7, wherein the separating means comprises a solid-jacket screen centrifuge and the floating means comprises pneumatic flotation plant without agitator connected to the centrifuge.

9. An apparatus as claimed in claim 8, wherein the solid-jacket screen centrifuge comprises a screen with a gauge of 0.3 mm.

10. An apparatus as claimed in claim 8 or 9, wherein the flotation plant is constructed in the form of a free-jet flotation plant wherein the air - entrained by the free-jet - is introduced into the flotation cells.

11. An apparatus as claimed in claim 8,9 or 10, wherein the flotation plant comprises a plurality of flotation cells with connecting pipelines for the repeated feed of the solid fraction.

12. A method of preparing very fine grain material substantially as described herein with reference to the drawings.

13. An apparatus for preparing very fine grain material substantially as described herein with reference to the drawings.