GB2142065A - Improvements relating to mine waste disposal - Google Patents

Abstract

Mine waste is encapsulated in box-like containers (1) of sheet steel. These containers are then used to build a pack to support the inner leg of a roadway arch. The waste is an extruded mixture of ground-up coarse discard (which is dry) and fine wet tailings to give controlled density and moisture content. <IMAGE>

Description

SPECIFICATION Improvements relating to mine waste disposal This invention relates to mine waste disposal.

One of the major problems associated with the mining of coal is that of "dirt" disposal, dirt, in this context being the waste material produced in the mining and subsequent cleaning for market of the coal.

Mining waste falls into three main categories as follows: a) Coarse waste material, derived from the driving of so called "development headings" in the mine and consisting of the broken up material of the strata immediately overlying the particular coal seams being worked. This material, usually described as "development dirt", is usually of size range below 200 mm.

b) Coarse washery discard, being shaley material of usual size range 100 mm to 1 mm and possessing an overall moisture content of 8% to 10%, depending very much on the washery facilities for dewatering and the efficiency of use of those facilities.

c) Fine washery discard, being a coaly, silty clay of size range below 1 mm and possessing a high moisture content, the percentage value of which depends upon the type of process employed in the particular washery for dewatering the fine discard.

The process most often used is pressure filtration and this produces a "filter cake" of about 35% moisture content expressed as a percentage of total weight.

The proportions of these types of material vary from colliery to colliery but generally speaking, development dirt comprises only about 10% of the total, much the greater part of the material having to be discarded consisting of washery discard. Again the proportion of coarse washery discard to fine varies from washery to washery but is usually of the order of 7 1 on a dry-weight basis.

Present day mining methods are based upon the operation of fully-mechanised long-wall faces, in which all the coal is extracted between a pair of access headings one at each end of the face. As the coal is cut and removed, the hydraulic roof supports, together with all the associated coal cutting and conveying equipment, move forward and in this way the process is continued until all the coal in the particular panel being worked is eventually removed. The access roads have also to be continuously extended as the face advances, unless they have been previously driven to their full extent, in which case coal extraction commences at the remote ends of the roadways, a method known as "retreat" mining.

As the coal is extracted and the machinery moves forward, the "roof" behind is allowed to collapse or "cave", this movement being transmitted to the surface and resulting in subsidence of the ground and damage to surface buildings.

The total extraction of the coal between the roadways, and the subsequent caving of the roof, throws heavy loads on to the inner legs of the roadway arches and these have to be supported by the building of "packs" which are constructed so as to support these inner legs. Various methods of pack building are in current use, including the use of concrete blocks, but the most sophisticated is probably the "pump-pack" system in which a proportion of the raw coal from the face is mixed with ultra rapid-harderling cement and poured between a pair of continuously advancing shutters, linked to the advancing face machinery. Roadside pack construction forms an essential but expensive part of modern coal mining technology.

The only method currently available for returning at least a proportion of the mining waste below ground, is that of pneumatic stowing, which is a process in which dry-screened material (maximum size 50 mm) is introduced into a pressurised delivery pipe and "blown" through the system, to the delivery end behind the coal-face, where the void left behind the advancing face machinery is filled, or partly filled, by ejecting the material into it. Under operating conditions the average bulk density of the material within the pipeline is very low, so that the power requirement is disproportionately high. In addition the process is inherently dust-making, which is in direct opposition to the requirement to provide a dust-controlled environment at the coal face.

Furthermore, for technical reasons, the application of the process unavoidably restricts the operation of the coal-getting face associated with it, resulting in a reduction in the production of coal from that face, the cost of the loss of that coal having to be added to the other, very high, costs of operating the system.

The result is that pneumatic stowing has been abandoned, in the United Kingdom, as a viable method of conveying mine waste back underground, but apparently a few German mines still operate the system.

On the other hand, pneumatic stowing has one great advantage, namely that the material is delivered within a pipeline so that demands on the very limited space available within the mine are minimal.

Any other system, to have any possibility of success, must approximate as closely as possible to a pipeline system.

According to the present invention there is provided a mine waste disposal system in which mine waste is encapsulated in box-like containers which are then used as blocks for building packs alongside roadway arches.

The containers are preferably of sheet steel, and will generally be of elongated box-like form. It is then possible to convey them end-on along a guide system which can approximate to the desired pipeline form.

The waste will preferably be an extruded mixture of relativly dry ground-up tailings and fine washery discard. The extrusion may have a longitudinal passage for the drainage of moisture within the container, or moisture absorbent elements may be added to the extrusion.

For a better understanding of the present invention one embodiment will now be described, by way of example, with reference to the accompanying drawing, in which Figure 1 is a perspective view of a mine waste container, Figure 2 is a cross section of part of a mine showing how such containers can form a support pack, and Figure 3 is a diagram illustrating distortion of the containers under load.

The first step in this waste disposal system is the conversion of the various discards into a form suitable for such disposal i.e. into a dense, hard material capable of being quality controlled as to moisture and overall size grading. A proportion of the coarse waste material is ground up and this dry material is added to the tailings. The mixture is extruded through a screw-auger of the type used in brick-making for extruding the "green" bricks, prior to firing. This type of machine is fitted with a vacuum pump for de-aering the extruded material. Trials have indicated that, when three parts of ground washery shale is mixed with one part of "cone" tailings and put through such a machine, there is produced a stiff extrusion, of bulk density about 1900 kg m3, with a moisture content of about 12% (total weight basis).It may be described, in soil mechanics terms, as a very stiff clay and it could be expected to have an unconfined compressive strength of about 80 kN m2.

Such a machine is thus capable of producing material the moisture content of which can be closely controlled, and can produce itto precise dimensions. The product of such a machine can be regarded, therefore, as an ideal feed material for the waste containers to be described.

In order to assist the eventual drying out and solidifying of the material, moisture absorbent pellets may be added to the extrusion, or the latter may be arranged to have one or more longitudinal passages into which moisture will drain. Both measures may be adopted.

Immediately following the extrusion process, controlled amounts are wrapped or encapsulated in thin sheet steel box-like containers 1, as shown in Figure 1.

The dimensions of these may be of the order of 30 mm x30 mmx 120 mm, although practice may show different dimensions, particularly an increase in length, to be advantageous.

The containers are presented to the delivery end of the extruder complete apart from an open lid portion 2, and a regulated amount of material is chopped off and falls into each container as it is progressed by.

The lid portion is then folded down. It may be welded in position along the previously free edges or there may be abutting edge portions which can be folded over to make tight seams.

Instead of using a long side of the container as a lid, just one end may be left open, with a closable vent at the other end if necessary. The extrusion would then be injected endwise into the container, and the open end would be closed to complete the encapsulation.

The steel may be fairly thin sheet and does not have to be galvanised or otherwise treated. It may well eventually rust away, but by that time the contents will have solidified and the role of the outer skin will have become redundant.

The containers are taken on a delivery system along the access roads to just behind the face, where they are built into packs to support the roadway arches, as shown in Figure 2. They are simply stacked on one another, without mutual attachment or adhesive, and the number used in any one cross-section and the nature of the lay can be chosen to suit conditions. As shown, there are two stacks without overlap, but there may need to be three or more, and "bonding" the blocks brick fashion may be adopted.

It cannot be guaranteed that the roof height will correspond to an exact multiple of container heights, and so there will usually be a gap at the top. It may be satisfactory to leave this and allow the roof to settle onto the pack, or it may be advisable to insert filler elements.

Once the weight of the roof comes on the pack, the compression will tend to flatten the containers, bulging their sides. However, the tensile strength of the steel sheet is very substantial and this flattening will only progress so far before a state of equilibrium is achieved, as shown diagrammatically in Figure 3.

Over a period of months, the compacted waste within the containers will solidify and, as mentioned above, the rusting away of the encapsulating skin, which will take longer, will not matter.

Claims (7)

1. A mine waste disposal system in which mine waste is encapsulated in box-like containers which are then used as blocks for building packs alongside roadway arches.

2. A system as claimed in Claim 1, wherein the containers are of sheet steel.

3. A system as claimed in Claim 1 or 2, wherein the containers are of elongated rectangular box-like form.

4. A system as claimed in any preceding claim, wherein the waste is an extruded mixture of relatively dry, ground-up tailings and fine washery discard.

5. A system as claimed in Claim 4, wherein the extrusion has a longitudinal passage for the drainage of moisture within the container.

6. A system as claimed in Claim 4 or 5, wherein moisture absorbent elements are added to the extrusion.

7. A mine waste disposal system substantially as hereinbefore described with reference to the accompanying drawing.