GB2227311A - Method and apparatus for sampling and measuring tailings - Google Patents

Abstract

Tailings are selectively fed from a bank of froth flotation cells, for example for coal washing, via a filter through a de-aerator 11 to a vertical measurement column 13. An X-ray ash content monitor 33 and a gamma ray solids content monitor 34, 35 are located at an intermediate part of the column 13 which diverges at an angle not exceeding 8 DEG . The tailings are fed to the base of the column through a gently curved pipe 36 followed by a straight section which establishes laminar flow. <IMAGE>

Description

METHOD AND APPARATUS OF SAMPLING AND MEASURING TAILINGS This invention relates to a method of sampling and measuring tailings fran an industrial process such as a coal washing plant.

In order to control effectively an industrial process particularly a fine coal washing plant, such as coal flotation, it is necessary to know the composition of the feed coal and resultant products particularly the tailings with regards to pulp density, ie solids content, and also with regard to the ash content. Techniques have been developed for measuring both components by a sampling system and naturally this has been developed into an on-line system. However, the accuracy of the on-line system has not been as high as is desired and it is an object of the present invention to provide an improved method and apparatus for sampling and measuring tailings, which is also capable of measuring plant feed and clean coal suspensions.

The method uses known forms of radio isotope detector systems for both the solid content and the ash content of tailings which are presented in a specially designed columnar cell. The isotopes emit gamma-rays and x-rays respectively, the transmitted gamma radiation and back-scattered x-radiation are assessed to give an indication of the solids and ash-content of the material.

The known system is set out for example in a paper entitled "A Technique for Measuring the Ash Content of Coal in a Tailings Stream" by I S Boyce published in the Institute Journal of the Application of Radiation Isotopes Volume 34 Number 1 1983 pages 45-54. It has been found in practice that the flowcell type of measurement which is described therein is not wholly accurate since due to turbulence which occurs in the flowcell, there may be some swirling and recirculation of solid particles during the presentation of the tailings sample, and particularly some degree of classification may occur. Further, the presence of variable quantities of air entrained within the sample strongly influences subsequent measurements. Both of these effects can give rise to erroneous readings.

According to a first aspect of the present invention the method of sampling and measuring tailings from a coal flotation process comprises the steps of extracting a sample of the tailings from the last stage of the flotation process, feeding the sample through a de-aerator device to a vertical measurement column, and presenting the sample serially in laminar flow form to ash content measuring means and to solids content measuring means, and returning the sample to the main stream of treated tailings.

According to a second aspect of the present invention apparatus for sampling and measuring tailings from a coal flotation process comprises filtering and valve means for isolating the flow of a sample of tailings from froth flotation means, a de-aerator device, a vertical measuring column, on/off valve means for isolating the flow of sampled tailings from the de-aerator device to the measuring column through a gradually curved input pipe and constructing the column to give a laminar flow of tailings in the column, and measuring and water calibration means associated with the column.

The tailings may be filtered to remove extraneous oversize material and they may be de-aerated in a cyclonic device prior to presentation to the measurement column.

Preferably the tailings are presented to the vertical measurement column along a gradually curved path to prevent classification and recirculation of solids in the tailings which can occur if there is a sharp bend through which the tailings have to pass before entering into the column.

Preferably the measuring means are located along the length of column with the ash content measuring means separated below the solid content means by a distance of at least 300nan. The measuring column is preferably provided mainly with tapered sides which diverge upwardly in the direction of flow of the sample.

The column may be in the form of a divergent cone in the direction of progression of the tailings, and may have two parallel sides and two sloping sides. The angle of divergence is at maximum of 80 but is preferred to be of the order of 60. The section of the column in which the ash is measured is tapered with the ash measurement means presented to one of the parallel faces. The solids content is measured at the top of the tapered section to provide the maximum available attenuation distance preferably 150mm.

The x-ray source is preferably a cadmium source which has an associated detector which measures the back scatter of radiation reflected from the solids in the tailings. The gamma ray radiation source is preferably a 137 Caesium source with an associated detector which measures the attenuation of gamma rays passing through the tailings in the pipe.

Both sources are connected to give an electrical signal output which is shown on a recorder or monitor associated with the apparatus.

In order that the radiation level can be kept within safe boundaries it is preferable that the Caesium source has a maximum energy level at the order of 20mCi and the cadmium source has a maximum energy level at the order of 0.25mCi.

Preferably the tailings sampled are fed to the apparatus through valve means connected to the end cell of a froth flotation bank of cells and thence to a de-aerator. Prior to the de-aerator a purpose designed filter may be used to prevent any extraneous coarse particles entering the monitor which would block the cell or distort the sampling, and this filter may be in the form of a swan necked pipe including a grid on the off-take from the end cell sample point and prior to the de-aerator. The outlet from the de-aerator conveniently feeds to the vertical column through an isolation valve. A dump valve may also be incorporated in the circuit to drain the tailings from the system at any stage of the process.

In order to enable the apparatus to be calibrated, water spray means are preferably situated above the column so as to be capable of purging the column using the drain valve means and then filling the column with water to allow it to be calibrated.

The apparatus preferably has an overflow for the discharged tailings sample which can either lead back to the main stream of processed tailings or, when for example flotation feed suspension is sampled, back to the flotation cells.

In order that the invention may be readily understood one example of apparatus in accordance therewith and using the method thereof will now be described with reference to the accompanying drawings. In the drawings Figure 1 shows in schematic form the apparatus of the invention connected with froth flotation cells.

Figure 2 shows in detail a front view of the apparatus. Figure 3 shows detail side view of the apparatus illustrating the discharge aspect of the apparatus, Figure 4 is a top plan view and Figure 5 shows a detail of a filter arrangement.

The apparatus is used in this example to sample and measure the ash content and the solids content of coal flotation tailings. The apparatus may also sample and measure the ash content and solids content of the flotation plant feed.

Referring first to Figure 1 this shows the raw coal feed suspension head tank (1). The raw feed is allowed to pass from the tank (1) selectively through valves (2) and (3) to two banks of froth flotation cells (4) and (5). The banks of cells are arranged such that the raw coal feed passes serially through them and the final cell discharges along the lines (6) and (7) respectively to a disposal plant. The final cell in each bank (4) and (5) is connected through on/off valves (8), (9) respectively to a feed line (10) which feeds the tailings into a de-aerator (11). The de-aerator has an outlet to a valve (12) which leads to the base of a vertically mounted measurement column (13).

A discharge conduit (14) is connected to the top of the column (13) and is able to discharge tailings preferably to the disposal area, but in the case of raw coal feed or cleaned coal concentrate into a sump (15).

The sump (15) has its contents taken off via a pump (16) and fed back to the banks of cells through a conduit (17) and nonreturn valves (18), (19) connected to the inputs of the banks (4), (5) respectively.

A water valve (21) is connected to a water source (22) to feed water to a spray (23) situated at the top of the column (13). The water is used for washing out the column and for filling it prior to calibration or plantshutdown.

A dump valve (24) is connected on the outlet side of the valve (12) in between it and the base of the column (13). The dump valve is for draining tailings or water from the column and this is passed through a conduit (25) to a thickener (not shown). Referring now also to Figures 2 and 3 it can be seen from these figures that the vertical column (13) has a lower part (30) which is of constant cross sectional area and converts from round to rectangular in cross section. This leads into an intermediate part (31) which is tapered in one elevation upwards and outwards at an angle in this example of 60. This is the optimum angle although angles up to 80 may be used. The upper part (32) of the column is again straight-sided.

An ash content monitor (33) comprising a cadmium 109 source which emits x-rays which are back scattered to a scintillation counter is situated adjacent the intermediate part (31) of the column (13). Spaced above this by a distance of 300mm is a Caesium 137 source (34) which emits gamma rays through the flow of tailings upward through the column (13) and the gamma rays are attenuated by the tailings depending on their solids content.

This attenuation is measured by a detector (35). These sources and detectors operate in known manner.

The base of the column (13) is connected by a gently curved section of pipe (36) to the valve (12). The purpose of the gently curving section is to enable there to be a gradual change of direction of tailings passing through the valve (12) into the vertical column. The absence of a sudden junction means that the tailings are sensibly presented in a laminar flow configuration and that classification due to turbulence at the base of the column is minimised.

Valve (12) is connected to the de-aerator (11), and material to the de-aerator (11) is fed through a pipe (10) which is controlled by fail safe on/off valves (8) and (9) which draw material from the froth flotation cells (4) and (5).

It is necessary to prevent blockage of the system which would occur if oversize or extraneous solids entered the pipe (10). Therefore as can be seen from Figure 5 swan-necks (40) and coarse, typically 25mm square, filters (41) are mounted inside the froth cells (4) and (5) to eliminate blockage.

The de-aerator (11) acts as a cyclone to remove air bubbles under centrifugal force. These bubbles may occasionally be entrained in water with a low solids concentration and therefore pipe (44) feeds to discharge conduit (14).

As can be seen in Figure 3 water in that pipe (23) terminates in a jet (42) which is located immediately at the top of the column (13). Water is introduced here to assist in calibrating the instrument as will be described later. The top of the column (23) overflows at a weir (43) into the discharge conduit (14).

An electronics cabinet/mimic board (45) is situated adjacent column (13) and this shows in diagram and light form the state of the various valves in the apparatus and also gives a digital read out on indicators (46), (47) of the signal derived from the detectors at (35) and (33) to give an indication of solids and ash contents respectively. This indication is usually given as a percentage of the whole.

In use the raw coal feed from the head tank (1) is fed through valves (2) and (3) to the banks (4) and (5) of froth flotation cells. If it is decided to sample the tailings in bank (4) then the control valve (8) is opened and control valve (9) is closed and a sample of tailings is taken over feed line (10) into the de-aerator (11). The valve (12) is then opened and tailings are forced up the column (13). They first enter the lower part (30) of the column and are aligned in a laminar linear mode.

After this they are dispersed in a divergent direction in the intermediate part (31) and are subjected to the x-rays of the ash monitor (33). They then pass between the source (34) and detector (35) to determine the density or solid content of the material. The tailings then pass through the upper part (32) of column (13) over the weir (43) and into the discharge conduit (14).

Signals from the detector (35) and (33) are fed to the electronics cabinet/mimic monitor board (45) and are shown there on indicators (46), (47).

The apparatus can run continuously for a long time without being calibrated. When automatic calibration is initiated the valve (12) is closed and the dump valve (24) is opened to discharge the content of the column through line (25). The dump valve (24) is then closed and the valve (21) is opened to feed water from the source (22) to conduit (23) and spray jet (42) to fill column (13). The column is flushed through by opening dump valve (24) and when it is clean the valve is again closed and the column filled with water. The detectors are then operated to give a datum signal and this is calibrated into the electronics cabinet.

The material discharged through the discharge conduit (14) joins the main stream of processed tailings to the thickener or in the case of raw feed or cleaned coal concentrate being monitored into the sump (15) pumped back through pump (16) and conduit (17) to a selected one of the banks of cells (4), (5) where it is re-processed. The selected cell is determined by operating valve (18) or (19) as required.

It will be appreciated that all that is necessary to change the sampling from bank (4) to bank (5) is to close the valve (8) and to open valve (9).

The sampling from the last cell of bank (5) is now discharged into the feed line (10) through the de-aerator and valve (12) when opened in to the column (13).

The use of the invention enables a continuous monitoring of state of the tailings to be achieved and the monitoring can be swiftly changed between cells. The use of the specially structured column (13) ensures that the distortion of readings due to turbulence or classification within the column is kept to a minimum and is an improvement over previously available systems.

Claims (32)

1. A method of sampling and measuring tailings from a coal flotation process comprising the steps of; extracting a sample of the tailings from the last stage of the flotation process, feeding the sample through a de-aerator to a vertical measurement column and presenting the sample serially in a laminar flow form to ash content measuring means and to solids content measuring means and returning the sample to the main stream of treated tailings.

2. A method as claimed in claim 1 in which the tailings to be sampled are filtered to remove extraneous oversize material.

3. A method as claimed in claim 1 or claim 2 in which the tailings are de-aerated in a cyclonic device prior to their presentation to the measuring column.

4. A method as claimed in any preceding claim in which the de-aerated tailings are presented to the vertical measurement column along a gradually curved path which prevents turbulence of solids in the tailings.

5. A method as claimed in any preceding claim in which the tailings are presented to the ash content measuring means in a part of the vertical measurement column which has two parallel and two sloping sides.

6. A method as claimed in any preceding claim in which the tailings are subjected to x-ray radiation from an x-ray source to determine by back scatter of radiation the ash content of the tailings.

7. A method as claimed in claim 6 wherein the source of x-rays is a cadmium source.

8. A method as claimed in claim 6 or claim 7 wherein the cadmium source has an energy level of the order of 0.25mCi.

9. A method as claimed in any preceding claim in which the tailings are presented to the solids content measuring means in a diverging conical configuration.

10. A method as claimed in claim 9 wherein the angle of divergence of the conical configuration does not exceed 80.

11. A method as claimed in claim 9 or claim 10 wherein the angle of divergence of the conical configuration is substantially of the order of 60.

12. A method as claimed in any preceding claim in which the tailings are arranged to pass through radiation from a gamma ray source located on one side of the vertical measuring column to a detector located on the other, the detector being arranged to measure the attenuation of radiation dependent on the solids content of the tailings material passing through the radiation from the source.

13. A method as claimed in claim 12 in which the source is a Caesium source.

14. A method as claimed in claim 10 or 11 in which the source has an energy level of the order of 20mCi.

15. A method as claimed in any preceding claim including calibrating the measuring means by shutting off the flow of tailings and feeding a calibrating fluid through the column to enable the measuring means to produce reference measurements.

16. A method as claimed in claim 15 and including using water as a calibrating fluid.

17. Apparatus for sampling and measuring tailings from a coal flotation process comprising filtering and valve means for controlling the flow of the sample of tailings from a flotation means, a de-aerator device, a vertical measuring column, on/off valve means for isolating the flow of sample tailings from the de-aerator device to the column through a gradually curved input pipe and constructing the column to give laminar flow of tailings in the column, and measuring means associated with the column.

18. Apparatus as claimed in claim 17 in which the measuring means includes an x-ray source and detector situated adjacent to first part of the column to determine the ash content of the tailings by back scatter, the said first part of the column being in the form of a divergent cone in the direction of progression of the tailings.

19. Apparatus as claimed in claim 17 or claim 18 in which the measuring means includes a gamma ray source situated on one side of the column as a second part thereof, and a detector situated on the other side to receive gamma rays attenuated by tailings passing through the column, the said second part of the column being at the top of a divergent cone in the direction of progression of the tailings, the angle of divergence not exceeding 80.

20. Apparatus as claimed in claim 19 in which the angle of divergence is substantially of the order of 60.

21. Apparatus as claimed in claims 18 and 19 in which the gamma ray source is situated above the x-ray source.

22. Apparatus as claimed in claim 21 in which the sources are separated by at least 300mm in a vertical direction.

23. Apparatus as claimed in any one of claims 17 to 22 in which a dump valve is included in the line to the vertical measuring column to remove excess sample tailings.

24. Apparatus as claimed in any one of claims 17 to 23 and including a shut off valve in the flow line from the valve control means to the column.

25. Apparatus as claimed in any one of claims 17 to 24 including de-aerator means located between the valve control means and the column.

26. Apparatus as claimed in claim 25 in which a filter means for restraining the particle size of tailings to the column is included prior to the de-aerator means.

27. Apparatus as claimed in claim 26 in which the filter means comprises of a grid basket.

28. Apparatus as claimed in any one of claims 17 to 27 and including water spray means located at the top of the column to fill the column with water for calibration purposes.

29. Apparatus as claimed in any one of claims 17 to 28 and including a discharge conduit connected to the top of the column to discharge measured tailings from the column.

30. Apparatus as claimed in any preceding claim in which the measuring means are operated along the length of the column by a distance of at least 300mm.

31. Apparatus substantially as hereinbefore described and with reference to the accompanying drawings.

32. A method of sampling and measuring tailings substantially as hereinbefore described with reference to the accompanying drawings.