GB2182172A - Control system for froth flotation process - Google Patents

Abstract

A method of monitoring the operation of a froth flotation process having an input stream consisting of raw feed, a first output stream consisting of frothed product to be reclaimed and a second output stream consisting of tailings to be discarded, comprises determining the solids content of at least the input and the tailings stream and varying the process conditions, such as the amount of flotation reagent added in response to the readings attained. The readings are preferably effected by turbidity measurement of samples withdrawn from the streams which are diluted in proportion to reduce their solids content prior to turbidity measurement. <IMAGE>

Description

SPECIFICATION Control system for froth flotation processes This invention relates to control systems for froth flotation processes and is particularly but not exclusively applicable to the control of processes employed in the froth flotation of coal.

Froth flotation processes are employed in mining to separate constituents of the mined material and in particularto extract products which may be present in relativelysmall quantities and of small size, separation and extraction of which is difficult by other processes. In the mining of coal large quantities of small coal particles or "fines", generally less than about 0.5 mm in diameterare produced.

These fines together with quantities of dirt and particles of other minerals (together referred to as "ash") are mixed with water and it is desirableto extract as much of the coal fines as possible since these can be put to commercial use.

Froth flotation processes effect separation by subjecting the slurry of particles and water to action by flotation reagents. These are chemicals having a particular affinity to the particles to be extracted, which in the case of coal mining are the coal fines, and which render them hydrophobic. In the course of the flotation process the coal fines rise to the surface from where they may be separated from other particles which sink. There are thus two output streams from flotation processes of this kind, one being the frothed product incorporating the particles to be recovered and the other being the tailings consisting ofthe remaining unwanted material. The input slurry which is separated by the flotation process is referred to herein as the " raw feed".

The raw feed to be treated by froth flotation varies considerably from one mining operation to another and can vary greatly in a relatively short period of time. Variable factors include the proportion of extractable particles to unwanted tailings, the proportion of ask entrained with the coal particles, the size distribution of particles and the chemical nature of the surface ofthe extractable particles. The ash content ofthe frothed product requires to be maintained below certain levels if if the frothed product is to be commercially acceptable.In order to obtain the best possible results the quanity and in some cases the type of flotation reagent added requires to be varied dependent on variations in the raw feed and hitherto this has conventionally been effected by an operator in response to periodic visual inspection. However in many instances a dosage of reagent giving the best average result over a wide rangeofrawfeed inputs is maintainedthroughout the process due to the difficulty of effecting adjustments which respond quickly and effectively to variations in raw feed quality.

With a view to improving performance attempts have been made to continuously monitor the condition ofthe frothed product and/orthe rawfeed and to vary the addition offlotation reagent dependent on the readings attained. Hitherto this has been effected by measurement ofthe ash content in the frothed product and this requires typically the use of radioactive means involving the use of complex and expensive equipment. It is an object of the present invention to provide an improved means of controlling a froth flotation process employing less sophisticated and expensive equipment and producing equivalent or improved results. To this end we have found that it is possible to attain the desired result without the need to distinguish between ash content and other solids thereby rendering the use of radioactive means for nucleonic ash determination unnecessary.

Thus the invention provides a method for monitoring the operation of a froth flotation process having an input stream consisting of raw feed, afirst output stream consisting offrothed productto be reclaimed and a second output stream consisting of tailingsto be discarded, the method comprising determining the solids content of at least the input and tailings stream and varying the process conditions in response to the readings attained.

Preferably the solids content of the input stream and both of said output streams is determined and the quantity of flotation reagent employed in the process altered in accordance with variations in solids content readings.

The solids contents in said streams may be assessed by measuring turbidity of samples withdrawn from the streams continuously or periodically. In orderto compensate for limitations of currently available turbidity measuring apparatus the samples are preferably diluted by addition of water to reduce the solids contents of each sample in proportion to levels readable by the turbidity measuring apparatus. Where measuring is effected on continuously flowing samples the rate offlowof the samples is preferably maintained constant.

The invention also provides apparatus for monitoring the operation of a froth flotation process having an input stream consisting of raw feed, a first output stream consisting of frothed product to be reclaimed and a second output stream consisting of tailingsto be discarded, the apparatus comprising means for measuring the solids content of at leastthe input and tailings streams and control means operable in response to signals derived from said measurements to vary the dosage offlotation reagents employed in the process.

Preferably means is provided for effecting measurement of solids content of said input stream and both of said output streams, said control means incorporating comparator means adapted to receive signals generated in response to said measurements, to compare same with optimum readings andto generate outputsignals proportional to the deviation from said optimum readings.

Preferably the comparator means comprises a computer. The flotation reagents are preferably added to the raw feed by metering means operation of which is controlled bytheoutputsignalsfromsaid control means.

Preferably also said means for measuring solids content comprises turbidity sensing means associated with said streams. The turbidity sensing means may comprise light sensing units through which samples drawn from said streams are directed. Means is preferably provided to effect dilution of the samples to less than 2% solids content before passing through said sensing units.

Advantageously additional sensing means may be employed to determine ash content in the frothed product said additional sensing means being adapted to transmit control signals to said computer operable to vary the dosage of flotation reagent to maintain ash level in the frothed product between predetermined parameters An embodimentofthe invention will now be described, by way of example only, with reference to the accompanying diagrammatic drawings in which Figure lisa flow diagram of a froth flotation system for coal recovery incorporating the invention; Figure 2 is an enlarged diagrammatic cross-sectional view th rough a sampling and sensing unit.

Referring to Figure 1,the rawfeed material to be processed consists of a liquid or slurry incorporating coal fines and other particles or "ash" in water and is introduced through a supply pipe 8to a froth flotation unit 9. The froth flotation unit consists of a series of treatment cells through which the raw feed passes and is brought into contact with froth floation reagents having an affinityforthe coal particles, rendering them hydrophobic. Air bubbles are generated in the cells and pass through the slurry and the hydrophobic coal particles attach themselves to the bubbles and rise to the surface from whence they are withdrawn into a discharge pipe 10 reading to a dewatering system.The tailings or unwanted material remaining after separation of the coal fines by the flotation process are drawn off through pipework 11 for disposal or subsequent treatment. Movement of the material through the system is effected by means of pumps (not shown) which maintain a uniform metered flow.

A sample of the rawfeed is withdrawn from the pipe 8 prior to entry of the slurry into the froth flotation unit 9. Withdrawal is effected through a pipe 12 of predetermined diameter and is directed to a monitoring and control system referred to hereafter.

Theflowthrough the pipe 12 may be controlled buy a flow rate splitter and/or a flow rate reducing or restrictorvalve or by a metering pump (not shown).

Samplesofthefrothed productandofthetailingsare withdrawn from the pipe 10 and discharge pipe 11 in a similar mannerthrough pipes 13 and 14 respectively.

The raw feed withdrawn through pipe 12 is diluted by water introduced through auxiliary pipes 17 and 26 and deliveredthrough an overflow unit 18 and pipe 19 to a sensing unit 20. The frothed product withdrawn from the pipe 10 is delivered through an overflow unit 22 and pipe 23 to a similar sensing unit 25. Priorto introduction to the sensing unit 25 water is addedtothe samplethrough a supply line27 connected to the line 17. In a similarfashionthe tailings withdrawn through pipe 14 are delivered through an overflow unit 28 and pipe 29, diluted by water introduced from an auxiliary pipe 31 and delivered to a sensing unit 30 similarto the units 20 and 25.The material discharged from all three of the sensing units is directed through a common discharge pipe 32 to a collecting tank (not shown) from whence it is returned to the flotation system.

Waterfordilution is supplied to the lines 26,27,31 and 17 from a common supply tank37 through a flow/pressure regulator38 and individual manually adjustable flow control valves 40.

Referring now to Figure 2 ofthe drawings, each of the sensing units 20,25 and 30 comprises a vessel or container45 having a dilution water inlet46 and a sample inlet 47 both connected to a mixing chamber 48 from which the diluted slurry is introduced into the vessel 45. Aturbidity sensor 49 projects into the vessel adjacent to the outlet ofthe mixer 48. The turbidity sensor incorporates light emitting devices and light sensitive cells between which the diluted slurry passes. The sensor 49 produces an output signal dependent on the light intensity sensed by the cells and hence related to the solids content ofthe sample. Uniform dispersion of solids throughout the sample is ensured by the action ofthe mixing chamber48.Thesample is dischargedfrom the vessel 45 to the discharge pipe 32 through an outlet pipe 50.

The turbidity sensors 49 are designed to measure turbidity or cloudiness of liquids and are not capable of operating satisfactorily where the liquid is contaminated by greaterthan about 2% solids content. Dilution of the various samples is therefore effected to reduce the solids content to a range capable of being handled by the turbidity sensors.

For this purpose the maxmum solids content likely to be encountered in each stream requires to be known in advance. In most froth flotation processes employed in coal reclamation the solids content in the raw feed will be in the region of 5% to 20%, unable raw feed will not produce a solids content in the tailings of greater than 15% and the solids content in the frothed product will typically be less than 40% otherwise the frothed product cannot be removed efficiently from the slurry surface in the froth flotation unit.It is thus possible from a knowledge of the maximum solids contents which will be encountered in each of the three streams forthe poorest condition processable raw feed derived from any given mined material to calculate the extent of dilution required to produce samples forturbidity testing having a maximum solids content within the workable range ofthe turbidity sensors. Provided the rate offlow of each of the samples to be tested is maintained constant and the condition of the raw feed material does not become poorerthantheworst material allowed for, the quantity of water be added to each of the samples will remain constant throughout operation ofthe process and once determined can be preset by means ofthe manual flow control valves 40 in the main water supply line 17.

The signals from the turbidity sensor 49 in each of the units 20,25 and 30 are transmitted to a computer indicated diagrammatically at 55. The computer is programmed to produce output signals dependent on the input from the sensors to control metering pumps indicated at 56 supplying flotation reagents to the flotation unit 9. For this purpose the computer is preprogrammed with data relating the yield of frothed product to the reagent dosage and operates to calculate actual yield of frothed product determined from the input signals from the three sensors and adjust the reagent supply pumps 56 to alterthe reagent dosage in a mannerto increase yield of frothed product.The yield/reagent dosage data with which the computer is programmed is preferably determined by prior planttrials and covers the full spectrum of raw feeds normally associated with the plant. The system operates to adjust reagent dosage at predetermined time intervals, for example every twenty seconds, for the purpose of increasing yield compared with that calculated from the previously generated signals.

In construction ofthe system it is important that the pipework should be designed to prevent blockage and permit smooth flow ofslurrythrough the system. Forthis purpose the pipework diameter must be related to the nature ofthe slurry being treated and particularly to the maximum particle size.

Changes in diameter of pipe are preferably effected by conical reducers and right angle bends avoided.

Valves and other obstructions are preferably keptto a minimum and valves located in vertical or inclined pipe runs in orderto ensure recommencementof flow by action of gravity following valve operation.

Pipe runs are preferably vertical and inclined rather than horizontal to enableflowto be assisted by the action of gravity although flow may be controlled by use of metering pumps rather than being dependent solely on gravity feed ifdesired.

By virtue of the arrangement described the dosage offlotation reagents is continuously monitored and adjusted in accordance with readings taken on a continuous basisfromthe inputandoutputflowsof the flotation system thereby enabling the system to "hunt" continuously for maximum yield by automatically and regularly altering reagent dosage in accordance with variations in processing conditions. The use of diluted samplesfortesting enables measurement of solids content by sensing turbidity, thereby eliminating much ofthe complication and hence expense of previously proposed system referred to above.

Various modifications may be made without departing from the invention. For example an additional sensor may be provided to monitor the ash content in the frothed product thus enabling the reagent dosage to be adjusted automatically to maintain ash content within predetermined limits.

This provides a control over the quality of the resulting product in addition to the optimum yield of coal fines. Solids content of the sample streams may be measured by means other than turbidity sensing if desired. The system may also be adapted to vary the quantities and/or proportions of a number of different flotation reagents instead of a single reagent if required. Moreover while reference has been made herein primarily to the flotation separation of coal products, the invention is also applicable to the flotation separation of other minerals.

Whilst endeavouring in the foregoing specification to draw attention to those features ofthe invention believed to be of particular importance it should be understood that the Applicant claims protection in respect of any patentable feature or combination of features hereinbefore referred to whether or not particular emphasis has been placed thereon.

Claims (23)

1. A method for monitoring the operation of a froth flotation process having an input stream consisting of raw feed, a first output stream consisting offrothed product to be reclaimed and a second output stream consisting oftailingsto be discarded, the method comprising determining the solids content of at leastthe input and tailings stream and varying the process conditions in response to the readings attained.

2. A method according to claim 1 wherein the solids content ofthe input stream and both of said output streams is determined.

3. A method according to claim 1 or 2 wherein the quantity offlotation reagent employed in the process is altered in accordance with variations in solids content readings.

4. A method according to any preceding claim wherein the solids content in said streams is assessed by measuring turbidity of samples withdrawn from the streams.

5. A method according to claim 4wherein said samples are diluted in proportion to reduce their solids content prior to measurement ofturbidity.

6. A method according to claim 4 or 5 wherein said samples are continuously flowing and maintained at a constant flow rate.

7. A method for monitoring the operation ofa froth flotation process substantially as hereinbefore described.

8. Apparatus for monitoring the operation of a froth flotation process having an input stream consisting of raw feed, a first output stream consisting of frothed product to be reclaimed and a second output stream consisting oftailingsto be discarded, the apparatus comprising means for measuring the solids content of at least the input and tailingsstreams and control means operable in response to signals derived from said measurements to vary the dosage of flotation reagents employed in the process.

9. Apparatus according to claim 8 wherein means is provided for effecting measurement of solids content of said input stream and both of said output streams.

10. Apparatus according to claim 8 or9 wherein said means for measuring solids content comprises turbidity sensing means associated with said streams.

11. Apparatus according to claim 10 including meansforwithdrawing samples from said streams forturbidity sensing.

12. Apparatus according to claim 11 wherein said turbidity sensing means incorporates light sensing units through which said samples are directed.

13. Apparatus according to claim 11 or 12 including meansfor reducing the solids contents of said samples prior to turbidity sensing.

14. Apparatus according to claim 13 wherein the solids contents of said samples are reduced by dilution.

15. Apparatus according to claim 13or14 wherein the solids contents of said samples are reduced to not more than 2% priorto turbidity sensing.

16. Apparatus according to any of claims 8 to to claims 8 to 15 wherein said control means incorporates comparator means adapted to receive signals generated in response to said measurements, to compare same with optimum readingsandto generate outputsignals proportional to the deviation from said optimum readings.

17. Apparatus according to claim 16wherein said comparator means comprises a computer.

18. Apparatus according to claim 17 wherein flotation reagents are added to the raw feed by metering means operation ofwhich is controlled by the output signals from said control means.

19. Apparatus according to anyofclaims 8to 18 including additional sensing means to determine ash content in the frothed product.

20. Apparatus according to any of claims 8 to 19 wherein said additional sensing means is adapted to transmit control signals to said computer operable to vary the dosage offlotation reagentto maintain ash level in the frothed product between predetermined parameters.

21. Apparatus for monitoring the operation of a froth flotation process substantially as hereinbefore described with reference to Figure 1 ofthe accompanying drawings.

22. Apparatus for monitoring the operation of a froth flotation process incorporating turbidity sensing means substantially as hereinbefore described with reference to Figure2 ofthe accompanying drawings.

23. Any novel subject matter of combination including novel subject matter disclosed in the foregoing specification or claims and/or shown in the drawings, whether or not within the scope of or reiating to the same invention as any ofthe preceding claims.