Abstract:
A method and apparatus for processing particulate material such as coal, and also for measuring the efficiency of separation of the coal is disclosed. Particulate material is supplied to a separator such as a heavy medium device containing a dense medium ( 6 ). A parameter of the device ( 6 ) indicative of separation cut point is measured. The parameter may be density of the medium, flow rate of material or pressure of feed as well as medium to coal ratio. Measurements of these parameters are made over a time period and, from the measurements, an induced value indicative of separating efficiency is determined. The induced value provides a measure of separation efficiency and also provides a value which can be compared with a predetermined value so that an alarm can be generated if the value departs from the predetermined value by a predetermined amount.

Description:
FIELD OF THE INVENTION  
       [0001]     This invention relates to a method and apparatus for processing particulate material and, in particular, minerals and carbonaceous solids such as coal, iron ore, manganese, diamonds and other materials. The invention has particular application to the processing of coal, and will be further described in relation to the processing of coal. However, it should be understood that the invention is applicable to processing other materials including but not restricted to those mentioned above.  
       BACKGROUND OF THE INVENTION  
       [0002]     Raw coal is mined from the ground and is processed to provide a desirable commercial product. Raw coal includes a certain amount of gangue mineral content which, following combustion under standard conditions, leaves a solid ash residue.  
         [0003]     For some applications (eg coke making) saleable coal most preferably has a fixed ash specification limit which is normally specified in contractual agreements between the producer and the purchaser. A typical example of an ash specification for high quality coking coal is 10% (air dried basis). If the ash level of produced coal increases above this level, the product may still be saleable but its price is deleteriously affected and/or some penalties for the producer may be incurred.  
         [0004]     For other applications, saleable coal most preferably has a minimum or fixed specific energy content limit which is normally specified in contractual agreements between the producer and the purchaser. A typical example of an energy specification for high quality thermal coal is 6000 kCal/kg (net as received basis). If the specific energy level of produced coal decreases below this level, the product may still be saleable but its price is deleteriously affected and/or some penalties for the producer may be incurred.  
         [0005]     Raw coal after mining may be comminuted to a required size and separated into a particular particle size by a screen mesh type or other classification-type device to separate the raw coal into predetermined particle sizes defined by, for example, the screen aperture size of the screen separator and other operating characteristics such as state of screen wear, solids loading level, water addition rate etc.  
         [0006]     The separated coal of the desired size is then supplied to a dense medium separator. There are a number of different dense medium separators currently in use depending on the size of particles being treated. For example, large lumps may be processed in heavy medium drums, heavy medium baths, heavy medium vessels, larcodems etc, and smaller but still coarse particles may be processed in heavy medium cyclones, heavy medium cycloids etc. Note that the words “heavy” and “dense” can be used interchangeably in this context. These types of heavy medium devices use a benign or inert finely ground powder of medium solids (such as magnetite or ferro-silicon) slurried in water to form a dense medium whose density can be automatically controlled by the proportion of solids in the slurry. Mixing the raw coal with the dense medium enables separation on the basis of its density relative to the density of the dense medium. For example, coal with an ash level of 10% may be separable from higher ash components of the raw coal by adding the raw coal to a dense medium of, for example, 1400 kg/m 3 . In this example, the 10% ash product coal might float clear of the higher ash material which might tend to sink in the dense medium. The material that floats would report to the overflow outlet of a separator and that which sinks would report to the underflow outlet.  
         [0007]     For the specific case of a dense medium cyclone, it is separating efficiency of the coal particles that is often critical to maximising yield and recovery. The accepted industry standard for measuring efficiency is the partition coefficient curve with its characteristic D 50  and Ep parameters. The D 50  is the separating density of the particles and the Ep is a measure of the sharpness the separation (a higher value of Ep indicates more misplacement of particles and hence a lower efficiency).  
         [0008]     Whilst the D 50  of a separation is strongly related to the medium density, there are machine effects that lead to, almost invariably, the D 50  being a little higher than the medium density. The difference between D 50  and the medium is conventionally termed “offset”. The extent to which it is greater is dependent on a number of parameters, including, but not limited to, medium density, dense medium cyclone pressure, raw coal feed rate, medium to coal ratio, and variations therein. The overall sharpness of separation is a strong function of variations in each of these parameters (medium density, pressure, feed rate and medium to coal ratio).  
         [0009]     Measurement of the density of medium slurry is performed by, for example, nucleonic gauges or differential pressure transducers. Measurement of pressure of the material feeding a dense medium cyclone is performed with pressure transducers and the like, while plant feed rate is determined with weightometers on the conveyor belt feeding the plant. Medium to coal ratio is not conventionally measured on-line and plant feed rate may be used as a proxy. However, it is conceivable that such measurement may be made in the future when the measurement technology is developed.  
         [0010]     Each of these parameters may be incorporated into individual control systems which attempt to maintain operational values of these parameters within acceptable limits. However, control systems are imperfect and variations occur during normal industrial operations. Variations in the medium density, pressure, feed rate and medium to coal ratio cause separations to occur at densities (D 50 &#39;s) different from those desired. Momentary fluctuations that lead to higher D 50 &#39;s than desired will result in higher proportions of the raw coal being collected at the separator floats or overflow outlet. A momentary change in product quality will occur with a higher ash material separated. Similarly, the momentary changes in product quality will occur when fluctuations lead to lower D 50&#39;s , which result in decreases in the ash of the separated material.  
         [0011]     Whilst plant control systems almost invariably allow overall consignment product within ash specification to be separated, this is often achieved at the expense of yield and recovery. Maximum yield or recovery at a given product quality is achieved when fluctuations in each of medium density, pressure,feed rate and medium to coal ratio are minimised.  
         [0012]     Typically, in order to obtain an Ep value, samples of the material which are being processed (such as coal) are acquired representatively following strict sampling procedures. This typically involves concurrent taking of a sample from the feed line to the separator, and also samples which have reported to product and reported to reject. Those three samples are then forwarded to a laboratory for analysis and raw data is obtained which is then analysed to produce the partition curve. Typically, the taking of the samples involves a number of people who may, for example, take sample increments over a nine hour period. Furthermore, typically the analysis of the samples and then the preparation of the partition curve may take several weeks. Thus, results are not available in accordance with the prior art teaching for some weeks or the like after the sample material is actually acquired.  
       SUMMARY OF THE INVENTION  
       [0013]     The object of the invention is to provide a method and apparatus for processing particulate material, such as coal, in which yield or recovery losses can be reduced.  
         [0014]     The present invention provides a method of processing particulate material, including the steps of:  
         [0015]     supplying the particulate material to a separator;  
         [0016]     monitoring a parameter or parameters of the separator indicative of a separation value of the material;  
         [0017]     determining from said parameter an induced value indicative of the separating efficiency of the material that passed through said separator;  
         [0018]     comparing said value with a predetermined value; and  
         [0019]     generating an alarm condition if the said value departs from the predetermined value by a predetermined amount.  
         [0020]     Thus, according to the invention, if the effective separating efficiency departs from the required separating efficiency by a predetermined amount an alarm signal is generated. This enables remedial action to be taken to correct whatever fault has caused the change in the separating efficiency of the dense medium device, thereby returning the separating efficiency to its desired level to decrease the loss due to fluctuations in the separating density of the material. In other words, the fluctuation cycle of the cut point and other partition coefficient-based characteristics can be more quickly responded to so as to reduce both the magnitude and time of the fluctuations to reduce yield and recovery losses caused by those fluctuations.  
         [0021]     The separation value may comprise the separating density if the separator is a medium dense separator or may be size of material if the separator is a classifying separator based on size of the material.  
         [0022]     Preferably the separator comprises a heavy medium device containing a dense medium.  
         [0023]     Preferably the step of determining the induced value comprises determining an induced set of values indicative of the separating efficiency of the material that passed through the device, the step of comparing said value comprises comparing said set of values with a predetermined range for the set of values, and the step of generating the alarm condition comprises generating the alarm condition if the said set of values departs from the predetermined range for the set of values by a predetermined amount.  
         [0024]     The set of values may be in the form of a partition coefficient curve and parameters derived therefrom.  
         [0025]     In the preferred embodiment of the invention, the parameter which is monitored is the actual density of the medium.  
         [0026]     However, in another embodiment, the parameter is pressure of the medium and particle mixture which is supplied to the device.  
         [0027]     In a still further embodiment the parameter is the feed rate of the medium and particle mixture supplied to the device. A practical proxy for this is the overall processing plant feed rate.  
         [0028]     In a still further embodiment the parameter is the ratio of volume or mass flow rate of medium to the volume or mass flow rate of the raw coal, commonly referred to as “Medium to Coal Ratio”. Direct measurement of this parameter is preferable, but a practical proxy is processing plant feed rate.  
         [0029]     In a still further embodiment of the invention, two or more of the medium density, pressure of the medium and particle mixture, feed rate of the medium and particle mixture, and Medium to Coal Ratio are monitored.  
         [0030]     In the preferred embodiment of the invention, the density of the medium is measured at predetermined time intervals, and for a predetermined time period, the number of measurements at each measured value is determined to produce a cumulative normalised frequency distribution of the length of time the particle spends at each measured density, and said set of values characterising separating efficiency is determined as a medium induced partition coefficient curve and/or a parameter derived therefrom, for example medium induced Ep value (MIEp value) by taking the absolute value of the difference in density at the 75 th  and 25 th  percentiles, and dividing by 2000 so as to produce an MIEp value which is a theoretical value solely dependent on medium density variations, and comparing the MIEP value with the said predetermined value, or medium induced partition coefficient curve with a predetermined partition coefficient curve. When making the necessary measurements to calculate the said separating efficiency characteristics, the predetermined time interval should be small in relation to the predetermined time period. A further assumption implicit in this approach is that offset is constant over the range of density values encountered.  
         [0031]     In the other embodiments of the invention a feed rate induced partition coefficient curve and/or a parameter derived therefrom, for example feed rate induced Ep(FRIEp) value is determined in the same manner from the feed rate measurements made over the predetermined time period.  
         [0032]     However a theoretical and/or empirical calibration will be required to convert feed rate variation to D 50  variation so as to produce a cumulative normalised frequency distribution of separating densities and so provide the length of time spent at each separating density. However, a pseudo-feed rate induced partition coefficient curve and derivatives therefrom may be calculated without the need for a theoretical and/or empirical calibration. In such case the cumulative normalised frequency distribution curve would be plotted against feed rate as the abscissa and a pseudo FRIEp calculated in a similar manner to MIEp. As the pseudo variation on the concept does not require calibration, is easier to measure and use, and it is the preferred method of efficiency assessment if the parameter is feed rate. In the case of measuring the pressure of the medium and particle mixture, a pressure induced partition coefficient curve and a derived pressure induced Ep(PIEp) value is determined so that individual values over the predetermined time period are used to calculate a cumulative normalised frequency distribution of separating densities, giving the length of time spent at each separating density. Once again a theoretical and/or empirical calibration is required to convert pressure measurements to separating density (D 50 ) In a similar manner to the case for feed rate, a pseudo curve and pseudo PIEp may be calculated. As the pseudo variation on the concept does not require calibration, is easier to measure and use, and it is the preferred method of efficiency assessment if the parameter is pressure. In the case of measuring the Medium to Coal Ratio of the medium and particle mixture, a Medium to Coal Ratio induced partition coefficient curve and a derived Medium to Coal Ratio induced Ep(MCRIEp) value is determined so that individual values over the predetermined time period are used to calculate a cumulative normalised frequency distribution of separating densities, giving the length of time spent at each separating density. Once again a theoretical and/or empirical calibration is required to convert Medium to Coal Ratio measurements to separating density (D 50 ). In a similar manner to the case for feed rate and pressure, a pseudo curve and pseudo MCRIEp may be calculated. As the pseudo variation on the concept does not require calibration, is easier to measure and use, and it is the preferred method of efficiency assessment if the parameter is medium to coal ratio.  
         [0033]     The present invention may be said to reside in an apparatus for processing particulate material, comprising:  
         [0034]     means for supplying the particulate material to a separator;  
         [0035]     means for monitoring a parameter of the separator indicative of a separation value of the material;  
         [0036]     processing means for determining from said parameter an induced value indicative of the separating efficiency of the material that passed through said separator;  
         [0037]     comparing means for comparing said value with a predetermined value; and  
         [0038]     alarm means for producing an alarm condition if the said value departs from the predetermined value set by a predetermined amount.  
         [0039]     Preferably the separator comprises a heavy medium device.  
         [0040]     Preferably the processing means determines from said parameter an induced set of values indicative of the separating efficiency of the material that passed through the device, the comparing means compares the said value set with a predetermined value set and the alarm means is for producing the alarm condition if the set of values departs from the predetermined value set by a predetermined amount.  
         [0041]     The set of values may be in the form of an induced partition coefficient curve and parameters derived therefrom.  
         [0042]     In the preferred embodiment of the invention, the monitoring means measures the density of the medium at predetermined time intervals, and for a predetermined time period, such that the predetermined time intervals are small compared to the predetermined time and the processing means determines the number of measurements at each measured value to produce a cumulative normalised frequency distribution of the length of time the particle spends at each measured density, and determines said value set as a medium induced partition coefficient curve and/or parameters derived therefrom, for example medium induced Ep value (MIEp value) by taking the absolute value of the difference in relative density at the 75 th  and 25 th  percentiles, and dividing by 2000 so as to produce an MIEp value which is a theoretical value solely dependent on medium density variations, and comparing the partition coefficient curve and parameters derived therefrom, for example, MIEp value set with the said predetermined value set.  
         [0043]     In the other embodiments of the invention a feed rate induced partition coefficient curve and parameters derived therefrom, for example Ep(FRIEp) value set is determined in a similar manner from the feed rate measurements made over the predetermined time period. As feed rate to dense medium separators is not commonly measured directly, overall processing plant feed rate is used as a proxy. However a theoretical and/or empirical calibration will be required to convert feed rate variation to D 50  variation so as to produce a cumulative normalised frequency distribution of separating densities and so provide the length of time spent at each separating density. However, a pseudo-feed rate induced partition coefficient curve and derivatives there from may be calculated without the need for a theoretical and/or empirical calibration. In such case the cumulative normalised frequency distribution curve would be plotted against feed rate as the abscissa and a pseudo FRIEp calculated in a similar manner to MIEp. As the pseudo variation on the concept does not require calibration, is easier to measure and use, and it is the preferred method of efficiency assessment. In the case of measuring the pressure of the medium and particle mixture, a pressure induced partition coefficient curve and parameters derived therefrom, for example, pressure induced Ep(PIEP) value set is determined in a similar manner from the pressure measurements made over the predetermined time period. However a theoretical and/or empirical calibration will be required to convert pressure variation to D 50  variation so as to produce a cumulative normalised frequency distribution of separating densities and so provide the length of time spent at each separating density. In a similar manner to the case for feed rate, a pseudo curve and pseudo PIEp may be calculated. As the pseudo variation on the concept does not require calibration, is easier to measure and use, and it is the preferred method of efficiency assessment. In the case of measuring the Medium to Coal Ratio, a Medium to Coal Ratio induced partition coefficient curve and parameters derived therefrom, for example, Medium to Coal Ratio induced Ep(MCRIEp) value set is determined in a similar manner from the Medium to Coal Ratio measurements made over the predetermined time period. However a theoretical and/or empirical calibration will be required to convert Medium to Coal Ratio variation to D 50  variation so as to produce a cumulative normalised frequency distribution of separating densities and so provide the length of time spent at each separating density. In a similar manner to the case for feed rate and pressure, a pseudo curve and pseudo MCRIEp may be calculated. As the pseudo variation on the concept does not require calibration, is easier to measure and use, and it is the preferred method of efficiency assessment.  
         [0044]     A second aspect of the invention provides a method of determining the efficiency of separation of particulate material supplied to a separator, comprising the steps of:  
         [0045]     monitoring a parameter of the separator indicative of a separation value of the material;  
         [0046]     determining from said parameter an induced value indicative of the separating efficiency of the material that pass through the separator; and  
         [0047]     using the induced value to provide a measure of the efficiency of separation.  
         [0048]     Thus, according to this aspect of the invention, because a parameter of the separator, rather than the material which is being separated is monitored, the data required to determine efficiency can be acquired much more quickly and also much less expensively because the equipment needed to measure the parameters of the separator, rather than analysis actual sample material can be performed much quicker and less expensively. In addition, in the case of medium induced Ep, the density measurements required are readily available as they comprise those used to as part of a density control system. The same can be said for pressure and feed rate. Thus, an efficiency measure of the separation of the coal can be produced almost in real time, thereby enabling remedial action to be taken should the efficiency of separation deteriorate. This in turn enables a processing plant for processing the material to be corrected where necessary to ensure that separation is efficiently performed, thereby producing better product and economic results.  
         [0049]     Preferably the step of determining the induced value comprises determining an induced set of values indicative of the separating efficiency of the material that passed through the device, the step of comparing said value comprises comparing said set of values with a predetermined range for the set of values, and the step of generating the alarm condition comprises generating the alarm condition if the said set of values departs from the predetermined range for the set of values by a predetermined amount.  
         [0050]     The set of values may be in the form of an induced partition coefficient curve and parameters derived therefrom.  
         [0051]     In the preferred embodiment of the invention, the parameter which is monitored is the actual density of the medium.  
         [0052]     However, in another embodiment, the parameter is pressure of the medium and particle mixture which is supplied to the device.  
         [0053]     In a still further embodiment the parameter is the feed rate of the medium and particle mixture supplied to the device. A practical proxy for this is the overall processing plant feed rate.  
         [0054]     In a still further embodiment the parameter is the ratio of volume or mass flow rate of medium to the volume of mass flow rate of the raw coal, commonly referred to as “Medium to Coal Ratio”. Direct measurement of this parameter is preferable, but a practical proxy is processing plant feed rate.  
         [0055]     In a still further embodiment of the invention, two or more of the medium density, pressure of the medium and particle mixture, feed rate of the medium and particle mixture, and Medium to Coal Ratio are monitored.  
         [0056]     In the preferred embodiment of the invention, the density of the medium is measured at predetermined time intervals, and for a predetermined time period, the number of measurements at each measured value is determined to produce a cumulative normalised frequency distribution of the length of time the particle spends at each measured density, and said set of values characterising separating efficiency is determined as a medium induced partition coefficient curve and/or a parameter derived therefrom, for example medium induced Ep value (MIEp value) by taking the absolute value of the difference in density at the 75 th  and 25 th  percentiles, and dividing by 2000 so as to produce an MIEp value which is a theoretical value solely dependent on medium density variations, and comparing the MIEp value with the said predetermined value, or medium induced partition coefficient curve with a predetermined partition coefficient curve. When making the necessary measurements to calculate the said separating efficiency characteristics, the predetermined time interval should be small in relation to the predetermined time period. A further assumption implicit in this approach is that offset is constant over the range of density values encountered.  
         [0057]     In the other embodiments of the invention a feed rate induced partition coefficient curve and/or a parameter derived therefrom, for example feed rate induced Ep(FRIEp) value is determined in the same manner from the feed rate measurements made over the predetermined time period. However a theoretical and/or empirical calibration will be required to convert feed rate variation to D 50  variation so as to produce a cumulative normalised frequency distribution of separating densities and so provide the length of time spent at each separating density. However, a pseudo feed rate induced partition coefficient curve may be derived without the need for a theoretical and/or empirical calibration. In such case the cumulative normalised frequency distribution curve would be plotted against feed rate as abscissa and the pseudo FRIEP calculated in a similar way to FRIEP. As the pseudo variation on the concept does not require calibration, is easier to measure and use, and it is the preferred method of efficiency assessment. In the case of measuring the pressure of the medium and particle mixture, a pressure induced partition coefficient curve and a derived pressure induced Ep(PIEp) value is determined so that individual values over the predetermined time period are used to calculate a cumulative normalised frequency distribution of separating densities, giving the length of time spent at each separating density. Once again a theoretical and/or empirical calibration is required to convert pressure measurements to separating density (D 50 ) However, a pseudo pressure induced partition coefficient curve may be derived without the need for a theoretical and/or empirical calibration. In such case the cumulative normalised frequency distribution curve would be plotted against feed rate as abscissa and the pseudo PIEp calculated in a similar way to PIEp. As the pseudo variation on the concept does not require calibration, is easier to measure and use, and it is the preferred method of efficiency assessment. In the case of measuring the Medium to Coal Ratio of the medium and particle mixture, a Medium to Coal Ratio induced partition coefficient curve and a derived Medium to Coal Ratio induced Ep(MCRIEp) value is determined so that individual values over the predetermined time period are used to calculate a cumulative normalised frequency distribution of separating densities, giving the length of time spent at each separating density. Once again a theoretical and/or empirical calibration is required to convert Medium to Coal Ratio measurements to separating density (D 50 ) However, a pseudo Medium to Coal Ratio induced partition coefficient curve may be derived without the need for a theoretical and/or empirical calibration. In such case the cumulative normalised frequency distribution curve would be plotted against feed rate as abscissa and the pseudo MCRIEP calculated in a similar way to MCRIEp. As the pseudo variation on the concept does not require calibration, is easier to measure and use, and it is the preferred method of efficiency assessment.  
         [0058]     This aspect of the invention also provides using the measure of efficiency determined according to the above method to adjust a processing plant to more efficiently separate the material.  
         [0059]     This aspect of the invention also provides an apparatus for processing particulate material, comprising:  
         [0060]     means for supplying the particulate material to a separator;  
         [0061]     means for monitoring a parameter of the separator indicative of a separation value of the material; and  
         [0062]     processing means for determining from said parameter an induced value indicative of the separating efficiency of the material that pass through said separator to thereby provide a measure of the efficiency of the apparatus.  
         [0063]     Preferably the separator comprises a heavy medium device. Preferably the processing means determines from said parameter an induced set of values indicative of the separating efficiency of the material that passed through the device, the comparing means compares the said value set with a predetermined value set and the alarm means is for producing the alarm condition if the set of values departs from the predetermined value set by a predetermined amount.  
         [0064]     The set of values may be in the form of a partition coefficient curve and parameters derived therefrom.  
         [0065]     In the preferred embodiment of the invention, the monitoring means measures the density of the medium at predetermined time intervals, and for a predetermined time period, and the processing means determines the number of measurements at each measured value to produce a cumulative normalised frequency distribution of the length of time the particle spends at each measured density, and determines said value set as a medium induced partition coefficient curve and/or parameters derived therefrom, for example medium induced Ep value (MIEp value) by taking the absolute value of the difference in relative density at the 75 th  and 25 th  percentiles, and dividing by 2000 so as to produce an MIEp value which is a theoretical value solely dependent on medium density variations, and comparing the partition coefficient curve and parameters derived therefrom, for example, MIEp value set with the said predetermined value set.  
         [0066]     In the other embodiments of the invention a feed rate induced partition coefficient curve and parameters derived therefrom, for example Ep(FRIEp) value set is determined in a similar manner from the feed rate measurements made over the predetermined time period. As feed rate to dense medium separators is not commonly measured directly, overall processing plant feed rate is used as a proxy. However a theoretical and/or empirical calibration will be required to convert feed rate variation to D 50  variation so as to produce a cumulative normalised frequency distribution of separating densities and so provide the length of time spent at each separating density. However, a pseudo-feed rate induced partition coefficient curve and derivatives there from may be calculated without the need for a theoretical and/or empirical calibration. As the pseudo variation on the concept does not require calibration, is easier to measure and use, and it is the preferred method of efficiency assessment. In the case of measuring the pressure of the medium and particle mixture, a pressure induced partition coefficient curve and parameters derived therefrom, for example, pressure induced Ep(PIEp) value set is determined in a similar manner from the pressure measurements made over the predetermined time period. However a theoretical and/or empirical calibration will be required to convert pressure variation to D 50  variation so as to produce a cumulative normalised frequency distribution of separating densities and so provide the length of time spent at each separating density. In a similar manner to the case for feed rate, a pseudo curve and pseudo PIEp may be calculated. As the pseudo variation on the concept does not require calibration, is easier to measure and use, and it is the preferred method of efficiency assessment. In the case of measuring the Medium to Coal Ratio, a Medium to Coal Ratio induced partition coefficient curve and parameters derived therefrom, for example, Medium to Coal. Ratio induced Ep(MCRIEp) value set is determined in a similar manner from the Medium to Coal Ratio measurements made over the predetermined time period. However a theoretical and/or empirical calibration will be required to convert Medium to Coal Ratio variation to D 50  variation so as to produce a cumulative normalised frequency distribution of separating densities and so provide the length of time spent at each separating density. In a similar manner to the case for feed rate and pressure, a pseudo MCRIEP may be calculated. As the pseudo variation on the concept does not require calibration, is easier to measure and use, and it is the preferred method of efficiency assessment.  
         [0067]     Conventionally, the partition coefficient curve is measured by determining how coal particles entering the separating device separate. This invention separates the impact of separator design, operational configuration and wear condition from the impact of processing operating variables such as medium density, pressure and flow rates. In essence, the invention separates in to distinct measurable entities inefficiencies due to variations in process variables such as medium density, pressure and flow rates. The overall separating Ep for coal will be the combination of the Ep due to the separator design, configuration and wear condition (which has a relatively slow temporal change rate), Ep due to medium density variation, Ep due to pressure variation, Ep due to feed rate variation etc. The later factors will have a much higher temporal change rate. Furthermore, whilst conventional measurement of coal partition coefficient curve is laborious and time consuming, quantification of the process variables, particularly medium density, pressure and feed rate is rapid, easy and cheap to achieve on-line utilising systems and equipment commonly existing in modern processing facilities. 
     
    
     BRIEF DESCRIPTION OF THE DRAWINGS  
       [0068]     A preferred embodiment of the invention will be described, by way of example, with reference to the accompanying drawings in which:  
         [0069]      FIG. 1  is an illustrative diagram illustrating apparatus for processing coal;  
         [0070]      FIG. 2  is a block diagram illustrating the operation of the preferred embodiment of the invention;  
         [0071]      FIG. 3  is a graph showing the accumulative normalised frequency distribution for an ideal situation; and  
         [0072]      FIG. 4  is a graph of the type of  FIG. 3  exemplifying what may occur in actual practice. 
     
    
     DETAILED DESCRIPTION OF THE PREFERRED EMBODIMENTS  
       [0073]     The following is a specific example of a generic dense medium cyclone circuit. It is given as a means only of explaining how the invention can be applied and does not limit the coverage of the invention to the specific example given.  
         [0074]     Prior to entering the process depicted in  FIG. 1 , raw coal may be reduced to 50 mm top size. With reference to  FIG. 1 , raw coal is separated on a sieve bend  1  followed by a vibratory screen  2  with wash water addition  3 . This device removes fine particles, typically less than 2-0.2 mm, from the raw coal and all the undersize is processed in devices not mentioned here. The oversize material gravitates to sump  4  from which it is pumped  5  to the dense medium cyclone  6 . It will be noted on  FIG. 1  that dense medium is added to the coarse coal particles in the dense medium cyclone feed sump  4 . The coarse raw coal is separated in the dense medium cyclone  6  to produce a lower ash product and a higher ash reject. The product is separated from the dense medium on sieve bend  7  and drain  8  and rinse screen  9 . The sieve bend and drain screens remove the bulk of the dense medium which can then recycled to the dense medium sump  14 . The rinse screen  9  uses water addition  21 ,  22  (dirty and clarified) to aid the removal of medium adhering to the coal particles. Rinse screen underflow is significantly diluted and must be concentrated such that the water is removed before it can be reused in the operation of the dense medium cyclone. Similar sieve bend  10 , drain  11  and rinse  12  screen recovery of dense medium occurs for the dense medium cyclone underflow material.  
         [0075]     The diluted dense medium is dewatered with magnetic separators  16  and  17 . The recovered dense medium is passed to the over-dense sump  18  from where it is pumped  15  to the dense medium sump  14 . The separated water is recycled for use elsewhere in the plant, including water addition to the screening operations described above.  
         [0076]     Also shown on  FIG. 1  are the locations of measuring devices for medium density D, pressure P, Medium to Coal Ratio (MCR) and feed rate F.  
         [0077]     It should be noted once again that this is a very brief and simplified description of the generic circuitry for coal processing.  
         [0078]     The density of the dense medium supplied to the mixture with the particulate material is measured with a nucleonic or differential pressure transducer D. Two indicative locations for measuring this parameter are indicated on  FIG. 1 .  
         [0079]     The pressure of the medium density and particulate mixture supplied to the dense medium cyclone is also measured by pressure transducer P.  
         [0080]     The location of Medium to Coal Ratio measurement is also shown and could be measured by the emerging electro-impedance spectrometry technology which is not yet common place in the industry.  
         [0081]     In the preferred embodiment of the invention, the density measurements made by the nucleonic or differential pressure transducer D are used to generate an alarm condition, should the medium induced partition coefficient curve and/or parameters derived therefrom change from the desired values so that remedial action can be taken to restore the desired density control and thereby minimise losses caused by fluctuations or variations in the density of the medium density. However, as has been previously described, the pressure measurements, Medium to Coal Ratio measurements or feed rate measurements may be used in combination with the density measurements or instead of the density measurements in order to continually monitor the fluctuations in medium induced partition coefficient curve and/or parameters derived therefrom to enable the alarm condition to be generated and remedial action immediately taken to restore the required level of control of the dense medium separation.  
         [0082]     With reference to  FIG. 2 , the density measurements from the nucleonic or differential pressure transducer D are fed to a processor  50 , typically maintained in, but not limited to, the coal plant operation room when in the desired location, or any other suitable location. The pressure and feed rate measurements from the pressure transducer P and weightometers F are also fed to the processor  50 . Medium to Coal Ratio measurements from electro-impedance spectrometry technology would also be fed to the processor  50 .  
         [0083]     According to the preferred embodiment of the invention, measurements are read frequently, for example every 1 minute, and those measurements are taken over a predetermined time period of, for example 30 minutes to 2.5 hours, may be used to determine the value set for comparison with the predetermined value set in order to determine whether the alarm condition needs to be generated.  
         [0084]     Table 1 below shows exemplary measurements which may be taken over a time period of 9 hours and used for processing in the processor  50 .  
                                         TABLE 1                                   Time   Density                                         7:21:54   1571.48            7:22:29   1571.29            7:23:05   1568.14            7:23:41   1565.46            7:24:17   1560.24            7:24:53   1557.2            7:25:29   1557.36            7:26:05   1555.98            7:26:41   1552.94            7:27:17   1541.99            7:27:53   1535.55            7:28:29   1530.52            7:29:05   1524.52            7:29:41   1518.36            7:30:17   1508.26            7:30:53   1509.17            7:31:29   1524.88            7:32:05   1550.78            7:32:41   1563.68            7:33:17   1565.84            7:33:53   1563.41            7:34:29   1555.61            7:35:05   1552.5            7:35:41   1544.18            7:36:17   1539.94            7:36:53   1532.69            7:37:28   1526.97            7:38:04   1521.66            7:38:40   1519.88            7:39:16   1516.89            7:39:52   1501.46            7:40:28   1480.52            7:41:04   1471.89            7:41:40   1473.86            7:42:16   1490.65            7:42:52   1511.69            7:43:28   1524.97            7:44:04   1548.59            7:44:40   1580.46            7:45:16   1595.15            7:45:52   1611.78            7:46:28   1618.13            7:47:04   1622.66            7:47:40   1622.54            7:48:16   1618.63            7:48:52   1587.34            7:49:28   1577.82            7:50:04   1568.54            7:50:40   1562.07            7:51:16   1554.97            7:51:52   1549.87            7:52:27   1544.62            7:53:03   1537.75            7:53:39   1526.34            7:54:15   1522.88            7:54:51   1521.17            7:55:27   1522.5            7:56:03   1521.06            7:56:39   1523.56            7:57:15   1524.7            7:57:51   1526.32            7:58:27   1525.81            7:59:03   1524.35            7:59:39   1522.54            8:00:15   1518.14            8:00:51   1513.85            8:01:27   1514.7            8:02:03   1525.43            8:02:39   1533.79            8:03:15   1543.44            8:03:51   1549.9            8:04:27   1548.61            8:05:03   1547.15            8:05:39   1545.95            8:06:15   1543.43            8:06:51   1539.92            8:07:26   1536.66            8:08:02   1531.5            8:08:38   1525.81            8:09:14   1519.66            8:09:50   1513.08            8:10:26   1512.24            8:11:02   1515.62            8:11:38   1530.43            8:12:14   1546.59            8:12:50   1547.2            8:13:26   1546.7            8:14:02   1545.82            8:14:38   1543.18            8:15:14   1541.39            8:15:50   1536.15            8:16:26   1532.97            8:17:02   1530.05            8:17:38   1523.18            8:18:14   1520.75            8:18:50   1514.17            8:19:26   1523.2            8:20:02   1533.14            8:20:38   1532.79            8:21:14   1528.03            8:21:50   1521.08            8:22:25   1522.11            8:23:01   1520.89            8:23:37   1510.81            8:24:13   1498.6            8:24:49   1486.71            8:25:25   1464.58            8:26:01   1455.65            8:26:37   1446.62            8:27:13   1442.86            8:27:49   1463.41            8:28:25   1488.11            8:29:01   1508.38            8:29:37   1518.74            8:30:13   1529.76            8:30:49   1537.17            8:31:25   1536.6            8:32:01   1533.14            8:32:37   1525.17            8:33:13   1524.33            8:33:49   1522.95            8:34:25   1521.1            8:35:01   1519.82            8:35:37   1518.87            8:36:13   1517.45            8:36:49   1515.65            8:37:24   1515.39            8:38:00   1518.52            8:38:36   1528.5            8:39:12   1541.7            8:39:48   1540.91            8:40:24   1540.16            8:41:00   1537.56            8:41:36   1532.68            8:42:12   1523.01            8:42:48   1514.37            8:43:24   1512.51            8:44:00   1515.4            8:44:36   1528.01            8:45:12   1549.12            8:45:48   1566.6            8:46:24   1591.5            8:47:00   1582.88            8:47:36   1579.59            8:48:12   1572.02            8:48:48   1567            8:49:24   1566.1            8:50:00   1563.72            8:50:36   1559.59            8:51:12   1559.19            8:51:48   1553.49            8:52:23   1549.28            8:52:59   1543.38            8:53:35   1538.93            8:54:11   1531.98            8:54:47   1527.54            8:55:23   1520.06            8:55:59   1518.66            8:56:35   1512            8:57:11   1510.46            8:57:47   1516.8            8:58:23   1538.85            8:58:59   1556.67            8:59:35   1566.7            9:00:11   1560.83            9:00:47   1555.12            9:01:23   1553.18            9:01:59   1549.47            9:02:35   1549.32            9:03:11   1550.1            9:03:47   1551.14            9:04:23   1552.42            9:04:59   1550.17            9:05:35   1541.97            9:06:11   1539.53            9:06:47   1534.76            9:07:22   1532.91            9:07:58   1525.5            9:08:34   1520.57            9:09:10   1518.59            9:09:46   1512.5            9:10:22   1510.54            9:10:58   1509.42            9:11:34   1511.09            9:12:10   1528.41            9:12:46   1533.87            9:13:22   1566.18            9:13:58   1591.25            9:14:34   1573.89            9:15:10   1572.24            9:15:46   1570.41            9:16:22   1562.4            9:16:58   1561.26            9:17:34   1560.41            9:18:10   1559.66            9:18:46   1558.07            9:19:22   1548.05            9:19:58   1542.21            9:20:34   1538.82            9:21:10   1531.64            9:21:46   1524.34            9:22:21   1521.97            9:22:57   1515.61            9:23:33   1509.27            9:24:09   1508.49            9:24:45   1517.54            9:25:21   1535.31            9:25:57   1546.61            9:26:33   1554.74            9:27:09   1562.12            9:27:45   1564.06            9:28:21   1574.38            9:28:57   1574.84            9:29:33   1566.97            9:30:09   1566.28            9:30:45   1561.85            9:31:21   1558.69            9:31:57   1549.33            9:32:33   1546.23            9:33:09   1539.1            9:33:45   1533.81            9:34:21   1525.34            9:34:57   1516.18            9:35:33   1507.14            9:36:09   1505.81            9:36:45   1518.01            9:37:20   1531.86            9:37:56   1554.32            9:38:32   1563.99            9:39:08   1576.83            9:39:44   1590            9:40:20   1583.98            9:40:56   1583.16            9:41:32   1579.93            9:42:08   1577.61            9:42:44   1578.47            9:43:20   1578.01            9:43:56   1573.13            9:44:32   1567.29            9:45:08   1564.71            9:45:44   1560.32            9:46:20   1554.06            9:46:56   1545.22            9:47:32   1536.95            9:48:08   1531.57            9:48:44   1520.58            9:49:20   1514.83            9:49:56   1514.19            9:50:32   1526.09            9:51:08   1541.41            9:51:44   1544.95            9:52:19   1544.7            9:52:55   1543.15            9:53:31   1536.54            9:54:07   1532.97            9:54:43   1522.12            9:55:19   1501            9:55:55   1504.86            9:56:31   1515.49            9:57:07   1554.31            9:57:43   1594.72            9:58:19   1581.69            9:58:55   1578.96            9:59:31   1577.34           10:00:07   1571.28           10:00:43   1570.39           10:01:19   1569.2           10:01:55   1569.02           10:02:31   1568.81           10:03:07   1564.34           10:03:43   1557.1           10:04:19   1551.67           10:04:55   1547.28           10:05:31   1531.81           10:06:07   1530.39           10:06:43   1519.56           10:07:18   1514.21           10:07:54   1512.76           10:08:30   1519.42           10:09:06   1530.69           10:09:42   1544.09           10:10:18   1550.81           10:10:54   1550.33           10:11:30   1548.65           10:12:06   1542.8           10:12:42   1541.02           10:13:18   1537.74           10:13:54   1530.19           10:14:30   1528.48           10:15:06   1528.96           10:15:42   1529.01           10:16:18   1529.75           10:16:54   1530.13           10:17:30   1526.86           10:18:06   1521.66           10:18:42   1512.05           10:19:18   1510.26           10:19:54   1516.46           10:20:30   1529.82           10:21:06   1548.4           10:21:42   1561.94           10:22:17   1572.51           10:22:53   1569.01           10:23:29   1563.45           10:24:05   1562.52           10:24:41   1562.84           10:25:17   1564.35           10:25:53   1563.21           10:26:29   1561.2           10:27:05   1557.38           10:27:41   1554.12           10:28:17   1548.84           10:28:53   1545.58           10:29:29   1541.8           10:30:05   1539.85           10:30:41   1532.89           10:31:17   1526.82           10:31:53   1521.66           10:32:29   1519.89           10:33:05   1517.12           10:33:41   1508.57           10:34:17   1502.52           10:34:53   1510.72           10:35:29   1529.87           10:36:05   1554.8           10:36:41   1568.52           10:37:16   1570           10:37:52   1569.09           10:38:28   1567.52           10:39:04   1567.26           10:39:40   1576.85           10:40:16   1581.32           10:40:52   1578.59           10:41:28   1570.35           10:42:04   1568.94           10:42:40   1567.89           10:43:16   1563.15           10:43:52   1561.13           10:44:28   1557.47           10:45:04   1555.12           10:45:40   1548.41           10:46:16   1540.41           10:46:52   1536.24           10:47:28   1524.24           10:48:04   1514.32           10:48:40   1513.28           10:49:16   1513.98           10:49:52   1531.54           10:50:28   1555.78           10:51:04   1563.7           10:51:40   1581.18           10:52:15   1590.08           10:52:51   1575.13           10:53:27   1573.64           10:54:03   1571.91           10:54:39   1569.33           10:55:15   1565.4           10:55:51   1565.82           10:56:27   1564.85           10:57:03   1563.39           10:57:39   1552.9           10:58:15   1544.92           10:58:51   1539.92           10:59:27   1533.3           11:00:03   1527.51           11:00:39   1526.38           11:01:15   1521.48           11:01:51   1518.69           11:02:27   1508.63           11:03:03   1508.76           11:03:39   1510.07           11:04:15   1521.7           11:04:51   1534.43           11:05:27   1560.22           11:06:03   1570.76           11:06:39   1581.18           11:07:14   1575.61           11:07:50   1571.99           11:08:26   1570.68           11:09:02   1570.05           11:09:38   1567.74           11:10:14   1567.49           11:10:50   1566.11           11:11:26   1564.54           11:12:02   1561.24           11:12:38   1556.06           11:13:14   1549.86           11:13:50   1548.67           11:14:26   1533.39           11:15:02   1532.13           11:15:38   1527.21           11:16:14   1520.99           11:16:50   1514.18           11:17:26   1510           11:18:02   1510.96           11:18:38   1526.43           11:19:14   1548.92           11:19:50   1559.01           11:20:26   1559.8           11:21:02   1559.88           11:21:38   1557.63           11:22:13   1546.76           11:22:49   1522.9           11:23:25   1513.58           11:24:01   1501.81           11:24:37   1491.13           11:25:13   1511.48           11:25:49   1525.25           11:26:25   1547.59           11:27:01   1587.49           11:27:37   1615.3           11:28:13   1622.86           11:28:49   1623.28           11:29:25   1629.42           11:30:01   1627.97           11:30:37   1627.81           11:31:13   1610.47           11:31:49   1588.57           11:32:25   1580.53           11:33:01   1569.3           11:33:37   1561.99           11:34:13   1556.57           11:34:49   1546.36           11:35:25   1539.22           11:36:01   1532.02           11:36:37   1517.79           11:37:12   1504.21           11:37:48   1502.88           11:38:24   1508.15           11:39:00   1534.92           11:39:36   1542.27           11:40:12   1560.12           11:40:48   1561.58           11:41:24   1569.31           11:42:00   1602.57           11:42:36   1630.03           11:43:12   1623.15           11:43:48   1614.47           11:44:24   1611.08           11:45:00   1610.18           11:45:36   1608.51           11:46:12   1607.48           11:46:48   1598.75           11:47:24   1591.39           11:48:00   1585.69           11:48:36   1580.62           11:49:12   1576.74           11:49:48   1571.49           11:50:24   1565.49           11:51:00   1557.92           11:51:36   1549.07           11:52:11   1542.65           11:52:47   1540.23           11:53:23   1531.1           11:53:59   1529.78           11:54:35   1520.32           11:55:11   1517.97           11:55:47   1513.61           11:56:23   1513.7           11:56:59   1515.11           11:57:35   1533.13           11:58:11   1550.87           11:58:47   1564.56           11:59:23   1587.36           11:59:59   1588.18           12:00:35   1581.23           12:01:11   1580.27           12:01:47   1578.79           12:02:23   1573.9           12:02:59   1567.59           12:03:35   1567.47           12:04:11   1567.51           12:04:47   1565.16           12:05:23   1554.35           12:05:59   1551.26           12:06:35   1544.48           12:07:10   1540.49           12:07:46   1528.76           12:08:22   1523.15           12:08:58   1520.7           12:09:34   1517.39           12:10:10   1510.07           12:10:46   1516.29           12:11:22   1531.6           12:11:58   1548.3           12:12:34   1552.85           12:13:10   1554.14           12:13:46   1554.02           12:14:22   1550.23           12:14:58   1542.21           12:15:34   1540.48           12:16:10   1533.69           12:16:46   1528.04           12:17:22   1507.88           12:17:58   1533.74           12:18:34   1544.35           12:19:10   1545.04           12:19:46   1542.53           12:20:22   1538.79           12:20:58   1539.43           12:21:34   1537.63           12:22:09   1533.7           12:22:45   1526.92           12:23:21   1522.59           12:23:57   1519.81           12:24:33   1516.35           12:25:09   1509.23           12:25:45   1508.19           12:26:21   1520.57           12:26:57   1552.97           12:27:33   1568.78           12:28:09   1582.35           12:28:45   1574.04           12:29:21   1574.23           12:29:57   1571.59           12:30:33   1570.09           12:31:09   1553.8           12:31:45   1548.23           12:32:21   1548.2           12:32:57   1548.62           12:33:33   1547.59           12:34:09   1544.93           12:34:45   1538.97           12:35:21   1536.45           12:35:57   1530.41           12:36:33   1528.81           12:37:08   1525.79           12:37:44   1524.42           12:38:20   1512.65           12:38:56   1513.54           12:39:32   1525.07           12:40:08   1541.86           12:40:44   1563.75           12:41:20   1569.69           12:41:56   1569.45           12:42:32   1568.11           12:43:08   1561.01           12:43:44   1555.42           12:44:20   1551.74           12:44:56   1544.76           12:45:32   1540.13           12:46:08   1538.53           12:46:44   1529.59           12:47:20   1523.21           12:47:56   1519.08           12:48:32   1514.1           12:49:08   1513.1           12:49:44   1502.05           12:50:20   1526.46           12:50:56   1586.25           12:51:32   1620.56           12:52:07   1614           12:52:43   1601.39           12:53:19   1601.76           12:53:55   1603.86           12:54:31   1602.71           12:55:07   1601.32           12:55:43   1593.09           12:56:19   1585.93           12:56:55   1579.51           12:57:31   1574.21           12:58:07   1566.15           12:58:43   1556.04           12:59:19   1554.77           12:59:55   1553.03           13:00:31   1545.92           13:01:07   1539.03           13:01:43   1532.93           13:02:19   1531.59           13:02:55   1529.45           13:03:31   1522.97           13:04:07   1517.31           13:04:43   1514.11           13:05:19   1514.84           13:05:55   1520.18           13:06:31   1527.69           13:07:06   1538.51           13:07:42   1551.43           13:08:18   1568.34           13:08:54   1576.6           13:09:30   1567.74           13:10:06   1565.52           13:10:42   1563.96           13:11:18   1554.28           13:11:54   1553.32           13:12:30   1552.24           13:13:06   1545.65           13:13:42   1538.04           13:14:18   1531.52           13:14:54   1526.32           13:15:30   1516.27           13:16:06   1513.4           13:16:42   1514.22           13:17:18   1524.64           13:17:54   1541.47           13:18:30   1558.07           13:19:06   1560.21           13:19:42   1559.52           13:20:18   1558.59           13:20:54   1557.39           13:21:30   1556.18           13:22:05   1555.23           13:22:41   1551.83           13:23:17   1540.64           13:23:53   1540.09           13:24:29   1538.82           13:25:05   1533.68           13:25:41   1526.91           13:26:17   1521.88           13:26:53   1513.14           13:27:29   1508.49           13:28:05   1514.39           13:28:41   1523.07           13:29:17   1546.83           13:29:53   1556.79           13:30:29   1567.5           13:31:05   1570.72           13:31:41   1559.43           13:32:17   1558.85           13:32:53   1558.8           13:33:29   1557.27           13:34:05   1555.6           13:34:41   1553.93           13:35:17   1551.62           13:35:53   1541.33           13:36:29   1539.14           13:37:04   1531.42           13:37:40   1527.56           13:38:16   1523.44           13:38:52   1514.91           13:39:28   1512.32           13:40:04   1513.59           13:40:40   1528.29           13:41:16   1547.55           13:41:52   1554.59           13:42:28   1556.7           13:43:04   1555.7           13:43:40   1555.02           13:44:16   1553.05           13:44:52   1544.86           13:45:28   1535.24           13:46:04   1534.7           13:46:40   1527.93           13:47:16   1526.32           13:47:52   1526.17           13:48:28   1521.69           13:49:04   1512.85           13:49:40   1511.38           13:50:16   1515.48           13:50:52   1541.15           13:51:28   1559.98           13:52:03   1564.4           13:52:39   1565.1           13:53:15   1564.1           13:53:51   1549.58           13:54:27   1538.78           13:55:03   1542.46           13:55:39   1530.63           13:56:15   1528.54           13:56:51   1529.15           13:57:27   1526.71           13:58:03   1517.29           13:58:39   1515.54           13:59:15   1513.46           13:59:51   1520.17           14:00:27   1538.61           14:01:03   1554.4           14:01:39   1554.12           14:02:15   1554.73           14:02:51   1555.26           14:03:27   1549.32           14:04:03   1542.55           14:04:39   1540.98           14:05:15   1539.91           14:05:51   1539.78           14:06:27   1538.13           14:07:02   1529.42           14:07:38   1524.8           14:08:14   1515.33           14:08:50   1514.53           14:09:26   1518.01           14:10:02   1535.99           14:10:38   1550.72           14:11:14   1550.79           14:11:50   1545.1           14:12:26   1535.62           14:13:02   1529.48           14:13:38   1525.68           14:14:14   1514.88           14:14:50   1513.7           14:15:26   1515.88           14:16:02   1528.14           14:16:38   1561.81           14:17:14   1568.32           14:17:50   1557.94           14:18:26   1558.18           14:19:02   1555.92           14:19:38   1556.49           14:20:14   1556.02           14:20:50   1555.68           14:21:26   1550.04           14:22:01   1543.23           14:22:37   1537.92           14:23:13   1528.89           14:23:49   1525.98           14:24:25   1519.11           14:25:01   1515.97           14:25:37   1512.44           14:26:13   1511.67           14:26:49   1516.37           14:27:25   1531.43           14:28:01   1547.17           14:28:37   1562.37           14:29:13   1569.31           14:29:49   1573.25           14:30:25   1572.26           14:31:01   1570.36           14:31:37   1564.07           14:32:13   1557.66           14:32:49   1557.39           14:33:25   1557.44           14:34:01   1557.17           14:34:37   1556.64           14:35:13   1555.3           14:35:49   1551.1           14:36:25   1543.87           14:37:00   1529.51           14:37:36   1526.11           14:38:12   1521.3           14:38:48   1514.25           14:39:24   1512.46           14:40:00   1509.48           14:40:36   1512.16           14:41:12   1521.87           14:41:48   1557           14:42:24   1605.18           14:43:00   1613.52           14:43:36   1601.23           14:44:12   1597.73           14:44:48   1594.25           14:45:24   1593.59           14:46:00   1585.3           14:46:36   1582.45           14:47:12   1581.75           14:47:48   1574.28           14:48:24   1569.78           14:49:00   1560.16           14:49:36   1552.86           14:50:12   1541.55           14:50:48   1538.76           14:51:24   1530.33           14:51:59   1523.89           14:52:35   1520.8           14:53:11   1515.33           14:53:47   1509.78           14:54:23   1508.79           14:54:59   1516.99           14:55:35   1539.54           14:56:11   1561.1           14:56:47   1570.26           14:57:23   1579.62           14:57:59   1586.85           14:58:35   1587.4           14:59:11   1586           14:59:47   1584.18           15:00:23   1564.69           15:00:59   1542.28           15:01:35   1533.94           15:02:11   1522.08           15:02:47   1520.29           15:03:23   1516.89           15:03:59   1511.1           15:04:35   1504.9           15:05:11   1499.99           15:05:47   1517.2           15:06:23   1521.46           15:06:58   1529.45           15:07:34   1545.4           15:08:10   1576.52           15:08:46   1610.76           15:09:22   1619.6           15:09:58   1635.18           15:10:34   1642.76           15:11:10   1641.49           15:11:46   1640.13           15:12:22   1632.55           15:12:58   1631.12           15:13:34   1629.79           15:14:10   1626.76           15:14:46   1620.1           15:15:22   1612.22           15:15:58   1603.53           15:16:34   1596.14           15:17:10   1586.7           15:17:46   1577.42           15:18:22   1568.21           15:18:58   1563.21           15:19:34   1561.99           15:20:10   1550.79           15:20:46   1543.95           15:21:22   1537.67           15:21:57   1530.23           15:22:33   1521.37           15:23:09   1513.18           15:23:45   1512.23           15:24:21   1519.37           15:24:57   1530.3           15:25:33   1558.55           15:26:09   1569.79           15:26:45   1571.16           15:27:21   1576.17           15:27:57   1575.97           15:28:33   1569.29           15:29:09   1565.26           15:29:45   1557.01           15:30:21   1550.25           15:30:57   1547.64           15:31:33   1546.99           15:32:09   1540.65           15:32:45   1532.65           15:33:21   1526.54           15:33:57   1519.66           15:34:33   1513.74           15:35:09   1516.67           15:35:45   1520.25           15:36:21   1533.79           15:36:56   1548.99           15:37:32   1548.27           15:38:08   1541.54           15:38:44   1536.82           15:39:20   1529.14           15:39:56   1518.88           15:40:32   1512.68           15:41:08   1508.48           15:41:44   1514.94           15:42:20   1551.58           15:42:56   1597.5           15:43:32   1580.9           15:44:08   1577.17           15:44:44   1576.19           15:45:20   1575.9           15:45:56   1574.46           15:46:32   1572.2           15:47:08   1571.52           15:47:44   1570.77           15:48:20   1560.67           15:48:56   1554.55           15:49:32   1549.06           15:50:08   1543.45           15:50:44   1537.69           15:51:20   1531.33           15:51:55   1523.09           15:52:31   1511.24           15:53:07   1513.81           15:53:43   1521.84           15:54:19   1539.68           15:54:55   1557.55           15:55:31   1558.06           15:56:07   1557.15           15:56:43   1555.45           15:57:19   1553.53           15:57:55   1544.92           15:58:31   1531.07           15:59:07   1529.55           15:59:43   1525.89           16:00:19   1517.64           16:00:55   1514.72           16:01:31   1514.73           16:02:07   1515.93           16:02:43   1546.66           16:03:19   1562.99           16:03:55   1554.84           16:04:31   1554.78           16:05:07   1554.41           16:05:43   1554           16:06:19   1551.15           16:06:54   1550.61           16:07:30   1550.99           16:08:06   1549.3           16:08:42   1544.41           16:09:18   1539.01           16:09:54   1531.55           16:10:30   1525.98           16:11:06   1521.31           16:11:42   1513.79           16:12:18   1509.34           16:12:54   1523.44           16:13:30   1539.94           16:14:06   1556.73           16:14:42   1557.62           16:15:18   1554.25           16:15:54   1547.7           16:16:30   1543.48           16:17:06   1530.16           16:17:42   1523.43           16:18:18   1521.88           16:18:54   1520.07           16:19:30   1511.82           16:20:06   1511.38           16:20:42   1516.9           16:21:18   1547.85           16:21:53   1594.85                      
 
         [0085]     In table 2 set out below, the normalised frequency distribution of the densities given in Table 1 are set out.  
         [0086]     The normalised frequency is obtained by multiplying the frequency value by 100 and dividing by the sum of the normalised frequency column. The cumulative normalised frequency is the addition of the particular normalised frequency by the sum of the previous normalised frequencies.  
                                                                                   TABLE 2                           Frequency                       Distribution       Density Range               Cumulative            Lower   Upper   Mean       Normalised   Normalised       kg/m3   kg/m3   Density   Frequency   Frequency   frequency                        1442       0   0.000   0.000       1442   1443   1442.5   1   0.111   0.111       1443   1444   1443.5   0   0.000   0.111       1444   1445   1444.5   0   0.000   0.111       1445   1446   1445.5   0   0.000   0.111       1446   1447   1446.5   1   0.111   0.222       1447   1448   1447.5   0   0.000   0.222       1448   1449   1448.5   0   0.000   0.222       1449   1450   1449.5   0   0.000   0.222       1450   1451   1450.5   0   0.000   0.222       1451   1452   1451.5   0   0.000   0.222       1452   1453   1452.5   0   0.000   0.222       1453   1454   1453.5   0   0.000   0.222       1454   1455   1454.5   0   0.000   0.222       1455   1456   1455.5   1   0.111   0.333       1456   1457   1456.5   0   0.000   0.333       1457   1458   1457.5   0   0.000   0.333       1458   1459   1458.5   0   0.000   0.333       1459   1460   1459.5   0   0.000   0.333       1460   1461   1460.5   0   0.000   0.333       1461   1462   1461.5   0   0.000   0.333       1462   1463   1462.5   0   0.000   0.333       1463   1464   1463.5   1   0.111   0.443       1464   1465   1464.5   1   0.111   0.554       1465   1466   1465.5   0   0.000   0.554       1466   1467   1466.5   0   0.000   0.554       1467   1468   1467.5   0   0.000   0.554       1468   1469   1468.5   0   0.000   0.554       1469   1470   1469.5   0   0.000   0.554       1470   1471   1470.5   0   0.000   0.554       1471   1472   1471.5   1   0.111   0.665       1472   1473   1472.5   0   0.000   0.665       1473   1474   1473.5   1   0.111   0.776       1474   1475   1474.5   0   0.000   0.776       1475   1476   1475.5   0   0.000   0.776       1476   1477   1476.5   0   0.000   0.776       1477   1478   1477.5   0   0.000   0.776       1478   1479   1478.5   0   0.000   0.776       1479   1480   1479.5   0   0.000   0.776       1480   1481   1480.5   1   0.111   0.887       1481   1482   1481.5   0   0.000   0.887       1482   1483   1482.5   0   0.000   0.887       1483   1484   1483.5   0   0.000   0.887       1484   1485   1484.5   0   0.000   0.887       1485   1486   1485.5   0   0.000   0.887       1486   1487   1486.5   1   0.111   0.998       1487   1488   1487.5   0   0.000   0.998       1488   1489   1488.5   1   0.111   1.109       1489   1490   1489.5   0   0.000   1.109       1490   1491   1490.5   1   0.111   1.220       1491   1492   1491.5   1   0.111   1.330       1492   1493   1492.5   0   0.000   1.330       1493   1494   1493.5   0   0.000   1.330       1494   1495   1494.5   0   0.000   1.330       1495   1496   1495.5   0   0.000   1.330       1496   1497   1496.5   0   0.000   1.330       1497   1498   1497.5   0   0.000   1.330       1498   1499   1498.5   1   0.111   1.441       1499   1500   1499.5   1   0.111   1.552       1500   1501   1500.5   0   0.000   1.552       1501   1502   1501.5   3   0.333   1.885       1502   1503   1502.5   3   0.333   2.217       1503   1504   1503.5   0   0.000   2.217       1504   1505   1504.5   3   0.333   2.550       1505   1506   1505.5   1   0.111   2.661       1506   1507   1506.5   0   0.000   2.661       1507   1508   1507.5   2   0.222   2.882       1508   1509   1508.5   11   1.220   4.102       1509   1510   1509.5   7   0.776   4.878       1510   1511   1510.5   9   0.998   5.876       1511   1512   1511.5   9   0.998   6.874       1512   1513   1512.5   14   1.552   8.426       1513   1514   1513.5   18   1.996   10.421       1514   1515   1514.5   20   2.217   12.639       1515   1516   1515.5   14   1.552   14.191       1516   1517   1516.5   12   1.330   15.521       1517   1518   1517.5   10   1.109   16.630       1518   1519   1518.5   11   1.220   17.849       1519   1520   1519.5   11   1.220   19.069       1520   1521   1520.5   15   1.663   20.732       1521   1522   1521.5   19   2.106   22.838       1522   1523   1522.5   10   1.109   23.947       1523   1524   1523.5   12   1.330   25.277       1524   1525   1524.5   11   1.220   26.497       1525   1526   1525.5   13   1.441   27.938       1526   1527   1526.5   17   1.885   29.823       1527   1528   1527.5   6   0.665   30.488       1528   1529   1528.5   13   1.441   31.929       1529   1530   1529.5   15   1.663   33.592       1530   1531   1530.5   13   1.441   35.033       1531   1532   1531.5   16   1.774   36.807       1532   1533   1532.5   11   1.220   38.027       1533   1534   1533.5   14   1.552   39.579       1534   1535   1534.5   4   0.443   40.022       1535   1536   1535.5   5   0.554   40.576       1536   1537   1536.5   8   0.887   41.463       1537   1538   1537.5   8   0.887   42.350       1538   1539   1538.5   13   1.441   43.792       1539   1540   1539.5   16   1.774   45.565       1540   1541   1540.5   11   1.220   46.785       1541   1542   1541.5   13   1.441   48.226       1542   1543   1542.5   9   0.998   49.224       1543   1544   1543.5   10   1.109   50.333       1544   1545   1544.5   13   1.441   51.774       1545   1546   1545.5   9   0.998   52.772       1546   1547   1546.5   9   0.998   53.769       1547   1548   1547.5   10   1.109   54.878       1548   1549   1548.5   15   1.663   56.541       1549   1550   1549.5   13   1.441   57.982       1550   1551   1550.5   14   1.552   59.534       1551   1552   1551.5   10   1.109   60.643       1552   1553   1552.5   8   0.887   61.530       1553   1554   1553.5   8   0.887   62.417       1554   1555   1554.5   22   2.439   64.856       1555   1556   1555.5   15   1.663   66.519       1556   1557   1556.5   11   1.220   67.738       1557   1558   1557.5   19   2.106   69.845       1558   1559   1558.5   9   0.998   70.843       1559   1560   1559.5   9   0.998   71.840       1560   1561   1560.5   9   0.998   72.838       1561   1562   1561.5   12   1.330   74.169       1562   1563   1562.5   7   0.776   74.945       1563   1564   1563.5   12   1.330   76.275       1564   1565   1564.5   11   1.220   77.494       1565   1566   1565.5   9   0.998   78.492       1566   1567   1566.5   8   0.887   79.379       1567   1568   1567.5   12   1.330   80.710       1568   1569   1568.5   10   1.109   81.818       1569   1570   1569.5   13   1.441   83.259       1570   1571   1570.5   12   1.330   84.590       1571   1572   1571.5   9   0.998   85.588       1572   1573   1572.5   5   0.554   86.142       1573   1574   1573.5   5   0.554   86.696       1574   1575   1574.5   7   0.776   87.472       1575   1576   1575.5   4   0.443   87.916       1576   1577   1576.5   7   0.776   88.692       1577   1578   1577.5   5   0.554   89.246       1578   1579   1578.5   5   0.554   89.800       1579   1580   1579.5   4   0.443   90.244       1580   1581   1580.5   5   0.554   90.798       1581   1582   1581.5   6   0.665   91.463       1582   1583   1582.5   3   0.333   91.796       1583   1584   1583.5   2   0.222   92.018       1584   1585   1584.5   1   0.111   92.129       1585   1586   1585.5   3   0.333   92.461       1586   1587   1586.5   4   0.443   92.905       1587   1588   1587.5   4   0.443   93.348       1588   1589   1588.5   2   0.222   93.570       1589   1590   1589.5   0   0.000   93.570       1590   1591   1590.5   2   0.222   93.792       1591   1592   1591.5   3   0.333   94.124       1592   1593   1592.5   0   0.000   94.124       1593   1594   1593.5   2   0.222   94.346       1594   1595   1594.5   3   0.333   94.678       1595   1596   1595.5   1   0.111   94.789       1596   1597   1596.5   1   0.111   94.900       1597   1598   1597.5   2   0.222   95.122       1598   1599   1598.5   1   0.111   95.233       1599   1600   1599.5   0   0.000   95.233       1600   1601   1600.5   0   0.000   95.233       1601   1602   1601.5   4   0.443   95.676       1602   1603   1602.5   2   0.222   95.898       1603   1604   1603.5   2   0.222   96.120       1604   1605   1604.5   0   0.000   96.120       1605   1606   1605.5   1   0.111   96.231       1606   1607   1606.5   0   0.000   96.231       1607   1608   1607.5   1   0.111   96.341       1608   1609   1608.5   1   0.111   96.452       1609   1610   1609.5   0   0.000   96.452       1610   1611   1610.5   3   0.333   96.785       1611   1612   1611.5   2   0.222   97.007       1612   1613   1612.5   1   0.111   97.118       1613   1614   1613.5   1   0.111   97.228       1614   1615   1614.5   2   0.222   97.450       1615   1616   1615.5   1   0.111   97.561       1616   1617   1616.5   0   0.000   97.561       1617   1618   1617.5   0   0.000   97.561       1618   1619   1618.5   2   0.222   97.783       1619   1620   1619.5   1   0.111   97.894       1620   1621   1620.5   2   0.222   98.115       1621   1622   1621.5   0   0.000   98.115       1622   1623   1622.5   3   0.333   98.448       1623   1624   1623.5   2   0.222   98.670       1624   1625   1624.5   0   0.000   98.670       1625   1626   1625.5   0   0.000   98.670       1626   1627   1626.5   1   0.111   98.780       1627   1628   1627.5   2   0.222   99.002       1628   1629   1628.5   0   0.000   99.002       1629   1630   1629.5   2   0.222   99.224       1630   1631   1630.5   1   0.111   99.335       1631   1632   1631.5   1   0.111   99.446       1632   1633   1632.5   1   0.111   99.557       1633   1634   1633.5   0   0.000   99.557       1634   1635   1634.5   0   0.000   99.557       1635   1636   1635.5   1   0.111   99.667       1636   1637   1636.5   0   0.000   99.667       1637   1638   1637.5   0   0.000   99.667       1638   1639   1638.5   0   0.000   99.667       1639   1640   1639.5   0   0.000   99.667       1640   1641   1640.5   1   0.111   99.778       1641   1642   1641.5   1   0.111   99.889       1642   1643   1642.5   1   0.111   100.000       1643   1644   1643.5   0   0.000   100.000       1644   1645   1644.5   0   0.000   100.000       1645                                   Total = 902   Total =                       100.000                  
 
         [0087]     The processor  50  then lines up the measured density values from lowest to highest so that the frequency of each measured value can be determined.  
         [0088]     A chart is then prepared whereby the mid point of each density range is plotted against the density to give the partition coefficient curve.  
         [0089]     The processor  50  then determines an induced value, which in the preferred embodiment uses the density measurements, is a medium induced Ep value from the cumulative frequency distribution of the length of time spent at each density by taking the absolute value of the difference in density at the 75 th  and 25 th  percentiles and dividing by 2000 as shown by the following equation: 
 
 Ep =absolute value (Density at 75 th  percentile−Density at 25 th  percentile)/2000   Equation 
 
         [0090]     By way of further explanation, the inefficiency of the processing is generally given by the Ep value.  FIG. 3  is a graph in an ideal situation where perfect separation results in correct placement of all material in the feed that should report to product reporting to product and all material in feed that should report to reject reporting to reject. If the above equation is applied to the data in  FIG. 3 , it will be seen that the Ep value is 0, which gives a theoretically perfect result. However, in real operating conditions, the graph of  FIG. 3  is more likely to look like that shown in  FIG. 4  Using the data. supplied in Table 2 and  FIG. 4 , the Ep value is (1562.5−1523.5)/2000, which equals 0.0195. The processor  50  is programmed to generate an alarm, should the calculated Ep value become, for example, 0.025. Thus, the graph shown in  FIG. 4  is indicative of a acceptable MIEp value in this context indicating that remedial action does not need to be taken. If the value was above 0.025, an alarm condition would be generated. As shown in  FIG. 2 , the processor may output a signal to an alarm  52  to generate the alarm, which could be an audible alarm or simply a visual indication on a monitor or a combination of both to alert operators in the control room that fluctuations have exceeded a desired value and that remedial action should be taken to correct the situation to restore the proper medium density, and thereby restore maximum yield operation to the processing plant.  
         [0091]     The remedial action which may be taken may be to dispatch workmen to inspect valves in the system to ensure that they are operating properly and have not jammed or closed, pipelines to ensure that there are no leakages, and other operating parameters of the equipment. Action can be taken by workmen to correct any fault which may be observed immediately, rather than awaiting routine inspections or the like which may result in a fault continuing for a continued period of time, and thereby resulting in significant loss in the yield from the plant until the remedial action is identified and taken.  
         [0092]     The remedial action may also take the form of an automated response, for example the remedial action may be to invoke a control system retune algorithm to optimise PID control system values.  
         [0093]     MIEp values are periodically determined after an initial period of 9 hours by simply dropping off the first measurement made and adding to the total of measurements the next successive measurement made. For example, in Table 1, the next MIEp value may be calculated by dropping off the density reading for the time 7:21:54 and adding to the list of density values measured that for time period 16:21:53. This would provide a new MIEP value for comparison with the predetermined value every 36 seconds. Obviously, if a greater period is desired, then additional earlier readings can be ignored and more subsequent measurements made before a further MIEp value is calculated. Also, if measurements of MIEp over a shorter period are desired, density data would be collected for the shorter period and used in a manner similar to that presented above.  
         [0094]     An additional example is given with the same data as shown in Table 1 for the situation where measurements of MIEp over a shorter period are required. For a rolling period of 90 minutes a rolling MIEp can be calculated. It is then possible to plot rolling MIEp as ordinate and time as abscissa.  
         [0095]     In accordance with the preferred embodiment of the invention, the processing plant can be monitored to determine when its separating performance drops below required levels, thereby enabling remedial action to be immediately taken, and this could be worth millions of dollars per annum to the operation. The monitoring can take the form of a run chart of MIEp in which upper and lower control limits can be derived. Derivation above the upper control limit can be used as the signal for corrective action in the processor  50 . Also, the run charts of MIEp can be used as a benchmarking tool to compare control systems within a given plant, and also between plants.  
         [0096]     In the second embodiment of the invention in which the pressure measurements are taken so as to produce a pressure induced Ep value, a similar algorithm to that described above is used with the inclusion of a theoretically and/or empirically determined relationship between pressure and separating density. Alternatively, the pseudo PIEp concept can be used. The pressure values are measured at the time intervals similar to that in  FIG. 1 . The separating density is a function of the pressure and therefore the pressure values can be converted to separating density values via an appropriate empirical or theoretical calibration which are accumulated in the same manner as described with reference to Table 2 so as to enable the Ep value to be calculated.  
         [0097]     Similarly, in the embodiment which uses feed rate, the feed rate of material is measured as, for example, weight in tonnes per hour, and these values are again converted to separation density values so that an accumulation of separation densities can be used to enable the feed rate induced Ep value to be determined. Alternatively, the pseudo FRIEP concept can be used.  
         [0098]     Similarly, in the embodiment which uses Medium to Coal Ratio, the Medium to Coal Ratio is measured as, for example, cubic meters of medium per hours divided by weight in tonnes per hour of dense medium cyclone feed, and these values are again converted to separation density values so that an accumulation of separation densities can be used to enable the Medium to Coal Ratio induced Ep value to be determined. Alternatively, the pseudo MCRIEP concept can be used.  
         [0099]     For the example given above, the detailed calculations presented indicated that the medium induced Ep was 0.0195. Following similar lines, it is possible to calculate a pressure induced Ep=0.002. At the same time, the measured Ep for coal was 0.026. This is interpreted as about 70% of the Ep was due to medium density variation and about 7% was due to pressure variation.  
         [0100]     The additional interpretation of the invention is that the large proportion of the actual separating inefficiencies of the dense medium separator is due to process variation and can be measured with relative ease in most modern processing facilities. Also, if the MIEp=0.0195 then the Ep of the coal cannot be smaller than 0.0195, and so the invention also permits the lower limit of coal separating efficiency to be measured with relative ease on-line.  
         [0101]     Since modifications within the spirit and scope of the invention may readily be effected by persons skilled within the art, it is to be understood that this invention is not limited to the particular embodiment described by way of example hereinabove.  
         [0102]     In the claims which follow and in the preceding description of the invention, except where the context requires otherwise due to express language or necessary implication, the word “comprise”, or variations such as “comprises” or “comprising”, is used in an inclusive sense, ie. to specify the presence of the stated features but not to preclude the presence or addition of further features in various embodiments of the invention.