AU702355B2 - On-line assay and method - Google Patents

Description

The present invention relates to a method and apparatus for the continuous sampling and analysis of a liquid sample stream. The invention has particular application in monitoring the level of a chemical in a sample stream, where the level of the chemical being monitored may vary with time.

The method and apparatus of the present invention have been developed with a view to use for the determination of cyanide levels in gold plant solutions.

Indeed, the majority of the following description will relate to that use. However, it must be appreciated that the invention is not to be limited only to that use.

The traditional method for determining cyanide levels in gold plant solutions is a manual titration using a standard silver nitrate solution. On most minesites this titration is carried out by plant operators who have had little or no chemical training. As a result, the accuracy obtained is low and in many cases there is a S. marked change in the results at the end of a shift when a new operator takes over.

The next step up from this procedure is to use some form of automatic titrator to remove the human factor and this usually produces a marked improvement in reproducibility. However most automatic instruments presently available are expensive and complex.

Also, the automatic titrators currently in use still require manual operations to produce a suitable sample solution, interpret the results and make any necessary corrections to plant conditions. They also require a continuing supply of a standard silver nitrate solution. In addition, due to the fact that plant operators have other duties to perform, it is usually not practicable to carry out determinations at less than perhaps two hourly intervals.

It is an object of this invention to provide an apparatus which may be used as a portable unit, or which may be incorporated as a part of a plant, where the concentration of certain chemicals in a solution is required to be determined.

3 The present invention provides a sampling apparatus having a sample stream inlet and a sample stream outlet, a reservoir of fixed predetermined volume having an inlet at one end and an outlet at the other, the outlet being arranged to overflow to a drain connecting to the sample stream outlet, and a valve assembly located between said inlet and said sample stream inlet, said valve assembly having a sample drain outlet, said valve assembly being selectable between a normal position in which sample solution proceeds from the sample stream inlet to the reservoir, and a sampling position in which sample solution flow from the sample stream inlet is shut off and sample solution in the sample stream inlet is isolated from sample solution in the reservoir and sample solution in the reservoir drains under gravity via said sample drain outlet for assay.

Preferably the valve assembly comprises a three connector valve where any two connector passages thereof are in fluid communication at any time. It is most preferred that the three connector valve is a rotary type, the operation of which S 15 can readily be controlled using a stepper motor or other electric rotary motor.

Preferably the reservoir comprises a measuring column. In some applications it may be preferred that the fluid flow approximates a "plug flow" model, in which case the measuring column should be of relatively narrow cross-section relative to height so that mixing of sample in the sample stream will be minimised. This 20 will result in sample contained within the column being continuously representative of the fluid being sampled.

*o Most preferably the reservoir comprises an upwardly directed glass pipette. The pipette is most preferably of relatively narrow cross-section relative to height, in order to retain the aforementioned advantages. Use of a pipette also ensures that sample volumes are consistent.

It will also be understood that where it is desired to have a time averaged sample, the considerations relating to narrow reservoir cross-section do not apply.

The present invention also provides an assay apparatus comprising a sampling apparatus as described above and an assay chamber in which analysis of sample solution may take place.

Preferably the assay chamber comprises a coulometer or coulometric titration cell. Such a technique allows the assay apparatus to be calibrated for the intended assay at the point of manufacture, obviating the need for in-field calibration.

The apparatus preferably includes a coulometer or coulometric titration cell having a generator electrode in a main compartment of the coulometer or cell, and a second electrode completing a circuit with the generator electrode, in a second electrode compartment, where the second electrode compartment includes an aperture to allow overflow of excess electrolyte from the second o electrode compartment to the main compartment, as fresh electrolyte is introduced. Such an arrangement prevents accumulation of electrolysis products in the generator electrode compartment.

The present invention also provides an apparatus suitable for performing an assay to determine concentration of a chemical in a solution, the method comprising continuously drawing the solution through a reservoir and discharging overflow from the reservoir, so that the reservoir contains a predetermined volume of a sample solution representative of the solution at any 20 point in time, and cutting flow of the solution to the reservoir and discharging the *i sample solution from the reservoir to suitable assay apparatus.

The apparatus according to the invention is intended for on-line determination of any components in a solution which are capable of reacting with an electrolytically generated titrant. This includes ionic moieties in solution or suspensions of solids, although the latter may need to be solubilised depending upon the assay method being utilised.

Typical determinations which may be carried out using the invention include the following:-

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Acid-base titrations using electrolytically generated hydrogen or hydroxide ions. This procedure is suitable for the determination of both strong and weak bases and acids.

Oxidation-reduction titrations using electrolytically generated reagents including chlorine, bromine, iodine, ferric ions and ceric ions. Typical reactions include the determination of olefins with bromine, the determination of trivalent arsenic with iodine and the determination of hexavalent chromium with ferrous ions.

Complex-forming reactions using electrolytically generated materials such as EDTA and silver ions. Typical reactions include the determination of zinc by EDTA released electrolytically from its mercury amine complex and the determination of cyanide by complexing with silver ions.

The invention will now be described in the following description of one specific 15 embodiment thereof, made with reference to the drawings in which:- Figure 1 is a schematic view of an assay apparatus, showing fluid flow paths and the coulometric titration cell; Figure 2 is a part view of the fluid flow path of the assay apparatus of figure 1, showing a different selected state; Figure 3 is a block diagram of the assay apparatus according to the embodiment; and Figure 4 is a diagram showing the front panel layout for the user control interface of the assay apparatus according to the embodiment.

The construction and operation of the assay apparatus depend to some extent on the determination being carried out, but the embodiment described in the following text demonstrates the general principle which is applicable in all cases.

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The apparatus 11 includes a sample stream inlet 13 and a sample stream outlet The sample stream inlet 13 feeds to a reservoir 17 via a 3-way connector valve 19. The reservoir has a discharge outlet 21 which overflows to a drain 23 which connects to the outlet 15. The reservoir 17 comprises an upturned glass pipette of fixed predetermined volume. The 3-way connector valve 19 has a fluid flow path which is selectable to extend between the sample stream inlet connector passage 13 and the inlet connector passage 25 of the reservoir 17 (as shown in figure and to extend between the inlet connector passage 25 and a feed inlet connector passage 27 to the assay chamber 29 (as shown in Figure 2).

The assay chamber 29 in this embodiment is a coulometric titration cell 31. The o.

coulometric titration cell 31 comprises a sensing electrode 33, a reference ::electrode 35, and a generator electrode 37a contained in a main compartment 36, and another electrode 37b to complete the current path with the generator 15 electrode 37a contained in a second generator electrode compartment 38, separated by a glass wall 39. The glass wall 39 includes a sintered glass disc 41 which provides an electrical connection between the two cell halves while separating the generator electrode 37, which in the determination described in this embodiment is a cathode, from the reaction zone. In other determinations ~20 the generator electrode 37b may be an anode, depending upon the chemistry of the determination.

The glass wall 39 also includes an aperture 43 for overflow of electrolyte. The titration cell 31 includes an inlet 45 connected to a metering pump such as a peristaltic pump 47, for feeding a measured amount of electrolyte to the titration cell 31. The titration cell 31 also includes a drain 49 controlled by a solenoid valve 51. The three connector valve 19, which controls flow of sample fluid to and from the reservoir 17, is connected to a motor (not shown), to control actuation thereof. Thorough mixing within the titration cell 31 is ensured by use of a magnetic stirrer apparatus With the inclusion of the peristaltic pump 47, solenoid valve 51, and motor 53 controlling the 3-way connector valve 19, the apparatus is well suited to electronic control. A preferred type of electronic controller is shown in the block diagram of Figure 3, the heart of the electronic controller being a timer circuit 57 comprising monostables or 555 timers, or alternatively comprising a microprocessor having the necessary timing sequences programmed.

The preferred operation of the apparatus will now be described. A sample stream flows continuously from the sample stream inlet 13, through the 3-way connector valve 19 and up through the reservoir 17, where it is discharged via the discharge outlet 21 into the drain 23, where it drains through the output to waste. Due to the sample stream overflowing the discharge outlet 21 of the reservoir 17, a constant volume of sample stream is always held within the *reservoir 17. This sample is representative of solution being sampled at any point in time.

The reservoir 17 preferably comprises an upturned pipette having a total volume of 5ml, although the volume may be varied as required.

At the start of a timing cycle, the solenoid valve 51 opens to drain part of the previously titrated solution, or any excess solution in the titration cell 31, to bring the electrolyte level down to a fixed point. Typically, the solenoid valve 51 should be operated for a period of 18 seconds, to ensure that all excess fluid is "--"drained from the titration cell 31.

The peristaltic pump 47 may then be operated for a fixed time to deliver a predetermined fixed volume of a supporting electrolyte to the generator electrode compartment 38 of the titration cell 31. A typical time period for operation of the peristaltic pump may be in the order of 5 to 6 seconds, although it will be understood that the timing will be determined by the concentration and volume of the electrolyte being delivered to the titration cell 31. The supporting electrolyte provides electrical conductivity in the cell, and chemical conditions 8 under which the dissolution of the working (generator) electrodes 37 can proceed with greatest efficiency.

The timer circuit 57 then actuates the motor 53 controlling the three connector valve 19, by providing a 3 second pulse to trigger the motor 53. After a nominal 12 second delay, the reservoir 17 will have emptied, whereafter a further 3 second pulse is provided by the timer circuit 57 to reverse the motor 53 to return the three connector valve back to the position shown in Figure 1, whereupon the sample flow will resume through the sample stream inlet 13 to the outlet The constant current supply 59 is then connected to the generator electrodes 37 in the titration cell 31, to commence the titration. At the same time, clock pulses are fed to the decimal counter section of counters 60 and the counter output is S.fed to display 61. The period of the clock pulses is set, so that the display 61 provides a direct readout of concentration of the ionic moiety being titrated.

When the end point is detected by the sensing electrode, the cell current is cut off and the counter output is latched, to hold the display 61. The displayed value is held until it is cleared at the start of the next measurement cycle. The output of a binary counter in the counters 67 is also fed to a digital to analogue converter 63 and a 4 to 20 milliamp converter 65 to provide a current output for equipment in the plant control system, if this is required. The value of output 20 current from the 4 to 20 milliamp converter 65 is maintained latched at its current value until it is updated at the end of the next measurement cycle.

The titrant is preferably generated directly in the titration cell using the constant current source 59, and the time taken to complete the reaction is a direct measure of the concentration of the material being titrated. The end- point of the titration may be determined by any convenient method such as a colour change, electrode potential change or conductivity change.

A fresh sample of the analyte stream may be taken each five minutes and the results appear directly in concentration units on a digital display. At the same time the instrument preferably also converts the result into a 4-20mA output signal which may be used to control a plant process.

The operation of the apparatus and method of the invention may be illustrated by considering a typical operation such as the determination of free cyanide in a gold plant solution.

In this case the sensing electrode system which is used to detect the end-point of the titration preferably consists of a metallic silver electrode working in conjunction with a calomel reference electrode. This combination acts effectively to indicate the free cyanide concentration during the course of the titration.

The generator electrodes consist of a metallic silver anode in the titration cell and a carbon cathode located in a separate section of the cell which is isolated ol:. by the sintered glass disc 41. The purpose of the sintered disc 41 is to allow electrical connection between the two halves of the cell while at the same time 15 keeping the cathode away from the reaction zone.

*,oS S In the determination of free cyanide in this example, the preferably supporting -:ooo electrolyte contains 5 grams per litre of ammonium chloride and 30ml per litre of concentrated ammonia solution. This maintains the correct pH to enable the titration to be carried out satisfactorily and allows the silver generator electrode 20 to operate at 100% efficiency. It will be understood that this combination of ammonium salts and alkali may be changed in order to provide satisfactory performance with sample streams of various compositions.

When the peristaltic pump 47 feeds the next charge of supporting electrolyte to the cell, the level rises and the solution flows through the hole 43 above the sintered disc 41 to enter the main section of the cell 31. This ensures that there is no accumulation of electrolysis products in the right hand section of cell 31.

At the end of one titration cycle the solenoid valve operates to lower the liquid level to prepare for the next cycle. The cell is not completely emptied, but the previously titrated solution does not interfere with the next measurement. This procedure ensures that the silver generator electrode has a sufficiently large surface area exposed to the solution to operate correctly.

Referring to Figure 3, for titration of cyanide as described for the above application, the counter section 67 includes a binary counter which generates an output 304 seconds after the start of a measurement cycle, and the measurement system also generates an output at the end of its cycle. These two signals are combined in an AND gate to provide a cycle time with a minimum value of 304 seconds, but which may be longer in the case where very high cyanide concentrations are being measured.

The timer 57 is driven by the cycle as determined by the counter section 67 and I' .I ocontains a re-triggerable monostable input. The monostable output will then go low only after the measurement system output signal has been stable for at least 4.7 seconds after the end-point has been reached. This feature is provided to stop the monostable from re- setting until it is certain that the true end-point has been reached and that further stirring of the solution in the titration cell will not °9* reverse the output signal.

When the monostable re-sets, it generates the following outputs in sequence: A nominal 0.1 second pulse to latch binary counter readings into 20 the digital-to-analogue converter.

A nominal 0.1 second pulse to reset all counters ready for the start of the next cycle.

A nominal 18 second pulse to operate the dump valve to allow surplus liquid to drain from the cell.

A nominal 5.6 second pulse to drive the peristaltic pump to deliver an aliquot of support electrolyte to the titration cell.

11 A nominal 3 second pulse to trigger the rotor to drive forward 1200 to allow the sample pipette to drain to the titration cell.

A nominal 12 second delay to allow the pipette to drain completely.

A nominal 3 second pulse to trigger the rotor to reverse 1200 to return to the sampling mode.

The measurement system consists of four sections as listed below:- A metallic silver electrode used in conjunction with a saturated calomel electrode and located in the titration cell. This combination, which actually measures the silver concentration in S 10 the cell, acts effectively as a cyanide electrode system and produces a rising voltage as the end-point is approached.

The electrode signal feeds directly into a high input impedance CA3140 amplifier connected as a voltage follower. The output of this stage feeds an LM348 unity gain buffer amplifier.

The amplifier output and a reference voltage derived from an adjustable supply are then fed to an LM339 comparator. The system is adjusted so that the comparator output voltage changes when the end-point is reached.

The comparator output then goes to a 74HC14 Schmitt trigger which in turn drives a single transistor stage to operate the relay in the counter stage.

The counter stage is driven by a 4MHz crystal oscillator whose output is divided down to produce outputs at 2Hz and 8Hz.

The 2Hz signal is fed to an ICM 7224 4 digit counter via the counter relay in order to determine the time taken to complete the titration. The controlled current fed to the generator electrodes is maintained at a level which produces a counter readout which represents the cyanide concentration expressed in parts per million. At the end of the count the readout is held until it is cleared at the start of the next cycle. This allows display 69 to give a progressive readout during the course of the titration.

In an alternative form of the instrument, the output of the decade counter in counters 67 may be latched into the p.p.m. display 61 at the end of a measurement cycle, and held until it is updated at the end of the next cycle.

When this is done, display 61 always shows the result of the previous titration until it is updated at the end of the current cycle.

The 8Hz signal is fed to a binary counter whose output is latched into an AD694 digital-to-analogue converter 63 at the end of the titration. The digital output then goes to an AD7545 4-20 mA converter 65 to provide a process control output signal.

15 The 8Hz signal is also fed to a second binary counter system which is used to provide the basic cycle time of 304 seconds as mentioned above. When the required count is reached, the counter is stopped and its output together with an output from the counter relay goes to an AND gate to provide a start signal for the timer system. As a result, the timer starts only when the titration has finished 20 and the binary counter has reached its required count.

The rotor and stirrer control unit provides drive and reversing circuitry to control the 3-way valve motor 53. The motor 53 is stopped at the required positions by micro- switches operated by detents on the rotor assembly.

The stirrer is controlled by signals from the measuring unit and the timer so that it only operates while a titration is under way.

The power supply 71 is a conventional type which supplies regulated d.c. at +12, and -5 volt levels.

13 A separate floating supply 59 is used to provide the constant current required for the generator electrodes. The theoretical current required to make the counter display read directly in parts per million of sodium cyanide is 1.9683 v mA where v is the volume in mis delivered by the sampling system but due to manufacturing difficulties in the glassware it is not practical to attempt to reach the exact value. However the actual value is easily determined by weighing the amount of water delivered to the cell and this is done at the manufacturing stage.

The required current is then calculated as mentioned above and this value is marked permanently on the front of the assembly.

The front panel is preferably as shown in Figure 4 and the functions of the various items are as listed below:a I' i The digital panel meter 73 is used to indicate the electrode voltage ***during the course of the titration and for calibration purposes when the instrument is switched to the manual mode.

15 The 4 digit counter display 69 is used to indicate the last measured cyanide concentration when the instrument is in the automatic mode and for progressive readings during a calibration run.

The AUTO/MANUAL control 75 switches the instrument between normal operation and calibration modes.

The four position calibration switch 77 is only operative in the manual mode.

The first position "CAL" is used when carrying out a manual titration to determine the correct setting for the reference voltage so that the comparator output voltage charge occurs at the endpoint.

The second position "mV" is used to set the variable voltage determined in the manual titration. The adjustment is made using the screw driver operated turn "SET mV" potentiometer.

The third position "mA" is used to set the cell current to the calculated value as described above. The adjustment is made using the screw driver operated turn "Set mA" potentiometer.

The fourth position "LOAD" is used to set the value of a dummy load using the "SET LOAD" potentiometer. The dummy load receives the controlled current during periods when no titration is being carried out. This ensures that the internal temperatures in the current source are maintained at a constant value in order to provide maximum stability.

The CAL control 79 is a momentary action push button which is used to switch the controlled current source to the cell during the manual calibration titration.

The RESET button 81 is used to reset the counter display to zero prior to commencing the calibration titration.

Four momentary action push buttons are located near the assembly of the motor S: 15 and the three connector valve 19. These provide the following functions: The INITIALISE button is only used during the start-up of the instrument. It brings the rotor to the normal sampling position as shown in figure 1 and places the drive system in the forward mode prior to the start of automatic operation.

The SUPPORT button allows the support pump to be manually operated during the calibration procedure.

The DUMP button allows the dump valve to be manually operated.

The START button is used to start the instrument when it is switched to the automatic mode. This is necessary since the normal start pulse to the timer circuit is only generated at the end of a previous automatic titration sequence.

It should be appreciated that the scope of the invention is not limited to the scope of the embodiment described herewith. It will be understood that the electronic control circuitry, and the valve arrangements may be altered without departing from the spirit and scope of the invention.

S o 1P 16 The Claims defining the invention are as follows:- 1. A sampling apparatus having a sample stream inlet and a sample stream outlet, a reservoir of fixed predetermined volume having an inlet at one end and an outlet at the other, the outlet being arranged to overflow to a drain connecting to the sample stream outlet, and a valve assembly located between said inlet and said sample stream inlet, said valve assembly having a sample drain outlet, said valve assembly being selectable between a normal position in which sample solution proceeds from the sample stream inlet to the reservoir, and a sampling position in which sample solution flow from the sample stream inlet is shut off and sample solution in the sample stream inlet is isolated from sample solution in the reservoir and sample solution in the reservoir drains under gravity via said sample drain outlet for o• assay.

9 o• ego• 2. A sampling apparatus according to claim 1 wherein the valve assembly 15 comprises a three connector valve where any two connector passages thereof are in fluid communication at any time.

999* 9* 3. A sampling apparatus according to claim 1 or claim 2 wherein the reservoir comprises a measuring column.

9 4. A sampling apparatus according to claim 3 wherein the reservoir comprises 20 an upwardly directed glass pipette.

An assay apparatus comprising a sampling apparatus according to any one of claims 1 to 4 and an assay -chamber in which analysis of sample solution may take place.

6. An assay apparatus according to claim 5 wherein the assay chamber comprises a coulometer or coulometric titration cell.

7. An assay apparatus according to claim 6 wherein the coulometer or '4 /AN coulometric titration cell has a generator electrode a main compartment of

Claims (2)

1. 8. A sample apparatus according to claim 1 substantially as herein described in relation to the accompanying drawings.
2. 9. An assay apparatus according to claim 5 substantially as herein described in relation to the accompanying drawings. DATED THIS twenty-fourth day of DECEMBER 1998. o* ALFRED BERTRAM HOLLEBON 20 Applicant to to t Wray Associates, Perth, Western Australia, Patent Attorneys for Applicant. a 18 ABSTRACT An assay apparatus for performing continuous sampling of a fluid, comprising a sample stream inlet 13 and a sample stream outlet 15. The sample stream inlet 13 feeds to a reservoir 17 via a 3-way connector valve 19. The reservoir has a discharge outlet 21 which overflows to a drain 23 which connects to the outlet The reservoir 17 comprises an upturned glass pipette of fixed predetermined volume. The 3-way connector valve 19 has a fluid flow path which is selectable to extend between the sample stream inlet connector in. oo 10 passage 13 and the inlet connector passage 25 of the reservoir 17, and to 0.0 extend between the inlet connector passage 25 and a feed inlet connector passage 27 to the assay chamber 29. The assay chamber 29 comprises a coulometric titration cell 31 having a sensing electrode 33, a reference electrode °ooo and a generator electrode 37a contained in a main compartment 36, and a 15 second electrode 37b to complete the circuit with the generator electrode 37a, contained in a second electrode compartment 38, separated by a glass wall 39 including a sintered glass disc 41 which provides an electrical connection between the two cell halves. An aperture 43 for overflow of electrolyte from the second electrode compartment to the main compartment. 0 O2 oOOOO ooo