SENSING METHOD AND DEVICE FOR AGGREGATION CONTROL

Abstract: Devices and methods for sensing the charge condition of a mixed stream of aggregant and aqueous dispersion of charged particles, are based on sensing changes in the potential of an electrode contacting the stream. Sensing the changes in potential permits better control of aggregation, e.g. in raw water and effluent treatments.

Description: SENSING METHOD AND DEVICE FOR AGGREGATION CONTROL This Vi.er.tion ιε ccncerned with a method cf fcl 'owing the influence cf an aggregant (coagulant anα/or flocculant; on a stream cf an aqueous dispersion cf charged solid or iiuu.d pai ticks and a αe ice suitable as one component of a control loop process governing dosing of the agg r egant. THE FIELD OF THE INVENTION The process of aggregating aqueous dispersions of charged particles is widespread in man industries, includ- my. fcr e..ample, the treatment of raw water in making αrini.in water, effluent and sludge treatment and paper- ma..- n . The stability of an aαueous cispersion deoenαs on the electrical double layers which exist at the surfaces of tne particles and extend into the liquid phase. Coagulaticn is caused by changes in the structure of the double layers. The most widely used method of coagulating a disperse system, is tc add colloids having an effective charge opposite in sign to that carried at the surface of the disperse phase: thus "solutions' of alum or ferric salts ;ιπ fact dispersions of colloidal cationic hydroxylic poiymerc- of aluminium or of ferric iron ) are effective coagulating agents for natural dispersions such as raw water. sludge and effluent and some man-made dispersions such as pulp cispersions used in paper-making, al ' ; of which are anionic. Maintaining ideal coagulation conditions in a stream has neve r been eas;, . The ar test which is in widespread use is ac best an uncertain guide: the process is so lengthy that b;, the time the result is available, the condition of the stream may have changed sc significantly that tne test is no longer appropriate to determine the action to be taken. :- i ..ecu1atιon involves the interaction of a colicIda ' sou-tier of a flocculant with the disperse phase. DI3CUSSI0U OF THE PRIOR ART Tne first, and hitherto the only, εteo towards auto- 5 mating the maintenance of a chosen aggregation condition in a ni.-.ed stream cf an aqueous dispersion and an aggregant, was to use devices utilising the electrc inetic phenomenon o- streaming current. An account of the competence or these devices generally known as Streaming Current Detec- ■ 0 tcrs. cr SCDs, is contained in the speci ica ion cf Euro¬ pean Patent Application 88901621.8. SCDs have a snared essential characteristic, namely that samples o~ the stream are caused to fl w back and fore through a capillary, limiting the in-line application of SCDs tc monitoring the T reatment of very dilute dispersions of colloids of smal " : particle size, like some raw waters in making drinking water. The range of applications of SCDs was increased by the discovery that the charged condition of the filtered mixed stream was identical with that of the unfiltered 0 mixed stream, but that introduced the additional step of filtration and a delay in the analysis. The addition of alum to a pulp dispersion was thought to be controllable indirectly by the acidity of the mixed stream using a pH meter with a hydrogen-selective elec- 5 trode . but the method tool. insufficient account of the accumulation of the sulphuric acid in the mill's recycled water and has been displaced by SCDs used off-line. There is a need for an aggregation control method able tt operate in-line on such aqueous dispersions as sewage, 30 effluent, pulp dispersions and raw waters of all kinds. One object of the invention is to provide such an in¬ line method and a device fcr employment in the method. -UMMAR . F _THE .. I . !{ . VENTI_pf_. According to one aspect of the present invention there i- pro 1 . -.ded a potent i omεtr . z method :c r sensing the cnargε cf a mixed stream cf an aggregant and an aqueous disper- . ι or.. suitable fcr use in an aggregation control loop process, which comprises flowing the mixec stream in contact with a metal " ' ic inoicator electrode, measuring the potential difference between the indicator electroce and a second electrode of substantially stable potential dif- ferent from that cf the indicator electrode, whilst pre ¬ venting an.. substantial electrical current from flowing between the electrodes. For the purposes of this specification, " aggregation. ' means the process cf coagulation, or that of f1occul atlon . Desirably the potential difference is measured with a device for measuring potential which allows only negligible electric current (e.g. less than 10 amps) to flow between the electrodes. The second electrode may be a reference electrode, or i and preferably, when the mixed stream is earthed) an earthed electrode or an earthed connection to the reference terminal of the device for measuring potential . It is common practice to classify electrodes as either metal lic electrodes or membrane electrodes; a carbon electrode is classed as a metallic electrode. The method may be carried out using a direct-reading pH meter c r potentiometer equipped with the metallic in ¬ dicato r electrode. Such meters, hitherto, have been cens'dered to respond solely to solute ions, their reac- tions and concentrations and are generally equipped with ion-specific membrane electrodes. No such electrochemical cell has been proposed for use in directly discriminating the reaction of an insoluble colloid with another opposite- ly cnargeα colloid or colloidal solution. Eartn-.ng one electrode of a pH meter has never been proposed but surprisingly if such earthing is effected an: an approp¬ riate choice of indicator electrode is made, highly effec- t e aggregant addition control of an eartned nm <ed stream can be achieved using such a pH meter. There is no accepted theory which explains the present method. Tne inventors would not wish to be held tc either of the following possible explanations, but it appears to them that the negat"vely charged surface of a cathode is a cite exerting electrostatic attraction or. the free col¬ loidal cationι c aggregant which appears wher the interac¬ tion of the co loids is substantiall complete: there ma 1 , fellow a close association of the aggregant and the elec- trade surface, or a compression of the he1mhoIt:: layer. Alternatively, an amonic aggregant (for example the copo yacryl i c acids used in paper-making) may act as a ligand for etal ions released at the anode. Any such process would affect the potential of the electrode and the potential difference reading. The interaction of an electrode surface and the agg-egant will depend on the nature of the aggregant, the relative proportions of disperse phase and aggregant i the i ed stream, as well as on the electrode potential, electrode material, pH , temperature and flow rate. The latter three variables may be compensated, or so reαuced in consequence that no compensation is required, for example when the aqueous dispersion is subject to pH, temperature or flow control . Thus the method is directly sensitive to the charge condition of the mixed stream. No sensor, apart from SCDs, has been suggested for this use and none, including tne CCDs. has been capable of in-line use in high solid streams such as sewage, effluent and pulp dispersions. The indicator electrode εnould have no particular lon- specificity and a stable surface substantial!;, cnemical " ι;> inert to the electrolytes in the mixed stream. Suitable metallic electrodes include stainless steel, gun metal oronze. carbon, gold, lead, platinum and si lver, cf which the first four e proven tc be the most widely useful lr the various mixed streams tc which the method and ue.'ice may oe applied: the others are selectively useful , depend¬ ing en the chemical nature of tiie stream. For example, lead and silver are changed at their surfaces py streams which contain hydrogen sulphide (sewage effluents; and platinum, from its sensitivity to Hydrogen ion, is net preferred in streams with variable acidity. Specifically excluded as indicator electrodes are membrane and ion-selective electrodes such as hydrogen electrodes used in pH measurement, where tne response to a particular ion is dominant and obscures whatever response such an electrode may have to the charge condition of the stream. According to a further aspect of the invention a sensor for use in the aggregation of an aqueous dispersion of cnarged particles in an aggregation control loop process, comprises a galvanic cell having an indicator electrode without any particular ion-specificity and a second electrode, the electrolyte of the cell being a mixed stream of the dispersion and an aggregant for the charged particles, and a voltmeter sensing the potential difference appearing between the electrodes. The potential difference of the galvanic cell may arise spontaneously from the difference in the potentials of the two electrodes, or the potential difference may oe impressed on the cell by an external source of stable e.m.f. pridging the electrodes. In the latter case the electrodes may be of similar composition. In a eel " ; with an impressed potential difference, preferably both electrodes are metallic. In a cell in which the potential difference arises spontaneous1 > , the second electrode acts as a reference electrode developing a substantiall stable potential sc that the changes in the potential of the indicator elec¬ trode may be recognised by changes in the cell e.m.f.. The I nown reference electrodes comprising half cells of a metal l ic electrode in a standard solution, for example the si 1\ er si 1vet chlor idei sol id . /hCl (solution) composite are useful , as are a sil er/silver chloride solid electrode with no surrounding standard solution. and, particularly for use in the aggregation of particles in effluent and sewage, a silver/silver sulphide electrode. Whi lst the indicator electrode must contact the mixed stream, it is not always necessary that the second elec¬ trode should do so. If there is no extraneous, signifi¬ cant, variable electrical field, such as would form a ground loop affecting the potential difference, and the mixed stream i -> earthed, it is sufficient that the second electrode is an earthed connection from the voltmeter directly to earth or to a metal conduit carrying the stream. Preferably such earthed connection is the anode. The indicator electrode or both electrodes may be housed in a conduit which determines the path of the mixed stream, or they may be mounted externally on a Dody for immersion in the mixed stream. The potential difference is desirably measured by a high impedance voltmeter drawing insufficient electric current tc affect the potential difference between the electrodes - available electronic voltmeters operating with _ ι - > -14 a current of from 10 " to 10 amperes are ideal. preferably the voltmeter incorporates electronic amplifica¬ tion tc produce a signal which can be displayed in digital and/or analog form. Tne conductivity of the mixed stream is of no consequence to the operation of a sensor cell comprising such a voltmeter. There is, in the vicinity of aggregation, for each substantial change in the electrical double layers of the particles, a corresponding and substantially unique poten¬ tial difference which is sensed by the method and device of this indention. If there is efficient aggregation of t'he particles, the corresponding measured potential difference can be used as a datum signal for the preservation cf that condition despite changes in the concentration of particles and,'or the charges they carry. Furthermore, the indicator electrode may be situated no more than 30 seconds in real time downstream of an ag- gregant dosing point: this short response time is valuable to close control and compares with 40 minutes real time delay for a ar test and 5 to 8 minutes delay for an SCD operating on a filtrate derived from the stream. It should be understood that a control loop method based on the method of the invention does not determine the best coagulation conditions, but will preserve a selected coagulation condition established by other means by pre¬ serving the datum signal generated b> the selected condi¬ tion. The method of the invention may discriminate the effect of a trace, say less than 1 part per million, of coagulants such as hydroxylic polymers of aluminium and of ferric iron in the vicinity of efficient coagulation, and flocculants such as synthetic polycations. The method may also, in similar circumstances, discriminate traces of polyamonic substances, for example copol yacryl ic acids and po 1 yphαsphates . used in paper-ma ing. When it is intended that the indicator electrode and a reference electrode both contact the mixed stream, the electrodes may be disposed relative!) to each other trans¬ versely or longitudinally of the path of the stream. A range of useful datum potential differences is avai lable from the choices of cell conformations, eiectrode materials and/or externally applied e.m.f.. Monitoring aqueous streams in conduits from capil ¬ laries to pipes of several hundred millimetres diameter would encompass most practical applications, but no limit has been found on the size of the conduit or the rate of flow of the liquid. In dealing with some mixed streams, it is seldom possible to use a sampling pump to abstract a side stream from the very large works' pipes (frequently 700 mm diameter) to feed to the sensor cell, firstly because such mixed streams are prone to block the pump and secondly such fast acting pumps wear quickly from contact with the abrasive material such mixed streams frequently contain. In these circumstances and the absence of interfering ground loops, the indicator electrode may be let into the pipe (electrically insulated from the pipe if it is metal ¬ lic) and the second electrode may be a connection between the voltmeter and earth or to the pipe as appropriate. The surface of an electrode in contact with some mixed streams, will be kept in working condition by abrasion from components in the mixed stream. In other cases the surface may become fouled and then must be cleaned by mechanical abrasion, electrolytical 1y, or ultrasomcal 1y . The or each electrode may be placed or shaped to reduce the incidence of fouling - flush fitting in the inner wall cf the conduit, cambering, or streamlining. BRIEF DESCRIPTION OF THE DRAWINGS The invention will now be further described, by way o- g example, with reference to the accompanying drawings, in whieh: Figures 1 to 5 are schematic cross-sectional represen¬ tations of a series of devices according to the invention showing variations in a measuring cell. Figure 6 is a cross-sectional representation of a cell with external electrodes. Figure 7 is a reproduction of a chart recording of an inefficiently coagulated effluent stream monitored by a de/ice according to the invention, Figure 8 is a reproduction of a chart recording of an effluent stream dosed in accordance with the output of a device according to the invention, Figures 9 and 10 show alternative arrangements of electrodes in sensing devices according to the invention, and Figure 11 shows a portable cell for use in the method of the invention. DESCRIPTION OF PREFERRED EMBODIMENTS Referring to Figure 1 , a length of horizontal works' pipe 1 (through which flows the grounded works' effluent) shown sectioned on a vertical diameter, has a port 2 closed by an insulating rubber collar 3 surrounding a silver indicator electrode 4. An earthed connection 5 serves as a reference electrode. The electrodes 4 and 5 are bridged by a high impedance voltameter 6 via screened cables 7a and 7b respectively. The output of the voltameter is connected to a signal processor 8 from which a control signal is relayed to a chemical dosing pump 9 via a control line 10. Apparatus according to Figure 1 was installed at an effluent works using a commercial polyouaternaryammomurn salt as floceulant. The works' chemist established the dose of floceulant to give satisfactory flocculation and the datum reading of the voltmeter 6 was established as 37. Tho control loop was put in control of the floceulant dosing pump 9 to preserve a datum reading cf 97 and through three weeks of continuous operation, the quality of floc¬ culation remained constant without need of recal ioration of the datum reading. During a period of about two hours. the dispersion was of low solids content and the dosing pump stalled under the low revolutions demanded by the automatic control. Manual control was instituted to maintain the datum reading. The quality of the floccula¬ tion continued satisfactory and was later returned to automatic control. Over the three week period a 25% saving of floceulant was logged and the plant had not previously enjoyed such a sustained period of acceptable flocculation. Referring to Figure 2, a polypropylene pipe 11 shown in longitudinal vertical cross-section, conducts an un- grounded mixed stream. A port 12 in the pipe is sealed by a collar 12 of synthetic rubber surrounding a stainless steel indicator electrode 14. A reference electrode 15 of £.i 1ver /si 1ver sulphide is housed within an insulating collar 13a sealing port 12a. The electrodes 14 and 15 are bridged by a high impedance voltameter 16 via screened cables 17a and 17b and the output of the voltmeter is connected to a signal processor 18. The processor 18 is used to control the addition of aggregant to the stream flowing in pipe 11 upstream of the region shown. Referring to Figure 3, a right-sectioned cylindrical conduit 21, of polyvinyl chloride, shown end-on, has a screwed-in indicator electrode 24 and a reference electrode 25, both of stainless steel. The electrodes are bridged firstly by a high impedance voltmeter 26 through cables 27a and 27b, and secondly by a battery B having an e.m.f. of 2 volts connected in series with a resistor R of 0.4 megohms. The voltmeter 26 is connected to a signal processor 28. The processor 28 is used to control the addition of ag¬ gregant to the stream flowing in pipe 21. Figure 4 shows an alternative arrangement to Figure 2, the electrodes 14 and 15 being opposite each other and differently mounted in the tube. Referring to Figure 5, the numbered items 11 to 18 have the meaning given to items 11 to 18 in Figure 4, but an additional electrode 19 is set upstream of the other electrodes and a screened cable 20 connects electrodes 14 and 19. The third electrode 19 increased the potential difference available from the cell and may make amplifying of the output signal unnecessary. In a trial at a slaughterhouse, unfiltered effluent from the slaughterhouse was flowed through the conduit 11 of Figure 5 (diameter 5 centimetres) and the characteristic reading was exhibited on a digital display 18A. Figure 7 shows the record of the display over a twelve hour period with dosing of a suitable coagulant determined under manual control guided by jar tests. During this period the coagulation was more cften poor than good. Figure 8 shows the record of continuous manual adjustment of the coagulant dose guided by the digital display 18A. Good coagulation was known to occur at a reading of 450 and the dosing adjustments were undertaken to seek to maintain this figure. The coagulation was of consistently good quality throughout this period and it was found that there was a display band width of from 443 to 455 in which this quality was assured. This performance would be guaranteed if the output signal of the processor 18 was useα to control coagulant dosing in a coagulant dose control loop. Referring to Figure 6, a portable cell for use in the method of the present invention comprises a T-piece 31 cf polyvinyl chloride which carries external electrodes 32, 33 and 3 of M316 stainless steel. Electrodes 22 and S3 are connected by screened cable 35 and electrode 34 is con¬ nected tc a cable 36. Cables 25 and 36 issue from the T- piece 31 for connection to a high impedance amplifier and signal processor (not shown ) . This form of device is useful for occasional or permanent examinations cf in¬ dustrial effluent streams running in otherwise inaccessible gullies. The device of Figure 6 has been tested success- full,, in a gull 3.7 meters deep in which ran effluent from a papei mill. This effluent had routinely blocked a sampling pump meant to lift a continuous sample to the filter of a filter/SCD sensing device used for monitoring a parameter of the effluent. The cell shown m Figure 6 enabled effective parameter monitoring to be acmeved without blockage problems. Referring to Figure 9, a conduit 41 of partial frusto- conical shape is shown, housing a larger annular electrode 42 having an internal diameter of 12.5 millimeters, at the wide end of the frusto-conical part and a smaller annular electrode 43 having an internal diameter of 6.25 milli¬ meters at the narrow end; each of the electrodes 42 and 43 is equipped with an extension 45 or 46 respectively, which penetrates the wal 1 of the conduit for electrical connec¬ tion to a voltmeter (not shown). An advantage of a venturi-shaped zone with the electrodes situated at the narrowest bore is that there is a self-cleaning effect achieved with this set-up caused by the acceleration of flow of the liquid as it streams through the constricted bore. Referring to Figure 10, a conduit 61 of polypropylene lias a waisted bore 67 and houses stainless steel electrodes 62 and 63 with non-conducting spacers 68 and 69 all located at the waist, flush with the boundary cf the bore. Stain¬ less steel lugs 55 and 66 are secured to the electrodes 63 and 62 respectively for connection to other elements of the eel Figure 11 shows a modified form of portable test device for use in the method of the invention. Hollow tube 71 includes an electrode pair (e.g. such as shown in Figure 10) which can be dipped into a flowing stream using handle 72 and hollow support shaft 73. The potential created across the electrode pair is sensed by a mi 111voltmeter 74 via wires passing through the hollow shaft 73. An eartmng spike 75 can be clipped to the shaft 73 when not required, but is used to ground one side of the voltmeter if a grounded effluent stream is being monitored, in which case only the indicator electrode (62) need be connected tc the other side of the voltmeter.

Claims: CLAIMS 1. A method of sensing the charge cf a mixed stream of an aggregant and an aqueous dispersion, suitable for use in an aggregation control loop process, which comprises flowing the mixed stream in contact with a metallic in ¬ dicator electrode, measuring the potential difference between the indicator electrode and a second electrode of substantially stable potential different from that of the indicator electrode, whilst preventing any substantial electrical current from flowing between the electrodes. 2. A method according to claim 1 , in which the potential difference is measured with a voltmeter which allows a negligible electrical current to flow between the electrodes. 3. A method according to claim 2, in which the addition of aggregant is controlled to maintain the measured potential difference substantially constant. 4. A method according to claim 1, in which the indicator electrode has no particular ion-specificity and is selected from the group consisting of stainless steel, gun metal bronze, carbon and gold. 5. A method according to claim 1, in which the second electrode is a reference electrode. 6. A method according to claim 1, in which, when the stream is earthed, the second electrode is an earthed connection to the reference terminal of the device to measure potential. 7. A method according to claim 1, in which both electrodes contact the mixed stream. . A method according to claim 1, in which at least the indicator electrode is housed in a conduit which determines the path of the stream. 9. A sensor for use in the aggregation of an aqueous dispersion of charged particles in an aggregation control loop process, comprising a galvanic cell having an in¬ dicator electrode without any particular ion-specificity and a second electrode, the electrolyte of the cell being a mixed stream of the dispersion and an aggregant for the charged particles, and a device for measuring electrical potential bridging the electrodes. 10. A sensor as claimed in claim 9, in which the second electrode is a reference electrode. 11. A sensor as claimed in claim 9, in which the second electrode is an earthed connection to the device to measure electrical potential. 12. A sensor as claimed in claim 10 or 11, comprising a direct-reading pH meter having a metallic indicator electrode. 13. A sensor as claimed in claim 9, in which the electrodes are located in a conduit which determines the path of the stream. 14. A sensor as claimed in claim 9, in which the electrodes are located on a body for immersion in the

Inventors: BLANCHARD MICHAEL JOHN, COLLINS BRIAN, BLANCHARD, MICHAEL, JOHN, COLLINS, BRIAN
Applicants: ANALAQ LTD, ANALAQ LIMITED
Application year: 1990
Main classification: G01N27
IPC codes: G01N27, G01N33, G05D21