Electricity: measuring and testing – Electrolyte properties – Using a conductivity determining device
Patent
1992-05-14
1996-01-16
Regan, Maura K.
Electricity: measuring and testing
Electrolyte properties
Using a conductivity determining device
324693, 162198, G01R 2926
Patent
active
054850991
DESCRIPTION:
BRIEF SUMMARY
This invention is concerned with a method of following the influence of a coagulant and/or flocculant (hereafter referred to as an "aggregant") on a stream of an aqueous dispersion of charged solid or liquid particles and a device suitable as one component of a control loop process governing dosing of the aggregant.
THE FIELD OF THE INVENTION
The process of aggregating aqueous dispersions of charged particles is widespread in many industries, including, for example, the treatment of raw water in making drinking water, effluent and sludge treatment and paper-making.
The stability of an aqueous dispersion depends on the electrical double layers which exist at the surfaces of the particles and extend into the liquid phase. Coagulation is caused by changes in the structure of the double layers.
The most widely used method of coagulating a disperse system, is to add colloids having an effective change opposite in sign to that carried at the surface of the disperse phase: thus "solutions" of alum or ferric salts (in fact dispersions of colloidal cationic hydroxylic polymers of aluminum 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 dispersions used in paper-making, all of which are anionic.
Maintaining ideal coagulation conditions in a stream has never been easy. The jaw test which is in widespread use is at best an uncertain guide; the process is so lengthy that by the time the result is available, the condition of the stream may have changed so significantly that the test is no longer appropriate to determine the action to be taken.
Flocculation involves the interaction of a colloidal solution of a flocculant with the disperse phase.
DISCUSSION OF THE PRIOR ART
The first, and hitherto the only, step towards automating the maintenance of a chosen aggregation condition in a mixed stream of an aqueous dispersion and an aggregant, was to use devices utilising the electrokinetic phenomenon of streaming current. An account of the competence of these devices generally known as Streaming Current Detectors, or SCDs, is contained in the specification of European Patent Application 88901621.8 SCDs have a shared essential characteristic, namely that samples of the stream are caused to flow back and fore through a capillary, limiting the in-line application of SCDs to monitoring the treatment of very dilute dispersions of colloids of small 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 mixed stream, but that introduced the addition 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 electrode, but the method took 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 to operate in-line on such aqueous dispersions as sewage, 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 for employment in the method.
SUMMARY OF THE INVENTION
According to one aspect of the present invention there is provided a potentiometric method for sensing the charge of 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 indicator 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.
For the purposes of this specification, aggregation means th
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Blanchard Michael J.
Collins Brian
Blanchard Marketing Services Limited
Regan Maura K.
Skillman Henry H.
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